(285b) Mass Transport through Zeolite Membranes: Investigating Framework Polarity
The polar interactions between the zeolite host and the permeating guest molecules determines the transport behavior of permeating compounds . There exists an optimum between high adsorptive loading of the zeolite along with a low diffusivity on the one hand and weaker adsorption but higher diffusivity on the other hand. In this contribution we systematically investigate the framework polarity of zeolite membranes. Tuning the zeolite polarity allows the optimization of membrane processes.
There are several ways of modifying the zeolite polarity. The polarity is determined by the silica to alumina ratio, the cation type, and the amount of silanol groups. We here concentrate on silanol groups (Si-OH), since the number of silanol groups can gradually be reduced for one given zeolite membrane. Working with only one membrane has the advantage, that all other membrane characteristics remain constant supporting thus a systematic approach. This is different if one modifies the silica to alumnia ratio or the cations, since different membranes with different layer thicknesses and effective crystal thicknesses are obtained.
Silanol groups are defects in the crystal structure. When a Si-O-Si bond is broken, a Si-OH group can be formed. These silanol groups are present in every real zeolite, and provide a certain degree of polarity. At high temperatures the silanol groups are removed through a condensation reaction  and the zeolite becomes more hydrophobic. Once a membrane containing silanol groups is available one can tune the polarity by a post synthesis calcination at different temperatures, leaving different amounts of silanol groups . De Ruiter et al. have shown that when [B]MFI crystals are calcined under ammonia atmosphere, the template is removed, but the boron remains in the zeolite framework. The boron can then be easily removed subsequently, leaving silanol nests . Up to five boron molecules per unit cell can be isomorphically substituted in an MFI zeolite . The deboronation method does not suffer from diffusive limitations, however, the synhesis procedure is elaborate and it is difficult to synthesize membranes at high boron content.
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 T. Masuda, S. Otani, T. Tsuji, M. Kitamura, and S. R. Mukai, Separation and Purifi-cation Technology, vol.32, 181 (2003)
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