In Situ Synthesis of MIL-160 Tubular Membrane with High Selectivity for Gas Separation

Abstract

Metal–organic frameworks (MOFs) are a rapidly growing class of crystalline porous materials known for their high surface area and tunable porosity, making them ideal for various applications, including gas separation. While the utility of MOFs primarily stems from their intrinsic micropores, fabricating MOF-based membranes further enhances their applicability, particularly in CO₂ separation from flue gas (CO₂/N₂) and natural gas (CO₂/CH₄). In this work, we developed an in situ synthesis method to fabricate MIL-160 membranes on ceramic tubular substrates for gas separation. MIL-160, with its three-dimensional interconnected channels and a pore-limiting diameter of 4.3 Å, is well-suited for separating small gas molecules. Through multiple synthesis trials, we produced MIL-160 membranes with distinct crystal morphologies─ball, flake, and cuboid─and characterized them using X-ray diffraction, scanning electron microscopy, nitrogen physisorption, gas adsorption, thermogravimetric analysis, and confocal microscopy. The crystal morphology was found to significantly influence membrane quality, particularly in reducing grain boundaries and pinholes. Confocal microscopy revealed substantial defects in the ball- and flake-shaped membranes, while the cuboid-shaped membrane showed minimal dye infiltration, indicating fewer defects and a more uniform structure. Single-gas permeation tests confirmed the superior performance of the cuboid-shaped MIL-160 membrane, achieving ideal CO₂/N₂ and CO₂/CH₄ selectivities of 56.8 and 130, respectively, with a CO₂ permeance of 75.5 GPU. In mixed-gas tests, the membrane reached a CO₂/N₂ selectivity of 259 at X_CO2 = 0.5, and a CO₂/CH₄ selectivity of 224 at X_CO2 = 0.2. Additionally, molecular simulations of binary gas adsorption supported these findings, demonstrating competitive CO₂ adsorption in the presence of N₂ and CH₄. This study highlights the potential of in situ synthesis of MIL-160 membranes on tubular substrates as a scalable and effective solution for CO₂ removal from flue gas and natural gas.