Gas separation performance and physical aging of tubular thin-film composite carbon molecular sieve membranes based on a polyimide of intrinsic microporosity precursor

Abstract

We present a study on the fabrication of tubular thin-film composite CMS membranes based on an intrinsically microporous polyimide of intrinsic microporosity (PIM-PI), PIM-6FDA-OH. Besides the inherent structural similarity between the PIM-PI and CMS membranes (i.e. microporosity with pores <20 Å), the unique feature of the chosen precursor is its ability to undergo a thermal rearrangement (TR) reaction which constitutes an additional mechanism of microporosity evolution in addition to the pyrolysis process. By using Raman spectroscopy and in-situ thermal spectroscopic ellipsometry we tracked the structural TR- and pyrolysis-related evolution in CMS films as thin as 100 nm. Our study revealed a pronounced acceleration of the microstructure collapse (densification) due to physical aging that occurred in ultra-thin films. These, and our previous findings, suggest that excessive reductions in selective layer thickness in microporous amorphous materials, such as PIMs or CMS, may not be beneficial to obtaining highly efficient membranes. Instead, we have shown that excellent and stable separation properties could be achieved by PIM-PI-derived CMS membranes with thicker, ∼3 μm, selective layers (e.g. CO2, H2 permeances of >200 GPU, with CO2/CH4, CO2/N2, and O2/N2 selectivities of 43.0, 41.0, and 7.5, respectively) even after 3 months of aging.