Significant adsorption enhancements driven by pore microenvironment tuning for efficient C2H2/C2H4 separation in a chemically stable Ni7-cluster-based framework

Abstract

In the activation process of MOF materials, the formation of open metal sites (OMSs) generally enhances the binding force between the framework and C₂H₂/C₂H₄, while it can also lead to co-adsorption and significantly reduce separation efficiency. In actual working environments, OMSs are susceptible to water attacks, rendering the material more prone to damage. Herein, a heptanuclear cluster-based MOF material denoted as Ni7-MOF is reported, which is constructed using extremely inexpensive organic linkers under pure water conditions. Two guest-free materials, Ni7-100 with coordinated water and Ni7-250 with OMSs, were obtained to investigate the C₂H₂/C₂H₄ separation properties and mechanisms. The enhanced adsorption and separation performance of Ni7-250 compared to Ni7-100 is primarily attributed to the appropriate pore environment within the framework, leading to the dense packing of guest molecules, and is independent of OMSs. This conclusion is further supported by grand canonical Monte Carlo (GCMC) calculations. Due to the stability of nickel ions in the pentagonal pyramid structure, Ni7-250 with OMSs exhibits high stability and can achieve structural restoration under water vapor conditions. Although environmental factors can disrupt MOF materials through OMSs, such disruption can be avoided by selecting metal ions for MOF materials based on the stability of metal ion coordination geometry. Concurrently, the regeneration of materials through the adsorption of water vapor by OMSs in the air can achieve a longer service life.