Phosphorus modified molecular sieve as well as preparation method and application thereof

Abstract

The invention provides a phosphorus-modified molecular sieve as well as a preparation method and application thereof, and relates to the technical field of molecular sieves. The preparation method comprises the following steps: mixing a molecular sieve, water and 3-aminopropyl phosphoric acid, and sequentially concentrating and drying the obtained mixed solution to obtain a molecular sieve loaded with 3-aminopropyl phosphoric acid; and roasting the molecular sieve loaded with 3-aminopropyl phosphoric acid to obtain the phosphorus modified molecular sieve. According to the invention, 3-aminopropyl phosphoric acid is used as a phosphorus source to modify the molecular sieve, amino group functionalization endows the 3-aminopropyl phosphoric acid and the molecular sieve with more binding sites, and after roasting, phosphate group is broken to generate small-size phosphorus oxide and generate stable tetra-coordinated phosphorus species, and the tetra-coordinated phosphorus species are more firmly combined with a molecular sieve skeleton structure through Al-O-P chemical bonds. The phosphorus-modified molecular sieve prepared by the method has excellent catalytic cracking performance, and the raw material conversion rate, the diene selectivity and the cracking stability are remarkably improved when the phosphorus-modified molecular sieve is used as a catalyst in the reaction of preparing low-carbon olefin by catalytic cracking.

Description

Technical Field

[0001] The present invention relates to the technical field of molecular sieves, and in particular to a phosphorus-modified molecular sieve and a preparation method and application thereof. Background Art

[0002] "Light olefins" primarily include ethylene and propylene, which are key petrochemical raw materials. Ethylene is used as a raw material for over 75% of petrochemical products, and propylene, the second largest chemical raw material after ethylene, continues to see growing demand. Light olefins are primarily produced through processes such as steam cracking, catalytic cracking, and methanol-to-olefins. Steam cracking typically requires very high reaction temperatures (>800°C), resulting in significant greenhouse gas emissions during production. While the methanol-to-olefins process has, to a certain extent, eliminated its dependence on petroleum raw materials, the upstream coal-to-methanol process produces significant amounts of CO2. Catalytic cracking, on the other hand, operates at a reaction temperature approximately 200°C lower than steam cracking, significantly reducing energy consumption and offering higher light olefin selectivity and flexible feedstock adaptability. The catalytic cracking process demonstrates significantly greater economic efficiency and sustainability, offering broad prospects for development and application.

[0003] Catalysts are key factors influencing the effectiveness of catalytic cracking to produce light olefins. Molecular sieves, due to their unique pore structure, precisely tunable surface acidity, and strong resistance to carbon deposition, are the catalyst of choice for catalytic cracking reaction systems. Representative molecular sieves currently used in catalytic cracking reaction systems include MFI (double ten-membered intersecting pores), FAU (three-dimensional intersecting pores of twelve-membered and six-membered rings), *BEA (three-dimensional intersecting pores of twelve-membered rings), CHA (three-dimensional linear intersecting pores of eight-membered rings), and MSE (three-dimensional intersecting pores of ten-membered and twelve-membered rings). The unique cage structure and acid density of molecular sieves are key factors in achieving the confined catalytic effect. Optimizing acid strength is a common modification method to enhance the catalytic cracking performance of molecular sieves, typically involving phosphorus modification. The mechanism of phosphorus action on the acidic sites of molecular sieves is primarily to promote the hydrolysis of Brønsted acid sites (bridging hydroxyl groups Si-O-Al). Phosphorus replaces hydrogen atoms and interacts with oxygen atoms in the Si-O-Al bond, resulting in Si-O-Al cleavage and the replacement of hydroxyl groups by P-OH. Since the P-OH strength is less than the Si-OH strength, the Brønsted acid strength, especially the strong Brønsted acid strength, is reduced. In contrast, phosphorus has a relatively small effect on Lewis acid sites, which leads to a decrease in the total acid content and Brønsted / Lewis acid ratio of the molecular sieve.

[0004] Phosphorus modification generally uses inorganic phosphorus compounds as precursors, primarily phosphoric acid (H3PO4), ammonium dihydrogen phosphate, and trimethyl phosphate (TMP). Phosphoric acid incorporation into zeolites can significantly enhance their catalytic activity and selectivity. Phosphoric acid interacts with Si-OH-Al groups, potentially forming new acidic sites from the P-OH groups, converting them into weaker Brønsted acid sites, resulting in a decrease in acid strength, while weaker acids remain largely unaffected. However, due to its strong acidity, phosphoric acid can cause significant dealumination of the zeolite framework. Modification of zeolites with phosphate compounds (such as ammonium dihydrogen phosphate) can neutralize these strong acidic sites, limiting the conversion of light olefins to aromatics. Some believe that the change in selectivity is primarily due to the phosphate compound bonding to the zeolite framework, narrowing the pore size. Trimethyl phosphate can improve the structural integrity and specific surface area of ​​steam-treated zeolites and regulate the distribution of surface acid sites. The researchers compared the performance of ZSM-5 zeolites impregnated with trimethyl phosphate and ammonium dihydrogen phosphate. The results showed that the trimethyl phosphate-modified ZSM-5 maintained high crystallinity, large specific surface area and micropore volume, while retaining abundant surface acid sites, which is more advantageous than the ammonium dihydrogen phosphate-modified ZSM-5. However, after calcination, phosphoric acid and trimethyl phosphate easily form large-sized phosphates through condensation reaction. Their kinetic diameter can reach more than 1.0 nm, which is close to the size of the molecular sieve pores. This causes phosphorus species to be enriched on the outer surface of the molecular sieve and difficult to penetrate into the pores. This will block the reactant molecules from entering the molecular pores, resulting in the inability to contact the acid active sites inside the pores, which significantly reduces the catalytic cracking performance of the molecular sieve. Summary of the Invention

[0005] In view of this, the object of the present invention is to provide a phosphorus-modified molecular sieve and its preparation method and application. The phosphorus-modified molecular sieve prepared by the present invention has excellent catalytic cracking performance.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions: The present invention provides a method for preparing a phosphorus-modified molecular sieve, comprising the following steps: Mixing molecular sieves, water and 3-aminopropyl phosphoric acid, and sequentially concentrating and drying the resulting mixture to obtain a molecular sieve loaded with 3-aminopropyl phosphoric acid; the molecular sieve comprises one or more of MFI, MSE, *BEA and FAU type molecular sieves; The molecular sieve loaded with 3-aminopropyl phosphoric acid is calcined to obtain the phosphorus-modified molecular sieve.

[0007] Preferably, the mass of the phosphorus element in the 3-aminopropyl phosphoric acid is 0.1-1.0% of the mass of the molecular sieve.

[0008] Preferably, the mass ratio of water to molecular sieve is 10:1 to 50:1.

[0009] Preferably, the mixing method is: first mixing the molecular sieve and water to obtain a molecular sieve dispersion; The molecular sieve dispersion is mixed with 3-aminopropyl phosphoric acid for a second time to obtain the mixed solution.

[0010] Preferably, the temperature of the second mixing is 40-60°C.

[0011] Preferably, the concentration method is rotary evaporation, and the rotary evaporation temperature is 80-100°C; the drying temperature is 80-100°C.

[0012] Preferably, the calcination temperature is 500-600° C. and the calcination time is 6-10 hours.

[0013] The present invention provides a phosphorus-modified molecular sieve prepared by the preparation method described in the above technical solution.

[0014] The present invention provides the use of the phosphorus-modified molecular sieve described in the above technical solution in the catalytic cracking reaction of normal alkanes to produce light olefins.

[0015] Preferably, the n-alkane has 5 to 9 carbon atoms.

[0016] The present invention provides a method for preparing a phosphorus-modified molecular sieve. Compared with the prior art, the present invention has the following beneficial effects: the present invention uses 3-aminopropyl phosphoric acid as a phosphorus source to modify the molecular sieve. After calcination, the phosphate group of 3-aminopropyl phosphoric acid breaks to produce small-sized phosphorus oxides, generating stable tetracoordinate phosphorus species with small molecular size that easily enters the pores of the molecular sieve. In addition, 3-aminopropyl phosphoric acid contains both amino and phosphate groups, which can strongly interact with the molecular sieve framework, more easily forming Al-OP bonds, and more firmly bonding to the molecular sieve framework structure through Al-OP chemical bonds, thereby converting some strong Brønsted acid sites into weak Brønsted acid sites. The phosphorus-modified molecular sieve prepared by the present invention has excellent catalytic cracking performance. In a reaction system for catalytic cracking to produce light olefins, the use of the phosphorus-modified molecular sieve as a catalyst significantly improves the raw material conversion rate, diene selectivity, and cracking stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The unmodified UZM-35, 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples 27 Al MAS NMR ( 27 Al magic angle spinning solid-state NMR) spectrum, Figure 1 (a) is the unmodified UZM-35 and 3APPi-0.3~3.0-UZM samples 27Al MAS NMR spectra, (b) unmodified UZM-35 and TMP-0.3~3.0-UZM samples 27 Al MAS NMR spectrum; Figure 2 The unmodified UZM-35, 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples 31 P MAS NMR ( 31 P magic angle spinning solid-state NMR) spectrum, Figure 2 (a) is the unmodified UZM-35 and 3APPi-0.3~3.0-UZM samples 31 P MAS NMR spectra, (b) unmodified UZM-35 and TMP-0.3~3.0-UZM samples 31 P MAS NMR spectrum; Figure 3 The conversion results of n-heptane catalytic cracking by unmodified UZM-35, 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples are shown in Table 1. Figure 3 (a) shows the conversion rate of n-heptane by catalytic cracking of unmodified UZM-35 and 3APPi-0.3~3.0-UZM samples, and (b) shows the conversion rate of n-heptane by catalytic cracking of unmodified UZM-35 and TMP-0.3~3.0-UZM samples. Figure 4 The product selectivity results of the catalytic cracking of n-heptane by unmodified UZM-35, 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples are shown; Figure 5 The stability results of the catalytic cracking of n-heptane by unmodified UZM-35, 3APPi-1.0-UZM and TMP-1.0-UZM samples are shown; Figure 6 The conversion results of n-heptane catalytic cracking by unmodified ZSM-5 and 3APPi-0.3~3.0-ZSM samples are shown; Figure 7 The product yield and selectivity results of the catalytic cracking of n-heptane by unmodified ZSM-5 and 3APPi-0.3~3.0-ZSM samples are shown; Figure 8 The stability results of the catalytic cracking of n-heptane by unmodified ZSM-5 and 3APPi-1.0-ZSM samples are shown. DETAILED DESCRIPTION

[0018] The present invention provides a method for preparing a phosphorus-modified molecular sieve, comprising the following steps: Mixing molecular sieves, water and 3-aminopropyl phosphoric acid, and sequentially concentrating and drying the resulting mixture to obtain a molecular sieve loaded with 3-aminopropyl phosphoric acid; the molecular sieve comprises one or more of MFI, MSE, *BEA and FAU type molecular sieves; The molecular sieve loaded with 3-aminopropyl phosphoric acid is calcined to obtain the phosphorus-modified molecular sieve.

[0019] In the present invention, unless otherwise specified, all raw materials involved are commercially available products well known in the art.

[0020] The invention mixes molecular sieve, water and 3-aminopropyl phosphoric acid, and sequentially concentrates and dries the obtained mixed solution to obtain the molecular sieve loaded with 3-aminopropyl phosphoric acid.

[0021] In the present invention, the molecular sieve comprises one or more of MFI, MSE, *BEA, and FAU molecular sieves. There are no specific requirements for these MFI, MSE, *BEA, and FAU molecular sieves; any molecular sieve known to those skilled in the art may be used. For example, the MSE molecular sieve may be UZM-35, and the MFI molecular sieve may be ZSM-5. In the present invention, the molecular sieve is a powder, and there are no specific requirements for the particle size of the powder. In the present invention, the water is preferably deionized water. The mass ratio of water to molecular sieve (i.e., liquid-to-solid ratio) is preferably 10:1 to 50:1, and may be 10:1:20:1, 30:1, 40:1, or 50:1. Controlling the water-to-molecular sieve mass ratio within this range facilitates the dispersion and diffusion of 3-aminopropyl phosphoric acid, preventing agglomeration, and increases the effective loading of 3-aminopropyl phosphoric acid.

[0022] In the present invention, 3-Aminopropylphosphonic Acid (3APPi) has a molecular formula of H2N(CH2)3P(O)(OH)2 and a CAS number of 13138-33-5. 3APPi is a bifunctional organophosphorus compound containing both an amino group (-NH2) and a phosphate group (-PO(OH)2). Its unique molecular structure imparts it with excellent chemical stability, chelating ability, and surface modification potential. The mass of phosphorus in 3APPi is preferably 0.1-1.0% of the molecular sieve mass, and may be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1.0%, more preferably 0.6-1.0%. Controlling the mass ratio of 3APPi within this range reduces the acid strength of the molecular sieve, inhibits hydrogen transfer reactions, and significantly improves the ethylene selectivity and diene yield of the molecular sieve, while maintaining the diffusion properties of the molecular sieve's pore structure.

[0023] In the present invention, the mixing method is preferably: first mixing the molecular sieve and water to obtain a molecular sieve dispersion; The molecular sieve dispersion is mixed with 3-aminopropyl phosphoric acid for a second time to obtain the mixed solution.

[0024] In the present invention, the first mixing is preferably performed by stirring, and the stirring is performed to ensure uniform mixing. The first mixing can be performed at room temperature (i.e., without additional heating or cooling). In the present invention, 3-aminopropyl phosphoric acid is preferably added to the molecular sieve dispersion for second mixing. In the present invention, the temperature of the second mixing is preferably 40-60°C, and may be 40, 50, or 60°C, and the duration is preferably 6 hours or more, and may be 10 hours. The second mixing is preferably performed under stirring.

[0025] In the present invention, the concentration method is preferably rotary evaporation, and the temperature of the rotary evaporation is preferably 80-100° C., and can be 80, 90 or 100° C. The rotary evaporation is carried out when the liquid level no longer drops.

[0026] In the present invention, the drying temperature is preferably 80-100° C., and may be 80, 90 or 100° C., and the drying time is preferably more than 12 hours, and may be 12 hours; the drying is preferably carried out in a vacuum oven.

[0027] The present invention adopts a wet impregnation method, that is, a molecular sieve, water and 3-aminopropyl phosphoric acid are mixed, and then concentrated and dried. The 3-aminopropyl phosphoric acid is loaded on the molecular sieve in a chemically modified manner (the presence of the amino group can strengthen the binding of the 3-aminopropyl phosphoric acid to the molecular sieve framework), thereby obtaining a molecular sieve loaded with 3-aminopropyl phosphoric acid.

[0028] After obtaining the molecular sieve loaded with 3-aminopropyl phosphoric acid, the present invention calcines the molecular sieve loaded with 3-aminopropyl phosphoric acid to obtain the phosphorus-modified molecular sieve.

[0029] Before calcination, the present invention preferably grinds the molecular sieve loaded with 3-aminopropyl phosphoric acid to obtain a uniform powder.

[0030] In the present invention, the calcination (also known as calcination) is preferably performed at a temperature of 500-600°C, preferably 500, 550, or 600°C, and for a time of 6-10 hours, preferably 6, 7, 8, 9, or 10 hours. The calcination can be performed in an air atmosphere. In an embodiment of the present invention, the calcination is performed in a muffle furnace. During the calcination process, excess phosphorus source is removed and phosphorus is incorporated into the molecular sieve framework. Specifically, the amino group (-NH2) of 3-aminopropylphosphoric acid is converted to NH3. After calcination, the phosphate group (-PO3) breaks down to produce small-sized phosphorus oxides, forming a stable tetracoordinate phosphorus species, which is bound to the molecular sieve framework via PO-Al bonds.

[0031] This invention, for the first time, uses 3-aminopropyl phosphoric acid as a molecular sieve modifier. It is loaded onto the molecular sieve via an impregnation method and then calcined to produce a phosphorus-modified molecular sieve with adjustable phosphorus loading. In this invention, 3-aminopropyl phosphoric acid is a highly efficient molecular sieve modifier, significantly improving the catalytic cracking performance of the modified molecular sieve, providing new insights into the design and modification of molecular sieves.

[0032] The present invention provides a phosphorus-modified molecular sieve prepared by the preparation method described in the above technical solution. 31 The P MAS NMR spectrum shows resonance signal peaks with chemical shifts of 0±3ppm, -5±3ppm, -10±3ppm, -22±3ppm, and -32±3ppm. The peak near δ=0±3ppm is attributed to the orthophosphoric acid (PO4) structure, and the peak at δ=-5±3ppm can be attributed to two structures: oligomeric phosphorus species or phosphorus-oxygen tetrahedral structures bonded to a single aluminum atom. Phosphate species bonded to aluminum passivate Brønsted acid sites, providing relatively weak acid sites and gradually transforming into more highly polymerized phosphorus species as the phosphorus loading increases. The peak at δ=-10±3ppm corresponds to moderately polymerized phosphorus, which stabilizes the skeleton aluminum and reduces high-temperature dealumination. The peak at δ=-22±3ppm is attributed to the presence of polymerized phosphorus with bidentate Al-OP bonds, and the peak at δ=-32±3ppm is attributed to polyphosphorus or branched polymeric phosphorus. With the increase of phosphorus content, the peak area and intensity between 0 and -50 ppm shift increase, indicating that the bonding form of phosphorus species and aluminum increases. 27In the Al MASNMR spectrum, resonance signal peaks with chemical shifts of 57±3ppm, 0±3ppm and -10±3ppm appear; among them, the characteristic peak at δ=57±3ppm corresponds to the tetrahedral coordinated aluminum structure in the molecular sieve framework; the characteristic peak at δ=0±3 ppm corresponds to the non-framework octa-coordinated aluminum species, generally non-framework Al-(H2O)6, and the coordination effect of hexa-coordinated aluminum with phosphorus will cause its resonance peak shift to gradually decrease; the characteristic peak at δ=-10±3ppm belongs to the distorted framework octahedral aluminum.

[0033] The present invention provides the use of the phosphorus-modified molecular sieve described in the above technical solution in the catalytic cracking reaction of normal alkanes to produce light olefins.

[0034] In the present invention, the carbon number of the normal alkane is preferably 5 to 9, and the normal alkane can be derived from naphtha. In the embodiment of the present invention, the normal alkane is n-heptane as an example for illustration. In the present invention, the temperature of the catalytic cracking reaction of normal alkane to produce light olefins is preferably 520 to 680°C, and can be 540, 560, 580, 600, 620, 640 or 660°C, and the mass space velocity is preferably 2 to 8 g·g Cat. -1 ·h -1 , can be 2, 3, 4, 5, 6, 7 or 8 g g Cat. -1 ·h -1 , preferably at atmospheric pressure. In the present invention, the light olefins primarily include ethylene and propylene. The phosphorus-modified molecular sieve provided by the present invention has excellent catalytic cracking performance. In a reaction system for catalytic cracking to produce light olefins, the use of the phosphorus-modified molecular sieve as a catalyst significantly improves feedstock conversion, diene selectivity, and cracking stability.

[0035] To further illustrate the present invention, the phosphorus-modified molecular sieve provided by the present invention, its preparation method and application are described in detail below with reference to examples, but they should not be construed as limiting the scope of protection of the present invention.

[0036] In the embodiment, 31 P MAS NMR ( 31 The local chemical environment of phosphorus atoms in the molecular sieve samples was determined using an Avance 600 AV solid-state NMR spectrometer from Bruker, Germany. The test was performed at a magnetic field strength of 11.7 T, an operating frequency of 161.9 MHz, a rotation speed of 6-20 kHz, a pulse interval of 2 s, and chemical shift calibration using 85% HPO3 solution as the reference.

[0037] 27 Al MAS NMR ( 27Al magic angle spinning solid-state nuclear magnetic resonance (MAS) was performed using an Avance 600AV solid-state nuclear magnetic resonance spectrometer from Bruker, Germany, to determine the local chemical environment of aluminum atoms in the molecular sieve samples. The test was performed at a magnetic field strength of 11.7 T, an operating frequency of 130.2 MHz, a sample spin rate of 13 kHz, a pulse interval of 2 s, and chemical shift calibration using 0.1 mol / L Al(NO3)3 solution as a reference.

[0038] X-ray diffraction analysis (XRD) was performed using a D8 Advance X-ray polycrystal diffractometer from Bruker, Germany, using Cu target Kα radiation (wavelength λ = 1.5418 Å), tube current and tube voltage of 40 mA and 40 kV, diffraction angle 2θ scanning range of 5°–50°, and scanning rate of 10° / min.

[0039] Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to quantitatively analyze the contents of silicon, aluminum, and phosphorus in the molecular sieve samples using an Agilent 5800 VDV inductively coupled plasma optical emission spectrometer. The samples were pretreated and prepared to a concentration of 0.01-10 mg·g -1 The detection wavelength range is set to 167~785nm.

[0040] The catalytic cracking performance of the prepared phosphorus-modified molecular sieve was evaluated by catalytic cracking of n-heptane using a CEL-GPPCH fixed-bed reactor with an inner diameter of 1 cm and a length of 110 cm. The reaction conditions were: reaction temperature range 520-680°C, catalyst loading 0.2 g, carrier gas nitrogen flow rate 20 mL / min, and mass space velocity (WHSV) of 5 g n-heptane ·g Cat. -1 ·h -1 The product composition was analyzed online using a Shimadzu GC-2014 gas chromatograph (50 m × Φ0.32 mm × 0.5 μm) with a capillary column (50 m × Φ0.32 mm × 0.5 μm). Composition was determined using a flame ionization detector (FID). The catalyst stability evaluation reaction conditions were as follows: a continuous reaction temperature of 660°C for 10 h, a catalyst loading of 0.2 g, and an n-heptane injection flow rate of 0.025 mL / min. -1 , the carrier gas nitrogen flow rate is 20mL / min, and the corresponding WHSV is 5g n-heptane ·g Cat. -1 ·h -1 .

[0041] Example 1 3-Aminopropyl phosphoric acid (3APPi) was added to an aqueous dispersion of UZM-35 molecular sieves (MSE type molecular sieve, powdered), and the mixture was stirred at 40°C for 10 hours to obtain a mixed solution; wherein the liquid-to-solid ratio of the UZM-35 molecular sieve aqueous dispersion (i.e., the mass ratio of water to UZM-35 molecular sieve in the UZM-35 molecular sieve aqueous dispersion) was 10:1, and the masses of phosphorus in the 3-aminopropyl phosphoric acid were 0.3%, 0.6%, 1.0%, 2.0%, and 3.0% of the mass of the UZM-35 molecular sieve, respectively; The mixed liquid was rotary evaporated at 80° C. until the liquid level stopped dropping. The obtained solid was dried in a vacuum oven at 80° C. for 12 h. After being fully ground, it was transferred to a muffle furnace and calcined at 550° C. for 6 h to obtain a phosphorus-modified molecular sieve.

[0042] According to the difference in phosphorus source and its loading amount, the modified UZM-35 molecular sieve (i.e., the phosphorus-modified molecular sieve) is named in the form of "phosphorus source-loading amount (wt%)-molecular sieve", where the loading amount is calculated as the mass percentage of phosphorus element in 3-aminopropyl phosphoric acid relative to UZM-35 molecular sieve. The 3APPi-modified UZM-35 samples are named: 3APPi-0.3-UZM, 3APPi-0.6-UZM, 3APPi-1.0-UZM, 3APPi-2.0-UZM and 3APPi-3.0-UZM.

[0043] Comparative Example 1 Trimethyl phosphate (TMP) was added to an aqueous dispersion of UZM-35 molecular sieve (MSE type molecular sieve) and stirred at 40°C for 10 hours to obtain a mixed solution; wherein the liquid-to-solid ratio of the UZM-35 molecular sieve aqueous dispersion was 10:1, and the mass of phosphorus in trimethyl phosphate was 0.3%, 0.6%, 1.0%, 2.0% and 3.0% of the mass of the UZM-35 molecular sieve, respectively; The mixed liquid was rotary evaporated at 80° C. until the liquid level stopped dropping. The obtained solid was dried in a vacuum oven at 80° C. for 12 h. After being fully ground, it was transferred to a muffle furnace and calcined at 550° C. for 6 h to obtain a phosphorus-modified molecular sieve.

[0044] According to the difference in phosphorus source and its loading amount, the modified UZM-35 molecular sieve (i.e., the phosphorus-modified molecular sieve) is named in the form of "phosphorus source-loading amount (wt%)-molecular sieve", where the loading amount is calculated as the mass percentage of phosphorus element in trimethyl phosphate relative to UZM-35 molecular sieve. The TMP-modified UZM-35 samples are named: TMP-0.3-UZM, TMP-0.6-UZM, TMP-1.0-UZM, TMP-2.0-UZM and TMP-3.0-UZM.

[0045] Table 1 shows the phosphorus loading (the loading amount of phosphorus element relative to the molecular sieve) and relative crystallinity of the UZM-35 molecular sieve before and after phosphorus modification in Example 1 and Comparative Example 1 measured by ICP and XRD.

[0046] Table 1 Phosphorus loading and relative crystallinity of UZM-35 molecular sieve before and after phosphorus modification in Example 1 and Comparative Example 1

[0047] As shown in Table 1, different concentrations of phosphorus sources were successfully loaded onto the UZM-35 molecular sieve via the wet impregnation method. The peak intensities of the phosphorus-modified 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples decreased, with the relative crystallinity of the 3APPi-0.3~3.0-UZM sample decreasing from 94% to 70%, and that of the TMP-0.3~3.0-UZM sample decreasing from 94% to 63%. This is because the added phosphorus source coordinates with aluminum, disrupting the coordination between silicon and aluminum in the bridging hydroxyl Si-OH-Al groups, preventing aluminum from entering the molecular sieve framework and ultimately causing a decrease in the relative crystallinity of the molecular sieve.

[0048] Figure 1 The unmodified UZM-35, 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples 27 Al MAS NMR ( 27 Al magic angle spinning solid-state NMR) spectrum, Figure 1 (a) is the unmodified UZM-35 and 3APPi-0.3~3.0-UZM samples 27 Al MAS NMR spectra, (b) unmodified UZM-35 and TMP-0.3~3.0-UZM samples 27 Al MAS NMR spectrum. Figure 1Among them, the unmodified UZM-35 sample, 3APPi-0.3~3.0-UZM, and TMP-0.3~3.0-UZM samples all exhibit a prominent aluminum atomic signal peak at δ = 57 ppm. This characteristic peak corresponds to the tetrahedrally coordinated aluminum structure within the zeolite framework, confirming that the majority of the aluminum atoms in the samples exist within the framework system as AlO4-. However, with increasing phosphorus loading, the peak intensity gradually weakens and shifts, indicating that the introduction of phosphorus causes the Si-O-Al bonds in the zeolite framework to break, resulting in the partial separation of the tetracoordinate aluminum from the framework structure. The peak near δ = 0 ppm corresponds to a non-framework octacoordinate aluminum species, typically non-framework Al-(H2O)6. The coordination of the hexacoordinate aluminum with phosphorus causes its resonance peak shift to gradually decrease. The peak at δ = -10 ppm belongs to the distorted framework octahedral aluminum, which is located on the outer surface or in the defect sites of the molecular sieve framework, indicating that the introduction of phosphorus causes a small part of the internal aluminum of the framework to form chemical bonds with phosphorus, and part of the external aluminum combines with phosphorus oxygen tetrahedron and is transformed into distorted framework octahedral aluminum.

[0049] Figure 2 The unmodified UZM-35, 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples 31 P MAS NMR ( 31 P magic angle spinning solid-state NMR) spectrum, Figure 2 (a) is the unmodified UZM-35 and 3APPi-0.3~3.0-UZM samples 31 P MAS NMR spectra, (b) unmodified UZM-35 and TMP-0.3~3.0-UZM samples 31 P MAS NMR spectrum. Figure 2The peak near δ = 0 ppm is associated with orthophosphoric acid (PO4) structures. The signal persists after calcination, indicating that not all loaded phosphorus participates in the zeolite framework bonding process. The peak at δ = -5 ppm can be attributed to two structures: oligomeric phosphorus species or phosphorus-oxygen tetrahedra bonded to single aluminum atoms. Phosphate species bonded to aluminum passivate Brønsted acid sites, providing relatively weak acid sites. As phosphorus loading increases, they gradually transform into more highly polymerized phosphorus species. The peak at δ = -10 ppm corresponds to moderately polymerized phosphorus, which stabilizes the framework aluminum and reduces high-temperature dealumination. The peak at δ = -22 ppm is attributed to polymerized phosphorus with bidentate Al-OP bonds, while the peak at δ = -32 ppm is attributed to polymeric or branched polymeric phosphorus. When the phosphorus loading exceeds 1.0 wt%, the peaks at δ = -22 ppm and -32 ppm become dominant, indicating that the excess phosphorus forms large polyphosphates, which block the pores and reduce mass transfer efficiency. With increasing phosphorus content, the peak area and intensity between 0 and -50 ppm increase, indicating an increase in the number of phosphorus species bonding to aluminum. The characteristic peaks of 3APPi-1.0-UZM are primarily concentrated between 0 and -10 ppm, while TMP-1.0-UZM tends to form medium- and high-polymerization phosphorus species, indicating that the 3APPi phosphorus source can chemically bond with more internal and external aluminum on the molecular sieve, forming low-polymerization Al-OP bonds.

[0050] Figure 3 Table 2 shows the conversion results of catalytic cracking of n-heptane by unmodified UZM-35, 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples. Figure 3(a) shows the n-heptane conversion results for the catalytic cracking of unmodified UZM-35 and 3APPi-0.3~3.0-UZM samples, and (b) shows the n-heptane conversion results for the catalytic cracking of unmodified UZM-35 and TMP-0.3~3.0-UZM samples. With increasing phosphorus loading, the n-heptane conversion of UZM-35 initially increases and then decreases. When the phosphorus loading increases to 1.0 wt%, the n-heptane conversions of 3APPi-1.0-UZM and TMP-1.0-UZM at 680°C reach their optimal values ​​of 99.82% and 99.50%, respectively. However, when the phosphorus loading is further increased to 3.0 wt%, the n-heptane conversions of 3APPi-3.0-UZM and TMP-3.0-UZM at 680°C decrease to 91.3% and 81.05%, respectively. This indicates that 1.0 wt% is the optimal loading of both 3-aminopropyl phosphate and trimethyl phosphate, both of which significantly improve the conversion of n-heptane catalytic cracking on UZM-35 zeolite, with 3APPi modification showing slightly better results than TMP. However, excessive phosphorus loading can severely disrupt the topological structure of the UZM-35 zeolite, leading to a decrease in conversion.

[0051] Table 2 Conversion results of catalytic cracking of n-heptane by unmodified UZM-35, 3APPi-0.3~3.0-UZM, and TMP-0.3~3.0-UZM samples (%)

[0052] Figure 4 Table 3 shows the product selectivity results of the catalytic cracking of n-heptane by unmodified UZM-35, 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples (reaction temperature 660℃). In Table 3, C1, C2, C3, C4 and C5 represent alkanes with corresponding carbon atoms, C2 = 、C3 = 、C4 = and C5 = Respectively represent the corresponding number of carbon atoms of monoolefins, 1,3C4 =represents 1,3-butadiene, and BTX represents a mixture of benzene, toluene, and xylenes. With increasing phosphorus loading, the yields and selectivities of ethylene and propylene for UZM-35, 3APPi-0.3~3.0-UZM, and TMP-0.3~3.0-UZM samples initially increase and then decrease. Both 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples exhibit optimal catalytic cracking performance at a phosphorus loading of 1.0 wt%. The diene yield (ethylene and propylene) and diene selectivity for 3APPi-1.0-UZM were 64.6% and 65.0%, respectively, representing increases of 12.7% and 8.8% over unmodified UZM-35. The diene yield and selectivity for TMP-1.0-UZM were 56.8% and 57.5%, respectively, representing increases of 4.9% and 1.3% over unmodified UZM-35. Under the same loading conditions, 3APPi-0.3~3.0-UZM and TMP-0.3~3.0-UZM samples showed better catalytic cracking performance of 3-aminopropyl phosphoric acid than trimethyl phosphate, indicating that 3APPi is a more efficient modified phosphorus source.

[0053] Table 3 Product selectivity results (%) of catalytic cracking of n-heptane by unmodified UZM-35, 3APPi-0.3~3.0-UZM, and TMP-0.3~3.0-UZM samples

[0054] Figure 5 The stability results for the catalytic cracking of n-heptane (reaction temperature: 660°C) using unmodified UZM-35, 3APPi-1.0-UZM, and TMP-1.0-UZM samples are shown. At 660°C, after 10 hours of continuous reaction, the conversion of UZM-35 decreased by 31.4%, while the conversion of 3APPi-1.0-UZM decreased by only 1.0%, and the conversion of TMP-1.0-UZM decreased by 4.2%. This indicates that the UZM-35 molecular sieve modified with 3-aminopropylphosphoric acid exhibits more stable catalytic cracking performance.

[0055] Example 2 3-Aminopropyl phosphoric acid (3APPi) was added to an aqueous solution of ZSM-5 molecular sieve (MFI type molecular sieve) and stirred at 40°C for 10 hours to obtain a mixed solution. The liquid-to-solid ratio of the ZSM-5 molecular sieve aqueous solution was 10:1, and the mass of phosphorus in 3-aminopropyl phosphoric acid was 0.3%, 0.6%, 1.0%, 2.0%, and 3.0% of the mass of the ZSM-5 molecular sieve, respectively. The mixed liquid was rotary evaporated at 80° C. until the liquid level stopped dropping. The obtained solid was dried in a vacuum oven at 80° C. for 12 h. After being fully ground, it was transferred to a muffle furnace and calcined at 550° C. for 6 h to obtain a phosphorus-modified molecular sieve.

[0056] According to the difference in phosphorus source and its loading amount, the modified ZSM-5 molecular sieve (i.e., the phosphorus-modified molecular sieve) is named in the form of "phosphorus source-loading amount (wt%)-molecular sieve", where the loading amount is calculated as the mass percentage of phosphorus element in 3-aminopropyl phosphoric acid relative to ZSM-5 molecular sieve. The 3APPi-modified ZSM-5 samples are named: 3APPi-0.3-ZSM, 3APPi-0.6-ZSM, 3APPi-1.0-ZSM, 3APPi-2.0-ZSM and 3APPi-3.0-ZSM.

[0057] Table 4 shows the phosphorus loading and relative crystallinity of ZSM-5 molecular sieve before and after phosphorus modification measured by ICP and XRD.

[0058] Table 4 Phosphorus loading and relative crystallinity of ZSM-5 molecular sieve before and after phosphorus modification

[0059] Figure 6 Table 5 and Table 5 show the conversion results of n-heptane catalytic cracking using unmodified ZSM-5 and 3APPi-0.3~3.0-ZSM samples. The conversions of ZSM-5 and 3APPi-0.3~3.0-ZSM gradually increased with increasing reaction temperature. At 520°C, the initial conversions of ZSM-5 and 3APPi-1.0-ZSM were 56.9% and 82.1%, respectively. At 680°C, the conversions increased to 97.5% and 99.9%, respectively. This indicates that phosphorus source modification promotes n-heptane conversion on ZSM-5 zeolites. However, excessive phosphorus loading significantly reduces the catalytic activity of ZSM-5 zeolites, with 3APPi-3.0-ZSM achieving a conversion of only 86.2% at 680°C.

[0060] Table 5 Conversion results of n-heptane catalytic cracking by unmodified ZSM-5 and 3APPi-0.3~3.0-ZSM samples (%)

[0061] Figure 7 Table 6 shows the product yield and selectivity results of the catalytic cracking of n-heptane (reaction temperature 660℃) by unmodified ZSM-5 and 3APPi-0.3~3.0-ZSM samples. Figure 7 C7 in represents n-heptane, Figure 7The middle curve corresponds to diene selectivity, while the bar graph corresponds to ethylene and propylene yields. The diene yield and diene selectivity of ZSM-5 at 660°C were 50.6% and 52.4%, respectively. As the phosphorus loading increased from 0.3 wt% to 1.0 wt%, the diene yield of the 3APPi-0.3~1.0-ZSM samples gradually increased. However, as the phosphorus loading further increased, the diene yield of the ZSM-5 samples decreased significantly. All phosphorus-loaded samples exhibited optimal catalytic cracking performance at a phosphorus loading of 1.0 wt%, with 3APPi-1.0-ZSM achieving a diene yield and selectivity of 60.5% and 60.8%, respectively.

[0062] Table 6 Product selectivity results of catalytic cracking of n-heptane by unmodified ZSM-5 and 3APPi-0.3~3.0-ZSM samples

[0063] Figure 8 Figure 3 shows the stability of the catalytic cracking of n-heptane by unmodified ZSM-5 and 3APPi-1.0-ZSM samples. At 660°C, after 10 hours of continuous reaction, the conversion of ZSM-5 decreased by 35.0%, while the conversion of 3APPi-1.0-ZSM decreased by only 9.1%.

[0064] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation thereto. It should be noted that those skilled in the art may make various improvements and modifications without departing from the principles of the present invention, and such improvements and modifications shall also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a phosphorus-modified molecular sieve, characterized in that: The following steps are involved: Mixing molecular sieves, water and 3-aminopropyl phosphoric acid, and sequentially concentrating and drying the resulting mixture to obtain a molecular sieve loaded with 3-aminopropyl phosphoric acid; the molecular sieve comprises one or more of MFI, MSE, *BEA and FAU type molecular sieves; The molecular sieve loaded with 3-aminopropyl phosphoric acid is calcined to obtain the phosphorus-modified molecular sieve.

2. The preparation method according to claim 1, characterized in that The mass of the phosphorus element in the 3-aminopropyl phosphoric acid is 0.1-1.0% of the mass of the molecular sieve.

3. The preparation method according to claim 1 or 2, characterized in that The mass ratio of water to molecular sieve is 10:1 to 50:

1.

4. The preparation method according to claim 1, characterized in that The mixing method comprises: first mixing the molecular sieve and water to obtain a molecular sieve dispersion; The molecular sieve dispersion is mixed with 3-aminopropyl phosphoric acid for a second time to obtain the mixed solution.

5. The preparation method according to claim 4, characterized in that The temperature of the second mixing is 40-60°C.

6. The preparation method according to claim 1, characterized in that The concentration method is rotary evaporation, and the rotary evaporation temperature is 80-100°C; the drying temperature is 80-100°C.

7. The preparation method according to claim 1, characterized in that The calcination temperature is 500-600° C. and the calcination time is 6-10 hours.

8. The phosphorus-modified molecular sieve prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the phosphorus-modified molecular sieve according to claim 8 in the catalytic cracking reaction of normal alkanes to produce light olefins.

10. The use according to claim 9, characterized in that The carbon number of the normal alkane is 5 to 9.

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