Spatial functionality gradient in a ZIF-8 thin film membrane

Zeolitic imidazolate framework-8 (ZIF-8), constructed from Zn²⁺ ions and 2-methylimidazole (mIm) linkers, is widely recognized for its excellent thermal and chemical stability, making it a strong candidate for chemical separation and heterogeneous catalysis. In this study, we report a new crystalline phase of functionalized ZIF-8 featuring a spatially graded distribution of chemical functionalities. This anisotropic functionalization is achieved <i>via</i> a vapor-phase process applied to a ZIF-8 monolithic film. Anisotropic diffusion of a reactive linker-imidazole-2-carboxaldehyde (CHO-Im)-into the ZIF-8 monolith results in a gradient incorporation of CHO-Im, yielding a ZIF-8-CHO monolith. Spatial anisotropy of the CHO functional groups is confirmed through X-ray diffraction, scanning electron microscopy, and vibrational spectroscopy. To demonstrate its potential, we fabricated an anodic aluminium oxide-supported membrane and highlighted its enhanced gas permselectivity compared to pristine membranes.