CN110856819B - Surface aluminum-rich molecular sieve, preparation method and application thereof, isomerization reaction catalyst and application thereof - Google Patents

Abstract

The invention relates to the field of molecular sieves, and discloses a method for preparing a surface aluminum-rich molecular sieve. The method prepares two molecular sieve precursor gels with different silicon-alumina ratios, and controls the crystallization conditions so that the two molecular sieve precursor gels are respectively subjected to the first step of crystallization. and mixing the first-step crystallization products according to a predetermined ratio to carry out the second-step crystallization to finally obtain a surface aluminum-rich molecular sieve. The obtained molecular sieve not only has high crystallinity, but also has effective acid centers concentrated on the outer surface, showing high catalytic activity and high catalytic selectivity in the isomerization catalytic reaction of long-chain alkanes.

Description

Surface aluminum-rich molecular sieve, preparation method and application, isomerization reaction catalyst and application thereof

technical field

The invention relates to the field of molecular sieves, in particular to a surface aluminum-rich molecular sieve, a preparation method and application, an isomerization reaction catalyst and application thereof.

Background technique

Molecular sieves are often used as catalysts or catalyst carriers due to their pore structure. Among them, ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-48 and SSZ-32 molecular sieves are used more frequently. ZSM-48 is a kind of high-silica zeolite, which belongs to the orthorhombic structure. It has 10-membered pore openings without interlaced linear channels. The pores are connected by 5-membered rings, and the pore diameter is 0.53nm×0.56nm. Generally, pure silica ZSM-48 molecular sieve has good selectivity of light olefins in the synthesis gas to olefin reaction, while ZSM-48 molecular sieve with low silicon to aluminum ratio (SiO ₂ /Al ₂ O ₃ ) has good isomerization Cracking catalytic performance.

"Synthesis and Characterization of High Silica Zeolite ZSM-48" (Natural Gas Chemical Industry, 1993, 18(1): 8-12) discloses the preparation of high silica by hydrothermal crystallization and using 1,6-hexanediamine as a structure directing agent The method for zeolite ZSM-48, the raw materials used are silica sol, NaOH, 1,6-hexanediamine and deionized water. The reactants were added to a 100 mL stainless steel high-pressure synthesis kettle in a certain proportion and order, stirred for 10-15 minutes, sealed and crystallized at 165-185 ° C until the crystallization was completed, and the crystallization time was selected according to the temperature. After separation, washing and drying, the original powder of NaZSM-48 is obtained, but the synthesis and application of aluminum-containing ZSM-48 are not involved in the text. Compared with ZSM-48 molecular sieve, the synthesized high-silica-alumina has too large and aggregated crystallites, large crystallite size and poor catalyst performance; high water content and low single-pot yield.

"Synthesis and Characterization of ZSM-48 Molecular Sieve with Low Silicon-Aluminum Ratio" (Modern Chemical Industry, 2014, 34, 3:97-102) discloses a synthesis method of ZSM-48 molecular sieve with low silicon-aluminum ratio. Through orthogonal experimental design, the influence of various factors on the synthesis of ZSM-48 was studied. By changing the amount of template agent, silicon source, alkali source and water to optimize the synthesis system of ZSM-48 molecular sieve with low silicon-aluminum ratio, silicon-alumina was finally obtained. The ZSM-48 molecular sieve with a ratio as low as 56.7, however, the test results show that the low silica-alumina ratio ZSM-48 is a rod-like and flake-like agglomerated form with a lower relative crystallinity.

The applicant found that the high-silicon ZSM-48 molecular sieve has too low activity in isomerization catalysis, while the low-silicon-aluminum ratio ZSM-48 molecular sieve improves the catalytic activity, but deteriorates the isomerization selectivity. At present, many synthetic methods of ZSM-48 molecular sieves have been reported in the literature, but none of the obtained molecular sieves can achieve the co-optimization of catalytic activity and isomeric selectivity, that is, to obtain higher catalytic activity and ensure higher selectivity.

The applicant has also found that during the isomerization of long-chain alkanes, the content of framework aluminum on the molecular sieve, corresponding to the acidity of the framework, determines the catalytic activity. On the premise that the metal function remains unchanged, increasing the acid function (low silicon-aluminum ratio) is beneficial to improve the conversion rate of raw materials, but is not conducive to the isomerization rate; although reducing the acid function (high silicon-aluminum ratio) can reduce unwanted cracking reactions , but at the same time, the conversion rate of raw materials is also declining, and the catalytic activity is reduced. Molecular sieves synthesized by conventional methods usually result in an inability to balance activity and selectivity. This is mainly because the distribution of acid centers of the molecular sieve synthesized by one crystallization is unreasonable, and the inner surface of the molecular sieve contains aluminum acid sites, which is easy to cause further cracking and loss of isomer products. .

Therefore, there is an urgent need for a method for preparing a molecular sieve. The surface of the obtained molecular sieve has sufficient aluminum acid sites, and the inner surface does not contain or contains less framework aluminum acid sites, so that the surface catalytic activity can be ensured under the premise of, Side reactions caused by framework acid sites contained on the inner surface of the molecular sieve can be avoided or reduced.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the problem that high activity and high isomerization selectivity cannot be obtained simultaneously when the molecular sieve existing in the prior art is used for the isomerization of long-chain alkanes, and provides a surface aluminum-rich molecular sieve and its preparation method, application and isomerization Reaction catalysts and their applications.

In order to achieve the above object, a first aspect of the present invention provides a method for preparing a surface aluminum-rich molecular sieve, the method comprising:

(1) Precursor gels A and B are prepared, the silicon-alumina ratio of the precursor gel A is 40-100, and the silicon-aluminum ratio of the precursor gel B is 100-400, and the silicon-alumina ratios of the two are different;

(2) carrying out the first-step crystallization of the precursor gels A and B respectively;

(3) The first-step crystallization products obtained from the precursor gels A and B, respectively, are mixed in a mass ratio of 1:(3-50) to carry out the second-step crystallization, and finally the surface aluminum-rich molecular sieve is obtained.

The second aspect of the present invention provides a surface aluminum-rich molecular sieve prepared by the method described in the first aspect of the present invention.

The third aspect of the present invention provides the use of the molecular sieve described in the second aspect of the present invention in a catalytic reaction.

A fourth aspect of the present invention provides an isomerization reaction catalyst, the catalyst comprising a binder, a metal active component and the molecular sieve described in the second aspect of the present invention.

The fifth aspect of the present invention provides the application of the isomerization reaction catalyst described in the fourth aspect of the present invention in alkane isomerization reaction.

In the present invention, two parts of molecular sieve precursor gels with different silicon-aluminum ratios are prepared, and the silicon-aluminum ratios are both in the range of 40-400, and the above-mentioned molecular sieve precursor gels are respectively crystallized for a period of time to obtain semi-crystallization through fractional crystallization. Or a completely crystallized product, and then mixed in a certain proportion and crystallized again for a period of time to obtain a molecular sieve with a gradient distribution of aluminum elements from outside to inside with controllable acid distribution.

The method of the invention can be used for the synthesis of ZSM-48 molecular sieves, and can also be used for the synthesis of ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-48 and SSZ-32 molecular sieves, and the obtained molecular sieves not only crystallize It has a high degree of concentration, and the effective acid centers are concentrated on the outer surface, which avoids the adverse effects brought by the acid centers on the inner surface, and can have higher isomer selectivity while maintaining high catalytic activity. The surface aluminum-rich molecular sieve of the invention has good application prospects in the fields of synthesis gas preparation of low-carbon olefins, normal paraffin isomerization and wax oil hydroisomerization preparation of lubricating oil base oil and the like.

Description of drawings

FIG. 1 is a transmission electron microscope (TEM) section analysis diagram of the molecular sieve described in Example 2. FIG.

FIG. 2 is the silicon-alumina ratio within the thickness range of 200 nm on the surface of the molecular sieve described in Example 2, measured by TEM section analysis.

Detailed ways

The endpoints of ranges and any values disclosed herein are not limited to the precise ranges or values, which are to be understood to encompass values proximate to those ranges or values. For ranges of values, the endpoints of each range, the endpoints of each range and the individual point values, and the individual point values can be combined with each other to yield one or more new ranges of values that Ranges should be considered as specifically disclosed herein.

The present invention provides a first aspect of the present invention provides a method for preparing a surface aluminum-rich molecular sieve, the method comprising:

(1) Precursor gels A and B are prepared, the silicon-alumina ratio of the precursor gel A is 40-100, and the silicon-aluminum ratio of the precursor gel B is 100-400, and the silicon-alumina ratios of the two are different;

(2) carrying out the first-step crystallization of the precursor gels A and B respectively;

(3) The first-step crystallization products obtained from the precursor gels A and B, respectively, are mixed in a mass ratio of 1:(3-50) to carry out the second-step crystallization, and finally the surface aluminum-rich molecular sieve is obtained.

Herein, the silicon-alumina ratio refers to the molar ratio of silicon oxide to aluminum oxide.

In the present invention, the silicon-alumina ratio of the precursor gel A may be 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 and 100 and any two of the above point values Any value in the constituted range, preferably, the silicon-alumina ratio of the precursor gel A is 50-100.

In the present invention, the silicon-alumina ratio of the precursor gel B can be 105, 110, 120, 150, 170, 200, 230, 250, 270, 300, 320, 350, 380 and 400 and any of the above point values Any value in the two constituted ranges, preferably, the silicon-alumina ratio of the precursor gel B is 150-350, more preferably 150-300.

In the present invention, in step (1), the precursor gels A and B are prepared by a template agent, a silicon source, an aluminum source, water and sodium hydroxide, respectively.

In the present invention, in the precursor gel A, the molar ratio of the silicon source, aluminum source, water, alkali and template agent is 1:(0.01-0.025):(10-40):(0.11-0.19) : (0.0175-0.0215), preferably 1: (0.015-0.025): (20-40): (0.14-0.19): (0.0195-0.0215), more preferably 1: (0.015-0.02): (20-30 ): (0.15-0.17): (0.0195-0.0215). Here, the silicon source is calculated as SiO ₂ , the aluminum source is calculated as Al ₂ O ₃ , and the base is calculated as OH ⁻ .

In the precursor gel B, the molar ratio of the silicon source, aluminum source, water, alkali and template agent is 1:(0.0025-0.01):(10-40):(0.11-0.19):(0.0175- 0.0215), preferably 1:(0.0025-0.008):(10-20):(0.11-0.15):(0.0175-0.0195), more preferably 1:(0.0035-0.005):(10-20):(0.13 -0.15): (0.0175-0.0195). Here, the silicon source is calculated as SiO ₂ , the aluminum source is calculated as Al ₂ O ₃ , and the base is calculated as OH ⁻ .

In the present invention, the silicon source and aluminum source used can be selected according to the prior art, for example, the silicon source can be silica sol, solid silica gel, sodium silicate, tetraethyl silicate, etc., and the aluminum source can be For sodium metaaluminate, aluminum sulfate, aluminum chloride, aluminum sol and so on. According to a preferred embodiment, the silicon source is silica sol, and the aluminum source is sodium metaaluminate. The content of silicon oxide in the silica sol can be selected according to actual needs, for example, 20-40 wt %.

In the present invention, the base can be selected according to the prior art. Preferably, the base is an alkali metal and/or alkaline earth metal hydroxide, such as at least one of sodium hydroxide, potassium hydroxide and calcium hydroxide.

In the present invention, the template agent can be selected according to the molecular sieves to be prepared. In one embodiment of the present invention, the template agent is selected from at least one of 1,6-hexanediamine, 1,8-octanediamine and hexamethylammonium bromide, and the molecular sieve prepared by using the template agent is ZSM-48 molecular sieve.

In the present invention, in step (2), the first crystallization process of the precursor gel A makes it crystallize for a period of time to obtain a gel state in an amorphous state before the crystal phase is formed, so it can be The actual operating conditions adjust the crystallization conditions, for example, the conditions for the first crystallization of the precursor gel A include: the temperature is 150-190°C, preferably 150-160°C; the time is 3-120h, preferably 4- 24h, more preferably 8-16h.

In the present invention, in step (2), the first crystallization process of the precursor gel B makes it crystallize for a period of time to obtain the gel state of the crystalline phase, so the crystallization can be adjusted according to the actual operating conditions. Conditions, for example, the conditions for the first crystallization of the precursor gel B include: the temperature is 150-190°C, preferably 150-160°C; the time is 3-120h, preferably 12-48h, more preferably 16- 32h.

In the present invention, in step (3), the first-step crystallization products of the precursor gels A and B are mixed in a certain mass ratio to carry out the second-step crystallization. The mixing ratio of the first-step crystallization products of the precursor gels A and B can be adjusted according to the silicon-aluminum ratio of both A and B, the conditions of the first-step crystallization, etc., for example, it can be 1:3, 1:5, 1:10, 1:15, 1:20, 1:25, 1:30, 1:35, 1:40, 1:45 and 1:50, preferably, the first The mass ratio of the step-crystallized product is 1:(5-30); more preferably, it is 1:(5-20).

In the present invention, after the first-step crystallization products of the precursor gels A and B are mixed according to the above ratio, the second-step crystallization is performed. The conditions of the second step of crystallization include: the temperature is 150-190°C, preferably 160-170°C; the time is 6-48h, preferably 8-12h.

In the present invention, the first-step crystallization and the second-step crystallization are carried out in a reactor, such as a reaction kettle or a crystallization kettle. The first-step crystallization and the second-step crystallization may be selected from dynamic crystallization or static crystallization, respectively. In the present invention, the static crystallization means that there is no stirring process or a mixing process similar to stirring in the system during the crystallization process; the dynamic crystallization means that there is always stirring in the system during the crystallization process process or a mixing process similar to stirring.

In the present invention, preferably, the first step of crystallization of the precursor gels A and B is dynamic crystallization. Moreover, the second-step crystallization of the mixture of the first-step crystallization products of the precursor gels A and B is static crystallization. During the first-step dynamic crystallization process of the precursor gels A and B, the stirring speed can be adjusted as required, for example, 100-1000 rpm.

In the present invention, after the second step of crystallization, the obtained product is separated, washed, dried, ammonium crossed and calcined to obtain a molecular sieve with rich surface aluminum. The specific operations of the separation and washing, drying, ammonium transfer and roasting can be selected according to the prior art, for example, vacuum filtration or centrifugal washing is adopted for separation and washing, until the pH of the filtrate drops below 10; The ammonium nitrate solution was stirred at 80 °C for 1 h, filtered and washed, and repeated 2-3 times; the calcination was carried out at 550 °C for 4 h in an air atmosphere.

The second aspect of the present invention provides a surface aluminum-rich molecular sieve prepared by the method of the first aspect of the present invention, and the molecular sieve can be ZSM-5, ZSM-12, ZSM-22, ZSM-23, ZSM-48 or SSZ-32 Any of the molecular sieves.

According to one embodiment, the ZSM-48 molecular sieve prepared by the method, the overall silicon-alumina ratio of the molecular sieve is 150-300, and the silicon-alumina ratio within 50 nm of the particle surface of the molecular sieve is 40-150, preferably 40-90. Here, the "within 50 nm thickness of the particle surface" should be understood as a thickness range of 50 nm from the particle surface.

According to one embodiment, the molecular sieve prepared by the method has a specific surface area of not less than 200 m ² /g, preferably 200-1000 m ² /g; a pore size of 0.53-0.56 nm; a pore volume of not less than 0.1 cm ³ /g, preferably 0.1-1 cm ³ /g.

In the present invention, the molecular sieve prepared by the method has high crystallinity, and the crystallinity is more than 98%.

The third aspect of the present invention provides the application of the molecular sieve described in the second aspect of the present invention in a catalytic reaction.

The fourth aspect of the present invention provides an isomerization reaction catalyst, the catalyst comprising the molecular sieve described in the second aspect of the present invention.

In an embodiment of the present invention, the catalyst further contains a binder and a metal active component, and the metal active component can be selected according to the prior art, for example, one selected from Pt, Pd and Ni Or more, the binder can also be selected according to the prior art, for example, it can be alumina or silica and the like.

In the present invention, the isomerization reaction catalyst comprises the molecular sieve prepared by the method described in the first aspect of the present invention, the molecular sieve has a relatively high degree of crystallinity (above 98%), and the distribution of aluminum elements decreases gradient from outside to inside, Such a structure and composition make it possible to obtain both high catalytic activity and high isomerization selectivity in the isomerization reaction of long-chain alkanes (10 or more carbon atoms, such as n-dodecane or n-hexadecane).

The fifth aspect of the present invention provides the application of the isomerization reaction catalyst described in the fourth aspect of the present invention in alkane isomerization reaction.

The present invention will be described in detail below by means of examples.

Example 1

(1) 7.78g of hexamethylammonium bromide, 3.28g of sodium metaaluminate, 6g of sodium hydroxide were dissolved in 540g of water to form a solution; 200g of silica sol (concentration of 30wt%) was slowly stirred under stirring Slowly add to the solution of the above system and stir evenly to prepare molecular sieve precursor gel A, wherein the molar ratio in terms of SiO ₂ : Al ₂ O ₃ : H ₂ O: OH ⁻ : hexamethylammonium bromide is 1:0.02:30 :0.15:0.0215;

(2) 6.34g of hexamethylammonium bromide, 0.82g of sodium metaaluminate, 5.2g of sodium hydroxide were dissolved in 270g of water to form a solution; 200g of silica sol (concentration of 30wt%) was stirred under stirring Slowly add to the solution of the above system and stir evenly to prepare molecular sieve precursor gel B, wherein the molar ratio in terms of SiO ₂ : Al ₂ O ₃ : H ₂ O: OH ⁻ : hexamethylammonium bromide is 1:0.005: 15:0.13:0.0175;

(3) Put the molecular sieve precursor gels A and B into different reaction kettles, seal them, and perform the first step of crystallization for A at 150°C and a rotational speed of 500 rpm for 12 hours; B at 160°C and a rotational speed of 500 rpm The first step of crystallization is carried out, and the crystallization is carried out for 24h;

(4) cooling the above-mentioned reaction kettle to below 100°C, then mixing the first-step crystallization products of A and B according to a mass ratio of 1:5, and stirring them evenly, and performing the second-step crystallization at 170°C, After static crystallization for 12 hours, the obtained product was filtered, washed, dried, and ammonium crossed, and calcined at 550° C. for 4 hours to obtain molecular sieves with a crystallinity of 98%, denoted as A1.

Example 2

The molecular sieve was prepared with reference to the method described in Example 1. The difference was that the first-step crystallization products of A and B were mixed according to the mass ratio of 1:10, and then the second-step crystallization was carried out to finally obtain molecular sieve. 99%, denoted as A2.

Example 3

The molecular sieve was prepared with reference to the method described in Example 1. The difference was that the first-step crystallization products of A and B were mixed in a ratio of 1:20 by mass, and then the second-step crystallization was performed to finally obtain molecular sieves with a degree of crystallinity. 98%, denoted as A3.

Example 4

The molecular sieve was prepared with reference to the method described in Example 1. The difference was that the first-step crystallization products of A and B were mixed according to the mass ratio of 1:30, and then the second-step crystallization was carried out to finally obtain molecular sieve. 98%, denoted as A4.

Example 5

The molecular sieve was prepared with reference to the method described in Example 1. The difference was that the first-step crystallization products of A and B were mixed according to the mass ratio of 1:50, and then the second-step crystallization was carried out to finally obtain molecular sieve. 98%, denoted as A5.

Example 6

The molecular sieve was prepared with reference to the method described in Example 1. The difference was that in step (4), A was subjected to the first step of crystallization at 150° C. and a rotational speed of 500 rpm for 24 hours, and the molecular sieve was finally obtained with a crystallinity of 98%. Record it as A6.

Comparative Example 1

The molecular sieve was prepared by referring to the method described in Example 1. The difference was that the first-step crystallization products of A and B were mixed in a mass ratio of 1:80 to obtain a molecular sieve with a crystallinity of 96%, denoted as D1.

Comparative Example 2

Molecular sieves were prepared by referring to the method described in Example 1. The difference was that the first-step crystallization products of A and B were respectively crystallized for 12 h according to the second-step crystallization conditions described in Example 1; According to the mass ratio of 1:5, molecular sieve is finally obtained, and the crystallinity is 91%, which is recorded as D2.

Example 7

(1) 7.78g of hexamethylammonium bromide, 1.64g of sodium metaaluminate, and 6g of sodium hydroxide were dissolved in 540g of water to form a solution; 200g of silica sol (concentration of 30wt%) was slowly stirred under stirring Slowly add to the solution of the above system and stir evenly to prepare molecular sieve precursor gel A, wherein the molar ratio in terms of SiO ₂ : Al ₂ O ₃ : H ₂ O: OH ⁻ : hexamethylammonium bromide is 1:0.01:30 :0.15:0.0215;

(3) The molecular sieve precursor gel B was prepared according to the same method as in Example 1, wherein the molar ratio in terms of SiO ₂ : Al ₂ O ₃ : H ₂ O: OH ⁻ : hexamethylammonium bromide was 1:0.005:15: 0.13:0.0175;

(4) Put the molecular sieve precursor gels A and B into the reaction kettle respectively, and seal them. A performs the first crystallization for 10 hours at 150°C and a rotational speed of 500 rpm, and B performs the first step of crystallization at 160°C and a rotational speed of 500 rpm for 10 hours. One-step crystallization 24h;

(5) Cool the above reaction kettle to below 100°C, mix the first-step crystallization products of A and B according to the mass ratio of 1:10, stir evenly, and statically crystallize the final product at 165°C for 12 hours again. After filtering, washing, drying, ammonium transfer, and calcining at 550°C for 4 hours, molecular sieves were obtained, and the crystallinity was 98%, which was denoted as A7.

Comparative Example 3

The molecular sieve was prepared by referring to the method described in Example 5. The difference was that a molecular sieve precursor gel with a silicon-alumina ratio of 162 was directly prepared, and it was statically crystallized at 165 ° C for 12 h, and the final product was directly obtained by filtration, washing, drying, ammonium The obtained molecular sieve has a crystallinity of 95%, and is denoted as D3.

Example 8

(1) 7.06g of hexamethylammonium bromide, 3.28g of sodium metaaluminate, and 6g of sodium hydroxide are dissolved in 540g of water to form a solution; 200g of silica sol (concentration is 30wt%) is slowly stirred under stirring Slowly add to the solution of the above system and stir evenly to prepare molecular sieve precursor gel A, wherein the molar ratio in terms of SiO ₂ : Al ₂ O ₃ : H ₂ O: OH ⁻ : hexamethylammonium bromide is 1:0.02:25 :0.17:0.0195;

(3) 6.34g of hexamethylammonium bromide, 0.41g of sodium metaaluminate, and 5.2g of sodium hydroxide were dissolved in 270g of water to form a solution; 200g of silica sol (concentration of 30wt%) was stirred under stirring Slowly add it to the solution of system (1) and stir it evenly to prepare molecular sieve precursor gel A, wherein the molar ratio in terms of SiO ₂ : Al ₂ O ₃ : H ₂ O : OH ⁻ : hexamethylammonium bromide is 1: 0.0025:20:0.15:0.0195;

(4) Put the molecular sieve precursor gels A and B into the reaction kettle respectively, and seal them. A is at 150°C and the rotation speed is 500rpm for the first step of crystallization for 15h, and B is at 170°C and the rotation speed is 500rpm for the first step of crystallization. One-step crystallization 20h;

(5) Cool the temperature, mix the first-step crystallization products of A and B according to the mass ratio of 1:10, stir evenly, and statically crystallize again at 170 ° C for 12 hours, the final product is filtered, washed, dried, and dried. Ammonium cross, calcined at 550 for 4 hours to obtain molecular sieve with a crystallinity of 98%, denoted as A8.

test case

1. XRD test

XRD tests were carried out on the molecular sieves A1-A8 and D1-D3 obtained in Examples 1-8 and Comparative Examples 1-3, respectively, and the tests showed that the XRD characteristic peaks of all samples belonged to the MRE topology (the spatial structure code of ZSM-48 molecular sieves) , indicating that the molecular sieves A1-A8 and D1-D3 are all ZSM-48 molecular sieves, and the crystallinity of the molecular sieves A1-A8 is above 98%.

2. X-ray fluorescence spectrometry (XRF) test

The molecular sieves A1-A8 and D1-D3 obtained in Examples 1-8 and Comparative Examples 1-3 were subjected to XRF test to detect the elemental composition test of the molecular sieves. The results are shown in Table 1 as the overall silicon-alumina ratio.

Table 1

3. Distribution test of Al element

The molecular sieves A1-A8 and D1-D3 obtained in Examples 1-8 and Comparative Examples 1-3 were subjected to transmission electron microscopy (TEM) section analysis and detection, and the distribution of Al elements in the molecular sieve from the outside to the inside was detected. The silicon aluminum ratio is shown in Table 1. Figure 1 shows the TEM section analysis and detection diagram of the molecular sieve described in Example 2. Figure 2 shows the silicon-alumina ratio of the molecular sieve described in Example 2 within the surface thickness of 200 nm. It can be seen from Figure 2 that the molecular sieve The Al element is distributed in a gradient.

4. Test of specific surface area and pore volume

The specific surface area and pore volume of molecular sieves A1-A8 and D1-D3 were tested by low temperature liquid nitrogen physical adsorption method, and the test results are shown in Table 1.

5. Test of skeleton acidity

Take 20 mg of molecular sieve sample with a tablet diameter of 1.3 cm, put it into the infrared test sample cell, vacuumize to below 10 ^-3 Pa, heat up to 400 ° C for pretreatment for 30 min, reduce to room temperature to adsorb pyridine, vacuum for desorption, and desorb at 350 The desorption equilibrium was reached at a constant temperature of ℃, the infrared signal was measured, the background signal was deducted, and the final acid amount was calculated (reference "Preparation of the surface Ti, Al rich ETS-10 and modification of its pore structure and acidity by desilication and realumation", Microporous and Mesoporous Materials, Volume 145, Pages 224-230).

Catalyst Preparation and Isomerization Reaction Testing

The preparation of the catalyst refers to US8790507B2, and the composition ratio of the catalyst is molecular sieve:alumina:Pt=65:35:0.35.

The isomerization test of the catalyst was carried out on a micro-reaction device, with n-hexadecane as the model compound, the reaction conditions were normal pressure, hydrogen-oil ratio 500:1, mass space velocity 2.3h ^-1 , and the total conversion was adjusted by changing the temperature. 94%, and test the isohexadecane selectivity data when the total conversion is 94%, the results are shown in Table 2.

Table 2

From the results in Table 2, it can be seen that the embodiment of the present invention has significantly better effects. Under the condition of the same conversion rate, the reaction temperature of the sample obtained by the embodiment of the present invention is lower, indicating that the catalyst activity is higher, wherein The reaction temperature of A1, A2, A7 and A8 is not more than 290°C, and the isomeric selectivity is higher, not less than 95%.

The preferred embodiments of the present invention have been described above in detail, however, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, a variety of simple modifications can be made to the technical solutions of the present invention, including combining various technical features in any other suitable manner. These simple modifications and combinations should also be regarded as the content disclosed in the present invention. All belong to the protection scope of the present invention.

Claims (14)

1. a preparation method of the molecular sieve rich in surface aluminum, the method comprises: (1) Precursor gels A and B are prepared, the silicon-alumina ratio of the precursor gel A is 40-100, and the silicon-aluminum ratio of the precursor gel B is 100-400, and the silicon-alumina ratios of the two are different; (2) carrying out the first-step crystallization of the precursor gels A and B respectively; (3) mixing the first-step crystallization products obtained from the precursor gels A and B respectively in a mass ratio of 1:(3-50) to carry out the second-step crystallization, and finally obtains a molecular sieve rich in aluminum on the surface; In step (2), the conditions for the first crystallization of the precursor gel A include: the temperature is 150-190°C, and the time is 3-120h; In step (2), the first crystallization conditions of the precursor gel B include: the temperature is 150-190°C, and the time is 3-120h; The conditions of the second step of crystallization include: the temperature is 150-190°C, and the time is 6-48h; The molecular sieve is ZSM-48 molecular sieve, the silicon-aluminum ratio within 50 nm of the particle surface of the molecular sieve is 40-150, and the overall silicon-aluminum ratio of the molecular sieve is 150-300. 2 . The method according to claim 1 , wherein, in step (1), the precursor gels A and B are prepared by a template agent, a silicon source, an aluminum source, water and sodium hydroxide, respectively. 3 . 3. The method according to claim 2, wherein, in the precursor gel A, the molar ratio of the silicon source, aluminum source, water, alkali and template agent is 1:(0.01-0.025):(10- 40): (0.11-0.19): (0.0175-0.0215); In the precursor gel B, the molar ratio of the silicon source, aluminum source, water, alkali and template agent is 1:(0.0025-0.01):(10-40):(0.11-0.19):(0.0175-0.0215 ); Wherein, the silicon source is calculated as SiO ₂ , the aluminum source is calculated as Al ₂ O ₃ , and the alkali is calculated as

count. 4. The method according to claim 2 or 3, wherein the silicon source is silica sol, and the aluminum source is sodium metaaluminate. 5 . The method according to claim 1 , wherein, in step (2), the conditions for the first crystallization of the precursor gel A include: a temperature of 150-160° C. and a time of 4-24 h. 6 . 6. The method according to claim 1 or 5, wherein, in step (2), the conditions for the first crystallization of the precursor gel A include: time is 8-16h. 7 . The method according to claim 1 , wherein, in step (2), the first crystallization conditions of the precursor gel B include: a temperature of 150-160° C. and a time of 12-48 h. 8 . 8. The method according to claim 1 or 7, wherein, in step (2), the conditions for the first step of crystallization of the precursor gel B include: a time of 16-32 h. 9 . The method according to claim 1 , wherein the conditions of the second step of crystallization include: a time of 8-12 h. 10 . 10. The surface aluminum-rich molecular sieve prepared by the method of any one of claims 1-9. 11 . The molecular sieve according to claim 10 , wherein the ratio of silicon to aluminum within 50 nm of the particle surface of the molecular sieve is 40-90. 12 . 12. The application of the molecular sieve according to claim 10 or 11 in alkane isomerization reaction. 13. An isomerization reaction catalyst comprising a binder, a metal active component and the molecular sieve of claim 10 or 11. 14. The application of the isomerization catalyst of claim 13 in alkane isomerization.

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