MOF Instability and Polymer Infiltration in Amine-Functionalized UiO MOFs/PIM-COOH-Based MMMs for Solvent-Based Separations

Organic solvent-based separations with membranes hold promise for reducing the energy demands of traditional thermally driven processes. However, their widespread adoption is hindered by the limited availability of commercial membranes and challenges related to their long-term stability and molecular weight selectivity. Mixed-matrix membranes (MMMs), which incorporate fillers such as metal-organic frameworks (MOFs) with well-defined pore structures, offer a pathway to improve selectivity while maintaining high throughput. Despite this promising opportunity, challenges with MOF stability and nonideal polymer-MOF interfaces persist and remain understudied for organic solvent nanofiltration (OSN) MMMs. In this study, MMMs using carboxylic acid-functionalized PIM-1 (PIM-COOH) and amine-functionalized UiO MOFs with varying pore sizes (UiO-6x-NH₂, x = 6, 7, 8) were developed. The functional groups allowed for facile polymer-MOF cross-linking to ensure membrane structural integrity. However, larger-pore MOFs faced stability issues, with the UiO-68-NH₂ framework collapsing when activated from solvents that had a high affinity to the MOF or solvents that had a high surface tension. Additionally, the UiO-67-NH₂ framework lost some of its crystalline character when cross-linked to PIM-COOH. The OSN performance of the MMMs was evaluated through solvent permeation and dye rejection. While the incorporation of UiO-66-NH₂ into PIM-COOH increased solvent permeability (a 36-143% increase compared to PIM-COOH at 21 wt % loading), the performance of MMMs incorporating larger-pore MOFs was hindered by MOF collapse and nonideal polymer-MOF interfaces that resulted in polymer infiltration, leading to up to a 78% decrease in permeability. Polymer infiltration effects were further investigated through CO₂ sorption and solid-state NMR experiments. This work underscores the critical challenges in the design of OSN MMMs, providing insights to guide the design of robust, high-performing materials for energy-efficient separations.