Deactivation of VMgO x Catalysts by Coke in the Process of Isobutane Dehydrogenation with Carbon Dioxide
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Deactivation of VMgOx Catalysts by Coke in the Process of Isobutane Dehydrogenation with Carbon Dioxide Jan Ogonowski Æ El_zbieta Skrzyn´ska
Received: 15 March 2007 / Accepted: 22 October 2007 / Published online: 13 November 2007 Ó Springer Science+Business Media, LLC 2007
Abstract The process of isobutane dehydrogenation in the presence or absence of carbon dioxide was carried out over VMgOx catalysts with different vanadium loading. The performed tests show that both the reaction atmosphere and physicochemical properties of the catalysts (related to vanadium content) have a great influence on the activity decrease and the carbonaceous deposit formation. Despite small ability of carbon dioxide to remove coke in the Boudouard reaction, the amounts of carbonaceous species deposited on the catalysts after the isobutane dehydrogenation under CO2 atmosphere were even twice greater in comparison to those deposited in helium stream. Moreover, the rate of coke deposition during the dehydrogenation in the inert gas flow was only slightly dependent on the reaction time, in contrast to the process in carbon dioxide atmosphere. The results show that the coke formation on VMgOx is enhanced predominantly by surface acidity of the catalysts, which grows with the vanadium content and the presence of CO2 in the feed. Keywords Isobutane Dehydrogenation Carbon dioxide VMgOx catalysts deactivation
1 Introduction The catalysts deactivation is one of the most serious problems in modern chemical technology. Depending on the process, the catalyst life cycle can vary from a few
J. Ogonowski E. Skrzyn´ska (&) Institute of Organic Chemistry and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Krakow, Poland e-mail: [email protected]
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seconds or minutes, as in the fluid catalytic cracking (FCC) and the dehydrogenation reactions, respectively, to several months or even years, as in the case of ammonia synthesis [1]. The nature of catalyst deactivation is complex, however there are six main causes of the activity loss in the catalytic processes: (a) fouling by coke, (b) poisoning, (c) leaching of the active elements by the reaction mixture, (d) chemical corrosion (vapor–solid and/or solid–solid reactions), (e) thermal degradation (sintering and/or evaporation), and (f) mechanical damage (attrition and/or crushing) [1–3]. The coke deposition on the catalyst surface is one of the main reasons of the activity loss in the case of dehydrogenation reactions. It is well known, that alkenes, aromatics and cyclic compounds are more reactive than saturated hydrocarbons, thus their interaction with acidic centers on the catalyst surface may cause undesirable side reactions, such as oligomerization, aromatization or alkylation to heavier compounds, which are most important coke precursors [3–5]. In most of the papers on the catalytic reactions of hydrocarbons with carbon dioxide an observation prevails, that introduction of CO2 into the feed can decrease significantly (or even suppress) the coke deposition in com
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