Accessing accelerated molecular diffusion by nanopore alignment in a MOF thin film

Molecular diffusion through a porous membrane can be accelerated by adjusting the nanopore windows and channels along the diffusion path, i.e., across the membrane thickness. We evaluated this process in a zeolitic imidazolate framework (ZIF-8) thin film. For the pore windows that were orthogonal to the plane of the thin film (membrane), we could realize a higher methanol vapour diffusivity (∼2×) with a lower energy of activation (∼3×), compared to a mixed-oriented ZIF-8 thin film. The variation in the diffusivities enhanced the structural isomer (1 and 2-propanol) selectivities (>7×), as indicated by vapour-phase and mass uptake studies using quartz crystal microbalance.