A modified zsm-5 molecular sieve and its use

Abstract

The application discloses a modified molecular sieve, characterized in that the molecular sieve is obtained by modifying ZSM-5 molecular sieve with elemental phosphorus; in a thermogravimetric analysis under the condition that an atmosphere environment is air, the elemental phosphorus has weight gain within 330 DEG C-420 DEG C and weight loss within 420 DEG C-500 DEG C; in a thermogravimetric analysis under the condition that an atmosphere environment is air and temperature is increased from 20 DEG C to 800 DEG C, the ratio of the mass of the molecular sieve at 800 DEG C to the minimum mass in the temperature increasing process is (1.05-1.25). The molecular sieve has excellent hydrothermal stability and can be used as an active component of a catalyst for catalytic cracking.

Description

Technical Field

[0001] This invention relates to a modified ZSM-5 molecular sieve and its application, and more particularly to a phosphorus-modified ZSM-5 molecular sieve and its application in catalytic reactions. 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] CN1211469A 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-containing 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 experimentation, the inventors discovered that modifying ZSM-5 molecular sieves with elemental phosphorus such as red phosphorus as a phosphorus source can promote the activation and coordination of phosphorus with the aluminum framework of the molecular sieve. The resulting modified molecular sieve exhibits physicochemical characteristics different from those of conventionally phosphorus-modified ZSM-5 molecular sieves. Furthermore, the phosphorus retention rate is high after water washing. Particularly in industrial applications, the hydrothermal stability, pyrolysis activity, and low-carbon olefin selectivity of the modified molecular sieve are improved after aging. Based on these findings, this invention was developed.

[0011] The purpose of this invention is to address the problem of unsatisfactory hydrothermal stability in existing technologies by providing a modified ZSM-5 molecular sieve that differs from existing technologies and exhibits better catalytic cracking activity and selectivity for low-carbon olefins when applied to catalytic cracking.

[0012] To achieve the above objectives, the present invention provides a modified molecular sieve, characterized in that the molecular sieve is obtained by modifying ZSM-5 molecular sieve with elemental phosphorus; in thermogravimetric analysis under air atmosphere, the elemental phosphorus shows weight gain within 330℃-420℃ and weight loss within 420-500℃; in thermogravimetric analysis under air atmosphere from 20℃ to 800℃, the ratio of the mass of the molecular sieve at 800℃ to the minimum mass during the heating process is (1.05-1.25).

[0013] Thermogravimetric analysis (TGA) is a technique that measures the relationship between the mass and temperature of a sample under programmed temperature control and in different atmospheres. It can be used to characterize changes in the properties of substances, such as physical phenomena like melting, evaporation, sublimation, and adsorption, or chemical phenomena like dehydration, dissociation, oxidation, and reduction. In this invention, the weight gain and loss of elemental phosphorus in TGA represent the oxidation and sublimation of the phosphorus source used for modification, i.e., elemental phosphorus. The weight gain indicates the degree of oxidation and valence state change of elemental phosphorus, while the weight loss indicates the sublimation of elemental phosphorus, such as red phosphorus, at approximately 416°C. The modified molecular sieve of this invention, in a thermogravimetric analysis in an air atmosphere, shows a weight gain of approximately 5-25% at 800°C compared to the minimum mass during the heating process from 20°C to 800°C. In contrast, molecular sieves modified with phosphorus sources such as phosphoric acid or ammonium hydrogen phosphate in the prior art do not exhibit a significant percentage increase in mass during TGA because the phosphorus source does not undergo a significant oxidation and weight gain process after heating.

[0014] Phosphorus species on the outer surface of the modified ZSM-5 molecular sieve are more easily washed away with water; therefore, the phosphorus retention rate can be determined by measuring the phosphorus content before and after washing. The modified molecular sieve of this invention has a phosphorus retention rate higher than 50%. The phosphorus retention rate is defined as the mass percentage of phosphorus after washing to that before washing. Washing refers to placing the modified molecular sieve in water at 70°C and stirring thoroughly for 2 hours. The phosphorus is calculated as P2O5. Compared to the existing technology using diammonium hydrogen phosphate modification, which achieves a phosphorus retention rate of less than 50%, this invention uses elemental phosphorus modification. Elemental phosphorus more easily enters the pores of the ZSM-5 molecular sieve during the modification process, resulting in phosphorus retention rates consistently above 50%, for example, 52-59%, close to 60%, indicating that elemental phosphorus modification has a high phosphorus utilization rate.

[0015] The phosphorus retention rate was determined by XRF characterization. During the phosphorus modification process of ZSM-5 molecular sieve, phosphorus migrates into the sieve channels, but some phosphorus remains attached to the surface of the molecular sieve and is oxidized to phosphorus pentoxide at high temperatures. After washing the modified molecular sieve sample with water, the phosphorus pentoxide on the surface of the molecular sieve is converted into phosphoric acid and washed away. Therefore, the phosphorus content measured after washing and drying is the phosphorus content within the crystal.

[0016] The modified molecular sieve of the present invention has a ratio of 0.01 to 5 when both phosphorus and aluminum are measured in moles, preferably a ratio of 0.1 to 3 when both phosphorus and aluminum are measured in moles.

[0017] The modified ZSM-5 molecular sieve provided by this invention can be prepared by two methods: solvent mixing and direct mixing. More specifically, the modified molecular sieve of this invention is prepared by mixing elemental phosphorus with ZSM-5 molecular sieve using a solvent, filtering, and then drying or directly evaporating to obtain a mixture of ZSM-5 molecular sieve containing elemental phosphorus; or the modified molecular sieve of this invention is prepared by directly mixing elemental phosphorus with ZSM-5 molecular sieve to obtain a mixture of ZSM-5 molecular sieve containing elemental phosphorus.

[0018] This invention uses elemental phosphorus with a valence of 0 as the phosphorus source to modify ZSM-5 molecular sieves. By selecting elemental phosphorus with a relatively small molecular weight of 0, the migration of phosphorus species into the pores of ZSM-5 molecular sieves is accelerated via gas-solid two-phase mass transfer, thereby increasing the activation efficiency of phosphorus-aluminum species. The elemental phosphorus is selected from one or more of red phosphorus, white phosphorus, and black phosphorus, with red phosphorus being preferred. Using elemental phosphorus as the phosphorus source, after calcination, the phosphorus species are converted from 0 to a higher oxidation state, enabling them to stabilize the skeletal aluminum framework during the aging process.

[0019] In the preparation of the modified molecular sieve, the solvent is preferably water, methanol, or ethanol. The calcination is preferably carried out at 200–800°C for 0.5–12 hours in an air atmosphere. The molar ratio of the ZSM-5 molecular sieve to elemental phosphorus is (0.01–5):1, preferably (0.1–3):1, wherein the ZSM-5 molecular sieve is calculated as aluminum, and the phosphorus-containing compound is calculated as phosphorus.

[0020] In the modified sieve of the present invention, the coordination effect between phosphorus species and skeletal aluminum is significant, which fully protects the skeletal aluminum and has a higher crystallinity retention. After hydrothermal aging at 800°C and 100% steam for 17 hours, the relative crystallinity is ≥90%, preferably 91-99%, and it has excellent hydrothermal stability.

[0021] The modified molecular sieve of this invention can be used as an active component of catalysts in catalytic cracking reactions. In the cracking reaction of n-tetradecane, the main indicators are improved compared with the control sample, such as higher conversion rate and liquefied gas yield, while the yields of trienes (ethylene, propylene and butene) are significantly improved. Attached Figure Description

[0022] Figure 1 Thermogravimetric analysis diagram of phosphorus precursor.

[0023] Figure 2 This is a thermogravimetric analysis diagram of Example 1.

[0024] Figure 3 The thermogravimetric analysis diagram is for Comparative Example 1. Detailed Implementation

[0025] The present invention will be further illustrated by the following examples, but these examples do not limit the scope of the invention.

[0026] Thermogravimetric analysis (TG) was used to characterize the physicochemical phenomena of phosphorus precursors, such as oxidation and sublimation. A STA409PC thermogravimetric analyzer from Netzsch GmbH, Germany, was used. The test conditions were: air as the carrier gas, and the temperature was increased from 20°C to 800°C at a rate of 10 K / min.

[0027] 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.

[0028] X-ray fluorescence spectroscopy (XRF) was used to analyze the phosphorus content of molecular sieves and to examine the phosphorus retention rate after water washing. A Rigaku ZSX100E X-ray fluorescence spectrometer was used. Test conditions: the sample was tested after pelleting, with a tungsten target, an excitation voltage of 40 kV, and an excitation current of 250 mA.

[0029] In the following embodiments of the present invention, unless otherwise specified, all chemical reagents used are commercially available products.

[0030] Example 1

[0031] Red phosphorus and ZSM-5 molecular sieve (nSiO2 / nAl2O3=30) were stoichiometrically mixed at a phosphorus-aluminum molar ratio of 0.5:1, and water was added to make a slurry. After filtration and drying, a filter cake was obtained. The filter cake was placed in a crucible and heated in a muffle furnace at 450℃ for 2 hours and recovered to obtain a phosphorus-modified ZSM-5 molecular sieve sample, numbered A1.

[0032] Comparative Example 1

[0033] Comparative Example 1 used the same parent ZSM-5 molecular sieve as Example 1. Diammonium hydrogen phosphate (DHP) with a phosphorus-aluminum molar ratio of 0.5:1 was dissolved in deionized water. An equal volume of the DHP solution was impregnated with the molecular sieve and mixed thoroughly. After drying, the mixture was heated in a muffle furnace at 550°C for 2 hours. This method is the industrially applied method for phosphorus modification of ZSM-5 molecular sieve. A phosphorus-modified ZSM-5 molecular sieve sample was recovered and designated DA1.

[0034] Example 2

[0035] Red phosphorus and ZSM-5 molecular sieve (nSiO2 / nAl2O3 = 30) were stoichiometrically mixed at a phosphorus-aluminum molar ratio of 1:1, and water was added to form a slurry. The mixture was then filtered and dried to obtain a filter cake. The filter cake was placed in a crucible and heated in a muffle furnace at 450°C for 2 hours. The resulting phosphorus-modified ZSM-5 molecular sieve sample was then recovered and designated as A2.

[0036] Comparative Example 2

[0037] Comparative Example 2 used the same parent ZSM-5 molecular sieve as Example 1. Diammonium hydrogen phosphate (DHP) with a phosphorus-aluminum molar ratio of 1:1 was dissolved in deionized water. An equal volume of the DHP solution was impregnated with the molecular sieve and mixed thoroughly. After drying, the mixture was heated in a muffle furnace at 550°C for 2 hours. This method is the industrially applied method for phosphorus modification of ZSM-5 molecular sieve. A phosphorus-modified ZSM-5 molecular sieve sample was recovered and designated DA2.

[0038] Example 3

[0039] Red phosphorus and ZSM-5 molecular sieve (nSiO2 / nAl2O3 = 30) were stoichiometrically mixed at a phosphorus-aluminum molar ratio of 1.5:1, and then water was added to form a slurry. After filtration and drying, a filter cake was obtained. The filter cake was placed in a crucible and heated in a muffle furnace at 450°C for 2 hours. The phosphorus-modified ZSM-5 molecular sieve sample was recovered and designated as A3.

[0040] Comparative Example 3

[0041] Comparative Example 3 used the same parent ZSM-5 molecular sieve as Example 1. Diammonium hydrogen phosphate (DHP) with a phosphorus-aluminum molar ratio of 1.5:1 was dissolved in deionized water. An equal volume of the DHP solution was impregnated with the molecular sieve and mixed thoroughly. After drying, the mixture was heated in a muffle furnace at 550°C for 2 hours. This method is the industrially applied method for phosphorus modification of ZSM-5 molecular sieves. A phosphorus-modified ZSM-5 molecular sieve sample was recovered and designated DA3.

[0042] Samples A1, A2, A3 and control samples DA1, DA2, DA3 were stirred in water at 70℃ for 2 h and filtered. The dried samples were then characterized by XRF to obtain the phosphorus-aluminum ratio after water washing. The phosphorus retention rate was (P2O5 after water washing / P2O5 after calcination)*100%. The XRF and XRD characterization data are listed in Table 1.

[0043] The relative crystallinity of samples A1, A2, A3 and control samples DA1, DA2, DA3 was measured after hydrothermal aging at 800℃ for 17 hours. The data are listed in Table 1.

[0044] Table 1

[0045]

[0046] Compared to diammonium hydrogen phosphate modification, elemental phosphorus is more likely to penetrate the pores of ZSM-5 molecular sieves during modification. Washing the calcined samples with water removes phosphorus species from the outer surface of the ZSM-5 molecular sieve more easily; therefore, the phosphorus retention rate can be determined by measuring the phosphorus content before and after washing. As shown in Table 1, the phosphorus retention rates of the elemental phosphorus modified samples with different phosphorus-aluminum ratios in Examples 1-3 ranged from 52% to 60%, with all retention rates exceeding 50%, reaching nearly 60% in some cases. This indicates that elemental phosphorus modification has a high phosphorus utilization rate.

[0047] Compared to diammonium hydrogen phosphate modification, the elemental phosphorus modification of this invention exhibits a significant coordination effect between phosphorus species and skeletal aluminum, thus fully protecting the skeletal aluminum and resulting in higher crystallinity retention. As shown in Table 1, the relative crystallinity is 96-98%, while the comparative sample only has 83-86%, demonstrating that the modified molecular sieve of this invention has excellent hydrothermal stability.

[0048] Figure 1 Thermogravimetric analysis diagram of elemental phosphorus as a phosphorus precursor when it is red phosphorus. Figure 1 The weight gain at 300-420℃ is the oxidation process of red phosphorus, and the weight loss at 420-500℃ is the sublimation process of red phosphorus.

[0049] Figure 2 The thermogravimetric analysis (TGA) chromatogram for sample A1 shows a weight gain of approximately 18% (M800) at 800℃ compared to the minimum mass value observed during this process. This weight gain is attributed to the oxidation of elemental red phosphorus in the modified sample. Similarly, the TGA chromatograms for samples A2 and A3 show... Figure 2 The characteristics of these materials are that the ratios of their mass at 800℃ (M800) to the minimum mass during the heating process are 1.15 and 1.21, respectively, with weight gains of 11.9% and 16.8% by mass.

[0050] Figure 3 To compare the thermogravimetric analysis of sample DA1, it can be seen that there is no obvious weight gain, and the weight loss is only caused by dehydration.

[0051] Test Example 1

[0052] Evaluation of n-tetradecane cracking was conducted under the following microreactor conditions: molecular sieve loading 2g, feedstock n-tetradecane, feed rate 1.56g, reaction temperature 550℃, and regeneration temperature 600℃. The evaluation results are listed in Table 2.

[0053] Table 2

[0054] Material balance / m% dry air 4.02 3.25 4.15 3.37 4.59 3.41 Liquefied gas 51.26 38.66 52.02 40.76 53.66 40.94 gasoline 24.93 23.78 25.87 22.72 26.64 23.19 diesel fuel 19.11 33.89 18.91 32.93 17.73 32.22 m% of main products in cracked gas ethylene 3.37 2.74 3.41 2.77 3.72 2.72 propylene 17.15 14.49 17.93 14.58 18.65 14.97 Total butene 14.69 13.67 15.57 13.65 16.56 14.08 Conversion rate / m% 82.11 65.57 83.32 66.15 84.27 67.01

[0055] As can be seen from the data in Table 2, compared with the phosphorus-modified control sample DA1 prepared by the industrially widely used diammonium hydrogen phosphate modification method in Comparative Example 1, sample A1 obtained by elemental phosphorus modification exhibits better catalytic cracking activity after hydrothermal aging. When the phosphorus-aluminum ratio is 0.5, the conversion rate of n-tetradecane and the yield of low-carbon olefins are significantly higher than those in Comparative Example 1; as the phosphorus-aluminum ratio increases, the activity and selectivity of the elemental phosphorus-modified sample are still significantly better than the corresponding control samples.

[0056] Based on the above reaction evaluation test results, it can be concluded that elemental phosphorus modification can effectively enhance the hydrothermal stability of ZSM-5 molecular sieve, and its effect is better than the diammonium hydrogen phosphate modification technology widely used in industry.

[0057] Example 4

[0058] Red phosphorus and ZSM-5 molecular sieve (nSiO2 / nAl2O3=60) were stoichiometrically mixed at a phosphorus-aluminum molar ratio of 0.5:1, and water was added to make a slurry. After filtration and drying, a filter cake was obtained. The filter cake was placed in a crucible and heated in a muffle furnace at 450℃ for 2 hours and recovered to obtain a phosphorus-modified ZSM-5 molecular sieve sample, numbered A4.

[0059] Comparative Example 4

[0060] Comparative Example 4 used the same parent ZSM-5 molecular sieve as Example 4. Diammonium hydrogen phosphate (DHP) with a phosphorus-aluminum molar ratio of 0.5:1 to the molecular sieve was dissolved in deionized water. An equal volume of the DHP solution was impregnated with the molecular sieve and mixed thoroughly. After drying, the mixture was heated in a muffle furnace at 550°C for 2 hours. This method is the industrially applied method for phosphorus modification of ZSM-5 molecular sieve. A phosphorus-modified ZSM-5 molecular sieve sample was recovered and designated DA4.

[0061] Example 5

[0062] Red phosphorus and ZSM-5 molecular sieve (nSiO2 / nAl2O3=120) were stoichiometrically mixed at a phosphorus-aluminum molar ratio of 0.5:1, and water was added to make a slurry. After filtration and drying, a filter cake was obtained. The filter cake was placed in a crucible and heated in a muffle furnace at 450℃ for 2 hours and recovered to obtain a phosphorus-modified ZSM-5 molecular sieve sample, numbered A5.

[0063] Comparative Example 5

[0064] Comparative Example 5 used the same parent ZSM-5 molecular sieve as Example 5. Diammonium hydrogen phosphate (DHP) with a phosphorus-aluminum molar ratio of 0.5:1 to the molecular sieve was dissolved in deionized water. An equal volume of the DHP solution was impregnated with the molecular sieve and mixed thoroughly. After drying, the mixture was heated in a muffle furnace at 550°C for 2 hours. This method is the industrially applied method for phosphorus modification of ZSM-5 molecular sieve. A phosphorus-modified ZSM-5 molecular sieve sample was recovered and designated DA5.

[0065] Samples A4 and A5, and control samples DA4 and DA5 were stirred in water at 70℃ for 2 hours and then filtered. The dried samples were characterized by XRF to obtain the phosphorus-aluminum ratio after water washing. The phosphorus retention rate was (P2O5 after water washing / P2O5 after calcination)*100%. The XRF and XRD characterization data are listed in Table 3.

[0066] By performing the same thermogravimetric analysis as in Example 1, the ratios of the mass M800 of samples A4 and A5 at 800°C to the minimum mass during the heating process were 1.13 and 1.19, respectively, with weight gains of 10.7% and 15.2% by mass.

[0067] Table 3

[0068]

[0069] Test Example 2

[0070] Evaluation of n-tetradecane cracking was conducted under the following microreactor conditions: molecular sieve loading 2g, feedstock n-tetradecane, feed rate 1.56g, reaction temperature 550℃, and regeneration temperature 600℃. The evaluation results are listed in Table 4.

[0071] Table 4

[0072] Material balance / m% dry air 3.02 2.88 2.59 2.08 Liquefied gas 28.42 21.16 19.42 18.65 gasoline 30.15 36.07 30.64 29.15 diesel fuel 38.41 40.59 47.37 50.11 m% of main products in cracked gas ethylene 2.98 2.45 2.47 2.36 propylene 11.54 10.37 9.65 8.51 Total butene 13.68 12.14 11.56 10.52 Conversion rate / m% 52.68 45.03 48.48 42.41

Claims

1. A modified molecular sieve, characterized in that, This molecular sieve is obtained by modifying ZSM-5 molecular sieve with elemental phosphorus. In thermogravimetric analysis (TGA) under air conditions, the elemental phosphorus shows weight gain between 330℃ and 420℃ and weight loss between 420℃ and 500℃. In TGA analysis under air conditions, from 20℃ to 800℃, the ratio of the molecular sieve's mass at 800℃ to its minimum mass during the heating process is 1.05-1.

25. The preparation process of this modified molecular sieve includes mixing elemental phosphorus with ZSM-5 molecular sieve using a solvent, filtering, and then drying or directly evaporating to obtain a mixture of ZSM-5 molecular sieve containing elemental phosphorus, followed by calcination. Alternatively, the preparation process includes directly mixing elemental phosphorus with ZSM-5 molecular sieve to obtain a mixture of ZSM-5 molecular sieve containing elemental phosphorus, followed by calcination.

2. The modified molecular sieve according to claim 1, characterized in that, The phosphorus content retention rate is higher than 50%, which is defined as the mass percentage of phosphorus after the modified molecular sieve is washed with water to that before washing. The washing process involves placing the modified molecular sieve in water at 70°C and stirring for 2 hours. The phosphorus content is calculated as P2O5.

3. The modified molecular sieve according to claim 2, characterized in that, The phosphorus content retention rate is 52-59%.

4. The modified molecular sieve according to claim 1, characterized in that, When both phosphorus and aluminum are measured in moles, their ratio is 0.01 to 5.

5. The modified molecular sieve according to claim 1, characterized in that, When both phosphorus and aluminum are measured in moles, their ratio is 0.1 to 3.

6. The modified molecular sieve according to claim 1, characterized in that, The elemental phosphorus is selected from one or more of red phosphorus, white phosphorus, and black phosphorus.

7. The modified molecular sieve according to claim 1, wherein, The solvent is water, methanol, or ethanol.

8. The modified molecular sieve according to claim 1, characterized in that, The molar ratio of the ZSM-5 molecular sieve to the elemental phosphorus is (0.01-5):1, wherein the ZSM-5 molecular sieve is calculated as aluminum.

9. The modified molecular sieve according to claim 8, characterized in that, The molar ratio of the ZSM-5 molecular sieve to the elemental phosphorus is (0.1-3):1, wherein the ZSM-5 molecular sieve is calculated as aluminum.

10. The modified molecular sieve according to claim 1, characterized in that, After hydrothermal aging at 800℃ and 100% water vapor for 17 hours, the relative crystallinity is ≥90%.

11. The modified molecular sieve according to claim 10, characterized in that, After hydrothermal aging at 800℃ and 100% water vapor for 17 hours, the relative crystallinity is 91-99%.

12. The application of the modified molecular sieve according to any one of claims 1-11 in catalytic cracking.

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