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Changes in properties of V2O5–K2SO4–SiO2 catalysts in air, hydrogen and toluene vapors
Applied Catalysis A: General 184 (1999) 103±113
Changes in properties of V2O5±K2SO4±SiO2 catalysts in air, hydrogen and toluene vapors A. Kaszonyia,*, M. Hroneca, G. Delahay1,b, D. Ballivet-Tkatchenkob a
Department of Organic Technology, Slovak University of Technology, 812 37 Bratislava, Slovak Republic Laboratoire de Chimie de Coordination, CNRS, 205 Route de Narbonne, 31077, Toulouse cedex, France
b
Received 29 June 1998; received in revised form 24 March 1999; accepted 30 March 1999
Abstract V2O5±K2SO4±SiO2 catalysts were studied by TG, DTA, TPR and XRD methods. During activation NH4VO3 and (COOH)2, used for impregnation of the support, decompose in air below a temperature of 3508C. Over 3508C oxidation of vanadium up to V5 and changes in the catalyst reducibility occur. In TPR using pure hydrogen, the freshly activated catalyst is reduced in three steps. The ®rst peak with a maximum around 4308C is a result of reduction of the vanadium oxide species. The second and the third TPR peaks correspond mainly to the reduction of sulfate species to H2S. It was found that vanadium oxides noticeably enhanced the reduction of sulfate to H2S. Similar TPR pro®les were found after catalytic tests (vapor phase oxidation of toluene by air) and after reduction of the same catalyst by H2, and its reoxidation by air in some particular conditions. By increasing temperature and prolonging time of activation, or by prolonging time-on-stream of toluene oxidation the reducibility of vanadium oxides decreases and the easily reducible part of sulfate species is converted into a less reducible part. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Supported V2O5 catalyst; TG; DTA; TPR; Catalyst deactivation; Toluene oxidation
1. Introduction Vanadium oxides are widely used as catalysts in oxidation reactions, (e.g. oxidation of sulfur dioxide, carbon monoxide, hydrocarbons) or for the selective catalytic reduction of nitrogen oxides by ammonia. Vanadium oxides supported on silica have been tested in numerous reactions and studied by several methods to ®nd a relationship between catalytic activity and *Corresponding author. E-mail: [email protected] 1 Present address: Laboratorie de MateÂriaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS, ENSCM, 8 rue de l'Ecole Normale, 34296 Montpellier Cedex, France.
properties of vanadium species present on the catalysts surface, but many details of this relation are unknown. The most important informations about vanadium species were observed by TPR, DTA, TGA, XRD, ESCA, UV±visible, IR and Raman spectroscopy [1± 7]. An addition of K to vanadia containing catalysts changes the acidity of catalyst and the V±O bond length of the surface vanadia species [1,8]. Moreover K2SO4 can form surface compounds with vanadia and can change the mobility of oxygen in V2O5 and has a promoting action mainly on the oxidation of alcohols [8,9] and alkylaromatics [10]. For instance in the vapor-phase oxidation of toluene to benzaldehyde [10], the highest activity and selectivity obey vana-
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 0 9 9 - X
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dium oxides supported on a mixture of K2SO4 and SiO2. However, the oxidation activity of these catalysts decreases with time-on-stream. In this work, changes in the physico-chemical properties of V2O5±K2SO4±SiO2 catalysts after activation in air, reduction in hydrogen, pretreatment in nitrogen±toluene mixture and oxidation of toluene at temperatures up to 6508C, were studied by TG, DTA, TPR, IR and XRD techniques. 2. Experimental 2.1. Materials SiO2 was in the form of a 30% aqueous solution (silica sol). K2SO4, NH4VO3, (COOH)22H2O and KOH were analytical grade. 2.2. Catalyst preparation An appropriate amount of KOH was added to the aqueous solution of SiO2. The formed solution of potassium silicate was adjusted to a pH of 5.5±6.0 by addition of 10% H2SO4. The precipitated gel was dried at 1108C and used as a support (speci®c surface area 143 m2/g). 1.35 g of NH4VO3 and 0.675 g of K2SO4 were added in appropriate amounts to a warm solution of 2.2 g of oxalic acid in 15 cm3 of water for the preparation of the catalyst A (0.5 g of K2SO4 was used for catalyst B). Then 6 g of dry support (particle size 0.3±1.0 mm) was added and well mixed. After evaporation of water the catalyst was dried at 1108C and then activated in air at 4508C for 4 h. The chemical composition of the prepared catalyst was as follows: 9.3 wt% V2O5, 25.7 wt% K2SO4 and 65 wt% SiO2. 2.3. Characterization of the catalysts 2.3.1. Chemical analysis of vanadium The total content of vanadium in the catalysts was determined after their dissolution in a mixture of 25 cm3 of 4 M H2SO4 and 25 cm3 of 4 M HF. The weakly interacting vanadium was removed from the support by 50 cm3 of 4 M H2SO4 [11]. The prepared solutions were diluted to 100 cm3 with water, and 20 cm3 portions of the diluted solution was titrated with 0.02 M KMnO4, and 0.1 M iron(II) ammonium sulfate solution. Both the content of vanadia and its
average oxidation number were determined by the method published by Niwa [12]. SETARAM TG-DTA92 was used in TG and DTA experiments. Outlet gases from TG were analyzed by a QX 2000 mass spectrometer (Leybold). 2.3.2. Temperature-programmed reduction Catalyst samples (50 mg) were reduced in a quartz reactor [13] with pure hydrogen (¯ow rate 50 cm3/ min). The difference between thermal conductivity of fed hydrogen and outlet gases from the reactor was measured in the temperature range 20±6108C (rate of temperature increase was 78C/min). Then the temperature was kept at 6108C for 30 min to complete the reduction of the catalyst. The TPR apparatus was calibrated with CuO and V2O5 in order to obtain a correlation between the amount of formed water and the TPR peak surface areas. IR spectra of catalysts were measured by SPECORD IR 75 using KBr technique. The surface area of catalyst was measured by Pulse Chemisorb 2700 (Micromeritics). 2.3.3. Oxidation of toluene A tubular steal microreactor with I.D. 12 mm (OL 115/10, FOK GYEM, Hungary) was used in experiments. Toluene was pumped to the upper part of the reactor, where it was evaporated and mixed with air in the layer of small glass balls situated over the catalyst layer. The outlet gas from the reactor was cooled in two successive coolers, the ®rst was kept at ÿ208C and the second (a cold trap) at ÿ708C. The catalyst (5 g, particle size 0.3±0.1 mm, speci®c surface area 31 m2/g) was diluted with a support (1:1) to avoid adverse thermal effects. No reaction of toluene on the pure support at reaction conditions used in the oxidation of toluene was observed. The liquid products were analyzed by a GC Chrompack 9000 using a column packed with 10% of APIEZON L 1% of H3PO4 on CHROMOSORB W 0.125±0.16 mm. 3. Results and discussion 3.1. Activation of the catalyst in air The most ef®cient temperature for the activation of the V2O5±K2SO4±SiO2 catalyst is at 4508C [2,10].
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Fig. 1. Thermogravimetric and differential thermal analysis of impregnated V2O5±K2SO4±SiO2 catalyst A, measured in air. Amount of sample 23.76 mg, temp. rise 108C/min.
The TG and DTA curves of the catalyst A were measured in air between 208C and 6508C (Fig. 1) and show that the decomposition of ammonium metavanadate and oxalic acid proceeds at around 2808C. But these TG and DTA curves do not allow to recognize the successive steps of NH4VO3 thermal decomposition [14], because of parallel oxalic acid oxidation. Furthemore, both the chemical analysis of the catalyst after TG experiments in air (up to
4508C) and of the same catalyst activated at 4508C also in air con®rms the absence of nitrogen and carbon. In the IR spectra the characteristic bands of NH4, ±COOH and ±COOÿ disappear for both catalysts at 4508C (Fig. 2). The decrease of the sample weight during TG analysis (15.7 wt%) is in agreement with the initial amount of NH4VO3, (COOH)2 and H2O in the catalyst. Therefore, all these results show the complete decomposition of ammonium metavanadate
Fig. 2. IR spectrum of (1) impregnated catalyst A before activation, (2) catalyst activated at 4508C in air for 4 h and, (3) support. The bands in ÿ1 the ranges 3100±3700, 1650±1750 and 1350±1450 cmÿ1 belongs to ±COOÿ, ±COOH and NH 4 . The band of sulfate is at 619 cm .
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and oxalic acid at a temperature above 3508C. In the temperature range from 4108C to 6508C, no further weight loss (TG curve, Fig. 1) is observed in air. Moreover, mass spectrometric analysis of the outlet gases in this temperature range veri®es the nonappearance of the decomposition product. Besides a possible increase in the weight of the catalyst due to the reoxidation of vanadium, the observed rise in the weight of the catalyst can be accounted for the in¯uence of Archimedes's law. Before activation in air, the average oxidation number of vanadium was 4.3±4.4, as a result of partial reduction of vanadium by oxalic acid during impregnation. After activation, the average oxidation number of vanadium was 4.9±5.0, i.e. vanadium was almost totally reoxidized. However, it is necessary to mention that the average oxidation number of vanadium depends on the method and duration of activation. If there is not enough oxygen for the oxidation of vanadium during the decomposition of ammonium metavanadate and oxalic acid in the temperature interval 150±3508C, then reduction of vanadium occurs to some extent and the reduced vanadium species are localized mainly inside the catalysts particles. According to the results of Centi et al. [11] the interaction of vanadium and supports was characterized also by reaction with H2SO4. With the fresh catalyst, it is possible to remove all vanadium from the support with 4 M H2SO4. However, after activation
in air at temperatures of 4508C or 6008C a solution of 4 M H2SO4 removes only 91.5% or 90.2%, respectively, of the vanadium species initially present. Therefore only a weak interaction exists between vanadium species and the support before activation [11]. After pretreatment in air at 4508C, about 8.5% of vanadium is in strong interaction with the support. However these species represent a small part of the total amount of vanadium and their portion is not modi®ed by an increase in the temperature of activation up to 6008C. 3.2. Reduction of the catalyst by hydrogen In the TG and DTA curves obtained in hydrogen below 3508C (Fig. 3), no changes of weight or heat ¯ow were observed for the freshly activated catalyst A. In addition, no formation of decomposition products from the catalyst was detected by mass spectrometry. The reduction of V2O5 starts above 4008C, and above 4508C the reduction of sulfate to H2S is observed (Figs. 3 and 4). In comparison with the decomposition of NH4VO3 and (COOH)2 (Fig. 1), the reduction of sulfate and V2O5 has a very low thermal effect. From mass spectroscopic analysis of the TG outlet gases, the formation of H2S is signi®cant above 5008C. The maximum rate of H2S formation occurs at 6008C and the end of reaction is not achieved at 6508C (Figs. 3 and 4). The signals corresponding to
Fig. 3. Thermogravimetric and differential thermal analysis of freshly activated V2O5±K2SO4±SiO2 catalyst A measured in hydrogen. Amount of sample 26.5 mg, temp. rise 108C/min.
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Fig. 4. Changes of signal intensities at m/z32, 33 and 34 in mass spectrum of outlet gases during TGA and DTA of V2O5±K2SO4±SiO2 catalyst A in hydrogen.
m/z48, 64, 80 (SOx) have the same intensities as the background signals. Therefore, the mass spectra of outlet gases do not con®rm the formation of SOx or decomposition of sulfate to SOx during the reduction process. The absence of sulfate decomposition products may be attributed to the fact, that the reduction is performed under pure hydrogen. More information about the reducibility of catalysts can be gained by temperature-programmed reduction in hydrogen. Our TPR experiments were performed in pure hydrogen and therefore are based on the thermal conductivity of the gaseous products of the reduction. The reduction of a freshly activated catalyst takes place in three steps (three peaks in the TPR curve ± Fig. 5). The variation of the particle size of the catalyst has no in¯uence on the TPR pro®le (Fig. 5), therefore intraparticular mass transfer during TPR does not shadow the chemical processes. The ®rst peak can be strictly attributed to the reduction of vanadium oxide species according to the TG and DTA experiments presented above. In the second and third peaks, reduction of sulfate species to H2S and H2O occurs, and therefore the surface area of this peak does not allow us to make an accurate quantitative analysis. Nevertheless, after integrating the TPR curves, the areas corresponding to the reduction of V2O5 to V2O3 were calculated. The endpoints of these areas are
marked in Figs. 5 and 6 by arrows. Roozeboom et al. [3] found that at comparable conditions, reduction of V2O5 on silica proceeded in two steps, Koranne et al. [2] found three steps for this process. Roozeboom et al. [3] used 66% of H2 in N2 and they found the ®rst peak around 4308C, and the second at around 5008C. Koranne et al. [2] used 5% of H2 in Ar and found TPR peaks around 4608C, 5508C and 5908C. In the TPR curves of catalyst A we can ®nd three peaks
Fig. 5. TPR curve of freshly activated V2O5±K2SO4±SiO2 catalyst A at different particle size. Amount of catalyst 50 mg, increase of temp.(78C/min) from 208C to 6108C ; at 6108C isotherm for 30 min.
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Fig. 6. TPR curve of V2O5±K2SO4±SiO2 catalysts A activated in air at (1) 4508C for 4 h, (2) 4508C for 36 h and (3) 6008C for 1 h. For conditions see Fig. 5.
(Fig. 6), i.e. all peaks in pure hydrogen can be connected with the partial reduction of V2O5. During reduction of our catalysts the ®rst signi®cant amounts of H2S were observed in outlet gases nearby arrows, i.e. at the beginning of the ®rst high peak in all the cases in the pro®les. Therefore, the highest peaks in the TPR curve of our catalysts are mainly the result of sulfate reduction to H2S.
It is known, that the color of vanadium oxides changes signi®cantly with its average oxidation number. After the ®rst temperature-programmed reduction in hydrogen the color of the catalyst is black, indicating a reduction of vanadium probably up to V3 in agreement with the results of Koranne et al. [2]. The content of sulfate is markedly decreased, but it is not zero. An elemental analysis con®rms traces of sulfur in the reduced catalyst and furthermore in an IR spectrum of it, there is a small peak belonging to sulfate (Table 1). When the reduced catalyst is reoxidized at 4508C, the color of the catalyst returns to orange-brown, which is characteristic for bulk V2O5. During following reductions and reoxidations both, the color of the catalyst and the area of TPR curves change signi®cantly (Table 1, Fig. 7). Horvath et al. [15] observed, that at high temperatures vanadium oxides react with silica to form colorless surface species, which can be reduced to violet species containing V3. This can explain our observations. During redox cycles initially orange-brown bulk V2O5 located on the surface of support reacts with silica to form white (colorless) species. The white color of the catalyst was observed mainly after reoxidation at 6308C. After following TPR the color of the catalyst
Table 1 Some properties of catalyst A Color
Unused catalyst
Used catalystb
Surface area (m2/g)
IR band intensitya SO2ÿ 4 =SiO2
Area under the TPR curve
Support
White
143
0.410
0.73y
(0) Impregnated (1) Activated at 4508C (2) Activated at 6008C (3) (1) After first TPR (4) (3) Reox. at 4508C (5) (4) After TPR (6) (5) Reox. at 4508C (7) (6) After TPR (8) (7) Reox. at 6308C (9) (8) After TPR Orange-brown particles Dark-green particles Grey-violet particles White particles Mixture from the reactor
Dark green Orange-brown Orange-brown Black Orange-brown Dark grey White and orange-brown Grey and violet White Violet
± 31 8 33 ± ± ± ± ± 6.8 22.7 28.2 17.2 9.9 21.2
0.333 0.348 0.388 0.062 ± ± ± ± ± 0.000 0.250 0.260 0.312 0.150 ±
± 5.12y 5.14y ± 1.54y ± 1.35y ± 1.02y 4.99y 5.08y ± ± ±
y: The equivalent area corresponding to the reduction of V2O5 to V2O3. Ratio of IR band intensities in absorbance units at 619 and 809 cmÿ1 assigned to sulfate and silica, respectively. b The catalyst was used in vapor phase oxidation of toluene. During the oxidation it was overheated above 6008C. The catalyst particles were seperated manually according to it colour. a
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Fig. 7. TPR curve of V2O5±K2SO4±SiO2 catalyst A activated at 4508C for 4 h and then several times reoxidized and reduced by TPR. For conditions see Table 1 and Fig. 5.
turns to light violet, which is in agreement with the observations of Horvath et al. [15]. By increasing the number of redox cycles the area of the TPR curves decreases (Table 1, Fig. 7). For a better comparison the TPR areas of catalysts are given in unit area which is equivalent to the reduction of V2O5 to V2O3. From the data listed in Table 1, it follows that the greatest decrease of TPR area is observed between the ®rst and the second TPR, because most of the sulfate is reduced during the ®rst TPR and the regeneration of sulfate from H2S is not possible. After several redox cycles sulfate is totally decomposed and as a result, the peak due to the sulfate in IR spectrum of the catalyst disappears (Table 1). The reoxidation of vanadium oxides is still possible. After complete decomposition of sulfate the TPR pro®le of our catalyst (Fig. 7) is in very good agreement with the TPR pro®le of 8.2% V2O5/SiO2 found by Koranne et al. [2]. During redox cycles the surface area of the catalyst A is reduced from 31 m2/g (freshly activated catalyst) to 6.8 m2/g (violet reduced catalyst).) In a blank TPR experiment a support, which contains the same amount of sulfate as the real catalyst, was reduced (Fig. 8). The maximum for the observed small TPR peak (around 6058C) shifts by 308C to the higher temperatures in comparison with the last TPR peak of catalyst A (around 5758C). The area under the TPR curve of support is made up from 14% of the freshly activated catalyst. This means that in the presence of vanadium oxides the sulfate is reduced more easily, i.e. vanadium oxides have to enhance the
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Fig. 8. TPR curve of V2O5±K2SO4±SiO2 catalysts A (1) activated in air at 6008C for 1 h, (2) activated in air at 4508C for 4 h and then heated in helium up to 6008C for 1 h, (3) used in vapor phase oxidation of toluene between the temperature 3808C and 4508C for several tenths of hours, (4) TPR of support. For conditions see Fig. 5.
reduction of sulfate to H2S. This explains why the sulfate is not completely reduced during the ®rst TPR of the studied catalysts. After impregnation of K2SO4± SiO2 support and activation of catalyst, vanadium oxides are located on the surface of a support and a part of the sulfate remains inside of the support as crystalline K2SO4 without vanadium oxides. During redox cycles vanadium oxides can migrate into the inner bulk part of the support, where they react with sulfate and thus enhance its reduction to H2S. In the second blank TPR experiment with freshly activated catalyst helium was used in place of hydrogen. The observed curve was practically the same as in the blank experiment with the support described above. The area of the observed TPR curve (with the maximum around 6058C) is about 16% of the TPR area measured in hydrogen for the same catalyst. Therefore, probably in both blank experiments decomposition of a part of the catalyst proceeds. As it was mentioned above, mass spectra of outlet gases do not con®rm formation of SOx, but formation of oxygen (m/z32) is possible (Fig. 4). 3.3. Changes of properties of catalyst during oxidation of toluene The shape of the TPR curves of catalysts used in toluene oxidation varies with the time-on-stream (Fig. 8), but also with the time and the temperature
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of activation (Fig. 6). A longer time of activation at a given temperature involves a decrease of the ®rst TPR peak and a small increase and a temperature shift of the peaks observed at temperatures over 5008C. On the other hand a higher activation temperature leads to the decrease of the ®rst two peaks and increase of area of the third peak. However, the total area under the TPR curves remains practically unchanged (Table 1). The resulting TPR pro®le of a pretreated catalyst at 6008C or used in toluene oxidation at 4508C are similar (Fig. 8). Some minor differences are observed in the TPR curve of the catalyst freshly activated at 6008C in air. It is possible to distinguish two small peaks below 5308C. One around 4308C, which probably can be assigned to a certain surface V2O5 phase and another around 5008C belonging to a crystalline V2O5 phase [1,2]. The reducibility of vanadium decrease and the most reducible part of sulfate is converted to the less reducible part by prolonging time and by increasing temperature of activation, or by prolonging time-on-stream of toluene oxidation. So in order to obtain a catalyst with a stable TPR pro®le i.e. composed of one single peak around 5808C, we need for instance to activate the catalyst at 6008C in air for 1 h, or to apply it in the oxidation of toluene at temperatures 400±4508C or to activate at 4508C sev-
eral tenths of hours. If before TPR analysis the freshly activated catalyst is heated in helium up to 6008C for 1 h, the shape of its TPR curve is practically the same as the TPR pro®le of the activated catalyst in air under the same conditions (Fig. 8). After heating in helium, however, it is not possible to distinguish TPR peaks belonging to the surface and crystalline V2O5 phases. Fig. 9 shows an XRD spectrum of the support and catalyst A (containing 9.3 wt% V2O5). The XRD spectrum of V2O5 on different supports was published by Roozeboom et al. [3] and Owens et al. [16]. From Fig. 9 it follows that it is not possible to ®nd characteristic XRD lines of the crystalline V2O5 phase in the XRD pattern of the catalyst activated at 4508C, or at 6008C. The observed lines in the XRD pattern of the catalysts and support belong to K2SO4. Differences between XRD patterns of the catalysts can be put down to experimental error. Therefore, from the measured XRD lines no correlation between the crystallinity of the catalyst and the above-mentioned changes in the reducibility of the sulfate and V2O5 can be postulated. Catalyst B was activated by standard procedure in air or in nitrogen. The catalyst after treatment in nitrogen was black (instead of orange-brown), and inactive at the beginning of the experiment in the
Fig. 9. XRD pattern of (1) K2SO4±SiO2 support, (2) V2O5±K2SO4±SiO2 catalysts A activated in air at 6008C for 4 h and (3) at 4508C for 4 h.
A. Kaszonyi et al. / Applied Catalysis A: General 184 (1999) 103±113 Table 2 Performance of catalyst B in the vapor phase oxidation of toluene at the start of the reaction Temperature (8C)
390 400 410 420 430 440 435 440
Rate of benzaldehyde formation on catalyst B activated in (mmol/g h) Nitrogen
Air
0.020 0.049 0.073 0.088 0.080 0.110 0.230a 0.260a
0.260 0.195 0.238 0.591 0.668 1.006 0.270b 0.265b
Toluene feed 11.5 mmol/g h, air flow 120 dm3/h, O2:toluene16:1. The catalyst was activated in nitrogen or in air for 10 h. a Data measured after 30 and 33 h of time on stream at 4358C and 4408C, respectively. b Data measured after 52 and 55 h of time on stream at 4358C and 4408C, respectively.
vapor phase oxidation of toluene. After reoxidation at 4508C in air for 10 h its activity gradually increased. However, when subjected to a stream of toluene and air for 30 h, it is suf®cient enough to charge the catalyst to the same level of activity, as if the catalyst had been charged by standard procedure in air (Table 2). The TPR pro®le of the orange-brown catalyst (activation in air) and of the black catalyst (activation in nitrogen) are quite different (Fig. 10).
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The peak of V2O5 reduction (®rst peak) observed for the catalyst pretreated in air is absent from the TPR curve of the catalyst pretreated in nitrogen. Therefore, according to the color and the absence of the ®rst peak, vanadium oxides in this latter catalyst are probably already in the form of V2O3. It is assumed that the reduction of V5 takes place during decomposition of NH4VO3 and (COOH)2. Practically the whole TPR curve of the black catalyst is a result of sulfate reduction to H2S. Moreover, reduction of sulfate is easier after activation of catalyst in nitrogen than after activation in air, since the maximum rate of its reduction is assigned to the maximum in the TPR curve. It shifts by 248C to a lower temperature in the case of the nitrogen pretreatment. The TPR curve of the black colored catalyst after its reoxidation and use in oxidation of toluene for 60 h has the same stabilized shape as for the catalysts activated in air (Figs. 8 and 10). Therefore, 60 h of catalyst operation in toluene oxidation is enough to rebuild the structure and reducibility of the catalyst to the stable state. The color of catalysts A and B after vapor phase oxidation of toluene in the temperature range 400± 4508C is dark green, and the average oxidation number of vanadium is 4.6±4.8, depending on the concentration of toluene and the ¯ow rate of the air used. If in this range of temperature during the catalytic test the toluene feed is stopped, the average oxidation number of vanadium gradually increases up to
Fig. 10. TPR curve of V2O5±K2SO4±SiO2 catalysts B activated at 4508C for 4 h (1) in nitrogen, (2) in air. Both catalysts were used in vapor phase oxidation of toluene at temperatures 380±4508C for 60 h. (3) TPR curve of used catalyst B activated in nitrogen and (4) in air. For conditions see Fig. 5.
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4.95±5.0 and the initial orange±brown color is restored by reoxidation. The TPR area of this catalyst is indeed practically the same as of the TPR pro®le of the freshly activated catalyst. The activity of catalyst decreases very slowly by increasing time on the stream. If the catalytic experiment is carried out without dilution of the catalyst by clean support, a part of the catalyst can be overheated by reaction heat. At the beginning of the autoheating the reaction temperature increases slowly up to 4608C due to the selective oxidation and then very quickly due to total oxidation of toluene it increases above 6008C. After this overheating the particles which form the bed of the catalysts are different colors. The activity of such a catalyst becomes notably worse. Colors, speci®c surface areas and the relative IR band intensities (sulfate/silica) obtained for the particles are given in Table 1. As we can see, the speci®c surface areas of these used catalyst particles have become smaller. The white colored particles were in the range of the highest temperature and therefore they have the lowest speci®c surface area. The IR band intensities of the overheated particles show that the amount of sulfate has decreased in comparison with the freshly activated catalyst. The greatest decrease is also observed at the white particles. It follows from above that if a part of the catalyst undergoes an overheating during oxidation of toluene, sulfate species can start to decompose. The selective oxidation of toluene proceeds on the surface of the catalyst. Therefore the catalyst deactivation can be attributed to: 1. the decrease of the catalysts speci®c surface area 2. the decomposition of sulfate species; involved in both cases by an overheating during the total oxidation of toluene. According to GuÈnduÈz et al. [10], an increase of the potassium sulfate concentration up to 30±40 wt% in the catalyst enhances the selectivity towards the formation of benzaldehyde. TPR by 2 vol% toluene in nitrogen has been carried out from 208C to 6008C with a rate of 108C/min. During these experiments the concentration of SOx and H2S in the outlet gases were below the detection limit of the mass spectrometer which was used. When the TPR experiment was stopped at 4508C, the color of the upper part of catalyst bed was dark-green (initial color was orange-brown) and the average oxidation number of vanadium was 4.6. Toluene was partially
oxidized mainly to benzaldehyde and benzoic acid. When the TPR experiment was carried out up to 6008C, the upper part of the catalyst bed was black, below it was dark-green and the lower par remained orange-brown. This means, that toluene in nitrogen can reduce vanadium oxides at nearly the same temperature that pure hydrogen can. 4. Conclusion The activity and reducibility of V2O5±K2SO4±SiO2 catalysts prepared by impregnation of K2SO4±SiO2 support is in¯uenced by the conditions of its activation and by the reaction temperature. If a fresh catalyst is heated up to the reaction temperature in the absence of oxygen, it is inactive, because vanadium is strongly reduced by the components, which are used for impregnation. In the presence of oxygen these components are oxidized and the oxidation state of vanadium is near 5. During reduction of an active catalyst by hydrogen or toluene, V5 and also sulfate are reduced. Moreover vanadium oxides enhance the reduction of sulfate to H2S. The reducibility and activity of the catalyst decrease by increasing temperature of activation or catalyst operation. Deactivation of V2O5±K2SO4±SiO2 during oxidation of toluene is probably due to the decrease of the surface area, or to the loss of the sulfate species, both generated by heat transfer problem or hot points during the reaction.
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[13] D. Ballivet-Tkatchenko, G. Delahay, J. Therm. Anal. 41 (1994) 1141. [14] A. Lengrouri, T. Baird, J.R. Fryer, React. Solids 5 (1988) 53. [15] B. Horvath, J. Geyer, H.L. Kraus, Z. Anorg. Allg. Chem. 426 (1976) 141. [16] L. Owens, H.H. Kung, J. Catal. 148 (1994) 587.
Changes in properties of V2O5±K2SO4±SiO2 catalysts in air, hydrogen and toluene vapors A. Kaszonyia,*, M. Hroneca, G. Delahay1,b, D. Ballivet-Tkatchenkob a
Department of Organic Technology, Slovak University of Technology, 812 37 Bratislava, Slovak Republic Laboratoire de Chimie de Coordination, CNRS, 205 Route de Narbonne, 31077, Toulouse cedex, France
b
Received 29 June 1998; received in revised form 24 March 1999; accepted 30 March 1999
Abstract V2O5±K2SO4±SiO2 catalysts were studied by TG, DTA, TPR and XRD methods. During activation NH4VO3 and (COOH)2, used for impregnation of the support, decompose in air below a temperature of 3508C. Over 3508C oxidation of vanadium up to V5 and changes in the catalyst reducibility occur. In TPR using pure hydrogen, the freshly activated catalyst is reduced in three steps. The ®rst peak with a maximum around 4308C is a result of reduction of the vanadium oxide species. The second and the third TPR peaks correspond mainly to the reduction of sulfate species to H2S. It was found that vanadium oxides noticeably enhanced the reduction of sulfate to H2S. Similar TPR pro®les were found after catalytic tests (vapor phase oxidation of toluene by air) and after reduction of the same catalyst by H2, and its reoxidation by air in some particular conditions. By increasing temperature and prolonging time of activation, or by prolonging time-on-stream of toluene oxidation the reducibility of vanadium oxides decreases and the easily reducible part of sulfate species is converted into a less reducible part. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Supported V2O5 catalyst; TG; DTA; TPR; Catalyst deactivation; Toluene oxidation
1. Introduction Vanadium oxides are widely used as catalysts in oxidation reactions, (e.g. oxidation of sulfur dioxide, carbon monoxide, hydrocarbons) or for the selective catalytic reduction of nitrogen oxides by ammonia. Vanadium oxides supported on silica have been tested in numerous reactions and studied by several methods to ®nd a relationship between catalytic activity and *Corresponding author. E-mail: [email protected] 1 Present address: Laboratorie de MateÂriaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS, ENSCM, 8 rue de l'Ecole Normale, 34296 Montpellier Cedex, France.
properties of vanadium species present on the catalysts surface, but many details of this relation are unknown. The most important informations about vanadium species were observed by TPR, DTA, TGA, XRD, ESCA, UV±visible, IR and Raman spectroscopy [1± 7]. An addition of K to vanadia containing catalysts changes the acidity of catalyst and the V±O bond length of the surface vanadia species [1,8]. Moreover K2SO4 can form surface compounds with vanadia and can change the mobility of oxygen in V2O5 and has a promoting action mainly on the oxidation of alcohols [8,9] and alkylaromatics [10]. For instance in the vapor-phase oxidation of toluene to benzaldehyde [10], the highest activity and selectivity obey vana-
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 0 9 9 - X
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dium oxides supported on a mixture of K2SO4 and SiO2. However, the oxidation activity of these catalysts decreases with time-on-stream. In this work, changes in the physico-chemical properties of V2O5±K2SO4±SiO2 catalysts after activation in air, reduction in hydrogen, pretreatment in nitrogen±toluene mixture and oxidation of toluene at temperatures up to 6508C, were studied by TG, DTA, TPR, IR and XRD techniques. 2. Experimental 2.1. Materials SiO2 was in the form of a 30% aqueous solution (silica sol). K2SO4, NH4VO3, (COOH)22H2O and KOH were analytical grade. 2.2. Catalyst preparation An appropriate amount of KOH was added to the aqueous solution of SiO2. The formed solution of potassium silicate was adjusted to a pH of 5.5±6.0 by addition of 10% H2SO4. The precipitated gel was dried at 1108C and used as a support (speci®c surface area 143 m2/g). 1.35 g of NH4VO3 and 0.675 g of K2SO4 were added in appropriate amounts to a warm solution of 2.2 g of oxalic acid in 15 cm3 of water for the preparation of the catalyst A (0.5 g of K2SO4 was used for catalyst B). Then 6 g of dry support (particle size 0.3±1.0 mm) was added and well mixed. After evaporation of water the catalyst was dried at 1108C and then activated in air at 4508C for 4 h. The chemical composition of the prepared catalyst was as follows: 9.3 wt% V2O5, 25.7 wt% K2SO4 and 65 wt% SiO2. 2.3. Characterization of the catalysts 2.3.1. Chemical analysis of vanadium The total content of vanadium in the catalysts was determined after their dissolution in a mixture of 25 cm3 of 4 M H2SO4 and 25 cm3 of 4 M HF. The weakly interacting vanadium was removed from the support by 50 cm3 of 4 M H2SO4 [11]. The prepared solutions were diluted to 100 cm3 with water, and 20 cm3 portions of the diluted solution was titrated with 0.02 M KMnO4, and 0.1 M iron(II) ammonium sulfate solution. Both the content of vanadia and its
average oxidation number were determined by the method published by Niwa [12]. SETARAM TG-DTA92 was used in TG and DTA experiments. Outlet gases from TG were analyzed by a QX 2000 mass spectrometer (Leybold). 2.3.2. Temperature-programmed reduction Catalyst samples (50 mg) were reduced in a quartz reactor [13] with pure hydrogen (¯ow rate 50 cm3/ min). The difference between thermal conductivity of fed hydrogen and outlet gases from the reactor was measured in the temperature range 20±6108C (rate of temperature increase was 78C/min). Then the temperature was kept at 6108C for 30 min to complete the reduction of the catalyst. The TPR apparatus was calibrated with CuO and V2O5 in order to obtain a correlation between the amount of formed water and the TPR peak surface areas. IR spectra of catalysts were measured by SPECORD IR 75 using KBr technique. The surface area of catalyst was measured by Pulse Chemisorb 2700 (Micromeritics). 2.3.3. Oxidation of toluene A tubular steal microreactor with I.D. 12 mm (OL 115/10, FOK GYEM, Hungary) was used in experiments. Toluene was pumped to the upper part of the reactor, where it was evaporated and mixed with air in the layer of small glass balls situated over the catalyst layer. The outlet gas from the reactor was cooled in two successive coolers, the ®rst was kept at ÿ208C and the second (a cold trap) at ÿ708C. The catalyst (5 g, particle size 0.3±0.1 mm, speci®c surface area 31 m2/g) was diluted with a support (1:1) to avoid adverse thermal effects. No reaction of toluene on the pure support at reaction conditions used in the oxidation of toluene was observed. The liquid products were analyzed by a GC Chrompack 9000 using a column packed with 10% of APIEZON L 1% of H3PO4 on CHROMOSORB W 0.125±0.16 mm. 3. Results and discussion 3.1. Activation of the catalyst in air The most ef®cient temperature for the activation of the V2O5±K2SO4±SiO2 catalyst is at 4508C [2,10].
A. Kaszonyi et al. / Applied Catalysis A: General 184 (1999) 103±113
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Fig. 1. Thermogravimetric and differential thermal analysis of impregnated V2O5±K2SO4±SiO2 catalyst A, measured in air. Amount of sample 23.76 mg, temp. rise 108C/min.
The TG and DTA curves of the catalyst A were measured in air between 208C and 6508C (Fig. 1) and show that the decomposition of ammonium metavanadate and oxalic acid proceeds at around 2808C. But these TG and DTA curves do not allow to recognize the successive steps of NH4VO3 thermal decomposition [14], because of parallel oxalic acid oxidation. Furthemore, both the chemical analysis of the catalyst after TG experiments in air (up to
4508C) and of the same catalyst activated at 4508C also in air con®rms the absence of nitrogen and carbon. In the IR spectra the characteristic bands of NH4, ±COOH and ±COOÿ disappear for both catalysts at 4508C (Fig. 2). The decrease of the sample weight during TG analysis (15.7 wt%) is in agreement with the initial amount of NH4VO3, (COOH)2 and H2O in the catalyst. Therefore, all these results show the complete decomposition of ammonium metavanadate
Fig. 2. IR spectrum of (1) impregnated catalyst A before activation, (2) catalyst activated at 4508C in air for 4 h and, (3) support. The bands in ÿ1 the ranges 3100±3700, 1650±1750 and 1350±1450 cmÿ1 belongs to ±COOÿ, ±COOH and NH 4 . The band of sulfate is at 619 cm .
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and oxalic acid at a temperature above 3508C. In the temperature range from 4108C to 6508C, no further weight loss (TG curve, Fig. 1) is observed in air. Moreover, mass spectrometric analysis of the outlet gases in this temperature range veri®es the nonappearance of the decomposition product. Besides a possible increase in the weight of the catalyst due to the reoxidation of vanadium, the observed rise in the weight of the catalyst can be accounted for the in¯uence of Archimedes's law. Before activation in air, the average oxidation number of vanadium was 4.3±4.4, as a result of partial reduction of vanadium by oxalic acid during impregnation. After activation, the average oxidation number of vanadium was 4.9±5.0, i.e. vanadium was almost totally reoxidized. However, it is necessary to mention that the average oxidation number of vanadium depends on the method and duration of activation. If there is not enough oxygen for the oxidation of vanadium during the decomposition of ammonium metavanadate and oxalic acid in the temperature interval 150±3508C, then reduction of vanadium occurs to some extent and the reduced vanadium species are localized mainly inside the catalysts particles. According to the results of Centi et al. [11] the interaction of vanadium and supports was characterized also by reaction with H2SO4. With the fresh catalyst, it is possible to remove all vanadium from the support with 4 M H2SO4. However, after activation
in air at temperatures of 4508C or 6008C a solution of 4 M H2SO4 removes only 91.5% or 90.2%, respectively, of the vanadium species initially present. Therefore only a weak interaction exists between vanadium species and the support before activation [11]. After pretreatment in air at 4508C, about 8.5% of vanadium is in strong interaction with the support. However these species represent a small part of the total amount of vanadium and their portion is not modi®ed by an increase in the temperature of activation up to 6008C. 3.2. Reduction of the catalyst by hydrogen In the TG and DTA curves obtained in hydrogen below 3508C (Fig. 3), no changes of weight or heat ¯ow were observed for the freshly activated catalyst A. In addition, no formation of decomposition products from the catalyst was detected by mass spectrometry. The reduction of V2O5 starts above 4008C, and above 4508C the reduction of sulfate to H2S is observed (Figs. 3 and 4). In comparison with the decomposition of NH4VO3 and (COOH)2 (Fig. 1), the reduction of sulfate and V2O5 has a very low thermal effect. From mass spectroscopic analysis of the TG outlet gases, the formation of H2S is signi®cant above 5008C. The maximum rate of H2S formation occurs at 6008C and the end of reaction is not achieved at 6508C (Figs. 3 and 4). The signals corresponding to
Fig. 3. Thermogravimetric and differential thermal analysis of freshly activated V2O5±K2SO4±SiO2 catalyst A measured in hydrogen. Amount of sample 26.5 mg, temp. rise 108C/min.
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107
Fig. 4. Changes of signal intensities at m/z32, 33 and 34 in mass spectrum of outlet gases during TGA and DTA of V2O5±K2SO4±SiO2 catalyst A in hydrogen.
m/z48, 64, 80 (SOx) have the same intensities as the background signals. Therefore, the mass spectra of outlet gases do not con®rm the formation of SOx or decomposition of sulfate to SOx during the reduction process. The absence of sulfate decomposition products may be attributed to the fact, that the reduction is performed under pure hydrogen. More information about the reducibility of catalysts can be gained by temperature-programmed reduction in hydrogen. Our TPR experiments were performed in pure hydrogen and therefore are based on the thermal conductivity of the gaseous products of the reduction. The reduction of a freshly activated catalyst takes place in three steps (three peaks in the TPR curve ± Fig. 5). The variation of the particle size of the catalyst has no in¯uence on the TPR pro®le (Fig. 5), therefore intraparticular mass transfer during TPR does not shadow the chemical processes. The ®rst peak can be strictly attributed to the reduction of vanadium oxide species according to the TG and DTA experiments presented above. In the second and third peaks, reduction of sulfate species to H2S and H2O occurs, and therefore the surface area of this peak does not allow us to make an accurate quantitative analysis. Nevertheless, after integrating the TPR curves, the areas corresponding to the reduction of V2O5 to V2O3 were calculated. The endpoints of these areas are
marked in Figs. 5 and 6 by arrows. Roozeboom et al. [3] found that at comparable conditions, reduction of V2O5 on silica proceeded in two steps, Koranne et al. [2] found three steps for this process. Roozeboom et al. [3] used 66% of H2 in N2 and they found the ®rst peak around 4308C, and the second at around 5008C. Koranne et al. [2] used 5% of H2 in Ar and found TPR peaks around 4608C, 5508C and 5908C. In the TPR curves of catalyst A we can ®nd three peaks
Fig. 5. TPR curve of freshly activated V2O5±K2SO4±SiO2 catalyst A at different particle size. Amount of catalyst 50 mg, increase of temp.(78C/min) from 208C to 6108C ; at 6108C isotherm for 30 min.
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Fig. 6. TPR curve of V2O5±K2SO4±SiO2 catalysts A activated in air at (1) 4508C for 4 h, (2) 4508C for 36 h and (3) 6008C for 1 h. For conditions see Fig. 5.
(Fig. 6), i.e. all peaks in pure hydrogen can be connected with the partial reduction of V2O5. During reduction of our catalysts the ®rst signi®cant amounts of H2S were observed in outlet gases nearby arrows, i.e. at the beginning of the ®rst high peak in all the cases in the pro®les. Therefore, the highest peaks in the TPR curve of our catalysts are mainly the result of sulfate reduction to H2S.
It is known, that the color of vanadium oxides changes signi®cantly with its average oxidation number. After the ®rst temperature-programmed reduction in hydrogen the color of the catalyst is black, indicating a reduction of vanadium probably up to V3 in agreement with the results of Koranne et al. [2]. The content of sulfate is markedly decreased, but it is not zero. An elemental analysis con®rms traces of sulfur in the reduced catalyst and furthermore in an IR spectrum of it, there is a small peak belonging to sulfate (Table 1). When the reduced catalyst is reoxidized at 4508C, the color of the catalyst returns to orange-brown, which is characteristic for bulk V2O5. During following reductions and reoxidations both, the color of the catalyst and the area of TPR curves change signi®cantly (Table 1, Fig. 7). Horvath et al. [15] observed, that at high temperatures vanadium oxides react with silica to form colorless surface species, which can be reduced to violet species containing V3. This can explain our observations. During redox cycles initially orange-brown bulk V2O5 located on the surface of support reacts with silica to form white (colorless) species. The white color of the catalyst was observed mainly after reoxidation at 6308C. After following TPR the color of the catalyst
Table 1 Some properties of catalyst A Color
Unused catalyst
Used catalystb
Surface area (m2/g)
IR band intensitya SO2ÿ 4 =SiO2
Area under the TPR curve
Support
White
143
0.410
0.73y
(0) Impregnated (1) Activated at 4508C (2) Activated at 6008C (3) (1) After first TPR (4) (3) Reox. at 4508C (5) (4) After TPR (6) (5) Reox. at 4508C (7) (6) After TPR (8) (7) Reox. at 6308C (9) (8) After TPR Orange-brown particles Dark-green particles Grey-violet particles White particles Mixture from the reactor
Dark green Orange-brown Orange-brown Black Orange-brown Dark grey White and orange-brown Grey and violet White Violet
± 31 8 33 ± ± ± ± ± 6.8 22.7 28.2 17.2 9.9 21.2
0.333 0.348 0.388 0.062 ± ± ± ± ± 0.000 0.250 0.260 0.312 0.150 ±
± 5.12y 5.14y ± 1.54y ± 1.35y ± 1.02y 4.99y 5.08y ± ± ±
y: The equivalent area corresponding to the reduction of V2O5 to V2O3. Ratio of IR band intensities in absorbance units at 619 and 809 cmÿ1 assigned to sulfate and silica, respectively. b The catalyst was used in vapor phase oxidation of toluene. During the oxidation it was overheated above 6008C. The catalyst particles were seperated manually according to it colour. a
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Fig. 7. TPR curve of V2O5±K2SO4±SiO2 catalyst A activated at 4508C for 4 h and then several times reoxidized and reduced by TPR. For conditions see Table 1 and Fig. 5.
turns to light violet, which is in agreement with the observations of Horvath et al. [15]. By increasing the number of redox cycles the area of the TPR curves decreases (Table 1, Fig. 7). For a better comparison the TPR areas of catalysts are given in unit area which is equivalent to the reduction of V2O5 to V2O3. From the data listed in Table 1, it follows that the greatest decrease of TPR area is observed between the ®rst and the second TPR, because most of the sulfate is reduced during the ®rst TPR and the regeneration of sulfate from H2S is not possible. After several redox cycles sulfate is totally decomposed and as a result, the peak due to the sulfate in IR spectrum of the catalyst disappears (Table 1). The reoxidation of vanadium oxides is still possible. After complete decomposition of sulfate the TPR pro®le of our catalyst (Fig. 7) is in very good agreement with the TPR pro®le of 8.2% V2O5/SiO2 found by Koranne et al. [2]. During redox cycles the surface area of the catalyst A is reduced from 31 m2/g (freshly activated catalyst) to 6.8 m2/g (violet reduced catalyst).) In a blank TPR experiment a support, which contains the same amount of sulfate as the real catalyst, was reduced (Fig. 8). The maximum for the observed small TPR peak (around 6058C) shifts by 308C to the higher temperatures in comparison with the last TPR peak of catalyst A (around 5758C). The area under the TPR curve of support is made up from 14% of the freshly activated catalyst. This means that in the presence of vanadium oxides the sulfate is reduced more easily, i.e. vanadium oxides have to enhance the
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Fig. 8. TPR curve of V2O5±K2SO4±SiO2 catalysts A (1) activated in air at 6008C for 1 h, (2) activated in air at 4508C for 4 h and then heated in helium up to 6008C for 1 h, (3) used in vapor phase oxidation of toluene between the temperature 3808C and 4508C for several tenths of hours, (4) TPR of support. For conditions see Fig. 5.
reduction of sulfate to H2S. This explains why the sulfate is not completely reduced during the ®rst TPR of the studied catalysts. After impregnation of K2SO4± SiO2 support and activation of catalyst, vanadium oxides are located on the surface of a support and a part of the sulfate remains inside of the support as crystalline K2SO4 without vanadium oxides. During redox cycles vanadium oxides can migrate into the inner bulk part of the support, where they react with sulfate and thus enhance its reduction to H2S. In the second blank TPR experiment with freshly activated catalyst helium was used in place of hydrogen. The observed curve was practically the same as in the blank experiment with the support described above. The area of the observed TPR curve (with the maximum around 6058C) is about 16% of the TPR area measured in hydrogen for the same catalyst. Therefore, probably in both blank experiments decomposition of a part of the catalyst proceeds. As it was mentioned above, mass spectra of outlet gases do not con®rm formation of SOx, but formation of oxygen (m/z32) is possible (Fig. 4). 3.3. Changes of properties of catalyst during oxidation of toluene The shape of the TPR curves of catalysts used in toluene oxidation varies with the time-on-stream (Fig. 8), but also with the time and the temperature
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of activation (Fig. 6). A longer time of activation at a given temperature involves a decrease of the ®rst TPR peak and a small increase and a temperature shift of the peaks observed at temperatures over 5008C. On the other hand a higher activation temperature leads to the decrease of the ®rst two peaks and increase of area of the third peak. However, the total area under the TPR curves remains practically unchanged (Table 1). The resulting TPR pro®le of a pretreated catalyst at 6008C or used in toluene oxidation at 4508C are similar (Fig. 8). Some minor differences are observed in the TPR curve of the catalyst freshly activated at 6008C in air. It is possible to distinguish two small peaks below 5308C. One around 4308C, which probably can be assigned to a certain surface V2O5 phase and another around 5008C belonging to a crystalline V2O5 phase [1,2]. The reducibility of vanadium decrease and the most reducible part of sulfate is converted to the less reducible part by prolonging time and by increasing temperature of activation, or by prolonging time-on-stream of toluene oxidation. So in order to obtain a catalyst with a stable TPR pro®le i.e. composed of one single peak around 5808C, we need for instance to activate the catalyst at 6008C in air for 1 h, or to apply it in the oxidation of toluene at temperatures 400±4508C or to activate at 4508C sev-
eral tenths of hours. If before TPR analysis the freshly activated catalyst is heated in helium up to 6008C for 1 h, the shape of its TPR curve is practically the same as the TPR pro®le of the activated catalyst in air under the same conditions (Fig. 8). After heating in helium, however, it is not possible to distinguish TPR peaks belonging to the surface and crystalline V2O5 phases. Fig. 9 shows an XRD spectrum of the support and catalyst A (containing 9.3 wt% V2O5). The XRD spectrum of V2O5 on different supports was published by Roozeboom et al. [3] and Owens et al. [16]. From Fig. 9 it follows that it is not possible to ®nd characteristic XRD lines of the crystalline V2O5 phase in the XRD pattern of the catalyst activated at 4508C, or at 6008C. The observed lines in the XRD pattern of the catalysts and support belong to K2SO4. Differences between XRD patterns of the catalysts can be put down to experimental error. Therefore, from the measured XRD lines no correlation between the crystallinity of the catalyst and the above-mentioned changes in the reducibility of the sulfate and V2O5 can be postulated. Catalyst B was activated by standard procedure in air or in nitrogen. The catalyst after treatment in nitrogen was black (instead of orange-brown), and inactive at the beginning of the experiment in the
Fig. 9. XRD pattern of (1) K2SO4±SiO2 support, (2) V2O5±K2SO4±SiO2 catalysts A activated in air at 6008C for 4 h and (3) at 4508C for 4 h.
A. Kaszonyi et al. / Applied Catalysis A: General 184 (1999) 103±113 Table 2 Performance of catalyst B in the vapor phase oxidation of toluene at the start of the reaction Temperature (8C)
390 400 410 420 430 440 435 440
Rate of benzaldehyde formation on catalyst B activated in (mmol/g h) Nitrogen
Air
0.020 0.049 0.073 0.088 0.080 0.110 0.230a 0.260a
0.260 0.195 0.238 0.591 0.668 1.006 0.270b 0.265b
Toluene feed 11.5 mmol/g h, air flow 120 dm3/h, O2:toluene16:1. The catalyst was activated in nitrogen or in air for 10 h. a Data measured after 30 and 33 h of time on stream at 4358C and 4408C, respectively. b Data measured after 52 and 55 h of time on stream at 4358C and 4408C, respectively.
vapor phase oxidation of toluene. After reoxidation at 4508C in air for 10 h its activity gradually increased. However, when subjected to a stream of toluene and air for 30 h, it is suf®cient enough to charge the catalyst to the same level of activity, as if the catalyst had been charged by standard procedure in air (Table 2). The TPR pro®le of the orange-brown catalyst (activation in air) and of the black catalyst (activation in nitrogen) are quite different (Fig. 10).
111
The peak of V2O5 reduction (®rst peak) observed for the catalyst pretreated in air is absent from the TPR curve of the catalyst pretreated in nitrogen. Therefore, according to the color and the absence of the ®rst peak, vanadium oxides in this latter catalyst are probably already in the form of V2O3. It is assumed that the reduction of V5 takes place during decomposition of NH4VO3 and (COOH)2. Practically the whole TPR curve of the black catalyst is a result of sulfate reduction to H2S. Moreover, reduction of sulfate is easier after activation of catalyst in nitrogen than after activation in air, since the maximum rate of its reduction is assigned to the maximum in the TPR curve. It shifts by 248C to a lower temperature in the case of the nitrogen pretreatment. The TPR curve of the black colored catalyst after its reoxidation and use in oxidation of toluene for 60 h has the same stabilized shape as for the catalysts activated in air (Figs. 8 and 10). Therefore, 60 h of catalyst operation in toluene oxidation is enough to rebuild the structure and reducibility of the catalyst to the stable state. The color of catalysts A and B after vapor phase oxidation of toluene in the temperature range 400± 4508C is dark green, and the average oxidation number of vanadium is 4.6±4.8, depending on the concentration of toluene and the ¯ow rate of the air used. If in this range of temperature during the catalytic test the toluene feed is stopped, the average oxidation number of vanadium gradually increases up to
Fig. 10. TPR curve of V2O5±K2SO4±SiO2 catalysts B activated at 4508C for 4 h (1) in nitrogen, (2) in air. Both catalysts were used in vapor phase oxidation of toluene at temperatures 380±4508C for 60 h. (3) TPR curve of used catalyst B activated in nitrogen and (4) in air. For conditions see Fig. 5.
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4.95±5.0 and the initial orange±brown color is restored by reoxidation. The TPR area of this catalyst is indeed practically the same as of the TPR pro®le of the freshly activated catalyst. The activity of catalyst decreases very slowly by increasing time on the stream. If the catalytic experiment is carried out without dilution of the catalyst by clean support, a part of the catalyst can be overheated by reaction heat. At the beginning of the autoheating the reaction temperature increases slowly up to 4608C due to the selective oxidation and then very quickly due to total oxidation of toluene it increases above 6008C. After this overheating the particles which form the bed of the catalysts are different colors. The activity of such a catalyst becomes notably worse. Colors, speci®c surface areas and the relative IR band intensities (sulfate/silica) obtained for the particles are given in Table 1. As we can see, the speci®c surface areas of these used catalyst particles have become smaller. The white colored particles were in the range of the highest temperature and therefore they have the lowest speci®c surface area. The IR band intensities of the overheated particles show that the amount of sulfate has decreased in comparison with the freshly activated catalyst. The greatest decrease is also observed at the white particles. It follows from above that if a part of the catalyst undergoes an overheating during oxidation of toluene, sulfate species can start to decompose. The selective oxidation of toluene proceeds on the surface of the catalyst. Therefore the catalyst deactivation can be attributed to: 1. the decrease of the catalysts speci®c surface area 2. the decomposition of sulfate species; involved in both cases by an overheating during the total oxidation of toluene. According to GuÈnduÈz et al. [10], an increase of the potassium sulfate concentration up to 30±40 wt% in the catalyst enhances the selectivity towards the formation of benzaldehyde. TPR by 2 vol% toluene in nitrogen has been carried out from 208C to 6008C with a rate of 108C/min. During these experiments the concentration of SOx and H2S in the outlet gases were below the detection limit of the mass spectrometer which was used. When the TPR experiment was stopped at 4508C, the color of the upper part of catalyst bed was dark-green (initial color was orange-brown) and the average oxidation number of vanadium was 4.6. Toluene was partially
oxidized mainly to benzaldehyde and benzoic acid. When the TPR experiment was carried out up to 6008C, the upper part of the catalyst bed was black, below it was dark-green and the lower par remained orange-brown. This means, that toluene in nitrogen can reduce vanadium oxides at nearly the same temperature that pure hydrogen can. 4. Conclusion The activity and reducibility of V2O5±K2SO4±SiO2 catalysts prepared by impregnation of K2SO4±SiO2 support is in¯uenced by the conditions of its activation and by the reaction temperature. If a fresh catalyst is heated up to the reaction temperature in the absence of oxygen, it is inactive, because vanadium is strongly reduced by the components, which are used for impregnation. In the presence of oxygen these components are oxidized and the oxidation state of vanadium is near 5. During reduction of an active catalyst by hydrogen or toluene, V5 and also sulfate are reduced. Moreover vanadium oxides enhance the reduction of sulfate to H2S. The reducibility and activity of the catalyst decrease by increasing temperature of activation or catalyst operation. Deactivation of V2O5±K2SO4±SiO2 during oxidation of toluene is probably due to the decrease of the surface area, or to the loss of the sulfate species, both generated by heat transfer problem or hot points during the reaction.
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