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Immobilized catalysts for the oxidation of hydrocarbons based on triazole complexes of ruthenium

Leckie, Laura (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: The synthesis of model and siloxane functionalized ruthenium complexes and the subsequent immobilization of the latter onto mesoporous silica, as well as the application of the prepared substances in the oxidation of hydrocarbons, is described in this thesis. Model and siloxane functionalized pyridine triazole (ML1 and SL1), pyridine N-oxide triazole (ML2 and SL2) and quinoline triazole (ML3 and SL3) ligands were successfully synthesized utilising a copper mediated click-type cycloaddition reactions. These ligands were reacted with the ruthenium arene dimer, [RuCl2(p-cymene)]2, to produce mononuclear cationic complexes (MC1-MC3) which were stabilized with tetraphenylborate as counterion. The ligands together with the model and siloxane functionalized complexes, MC1-MC3 and SC1-SC3, were fully characterized using FT-IR spectroscopy, NMR (1H and 13C) spectroscopy, ESI-MS analysis and microanalysis. The siloxane functionalized complexes, SC1-SC3, were immobilized onto mesoporous silica supports, MCM-41 and SBA-15, to afford the immobilized catalysts IC1-IC6. The immobilization is effected by a condensation reaction between the siloxane functionality of the complexes and the surface silanols on the MCM-41 and SBA-15 supports, thereby affording a ruthenium complex that is covalently immobilized onto the support. Furthermore the model complex, MC1, was physically adsorbed onto MCM-41 and SBA-15 to afford the adsorbed catalysts AC1 and AC2. The native silicas as well as the supported catalysts were fully characterized using a variety of solid state characterization techniques including infrared spectroscopy, nitrogen adsorption/desorption (Brunauer–Emmett–Teller, BET) surface analysis, low-angle powder X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and ICP-OES. ICP-OES was used to determine the ruthenium loading on the silica supports, thereby facilitating an accurate and direct comparison between the model, adsorbed and immobilized systems during catalysis. The model (MC1-MC3) and immobilized (IC1-IC6) ruthenium catalysts were employed in the oxidative cleavage of 1-octene. The catalysts were successful in transforming 1-octene to heptaldehyde and subsequently to heptanoic acid at extended reaction times. The immobilized N,N (pyridine triazole and quinoline triazole) catalysts were significantly more active than their model counterparts at a low catalyst loading of 0.1 mol%. At this same catalyst loading the N,O (pyridine N-oxide triazole) complexes gave similar results when comparing the model and immobilized systems. The N,O catalyst systems performed better in the oxidative cleavage of 1-octene than the N,N systems. The role of the mesoporous silica supports in the oxidative cleavage reaction was investigated using various catalytic systems including: model complex MC1, model complex MC1 in the presence of either MCM-41 or SBA-15 and model complex MC1 adsorbed on MCM-41 and SBA-15 (AC1 and AC2, respectively). From the results of this investigation it was noted that the immobilized catalysts employed appear to be hydrophilic in nature. This was deduced from the fact that in these biphasic systems, the silica material was found closely associated with the water layer. Thus the immobilization of the ruthenium complex on the silica support facilitates the transfer of the ruthenium pre-catalyst from the organic phase into the aqueous phase which contains the oxidant. This promotes the oxidation of the precursor to RuO4 which is the actual active species. This phase transfer allows thus the proposed active species, RuO4, to form at a faster rate and leads to enhanced reaction rates for the immobilized catalysts compared to their model analogues in the oxidative cleavage of 1-octene. The presence of RuO4 as an important intermediate in the oxidative cleavage reaction was confirmed using UV-Vis studies. The ruthenium model complexes (MC1-MC3) and their immobilized counterparts (IC1-IC6) were also applied in the oxidation of n-octane. The catalysts were successful in transforming n-octane into octanones and octanols in the presence of tert-Butyl hydroperoxide (TBHP), with a catalyst loading of only 0.01 mol% being required for the catalytic oxidation. Reaction mixtures were reduced with an excess of triphenylphosphine after the oxidation reaction, from this an estimate of the selectivity for octyl hydroperoxides could be obtained. Hydrogen peroxide was found to be ineffective as terminal oxidant for these reactions. The immobilized catalysts IC4 and IC5 could be recycled once, while IC3 could be recycled twice. Very little drop in product yield was observed in the recycling of the immobilized catalysts.

AFRIKAANSE OPSOMMING: In hierdie tesis word die sintese van model en siloksaan gefunksioneerde ruteniumkomplekse, die daaropvolgende immobilisering van die laasgenoemde komplekse op mesoporiëuse silika, sowel as die toepassing van die komplekse in die oksidasie van koolwaterstowwe beskryf. Model en siloksaan gefunksioneerde piridien-triasool (ML1 en SL1), piridien N-oksied triasool (ML2 en SL2) en kinolien triasool (ML3 en SL3) ligande is sukselvol gesintetiseer deur van koper-bemiddelde klik-tipe sikloaddisiereaksies gebruik te maak. Hierdie ligande is gereageer met die rutenium areen dimeer, [RuCl2(p-simeen)]2, gereageer om mononukluêre kationiese komplekse wat met tetrafenielboraat as teenioon gestabiliseer is, te lewer. Die ligande sowel as die model en siloksaan gefunsioneerde komplekse MC1-MC3 en SC1-SC3 is volledig gekarakteriseer deur van FT-IR spektroskopie, KMR (1H en 13C) spektroskopie, ESI massaspektrometrie en mikroanalise gebruik te maak. Die siloksaan gefunksioneerde komplekse, SC1-SC3, is op die mesoporiëuse silica, MCM- 41 en SBA-15, geïmmobiliseer om die geïmmobiliseerde komplekse IC1-IC6 te lewer. Die immobilisering vind plaas deur ‘n kondenseringsreaksie tussen die siloksaan funksionaliteit van die komplekse en die silanol groepe op die oppervlak van die MCM-41 and SBA-15 draers. Die ruteniumkomplekse is dus op kovalente wyse aan die draer gebind. Die modelkompleks MC1 is verder fisies geabsorbeer op MCM-41 en SBA-15 om die geabsorbeerde katalisators AC1 en AC2 te lewer. Die natuurlike silikas sowel as die gedraagde katalisators is volledig gekarakteriseer deur van verskeie vastestof analise tegnieke gebruik te maak, insluitende infrarooi spektroskopie, stikstof adsorpsie/desorpsie (BET) oppervlakanalise, lae-hoek poeier X-straal diffraksie, transmissie elektron mikroskopie (TEM), skandeer elektron mikroskopie (SEM), termiese gravimetriese analisie (TGA) en IKPOES. IKP-OES is gebruik om die hoeveelheid rutenium op die silika draers te bepaal om sodoende ‘n akkurate vergelyking tussen die model, geabsorbeerde en geïmmobiliseerde stelsels tydens katalise moontlik te maak. Die model (MC1-MC3) en geïmmobiliseerde (IC1-IC6) rutenium katalisators is gebruik in die oksidatiewe splitsing van 1-okteen. Die katalisators kon 1-okteen suksesvol transformeer na heptaldehied en daarna, oor lang reaksietye, na heptanoësuur. Die geïmmobiliseerde N,N (piridien triasool en kinolien triasool) katalisators was aansienlik meer aktief as hulle model ewekniëe by ‘n lae katalisator-lading van 0.1 mol%. Die N,O (piridien N-oksied triasool) komplekse het, by hierdie lading, vergelykbare resultate vir die model en geïmmobiliseerde stelsels getoon. Die N,O stelsels het oor die algemeen beter resultate as die N,N stelsel in die oksidatiewe splitsing van 1-okteen gelewer. Die rol van die mesoporiëuse silika draers in die oksidatiewe splytingsreaksie is ondersoek deur van ‘n verskeidenheid katalitise stelsels gebruik te maak. Dit sluit modelkompleks MC1, modelkompleks MC1 in die teenwoondigheid van óf MCM-41 óf SBA-15, en modelkompleks MC1 wat op MCM-41 en SBA-15 geabsorbeer is (AC1 en AC2 onderskeidelik) in. Daar is in hierdie ondersoek waargeneem dat die geïmmobiliseerde katalisators hidrofilies is. Hierdie insig is aflei vanaf die feit dat, in die tweefasige stelsels, die silikamateriaal in die waterlaag gevind is. Die immobilisering van die ruteniumkompleks op die silika draer bevorder dus die oordrag van die rutenium prekatalisator vanaf die organise laag na die waterlaag – waarin die oksidant gevind word. Die oksidasie van die voorloper na RuO4, die werklike aktiewe spesie, word dus bevorder. Die fase-oordrag veroorsaak dus dat die aktiewe spesie, RuO4, vinniger vorm en dit lei tot hoër reaksietempos in die geval van die geïmmobiliseerde katalisators in vergelyking met dit van hul model ewekniëe in die oksidatiewe splitsing van 1-okteen. UV-Vis is gebruik om die teenwoordigheid van RuO4, ‘n belangrike tussenspesie in die oksidatiewe splitsingsreaksie, te bevestig. Die rutenium modelkomplekse (MC1-MC3) en hulle geïmmobiliseerde ewekniëe (IC1-IC6) is ook as katilisator voorganger in die oksidasie van n-oktaan gebruik. Die katalisators kon noktaan sukselvol omskep in oktanone en oktanole in die teenwoordigheid van TBHP. ‘n Katalisator-lading van slegs 0.01 mol% is gebruik vir die katalitiese n-oktaan oksidasie. Die reaksiemengsels is met ‘n oormaat van trifenielfosfien gereduseer na afloop van die oksidasiereaksie. Vanaf laasgenoemde reaksie kon ‘n skatting van die selektiwiteit vir oktiel waterperoksiede bepaal word. Daar is gevind dat waterstofperoksied nie ‘n effektiewe oksidant vir hierdie reaksies is nie. Die geïmmobiliseerde katalisators IC4 en IC5 kon een keer herwin word, terwyl IC3 twee keer herwin is. Daar is slegs ‘n baie klein vermindering in die opbrengs opgelet tydens die herwinning van die geïmmobiliseerde katalisators.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101411
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