Phenyl-incorporated polyorganosilica membranes with enhanced hydrothermal stability for H2/CO2 separation

Ultrathin silica membranes can be prepared by oxygen plasma treatment of polysiloxane-based membranes and exhibit excellent H-2/CO2 separation properties. However, silica often faces hydrothermal instability, reducing gas selectivity. Here, we incorporate hydrophobic phenyl groups into polysiloxane precursors by copolymerizing with vinyl-terminated polyphenylmethylsiloxane (vPPMS) and thoroughly investigate the chemical and structural properties and H-2/CO2 separation stability of the resulting polyorganosilica membranes. Significantly, adding phenyl groups enhances the hydrothermal stability of the polyorganosilica membranes, as corroborated by their relatively stable surface atomic compositions. For instance, the hydrothermal challenge decreases the H-2/CO2 selectivity by 74% for a phenyl-free membrane and the O/Si molar ratio from 1.97 to 1.72; by contrast, it reduces the selectivity by only 17% for a membrane derived from a polysiloxane containing 60 mass% PPMS, which is accompanied by a smaller change of the O/Si molar ratio from 2.0 to 1.85. This study unravels the mechanism of hydrothermal instability of silica membranes and demonstrates that enhancing hydrophobicity can effectively improve their potential applicability involving water vapor.