Porous Solids Impregnated with Task-Specific Ionic Liquids as Composite Sorbents

Abstract

This work examines six task-specific ionic liquids (TSILs), comprised of the taurinate anion paired with five tetraalkylammonium cations (where alkyl = methyl, ethyl, propyl, butyl, and hexyl) and a tetrabutylphosphonium cation, impregnated in ordered mesoporous silica, SBA-15. The composites showed significantly increased CO2 uptakes relative to the parent SBA-15 materials. The surface area of these materials varies from approximately 5 to 1000 m2/g depending on the amount of IL loaded into the silica pores. The presence of n-hexyl side chains on the TSIL significantly reduces water loading, indicating that judicious IL selection may provide a means of controlling water uptake. After exposure to water vapor, three of the six cation-taurinate composites displayed an increase in CO2 capacity. X-ray diffraction data of the composite of tetramethylammonium taurinate and SBA-15 shows that the ionic liquid is crystalline inside the pores of the silica. Isotherms are measured at several different temperatures and the results show that storage at ambient humidity significantly impacts the capacity of these materials. By comparison, the TSILs supported on an amorphous porous support, BPL activated carbon, showed no increase in CO2 adsorption capacity. The results provide physical insight into the synthesis, structure, porosity, and sorption capacity of composites of adsorbents and ionic liquids that need to be considered prior to application.