Unexpected high CO2 over C2H2 separation performance by high-silica CHA zeolite membranes

Energy-efficient CO2/C2H2 separation by membranes is critical for producing acetylene by partial combustion of natural gas. Separating CO2 and C2H2 by membranes is challenging because they have the same kinetic diameter and similar dimensions. Here we demonstrate for the first time high-silica chabazite (CHA) zeolite membranes with surprisingly high CO2 selectivity over C2H2. Both unary permeation and binary mixture permeation data demonstrated consistent high CO2 permselectivity >30. Equilibrium adsorption isotherms indicate only a slight adsorption selectivity for C2H2, implying that a high diffusion selectivity for CO2 is required for the effective separation. The dynamic uptake data, however, points to slight C2H2 diffusion selectivity in CHA zeolite crystals. Hypotheses are proposed to explain the inconsistency between crystal and membrane diffusion results.