CO2 and CH4 sorption and selectivity by solid-state [P2 4 4 4][PF6], [P4 4 4 4][PF6] and [P6 4 4 4][PF6] ionic liquids under different pressures

Abstract

CO2 and CH4 capacities trapped by three hydrophobic solid-state ionic liquids, ethyltributylphosphonium hexafluorophosphate [P2 4 4 4][PF6], tetrabutylphosphonium hexafluorophosphate [P4 4 4 4][PF6], and tributylhexylphosphonium hexafluorophosphate [P4 4 4 4][PF6], were measured by a Quartz Crystal Microbalance (QCM) measurement system under pressures from 0.1 to 4 MPa at 303.15 K. [P4 4 4 4][PF6] shows the least capacity for CO2 and CH4 due to its strongest cation-anion interaction. CO2 capacity in [P6 4 4 4][PF6] is slightly larger than that in [P2 4 4 4][PF6], while opposite for CH4. The stronger polarity of [P6 4 4 4][PF6], caused by its longer alkyl chain, would be the main reason for its less CH4 but larger CO2 capacity than [P2 4 4 4][PF6]. CO2/CH4 selectivity does not obey the linearity with increasing pressures. The compactly structured [P4 4 4 4][PF6] shows a satisfactory CO2/CH4 mass selectivity, even higher than those fluid and large phosphonium counterparts.