Gas diffusion in functionalized mesoporous membranes

Abstract

This work investigates the fundamentals of gas diffusion in functionalized mesoporous structures with pore diameters of around 20 nm. For this purpose, an extrusion process based on a yttria stabilized zirconia nanopowder is optimized to shape defect-free capillary membranes. The membranes sintered at 1050 degC for 2 h show a highly homogeneous microstructure with an open porosity of around 40% and a monomodal pore size distribution with mean pore diameters between 23 and 26 nm. To investigate the influence of surface functionalizations on the gas flow, functional groups are covalently bond onto the pore walls using a wet-chemical silanization process. Hexadecyltrimethoxysilane, a silane with a C16 alkylchain as functional group, is chosen as functional model silane. After successful surface functionalization, the membranes show a functional group density between 2 and 4 groups nm-2 depending on the silane concentration during functionalization. Structural analysis reveal decreased open porosities (27 %) and slightly smaller mean pore diameters of around 20 nm which indicate a monolayer of immobilized C16-chains. The gas diffusion properties are analyzed via single gas permeation measurements using a setup operating in dead-end mode. Measurements are performed under different temperature conditions (0-80 degC) using nitrogen (N2), argon (Ar), methane (CH4) and carbon dioxide (CO2). Non-functionalized structures show ideal Knudsen diffusion behavior, independent of gas type and temperature. In contrast, the gas permeation of alkyl-functionalized structures is decreasing up to one order of magnitude with increasing alkyl-chain density on the pore walls. Furthermore, the ideal selectivities show an increased deviation from Knudsen theory, having the highest influence on CO2. These deviations are further increased with increasing operating temperature. It is assumed that sterical hinderance due to the long C16-chains on the material surface is responsible for the determined gas flow characteristics. In addition, it is hypothesized that the deviations in ideal selectivity are caused by the difference in molecular size of the gas species and that the increased deviation with increased temperature is caused by the temperature movement of the surface functional groups. The results contribute to the fundamental understanding of gas diffusion in functionalized structures as present in many applications, ranging from gas separation membranes to gas chromatography.