High-activity catalyst for synthesizing propylene carbonate from CO2 and epoxypropane as well as preparation method and application of high-activity catalyst

Abstract

The invention relates to a high-activity catalyst for synthesizing propylene carbonate from CO2 and epoxypropane and a preparation method and application thereof.The catalyst comprises a molecular sieve and metal oxide loaded on the molecular sieve, the molecular sieve is a pure silicon molecular sieve, the metal oxide is one or a combination of more of ZnO, Fe2O3, NiO, CoO and Cr2O3, the metal content accounts for 1-7 wt% of the mass of the catalyst, and the metal content accounts for 1-5 wt% of the mass of the catalyst. The particle size of the catalyst is 0.1-4 [mu] m. The preparation method comprises the following steps: treating a molecular sieve with an organic alkali, dipping the treated molecular sieve in a metal precursor solution subjected to acid treatment, controlling the pH value of the prepared metal precursor solution to be 2-4, shaking with an orbital shaking table, standing, drying, and roasting to obtain the catalyst. According to the invention, rich silicon hydroxyl nest sites on the surface of the molecular sieve with small particle size are utilized to effectively anchor metal species, and under the appropriate metal loading capacity, the synergistic catalysis of the metal and the adjacent silicon hydroxyl sites enables the molecular sieve to have high reaction performance for synthesizing propylene carbonate from CO2 and epoxypropane.

Description

Technical Field

[0001] This invention relates to a highly active catalyst for the reaction of CO2 with propylene oxide to synthesize propylene carbonate, its preparation method and application, belonging to the field of green catalysis technology. Background Technology

[0002] Propylene carbonate is an important chemical raw material, used as a solvent in organic reactions and an electrolyte solution in lithium-ion batteries. It can also be used to synthesize dimethyl carbonate, polycarbonate, and other chemical products. With the rise of the electrochemical and battery industries, the market demand for propylene carbonate is increasing. In 2019, the global supply of propylene carbonate increased to 1.7 million tons, indicating broad application prospects.

[0003] Currently, the main methods for synthesizing propylene carbonate are the phosgene method, transesterification method, chlorohydrin method, urea alcoholysis method, and CO2-propylene oxide cycloaddition method. The phosgene method has been gradually phased out due to the use of highly toxic phosgene and the corrosive effects of the hydrochloric acid produced. The transesterification method uses expensive propylene glycol and dimethyl carbonate as raw materials, resulting in a low cost-effectiveness ratio for propylene carbonate production. Furthermore, the commonly used catalyst in this process is an organotin catalyst, which is highly toxic and not a green method for synthesizing propylene carbonate. The chlorohydrin method produces many byproducts, and the separation and purification process is complex, thus preventing its industrial-scale production. The urea alcoholysis method uses expensive propylene glycol as a raw material; low-temperature synthesis is thermodynamically limited, while high-temperature synthesis suffers from high energy consumption, thus limiting its widespread application. The CO2-propylene oxide cycloaddition method is currently the most commonly used industrial process for synthesizing propylene carbonate. This process features a wide range of raw material sources, no byproduct generation, and an atom utilization efficiency of up to 100%. It is also an important technology for CO2 catalytic conversion and utilization, attracting widespread attention.

[0004] Currently, catalysts used in the industrial CO2-propylene oxide cycloaddition process are typically homogeneous catalysts (e.g., KI, ZnBr2). These catalysts require relatively harsh reaction temperatures and pressures to achieve good catalytic performance. For example, CN 103752344A reported that using ZnBr2 / DMF as a catalyst, the yield of propylene carbonate was only 46.4% at a reaction temperature of 150℃ and a reaction pressure of 4MPa. Furthermore, this type of catalyst is difficult to separate from the product propylene carbonate, resulting in low product purity. This limits the application of such catalysts. CN 101844090 A reported an imidazole-based grafted ionic liquid catalyst for the CO2-propylene oxide cycloaddition reaction. It was reported that modified MCM-41 or SBA-15 catalyzed the synthesis of propylene carbonate under milder conditions, but the catalyst and its preparation process are relatively complex and require further improvement. CN104448381 A reports an imidazole-functionalized ordered mesoporous phenolic resin material for the CO2-propylene oxide cycloaddition reaction. This catalyst has advantages such as good selectivity, easy separation, and recyclability. At a reaction temperature of 110℃ and a CO2 pressure of 1MPa, its propylene carbonate yield reaches 99%. However, the stability of this type of catalyst still needs improvement. CN104876906A reports a molecular sieve catalyst containing an organic template agent for the CO2-propylene oxide cycloaddition reaction. This catalyst has high reactivity, and its carbonate yield can reach 96.6%. However, the loss of the organic template agent during the reaction is still unavoidable. For example, the Beta catalyst containing the organic template agent prepared by Liao et al. (Fuel, 2022, 323:124389) showed a decrease in carbonate yield from 98% to about 80% after the second recycling. Zhang Jing et al. (Journal of Catalysis, 29(7), 2008:589-591) used titanium silicate molecular sieves to support quaternary ammonium salts for the catalytic synthesis of propylene carbonate, but the yield of propylene carbonate was only 48%.

[0005] In summary, current homogeneous catalysts used in the synthesis of propylene carbonate from CO2 and propylene oxide suffer from problems such as harsh reaction conditions, difficulty in separating the catalyst from the product, and environmental pollution. Heterogeneous catalysts developed for this reaction, on the other hand, face challenges such as difficult preparation conditions, easy loss of active sites, and poor reaction cycle stability. Therefore, it is crucial to research and develop efficient, inexpensive, and simple catalysts for the cycloaddition reaction of CO2 and propylene oxide. Summary of the Invention

[0006] To address the aforementioned problems in the existing technology, the purpose of this invention is to provide a highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide, its preparation method, and its application, thereby solving the problems of harsh reaction conditions, difficult catalyst preparation conditions, and low catalyst activity in the current synthesis of propylene carbonate from CO2 and propylene oxide.

[0007] A highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide is characterized in that the catalyst comprises a molecular sieve and a metal oxide supported thereon; the metal oxide is one or a combination of ZnO, Fe2O3, NiO, CoO, and Cr2O3, and the metal oxide is uniformly distributed in the molecular sieve, with the metal accounting for 1-7 wt% of the catalyst mass, preferably 2-4 wt%, and more preferably 3 wt%. The suitable metal loading ensures that the catalyst simultaneously contains appropriate amounts of Lewis acidic sites (i.e., metal oxides) and hydrogen bond donor sites (i.e., silanol groups). Notably, the metal oxides interact with the silanol sites in the molecular sieve, thereby increasing the dispersion of the metal oxides. Furthermore, the synergistic catalytic effect between the metal oxides anchored in the silanol nests of the molecular sieve and adjacent silanol sites can effectively improve the catalyst's performance in the propylene oxide cycloaddition reaction.

[0008] The molecular sieve is a pure silicon molecular sieve with a specific surface area of ​​120–900 m². 2 The molecular sieve particles have an average particle size of 0.1–4 μm, more preferably 0.1–0.5 μm, and exist in the form of aggregates. The surface of the pure silicon molecular sieve with its small particle size has a large number of silanol sites. On the one hand, the silanol sites can interact with metal oxides, thereby improving the dispersion of metal oxides. On the other hand, the silanol sites in the molecular sieve can act as hydrogen bond donors to catalyze the cycloaddition reaction of CO2 and propylene oxide. The synergistic catalytic effect between the metal oxides anchored in the silanol nests of the molecular sieve and the adjacent silanol sites can effectively enhance its cycloaddition reaction activity.

[0009] Preferably, the pure silicon molecular sieve is one or more combinations of Silicate-1 molecular sieve, Beta-type molecular sieve, MWW-type molecular sieve and CHA-type molecular sieve.

[0010] This invention also provides a method for preparing the above-mentioned catalyst, wherein the catalyst is obtained by impregnating a molecular sieve in an acid-treated metal precursor solution after treatment with an organic base, followed by shaking on an orbital shaker and then allowing it to stand, and finally drying and calcining. The specific method is as follows:

[0011] (1) First, add the molecular sieve to an organic base solution with a pH of 10 to 13, wherein the mass ratio of the molecular sieve to the organic base solution is 1:10 to 1:20. Treat the molecular sieve at 30 to 60°C for 2 to 5 hours. After filtering and washing, place the molecular sieve in an oven at 50 to 80°C to dry for 2 to 12 hours.

[0012] (2) Then place it in a vacuum drying oven and keep it at 80-150℃ for 2-12 hours to obtain the alkali-treated molecular sieve;

[0013] (3) Prepare a metal precursor solution of a certain concentration, and then weigh the metal precursor solution and the alkali-treated molecular sieve at a mass ratio of 1:1 to 3:1; adjust the pH of the metal precursor solution to 2 to 4 with acid; slowly add the acid-treated metal precursor solution to the alkali-treated molecular sieve at room temperature, while shaking it on a track shaker. After the addition is completed, continue shaking for 30 to 90 minutes, and let it stand at room temperature for 2 to 12 hours to obtain the molecular sieve loaded with metal precursor.

[0014] (4) The molecular sieve loaded with metal precursor is dried at 80-120℃ for 2-8h and then calcined at 300℃ for 4-6h to obtain a highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide.

[0015] In the above method, the organic base is one or a combination of several of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, triethylamine, and triethanolamine. The organic base solution refers to an aqueous solution of the above-mentioned organic base.

[0016] In the above method, the molecular sieve is treated with an organic alkali solution, and the pH value of the organic alkali solution is between 10 and 13. The mass ratio of molecular sieve to organic alkali solution is 1:10 to 1:20, more preferably 1:10. The treatment temperature is 30 to 60°C. Treating the molecular sieve with an organic alkali under relatively mild conditions is beneficial to increasing the silanol nesting sites in the catalyst without destroying the molecular sieve topology. The more silanol nesting sites in the molecular sieve, the better it is for anchoring the supported metal. On the other hand, the hydroxyl groups in the silanol nests will form synergistic catalytic sites with metal oxides, thereby improving the catalyst reaction performance.

[0017] In the above method, the metal precursor solution is one or a combination of several of the nitrate and / or sulfate solutions of Zn, Fe, Ni, Co, and Cr; the concentration of the metal precursor solution is 0.01-1 mol / L; and the mass ratio of the metal precursor solution to the alkali-treated molecular sieve is 1:1 to 3:1, more preferably 1.5:1.

[0018] In the above method, the acid is one or a combination of several of nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid, phosphoric acid, and acetic acid.

[0019] In the above method, acid is added to the metal precursor solution to adjust the pH, resulting in a pH value of 2 to 4 for the acid-treated metal precursor solution, more preferably 3. When the pH value of the metal precursor solution is low, the metal salt solution interacts more readily with the silanol sites in the molecular sieve, thereby enhancing the interaction between the metal and silanol sites in the catalyst.

[0020] In the above method, the pretreatment in the vacuum drying oven in step (2) can effectively remove impurities in the molecular sieve channels, thereby allowing more metal precursor solution to enter the molecular sieve channels, thus increasing the contact probability between the metal precursor solution and the silanol species in the molecular sieve.

[0021] The present invention also provides the application of the above-mentioned catalyst in the reaction of CO2 and propylene oxide to synthesize propylene carbonate, characterized in that the reaction temperature of CO2 and propylene oxide to synthesize propylene carbonate is 80-140°C, the reaction pressure is 1-5 MPa, and the yield of propylene carbonate is 70%-99%.

[0022] Compared with the prior art, the highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide provided by the present invention has the following significant characteristics:

[0023] (1) The catalyst prepared by the present invention has two active sites: Lewis acid and hydrogen bond donor. The synergistic catalytic effect of Lewis acid site and hydrogen bond donor in the catalyst effectively improves the reaction performance of the catalyst in the synthesis of propylene carbonate from CO2 and propylene oxide.

[0024] (2) Propylene carbonate is synthesized efficiently from CO2 and propylene oxide under relatively mild conditions. At 120°C and a CO2 pressure of only 1 MPa, the yield of propylene carbonate can reach over 99%. The catalyst used is a heterogeneous catalyst, which can be recycled. The preparation method is simple, inexpensive, and easy to carry out industrial production. Attached Figure Description

[0025] Figure 1 SEM image of the catalyst prepared in Example 1.

[0026] Figure 2 XRD pattern of the catalyst prepared in Example 1.

[0027] Figure 3 N2 adsorption-desorption isotherm of the catalyst prepared in Example 1.

[0028] Figure 4 hydroxyl infrared spectra of the catalysts prepared in Example 1 and Comparative Examples 1-2.

[0029] Figure 5 The pyridine infrared spectrum of the catalyst prepared in Example 1.

[0030] Figure 6 SEM image of the catalyst prepared in Example 2.

[0031] Figure 7 XRD pattern of the catalyst prepared in Example 2.

[0032] Figure 8 N2 adsorption-desorption isotherm of the catalyst prepared in Example 2.

[0033] Figure 9 The hydroxyl infrared spectrum of the catalyst prepared in Example 2.

[0034] Figure 10 The pyridine infrared spectrum of the catalyst prepared in Example 2.

[0035] Figure 11 SEM image of the catalyst prepared in Example 3.

[0036] Figure 12 XRD pattern of the catalyst prepared in Example 3.

[0037] Figure 13 N2 adsorption-desorption isotherm of the catalyst prepared in Example 3.

[0038] Figure 14 The hydroxyl infrared spectrum of the catalyst prepared in Example 3.

[0039] Figure 15 The pyridine infrared spectrum of the catalyst prepared in Example 3. Detailed Implementation

[0040] To facilitate understanding of the present invention, the following embodiments are provided. These embodiments are merely illustrative and should not be considered as specific limitations of the invention. Because the present invention can also be described and explained through other solutions that do not depart from its technical features, all modifications within the scope of the present invention or its equivalents should fall within the protection scope of the present invention. The present invention is further illustrated below with reference to the embodiments.

[0041] Example 1

[0042] First, prepare 30g of tetrapropylammonium hydroxide solution with a pH of approximately 11. Then, weigh 3g of Silicate-1 molecular sieve and add it to the prepared tetrapropylammonium hydroxide solution. Treat at 50℃ for 2 hours, filter and wash, then dry in an 80℃ oven for 12 hours. Next, place it in a vacuum drying oven and maintain at 80℃ for 12 hours to obtain the alkali-treated Silicate-1 molecular sieve. Then, prepare a 0.24mol / L zinc nitrate solution. Take 3g of the prepared zinc nitrate solution and adjust the pH to approximately 3 using 0.1mol / L nitric acid. The acid-treated zinc nitrate solution was added dropwise to 2g of alkali-treated Silicate-1 molecular sieve while the mixture was shaken on a track shaker. After the addition was complete, the mixture was shaken for 90min, sealed, and allowed to stand at room temperature for 5h to obtain a molecular sieve loaded with a metal precursor. The sieve was then dried at 80℃ for 4h and finally calcined at 300℃ for 5h to obtain a highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide. The Zn content in the catalyst was 3wt%, and it was named 3wt%Zn / S-1.

[0043] Next, 0.2 g of 3wt% Zn / S-1 catalyst and 0.02 g of tetrapropylammonium bromide co-catalyst were added to a 100 mL reactor. The reactor was placed in an ice-water bath, and 0.58 g of propylene oxide reactant was added, followed by 20 mL of solvent. The reactor was then sealed. A certain amount of carbon dioxide was introduced into the reactor, and then the gas was vented. Carbon dioxide was then introduced again, and the gas was vented. The gas in the reactor was replaced three times, and then carbon dioxide was introduced again, maintaining an initial carbon dioxide pressure of 1 MPa. The reactor temperature was then raised to 120 °C, and the reaction was carried out for 3 h. The reactor was then placed in an ice-water bath for cooling, and the resulting product was further analyzed by chromatography.

[0044] Figure 1 The image shows a SEM image of the 3wt% Zn / S-1 catalyst prepared in Example 1, which shows that the average particle size of the molecular sieve is 300 nanometers. Figure 2 The figure shows the XRD pattern of the 3wt% Zn / S-1 catalyst prepared in Example 1. As can be seen from the figure, all the peaks appearing in the figure belong to the characteristic diffraction peaks of MFI type molecular sieves. No diffraction peaks of ZnO are present, indicating that ZnO has a high degree of dispersion. Figure 3 The N2 adsorption-desorption isotherm of the 3wt% Zn / S-1 catalyst prepared in Example 1 shows that the pore volume of the catalyst, measured by nitrogen adsorption-desorption, is 0.25 cm⁻¹. 3 / g, specific surface area 380m² 2 / g. The Zn mass content was 3% as measured by inductively coupled plasma atomic emission spectrometry. Figure 4The hydroxyl infrared spectra of the catalysts prepared in Example 1 and Comparative Examples 1-2 were used to test the content of silanol clusters in the catalysts. The tests showed that the content of silanol clusters in Example 1 was higher than that in Comparative Example 2, but lower than that in Comparative Example 1. Figure 5 This is the pyridine infrared spectrum of the catalyst prepared in Example 1, where 1453 cm⁻¹ is the value of the peak value. -1 The wavenumber peaks are attributed to the adsorption of pyridine by Lewis acids in the catalyst. Studies indicate that Example 1 contains appropriate amounts of silanol nests and Lewis acid sites.

[0045] Example 2

[0046] First, prepare 30g of tetrapropylammonium hydroxide solution with a pH of approximately 12. Then, weigh 3g of Silicate-1 molecular sieve and add it to the prepared tetrapropylammonium hydroxide solution. Treat at 60℃ for 2 hours, filter and wash, then dry in a 60℃ oven for 12 hours. Place the treated Silicate-1 molecular sieve in a vacuum drying oven and maintain at 120℃ for 4 hours to obtain the alkali-treated Silicate-1 molecular sieve. Next, prepare a 0.08mol / L zinc nitrate solution. Take 3g of the prepared zinc nitrate solution and adjust the pH of the solution to approximately 2 using 0.2mol / L nitric acid. The acid-treated zinc nitrate solution was added dropwise to 2g of alkali-treated Silicate-1 molecular sieve while the mixture was shaken on an orbital shaker. After the addition was complete, the mixture was shaken for 60min, sealed, and allowed to stand at room temperature for 3h to obtain a molecular sieve loaded with a metal precursor. The sieve was then dried at 120℃ for 4h and finally calcined at 300℃ for 5h to obtain a highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide. The Zn content in the catalyst was 1wt%, and it was named 1wt%Zn / S-1.

[0047] Next, 0.2 g of 1 wt% Zn / S-1 catalyst and 0.02 g of tetrapropylammonium bromide co-catalyst were added to a 100 mL reactor. The reactor was placed in an ice-water bath, and 0.58 g of propylene oxide reactant was added, followed by 20 mL of solvent. The reactor was then sealed. A certain amount of carbon dioxide was introduced into the reactor, and then the gas was vented. Carbon dioxide was then introduced again, and the gas was vented. The gas in the reactor was replaced three times, and then carbon dioxide was introduced again, maintaining an initial carbon dioxide pressure of 1 MPa. The reactor temperature was then raised to 120 °C, and the reaction was carried out for 3 h. The reactor was then placed in an ice-water bath for cooling, and the resulting product was further analyzed by chromatography.

[0048] Figure 6 The image shows an SEM image of the 1wt% Zn / S-1 catalyst prepared in Example 2, which shows that the average particle size of the molecular sieve is 330 nm. Figure 7The figure shows the XRD pattern of the 1wt% Zn / S-1 catalyst prepared in Example 2. As can be seen from the figure, all the peaks appearing in the figure belong to the characteristic diffraction peaks of MFI type molecular sieves. No diffraction peaks of ZnO are present, indicating that ZnO has a high degree of dispersion. Figure 8 The N2 adsorption-desorption isotherm of the 1 wt% Zn / S-1 catalyst prepared in Example 2 shows that the pore volume of the catalyst, measured by nitrogen adsorption-desorption, is 0.22 cm³. 3 / g, specific surface area is 401m² 2 / g. The Zn mass content was 1% as measured by inductively coupled plasma atomic emission spectrometry. Figure 9 The hydroxyl infrared spectrum of the catalyst prepared in Example 2 was used to test the content of silanol clusters in the catalyst. Figure 10 This is the pyridine infrared spectrum of the catalyst prepared in Example 2. Studies show that Example 2 contains a certain amount of silanol clusters, but its Lewis acidic site content is relatively low.

[0049] Example 3

[0050] First, prepare 30g of tetrapropylammonium hydroxide solution with a pH of approximately 10. Then, weigh 3g of Silicate-1 molecular sieve and add it to the prepared tetrapropylammonium hydroxide solution. Treat at 50℃ for 2 hours, filter and wash, then dry in an 80℃ oven for 12 hours. Next, place it in a vacuum drying oven and maintain at 80℃ for 12 hours to obtain the alkali-treated Silicate-1 molecular sieve. Then, prepare a 0.56mol / L zinc nitrate solution. Take 3g of the prepared zinc nitrate solution and adjust its pH to approximately 4 using 0.1mol / L sulfuric acid. The acid-treated zinc nitrate solution was added dropwise to 2g of alkali-treated Silicate-1 molecular sieve while the mixture was shaken on an orbital shaker. After the addition was complete, the mixture was shaken for 90 minutes, sealed, and allowed to stand at room temperature for 5 hours to obtain a molecular sieve loaded with a metal precursor. The sieve was then dried at 80℃ for 4 hours and finally calcined at 300℃ for 5 hours to obtain a highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide. The Zn content in the catalyst was 7wt%, and it was named 7wt%Zn / S-1.

[0051] Next, 0.2 g of 7wt% Zn / S-1 catalyst and 0.02 g of tetrapropylammonium bromide co-catalyst were added to a 100 mL reactor. The reactor was placed in an ice-water bath, and 0.58 g of propylene oxide reactant was added, followed by 20 mL of solvent. The reactor was then sealed. A certain amount of carbon dioxide was introduced into the reactor, and then the gas was vented. Carbon dioxide was then introduced again, and the gas was vented. The gas in the reactor was replaced three times, and then carbon dioxide was introduced again, maintaining an initial carbon dioxide pressure of 1 MPa. The reactor temperature was then raised to 120 °C, and the reaction was carried out for 3 h. The reactor was then placed in an ice-water bath for cooling, and the obtained product was further analyzed by chromatography.

[0052] Figure 11 The image shows a SEM image of the 7wt% Zn / S-1 catalyst prepared in Example 3, which shows that the average particle size of the molecular sieve is 310 nm. Figure 12 The figure shows the XRD pattern of the 7wt% Zn / S-1 catalyst prepared in Example 3. As can be seen from the figure, all the peaks appearing in the figure belong to the characteristic diffraction peaks of MFI molecular sieves. No diffraction peaks of ZnO are present, indicating that ZnO has a high degree of dispersion. Figure 13 The N2 adsorption-desorption isotherm of the 3wt% Zn / S-1 catalyst prepared in Example 3 shows that the pore volume of the catalyst, measured by nitrogen adsorption-desorption, is 0.21 cm³. 3 / g, specific surface area 355m² 2 / g. The Zn mass content was 7% as measured by inductively coupled plasma atomic emission spectrometry. Figure 14 The hydroxyl infrared spectrum of the catalyst prepared in Example 3 was used to test the content of silanol clusters in the catalyst. Figure 15 This is the pyridine infrared spectrum of the catalyst prepared in Example 3. Studies show that Example 3 contains a certain amount of Lewis acidic sites, but the content of silanol nests is low.

[0053] Example 4

[0054] First, prepare 30g of tetrapropylammonium hydroxide solution with a pH of approximately 10. Then, weigh 3g of pure silicon Beta molecular sieve and add it to the prepared tetrapropylammonium hydroxide solution. Treat at 30℃ for 2 hours, filter and wash, then dry in a 70℃ oven for 5 hours. Next, place it in a vacuum drying oven and maintain at 120℃ for 4 hours to obtain alkali-treated pure silicon Beta molecular sieve. Then, prepare a 0.08mol / L zinc nitrate solution. Take 3g of the prepared zinc nitrate solution and adjust the pH to approximately 2.5 using 0.1mol / L nitric acid. The acid-treated zinc nitrate solution was added dropwise to 2g of alkali-treated pure silicon Beta molecular sieve while the solution was being added dropwise using a track shaker. After the addition was complete, the solution was shaken for 60 minutes, sealed, and allowed to stand at room temperature for 3 hours to obtain a molecular sieve loaded with a metal precursor. The solution was then dried at 120℃ for 4 hours and finally calcined at 300℃ for 5 hours to obtain a highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide. The Zn content in the catalyst was 1wt%, and it was named 1wt%Zn / Beta.

[0055] Next, 0.2 g of the catalyst 1 wt% Zn / Beta and 0.02 g of the co-catalyst tetrapropylammonium bromide were added to a 100 mL reactor. The reactor was placed in an ice-water bath, and 0.58 g of the reactant propylene oxide was added, followed by 20 mL of solvent. The reactor was then sealed. A certain amount of carbon dioxide was introduced into the reactor, and then the gas was vented. Carbon dioxide was then introduced again, and the gas in the reactor was vented. This process was repeated three times, and then carbon dioxide was introduced again, maintaining an initial carbon dioxide pressure of 1 MPa. The reactor temperature was then raised to 120 °C, and the reaction was carried out for 3 h. The reactor was then placed in an ice-water bath for cooling, and the resulting product was further analyzed by chromatography.

[0056] Example 5

[0057] First, prepare 30g of tetrapropylammonium hydroxide solution with a pH of approximately 13. Then, weigh 3g of pure silicon Beta molecular sieve and add it to the prepared tetrapropylammonium hydroxide solution. Treat at 50℃ for 2 hours, filter and wash, then dry in an 80℃ oven for 12 hours. Next, place it in a vacuum drying oven and maintain at 80℃ for 12 hours to obtain alkali-treated pure silicon Beta molecular sieve. Then, prepare a 0.56mol / L zinc nitrate solution. Take 3g of the prepared zinc nitrate solution and adjust the pH to approximately 3 using 0.1mol / L nitric acid. The acid-treated zinc nitrate solution was added dropwise to 2g of alkali-treated pure silicon Beta molecular sieve while the solution was being added dropwise using a track shaker. After the addition was complete, the shaking continued for 90 minutes. The sieve was then sealed and allowed to stand at room temperature for 5 hours to obtain a molecular sieve loaded with a metal precursor. It was then dried at 80℃ for 4 hours and finally calcined at 300℃ for 5 hours to obtain a highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide. The Zn content in the catalyst was 7wt%, and it was named 7wt%Zn / Beta.

[0058] Next, 0.2 g of the catalyst 7wt% Zn / Beta and 0.02 g of the co-catalyst tetrapropylammonium bromide were added to a 100 mL reactor. The reactor was placed in an ice-water bath, and 0.58 g of the reactant propylene oxide was added, followed by 20 mL of solvent. The reactor was then sealed. A certain amount of carbon dioxide was introduced into the reactor, and then the gas was vented. Carbon dioxide was then introduced again, and the gas in the reactor was vented. This process was repeated three times, and then carbon dioxide was introduced again, maintaining an initial carbon dioxide pressure of 1 MPa. The reactor temperature was then raised to 120 °C, and the reaction was carried out for 3 h. The reactor was then placed in an ice-water bath for cooling, and the resulting product was further analyzed by chromatography.

[0059] Comparative Example 1

[0060] First, prepare 30g of tetrapropylammonium hydroxide solution with a pH of approximately 11. Then, weigh 3g of Silicate-1 molecular sieve and add it to the prepared tetrapropylammonium hydroxide solution. Treat at 50℃ for 2h, filter and wash, then dry in an 80℃ oven for 12h. Next, place it in a vacuum drying oven and maintain at 80℃ for 12h to obtain the alkali-treated Silicate-1 molecular sieve. Then, prepare a 0.24mol / L zinc nitrate solution and add it dropwise to 2g of the alkali-treated Silicate-1 molecular sieve while simultaneously shaking on a track shaker. After the addition is complete, continue shaking for 90min, seal, and let stand at room temperature for 5h. Dry at 80℃ for 4h, and finally calcine at 300℃ for 5h to obtain the catalyst. At this point, the Zn content in the catalyst is 3wt%, named 3wt%Zn / S-1-Comparative Example 1.

[0061] Next, 0.2 g of catalyst 3wt% Zn / S-1-Comparative Example 1 and 0.02 g of co-catalyst tetrapropylammonium bromide were added to a 100 mL reactor. The reactor was placed in an ice-water bath, and 0.58 g of reactant propylene oxide was added, followed by 20 mL of solvent. The reactor was then sealed. A certain amount of carbon dioxide was introduced into the reactor, and then the gas was vented. Carbon dioxide was then introduced again, and the gas in the reactor was vented. This process was repeated three times, and then carbon dioxide was introduced again, maintaining an initial pressure of 1 MPa. The reactor temperature was then raised to 120 °C, and the reaction was carried out for 3 h. The reactor was then placed in an ice-water bath for cooling, and the resulting product was further analyzed by chromatography.

[0062] Comparative Example 2

[0063] First, Silicate-1 molecular sieve was placed in a drying oven and kept at 80℃ for 12 hours. Then, a zinc nitrate solution with a concentration of 0.24 mol / L was prepared, and 3 g of zinc nitrate solution was added dropwise to 2 g of Silicate-1 molecular sieve while shaking on a track shaker. After the addition was completed, shaking was continued for 90 minutes. After sealing, it was allowed to stand at room temperature for 5 hours, dried at 80℃ for 4 hours, and finally calcined at 300℃ for 5 hours to obtain the catalyst. At this time, the Zn content in the catalyst was 3 wt%, named 3 wt% Zn / S-1-Comparative Example 2.

[0064] Next, 0.2 g of catalyst 3wt% Zn / S-1-Comparative Example 2 and 0.02 g of co-catalyst tetrapropylammonium bromide were added to a 100 mL reactor. The reactor was placed in an ice-water bath, and 0.58 g of reactant propylene oxide was added, followed by 20 mL of solvent. The reactor was then sealed. A certain amount of carbon dioxide was introduced into the reactor, and then the gas was vented. Carbon dioxide was then introduced again, and the gas in the reactor was vented. This process was repeated three times, and then carbon dioxide was introduced again, maintaining an initial carbon dioxide pressure of 1 MPa. The reactor temperature was then raised to 120 °C, and the reaction was carried out for 3 h. The reactor was then placed in an ice-water bath for cooling, and the resulting product was further analyzed by chromatography.

[0065] Table 1 shows the reaction results of the catalysts prepared in Examples 1-5 and Comparative Examples 1-2 in the cycloaddition of CO2 with propylene oxide to prepare propylene carbonate. The study shows that all prepared catalysts have high reactivity; at 120°C and a CO2 pressure of only 1 MPa, the yield of propylene carbonate can reach over 80%, with a maximum of 99%. Combining the hydroxyl infrared spectra and pyridine infrared spectra of the different catalysts used, it can be found that the 3wt% Zn / S-1 catalyst used in Example 1 has a greater number of Lewis acid sites (metal oxides) and hydrogen bond donor active sites (silanol groups), exhibiting the highest reactivity and a propylene carbonate yield of 99%. The 1wt% Zn / S-1 catalyst used in Example 2 has a relatively lower silanol content, resulting in a propylene carbonate yield of 85%. The 7wt% Zn / S-1 catalyst used in Example 3 has a relatively low Lewis acid content, leading to a propylene carbonate yield of only 80%. This further illustrates that the synergistic catalytic effect of silanol species and Lewis acidic sites can effectively improve catalyst reaction performance. Compared with Comparative Examples 1 and 2, Example 1 shows a significant improvement in reaction performance. In Comparative Example 1, no acid was used to adjust the acidity of the metal salt solution, which would make it difficult for the metal salt solution to interact with the silanol sites in the molecular sieve during metal loading. Figure 4 As shown, although Comparative Example 1 and Example 1 used the same molecular sieve as the support and had the same metal loading, the content of silanol clusters in Comparative Example 1 was significantly higher than that in Example 1 after metal loading. This is because the metal did not react with the silanol clusters during the impregnation process. Comparative Example 2, however, did not use an organic base to treat the molecular sieve, resulting in fewer silanol clusters in the catalyst, thus affecting its propylene oxide cycloaddition reaction performance.

[0066] Table 1 Catalytic performance of Examples 1-5

[0067] catalyst propylene oxide conversion rate / % propylene carbonate selectivity / % Propylene carbonate yield / % Example 1 99 >99 99 Example 2 85 >99 85 Example 3 80 >99 80 Example 4 82 >99 82 Example 5 96 >99 96 Comparative Example 1 52 >99 52 Comparative Example 2 35 >99 35

Claims

1. A highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide, characterized in that, The catalyst comprises a molecular sieve and a metal oxide supported thereon; the metal oxide is one or a combination of several of ZnO, Fe2O3, NiO, CoO, and Cr2O3, and the metal oxide is uniformly distributed in the molecular sieve, with the metal accounting for 1-7 wt% of the catalyst mass; the molecular sieve is a pure silicon molecular sieve with a specific surface area of ​​120-900 m². 2 / g, the molecular sieve particles have an average particle size of 0.1 to 4 μm and exist in the form of aggregates.

2. The highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide according to claim 1, characterized in that, The pure silicon molecular sieve is one or more combinations of Silicate-1 molecular sieve, pure silicon Beta type molecular sieve, pure silicon MWW type molecular sieve and pure silicon CHA type molecular sieve.

3. The highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide according to claim 1, characterized in that, The metal constitutes 2 to 4 wt% of the catalyst by mass.

4. The highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide according to claim 3, characterized in that, The metal constitutes 3 wt% of the catalyst mass.

5. The highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide according to claim 1, characterized in that, The average particle size of the molecular sieve particles is 0.1–0.5 μm.

6. A method for preparing a highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide according to any one of claims 1-5, characterized in that, The specific steps include the following: (1) First, add the molecular sieve to an organic base solution with a pH of 10 to 13, wherein the mass ratio of the molecular sieve to the organic base solution is 1:10 to 1:

20. Treat the molecular sieve at 30 to 60°C for 2 to 5 hours. After filtering and washing, place the molecular sieve in an oven at 50 to 80°C to dry for 2 to 12 hours. (2) Then place it in a vacuum drying oven and keep it at 80-150℃ for 2-12 hours to obtain the alkali-treated molecular sieve; (3) Prepare a metal precursor solution of a certain concentration; then weigh the metal precursor solution and the alkali-treated molecular sieve at a mass ratio of 1:1 to 3:1; adjust the pH of the metal precursor solution to 2 to 4 using acid; slowly add the acid-treated metal precursor solution to the alkali-treated molecular sieve at room temperature, while shaking it on a track shaker. After the addition is complete, continue shaking for 30 to 90 minutes, and let it stand at room temperature for 2 to 12 hours to obtain the molecular sieve loaded with metal precursor. (4) The molecular sieve loaded with metal precursor is dried at 80-120℃ for 2-8h and then calcined at 300℃ for 4-6h to obtain a highly active catalyst for the synthesis of propylene carbonate from CO2 and propylene oxide.

7. The method according to claim 6, characterized in that, The organic base is one or a combination of several of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, triethylamine, and triethanolamine.

8. The method according to claim 6, characterized in that, The metal precursor solution is one or a combination of several of the nitrate and / or sulfate solutions of Zn, Fe, Ni, Co, and Cr, and the concentration of the metal precursor solution is 0.01 to 1 mol / L. The acid is one or a combination of several of the following: nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid, phosphoric acid, and acetic acid.

9. The method according to claim 6, characterized in that, The mass ratio of molecular sieve to organic alkali solution is 1:10, the mass ratio of metal precursor solution to alkali-treated molecular sieve is 1.5:1, and the pH value of the acid-treated metal precursor solution is 3.

10. The application of the highly active catalyst according to any one of claims 1-5 for the synthesis of propylene carbonate from CO2 and propylene oxide in the reaction of CO2 and propylene oxide to propylene carbonate, characterized in that, The reaction temperature for synthesizing propylene carbonate from CO2 and propylene oxide is 80–140℃, and the reaction pressure is 1–5 MPa, with a propylene carbonate yield of 70%–99%.

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