Diffusion of Xylene Isomers in the MIL-47(V) MOF Material: A Synergic Combination of Computational and Experimental Tools

The dynamics of xylene isomers in the metal-organic framework MIL-47(V) has been investigated by combining molecular dynamics (MD) simulations and experimental tools including quasi-elastic neutron scattering (QENS) and deuterium nuclear magnetic resonance (H-2 NMR). The experimental and simulated self-diffusion coefficients (D-s) values for each single component isomer are in reasonable agreement in the whole range of temperatures. More interestingly, the simulations predict a nonmonotonous evolution of D-s with the temperature for all xylenes. Such an unusual trend is experimentally confirmed for p-xylene. Two distinct diffusion regimes are elucidated at the microscopic level: a low-temperature regime where the xylene molecules are close to the MIL-47(V) pore wall with a high activation energy barrier for the diffusion and a high-temperature regime where the xylene molecules are mainly located in the center of the channel associated with a lower activation energy for the diffusion. This dynamic behavior remains also true whatever the degree of pore filling. It has been further shown that the diffusivity is only slightly affected when one compares the case of xylenes in single component and in mixture. H-2 NMR experiments enlighten that packing effects and guest-guest interactions are crucial when considering the dynamics of confined xylenes in MIL-47(V).