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Butane Dehydrogenation over Pt/Alumina: Activation, Deactivation and the Generation of Selectivity. S. D. Jackson, D. Lennon, and J. M. McNamara. Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland U.K. Introduction In this study the dehydrogenation of butane over a Pt/alumina catalyst has been studied using a pulse flow technique. The dehydrogenation of alkanes to alkenes is an important industrial process that has been operated for years however none of the current industrial processes are ideal and ways of improving the process are always being sought [1 ­ 4]. The catalysis is complex with a series of competing reactions occurring simultaneously including significant carbon laydown resulting in catalyst deactivation. In this study we have examined the first few seconds of catalyst life to try to understand the generation of selectivity during process of catalyst activation and the initial stages of deactivation. Results and Discussion The conversion of butane was studied over a reduced and unreduced Pt/alumina catalyst under dehydrogenation conditions of 673 K and 0.101 MPa. With both the reduced and unreduced catalysts 100% conversion was observed on the first pulse with zero selectivity to the formation of alkenes. Under reaction conditions the unreduced catalyst was reduced. However the rates of dehydrogenation and butene isomerisation were sensitive to whether the catalyst had been activated by hydrogen reduction or by in situ reduction by butane. For example over the unreduced catalyst the formation of 1-butene and trans-but-2-ene is favoured whereas over the reduced catalyst cis-but-2-ene is favoured. Over the unreduced catalyst the initial process is reduction of the platinum by the butane. As carbon is used to remove oxygen as carbon dioxide the extent of carbon deposition is less over the unreduced catalyst. This has a negative effect on the selectivity of the dehydrogenation reaction and generates a catalyst that has a different selectivity pattern to that of a catalyst reduced in hydrogen before being subjected to butane (Figures). At a similar conversion, the catalyst that has been reduced in hydrogen has a 50% higher yield of butenes, than the comparable catalyst that has been reduced in situ with butane. Therefore by using a pulse methodology we have been able to show that the yield of butenes increases even as the amount of carbon on the catalyst increases. Indeed the selectivity of the catalyst to dehydrogenation rests upon the deposition of carbon to eliminate unselective sites.

Dehydrogenation yield (%)

20 15 10 5 0

3E+19 2.5E+19 2E+19 1.5E+19 1E+19 5E+18 1 2 3 4 Pulse Number 5 6 0

Unreduced, dehydrogenation

Unreduced, carbon deposition

Dehydrogenation yield (%)

25 20 15 10 5 0 1 2 3 4 Pulse Number 5 6

3.5E+19 3E+19 2.5E+19 2E+19 1.5E+19 1E+19 5E+18 0

Reduced, dehydrogenation

Reduced, carbon deposition

Therefore the activation and the deactivation processes occurring in the butane dehydrogenation reaction are fundamental in obtaining the desired selectivity. The links between propane and butane dehydrogenation over platinum catalysts will also be discussed as will the link between dehydrogenation and oxidative dehydrogenation.

1. 2. 3. 4. K. Kearby, Catalysis Vol. 3, P. H. Emmet (ed.), Reinhold, New York, p.453, 1955. M. M. Bhasin, J. H. McCain, B. V. Vora, T. Imai, and P. R. Pujado, Appl. Catal. A, 221 (2001) 397. T. Hutson, Jr., and W. C. McCarthy, Handbook of Petroleum Refining Processes, R. A. Meyers (ed.), McGraw-Hill, London, 1986. P. R. Pujado and B. V. Vora, Hydrocarbon Process., March, (1990) 65.

Cumulative carbon deposition

Cumulative carbon deposition

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Microsoft Word - 307-Butane Dehydrogenation over Pt-Alumina= Activation, De­