The hydrophilic nature trade-off of supported ionic liquid membranes on CO2/CH4 separation performance

Abstract

Biomethane has been highlighted as an energy transition carrier in the quest for a resilient bioeconomy. Advances in its production process are accompanied by the need for new efficient separation methods for purification and carbon capture. However, biogas is fully saturated with water, jeopardising the expected performance of proposed novel materials. In this work, imidazolium-based Supported Liquid Membranes (SLM) with different hydrophilicity levels were proven to separate mixed CO₂/CH₄ successfully. IL-based SLMs were assessed in terms of stability, transport kinetics, and phase equilibria. Gas diffusion in dry and humid environments was estimated using the time-resolved FTIR-ATR spectroscopy technique. Henry's law constants of CO₂ and CH₄ and molecular interactions were estimated using the quantum chemical COnductor like Screening MOdel for Real Solvents (COSMO-RS) method. The SLM stability diagram is proposed to represent the effect of the gas water content on the total filled pores as a function of the components' physicochemical properties, support morphology, and operational pressure gradient. Diffusion coefficients of CO₂ in the hydrophilic SLMs are substantially altered under wet environments. However, it has been shown that gas solubility mainly dominates the gas separation. The SLM's water sorption increases this effect by further rejecting methane. While the membrane hydrophilicity improves the gas selectivity, its stability diminishes in humid gas processing. The trade-off between the separation performance using highly hydrophilic solvents and the SLM module stability under real and variable conditions of pressure and humidity constrains the technology feasibility. Therefore, its study must be considered in the development and application of SLMs in sustainable biofuel production routes.