Modifying Inorganic Structure through Hydration in Vapor Phase Infiltrated AlOxHy-PIM-1 Hybrid Membranes: Implications for Solvent Stability, Permeance, and Selectivity

Vapor-phase infiltration (VPI) presents a promising approach to enhancing the stability of organic membranes in organic solvents while maintaining critical properties, such as membrane permeance and selectivity. However, the precise chemical structure of the infiltrated inorganics and their impact on solvent stability remain poorly understood, limiting efforts to improve VPI treated hybrid membrane technology for organic solvent reverse osmosis (OSRO). This study uses X-ray absorption spectroscopy alongside X-ray photoelectron spectroscopy (XPS) to elucidate the inorganic cluster structure within PIM-1/AlO_xH_y. By analyzing the H₂O ratio via XPS and assessing the first and second shell Al coordination numbers from Al K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, we propose that aluminum oxyhydroxide tends to form nonlinear networked structures. X-ray absorption near-edge spectroscopy (XANES) analysis confirms a predominantly six-coordinate structure in the first shell, while EXAFS analysis of the second shell reveals the presence of three aluminum atoms, suggesting clusters significantly larger than simple dimers or trimers, similar to larger aluminum hydroxide and oxyhydroxide crystal structures. Furthermore, we demonstrate that postprocessing techniques, such as dehydration and rehydration, can be utilized to control this network structure and membrane permeance and selectivity without compromising solvent stability.