Hierarchically Nanostructured Zeolites of Tunable Porosities with Aerogel Templating
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Hierarchically Nanostructured Zeolites of Tunable Porosities with Aerogel Templating Yousheng Tao, Morinobu Endo and Katsumi Kaneko Institute of Carbon Science and Technology, Shinshu University, Nagano 380-8553, Japan ABSTRACT We present the synthesis of resorcinol-formaldehyde aerogels and carbon aerogels of different nanoporosities, emphasizing on the recent developments in fabrication pathways of lower cost. Recent results showed a simple way to the production of highly nanoporous carbon xerogels. While using an approach combined colloidal silica nanocasting and carbon dioxide supercritical drying, hydrophilicity-controlled carbon aerogels with high mesoporosity were synthesized. Then, we demonstrate the functions of these aerogels for template synthesis of hierarchically nanostructured zeolites having micropores and mesopores. INTRODUCTION Zeolite molecular sieves are representative of crystalline microporous solids. They have been found to catalyze a wide variety of chemical reactions. In many cases, the sole presence of micropores results in intracrystalline diffusion limitation as well as high backpressure on flow systems, which hinders the performance of industrial catalysts. Hierarchically nanostructured zeolites having micropores and mesopores are demanded for efficiently energy-consuming chemical process [1-5]. These materials are chemically and thermally stable and have high surface areas and large nanopore volumes. Moreover, the interconnected mesopore channels which provide fast mass transfer could lead to significantly improved properties as compared to conventional zeolite catalysts. It was reported that in the alkylation of benzene with ethane using mesoporous zeolite single crystals have improving activity and selectivity [6,7]. Catalytic conversion of bulky molecules over mesoporous zeolites was much higher than those of conventional zeolite because of its larger external surface area [8,9]. Recently there have been active developments in the template synthesis for the preparation of nanoporous carbons and nanoporous silica. In this study, we present the synthesis of resorcinol-formaldehyde aerogels and carbon aerogels of different nanoporosities, emphasizing on the recent developments in fabrication pathways. Most recently hydrophilicitycontrolled carbon aerogels with high mesoporosities were synthesized using an approach combined colloidal silica nanocasting and carbon dioxide supercritical drying. Then, we demonstrate the functions of these aerogels for template synthesis of hierarchically nanostructured zeolites having micropores and mesopores, while other applications of these aerogels such as in high-conductivity nano-composites and in electrical energy storage can be found in literature [10-12]. EXPERIMENTAL Resorcinol-formaldehyde aerogels were prepared from a sol-gel polymerization process of resorcinol and formaldehyde with sodium carbonate as a basic catalyst according to the
method reported by Pekala et al. [13]. We directly prepared resorcinol-formaldehyde xerogels using a narrow s
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