Direct Structural Evidence of Molecular Packing Effects of Xylene Isomers Adsorbed in BIF-20

Abstract

Improved xylene separation performance through selective sorption of para-xylene on an industrial scale could significantly reduce the energy cost associated with purification and recovery. Understanding the host–guest interactions can aid in the design of more selective sorbent materials. However, only a few—primarily computational—metal–organic framework crystal structures containing xylene isomers have been reported. Here, we report a boron imidazolate framework material that displays local framework flexibility, with high xylene uptake and para-xylene/ortho-xylene single component diffusion selectivity of 3. Single-crystal X-ray diffraction studies show the different molecular packing arrangements between the xylene isomers, revealing the presence of steric effects in ortho-xylene adsorption. These results provide direct structural insight into flexibility-assisted xylene sorption within the framework and the location of guest binding sites, which could help contribute to the design of a flexible system for selective separation applications.