Modified molecular sieve catalyst, preparation method thereof and method for preparing low-carbon olefin by co-feeding methanol and C4 alkane

Abstract

The invention belongs to the technical field of low-carbon olefin preparation, and relates to a modified molecular sieve catalyst and a preparation method thereof, and a method for preparing low-carbon olefin by co-feeding methanol and C4 alkane. The modified molecular sieve catalyst comprises a molecular sieve catalyst as well as a phosphorus element, a lanthanum element and a zinc element which are loaded on the molecular sieve catalyst. The preparation method comprises the following steps: (1) loading a solution of a phosphorus oxide precursor on a molecular sieve catalyst, and carrying out first drying and roasting to obtain a phosphorus modified molecular sieve catalyst; and (2) loading a mixed solution of a lanthanum oxide precursor and a zinc oxide precursor on the phosphorus-modified molecular sieve catalyst obtained in the step (1), and carrying out secondary drying and roasting to obtain the modified molecular sieve catalyst. The method has the following advantages that the phosphorus-zinc-lanthanum modified molecular sieve is adopted as the catalyst, and compared with an unmodified molecular sieve, the yield of low-carbon olefins is obviously improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparing light olefins, and in particular, relates to a modified molecular sieve catalyst and a preparation method thereof, and a method for preparing light olefins by co-feeding methanol and C4 alkane. Background Art

[0002] Low-carbon olefins include ethylene, propylene and butene, which are an important class of organic chemicals and are widely used in the fields of chemical industry, medicine, materials, etc. The industrial production of low-carbon olefins mainly relies on tubular furnace steam cracking. The reaction needs to be carried out at above 800°C, which consumes huge energy and has a high methane yield. Catalytic cracking to produce olefins can overcome the above shortcomings. Methanol and butane are common chemical raw materials. Methanol mainly comes from coal chemical industry, and the current production capacity is in excess. C4 alkanes come from by-products of natural gas processing and petroleum processing and liquefied petroleum gas. The current utilization rate is low and they are mainly used as fuel. Methanol to olefins is an exothermic reaction, while C4 alkane to olefins is an endothermic reaction. If methanol and C4 alkane are co-fed to produce low-carbon olefins, thermal coupling can be achieved, and even the reaction can be made thermally neutral. In addition to thermal coupling, the two reaction processes will also be coupled, and the effect of promoting the production of low-carbon olefins can be achieved under suitable catalytic systems and reaction conditions.

[0003] Nowak et al. (Appl. Catal. A 1989, 50:149) achieved thermal coupling by co-feeding methanol and C4 alkane. When the reaction temperature was 600-700°C and the molecular molar ratio of methanol to n-butane was 3:1, the reaction reached thermal neutrality on HZSM-5 molecular sieve.

[0004] The journal CHEM ENG J (2010, 160: 761) reported the methanol-n-butane coupling process to olefins, using ZSM-5 molecular sieve as catalyst, the molar ratio of methanol to n-butane was 3:1, the reaction temperature was 500-550℃, and the space velocity was 0.1-0.2h -1 The propylene yield obtained by the reaction is higher than when the two raw materials are fed separately.

[0005] CN102531821B provides a method for producing low-carbon olefins by catalytic cracking of methanol-coupled naphtha, using a modified ZSM-5 molecular sieve, which includes 25wt%-80wt% of ZSM-5 molecular sieve, 15wt%-70wt% of binder, 2.6wt.%-6.0wt% of lanthanum and 1.0wt%-2.8wt% of phosphorus in terms of weight percentage. The ethylene yield is 10%-25%, the propylene yield is 15%-28%, the butene yield is 5%-15%, and the BTX (benzene, toluene and xylene) yield is 4%-20%.

[0006] CN102050691A provides a method for preparing low-carbon olefins by coupling C4 hydrocarbons with methanol, wherein phosphorus-containing solution, preferably phosphorus-containing ammonium salt aqueous solution, is used to modify HZSM molecular sieve with phosphorus. Under the conditions of a methanol to C4 hydrocarbon molar ratio of 0.5-5:1, a temperature of 350-650°C, and a reaction pressure of 0.01-0.2Mpa, the propylene yield can reach up to 44.0%. However, this patent mainly couples C4 olefins with methanol, and when the main raw material is C4 alkane, the propylene yield is relatively low.

[0007] Researching and developing efficient catalysts for the reaction process of methanol-coupled catalytic cracking of C4 alkanes is of great practical significance for improving the utilization rate of methanol and C4 alkanes. It also increases the yield of low-carbon olefins and reduces energy consumption in the reaction process, which is beneficial for petrochemical and oil companies to improve their economic benefits. Summary of the invention

[0008] The main technical problem solved by the present invention is to provide a method for preparing light olefins from methanol and C4 alkane by using phosphorus zinc lanthanum modified molecular sieve as a catalyst. The method can produce light olefins with low energy consumption and make full use of abundant C4 alkane raw materials through the selection of catalyst. The method converts C4 alkane with lower economic value into light olefins with higher economic value, and improves the yield of light olefins compared with feeding C4 alkane alone.

[0009] In order to achieve the purpose of the present invention, the first aspect of the present invention provides a modified molecular sieve catalyst, which comprises: a molecular sieve catalyst and phosphorus, lanthanum and zinc elements carried thereon.

[0010] The second aspect of the present invention provides a method for preparing a modified molecular sieve catalyst, comprising the following steps:

[0011] (1) loading a solution of a phosphorus oxide precursor onto a molecular sieve catalyst, performing a first drying and calcining to obtain a phosphorus-modified molecular sieve catalyst;

[0012] (2) Loading the mixed solution of the lanthanum oxide precursor and the zinc oxide precursor on the phosphorus-modified molecular sieve catalyst obtained in step (1), performing a second drying and calcining to obtain the modified molecular sieve catalyst.

[0013] The third aspect of the present invention provides a modified molecular sieve catalyst prepared by the preparation method.

[0014] The fourth aspect of the present invention provides a method for preparing light olefins by co-feeding methanol and C4 alkane, so that methanol, C4 alkane and the modified molecular sieve catalyst are contacted and reacted to obtain the light olefins.

[0015] The present invention has the following advantages:

[0016] (1) The present invention uses phosphorus zinc lanthanum modified molecular sieve as a catalyst, and compared with the unmodified molecular sieve, the yield of light olefins is significantly improved.

[0017] (2) The molecular sieve catalyst used in the present invention can produce light olefins with low energy consumption and make full use of the abundant C4 alkane raw materials, which is conducive to the industrial production of light olefins by catalytic cracking.

[0018] Other features and advantages of the present invention will be described in detail in the following detailed description. DETAILED DESCRIPTION

[0019] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0020] In order to achieve the purpose of the present invention, the first aspect of the present invention provides a modified molecular sieve catalyst, which comprises: a molecular sieve catalyst and phosphorus, lanthanum and zinc elements carried thereon.

[0021] According to the present invention, preferably, in the modified molecular sieve catalyst, the mass ratio of phosphorus element to molecular sieve catalyst is 0.01-5:100, preferably 0.5-2:100, the mass ratio of zinc element to molecular sieve catalyst is 0.01-8:100, preferably 1-3:100, and the mass ratio of lanthanum element to molecular sieve catalyst is 0.01-2:100, preferably 0.2-1.3:100.

[0022] According to the present invention, preferably, the silicon-aluminum ratio of the molecular sieve catalyst is 15-250.

[0023] Further preferably, the molecular sieve catalyst is selected from at least one of HZSM-5, HZSM-11, HBeta and HY.

[0024] In the present invention, the molecular sieve catalyst is any one of HZSM-5, HZSM-11, HBeta, and HY, or a eutectic or physical mixture of several of them.

[0025] The second aspect of the present invention provides a method for preparing a modified molecular sieve catalyst, comprising the following steps:

[0026] (1) loading a solution of a phosphorus oxide precursor onto a molecular sieve catalyst, performing a first drying and calcining to obtain a phosphorus-modified molecular sieve catalyst;

[0027] (2) Loading the mixed solution of the lanthanum oxide precursor and the zinc oxide precursor on the phosphorus-modified molecular sieve catalyst obtained in step (1), performing a second drying and calcining to obtain the modified molecular sieve catalyst.

[0028] According to the present invention, preferably, in step (1) and step (2), the loading method is independently an ion exchange method or an equal volume impregnation method.

[0029] According to the present invention, preferably, the concentration of the solution of the phosphorus oxide precursor is such that the mass ratio of phosphorus element to molecular sieve catalyst in the modified molecular sieve catalyst is 0.01-5:100, preferably 0.5-2:100.

[0030] According to the present invention, preferably, the concentration of the solution of the zinc oxide precursor is such that the mass ratio of the zinc element to the molecular sieve catalyst in the modified molecular sieve catalyst is 0.01-8:100, preferably 1-3:100.

[0031] According to the present invention, preferably, the concentration of the solution of the lanthanum oxide precursor is such that the mass ratio of the lanthanum element to the molecular sieve catalyst in the modified molecular sieve catalyst is 0.01-2:100, preferably 0.2-1.3:100.

[0032] According to the present invention, preferably, the silicon-aluminum ratio of the molecular sieve catalyst is 15-250.

[0033] Further preferably, the molecular sieve catalyst is selected from at least one of HZSM-5, HZSM-11, HBeta and HY.

[0034] According to the present invention, preferably, the phosphorus oxide precursor is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and ammonium phosphate.

[0035] According to the present invention, preferably, the lanthanum oxide precursor and the zinc oxide precursor are selected from at least one of nitrates, sulfates, chlorides and acetates of lanthanum and zinc.

[0036] According to the present invention, preferably, step (1) further comprises: placing the molecular sieve catalyst in a solution of a phosphorus oxide precursor, stirring at 60-100° C. for 1-2 hours to obtain a suspension, centrifuging, and then performing a first drying and calcining to obtain a phosphorus-modified molecular sieve catalyst.

[0037] Further preferably, the conditions for the first drying include: a temperature of 100 to 140° C. and a time of 2 to 4 hours.

[0038] Further preferably, the conditions of the first calcination include: temperature of 400-650° C. and time of 3-8 h.

[0039] According to the present invention, preferably, step (2) further comprises: crushing the phosphorus-modified molecular sieve catalyst obtained in step (1) into tablets, placing it in a mixed solution of a lanthanum oxide precursor and a zinc oxide precursor, and stirring it at 10 to 25° C. for 1 to 2 hours to obtain a suspension, centrifuging it, and then drying and calcining it for a second time to obtain a powdered modified molecular sieve catalyst, and tableting and grinding it to obtain the modified molecular sieve catalyst.

[0040] Further preferably, the conditions for the second drying include: a temperature of 100 to 140° C. and a time of 10 to 14 hours.

[0041] Further preferably, the conditions for the second calcination include: a temperature of 400-650° C. and a time of 3-8 h.

[0042] More preferably, the grinding particle size is 20-40 mesh.

[0043] The third aspect of the present invention provides a modified molecular sieve catalyst prepared by the preparation method.

[0044] The fourth aspect of the present invention provides a method for preparing light olefins by co-feeding methanol and C4 alkane, so that methanol, C4 alkane and the modified molecular sieve catalyst are contacted and reacted to obtain the light olefins.

[0045] According to the present invention, preferably, the molar ratio of methanol to C4 alkane is 0.2-5.5:1, and the contact reaction conditions include: reaction temperature of 480-680°C, reaction pressure of 0.01-0.50Mpa, mass space velocity of 0.3-8h -1 .

[0046] According to the present invention, preferably, the C4 alkane is n-butane and / or isobutane; the low-carbon olefins include ethylene, propylene and butene.

[0047] The present invention will be further described below in conjunction with embodiments, but the scope of the present invention is not limited to these embodiments.

[0048] Example 1

[0049] 25g of HZSM-5 molecular sieve with a silicon-aluminum ratio of 35 was placed in 500ml of 1.5mol / L diammonium hydrogen phosphate solution and mechanically stirred at 80℃ for 2h. The obtained suspension was centrifuged and dried at 120℃ for 4h, then calcined at 450℃ for 3h, pressed into tablets and crushed. The above-prepared sample was placed in a flask, and a mixed solution of lanthanum nitrate and zinc nitrate was added thereto while stirring with a glass rod at room temperature, the mass ratio of the added lanthanum element to the HZSM-5 molecular sieve was controlled to be 1.0%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve was controlled to be 2.0%, and the mixture was stirred continuously for 0.5h, then dried at 120℃ for 12h, and then calcined at 500℃ in a muffle furnace for 5h. The prepared powdered modified molecular sieve is subjected to tabletting, grinding and screening to obtain a product with a particle size of 20 to 40 mesh, which is used as a catalyst for catalytic cracking reactions. The mass ratio of the phosphorus element introduced into the catalyst to the HZSM-5 molecular sieve is 1.0%, the mass ratio of the lanthanum element to the HZSM-5 molecular sieve is 1.0%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve is 2.0%.

[0050] The reaction conditions are as follows: 5 g of the phosphorus lanthanum zinc modified HZSM-5 molecular sieve catalyst is loaded into a fixed bed reactor, the reaction temperature is 550°C, the reaction pressure is 0.1 MPa, and the space velocity is 1 h -1 The reaction raw materials are methanol and n-butane, and the feed molar ratio of the two raw materials is 1:1. The reaction results and main product yields are shown in Table 1.

[0051] Example 2

[0052] 25g of HZSM-5 molecular sieve with a silicon-aluminum ratio of 80 was placed in 500ml of a 1.0mol / L phosphoric acid solution and mechanically stirred at 80°C for 2h. The obtained suspension was centrifuged and dried at 120°C for 4h, then calcined at 500°C for 5h, pressed into tablets, and then crushed. The sample prepared above was placed in a flask, and a mixed solution of lanthanum nitrate and zinc nitrate was added thereto while stirring with a glass rod at room temperature, the mass ratio of the added lanthanum element to the HZSM-5 molecular sieve was controlled to be 0.8%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve was controlled to be 2.5%, and the mixture was stirred continuously for 1h, then dried at 120°C for 8h, and then placed in a muffle furnace and calcined at 500°C for 5h. The prepared powdered modified molecular sieve is subjected to tabletting, grinding and screening to obtain a product with a particle size of 20 to 40 mesh, which is used as a catalyst for catalytic cracking reactions. The mass ratio of the phosphorus element introduced into the catalyst to the HZSM-5 molecular sieve is 0.8%, the mass ratio of the lanthanum element to the HZSM-5 molecular sieve is 0.8%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve is 2.5%.

[0053] The reaction conditions are as follows: 5 g of the phosphorus lanthanum zinc modified HZSM-5 molecular sieve catalyst is loaded into a fixed bed reactor, the reaction temperature is 550°C, the reaction pressure is 0.1 MPa, and the space velocity is 1 h -1The reaction raw materials are methanol and n-butane, and the feed molar ratio of the two raw materials is 3:1. The reaction results and main product yields are shown in Table 1.

[0054] Example 3

[0055] 25g of HZSM-5 molecular sieve with a silicon-aluminum ratio of 80 was placed in a beaker, and phosphoric acid solution was added thereto while stirring with a glass rod at room temperature, and the mass ratio of the added phosphorus element to the HZSM-5 molecular sieve was controlled to be 1.0%, and then dried at 120°C for 4h, and then calcined at 500°C for 5h and crushed into tablets. The above-prepared sample was placed in a flask, and a mixed solution of lanthanum nitrate and zinc nitrate was added thereto while stirring with a glass rod at room temperature, and the mass ratio of the added lanthanum element to the HZSM-5 molecular sieve was controlled to be 0.8%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve was controlled to be 2.5%, and the mixture was continuously stirred for 1h, and then dried at 120°C for 8h, and then placed in a muffle furnace and calcined at 500°C for 5h. The prepared powdered modified molecular sieve is subjected to tabletting, grinding and screening to obtain a product with a particle size of 20 to 40 mesh, which is used as a catalyst for catalytic cracking reactions. The mass ratio of the phosphorus element introduced into the catalyst to the HZSM-5 molecular sieve is 0.8%, the mass ratio of the lanthanum element to the HZSM-5 molecular sieve is 0.8%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve is 2.5%.

[0056] The reaction conditions are as follows: 5 g of the phosphorus lanthanum zinc modified HZSM-5 molecular sieve catalyst is loaded into a fixed bed reactor, the reaction temperature is 550°C, the reaction pressure is 0.1 MPa, and the space velocity is 1 h -1 The reaction raw materials are methanol and n-butane, and the feed molar ratio of the two raw materials is 3:1. The reaction results and main product yields are shown in Table 1.

[0057] Example 4

[0058] 25g of HZSM-5 molecular sieve with a silicon-aluminum ratio of 80 was placed in 500ml of a 1.0mol / L phosphoric acid solution and mechanically stirred at 80°C for 2h. The obtained suspension was centrifuged and dried at 120°C for 4h, then calcined at 500°C for 5h, pressed into tablets, and crushed. The sample prepared above was placed in a flask, and a mixed solution of lanthanum nitrate and zinc nitrate was added thereto while stirring with a glass rod at room temperature, the mass ratio of the added lanthanum element to the HZSM-5 molecular sieve was controlled to be 1%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve was controlled to be 2.5%, and the mixture was stirred continuously for 1h, then dried at 120°C for 8h, and then placed in a muffle furnace and calcined at 500°C for 5h. The prepared powdered modified molecular sieve is subjected to tabletting, grinding and screening to obtain a product with a particle size of 20 to 40 mesh, which is used as a catalyst for catalytic cracking reactions. The mass ratio of the phosphorus element introduced into the catalyst to the HZSM-5 molecular sieve is 1%, the mass ratio of the lanthanum element to the HZSM-5 molecular sieve is 0.8%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve is 2.5%.

[0059] The reaction conditions are as follows: 5 g of the phosphorus lanthanum zinc modified HZSM-5 molecular sieve catalyst is loaded into a fixed bed reactor, the reaction temperature is 550°C, the reaction pressure is 0.1 MPa, and the space velocity is 1 h -1 The reaction raw materials are methanol and n-butane, and the feed molar ratio of the two raw materials is 3:1. The reaction results and main product yields are shown in Table 1.

[0060] Example 5

[0061] 25g of HY molecular sieve with a silicon-aluminum ratio of 100 was placed in 500ml of a 1.0mol / L phosphoric acid solution and mechanically stirred at 80°C for 1.5h. The obtained suspension was centrifuged and dried at 120°C for 3h, then calcined at 500°C for 4h, pressed into tablets, and crushed. The above-prepared sample was placed in a flask, and a mixed solution of lanthanum nitrate and zinc nitrate was added thereto while stirring with a glass rod at room temperature, and the mass ratio of the added lanthanum element to the HZSM-5 molecular sieve was controlled to be 1.5%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve was controlled to be 3.5%, and the mixture was stirred continuously for 0.5h, then dried at 120°C for 8h, and then calcined at 500°C in a muffle furnace for 4h. The prepared powdered modified molecular sieve is subjected to tabletting, grinding and screening to obtain a product with a particle size of 20 to 40 mesh, which is used as a catalyst for catalytic cracking reactions. The mass ratio of the phosphorus element introduced into the catalyst to the HZSM-5 molecular sieve is 0.8%, the mass ratio of the lanthanum element to the HZSM-5 molecular sieve is 1.5%, and the mass ratio of the zinc element to the HZSM-5 molecular sieve is 3.5%.

[0062] The reaction conditions are as follows: 5 g of the phosphorus lanthanum zinc modified HZSM-5 molecular sieve catalyst is loaded into a fixed bed reactor, the reaction temperature is 500°C, the reaction pressure is 0.2 MPa, and the space velocity is 2 h -1 The reaction raw materials are methanol and C4 alkane, the feed molar ratio of the two raw materials is 3:1, the molar ratio of n-butane to isobutane in C4 alkane is 1:1, and the reaction results and main product yields are shown in Table 1.

[0063] Example 6

[0064] The only difference from Example 2 is that the mass ratio of phosphorus element to HZSM-5 molecular sieve introduced into the catalyst is 3.5%, the mass ratio of lanthanum element to HZSM-5 molecular sieve is 0.8%, and the mass ratio of zinc element to HZSM-5 molecular sieve is 2.5%.

[0065] Example 7

[0066] The only difference from Example 2 is that the mass ratio of phosphorus element to HZSM-5 molecular sieve introduced into the catalyst is 0.8%, the mass ratio of lanthanum element to HZSM-5 molecular sieve is 1.5%, and the mass ratio of zinc element to HZSM-5 molecular sieve is 5.0%.

[0067] Comparative Example 1

[0068] The catalyst and reaction conditions used in this comparative example are exactly the same as those in Example 2, except that the reaction raw material is pure n-butane. The reaction results and main product yields are shown in Table 1.

[0069] Comparative Example 2

[0070] The reaction conditions used in this comparative example are exactly the same as those in Example 2, except that the catalyst used is an unmodified HZSM-5 molecular sieve with a silicon-aluminum ratio of 80. The reaction results and the yields of the main products are shown in Table 1.

[0071] Comparative Example 3

[0072] The only difference between this comparative example and Example 2 is that zinc nitrate solution is not added during the catalyst modification process, and other reaction conditions are exactly the same as those in Example 2. The reaction results and main product yields are shown in Table 1.

[0073] Comparative Example 4

[0074] The only difference between this comparative example and Example 2 is that zinc nitrate is not added during the catalyst modification process, and the mass ratio of the added phosphorus element to the HZSM-5 molecular sieve is controlled to be 2.8%, and the mass ratio of the added lanthanum element to the HZSM-5 molecular sieve is controlled to be 2.0%. The other reaction conditions are exactly the same as those in Example 2. The reaction results and the yields of the main products are shown in Table 1.

[0075] Comparative Example 5

[0076] The difference between this comparative example and Example 2 is that the mixed solution of lanthanum nitrate and zinc nitrate is not added during the catalyst modification process, and the mass ratio of the added phosphorus element to the HZSM-5 molecular sieve is controlled to be 3%. The other reaction conditions are exactly the same as those in Example 2. The reaction results and the yields of the main products are shown in Table 1.

[0077] Table 1 Test results

[0078]

[0079]

[0080] It can be seen from Table 1 that, relative to Comparative Examples 1-5, in Examples 1-7, by simultaneously adding phosphorus, lanthanum and zinc, the catalyst obtained has a significantly improved yield of light olefins.

[0081] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

[0082] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

Claims

1. A modified molecular sieve catalyst, characterized in that: The modified molecular sieve catalyst comprises: a molecular sieve catalyst and phosphorus elements, lanthanum elements and zinc elements carried thereon.

2. The modified molecular sieve catalyst according to claim 1, wherein In the modified molecular sieve catalyst, the mass ratio of phosphorus element to molecular sieve catalyst is 0.01-5:100, preferably 0.5-2:100, the mass ratio of zinc element to molecular sieve catalyst is 0.01-8:100, preferably 1-3:100, and the mass ratio of lanthanum element to molecular sieve catalyst is 0.01-2:100, preferably 0.2-1.3:

100.

3. The modified molecular sieve catalyst according to claim 1, wherein The silicon-to-aluminum ratio of the molecular sieve catalyst is 15-250; Preferably, the molecular sieve catalyst is selected from at least one of HZSM-5, HZSM-11, HBeta and HY.

4. A method for preparing a modified molecular sieve catalyst, characterized in that: The steps include: (1) loading a solution of a phosphorus oxide precursor onto a molecular sieve catalyst, performing a first drying and calcining to obtain a phosphorus-modified molecular sieve catalyst; (2) Loading the mixed solution of the lanthanum oxide precursor and the zinc oxide precursor on the phosphorus-modified molecular sieve catalyst obtained in step (1), performing a second drying and calcining to obtain the modified molecular sieve catalyst.

5. The preparation method according to claim 4, wherein In step (1) and step (2), the loading method is independently an ion exchange method or an equal volume impregnation method.

6. The preparation method according to claim 4, wherein The concentration of the solution of the phosphorus oxide precursor is such that the mass ratio of phosphorus element to molecular sieve catalyst in the modified molecular sieve catalyst is 0.01-5:100, preferably 0.5-2:100; The concentration of the zinc oxide precursor solution is such that the mass ratio of zinc element to molecular sieve catalyst in the modified molecular sieve catalyst is 0.01-8:100, preferably 1-3:100; The concentration of the solution of the lanthanum oxide precursor is such that the mass ratio of the lanthanum element to the molecular sieve catalyst in the modified molecular sieve catalyst is 0.01-2:100, preferably 0.2-1.3:

100.

7. The preparation method according to claim 4, wherein The silicon-to-aluminum ratio of the molecular sieve catalyst is 15-250; Preferably, the molecular sieve catalyst is selected from at least one of HZSM-5, HZSM-11, HBeta and HY.

8. The preparation method according to claim 4, wherein The phosphorus oxide precursor is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and ammonium phosphate; The lanthanum oxide precursor and the zinc oxide precursor are selected from at least one of nitrates, sulfates, chlorides and acetates of lanthanum and zinc.

9. The preparation method according to any one of claims 4 to 8, wherein: Step (1) further comprises: placing the molecular sieve catalyst in a solution of a phosphorus oxide precursor, stirring at 60 to 100° C. for 1 to 2 hours to obtain a suspension, centrifuging, and then performing a first drying and calcining to obtain a phosphorus-modified molecular sieve catalyst; The conditions of the first drying include: temperature of 100-140°C and time of 2-4h; The conditions for the first calcination include: a temperature of 400-650° C. and a time of 3-8 hours.

10. The preparation method according to any one of claims 4 to 8, wherein: Step (2) further comprises: crushing the phosphorus-modified molecular sieve catalyst obtained in step (1) into tablets, placing the tablets in a mixed solution of a lanthanum oxide precursor and a zinc oxide precursor, stirring the tablets at 10 to 25° C. for 1 to 2 hours to obtain a suspension, centrifuging the suspension, drying the suspension for a second time, and calcining the suspension to obtain a powdered modified molecular sieve catalyst, and tableting and grinding the tablets to obtain the modified molecular sieve catalyst; The conditions of the second drying include: temperature of 100-140°C and time of 10-14h; The conditions of the second calcination include: temperature of 400-650°C and time of 3-8h; The grinding particle size is 20 to 40 meshes.

11. A modified molecular sieve catalyst prepared by the preparation method according to any one of claims 4 to 10.

12. A method for preparing light olefins by co-feeding methanol and C4 alkanes, characterized in that: Methanol, C4 alkane and the modified molecular sieve catalyst according to any one of claims 1 to 3 and 11 are contacted to react to obtain the light olefins.

13. The method for preparing light olefins by co-feeding methanol and C4 alkanes according to claim 12, wherein: The molar ratio of methanol to C4 alkane is 0.2-5.5:

1. The contact reaction conditions include: reaction temperature of 480-680°C, reaction pressure of 0.01-0.50Mpa, mass space velocity of 0.3-8h -1 .

14. The method for preparing light olefins by co-feeding methanol and C4 alkanes according to claim 12, wherein: The C4 alkane is n-butane and / or isobutane; the low-carbon olefins include ethylene, propylene and butene.

Images