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School of Chemistry and Physics

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Browsing School of Chemistry and Physics by Subject "Activation (Chemistry)"

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    Oxidation of ɳ-octane over molybdenum oxide based catalysts.
    (2014) Golandaj, Ajij Jahangeer.; Friedrich, Holger Bernhard.; Singh, Sooboo.
    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.
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    The synthesis of xanthene-based transition metal complexes and their application in the oxidation reaction of n-octane.
    (2014) Nyalungu, Nonjabulo.; Friedrich, Holger Bernhard.; Bala, Muhammad Dabai.
    The oxidation of alkanes into valuable products such as alcohols, ketones and aldehydes is very important to industry for detergents and perfumes. One of the challenges with alkanes is their inertness which results from the strong and localized C-C and C-H bonds. There are few methods that are known to transform alkanes into products of value. Therefore in this study xanthene-based ligands were used in an attempt to transform alkanes into products of value. Xanthene-based ligands are known to produce catalysts that are highly active and selective in reactions such as hydroformylation and hydrocyanation. These ligands are bidentate and their structure consists of a xanthene backbone with two phosphorus donor atoms and a rigid backbone. Five xanthene-based ligands were synthesized, characterized and complexed to cobalt and nickel. In this study modification at position X was done by using a sulphur atom, a methyl group as well as an isopropyl group in order to observe the effect this has on the activation of noctane. Crystal structures of ligand (4,5-bis(di-p-tolylphosphino)-9,9-dimethyl xanthene) and complex (Co(4,5-bis(di-p-tolylphosphino)-9,9-dimethylxanthene)Cl2) and (Ni(4,5-bis(di-ptolylphosphino)- 9,9-dimethylxanthene)Cl2) were obtained. The five cobalt and five nickel complexes were catalytically tested in the oxidation of n-octane, using three oxidants tert-butyl hydroperoxide, hydrogen peroxide and meta-chloroperbenzoic acid. This was carried out in tetrahydrofuran solvent at varying temperatures. Hydrogen peroxide and meta-chloroperbenzoic acid gave no substantial activation, while tertbutyl hydroperoxide showed activity. Modifications to the backbone at position X brought changes to the bite angle and minor changes to activity. Selectivity at 50 and 60 °C favoured the C-2 position with 2-octanone as the dominant product. Terminal position showed no products of alkane oxygenation. Alcohols (3-octanol and 4-octanol) were observed at higher temperatures. Steric factors had no significance effect on activity while temperature had a greater effect. The temperature that was best to work with was 50 °C since all catalysts were active. Sulphur had a deactivating effect on efficacy of both cobalt and nickel catalysts.