Dynamics of xylene isomers in MIL-53 (Al) MOF probed by solid state 2H NMR

Abstract

Abstract With regard to understanding the effect of the dynamics of xylenes confined in the MOFs pores on the origin of xylene isomers separation, the mobility of para and ortho isomers of xylene in MIL-53(Al) MOF has been studied by solid-state 2H NMR. It is inferred that both isomers perform rotational and librational motions of the aromatic ring plane. The geometry, rates and energy barriers of the detected rotational modes for the molecules confined in the channels have been determined. The rotation around the axis perpendicular to the plane of aromatic ring is obliged with pairwise arrangement of xylene molecules along 1D channels of the MOF. The evolution of motional patterns with temperature shows that the pairwise arrangement of ortho isomer in the channels is held till 300 K, whereas the pairs are stable up to 405 K for the para isomer. This was rationalized by stronger interaction of o-xylene with the channel walls, while π–π intermolecular interaction is stronger for p-xylene. Due to stronger interaction between the unpaired molecules and MOF walls o-xylene exhibits notably higher temperature of the molecules desorption from the channels (494 K). In contrast, the p-xylene is capable to leave the channels of MIL-53 already at 405 K, moving in the channels predominantly in pairs. The different motional behavior of xylene isomers provides the conditions for a faster translational mobility along the MOF channel of para compared to the ortho isomer, thus accounting for the molecular mechanism behind the separation selectivity for xylene isomers.