SAPO-34 molecular sieve, its preparation method and application

Abstract

The present invention relates to the field of synthesis of catalytic materials, and discloses a SAPO-34 molecular sieve, a preparation method thereof and an application thereof. The method comprises the following steps: providing a raw material mixture containing an aluminum source, water, a phosphorus source, a silicon source, a template agent and modified starch, wherein the dosage of the modified starch is 2-10 g relative to every 100 g of the aluminum source calculated as Al2O3; carrying out crystallization on the raw material mixture under crystallization conditions; carrying out solid-liquid separation on the crystallized product, and taking the solid phase to carry out drying and calcination in sequence. The SAPO-34 molecular sieve prepared by the method of the present invention has a lower strength (desorption temperature) of medium / strong acid and a lower amount of medium strong acid / strong acid.

Description

Technical Field

[0001] The present invention relates to the field of synthesis of catalytic materials, and particularly to a SAPO-34 molecular sieve, a preparation method thereof, and an application thereof. Background Art

[0002] In 1984, UCC Company in the United States reported the first synthesis of SAPO-34 molecular sieve (US4440871). Since the SAPO-34 molecular sieve has a chabazite type (CHA structure), with unique pore size, pore structure, acid strength, as well as good thermal stability and hydrothermal stability, it has a high selectivity for ethylene and propylene in the reaction of methanol to olefins (MTO), showing excellent catalytic performance, and has currently become the preferred catalyst for the MTO process.

[0003] Although the SAPO-34 molecular sieve has currently become the preferred catalyst for MTO technology and shows excellent catalytic performance in the MTO reaction, the SAPO-34 molecular sieve is prone to coking during the catalytic MTO reaction. The coking products block the pores of the SAPO-34 molecular sieve, causing the SAPO-34 molecular sieve to quickly deactivate. Generally speaking, the too high acid strength and acid amount of the SAPO-34 molecular sieve will cause a decrease in the selectivity of light olefins in the MTO reaction, an increase in hydrogen transfer reactions, and the catalyst to quickly coke and deactivate, which has an adverse effect on the MTO reaction. This is because too high acid strength will accelerate the coking formation rate of the MTO reaction, and the coke will cover the acidic sites or block the pores, thus causing the rapid coking deactivation of the SAPO-34 molecular sieve. In addition, too high acid strength will increase side reactions such as hydrogen transfer reactions in the MTO reaction, resulting in a decrease in the selectivity of the target product light olefins and an increase in the selectivity of by-product alkanes. Therefore, the SAPO-34 molecular sieve with appropriately reduced acid strength and reduced acid amount has more excellent MTO catalytic performance.

[0004] At present, the main strategy for synthesizing SAPO-34 molecular sieves with weak acid strength and low acid amount is to reduce the silicon content of SAPO-34 molecular sieves to synthesize low-silicon SAPO-34 molecular sieves. The literature (The Journal of Physical Chemistry C, 2011, 115(45): 22505-22513.) found that low-silicon SAPO-34 molecular sieves have lower Brønsted acid strength than high-silicon SAPO-34 molecular sieves. Generally, when the SiO2 / Al2O3 molar ratio of SAPO-34 molecular sieves is less than 0.3, it is called a low-silicon SAPO-34 molecular sieve, and when the SiO2 / Al2O3 molar ratio is higher than 0.3, it is called a high-silicon SAPO-34 molecular sieve. US6793901 mentions that reducing the silicon content of SAPO-34 molecular sieves can effectively reduce their acid strength and acid amount, and at the same time can reduce the coke deposition rate of SAPO-34 molecular sieves in the MTO reaction. The literature (Microporous and Mesoporous Materials, 1999, 29(1-2): 117-126.) also reported that low-silicon SAPO-34 molecular sieves can reduce the propane selectivity and the molecular sieve deactivation rate in the MTO reaction. The literature (Microporous and Mesoporous Materials, 2008, 115(3): 332-337.) found that when the silicon content in the synthesis system is low, the synthesis system is prone to direct the formation of silicon aluminophosphate molecular sieves such as SAPO-5, SAPO-11, and SAPO-18.

[0005] CN108328623A discloses a preparation method of a SAPO-34 molecular sieve with low acidity. After adding a cationic surfactant (such as dodecyltrimethylammonium bromide, cetyltrimethylammonium bromide, or octadecyltrimethylammonium bromide) to water and stirring, a template agent I is added and stirred for 0.5-1.0 h, and finally a silicon source is added and stirred to form solution A; an aluminum source is added to a template agent II, stirred, and then a phosphorus source is added and stirred. Finally, an anionic surfactant (such as sodium dodecylsulfonate, sodium dodecylcarboxylate, or cetylmethionine) is added and stirred to form solution B; solution A and solution B are respectively crystallized, and after crystallization, solution A and solution B are mixed and stirred to form solution C for crystallization to obtain a SAPO-34 molecular sieve with low acidity. This method adds two additional surfactants, resulting in high synthesis costs, a high water addition amount (the H2O / Al2O3 molar ratio is 30-60), and cumbersome operation steps, which is not conducive to industrial scale-up production.

[0006] CN108163872B discloses a method for preparing a SAPO-34 molecular sieve with a low acid density. First, water, a template agent, and a silicon source are stirred for 1 - 24 hours, and then phosphoric acid is quickly added to form a mixed solution A; the mixed solution A is heated to 60 - 80 °C and then an aluminum source is added, and stirring is continued for 0.5 - 4 hours to form a mixed solution B; then, low-temperature liquid nitrogen is introduced into the mixed solution B for refrigeration, and the temperature of the mixed solution B is reduced to 0 - 20 °C within 0.5 - 2 hours; then, the mixed solution B is crystallized at 130 - 200 °C for 12 - 60 hours to obtain a SAPO-34 molecular sieve with a low acid density. This method requires heating the raw material mixture to 60 - 80 °C and then quickly cooling it with liquid nitrogen. The water addition amount (the molar ratio of H2O / Al2O3 is 20 - 90) is high, resulting in high energy consumption, large water consumption, and large wastewater discharge, and the method of quickly cooling with liquid nitrogen makes it impossible to carry out large-scale production in industry.

[0007] In summary, the range of silicon content reduction in the SAPO-34 molecular sieve is limited, and the range of acid amount reduction by reducing the silicon content is also limited. Therefore, it is of great significance to develop a synthesis method with simple feeding and suitable for industrial production through the improvement and modulation of the synthesis method to further reduce the acid amount of the SAPO-34 molecular sieve with low silicon content and synthesize a SAPO-34 molecular sieve with weak acid strength and less acid amount.

[0008] In addition, due to its small pore size, the SAPO-34 molecular sieve will also cause problems such as easy coke formation and rapid deactivation during the catalytic MTO reaction. To solve this problem, researchers have found that introducing appropriate mesopores or (and) macropores into the microporous structure of the SAPO-34 molecular sieve can effectively improve the diffusion efficiency of reactants and products, reduce reaction coke formation, and thus extend the life of the catalyst. Summary of the Invention

[0009] To solve the above technical problems, the purpose of the present invention is to provide a SAPO-34 molecular sieve, its preparation method and application. The obtained SAPO-34 molecular sieve has weak acid strength and less acid amount. The preparation method has a low water addition amount, and the raw material preparation process is simple and fast, which is easy for industrial scale production.

[0010] To solve the above technical problems, the first aspect of the present invention provides a method for preparing a SAPO-34 molecular sieve, which includes the following steps:

[0011] (1) Provide a raw material mixture containing an aluminum source, water, a phosphorus source, a silicon source, a template agent, and modified starch. The dosage of the modified starch is 2 - 10 g relative to every 100 g of the aluminum source calculated as Al2O3;

[0012] (2) Under crystallization conditions, crystallize the raw material mixture.

[0013] (3) Separate the solid and liquid of the crystallized product, and take the solid phase to carry out drying and calcination in sequence.

[0014] The second aspect of the present invention provides a SAPO-34 molecular sieve, and this SAPO-34 molecular sieve is prepared by the method described in the first aspect;

[0015] Alternatively, the total pore volume of the SAPO-34 molecular sieve is 0.36 - 0.43 cm 3 / g, the mesopore volume is 0.20 - 0.26 cm 3 / g, the micropore volume is 0.15 - 0.19 cm 3 / g, and the proportion of the mesopore volume in the total pore volume is 55 - 63%; the total acid amount of the SAPO-34 molecular sieve is less than 1645 μmol g -1 , the desorption temperature of medium-strong / strong acid is lower than 351 °C, and the medium-strong / strong acid amount is less than 590 μmol g -1 .

[0016] The third aspect of the present invention provides the application of the SAPO-34 molecular sieve described in the second aspect as a catalyst in acid-catalyzed reactions.

[0017] Through the above technical solutions, the present invention has obtained the following beneficial effects:

[0018] (1) By using the method of the present invention, a pure-phase SAPO-34 molecular sieve can be obtained, and the strength (desorption temperature) of the medium-strong / strong acid of the SAPO-34 molecular sieve is relatively low, and the medium-strong acid / strong acid amount is relatively low.

[0019] (2) The SAPO-34 molecular sieve prepared by using the method of the present invention has a relatively large pore volume, and the proportion of the mesopore volume in the total pore volume is relatively high.

[0020] (3) The preparation method of the present invention is simple, easy for industrial scale production, and the water consumption in the molecular sieve preparation process is relatively small. Description of the Drawings

[0021] Figure 1 It is the XRD pattern of the calcined sample of the molecular sieve in Example 1;

[0022] Figure 2 It is the XRD pattern of the calcined sample of the molecular sieve in Example 2;

[0023] Figure 3 It is the XRD pattern of the calcined sample of the molecular sieve in Example 3;

[0024] Figure 4 It is the XRD pattern of the calcined sample of the molecular sieve in Example 4;

[0025] Figure 5SEM photograph of the calcined zeolite sample of Example 1;

[0026] Figure 6 SEM photograph of the calcined zeolite sample of Example 2;

[0027] Figure 7 SEM photograph of the calcined zeolite sample of Example 3;

[0028] Figure 8 SEM photograph of the calcined zeolite sample of Example 4;

[0029] Figure 9 XRD pattern of the calcined zeolite sample of Comparative Example 1;

[0030] Figure 10 SEM photograph of the calcined zeolite sample of Comparative Example 1;

[0031] Figure 11 NH₃-TPD spectrum of the synthesized zeolite sample, where S1 - S4 are the NH₃-TPD curves of the synthesized zeolite samples of Examples 1 - 4 in sequence, and D1 is the NH₃-TPD curve of the synthesized zeolite sample of Comparative Example 1. Detailed implementation manners

[0032] The endpoints and any values within the ranges disclosed herein are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoints of each range, between the endpoints of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0033] For the technical terms in the present invention, those with definitions are understood according to their definitions, and those without definitions are understood in accordance with the common meanings in the art.

[0034] The first aspect of the present invention provides a method for preparing SAPO-34 zeolite, the method comprising the following steps:

[0035] (1) Provide a raw material mixture containing an aluminum source, water, a phosphorus source, a silicon source, a template agent, and modified starch. With respect to every 100 g of the aluminum source calculated as Al₂O₃, the amount of the modified starch used is 2 - 10 g (such as 2.5, 2.8, 3, 3.2, 4, 4.8, 5, 5.2, 6, 7, 7.8, 8, 8.2, 9, 9.5 g or any value between the above values);

[0036] (2) Under crystallization conditions, crystallize the raw material mixture;

[0037] (3) Carry out solid-liquid separation on the crystallized product, and take the solid phase to carry out drying and calcination in sequence.

[0038] According to the present invention, the type of the template agent can be selected within a relatively wide range. Preferably, the template agent includes template agent R1 and template agent R2. The template agent R1 is triethylamine, and the template agent R2 is tetraethylammonium hydroxide.

[0039] According to the present invention, the ratio of each raw material can be a conventional selection. According to the methods reported in the existing literature, when the silicon content in the synthesis system is low, the synthesis system is likely to direct the formation of silicoaluminophosphate molecular sieves such as SAPO-5, SAPO-11, and SAPO-18, and it is difficult to obtain pure-phase SAPO-34 molecular sieve. However, the method of the present invention can obtain pure-phase SAPO-34 molecular sieve when the molar ratio of the silicon source to the aluminum source is less than 0.3. Therefore, in the raw material mixture of the present invention, the molar ratio of the silicon source calculated as SiO2 to the aluminum source calculated as Al2O3 is less than 0.3. Preferably, the aluminum source is calculated as Al2O3, the phosphorus source is calculated as P2O5, and the silicon source is calculated as SiO2. In the raw material mixture, the molar ratio of the aluminum source, the phosphorus source, the silicon source, the template agent, and water is 1:0.8 - 1:0.05 - 0.17:0.4 - 3:15 - 20. More preferably, as described above, the template agent includes template agent R1 and template agent R2. The aluminum source is calculated as Al2O3, the phosphorus source is calculated as P2O5, and the silicon source is calculated as SiO2. In the raw material mixture, the molar ratio of the aluminum source, the phosphorus source, the silicon source, template agent R1, template agent R2, and water is 1:0.8 - 1:0.05 - 0.17:1.8 - 2.1:0.4 - 0.6:15 - 20. By using the method of the present invention, molecular sieves with excellent performance can be prepared under the condition of low water consumption. Therefore, the method of the present invention has a low wastewater discharge and is more suitable for industrial scale production.

[0040] According to the present invention, preferably, the modified starch is cross-linked starch; more preferably, the cross-linked starch is non-ionic cross-linked starch, and more preferably epichlorohydrin cross-linked starch. The modified starch can be obtained by commercial purchase, for example, purchased from Hangzhou Chemical Industry Research Institute Co., Ltd.; it can also be prepared by itself, for example, according to the following method: Add sodium hydroxide solution (the concentration can be 0.5 - 1.5 mol / L) to the mixture of starch, water, and sodium chloride until the pH is 10 - 12, then add epichlorohydrin, react at 40 - 60 °C for 3 - 7 h, then add hydrochloric acid (the concentration can be 0.5 - 1.5 mol / L) to neutralize the pH to 6 - 7, let it stand for 25 - 40 min, and then wash and dry (such as drying at 70 - 100 °C for 3 - 5 h) to obtain the modified starch, wherein, relative to 50 g of starch, the amount of water used is preferably 30 - 100 g, the amount of sodium chloride used is preferably 1.5 - 5 g, and the amount of epichlorohydrin used is preferably 1 - 3 g.

[0041] According to the present invention, the aluminum source is at least one of aluminum oxide, aluminum hydroxide, and pseudo-boehmite.

[0042] According to the present invention, the phosphorus source is orthophosphoric acid.

[0043] According to the present invention, the silicon source is silicon dioxide, preferably fumed silica and / or silica gel.

[0044] According to the present invention, in the present invention, there is no particular requirement for the crystallization method. One-stage crystallization or two-stage crystallization can be adopted. The crystallization conditions can be conventional conditions in the art. Preferably, the crystallization is a two-stage crystallization process, including a first-stage crystallization and a second-stage crystallization. The temperature of the first-stage crystallization is lower than that of the second-stage crystallization. More preferably, the temperature of the first-stage crystallization is 10-50 °C lower than that of the second-stage crystallization, and more preferably 20-40 °C lower. More preferably, the time of the first-stage crystallization is 1-10 h shorter than that of the second-stage crystallization, and further preferably 4-9 h shorter.

[0045] According to the present invention, in order to further reduce the acid amount of the SAPO-34 molecular sieve and increase the pore volume and the proportion of mesopore volume, preferably, the first-stage crystallization is carried out at autogenous pressure and 140-160 °C for 33-40 hours, and the second-stage crystallization is carried out at autogenous pressure and 170-190 °C for 35-50 hours. More preferably, the first-stage crystallization is carried out at autogenous pressure and 145-155 °C for 35-39 hours, and the second-stage crystallization is carried out at autogenous pressure and 175-185 °C for 40-50 hours.

[0046] In the present invention, the crystallization can adopt static crystallization or dynamic crystallization, preferably dynamic crystallization. As the conditions for dynamic crystallization, the rotation speed is preferably 15-40 r / min.

[0047] According to the present invention, the drying conditions can be conventional conditions in the art. Preferably, the drying temperature is 70-110 °C, and the drying time is 4-12 h.

[0048] According to the present invention, the calcination conditions can be conventional conditions in the art. Preferably, the calcination temperature is 500-600 °C, and the calcination time is 3-5 h.

[0049] The raw material mixture is obtained by first stirring and dissolving the aluminum source and deionized water, then dropping the phosphorus source, stirring evenly and then adding the silicon source and mixing evenly, then adding the template agents R1 and R2 and stirring evenly, and then adding the modified starch to stir to form a raw material mixture (initial raw material mixture).

[0050] In the second aspect of the present invention, a SAPO-34 molecular sieve is provided, and the SAPO-34 molecular sieve is prepared by the method described in the first aspect;

[0051] Alternatively, the total pore volume of the SAPO-34 molecular sieve is 0.36 - 0.43 cm 3 / g, the mesopore volume is 0.20 - 0.26 cm 3 / g, the micropore volume is 0.15 - 0.19 cm 3 / g, and the proportion of the mesopore volume in the total pore volume is 55 - 63%; the total acid amount of the SAPO-34 molecular sieve is less than 1645 μmol g -1 , the desorption temperature of medium-strong / strong acid is lower than 351 °C, and the amount of medium-strong / strong acid is less than 590 μmol g -1 .

[0052] Preferably, the total acid amount of the SAPO-34 molecular sieve is 1500 - 1645 μmol·g -1 , the amount of medium-strong / strong acid is 490 - 590 μmol·g -1 , and the desorption temperature of medium-strong / strong acid is 341 - 351 °C.

[0053] Preferably, the crystal grain size of the SAPO-34 molecular sieve is in the range of 200 - 900 nm.

[0054] In the third aspect of the present invention, an application of the SAPO-34 molecular sieve described in the second aspect as a catalyst in an acid-catalyzed reaction is provided.

[0055] The present invention will be described in detail below through examples. In the following examples and comparative examples,

[0056] X-ray powder diffraction phase analysis (XRD) is performed using a PANalytical Empyrean diffractometer from the Netherlands, which is equipped with a PIXcel 3D detector. Test conditions: Cu target, Kα radiation, Ni filter, tube voltage 40 kV, tube current 40 mA, scanning range 5° - 50°.

[0057] Scanning electron microscope morphology analysis (SEM) is performed using a Hitachi S4800 scanning electron microscope from Japan. Test conditions: After the sample is dried and ground, it is adhered to the conductive adhesive. The accelerating voltage of the analysis electron microscope is 5.0 kV, and the magnification is 20 - 800000 times.

[0058] Temperature-programmed desorption of ammonia (NH3-TPD analysis) is performed using an Autochem II 2920 temperature-programmed desorption instrument from Micromeritics, USA. Test conditions: Weigh 0.232 g of 20 - 40 mesh molecular sieve and load it into the sample tube, place it in the thermal conductivity cell heating furnace, and use He gas as the carrier gas (25 mL·min -1), heating up to 600 °C at a rate of 20 °C·min -1 . Purge for 60 min to remove the impurities adsorbed on the surface of the molecular sieve. Then cool down to 100 °C, keep the temperature constant for 10 min, switch to NH3-He mixed gas (10.02% NH3 + 89.98% He) for adsorption for 30 min, and then continue to purge with He gas for 90 min until the baseline is stable to desorb the physically adsorbed ammonia. Heat up at a rate of 10 °C·min -1 for programmed temperature rise to 600 °C for desorption, keep for 30 min, and the desorption ends. Use a TCD detector to detect the change of gas components, and the instrument automatically integrates to obtain the acid amount.

[0059] For physical adsorption (BET analysis), a Micromeritics ASAP 2010 type static nitrogen adsorption instrument is used. Test conditions: The weighed sample is evacuated to 1.33×10 -2 Pa at 350 °C, keep the temperature and pressure constant for 15 h for sample pretreatment. At the liquid nitrogen temperature of -196 °C, measure the adsorption amount and desorption amount of nitrogen by the sample under different relative pressures p / p0 to obtain the nitrogen adsorption-desorption isotherm curve. Then use the BET formula to calculate the BET specific surface area, use the t-plot method to calculate the micropore specific surface area and micropore volume of the sample, and calculate the total pore volume based on the adsorption amount at P / P0 = 0.98.

[0060] Let R represent the template agent, R1 is triethylamine (TEA), and R2 is tetraethylammonium hydroxide (TEAOH).

[0061] Preparation Example

[0062] Add 50 g of cassava starch, 50 g of water, and sodium chloride accounting for 6% of the starch weight into the flask in sequence, stir evenly and keep stirring, then slowly drop 1 mol / L sodium hydroxide solution until the pH is 10.5. After stirring for 10 min, drop 1.2 g of epichlorohydrin, stir and react at 50 °C for 5 h under closed conditions, then drop 1 mol / L hydrochloric acid to neutralize the pH to 6.5, and let it stand for 30 min. Filter the reaction mixture, wash it to neutral, dry it at 80 °C for 4 h, and grind it to obtain epichlorohydrin cross-linked starch.

[0063] Example 1

[0064] 4.248 g of pseudo-boehmite (mass fraction of Al2O3: 72%) was added to deionized water and stirred. Orthophosphoric acid (H3PO4 mass fraction: 85%) was added dropwise to the aqueous solution of pseudo-boehmite. After stirring evenly, solid silica gel (SiO2 mass fraction: 90%) was added and mixed evenly. Then, triethylamine (TEA mass fraction: 99%) and tetraethylammonium hydroxide (TEAOH mass fraction: 27.2%) were added in sequence and stirred evenly. Finally, the epichlorohydrin cross-linked starch prepared in the preparation example was added and stirred evenly to form a raw material mixture. In the raw material mixture, the molar ratio of the aluminum source calculated as Al2O3, the phosphorus source calculated as P2O5, the silicon source calculated as SiO2, template agent R1, template agent R2 and water was 1:1:0.15:2:0.5:20, and the addition amount of the epichlorohydrin cross-linked starch was 3% by weight of the weight of the aluminum source calculated as Al2O3.

[0065] The polytetrafluoroethylene inner liner containing the above raw material mixture was covered and placed in a stainless steel autoclave for sealing, and then placed in a rotating convection oven with the rotation speed set at 20 r / min. The first-stage crystallization was carried out under autogenous pressure: crystallization at 150 °C for 36 hours, then the temperature was raised to 180 °C for the second-stage crystallization: crystallization at 180 °C for 45 hours. After the crystallization was completed, the temperature was lowered to room temperature and the crystallization product was taken out. After filtration and washing, it was dried at 80 °C for 10 hours and calcined at 550 °C for 3 hours to obtain the molecular sieve sample.

[0066] The obtained calcined molecular sieve sample was subjected to X-ray diffraction analysis. The XRD pattern is shown in Figure 1 , which is a pure-phase SAPO-34 molecular sieve. The morphology of the molecular sieve was observed by SEM. The SEM photograph is shown in Figure 5 , showing a blocky morphology with the grain size in the range of 400 - 600 nm.

[0067] Example 2

[0068] 3.121 g of activated alumina (mass fraction of Al2O3: 98%) was added to deionized water and stirred. Orthophosphoric acid (H3PO4 mass fraction: 85%) was added dropwise to the aqueous solution of activated alumina. After stirring evenly, white carbon black (SiO2 mass fraction: 89%) was added and mixed evenly. Then, triethylamine (TEA mass fraction: 99%) and tetraethylammonium hydroxide (TEAOH mass fraction: 27.2%) were added in sequence and stirred evenly. Finally, the epichlorohydrin cross-linked starch prepared in the preparation example was added and stirred evenly to form a raw material mixture. In the raw material mixture, the molar ratio of the aluminum source calculated as Al2O3, the phosphorus source calculated as P2O5, the silicon source calculated as SiO2, template agent R1, template agent R2 and water was 1:0.9:0.1:2.1:0.4:15, and the addition amount of the epichlorohydrin cross-linked starch was 8% by weight of the weight of the aluminum source calculated as Al2O3.

[0069] Cover the polytetrafluoroethylene inner liner filled with the above raw material mixture, place it in a stainless-steel autoclave, seal it, and put it in a rotating convection oven with the rotation speed set at 20 r / min. Conduct the first-stage crystallization under autogenous pressure: crystallize at 155 °C for 35 hours, then raise the temperature to 185 °C and conduct the second-stage crystallization: crystallize at 185 °C for 40 hours. After the crystallization is completed, cool it to room temperature and take out the crystallization product. Filter, wash it, and then dry it at 80 °C for 10 hours and calcine it at 550 °C for 3 hours to obtain the molecular sieve sample.

[0070] Perform X-ray diffraction analysis on the obtained calcined molecular sieve sample. The XRD pattern is shown in Figure 2 , which is a pure-phase SAPO-34 molecular sieve. Observe the morphology of the molecular sieve by SEM. The SEM photo is shown in Figure 6 , presenting a blocky morphology with a grain size of 300 - 600 nm.

[0071] Example 3

[0072] Prepare the molecular sieve according to the method of Example 1, except that the addition amount of epichlorohydrin cross-linked starch is 8 wt% of the weight of the aluminum source calculated as Al2O3.

[0073] Perform X-ray diffraction analysis on the obtained calcined molecular sieve sample. The XRD pattern is shown in Figure 3 , which is a pure-phase SAPO-34 molecular sieve. Observe the morphology of the molecular sieve by SEM. The SEM photo is shown in Figure 7 , presenting a blocky morphology with a grain size of 200 - 600 nm.

[0074] Example 4

[0075] Prepare the molecular sieve according to the method of Example 1, except that the addition amount of epichlorohydrin cross-linked starch is 5 wt% of the weight of the aluminum source calculated as Al2O3.

[0076] Perform X-ray diffraction analysis on the obtained calcined molecular sieve sample. The XRD pattern is shown in Figure 4 , which is a pure-phase SAPO-34 molecular sieve. Observe the morphology of the molecular sieve by SEM. The SEM photo is shown in Figure 8 , presenting a blocky morphology with a grain size of 200 - 600 nm.

[0077] Example 5

[0078] 3.121 g of activated alumina (mass fraction of Al2O3: 98%) was added to deionized water and stirred. Orthophosphoric acid (H3PO4 with a mass fraction of 85%) was added dropwise to the aqueous solution of activated alumina. After stirring evenly, silica white (SiO2 with a mass fraction of 89%) was added and mixed evenly. Then, triethylamine (TEA with a mass fraction of 99%) and tetraethylammonium hydroxide (TEAOH with a mass fraction of 27.2%) were added successively and stirred evenly. Finally, the epichlorohydrin cross-linked starch prepared in the preparation example was added and stirred evenly to form a raw material mixture. In the raw material mixture, the molar ratio of the aluminum source calculated as Al2O3, the phosphorus source calculated as P2O5, the silicon source calculated as SiO2, the template agent R1, the template agent R2 and water was 1:1:0.07:2:0.6:17, and the addition amount of the epichlorohydrin cross-linked starch was 5% by weight of the weight of the aluminum source calculated as Al2O3.

[0079] The polytetrafluoroethylene inner liner containing the above raw material mixture was covered and placed in a stainless steel autoclave, sealed, and placed in a rotating convection oven. The rotation speed was set at 20 r / min, and the first-stage crystallization was carried out under autogenous pressure: crystallization at 145 °C for 39 hours, then the temperature was raised to 180 °C, and the second-stage crystallization was carried out: crystallization at 180 °C for 45 hours. After the crystallization was completed, the crystallization product was taken out after cooling to room temperature, filtered, washed, and then dried at 80 °C for 10 hours and calcined at 550 °C for 3 hours to obtain the molecular sieve sample.

[0080] The obtained calcined molecular sieve sample was subjected to X-ray diffraction analysis, and it was a pure-phase SAPO-34 molecular sieve. The morphology of the molecular sieve was observed by SEM, showing a massive morphology, and the crystal grain size was 200 - 600 nm.

[0081] Example 6

[0082] 4.078 g of aluminum hydroxide (mass fraction of Al2O3: 75%) was added to deionized water and stirred. Orthophosphoric acid (H3PO4 with a mass fraction of 85%) was added dropwise to the aqueous solution of aluminum hydroxide. After stirring evenly, silica white (SiO2 with a mass fraction of 89%) was added and mixed evenly. Then, triethylamine (TEA with a mass fraction of 99%) and tetraethylammonium hydroxide (TEAOH with a mass fraction of 27.2%) were added successively and stirred evenly. Finally, the epichlorohydrin cross-linked starch prepared in the preparation example was added and stirred evenly to form a raw material mixture. In the raw material mixture, the molar ratio of the aluminum source calculated as Al2O3, the phosphorus source calculated as P2O5, the silicon source calculated as SiO2, the template agent R1, the template agent R2 and water was 1:0.9:0.12:1.9:0.5:20, and the addition amount of the epichlorohydrin cross-linked starch was 3% by weight of the weight of the aluminum source calculated as Al2O3.

[0083] Cover the polytetrafluoroethylene inner lining containing the above raw material mixture, seal it in a stainless steel autoclave, place it in a rotating convection oven, set the rotation speed to 20 r / min, and carry out the first-stage crystallization under autogenous pressure: crystallize at 150 °C for 36 hours, then raise the temperature to 185 °C and carry out the second-stage crystallization: crystallize at 185 °C for 40 hours. After the crystallization is completed, cool it to room temperature and take out the crystallization product. Filter, wash it, and then dry it at 80 °C for 10 hours and calcine it at 550 °C for 3 hours to obtain the molecular sieve sample.

[0084] Perform X-ray diffraction analysis on the calcined molecular sieve sample obtained, and it is a pure-phase SAPO-34 molecular sieve. Use SEM to observe the morphology of the molecular sieve, and it shows a blocky morphology with a grain size of 200 - 600 nm.

[0085] Comparative Example 1

[0086] Prepare the molecular sieve according to the method of Example 1, except that epichlorohydrin cross-linked starch is not added.

[0087] Perform X-ray diffraction analysis on the molecular sieve sample obtained in Comparative Example 1. The XRD pattern is shown in Figure 9 and it is a pure-phase SAPO-34 molecular sieve. Use SEM to observe the morphology of the molecular sieve. The SEM photo is shown in Figure 10 and it shows a blocky morphology with a grain size of 700 - 900 nm.

[0088] Comparative Example 2

[0089] Prepare the molecular sieve according to the method of Example 1, except that epichlorohydrin cross-linked starch is replaced with ordinary unmodified edible starch.

[0090] The synthesis results show that due to the relatively high system temperature and pH value, the ordinary unmodified starch gelatinizes, affecting the mixing of system raw materials and making it impossible to synthesize the molecular sieve.

[0091] Test Example 1

[0092] Characterize the acid amount and pore structure of the molecular sieves prepared in the above examples and comparative examples. The test results of the acid amount are shown in Table 1, and the test results of the pore structure are shown in Table 2. Among them, the NH3-TPD spectra of the examples and comparative examples are as shown in Figure 11 shown.

[0093] Table 1

[0094]

[0095] As can be seen from Table 1, compared with Comparative Example 1, the weak acid strength (desorption temperature) of the SAPO-34 molecular sieve sample synthesized by the method of the present invention is basically the same, and the strength (desorption temperature) and acid amount of medium-strong / strong acids are significantly reduced. This shows that the method of the present invention can significantly reduce the strength (desorption temperature) and acid amount of medium-strong / strong acids. Although not shown, the total acid amount of the molecular sieves obtained in Examples 5-6 is less than 1645 μmol / g -1 , the desorption temperature of medium-strong / strong acids is lower than 351 °C, and the amount of medium-strong / strong acids is less than 590 μmol / g -1 .

[0096] Table 2

[0097]

[0098] As can be seen from Table 2, compared with the SAPO-34 molecular sieve synthesized in Comparative Example 1, the SAPO-34 molecular sieve sample synthesized by the preparation method of the present invention has a larger total pore volume, mesopore volume and a higher mesopore volume content.

[0099] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for preparing SAPO-34 molecular sieve, characterized in that, The method comprises the following steps: (1) Providing a raw material mixture containing an aluminum source, water, a phosphorus source, a silicon source, a templating agent, and modified starch, wherein, relative to every 100 g of the aluminum source calculated as Al2O3, the dosage of the modified starch is 2 - 10 g; (2) Crystallizing the raw material mixture under crystallization conditions; (3) Separating the solid and liquid of the crystallized product, and taking the solid phase to perform drying and calcination in sequence; The aluminum source is calculated as Al2O3, the phosphorus source is calculated as P2O5, the silicon source is calculated as SiO2, and in the raw material mixture, the molar ratio of the aluminum source, the phosphorus source, the silicon source, the templating agent, and water is 1:0.8 - 1:0.05 - 0.17:0.4 - 3:15 - 20; The modified starch is epichlorohydrin cross-linked starch.

2. The method according to claim 1, wherein, The templating agent includes templating agent R1 and templating agent R2, the templating agent R1 is triethylamine, and the templating agent R2 is tetraethylammonium hydroxide.

3. The method according to claim 2, wherein The aluminum source is calculated as Al2O3, the phosphorus source is calculated as P2O5, the silicon source is calculated as SiO2, and in the raw material mixture, the molar ratio of the aluminum source, the phosphorus source, the silicon source, templating agent R1, templating agent R2, and water is 1:0.8 - 1:0.05 - 0.17:1.8 - 2.1:0.4 - 0.6:15 - 20; 4. The method according to claim 1, wherein The aluminum source is at least one of alumina, aluminum hydroxide, and pseudo-boehmite; and / or, the phosphorus source is orthophosphoric acid; and / or, the silicon source is silicon dioxide.

5. The method according to claim 1 or 4, wherein The silicon source is silica and / or silica gel.

6. The method according to claim 1, wherein, The crystallization includes performing first-stage crystallization and second-stage crystallization in sequence, and the temperature of the first-stage crystallization is lower than that of the second-stage crystallization.

7. The method according to claim 6, wherein, The first-stage crystallization is carried out at autogenous pressure and at 140 - 160 °C for 33 - 40 hours, and the second-stage crystallization is carried out at autogenous pressure and at 170 - 190 °C for 35 - 50 hours.

8. The method according to claim 6, wherein The first-stage crystallization is carried out at autogenous pressure and at 145 - 155 °C for 35 - 39 hours, and the second-stage crystallization is carried out at autogenous pressure and at 175 - 185 °C for 40 - 50 hours.

9. The method according to claim 1, wherein The temperature of the drying is 70 - 110 °C, and the time of the drying is 4 - 12 h.

10. The method according to claim 1, wherein, The temperature of the calcination is 500 - 600 °C, and the time of the calcination is 3 - 5 h.

11. A SAPO-34 molecular sieve, characterized in that, This SAPO-34 molecular sieve is prepared by the method described in any one of claims 1 - 10; Alternatively, the total pore volume of the SAPO-34 molecular sieve is 0.36 - 0.43 cm 3 / g, the mesopore volume is 0.20 - 0.26 cm 3 / g, the micropore volume is 0.15 - 0.19 cm 3 / g, and the proportion of the mesopore volume in the total pore volume is 55 - 63%; the total acid amount of the SAPO-34 molecular sieve is less than 1645 μmol g -1 , the desorption temperature of medium-strong / strong acid is less than 351 °C, and the medium-strong / strong acid amount is less than 590 μmol g -1 .

12. Use of the SAPO-34 molecular sieve according to claim 11 as a catalyst in an acid-catalyzed reaction.

13. The application according to claim 12, wherein, The acid-catalyzed reaction is an oxygenate conversion to light olefins reaction.

14. The application according to claim 12, wherein, The acid-catalyzed reaction is a methanol conversion to light olefins reaction.

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