Adsorptive\nSeparation of Geometric Isomers of 2‑Butene\non Gallate-Based Metal–Organic Frameworks

Abstract

The\nseparation of mixed C₄ olefins is a highly energy-intensive\noperation in the chemical industry due to the close boiling points\nof the unsaturated C₄ isomers. In particular, the separation\nof <i>trans</i>/<i>cis</i>-2-butene is among the\nmost challenging separation processes for geometric isomers and is\nof prime importance to increase the added value of C₄ olefins.\nIn this work, we report a series of isostructural gallate-based metal–organic\nframeworks (MOFs), namely, M-gallate (M = Ni, Mg, Co), featuring oval-shaped\npores, that are ideally suitable for shape-selective separation of <i>trans</i>/<i>cis</i>-2-butene through their differentiation\nin minimum molecular cross-section size. Significantly, Mg-gallate\ndisplays a record high <i>trans</i>/<i>cis</i>-2-butene uptake selectivity of 3.19 at 298 K, 1.0 bar in single-component\nadsorption isotherms. These gallate-based MOFs not only exhibit the\nhighest selectivity for <i>trans</i>/<i>cis</i>-2-butene separation but also accomplish a highly efficient separation\nof 1,3-butadiene, 1-butene, and <i>iso</i>-butene. DFT-D\nstudy shows that Mg-gallate interacts strongly with <i>trans</i>-2-butene and 1,3-butadiene along with short distances of C···H–O\ncooperative supramolecular interaction of 2.57–2.83 and 2.45–2.79\nÅ, respectively. In breakthrough experiments, Mg-gallate not\nonly displays prominent separation performance for <i>trans</i>/<i>cis</i>-2-butene but also realizes the clean separation\nof a ternary mixture of 1,3-butadiene/1-butene/<i>iso</i>-butene and a binary mixture of 1-butene/<i>iso</i>-butene.\nThis work indicates that M-gallate are industrially promising materials\nfor adsorption separation of geometric isomers of C₄ hydrocarbons.