Heterogenization of Schiff base complexes on mesoporous silica and their application as catalysts in the oxidative transformation of alcohols

Joubert, Corli (2013-03)

Thesis (MSc)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: In this thesis the synthesis of a range of model and siloxane functionalized salicylaldimine ligands and [N-(n-propyl)-(2-pyridyl)] diimine ligands are described. The functionalized ligands were obtained by the reaction of salicylaldehyde and 2-pyridinecarboxaldehyde with 3-aminopropyltriethyoxysilane. All ligands were characterized by FT-IR and 1H NMR spectroscopy. The salicylaldimine ligands were reacted with either Cu(II) or Pd(II) salts to form both the model and functionalized Cu(II) and Pd(II) complexes. The Cu(II) complexes were characterized using FT-IR, EPR and UV-VIS spectroscopy while the Pd(II) complexes were characterized using FT-IR, 1H NMR and 13C{1H} NMR spectroscopy. The diimine ligands were reacted with Cu(II) salts to form functionalized pyridinyl complexes which were characterized used FT-IR spectroscopy. Two mesoporous silica supports, MCM-41 and SBA-15 were synthesized. The siloxane functionalized salicylaldimine Cu(II) and Pd(II) complexes were immobilized onto these supports to produce heterogenized catalysts. These catalysts were characterized using a range of solid-state techniques: BET nitrogen adsorption/desorption, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), ICP-AES, and powder X-ray diffraction (XRD) analysis. The powder XRD and SEM analysis showed that the structural integrity of the catalyst supports was kept intact during the immobilization process. BET analysis and ICP-AES showed that the complexes had been attached to the silica supports. Both the model complexes and heterogenized catalyst systems were tested in the oxidation of benzyl alcohol to benzaldehyde. The model copper catalyst showed high activity in this reaction with molecular oxygen used as oxidant and the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl radical as co-oxidant. The immobilized copper complexes showed low activity in the same reaction. This was attributed to steric hindrance around the active site dye to its entrapment with the silica matrix thus hindering the formation of a key intermediate in the oxidation cycle due to steric hindrance. The palladium complexes were not active in the oxidation reaction when molecular oxygen was used as oxidant, but showed slight activity when hydrogen peroxide was used.

AFRIKAANSE OPSOMMING: In hierdie tesis word die sintese van ‘n reeks model sowel as gefunksioneerde salisielaldemien ligande en [N-(n-propiel)-(2-piridiel)] di-imien ligande beskryf. Die gefunksioneerde ligande is verkry deur die reaksie van salisielaldehied en en 2-piridienaldehied met 3-aminopropieltriëtoksiesilaan. Al die ligande is gekarakteriseer deur FT-IR en 1H-KMR spektroskopie. Die salisielaldimien ligande is met óf Cu(II) óf Pd(II) soute reageer om beide die model en gefunksioneerde Cu(II) en Pd(II) komplekse te vorm. Die Cu(II) komplekse is gekarakteriseer deur FT-IR, EPR en UV-Vis spektroskopie terwyl die Pd(II) komplekse deur FT-IR, 1H-KMR en 13C{1H}-KMR spektroskopie gekarakteriseer is. Die di-imien ligande is met Cu(II) soute reageer om gefunksioneerde piridinielkompekse te vorm wat deur FT-IR spektroskopie gekarakteriseer is. Twee mesoporeuse silika draers, MCM-41 en SBA-15 is gesintetiseer. Die siloksaan-gefunksioneerde salisielaldemien Cu(II) en Pd(II) komplekse is op hierdie draers geimobiliseer om geheterogeniseerde katalisatore te vorm. Hierdie katalisatore is gekarakteriseer deur van ‘n wye reeks vaste toestand tegnieke gebruik te maak: BET stikstof adsorpsie/desorpsie, skandeer elektron mikroskopie (SEM), termiese gravimetriese analise (TGA), ICP-AES en poeier-XRD analise. Die poeier-XRD en SEM analisies het aangetoon dat die strukturele integriteit van die katalisator draers behoue gebly het tydens die immobiliseringsproses. BET analise en ICP-AES het aangetoon dat die komplekse aan die silika draers geheg is. Beide die model komplekse en geimobiliseerde katalisators is getoets in die oksidasie van bensielalkohol na bensaldehied. Die model Cu(II) katalisator het hoë aktiwiteit in hierdie reaksie getoon met molekulêre suurstof as oksideermiddel en die (2,2,6,6-tetrametielpipiridien-1-iel)oksiel radikaal as ko-oksideermiddel. Die geimobiliseerde Cu(II) komplekse het lae aktiwiteit in dieselfde reaksie getoon. Dit is toegeskryf aan steriese hindernis rondom die aktiewe sentrum as gevolg van die verstrikking binne die silika matrys, wat die vorming van ‘n belangrike tussentoestand in die oksidasie-siklus verhinder. Die Pd(II) komplekse was nie aktief in die oksidasie-reaksie in die geval waar molekulêre suurstof as oksideermiddel gebruik is nie, maar het effense aktiwiteit getoon waneer waterstofperoksied gebruik is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/79979
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