Sorption and Transport of Vapors in ZIF-11: Adsorption, Diffusion, and Linker Flexibility

The adsorption and diffusion of alcohol and hydrocarbon vapors in zeolitic imidazolate frameworks (ZIF)-11 are reported as a function of loading at 308 K. The hydrophobicity of the benzimidazole linkers provides moderate ideal adsorbed solution theory selectivities for adsorptive alcohol/water separations until high relative humidity conditions. Sorbates well beyond the crystallographic pore aperture are admitted into the framework at decreasing diffusivities with increasing molecular size. The diffusion of methane is also probed as a function of temperature, revealing a nonconstant activation energy of diffusion, which is linked to the temperature-dependent flexibility of the ligands comprising the six- and eight-membered aperture rings, as revealed through solid-state NMR. Compared to ZIF-8, ZIF-11 surprisingly demonstrates higher diffusivities and lower activation energies despite the smaller crystallographic aperture size. This phenomenon is explained via molecular dynamics simulations that highlight the greater flexibility of the benzimidazole linker in ZIF-11 relative to the 2-methylimidazolate linker in ZIF-8 and emphasizes the importance of structural flexibility in identifying diffusionally-selective materials.