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Partial Oxidation of N-Pentane Over Vanadium Phosphorus Oxide Supported on Hydroxyapatites

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Partial Oxidation of N-Pentane Over Vanadium Phosphorus Oxide Supported on Hydroxyapatites RESEARCH ARTICLE S. Singh, 1 S. Afr. J. Chem., 2016, 69, 1–8, <http://journals.sabinet.co.za/sajchem/>. Partial Oxidation of n-Pentane over Vanadium Phosphorus Oxide supported on Hydroxyapatites Sooboo Singh School of Chemistry and Physics, University of KwaZulu-Natal, Durban, 4000, South Africa. E-mail: [email protected] Received 14 August 2015, revised 13 November 2015, accepted 20 November 2015. ABSTRACT The selective oxidation of n-pentane to value-added products, maleic anhydride or phthallic anhydride by vanadium phosphorus oxide loaded on hydroxyapatites as catalysts and oxygen as oxidant was investigated. Hydroxyapatite (HAp) and cobalt- hydroxyapatite (Co-HAp) were prepared by the co-precipitation method and VPO with varying weight percentages (2.5–15.0 %) were loaded on the hydroxyapatite supports by the wet impregnation technique. The catalyst materials were characterized by surface area measurements, elemental analysis, powder X-ray diffraction (XRD), infrared spectroscopy (IR) and temperature- programmed reduction (TPR). VPO is present in two phases, viz.(VO)2P2O7 and VOPO4. With increase in the VPO loading on the hydroxyapatites, the (VO)2P2O7 phase also increased. From catalytic results, a conversion of 75 % of n-pentane and selectivity towards maleic anhydride, about 50 % and phthalic anhydride, about 25 %, were consistently achieved with loadings of 5.0 and 7.5 wt. % VPO at 360 °C for GHSVs of 1900 and 2300 h–1. Under optimum conditions, product yields of up to 40 % maleic anhydride and 20 % phthalic anhydride were obtained. It is proposed that the products formed through the diene intermediate. KEYWORDS Selective oxidation, n-pentane, vanadium phosphorus oxide, hydroxyapatite, maleic anhydride, phthalic anhydride. 1. Introduction produced pentenes and 1,4-pentadiene, whereas the 10 wt. % Maleic anhydride (MA) and phthalic anhydride (PA) are V2O5/SiO2 produced mainly carbon oxides. The Al2O3-based precursors used in the manufacture of synthetic fibres, plastics, catalysts produced dehydrogenated products. Here, MA was paints and adhesives1. The selective and controlled oxidation of produced with a selectivity of about 80 % at low conversion on hydrocarbons using heterogeneous catalysts to produce these the 23.4 wt. % V2O5 loaded catalyst, but was a minor product on compounds, occupy an important place in the chemical indus- 8.2 wt. % loaded catalyst. Small amounts of PA were observed 2,3 try . Vanadium phosphorus oxide (VPO) was found to be effi- on the Al2O3-based catalysts, and none was observed on the cient in converting n-pentane to maleic anhydride and phthalic supported SiO2 catalysts. This study was corroborated by a sepa- anhydride4,5. This catalytic reaction is promoted by the existence rate study conducted by Klose et al.11. of both the vanadyl pyrophosphate (VPP) as well as of VOPO4 Activation of hydrocarbons depend, to a large extent, on the phases in VPO5,6. Supported VPO catalysts showed an enhance- acid-base properties of the material or on the isolated cations ment in the selectivity towards maleic anhydride and phthalic capable of activating C-H bonds12,13. Hydroxyapatites (HAp) are 7 anhydride. Xiao et al. used fumed SiO2-supported VPO catalysts, bi-functional materials containing acidic and basic sites in the prepared by deposition-precipitation, for n-butane activation. A crystal lattice. HAps are useful as materials in bioceramics, conversion of 33–40 % of n-butane and maleic anhydride selec- adsorbents, catalysts and catalyst support materials. Its hexagonal tivity of 65–87 mol % were obtained over the temperatures structure is made up from columns of calcium ions and oxygen tested. In another study, Nie et al.8 compared the efficiency of atoms which are located parallel to its axis14–16. Three oxygen VPO supported on Al-MCM-41 with a large-pore silica-supported atoms from each PO4 tetrahedron are shared by one column and VPO catalyst in the activation of n-butane. The VPO supported the fourth oxygen atom is attached to the neighbouring column. on Al-MCM-41 showed an increase in maleic anhydride (MA) The lattice of HAp is structured to allow substitution of the selectivity with a slight decrease in n-butane conversion, attrib- calcium ions with other cations by ion exchange and ion adsorp- uted to the interaction between the VPO component and the tion or a combination of these methods to improve its catalytic Al-MCM-41 support. Recently, VPO supported on alumina was performance15,17–19. Co-HAp offers high stability and the OH used for propane ammoxidation and was hyped as a potentially group within the phosphate framework leads to the formation interesting catalytic system for the industrial synthesis of of active oxygen species which are essential for ODH reac- acrylonitrile 9. Michalakos et al.10 studied the variation in the tions14,20,21. In another study using Co-HAp as the catalyst, Opre selectivity for the selective oxidation n-pentane on vanadium et al.22 showed excellent results for the epoxidation of styrene oxide supported on Al2O3 and SiO2. The loadings for the Al2O3 with in dimethylformamide. Hydroxyapatites are efficient as samples were 8.2 and 23.4 wt. % V2O5 and for the SiO2 samples, supports, in particular its basic properties are useful for the 23 1.0 and 10.0 wt. % V2O5. The Al2O3-based catalysts were more Knoevenagel reaction in heterogenous media without solvent , active than the supported SiO2 catalysts, but for catalysts of the the water gas shift reaction with gold and ruthenium as the same support, the activity per mole depended slightly on the active components24 and the partial oxidation of methane over loading. At low pentane conversions, the 1 wt. % V2O5/SiO2 nickel-added strontium phosphate incorporating strontium ISSN 0379-4350 Online / ©2016 South African Chemical Institute / http://saci.co.za/journal DOI: http://dx.doi.org/10.17159/0379-4350/2015/v69a1 RESEARCH ARTICLE S. Singh, 2 S. Afr. J. Chem., 2016, 69, 1–8, <http://journals.sabinet.co.za/sajchem/>. hydroxyapatite25. In our previous study26, we reported the acti- FT-IR spectra of the samples were recorded with a Nicolet –1 vation of n-pentane using V2O5 supported on hydroxyapatite as Impact 420 spectrophotometer in the mid IR (400–4000 cm ) catalysts. V2O5 showed excellent redox capabilities. Selectivity region using the KBr disc technique (0.5 % dilution). TPR investi- towards the anhydrides (MA 40 % and PA 25 %) was obtained gations were carried out with a custom made reactor system with the 5.0 wt. % V2O5 at 360 °C at a conversion of about 60 %. At connected to a thermal conductivity unit. Hydrogen (5 %) in higher weight loadings, although the conversion increased, the argon was used for the analysis. For heating, a calibrated furnace selectivity towards the desired products decreased. was used and a temperature control unit monitored the temper- In this paper, we report the preparation and characterization ature to within 2 °C. The average vanadium oxidation state was of VPO supported on calcium and cobalt hydroxyapatites and determined by means of a redox titration28,29. the effect of the VPO loading and support interaction, together with different phases present on the catalyst, in the partial oxida- 2.3. Measurement of Catalytic Reactions tion of n-pentane. Catalytic testing was carried out using a continuous, fixed bed tubular stainless steel reactor (i.d. = 10 mm, l = 300 mm) in down 2. Experimental flow mode. The organic feedstock, n-pentane (Saarchem, South Africa) was injected into the reactor lines by means of a high 2.1. Preparation of Catalysts precision isocratic pump (LabAlliance Series II) and vapourized The catalytic materials were prepared using methods published on-line. The vapour was carried by the oxidant, an oxygen- 26,27 previously, with modifications . Concentrated ammonia helium mixture (20 % O2, balance He) (Afrox, BOC Special Prod- (BDH, Poole, England) was added to a 60 mL solution of ucts). Preliminary testing of VPO and the support materials, –2 Ca(NO3)2·4H2O (6.67 × 10 mol) (Merck KGa, Darmstadt, HAp and Co-HAp were initially conducted. After these prelimi- Germany) to increase its pH to 11. The calcium solution was nary runs, the VPO-supported catalysts were tested. In all test- subsequently diluted to twice its volume with distilled water. A ing, the catalyst bed (2 mL) was located at the centre of the –2 100 mL solution of (NH4)2HPO4 (4.00 × 10 mol) (Merck KGa, reactor, with carborundum (24 grit, Promark Chemicals, South Darmstadt, Germany) was also adjusted to pH 11 using ammonia, Africa) filling the spaces on either side of the bed and ends diluted to 160 mL. The phosphate solution was added drop-wise plugged with glass wool. The reactor was heated in an electric to the calcium solution over a period of 30 min with continuous furnace with K-type thermocouples monitoring the tempera- stirring at room temperature. A white, gelatinous precipitate ture of the furnace and the catalyst bed. The product stream was that formed was boiled for 10 min with stirring. Thereafter, the analysed by Gas Chromatograph (Varian Star 3400) equipped precipitate was filtered under vacuum and washed thoroughly with a FID module and GC-MS (Finnigan MAT GCQ) using with deionised water. The solid was left to dry overnight in an capillary columns (J&W HP5-MS, 250 µm diameter). The carbon oven set at 100 °C, followed by calcination under the flow of air at oxides and light organic compounds were intermittently moni- 500 °C. In the preparation of the cobalt hydroxyapatite, tored by a TCD Gas Chromatograph (Buck Scientific, SRI Instru- Co(NO3)2·6H2O (Merck KGa, Darmstadt, Germany) was used. ments, USA) using a packed column (6 silica gel/6 molecular The method of preparation of VPO has been extensively reported sieve). Water was quantified using a Karl Fischer titrator.
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