Gas separation membranes containing carbon immobilized by photochemical grafting onto polymers

Steady state permeabilities and diffusivities of carbon dioxide, oxygen, nitrogen, methane, ethane, ethylene and propylene have been measured as a function of temperature in the rage 298-373 K through membranes immobilizing 30 wt.% of active carbon, which were prepared by photochemically grafting an epoxy-diacrylate prepolymer onto cellulose. From the experimental data, solubility coefficients for the above gases in the membranes have been calculated as a function of temperature. Results show that mass transfer across photosynthetic membranes immobilizing carbon may occur either by a strongly activated process, seemingly involving surface migration of an adsorbed unimolecular layer of gas, or by Knudsen type diffusion. The first type of mechanism prevails for molecules, such as the hydrocarbons examined, for which relatively strong adsorption interactions with active sites occur. As a consequence of the observed permeability and diffusivity pattern, efficient separation of gas mixtures may be achieved, with high selectivities at high permeabilities between molecules possessing different transport mechanism. The role of sorption enthalpies is discussed in terms of these mechanisms, as well as in terms of the structure and performance of photosynthetic membranes immobilizing sorbents.