Solution-Diffusion in Polymers
Gas separation membranes work by the solution-diffusion mechanism: gas molecules dissolve into the polymer at the high-pressure face, diffuse through the dense matrix driven by a partial pressure gradient, and desorb at the low-pressure permeate face. Permeability — the product of solubility and diffusivity — determines how fast each gas species permeates. The ratio of permeabilities for two gases defines the membrane's ideal selectivity.
The Permeability-Selectivity Tradeoff
One of the most important findings in membrane science is the Robeson upper bound: polymers with high permeability tend to have low selectivity, and vice versa. This tradeoff arises because making a polymer more permeable (larger free volume, looser packing) allows both the fast and slow gas through more readily. The 2008 Robeson upper bound remains the benchmark against which all new materials are compared.
Breaking the Upper Bound
Several material strategies push beyond the Robeson limit. Polymers of intrinsic microporosity (PIMs) create rigid, contorted backbones that cannot pack efficiently, generating micropores with molecular sieving capability. Thermally rearranged (TR) polymers, carbon molecular sieve membranes, and mixed-matrix membranes incorporating zeolites or MOFs into polymer matrices all show promise for exceeding conventional polymer performance.
Industrial Membrane Gas Separation
Commercial gas separation membranes generate billions of cubic meters of nitrogen annually from air, recover hydrogen in petroleum refineries, remove CO2 from natural gas, and dehydrate compressed air. The simplicity of membrane systems — no moving parts, modular scale-up, low footprint — makes them attractive despite selectivity limitations that sometimes require multi-stage configurations or hybrid membrane-absorption processes.