Synthesis and gas permeation properties of tetraoxidethianthrene-based polymers of intrinsic microporosity

Abstract

A series of nine polymers of intrinsic microporosity (PIMs) derived from different bis-catechol monomers and 2,3,7,8-tetrafluoro-5,5′,10,10′-tetraoxidethianthrene (TOT) were synthesised and tested for their potential use as gas separation membranes. As powders, they demonstrate significant nitrogen adsorption at 77 K allowing apparent BET surface areas ranging from 432–785 m² g⁻¹ to be calculated. Six of the polymers were found to be soluble in quinoline facilitating the casting of self-standing films to allow the assessment of their gas separation properties. Spirobifluorene-based polymers exhibited the highest gas permeability, approaching the performance of the archetypal PIM-1, and the data for some are placed close to the 2008 Robeson upper bounds for O₂/N₂ and CO₂/CH₄. Ageing studies showed a gradual decrease in permeability, accompanied by an increase in selectivity that moved the data more-or-less parallel to the Robeson upper bounds. The two polymers with the lowest and highest gas permeability were both tested over the temperature range 25–55 °C and an enhancement in permeability for all gases, with the exception of CO₂, was observed along with decreased selectivity for almost all gas pairs. The latter seems to be due to the simultaneous drop in both diffusivity selectivity and solubility selectivity for all gas pairs, but especially those involving CO₂, due to a strong decrease in solubility with increasing temperature. The analysis of the energetic and entropic selectivity provides further insight into the remarkable transport properties of PIMs. Overall, the tetraoxidethianthrene unit proves to be a suitable building block for use in PIM synthesis for applications in gas separation membranes and these PIMs have a one to two orders of magnitude higher permeability than more common polysulfones.