Cerium Pyrazolates Grafted onto Mesoporous Silica SBA-15: Reversible CO2 Uptake and Catalytic Cycloaddition of Epoxides and Carbon Dioxide.

Abstract

The activation and catalytic conversion of CO2 is a current topic relating to molecular chemistry and materials science alike. As a transdisciplinary field of research, surface organometallic chemistry (SOMC) might be applicable to perform synergistically, thus striking a new path in sustainable chemistry. Both ceric and cerous rare-earth-metal pyrazolates, which were recently shown to reversibly insert CO2 and to promote the catalytic cycloaddition of epoxides and carbon dioxide, were grafted onto large-pore mesoporous silica SBA-15500, thermally pretreated at 500 °C. The obtained hybrid materials [Ce(Me2pz)4]2@SBA-15500, Ce(Me2pz)4(thf)@SBA-15500, Ce4(Me2pz)12@SBA-15500, and [Ce(Me2pz)3(thf)]2@SBA-15500 (Me2pz = 3,5-dimethylpyrazolato) were characterized by DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy), solid-state 1H/13C NMR spectroscopy, elemental analysis, ICP/OES, and N2 physisorption. The lanthanum(III)-based material [La(Me2pz)3(thf)]2@SBA-15500 was synthesized for better assessment of the cerous materials being highly sensitive to oxidation. To mimic ceric surface species, Ce[OSi(OtBu)3]3Cl was treated with 1 equiv of K(Me2pz), generating the mixed pyrazolyl/siloxy complex KCe[OSi(OtBu)3]4(Me2pz) featuring a cerium(IV)-bonded terminal pyrazolato ligand. All hybrid materials show efficient and reversible carbon dioxide uptake of maximum 20 wt % in the solid state. When combined with tetra-n-butylammonium bromide (TBAB), the hybrid materials catalyze the cycloaddition of CO2 and epoxides, displaying good conversion of various epoxides and reusability.