Transport in nanoporous carbon membranes: Experiments and analysis

Single-component permeances of six gases were measured on three different s upported nanoporous carbon membranes prepared by spray coating and pyrolysi s of poly(furfulyl alcohol) on porous stainless-steel disks. Global activat ion energies were regressed from data collected as a function of temperatur e. Permeances and global activation energies were correlated to molecular s ize, assuming that entropic affects dominated the transport. The permeance was best correlated to the minimum projected area of the molecule computed from first principles. The free-energy barriers to transport within the mem branes were derived from the temperature dependence of the permeance data, after accounting for porosity differences between the membranes and differe nces in molecular adsorption. Using transition-state theory and an entropic model derived, the free energy, enthalpy, and entropic barriers to transpo rt within the membrane were examined as a function of molecular size. Compu ted on the basis of size, the entropic component of this barrier did not ac count for the large differences in the transition-state free energies. Howe ver, when these entropic barrier values were used to compute the enthalpic portion of the barrier free energies, the minimum projected area of each mo lecule correlated strongly. Furthermore, these enthalpic components of the barriers were fitted nicely by the Everett-Powl mean field potential, using only the pore size as the adjustable parameter. These results shed light o n the underlying mechanism by which shape-selective transport takes place i n the NPC membranes and small molecules are separated.