Room Temperature Biomimetic Catalyst for Aliphatic C–H Bond Oxidation: Copper(II) Complexes Immobilized in Functionalized Mesoporous Silica Nanoparticles
Date Issued
2015
Date
2015
Author(s)
Liu, Chih-Cheng
Abstract
In this dissertation, two kinds of room temperature biomimetic catalysts were developed by immobilizing the active copper model complexes into the nanochannels of functionalized MSNs. These heterogeneous catalysts can efficiently catalyze aliphatic C-H bond oxidation with high turnover number and selectivity. In chapter 3, a tripodal tridentate copper(II) complex, CuImph (Imph = bis(4-imidazolyl methyl)benzylamine), is synthesized to mimic the active site of copper enzymes that mediate the oxidation of aliphatic C–H bonds under mild condition. The stability and catalytic activity of the formation of copper-dioxygen complex are moderate in solution at room temperature. In contrast, by immobilizing the model complex in the nanochannels of functionalized mesoporous silica nanoparticles (MSNs), we observed high stability and good reactivity at ambient temperature. We propose the mimic system promotes the formation of bis-μ-oxo species ([{CuIIIImph}2(μ-O2-)2]) in the presence of O2 or air at ambient temperature. The dioxygen-activated CuImph@MSN samples show high reactivity and selectivity toward toluene aliphatic C−H bond oxidation, converting the toluene initially to benzyl alcohol and subsequently to benzaldehyde as the major product in a kinetic consecutive reaction. No evidence for benzoic acid is obtained, unlike the over-oxidation typically associated with present-day industrial processes operating at high temperatures. In addition, the process is self-sustaining without the requirement for a sacrificial reductant to drive the catalytic turnover. The catalyst can be also fully recovered and re-used for several cycles without decay of activity. This unprecedented catalytic system should have potential industrial applications for the direct oxidation of toluene to benzaldehyde under ambient conditions with high efficiency and selectivity. In chapter 4, the single-turnover study was performed and revealed that the toluene is first converted into benzyl alcohol with the formation of the [{CuIIImph}2(μ-O2−)]2+ intermediate. However, further oxidation of the benzyl alcohol to benzaldehyde requires participation of a molecule of O2 to activate the CuII−O−CuII species for completion of the catalytic cycle. The implication is that the CuII−O−CuII species is in fact relatively stable and inert at room temperature. The mechanism is well clarified that our catalytic system of toluene oxidation is a toluene consecutive reaction involve the oxene O-atom insertion, not a typical radical oxidation mechanism. In chapter 5, two different pore diameter of functionalized MSN material (Al-MSN-ex, and MSN-TP) were synthesized and used to immobilize tricopper (II) complexes (Etppz, and Ethppz) for difficult methane oxidation reaction. By immobilizing Ethppz into the nanochannels of MSN material, the nano-confinement effect make the inactive Ethppz complex allows to catalyze the methane oxidation. The complexes immobilized in MSNs show almost non-leaching due to the strong electrostatic interaction between complex and silica surface, and can suitable to develop a new heterogeneous catalyst for methane oxidation reaction. Due to the high solubility of methane in the silica mesopores reported in the literature, the abortive cycle can be inhibited result for the increasing of the TONs of methanol. Therefore, we report an unprecedented case of active tricopper complexes immobilized in functionalized MSN samples to mimic the native enzyme pMMO that can efficiently catalyze the difficult methane oxidation reaction with high TONs. With the properties of little complex leaching and easy separation from the reaction mixture, we believe that the CuEtppz@MSN (Al-MSN-ex and MSN-TP) catalysts should also have high potential for industrial applications.
Subjects
mesoporous silica nanoparticles
bio-mimic catalysis
copper complex
aliphatic C-H bond oxidation
toluene oxidation
benzaldehyde
benzyl alcohol
methane oxidation
methanol
confinement effect
Type
thesis
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