Mg2(dobdc) crystals adhere to Matrimid matrix membranes bridged by diethylenetriamine (DETA) as an adhesion agent for efficient CO2 separation

Abstract

Diethylenetriamine (DETA)-bridged Mg2(dobdc) crystals were first synthesized and DETA molecule was acted as adhesion agent to stick Matrimid matrix together, fabricating the mixed matrix membranes (MMMs) with superior CO2 separation performance from the biomethane (CO2/CH4) and flue gas (CO2/N2). The detected CO2 adsorption isotherms (at 298 K and 1 bar) of Mg2(dobdc) crystals and DETA-bridged Mg2(dobdc) crystals demonstrated excellent CO2 uptake of 7.15 and 6.45 mmol/g, as well as superior separation selectivity. The XPS and FTIR spectra revealed that unsaturated Mg2+ sites in Mg2(dobdc) tightly grasped DETA molecule and alter the surrounding binding environment. Density functional theory (DFT) calculation implied that active amine group (–NH2) on DETA was bonded to unsaturated Mg2+ sites through Mgδ+···Nδ- interaction with Mg-N distance of 2.22 Å and binding energy of 160.8 kJ/mol. For Mg2(dobdc) crystals, one CO2 molecule was firmly attracted by unsaturated Mg2+ site via Mgδ+···Oδ- and Hδ+···Oδ- interaction with the calculated Mg-O distance of 2.32 Å and binding energy of 46.9 kJ/mol. For DETA-bridged Mg2(dobdc) crystals, one CO2 molecule was simultaneously caught by unsaturated Mg2+ site of Mg2(dobdc) and active amine group of DETA through Mgδ+···Oδ- (distance = 4.13 Å) and Nδ-···Cδ+ (distance = 3.19 Å) interactions. DETA molecule as adhesion agent stick Mg2(dobdc) crystals and Matrimid matrix together to eliminate interface defects and voids in MMMs, could dramatically increase CO2 permeability as well as the superior CO2/CH4 and CO2/N2 selectivity. Therefore, the CO2 permeability of Matrimid-based MMMs incorporated with DETA-bridged Mg2(dobdc) crystals first increased to a peak of 41.1 Barrers and then decreased, when Mg2(dobdc) crystals loading increased from 2.5 wt% to 10 wt% and further to 15 wt%. This CO2 permeability was 3.03 times higher than that (8.19 Barrers) of pristine Matrimid membrane and 17.8% higher than that (34.9 Barrers) of Matrimid-based MMMs incorporated with Mg2(dobdc) crystals (without DETA). Positron annihilation lifetime spectroscopy experiments revealed the different dispersion states of crystals in the MMMs with 15 wt% crystals, compared to the MMMs with 2.5 to 10 wt% crystals.