Comprehensive Computational Analysis Exploring the Formation of Caprolactam-Based Deep Eutectic Solvents and Their Applications in Natural Gas Desulfurization

Abstract

Several deep eutectic solvents (DESs) have been recently developed for extraction of hydrogen sulfide from natural gas. Among these newly designed DESs, a combination of caprolactam and tetrabutylammonium halides at a molar ratio of 1:1, as the DESs, has the highest desulfurization efficiencies. In this work, we explored the formation of caprolactam-based DESs using molecular dynamics (MD) simulations and ab initio computations. The results, based on the time average of the equilibrated production run of MD simulations, revealed ∼15% decrease in the ionic interactions of tetrabutylammonium halides and, more interestingly, ∼92% decrease in the hydrogen bonds between caprolactam, thereby explaining the strong depression in the melting point observed in experiments during the formation of the DES. Next, simulations of the DES with mixtures of methane and hydrogen sulfide were performed to mimic the natural gas desulfurization process. Efficient absorption of hydrogen sulfide from natural gas under different operating conditions (5000 and 10,000 ppm H2S, at 25 and 60 °C, and at 1 and 10 bar) can be observed from the simulations. The results further unveiled strong interactions between the anions of the DESs and hydrogen sulfide, with interaction energies 10-fold higher than those of methane/hydrogen sulfide, explaining the mechanism of desulfurization by these DESs. Additionally, two different DESs composed of monoethanolamines/methyltriphenylphosphonium bromide and urea/choline chloride were used to evaluate and compare their capacity to absorb hydrogen sulfide; however, we observed that the caprolactam-based DESs are highly efficient, particularly at low temperatures, low pressures, and low fuel/DES mole ratios. As the existing literature covers only experiments on the removal of hydrogen sulfide via the DESs, the simulations presented herein provide insights into the mechanism of absorption, thereby paving a way forward for better design of the DESs.

Rational Design of Caprolactam-Based Deep Eutectic Solvents for Extractive Desulfurization of Diesel Fuel and Mechanism Study

“…As shown in Figure a, the peak of CPL located at 3207 cm –1 was attributed to the N–H stretching vibration, which shifted to 3277 cm –1 . According to structure optimization by Gaussian, it could be deduced that hydrogen bonds, such as the C–N···H–C and the N–H···Br bonds between CPL and TBAB, were formed. , The hydrogen bonding interaction also caused the shift of the stretching vibration of the C–N bond from 1364 to 1353 cm –1 . In addition, it could be observed that the peak of CO in CPL red-shifted from 1653 to 1648 cm –1 due to the hydrogen bonding interaction between CO and C–H in TBAB.…”

Computer Simulations of Deep Eutectic Solvents: Challenges, Solutions, and Perspectives

“…Parameters are given Antibiotics in DES [ 151 ] 14 Benzene-1,4-diol:Urea Different molar ratios AMBER99 Investigation of DES structure and dynamics and water effect on it [ 371 ] 15 Betaine:Lactic acid (1:1) No name of FF. Parameters are given Investigation of DES structure [ 153 ] 16 Betaine Monohydrate:Glycerol Different molar ratios GAFF/no corrections Extraction of palmitic acid by DES [ 372 ] 17 Betaine Monohydrate:Glycerol (1:2) GAFF/no corrections Investigation of DES structure [ 373 ] 18 Bis(trifluoromethanesulfonyl)imide:Methanesulfonamide Bis(trifluoromethanesulfonyl)imide:Dimethylmethanesulfonamide Different molar ratios OPLS-AA/no corrections Characterization of the transport properties of binary DESs [ 374 ] 19 Caprolactam:Tetrabutylammonium bromide (1:1) Caprolactam:Tetrabutylammonium bromide (1:1) ChCl:Urea (1:2) Methyltriphenylphosphonium bromide:Monoethanolamine (1:6) Gromos54a7, the optimized forcefield parameters were obtained from the Automated Topology Builder (ATB) database Natural Gas Desulfurization using DES [ 375 …”

Rotten Eggs Revaluated: Ionic Liquids and Deep Eutectic Solvents for Removal and Utilization of Hydrogen Sulfide

“…114 Moreover, the IL with the lowest Henry's constant has the weakest interaction energy between anion and cation, which would lead to an increase of free volume for H 2 S to occupy and thus improve H 2 S solubility. Amhamed et al 116 RDFs between H 2 S with CH 4 and the components of caprolactam (CPL)-based DES (CPL/TBABr with a molar ratio of 1:1) were computed by Karibayev et al, 117 to explore the interactions within the DES, and to examine the interactions between the DES with H 2 S and CH 4 in natural gas. As shown in Figure 8, the highest peak at a distance of 3.9 Å indicated the strong interaction between the H 2 S molecule and the Br of the DES.…”

“…RDFs between H 2 S with CH 4 and the components of caprolactam (CPL)-based DES (CPL/TBABr with a molar ratio of 1:1) were computed by Karibayev et al, to explore the interactions within the DES, and to examine the interactions between the DES with H 2 S and CH 4 in natural gas. As shown in Figure , the highest peak at a distance of 3.9 Å indicated the strong interaction between the H 2 S molecule and the Br of the DES.…”