Wenjie Xiong,Mingzhen Shi,Yan Lu,Xiaomin Zhang,2,*,Xingbang Hu,2,Zhuoheng Tu,*,Youting Wu,*

1 Key Laboratory of Mesoscopic Chemistry of MOE,School of Chemistry and Chemical Engineering,Nanjing University,Nanjing 210023,China

2 Institute of Green Chemistry and Engineering,Nanjing University,Suzhou 215163,China

Keywords:Ionic liquids Reaction Catalysis Hydrogen sulfide DFT calculation

ABSTRACT The resource utilization of hydrogen sulfide(H2S)is of great significance in natural gas chemical industry.Described herein have developed a novel method mediated in tertiary amine-functionalized ionic liquids(ILs)to convert H2S into mercaptan alcohols with enols.The effect of ILs,substrate scope,and regeneration experiments have been investigated.It is found that the conversion of 3-methyl-2-buten-1-ol by H2S can reach 52% with a 50% (mol) catalyst loading of bis(2-dimethylaminoethyl) ether methoxyacetate within 12 h at 90°C.The reaction mechanism was speculated based on theoretical calculation.Besides,a plausible reaction-separation-integrated strategy was further proposed.This work reveals an effective insight into the capture and catalytic conversion of H2S to high valuable mercaptan alcohol,which makes the utilization method of H2S resource universal and has the potentiality for industrial application.

1.Introduction

Hydrogen sulfide(H2S),flammable gas with rotten eggs smell,is one of the most poisonous gases in the air[1].It is well known that H2S widely exists in energy gas,power plant gas,and refinery gas,and is considered to be the main pollutant to decline the operation safety.Excessive exposure to high concentrations of H2S is a great threat to the health of human beings and can lead to even death[2].In engineering applications,pipelines and process equipment will be corroded by the presence of H2S,which will also lead to the deactivation of catalyst and promote unnecessary side reactions [3,4].Therefore,there is a pressing demand to find a novel reaction medium with a favorable conversion efficiency,low volatility and toxicity,high stability,as well as excellent regeneration performance for efficient utilization of H2S.

Recent related studies including capture,separation,utilization,and conversion of H2S have been applied to electrochemistry[5-7],membrane separation[8-10],absorption[11-14],adsorption[15-17],etc.Electrochemical conversion of H2S is an energy-efficient technology,which is carried out at room temperature [5].However,directly conversing H2S traps by the drawback of anode passivation due to the accumulation of sulfur.By introducing Fe3+/Fe2+redox couples,the anode is prevented from being passivated by sulfur in an indirect electrochemical manner,but the separation of sulfur from the electrolyte is full of challenges [6].As energyefficient technology,membrane separation is restricted by its high modules cost,poor corrosion resistance,and short service lifetime,which is unfavorable to its industrial application[18].The absorption process is usually accompanied by a corresponding desorption(regeneration) unit.However,the regeneration of absorbents requires not only intense heat conditions but also cumbersome supplementary process units to handle the released H2S.In accordance with the concept of green sustainable chemistry,coupling of H2S capture and conversion should be considered,which can avoid the energy-intensive desorption process and tedious units [19].Therefore,both capture and further conversion of H2S are of great significance for environmental protection,human health,and resource reutilization.Overall,it is a promising strategy to make H2S converse into value-added organic compounds,which requires chemical complexation to activate the free H2S.

Ionic liquids (ILs) as molten salts under ambient conditions have attracted extensive attention[20-23].They have been considered as ‘‘green solvents” for a variety of applications attributed to their many unique advantages,such as negligible vapor pressure,high thermal stability,wide liquid range,etc.[24-26].In addition,selecting appropriate cations and anions or introducing functional groups into ILs can easily adjust their physicochemical properties to meet specific application demands,resulting in so-called taskspecific ILs.Related studies on ILs have mainly focused on catalytic synthesis[27-29],energy[30-32],materials[33-36],etc.Tertiaryamine functionalized ILs have a good affinity for H2S due to the existence of alkaline amine groups.It has played a significant role in gas absorption and separation,which is worthy of attention.

After years of in-depth research in the field of H2S capture by ILs,exploring the resource utilization of H2S has begun to be investigated in a green medium.On the other hand,H2S is an important feedstock of the productions such as sulfur,sulfuric acid,and thiolorganic chemicals,etc.Liquid-phase Claus process with creditable H2S conversion into sulfur mediated in ILs and deep eutectic solvents (DESs) under mild conditions have been disclosed in our group [20,37],without adding any additives,the absorbed SO2can be quickly converted into sulfurin situin the presence of H2S.This groundbreaking attempt makes it possible to capture and convert H2S through green and sustainable media.Yuet al.developed the efficient method of H2S absorption and oxidation mediated in Fe3+-based ILs [38].Wu and co-workers reported ILs/ethylene glycol catalyzed liquid-phase Claus reaction [39].However,the H2S conversion target discussed above is low valueadded chemical sulfur.Converting H2S into value-added chemicals is another option that makes more sense.Mishraet al.[40] used trihexyltetradecylphosphonium chloride (THTDPC) as the phase transfer catalyst for transferring H2S to bis(2-phenylethyl) sulfide(PES) with a high product selectivity.More specifically,H2S is absorbed firstly into the methyldiethanolamine (MDEA) aqueous solution,and then the H2S-rich MDEA solution reacts with 2-bromoethylbenzene (2-BEB) in the presence of THTDPC to form PES.When the concentration of IL is 0.03 kmol·m-3,the initial reaction rate is three times of that without IL catalyst.Finally,the selectivity of PES can reach 100%.Zhanget al.[19] pioneered a series of task-specific hydrophobic PILs catalyzed H2S conversion with unsaturated acids into value-added mercaptan acids.The conversion of α-methacrylic acid is up to 98%at 90°C.The product can be separated by water extraction without adding any organic solvent.This method has the advantages of solvent-free,high conversion,and good recoverability of catalyst.Xionget al.[27]developed a novel method for H2S conversion with epoxides to produce mercaptan alcohols with protic ILs serving as both solvents and catalysts under mild conditions.The conversion of styrene oxide can reach 99% at 2 h and 30 °C in H2S atmosphere.The conditions of whole reaction process are mild and the substrates have good universality.These recent studies demonstrate that ILs can provide new opportunities as promising media for the capture and conversion of H2S into organic sulfur compounds.Therefore,it is particularly meaningful to find a new reaction for the resource utilization of H2S in ILs media,especially through the coupled process of capture and conversion.

Herein,a catalytic conversion of H2S with unsaturated alcohols into high value-added products mediated by a series of tertiaryamine functionalized ILs has been developed.The effects of cation and anion,water content,and substrate scopes were investigated.The theoretical calculation was performed to investigate the reaction mechanism.A promising reaction mediated in task-specific ILs to utilize H2S has been demonstrated through the combination of H2S capture andin situconversion.

2.Experimental

2.1.Preparation of ILs

The specifications and sources of the chemicals used in this work were summarized in Table 1 (more details in Table S1 in Supplementary Materials).All reagents were used without further purification.These ILs were synthesized by a neutralization reaction according to our previous work [27,41].Taking a typical run,50 ml of the ethanol solution of methoxyacetic acid (MeOAc)(20 mmol) was added dropwise to the ethanol solution of bis(2-dimethylaminoethyl) ether (BDMAEE) (25 mmol).The reaction was stirred at ambient temperature for 12 h.Then,ethanol was removed by vacuum distillation and the remaining crude product was washed three times by diethyl ether to remove the excess reactant.The product was dried in a vacuum at 60°C for 24 h to remove trace solvents,and finally,a clear liquid bis(2-dimethylaminoethyl)ether methoxyacetate([BDMAEEH][MeOAc])was obtained.

2.2.Characterization

Nuclear magnetic resonance(NMR)spectra were obtained on a DPX 400 MHz spectrometer (Bruker,Switzerland) with CDCl3as solvent at ambient temperature.Fourier transform infrared spectroscope (FT-IR) spectra were carried out on Nicolet iS50 infrared spectrometer(Thermo Scientific,USA),with the spectral resolution and the number of scans of 4 cm-1and 32,respectively.The conversions of enols were determined by high performance liquid chromatography (HPLC,Agilent Technologies 1260 Infinity II,USA).Electro spray ionization-mass spectroscopy(ESI-MS,Thermo Fisher,Germany)was used to characterize the molecular weight of the product atm/z.

3.Results and Discussion

3.1.Optimization of reaction condition

It is known that the H2S molecule can be activated by the tertiary amine group to split into hydrogen proton and nucleophilic SH-group.The C=C double bond is easy to be attacked by the strong nucleophilic group SH-,especially when the chemical environment of the C=C double bond is influenced by the surrounding groups such as aryl,ester,carboxyl,and hydroxyl,etc.Hence,taking enols as substrates and ILs as catalysts,the addition reaction between enol and H2S has been investigated through the coupling of capture and conversion.The calculation formulas of conversion rate and isolated yield have been presented in Supplementary Materials.3-Methyl-2-buten-1-ol was taken as a typical substrate of the enols.Firstly,an investigation on the substrate in the absence of any catalyst was performed.It is found that no product can be detected (Entry 1,Table 2),indicating that its autocatalysis is fairly weak.[Bmim][BF4] was selected as a representative conventional IL to act as the catalyst to explore the addition reaction of H2S and 3-methyl-2-buten-1-ol (Entry 2,Table 2).It is found that the conversion of the substrate is significantly restricted with a catalyst loading of 50%(mol)at 90°C for 12 h.This result is probably since the alkalinity of[Bmim][BF4]is too weak to ensure effectively activate H2S.Subsequently,[BDMAEEH][MeOAc] as the catalyst was fed into the H2S/enol system to explore the role of ILs (Fig.1).

As is well known,dept135 NMR presents an upward and downward peak for -CH (or -CH3) and -CH2,respectively.For quaternary carbon,the signal cannot be collected.Dept135 and full spectrum (13C NMR) are very efficient means to distinguish C,CH,CH2,and CH3.As shown in Fig.1(a),through the characterization of13C NMR and dept135 NMR,it is found that the double bond signals of the substrate did not disappear after the reaction,which were labeled with c and d.Besides,a quaternary carbon signal marked m appeared,suggesting that the addition reaction is realized in favor of the formation of the Markovnikov addition product.

To evaluate the influence of ILs with different cations on the conversion of H2S,N,N,N’,N’-tetramethylethylenediaminemethoxyacetate ([TMEDAH][MeOAc]),N,N,N’,N’-tetramethyl-1,3-propanediamine methoxyacetate ([TMPDAH][MeOAc]),N,N,N’,N’-tetramethyl-1,6-hexanediamine methoxyacetate ([TMHDAH][MeOAc]),and [BDMAEEH][MeOAc] were investigated for the H2S addition reaction (Entries 3-6,Table 2) and the conversions of H2S mediated in the ILs are 43%,45%,47%,and 52%,respectively.[TMEDAH][MeOAc] and [TMHDAH][MeOAc] exhibit the lowest and highest catalytic performance because of the weakest and strongest alkalinity,respectively [27].ILs bearing the primary amine group on the cations were also investigated as both catalysts and solvents (Entries 7-9,Table 2),and a substrate conversion ranging from 41% to 49% was achieved,indicating primary amine group can also catalyze this H2S addition reaction.The influences of ILs with different anions have also been investigated based on[BDMAEEH]+cation,such as [BDMAEEH][4-F-PhO] and[BDMAEEH][NIA] (Entries 10 and 11,Table 2).The conversioncan also reach 41%,reflecting the catalytic activity of these alkaline ILs.

Table 1 Specifications and sources of chemicals

Table 2 The conversion of 3-methyl-2-buten-1-ol (3a) mediated in different ILs catalysts at 90 °C①

In the catalyst recycling experiment,IL catalysts and products were split into two phases with the addition of diethyl ether.After removing the ionic liquid catalyst by extraction,the product and the original substrate are presented in Fig.1(b).Besides,the lower phase was reused to feed into the next catalytical reaction.It is found that the reactivity of the catalyst was not significantly decreased.In the catalyst recycling experiment[42],the reactivity of the catalyst is of stabilization after two cycles(Entries 12 and 13,Table 2).It is well known that H2S in almost industrial gases is inevitably accompanied by humidity.To investigate the effect of water on H2S conversion,[BDMAEEH][MeOAc] containing 10%,20%,and 30% (mass) H2O were employed as the reaction medium(Entries 14-16,Table 2).It is found that the conversion of 3-methyl-2-buten-1-ol decreases slightly with the increasing water content,which should be attributed to the decrease of active site.

3.2.Proposal of the reaction mechanism

Based on the above results,a plausible mechanism is proposed in Fig.2.Firstly,the free tertiary amine group of [BDMAEEH][MeOAc] (1) interacts with H2S to generate a nucleophilic SHgroup.The charge distribution of C=C double bond in 3-methyl-2-buten-1-ol presents a positive and negative charges tendency due to the inductive effect of electron-withdrawing hydroxyl group and the electron-donating methyl groups [43,44].Therefore,it is easier to be attacked by sulfhydryl group,which can be intuitively proved by the distribution of Mulliken charge and electrostatic potential (ESP) (Fig.3) [45].With the addition of enol,the nucleophilic SH-group will attack the C=C double bond to form the transition state from component (4).At the same time,hydrogen protons generated by H2S activated by tertiary amine groups are transferred to the electronegative carbon on the other side of the enol to generate the target product (3b) due to an equilibrium occurred at a low energy barrier between components (1+3b)and (4).Finally,the product (3b) is separated from the system by liquid-liquid extraction resulted from the shift of the equilibrium reaction and meanwhile the ILs can also be recycled for the next run.The addition reaction is realized in favor of the formation of the Markovnikov product.

Fig.1.Dept135 and 13C NMR spectra of 3-methyl-2-buten-1-ol after the reaction with H2S in[BDMAEEH][MeOAc]at 90°C for 12 h(a);dept135 and 13C NMR spectra after removal of ILs by extraction (b).

Fig.2.The plausible mechanism for the conversion of H2S.

Fig.3.(a) The distribution of Mulliken charges of 3-methyl-2-buten-1-ol,(b)electrostatic potential (ESP).

To figure out the reaction mechanism of the H2S addition reaction,DFT-based theoretical calculations were further investigated to analyze the bond length and transition state of the structures(for more details please see SI).All calculations were fully optimized with Gaussian 09 program using the B3LYP method including the dispersion corrections method using the Empirical Dispersion=GD3BJ keyword.6-311++G(d,p) basis set was used for all atoms.As shown in Fig.4,taking ILs as a start(0.0 kcal·mol-1,1 kcal·mol-1=4.18 kJ·mol-1),compound(4)is formed by the addition of H2S and compound(3a),and the reaction enthalpy is -13.9 kcal·mol-1.The compound (4) is activated at a low barrier of 33.1 kcal·mol-1,so that the proton can be easily extracted from H2S molecule[46].Finally,the enthalpy charge of the H2S addition reaction of compound (1+3b) is 15.4 kcal·mol-1.To cross the energy barrier,since H2S is activated by the tertiary amine,its bond length has changed(0.135 and 0.135 nmvs0.135 and 0.314 nm).A proton from H2S is extracted near the N atom on the other side of the tertiary amine (the distance is 0.142 nm),and the distance between this proton and the negatively charged secondary carbon of compound 4 is 0.132 nm.Finally,the distance between the remaining SH-group and the positively charged tertiary carbon of compound (4) is 0.287 nm in the transition state,which will eventually contribute to the formation of compound(3b).The corresponding optimized geometry (Fig.S1) and the corresponding Cartesian coordinates can be found in Supplementary Material.

3.3.Investigation of substrate universality

With the optimized conditions in hand,diverse enols were selected to examine the substrate scope in the presence of[BDMAEEH][MeOAc] at 90 °C for 12 h (Table 3).High conversions were realized among all the substrates.2-Butene-1,4-diol and perillyl alcohol presented the excellent conversion of 92% and 95%,respectively.The reaction temperature for the conversion of perilla alcohol is 120 °C,which is higher than those for other substrates.High temperature will promote the conversion of perilla alcohol into corresponding sulfhydryl products.For 2-butene-1,4-diol,two hydroxyl groups endow themselves with higher hydrophilicity and polarity compared with other substrates,which would improve its compatibility with IL catalyst.For all of the systems,thin layer chromatography(TLC)was utilized to separate the products with EA/PE as eluent.NMR and ESI-MS spectra of all products are presented in Figs.S2-S27.

Fig.4.The optimized structures calculated by the B3LYP/6-311++G(d,p) method (numbers in pink and black are in the units of nm and kcal·mol-1,respectively.1 kcal·mol-1=4.18 kJ·mol-1).

Table 3 Investigation of the scope of the substrates①

Fig.5.A plausible reaction-separation-integrated(RSI)strategy proposed for the capture and conversion of H2S.T 01:absorption/reaction unit;T 02:extraction unit;T 03:multiple distillation system (MDS).

3.4.Proposal of technological process

In addition,a reaction separation coupling process was proposed as schematically shown in Fig.5.Firstly,enol together with H2S-containing gas stream were fed into the absorption/reaction unit (T 01).Then,the gas-liquid mixture in the T 01 was fed into the flash evaporation unit (Flash 1),and the released H2S by flash evaporation can be recycled into T 01.Whereafter,the liquid-liquid mixtures from Flash 1 are pumped into the extraction unit (T 02) [47,48].Notably,the reaction between H2S and the enol replaces the process of H2S desorption to avoid high energy consumption,which is in line with our envisaged green and sustainable concept.The product and unreacted enol are extracted by diethyl ether from the T 02.IL phase collected from T 02 was returned into T 01 after the removal of extractant by flash evaporation to re-catalyze H2S addition reaction.Raffinate,including product,unreacted enol,and extractant,are obtained from T 02 and then fed into the multiple distillation system (T 03,MDS,are composed of one or more groups of atmospheric distillation columns and vacuum distillation columns)to separate unreacted enol and value-added mercaptan alcohol products [49-52].The extractant is separated firstly from T 03 due to its lowest boiling point.Together with the extractant removed by flash evaporation,the extractant in ILs phase can also be reused in the T 02 [53,54].Besides,there is rectification residue (such as the dissolved IL in the extractant) at the end of T 03 that can be recovered by proper technique for reutilization.

4.Conclusions

In summary,an efficient capture and conversion of H2S into high valuable mercaptan alcohols were developed in this work.The reaction of H2S with the enols can be carried out catalyzed by tertiary amine-functionalized ILs in the absence of any organic solvents.Among different tertiary-amine functionalized IL catalysts,[BDMAEEH][MeOAc] demonstrates the highest catalytic performance to achieve a 52% conversion of 3-methyl-2-buten-1-ol with a 50% (mol) loading at 90 °C for 12 h.With the increasing water content in [BDMAEEH][MeOAc] (from 0 to 30% (mass)),the conversion of 3-methyl-2-buten-1-ol decreases from 52% to 46%,which should be attributed to the decreasing active site.The highest conversion of the substrate can reach as high as 95% with the H2S pressure of 1.0 MPa at 120 °C.The theoretical calculation was utilized to analyze the reaction mechanism.Furthermore,a plausible reaction-separation-integrated strategy is proposed to realize the H2S utilization.It provides a new potential prospect for future industrial applications.

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 (22078145 and 22208140).

Supplementary Material

Supplementary material to this article can be found online at https://doi.org/10.1016/j.cjche.2022.07.001.