Hierarchically Porous Oxides, Hybrids and Polymers via Sol-gel Accompanied by Phase Separation
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1007-S03-01
Hierarchically Porous Oxides, Hybrids and Polymers via Sol-gel Accompanied by Phase Separation Kazuki Nakanishi Graduate School of Science, Dept. Chemistry, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto, 6068502, Japan ABSTRACT In various crosslinking systems containing metal oxides, organo-siloxane polymers and pure hydrocarbons, monolithic materials with hierarchical well-defined macropores and controlled mesopores have been synthesized. Synthetic progress in alkoxy-derived macroporous silica lead to the preparation of long-range ordered mesoporous skeletons in well-defined macroporous framework. Alkylene-bridged silicon alkoxides can also be prepared into similarly hierarchical porous structures with broadened variations in framework morphology. Macromesoporous alkoxy-derived pure titania and zirconia have been prepared using hydrochloric acid – mediated processes. Compared with those prepared from colloidal dispersions, alkoxy-derived macroporous titania exhibited much higher mechanical strength. Titania monolith is a promising candidate as a separation medium to discriminate phosphorylated compounds in a liquid chromatography mode. Pure alumina macroporous monolith has been first synthesized from aluminum salt using propylene glycol as a proton scavenger to thrust the solution pH from acidic into neutral conditions. Alumina-based complex oxides such as garnets and spinels can also be prepared in pure phases. Polymerization and phase separation in organic crosslinker system was also controlled to obtain well-defined co-continuous macro-frameworks instead of those composed of aggregated particles. These examples demonstrate the versatility of using phaseseparation in gelling systems to obtain well-defined macroporous structures. INTRODUCTION The phase separation in gelling silica system to give well-defined micrometer-range heterogeneity was first reported nearly twenty years ago [1, 2]. Porous silica gels and glasses very similar in morphology to so-called “controlled pore glasses (CPG)” [3] could be obtained via the low-temperature solution process. Regarding the morphology formation process as a competition between dynamics of polymerization-induced phase separation and of steep chemical sol-gel transition, dependence of macroporous morphology on various synthesis parameters has been consistently interpreted [4, 5]. The experimental system has been readily extended to organo-silicon and silica-based multi-component oxide systems. It has been found only recently, however, pure titanium, zirconium and aluminum oxides with well-defined macropores can also be fabricated [6, 7]. Examination through various reaction systems revealed that the kind of gel-forming reaction, whether it is chemical polymerization, colloidal aggregation or oxolation of hydrated species, has a minor effect on the structure-formation process [8]. Compared with the time-consuming process of CPG including the leaching of solid glass phase, the chemically-polymerized phase-separated structure forms a solid gel phase and an easi
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