Reverse shape selectivity of hexane isomer in ligand inserted MOF-74

Abstract

Separation of linear, mono-branched, and di-branched isomers is critically important in the petrochemical industry. In this computational study, we demonstrate that the ligand inserted Mg-MOF-74 structure leads to a reverse selectivity effect (i.e. phenomenon that preferentially allows larger species molecules to permeate in a gas mixture) of hexane isomers in the resulting material. Molecular dynamics simulations suggest that strong confinement of the di-branched hydrocarbons in the small pores lead to reverse selectivity. Over a magnitude difference in diffusivity between linear alkanes and their di-branched isomers was observed, clearly showing the steric effects imposed by the pore structure.

Fig. 1. Illustration of pore space partitioned Mg-MOF-74 using ligands 2,4-di(4-pyridinyl)-1,3,5-triazine. (a) Viewed along z axis and (b) side view of the channels showing the cylindrical channel.

Fig. 2. Mean-squared displacement (MSD) of hexane isomer at 300 K for 2 ns. Linear hexane (nHEX, red), 2-methylpentane (2MP, purple), 3-methylpentane (3MP, blue), 2,3-dimethylpentane (23DMB, dark green), and 2,2-dimethylpentane (22DMB, green) are shown respectively.

Fig. 3. (a) Snapshot of nHEX in dpt-Mg-MOF-74 at 300 K. (b) Snapshot of 3MP in dpt-Mg-MOF-74 at 300 K. (c) Snapshot of 22DMB in dpt-Mg-MOF-74. C (nHEX), C (3MP), C (22DMB), and H are shown in red, blue, green and white colors, respectively. To better illustrate the diffuse configurations, the cluster was zoomed in and exterior atoms were omitted.

Fig. 4. Diffusivity versus inserted dpt ligand proportion in Mg-MOF-74. The linear hexane (nHEX, red), 2-methylpentane (2MP, purple), 3-methylpentane (3MP, blue), 2,3-dimethylpentane (23DMB, dark green), and 2,2-dimethylpentane (22DMB, green), respectively.