Abstract:

The fabrication of compact, continuous, and large-scale metal organic framework (MOF) membranes with high permeability and H₂/CO₂ selectivity remains challenging because of the wake interaction between the MOF membrane and the substrate. In addition, substrates with smooth and plain surfaces and suitable pore size are required to prepare high-quality MOF membranes because it is difficult to obtain dense and continuous MOF membranes on a substrate with large pores and rough surfaces. To overcome these challenges, numerous MOF membrane growth methods have emerged, including in situ (direct) growth, secondary (seeded) growth, and layer-by-layer growth methods as well as electrostatic spinning and the chemical modification of the substrate. Among these methods, usage of substrates suitable for surface-functionalization is a promising technique. Herein, Al₂O₃ was selected as the substrate and was coated with PIM-1 (one polymer of intrinsic microporosity), followed by carboxylation of PIM-1 to furnish a large number of carboxyl groups on the surface. In situ growth of the MOF membrane using the interactions between the carboxyl group and the metal yielded two types of compact, continuous, and large-scale polymer-supported MOF membranes (PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1). Furthermore, the fabricated polymer-supported MOF membrane structures were investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Gas separation experiments were performed to explore the gas permeability and selectivity of the prepared MOF membranes. The XRD characterization confirmed the pure phase and high crystallinity of the MOF membranes. The SEM images showed that the MOF membranes were compact and continuous with a tight combination between the MOF crystal membrane and the substrate. Gas separation measurements showed that both MOF membranes exhibited high H₂ permeability and selectivity for H₂/CO₂. For the PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1 membranes, the 1 : 1 binary mixtures gas separation factors of H₂/CO₂ calculated as the gas molar ratios in the permeate and retentate side 7.32 and 9.69, respectively, at room temperature and atmospheric pressure. The H₂/CO₂ mixture separation factors of the two MOF membranes exceeded the corresponding Knudsen constants (4.7), with H₂ permeances higher than 3.16 × 10^-6 and 1.14 × 10^-6 mol·m^-2·s^-1·Pa^-1, respectively. The ideal separation factors of H₂/CO₂ of both MOF membranes calculated as the ratio of single gas permeances were 7.70 and 12.04, respectively, with the respective H₂ permeances of up to 3.73 × 10^-6 and 3.86 × 10^-6 mol·m^-2·s^-1·Pa^-1. Because of their outstanding characteristics, these novel MOF membranes can be widely used in the fields of H₂ purification and separation.

Key words: Membrane, Polymer, Metal organic frameworks, Gas separation, Hydrogen purity