Surface silicon-poor nano SAPO-34 molecular sieve as well as preparation method and application thereof

Abstract

The invention relates to the field of molecular sieves, in particular to a surface silicon-poor nano SAPO-34 molecular sieve as well as a preparation method and application thereof. The method comprises the following steps: (1) mixing an SAPO-34 molecular sieve seed crystal with a template agent, and then carrying out first pretreatment to obtain a mixture A; (2) mixing an aluminum source, a silicon source, a phosphorus source and water, and then performing second pretreatment to obtain a mixture B; (3) mixing the mixture A with the mixture B to obtain initial gel; and (4) carrying out rapid heating and hydrothermal crystallization on the initial gel, and then washing and roasting the obtained solid product to obtain the surface silicon-poor nano SAPO-34 molecular sieve. According to the preparation method, an expensive template agent or other organic additives do not need to be added, the process is easy to operate, and the SAPO-34 molecular sieve obtained by the method has the characteristics of nanoscale particle size and poor silicon on the surface of a crystal phase; the catalyst has the advantages of long service life, high selectivity of ethylene and propylene and high ratio of ethylene selectivity to propylene selectivity when catalyzing methanol to prepare olefin.

Description

Technical Field

[0001] The present invention relates to the field of molecular sieves, and in particular to a surface silicon-poor nano SAPO-34 molecular sieve and a preparation method and application thereof. Background Art

[0002] The methanol-to-olefins reaction typically uses a solid acid catalyst, and a common catalyst is a catalyst containing the active species SAPO-34 molecular sieve. The unique CHA cage structure of SAPO-34 molecular sieve provides a venue for the methanol conversion reaction. At the same time, because the pore diameter of the eight-membered ring entering and exiting the cage is 0.38nm, which is close to the molecular dynamic diameter of ethylene and propylene, it exhibits high ethylene and propylene selectivity. However, on the other hand, the excessively small pore size prevents the product molecules from diffusing out of the pores in time during the methanol conversion reaction, prolonging the residence time of the product in the SAPO-34 molecular sieve crystals. This leads to side reactions such as hydrogen transfer and cyclization, forming carbon deposits that cover the acidic sites and cause catalyst deactivation.

[0003] Reducing the molecular sieve grain size, thereby shortening the diffusion path of the product molecular sieve, is an effective means in the prior art to improve the diffusion limitation of SAPO-34 molecular sieve and reduce the carbon deposition rate.

[0004] CN 102275948 A discloses a method for synthesizing a small-grain silicoaluminophosphate molecular sieve SAPO-34. During gel preparation, an aluminum source, a silicon source, an organic amine, and water are first mixed. The mixed solution is then pretreated at high temperature for a certain period of time. A phosphorus source and water are then added in situ at high temperature to crystallize and synthesize small-grain SAPO-34. This method requires that the phosphorus source must be added using a corrosion-resistant liquid pump and in situ at high temperature to obtain the small-grain molecular sieve, increasing the difficulty and cost of preparation. Furthermore, when the SAPO-34 molecular sieve prepared is used in methanol to olefins, the selectivity for ethylene is less than 45%, and the ratio of the selectivity for ethylene to the selectivity for propylene (ethylene to propylene ratio) is only about 1.1.

[0005] CN 103641131 A discloses a method for preparing flaky nano-SAPO-34 molecular sieves. This method uses tetraethylammonium hydroxide as a template and synthesizes the sieves via a hydrothermal or solvothermal synthesis method. The synthesized nano-SAPO-34 molecular sieves have a flaky morphology, an average crystal size of 50-250 nm, and a thickness of 50-100 nm, making them suitable for industrial scale-up applications. However, when used in a methanol-to-olefins reaction, the total yield of ethylene and propylene is only 83% at most, the ethylene selectivity is 42% at most, and the ethylene:propylene selectivity ratio is only 1.1 at most.

[0006] In addition, the crystal phase of SAPO-34 molecular sieve synthesized by conventional methods generally has the characteristic of being silicon-rich on the outer surface. The Si-O-Al skeleton will introduce acidic sites into the molecular sieve, resulting in a large number of acidic sites on the outer surface of the molecular sieve. In the methanol conversion to olefins reaction, the acidic sites on the outer surface of the molecular sieve will increase the side reactions of methanol to olefins, including hydrogen transfer reactions and methylation reactions, ultimately reducing the overall selectivity of ethylene and propylene and the ethylene-propylene ratio. In particular, after reducing the molecular sieve grain size, the acidic sites on the outer surface of the molecular sieve are more exposed, resulting in an increase in non-selective side reactions. Therefore, while reducing the molecular sieve particle size, it is necessary to suppress the silicon-rich outer surface of the molecular sieve. Summary of the Invention

[0007] The purpose of the present invention is to solve the problems faced in the background technology, thereby providing a molecular sieve with small size and surface silicon-poor, which has the characteristics of long catalytic life, high olefin selectivity and high ethylene-propylene ratio when catalyzing methanol to olefins.

[0008] In order to achieve the above object, the first aspect of the present invention provides a method for preparing surface silicon-deficient nano SAPO-34 molecular sieve, the method comprising the following steps:

[0009] (1) mixing SAPO-34 molecular sieve seed crystals with a template, and then performing a first pretreatment to obtain a mixture A;

[0010] (2) mixing an aluminum source, a silicon source, a phosphorus source, and water, and then performing a second pretreatment to obtain a mixture B;

[0011] (3) mixing the mixture A with the mixture B to obtain an initial gel;

[0012] (4) The initial gel is rapidly heated and hydrothermally crystallized, and then the obtained solid product is washed and calcined to obtain the surface silicon-poor nano-SAPO-34 molecular sieve.

[0013] The second aspect of the present invention provides a SAPO-34 molecular sieve obtained by the above preparation method, wherein:

[0014] The SAPO-34 molecular sieve is a pure phase CHA structure;

[0015] The bulk silicon content of the SAPO-34 molecular sieve is 4.5-10 wt%;

[0016] The ratio of the surface silicon content to the bulk silicon content of the SAPO-34 molecular sieve is not higher than 1.20;

[0017] The average particle size of the SAPO-34 molecular sieve is 600-900 nm.

[0018] A third aspect of the present invention provides use of the SAPO-34 molecular sieve in methanol to olefins technology.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The present invention uses a small amount of commercially available SAPO-34 molecular sieve as a guide crystal and performs high-temperature pretreatment in the presence of a template agent to decompose it into Si(OH) n+ The secondary structural unit fragments formed by the phosphorus-aluminum skeleton can induce the added aluminum source, silicon source and phosphorus source to quickly generate SAPO-34 molecular sieve, and the obtained SAPO-34 molecular sieve has a higher yield and a smaller particle size.

[0021] 2. The present invention performs high-temperature pretreatment on a mixture of an aluminum source, a silicon source, a phosphorus source and water so that they can be crystallized synchronously at the same rate after rapid heating, thereby solving the problem of slow silicon conversion rate at lower temperatures, avoiding silicon-rich molecular sieve crystal surface caused by uneven distribution of silicon source in the molecular sieve, and improving the ethylene conversion rate in methanol to olefins.

[0022] 3. The initial gel of the present invention is rapidly heated before hydrothermal crystallization, so that the active species obtained by high-temperature pretreatment can reach the temperature range suitable for crystal nucleation and crystal growth as soon as possible, further ensuring that the aluminum source, silicon source, and phosphorus source crystallize synchronously at the same rate.

[0023] 4. The SAPO-34 molecular sieve obtained by the present invention has the characteristic of being silicon-poor on the surface of the crystal phase, specifically, the ratio of the surface silicon content of the crystal phase to the bulk silicon content is not higher than 1.20.

[0024] 5. When the SAPO-34 molecular sieve obtained by the present invention is used to catalyze methanol to olefins, it has the advantages of long catalytic life, high selectivity for ethylene + propylene, and a high ratio of ethylene selectivity to propylene selectivity (ethylene to propylene ratio). BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a scanning electron microscope image of the SAPO-34 molecular sieve in Example 1 of the present invention.

[0026] Figure 2 This is a scanning electron microscope image of the SAPO-34 molecular sieve in Example 2 of the present invention.

[0027] Figure 3 This is a scanning electron microscope image of the SAPO-34 molecular sieve in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described below through specific examples. The examples of the present invention are only for illustration of the present invention and do not mean that the scope of the present invention is limited thereto.

[0029] The first aspect of the present invention provides a method for synthesizing surface silicon-poor nano-sized SAPO-34 molecular sieves, the method comprising the following steps:

[0030] (1) mixing SAPO-34 molecular sieve seed crystals with a template, and then performing a first pretreatment to obtain a mixture A;

[0031] (2) mixing an aluminum source, a silicon source, a phosphorus source, and water, and then performing a second pretreatment to obtain a mixture B;

[0032] (3) mixing the mixture A with the mixture B to obtain an initial gel;

[0033] (4) The initial gel is rapidly heated and hydrothermally crystallized, and then the obtained solid product is washed and calcined to obtain the surface silicon-poor nano-SAPO-34 molecular sieve.

[0034] In some embodiments, in step (1), the SAPO-34 molecular sieve seed crystals have a pure phase CHA structure without impurity crystals and are commercially available.

[0035] Furthermore, the usage amount of the SAPO-34 molecular sieve seed crystals is 0.1-5 wt% of the total mass of the initial gel;

[0036] In addition, the inventors calcinated the initial gel, and 1.0-20.0 wt% of Al (calculated as Al2O3) in the obtained gel dry basis came from SAPO-34 molecular sieve seed crystals;

[0037] 1.0-20.0 wt% of Si (calculated as SiO2) comes from SAPO-34 molecular sieve seed crystals;

[0038] 1.0-20.0 wt% of P (calculated as P2O5) comes from SAPO-34 molecular sieve seed crystals.

[0039] Furthermore, the bulk silicon content (calculated as SiO2) of the SAPO-34 molecular sieve seed crystal is not less than 5.0 wt%, preferably 7.0-10.0 wt%.

[0040] In the present invention, the template agent is selected from at least one of triethylamine, diethylamine, morpholine and diisopropylamine.

[0041] The present inventors found that in mixture A, the template agent provides an alkaline environment for the SAPO-34 molecular sieve seed crystals, which is conducive to the shedding and activation of the silicon skeleton of the molecular sieve. At this time, the shedding silicon skeleton is converted into Si(OH) in an alkaline environment. n+ The remaining phosphorus-aluminum framework of the molecular sieve is dispersed into small secondary structural unit fragments.

[0042] Furthermore, when mixture A is mixed with mixture B, the Si(OH) n+ The secondary structural unit fragments will quickly combine with the silicon source, aluminum source and phosphorus source added to the mixture B to form the crystal nucleus of the SAPO-34 molecular sieve.

[0043] Among them, Si(OH) n+ The secondary structural unit fragments can induce the formation of surface silicon-poor nano-SAPO-34 molecular sieves, and the average particle size of the obtained SAPO-34 molecular sieve is significantly reduced, thereby shortening the diffusion path of the products in the methanol to olefins reaction, reducing the probability of carbon deposition and deactivation, and thus improving the catalytic activity of the molecular sieve and extending its life.

[0044] It is worth noting that no water is added to the mixture A. This is because water will increase the degree of ionization of the template (especially diethylamine), and excessive alkalinity will excessively dissolve the skeleton, causing the secondary structural unit fragments of the phosphorus aluminum skeleton to further dissolve and lose the ability to rapidly crystallize and nucleate.

[0045] In some embodiments, in step (1), the temperature of the first pretreatment is 120-220° C., the pressure is 0.2-4.0 MPa, and the time is 0.5-5 h.

[0046] In the present invention, the aluminum source is selected from at least one of pseudo-boehmite and alumina.

[0047] In the present invention, the silicon source is at least one of silica sol, solid silica gel, white carbon black, and tetraethyl orthosilicate.

[0048] In the present invention, the phosphorus source is selected from at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0049] In some embodiments, in step (2), the temperature of the second pretreatment is 120-220° C., the pressure is 0.2-4.0 MPa, and the time is 0.5-5 h.

[0050] The inventors discovered that in the process of preparing surface silicon-poor nano SAPO-34 molecular sieves, the crystallization activity of aluminum sources, silicon sources, and phosphorus sources that have not undergone high-temperature pretreatment is low. In addition, the conversion rate of the silicon source during temperature-increased crystallization is slower than that of the aluminum and phosphorus sources. Therefore, the SAPO-34 molecular sieve often exhibits silicon-rich outer surface of the crystal phase (specifically, the ratio of the outer surface silicon content to the bulk silicon content is often greater than 2). This will cause the outer surface acid content of the SAPO-34 molecular sieve to increase, ultimately leading to an increase in non-selective catalytic side reactions. The consequence in the methanol to olefins reaction is a decrease in the proportion of ethylene products and an increase in the proportions of propylene and butene products.

[0051] In step (2) of the present invention, the aluminum source, silicon source, and phosphoric acid undergo activation transformation after high-temperature pretreatment. After rapid heating, they can be crystallized synchronously at the same rate, solving the problem of slow silicon conversion rate and avoiding uneven distribution of the silicon source in the molecular sieve.

[0052] In some embodiments, in step (3), the mixing is performed at 65-95°C.

[0053] According to the present invention, the amounts of the aluminum source, phosphorus source, silicon source, template, and water added are such that the molar ratio of aluminum source (calculated as Al2O3):phosphorus source (calculated as P2O5):silicon source (calculated as SiO2):template:H2O is 1.0:(0.5-1.5):(0.12-0.5):(2.0-5.0):(20-100). This molar ratio is applicable to both mixture A and mixture B, as well as the initial gel. Those skilled in the art will appreciate that, in actual operation, the molar ratio of Al2O3:P2O5:SiO2:template:H2O is 1.0:(0.5-1.5):(0.12-0.5):(2.0-5.0):(20-100), which defines the ratio of the silicon source, aluminum source, phosphorus source, water, template, and the proportion of SAPO-34 molecular sieve guiding crystals. Based on this, the usage amounts of the silicon source, aluminum source, phosphorus source, water, template, and molecular sieve can be calculated. The initial gel can be obtained by mixing all the molecular sieves and all the templates as mixed solution A, and the remaining silicon source, aluminum source, phosphorus source, and water as mixed solution B.

[0054] Furthermore, by controlling the molar ratio of Al2O3:P2O5:SiO2:template:H2O, the structure of the surface silicon-poor nano-SAPO-34 molecular sieve can be controlled, ensuring the preparation of pure SAPO-34 molecular sieves with excellent activity.

[0055] In some embodiments, in step (4), the rapid heating rate is 5-10°C / min.

[0056] Those skilled in the art will understand that, since mixture A and mixture B are respectively pretreated at high temperature, the components in mixture A and mixture B are converted into active species, and the active species need to reach a suitable crystal nucleation and crystal growth temperature range as quickly as possible.

[0057] Furthermore, a too slow heating rate will cause the active species to be converted back into inert species, which will still result in the SAPO-34 molecular sieve having a silicon-rich outer surface.

[0058] In some embodiments, in step (4), the hydrothermal crystallization conditions include: a hydrothermal crystallization temperature of 160-220° C., and a hydrothermal crystallization time of 5-24 h.

[0059] Furthermore, the above hydrothermal crystallization conditions can not only ensure that no new SAPO-5 impurity crystals are generated, but also ensure that all raw materials react fully to generate pure SAPO-34 molecular sieve crystals.

[0060] The second aspect of the present invention provides a surface silicon-poor nano SAPO-34 molecular sieve obtained by the above preparation method, wherein:

[0061] The surface silicon-poor nano SAPO-34 molecular sieve is a pure phase CHA structure well known in the art;

[0062] The bulk silicon content of the surface silicon-poor nano SAPO-34 molecular sieve is 4.5-10 wt %;

[0063] The ratio of the surface silicon content to the bulk silicon content of the surface silicon-poor nano SAPO-34 molecular sieve is not higher than 1.20;

[0064] The average particle size of the surface silicon-poor nano SAPO-34 molecular sieve is 600-900 nm.

[0065] The third aspect of the present invention provides the use of the above-mentioned surface silicon-poor nano SAPO-34 molecular sieve in methanol to olefins technology.

[0066] The present invention will be described in detail below through specific examples. In the following examples and comparative examples,

[0067] The surface morphology of the molecular sieve was characterized using a Nova Nano SEM 450 scanning electron microscope (SEM).

[0068] The ratio of surface silicon content to bulk silicon content was used as an indicator of the silicon-rich degree of the molecular sieve surface. The surface silicon content of the molecular sieve was measured using a Thermo Fisher ESCALAB 250Xi thermo X-ray photoelectron spectroscopy (XPS), and the bulk silicon content of the molecular sieve was measured using a Rigaku ZSX Primus II X-ray fluorescence spectrometer (XRF).

[0069] The crystal structure of SAPO-34 molecular sieve was characterized by Bruker D8A X-ray powder diffractometer (XRD).

[0070] The yield of molecular sieve is calculated by the formula: molecular sieve synthesis yield = product dry basis weight / raw material oxide dry basis weight × 100%;

[0071] All raw materials used are commercially available, including:

[0072] The aluminum source has an Al2O3 content of 71 wt% after conversion from pseudo-boehmite and an Al2O3 content of 99 wt% from alumina;

[0073] In the phosphorus source, the H3PO4 content of phosphoric acid is 85wt%, the converted P2O5 content is 62wt%, and the converted P2O5 content of diammonium hydrogen phosphate is 54wt%;

[0074] In the silicon source, the SiO2 content after conversion of silica sol is 30wt%; the SiO2 content of solid silica gel is 99wt%; and the SiO2 content after conversion of tetraethyl orthosilicate is 28wt%.

[0075] Example 1

[0076] (1) 1 g of SAPO-34 molecular sieve seed crystals and a template (35 g of triethylamine) were uniformly mixed and placed in a polytetrafluoroethylene-lined crystallization reactor. The mixture was maintained at 160°C and 1.1 MPa for 2 h, and then cooled to 70°C to obtain a mixed solution A.

[0077] The bulk element compositions (calculated as oxides) of the SAPO-34 molecular sieve seed crystals are 40.0 wt %, 52.3 wt % and 7.7 wt % of Al 2 O 3 , P 2 O 5 and SiO 2 , respectively.

[0078] (2) An aluminum source (13 g pseudo-boehmite), a phosphorus source (21 g phosphoric acid), a silicon source (7.3 g silica sol) and 60 g deionized water were uniformly mixed and placed in a polytetrafluoroethylene-lined crystallization reactor. The mixture was maintained at 200°C and 2.7 MPa for 2 h, and then cooled to 70°C to obtain a mixed solution B.

[0079] (3) uniformly mixing the mixed solution A and the mixed solution B to obtain an initial gel having a molar composition of Al2O3:P2O5:SiO2:triethylamine:H2O=1:1:0.4:3.8:45, wherein the molar composition ratio does not take into account the effect of adding seed crystals;

[0080] (4) The initial gel was placed in a polytetrafluoroethylene-lined crystallization reactor, heated to 200°C at a heating rate of 5°C / min, maintained for 24 hours, cooled to 25°C, filtered, and then washed with deionized water until neutral. After drying at 110°C for 12 hours, it was calcined in a muffle furnace at 650°C for 5 hours. The obtained surface silicon-poor nano-SAPO-34 molecular sieve was recorded as A1.

[0081] Example 2

[0082] The difference from Example 1 is that in step (1), the template agent is a mixture of 35 g triethylamine and 4 g diethylamine;

[0083] In step (2), the silicon source is 9 g of silica sol and 70 g of deionized water;

[0084] In step (3), the molar composition of the initial gel is Al2O3:P2O5:SiO2:triethylamine:diethylamine:H2O=1:1:0.5:3.8:0.6:50;

[0085] The obtained surface silicon-poor nano SAPO-34 molecular sieve is recorded as A2.

[0086] Example 3

[0087] The difference from Example 1 is that in step (1), the template agent is 30 g of triethylamine;

[0088] In step (2), the silicon source is 1.5 g of solid silica gel and 80 g of deionized water;

[0089] In step (3), the molar composition of the initial gel is Al2O3:P2O5:SiO2:triethylamine:H2O=1:1:0.27:3.27:53;

[0090] The obtained surface silicon-poor nano SAPO-34 molecular sieve is recorded as A3.

[0091] Example 4

[0092] The difference from Example 1 is that in step (1), the weight of the SAPO-34 molecular sieve seed crystal is 2 g, and the template agent is 26 g of diisopropylamine;

[0093] In step (2), the phosphorus source is 30 g of diammonium hydrogen phosphate and 80 g of deionized water;

[0094] In step (3), the molar composition of the initial gel is Al2O3:P2O5:SiO2:diisopropylamine:H2O=1:1.24:0.40:2.84:52;

[0095] The obtained surface silicon-poor nano SAPO-34 molecular sieve is marked as A4.

[0096] Example 5

[0097] The difference from Example 1 is that in step (1), the weight of the SAPO-34 molecular sieve seed crystal is 1.5 g, and the template agent is 26 g of triethylamine;

[0098] In step (2), the aluminum source is 9.5 g of aluminum oxide, the silicon source is 7.8 g of tetraethyl orthosilicate, and 70 g of deionized water;

[0099] In step (3), the molar composition of the initial gel is Al2O3:P2O5:SiO2:triethylamine:H2O=1:1.06:0.41:2.82:46;

[0100] The obtained surface silicon-poor nano SAPO-34 molecular sieve is marked as A5.

[0101] Comparative Example 1 (conventional method)

[0102] (1) An aluminum source (13 g pseudo-boehmite) was mixed evenly with 60 g deionized water, and a phosphorus source (21 g phosphoric acid) and a silicon source (7.3 g silica sol) were added in sequence. After stirring at room temperature for 0.5 h, a template (35 g triethylamine) was added to obtain an initial gel with a molar composition of Al2O3:P2O5:SiO2:triethylamine:H2O=1:1:0.4:3.8:45.

[0103] (2) The initial gel was placed in a polytetrafluoroethylene-lined crystallization reactor, heated to 200°C at a rate of 1°C / min, maintained for 24 h, cooled to 25°C, filtered, washed with deionized water until neutral, dried at 110°C for 12 h, and calcined in a muffle furnace at 650°C for 5 h;

[0104] The obtained SAPO-34 molecular sieve was recorded as D1.

[0105] Comparative Example 2 (low temperature pretreatment)

[0106] The difference from Example 1 is that in step (1), the mixed solution A is obtained by directly and evenly mixing the same molecular sieve seed crystals and template at 25°C;

[0107] In step (2), the mixed solution B is obtained by uniformly mixing the same aluminum source, phosphorus source, silicon source and deionized water at 25°C;

[0108] The obtained SAPO-34 molecular sieve was recorded as D2.

[0109] Comparative Example 3 (low temperature pretreatment mixed solution B only)

[0110] The difference from Example 1 is that in step (2), the mixed solution B is obtained by uniformly mixing the same aluminum source, phosphorus source, silicon source and deionized water at 25°C;

[0111] The obtained SAPO-34 molecular sieve was recorded as D3.

[0112] Comparative Example 4 (low heating rate)

[0113] The difference from Example 1 is that in step (4), the heating rate is 0.5°C / min;

[0114] The obtained SAPO-34 molecular sieve was recorded as D4.

[0115] Comparative Example 5 (addition of water to mixture A)

[0116] The difference from Example 1 is that in step (1), 10 g of deionized water is additionally added to the crystallization kettle;

[0117] The obtained SAPO-34 molecular sieve was recorded as D5.

[0118] Comparative Example 6 (Low-silicon SAPO-34 as seed crystal)

[0119] The difference from Example 1 is that in step (1), the bulk elemental composition (calculated as oxides) of the SAPO-34 molecular sieve seed crystals is 42.1 wt %, 53.4 wt % and 4.5 wt % of Al2O3, P2O5 and SiO2, respectively (i.e., the bulk silicon content of the seed crystals is less than 5.0 wt %);

[0120] The obtained SAPO-34 molecular sieve was recorded as D6.

[0121] The SAPO-34 molecular sieves obtained in Examples 1-5 and Comparative Examples 1-6 were characterized, and the results are shown in Table 1 below.

[0122] Table 1:

[0123]

[0124] As shown in Table 1, the SAPO-34 molecular sieve prepared using the method of the present invention has a high yield and a low average particle size, and is a pure SAPO-34 phase. Furthermore, compared to conventional methods, the surface silicon enrichment of the SAPO-34 molecular sieve prepared using the method of the present invention is significantly reduced and is no greater than 1.20.

[0125] Test Case

[0126] In the present invention, the performance of the obtained SAPO-34 molecular sieve was evaluated using a fixed bed reactor, the feed was a methanol aqueous solution with a concentration of 80%, the reaction temperature was 450°C, and the space velocity was 3.3h -1 , the molecular sieve filling amount is 0.63g.

[0127] The specific steps are:

[0128] Molecular sieves were placed in a stainless steel reaction tube, heated to 500°C for activation for 1 h, then cooled to 450°C and introduced into a methanol-water solution. Online sampling was used, and the product was separated by condensation. The gaseous components in the product were analyzed by a gas chromatograph (Agilent, model 7890A). The gas chromatograph was equipped with an HP PLOTAl2O3 / KCl column (50 m × 0.53 mm × 15 μm) (for separation of C1 C6 hydrocarbons), an HP PLOTQ column (30 m × 320 μm × 20 μm) (for separation of alcohols and ethers), a HayesepQ column and an X molecular sieve column (for separation of permanent gases such as CO, CO2, H2, and N2), two FID detectors, and a TCD detector.

[0129] Methanol conversion (X) and product selectivity (Si is calculated based on the number of moles of carbon, based on carbon-based selectivity) are calculated by the following equations:

[0130]

[0131]

[0132] Where X is methanol conversion; S is product selectivity; i is species entering the reactor; o is species exiting the reactor; C x H y - olefin (x-carbon atom number of hydrocarbon species, y-hydrogen atom number of hydrocarbon species); m-corresponding substance C x H y The number of carbon atoms; n-the number of moles of the corresponding substance; MeOH-methanol; DME-dimethyl ether.

[0133] When the methanol conversion rate in the test component is lower than 99%, the catalyst is considered to be deactivated, and the catalyst life is the time during which the methanol conversion rate is maintained above 99%.

[0134] The molecular sieves obtained in Examples 1-5 and Comparative Examples 1-6 were subjected to performance tests, and the results are shown in Table 2 below.

[0135] Table 2:

[0136]

[0137] Compared with Comparative Examples 1-4, the molecular sieves in Examples 1-5 have excellent catalytic performance, long reaction life, higher selectivity of ethylene + propylene, and higher ethylene to propylene ratio.

[0138] In addition, the molecular sieve obtained in Comparative Examples 5-6 also contains impurity crystalline SAPO-5, which results in a longer lifespan but a significant decrease in the selectivity of ethylene + propylene and the ethylene + propylene ratio.

[0139] The present invention has described in detail the preferred embodiments of the present invention, but the embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Those skilled in the art may make various other simple modifications and combinations within the scope of the present invention, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and fall within the scope of protection of the present invention.

Claims

1. A method for preparing surface silicon-poor nano SAPO-34 molecular sieve, characterized in that: The method comprises the following steps: (1) mixing SAPO-34 molecular sieve seed crystals with a template, and then performing a first pretreatment to obtain a mixture A; (2) mixing an aluminum source, a silicon source, a phosphorus source, and water, and then performing a second pretreatment to obtain a mixture B; (3) mixing the mixture A with the mixture B to obtain an initial gel; (4) The initial gel is rapidly heated and hydrothermally crystallized, and then the obtained solid product is washed and calcined to obtain the surface silicon-poor nano-SAPO-34 molecular sieve.

2. The preparation method according to claim 1, characterized in that The bulk silicon content of the SAPO-34 molecular sieve seed crystal is not less than 5 wt % calculated as SiO 2 ; Furthermore, the usage amount of the SAPO-34 molecular sieve seed crystals accounts for 0.1-5 wt% of the total mass of the initial gel.

3. The preparation method according to claim 1 or 2, characterized in that The template agent is selected from at least one of triethylamine, diethylamine, morpholine, and diisopropylamine; The aluminum source is selected from at least one of pseudo-boehmite and alumina; The silicon source is selected from at least one of silica sol, solid silica gel, white carbon black, and tetraethyl orthosilicate; The phosphorus source is selected from at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step (1), the temperature of the first pretreatment is 120-220°C; The pressure of the first pretreatment is 0.2-4.0 MPa; The time of the first pretreatment is 0.5-5h.

5. The preparation method according to any one of claims 1 to 4, characterized in that In step (2), the temperature of the second pretreatment is 120-220°C; The pressure of the second pretreatment is 0.2-4.0 MPa; The second pretreatment time is 0.5-5h.

6. The preparation method according to any one of claims 1 to 5, characterized in that In the initial gel, the added amounts of the aluminum source, phosphorus source, silicon source, template and water are such that the molar ratio of aluminum source (calculated as Al2O3):phosphorus source (calculated as P2O5):silicon source (calculated as SiO2):template:H2O is 1.0:(0.5-1.5):(0.12-0.5):(2.0-5.0):(20-100).

7. The preparation method according to any one of claims 1 to 6, characterized in that The rapid heating rate is 5-10°C / min.

8. The preparation method according to any one of claims 1 to 7, characterized in that The temperature of the hydrothermal crystallization is 160-220°C; The hydrothermal crystallization time is 5-24h.

9. A SAPO-34 molecular sieve obtained by the preparation method according to any one of claims 1 to 8, wherein the SAPO-34 molecular sieve has a pure phase CHA structure; The bulk silicon content of the SAPO-34 molecular sieve is 4.5-10 wt%; The ratio of the surface silicon content to the bulk silicon content of the SAPO-34 molecular sieve is not higher than 1.20; The average particle size of the SAPO-34 molecular sieve is 600-900 nm.

10. Use of the SAPO-34 molecular sieve obtained by the preparation method according to any one of claims 1 to 8 or the SAPO-34 molecular sieve according to claim 9 in methanol to olefins technology.

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