Modified zsm-5 molecular sieve, method for preparing same, and use thereof

Abstract

The application provides a modified ZSM-5 molecular sieve, a preparation method and application thereof. The preparation method comprises the following steps: mixing and impregnating a solution containing a phosphorus source with raw ZSM-5 molecular sieve to obtain P-ZSM-5 molecular sieve; mixing the P-ZSM-5 molecular sieve with a solution of an organic acid, reacting, drying and calcining to obtain the modified ZSM-5 molecular sieve. The application also provides the modified ZSM-5 molecular sieve obtained by the above preparation method and application of the molecular sieve. The preparation method can improve the single aluminum ratio of the ZSM-5 molecular sieve, thereby significantly improving the light hydrocarbon conversion capacity, low-carbon olefin yield and selectivity of the molecular sieve.

Description

Technical Field

[0001] This invention relates to the field of oil refining catalysis technology, and in particular to a modified ZSM-5 molecular sieve, its preparation method, and its application. Background Technology

[0002] Ethylene and propylene, as key raw materials for many products, are indicators of the level of development of the petrochemical industry, and their output and technology are among the key factors. Ethylene is the core of the petrochemical industry, used to produce polyethylene, ethylene oxide, dichloroethane, ethylene glycol, styrene monomer, and ethylene derivatives such as polyvinyl chloride. Ethylene and its downstream derivatives have extremely wide applications, spanning all aspects of human life, including clothing and textiles, agricultural production, building materials, and automotive parts. Propylene is the second most important basic petrochemical raw material after ethylene. Its downstream derivatives mainly include polypropylene and propylene oxide, acrylonitrile, acrylic acid, cumene / phenol / acetone, carbonyl alcohols, isopropanol, and other organic raw materials. In recent years, the demand growth rate of propylene has consistently exceeded that of ethylene, and both the demand and production capacity of propylene and its derivatives have grown at high rates. In the future, the market demand for low-carbon olefins will continue to grow, and reducing energy consumption and increasing the yield of low-carbon olefins will be a significant challenge.

[0003] ZSM-5 molecular sieve is a catalytic material developed by Mobil Chemical Company in the United States. Due to its relatively high activity, strong resistance to carbon deposition, excellent thermal stability, flexible and tunable acidity, and unique pore structure, it is currently a core catalytic material used as a catalyst and promoter for catalytic cracking to increase the production of low-carbon olefins. Its main function is to increase the production of key low-carbon olefin products such as ethylene and propylene. The acid properties of molecular sieves are mainly divided into macroscopic acidity and microscopic acidity. Macroscopic acidity includes acid type, acid strength, and acid quantity, while microscopic acidity includes factors such as Al deposition sites. In the petrochemical and fine chemical fields, many reactions use acid centers as active sites, and different acid properties of molecular sieves can directly affect the activity and selectivity of acid-catalyzed reactions. Therefore, the rational control of the acid properties of zeolite molecular sieves is a key scientific issue in the field of catalytic reaction research.

[0004] The spatial distance between adjacent acid sites in the molecular sieve framework significantly affects its performance in catalytic cracking reactions. The spatial distance between adjacent acid sites is defined as follows: when n≤2 in Al-O-(Si-O)n-Al, it is defined as Close Al, and according to Lowenstein's rule, the value of n is not zero; when n>2, it is defined as Single Al. Sazama et al., using n-butene as a model compound, studied the effect of single and adjacent aluminum in molecular sieves on catalytic cracking reactions. They found that single aluminum tends to undergo unimolecular cracking reactions, yielding more low-carbon olefins such as ethylene and propylene, while adjacent aluminum tends to undergo bimolecular reactions, yielding alkanes, benzene, and other products. Chen et al. found that in the catalytic cracking of n-octane, reducing the content of Close Al in the molecular sieve through different treatments can effectively inhibit hydrogen transfer reactions, thereby improving the selectivity for low-carbon olefins. Rational control of the Al distribution in the framework of zeolite molecular sieves is a key issue in the research of catalytic cracking reactions. Studies have shown that in catalytic cracking reactions, isolated acid sites are favorable for protonation cracking reactions of hydrocarbons, while adjacent acid sites are favorable for bimolecular hydrogen transfer reactions.

[0005] CN110372004A discloses a method for loading cobalt ions onto Na-type ZSM-5 molecular sieves using an ion exchange method to obtain Co-type ZSM-5 molecular sieves; post-treatment of the Co-type ZSM-5 molecular sieves with ammonium hexafluorosilicate and ammonium acetate to obtain product B; washing, drying, and calcining product B to obtain ZSM-5 molecular sieves with microscopic aluminum distribution control. The prepared molecular sieve is suitable for C4 hydrocarbon catalytic cracking reactions. The above method uses Co ions to protect the ortho-aluminum sites of ZSM-5 molecular sieves before post-treatment, which can control the relative content of ortho-aluminum and monoaluminum in ZSM-5 molecular sieves, thereby increasing the relative content of ortho-aluminum. The defects of this technology or the shortcomings of this invention are: the operation steps are cumbersome, requiring the molecular sieve to be exchanged to Na-type before the exchange treatment; in addition, this method increases the ortho-aluminum content but cannot increase the monoaluminum content.

[0006] CN 115007195A discloses a method in which zeolite molecular sieves are treated with modified citrate diester to preferentially remove non-framework aluminum and replenish aluminum through complexation, which can be applied to the catalytic alkylation reaction of toluene and tert-butanol. The preparation method of the low silica-to-alumina ratio zeolite molecular sieve includes the following steps: (1) calcination of zeolite molecular sieve powder; (2) preparation of modified citrate diester; (3) purification of modified citrate diester; (4) acid treatment and calcination. This invention treats zeolite molecular sieves with modified citrate diester, which can replenish aluminum while removing aluminum, not only making the pores more open and increasing the mass transfer efficiency, but also exposing more acidic centers. In addition, the improved hydrophobicity of the catalyst avoids the molecular sieve from deliquescence in the air. It can also be recycled after recovery, reducing costs. The defects of this technology or the shortcomings of this invention are: the above method uses organic acid synthesis to treat molecular sieves, and replenishes aluminum after removing non-framework aluminum to achieve the hydrophilicity-hydrophobicity modulation of molecular sieves and the increase of pore exposure.

[0007] CN116409795 A discloses a method for modifying a molecular sieve using an inorganic acid and an organic dicarboxylic acid to obtain the modified molecular sieve. The organic dicarboxylic acid contains a carbon-carbon double bond with two carboxyl groups arranged on the same side of the double bond, exhibiting a cis configuration. The modification using the organic dicarboxylic acid (maleic acid) is not performed earlier than the modification using the inorganic acid. This method can achieve deep removal of framework aluminum from the molecular sieve while effectively controlling the deposition of non-framework aluminum within the sieve channels. The drawbacks of this technology, or its shortcomings compared to the present invention, are: firstly, this technology mainly uses an organic acid containing both double bonds and carboxyl groups in combination with an inorganic acid to remove framework aluminum from the molecular sieve, but it does not consider issues such as the introduction of hydroxyl groups through aluminum supplementation.

[0008] Therefore, it is necessary to further improve the conversion rate of light hydrocarbon feedstock, the yield of low-carbon olefins, and the selectivity in the process of catalytic cracking of light hydrocarbons to produce low-carbon olefins. Summary of the Invention

[0009] To address the aforementioned problems, the present invention aims to provide a modified ZSM-5 molecular sieve, its preparation method, and its applications. This preparation method can regulate the microscopic aluminum distribution of the acidic centers on the outer surface of the ZSM-5 molecular sieve, increasing the proportion of single aluminum atoms, thereby significantly improving the molecular sieve's light hydrocarbon conversion capacity and the yield and selectivity of low-carbon olefins, and increasing the production of low-carbon olefins in catalytic cracking reactions.

[0010] To achieve the above objectives, the present invention provides a method for preparing modified ZSM-5 molecular sieves, the method comprising:

[0011] S1. Mix and impregnate the phosphorus source solution with the raw material ZSM-5 molecular sieve to obtain P-ZSM-5 molecular sieve;

[0012] S2. The P-ZSM-5 molecular sieve is mixed with a solution of organic acid, reacted, dried, and calcined to obtain the modified ZSM-5 molecular sieve; wherein the mass ratio of the P-ZSM-5 molecular sieve to the molar amount of the organic acid is 1000g:0.5-24mol.

[0013] The preparation method provided by this invention is a method for controlling the microscopic acidic centers of ZSM-5 molecular sieves. This method first modifies the molecular sieve channels with a phosphorus source, then treats the molecular sieve with an organic acid of a specific structural size and functional group type, preferentially treating the outer surface acidic centers that are more prone to bimolecular hydrogen transfer to rearrange the aluminum atom framework. By utilizing the instability of phosphorus-containing substances in the framework, this method achieves a significant improvement in the conversion capacity of light hydrocarbons and the yield and selectivity of low-carbon olefins of the molecular sieve without destroying the molecular sieve framework structure, adjusting the number of acidic centers, or introducing metal species.

[0014] In the above preparation method, the phosphorus source is unstable in the molecular sieve framework, which allows modification of the molecular sieve channels without damaging the framework structure. This guides the subsequent addition of organic acid macromolecules to react on the outer surface of the molecular sieve, preventing the organic acid from entering the interior. The phosphorus source is a neutral phosphorus-containing substance to avoid damaging the molecular sieve channels. Specifically, the phosphorus source may include one or a combination of two or more of ammonium phosphate, sodium phosphate, zinc phosphate, and iron phosphate.

[0015] In the above preparation method, the ratio of the mass of the raw material ZSM-5 molecular sieve to the molar ratio of phosphate in the phosphorus source can be 1000g:8-60mol, for example, specific values ​​such as 1000g:8mol, 1000g:10mol, 1000g:20mol, 1000g:30mol, 1000g:40mol, 1000g:50mol, 1000g:60mol, and any two of the above specific values ​​as endpoints.

[0016] In the above preparation method, the phosphorus source is typically prepared in the form of an aqueous solution containing the phosphorus source to impregnate the ZSM-5 molecular sieve raw material. The phosphorus source-containing solution contains phosphate (PO4) ions. 3- The concentration can be controlled from 0.8 to 3.0 mol / L, for example, specific values ​​such as 0.8 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, etc., and a range with any two of the above specific values ​​as endpoints; it can be further controlled from 0.05 to 0.5 mol / L.

[0017] In the above preparation method, the mixing ratio of the raw material ZSM-5 molecular sieve to the phosphorus source can be: based on the phosphate ions (PO4) in the phosphorus source-containing solution.3- With a concentration of 0.8-3.0 mol / L, the mass ratio of the raw material ZSM-5 molecular sieve to the phosphorus-containing source solution can be controlled to be 1:10-1:20, for example, specific values ​​such as 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19, 1:20, and any two of the above specific values ​​as endpoints.

[0018] In the above preparation method, the SiO2 / Al2O3 molar ratio of the raw material ZSM-5 molecular sieve is generally 20-100, for example, specific values ​​such as 20, 30, 40, 50, 60, 70, 80, 90, 100, etc., and a range with any two of the above specific values ​​as endpoints, and can further be 20-50.

[0019] In the above preparation method, the raw material ZSM-5 molecular sieve includes one or more of hydrogen-type ZSM-5 molecular sieve, sodium-type ZSM-5 molecular sieve, and ammonium-type ZSM-5 molecular sieve.

[0020] In the above preparation method, in S1, the impregnation temperature is generally room temperature (20-35℃), and the impregnation time is generally 1-6 hours.

[0021] In the above preparation method, S1 further includes washing, separating, and drying the impregnated ZSM-5 molecular sieve after impregnation. The drying temperature can be 60-120℃, and the drying time is 4-12 hours.

[0022] In the above preparation method, the organic acid can regulate the aluminum distribution at the acidic centers on the outer surface of the molecular sieve. The confinement effect near aluminum sites at the acidic centers on the outer surface of the molecular sieve is weakest, making it more suitable for bimolecular reactions to form large molecular products such as aromatics and gasoline. Conversely, the inner aluminum sites are more suitable for small molecule hydrogen transfer, which helps promote hydrocarbon molecule conversion. This invention utilizes a phosphorus source to reduce the pore size, avoiding the impact of subsequent organic acid treatment on the aluminum-adjacent active sites within the pores. Therefore, the modified ZSM-5 molecular sieve treated with organic acid exhibits improved conversion rate and higher ethylene + propylene yield.

[0023] The organic acids used in this invention are generally macromolecular organic acids, designed to prevent them from entering the internal pores of the molecular sieve and to promote the regulation of acidic centers on the outer surface, thereby altering the aluminum distribution. The ratio of hydroxyl to carboxyl groups in the organic acids used in this invention is generally greater than or equal to 1:2. The carboxyl groups can be used to remove skeletal and non-skeletal aluminum from the molecular sieve, and the hydroxyl groups can replenish the removed aluminum into the molecular sieve's framework.

[0024] Specifically, the organic acid may include one or more of the following: 2,5-dihydroxyterephthalic acid, 2,4-dihydroxybenzoic acid, 2,4,6-trihydroxybenzoic acid (e.g., 2,4,6-trihydroxybenzoic acid monohydrate), tartaric acid, malic acid, and lactic acid.

[0025] In the above preparation method, the dealuminization and aluminization rate of the organic acid on the molecular sieve can be controlled by adjusting the size of the organic acid, the content of hydroxyl and carboxyl groups, and the amount of organic acid used.

[0026] In the above preparation method, the ratio of the mass of the P-ZSM-5 molecular sieve to the molar amount of the organic acid can be 1000g:0.5-24mol, for example, specific values ​​such as 1000g:0.5mol, 1000g:1mol, 1000g:2mol, 1000g:5mol, 1000g:10mol, 1000g:15mol, 1000g:20mol, 1000g:24mol, and any two of the above specific values ​​as endpoints.

[0027] In the above preparation method, the organic acid can be prepared in solution form. The concentration of the organic acid solution is generally 0.05-0.8 mol / L, for example, specific values ​​such as 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, etc., and a range with any two of the above specific values ​​as endpoints; further, the concentration can be 0.05-0.5 mol / L.

[0028] In the above preparation method, the mixing ratio of the organic acid and the P-ZSM-5 molecular sieve can be: based on the solution concentration of the organic acid being 0.05-0.8 mol / L, the mass ratio of the P-ZSM-5 molecular sieve to the organic acid solution can be 1:10-1:30, for example, specific values ​​such as 1:10, 1:15, 1:20, 1:25, 1:30, etc., and a range with any two of the above specific values ​​as endpoints.

[0029] In the above preparation method, in step S2, the mixing method includes mechanical mixing methods such as stirring, ultrasonication, and high-speed dispersion to ensure that the organic acid and P-ZSM-5 molecular sieve are mixed evenly.

[0030] In the above preparation method, in S2, the reaction temperature is generally 40-90℃, for example, specific values ​​such as 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, etc., and a range with any two of the above specific values ​​as endpoints.

[0031] In the above preparation method, in S2, the reaction time is generally 2-10h, for example, specific values ​​such as 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, and a range with any two of the above specific values ​​as endpoints.

[0032] In the above preparation method, in step S2, the drying temperature can be 60-120℃, and the drying time can be 4-12h.

[0033] In the above preparation method, in step S2, the calcination temperature is generally 450-600℃, and the calcination time is generally 2-6h.

[0034] In specific implementation schemes, hydrogen-form modified ZSM-5 molecular sieves are generally used for catalytic cracking reactions and the production of low-carbon olefins. Accordingly, when the molecular sieve obtained after the reaction is a Na-type molecular sieve, S2 may further include ammonium exchange of the dried molecular sieve to convert it into a hydrogen-form molecular sieve. The ammonium exchange process can be as follows: mixing the calcined molecular sieve with an ammonium salt solution for ammonium exchange, separating the molecular sieve after exchange, and drying it to obtain the hydrogen-form modified ZSM-5 molecular sieve. The ammonium exchange temperature can be 60-80℃, and the ammonium exchange time is 4-6 hours. The ammonium salt can include one or more of NH4Cl, NH4NO3, and (NH4)2SO4. The drying temperature during the ammonium exchange process can be 60-120℃, and the drying time can be 4-12 hours.

[0035] According to a specific embodiment of the present invention, the monoaluminum content in the modified ZSM-5 molecular sieve obtained by the above preparation method is higher than the medium aluminum content in the raw ZSM-5 molecular sieve.

[0036] The present invention also provides a modified ZSM-5 molecular sieve, which is obtained by the above preparation method.

[0037] According to a specific embodiment of the present invention, the mass content of P2O5 in the modified ZSM-5 molecular sieve is ≤0.50%.

[0038] According to a specific embodiment of the present invention, the SiO2 / Al2O3 molar ratio of the modified ZSM-5 molecular sieve can be 20-100.

[0039] According to a specific embodiment of the present invention, the total specific surface area of ​​the modified ZSM-5 molecular sieve is 340 m². 2 / g or more, for example, 340-380m 2 / g.

[0040] According to a specific embodiment of the present invention, the mesoporous specific surface area of ​​the modified ZSM-5 molecular sieve is 131 m².2 / g or higher, for example, 131-210m 2 / g.

[0041] This invention also provides the application of the modified ZSM-5 molecular sieve in the catalytic cracking of light hydrocarbons to produce low-carbon olefins. The modified ZSM-5 molecular sieve exhibits improved aluminum distribution, which can increase the production of low-carbon olefins in the catalytic cracking reaction. The light hydrocarbons may include one or more combinations of alkane molecules with ≤10 carbon atoms, such as one or more combinations of n-hexane, n-butane, n-pentane, n-heptane, and n-octane. The low-carbon olefins may include ethylene, propylene, etc.

[0042] The beneficial effects of this invention include:

[0043] 1. In the preparation method provided by the present invention, during the organic acid treatment of phosphorus-containing molecular sieves, a specific organic acid will simultaneously remove non-framework aluminum and framework aluminum from ZSM-5 molecular sieves, and then the removed aluminum will be added back to the molecular sieve framework by means of hydroxyl groups. Under the premise of constant total acid content, aluminum distribution can be regulated. By increasing the proportion of single aluminum, the strong Brønsted acid can be increased, and the yield of low-carbon olefins of ZSM-5 molecular sieve can be greatly improved.

[0044] 2. The preparation method provided by the present invention uses a phosphorus source to treat the ZSM-5 molecular sieve, which has narrower pores, preventing the diffusion of large molecular organic acids into the pores of the molecular sieve. Therefore, it is easier to guide the organic acid to treat the aluminum atoms on the acid centers on the outer surface of the molecular sieve. Because of its poor shape selectivity, the treatment process will inhibit the role of the adjacent aluminum centers on the outer surface, thereby inhibiting the secondary reaction of low carbon olefins and helping to significantly improve the selectivity of the molecular sieve for low carbon olefins. Attached Figure Description

[0045] Figure 1 This is a SEM image of the molecular sieve sample from Example 1.

[0046] Figure 2 The image shows the XRD pattern of the molecular sieve sample from Example 1.

[0047] Figure 3 The image shows the SEM image of the molecular sieve sample from Comparative Example 1.

[0048] Figure 4 The image shows the XRD pattern of the molecular sieve sample from Comparative Example 1. Detailed Implementation

[0049] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0050] Source of raw materials or equipment:

[0051] (1) Organic acid treatment raw materials: 2,5-dihydroxyterephthalic acid, 2,4-dihydroxybenzoic acid, 2,4,6-trihydroxybenzoic acid monohydrate, tartaric acid, malic acid, lactic acid, ammonium phosphate [(NH4)3PO4], sodium phosphate, zinc phosphate and iron phosphate hydrate were all purchased from Sinopharm Chemical Reagent Co., Ltd.

[0052] (2) ZSM-5 molecular sieve raw materials: ZSM-5 molecular sieves 1-3#: ZSM-5 molecular sieve 1# is in the hydrogen form (SiO2 / Al2O3 ratio of 20), ZSM-5 molecular sieve 2# is in the sodium form (SiO2 / Al2O3 ratio of 50), and ZSM-5 molecular sieve 3# is in the hydrogen form (SiO2 / Al2O3 ratio of 100). All three types of industrial-grade ZSM-5 molecular sieves are sourced from Lanzhou Petrochemical Catalyst Plant. The SiO2 / Al2O3 ratios in this invention are all molar ratios.

[0053] (3) Molecular sieve micro-reflection evaluation device

[0054] Evaluation method: The ZSM-5 molecular sieves before and after treatment were used in the catalytic cracking reactions of n-butane and n-hexane. The specific evaluation process is as follows:

[0055] ① n-Butane Evaluation Process: n-Butane was used as the reactant in a fixed-bed reactor using a mixed feed of N2 and n-butane. The fixed-bed reactor consisted of a heater, a reaction quartz tube, and a temperature sensor. Before the reaction began, the prepared molecular sieve catalyst was pressed into tablets and sieved to form 40-60 mesh particles. An appropriate amount of catalyst was weighed and placed inside the reaction quartz tube, with quartz wool used as auxiliary material to support and fix the catalyst in place. The fixed-bed reactor was then controlled at a suitable reaction temperature using a temperature controller. A mixed gas of N2 and n-butane was then introduced to contact the catalyst for reaction. The n-butane flow rate was 2 mL / min, the nitrogen flow rate was 38 mL / min, and the catalyst dosage was 0.2 g. The product was directly analyzed online by gas chromatography.

[0056] ② Evaluation process of n-hexane: n-hexane was used as the reaction raw material. While ensuring a consistent packing height, 1-2g of molecular sieve was mixed with 4g of quartz sand to alter the mass hourly space velocity (MHV). The reaction temperature was 600℃, and the n-hexane feed rate was 0.98-1.15g. After feeding, nitrogen was purged at 100mL / min for 15min. The product was collected for gaseous and liquid product analysis.

[0057] (4) Characterization of the content of close Al and single Al in molecular sieves

[0058] ZSM-5 molecular sieves were subjected to sodium and cobalt ion exchange (exchange temperature 35-80℃) until cobalt ions were saturated and adsorbed at adjacent Al adsorption sites on the molecular sieve surface. The exchanged molecular sieves were then subjected to XRF testing to determine the aluminum content [Al]. total ] and cobalt content [Co max ].

[0059] The contents of Single Al and Close Al in molecular sieves can be calculated using the following formula:

[0060] Close Al=2[Co max ];

[0061] Single Al = [Al] total ]-2[Co max ].

[0062] Example 1

[0063] This embodiment provides a modified ZSM-5 molecular sieve, the preparation method of which includes:

[0064] S1. Dissolve a certain amount of triammonium phosphate in a certain amount of deionized water to prepare 110 mL of 3.0 mol / L triammonium phosphate aqueous solution. Add 10.0 g of hydrogen-form ZSM-5 molecular sieve 1# (SiO2 / Al2O3=20) to the prepared triammonium phosphate aqueous solution and stir at room temperature for 4 h. After stirring, filter and wash. Place the filtered residue in an oven at 100℃ and dry for 6 h to obtain molecular sieve sample P-ZSM-5-1#.

[0065] S2. Dissolve a certain amount of 2,5-dihydroxyterephthalic acid in a certain amount of deionized water to prepare 150 mL of 0.2 mol / L 2,5-dihydroxyterephthalic acid aqueous solution. Add 5.0 g of P-ZSM-5-1# to the prepared 2,5-dihydroxyterephthalic acid solution and transfer it to an ultrasonic disperser to disperse for 30 min. Then, stir in a water bath at 70℃ for 4 h to carry out the reaction. After stirring, filter and wash the sieve. Place the filtered residue in an oven at 100℃ and dry it for 4 h. Exchange the dried molecular sieve in a 1.0 mol / L NH4Cl aqueous solution at 80℃ for 4 h, then filter and wash it. Place the filtered residue in an oven at 100℃ and dry it for 4 h. Calcine it in a muffle furnace at 550℃ for 4 h to obtain the acid-treated molecular sieve sample ZSM-5-1#.

[0066] Example 2

[0067] This embodiment provides a modified ZSM-5 molecular sieve, the preparation method of which includes:

[0068] S1. Dissolve a certain amount of zinc phosphate in a certain amount of deionized water to prepare 100 mL of 0.8 mol / L zinc phosphate aqueous solution. Add 10.0 g of hydrogen-form ZSM-5 molecular sieve 2# (SiO2 / Al2O3=20) to the prepared zinc phosphate aqueous solution and stir at room temperature for 4 h. After stirring, filter and wash. Place the filtered residue in an oven at 80℃ and dry for 6 h to obtain molecular sieve sample P-ZSM-5-2#.

[0069] S2. Dissolve a certain amount of 2,4-dihydroxybenzoic acid in a certain amount of deionized water to prepare 150 ml of 0.1 mol / L 2,4-dihydroxybenzoic acid aqueous solution. Add 15.0 g of P-ZSM-5-2# to the prepared 2,4-dihydroxybenzoic acid solution and transfer it to a mechanically stirred container for 45 min. Then, stir in a water bath at 60℃ for 5 h to carry out the reaction. After stirring, filter and wash the mixture. Place the filtered residue in an oven at 80℃ and dry it for 10 h. Then, calcine it in a muffle furnace at 500℃ for 6 h to obtain the acid-treated molecular sieve sample ZSM-5-2#.

[0070] Example 3

[0071] This embodiment provides a modified ZSM-5 molecular sieve, the preparation method of which includes:

[0072] S1. Dissolve a certain amount of ferric phosphate (Fe III) in a certain amount of deionized water to prepare 200 mL of 1.65 mol / L ferric phosphate aqueous solution. Add 10.0 g of hydrogen-form ZSM-5 molecular sieve 2# (SiO2 / Al2O3=50) to the prepared ferric phosphate aqueous solution and stir at room temperature for 2 h. After stirring, filter and wash the solution. Place the filtered residue in an oven at 120℃ and dry for 2 h to obtain molecular sieve sample P-ZSM-5-3#.

[0073] S2. Dissolve a certain amount of 2,4,6-trihydroxybenzoic acid monohydrate in a certain amount of deionized water to prepare 150 ml of 0.05 mol / L 2,4,6-trihydroxybenzoic acid aqueous solution. Add 10.0 g of P-ZSM-5-3# to the prepared 2,4,6-trihydroxybenzoic acid aqueous solution and transfer to ultrasonic dispersion for 30 min. Then stir in a 50℃ water bath for 8 h to react. After stirring, filter and wash. Place the filtered residue in an 80℃ oven to dry for 10 h. Exchange the dried molecular sieve in a 1.0 mol / L NH4Cl aqueous solution at 80℃ for 4 h, then filter and wash. Place the filtered residue in an 100℃ oven to dry for 4 h, and calcine in a muffle furnace at 500℃ for 6 h to obtain the acid-treated molecular sieve sample ZSM-5-3#.

[0074] Example 4

[0075] This embodiment provides a modified ZSM-5 molecular sieve, the preparation method of which includes:

[0076] S1. Dissolve a certain amount of sodium phosphate in a certain amount of deionized water to prepare 150 mL of 2.6 mol / L sodium phosphate aqueous solution. Add 10.0 g of hydrogen-form ZSM-5 molecular sieve 1# (SiO2 / Al2O3=20) to the prepared sodium phosphate aqueous solution and stir at room temperature for 4 h. After stirring, filter and wash. Place the filtered residue in an oven at 100℃ and dry for 6 h to obtain molecular sieve sample P-ZSM-5-4#.

[0077] S2. A certain amount of tartaric acid and malic acid were dissolved in a certain amount of deionized water at a molar ratio of 3:2 to prepare 150 ml of a 0.5 mol / L mixed aqueous solution of tartaric acid and malic acid. 15.0 g of P-ZSM-5-4# was added to the prepared mixed aqueous solution of tartaric acid and malic acid, and then ultrasonically dispersed for 30 min. The mixture was then stirred in a water bath at 50 ℃ for 8 h to carry out the reaction. After stirring, the mixture was filtered and washed. The filter residue was placed in an oven at 80 ℃ and dried for 10 h. The dried molecular sieve was then exchanged in a 1.0 mol / L NH4Cl aqueous solution at 80 ℃ for 4 h, filtered and washed, and the filter residue was placed in an oven at 100 ℃ and dried for 4 h. Finally, it was calcined in a muffle furnace at 500 ℃ for 6 h to obtain the acid-treated molecular sieve sample ZSM-5-4#.

[0078] Example 5

[0079] This embodiment provides a modified ZSM-5 molecular sieve, the preparation method of which includes:

[0080] S1. Dissolve a certain amount of sodium phosphate in a certain amount of deionized water to prepare 300 mL of 1.1 mol / L sodium phosphate aqueous solution. Add 20.0 g of hydrogen-form ZSM-5 molecular sieve 1# (SiO2 / Al2O3=100) to the prepared sodium phosphate aqueous solution and stir at room temperature for 4 h. After stirring, filter and wash. Place the filtered residue in an oven at 100℃ and dry for 6 h to obtain molecular sieve sample P-ZSM-5-5#.

[0081] S2. Dissolve 1.5g of 2,5-dihydroxyterephthalic acid in a certain amount of deionized water to prepare 150ml of 0.05mol / L 2,5-dihydroxyterephthalic acid aqueous solution. Add 5.0g of P-ZSM-5-5# to the prepared 2,5-dihydroxyterephthalic acid solution and transfer it to an ultrasonic disperser for 30min. Then, stir in a water bath at 60℃ for 4h to react. After stirring, filter and wash the sieve. Place the filter residue in an oven at 100℃ and dry it for 4h. Exchange the dried molecular sieve in a 1.0mol / L NH4Cl aqueous solution at 80℃ for 4h, filter and wash it, and place the filter residue in an oven at 100℃ and dry it for 4h. Then, calcine it in a muffle furnace at 550℃ for 4h to obtain the acid-treated molecular sieve sample ZSM-5-5#.

[0082] Example 6

[0083] This embodiment provides a modified ZSM-5 molecular sieve, the preparation method of which includes:

[0084] S1. Dissolve a certain amount of sodium phosphate in a certain amount of deionized water to prepare 200 mL of 0.8 mol / L sodium phosphate aqueous solution. Add 10.0 g of hydrogen-form ZSM-5 molecular sieve 1# (SiO2 / Al2O3=20) to the prepared sodium phosphate aqueous solution and stir at room temperature for 4 h. After stirring, filter and wash. Place the filtered residue in an oven at 100℃ and dry for 6 h to obtain molecular sieve sample P-ZSM-5-5#.

[0085] S2. Dissolve 18.0g of tartaric acid in a certain amount of deionized water to prepare 150ml of 0.8mol / L tartaric acid mixed aqueous solution. Add 15.0g of P-ZSM-5-5# to the above-prepared tartaric acid aqueous solution, and then transfer to ultrasonic dispersion for 30min. Stir in a 90℃ water bath for 2h to carry out the reaction. After stirring, filter and wash. Place the filtered residue in an 80℃ oven to dry for 10h, and calcine in a muffle furnace at 500℃ for 6h to obtain acid-treated molecular sieve sample ZSM-5-6#.

[0086] Comparative Example 1

[0087] This comparative example provides a ZSM-5 molecular sieve, the preparation method of which includes:

[0088] 5.0 g of hydrogen-form ZSM-5 molecular sieve 1# (SiO2 / Al2O3=20) was added to 150 mL of deionized water solution and stirred at 80 °C for 4 h. After stirring, the mixture was filtered and washed. The filtered residue was placed in an oven at 100 °C and dried for 4 h. Then, it was calcined in a muffle furnace at 550 °C for 4 h to obtain the comparative molecular sieve sample ZSM-5-7#.

[0089] Comparative Example 2

[0090] This comparative example provides a ZSM-5 molecular sieve, the preparation method of which includes:

[0091] Dissolve 1.5g of tartaric acid in a certain amount of deionized water to prepare 100mL of 0.1mol / L tartaric acid aqueous solution. Add 5g of hydrogen-form ZSM-5 molecular sieve 1# (SiO2 / Al2O3=20) to the prepared tartaric acid solution and transfer it to an ultrasonic disperser to disperse for 30min. Then, stir in a water bath at 60℃ for 4h to carry out the reaction. After stirring, filter and wash the sieve. Place the filtered residue in an oven at 100℃ and dry for 4h. Calcine the dried molecular sieve in a muffle furnace at 550℃ for 4h to obtain the comparative molecular sieve sample ZSM-5-8#.

[0092] Comparative Example 3

[0093] This comparative example provides a ZSM-5 molecular sieve, the preparation method of which includes:

[0094] 10.0g of sodium-type ZSM-5 molecular sieve 2# was added to 150mL of 1.0mol / L ammonium chloride aqueous solution and stirred at 80℃ for 4h for ammonium exchange. After stirring, the mixture was filtered and washed. The filtered residue was placed in an oven at 100℃ and dried for 4h. Then, it was calcined in a muffle furnace at 550℃ for 4h to obtain the comparative molecular sieve sample ZSM-5-9#.

[0095] Comparative Example 4

[0096] This comparative example provides a ZSM-5 molecular sieve, the preparation method of which includes:

[0097] 5.0 g of hydrogen-form ZSM-5 molecular sieve 3# was added to 150 mL of deionized water solution and stirred at 80 °C for 4 h. After stirring, the mixture was filtered and washed. The filtered residue was dried in an oven at 100 °C for 4 h and then calcined in a muffle furnace at 550 °C for 4 h to obtain the comparative molecular sieve sample ZSM-5-10#.

[0098] Comparative Example 5

[0099] This comparative example provides a ZSM-5 molecular sieve, the preparation method of which includes:

[0100] A certain amount of tartaric acid and malic acid (molar ratio 3:2) were dissolved in a certain amount of deionized water to prepare 150 ml of a 1.0 mol / L mixed aqueous solution of tartaric acid and malic acid. 20.0 g of ZSM-5 molecular sieve 1# (SiO2 / Al2O3=20) was added to the above-prepared mixed aqueous solution and ultrasonically dispersed for 30 min. After stirring in a water bath at 50℃ for 8 h, the mixture was filtered and washed. The filtered residue was placed in an oven at 100℃ and dried for 4 h. Then, it was calcined in a muffle furnace at 500℃ for 6 h to obtain the acid-treated molecular sieve sample ZSM-5-11#.

[0101] Comparative Example 6

[0102] This comparative example provides a modified ZSM-5 molecular sieve, the preparation method of which is similar to that of Example 4, the difference being:

[0103] This comparative example directly treated hydrogen-form ZSM-5 molecular sieve 1# with a 150 ml mixed aqueous solution of 0.5 mol / L tartaric acid and malic acid prepared in Example 4. This molecular sieve was not treated with sodium phosphate or other phosphorus sources. Other steps and parameters were the same as in Example 4. The resulting acid-treated molecular sieve sample was named ZSM-5-12#.

[0104] Comparative Example 7

[0105] This comparative example provides a modified ZSM-5 molecular sieve, the preparation method of which includes:

[0106] 1. Dissolve a certain amount of citric acid in a certain amount of deionized water to prepare 150 ml of 0.5 mol / L citric acid aqueous solution. Add 15.0 g of P-ZSM-5-4# prepared according to Example 4 to the above-prepared citric acid aqueous solution, and then transfer it to ultrasonic dispersion for 30 min. After stirring in a 50℃ water bath for 8 h, the mixture is reacted. After stirring, the mixture is filtered and washed. The filtered residue is placed in an 80℃ oven and dried for 10 h. The dried molecular sieve is exchanged in a 1.0 mol / L NH4Cl aqueous solution at 80℃ for 4 h, and then filtered and washed. The filtered residue is placed in an 100℃ oven and dried for 4 h. Finally, it is calcined in a muffle furnace at 500℃ for 6 h to obtain the acid-treated molecular sieve sample ZSM-5-13#.

[0107] Test Example 1

[0108] The molecular sieve samples prepared in the above examples and comparative examples were characterized structurally and tested for performance. Table 1 shows the relative crystallinity and aluminum distribution of ZSM-5 molecular sieves prepared by different treatment methods; Table 2 shows the n-butane evaluation results of ZSM-5 molecular sieves prepared by different treatment methods; Table 3 shows the n-hexane evaluation results of ZSM-5 molecular sieves prepared by different treatment methods; Table 4 shows the typical texture parameters of ZSM-5 molecular sieves prepared by different treatment methods; Table 5 shows the pyridine infrared spectroscopy results of ZSM-5 molecular sieves prepared by different treatment methods, which were used for total acid content analysis.

[0109] Table 1. Relative crystallinity and aluminum distribution of ZSM-5 molecular sieves prepared by different treatment methods.

[0110] Sample Name Relative crystallinity <![CDATA[SiO2 / Al2O3]]> aluminum content % % of ortho-aluminum content <![CDATA[P2O5 content]]> <![CDATA[Comparative Example 1 (ZSM-5-7#) 1 > 100.0％ 20.2 79.0 21.0 0.t0％0 Comparative Example 2 (ZSM-5-8#) 99.5％ 20.3 81.0 19.0 0.00 Example 1 (ZSM-5-1#) 98.2％ 20.9 83.0 17.0 0.15 Example 2 (ZSM-5-2#) 98.1％ 20.7 91.0 9.0 0.08 Example 4 (ZSM-5-4#) 98.9％ 21.0 84.1 15.9 0.07 Example 6 (ZSM-5-6#) 97.0％ 23.0 86.0 14.0 0.04 Comparative Example 5 (ZSM-5-11#) 95.0％ 24.1 83.5 16.5 0.00 Comparative Example 6 (ZSM-5-12#) 98.0％ 21.5 82.0 18.0 0.00 Comparative Example 7 (ZSM-5-13#) 96.0％ 23.5 80.0 20.0 0.05 <![CDATA[Comparative Example 3 (ZSM-5-9#) 2 > 100.0％ 49.8 83.0 17.0 0.00 Example 3 (ZSM-5-3#) 97.7％ 50.2 87.0 13.0 0.00 <![CDATA[Comparative Example 4 (ZSM-5-10#) 3 > 100.0％ 99.8 91.0 9.0 0.00 Example 5 (ZSM-5-5#) 99.5％ 100.1 93.0 7.0 0.00

[0111] Note: Superscripts 1-3 are the relative crystallinity test benchmarks under this silicon-aluminum ratio. That is, the relative crystallinity of Comparative Examples 2, 5, 6, 7 and Examples 1, 2, 4, 6 is calculated based on Comparative Example 1; the relative crystallinity of Example 3 is calculated based on Comparative Example 3; and the relative crystallinity of Example 5 is calculated based on Comparative Example 4.

[0112] The relative crystallinity is calculated by dividing the characteristic peak diffraction area of ​​the molecular sieve crystal to be tested by the characteristic peak diffraction area of ​​the molecular sieve crystal used as a reference.

[0113] Table 2 Evaluation results of ZSM-5 molecular sieve n-butane prepared by different treatment methods

[0114]

[0115] Table 3 Evaluation results of ZSM-5 molecular sieve n-hexane prepared by different treatment methods

[0116]

[0117] Table 4 Typical texture properties of ZSM-5 molecular sieves prepared by different treatment methods

[0118]

[0119] Table 5 Infrared spectral analysis results of ZSM-5 molecular sieve pyridine prepared by different treatment methods

[0120]

[0121] The microstructure of the samples from Example 1 and Comparative Example 1 was characterized using SEM, and the results are as follows: Figure 1 and Figure 3 As shown. The morphology of ZSM-5 molecular sieves before and after organic acid treatment was not damaged, and the average diameter of both was 200-400 nm. The crystal phase structure of the samples of Example 1 and Comparative Example 1 was characterized by XRD. Figure 2 and Figure 4Both ZSM-5 molecular sieves before and after organic acid treatment exhibited typical MFI structural diffraction peaks in the 2θ = 5-50° range. Since the same ZSM-5 molecular sieve raw material was used and the post-treatment caused minimal damage to the molecular sieve crystals, the characteristic peak positions and peak areas of the XRD patterns did not change significantly. Further analysis of the relative crystallinity (compared to molecular sieves with the same Si / Al ratio), XRF elemental analysis, and aluminum distribution (Table 1) of the molecular sieve samples from Examples 1-6 and Comparative Examples 1-7 revealed that when ZSM-5 molecular sieves containing P with a SiO2 / Al2O3 = 20 were treated with organic acids within a reasonable range, the relative crystallinity loss of the molecular sieves was within 3%, while the single aluminum content of the molecular sieves increased significantly (the single aluminum content in Example 2 increased by 12% compared to Comparative Example 1). When the concentration of organic acid treatment was too high (Comparative Example 5), the relative crystallinity loss was higher, causing a certain degree of damage to the molecular sieve framework, especially the acidic centers, leading to the loss of acidic centers. Furthermore, referring to the experimental results of Comparative Examples 1 and 2, the phosphorus source is gradually washed away during the preparation process after entering the molecular sieve channels, and has little impact on the catalyst activity.

[0122] The effect is similar for ZSM-5 molecular sieves with SiO2 / Al2O3 = 50 and SiO2 / Al2O3 = 20, but for ZSM-5 molecular sieves with a higher SiO2 / Al2O3 = 100, the effect is not obvious due to the lower aluminum content and the higher single aluminum content. Therefore, the preparation method of the present invention is suitable for raw material ZSM-5 molecular sieves with a SiO2 / Al2O3 molar ratio of 20-100, and the effect is better for raw material ZSM-5 molecular sieves with a SiO2 / Al2O3 molar ratio of 20-50.

[0123] Textural properties (Table 4), pyridine infrared spectroscopy (Table 5), and n-butane catalytic cracking (Table 2) were evaluated for ZSM-5 molecular sieves with SiO2 / Al2O3 = 20 before and after organic acid treatment. The results showed that the total pore volume and total specific surface area of ​​the molecular sieve increased slightly after organic acid treatment, while the total acid content remained basically unchanged. Comparing Example 2 and Comparative Example 1, it can be found that the strong Brønsted acid centers in the molecular sieve sample of Example 2 increased slightly, especially the strong Brønsted acid / L ratio, which in turn proves that the proportion of strongly acidic single aluminum centers in the molecular sieve has increased.

[0124] The n-butane evaluation results showed that the activity and ethylene + propylene product selectivity of the ZSM-5 molecular sieve treated with organic acid were significantly increased compared with the molecular sieve sample without organic acid treatment. Compared with Comparative Example 1, the n-butane conversion rate of Example 2 increased from 21.2 wt% to 40.8 wt% (an increase of 19.6 percentage points), and the ethylene + propylene yield increased from 9.7 wt% to 20.1 wt% (an increase of 10.4 percentage points), which is nearly double that of the original molecular sieve sample. Compared with Comparative Example 2, Example 1 showed that the n-butane conversion increased from 31.0 wt% to 37.7 wt% (an increase of 6.7 percentage points), and the ethylene + propylene yield increased from 14.8 wt% to 18.4 wt% (an increase of 3.6 percentage points, accounting for 53.7% of the conversion increase). Compared with Comparative Example 6, Example 4 showed that the n-butane conversion increased from 34.2 wt% to 39.6 wt% (an increase of 5.4 percentage points), and the ethylene + propylene selectivity increased from 45.9 wt% to 48.9 wt% (an increase of 3.0 percentage points). These results indicate that the method of using phosphorus-containing pore blockage can effectively promote the conversion of n-butane while effectively improving the selectivity for ethylene and propylene. Compared with Comparative Example 7, the n-butane conversion rate of Example 4 increased from 29.0 wt% to 39.6 wt% (an increase of 10.6 percentage points), and the ethylene + propylene yield increased from 13.1 wt% to 19.4 wt% (an increase of 6.3 percentage points, accounting for 59.4% of the conversion rate increase). This result indicates that the treatment of ZSM-5 molecular sieve with an organic acid with a hydroxyl:carboxyl ratio of 1:3 resulted in more severe dealuminization due to the excessive number of carboxyl groups (see Table 1, the molecular sieve product of Example 4 has a SiO2 / Al2O3 ratio of 21, while the molecular sieve product of Comparative Example 7 has a SiO2 / Al2O3 ratio of 23.5). The ZSM-5 molecular sieves with weaker acidity (SiO2 / Al2O3 = 50 and SiO2 / Al2O3 = 100) were evaluated using hexane catalytic cracking (Table 3). The evaluation results showed that the activity and ethylene + propylene product selectivity of the ZSM-5 molecular sieve treated with organic acid were improved compared with the molecular sieve without organic acid treatment. In Example 3, compared with Comparative Example 3, the n-butane conversion rate increased from 66.7 wt% to 75.9 wt% (an increase of 9.2 percentage points), and the ethylene + propylene yield increased from 24.4 wt% to 28.5 wt% (an increase of 4.1 percentage points). In Example 5, compared with Comparative Example 4, the n-butane conversion rate increased from 50.7% to 54.8% (an increase of 4.1 percentage points), and the ethylene + propylene yield increased from 21.5% to 23.6% (an increase of 2.1 percentage points).

[0125] Analysis of the molecular sieve evaluation results with different silica-to-alumina ratios showed that, under the premise of increased ethylene + propylene yield, Example 2 showed a 1.7 percentage point increase in ethylene + propylene selectivity during the catalytic cracking of n-butane at 600℃ compared to Comparative Example 2; Example 3 showed a 0.9 percentage point increase in ethylene + propylene selectivity during the catalytic cracking of n-hexane at 600℃ compared to Comparative Example 3; and Example 5 showed a 0.7 percentage point increase in ethylene + propylene selectivity during the catalytic cracking of n-hexane at 600℃ compared to Comparative Example 4.

[0126] The above evaluation results all indicate that the ZSM-5 molecular sieve treated by this method, with the protection of phosphorus-containing molecular channels, has improved activity for light hydrocarbons and selectivity for low-carbon olefins with a low degree of structural damage.

Claims

1. A method for preparing modified ZSM-5 molecular sieve, the method comprising: S1. Mix and impregnate the phosphorus source solution with the raw material ZSM-5 molecular sieve to obtain P-ZSM-5 molecular sieve; S2. The P-ZSM-5 molecular sieve is mixed with a solution of organic acid, reacted, dried, and calcined to obtain the modified ZSM-5 molecular sieve. The ratio of the mass of the P-ZSM-5 molecular sieve to the molar amount of the organic acid is 1000g:0.5-24mol.

2. The preparation method according to claim 1, wherein, The phosphorus source includes one or more of the following: ammonium phosphate, sodium phosphate, zinc phosphate, and iron phosphate.

3. The preparation method according to claim 1, wherein, The mass ratio of the ZSM-5 molecular sieve to the molar ratio of phosphate in the phosphorus source is 1000g:8-60mol.

4. The preparation method according to claim 1, wherein, Based on the concentration of phosphate ions in the phosphorus source solution being 0.8-3.0 mol / L, the mass ratio of the raw material ZSM-5 molecular sieve to the phosphorus source solution is 1:10-1:

20.

5. The preparation method according to claim 1, wherein, The SiO2 / Al2O3 molar ratio of the raw material ZSM-5 molecular sieve is 20-100.

6. The preparation method according to claim 1, wherein, The immersion temperature is room temperature, and the immersion time is 1-6 hours.

7. The preparation method according to claim 1, wherein, The ratio of hydroxyl to carboxyl groups in the organic acid is greater than or equal to 1:

2.

8. The preparation method according to claim 1 or 7, wherein, The organic acid includes one or more of 2,5-dihydroxyterephthalic acid, 2,4-dihydroxybenzoic acid, 2,4,6-trihydroxybenzoic acid, tartaric acid, malic acid, and lactic acid.

9. The preparation method according to claim 1, wherein, Based on the organic acid solution concentration range of 0.05-0.8 mol / L, the mass ratio of the P-ZSM-5 molecular sieve to the organic acid solution is 1:10-1:

30.

10. The preparation method according to claim 1, wherein, The reaction temperature is 40-90℃, and the reaction time is 2-10h.

11. A modified ZSM-5 molecular sieve, which is obtained by the preparation method according to any one of claims 1-10.

12. The modified ZSM-5 molecular sieve according to claim 11, wherein, The modified ZSM-5 molecular sieve contains ≤0.50% P2O5 by mass.

13. The application of the modified ZSM-5 molecular sieve according to any one of claims 11-12 in the catalytic cracking of light hydrocarbons to produce low-carbon olefins.

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