(551j) Mixed Matrix Membranes Formed from Branched HKUST-1 for Improved Plasticization Resistance and Transport Performance

Won Seok Chi¹, Benjamin Sundell,² Ke Zhang,² Daniel Harrigan,² Steven C. Hayden,² Zachary P. Smith^1,*

1. MIT, Department of Chemical Engineering, Cambridge, MA, USA
2. Aramco Services Company, Aramco Research Company, Cambridge, MA, USA

Polymer membranes are promising systems for addressing energy-efficiency and the environmental concerns related to gas separations. However, polymer membranes have property sets limited by the Robeson upper bound, which describes the tradeoff relationship between permeability and selectivity. Additionally, glassy polymers, which are often used as gas separation membranes, exhibit plasticization behavior for polarizable gases (e.g., CO₂) at high gas feed pressure, resulting in decreased selectivity when tested under realistic conditions. Mixed-matrix membranes (MMMs), which incorporate inorganic fillers in polymer matrixes, could potentially address these current limitations for polymer membranes. As an effective inorganic filler, we have synthesized a metal-organic framework (MOF) containing coordinatively unsaturated metal sites (i.e., open metal sites), which can increase gas transport properties because of their high surface area, proper pore dimensions, and Lewis acidic metal sites. The MOF, known as HKUST-1, was transformed from its typical octahedral, micron-sized structure into a branched nanoparticle structure by introducing a modulator to control the nucleation and growth mechanism. The branched architecture of HKUST-1 nanoparticles shows an interconnected, high aspect ratio structure that is a few hundreds of nanometers in length but with branch widths of only a few tens of nanometers. When dispersed in a 6FDA-DAM polymer matrix, 3D reconstructed images from FIB-SEM tomography demonstrate that well-dispersed, high loadings of 30 wt% MOF lead to a partially percolated MOF network. In contrast, bulk HKUST-1 particles do not have high aspect ratios and settle in the polymer solution, leading to undesirably phase-separated membranes into MOF-rich and MOF-lean domains. The 30 wt% branched HKUST-1 nanoparticles in 6FDA-DAM significantly increase H₂, CH₄, N₂, O₂, and CO₂ gas transport properties without a large loss in selectivity for gas pairs of interest (i.e., CO₂/CH₄ and CO₂/N₂). Of particular interest, the MMMs with branched HKUST-1 nanoparticles demonstrate exceptional plasticization resistance compared to the pure polymer for high-pressure CO₂ permeation measurements. This study describes the benefits of using branched MOF nanoparticle structures for gas separation membranes compared to conventional, nearly spherical bulk MOFs. Additionally, extensive results are presented on the design, synthesis, and characterization of these MOFs and their corresponding MMMs.