SAPO-34 molecular sieve, method for preparing same, and use thereof

Abstract

The application relates to the field of molecular sieve preparation, and discloses a SAPO-34 molecular sieve, a preparation method and application thereof. The SAPO-34 molecular sieve has a relative amount of external surface acid sites of less than 50%. The SAPO-34 molecular sieve provided by the application has a relatively small relative amount of external surface acid sites, is applied to a reaction of methanol to olefins, and has the characteristics of high selectivity of low-carbon olefins.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve preparation, specifically to a SAPO-34 molecular sieve, its preparation method, and its application. Background Technology

[0002] Ethylene and propylene are essential basic organic chemical raw materials in the chemical industry, playing a crucial role in modern petroleum and chemical industries. In recent years, with the increasing scarcity of global oil resources, the supply and demand imbalance caused by traditional petroleum-based ethylene and propylene production has intensified, prompting countries to focus on developing new non-petroleum-based olefin production processes. Among these, the process of producing low-carbon olefins from methanol via syngas (CO+H2) from coal or natural gas is currently recognized as the most promising technology. my country's energy structure, characterized by abundant coal resources and relatively scarce petroleum resources, dictates that once the coal-to-olefins process is successfully implemented, it will have significant and far-reaching implications for expanding the development space of my country's traditional coal chemical industry and ensuring national energy security. Currently, coal gasification and syngas-to-methanol processes in the coal-to-olefins flow have developed into mature coal chemical technologies, while the industrial-scale development of methanol-to-olefins (MTO) technology is a key step in realizing this process.

[0003] The core of methanol-to-olefins (MTO) technology lies in the development of molecular sieve catalysts. Early catalysts used in MTO were mostly silica-alumina zeolite molecular sieves, such as ZSM-5. However, their relatively large pore size and strong acidity resulted in low yields of low-carbon olefins. In 1982, Union Carbide Corporation (UCC) first synthesized the SAPO series of silica-alumina phosphate molecular sieves. Among them, SAPO-34 molecular sieve is the most noteworthy. It possesses a chalcogenide-like structure, along with small pore size, moderate acidity, and strong hydrothermal stability. It exhibits excellent selectivity for low-carbon olefins in the catalytic reaction of methanol to low-carbon olefins, thus attracting widespread attention from researchers both domestically and internationally.

[0004] US4440871 discloses various methods for preparing phosphorus-containing molecular sieves. This patent specifically describes methods for producing various SAPO molecular sieves using silicon sources, phosphorus sources, and organic template agents.

[0005] Wilson et al. [Wilson S, Barger P. The characteristics of SAPO-34 which influence the conversion of methanol to light olefins[J]. Microporous and Mesoporous Materials, 1999, 29: 117-126] modified the acidity of SAPO-34 molecular sieve by adjusting the Si content. In the range of 0.016 to 0.14 molar Si content, reducing the Si content was beneficial to reducing the propane yield, while controlling the Si molar Si content below 0.05 molar Si content was beneficial to increasing the yield of low-carbon olefins and extending the catalyst lifetime.

[0006] The acidity can also be controlled by changing the template agent. The research results of Li Lisheng et al. [Li Lisheng, Li Jun, Zhang Fengmei. Effect of template agent on the synthesis and catalytic performance of SAPO-34 [J]. Petroleum Refining and Chemical Industry, 2008, 39(4): 1-5] on the synthesis of SAPO-34 molecular sieve using DEA+TEA as template agent showed that as the proportion of DEA in the template agent increased, the silicon content in the crystallized product increased, the pore volume and specific surface area decreased, the crystal size gradually increased, the number of acid centers decreased, and the acid strength weakened. When the obtained molecular sieve was used for MTO reaction, the yield of ethylene decreased, the yield of propylene increased, and the selectivity of ethylene to propylene gradually increased.

[0007] In addition, the acidity of SAPO-34 can also be modulated by metal modification. There are two methods of metal modification: one is to introduce metal ions into the molecular sieve framework or cation sites by changing the starting materials during the synthesis process, and the other is to modify the molecular sieve with metal ions after the synthesis of the molecular sieve. Delphine et al. [Delphine RD, Daniel LO, Liu Jing. Conversion of ethanol to olefins over cobalt-, manganese- and nikel-incorporated SAPO-34 molecular sieves[J]. Fuel Processing Technology, 2003, 83(1-3): 203-218] studied Ni-SAPO-34 and Co and Mn modified SAPO-34 prepared by two different methods. The results showed that the catalysts modified by Ni, Mn and Co did not change much in activity and C2-C4 selectivity compared with unsupported SAPO-34, but the introduction of these transition metals improved the catalyst lifetime to varying degrees. Among them, Mn-SAPO-34 had the strongest resistance to carbon deposition. Ni-SAPO-34 exhibits a low carbon deposition rate in MTO, a methanol conversion rate of 100%, and an ethylene selectivity of up to 88%. Researchers attribute its superior MTO catalytic performance to the reduction of acidic sites due to the intervention of Ni.

[0008] Previous studies have yielded numerous fruitful results in modulating the acidity within SAPO-34 molecular sieves to achieve optimal MTO reaction performance. Currently, there are few reports on reducing the acidic sites on the outer surface of SAPO-34 molecular sieves. Summary of the Invention

[0009] The purpose of this invention is to overcome the problems existing in the prior art and provide a SAPO-34 molecular sieve, its preparation method and application. The SAPO-34 molecular sieve provided by this invention has a relatively small amount of acidic sites on its outer surface. When applied to the methanol-to-olefins reaction, it has the characteristics of high selectivity for low-carbon olefins.

[0010] To achieve the above objectives, the present invention provides a SAPO-34 molecular sieve, wherein the relative amount of acidic sites on the outer surface of the molecular sieve is less than 50%.

[0011] In existing technologies, during the hydrothermal crystallization of SAPO-34 molecular sieves, the initial gel system is acidic or near-neutral. As crystallization progresses, phosphoric acid is gradually consumed, leading to a continuous increase in the pH of the synthesis system. The silicon source typically exists in a polymerized state in the early stages of crystallization. Due to its low isoelectric point, silicon oxide gradually depolymerizes as the pH of the synthesis system increases, resulting in silicon-rich surfaces on the molecular sieve crystals. During our research, we discovered that after 80% crystallization of the raw materials in the initial gel mixture, the system pH increases, making it easier for Si to replace phosphorus in the framework. This results in the molecular sieve having more acidic sites on its outer surface, which increases the selectivity of byproducts in the MTO reaction. Reducing the number of acidic sites on the outer surface of SAPO-34 molecular sieves is key to further improving the selectivity of the main product in the MTO reaction. We further discovered that by controlling the type of materials and the acid-base environment during the second crystallization, SAPO-34 molecular sieves with a relatively low amount of acidic sites on their outer surface can be obtained. When applied to the reaction of oxygen-containing compounds to olefins, this achieves higher selectivity for low-carbon olefins.

[0012] A second aspect of this invention provides a method for preparing SAPO-34 molecular sieve, the method comprising the following steps:

[0013] a) A mixture A containing a silicon source, a first phosphorus source, a first aluminum source, a template agent and water is subjected to a first hydrothermal crystallization to obtain solid product I and mother liquor B;

[0014] b) A mixture C containing a second phosphorus source, a second aluminum source, solid product I, at least a portion of mother liquor B, and water is subjected to a second hydrothermal crystallization to obtain the SAPO-34 molecular sieve.

[0015] The third aspect of this invention provides the application of the SAPO-34 molecular sieve described in the first aspect or the SAPO-34 molecular sieve prepared by the preparation method described in the second aspect in the reaction of oxygen-containing compounds to olefins.

[0016] The beneficial effects of the present invention through the above technical solution include:

[0017] The SAPO-34 molecular sieve provided by this invention has a relatively small amount of acidic sites on its outer surface. When applied to the reaction of producing olefins from oxygen-containing compounds, it effectively suppresses side reactions and has the characteristics of high selectivity for low-carbon olefins. Attached Figure Description

[0018] Figure 1 These are the XRD patterns of SAPO-34 molecular sieves prepared in the examples and comparative examples;

[0019] Figure 2 The images show the infrared spectra of the SAPO-34 molecular sieve before and after PN adsorption prepared in Example 1. Detailed Implementation

[0020] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0021] In one aspect, the present invention provides a SAPO-34 molecular sieve, wherein the relative amount of acidic sites on the outer surface of the molecular sieve is less than 50%.

[0022] According to the present invention, preferably, the relative amount of acidic sites on the outer surface of the molecular sieve is 20-48%.

[0023] In this invention, the relative amount of acidic sites on the outer surface of the molecular sieve refers to the ratio of the amount of acidic sites on the surface of the molecular sieve to that of the reference molecular sieve, wherein the amount of acidic sites on the surface of the reference molecular sieve is defined as 100%.

[0024] The reference molecular sieve described in this invention is a molecular sieve prepared by a conventional method. Specifically, the molecular sieve prepared in Comparative Example 1 below is defined as the reference molecular sieve, and its specific preparation method is described below.

[0025] The relative abundance of acidic sites on the outer surface of the SAPO-34 molecular sieve described in this invention was determined in an ultra-high vacuum in-situ infrared system. Specifically, 2,2-dimethylpropionitrile (Pivalonitrile, PN) was used as the probe molecule. After the SAPO-34 molecular sieve was vacuum dehydrated and evacuated to a high vacuum, infrared spectra were acquired. Then, PN vapors at concentrations of 0.01 mbar, 0.10 mbar, 0.50 mbar, and 1.0 mbar were sequentially introduced, and infrared spectra were acquired. Figure 2 As an example, the 3680cm² infrared spectrum was collected. -1 The peak at the position represents the hydroxyl absorption peak at the acidic sites on the outer surface of the molecular sieve. With the introduction of PN, PN interacts with the surface hydroxyl groups. The amount of acidic sites on the outer surface of the molecular sieve can be obtained by using the difference between the infrared peaks before and after PN vapor saturation adsorption.

[0026] According to the present invention, preferably, the molar ratio of SiO2 / Al2O3 in the molecular sieve is 1:2-5, more preferably 1:2.5-4.

[0027] The SiO2 / Al2O3 molar ratio of the molecular sieve described in this invention was determined by elemental analysis using ICP.

[0028] According to the present invention, preferably, the P2O5 content is 40-60% by weight, more preferably 45-55% by weight, based on the total weight of the molecular sieve.

[0029] The P2O5 content of the molecular sieve described in this invention was determined by elemental analysis using ICP.

[0030] A second aspect of this invention provides a method for preparing SAPO-34 molecular sieve, the method comprising the following steps:

[0031] a) A mixture A containing a silicon source, a first phosphorus source, a first aluminum source, a template agent and water is subjected to a first hydrothermal crystallization to obtain solid product I and mother liquor B;

[0032] b) A mixture C containing a second phosphorus source, a second aluminum source, solid product I, at least a portion of mother liquor B, and water is subjected to a second hydrothermal crystallization to obtain the SAPO-34 molecular sieve.

[0033] According to the present invention, preferably, the molar ratio of the first aluminum source to the second aluminum source is 5-30, more preferably 10-20, wherein both the first aluminum source and the second aluminum source are calculated as Al2O3. This preferred embodiment can effectively reduce the amount of acidic sites on the outer surface of the SAPO-34 molecular sieve.

[0034] According to the present invention, preferably, the molar composition of mixture C is: second phosphorus source: second aluminum source: solid product I: water = 0.9-1.1:1:5-30:200-600, more preferably 0.95-1.05:1:10-20:300-500, wherein the second phosphorus source is calculated as P2O5, the second aluminum source is calculated as Al2O3, and solid product I is calculated as Al2O3. Using this preferred embodiment, the acid-base environment of the second crystallization can be flexibly controlled to reduce the amount of acidic sites on the outer surface of the SAPO-34 molecular sieve.

[0035] According to the present invention, preferably, the amount of mother liquor B used in step (b) is 3-25 parts by weight, more preferably 5-15 parts by weight, relative to 100 parts by weight of the mother liquor B described in step (a).

[0036] According to the present invention, preferably, the conditions for the second hydrothermal crystallization include: a temperature of 180-220°C, more preferably 190-210°C; and a time of 5-24 hours, more preferably 12-24 hours. Using this preferred embodiment, the amount of acidic sites on the outer surface of the SAPO-34 molecular sieve can be further reduced.

[0037] Preferably, the second hydrothermal crystallization is carried out under stirring conditions. The present invention does not particularly limit the stirring rate and time, which can be appropriately selected according to specific circumstances.

[0038] The present invention does not particularly limit the method of adding the second phosphorus source, the second aluminum source, solid product I, at least a portion of the mother liquor B, and water; they can be added separately or together. According to a specific embodiment of the present invention, the second aluminum source and the second phosphorus source are first mixed with a portion of water, then the two are mixed together, and then solid product I and at least a portion of the mother liquor B are added to obtain mixture C.

[0039] Preferably, the mixture C can be formed by ultrasound or stirring, and the present invention does not have any particular limitation on this.

[0040] According to the present invention, preferably, the general formula of the mixture A is mR(Si) a Al b P c O2, where m = 0.03-0.8, a = 0.02-0.4, b = 0.3-0.6, c = 0.3-0.6, and a+b+c = 1, m, a, b, and c are the molar numbers of R, Si, Al, and P, respectively, and R is the template agent.

[0041] According to the present invention, preferably, m = 0.1-0.7, a = 0.04-0.3, b = 0.3-0.5, c = 0.3-0.5, and a+b+c = 1.

[0042] According to the present invention, preferably, the molar ratio of the first aluminum source to water is 1:200-600, more preferably 1:300-500.

[0043] The present invention does not particularly limit the conditions for the first hydrothermal crystallization, and can be carried out with reference to conventional methods in the art. Preferably, the conditions for the first hydrothermal crystallization include: a temperature of 180-220°C, more preferably 190-210°C; and a time of 12-36 hours, more preferably 16-24 hours.

[0044] Preferably, the first hydrothermal crystallization is carried out under stirring conditions. The present invention does not particularly limit the stirring rate and time, which can be appropriately selected according to specific circumstances.

[0045] The present invention does not particularly limit the method of adding the silicon source, the first phosphorus source, the first aluminum source, the template agent, and the water; they can be added separately or together. According to a specific embodiment of the present invention, the first aluminum source and the first phosphorus source are first mixed with a portion of water, then the two are mixed together, and then the template agent, the silicon source, and the remaining water are added to obtain mixture A.

[0046] Preferably, the mixture A can be ultrasonicated or stirred during its formation, and the present invention does not have any particular limitation on this.

[0047] The present invention allows for a wide range of choices for the first and second phosphorus sources. Preferably, the first and second phosphorus sources are each independently selected from at least one of phosphoric acid, triethyl phosphate, phosphorous acid, and phosphates, and more preferably from at least one of phosphoric acid, phosphates, and phosphorous acid. Phosphoric acid is used as an example in the embodiments of the present invention.

[0048] Preferably, the first phosphorus source and the second phosphorus source can be the same or different. The present invention does not have any particular limitation on this, and will not elaborate further here.

[0049] The present invention allows for a wide range of choices for the first and second aluminum sources, as long as aluminum can be provided. Preferably, the first and second aluminum sources are each independently selected from at least one of aluminum isopropoxide, aluminum phosphate, aluminum hydroxide, sodium aluminate, boehmite, alumina, and aluminum trichloride, and more preferably from at least one of aluminum isopropoxide, boehmite, and alumina.

[0050] Preferably, the first aluminum source and the second aluminum source can be the same or different. The present invention does not have any particular limitation on this, and will not be elaborated here.

[0051] The present invention allows for a wide range of silicon sources, which can be various silicon sources conventionally used in the art. Specifically, the silicon source is an organosilicon source and / or an inorganic silicon source. Preferably, the silicon source is selected from at least one of silicates, silica, tetraalkyl silicates, silica sol, silicic acid, and alkali metal silicates, and more preferably from at least one of tetraalkyl silicates, silica, and silica sol.

[0052] The present invention allows for a wide range of template agents, which can be various template agents conventionally used in the art. Preferably, the template agent is selected from at least one of tetraalkylammonium compounds and their salts, cyclohexylamine, morpholine, di-n-propylamine, tripropylamine, triethylamine, diethylamine, triethanolamine, and piperidine, and more preferably from at least one of tetraethylammonium hydroxide, tetrapropylammonium bromide, triethylamine, diethylamine, and morpholine.

[0053] More preferably, the template agent is tetraethylammonium hydroxide and triethylamine.

[0054] Preferably, the weight ratio of tetraethylammonium hydroxide to triethylamine is 1:5-10.

[0055] According to a preferred embodiment of the present invention, the product of the first hydrothermal crystallization is subjected to solid-liquid separation to obtain solid product I and mother liquor B. The present invention does not particularly limit the method of solid-liquid separation, and it can be carried out with reference to commonly used techniques in the art, which will not be elaborated further here.

[0056] According to the present invention, preferably, the method further includes: drying and optionally calcining the crystallized product obtained by the second hydrothermal crystallization to obtain the SAPO-34 molecular sieve.

[0057] The present invention does not particularly limit the drying conditions, and can be carried out according to conventional conditions in the art. Preferably, the drying conditions include: a temperature of 40-250°C, more preferably 60-150°C; and a time of 8-30 hours, more preferably 10-20 hours. The drying can be carried out under normal pressure or under reduced pressure.

[0058] The present invention does not impose any particular limitation on the roasting process, and methods commonly used in the art can be referred to. Preferably, the roasting conditions include: a temperature of 300-800℃, more preferably 400-650℃; and a time of 1-15 hours, more preferably 5-10 hours. The roasting is generally carried out in an air atmosphere, which may include a flowing atmosphere or a stagnant atmosphere.

[0059] Preferably, the method further includes: separating the crystallized product obtained by the second hydrothermal crystallization before drying and optionally calcining it.

[0060] This invention does not particularly limit the separation method and can refer to commonly used techniques in the art. This invention preferably employs a vacuum filtration method. The vacuum filtration can be performed under conventional conditions, which will not be elaborated upon here.

[0061] Preferably, the method further includes washing the separated materials.

[0062] The present invention does not impose any particular limitations on the specific conditions for washing, which can be carried out under conventional conditions, and will not be elaborated further here.

[0063] The third aspect of this invention provides the application of the SAPO-34 molecular sieve described in the first aspect or the SAPO-34 molecular sieve prepared by the preparation method described in the second aspect in the reaction of oxygen-containing compounds to olefins.

[0064] According to the present invention, preferably, the oxygen-containing compound is selected from at least one of methanol, ethanol, n-propanol, isopropanol, C4-C20 alcohol, methyl ethyl ether, dimethyl ether, diethyl ether, diisopropyl ether, formaldehyde, dimethyl carbonate and dimethyl ketone, preferably methanol and / or dimethyl ether, more preferably methanol.

[0065] According to a preferred embodiment of the present invention, the oxygen-containing compound is preheated before being reacted.

[0066] The present invention does not impose any particular limitation on the preheating method, and conventional technical means in the field can be referred to.

[0067] The present invention does not impose any particular limitation on the preheating conditions, and can refer to conventional methods in the field, as long as the purpose of vaporizing oxygen-containing compounds can be achieved.

[0068] According to the present invention, preferably, the olefin is a C2-C20 olefin, more preferably a C2-C8 olefin, further preferably a C2-C6 olefin, more preferably a C2-C4 olefin, and particularly preferably ethylene and / or propylene.

[0069] According to the present invention, preferably, the reaction conditions include: a temperature of 200-700°C, more preferably 250-600°C, and more preferably 300-500°C; a pressure of 0.1 kPa-5 MPa, more preferably 5 kPa-1 MPa, and more preferably 20 kPa-500 kPa; and a weight hourly space velocity (WHSV) of 1-5000 h⁻¹ for the oxygen-containing compound. -1 Preferably 2-3000h -1 More preferably 5-1500h -1 .

[0070] According to the present invention, preferably, the SAPO-34 molecular sieve can be used alone or formulated into a molecular sieve catalyst composition for use.

[0071] Preferably, the preparation method of the molecular sieve catalyst composition is as follows: the SAPO-34 molecular sieve is mixed with a binder precursor to form a slurry-like mixture, and then shaped to obtain the final product.

[0072] The present invention does not particularly limit the molding method, and can use techniques commonly used in the art, such as spray drying, granulation and extrusion, to obtain the desired shape and size of the composition.

[0073] The present invention allows for a wide range of choices of the adhesive, which can be various adhesives commonly used in the art. Preferably, the adhesive precursor is alumina and / or silica sol.

[0074] According to the present invention, preferably, the reactor for the reaction is a fixed-bed reactor or a fluidized-bed reactor, and more preferably a fluidized-bed reactor.

[0075] During the conversion of oxygen-containing compounds to olefins, carbonaceous deposits accumulate on the catalyst used to promote the conversion reaction. In some cases, this accumulation of carbonaceous deposits can lead to a decrease in the catalyst's catalytic activity, resulting in partial loss of catalyst activity. When the catalyst can no longer convert oxygen-containing compounds to olefin products, it is considered to have completely lost its activity. At this point, the catalyst needs to be regenerated to reduce the amount of carbon deposits on the catalyst and improve its activity.

[0076] The regeneration conditions described in this invention are not particularly limited and can be carried out with reference to conventional methods in the art, such that the amount of carbon deposit accounts for 0.01-15% by weight of the catalyst.

[0077] Preferably, the regeneration temperature is 250-750℃, and more preferably 500-700℃.

[0078] The present invention will be described in detail below through embodiments.

[0079] In the following embodiments, the crystal phase parameters of the SAPO-34 molecular sieve were measured by XRD method;

[0080] The specific XRD measurement method was as follows: it was performed on a German Bruker D8 polycrystalline X-ray diffractometer with a graphite monochromator, using a Cu-Ka X-ray source (Kα1 wavelength λ = 0.15406 nm), a scanning angle 2θ of 5-50°, and a scanning rate of 1° / min.

[0081] The method for determining the relative amounts of acidic sites on the outer surface of the SAPO-34 molecular sieve is as described above.

[0082] The reagents used in the following examples are all commercially available products.

[0083] Example 1

[0084] 12 g of γ-Al₂O₃ and 35 g of deionized water were mixed evenly to form solution a; 23.1 g of phosphoric acid (85 wt%) and 37.5 g of deionized water were mixed evenly to form solution b; after mixing a and b, the mixture was stirred at room temperature for 2 hours to form a homogeneous solution c; while stirring, 20.2 g of triethylamine, 15 g of tetraethylammonium hydroxide (TEAOH) (25 wt%), 4.5 g of silica sol (40 wt% SiO₂) and 10 g of deionized water were added to c in sequence, and the mixture was stirred thoroughly to obtain mixture A.

[0085] Mixture A was placed in a crystallization vessel, stirred, and heated to 200°C. After crystallization for 18 hours, the solid and liquid were separated to obtain solid product I and 35g of mother liquor B.

[0086] 1.2 g of γ-Al2O3 and 35 g of deionized water were mixed evenly to form solution a. 2.31 g of phosphoric acid (85 wt%) and 37.5 g of deionized water were mixed evenly to form solution b. After mixing a and b, 3.5 g of mother liquor B and all solid product I were added and stirred at room temperature for 2 hours to form mixture C. The solid product is calculated as Al2O3.

[0087] Mixture C was placed in a crystallization vessel, stirred, heated to 200°C, and crystallized for 15 hours. After filtration, washing, drying at 110°C for 12 hours, and calcining in air at 550°C for 5 hours, SAPO-34 molecular sieve was obtained. The composition and content are shown in Table 1.

[0088] XRD characterization was performed, see Figure 1 As can be seen, characteristic diffraction peaks are present at positions 2θ = 9.5, 13, 20.7, and 25.4°, indicating that the obtained product is SAPO-34 molecular sieve.

[0089] The relative amounts of acidic sites on the outer surface of SAPO-34 molecular sieve were determined, and the specific results are shown in Table 1.

[0090] Example 2

[0091] The procedure was carried out according to Example 1, except that in the preparation of mixture C, 0.96 g of γ-Al₂O₃, 1.85 g of phosphoric acid, and 3 g of mother liquor B were weighed out, and the crystallization temperature was 190 °C, while the rest remained unchanged. SAPO-34 molecular sieve was obtained, and its composition and content are shown in Table 1.

[0092] XRD characterization was performed, see Figure 1 This indicates that the obtained product is SAPO-34 molecular sieve.

[0093] The relative amounts of acidic sites on the outer surface of SAPO-34 molecular sieve were determined, and the specific results are shown in Table 1.

[0094] Example 3

[0095] The procedure is carried out according to the method of Example 1, except that...

[0096] The amount of mother liquor added to mixture C was 5 grams, the crystallization temperature was 210℃, and the rest remained unchanged. SAPO-34 molecular sieve was obtained, and its composition and content are shown in Table 1.

[0097] XRD characterization was performed, see Figure 1 This indicates that the obtained product is SAPO-34 molecular sieve.

[0098] The relative amounts of acidic sites on the outer surface of SAPO-34 molecular sieve were determined, and the specific results are shown in Table 1.

[0099] Example 4

[0100] The procedure is carried out according to the method of Example 1, except that...

[0101] The crystallization time for the first hydrothermal crystallization was 24 hours, and the crystallization time for the second hydrothermal crystallization was 12 hours, with other parameters remaining unchanged. SAPO-34 molecular sieve was obtained, and its composition and content are shown in Table 1.

[0102] XRD characterization was performed, see Figure 1This indicates that the obtained product is SAPO-34 molecular sieve.

[0103] The relative amounts of acidic sites on the outer surface of SAPO-34 molecular sieve were determined, and the specific results are shown in Table 1.

[0104] Example 5

[0105] The procedure is carried out according to the method of Example 1, except that...

[0106] The crystallization time for the first hydrothermal crystallization was 16 hours, and the crystallization time for the second hydrothermal crystallization was 18 hours, with the other parameters remaining unchanged. SAPO-34 molecular sieve was obtained, and its composition and content are shown in Table 1.

[0107] XRD characterization was performed, see Figure 1 This indicates that the obtained product is SAPO-34 molecular sieve.

[0108] The relative amounts of acidic sites on the outer surface of SAPO-34 molecular sieve were determined, and the specific results are shown in Table 1.

[0109] Example 6

[0110] The procedure is carried out according to the method of Example 1, except that...

[0111] When preparing mixture A, 4 grams of silica sol (40% by weight SiO2) were added. SAPO-34 molecular sieve was obtained, and its composition and content are shown in Table 1.

[0112] XRD characterization was performed, see Figure 1 This indicates that the obtained product is SAPO-34 molecular sieve.

[0113] The relative amounts of acidic sites on the outer surface of SAPO-34 molecular sieve were determined, and the specific results are shown in Table 1.

[0114] Example 7

[0115] The procedure was carried out according to Example 1, except that after crystallization of mixture A, solid-liquid separation was not performed. Instead, 1.2 g of γ-Al₂O₃ and 2.31 g of phosphoric acid (85% by weight) were added to obtain mixture C. SAPO-34 molecular sieve was obtained, and its composition and content are shown in Table 1.

[0116] XRD characterization was performed, see Figure 1 This indicates that the obtained product is SAPO-34 molecular sieve.

[0117] The relative amounts of acidic sites on the outer surface of SAPO-34 molecular sieve were determined, and the specific results are shown in Table 1.

[0118] Comparative Example 1

[0119] SAPO-34 molecular sieve prepared by existing technology

[0120] 12 g of γ-Al₂O₃ and 35 g of deionized water were mixed evenly to form solution a; 23.1 g of phosphoric acid (85 wt%) and 37.5 g of deionized water were mixed evenly to form solution b; after mixing a and b, the mixture was stirred at room temperature for 2 hours to form a homogeneous solution c; while stirring, 20.2 g of triethylamine, 15 g of tetraethylammonium hydroxide (TEAOH), 4.5 g of silica sol (40 wt% SiO₂) and 10 g of deionized water were added to c in sequence, and the mixture was stirred thoroughly to obtain mixture A.

[0121] Mixture A was placed in a crystallization vessel, stirred, and heated to 200°C. After crystallization for 24 hours, the product was recovered. SAPO-34 molecular sieve was obtained, and its composition and content are shown in Table 1.

[0122] XRD characterization was performed, see Figure 1 This indicates that the obtained product is SAPO-34 molecular sieve.

[0123] The relative amounts of acidic sites on the outer surface of SAPO-34 molecular sieve were determined, and the specific results are shown in Table 1.

[0124] Table 1

[0125]

[0126] Test Example 1

[0127] The SAPO-34 molecular sieves prepared in the examples and comparative examples were subjected to methanol-to-olefins reaction in a fixed bed. The reaction conditions were: 2g of molecular sieve, pure methanol feed, methanol preheating temperature 200°C, reaction temperature 460°C, atmospheric pressure, and weight hourly space velocity (WHSV) 6h. -1 The results are shown in Table 2.

[0128] Table 2

[0129]

[0130]

[0131] As can be seen from the results in Table 2, applying the SAPO-34 molecular sieve described in this invention to the methanol-to-olefins reaction yields significantly better results. With current technological advancements, the diene yield (ethylene + propylene) has reached approximately 80%. Furthermore, increasing the yield by just 1 percentage point would result in substantial economic benefits for a 10,000-ton-scale plant.

[0132] 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 inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A SAPO-34 molecular sieve, characterized in that, The relative amount of acidic sites on the outer surface of the molecular sieve is less than 50%; The preparation method of the SAPO-34 molecular sieve includes the following steps: a) A mixture A containing a silicon source, a first phosphorus source, a first aluminum source, a template agent and water is subjected to a first hydrothermal crystallization to obtain solid product I and mother liquor B; b) A mixture C containing a second phosphorus source, a second aluminum source, solid product I, at least a portion of mother liquor B, and water is subjected to a second hydrothermal crystallization to obtain the SAPO-34 molecular sieve; The amount of at least a portion of the mother liquor B used in step (b) is 3-25 parts by weight, relative to 100 parts by weight of the mother liquor B described in step (a).

2. The molecular sieve according to claim 1, wherein, The relative abundance of acidic sites on the outer surface of the molecular sieve is 20-48%.

3. The molecular sieve according to claim 1 or 2, wherein, In the molecular sieve, the molar ratio of SiO2 / Al2O3 is 1:2-5.

4. The molecular sieve according to claim 3, wherein, In the molecular sieve, the molar ratio of SiO2 / Al2O3 is 1:2.5-4.

5. The molecular sieve according to claim 1 or 2, wherein, Based on the total weight of the molecular sieve, the P2O5 content is 40-60% by weight.

6. The molecular sieve according to claim 5, wherein, Based on the total weight of the molecular sieve, the P2O5 content is 45-55% by weight.

7. A method for preparing SAPO-34 molecular sieve, the method comprising the following steps: a) A mixture A containing a silicon source, a first phosphorus source, a first aluminum source, a template agent and water is subjected to a first hydrothermal crystallization to obtain solid product I and mother liquor B; b) A mixture C containing a second phosphorus source, a second aluminum source, solid product I, at least a portion of mother liquor B, and water is subjected to a second hydrothermal crystallization to obtain the SAPO-34 molecular sieve; The amount of at least a portion of the mother liquor B used in step (b) is 3-25 parts by weight, relative to 100 parts by weight of the mother liquor B described in step (a).

8. The method according to claim 7, wherein, The molar ratio of the first aluminum source to the second aluminum source is 5-30, where both the first and second aluminum sources are calculated as Al2O3.

9. The method according to claim 8, wherein, The molar ratio of the first aluminum source to the second aluminum source is 10-20, where both the first and second aluminum sources are calculated as Al2O3.

10. The method according to claim 7, wherein, The molar composition of mixture C is: second phosphorus source: second aluminum source: solid product I: water = 0.9-1.1:1:5-30:200-600, wherein the second phosphorus source is calculated as P2O5, the second aluminum source is calculated as Al2O3, and solid product I is calculated as Al2O3.

11. The method according to claim 10, wherein, The molar composition of mixture C is: second phosphorus source: second aluminum source: solid product I: water = 0.95-1.05: 1: 10-20: 300-500, wherein the second phosphorus source is calculated as P2O5, the second aluminum source is calculated as Al2O3, and solid product I is calculated as Al2O3.

12. The method according to claim 7, wherein, The amount of at least a portion of the mother liquor B used in step (b) is 5-15 parts by weight, relative to 100 parts by weight of the mother liquor B described in step (a).

13. The method according to claim 7, wherein, The conditions for the second hydrothermal crystallization include: a temperature of 180-220℃ and a time of 5-24 hours.

14. The method according to claim 13, wherein, The conditions for the second hydrothermal crystallization include: a temperature of 190-210℃ and a time of 12-24 hours.

15. The method according to any one of claims 7-14, wherein, The general formula for the composition of mixture A is mR(Si) a Al b P c O2, where m=0.03-0.8, a=0.02-0.4, b=0.3-0.6, c=0.3-0.6, and a+b+c=1, m, a, b, and c are the molar numbers of R, Si, Al, and P, respectively, and R is the template agent.

16. The method according to claim 15, wherein, m = 0.1 - 0.7, a = 0.04 - 0.3, b = 0.3 - 0.5, c = 0.3 - 0.5, and a + b + c = 1.

17. The method according to any one of claims 7-14, wherein, The conditions for the first hydrothermal crystallization include: a temperature of 180-220℃ and a time of 12-36 hours.

18. The method according to claim 17, wherein, The conditions for the first hydrothermal crystallization include: a temperature of 190-210℃ and a time of 16-24 hours.

19. The method according to any one of claims 7-14, wherein, The first phosphorus source and the second phosphorus source are each independently selected from at least one of phosphoric acid, triethyl phosphate, phosphorous acid, and phosphate; The first aluminum source and the second aluminum source are each independently selected from at least one of aluminum isopropoxide, aluminum phosphate, aluminum hydroxide, sodium aluminate, boehmite, alumina and aluminum trichloride; The silicon source is selected from at least one of silicates, silica, tetraalkyl silicates, silica sol, silicic acid, and alkali metal silicates; The template agent is selected from at least one of tetraalkylammonium compounds and their salts, cyclohexylamine, morpholine, di-n-propylamine, tripropylamine, triethylamine, diethylamine, triethanolamine, and piperidine.

20. The method according to claim 19, wherein, The first phosphorus source and the second phosphorus source are each independently selected from at least one of phosphoric acid, phosphate, and phosphorous acid; The first aluminum source and the second aluminum source are each independently selected from at least one of aluminum isopropoxide, boehmite, and alumina; The silicon source is selected from at least one of tetraalkyl silicate, silica fume and silica sol; The template agent is selected from at least one of tetraethylammonium hydroxide, tetrapropylammonium bromide, triethylamine, diethylamine, and morpholine.

21. The method according to any one of claims 7-14, wherein, The method further includes drying and optionally calcining the crystallized product obtained by the second hydrothermal crystallization to obtain the SAPO-34 molecular sieve.

22. The method according to claim 21, wherein, The drying conditions include: a temperature of 40-250℃ and a time of 8-30 hours; The roasting conditions include: a temperature of 300-800℃ and a time of 1-15 hours.

23. The method according to claim 22, wherein, The drying conditions include: a temperature of 60-150℃ and a time of 10-20 hours; The roasting conditions include: a temperature of 400-650℃ and a time of 5-10 hours.

24. The use of the SAPO-34 molecular sieve according to any one of claims 1-6 or the SAPO-34 molecular sieve prepared by any one of claims 7-23 in the reaction of oxygen-containing compounds to olefins.

25. The application according to claim 24, wherein, The oxygen-containing compound is selected from at least one of methanol, ethanol, n-propanol, isopropanol, C4-C20 alcohol, methyl ethyl ether, dimethyl ether, diethyl ether, diisopropyl ether, formaldehyde, dimethyl carbonate, and dimethyl ketone; The olefin is a C2-C20 olefin.

26. The application according to claim 25, wherein, The oxygen-containing compound is methanol and / or dimethyl ether; The olefin is a C2-C8 olefin.

27. The application according to claim 26, wherein, The oxygen-containing compound is methanol; The olefin is a C2-C6 olefin.

28. The application according to claim 27, wherein, The olefin is a C2-C4 olefin.

29. The application according to claim 28, wherein, The olefin is ethylene and / or propylene.

30. The application according to claim 24, wherein, The reaction conditions include: a temperature of 200-700℃; a pressure of 0.1 kPa-5 MPa; and a weight hourly space velocity (WHSV) of 1-5000 h⁻¹ for the oxygen-containing compounds. -1 .

31. The application according to claim 30, wherein, The reaction conditions include: a temperature of 250-600℃; a pressure of 5 kPa-1 MPa; and a weight hourly space velocity (WHSV) of 2-3000 h⁻¹ for the oxygen-containing compounds. -1 .

32. The application according to claim 31, wherein, The reaction conditions include: a temperature of 300-500℃; a pressure of 20kPa-500kPa; and a weight hourly space velocity (WHSV) of 5-1500 h⁻¹ for the oxygen-containing compounds. -1 .

33. The application according to claim 24, wherein, The reactor used for the reaction is a fixed-bed reactor or a fluidized-bed reactor.

34. The application according to claim 33, wherein, The reactor used for the reaction is a fluidized bed reactor.

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