The development of effective solid acid-base bifunctional catalysts remains a challenge because of the difficulty associated with designing and controlling their active sites. In the present study, highly pure perovskite oxide nanoparticles with d0-transition-metal cations such as Ti4+, Zr4+, and Nb5+ as B-site elements were successfully synthesized by a sol-gel method using dicarboxylic acids. Moreover, the specific surface area of SrTiO3 was increased to 46 m2 g-1 by a simple procedure of changing the atmosphere from N2 to air during calcination of an amorphous precursor. The resultant SrTiO3 nanoparticles showed the highest catalytic activity for the cyanosilylation of acetophenone with trimethylsilyl cyanide (TMSCN) among the tested catalysts not subjected to a thermal pretreatment. Various aromatic and aliphatic carbonyl compounds were efficiently converted to the corresponding cyanohydrin silyl ethers in good-to-excellent yields. The present system was applicable to a larger-scale reaction of acetophenone with TMSCN (10 mmol scale), in which 2.06 g of the analytically pure corresponding product was isolated. In this case, the reaction rate was 8.4 mmol g-1 min-1, which is the highest rate among those reported for heterogeneous catalyst systems that do not involve a pretreatment. Mechanistic studies, including studies of the catalyst effect, Fourier transform infrared spectroscopy, and temperature-programmed desorption measurements using probe molecules such as pyridine, acetophenone, CO2, and CHCl3, and the poisoning effect of pyridine and acetic acid toward the cyanosilylation, revealed that moderate-strength acid and base sites present in moderate amounts on SrTiO3 most likely enable SrTiO3 to act as a bifunctional acid-base solid catalyst through cooperative activation of carbonyl compounds and TMSCN. This bifunctional catalysis through SrTiO3 resulted in high catalytic performance even without a heat pretreatment, in sharp contrast to the performance of basic MgO and acidic TiO2 catalysts.
Keywords: acid−base bifunctional catalysts; cyanosilylation; nanoparticle; perovskite-type oxides; strontium titanate.