CN113955767B

CN113955767B - Method for synthesizing nano SAPO-34 molecular sieve with assistance of heterogeneous seed crystal

Info

Publication number: CN113955767B
Application number: CN202111456017.4A
Authority: CN
Inventors: 郜贝贝; 刘志攀
Original assignee: Zhengzhou University
Current assignee: Zhengzhou University
Priority date: 2021-12-02
Filing date: 2021-12-02
Publication date: 2023-08-04
Anticipated expiration: 2041-12-02
Also published as: CN113955767A

Abstract

The invention relates to a method for synthesizing a nano SAPO-34 molecular sieve with the assistance of heterogeneous seed crystals, and belongs to the technical field of molecular sieves. The invention uses cheap template agent and adopts traditional hydrothermal synthesis method, which is characterized in that gel formed by mixing aluminum source, phosphorus source, silicon source, template agent and water is evenly stirred, and then heterogeneous molecular sieve mixed solution treated by alkali solution is added as seed crystal guiding solution to prepare nano SAPO-34 molecular sieve by induction. The method is simple to operate, and the prepared SAPO-34 molecular sieve has excellent performance in the reaction of preparing olefin from methanol.

Description

Method for synthesizing nano SAPO-34 molecular sieve with assistance of heterogeneous seed crystal

Technical Field

The invention relates to a preparation method of a SAPO-34 molecular sieve with a nanometer morphology. The invention also relates to application of the SAPO-34 molecular sieve synthesized by the method in acid catalysis reaction.

Background

Low-carbon olefins, in particular ethylene and propylene, are important platform compounds as a base stone for the modern chemical industry. At present, the traditional olefin is mainly prepared by naphtha pyrolysis, but petroleum resources in China are relatively deficient. In recent years, the price of petroleum has continuously risen, and conventional petroleum routes have failed to meet the production needs. Therefore, the development of chemical technology for preparing low-carbon olefin by a non-petroleum route has great significance. The methanol has wide sources and can be produced from abundant non-petroleum resources (such as natural gas, coal, biomass and the like). Currently, the methanol-to-olefins (MTO) process has proven to be a successful route to lower olefins rather than petroleum routes. The catalyst is used as a core technology of an MTO process and is a key for mastering and developing a complete technology for preparing olefin from methanol.

The crystal framework structure of the SAPO-34 molecular sieve is of the CHA type, and belongs to a trigonal system and R-3m space group. The structure is formed by arranging double six-membered ring structural units according to an ABC stacking mode, and an ellipsoidal cage structure with eight-membered ring openings and a three-dimensional cross duct are formed. Due to its particular pore structure and suitable acidity, it exhibits excellent catalytic performance in methanol-to-olefins (MTO) reactions (Applied Catalysis,1990, 64:31-40). However, conventional SAPO-34 molecular sieve catalysts are prone to deactivation and have a short single pass life. Studies have shown that decreasing the SAPO-34 crystallite size can effectively improve the diffusion limit and extend the catalyst life (Microporous and Mesoporous Materials,1999, 29:191-203).

The product obtained by the prior art for synthesizing the SAPO-34 molecular sieve has larger crystal grains, and the prepared catalyst is easy to deactivate, so that the research and development of the method for synthesizing the small crystal grain SAPO-34 molecular sieve has important significance, and particularly, the synthesis of the small crystal grain SAPO-34 by adopting a cheaper template agent has important industrial application significance. Patent WO00/06493 describes obtaining phosphorus-containing molecular sieves of smaller particle size and narrower particle size distribution by agitation, such as stirring or tumbling. WO2003/048042 reports a method for obtaining small particle size SAPO-34 molecular sieves by using ethyl orthosilicate as the silicon source, the structure directing agent employed being TEAOH or a mixture of TEAOH and DPA. Patent CN101462742 describes a synthesis method of small-grain SAPO-34, in which triethylamine and fluoride as structure directing agents are added in the process of forming a colloid mixture, and the colloid mixture is subjected to an aging process and then a crystallization process to prepare the small-grain SAPO-34.CN101823728 is formed by hydrothermal synthesis and aging treatment, then oxydol oxidation treatment, finally ultrasonic dispersion and vacuum drying to obtain small crystal SAPO-34 molecular sieve. Patent CN10124974 discloses a method for preparing small-grain SAPO-34 molecular sieve, which uses cheap triethylamine as template agent, synthesizes small-grain SAPO-34 under the condition of ultrasonic aging, and can reduce the grain size to one half of the original one. CN103332702 adds an aging step to the gel formed by aluminum source, phosphorus source, silicon source, solvent and template agent before crystallization, and the average particle size of the prepared SAPO-34 product is less than 2 μm. CN102275948 is prepared by mixing an aluminum source, a silicon source, an organic amine and a portion of water, pretreating the mixed solution for a certain time under a high temperature autogenous pressure, then adding a mixed solution of a phosphorus source and the rest of water at a high temperature in situ, and crystallizing to synthesize small-grain SAPO-34.CN103641131a and CN102616810a disclose a process for preparing flaky SAPO-34, but the process uses expensive tetraethylammonium hydroxide as a template, limiting its industrial application.

In summary, the prior art generally requires adding inorganic substances such as fluoride into the system, using expensive tetraethylammonium hydroxide as a template agent, or using methods such as ultrasound to reduce the grain size. However, the ultrasonic aging technique is difficult to apply industrially, the addition of HF and the like is disadvantageous for industrial production, and the effect of reducing the grain size that can be achieved is limited.

The seed crystal auxiliary method is a simple, economical and efficient preparation method of the nano molecular sieve. Patent CN101555024 reports that adding SAPO-34 molecular sieve raw powder solid seed crystals to a mixed solution containing a template agent, an aluminum source, a silicon source and water can reduce the amount of the template agent, but does not mention the effect of seed crystal addition on the grain size of the product. Liu Gongxing et al in the CN101284673 patent provide a method of adding small grains having an average size of less than 1 μm as seed crystals to the crystallized liquid, but the particle size of the obtained product is usually larger than that of the added seed crystals, and when seed crystals having a particle size of 200nm are added, the particle size of the obtained product can reach 900nm. The patent CN102464338 prepares the small crystal particle SAPO-34 by preparing an initial gel mixture for synthesizing the SAPO-34 molecular sieve from a phosphorus source, an aluminum source, a silicon source, a template agent, HF and water, taking a solution obtained by hydrothermal crystallization at 170-220 ℃ as a crystallization guiding agent, and adding the solution into the initial gel mixture to perform a hydrothermal crystallization reaction. However, in the method, HF exists in the system, and the addition amount of the seed crystal guiding agent needs to reach 30-60% of the total volume of the solution. The SAPO-34 catalyst with the crystal size of 300-800 nm is prepared by adopting a seed crystal auxiliary method by taking cheap triethylamine and morpholine as unique templates in Ji Gong and the like (Journal of Materials Chemistry A,2016,4:14978-14982;Chemical Communications,2017,53:13328-13331). However, the nano-seed crystal adopted in the method is prepared by taking expensive tetraethylammonium hydroxide as a template agent, and the requirement on the seed crystal is high. Yang Miao et al (Chemical Communications,2014, 50:1845-1847) adopts a "top-down" synthesis route, and adopts micro SAPO-34 crystals hydrothermally synthesized by taking diethylamine as a template agent, ball milling and crushing the micro SAPO-34 crystals as seed crystals, and re-crystallizing by taking triethylamine as the template agent, so as to prepare the SAPO-34 with the size of 50-350 nm. The patent CN104340986 adopts particles with 10-800 nm particle size and reduced crystallinity, which are obtained by pre-treating (mechanical crushing is the first choice) the SAPO-34 molecular sieve, as crystallization precursors, and the crystal structure is repaired by secondary crystallization to prepare the nano molecular sieve. Wang Yaquan the micro SAPO-34 synthesized by triethylamine is crushed by morpholine normal pressure reflux treatment, and 200-500 nm nano SAPO-34 (Applied Catalysis A: general,2020,589,117314) is prepared as seed crystal. However, mixed or seeding is often encountered in the synthesis of SAPO-34 molecular sieves, with the usual accompanying phases being SAPO-5, SAPO-18, DNL-6, etc., which provides the possibility of using heterogeneous seed crystals for directed synthesis of SAPO-34.

Disclosure of Invention

The invention aims to provide a method for preparing a nano SAPO-34 molecular sieve by using a cheap template agent through heterogeneous seed crystal assistance.

It is still another object of the present invention to provide an acid catalyzed reaction catalyst or an oxygenate conversion to olefins catalyst.

The invention adopts a traditional hydrothermal synthesis mode, firstly, conventional heterogeneous seed crystals are treated into seed crystal guide liquid through alkali solution, then the seed crystal guide liquid is introduced into a synthesis system, and the nano SAPO-34 molecular sieve is prepared through autogenous pressure in a high-pressure hydrothermal kettle.

The SAPO-34 molecular sieve prepared by the invention has the grain size of 20-800 nm, and can be used as a catalyst for the reaction of preparing olefin from methanol, so that the diffusion limit can be greatly improved, the generation of carbon deposit can be slowed down, the catalytic life of the catalyst can be prolonged, and the selectivity of low-carbon olefin can be improved.

The invention is characterized in that the synthesis steps are as follows:

1. adding heterogeneous seed crystal and alkali solution into a hydro-thermal synthesis kettle, uniformly stirring, sealing the hydro-thermal synthesis kettle, transferring the hydro-thermal synthesis kettle into a rotary oven, heating to 40-100 ℃ and keeping the temperature for 0.5-24 h to obtain seed crystal guide liquid A;

2. mixing an aluminum source, a phosphorus source, a silicon source, an organic template agent and water, and uniformly stirring to obtain an initial gel mixture B for synthesizing the SAPO-34 molecular sieve;

3. adding the seed crystal guide liquid A into the mixture B, uniformly stirring, and aging for 0-24 h, wherein the molar ratio of each component in the obtained gel precursor is as follows: (1.5-6.0) R (0.003-0.25) S:1.0Al ₂ O ₃ :(0.5～3.0)P ₂ O ₅ :(0.05～2.0)SiO ₂ :(20～200)H ₂ O, wherein R is an organic template agent, and S is an inorganic base;

4. and (3) placing the gel precursor obtained in the step (3) in a high-pressure reaction kettle, crystallizing at constant temperature under autogenous pressure, centrifuging a solid product after crystallization is finished, washing the solid product with deionized water to be neutral, and drying the solid product in air at 80-120 ℃ to obtain the SAPO-34 molecular sieve raw powder.

5. And (3) roasting the raw powder of the SAPO-34 molecular sieve at a high temperature to remove the template agent contained in the raw powder, adding the raw powder into an ammonium nitrate aqueous solution for ammonium exchange, and obtaining the hydrogen type SAPO-34 molecular sieve with nanometer size through centrifugation, washing and roasting.

The heterogeneous seed crystal in the step 1 is one or a mixture of any more of baked SAPO-44, SAPO-47, SAPO-5, SAPO-18, SAPO-56 and DNL-6; the mass of the heterogeneous seed crystal is 0.05-8% of the mass of the dry oxide in the final gel precursor; preferably, the mass of the heterogeneous seed crystal is 0.1-4% of the mass of the oxide dry basis in the final gel precursor; the alkali solution is one or a mixture of several of sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, cesium hydroxide aqueous solution and ammonia water.

The aluminum source in the step 2 is one or a mixture of any one or more of aluminum salt, activated alumina, aluminum isopropoxide, aluminum hydroxide, pseudo-boehmite or metakaolin; the phosphorus source is one or a mixture of any of orthophosphoric acid, metaphosphoric acid, phosphate or phosphite; the silicon source is one or a mixture of several of silica sol, active silicon dioxide, tetrasilicate, white carbon black, sodium silicate or metakaolin; the organic template agent R is any one or a mixture of any more of triethylamine, diethylamine, morpholine, diisopropylamine, di-n-propylamine, isopropylamine, n-butylamine, piperazine, diethanolamine or triethanolamine.

The step 3 of constant temperature crystallization refers to that the reaction kettle is placed in an oven to carry out constant temperature crystallization under autogenous pressure, the crystallization temperature is 140-230 ℃, and the crystallization time is 10-72 h.

In the step 5, when ammonia water is used as the inorganic base, the ammonium exchange step may be omitted.

In another aspect, the invention provides a nano SAPO-34 molecular sieve prepared by the method, which is characterized in that the particle size of the nano SAPO-34 molecular sieve is 20-800 nm, the crystal morphology is cubes or aggregates of cubes, and preferably, the particle size of the nano SAPO-34 molecular sieve is 50-500 nm.

In another aspect, the invention provides the use of the nano SAPO-34 molecular sieve calcined in air at 450-700 ℃ as a catalyst for acid catalyzed reactions, particularly in reactions of converting alcohol compounds, halogenated compounds to olefins and olefin conversion reactions.

Compared with the prior art, the invention has the following advantages due to the adoption of the technical scheme: (1) The nanometer molecular sieve is obtained by adopting cheap templates such as triethylamine, diethylamine, morpholine and the like, and tetraethylammonium hydroxide, HF and the like are not needed to be added, thereby being beneficial to the industrial application.

(2) The mixed solution after the heterogeneous seed crystal is treated by alkali is used as seed crystal guiding solution to be directly added into molecular sieve synthetic gel, and the molecular sieve structural unit in the liquid phase of the guiding solution and the molecular sieve particles in the solid phase can induce nucleation, so that the nano SAPO-34 seed crystal is not required to be prepared by tetraethylammonium hydroxide, and the particle size, the purity and the source of the molecular sieve seed crystal are not required to be special.

(3) The particle size of the SAPO-34 molecular sieve can be effectively regulated and controlled by changing the conditions of alkali treatment of the seed crystal or the addition amount of the seed crystal, so that the grain size of the SAPO-34 molecular sieve is changed between 0.2 and 0.8 mu m, and the limit that the particle size of a product in conventional seed crystal-assisted synthesis is generally larger than that of the added seed crystal is broken.

(4) Compared with the conventional SAPO-34, the prepared SAPO-34 molecular sieve has obviously prolonged service life in the reaction of converting methanol or dimethyl ether into low-carbon olefin, and the total selectivity of ethylene and propylene can be up to more than 85%.

Drawings

FIG. 1 is a scanning electron microscope image of a sample of SAPO-34 synthesized in example 1.

FIG. 2 is a scanning electron microscope image of a sample of SAPO-34 synthesized in comparative example 1.

FIG. 3 is a scanning electron microscope image of a sample of SAPO-34 synthesized in comparative example 2.

FIG. 4 is a scanning electron microscope image of a sample of SAPO-34 synthesized in example 2.

Detailed Description

Example 1

Preparing 0.12g of NaOH and 5g of deionized water into 0.6mol/L NaOH aqueous solution, adding 0.25g of baked SAPO-5 seed crystal into the mixed solution, transferring the mixed solution to a crystallization kettle, and treating the mixed solution in an oven at 80 ℃ for 6 hours to obtain liquid seed crystal guide liquid A; 3.45g of pseudo-boehmite (Al ₂ O ₃ 67.5 wt%) and 22.5g of water were mixed and stirred for 15min, 5.27g of phosphoric acid was added dropwise, and stirring was continued at room temperature for 1h to form a white uniform gel. Then, 2.86g of ethyl orthosilicate was added, and after stirring for 1h, 3.3g of diethylamine was added, and stirring was continued to be uniform, to obtain gel precursor B. Subsequently, the cooled seed crystal guide liquid A was added to the gel precursor B, stirring was continued at room temperature for 2 hours, and then the obtained gel was transferred to a 100mL stainless steel crystallization kettle, and subjected to hydrothermal crystallization at 200℃for 2d. After crystallization, the solid product is centrifuged, washed and dried in air at 100 ℃ to obtain the raw powder. The samples were characterized by XRD and SEM and the results are shown in table 1 and figure 1, respectively. The result shows that the synthesized product is nano SAPO-34 molecular sieve, and the average grain size of the nano SAPO-34 molecular sieve is about 200nm.

TABLE 1 XRD results for example 1 samples

Adding the obtained raw powder into 1mol/L ammonium nitrate solution, and adding water at 80 DEG CStirring for 3h under bath condition, repeating exchange for 3 times, drying, introducing air at 600 ℃ for roasting for 4h, tabletting, and crushing to 40-80 meshes. 1.0g of the sample was weighed and charged into a fixed bed reactor, and MTO reaction evaluation was performed. Nitrogen was introduced at 550 ℃ for activation for 1 hour, and then the temperature was lowered to 450 ℃. The nitrogen was turned off and 40wt% aqueous methanol was fed by a plunger pump at a methanol weight space velocity of 2.0h ^-1 . The reaction products were analyzed by on-line gas chromatography (Tianmei GC7900, FID detector, capillary column PoraPLOT Q-HT) and the results are shown in Table 2.

Table 2 results of the methanol to olefins reaction for the example 1 samples

* Lifetime is the time that the conversion of alcohol is maintained above 99%.

Selectivity is the highest selectivity when the conversion of methanol is maintained above 99%.

Comparative example 1

The batch ratio and crystallization process were the same as in example 1, except that the seed crystals added were not subjected to dynamic alkali treatment.

Preparing 0.12g of NaOH and 5g of deionized water into 0.6mol/L NaOH aqueous solution, adding 0.25g of baked SAPO-5 seed crystal into the mixed solution, and directly obtaining a mixed solution A without heating treatment; 3.45g of pseudo-boehmite and 22.5g of water were mixed and stirred for 15min, 5.27g of phosphoric acid was added dropwise, and stirring was continued at room temperature for 1h to form a white uniform gel. Then, 2.86g of ethyl orthosilicate was added, and after stirring for 1h, 3.3g of diethylamine was added, and stirring was continued to be uniform, to obtain gel precursor B. Subsequently, the mixed solution A was added to the gel precursor B, stirring was continued at room temperature for 2 hours, and then the obtained gel was transferred to a 100mL stainless steel crystallization kettle, and hydrothermal crystallization was performed at 200℃for 2d. After crystallization, the solid product is centrifuged, washed and dried in air at 100 ℃ to obtain the raw powder.

The grains of comparative sample 1 were cubic, with an average size of about 3 μm, and XRD and SEM characterization results are shown in Table 3 and FIG. 2, and catalytic evaluation results are shown in Table 4.

TABLE 3 XRD results for the sample of comparative example 1

Table 4 results of the methanol to olefins reaction for comparative example 1 samples

* Lifetime is the time that the conversion of alcohol is maintained above 99%.

Comparative example 2

The blending ratio and crystallization process were the same as in example 1, except that no seed crystal was added.

Preparing 0.12g of NaOH and 5g of deionized water into 0.6mol/L NaOH aqueous solution to obtain a mixed solution A; 3.45g of pseudo-boehmite and 22.5g of water were mixed and stirred for 15min, 5.27g of phosphoric acid was added dropwise, and stirring was continued at room temperature for 1h to form a white uniform gel. Then, 2.86g of ethyl orthosilicate was added, and after stirring for 1h, 3.3g of diethylamine was added, and stirring was continued to be uniform, to obtain gel precursor B. Subsequently, the mixed solution A was added to the gel precursor B, stirring was continued at room temperature for 2 hours, and then the obtained gel was transferred to a 100mL stainless steel crystallization kettle, and hydrothermal crystallization was performed at 200℃for 2d. After crystallization, the solid product is centrifuged, washed and dried in air at 100 ℃ to obtain the raw powder.

The grains of comparative sample 2 were cubic, with an average size of about 12 μm, and XRD and SEM characterization results are shown in Table 5 and FIG. 3, and catalytic evaluation results are shown in Table 6.

TABLE 5 XRD results for the comparative example 2 sample

Table 6 results of the methanol to olefins reaction for comparative example 2 samples

* Lifetime is the time that the conversion of alcohol is maintained above 99%.

Examples 2 to 10

The same procedure as in example 1 was followed, and the specific compounding ratio and aging and crystallization conditions are shown in Table 7. The XRD and SEM characterization results of example 2 are shown in Table 8 and FIG. 4, and the catalytic evaluation results are shown in Table 9.

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TABLE 8 XRD results for example 2 samples

Table 9 results of the methanol to olefins reaction for the example 2 samples

* Lifetime is the time that the conversion of alcohol is maintained above 99%.

The foregoing description is only a few examples of the present application and is not intended to limit the present application in any way, and although the present application is disclosed in the preferred examples, it is not intended to limit the present application, and any person skilled in the art may make some changes or modifications to the disclosed technology without departing from the scope of the technical solution of the present application, and the technical solution is equivalent to the equivalent embodiments.

Claims

1. A method for synthesizing a nano SAPO-34 molecular sieve with the assistance of heterogeneous seed crystals is characterized in that a seed crystal guiding liquid obtained by treating the heterogeneous seed crystals is adopted to synthesize the nano SAPO-34 molecular sieve with the assistance of the seed crystal guiding liquid:

the synthesis steps are as follows:

a) Adding heterogeneous seed crystal and alkali solution into a hydro-thermal synthesis kettle, uniformly stirring, sealing the hydro-thermal synthesis kettle, transferring the hydro-thermal synthesis kettle into a rotary oven, heating to 40-100 ℃ and keeping the temperature for 0.5-24 h to obtain seed crystal guide liquid A;

the alkali solution in the step a) is one or a mixture of any one or more of sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, cesium hydroxide aqueous solution and ammonia water;

the heterogeneous seed crystal in the step a) is one or a mixture of any of calcined SAPO-44, SAPO-47, SAPO-5, SAPO-18, SAPO-56 and DNL-6;

b) Mixing an aluminum source, a phosphorus source, a silicon source, an organic template agent and water, and uniformly stirring to obtain an initial gel mixture B for synthesizing the SAPO-34 molecular sieve;

c) Adding the seed crystal guide liquid A into the mixture B, uniformly stirring, and aging for 0-24 h, wherein the molar ratio of each component in the obtained gel precursor is as follows: (1.5-6.0) R: 0.003-0.25) S:1.0Al ₂ O ₃ ∶(0.5～3.0)P ₂ O ₅ ∶(0.05～2.0)SiO ₂ ∶(20～200)H ₂ O, wherein R is an organic template agent, and S is an inorganic base;

d) Placing the gel precursor obtained in the step c) into a high-pressure reaction kettle to carry out constant-temperature crystallization under autogenous pressure, after crystallization is completed, centrifugally separating a solid product, washing the solid product with deionized water to be neutral, and drying the solid product in air at 80-120 ℃ to obtain SAPO-34 molecular sieve raw powder;

e) And (3) roasting the raw powder of the SAPO-34 molecular sieve at a high temperature to remove the template agent contained in the raw powder, adding the raw powder into an ammonium nitrate aqueous solution for ammonium exchange, and obtaining the hydrogen type SAPO-34 molecular sieve with nanometer size through centrifugation, washing and roasting.

2. The method for synthesizing the nano SAPO-34 molecular sieve with the assistance of the heterogeneous seed crystal according to claim 1, wherein the method comprises the following steps: the mass of the heterogeneous seed crystal in the step a) is 0.05-8% of the mass of the dry oxide in the final gel precursor.

3. The method for synthesizing the nano SAPO-34 molecular sieve with the assistance of the heterogeneous seed crystal according to claim 2, wherein the method comprises the following steps: the mass of the heterogeneous seed crystal in the step a) is 0.1-4% of the mass of the dry oxide in the final gel precursor.

4. The method for synthesizing the nano SAPO-34 molecular sieve with the assistance of the heterogeneous seed crystal according to claim 1, wherein the method comprises the following steps: the aluminum source in the step b) is one or a mixture of any several of aluminum salt, activated alumina, aluminum isopropoxide, aluminum hydroxide, pseudo-boehmite or metakaolin; the phosphorus source is one or a mixture of any of orthophosphoric acid, metaphosphoric acid, phosphate or phosphite; the silicon source is one or a mixture of several of silica sol, active silicon dioxide, tetrasilicate, white carbon black, sodium silicate or metakaolin; the organic template agent R is any one or a mixture of any more of triethylamine, diethylamine, morpholine, diisopropylamine, di-n-propylamine, isopropylamine, n-butylamine, piperazine, diethanolamine or triethanolamine.

5. The method for synthesizing the nano SAPO-34 molecular sieve with the assistance of the heterogeneous seed crystal according to claim 1, wherein the method comprises the following steps: the step c) of constant temperature crystallization is to put the reaction kettle in an oven to perform constant temperature crystallization under autogenous pressure, the crystallization temperature is 140-230 ℃, and the crystallization time is 10-72 h.

6. The method for synthesizing the nano SAPO-34 molecular sieve with the assistance of the heterogeneous seed crystal according to claim 1, wherein the method comprises the following steps: when ammonia is used as the inorganic base, the ammonium exchange step in step e) may be omitted.