MFI structure molecular sieve with high and low carbon olefin yield in naphtha catalytic cracking and preparation method of MFI structure molecular sieve

Abstract

The invention discloses an MFI structure molecular sieve used for naphtha catalytic cracking and having high and low carbon olefin yields and a preparation method thereof, the MFI structure molecular sieve contains 0.1-2% by mass of niobium, the silica-alumina ratio is 10-50 in terms of SiO2 / Al2O3 molar ratio, and compared with a hydrogen type MFI structure molecular sieve, the strong L acid is increased by 10 [mu] mol / g or more, the weak L acid is increased by 10 [mu] mol / g or more, the strong B acid is increased by 1 [mu] mol / g or more, and the weak B acid is increased by 1 [mu] mol / g or more. The preparation method comprises the following steps: dipping the hydrogen type MFI structure molecular sieve in a niobium compound solution dissolved in a low-concentration oxalic acid solution, drying and roasting. The modified MFI structure molecular sieve is high in naphtha cracking activity and high in low-carbon olefin yield.

Description

Technical Field

[0001] This invention relates to a molecular sieve with an MFI structure for catalytic cracking of naphtha, which has high and low carbon olefin yields, and its preparation method. Background Technology

[0002] MFI-structured molecular sieves, such as ZSM-5, are among the most widely used molecular sieves, serving as the main active component in catalytic cracking catalysts for the production of low-carbon olefins. To improve their catalytic performance, they are typically modified using methods such as hydrothermal modification, acid-base modification, and metal modification. Metal modification allows for the control of the acid center distribution, pore size, and pore structure distribution within the molecular sieve, thereby enhancing its catalytic performance. Currently, modified ZSM-5 molecular sieves exhibit low yields and low conversion rates of low-carbon olefins when used in naphtha catalytic cracking.

[0003] Low-carbon olefins, including ethylene, propylene, and butene, are essential chemical raw materials. Ethylene, in particular, is one of the most important synthetic raw materials in the petrochemical industry. It is mainly used to produce polyethylene, ethylene oxide, dichloroethane, and ethylbenzene / styrene, among others. Steam cracking units for ethylene production are typically the core units of petrochemical enterprises. Currently, approximately 90% of ethylene is produced from naphtha steam cracking, with propylene as a co-product accounting for 30%. Propylene is mainly used to produce polypropylene, acrylonitrile, propylene oxide, and other chemical products. In recent years, the demand for propylene has grown rapidly. According to IHS forecasts, global propylene consumption will grow at an average rate of about 5%, exceeding the ethylene growth rate of 3.4%. However, the propylene-to-ethylene yield ratio from steam cracking cannot be flexibly adjusted. Moreover, the reaction temperature is as high as 840-860℃, and energy consumption accounts for about 40% of the petrochemical industry's energy consumption. The high temperature places high demands on the materials used in the reaction equipment, requiring expensive high-temperature resistant alloy materials. Furthermore, coke has high selectivity, stringent requirements for raw materials, and releases large amounts of carbon dioxide. Moreover, the process of ethane dehydrogenation to ethylene has been industrialized in many countries, which will reduce the proportion of naphtha steam cracking and reduce the propylene produced by it. Therefore, countries around the world are actively developing alternative technologies for steam cracking to produce high value-added products such as low-carbon olefins. Among them, catalytic cracking technology for naphtha and related hydrocarbons is the most competitive. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide an MFI-structured molecular sieve for naphtha catalytic cracking with higher yields of propylene and butene, which significantly increases the conversion rate and low-carbon olefin yield during naphtha catalytic cracking. Another technical problem to be solved by this invention is to provide a method for preparing the aforementioned MFI-structured molecular sieve.

[0005] This invention provides an MFI-structured molecular sieve for catalytic cracking of naphtha to improve the yield of high and low carbon olefins. The sieve has a silica-to-alumina ratio of 10-50 and contains 0.1-2% by mass of niobium. Compared with hydrogen-type MFI-structured molecular sieves, its L... 强铌 -L 强氢 The concentration is 10 μmol / g or higher, preferably 20 μmol / g or higher, for example, 10–55 μmol / g or 20–40 μmol / g, L 弱铌 -L 弱氢 For example, 10–45 μmol / g or 10–30 μmol / g, B 强铌 -B 强氢 B is 1 μmol / g or higher, for example, 2 μmol / g or higher, or 2–4 μmol / g or 1–5 μmol / g. 弱铌 -B 弱氢 The concentration is preferably 1 μmol / g or higher, or 3 μmol / g or higher, for example, 1–9 μmol / g or 3–8 μmol / g, wherein L 强氢 The strong L acid content of hydrogen-form MFI molecular sieves, L 强铌 The strong L acid content of niobium-containing MFI molecular sieves, L 弱氢 The weak L acid content of the hydrogen-form MFI molecular sieve, L 弱铌 The weak L acid content of niobium-containing MFI molecular sieves, B 强氢 The strong Brønsted acid content of the hydrogen-form MFI molecular sieve, B 强铌 The strong Brønsted acid content of niobium-containing MFI molecular sieves, B 弱氢 The weak Brønsted acid content of the hydrogen-form MFI molecular sieve, B 弱铌 The amount of weak Brønsted acid in the niobium-containing MFI structured molecular sieve is given. The niobium-containing MFI structured molecular sieve is the same MFI structured molecular sieve used for the catalytic cracking of naphtha to increase the yield of high and low carbon olefins. The silica-alumina ratio of the molecular sieve is expressed as the SiO2 / Al2O3 molar ratio.

[0006] The acid content was measured using pyridine adsorption infrared spectroscopy, and the Brønsted acid content was measured at 1540 cm⁻¹. -1 The absorption peak at 1450 cm⁻¹ was measured, and the acid content of L acid was measured at 1450 cm⁻¹. -1 The absorption peaks were measured. The total acid content was the acid content measured after vacuum desorption at 200℃, the strong acid content was the acid content measured after vacuum desorption at 350℃, and the weak acid content was the difference between the total acid content and the acid content measured after vacuum desorption at 350℃.

[0007] This invention further provides a method for preparing the MFI-structured molecular sieve for naphtha catalytic cracking with high and low carbon olefin yields, the method comprising:

[0008] (1) Dissolve Nb(OH)5 in a low-concentration oxalic acid solution to obtain an impregnation solution, wherein the molar ratio of Nb to oxalic acid in the low-concentration oxalic acid solution is greater than 0.5:1 and less than 1.1:1, and the concentration of the low-concentration oxalic acid solution is 0.1 to 0.3 mol / L;

[0009] (2) Impregnate the hydrogen-form MFI structured molecular sieve with the impregnation solution, wherein the sodium oxide content of the hydrogen-form MFI structured molecular sieve is less than 0.2% by mass;

[0010] (3) Drying, roasting, for example, hydrothermal roasting.

[0011] This invention also provides the application of the MFI-structured molecular sieve for high and low carbon olefin yields in naphtha catalytic cracking in naphtha catalytic cracking. The application method involves contacting naphtha-containing oil with the molecular sieve under catalytic cracking conditions. The naphtha can be a petroleum fraction with a distillation range of 30–80°C or a hydrocarbon or hydrocarbon mixture with a boiling point of 30–80°C. The catalytic cracking reaction conditions are, for example, a reaction temperature of 520–650°C, a reaction time of, for example, 1–100 seconds, and a mass hourly space velocity of, for example, 0.1–100 h⁻¹. -1 .

[0012] The MFI-structured molecular sieve provided by this invention for catalytic cracking of naphtha to achieve high and low carbon olefin yields has at least one or all of the following advantages compared to the hydrogen-form ZSM-5 molecular sieve:

[0013] 1) It has higher propylene and butene yields.

[0014] 2) It has a higher yield of low-carbon olefins.

[0015] 3) The propylene / ethylene ratio is relatively high.

[0016] 4) Higher conversion rate.

[0017] The present invention provides a method for preparing MFI-structured molecular sieves for naphtha catalytic cracking and yielding high and low carbon olefins. Through appropriate methods, Nb can be... The acid and Lewis (L) acid are redistributed, and the amounts of both Brønsted (B) acid and Lewis (L) acid are increased. Compared with the niobium-modified ZSM-5 molecular sieve obtained by existing methods, it has a higher conversion rate, higher total yield of low carbon olefins, higher yield of propylene, higher yield of butene, and a higher propylene / ethylene ratio when catalytically cracking naphtha. Detailed Implementation

[0018] According to the present invention, in the MFI structured molecular sieve for naphtha catalytic cracking with a high yield of low-carbon olefins, the niobium content (Nb) is preferably 0.2-1% by mass, for example 0.3-0.8% by mass. This can result in a higher butene yield and a higher propylene / ethylene ratio.

[0019] In one embodiment, the niobium content in the MFI structured molecular sieve used for naphtha catalytic cracking with a high yield of low-carbon olefins is 0.8 to 1.5% by mass, which can result in higher butene selectivity.

[0020] In one embodiment, the niobium content in the MFI structured molecular sieve for naphtha catalytic cracking with a high yield of low-carbon olefins is 1.5-2% by mass, which can result in a higher total yield of low-carbon olefins, a higher yield of ethylene, and higher selectivity for propylene and ethylene.

[0021] In one embodiment, the niobium content in the MFI structured molecular sieve used for naphtha catalytic cracking with a high yield of low-carbon olefins is 0.4-0.8% by mass, which can result in higher yields of propylene and butene, and a higher propylene / ethylene ratio.

[0022] According to the present invention, the sodium oxide content in the MFI structured molecular sieve used for catalytic cracking of naphtha to achieve high and low carbon olefin yields is preferably less than 0.2% by mass.

[0023] According to the present invention, the sodium oxide content of the hydrogen-form MFI structured molecular sieve is preferably less than 0.2% by mass.

[0024] In one embodiment, the niobium-containing MFI structured molecular sieve has the same silicon-to-aluminum ratio as the hydrogen-type MFI structured molecular sieve.

[0025] In one embodiment, the ratio of sodium oxide content in the niobium-containing MFI structured molecular sieve to that in the hydrogen-type MFI structured molecular sieve is 0.9 to 1.1 by mass.

[0026] According to the present invention, the MFI structured molecular sieve is preferably a ZSM-5 molecular sieve.

[0027] According to the present invention, the MFI structured molecular sieve for naphtha catalytic cracking of high and low carbon olefins yields has a weak L acid content (L... 弱铌 The value can be 20–70 μmol·g -1 For example, 20–55 μmol·g -1 Or 20–50 μmol·g -1 Or 30-70 μmol·g -1 Or 35-45 μmol·g -1 .

[0028] According to the present invention, the MFI structured molecular sieve for naphtha catalytic cracking of high and low carbon olefins yield has a strong L acid content (L... 强铌 The value can be 50–120 μmol·g. -1 For example, 50–105 μmol·g -1 Or 70–116 μmol·g -1 Or 50–100 μmol·g -1 Or 80–105 μmol·g -1 Or 80–90 μmol·g -1 .

[0029] According to the present invention, the MFI structured molecular sieve for naphtha catalytic cracking of high and low carbon olefins yields has a weak Brønsted acid content (B... 弱铌 The value can be 20–50 μmol·g. -1 For example, 25–35 μmol·g -1 Or 26–31 μmol·g -1 .

[0030] According to the present invention, the MFI structured molecular sieve for naphtha catalytic cracking of high and low carbon olefins yields has a strong Brønsted acid content (Brønsted acid content). 强铌 The value can be 40–80 μmol·g -1 For example, 45–50 μmol·g -1 Or 46–48 μmol·g -1 .

[0031] According to the present invention, in one embodiment of the MFI structured molecular sieve for catalytic cracking of naphtha to achieve high and low carbon olefin yields, the ratio of the amount of strong Brønsted acid to strong Lewis acid (B... 强铌 / L 强铌 The ratio can be 0.4 to 0.65:1, for example, 0.4 to 0.55:1.

[0032] According to the present invention, in one embodiment of the MFI structured molecular sieve for catalytic cracking of naphtha to achieve high and low carbon olefin yields, the weak Brønsted acid and the weak Lewis acid (Brønsted acid) are present. 弱铌 / L 弱铌 The ratio of acid content can be 0.4–0.85:1, 0.85–1.25:1, 0.58–0.75:1, 0.55–0.85:1, or 0.43–0.75:1.

[0033] According to the present invention, in one embodiment of the MFI structured molecular sieve for catalytic cracking of naphtha to increase the yield of high and low carbon olefins, the total acidity is 180–270 μmol·g. -1 For example, 195–260 μmol·g -1 .

[0034] According to the present invention, in the method for preparing MFI structured molecular sieves for naphtha catalytic cracking of high and low carbon olefins, in step (1), the molar ratio of Nb to oxalic acid in the oxalic acid solution is 0.7 to 1.05:1, for example, 0.9 to 1:1 or 1:1.

[0035] According to the present invention, a method for preparing MFI structured molecular sieves for naphtha catalytic cracking with high and low carbon olefin yields involves dissolving Nb(OH)5 in a low-concentration oxalic acid solution, for example, in an oxalic acid solution with a concentration of 0.1–0.3 mol / L. The dissolution of Nb(OH)5 in the oxalic acid solution can be carried out at a certain temperature, for example, 70–90°C, such as 75°C, 80°C, or 85°C, under stirring. The Nb(OH)5 is then dissolved to obtain an oxalic acid solution containing dissolved Nb(OH)5, which is the impregnation solution. The hydrogen-form MFI structured molecular sieve is then impregnated with the obtained impregnation solution.

[0036] The impregnation is, for example, a supersaturated impregnation. The impregnation can be performed at room temperature for a duration of 15–35 h or 20–30 h, for example, 18 h, 21 h, 23 h, 25 h, 27 h, 29 h, or 32 h, or any two values ​​in between, forming a range, followed by drying.

[0037] According to the present invention, a method for preparing MFI structured molecular sieves for increasing the yield of high and low carbon olefins in naphtha catalytic cracking is provided. The hydrogen-form MFI structured molecular sieve can be commercially available or prepared using existing methods. For example, it can be obtained by calcining an MFI structured molecular sieve with a sodium oxide content higher than 0.2% by mass after exchange washing. The MFI structured molecular sieve with a sodium oxide content higher than 0.2% by mass can be a Na-type MFI structured molecular sieve, an ammonium-type MFI structured molecular sieve, or a hydrogen-form MFI structured molecular sieve with a sodium content higher than 0.2% by mass. The Na-type MFI structured molecular sieve can be obtained by filtering the mother liquor from the crystallized MFI structured molecular sieve, followed by washing, drying, and calcining. In one embodiment, the sodium washing exchange is performed by exchanging the molecular sieve, ammonium salt, and H₂O at a mass ratio of 1:(0.1-1):(5-10) at room temperature to 60°C for 0.5-3 hours, followed by filtration. The ammonium salt can be a commonly used inorganic ammonium salt, for example, selected from at least one of ammonium chloride, ammonium sulfate, and ammonium nitrate. The sodium washing exchange can be repeated once or multiple times, for example, 1 to 3 times, to ensure that the sodium oxide content in the molecular sieve is less than 0.2% by mass. The hydrogen-form MFI structured molecular sieve is preferably an H-ZSM-5 molecular sieve.

[0038] In one embodiment, the hydrogen-form MFI structured molecular sieve can be obtained by the following method:

[0039] a. After filtering out the mother liquor from the crystallized MFI structure molecular sieve, such as crystallized ZSM-5 molecular sieve, the sieve is washed with water to obtain a washed molecular sieve. The washed molecular sieve is dried and optionally calcined in air to remove the template agent. The crystallized ZSM-5 molecular sieve can be a molecular sieve prepared by the template agent method, which can be commercially available or prepared according to existing methods. The crystallized ZSM-5 molecular sieve can be prepared by existing methods, for example, according to the methods disclosed in patent documents US3702 886 or CN1194181A (CN97103635.7). The calcination treatment is well known to those skilled in the art; for example, the molecular sieve can be calcined in air to remove the template agent. The calcination temperature is 350℃~650℃, and the calcination time is 3~10h.

[0040] b. The molecular sieve obtained in step a is subjected to sodium exchange with an ammonium salt solution to ensure that the sodium oxide content does not exceed 0.2% by mass, thereby obtaining a hydrogen-form MFI structured molecular sieve; one method of sodium exchange in step b is to exchange at room temperature to 60°C for 0.5 to 3 hours according to the mass ratio of molecular sieve: ammonium salt: H2O = 1:(0.1~1):(5~10), and then filter. The ammonium salt can be a commonly used inorganic ammonium salt, for example, at least one selected from ammonium chloride, ammonium sulfate, and ammonium nitrate; the sodium exchange can be repeated 1 to 3 times to ensure that the sodium oxide content in the molecular sieve is less than 0.2% by mass.

[0041] The MFI structured molecular sieve obtained in step b can be achieved by drying and / or calcining an MFI structured molecular sieve with a sodium oxide content not exceeding 0.2% by mass obtained through sodium exchange after washing. The calcination temperature can be, for example, 450–650°C, and the calcination time can be 2 hours or more, for example, 2–6 hours.

[0042] According to the method for preparing MFI structured molecular sieves for naphtha catalytic cracking and yielding high and low carbon olefins, in step (3) of the present invention, the drying is carried out, for example, at 100-150°C for 0.5-5 hours, for example, drying at 110°C for 2 hours, to obtain the dried molecular sieve. The dried molecular sieve is then heated to a calcination temperature for calcination. The heating to the calcination temperature can be achieved by directly heating the dried molecular sieve or by cooling the dried molecular sieve, for example, to room temperature before heating. For example, the calcination temperature can be reached from the initial heating temperature, for example, from room temperature, over 1-3 hours, for example, 2 hours.

[0043] According to the preparation method of the MFI structured molecular sieve for naphtha catalytic cracking and high and low carbon olefin yield according to the present invention, in step (3), the calcination temperature can be 400-700℃, and the calcination time can be 0.5-8 hours. The calcination temperature is, for example, 450-600℃ or 500-600℃, and the calcination time is 1-6 hours, for example, it can be calcined at a constant temperature of 550℃ for 3 hours. Calcination can be carried out in an air atmosphere, or in an atmosphere containing water vapor or an inert atmosphere.

[0044] In this invention, the room temperature is 15–40°C.

[0045] The following embodiments further illustrate the present invention. The present invention is not limited thereto. Unless otherwise specified, the instruments and reagents used in the embodiments of the present invention are instruments and reagents commonly used by those skilled in the art.

[0046] The effect of molecular sieves on propylene yield in naphtha catalytic cracking was evaluated using a microreactor. Molecular sieves were aged at 800℃ in 100% steam for 17 hours, and the evaluation was conducted in a microreactor. The feedstock was naphtha (distillation range 30–80℃, specific gravity 0.703). 5g of molecular sieve with a particle size of 20–40 mesh was weighed and loaded into a fixed-bed reactor. Before the reaction, the reactor was purged with high-purity nitrogen at 30 mL / min for at least 20 min. Automatic feeding was performed using a micro-injection pump at a rate of 1.56g of naphtha over a time of 70 s. The reaction temperature was 620℃, and the mass hourly space velocity (HHSV) was 16 h⁻¹. -1 .

[0047] The micro-reaction method of this invention can refer to the standard method Q / SH 3360-211. The PONA analysis of the micro-reaction product is determined by the RIPP 85-90 method (Petrochemical Analysis Methods (RIPP Test Methods)", Science Press, 1990, edited by Yang Cuiding et al.).

[0048] The type of acid center and its acid content were analyzed and determined using the pyridine adsorption infrared in-situ measurement method. Experimental instrument: Bruker IFS113V FT-IR (Fourier Transform Infrared) spectrometer, USA. Experimental method for determining acid content using the pyridine adsorption infrared method: The sample was self-supported and pressed into a pellet, then sealed in the in-situ cell of the infrared spectrometer. The temperature was raised to 400℃, and a vacuum was drawn to 10℃. -3 The sample was heated to 280 Pa and kept at a constant temperature for 2 hours to remove adsorbed gas molecules. The temperature was then lowered to room temperature, and pyridine vapor was introduced at a pressure of 2.67 Pa to maintain adsorption equilibrium for 30 minutes. The temperature was then raised to 200 °C and evacuated to a vacuum of 10 °C. -3 Desorption was performed at Pa for 30 min, followed by spectral analysis at room temperature. The wavenumber range was 1400 cm⁻¹. -1 -1700cm -1The pyridine adsorption infrared spectrum of the sample after desorption at 200℃ was obtained. Based on the pyridine adsorption infrared spectrum at 1540 cm⁻¹... -1 and 1450cm -1 The intensity of the characteristic adsorption peaks yields the total adsorption capacity of the molecular sieve. The relative amounts of acid centers (B acid centers) and Lewis acid centers (L acid centers).

[0049] After adsorption, the temperature is raised to 350℃ and then evacuated to 10℃. -3 After desorption at Pa for 30 min, the measured acid amounts are strong Brønsted acid (B acid) and strong Lewis acid (L acid). The total B acid amount minus the strong B acid amount equals the weak B acid amount, and the total L acid amount minus the strong L acid amount equals the weak L acid amount.

[0050] The metal content of the method of the present invention was determined using the standard method Q / SH 3360-205.

[0051] In the following examples and comparative examples, the room temperature is 25°C.

[0052] Example 1

[0053] The crystallized ZSM-5 molecular sieve (produced by Sinopec Catalyst Co., Ltd. Qilu Branch, synthesized by template agent method, with a silicon-to-aluminum ratio n(SiO2) / n(Al2O3) = 27 and a sodium oxide content of 0.45% by mass) was calcined in air for 6 hours at a temperature of 550℃ to remove the template agent, thus obtaining the calcined molecular sieve. The calcined molecular sieve was then added to 10 times its mass of water and 0.5 times its mass of ammonium chloride for sodium exchange at a temperature of 60℃ for 2 hours. After the exchange, the mixture was filtered, and the resulting filter cake was subjected to the same exchange process once more. The mixture was then filtered, dried, and calcined at 500℃ for 2 hours to obtain H-ZSM-5 molecular sieve. A 0.2 mol / L oxalic acid solution was prepared and Nb(OH)5 (with a molar ratio of Nb to oxalic acid of 1:1) was dissolved by stirring at 80 °C to obtain an impregnation solution. This impregnation solution was then supersaturated to impregnate H-ZSM-5 molecular sieve (with a mass ratio of niobium to H-ZSM-5 molecular sieve of 0.5:100). After impregnation at room temperature for 24 h, the solution was placed in an oven and dried at 110 °C for 2 h. The sieve was then ground and placed in a muffle furnace, heated from room temperature to 550 °C over 2 h, and calcined at 550 °C for 3 h to obtain niobium-containing ZSM-5 molecular sieve. The theoretical niobium loading mass fraction (based on Nb) was 0.5% by mass, the Na2O content was 0.15% by mass, and the silicon-to-aluminum ratio (based on the SiO2 / Al2O3 molar ratio) was 27. Its acid properties are shown in Table 1. The prepared niobium-containing ZSM-5 molecular sieve was evaluated using microreactors, and the results are shown in Table 2.

[0054] Example 2

[0055] The crystallized ZSM-5 molecular sieve (same as in Example 1) was calcined in air for 6 hours at a temperature of 550°C to remove the template agent, resulting in a calcined molecular sieve. The calcined molecular sieve was then added to 10 times its mass of water and 0.5 times its mass of ammonium chloride for sodium exchange at a temperature of 60°C for 2 hours. After the exchange was completed, the sieve was filtered, and the resulting filter cake was used to repeat the exchange process once more. The sieve was then filtered, dried, and calcined to obtain H-ZSM-5 molecular sieve. An oxalic acid aqueous solution with a concentration of 0.2 mol / L was prepared and Nb(OH)₅ was dissolved by stirring at 80 °C, wherein the molar ratio of Nb to oxalic acid was 1:1. This solution was then supersaturated and impregnated with H-ZSM-5 molecular sieve, wherein the mass ratio of niobium to H-ZSM-5 molecular sieve was 1:100. After impregnation at room temperature for 24 h, the solution was placed in an oven and dried at 110 °C for 2 h. After grinding, the solution was placed in a muffle furnace and heated to 550 °C at room temperature for 2 h, and then calcined at that temperature for 3 h. The theoretical niobium loading mass fraction (based on Nb) was 1 wt%, the Na₂O content was 0.14 wt%, and the silicon-to-aluminum ratio was 27. The prepared molecular sieve was analyzed, characterized, and evaluated using microreactors. The results are shown in Tables 1 and 2.

[0056] Example 3

[0057] Prepared according to the method of Example 1, except that the Nb content in the obtained product is 0.8% by weight.

[0058] Example 4

[0059] Prepared according to the method of Example 1, except that the Nb content in the obtained product is 0.3% by weight.

[0060] Example 5

[0061] Prepared according to the method of Example 1, except that the Nb content in the obtained product is 2% by weight.

[0062] Comparative Example 1

[0063] After drying the crystallized ZSM-5 molecular sieve, it was calcined in air for 6 hours at 550℃ to remove the template agent. The molecular sieve was then added to 10 times its mass of water and 0.5 times its mass of ammonium chloride for sodium exchange at 60℃ for 2 hours. After the exchange, the mixture was filtered, and the resulting filter cake was used for the same exchange process once more. The mixture was then filtered, dried, and calcined to obtain HZSM-5 molecular sieve with a Na₂O content of 0.15% by mass and a silicon-to-aluminum ratio of 27. The prepared molecular sieve was then evaluated using microreactors.

[0064] Comparative Example 2

[0065] Modified ZSM-5 molecular sieves were prepared according to the method in Example 1, except that the mass ratio of Nb to the exchanged Y molecular sieve was 3:100.

[0066] Comparative Example 3

[0067] Modified ZSM-5 molecular sieves were prepared according to the method in Example 2, except that the concentration of oxalic acid solution was 0.5 mol / L.

[0068] Comparative Example 4

[0069] Modified ZSM-5 molecular sieves were prepared according to the method in Example 2, except that oxalic acid was not added.

[0070] Table 1 Total Acidity and Acid Distribution

[0071]

[0072] In Table 1, the molecular sieve designation is represented by HZSM-5-X%Nb, where X% represents the theoretical mass percentage of Nb. For example, in HZSM-5-0.5%Nb, 0.5% indicates that the theoretical Nb loading is 0.5 mass.

[0073] Table 2 Evaluation results of the reaction

[0074]

[0075] As shown in Table 2, the niobium-containing MFI molecular sieve provided by this invention, when used for naphtha conversion, results in a higher naphtha conversion rate, higher propylene yield from naphtha catalytic cracking, higher butene yield, and a higher total yield of low-carbon olefins. Preferably, it can have a higher propylene / ethylene ratio.

Claims

1. A niobium-containing MFI molecular sieve for improving the yield of high and low carbon olefins in naphtha catalytic cracking, wherein the silica-alumina ratio (SiO2 / Al2O3 molar ratio) is 10-50, and it contains 0.1-2% by mass of niobium, and compared with hydrogen-type MFI molecular sieves, its L... 强铌 -L 强氢 The concentration is preferably 20 μmol / g or higher. 弱铌 -L 弱氢 B is above 10 μmol / g. 强铌 -B 强氢 For example, 2 μmol / g or higher, B 弱铌 -B 弱氢 The concentration is preferably 1 μmol / g or higher, and more preferably 3 μmol / g or higher. Where L 强氢 The strong L acid content of hydrogen-form MFI molecular sieves, L 强铌 The strong L acid content of niobium-containing MFI molecular sieves, L 弱氢 The weak L acid content of the hydrogen-form MFI molecular sieve, L 弱铌 The weak L acid content of niobium-containing MFI molecular sieves, B 强氢 The strong Brønsted acid content of the hydrogen-form MFI molecular sieve, B 强铌 The strong Brønsted acid content of niobium-containing MFI molecular sieves, B 弱氢 The weak Brønsted acid content of the hydrogen-form MFI molecular sieve, B 弱铌 The amount of weak Brønsted acid in niobium-containing MFI molecular sieves; The acid content was measured using pyridine adsorption infrared spectroscopy, and the Brønsted acid content was measured at 1540 cm⁻¹. -1 The absorption peak at 1450 cm⁻¹ was measured, and the acid content of L acid was measured at 1450 cm⁻¹. -1 The absorption peaks were measured. The total acid content was measured after vacuum desorption at 200℃. The weak acid content was the difference between the total acid content and the acid content measured after vacuum desorption at 350℃. The strong acid content was measured after vacuum desorption at 350℃.

2. The MFI-structured molecular sieve for catalytic cracking of naphtha to achieve high and low carbon olefin yields according to claim 1, characterized in that, The niobium content, calculated as Nb, is, for example, 0.2–1% by mass, 0.3–0.8% by mass, 0.8–1.5% by mass, or 1.5–2% by mass; the sodium oxide content in the MFI structured molecular sieve used for naphtha catalytic cracking to increase the yield of high and low carbon olefins is preferably less than 0.2% by mass, and the sodium oxide content in the hydrogen-form MFI structured molecular sieve is preferably less than 0.2% by mass; in one embodiment, the niobium-containing MFI structured molecular sieve and the hydrogen-form MFI structured molecular sieve have the same silica-alumina ratio, and the ratio of their sodium oxide contents is 0.9–1.1 by mass; the MFI structured molecular sieve is a ZSM-5 molecular sieve.

3. The MFI-structured molecular sieve for catalytic cracking of naphtha to achieve high and low carbon olefin yields according to claim 1 or 2, characterized in that, The MFI-structured molecular sieve used for the catalytic cracking of naphtha to increase the yield of high and low carbon olefins: its weak L acid content (L 弱铌 The value is 20–70 μmol·g. -1 Strong L acid (L) 强铌 The value is 50–120 μmol·g. -1 The acidity of weak β-acids (B 弱铌 The value is 20–50 μmol·g. -1 For example, 25–35 μmol·g -1 Strong β-acid acidity (B 强铌 The value is 40–80 μmol·g -1 For example, 45–50 μmol·g -1 ; In one embodiment, the ratio of the amount of strong Brønsted acid to the amount of strong Lewis acid (B0) is... 强铌 / L 强铌 The ratio of weak Brønsted acid to weak Lewis acid (B) is 0.4–0.65:

1. 弱铌 / L 弱铌 The ratio of acid content to total acid content is: 0.4-0.85:1 or 0.85-1.25:1 or 0.43-0.75:

1.

4. A method for preparing MFI-structured molecular sieves for catalytic cracking of naphtha to achieve high and low carbon olefin yields, the method comprising: (1) Dissolve Nb(OH)5 in a low-concentration oxalic acid solution to obtain an impregnation solution, wherein the molar ratio of Nb to oxalic acid in the low-concentration oxalic acid solution is greater than 0.5:1 and less than 1.1:1, and the concentration of the low-concentration oxalic acid solution is 0.1 to 0.3 mol / L; (2) Impregnate the hydrogen-form MFI structured molecular sieve with the impregnation solution, wherein the sodium oxide content of the hydrogen-form MFI structured molecular sieve is less than 0.2% by mass; (3) Drying and roasting.

5. The method according to claim 4, characterized in that, In step (1), the molar ratio of Nb to oxalic acid in the oxalic acid solution is 0.7 to 1.05:

1.

6. The method according to claim 4, characterized in that, In step (2), the impregnation is carried out at room temperature for 15 to 35 hours, and the impregnation results in a niobium content of 0.1 to 2% by mass in the obtained molecular sieve; the impregnation is, for example, supersaturated impregnation.

7. The method according to claim 4, characterized in that, In step (3), the roasting temperature is 400-700℃ and the roasting time is 0.5-8 hours.

8. The method according to claim 4, characterized in that, The hydrogen-form MFI structured molecular sieve is obtained by the following method: a. After filtering out the mother liquor from the crystallized MFI structured molecular sieve, such as crystallized ZSM-5 molecular sieve, wash with water. The molecular sieve is obtained after being washed with water. After drying, the molecular sieve is optionally calcined in air to remove the template agent. b. The molecular sieve obtained in step a is washed with an ammonium salt solution to exchange sodium, so that the sodium oxide content does not exceed 0.2% by mass, thereby obtaining a hydrogen-form MFI structured molecular sieve. In one embodiment, the sodium washing and exchange in step b is carried out at room temperature to 60°C for 0.5 to 3 hours according to the mass ratio of molecular sieve: ammonium salt: H2O = 1:(0.1~1):(5~10), and then filtered. The ammonium salt can be a commonly used inorganic ammonium salt, for example, at least one selected from ammonium chloride, ammonium sulfate and ammonium nitrate. The sodium washing and exchange can be repeated 1 to 3 times to make the sodium oxide content in the molecular sieve less than 0.2% by mass.

9. The method according to claim 4, wherein, In step (3), the molecular sieve obtained after drying is heated to the calcination temperature and then calcined. The heating to the calcination temperature can be carried out from room temperature for 1 to 3 hours. The calcination temperature is preferably 450-600℃ and the calcination time is preferably 1 to 6 hours.

10. The application of the modified molecular sieve according to any one of claims 1 to 3 in naphtha catalytic cracking catalyst.

11. The application of the catalytic cracking catalyst of claim 10 in naphtha catalytic cracking.

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