Transport phenomena in gas-selective silica membranes

Upcoming technology platforms for green fuel production require the development of advanced molecular separation processes for recovering dry liquid biofuels [1,2], biomethane [2] and hydrogen [3]. Replacement of extractive distillation, cryodistillation and adsorption processes by membrane units may lead to vast energy savings [2,3]. In this context, ultramicroporous silica membranes, that is, silica membranes with pores smaller than 1 nm [4], appear to be able to play a determinant role. Indeed, in reason of their extremely small pore size, these membranes can be used as sieves to recover, for instance, pure hydrogen from gaseous mixtures, or to dehydrate ethanol and other fuels produced in biological processes. Moreover, they can be fabricated by a facile procedure, they are more thermally, chemically and mechanically stable than their organic counterparts and they commonly show higher permeate fluxes than zeolite membranes.
Ultramicroporous silica membranes typical typically an asymmetric structure, consisting of few millimeters thick macroporous tubes or disks, which confer mechanical strength to the membrane, and one or more mesoporous intermediate layers with subsequently smaller pore sizes to provide a smooth deposition surface for the final ultramicroporous selective layer. This asymmetric structure has been developed in order to minimize the membrane thickness and thus to reduce the resistance of the membrane to the permeate flow.
This lecture will review the methods for the fabrication of ultramicroporous silica membranes [5-6] and the transport mechanisms occurring in the different membrane layers [5-7] including viscous flow, Knudsen diffusion and activated transport.