An Evaluation of Alumina Supported Platinum Catalysts for the Oxidative Dehydrogenation of n-Butane

McNamara, John Martin (2000) An Evaluation of Alumina Supported Platinum Catalysts for the Oxidative Dehydrogenation of n-Butane. PhD thesis, University of Glasgow.

Abstract

The oxidative dehydrogenation of n-butane has been studied as a potential alternative to conventional catalytic straight dehydrogenation for the production of butene olefin species. This work was inspired by recent advances in catalytic oxidative dehydrogenation studies utilising short contact times and surface science measurements highlighting the importance of 'hot oxygen' surface chemistry. Two low loading Pt/Al2O3 catalysts have been extensively characterised and used throughout this work. Experimental parameters such as temperature, flow-rate, catalyst mass and hydrocarbon:oxygen ratio were manipulated in an attempt to build a greater understanding of the influential factors responsible for the activity and selectivity of system. The results have shown that catalytic oxidative dehydrogenation is not feasible under the conditions used and that combustion reactions forming carbon dioxide dominate. In contrast, in the absence of the catalyst, relatively high olefin selectivity was observed as a result of homogeneous radical processes. The results from catalytic straight dehydrogenation reactions were dependent on the substrate used. With the 0 65wt% Pt/Al2O3, known from CO chemisorption data to consist of very small platinum crystallites, high dehydrogenation selectivity was observed at modest conversion. Further studies also demonstrated this to be the case regardless of the depth of the catalyst bed. On the contrary, the 0.5wt% Pt/Al2O3 catalyst displayed very poor dehydrogenation selectivity as well as low activity. This was due to the larger platinum crystallites contained in this catalyst, yielding unwanted isomerisation, cracking and hydrogenolysis products, which are known to be structure sensitive favouring larger particle sizes. The low activity observed in the straight dehydrogenation reactions on both catalysts occurred as a result of a combination of carbonaceous blocking of the active sites and pore blocking, restricting access to the active sites. Flow-rate studies demonstrated that the dehydrogenation selectivity could be controlled by manipulation of reactant contact time with the catalyst. Again, the trends were dependent on the substrate used. With the small particles of the 0.65wt% catalyst extended contact times were found to be more favourable for high olefin selectivity, whereas short contact times were found to give the best results for the larger particles of the 0.5wt% catalyst. These observations are consistent with the proposed reaction sequence and the contrasting surface chemistry of the two catalysts, with respect to their particle sizes. On the basis of 'hot oxygen' chemistry, where it was demonstrated that very low concentrations of oxygen could have a profound effect on oxidation reactions involving p-hydrogen abstraction on single crystal surfaces, a study was carried out to see if the dehydrogenation selectivity could be enhanced by manipulation of the hydrocarbon:oxygen ratio. It was found that increasing the oxygen concentration from zero to 1:2 resulted in a dramatic decrease in olefm selectivity with a subsequent increase in butane conversion. It was concluded that the maintenance of a 'hot oxygen' concentration was not possible on the supported catalyst systems studied in a plug flow reactor and that combustion processes were dominant. These combustion reactions being responsible for the increased conversion and decreased selectivity. Pulse-flow measurements were used in order to investigate the initial conditioning of the catalyst surface in an attempt to identify the processes responsible for the effects seen in the continuous-flow (steady state) experiments. A direct correlation was found between the deposition of carbonaceous material on the catalyst surface and the dehydrogenation selectivity. It is observed that the selectivity towards the olefm products increased as carbonaceous material was deposited. It was proposed from this that the presence of the carbonaceous material invoked a change in the electronic nature of the metal surface in a similar manner to that for platinum/tin alloys. The presence of oxygen was found to remove some of the carbonaceous material necessary for olefin formation. In conclusion, the presence of oxygen in the catalytic dehydrogenation of butane using supported platinum catalyst is detrimental to the production of the favoured butene species and that the particle size of the platinum crystallites contained in the catalyst is crucial to obtaining high olefm yields. Dehydrogenation being favoured by small platinum crystals. We also conclude that bridging the gap between single crystal surface science studies and supported catalyst chemistry is not feasible in a microreactor environment operating in steady and non-steady state regimes to achieve the clean, controlled environment compatible with a UHV study.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Dave Lennon
• Keywords: Organic chemistry
• Date of Award: 2000
• Depositing User: Enlighten Team
• Unique ID: glathesis:2000-76237
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Dec 2019 09:15
• Last Modified: 19 Dec 2019 09:15
• URI: https://theses.gla.ac.uk/id/eprint/76237