Selective adsorption of liquid long-chain alpha-olefin/paraffin on Mg-MOF-74: Adsorption behavior and interaction mechanism

The liquid products of Fischer-Tropsch synthesis with a high content of linear alpha-olefins can act as valuable raw materials for increasing high added-value alpha-olefin production if the challenging separation of long-chain alpha-olefin/paraffin is achieved. Adsorption separation is an efficient alternative to energy-intensive distillation. Herein, the selective adsorption behavior and interaction mechanism of liquid alpha-olefin/paraffin on Mg metal-organic framework (MOF)-74 were investigated using a combination of batch adsorption experiments and molecular simulation techniques. Mg-MOF-74 exhibited 301 and 333 mg/g olefin adsorption capacities for C-6 and C-8 linear alpha-olefins in binary olefin/paraffin mixtures, respectively, and was still unsaturated at high olefin concentrations. The adsorption isotherms were analyzed and compared with the simulated results by configurational-bias grand canonical Monte Carlo (CB-GCMC) simulation. The visualized adsorption sites by CB-GCMC simulation indicated that all adsorbates were arranged in hexagonal shapes and preferentially adsorbed by the vertex of the hexagon, where the metal node magnesium is located. The adsorption energies were -1.456 and -0.378 eV for C-8 linear alpha-olefin and paraffin, respectively, calculated by density functional theory simulation based on the visualized adsorption sites. The charge transfer was analyzed, and the contributions of different kinds of interactions to the overall adsorption energy were quantified by principle orbital interaction analysis to further reveal the difference in adsorption energy between alpha-olefin/paraffin and Mg-MOF-74. This work also provides a general means to investigate the liquid adsorption performance and host-guest interactions in the adsorption or catalytic processes of nanoporous materials.