Gas permeation properties of amorphous zeolitic imidazolate framework membranes made by atomic/molecular layer deposition

Amorphous metal-organic framework (MOF) membranes are desirable because they may retain some of the molecular sieving properties of their crystalline counterparts while being free of grain boundary defects, which often hinder the consistent achievement of high membrane performance. However, current methods, like melting and compression, for fabricating amorphous MOF membranes involve multi-step processes that require the formation of a crystalline membrane first, that is then amorphized, and therefore, could be challenging to scale. Here, we utilize atomic/molecular layer deposition (ALD/MLD) of diethylzinc (DEZ) and 2-methylimidazole (2mIm) to directly synthesize ultrathin amorphous zeolitic imidazolate framework (aZIF) deposits on γ-alumina-coated α-alumina supports. As the number of ALD/MLD cycles increased from 10 to 300, gas permeances decreased while ideal selectivities increased. Mixture separation factors for C₃H₆/C₃H₈, CO₂/N₂, and H₂/C₃H₈ as high as 4, 37, and 194, respectively, were obtained. At 200 °C and 2.5 bar equimolar feed of H₂ and C₃H₈, a H₂/C₃H₈ mixture separation factor of 185 is obtained with an H₂ permeance of ca. 4.15 × 10⁻⁸ mol/m²- s -Pa (124 GPU). Additionally, the membrane achieves a CO₂/N₂ mixture separation factor of 37 at 25 °C and 1 bar with a CO₂ permeance of ca. 3.96 × 10⁻⁸ mol/m²- s -Pa (118 GPU). Considering the vast array of compositionally distinct aZIFs that can be potentially deposited by this approach, an enormously large parameter space for membrane design is emerging to be explored.