159r 2010 American Chemical Society pubs.acs.org/EF

Energy Fuels 2011, 25, 159–161 : DOI:10.1021/ef1012006Published on Web 12/07/2010

Absorption and Oxidation of H2S in Caprolactam Tetrabutyl Ammonium Bromide

Ionic Liquid

Bin Guo,† Erhong Duan,*,† Yongfei Zhong,† Liang Gao,† Xuesong Zhang,† and Dishun Zhao‡

†School of Environmental Science and Engineering, Hebei University of Science and Technology, Shijiazhuang Hebei 050018,People’s Republic of China, and ‡College of Chemical and Pharmaceutical Engineering, Hebei University of Science and

Technology, Shijiazhuang Hebei 050018, People’s Republic of China

Received September 5, 2010. Revised Manuscript Received November 21, 2010

To explore environmentally benign solvents for absorbing and using H2S, a series of caprolactamtetrabutyl ammonium bromide ionic liquids were synthesized, the solubilities of H2S in which weremeasured at 303.2-363.2 K and atmospheric pressure. The solubility of H2S in the ionic liquid (1:1 mole ratio)was 5.40% at 303.2 K and ambient pressure, decreased sharply as temperature increased, and increasedwith the increasing mole ratio of caprolactam. The absorption and desorption of H2S were practicallyreversible in the ionic liquids, whichwas characterized by nuclearmagnetic resonance.Using air, hydrogensulfide could be oxidized to elemental S in the ionic liquids, which makes it easier to recycle hydrogensulfide. Caprolactam tetrabutyl ammonium bromide ionic liquids would be useful for removing andreusing H2S in pollution control and could be regarded as the most potential absorbent and recoverer ofH2S.

Introduction

H2S is produced along with methane and other hydrocar-bons in many gas fields as well as in hydrodesulfurizationprocesses of crude oils containing sulfur compounds. Indus-trial natural gas treating plants use aqueous solutions mainlyconsisting of alkanolamines, especially monoethanolamine,diethanolamine, and methyldiethanolamine.1,2 There are somedisadvantages associated with the commercial use of thesealkanolamine solutions, including transfer of water into thegas stream during the desorption stage and degradation ofalkanolamines to form corrosive byproducts, which make theprocess economically expensive.3

Ionic liquids (ILs) are low-melting salts with extremely lowvapor pressures, high thermal and chemical stability, andtunable solvent power for many organic and inorganic com-pounds. So they can be used as environmentally benignsolvents for a number of applications including gas solubilityand separations, cellulose processing, catalysis, extraction,

and high-temperature pyrochemical processing, etc.4,5 Forexample, acidic ILs have been proven to be efficient catalystsfor many acid-catalyzed organic reactions.6 Basic ILs withamino groups were synthesized and used to capture CO2 andH2S

7,8 and to promote hydrogenation of CO2.9 ILs can poten-

tially be used as liquid absorbents for permanent gases and assolvents for gas separations. Nowadays, one of the areas ofactive research is to explore the possibility of replacing task-specific ionic liquids for conventional alkanolamine solutionsin removal of acid gases (CO2 and H2S) in gas sweeteningprocesses.10

One of the properties which is important in the evaluationof ionic liquids as potential substitutes for alkanolamines inindustrial natural gas treating processes is the knowledge ofgas solubility in ionic liquids at various temperature andpressure conditions. In the past few years, a growing numberof measurements reporting CO2 solubility in various ILs havebecome available.11 However, experimental data for thesolubility of hydrogen sulfide in ionic liquids are scarce. Jouhas reported the solubility of H2S in [Bmim][PF6] at tempera-tures from 298.15 to 403.15 K and pressures up to 9.6MPa.12

At 9 MPa, the mole fraction of H2S in the liquid is about 0.7.At 2MPa, the solubility (mole fraction ofH2S) decreases from

*Towhomcorrespondenceshouldbeaddressed.E-mail: deh@tju.edu.cn.Fax: þ86-311-88632361.(1) Kohl,A. L.;Nielsen,R.B.GasPurification, 5th ed.; Gulf Publishing

Company: TX, 1997.(2) Huang, C. C.; Chen, C. H.; Chu, S. M. Effect of moisture on H2S

adsorption by copper impregnated activated carbon. J. Hazard. Mater.B 2006, 136, 866–873.(3) Galan Sanchez, L. M.; Meindersma, G. W.; de Haan, A. B.

Solvent properties of functionalized ionic liquids for CO2 absorption.Chem. Eng. Res. Des. 2007, 85, 31–39.(4) Liu, F. S.; Li, Z.;Yu, S. T.;Cui,X.;Ge,X. P.Environmentally benign

methanolysis of polycarbonate to recover bisphenol A and dimethylcarbonate in ionic liquids. J. Hazard. Mater. 2010, 174, 872–875.(5) Anderson, J. L.; Dixon, J. K.; Brennecke, J. F. Solubility of CO2,

CH4, C2H6, C2H4, O2, and N2 in 1-hexyl-3-methylpyridinium bis-(trifluoromethylsulfonyl)imide: comparison to other ionic liquids. Acc.Chem. Res. 2007, 40, 1208–1216.(6) Cole, A. C.; Jensen, J. L. I.; Ntai, K. L.; Tran, T.; Weaver, K. J.;

Forbes, D. C.; Davis, J. H., Jr. Novel brønsted acidic ionic liquids andtheir use as dual solvent-catalysts. J. Am. Chem. Soc. 2002, 124, 5962–5963.

(7) Bates, E. D.;Mayton, R.D.; Ntai, I.; Davis, J. H., Jr. CO2 captureby a task-specific ionic liquid. J. Am. Chem. Soc. 2002, 124, 926–927.

(8) Li, W. J.; Zhang, Z. F.; Han, B. X.; Hu, S. Q.; Song, J. L.; Xie, Y.;Zhou, X. S. Switching the basicity of ionic liquids by CO2. Green Chem.2008, 10, 1142–1145.

(9) Zhang, Z. F.; Xie, Y.; Li, W. J.; Hu, S. Q.; Song, J. L.; Jiang, T.;Han, B. X. Hydrogenation of carbon dioxide is promoted by a task-specific ionic liquid. Angew. Chem., Int. Ed. 2008, 47, 1127–1129.

(10) Crowhurst, L.; Lancaster, N. L.; Arlandis, J. M. P.; Welton, T.Manipulating solute nucleophilicity with room temperature ionic liquidsJ. Am. Chem. Soc. 2004, 126, 11549–11555.(11) Shariati, A.; Peters, C. J. High-pressure phase equilibria of

systems with ionic liquids. J. Supercrit. Fluids 2005, 34, 171–176.(12) Jou, F. Y.; Mather, A. E. Solubility of hydrogen hulfide in

[bmim][PF6]. Int. J. Thermophys. 2007, 28, 490–495.

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about 0.84 at 298 K to about 0.2 at 403 K. The solubility ofH2S in 1-butyl-3-methylimidazolium-based ILs containingdifferent anions and in a series of bis(trifluoromethyl) sulfo-nylimide ILswith different cations at 298.15K and 1.4MPa isreported by Pomelli.13 Jalili has reported the solubility of H2Sin [Bmim][PF6], [Bmim][BF4], and [Bmim][Tf2N] at tempera-tures ranging from 303.15 to 343.15 K and pressure increasesto 1MPa.14 The solubility of H2S in the studied ILs is in theorder [Bmim][Tf2N] > [Bmim][BF4] > [Bmim][PF6].

15

A better understanding of the solubility of H2S in ILs isnecessary. ILs based on different mole ratios of caprolactam(CPL) and tetrabutyl ammonium bromide (TBAB) wereprepared, and the solubilities ofH2S in themwere determined.The effects of differentmole ratios of CPL andTBABon theircapacity for absorption of H2S were systematically investi-gated, and the recovery ofH2S and recycle of the ILswere alsoconducted by increasing temperature anddecreasing pressure.Bubbling air into the ionic liquids absorbedH2S, andH2Swasoxidized to elemental S.

Experimental Methods

Materials. White crystalline caprolactam powder (CAS No.105-60-2, C6H11NO) was obtained from Shijiazhuang Refinery,China, the purity of which was >99%. Tetrabutyl ammoniumbromide (CAS No. 1643-19-2, C16H36NBr) was bought fromJintan Huadong Chemical Research Institute, China, the purityof which was >99.5%.16 Water was deionized before use.The water contamination of ILs was determined using theKarl Fischer technique.

Absorption Measurements. The ILs used here were driedunder vacuum for at least 24 h under vacuum at 323.15 K priorto use, and the water content determined by Karl Fischertitration was less than 100 ppm. H2S with a purity of 99.9%was supplied by Beijing Analytical Instrument Factory. Theapparatus of absorption of H2S in these ILs was preparedfollowing procedures reported elsewhere.17 H2S gas was bubbledwith a flow rate of 10 mL/min through predetermined amounts ofthe ILs (about 5 g) loaded in a glass vessel, respectively. After2 h, the weight of the vessel and H2S was not changed, and theequilibrium was considered to be reached. The off H2S wastreated by sodium hydroxide. These valves were closed, and theglass vessel was weighed using a balance with an uncertainty of(0.0001 g to get the mass of H2S absorbed, through which thesolubility of H2S in the ILs could be calculated.

Recycle of CPL-TBAB ILs. The absorption and desorptioncycles were conducted to study the recovery of H2S and recycleof these synthesized ILs. Experiments showed that the solubilityof H2S in these synthesized ILs is almost zero at the atmosphericpressure when the temperature rises to 373.2 K at 10.1 kPavacuum. Consecutive absorption (308.2 K, 101.3 kPa) and desorp-tion (383.2 K, 10.1 kPa vacuum) of H2S gas in recycled ILs werestudied against times. Standard deviations of the ratios are(0.03.

Characterization of CPL-TBAB ILs.1H NMR spectra were

measuredon aBrukerAM400MHz spectrometer, usingDMSOasa solvent with TMS as the internal standard. Seven different moleratios of caprolactam and tetrabutyl ammonium bromide ILswere synthesized and characterized. 1H NMR spectra of capro-lactam and tetrabutyl ammonium bromide (2:1) are presented.1H NMR (400 MHz, DMSO, δ): 7.4 ppm (s, 1H, H-N);3.33-3.17 ppm (t, 8H, Nþ(CH2)4); 3.06-3.03 ppm (t, 2H,HNCH2); 2.29-2.27 ppm (t, 2H, OCCH2); 1.67-1.65 ppm(t, 2H, OCCH2-CH2); 1.59-1.56 ppm (m, 8H, Nþ(CH2CH2)4);1.54-1.48 ppm (t, 4H, OCCH2CH2CH2CH2); 1.34-1.29 ppm(m, 8H, Nþ(CH2CH2CH2)4); 0.94-0.92 ppm (t, 12H, Nþ(CH2

CH2 CH2CH3)4).

Results and Discussion

Effect of Temperature on the Solubility of H2S in CPL-TBAB ILs.The absorption concentration of H2S in differentmole ratios from 1:1 to 7:1 CPL-TBAB ILs versus tempera-ture from 303.15 to 363.15 K was determined at 101.3 kPa(Figure 1). The solubility of H2S in these ILs decreases whenthe temperature increases. With the concentration of CPL inILs increasing, the solubility of H2S decreases slowly. As anoverall result of the different mole ratios of CPL and TBAB,CPL-TBAB ILs (1:1, mole ratio) show the highest capacityfor absorption of H2S, the weight percent of H2S in which is5.40% at 303.15 K and 3.45% at 363.15 K.

Oxidation of H2S in CPL-TBAB ILs. Clearly, the CPL-TBABILis colorless andhighviscosity (Figure2a).The colorofthe IL-absorbed H2S changed to bluish-green (Figure 2 b).

Figure 1. Weight percent of H2S as a function of temperature indifferentmole ratios of CPL andTBAB:9, 1:1;b, 2:1;2, 3:1;1, 4:1;[, 5:1; O, 6:1; 0, 7:1.

Figure 2. Color changes of H2S in caprolactam tetrabutyl ammo-nium bromides ionic liquids. (a) Caprolactam tetrabutyl ammo-nium bromides ionic liquids, (b) after bubbling H2S for 20 min, and(c) after bubbling air for 20 min at 303.2 K.

(13) Pomelli, C. S.; Chiappe,C.; Vidis, A.; Laurenczy,G.;Dyson, P. J.Influence of the interaction between hydrogen sulfide and ionic liquidson solubility: experimental and theoretical investigation. J. Phys. Chem.B 2007, 111, 13014–13019.(14) Jalili,A.H.;Rahmati-Rostami,M.;Ghotbi,C.;Hosseini-Jenab,M.;

Ahmadi, A.N. Solubility ofH2S in ionic liquids [bmim][PF6], [bmim][BF4],and [bmim][Tf2N]. J. Chem. Eng. Data 2009, 54, 1844–1849.(15) Rahmati-Rahmati, M.; Ghotbi, C.; Hosseini-Jenab, M.; Ahma-

di, A. N.; Jalili, A. H. Solubility of H2S in ionic liquids [hmim][PF6],[hmim][BF4], and [hmim][Tf2N]. J. Chem. Thermodyn. 2009, 41, 1052–1055.(16) Guo, B.;Duan, E.H.; Ren,A. L.;Wang,Y.; Liu,H.Y. Solubility

of SO2 in caprolactam tetrabutyl ammonium bromide ionic liquidsJ. Chem. Eng. Data 2010, 55, 1398–1401.(17) Yuan, X. L.; Zhang, S. J.; Lu, X. M. Hydroxyl ammonium ionic

liquids: synthesis, properties, and solubility of SO2. J. Chem. Eng. Data2007, 52, 596–599.

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After bubbled air in the IL absorbed H2S, the color of the IL(Figure 2 b) changed to yellowish-green (Figure 2 c). Then,the IL was filtered and vacuum-dried. We get an amount ofyellow powder.

TheXRDpatterns of the yellow powder and Swere shownin Figure 3. The conventional XRD pattern from the yellowpowder sample is consistent with S. Powder was observedwith significant S (23) peak reflection. According to theexpanded (23) XRD reflections, powder was S major. Thisproved that H2S was oxidized into S by O2 in the CPL-TBAB ILs.

Reusability of CPL-TBAB ILs. Both absorption anddesorption of H2S gas in the three examined ILs (the moleratio of CPL and TBAB = 1:1, 3:1, and 5:1) were relativelyfast, providing complete absorption in 1 h with pure H2S gas

(10 mL 3min-1) and complete gas desorption in 50 min at303.2Kand 10.1 kPa. InFigure 4, six consecutive absorptioncycles with the ILs were shown.Using ILsmultiple times hadlittle influence on the solubility of H2S. Moreover, the ILscould be reused six times without any loss of absorptioncapability. Therefore, the ionic liquids have excellent reus-able performance in H2S. Because ionic liquid CPL-TBABitself is of a good thermal stability, it is reasonable thatCPL-TBAB has good reusable performance in absorptionand desorption of H2S gas.

Characterization of CPL-TBAB ILs. The chemical shiftof the CPL-TBAB IL and CPL-TBAB IL saturated withH2S gas in the 1H NMR spectra (Figure 5) shows that thehydrogen bond influence is small and insufficient to demon-strate in the 1H NMR. From these results, it is evident thatthe H2S gas must be physically absorbed in the IL, providingno chemical bond between the IL and H2S.

Conclusions

In summary, the CPL-TBAB (1:1, mole ratio) IL couldphysically absorb large amounts of H2S gas, 5.40% (weightpercent) at 303.15 K and 3.45% (weight percent) at 363.15K.After H2S was absorbed, the color of the IL was changed.When H2S was oxidized, the color of the IL was yellowishgreen. H2Swas oxidized into S byO2. TheH2S absorbed in ILremained in the molecular state without any chemical reac-tion, allowing the IL to be reused six times without loss ofcapability. It is believed that the CPL-TBAB IL may beuseful for H2S removal regarding pollution control. Thisapproach will be studied in future work.

Acknowledgment. This research is supported by the NationalHigh Technology Research andDevelopment Program of China(863 Program, the Project No. 2007AA061702).

Figure 3. XRD spectra of the yellow powder (top) and S standard(bottom).

Figure 4.H2S desorption by heating shown asweight percent ofH2Sto different mole ratios of CPL and TBAB: 9, 1:1; b, 3:1; 2, 5:1.

Figure 5.1HNMR spectra ((CD3)2SO, 500 Hz) of CPL-TBAB ILs

(top) and CPL-TBAB ILs with H2S gas (bottom).