Experimental and theoretical analysis of the mass transport through porous glass membranes with different pore diameters

The understanding of mass transport in narrow pores, it is an important problem of long standing interest in heterogeneous catalysis, gas-solid reactions and adsorptive separations. In recent years intense worldwide activity in applying newly developed porous materials has led to renewed interest in this subject. Porous glass membranes characterized by a wide range of different pore sizes and good accessibility to the active sites provided suitable object of systematic investigation of gas transport mechanisms. Due to their large surface areas, molecular sieving ability and controlled host-sorbate interactions, micro- and mesoporous glass membranes can be applied as a medium for gas separation. This work was focused on studying single gas and competitive diffusion through porous glass membranes with different mean pore sizes in the range from 1.4 to 4.2 nm and through a Vycor glass membrane with mean pore size of 4 nm. The transport in mesoporous membranes has been widely studied and is reasonably well understood. Therefore the interest was focused on the membranes with pores sizes in transition regime between micro- and mesopores, where the basic understanding of the mass transport is quite limited. Diffusion in this regime is governed by both molecule-molecule and molecule-wall interactions. Thus, viscous flow, Knudsen and/or configurational diffusion have to be considered for quantification of gas phase transport. Macroscopic models based on well-established theories of combined gas and surface diffusion transport for adsorbable gases, were used to describe the diffusion behavior. For understanding the competitive diffusion in the investigated porous membranes, Maxwell-Stefan theory was employed to account for interactions between counter-diffusing species. In this thesis a comprehensive experimental data basis of pure component diffusion and counter-diffusion of different test gases was provided. Besides studying diffusion, the adsorption equilibrium measurements were performed to determine pure and binary mixture isotherms. The possibility to use the characterized membranes for the gas separation proposes was also considered. The large contributions of the surface flows allow for adsorbable gases the enhancement of the selectivities by selective surface transport. Competitive behavior of gases with the same molecular weights (e.g. carbon dioxide and propane) was very interesting to study, because no separation is possible due to gas phase diffusion. Further, due to a very reactive surface area of porous glasses, chemical surface modification, as one of the possibilities to enhance selectivities, was also considered in this work. Interesting properties of the prepared membranes provide a rational framework to meet current and future challenges in separation technology. In a final short overview, based on a comparison of all membranes examined, some suggestions are given regarding an exploitation of the mass transport through porous media in order to improve separation performances