Cellulose Triacetate Doped with Ionic Liquids for CO2/CH4 Separation

Polymeric membranes have been widely used for the removal of CO2 from natural gas.  It is a leading technology for offshore platform applications and is gaining more competitive advantages over conventional amine absorption for onshore applications.  The work horse membrane material for industrial CO2/CH4 separation is cellulose acetate/triacetate (CTA).  The more widely use of CTA is, however, limited by its high crystallinity, leading to low CO₂ permeability.  The goal of this work is to reduce the crystallinity in CTA by doping with ionic liquids such as [emim](BF₄) and [emim](DCA), to improve the CO2/CH4 separation properties.  Both ionic liquids have CO₂/CH₄ solubility selectivity higher than CTA.  In this project, we have prepared series of CTAs doped with various contents of [emim](BF₄) or [emim](DCA).  The effect of ionic liquids on the crystal formation in CTAs was characterized using DSC and WAXD.  Doping of ionic liquids can reduce or completely inhibit the crystal formation in CTA, resulting in an increase in gas solubility and permeability.  The presentation will discuss the effect of the doping of ionic liquids on the physical properties of the blends, including morphology and pure-gas solubility, diffusivity and permeability.  The blends exhibit the highest CO2/CH4 solubility selectivity among the polymers known.  The addition of ILs also moves the separation performance towards the upper bound, which makes them interesting platform for CO2/CH4 separation.