Chiara CORSINI1, Agilio PADUA1, Margarida COSTA GOMES1
1Laboratoire de Chimie, École normale supérieure de Lyon and CNRS, Lyon, France
Improving the solubility of weakly-interacting gases, such as CO2, N2 and H2, in the liquid phase is necessary to employ them for sustainable purposes, namely green-house gas capture and new energy sources. Different techniques are currently used for gas capture and separation but the sustainability of such processes is still insufficient. Chemical absorption of CO2 by amine solutions is highly energy intensive and polluting due to the degradation and evaporation of amines during the process. Electrochemical reduction of nitrogen to ammonia, the most promising hydrogen energy carrier, is a good substitute for the onerous Haber-Bosch process, but still highly inefficient. In this context, the design of new sustainable solvents for the capture and separation of these gases appears as fundamental for a sustainable chemistry.
Porous liquids are a new family of materials that provide permanent free volume in the liquid phase, which is particularly favorable for the capture of weakly-interacting gases. Dispersing metal-organic framework (MOF) particles into ionic liquids (ILs) can result in stable suspensions with permanent porosity. The performance in gas absorption of the porous ionic liquid overcomes that of the pure ionic liquid by 60% for CO2 and by over 100% for nitrogen, just by adding 5% of porous particles [1]. In this work, we improved the sustainability of these solvents while maintaining the same favorable properties by substituting the IL with a deep-eutectic solvent (DES). Nanoparticles of a metal-organic framework, ZIF-8, where dispersed in an ionic DES, methyl-triphenyl-phosphonium bromide/glycerol, producing stable suspensions.
Molecular dynamics simulations were used to investigate the properties of these heterogeneous systems and to tune them to improve gas capture and separation. The interactions between the DES, the MOF and different gases were modelled employing the polarizable CL&Pol force-field [2]. To truthfully model the structure of the explicit solid-liquid interface, the MOF was described as flexible and polarizable.
The structural and dynamic analysis of the DES showed the complex interplay between H-bonds and polar interactions. The analysis at the interface showed that neither the ion pair nor the molecular compound present in the DES enter the pores of the MOF, establishing the existence of the first porous DES. To test the gas capture properties of the porous liquid, gases were included in the pure DES and at the interface in a composite system. Simulations showed relatively weak affinity between the gas and the liquid, leading the gas molecules to prefer the free volume provided by the porous material. Thus, we demonstrated the possibility of employing this system for gas capture and separation.
[1] Jocasta Avila, Ctirad ?ervinka, Pierre-Yves Dugas, Agílio Pádua, and Margarida
Gomes. Porous ionic liquids: Structure, stability, and gas absorption mechanisms.
Advanced Materials Interfaces, March 2021.
[2] Kateryna Goloviznina, Zheng Gong, and Agílio Pádua. The CL &Pol polarizable
force field for the simulation of ionic liquids and eutectic solvents. WIREs
Computational Molecular Science, August 2021.