Selective catalytic oxidation of H2S to elemental sulfur

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Selective catalytic oxidation of H2S to elemental sulfur over titanium based TieFe, TieCr and TieZr catalysts H.Mehmet Tasdemir*, Sena Yasyerli, Nail Yasyerli Department of Chemical Engineering, Gazi University, 06570 Ankara, Turkey

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abstract

Article history:

In this study, titanium oxide catalyst was incorporated with iron, chromium and zirconium

Received 23 January 2015

to improve catalytic activity for selective catalytic oxidation of H2S to elemental sulfur.

Received in revised form

Equimolar titanium based iron (TieFe), chromium (TieCr) and zirconium (TieZr) catalysts

8 June 2015

were synthesized by the complexation method and tested in oxidation between the tem-

Accepted 10 June 2015

perature range of 200e300 C and using different O2/H2S ratios. TieFe catalyst with Fe2TiO5

Available online 2 July 2015

crystalline phase and TieCr catalyst with mainly Cr2O3 crystalline phase showed complete conversion of H2S and high sulfur selectivity (close to one) at 250 C. TieZr catalyst having

Keywords:

relatively high surface and small pore diameter could not prevent sulfur deposition on the

Hydrogen sulfide

surface and lost in catalytic activity at the same temperature. TieFe catalyst had high

Selective catalytic oxidation

activity with 100% conversion and sulfur selectivity in the reaction period of an experi-

Elemental sulfur

mental run (150 min) even at lower oxidation temperature (200 C). It was concluded that

Titanium

incorporation of iron into TieFe catalyst structure improved the redox ability and surface

Iron

acidity of the catalyst. Fe2TiO5 mixed metal oxide in the TieFe catalyst was responsible and

Chromium

active phase resulting in complete conversion of H2S and high sulfur selectivity in the selective oxidation of H2S to elemental sulfur. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Due to environmental concerns and economic considerations, sulfur recovery from hydrogen sulfide has attracted significant attention by researchers and fuel producers. High amount of H2S is produced as a side product, in the operation of an integrated gasification combined cycle system (IGCC) power plant as well as during gasification of fossil fuels and also in petroleum refinery. The integrated gasification combined cycle, which is an environmentally clean sustainable technology, involves coal gasifier, acid gas cleanup unit, and

power generation facilities. Sulfur in fuel can be converted to H2S due to reducing atmosphere during coal gasification. Hydrogen sulfide is usually removed from the sour gas by absorption in ammonia, alkanolamine or alkaline salts. The main disadvantages of the removal of H2S by absorption are its relatively high cost and the use of solvent/sorbent [1e3]. Claus process is a well-known process to convert H2S from tail gases to elemental sulfur. There are a number of publications in the literature on the modeling of the Claus process [4,5]. In the study of Jones et al., the modified Claus Process, when part of IGCC power plant, was proposed to destroy ammonia completely and recover sulfur thoroughly from a relatively

* Corresponding author. Tel.: þ90 312 582 3516; fax: þ90 312 230 8434. E-mail addresses: [email protected] (H.Mehmet Tasdemir), [email protected] (S. Yasyerli), [email protected] (N. Yasyerli). http://dx.doi.org/10.1016/j.ijhydene.2015.06.056 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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low purity acid gas stream [6]. In the conventional Claus process, 1/3 of the H2S in the feed gas is burned to SO2 (R1 denoted as thermal oxidation) at high temperatures,

H2S þ 3/2 O2 / SO2 þ H2O

(R1)

SO2 is then reacted with the unburned H2S to produce elemental sulfur at lower temperatures (R2 donated as catalytic step or Claus reaction)

2 H2S þ SO2 / 3/n Sn þ 2 H2O

(R2)

The main disadvantage of Claus process is thermodynamic restriction (97e98%) in the catalytic step (R2). Catalytic selective oxidation of H2S is an attractive alternate to conventional Claus process, to produce elemental sulfur in a single step (R3).

H2S þ 1/2 O2 / 1/nSn þ H2O

(200 C T 350 C)

(R3)

Direct oxidation of H2S (R3) does not have any equilibrium limitations and it is essentially irreversible. Some side reactions may reduce elemental sulfur yield; such as deep oxidation of H2S (R4) and oxidation of produced sulfur (R5).

2 H2S þ 3 O2 / 2 SO2 þ 2 H2O

(R4)

1/n Sn þ O2 / SO2

(R5)

Therefore, development of a selective catalyst for selective oxidation of H2S to elemental sulfur is crucial in producing elemental sulfur with a high yield. A number of investigations have been focused in the literature on the development of active, selective and stable catalyts for catalytic selective oxidation of H2S. Iron-, titanium-, chromium-, and vanadium based catalysts have been reported to have high potential in catalytic oxidation of H2S to produce elemental sulfur [7e10]. Also, catalysts with different types of supports (for example, MCM-41, SBA-15, alumina, actived carbon, alumina intercalated laponite etc.) have been tried to improve catalytic activity in a single step catalytic oxidation of H2S [11e15]. Iron oxide, which is Super-Claus catalyst, is known one of the oldest catalysts tested in oxidation of H2S. Also, its lower cost and availability are advantages of iron oxide catalyst. Additionaly, relatively high activity of iron based catalyst for oxidation of H2S was reported in the literature. However, iron oxide catalyst requires excess amount of oxygen in order to obtain elemental sulfur [16]. Number investigations were tried to modify iron oxide catalyst to achieve high sulfur selectivity. Nguyen et al. studied on thermal conductivity of silicon carbide (b-SiC) supported Fe2O3 catalyst. They have also reported silica supported Fecontaining catalysts [17,18]. Our previous study showed that sulfur selectivity and stability of iron oxide catalyst could be significantly enhanced by the incorporation of cerium into the catalyst structure [16]. Vanadium based catalysts with good

redox properties are also known to be highly active in partial oxidation of hydrocarbon and selective oxidation of hydrogen sulfide. Our earlier studies indicated that oxidation state of vanadium in the catalyst structure had a major role to achieve high sulfur yield. Bimetallic CueV catalysts (Cu/V ratio of 1/1) containing partially reduced vanadium in Vþ4 state were highly selective to elemental sulfur. However, elemental sulfur yield was significantly decreased when catalyst was in Vþ5 state [19,20]. In the study performed by Barba et al., catalytic performances of vanadium-based materials, supported on mixed metal oxides (CeO2, TiO2, CuFe2O4) were investigated for partial selective oxidation of H2S, at low temperatures (50e250 C). In that study, high H2S conversion was obtained with V2O5/CeO2 and V2O5/CuFe2O4 catalysts [21]. In the recent work of Palma and Barba, V2O5/CeO2 catalysts were investigated for low temperature catalytic oxidation of H2S. They proposed possible surface reaction mechanism for H2S oxidation to sulfur [22]. In another work of Tan et al., a series of Mn-substituted LaCrO3 were prepared by self combustion technique and tested as sulfur tolerant anode catalysts in solid oxide fuel cell using fuel gas containing H2S [23]. It was reported by Mobil Oil researchers that TiO2-based catalysts can be used to oxidize H2S to elemental sulfur using stoichiometric amount of O2 in the MODOP (Mobil Direct Oxidation Process). However, this type catalyst was reported to deactivate in the presence of water [17]. In addition to the catalysts mentioned above, chromium oxide based catalysts which were industrially used in dehydrogenation reaction [24,25], may also have some potential in this reaction. In the literature, studies with the titanium based catalysts are quite limited, as compared to iron and vanadium based catalysts, for selective oxidation of H2S to elemental sulfur [8,10]. The objective of the present study is to improve the selectivity and catalytic performance of titanium dioxide based catalysts with the aid of the synergistic effects of bimetallic titanium/iron, titanium/chromium and titanium/ zirconium. New bimetallic catalysts, namely TieFe, TieCr, and TieZr were synthesized by the complexation method and their catalytic performances were investigated in selective catalytic oxidation of H2S to elementel sulfur, in the temperature range of 200e300 C and at different O2/H2S ratios.

Experimental method Catalyst preparation and characterization In this study, titanium based iron (TieFe), chromium (TieCr) and zirconium (TieZr) catalysts, having equimolar ratios were synthesized by the modification of complexation method, which was originally described by Marcilly et al. [26]. In this synthesis method, equimolar ratio of citric acid and metal salt were mixed in a solution, where ethanol (Merck) was used as solvent. Then, this solution was evaporated at 65 C for about 3 h, with continuosly stirring, until its viscosity had noticeable increased. Evaporation time of solution is markedly decreased due to the use of ethanol. In the second step of synthesis, dehydration was completed in an oven at 65 C, by placing the viscous solution as a thin layer in a glass dish. The solid foam formed in this step was then calcined at


550 C for 8 h. In this synthesis route, titanium isopropoxide (C12H28O4Ti, %98, Merck), iron (III) nitrate nanohydrate (Fe (NO3)3.9H2O, %99, Merck), chromium (III) nitrat nanohydrate (Cr (NO3)3.9H2O), %98, Merck), zirkonium (IV) oxide chloride decahydrate ((ZrOCl2.8H2O), %99, Merck) were used as titanium, iron, chromium and zirconium sources, respectively. Citric acid monohydrate was used as the complexation agent. The catalysts synthesized in this work were characterized by X-ray powder diffraction (XRD, Riguka D/MAX 2200 employing a Cu KR radiation source), N2 adsorption-desorption (Quantachrome Autosorbe1), temperature programmed reduction (TPR, Quantachrome Chembet 3000), scanning electron microscopy (SEM, JEOL), energy dispersive X-ray spectroscopy (EDS, JEOL, JSMe6400), and X-ray photoelectron spectroscopy (XPS, SPECS), techniques. Fourier Transform Infrared (FT-IR) analyses of pyridine adsorbed catalysts were performed by using a Perkin Elmer Spectrum One instrument, scanned from 1690 to 1390 cm1. Before this analysis, the samples were dried at 110 C for 12 h. Pyridine adsorbed samples were than kept at 40 C for 2 h before obtaining the FT-IR spectra. Differences of the spectra obtained with and without pyridine adsorbtion were than analyzed for the characterization of the acid sites of the catalysts.

Catalytic tests H2S selective oxidation experiments were carried out in a fixedbed quartz reactor having a 0.6 cm inside diameter. In all experiments, total flow rate of the gas mixture was kept constant at 100 cm3/min (measured at 25 C) and helium was used as the carrier gas. 0.2 g catalyst, which was supported by quartz wool from both sides, was placed into the quartz tubular reactor. H2S concentration was kept as 1%, in all experiments. Experiments were performed with different O2/H2S ratios, ranging between 0 and 2, in a temperature range of 200e300 C. The effluent stream was continuously analyzed by an FT-IR (PerkineElmer Spectrum One, containing a flow gas cell) spectrophotometer, connected online to the exit of the reactor. A sulfur condenser was placed between the reactor and the FT-IR, to collect most of the produced elemental sulfur. The temperature between reactor and sulfur condenser was maintained at about 200 C, to prevent sulfur condensation. Similarly, the temperature between sulfur condenser to FT-IR was adjusted to about 100 C to prevent water condensation. The analysis procedure and the reaction system were detailed in the earlier study of Yasyerli et al. [19]. The conversion of H2S and sulfur selectivity were defined as follows; Conversion of H2 S ð%Þ ¼ Sulfur Selectivity ð%Þ ¼

½H2 Sinlet ½H2 Soutlet 100 ½H2 Sinlet

½H2 Sinlet ½H2 Soutlet ½SO2 outlet 100 ½H2 Sinlet ½H2 Soutlet

Results and discussion Characterization of fresh catalysts Nitrogen adsorption-desorption isotherms of fresh TieFe, TieCr and TieZr catalysts are given in Fig. 1.

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The isotherms of TieFe and TieCr are consistent with type IV according to IUPAC classification, which are typical for mesoporous materials with ordered pore structure. These two isotherms show H1 type adsorption hysteresis [27]. The isotherm of TieZr shows some difference from the other two catalysts. Absence of the major hysteresis loop in the isotherm of TieZr catalyst is an indication of loss of longrange order of mesopores in the synthesized material. Some physical properties of the catalysts prepared by complexation method are given in Table 1. TieZr catalyst has the highest surface area among the others due to its microporous structure and smaller average pore diameter (about 2.4 nm) compared to the other catalysts. XRD patterns of TieFe, TieCr, and TieZr catalysts prepared by the complexation method are shown in Fig. 2. As seen in Fig. 2a, XRD pattern of fresh TieZr catalyst showed a broad peak between 25 and 35 . No sharp peaks were observed corresponding to the Ti and/or Zr oxides. However, as reported by Stefanic et al., x-ray diffraction pattern scanned over the Bragg angle (2q) range from 26 to 38 , should contains the most prominent diffraction lines of m-, t-, c-ZrO2 [29]. This result indicated that metal oxides in the structure of the TieZr catalyst might have small particles (