Catalytic oxidation of lignin model compounds

Maggott, Emile Darron (2019-03)

Chemistry and Polymer Science

Thesis

ENGLISH ABSTRACT: This thesis pertains to the synthesis of a range of model and siloxane functionalized complexes containing salicylaldimine ligands. The functionalized complexes were immobilized onto two types of mesoporous silica viz. MCM-41 and SBA-15 which were employed in the oxidation of veratryl alcohol. Both model (L1) and functionalized (L2) were fully characterized by a range of analytical techniques such as FT-IR (ATR) and 1H NMR spectroscopy. The model and siloxane functionalized salicylaldimine ligands were complexed to metal precursors based on copper, cobalt and palladium salts in a bis-bidentate fashion. Full characterization of the model (MC1- MC3) and functionalized (FC1-FC3) complexes was achieved by mass spectrometry, FT-IR (ATR) spectroscopy, 1H NMR spectroscopy (MC3 and FC3), UV-Vis spectroscopy, EPR spectroscopy (MC1, MC2, FC1 and FC2), single crystal X-ray diffraction (MC3), elemental analysis and melting point determinations. The native silica supports, MCM-41 and SBA-15 and the functionalized complexes were reacted with each other to afford the immobilized complexes (IC1-IC6). The immobilization process involves a condensation reaction between the surface silanol groups of the support and the siloxane functionality of the complexes. These complexes are covalently bound to their respective supports, which ultimately led to “heterogenized” catalysts. The immobilized complexes (IC1-IC6) were fully characterized by a range of solid-state analytical techniques which include FT-IR (ATR) spectroscopy, low angle powder X-ray diffraction, thermal gravimetric analysis (TGA), solid state 29Si NMR spectroscopy, nitrogen adsorption/ desorption (BET) surface analysis, scanning and transmission electron microscopy (SEM and TEM). Inductive coupled plasma optical emission spectroscopy (ICP-OES) was used to quantitatively determine the metal loading of the immobilized complexes. Both the model (MC1-MC3) and immobilized (IC1-IC6) complexes were evaluated in the oxidation of veratryl alcohol. A comparison of the effect of different oxidants on the activity was investigated using either molecular oxygen, hydrogen peroxide or tert-butyl hydroperoxide (TBHP). Model complex, MC1, exhibited better activity than the other model catalysts (MC2 and MC3) in the oxidation of veratryl alcohol at 25 °C under 1 atm oxygen pressure. This catalyst managed to selectively produce veratrylaldehyde. Amongst the three oxidants, hydrogen peroxide exhibited the least activity when used in conjunction with MC1, which was used to optimize reaction conditions. Experiments with catalyst, MC1, were also conducted in the absence of oxygen, which resulted in only moderate activity of about 20% with 100% aldehyde formation. This most likely suggests that the reaction undergoes a β-hydride elimination reaction with subsequent generation of molecular hydrogen. The use of tert-butyl hydroperoxide as oxidant in the oxidation of veratryl alcohol, exhibited relatively high activity compared when molecular oxygen is used. The high activity can be attributed to the higher stability of TBHP as well as its solubility in the reaction mixture. This suggests that the active species forms faster with TBHP as oxidant. The best conversion was obtained for the cobalt catalyst, MC2, which was 85% followed by MC1 and MC3 respectively. The higher activity of TBHP resulted in higher veratrylaldehyde yield but unfortunately it also led to over-oxidation to veratric acid. The immobilized catalysts, IC1-IC6, were also evaluated in the oxidation of veratryl alcohol using TBHP as oxidant. These catalysts generally exhibited moderate activity although the cobalt analogues (IC3 and IC4) showed moderate conversion of up to 65%. Molecular oxygen when used as oxidant with the immobilized catalysts seemed to be ineffective in the oxidation of veratryl alcohol. All the immobilized catalysts except IC5 were recycled and reused for up to five catalytic runs. IC3 and IC4 exhibited moderate activity over the five cycles with only a slight decrease in activity. The nature of the support had an effect on the activity as well as the selectivity. The immobilized catalysts based on MCM-41 supports, showed enhanced activity. The selectivity was slightly altered when using SBA-15 as a support. Overall, the immobilized catalysts exhibited reasonable to moderate activity in the presence of TBHP while no activity is observed when using molecular oxygen as oxidant. However, the immobilized catalysts were outclassed by their model counterparts when employing both oxidants.

AFRIKAANSE OPSOMMING: Hierdie proefskrif behels die sintese van 'n verskeidenheid model- en siloksaan gefunksionaliseerde komplekse wat salisielaldemien ligande bevat. Die gefunksionaliseerde komplekse is geïmmobiliseer op twee tipes mesoporeuse silika draers, naamlik, MCM-41 en SBA-15 wat daarna in die oksidasie van veratryl alkohol geëvalueer was. Beide model (L1) en siloksaan gefunksionaliseerde (L2) ligande is ten volle gekaraktiseer deur middel van 'n reeks analitiese tegnieke soos FT-IR (ATR) en 1H KMR spektroskopie gebruik te maak. Die model- en siloksaan gefunksionaliseerde salisielaldimien ligande was met metaalvoorlopers gebaseer op koper-, kobalt- en palladiumsoute in 'n bis-bidentate wyse gereageer. Volle karakterisering van die model (MC1-MC3) en gefunksionaliseerde (FC1- FC3) komplekse is bereik deur massa spektrometrie, FT-IR (ATR) spektroskopie, 1H KMR spektroskopie (MC3 en FC3), UV-Sig spektroskopie, EPR spektroskopie (MC1 , MC2, FC1 en FC2), enkel kristal X-straal diffraksie (MC3), elementêre analise en smeltpunt bepalings. Die onveranderde-silika draers, MCM-41 en SBA-15 en die gefunksionaliseerde komplekse is met mekaar gereageer om die geïmmobiliseerde komplekse (IC1-IC6) te vorm. Die immobiliserings proses behels 'n kondensasie reaksie tussen die oppervlaksilanol groepe van die draers en die siloksaanfunksionaliteit van die komplekse. Hierdie komplekse word op ‘n kovalente manier aan hul onderskeie draers gebind, wat uiteindelik lei tot die vorming van "heterogeniseerde" katalisators. Die geïmmobiliseerde komplekse (IC1-IC6) is ten volle gekarakteriseer deur 'n verskeidenheid analitiese tegnieke in die vaste toestand wat FT-IR (ATR) spektroskopie, laë-hoek poeier X-straal diffraksie, termiese gravimetriese analise (TGA), vaste toestand 29Si KMR spektroskopie, stikstof adsorpsie/ desorpsie (BET) oppervlakanalise, skandeer en transmissie-elektron mikroskopie (SEM en TEM) insluit. Induktiewe gekoppelde plasma optiese emissie spektroskopie (ICP-OES) was gebruik om die metaal inhoud van die geïmmobiliseerde komplekse te bepaal. Beide die model (MC1-MC3) en geïmmobiliseerde (IC1-IC6) komplekse was geëvalueer in die oksidasie van veratryl alkohol. 'n Vergelyking van die effek van verskillende oksideermiddels op die aktiwiteit is ondersoek deur gebruik te maak van molekulêre suurstof, waterstofperoksied of tert-butielhidroperoksied as oksideermiddels. Model kompleks, MC1, het beter aktiwiteit getoon as die ander model katalisators (MC2 en MC3) in die oksidasie van veratryl alkohol by 25 °C onder 1 atm atmosferiese druk. Hierdie katalisator het selektief veratryl aldehied geproduseer. Van die drie oksideermiddels, het waterstof peroksied die laagste aktiwiteit getoon wanneer dit gebruik word in samewerking met MC1 gebruik was. Die laasgenoemde is gebruik om die reaksietoestande te optimaliseer. Eksperimente met katalisator, MC1, was ook in die afwesigheid van suurstof uitgevoer, wat slegs matige aktiwiteit van ongeveer 20% met 100% aldehied selektiwiteit getoon het. Dit stel waarskynlik voor dat die reaksie 'n β-hidried eliminasie reaksie ondergaan met daaropvolgende vorming van molekulêre waterstof. Die gebruik van tert-butielhidroperoksied as oksideermiddel in die oksidasie van veratryl alkohol het hoë aktiwiteit getoon in vergelyking met die gebruik van molekulêre suurstof. Die hoë aktiwiteit kan toegeskryf word aan die hoër stabiliteit van TBHP sowel as die oplosbaarheid daarvan in die reaksiemengsel. Dit dui daarop dat die aktiewe spesie vinniger met TBHP as oksideermiddel vorm. Die beste omsetting van substraat was verkry vanaf die kobalt katalisator, MC2, met ‘n waarde van 85%. Dit is dan gevolg deur MC1 en MC3. Die hoër aktiwiteit van TBHP, het tot hoër aldehied opbrengs gelei, maar dit het ongelukkig ook tot oor-oksidasie van veratrylaldehied na veratriese suur gelei. Die geïmmobiliseerde katalisators, IC1-IC6, was ook geëvalueer in die oksidasie van veratryl alkohol met behulp van TBHP as oksideermiddel. Hierdie katalisators het oor die algemeen redelik to matige aktiwiteit vertoon, terwyl die kobalt analoë (IC3 en IC4) matige omsetting van tot 65% getoon het. Die gebruik van molekulêre suurstof as oksideermiddel tesame met die geïmmobiliseerde katalisators was ondoeltreffend in die oksidasie van veratryl alkohol. Al die geïmmobiliseerde katalisators behalwe IC5 was herwinbaar en hergebruik vir vyf katalitiese lopies. IC3 en IC4 het matige aktiwiteit oor die vyf siklusse vertoon met slegs 'n effense afname in aktiwiteit. Die aard van die silika draers het 'n uitwerking op die aktiwiteit sowel as die selektiwiteit gehad. Die geïmmobiliseerde katalisators gebaseer op MCM-41 draers, het beter aktiwiteit getoon. Die selektiwiteit was effens verandered toe SBA-15 as draer gebruik word. In die algeheel het die geïmmobiliseerde katalisators redelik tot matige aktiwiteit getoon in die teenwoordigheid van TBHP, terwyl geen aktiwiteit waargeneem is nie wanneer molekulêre suurstof as oksideermiddel gebruik word. Die geïmmobiliseerde katalisators was egter deur hul model-analoë oortref wanneer beide oksideermiddels aangewend was.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105686
This item appears in the following collections: