Oxidative dehydrogenation of n-octane over molybdate based catalysts.

Abstract

The oxidative dehydrogenation of n-octane over different molybdates was investigated using a continuous flow fixed bed reactor in the temperature range of 350-550 °C at 50 °C intervals. Molybdates investigated in this study were synthesized by the co-precipitation method and characterized by powder and in situ (oxidation and reduction) X-Ray diffraction (XRD), BETsurface area measurements, inductively coupled plasma-optical emission spectroscopy (ICPOES), Raman spectroscopy, scanning electron microscopy (SEM), temperature programmed reduction (TPR) and temperature programmed oxidation (TPO). Molybdates focus of this study was magnesium molybdate (MM) and cobalt molybdate (CM). In the case of magnesium molybdate, catalysts with different magnesium : molybdenum ratios were synthesized (i.e. 0.87, 0.98, 1.06 and 1.25 magnesium : molybdenum). While for cobalt molybdate the ratio of cation to molybdenum was kept near the stoichiometric ratio. The influence of the synergistic effect between molybdenum trioxide and molybdate was investigated using MM. An increase of the molybdenum content in the catalysts resulted in an increase in the surface area of the catalysts and in the TPR results the intensity of the reduction peak corresponding to molybdenum trioxide increased as the molybdenum content increased, which marked an increase in the n-octane conversion. The preliminary catalytic testing was at a gas hourly space velocity (GHSV) of 4000 h-1 and carbon to oxygen ratio of 8:3 C:O. The highest conversion of n-octane and selectivity to value added products (i.e. octenes and aromatics) was obtained over the two catalysts with near stoichiometric ratio of molybdenum / magnesium (i.e. MM 0.98 and 1.06). The surface acidity of the catalysts was altered by varying the molybdenum content, which in return influenced the selectivity of the catalyst. Used catalyst characterization by Raman spectroscopy showed all catalysts were still dominated by the magnesium molybdate phase after the reaction. Both molybdates (i.e. MM and CM) with a near stoichiometric ratio of cation : molybdenum were tested under different oxidation environments ranging from oxygen lean to oxygen rich environments (i.e. carbon : oxygen ratio of 8:0, 8:1, 8:2, 8:3 and 8:4). The conversion of noctane over all molybdates increased as the oxygen concentration in the reaction feed increased. The carbon to oxygen ratio also greatly influences the selectivity of the catalyst. In general terms as the oxygen concentration increased the selectivity to octenes decreased and selectivity to aromatics increased, while the selectivity to COx increased and peaked at the reaction temperature close to the onset reduction temperature of the catalyst. The chemical stability of the catalyst was also altered by the oxygen concentration as determined by characterization of the used catalyst by powder XRD and Raman spectroscopy. In the case of MM and CM the initial phases of the catalyst was maintained and stable under moderate to oxygen rich environments (i.e. 8:2, 8:3 and 8:4 carbon : oxygen), while under oxygen lean environments (i.e. 8:0 and 8:1 carbon : oxygen) phase segregation takes place and molybdenum oxide dominates the catalysts. The effect of the cation in the molybdate structure was highlighted by comparing the activity and selectivity of the magnesium molybdate catalyst and the cobalt molybdate catalyst under isoconversion and iso-thermal conditions. Magnesium molybdate seems to favor olefin formation, while cobalt molybdate favors aromatics, based on the iso-conversion results. Considering the iso-thermal comparison between the two molybdates, the data indicate that cobalt molybdate is more active than magnesium molybdate.