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

Novel organic/inorganic gas-separation membranes were fabricated by modific ation of mesoporous gamma -alumina ultrafilters with octadecyltrichlorosila ne (ODS). Based on ellipsometry measurements and XPS analysis, our hypothes is 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. Pu re gas permeance of the alumina membrane decreased by 2 to 3 orders of magn itude 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 appr oximately 34.5 kPa. Following silane modification the membrane exhibited re verse 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 pur e gas permeance of various gases fit an exponential relationship with criti cal temperature that was consistent with transport based on preferential so rption and solution diffusion often observed in rubbery polymers. A model f or surface diffusion enhanced permeation provided a parametric fit to the p ure 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 activat ion energy of diffusion, distinguished the sorption and surface flow of n-C 4H10, from the transport of non-condensable gases and CO2. (C) 2001 Elsevie r Science B.V. All rights reserved.