Multivariate Metal-Organic Frameworks Prepared by Simultaneous Metal/Ligand Exchange for Enhanced C2-C3 Selective...

Recovering light alkanes from natural gas is a critical but challenging process in petrochemical production. Herein, we propose a postmodification strategy via simultaneous metal/ligand exchange to prepare multivariate metal-organic frameworks with enhanced capacity and selectivity of ethane (C₂H₆) and propane (C₃H₈) for their recovery from natural gas with methane (CH₄) as the primary component. By utilizing the Kuratowski-type secondary building unit of CFA-1 as a scaffold, namely, {Zn₅(OAc)₄}⁶⁺, the Zn²⁺ metal ions and OAc^- ligands were simultaneously exchanged by other transition metal ions and halogen ligands under mild conditions. Inspiringly, this postmodification treatment can give rise to improved capacity for C₂H₆ and C₃H₈ without a noticeable increase in CH₄ uptake, and consequently, it resulted in significantly enhanced selectivity toward C₂H₆/CH₄ and C₃H₈/CH₄. In particular, by adjusting the species and amount of the modulator, the optimal sample CFA-1-NiCl₂-2.3 demonstrated the maximum capacities of C₂H₆ (5.00 mmol/g) and C₃H₈ (8.59 mmol/g), increased by 29 and 32% compared to that of CFA-1. Moreover, this compound exhibited excellent separation performance toward C₂H₆/CH₄ and C₃H₈/CH₄, with high uptake ratios of 6.9 and 11.9 at 298 K and 1 bar, respectively, superior to the performance of a majority of the reported MOFs. Molecular simulations were applied to unravel the improved separation mechanism of CFA-1-NiCl₂-2.3 toward C₂H₆/CH₄ and C₃H₈/CH₄. Furthermore, remarkable thermal/chemical robustness, moderate isosteric heat, and fully reproducible breakthrough experiments were confirmed on CFA-1-NiCl₂-2.3, indicating its great potential for light alkane recovery from natural gas.