W.S. Chi, B.J. Sundell, K. Zhang, D.J. Harrigan, S.C. Hayden, Z.P. Smith
Keywords: metal-organic frameworks, gas separations
Summary:Natural gas frequently contains high concentrations of CO2 that need to be removed to increase heating value and prevent pipeline corrosion. Today, CO2 removal from natural gas is predominantly accomplished via amine absorption, but membrane-based separations offer an attractive energy-efficient and environmentally benign alternative. Unfortunately, the polymer membranes that are used today often lack the permeability, selectivity, and resistance to plasticization that are required to deploy membranes in many natural gas streams. One approach to overcoming these current performance limitations is to combine polymers with high performance materials known as metal-organic frameworks (MOFs) to make composite systems known as mixed-matrix membranes (MMMs). However, many MMMs are formed with MOF nanoparticles that are roughly spherical in shape, so high MOF loadings are required to improve membrane performance. In this work, two different crystalline MOFs have been formed into unique branched structures. These structures have rod-like branches approximately 10 nm in diameter that bifurcate into an interconnected hierarchical structure approximately 200 nm in end-to-end distance. When combined with polymers to form MMMs, the branched MOFs retain their morphology, creating an interconnected MOF network that allows for improved permeabilities at lower MOF loadings compared to traditional spherical nanoparticles. Additionally, these branched MOFs have significantly higher surface area interactions with their polymer supports, significantly improving the stability of these MMMs to plasticization compared to pure polymers. Plasticization resistance enables these MMMs to operate without significant performance decline when exposed to high pressure CO2 up to 50 bar at 35°C. An in-depth description of synthesis, materials characterization, and transport characterization is presented to discuss the role of these unique MOF structures for applications in natural gas purification and related gas separations.