Phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst as well as preparation method and application thereof

Abstract

The invention particularly relates to a phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst as well as a preparation method and application thereof. The preparation method comprises the following steps: dissolving zircon salt, terephthalic acid and 2-aminoterephthalic acid in an organic solvent; adding a regulator acetic acid, and activating to prepare amino-functionalized UiO-66; and slowly dropwise adding a phosphotungstic acid acetonitrile solution into the amino-functionalized UiO-66 acetonitrile dispersion liquid, stirring and reacting at room temperature, and separating and drying to obtain the catalyst HPW at amino-functionalized UiO-66. The bifunctional catalyst (phosphotungstic acid / amino-functionalized UiO-66) is coordinated through an acid-base site at a lower temperature (170 DEG C vsgt; according to the present invention, the reaction is performed at the temperature of 200 DEG C, the formation of carbon deposition can be substantially inhibited at the low temperature, and the catalyst has characteristics of high catalytic activity after five cycles, extraordinary hydrothermal stability and chemical stability, can maintain the stability of the catalyst in the reaction environment with the existence of a small amount of water, and has good and broad application prospects.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, specifically to a phosphotungstic acid / aminofunctionalized UiO-66 bifunctional catalyst, its preparation method, and its application. Background Technology

[0002] N-Methylpyrrolidone is miscible with water, alcohols, ethers, esters, ketones, halogenated hydrocarbons, aromatics, and castor oil. It has low volatility, excellent thermal and chemical stability, and evaporates with water vapor. It is hygroscopic. N-Methylpyrrolidone is widely used in the lithium battery, pharmaceutical, pesticide, pigment, cleaning agent, and insulating material industries. It is widely used in aromatic hydrocarbon extraction, acetylene purification, and lubricant refining. It is an extractant for butadiene and isoprene, and in the semiconductor industry, it can be used for cleaning precision instruments and circuit boards. It is also an electrode auxiliary material for lithium-ion batteries. Current industrial production methods for N-methylpyrrolidone mainly involve synthesis using γ-butyrolactone and monomethylamine (monomethylamine) as starting materials. The industrial reactor type used is a tubular adiabatic reactor at a pressure of 30-90 atmospheres. The overall reaction process consists of two steps: addition and cyclization. After the reaction is complete, the pure product is obtained through separation steps such as flash evaporation and distillation. High reaction temperatures and pressures are required, placing high demands on equipment and consuming significant energy. Adding a suitable catalyst can lower the activation energy, enabling the stable synthesis of NMP in high yields under milder reaction conditions.

[0003] Patent CN112142641A discloses a method using a solid strong acid catalyst (SO4). 2- / ZrO2、SO4 2- A process for synthesizing NMP using TiO2, etc., is available. This process can synthesize high-purity NMP, but during the synthesis reaction, water or water vapor reacts with SO42- on the surface. 2- Contact makes it easy for SO4 on its surface to adhere. 2- Loss of acid centers on the catalyst surface reduces the number of acid centers and decreases catalyst activity. CN114011456B discloses a copper-doped ZSM-5 molecular sieve catalyst for the synthesis of N-methylpyrrolidone from γ-butyrolactone and dimethylamine. The catalyst has a reaction yield of 98%, but it has the disadvantage that the copper-doped ZSM-5 molecular sieve catalyst has a limited surface area, and the intermediate is over-activated, resulting in side reactions. It is also prone to carbon deposition and deactivation, which leads to a shortened catalyst life.

[0004] This is because traditional molecular sieve acid catalysts have a high density and uneven distribution of strong acid sites, which can easily trigger deep reactions, leading to olefin polymerization, cyclization, and the formation of carbon precursors that clog the pores.

[0005] To address the aforementioned problems, there is an urgent need for a catalyst with high stability and high catalytic activity. Summary of the Invention

[0006] To address the problems existing in current technologies, this invention proposes a catalyst that possesses a certain degree of size selectivity, effectively excluding some large molecular impurities. Overcoming issues such as carbon buildup and deactivation in existing catalysts, the catalyst of this invention, when applied to the synthesis of NMP, effectively improves catalytic efficiency and catalyst stability.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A method for preparing a phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst includes the following steps: (1) Synthesis of amino-functionalized UiO-66 (amino-functionalized UiO-66): Zirconium salt (zirconium chloride ZrCl4 or zirconium dichloride octahydrate ZrOCl2·8H2O), terephthalic acid, and 2-aminoterephthalic acid (NH2-BDC) were ultrasonically dissolved in an organic solvent, which was one of N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMA). A small amount of acetic acid is added as a regulator to promote the formation of crystals with fewer defects and higher crystallinity.

[0008] The mixed solution was subjected to a solvothermal reaction at 110-130℃ for 12-24 hours.

[0009] After the reaction was completed, the solid product was collected by centrifugation and washed repeatedly with DMF and methanol to remove unreacted ligands and solvent. Finally, it was activated under vacuum (e.g., 130-170℃) for 10-14 h to obtain amino-functionalized UiO-66 white powder.

[0010] (2) Encapsulating phosphotungstic acid (HPW) in a "ship-in-a-bottle" method: The activated amino-functionalized UiO-66 powder was dispersed in acetonitrile.

[0011] A certain amount of phosphotungstic acid (HPW) was dissolved in acetonitrile and slowly added dropwise to a suspension of amino-functionalized UiO-66.

[0012] Acetonitrile has good solubility in HPW and good stability and wettability in MOF, which allows it to form a stable suspension under ultrasound or stirring, without causing its skeleton to collapse or dissolve.

[0013] Phosphotungstic acid (HPW) exhibits excellent solubility in acetonitrile, ensuring that HPW molecules can move freely in the solvent as monomolecules or small aggregates, thus having the opportunity to diffuse and enter the pore windows of MOFs. Being aprotic, it does not compete for adsorption, does not cause hydrolytic collapse of the MOF structure, and has moderate polarity, making it an ideal solvent for the "shipbuilding in a bottle" method.

[0014] DMF can readily dissolve metal salts and organic ligands at high temperatures. However, DMF has a high boiling point (~152°C), making it difficult to completely remove completely. In post-encapsulation steps, acetonitrile, with its low boiling point (~82°C), facilitates subsequent washing and drying processes.

[0015] The mixture was stirred continuously at room temperature for 20–28 hours. Because the window size of amino-functionalized UiO-66 (approximately 6 Å) is smaller than the kinetic diameter of HPW (approximately 10 Å), but the MOF structure exhibits some flexibility, HPW molecules were "captured" into the pores of the MOF during synthesis. The mixture was then centrifuged and repeatedly washed with acetonitrile to thoroughly remove the physically adsorbed HPW from the surface. Finally, it was dried under vacuum to obtain the final catalyst HPW@amino-functionalized UiO-66.

[0016] After amino-functionalization of UiO-66, the -NH2 group forms an acid-base / hydrogen bond interaction with methylamine, enhancing the adsorption of raw materials such as methylamine. Methylamine adsorption / conversion is the rate-determining step. Amino-functionalization can also enhance the anchoring of HPW, achieving high NMP selectivity at low pressure and 140-170℃, while the parent UiO-66 has insufficient activity.

[0017] Further, in step (1), the zirconium salt is zirconium chloride or zirconium oxychloride octahydrate; the molar ratio of terephthalic acid and 2-aminoterephthalic acid is (6-8):(2-4), the sum of the molar ratios of terephthalic acid and 2-aminoterephthalic acid and the molar ratio of zirconium salt is (1-1.5):1; the concentration of zirconium salt in the mixed solution is 0.01-0.03 mmol / mL, and the amount of acetic acid added is 5-10 wt% of the total weight of the solution.

[0018] Preferably, the molar ratio of terephthalic acid to 2-aminoterephthalic acid is (7-8):(2-3), with about 20%-30% of 2-aminoterephthalic acid replacing H2BDC, ensuring that "0.5-1 -NH2 per cage" can balance acidity and diffusion; an excessively high proportion (≥50%) will reduce the catalyst activity.

[0019] Furthermore, the solvothermal reaction conditions in step (1) are 110-130℃ and the time is 12-24h.

[0020] Furthermore, the heating activation temperature is 130-170℃, and the time is 10-14h.

[0021] Further, in step (2), the mass ratio of amino-functionalized UiO-66 powder to phosphotungstic acid is 3-5.7:1, the concentration of amino-functionalized UiO-66 in the mixed solution is 5-9 mg / mL, and the theoretical loading of phosphotungstic acid in the catalyst is 15-25 wt%.

[0022] Furthermore, in step (2), the stirring time at room temperature is 20-28 hours, and the solvent used for washing is acetonitrile.

[0023] Furthermore, in step (2), the vacuum drying temperature is 70-90℃ and the time is 5-7h.

[0024] Furthermore, the phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst prepared by any of the above methods is further described in its application in the synthesis of N-methylpyrrolidone.

[0025] Furthermore, in the application described, the amount of the phosphotungstic acid / aminofunctionalized UiO-66 bifunctional catalyst is 3-5 wt% of the raw material γ-butyrolactone, the catalytic reaction temperature is 130-180℃, preferably 150-170℃, the pressure is 1.5-2.5 MPa, and the time is 2-4 h.

[0026] Catalytic reaction mechanism: Acid activation: Phosphotungstic acid (H3PW) 12 O 40 HPW, as a strong Brønsted acid, is responsible for activating the carbonyl group of γ-butyrolactone (GBL), and the proton (H) + The carbonyl oxygen (C=O) in the GBL molecule combines with the carbonyl oxygen, which enhances the positive charge of the carbon atom and makes it more susceptible to nucleophilic attack.

[0027] Basic activation and nucleophilic attack: The amino group (-NH2) on amino-functionalized UiO-66 extracts a proton from methylamine (MEA) to generate the stronger nucleophile methylamine anion (CH3NH). - The anion rapidly attacks the activated GBL carbonyl carbon.

[0028] Synergistic and confinement effects: The acid-base activation steps described above occur simultaneously and synergistically within the nanoscale pores of the MOF, significantly reducing the energy barrier of the entire reaction pathway from ring-opening to amination to cyclization. The pores of the MOF restrict the movement of reactants, increasing their local concentration and residence time near the active sites, further enhancing the reaction probability.

[0029] Product and byproduct water desorption: The generated NMP and byproduct water are desorbed from the catalyst channels, the active sites are regenerated, and the next catalytic cycle begins.

[0030] The beneficial effects of this invention are: 1. This invention designs a "bifunctional" catalyst with both acidic and basic sites. Stable basic sites are introduced by modifying a MOF with amino groups (-NH2). Furthermore, a highly active homogeneous catalyst, phosphotungstic acid, is encapsulated in an amino-functionalized UiO-66 via an in-bottle method to improve HPW dispersion: the amino group forms a "-NH3" group with the protons of Keggin-type HPW. + [PW 12 O 40 ] - "Ion pairs enhance the anchoring strength of heteropolyacids within the cage, reducing dissolution and desorption; the amino-functionalized UiO-66 in the catalyst interacts with methylamine via acid-base / hydrogen bonding, enhancing adsorption of the feedstock and increasing local amine concentration; it promotes lactone ring-opening, and the amino group can synergistically stabilize the ring-opening transition state with the acid site, lowering the energy barrier of the entire ring-opening-amine-cyclization reaction pathway. The synergistic effect of these two sites in the reaction greatly improves catalytic efficiency, thereby achieving high conversion rates under milder conditions; and the 'confined effect' enhances catalytic performance." 2. In this invention, the pores of the MOF restrict the movement of reactants, and the regular channels facilitate the diffusion of reactants and products. The generated NMP and byproduct water are desorbed from the catalyst channels, the active sites are regenerated, and the next catalytic cycle begins. 3. The bifunctional catalyst (phosphotungstic acid / amino-functionalized UiO-66) of this invention, through acid-base synergy, allows the reaction to proceed at a lower temperature (170℃ vs >200℃), and the low temperature itself can greatly inhibit carbon deposition. Bifunctional catalysis may provide a more direct and efficient reaction pathway, reducing the residence time of side reactions and intermediates, thereby reducing the probability of carbon deposition. The catalyst of this invention still exhibits high catalytic activity after five cycles. 4. The Zr6O4(OH)4 metal cluster in the HPW@aminofunctionalized UiO-66 catalyst has an extremely high coordination number (12) and strong bond energy with the carboxylic acid linker, giving it exceptional hydrothermal and chemical stability. It remains stable in reaction environments with minimal water presence and can withstand high temperatures of 170°C in the reaction system. The aminofunctionalized UiO-66 framework itself is water-resistant. While HPW is a Brønsted acid, its protons can also bind to water, but this interaction is reversible to some extent. At the reaction temperature, water can be desorbed, and the active sites can be regenerated. Therefore, this catalyst can be used to catalyze the reaction of GBL and methylamine to prepare NMP in the presence of minimal water, achieving a conversion rate of 99% and a yield of 98%. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention. Example 1

[0032] This embodiment prepares a bifunctional catalyst of phosphotungstic acid / aminofunctionalized UiO-66, and the preparation steps are as follows: (1) Synthesis of amino-functionalized UiO-66: 0.233 g (1.0 mmol) zirconium chloride ZrCl4, 0.0543 g (0.3 mmol) 2-aminoterephthalic acid (NH2-BDC), and 0.116 g (0.7 mmol) terephthalic acid (H2BDC) were ultrasonically dissolved in 50 ml of N,N-dimethylformamide (DMF); 5 ml of acetic acid was added as a regulator to promote the formation of crystals with fewer defects and higher crystallinity. UiO-66 material with 30% 2-aminoterephthalic acid replacing H2BDC was obtained. The mixed solution was placed in a 100 mL polytetrafluoroethylene-lined high-pressure reactor, sealed, and then placed in a forced-air drying oven for a solvothermal reaction at 120°C for 20 hours.

[0033] After the reaction was complete, the mixture was allowed to cool naturally to room temperature. The resulting yellow solid was collected by centrifugation (8000 rpm, 5 min). The solid was washed three times with fresh DMF and then three times with methanol, centrifuged after each wash.

[0034] The product was activated in a vacuum drying oven at 150°C for 12 hours to obtain activated amino-functionalized UiO-66 powder, which was then stored in a desiccator for later use.

[0035] (2) Encapsulating phosphotungstic acid (HPW) in a "ship-in-a-bottle" method: Disperse 0.5 g of activated amino-functionalized UiO-66 powder in 50 ml of acetonitrile and sonicate for 30 minutes to ensure complete dispersion.

[0036] 0.125 g of phosphotungstic acid (HPW) was dissolved in 20 ml of acetonitrile to prepare a clear solution, which was then slowly added dropwise to a suspension of amino-functionalized UiO-66 under vigorous stirring and stirred continuously at room temperature for 24 hours.

[0037] Because the window size of amino-functionalized UiO-66 (approximately 6 Å) is smaller than the kinetic diameter of HPW (approximately 10 Å), but the MOF structure has a certain degree of flexibility, HPW molecules can be "captured" into the pores of the MOF during the synthesis process.

[0038] After the reaction is complete, the solid product is separated by centrifugation and washed repeatedly with acetonitrile (at least 5 times) until the characteristic absorption peak of HPW cannot be detected by ultraviolet spectroscopy in the supernatant.

[0039] Finally, the catalyst was dried under vacuum to obtain a bifunctional catalyst HPW@aminofunctionalizedUiO-66 with a final loading of 20 wt% phosphotungstic acid / aminofunctionalized UiO-66. Example 2

[0040] This comparative example provides a method for preparing a phosphotungstic acid / aminofunctionalized UiO-66 bifunctional catalyst, wherein the molar ratio of terephthalic acid and 2-aminoterephthalic acid in step (1) is 6:4, resulting in a UiO-66 material with 40% 2-aminoterephthalic acid replacing H2BDC; the rest is the same as in Example 1. Example 3

[0041] This comparative example provides a method for preparing a phosphotungstic acid / aminofunctionalized UiO-66 bifunctional catalyst, wherein the molar ratio of terephthalic acid and 2-aminoterephthalic acid in step (1) is 8:2, resulting in a UiO-66 material with 20% 2-aminoterephthalic acid replacing H2BDC; the rest is the same as in Example 1. Example 4

[0042] This comparative example provides a method for preparing a phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst, wherein the amount of phosphotungstic acid (HPW) in step (2) is 0.170 g, and the rest is the same as in Example 1. At this time, the loading of phosphotungstic acid in the catalyst is 25 wt%. Example 5

[0043] This comparative example provides a method for preparing a phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst, wherein the amount of phosphotungstic acid (HPW) in step (2) is 0.09 g, and the rest is the same as in Example 1. At this time, the loading of phosphotungstic acid in the catalyst is 15 wt%. Example 6

[0044] This embodiment prepares a bifunctional catalyst of phosphotungstic acid / aminofunctionalized UiO-66, and the preparation steps are as follows: (1) Synthesis of amino-functionalized UiO-66 (amino-functionalized UiO-66): 0.322 g (1.0 mmol) zirconium dichloride octahydrate ZrOCl2·8H2O, 0.1086 g (0.6 mmol) 2-aminoterephthalic acid (NH2-BDC), and 0.150 g (0.9 mmol) terephthalic acid (H2BDC) were dissolved in 100 ml N,N-dimethylformamide (DMF); 8 ml acetic acid was added as a regulator to promote the formation of crystals with fewer defects and higher crystallinity.

[0045] The mixed solution was placed in a 100 mL polytetrafluoroethylene-lined high-pressure reactor, sealed, and then placed in a forced-air drying oven for a solvothermal reaction at 110 °C for 24 hours.

[0046] After the reaction was complete, the mixture was allowed to cool naturally to room temperature. The resulting yellow solid was collected by centrifugation (8000 rpm, 5 min). The solid was washed three times with fresh DMF and then three times with methanol, centrifuged after each wash.

[0047] The product was activated in a vacuum drying oven at 130°C for 14 hours to obtain activated amino-functionalized UiO-66 powder, which was then stored in a desiccator for later use.

[0048] (2) Encapsulating phosphotungstic acid (HPW) in a "ship-in-a-bottle" method: Disperse 0.5 g of activated amino-functionalized UiO-66 powder in 50 ml of acetonitrile and sonicate for 30 minutes to ensure complete dispersion.

[0049] 0.102 g of phosphotungstic acid (HPW) was dissolved in 50 ml of acetonitrile to prepare a clear solution, which was then slowly added dropwise to a suspension of amino-functionalized UiO-66 under vigorous stirring and stirred continuously at room temperature for 20 hours.

[0050] After the reaction is complete, the solid product is separated by centrifugation and washed repeatedly with acetonitrile (at least 5 times) until the characteristic absorption peak of HPW cannot be detected by ultraviolet spectroscopy in the supernatant.

[0051] Finally, the catalyst was dried under vacuum to obtain a bifunctional catalyst HPW@aminofunctionalized UiO-66 with a final loading of 17 wt% phosphotungstic acid / aminofunctionalized UiO-66. Example 7

[0052] This embodiment prepares a bifunctional catalyst of phosphotungstic acid / aminofunctionalized UiO-66, and the preparation steps are as follows: (1) Synthesis of amino-functionalized UiO-66 (amino-functionalized UiO-66): 0.233 g (1.0 mmol) zirconium chloride ZrCl4, 0.0543 g (0.3 mmol) 2-aminoterephthalic acid (NH2-BDC), and 0.116 g (0.7 mmol) terephthalic acid (H2BDC) were dissolved in 50 ml of N,N-dimethylformamide (DMF); 3 ml of acetic acid was added as a regulator to promote the formation of crystals with fewer defects and higher crystallinity.

[0053] The mixed solution was placed in a 100 mL polytetrafluoroethylene-lined high-pressure reactor, sealed, and then placed in a forced-air drying oven for a solvothermal reaction at 130°C for 20 hours.

[0054] After the reaction was complete, the mixture was allowed to cool naturally to room temperature. The resulting yellow solid was collected by centrifugation (8000 rpm, 5 min). The solid was washed three times with fresh DMF and then three times with methanol, centrifuged after each wash.

[0055] The product was activated in a vacuum drying oven at 170°C for 10 hours to obtain activated amino-functionalized UiO-66 powder, which was then stored in a desiccator for later use.

[0056] (2) Encapsulating phosphotungstic acid (HPW) in a "ship-in-a-bottle" method: Disperse 0.5 g of activated amino-functionalized UiO-66 powder in 50 ml of acetonitrile and sonicate for 30 minutes to ensure complete dispersion.

[0057] Dissolve 0.15 g of phosphotungstic acid (HPW) in 10 ml of acetonitrile to prepare a clear solution. Add the solution slowly dropwise to the suspension of amino-functionalized UiO-66 under vigorous stirring and continue stirring at room temperature for 28 hours.

[0058] Because the window size of amino-functionalized UiO-66 (approximately 6 Å) is smaller than the kinetic diameter of HPW (approximately 10 Å), but the MOF structure has a certain degree of flexibility, HPW molecules can be "captured" into the pores of the MOF during the synthesis process.

[0059] After the reaction is complete, the solid product is separated by centrifugation and washed repeatedly with acetonitrile (at least 5 times) until the characteristic absorption peak of HPW cannot be detected by ultraviolet spectroscopy in the supernatant.

[0060] Finally, the catalyst was dried under vacuum to obtain a bifunctional catalyst HPW@aminofunctionalizedUiO-66 with a final loading of 23 wt% phosphotungstic acid / aminofunctionalized UiO-66. Example 8

[0061] In this embodiment, the bifunctional catalyst of phosphotungstic acid / amino-functionalized UiO-66 prepared in Example 1 is used to catalyze the reaction of γ-butyrolactone with methylamine to prepare N-methylpyrrolidone. The preparation process steps are as follows: Catalyst (HPW@aminofunctionalized UiO-66): 0.43 g (5 wt% of GBL mass) 8.6 g (0.1 mol) of γ-butyrolactone GBL and 0.43 g (5 wt% of GBL mass) of catalyst (HPW@aminofunctionalized UiO-66) were added to a 100 mL high-pressure reactor. The reactor was sealed, and nitrogen was used to pressurize and depressurize the reactor three times to remove air from the reactor. 34.5 g of a metered 90% monomethylamine (MEA) aqueous solution (containing approximately 0.138 mol of MEA, MEA:GBL = 1.38:1) was injected via a pressure pump, with excess MEA to improve GBL conversion. Magnetic stirring was started at approximately 500 rpm. The temperature was increased to the target reaction temperature of 170°C. As the temperature increased, the pressure inside the reactor gradually rose to 1.8 MPa, which is mainly the vapor pressure of the amine and water. Timing was started at the target temperature and pressure, and the reaction lasted for 3 hours.

[0062] After the reaction is complete, the reactor is placed in an ice-water bath to cool rapidly to room temperature. The pressure is slowly released, the reactor is opened, and the entire reaction mixture is removed. The solid catalyst is then separated by filtration or centrifugation. The solid catalyst is washed with a small amount of methanol, and the filtrates are combined.

[0063] Product analysis: The filtrate was quantitatively analyzed using gas chromatography (GC) equipped with an FID detector.

[0064] The conversion rate of GBL and the selectivity of NMP were calculated using the internal standard method (e.g., using cyclohexanone as the internal standard).

[0065] γ-Butyrolactone (GBL) conversion >99%, N-methylpyrrolidone (NMP) selectivity >99%, NMP yield >98%.

[0066] The catalyst separated after the reaction was thoroughly cleaned by Soxhlet extraction with methanol for 6 hours, and then dried under vacuum at 150°C for 3 hours.

[0067] Repeated use experiments were conducted under the same reaction conditions. After five cycles, the GBL conversion remained above 98%, and the NMP selectivity showed no significant decrease. PXRD characterization revealed that the crystal structure of the recovered catalyst remained unchanged, demonstrating its excellent stability. Example 9

[0068] This comparative example provides the catalyst of Example 1 to catalyze the reaction of γ-butyrolactone with anhydrous methylamine to prepare N-methylpyrrolidone. The raw material is replaced by anhydrous methylamine instead of 90% of the monomethylamine aqueous solution. The effective content of monomethylamine (MEA) is the same as in Example 8. The preparation process is as in Example 8. Comparative Example 1

[0069] This comparative example provides a method for preparing a phosphotungstic acid / aminofunctionalized UiO-66 bifunctional catalyst, wherein the molar ratio of terephthalic acid and 2-aminoterephthalic acid is 5:5, resulting in a UiO-66 material with 50% 2-aminoterephthalic acid replacing H2BDC; the rest is the same as in Example 1. Comparative Example 2

[0070] This comparative example provides a method for preparing a phosphotungstic acid / UiO-66 catalyst, wherein only terephthalic acid is used. In step (1) of Example 1, the amount of 2-aminoterephthalic acid used is replaced by an equal amount of terephthalic acid, resulting in 0% 2-aminoterephthalic acid replacing H2BDC UiO-66 material; the rest is the same as in Example 1. Comparative Example 3

[0071] This comparative example provides a method for preparing a phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst, wherein the amount of phosphotungstic acid (HPW) is 0.30 g, and the loading of phosphotungstic acid in the catalyst is 37.5 wt%, and the rest is the same as in Example 1. Comparative Example 4

[0072] This comparative example provides a method for preparing a phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst, wherein the amount of phosphotungstic acid (HPW) is 0.06 g, and the loading of phosphotungstic acid in the catalyst is 10 wt%, and the rest is the same as in Example 1. Comparative Example 5

[0073] This comparative example provides a method for preparing a phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst, wherein the encapsulation solvent for phosphotungstic acid (HPW) in step (3) is methanol, and the rest is the same as in Example 1. Comparative Example 6

[0074] This comparative example provides a method for preparing a phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst, wherein the activation temperature in step (2) is 110°C, and the rest is the same as in Example 1. Comparative Examples 7-12

[0075] Comparative Examples 7-12 used the catalysts prepared in Comparative Examples 1-6 to catalyze the reaction of γ-butyrolactone with methylamine to prepare N-methylpyrrolidone, and the preparation process was as described in Example 8.

[0076] After catalytic preparation of N-methylpyrrolidone, the conversion rate of GBL, the selectivity of NMP, and the selectivity of NMP after five catalyst recycling were tested.

[0077] The experimental results are shown in Table 1:

[0078] As shown in Table 1, the catalyst prepared by the scheme of the present invention in Example 8, as a catalyst for NMP synthesis, showed a high GBL conversion rate and good NMP selectivity when using 90% methylamine aqueous solution and GBL as raw materials.

[0079] As shown in Example 9, the raw material is an anhydrous system, which shifts the equilibrium to the right and prevents the acid sites from being hydrated. The GBL conversion rate, NMP selectivity, and NMP selectivity of the catalyst after 5 cycles all show good performance.

[0080] As shown in Comparative Example 7, excessive amino modification results in NH2-BDC having one more -NH2 group than H2BDC, with a van der Waals volume of approximately 15 Å, equivalent to permanently "attaching" a sphere to the inside of the pore window. Crystallographic refinement reveals that at 30% NH2-BDC, the window diameter decreases from 6.0 Å to 5.2 Å; at 50%, it further decreases to 4.7 Å, approaching the GBL kinetic diameter (5.3 Å), with a diffusion coefficient decrease of >40%. The more amino groups present, the larger the ligand "volume," the thicker the pore walls, and the smaller the effective pore size. When the NH2-BDC ratio exceeds 30%, the pore opening shrinks to the point of hindering substrate / product diffusion; thus, "diffusion-restricted" becomes the primary cause of performance degradation, replacing "insufficient acid sites."

[0081] According to Comparative Example 8, UiO-66 without amino functionalization modification has no anchoring effect, HPW is easily lost, resulting in poor catalyst cycle performance.

[0082] According to Comparative Example 9, excessive HPW loading caused the amino-functionalized UiO-66 to be basically filled with all the cage sites when HPW was loaded at 20 wt%. Further loading only resulted in accumulation on the outer surface, reducing the anchoring effect. After 5 cycles, the selectivity of NMP decreased.

[0083] According to Comparative Example 10, with an HPW loading of 10wt%, the acid content is insufficient, resulting in a low GBL conversion rate.

[0084] As shown in Comparative Example 11, ethanol molecules are smaller than acetonitrile molecules and are protic solvents (containing -OH). This may make it easier for ethanol to enter the MOF channels and compete with HPW for binding sites, or it may cause unnecessary solvation of HPW itself, thus slightly reducing encapsulation efficiency and consequently reducing catalytic activity.

[0085] According to Comparative Example 12, when the activation temperature is <130℃, residual DMF occupies Zr open sites, resulting in an artificially high amount of Brønsted acid but a decrease in the accessibility of the real active center. Compared with Example 8, it shows a decrease in GBL conversion rate and NMP selectivity.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst, characterized in that, Includes the following steps: (1) Aminofunctionalized UiO-66 was prepared by solvothermal reaction using zirconium salt, terephthalic acid and 2-aminoterephthalic acid; (2) Phosphotungstic acid and the obtained amino-functionalized UiO-66 were dispersed in acetonitrile to obtain amino-functionalized UiO-66 dispersion and phosphotungstic acid solution, respectively. The phosphotungstic acid solution was slowly added dropwise to the amino-functionalized UiO-66 dispersion, and the reaction was stirred at room temperature. The solid and liquid were separated, and the obtained solid was washed with solvent and dried under vacuum to obtain the phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst, denoted as HPW@amino-functionalized UiO-66.

2. The method for preparing the phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst according to claim 1, characterized in that: In step (1), the zirconium salt is zirconium chloride or zirconium oxychloride octahydrate; the molar ratio of terephthalic acid and 2-aminoterephthalic acid is (6-8):(2-4), and the molar ratio of the sum of the amounts of terephthalic acid and 2-aminoterephthalic acid to the zirconium salt is (1-1.5):

1.

3. The method for preparing the phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst according to claim 1, characterized in that: The reaction in step (1) is carried out in an organic solvent, which is one of N,N-dimethylformamide or N,N-dimethylacetamide; the concentration of zirconium salt in the mixed solution is 0.01-0.03 mmol / mL, the solvothermal reaction conditions are 110-130℃, and the time is 12-24h.

4. The method for preparing the phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst according to claim 1 or 2, characterized in that: In step (1), acetic acid is added as a regulator, and the amount of acetic acid added is 5-10 wt% of the total weight of the solution; after the solvothermal process is completed, heating activation is performed; the heating activation temperature is 130-170℃ and the time is 10-14h.

5. The method for preparing the phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst according to claim 1, characterized in that: In step (2), the mass ratio of amino-functionalized UiO-66 powder to phosphotungstic acid is 3-5.7:1, and the concentration of amino-functionalized UiO-66 in the mixed solution is 5-9 mg / mL.

6. The method for preparing the phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst according to claim 1, characterized in that: In step (2), the reaction time at room temperature is 20-28 hours, and the solvent used for washing is acetonitrile.

7. The method for preparing the phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst according to claim 1, characterized in that: The vacuum drying temperature in step (2) is 70-90℃ and the time is 5-7h.

8. The phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst prepared by the method according to any one of claims 1-7.

9. The application of the phosphotungstic acid / amino-functionalized UiO-66 bifunctional catalyst according to claim 8 in the synthesis of N-methylpyrrolidone.

10. The application according to claim 9, characterized in that, The amount of the phosphotungstic acid / aminofunctionalized UiO-66 bifunctional catalyst is 3-5 wt% of the raw material γ-butyrolactone. The catalytic reaction temperature is 130-180℃, the pressure is 1.5-2.5 MPa, and the time is 2-4 h.

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