Carbon dioxide in the dehydrogenation of isobutane over VMgOx

Catalysis Communications 11 (2009) 132–136

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Carbon dioxide in the dehydrogenation of isobutane over VMgOx _ Jan Ogonowski, Elzbieta Skrzyn´ska * Institute of Organic Chemistry and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Kraków, Poland

a r t i c l e

i n f o

Article history: Received 12 June 2008 Received in revised form 10 December 2008 Accepted 3 September 2009 Available online 10 September 2009 Keywords: Dehydrogenation Isobutane Carbon dioxide Vanadium–magnesium oxide catalyst

a b s t r a c t On the basis of catalytic tests and kinetic investigations, the role of carbon dioxide in the conversion of isobutane to isobutene over VMgOx catalyst was studied. It was shown that both the high temperature (>826 K) and the presence of VMgOx catalyst are required for activation of CO2 molecule. Presented results stay in agreement with thermodynamic analysis of the isobutane dehydrogenation process, where the two-step dehydrogenation pathway (i.e., simple dehydrogenation followed by reverse water gas shift reaction) was the most favorable way of isobutene synthesis. Moreover, insufficient oxidizing property of carbon dioxide allows to exclude possibility of the redox-cycle mechanism over VMgOx catalyst. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction For a long time carbon dioxide was traditionally considered as an inert gas in the gas phase oxidation reactions. This opinion resulted from relatively high thermodynamic stability of CO2 molecule under typical reaction conditions, (i.e., under atmospheric pressure and the temperature far below 1273 K). Conversion of carbon dioxide to CO and O2 at 2273 K is less than 2%, due to low value of the reaction enthalpy (DH° at 298 K is 293 kJ/mol) [1,2]. In the early nineties, however, it was found that addition of small amounts of CO2 (below 0.5%) to ethylbenzene feed in the presence of a commercial dehydrogenation catalyst decreased the catalyst deactivation by coke formation [3,4], and it allowed reducing the cost of styrene production by saving energy [3]. Further investigations showed that some catalysts were able to activate carbon dioxide, so these catalysts could be used in oxidative dehydrogenation reactions [4]. On that basis, the researchers from Korea Research Institute of Chemical Technology (KRICT) developed a process for production of styrene from ethylbenzene using carbon dioxide as a mild oxidant [5]. The KRICT–DECSO process was implemented at a pilot-scale plant in Korea, operating at Samsung General Chemicals Co., Ltd. (SGC), with the styrene production capacity of 100 tons per day [5]. Isobutane is the least intensively studied alkane in the dehydrogenation process with carbon dioxide. Nevertheless, from the available literature [4] it can be found that mechanism of the dehydrogenation strongly depends on such physicochemical properties of the catalyst as reducibility [1,2,6–12] and acidity [7,13–18]. For lower hydrocarbons, most often used catalysts contain such ele* Corresponding author. Tel.: +48 126282761; fax: +48 126325352. E-mail address: [email protected] (E. Skrzyn´ska). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.09.011

ments as chromium [6–8], vanadium [8–10], iron [11,12], molybdenum [1,2], manganese [6,13,14], and other easily reducible oxides [4]. Thus, the redox mechanism is possible even in the presence of weak oxidizing agents, such as carbon dioxide. According to the Mars and van Krevelen mechanism, which is commonly used for description of olefin production processes by oxidative dehydrogenation reactions [18–20], carbon dioxide should be able reoxidize the catalyst surface reduced by hydrocarbons. Therefore, the presence of CO2 in the feed is necessary to keep active metal oxide at high oxidation state. Our recent experiments over VMgOx catalysts [21] confirmed that reoxidation of the species deeply reduced by hydrogen with pure CO2 at 873 K was very hard and could not restore initial, high oxidation state of vanadium cations. Also, Michorczyk reported similar results for a silica supported chromium oxide catalyst [22]. The other reports claim that carbon dioxide influences the reaction course by modifying acid–base properties of the catalyst surface [7,8,15,16,23], or, it decreases the extent of the catalyst deactivation by gasification of coke [2,4,8]. There can be also another possibility, where CO2 does not take part in the dehydrogenation reaction, but acts as an inert gas (i.e., it decreases partial pressure of the alkane feed and delivers heat required for the endothermic reaction) [4,24]. The aim of study is analysis of the most probable pathways of the isobutane dehydrogenation reaction with carbon dioxide over vanadium–magnesium oxide catalyst. The experimental results are discussed and referred to available literature reports.

2. Experimental A series of catalytic tests were carried out using vanadium– magnesium oxide with V/Mg molar ratio equal 0.11. The catalyst

´ ska / Catalysis Communications 11 (2009) 132–136 J. Ogonowski, E. Skrzyn

precursor was prepared by the citrate method using citric acid as a complexing agent, and, ammonium metavanadate and magnesium nitrate as sources of vanadium and magnesium, respectively [25]. Heating at 923 K in static air for 6 h gave mixed oxide catalyst denoted as VMgOx. XRD and IR analysis showed that freshly prepared material consisted of two stable phases: magnesium ortowanadate – Mg3(VO4)2 and magnesium oxide (periclase) [25]. The specific surface area of the catalyst (BET method) was 21 m2/g. Detailed description of the catalyst preparation, characterization and activity of the Mg3(VO4)2 phase were reported elsewhere [25,26]. In order to keep comparable conditions of the experiments, total flow of 5400 cm3/h gcat was maintained while the initial molar ratio of CO2 (or He) to iC4H10 was varied from 1 to 26 (typically adjusted to 6). The catalytic tests were performed over 0.4 g of VMgOx catalyst (0.2–0.3 mm diameter fraction) under atmospheric pressure. Detailed description can be found in [23,25,26]. To avoid problems with the catalyst deactivation, every change in the temperature and/or molar ratio of reagents was accompanied by replacement of the catalytic bed with a fresh portion of VMgOx. The main products of isobutane dehydrogenation were: isobutene, propane, propene, methane, traces of ethane, ethylene and linear C4 hydrocarbons. Except theme and unreacted feed, the hydrogen, carbon oxides and water were also detected in the reaction mixture. Conversions of isobutane and carbon dioxide, the same as the isobutene yield and selectivity were defined previously [23,26]. An apparent energy of activation was calculated on the basis of Arrhenius equation. 3. Results and discussion Fig. 1 shows the results of the isobutane dehydrogenation reaction carried out in the presence or absence of carbon dioxide at 773 and 873 K. The experiments were done under atmospheric pressure and constant total flow velocity of the reagents (5400 cm3/ h gcat) while the initial molar ratio of CO2 (or He) to iC4H10 was varied from 1 to 26. As it can be seen from Fig. 1, at the temperature of 773 K the isobutane conversion and selectivity to isobutene in the presence of CO2 were almost equal to those in the presence of He. Increasing of carbon dioxide partial pressure affected the isobutane conver-

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sion, while the selectivity to isobutene stayed almost constant. Similarly, conversion of carbon dioxide almost did not change and was close to zero within a wide range of feed compositions. These results suggest that carbon dioxide was not activated at the temperature of 773 K. Thus, the role of carbon dioxide at steady-state conditions was limited to lowering the partial pressure of hydrocarbon feed. Quite different results were obtained at the temperature higher by 100°, where carbon dioxide was activated and consumed at 873 K over VMgOx catalyst (conversion of CO2 decreased from 12 to 4 with its increasing partial pressure). Moreover, conversion of isobutane in the presence of carbon dioxide was always higher than that with helium. Analogous results were obtained from the theoretical calculations of the two-step reaction pathway [27]. It has to be pointed out, that thermodynamical calculations distinguish between two possible reaction pathways [4,23,27]: direct isobutane conversion with participation of carbon dioxide (Eq. (1)), and, a two-step process, where simple dehydrogenation is followed by RWGS reaction (Eqs. (2) and (3), respectively).

iC4 H10 þ CO2 ¢ iC4 H8 þ H2 O þ CO

ð1Þ

iC4 H10 ¢ iC4 H8 þ H2

ð2Þ

H2 þ CO2 ¢ H2 O þ CO

ð3Þ

As it was shown in our previous paper [27], significant differences between the reaction pathways were found from thermodynamical calculations for the initial carbon dioxide (or helium) to isobutane molar ratios below three. For such feed compositions, the equilibrium conversion of isobutane for the one-step reaction pathway was lower than the conversion calculated for the process in the absence of CO2, within a wide temperature range. On the other hand, the two-step reaction pathway gave highest hydrocarbon conversions at all reaction temperatures and molar ratios of the reagents studied [27]. Michorczyk and Ogonowski [28] and Mimura and Saito [29] reported similar trends for the dehydrogenation of propane and ethylbenzene. Thus, the comparison of the experimental results over VMgOx catalyst at 873 K (Fig. 1) with the theoretical predictions suggests that the main role of CO2 at steady-state conditions should to be removal of hydrogen produced by the simple dehydrogenation

Fig. 1. The effect of feed composition on isobutane conversion (diamonds) and selectivity to isobutene (triangles) in isobutane dehydrogenation in the presence (solid symbols) or absence (open symbols) of carbon dioxide. Crossed symbols represent conversion of CO2. Reaction conditions: quartz microreactor loaded with 0.4 g of the VMgOx catalyst (grains with 0.2–0.3 mm diameter), 773 and 873 K, atmospheric pressure, CO2 (or He)/iC4H10 = 1–26, total flow rate: 5400 cm3/h gcat. The analyses were done on-line after 10 min since the start of each process.

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(Eqs. (2) and (3)). The presence of both hydrogen and carbon oxide in the reaction products seems to support that statement. H2/CO molar ratio was not constant and decreased from 0.3 to 0.1 with the increase of carbon dioxide partial pressure in the feed. From the thermodynamical point of view, such situation supports the two-step dehydrogenation pathway what stays in agreement with thermodynamics. On the other hand, Takahara et al. [30] reported that stable and ratio of hydrogen to carbon dioxide should be observed for coexisting reactions (1) and (2), regardless of the alkene yield. Because of observed significant differences between carbon dioxide behavior at 773 and 873 K, a series of catalytic tests were undertaken over a wide range of temperatures varied from 673 K to 923 K. To avoid problems with VMgOx deactivation, all experiments at different temperatures were done using always a fresh catalyst sample. The measurements were performed in fixed-bed microreactor with differential heating. The amount of catalyst was adjusted to limit axial and radial temperature gradients to minimum (less than 5°). The results of isobutane dehydrogenation in the presence of CO2 were compared with the results of RWGS reaction and the dehydrogenation in an inert gas atmosphere. Analogous experiments were carried out using empty microreactor. Fig. 2A and B shows the results. At it is shown, the highest temperature below which carbon dioxide was not activated and did not take part in the isobutane dehydrogenation process was around 800–826 K. Minimal conversion of CO2 resulted from negligible promotion of isobutane conversion in comparison to the process under helium atmosphere. Beneficial effect of CO2 on the dehydrogenation reaction was observed above 826 K, where conversion of carbon dioxide was measurable. It should be noted that the same temperature was terminal for RWGS reaction over VMgOx catalyst (Fig. 2A). Similarly, Michorczyk and Ogonowski [31] and Zheng et al. [32] concluded that the activation of carbon dioxide required temperatures higher than 823 K. The tests performed without catalyst (Fig. 2B) proved that carbon dioxide was not consumed in the homogenous dehydrogenation and RWGS reactions even at high temperatures above 873 K. Consequently, conversion of isobutane in the dehydrogenation reaction was independent of the presence of CO2 in the feed. On the basis of the results obtained, it can be concluded that both the catalyst and the high temperature are required for the

(A)

carbon dioxide activation. Below 800 K isobutene is produced mainly by simple dehydrogenation, while CO2 behaves as an inert gas. At higher temperatures, carbon dioxide is consumed and takes part in the dehydrogenation process, enhancing overall alkane conversion. Different values of the apparent activation energy shown in Fig. 3 confirm that the reaction mechanism changes. On the other hand, the differences observed at the temperature below 873 K can be caused by changes of the active center character. Influence of the temperature on reducibility and oxidation state of VMgOx catalyst was discussed in [22]. The Arrhenius plot for the isobutane dehydrogenation over VMgOx contains two different areas (Fig. 3). At the temperatures below 760 K, the apparent activation energy is 49 kJ/mol both in the presence and the absence of CO2. This suggests that both processes proceed in a similar way, (i.e., by the simple dehydrogenation – Eq. (2)). On the other hand, at the temperatures higher than 826 K, the apparent activation energy of the reaction in the presence of carbon dioxide is lower than that under helium atmosphere (71 and 84 kJ/mol, respectively). Therefore, the presence of carbon dioxide clearly promotes conversion of isobutane over VMgOx catalyst. The beneficial effect of CO2 can be related to its gasification properties. Nevertheless, as it was shown in [26], the amount of coke generated during the dehydrogenation in the presence of CO2 was always higher than that in helium. Carbon dioxide stream was able to gasify some part of the carbonaceous deposit formed on VMgOx catalyst surface, but it did not remove all the deposit under steady-state conditions, even though under atmospheric pressure at 873 K the time of Boudouard reaction was equal to the time of isobutane dehydrogenation in helium. Thus, the rate of coking was faster than the rate of decoking [26]. The isobutane dehydrogenation reaction at 873 K combined with periodical heating of the catalyst bed under carbon dioxide and oxygen flow at the same temperature was employed for examining the ability of CO2 to regenerate the catalyst. Fig. 4 shows the conversion of isobutane and carbon dioxide, and selectivity to isobutene as a function of time-on-stream over VMgOx catalyst. As it has been shown, the ability of carbon dioxide to regenerate VMgOx catalyst was insufficient, because heating of the used-up catalyst in pure CO2 flow did not recover its initial activity (grey areas in Fig. 4). Even large excess of carbon dioxide used in the regeneration cycle did not allow to observe an increase of the iso-

(B)

Fig. 2. The effect of reaction temperature on isobutane conversion (diamonds) and selectivity to isobutene (triangles) in dehydrogenation reaction in the presence (solid symbols) or absence (open symbols) of carbon dioxide. Crossed symbols represent conversion of CO2 in the dehydrogenation while dotted line corresponds to RWGS reaction (H2/CO2 molar ratio = 1/7). Reaction conditions: 0.4 g of VMgOx catalyst (A) or empty quartz microreactor (B), feed composition: CO2 (or He)/iC4H10 = 1/6, total flow rate: 5400 cm3/h gcat. On-line analysis of the products was carried out under atmospheric pressure after initial 10 min run over fresh catalyst sample.

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Fig. 3. Arrhenius plot for the dehydrogenation of isobutane in the presence (full symbols) or absence (open symbols) of carbon dioxide in the feed. Reaction conditions were the same as those in Fig. 2.

Fig. 4. The effect of time-on-stream on isobutane conversion (diamonds) and selectivity to isobutene (triangles) in dehydrogenation reaction with carbon dioxide. Crossed symbols represent conversion of CO2. Reaction conditions: 0.4 g of VMgOx catalyst, feed composition: CO2/iC4H10 = 1/6, total flow rate: 5400 cm3/h gcat. Regeneration by pure carbon dioxide and air at 873 K, total flow rate: 4500 cm3/h gcat.

butane conversion. On the other hand, even small volume of air (shading area in Fig. 4) was able to reoxidize VMgOx surface. Similarly, the thermodynamic analysis presented by Sakurai et al. [10] proves that reoxidation of vanadium–magnesium oxide by carbon

dioxide is rather difficult. Thus, it can be concluded that poor oxidizing power of carbon dioxide reduces significantly contribution of redox mechanism to the mechanism of the dehydrogenation reaction. Nonetheless, the majority of researchers still consider

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the redox cycle as a prevailing mechanism for dehydrogenation of light hydrocarbons in the presence of CO2 over various transition metal oxide catalysts [4,6–14]. Thus, the acid–base properties of both the catalyst and the reagents seem to be essential for the reaction course over VMgOx. The results obtained for isopropanol decomposition reaction carried out in the presence or absence of carbon dioxide over series of VMgOx catalysts [26] proved that introduction of weakly acidic CO2 into the reaction stream modified physicochemical properties of the catalyst surface by increasing its acidity. As the result, considerable differences in the reaction outcome (i.e., alkane conversion, selectivity to the desired olefin, and, rate of the catalyst deactivation by coke) could be observed. Analogous effect was reported previously in the literature mostly for hard and/or nonreducible catalysts, such as gallium [15,16] or sodium oxides [17], where the redox cycle was negligible and the improvement in the conversion of hydrocarbons and/or selectivity to the desired olefin was attributed to modification of acid–base properties the catalyst surface by carbon dioxide. In [8,15,30] we found also the opposite effect, where strong interaction of CO2 with the catalyst surface decreased the catalyst activity. Nevertheless, the results of our investigation of isobutane dehydrogenation over various vanadium oxide supported catalysts [33] have shown that the latter situation occurs only when the catalyst surface contains strong basic sites. Thus, medium-strength basicity and acidity, typical for VMgOx catalyst, is found to be most appropriate for the activation of both isobutane and carbon dioxide in the dehydrogenation process [33]. 4. Conclusions The catalytic tests and the kinetic investigations (Figs. 2 and 3, respectively) prove that carbon dioxide activation requires presence of the catalyst and starts at the temperature about 800– 826 K. This activation changes the overall reaction mechanism. At lower temperatures carbon dioxide behaves as an inert gas – it merely decreases the hydrocarbon partial pressure. The same situation can be observed in the absence of the catalyst, where the simple dehydrogenation reaction (2) is the main path to isobutene formation. On the other hand, above 826 K conversion of CO2 over VMgOx catalyst in both the dehydrogenation of isobutane and RWGS reaction becomes measurable. Moreover, H2/CO molar ratio depends on both the isobutene yield and the carbon dioxide partial pressure. Thus, it seems to be reasonable to assume that carbon dioxide removes hydrogen from the reaction mixture and enhances isobutane conversion to isobutene as predicted by the thermodynamic calculations done for the two-step pathway of the dehydrogenation process [27]. Moreover, acid–base properties of

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