Impact of MOF-5 on Pyrrolidinium-Based Poly(ionic liquid)/Ionic Liquid Membranes for Biogas Upgrading

Postprint Bearing in mind that Metal Organic Frameworks (MOFs) have remarkable CO2 adsorption selectivity and Mixed Matrix Membranes (MMMs) have been identified as potential solution for advancing the current state of the art of membrane separation technology, this work investigates the effect of combining a MOF, with high adsorption properties towards CO2 when compared to CH4 (MOF-5), with a blend of poly(ionic liquid)/ionic liquid (PIL/IL) for biogas upgrading. The blend system consisted of a pyrrolidinium-based PIL, poly([Pyr11][Tf2N]), and a free imidazolium-based IL, [C2mim][BETI]. The MOF-5 was incorporated at different loadings (10, 20, 30 wt%), and MMMs were prepared by solvent evaporation technique and characterized by diverse techniques (FTIR, SEM, TGA, puncture tests, water contact angle and single gas transport). The results showed that the free IL is miscible with the PIL, while MOF-5 particles were uniformly dispersed into the PIL/IL matrix. The formed PIL/IL/MOF-5 membranes revealed suitable thermal stability (Tonset up to 656 K) for biogas upgrading processes, but a loss of mechanical stability was found after the incorporation of MOF-5, and thus more rigid and fragile membranes were obtained. Besides, increasing MOF-5 content in the MMMs resulted in improved CO2 permeability. At 30 wt% of MOF-5 loading the CO2 permeability increased 133% when compared to that of the pristine PIL/IL membrane, while the ideal selectivity CO2/CH4 decreases. It was possible to demonstrate the relevance of studying different components within the polymeric matrix in order to assess not only thermal, mechanical and chemical properties, but also gas transport response. This work was supported by FCT (Fundação para Ciência e a Tecnologia) through the project PTDC/CTM-POL/2676/2014 and the Associate Laboratory for Green Chemistry - LAQV (UID/QUI/50006/2019). Ana R. Nabais and Luísa A. Neves are grateful to FCT/MCTES for their PhD grant (SFRH/BD/136963/2018) and FCT Investigador Contract (IF/00505/2014), respectively. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 745734.