Date-pit-shaped ZSM-11 molecular sieve as well as synthesis method and application thereof

Abstract

The invention discloses a date-pit-shaped ZSM-11 molecular sieve as well as a synthesis method and application thereof. According to the present invention, a silicon source, an aluminum source and a small amount of a template agent are adopted as raw materials, the pH value of the system is regulated by using a strong alkali and weak acid salt, and the jujube-pit-shaped ZSM-11 molecular sieve with the wide middle and the narrow two ends is prepared through hydrothermal crystallization, and has characteristics of good dispersion and particle size of 200-400 nm; the molecular sieve has the advantages of favorable crystallinity, high purity, abundant outer surface and appropriate acid content. The method for preparing the ZSM-11 molecular sieve is simple in synthesis step and low in preparation cost, and is an economical and feasible method for preparing the ZSM-11 molecular sieve. The ZSM-11 molecular sieve prepared by the method can be used as a catalyst for preparing cyclohexanol by cyclohexene hydration, can remarkably improve the cyclohexene conversion rate, and has a remarkable industrial application value.

Description

Technical Field

[0001] The invention relates to the field of molecular sieve catalysis, and in particular to a jujube-shaped ZSM-11 molecular sieve and a synthesis method and application thereof. Background Art

[0002] ZSM-11 molecular sieve is composed of three structural units: mor, mfi and mel, and has a MEL topological structure. It belongs to the same Pentasil type molecular sieve as ZSM-5 molecular sieve, and the skeleton density and pore size of the two are similar. The difference is that ZSM-5 molecular sieve has intersecting straight channels and sinusoidal channels, while ZSM-11 molecular sieve has straight channels parallel to the a-axis and b-axis (the channel size is ). It is the bidirectional parallel straight pores of ZSM-11 molecular sieve that give it a regular and orderly pore structure and a short pore distance, which is conducive to the diffusion of reactants and products inside it, reducing the occurrence of carbon deposition deactivation and side reactions. However, ZSM-11 molecular sieve has no openings along the c-axis direction, which to a certain extent hinders the reactant molecules from entering the internal pores, reduces the utilization rate of active sites, and leads to a decrease in reaction performance. Current research is to improve the performance of ZSM-11 molecular sieve by adjusting the molecular sieve particle size, component content, post-treatment and other methods, while there are fewer methods to change the grain morphology of ZSM-11 molecular sieve to give it excellent catalytic performance.

[0003] Patent application CN 117303396A discloses a method for synthesizing a ZSM-11 molecular sieve. The molecular sieve prepared by this method has the morphology of conical particles formed by orderly stacking of nanorods, with a particle size of 500nm to 2μm and a radial size of 20 to 60nm. When it is used for the alkylation reaction of benzene and methanol, the benzene conversion rate, toluene selectivity and xylene yield are improved. However, the template / SiO2 molar ratio of the particle morphology of this method is 0.4 to 2, and the template dosage is high.

[0004] Patent application CN 115010144A discloses a method for synthesizing a flaky ZSM-11 molecular sieve, wherein the flaky ZSM-11 molecular sieve is rapidly prepared by subjecting the gel obtained by fully mixing the raw materials to a short-time hydrothermal crystallization. The flaky ZSM-11 molecular sieve has a thickness of less than 10 nm and good diffusion performance. However, the method uses a template agent / SiO2 molar ratio of 0.01 to 1.0, and there is also the problem of a high amount of template agent.

[0005] Patent application CN 103011188A discloses a method for preparing nano-sized SAPO-34 molecular sieve. By controlling the microwave synthesis conditions through microwave heating, spherical SAPO-34 molecular sieves with a grain size below 30 nm or flaky SAPO-34 molecular sieves with a thickness below 70 nm can be obtained. However, this preparation method uses microwave heating, which is only applicable to small-scale experiments and not suitable for large-scale production, making it difficult to scale up industrially.

[0006] Patent application CN 105712378A discloses a method for synthesizing nano-sized ZSM-5 with high yield. By mixing and crystallizing precursor I containing a surfactant with precursor II containing an organic templating agent, nano-sized ZSM-5 molecular sieves with a particle size range of 10 - 80 nm and a yield greater than 95% are obtained, and they are applied to the methanol-to-propylene reaction, showing good catalytic performance. However, its synthesis steps are complex, inhibiting the growth of specific crystal planes through additives such as surfactants, resulting in high production costs.

[0007] The above technologies have made certain breakthroughs in the preparation of nano-molecular sieves, but the products obtained still have deficiencies such as small specific surface area, irregular particle morphology, and relatively high templating agent dosage. This not only increases the synthesis cost but also makes it difficult to achieve large-scale preparation. Currently, there is no report on ZSM-11 molecular sieve with a date pit-like morphology. On the other hand, when using ZSM-11 molecular sieve as a catalyst in the hydration of cyclohexene to prepare cyclohexanol industrially, there is a problem of low cyclohexene conversion rate, which is mainly due to the microporous channels of ZSM-11 molecular sieve restricting the diffusion of reactants. Therefore, it is particularly necessary to prepare ZSM-11 molecular sieve with a short diffusion path through a simple synthesis method under an extremely low templating agent system. Summary of the Invention

[0008] Aiming at the deficiencies in the existing technology, the purpose of the present invention is to provide a date pit-like ZSM-11 molecular sieve, its synthesis method, and its application in the hydration of cyclohexene to prepare cyclohexanol. By using a strong base weak acid salt to regulate the pH of the system under an extremely low templating agent dosage, a date pit-like ZSM-11 molecular sieve is synthesized, which has uniform particle size and a mesoporous structure. When the prepared date pit-like ZSM-11 molecular sieve is used in the hydration of cyclohexene to prepare cyclohexanol, the cyclohexene conversion rate can be significantly improved, having significant industrial application value.

[0009] To achieve the above purpose, the present invention adopts the following technical solutions:

[0010] A synthesis method of a date pit-like ZSM-11 molecular sieve, comprising the following steps:

[0011] (1) Mix the silicon source and the templating agent evenly to obtain gel A;

[0012] (2) Dissolve the aluminum source and the weak base strong acid salt in deionized water, and mix evenly to obtain solution B;

[0013] (3) Dropwise add solution B into gel A, stir evenly to obtain the initial gel, the pH of the initial gel is 9-14, and the molar ratio of the initial gel is: SiO2 / Al2O3 = 50-200, template agent / SiO2 = 0.001-0.005, H2O / SiO2 = 15-30, M2O / SiO2 = 0.6-2; where M2O is the weak base strong acid salt expressed as an oxide, and M represents an alkali metal ion;

[0014] (4) Transfer the above-mentioned initial gel to an autoclave with a polytetrafluoroethylene lining, and obtain spindle-shaped ZSM-11 molecular sieve after crystallization, washing, drying and calcination.

[0015] Further, the silicon source described in step (1) is selected from at least one of silica sol, white carbon black, sodium silicate, and water glass, and preferably silica sol.

[0016] Further, the template agent described in step (1) is selected from at least one of tetrabutylammonium hydroxide (TBAOH), tetrabutylammonium bromide, and 1,8-octanediamine, and preferably tetrabutylammonium hydroxide.

[0017] Further, the aluminum source described in step (2) is selected from at least one of aluminum sulfate, sodium metaaluminate, aluminum isopropoxide, and pseudoboehmite, and preferably aluminum sulfate hydrate.

[0018] Further, the weak base strong acid salt described in step (2) is selected from at least one of sodium bicarbonate and potassium bicarbonate, and preferably sodium bicarbonate.

[0019] Further, the stirring temperature in step (3) is 20-80 °C, and the stirring time is 0.5-12 h.

[0020] Further, the crystallization temperature in step (4) is 120-180 °C, and the crystallization time is 12-120 h.

[0021] Further, the calcination temperature in step (4) is 400-700 °C, and the calcination time is 2-8 h.

[0022] Further, the morphology of the obtained spindle-shaped ZSM-11 molecular sieve is spindle-shaped particles that are wide in the middle and narrow at both ends, and the particle size is 200-400 nm.

[0023] Further, the total specific surface area of the obtained spindle-shaped ZSM-11 molecular sieve is 400-510 m 2 / g, and the external specific surface area is 180-300 m 2 / g, and the total pore volume is 0.22 - 0.36 cm 3 / g, and the mesopore volume is 0.15 - 0.25 cm 3 / g.

[0024] The present invention also provides an application of the above-mentioned date-pit-shaped ZSM-11 molecular sieve in the preparation of cyclohexanol by cyclohexene hydration.

[0025] Furthermore, the date-pit-shaped ZSM-11 molecular sieve is first subjected to ammonium exchange to obtain a hydrogen-type ZSM-11 molecular sieve, and then the hydrogen-type ZSM-11 molecular sieve is used as a catalyst in the preparation of cyclohexanol by cyclohexene hydration.

[0026] Furthermore, when the date-pit-shaped ZSM-11 molecular sieve is used for the preparation of cyclohexanol by cyclohexene hydration, the conversion rate of cyclohexene is greater than 13.2%, and the selectivity of cyclohexanol is greater than 99%.

[0027] The present invention uses a silicon source, an aluminum source, and a small amount of template agent as raw materials, and regulates the pH of the system by using a strong base weak acid salt. After hydrothermal crystallization, a date-pit-shaped ZSM-11 molecular sieve with a wide middle and narrow ends is prepared. This molecular sieve has good crystallinity, high purity, a rich external surface, and an appropriate acid amount. Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. In the present invention, under an extremely low template agent system, OH is provided by the hydrolysis of a strong base weak acid salt - , enabling the silicon-aluminum species to dissolve and polymerize uniformly to form a precursor. Since the

[101] crystal plane has a relatively high surface energy, it preferentially adsorbs the weak acid salt anions, causing the silicon-aluminum species to deposit. As a result, the growth rate at both ends of the crystal is slow. As the alkalinity of the system decreases, the adsorption amount of the weak acid salt anions will decrease, leading to a large difference in the growth rate of different crystal planes and affecting the crystal morphology. By adding a strong base weak acid salt, the pH of the system is effectively regulated to obtain a date-pit-shaped ZSM-11 molecular sieve. Its synthesis steps are simple, the preparation cost is low, it is economically feasible, and it conforms to the development concept of green chemistry, and is expected to be industrially produced on a large scale.

[0029] 2. The particle size of conventional ZSM-11 molecular sieves is relatively large, and their morphology is mostly spherical or nano-rod stacked particles, and the accessibility of the active sites in the molecular sieve channels is very poor. Therefore, the present invention provides a new type of ZSM-11 molecular sieve, which is a date-pit-shaped particle with a wide middle and narrow ends. Its particles are evenly dispersed. The wide middle part provides a larger reaction space for the reactants and has more active sites, while the narrow ends structure promotes the rapid desorption of the product molecules along the pore direction, reducing the occurrence of secondary reactions. The obtained date-pit-shaped ZSM-11 molecular sieve has a rich external surface and an appropriate acidity, providing a sufficient reaction site for the catalytic reaction.

[0030] 3. In industrial production, the hydration of cyclohexene to cyclohexanol has problems such as slow reaction rate, fast deactivation of the catalyst, and relatively low single-pass conversion rate. The present invention provides a novel ZSM-11 molecular sieve as a catalyst for the hydration of cyclohexene to cyclohexanol, effectively improving the reaction rate and conversion rate. The conversion rate of cyclohexene reaches 13%, showing good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 XRD spectra of the ZSM-11 molecular sieves obtained in Examples 1-5 and Comparative Examples.

[0032] Figure 2 SEM image of the ZSM-11 molecular sieve obtained in Example 1.

[0033] Figure 3 SEM image of the ZSM-11 molecular sieve obtained in Comparative Example. DETAILED DESCRIPTION OF THE INVENTION

[0034] To make the present invention more understandable, the technical solutions of the invention will be further described below in conjunction with specific embodiments, but the relevant synthesis methods are not limited thereto.

[0035] Example 1

[0036] Synthesis method of a spindle-shaped ZSM-11 molecular sieve

[0037] Weigh 18 g of silica sol and 0.4 g of TBAOH, stir for 30 min to obtain gel A; then add 1 g of (Al2(SO4)3)·18H2O and 2 g of NaHCO3 to 20 g of deionized water, stir for 30 min to obtain solution B; subsequently, slowly add solution B dropwise to gel A, and vigorously stir and age the mixture at 30 °C for 12 h to obtain an initial gel with pH = 10. Then transfer the initial gel to a crystallization kettle and crystallize at 160 °C for 48 h. After centrifugation and drying, finally calcine at 550 °C for 4 h to obtain the synthesized product (ZSM-11-1).

[0038] From Figure 1 the XRD spectra shown, the synthesized product is a ZSM-11 molecular sieve with a relative crystallinity of 101%. From Figure 2 the SEM photos shown, it can be seen that the synthesized ZSM-11 molecular sieve has a uniform spindle-like morphology, and the particle size is 200 - 400 nm. The total specific surface area of the synthesized ZSM-11 molecular sieve is 510 m 2 / g, the external specific surface area is 300 m 2 / g, the total pore volume is 0.36 cm 3 / g, and the mesoporous pore volume is 0.22 cm 3 / g.

[0039] Example 2

[0040] Synthesis method of a jujube pit-shaped ZSM-11 molecular sieve

[0041] Weigh 18 g of silica sol and 0.3 g of TBAOH and stir for 30 min to obtain gel A; then add 1 g of (Al2(SO4)3)·18H2O and 6 g of NaHCO3 to 20 g of deionized water, stir for 30 min to obtain solution B; subsequently, add solution B dropwise to gel A, and vigorously stir and age the mixture at 30 °C for 12 h to obtain an initial gel with pH = 14; then transfer the initial gel to a crystallization kettle and crystallize at 160 °C for 48 h, centrifuge and dry, and finally calcine at 550 °C for 4 h to obtain the synthetic product (ZSM-11-2).

[0042] From Figure 1 the XRD pattern shown, it can be seen that the synthetic product is a ZSM-11 molecular sieve with a relative crystallinity of 100%. The synthesized ZSM-11 molecular sieve has a uniform jujube pit shape, and the particle size is 200 - 400 nm. The total specific surface area of the synthesized ZSM-11 molecular sieve is 486 m 2 / g, the external specific surface area is 265 m 2 / g, the total pore volume is 0.36 cm 3 / g, and the mesopore volume is 0.25 cm 3 / g.

[0043] Example 3

[0044] Synthesis method of a jujube pit-shaped ZSM-11 molecular sieve

[0045] Weigh 17 g of silica sol and 0.09 g of TBAOH and stir for 30 min to obtain gel A; then add 0.5 g of (Al2(SO4)3)·18H2O and 5 g of NaHCO3 to 32 g of deionized water, stir for 30 mim to obtain solution B; subsequently, add solution B dropwise to gel A, and vigorously stir and age the mixture at 30 °C for 0.5 h to obtain an initial gel with pH = 9; then transfer the initial gel to a crystallization kettle and crystallize at 170 °C for 100 h; centrifuge and dry, and finally calcine at 400 °C for 8 h to obtain the synthetic product (ZSM-11-3).

[0046] From Figure 1 the XRD pattern shown, it can be seen that the synthetic product is a ZSM-11 molecular sieve with a relative crystallinity of 95%. The morphology is similar to that of ZSM-11-1, both are jujube pit shapes, and the particle size is 200 - 400 nm. The total specific surface area of the synthesized ZSM-11 molecular sieve is 400 m 2 / g, the external specific surface area is 180 m2 / g, the total pore volume is 0.22 cm 3 / g, the mesopore volume is 0.15 cm 3 / g.

[0047] Example 4

[0048] A method for synthesizing spindle-shaped ZSM-11 molecular sieve

[0049] Weigh 17 g of silica sol and 0.4 g of TBAOH, stir for 30 min to obtain gel A; then add 0.4 g of (Al2(SO4)3)·18H2O and 4 g of NaHCO3 to 27 g of deionized water, stir for 30 min to obtain solution B; subsequently, add solution B dropwise to gel A, and vigorously stir and age the mixture at 30 °C for 12 h to obtain the initial gel, pH = 12; then transfer the initial gel to a crystallization kettle and crystallize at 120 °C for 120 h, centrifugate and dry, and finally calcine at 600 °C for 4 h to obtain the synthetic product (ZSM-11-4).

[0050] From Figure 1 As shown in the XRD pattern, the synthetic product is ZSM-11 molecular sieve, its relative crystallinity is 98%, the morphology is similar to that of ZSM-11-1, both are spindle-shaped morphologies, and the particle size is 200 - 400 nm. The total specific surface area of the synthesized ZSM-11 molecular sieve is 421 m 2 / g, the external specific surface area is 219 m 2 / g, the total pore volume is 0.35 cm 3 / g, the mesopore volume is 0.21 cm 3 / g.

[0051] Example 5

[0052] A method for synthesizing spindle-shaped ZSM-11 molecular sieve

[0053] Weigh 17 g of silica sol and 0.5 g of TBAOH, stir for 30 min to obtain gel A; then add 0.2 g of (Al2(SO4)3)·18H2O and 3 g of NaHCO3 to 10 g of deionized water, stir for 30 mim to obtain solution B; subsequently, add solution B dropwise to gel A, and vigorously stir and age the mixture at 30 °C for 0.5 h to obtain the initial gel, pH = 13; then transfer the initial gel to a crystallization kettle and crystallize at 150 °C for 80 h; centrifugate and dry, and finally calcine at 500 °C for 6 h to obtain the synthetic product (ZSM-11-5).

[0054] From Figure 1As can be seen from the XRD pattern shown, the synthesized product is ZSM-11 molecular sieve, with a relative crystallinity of 102%, and its morphology is similar to that of ZSM-11-1, both being jujube pit-like, and the particle size is 200 - 400 nm. The total specific surface area of the synthesized ZSM-11 molecular sieve is 438 m 2 / g, the external specific surface area is 227 m 2 / g, the total pore volume is 0.38 cm 3 / g, and the mesopore volume is 0.24 cm 3 / g.

[0055] Comparative Example

[0056] Weigh 20 g of silica sol, 5 g of TBAOH, 25 g of deionized water, 1 g of (Al2(SO4)3), and 3.5 g of NaOH, add them to a beaker and stir evenly. Then, age the mixture under vigorous stirring at 30 °C for 24 h to obtain an initial gel. Next, transfer the initial gel to a crystallization kettle and crystallize it at 160 °C for 48 h; after centrifugation and drying, finally calcine it at 550 °C for 6 h to obtain the synthesized product (ZSM-11-Ref).

[0057] From Figure 1 the XRD pattern shown, it can be seen that the synthesized product is ZSM-11 molecular sieve, with a relative crystallinity of 103%; from Figure 3 the SEM image shown, it can be seen that its morphology is spherical particles with a size of 800 nm - 1 μm, the total specific surface area is 278 m 2 / g, the external specific surface area is 153 m 2 / g, the total pore volume is 0.19 cm 3 / g, and the mesopore volume is 0.08 cm 3 / g.

[0058] Application Example

[0059] Perform ammonium exchange on the ZSM-11 molecular sieves obtained in the above Examples 1 - 5 and the comparative example respectively: Mix the molecular sieve with 0.1 mol / L ammonium chloride solution at a solid-liquid ratio of 1:10, stir at 80 °C for 60 min, filter, wash, and dry. Repeat the ammonium exchange step 3 times, and calcine at 550 °C for 4 h to obtain hydrogen-type ZSM-11 molecular sieve; use the prepared hydrogen-type ZSM-11 molecular sieve as a catalyst, and prepare a reaction mixture by mixing the catalyst, cyclohexene, and water at a volume ratio of water to alkene of 2:1 and a catalyst mass concentration of 20%. Then, place the reaction mixture in a reaction kettle, displace it with nitrogen 3 times, then raise the temperature of the reaction kettle to 125 °C, raise the reaction pressure to 0.25 MPa, and the stirring speed is 600 r / min. After reacting for 180 min, wait for the temperature of the reaction kettle to drop to room temperature, take out the material and centrifuge it, analyze the composition of the upper layer solution using a gas chromatograph, and calculate the reactant conversion rate and product selectivity. The results are shown in Table 1.

[0060] As can be seen from Table 1, the cyclohexanol selectivity of all catalysts is above 99%. The difference is that when the spindle-shaped ZSM-11 molecular sieve prepared by the present invention is used as the catalyst, the cyclohexene conversion rate is above 13.2%, and the highest can reach 14.2%. However, the cyclohexene conversion rate of the ZSM-11 molecular sieve obtained in the comparative example is only 7.8%. This shows that in the reaction of cyclohexene hydration to prepare cyclohexanol, the spindle-shaped ZSM-11 molecular sieve prepared by the present invention has more excellent catalytic activity than the conventional ZSM-11 molecular sieve. This is mainly because its particles are evenly dispersed, and the wider part in the middle provides a larger reaction space for the reactants, with more active sites. The narrow structures at both ends promote the rapid desorption of product molecules along the pore direction, reducing the occurrence of secondary reactions. In addition, the spindle-shaped ZSM-11 molecular sieve has a rich outer surface and appropriate acidity, with good diffusion performance, thereby improving the cyclohexene conversion rate.

[0061] Table 1 Evaluation results of cyclohexene hydration reaction with different catalysts

[0062]

Claims

1. A spindle-shaped ZSM-11 molecular sieve, characterized in that, Its morphology is that of a jujube pit-shaped particle with a wide middle and narrow ends, and the particle size is 200 - 400 nm.

2. A spindle-shaped ZSM-11 molecular sieve according to claim 1, wherein The total specific surface area of the spindle-shaped ZSM-11 molecular sieve is 400~510 m 2 / g, the external specific surface area is 180~300 m 2 / g, the total pore volume is 0.22~0.36 cm 3 / g, and the mesopore volume is 0.15~0.25 cm 3 / g.

3. The synthesis method of a spindle-shaped ZSM-11 molecular sieve according to claim 1 or 2, characterized in that, It includes the following steps: (1) Mix the silicon source and the template agent evenly to obtain gel A; (2) Dissolve the aluminum source and the strong base weak acid salt in water, and mix evenly to obtain solution B; (3) Dropwise add solution B into gel A, stir evenly to obtain the initial gel, the pH of the initial gel is 9 - 14, and the molar ratio of the initial gel is: SiO2 / Al2O3 = 50 - 200, template agent / SiO2 = 0.001 - 0.005, H2O / SiO2 = 15 - 30, M2O / SiO2 = 0.6 - 2; where M2O is the strong base weak acid salt expressed as an oxide, and M represents an alkali metal ion; (4) Transfer the above initial gel to an autoclave with a polytetrafluoroethylene lining, and obtain the jujube pit-shaped ZSM-11 molecular sieve after crystallization, washing, drying and calcination.

4. The synthesis method of a spindle-shaped ZSM-11 molecular sieve according to claim 3, wherein, The silicon source described in step (1) is at least one of silica sol, fumed silica, sodium silicate, and water glass; the template agent is at least one of tetrabutylammonium hydroxide, tetrabutylammonium bromide, and 1,8-octanediamine; the aluminum source is at least one of aluminum sulfate, sodium aluminate, aluminum isopropoxide, and pseudo-boehmite.

5. The synthesis method of a spindle-shaped ZSM-11 molecular sieve according to claim 3, characterized in that, The strong base weak acid salt described in step (1) is at least one of sodium bicarbonate and potassium bicarbonate.

6. The synthesis method of a spindle-shaped ZSM-11 molecular sieve according to claim 3, wherein The stirring temperature described in step (3) is 20~80 o °C, and the stirring time is 0.5~12 h.

7. The synthesis method of a spindle-shaped ZSM-11 molecular sieve according to claim 3, wherein The crystallization temperature described in step (4) is 120~180 o °C, and the crystallization time is 12~120 h; the calcination temperature is 400~700 o °C, and the calcination time is 2~8 h.

8. Application of the jujube pit-shaped ZSM-11 molecular sieve obtained by the preparation method according to any one of claims 3 - 7 in the hydration of cyclohexene to prepare cyclohexanol.

9. The application according to claim 8, wherein The jujube pit-shaped ZSM-11 molecular sieve is first subjected to ammonium exchange to obtain a hydrogen-type ZSM-11 molecular sieve, and then the hydrogen-type ZSM-11 molecular sieve is used as a catalyst in the hydration of cyclohexene to prepare cyclohexanol.

10. The application according to claim 9, wherein When the jujube pit-shaped ZSM-11 molecular sieve is used in the hydration of cyclohexene to prepare cyclohexanol, the conversion rate of cyclohexene is greater than 13.2%, and the selectivity of cyclohexanol is greater than 99%.

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