Mesoporous Titanium-Silicalite Zeolite Containing Organic Templates as a Bifunctional Catalyst for Cycloaddition of CO₂ and Epoxides

Mesoporous TS-1 (MTS-1) containing organic templates tetrapropylammonium hydroxide (TPAOH) and polydiallyldimethylammonium chloride is synthesized and treated with different concentrations of hydrochloric acid aqueous solution. The as-synthesized MTS-1 are characterized using X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy, transmission electron microscopy, UV–vis, 13C cross-polarization/magic angle spinning (CP/MAS) NMR, thermogravimetry analysis, CO2-temperature-programmed desorption (TPD), N2 adsorption–desorption, X-ray fluorescence, and elemental analysis. The results of 13C CP/MAS NMR show that the structures of organic templates are retained after acid washing treatment. Not required to be removed by calcination, the organic templates embedded in MTS-1 can take the role of basic sites and activate carbon dioxide (CO2), which is confirmed by FT-IR. Moreover, the amount of acid sites and basic sites in the samples before and after acid treatment is characterized by modified Hammett indicator method and CO2-TPD, respectively. The results show that both the acidity and the basicity in the material are improved after acid washing. Thus, the sample after acid treatment contains two active sites: basic sites stemming from organic species and acid sites coming from framework Ti, which is beneficial to be used as a bifunctional catalyst in the cycloaddition reaction of CO2 and epoxides. It is highly active in the cycloaddition reaction, in which the conversion of epichlorohydrin (ECH) could reach 97.8% and the selectivity of cyclic carbonate is 98.0% under 1.6 MPa at 393 K for 6 h when acetonitrile is used as a solvent. Moreover, the kinetics of the cycloaddition reaction are studied using ECH as a substrate by varying the reaction parameters. More importantly, the organic–inorganic hybrid catalyst is reusable and stable against leaching of organic species in the cycloaddition reaction.