The oxidative dehydrogenation of ɳ-octane using manganese oxide supported on SBA-15 catalysts.

Abstract

Supported metal oxides have been widely used in heterogeneous catalysis, particularly for the activation of paraffin’s. For this study, manganese oxide supported on SBA-15 was used to investigate the oxidative dehydrogenation of ɳ-octane. Wet impregnation was used firstly to synthesize catalysts with clusters of manganese oxide on the surface of the SBA-15 support. Thereafter, deposition precipitation was used to prepare catalysts with metal oxides in the pores of the SBA-15 support. Weight loadings of 2, 5 and 9 weight percent were synthesized for both groups of catalysts. Catalysts were characterized by inductively coupled plasma – optical emission spectroscopy, Raman spectroscopy, powder and in situ X-ray diffraction, temperature programmed analyses, oxygen chemisorption, nitrogen physiorption, Brunauer-Emmet-Teller surface analyses, transmission electron microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy. With most characterization techniques, clear differences were seen for catalysts with metal oxide on the surface of the support when compared to those with metal oxide in the pores of the SBA-15 support at the same weight loadings. However, the dominant phase present at room temperature was found to be Mn₃O₄ for both groups of catalysts, with a phase change to MnO taking place after reduction similarly for both groups of catalysts. The catalysts were tested in a continuous flow fixed-bed reactor system. The support and all catalysts were found to be active for the oxidative dehydrogenation of ɳ-octane. Dominant product groups obtained were octene isomers, aromatic compounds, cracked products and carbon oxides. Desired products groups were octene isomers and aromatic compounds and based on this and conversions of ɳ-octane, reaction conditions were optimized and all catalysts were tested and compared under optimum conditions. From the results obtained, insight into product pathways were obtained and a reaction mechanism was proposed for this metal oxide system. The 9 weight percent catalysts were found to provide the best conversions for both groups of catalysts, with catalysts having metal oxide on the surface displaying greater conversion than catalysts with metal oxides in the pores of the SBA-15 support, likely due to the enhanced accessibility of the active site. The lower accessibility of the active site did however lead to one product group having high selectivity for catalysts with metal oxide in the pores, with carbon oxides and octenes being the most selective product group for the 2 and 9 weight percent catalysts respectively.