Oxidative dehydrogenation of n-octane using morphologically different VMgO catalysts.

Abstract

ABSTRACT Gas phase paraffin oxidative dehydrogenation (ODH) has been explored extensively using medium chain paraffins such as n-hexane and n-octane as feedstock over various catalytic systems. Among these systems, the activity and stability of magnesium vanadates have been studied during the ODH of these paraffins. In most cases, it was found that 15 wt% vanadium was the optimum loading on the support, i.e. magnesium oxide. In addition, it was found that the morphology of the magnesium oxide influenced the performance of the catalyst. However, this has not been explored in much detail. In this work, magnesium oxide was synthesized using several methods to produce surfaces that differed morphologically, such as materials with a cubic morphology, denoted MgO-A, spherical morphology (MgO-B) and MgO with a nano-sheet type morphology (MgO-C). These supports were then loaded with 15 wt% vanadium by wet impregnation method using different synthesis methods. They were characterized using X-ray diffraction (XRD), Brunauer-Emmet-Teller technique, inductively-coupled plasma-optical emission spectroscopy, Raman spectroscopy, scanning and transmission electronic microscopy, temperature programed reduction (TPR) and temperature programed desorption (TPD). Electron micrographs generated for all the catalysts confirmed their respective morphologies. The XRD and Raman results showed the formation of magnesium orthovanadate and pyrovanadate phases for the cubic MgO. Only the orthovanadate phase was present for the other VMgO catalysts. This was confirmed by TPR which showed two reduction peaks for VMgO-A and one reduction peak for the other catalysts. The catalysts were tested for the ODH of n-octane at 450 °C using air as an oxidant. At an iso-conversion of 14 ± 1 %, VMgO-B (water), whose support had a spherical morphology was more selective towards ODH products, whereas VMgO-A (oxalic acid) where a cubic morphology was observed for the support, carbon oxides (COx) were the dominant products and this correlated well with the ammonium-TPD results which showed that catalysts with weak and medium acidic sites promoted the formation of ODH products, whereas catalysts with strong acidic site were more selective towards COx, in agreement with published results. Scanning electron microscopy images of the used catalyst showed agglomeration of particles while XRD showed a peak for the reduced phase of magnesium orthovanadate. This study has shown that catalysts with the same formulation but with different morphologies synthesised by methods modified to suit its physical and chemical properties, influenced the variance in the catalysis.