Short b-axis ZSM-5 molecular sieve as well as synthesis method and application thereof

Abstract

The invention discloses a short b-axis ZSM-5 molecular sieve as well as a synthesis method and application thereof, tetraethyl orthosilicate and aluminum isopropoxide are taken as an aluminum source, tetrapropylammonium hydroxide is taken as a template agent, ammonium salt is added as an auxiliary agent to limit the growth of particles along a b axis, after a hydrothermal crystallization reaction, the obtained product is washed, baked and calcined, and the short b-axis ZSM-5 molecular sieve is obtained. The method disclosed by the invention is simple to operate, has repeatability, small template agent dosage and low cost, the synthesized ZSM-5 molecular sieve has the advantages of good thermal stability, high specific surface area, strong acid center, slow carbon deposition speed in reaction and the like, meanwhile, more active sites can be exposed, the transmission path of reactants and products is shortened, the diffusion resistance is reduced, and the method is suitable for industrial production. The catalyst is used for catalytic synthesis of pyridine and derivatives thereof, and has high selectivity and high catalytic activity.

Description

Technical Field

[0001] The present invention belongs to the technical field of molecular sieve synthesis, and specifically relates to a short b-axis ZSM-5 molecular sieve and a synthesis method thereof, and provides application of the short b-axis ZSM-5 molecular sieve in catalytic synthesis of pyridine and its derivatives. Background Art

[0002] ZSM-5 molecular sieve is a microporous molecular sieve with MFI topological structure like TS-1 molecular sieve. ZSM-5 molecular sieve is widely used in petroleum refining process at home and abroad due to its unique shape selectivity, outstanding thermal stability and good hydrothermal stability, and has been valued by scholars in the field of chemical catalysis. With its unique microporous structure and strong acid center, ZSM-5 molecular sieve has become a new type of molecular sieve catalyst with acid catalytic shape selectivity, which not only greatly improves the carbon deposition problem, but also the structure of ZSM-5 molecular sieve begins to be destroyed after calcination at 1050℃, which shows that it has excellent thermal stability, which has a significant improvement on the yield of pyridine and its derivatives.

[0003] There are many methods for synthesizing ZSM-5 molecular sieves, such as organic amine and amine-free systems, hydrothermal synthesis and non-hydrothermal synthesis, alkaline conditions and non-alkaline conditions, etc. However, the most classic synthesis method is to use tetraethyl orthosilicate as the silicon source, tetrapropylammonium hydroxide as the template, and aluminum isopropoxide as the aluminum source, and then prepare ZSM-5 molecular sieves after high-temperature hydrothermal treatment. ZSM-5 molecular sieves have an MFI-type molecular sieve skeleton with an anisotropic structure. They are composed of two different ten-membered ring channels that intersect each other. One is a straight ten-membered ring channel that is linear and parallel to the b-axis. The other is a ten-membered ring channel that is S-shaped and parallel to the a-axis. The pore size of the straight channel is larger, which is conducive to the transmission and diffusion of molecules within it. The S-shaped channel size is relatively small, which limits the internal diffusion and transmission of molecules and is conducive to the realization of molecular shape selectivity. Therefore, by changing the morphology of the ZSM-5 molecular sieve, the length of the b axis can be controlled, thereby effectively controlling the adsorption and catalytic performance of the molecular sieve. Summary of the invention

[0004] The purpose of the present invention is to provide a short b-axis ZSM-5 molecular sieve and a synthesis method thereof, and to provide a new application for the molecular sieve.

[0005] To achieve the above-mentioned purpose, the technical solution adopted by the present invention comprises the following steps:

[0006] Step 1: weighing various raw materials according to the molar ratio of tetraethyl orthosilicate, aluminum isopropoxide, tetrapropylammonium hydroxide, deionized water and ammonium salt of 0.003-0.006:0.02-0.07:8-18:0.2-0.5:1; adding tetrapropylammonium hydroxide into deionized water, stirring and mixing evenly to obtain solution A; adding tetraethyl orthosilicate into solution A, stirring at room temperature to fully volatilize the water therein, to obtain solution B; adding aluminum isopropoxide into solution B, stirring at room temperature for 10-12 hours, then adding ammonium salt, continuing stirring at room temperature for 1-2 hours, to obtain solution C; the ammonium salt is any one of urea, ammonium chloride, ammonium carbonate, ammonium sulfate and ammonium nitrate.

[0007] Step 2: Place the solution C obtained in step 1 in a reactor and perform a hydrothermal reaction at 160-180°C for 36-72 hours. The obtained product is filtered, washed, dried, ground, and then calcined in a muffle furnace at 500-600°C for 4-10 hours to obtain a short b-axis ZSM-5 molecular sieve.

[0008] In the above step 1, preferably, the raw materials are weighed according to the molar ratio of tetraethyl orthosilicate, aluminum isopropoxide, tetrapropylammonium hydroxide, deionized water and ammonium salt of 0.004-0.005:0.03-0.04:14-16:0.3-0.4:1.

[0009] In the above step 1, tetrapropylammonium hydroxide is preferably added to deionized water and stirred for 30 to 60 minutes to obtain solution A.

[0010] In the above step 1, tetraethyl orthosilicate is preferably added to solution A, and the mixture is stirred at room temperature for 10 to 12 hours to fully volatilize the water therein, thereby obtaining solution B.

[0011] In the above step 2, the hydrothermal reaction is preferably carried out at 180° C. for 48 to 60 hours.

[0012] In the above step 2, the heating rate of calcination is preferably 1 to 3°C / min.

[0013] The a-axis and c-axis sizes of the short b-axis ZSM-5 molecular sieve obtained by the above synthesis method are 70 to 140 nm, and the b-axis size is 20 to 40 nm.

[0014] The present invention also provides the use of the short b-axis ZSM-5 molecular sieve for catalytic synthesis of pyridine and its derivatives. The specific method is: using a fixed bed reactor, the short b-axis ZSM-5 molecular sieve is loaded into a reaction tube, nitrogen is used as a protective gas, ammonia is used as a reaction gas, and a mixed aqueous solution of formaldehyde and acetaldehyde is used as a reaction liquid. The reaction liquid is vaporized by a vaporizer and enters the reaction tube in the form of gas for reaction. The molar ratio of ammonia to formaldehyde and acetaldehyde is 0.5-1:1-1.5:2-8, the reaction temperature is 180-350°C, the reaction pressure is 0.2-0.5MPa, the reaction time is 4-15h, and the gas phase space velocity is 200-280h -1 .

[0015] The reaction of synthesizing pyridine and its derivatives using ammonia, formaldehyde and acetaldehyde as raw materials is a condensation reaction, and its general reaction formula can be expressed as: nNH3+mHCHO+pCH3CHO→pyridine and its derivatives+H2O, where the ratio of n, m and p needs to be adjusted according to the specific reaction conditions and product requirements, roughly satisfying the molar ratio range of the raw materials mentioned above.

[0016] The beneficial effects of the present invention are as follows:

[0017] The present invention uses tetraethyl orthosilicate as a silicon source, aluminum isopropoxide as an aluminum source, tetrapropylammonium hydroxide as a template agent, and ammonium salt as an auxiliary agent. The tetraethyl orthosilicate is fully hydrolyzed by extending the stirring time, so as to better participate in the formation of molecular sieves during the crystallization process; secondly, the ammonium salt is added to ensure that the silicon source and the aluminum source are fully hydrolyzed, so that the particles can be restricted to grow along the b axis during the crystallization process, and calcined after the hydrothermal crystallization reaction to obtain a short b axis ZSM-5 molecular sieve. The method of the present invention is simple to operate, has repeatability, a small amount of template agent, and low cost. The synthesized ZSM-5 molecular sieve has the advantages of good thermal stability, high specific surface area, strong acid center, slow carbon deposition speed in the reaction, etc., and can also expose more active sites, shorten the transmission path of reactants and products, reduce diffusion resistance, and is used for catalytic synthesis of pyridine and its derivatives with high selectivity and higher catalytic activity. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is the SEM image of sample S1 prepared in Example 1.

[0019] Figure 2 is the XRD pattern of sample S1 prepared in Example 1. DETAILED DESCRIPTION

[0020] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments, but the protection scope of the present invention is not limited to these embodiments.

[0021] Example 1

[0022] Step 1: Weigh the raw materials according to the molar ratio of tetraethyl orthosilicate, aluminum isopropoxide, tetrapropylammonium hydroxide, deionized water and urea of ​​0.004:0.03:16:0.4:1; add 4.5g of tetrapropylammonium hydroxide to 4.6mL of water, stir in a magnetic stirrer for 1 hour to form a transparent and clear solution A; add 2.8g of tetraethyl orthosilicate to solution A, stir for 12 hours to fully evaporate the water therein, and obtain solution B; slowly add 0.028g of aluminum isopropoxide to solution B, continue stirring for 12 hours, then add 1g of urea, and continue stirring at room temperature for 1 hour to obtain solution C.

[0023] Step 2: Transfer the solution C obtained in step 1 into a 50mL stainless steel reactor and perform hydrothermal reaction at 180°C for 48 hours. After the reactor is naturally cooled to room temperature, filter the white suspension, wash it with water for more than 3 times, and then dry and grind it in a 100°C oven. Place the obtained white sample in a muffle furnace and heat it to 550°C at a heating rate of 2°C / min. Calcined at a constant temperature for 4 hours to obtain a short b-axis ZSM-5 molecular sieve powder (denoted as S1). Figure 1 and Figure 2 It can be seen that the b-axis size of the obtained ZSM-5 molecular sieve powder is 20-30 nm, and the a-axis and c-axis sizes are about 80-100 nm.

[0024] Example 2

[0025] Step 1: Weigh the raw materials according to the molar ratio of tetraethyl orthosilicate, aluminum isopropoxide, tetrapropylammonium hydroxide, deionized water and ammonium chloride of 0.005:0.032:16:0.4:1; add 9 g of tetrapropylammonium hydroxide to 9.2 mL of water, stir in a magnetic stirrer for 1 hour to form a transparent and clear solution A; add 5.6 g of tetraethyl orthosilicate to solution A, stir for 12 hours to fully evaporate the water therein, and obtain solution B; slowly add 0.056 g of aluminum isopropoxide to solution B, continue stirring for 12 hours, then add 1 g of ammonium chloride, and continue stirring at room temperature for 1 hour to obtain solution C.

[0026] Step 2: Transfer the solution C obtained in step 1 into a 50mL stainless steel reactor, and perform a hydrothermal reaction at 180°C for 48 hours. After the reactor is naturally cooled to room temperature, filter the white suspension, wash it with water for more than 3 times, and then dry and grind it in a 100°C oven. Place the obtained white sample in a muffle furnace, heat it to 550°C at a heating rate of 2°C / min, and calcine it at a constant temperature for 4 hours to obtain a short b-axis ZSM-5 molecular sieve powder (denoted as S2). The b-axis size of the obtained ZSM-5 molecular sieve powder is 25-30nm, and the a-axis and c-axis sizes are about 90-120nm.

[0027] Example 3

[0028] Step 1: Weigh the raw materials according to the molar ratio of tetraethyl orthosilicate, aluminum isopropoxide, tetrapropylammonium hydroxide, deionized water and ammonium sulfide of 0.004:0.03:16:0.4:1; add 12.5g of tetrapropylammonium hydroxide to 12.8mL of water, stir in a magnetic stirrer for 1 hour to form a transparent and clear solution A; add 8.4g of tetraethyl orthosilicate to solution A, stir for 12 hours to fully evaporate the water therein, and obtain solution B; slowly add 0.084g of aluminum isopropoxide to solution B, continue stirring for 12 hours, then add 1g of ammonium sulfide, and continue stirring at room temperature for 1 hour to obtain solution C.

[0029] Step 2: Transfer the solution C obtained in step 1 into a 50mL stainless steel reactor, and perform a hydrothermal reaction at 180°C for 48 hours. After the reactor is naturally cooled to room temperature, filter the white suspension, wash it with water for more than 3 times, and then dry and grind it in a 100°C oven. Place the obtained white sample in a muffle furnace, heat it to 550°C at a heating rate of 2°C / min, and calcine it at a constant temperature for 4 hours to obtain a short b-axis ZSM-5 molecular sieve powder (denoted as S3). The b-axis size of the obtained ZSM-5 molecular sieve powder is 30-40nm, and the a-axis and c-axis sizes are about 70-140nm.

[0030] Example 4

[0031] Application of the short b-axis ZSM-5 molecular sieve obtained in Examples 1 to 3 to catalyze the synthesis of pyridine

[0032] Weigh 1g of short b-axis ZSM-5 molecular sieve, then fill in 60-80 mesh quartz sand, mix well and load into the middle section of the reaction tube of the micro fixed bed reactor, and load the reaction tube with high-temperature rock wool, quartz sand, short b-axis ZSM-5 molecular sieve, quartz sand and high-temperature rock wool from bottom to top; nitrogen is used as the protective gas, ammonia is used as the reaction gas, and a mixed aqueous solution of formaldehyde and acetaldehyde is used as the reaction liquid. The reaction liquid is vaporized in a vaporizer and enters the reaction tube in the form of gas for reaction. The molar ratio of ammonia to formaldehyde and acetaldehyde is 0.8:1:3, the reaction temperature is 300°C, the reaction pressure is 0.3MPa, and the gas phase space velocity is 220h -1 After 4 hours of reaction, samples were taken for analysis, and the conversion rate of the raw materials and the yield of the target product were detected by gas chromatography (GC, Bruker 450-GC). The test results are shown in Table 1.

[0033] Table 1 Reaction performance of short b-axis ZSM-5 molecular sieve catalyst for synthesizing pyridine base and its derivatives

[0034] catalyst S1 S2 S3 Conversion rate 95.5％ 92.3％ 91.6％ Yield of pyridine base 67.25％ 63.52％ 60.48％ Pyridine yield 15.57％ 12.33％ 13.27％ 2-Methylpyridine yield 13.36％ 10.66％ 8.33％ Yield of 3-methylpyridine 31.10％ 29.86％ 28.56％ Yield of 4-methylpyridine 7.22％ 10.67％ 10.32％

[0035] It can be seen from the results in Table 1 that the short b-axis ZSM-5 molecular sieve prepared in the present invention has good selectivity and high catalytic activity for pyridine and its derivatives.

Claims

1. A method for synthesizing a short b-axis ZSM-5 molecular sieve, characterized in that: The synthesis method comprises the following steps: Step 1: Weigh the raw materials according to the molar ratio of tetraethyl orthosilicate, aluminum isopropoxide, tetrapropylammonium hydroxide, deionized water and ammonium salt of 0.003-0.006:0.02-0.07:8-18:0.2-0.5:1; add tetrapropylammonium hydroxide into deionized water, stir and mix evenly to obtain solution A; add tetraethyl orthosilicate into solution A, stir at room temperature to fully volatilize the water therein, and obtain solution B; add aluminum isopropoxide into solution B, stir at room temperature for 10-12 hours, then add ammonium salt, continue stirring at room temperature for 1-2 hours, and obtain solution C; the ammonium salt is any one of urea, ammonium chloride, ammonium carbonate, ammonium sulfate and ammonium nitrate; Step 2: Place the solution C obtained in step 1 in a reactor and perform a hydrothermal reaction at 160-180°C for 36-72 hours. The obtained product is filtered, washed, dried, ground, and then calcined in a muffle furnace at 500-600°C for 4-10 hours to obtain a short b-axis ZSM-5 molecular sieve.

2. The method for synthesizing the short b-axis ZSM-5 molecular sieve according to claim 1, characterized in that: In step 1, weigh the raw materials according to the molar ratio of tetraethyl orthosilicate, aluminum isopropoxide, tetrapropylammonium hydroxide, deionized water and ammonium salt of 0.004-0.005:0.03-0.04:14-16:0.3-0.4:

1.

3. The method for synthesizing the short b-axis ZSM-5 molecular sieve according to claim 1, characterized in that: In step 1, tetrapropylammonium hydroxide is added to deionized water and stirred for 30 to 60 minutes to obtain solution A.

4. The method for synthesizing the short b-axis ZSM-5 molecular sieve according to claim 1, characterized in that: In step 1, tetraethyl orthosilicate is added to solution A, and the mixture is stirred at room temperature for 10 to 12 hours to fully evaporate the water therein, thereby obtaining solution B.

5. The method for synthesizing the short b-axis ZSM-5 molecular sieve according to claim 1, characterized in that: In step 2, the hydrothermal reaction is carried out at 180° C. for 48 to 60 hours.

6. The method for synthesizing a short b-axis ZSM-5 molecular sieve according to claim 1, characterized in that: In step 2, the heating rate of calcination is 1 to 3°C / min.

7. The short b-axis ZSM-5 molecular sieve obtained by the synthesis method according to any one of claims 1 to 6, characterized in that: The a-axis and c-axis sizes of the ZSM-5 molecular sieve are 70-140 nm, and the b-axis size is 20-40 nm.

8. Use of the short b-axis ZSM-5 molecular sieve described in claim 7 for catalytic synthesis of pyridine and its derivatives.

9. Use of the short b-axis ZSM-5 molecular sieve for catalytic synthesis of pyridine and its derivatives according to claim 8, characterized in that: A fixed bed reactor is used, a short b-axis ZSM-5 molecular sieve is loaded into a reaction tube, nitrogen is used as a protective gas, ammonia is used as a reaction gas, and a mixed aqueous solution of formaldehyde and acetaldehyde is used as a reaction liquid. The reaction liquid is vaporized by a vaporizer and enters the reaction tube in the form of gas for reaction. The molar ratio of ammonia to formaldehyde and acetaldehyde is 0.5-1:1-1.5:2-8, the reaction temperature is 180-350°C, the reaction pressure is 0.2-0.5MPa, the reaction time is 4-15h, and the gas phase space velocity is 200-280h -1 .

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