Phosphorus-modified zsm-5 molecular sieve and preparation method thereof

Abstract

The application discloses a phosphorus-modified ZSM-5 molecular sieve, characterized in that, after drying at 100-120 DEG C in an air atmosphere, the binding energy of phosphorus in the surface phosphorus species of the molecular sieve is 134.5 eV; and after hydrothermal aging at 800 DEG C under 100% water vapor for 17 h, the binding energy of phosphorus in the surface phosphorus species of the molecular sieve is 135.2 eV.

Description

Technical Field

[0001] This invention relates to a modified ZSM-5 molecular sieve and its preparation method, and more particularly to a phosphorus-modified ZSM-5 molecular sieve and its preparation method. Background Technology

[0002] ZSM-5 molecular sieve (USP3702886) is a high-silica, three-dimensional, straight-channel mesoporous molecular sieve with an MFI structure, developed by Mobil Petroleum Corporation in the United States. Its unique pore structure endows it with excellent shape-selective catalysis and isomerization performance. It also features high thermal and hydrothermal stability, high specific surface area, a wide range of silica-to-alumina ratios, unique surface acidity, and low carbon deposition. ZSM-5 molecular sieve is widely used as a catalyst and catalyst support, and has been successfully applied in alkylation, isomerization, disproportionation, catalytic cracking, methanol-to-gasoline, and methanol-to-olefins production processes.

[0003] Since 1983, ZSM-5 molecular sieves have been used as octane enhancers in catalytic cracking processes to improve the octane number and selectivity of low-carbon olefins in catalytic cracked gasoline. US3758403 first reported the use of ZSM-5 molecular sieves as an active component for enhancing propylene production; it was used in conjunction with REY as an active component in FCC catalysts. US5997728 disclosed the use of unmodified ZSM-5 molecular sieves as an additive for enhancing propylene production, but the propylene yields were not high. While HZSM-5 molecular sieves possess good shape selectivity and isomerization properties, their drawback is poor hydrothermal stability; they are prone to deactivation under harsh high-temperature hydrothermal conditions, leading to reduced catalytic performance.

[0004] In the 1980s, Mobil discovered that phosphorus could improve the hydrothermal stability of ZSM-5 molecular sieves, and that phosphorus modification of ZSM-5 molecular sieves increased the yield of low-carbon olefins. Commonly used ZSM-5 additives, activated by phosphorus, selectively convert primary cracking products (such as gasoline olefins) into C3 and C4 olefins. Post-synthesis modification of ZSM-5 molecular sieves with appropriate amounts of inorganic phosphorus compounds can stabilize the skeletal aluminum framework under harsh hydrothermal conditions.

[0005] CN 1211469A discloses a five-membered ring molecular sieve composition that produces high yields of propylene and ethylene, comprising 85-95 wt% of a five-membered ring molecular sieve, 2-10 wt% of phosphorus (based on oxides), 0.3-5 wt% of alkaline earth metals (based on oxides), and 0.3-5 wt% of transition metal elements (based on oxides). This composition yields high ethylene yields when used in catalytic thermal cracking reactions.

[0006] US5171921 discloses a phosphorus-modified ZSM-5 molecular sieve with a silicon-to-aluminum ratio of 20-60. After being impregnated with a phosphorus-containing compound and treated with steam at 500-700°C, it exhibits higher activity than untreated HZSM-5 when used in the reaction of converting C3-C20 hydrocarbons into C2-C5 olefins.

[0007] CN102166533A discloses a method for preparing phosphorus-modified ZSM-5 molecular sieves. The method involves adding the molecular sieve to a phosphorus-containing aqueous solution and reacting it for a period of time under specific pH, temperature, and pressure conditions. The resulting product is then filtered, dried, and calcined to obtain the phosphorus-modified molecular sieve. Subsequently, the phosphorus-modified molecular sieve is added to an aqueous solution containing rare earth ions and reacted for a period of time under specific temperature and pressure conditions. This is followed by filtration, washing, drying, and calcination to obtain a composite modified molecular sieve. Model catalysts prepared using this composite modified molecular sieve exhibit higher hydrothermal stability and micro-activity compared to model catalysts containing unmodified molecular sieves or those modified by other methods.

[0008] CN106994364A discloses a method for preparing phosphorus-modified ZSM-5 molecular sieves. The method involves first mixing one or more phosphorus-containing compounds selected from phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate with a ZSM-5 molecular sieve containing high alkali metal ions to obtain a mixture with a phosphorus loading of at least 0.1 wt% (based on P2O5). The mixture is then dried, calcined, and subjected to ammonium ion exchange and water washing steps to reduce the alkali metal ion content to below 0.10 wt%. Finally, it undergoes drying and hydrothermal aging at 400-1000℃ under 100% steam conditions. The phosphorus-containing ZSM-5 molecular sieve obtained by this method has a high total acid content, excellent cracking conversion rate and propylene selectivity, and a high liquefied gas yield.

[0009] Although modifying ZSM-5 molecular sieves with organic / inorganic phosphorus compounds can inhibit framework dealuminization and thus improve hydrothermal stability, the introduction of phosphorus species also plays a role in regulating the acid center properties of ZSM-5 molecular sieves, thereby improving the cracking conversion rate of long-chain alkanes and the selectivity of low-carbon olefins. However, during the roasting process of existing technologies, phosphoric acid or ammonium phosphate salts will generate large molecular phosphoric acid species such as polyphosphoric acid due to dehydration and self-polymerization. These species are easily enriched on the outer surface of the molecular sieve, clogging the pores, reducing the pore volume and specific surface area, resulting in low phosphorus aluminum activation efficiency. Therefore, phosphorus modification cannot achieve the ideal effect of improving hydrothermal stability. Summary of the Invention

[0010] Based on extensive experiments, the inventors discovered that phosphorus-modified ZSM-5 molecular sieves prepared using solid phosphorus compounds with a +3 valence of phosphorus as precursors exhibit physicochemical characteristics different from those of molecular sieves prepared by conventional aqueous solution impregnation methods with +5 valence phosphorus sources. This improves the phosphorus-aluminum stabilization efficiency, and enhances the hydrothermal stability, pyrolysis activity, and low-carbon olefin selectivity of the ZSM-5 molecular sieve. Based on this, the present invention was developed.

[0011] Therefore, the purpose of this invention is to overcome the problems of unsatisfactory hydrothermal stability, pyrolysis activity and low-carbon olefin selectivity in the prior art, and to provide a phosphorus-modified ZSM-5 molecular sieve that is different from the prior art and to provide its preparation method.

[0012] To achieve the above objectives, the first aspect of the present invention provides a phosphorus-modified ZSM-5 molecular sieve, characterized in that, after drying in an air environment at 100-120°C, the electron binding energy of phosphorus in the phosphorus species on the surface of the molecular sieve is 134.5 eV; and after hydrothermal aging at 800°C and 100% water vapor for 17 h, the electron binding energy of phosphorus in the phosphorus species on the surface of the molecular sieve is 135.2 eV.

[0013] The molecular sieve, after hydrothermal aging at 800℃ and 100% steam for 17 hours, exhibits a crystallinity retention of 70%–110% in XRD analysis, preferably 95%–110%. Crystallinity retention is the ratio of relative crystallinity before and after aging; a higher crystallinity retention indicates a better effect of phosphorus species in stabilizing the aluminum framework.

[0014] The phosphorus content in the molecular sieve, when both phosphorus and aluminum are measured in molar amounts, has a ratio of 0.1 to 5, preferably 0.5 to 2.5.

[0015] The molecular sieve is preferably HZSM-5 molecular sieve, which is obtained by modifying it with a solid phosphorus-containing compound in which phosphorus has a +3 oxidation state.

[0016] To achieve the above objectives, a second aspect of the present invention provides a method for preparing the phosphorus-modified ZSM-5 molecular sieve, characterized in that a solid phosphorus-containing compound precursor with phosphorus having a +3 valence state and HZSM-5 molecular sieve are mixed and ground to obtain a solid mixture; the solid mixture is heated and melted; the heated and melted solid mixture is cooled to room temperature and then calcined to obtain the phosphorus-modified ZSM-5 molecular sieve.

[0017] Another embodiment of the preparation method of the present invention is aqueous solution impregnation, which involves contacting and impregnating HZSM-5 molecular sieve with an aqueous solution of a phosphorus-containing compound with a phosphorus oxidation state of +3, followed by drying and calcination to obtain phosphorus-modified ZSM-5 molecular sieve. In this embodiment, the water-to-sieve weight ratio is 0.5-2, and the process is carried out at room temperature (e.g., 20-25°C) for 0.5-10 hours; the drying is carried out in an air atmosphere at 100-120°C for 2-24 hours; and the calcination is carried out in an air atmosphere at 200-800°C for 0.5-12 hours.

[0018] In the preparation method described above, the solid phosphorus-containing compound precursor with a phosphorus oxidation state of +3 is selected from phosphorous acid and / or ammonium phosphite. The heating and melting impregnation treatment is performed at a temperature higher than the melting point of the solid phosphorus-containing compound precursor but lower than the temperature at which the precursor decomposes or dehydrates. Taking phosphorous acid as an example, the solid melting temperature of phosphorous acid is 78°C, and its decomposition temperature is 170°C. After decomposition, it produces phosphine and phosphoric acid. Therefore, during the melting and impregnation process, it is necessary to ensure that phosphorous acid does not undergo chemical changes, and the heating temperature must be controlled to be higher than the melting point but lower than the decomposition point. The heating and melting treatment time is 2–72 hours. This heating and melting impregnation method eliminates the influence of water on phosphorus species migration, accelerates the activation efficiency of aluminum phosphate, and is superior to conventional aqueous solution impregnation methods.

[0019] In the preparation method described above, the room temperature is typically 20–30°C.

[0020] In the preparation method described above, the calcination treatment is carried out in an air atmosphere at 200–800°C for 0.5–12 hours.

[0021] In the preparation method described above, the solid phosphorus-containing compound precursor with phosphorus in the +3 oxidation state is calculated as phosphorus, and the HZSM-5 molecular sieve is calculated as aluminum, with a molar ratio of 0.1 to 5, preferably 0.5 to 2.5.

[0022] The phosphorus-modified ZSM-5 molecular sieve provided by this invention exhibits reduced surface enrichment of phosphorus species, sufficient coordination between phosphorus species and framework aluminum, and quantitative phosphorus-containing compounds that can stabilize more framework aluminum. This results in excellent hydrothermal stability of the phosphorus-modified ZSM-5 molecular sieve, with higher crystallinity retention after hydrothermal aging at 800℃ and 100% steam conditions for 17 hours or under more stringent conditions. In the cracking reaction of n-tetradecane, the main indicators are improved compared to the control sample, such as higher conversion rate and liquefied gas yield, while the yield of trienes (ethylene, propylene, and butene) is significantly improved. Attached Figure Description

[0023] Figure 1This is the XPS (P2p Scan) spectrum of sample RYB-1. Detailed Implementation

[0024] The present invention will be further illustrated by the following examples, but these examples do not limit the scope of the invention.

[0025] X-ray diffraction (XRD) patterns were determined using a Rigaku TTR-3 powder X-ray diffractometer. Instrument parameters: copper target (tube voltage 40 kV, tube current 250 mA), scintillation counter, step width 0.02°, scan rate 0.4 (°) / min. ZSM-5 molecular sieve synthesized using the method in Example 1 of CN1056818C was used as a standard, and its crystallinity was defined as 100%. Relative crystallinity was expressed as a percentage, representing the sum of the peak areas of the five characteristic diffraction peaks between 2θ and 25.0° in the X-ray diffraction patterns of the obtained product and the standard.

[0026] X-ray photoelectron spectroscopy (XPS) was used for the analysis of phosphorus species on the surface of molecular sieves. The valence state changes of phosphorus compounds loaded on phosphorus-modified molecular sieves after drying and calcination were investigated using a Thermo Fisher-VG ESCALAB 250 X-ray photoelectron spectrometer. Instrument parameters: The excitation source was monochromatic AlKα X-rays with a power of 150 W; the charge shift was corrected using the C1s peak (284.8 eV) from contaminating carbon.

[0027] Example 1

[0028] 10.8g of solid phosphorous acid was added to 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30), and the mixture was ground and mixed for 0.5h. The mixture was then transferred to a Teflon-lined stainless steel high-pressure reactor for a melting process at 100℃ for 10h. After the process, the sample was allowed to cool to room temperature naturally before being removed. The sample was then calcined at 550℃ for 2h to obtain a phosphorus-modified ZSM-5 molecular sieve sample, denoted as RYB-1.

[0029] Example 1-1

[0030] 10.8g of solid phosphorous acid was dissolved in 145g of deionized water at 25℃ and stirred for 0.5h to obtain a phosphorus-containing aqueous solution. 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30) was added, and the solution was modified by impregnation. After impregnation at 25℃ for 0.5h, the solution was transferred to an oven and dried at 120℃ for 12h. The solution was then calcined in an air atmosphere in a muffle furnace at 550℃ for 2h. The resulting phosphorus-modified ZSM-5 molecular sieve control sample was designated as RYB-1s.

[0031] Example 2

[0032] 15.4g of solid diammonium hydrogen phosphite was added to 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30). After grinding and mixing for 0.5h, the mixture was transferred to a Teflon-lined stainless steel high-pressure reactor for melting at 100℃ for 10h. After the melting process, the sample was taken out after naturally cooling to room temperature. The sample was then calcined at 550℃ for 2h to obtain a phosphorus-modified ZSM-5 molecular sieve sample, denoted as RYB-2.

[0033] Comparative Example 1

[0034] 17.4 g of diammonium hydrogen phosphate was dissolved in 145 g of deionized water at 25 °C and stirred for 0.5 h to obtain a phosphorus-containing aqueous solution. 100 g of HZSM-5 molecular sieve (nSiO2 / nAl2O3 = 30) was added, and the solution was modified by impregnation. After impregnation at 25 °C for 0.5 h, the solution was transferred to an oven and dried at 120 °C for 12 h. The solution was then calcined in an air atmosphere in a muffle furnace at 550 °C for 2 h. The resulting phosphorus-modified ZSM-5 molecular sieve control sample was designated as D-1.

[0035] Comparative Example 2

[0036] 15.1 g of phosphoric acid solution (85 wt%) was dissolved in 145 g of deionized water at 25 °C and stirred for 0.5 h to obtain a phosphorus-containing aqueous solution. 100 g of HZSM-5 molecular sieve (nSiO2 / nAl2O3 = 30) was added, and the solution was modified by impregnation. After impregnation at 25 °C for 0.5 h, the solution was transferred to an oven and dried at 120 °C for 12 h. The solution was then calcined in an air atmosphere in a muffle furnace at 550 °C for 2 h. The resulting phosphorus-modified ZSM-5 molecular sieve control sample was designated as D-2.

[0037] In Examples 1, 1-1, 2 and Comparative Examples 1, 2, the phosphorus-aluminum ratio of the phosphorus-containing compound to the molecular sieve was 1.25.

[0038] The XPS (P2p Scan) spectrum of sample RYB-1 is shown below. Figure 1 . Figure 1 In the image, peaks of 134.5 eV and 135.2 eV represent the electron binding energies of phosphorus in the phosphorus species on the surface of sample RYB-1 after drying and calcination, respectively. The XPS (P2pScan) spectra of RYB-1s and RYB-2 both exhibit... Figure 1 Its characteristics.

[0039] The relative crystallinity and crystallinity retention of XRD values ​​for RYB-1, RYB-1s, RYB-2, D-1, and D-2 before and after hydrothermal aging treatment at 800℃ and 100% steam for 17 hours are shown in Table 1.

[0040] RYB-1, RYB-1s, RYB-2, D-1, and D-2 were evaluated for n-tetradecane cracking. The microreactor evaluation conditions were: molecular sieve loading 2g, feedstock n-tetradecane, feed rate 1.56g, reaction temperature 550℃, and regeneration temperature 600℃ (the same below). Evaluation data are shown in Table 2.

[0041] Table 1

[0042]

[0043] Table 2

[0044] RYB-1 RYB-1s RYB-2 D-1 D-2 Material balance / m% dry air 4.24 3.91 3.67 3.51 3.40 Liquefied gas 54.05 53.21 49.22 41.76 39.12 gasoline 24.54 21.73 21.14 20.72 21.66 diesel fuel 16.51 18.99 23.52 30.93 33.51 m% of main products in cracked gas ethylene 3.65 3.42 3.12 2.75 2.59 propylene 18.51 17.63 16.44 15.58 15.32 Total butene 18.58 17.41 16.14 14.65 14.37 Conversion rate / m% 83.04 81.27 76.18 68.17 66.06

[0045] Example 3

[0046] 6.5g of solid phosphorous acid was added to 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30), and the mixture was ground and mixed for 0.5h. The mixture was then transferred to a Teflon-lined stainless steel high-pressure reactor for a melting process at 100℃ for 10h. After the melting process, the sample was allowed to cool to room temperature naturally before being removed. The sample was then calcined at 550℃ for 2h to obtain a phosphorus-modified ZSM-5 molecular sieve sample, denoted as RYB-3.

[0047] Example 3-1

[0048] 6.5g of solid phosphorous acid was dissolved in 145g of deionized water at 25℃ and stirred for 0.5h to obtain a phosphorus-containing aqueous solution. 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30) was added, and the solution was modified by impregnation. After impregnation at 25℃ for 0.5h, the solution was transferred to an oven and dried at 120℃ for 12h. The solution was then calcined in an air atmosphere in a muffle furnace at 550℃ for 2h. The resulting phosphorus-modified ZSM-5 molecular sieve control sample was designated as RYB-3s.

[0049] Example 4

[0050] Take 9.3g of solid diammonium hydrogen phosphite and add it to 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30). Grind and mix it for 0.5h and then transfer it to a Teflon-lined stainless steel high-pressure reactor for a melting process at 100℃ for 10h. After the process, let the sample cool to room temperature naturally and then take it out. After calcining the above sample at 550℃ for 2h, a phosphorus-modified ZSM-5 molecular sieve sample is obtained, which is denoted as RYB-4.

[0051] Comparative Example 3

[0052] 10.5g of diammonium hydrogen phosphate was dissolved in 145g of deionized water at 25℃ and stirred for 0.5h to obtain a phosphorus-containing aqueous solution. 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30) was added, and the solution was modified by impregnation. After impregnation at 25℃ for 0.5h, the solution was transferred to an oven and dried at 120℃ for 12h. The solution was then calcined in an air atmosphere in a muffle furnace at 550℃ for 2h. The resulting phosphorus-modified ZSM-5 molecular sieve control sample was designated as D-3.

[0053] Comparative Example 4

[0054] 9.2 g of phosphoric acid solution (85 wt%) was dissolved in 145 g of deionized water at 25 °C and stirred for 0.5 h to obtain a phosphorus-containing aqueous solution. 100 g of HZSM-5 molecular sieve (nSiO2 / nAl2O3 = 30) was added, and the solution was modified by impregnation. After impregnation at 25 °C for 0.5 h, the solution was transferred to an oven and dried at 120 °C for 12 h. The solution was then calcined in an air atmosphere in a muffle furnace at 550 °C for 2 h. The resulting phosphorus-modified ZSM-5 molecular sieve control sample was designated as D-4.

[0055] In Examples 3, 3-1, 4 and Comparative Examples 3, 4, the phosphorus-aluminum ratio of the phosphorus-containing compound to the molecular sieve was 0.75.

[0056] XPS (P2p Scan) spectra of samples RYB-3, RYB-3s, and RYB-4 all have Figure 1 Its characteristics.

[0057] The relative crystallinity and crystallinity retention of XRD values ​​for RYB-3, RYB-3s, RYB-4, D-3, and D-4 before and after hydrothermal aging treatment at 800℃ and 100% steam for 17 hours are shown in Table 3.

[0058] RYB-3, RYB-3s, RYB-4, D-3, and D-4 were evaluated for n-tetradecane cracking. The microreactor evaluation conditions were: molecular sieve loading 2g, feedstock n-tetradecane, feed rate 1.56g, reaction temperature 550℃, and regeneration temperature 600℃ (the same below). Evaluation data are shown in Table 4.

[0059] Table 3

[0060]

[0061]

[0062] Table 4

[0063] RYB-3 RYB-3s RYB-4 D-3 D-4 Material balance / m% dry air 4.61 4.14 3.87 3.41 3.35 Liquefied gas 53.25 51.24 49.13 41.87 39.03 gasoline 23.24 22.19 21.64 19.92 21.46 diesel fuel 16.22 19.87 23.04 31.99 32.47 m% of main products in cracked gas ethylene 3.85 3.69 3.10 2.85 2.63 propylene 17.99 17.22 16.46 15.88 15.72 Total butene 18.14 17.54 16.04 14.25 14.17 Conversion rate / m% 82.51 79.34 75.80 67.33 65.54

[0064] Example 5

[0065] 15.0g of solid phosphorous acid was added to 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30), and the mixture was ground and mixed for 0.5h. The mixture was then transferred to a Teflon-lined stainless steel high-pressure reactor for a melting process at 100℃ for 10h. After the melting process, the sample was allowed to cool to room temperature naturally before being removed. The sample was then calcined at 550℃ for 2h to obtain a phosphorus-modified ZSM-5 molecular sieve sample, denoted as RYB-5.

[0066] Example 5-1

[0067] 15.0g of solid phosphorous acid was dissolved in 145g of deionized water at 25℃ and stirred for 0.5h to obtain a phosphorus-containing aqueous solution. 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30) was added, and the solution was modified by impregnation. After impregnation at 25℃ for 0.5h, the solution was transferred to an oven and dried at 120℃ for 12h. The solution was then calcined in an air atmosphere in a muffle furnace at 550℃ for 2h. The resulting phosphorus-modified ZSM-5 molecular sieve control sample was designated as RYB-5s.

[0068] Example 6

[0069] 21.7g of solid diammonium hydrogen phosphite was added to 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30). After grinding and mixing for 0.5h, the mixture was transferred to a Teflon-lined stainless steel high-pressure reactor for melting at 100℃ for 10h. After the melting process, the sample was taken out after naturally cooling to room temperature. The sample was then calcined at 550℃ for 2h to obtain a phosphorus-modified ZSM-5 molecular sieve sample, denoted as RYB-6.

[0070] Comparative Example 5

[0071] 24.5g of diammonium hydrogen phosphate was dissolved in 145g of deionized water at 25℃ and stirred for 0.5h to obtain a phosphorus-containing aqueous solution. 100g of HZSM-5 molecular sieve (nSiO2 / nAl2O3=30) was added, and the solution was modified by impregnation. After impregnation at 25℃ for 0.5h, the solution was transferred to an oven and dried at 120℃ for 12h. The solution was then calcined in an air atmosphere in a muffle furnace at 550℃ for 2h. The resulting phosphorus-modified ZSM-5 molecular sieve control sample was designated as D-5.

[0072] Comparative Example 6

[0073] 21.4 g of phosphoric acid solution (85 wt%) was dissolved in 145 g of deionized water at 25 °C and stirred for 0.5 h to obtain a phosphorus-containing aqueous solution. 100 g of HZSM-5 molecular sieve (nSiO2 / nAl2O3 = 30) was added, and the solution was modified by impregnation. After impregnation at 25 °C for 0.5 h, the solution was transferred to an oven and dried at 120 °C for 12 h. The solution was then calcined in an air atmosphere in a muffle furnace at 550 °C for 2 h. The resulting phosphorus-modified ZSM-5 molecular sieve control sample was designated as D-6.

[0074] In Examples 5, 5-1, 6 and Comparative Examples 5, 6, the phosphorus-aluminum ratio of the phosphorus-containing compound to the molecular sieve was 1.75.

[0075] XPS (P2p Scan) spectra of samples RYB-5, RYB-5s, and RYB-6 all have Figure 1 Its characteristics.

[0076] The relative crystallinity and crystallinity retention of XRD values ​​for RYB-5, RYB-5s, RYB-6, D-5, and D-6 before and after hydrothermal aging treatment at 800℃ and 100% steam for 17 hours are shown in Table 5.

[0077] RYB-5, RYB-5s, RYB-6, D-5, and D-6 were evaluated for n-tetradecane cracking. The microreactor evaluation conditions were: molecular sieve charge 2g, feedstock n-tetradecane, feed rate 1.56g, reaction temperature 550℃, and regeneration temperature 600℃ (the same applies below). Evaluation data are shown in Table 6.

[0078] Table 5

[0079]

[0080] Table 6

[0081] RYB-5 RYB-5s RYB-6 D-5 D-6 Material balance / m% dry air 4.53 3.99 3.45 3.22 3.17 Liquefied gas 53.55 50.49 49.42 41.95 39.73 gasoline 23.14 21.19 21.23 20.76 21.68 diesel fuel 16.11 21.98 23.85 30.43 32.58 m% of main products in cracked gas ethylene 3.41 3.18 3.75 2.22 2.41 propylene 17.42 16.92 16.41 15.82 15.74 Total butene 17.74 17.05 16.75 14.84 14.46 Conversion rate / m% 81.42 79.61 75.77 68.28 65.66

Claims

1. A method for preparing phosphorus-modified ZSM-5 molecular sieve, characterized in that, A solid phosphorus-containing compound precursor with a phosphorus oxidation state of +3 and HZSM-5 molecular sieve are mixed and ground to obtain a solid mixture; the solid mixture is heated and melted; the heated and melted solid mixture is cooled to room temperature and then calcined to obtain phosphorus-modified ZSM-5 molecular sieve; the solid phosphorus-containing compound precursor with a phosphorus oxidation state of +3 is phosphorous acid.

2. The preparation method according to claim 1, characterized in that, The heating and melting process is carried out at a temperature higher than the melting point of the solid phosphorus-containing compound precursor and lower than the temperature at which the solid phosphorus-containing compound precursor decomposes or dehydrates.

3. The preparation method according to claim 1 or 2, characterized in that, The heating and melting treatment time is 2 to 72 hours.

4. The preparation method according to claim 1, characterized in that, The roasting process is carried out in an air atmosphere at 200–800°C for 0.5–12 hours.

5. The preparation method according to claim 1, characterized in that, The molar ratio of solid phosphorus-containing compound precursors with a +3 oxidation state (calculated as phosphorus) and HZSM-5 molecular sieves (calculated as aluminum) is 0.1–5.

6. The preparation method according to claim 1, characterized in that, The molar ratio of solid phosphorus-containing compound precursors with a +3 oxidation state to phosphorus and HZSM-5 molecular sieves with aluminum is 0.5 to 2.

5.

7. The phosphorus-modified ZSM-5 molecular sieve obtained by any one of claims 1-6.

8. A catalytic cracking method, characterized in that, The phosphorus-modified ZSM-5 molecular sieve of claim 7 was used as a catalyst.

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