Engineering of Porosity in Amorphous Materials. Plasma Oxidation of Hydrocarbon Templates in Polysilsesquioxanes
- PDF / 342,489 Bytes
- 5 Pages / 414.72 x 648 pts Page_size
- 36 Downloads / 166 Views
KENNETH J.
ABSTRACT Arylene- and alkylene-bridged polysilsesquioxanes were prepared by sol-gel processing of bis(triethoxysilyl)-arylene monomers 1-4, and alkylene monomers 5-9. The arylene polysilsesquioxanes were porous materials with surface areas as high as 830 m2 /g (BET). Treatment with an inductively coupled oxygen plasma resulted in the near quantitative removal of the arylene bridging groups and a coarsening of the pore structure. Solid state 29 Si NMR was used to confirm the conversion of the sesquioxane silicons (T) to silica (Q). The alkylene-bridged polysilsesquioxanes were non-porous. Oxygen plasma treatment afforded silica gels with mesoporosity. The porosity in the silica gels appears to arise entirely from the oxidation of the alkylene spacers. INTRODUCTION Sol-gel processing of tetraalkoxysilanes leads to the formation of porous silica gels (xerogels and aerogels). Pore structure in these silica gels can be manipulated by changing the reaction and processing conditions used in their preparation [I]. However, the pore size distributions are too broad for application of these materials as molecular sieves. Another strategy for manipulated porosity is to build materials with molecular-sized building blocks [2-7]. Most of these attempts have afforded non-porous materials or demonstrated lack of control over pore distribution. For example, arylene-bridged polysilsesquioxanes are mostly microporous materials, yet the length of the arylene spacer or building block has no significant effect on the mean pore diameter [7]. An alternative method for generating porosity in amorphous materials is to use a template approach (Scheme 1). An organic molecule dispersed in a non-porous inorganic matrix is removed to create porosity. There are a few requirements for the template approach to be successful: First, the template must be removed without significantly disturbing the bulk inorganic phase. Second, the organic template must be compatible with the inorganic matrix. Third, the organic template-inorganic composite must be non-porous. It would also be desirable if a variety of different organic templates with different dimensions could be introduced. Disposable organic templates for porosity were first demonstrated by Chujo et. al. with silica-polyvinyloxazolines composites [8]. These non-porous hybrid organicinorganic materials were heated in air to bum out the organic polymer phase and afford porous silica. Roger et. al. demonstrated an alternative approach utilizing tin-carboxylate polymers in which the templates coordinated the tin through carboxylate groups [9]. The hydrocarbon templates were hydrolytically removed leaving mesoporous tin oxide. In this study, we use the organic component of bridged polysilsesquioxanes as a disposable pore template rather than simply a structural spacer. Porous silica gels are prepared by plasma oxidizing the organic bridging groups in arylene- and alkylene bridged polysilsesquioxanes (Scheme 2). * This research was supported by the United States Department of Energy under C
Data Loading...
