Small-grain ZSM-12 molecular sieve and preparation method thereof

Abstract

The present invention relates to the field of catalytic material synthesis, and discloses a small-grain ZSM-12 molecular sieve and a preparation method thereof, wherein the small-grain ZSM-12 molecular sieve adopts a silica-alumina gel (especially an acid-treated silica-alumina gel) with a large silicon-alumina ratio as a composite silicon-alumina source, and uses an acid-treated silica-alumina gel, an inorganic base, a template agent 1,4-bis(N-methylpyrrolidine)butane halide, and water as raw materials for crystallization. The synthesized ZSM-12 molecular sieve has high crystallinity, an adjustable silicon-alumina ratio, a regular morphology, and a grain size of 100-200nm.

Description

Technical Field

[0001] The invention relates to the field of catalytic material synthesis, and in particular to a method for preparing a small-grain ZSM-12 molecular sieve. Background Art

[0002] ZSM-12 molecular sieve is an important silicon-aluminum molecular sieve. It was first synthesized and reported in 1973 by US3832449. It has an MTW topological structure and a 1-dimensional 12-membered ring pore structure. The pore size of the 12-membered ring in the (010) direction is 0.56×0.60nm, which is a large-pore molecular sieve. Due to its good thermal stability, unique pore structure and adjustable acidity, it has shown good catalytic activity and application prospects in the cracking, isomerization, reforming of alkanes and the alkylation of aromatics.

[0003] Due to its small pore size distribution, ZSM-12 molecular sieve makes it difficult for reactant molecules to approach the active sites of the molecular sieve, affecting its utilization rate. Larger product molecules are difficult to leave the active sites, resulting in side reactions, which can easily lead to carbon deposition, low catalytic efficiency, and rapid catalyst deactivation. Small-grained ZSM-12 molecular sieves have the characteristics of large surface area, short diffusion path, and enhanced accessibility of acidic active centers, which makes them have higher catalytic activity.

[0004] CN102648158A discloses a method for synthesizing a small crystal aggregate ZSM-12 molecular sieve, wherein tetraethylammonium hydroxide is used as a template, silicon dioxide, and sodium aluminate are used to crystallize and synthesize the ZSM-12 molecular sieve at 160°C, and the synthesized ZSM-12 crystal morphology is an aggregate formed by the accumulation of small crystals with an average particle size of 0.05 μm. CN102666386B also discloses a ZSM-12 molecular sieve with an average particle size of 0.05 μm, which is synthesized by using two organic amines, methyltriethylammonium chloride and hexamethylammonium chloride, as a composite template. CN106698465 B improves the method based on CN102666386B to synthesize a nano ZSM-12 molecular sieve. The method firstly prepares a carbon material with calcium carbonate, NaOH, sucrose, water and hydrochloric acid through multiple steps, including ultrasonic treatment, filtration, evaporation, drying, washing and repeated high-temperature roasting (900°C constant temperature for 5 hours); then sodium hydroxide, sodium aluminate, white carbon black and methyltriethylammonium chloride are taken to prepare a carbon material mixture, ultrasonic treatment, drying, high-temperature stirring and evaporation of water, and then water is added to crystallize at 160°C for 100 hours to obtain a ZSM-12 molecular sieve with a grain size of less than 100 nm. The method has lengthy and cumbersome steps, and a high temperature of 900°C needs to be reached during the preparation process, and additional chemical reagents such as calcium carbonate, sucrose and hydrochloric acid need to be added. CN103435065B discloses a method for preparing a spherical nanocrystalline aggregate ZSM-12 molecular sieve. The method comprises the following steps: firstly, sodium aluminate, sodium hydroxide, tetraethylammonium bromide, water and silica sol are mixed by stirring for multiple steps to prepare pre-crystallized seeds; then, sodium aluminate, sodium hydroxide, tetraethylammonium bromide, water and silica sol are mixed by stirring for multiple steps to prepare a mixed gel; then, the pre-crystallized seeds are added to the mixed gel, and the mixture is crystallized at 165° C. for 24 hours. The prepared ZSM-12 molecular sieve is a spherical nanocrystalline aggregate, and the size of the nanocrystalline aggregate is about 1400 nm. Although the crystallization time of the synthesis by the method is greatly shortened, the operation steps are long and the operation process is relatively complicated. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects of the prior art that the steps are complicated and the grain size of the prepared ZSM-12 molecular sieve is still large, and to provide a small-grain ZSM-12 molecular sieve and a preparation method thereof. A silica-alumina gel with a large silica-alumina ratio is used as a composite silica-alumina source for synthesis, and 1,4-bis(N-methylpyrrolidine)butane halide is used as a template agent. The synthesized ZSM-12 molecular sieve has high crystallinity, an adjustable silica-alumina ratio, a regular morphology, and a grain size between 100-200nm.

[0006] In order to solve the above technical problems, the present invention provides a method for preparing a small-grain ZSM-12 molecular sieve, which comprises: crystallizing a mixture containing acid-treated silica-alumina gel, an inorganic base, a template and water, wherein the molar ratio of SiO2 to Al2O3 in the acid-treated silica-alumina gel is greater than or equal to 80; and the template is selected from at least one of the compounds shown in Formula I:

[0007]

[0008] In formula I, X is selected from halogen, and n=4.

[0009] The invention also provides a small-grain ZSM-12 molecular sieve.

[0010] Through the above technical scheme, the present invention can obtain a small-grain ZSM-12 molecular sieve based on a simple and easy-to-operate process, which has high crystallinity, adjustable silicon-aluminum ratio, regular morphology, and a grain size in the range of 100-200nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 XRD spectrum of the molecular sieve sample synthesized in Example 2;

[0012] Figure 2 This is a SEM photo of the molecular sieve sample synthesized in Example 2;

[0013] Figure 3 XRD spectrum of the molecular sieve sample synthesized in Example 3;

[0014] Figure 4 This is a SEM photo of the molecular sieve sample synthesized in Example 3;

[0015] Figure 5 XRD spectrum of the molecular sieve sample synthesized in Example 4;

[0016] Figure 6 This is a SEM photo of the molecular sieve sample synthesized in Example 4;

[0017] Figure 7 This is a SEM photo of the molecular sieve sample synthesized in Example 5;

[0018] Figure 8 XRD spectrum of the molecular sieve sample synthesized in Comparative Example 1;

[0019] Fig. 9 This is a SEM photo of the molecular sieve sample synthesized in Comparative Example 1;

[0020] Fig.10 XRD spectrum of the molecular sieve sample synthesized in Comparative Example 2;

[0021] Fig.11 This is a SEM photo of the molecular sieve sample synthesized in Comparative Example 2;

[0022] Fig.12 This is the SEM photograph of the molecular sieve sample synthesized in Comparative Example 3. DETAILED DESCRIPTION

[0023] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0024] Technical terms in the present invention are defined according to their definitions, and are understood according to the common meanings in the art if not defined. The template in the present invention is also called a structure directing agent or an organic structure directing agent in the art.

[0025] The present invention provides a method for preparing a small-grain ZSM-12 molecular sieve, characterized in that the method comprises: crystallizing a mixture containing acid-treated silica-alumina gel, an inorganic base, a template and water, wherein the molar ratio of SiO2 to Al2O3 in the acid-treated silica-alumina gel is greater than or equal to 80; and the template is selected from at least one of the compounds shown in formula I:

[0026]

[0027] In formula I, X is selected from halogen (such as Cl, Br), and n=4.

[0028] In the present invention, the molar ratio of SiO2 to Al2O3 in the acid-treated silica-alumina gel is preferably 90-200 (such as 92, 95, 98, 100, 110, 120, 122, 130, 135, 140, 150, 160, 164, 165, 170, 180, 190, 192, 193, 195, 198 or any value between the above values).

[0029] In a preferred embodiment of the present invention, in order to obtain acid-treated silica-alumina gel, the acid treatment method comprises: immersing silica-alumina gel having a molar ratio of SiO2 to Al2O3 less than 80 in an acid solution, and obtaining the acid-treated silica-alumina gel after washing and drying.

[0030] More preferably, the acid solution is selected from an aqueous hydrochloric acid solution having an HCl content of 3.5-5 wt%.

[0031] More preferably, the weight ratio of the silica-alumina gel with a molar ratio of SiO2 to Al2O3 less than 80 to the acid solution is 1:1.5-5.

[0032] More preferably, the soaking conditions include a temperature of 15-40° C. and a time of 1-2 days.

[0033] According to a preferred embodiment of the present invention, the preparation method of the silicon-aluminum source comprises: adding silicon-aluminum gel to an HCl aqueous solution with an HCl content of 3.5-4%, wherein the amount of the HCl aqueous solution is 1.5-2.5 g per gram of silicon-aluminum gel; stirring and mixing, sealing, and soaking at room temperature for 1-1.5 days; washing the soaked silicon-aluminum gel solid with deionized water until neutral, and drying at 100-110°C for 4-6 hours.

[0034] In the present invention, the amount of OH is relative to the amount of acid-treated alumina gel in terms of SiO2. - The amount of inorganic base used is 0.5-0.7 mol (such as 0.52, 0.54, 0.56, 0.58, 0.62, 0.64, 0.66, 0.68 mol or any value between the above values).

[0035] In the present invention, the amount of the template used is 0.1-0.3 mol (such as 0.12, 0.14, 0.16, 0.17, 0.18, 0.22, 0.26, 0.28 mol or any value in between) per mole of acid-treated silica-alumina gel calculated as SiO2.

[0036] In the present invention, the amount of water used is 10-50 mol (such as 12, 15, 18, 22, 28, 32, 35, 38, 42, 44, 46, 48 mol or any value in between) per mole of acid-treated silica-alumina gel calculated as SiO2.

[0037] In the present invention, there is no particular requirement for the selection of the inorganic base, which may be any common inorganic base in the art. Preferably, the inorganic base is selected from alkali metal hydroxides and / or alkaline earth metal hydroxides, more preferably sodium hydroxide.

[0038] In a preferred embodiment of the present invention, the template agent is selected from 1,4-bis(N-methylpyrrolidine)butane bromide (ie, X is Br and n=4).

[0039] In the present invention, there is no special requirement for the crystallization method. One-stage crystallization or two-stage crystallization can be used. In a preferred embodiment of the present invention, the crystallization includes sequentially performing a first stage crystallization and a second stage crystallization. More preferably, the temperature of the first stage crystallization is 40-60°C lower than the temperature of the second stage crystallization (such as 42, 45, 48, 52, 54, 58°C or any value between the above values). More preferably, the time of the first stage crystallization is 2-6 days shorter than the time of the second stage crystallization (such as 2.2, 2.8, 3.2, 3.5, 3.8, 4.2, 4.5, 4.8, 5.2, 5.5, 5.8 days or any value between the above values).

[0040] More preferably, the conditions of the first stage of crystallization include: a temperature of 110-130° C. (such as 111, 112, 115, 118, 122, 128, 129° C. or any value therebetween). More preferably, the conditions of the first stage of crystallization also include: a time of 1-2 days (such as 1.1, 1.2, 1.3, 1.4, 1.8, 1.9 days or any value therebetween).

[0041] More preferably, the conditions for the second stage of crystallization include: a temperature of 155-170° C. (e.g., 156, 158, 160, 162, 164, 168° C. or any value therebetween). More preferably, the conditions for the second stage of crystallization also include: a time of 4-8 days (e.g., 4.2, 4.5, 4.8, 5.2, 5.5, 5.8, 6.2, 6.5, 6.8, 7.2, 7.5, 7.8 days or any value therebetween).

[0042] In the present invention, the crystallization has no special requirements on the pressure, and can be autogenous pressure. The crystallization can be static crystallization or dynamic crystallization, preferably dynamic crystallization. As a condition for dynamic crystallization, the preferred rotation speed is 15-40r / min.

[0043] In the present invention, in order to obtain the molecular sieve (raw powder), the method may further include subjecting the crystallized product to solid-liquid separation, and then washing and drying the obtained solid phase in sequence to obtain the molecular sieve. Specifically, the solid-liquid separation method can be carried out by conventional methods, such as filtration, centrifugation, etc. In addition, the solid phase obtained by solid-liquid separation is washed before drying. The washing can be carried out by conventional methods, and in order to avoid the introduction of other impurities, it is preferably washed with deionized water to neutrality. The drying can be carried out at a temperature of 80-120°C, and the drying time can be selected according to the drying temperature, generally 4-15 hours.

[0044] The present invention also provides a small-grain ZSM-12 molecular sieve, characterized in that the small-grain ZSM-12 molecular sieve is prepared by the method as described above; or, the grain size of the small-grain ZSM-12 molecular sieve is 100-200nm, and the molar ratio of SiO2 to Al2O3 is 90-200.

[0045] The present invention also relates to the use of acid-treated alumina silica gel (or alumina silica gel with a molar ratio of SiO2 to Al2O3 greater than or equal to 80) in the preparation of small-grained ZSM-12 molecular sieve. The acid treatment method is as described above and will not be repeated here.

[0046] The present invention will be described in detail below by way of examples. In the following examples and comparative examples,

[0047] X-ray powder diffraction (XRD) was performed using a PANalytical Empyrean diffractometer equipped with a PIXcel 3D Detector. Test conditions: Cu target, Kα radiation, Ni filter, tube voltage 40kV, tube current 40mA, scanning range 5°-35°.

[0048] Scanning electron microscope morphology analysis (SEM) uses a Japanese Hitachi S4800 scanning electron microscope. Test conditions: After the sample is dried and ground, it is glued on a conductive adhesive. The acceleration voltage of the analysis electron microscope is 5.0 kV, and the magnification is 20-800000 times.

[0049] X-ray fluorescence analysis (XRF analysis) test instrument: Philips MagiX fluorescence spectrometer. Test conditions: tungsten target, excitation voltage 40kV, excitation current 50mA. The intensity of the characteristic spectrum line of each element is measured by a scintillation calculator and a proportional calculator to perform elemental composition analysis.

[0050] R represents the template 1,4-bis(N-methylpyrrolidino)butane bromide (1,4-MPB).

[0051] Silica gel was purchased from Dongying Yiming New Materials Co., Ltd.

[0052] Example 1

[0053] Example 1 is used to illustrate the preparation of the acid-treated silica-alumina gel used in the present invention.

[0054] Prepare 400g of 3.5% HCl aqueous solution in a beaker, add 200g of silica-alumina gel (SiO2 / Al2O3 molar ratio is 60), stir and mix, seal, and soak at room temperature for 1 day. Wash the soaked silica-alumina gel solid with deionized water until neutral, and dry at 110°C for 5 hours to obtain acid-treated silica-alumina gel.

[0055] Examples 2-7 are used to illustrate the synthesis of the small-grain ZSM-12 molecular sieve by the method of the present invention.

[0056] Example 2

[0057] Take 20.872 grams of deionized water and add it to the polytetrafluoroethylene liner, then add 1,4-bis(N-methylpyrrolidine)butane bromide (1,4-MPB) (mass fraction 69%) and NaOH (mass fraction 96%) in turn, stir and mix evenly, then add the acid-treated silica-alumina gel prepared in Example 1 (solid content 96.7% by weight, SiO2 mass fraction 97.6%, Al2O3 mass fraction 1.67%, SiO2 / Al2O3=99.18), stir evenly, wherein the added molar ratio of each component is: NaOH / SiO2=0.60, R / SiO2=0.15, H2O / SiO2=20.

[0058] The polytetrafluoroethylene liner containing the above reaction mixture is covered, placed in a stainless steel autoclave, sealed, and placed in a rotary convection oven with a rotation speed set to 20 r / min. Crystallization is carried out under autogenous pressure: first crystallize at 120°C for 1 day, then crystallize at 160°C for 5 days. After the crystallization is completed and the temperature is cooled to room temperature, the product is taken out, filtered, washed, and dried at 110°C for 12 hours to obtain a molecular sieve sample.

[0059] The obtained molecular sieve sample was subjected to X-ray diffraction analysis, and the XRD spectrum is shown in Figure 1 , which is ZSM-12 molecular sieve. The morphology of the molecular sieve was observed by SEM. The SEM photos are shown in Figure 2 The molecular sieve has a small granular morphology, a grain size of 100-200nm, and regular grains with a smooth surface, indicating that the molecular sieve has high crystallinity. The molecular sieve is analyzed by XRF, and the SiO2 / Al2O3 molar ratio of the molecular sieve is 90.58.

[0060] Example 3

[0061] The method is as in Example 2, except that the amount of NaOH added is changed to NaOH / SiO2=0.7, the amount of template added is changed to R / SiO2=0.2, and the crystallization conditions are changed to: first crystallize at 120°C for 1 day, and then crystallize at 160°C for 6 days.

[0062] The obtained molecular sieve sample was subjected to X-ray diffraction analysis, and the XRD spectrum is shown in Figure 3 , which is ZSM-12 molecular sieve. The morphology of the molecular sieve was observed by SEM. The SEM photos are shown in Figure 4 The molecular sieve has a small granular morphology, a grain size of 100-200nm, and regular grains with a smooth surface, indicating that the molecular sieve has high crystallinity. The molecular sieve is analyzed by XRF, and the SiO2 / Al2O3 molar ratio of the molecular sieve is 92.71.

[0063] Example 4

[0064] The method is as in Example 2, except that the amount of NaOH added is changed to NaOH / SiO2=0.65, the amount of template added is changed to R / SiO2=0.3, and the amount of water added is changed to H2O / SiO2=40.

[0065] The obtained molecular sieve sample was subjected to X-ray diffraction analysis, and the XRD spectrum is shown in Figure 5 , which is ZSM-12 molecular sieve. The morphology of the molecular sieve was observed by SEM. The SEM photos are shown in Figure 6 The molecular sieve has a small granular morphology, a grain size of 100-200nm, and regular grains with a smooth surface, indicating that the molecular sieve has high crystallinity. The molecular sieve is analyzed by XRF, and the SiO2 / Al2O3 molar ratio of the molecular sieve is 92.04.

[0066] Example 5

[0067] Prepare 400g of 4.0% HCl aqueous solution in a beaker, add 200g of silica-alumina gel (SiO2 / Al2O3=60), stir and mix, seal, and soak at room temperature for 1 day. Wash the soaked silica-alumina gel solid with deionized water until neutral, and dry at 110°C for 5 hours to obtain acid-treated silica-alumina gel.

[0068] Take 30.334 grams of deionized water and add it to the polytetrafluoroethylene liner, then add 1,4-bis(N-methylpyrrolidine)butane bromide (1,4-MPB) (mass fraction 69%) and NaOH (mass fraction 96%) in turn and stir to mix evenly, then add acid-treated silica-alumina gel (solid content 93.90%, SiO2 mass fraction 98.40%, Al2O3 mass fraction 1.38%, SiO2 / Al2O3=121.01), stir evenly, and the molar ratio of each component added is: NaOH / SiO2=0.60, R / SiO2=0.25, H2O / SiO2=30.

[0069] The polytetrafluoroethylene liner containing the above reaction mixture is covered, placed in a stainless steel autoclave, sealed, and placed in a rotary convection oven with a rotation speed set to 20 r / min. Crystallization is carried out under autogenous pressure: first crystallize at 110°C for 2 days, then crystallize at 170°C for 4 days. After the crystallization is completed and the temperature is cooled to room temperature, the product is taken out, filtered, washed, and dried at 110°C for 12 hours to obtain a molecular sieve sample.

[0070] The obtained molecular sieve sample was subjected to X-ray diffraction analysis and was found to be ZSM-12 molecular sieve. The morphology of the molecular sieve was observed by SEM. The SEM photos are shown in Figure 7The molecular sieve has a small granular morphology, a grain size of 100-200nm, and regular grains with a smooth surface, indicating that the molecular sieve has high crystallinity. The molecular sieve is analyzed by XRF, and the SiO2 / Al2O3 molar ratio of the molecular sieve is 113.68.

[0071] Example 6

[0072] Prepare 400g of 4.0% HCl aqueous solution in a beaker, add 200g of silica-alumina gel (SiO2 / Al2O3=60), stir and mix, seal, and soak at room temperature for 2 days. Wash the soaked silica-alumina gel solid with deionized water until neutral, and dry at 110°C for 5 hours to obtain acid-treated silica-alumina gel.

[0073] Take 47.510 grams of deionized water and add it to the polytetrafluoroethylene lining, then add 1,4-bis(N-methylpyrrolidine)butane bromide (1,4-MPB) (mass fraction 69%) and NaOH (mass fraction 96%) in turn and stir to mix evenly, then add acid-treated silica-alumina gel (solid content 94.20%, SiO2 mass fraction 98.10%, Al2O3 mass fraction 1.01%, SiO2 / Al2O3=164.83), stir evenly, and the molar ratio of each component added is: NaOH / SiO2=0.7, R / SiO2=0.2, H2O / SiO2=45.

[0074] The polytetrafluoroethylene liner containing the above reaction mixture is covered, placed in a stainless steel autoclave, sealed, and placed in a rotary convection oven with a rotation speed set to 20 r / min. Crystallization is carried out under autogenous pressure: first crystallize at 110°C for 1 day, then crystallize at 170°C for 5 days. After the crystallization is completed and the temperature is cooled to room temperature, the product is taken out, filtered, washed, and dried at 110°C for 12 hours to obtain a molecular sieve sample.

[0075] The obtained molecular sieve sample was subjected to X-ray diffraction analysis, which was ZSM-12 molecular sieve. The morphology of the molecular sieve was observed by SEM, which showed a small granular morphology with a grain size of 100-200nm, regular grains and a smooth surface, indicating that the molecular sieve had high crystallinity. The molecular sieve was analyzed by XRF, and the SiO2 / Al2O3 molar ratio of the molecular sieve was 149.10.

[0076] Example 7

[0077] Prepare 400g of 4.5% HCl aqueous solution in a beaker, add 200g of silica-alumina gel (SiO2 / Al2O3=60), stir and mix, seal, and soak at room temperature for 2 days. Wash the soaked silica-alumina gel solid with deionized water until neutral, and dry at 110°C for 5 hours to obtain acid-treated silica-alumina gel.

[0078] Take 26.426 grams of deionized water and add it to the polytetrafluoroethylene lining, then add 1,4-bis(N-methylpyrrolidine)butane bromide (1,4-MPB) (mass fraction 69%) and NaOH (mass fraction 96%) in turn and stir to mix evenly, then add acid-treated silica-alumina gel (solid content 95.50%, SiO2 mass fraction 98.60%, Al2O3 mass fraction 0.87%, SiO2 / Al2O3=192.33), stir evenly, and the molar ratio of each component added is: NaOH / SiO2=0.55, R / SiO2=0.15, H2O / SiO2=25.

[0079] The polytetrafluoroethylene liner containing the above reaction mixture is covered, placed in a stainless steel autoclave, sealed, and placed in a rotary convection oven with a rotation speed set to 20 r / min. Crystallization is carried out under autogenous pressure: first crystallize at 120°C for 1 day, then crystallize at 160°C for 7 days. After the crystallization is completed and the temperature is cooled to room temperature, the product is taken out, filtered, washed, and dried at 110°C for 12 hours to obtain a molecular sieve sample.

[0080] The obtained molecular sieve sample was subjected to X-ray diffraction analysis, which was ZSM-12 molecular sieve. The morphology of the molecular sieve was observed by SEM, which showed a small granular morphology with a grain size of 100-200nm, regular grains and a smooth surface, indicating that the molecular sieve had high crystallinity. The molecular sieve was analyzed by XRF, and the SiO2 / Al2O3 molar ratio of the molecular sieve was 176.49.

[0081] Comparative Example 1

[0082] The method is as in Example 2, except that the template agent is 1,4-bis(N-methylpyrrolidine)butane hydroxide (1,4-MPBOH) (mass fraction 35.58%).

[0083] The obtained solid was subjected to X-ray diffraction analysis, and the XRD spectrum is shown in Figure 8 , for other molecular sieves. The morphology of the molecular sieve was observed by SEM, and the SEM photos are shown in Fig. 9 , with lamellar and spherical morphologies coexisting, and the grain size of the spherical morphology is about 1μm.

[0084] Comparative Example 2

[0085] According to Example 4, the difference is that silica-alumina gel without acid treatment is used (solid content 97.1%, SiO2, mass fraction 98.44%, Al2O3 mass fraction 1.6%, SiO2 / Al2O3=104.41), and the silicon-aluminum ratio is similar to that of Example 4.

[0086] The obtained solid was subjected to X-ray diffraction analysis, and the XRD spectrum is shown in Fig.10, for other molecular sieves. The morphology of the molecular sieve was observed by SEM, and the SEM photos are shown in Fig.11 , not in the form of small grains.

[0087] Comparative Example 3

[0088] The method is as in Example 5, except that coarse pore silica gel (solid content 91.3%) is used as the silicon source, and sodium aluminate (specific gravity 1.241 g / ml, Al2O3 mass concentration 101.5 g / L, NaOH mass fraction 194.0 g / L) is used as the aluminum source.

[0089] The obtained solid was subjected to X-ray diffraction analysis and was found to be other molecular sieves. The morphology of the molecular sieve was observed by SEM. The SEM photos are shown in Fig.12 It can be seen that under the same ratio and crystallization conditions, ZSM-12 molecular sieve and small-grain ZSM-12 molecular sieve cannot be synthesized by using only silicon source and aluminum source.

[0090] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for preparing small-grain ZSM-12 molecular sieve, characterized in that: The method comprises: crystallizing a mixture containing acid-treated alumina silica gel, an inorganic base, a template and water, wherein the acid treatment method comprises immersing alumina silica gel having a molar ratio of SiO2 to Al2O3 less than 80 in an acid solution, washing and drying to obtain the acid-treated alumina silica gel; the acid solution is selected from a hydrochloric acid aqueous solution having an HCl content of 3.5-5% by weight; the molar ratio of SiO2 to Al2O3 in the acid-treated alumina silica gel is greater than or equal to 80; and the template is selected from at least one of the compounds shown in formula I: Formula I In formula I, X is selected from halogen, n=4; The soaking conditions include a temperature of 15-40° C. and a time of 0.5-2 days; The weight ratio of the silica-alumina gel having a molar ratio of SiO2 to Al2O3 less than 80 to the acid solution is 1:1.5-5; Relative to each mole of acid-treated silica-alumina gel, calculated as SiO2, OH - The amount of inorganic base is 0.5-0.7 mol, the amount of template is 0.1-0.3 mol, and the amount of water is 10-50 mol; The crystallization includes sequentially performing a first stage crystallization and a second stage crystallization; The conditions of the first stage of crystallization include: a temperature of 110-130° C. and a time of 1-2 days; The conditions of the second stage crystallization include: temperature of 155-170° C. and time of 4-8 days.

2. The method according to claim 1, wherein: The molar ratio of SiO2 to Al2O3 in the acid-treated silica-alumina gel is 90-200.

3. The method according to claim 1, wherein: The inorganic base is selected from alkali metal hydroxides and / or alkaline earth metal hydroxides.

4. The method according to claim 1, wherein: The inorganic base is selected from sodium hydroxide.

5. The method according to claim 1, wherein: The template agent is selected from 1,4-bis(N-methylpyrrolidine)butane bromide and / or 1,4-bis(N-methylpyrrolidine)butane chloride.

6. The method according to any one of claims 1 to 5, wherein: The method further comprises performing solid-liquid separation on the crystallized product, and then washing and drying the obtained solid phase in sequence to obtain molecular sieve raw powder.

7. A small-grain ZSM-12 molecular sieve, characterized in that: The small-grain ZSM-12 molecular sieve is prepared by the method described in any one of claims 1 to 6.

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