阅读说明：本技术 一种多产异构烃的高稳定性改性y型分子筛及其制备方法 (High-stability modified Y-type molecular sieve for producing more isomeric hydrocarbon and preparation method thereof ) 是由 袁帅 周灵萍 田辉平 陈振宇 张蔚琳 沙昊 于 2018-06-29 设计创作，主要内容包括：一种多产异构烃的高稳定性的改性Y型分子筛及其制备方法,该改性Y型分子筛的MgO含量为0.5～4.5重量％,Na<Sub>2</Sub>O含量为0.1～0.5重量％,总孔体积为0.33～0.39mL/g,2～100nm的二级孔的孔体积占总孔体积的10～25％,晶胞常数为2.440～2.455nm,非骨架铝含量占总铝含量比例不高于20％,晶格崩塌温度不低于1040℃,用吡啶吸附红外法在200℃时测定的B酸量与L酸量的比值不低于2.30；该分子筛紫外可见吸收光谱285nm～295nm处没有吸收峰。该分子筛的制备方法包括离子交换、在一定的温度和水蒸汽条件下改性处理以及与四氯化硅反应的步骤。该改性Y型分子筛,具有更高的重油转化活性和较低的焦炭选择性,具有更高的汽油收率、液化气收率,且汽油中具有更高的异构烃含量。(The modified Y-shaped molecular sieve with high stability for producing more isomeric hydrocarbon and the preparation method thereof, the MgO content of the modified Y-shaped molecular sieve is 0.5-4.5 wt%, Na 2 The content of O is 0.1-0.5 wt%, the total pore volume is 0.33-0.39 mL/g, the pore volume of secondary pores with the diameter of 2-100 nm accounts for 10-25% of the total pore volume, the unit cell constant is 2.440-2.455 nm, the proportion of non-framework aluminum content in the total aluminum content is not higher than 20%, the lattice collapse temperature is not lower than 1040 ℃, and the ratio of the B acid content to the L acid content determined by a pyridine adsorption infrared method at 200 ℃ is not lower than 2.30; the molecular sieve has no absorption peak at 285 nm-295 nm of ultraviolet visible absorption spectrum. The preparation method of the molecular sieve comprises ion exchange, at a certain temperature andmodification treatment under the condition of water vapor and reaction with silicon tetrachloride. The modified Y-type molecular sieve has higher heavy oil conversion activity, lower coke selectivity, higher gasoline yield and liquefied gas yield, and higher content of isomeric hydrocarbon in gasoline.)

1. A modified Y-type molecular sieve is characterized in that the magnesium oxide content of the modified Y-type molecular sieve is 0.5-4.5 wt%, the sodium oxide content is 0.1-0.5 wt%, the total pore volume is 0.33-0.39 mL/g, the pore volume of secondary pores with the pore diameter of 2-100 nm accounts for 10-25% of the total pore volume, the unit cell constant is 2.440-2.455 nm, the non-framework aluminum content of the modified Y-type molecular sieve accounts for not more than 20% of the total aluminum content, the lattice collapse temperature is not lower than 1040 ℃, and the ratio of the B acid amount to the L acid amount in the total acid amount of the modified Y-type molecular sieve determined by a pyridine adsorption infrared method at 200 ℃ is not lower than 2.30; when the molecular sieve is used for ultraviolet and visible light analysis, no absorption peak exists at the position of 285 nm-295 nm of an ultraviolet and visible absorption spectrum.

2. The modified Y-type molecular sieve of claim 1, wherein the modified Y-type molecular sieve has secondary pores with a pore size of 2nm to 100nm, the pore volume percentage of which is 15% to 21% of the total pore volume.

3. The modified Y-type molecular sieve of claim 1, wherein the non-framework aluminum content of the modified Y-type molecular sieve is 13-19% of the total aluminum content, and the framework silica-alumina ratio is SiO₂/Al₂O₃The molar ratio is 7.3-14.

4. The modified Y-type molecular sieve of claim 1, wherein the modified Y-type molecular sieve has a lattice collapse temperature of 1045 ℃ to 1080 ℃.

5. The modified Y-type molecular sieve of claim 1, wherein the ratio of the amount of B acid to the amount of L acid in the total acid amount of the modified Y-type molecular sieve measured at 200 ℃ by pyridine adsorption infrared is 2.4 to 4.2.

6. The modified Y-type molecular sieve of claim 1, wherein the modified Y-type molecular sieve has a relative crystal retention of 33% or more, for example, 33 to 45%, after aging at 800 ℃ under normal pressure in a 100% steam atmosphere for 17 hours.

7. The modified Y-type molecular sieve of claim 1, wherein the modified Y-type molecular sieve has a relative crystallinity of 58 to 70%.

8. The modified Y-type molecular sieve of any one of claims 1 to 7, wherein the modified Y-type molecular sieve has a magnesium oxide content of 0.6 to 4.3 wt%, a sodium oxide content of 0.2 to 0.5 wt%, a unit cell constant of 2.442 to 2.450nm, and a framework silicon-aluminum ratio of 8.4 to 12.6.

9. A preparation method of a modified Y-type molecular sieve comprises the following steps:

(1) contacting the NaY molecular sieve with a soluble magnesium salt solution to perform an ion exchange reaction, filtering, washing, and optionally drying to obtain a Y-type molecular sieve with a conventional unit cell size and reduced sodium oxide content and containing magnesium;

(2) roasting the Y-type molecular sieve with the reduced sodium oxide content and the conventional unit cell size containing magnesium at the temperature of 350-480 ℃ for 4.5-7 hours in the atmosphere of 30-90 vol% of water vapor, and optionally drying to obtain the Y-type molecular sieve with the reduced unit cell constant;

(3) according to SiCl₄: the Y-type molecular sieve with reduced unit cell constant is 0.1-0.7: 1, carrying out contact reaction on the Y-shaped molecular sieve with the reduced unit cell constant and silicon tetrachloride gas at the reaction temperature of 200-650 ℃ for 10 minutes to 5 hours, washing and filtering to obtain the modified Y-shaped molecular sieve.

10. The process of claim 9, wherein the reduced sodium oxide content magnesium-containing conventional unit cell size Y-type molecular sieve of step (1) has a unit cell constant of 2.465 to 2.472nm and a sodium oxide content of no more than 8.8 wt.%.

11. The process of claim 9, wherein in step (1), the magnesium-containing conventional unit cell size Y-type molecular sieve having a reduced sodium oxide content has a magnesium content of 0.6 to 5.5 wt.% as MgO, a sodium oxide content of 4 to 8.8 wt.%, such as 5.5 to 8.5 wt.%, and a unit cell constant of 2.465nm to 2.472 nm.

12. The method of claim 9, wherein step (1) comprises adding N to said mixtureContacting the aY molecular sieve with a soluble magnesium salt solution to perform an ion exchange reaction according to the following steps of: soluble magnesium salt: h₂O is 1: 0.005-0.19: 5-15, mixing NaY molecular sieve, soluble magnesium salt and water, and stirring.

13. The method of claim 10 or 13, wherein the step (1) of contacting the NaY molecular sieve with a solution of soluble magnesium salt for an ion exchange reaction comprises: mixing NaY molecular sieve with water, adding magnesium salt and/or magnesium salt solution under stirring for ion exchange reaction, filtering and washing; the conditions of the ion exchange reaction are as follows: the exchange temperature is 15-95 ℃, the exchange time is 30-120 minutes, and the soluble magnesium salt solution is a soluble magnesium salt aqueous solution; the soluble magnesium salt is preferably magnesium chloride and/or magnesium nitrate.

14. The method of claim 9, wherein the roasting temperature in the step (2) is 380-460 ℃, the roasting atmosphere is 40-80% water vapor atmosphere, and the roasting time is 5-6 hours.

15. The method of claim 9, wherein the unit cell constant of the Y-type molecular sieve with reduced unit cell constant obtained in step (2) is 2.450nm to 2.462nm, and the water content of the Y-type molecular sieve with reduced unit cell constant is not more than 1 wt%.

16. The method of claim 9, wherein the washing method in step (3) is washing with water under the washing conditions that the molecular sieve: h₂O is 1: 6-15, the pH value is 2.5-5.0, and the washing temperature is 30-60 ℃.

Technical Field

The invention relates to a high-stability modified Y-type molecular sieve for producing more isomeric hydrocarbon and a preparation method thereof.

Background

At present, the industrial preparation of the high-silicon Y-type zeolite mainly adopts a hydrothermal method, and the NaY zeolite is subjected to rare earth ion exchange for many times and high-temperature roasting for many times, so that the rare earth-containing high-silicon Y-type zeolite can be prepared, which is the most conventional method for preparing the high-silicon Y-type zeolite, but the hydrothermal method for preparing the rare earth high-silicon Y-type zeolite has the defects that: because the structure of the zeolite can be damaged by too harsh hydrothermal treatment conditions, the Y-type zeolite with high silica-alumina ratio can not be obtained; although the generation of extra-framework aluminum is beneficial to improving the stability of the zeolite and forming new acid centers, the excessive extra-framework aluminum reduces the selectivity of the zeolite, and in addition, a plurality of dealumination cavities in the zeolite cannot be timely supplemented by silicon migrated from the framework, so that the lattice defect of the zeolite is often caused, and the crystal retention of the zeolite is low. And because the conventional Y molecular sieve only contains rare earth, silicon, aluminum and other elements, the adjustment of the structure and the performance of the conventional Y molecular sieve is limited in a certain range, and the composition of a product is often stabilized in a certain range. Therefore, the thermal and hydrothermal stability of the rare earth-containing high-silicon Y-type zeolite prepared by the hydrothermal method is poor, which is shown in that the lattice collapse temperature is low, the crystallinity retention rate and the specific surface area retention rate are low after hydrothermal aging, and the selectivity is poor. Moreover, the content of the isomeric hydrocarbon in the gasoline produced by the catalyst prepared in the conventional Y molecular sieve is stable in a certain range and is difficult to increase. This limits the improvement of the quality of the catalytically cracked gasoline and reduces the competitiveness of the catalytically cracked gasoline product.

In U.S. Pat. Nos. 4,849,287 and 4,4429053, NaY zeolite is exchanged with rare earth ions and then treated with steam, the aluminum removal of the zeolite is difficult in the steam treatment process, the unit cell parameters of the zeolite before the steam treatment are increased to 2.465-2.475 nm, the unit cell parameters after the treatment are 2.420-2.464 nm, and the temperature required for reducing the unit cell parameters is higher (593-733 ℃). The heavy oil cracking activity of zeolite is not high and coke selectivity is not good.

In the processes provided in US5340957 and US5206194, SiO of NaY zeolite is used as the starting material₂/Al₂O₃The ratio is 6.0, and this method also has the disadvantages of the aforementioned U.S. Pat. Nos. 4,84287 and 4429053, in which NaY is subjected to rare earth exchange and then to hydrothermal treatment.

Gas phase chemical processes are another important process for preparing high silica zeolites first reported by Beyer and Mankui in 1980. The gas phase chemical method generally adopts SiCl under the protection of nitrogen₄Reacting with anhydrous NaY zeolite at a certain temperature. U.S. Pat. Nos. 4,42737,178, U.S. Pat. No. 4,4438178, Chinese patent Nos. CN1382525A, CN1194941A and CN1683244A disclose the use of SiCl₄A process for preparing ultra-stable Y-type zeolite by gas-phase chemical dealumination. The pore structure analysis shows that the gas phase super stable molecular sieve has almost no secondary pores.

Zhuhuayuan (Petroleum institute, 2001, 17(6):6-10) et al proposed the effect of magnesium-containing modified molecular sieve on the performance of FCC catalyst. Researches find that the FCC catalyst containing the Mg and Ca molecular sieves has strong heavy oil conversion capability, high hydrogen transfer reaction activity and higher isobutane product content. However, the Y molecular sieve prepared by the method has poor thermal and hydrothermal stability, and unsatisfactory reactivity and selectivity, can only increase the content of isobutane, and cannot effectively increase the content of isomeric hydrocarbon in gasoline.

Disclosure of Invention

One of the technical problems to be solved by the invention is to provide a Y-type molecular sieve (also called Y-type zeolite) which is suitable for heavy oil catalytic cracking processing and produces more isomeric hydrocarbons and has high stability. The second technical problem to be solved by the invention is to provide a preparation method of the modified Y-type molecular sieve.

The invention provides a modified Y-type molecular sieve, wherein the modified molecular sieve has the magnesium oxide content of 0.5-4.5 wt%, the sodium oxide content of 0.1-0.5 wt%, the total pore volume of 0.33-0.39 mL/g, and the pore volume of secondary pores with the pore diameter of 2-100 nm accounts for 10% of the total pore volume of the modified Y-type molecular sieve25%, unit cell constant of 2.440-2.455 nm, and framework silicon-aluminum ratio (SiO)₂/Al₂O₃Molar ratio) is: 7.3-14.0, the percentage of non-framework aluminum content in the molecular sieve to the total aluminum content is not higher than 20%, the lattice collapse temperature is not lower than 1040 ℃, and the ratio of the B acid amount to the L acid amount in the total acid amount of the modified Y-type molecular sieve measured by a pyridine adsorption infrared method at 200 ℃ is not lower than 2.30.

The modified Y-type molecular sieve provided by the invention has a lattice collapse temperature (also called structure collapse temperature) of not less than 1040 ℃, preferably, the lattice collapse temperature of the molecular sieve is 1045-1080 ℃, for example, 1047-1058 ℃, 1057-1075 ℃ or 1047-1065 ℃.

The ratio of the amount of B acid to the amount of L acid in the total acid amount of the modified Y-type molecular sieve, which is measured at 200 ℃ by using a pyridine adsorption infrared method, is preferably 2.4-4.2, for example, 2.4-3.5.

The unit cell constant of the modified Y-type molecular sieve provided by the invention is 2.440-2.455 nm, such as 2.442-2.452 nm.

The modified Y-type molecular sieve provided by the invention is a high-silicon Y-type molecular sieve, and the framework silicon-aluminum ratio (SiO) of the high-silicon Y-type molecular sieve₂/Al₂O₃Molar ratio) of 7.3 to 14.0, for example: 8.4-12.6.

According to the modified Y-type molecular sieve provided by the invention, the percentage of non-framework aluminum content in the molecular sieve in the total aluminum content is not higher than 20%, for example, 13-19 wt%.

The modified Y-type molecular sieve provided by the invention has a crystal retention of 33% or more, for example, 33-48% or 33-45% or 36-40% or 39-45% after aging for 17 hours at 800 ℃ under normal pressure and in a 100 volume% steam atmosphere. The normal pressure is 1 atm.

The relative crystallinity of the modified Y-type molecular sieve provided by the invention is not less than 58%, preferably, the relative crystallinity of the modified Y-type molecular sieve provided by the invention is 58-70%, for example, 59-68% or 59-64%.

The invention provides a modified Y-type molecular sieve, one embodiment of which has a specific surface area of 620-670 m²The/g is, for example, 630 to 660m²/g。

The modified Y-type molecular sieve provided by the invention has the preferable total pore volume of 0.35-0.39 mL/g, such as 0.355-0.375 mL/g.

The modified Y-type molecular sieve provided by the invention has the pore volume of secondary pores with the pore diameter (diameter) of 2.0-100 nm accounting for 10-25% of the total pore volume, and preferably 15-21%.

In one embodiment, the modified Y-type molecular sieve provided by the invention has a micropore volume of 0.25-0.35 mL/g, for example, 0.26-0.32 mL/g.

The modified Y-type molecular sieve contains magnesium element, and the magnesium content in the modified Y-type molecular sieve calculated by MgO can be 0.8-4.5 wt%, preferably 1-4.5 wt%, for example 1.2-4.3 wt%.

The ultraviolet visible light absorption spectrum of the modified Y-type molecular sieve provided by the invention has no absorption peak at the wavelength of 285 nm-295 nm.

The modified Y-type molecular sieve provided by the invention has the sodium oxide content of not more than 0.5 wt%, and can be 0.15-0.5 wt%, such as 0.30-0.5 wt% or 0.35-0.48 wt%.

The invention provides a preparation method of a modified Y-type molecular sieve, which comprises the following steps:

(1) contacting the NaY molecular sieve with a soluble magnesium salt solution to perform an ion exchange reaction, filtering and washing to obtain a Y-type molecular sieve with a conventional unit cell size containing magnesium and with reduced sodium oxide content; wherein the soluble magnesium salt solution is also called magnesium salt solution;

(2) modifying the Y-type molecular sieve with the reduced sodium oxide content and the conventional magnesium-containing unit cell size, and optionally drying to obtain the Y-type molecular sieve with the reduced unit cell constant, wherein the modifying treatment is to roast the Y-type molecular sieve with the reduced sodium oxide content and the conventional magnesium-containing unit cell size at the temperature of 350-480 ℃ in an atmosphere containing 30-90 vol% of water vapor (also called 30-90 vol% of water vapor atmosphere or 30-90 vol% of water vapor) for 4.5-7 hours;

(3) mixing the Y-type molecular sieve sample with SiCl, wherein the unit cell constant is reduced₄Gas at a temperature of 200 deg.CContact reaction at 650 deg.C, wherein SiCl₄: the weight ratio of the Y-type molecular sieve with reduced unit cell constant obtained in the step (2) on a dry basis is 0.1-0.7: 1, reacting for 10 minutes to 5 hours, and then washing and filtering to obtain the modified Y-type molecular sieve. Wherein the water content of the Y-type molecular sieve sample with reduced unit cell constant is preferably not more than 1 wt%; if the water content in the Y-type molecular sieve sample obtained by modification treatment in the step (2) (in the Y-type molecular sieve sample obtained by roasting) is not more than 1 wt%, the Y-type molecular sieve sample can be directly used for contacting silicon tetrachloride to carry out the reaction, and if the water content in the Y-type molecular sieve sample obtained by roasting in the step (2) exceeds 1 wt%, the Y-type molecular sieve sample with the reduced unit cell constant obtained by roasting in the step (2) is dried to enable the water content to be lower than 1 wt%.

The modified Y-type molecular sieve provided by the invention has high thermal and hydrothermal stability, is used for heavy oil catalytic cracking, has lower coke selectivity, higher gasoline yield and liquefied gas yield compared with the conventional Y-type molecular sieve, has higher content of isomeric hydrocarbon in gasoline, and increases monomethyl isomeric hydrocarbon in gasoline.

In the present invention, the isoparaffin refers to a chain isoparaffin and a chain isoolefin. The term "bis-methyl isomeric hydrocarbons" means that the carbon chain contains two methyl branches, and the term "mono-methyl isomeric hydrocarbons" means that the carbon chain contains one methyl branch.

The preparation method of the magnesium modified Y-shaped molecular sieve can prepare the high-silicon Y-shaped molecular sieve with high crystallinity, high thermal stability and high hydrothermal stability and a certain secondary pore structure, the magnesium-containing molecular sieve has uniform aluminum distribution and less non-framework aluminum content, the modified Y-shaped molecular sieve is used for heavy oil conversion, the coke selectivity is good, the heavy oil cracking activity is high, the gasoline yield, the liquefied gas yield and the total liquid yield of the molecular sieve used for heavy oil conversion can be improved, and the content of isomeric hydrocarbon (mainly monomethyl isomeric hydrocarbon) in gasoline is increased. Increasing the content of isomeric hydrocarbons (mainly monomethyl isomeric hydrocarbons) is beneficial for improving gasoline quality, e.g., maintaining gasoline octane while reducing the content of olefins or aromatics.

The modified Y-type molecular sieve provided by the invention can be used as an active component of a catalytic cracking catalyst and used for converting heavy oil or poor oil; the catalytic cracking catalyst with the molecular sieve as the active component has the advantages of strong heavy oil conversion capacity, high stability, good coke selectivity, high light oil yield and high gasoline yield, and the gasoline has high content of isomeric hydrocarbon, particularly methyl isomeric hydrocarbon. The modified Y-type molecular sieve provided by the invention can also be used as a carrier of a lubricating oil isomerization pour point depressing catalyst and used in the isomerization process of lubricating oil, and the isomerization pour point depressing catalyst taking the molecular sieve as the carrier has stronger isomerization performance and higher stability.

Detailed Description

The modified Y-type molecular sieve provided by the invention has an embodiment that the magnesium oxide content is 0.5-4.5 wt%, preferably 0.8-4.3 wt%, the sodium oxide content is 0.1-0.5 wt%, for example 0.2-0.48 wt%, the total pore volume is 0.33-0.39 mL/g, the percentage of the pore volume of secondary pores with the pore diameter of 2-100 nm in the total pore volume is 10-25%, preferably 15-21%, the unit cell constant is 2.440-2.455 nm, and the framework silicon-aluminum ratio (SiO is)₂/Al₂O₃Molar ratio) is: 7.3 to 14.0, for example, 8.5 to 12.6, the percentage of non-framework aluminum content in the molecular sieve to the total aluminum content is not higher than 20%, preferably 13 to 19, the relative crystallinity is not lower than 60%, the lattice collapse temperature is 1040 to 1080 ℃, for example, 1045 to 1060 ℃, and the ratio of the B acid amount to the L acid amount in the total acid amount of the modified Y-type molecular sieve measured by a pyridine adsorption infrared method at 200 ℃ is not lower than 2.30, preferably 2.4 to 4.2.

The preparation process of the modified Y-type molecular sieve comprises the step of contacting the Y-type molecular sieve with silicon tetrachloride to carry out dealuminization and silicon supplementation reaction.

In the preparation method of the modified Y-type molecular sieve, in the step (1), the NaY molecular sieve and a soluble magnesium salt solution are subjected to ion exchange reaction to obtain the Y-type molecular sieve with the conventional unit cell size of magnesium and with the content of sodium oxide reduced. The NaY molecular sieve can be purchased commercially or prepared according to the existing method, and in one embodiment, the unit cell constant of the NaY molecular sieve is 2.465-2.472 nm, and the framework silicon-aluminum ratio (SiO)₂/Al₂O₃Molar ratio) of 4.5 to 5.2, a relative crystallinity of 85% or more, for example, 85 to 95%, and a sodium oxide content of 13.0 to 13.8% by weight. The NaY molecular sieve and the soluble magnesium salt solution are subjected to ion exchange reaction, the exchange temperature is preferably 15-95 ℃, for example 65-95 ℃, and the exchange time is preferably 30-120 minutes, for example 45-90 minutes. NaY molecular sieve (dry basis), magnesium salt (MgO basis), H₂The weight ratio of O to O is 1:0.005 to 0.19:5 to 15, for example, 1: 0.01-0.15: 6-12 weight ratio or 1: 0.05-0.12: 6 to 12 weight ratio. The soluble magnesium salt solution is a soluble magnesium salt aqueous solution. In one embodiment, the ion exchange reaction of the NaY molecular sieve and the magnesium salt solution comprises the following steps of mixing the NaY molecular sieve, the magnesium salt and H₂Forming a mixture of NaY molecular sieve (also called NaY zeolite), magnesium salt and water in a weight ratio of 1: 0.005-0.19: 5-15, and stirring at 15-95 ℃, for example, 65-95 ℃, preferably for 30-120 minutes to exchange magnesium ions and sodium ions, wherein the water is decationized water, deionized water or a mixture thereof. The method comprises the steps of mixing NaY molecular sieve, magnesium salt and water to form a mixture, forming slurry by the NaY molecular sieve and the water, and then adding an aqueous solution of magnesium salt and/or magnesium salt into the slurry, wherein the magnesium salt is soluble magnesium salt, and the magnesium salt can be one or more of magnesium chloride, magnesium nitrate and magnesium sulfate, preferably magnesium chloride and/or magnesium nitrate. The washing in step (1) is intended to wash out exchanged sodium ions, and for example, deionized water or decationized water may be used for washing. Preferably, the magnesium content of the magnesium-containing Y-type molecular sieve with conventional unit cell size and reduced sodium oxide content obtained in step (1) is 0.5-4.5 wt% such as 0.7-4.5 wt% or 0.55-4.5 wt% calculated on MgO, the sodium oxide content is not more than 9 wt% such as 5.5-8.5 wt% or 5.5-7.5 wt%, and the unit cell constant is 2.465 nm-2.472 nm.

In the preparation method of the modified Y-type molecular sieve, in the step (2), the Y-type molecular sieve containing magnesium and having a conventional unit cell size is roasted for 4.5-7 hours at the temperature of 350-480 ℃ in a 30-90 vol% steam atmosphere, preferably, the roasting temperature in the step (2) is 380-460 ℃, the roasting atmosphere is 40-80 vol% steam atmosphere, and the roasting time is 5-6 hours. The water vapor atmosphere contains 30-90% by volume, preferably 40-80% by volume of water vapor, and also contains other gases, such as one or more of air, helium or nitrogen. The Y-type molecular sieve with the reduced unit cell constant in the step (2) has the unit cell constant of 2.450 nm-2.462 nm. Preferably, the calcined molecular sieve is also dried in step (2) so that the water content in the Y-type molecular sieve having a reduced unit cell constant is preferably not more than 1 wt%.

In the preparation method of the modified Y-type molecular sieve, SiCl is adopted in the step (3)₄: the weight ratio of the Y-type zeolite (on a dry basis) is preferably 0.3-0.6: 1, the reaction temperature is preferably 350-500 ℃, and the washing method in the step (3) can adopt a conventional washing method, and can be washed by water, such as decationized water or deionized water, so as to remove Na remained in the zeolite⁺，Cl^-And Al³⁺Etc. soluble by-products, for example the washing conditions may be: the weight ratio of the washing water to the molecular sieve can be 5-20: 1, typically molecular sieve: h₂The weight ratio of O is 1: 6-15, the pH value is preferably 2.5-5.0, and the washing temperature is 30-60 ℃. Preferably, the washing is performed such that no free Na is detected in the washing solution after washing⁺，Cl^-And Al³⁺Plasma, Na in the washing liquid after washing in general⁺，Cl^-And Al³⁺The respective contents of ions do not exceed 0.05 wt.%.

The preparation method of the modified Y-type molecular sieve provided by the invention comprises the following steps:

(1) carrying out ion exchange reaction on a NaY molecular sieve (also called NaY zeolite) and a soluble magnesium salt solution, filtering and washing to obtain a Y-type molecular sieve with reduced sodium oxide content and containing magnesium and with a conventional unit cell size; the ion exchange is carried out for 30-120 minutes under the conditions of stirring and the temperature of 15-95 ℃, preferably 65-95 ℃;

(2) roasting the rare earth-containing Y-type molecular sieve with the normal unit cell size and the reduced sodium oxide content for 4.5-7 hours at the temperature of 350-480 ℃ in the atmosphere containing 30-90 vol% of water vapor, and drying to obtain the Y-type molecular sieve with the reduced unit cell constant and the water content of less than 1 wt%; the unit cell constant of the Y-type molecular sieve with the reduced unit cell constant is 2.450 nm-2.462 nm;

(3) mixing the Y-type molecular sieve with water content lower than 1 wt% and SiCl vaporized by heating₄Gas contact of SiCl₄: the weight ratio of the Y-type molecular sieve with the water content lower than 1 wt% and the reduced unit cell constant (calculated by dry basis) is 0.1-0.7: 1, carrying out contact reaction for 10 minutes to 5 hours at the temperature of 200-650 ℃, and washing and filtering to obtain the modified Y-type molecular sieve provided by the invention.

The following examples further illustrate the invention but are not intended to limit the invention thereto.