Oxidative dehydrogenation of n-octane over molybdate based catalysts.
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Date
2014
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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.
Description
Ph. D. University of KwaZulu-Natal, Durban 2014.
Keywords
Dehydrogenation., Molybdates., Catalysts., Theses--Chemistry.