CN118847200A - Preparation method and application of Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone - Google Patents

Abstract

The present invention belongs to the field of petrochemical catalysis technology, and relates to a preparation method and application of a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone. The main technical feature of the present invention is that a dealuminated Beta zeolite is used as a carrier, and copper is loaded in the pores of the zeolite carrier by an improved ammonia evaporation method. The core of the improved ammonia evaporation method is to impregnate the zeolite carrier with an equal volume of a copper-ammine complex solution, so that the copper-ammine complex mainly reacts to deposit copper hydroxide in the pores of the zeolite carrier during the ammonia evaporation process. After calcination and hydrogen reduction treatment, the copper hydroxide deposited in the zeolite pores can directly form highly dispersed nano and sub-nano copper particles at the lattice defect sites of the hydroxyl nests in the pores, and the latter obtains anti-sintering ability by closely interacting with the hydroxyl nests. When the Cu-Beta zeolite catalyst is prepared by the method of the present invention and applied to the reaction of preparing caprolactam by gas-solid phase hydrogen amination of caprolactone, it has high activity and selectivity and good stability.

Description

Preparation method and application of Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone

Technical Field

The invention belongs to the technical field of petrochemical catalysis, and relates to a preparation method and application of a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone.

Background Art

ε-Caprolactam (CPL) is a white solid organic compound, most of which is used to produce polycaprolactam chips, and a small part is used to produce lysine and pharmaceutical intermediates. Among the downstream products of polycaprolactam chips, nylon-6 fiber and engineering plastics consume about 70% and 20% of polycaprolactam chips respectively. The remaining polycaprolactam chips are processed into packaging films and food cling films.

Nylon-6 is the first synthetic fiber product developed in the world. The most outstanding advantage of nylon-6 fiber is that it has higher wear resistance than all other fibers. It is 10 times more wear-resistant than cotton and 20 times more wear-resistant than wool. At the same time, the strength of nylon-6 fiber is 1-2 times higher than cotton, 4-5 times higher than wool, and 3 times higher than viscose fiber. Adding a small amount of polyamide fiber to blended fabrics can greatly improve its wear resistance, elastic recovery rate and flexural fracture resistance. In addition, nylon-6 fiber also has good moisture absorption and dyeability. Nylon-6 fiber can be used as civilian silk and industrial silk. Nylon civilian silk is used to make shirts, pullovers, pajamas, carpets, blankets, cords and bags, etc.; industrial silk is used to make tents, car tires, transmission belts, hoses, cables, fishing nets, ropes, insulation materials, etc.

At present, the benzene-based caprolactam process has become the mainstream production process of caprolactam. The process mainly includes three basic processes: benzene to cyclohexanone, cyclohexanone to cyclohexanone oxime, and cyclohexanone oxime rearrangement to caprolactam.

Cyclohexanone from benzene is the first step in the benzene-based caprolactam process. There are two main process routes in the industry, namely cyclohexane oxidation and cyclohexene hydration. Since 2017, cyclohexene hydration has surpassed cyclohexane oxidation to become the world's largest source of cyclohexanone.

The liquid phase oxidation method is mainly used to produce cyclohexanone from cyclohexane. This method uses benzene and hydrogen as starting materials. First, cyclohexane is obtained by hydrogenating benzene, and then cyclohexanone and cyclohexanol are generated by air oxidation, decomposition and saponification of cyclohexane, and then cyclohexanol is dehydrogenated to cyclohexanone. This method has mild process conditions, simple operation and mature technology. Its main disadvantages are low product yield (only 75%-80%), high raw material consumption, high energy consumption, more by-products and difficult separation, and environmental unfriendly.

Cyclohexene hydration is a cyclohexanone production technology developed by Asahi Chemical Corporation of Japan in the 1980s (Japanese Patent JP 60104031A, 1985), which can make the utilization rate of benzene reach 99.5%. The first step of this technology is to partially hydrogenate benzene to generate cyclohexene (about 80%) with high selectivity in the presence of a ruthenium catalyst, and the by-product is cyclohexane. The second step is to separate cyclohexene and hydrate cyclohexene with water under the action of a solid acid catalyst to generate cyclohexanol. The third step is to dehydrogenate cyclohexanol in the presence of a Cu-Si catalyst to generate cyclohexanone. Compared with the cyclohexane oxidation method, the reaction process of cyclohexene hydration method for producing cyclohexanone is simple, avoiding the problems of coking and producing waste alkali liquid. However, the process has low production efficiency and high energy consumption.

Cyclohexanone to cyclohexanone oxime is the second and most important step in the benzene-based caprolactam process. Common production processes for this step include hydroxylamine sulfate method (HSO), hydroxylamine phosphate method (HPO), nitric oxide reduction method (NO) and ammoximation method (HAO).

The process principle of the hydroxylamine sulfate method is: first, _NH3 is catalytically oxidized to a mixture of NO and _NO2 , and the gas mixture is absorbed by ( _NH4 ) _{2CO3 solution} to obtain _NH4NO2 , then _NH4NO2 is reduced with _SO2 at low temperature to generate _{hydroxylamine} disulfonate, and the disulfonate is hydrolyzed to hydroxylamine sulfate. Secondly, cyclohexanone and hydroxylamine _sulfate are subjected to oximation reaction to produce cyclohexanone oxime. This process requires the use of _NH3 to neutralize free sulfuric acid, resulting in the generation of ammonium sulfate by-product. The amount of ammonium sulfate by-product generated is 2.5-2.7 tons of ammonium sulfate/ton of cyclohexanone oxime. The main disadvantage of this process is that a large amount of _SO2 is consumed and ammonium sulfate is produced as a by-product, which is not conducive to the production efficiency of caprolactam. In addition, the hydroxylamine sulfate method also has the disadvantages of a long production process, high energy consumption and large discharge of three wastes. These disadvantages limit the development of the hydroxylamine sulfate method.

The hydroxylamine phosphate method was developed by Royal DSM of the Netherlands. The first step of this method is to prepare hydroxylamine phosphate salt by reducing phosphoric acid and ammonium nitrate with hydrogen under the catalysis of Pd/C, and the second step is to react cyclohexanone with hydroxylamine phosphate salt in a buffer solution of ammonium dihydrogen phosphate to generate cyclohexanone oxime. After the cyclohexanone oxime is separated, 60% nitric acid solution is added to replenish the consumed nitrate ions, and phosphoric acid and ammonium nitrate can be returned to the hydroxylamine synthesis process for reuse. The disadvantage of the hydroxylamine phosphate method is that it requires the use of precious metal catalysts and the process operation is difficult.

The nitric oxide reduction method is jointly developed by BASF of Germany, Inventa of Switzerland and Zaklady Azotowe of Poland. The first step of the method is to generate nitric oxide with ammonia and oxygen reaction, and to hydrogenate nitric oxide in aqueous sulfuric acid solution under the action of platinum catalyst to generate hydroxylamine sulfate. The second step is to make hydroxylamine sulfate and cyclohexanone undergo oximation reaction to generate cyclohexanone oxime. The technology of the method is relatively mature, and the consumption of ammonia and hydrogen is low, and ammonium sulfate will not be by-produced in the generation of hydroxylamine, but oxygen and hydrogen are used in the production, which is prone to accidents. In addition, the platinum catalyst used in the process is easily poisoned by metal ions such as arsenic, mercury, and aluminum contained in the aqueous sulfuric acid solution.

The cyclohexanone ammoximation method (HAO method) was proposed by Armor, a United Chemical Company in the United States in 1980. The method initially used oxygen as an oxidant and amorphous silica as a catalyst to catalyze the reaction of cyclohexanone, oxygen and ammonia at 467K to prepare cyclohexanone oxime. This reaction method is applicable to other ketones. For example, acetone, 3-pentanone, 2-methylcyclohexanone and acetophenone can all generate corresponding ketone oximes under certain conditions. The preparation method is simple and does not require additional synthesis of hydroxylamine salts. However, the cyclohexanone conversion rate is not ideal (54%) and the cyclohexanone oxime selectivity is low (51%). In addition, the by-products generated by the reaction remain on the catalyst, accelerating the coking and deactivation of the catalyst.

In 1987, U.S. Patent No. 4745221 first disclosed a new technology route for the liquid phase ammoximation of cyclohexanone catalyzed by TS-1, which is the ammoximation method. Specifically, this technology route uses tert-butyl alcohol and water as solvents, and reacts cyclohexanone with ammonia and hydrogen peroxide under the catalysis of TS-1 to generate cyclohexanone oxime, which can achieve a high yield of more than 99%, while avoiding the generation of low-value ammonium sulfate, and is environmentally friendly. At present, large-scale industrial applications have been achieved. Compared with the hydroxylamine sulfate method and the hydroxylamine phosphate method, the ammoximation method has the advantages of low hydrogen consumption, short production process, simple control, low requirements for equipment and pipeline materials, and small investment and land occupation.

Those familiar with the field know that the ammoximation method has achieved great results in industrial applications. However, the process still faces some challenges. For example, the recycling of a large amount of tert-butyl alcohol solvent leads to increased separation costs; the by-products produced during the reaction cause the TS-1 catalyst to be deactivated; the titanium silicon molecular sieve catalyst is expensive and the cost sharing is high; the ammoximation of cyclohexanone to produce cyclohexanone oxime is a liquid phase reaction, and the titanium silicon molecular sieve catalyst is in a strong alkaline environment for a long time, and the dissolution of the skeleton silicon on the catalyst cannot be avoided. This not only causes the ineffective loss of the catalyst, but also shortens the service life of the catalyst, affecting the stable operation of the ammoximation reaction system; in addition, the liquid-solid phase reaction has the disadvantage of large mass transfer resistance, and the internal diffusion problem of the catalyst has a serious impact.

The Beckmann rearrangement of cyclohexanone oxime to produce caprolactam is the third step of the benzene-based caprolactam process, and it is also a key step. Currently, about 90% of the industrial production of caprolactam requires the Beckmann rearrangement to complete. The Beckmann rearrangement technology for producing caprolactam has two process routes: liquid phase rearrangement and gas phase rearrangement.

Liquid-phase Beckmann rearrangement is currently the most important caprolactam production process. This process has been industrialized for a long time and the technology is relatively mature. Liquid-phase Beckmann rearrangement is a rearrangement process using fuming sulfuric acid as a catalyst. The reaction conditions are mild, the selectivity is high, the caprolactam yield is high, and the product quality is relatively stable. However, due to the use of highly corrosive fuming sulfuric acid, there is corrosion to the equipment, and 1.5 to 1.8 tons of ammonium sulfate are produced as a by-product per ton of caprolactam. In addition, the process consumes a large amount of liquid ammonia, not only has the problem of low-value ammonium sulfate as a by-product, but also has the difficulty of handling a large amount of neutralization heat, which to a certain extent affects the technical and economic feasibility of the process.

Compared with the liquid phase Beckmann rearrangement process, the gas phase Beckmann rearrangement is a more advanced process. Since the late 1980s, people have been committed to using solid acid catalysts instead of oleum to carry out the Beckmann rearrangement reaction of cyclohexanone oxime. The public document AFINIDAD (Spain), 1981, 38 (373) 225-227 reported the research work on the Beckmann rearrangement reaction of cyclohexanone oxime on a series of AlPO ₄ /γ-Al ₂ O ₃ catalysts. The results show that as the acidity of the catalyst increases, its reaction activity increases. However, the selectivity of the rearrangement reaction decreases, and the yield of the gas phase Beckmann rearrangement reaction of cyclohexanone oxime can reach up to 73.1%. The public document Shokubai, 1989, 31 (6): 365-368 reported in 1989 the gas phase Beckmann rearrangement reaction of cyclohexanone oxime catalyzed by high silicon ZSM-5 molecular sieve. At present, the vapor-phase Beckmann rearrangement catalyzed by high-silicon ZSM-5 molecular sieve has become a new caprolactam production technology. The vapor-phase Beckmann rearrangement technology can complete the conversion process of cyclohexanone oxime to caprolactam under the action of a solid acid catalyst. This process does not use fuming sulfuric acid, omits the process of neutralizing sulfuric acid, and thus does not produce by-product ammonium sulfate, greatly reducing the consumption of liquid ammonia in the caprolactam production process, avoiding equipment corrosion and environmental pollution, and has aroused widespread interest.

U.S. Pat. No. 4,717,769 (1987) discloses a gas-solid phase rearrangement reaction method of cyclohexanone oxime using a high silicon-aluminum oxide molar ratio (>500) MFI type molecular sieve as a catalyst. At a weight hourly space velocity of 11.7 h ^-1 , the conversion rate of cyclohexanone oxime was still 100% after 15.3 hours of reaction. However, the selectivity for caprolactam was very low (83.5%).

U.S. Patent No. 6303099 (2000) discloses a method for Beckmann rearrangement of cyclohexanone oxime using modified high-silicon zeolite as a catalyst. Its technical feature is that the modified catalyst is obtained by post-treating molecular sieve powder with a high silicon-aluminum oxide molar ratio with a nitrogen-containing base. When the reaction is carried out for 5.5 hours at a weight hourly space velocity of 8h ^-1 , the cyclohexanone oxime conversion rate is 99.5% and the caprolactam selectivity is 96.2%.

Chinese patent CN1883803A (2005) also discloses a method for Beckmann rearrangement of cyclohexanone oxime using modified high-silicon zeolite as a catalyst. Its technical feature is that the modified catalyst is obtained by post-treating pure silicon and high silicon-aluminum oxide molar ratio molecular sieve with hydrofluoric acid. Under the conditions of reaction temperature of 370°C, normal pressure, space velocity of 8h ^-1 , and carrier gas flow rate of 60m1/min, after 20 hours of reaction, the raw material conversion rate is 98.3% and the product selectivity is 98.5%. Basically, the best reaction result is achieved.

In addition to the patent literature, there are many journal articles dealing with the vapor-phase Beckmann rearrangement of cyclohexanone oxime to caprolactam.

For example:

The open document Studies in Surface Science and Catalysis, 1993, 78: 615-622 reported the catalytic performance of AlPO ₄ and AlPO ₄ /TiO ₂ catalysts for the vapor-phase Beckmann rearrangement reaction of cyclohexanone oxime. The results show that as the acidity of the catalyst increases, the reaction activity increases, but the caprolactam selectivity decreases; within the range of 200-400°C, as the temperature increases, the activity and selectivity of both AlPO ₄ and AlPO ₄ /TiO ₂ catalysts increase. Under the same reaction conditions, the activity and selectivity of the AlPO ₄ /TiO ₂ catalyst are better than those of the AlPO ₄ catalyst. In the AlPO ₄ /TiO ₂ catalyst, increasing the TiO ₂ content causes the catalyst surface acidity to decrease, thereby resulting in a decrease in catalyst activity and an increase in selectivity.

The open document Applied Catalysis A: General, 1999: 99-108 reported the catalytic performance of β molecular sieve catalyst modified with Al and B elements for the gas-phase Beckmann rearrangement reaction of cyclohexanone. The study found that β molecular sieve is similar to [B]-ZSM-5. When the reaction temperature and pressure are reduced, the selectivity of [B]-β and [Al]-β molecular sieves for caprolactam is significantly improved. However, at lower reaction temperatures and pressures, the catalyst deactivation rate is accelerated. In contrast, the deactivation of [B]-β molecular sieve is more serious, about fifteen times the deactivation rate of [Al]-β molecular sieve. The study also found that the active center of the [Al]-β and [B]-β molecular sieves in the Beckmann rearrangement reaction of cyclohexanone oxime is the ortho-silanol group.

In 2003, Sumitomo Chemical Co., Ltd. of Japan took the lead in developing the cyclohexanone oxime gas-phase Beckmann rearrangement reaction technology and realized industrial application. According to the public document Contemporary Petroleum and Petrochemicals, 2019, 27(04):32-36., Sumitomo Chemical Co., Ltd.'s gas-phase rearrangement technology uses a fluidized bed reactor, high-silicon MFI molecular sieve as a catalyst, methanol as a solvent, and nitrogen as a carrier gas. Under the conditions of a reaction temperature of 350-400°C and a weight hourly space velocity of 8h ^-1 , the conversion rate of cyclohexanone oxime can reach more than 99%, and the selectivity of caprolactam can reach more than 95%. Sinopec Research Institute of Petroleum Processing has also developed a similar gas-phase rearrangement technology, but the reactor is a radial moving bed, and its cyclohexanone oxime conversion rate is as high as more than 99.9%, and the average selectivity of caprolactam is as high as 96.5%. As of the end of 2020, small-scale and pilot-scale technical research has been completed.

The open document Applied Catalysis A: General, 2005: 145-153 reports the catalytic performance of a series of composite metal oxide catalysts for the gas-phase Beckmann rearrangement reaction of cyclohexanone oxime. The study found that the conversion rate of the Beckmann rearrangement reaction can reach 100% and the selectivity can reach 97.4% after the composite oxide TiO ₂ -ZrO ₂ prepared by coprecipitation method using ammonia water as coprecipitant and then prepared by equal volume impregnation method to prepare B ₂ O ₃ /TiO ₂ -ZrO ₂ catalyst. The results are significantly better than those of B ₂ O ₃ /SiO ₂ -Al ₂ O ₃ , B ₂ O ₃ /SiO ₂ -TiO ₂ , B ₂ O ₃ /SiO ₂ -ZrO ₂ , B ₂ O ₃ /Al ₂ O ₃ -TiO ₂ , and B ₂ O ₃ /Al ₂ O ₃ -ZrO ₂ catalysts. In the range of 500-700℃, increasing the calcination temperature is conducive to the formation of medium-strength acid centers, thereby increasing the conversion rate and product selectivity of cyclohexanone oxime; but when the calcination temperature is higher than this range, a large number of B ₂ O ₃ crystal phases will appear, which is not conducive to the catalytic activity and selectivity of B ₂ O ₃ /TiO ₂ -ZrO _2. Studies have found that polar solvents are conducive to the desorption of the reaction product caprolactam from the active center, improving the selectivity of caprolactam and also prolonging the service life of the catalyst. Acetonitrile is the most polar and therefore the best solvent in the research solution. In the deactivation and regeneration study of B ₂ O ₃ /TiO ₂ -ZrO ₂ catalyst, it was found that the pore size distribution and structure of the deactivated catalyst did not change, but the acid center was reduced. Surface carbon accumulation is the main reason for catalyst deactivation. The deactivated catalyst can be fully restored to its activity by calcining at 600℃ for 8h.

The open document Catalysis Communications, 2005: 53-56 reports the catalytic performance of different β zeolite catalysts for the gas-phase Beckmann rearrangement reaction of cyclohexanone oxime. The different β zeolite catalysts include a boric acid impregnated modified Hβ molecular sieve catalyst (B ₂ O ₃ /Hβ), a series of Ti-Hβ molecular sieves synthesized by liquid-solid isomorphous substitution of Hβ molecular sieves with Ti(SO ₄ ) ₂ solution, and a modified catalyst obtained by treating Hβ molecular sieves with ammonia water. The results show that compared with the Hβ molecular sieve matrix synthesized by hydrothermal synthesis, the BET surface area of Ti-Hβ molecular sieve and Hβ molecular sieve modified with ammonia water increases. The BET area of the B ₂ O ₃ /Hβ catalyst modified by 9.09w% boron oxide loading decreases. However, the total acid content and weak acid content of the three modified catalysts have increased, especially the amount of weak B acid has increased significantly. In the Beckmann rearrangement reaction, the activity and selectivity of the above modified catalysts are increased, and the deactivation rate is reduced. Based on the above results, the authors speculate that in the gas-phase Beckmann rearrangement reaction, the weak B acidic site is the real catalytic active center.

The open document Catalysis Today, 2012: 289-299 reported the research work of synthesizing caprolactam under different conditions using amorphous silica gel impregnated with niobium pentoxide (Nb ₂ O ₅ ) as a cheap catalyst (Nb ₂ O ₅ /SiO ₂ ). The results showed that with Nb ₂ O ₅ /SiO ₂ as the catalyst, the conversion rate of cyclohexanone oxime was close to 100%, and the selectivity of caprolactam was as high as 98%.

People familiar with the art know that there are two difficulties in the process of technology development for the gas-phase Beckmann rearrangement reaction. One is that cyclohexanone oxime needs to participate in the reaction in the gas phase, but since the boiling point of cyclohexanone oxime itself is relatively high (the boiling point at normal pressure is 203°C), and the substance has poor high-temperature thermal stability, it will condense and deteriorate at 160°C. Therefore, how to gasify cyclohexanone oxime without deteriorating is a challenging problem that must be faced in the process of gas-phase rearrangement process development. In addition, the catalyst is easily deactivated in the gas-phase rearrangement reaction. In order to ensure the continuous and stable progress of the reaction, how to restore the activity of the catalyst online has also become a difficulty that needs to be solved in the process of gas-phase Beckmann rearrangement development. The above two are still obstacles to the large-scale industrialization of the cyclohexanone oxime gas-phase Beckmann rearrangement method.

In view of the technical status of the existing benzene-based caprolactam production process and the problems faced in the development of new processes, it is imperative to find a new way to obtain a new process for the efficient preparation of caprolactam without low-value by-products (such as ammonium sulfate), equipment corrosion and environmental pollution problems, high process atomic utilization, and low energy consumption (low carbon emissions).

In the literature research, it was found that as early as 1957, Shell Company first disclosed a method for preparing caprolactam from caprolactone in US Patent US2817646. Specifically, the method uses a hydrogenation catalyst (such as neutral Raney nickel) to prepare caprolactam from ammonia, hydrogen and caprolactone raw materials at 175-200°C and 7-40 standard atmospheres. However, the main products of the reaction are actually various amides, such as polyamides, amine amides and hydroxyamides. The yield of caprolactam is only about 4%. Obviously, the selectivity of this method is so poor that it has no application value.

In 1961, the Union Carbide Corporation (UCC) of the United States disclosed a method for preparing caprolactam in US Pat. No. 3,000,879. The method is to heat a 25% aqueous solution of 6-hydroxycaproamide to 300-475°C in a closed container, and prepare caprolactam through a non-catalytic reaction under high pressure (pressure up to 15 MPa), and a 30% single-pass caprolactam yield can be obtained. It is undoubtedly unimaginable for a bulk chemical to achieve industrial production under high temperature and high pressure using an intermittent reactor.

In the same year, Union Carbide Corporation of the United States once again disclosed a process for preparing caprolactam from caprolactone in US Patent US3000800. Specifically, the process is a process of preparing caprolactam by mixing caprolactone with ammonia water or primary amine aqueous solution under high pressure (P>22.1MPa) and above the critical temperature of ammonia and water (373℃＜T>473℃) through non-catalytic reaction. This method has been successfully industrialized, but the process can only be carried out under high temperature and high pressure, and the energy consumption is huge. Moreover, the non-catalytic reaction carried out under high temperature and high pressure is easy to generate polymers, so the actual yield of caprolactam is low, not exceeding 50%. This may be the main reason why Union Carbide Corporation of the United States later stopped the production line of the process for preparing caprolactam from caprolactone. On the other hand, the raw material of caprolactone at that time came from the oxidation reaction of cyclohexanone and peracetic acid. Peracetic acid is a strong oxidant and is extremely unstable. Peracetic acid will explode when it encounters high heat, reducing agents or metal ions. In fact, peracetic acid is explosive when the concentration is greater than 45%, and it will explode even at -20℃. Therefore, the production process of caprolactone was extremely dangerous at that time. This may also be an important reason why Union Carbide later stopped the production line of caprolactam from caprolactone.

In 1964, Teijin Co., Ltd. of Japan disclosed a catalytic method for preparing caprolactam from amide derivatives of caprolactone, 6-hydroxycaproamide or 6-hydroxycaproic acid in U.S. Patents US3317516 and US3317517. Specifically, the catalytic method disclosed in the above patents is to heat caprolactone, 6-hydroxycaproamide or amide derivatives of 6-hydroxycaproic acid and ammonia water to 200-400° C. in a high-pressure reactor under the catalysis of at least one or a combination of hydrogenation catalysts containing precious metals, cobalt, and nickel to obtain caprolactam with less coloration. The method can be used under a hydrogen atmosphere. Obviously, the reaction conditions of the catalytic method are relatively mild. However, the single-pass yield of caprolactam obtained by the method is also not high, with a maximum of only 45.1%, so the economic efficiency of the method is not high.

In 1965, Union Carbide Corporation of the United States disclosed a continuous two-stage process for preparing caprolactam in U.S. Patent No. 3320241. The technical background of the process is that people already knew that the non-catalytic reaction of caprolactone and ammonia under high temperature and high pressure could produce caprolactam. However, the reaction under high temperature and high pressure is not suitable for industrial application. First, due to the limitation of reaction equilibrium, the single-pass yield of the reaction is relatively low. Secondly, the reaction process will produce a large number of irreversible by-products. Since the yield of each process is very low, the reaction intermediates and unconverted caprolactone must be recycled, which will increase energy consumption and carbon emissions. In addition, since the reaction generates a large number of irreversible by-products, the separation process is also very complicated. All of these make the operating cost too high. Accordingly, the new process proposed in the patent is that in the first stage of conversion, a mixture of caprolactone, ammonia and water is reacted at a relatively low temperature, and the reaction time is controlled so that a large amount of caprolactone is converted into a reaction intermediate. Then, in the second stage conversion, the product of the first stage conversion reaction is converted under the high temperature (300-400°C) and ultra-high pressure (136-680 standard atmospheres) required to form caprolactam. The downstream of the two-stage conversion reaction is the separation stage. The task of the separation stage is to separate caprolactam from the mixture containing caprolactam and reaction intermediates, and to recycle the unreacted products and intermediates back to the first stage for reconversion. Since the intermediates can be converted through the entire two-stage process after returning to the first stage, the reaction time is long, which is conducive to the maximum conversion into the target product caprolactam. The caprolactam separated from the second stage reaction mixture must undergo multiple steps of purification to become a pure product. All the residues produced during the product purification process can be recycled back to the first stage for reconversion to become the target product as much as possible. The use of this process can achieve a caprolactam yield of 90.2%. The process of this process is complicated and the second step is still a high-pressure reaction, which has extremely high requirements for production equipment and management, and the difficulty of industrialization is also relatively large.

In 1966, Kanegafuchi, BOSEKI KABUSHIKI KAISHA of Japan first disclosed a catalytic method for preparing caprolactam from caprolactone in the gas phase in British patent GB1109540. The method is to first vaporize caprolactone and a certain amount of water, then mix with ammonia and hydrogen, and the mixed gas is catalytically reacted by a copper chromite catalyst at 120-350°C and normal pressure. The conversion rate of caprolactone in this method can reach 100%, and the selectivity of caprolactam can reach 97%. However, the resin generated by the polymerization side reaction is deposited on the catalyst, resulting in rapid deactivation of the catalyst. This has become a major obstacle to the industrial application of the method. In addition, the copper chromite catalyst used in the method is toxic and will produce a large amount of chromium-containing wastewater during the preparation process, which is unfavorable to environmental protection.

In 1967, DuPont Canada disclosed a non-catalytic method for preparing caprolactam from caprolactone in Canadian Patent CA770148. The method involves reacting caprolactone or polycaprolactone with aqueous ammonia at 305-365°C in a stainless steel reactor at 18-45 MPa, and a maximum single-pass yield of caprolactam of 85% can be obtained. The method provided in the patent shows that polycaprolactone is a byproduct that can be reversed to the target product.

In 1968, Stamicarbon of the Netherlands disclosed in U.S. Patent No. 3401161 a non-catalytic process for preparing caprolactam by reacting caprolactone with ammonia in an inert organic solvent. The reaction is carried out at very high temperature and pressure (T>330°C, 125atm>P>90atm), and the maximum yield of caprolactam can be 60%. Organic solvents that can be used include pyridine, dibutyl ether, diacyl ether, dioxane, toluene, xylene, decalin, heptane and octane.

In 1970, Union Carbide Corporation of the United States also disclosed in U.S. Patent No. 3,497,500 a non-catalytic method for preparing caprolactam from caprolactone at high temperature and high pressure. The method emphasizes the importance of removing a portion of the carbon dioxide produced by the reaction from the reaction system. In short, according to the patent, in the reaction of caprolactone to caprolactam at high temperature and high pressure, the presence of an excess of carbon dioxide atmosphere in the reaction system is not conducive to obtaining a satisfactory caprolactam yield.

In 1972, Kanegafuchi, BOSEKI KABUSHIKI KAISHA of Japan disclosed a method for preparing caprolactam using copper chromite as a catalyst in U.S. Patent 3652549. The copper chromite catalyst is obtained by coprecipitation using copper nitrate and ammonium dichromate as raw materials and ammonia water as a precipitant. The technical features of the catalyst preparation process include that the precipitate generated by the coprecipitation reaction is filtered, dehydrated, low-temperature dried (75-80°C, 20h) and high-temperature decomposed, and then soaked in a dilute acetic acid solution. The catalyst precursor after soaking is filtered, washed with water, and dried (125°C, 12h) to become a catalyst. Before the catalyst is used in a fixed bed reactor, it needs to be pressed into tablets. Before the reaction, the catalyst needs to be reduced with hydrogen at 200°C. The atomic ratio of chromium to copper in the copper chromite catalyst is 0.1-5, preferably 0.1-3. The catalyst may also contain a third metal component (Ba, Ca, Mg, Sr, Al, Ga, Ti, V, Mn, Fe, Co, Ni, Zn, Mo, Ru, Rh, Pd, Ag, Cd, Sn, Pd, As, Bi, Sb). The atomic ratio of the third metal component to copper is 0.001-1, preferably 0.01-0.2. The raw material for preparing caprolactam conforms to the general formula X-(CH ₂ ) ₄ -COY. Wherein, X＝CHO, -CH(OR)(OR ₁ ), -COOH, -COONH ₄ , -CONH ₂ , or COR ₂ ; Y＝OH, ONH ₄ , NH ₂ , or OR ₃ . The reaction is carried out in a fixed bed reactor in a gas-solid phase, the reaction temperature ranges from 170-300°C, and the hydrogen partial pressure is 0.1-1.5atm. The feed also contains ammonia and water vapor, and the preferred usage ranges (molar ratio to the raw material) are 2-50 and 10-100 respectively. When dimethyl adipate is used as the raw material, the reaction results of the copper chromite catalyst containing a small amount of zinc are: the raw material conversion rate is 99%, and the caprolactam selectivity is 95%; the reaction results of the copper chromite catalyst containing a small amount of Mo are: the raw material conversion rate is 100%, and the caprolactam selectivity is 96%.

In 1975, Teijin Ltd. of Japan disclosed in US Pat. No. 3,888,845 a catalytic process for preparing caprolactam from C1-C4 alkyl esters of caprolactone or 6-hydroxycaproic acid. Specifically, the patent disclosed a process for producing caprolactam by gas-solid phase catalytic reaction using C1-C4 alkyl esters of caprolactone or 6-hydroxycaproic acid, hydrogen and ammonia as raw materials. The characteristics of the process are low reaction temperature and reaction pressure, high conversion rate of C1-C4 alkyl esters of caprolactone or 6-hydroxycaproic acid, and high selectivity of caprolactam. The solid catalyst used in the process consists of three parts: A, B and C: A is an oxide carrier selected from titanium oxide, aluminum oxide, silicon oxide and a composite of aluminum oxide and silicon oxide; B is the main metal component of the catalyst - copper; C is the trace metal component of the catalyst - nickel or chromium. The catalyst can be prepared by a deposition precipitation method. The carrier is preferably anatase titanium oxide, the weight ratio of copper to the carrier can be in the range of 0.5-200, preferably 5-100, more preferably 10-70; the atomic ratio of Ni(Cr) to Cu can be in the range of 0.001-1, preferably 0.005-0.25. The gas-solid phase catalytic reaction of preparing caprolactam using caprolactone or C1-C4 alkyl ester of 6-hydroxycaproic acid can be carried out at 200-320°C and 0.01-2atm, preferably 220-310°C and 0.1-1.2atm. The amount of hydrogen and ammonia can be in the range of 5-70 ( _H2 /ester molar ratio) and 1-50 ( _NH3 /ester molar ratio), respectively, and the preferred ranges are 10-50 ( _H2 /ester molar ratio) and 2-25 ( _NH3 /ester molar ratio), respectively. In addition, the process also emphasizes the importance of the hydrogen to ammonia molar ratio and the addition of water in the reactor feed. In general, the use of a suitable hydrogen to ammonia molar ratio is conducive to improving the selectivity of the reaction. Adding water to the feed of the reactor can not only reduce side reactions and improve the selectivity of caprolactam, but also slow down the deactivation rate of the catalyst. The optional range of the hydrogen to ammonia molar ratio is 0.2-30, and the preferred range is 0.5-15; the optional range of the water/ester molar ratio is 0-50, and the preferred range is 5-30. Under preferred conditions, when caprolactam is prepared from caprolactone, the conversion rate of caprolactone can reach up to 99%, and the caprolactam selectivity can reach up to 90%. The problem is that the catalyst is deactivated quickly by carbon deposition. However, the patent provides two methods for catalyst regeneration. One method is redox treatment, and the other method is steam treatment. The redox treatment is actually to regenerate the catalyst by burning carbon with molecular oxygen first, and then reduce the catalyst with hydrogen. The molecular oxygen burning carbon can be carried out in the temperature range of 100-800°C, preferably in the temperature range of 150-500°C. The charcoal burning time is 20 minutes to 20 hours; the hydrogen reduction after charcoal burning can be carried out in the temperature range of 170-350°C, preferably in the temperature range of 170-270°C. The water vapor treatment can be carried out in the temperature range of 100-500°C, preferably in the temperature range of 200-400°C. The water vapor treatment time is 20 minutes to 20 hours. The water vapor treatment can also be carried out in the presence of hydrogen, and it is best to reduce the catalyst with hydrogen after the water vapor treatment. The hydrogen reduction can be carried out in the temperature range of 170-350°C, preferably in the temperature range of 170-270°C.

In 2012, German patent DE102012006946A1 disclosed a new catalytic process for preparing caprolactam from D-glucose raw material via adipic acid and caprolactone. In the reaction of caprolactone to caprolactam, the patent used a Cu-Mo-Ti catalyst, and the raw materials for the reaction included ammonia and hydrogen in addition to caprolactone. The method can achieve a caprolactam yield of 80%.

In 2018, Chinese patent CN108774172A disclosed a catalytic method for preparing caprolactam and N-substituted caprolactam using caprolactone and ammonia (amine) as raw materials. Its technical feature is that the reactor is a fixed bed. The catalyst loaded in the fixed bed reactor is a granular SO ₄ ^2- /M _X O _Y solid superacid with a diameter of Φ=4.0-6.0 mm. The patent emphasizes that the superacid catalyst used should always be in a nitrogen protection state. The reaction is carried out under normal pressure and nitrogen protection. The optional range of the reaction temperature is 180-320°C, and the preferred range is 220-280°C; the molar ratio of caprolactone to ammonia (amine) is 1-1.5, and the preferred range is 1.1-1.3. The method also requires the use of a solvent. The solvent refers to water, benzene, toluene, xylene and cyclohexane. The amount of solvent added is 1-2 times the total weight of caprolactone and ammonia (amine).

In addition to the above patents, there are several journal articles on the research of caprolactone to caprolactam. For example:

In 1977, the open document Journal of the Chemical Society of Japan, 1977, (7), p. 1013-1017 reported the catalytic effect of a Cu- _TiO2 catalyst on the gas phase ammonolysis of caprolactone to caprolactam. The ammonolysis reaction was carried out at normal pressure, and the products, in addition to caprolactam, included 6-hydroxycapronitrile, 6-hydroxycapronamide, adiponitrile and polymers. The article proved through a blank experiment that a simple solid acid catalyst resulted in the production of 6-hydroxycapronitrile, while a simple copper catalyst had almost no catalytic activity for the conversion of caprolactone. The Cu- _TiO2 catalyst needs to be reduced with hydrogen before use, and hydrogen is required during the reaction. The activity of the catalyst decreases rapidly over time, presumably because the catalyst surface is covered with a polymer.

In 2001, the Japanese document Kobunshi Ronbunshu, 58 (12), 679-684 (2001) reported a study on the preparation of caprolactam by non-catalytic reaction of caprolactone and ammonia in supercritical water (T>374°C, P>22.1MPa). The study proposed a reaction mechanism for preparing caprolactam from caprolactone, and believed that 6-hydroxycaproamide was an intermediate in the reaction. 6-hydroxycaproamide was dehydrated and ring-closed to form caprolactam. The article also studied the effects of reaction temperature, water density and ammonia concentration. The results showed that at 380°C and 38MPa (water density was 0.5g/ ^cm3 ), the conversion rate of caprolactone and ammonia and the yield of caprolactam increased with the increase of reaction time. The yield of caprolactam can reach 79.2% when the reaction time is 60min. However, the high temperature and high pressure conditions of supercritical water will increase equipment cost and operation difficulty, and also increase safety risks.

In 2022, the public document ChemSusChem, 2022, 15 (16) reported a reaction process for preparing caprolactam from 6-hydroxyhexanoic acid catalyzed by biological enzymes, which has little practical value.

From the above, it can be seen that the preparation of caprolactam from caprolactone can save the cyclohexanone oxime Beckmann rearrangement step in the existing benzene method caprolactam process, and can avoid the problems existing in the cyclohexanone ammoximation step, which is a new caprolactam production route with great application potential. However, at present, the technical route for preparing caprolactam from caprolactone raw materials has not received much attention. The existing process methods and catalysts for preparing caprolactam from caprolactone are mainly disclosed by patents and papers in the 1970s and before. In general, the reaction process methods proposed in the early stage are mainly divided into two types: non-catalytic method and catalytic method. Among them, the non-catalytic method requires high temperature and high pressure reaction conditions. Due to the problems of thermodynamic limitations and the generation of by-products, the caprolactam yield of the non-catalytic method is low. In contrast, the reaction conditions of the catalytic method are mild, which is not only favorable in terms of reaction thermodynamics, but also conducive to avoiding the occurrence of side reactions, and is also conducive to reducing equipment investment, reducing energy consumption and reducing production costs. Therefore, it is conducive to industrial application. However, the catalytic method requires a catalyst with high activity, high selectivity and strong anti-deactivation ability, and the existing catalysts cannot meet the needs of industrial application.

Summary of the invention

The purpose of the present invention is to provide a preparation method and application of a Cu-Beta zeolite catalyst for gas-solid phase catalytic reaction of preparing caprolactam by hydroamination of caprolactone.

Specifically, the Cu-Beta zeolite catalyst for the gas-solid phase catalytic reaction of producing caprolactam from caprolactone provided by the present invention is a catalyst prepared by using dealuminated Beta zeolite as a carrier and loading copper in the pores by an improved ammonia distillation method.

After research, it was found that for the gas-solid phase catalytic reaction of caprolactone hydroamination to caprolactam, from the perspective of catalytic activity and selectivity, supported copper-based catalysts have the most promising prospects for industrial application. However, from the perspective of catalyst stability, the deactivation problem is the biggest challenge for its industrial application. The deactivation of copper-based catalysts in the gas-solid phase catalytic reaction of caprolactone hydroamination to caprolactam is not limited to carbon deposition. The sintering problem of highly dispersed copper particles is also an important cause of catalyst deactivation. People familiar with this field know that carbon deposition deactivation is a temporary deactivation of the catalyst, and its catalytic activity can generally be restored by various regeneration methods, thereby extending the service life of the catalyst. In contrast, sintering deactivation is generally a permanent deactivation of the catalyst, which has the greatest impact on the service life of the catalyst.

The main benefit of the Cu-Beta zeolite catalyst provided by the present invention is that the copper particles therein are stabilized by the hydroxyl pits of the zeolite carrier and thus have the ability to resist sintering, so that the copper-based catalyst can be used without adding anti-sintering aids such as chromium and nickel.

Firstly, the Cu-Beta zeolite catalyst provided by the present invention is mainly characterized in that it uses dealuminated Beta zeolite as a carrier.

The catalyst for preparing caprolactam from caprolactone by catalytic method, as recorded in the existing related patents and academic papers, is mainly a non-loaded bulk copper chromite catalyst and a copper catalyst (adding a second metal component of nickel or chromium) loaded with an amorphous single oxide carrier (such as titanium oxide, aluminum oxide, silicon oxide) and a binary composite oxide carrier (such as silicon oxide and aluminum oxide). People familiar with the art know that the non-loaded copper chromite catalyst has a small specific surface area, and the exposed metal active sites are few, so the metal dosage is large and the catalytic efficiency is low. The problem of non-loaded catalyst can be overcome by loading copper, copper-nickel and copper-chromium with an amorphous oxide carrier (single oxide carrier and binary composite oxide carrier), but the problem of easy sintering deactivation exists in the loaded copper catalyst, which is a huge challenge for industrial application. Chromium is added to the loaded copper catalyst to form a copper chromite phase, which can improve the anti-sintering ability of the loaded copper catalyst. However, chromium is a metal with limited use. Clinically, chromium and its compounds mainly infringe on people's skin, respiratory and digestive systems, and even if the chromium content is very low, it will produce a strong toxic effect on the human body. Therefore, the catalysts added with chromium will encounter great troubles in the processes of preparation, use and harmless treatment of waste catalysts. Adding nickel to the supported copper catalyst can also improve the anti-sintering performance of copper. However, our research results show that for the reaction of preparing caprolactam from caprolactone, the introduction of a large amount of nickel into the supported copper catalyst will significantly reduce the ability of the catalyst to catalyze the gas-solid phase hydroamination of caprolactone into caprolactam. For example, under the same conditions, the copper-amorphous silicon oxide catalyst (10wt.%Cu) prepared with white carbon black silica (fumed silica) as a carrier, when a small amount of nickel (Ni:Cu=0.3) is added, its ability to catalyze the hydroamination of caprolactone into caprolactam has decreased by 15-20%. In the relevant patent (US3888845), although nickel is used as a preferred auxiliary agent for the supported copper catalyst, its addition amount is strictly limited to the range of Ni:Cu=0.001-1 (atomic ratio), preferably within the range of 0.005-0.25. Undoubtedly, the addition of a small amount of nickel can improve the sintering resistance of supported copper catalysts, but the effect of a small amount of nickel alone is not enough to properly solve the sintering deactivation problem of supported copper catalysts.

In fact, the sintering deactivation problem of supported copper catalysts is a universal problem. Copper-based catalysts are cheap and environmentally friendly, so they are widely used in alcohol dehydrogenation, carbonyl hydrogenation, ester hydrogenolysis, amination, hydrocarbon hydrogenation, isomerization, and C-C and C-Si bond hydrogenolysis reactions. The main reason why highly dispersed copper particles are easy to sinter and grow is that the ionic radius of copper metal is large, the melting point is low (1083℃), and the Tammann temperature and Hüttig temperature are low. Supported copper catalysts can sinter at a temperature of 170℃. Some people have summarized the thermal stability of common metal catalysts and given the following order: Ag<Cu<Pd<Fe<Ni<Co<Pt<Rh<Ru<Ir<Os<Re. It can be seen that the thermal stability of copper is lower than that of most common metal catalysts.

The invention uses dealuminated Beta zeolite as a carrier to prepare a loaded copper catalyst, and aims to utilize a large number of hydroxyl nest lattice defect sites in the dealuminated Beta zeolite to disperse and stabilize the loaded copper particles. The large number of hydroxyl nest lattice defect sites in the dealuminated Beta zeolite can be produced by removing framework aluminum from the crystal of Beta zeolite using a conventional acid treatment method. When the Beta zeolite is subjected to an acid dealumination treatment, each time one framework aluminum is removed, four Si-O-Al bonds need to be acid-lyzed ([Al-(OSi) ₄ ] ^- +4H ₂ O＝[Al(OH) ₄ ] ^- +4≡Si-OH, [Al(OH) ₄ ] ^- +4H ⁺ ＝Al ³⁺ +4H ₂ O, and the overall reaction equation is [Al-(OSi) ₄ ] ^- +4H ⁺ ＝Al ³⁺ +4≡Si-OH), thereby producing a hydroxyl nest lattice defect site surrounded by four silanol groups (≡Si-OH).

The main idea of the present invention comes from the inventor's previous work. In the previous work, the inventor has conducted in-depth research on the physicochemical properties and catalytic functions of hydroxyl nest lattice defects in the MFI zeolite family (ZSM-5, B-ZSM-5, Silicalite-1 (S-1) and TS-1). Some representative research works are recorded in the following public documents: Silicalite-1zeolite acidification by zinc modification and its catalytic properties for isobutane conversion, RSC Advances, 2018, 33 (8), p. 18663-1867; Pt supported on Znmodified silicalite-1zeolite as a catalyst for n-hexane aromatization, JOURNAL OF ENERGY CHEMISTRY, 2018, (36), p .96-103; Operando Dual Beam FTIR Study of Hydroxyl Groups and Zn Species over Defective HZSM-5Zeolite Supported ZincCatalysts,Catalysts,2019,1(9),p.100; Effect of Zeolitic Hydroxyl Nests on theAcidity and Propane Aromatization Performance of Zinc Nitrate Impregnation-Modified HZSM-5 Zeolite, Industrial & Engineering Chemistry Research, 2020, 37(59), p.16146-16160. Liu Guodong. Study on the surface acidity and catalytic performance of ZnO-modified nano-silicalite-1 zeolite [D]. Dalian University of Technology, 2020.; Lin Long. Characterization, modification and catalytic performance of defective ZSM-5 zeolite [D]. Dalian University of Technology, 2022.). In short, the above research results show that the silanols in the hydroxyl nest lattice defect of zeolite are chemically more active than the isolated silanols on the outer surface of the zeolite crystal because they are easy to form hydrogen bonds. In addition, it is particularly worth mentioning that the inventors found the important phenomenon that zinc oxide preferentially lands on the hydroxyl nest lattice defect of zeolite in the study of zinc nitrate impregnation and preparation of zinc oxide modified catalysts for defective all-silica zeolite S-1 and defective ZSM-5 zeolite. Moreover, the zinc oxide located in the lattice defect sites of the zeolite hydroxyl nests is highly dispersed sub-nanometer zinc oxide species. These research experiences provide important scientific guidance for the present invention.

However, the present invention does not use defective MFI zeolite (e.g., defective all-silica zeolite S-1 and deborated B-ZSM-5 zeolite) as a carrier for preparing a loaded copper catalyst, but uses dealuminated Beta zeolite as a carrier for preparing a loaded copper catalyst. This is not because the hydroxyl nest lattice defect sites in the defective MFI molecular sieve (e.g., defective all-silica zeolite S-1 and deborated B-ZSM-5 zeolite) cannot disperse and stabilize copper particles, nor is it because the loaded copper catalyst prepared using the defective MFI molecular sieve (e.g., defective all-silica zeolite S-1 and deborated B-ZSM-5 zeolite) carrier is ineffective for catalyzing the gas-solid phase reaction of preparing caprolactam from caprolactone. This is because the cylindrical pores of the MFI family zeolites are ten-membered rings. When the loading amount of metallic copper is slightly larger, the effective size of the pores will be significantly reduced, which is not conducive to the intrapore diffusion of the reactant caprolactone (seven-membered ring), nor is it conducive to the formation and intrapore diffusion of the caprolactam product, which is also a seven-membered ring. Therefore, it is not conducive to the preparation of catalysts with high activity, high selectivity and strong resistance to deactivation.

People familiar with the art know that as a crystalline porous catalytic material, Beta zeolite and MFI zeolite have similar advantages such as: (1) Both are high-silicon zeolites, so they have high thermal stability and hydrothermal stability, good regeneration performance, and can be repeatedly regenerated and reused after being made into catalysts; (2) Both are cylindrical channels, and both have a three-dimensional cross-channel system, which can provide a networked channel for molecular diffusion, so the channel diffusivity is good and the anti-clogging ability is strong, which is conducive to the catalyst to maintain long-term activity stability during continuous reaction. In addition, compared with MFI zeolite, Beta zeolite is unique. On the one hand, the three-dimensional cylindrical channels of Beta zeolite are all large pores with twelve-membered rings. In the three-dimensional pore system of Beta zeolite, there is a group of "Z"-shaped curved pores parallel to the [001] direction and with an elliptical cross section, with a pore size of 0.56nm×0.65nm; there are also two groups of straight pores parallel to the [100] and [010] directions, with elliptical cross sections, and their pore sizes are both 0.66nm×0.77nm. In contrast, the three-dimensional cylindrical pores of MFI zeolite are all mesopores with ten-membered rings. In the pore system of MFI zeolite, there is a group of straight pores parallel to the (100) crystal plane and with a nearly circular cross section (pore size of 0.53nm×0.56nm) and two groups of "Z"-shaped curved pores parallel to the (010) crystal plane, with opposite folds and elliptical cross sections (pore size of 0.51nm×0.55nm). It is conceivable that the pore system of Beta zeolite is more suitable for the diffusion of the reactant caprolactone (seven-membered ring) in the pores due to its relatively loose macropore characteristics, and is also more suitable for the formation and diffusion of the caprolactam product, which is also a seven-membered ring, and is therefore more conducive to the preparation of catalysts with high activity, high selectivity and strong resistance to deactivation. In fact, Beta zeolite is the only zeolite molecular sieve among industrial zeolite catalytic materials that has the advantages of a high silicon-aluminum oxide molar ratio, a three-dimensional cross-pore system, and all pores are twelve-membered ring macropores.

On the other hand, the framework aluminum of Beta zeolite can be easily removed by acid treatment, thereby generating high-density hydroxyl nest defect sites on the crystal framework. This feature is rare among industrial zeolite catalytic materials and is unmatched by MFI zeolite. When Beta zeolite is subjected to acid dealumination treatment, each time one framework aluminum is removed, four Si-O-Al bonds need to be acid-lyzed ([Al-(OSi) ₄ ] ^- +4H ₂ O＝[Al(OH) ₄ ] ^- +4≡Si-OH, [Al(OH) ₄ ] ^- +4H ⁺ ＝Al ³⁺ +4H ₂ O, the overall reaction equation is [Al-(OSi) ₄ ] ^- +4H ⁺ ＝Al ³⁺ +4≡Si-OH), generating a hydroxyl nest lattice defect site surrounded by four silanol groups (≡Si-OH). In MFI zeolite, the hydroxyl pit lattice defect sites of defective all-silicon zeolite S-1 are randomly formed during the hydrothermal synthesis of S-1 zeolite in an alkaline medium, and their number and distribution are poorly controllable; although the framework aluminum content of ZSM-5 zeolite has a wide adjustable range, the lower limit of its silicon-aluminum molar ratio (Si/Al) can reach about 10, and the upper limit can reach all-silicon zeolite, namely Silicalite-1 (S-1). However, it is difficult to completely remove the framework aluminum of ZSM-5 zeolite. Therefore, in the research on the preparation of titanium atom hybridized ZSM-5 zeolite by post-synthesis method, the general practice is to first try to synthesize boron-containing ZSM-5 zeolite (B-ZSM-5), and then remove boron from B-ZSM-5 zeolite to obtain a ZSM-5 zeolite carrier with a high density of hydroxyl pit defect sites on the framework.

In summary, the present invention does not use MFI zeolite with hydroxyl nest lattice defect sites (for example, defective all-silicon zeolite S-1 and deborated B-ZSM-5 zeolite) as a carrier for preparing a loaded copper catalyst, but selects Beta zeolite with hydroxyl nest lattice defect sites as a carrier for preparing a loaded copper catalyst. The main reason is that for the purpose of the present invention, Beta zeolite combines the three advantages of a high silicon-aluminum oxide molar ratio framework, a three-dimensional twelve-membered ring cross-channel system, and easy and complete removal of framework aluminum, which is an unmatched material advantage. In addition, Beta zeolite is a catalytic material that has been industrially synthesized earlier, that is, it can be obtained in large quantities through commercial sales, and it can also be easily prepared by hydrothermal synthesis.

Secondly, the preparation method of the Cu-Beta zeolite catalyst provided by the present invention is mainly characterized in that the copper loading on the dealuminated Beta zeolite carrier is achieved by an improved ammonia evaporation method. The core of the improved ammonia evaporation method for loading copper is to impregnate the zeolite carrier with an equal volume of a copper-ammonia complex solution. In this process, the capillary condensation of the zeolite carrier is relied on to absorb most of the complex solution into the pores, so that the copper hydroxide generated during the ammonia evaporation process is directly deposited in the pores of the Beta zeolite. Therefore, in the subsequent drying, roasting and hydrogen reduction treatment processes, the copper hydroxide can be first converted into copper oxide in the pores of the zeolite, and then further converted into sub-nano and nanoparticles of metallic copper. The sub-nano and nanoparticles of metallic copper can be directly captured by the hydroxyl nest lattice defect sites mainly present in the pores of the zeolite during the formation process, so as to be dispersed and stabilized in time.

People familiar with the art know that ammonia evaporation is one of the most commonly used methods for preparing copper-based catalysts. Ube Industries Ltd. (US4 440 873 (1984), EP0 064 241B1 (1985)) first proposed the use of ammonia evaporation to prepare Cu/ _SiO2 catalysts for the purpose of gas-solid phase hydrogenation of dimethyl oxalate to produce ethylene glycol and glycolate. The earliest proposed ammonia evaporation method is as follows: the first step is to prepare a copper-ammine complex solution. First, a soluble copper-containing compound is dissolved in water to obtain an aqueous solution containing copper ions, and then an appropriate amount of concentrated ammonia water is added to the aqueous solution containing copper ions to make the pH value greater than 10, for example, to make the pH value reach 10-12. Thus, a dark blue transparent solution containing a copper-ammine complex can be obtained; the second step is to use silica sol as a precursor of the _SiO2 carrier and mix it with the copper-ammine complex. That is, silica sol is added to the dark blue transparent solution containing the copper-ammine complex and stirred thoroughly to mix them evenly. This stirring mixing process can be carried out under normal pressure and pressurized conditions, from room temperature to 150°C; the third step is ammonia evaporation treatment. That is, the mixture containing the copper ammonia complex is subjected to ammonia evaporation treatment to obtain a solid catalyst precursor. The ammonia evaporation treatment can be carried out under pressurized and reduced pressure, and the preferred temperature range is 60-90°C; the fourth step is the pretreatment of the solid catalyst precursor. This step refers to the pretreatment of the solid catalyst precursor before hydrogen reduction, including drying and water washing. In addition, pre-calcination treatment can also be selected. The temperature range of the pre-calcination treatment is 400-800°C, preferably 500-750°C; the fifth step is hydrogen reduction treatment. The pretreated solid catalyst precursor is subjected to hydrogen reduction treatment. The hydrogen reduction time is 1-15h, and the reduction temperature range is 150-500°C, preferably 200-400°C.

In US Pat. No. 4,440,873, examples of soluble copper-containing compounds that can be used to prepare copper-ammine complex solutions are given, including copper nitrate, copper sulfate, copper oxalate, copper chloride and copper acetate, and copper nitrate is pointed out as a preferred option. In Example 1 _{, the preparation of Cu/SiO2 catalyst by ammonia evaporation method is described as follows: (1) 19.0 g of copper nitrate (Cu(NO3)2.3H2O ) is dissolved} in 200 ml of water to obtain an aqueous solution containing copper ions, and then 60 ml of concentrated ammonia solution is added thereto, and the pH value is adjusted to 11-12, thereby obtaining a dark blue solution containing copper-ammine complex; (2) 66.6 g of silica sol (30 wt.% _SiO2 ) is added to the copper-ammine complex solution and stirred at room temperature for several hours; (3) The reaction mixture of step (2) is subjected to ammonia evaporation treatment by heating. Ammonia is evaporated until most of the water is also evaporated to obtain a solid product; (4) The solid product is dried at 120°C for 12 hours. The dried product was fully washed with water and then dried. The drying condition was 140℃×14h; (5) The dried product was subjected to hydrogen reduction treatment. The reduction condition was 350℃×2-3h. The prepared Cu/ _SiO2 catalyst contained about 20wt.% copper.

From the above, it can be seen that the earliest proposed process for preparing Cu/ _SiO2 catalyst by ammonia evaporation has the following characteristics: On the one hand, the amorphous silica carrier is not prefabricated, but is generated in situ during the ammonia evaporation process using silica sol as a precursor. Specifically, during the ammonia evaporation process, the silica sol is converted into silica gel. At the same time, the copper ammonia complex loses ammonia to generate copper hydroxide precipitate, which is deposited on the surface of the silica gel. This process has dynamic characteristics. That is, the silica gel particles continue to grow after being generated, and at the same time, the copper hydroxide precipitate is also continuously generated. While the silica gel particles grow, the copper hydroxide precipitate is deposited and reacted on its surface, resulting in a loading state in which silica gel and copper hydroxide are mixed layer by layer. Later, researchers in this field pointed out (J.Catal.257(2008)172–180) that this ammonia evaporation method is essentially a homogeneous deposition-precipitation method, and the prepared Cu/ _SiO2 catalyst is a layered copper silicate. On the other hand, a diluted copper ammonia complex solution is prepared and used. The volume of this copper-ammine complex solution is greatly in excess relative to the liquid holding capacity (pore volume) of the silica gel finally generated. The water solvent therein needs to be removed by post-treatment operations such as filtration or evaporation, and the mechanism of copper loading and dispersion on the silica carrier is deposition and precipitation, that is, the silica gel particles grow while copper hydroxide precipitates and reacts on its surface, and a uniform loading state is achieved by the layer-by-layer mixing of silica gel and copper hydroxide. It is conceivable that if the silica carrier is not generated during the ammonia evaporation process but is prefabricated, then the use of this diluted and excessive volume (the solution volume is greatly excessive relative to the total pore volume of the silica carrier) copper-ammine complex solution for ammonia evaporation will inevitably lead to a large amount of copper hydroxide being deposited on the outer surface of the carrier particles, resulting in the uneven result that less copper is loaded in the carrier pores and more copper is loaded outside the carrier pores.

Some scholars have prepared amorphous oxide carrier-supported copper-based catalysts for the purpose of gas-solid phase hydrogenation of dimethyl oxalate to ethylene glycol according to the earliest proposed ammonia evaporation method. For example, in the public literature J. Catal. 257 (2008) 172-180 and Appl. Catal. A: Gen. 458 (2013) 82-89, there are reports of related studies, and the amorphous oxide carriers involved are silicon dioxide and a binary composite of silicon dioxide and titanium dioxide. When silicon dioxide is used as a carrier, silica sol (Ludox AS-40) is used as a precursor of the carrier. When a binary composite of silicon dioxide and titanium dioxide is used as a carrier, silica sol (JN30, Qingdao Haiyang Chem. Co., Ltd.) and titanium dioxide sol are used as precursors of the carrier. After the ammonia evaporation is completed (the pH value of the slurry drops to 6-7), the amorphous oxide-supported copper hydroxide solid product is obtained by filtration.

Some scholars have improved the ammonia evaporation method proposed earlier when preparing Cu/ _SiO2 catalysts for gas-solid phase hydrogenation of dimethyl oxalate. Among them, in the public literature J.Am.Chem.Soc.2012,134,13922-13925 and J.Catal.297(2013)142-150, researchers reported the ammonia evaporation hydrothermal (AEH) method. This method actually transfers the ammonia evaporation product (slurry containing copper hydroxide/silica gel precipitate, pH=6-7) of the traditional ammonia evaporation method (the earliest proposed ammonia evaporation method) to a high-pressure synthesis reactor for hydrothermal treatment at 190-210℃ for 12h, and then performs conventional filtration, washing, drying, roasting and hydrogen reduction treatment on the solid product. In other words, this method is not an improvement on the ammonia evaporation method itself, but a hydrothermal post-treatment step is inserted into the product of the ammonia evaporation method before the conventional post-treatment. It should be emphasized that the operation of the ammonia evaporation hydrothermal method before the hydrothermal post-treatment is the same as that of the traditional ammonia evaporation method. The silica carrier is generated in situ using silica sol as a precursor, and the water solvent of the diluted and excess copper ammonia complex solution is finally removed by filtration; in the public document J.Phys.Chem.C 2015,119,13758-13766, scholars added urea as a precipitation aid to the solution when preparing the copper ammonia complex solution. The other methods are the same as those of the traditional ammonia evaporation method. The silica carrier is in situ generated using silica sol (Ludox AS-40, 40wt.% _SiO2 ) as a precursor, and the water solvent of the diluted and excess copper ammonia complex solution is finally removed by filtration. In the open literature Natural Gas Chemical Industry (C1 Chemistry and Chemical Industry), 2013, 38(3):43-47, Natural Gas Chemical Industry (C1 Chemistry and Chemical Industry), 2014, 39(5):31-34 and Journal of Shenyang University of Chemical Technology, 2016, 30(3):212-216., the researchers used prefabricated JN-25 alkaline silica gel (primary particle size 10nm, Qingdao Ocean Chemical Co., Ltd.) as the carrier of the Cu/ _SiO2 catalyst prepared by the ammonia evaporation method. In order to achieve a uniform deposition and precipitation effect, a certain amount of silica sol is also added as a precursor for the in situ generation of the silica gel carrier. In order to overcome the problem caused by the reduced amount of silica sol when using prefabricated JN-25 alkaline silica gel as the carrier of Cu/ _SiO2 catalyst, researchers have also tried to add hexadecyltrimethylammonium bromide (CTAB) surfactant to the prepared copper ammonia complex aqueous solution to disperse the silica sol and generate mesopores in the in-situ generated silica gel. The other methods are no different from the traditional ammonia evaporation method; in the public literature RSC Adv., 2015, 5, 29040-29047 and Applied Catalysis A: General 509 (2016) 66-74, the researchers used prefabricated titanium dioxide (P25, Degussa Co., Ltd) as the carrier of the Cu/ _TiO2 catalyst prepared by the ammonia evaporation method, and the other methods are no different from the traditional ammonia evaporation method. It should be noted that the P25 type _TiO2 carrier is a low specific surface area carrier with underdeveloped pores. Therefore, in the prepared Cu/ _TiO2 catalyst, it does not have a uniform deposition and precipitation effect, that is, the loaded copper hydroxide mainly exists on the outer surface of the titanium dioxide carrier. The calcined sample was analyzed by X-ray diffraction, and there were obvious diffraction characteristic peaks of the CuO phase at 2θ=35.5°, 38.7° and 48.7°, indicating that its hydrogen reduction product - metallic copper has poor dispersion.

In addition, it is worth mentioning that in the open document Applied Catalysis A, General 539 (2017) 59–69, the researchers used prefabricated ordered mesoporous silica (OMS) as the carrier of the Cu/OMS catalyst prepared by the ammonia evaporation method. In order to reduce the destructive effect of the alkalinity of the copper-ammine complex solution on the ordered mesopores of the prefabricated silica carrier, the researchers also appropriately reduced the concentration of ammonia water in the prepared copper-ammine complex solution (the researchers regarded it as an improvement on the ammonia evaporation method). In addition to the above two points, there is no other difference between the improved ammonia evaporation method described in this study and the traditional ammonia evaporation method. After the ammonia evaporation is completed (the pH value of the slurry drops to 6-7), the water solvent of the diluted and excess volume of the copper-ammine complex solution is finally removed by filtration to obtain a solid product loaded with copper hydroxide. The research results show that the ordered mesoporous structure of the prefabricated silica carrier used in this study has been largely destroyed after being loaded with metallic copper by the ammonia evaporation method, and a large amount of layered copper silicate exists in the catalyst, indicating that the prefabricated silica carrier used was dissolved in large quantities during the contact with the copper-ammine complex solution and turned into silica sol, which produced a uniform deposition and precipitation effect with copper hydroxide during the ammonia evaporation process; in the open document Journal of Catalysis 280 (2011) 77–88, the researchers also used prefabricated mesoporous silica (HMS) as the carrier of the Cu/HMS catalyst prepared by the ammonia evaporation method. In addition, the researchers also added water-soluble nickel salt (nickel nitrate) to the prepared copper-ammine complex aqueous solution, so that the prepared copper-based catalyst contained metallic nickel (CuxNi/HMS). The ammonia evaporation method used in this study is the same as the traditional ammonia evaporation method except that a prefabricated mesoporous silica carrier is used and a water-soluble nickel salt (nickel nitrate) is added to the prepared copper-ammine complex aqueous solution so that the prepared copper-based catalyst contains a metal nickel additive. The ammonia evaporation operation is carried out at 90°C. After the ammonia evaporation is completed (the pH value of the slurry drops to 7-8), the diluted and excess volume of the copper-ammine complex solution is finally removed by filtration to obtain a solid product loaded with copper hydroxide and nickel hydroxide. Similarly, in this study, the ordered mesoporous structure of HMS silica has been mostly destroyed after the ordered mesopores of HMS silica are loaded with metal copper and nickel by the ammonia evaporation method (the specific surface area decreases by more than 50%). In addition, the XRD characterization results show that the prepared supported catalyst sample has characteristic diffraction peaks of the metal oxide phase before hydrogen reduction (calcined at 450°C for 4h), and has characteristic diffraction peaks of the metal phase after hydrogen reduction, indicating that the metal copper and nickel are unevenly loaded on the HMS carrier and have poor dispersion.

According to the results of literature survey, except for a recent open article in Science (Science10.1126/science.adj1962(2023).) reporting the use of ammonia evaporation to prepare a supported copper catalyst on a dealuminated Beta zeolite support for the purpose of gas-solid phase hydrogenation of dimethyl oxalate, no other research work on the use of ammonia evaporation to prepare supported metal catalysts on zeolite support has been found at home and abroad. It should be noted that the relevant research work recently published in Science used the traditional ammonia evaporation method to prepare supported copper catalysts on dealuminated Beta zeolite support. The specific method is as follows: First, 0.23g Cu(NO ₃ ) ₂ ·3H ₂ O was dissolved in 100ml ammonia solution (containing 0.75g NH ₃ ·H ₂ O) and stirred at room temperature for 10min to prepare a copper-ammine complex aqueous solution; Second, 1.94g dealuminated Beta zeolite support (Beta-deAl) was added to the copper-ammine complex solution and ammonia evaporation was performed under vigorous stirring. The temperature of ammonia evaporation is 80℃, and the time of ammonia evaporation is 6h; in the third step, after the ammonia evaporation is completed, the water solvent of the diluted and excessive volume copper-ammine complex solution is finally removed by filtration; in the fourth step, the solid product is dried at 100℃ overnight and calcined at 400℃ for 3h to obtain a catalyst; in the fifth step, in order to catalyze the hydrogenation reaction of dimethyl oxalate with the catalyst, it is reduced with hydrogen at 400℃ for 3h. It is not difficult to see that in this study, except that the catalyst carrier is prefabricated dealuminated Beta zeolite (obtained by acid dealumination treatment of Al-Beta zeolite matrix with Si/Al=13 with 13MHNO ₃ solution at 80℃ for 12h), the other methods are the same as the traditional ammonia evaporation method. In the prepared copper-ammine complex solution, the copper ion concentration is very dilute (only about 9.5mmol/L); when the catalyst is prepared by the ammonia evaporation method, the initial liquid-solid ratio is as high as about 51.5 (ml/g), that is, the volume of the copper-ammine complex solution is greatly excessive to the zeolite carrier. The results show that although the prepared dealuminated Beta zeolite-supported copper catalyst Cu/Beta-deAl has a very low copper content (about 3wt.% Cu), the specific surface area loss is as high as 15% (due to the excessive copper-ammine complex solution (NH ₃ /Cu molar ratio 22.5) causing a large amount of dissolution and desiliconization of the dealuminated Beta zeolite, destroying the skeleton structure), and its XRD pattern still has obvious characteristic diffraction peaks of metallic copper (2θ=43.3°). Transmission electron microscopy studies show that the copper in the fresh catalyst is mainly supported on the outer surface of the dealuminated Beta zeolite, and the particle size is relatively large (the use of a diluted and excessive copper-ammine complex solution causes excessive deposition of copper hydroxide outside the zeolite pores during the ammonia evaporation process), and requires methanol vapor post-treatment through a reverse Ostwald ripening process for redispersion to transfer it into the zeolite pores. This research work fully demonstrates that when using dealuminated Beta zeolite as a carrier to prepare molecular sieve-loaded copper catalysts through copper-ammine complexes, the traditional ammonia evaporation method reported in the literature for silica carriers cannot be used. Otherwise, the following problems will occur: (1) Excess copper-ammine complex solution will deposit a large amount of copper outside the zeolite pores during the ammonia evaporation process; (2) Dealuminated Beta zeolite will undergo a desiliconization reaction (ammonia evaporation temperature 80°C) in an excess of copper-ammine complex solution (pH = 10-12), destroying the crystal structure.

The present invention therefore proposes to use an improved ammonia distillation process different from the known practices to meet the need for supporting metallic copper in the pores of a zeolite support, especially a high-silica zeolite support that is easily desiliconized, such as dealuminated Beta zeolite.

In addition, the main feature of the present invention is that the provided Cu-Beta zeolite catalyst is used for the purpose of preparing caprolactam from caprolactone in a gas-solid phase reaction state. So far, neither the disclosed invention patents nor other published documents have involved this application purpose of the Cu-Beta zeolite catalyst. The reaction system is unique. This is mainly because the reaction of preparing caprolactam from caprolactone in a gas-solid phase reaction state involves the use of water vapor, hydrogen and ammonia. For copper-based catalysts, this is a harsh application scenario.

As mentioned above, in the catalytic methods for preparing caprolactam from caprolactone recorded in the existing relevant patents and academic papers, the catalysts used are mainly two types, one is a non-supported bulk copper chromite catalyst, and the other is a copper catalyst (with a second metal component of nickel or chromium added) supported by a single oxide carrier (such as titanium oxide, aluminum oxide, silicon oxide) or a binary composite oxide carrier (such as silicon oxide and aluminum oxide). The single oxide and the binary composite oxide used as the carrier in the supported copper catalyst (with a second metal component of nickel or chromium added) are both amorphous.

The gas-solid phase reaction state is a suitable way to convert caprolactone into caprolactam by the approach of catalytic method. As mentioned above, the catalytic methods for preparing caprolactam from caprolactone disclosed by Kanegafuchi, Bosekikabushiki Kaisha in British Patent GB1109540 (1966) and U.S. Patent 3652549 (1972), and Teijin Co., Ltd. in U.S. Patent 3888845 (1975) all adopt the gas-solid phase reaction state.

As is known to all, the gas-solid phase reaction state is a common form of heterogeneous catalysis, specifically referring to the catalytic reaction in which the reactants contact the solid catalyst in the form of gas. In the field of heterogeneous catalysis, the gas-solid phase reaction is sometimes referred to as the gas-phase reaction. The gas-solid phase reaction state is a reaction form with mild reaction conditions, high mass transfer and heat transfer efficiency, and very simple operation. For the gas-solid phase reaction, the process of the reactants being converted into products on the catalyst consists of seven elementary steps: (1) external diffusion of reactants. In this step, the reactants pass through the adsorption film on the surface of the solid catalyst and contact the outer surface of the catalyst; (2) internal diffusion of reactants. In this step, the reactants diffuse into the pores through the pores on the surface of the solid catalyst to approach the catalytic active centers in the pores; (3) chemical adsorption of reactants on the catalytic active centers. In this step, the reactant molecules are activated and become activated molecules; (4) surface reaction. In this step, the reactants are converted into adsorbed product forms on the catalyst active centers; (5) product desorption. In this step, the adsorbed product detaches from the catalytic active centers; (6) internal diffusion of products. This step is the process of product molecules moving from the pores to the outer surface of the catalyst after they have separated from the catalytic active centers; (7) External diffusion of products. In this step, product molecules leave the pores on the outer surface of the solid catalyst, pass through the adsorption film on the surface of the solid catalyst, separate from the solid catalyst particles, and become reaction products.

The Cu-Beta catalyst provided by the present invention is suitable for the gas-solid phase hydrogenation catalytic reaction conditions of preparing caprolactam from caprolactone recorded in existing relevant patents and academic papers. As mentioned above, in 1966, Kanegafuchi, BOSEKI KABUSHIKI KAISHA first disclosed a gas-solid phase catalytic method for preparing caprolactam in British patent GB1109540, specifically, firstly vaporizing caprolactone and a certain amount of water, then mixing with ammonia and hydrogen, and the mixed gas is catalytically reacted by copper chromite catalyst at 120-350°C and normal pressure; in 1972, Kanegafuchi, BOSEKI KABUSHIKI KAISHA disclosed a method for preparing caprolactam again in U.S. Patent 3652549, which is also a gas-solid phase catalytic method. Specifically, the method uses a fixed bed reactor, the reaction temperature range is 170-300°C, and the hydrogen partial pressure is 0.1-1.5atm. The feed also contains ammonia and water vapor, and the preferred ranges of their dosage (molar ratio to the raw material) are 2-50 and 10-100 respectively; in 1975, Teijin Co., Ltd. of Japan disclosed a method for preparing caprolactam in US Pat. No. 3,888,845, which is also a gas-solid phase catalytic method. Specifically, the gas-solid phase catalytic reaction can be carried out at 200-320°C and 0.01-2atm, preferably at 220-310°C and 0.1-1.2atm. The optional ranges of hydrogen and ammonia dosage are 5-70 (H ₂ /ester molar ratio) and 1-50 (NH ₃ /ester molar ratio), respectively, and the preferred ranges are 10-50 (H ₂ /ester molar ratio) and 2-25 (NH ₃ /ester molar ratio). In addition, the process also emphasizes the importance of the hydrogen to ammonia molar ratio and the addition of water in the reactor feed. In general, the use of an appropriate hydrogen to ammonia molar ratio is conducive to improving the selectivity of the reaction. Adding water to the reactor feed can not only reduce side reactions and improve caprolactam selectivity, but also slow down the deactivation rate of the catalyst. The optional range of the hydrogen to ammonia molar ratio is 0.2-30, preferably 0.5-15; the optional range of the water to ester molar ratio is 0-50, preferably 5-30.

In summary, according to the records in the existing relevant patents and academic papers, the gas-solid phase hydroamination catalytic reaction of preparing caprolactam from caprolactone, in addition to the caprolactone raw material, the reactor feed also includes water, ammonia and hydrogen. It is not difficult to see from the molecular formula of the caprolactone raw material (C ₆ H ₁₀ O ₂ ) and the molecular formula of the caprolactam product (C ₆ H ₁₁ NO) that ammonia and hydrogen are also reaction raw materials, and water vaporization becomes water vapor as a diluent gas. The optional range of reaction temperature is 120-350°C, the optional range of reaction pressure is 0.01-2atm, and the optional range of ammonia-ester, hydrogen-ester and water-ester molar ratios is 1-50, 5-70 and 0-100 respectively; the preferred range of reaction temperature is 220-300°C, the preferred range of reaction pressure is 0.1-1.2atm, and the preferred range of ammonia-ester, hydrogen-ester and water-ester molar ratios is 2-25, 10-50 and 5-30 respectively. In order to facilitate people in the field to better understand the implementation effect of the present invention, in the present invention, the catalytic performance of the provided Cu-Beta zeolite catalyst in the gas-solid phase hydroamination reaction of caprolactone to caprolactam is evaluated, and the reaction conditions adopted are within the above range.

The technical solution of the present invention:

A method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone, comprising the following steps:

The first step is to prepare the dealuminated Beta zeolite carrier

Engineers familiar with the art can prepare a dealuminated Beta zeolite carrier from a Beta zeolite matrix according to the requirements of the present invention, combined with their own work experience and reference to conventional acid dealumination methods in relevant literature. The requirements of the present invention are as follows:

(1) Select Beta zeolite matrix

The Beta zeolite matrix refers to silicon-aluminum Beta zeolite. The present invention has no restrictions on the crystal size of the Beta zeolite matrix, nor on the production process of the Beta zeolite matrix. However, in order to facilitate the implementation of the present invention, the Beta zeolite matrix has the following restrictions: 1) there are no impurity crystals in the Beta zeolite matrix; 2) the crystallization of the Beta zeolite matrix is good; 3) the molar ratio of silicon-aluminum oxide (the molar ratio of SiO ₂ to Al ₂ O ₃ ) of the Beta zeolite matrix is appropriate.

Among them, whether there are impurity crystals in the Beta zeolite matrix can be checked and confirmed by X-ray polycrystalline powder diffraction (XRD) method. People familiar with the art know that the molar ratio of silicon aluminum oxide (molar ratio of _SiO2 to _{Al2O3 )} of Beta zeolite produced by hydrothermal synthesis is usually between 10-200 (US3308069 (1967)). In Beta zeolite products with lower molar ratio of silicon aluminum oxide (molar ratio of _SiO2 to _{Al2O3 )} , there may generally be impurity crystals of mordenite (MOR), while in Beta zeolite with higher molar ratio of silicon aluminum oxide (molar ratio of _SiO2 to _{Al2O3 )} , there may generally be impurity crystals of ZSM-5 zeolite. By sampling and performing XRD analysis on the Beta zeolite matrix and comparing the XRD pattern of the sample with the standard diffraction cards of Beta zeolite, MOR zeolite and ZSM-5 zeolite, it can be determined whether there are characteristic peaks of MOR zeolite and ZSM-5 zeolite impurities in the XRD pattern of the sample, thereby knowing whether the Beta zeolite matrix is a pure Beta zeolite phase.

Theoretically, the crystallization of the Beta zeolite matrix can also be analyzed by XRD and measured by the relative crystallinity index. However, in view of the fact that the XRD relative crystallinity index needs to be obtained by comparing the sum of the intensities of the medium-intensity characteristic diffraction peak of the Beta zeolite matrix located between 2θ=7.6-8° and the highest intensity characteristic diffraction peak located at 2θ=22-23° with the sum of the intensities of the corresponding diffraction peaks of the reference sample (standard Beta zeolite with a crystallinity of 100%), and the reference sample has no unified definition; and in view of the fact that the intensity of the medium-intensity characteristic diffraction peak of the Beta zeolite located between 2θ=7.6-8° and the highest intensity characteristic diffraction peak located at 2θ=22-23° is greatly affected by the process and conditions of post-processing such as calcination. Therefore, using the XRD relative crystallinity index to determine whether the crystallization of the purchased or synthesized Beta zeolite matrix is good has poor universality. For this reason, the present invention recommends using the specific surface area index of the Beta zeolite matrix to measure whether the crystallization of the purchased or synthesized Beta zeolite matrix meets the requirements. According to our statistical results of the literature reports on the specific surface area data of Beta zeolite, the BET specific surface area value of the well-crystallized Beta zeolite produced by hydrothermal synthesis is generally not less than ^450m2 /g. Engineers familiar with the art can use conventional nitrogen physical adsorption method to first measure the nitrogen adsorption isotherm data of the Beta zeolite matrix, and then calculate its BET specific surface area value according to the BET model. In short, the present invention requires that the BET specific surface area value of the Beta zeolite matrix used is ≧ ^450m2 /g, indicating that its crystallization is good.

The molar ratio of silicon-aluminum oxide (the molar ratio of SiO ₂ to Al ₂ O ₃ ) is a key indicator of the Beta zeolite matrix. This is because, on the one hand, the lower the molar ratio of silicon-aluminum oxide (the molar ratio of SiO ₂ to Al ₂ O ₃ ) of the Beta zeolite matrix, that is, the higher the content of framework aluminum, the more hydroxyl nest lattice defect sites that the dealuminated Beta zeolite carrier can use to disperse and stabilize nano and subnano copper particles; on the other hand, pure phase Beta zeolite with a very low molar ratio of silicon-aluminum oxide (the molar ratio of SiO ₂ to Al ₂ O ₃ ) is difficult to synthesize using a hydrothermal method. Moreover, after the Beta zeolite matrix with a very low molar ratio of silicon-aluminum oxide (the molar ratio of SiO ₂ to Al ₂ O ₃ ) is made into a dealuminated Beta zeolite carrier by an acid dealumination method, the framework thermal stability is poor, and the crystallinity will be lost in the subsequent calcination step of preparing the Cu-Beta zeolite catalyst, resulting in poor performance of the catalyst. Therefore, the molar ratio of silicon-aluminium oxide (molar ratio of _SiO2 to _{Al2O3 )} of the Beta zeolite matrix required by the present invention is suitably in the range of 10-200, preferably in the range of 20-100, and more preferably in the range of 25-60. The molar ratio of silicon-aluminium oxide (molar ratio of _SiO2 to _{Al2O3 )} of the Beta zeolite matrix can be analyzed by conventional chemical analysis (titration), or by X-ray fluorescence spectroscopy (XRF) or inductively coupled plasma emission spectroscopy (ICP). The present invention recommends the use of the simple and fast XRF method.

The Beta zeolite matrix that meets the requirements of the present invention can be obtained from commercial sources or synthesized by itself. Engineers familiar with the art can also synthesize the Beta zeolite matrix that meets the requirements of the present invention based on their own experience and other literature reports. If Beta zeolite matrix is synthesized by oneself, the methods reported in the following invention patents and public documents can be selected: US3308069 (1967), EP187522A2 (1986), US4847055 (1989), CN1086792A (application date 1993.9.20), CN1108213A (application date 1994.3.11), CN1108214A (application date 1994.3.11), CN1154341A (application date 1996.1.11), CN1154242A (application date 1996.1.9), CN1154342A (application date 1996.1.11), CN1268 545A (application date 1999.3.30), CN1 133 497C (application date 1999.3.30), CN1108 275C (application date 1999.9.10), CN1 100 004C (application date 2000.5.19), CN1 335 258A (application date 2001.2.28), CN1 116 227C (application date 2001.3.12), CN101 205 072B (application date 2006.12.18), Chem. Comm., 1996, 625; J. Mater. Chem., 1998, 8 (9), 2137-2145; Microporous and Mesoporous Materials 21 (1998) 305-313; Applied Catalysis A-GENERAL, 166 (1998), 97–103; Microporous and Mesoporous Materials 48 (2001) 23-29; Microporous and Mesoporous Materials 56 (2002) 1–10.; Journal of Molecular Catalysis A: Chemical 252 (2006) 76–84; Microporous and Mesoporous Materials 94 (200 6)1–8; J.Mater.Sci.41(2006)1861-1864; Cryst.Res.Technol.44,No.4,379-385(2009)DOI10.1002/crat.200800474; Microporous and Mesoporous Materials 143(2011)97-103; RSC Adv.2019,9,3653-3660.

(2) Preparation of dealuminated Beta zeolite carrier

As mentioned above, a dealuminated Beta zeolite carrier can be prepared on the basis of a Beta zeolite matrix by a conventional acid dealumination method. The present invention requires that the silicon-aluminum oxide molar ratio (the molar ratio of SiO ₂ to Al ₂ O ₃ ) of the dealuminated Beta zeolite carrier be as high as possible, that is, the framework aluminum of the Beta zeolite matrix should be completely removed as much as possible. The dealuminated Beta zeolite carrier that meets the requirements of the present invention has a silicon-aluminum oxide molar ratio (the molar ratio of SiO ₂ to Al ₂ O ₃ ) in a suitable range of ≧700, a preferred range of ≧800, and a more preferred range of ≧900. Because the silicon-aluminum oxide molar ratio (the molar ratio of SiO ₂ to Al ₂ O ₃ ) of the dealuminated Beta zeolite is very high and the aluminum content is very low, the inductively coupled plasma emission spectroscopy (ICP) method or the atomic absorption (AA) method is required to accurately determine the silicon-aluminum oxide molar ratio (the molar ratio of SiO ₂ to Al ₂ O ₃ ). The present invention recommends the use of the ICP method.

When the Beta zeolite matrix is subjected to acid dealumination treatment, efforts should be made to remove all of its framework aluminum. The harm of too much framework aluminum remaining on the dealumination Beta zeolite carrier is that the strong acidity of the framework aluminum will accelerate the coking and deactivation of the Cu-Beta zeolite catalyst and reduce the selectivity of the catalyst for the main product of caprolactam.

Although the framework aluminum of Beta zeolite is easy to remove, so that a dealuminized Beta zeolite carrier that meets the requirements of the present invention can be prepared on the basis of Beta zeolite matrix by high-temperature steam dealumination, EDTA and other complexing agent dealumination, organic acid solution dealumination, inorganic acid (concentrated hydrochloric acid, concentrated nitric acid) solution dealumination, or a dealumination method formed by any combination of the above different methods, but considering the production cost of the dealuminized Beta zeolite carrier, the complexity of the process and the difficulty of treating the waste liquid generated by dealumination, the present invention recommends using a concentrated nitric acid aqueous solution dealumination method to prepare a dealuminized Beta zeolite carrier that meets the requirements of the present invention.

Engineers familiar with the art can, based on their own experience or referring to the specific practices disclosed in the following documents, carry out acid dealumination treatment on the Beta zeolite matrix with concentrated nitric acid aqueous solution to prepare the dealumination Beta zeolite carrier that meets the requirements of the present invention: Chemical Communications, 1998, 1: 87-88; Micropor. Mesopor. Mater., 1999, 31: 163-173; Micropor. Mesopor. Mater., 2001, 49: 103–109; Micropor. Mesopor. Mater., 2008, 110: 480–487; Micropor. Mesopor. Mater., 2012, 163: 122-130; ACS Catalysis, 2014, 4(8): 2801-2810.

When a Beta zeolite matrix is acid-dealuminated with a concentrated nitric acid aqueous solution to prepare a dealuminated Beta zeolite carrier, the concentration of the nitric acid aqueous solution, the ratio of the acid solution to the zeolite (liquid-to-solid ratio), and the temperature and time of the acid treatment are all important factors affecting the degree of acid dealumination of the Beta zeolite matrix. The influence of the above-mentioned factors on the dealumination of the Beta zeolite matrix is ultimately reflected in the residual aluminum content of the dealuminated Beta zeolite carrier. However, if a dealuminated Beta zeolite carrier having a silicon-aluminum oxide molar ratio (SiO ₂ to Al ₂ O ₃ molar ratio) that meets the requirements cannot be obtained after a single dealumination, the silicon-aluminum oxide molar ratio (SiO ₂ to Al ₂ O ₃ molar ratio) of the dealuminated Beta zeolite can be made to meet the requirements of the present invention by secondary or even multiple additional dealumination. The present invention recommends using 13M concentrated nitric acid as the dealuminated acid solution, and using the acid solution amount according to a liquid-to-solid ratio (ml/g) of 20:1. Under this premise, the dealumination reaction is carried out at 95°C for 20 hours. After the dealumination reaction is completed, the solid product is first recovered by solid-liquid separation, and then the solid product is washed with water until the pH value is neutral, and then dried at 80-200°C for 3-24 hours, and calcined at 500°C-600°C for 3-8 hours to obtain the dealumination Beta zeolite. After the Beta zeolite matrix is dealumination, a large number of hydroxyl nest lattice defect sites are generated, and its water and moisture absorption capacity is stronger, so it should be sealed and stored for future use.

In the second step, copper is loaded in the pores of the dealuminated Beta zeolite carrier using an improved ammonia evaporation method to prepare a Cu-Beta zeolite catalyst.

As mentioned above, the core of the improved ammonia distillation method of the present invention is to use a copper-ammonia complex solution to impregnate an equal volume of a dealuminated Beta zeolite carrier. In this process, the capillary condensation of the zeolite pores is relied on to absorb most of the complex solution into the pores, so as to achieve the purpose of depositing copper hydroxide and loading metallic copper in the pores. The specific method is as follows:

(1) Prepare a dilute ammonia basic solution and a saturated solution of a copper ammonia complex: prepare a dilute ammonia basic solution with a pH value of 11-12 according to a ratio of 4.4 g industrial ammonia (containing 25-28 wt.%) of NH ₃ to 100 ml of deionized water, and seal and store for later use; then, according to a molar ratio of copper ions (Cu ²⁺ ) to ammonia molecules of 1:4, use copper nitrate trihydrate (Cu(NO ₃ ) ₂ ·3H ₂ O) as a soluble copper-containing compound to react with industrial ammonia to synthesize a copper ammonia complex; finally, dissolve the copper ammonia complex with a dilute ammonia basic solution at room temperature to prepare a saturated solution of the copper ammonia complex, and seal and store for later use. The copper ammonia ion concentration in the saturated solution of the copper ammonia complex is about 0.4 mol/L (0.4 M), and the color is dark blue and clear.

It should be noted that although U.S. Pat. No. 4,440,873 (1984) has recorded that soluble copper-containing compounds that can be used to prepare copper-ammine complex solutions include copper nitrate, copper sulfate, copper oxalate, copper chloride and copper acetate, considering that sulfate and chloride ions will increase the burden of subsequent water washing, and oxalate and acetate have corrosive problems, the present invention recommends the use of copper nitrate (Cu(NO ₃ ) ₂ ·3H ₂ O).

(2) Impregnation of the zeolite carrier with an equal volume of copper-ammine complex solution: First, the saturated water absorption rate of the zeolite carrier is measured, and the amount of copper-ammine complex solution used for impregnation of the zeolite carrier with an equal volume is calculated. Then, the concentration of the required copper-ammine complex solution is calculated according to the copper loading of the Cu-beta zeolite catalyst to be prepared. When the calculated concentration is equal to 0.4M, the zeolite carrier is directly impregnated with an equal volume of the saturated solution of the copper-ammine complex; when the calculated concentration is lower than 0.4M, the saturated solution of the copper-ammine complex is appropriately diluted with a dilute ammonia basic solution, and then the zeolite carrier is impregnated with an equal volume; when the calculated value is higher than 0.4M, the concentration of the copper-ammine complex solution for a single equal volume impregnation should be recalculated according to multiple equal volume impregnations, and the dilute ammonia basic solution and the saturated solution of the copper-ammine complex are used to prepare the copper-ammine complex solution of the required concentration for each equal volume impregnation. After each impregnation, the dealuminated Beta zeolite carrier is subjected to ammonia evaporation.

The equal volume impregnation is carried out at room temperature in a closed container. In this process, the zeolite carrier absorbs the copper ammine complex solution into the zeolite pores by capillary condensation, so that the copper ammine complex ions contact and interact with the hydroxyl nest lattice defect sites in the pores. The suitable range of the equal volume impregnation time is 0.5-24h, the preferred range of the equal volume impregnation time is 1-12h, and the more preferred range of the equal volume impregnation time is 2-6h;

(3) Ammonia evaporation treatment: The ammonia evaporation process can be carried out under normal pressure or under reduced pressure. The suitable range of ammonia evaporation temperature and time is 50-100°C and 0.5-48h, the preferred range of ammonia evaporation temperature and time is 60-90°C and 1-24h, and the more preferred range of ammonia evaporation temperature and time is 65-85°C and 3-12h. During the ammonia evaporation process, the copper ammonia complex decomposes to generate ammonia gas and copper hydroxide, the former is absorbed by water, and the latter is deposited in the zeolite pores and hydroxyl nest lattice defect sites.

(4) Dehydration and drying treatment after ammonia evaporation: The suitable ranges of drying temperature and time are 100-200°C and 0.5-48h, respectively. The preferred ranges of drying temperature and time are 110-170°C and 1-24h, respectively. The more preferred ranges of drying temperature and time are 120-150°C and 3-12h, respectively.

(5) Calcination after ammonia evaporation: This step is used to convert the copper hydroxide precipitates deposited in the zeolite pores and hydroxyl nest lattice defect sites into nano and sub-nano copper oxide particles, thereby obtaining a catalyst precursor. The calcination is carried out in an air atmosphere, and the appropriate ranges of calcination temperature and time are 350-650°C and 0.5-24h, respectively, the preferred ranges of calcination temperature and time are 400-600°C and 1-12h, respectively, and the more preferred ranges of calcination temperature and time are 450-550°C and 2-6h, respectively;

(6) Hydrogen reduction treatment of catalyst precursor: to obtain finished Cu-Beta zeolite catalyst. The suitable ranges of reduction temperature, time and hydrogen flow rate (expressed as hydrogen volume space velocity, defined as the volume of hydrogen passing through the catalyst per unit volume per unit time, calculated as ideal gas) are 280-600°C, 0.5-20h and 1-2000h ^-1 , respectively, preferably 300-550°C, 1-15h and 10-1500h ^-1 , respectively, and more preferably 350-500°C, 2-8h and 20-1000h ^-1 , respectively.

The Cu-Beta zeolite catalyst obtained by the preparation method is used to catalyze the gas-solid phase hydroamination of caprolactone to produce caprolactam.

As mentioned above, one of the main features of the present invention is that the provided Cu-Beta zeolite catalyst is used for catalyzing the gas-solid phase hydroamination reaction of caprolactone to produce caprolactam.

However, the present invention does not limit the specific method of preparing caprolactam by gas-solid phase hydroamination of caprolactone. Engineers familiar with the art can refer to the methods disclosed in relevant patents and other literature to implement the gas-solid phase hydroamination of caprolactone. According to relevant patents and other literature materials, the present invention sorts out the following ranges of reaction conditions for gas-solid phase hydroamination of caprolactone for reference: the suitable range of reaction temperature is 120-350°C, the suitable range of reaction pressure is 0.01-2atm, the suitable range of feed space velocity (WHSV) of caprolactone is 0.1-5h ^-1 , and the suitable ranges of molar ratios of ammonia-ester, hydrogen-ester and water-ester are 1-50, 5-70 and 0-100, respectively; the preferred range of reaction temperature is 220-300°C, the preferred range of reaction pressure is 0.1-1.2atm, the preferred range of feed space velocity (WHSV) of caprolactone is 0.2-2h ^-1 , and the preferred ranges of molar ratios of ammonia-ester, hydrogen-ester and water-ester are 2-25, 10-50 and 5-30, respectively.

In order to facilitate the explanation of the implementation effect of the catalyst and the preparation method of the present invention and to avoid unnecessary complexity, a typical method for preparing caprolactam by gas-solid hydrogenation reaction of caprolactone on Cu-Beta zeolite catalyst is introduced as follows, taking a small fixed bed reactor in the laboratory as an example: the small fixed bed reactor adopts the upper feeding and lower discharging operation mode, and the Cu-Beta zeolite catalyst is installed in the constant temperature zone of the reactor. The upper and lower spaces of the catalyst bed are filled with inert porcelain balls. Among them, the upper porcelain ball area of the reactor serves as the vaporization and preheating zone of the raw materials. For convenience, caprolactone, water and ammonia can be prepared into a mixed feed, which is transported by a micro-metering pump, and the hydrogen is fed by a mass flow meter. The reaction is carried out under fixed conditions: reaction temperature 280°C, reaction pressure 1atm, caprolactone feed space velocity (WHSV) 0.6h ^-1 , ammonia-ester, hydrogen-ester and water-ester molar ratios are 6, 50 and 30 respectively.

Beneficial effects of the present invention:

First, the present invention uses an improved ammonia evaporation method to prepare a Cu-Beta zeolite catalyst on a dealuminated Beta zeolite carrier, which can cause the copper-ammine complex to react with copper hydroxide to be deposited mainly in the zeolite pores during the ammonia evaporation process. The copper hydroxide deposited in the zeolite pores can directly form highly dispersed nano and sub-nanometer copper particles at the hydroxyl nest lattice defect sites in the pores after calcination and hydrogen reduction treatment. The highly dispersed nano and sub-nanometer copper particles formed in this way can interact closely with the hydroxyl nest lattice defect sites, thereby being stabilized by the active silicon hydroxyl groups in the hydroxyl nests, and can be used under harsh conditions where water vapor, hydrogen and ammonia coexist without the aid of anti-sintering aids such as chromium and nickel. Secondly, the core of the improved ammonia evaporation method used in the present invention is to impregnate the dealuminated Beta zeolite carrier with an equal volume of a copper-ammine complex solution. Since the amount of copper-ammine complex solution is small, it is beneficial to inhibit the destruction of the dealuminated Beta zeolite framework by the silicon dissolving effect of the copper-ammine complex alkaline solution (pH=10-12), thereby facilitating the dispersion and stabilization effect of the dealuminated Beta zeolite hydroxyl nests on highly dispersed nanometer and sub-nanometer copper particles, and is beneficial to the preparation of a Cu-Beta zeolite catalyst with high activity, high selectivity and high stability. In addition, the present invention uses the Cu-Beta zeolite catalyst for the purpose of gas-solid phase hydroamination of caprolactone, which will greatly reduce the industrialization difficulty of the technical route of using caprolactone to prepare caprolactam.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is the XRD diagram of the dealuminated Beta zeolite carrier (Beta24c) produced by acid dealumination of a Beta zeolite matrix having a silicon-aluminum oxide molar ratio (molar ratio of _SiO2 to _{Al2O3 )} of 24, as well as the XRD diagram of a Cu-Beta zeolite catalyst (Cu3-Beta24c-1) having a copper content of 3 wt.% prepared using Beta24c as a carrier and an improved ammonia distillation method.

Figure 2 is an infrared spectrum in the hydroxyl region of a dealuminated Beta zeolite carrier (Beta24c) produced by acid dealumination of a Beta zeolite matrix having a silicon-aluminum oxide molar ratio (molar ratio of _SiO2 to _{Al2O3 )} of 24, and an infrared spectrum in the hydroxyl region of a Cu-Beta zeolite catalyst (Cu3-Beta24c-1) having a copper content of 3 wt.% prepared using Beta24c as a carrier and an improved ammonia distillation method.

FIG3 is a high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) photograph of the Cu3-Beta24c-1 catalyst.

FIG4 is a high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) photograph of a Cu3-Beta24c-1 catalyst sample calcined at high temperature (550°C×3h).

Figure 5 is the XRD pattern of the dealuminated Beta zeolite carrier (Beta24c) generated by acid dealumination of a Beta zeolite matrix having a silicon-aluminum oxide molar ratio (molar ratio of _SiO2 to _{Al2O3 )} of 24, as well as the XRD pattern of a Cu-Beta zeolite catalyst (Cu3-Beta24c-CE1) having a copper content of 3 wt.% prepared using Beta24c as a carrier and conventional ammonia evaporation method.

DETAILED DESCRIPTION

The implementation effect of the present invention can be evaluated by characterizing the physicochemical properties of the prepared Cu-Beta zeolite catalyst and detecting its catalytic performance in the gas-solid phase hydroamination reaction of caprolactone to produce caprolactam.

In characterizing the physicochemical properties of Cu-Beta zeolite catalysts, the focus can be on characterizing the damage of the crystal structure, the occupation of hydroxyl nests, the high dispersion of the loaded metallic copper, and the resistance to sintering of copper particles.

Among them, the damage of the Beta zeolite crystal structure can be characterized by the X-ray polycrystalline powder diffraction (XRD) method. If the ammonia evaporation treatment leads to significant damage to the Beta zeolite crystal structure, the intensity of the characteristic diffraction peak at 2θ=22-23° in the XRD diagram of the catalyst will be significantly reduced. If the dealuminated Beta zeolite carrier is used as a reference sample, the relative crystallinity reduction degree of the zeolite carrier in the Cu-Beta zeolite catalyst can also be estimated.

The occupancy of the hydroxyl cavity in the Cu-Beta zeolite catalyst can be determined by using the Fourier transform infrared spectroscopy (FT-IR) method to obtain the hydroxyl vibration infrared spectrum of the catalyst and comparing it with the hydroxyl vibration infrared spectrum of the dealuminated Beta zeolite carrier for qualitative judgment. The more nano and sub-nano copper particles are located in the hydroxyl cavity, the weaker the intensity of the infrared characteristic band of the hydroxyl cavity of the catalyst (the broad absorption band between 3300-3600 cm ^-1 ), and vice versa.

In addition, the high dispersion of the metal copper loaded in the Cu-Beta zeolite catalyst can be observed by transmission electron microscopy (TEM), and the anti-sintering of the copper particles can be concluded by calcination treatment combined with TEM observation. It can also be judged by the attenuation of the catalytic activity (caprolactam yield) of the catalyst in the gas-solid phase hydroamination of caprolactone to produce caprolactam.

As for the catalytic performance of Cu-Beta zeolite catalyst in the gas-solid phase hydroamination of caprolactone to produce caprolactam, it can be evaluated using a small laboratory fixed bed reactor. Its operating method and reaction conditions are as described above. The reaction product composition is analyzed by gas chromatography (GC). The GC uses an FID detector and is equipped with an OV-1701 chromatographic column. The conversion rate of caprolactone and the selectivity of caprolactam are calculated by the internal standard method (the internal standard is 1,4-dioxane). The product of the caprolactone conversion rate and the caprolactam selectivity is used to obtain the caprolactam yield data as an evaluation index of the catalyst activity.

The present invention will be further described below by way of examples, but the present invention is not limited by these examples.

Example 1: This example is used to illustrate that the Cu-Beta zeolite catalyst prepared by using the dealuminated Beta zeolite as a carrier and loading copper in the pores by the improved ammonia evaporation method according to the present invention can better maintain the crystal structure of the dealuminated Beta zeolite carrier, and can make the loaded metal copper mainly in the form of highly dispersed nano and sub-nano copper particles fall on the hydroxyl nest lattice defect sites in the zeolite pores, thereby being stabilized by the active silicon hydroxyl groups in the hydroxyl nests, thereby improving the anti-sintering ability. The prepared Cu-Beta zeolite catalyst is suitable as a catalyst for the gas-solid phase hydroamination of caprolactone to produce caprolactam.

First, a Cu-Beta zeolite catalyst is prepared according to the embodiment provided by the present invention:

The first step is to prepare the dealuminated Beta zeolite carrier

(1) According to the hydrothermal crystallization method provided in U.S. Pat. No. 3,308,069 (1967), a Beta zeolite matrix with a molar ratio of _SiO2 to _Al2O3 of ₂₅ was synthesized as a raw material for preparing a dealuminated Beta zeolite carrier. After conventional filtration, washing, drying (110°C, 12h) and calcination to remove the template (540°C, 6h), the synthesized Beta zeolite matrix was observed to have a grain size of less than 100 nanometers by TEM, belonging to nano Beta zeolite; no impurities were found in the matrix by XRD, and its BET specific surface area was _calculated to be about 550 ^m2 /g by using its nitrogen physical adsorption data. Its molar ratio of _SiO2 to _Al2O3 was measured by XRF to be about 24, which met the technical requirements of the present invention for the Beta zeolite matrix.

(2) The Beta zeolite matrix was dealuminated with concentrated nitric acid to prepare a dealuminated Beta zeolite carrier.

First, a concentrated nitric acid solution with a molar concentration of 13M was prepared. Then, according to a liquid-solid ratio of 20:1 (ml/g), 20g of the Beta zeolite matrix that had been dried and calcined as described above was added to a three-necked flask containing 400ml of 13M concentrated nitric acid solution under stirring for dealumination. The dealumination temperature was 95°C and the dealumination time was 20h. During the dealumination reaction, the three-necked flask was kept in a reflux state. After the dealumination reaction was completed, the feed liquid was cooled to room temperature and the solid product was filtered and recovered. Then, conventional water washing, drying (overnight at 110°C) and calcination (550°C, 3h) were used to obtain a dealumination Beta zeolite carrier (codenamed Beta24c, where the lowercase "c" represents the hydroxyl pits produced by dealumination in Beta zeolite). The silicon aluminum oxide molar ratio (the molar ratio of _SiO2 to _{Al2O3 )} of the dealumination Beta zeolite carrier was measured by ICP method to be 980. It is suitable as a carrier for the catalyst of the present invention. Keep sealed for future use to avoid moisture absorption.

In the second step, copper was loaded into the pores of dealuminated Beta zeolite using an improved ammonia evaporation method to prepare a Cu-Beta zeolite catalyst.

(1) Prepare a dilute ammonia basic solution, and synthesize a copper ammonia complex with copper nitrate trihydrate (Cu(NO ₃ ) ₂ ·3H ₂ O), and then prepare a saturated solution of the copper ammonia complex at room temperature. The pH value of the dilute ammonia basic solution is 11-12, and industrial ammonia (containing 25-28 wt.% NH ₃ ) is prepared in the ratio of 100 ml of deionized water to 4.4 g of industrial ammonia; the copper ammonia complex is obtained by reacting Cu(NO ₃ ) ₂ ·3H ₂ O with industrial ammonia in the ratio of 1:4 of copper ions to ammonia molecules; the saturated solution of the copper ammonia complex is obtained by dissolving the copper ammonia complex with the dilute ammonia basic solution, wherein the copper ammonia complex contains about 0.4 M.

(2) The zeolite carrier was impregnated with an equal volume of a copper-ammine complex solution to prepare a Cu-Beta zeolite catalyst with a copper loading of 3 wt.%. First, 5 g of the dealuminated Beta zeolite carrier (Beta24c) that was sealed and stored after calcination was taken, and titrated with deionized water until all samples were evenly soaked but no free liquid water appeared. A total of 6 ml of deionized water was consumed, and the water absorption rate of the dealuminated Beta zeolite carrier (Beta24c) was calculated to be 1.2 ml/g. According to the feed amount of 10 g of the carrier, a total of 12 ml of copper-ammine complex solution was required. According to the copper loading of 3 wt.%, the concentration of the required copper-ammine complex solution was calculated to be approximately 0.39 M. That is, the calculated value of the required copper-ammine complex solution concentration was very close to the concentration of the saturated solution of the copper-ammine complex. Therefore, 10 g of the dealuminated Beta zeolite carrier (Beta24c) was directly impregnated with an equal volume of 12 ml of a saturated solution of the copper-ammine complex. The equal volume impregnation was carried out at room temperature for 4 hours.

(3) Equal volume of impregnated material is treated with ammonia evaporation at normal pressure. The ammonia evaporation temperature is 80°C and the ammonia evaporation time is 10 hours. In this process, the copper-ammonia complex that enters the zeolite pores due to capillary condensation gradually precipitates in the zeolite pores in the form of copper hydroxide due to the loss of ammonia.

(4) Dehydrate and dry the material after ammonia evaporation. Drying temperature: 110℃, drying time: 12h

(5) The dried material is calcined at a temperature of 500°C for 3 hours. After calcination, the copper hydroxide deposited in the zeolite pores is converted into nano and sub-nano copper oxide particles, thereby obtaining a catalyst precursor.

(6) The catalyst precursor is subjected to hydrogen reduction treatment. The reduction temperature is 400°C, the reduction time is 4 hours, and the hydrogen flow rate (expressed as hydrogen volume space velocity, defined as the volume of hydrogen passing through a unit volume of catalyst per unit time, calculated as an ideal gas) is 300 h ^-1 . After hydrogen reduction treatment, a finished Cu-Beta zeolite catalyst is obtained, codenamed Cu3-Beta24c-1.

Secondly, in order to understand the implementation effect of the catalyst preparation method provided by the present invention from the aspect of the physicochemical properties of the catalyst, the XRD pattern and hydroxyl vibration infrared spectrum of Cu3-Beta24c-1 and its dealuminated Beta zeolite carrier (Beta24c) were measured in parallel by XRD method and FT-IR method, as shown in Figures 1 and 2. In addition, TEM photos of Cu3-Beta24c-1 catalyst and its high temperature calcined sample (550°C×3h) were taken by transmission electron microscopy, as shown in Figures 3 and 4.

As can be seen from FIG1, the Cu3-Beta24c-1 catalyst prepared by the improved ammonia distillation method provided by the present invention retains the Beta zeolite crystal structure well, and the relative crystallinity of the zeolite in the catalyst calculated based on the dealuminated Beta zeolite carrier (Beta24c) is 79%. As can be seen from FIG2, the intensity of the infrared characteristic band of the zeolite hydroxyl nest of the Cu3-Beta24c-1 catalyst prepared according to the method of the present invention is greatly reduced compared with the dealuminated Beta zeolite carrier (Beta24c), indicating that the metal copper occupies a large number of hydroxyl nest lattice defect sites. In addition, as can be seen from FIG3 and FIG4, the metal copper particles in the Cu3-Beta24c-1 catalyst exist in the state of highly dispersed nanometer and sub-nanometer particles, with an average particle size of about 6nm. After calcination at a high temperature of 550°C for 3h, the dispersion state of the metal copper particles is good, with an average particle size of about 10nm. These data illustrate that the hydroxyl nest lattice defect sites of the dealuminated Beta zeolite carrier have the effect of dispersing and stabilizing nanometer and sub-nanometer copper particles.

On this basis, the catalytic performance of Cu3-Beta24c-1 catalyst and its 550℃ high temperature calcined sample were evaluated by gas-solid phase hydroamination of caprolactone to produce caprolactam. The reaction was carried out in a small fixed bed reactor. The inner diameter of the stainless steel reaction tube was 9mm, and the upper feeding and lower discharging operation mode was adopted. 2g of tablet-formed catalyst (20-40 mesh sample was taken after sieving) was loaded in the constant temperature zone of the reactor. The upper and lower spaces of the catalyst bed were filled with inert ceramic balls. Among them, the upper ceramic ball area of the reactor served as the vaporization and preheating zone of the raw materials. The temperature of the reaction constant temperature zone was 280℃, the reaction pressure was 1atm, and the feed space velocity (WHSV) of caprolactone was 0.6h ^-1 . For convenience, caprolactone, ammonia water raw materials (analytical grade, ammonia concentration of 25-28wt.%) and deionized water were prepared into raw material liquid according to the ammonia-ester molar ratio of 6 and the water-ester molar ratio of 30, and the reactor was fed with a micro-metering pump, and the hydrogen was fed with a mass flow meter according to the hydrogen-ester molar ratio of 50. The reaction product was continuously collected by a stainless steel collection tank connected to the reactor outlet and equipped with a cooling water jacket, and the product liquid was collected at fixed time intervals and analyzed on Shimadzu gas chromatograph GC-2014 (FID detector, OV-1701 chromatographic column), and the caprolactone conversion rate and caprolactam selectivity were calculated by the internal standard method (internal standard substance was 1,4-dioxane). Under the above conditions, when the hydroamination reaction of caprolactone was continuously carried out for 6 hours, the caprolactam yield of Cu3-Beta24c-1 catalyst was about 84%; the caprolactam yield of its 550°C high temperature roasted sample was 81%. The above reaction results show that the Cu-Beta zeolite catalyst provided by the present invention is a catalyst with excellent performance for the hydroamination of caprolactone to produce caprolactam.

Comparative Example 1: This example is used to illustrate that when a Cu-Beta zeolite catalyst is prepared by loading copper with dealuminated Beta zeolite as a carrier using the traditional ammonia evaporation method, the crystal structure of the Beta zeolite carrier is greatly damaged, the loaded metallic copper is mainly located outside the zeolite pores, the dispersion is low, the copper particles are relatively large, and the copper particles have poor anti-sintering ability because they are not protected by the active silanol groups in the hydroxyl nests of the dealuminated Beta zeolite.

Example 1 was repeated, but after preparing a qualified dealuminated Beta zeolite support (Beta24c) in the first step, copper hydroxide was deposited on the dealuminated Beta zeolite support using the same conventional ammonia evaporation method as disclosed in the open document Science 10.1126/science.adj1962 (2023)., as follows:

(1) 1.18 g Cu(NO ₃ ) ₂ ·3H ₂ O was dissolved in 515 ml ammonia solution (containing 3.86 g NH ₃ ·H ₂ O, equivalent to 8.1 ml 26 wt.% industrial ammonia solution, with a molar ratio of copper ions to ammonia molecules of about 1:23) and stirred at room temperature for 10 min to prepare a copper-ammonia complex aqueous solution (complex ion concentration of about 9.5 mmol/L, i.e., 9.5 mM);

(2) Add 10 g of dealuminated Beta zeolite carrier (Beta24c) to 515 ml of copper-ammonia complex solution and perform ammonia evaporation under vigorous stirring. The ammonia evaporation temperature is 80°C and the ammonia evaporation time is 6 h;

(3) After the ammonia evaporation is completed, the water solvent of the diluted and excess copper-ammine complex solution is finally removed by filtration. The filter cake obtained is dried, calcined and hydrogen-reduced in the same post-treatment method as in Example 1 to obtain a Cu-Beta zeolite catalyst, code-named Cu3-Beta24c-CE1 (CE=Comparative Example).

In order to understand the characteristics of the Cu-Beta zeolite catalyst prepared by the traditional ammonia distillation method from the perspective of the physicochemical properties of the catalyst, the Beta zeolite crystal structure of the Cu3-Beta24c-CE1 catalyst was characterized by XRD and compared with its carrier (Beta24c), as shown in Figure 5. In addition, the Cu metal dispersion of the Cu3-Beta24c-CE1 catalyst and its high-temperature calcined sample (550℃×3h) was characterized by transmission electron microscopy, and the catalytic performance of the Cu3-Beta24c-CE1 catalyst and its high-temperature calcined sample was evaluated by the gas-solid phase hydroamination of caprolactone to produce caprolactam.

As can be seen from Figure 5, the Beta zeolite crystal structure of the Cu3-Beta24c-CE1 catalyst prepared by the traditional ammonia evaporation method is relatively damaged, and the relative crystallinity of the zeolite in the catalyst calculated based on its carrier (Beta24c) is 62%. According to the characterization results of transmission electron microscopy, the average particle size of Cu metal of the Cu3-Beta24c-CE1 catalyst and its high-temperature calcined sample is 11nm and 20nm, respectively, that is, the Cu metal dispersion on the catalyst is low and easy to sinter. The reaction evaluation results show that under the same reaction conditions, the caprolactam yield of the Cu3-Beta24c-CE1 catalyst is about 77%; the caprolactam yield of its high-temperature calcined sample is about 72%. The reaction results show that on the dealuminated Beta zeolite carrier, the Cu-Beta zeolite catalyst prepared by the traditional ammonia evaporation method has low catalytic activity for the gas-solid phase hydroamination of caprolactone to produce caprolactam and has poor resistance to sintering deactivation.

Comparative Example 2: This example is used to illustrate that the copper-silicon dioxide catalyst prepared by using amorphous fumed silicon dioxide (white carbon black, BET specific surface area of 286 m ² /g) as a carrier and copper supported by the traditional ammonia evaporation method has poor sintering resistance.

The conventional ammonia distillation method described in this example is based on the method of Example 1 in U.S. Pat. No. 4,440,873 (1984), which is as follows:

(1) 1.14 g of copper nitrate (Cu(NO ₃ ) ₂ .3H ₂ O) was dissolved in 100 ml of water to obtain an aqueous solution containing copper ions, and then 3.6 ml of concentrated ammonia solution (industrial ammonia water with an NH ₃ content of 26 wt.%, density 0.89 g/ml) was added thereto according to a molar ratio of copper ions to ammonia molecules of about 1:10, and 50 ml of water was added to obtain a dark blue copper-ammonia complex solution with a pH value of 11-12 (complex ion concentration of about 30.8 mM. The purpose of adding 50 ml of water was to keep the ratio of solution volume to silica dry basis consistent with the literature);

(2) adding 10 g of fumed silica (dry basis) to the copper ammonia complex solution and stirring at room temperature for 2 h;

(3) heating the reaction mixture of step (2) to evaporate ammonia (80° C., 6 h), and when the pH of the mixture drops to 6-7, filtering to obtain a solid, washing it three times with deionized water to obtain a solid product;

(4) drying the solid product at 120°C for 12 h and calcining at 450°C for 4 h;

(5) The calcined solid product was subjected to hydrogen reduction treatment at 350°C for 2h to obtain a copper-silicon dioxide catalyst, codenamed Cu3-SiO ₂ -CE2.

The evaluation results of the gas-solid phase hydroamination of caprolactone to caprolactam showed that under the same reaction conditions, the caprolactam yield of the Cu3-SiO2- _CE2 catalyst was 77%; the caprolactam yield of its high-temperature calcined sample (550℃×3h) was 69%. The reaction results show that the copper-based catalyst prepared with amorphous silica as a carrier has poor sintering resistance and its catalytic activity decreases significantly after high-temperature treatment.

Comparative Example 3: This example is used to further illustrate that the copper-silicon dioxide catalyst prepared by the traditional ammonia evaporation method with copper loading has poor sintering resistance.

Comparative Example 2 was repeated, but 33.3 g of silica sol (30 wt.% SiO ₂ ) was used as a precursor for in-situ generation of 10 g of silica support. In order to keep the ratio of the volume of the copper-ammine complex solution to the dry basis of silica consistent with that of Comparative Example 2, the amount of water added in step (1) when preparing the copper-ammine complex solution was changed to 26.7 ml. The prepared copper-silicon dioxide catalyst was coded as Cu3-SiO ₂ -CE3

The evaluation results of the gas-solid phase hydroamination of caprolactone to caprolactam showed that under the same reaction conditions, the caprolactam yield of the Cu3-SiO2 _- CE3 catalyst was 80%; the caprolactam yield of its high-temperature calcined sample (550℃×3h) was 68%. The reaction results also show that the copper-based catalyst prepared with amorphous silica as a carrier has poor sintering resistance and its catalytic activity drops significantly after high-temperature treatment.

Comparative Example 4: This example is used to illustrate that the amorphous nature of the silica carrier determines that the copper-silica catalyst has poor sintering resistance.

In this example, the copper-silicon dioxide catalyst was prepared on a fumed silicon dioxide carrier using the improved ammonia evaporation method provided by the present invention. The details are as follows:

Example 1 was repeated, but 10 g of the dealuminated Beta zeolite carrier was replaced with 10 g (dry basis) of fumed silica (white carbon black, BET specific surface area 286 m ² /g, saturated water absorption rate 2.5 ml/g). 10 g of fumed silica required 25 ml of copper ammonia complex solution. The concentration of the required copper ammonia complex solution was calculated to be about 0.19 M based on the copper loading of 3 wt.%. 11.9 ml of the saturated copper ammonia complex solution was diluted to 25 ml with a dilute ammonia base solution to obtain 25 ml of a copper ammonia complex solution with a concentration of 0.19 M. The prepared copper-silica catalyst was codenamed Cu3-SiO ₂ -CE4.

The evaluation results of the gas-solid phase hydroamination of caprolactone to caprolactam showed that under the same reaction conditions, the caprolactam yield of the Cu3-SiO2 _- CE4 catalyst was 78%; the caprolactam yield of its high-temperature calcined sample (550℃×3h) was 69%. The reaction results also show that the copper-based catalyst prepared with amorphous silica as a carrier has poor sintering resistance and its catalytic activity drops significantly after high-temperature treatment.

Example 2: This example is used to illustrate that according to the method for preparing a Cu-Beta zeolite catalyst provided by the present invention, which uses dealuminated Beta zeolite as a carrier and uses an improved ammonia evaporation method to load copper in the zeolite pores, Cu-Beta zeolite catalysts with different metallic copper loadings can be prepared.

Example 1 was repeated, but the copper loading in the prepared Cu-Beta zeolite catalyst was reduced to 1wt.% and 2wt.% respectively, and the concentration of the required copper ammonia complex solution was about 0.13M and 0.26M respectively. 3.9ml and 7.8ml of the saturated copper ammonia complex solution were diluted to 12ml with a dilute ammonia base solution to obtain equal volume impregnation solutions of Cu-Beta zeolite catalysts with copper loadings of 1wt.% and 2wt.%. When the Cu-Beta zeolite catalyst was prepared by the improved ammonia evaporation method, the time for equal volume impregnation treatment of the dealuminated Beta carrier at room temperature was changed to 6h, the temperature and time of ammonia evaporation treatment were changed to 65℃ and 12h respectively, the temperature and time of dehydration and drying were changed to 150℃ and 3h respectively, the subsequent calcination temperature and time were changed to 450℃ and 6h respectively, the final hydrogen reduction temperature and time were changed to 350℃ and 8h respectively, and the hydrogen flow rate (volume space velocity) was changed to 1000h ^-1 . The prepared Cu-Beta zeolite catalysts are coded Cu1-Beta24c-2 and Cu2-Beta24c-2 respectively.

The evaluation results of the gas-solid phase hydroamination of caprolactone to produce caprolactam showed that under the same reaction conditions, the caprolactam yield of Cu1-Beta24c-2 catalyst was 81%; the caprolactam yield of Cu2-Beta24c-2 was 82%.

Example 3: This example is used to further illustrate that according to the method for preparing a Cu-Beta zeolite catalyst provided by the present invention, which uses a dealuminated Beta zeolite as a carrier and uses an improved ammonia distillation method to load copper in the zeolite pores, a Cu-Beta zeolite catalyst with different metallic copper loadings can be prepared. However, when preparing a Cu-Beta zeolite catalyst with a copper loading of more than 3 wt.%, it is preferable to use a multiple loading scheme to prepare the catalyst.

Example 1 was repeated, but the copper loading in the prepared Cu-Beta zeolite catalyst was increased to 4wt.% and 6wt.%, respectively, and the concentrations of the required copper ammonia complex solutions were calculated to be approximately 0.52M and 0.79M, respectively. Obviously, the required copper ammonia complex concentrations all exceeded the concentration of the copper ammonia complex saturated solution prepared with a dilute ammonia base solution. We tried to use industrial ammonia instead of a dilute ammonia base solution to dissolve the copper ammonia complex, and the maximum concentration of the copper ammonia complex solution was approximately 0.5M. Although a high-concentration copper ammonia complex solution with a concentration of approximately 0.5M can be prepared by dissolving the copper ammonia complex with industrial ammonia, the ammonia/copper ion molar ratio of the solution exceeds 24:1, the alkalinity is strong, and the ammonia volatilization is serious, which is not conducive to protecting the crystal structure of the dealuminated Beta zeolite and is not conducive to operation. This shows that it is impossible to obtain a Cu-Beta zeolite catalyst with a high copper loading by a single equal volume impregnation and ammonia evaporation operation on a dealuminated Beta zeolite carrier using the improved ammonia evaporation method. In view of this, the present invention uses a dilute ammonia base solution to dilute a saturated solution of a copper-ammine complex, and prepares a Cu-Beta zeolite catalyst with a copper loading increased to 4wt.% and 6wt.% respectively according to multiple equal volume impregnation and ammonia evaporation operations. For the preparation of a catalyst with a copper loading of 4wt.%, the equal volume impregnation and ammonia evaporation operations can be carried out twice, such as 1wt.%+3wt.%, 2wt.%+2wt.%; for the preparation of a catalyst with a copper loading of 6wt.%, it can be carried out twice (3wt.%+3wt.%), or it can be carried out three times (2wt.%+2wt.%+2wt.%). For simplicity, in this example, the preparation of a Cu-Beta zeolite catalyst with a copper loading increased to 4wt.% and 6wt.% respectively is carried out twice (2wt.%+2wt.%, 3wt.%+3wt.%). To load 2 wt.% of copper metal on a dealuminated Beta zeolite carrier by the improved ammonia evaporation method, 12 ml of an impregnation solution with a copper-ammine complex concentration of 0.26 M is required; to load 3 wt.% of copper metal on a dealuminated Beta zeolite carrier by the improved ammonia evaporation method, 12 ml of an impregnation solution with a copper-ammine complex concentration of 0.40 M is required. The 0.26 M impregnation solution is obtained by diluting 7.8 ml of a saturated copper-ammine complex solution with a dilute ammonia base solution, and the 0.40 M impregnation solution is directly a saturated copper-ammine complex solution. When the dealuminated Beta carrier was treated with equal volume impregnation at room temperature, the impregnation time was changed to 6 hours, the temperature and time of ammonia evaporation were changed to 85°C and 3 hours respectively, the temperature and time of dehydration and drying were changed to 120°C and 5 hours respectively, the subsequent calcination temperature and time were changed to 550°C and 2 hours respectively, the final hydrogen reduction temperature and time were changed to 500°C and 2 hours respectively, and the hydrogen flow rate (volume space velocity) was changed to 20h ^-1 (after mixing with an appropriate amount of nitrogen to control the feed with a mass flow meter). The prepared Cu-Beta zeolite catalysts are coded Cu4-Beta24c-3 and Cu6-Beta24c-3, respectively.

The evaluation results of the gas-solid phase hydroamination of caprolactone to produce caprolactam showed that under the same reaction conditions, the caprolactam yield of Cu4-Beta24C-3 catalyst was 80%; the caprolactam yield of Cu6-Beta24C-3 was 79%.

Example 4: This example is used to illustrate that when preparing Cu-Beta zeolite catalyst according to the improved ammonia distillation method provided by the present invention, it is allowed to use Beta zeolite matrix with different particle sizes to prepare dealuminated Beta zeolite carrier.

Example 1 was repeated, but in the first step of preparing the dealuminated Beta zeolite carrier, a Beta zeolite precursor with a silicon-aluminum oxide molar ratio (molar ratio of _SiO2 to _{Al2O3 )} of 22 was synthesized by itself according to the hydrothermal crystallization method (with ammonium fluoride as an auxiliary agent) provided in the public document J. Mater. Sci. 41 (2006) 1861-1864 as the raw material for preparing the dealuminated Beta zeolite carrier. After conventional filtration, washing, drying (80°C, 24h) and calcination to remove the template agent (600°C, 3h), the synthesized Beta zeolite matrix was observed to have an average grain size of 1μm by TEM, belonging to large-grain Beta zeolite; no impurities were found in the matrix by XRD method, and its BET specific surface area was calculated to be about ^480m2 /g by using its nitrogen physical adsorption data, and its silicon-aluminum oxide molar ratio (the molar ratio of _SiO2 to _{Al2O3 )} was about 23 by XRF method, which met the technical requirements of the present invention for the Beta zeolite matrix. The Beta zeolite matrix was dealuminated with 13M concentrated nitric acid to become a large-grain dealuminated Beta zeolite carrier (codenamed Beta23c), and its silicon-aluminum oxide molar ratio (the molar ratio of _SiO2 to _{Al2O3 )} was 748 (>700), which met the technical requirements of the present invention for the dealuminated Beta zeolite carrier. On this basis, copper was loaded in the zeolite pores by using an improved ammonia evaporation method to produce a Cu-Beta zeolite catalyst with a copper content of 3wt.%, code-named Cu3-Beta23c-4.

The evaluation results of the gas-solid phase hydroamination of caprolactone to produce caprolactam showed that under the same reaction conditions, the caprolactam yield of the Cu3-Beta23c-4 catalyst was 82%.

Example 5: This example is used to illustrate that when preparing Cu-Beta zeolite catalyst according to the improved ammonia distillation method provided by the present invention, it is allowed to use Beta zeolite matrix with different silicon aluminum oxide molar ratios (molar ratio of _SiO2 to _{Al2O3 )} to prepare dealuminated Beta zeolite carrier.

Example 1 was repeated, but in the first step of preparing the dealuminated Beta zeolite carrier, a Beta zeolite matrix with a molar ratio of _SiO2 to _Al2O3 of ₆₀ was synthesized by itself according to the hydrothermal crystallization method provided in U.S. Pat. No. 3,308,069 (1967) as the raw material for preparing the dealuminated Beta zeolite carrier. After the synthesized Beta zeolite matrix was subjected to conventional filtration, washing, drying (170°C, 3h) and calcination to remove the template agent (500°C, 8h), the average grain size of the synthesized Beta zeolite matrix was close to 100 nanometers by TEM observation, belonging to nano Beta zeolite; no impurity crystals were found in the matrix by XRD method, and the BET specific surface area was calculated to be about 530 ^m2 /g by using the nitrogen physical adsorption data, and the molar ratio of _SiO2 to _Al2O3 was about ₅₇ by XRF method, which met the technical requirements of the present invention for the Beta zeolite matrix.

The Beta zeolite matrix was used for dealumination with concentrated nitric acid to prepare a dealumination Beta zeolite carrier, and a dealumination Beta zeolite carrier (Beta57c) with a silicon aluminum oxide molar ratio (molar ratio of _SiO2 to _{Al2O3 )} of 861 (>800) was obtained. The dealumination degree of the carrier meets the requirements of the present invention. On this basis, a Cu-Beta zeolite catalyst with a copper content of 3wt.% was prepared by an improved ammonia evaporation method. Among them, when the carrier was impregnated with an equal volume of a saturated solution of a copper ammonia complex at room temperature, the impregnation time was changed to 2h. When ammonia was evaporated, a slight negative pressure was used for ammonia evaporation, the ammonia evaporation temperature was 50°C, and the ammonia evaporation time was 48h. When the material after ammonia evaporation was dehydrated and dried, the drying temperature and time were changed to 100°C and 48h respectively. The subsequent roasting temperature and time were changed to 350°C and 24h respectively. The final hydrogen reduction treatment temperature, time and hydrogen flow rate (volume space velocity) were changed to 300℃, 20h and 2000h ^-1 respectively. The catalyst code was Cu3-Beta57c-5

The evaluation results of the gas-solid phase hydroamination of caprolactone to produce caprolactam showed that under the same reaction conditions, the caprolactam yield of the Cu3-Beta57c-7 catalyst was 80%.

Example 6: This example is used to further illustrate that when preparing Cu-Beta zeolite catalyst according to the improved ammonia distillation method provided by the present invention, it is allowed to use Beta zeolite matrix with different silicon aluminum oxide molar ratios (molar ratio of _SiO2 to _{Al2O3 )} to prepare dealuminated Beta zeolite carrier.

Example 1 was repeated, but in the first step of preparing the dealuminated Beta zeolite carrier, the Beta zeolite matrix with a molar ratio of _SiO2 to _Al2O3 of 40, 80, 100, 150 and ₂₀₀ was synthesized by itself as the raw material for preparing the dealuminated Beta zeolite carrier according to the hydrothermal crystallization method provided in Chinese invention patent CN1108275C (application date 1999.9.10). After the synthesized Beta zeolite matrix was subjected to conventional filtration, washing, drying (110°C, 12h) and calcination to remove the template agent (540°C, 6h), the average grain size of the Beta zeolite matrix was observed by TEM and was of nanometer level and small grain (less than 1μm) level. As the molar ratio of silicon-aluminum oxide (the molar ratio of SiO ₂ to Al ₂ O ₃ ) increases, the grain size increases; no impurity crystals are found in the ore by XRD inspection, and the BET specific surface areas calculated by nitrogen physical adsorption data are all higher than 500 m ² /g. The molar ratio of silicon-aluminum oxide (the molar ratio of SiO ₂ to Al ₂ O ₃ ) measured by XRF method is 38, 72, 94, 136 and 189, respectively, which meets the technical requirements of the present invention for Beta zeolite matrix.

The above five Beta zeolite precursors were used for dealumination with concentrated nitric acid to prepare dealumination Beta zeolite carriers, and five dealumination Beta zeolite carriers Beta38c, Beta72c, Beta94c, Beta136c and Beta189c were obtained, and the molar ratio of silicon aluminum oxide (the molar ratio of _SiO2 to _{Al2O3 )} was 870, 855, 932, 1088 and 960 respectively, which all met the technical requirements of dealumination Beta zeolite carriers. On this basis, a Cu-Beta zeolite catalyst with a copper content of 3wt.% was prepared by an improved ammonia evaporation method. Among them, when the carrier was impregnated with an equal volume of a saturated solution of a copper ammonia complex at room temperature, the impregnation time was changed to 1h. When ammonia was evaporated, the ammonia evaporation temperature was 90℃ and the ammonia evaporation time was 1h. When the material after ammonia evaporation was dehydrated and dried, the drying temperature and time were changed to 200℃ and 1h respectively. The subsequent calcination temperature and time were changed to 550℃ and 1h respectively. The final hydrogen reduction treatment temperature, time and hydrogen flow rate were changed to 550℃, 1h and 5h ^-1 respectively (the feed was controlled by a mass flow meter after preparing a mixed gas with an appropriate amount of nitrogen). The catalyst codes were Cu3-Beta38c-6, Cu3-Beta72c-6, Cu3-Beta94c-6, Cu3-Beta136c-6 and Cu3-Beta189c-6 respectively.

The anti-sintering deactivation performance of the above catalysts and their samples calcined at 550℃ (3h) was evaluated by the gas-solid phase hydroamination of caprolactone to produce caprolactam. The results showed that under the same reaction conditions, the catalytic activity (caprolactam yield) of Cu3-Beta38c-6, Cu3-Beta72c-6, Cu3-Beta94c-6, Cu3-Beta136c-6 and Cu3-Beta189c-6 catalysts after high-temperature calcination decreased by approximately 5%, 8%, 10%, 14% and 17% respectively.

Example 7: This example is used to illustrate that when preparing Cu-Beta zeolite catalysts according to the improved ammonia distillation method provided by the present invention, Beta zeolite precursors with different silicon-aluminum oxide molar ratios (molar ratios of SiO ₂ to Al ₂ O ₃ ) are allowed to be used to prepare dealuminated Beta zeolite carriers. However, using Beta zeolite precursors with higher silicon-aluminum oxide molar ratios (molar ratios of SiO ₂ to Al ₂ O ₃ ) to prepare dealuminated Beta zeolite carriers is suitable for preparing Cu-Beta zeolite catalysts with lower copper loading.

Example 1 was repeated, but in the first step of preparing the dealuminated Beta zeolite carrier, a Beta zeolite matrix with a molar ratio of _SiO2 to _Al2O3 of ₁₀₀ was synthesized by itself according to the hydrothermal crystallization method provided by Chinese invention patent CN1108275C (application date 1999.9.10) as the raw material for preparing the dealuminated Beta zeolite carrier. After the synthesized Beta zeolite matrix was subjected to conventional filtration, washing, drying (200°C, 3h) and calcination to remove the template agent (500°C, 8h), its average grain size was observed by TEM to be small grain (less than 1μm). No impurities were found in the matrix by XRD method, and its BET specific surface area was _calculated to be higher than ^530m2 /g by using its nitrogen physical adsorption data. Its molar ratio of _SiO2 to _Al2O3 was measured by XRF method to be 94, which met the technical requirements of the present invention for the Beta zeolite matrix.

The Beta zeolite matrix was used for dealumination with concentrated nitric acid to prepare a dealumination Beta zeolite carrier (Beta94c) with a silicon-aluminum oxide molar ratio ( _SiO2 to _Al2O3 molar ratio) of 932 (>900), and the silicon-aluminum oxide molar ratio of the carrier met the technical requirements of the present invention. On this basis, a _Cu -Beta zeolite catalyst with a copper content of 6 wt.% was prepared by using an improved ammonia evaporation method according to the method of two equal volume impregnations and two ammonia evaporations. The concentration of the copper-ammine complex solution used in the two equal volume impregnations was a saturated copper-ammine complex solution of 0.4 M. The prepared catalyst was code-named Cu6-Beta94c-7.

The catalyst and its 550℃ calcined (3h) samples were evaluated for their anti-sintering deactivation performance by gas-solid phase hydroamination of caprolactone to produce caprolactam. The results showed that under the same reaction conditions, the catalytic activity (caprolactam yield) of the Cu6-Beta94c-7 catalyst decreased by 15% after high-temperature calcination.

Claims (10)

1. A method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone, characterized in that the steps are as follows: The first step is to prepare the dealuminated Beta zeolite carrier (1) Select Beta zeolite matrix The Beta zeolite matrix refers to a silicon-aluminum Beta zeolite that meets the following requirements: 1) there are no impurity crystals in the Beta zeolite matrix; 2) the crystallization of the Beta zeolite matrix is good, that is, the BET specific surface area value of the Beta zeolite matrix measured by nitrogen physical adsorption method is ≥ ^450m2 /g; 3) the molar ratio of silicon-aluminum oxide in the Beta zeolite matrix, that is, the molar ratio of _SiO2 to _Al2O3 is in the range of 10-200 _; (2) Preparation of dealuminated Beta zeolite carrier A dealuminated Beta zeolite carrier is prepared by acid dealumination on the basis of Beta zeolite matrix; the molar ratio of silicon-aluminum oxide, i.e., the molar ratio of SiO ₂ to Al ₂ O ₃ of the dealuminated Beta zeolite carrier is required to be ≥ 700; In the second step, copper is loaded in the pores of the dealuminated Beta zeolite carrier by an improved ammonia evaporation method to prepare a Cu-Beta zeolite catalyst. The specific steps are as follows: (1) preparing a dilute ammonia basic solution and a saturated solution of a copper ammonia complex: preparing a dilute ammonia basic solution with a pH value of 11-12 by mixing 4.4 g of industrial ammonia containing 25-28 wt.% _NH3 with 100 ml of deionized water, and sealing and storing it for later use; then, using copper nitrate trihydrate as a soluble copper-containing compound and reacting it with industrial ammonia to synthesize a copper ammonia complex in a molar ratio of copper ions to ammonia molecules of 1:4; finally, dissolving the copper ammonia complex with the dilute ammonia basic solution at room temperature to prepare a saturated solution of the copper ammonia complex, and sealing and storing it for later use; the concentration of copper ammonia ions in the saturated solution of the copper ammonia complex is 0.4 M; (2) Impregnating the zeolite carrier with an equal volume of the copper-ammine complex solution: First, the saturated water absorption rate of the zeolite carrier is measured, and the amount of the copper-ammine complex solution used to impregnate the zeolite carrier with an equal volume is calculated; then, the concentration of the required copper-ammine complex solution is calculated according to the copper loading of the Cu-Beta zeolite catalyst to be prepared; when the calculated concentration is equal to 0.4 M, the zeolite carrier is directly impregnated with an equal volume of the saturated solution of the copper-ammine complex; when the calculated concentration is lower than 0.4 M, the saturated solution of the copper-ammine complex is appropriately diluted with a dilute ammonia base solution. After dilution, the zeolite carrier is impregnated with equal volume; when the calculated value is higher than 0.4M, the concentration of the copper ammonia complex solution for a single equal volume impregnation should be recalculated according to multiple equal volume impregnations, and a copper ammonia complex solution of the required concentration is prepared with a dilute ammonia base solution and a saturated solution of the copper ammonia complex for each equal volume impregnation; after each impregnation, the dealuminated Beta zeolite carrier is subjected to ammonia evaporation; the equal volume impregnation is carried out at room temperature in a closed container; the equal volume impregnation time ranges from 0.5 to 24 hours; (3) Ammonia evaporation: The ammonia evaporation process is carried out under normal pressure or reduced pressure; the ammonia evaporation temperature and time range from 50-100°C and 0.5-48h; (4) Dehydration and drying after ammonia evaporation: the drying temperature and time ranges are 100-200°C and 0.5-48h respectively; (5) Calcination after ammonia evaporation: Calcination is carried out in an air atmosphere, and the calcination temperature and time ranges from 350-650° C. and 0.5-24 h, respectively; after calcination, a catalyst precursor is obtained; (6) Hydrogen reduction treatment of the catalyst precursor: the reduction temperature, time and hydrogen volume space velocity are in the range of 280-600°C, 0.5-20h and 1-2000h ^-1 , respectively; the catalyst precursor becomes a Cu-Beta zeolite catalyst after hydrogen reduction treatment. 2. The method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone according to claim 1, characterized in that: In the first step (1), the molar ratio of silicon to aluminum oxide in the Beta zeolite matrix, that is, the molar ratio of SiO ₂ to Al ₂ O _3, is in the range of 20-100. 3. The method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone according to claim 2, characterized in that: In the first step (1), the molar ratio of silicon to aluminum oxide in the Beta zeolite matrix, that is, the molar ratio of SiO ₂ to Al ₂ O _3, is in the range of 25-60. 4. The method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone according to claim 1, characterized in that: In the first step (2), the molar ratio of silicon aluminum oxide, that is, the molar ratio of SiO ₂ to Al ₂ O ₃ of the prepared dealuminated Beta zeolite carrier is required to be in the range of ≧800. 5. The method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone according to claim 4, characterized in that: In the first step (2), the molar ratio of silicon aluminum oxide, that is, the molar ratio of SiO ₂ to Al ₂ O ₃ of the prepared dealuminated Beta zeolite carrier is required to be in the range of ≧900. 6. The method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone according to claim 1, characterized in that: In the first step (2), the Beta zeolite matrix is subjected to an acid dealumination treatment using a concentrated nitric acid aqueous solution, as follows: 13M concentrated nitric acid is used as dealuminous acid solution, and the amount of acid solution is used according to a liquid-to-solid ratio of 20:1, and the unit of the liquid-to-solid ratio is ml/g; the dealuminous reaction is carried out at 95°C, and the dealuminous reaction time is 20h; after the dealuminous reaction is completed, the solid product is first recovered by solid-liquid separation, and then the solid product is washed with water to a neutral pH value, and then dried at a temperature of 80-200°C for 3-24h, and calcined at a temperature of 500°C-600°C for 3-8h to obtain the dealuminous Beta zeolite. 7. The method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone according to claim 1, characterized in that: In the second step (2), the equal volume immersion time ranges from 1 to 12 h; In the second step (3), the temperature and time of ammonia evaporation are in the range of 60-90°C and 1-24h; In the second step (4), the drying temperature and time ranges from 110-170°C and 1-24h, respectively; In the second step (5), the calcination temperature and time range from 400 to 600°C and 1 to 12 h, respectively; In the second step (6), the reduction temperature, time and hydrogen volume space velocity are in the range of 300-550° C., 1-15 h and 10-1500 h ^-1 , respectively. 8. The method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone according to claim 7, characterized in that: In the second step (2), the equal volume immersion time ranges from 2 to 6 h; In the second step (3), the temperature and time of ammonia evaporation are in the range of 65-85°C and 3-12h; In the second step (4), the drying temperature and time ranged from 120-150°C and 3-12h, respectively; In the second step (5), the calcination temperature and time range from 450-550°C and 2-6h, respectively; In the second step (6), the reduction temperature, time and hydrogen volume space velocity are in the range of 350-500° C., 2-8 h and 20-1000 h ^-1 , respectively. 9. The Cu-Beta zeolite catalyst prepared by the method for preparing a Cu-Beta zeolite catalyst for preparing caprolactam from caprolactone according to any one of claims 1 to 8 is used for catalyzing the gas-solid phase hydroamination of caprolactone to prepare caprolactam. 10. The use according to claim 9, characterized in that the reaction conditions are as follows: The reaction temperature ranges from 120-350°C, the reaction pressure ranges from 0.01-2 atm, the feed space velocity of caprolactone ranges from 0.1-5 h ^-1 , and the molar ratios of ammonia-ester, hydrogen-ester and water-ester range from 1-50, 5-70 and 0-100 respectively.