Control of pore structure and characterization of plasma-polymerized SiOCH films deposited from octamethylcyclotetrasiloxane (OMCTS)

On the Mixed Gas Behavior of Organosilica Membranes Fabricated by Plasma-Enhanced Chemical Vapor Deposition (PECVD)

Selective, nanometer-thin organosilica layers created by plasma-enhanced chemical vapor deposition (PECVD) exhibit selective gas permeation behavior. Despite their promising pure gas performance, published data with regard to mixed gas behavior are still severely lacking. This study endeavors to close this gap by investigating the pure and mixed gas behavior depending on temperatures from 0 °C to 60 °C for four gases (helium, methane, carbon dioxide, and nitrogen) and water vapor. For the two permanent gases, helium and methane, the studied organosilica membrane shows a substantial increase in selectivity from α_He/CH4 = 9 at 0 °C to α_He/CH4 = 40 at 60 °C for pure as well as mixed gases with helium permeance of up to 300 GPU. In contrast, a condensable gas such as CO₂ leads to a decrease in selectivity and an increase in permeance compared to its pure gas performance. When water vapor is present in the feed gas, the organosilica membrane shows even stronger deviations from pure gas behavior with a permeance loss of about 60 % accompanied by an increase in ideal selectivity α_He/CO2 from 8 to 13. All in all, the studied organosilica membrane shows very promising results for mixed gases. Especially for elevated temperatures, there is a high potential for separation by size exclusion.