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Oxidative absorption of hydrogen sulfide by iron-containing ionic liquids Jianhong Wang, and Weidong Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef500527w • Publication Date (Web): 25 Aug 2014 Downloaded from http://pubs.acs.org on August 26, 2014
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Oxidative absorption of hydrogen sulfide by iron-containing ionic liquids Jianhong Wang, Weidong Zhang Beijing Key Laboratory of Membrane Science and Technology, State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing100029, People's Republic of China
ABSTRACT: Hydrogen sulfide was oxidatively absorbed by iron-containing ionic liquids which were synthesized in different molar ratios of FeCl3•6H2O to 1-butyl-3-methylimidazolium chloride. It was found 1-butyl-3-methylimidazolium tetrachloroferrate ([bmim]Fe(III)Cl4), being highly reactive toward H2S, was the dominating iron(III) species independent of molar ratios, but the concentration of iron and chloride as well as the acidic strength lowered with the decrease of molar ratio from 2 to 0.3. Moreover, the acidity and iron(III) concentration had important influence on oxidation of H2S, but their significance varied with the temperature. HCl emissions were also discovered in the oxidation of H2S to S8 and the reduction of [bmim]Fe(III)Cl4 to [bmim]Fe(II)Cl4H. A way for controlling HCl emissions was proposed, involving the decrease in the chloride concentration of iron-containing ionic liquids in presence of water.
1. INTRODUCTION Hydrogen sulfide mainly comes from natural gas plants, petroleum refinery and coal gasification. Because of its toxic and corrosive characters, hydrogen sulfide should be Received:
March 06 , 2014
Revised:
August
Published:
19 , 2014 , 2014 1
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removed down to very low concentration levels. Among various approaches developed for H2S removal, liquid phase oxidation process using an iron(III) chelate catalytic solution (LO-CAT) has remarkable advantages such as converting H2S direct to elemental sulfur at alkali pH and high H2S removal efficiencies.1,2 However, there are some disadvantages associated with the commercial use of LO-CAT, including: (1) iron(III) chelate degradation, (2) side reaction resulting in formation of sulfur oxo-acid salt, (3) economic advantages only for 850-1050 kg/h of sulfur production due to low concentrations of the ferric complex in solution.1-4 For these reasons, 1-butyl-3-methylimidazolium tetrachloroferrate ([bmim]Fe(III)Cl4 ) ionic liquid has been proposed for the oxidation of H2S.5 The potential of [bmim]Fe(III)Cl4 for replacing the alkali iron(III) chelate in the catalytic oxidation of H2S is attributed to its good gas solubility and efficient catalytic activity. [bmim]Fe(III)Cl4 is one of magnetic ionic liquids with a very low volatility, superior hydrophobicity, low viscosity and low surface tension,6-8 which makes it suitable for gas separation. The substitution of conventional solvents by [bmim]Fe(III)Cl4 in gas absorption is one of the active research areas today.9 Jiang et al. first reported a gram of [bmim]Fe(III)Cl4 could absorb 0.68 g benzene emissions.10 Albo et al. then indicated [bmim]Fe(III)Cl4 could recover CO2 from a gas stream.11 Furthermore, a growing number of measurements reporting catalyst of [bmim]Fe(III)Cl4 in various reactions have become available. Bica et al. reported [bmim]Fe(III)Cl4 was a recyclable catalyst.12 Nguyen et al. then demonstrated [bmim]Fe(III)Cl4 was highly effective for the dimerization.13 Afterwards, Wang et al. pointed out [bmim]Fe(III)Cl4 exhibited higher catalytic activity for the glycolysis of PET.14 More recent studies by Gao et al. showed iron(III)-containing
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ionic liquid was a recyclable and efficient heterogeneous catalyst for converting CO2 into cyclic carbonate.15 However, so far, there has been a little work published about use of [bmim]Fe(III)Cl4 as both a solution and a regenerable catalyst for the conversion of H2S to S. In this catalytic oxidation process, [bmim]Fe(III)Cl4 was firstly reduced by H2S and then regenerated by aeration or electrolysis in the same or a separate vessel. He et al firstly investigated the effects of the inlet H2S concentration (142816-285714 mg/m3), the gas containing H2S flow rate (30-60 mL/min), the O2 flow rate (10-70 mL/min) and temperature (30-90℃) on the catalytic oxidation of H2S by [bmim]Fe(III)Cl4.5 Yao et al further found the regeneration rate of [bmim]Fe(III)Cl4 by air or oxygen was very slow without mass transfer equipment.16 The remaining studies have been conducted in our laboratory, which demonstrated that [bmim]Fe(III)Cl4 could be reused four times without any loss of H2S oxidative-absorption capability.17 The above researches give a clear conclusion. The catalytic oxidation of H2S by acidic [bmim]Fe(III)Cl4 has high H2S removal efficiency (over 99%),5 good regeneration due to the formation of Fe-Cl complex, no side reaction which can’t take place in an acid environment,18 and no degradation without water.1 Thus, the loss of ionic liquids mainly came from the separation of sulfur. Considering a loss of 0.5%, the cost of [bmim]Cl (22000$/t quoted by Shanghai Cheng Jie Chemical Co.LTD) during separation of sulfur (10-15%), was approximated at 32-51$ t-1 H2S removed. The market value of sulfur was higher than 167 $ per ton. So, it could be seen that the catalytic oxidation of H2S by [bmim]Fe(III)Cl4 was the process that generated profit. In contrast, the cost of chemicals in LO-CAT process was about 206 $ t-1 H2S removed.19 However, Kazemi et al showed
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LO-CAT was the best process for natural gas sweetening among Sulfinol-M, LO-CAT, shell and mixed amine processes when capital and operating costs were taken into account.20 Since the equipment applied in the catalytic oxidation of H2S by [bmim]Fe(III)Cl4 are similar to that in LO-CAT, their capital costs are the same, but the catalytic oxidation of H2S by [bmim]Fe(III)Cl4 has lower operating costs than LO-CAT. Consequently, the catalytic oxidation of H2S by [bmim]Fe(III)Cl4 is more economical than other commercial processes, but it still has many unsolved problems. For example, it was well-known that solution pH and concentration of iron(III) were widely recognized as key variables for H2S oxidation processes using iron(III) solutions.2,21-24 However, to our knowledge, the effects of acidity and iron(III) concentration on the oxidation of H2S using [bmim]Fe(III)Cl4 have never previously been reported. Moreover, acidity of aqueous solutions was adjusted by adding acid or alkali， unfortunately, which was not applicable to alter the acidity of ionic liquids. In contrast, Lewis acidic strength of [bmim]Cl(1-butyl-3-methylimidazolium chloride) -AlCl3 and [bmim]Cl-ZnCl2 ionic liquids could be modified by varying the molar fraction of AlCl3 or ZnCl2 to [bmim]Cl.25 Accordingly, in this paper, we tentatively altered acidity and iron(III) concentration by varying molar ratios of FeCl3•6H2O to [bmim]Cl in the synthesis of iron(III)-containing ionic liquids. The effects of the molar ratio on properties of iron(III)-containing ionic liquids and the concentration of absorbed H2S in iron(III)-containing ionic liquids were also described. 2. EXPERIMENTAL METHODS 2.1 Materials. [bmim]Cl (purity>99.0%) was obtained from the Henan Lihua Pharmaceutical Co., Ltd and used as received. FeCl3•6H2O (AR grade reagent) was
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purchased from Tianjin Fu Chen Chemical Reagent Factory and used without further purification. H2S with a purity of 99.9% was supplied by Beijing Bei temperature gas plant. 2.2 Synthetic procedure of iron(III)-containing ionic liquids. Iron(III)-containing ionic liquids were synthesized in M=0.3, 0.4, 05, 0, 7, 1 and 2, respectively, where M was the molar ratio of FeCl3•6H2O to [bmim]Cl. One hydrophilic iron(III)-containing ionic liquid at M
