A nano-SAPO-34 / SAPO-5 composite molecular sieve, its synthesis method and application

Abstract

This invention provides a nano-SAPO-34 / SAPO-5 composite molecular sieve, its synthesis method, and its application, comprising: (1) adding an aluminum source to water and mixing and stirring, then adding a phosphorus source, a silicon source, and a template agent to obtain a mixed gel; the molar ratio of each component in the mixed gel is Al2O3:P2O5:SiO2:template agent:H2O=1.0:(0.6~1.4):(0.2~0.8):(1.0~3.0):(30~100); (2) aging the mixed gel under stirring, and then crystallizing it; (3) separating the crystallized product into solid and liquid phases, and washing, drying, and calcining the solid after solid-liquid separation to obtain a nano-SAPO-34 / SAPO-5 composite molecular sieve; wherein, the silicon source in step (1) has the structure shown in Formula I. The composite molecular sieve prepared by the synthesis method of this invention exhibits excellent catalytic performance in the MTO reaction, significantly prolonging its service life and improving the selectivity of low-carbon olefins;

Description

Technical Field

[0001] This invention belongs to the technical field of molecular sieve synthesis, specifically relating to a nano-SAPO-34 / SAPO-5 composite molecular sieve, its synthesis method, and its application. Background Technology

[0002] SAPO-34 molecular sieves belong to the eight-membered ring chamazolithite CHA structure, with an eight-membered ring pore size of 0.38 nm, which is similar to the molecular dynamic diameter of low-carbon olefins. SAPO-34 molecular sieves possess suitable acid properties and pore structure, exhibiting excellent catalytic performance in methanol-to-olefins (MTO). However, precisely because the micropores of SAPO-34 molecular sieves are too small, as the reaction time increases in the methanol-to-olefins reaction, the product molecular sieve cannot diffuse out in time, leading to side reactions such as hydrogen transfer or cyclization, generating carbon deposits, and causing a decrease in catalyst activity.

[0003] SAPO-5 molecular sieve has a one-dimensional channel structure with twelve-membered rings and a pore size of 0.8 nm. Compared with SAPO-34 molecular sieve, it has less restriction on the diffusion of low-carbon olefins in the methanol conversion reaction, resulting in a slower carbon deposition rate and a longer lifespan. However, its disadvantage is that it has lower selectivity for low-carbon olefins.

[0004] Existing technologies have already prepared these two types of composite molecular sieves: Patent CN104828842 discloses a method for preparing a composite molecular sieve coexisting with SAPO-5 and SAPO-34. This method uses natural or synthetic clay minerals with a SiO2 / Al2O3 molar ratio ranging from 0.05 to 1.5 as a composite of silicon and aluminum sources, and obtains the SAPO-5 and SAPO-34 coexisting composite molecular sieve through a single hydrothermal synthesis and crystallization. The numerous macropores and mesopores in the SAPO-5 molecular sieve effectively compensate for the deficiencies in macropores and mesopores in the SAPO-34 molecular sieve, resulting in improved yields of propylene and butene when applied to the methanol-to-olefins reaction. However, the composite molecular sieve provided by this patent has a short catalytic lifetime and does not offer a significant advantage.

[0005] Patent CN105642342 discloses a method for preparing SAPO-5 / SAPO-34 composite molecular sieves. The inventors achieved precise control over the phase ratio in the SAPO-5 / SAPO-34 composite molecular sieves by optimizing the basic formulation of the synthetic gel, crystallization conditions, and pre-adding a physically mixed SAPO-5 and SAPO-34 molecular sieves in a specific ratio to the mixed gel. The SAPO-34 molecular sieve content was controlled at 40-60%. This method is used to produce low-carbon olefins from organic oxygen-containing compounds, exhibiting high selectivity for propylene and butene, but low coke yield. However, this patent requires the pre-preparation of SAPO-5 and SAPO-34 molecular sieves, resulting in a high preparation process and cost. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, this invention provides a method for synthesizing nano-SAPO-34 / SAPO-5 composite molecular sieves. The catalyst prepared exhibits excellent catalytic performance in the MTO reaction, significantly extends its service life, and improves the selectivity of low-carbon olefins.

[0007] The present invention provides a method for synthesizing nano-SAPO-34 / SAPO-5 composite molecular sieves in a first aspect, characterized by comprising the following steps:

[0008] (1) Add aluminum source to water and mix and stir, then add phosphorus source, silicon source and template agent to obtain mixed gel;

[0009] The molar ratio of each component in the mixed gel is Al2O3:P2O5:SiO2:template agent:H2O = 1.0:(0.6~1.4):(0.2~0.8):(1.0~3.0):(30~100); for example, 1.0:(0.7~1.3):(0.3~0.7):(1.5~2.5):(35~80).

[0010] (2) The mixed gel is aged under stirring and then crystallized.

[0011] (3) The crystallized product is subjected to solid-liquid separation, and the solid after solid-liquid separation is washed, dried and calcined to obtain nano-SAPO-34 / SAPO-5 composite molecular sieve;

[0012] The silicon source described in step (1) has the structure shown in Formula I below;

[0013]

[0014] In the structure of Formula I, R1, R2, and R3 are independently selected from C1 to C4 alkyl groups; R4 and R5 are independently selected from H or C1 to C3 alkyl groups, respectively.

[0015] In some specific embodiments, in the structure of Formula I, R1, R2, and R3 are selected from -CH3, -CH2CH3, -(CH2)2CH3, and -(CH2)3CH3, preferably -CH3 and -CH2CH3;

[0016] R4 and R5 are independently selected from H or -CH3, -CH2CH3, and -(CH2)2CH3, respectively, wherein R4 is preferably H or -CH3, and R5 is preferably -CH3 or -CH2CH3.

[0017] In the composite molecular sieve synthesis method provided by this invention, an organosilane with the structure shown in Formula I is used as the silicon source. During the preparation of the composite molecular sieve, the silicon source with the above structure is hydrolyzed to remove aromatic and alkyl groups to obtain monosilicon (Si(OH)4). Then, the monosilicon undergoes a polymerization reaction to obtain polysilicon, which forms a framework structure after a certain reaction time. Compared with conventional silicon sources such as silica sol and solid silica gel, the organosilicon source used in this invention has a slower depolymerization-polymerization rate and a longer reaction time. During the nucleation process of the composite molecular sieve, the growth rate of crystal nuclei is slow, reducing the size of the composite molecular sieve crystals. In addition, the organosilicon source containing aromatic and alkane groups used in this invention has a larger molecular size, which enables the molecular sieve to have a specific structure-directing effect during synthesis. Under specific crystallization conditions, it guides the formation of a composite molecular sieve co-existing with SAPO-34 and SAPO-5 together with the template agent.

[0018] In step (1) of the preparation method of the present invention, the molar ratio of each component in the mixed gel is Al2O3:P2O5:SiO2:template agent:H2O = 1.0:(0.8~1.2):(0.4~0.6):(2.0~3.0):(40~60), for example, 1.0:0.9:0.5:2.6:50, 1.0:1.0:0.55:2.8:55, 1.0:1.0:0.45:2.2:58. In the present invention, the molar ratio of aluminum source, phosphorus source and silicon source in the molecular sieve preparation process is usually expressed in oxide form, and the template agent and water are calculated in molar parts. The water in the gel includes water added during the mixing process and water introduced from other components.

[0019] In some specific embodiments, the aluminum source in step (1) is selected from one or more of boehmite, aluminum isopropoxide, or alumina; the phosphorus source is selected from one or more of phosphoric acid, metaphosphoric acid, or phosphate; and the template agent is selected from one or more of triethylamine, diethylamine, morpholine, tetraethylammonium hydroxide, diisopropylamine, or di-n-propylamine.

[0020] In step (2) of the preparation method of the present invention, the mixed gel is stirred at 20-60°C for 2-10 hours; then transferred to a crystallization vessel for crystallization treatment at 160-210°C for 12-60 hours.

[0021] In the preparation method of the present invention, the solid after washing in step (3) is dried at 110°C for 6 to 12 hours and then calcined at 400 to 700°C to obtain nano-SAPO-34 / SAPO-5 composite molecular sieve.

[0022] In a second aspect, this invention provides a nano-SAPO-34 / SAPO-5 composite molecular sieve prepared by the above method, wherein the proportion of SAPO-34 molecular sieve in the nano-SAPO-34 / SAPO-5 composite molecular sieve is 40-80 wt%. The nano-SAPO-34 / SAPO-5 symbiotic composite molecular sieve provided by this invention solves the diffusion limitation of existing SAPO-34 molecular sieves in methanol conversion reactions, avoiding the problem of rapid deactivation rate in MTO reactions.

[0023] In some specific embodiments, the particle size distribution of the nano-SAPO-34 / SAPO-5 composite molecular sieve is between 100 nm and 200 nm, and the external specific surface area is 50 to 100 m². 2 / g.

[0024] In a third aspect, the present invention provides an application of a nano-SAPO-34 / SAPO-5 composite molecular sieve. In the presence of the above-mentioned nano-SAPO-34 / SAPO-5 composite molecular sieve prepared by the above-mentioned synthesis method, a methanol-to-low-carbon olefin reaction is carried out in a fixed-bed reactor using a methanol solution with a mass concentration of 80%-99%. The resulting product has high selectivity for ethylene and propylene, and the composite molecular sieve has a long reaction life.

[0025] For example, the selectivity of the ethylene and propylene obtained by the reaction is greater than 84%, and the lifetime of the composite molecular sieve is greater than 310 min.

[0026] In some specific applications, the nano-SAPO-34 / SAPO-5 composite molecular sieve is activated at 450–550 °C for 1–3 h, and then further activated at 450–550 °C with a mass hourly space velocity (HHSV) of 1–5 h⁻¹. -1 The methanol-to-olefin reaction is carried out below.

[0027] The above technical solution achieves the following technical effects:

[0028] The synthesis method of nano-SAPO-34 / SAPO-5 composite molecular sieve provided by this invention increases the proportion of SAPO-34 molecular sieve in the composite molecular sieve to more than 40%, effectively extending the catalyst lifetime.

[0029] The nano-SAPO-34 / SAPO-5 composite molecular sieve provided by this invention can achieve a selectivity of over 84% for ethylene and propylene in the methanol-to-olefins (MTO) reaction, and the lifetime of the composite molecular sieve can reach over 310 min. Attached Figure Description

[0030] Figure 1SEM surface morphology of the SAPO-34 / SAPO-5 mixed molecular sieve obtained in Example 1 of this invention;

[0031] Figure 2 SEM surface morphology of the SAPO-34 / SAPO-5 mixed molecular sieve obtained in Comparative Example 1 of this invention. Detailed Implementation

[0032] To facilitate understanding of the present invention, the following description, in conjunction with embodiments, will further illustrate the invention. It should be understood that the following embodiments are merely for a better understanding of the invention and do not imply that the invention is limited to these embodiments.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0034] Where specific experimental steps or conditions are not specified in the examples, the corresponding conventional experimental steps or conditions in this technical field can be followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0035] The methods for testing the performance of the composite molecular sieves prepared in the following embodiments and comparative examples of the present invention are as follows:

[0036] (1) Surface morphology detection of composite molecular sieves: The morphology of the obtained samples was characterized by scanning electron microscopes (SEM) of FEI Nova Nano SEM450 from the United States. The test conditions were accelerating voltage of 30V-30kV.

[0037] (2) Pore structure: The pore structure was characterized by low-temperature nitrogen physical adsorption using a Micromeritics ASAP2460 (USA). Helium was used as the inert gas to measure the dead volume of the sample tube, and nitrogen was used as the adsorbent to measure the pore volume and specific surface area of ​​the sample. The specific surface area was calculated using the BET formula, and the specific surface area and micropore volume of the micropores were calculated using the t-plot method. The mesopore volume was the difference between the total pore volume and the micropore volume.

[0038] (3) Particle size distribution: The obtained samples were characterized by scanning electron microscopes (SEM) of FEI Nova Nano SEM 450 from the United States. The test conditions were 30V-30kV acceleration voltage. The molecular sieve particle size was statistically analyzed (the number of samples was not less than 50) and the average particle size was calculated.

[0039] (4) XRD measurement: The German Bruker D8 ADVANCE X-ray diffractometer was used. The test conditions were CuKα ray source, tube voltage of 40kV, tube current of 40mA; 5-50° scan, scan rate of 4° / min.

[0040] Example 1

[0041] (1) Mix 11g of pseudoboehmite (Al2O3 content of 76wt%) with 65g of deionized water and stir for 0.5h. Then add 18g of phosphoric acid aqueous solution (85wt%) and 12g of phenyltriethoxysilane (98wt%, molecular structure shown below) in sequence. After stirring at room temperature for 0.5h, add 97g of tetraethylammonium hydroxide (25wt%) to obtain a mixed gel.

[0042] The molar ratio of each component in the mixed gel is Al2O3:P2O5:SiO2:template:H2O = 1:0.95:0.6:2.0:90;

[0043]

[0044] (2) After stirring the mixed gel for 4 hours, it was transferred to a stainless steel crystallization kettle with a polytetrafluoroethylene liner and crystallized at 200°C for 24 hours under dynamic conditions in a rotating homogeneous reactor.

[0045] (3) The crystallized product after cooling to room temperature was filtered, washed, dried at 110°C overnight, and finally calcined at 650°C for 5 hours in a muffle furnace to obtain a molecular sieve sample.

[0046] XRD analysis determined the molecular sieve sample to be a SAPO-34 / SAPO-5 composite molecular sieve. Scanning electron microscopy (SEM) was used to characterize the morphology of the sample, and the SEM images are attached. Figure 1 As shown, the obtained sample consists of nanocrystals aggregated into spherical micron-sized particles.

[0047] The average grain size of the nano-SAPO-34 / SAPO-5 composite molecular sieve was determined to be 137 nm, and the external specific surface area, as determined by low-temperature nitrogen physical adsorption-desorption characterization, was 78.6 m². 2 / g.

[0048] Examples 2-8

[0049] The synthesis process of Examples 2 to 8 is the same as that of Example 1, except for the proportion of each component and the crystallization conditions, as shown in Table 1.

[0050] Table 1

[0051]

[0052]

[0053]

[0054] Note: In Table 1, the percentage of each template agent is a molar percentage.

[0055] Comparative Example 1

[0056] (1) 11g of boehmite (76wt% Al2O3) was mixed with 53g of deionized water and stirred for 0.5h. Then, 18g of phosphoric acid (85wt%) and 8.2g of silica sol (30wt% SiO2) were added sequentially and stirred at room temperature for 0.5h. Finally, 48.3g of tetraethylammonium hydroxide (25wt%) and 16.6g of triethylamine (99wt%) were added to obtain a mixed gel.

[0057] The molar ratio of each component in the mixed gel is Al2O3:P2O5:SiO2:template:H2O = 1:0.95:0.5:3.0:65; wherein, the molar ratio of triethylamine to tetraethylammonium hydroxide in the template is 2:1.

[0058] (2) After stirring the mixed gel for 4 hours, it was transferred to a stainless steel crystallization kettle with a polytetrafluoroethylene liner and crystallized at 200°C for 24 hours under dynamic conditions in a rotating homogeneous reactor.

[0059] (3) The crystallized product after cooling to room temperature was filtered, washed, dried at 110°C overnight, and finally calcined at 650°C for 5 hours in a muffle furnace to obtain a molecular sieve sample.

[0060] XRD analysis confirmed that the molecular sieve sample was a pure-phase SAPO-34 molecular sieve. Scanning electron microscopy (SEM) was used to characterize the morphology of the sample, and the SEM images are attached. Figure 2 As shown, the obtained sample consists of cubic particles.

[0061] The average particle size of the nano-SAPO-34 / SAPO-5 composite molecular sieve was determined to be 2108 nm, and the external specific surface area, as determined by low-temperature nitrogen physical adsorption-desorption characterization, was 18.1 m². 2 / g.

[0062] Comparative Example 2

[0063] (1) 11g of boehmite (76wt% Al2O3) was mixed with 58g of deionized water and stirred for 0.5h. Then, 18g of phosphoric acid aqueous solution (85wt%) and 4.4g of silica sol (30wt% SiO2) were added sequentially and stirred at room temperature for 0.5h. Finally, 7.6g of diethylammonium (99wt%) and 10.3g of triethylamine (99wt%) were added to obtain a mixed gel.

[0064] The molar ratio of each component in the mixed gel is Al2O3:P2O5:SiO2:template:H2O = 1:0.95:0.27:2.5:45; wherein, the molar ratio of diethylamine to triethylamine in the template is 1:1.

[0065] (2) After stirring the mixed gel for 4 hours, it was transferred to a stainless steel crystallization kettle with a polytetrafluoroethylene liner and crystallized at 200°C for 24 hours under dynamic conditions in a rotating homogeneous reactor.

[0066] (3) The crystallized product after cooling to room temperature was filtered, washed, dried at 110°C overnight, and finally calcined at 650°C for 5 hours in a muffle furnace to obtain a molecular sieve sample.

[0067] The molecular sieve samples were determined by XRD for a composite molecular sieve of SAPO-34 and SAPO-18. The morphology of the samples was characterized by scanning electron microscopy, and the samples were found to be cubic particles.

[0068] The average particle size of the nano-SAPO-34 / SAPO-5 composite molecular sieve was determined to be 3568 nm, and the external specific surface area, as determined by low-temperature nitrogen physical adsorption-desorption characterization, was 12.0 m². 2 / g.

[0069] The molecular sieve samples obtained above were tested for specific surface area, average particle size and crystal phase ratio. The test results are shown in Table 2.

[0070] Table 2

[0071]

[0072] The catalytic effect of the obtained molecular sieve samples was evaluated in a fixed-bed reactor. The experimental conditions were as follows: feed was an 80% methanol-water solution, reaction temperature was 450℃, and mass hourly space velocity (H₂Sv) was 3.4 h⁻¹. -1 The molecular sieve loading amount is 0.8g.

[0073] Specific steps: Molecular sieves are loaded into a stainless steel reaction tube as a catalyst, heated to 500℃ for 1 hour, cooled to 450℃, and methanol-water solution is introduced. Online sampling is used, and the product is separated by condensation. The gaseous components are analyzed by gas chromatography (Agilent, 7890A type). The chromatography is equipped with an HP-PLOT Al2O3 / KCl column (50m×0.53mm×15μm) (for separating C1-C6 hydrocarbons), an HP-PLOT Q column (30m×320μm×20μm) (for separating alcohols and ethers), a Hayesep Q column and an X molecular sieve column (for separating permanent gases such as CO, CO2, H2, and N2), two FID detectors and one TCD detector.

[0074] The methanol conversion rate (X) and product selectivity (Si is expressed in mol of carbon, based on carbon-based selectivity) during the reaction are calculated by the following equations:

[0075]

[0076]

[0077] Where X is the methanol conversion rate; S is the product selectivity; i is the species entering the reactor; o is the species leaving the reactor; C is the product selectivity. x H y -Alkene (x - number of carbon atoms in the hydrocarbon species, y - number of hydrogen atoms in the hydrocarbon species); m - corresponding substance C x H y The number of carbon atoms; n - the corresponding molar number of substances; MeOH - methanol; DME - dimethyl ether.

[0078] When the conversion rate of methanol in the tested component is less than 99%, it is considered that the catalyst is deactivated. The results are shown in Table 3.

[0079] Table 3

[0080]

[0081]

[0082] Note: Catalyst lifetime refers to the time during which methanol conversion is maintained at 99% or higher.

[0083] As can be seen from the data in the table above and the accompanying drawings, the nano-SAPO-34 / SAPO-5 composite molecular sieve provided by this invention consists of nanocrystals aggregated into spherical micron-sized particles with a particle size distribution between 100 nm and 200 nm and an external specific surface area of ​​50–100 m². 2 / g; Meanwhile, the proportion of SAPO-34 molecular sieve in the nano-SAPO-34 / SAPO-5 composite molecular sieve prepared by the synthesis method of the present invention can reach 40-80 wt%, which makes it have excellent catalytic performance and long reaction life in methanol to olefins (MTO reaction), and improves the selectivity of ethylene and propylene in the reaction process.

Claims

1. A method for synthesizing a nano SAPO-34 / SAPO-5 composite molecular sieve, characterized in that, The method comprises the following steps: (1) adding an aluminum source into water and mixing and stirring, then adding a phosphorus source, a silicon source and a template agent to obtain a mixed gel; The molar ratio of the components in the mixed gel is Al2O3:P2O5:SiO2:template agent:H2O=1.0:(0.6-1.4):(0.2-0.8):(1.0-3.0):(30-100); (2) aging the mixed gel under mixing and stirring, and then performing a crystallization treatment; (3) performing solid-liquid separation on the crystallization product, and then washing, drying and calcining the solid after the solid-liquid separation to obtain a nano SAPO-34 / SAPO-5 composite molecular sieve; In step (1), the silicon source has a structure shown in Formula I. Formula I In the structure of Formula I, R1, R2 and R3 are independently selected from C1-C4 alkyl; R4 and R5 are independently selected from H or C1-C3 alkyl.

2. The method of synthesis of claim 1, wherein, In the structure of Formula I, R1, R2 and R3 are selected from -CH3, -CH2CH3, -(CH2)2CH3 and -(CH2)3CH3. R4 and R5 are independently selected from H or -CH3, -CH2CH3 and -(CH2)2CH3.

3. The method of synthesis of claim 2, wherein, In the structure of Formula I, R1, R2 and R3 are selected from -CH3 and -CH2CH3.

4. The method of synthesis of claim 2, wherein, In the structure of Formula I, R4 is selected from H and -CH3.

5. The method of synthesis of claim 2, wherein, In the structure of Formula I, R5 is selected from -CH3 and -CH2CH3.

6. The method of synthesis according to any one of claims 1 to 5, wherein, In step (1), the molar ratio of the components in the mixed gel is Al2O3:P2O5:SiO2:template agent:H2O=1.0:(0.8-1.2):(0.4-0.6):(2.0-3.0):(40-60).

7. The method of synthesis of claim 6, wherein, The aluminum source in step (1) is selected from pseudo-boehmite and / or aluminum isopropoxide; The phosphorus source is selected from one or more of phosphoric acid, metaphosphoric acid or a phosphate salt; The template agent is selected from one or more of triethylamine, diethylamine, morpholine, tetraethylammonium hydroxide, diisopropylamine or di-n-propylamine.

8. The method of synthesis according to claim 7, wherein, In step (2), the mixed gel is stirred at 20-60°C for 2-10 h, and then transferred to a crystallization kettle for a crystallization treatment at 160-210°C for 12-60 h.

9. The method of synthesis according to claim 8, wherein, In step (3), the solid after washing is dried at 110°C for 6-12 h, and then calcined at 400-700°C to obtain the nano SAPO-34 / SAPO-5 composite molecular sieve.

10. A nano SAPO-34 / SAPO-5 composite molecular sieve prepared by the synthesis method of any one of claims 1-9, characterized in that, In the nano SAPO-34 / SAPO-5 composite molecular sieve, the proportion of SAPO-34 molecular sieve is 40-80 wt%.

11. The method of synthesis according to claim 10, wherein, The nano SAPO-34 / SAPO-5 composite molecular sieve has a particle size distribution of 100 nm to 200 nm, and an external specific surface area of 50 to 100 m 2 / g.

12. Use of a nano SAPO-34 / SAPO-5 composite molecular sieve, characterized in that, In the presence of the nano SAPO-34 / SAPO-5 composite molecular sieve prepared by the synthesis method in any one of claims 1-9 or the nano SAPO-34 / SAPO-5 composite molecular sieve in claim 10 or 11, a methanol-to-light olefins reaction is performed on a fixed bed reactor using a methanol solution with a mass concentration of 80%-99%, and the selectivity of ethylene and propylene in the obtained product is high, and the reaction life of the composite molecular sieve is long. The selectivity of ethylene and propylene obtained in the reaction is greater than 84%, and the life of the composite molecular sieve is greater than 310 min.

13. Use according to claim 12, characterized in that, The nano SAPO-34 / SAPO-5 composite molecular sieve is activated at 450-550 ℃ for 1-3 h, and then the methanol-to-olefin reaction is carried out at 450-500 ℃, the mass space velocity is 1-5 h -1 The methanol-to-olefin reaction is carried out below.

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