Synthesis and performance of polymerizable room-temperature ionic liquids as gas separation membranes

Room-temperature ionic liquids (RTILs) with polymerizable groups can be readily converted into solid, dense poly(RTILs) for use as gas separation membranes. A series of RTIL monomers with varying length n-alkyl substituents were synthesized and converted into polymer films. These membranes were tested for their performance in separations involving CO2, N-2, and CH4. CO2 permeability was observed to increase in a nonlinear fashion as the n-alkyl substituent was lengthened. CO2/N-2 separation performance was relatively unaffected as CO2 permeability increased. Plotting the performance of these membranes on a Robeson plot for CO2/N-2 shows that first-generation poly(RTILs) hug the upper bound of the chart, indicating that they perform as well or better than many other polymers for this separation. The CO2/CH4 separation is less impressive when compared to other polymer membranes on a Robeson plot, but poly(RTILs) perform as well or better than molten RTILs do in bulk fluid gas absorptions for that gas pair. Furthermore, poly(RTILs) were determined to be able to absorb about twice as much CO2 as their liquid analogues, an important factor which may give them potential use as gas and vapor sorbents.