Transition metal-containing carbon molecular sieve membranes for advanced olefin/paraffin separations
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In this work, carbon molecular sieve (CMS) dense film membranes derived from 6FDA-DAM:DABA (3:2) polyimide precursor were studied for separation of mixed olefins (C2H4 and C3H6) from paraffins (C2H6 and C3H8). Olefin-selective CMS membranes with high performance can be made by pyrolysis of metal-containing polymeric precursors. Pyrolyzed at 550°C with a fast ramp rate, CMS membranes with integrated Fe2+ (2.2 wt% in the precursor) showed 19% higher C2H4/C2H6 and 11% higher C3H6/C3H8 sorption selectivity than that of the neat CMS membrane. Additional investigations with a quaternary mixture feed (C2 and C3 hydrocarbons) show that C2H4 permeability above 10 Barrers with C2H4/C2H6 permselectivity near 11 were achieved for the 3.2 wt% Fe loading case. Although Fe incorporation did not appear to promote C3H6/C2H6 permselectivity, Fe is useful to achieve impressive C2 pair olefin/paraffin separation. Deconvolution of the C2H4/C2H6 permselectivity for the more extensively studied 2.2 wt% loading case was also revealing. While both sorption and diffusion selectivity increased due to the Fe incorporation, a larger influence is seen on the diffusion selectivity versus the sorption selectivity. This added diffusion selectivity was dominated by a contribution from an entropic factor with Fe, which is the feature for CMS materials to surpass conventional polymer membranes.