Effect of Surface Characteristics of Graphene Aerogels and Hydrophilicity of Ionic Liquids on the CO₂/CH₄ Separation Performance of Ionic Liquid/Reduced Graphene Aerogel Composites

Abstract

High Resolution Image Download MS PowerPoint Slide Two ionic liquids (ILs) having the same cation with different anions offering opposite hydrophilic/hydrophobic characters, 1- n -butyl-1-methylpyrrolidinium dicyanamide ([BMPyr][DCA]) and 1- n -butyl-1-methylpyrrolidinium hexafluorophosphate ([BMPyr][PF 6 ]), were impregnated onto two different reduced graphene aerogels (rGAs) prepared by the thermal treatment of GAs at 300 and 500 °C to investigate the consequences of the changes in the hydrophilic character of ILs and the reduction temperature of the GAs on the corresponding gas sorption and separation performance of the IL/rGAs. The structural analyses of nanoporous rGAs and IL/rGAs pointed to a change in the quantity of oxygenated functional groups upon thermal treatment and a change in the direct interactions between IL molecules and the host rGA surface upon IL deposition. Single-component CO 2 and CH 4 sorption measurements were performed for each rGA and IL/rGA composite, and both ideal and mixture CO 2 /CH 4 selectivities were calculated. The samples prepared by reducing the GA at 300 and 500 °C yielded ideal CO 2 /CH 4 selectivities of 3.6 and 18 at 1 mbar and 25 °C, respectively. Among IL/rGA composites, the one prepared at 300 °C displayed a remarkable CO 2 /CH 4 separation performance when combined with the hydrophobic [BMPyr][PF 6 ], offering an ideal selectivity of 450.9 at 1 mbar and 25 °C, whereas the composite prepared with rGA500 yielded a substantially high CO 2 /CH 4 selectivity of 173.5 after the incorporation of the hydrophilic [BMPyr][DCA] at 1 mbar and 25 °C. The ideal CO 2 /CH 4 selectivities of [BMPyr][PF 6 ]/rGA300 and [BMPyr][DCA]/rGA500 surpassed most of the previously reported selectivities of carbon-based materials in the literature. These results demonstrate the broad potential of IL/rGAs in sorption-based gas separations owing to the highly tunable nature of both the structure of IL and the surface characteristics of rGA.