Theoretical Study of Nanoporous Graphene Membranes for Natural Gas Purification

Gas filtration by means of membranes is becoming increasingly important for industrial processes due to its low cost. In particular, membranes can be applied to separate methane in natural gas from pollutants such as hydrogen sulfide and carbon dioxide. The recent advent of nanoporous graphene as material for membranes helped to overcome the current problems of polymeric membranes, namely the permeability/selectivity tradeoff. However, the factors that determine gas filtration through nanoporous graphene are not completely clear yet. In this work, we show that pore size, shape and functionalization severely affect the selectivity of the membrane toward CO 2 and H 2 S with respect to CH 4 . We identified that the critical diameter of circular pore for the separation of contaminants from methane with graphene membranes is 5.90 Å. An elliptical pore is discovered to select gas species having similar sizes on the basis of their shape. The more elongated CO 2 is allowed to pass though the pore while the more spherical H 2 S and CH 4 are rejected. Finally, the gas-membrane interactions are found to decisively affect the filtration performances. Functionalization with hydroxyl groups led to a higher permeability of the gas species with polar bonds while keeping an excellent selectivity.