Pulse Mycrocatalytic Investigation of Model Hydrogenation Reaction on Platinium-Rhnium\Aluminium Oxide Catalyst
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Date
1985
Authors
Daniel, B.A
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Publisher
University of Lagos
Abstract
The hydrogenation and dehydrogenation of benzene, cyclohexene and cyclohexane were investigated on a 0.3% Pt- 0.3% Re on Al203 catalyst (Catalyst (A)) using the pulse micro-catalytic technique. Reactions were carried out in hydrogen and in nitrogen containing 0.66% oxygen (labelled N2/O2) under the following conditions: 4 atmospheres total pressure, 500C to 4000C and 120cm3 min-1. Kinetic data were obtained on clean coked and CS2-poisoned catalyst. The theory of Sica et al (1) was applied to extract kinetic parameters. The activation energies were found to be identical for reactions carried out on coked, CS2-poisoned (at 3730C) and clean catalyst, thus suggesting that deactivation of catalysts by CS2 and coke occur predominantly by geometrical blockage of active sites. A slight enhancement of the selectivity for hydrogenation of cyclohexene to cyclohexane over dehydrogenation to benzene by CS2 poisoning suggested that the poison also exerted some electronic effect on the reactions. The u-6 bond shift mechanism of Ruiz - vizcaya et al (2) for the dehydrogenation of cyclohexan on Pt/Pd catalysts was found to account for most of the observed kinetic phenomena. The kinetic parameters determined by the pulse experiments were largely confirmed by continuous (plug) flow microreactor experiments. The order of reaction was found to be zero and the activation energy was found to be about 8 kcal/mole for benzene and cyclohexene hydrogenation to cyclohexane. For cyclohexane dehydrogenation, the order of reaction ranged between 0.7 to 0.9 while the activation energy varied more significantly. For the pulse experiments, it was 33 kcal/gmol (30000 - 3350C) and 16.5 kcal/gmol. (3350 - 3900C) while a single value of 24 kcal/gmol (3150 - 3750C) was calculated for the flow experiments. The deactivation of catalyst (A) by cyclohexene and cyclohexane in very dry and high purity helium in the temperature range 3000C to 4000C and 4 atmosphere total pressure was studied. A model developed in this thesis was used for data analysis. The deactivation of catalyst (A) by cyclohexane and cyclohexene was found to be described by the relation. da = kda dt With zero activation energy. Deactivation was rapid when catalyst surface was contacted with oxygen before reaction. Catalyst (A) was charaterised by means of the conventional H2/O2 titration in helium and by the titration of H2 and O2 in N2/O2 carrier (a method proposed here). Charaterization was also done by monitoring the Temperature Programmed Desorption (TPD) profile of oxygen adsorbed on the catalyst. The H2/O2titration and the TPD results suggest the existence of bimetallic clusters of Pt and Re which segregates to Re-rich and Pt-rich clusters in the presence of oxygen. The observed increase in the hydrogenolysis of cylohexane on oxidised catalyst is also consistent with this concept.
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Keywords
Pulse Mycrocatalytic Investigation , Model Hydrogenation Reaction , Platinium-Rhnium\Aluminium Oxide Catalyst
Citation
Daniel, B.A (1986) Pulse Mycrocatalytic Investigation of Model Hydrogenation Reaction on Platinium-Rhnium\Aluminium Oxide Catalyst. University of Lagos School of Postgraduate Studies Phd Chemical Engineering Thesis and Dissertation Abstracts, 565p.