Propane Dehydrogenation on Metal and Metal Oxide Catalysts

Abstract

In this PhD thesis, the catalytic performance and deactivation of various propane dehydrogenation catalysts is studied. First of all, a literature study is performed, where the three most commonly used formulations, namely Pt-, CrOx- and GaOx-based catalysts are compared in terms of yield relative to space velocity and deactivation rate. It was found that a supported Pt-Sn catalyst provided the more active and stable formulation at similar reaction conditions. Here, a novel catalyst based on 1000 ppm Pt and 3 wt% Ga supported on Al2O3 is reported, that provides a similar excellent activity and stability. A synergy between the two metals is observed, as the catalysts containing either Ga or Pt display significantly lower activities. After extensively characterizing the material, the synergy between the two metals is explained by the presence of a highly stable mixed Al-Ga oxide, on which the Pt is highly dispersed in sub-nanometer clusters. Surface Ga Lewis acid sites are believed to be responsible for the dehydrogenation activity, with the Pt providing a promoting function of facilitating the rate-limiting step of the reaction, which is the recombination of adsorbed hydrogen atoms. A second point of interest, was to use spectroscopic tools such as Raman spectroscopy and Scanning Transmission X-ray Microscopy (STXM) to study differences in the carbon deposits that are formed as a byproduct during the dehydrogenation reaction on the catalysts. Effects, such as the partial pressure of propane, propylene and hydrogen, temperature and catalyst material used were investigated. For example, bands corresponding with coke deposits change in the Raman spectra, as the partial pressure of hydrogen is increased. This suggests the formation of smaller, more graphitic carbon crystallites. As a second example, the coke formed on Pt, Pt-Sn, Ga and Pt-Ga/Al2O3 was compared. On the latter two catalyst, more coke was formed, which had a more graphitic nature. Finally, using STXM, heterogeneities in the coke formed on individual catalyst particles were observed. Specifically, coke formed on the outside of a catalyst particle has a different chemical nature than coke formed on the inside. Thirdly, the design, construction and testing of special probes to study the process of coke deposition and the consecutive combustion of the coke deposits, inside a propane dehydrogenation pilot scale plant was discussed. The probes are designed as such that they are able to withstand the harsh conditions of the propane dehydrogenation reaction, and that the tip does not foul during the reaction. The process of coke formation was studied at different heights of the catalyst bed, and it was found that the coke deposition is more rapid at the top of the catalyst bed, which is related to the temperature of the catalyst bed. Furthermore, by combining the information obtained by Raman and UV-Vis spectroscopy, quantitative information of the process of coke deposition was obtained during the first hour of the dehydrogenation reaction.