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Ind. Eng. Chem. Res. 1997, 36, 1480-1484
Catalytic Oxidation of Hydrogen Sulfide to Sulfur on Vanadium Antimonate Kuo-Tseng Li* and Ni-Shen Shyu Department of Chemical Engineering, Tunghai University, Taichung, Taiwan, Republic of China
The catalytic oxidation of hydrogen sulfide to sulfur on three vanadium antimonate catalysts (with bulk V/Sb atomic ratio at 5/1, 1/1, and 1/5) was studied with a flow reactor in the temperature range of 180-280 °C. Strong synergistic phenomena in catalytic activity and selectivity were observed for the vanadium antimonate catalysts. The catalyst with equal vanadium atoms and antimony atoms (abbreviated as VSB11 catalyst) was found to be the most effective, and the X-ray diffractometric data showed that the major phase present in the VSB11 catalyst was rutile VSbO4 which contained antimony in the oxidized state (Sb5+) and vanadium in the reduced state. X-ray photoelectron spectroscopic data suggested that both surface vanadium sites and surface antimony sites were in the reduced state after the oxidation of hydrogen sulfide. Under the condition of dilute H2S and O2/H2S molar ratio g1, the oxidation rate on the VSB11 catalyst was determined to be first order in hydrogen sulfide and zero order in oxygen, which suggested that the reaction followed the redox mechanism and the rate-limiting step was the reduction of the oxidized catalyst (probably VSbO4) by hydrogen sulfide. Introduction
selectivity have been observed for the binary oxide catalysts of vanadium-antimony.
Conversion of H2S to elemental sulfur is presently considered the most effective means to remove the pollutant from petroleum refinery gases and from natural gases. In sulfur recovery plants, the oxidation of hydrogen sulfide by sulfur dioxide (obtained from incomplete combustion of hydrogen sulfide) to form elemental sulfur and water (called the Claus process) is incomplete due to the thermodynamics limitation. Therefore, tail gas treating (TGT) units are required for the sulfur recovery plants, and amines are usually used in TGT to absorb the remaining hydrogen sulfide in the Claus tail gas. Recently, a new tail gas treating system (Lagas et al., 1988; Therorde et al., 1993; Van Nisselrooy and Lagas, 1993) has been developed (called SuperClaus) which uses an R-alumina-supported iron oxide/ chromium oxide catalyst to catalyze the reaction between hydrogen sulfide and oxygen to form elemental sulfur. The reaction is irreversible; therefore, maximum sulfur recovery can be achieved. Several catalysts have been employed for catalyzing the reaction between hydrogen sulfide and oxygen to form elemental sulfur, including iron (Thomas, 1970), iron-chromium (Lagas et al., 1988), binary oxides of vanadium-bismuth, vanadium-molybdenum, vanadium-magnesium (Li et al., 1996), and bismuth molybdate (Li and Cheng, 1996). Vanadium antimonate catalysts have been used for catalyzing the ammoxidation of propane or propylene to acrylonitrile (Catani et al., 1992; Nilsson et al., 1994a,b; Anderson et al., 1994; Centi and Perathoner, 1995; Nilsson et al., 1996). However, no literature reported the use of vanadium antimonate catalysts for catalyzing the reaction between hydrogen sulfide and oxygen to form elemental sulfur. The present work deals with the use of vanadium antimonate catalysts for catalyzing the oxidation of hydrogen sulfide to sulfur, from which strong synergistic phenomena in both catalytic activity and catalytic * To whom correspondence should be addressed. FAX: 8864-3590009. S0888-5885(96)00459-9 CCC: $14.00
Experimental Section Catalyst Samples. The binary oxides of vanadiumantimony with five different atomic ratios (V/Sb ) 1/0, 5/1, 1/1, 1/5, and 0/1) were prepared by mixing antimony(III) oxide Sb2O3 (Janssen Chimica, Beerse, Belgium) with ammonium vanadate (Showa Chemicals, Tokyo, Japan) in a 500 mL 0.1 N oxalic acid solution (Janssen Chimica, Belgium) at 80 °C, followed by evaporation, drying at 100 °C for 72 h, and then calcination at 500 °C for 8 h. The catalysts were then crushed and 20-40 mesh particles were screened for catalytic studies. The specific surface areas of the catalyst samples were determined with a Micromeritics BET surface analyzer (Model Gemini). The phase analysis was performed by X-ray diffraction crystallography (XRD) (Shimadzu XD5), and the microscopic aspect of the catalysts was examined under the scanning electron microscopy (SEM) (Banseh & Lomb ARL. Nonol 2100). The X-ray photoelectron spectroscopy (XPS) spectra were acquired on a ESCA 210 VZ spectrometer (Fission Surface Science) using Mg KR X-ray radiation. Apparatus and Procedure. The catalytic reaction between hydrogen sulfide and oxygen was carried out in a tubular reactor. The reactor was a Pyrex tube with 0.007 m inside diameter, and the weight of catalyst packing was 0.2 g unless specified otherwise. The packing was supported on quartz wool. Above the catalyst packing, there was a preheating zone packed with quartz chips. The reactor was heated externally with a tube furnace. Before the measurements of catalytic properties, catalysts were presulfurized in an environment of 15 vol % hydrogen sulfide, 25 vol % oxygen, and 60 vol % nitrogen at 250 °C for 8 h. After presulfurization, reactor tempeature was decreased to 180 °C and a gaseous feed consisting of 1 vol % hydrogen sulfide, 5 vol% oxygen, and 94 vol % nitrogen was introduced into the top of the reactor. The flow rate of the gaseous feed was 200 mL/min and the catalyst contact time was about 0.03 s (reactor temperature was © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1481 Table 1. Catalytic Properties of Vanadium Antimonate Catalysts for Selective Oxidation of Hydrogen Sulfidea temp (°C) 180 conv. (%) select. (%) 200 conv. (%) select. (%) 220 conv. (%) select. (%) 240 conv. (%) select. (%) 260 conv. (%) select. (%) 280 conv. (%) select. (%) BET surface area (m2/g) fresh catalyst spent catalyst
V/Sb ) 1/0
V/Sb ) 5/1
V/Sb ) 1/1
V/Sb ) 1/5
V/Sb ) 0/1
35.1 100
39.0 100
71.1 100
31.4 100
74.3 92.3
70.4 94.8
90.8 100
54.2 100
85.2 82.7
94.1 91.4
99.2 100
75.0 100
4.3 100
93.9 70.9
96.3 77.1
100 100
89.7 100
5.1 100
100 51.8
100 72.1
100 91.3
91.3 100
7.0 100
100 35.1
100 58.1
100 86.2
93.0 97.6
16.7 100
1.44 2.47
11.97 8.39
3.05 0.42
0.13
0.61 0.03
a Reaction conditions: gas flow rate ) 200 mL/min with 1 vol % hydrogen sulfide, catalyst weight ) 0.2 g. Conv. represents the conversion of hydrogen sulfide; select. represents sulfur selectivity.
in the range between 180-280 °C). The reaction product exiting the reactor was introduced into a gassolid separator in which the reaction product was separated into a solid sulfur product and a gaseous product. The data at 180 °C were taken at 12 h after the presulfurization stage, and the data at 280 °C were taken at 22 h after presulfurization. After 30 h on stream (including the 8 h presulfurization stage), the spent catalyst was taken for analysis (including the analysis of surface area, SEM, and XPS). Gas product analyses were performed with a gas chromatograph using a 9-m-long Porapak Q column. The conversion of hydrogen sulfide and the sulfur selectivity were calculated using the following equations:
conversion (%) ) (moles of hydrogen sulfide reacted)/ (moles of hydrogen sulfide fed) × 100% sulfur selectivity ) (moles of hydrogen sulfide reacted moles of sulfur dioxide produced)/ (moles of hydrogen sulfide reacted) × 100% Sulfur balance was not conducted because the amount of sulfur produced was too little to be recovered completely. Results and Discussion Oxidation of Hydrogen Sulfide. Five vanadium/ antimony atomic ratios were studied for the binary oxide catalysts, including V/Sb ) 1/0,5/1,1/1,1/5,and 0/1. Table 1 presents the performances of these catalysts for the selective oxidation of hydrogen sulfide when the gas flow rate was 200 mL/min and the catalyst weight was 0.2 g. At 180 °C, the conversion of hydrogen sulfide for the binary oxide catalyst with V/Sb atomic ratio ) 1/1 (abbreviated as VSB11) was 71.1%, which was much higher than catalysts with other V/Sb ratios (conversion was 35.1% for single vanadium oxide, 39.0% for V/Sb ) 5/1, 31.4% for V/Sb ) 1/5, and negligible for single antimony oxide). The low activity of the single anti-
Figure 1. Sulfur yield as a function of reaction temperature.
mony oxide should be due to the fact that Sb2O4 could not be reoxidized by gaseous oxygen to Sb5+ oxide (Centi and Perathoner, 1995). However, the addition of a small amount of vanadium oxide into antimony oxide dramatically increased the catalyst activity (hydrogen sulfide conversion increased from 0 to 31.4% when the V/Sb atomic ratio was changed from 0/1 to 1/5). The role of vanadium was probably to enhance the rate of reoxidation of reduced antimony by forming VSbO4 rutile phase (Centi and Perathoner, 1995) and therefore increased the rate of hydrogen sulfide oxidation. XRD patterns (given in Figures 2 and 3) showed that VSB11 catalyst had the highest concentration of VSbO4 rutile phase; therefore, the VSB11 catalyst had the highest activity among the five catalysts studied in Table 1. Figure 1 shows the sulfur yield as a function of reaction temperature for four catalysts listed in Table 1 (single antimony oxide was eliminated due to its low activity). The maximum sulfur yield obtained for the binary oxide catalysts with V/Sb ) 5/1, 1/1, and 1/5 was 86.1%, 100%, and 92%, respectively. The maximum sulfur yield obtained with single vanadium oxide was
1482 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997
Figure 2. X-ray diffractometry patterns of fresh catalysts with V/Sb ) (A) 1/0, (B) 5/1, (C) 1/1, (D) 1/5, and (E) 0/1. Letters a, b, and c represent the characteristic peaks of Sb 2O4, SbVO4, and V2O5, respectively.
Figure 3. X-ray diffractometry patterns of spent catalysts with V/Sb ) (A) 1/0, (B) 5/1, (C) 1/1, (D) 1/5, and (E) 0/1. Letters a, b, and c represent the characteristic peaks of Sb 2O4, SbVO4, and V2O5, respectively.
only 70.65%. Therefore, the sulfur yield of the vanadium antimonate catalysts was significantly better than that of the corresponding single oxide catalysts. The results suggest that the mixture of vanadium oxide and antimony oxide exhibits strong synergistic behavior in the catalytic performances for hydrogen sulfide selective oxidation and the catalyst with V/Sb ) 1/1 had the best catalytic activity and selectivity. The last rows of Table 1 show the specific surface areas obtained for the fresh catalysts (after calcination) and for the spent catalysts (after the oxidation reaction). The surface areas of the three vanadium antimonate catalysts decreased significantly after the oxidation of hydrogen sulfide, and the surface area of the spent catalyst with V/Sb ) 1/5 was too small to be measured by the surface area analyzer. Scanning electron microscopy of the VSB11 catalyst showed that the grain size and the pore size of the spent catalyst were significantly larger than those of the fresh catalyst. The increase of grain size and pore size should be the reason for the significant decrease of surface area after the reaction. The surface area of the spent VSB11 catalyst was only 0.42 m2/g, which was much smaller than the surface areas of the catalysts with V/Sb ) 5/1 and 1/0. The low surface area of the spent VSB11 catalyst indicates that the high activity of the VSb11 catalyst was not due to its surface area. The cause for the dramatic increase of the VSB11 catalyst activity should be due to the formation of the new VSbO4 rutile phase. XRD Studies. The X-ray diffractometric patterns of five vanadium-antimony catalysts are presented in Figure 2 (fresh catalyst) and in Figure 3 (spent catalyst). Figure 2 shows that the major component in the VSB11 catalyst was VSbO4 and the minor component was Sb2O4. In addition, the two figures show that the concentration of VSbO4 in VSB11 catalyst was much higher that the VSbO4 concentration in the catalyst with V/Sb ) 5/1 and 1/5. The results in Table 1 and in Figures 2 and 3 suggest that the compound VSbO4 is a much better catalyst than single vanadium oxide and single antimony oxide for the selective oxidation of hydrogen sulfide. The coexistence of VSbO4 and Sb2O4 in the VSB11 catalyst and in the catalyst with V/Sb ) 1/5 might be part of the reasons for causing the synergy effect in hydrogen sulfide oxidation because Nilsson et
al. (1994a) proposed that the migration of antimony over SbVO4, modifying its surface, most probably caused the synergy effect for propane ammoxidation. In addition, the coexistence of Sb2O4 with SbVO4 could decrease the extent of surface reduction (Centi and Perathoner, 1995). After the use of catalysts in hydrogen sulfide oxidation, the characteristic lines from crystal V2O5 disappeared (which can be seen from the comparisons between XRD patterns 2A and 3A), while the characteristic lines from VSbO4 and Sb2O4 remained, but the intensity of VSbO4 and Sb2O4 characteristic peaks decreased significantly after reaction (which can be seen from the comparisons between XRD patterns 2C and 3C, 2D and 3D, and 2E and 3E). The absence of XRD bands for V2O5 suggests that vanadia was reduced to an amorphous structure during H2S oxidation. The decrease of VSbO4 peak intensity for the spent catalyst suggests that some antimony sites in SbVO4 (mostly in the surface region) were reduced during the oxidation of hydrogen sulfide. The reduction of antimony sites in VSbO4 caused the destruction of VSbO4 phase and decreased the VSbO4 peak intensity because the formation of the VSbO4 phase requires the presence of antimony in the oxidized state (Sb5+) and vanadium in the reduced state (Centi and Perathoner, 1995). XPS Studies. Figure 4 shows the XPS spectra recorded from the used VSB11 catalyst. The binding energies for the Sb(3d3/2) and V(2p3/2) peaks were 540.08 and 516.68 eV, respectively, which were similar to those obtained by Anderson et al. (1994) (their Sb(3d3/2) binding energy was 540.4 eV, and their V(2p3/2) binding energy was 516.5 eV). Since the binding energies for V5+, V3+, Sb5+, and Sb 3+ were 517.9, 516.1, 540.8, and 540 eV, respectively (Berry and Brett, 1984; Anderson et al., 1994), the binding energies obtained here for the used catalyst suggested that Sb5+ was reduced during the oxidation of hydrogen sulfide and that vanadium was in the reduced state. Since the reduced state of vanadium was already present in VSbO4 (Centi and Perathoner, 1995), antimony sites in VSbO4 should be responsible for the oxidation of hydrogen sulfide. The surface Sb/V atomic ratio of the used VSB11 catalyst was determined using XPS data from the Sb(3d3/2) and V(2p3/2) peaks. The surface atomic ratio
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1483
Figure 5. First-order plot for the mixed-oxide catalyst with V/Sb ) 1/1 at 180 °C.
Figure 4. XPS spectra of (A) Sb(3d3/2) and (B) V(2p3/2) for the used VSB11 catalyst.
Sb/V was found to be 2.52, which was significantly higher than the bulk Sb/V atomic ratio ()1). The results suggest that antimony had a stronger tendency to stay on the catalyst surface than vanadium. Our results are in agreement with the results obtained by Bower et al. (1996). They studied the ammoxidation of propane to acrylonitrile on FeSbO4, and they found that the surface Sb/Fe atomic ratio was 4 when the bulk Sb/ Fe atomic ratio was 1 and the surface Sb/Fe atomic ratio was 8 when the bulk Sb/Fe atomic ratio was 2. XPS spectra also indicated that sulfur existed on the surface of the used VSB11 catalyst. The surface S/V atomic ratio was determined to be 0.31 using the XPS data from the V(2p3/2) and S(2p) peaks (the binding energy of S(2p) peak was 169.8 eV). Kinetics and Mechanism. It was desired to establish a rate equation and determine the kinetic parameters for the catalytic oxidation of H2S on the VSB11 catalyst. To determine the rate dependence on the H2S concentration, the inlet concentration was kept at 1 vol % H2S, 5 vol % oxygen, and 94 vol % nitrogen, and the concentration of oxygen could be assumed constant during the reaction because of a large excess of oxygen compared to the stoichiometric value (the stoichiometric ratio of O2/H2S is 0.5). The following differential equation was established to describe the reaction system in a plug flow reactor by assuming a pseudo-first-order equation for H2S:
dX/d(W/FAo) ) kCA
(1)
Integration of eq 1 yields
-ln(1 - X) ) kCAoW/FAo
(2)
The H2S dependence was established by varying the total inlet gaseous flow rate between 150 and 300 mL/ min STP, and the catalyst weight was kept at 0.1 g so that the values of W/FAo in eq 2 was varied between 816.6 and 1633.3 (g of cat‚min/mol H2S fed). Experi-
Figure 6. Arrhenius plot for the mixed-oxide catalyst with V/Sb ) 1/1.
mental results were plotted according to eq 2, and a straight line passing through zero was obtained, as illustrated in Figure 5. Therefore, under the condition of dilute H2S, the rate of H2S oxidation to sulfur is firstorder with respect to H2S concentration. For determining the effect of oxygen, the feed concentration was kept at 1 vol % H2S, 1 vol % oxygen, and 98 vol % nitrogen. It was observed that the change of oxygen concentration from 5 to 1 vol % caused little change in the H2S conversion. Therefore, the reaction rate is independent of the concentration of oxygen, and the rate is zero-order with respect to oxygen concentration when O2/H2S molar ratio g 1. The reaction rates measured here were not affected by internal mass transfer (i.e., pore diffusion) because the calculated values of (rs)2(-rH2S)Fp/ (DeCA) were less than 1 (Smith, 1981). The rate equation can therefore be written as -rH2S ) kCH2S with k ) A exp(-E/RT). An Arrhenius plot of ln k vs 1000/T is shown in Figure 6, from which the activation energy was evaluated as 53 KJ/mol. Van den Brink et al. (1992) mentioned that the activation energy for the catalytic oxidation of H2S to sulfur was about 60 kJ/mol, which is similar to the one we obtain here.
1484 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997
The kinetics results obtained above suggest that the selective oxidation of hydrogen sulfide over the catalyst follows a reduction-oxidation cycle and the reduction of the oxide catalyst by hydrogen sulfide is the ratelimiting step. The zero-order dependency on oxygen concentration suggested that the rate of the reoxidation of the reduced VSB11 was very fast compared to the reduction of the oxide catalyst. However, Van den Brink et al. (1992) studied the catalytic oxidation of H2S to sulfur over supported iron sulfate catalysts and found that the rate of the reoxidation of the catalyst mainly determines the activity of the catalysts. The difference between our results and Van den Brink et al. results suggest that the rate-limiting step for hydrogen sulfide oxidation strongly depends on the catalyst types. For VSB11 catalyst, vanadium significantly enhanced the rate of reoxidation of reduced antimony by forming VSbO4 rutile phase (Centi and Perathoner, 1995); therefore, the rate of the reoxidation of the catalyst became much faster than the rate of the reduction of the catalyst in oxidized form by hydrogen sulfide. Conclusions Vanadium antimonate catalysts have been studied for the selective oxidation of hydrogen sulfide to sulfur. Synergistic phenomena in catalytic performances were observed for the binary oxides of vanadium-antimony, and the best catalytic performance was observed for the catalyst with V/Sb atomic ratio ) 1. X-ray diffractometry study suggested that the synergistic phenomena were due to the formation of a new compound of VSbO4 in the binary oxides. X-ray photoelectron spectroscopic study of the catalyst with V/Sb ) 1/1 suggested that the antimony site in VSbO4 was responsible for the oxidation of hydrogen sulfide. Kinetic study (under the condition of dilute H2S and O2/H2S molar ratio g1) indicated that the selective oxidation on the vanadiumantimony catalyst with V/Sb ) 1/1 was first-order in hydrogen sulfide and zero-order in oxygen, which suggested that the reaction followed the redox mechanism and the reaction between hydrogen sulfide and the catalyst in its oxidized form was the rate-limiting step. Acknowledgment We gratefully acknowledge the National Science Council of the Republic of China for financial support (Grant No. NSC-84-2214-E-029-002). Nomenclature CA ) concentration of H2S, mol/cm3 De ) effective pore diffusivity, cm2/min E ) activation energy, J/mol FAo ) molar flow rate of H2S in feed, mol/min k ) rate constant, cm3/(g of cat‚min) -rH2S ) reaction rate of hydrogen sulfide, mol/(g of cat‚min) rS ) radius of catalyst particle, cm
R ) gas constant, J/(mol‚K) T ) reaction temperature, K W ) amount of catalyst in the reactor, g X ) conversion of hydrogen sulfide Fp ) catalyst particle density, g/cm3
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Received for review July 30, 1996 Revised manuscript received January 13, 1997 Accepted January 27, 1997X IE960459V
X Abstract published in Advance ACS Abstracts, March 15, 1997.
