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Selective Oxidation of Hydrogen Sulfide to Sulfur on Vanadium-Based Catalysts Containing Tin and Antimony Kuo-Tseng Li* and Kuo-Shan Wu Department of Chemical Engineering, Tunghai University, Taichung, Taiwan, Republic of China
Oxidation of hydrogen sulfide to sulfur was investigated over a series of mixed-metal oxide catalysts containing vanadium, tin, and antimony. These catalysts were prepared by a coprecipitation method. V/Sn/Sb ternary oxides exhibited 100% sulfur yield over a wide range of temperature (180-240 °C when weight hourly space velocity was 0.0245 mol of H2S/g of catalyst per h), which was superior to the binary oxides of V/Sn, V/Sb, and Sn/Sb. In addition, V/Sn binary oxides had a better sulfur yield than the corresponding single oxides. For V/Sn and V/Sn/Sb catalysts, Brunauer-Emmett-Teller measurements indicated that catalyst surface areas increased linearly with the Sn/V atomic ratio and temperature-programmed reduction studies showed that the reducibility of VOx species increased with the tin content. These two factors should contribute to the better catalytic behaviors of the V/Sn and V/Sn/Sb catalysts in H2S oxidation. The presence of 30 vol % water vapor decreased the catalyst activity but had little effect on the maximum sulfur yield. Results of X-ray diffraction and scanning electron microscopy suggested that the ternary oxide catalysts were stable during the reaction. Introduction Hydrogen sulfide is a byproduct of many industrial operations, such as hydrodesulfurization of crude oil, coal, and natural gas.1 It is usually converted to elemental sulfur in sulfur-recovery plants or so-called Claus plants. Because of thermodynamic limitation, the sulfur recovery offered by a Claus process is limited in practice to about 97%. Because of the strict air pollution regulations, a variety of Claus tail gas treatment (TGT) processes have been developed to increase the total sulfur-recovery efficiency.2 Conventional Claus TGT processes are wet processes, in which a tail gas containing unreacted hydrogen sulfide is absorbed by an alkaline liquid solvent. Removing the last percentages of H2S by means of these wet Claus TGT processes is relatively expensive. Two dry Claus TGT processes have been developed to avoid the shortcoming of the wet types of Claus TGT processes. They are Mobil’s direct-oxidation process developed by Mobil AG Co. in Germany3 and a superClaus process developed by Comprimo Co. in The Netherlands.4 Both comprise a step of recovering elemental sulfur from the Claus tail gas by selective oxidation of hydrogen sulfide using the following catalytic reaction:
H2S + 1/2O2 f (1/n)Sn + H2O (n ) 6-8)
(1)
The catalyst used by Comprimo’s super-Claus process is R-alumina-supported iron oxide/chromium oxide, and the catalyst used by Mobil’s direct-oxidation process is a TiO2-based catalyst. Research continues to obtain a more active catalyst and to exclude chromium oxide as an additive in the active phase because of the toxic nature of chromium-containing materials.5,6 Reaction (1) is irreversible, so the maximum recovery of hydrogen sulfide can be obtained. However, SO2 can * To whom correspondence should be addressed. Fax: +8864-3590009. E-mail:
[email protected].
also be generated simultaneously because of the following side reactions:7
H2S + 3/2O2 f SO2 + H2O
(2)
(1/n)Sn + O2 f SO2
(3)
(3/n)Sn + 2H2O S 2H2S + SO2
(4)
Therefore, an optimum catalyst should be able to maximize the sulfur yield and to minimize the sulfur dioxide generation. Earlier work from our laboratory showed that vanadium-antimony binary oxides had a better sulfur yield than vanadium oxide alone for the selective oxidation of hydrogen sulfide to sulfur.8,9 However, vanadiumantimony binary oxides had narrow operation temperature windows because their sulfur selectivities were sensitive to temperature change. It is desired to develop catalysts with a wide range of operation temperatures because reaction (1) is highly exothermic with a heat of -48.4 kcal/mol at 25 °C.10 In this work, selective oxidation of hydrogen sulfide to sulfur was carried out over a series of V-Sn-Sb ternary oxides and V-Sn binary oxides with various Sn/V atomic ratios. It was found that V-Sn-Sb catalysts had 100% sulfur yield in a wide range of temperatures, which was much better than binary oxide catalysts and single oxide catalysts. Experimental Section Catalyst Preparation. Six vanadium-tin-antimony ternary oxides (V/Sn/Sb atomic ratio ) 1/n/1, where n ) 0.3, 0.5, 0.7, 0.85, 1, and 1.5) and four vanadium-tin binary oxides (Sn/V atomic ratio ) 0.3, 0.7, 1, and 1.5) were prepared by a coprecipitation method from an ethanolic phase.11,12 The main preparation steps were as follows: appropriate amounts of SnCl4, vanadyl(IV) acetylacetonate, and SbCl5 (all sup-
10.1021/ie0007015 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/30/2001
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plied by ACROS Chemicals, USA) were mixed in ethanol. The above solution was added dropwise into a 2 M CH3COONH4 solution (Showa Chemicals, Japan), and precipitation occurred while the solution pH value was maintained at 5-7.5 with an ammonia solution. The precipitates were washed with deionized water and dried at 120 °C overnight. The obtained solids were then heated at 350 °C for 1 h and finally calcined at 700 °C for 3 h. For comparison, two binary oxide samples (V/ Sb atomic ratio ) 1/1 and Sb/Sn atomic ratio ) 1/1) and two single oxide samples (Sb2O4 and SnO2) were also prepared by the same procedure as that listed above. V2O5 was obtained from Riedel-deHaen, Sleeze, Germany. All of the oxides were screened, and 20-40 mesh particles were used for catalytic studies. Catalyst Characterization. Catalyst phase analysis was performed by X-ray diffraction (XRD) crystallography on a Shimadzu XD-5 diffractometer with Cu KR radiation. The specific surface areas of the catalyst samples were determined by nitrogen adsorption with a Micromeritics Brunauer-Emmett-Teller (BET) surface area analyzer (model Gemini). The catalyst reducibility was studied with a temperature-programmed reduction (TPR) method, which was conducted using 0.15 g of catalyst in a stream of 10% hydrogen in argon and with a heating rate of 10 °C/min. Microscopic aspects of the catalysts were examined under a scanning electron microscope (SEM; Bausch and Lomb ARL Nonol 2100). Reaction Studies. Selective oxidation of hydrogen sulfide was carried out in a continuous flow reactor (made of Pyrex glass) packed with 0.2 g of catalyst at atmospheric pressure. Before the catalytic studies, catalysts were pretreated in an environment of 9 vol % hydrogen sulfide and 91 vol % nitrogen at 250 °C for 8 h. The feed composition was then changed to the following reaction conditions: flow rate ) 200 mL/min with H2S/O2/N2 ) 1/5/94 (by volume), which corresponded to the weight hourly space velocity of 0.0245 mol of H2S/g of catalyst per h. Water vapor of 30 vol % was also added in the feed stream to study the effect of water on the catalytic performances of V/Sn and V/Sn/ Sb catalysts. When the activities among V/Sn/Sb catalysts were compared, the flow rate of dry gas was increased to 400 mL/min and the concentration was changed to H2S/O2/N2 ) 1/1/98 (by volume). Experimental data at 180 °C (the initial data point) were taken 12 h after the pretreatment stage. Good reproducibility of the experimental data was achieved when the same reaction temperature was repeated at different reaction times. The differences in sulfur yield obtained were within 2.7% when the same reaction conditions were repeated. The gas products were dried and analyzed by a HP5890 gas chromatograph with a 9-m-long Porapak Q column. The reaction conversion was defined as (moles of hydrogen sulfide reacted)/(moles of hydrogen sulfide fed) × 100%. The sulfur selectivity was calculated as (moles of hydrogen sulfide reacted - moles of sulfur dioxide produced)/(moles of hydrogen sulfide reacted) × 100%. The sulfur yield was defined as hydrogen sulfide conversion times sulfur selectivity. Because the conversion increased with temperature while the selectivity decreased with temperature, a maximum sulfur yield occurred when the sulfur yield was plotted as a function of the reaction temperature.
Figure 1. H2S conversion as a function of the reaction temperature for single oxide catalysts: (a) V2O5; (b) SnO2; (c) Sb2O4.
Results and Discussion Catalytic Properties of Single Oxides. Figure 1 presents hydrogen sulfide conversion as a function of the reaction temperature for three single oxide catalysts (V2O5, SnO2, and Sb2O4) when the gas flow rate was 200 mL/min with a composition of H2S/O2/N2 ) 1/5/94 (by volume). At 180 °C, the conversion of vanadium oxide alone was 90%, which was much higher than those of tin oxide (14.2%) and antimony oxide (11.2%). However, the sulfur selectivity of V2O5 (93.5% at 180 °C and 26% at 210 °C) was very sensitive to temperature change. The selectivity decreased rapidly at higher temperatures after most H2S was consumed. That is, the H2S reaction occurred at the front part of the catalyst bed. After it was all consumed, the remaining catalyst bed catalyzed the oxidation of S to SO2, thereby lowering the overall selectivity. The maximum sulfur yield obtained with vanadium oxide alone was only 84%, which occurred at 180 °C. Catalytic Properties of Binary Oxides. Figure 2 shows the relationships between hydrogen sulfide conversion and reaction temperature for binary oxide catalysts with (a) V/Sn atomic ratio ) 1/1, (b) V/Sb atomic ratio ) 1/1, and (c) Sn/Sb atomic ratio ) 1/1. The gas flow rate and feed composition were identical to those used for single oxide catalysts. At 180 °C, hydrogen sulfide conversions for the V/Sn, V/Sb, and Sn/Sb catalysts were 95%, 24.9%, and 8.3%, respectively. That is, the V/Sn catalyst had the highest activity among these binary oxides. Figure 3 and Table 1 show the effect of 30 vol % water vapor in the feed stream on the catalytic performances of the V/Sn catalysts. The presence of water vapor decreased the catalyst activity (higher reaction temperatures were needed to reach the maximum sulfur yield) but had little effect on the maximum sulfur yield obtained. In Table 1, the maximum sulfur yields obtained with V/Sn binary oxides were all above 93%, which were significantly higher than that obtained with vanadium oxide alone (84%). Catalytic Properties of Ternary Oxides. Table 2 presents sulfur yield as a function of the reaction
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Table 1. Maximum Sulfur Yields Obtained with V/Sn Binary Oxidesa Sn/V atomic ratio without water in the feed with 30 vol % water vapor in the feed a
0.3
0.7
1.0
1.5
98.6% (at 210 °C) 93% (at 240 °C)
96.2% (at 200 °C) 98.4% (at 260 °C)
95.6% (at 190 °C) 99% (at 230 °C)
93.3% (at 210 °C) 96.8% (at 260 °C)
Reaction conditions: dry gas flow rate ) 200 mL/min with a composition of H2S/O2/N2 ) 1/5/94 vol %. Table 2. Sulfur Yields (%) Obtained for the V/Sn/Sb Catalystsa temp (°C)
V/Sn/Sb ) 1/0.3/1
V/Sn/Sb ) 1/0.5/1
V/Sn/Sb ) 1/0.7/1
V/Sn/Sb ) 1/1/1
V/Sn ) 1/1
180 200 220 230 240 250 260 270 280 290
42.4 51.7 67.2 73.7 86.4 88.2 92.6 97.4 100 95.2
100 100 100 100 100 98.2 75.6 19.8 0
100 100 100 100 100 82.3 61.8 25.2 0
100 100 100 100 100 82.5 77.1 53 22 0
63.8 86.4 94.4 99.0 83.3 46.8 22.8 0
a Reaction conditions: dry gas flow rate ) 200 mL/min with a composition of H2S/O2/N2 ) 1/5/94 vol % and in the presence of 30 vol % water vapor.
Figure 2. H2S conversion as a function of the reaction temperature for binary oxide catalysts: (a) Sn/V atomic ratio ) 1/1; (b) Sb/V atomic ratio ) 1/1; (c) Sn/Sb atomic ratio ) 1/1.
Figure 4. Reaction rates as a function of the Sn/V atomic ratio for V/Sn/Sb ternary oxide catalysts at (a) 180 °C and (b) 220 °C.
Figure 3. Influence of 30 vol % water vapor on sulfur yield for the catalyst with a Sn/V atomic ratio of 1.
temperature for ternary oxide catalysts when the feed contained 200 mL/min dry gas and 30 vol % water vapor. The results indicate that V/Sn/Sb catalysts with certain Sn/V atomic ratios had 100% sulfur yield in a wide range of reaction temperatures, which makes them very suitable for the industrial use to catalyze the oxidization of hydrogen sulfide to sulfur. The width of the usable window should depend partly on the inactivity of the ternary catalysts for the unwanted reactions (mainly the reaction of sulfur with oxygen), which
should have higher activation energies. It was also found that the sulfur yields obtained with these V/Sn/ Sb catalysts were insensitive to the water content. Comparisons of Tables 1 and 2 and Figure 2 clearly indicate that V/Sn/Sb ternary oxide catalysts were superior to the corresponding binary oxides (V/Sn, V/Sb, and Sn/Sb) in catalytic performances. To compare the catalytic activity among V/Sn/Sb ternary oxides, H2S conversions were decreased by increasing the dry gas flow rate to 400 mL/min and decreasing the oxygen concentration to O2/H2S/N2 volumetric ratio ) 1/1/98. In addition, 30 vol % water vapor was also added in the feed stream. Figure 4 shows the reaction rates of V/Sn/Sb catalysts at 180 °C (curve a) and at 220 °C (curve b) as a function of the Sn/V atomic ratio. The results in Figure 4 indicate that the activity of the V/Sn/Sb ternary oxides increased with the Sn/V atomic ratio when Sn/V atomic ratio < 1 and then leveled off when Sn/V atomic ratio g 1.
Ind. Eng. Chem. Res., Vol. 40, No. 4, 2001 1055 Table 3. Sulfur Yields (%) Obtained for the V/Sn/Sb Catalystsa temp V/Sn/Sb ) V/Sn/Sb ) V/Sn/Sb ) V/Sn/Sb ) V/Sn/Sb ) (°C) 1/0.5/1 1/0.7/1 1/0.85/1 1/1/1 1/1.5/1 180 220 230 240 250 260 270 280 290 300 310 320 330 350
45.6 82.9 84.7 89.4 97.5 100 100 100 100 100 100 79.5 38.6 0
52.1 91.3 94.0 97.5 100 100 100 100 100 100 89.2 54.1 31.9 0
50.5 95.4 100 100 100 100 100 85.6 64.3 37.6 0
53.9 94.3 98.1 100 100 100 100 100 100 90.2 70.7 68.2 41.1 0
57.9 94.9 100 100 100 100 100 100 100 71.4 43.4 29.9 0
a Reaction conditions: dry gas flow rate ) 400 mL/min with a composition of H2S/O2/N2 ) 1/1/98 vol % and in the presence of 30 vol % water vapor.
Figure 6. Specific reaction rates at 180 °C as a function of the Sn/V atomic ratio for the V/Sn/Sb ternary oxide catalysts.
Figure 5. Specific surface area as a function of the Sn/V atomic ratio for the used (a) V/Sn/Sb ternary oxides and (b) V/Sn binary oxides.
Table 3 shows sulfur yield as a function of the reaction temperature for V/Sn/Sb catalysts when the feed contained 400 mL/min dry gas (with composition H2S/O2/ N2 ) 1/1/98 vol %) and 30 vol % water vapor. The width of temperature range for obtaining 100% sulfur yield was at least 50 °C even with the increase of the flow rate and the decrease of the oxygen concentration. The results further indicate that the sulfur yields of these catalysts were not sensitive to the temperature change and these catalysts should be very suitable for the industrial use to catalyze the oxidization of hydrogen sulfide to sulfur. Comparisons of Tables 2 and 3 indicate that a higher reaction temperature was needed to reach 100% sulfur yield when the flow rate was increased and the oxygen concentration was decreased. Catalyst Characterization. Figure 5 shows the relationships between the surface area of used catalysts and the Sn/V atomic ratio for (a) V/Sn/Sb ternary oxides and (b) V/Sn binary oxides. It is interesting to note that catalyst surface areas increased linearly with the Sn/V atomic ratio for both the ternary oxides and the binary oxides, which suggests that tin oxide might work as a
role of support in V/Sn and V/Sn/Sb catalysts. Albonetti et al. also suggested that tin acted as a dispersed matrix in the Sn/V/Sb catalysts they studied.11 The results in Figure 5 also indicate that V/Sn/Sb ternary oxides had higher surface areas than V/Sn binary oxide at the same Sn/V atomic ratio. The differences in the surface areas should contribute to the improvement of the catalyst activity for the V/Sn/Sb ternary oxide catalysts compared to the binary oxide catalysts of V/Sb and V/Sn. On the basis of the reaction rate data in Figure 4 and the surface area data in Figure 5, the specific rates (reaction rate per unit surface area of the catalyst) at 180 °C are presented in Figure 6 for the V/Sn/Sb ternary catalysts. It can be clearly seen from Figure 6 that the specific rate decreased with increasing Sn/V atomic ratio, which suggests that vanadium should be the major active site for H2S oxidation in the mixed-oxide catalysts. The combination of Figures 4-6 indicates that the surface area is responsible for the high activity observed (as shown in Figure 4) for the catalysts with high Sn/V atomic ratio. Catalyst reducibility was measured using a TPR method with hydrogen as the reductant. Figure 7 shows the TPR profiles of V-Sn binary oxide catalysts with Sn/V atomic ratios of 0.3 (curve a), 0.7 (curve b), and 1.5 (curve c). Two peak maxima were observed in these TPR profiles. It is generally accepted that two forms of vanadia structure formed on metal oxide supported samples: a highly dispersed structure (isolated VO4 tetrahedra) and crystalline V2O5.13,14 The peak maximum appearing at temperatures less than 600 °C (denoted as Tmax1) should correspond to the reduction of isolated V5+ (a highly dispersed form of the structure (Sn-O)3tVdO) to V4+. Because the incipient reduction temperatures of these isolated V5+ were close to the reaction temperature used for H2S oxidation (180-350 °C), these isolated V5+ species should be the major sites for the oxidation of hydrogen sulfide to sulfur. Figure 7 shows that Tmax1 decreased with an increase of the Sn/V atomic ratio (Tmax1 was 588, 475, and 340 °C for samples with Sn/V atomic ratios of 0.3, 0.7, and 1.5, respectively), which indicates that strong interactions occurred between vanadium oxide and tin oxide.
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Figure 7. TPR profiles of V-Sn binary oxide catalysts with Sn/V atomic ratio ) (a) 0.3, (b) 0.7, and (c) 1.5.
It is known that Tmax of supported vanadia catalysts depends strongly on the loading of vanadium oxide on the support,14 and Jehng found the formation of VOSnO bonds when he studied V2O5 supported on a series of SnO2/SiO2 mixed oxide for methanol oxidation.15 The areas of the peaks around Tmax1 and the amounts of vanadium atoms reduced in these peaks (denoted as V1) were calculated by deconvoluting the TPR spectra in Figure 7. The ratios of V1/Vtotal (Vtotal denotes the total amounts of vanadium atoms in the TPR samples) were 0.66, 0.24, and 0.21 for samples with Sn/V atomic ratios of 0.3, 0.7, and 1.5, respectively. If the vanadium atoms reduced in these low-temperature peaks were responsible for catalyzing H2S oxidation, the turnover frequencies (expressed as H2S atoms converted/vanadium atom per h) at 180 °C were 1.75, 11.25, and 16.54 for catalysts with Sn/V atomic ratios of 0.3, 0.7, and 1.5, respectively. The results indicate that the turnover frequency increased with increasing reducibility (i.e., decreasing reduction temperature) of vanadium atoms. In Figure 7, the peak maximum occurring at temperatures greater than 800 °C (denoted as Tmax2) should correspond to the reduction of both crystalline V2O5 and SnO2. Tmax2 was 818, 834, and 860 °C for samples with Sn/V atomic ratios of 0.3, 0.7. and 1.5, respectively, which indicate that Tmax2 increased with Sn/V atomic ratio. This is due to the fact that the TPR peak maximum temperature for SnO2 alone was around 875 °C, which was significantly higher than the TPR peak maximum temperature for V2O5 alone. For V2O5 alone, three TPR peak maxima were observed in the temperature range of 650-790 °C.9 Figure 8 shows TPR profiles for (a) ternary oxide with a V/Sn/Sb atomic ratio of 1/0.7/1 and (b) binary oxide with a V/Sb atomic ratio of 1/1. These two profiles were similar to each other except for a shift of 50 °C to a lower temperature of the peak maximum observed for the ternary oxide (Tmax was 680 °C for the ternary oxide and was around 730 °C for the binary oxide). In addition, the onset of the ternary oxide reduction occurred at 240 °C, which was much lower than that for the binary oxide. The lower reduction temperature for the V/Sn/Sb ternary oxides should be the major
Figure 8. TPR patterns of (a) ternary oxide with V/Sn/Sb atomic ratio ) 1/0.7/1 and (b) binary oxide with V/Sb atomic ratio ) 1/1.
Figure 9. SEM of the used catalysts with (a) V/Sn/Sb atomic ratio ) 1/0.7/1 and (b) V/Sb atomic ratio ) 1/1.
reason that they had better activity than the V/Sb binary oxide. Figure 9 shows the SEM pictures of the used catalysts with (a) V/Sn/Sb atomic ratio ) 1/0.7/1 and (b) V/Sb atomic ratio ) 1/1. The phase structure of the V/Sn/Sb catalyst was a discrete tiny particle form with particle
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areas increased linearly with the Sn/V atomic ratio, which suggests that tin oxide acts as a role of support in these oxides. TPR studies revealed that the ease of vanadium reduction increased with the Sn/V atomic ratio, which indicated that strong interactions occurred between vanadium oxide and tin oxide. The increase of the surface area and the increase of the vanadium reducibility should be responsible for the improvements of catalyst performances in the V/Sn and V/Sn/Sb catalysts. Results of XRD and SEM suggested that the V/Sn/Sb catalysts were stable under the reaction conditions in hydrogen sulfide oxidation. Acknowledgment Figure 10. XRD patterns of (a) fresh catalyst and (b) spent catalyst with V/Sn/Sb atomic ratio ) 1/0.85/1. b: SnO2.
sizes much smaller than that of the V/Sb catalyst. This should be one of the reasons that the catalytic activity of the V/Sn/Sb catalyst was higher than that of the V/Sb catalyst. For all of the V/Sn/Sb catalysts, the morphologies of the used catalysts were similar to those of the fresh catalysts, indicating that the catalysts were stable during the reaction. Figure 10 shows the XRD patterns of the catalyst with a V/Sn/Sb atomic ratio of 1/0.85/1 before the reaction (pattern a) and after the reaction (pattern b). These patterns exhibited only the characteristic peaks of SnO2. It is known that VSbO4 might be formed from the reaction of vanadium and antimony and the strongest XRD characteristic peak of VSbO4 occurred at 2θ ) 27°.8 In Figure 10, the characteristic peaks of the VSbO4 compound might be overlapped by the characteristic peak of SnO2 at the same position. For V/Sn/Sb catalysts, the used catalysts had XRD patterns and XRD peak areas similar to those of the fresh catalysts, which also indicates that the V/Sn/Sb catalysts were stable during the oxidation of hydrogen sulfide to sulfur. Conclusions Vanadium-tin binary oxides and vanadium-tinantimony ternary oxides with different Sn/V atomic ratios were prepared with a coprecipitation method from an ethanolic phase and were used to catalyze the reaction between hydrogen sulfide and molecular oxygen. The vanadium-tin-antimony ternary oxide had much higher catalytic performances than the binary oxides of vanadium-tin, vanadium-antimony, and tinantimony. The vanadium-tin-antimony ternary oxides with a Sn/V atomic ratio in the range of 0.5-1 and with a Sb/V atomic ratio of 1 exhibited 100% sulfur yield in the temperature range of 180-240 °C when the weight hourly space velocity was 0.0245 mol of H2S/g of catalyst per h. The wide range of the operation temperature window makes these ternary oxide catalysts very suitable for industrial use to catalyze the highly exothermic reaction-selective oxidation of hydrogen sulfide to sulfur. For V/Sn binary oxides and V/Sn/Sb ternary oxides, BET measurements showed that the catalyst surface
The authors gratefully acknowledge the National Science Council of the Republic of China for financial support (Grant NSC-88-2214-E-029-003). Literature Cited (1) Duncan, M. P. Sulfur Compounds. In Encyclopedia of Chemical Technology; Kroschwitz, J. K., Howe-Grant, M., Eds.; Wiley: New York, 1997; Vol. 23, p 276. (2) Capone, M. Sulfur Removal and Recovery. In Encyclopedia of Chemical Technology; Kroschwitz, J. K., Howe-Grant, M., Eds.; Wiley: New York, 1997; Vol. 23, p 432. (3) Ketter, R.; Liermann, N. New Claus Tail-gas Process Proved in German Operation. Oil Gas J. 1988, 86, 63. (4) Lagas, J. A.; Borsboom, J.; Berben, P. H. Selective Oxidation Catalyst Improve Claus Process. Oil Gas J. 1988, 86, 68. (5) Pieplu, A.; Saur, O.; Lavalley, J. C. Claus Catalysis and H2S Selective Oxidation. Catal. Rev.sSci. Eng. 1998, 40, 409. (6) Billie, J. P.; Gary, W. L. Chromium Compounds. In Encyclopedia of Chemical Technology; Kroschwitz, J. K., Howe-Grant, M., Eds.; Wiley: New York, 1993; Vol. 6, p 284. (7) Terorde, R. F. A. A.; Van den Brink, P. J.; Visser, L. M.; Van Dillen, A. J.; Geus, J. W. Selective Oxidation of Hydrogen Sulfide to Elemental Sulfur Using Iron Oxide Catalysts on Various Supports. Catal. Today 1993, 17, 217. (8) Li, K. T.; Shyu, N. S. Catalytic Oxidation of Hydrogen Sulfide to Sulfur on Vanadium Antimonate. Ind. Eng. Chem. Res. 1996, 35, 621. (9) Li, K. T.; Chien, T. Y. Effect of Supports in Hydrogen Sulfide Oxidation on Vanadium-based Catalysts. Catal. Lett. 1999, 57, 77. (10) Liley, P. E.; Gambill, W. R. Physical and Chemical Data. In Chemical Engineerings’ Handbook; Perry, R. H., Chilton, C. H., Eds.; McGraw-Hill: New York, 1973. (11) Albonetti, S.; Blanchard, G.; Burattin, P.; Masetti, S.; Trifiro, F. A New Ternary Mixed-Oxide Catalyst for Ammoxidation of Propane-Sn/V/Sb. Catal. Lett. 1998, 50, 17. (12) Albonetti, S.; Blanchard, G.; Burattin, P.; Cavani, F.; Masetti, S.; Trifiro, F. Propane Ammoxidation over a Tin-Based Mixed-Oxide Catalyst. Catal. Today 1998, 42, 283. (13) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier: Amsterdam, The Netherlands, 1989. (14) Bond, G. C.; Tahir, S. F. Vanadium Oxide Monolayer Catalysts, Preparation, Characterization and Catalytic Activity. Appl. Catal. 1991, 71, 1. (15) Jehng, J. M. Dynamic States of V2O5 Supported on SnO2/ SiO2 and CeO2/SiO2 Mixed-oxide Catalysts during Methanol Oxidation. J. Phys. Chem. 1998, 102, 5816.
Received for review July 27, 2000 Revised manuscript received December 6, 2000 Accepted December 20, 2000 IE0007015
