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A Study of Nickel Molybdenum Oxide Catalysts for The A STUDY OF NICKEL MOLYBDENUM OXIDE CATALYSTS FOR THE OXIDATIVE DEHYDROGENATION OF n-HEXANE By Bavani Pillay Submitted in fulfilment of the academic requirements for the degree of Doctor of Philosophy in the School of Chemistry University of KwaZulu-Natal Durban, South Africa June 2009 As the candidate’s supervisor I have/have not approved this thesis/dissertation for submission. Signed: _____________ Name: _____________ Date: ______________ ii ABSTRACT Nickel molybdenum oxide catalysts with different chemical compositions have been synthesized and tested for the oxidative dehydrogenation of n-hexane. The co-precipitation method was used for the synthesis and several methods were used to characterize these catalysts. These include inductively-coupled plasma optical emission spectroscopy, Raman spectroscopy, infrared spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy, temperature programmed reduction, temperature programmed desorption, X-ray photo-electron spectroscopy and X-ray diffraction spectroscopy techniques as well as the Brauner-Emmet-Teller technique for surface area determination. The phase composition of the catalysts was largely dependent on the chemical composition. Catalyst testing on n-hexane feed was done with a fixed bed continuous flow reactor and experiments were performed with feed/air ratios above and below the flammability limit. Varied reaction conditions were used for the catalytic testing. Prior to the catalytic testing, blank experiments were performed. Analysis of the products were done both online and offline in conjunction with gas chromatography employing FID and TCD detectors. The influence of the catalyst on the conversion of n-hexane and selectivity to dehydrogenation products is reported. Products observed were the carbon oxides (CO and CO 2), isomers of hexene (1-hexene, 2-hexene and 3-hexene), cyclic C 6 products (cyclohexene and benzene), cracked products: alkanes/alkenes (propane/ene, butane/ene) and oxygenates (ethanal, acetic acid and propanoic acid). β-NiMoO 4 was most selective to the hexenes, especially, 1-hexene and a reaction scheme is proposed. iii PREFACE The experimental work described in this thesis was carried out in the School of Chemistry, University of KwaZulu-Natal, Westville Campus, Durban from July 2003 to December 2007, under the supervision of Prof. H. B. Friedrich. The studies represent original work by the author and have not otherwise been submitted in any form or degree or diploma to any tertiary institution. Where use has been made of the work by other authors it is duly acknowledged in the text. iv DECLARATION 1- PLAGARISM I, Bavani Pillay declare that 1. The research reported in this thesis, except where otherwise indicated, is my original research. 2. This thesis has never been submitted, in part or in whole for any degree or examination at any university. 3. This thesis does not contain any person’s data, pictures, graphs or other information unless specifically acknowledged as being sourced from other persons. 4. This thesis does not contain any person’s writing, unless specifically acknowledged as being sourced from other researchers. Where other written sources have been quoted, then: a. Their words have been re-written but the general information attributed to them has been referenced. b. Where their exact words have been used, then their writing has been place in italics and inside quotation marks and referenced. 5. This thesis does not contain text, graphics and tables copied and pasted from the internet, unless specifically acknowledged, and the source being detailed in the thesis and in the reference sections. Signed………………………………………. v DECLARATION 2- PUBLICATIONS Publication 1 : The oxidative dehydrogenation of n-hexane over Ni-Mo-O catalysts, First author: Bavani Pillay, Other authors: Mfanuwenkosi R. Mathebula and Holger B. Friedrich, Appl. Catal. A, 361 (2009) 57. Publication 2 : The oxidative dehydrogenation of n-hexane over β-NiMoO 4 catalysts, First author: Bavani Pillay, Other authors: Mfanuwenkosi R. Mathebula and Holger B. Friedrich submitted for publication in Applied Catalysis A. Signed:………………………………. vi CONFERENCE CONTRIBUTIONS Catalysis Society of South Africa conference 2008, Parys, Poster presentation, “Effect of promoters on nickel molybdate catalysts on n-hexane oxidation”. Catalysis Society of South Africa conference 2007, Richard’s Bay, Poster presentation, “A catalytic study of nickel molybdate on n-hexane above and below the flammability range”. 38 th South African Chemical Institute Convention 2006, Durban, South Africa, oral presentation, “Investigation of nickel molybdate in hexane oxidation”. Catalysis Society of South Africa conference 2006, Mossel Bay, Oral presentation, “A Study of nickel molybdate in hexane oxidation”. 1st International IUPAC conference on Green-sustainable chemistry, Dresden, Germany, September 10-15 2006, poster presentation, “Investigation of nickel molybdate in hexane oxidation”. Catalysis society of South Africa conference 2005, Eskom Convention Centre, Midrand, South Africa, Oral presentation, “An Investigation of NiMoO 4 in hexane oxidation”. Catalysis society of South Africa conference 2004, North-West University, Potchefstroom, South Africa, poster presentation, “Investigation of molybdates in the oxidative dehydrogenation of paraffins”. vii ABBREVIATIONS ATR attenuated total reflection BET Brunauer, Emmet and Teller BUT butane/ene CAT catalyst CO x carbon oxides CT contact time Cyc C6 cyclic C 6 EDX energy dispersive X-ray analysis ESR electron spin resonance FID flame ionization detector FTIR Fourier transform infrared GC gas chromatograph GC-MS gas chromatography mass spectrometry HPLC high performance liquid chromatography HT-XRD high temperature x-ray diffraction ICP-OES inductively-coupled plasma optical emission spectroscopy LFL lower flammability limit ODH oxidative dehydrogenation 1-HEX 1-hexene PROP propane/ene SEM scanning electron microscopy TCD thermal conductivity detector THF tetrahydrofuran T cracked total cracked TPD temperature programmed desorption TPO-MS temperature programmed oxidation-mass spectrometry TPR temperature programmed reduction 2-HEX (c) 2-hexene ( cis ) 2-HEX (t) 2-hexene ( trans ) UFL upper flammability limit X conversion XRD X-ray diffraction viii XPS X-ray photo-electron spectroscopy ix DEFINITIONS AND CALCULATIONS 1. % Hexane conversion: (N Hin − N Hout ) X H = x 100 N Hin (where X = conversion, N = moles of hexane, Hin and Hout referring to concentration of hexane in the feed and concentration of hexane in product stream) 2. % Selectivity to specific product: (where S = selectivity of product, Np = moles of carbon product, N Hin -NHout = Hexane conversion) N P SP = x 100 (N Hin − N Hout ) 3. % Yield of a specific product: (where Yp = yield of product, Sp = selectivity of product, X = conversion) N P YP = x 100 Sp − X p 4. Contact time (s): Volume of catalyst (mL) CT = x 60 (s) flow rate of gaseous feed (mL/hr) 5. % Carbon balance: (∑NCp)/NCHin x 100 (where NCp = mol carbon product x carbon number, NHin = moles hexane in feed x carbon number) ∑ NC P NC P = x 100 NC Hin x LIST OF TABLES Table 1.1: Present and future routes to organic chemicals and hydrocarbon fuels. 2 Table 1.2: Classification of heterogeneous catalysts. 3 Table 1.3: Compatibility between reaction systems, reactor types and catalyst forms. 4 Table 1.4: Classification of catalyzed reactions of hydrocarbons with oxygen. 7 Table 3.1: Chemicals and reagents used for compound identification and quantification. 60 Table 4.1: Labelling of synthesized precursors. 64 Table 4.2: Atomic ratios of catalysts synthesized (ICP). 65 Table 4.3: BET surface areas of catalysts synthesized. 66 Table 4.4: XPS data for NiMoO 4 samples used in catalytic testing. 73 Table 4.5: Assignment of Raman bands (cm -1) for catalyst A and B. 77 Table 4.6: Chemical composition and surface of areas of Cs-promoted catalysts. 78 Table 4.7: XPS data for unpromoted and promoted NiMoO 4 samples. 79 Table 5.1: Actual fuel/air % and fuel/O2 ratios used in experiments. 84 Table 6.1: Catalytic results for hexane oxidation over NiO-MoO 3 catalysts at CT = 1.8 s. 100 Table 6.2: The effect of temperature on conversion, selectivity and yield of products for 108 the ODH of n-hexane over β-NiMoO 4 above the UFL for a CT of 0.61 s. Table 6.3: The effect of temperature on conversion, selectivity and yield of products for 108 the ODH of n-hexane over β-NiMoO 4 above the UFL for a CT of 1.04 s. Table 6.4: The effect of temperature on conversion, selectivity and yield of products for 109 the ODH of n-hexane over β-NiMoO 4 above the UFL for a CT of 1.47 s. Table 6.5: The effect of temperature on conversion, selectivity and yield of products for 109 the ODH of n-hexane over β-NiMoO 4 above the UFL for a CT of 1.82 s. Table 6.6: The effect of temperature on conversion, selectivity and yield of products for 110 the ODH of n-hexane over β-NiMoO 4 above the UFL for a CT of 2.40 s. Table 6.7: The effect of temperature on conversion, selectivity and yield of products for 117 the ODH of n-hexane over β-NiMoO 4 below the LFL for a CT of 0.40 s. Table 6.8: The effect of temperature on conversion, selectivity and yield of products for 117 xi the ODH of n-hexane over β-NiMoO 4 below the LFL for a CT of 0.91 s. Table 6.9: The effect of temperature on conversion, selectivity and yield of products for 118 the ODH of n-hexane over β-NiMoO 4 below the LFL for a CT of 1.2 s. Table 6.10: The effect of temperature on conversion, selectivity and yield of products for 118 the ODH of n-hexane over β-NiMoO 4 below the LFL for a CT of 1.8 s. Table 6.11: The effect of temperature on conversion, selectivity and yield of products for 125 the ODH of n-hexane over undoped β-NiMoO 4 for CT 1.04 s.
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