A method for preparing nano-eutectic silica-alumina molecular sieves and its application

Abstract

This application discloses a method for synthesizing and preparing nano-eutectic aluminosilicate phosphate molecular sieves and their applications. The preparation method includes: mixing aluminosilicate phosphate molecular sieve with a C-type topology with an organic template agent suitable for D-type molecular sieve crystallization, followed by hydrothermal crystallization to obtain nano-eutectic aluminosilicate phosphate molecular sieves; C and D are not the same molecular sieve structure type. This preparation method is simple, has a fast crystallization rate, and is an effective method for synthesizing novel eutectic molecular sieves. The synthesized molecular sieve has a particle size at the nanoscale, which is beneficial for mass transport and makes it an excellent adsorbent, acid catalyst, and catalyst support.

Description

Technical Field

[0001] This application belongs to the field of synthesis of silica-alumina molecular sieves, and specifically relates to a method for preparing nano-eutectic silica-alumina molecular sieves and their applications. Background Technology

[0002] SAPO-n molecular sieves are a class of microporous crystalline materials developed by Union Carbide Corporation (UCC) in 1984 (USP 4,440,871). Their infinitely open framework structure consists of [SiO4]. 0 [AlO4] - [PO4] + SAPO molecular sieves are composed of three tetrahedral units connected at common vertices according to a certain symmetry. Si atoms isomorphously replace some P atoms in the neutral aluminum phosphate framework, generating a net negative charge in the framework and introducing Brønsted acid centers, thus endowing SAPO molecular sieves with acid catalytic properties. Currently, approximately 40 types of SAPO molecular sieve structures have been synthesized and their structures have been clearly identified. Due to the unclear understanding of the crystallization mechanism and the limitations of the molecular sieve framework composition, expanding the structural diversity of SAPO molecular sieves remains a significant challenge (USP 10,336,622 B1). The unique structural types and acidic properties of SAPO molecular sieves may lead to more applications with superior catalytic performance and adsorption separation.

[0003] Eutectic molecular sieves refer to the coexistence of two or more topological structures within a single molecular sieve crystal. Different topological structures grow together through shared structural units or cross-sections. These crystalline materials typically have a uniform morphology, but their XRD diffraction patterns show diffraction peaks corresponding to more than one structure. Depending on the degree of order in the coexisting structures, the intensity and width of the XRD diffraction peaks vary. Currently, the most common coexisting SAPO molecular sieves are SAPO-18 / 34. These two molecular sieves share many structural similarities and both belong to the ABC-6 family. To date, disordered coexisting material systems in the ABC-d6r family include Linde T (ERI / OFF); Babelite (random stacking); Linde D (disordered CHA); Phi (disordered CHA); ZK-14 (disordered CHA); LZ-276 (disordered CHA); and LZ-277 (disordered CHA), among others. These structures can be formed by connecting common d6r structural units in different symmetry patterns, thus offering the potential for complex structures with multiple coexisting structures. Recently, integrated differential phase-contrast electron microscopy (iDPC-STEM) has been applied to the characterization of eutectic materials, revealing the microscopic atomic arrangement of the coexisting phase at the atomic scale. Studies have shown that different preparation methods can lead to drastically different microscopic atomic arrangements in eutectic materials (Chen X and Wei F et al, Nat. Commun. 2021, 12, 2212).

[0004] The most common method for synthesizing eutectic molecular sieve materials is to simultaneously introduce a mixture of template agents that can guide two structures. Yuan Long et al. from Tianjin University synthesized a molecular sieve material with a SAPO-56 / 34 composite by mixing N,N,N',N'-tetramethyl-1,6-hexanediamine (TMHDA), a typical template agent for SAPO-56, with triethylamine, a template agent for SAPO-34 (CN108862315A). Tian Peng et al. from the Dalian Institute of Chemical Physics synthesized a symbiotic phase of SAPO-17 / 56 using a mixed template agent of triethanolamine (TEOA) and TMDHA (CN105984877A); they also synthesized a novel SAPO molecular sieve with GME and CHA symbiosis using a mixed system of diisopropanolamine (diethanolamine) and trimethylamine (CN107032363A). Unlike the above synthesis, SAPO-18 / 34 eutectic molecular sieves can usually be synthesized using a single template agent. Using triethylamine (TEA) as a template agent, hydrothermal synthesis often yields a symbiotic phase of SAPO-34 and SAPO-18. Chinese patent CN101450806A discloses an AEI / CHA eutectic molecular sieve containing TEA and its synthesis method, reducing preparation costs. The prepared eutectic molecular sieve is dominated by the CHA phase. Under amaminedothermal conditions, TEA acts as both a solvent and a template agent, and the water content corresponds to the reactant dosage, resulting in the synthesis of a relatively pure SAPO-18 phase with a maximum silicon content of 11.8% (Fan D. and Tian P. et al. J. Mater. Chem., 2012, 22, 6568). In a hydrothermal system using TEAOH as a template agent, the SAPO-18 phase readily emerges under low silicon feed conditions. As the silicon feed increases, the crystalline phase gradually transitions towards SAPO-34, resulting in a symbiotic product state. Chinese patent CN102372291A discloses a CHA / AEI symbiotic molecular sieve product synthesized using TEAOH as a template agent, in which the SAPO-34 content, calculated by mass percentage, ranges from 40% to 98%. Mixed template agents can also be used in the synthesis of the SAPO-34 / SAPO-18 eutectic phase. Chinese patent CN103878018A discloses a method using triethylamine (TEA) and tetraethylammonium hydroxide (TEAOH) as mixed template agents, and adding a crystal growth inhibitor to the synthesis solution to obtain small-crystal SAPO-18 / SAPO-34 eutectic molecular sieves. The eutectic molecular sieves obtained by this synthesis method have a low SAPO-18 content. US patent US2005 / 0233895 discloses a silica-aluminophosphate molecular sieve comprising a CHA / AEI framework cross-phase growth; X-ray diffraction (DIFFaX) analysis of calcined samples determined the symbiotic ratio of the two crystal phases to be between 5 / 95 and 40 / 60. Summary of the Invention

[0005] The purpose of this invention is to provide a novel method for synthesizing nano-eutectic SAPO molecular sieves. This method effectively promotes the formation of the eutectic structure, avoids phase separation of the two structures, and allows for effective control of the proportion of the symbiotic crystalline phases. The synthesis method is simple and convenient to operate, enabling rapid synthesis of the target eutectic SAPO molecular sieve with crystal particle sizes at the nanoscale.

[0006] According to a first aspect of this application, a method for preparing nano-eutectic silica-alumina molecular sieves is provided, the method comprising:

[0007] Step S1: Mix the C-type topological structure of the silica-alumina molecular sieve with an organic template agent solution suitable for D-type molecular sieve crystallization, and perform hydrothermal crystallization to obtain the nano-eutectic silica-alumina molecular sieve.

[0008] The particle size of the silicon-aluminum phosphate molecular sieve with C-type topology is less than 300 nm.

[0009] The silicon aluminum phosphate sieve with C-type topology is selected from at least one of SAPO-18, SAPO-56, SAPO-34, SAPO-35 and SAPO-17; D and C are not the same type of molecular sieve structure, and the organic template agent used to synthesize D-type molecular sieves cannot synthesize C-type molecular sieves.

[0010] Optionally, the D-type molecular sieve is selected from at least one of the following molecular sieves having the following structures: GME, AEI, CHA, AFN, LEV; the combination of C and D cannot be the same molecular sieve structure.

[0011] Optionally, the organic template agent suitable for D-type molecular sieve crystallization should not be a template agent of the C-phase.

[0012] Optionally, the C-type topological aluminum silicate phosphate molecule is screened from at least one of SAPO-18 (AEI); SAPO-56 (AFX); SAPO-34 (CHA); SAPO-35 (LEV) and SAPO-17 (ERI).

[0013] Optionally, the organic template agent suitable for D-type molecular sieve crystallization is selected from at least one of N,N,N',N'-tetramethyl-1,6-hexanediamine, N,N-diisopropylethylamine, N,N-dimethylisopropylamine, tripropylamine, triethylamine, diethylamine, diisopropylamine, isopropylamine, diisoolamine, benzyltrimethylammonium chloride, triethylenediamine, tetraethylammonium hydroxide, cycloheximine, cyclohexylamine, tert-butylamine, and piperidine; the organic template agent suitable for D-type molecular sieve crystallization cannot be used to synthesize C-type molecular sieves.

[0014] Optionally, the nano-eutectic silica-alumina phosphate molecular sieve in this application has a crystal particle size of less than 1 micrometer and a complex and uniform crystal morphology.

[0015] Optionally, the crystallization conditions are: reaction at 120-240°C for 3-72 hours in a closed hydrothermal reactor;

[0016] Optionally, the organic template agent solution suitable for D-type molecular sieve crystallization has a mass concentration of 10-50 wt%; the solid-liquid mass ratio of the C-type topological structure phosphate aluminum molecular sieve to the organic template agent solution suitable for D-type molecular sieve crystallization is 1:2 to 1:20.

[0017] Optionally, the upper limit of the mass concentration of the organic template agent solution suitable for D-type molecular sieve crystallization is independently selected from 50wt%, 40wt%, 30wt%, 20wt%, and 15wt%, and the lower limit is independently selected from 10wt%, 40wt%, 30wt%, 20wt%, and 15wt%.

[0018] Optionally, the upper limit of the solid-liquid mass ratio of the C-type topological structured silica-alumina molecular sieve to the organic template agent solution suitable for D-type molecular sieve crystallization is independently selected from 1:2, 1:5, 1:10, and 1:15, and the lower limit is independently selected from 1:20, 1:5, 1:10, and 1:15.

[0019] Optionally, step S0 may be included before step S1;

[0020] Step S0: The silica-alumina molecular sieve with C-type topology is pretreated to destroy its crystal structure.

[0021] The pretreatment is selected from at least one of ball milling, acid treatment, and alkali treatment.

[0022] Optionally, the pretreatment includes one of ball milling followed by alkali treatment or ball milling followed by acid treatment.

[0023] Optionally, in step S0, D-type molecular sieves are added during pretreatment. Pretreatment with the mixed molecular sieves of these two target eutectic phases can increase the success rate of eutectic phase formation.

[0024] Optionally, in step S1, a phosphorus source and / or an aluminum source are added together to carry out the hydrothermal crystallization. The phosphoric acid and / or aluminum source can control the composition of the crystal product.

[0025] Optionally, the phosphorus source is selected from at least one of H3PO4 (80wt%), NH4H2PO4, and (NH4)2HPO4;

[0026] The aluminum source is selected from at least one of boehmite, Al(OH)3, and aluminum sol.

[0027] Optionally, the above-mentioned phosphorus aluminum source and organic amine solution are premixed in a certain proportion and then added to the reaction system for the synthesis of eutectic molecular sieves.

[0028] Optionally, the crystallization process can be carried out under static or dynamic conditions, without special requirements.

[0029] Optionally, the preparation method further includes step S2, in which the product obtained after the reaction is completed is cooled to room temperature, solid-liquid separation is performed, the product is washed with deionized water and dried to obtain the molecular sieve with two coexisting C and D structures; where C and D refer to two different topological structures respectively.

[0030] According to a second aspect of this application, a nano-eutectic silica-alumina phosphate molecular sieve is provided, wherein the nano-eutectic silica-alumina phosphate molecular sieve is selected from at least one of the nano-eutectic silica-alumina phosphate molecular sieves prepared according to the above method.

[0031] According to a third aspect of this application, an application is provided for the nano-eutectic silica-alumina phosphate molecular sieve prepared according to the above method in an adsorbent, acid catalyst or catalyst support.

[0032] Optionally, the nano-eutectic silica-alumina molecular sieve is used after being calcined in air at 500–800°C.

[0033] Optionally, the acid catalyst is a molecular sieve catalyst for methanol-to-olefins.

[0034] Optionally, the nano-eutectic silica-alumina molecular sieve can be calcined in air at 500–800°C and used as a catalyst for methanol-to-olefins reaction, or further subjected to copper ion loading for use as a catalyst for ammonia selective reduction denitration reaction.

[0035] The beneficial effects that this application may produce include at least the following:

[0036] (1) This application provides a novel method for synthesizing nano-eutectic SAPO molecular sieves, which can effectively control the eutectic ratio (by controlling the ratio of symbiotic crystalline phases by controlling the temperature and time of the crystallization reaction), crystal morphology, and acid properties, thus facilitating the application of SAPO molecular sieves in methanol-to-olefins (MTO) and DeNOx reactions. x Catalytic applications, such as selective catalytic reduction, offer room for further improvement.

[0037] (2) This method is simple and easy to operate, and it also has the advantages of short crystallization time and high product yield, which provides convenience for large-scale industrial production. Attached Figure Description

[0038] Figure 1 The X-ray diffraction pattern of the SAPO-AFX / CHA (Example 3) sample.

[0039] Figure 2 Scanning electron microscope image of SAPO-AFX / CHA (Example 3) sample.

[0040] Figure 3 The X-ray diffraction pattern of the SAPO-CHA / AEI (Example 2) sample is shown.

[0041] Figure 4 Scanning electron microscope image of SAPO-CHA / AEI (Example 2) sample.

[0042] Figure 5 The X-ray diffraction pattern of the SAPO-CHA / GME (Example 1) sample is shown.

[0043] Figure 6 A scanning electron microscope image of the SAPO-CHA / GME (Example 1) sample.

[0044] Figure 7 The X-ray diffraction pattern of the SAPO-ERI / LEV (Example 10) sample is shown.

[0045] Figure 8 The results of the NH3-SCR reaction of the SAPO-CHA / AEI (Example 2) sample after Cu ion exchange and calcination are shown.

[0046] Figure 9 The results of the catalytic NH3-SCR reaction of the SAPO-AFX / CHA (Example 3) sample after Cu ion exchange and calcination are shown. Detailed Implementation

[0047] The present invention is further illustrated below with reference to embodiments, but the present invention is not limited to these embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer. Unless otherwise specified, the raw materials used in this application are all purchased commercially and used directly without special treatment.

[0048] The analysis method in the embodiments of this application is as follows:

[0049] X-ray powder diffraction (XRD) phase analysis was performed using an X'Pert PRO X-ray diffractometer from PANalytical, Netherlands, with a Cu target, Kα radiation source (λ = 0.15418 nm), voltage 40 kV, and current 40 mA.

[0050] The scanning electron microscope (SEM) used for testing was a Hitachi SU8020 field emission scanning electron microscope with an accelerating voltage of 2kV.

[0051] In the examples, the bulk elemental composition was determined using a Philips Magix 2424 X-ray fluorescence analyzer (XRF).

[0052] The precursor was prepared by mechanical crushing on a QM-3SP2 planetary ball mill.

[0053] The sample yield was calculated as follows: product mass × 85% ÷ (dry basis mass of inorganic matter in the initial gel product + mass of SAPO-34 precursor) × 100%.

[0054] The test conditions for the MTO catalytic reaction in this application are as follows:

[0055] The catalyst was first compressed into tablets and crushed to 20-40 mesh, then calcined at 600℃ with air for 4 hours. 0.3 g of this sample was loaded into a self-made fixed-bed reactor (a quartz tube reactor) for MTO reaction evaluation. The reactor was activated at 550℃ with nitrogen for 1 hour, then cooled to the target temperature for the reaction. Methanol was carried by nitrogen. The reaction products were analyzed by online gas chromatography (Agilent A7890) using a PoraPlot-Q-HT column.

[0056] The specific experimental procedures and test conditions for the NH3-SCR catalytic reaction in this application are as follows:

[0057] First, 5g of molecular sieve powder was added to 100g of 0.01mol / L copper acetate solution, and ion exchange was performed at 80℃ for 5h. The sample was then centrifuged and washed with deionized water until neutral. After drying in a 120℃ oven, it was calcined at 600℃ for 5h. The calcined sample was further pressed into tablets and sieved. 0.3g of 60-80 mesh sample was weighed and mixed with 1.9g of quartz sand (60-80 mesh), and loaded into a fixed-bed reactor. Nitrogen gas was introduced at 600℃ for 40min for activation, then the temperature was lowered to 120℃ to begin the reaction, and then raised to 550℃. The reactant gases were: NO: 500ppm, NH3: 500ppm, O2: 5%, H2O: 5%, with N2 as the equilibrium gas. The gas flow rate was 1000mL / min, corresponding to a space velocity of 180,000h⁻¹. -1 The reaction tail gas was analyzed online using a Bruker Tensor 27 instrument; the results are shown below. Figure 8 .

[0058] The high-temperature hydrothermal aging temperature is 800℃, corresponding to a space velocity of 100,000 h⁻¹. -1 The aging time is 16 hours.

[0059] The initial synthesis of molecular sieve materials was based on classic synthetic literature, but the materials were not limited to these methods; some were purchased directly from companies. The silicon content of these molecular sieves is indicated by a percentage in parentheses, and there are no special requirements for them, nor are there any special requirements for other properties of the molecular sieves.

[0060] Examples 1-10: Synthesis of eutectic SAPO molecular sieves

[0061] 1) First, molecular sieve C was synthesized. The specific synthesis method and results are shown in Table 1. The general SAPO molecular sieve C synthesis steps are as follows: First, the aluminum source is dissolved in water, and then optional phosphorus source, silicon source, and organic template agent R are added sequentially. After stirring evenly at room temperature, the gel is transferred to a stainless steel reactor. After placing the reactor in an oven, the temperature is raised to 160-200℃ and the reaction is carried out for 0.5-72 hours until crystallization is complete. The solid product is centrifuged, washed, and dried in air at 120℃ to obtain the molecular sieve powder sample. XRD, SEM, and XRF analyses were performed on the synthesized sample.

[0062] Table 1 Synthesis conditions and results of molecular sieve C

[0063]

[0064] 2) For molecular sieve C with AFX, CHA, and LEV structures, 10g of molecular sieve C was dispersed in 30g of H2O, placed in an agate jar, and a certain amount of agate microspheres were added. The mixture was ball-milled in a ball mill for 6 hours at a speed of 550rpm to obtain precursor P, named SAPO-CP. C represents the specific structural code. Please refer to Table 1 and Table 2 for details.

[0065] 3) For molecular sieve precursor C with ERI and AEI structures, since the crystals of these two types of molecular sieves are not easy to grow and the particle size is less than 300 nm, the synthesized molecular sieve sample can be directly selected as the precursor.

[0066] 4) Synthesis of eutectic molecular sieves: Prepare an aqueous template agent solution R of a certain concentration (the mass content of each substance in the aqueous solution is detailed in Table 2; for example, in Example 1, 30 wt% diethanolamine and 5 wt% benzyltrimethylammonium chloride were used simultaneously, and 10 times the mass of the precursor organic amine solution was added to each). Mix the precursor P with it at a certain mass ratio. If necessary, add a certain amount of phosphoric acid and aluminum source simultaneously, stir evenly, and seal in a hydrothermal reactor for hydrothermal crystallization. The obtained solid product is centrifuged, washed with deionized water until neutral, and dried in air at 120°C to obtain the raw powder, denoted as sample SAPO-C / D, where C and D refer to the corresponding crystal phases. The sample was characterized by XRD diffraction analysis, compositional analysis, and SEM scanning electron microscopy.

[0067] Table 2 Synthesis conditions and results of Examples 1-10

[0068]

[0069]

[0070] a: The gel composition of the reactants is n(Al2O3):n(P2O5):n(SiO2):n(R):n(H2O) = 1:0.8:0.25:4.0:50. Using SAPO-LEV-P as the single silicon source, boehmite as the aluminum source, and phosphoric acid (80%) as the phosphoric acid source, calculate the feed amount.

[0071] b: The gel composition of the reactants is n(Al2O3):n(P2O5):n(SiO2):n(R):n(H2O) = 1:0.8:0.08:4.0:50. Using SAPO-AEI-P as the single silicon source, Al(OH)3 as the aluminum source, and NH4H2PO4 as the phosphorus source, calculate the feed amounts.

[0072] c: The reactant gel composition is n(Al2O3):n(P2O5):n(SiO2):n(R):n(H2O) = 1:0.8:0.15:4.0:50. Using SAPO-CHA-P as the single silicon source, aluminum source: aluminum sol, and phosphorus source: (NH4)2HPO4, calculate the feed amounts.

[0073] Example 11: Selective catalytic reduction of NO by NH3- in SAPO-CHA / AEI (Example 2) x (NH3-SCR) performance

[0074] Specific experimental procedures and reaction conditions are detailed in the detailed implementation method section. The CuO content in the sample was 3.0%. The reaction results are shown below. Figure 8 It can be seen that the sample has a wide active temperature window and good hydrothermal stability.

[0075] Example 12: Selective catalytic reduction of NO by NH3- in SAPO-AFX / CHA (Example 3) x (NH3-SCR) performance

[0076] Specific experimental procedures and reaction conditions are detailed in the detailed implementation method section. The CuO content in the sample was 3.5%. Specific reaction results are shown below. Figure 9 As can be seen, the samples exhibit a wide active temperature window and good hydrothermal stability. Examples 11 and 12 demonstrate that both samples possess a wide active temperature window and good hydrothermal stability.

[0077] Examples 13-15: The MTO catalytic performance of the three low-silicon-content SAPO eutectic molecular sieves synthesized by this method was tested, and the specific results are shown in Table 3.

[0078] Table 3: MTO catalytic performance of some synthesized samples a

[0079]

[0080] a Here, we selected the result with a methanol conversion rate greater than 99% and the highest propylene selectivity.

[0081] Figures 1 to 7 The figures show the X-ray diffraction patterns and scanning electron microscope images of the eutectic molecular sieves prepared in Examples 1-3 and Example 10. As can be seen from the figures, the XRD diffraction peaks vary in width, exhibiting typical characteristics of eutectic molecular sieves. In addition, the molecular sieves have uniform morphology, rough crystal surfaces, and nanoscale particle sizes.

[0082] As can be seen from Examples 13-15, the eutectic samples can have excellent MTO catalytic performance.

[0083] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for preparing a nano-eutectic silica-alumina molecular sieve, characterized in that, The preparation method includes: Step S1: The silica-alumina molecular sieve with a C-type topology is mixed with an organic template agent solution suitable for D-type molecular sieve crystallization and hydrothermally crystallized to obtain a eutectic molecular sieve with nanoscale C and D structures. The particle size of the silica-alumina molecular sieve with C-type topology is less than 300 nm. The silicon aluminum phosphate sieve with C-type topology is selected from at least one of SAPO-18, SAPO-56, SAPO-34, SAPO-35 and SAPO-17; D and C are not the same type of molecular sieve structure, and the organic template agent used to synthesize D-type molecular sieves cannot synthesize C-type molecular sieves; The D-type molecular sieve is selected from at least one of the following molecular sieves having the following structures: GME, AEI, CHA, AFN, LEV; The crystallization conditions are: reaction at 120-240℃ for 3-72 h in a closed hydrothermal reactor; The organic template agent solution suitable for D-type molecular sieve crystallization has a mass concentration of 10-50 wt%; the solid-liquid mass ratio of the C-type topological structure phosphate aluminum molecular sieve to the organic template agent solution suitable for D-type molecular sieve crystallization is 1:2 to 1:

20.

2. The preparation method according to claim 1, characterized in that, The organic template agent suitable for D-type molecular sieve crystallization is selected from at least one of N,N,N',N'-tetramethyl-1,6-hexanediamine, N,N-diisopropylethylamine, N,N-dimethylisopropylamine, tripropylamine, triethylamine, diethylamine, diisopropylamine, isopropylamine, diisoolamine, benzyltrimethylammonium chloride, triethylenediamine, tetraethylammonium hydroxide, cycloheximine, cyclohexylamine, tert-butylamine, and piperidine; the organic template agent suitable for D-type molecular sieve crystallization cannot be used to synthesize C-type molecular sieves.

3. The preparation method according to claim 1, characterized in that, Before step S1, step S0 is also included; Step S0: The silica-alumina molecular sieve with C-type topology is pretreated to destroy its crystal structure. The pretreatment is selected from at least one of ball milling, acid treatment, and alkali treatment.

4. The preparation method according to claim 3, characterized in that, In step S0, D-type molecular sieves are added during pretreatment.

5. The preparation method according to claim 1, characterized in that, In step S1, a phosphorus source and / or an aluminum source are added together to carry out the hydrothermal crystallization.

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