2024 AIChE Annual Meeting
(537e) Exploring Cation Effects on Transport Properties and Reaction Thermodynamics with CO2 in Aprotic N-Heterocyclic Anion-Based Ionic Liquids
Authors
Thacker, P. - Presenter, The University of Texas at Austin
Canada, L., The University of Texas at Austin
Brennecke, J., The University of Texas At Austin
In 2019 alone, 36.8 billion tonnes of carbon dioxide (CO2) were generated, and CO2 emissions contributed approximately 74% of total greenhouse gas emissions, resulting in the CO2 concentration in the atmosphere reaching 421 ppm. To mitigate and control increasing CO2 emissions, researchers have singled out carbon capture, utilization, and storage (CCSU) as a pivotal technology. This project targets post-combustion CO2 capture from flue gas utilizing chemisorbing ionic liquids (ILs). They may be a greener alternative to aqueous amine solvents, offering negligible volatility and the ability to tune the properties of the IL for specific applications. The focus is placed on a special class of ILs with aprotic N-heterocyclic anions (AHAs) that are insensitive to undesirable viscosity increase post-CO2 capture. By varying the anion base group, heterogeneity of substituents, and the number of ring structures, AHA ILs render enthalpic tunability and have been shown to have a prominent effect on CO2 solubility. However, investigations of the role of the cation has been limited to phosphonium- and imidazolium-based AHA ILs. In this work, we report transport properties (i.e., density, viscosity, and ionic conductivity) and CO2 reaction thermodynamics of triethylhexylammonium ([N2226]+), 1-butyl-1-methylpyrrolidinium ([BMpyrr]+), and 1,1-dibutyl-2,2,3,3-tetramethylguanidin-2-ium ([B2TMG]+) cations paired with two common AHAs—2-cyanopyrrolide ([2-CNPyr]–) and 3-(trifluoromethyl)pyrazolide ([3-CF3Pyra]–). Their performance will be compared to the benchmark triethyloctylphosphonium ([P2228]+) cation. The solubility of CO2 will be measured at temperature of 25 °C, 40 °C, and 60 °C, with subsequent fitting of a temperature-dependent Langmuir model to determine the standard enthalpy and entropy of the reaction of the IL with CO2 across different cation moieties. In addition, the mechanism of the reaction of these AHA ILs with CO2 will be explored using NMR spectroscopy. A custom-build in-situ NMR (iNMR) preparation setup saturates the ILs with CO2 inside an NMR tube, while sonication helps with the absorption kinetics. A Teflon™ valve on the NMR tube is then closed, and the tube is placed in a larger tube containing dmso-d6 (i.e., coaxial). We can then use conventional NMR instruments to determine the fraction of the total CO2 uptake that results from reaction with the anion to form carbamate versus reaction with the cation to form carboxylates. As a result, this study widens the design space for specific applications where ILs that chemically react with CO2 may be attractive.