Adsorption of propylene, propane, ethylene and ethane in an isoreticular series of MOF-74 structures

Alkene/alkane separation is the most expensive stage in the process of propylene or ethylene producing. Adsorptive separation with porous solid adsorbents is an alternative way to currently used cryogenic distillation process. In this work by GCMC simulation we investigate propane, propylene, ethane and ethylene adsorption in an isoreticular series of IRMOF-74-I (Mg-MOF-74) structures with different pore size. The adsorption isotherms of pure components as well as the selectivity of binary mixtures of propylene/propane and ethylene/ethane at the pressure fixed at 1 bar with variable composition are determined at 318 K. Also, the snapshots of guest adsorption are used to elucidate the adsorption mechanisms. At low pressure, ethane uptake is the same for all studied structures. IRMOF-74-I has the highest ethylene uptake at low pressure and also ethylene/ethane selectivity among studied MOFs. Open metal site is the first preferential site for adsorbing the guest in all structures. IRMOF-74-II, IRMOF-74-III and IRMOF-74-IV have additional site for adsorbing guest molecules. This additional adsorption site makes propylene and propane uptakes higher than that of IRMOF-74-I at low pressure. The second preferential site of IRMOF-74-II, IRMOF-74-III and IRMOF-74-IV, organic linker, interact with propane and propylene as the same and can’t separate them so IRMOF-74-II, IRMOF-74-III and IRMOF-74-IV have lesser selectivity than IRMOF-74-I. At high pressure, IRMOF-74-IV with largest pore size has the highest uptake for all studied hydrocarbons.