Conformational-change-induced selectivity enhancement of CAU-10-PDC membrane for H2/CH4 and CO2/CH4 separation

The separation of H₂/CH₄ or CO₂/CH₄ is critical to the purification of natural gas. Herein, we report on novel membranes with a metal-organic framework of CAU-10-PDC for the separation of these two mixtures. The dense CAU-10-PDC membranes are fabricated on a porous alumina support using the seeded growth method. An unexpected increase in selectivity was observed while testing mixed gas permeation with either H₂/CH₄ or CO₂/CH₄ at a molar ratio of 50:50. Steady-state selectivity reached 101 for H₂/CH₄ and 62 for CO₂/CH₄. Ideal selectivity measured from single gas permeation reached 475 for H₂/CH₄ and 288 for CO₂/CH₄. Molecular dynamics simulations and time-resolved X-ray diffraction with a synchrotron radiation source were used to probe conformational changes in CAU-10-PDC induced by exposure to CH₄. When exposed to an atmosphere containing CH₄, CAU-10-PDC presented a change in the space group (from I4₁/amd to I4₁), which drastically reduced the pore limiting diameter from 4.15 to 2.95 Å, rendering the channel nearly impermeable to CH₄.