CN112675830A

CN112675830A - Aldol condensation catalyst, preparation method and method for preparing methyl methacrylate by using aldol condensation catalyst

Info

Publication number: CN112675830A
Application number: CN202110033582.3A
Authority: CN
Inventors: 易光铨; 殷艳欣; 孙康; 纪勇强; 黎源
Original assignee: Wanhua Chemical Group Co Ltd
Current assignee: Wanhua Chemical Group Co Ltd
Priority date: 2021-01-12
Filing date: 2021-01-12
Publication date: 2021-04-20
Anticipated expiration: 2041-01-12
Also published as: CN112675830B

Abstract

The invention provides an aldol condensation catalyst, a preparation method and a method for preparing methyl methacrylate by using the aldol condensation catalyst. The catalyst comprises a modified silicon dioxide carrier and an alkali metal element loaded on the carrier, wherein the modified silicon dioxide carrier is a foam porous structure and has a first range consisting of macropores with the diameter of 10-1000 mu m and a second range consisting of submicron pores with the diameter of 2-500 nm; the thickest part at any position of the carrier is less than 1 mm; the loading amount of alkali metal on the catalyst is 4-9 wt% (calculated by alkali metal elements), and the loading amount of free alkali metal is less than 0.5 wt%; the total loading amount of the modifier is 0.5-3 wt% (calculated by modifier elements). The catalyst provided by the invention has excellent water resistance, carbon deposition resistance and long-period activity stability, and is suitable for industrial application of preparing methyl methacrylate by performing aldol condensation on methyl propionate and formaldehyde.

Description

Aldol condensation catalyst, preparation method and method for preparing methyl methacrylate by using aldol condensation catalyst

Technical Field

The invention belongs to the technical field of catalysts, and particularly relates to an aldol condensation catalyst, a preparation method thereof and a method for preparing methyl methacrylate by performing an aldol condensation reaction on methyl propionate and formaldehyde.

Background

Methyl Methacrylate (MMA) is an important organic chemical raw material, is mainly used for producing polymethyl methacrylate (PMMA) and acrylic resin materials, and is also widely used for manufacturing other resins, plastics, coatings, adhesives, lubricants, wetting agents, glazing agents, printing and dyeing auxiliaries, insulating filling materials and the like.

The process route of preparing methyl propionate by using ethylene, methanol and carbon monoxide as raw materials through oxo synthesis and then preparing methyl methacrylate by aldol condensation reaction with formaldehyde is also called Alpha process, and has the comprehensive advantages of environmental friendliness, safety, mild reaction, low corrosion of devices, low production cost and the like, so that the process has attracted more and more attention. One of the difficulties of the Alpha process is the development of aldol condensation catalysts with high activity and high stability.

The most effective aldol condensation catalyst for methyl propionate and formaldehyde at present is known to be a supported catalyst with silicon dioxide as a main carrier and cesium as a main active component, and the catalyst has the phenomena that the specific surface area is gradually reduced and the strength is gradually weakened along with the prolonging of time in a reaction system, and simultaneously, the problem of rapid reduction of the activity is caused by the accumulation of carbon deposit.

To improve the catalyst performance, researchers have conducted a great deal of work. CN103551148B discloses a water-resistant catalyst for aldol condensation, the main active componentComprises one or more of oxides or salts of Cs, the active auxiliary agent is one or more of oxides or salts of Sb, Nb, Ag, Al and Zr, and the carrier comprises SiO₂And a carrier auxiliary agent, wherein the single service life of the catalyst is over 400 h.

CN106423159B discloses an anti-carbon-deposition aldol condensation catalyst, which is characterized by high activity, good selectivity, strong anti-carbon-deposition capability and high stability by introducing one or more of anti-carbon-deposition auxiliary agents Pt, Rh, Ru, Pd, Ir, Fe, Co and Ni oxides, chlorides, nitrates, carbonates and organic acid salts.

CN104525176B discloses a microspherical catalyst for fluidized bed, which has the characteristic of low attrition, and can realize the cyclic regeneration of the catalyst by combining with the control of reaction process, but it still has the disadvantage that the catalyst strength gradually deteriorates and the final attrition is too high.

Although the methods disclosed in the above patents can improve certain performances of the catalyst to a certain extent, the industrial requirements are still insufficient, and further improvement of the water resistance of the catalyst, reduction of carbon deposition and finally improvement of the service life of the catalyst are required.

Disclosure of Invention

In order to overcome the defects in the prior art, the invention mainly aims to provide an aldol condensation catalyst which has excellent comprehensive properties including water resistance, carbon deposition resistance and long-period activity stability when methyl propionate is catalyzed to react with formaldehyde to prepare methyl methacrylate.

The invention also aims to provide a preparation method of the catalyst, which has simple preparation process, leads the active components of the catalyst to be dispersed and the free harmful components to be selectively removed by controlling the drying and roasting conditions and the post-treatment process of the carrier and the catalyst, and finally leads the catalyst to have excellent structure and catalytic performance.

The invention further aims to provide the application of the catalyst, and the catalyst has the characteristics of high activity and high stability when being used for preparing methyl methacrylate by reacting methyl propionate with formaldehyde, and has industrial application value.

In order to achieve the purpose, the invention adopts the following technical scheme:

the invention provides a catalyst for aldol condensation reaction of methyl propionate and formaldehyde, which comprises a modified silica carrier and an alkali metal element loaded on the carrier, wherein:

1) the support is a foamed porous structure having two different ranges of porosity, including a first range consisting of macropores having a diameter of 10-1000 μm, and a second range consisting of submicron pores having a diameter of 2-500 nm; preferably, the pore volume of the macropores is 0.5-1.5 mL/g, and the pore volume of the sub-micropores is 0.2-1.5 mL/g;

2) the thickness of any part of the carrier is less than 1mm, namely the thickest part does not exceed 1 mm;

3) the alkali metal loading amount is 4-9 wt% (calculated by alkali metal elements), and preferably 5-8 wt%; the free alkali metal loading of the catalyst is <0.5 wt%, preferably <0.2 wt%;

4) the total loading amount of the modifier is 0.5-3 wt% (calculated by modifier elements), and preferably 0.6-2.5 wt%.

The catalyst has a porous structure, and the inventor finds that the catalyst has outstanding performance, particularly anti-carbon performance, when catalyzing the aldol condensation reaction of methyl propionate and formaldehyde under the structure. The catalyst of the invention may be a monolithic catalyst depending on the shape of the silica support, but may also be any other shape, such as spherical, cylindrical, platelet or irregular.

In the catalyst of the present invention, an alkali metal is an active component, and preferably, the alkali metal element is at least any one of sodium, potassium or cesium, that is, any one, any two combinations and a combination of three of sodium, potassium or cesium, and more preferably, one or two combinations of potassium and/or cesium, and still more preferably, cesium. The alkali metal element of the present invention is in a uniformly dispersed state in the catalyst, because the inventors have surprisingly found in the course of the test that a catalyst having a small content of free alkali metal has a surprisingly excellent effect, and therefore it is necessary to control the content of free alkali metal. It is particularly desirable in the present invention to control the free alkali metal loading of the catalyst to be <0.5 wt%, preferably <0.2 wt%.

In the catalyst of the present invention, preferably, the modifier element is selected from at least any two of aluminum, zirconium, bismuth, phosphorus, boron, magnesium, titanium, hafnium, lanthanum, cerium, iron, vanadium, antimony, tungsten, or silver, i.e., may be selected from a combination of any two of the foregoing and a combination of any three or more of the foregoing, more preferably two or more of aluminum, zirconium, bismuth, boron, lanthanum, iron, tungsten, and silver, and still more preferably two or more of aluminum, zirconium, boron, bismuth, and silver. The state of the modifier element in the catalyst according to the invention is preferably its corresponding oxide state, more preferably its oxide state in the highest valence state. The addition of the modifier element can improve the water resistance of the silica carrier, which is crucial to the long-term use of the catalyst.

The catalyst of the invention preferably has a BET specific surface area of 20 to 500m²Preferably 50 to 400 m/g²A more preferable range is 70 to 350 m/g²The average pore diameter of BJH is preferably 4-30 nm, and more preferably 5-25 nm. The fact that the aldol condensation catalyst of the present invention has a suitable specific surface area and pore size is the basic condition for its high activity and high stability.

In a second aspect of the present invention, the preparation method of the aldol condensation catalyst comprises the following steps:

1) preparing a foamed silica support comprising a modifier element;

2) roasting the foamed silicon dioxide carrier prepared in the step 1) to obtain a modified silicon dioxide carrier;

3) dissolving water-soluble salt containing alkali metal elements in water, and soaking the water-soluble salt on a modified silicon dioxide carrier to obtain a catalyst precursor A;

4) drying and roasting the catalyst precursor A to obtain a catalyst precursor B;

5) and washing the catalyst precursor B by using a mixed solution of methanol and methyl propionate to remove free alkali metal elements, thus obtaining the catalyst.

In a preferred embodiment, the process for the preparation of the aldol condensation catalyst comprises the steps of:

1) preparing a foamed silicon dioxide carrier;

2) dissolving water-soluble salt containing modifier elements in water, and soaking the water-soluble salt on a foam silicon dioxide carrier to obtain a modified silicon dioxide precursor;

3) drying and roasting the modified silicon dioxide precursor to obtain a modified silicon dioxide carrier;

4) dissolving water-soluble salt containing alkali metal elements in water, and soaking the water-soluble salt on a modified silicon dioxide carrier to obtain a catalyst precursor A;

5) drying and roasting the catalyst precursor A to obtain a catalyst precursor B;

6) and washing the catalyst precursor B with a mixed solution of methanol and methyl propionate to remove free alkali metal elements, and drying to obtain the catalyst.

The impregnation method in the preparation method of the present invention is not particularly limited, and any impregnation method known to those skilled in the art may be used, for example, an equal volume impregnation method, an excess impregnation method, an ion exchange method, a precipitation impregnation method, etc. may be used. Soluble salts of the desired elements can be impregnated into the catalyst support and then dried, and the above steps can be repeated as many times as necessary to achieve the desired loading. The soluble salt of the present invention is preferably a water-soluble salt thereof, because such a preparation method is most common and convenient based on the impregnation effect.

In the preparation method of the present invention, preferably, the water-soluble salt of the modifier element is selected from a nitrate, a sulfate, a carbonate, a chloride, an ammonium salt or an organic acid salt thereof, and preferably, the nitrate, the ammonium salt or the organic acid salt thereof; the water-soluble salt of the alkali metal element is selected from nitrate, sulfate, carbonate, chloride, organic acid salt or hydroxide thereof, and preferably from nitrate, carbonate, organic acid salt or hydroxide thereof.

According to the preparation method provided by the invention, preferably, the drying temperature of the silicon dioxide precursor and the catalyst precursor A is 60-120 ℃, and the drying time is 2-20 h.

In the preparation method of the invention, preferably, the modified silica carrier is roasted before being impregnated with alkali metal to obtain an oxide corresponding to a modifier element; the roasting atmosphere is preferably an oxygen-containing atmosphere, and is further preferably an air atmosphere; the roasting temperature is determined according to the decomposition temperature of the modifier element precursor, but is preferably less than 600 ℃ so as to prevent the occurrence of large change of a carrier structure or specific surface area, and the roasting time is preferably 1-10 h.

In the preparation method of the invention, preferably, the roasting atmosphere of the catalyst precursor A is N containing 5-20% (mol ratio) of water₂The roasting temperature is 400-600 ℃, and the roasting time is 2-20 h. The highly dispersed alkali metal elements are more tightly bound to the support by the calcination treatment in an aqueous atmosphere, and also selectively expose free alkali metals, which are detrimental to the reaction and need to be selectively removed.

According to the preparation method provided by the invention, preferably, the mass of methanol in the mixed liquid of methanol and methyl propionate for washing the catalyst precursor B accounts for 10-90 wt%, and more preferably 20-70%; the washing is preferably performed at normal temperature. Most of the free alkali metal on the washed catalyst is selectively removed, and the number of acid groups on the surface of the catalyst is not influenced significantly.

In a third aspect of the present invention, there is provided a process for preparing methyl methacrylate by subjecting methyl propionate and formaldehyde to aldol condensation in the presence of a catalyst, wherein a stream containing methyl propionate, formaldehyde and methanol is subjected to a gas phase aldol condensation reaction to obtain methyl methacrylate, and the catalyst is the catalyst described above or the catalyst prepared by the preparation process described above.

In the preparation method of methyl methacrylate, the molar ratio of methyl propionate to formaldehyde is 10: 1-1: 2, the molar ratio of methanol to formaldehyde is 10: 1-1: 2, the reaction temperature is 290-370 ℃, and the pressure is 0.05-1 MPa; the hourly space velocity of the reaction liquid is 0.1-5 h based on the sum of methyl propionate, formaldehyde and methanol^-1。

As a feed stream of the methyl methacrylate production process of the present invention, in addition to methyl propionate, formaldehyde and methanol, it may optionally contain a certain amount of propionic acid, methacrylic acid, water and the like. The propionic acid accounts for less than 2 wt% of the total mass of the raw materials, the methacrylic acid accounts for less than 1 wt% of the total mass of the raw materials, and the water accounts for less than 5 wt% of the total mass of the raw materials. The performance advantages of the catalyst of the invention in the case of use of a catalyst comprising the above-mentioned materials can be further enhanced.

Compared with the prior art, the invention has the beneficial effects that:

1) the catalyst has the characteristics of specific porous structure and thin wall thickness, hot spots are not easy to form in the reaction process, reaction products are quickly desorbed, the chance of further side reaction is reduced, and carbon deposition is reduced.

2) According to the preparation method of the catalyst, in the preparation process, the catalyst precursor is roasted under a certain water vapor condition, free alkali metal with weaker action with the carrier is exposed, then washing is carried out under a non-aqueous system to remove the free alkali metal, the water resistance of the finally obtained catalyst is improved, the side reaction catalyzed by the free alkali metal is avoided, carbon deposition is reduced, and the catalyst has excellent stability due to the combination of the free alkali metal and the free alkali metal.

3) The catalyst is used for preparing methyl methacrylate by performing aldol condensation reaction on methyl propionate and formaldehyde, has a single service life of 800h and an activity retention rate of more than 70 percent, and has a significant industrial application value.

Drawings

FIG. 1 shows mercury intrusion test results for catalysts according to various embodiments of the present invention.

FIG. 2 shows the stability test results of catalysts according to various embodiments of the present invention.

Detailed Description

Embodiments of the present invention are further illustrated by the following figures and examples. The invention is not limited to the embodiments listed but also comprises any other known variations within the scope of the invention as claimed.

1. Method for measuring specific surface area and pore structure information

BET specific surface area, BJH desorption pore volume and pore diameter of carrier and catalyst are measured by N₂The test result is obtained by physical adsorption method, and the model of the test instrument is Micromeritics ASAP 2460. The macroporous information is obtained by mercury intrusion method measurement, and the model of the test instrument is Micromeritics AutoPore IV 9500.

2. Method for measuring content of carbon deposit

The carbon deposition content of the used catalyst is obtained by measuring with a carbon hydrogen nitrogen element analyzer, and the model of the testing instrument is Rieman EA 3000.

3. Determination of free alkali Metal content

The free alkali metal content of the fresh catalyst was determined as follows: 1g of catalyst is taken and dispersed in 10g of deionized water, after the catalyst is soaked for 1h, the catalyst is taken out, 30g of deionized water is used for washing the catalyst for three times, all liquid is collected, and the total amount of alkali metal in the liquid is measured by adopting ICP (inductively coupled plasma), namely the content of free alkali metal in the catalyst.

4. Measurement of catalyst Performance

In the examples and comparative examples, fixed bed reactors were used, the reactor had an inner diameter of 30mm and a height of 600mm, and a temperature measuring sleeve having an outer diameter of 3mm was included in the reactor to monitor the temperature inside the reactor. The loading amount of the catalyst is 60mL, the flow rate of raw materials such as methanol, methyl propionate, formaldehyde and a small amount of water (the molar ratio is 1: 1: 0.3: 0.06) is controlled by a booster pump, the raw materials enter a reactor after being gasified by heat, and the hourly space velocity of the raw material liquid is 3.0h^-1And the reaction temperature is 340 ℃, the pressure is 0.25MPa (gauge pressure), and after about 20 hours of reaction, sampling is carried out to obtain information such as conversion rate, selectivity, yield and the like.

5. Foamed silica preparation

The foamed silica prepared in example 2 of reference patent No. CN201280044614.9 was used as the source of the foamed silica support used in the present invention.

The preparation method comprises the following steps: the precipitated silica of Qingdao Meigao company is dispersed in water, soluble salt of a modification auxiliary agent is added for impregnation, and then polyacrylamide anionic surfactant, glycerol and silica sol are added, and mechanical stirring is carried out until a large amount of wet foam is generated. The wet silica foam was cast in a square mold and dried in an oven at 24 ℃ and 80% relative humidity for 3 days to give a solid. And drying the obtained solid at 80 ℃ for 5h, and then drying at 120 ℃ for 3h to obtain the required foamed silicon dioxide carrier. The foamed silica with different pore structure properties can be obtained by adjusting the pore structure of the raw material silica, the proportion of each raw material (including water, surfactant, glycerol, silica sol) and the like. The physical properties of the prepared foamed silica samples are shown in Table 1 below.

TABLE 1 physical Properties data for different foamed silica supports

The main raw material sources used in the examples or comparative examples of the present invention are:

methanol was purchased from Tianjin Kemiuiou Chemicals, Inc.;

the formaldehyde is obtained by dehydrating and concentrating formalin solution, and the final water content is about 17%;

methyl propionate was purchased from chemical ltd, jon shou de jen;

cesium carbonate, cesium hydroxide, cesium nitrate, zirconium nitrate, aluminum nitrate, potassium nitrate, and the like are all available from shanghai alading biochemical science and technology ltd;

other raw materials are all commercially available materials unless otherwise specified.

Example 1

6.82g of zirconium nitrate, 1.16g of bismuth nitrate, 2.78g of aluminum nitrate and 5g of concentrated nitric acid are dissolved in 90g of deionized water, 100g of foamed silica PSI-1 is added, the mixture is soaked for 12 hours and then dried for 3 hours at 90 ℃, and then dried for 2 hours at 120 ℃ and roasted for 3 hours at 450 ℃, so as to prepare the modified foamed silica carrier.

2.93g of cesium nitrate and 0.646g of potassium nitrate were dissolved in 45g of deionized water, 50g of the above modified silica carrier was added, and the mixture was immersed for 12 hours, then dried at 90 ℃ for 3 hours, and then dried at 120 ℃ for 2 hours, to obtain a catalyst precursor a. Catalyst precursor A is put in 100mL/min N containing 13% of water vapor₂And then, heating to 500 ℃ at the speed of 2 ℃/min, treating for 5h, taking out and cooling to room temperature to obtain a catalyst precursor B. Soaking the catalyst precursor B in 100g of a mixed solution of methanol and methyl propionate (methanol accounts for 40%) for 20h, removing the liquid, and drying at 110 ℃ for 2h to obtain the catalyst.

The catalyst obtained was found to have a free alkali metal content (Cs + K) of 0.17% and a BET specific surface area of 355m²(ii)/g, average pore diameter 15.14 nm; the mercury intrusion test of the catalyst is carried out, the result is shown in figure 1, the test results show that the pore volume of the large pores of the catalyst is 0.66mL/g, and the pore volume of the submicron pores of the carrier is 0.75 mL/g.

The performance and stability tests were performed on the catalysts and the results are shown in figure 2 and table 3.

Carbon deposition determination was performed on the catalyst disassembled for 800h, and the carbon deposition amount was determined to be 6.75%.

Example 2

Heating and dissolving 2.36g of zirconium nitrate, 0.29g of ammonium metavanadate and 0.24g of antimony acetate in 90g of deionized water, adding 100g of foam silica PSI-1, soaking for 12h, drying at 90 ℃ for 3h, drying at 110 ℃ for 10h, and roasting at 600 ℃ for 1h to obtain the modified foam silica carrier.

Dissolving 3.22g of cesium hydroxide and 0.19g of sodium hydroxide in 45g of deionized water, adding 50g of the modified carrier, soaking for 12h, drying at 90 ℃ for 3h, and then drying at 110 ℃ for 10h to obtain a catalyst precursor A. Catalyst precursor A is placed in 200mL/min N containing 5% of water vapor₂And then, heating to 600 ℃ at the speed of 2 ℃/min, treating for 1h, taking out and cooling to room temperature to obtain a catalyst precursor B. Soaking the catalyst precursor B15h in 100g of a mixed solution of methanol and methyl propionate (methanol accounts for 10%), removing the liquid, and drying at 100 ℃ for 2h to obtain the catalyst.

The free alkali metal content (Cs + Na) of the obtained catalyst was determined to be 0.16%BET specific surface area of 361m²G, average pore diameter 14.37 nm.

The catalysts were tested for performance and stability and the results are shown in table 3.

Carbon deposition determination was performed on the catalyst disassembled for 800h, and the carbon deposition amount was determined to be 4.27%.

Example 3

And (3) taking 100g of foam silica PSI-2, and roasting at 300 ℃ for 4h to obtain the modified foam silica carrier.

5.05g of cesium hydroxide is dissolved in 45g of deionized water, 50g of the modified carrier is added, the modified carrier is soaked for 12 hours, then dried at 90 ℃ for 3 hours, and then dried at 120 ℃ for 2 hours, so that a catalyst precursor A is obtained. Catalyst precursor A is put in 200mL/min N containing 6% of water vapor₂And then, heating to 550 ℃ at the speed of 2 ℃/min, treating for 3h, taking out and cooling to room temperature to obtain a catalyst precursor B. Soaking the catalyst precursor B in 100g of a mixed solution of methanol and methyl propionate (methanol accounts for 20%) for 30h, removing the liquid, and drying at 100 ℃ for 2h to obtain the catalyst.

The catalyst obtained by measurement has a free Cs content of 0.09% and a BET specific surface area of 209m²G, average pore diameter 11.35 nm.

Carbon deposition determination was performed on the catalyst disassembled for 800h, and the carbon deposition amount was determined to be 5.68%.

Example 4

Dissolving 0.315g of silver nitrate in 80g of deionized water, adding 100g of foam silica PSI-3, soaking for 12h, drying at 90 ℃ for 3h, drying at 120 ℃ for 2h, and roasting at 450 ℃ for 3h to obtain the modified foam silica carrier.

3.68g of cesium carbonate is dissolved in 50g of deionized water, 50g of the modified carrier is added, the cesium carbonate is soaked for 12 hours, then the cesium carbonate is dried at 90 ℃ for 3 hours, and then the cesium carbonate is dried at 120 ℃ for 2 hours to obtain a catalyst precursor A. Catalyst precursor A is put in 100mL/min N containing 20% of water vapor₂And then, heating to 400 ℃ at the speed of 2 ℃/min, treating for 6h, taking out and cooling to room temperature to obtain a catalyst precursor B. The catalyst precursor B12h was soaked in 80g of a mixture of methanol and methyl propionate (methanol: 70%)And drying the catalyst for 2 hours at 100 ℃ after the liquid is removed to obtain the catalyst.

The content of free Cs in the obtained catalyst is measured to be 0.05 percent, and the BET specific surface area is 310m²G, average pore diameter of 13.64 nm.

Carbon deposition determination was performed on the catalyst disassembled for 800h, and the carbon deposition amount was determined to be 4.92%.

Example 5

2.85g of boric acid, 9.18g of aluminum nitrate and 1.56g of lanthanum nitrate are dissolved in 90g of deionized water, 100g of foam silica PSI-1 is added, the mixture is soaked for 12h and then dried at 90 ℃ for 3h, then dried at 120 ℃ for 2h and roasted at 450 ℃ for 3h, and the modified foam silica carrier is prepared.

5.06g of cesium acetate and 1.26g of potassium acetate are dissolved in 45g of deionized water, 50g of the modified carrier is added, the obtained product is immersed for 12 hours, dried at 90 ℃ for 3 hours and then dried at 120 ℃ for 2 hours, and a catalyst precursor A is obtained. Catalyst precursor A is placed in 300mL/min N containing 10% of water vapor₂And then, heating to 500 ℃ at the speed of 2 ℃/min, treating for 4h, taking out and cooling to room temperature to obtain a catalyst precursor B. Soaking the catalyst precursor B10h in 100g of a mixed solution of methanol and methyl propionate (methanol accounts for 50%), removing the liquid, and drying at 100 ℃ for 2h to obtain the catalyst.

The catalyst obtained was found to have a free alkali metal content (Cs + K) of 0.11% and a BET specific surface area of 342m²G, average pore diameter 7.21 nm.

Carbon deposition determination was performed on the catalyst disassembled for 800h, and the carbon deposition amount was determined to be 7.24%.

Example 6

And (2) dissolving 9.21g of zirconium nitrate, 3.69g of magnesium nitrate and 0.46g of phosphoric acid in 90g of deionized water, adding 100g of foamed silicon dioxide PSI-5, soaking for 12h, drying at 90 ℃ for 3h, drying at 120 ℃ for 2h, and roasting at 400 ℃ for 15h to obtain the modified foamed silicon dioxide carrier.

5.11g of cesium hydroxide and 0.53g of cesium carbonate were dissolved in 45g of deionized water, and added50g of the modified carrier is soaked for 12h, dried for 3h at 90 ℃ and then dried for 2h at 120 ℃ to obtain a catalyst precursor A. Catalyst precursor A is placed in 400mL/min N containing 17% of water vapor₂And then, heating to 400 ℃ at the speed of 2 ℃/min, treating for 15h, taking out and cooling to room temperature to obtain a catalyst precursor B. Soaking the catalyst precursor B10h in 100g of a mixed solution of methanol and methyl propionate (the methanol accounts for 90%), removing the liquid, and drying at 100 ℃ for 2h to obtain the catalyst.

The catalyst obtained by measurement had a free alkali metal content Cs of 0.21% and a BET specific surface area of 184m²G, average pore diameter 11.33 nm.

Carbon deposition determination was performed on the catalyst disassembled for 800h, and the carbon deposition amount was determined to be 5.82%.

Comparative example 1

A catalyst was prepared with reference to example 1, except that a carrier of a general structure was used, and the obtained catalyst was not subjected to a washing step with a mixed solution of methanol and methyl propionate.

6.82g of zirconium nitrate, 1.16g of bismuth nitrate, 2.78g of aluminum nitrate and 5g of concentrated nitric acid are dissolved in 95g of deionized water, 100g of Qingdaomei high silica carrier (particles of 8-16 meshes) is added, the obtained product is immersed for 12 hours, dried at 90 ℃ for 3 hours, dried at 120 ℃ for 2 hours and roasted at 450 ℃ for 3 hours, and the modified silica carrier is prepared.

2.93g of cesium nitrate and 0.646g of potassium nitrate were dissolved in 47g of deionized water, 50g of the above modified silica carrier was added, and the mixture was immersed for 12 hours, then dried at 90 ℃ for 3 hours, and then dried at 120 ℃ for 2 hours, to obtain a catalyst precursor a. Catalyst precursor A is put in 100mL/min N containing 13% of water vapor₂Then, the temperature is raised to 500 ℃ at the speed of 2 ℃/min, and the catalyst is obtained after being taken out and cooled to room temperature after being treated for 5 hours.

The catalyst obtained was found to have a free alkali metal content (Cs + K) of 0.83% and a BET specific surface area of 372m²(ii)/g, average pore diameter 12.71 nm; the mercury intrusion test of the catalyst is carried out, the result is shown in figure 1, the test results show that the pore volume of the macropores of the catalyst is almost 0mL/g, and the pore volume of the submicron pores of the carrier is 0.77 mL/g.

Comparative example 2

A catalyst was prepared with reference to example 1 except that only a support of a general structure was used.

The other steps are exactly the same as in example 1.

The catalyst obtained by measurement had a free alkali metal content (Cs + K) of 0.29% and a BET specific surface area of 380m²G, average pore diameter of 12.82 nm.

Carbon deposition determination was performed on the catalyst disassembled for 800h, and the carbon deposition amount was determined to be 23.28%. Comparative example 3

The catalyst was prepared with reference to example 1, except that after obtaining the catalyst precursor B, the washing step of the mixed solution of methanol and methyl propionate was eliminated.

The catalyst obtained was found to have a free alkali metal content (Cs + K) of 0.69% and a BET specific surface area of 343m²In terms of/g, the mean pore diameter is 15.09 nm. The performance and stability tests were performed on the catalysts and the results are shown in figure 2 and table 3.

Carbon deposition was measured on the 800h dismantled catalyst and the carbon deposition was found to be 25.41%.

Comparative example 4

The catalyst was prepared according to the formulation of example 4 except that foamed silica PSI-4 was used as the carrier, and the specific procedure was as follows.

3.11g of zirconium nitrate, 9.18g of aluminum nitrate and 0.315g of silver nitrate are dissolved in 80g of deionized water, 100g of foam silica PSI-4 is added, the mixture is soaked for 12 hours and then dried at 90 ℃ for 3 hours, then dried at 120 ℃ for 2 hours and roasted at 450 ℃ for 3 hours, and the modified foam silica carrier is prepared.

3.68g of cesium carbonate is dissolved in 50g of deionized water, 50g of the modified carrier is added, the cesium carbonate is soaked for 12 hours, then the cesium carbonate is dried at 90 ℃ for 3 hours, and then the cesium carbonate is dried at 120 ℃ for 2 hours to obtain a catalyst precursor A. Catalyst precursor A is put in 100mL/min N containing 20% of water vapor₂And then, heating to 400 ℃ at the speed of 2 ℃/min, treating for 6h, taking out and cooling to room temperature to obtain a catalyst precursor B. The catalyst precursor B12h was soaked in 80g of a mixture of methanol and methyl propionate (methanol accounts for 70%), and after removal of the liquid, dried at 100 ℃ for 2 hours to obtain the catalyst.

The catalyst obtained by measurement has a free Cs content of 0.19% and a BET specific surface area of 290m²G, average pore diameter of 12.41 nm.

Carbon deposition determination was performed on the 800h disassembled catalyst and the carbon deposition was found to be 19.84%.

Comparative example 5

A catalyst was prepared with reference to example 5, except that the calcination atmosphere of the catalyst precursor a was air.

The catalyst was determined to have a free alkali metal content (Cs + K) of 0.54% and a BET specific surface area of 336m²G, average pore diameter of 7.03 nm.

TABLE 2 preparation and physical Properties of different examples/comparative examples catalysts

TABLE 3 Performance test data for different examples/comparative examples

By comparing the example 1 with the comparative example 1 and the comparative example 2, the catalyst prepared by the common carrier is easy to deposit carbon, and the activity is reduced quickly after long-term use; compared with the comparative example 3, the catalyst which is not washed by the mixed solution of methanol and methyl propionate has high content of free alkali metal, so that the catalyst is more prone to carbon deposition and the activity is reduced too fast in long-term use; comparing example 4 with comparative example 4, it can be seen that the thickness of the catalyst exceeds 1mm, and the carbon deposit amount after 800h use is increased from 4.92% to 19.84%; and the wall thickness of the comparative example 4 is larger, the free alkali metal loading capacity is still higher than that of the example 4 after the same mixed solution of methanol and methyl propionate is adopted for washing, and the overall performance of the catalyst is lower than that of the example 4. Comparing example 5 with comparative example 5, it can be seen that the catalyst is calcined by air without water, the content of free alkali metal is too high after being washed by the same methanol and methyl propionate mixed solution, the structure of the catalyst collapses after long-term use, the pressure drop of the bed layer is too large, and the catalyst cannot continue to operate.

The aldol condensation catalyst prepared by the preparation method of the catalyst has a pore structure with a specific size, a thickness not more than 1mm and loading amounts of alkali metal and a modifier, particularly the loading amount of free alkali metal is less than 0.5 wt%, so that the catalyst has excellent long-period activity stability, good carbon deposition resistance and water resistance, and can be suitable for industrial application of methyl propionate and formaldehyde to prepare methyl methacrylate through aldol condensation.

It will be appreciated by those skilled in the art that modifications or adaptations to the invention may be made in light of the teachings of the present specification. Such modifications or adaptations are intended to be within the scope of the present invention as defined in the claims.

Claims

1. An aldol condensation catalyst comprising a modified silica carrier, and an alkali metal element supported on the carrier, characterized in that:

1) the support is a foamed porous structure having two different ranges of porosity, including a first range consisting of macropores having a diameter of 10-1000 μm, and a second range consisting of submicron pores having a diameter of 2-500 nm;

2) the thickness of any part of the carrier is less than 1 mm;

3) the alkali metal loading is 4-9 wt% (calculated by alkali metal elements), and the free alkali metal loading is less than 0.5 wt%;

4) the total loading amount of the modifier is 0.5-3 wt% (based on modifier elements).

2. The aldol condensation catalyst according to claim 1, characterized in that the alkali metal element is at least any one of sodium, potassium or cesium, preferably potassium and/or cesium; more preferably, the alkali metal loading is 5-8% (calculated as alkali metal element) and the free alkali metal loading is <0.2 wt%.

3. The aldol condensation catalyst according to claim 1, wherein the modifier is selected from at least any two of aluminium, zirconium, bismuth, phosphorus, boron, magnesium, titanium, hafnium, lanthanum, cerium, iron, vanadium, antimony, tungsten or silver, preferably any two or more of aluminium, zirconium, bismuth, boron, lanthanum, iron, tungsten or silver; more preferably, the total loading amount of the modifier is 0.6-2.5 wt% (calculated by modifier elements).

4. The aldol condensation catalyst according to any one of claims 1 to 3, wherein the macropore has a pore volume of 0.5 to 1.5mL/g, and the submicron pores has a pore volume of 0.2 to 1.5 mL/g; preferably, the BET specific surface area of the aldol condensation catalyst is 20-500 m²/g。

5. A process for preparing an aldol condensation catalyst according to any one of claims 1 to 4, comprising the steps of:

1) preparing a foamed silica support comprising a modifier element;

6. The method for preparing an aldol condensation catalyst according to claim 5, wherein the step 1) is a method for preparing a foamed silica support; and dissolving the water-soluble salt containing the modifier element in water, and soaking the water-soluble salt on the foamed silicon dioxide carrier to obtain the foamed silicon dioxide carrier containing the modifier element.

7. The method for producing an aldol condensation catalyst according to claim 5 or 6, characterized in that the water-soluble salt of the modifier element is selected from a nitrate, a sulfate, a carbonate, a chloride or an organic acid salt of the modifier; the water-soluble salt of the alkali metal element is selected from nitrate, sulfate, carbonate, chloride, organic acid salt or hydroxide of the alkali metal element.

8. The preparation method of the aldol condensation catalyst according to claim 5 or 6, wherein the drying temperature of the catalyst precursor A in the step 4) is 60 to 120 ℃, and the drying time is 2 to 20 hours; the roasting atmosphere is N containing 5-20% (mol ratio) of water₂The roasting temperature is 400-600 ℃, and the roasting time is 2-20 h.

9. The method for preparing an aldol condensation catalyst according to claim 5 or 6, wherein the mass of methanol in the mixed solution of methanol and methyl propionate for washing the catalyst precursor 2 in step 5) is 10 to 90 wt%.

10. A process for preparing methyl methacrylate by aldol condensation of methyl propionate and formaldehyde is characterized in that a material flow containing methyl propionate, formaldehyde and methanol is catalyzedPreparing methyl methacrylate by gas-phase aldol condensation in the presence of a catalyst, wherein the catalyst is the catalyst according to any one of claims 1 to 4 or the catalyst prepared by the preparation method according to any one of claims 5 to 9; preferably, the molar ratio of the methyl propionate to the formaldehyde is 10: 1-1: 2, the molar ratio of the methanol to the formaldehyde is 10: 1-1: 2, the reaction temperature is 290-370 ℃, and the pressure is 0.05-1 MPa; the hourly space velocity of the reaction liquid is 0.1-5 h based on the sum of methyl propionate, formaldehyde and methanol^-1。