(Abstract)

Mesocellular siliceous foam (MCF) accommodates the fixation of various objects including homogeneous organometallic or organic catalysts and bulky biocatalysts and facilitates various transformations of numerous types of substrates with high catalytic activity and recyclability, associated with its uniform, ultralarge, and 3-dimensionally interconnected pores. Particularly, its good mechanical strength and high porosity make it highly suitable for a packing material in a packed bed reactor, potentially utilized as a continuous flow reactor which is environmentally friendly. For immobilizing homogeneous organometallic or organic catalysts, silica is not a popular support material due to the reactive silanol groups on surface, which may lead to lowering catalytic activity and selectivity.

Spherical MCF microparticles have been successfully prepared by a simple modification of conventional method. They show excellent performance as a stationary phase in reverse-phase column chromatography and size exclusion chromatography (SEC) in conjunction with a potential usage as a good packing material. Using the spherical MCF, we have successfully developed various heterogeneous catalysts by immobilizing several homogeneous organometallic catalysts, organocatalysts and biocatalysts. To minimize the influence of free silanol groups, we have developed a simple method of partial pre-capping. Using this efficient method, MCF-supported chiral bisoxazoline catalysts show comparable selectivity and reactivity to the homogeneous counterparts through immobilization. The immobilized homogeneous catalysts have been applied to a packed bed reactor for flow-type reactions. The partial pre-capping method has been successfully applied to immobilization of homogeneous ruthenium-based metathesis catalysts achieving high reactivity and good recyclability and then flow-type reaction with a packed bed. Another method has been developed by using simply the pressure to immobilize enzymes in hydrophobically modified cage-like nanopores. The immobilized enzymes show excellent catalytic activity and selectivity as well as reduced leaching and excellent thermal selectivity. This system has been also applied to a continuous flow reaction. Recently, we developed a dual fixed bed reactor system with metal oxide catalysts highly dispersed on the MCF surface for efficient conversion of ethanol to 1,3-butadiene through gas phase reactions in packed beds. Our results demonstrate that MCF is a very useful support material for flow-type reaction systems with packed beds.