Some time ago you asked me about the existence of materials that might be
semi-permeable to gases (i.e, for selection of He vs. CO2). Interestingly,
I read in the July issue of Geotimes (of all places) about a new process
developed at Los Alamos in collaboration with Dow Chemical. They are using
laser thin-film deposition to deposit thin layers of zeolites onto
permeable substrates. The resultant materials can be used for processing
of gases. Specifically, the zeolite structure can be used to selectively
trap molecules by size in the gas phase.
Might be worth checking into...