Effects of structural crystallinity and defects in microporous Al-MOF filled chitosan mixed matrix membranes for pervaporation of water/ethanol mixtures

A new two-dimensional microporous metal organic framework (MOF), Al-MOF, [Al(OH)(MBA)] (CYCU-7, MBA = diphenylmethane-4,4′-dicarboxylate anion) and its reported analogue, [Al(OH)(SBA)] (CAU-11, SBA = 4,4′-sulfonyldibenzoate anion), have been synthesized using hydrothermal and solvothermal methods, respectively, and their structural crystallinities and defect porosities were carefully controlled and characterized by N₂ sorption isotherms and ²⁷Al solid-state nuclear magnetic resonance measurements. Interestingly, the MOF synthesized by the ethanol-based solvothermal method (CYCU-7) show a significant degree of linker-missing defects compared to that synthesized by the water-based hydrothermal method (CAU-11). We further incorporated the synthesized CYCU-7 and CAU-11 with chitosan (CS) biopolymer to make CYCU-7@CS and CAU-11@CS mixed matrix membranes (MMMs) with the loading amount of MOF 2.5, 5.0, or 10 wt%. The as-prepared CYCU-7@CS and CAU-11@CS MMMs were applied for separation of water/ethanol mixtures through the pervaporation process, and the effects of the structural properties (e.g. crystallinity and defects) of CYCU-7 and CAU-11 on the separation performance are studied. It is found that defect-rich CYCU-7@CS MMMs exhibit higher flux, while CAU-11@CS MMMs with higher crystallinity exhibit a higher separation factor. In addition, the CAU-11@CS MMM with 5.0 wt% loading of CAU-11 displays the best separation performance (separation factor = 2741 and flux = 458 g/m² h).