Three inter-linked active sites in the dehydrogenation of n-octane over magnesium molybdate based catalysts and their influences on coking and cracking side reactions

Abstract

Dehydrogenation of paraffins is an industrially practiced technique for the production of value added olefins. Molybdenum based catalysts are known for this application. This study focuses on the application of magnesium molybdate based catalysts with different Mg:Mo ratios for the dehydrogenation of n-octane. Several characterisation techniques were utilised to investigate the physicochemical properties of the bulk as well as the surfaces of the fresh and used catalysts. Three active sites with different functionalities were identified for these materials in the chosen catalytic reaction. Reduced molybdenum oxide (Mo4+ and Mo5+) sites were responsible for dehydrogenation, whereas MgO (involved in the MgMoO4 lattice or as the MgO phase) modified the density of basic sites that strongly controlled the adsorption of n-octane. Finally, acidic sites (likely from molybdenum oxides) promoted cracking and played an indirect role in increasing coke deposition over these materials. A mechanism that involves the simultaneous operation of these three active sites is proposed. The ratio between MgMoO4 and MoO3 was found to strongly influence the redox transition and thermal stability of these phases. The discussed science in this study can be used as a tool for preparation of advanced molybdenum based catalysts for different catalytic applications.