Diffusive transport through mesoporous silica membranes

The results of a detailed study of the permeation of several light gases through unmodified and modified mesoporous silica membranes are reported. The base membranes which were synthesized by dip-coating multiple layers of a templated silica onto a macroporous alumina support showed relatively high permeances with evidence of both support resistance and a contribution from viscous flow, in addition to Knudsen diffusion, through the active layer. The behavior of the modified membranes, which were prepared by silanation of the original membranes with octadecyldimethylchlorosilane, was more interesting. Permeances were lower and there was no evidence of support resistance or viscous flow. Permeation through the active layer appeared to occur by a “Knudsen-like” process. Permeance and permeance ratios measured in both single component and binary systems showed the characteristic inverse dependence on the square root of the molecular weight (or the molecular weight ratio) but the temperature dependence was much stronger than expected for classical Knudsen flow. The behavior of CO2 was somewhat anomalous yielding permeances that were about 25% larger than those for propane which has the same molecular weight.