Hydrocarbon oxidations using metal-containing polypropylene-imine dendrimers

Wilbers, Derik (2019-12)

Thesis (PhD)--Stellenbosch University, 2019

Thesis

ENGLISH ABSTRACT: The production of oxygenates from hydrocarbon feedstocks is an important industrial process. However, this process suffers from various weaknesses such as poor catalytic activity, chemoselectivity, and often requires the use of harsh reaction conditions. In this thesis attempts to address these shortcomings are described. The synthesis and characterization of a number of transition metal complexes, many of which are novel, as well as their application in the oxidation of hydrocarbons (both unsaturated and saturated) are described. The complexes include both iron metallodendrimers as well as iron mononuclear complexes (model complexes) based on pyridine-imine ligands. In addition to the iron complexes, a number of cobalt salicylaldimine complexes (metallodendrimers and model complexes) as well as cobalt thioether-amine complexes were synthesized. The complexes were then employed as catalyst precursors in the catalytic oxidation of cyclohexene. Both the cobalt model complexes C1 and C2 as well as their dendritic counterparts (MD1 and MD2) performed poorly as catalyst precursors in the oxidation of cyclohexene when using H2O2 as oxidant. The poor performance of the model complexes was attributed to the disproportionation of the oxidant, a process catalyzed by the model cobalt complexes C1 and C2. When H2O2 was replaced by TBHP as oxidant, virtually no disproportionation of this oxidant was observed and an improvement in catalytic activity was obtained, however, turnover numbers (20 mol product/ mol catalyst) remain relatively low. The cobalt complexes exhibited poor activity in the oxidation of n-octane obtaining product yields of around 2% under the optimized conditions (2 mmol cyclohexene, 1 mol% catalyst, 4 mmol TBHP, 40°C, MeCN). The iron complexes were more active for catalytic cyclohexene oxidation than the cobalt complexes. C4 and MD5 (iron pyridine-imine complexes) gave cyclohexene conversions of around 50% with turnover numbers around 60. In all cases high selectivity towards the allylic oxidation product 2-cyclohexen-1-one was obtained. The iron complexes were also more active than the cobalt complexes in n-octane oxidation achieving product yields of around 7% (2 mmol octane, 0.5 mol% iron, 4 mmol TBHP, 60°C, MeCN). The kinetics of the reaction of selected complexes with peroxide-based oxidants were investigated in an attempt to rationalize the observed differences in catalytic activity for these systems. It was found that the cobalt salicylaldimine complex MD1 has a significantly higher activation energy than the iron complex MD5 which might serve as a possible explanation for the observed differences in catalytic activity of the two catalyst systems. Further investigations attempted to ascertain the most likely mechanism by which hydrocarbon oxidation, catalyzed by these complexes, is achieved. It was found that these complexes are likely to operate via a free radical-centered mechanism.

AFRIKAANSE OPSOMMING: Die produksie van suurstofbevattende produkte vanuit koolwaterstowwe is ‘n belangrike industriële proses. Hierdie proses het egter verskeie nadele, soos onder andere, lae katalitiese aktiwiteit, swak produkselektiwiteit, asook die gebruik van energie-intensiewe reaksie toestande. In hierdie tesis word pogings om die voorgenoemde tekortkominge en nadele aan te spreek, beskryf. Die sintese en karakterisering van verskeie oorgangsmetaal-komplekse asook hul gebruik as katalisators in die oksidasie van verskeie koolwaterstowwe (beide versadigde sowel as onversadigde) word hier beskryf. Hierdie komplekse sluit in beide yster metallodendrimere sowel as mononukleêre yster modelkomplekse van piridien-imien ligande. Benewens die ysterkomplekse is ‘n reeks kobaltsalisielaldemien komplekse (metallodendrimere en modelkomplekse) sowel as kopertioeter-amien komplekse gesintetiseer. Die komplekse is getoets as katalisators in die oksidasie van siklohekseen. Beide die kobalt metallodendrimere en die kobalt modelkomplekse het lae katalitiese aktiwiteit vir hierdie transformasie getoon in reaksies waar waterstofperoksied gebruik is as oksideermiddel. Die swak katalitiese aktiwiteit is toe geskryf aan die disproporsionering van waterstofperoksied na water en suurstof. As die H2O2 oksideermiddel met TBHP vervang word, is feitlik geen disproporsionering waargeneem nie. In hierdie geval is die katalitiese aktiwiteit hoër, maar omset getalle (20 mol produk/ mol katalis) is steeds relatief laag. Hierdie kobalt komplekse toon uiters lae aktiwiteit in die katalitiese oksidasie van oktaan waar omsette van slegs 2% onder optimale kondisies verkry is (2 mmol oktaan, 1 mol% kobalt, 4 mmol TBHP, 60°C). Die ysterkomplekse was relatief meer aktief in die oksidasie van siklohekseen as die kobaltkomplekse. Komplekse C4 en MD5, beide piridienimien-komplekse, kon omsette van ongeveer 50% behaal, met TON waardes bo 60. In alle gevalle is hoë selektiwiteit na die produkte, 2-sikloheksen-1-oon asook 2-sikloheksen-1-ol verkry. Die ysterkomplekse was veel meer aktief as die kobaltkomplekse in die oksidasie van oktaan en omsette van 7% is verkry. (2 mmol oktaan, 0.5 mol% yster, 4 mmol TBHP 4 ure, 60°C in MeCN). Die kinetika van die reaksies van geselekteerde komplekse met waterstofperoksied as oksideermiddel is ondersoek in ‘n poging om die waargenome verskille in katalitiese aktiwiteit vir hierdie sisteme te rasionaliseer. Daar is gevind dat die kobaltsalisielaldemien kompleks MD1, ‘n beduidend hoër aktiveringsenergie as die yster kompleks MD5 het. Dit mag ‘n moontlike verduideliking wees vir die waargenome verskille in katalitiese aktiwiteit van die twee katalisatorsisteme. In verdere ondersoeke is daar gepoog om vas te stel wat die mees waarskynlike meganisme is waarvolgens oksidasie reaksies, gekataliseer deur hierdie komplekse, plaasvind. Daar is gevind dat hierdie komplekse waarskynlik oksidasie bewerkstellig via ‘n vrye radikaalgesentreerde meganisme.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106919
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