Effect of Catalyst Prereduction on the Dehydrogenation of Isobutane

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Effect of Catalyst Prereduction on the Dehydrogenation of Isobutane over Chromia/Alumina Sanna M. K. Airaksinen* and A. Outi I. Krause Department of Chemical Technology, Helsinki University of Technology, P.O. Box 6100, FI-02015 HUT, Espoo, Finland

The effect of hydrogen and carbon monoxide prereduction on the initial activity and deactivation of chromia/alumina was investigated in isobutane dehydrogenation. Measurements were done at 580 °C in a fixed bed reactor and by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with mass spectrometry. Prereduction with hydrogen decreased the dehydrogenation activity compared to an isobutane-reduced catalyst, and prereduction by carbon monoxide increased the cracking activity. The catalysts deactivated with time on stream due to formation of carbon-containing deposits: carboxylates and aliphatic and unsaturated/ aromatic hydrocarbon species. Prereduction affected the rate of coke deposition but not the nature of the species formed. The observed effects were attributed mainly to the hydroxyl groups formed during hydrogen prereduction and to the carbonate and formate species formed during carbon monoxide prereduction. Furthermore, the carbon monoxide-prereduced surface possibly contained a higher number of active chromium sites unselective for dehydrogenation. Introduction Chromia supported on alumina is an active catalyst in the dehydrogenation of light alkanes to alkenes.1 Oxidized chromia/alumina contains chromium mainly as Cr3+ and Cr6+ and in trace amounts as Cr5+. The relative amounts and the structures of the Cr3+ and Cr6+ oxides depend on the chromium content of the catalyst.1-4 At low chromium loadings below about 5 atCr/nm2 (4-8 wt % depending on the catalyst surface area) Cr6+ dominates and forms mono- and polychromates. With increasing chromium content, the amount of Cr6+ stabilizes whereas the amount of Cr3+ increases. The Cr3+ oxide phase is first amorphous; crystalline Cr2O3 has been detected, for example, by X-ray diffraction above about 8-10 atCr/nm2 (6-16 wt %). Under dehydrogenation conditions the high oxidation state chromium species are reduced by the alkane with release of carbon oxides and water. Thereafter, dehydrogenation products start to form. Coordinatively unsaturated (cus) Cr3+ ions formed in the reduction or present already in the oxidized catalyst are generally considered to be the active sites in dehydrogenation.1-3,5 The initial unselective combustion period can be avoided by prereducing the catalyst for example with hydrogen or carbon monoxide. However, these gases have been observed to affect the activity in dehydrogenation compared to reduction with the alkane feed.5-9 Prereduction with hydrogen decreases the dehydrogenation activity, and prereduction by carbon monoxide increases side reactionsscracking and coke formations during dehydrogenation. These effects have been speculated to be caused by the formation of different adsorbed surface species or chromium oxidation states during reduction with different gases.1,5,6 Earlier, we investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) the nature of the surface species formed during reduction of chromia/ * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +358 9 451 2622.

alumina by hydrogen,10-12 carbon monoxide,10 propane,11 and isobutane.12 Hydroxyl groups were formed in the reduction by hydrogen or the alkanes, and oxygen-containing carbon species were formed in reduction by carbon monoxide or the alkanes. The purpose of the present study was to evaluate the effect of these species on the initial activity of chromia/alumina in isobutane dehydrogenation. Another aim in the study was to determine the effect of carbon monoxide prereduction on the deactivation of chromia/alumina. Carbon-containing deposits are formed during dehydrogenation that decrease the catalytic activity and necessitate periodic regeneration of the catalyst. Earlier, we investigated coke deposition on calcined and hydrogen-prereduced chromia/alumina catalysts during propane11 and isobutane12 dehydrogenation by in situ DRIFT and Raman spectroscopies. These two complementary methods give information on different types of carbon-containing deposits. Infrared spectroscopy can be used to follow the formation of aliphatic and aromatic hydrocarbon-type species and oxygen-containing deposits (e.g., carbonates and carboxylates) on oxide samples.13 Raman spectroscopic measurements may reveal aromatic hydrocarbon species14 and graphite-like deposits.14,15 Our DRIFTS measurements indicated that on calcined chromia/alumina first carboxylates and aliphatic hydrocarbon deposits and then with increasing time on stream unsaturated/ aromatic species formed.11,12 Moreover, the Raman spectroscopic measurements11 showed formation of graphite-like deposits with longer times on stream. Hydrogen prereduction decreased the rate of coke deposition but did not influence the nature of the deposits.11,12 Isobutane dehydrogenation was investigated by activity measurements at 580 °C and by in situ DRIFTS combined with mass spectrometry (MS). The methods used allowed us to elucidate more reliably the effect of prereduction on the initial stages of dehydrogenation and on the deactivation of the catalyst. The activity of the catalyst could be measured almost continuously, and

10.1021/ie050060j CCC: $30.25 © 2005 American Chemical Society Published on Web 04/27/2005

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because DRIFTS was chosen as the in situ spectroscopic method, the reduction of Cr6+ and the formation of hydroxyls, oxygen-containing, and hydrocarbon-type carbonaceous species during reduction and dehydrogenation could be followed. Experimental Section Samples Used in the Study. Three chromia/ alumina catalysts prepared by the atomic layer deposition (ALD) technique were used in the study. In the ALD technique, the precursor of the metal oxide is deposited on the support from the gas phase through saturating gas-solid reactions.2 The γ-alumina support (AKZO 000-1.5E) was crushed, sieved, and calcined with air at 600 °C for 16 h. The catalysts were prepared in a flow-type ALD reactor. The chromia precursor, chromium(III) acetylacetonate, Cr(acac)3 (Riedel-de Hae¨n, 99%), was vaporized and directed through the support bed held at 200 °C. After the Cr(acac)3 chemisorption, excess precursor was flushed from the reactor with nitrogen, and the acac ligands were removed by air at 520 °C. The chemisorption-ligand removal cycles were repeated 1, 6, or 12 times to obtain different chromium loadings, after which the samples were calcined with air at 600 °C for 4 h. According to earlier analyses,10,16 the catalysts contained chromium 1.2, 7.5, and 13.5 wt % (0.7, 2.9, and 8.2 atCr/nm2support), and Cr6+ 0.9, 2.4, and 3.0 wt %. The samples are referred to in the text according to their chromium contents. No crystalline Cr2O3 was detected by X-ray diffraction, indicating that the chromia species were well-dispersed. X-ray photoelectron spectroscopic (XPS) measurements indicated that the 13.5CrAl catalyst contained Cr3+ and Cr6+ after oxidation and mainly Cr3+ after reduction with hydrogen, carbon monoxide, or n-butane. The treatments were done in a reactor connected directly to the XPS system enabling sample transfer in a vacuum. For comparison, also the alumina support and a bulk chromia sample (Cr2O3, Aldrich, 98+) were investigated. The chromia sample was calcined with air at 600 °C for 4 h before use and contained Cr6+ 0.2 ( 0.1 wt %.10 Isobutane Dehydrogenation Activity Measurements. The isobutane dehydrogenation activity of the 13.5CrAl catalyst was measured in a continuous flow reaction system consisting of a fixed-bed microreactor along with a Fourier transform infrared (FTIR) gas analyzer and a gas chromatograph (GC) for on-line product analysis. The activity measurements were carried out at 580 °C under atmospheric pressure. The catalyst was heated to the reaction temperature under 5% O2/N2 (AGA, air 99.99%, N2 99.999%). Nitrogen used in the experiments was purified with Oxisorb (Messer Griesheim GmbH). In the dehydrogenation experiments aimed to compare the behavior of calcined and prereduced chromia/ alumina, the reduction of the catalyst (0.2 g) was accomplished either during the first minutes on isobutane stream or during a 30 min prereduction with 10% H2/N2 (H2, AGA 99.999%) or 5% CO/N2 (CO, Messer Griesheim GmbH, 99.997%). Dehydrogenation was carried out for 15 min using isobutane feed (AGA 99.95%) with a weight hourly space velocity (WHSV) of 5 h-1 and diluted with nitrogen at a molar ratio of 1:9. After the dehydrogenation, the catalyst was flushed with nitrogen and regenerated with diluted air. The reduction and regeneration productsscarbon oxides and waterswere measured by FTIR. The dehydrogenation

products were monitored by FTIR for the first 6 min on stream, a GC sample was taken after 10 min, and the FTIR analysis was then continued. In selected measurements the catalyst was regenerated directly after the prereduction without implementing the dehydrogenation stage. The amount of coke as a function of time on isobutane stream was measured for a hydrogen-prereduced catalyst (0.1 g, 15-min prereduction with 10% H2/N2). The isobutane flow with a WHSV of 30 h-1 was diluted with nitrogen at a molar ratio of 1:1. Dehydrogenation was carried out for 2, 5, 10, or 15 min and was followed with the FTIR. The amount of coke deposited on the catalyst during dehydrogenation was calculated from the amounts of carbon oxides measured by FTIR during regeneration. The gaseous products were analyzed on-line with a Gasmet FTIR gas analyzer (Temet Instruments Ltd.) equipped with a Peltier-cooled mercury-cadmiumtelluride detector and with an HP 6890 gas chromatograph equipped with an HP PLOT/Al2O3 “M” column and a flame ionization detector. The FTIR spectra were recorded in the wavenumber range of 4000-850 cm-1 with a resolution of 8 cm-1 and a scanning rate of 10 scans/s. The analysis cuvette (9 cm3) was maintained at constant temperature (180 °C) and pressure (103 kPa). The spectra were measured every 2 s during the first 2 min on stream, every 5 s during the next 2 min, and thereafter every 30 or 60 s. Further details of the FTIR gas analysis method and of the determination of the product distribution based on the measured spectra can be found elsewhere.9,17 The conversions, selectivities, and yields were calculated on a molar basis. In Situ DRIFTS-MS Measurements. The formation of hydroxyl and carbon-containing species during catalyst reduction and isobutane dehydrogenation was studied by in situ DRIFTS combined with MS. The DRIFTS measurements were performed using a Nicolet Nexus FTIR spectrometer and a Spectra-Tech hightemperature/high-pressure reaction chamber. Gaseous products were monitored on-line by a Pfeiffer Vacuum OmniStar mass spectrometer. Nitrogen used in the measurements was purified with Oxisorb (MesserGriesheim GmbH). Isobutane dehydrogenation was investigated as a function of time on stream at 580 °C for the three chromia/alumina catalysts and for the pure chromia and alumina samples. A fresh sample was used in each experiment and was pretreated by in situ calcination with 10% O2/N2 (AGA, air 99.99%, N2 99.999%) at 580 °C for 2 h. Dehydrogenation was carried out directly after calcination and for the 13.5CrAl catalyst also after a 15-min prereduction with 5% H2/N2 (H2, AGA 99.999%) or 5% CO/N2 (CO, Messer Griesheim GmbH, 99.997%). Spectra were collected once every minute (4 cm-1, 30 scans) with the spectrum of an aluminum mirror measured under nitrogen flow as the background, and the product gas was analyzed continuously by MS. The total gas flow rate was kept at 50 cm3/min. After 3 min on stream of 5% i-C4H10/N2 (i-C4H10, AGA 99.95%), the sample cell was flushed with nitrogen to obtain a spectrum without the contribution by gas-phase isobutane. Thereafter, the isobutane flow was continued for another 3 min, making the total time on stream 6 min, and the sample cell was again flushed with nitrogen. The experiment was continued thus with nitrogen flushes after a total of 10 and 15 min on isobutane stream. After the dehydrogenation the sample was

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flushed with nitrogen and regenerated with diluted air at 580 °C. Interaction of the calcined 13.5CrAl catalyst with 5% i-C4H8/N2 (i-C4H8, AGA 99%) was investigated in a separate measurement carried out as described above. Results Isobutane Dehydrogenation Activity Measurements. The dehydrogenation activity of the 13.5CrAl catalyst was measured in a fixed bed reactor after calcination and after prereduction by hydrogen or carbon monoxide. This sample was chosen for closer examination because it had the highest chromium loading, and thus the contribution by the alumina support was lowest. The reduction of the catalyst by isobutane released carbon oxides, by hydrogen released water, and by carbon monoxide released carbon dioxide. The reduction was fast: gaseous reduction products were detected for less than 1 min on stream. As has been observed earlier,5,8,9 no water was released from isobutane although it was expected to form, and the amount from hydrogen was lower than anticipated based on the measured amount of reducible Cr6+ in the catalyst. It has been estimated that about 30-50% of the theoretical amount of water formed from hydrogen remains on the catalyst.5,8,18,19 Retainment of hydrogen- or carboncontaining species from the prereduction gases was verified by regenerating the samples by air directly after the prereduction. Water was released from the hydrogenprereduced catalyst, and carbon dioxide was released from the carbon monoxide-prereduced one. The average oxidation state (aos) of reduced chromium was estimated based on the amount of Cr6+ on the fresh catalyst and the measured amounts of gaseous reduction products. In the calculation it was assumed that 50%5,18,19 of the water formed in prereduction by hydrogen remains on the catalyst. The aos was calculated to be close to +3 after reduction by isobutane, hydrogen, or carbon monoxide; no clear difference was observed between the three gases. This is in accordance with the separate XPS measurements, which indicated that mainly Cr3+ was present after the reductions, and also complies with the results by others.1,5 Figure 1 shows the conversion of isobutane and the selectivity to isobutene obtained with the 13.5CrAl catalyst after the different pretreatments. Results are presented starting from 1 min on stream because before this carbon oxides were formed on the calcined catalyst due to reduction by the isobutane feed. The main product on the reduced catalysts was isobutene. Cracking of C4 hydrocarbons to C1-C3 hydrocarbons took place as a side reaction. The yields of isobutene and C1C3 hydrocarbons are presented in Figure 2. The conversion of isobutane at 1 min on stream was highest after prereduction by carbon monoxide. However, this catalyst had high cracking activity, which lowered the yield of isobutene, as was observed earlier by Hakuli et al.5 The calcined catalyst had the highest dehydrogenation activity. The activity decreased with time on stream for all three samples with an increase in the selectivity to isobutene, although hydrogen prereduction seemed to inhibit the deactivation. The cracking activities of the samples reached similar values after about 5 min on stream. The coke contents of the calcined, hydrogen-prereduced, and carbon monoxideprereduced catalysts after the 15 min dehydrogenation were 2.9, 2.3, and 3.6 mmol/gcat, respectively.

Figure 1. Conversion of isobutane (X) and selectivity to isobutene (S) obtained during isobutane dehydrogenation on the 13.5CrAl catalyst after calcination and after prereduction by hydrogen or carbon monoxide. Conditions: mcat ) 0.2 g, T ) 580 °C, WHSV ) 5 h-1, feed 1 mol of i-C4H10/9 mol of N2.

Figure 2. Yield of isobutene and C1-C3 hydrocarbons obtained during isobutane dehydrogenation on the 13.5CrAl catalyst after calcination and after prereduction by hydrogen or carbon monoxide. Conditions: mcat ) 0.2 g, T ) 580 °C, WHSV ) 5 h-1, feed 1 mol of i-C4H10/9 mol of N2.

Figure 3. Yield of isobutene and amount of coke with time on stream on the hydrogen-prereduced 13.5CrAl catalyst during isobutane dehydrogenation. Conditions: mcat ) 0.1 g, T ) 580 °C, WHSV ) 30 h-1, feed 1 mol of i-C4H10/1 mol of N2.

The deposition of coke as a function of time on stream was investigated after hydrogen prereduction because here no carbon-containing material was formed during the reduction of the catalyst. The amount of coke and the yield of isobutene are presented in Figure 3. The different values compared to the results described above


Figure 4. DRIFT spectra measured for the 13.5CrAl catalyst at 580 °C (a) after calcination and after (b) 10 s, (c) 1 min 10 s, (d) 3 min, (e) 6 min, (f) 10 min, and (g) 15 min on isobutane stream. Spectra b and c were measured under isobutane flow, and spectra d-g were measured after nitrogen flush.

are explained by the different conditions used in the experiments. The amount of coke increased fairly linearly with time on stream, and the H/C ratio of the coke decreased from 0.82 to 0.03 indicating loss of hydrogen from the coke. At the same time the dehydrogenation activity decreased with an increase in the selectivity to isobutene, suggesting that the initially unselective sites were deactivated by coke. In Situ DRIFTS-MS Measurements. The formation of hydroxyl and carbon-containing species during isobutane dehydrogenation on calcined and hydrogenor carbon monoxide-prereduced chromia/alumina was investigated by in situ DRIFTS at 580 °C for the 13.5CrAl catalyst. Selected spectra measured for the calcined catalyst are shown in Figure 4. Spectra b and c were measured under isobutane flow during the first 2 min on stream, whereas spectra d-g were measured after flushing the sample cell with nitrogen after selected times on stream. Thus the strong C-H stretching band of gas-phase isobutane at 2970 cm-1 does not dominate the C-H area, allowing the changes between 3100 and 2800 cm-1 to be observed. Since the nitrogen flush could result in desorption of adsorbed species, reference measurements were done without the flushes. The results were not noticeably different from the ones obtained by stopping the alkane feed. Figures 5 and 6 present selected spectra measured for the hydrogen- and carbon monoxide-prereduced samples, respectively. In these, the b spectrum was measured after prereduction, and spectra c-h were measured after different times on isobutane stream. The calcined 13.5CrAl catalyst exhibited bands at 3710 and 3620 cm-1 due to Al-OH20 and Cr-OH2 species, respectively. The broad band at 1985 cm-1 is assigned to the chromate CrdO overtone vibrations10 and indicates the presence of oxidized chromium (Cr6+). The sharp peak at 2340 cm-1 is characteristic for the γ-alumina used as the support and belongs to carbon dioxide species trapped in the material possibly during its preparation.21,22 The reduction of the calcined 13.5CrAl catalyst resulted in fast disappearance of the chromate band.

Figure 5. DRIFT spectra measured for the 13.5CrAl catalyst at 580 °C (a) after calcination, (b) after hydrogen prereduction, and after (c) 10 s, (d) 1 min 10 s, (e) 3 min, (f) 6 min, (g) 10 min, and (h) 15 min on isobutane stream. Spectra c and d were measured under isobutane flow, and spectra e-h were measured after nitrogen flush.

Figure 6. DRIFT spectra measured for the 13.5CrAl catalyst at 580 °C (a) after calcination, (b) after carbon monoxide prereduction, and after (c) 10 s, (d) 1 min 10 s, (e) 3 min, (f) 6 min, (g) 10 min, and (h) 15 min on isobutane stream. Spectra c and d were measured under isobutane flow, and spectra e-h were measured after nitrogen flush.

Gaseous reduction products formed as in the activity measurements. Reduction by isobutane released gaseous carbon dioxide (splitting of the 2340 cm-1 peak, MS). Unstable formate species were seen briefly as a shoulder at 2880 cm-1 (ν(CH)),23,24 and acetates, or carboxylates in general, appeared at 1530, 1430, and 1345 cm-1 (νas(COO), νs(COO), δs(CH3)).13,23 Al-OH and Cr-OH bands increased at 3710 and 3620 cm-1, respectively, and a band due to hydrogen-bonded hydroxyls25 at 3480 cm-1. In preliminary in situ DRIFTS measurements with isobutane,12 the reduction resulted, possibly via an intermediate tert-butoxide species, in formation of adsorbed acetone, formates, and acetates, of which only the acetates were stable at 580 °C.

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Prereduction of the catalyst with hydrogen released gaseous water (MS) and resulted in formation of adsorbed hydroxyl species in accordance with earlier measurements.10-12 The hydroxyls were present at similar wavenumbers and, thus, most likely had similar structures as those formed during reduction by isobutane. Since the intensity of the Cr-OH band and, thus, the number of these species increased, the hydroxyl groups saturated some of the cus chromium ions formed in the reductions. This most likely decreased their potential activity in dehydrogenation. No Cr-H species, which should appear at 1714 and 1697 cm-1,26 were detected but their presence is not ruled out. Monodentate carbonates were present on the carbon monoxide-prereduced catalyst at about 1530 and 1310 cm-1 (νas(COO), νs(COO)),13 and at 2870 and 2580 cm-1 (overtones and combinations).22 Furthermore, during the prereduction formate species bonded to the chromia, and the alumina phases were observed at 2970 and 2880 cm-1 (νs(COO) + νas(COO), ν(CH))24 and at 3009 and 2930 cm-1,27 respectively. The carbonates and the formates were most likely formed in the reduction of the catalyst, and the formates were formed also in the reaction of gaseous carbon monoxide with surface hydroxyl groups.10 The carbonate and the formate bands overlap in the wavenumber region 1600-1300 cm-1, which is why the carbonate bands in this area were earlier10 attributed to formates. Hydroxyl species appeared also on the carbon monoxide-prereduced surface. However, since hydroxyls cannot form from carbon monoxide, they were probably present already before the prereduction or resulted from impurities in the feed. Therefore, their amount was most likely low, which is supported by the absence of hydrogen-bonded hydroxyls, and the carbon monoxide-prereduced surface contained the highest number of cus sites able to act as active species. Some of these sites may have been covered by the carbon-containing species formed from carbon monoxide. During the first minutes on isobutane stream, aliphatic C-H species25 at 2935 cm-1 formed on the calcined catalyst in addition to the oxygen-containing species. With increasing time on stream, a broad band increased at 3060 cm-1 assigned to unsaturated or aromatic hydrocarbon deposits, or both, strongly bonded to the catalyst surface.25,28,29 A shoulder at 1585 cm-1 due to the same species was present too. At the same time, the dehydrogenation activity of the catalyst decreased (MS). Isobutane dehydrogenation on the hydrogen- or carbon monoxide-prereduced surfaces resulted in formation of acetates/carboxylates (1530 and 1430 cm-1), although not as rapidly as with the calcined catalyst. On the hydrogen-prereduced catalyst the aliphatic species (2925 cm-1) appeared less rapidly than on the alkane-reduced surface. Otherwise the behavior was similar to that observed after calcination with gradual formation of the unsaturated/aromatic deposits (3045 and 1585 cm-1). On the carbon monoxide-prereduced surface the carbonate species disappeared rapidly at the beginning of the dehydrogenation, indicating that they reacted further either to gaseous or adsorbed species. On this surface, both the aliphatic and the unsaturated/aromatic species formed at the beginning of the isobutane feed. Thus, coke deposition was most rapid after carbon monoxide prereduction. The catalysts deactivated with increasing time on stream (MS). The effect of catalyst chromium content on the formation of the carbon-containing species was studied for the

Figure 7. DRIFT spectra measured for the pure (a) chromia (area 1730-1300 cm-1 × 4) and (b) alumina samples, and for the (c) 1.2CrAl, (d) 7.5CrAl, and (e) 13.5CrAl catalysts at 580 °C after 15 min on isobutane stream. Samples pretreated by calcination. Spectra measured under nitrogen flow.

calcined 1.2CrAl, 7.5CrAl, and 13.5CrAl catalysts and the pure chromia and alumina samples. The spectra measured after 15 min on isobutane stream are compared in Figure 7. The 1.2CrAl and 7.5CrAl catalysts behaved similarly as the 13.5CrAl catalyst: acetates/ carboxylates and aliphatic hydrocarbon species formed first and unsaturated/aromatic species with increasing time on stream with simultaneous decrease in the dehydrogenation activity. The initial dehydrogenation activity increased with the chromium content of the catalyst. Acetates/carboxylates formed on the bulk chromia and alumina samples also, which had negligible dehydrogenation activities. The chromium content of the catalyst affected the wavenumbers of the acetate/ carboxylate bands. The shift in the band positions indicated a change from alumina-bonded to chromiabonded species with increasing coverage of the support by the chromia phase. The intensities of the hydrocarbon bands and therefore the amount of these deposits increased slightly with the chromium content and the dehydrogenation activity of the catalyst. Separate in situ measurements were performed with isobutene for the calcined 13.5CrAl catalyst. In these, in addition to the acetates/carboxylates and the aliphatic hydrocarbon species, also the unsaturated/aromatic species formed fast during the first minutes on isobutene stream. Discussion Formation of Carbon-Containing Species during Isobutane Dehydrogenation. Oxygenated and hydrocarbon-type surface species were deposited on the chromia/alumina catalysts during isobutane dehydrogenation. At the same time the dehydrogenation activities decreased indicating that the carbon-containing species caused deactivation. Acetates/carboxylates appeared on the calcined and on the prereduced chromia/ alumina catalysts and on the pure chromia and alumina samples. Furthermore, these species formed from both isobutane and also isobutene. This is in accordance with our earlier conclusion11,12 that the acetates/carboxylates form not only in the reduction of the chromia phase by the alkane but also in other reactions, for example, in reaction of gaseous hydrocarbons with surface hydroxyl groups.30,31 The ratio of the unsaturated/aromatic hydrocarbon species to the aliphatic ones increased with time on


stream. Similar behavior was observed during propane dehydrogenation on the 13.5CrAl catalyst leading to graphite-like deposits with increasing time on stream.11 A gradual graphitization of the coke formed on chromia/ alumina during alkane dehydrogenation has been observed also by others.14 The fast formation of the unsaturated/aromatic deposits from isobutene and the decrease in the H/C ratio of coke with increasing time on stream are in accordance with our earlier conclusion11 that the hydrocarbon deposits form mainly from adsorbed alkene molecules or dehydrogenation intermediates in successive polymerization, cyclization, and dehydrogenation reactions. The DRIFTS results suggested that the chromia and the alumina phases were both involved in the formation of the carbon-containing deposits. Active sites for this may have been Lewis acidic cations32 or Brønsted acidic hydroxyl species,33 which were present on the catalysts. On the Lewis acid sites the formation of coke from alkenes may have taken place via allylic intermediate species.32 However, in general the formation of carboncontaining deposits on oxide catalysts is thought to occur on Brønsted acid sites via carbocation intermediates.33 In addition to coke formation, the alumina-bonded hydroxyls detected by DRIFTS, although weakly acidic, may have contributed to the cracking of isobutane31 observed as another side reaction during dehydrogenation. Alkali metal promoters are often used in industrial chromia catalysts to neutralize the acid sites of alumina.1 Effect of Prereduction on the Dehydrogenation Behavior. The lower dehydrogenation activity after hydrogen prereduction has been suggested to be related to a different structure or concentration of the -OH/H groups formed during hydrogen prereduction compared to those on the alkane-reduced surface.6 These groups could affect the activity by lowering the coordinative unsaturation of the chromium ions and thereby decreasing the number of active sites. In addition, Cr-H species have been suggested to be involved in the mechanism of dehydrogenation.1,9,34 Therefore, the Cr-H groups formed during prereduction could influence the dehydrogenation activity by affecting the concentration of intermediate adsorbed species. Since similar Al-OH, Cr-OH, and hydrogen-bonded hydroxyls formed during hydrogen and alkane reductions, the quality of the hydroxyl groups is not the primary cause for the different activities. The quantities of the different hydroxyl groups or the presence of adsorbed hydrogen molecules or ions may be more important. However, it is not possible to conclude from the DRIFTS results whether the hydroxyl species were present in different amounts or whether, for example, Cr-H species existed on the surfaces. On the other hand, the main difference observed by DRIFTS between the hydrogen- and the alkane-reduced surfaces was the more rapid formation of acetates/ carboxylates after calcination. This combined with the higher activity after alkane reduction could suggest the involvement of the acetates/carboxylates in the dehydrogenation reaction. However, these species formed on the pure chromia and alumina samples too, which had negligible dehydrogenation activities. Therefore, the acetates/carboxylates were not active sites in dehydrogenation, and the effect of hydrogen prereduction is more likely related to the -OH/H species as discussed above. It must be noted, though, that Nijhuis et al.35 speculated that coke formed during propane dehydrogenation on chromia/alumina could facilitate the ad-

sorption of the alkane and, in this way, explain the increase in activity they observed with increasing amount of coke with time on stream. In our experiments, the dehydrogenation activity decreased continuously with time on stream. Several possible explanations have been proposed for the difference in the dehydrogenation behavior between the alkane-reduced and the carbon monoxide-prereduced surfaces, observed primarily as different cracking and coke formation activities. The absence of -OH/H species and the presence of carbon-containing deposits on the carbon monoxide-prereduced surface have been postulated as possible causes.6 In addition, the reduction of Cr6+/Cr5+ by carbon monoxide to Cr2+, unselective in dehydrogenation, has also been suggested.6 However, in general Cr2+ has not been detected on reduced chromia/alumina,1 and the present results did not support its existence either. The DRIFTS results suggested that the carbon monoxide-prereduced surface contained the highest number of cus chromium sites. This possibly contributed to the high initial conversion of isobutane observed with this sample. However, the high cracking activity suggested that the additional active sites were unselective for dehydrogenation and contributed to side reactions. Furthermore, the carbonate species, which disappeared rapidly after the start of the isobutane feed, most likely increased the side reactions. The carbonates may have reacted to methane in the gas phase or to the unsaturated/aromatic deposits detected by DRIFTS already during the first minutes on isobutane stream. Prereduction of the chromia/alumina catalyst affected the amount of coke deposited but not the nature of the species formed. The lower amount of coke on the hydrogen-prereduced catalyst and, thus, the slower deactivation with time on stream compared to the calcined catalyst can be explained by two observations. First, by the less rapid formation of the acetates/ carboxylates and the aliphatic species and, second, by the lower initial dehydrogenation activity, which most likely decreased the amount of adsorbed isobutene on the surface and, therefore, the amount of coke intermediates. On the carbon monoxide-prereduced surface the carbonates and the formates most likely increased the amount of coke with their intrinsic amount and by increasing the formation rate of the aliphatic and the unsaturated/aromatic species by acting as coke intermediates. Conclusions Prereduction of chromia/alumina affected the activity and deactivation of the catalyst in isobutane dehydrogenation. Preeduction with hydrogen decreased the dehydrogenation activity compared to an isobutanereduced catalyst, whereas prereduction by carbon monoxide increased the initial cracking activity. The catalysts deactivated with time on stream due to formation of carbon-containing deposits. On the calcined catalyst, acetates/carboxylates and aliphatic hydrocarbon species formed first, and unsaturated/aromatic deposits formed with increasing time on stream. Prereduction of the catalyst affected the rate of coke formation but not the nature of the species formed. Both hydrogen and carbon monoxide decreased the formation of the oxygencontaining species, whereas the effect on the hydrocarbon-type deposits was different. Hydrogen decreased the formation of the aliphatic deposits but did not influence the unsaturated/aromatic species noticeably, whereas

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carbon monoxide increased the formation of the aliphatic and the unsaturated/aromatic species. The adsorbed surface species formed during reduction were suggested to contribute to the behavior of the reduced surfaces. The effect of hydrogen was possibly related to the quantities of the different hydroxyl species formed during reduction, and the effect of carbon monoxide to the carbonates and formates present on the reduced surface. Furthermore, the carbon monoxide-prereduced surface possibly contained a higher number of cus chromium sites unselective for dehydrogenation. Acknowledgment Financial support received from the Academy of Finland is gratefully acknowledged. We thank Dr. Arla Kyto¨kivi and Ms. Mirja Rissanen at Fortum Oil Oy for the preparation of the chromia/alumina samples and Ms. Johanna Lempiaïnen for performing some of the DRIFTS measurements. Literature Cited (1) Weckhuysen, B. M.; Schoonheydt, R. A. Alkane dehydrogenation over supported chromium oxide catalysts. Catal. Today 1999, 51, 223. (2) Hakuli, A.; Kyto¨kivi, A.; Krause, A. O. I. Dehydrogenation of i-butane on CrOx/Al2O3 catalysts prepared by ALE and impregnation techniques. Appl. Catal. A: Gen. 2000, 190, 219. (3) Cavani, F.; Koutyrev, M.; Trifiro`, F.; Bartolini, A.; Ghisletti, D.; Iezzi, R.; Santucci, A.; Del Piero, G. Chemical and physical characterization of alumina-supported chromia-based catalysts and their activity in dehydrogenation of isobutane. J. Catal. 1996, 158, 236. (4) Grzybowska, B.; Sloczynski, J.; Grabowski, R.; Wcislo, K.; Kozlowska, A.; Stoch, J.; Zielinski, J. Chromium oxide/alumina catalysts in oxidative dehydrogenation of isobutane. J. Catal. 1998, 178, 687. (5) Hakuli, A.; Kyto¨kivi, A.; Krause, A. O. I.; Suntola, T. Initial activity of reduced chromia/alumina catalyst in n-butane dehydrogenation monitored by on-line FT-IR gas analysis. J. Catal. 1996, 161, 393. (6) Hakuli, A. Ph.D. Thesis. Helsinki University of Technology, Espoo, Finland, 1999. (7) Gorriz, O. F.; Corte´s Corberań, V.; Fierro, J. L. G. Propane dehydrogenation and coke formation on chromia-alumina catalysts: effect of reductive pretreatments. Ind. Eng. Chem. Res. 1992, 31, 2670. (8) Harlin, E. Ph.D. Thesis. Helsinki University of Technology, Espoo, Finland, 2001. (9) Airaksinen, S. M. K.; Harlin M. E.; Krause, A. O. I. Kinetic modeling of dehydrogenation of isobutane on chromia/alumina catalyst. Ind. Eng. Chem. Res. 2002, 41, 5619. (10) Airaksinen, S. M. K.; Krause, A. O. I.; Sainio, J.; Lahtinen, J.; Chao, K.-j.; Guerrero-Pe´rez, M. O.; Banãres, M. A. Reduction of chromia/alumina catalyst monitored by DRIFTS-mass spectrometry and TPR-Raman spectroscopy. Phys. Chem. Chem. Phys. 2003, 5, 4371. (11) Airaksinen, S. M. K.; Banãres, M. A.; Krause, A. O. I. In situ characterisation of carbon-containing species formed on chromia/alumina during propane dehydrogenation. J. Catal. 2005, 230, 507. (12) Airaksinen, S. M. K.; Krause, A. O. I. Formation of carboncontaining deposits on chromia/alumina during isobutane dehydrogenation. Proceedings of the DGMK Conference “C4-C5 Hydrocarbons: Routes to Higher Value-Added Products”, Munich, 2004. (13) Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides; John Wiley & Sons: Chichester, 1984. (14) Tinnemans, S. J.; Kox, M. H. F.; Nijhuis, T. A.; Visser, T.; Weckhuysen, B. M. Real time quantitative Raman spectroscopy of supported metal oxide catalysts without the need of an internal standard. Phys. Chem. Chem. Phys. 2005, 7, 211. (15) Mul, G.; Banãres, M. A.; Garcia Corte´z, G.; van der Linden, B.; Khatib, S. J.; Moulijn, J. A. MultiTRACK and Operando Raman-GC study of oxidative dehydrogenation of propane over

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Received for review January 17, 2005 Revised manuscript received March 24, 2005 Accepted April 1, 2005 IE050060J

Effect of Catalyst Prereduction on the Dehydrogenation of Isobutane

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