Development of a model surface flow membrane by modification of porous γ-alumina with octadecyltrichlorosilane

Novel organic/inorganic gas-separation membranes were fabricated by modification of mesoporous gamma -alumina ultrafilters with octadecyltrichlorosilane (ODS). Based on ellipsometry measurements and XPS analysis, our hypothesis is that the membranes were composed of a very thin, approximately 11 nm, layer of ODS oligimers grafted to the surface of the mesoporous substrate. However, the microstructure of the silane layer is not well understood. Pure gas permeance of the alumina membrane decreased by 2 to 3 orders of magnitude after modification with ODS. Permeance was history dependent, however the flow could be recovered by rinsing in toluene and drying at 333 K, or by exposure to flowing N-2 with a pressure drop across the membrane of approximately 34.5 kPa. Following silane modification the membrane exhibited reverse selectivity, or selectivity for heavier gases such as CO2 and n-C4H10, over lighter gases such as H-2, N-2, CH4 and C2H6. Reverse selectivities were measured as high as 48 for n-C4H10/N-2 and 24 for n-C4H10/CH4. The pure gas permeance of various gases fit an exponential relationship with critical temperature that was consistent with transport based on preferential sorption and solution diffusion often observed in rubbery polymers. A model for surface diffusion enhanced permeation provided a parametric fit to the pure gas permeance of the ODS membrane. A maximum in permeance as a function of pressure and temperature, and a change in sign for the apparent activation energy of diffusion, distinguished the sorption and surface flow of n-C4H10, from the transport of non-condensable gases and CO2. (C) 2001 Elsevier Science B.V. All rights reserved.