Nanosized Ti-Based Perovskite Oxides as Acid–Base Bifunctional Catalysts for Cyanosilylation of Carbonyl Compounds
Takeshi Aihara, Wataru Aoki, Shin Kiyohara, Yu Kumagai, Keigo Kamata, Masahiko Hara
Abstract
High Resolution Image Download MS PowerPoint Slide 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 d 0 -transition-metal cations such as Ti 4+, Zr 4+, and Nb 5+ as B -site elements were successfully synthesized by a sol–gel method using dicarboxylic acids. Moreover, the specific surface area of SrTiO 3 was increased to 46 m 2 g –1 by a simple procedure of changing the atmosphere from N 2 to air during calcination of an amorphous precursor. The resultant SrTiO 3 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, CO 2, and CHCl 3, 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 SrTiO 3 most likely enable SrTiO 3 to act as a bifunctional acid–base solid catalyst through cooperative activation of carbonyl compounds and TMSCN. This bifunctional catalysis through SrTiO 3 resulted in high catalytic performance even without a heat pretreatment, in sharp contrast to the performance of basic MgO and acidic TiO 2 catalysts.