The effect of pore shape on hydrocarbon selectivity on UiO-66(Zr), HKUST-1 and MIL-125(Ti) metal organic frameworks: Insights from molecular simulations and chromatography

Abstract

Configurational Bias Grand Canonical Monte Carlo simulations have been used to show that the alkane isomer adsorption selectivity of porous MOF materials containing two pore types depends on the orientation of organic linkers’ phenyl groups. These simulations were performed at low pressure (0.1kPa) using mixtures of n-hexane and its branched isomers (2,2-dimethylbutane, 2,3-dimethylbutane and 2-methylpentane). Where possible, we compared the results with our gas chromatography results. In typical 1D narrow pore materials, the linear isomer is usually preferentially adsorbed over its branched isomers. In MOF materials exhibiting a 3D pore system with two pore types, a large one interconnected by smaller pores, the selectivity order is the inverse. Here, we show that this depends on the degree of opening of the access windows, which can allow it to be either “closed”, or to mimic a small pore channel. The consequence of this is the possibility (in the linear/branched mixture case) for the linear alkane to remain linear and thus maximize its interactions with the pore. The linear/aromatic mixture case considers a mixture of benzene and n-hexane, to show that a more favorable packing efficiency pushes the selectivity towards the aromatic molecule, regardless of the degree of the pore opening, although n-hexane can increase its competitiveness for the adsorption sites in materials where it can remain in mostly linear conformations.