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SiO2 and V2O5 catalysts
Applied Catalysis A: General 211 (2001) 213–225
Selective oxidation of H2 S to elemental sulfur over VOx /SiO2 and V2 O5 catalysts Moon Young Shin a , Chang Mo Nam a , Dae Won Park b , Jong Shik Chung a,∗ a
Department of Chemical Engineering, School of Enviromental Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja Dong, Pohang 790-784, South Korea b Department of Chemical Engineering, Pusan National University, Pusan 609-735, South Korea Received 31 August 2000; received in revised form 30 October 2000; accepted 30 October 2000
Abstract The selective oxidation of H2 S was investigated over VOx /SiO2 catalysts (1–100% V contents) using a packed-bed flow reactor in the temperature range 200–350◦ C. When a stoichiometric ratio of O2 /H2 S is used in the feed, high conversion of H2 S and high selectivity to sulfur (>90%) can be achieved at various V loadings (5–100 wt.%) if the space velocity is kept below 4000 h−1 and the reaction temperature above 270◦ C. About 10–30% decrease of the sulfur yield is observed in the presence of 30% water in the feed. The conversion of H2 S and selectivity to sulfur exceed the equilibrium values. Reaction tests and characterization using XRD, XPS and XANES have revealed that during the reaction, the V2 O5 catalysts are reduced severely to VO2 and/or V2 O3 , resulting in changes in the surface area and pore size. The sulfur selectivity can be kept high (>90%) only at such a highly reduced condition. Oxidized catalyst (with O/V ratio > 2.26) exhibits complete oxidation of H2 S to SO2 (zero sulfur selectivity). As the reaction proceeds via redox mechanism, high space velocities and low V loadings at low reaction temperatures (<250◦ C) result in eventual decrease of the conversion with time on stream due to depletion of labile oxygen on the surface. © 2001 Elsevier Science B.V. All rights reserved. Keywords: H2 S; Selective oxidation; Sulfur; V2 O5 ; Deactivation
1. Introduction In petroleum refineries and natural gas plants, hydrogen sulfide (H2 S) is typically collected by contact with amine-based solutions, and subsequently fed to a well known Claus plant [1,2]. The Claus process consists of two steps: thermal oxidation and catalytic reaction. In the thermal oxidation, one-third of H2 S is first burned with air to produce sulfur dioxide in a waste heat furnace, while unconverted H2 S reacts with SO2 to elemental sulfur through the subsequent ∗ Corresponding author. Tel.: +82-54-279-2267; fax: +82-54-279-5799. E-mail address: [email protected] (J.S. Chung).
catalytic reaction over Al2 O3 catalysts. However, due to thermodynamic limitations, 3–5% of H2 S is typically not converted to sulfur. As the environmental regulations become more stringent, it is necessary to further treat the residual (tail) gas of the Claus plants. H2 S in the tail gas of Claus plants or from other emission sources has been conventionally treated by various techniques such as adsorption, absorption, and wet oxidation [1]. Recently, a dry catalytic process has been developed for the selective catalytic oxidation of H2 S to elemental sulfur. Examples of commercially developed catalysts for this purpose are the titanium-based catalysts in the MODOP process [3,4] and the iron-based catalysts in the Super Claus process [5–7].
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These processes are based on the following irreversible selective oxidation of H2 S to S as a main reaction (Eq. (1)), the other oxidation reactions (Eqs. (2) and (3)) and the Claus reaction (Eq. (4)). 1 1 H2 S + O2 → Sn + H2 O 2 n
(1)
1 Sn + O2 → SO2 n
(2)
H2 S +
(3)
3 2 O2
→ SO2 + H2 O
3 2H2 S + SO2 ↔ Sn + 2H2 O n
(4)
In the MODOP process, H2 S is oxidized into elemental sulfur with a stoichiometric amount of oxygen over TiO2 catalysts. One drawback of the MODOP process is that water should be removed from the tail gas before the reaction, due to the deactivation of catalysts in the presence of water. In the Super Claus process, tail gas can be treated without a dehydration step, since the Fe2 O3 /SiO2 catalyst is not deactivated even in the presence of 30 vol.% of water vapor. However, the Super Claus process cannot treat high concentrations of H2 S above 2 vol.% because it is necessary to supply excess oxygen (usually 10 times the stoichiometric amount) to overcome the catalytic deactivation caused by water. Catalysts containing vanadium oxide have also been reported to be active for the oxidation of H2 S to sulfur both with a stoichiometric and an excess amount of oxygen [8–15]. VBiOx /SiO2 was used to improve a long-term deactivation problem noticed in V/SiO2 , but it needed a dehydration step before being sent to a catalytic reactor to increase H2 S conversion (BSR Selectox process) [9,10]. It has been suggested that the formation of less active forms of vanadium such as vanadyl sulfate (VOSO4 ) is one cause of the deactivation. Various binary oxides such as V–Mg, V–Bi, V–Mo, V–Sb, Fe–Sn and Bi–Mo were tested in excess oxygen without water [11–13]. Solid solutions of vanadium, A4 ± y V2 ± x O9 (A = Mg, Ca or Zn, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.5) [14,15] were also reported as active catalysts, even in the presence of 30 vol.% of water. This study presents the catalytic behaviors of VOx /SiO2 at various V loading amounts (1–100%) for the selective oxidation of H2 S. Various reaction
parameters such as V loading, O2 /H2 S ratio in the feed, temperature, space velocity and reaction time are investigated to obtain a high yield of sulfur at a wide window of operation condition. Physical and structural characterization are also performed for understanding the reaction mechanism and the causes of catalyst deactivation observed at specific conditions. 2. Experimental The supported catalysts, VOx /SiO2 , were prepared by an evaporation method with 1 to 30 wt.% of V loading as was previously described elsewhere [25,26]. Bulk V2 O5 was purchased from Junsei Co. Ltd. (99.9%) and used after calcination at 400◦ C for 5 h using air. The catalytic reaction was performed using a vertical packed-bed reactor made of 2.54 cm i.d., Pyrex tube. Hydrogen sulfide (Solkatronic Chemical, 1–5 vol.%) and air (2–5 vol.%) were mixed and diluted with helium by mass-flow-controllers. Total flow rate ranged from 50 to 160 ml/min, and hence the gas hourly space velocity (GHSV) was between 500–140,000 h−1 . About 80–100 mesh of catalyst powder was diluted five times with the same mesh size of glass powder. Water was injected using a syringe pump to a vaporizer located before the reactor. Sulfur in the product was separated using a sulfur condenser placed at the effluent side of the reactor. The temperature of the sulfur condenser was maintained at 110◦ C. A line filter was also installed after the sulfur condenser to trap any sulfur mist that was not collected by the condenser. To prevent the condensation of sulfur and water vapor from the line filter up to the gas chromatograph, all the lines and fittings were heated above 150◦ C. The gaseous products, H2 S and SO2 were analyzed by a gas chromatograph (Hewlett Packard 5890) with a Porapak T column (80/100 mesh, 1.5 m) and a thermal conductivity detector. The conversion of hydrogen sulfide, the selectivity to sulfur, and the yield of sulfur were defined, respectively, as follows: Conversion of H2 S(%)=
[H2 S]inlet −[H2 S]outlet × 100 [H2 S]inlet
Selectivity of sulfur(%) [H2 S]inlet − [H2 S]outlet − [SO2 ]outlet × 100 = [H2 S]inlet [H2 S]outlet
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Yield of sulfur(%) Conversion of H2 S × Selectivity of sulfur = 100 The calculation of the chemical equilibria of a multi-component system was performed by using the SOLGASMIX-PV code, developed by Eriksson [16]. It takes the atoms of a particular chemical mixture and determines the product assemblage, which has the minimum Gibbs free energy subject to the mass-balance constraint [17,18]. With this program, numerous chemical equilibrium analyses of solid–gas systems have already proven useful in understanding the chemical processes [19]. The thermodynamics of an oxygen-based Claus process was also evaluated by Khudenko et al. [20], who used the STANJAN chemical equilibrium solver program, which utilizes the principle of minimization of Gibbs free energy. Calculated equilibrium conversion and selectivity are verified with those obtained following the method suggested by Gamson et al. [21], which considers the partial and complete oxidation of H2 S (Eqs. (1)–(4)) and following equilibrium reactions between the produced sulfur species below. S6 ↔ 3S2
(5)
S8 ↔ 4S2
(6)
X-ray diffraction patterns of catalysts were obtained using a X-ray analyzer (M18XHF, MAC Science Co.). Ni-filtered Cu K␣ radiation (λ = 1.5415 Å) was used with an X-ray gun operated at 40 kV and 200 mA. Diffraction patterns were obtained within the range of 2θ = 10–90◦ at a scan rate of 4◦ /min. To observe changes in the catalyst surface after reactions, XPS spectra were obtained using a Kratos XSAM 800pci X-ray photoelectron spectrometer with Al K␣ monochromatic X-ray (1487 eV) radiation. Catalysts were uniformly ground into powder and then pressed into self-supporting wafers without any binders, followed by a pre-treatment at an ultra-high vacuum of 10–12 mmHg for 10 h. The charging effect of XPS spectra was carefully corrected with a carbon peak at 284.6 eV as a standard. X-ray absorption near-edge spectra, XANES, were performed on 3C1 EXAFS beamline at the Pohang Acceleration Laboratory (PAL) with ring energy of 2.0 GeV. XANES were recorded using the EXAFS
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facilities installed in the transmission mode at room temperature with a Si(1 1 1) two-crystal monochrometer. The detector gases for the incident beam was N2 (100 vol.%), and the gas for the transmitted beam was N2 (85 vol.%) + Ar(15 vol.%). The energy resolution of ca. 0.56 eV can be achieved at the V K edge, 5471.0 eV. The obtained XANES data were analyzed by the R-space method with the UWXAFS 3.0 package. The specific surface areas and the pore-size distribution of the catalysts were measured using a BET apparatus (Accusorb 2100E, Micromeritics).
3. Results 3.1. Reaction measurements Fig. 1 shows the effect of the O2 /H2 S mole ratio on the conversion of H2 S and the selectivity to sulfur over the V(30)/SiO2 catalyst. The oxygen concentration was varied from 2 to 5 vol.%, while the H2 S and H2 O concentrations were kept constant at 5 and 0 or 30 vol.%, respectively. As the O2 /H2 S ratio increases from 0.4 to 1.0, the conversion increases from 72 to 99%, while the selectivity decreases from 98 to 42%,
Fig. 1. Effect of O2 /H2 S feed ratio on the conversion of H2 S (䊉) and selectivity to sulfur (䊊) for V(30)/SiO2 . Reaction conditions: H2 S 5 vol.%, H2 O 30 vol.%, T = 225◦ C, GHSV = 96,000 h−1 .
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Table 1 The conversion of H2 S and the selectivity to sulfur with different vanadium loadingsa V loading (wt.%)
1.0 5.0 10.0 30.0 100.0 Equilibrium value a b
Conversion and selectivity (%)
Without H2 O, temperature (o C) 225
250
275
300
325
225
250
275
300
325
Conversion Selectivity Conversion Selectivity Conversion Selectivity Conversion Selectivity Conversion Selectivity Conversion Selectivity
50.1b
75.2 91.0 96.0 96.5 97.3 97.5 95.5 98.8 97.2 98.0 95.0 97.4
89.0 94.1 94.1 95.5 95.0 97.0 93.4 98.0 96.0 97.5 93.0 96.3
91.5 95.2 92.0 94.0 93.2 96.6 91.7 96.2 94.1 96.6 90.6 94.9
92.0 94.0 92.2 92.1 90.1 94.5 91.0 94.0 92.0 94.5 87.8 93.1
– – – – 82.1 89.3 92.0 95.0 93.2 94.5 87.8 93.0
– – – – 85.0 91.0 88.2 92.0 88.0 92.3 82.0 89.0
– – – – 83.8 90.2 84.0 89.3 84.0 90.0 75.1 83.4
– – – – 79.0 83.1 78.3 84.0 80.0 85.0 67.1 75.5
– – – – 73.3 78.0 74.0 78.0 75.0 78.6 58.4 64.1
91.0b 96.0 96.5 98.0 98.0 98.0 99.1 98.2 98.6 96.7 98.3
With 30 vol.% H2 O, temperature (o C)
Reaction conditions; GHSV 3000 h−1 , 5 vol.% of H2 S, 2.5 vol.% of O2 balanced with He. Conversion measured after 20 h reaction at 225◦ C, where the conversion is steadily decreased from initial value of 95.0%.
indicating that the highest yield of sulfur is near the stoichiometric ratio of O2 /H2 S (0.5). Therefore, most of the reactions hereafter were conducted at these conditions, 5 vol.% H2 S, 2.5 vol.% O2 , and 0 or 30 vol.% H2 O balanced with He (O2 /H2 S = 0.5), unless specified. The effects of vanadium loading and temperature on the conversion of H2 S and selectivity to sulfur are summarized in Table 1. High values (more than 90%) of the conversion and selectivity can be achieved over wide ranges of vanadium loading and temperature, except for a few data points where the vanadium loading and reaction temperature are kept very low. As mentioned in the table, the data points obtained with V(1)/SiO2 catalyst are not steady state values but dynamic values obtained after 20 h of reaction as the catalyst exhibits a continuous decrease in the conversion as reaction proceeds at temperatures below 250◦ C. Catalysts having high vanadium loading (higher than 10%) do not show any deactivation within 20 h of reaction even at low temperatures, indicating to us that the bulk phase of vanadium is more active than the dispersed phase. According to Bars et al. [22], 10 wt.% V loading is the theoretical monolayer coverage of vanadium oxide over SiO2 (300 m2 /g) in the form of VO4 . The presence of water in the feed also decreases the sulfur yield, 10–30%, indicating that in addition to the irreversible selective oxidation
reaction (1), total oxidation reactions ((2) and (3)) and the reversible Claus reaction occur to some extent. The effect of space velocity was investigated over V(30)/SiO2 as functions of reaction temperature and time, as shown in Fig. 2. Conversion and selectivity measured at 20 h of reaction exceed the equilibrium values. As the space velocity increases up to 135,000 h−1 , there is an eventual decrease of the conversion with time on stream and a steady state in the conversion cannot be obtained at low temperature (i.e. 230◦ C). In contrast to this, the selectivity always remains constant with time on stream. 3.2. Deactivation and regeneration of catalysts In order to study the cause of deactivation, the reaction was carried out intentionally at a low temperature of 230◦ C. Fig. 3 shows the decreasing patterns in the conversion with time on stream, which can be observed at high space velocities. It is noteworthy to observe that at low space velocities, the selectivity at the beginning stage of the reaction is very low. We conclude, therefore, that fresh vanadium in its fully oxidized state is very active for the total oxidation of H2 S and not for the selective oxidation. The effect of vanadium loading on the catalyst deactivation is shown in Fig. 4. The lower the vanadium loading is, the faster the deactivation of catalyst is. These
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Fig. 2. Effect of space velocity on the conversion of H2 S and selectivity to sulfur over V(30)/SiO2 : (䊉) 500; (䊏) 4000; (䉱) 8000; (䉲) 65,000; (䉬) 135,000 h−1 ; equilibrium conversion and selectivity (dashed line). Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%.
results lead us to assume that H2 S consumes oxygen in vanadium oxide and gas phase oxygen replenishes the reduced sites of vanadium oxide, i.e. the reaction occurs via the redox mechanism. Faster deactivation at lower vanadium loading or at higher space velocity supports the conclusion that the reoxidation step by gas phase O2 proceeds at a much slower rate than the reduction step by H2 S, especially when the temperature is low. In other words, the reoxidation of reduced VOx by gas-phase O2 requires higher activation energy. This was confirmed by regeneration experiments
with flowing air at different temperatures over the deactivated catalyst, as shown in Fig. 5. Reoxidation (step (b)) of the catalyst deactivated for 10 h at 225◦ C (during step (a)) does not show any regeneration effect. When the reoxidation temperature is raised to 335◦ C (step (d)), the same period of the oxygen treatment regenerates the deactivated catalyst to a certain degree; i.e. 40% conversion at the end of the reaction step (c) is increased to 50% at the beginning of reaction step (e). Moreover, during the reaction at 335◦ C (step (e)), the conversion is increased (the reduced
Fig. 3. Deactivation patterns of V(30)/SiO2 catalysts with time on stream at various space velocities: (䊉, 䊊) 500; (䊏, 䊐) 4000; (䉱, 4) 8000; (䉲, 5) 65,000; (䉬, 䉫) 135,000 h−1 . Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, T = 230◦ C.
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Fig. 4. Effect of V loading on catalyst deactivation: (䊉) V(5)/SiO2 ; (䊏) V(10)/SiO2 ; (䉱) V(20)/SiO2 ; (䉲) V(30)/SiO2 ; (䉬) V2 O5 . Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 30,000 h−1 for V(5)/SiO2 and V(10)/SiO2 , GHSV = 65,000 h−1 for V(20)/SiO2 and V(30)/SiO2 , GHSV = 106,000 h−1 for V2 O5 , T = 230◦ C.
catalyst is oxidized) and reaches a steady state. This implies that, with temperature increase, increased oxidation rate is now balanced with the reduction rate by H2 S so that the catalyst keeps its reduction state constant during the reaction. Fig. 6 presents the oxygen effect in the feed on the conversion and selectivity at 254◦ C over bulk V2 O5 . We used unsupported V2 O5 for the transient reaction test simply because, compared with vanadium dispersed on SiO2 , V2 O5 will have the largest amount of oxygen storage per gram of catalyst. When there is a change in the feed composition, a larger amount of the oxygen storage will give a longer transition period. This allows us to detect transition states more easily. The reaction was first carried out with 1 vol.% H2 S balanced with He at 254◦ C and at GHSV of 3000 h−1 for 180 min (step (a)). After purging the reactor with inert He for 100 min (step
Fig. 5. Reaction and regeneration at various conditions, conversion (䊉) and selectivity (䊊): (a) reaction at 225◦ C; (b) regeneration in flowing air 50 ml/min for 10 h at 225◦ C; (c) reaction at 225◦ C; (d) regeneration in flowing air 50 ml/min for 5 h at 335◦ C; (e) reaction at 335◦ C. Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 65,000 h−1 .
(b)), the feed was changed into a stoichiometric mixture of H2 S/O2 (step (c)). The point 1 shown in the figure corresponds to the case where vanadium oxide loses the monolayer amount of oxygen during the reaction. The points 2, 3 and 4 represent vanadium oxides in reduced states having the O/V ratios 2.26, 1.94 and 1.87, respectively. These values were calculated by integrating the oxygen consumption that was obtained from the conversion as a function of reaction period. The monolayer amount of oxygen was calculated based on the assumption that the surface area of VO4 is 0.1 nm2 at the surface [22]. When oxygen is relatively abundant on the surface of vanadium oxide (O/V ratio > 2.26), H2 S is completely oxidized to SO2 without the production of sulfur. When catalyst is in a severely reduced state (O/V ratio < 1.94), the conversion begins to decrease but the sulfur selectivity becomes almost 100%. Passing He and then oxygen-containing feed increases the conversion abruptly, and eventually the conversion reaches a steady value.
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3.3. Characterization of catalysts The BET surface areas of the catalysts are shown in Table 2. With increased V loading from 1 to 30 wt.%, the surface area is decreased substantially from 291.4 to 142.2 m2 /g. After reaction, supported catalysts show decreases in the surface area, whereas unsupported catalysts show increased surface area. XRD patterns of VOx /SiO2 before and after the reaction are shown in Fig. 7. After the reaction, the XRD peaks disappeared or became weakened, implying that the catalysts become either amorphous or their crystallite sizes become smaller. Since, we failed to detect any phase change with the supported catalysts owing to the X-ray transparency, unsupported V2 O5 catalysts were tested at several reaction conditions; the results are shown in Fig. 8. Although not shown here, the diffraction peaks of fresh V2 O5 were very sharp and strong. Compared with fresh V2 O5 , the diffraction peaks of all the reacted catalysts exhibit decreases in their intensities
Fig. 6. Reactions with and without oxygen supply and regeneration over V2 O5 , conversion (䊉) and selectivity (䊊): (a) 1 vol.% H2 S only; (b) He; (c) 1 vol.% H2 S and 0.5 vol.% O2 , GHSV = 3000 h−1 , T = 254◦ C; (1) monolayer amount of oxygen (O), 20 min, (2) V:O = 1:2.26, (3) V:O = 1:1.94, (4) V:O = 1:1.87.
Table 2 Changes of BET surface area before and after reactiona Catalyst
Before reaction (m2 /g)
After reaction (m2 /g)
SiO2 Davisil V(1)/SiO2 V(5)/SiO2 V(10)/SiO2 V(30)/SiO2 Bulk V2 O5 Bulk V2 O5 Bulk V2 O5
306.6 291.4 264.1 227.5 142.2 6.7 – –
264.6b 232.9b 224.7b 108.5b 17.3b 12.1c 20.5d
a Feed gas; 5 vol.% of H2 O balanced with 84,000 h−1 . b Measured after the c Measured after the d Measured after the
of H2 S, 2.5 vol.% of O2 and 30 vol.% He. Rection period = 70 h, GHSV = reaction shown in Table 1. reaction at 235◦ C. reaction at 335◦ C.
Fig. 7. XRD patterns of various VOx /SiO2 catalysts before (dotted) and after the reaction (solid line). Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 106,000 h−1 , T = 335◦ C.
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Fig. 8. XRD patterns of unsupported V2 O5 catalysts after the reaction at various temperatures and time periods: (a) blank scotch tape; (b) 2.3 h at 235◦ C; (c) 12 h at 235◦ C; (d) 40 h at 235◦ C; (e) 70 h at 235◦ C; (f) 3.5 h at 335◦ C; (g) 12 h at 335◦ C; (h) 40 h at 335◦ C; (i) 70 h at 335◦ C. Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 106,000 h−1 , T = 335◦ C.
by an order of magnitude, indicating to us that there is a substantial amount of change in either structure or morphology of V2 O5 after reaction. At the mild reaction condition of 235◦ C ((b) through (f)), fresh catalyst of V2 O5 is reduced to VO2 . At high temperature of 335◦ C ((g), (h) and (i)), a slow but additional phase transition from VO2 to V2 O3 occurs. In the case of (i), no X-ray peak is observed probably due to severe change in the crystal structure and morphology. These phenomena can be further observed through the SEM images (not shown) [25]. XANES were carried out for both fresh and used catalysts to observe the amorphous state of catalyst. The results in Fig. 9 confirm again that VOx /SiO2 and unsupported V2 O5 are transformed to VO2 phase after the reaction at 335◦ C. Fig. 10 shows the XPS spectra of the V2 O5 catalysts before and after reaction. The standard XPS peaks of V 2p3/2 for V+5 , V+4 ,
Fig. 9. XANES analyses of various V/SiO2 and bulk V2 O5 before and after reaction: (a) reference V2 O3 ; (b) reference V2 O4 ; (c) reference V2 O5 ; (d) V(5)/SiO2 before reaction; (e) V(5)/SiO2 after reaction; (f) V(10)/SiO2 before reaction; (g) V(10)/SiO2 after reaction; (h) V2 O5 before reaction; (i) V2 O5 after reaction. Conditions are the same as in Table 1.
and V+3 are located at 517.2, 515.9 and 515.3 eV, respectively [23,24]. The results in case (a) confirm previous observations that fresh V2 O5 is transformed into VO2 after the reaction at 335◦ C. There is also a change in the O1s peak (case (b)) after the reaction. Since, the oxygen 1s peaks for oxide and sulfate form are, respectively, 529.9–530.5 and 531.2–532.4 eV, it is believed that used catalyst contains some portion of vanadyl sulfate on the surface. The sulfur 2p peak (case (c)) also confirms the existence of elemental sulfur and vanadyl sulfate species that are located at 163.8–164.0 and 166–170 eV, respectively. Pore size distributions (PSD) were measured for fresh V2 O5 catalyst and for those obtained after reactions at 235–335◦ C, the results are shown in Fig. 11. The original pore size shown in the fresh V2 O5 (28 Å in diameter) decreased or disappeared after reactions. Pores of about 38 Å are generated after the reaction at 235◦ C, and pores around 200 Å are additionally generated after the reaction at 335◦ C. Adsorption and desorption patterns observed during the PSD are shown in Fig. 12. According to the classification of Brunauer
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Fig. 10. XPS analyses of V2 O5 before (solid line) and after reaction (dashed line for 235◦ C and dotted line for 335◦ C): (a) V 2p3/2 ; (b) O 1s; (c) S 2p. Conditions; H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 106,000 h−1 .
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Fig. 11. Pore size distribution (PSD) of V2 O5 measured before (a, fresh) and after reaction (b, 235◦ C; c, 335◦ C). Conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, t = 70 h, GHSV = 84,000 h−1 .
[27], the shape of the curve observed after the reaction corresponds to type II, which is commonly encountered on macro-porous material. In terms of De Boer classification [28], there are five types of hysteresis loops which represent the nature of the condensation process in the pores, depending on the associated pore shapes. The adsorption isotherm of V2 O5 exhibits only mild hysteresis, and the shape of hysteresis for our catalysts does not belong to any of the five types.
4. Discussion The results of the present work have shown that the catalytic activity has relevance to the bulk property of V2 O5 . The H2 S conversion increases with increased VOx loading (Table 1, Fig. 5) and decreases with an increase of space velocity (Figs. 2 and 3), especially at low temperatures. The conversions measured at temperatures above 260◦ C are higher than the equilibrium values. The selectivities to S, however, remain high enough above 90% even at low temperatures
Fig. 12. Adsorption and desorption branch of the pore size distribution for V2 O5 after reaction: (a) 235◦ C; (b) 335◦ C. Adsorption branch (solid line) and desorption (dotted line). Conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, t = 70 h, GHSV = 84,000 h−1 .
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regardless of the change of V loading and space velocity. Steijns et al. [29] have found that, using coal as a catalyst, that the oxidation rate of sulfur to SO2 was quite slow compared with that of H2 S to sulfur when the reaction temperature was kept below 200◦ C. This is because the activation energy for the sulfur oxidation (125 ± 10 kJ mol−1 ) is much higher than that for the H2 S oxidation (∼40 kJ mol−1 ). As the reaction temperature increases above 300◦ C, however, the order of the magnitude of the rates for the two reactions becomes about equal. Then, a new reaction route, a so-called Claus reaction occurs; SO2 that is produced from the oxidation of sulfur reacts with H2 S to form sulfur and water. Considering the results of Steijns et al. [29] and our experiments, we conclude that the irreversible selective oxidation of H2 S to sulfur occurs dominantly and rapidly at low temperature (below 260◦ C), where both conversion and selectivity are kept above 90%. As temperature increases, the Claus reaction that removes SO2 by reaction with H2 S to form sulfur begins to dominate the complete oxidation of the product sulfur to SO2 , which keeps the sulfur selectivity still high. Observation of higher values of the conversion and selectivity than equilibrium values is possible when the rate of the irreversible selective oxidation proceeds at a much faster rate than the reversible Claus reaction. The fact that V2 O5 catalysts exist in reduced forms during the reaction leads us to assume that the selective oxidation reaction occurs via the redox mechanism with participation of the lattice oxygen in vanadium oxide during the reaction [30]. This is indirectly supported from the results in Figs. 3 and 6; oxidized catalyst is very active to yield 100% conversion but results in the total oxidation. It is natural to assume that oxidized catalyst has more labile oxygen (both chemisorbed and lattice oxygen) than reduced one. The selective oxidation of H2 S to sulfur proceeds only when V2 O5 is substantially reduced (more than 20%). The degree of reduction of catalyst during the reaction is determined by comparing the relative magnitudes of the reduction force due to H2 S with the oxidation force due to the gas phase oxygen. A redox mechanism on V2 O5 catalysts is also suggested by others in hydrocarbon oxidation and SO2 oxidation reactions [29,31]. As shown in Figs. 3–6, deactivation occurs at temperatures below 250◦ C. Hass et al. [10] have
Fig. 13. Effect of temperature on catalyst deactivation of unsupported V2 O5 : conversion (䊉) and selectivity (䊊) at 235◦ C; conversion (䊏) and selectivity (䊐) at 335◦ C. Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 106,000 h−1 .
suggested that the formation of vanadyl sulfate may be the cause of deactivation in the presence of water (above 1.5 psia) at 120–320◦ C. There is doubt, however, about whether to attribute the cause of deactivation solely to the formation of vanadyl sulfate. The results in Fig. 13 confirm that no catalyst deactivation is observed with time on stream during the reaction at 335◦ C, although XPS measurements in Fig. 10 detect the existence of sulfur and vanadyl sulfate species on the surface of the catalyst used in Fig. 13. We suggest that the deactivation is caused by oxygen unbalance in the catalyst due to slower oxidation rate by gas phase oxygen, compared with the reduction rate by H2 S. The reduction becomes faster and faster when the reaction temperature is decreased or when more H2 S is introduced (at higher space velocity). This results in an irreversible deactivation (reduction) of catalyst due to the oxygen unbalance as shown in Figs. 3, 4 and 13. It is possible to regenerate severely deactivated catalyst by flowing oxygen at a high temperature of 335◦ C (Fig. 4). These observations imply that the re-oxidation of reduced VOx by gas-phase molecular oxygen has higher activation
M.Y. Shin et al. / Applied Catalysis A: General 211 (2001) 213–225
energy, and acts as a rate-determining step at low temperatures. A similar observation has been made by Al’kaeva et al. [32] for the oxidation of propylene over Mo–V–Si oxide catalysts. They have observed that the re-oxidation of catalyst surface becomes the limiting step at low temperatures. As temperature increases, the rate of re-oxidation increases at faster rate than that of reduction, and this changes the rate-determining step. They have also observed the increase in the diffusion rate of lattice oxygen with increased temperature, which facilitates re-oxidation of the reduced surface [33–35]. Bosch et al. [36] have observed that the reduction of V2 O5 proceeds through several stages: V2 O5 → V6 O13 → VO2 → V2 O3 . Colpaert et al. [37] have reported the temperature dependence for the reduction of V2 O5 . It is interesting to observe that the catalyst is in a severely reduced state during the reaction. Fresh V2 O5 is reduced usually to the VO2 phase during the reaction as shown in Figs. 6–8. At a high severity condition of T = 335◦ C and space velocity = 106, 000 h−1 , the reduction proceeds further toward V2 O3 (Fig. 8), but the conversion keeps a steady state as a function of reaction time as shown in Fig. 13. A fast rate of re-oxidation and increased mobility of lattice oxygen at high temperatures prevent the irreversible reduction of the catalyst at such a high severity condition. The product sulfur seems to be oxidized easily to SO2 when there is labile oxygen on the catalyst surface. The results in Fig. 6 have shown that only SO2 is produced if vanadium oxide keeps the O/V ratio greater than 2.26 (9.6% reduction of V2 O5 ). It would be natural to assume that the reaction surface on which H2 S chemisorbs has a concentration gradient of oxygen from the bulk to the surface (or from one face of vanadium crystal to the reaction face). In this case, the reaction surface must be in a more highly reduced state than the averaged value of 9.6%, which assumes no concentration gradient throughout the bulk and surface of vanadium oxide particle. This can be confirmed from the results in Fig. 6. It must be remembered, at the present moment, that the reduction step (a) in Fig. 6 with only H2 S is carried out at low temperature of 254◦ C where the oxygen diffusion is very much limited. The complete oxidation of H2 S to SO2 (zero selectivity to sulfur) is maintained up to the point at which about three times of monolayer oxygen in vanadium oxide is lost. Although, we do not know
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quantitatively how deep inside the bulk phase of the catalyst is reduced in the above case, it is appropriate to assume that the surface is severely reduced due to the limited diffusion of oxygen from the bulk at such a low temperature. High selectivity to sulfur is obtained when the catalyst keeps the O/V ratio smaller than 1.94 (22.4% reduction of V2 O5 ). In such circumstances, the product sulfur on the catalyst surface probably cannot find any reactive oxygen nearby due to depletion of labile oxygen on the surface. The severe reduction of vanadium oxide accompanies changes in morphology, surface area, and pore structure at the same time. From the analyses with XRD, XPS, and XANES, it can be observed that, after the reaction, the original V2 O5 crystals disintegrate into micro-cystalline clusters. Thus, the surface areas of supported catalysts decrease after the reaction, probably due to the blocking of support SiO2 pores by micro-crystalline VOx clusters, while the surface area of bulk V2 O5 increases. In the pore-size distribution (Fig. 11), the generation of meso- and macro-pores are observed after the reaction of vanadium oxide. The structure and mechanism of the V2 O5 reduction and the resulting transformations into different vanadium oxides have been studied by several groups [37–39]. The stereochemistry of vanadium ions in V2 O5 may be considered to be either a distorted trigonal bipyramid (five V–O bond lengths of 1.58–2.02 Å), a distorted tetragonal pyramid, or a distorted octahedron (the sixth V–O bond length of 2.79 Å) [39]. Three kinds of oxygen atoms are present in the V2 O5 structure: vanadyl oxygen O(1) atoms having only one strong bond (V–O = 1.58 Å), bridging oxygen O(2) atoms forming two bonds (V–O = 1.77 Å) with the V–O(2)–V angle of 125◦ and bridging oxygen O(3) atoms forming three bonds, two at 1.88 Å and one at 2.02 Å. Based on experiments (electrical conductivity, EPR and IR spectra) and quantum-chemical calculations, Harber et al. [39] have suggested that the V–O(3) bond at 2.02 Å will be broken most easily among V–O bonds, and such a break will facilitate the cleavage along the [1 0 0] direction. Short distances of O(3)–O(3) indicates strong repulsive forces between oxygen atoms, and this is in agreement with recent experimental observations [31,39,40]. The following two conditions make vanadium oxides function as a catalyst in the selective oxidation. First, the crystal structure of V2 O5 is characterized by the existence of
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channels in the [0 0 1] and [0 1 0] directions, which have the effective diameter of 2 Å and, thus, permit a facile diffusion of oxygen. Second, the ability of vanadium atoms to possess multiple stable oxidation states from two to five results in an easy conversion between oxides of different stoichiometry by oxidation or reduction. The presence of shear planes, initiated by the removal of oxygen (i.e. anion vacancies or defects), at the boundary between two next neighboring oxide nuclei (such as V2 O5 and V6 O13 nuclei) locally enhances the rate of oxygen loss and allows the reduced phase (V6 O13 ) to grow into the bulk generating micro-pores [37–39]. Considering the reduction characteristics of vanadium oxide and the mismatch of the pore structure with the De Boer classification [28] (shown in Fig. 12), the generation of meso- and macro-pores after the reaction may originate from the agglomeration of micro-pores generated in the reduction process.
5. Conclusion The present results show that VOx /SiO2 and bulk V2 O5 catalysts are effective for the selective oxidation of H2 S to elemental sulfur over a wide temperature range, 270–350◦ C. Very high values (>90%) of the H2 S conversion and selectivity to sulfur can be obtained over various V loadings (5–100 wt.%) if the reaction temperature is kept above 270◦ C and the space velocities below 4000 h−1 . The conversion and selectivity exceed the equilibrium values probably because the irreversible selective oxidation of H2 S to sulfur occurs at much faster rate compared with the reversible Claus reaction. Reaction tests and characterization of catalysts using XRD, XPS and XANES have revealed that V2 O5 catalysts exist in a severely reduced state of VO2 or V2 O3 during the reaction. Oxidized form of vanadium oxide with O/V ratio greater than 2.26 yields zero selectivity to sulfur and gives only the complete oxidation product, SO2 . The reaction proceeds via a redox mechanism; the reduction force by H2 S is balanced with the oxidation force by gas phase oxygen during the reaction. For this reason, the catalyst shows eventual deactivation with time on stream when the reduction rate is faster than the reoxidation rate, and this is the case where the space velocity is kept high or the V loading is kept low at low reaction
temperatures (<250◦ C). Even for the stable reaction at high temperatures, the severely reduced state of vanadium oxide during the reaction accompanies an increase of surface area and the generation of mesoand macro-pores, due to disintegration of vanadium oxide crystal and structural change.
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Selective oxidation of H2 S to elemental sulfur over VOx /SiO2 and V2 O5 catalysts Moon Young Shin a , Chang Mo Nam a , Dae Won Park b , Jong Shik Chung a,∗ a
Department of Chemical Engineering, School of Enviromental Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja Dong, Pohang 790-784, South Korea b Department of Chemical Engineering, Pusan National University, Pusan 609-735, South Korea Received 31 August 2000; received in revised form 30 October 2000; accepted 30 October 2000
Abstract The selective oxidation of H2 S was investigated over VOx /SiO2 catalysts (1–100% V contents) using a packed-bed flow reactor in the temperature range 200–350◦ C. When a stoichiometric ratio of O2 /H2 S is used in the feed, high conversion of H2 S and high selectivity to sulfur (>90%) can be achieved at various V loadings (5–100 wt.%) if the space velocity is kept below 4000 h−1 and the reaction temperature above 270◦ C. About 10–30% decrease of the sulfur yield is observed in the presence of 30% water in the feed. The conversion of H2 S and selectivity to sulfur exceed the equilibrium values. Reaction tests and characterization using XRD, XPS and XANES have revealed that during the reaction, the V2 O5 catalysts are reduced severely to VO2 and/or V2 O3 , resulting in changes in the surface area and pore size. The sulfur selectivity can be kept high (>90%) only at such a highly reduced condition. Oxidized catalyst (with O/V ratio > 2.26) exhibits complete oxidation of H2 S to SO2 (zero sulfur selectivity). As the reaction proceeds via redox mechanism, high space velocities and low V loadings at low reaction temperatures (<250◦ C) result in eventual decrease of the conversion with time on stream due to depletion of labile oxygen on the surface. © 2001 Elsevier Science B.V. All rights reserved. Keywords: H2 S; Selective oxidation; Sulfur; V2 O5 ; Deactivation
1. Introduction In petroleum refineries and natural gas plants, hydrogen sulfide (H2 S) is typically collected by contact with amine-based solutions, and subsequently fed to a well known Claus plant [1,2]. The Claus process consists of two steps: thermal oxidation and catalytic reaction. In the thermal oxidation, one-third of H2 S is first burned with air to produce sulfur dioxide in a waste heat furnace, while unconverted H2 S reacts with SO2 to elemental sulfur through the subsequent ∗ Corresponding author. Tel.: +82-54-279-2267; fax: +82-54-279-5799. E-mail address: [email protected] (J.S. Chung).
catalytic reaction over Al2 O3 catalysts. However, due to thermodynamic limitations, 3–5% of H2 S is typically not converted to sulfur. As the environmental regulations become more stringent, it is necessary to further treat the residual (tail) gas of the Claus plants. H2 S in the tail gas of Claus plants or from other emission sources has been conventionally treated by various techniques such as adsorption, absorption, and wet oxidation [1]. Recently, a dry catalytic process has been developed for the selective catalytic oxidation of H2 S to elemental sulfur. Examples of commercially developed catalysts for this purpose are the titanium-based catalysts in the MODOP process [3,4] and the iron-based catalysts in the Super Claus process [5–7].
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 0 ) 0 0 8 6 6 - 8
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These processes are based on the following irreversible selective oxidation of H2 S to S as a main reaction (Eq. (1)), the other oxidation reactions (Eqs. (2) and (3)) and the Claus reaction (Eq. (4)). 1 1 H2 S + O2 → Sn + H2 O 2 n
(1)
1 Sn + O2 → SO2 n
(2)
H2 S +
(3)
3 2 O2
→ SO2 + H2 O
3 2H2 S + SO2 ↔ Sn + 2H2 O n
(4)
In the MODOP process, H2 S is oxidized into elemental sulfur with a stoichiometric amount of oxygen over TiO2 catalysts. One drawback of the MODOP process is that water should be removed from the tail gas before the reaction, due to the deactivation of catalysts in the presence of water. In the Super Claus process, tail gas can be treated without a dehydration step, since the Fe2 O3 /SiO2 catalyst is not deactivated even in the presence of 30 vol.% of water vapor. However, the Super Claus process cannot treat high concentrations of H2 S above 2 vol.% because it is necessary to supply excess oxygen (usually 10 times the stoichiometric amount) to overcome the catalytic deactivation caused by water. Catalysts containing vanadium oxide have also been reported to be active for the oxidation of H2 S to sulfur both with a stoichiometric and an excess amount of oxygen [8–15]. VBiOx /SiO2 was used to improve a long-term deactivation problem noticed in V/SiO2 , but it needed a dehydration step before being sent to a catalytic reactor to increase H2 S conversion (BSR Selectox process) [9,10]. It has been suggested that the formation of less active forms of vanadium such as vanadyl sulfate (VOSO4 ) is one cause of the deactivation. Various binary oxides such as V–Mg, V–Bi, V–Mo, V–Sb, Fe–Sn and Bi–Mo were tested in excess oxygen without water [11–13]. Solid solutions of vanadium, A4 ± y V2 ± x O9 (A = Mg, Ca or Zn, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.5) [14,15] were also reported as active catalysts, even in the presence of 30 vol.% of water. This study presents the catalytic behaviors of VOx /SiO2 at various V loading amounts (1–100%) for the selective oxidation of H2 S. Various reaction
parameters such as V loading, O2 /H2 S ratio in the feed, temperature, space velocity and reaction time are investigated to obtain a high yield of sulfur at a wide window of operation condition. Physical and structural characterization are also performed for understanding the reaction mechanism and the causes of catalyst deactivation observed at specific conditions. 2. Experimental The supported catalysts, VOx /SiO2 , were prepared by an evaporation method with 1 to 30 wt.% of V loading as was previously described elsewhere [25,26]. Bulk V2 O5 was purchased from Junsei Co. Ltd. (99.9%) and used after calcination at 400◦ C for 5 h using air. The catalytic reaction was performed using a vertical packed-bed reactor made of 2.54 cm i.d., Pyrex tube. Hydrogen sulfide (Solkatronic Chemical, 1–5 vol.%) and air (2–5 vol.%) were mixed and diluted with helium by mass-flow-controllers. Total flow rate ranged from 50 to 160 ml/min, and hence the gas hourly space velocity (GHSV) was between 500–140,000 h−1 . About 80–100 mesh of catalyst powder was diluted five times with the same mesh size of glass powder. Water was injected using a syringe pump to a vaporizer located before the reactor. Sulfur in the product was separated using a sulfur condenser placed at the effluent side of the reactor. The temperature of the sulfur condenser was maintained at 110◦ C. A line filter was also installed after the sulfur condenser to trap any sulfur mist that was not collected by the condenser. To prevent the condensation of sulfur and water vapor from the line filter up to the gas chromatograph, all the lines and fittings were heated above 150◦ C. The gaseous products, H2 S and SO2 were analyzed by a gas chromatograph (Hewlett Packard 5890) with a Porapak T column (80/100 mesh, 1.5 m) and a thermal conductivity detector. The conversion of hydrogen sulfide, the selectivity to sulfur, and the yield of sulfur were defined, respectively, as follows: Conversion of H2 S(%)=
[H2 S]inlet −[H2 S]outlet × 100 [H2 S]inlet
Selectivity of sulfur(%) [H2 S]inlet − [H2 S]outlet − [SO2 ]outlet × 100 = [H2 S]inlet [H2 S]outlet
M.Y. Shin et al. / Applied Catalysis A: General 211 (2001) 213–225
Yield of sulfur(%) Conversion of H2 S × Selectivity of sulfur = 100 The calculation of the chemical equilibria of a multi-component system was performed by using the SOLGASMIX-PV code, developed by Eriksson [16]. It takes the atoms of a particular chemical mixture and determines the product assemblage, which has the minimum Gibbs free energy subject to the mass-balance constraint [17,18]. With this program, numerous chemical equilibrium analyses of solid–gas systems have already proven useful in understanding the chemical processes [19]. The thermodynamics of an oxygen-based Claus process was also evaluated by Khudenko et al. [20], who used the STANJAN chemical equilibrium solver program, which utilizes the principle of minimization of Gibbs free energy. Calculated equilibrium conversion and selectivity are verified with those obtained following the method suggested by Gamson et al. [21], which considers the partial and complete oxidation of H2 S (Eqs. (1)–(4)) and following equilibrium reactions between the produced sulfur species below. S6 ↔ 3S2
(5)
S8 ↔ 4S2
(6)
X-ray diffraction patterns of catalysts were obtained using a X-ray analyzer (M18XHF, MAC Science Co.). Ni-filtered Cu K␣ radiation (λ = 1.5415 Å) was used with an X-ray gun operated at 40 kV and 200 mA. Diffraction patterns were obtained within the range of 2θ = 10–90◦ at a scan rate of 4◦ /min. To observe changes in the catalyst surface after reactions, XPS spectra were obtained using a Kratos XSAM 800pci X-ray photoelectron spectrometer with Al K␣ monochromatic X-ray (1487 eV) radiation. Catalysts were uniformly ground into powder and then pressed into self-supporting wafers without any binders, followed by a pre-treatment at an ultra-high vacuum of 10–12 mmHg for 10 h. The charging effect of XPS spectra was carefully corrected with a carbon peak at 284.6 eV as a standard. X-ray absorption near-edge spectra, XANES, were performed on 3C1 EXAFS beamline at the Pohang Acceleration Laboratory (PAL) with ring energy of 2.0 GeV. XANES were recorded using the EXAFS
215
facilities installed in the transmission mode at room temperature with a Si(1 1 1) two-crystal monochrometer. The detector gases for the incident beam was N2 (100 vol.%), and the gas for the transmitted beam was N2 (85 vol.%) + Ar(15 vol.%). The energy resolution of ca. 0.56 eV can be achieved at the V K edge, 5471.0 eV. The obtained XANES data were analyzed by the R-space method with the UWXAFS 3.0 package. The specific surface areas and the pore-size distribution of the catalysts were measured using a BET apparatus (Accusorb 2100E, Micromeritics).
3. Results 3.1. Reaction measurements Fig. 1 shows the effect of the O2 /H2 S mole ratio on the conversion of H2 S and the selectivity to sulfur over the V(30)/SiO2 catalyst. The oxygen concentration was varied from 2 to 5 vol.%, while the H2 S and H2 O concentrations were kept constant at 5 and 0 or 30 vol.%, respectively. As the O2 /H2 S ratio increases from 0.4 to 1.0, the conversion increases from 72 to 99%, while the selectivity decreases from 98 to 42%,
Fig. 1. Effect of O2 /H2 S feed ratio on the conversion of H2 S (䊉) and selectivity to sulfur (䊊) for V(30)/SiO2 . Reaction conditions: H2 S 5 vol.%, H2 O 30 vol.%, T = 225◦ C, GHSV = 96,000 h−1 .
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Table 1 The conversion of H2 S and the selectivity to sulfur with different vanadium loadingsa V loading (wt.%)
1.0 5.0 10.0 30.0 100.0 Equilibrium value a b
Conversion and selectivity (%)
Without H2 O, temperature (o C) 225
250
275
300
325
225
250
275
300
325
Conversion Selectivity Conversion Selectivity Conversion Selectivity Conversion Selectivity Conversion Selectivity Conversion Selectivity
50.1b
75.2 91.0 96.0 96.5 97.3 97.5 95.5 98.8 97.2 98.0 95.0 97.4
89.0 94.1 94.1 95.5 95.0 97.0 93.4 98.0 96.0 97.5 93.0 96.3
91.5 95.2 92.0 94.0 93.2 96.6 91.7 96.2 94.1 96.6 90.6 94.9
92.0 94.0 92.2 92.1 90.1 94.5 91.0 94.0 92.0 94.5 87.8 93.1
– – – – 82.1 89.3 92.0 95.0 93.2 94.5 87.8 93.0
– – – – 85.0 91.0 88.2 92.0 88.0 92.3 82.0 89.0
– – – – 83.8 90.2 84.0 89.3 84.0 90.0 75.1 83.4
– – – – 79.0 83.1 78.3 84.0 80.0 85.0 67.1 75.5
– – – – 73.3 78.0 74.0 78.0 75.0 78.6 58.4 64.1
91.0b 96.0 96.5 98.0 98.0 98.0 99.1 98.2 98.6 96.7 98.3
With 30 vol.% H2 O, temperature (o C)
Reaction conditions; GHSV 3000 h−1 , 5 vol.% of H2 S, 2.5 vol.% of O2 balanced with He. Conversion measured after 20 h reaction at 225◦ C, where the conversion is steadily decreased from initial value of 95.0%.
indicating that the highest yield of sulfur is near the stoichiometric ratio of O2 /H2 S (0.5). Therefore, most of the reactions hereafter were conducted at these conditions, 5 vol.% H2 S, 2.5 vol.% O2 , and 0 or 30 vol.% H2 O balanced with He (O2 /H2 S = 0.5), unless specified. The effects of vanadium loading and temperature on the conversion of H2 S and selectivity to sulfur are summarized in Table 1. High values (more than 90%) of the conversion and selectivity can be achieved over wide ranges of vanadium loading and temperature, except for a few data points where the vanadium loading and reaction temperature are kept very low. As mentioned in the table, the data points obtained with V(1)/SiO2 catalyst are not steady state values but dynamic values obtained after 20 h of reaction as the catalyst exhibits a continuous decrease in the conversion as reaction proceeds at temperatures below 250◦ C. Catalysts having high vanadium loading (higher than 10%) do not show any deactivation within 20 h of reaction even at low temperatures, indicating to us that the bulk phase of vanadium is more active than the dispersed phase. According to Bars et al. [22], 10 wt.% V loading is the theoretical monolayer coverage of vanadium oxide over SiO2 (300 m2 /g) in the form of VO4 . The presence of water in the feed also decreases the sulfur yield, 10–30%, indicating that in addition to the irreversible selective oxidation
reaction (1), total oxidation reactions ((2) and (3)) and the reversible Claus reaction occur to some extent. The effect of space velocity was investigated over V(30)/SiO2 as functions of reaction temperature and time, as shown in Fig. 2. Conversion and selectivity measured at 20 h of reaction exceed the equilibrium values. As the space velocity increases up to 135,000 h−1 , there is an eventual decrease of the conversion with time on stream and a steady state in the conversion cannot be obtained at low temperature (i.e. 230◦ C). In contrast to this, the selectivity always remains constant with time on stream. 3.2. Deactivation and regeneration of catalysts In order to study the cause of deactivation, the reaction was carried out intentionally at a low temperature of 230◦ C. Fig. 3 shows the decreasing patterns in the conversion with time on stream, which can be observed at high space velocities. It is noteworthy to observe that at low space velocities, the selectivity at the beginning stage of the reaction is very low. We conclude, therefore, that fresh vanadium in its fully oxidized state is very active for the total oxidation of H2 S and not for the selective oxidation. The effect of vanadium loading on the catalyst deactivation is shown in Fig. 4. The lower the vanadium loading is, the faster the deactivation of catalyst is. These
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Fig. 2. Effect of space velocity on the conversion of H2 S and selectivity to sulfur over V(30)/SiO2 : (䊉) 500; (䊏) 4000; (䉱) 8000; (䉲) 65,000; (䉬) 135,000 h−1 ; equilibrium conversion and selectivity (dashed line). Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%.
results lead us to assume that H2 S consumes oxygen in vanadium oxide and gas phase oxygen replenishes the reduced sites of vanadium oxide, i.e. the reaction occurs via the redox mechanism. Faster deactivation at lower vanadium loading or at higher space velocity supports the conclusion that the reoxidation step by gas phase O2 proceeds at a much slower rate than the reduction step by H2 S, especially when the temperature is low. In other words, the reoxidation of reduced VOx by gas-phase O2 requires higher activation energy. This was confirmed by regeneration experiments
with flowing air at different temperatures over the deactivated catalyst, as shown in Fig. 5. Reoxidation (step (b)) of the catalyst deactivated for 10 h at 225◦ C (during step (a)) does not show any regeneration effect. When the reoxidation temperature is raised to 335◦ C (step (d)), the same period of the oxygen treatment regenerates the deactivated catalyst to a certain degree; i.e. 40% conversion at the end of the reaction step (c) is increased to 50% at the beginning of reaction step (e). Moreover, during the reaction at 335◦ C (step (e)), the conversion is increased (the reduced
Fig. 3. Deactivation patterns of V(30)/SiO2 catalysts with time on stream at various space velocities: (䊉, 䊊) 500; (䊏, 䊐) 4000; (䉱, 4) 8000; (䉲, 5) 65,000; (䉬, 䉫) 135,000 h−1 . Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, T = 230◦ C.
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Fig. 4. Effect of V loading on catalyst deactivation: (䊉) V(5)/SiO2 ; (䊏) V(10)/SiO2 ; (䉱) V(20)/SiO2 ; (䉲) V(30)/SiO2 ; (䉬) V2 O5 . Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 30,000 h−1 for V(5)/SiO2 and V(10)/SiO2 , GHSV = 65,000 h−1 for V(20)/SiO2 and V(30)/SiO2 , GHSV = 106,000 h−1 for V2 O5 , T = 230◦ C.
catalyst is oxidized) and reaches a steady state. This implies that, with temperature increase, increased oxidation rate is now balanced with the reduction rate by H2 S so that the catalyst keeps its reduction state constant during the reaction. Fig. 6 presents the oxygen effect in the feed on the conversion and selectivity at 254◦ C over bulk V2 O5 . We used unsupported V2 O5 for the transient reaction test simply because, compared with vanadium dispersed on SiO2 , V2 O5 will have the largest amount of oxygen storage per gram of catalyst. When there is a change in the feed composition, a larger amount of the oxygen storage will give a longer transition period. This allows us to detect transition states more easily. The reaction was first carried out with 1 vol.% H2 S balanced with He at 254◦ C and at GHSV of 3000 h−1 for 180 min (step (a)). After purging the reactor with inert He for 100 min (step
Fig. 5. Reaction and regeneration at various conditions, conversion (䊉) and selectivity (䊊): (a) reaction at 225◦ C; (b) regeneration in flowing air 50 ml/min for 10 h at 225◦ C; (c) reaction at 225◦ C; (d) regeneration in flowing air 50 ml/min for 5 h at 335◦ C; (e) reaction at 335◦ C. Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 65,000 h−1 .
(b)), the feed was changed into a stoichiometric mixture of H2 S/O2 (step (c)). The point 1 shown in the figure corresponds to the case where vanadium oxide loses the monolayer amount of oxygen during the reaction. The points 2, 3 and 4 represent vanadium oxides in reduced states having the O/V ratios 2.26, 1.94 and 1.87, respectively. These values were calculated by integrating the oxygen consumption that was obtained from the conversion as a function of reaction period. The monolayer amount of oxygen was calculated based on the assumption that the surface area of VO4 is 0.1 nm2 at the surface [22]. When oxygen is relatively abundant on the surface of vanadium oxide (O/V ratio > 2.26), H2 S is completely oxidized to SO2 without the production of sulfur. When catalyst is in a severely reduced state (O/V ratio < 1.94), the conversion begins to decrease but the sulfur selectivity becomes almost 100%. Passing He and then oxygen-containing feed increases the conversion abruptly, and eventually the conversion reaches a steady value.
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3.3. Characterization of catalysts The BET surface areas of the catalysts are shown in Table 2. With increased V loading from 1 to 30 wt.%, the surface area is decreased substantially from 291.4 to 142.2 m2 /g. After reaction, supported catalysts show decreases in the surface area, whereas unsupported catalysts show increased surface area. XRD patterns of VOx /SiO2 before and after the reaction are shown in Fig. 7. After the reaction, the XRD peaks disappeared or became weakened, implying that the catalysts become either amorphous or their crystallite sizes become smaller. Since, we failed to detect any phase change with the supported catalysts owing to the X-ray transparency, unsupported V2 O5 catalysts were tested at several reaction conditions; the results are shown in Fig. 8. Although not shown here, the diffraction peaks of fresh V2 O5 were very sharp and strong. Compared with fresh V2 O5 , the diffraction peaks of all the reacted catalysts exhibit decreases in their intensities
Fig. 6. Reactions with and without oxygen supply and regeneration over V2 O5 , conversion (䊉) and selectivity (䊊): (a) 1 vol.% H2 S only; (b) He; (c) 1 vol.% H2 S and 0.5 vol.% O2 , GHSV = 3000 h−1 , T = 254◦ C; (1) monolayer amount of oxygen (O), 20 min, (2) V:O = 1:2.26, (3) V:O = 1:1.94, (4) V:O = 1:1.87.
Table 2 Changes of BET surface area before and after reactiona Catalyst
Before reaction (m2 /g)
After reaction (m2 /g)
SiO2 Davisil V(1)/SiO2 V(5)/SiO2 V(10)/SiO2 V(30)/SiO2 Bulk V2 O5 Bulk V2 O5 Bulk V2 O5
306.6 291.4 264.1 227.5 142.2 6.7 – –
264.6b 232.9b 224.7b 108.5b 17.3b 12.1c 20.5d
a Feed gas; 5 vol.% of H2 O balanced with 84,000 h−1 . b Measured after the c Measured after the d Measured after the
of H2 S, 2.5 vol.% of O2 and 30 vol.% He. Rection period = 70 h, GHSV = reaction shown in Table 1. reaction at 235◦ C. reaction at 335◦ C.
Fig. 7. XRD patterns of various VOx /SiO2 catalysts before (dotted) and after the reaction (solid line). Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 106,000 h−1 , T = 335◦ C.
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Fig. 8. XRD patterns of unsupported V2 O5 catalysts after the reaction at various temperatures and time periods: (a) blank scotch tape; (b) 2.3 h at 235◦ C; (c) 12 h at 235◦ C; (d) 40 h at 235◦ C; (e) 70 h at 235◦ C; (f) 3.5 h at 335◦ C; (g) 12 h at 335◦ C; (h) 40 h at 335◦ C; (i) 70 h at 335◦ C. Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 106,000 h−1 , T = 335◦ C.
by an order of magnitude, indicating to us that there is a substantial amount of change in either structure or morphology of V2 O5 after reaction. At the mild reaction condition of 235◦ C ((b) through (f)), fresh catalyst of V2 O5 is reduced to VO2 . At high temperature of 335◦ C ((g), (h) and (i)), a slow but additional phase transition from VO2 to V2 O3 occurs. In the case of (i), no X-ray peak is observed probably due to severe change in the crystal structure and morphology. These phenomena can be further observed through the SEM images (not shown) [25]. XANES were carried out for both fresh and used catalysts to observe the amorphous state of catalyst. The results in Fig. 9 confirm again that VOx /SiO2 and unsupported V2 O5 are transformed to VO2 phase after the reaction at 335◦ C. Fig. 10 shows the XPS spectra of the V2 O5 catalysts before and after reaction. The standard XPS peaks of V 2p3/2 for V+5 , V+4 ,
Fig. 9. XANES analyses of various V/SiO2 and bulk V2 O5 before and after reaction: (a) reference V2 O3 ; (b) reference V2 O4 ; (c) reference V2 O5 ; (d) V(5)/SiO2 before reaction; (e) V(5)/SiO2 after reaction; (f) V(10)/SiO2 before reaction; (g) V(10)/SiO2 after reaction; (h) V2 O5 before reaction; (i) V2 O5 after reaction. Conditions are the same as in Table 1.
and V+3 are located at 517.2, 515.9 and 515.3 eV, respectively [23,24]. The results in case (a) confirm previous observations that fresh V2 O5 is transformed into VO2 after the reaction at 335◦ C. There is also a change in the O1s peak (case (b)) after the reaction. Since, the oxygen 1s peaks for oxide and sulfate form are, respectively, 529.9–530.5 and 531.2–532.4 eV, it is believed that used catalyst contains some portion of vanadyl sulfate on the surface. The sulfur 2p peak (case (c)) also confirms the existence of elemental sulfur and vanadyl sulfate species that are located at 163.8–164.0 and 166–170 eV, respectively. Pore size distributions (PSD) were measured for fresh V2 O5 catalyst and for those obtained after reactions at 235–335◦ C, the results are shown in Fig. 11. The original pore size shown in the fresh V2 O5 (28 Å in diameter) decreased or disappeared after reactions. Pores of about 38 Å are generated after the reaction at 235◦ C, and pores around 200 Å are additionally generated after the reaction at 335◦ C. Adsorption and desorption patterns observed during the PSD are shown in Fig. 12. According to the classification of Brunauer
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Fig. 10. XPS analyses of V2 O5 before (solid line) and after reaction (dashed line for 235◦ C and dotted line for 335◦ C): (a) V 2p3/2 ; (b) O 1s; (c) S 2p. Conditions; H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 106,000 h−1 .
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Fig. 11. Pore size distribution (PSD) of V2 O5 measured before (a, fresh) and after reaction (b, 235◦ C; c, 335◦ C). Conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, t = 70 h, GHSV = 84,000 h−1 .
[27], the shape of the curve observed after the reaction corresponds to type II, which is commonly encountered on macro-porous material. In terms of De Boer classification [28], there are five types of hysteresis loops which represent the nature of the condensation process in the pores, depending on the associated pore shapes. The adsorption isotherm of V2 O5 exhibits only mild hysteresis, and the shape of hysteresis for our catalysts does not belong to any of the five types.
4. Discussion The results of the present work have shown that the catalytic activity has relevance to the bulk property of V2 O5 . The H2 S conversion increases with increased VOx loading (Table 1, Fig. 5) and decreases with an increase of space velocity (Figs. 2 and 3), especially at low temperatures. The conversions measured at temperatures above 260◦ C are higher than the equilibrium values. The selectivities to S, however, remain high enough above 90% even at low temperatures
Fig. 12. Adsorption and desorption branch of the pore size distribution for V2 O5 after reaction: (a) 235◦ C; (b) 335◦ C. Adsorption branch (solid line) and desorption (dotted line). Conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, t = 70 h, GHSV = 84,000 h−1 .
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regardless of the change of V loading and space velocity. Steijns et al. [29] have found that, using coal as a catalyst, that the oxidation rate of sulfur to SO2 was quite slow compared with that of H2 S to sulfur when the reaction temperature was kept below 200◦ C. This is because the activation energy for the sulfur oxidation (125 ± 10 kJ mol−1 ) is much higher than that for the H2 S oxidation (∼40 kJ mol−1 ). As the reaction temperature increases above 300◦ C, however, the order of the magnitude of the rates for the two reactions becomes about equal. Then, a new reaction route, a so-called Claus reaction occurs; SO2 that is produced from the oxidation of sulfur reacts with H2 S to form sulfur and water. Considering the results of Steijns et al. [29] and our experiments, we conclude that the irreversible selective oxidation of H2 S to sulfur occurs dominantly and rapidly at low temperature (below 260◦ C), where both conversion and selectivity are kept above 90%. As temperature increases, the Claus reaction that removes SO2 by reaction with H2 S to form sulfur begins to dominate the complete oxidation of the product sulfur to SO2 , which keeps the sulfur selectivity still high. Observation of higher values of the conversion and selectivity than equilibrium values is possible when the rate of the irreversible selective oxidation proceeds at a much faster rate than the reversible Claus reaction. The fact that V2 O5 catalysts exist in reduced forms during the reaction leads us to assume that the selective oxidation reaction occurs via the redox mechanism with participation of the lattice oxygen in vanadium oxide during the reaction [30]. This is indirectly supported from the results in Figs. 3 and 6; oxidized catalyst is very active to yield 100% conversion but results in the total oxidation. It is natural to assume that oxidized catalyst has more labile oxygen (both chemisorbed and lattice oxygen) than reduced one. The selective oxidation of H2 S to sulfur proceeds only when V2 O5 is substantially reduced (more than 20%). The degree of reduction of catalyst during the reaction is determined by comparing the relative magnitudes of the reduction force due to H2 S with the oxidation force due to the gas phase oxygen. A redox mechanism on V2 O5 catalysts is also suggested by others in hydrocarbon oxidation and SO2 oxidation reactions [29,31]. As shown in Figs. 3–6, deactivation occurs at temperatures below 250◦ C. Hass et al. [10] have
Fig. 13. Effect of temperature on catalyst deactivation of unsupported V2 O5 : conversion (䊉) and selectivity (䊊) at 235◦ C; conversion (䊏) and selectivity (䊐) at 335◦ C. Reaction conditions: H2 S 5 vol.%, O2 2.5 vol.%, H2 O 30 vol.%, GHSV = 106,000 h−1 .
suggested that the formation of vanadyl sulfate may be the cause of deactivation in the presence of water (above 1.5 psia) at 120–320◦ C. There is doubt, however, about whether to attribute the cause of deactivation solely to the formation of vanadyl sulfate. The results in Fig. 13 confirm that no catalyst deactivation is observed with time on stream during the reaction at 335◦ C, although XPS measurements in Fig. 10 detect the existence of sulfur and vanadyl sulfate species on the surface of the catalyst used in Fig. 13. We suggest that the deactivation is caused by oxygen unbalance in the catalyst due to slower oxidation rate by gas phase oxygen, compared with the reduction rate by H2 S. The reduction becomes faster and faster when the reaction temperature is decreased or when more H2 S is introduced (at higher space velocity). This results in an irreversible deactivation (reduction) of catalyst due to the oxygen unbalance as shown in Figs. 3, 4 and 13. It is possible to regenerate severely deactivated catalyst by flowing oxygen at a high temperature of 335◦ C (Fig. 4). These observations imply that the re-oxidation of reduced VOx by gas-phase molecular oxygen has higher activation
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energy, and acts as a rate-determining step at low temperatures. A similar observation has been made by Al’kaeva et al. [32] for the oxidation of propylene over Mo–V–Si oxide catalysts. They have observed that the re-oxidation of catalyst surface becomes the limiting step at low temperatures. As temperature increases, the rate of re-oxidation increases at faster rate than that of reduction, and this changes the rate-determining step. They have also observed the increase in the diffusion rate of lattice oxygen with increased temperature, which facilitates re-oxidation of the reduced surface [33–35]. Bosch et al. [36] have observed that the reduction of V2 O5 proceeds through several stages: V2 O5 → V6 O13 → VO2 → V2 O3 . Colpaert et al. [37] have reported the temperature dependence for the reduction of V2 O5 . It is interesting to observe that the catalyst is in a severely reduced state during the reaction. Fresh V2 O5 is reduced usually to the VO2 phase during the reaction as shown in Figs. 6–8. At a high severity condition of T = 335◦ C and space velocity = 106, 000 h−1 , the reduction proceeds further toward V2 O3 (Fig. 8), but the conversion keeps a steady state as a function of reaction time as shown in Fig. 13. A fast rate of re-oxidation and increased mobility of lattice oxygen at high temperatures prevent the irreversible reduction of the catalyst at such a high severity condition. The product sulfur seems to be oxidized easily to SO2 when there is labile oxygen on the catalyst surface. The results in Fig. 6 have shown that only SO2 is produced if vanadium oxide keeps the O/V ratio greater than 2.26 (9.6% reduction of V2 O5 ). It would be natural to assume that the reaction surface on which H2 S chemisorbs has a concentration gradient of oxygen from the bulk to the surface (or from one face of vanadium crystal to the reaction face). In this case, the reaction surface must be in a more highly reduced state than the averaged value of 9.6%, which assumes no concentration gradient throughout the bulk and surface of vanadium oxide particle. This can be confirmed from the results in Fig. 6. It must be remembered, at the present moment, that the reduction step (a) in Fig. 6 with only H2 S is carried out at low temperature of 254◦ C where the oxygen diffusion is very much limited. The complete oxidation of H2 S to SO2 (zero selectivity to sulfur) is maintained up to the point at which about three times of monolayer oxygen in vanadium oxide is lost. Although, we do not know
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quantitatively how deep inside the bulk phase of the catalyst is reduced in the above case, it is appropriate to assume that the surface is severely reduced due to the limited diffusion of oxygen from the bulk at such a low temperature. High selectivity to sulfur is obtained when the catalyst keeps the O/V ratio smaller than 1.94 (22.4% reduction of V2 O5 ). In such circumstances, the product sulfur on the catalyst surface probably cannot find any reactive oxygen nearby due to depletion of labile oxygen on the surface. The severe reduction of vanadium oxide accompanies changes in morphology, surface area, and pore structure at the same time. From the analyses with XRD, XPS, and XANES, it can be observed that, after the reaction, the original V2 O5 crystals disintegrate into micro-cystalline clusters. Thus, the surface areas of supported catalysts decrease after the reaction, probably due to the blocking of support SiO2 pores by micro-crystalline VOx clusters, while the surface area of bulk V2 O5 increases. In the pore-size distribution (Fig. 11), the generation of meso- and macro-pores are observed after the reaction of vanadium oxide. The structure and mechanism of the V2 O5 reduction and the resulting transformations into different vanadium oxides have been studied by several groups [37–39]. The stereochemistry of vanadium ions in V2 O5 may be considered to be either a distorted trigonal bipyramid (five V–O bond lengths of 1.58–2.02 Å), a distorted tetragonal pyramid, or a distorted octahedron (the sixth V–O bond length of 2.79 Å) [39]. Three kinds of oxygen atoms are present in the V2 O5 structure: vanadyl oxygen O(1) atoms having only one strong bond (V–O = 1.58 Å), bridging oxygen O(2) atoms forming two bonds (V–O = 1.77 Å) with the V–O(2)–V angle of 125◦ and bridging oxygen O(3) atoms forming three bonds, two at 1.88 Å and one at 2.02 Å. Based on experiments (electrical conductivity, EPR and IR spectra) and quantum-chemical calculations, Harber et al. [39] have suggested that the V–O(3) bond at 2.02 Å will be broken most easily among V–O bonds, and such a break will facilitate the cleavage along the [1 0 0] direction. Short distances of O(3)–O(3) indicates strong repulsive forces between oxygen atoms, and this is in agreement with recent experimental observations [31,39,40]. The following two conditions make vanadium oxides function as a catalyst in the selective oxidation. First, the crystal structure of V2 O5 is characterized by the existence of
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channels in the [0 0 1] and [0 1 0] directions, which have the effective diameter of 2 Å and, thus, permit a facile diffusion of oxygen. Second, the ability of vanadium atoms to possess multiple stable oxidation states from two to five results in an easy conversion between oxides of different stoichiometry by oxidation or reduction. The presence of shear planes, initiated by the removal of oxygen (i.e. anion vacancies or defects), at the boundary between two next neighboring oxide nuclei (such as V2 O5 and V6 O13 nuclei) locally enhances the rate of oxygen loss and allows the reduced phase (V6 O13 ) to grow into the bulk generating micro-pores [37–39]. Considering the reduction characteristics of vanadium oxide and the mismatch of the pore structure with the De Boer classification [28] (shown in Fig. 12), the generation of meso- and macro-pores after the reaction may originate from the agglomeration of micro-pores generated in the reduction process.
5. Conclusion The present results show that VOx /SiO2 and bulk V2 O5 catalysts are effective for the selective oxidation of H2 S to elemental sulfur over a wide temperature range, 270–350◦ C. Very high values (>90%) of the H2 S conversion and selectivity to sulfur can be obtained over various V loadings (5–100 wt.%) if the reaction temperature is kept above 270◦ C and the space velocities below 4000 h−1 . The conversion and selectivity exceed the equilibrium values probably because the irreversible selective oxidation of H2 S to sulfur occurs at much faster rate compared with the reversible Claus reaction. Reaction tests and characterization of catalysts using XRD, XPS and XANES have revealed that V2 O5 catalysts exist in a severely reduced state of VO2 or V2 O3 during the reaction. Oxidized form of vanadium oxide with O/V ratio greater than 2.26 yields zero selectivity to sulfur and gives only the complete oxidation product, SO2 . The reaction proceeds via a redox mechanism; the reduction force by H2 S is balanced with the oxidation force by gas phase oxygen during the reaction. For this reason, the catalyst shows eventual deactivation with time on stream when the reduction rate is faster than the reoxidation rate, and this is the case where the space velocity is kept high or the V loading is kept low at low reaction
temperatures (<250◦ C). Even for the stable reaction at high temperatures, the severely reduced state of vanadium oxide during the reaction accompanies an increase of surface area and the generation of mesoand macro-pores, due to disintegration of vanadium oxide crystal and structural change.
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