CMS membranes from rigid diphenyl pyrene polyimides: Effect of precursor structure in the final gas permeability and separation properties

Abstract

Carbon molecular sieve membranes CMSM were prepared by pyrolysis at 600 °C final temperature under Ar from 4 high temperature resistant polyimides, DP-TMPD, DP-MBDAM, DPt-TMPD and DPt-MBDAM, based in 3,8-diphenylpyrene-1,2,6,7-tetracarboxylic dianhydride (DP) and tert-butyl lateral group modified 3,8-di(4-tert-butylphenyl)pyrene-1,2,6,7-tetracarboxylic dianhydride (DPt) reacted each with two different diamines 2,4,6-trimethyl-m-phenylenediamine (TMPD) and 4,4′-methylenebis(2,6-dimethyl-aniline) (MBDAM). As obtained CMSM show between 3 and 7 times higher selectivity compared to polyimide precursor. In these CMSM CO2/CH4 surpasses 2019 Robeson's upper bond selectivity, for H2/N2 and O2/N2 it also lies above 2015 Robeson's upper bond. There is also an increase in gas permeability coefficients, P, between 1.5 and 3 times those of the polyimide precursors. CMSM Ageing for 8040 h show only a slight decrease and P with a slight increase on gas selectivity. CMSM membranes characterization by wide angle X-ray diffraction (WADX) indicate that the presence, in the polyimide precursor, of tert-butyl lateral groups in the diphenyl pyrene structure (DPt) gives rise to a third intermediate d-space value, around 8 Å, between those presented by DP. As a result, DPt based CMSM's increase P while selectivity decreases slightly compared to DP based CMSMs. Diffusion coefficients, D, for pyrolyzed DP-TMPD-P, DPt-TMPD-P, DP-MBDAM-P and DPt-MBDAM-P are lower than those of the precursor membranes. In all cases reduction in, D, particularly for the large dk gases, CH4 up to 10 time lower, is the main reason for selectivity increases in CMSM. In general DP and DPt based CMS membranes studied in this work present an important increase in selectivity reaching or surpassing Robeson's latest upper bond limit with an increase or minimum loss in P. This increase in selectivity is comparable and in some instances it surpasses that of other high rigidity precursor polyimide or CMS membranes reported in the literature.