7. Hollow Core–Shell Titania Photocatalysts for Selective Organic Synthesis

Titania, as one of the most widely used material for heterogeneous semiconductor photocatalysts, shows exceptional characteristics such as high chemical stability, nontoxicity, low cost, and capacity to induce various kinds of chemical reactions. A common drawback, however, is its lack of selectivity. This problem has been addressed through the synthesis of hollow core–shell particles, which consisted of commercially available titania particles incorporated into a hollow silica shell, via a multistep process comprising carbon and silica coatings followed by calcination to remove the carbon layer. The silica shell was found to be porous, water-swollen, and not directly connected to the titania particles. In the photocatalytic redox-combined synthesis of pipecolinic acid (l-PCA) from l-lysine to obtain optically active PCA, the hollow core–shell particles showed improved stereoselectivity, maintaining the l-lysine conversion. The PCA selectivity was also almost unchanged from those observed in a bare titania particle system. It is deduced that the acidic microenvironment of the titania core induced by the silica shell effectively controlled the position of oxidation by positive holes as the first step of the redox-combined process, leading to the improved stereoselectivity.