Task-specific ionic liquids as absorbents and catalysts for efficient capture and conversion of H2S into value-added mercaptan acids
Graphical abstract
Introduction
Developing clean energy is an important task to improve energy structure, ensure energy security and promote ecological civilization construction [1], [2], [3]. Hydrogen sulfide (H2S), as a toxic and disgusting gas, widely exists in raw natural gas, coal gasification gas, biogas, and petrochemical refinery gas [4]. At present, absorption separation technique by means of aqueous solutions of alkanolamines for the capture of H2S have been screened over the past 100 years [5], [6], [7]. Corrosiveness, solvent volatilization/degradation, and energy-intensive input have restricted their application. In particular, the regeneration of absorbents requires not only intensive heat and high temperature (usually >100 °C) conditions, but also support of tedious supplementary flowsheet units to treat the released H2S from the solvent [8]. The high energy requirements of H2S desorption and transportation could be critical problems in industrial chemistry. Therefore, avoiding desorption process is highly desirable to reduce energy consumption.
Ionic liquids (ILs) have been paid worldwide attention in recent years due to their unique properties, such as extremely low vapor pressure, high thermal stability, designable structure, and outstanding affinity for acidic gases. A numerous study on the use of ILs has been emerging, such as catalysts [9], [10], [11], [12], [13], hypergolic fuels [14], [15], lubricants [16], solvents [17], [18], [19], [20], and electrolytes [21], [22], [23], [24], [25], [26] et al. There are also plenty of literatures reporting the use of ILs as effective absorbents for the removal of H2S [27], [28], [29], [30], [31], [32], [33], [34].
After years of researches on the capture of acidic gas (H2S, SO2, CO2 et al.) [28], [29], [30], [33], [35], [36], [37], [38], [39], [40], [41], [42], [43], we started to work on how to avoid the desorption process using new strategy. Capture and in-situ conversion of H2S should be an ideal way. However, except for the process to produce sulphur [33], [44], examples for the capture and transformation of H2S to valuable organic compounds is scarce, due to the following reasons: (1) achieving such processes highly depended on the developing of new systems which can serve as both absorbent and catalyst [45]. (2) There are only a few examples concerning the conversion of H2S to organic compounds [46]. The scarcity of catalysts for the conversion of H2S makes it more difficult to build a system for the capture and conversion of H2S.
Herein, we develop a catalytic transformation of H2S into high valuable product mediated by a series of hydrophobic ILs. The effect of different ILs, temperature, ILs loading, water content, and pressure on H2S conversion was systematically investigated. It is anticipated that the highly efficient capture and intu-conversion method mediated by task-specific ILs provides a promising avenue to recycle H2S.
Section snippets
Results and discussion
In the absorption of H2S by triethylbutylammonium maleate ([N2224]2[maleate]) S1, we found that [N2224]2[maleate] showed an absorption capacity of 1.43 mol of H2S per mol of IL at 1.0 bar and 40 °C (Fig. 1), which is the highest value compared with literatures [28], [29], [30], [31], [42], [47], [48], [49]. The previously reported highest value is 1.20 mol of H2S per mol of IL at the same condition. It can be seen that there is a 19% increment. 1H and 13C NMR analysis of [N2224]2[maleate]
Conclusions
In conclusions, an efficient capture and conversion of H2S into high valuable mercaptan acids were proposed in this work. These task-specific ILs can be used impressively as a dual role of H2S absorbent and catalyst. The reaction of H2S with the unsaturated acids can proceed very well in the absence of any organic solvents. Water extraction was employed to separate the product from the parent system. A plausible reaction strategy is reasonably proposed in this work. The conversion of substrate
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was sponsored by the National Natural Science Foundation of China (No. 21576129 and 21878141) and the Natural Science Foundation of Jiangsu Province (BK20190310).
X. Zhang acknowledges the Yuxiu Young Scholars Program of Nanjing University for partial financial support.
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These authors contributed equally.