High styrene/ethylbenzene selectivity in a metal-organic framework with coordinatively unsaturated cobalt(II) sites

Abstract Styrene (ST) is one of the most important feedstocks in the petrochemical industry and must be separated from a mixture of ST and ethylbenzene (EB). While the current technology comprises the energy-intensive vacuum distillation process, energy-saving adsorptive separation can be an alternative for ST/EB separation. The potential of a series of isostructural metal-organic frameworks (MOFs), M-MOF-74 (M = Mg, Co, Cu, or Zn), featured by highly dense unsaturated metal sites were evaluated for use in ST/EB separation using liquid-phase adsorption experiments. While all four M-MOF-74 materials showed selective adsorption of ST over EB, Co-MOF-74 exhibited the highest ST/EB selectivity (6.9) and the largest ST uptake (2.5 mmol/g), which were superior to those of benchmark MOF materials. This can be explained by the preferential interaction between unsaturated Co2+ sites and unsaturated ST molecules. Moreover, Co-MOF-74 retained its adsorption performance, porosity, and structural integrity after three consecutive adsorption and desorption experiments. These findings suggest that creating high density of Co2+ sites within a proper pore size would be an efficient strategy for developing an adsorbent for ST/EB separation.