Oxidation of ɳ-octane over molybdenum oxide based catalysts.

Abstract

The research into alternative feedstocks has gained importance in recent years to replace olefins by alkanes. The use of alkanes is viable since they are cheap, abundant and can be easily sourced from Gas to liquid (GTL) plants and oil refineries. Oxidative dehydrogenation (ODH) is preferred over dehydrogenation (DH) as the catalysts regeneration and higher conversion can be achieved at comparatively lower temperature, and thus it is less energy intensive. This study focussed on oxidative dehydrogenation of ɳ-octane using molybdenum based catalysts. The choice of these oxides was based on their ability to form different oxides, among them, bulk MoO₃, monomeric and polymeric MoOₓ, crystalline MoO₃ species, and cationic molybdates such as NiMoO₄, which were exploited for the ODH of ɳ-octane. The catalysts were synthesized by the wet impregnation and co-precipitation methods. The prepared catalysts were characterised using ICP-OES, XRD, Raman, N₂-physisorption, SEM, TEM and TPR. The catalytic testing was carried out with a continuous flow fixed bed reactor in the temperature range of 350 to 550 ºC and the C:O ratios studied were 8:0, 8:1, 8:2, 8:3 and 8:4. The study was divided in to three parts, consisting of the effect of C:O ratio over bulk MoO₃, the effect of different weight loadings of Mo on SBA-15 and the effect of different phases of bulk and SBA-15 supported NiMoO4 catalysts for the oxidative dehydrogenation of ɳ-octane. The catalytic and non-catalytic activation of ɳ-octane was studied in the presence and absence of bulk MoO₃. Only cracked products were formed in a carborundum packed reactor tube, whereas with the catalyst, high selectivity to octenes was observed. Initially, octenes formation appeared due to lattice oxygen, after which their formation was ascribed to dehydrogenation over MoO₂, after depletion of lattice oxygen. The conversion increased with an increase in the oxygen content in both cases i.e. non-catalytic and catalytic, but it was higher for the catalytic route due to the operating redox cycle between MoO₃ and Mo₄O₁₁. With increase in oxygen content, the octene selectivity decreased and formation of COₓ increased. Highest 1-octene selectivity (16%) was observed when MoO₃ was used a catalyst, as opposed to the non-catalytic reactions where 1-octene selectivity was 8%. When the molybdenum (4 to 18 wt%) was supported on SBA-15, formation of monomeric, polymeric MoOₓ and crystalline MoO₃ was observed. The conversion of ɳ-octane increased and octenes selectivity decreased with an increase in the molybdenum loading. The catalyst with the 10 wt% molybdenum loading showed the highest yield towards ODH products. Furthermore, when the effect of temperature was examined over the 10 wt% catalyst, an increase in temperature resulted in an increase in the selectivity to aromatics. The conversion and selectivity to non-selective products increased with an increase in the oxygen content in the feed. The GHSV studies revealed that the octenes, cracked products and CO₂ are the primary products of the reaction. For the study involving nickel molybdate (NiMoO₄) as a catalyst, the effect of C:O ratio (8:1, 8:2, and 8:3) at a GHSV of 4000 h⁻¹ was investigated over the α-phase of NiMoO₄. In which case, the C:O ratio of 8:1 was found optimum to produce octenes with a high selectivity. The unsupported β-NiMoO₄, α-NiMoO₄/SBA-15 and β-NiMoO₄/SBA-15 were then tested at the optimum C:O ratio of 8:1 at a GHSV of 4000 h⁻¹. When the conversions of all the catalysts were compared, the unsupported α-NiMoO₄ system showed the highest conversion. Octene was the dominant product observed and the highest octene selectivity was observed over the unsupported β-NiMoO₄ catalyst, whereas the highest aromatics selectivity was exhibited by the α-NiMoO₄/SBA-15 catalyst. In the case of supported catalysts, at temperatures of 350 and 400 ºC, the reactions were driven by surface oxygen, whereas at temperatures of 450 and 500 ºC, it is driven by the lattice oxygen.