CO2 separation of fluorinated 6FDA-based polyimides, performance-improved ZIF-incorporated mixed matrix membranes and gas permeability model evaluations

Abstract

In this study, we compare a newly introduced high free volume fluorinated 6FDA-polyimide, namely 6FDA-bisP, with excellent gas separation properties (CO2 permeability, PCO2 of 35.3 Barrer; CO2/CH4 selectivity, αCO2/CH4 of 25.5) to the well-investigated, lower free volume 6FDA-ODA (PCO2 = 25.9 Barrer; αCO2/CH4 = 20.6). Their gas separation performances were improved in the form of mixed matrix membranes (MMMs) with size sieving zeolitic imidazolate framework (ZIF-8, particle range of <160 nm) addition between 5 and 20 wt% loadings. XRD, BET, FTIR, TGA, DSC, SEM, and TEM were used to characterize the nanoparticles and MMMs. Solid density was obtained from He pycnometry analysis (pressurized cycle test between 2 and 20 bar at 20 °C) to calculate the free fractional volume (FFV). Using an equimolar CO2 and CH4 mixture, gas separation performance was evaluated at a constant 5 bar, at 25 °C. The optimum loading was achieved at 15 wt% for 6FDA-bisP with MMM performance of PCO2 = 81.2 Barrer and αCO2/CH4 = 35.0, whereas 10 wt% for 6FDA-ODA (PCO2 = 42.2 Barrer; αCO2/CH4 = 44.8). The MMMs showed superior performances than the currently relevant polymers in industrial separation processes. Four different gas permeability models: Maxwell, Maxwell-Wagner-Sillar, Higuchi and Lewis Nielsen, were utilized to analyze the obtained data.