Ethyl

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Experimental Investigation of the Interaction of Dimethyl Sulfide/ Ethyl Mercaptan with Nano-Manganese Dioxide Jie He,* Xiaotian Liu, Lin Li, Bin Wang, and Jinsong Hu School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, P. R. China ABSTRACT: Nano-MnO2 was prepared by a sol−gel technology, and the structure was characterized by means of powder Xray diffraction (XRD). The interaction of dimethyl sulfide (DMS) or ethyl mercaptan (EM) with the as-prepared MnO2 was investigated at ambient temperature with Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and laser Raman spectroscopy (LRS). The results showed that DMS and EM undergo different interactions with the surface of MnO2. The interaction of DMS and MnO2 bring about carbon−sulfur bond cleavage and the oxidation of C. However, there might be hydrogen bonding between EM and MnO2 in addition to chemical bonding, which results in a weaker S−H bond in the EM molecule, so that EM is easily oxidized to sulfonates and sulfates. Tentative interaction mechanisms for DMS and EM on MnO2 are suggested.

1. INTRODUCTION Environmental protection and sustainable economic development are the consensus of the international community today. Fuel cells are efficient energy-conversion devices, and their efficiency is not restricted by Carnot’s law. However, hydrocarbons as fuels for fuel cells usually contain sulfur compounds, including inorganic sulfur (such as H2S, COS, and CS2) and organic sulfur (such as sulfide, mercaptan, and thiophene). To ensure the catalytic stability of the anode catalysts, these sulfur compounds must be removed from fuels, and their concentrations must be lower than 0.1 ppmv, or even 20 ppbv. Inorganic sulfides are usually removed by adsorption methods. Organic sulfides can be removed by adsorption,1−6 hydrodesulfurization (HDS),7−11 photocatalytic oxidization,12,13 and selective catalytic oxidation.14 Adsorption desulfurization is considered to be a promising new approach for deep desulfurization of hydrocarbon fuels. In comparison with conventional HDS, adsorption can be carried out under milder conditions. There has been an intense worldwide search for better adsorbents during the past few years. Several adsorbents, such as zeolites, transition-metal oxides, mixedmetal oxides, and activated carbon (AC), have often been used in deep desulfurization.15−17 Important findings are that manganese-based adsorbents still have high efficiency and mechanical strength upon recycling and, under an oxidative atmosphere, the used adsorbents can also be easily regenerated.18 Pure MnO2 as an adsorbent has a high adsorption rate and high adsorption capacity of hydrogen sulfide,17,19 whereas it shows distinctly different adsorption characteristics of organic sulfur compounds. Compared with bulk materials, nanocrystalline metal oxide adsorbents have several advantages, including high surface area, high atom utilization, low diffusion resistance, and high pore volume.4,20 Therefore, research on the adsorption mechanisms of typical sulfides on nano-MnO2 adsorbent is very important for the development of efficient adsorption processes based on nano-MnO2 adsorbent. © 2012 American Chemical Society

Dimethyl sulfide (DMS) and ethyl mercaptan (EM) are typical sulfides with low contents in fuels. However, DMS and EM at very low concentration can emit an offensive odor. Moreover, among sulfur compounds, DMS is the most difficult to remove, although it is the major odorant found in all pipeline natural gas.2 Therefore, DMS and EM are often used as model compounds to study the desulfurization mechanism and process. However, to date, reports on the interaction between DMS or EM and nano-MnO2 are relatively rare, especially those on the adsorption and conversion mechanism. Therefore, it is important to study the interaction mechanism between DMS or EM and nano-MnO2, which is useful for the selective removal of related species from mixtures of mercaptan and sulfur ether. In this work, using prepared nano-MnO2 adsorbent, the adsorption states of DMS and EM have been studied by several spectra methods. Based on the experimental results combined with theoretical calculations, possible adsorption mechanisms of DMS and EM have been explored.

2. EXPERIMENTAL SECTION 2.1. Reagents and Physical Measurements. All chemicals were of reagent-grade quality from commercial sources and were used without further purification. X-ray diffraction (XRD) patterns were recorded using a XD-3 powder diffractometer (Beijing Purkinje General Instrument Co., Ltd.) with graphite-monochromatized Cu Kα radiation (λ = 0.15406 nm) operating at 40 kV and 30 mA at room temperature. Scanning electron microscopy (SEM) images of the nanoMnO2 adsorbent were obtained on a field-emission scanning electron micro-analyzer (Hitachi S-4800), employing an accelerating voltage of 10 kV. Fourier transform infrared (FTIR) spectroscopy of the samples dispersed in KBr were Received: Revised: Accepted: Published: 15912

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collected on a Bruker Vector 33 FT-IR spectrophotometer [deuterated triglycine sulfate (DTGS) detector] with a resolution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) measurements were performed with a JEOL JPS-9010 spectrometer using Mg Kα 1,2 radiation (1253.6 eV). The sample chamber vacuum was better than 2 × 10−6 Pa. Laser Raman spectrocopy (LRS) with a resolution of 1 cm−1 was performed at room temperature using a Via-Reflex confocal Raman microscope from Renishaw. All Raman spectra were excited by 532-nm laser. A laser power of about 1 mW was used. 2.2. Preparation of Nano-MnO2. Nano-MnO2 was prepared with a sol−gel method using manganese acetate as the precursor. A typical synthetic procedure was as follows: Mn(OOCCH3)4·4H2O (5.5 g), citrate (C6H8O7·H2O; 9.6 g), and (NH2)2CO (4.7 g) were dissolved in water with stirring. After being aged at 353 K for 50 h, the gel was dried at 383 K for 12 h. The powder was ground and calcined at 653 K for 10 h in air and then treated with 2 mol·L−1 aqueous sulfuric acid solution for 2 h at ambient temperature. The product was washed with distilled water and dried in air at 353 K for 12 h. 2.3. Adsorption Experiments. Adsorption features of the as-prepared samples for dimethyl sulfide (DMS) and ethyl mercaptan (EM) were evaluated under static conditions at room temperature. A 50-mg sample was placed at the center of the reactor, which consisted of a horizontal cylindrical quartz tube with a diameter of 48 mm and a length of 130 mm, and DMS (or EM) at an initial partial pressure of about 1.43 kPa was added. The surface-adsorbed species were analyzed by means of infrared spectroscopy and X-ray photoelectron spectroscopy.

Figure 2. SEM image of as-prepared nano-MnO2.

interplanar distance of 0.400 nm. Figure 2 shows that rodshaped nanocrystals of MnO2 with a uniform particle size distribution were synthesized successfully by a sol−gel technology. 3.2. Adsorption and Oxidation Characteristics of DMS and EM on MnO2. The adsorption features of DMS and EM on the as-prepared adsorbent were evaluated through the vibration spectra of species on the samples by FT-IR spectroscopy. The results are shown in Figures 3 and 4. As shown in Figure 3c, the IR spectrum of MnO2 after the adsorption of DMS can be confirmed by the occurrence of vibration peaks at 1418 and 1281.5 cm−1, which can be attributed to the CH2−S−C deformation vibration and the CH2−S wagging vibration,21 respectively. The peaks at 2924.5, 2854.1, and 1457.9 cm−1 can be assigned to the −CH2 asymmetric stretching, symmetric stretching, and asymmetric bending vibrations, respectively. In addition, the band at 1710.6 cm−1 is probably due to the CO stretching vibration. The peak at 1116.6 cm−1 can be attributed to the C−O−C stretching vibration. The two peaks at 1710.6 and 1116.6 cm−1 are not present in Figure 3b. These results indicate that DMS adsorbed on the surface of as-prepared MnO2 might be partially oxidized to formiate. Figure 4c includes peaks at 2921.6, 2851.2, and 1247.7 cm−1 that correspond to the −CH2 stretching and CH2−S wagging vibrations of adsorbed EM. The bands at 1165.8 and 1042.3 cm−1 can be attributed to the SO stretching vibration and the OSO symmetric stretching vibration,21 respectively, which are not present in the IR spectrum of pure EM. These results show that EM was oxidized to sulfonate, sulfate, and sulfonic acid (−SO3H) adsorbed on the surface of the as-prepared MnO2. 3.3. Interaction between Sulfides and the MnO2 Surface. To further explore the adsorption features of DMS and EM on the surface of nano-MnO2, XPS technology was used to characterize the surface species on MnO2 after adsorption of DMS or EM. The results are shown in Figures 5 and 6. The XPS spectra of Mn species are shown in Figure 7. As shown in Figure 5, for MnO2 with DMS adsorption, no significant S 2p peak was observed, indicating that the surface retention of the S components was below the detection limit under a high vacuum. This is consistent with the results of Figure 3. However, upon adsorption EM, an S 2p peak was observed. Fitting the peak yielded two peaks with binding energies of 167.5 and 168.8 eV. These results reveal that two

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology of the AsPrepared MnO2. The crystal structure of the as-prepared sample was characterized by powder X-ray diffraction (XRD), and the corresponding results are shown in Figure 1. An SEM image of the sample is presented in Figure 2.

Figure 1. XRD pattern of as-prepared nano-MnO2.

It can be seen that the as-synthesized MnO2 shows diffraction peaks at 2θ = 22.18°, 36.96°, 56.08°, 42.42°, 34.96°, and 38.64° that can be indexed to orthorhombic-phase MnO2 (JCPDS 44-0142). No other crystalline phase was observed in the pattern, indicating that the MnO2 was prepared as a pure phase. The main diffraction line for MnO2 is at 2θ = 22.18°, which corresponds to the (101) plane, giving an 15913

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Figure 3. FT-IR spectra of (a) MnO2, (b) DMS, and (c) MnO2 after adsorption of DMS.

Figure 4. FT-IR spectra of (a) MnO2, (b) EM, and (c) MnO2 after adsorption of EM.

Figure 5. S 2p XPS spectra of samples after the adsorption of (a) DMS and (b) EM.

Figure 6. C 1s XPS spectra of samples after the adsorption of (a) DMS and (b) EM. 15914

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Figure 7. Mn 2p XPS spectra of (a) fresh MnO2, (b) MnO2 after the adsorption of DMS, and (c) MnO2 after the adsorption of EM.

Figure 8. Raman spectra of (a) fresh MnO2, (b) MnO2 after the adsorption of DMS, and (c) MnO2 after the adsorption of EM.

forms of S species were present on the as-prepared MnO2, which can be attributed to sulfur sulfonate (S 2p at 167.5 eV)22 and sulfate (S 2p at 168.8 eV).23 However, a S 2p peak corresponding to the S−H group was not observed, which could be because adsorbed EM was removed from the MnO2 surface under a vacuum, indicating that the interaction between the EM molecule itself and MnO2 is not very strong. Figure 6 shows the C 1s lines of the adsorbed species. In the case of DMS, the signals of the ester (−CO2−) carbons and the likely relative shakeup satellite peaks of unsaturated functionalities were found at 291.7 and 294.4 eV,24 respectively, but were not observed upon adsorption of EM. These results reveal that DMS might be oxidized to an ester, which is in accordance with the IR spectral analyses of Figure 3. In the case of fresh MnO2, shown in Figure 7a, the Mn 2p3/2 peak can be observed at 642.1 eV, which corresponds to the binding energy of Mn4+−O.25 After adsorption of DMS, the binding energy (642.2 eV) did not change significantly, indicating that the oxidation state of manganese did not change. However, after the adsorption of EM, the width of the Mn 2p 3/2 peak clearly increased. As reported in the literature,26,27 the binding energies of Mn 2p3/2 for different valence states of manganese oxide lie within a narrow range, which causes difficulty in unambiguous identification. Therefore, we believe that Mn 2p3/2 peak broadening might be because the sample after adsorption of EM contains Mn atoms in different oxidation states. To further explore the difference in the interactions of DMS and EM with the MnO2 surface, the Raman spectra of the asprepared samples were investigated before and after adsorption of DMS or EM, and the results are shown in Figure 8. As shown in Figure 8, the Raman spectrum of fresh MnO2 features two main contributions at 643.5 and 359.3 cm−1, which can be attributed to the stretching and bending vibrations, respectively, of Mn−O bonds.28−31 After adsorption of DMS, a red shift was observed corresponding to the stretching and bending vibrations of MnO2. However, the red shift of the stretching vibration band was more obvious in the case of adsorption of EM, indicating that a greater interaction exists between MnO2 and adsorbed EM. In addition, in the case of adsorption of EM, Figure 8c also displays one sharp peak at 658.4 cm−1 along with three small bands recorded at 473.7, 373.7, and 319.4 cm−1, in agreement with the Raman results of Mn3O4 reported in the literature.29,32 These bands can be induced by electron transfer occurring between MnO2 and the

adsorbed species. The results reveal that there is a significant difference between the interactions of dimethyl sulfide and ethyl mercaptan with manganese dioxide. 3.4. Mechanisms of Interaction of MnO2 with DMS and EM. From thermodynamic considerations, the bond energies of C−S and S−H are 272 and 347 kJ·mol−1, respectively, so C−S bonds are easier to break and oxidize further than S−H bonds are. According to the preceding discussion, we know that both of the bonds were broken. DMS and EM are isomers, but an obvious difference in their structures leads to different interactions between the MnO2 surface and the sulfides, as shown. With density functional theory at the B3LYP level, the charge densities corresponding to atoms in DMS and EM were calculated for the optimized structures, and the results are shown in Figure 9. It can be observed that there is a significant difference in the atomic charge densities on S in DMS and EM. In DMS, S has a positive density of ca. 0.0653, whereas in EM, the value is ca. −0.1096. However, the C atoms have a more negative charge density in DMS than in EM. Combined with the experimental XPS and FT-IR results, it is believed that the interaction between DMS and MnO2 causes C−S bond cleavage. Meanwhile, the C atom is oxidized to ester. However, the positive charge density on S in DMS makes it difficult to transfer an electron to Mn4+ and be oxidized. EM adsorption is different. In addition to van der Waals attractive forces, there is an additional interaction caused by hydrogen bonds between EM and MnO 2 . Tentative adsorption mechanisms are suggested and shown in Figure 10. According to the tentative adsorption mechanisms, we suggest that the interaction between −CH3 (δ−) in DMS and Mn (δ+) in MnO2 leads to the oxidation of the C. At the same time, because the S atom in DMS has a positive charge density, it is difficult to oxidize through electron loss. Therefore, oxidation products of S are not observed on the MnO2 surface. However, the case of EM is different. The charge density on S is negative (δ−), so S can easily transfer its electron to Mn (δ+). Meanwhile, the hydrogen bond between the H atom in the S− H bond and the O atom in the Mn−O bond might weaken the S−H bond strength and promote the oxidation of the S−H group in EM. 15915

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Figure 9. Charge densities of DMS and EM. (4) Yang, X. X.; Erickson, L. E.; Hohn, K. L.; Jeevanandam, P.; Klabunde, K. J. Sol−Gel Cu−Al2O3 Adsorbents for Selective Adsorption of Thiophene out of Hydrocarbon. Ind. Eng. Chem. Res. 2006, 45, 6169. (5) Ito, E.; Veen, J. A. R. V. On novel processes for removing sulphur from refinery streams. Catal. Today 2006, 116, 446. (6) Hernández-Maldonado, A. J.; Yang, R. T. Desulfurization of Diesel Fuels by Adsorption via π-Complexation with Vapor-Phase Exchanged Cu(I)−Y Zeolites. J. Am. Chem. Soc. 2004, 126, 992. (7) Babich, I. V.; Moulijn, J. A. Science and technology of novel process for deep desulfurization of oil refinery streams: A review. Fuel 2003, 82, 607. (8) Bejenaru, N.; Lancelot, C.; Blanchard, P.; Lamonier, C.; Rouleau, L.; Payen, E.; Dumeignil, F.; Royer, S. Synthesis, Characterization, and Catalytic Performances of Novel CoMo Hydrodesulfurization Catalysts Supported on Mesoporous Aluminas. Chem. Mater. 2009, 21, 522. (9) Kogan, V. M.; Nikulshin, P. A.; Rozhdestvenskaya, N. N. Evolution and interlayer dynamics of active sites of promoted transition metal sulfide catalysts under hydrodesulfurization conditions. Fuel 2012, 100, 2. (10) Wang, L.; Zhang, Y. N.; Zhang, Y. L.; Liu, P.; Han, H. X.; Yang, M.; Jiang, Z. X.; Li, C. Hydrodesulfurization of 4,6-DMDBT on a multi-metallic sulfide catalyst with layered structure. Appl. Catal. A 2011, 394, 18. (11) Tawara, K.; Nishimura, T.; Iwanami, H.; Nishimoto, T.; Hasuike, T. New Hydrodesulfurization Catalyst for Petroleum-Fed Fuel Cell Vehicles and Cogenerations. Ind. Eng. Chem. Res. 2001, 40, 2367. (12) Na, P.; Zhao, B. L.; Gu, L. Y.; Liu, J.; Na, J. Y. Deep desulfurization of model gasoline over photoirradiated titaniumpillared montmorillonite. J. Phys. Chem. Solids 2009, 70, 1465. (13) Cai, W. M.; Lu, G. H.; He, J.; Lan, Y. X. The adsorption feature and photocatalytic oxidation activity of K1−2xMxTiNbO5 (M = Mn, Ni) for methyl mercaptan in methane. Ceram. Int. 2012, 38, 3167. (14) Lampert, J. Selective catalytic oxidation: A new catalytic approach to the desulfurization of natural gas and liquid petroleum gas for fuel cell reformer applications. J. Power Sources 2004, 131, 27. (15) Shimizu, K. I.; Komai, S. I.; Kojima, T.; Satokawa, S.; Satsuma, A. Mechanism of Adsorptive Removal of tert-Butanethiol under Ambient Conditions with Silver Nitrate Supported on Silica and Silica−Alumina. J. Phys. Chem. C 2007, 111, 3480. (16) Ko, T. H.; Chu, H.; Chaung, L. K. The sorption of hydrogen sulfide from hot syngas by metal oxides over supports. Chemosphere 2005, 58, 467. (17) Satokawa, S.; Kobayashi, Y.; Fujiki, H. Adsorptive removal of dimethylsulfide and t-butylmercaptan from pipeline natural gas fuel on Ag zeolites under ambient conditions. Appl. Catal. B 2005, 56, 51. (18) Alonso, L.; Palacios, J. M. Performance and recovering of a Zndoped manganese oxide as a regenerable sorbent for hot coal gas desulfurization. Energy Fuels 2002, 16, 1550. (19) Bakker, W. J. W.; Kapteijn, F.; Moulijn, J. A. A high capacity manganese-based sorbent for regenerative high temperature desulfurization with direct sulfur production: Conceptual process application to coal gas cleaning. Chem. Eng. J. 2003, 96, 223.

Figure 10. Tentative mechanisms of DMS and EM adsorption on MnO2.

4. CONCLUSIONS Although dimethyl sulfide (DMS) and ethyl mercaptan (EM) are constitutional isomers, there exists an obvious difference in the interactions of the two isomeric sulfides with nano-MnO2. This is mainly because of the significant disparities in the charge density distributions around the S and C atoms, which affect electron transfer and bond rupture in different ways, eventually leading to the results that the adsorption modes and oxidation products on the surface of MnO2 are markedly different.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-554-6668520. E-mail: [email protected]. Author Contributions

The manuscript was written with contributions from all of the authors. All of the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21271008) and the Natural Science Foundation of Anhui Province of China (11040606M38). We thank Professor Siwei Bi (School of Chemistry and Chemical Engineering, Qufu Normal University) for kind assistance with density functional theory calculations.

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ABBREVIATIONS DMS, dimethyl sulfide; EM, ethyl mercaptan; HDS, hydrodesulfurization REFERENCES

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