The diffusion of methane confined in nano-porous carbon aerogel with the average pore size 48Å and porosity of ∼60% was investigated as a function of pressure at T = 298K using quasi-elastic neutron scattering. The diffusivity of methane showed a clear effect of confinement: it was about two orders of magnitude lower than in bulk at the same thermodynamic conditions and was close to the diffusivity of liquid methane at 100K (i.e. ∼90K below the liquid-gas critical temperature TC ≈ 191K). The diffusion coefficient of methane initially increased with pressure by a factor of ∼2.5, from 3.47 x 10-10m2/s at 0.482MPa to 8.55 x 10-10m2/s at 2.75MPa and started to decrease at higher pressures. An explanation of the observed non-monotonic behavior of the diffusivity in the confined fluid was based upon the results of small-angle neutron scattering experiments of the phase behavior of methane in a similar carbon aerogel sample. The initial increase of the diffusion coefficient with pressure was explained as due to progressive filling of bigger pores in which molecular mobility in the internal pore volume was less affected by the sluggish liquid-like molecular mobility in the adsorbed phase. A subsequent decrease in diffusivity was associated with the effect of intermolecular collisions, which result in a lower total molecular mobility with pressure, as in the bulk state. The results were compared with the available quasi-elastic neutron scattering data on the methane diffusivity in zeolites, metal organic frameworks, and porous silica as well as with the molecular dynamics simulations of methane in nano-porous carbons and silica zeolites.

Diffusion and Adsorption of Methane Confined in Nano-Porous Carbon Aerogel: a Combined Quasi-Elastic and Small-Angle Neutron Scattering Study. Chathoth, S.M., Mamontov, E., Melnichenko, Y.B., Zamponi, M.: Microporous and Mesoporous Materials, 2010, 132[1-2], 148-53