A low-silicon saPO-34 molecular sieve, a preparation method and application thereof

Abstract

The application discloses a kind of low silicon SAPO-34 molecular sieve and its preparation method and application.It includes: a) aluminum source, phosphorus source, silicon source, organic amine template R, metal source A and water are mixed to obtain initial gel mixture;B) to the initial gel mixture is added nano SAPO-34 molecular sieve seed crystal;C) the mixture obtained in step b) is placed into reaction kettle, after sealing, in rotating oven, at 120~170 ℃, rotating crystallization 0.5~24h;D) temperature is raised to 180~220 ℃, rotating crystallization 1~24h;E) temperature is lowered to 20~100 ℃, rotating stays 0.1~6h;F) after crystallization is completed, solid product is separated, washed, dried, and high crystallinity and pure phase low silicon SAPO-34 molecular sieve is obtained, which shows excellent performance in methanol to olefin reaction, the method is easy to operate, repeatability is good, and efficiency is high.

Description

Technical Field

[0001] This invention belongs to the field of chemical materials technology, and in particular relates to a low-silica SAPO-34 molecular sieve, its preparation method and application. Background Technology

[0002] SAPO-34 is an eight-membered ring microporous silica-alumina-phosphorus molecular sieve with a chalcogenide (CHA) structure. It was first synthesized by Union Carbide Corporation in 1984, and its framework structure consists of [SiO4]. 0 [AlO4] - [PO4] + SAPO-34 is composed of three interconnected tetrahedral units. It possesses a three-dimensional pore structure with an orifice diameter of approximately 0.38 nm × 0.38 nm, allowing smaller gases such as ethylene, propylene, and butene to selectively pass through its pores. SAPO-34 molecular sieves exhibit moderate acidity, abundant pore structure, and good hydrothermal and thermal stability, demonstrating excellent catalytic performance in the methanol-to-olefins reaction.

[0003] Generally, low-silica SAPO-34 molecular sieves refer to crystals with a Si / Al atomic ratio of less than 0.17. Studies by Wilson et al. have shown that low-silica SAPO-34 exhibits excellent MTO catalytic performance (Microporous and Mesoporous Materials, 1999, 29, 117-126), especially its high ethylene selectivity, significantly superior to ZSM-5, thus promoting the industrialization of low-silica SAPO-34 molecular sieves. However, traditional hydrothermal synthesis methods often fail to synthesize pure-phase low-silica SAPO-34 molecular sieves, frequently resulting in SAPO-5 or SAPO-18 impurities, or appearing as composite molecular sieves. CN106564912A discloses a SAPO-34 / SAPO-18 composite molecular sieve and its preparation method, while CN104828842A discloses a preparation method for a SAPO-5 / SAPO-34 composite molecular sieve. The presence of impurities in these composite molecular sieves significantly limits their application range.

[0004] The literature (Microporous and Mesoporous Materials, 2009, 126, 1-7) mentions several methods for synthesizing low-silica SAPO-34 molecular sieves: First, adding fluorides to promote the depolymerization of silicon species; second, adding hydrochloric acid as a mineralizing agent; and third, first synthesizing high-silica molecular sieves and then desiliconizing them to synthesize low-silica SAPO-34 molecular sieves. CN106513036A discloses a low-silica SAPO-34 molecular sieve, its preparation method, and its applications, but this method requires the use of a seed crystal guiding liquid and is relatively cumbersome. The above methods often suffer from difficulties in synthesizing highly crystalline, pure-phase SAPO-34 molecular sieves, poor reproducibility, and numerous steps.

[0005] Therefore, there is a need to develop a more readily achievable method for synthesizing low-silica SAPO-34 molecular sieves in order to obtain high-purity SAPO-34 molecular sieves. Summary of the Invention

[0006] In view of this, the present invention provides a low-silica SAPO-34 molecular sieve, its preparation method and application, the main purpose of which is to solve the technical problem that it is difficult to synthesize high-crystallinity pure phase low silica-alumina ratio SAPO-34 molecular sieve and the synthesis method is complicated.

[0007] On one hand, the present invention provides a method for preparing low-silica SAPO-34 molecular sieve, the method comprising the following steps:

[0008] a) Mix aluminum source, phosphorus source, silicon source, organic amine template agent R, metal source A, and water to obtain an initial gel mixture with the following molar ratio:

[0009] R:A:Al2O3:P2O5:SiO2:H2O=1.5~5.0:0.01~0.15:1.0:0.5~2.0:0.08~0.3:20~200;

[0010] b) Add nano-SAPO-34 molecular sieve seed crystals to the initial gel mixture obtained in step a) to obtain gel mixture II; the amount of seed crystals added is 0.1 to 3 wt% of the dry weight of the initial gel mixture;

[0011] c) Place the gel mixture II obtained in step b) into a reaction vessel, seal it, and heat it to 120-170°C in a rotary oven for 0.5-24 hours for rotational crystallization;

[0012] d) After step c) is completed, raise the temperature to 180-220℃ and rotate to crystallize for 1-24 hours;

[0013] e) After step d) is completed, cool down to 20-100℃ and rotate for 0.1-6 hours;

[0014] f) After crystallization in step e) is completed, the obtained solid product is separated, washed and dried to obtain the low-silica SAPO-34 molecular sieve.

[0015] In the initial gel mixture of step a) of the present invention, the amount of aluminum source added is calculated in the number of moles of Al2O3, the amount of phosphorus source added is calculated in the number of moles of P2O5, and the amount of silicon source added is calculated in the number of moles of SiO2.

[0016] The method for preparing low-silica SAPO-34 molecular sieve provided by this invention can use inexpensive template agents such as triethylamine. By adding a metal source and adopting a two-step temperature-variable crystallization strategy, a low-silica SAPO-34 product with a pure CHA crystalline phase can be synthesized. This low-silica SAPO-34 product is characterized by being rich in metal internally and having a low metal content on the outer surface. The low-silica SAPO-34 product can then be further etched in situ by being kept at a specific temperature range of 20 to 100°C for a specific time. The metal-rich areas inside are etched into the liquid phase, ultimately yielding a hollow molecular sieve product.

[0017] Optionally, the molar ratio of the organic amine template agent R to the aluminum source is selected from any value of 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or any range between the two.

[0018] Preferably, the molar ratio of the organic amine template agent R to the aluminum source is R:Al2O3 = 2.5~4.0:1.0.

[0019] Optionally, the molar ratio of the metal source A to the aluminum source is any value selected from 0.01, 0.03, 0.05, 0.07, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15 or a range between any two.

[0020] Preferably, the molar ratio of the metal source A to the aluminum source is A:Al2O3 = 0.01 to 0.1:1.0.

[0021] Optionally, the metal source A in step a) is selected from at least one of magnesium oxide, magnesium acetate, magnesium nitrate, manganese oxide, manganese acetate, manganese nitrate, nickel oxide, nickel acetate, nickel nitrate, chromium oxide, chromium acetate, chromium nitrate, zinc oxide, zinc acetate, zinc nitrate, cobalt oxide, cobalt acetate, and cobalt nitrate.

[0022] Optionally, the metal content corresponding to metal source A in the low-silica SAPO-34 molecular sieve obtained in step f) is 0.01 to 1.2 wt%.

[0023] Optionally, the metal content corresponding to the metal source A of the low-silica SAPO-34 molecular sieve obtained in step f) is selected from any value or a range between 0.01wt%, 0.02wt%, 0.05wt%, 0.08wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.5wt%, 0.8wt%, 1.0wt%, and 1.2wt%.

[0024] Preferably, the metal content corresponding to metal source A in the low-silica SAPO-34 molecular sieve obtained in step f) is 0.01 to 0.15 wt%.

[0025] Optionally, the amount of seed crystals added in step b) is selected from any value or a range between any two of the following dry weights in the initial gel mixture: 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.5 wt%, 2.8 wt%, and 3.0 wt%.

[0026] Preferably, the amount of seed crystals added in step b) is 0.1 to 1.5 wt% of the dry weight of the initial gel mixture.

[0027] Optionally, the aluminum source in step a) is selected from at least one of boehmite, aluminum isopropoxide, sodium aluminate, aluminum foil, aluminum hydroxide, aluminum salt, activated alumina, aluminum alkoxy, and metakaolin.

[0028] Optionally, the aluminum salt is selected from at least one of aluminum sulfate, aluminum chloride, and aluminum nitrate.

[0029] Optionally, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0030] Optionally, the phosphoric acid is selected from at least one of orthophosphoric acid, metaphosphoric acid, and phosphite.

[0031] Optionally, the silicon source is selected from at least one of tetraethyl orthosilicate, silica sol, silica gel, silica fume, silicic acid, water glass, diatomaceous earth, and metakaolin.

[0032] Optionally, the organic amine template agent R is selected from at least one of cyclohexylimine, morpholine, diisopropylamine, diethanolamine, triethanolamine, diethylamine, pyridine, piperidine, cyclohexylamine, tetraethylammonium hydroxide, triethylamine, imidazole, methylimidazole, piperazine, and ethylenediamine.

[0033] Optionally, the particle size of the nano-SAPO-34 molecular sieve seed crystals in step b) does not exceed 2 μm.

[0034] Preferably, the particle size of the nano-SAPO-34 molecular sieve seed crystals in step b) does not exceed 1 μm.

[0035] The nano-SAPO-34 molecular sieve seed crystals of the present invention can be obtained by direct synthesis or by post-processing large-particle-size SAPO-34 molecular sieves, such as by ball milling.

[0036] Optionally, the silicon-aluminum molar ratio of the low-silicon SAPO-34 molecular sieve is Si / Al = 0.06 to 0.17.

[0037] Optionally, the silicon-to-aluminum atomic ratio of the low-silicon SAPO-34 molecular sieve obtained in step f) is selected from any value of 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16 and 0.17 or a range between any two.

[0038] Preferably, the silicon-aluminum molar ratio of the low-silicon SAPO-34 molecular sieve is Si / Al = 0.06 to 0.10.

[0039] Optionally, the temperature in step c) is selected from any value or a range between 120°C, 125°C, 130°C, 140°C, 145°C, 150°C, 155°C, 160°C, 165°C and 170°C.

[0040] Optionally, the time in step c) is selected from any value or a range between 0.5h, 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, and 24h.

[0041] Preferably, the temperature in step c) is 150–170°C and the crystallization time is 10–15 h.

[0042] Optionally, the temperature in step d) is selected from any value or a range between 180°C, 185°C, 190°C, 195°C, 200°C, 205°C, 210°C, 215°C and 220°C.

[0043] Optionally, the time in step d) is selected from any value or a range between 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, and 24h.

[0044] Preferably, the temperature in step d) is 180–200°C and the crystallization time is 10–12 h.

[0045] Optionally, the temperature in step e) is selected from any value or a range between 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C and 100°C.

[0046] Optionally, the time in step e) is selected from any value among 0.1h, 0.5h, 1.0h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, and 6h, or a range between any two.

[0047] Preferably, the temperature in step e) is 20–60°C and the residence time is 0.1–4 h.

[0048] Secondly, the present invention provides a low-silica SAPO-34 molecular sieve, which is prepared by the above-described preparation method.

[0049] Thirdly, the present invention provides a molecular sieve material, wherein the molecular sieve material is a low-silica SAPO-34 molecular sieve prepared by the above method or a material obtained by calcining the above low-silica SAPO-34 molecular sieve in air at 400°C to 700°C.

[0050] Optionally, the calcination temperature of the low-silica SAPO-34 molecular sieve is selected from any value or a range between 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, and 700℃.

[0051] Fourthly, the present invention provides the application of the above-mentioned molecular sieve materials in catalytic reactions or adsorption separation.

[0052] Optionally, the catalytic reaction includes an acid-catalyzed reaction or a reaction of oxygen-containing compounds to olefins.

[0053] Optionally, the adsorption separation includes the adsorption separation of methane and / or nitrogen with carbon dioxide.

[0054] Optionally, the molecular sieve material adsorbs carbon dioxide.

[0055] Compared with the prior art, the present invention has the following beneficial effects:

[0056] (1) The method for preparing low-silicon SAPO-34 molecular sieve provided by the present invention solves the problem that SAPO-5 or SAPO-18 impurities are easily generated during the synthesis of low-silicon SAPO-34 and the product quality is unqualified. Moreover, it does not require the addition of HF, etc. The process is simple, easy to operate, has a short crystallization time, and can synthesize low-silicon SAPO-34 molecular sieve with high crystallinity and pure phase.

[0057] (2) The method for preparing low-silicon SAPO-34 molecular sieve provided by the present invention can be achieved using inexpensive organic amines, such as triethylamine, and provides an economical method for synthesizing low-silicon SAPO-34.

[0058] (3) The low-silicon SAPO-34 molecular sieve provided by the present invention has a significantly increased single-pass lifetime and a greatly improved ethylene-propylene selectivity in the reaction of methanol or dimethyl ether to low-carbon olefins, and has industrial application prospects. Attached Figure Description

[0059] Figure 1 The X-ray diffraction pattern of sample 1 in Example 1 of this invention;

[0060] Figure 2 This is a scanning electron microscope image of sample 1 in Example 1 of the present invention;

[0061] Figure 3 This is a scanning electron microscope image of sample 2 in Comparative Example 1 of the present invention;

[0062] Figure 4 This is the X-ray diffraction pattern of sample 3 in Comparative Example 2 of this invention. Detailed Implementation

[0063] The present application is further illustrated below with reference to specific embodiments. The following descriptions are merely a few embodiments of the present application and are not intended to limit the present application in any way. Although the present application discloses preferred embodiments as follows, they are not intended to limit the present application. Any modifications or variations made by those skilled in the art without departing from the scope of the technical solution of the present application using the disclosed technical content are equivalent to equivalent implementation cases and all fall within the scope of the technical solution.

[0064] Unless otherwise specified, all raw materials used in the embodiments of this application were purchased through commercial channels.

[0065] The analysis method in the embodiments of this application is as follows:

[0066] 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.

[0067] The bulk elemental composition of the samples was determined using a Philips Magix 601X X-ray fluorescence spectrometer (XRF). The surface elemental composition of the samples was determined using a Thermo ESCALAB 250Xi photoelectron spectrometer (XPS).

[0068] The scanning electron microscope (SEM) used for testing was a Hitachi SU8020 field emission scanning electron microscope with an accelerating voltage of 2kV.

[0069] The MTO reaction performance of the catalyst was evaluated using an atmospheric pressure fixed-bed reactor. The specific steps were as follows: Granulated catalyst powder (40-60 mesh) was calcined in air at 600℃ for 4 hours to remove the template agent. 0.3 g of catalyst was weighed and loaded into a quartz tube reactor, activated at 550℃ under a nitrogen atmosphere at 30 mL / min for 1 hour, then cooled to 450℃, and feed gas was introduced for reaction. Methanol was fed by nitrogen carrying saturated methanol vapor, with a methanol mass hourly space velocity (MHSV) of 4 h⁻¹. -1 Product distribution was analyzed using an Agilent GC7890 online gas chromatograph. A PoraPLOTQHT column and an FID detector were used. Product distribution results are presented as mass percentages, with the total amount of hydrocarbons as 100%. Dimethyl ether generated during the reaction was not included in the product distribution (reflected in selectivity) but was treated as unreacted feedstock (reflected in conversion). The single-pass lifetime was calculated based on the time during which methanol conversion was above 99%.

[0070] Preparation of nano-SAPO-34 seeds

[0071] Following the method described in the literature (Chemical Communications, 2014, 50, 1845-1847), 15g of commercially available SAPO-34 molecular sieve was calcined to remove the template agent, followed by the addition of 15g of deionized water. The mixture was placed in a ball mill jar, and 3mm and 6mm mixed agate grinding beads were added. The mixture was then ball-milled for 360min at 550rpm / min using a planetary ball mill (QM-3SP2). The resulting sample was dried in an oven at 110℃. The particle size of the ball-milled sample was measured using a particle size analyzer, and the particle size was approximately 800nm. Seed preparation is not limited to this method. Nano-sized SAPO-34 molecular sieves (particle size less than 2μm, preferably less than 1μm) prepared by other methods can also be used directly as seed crystals.

[0072] Example 1: (Preparation of Sample 1)

[0073] 5g of boehmite (Al2O3 content 67.5wt%) and 22.9g of deionized water were mixed and stirred until homogeneous. Then, 8g of phosphoric acid (85wt%) and 1.8g of silica sol (27.3wt%) were added dropwise to the mixture, stirred until homogeneous, and then 10g of triethylamine was added. After thorough stirring, 0.054g of zinc oxide was added to obtain the initial gel mixture for synthesizing low-silica SAPO-34 molecular sieves. The specific proportions and crystallization conditions are shown in Table 1. Nano-SAPO-34 seed crystals, equivalent to 1.5wt% of the dry gel weight, were added to the initial gel. After stirring until homogeneous, the mixture was transferred to a stainless steel high-pressure reactor and dynamically crystallized at 160℃ for 15h, then heated to 200℃ for 10h, and finally cooled to 60℃ for 4h. After crystallization, the solid product was centrifuged, washed, and dried in air at 100℃ to obtain the low-silica SAPO-34 molecular sieve, designated as Sample 1. X-ray diffraction analysis of sample 1 showed that the synthesized product has the characteristics of a single pure-phase CHA structure (X-ray spectrum shown in [reference]). Figure 1 The elemental composition of the sample is listed in Table 2. The molar elemental composition of the solid detected by XRF is Al. 0.498 Si 0.071 P 0.431 The atomic mass fraction of O2 and Zn is 0.028 wt%. XPS analysis shows the surface elemental composition to be Al. 0.503 Si 0.101 P 0.396 O2 was detected, and no Zn was found, indicating an extremely low Zn content on the surface, with a Zn atomic mass fraction of less than 0.1%. The difference in elemental composition between the bulk and surface phases suggests that the synthesized low-silica SAPO-34 molecular sieve is Si-rich on the surface, while metallic Zn is mainly present inside the crystal. After being held at 60℃ for 4 hours, the metallic Zn was essentially completely etched away. Scanning electron microscopy (SEM) images show a cubic morphology of the molecular sieve and porosity on the crystal surface, indicating a certain degree of etching. The SEM spectra are shown below. Figure 2 .

[0074] Examples 1-1 to 1-7: (Preparation of 1-1 to 1-7)

[0075] The difference between Examples 1-1 to 1-7 and Example 1 is that the raw materials used are different, but otherwise they are the same as Example 1. The specific ingredient ratios and crystallization conditions are shown in Table 1.

[0076] X-ray diffraction and XRF elemental analysis were performed on the samples obtained in Examples 1-1 to 1-7 (sample 1-1, sample 1-2, sample 1-3, sample 1-4, sample 1-5, sample 1-6, and sample 1-7). The X-ray diffraction patterns of these samples have the same characteristics as those obtained in Examples 1-1 to 1-7. Figure 1Similar characteristics, and the solid elemental composition is close to that of Example 1; this indicates that a single pure phase low silica SAPO-34 molecular sieve has been successfully synthesized. The XRD patterns and elemental compositions of the above samples will not be repeated.

[0077] Examples 1-8: (Preparation of Samples 1-8)

[0078] The difference between Examples 1-8 and Example 1 is that the amount of nano-SAPO-34 molecular sieve seed crystals added is different, which is 0.1 wt% of the dry weight of the initial gel mixture. The other aspects are the same as in Example 1. The specific ingredient ratios and crystallization conditions are shown in Table 1.

[0079] X-ray diffraction analysis and XRF elemental analysis were performed on samples 1-8 obtained in Examples 1-8. The X-ray diffraction patterns of these samples have the same characteristics as... Figure 1 Similar characteristics, and the solid elemental composition is close to that of Example 1; this indicates that a single pure phase low silica SAPO-34 molecular sieve has been successfully synthesized. The XRD patterns and elemental compositions of the above samples will not be repeated.

[0080] SEM results showed that the particle size of samples 1-8 was slightly larger than that of sample 1, indicating that increasing the amount of nano-SAPO-34 seed crystals added can effectively reduce the particle size of the product.

[0081] Comparative Example 1 (Sample 2 Preparation)

[0082] The difference between Comparative Example 1 and Example 1 is that step e) is omitted in the synthesis method, and the resulting sample is designated as Sample 2. X-ray diffraction analysis was performed on Sample 2, and the X-ray diffraction pattern showed similarities to... Figure 1 Similar characteristics indicate that a single pure phase SAPO-34 molecular sieve was successfully synthesized. The XRD patterns of the above samples will not be repeated here.

[0083] XRF analysis revealed that the molar elemental composition of the solid was Al. 0.498 Si 0.067 P 0.435 The atomic mass fraction of O2 and Zn is 0.494 wt%. XPS analysis shows the surface elemental composition to be Al. 0.488 Si 0.089 P 0.423 O2 was detected, and Zn was not found, indicating an extremely low Zn content on the surface, with a Zn atomic mass fraction of less than 0.1 wt%. The difference in elemental composition between the bulk and surface phases suggests that the synthesized low-silica SAPO-34 molecular sieve is Si-rich on the surface, while metallic Zn is mainly present within the crystal. Scanning electron microscopy (SEM) images show a cubic morphology of the molecular sieve with a smooth, flat crystal surface. (See SEM spectra for details.) Figure 3 .

[0084] By comparing the results of Example 1 and Comparative Example 1, it can be seen that step e) in the synthesis method of the present invention mainly involves etching, which can remove the metal element Zn from sample 1.

[0085] Comparative Example 2 (Sample 3)

[0086] Comparative Example 2 differs from Example 1 in that the silicon dosage is increased, no metal source is added, and step e) is omitted. The resulting sample is designated as Sample 3. X-ray diffraction analysis of Sample 3 revealed an uneven baseline and broadened diffraction peaks, indicating that the synthesized product exhibits a co-existing structure of AEI and CHA (see X-ray spectrum). Figure 4 ).

[0087] By comparing the results of Example 1 and Comparative Example 2, it can be seen that adding a metal source to the synthetic raw materials of the present invention can correct the crystal phase and obtain a single pure SAPO-34 crystal phase.

[0088] Comparative Example 3 (Sample 4)

[0089] Comparative Example 3 differs from Example 1 in that: step d) involves heating to 180°C for 24 hours to crystallize, and step e) involves cooling to 20°C and holding for 6 hours. The resulting sample is designated as Sample 4. X-ray diffraction analysis was performed on Sample 4, and the X-ray diffraction pattern showed similarities to... Figure 1 Similar characteristics; this indicates that a single pure phase SAPO-34 molecular sieve has been successfully synthesized, and the XRD patterns of the above samples will not be repeated.

[0090] By comparing the results of Example 1 and Comparative Example 3, it can be seen that the yield of solid product in Comparative Example 3 is significantly reduced, indicating that in the synthesis method of the present invention, extending the etching time will sacrifice the product yield.

[0091] Comparative Example 4 (Sample 5)

[0092] Comparative Example 4 differs from Example 1 in that: the amount of metal source added is 0.027 g; step d) involves heating to 220°C for 6 hours of crystallization; and step e) involves cooling to 100°C for 1 hour of crystallization. The resulting sample is designated as Sample 5. X-ray diffraction analysis was performed on Sample 5, and the X-ray diffraction pattern showed similarities to... Figure 4 Similar characteristics indicate that the synthesized product has a symbiotic structure of AEI and CHA, which means that the amount of metal source added in this synthesis method is too low to obtain pure phase low silicon SAPO-34.

[0093] Comparative Example 5 (Sample 6)

[0094] Comparative Example 5 differs from Example 1 in that the amount of metal source added is 0.081 g, and step e) involves cooling to 60°C and holding for 4 hours. The resulting sample is designated as Sample 6. X-ray diffraction analysis was performed on Sample 6, and the X-ray diffraction pattern exhibits characteristics similar to... Figure 1Similar characteristics indicate that a single pure phase SAPO-34 molecular sieve was successfully synthesized. The XRD patterns of the above samples will not be repeated here.

[0095] By comparing the results of Example 1 and Comparative Example 5, it can be seen that the yield of solid product in Comparative Example 5 is significantly reduced, indicating that increasing the amount of metal source added requires extending the etching time, which will sacrifice the yield.

[0096] Comparative Example 6 (Preparation of Sample 7)

[0097] Comparative Example 6 differs from Example 1 in that: the amount of silica sol added is 0.36 g, the amount of nano-SAPO-34 seed crystals added is 1.2 g, and step c) involves heating to 120°C for crystallization for 24 h. The resulting sample is designated as Sample 7. X-ray diffraction analysis was performed on Sample 7, and the X-ray diffraction pattern showed similarities to... Figure 4 Similar characteristics indicate that the synthesized product has a symbiotic structure of AEI and CHA, which means that reducing the silicon feed and increasing the seed crystal addition in this synthesis method cannot obtain pure phase low-silicon SAPO-34.

[0098] Comparative Example 7 (Sample 8)

[0099] Comparative Example 7 differs from Example 1 in that: the amount of silica sol added is 1.44 g, the amount of nano-SAPO-34 seed crystals added is 0.42 g, and step c) involves heating to 170°C for crystallization for 10 h. The resulting sample is designated as Sample 8. X-ray diffraction analysis was performed on Sample 8, and the X-ray diffraction pattern showed similarities to... Figure 1 Similar characteristics indicate that a single pure-phase SAPO-34 molecular sieve was successfully synthesized. The XRD pattern of the sample is not repeated here. The XRD measurement showed that the silicon / aluminum atomic molar ratio Si / Al was 0.10, which means that by appropriately reducing the silicon dosage and increasing the amount of seed crystals added, a pure-phase SAPO-34 with lower silicon content can be obtained.

[0100] Example 2: (Preparation of Sample 9)

[0101] 6.3 g of boehmite (Al₂O₃ content 67.5 wt%) and 30.3 g of deionized water were mixed and stirred until homogeneous. Then, 10 g of phosphoric acid (85 wt%) was added dropwise to the mixture, and after stirring until homogeneous, 12.5 g of triethylamine was added. After thorough stirring, 0.671 g of zinc oxide was added to obtain the initial gel mixture for synthesizing ZnAPO-34 molecular sieve. Nano-SAPO-34 seed crystals, equivalent to 3 wt% of the dry gel weight, were added to the initial gel, stirred until homogeneous, and then transferred to a stainless steel high-pressure reactor. The mixture was heated to 160 °C and crystallized for 24 h. After crystallization, the solid product was centrifuged, washed, and dried in air at 100 °C to obtain ZnAPO-34 molecular sieve, designated as sample 9. X-ray diffraction analysis was performed on sample 9, and the X-ray diffraction pattern showed similarities to... Figure 1Similar characteristics are present, and the XRD spectra of the above samples will not be repeated. XRF analysis indicates the solid's molar elemental composition is Al. 0.378 Zn 0.146 P 0.476 O2 indicates that a single pure phase ZnAPO-34 molecular sieve was successfully synthesized.

[0102] Example 3: (Preparation of Sample 10)

[0103] 6.3 g of boehmite (Al₂O₃ content 67.5 wt%) and 28.7 g of deionized water were mixed and stirred until homogeneous. Then, 10 g of phosphoric acid (85 wt%) and 2.2 g of silica sol (27.3 wt%) were added dropwise to the mixture, and stirred until homogeneous. 12.5 g of triethylamine was then added, followed by thorough stirring and the addition of 0.067 g of zinc oxide, yielding the initial gel mixture for synthesizing low-silica SAPO-34 molecular sieves. Nano-SAPO-34 seed crystals, equivalent to 1.5 wt% of the dry gel weight, were added to the initial gel. After stirring until homogeneous, the mixture was transferred to a stainless steel high-pressure reactor and crystallized at 200 °C for 10 h, then cooled to 20 °C and held for 6 h. After crystallization, the solid product was centrifuged, washed, and dried in air at 100 °C to obtain the low-silica SAPO-34 molecular sieve, designated as sample 10. X-ray diffraction analysis was performed on sample 10, and the X-ray diffraction pattern showed similarities to... Figure 4 Similar characteristics indicate that the synthesized product possesses a co-existing structure of AEI and CHA. XRF analysis revealed that the molar elemental composition of the solid was Al. 0.489 Si 0.063 P 0.448 The presence of O2 and 0.565 wt% Zn indicates that without step c) the low-temperature crystallization stage, the metallic element Zn cannot be etched away from the framework in step e).

[0104] Comparative Example 8: (Sample 11)

[0105] Comparative Example 8 differs from Example 3 in that step e) is omitted, and the resulting sample is designated as Sample 11. X-ray diffraction analysis was performed on Sample 11, and the X-ray diffraction pattern exhibits the same characteristics as... Figure 4 Similar characteristics indicate that the synthesized product possesses a co-existing structure of AEI and CHA. XRF analysis revealed that the molar elemental composition of the solid was Al. 0.492 Si 0.061 P 0.447 It contains O2 and 0.604 wt% Zn, which is close to the metal content of sample 10 obtained in step e).

[0106] By comparing the results of Example 3 and Comparative Example 8, it can be seen that the presence or absence of step e) does not change the metal content in the solid product when there is no step c) low-temperature crystallization stage, which further indicates that the metal element Zn cannot be etched and removed from the skeleton when there is no step c) low-temperature crystallization stage.

[0107] Comparative Example 9: (Prepared Sample 12)

[0108] Comparative Example 9 differs from Example 3 in that no metal source is added and step e) is omitted; the resulting sample is designated as Sample 12. X-ray diffraction analysis was performed on Sample 12, and the X-ray diffraction pattern exhibits similar characteristics to... Figure 4 Similar characteristics indicate that the synthesized product has a symbiotic structure of AEI and CHA, which means that without step c) the low-temperature crystallization stage, without the addition of a metal source, and without step e), pure phase low-silicon SAPO-34 cannot be obtained.

[0109] Table 1. Molecular sieve synthesis ingredients and crystallization conditions

[0110]

[0111] Table 2. Elemental composition of the samples

[0112]

[0113] Example 4 (Performance Test of Methanol-to-Olefins Conversion Reaction)

[0114] The methanol-to-olefins reaction performance of samples 1, 2, and 10 was tested, and the results are shown in Table 3. It can be seen that, according to the method of this application, the pure-phase low-silica SAPO-34 molecular sieve prepared by adding a metal source and through a two-stage variable-temperature crystallization and a third-stage in-situ metal etching process, when used as a catalyst for methanol-to-olefins reaction, exhibits significantly improved lifetime and diene selectivity.

[0115] Table 3. Results of the methanol-to-olefins reaction*

[0116]

[0117] *The single-pass reaction life is calculated based on the time during which the methanol conversion rate is higher than 99%.

[0118] Selectivity refers to the highest selectivity when the methanol conversion rate is above 99%.

[0119] 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 low-silica SAPO-34 molecular sieve, characterized in that, The preparation method includes at least the following steps: a) Mix aluminum source, phosphorus source, silicon source, organic amine template agent R, metal source A, and water to obtain an initial gel mixture with the following molar ratio: R: A: Al2O3: P2O5: SiO2: H2O=1.5~5.0: 0.01~0.15: 1.0: 0.5~2.0: 0.08~0.3: 20~200; The metal source A is selected from at least one of magnesium oxide, magnesium acetate, magnesium nitrate, manganese oxide, manganese acetate, manganese nitrate, nickel oxide, nickel acetate, nickel nitrate, zinc oxide, zinc acetate, zinc nitrate, cobalt oxide, cobalt acetate, and cobalt nitrate. b) Add nano-SAPO-34 molecular sieve seed crystals to the initial gel mixture obtained in step a) to obtain gel mixture II; the amount of seed crystals added is 0.1~3 wt% of the dry weight of the initial gel mixture. c) Place the gel mixture II obtained in step b) into a reaction vessel, seal it, and heat it to 120~170 °C in a rotary oven for 0.5~24 h for rotational crystallization; d) After step c) is completed, raise the temperature to 180~220 °C and rotate to crystallize for 1~24 h; e) After step d) is completed, cool to 20~100 °C and rotate and hold for 0.1~6 h; f) After crystallization in step e) is completed, the obtained solid product is separated, washed, and dried to obtain the low-silica SAPO-34 molecular sieve.

2. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The metal content corresponding to metal source A in the low-silica SAPO-34 molecular sieve obtained in step f) is 0.01~1.2 wt%.

3. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The metal content corresponding to metal source A in the low-silica SAPO-34 molecular sieve obtained in step f) is 0.01~0.15 wt%.

4. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The aluminum source in step a) is selected from at least one of sodium aluminate, aluminum foil, aluminum hydroxide, aluminum salt, activated alumina, aluminum alkoxy, and metakaolin. The phosphorus source is selected from at least one of orthophosphoric acid, metaphosphoric acid, phosphite, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; The silicon source is selected from at least one of tetraethyl orthosilicate, silica sol, silica gel, silica fume, silicic acid, water glass, diatomaceous earth, and metakaolin. The organic amine template agent R is selected from at least one of cycloheximine, morpholine, diisopropylamine, diethanolamine, triethanolamine, diethylamine, pyridine, piperidine, cyclohexylamine, tetraethylammonium hydroxide, triethylamine, imidazole, methylimidazole, piperazine, and ethylenediamine.

5. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The particle size of the nano-SAPO-34 molecular sieve seed crystals in step b) does not exceed 2 μm; The silicon-aluminum molar ratio of the low-silicon SAPO-34 molecular sieve is Si / Al = 0.06~0.

17.

6. The method for preparing low-silica SAPO-34 molecular sieve according to claim 1, characterized in that, The particle size of the nano-SAPO-34 molecular sieve seed crystals in step b) does not exceed 1 μm.

7. A low-silica SAPO-34 molecular sieve, characterized in that, The molecular sieve is prepared using the preparation method described in any one of claims 1 to 6.

8. A molecular sieve material, characterized in that, The molecular sieve material is a low-silica SAPO-34 molecular sieve prepared by the method according to any one of claims 1 to 6, or a material obtained by calcining the low-silica SAPO-34 molecular sieve according to claim 7 in air at 400°C to 700°C.

9. The application of the molecular sieve material according to claim 8 in catalytic reactions or adsorption separation.

10. The application of the molecular sieve material according to claim 9 in catalytic reactions or adsorption separation, characterized in that, The catalytic reactions include acid catalysis or the conversion of oxygen-containing compounds into olefins.

11. The application of the molecular sieve material according to claim 9 in catalytic reactions or adsorption separation, characterized in that, The adsorption separation includes the adsorption separation of methane and / or nitrogen with carbon dioxide.

12. The application of the molecular sieve material according to claim 9 in catalytic reactions or adsorption separation, characterized in that, The molecular sieve material adsorbs carbon dioxide.

Images