MICROPOROUS SIO2 VYCOR MEMBRANES FOR GAS SEPARATION/

In this study, porous Vycor tubes with 40 Angstrom initial pore diamet er were modified using low pressure chemical vapor deposition (LPCVD) of SiO2. Diethylsilane (DES) in conjunction with O-2 Or N2O were used as precursors to synthesize the SiO2 films. Both ''single side'' (reac tants flowing on the same side of porous membrane) and ''counterflow'' (reactants flowing on both sides of porous membrane) reactant geometr ies have been investigated. The flow of H-2, He, N-2, Ar, and toluene (C7H8) was monitored in situ after each deposition period. Membranes m odified by the ''single side'' reactants geometry exhibited good selec tivities between small and large molecules. However, cracking in these membranes after prolonged deposition limited the maximum achievable s electivity values. Higher selectivities and better mechanical stabilit y were achieved with membranes produced using the ''counterflow'' reac tants geometry. Pore narrowing rate was observed to increase with oxid ant flow (O-2 Or N2O) For membranes prepared using both oxidants, sele ctivities on the order of 1000:1 were readily attained for H-2 and He over N-2, Ar, and C7H8 AS compared to O-2, the use of N2O caused impro vements in both the pore narrowing rate and N-2:C7H8 selectivity. Memb ranes prepared using the ''counterflow'' geometry showed no signs of d egradation or cracking after thermal cycling.