Continuous process for selective oxidation of H2S over SiC-supported iron catalysts into elemental sulfur above its dewpoint

Applied Catalysis A: General 217 (2001) 205–217

Continuous process for selective oxidation of H2 S over SiC-supported iron catalysts into elemental sulfur above its dewpoint Nicolas Keller, Cuong Pham-Huu, Marc J. Ledoux∗ Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse, UMR 7515 du CNRS, ECPM-ULP, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France Received 21 December 2000; received in revised form 23 March 2001; accepted 26 March 2001

Abstract Iron supported on silicon carbide (SiC) is shown to be a high conversion catalyst for the oxidation of H2 S with a high selectivity into elemental sulfur above sulfur dewpoint in the presence of large amounts of oxygen and steam in the feed. The active phase is probably an iron oxysulfide or a non-stoichiometric sulfate phase. It is stable for several weeks in an industrial micropilot plant, and no deactivation is observed even in the presence of a large amount of steam. These performances are due to the intrinsic physical properties of the SiC carrier but also because of the optimal dispersion of the active phase. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon carbide; Selective oxidation of H2 S; Iron-based catalyst; Iron oxysulfide

1. Introduction Over the last few decades, sulfur recovery from the H2 S-containing acid gases (generated by oil refineries or natural gas plants) has become more and more important due to the ever increasing standards of efficiency required by environmental protection pressures. The general trend is to selectively transform the H2 S into elemental sulfur, which is a valuable product, by the equilibrated Claus process: 2H2 S + SO2 ↔ (3/n)Sn + 2H2 O [1–3]. Thermodynamic limitations of the Claus equilibrium reaction led to the development of new processes to deal with the Claus tail gas, based on the direct oxidation of ∗ Corresponding author. Tel.: +33-390242633; fax: +33-390242674. E-mail address: [email protected] (M.J. Ledoux).

remaining traces (=1 vol.%) of H2 S by oxygen or H2 S absorption/recycling technologies, in order to meet ever stricter legislation requirements. Details concerning all these processes were recently summarized in a series of reviews published in the literature [4–6]. Up to now, two main catalytic processes dealing with the selective oxidation of H2 S by oxygen into elemental sulfur have been developed. The high temperature Superclaus process (Comprimo B.V.), working above the sulfur dewpoint (>180◦ C) with an overall sulfur removal efficiency of 99.5%, is based on Fe and Fe/Cr catalysts supported on alumina or silica [7–9]. Below the sulfur dewpoint, the Doxosulfreen process (Elf-Lurgi) based on Cu catalysts supported on modified alumina and working around 100◦ C in a discontinuous mode of reaction/regeneration reaches efficiencies of 99.9% [10–12].

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 6 0 1 - 9

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Recently, it has been reported by several research groups that high surface area silicon carbide (SiC) could be successfully used in place of the traditional oxidic supports such as alumina for several reactions [13–18]. In previous articles [19–23], it has been reported that the nickel sulfide-based catalyst supported on high surface area SiC was an active and very selective catalyst for the oxidation of H2 S into elemental sulfur at temperatures between 40 and 120◦ C, in the presence of an excess of oxygen and of a high humidity (=20 vol.% of steam). A high oxygen partial pressure was required to deal with the fluctuations of the H2 S flow while the steam was conditioned by the water content of the Claus tail gas. The catalyst remained 100% active and selective even with a sulfur storage of almost 200% in weight and could be totally regenerated by sulfur removal at 250–300◦ C under inert gas (He). It was nevertheless of interest to find new catalysts which could overcome the slight disadvantage of such a discontinuous process involving reaction and regeneration periods, and could also allow a continuous H2 S oxidation over the dewpoint of sulfur. In such conditions, the main problem in the catalytic oxidation of H2 S was linked to the presence of sulfur and water: most of the oxidic supports used and especially alumina reacted with the reactants leading to a decrease in the catalyst performances or even to the destruction of the catalyst (i.e. sulfatation). Furthermore, the formation of hot spots on the catalyst surface, due to the very exothermic nature of the H2 S oxidation (ca. 70◦ C temperature increase per percent of H2 S converted in an adiabatic mode), could lead to a decrease in the selectivity into elemental sulfur by formation of SO2 . The chemical inertness and the high thermal conductivity of SiC could make such a support a promising candidate for the substitution of oxidic supports for over-dewpoint H2 S oxidation reactions. The aim of the present article is to report new results for the selective oxidation of H2 S into elemental sulfur over SiC-supported iron-based catalysts in a continuous mode in the reaction temperature range 210–240◦ C [23,24]. The catalysts before and after reaction were characterized by different techniques, such as powder X-ray diffraction (XRD), surface area measurements, and magnetization measurements. Finally, the nature of the active phase is discussed by comparing the catalytic performances of different iron-based catalysts.

2. Experimental 2.1. Support and catalyst SiC was prepared by the gas–solid reaction between high surface area activated charcoal and SiO vapors under dynamic vacuum according to the shape memory synthesis method developed by Ledoux et al. [13,25]. The support used was in an extrudate form (6 mm length and 3 mm diameter), and detailed studies concerning the preparation and characterization of the material can be found elsewhere [26,27]. The SiC-supported iron catalyst was prepared by incipient wetness impregnation of the dry support with an aqueous solution of Fe(NO3 )3 ·9H2 O (Merck). The resulting material was dried in an oven at 110◦ C for 2 h and then calcined in air at 400◦ C for 2 h in order to decompose the iron nitrate precursor and to form its corresponding oxide. The impregnation was also performed with the iron salt dissolved in a same volume of water containing 20 vol.% of glycerol (Fluka puriss grade). In this case, the material was dried in an oven at 150◦ C for 14 h and then calcined in the same conditions as above. 2.2. H2 S oxidation test under standard conditions Selective oxidation of H2 S was carried out in an apparatus working isothermally at atmospheric pressure presented in Fig. 1. An amount of 3 g (5 ml) of catalyst was placed on silica wool in a Pyrex, heated, fixed-bed reactor (30 mm i.d. and 600 mm height). The gas mixture was passed downwards through the catalyst bed. The reactor was vertically mounted in an electric furnace, and the temperature was controlled by a K-type thermocouple and a Minicor regulator. The flow rate of gases (H2 S and O2 ) was monitored by Tylan FC280A flowmeters linked to a Tylan RC280 control unit. The composition of the reactant feed was H2 S (0.91 vol.%), O2 (2.29 vol.%), H2 O (20 vol.%), and balanced He, corresponding to a O2 /H2 S ratio of 2.5 and a gas hourly space velocity (GHSV) of 1050 h−1 which are typical industrial values. Steam was also present in the feed in a relatively large amount (20 vol.%) in order to accurately reproduce the real industrial reaction conditions (the water formed during the Claus process is not removed and remains in the tail gas). The steam was provided by a

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Fig. 1. Schematic diagram of the apparatus used for the H2 S catalytic oxidation test.

saturator kept at the required temperature. The sulfur produced condensed in the lower part of the reactor and the effluent flow passed through a vessel in which calcium chloride was used as drying agent. All the lines were maintained at 120◦ C with a heating tape to avoid condensation before the chromatographic analysis. Indeed, reactions between the flow and liquid water would lead to a wrong H2 S conversion and a wrong selectivity into sulfur. Analyses of the inlet and outlet gases were performed on-line using a Varian CX-3400 gas chromatograph equipped with a Chrompack JSQ capillary column allowing the separation of O2 , H2 S, H2 O and SO2 , a catharometer detector (TCD) and a calibrated six-port loop (500 ␮l). SO2 was measured in parallel by means of a specific detector (Dräger Model 81-01531) allowing the detection of SO2 in a range of concentrations between 10 and 5000 ppm and previously calibrated using known amounts of SO2 . Before the reaction, the reactor was purged with He

at room temperature until no trace of oxygen could be detected by gas chromatography at the exit of the reactor, then the dry helium flow was replaced by the one containing steam. The catalyst was heated from room temperature to the reaction temperature (heating rate = 10◦ C min−1 ) and the wet helium flow was replaced by the reactant flow. 2.3. Characterization techniques The metal loading, analyzed by atomic absorption spectroscopy (AAS) performed at the Service Central d’Analyse of the CNRS (Vernaison, France), was 5 wt.%. Structural characterization of the samples was done by powder XRD carried out with a Siemens Diffractometer Model D-5000, using a Cu K␣ radiation operated at 40 kV and 20 mA. The measurements were made with long time scan (10 s) and a small step scan (2θ = 0.02◦ ). The mean crystallite size was

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determined from the Scherrer equation with the normal assumption of spherical crystallites. The nature of the crystalline phase in the sample was checked using the database of the Joint Committee on Powder Diffraction Standards (JCPDS). The pore size and surface area measurements were performed on a Coulter SA-3100 porosimeter using N2 as adsorbent. Before each measurement, the sample was transferred into the BET cell via a glove-box under dry nitrogen. The cell was equipped with a greaseless valve in order to avoid air exposure of the sample during the transfer to the porosimeter. SBET is the surface area of the sample calculated from the nitrogen isotherm using the BET method. The desorption branch of the nitrogen isotherm allowed to obtain the pore size distribution following the Barrett et al. method [28] and the surface area of all the pores except micropores (SBJH ). The micropore surface area and volume were calculated using the t-plot method developed by de Boer [29]. A more detailed study has been published by Mikhail et al. [30] concerning the correctness of the different parameters used in the method. The t-plot consists of the analysis of the v l –t plot curve, where v l is the volume of nitrogen adsorbed as liquid at a given pressure P/P0 by the BET surface and t the statistical thickness obtained by dividing the volume of nitrogen adsorbed as liquid at a given pressure P/P0 by the BET surface. The combination of the t-plot and the BJH method for narrow and larger pores allowed an almost complete analysis of the pore volume and the pore surface distributions in the sample studied. Magnetization measurements were also used to characterize the nature and the particle size of the iron phases. Detailed explanations of this technique are reported elsewhere [31], and two types of magnetism can be defined: (1) non-cooperative magnetism in which the molecules have no spontaneous magnetism and therefore do not show any order at long distance, either because the atoms carrying moments are too diluted (paramagnetism, susceptibility χ = dM/dH > 0) or because the electronic configuration of the atoms results in no moment (diamagnetism, χ = dM/dH < 0); (2) cooperative magnetism in which the interactions in the material are strong and sufficient to develop a magnetic order at long distance. In this case, an arrangement of parallel spins in the carrier atoms will be found; these compounds

are ferromagnetic. The magnetization measurements were performed on a Foner magnetometer operating at room temperature.

3. Results and discussion 3.1. Catalyst characteristics 3.1.1. X-ray diffraction The XRD patterns of the support and iron oxide catalysts prepared without glycerol addition are presented in Fig. 2. The XRD pattern of the support only showed diffraction lines corresponding to ␤-SiC, no traces of other compounds such as SiO2 or Si were detected, meaning that such species if present were either in very small crystalline amounts or in a superficial amorphous form which could not be accurately detected by the XRD technique [13,27]. Previous

Fig. 2. XRD patterns of the SiC support (a) and SiC-supported iron oxide catalyst prepared without glycerol addition (b).

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characterization using high-resolution TEM and XPS have shown that the SiC support was partially covered by a thin 1–3 nm thick amorphous layer of SiOx Cy and SiO2 which was not detected by XRD [26,32,33]. The existence of such Si oxycarbide or/and SiO2 phases was in close agreement with the literature [34–38]. This oxidic upperlayer exhibits a totally different behavior as that of the conventional SiO2 material [13]. The influence of the underlayers of SiC are probably responsible for this. On the catalyst prepared by the purely aqueous impregnation method, diffraction lines attributed to the iron oxide in the hematite form were clearly observed (Fig. 2b) and the average particle size calculated using the X-ray line broadening method was about 30 nm. The XRD pattern of the catalyst prepared with a mixture of water and glycerol was the same to that of the fresh support, and no diffraction lines corresponding to the iron oxide phase have ever been detected. It meant that the use of glycerol was responsible for the formation of highly dispersed (<5 nm) or amorphous iron oxide particles on the support which were not detectable by the XRD technique. 3.1.2. Magnetization measurements Fig. 3a shows the ferromagnetic behavior and a magnetism at saturation of 0.06 emu g−1 of the pure SiC support. The presence of iron in the support (0.46 wt.%) coming from impurities in the charcoal and proved by energy dispersive X-ray spectroscopy (EDX) could explain this behavior; both iron oxide and iron carbide being ferromagnetic. The Fe2 O3 (5%)/SiC catalyst showed ferromagnetic behavior, but no saturation was reached for a field of 1.7 T. Iron oxide could be present either in the hematite phase (␣), weakly ferromagnetic (0.5 emu g−1 ) at ambient temperature or in the strongly ferromagnetic maghaemite phase (␥) (74 emu g−1 ). However, the maximum magnetism observed, 0.13 emu (g of material)−1 corresponding to 2.6 emu (g of Fe)−1 without taking into account the weak support magnetization, excluded the sole presence of ␣-Fe2 O3 . In addition, the non-saturation of the magnetization curve was explained by the presence of nanoparticles of ␥-Fe2 O3 . Indeed, the literature reported that if iron oxide particles had a smaller size than 20 nm, they would exhibit superparamagnetic behavior [39–41]. The apparent contradiction between XRD and magnetic

Fig. 3. (a) Magnetic behavior at room temperature of the SiC support (—), the SiC-supported iron oxide catalysts prepared without (䊊) and with glycerol addition (䊉); (b) the enlargement at low field shows the absence of hysteresis with the iron oxide particles obtained with glycerol addition.

measurements could also be attributed to the coexistence of ␥-Fe2 O3 nanoparticles (superparamagnetic behavior) and larger hematite ones (XRD characterization). However, this excluded the presence of amorphous iron oxide or of a small amount of highly dispersed hematite. The use of glycerol in the preparation of the SiC-supported iron oxide catalyst led to an increase in the superparamagnetic behavior of the material (Fig. 3a). Such impregnation method provided a higher dispersion of the iron oxide phase compared to that obtained by using pure water, in good agreement with the XRD diffraction pattern and the lack of any hysteresis behavior (Fig. 3b) [42]. Similar results have been reported by Piéplu [43] who linked the appearance of a superparamagnetic behavior to a decrease in the size of mixed Fe/Cr oxide particles using Mössbauer characterization. A more detailed

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Fig. 4. Pore size distribution and specific surface area of the support and of the SiC-supported iron oxide catalysts.

study will be devoted to the role of glycerol in this impregnation method [44]. 3.1.3. Surface area and pore size measurements Fig. 4 reports the specific surface area and the pore size distribution of the support and the SiC-supported iron oxide catalyst. The Fe2 O3 /SiC catalyst prepared with pure water exhibited the same surface area of 24 m2 g−1 as the sole support. The iron oxide presence led to a slight blockage of small mesopores and to the development of a porosity at around 20–30 nm. The increase in surface area from 24 to 28 m2 g−1 when the impregnation was carried out in the presence of glycerol was in close agreement with the high dispersion of the iron oxide phase on the support, which could induce pore size network formation during the calcination step. 3.2. Influence of the reaction temperature The desulfurization activity and selectivity obtained on the Fe2 O3 (5%)/SiC catalyst as a function of reaction temperature (210 and 240◦ C) and time-on-stream are presented in Fig. 5. At 210◦ C, the catalytic reactivity of the catalyst was not high enough to convert all the H2 S, and after a rapid deactivation during the first hours, the conversion remained stable at 91%. Increasing the temperature up to 240◦ C permitted to reach a conversion of 100% but a small drop in the sulfur selectivity was observed, the selectivity stabilizing at around 90% instead of 96% at 210◦ C. The amount of SO2 observed (ca. 10%) could be due

Fig. 5. H2 S conversion and selectivity into sulfur obtained as a function of time-on-stream at 210◦ C (䊏, 䊐) and 240◦ C (䊉, 䊊) on the Fe2 O3 (5%)/SiC catalyst in the presence of 20 vol.% of steam in the feed.

either to the formation of hot spots on the catalyst surface during H2 S oxidation or simply by the too high reaction temperature which thermodynamically favors extensive oxidation into SO2 . However, hot spots are not expected to occur on the SiC-supported catalyst because of the high thermal conductivity of the SiC support which rapidly disperses the heat through the whole surface of the catalyst, contrary to classical isolating supports such as alumina, silica, and other oxides, i.e. TiO2 , CrO3 . Such a positive effect of the high thermal conductivity of the support on the performances of SiC-supported catalysts has already been reported in the literature in the case of very exothermic reactions [45,46]. The SO2 formation in this temperature range, compared to the elemental sulfur selectivity obtained on similar catalysts at lower temperature reported in previous work, could be attributed to the increase in the rate of sulfur oxidation with increasing reaction temperature. The activation energy for sulfur oxidation (125 kJ mol−1 ; [47–49]) is much higher than that for H2 S oxidation (40 kJ mol−1 ; [49–57]). It was also significant to note that the steady state obtained on the Fe2 O3 /SiC catalyst was reached only after several hours on stream, corresponding at both temperatures to similar sulfur yields of 90% at 240◦ C and 88% at 210◦ C. Such a phenomenon could be attributed to the modification of the catalyst surface during the course of the reaction, i.e. oxysulfide formation by reaction between oxygen of the starting oxide and H2 S in the feed. The nature of the active phase formed

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be explained by the competitive adsorption of steam with sulfur on the catalyst surface. The competition on the active sites between steam and sulfur produced by the reaction, limits the sulfur adsorption and consequently diminishes its oxidation into SO2 according to the following reaction (2): S + O2 → SO2

Fig. 6. H2 S conversion and selectivity into sulfur obtained as a function of time-on-stream at 240◦ C in the presence of 0 vol.% (䊉, 䊊) and 20 vol.% (䊏, 䊐) of steam in the feed on the Fe2 O3 (5%)/SiC catalyst.

is discussed in Section 3.6 using different starting iron phases, i.e. sulfide and sulfate. 3.3. Influence of steam A strong resistance to steam should be exhibited by catalysts used for H2 S oxidation, since the acidic gas containing the H2 S released by the Claus process usually contains a high concentration of water (=20 vol.%). Fig. 6 shows the influence of steam on the catalytic performances of the Fe2 O3 (5%)/SiC catalyst at 240◦ C. The catalytic activity remained unchanged whatever the presence or absence of steam, meaning that water was not involved in an active manner in the catalytic activity itself, but the selectivity into sulfur was slightly decreased from 90 to 82% in the absence of steam. It was expected that steam would have no effect on the performances of the catalyst, because of the chemical inertness of SiC used as support and the absence of any basic sites at its surface. The literature reports that the presence of water at these temperatures leads to the setting of the Claus equilibrium catalyzed by basic sites [58–62]: 2H2 S + SO2 ↔ 3S + 2H2 O

(2)

On the other hand, the competitive adsorption of steam and H2 S could also block the superficial formation of a less-selective iron sulfide phase. This competitive adsorption of steam and H2 S has already been reported by several authors in the literature in the study of the oxidation of H2 S into elemental sulfur over sulfur dewpoint temperatures [43,63,64]. Piéplu explained the loss in sulfur selectivity observed on bulk iron or mixed iron-chromium oxide catalysts in the absence of steam in the feed by the formation of an iron sulfide phase. This sulfidation can also lead to the deactivation of the catalyst by the sintering of the active phase during the oxide to sulfur transformation [65,66]. 3.4. Influence of the O2 :H2 S ratio and of the GHSV Fig. 7 shows the influence of the O2 :H2 S ratio on the H2 S conversion and selectivity into sulfur obtained on the Fe2 O3 (5%)/SiC catalyst at 240◦ C. An excess of oxygen led to a slight decrease in the sulfur selectivity from 90% (O2 :H2 S = 1.5) to 85% (O2 :H2 S = 12.5), while the H2 S conversion remained complete. The loss in conversion (90%) when the reaction occurred

(1)

A rapid and strong deactivation of the TiO2 /SiO2 catalyst with a decrease in selectivity in the presence of water has been reported by Chun et al. [62]. This was attributed to the retro-Claus reaction on the basic sites of the catalyst. The positive effect of steam on the selectivity into sulfur observed in this work could

Fig. 7. H2 S conversion (䊉) and selectivity into sulfur (䊊) obtained at 240◦ C on the Fe2 O3 (5%)/SiC catalyst as a function of the O2 :H2 S ratio in the presence of 20 vol.% of steam in the feed.

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in stoichiometric conditions was remarkable. The decrease in selectivity when increasing the O2 :H2 S ratio could be attributed to the SO2 formation by the consecutive (2) and/or parallel (3) reactions with excess of oxygen: H2 S + 23 O2 → SO2 + H2 O

(3)

Nevertheless, the SiC-supported iron oxide catalyst exhibited a strong versatility according to the oxygen stoichiometry even at high excess of oxygen compared to other catalysts; this was certainly due to the absence of any basic sites on the catalyst surface and of any microporosity. Chun et al. [62] explained the total loss in selectivity on a TiO2 /SiO2 -based catalyst when the O2 :H2 S ratio was increased from stoichiometry to 2 by the presence of these basic sites. Berben and Geus [67,68] have proved that the decrease in selectivity due to excess of oxygen on alumina-based catalysts was directly linked to the presence of microporosity and to the basicity of the support. The selectivity loss could also be due to a temperature increase inside the catalyst bed at high O2 :H2 S ratio. Different explanations can be produced to explain the slight decrease in H2 S conversion observed in stoichiometric conditions. Firstly, the dissociation or adsorption of oxygen molecules could be weaker than that of H2 S, rendering necessary to work under an excess of oxygen. Secondly, the reaction of a small fraction of oxygen with sulfur or H2 S to form SO2 according to the reactions (2) and (3) can lead to a depletion of oxygen at the surface, limiting the H2 S conversion. This hypothesis has been suggested by Berben to explain similar results on a Fe2 O3 /␣-Al2 O3 catalyst [67]. The author also reported that the lack of oxygen gives a more reductor nature to the feed, leading to the sulfidation of the oxide phase. Such a phenomenon probably did not occur in this study due to the lower reaction temperature (240◦ C compared to 300◦ C in the work of Berben). Thirdly, it has been reported in previous work [22,69] that the active phase particles were probably located on the hydrophilic parts of the support in the pores of the catalyst, because the precursor salt was impregnated using an aqueous solution. The H2 S conversion in stoichiometric conditions could thus be limited by the diffusion of the reactants into the pore network of the support. This explanation would be consistent with the influence of the GHSV on the selectivity into elemental sulfur.

Fig. 8. H2 S conversion (䊉, 䊊) and selectivity into sulfur (䊏, 䊐) obtained at 240◦ C in the presence of 20 vol.% of steam in the feed on the Fe2 O3 (5%)/SiC according to the GHSV.

Fig. 8 shows the performances obtained at 240◦ C on the Fe2 O3 (5%)/SiC catalyst as a function of the GHSV. The increase in the GHSV from 1050 to 1500 h−1 significantly improved the selectivity into sulfur from 90 to 95%, while the H2 S conversion remained complete, leading to the obtention of a sulfur yield of 95%. Such result confirmed that SO2 was mainly produced according to the successive reaction between sulfur and oxygen (2) and not directly from H2 S. At high GHSV, i.e. low contact time, it was expected that sulfur, rapidly formed, escaped from the catalyst bed into the gas phase without any readsorption and consecutive reaction with the excess of oxygen, leading to the formation of SO2 . This was well observed at higher reaction temperatures, i.e. 300◦ C (not reported). The very low acidity of SiC compared to traditional oxidic supports could also explain the high sulfur selectivity obtained [70]. Terörde et al. [71] reported that the formation of sulfur radicals which can react with molecular oxygen could probably be attributed to the occurrence of highly acidic sites on the silica surface under the reaction conditions. The authors related that addition of basic sodium diminished the formation of sulfur radicals. 3.5. Influence of the active phase dispersion It is well known in catalysis that both overall activity and selectivity can be significantly influenced by the dispersion of the active phase. In this section, the influence of the starting oxide phase dispersion

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limited by the H2 S conversion at 210◦ C as is the case in the traditionally impregnated catalyst. 3.6. The nature of the active phase

Fig. 9. H2 S conversion (䊉, 䊊) and selectivity into sulfur (䊏, 䊐) obtained at 210◦ C in the presence of 20 vol.% of steam in the feed on the Fe2 O3 (5%)/SiC catalyst as a function of time-on-stream and of the impregnation media.

will be investigated and discussed. Desulfurization experiments were carried out over two iron oxidebased catalysts prepared with and without glycerol in the mother aqueous solution (see Section 3.1). The characterizations of both catalysts are also reported. The influence of the impregnation mode on the performances of the Fe2 O3 (5%)/SiC catalyst at a temperature of 210◦ C is reported in the Fig. 9. Total conversion was observed on the well-dispersed (ex glycerol-water) catalyst, while only 90% of conversion was reached on the conventional catalyst (ex pure water). The increase in the iron oxide dispersion via the mixed glycerol/water impregnation solution allowed to diminish the reaction temperature from 240 to 210◦ C keeping the H2 S conversion at 100%. van den Brink et al. [72] have related that H2 S conversion and selectivity into sulfur obtained on iron-based catalysts were closely dependent on the iron oxide particle size distribution. Similar results have been reported by Li et al. [73] on bulk Fe2 O3 catalysts. However, the high dispersion of the active phase led to a decrease in the sulfur selectivity from 95 to 85%. Such a decrease was explained by the successive oxidation of sulfur by the excess of oxygen due to the presence of a higher concentration of active sites because of the higher dispersion. These results confirmed the essential role played by the re-oxidation of elemental sulfur in the selectivity control above dewpoint temperatures. In addition, the use of glycerol led to a displacement of the temperature domain of total conversion, the yield into sulfur not being

The XRD pattern of the Fe2 O3 /SiC catalyst (aqueous impregnation) after reaction at 240◦ C only showed diffraction lines corresponding to the SiC support (not reported), the iron oxide lines having disappeared. Thus, it seemed clear that the starting iron oxide phase was modified at 240◦ C in the presence of the reactant feed during the first few hours on stream before the stabilization of the catalyst. The absence of diffraction lines corresponding to oxide, sulfide, or sulfate indicated that these phases, if present on the catalyst, were small particles and/or in a superficial amorphous form. The catalyst surface area and the pore size distribution remained almost unchanged, which indicated that no trace sulfur was trapped in the catalyst pores. In order to obtain more insight about the nature of the active phase, different starting iron phases supported on SiC were tested, i.e. iron oxide (Fe2 O3 ), iron sulfide (FeS2 ), and iron sulfate (FeII SO4 ). The FeS2 /SiC catalyst was obtained by sulfidation of Fe2 O3 (5%)/SiC by reaction with a H2 S/He flow at 300◦ C. This material could be subsequently calcined in air at 400◦ C for 2 h in order to form FeII SO4 . The obtention of both sulfur-containing phases supported on SiC was in good agreement with the literature [74,75]; detailed characterizations are reported elsewhere [69]. The desulfurization results obtained at 240◦ C in the presence of 20 vol.% of steam over these different catalysts are presented in Fig. 10. Both iron sulfideand iron oxide-based catalysts exhibited a H2 S conversion of 100% at the beginning of the test, while two catalytic regimes were observed on the FeSO4 /SiC catalyst. After a first H2 S adsorption period, the catalyst showed a long activating period before reaching a steady state at almost the same H2 S conversion as that obtained on the other two catalysts. The low selectivity into sulfur obtained on the FeS2 /SiC catalyst (60%) was in good agreement with the work of Pieplu [43] who observed a decrease in selectivity after the sulfidation of bulk iron oxide. The author also reported a significant deactivation of the catalyst, probably due to the sintering of bulk Fe2 O3 during the oxide → sulfur transformation detailed in the literature [65,66]. The

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formation essentially occurred on the topmost layers of the iron particles. The transformation on stream of the iron oxide phase, bulk or supported, has already been reported in the literature [43,60,63,64,67,68]. The poor sulfur yield obtained on the FeS2 catalyst excluded this phase as the final active phase in the reaction conditions used. The poor initial conversion on FeSO4 also excluded this phase as the active one. The active phase for the desulfurization reaction was probably an iron phase containing both sulfur and oxygen and not a pure oxide, sulfide nor sulfate species. van den Brink [60] showed that Fe2 III O3 supported on silica was transformed as a function of time-on-stream into a FeII SO4 phase via an iron-III intermediate observed by the Mössbauer technique. Thermodynamic calculations and magnetic studies allowed the authors to propose the following sequence during the transformation of Fe2 III O3 into FeII SO4 , according to the work of Bristoti et al. [76,77] and Kayo et al.[78]: Fe2 III O3 → Fe2 III O3 · xSO3 (x < 2) → FeII SO4

Fig. 10. Influence of the nature of the supported phase on the H2 S conversion (a) and selectivity into sulfur (b) obtained at 240◦ C in the presence of 20 vol.% of steam in the feed on the SiC-supported iron-based catalysts as a function of time-on-stream (Fe2 O3 (䊉); FeS2 (䊊); FeSO4 (䊏)).

use of a supported active phase allowed in the present study to preserve the required number of active sites to avoid any loss in activity of the sulfide catalyst. The selectivities into sulfur observed on these iron-based catalysts also exhibited modification before reaching their steady states (Fig. 10b). Among them, the iron sulfate phase exhibited the highest sulfur selectivity 96% instead of 90% for iron oxide and 60% for iron sulfide. These results indicated that the best phase in terms of yield (conversion times selectivity) was formed when starting from the iron sulfate phase. Magnetization measurements carried out on these different catalysts after test showed almost similar signatures, which indicated that the bulk structure was probably not modified during the tests, meaning that the starting iron phases were only superficially modified by the reactant feeds during the course of the reaction and the trans-

It is possible to imagine that this intermediate iron-III phase is stabilized on stream during the Fe2 O3 transformation in our reaction conditions. The formation of what can be called an oxysulfide or a non-stoichiometric sulfate phase required the diffusion of hexavalent sulfur ions and of O2− ions in the supported phase. The diffusions were reported to be the rate-limiting step during the Shrinking Core Model transformations, in which the sulfur ion diffused through the first topmost layers of the new formed phase [79,80]. The transformation of a hematite Fe2 O3 particle can lead to a large increase in the particle volume, i.e. 280% v/v, due to the diffusion of both ions. Such dilatation could explain the redispersion of the supported phase and/or the formation of an amorphous or badly crystallized phase, neither of which is detectable by the XRD technique. The absence of any modification in the magnetization behavior of the catalyst after test could also be due to the conservation of an iron oxidation number of III in this new supported phase. A similar hypothesis has been reported by van den Brink [60] in order to explain the high absorption of their catalysts by the Mössbauer technique. As mentioned above, the results obtained on the FeII SO4 /SiC showed that the new phase Fe2 III O3 · xSO3 (x < 2) was not transformed on stream into the corresponding iron sulfate. The FeSO4 catalyst

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required a long activating period before presenting high performances. The iron sulfate chemistry has been reported by several authors in the literature, which evidenced some contradictions [60,81–84]. It could be suggested that the activating period observed when starting from FeSO4 consisted of a reduction on stream of the sulfated phase by H2 S. The H2 S adsorbed during the first hours of reaction can initiate the superficial reduction of the FeSO4 phase. The reduction of the sulfate phase on stream has been proposed by Piéplu who has reported that pre-sulfated bulk Fe2 O3 showed an activating period [43]. It is also significant to note that the activation was longer when starting from iron sulfate as precursor than when starting from iron oxide, in similar reaction conditions. Such a phenomenon could be explained by the fact that sulfate species were more difficult to reduce by H2 S than the oxide species. Finally, the FeS2 phase was not able to be transformed into the active and selective iron oxysulfide under the reaction conditions, because high selectivity was never obtained in this case. Work is ongoing in order to explain in more detail why such an oxysulfide or a non-stoichiometric sulfate phase may be stabilized on stream in our reaction conditions. It has been proposed that the use of SiC as support, especially because of its chemical inertness, may allow the stabilization of such a phase. A similar role of SiC has been evidenced in the isomerization of light n-alkanes over MoO3 -supported catalysts [85]. The small interaction between MoO3 and the SiC support led to the formation, on stream, of a very active and selective oxycarbide phase, whereas the same oxide supported on high surface area ␥-Al2 O3 showed lower performances: strong interactions between the oxide and the alumina support inhibited the transformation of MoO3 into the real active phase, which resulted in an inactive catalyst for isomerization. A complete study has been devoted by Geus et al. [61] to the role of the oxidic support in the desulfurization of H2 S and led to the conclusion that the obtention of high performances was directly linked to the nature of the oxidic support. It has been recently reported by Li and Chien [86] that the nature of the interactions between the supported phase (vanadium or mixed V/Sb oxides) and the oxidic carrier (␥-Al2 O3 , ZrO2 , and TiO2 ) could significantly modify the H2 S conversion and the selectivity into sulfur. Recent work based on

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the XPS technique again confirmed the existence of a superficial iron oxysulfide phase; this will be published later. It should be mentioned that the existence of a SiOx Sy underlayer was not observed by XPS analysis. If such a phase was present and active, the blanck test performed on the support alone (SiOx Cy /SiC) should show some activity; this was never found.

4. Conclusion SiC-supported iron-based catalysts exhibited both a high H2 S conversion and a high selectivity into elemental sulfur above sulfur dewpoint in the presence of a large amount of oxygen and steam in the feed. The catalytic results have shown that the starting iron phase (oxide or sulfate) was subsequently modified under the reactant mixture, probably into an iron oxysulfide or a non-stoichiometric sulfate phase, very active and selective for H2 S oxidation. The SiC-supported iron-based catalyst also exhibited a high stability as a function of time-on-stream during several weeks in an industrial pilot plant, and no deactivation was ever observed even in the presence of a large amount of steam. The SiC support material provided the following advantages when compared to classical oxidic or carboneous supports: (i) high thermal conductivity which is useful in an adiabatic mode; (ii) the presence of a large fraction of macropores; (iii) the absence of any acidic and/or basic sites which can induce secondary reactions; (iv) chemical inertness preventing reactions between the reactants, the products and/or the active phase with the support. The optimal dispersion of the active particles could explain the high activity of the catalyst, small enough to present the required number of active sites and to diminish sulfur readsorption leading to SO2 formation, not large enough to be too reactive and less selective. Indeed, the preparation of a highly dispersed catalyst using glycerol as visquous agent allowed to significantly improve the activity, but led to a detrimental formation of SO2 . The high selectivity into elemental sulfur was attributed to the absence of microporosity, generally harmful to the selectivity at over-dewpoint temperatures. It is too early to announce the exact stoichiometry of the iron active phase due to the presence of both oxidant and reductor gases in the processing feed and

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