Optimization of continuous phase in amino-functionalized metal–organic framework (MIL-53) based co-polyimide mixed matrix membranes for CO2/CH4 separation

Abstract

Nano-size non- and amino-functionalized flexible crystalline metal-organic frameworks (MOFs) MIL-53(Al) and NH₂-MIL-53(Al), two co-polyimides 6FDA/ODA-DAM (1 : 1 and 1 : 4) and cross-linked co-polyimide 6FDA-ODA-DAM (1 : 1, 2% agent cross-linking APTMDS) were used to prepare mixed matrix membranes (MMMs) whilst membranes consisting of MIL-53(Al) and commercial polyimides (Matrimid 5218 and Ultem 1000) were also fabricated for comparison. Pure gases and blends of CO₂ and CH₄ permeation tests showed enhanced separation performance of MMMs with relatively high NH₂-MIL-53(Al) loadings (CO₂ permeability up to 65 Barrer, ideal selectivity up to 36.5). A detailed study of the relation between MMM properties and their morphology as affected by the nature of continuous polyimide phase was performed. The separation factor increased with MOF loading. Furthermore, these materials showed stable CO₂/CH₄ selectivity at increasing feed pressure, in contrast to the traditional polymer membranes. The separation performances as functions of operation temperature and composition of gas mixture were also studied. Experimental permeation data of MMMs with 6FDA-ODA-DAM (1 : 1, 2% agent cross-linking APTMDS) and up to 35 wt% NH₂-MIL-53(Al) loading are in excellent agreement with the modeling predictions by both the Maxwell model and the modified Maxwell model. An approximately ideal morphology was reached when preparing MMMs. A computational optimized process was proposed in order to estimate simultaneously three independent parameters, including gas permeability of dispersed filler, interphase thickness and polymer chain rigidification factors, which would be applied for non-ideal MMM systems.