Studies in Surface Science and Catalysis J.J. Spivey, E. Iglesia and T.H. Fleisch (Editors) �9 2001 Elsevier Science B.V. All rights reserved. 153

Deactivation of CrOx/Al203 Catalysts in the Dehydrogenation of/-Butane

S.M.K. Airaksinen, J.M. Kanervo, A.O.I. Krause

Helsinki University of Technology, Department of Chemical Technology, P.O.Box 6100, FIN-02015 HUT, Finland

The deactivation of two CrOx/A1203 catalysts containing different amounts of Cr 6§ was studied in the dehydrogenation of/-butane. The reduction-oxidation behaviour was also examined with H2-TPR. The catalyst with the lower Cr 6+ content was found to be more stable under the studied reaction conditions. The deactivation during a single dehydrogenation test run was caused by coke formation which was suggested to be related to the amount of redox Cr 3§ on the catalyst. The deactivation in several dehydrogenation-regeneration cycles was attributed to the decline in the amount of Cr 6+ and the clustering of the active species.

1. INTRODUCTION

Supported chromium oxide catalysts are widely used in various industrial processes. CrOx/A1203 is effective in the dehydrogenation of light alkanes such as propane and/-butane, and the Phillips catalyst CrOx/SiOe in ethene polymerisation. Because of their industrial importance, these catalysts have been extensively studied with different characterisation methods [ 1 ]. Especially CrOx/A1203 has been the object of great interest, as recently reviewed by Weckhuysen and Schoonheydt [2]. However, there are still unanswered questions related to the structure of the catalyst under dehydrogenation conditions and thus to the nature of the catalytic site(s), the reaction mechanism and the catalyst deactivation. It is notable that the studies done on CrOx/A1203 catalysts have often been performed on samples with a lower chromium content than what is used on the industrial dehydrogenation catalysts. These contain 12-14 wt-% chromium [3], which is also in the range of the chromium content at which the maximum activity in the dehydrogenation is reached [4,5]. Thus the studies on low chromium contents do not provide a proper description of the dehydrogenation catalyst.

An essential feature of the dehydrogenation process is the frequent regeneration of the catalyst. During the dehydrogenation the catalyst is fouled with coke. The short dehydrogenation-regeneration cycle enables the heat released in the burning of the coke to be used efficiently in the endothermic dehydrogenation reaction. Because of the demanding and rapidly changing conditions, the structure of the dehydrogenation catalyst should be stable. Therefore, information about the nature and the behaviour of the active sites is valuable in developing better and more durable catalysts.

A number of arguments can be given to support the idea that the Cr 3+ sites formed in the reduction of Cr 6+ (so called redox Cr 3§ [4] or dispersed phase [2]) might be the active sites. However, it was recently shown [4,5] that the dehydrogenation activity of the CrOx/A1203 catalysts reaches its maximum value at chromium loadings between 14 and 16 wt-% in the

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dehydrogenation of/-butane, even though the amount of redox Cr 3+ levels off already at a chromium loading of about 8 wt-%. This means that the activity cannot be explained only by the amount of redox Cr 3+. It has been suggested [4,7] that in addition to the redox Cr 3+, also some non-redox Cr 3§ could be responsible for the catalytic activity.

Irreversible alterations in the structure of the active phase have been proposed as potential causes for the long-term deactivation of the CrOx catalysts. It has been observed [6] that the deactivation in repeated dehydrogenation-regeneration cycles is connected with a decline in the amount of Cr 6+. Two processes have been postulated [6] as possible explanations for this: 1) some of the chromium sites could migrate into the alumina lattice becoming inactive, or 2) deactivation could occur due to irreversible clustering of the active chromium species.

This study was overtaken in order to clarify the behaviour of the less-studied high chromium loading CrOx/A1203 catalysts. Both the short-term and the long-term behaviour was of interest because the reasons for deactivation are still undetermined. With the aid of an on line FTIR equipment we can report the catalytic activity as a function of time on stream almost continuously during the dehydrogenation. Because the long-term deactivation is connected with the reduction-oxidation properties, it was examined ~ in addition to the consecutive dehydrogenation experiments m with temperature programmed reduction.

2. EXPERIMENTAL

The first catalyst was prepared by gas phase impregnation of Cr(acac)3 and will be referred to as the ALD catalyst. The calcined support (AKZO alumina 000-1.5E, calcination temperature 600 ~ was treated in 12 successive cycles with Cr(acac)3 at 200 ~ and with air between the cycles at 520 ~ After the 12 cycles the ALD catalyst was calcined in air at 600 ~ for 4 h. The second catalyst was a commercial CrOx/A1203 catalyst without the promoter and developed for the fluidised bed operation (referred to as the FB catalyst). The total Cr content of the catalysts was 13-14 wt-%. The amount of Cr 6§ was measured with UV-Vis spectrophotometry (UV-1201 Shimadzu) and was 2.9 wt-% for the ALD catalyst and 1.0 wt-% for the FB catalyst. XRD analysis (Siemens D500) showed no crystalline Cr203 phases. According to XPS measurements (SSX-100), the only chromium oxidation states present on the calcined samples were Cr 3§ and Cr 6§

The dehydrogenation experiments were carried out in a fixed-bed microreactor at 580 ~ under atmospheric pressure. The products were analysed on-line with an FTIR gas analyser (Gasmet, Temet Instruments Ltd.) and with gas chromatography (HP 6890). The point at time on stream of 10 minutes was measured with GC. The amount of catalyst used was 0.1 g. One test series consisted of 12 (pre)reduction-dehydrogenation-regeneration cycles. First six cycles were done without prereducing the catalyst, the next five with prereduction and the last one without prereduction. The catalyst was heated to the required temperature under the flow of 5% O2/N2. Prereduction was accomplished at 590 ~ with 10% H2/N2, using a reduction time of 15 minutes. The/-butane feed with weight hourly space velocity (WHSV) of 15 h "l was diluted with N2 at a mole ratio of 1:1. The catalyst was regenerated with 2-10% O2/N2 after each 15 minute dehydrogenation test run, until no carbon oxides were detected. The coke content of the catalysts was calculated based on the measured carbon oxides.

The H2-TPR measurements were performed with Altamira Instruments AMI-100 catalyst characterisation system. The catalyst samples (30 mg) were stabilised prior to reduction by heating to 600 ~ under the flow of 5% O2/Ar. TPR was performed at heating rates of 6, 11

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and 17 ~ up to 600 ~ under the flow of 11.2% H2/Ar (50 cm3/min). The hydrogen consumption was monitored using a thermal conductivity detector. The whole TPR procedure was also performed repeatedly five times in series for both ALD and FB samples to investigate their stability.

3. RESULTS AND DISCUSSION

3.1. Catalytic activity The activity of the catalysts was studied in the dehydrogenation of /-butane. The

behaviour of the ALD and FB catalysts was quite different during a single dehydrogenation test run. This can be seen from Figure 1, in which the conversion of/-butane and the selectivity to i-butene are shown as an example for the cycle no. 7 (the catalysts had been prereduced with H2). The ALD catalyst showed high initial activity. However, the activity was not constant during the 15 minute experiment but declined continuously. Despite the high initial conversion, the yield of i-butene was hampered because of the generation of marked amounts of cracking products (mostly methane and propene). The selectivity to i-butene improved with increasing time on stream. The FB catalyst was less active but more stable. Also, the selectivity was better and did not change during the experiment. A slight deactivation could also be observed with the FB catalyst, but not to the extent as with the ALD. The coke content after the dehydrogenation was higher on the ALD catalyst (4.5 wt-%) than on the FB catalyst (0.9 wt-%). The similar behaviour as a function of time was observed during the other cycles.

The catalysts were subjected to 12 consecutive experiments in order to study their behaviour in several (pre)reduction-dehydrogenation-regeneration cycles. The conversion of /-butane in the series at time on stream of 10 minutes as measured with GC is shown in Figure 2.

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Fig. 1. Activities of the catalysts in a single dehydrogenation (data from test run 7). �9 Conversion X (ALD) o Conversion X (FB) �9 Selectivity to i-butene S (ALD) A Selectivity to i-butene S (FB)

Fig. 2. The conversion of/-butane after 10 minutes during the test series. x Conversion (non-prereduced ALD) �9 Conversion (prereduced ALD) + Conversion (non-prereduced FB) o Conversion (prereduced FB)

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The activity decline in the series was fairly constant with both of the catalysts. The selectivity to i-butene did not change markedly as deactivation occurred. There was a clear decrease in the amount of coke deposited on the ALD catalyst. On the FB catalyst this process was not so obvious probably due to the low initial coke content. A decrease in the amount of reduction products was also noticed indicating a decline in the amount of redox Cr 3§ This is in accordance with the earlier results obtained by Hakuli et al. [6]. They attributed this decrease in the concentration of Cr 6§ rather to the irreversible clustering of the chromium species than to the migration of the active sites into the alumina lattice. Interestingly, in our study it was noticed that the amount of reduction products was slightly increased with longer oxidation times compared to the previous cycle. This behaviour could support the clustering explanation since it seems unlikely that the chromium species would re-emerge from the alumina lattice if they had migrated into it. The increase in Cr 6§ could indicate that the species clustered during reduction/dehydrogenation were better re-dispersed during longer oxidation treatments. However, the phenomenon was not correlated in the activities of the catalysts. This could support the idea that the redox Cr 3§ is not the only active site on the catalyst.

The high initial activity of the ALD catalyst during a single dehydrogenation test run could most likely be related to its high redox Cr 3§ content. However, the amount of side products was considerable. It has been proposed that the coke formation is related to the high alkene concentration in the gas phase [9]. Furthermore, it is possible that the redox Cr ~§ sites generate more coke and side products than the non-redox sites. The poisoning of the redox sites could thus account for the increase in the selectivity after the initial period of the experiment. The more stable behaviour of the FB catalyst could thus be explained by its lower redox Cr 3§ content which, however, decreases the overall activity in the beginning.

It was not possible to deduce clear differences between the stabilities of the catalysts based on the long-term deactivation experiments because of the coke formation on the ALD catalyst. So, it was necessary to study the stability in consecutive reduction-oxidation cycles with H2-TPR. The data obtained in the reduction studies might give an indication about the number and the properties of the reducible sites.

3.2. Temperature programmed reduction The reduction studies made for the ALD and FB catalysts indicated notable differences

between the samples. The ALD samples reduced at substantially lower temperatures than the FB samples. The reduction rate maximum of the ALD catalyst was around 315 ~ and that of the FB catalyst close to 340 ~ under similar reduction conditions. The FB sites were evidently more difficult to reduce. The reduction data of the both samples exhibited clearly a single-peak behaviour. This was interpreted that only one clearly dominant reduction process was taking place on the both catalysts.

The Kissinger peak analysis [10] was performed for H2 consumption data. It gave the apparent activation energy values of 99 and 73 kJ/mol for the ALD and the FB catalyst, respectively. Also the Friedman's method of constant conversion [ 11 ] was carried out. For the ALD sample data, this technique resulted in the activation energy values around 95 kJ/mol for the reduction degrees from 0.2 to 0.5 and then slightly higher values for the higher reduction degrees. Correspondingly, for the FB sample data, Friedman analysis gave the activation energy values around 75 kJ/mol for the reduction degrees below 0.6 and then again slightly higher values for the higher reduction degrees. The results suggest that the reduction behaviour of these two catalysts are fundamentally different.

157

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Fig. 3. Repeated TPR for the FB catalyst. Fig. 4. Repeated TPR for the ALD catalyst.

The kinetic modelling of the TPR patterns was then performed with nonlinear regression analysis to test various reduction mechanisms [12]. The nuclei growth models (Avrami- Erofeev models) were adequate to describe the data, but the conventional models suggested that the dimension of the nuclei growth was different for the studied ALD and FB samples. The Avrami-Erofeev type model was therefore re-derived into a form that included both the nucleation and the nuclei growth with temperature dependent rate coefficients. This model with the first order nucleation and the 2-dimensional nuclei growth now best described the measured data for the both catalysts. The activation energies of the nuclei growth were 76 kJ/mol and 89 kJ/mol for the ALD and FB catalysts, resp. The nucleation parameters were not well-identified, but obtained such values that the nucleation for the FB catalyst was fast and for the ALD catalyst practically instantaneous. The modelling results imply differences in chromia structure on the two catalysts. The overall description of the kinetic model was better for the ALD catalyst. The structure of the reducible sites on the ALD catalyst might be closer to the structure assumed in the model derivation (i.e. large 2-dimensional islands) compared to the structure of the FB catalyst. The activation energies of nuclei growth deviate from the values received from the Kissinger and the Friedman analysis, which is likely due to the fact that these methods do not take into account any mechanistic assumptions. The two- dimensional nuclei growth model is adequate due to the assumed location of Cr 6+ on the catalyst surface.

The repeated H2-TPR procedures revealed qualitative differences between the studied catalyst samples. The reduction rate maxima of the ALD catalyst shifted to higher temperatures, whereas the reduction rate maxima of the FB catalyst remained within a temperature range of 3 ~ for all the five TPR-runs, as seen in Figures 3 and 4. Furthermore, the five H2-TPR curves, between which the oxidation at 590 ~ was performed, showed that the decline in the amount of Cr 6+ was different for the two catalysts: the decline was faster with the catalyst prepared by ALD. The ALD catalyst contained more reducible chromia than the fluidised bed catalyst with the equal chromium loading, and the reduction studies indicated a higher stability of the fluidised bed catalyst. The repeated reduction treatments were obviously able to change both the nature and the amount of reducible chromia species of the ALD catalysts.

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4. CONCLUSIONS

An essential difference was found in the behaviour of the two catalysts during dehydro- genation. The activity of the ALD catalyst declined rapidly with an increase in the selectivity. The deactivation during one dehydrogenation test run was mainly caused by coke formation, which was more advanced on this catalyst because of the higher amount of redox Cr 3+.

The repeated TPR measurements showed a dissimilarity in the reduction behaviour of the catalysts. The ALD catalyst was easily reducible but the amount of Cr 6§ decreased faster compared to the FB catalyst and the temperature of the maximum reduction rate shifted to higher temperatures. However, the observed decline in the amount of redox Cr 3§ was faster than the decrease in the dehydrogenation activity on the two catalysts. In addition, the kinetic modelling indicated that the structure of the reducible sites on the ALD catalyst was closer to large 2-dimensional islands than the structure on the FB catalyst. From all these results we may conclude that on the FB catalyst the redox sites are more scattered and structurally stable. On the ALD catalyst the Cr 6§ sites seem to be more mobile and thus susceptible to clustering, which could transform the reducible sites into more scattered non-redox species, approaching the structure of the FB catalyst. In that case the long-term deactivation of the catalysts could be explained by the decline in Cr 6§ in addition to the cluster formation on the ALD catalyst.

In a summary, it seems that the behaviour of the catalysts in the dehydrogenation cannot be explained by only one active site. This is in an agreement with the earlier observations that the redox and the non-redox Cr 3§ sites are both active in dehydrogenation [4,7], but with different activities [7].

ACKNOWLEDGEMENTS

The Academy of Finland is gratefully acknowledged for supporting this study. We thank also Dr. Aria Kyt6kivi at Fortum Oil and Gas Oy for preparing the ALD catalyst.

REFERENCES

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