Marked inducing effects of metal oxide supports on the hydrothermal stability of zeolitic imidazolate framework (ZIF) membranes

The thermal and hydrothermal stabilities and the structural degradation characteristics of a supported ZIF-7 membrane were studied at various temperatures (423-673 K), H₂O vapor concentrations (10-40 mol%), and metal oxide supports (alumina, silica-alumina, silica, magnesia). The α-Al₂O₃ supported ZIF-7 membrane prepared by the seeding and secondary growth method exhibited a high H₂ permeance (4.0 × 10^-7 mol m^-2 s^-1 Pa^-1) with high H₂ selectivities (∼10) over larger CO, CH₄, and CO₂ gases at an elevated temperature of 573 K. The thermal stability of the membrane in a dry atmosphere was determined primarily by the intercrystalline grain boundaries on the ZIF-7 overlayer, exhibiting a high thermal stability at 573 K. However, considerable cracks developed through the grain boundaries at 673 K due to excessive thermal stress. It was found that the metal oxide supports markedly affect the hydrothermal stability of the ZIF membranes. The α-Al₂O₃ supported membrane suffered from fatal hydrothermal degradation of the ZIF-7 membrane layer even at a relatively low temperature of 473 K, and structural degradation was facilitated as the temperature and H₂O vapor concentration increased. Surface acid/base properties of the metal oxide supports had a critical impact on the stability of the membrane; the ZIF-7 structure exhibited severe hydrothermal degradation on the acidic Al₂O₃ and SiO₂-Al₂O₃ supports. Conversely, the ZIF-7 crystalline structure remained intact on the neutral SiO₂ and basic MgO supports even under highly antagonistic hydrothermal conditions (573 K, 20 mol% H₂O). These findings provide important new insights for the effective construction of ZIF membranes with improved structural stabilities under practically relevant thermal and hydrothermal operation conditions.