Synthesis of Monolithic Microhoneycombs Equipped with Unique Hierarchical Pore Systems Using Ice Templating

Abstract

Various porous materials with unique functions can be obtained through sol-gel synthesis. In many cases the performance of such materials can be significantly improved by controlling the morphology of them. Recently, we found that sol-gel derived materials can be molded into the form of monolithic microhoneycombs by freezing their parent hydrogel unidirectionally. In this process, the ice crystals which are formed within the hydrogel during freezing act as the template. The sizes of the monolith channels, the traces of the ice crystals, are in the micrometer range, therefore can be considered as macropores. The walls which form the channels have thicknesses around 1 μm, and have developed nanopores within them. Therefore, such monoliths are equipped with a unique hierarchical pore system in which short nanopores are directly connected to straight macropores. Due to this unique structure, such monoliths do not cause severe pressure drops when fluids are passed through them, even though the lengths of the diffusion paths within them are extremely short. This method can be applied to various hydrogels, either organic or inorganic, therefore monoliths having a wide variety of functions along with this unique structure can be easily obtained. This method can also be used to assemble fine particles into the form of a monolithic microhoneycomb by using a proper hydrogel as the binder.