Selectivity enhancement by the presence of grain boundary in chabazite zeolite membranes investigated by non-equilibrium molecular dynamics

Abstract

The effects of a sub-nanometer grain boundary in all-silica chabazite (CHA) zeolite membranes on their permeation properties were investigated by non-equilibrium molecular dynamics (NEMD). The results indicated that at the total pressure of 0.5 MPa, at 298 K, the CO2 selectivity toward CH4 drastically increased due to the presence of a grain boundary region inside the membrane. This was a consequence of the selective condensation of CO2 in the grain boundary. The calculated CO2 selectivity for the perfect crystal CHA membranes was in the range of 183–278 and depended on the surface orientation of the modeled membranes. Notably, the selectivity was comparable to the previously reported values. Two different membrane structures containing a grain boundary of approximately 0.6 nm were modeled using NEMD. In the case of the grain boundary located inside the membrane, the condensed CO2 molecules in the grain boundary region prevented permeation of CH4, resulting in significantly increased CO2 selectivity. The NEMD results suggest that compared to perfect zeolite crystal membranes, the presence of a grain boundary inside a CHA membrane increases the membrane performance, selectivity, and permeability.