How is mixed-gas permeation through poly(1-trimethylsilyl-1-propyne) membranes influenced by elevated temperatures?

Abstract

Poly(1-trimethylsilyl-1-propyne) (PTMSP), one of the most permeable membrane materials, has been extensively studied for gas permeation in the past. Even though it is in a glassy state, PTMSP permeates larger molecules selectively over small inert gases. Its mixed-gas selectivity in gas mixtures containing condensable gases is preferential towards the condensable components. This peculiar behavior is due to its extremely high non-equilibrium frozen-in excess free volume. This behavior also makes PTMSP and its transport performance highly relevant to study the effect of the high free volume - often called intrinsic microporosity - for applications in natural gas treatment, petroleum processing, or membrane reactors. While condensability strongly depends on the temperature, a detailed analysis of the temperature behavior in terms of the enhanced mixed-gas selectivity has not yet been performed for polymers with intrinsic porosity.Within this work, pure- and mixed-gas experiments with 1-butene (C4H8) and nitrogen (N2) were conducted at different C4H8 partial pressures and for temperatures ranging from 20∘C up to 120∘C. At 20∘C, a minimal C4H8 partial pressure of 0.5 mbar already depressed the N2 flux through PTMSP membranes in C4H8/N2 mixtures. With increasing temperatures, the extent of the C4H8 influence on N2 permeation vanishes. Elevated temperatures caused the mixed-gas selectivity αC4H8/N2 to decrease, but even at 120∘C it remained above the pure-gas selectivity.PTMSP is a membrane material with high-excess non-equilibrium free volume, which makes it prone to aging. Therefore, the physical aging of PTMSP membranes was recorded throughout the experiments. For temperatures exceeding 50∘C, pronounced aging occurred. Although the physical aging was reversed to a small degree when contacted by C4H8, the resulting permeabilities remained below the initial values. For temperatures ≤50∘C however, an in-situ rejuvination of membrane permeabilities was possible with small concentrations of C4H8 in the gas mixture. This in-situ rejuvination makes the long-term use of PTMSP membranes possible at low temperatures where condensable gases are present at sufficiently high concentration in the polymer.