Mesoporous Ta–W Composite Oxides: A Highly Effective and Reusable Acid–Base Catalysts for the Cycloaddition Reaction of Carbon Dioxide with Epoxides

Cycloadditions of epoxides and carbon dioxide into corresponding cyclic carbonates were performed over mesoporous Ta–W composite oxides prepared by a modified hydrolytic method. The best yields of styrene carbonate were obtained when the Ta/W mole ratio was 2:1 (labeled as Ta0.67W0.33Os). Under optimal reaction conditions, the conversion of styrene oxide and selectivity of styrene carbonate reached 95 and 97%, respectively. These Ta–W composite oxides have been extensively characterized by several techniques. X-ray diffraction (XRD) patterns and Transmission Electron Microscope (TEM) revealed that Ta2O5 was completely dispersed in WOx. Scanning electron microscopy (SEM) exhibited that the particle size distributions become more and more uniform with increment of tungsten content. CO2 and NH3 temperature-programmed desorption (CO2 and NH3-TPD) revealed that Ta0.67W0.33Os catalyst had the strongest acid and base strength. X-ray photoelectron spectroscopy (XPS) shows that the strongest acid–base sites of Ta0.67W0.33Os catalyst origin from its highest lattice oxygen concentration and W 4f5/2 species with Bronsted acidity. We discussed the reaction kinetics and proposed a possible mechanism, indicating the excellent catalytic activity is attributed to the cooperative action of acidic and neighboring basic sites on the catalyst surface. Cycloaddition reactions of carbon dioxide with epoxides into corresponding cyclic carbonates were performed over mesoporous Ta–W composite oxides prepared by a modified hydrolytic method. The best yields for cyclic carbonates were obtained when the Ta/W mole ratio was 2:1 (denoted as Ta0.67W0.33Os). Acid-base synergy, specific surface area and mesoporous structure could be ascribed to the main reasons for the highest catalytic activity of Ta0.67W0.33Os catalyst. Meanwhile, reaction kinetics was discussed and a possible reaction pathway was proposed.