Influence of confinement in mesoporous silica on diffusion of a mixture of carbon dioxide and an imidazolium-based ionic liquid by high field diffusion NMR

Abstract

Proton and carbon-13 pulsed field gradient (PFG) NMR was applied to study diffusion of the mixture of carbon dioxide and the ionic liquid 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide under condition of confinement in the mesopores of KIT-6 silica. The results of these diffusion studies were compared with the results obtained for the corresponding bulk mixture and pure ionic liquid (IL) as well as for the confined pure IL. It was observed that the confinement in mesopores, which were fully loaded with the IL, reduces the cation diffusivity by around a factor of two in comparison to that in the corresponding bulk CO2/IL mixture or the pure IL. This result was obtained for short diffusion times when possible transport resistances at the external surface of KIT-6 particles are not expected to change the intraparticle diffusivity of the cations. The large experimental uncertainty of the measurements of carbon dioxide diffusivity in the confined CO2/IL mixture at the smallest diffusion time prevented making a definitive conclusion about the influence of the confinement on CO2 diffusivity. The reported experimental data suggest that the CO2 diffusivity under the conditions of confinement can be comparable to or even larger than that in the bulk CO2/IL mixture. For larger diffusion times, both the carbon dioxide and cation diffusivities in the confined mixture were found to decrease with increasing diffusion time. This diffusion time dependence was attributed to the influence of transport resistances at the external surface of KIT-6 particles.