Doctoral Degrees (Chemical Engineering)

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Browsing Doctoral Degrees (Chemical Engineering) by Subject "Alkenes"

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    Hydroformylation of post-metathesis product using rhodium-based catalysts
    (Stellenbosch : Stellenbosch University, 2019-12) Breckwoldt, Nicholas Claus Carl; Goosen, N. J.; Smith, G.; Van der Gryp, Percy; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: This study forms part of the overall scope of the RSA Olefins Programme for the upgrading of low-value α-alkene feedstocks to higher value detergent-range products within the South African context. The programme is motivated by the unique α-alkene market position in South Africa, as a producer of both odd- and even-numbered α-alkenes via Fischer-Tropsch conversion of syngas. Based on currently available technologies, the beneficiation of these low-value short-chain α-alkenes (C5-C9) via consecutive transition-metal-catalysed alkene metathesis and hydroformylation reactions is under consideration in South Africa. This study focused on evaluating the hydroformylation reaction within the scope of the proposed catalytic beneficiation process. The contributions of the study were thus two-fold in firstly describing the application of commercially available rhodium-based catalysts and secondly the application of non-commercial Schiff base derived rhodium-based precatalysts for the identified model hydroformylation reaction of the post-metathesis product 7-tetradecene. For both sets of catalyst systems, the study aimed to i) understand the catalytic performances of different rhodium-based catalysts for model reaction system, ii) to evaluate the effect of process conditions on the hydroformylation performance and iii) to evaluate and describe the reaction kinetics through phenomenological rate law model development that can be used in the reaction-engineering context. In terms of commercial rhodium-based catalysts: The performance of three commercially available catalyst systems, Rh-tris(2,4-ditertbutylphenyl)phosphite (1), Rh-triphenylphosphine (2) and Rh-triphenylphosphite (3) were evaluated for the model reaction by varying operating conditions such as temperature (60-90°C) and pressure (10-30 bar). Catalyst performance was characterised according to the activity (turnover numbers) and selectivity. It was found that all three commercial catalysts showed hydroformylation and isomerisation behaviour that was largely temperature- and pressure-dependent. Optimal conditions were established at 70°C (30 bar, CO:H2, 1:1) within the investigated ranges, for which 1 was exclusively hydroformylation-selective, while 2 and 3 were both hydroformylation- and isomerisation-selective. Overall, it was found that 1 was the most effective commercial catalyst system in terms of both activity (TON of 980) and regioselectivity toward targeted branched aldehyde product 2-hexylnonanal (>99%). It was further proposed and found that the reaction kinetics of the model reaction with 1 could be accurately described by a set of three interdependent first-order ordinary differential mole-balance equations. A mechanism-based rate-equation derived for bulky phosphite ligands was found to be consistent with the rate data (first-order in alkene and rhodium concentration, zero-order in hydrogen and negative order in carbon monoxide) over a wide alkene conversion range. In terms of non-commercial Schiff base derived rhodium-based catalysts: The performance of the monometallic rhodium-aryl (4) and heterobimetallic rhodium-ferrocenyl (5) Schiff base derived precatalysts bearing N’O chelate ligands were evaluated for the model reaction by varying operating conditions such as temperature (75-115°C), pressure (30-50 bar) and catalyst loading (7-tetradecene-to-precatalyst molar ratio from 1000:1 to 6000:1). It was found that the optimal reaction temperature for both 4 and 5 was 95°C (40 bar, CO:H2, 1:1). Even though precatalysts 4 and 5 were less regioselective (40:60 split in favour of isomeric aldehydes) compared to the commercial catalyst systems, significantly higher turnover numbers (up to 4310) were recorded using the Schiff base derived precatalyst systems at lower rhodium loadings. Evidence of a cooperative effect by including the second metal (ferrocene) in the heterobimetallic catalyst system was also observed due to improved catalytic activity compared to the monometallic catalyst systems under low temperature conditions. It was further found that the reaction kinetics of the model reaction with 5 could be accurately described by a set of four interdependent first-order ordinary differential mole-balance equations. A mechanism-derived rate equation derived for the hydroformylation using conventional monometallic rhodium-based catalyst was found to be consistent with the parametric influences of different reaction conditions affecting the reaction rate using the heterobimetallic precatalyst, with minor modification to account for observed fractional order dependence in precatalyst concentration. Thus, the rate of reaction was found to be first-order in alkene concentration, positive fractional-order in precatalyst concentration and first-order in both hydrogen and carbon monoxide.