| For microporous silica membranes prepared from polysiloxane precursors, the competing effects of capillary stresses (that cause the collapse of the network) and continued condensation reactions (that stiffen the network preventing collapse) establish the final pore size during drying. Under these conditions, small pores necessary for gas separation are achieved by promoting drying shrinkage and hence selectivity is obtained at the expense of pore volume and gas flux.; We describe a "template" approach that overcomes the above limitation through synthesis of tetraethoxysilane (TEOS) and organoalkoxysilane copolymers and processing into xerogels, thin films, or supported membranes, followed by thermal treatment to create a dense organic/inorganic matrix, and subsequent pyrolysis to create a microporous network.; Microstructure of organic/inorganic xerogels, in addition to the overall extent of shrinkage (aging and drying), depends on polymer/solvent interactions, drying rates, and the nature of the template ligand. Competing effects of syneresis (that occurs during aging) and drying shrinkage resulted in the overall linear shrinkage of gels to be constant at {dollar}sim{dollar}50%. Shrinkage, density, contact angle, and N{dollar}sb2{dollar} sorption experiments, revealed that with increasing hydrophobicity of the gel network the driving force for syneresis changes from primarily condensation reactions to a combination of condensation and solid/liquid interfacial free energy. For the same overall extent of shrinkage ({dollar}sim{dollar}50%) xerogels exhibited vastly different microstructures due to the effects of microphase separation.; In xerogels, the disruption of gel network caused by the pyrolysis of organic ligands collapsed the pore structure after the organic groups were pyrolyzed. But the constraint to sintering imposed by the substrate stabilized the microporosity to higher temperatures in thin film/membrane samples. Kinetic analysis of thin film stress relaxation experiments suggests that pore size indeed increases with template size for methyl and phenyl templated films but the average pore radii somewhat exceeds the template size due to residual matrix porosity or some incipient phase separation.; Resulting microporous inorganic membranes exhibited high flux combined with high selectivity overcoming limitations inherent to both inorganic (sol-gel, CVD) and organic membranes. Single layer membranes (thickness {dollar}le{dollar} 750A) exhibited very high CO{dollar}sb2{dollar} permeance values {dollar}rm(2.57times10sp{lcub}-3{rcub} cmsp3/cmsp2{dollar}-s-cm Hg) and moderate CO{dollar}rmsb2/CHsb4{dollar} selectivities (12.2). Subsequent derivatization of the pore surfaces with monomeric TEOS significantly increased {dollar}rm COsb2/CHsb4{dollar} separation factors (71.5) with only a moderate reduction in CO{dollar}sb2{dollar} permeance {dollar}rm(2.04times10sp{lcub}-4{rcub} cmsp3/cmsp2{dollar}-s-cm Hg).; Hydrolytic and thermal stability of microporous thin films and membranes were improved by pyrolyzing the templates under reducing conditions. Cantilever beam bending experiments, FTIR, and N{dollar}sb2{dollar} sorption were used to characterize the microstructure of hydrophobic films and xerogels, which exhibited a water contact angle = 105{dollar}spcirc{dollar} and a surface concentration of 2.8 CH{dollar}sb3{dollar} groups/nm{dollar}sp2.{dollar}... |
