Room Temperature Biomimetic Catalyst for Aliphatic C–H Bond Oxidation: Copper(II) Complexes Immobilized in Functionalized Mesoporous Silica Nanoparticles

本論文發展了兩種常溫下催化的仿生觸媒，將具活性的銅模型錯合物固定於功能性的二氧化矽奈米球的管道中而得。這些異相觸媒可以很有效率的催化脂肪族碳氫鍵活化反應，並得到很高的轉換率以及反應選擇性。主要實驗分為兩部分： (一)在第三章我們合成了一個三配位的二價銅錯合物用以模擬酵素催化行為，在常溫下執行甲苯碳氫鍵活化的反應。而這些錯合物與氧形成的活性中間體本來在常溫下並不穩定並且催化性也不太佳，然而透過固定於二氧化矽奈米球的管道中，我們卻發現在常溫下錯合物可以得到很高的穩定性以及良好的反應性( TON可達550, 最終產物選擇性可高達95%)。我們推測此模擬系統可以促進活性中間體bis-μ-oxo dicopper (III) species 在常溫下形成並穩定存在。此活性中間體固定在孔洞中，具有很高的反應性與選擇性，先將甲苯轉化成苯甲醇，再進一步轉化成最終產物苯甲醛。而現今工業上高溫催化過程中過度氧化形成的苯甲酸，在此反應中並沒有產生。此外, 此催化反應為無限循環且不需要額外的犧牲還原試劑來驅動反應, 並且催化劑可以重複使用好幾次卻不失去催化活性。在此, 我們提出一個成功的仿生系統, 相信在未來應用上有很大潛力。 延續此工作,我們在第四章進行反應機制的研究。單一轉換實驗證實甲苯第一步被轉化成苯甲醇,並且產生μ-oxo dicopper (II) species (CuII−O−CuII) 中間體, 接著透過氧氣的活化進行第二步苯甲醇氧化成苯甲醛的反應, 來完成此催化循環。在此, 我們的甲苯催化系統的反應機制已經被完整的澄清是屬於氧原子插入脂肪族碳氫鍵的甲苯連續氧化反應, 而非典型的自由基氧化反應。 (二)在第五章我們合成了兩種不同孔徑大小的二氧化矽奈米球來負載具有活性的三銅錯合物，用於催化較困難的甲烷碳氫鍵活化反應。藉由孔洞的限制空間效應，使得原來在勻相反應中不具活性的Ethppz錯合物，可以在孔道中催化甲烷為甲醇。由於在錯合物與矽材表面有著強大的靜電作用力，因此固定在孔道中的錯合物幾乎不會外漏，非常適合發展成新穎的異相觸媒來推動甲烷氧化反應。由於有文獻報導甲烷分子在二氧化矽奈米材料的中孔洞有很高的溶解度，因此可以抑制失敗的反應循環(abortive cycle)發生使得甲醇的轉換數大大提升。在此我們提出了一個史無前例的例子，將具活性的三銅錯合物固定在功能性的二氧化矽奈米球中用以模擬微粒體甲烷單氧化酶的催化行為並具有很高的甲醇轉化數。而這種異相觸媒不僅固定在內的錯合物幾乎不外漏，並且反應完成後也很容易地從反應系統中分離出來，我們相信這將會有非常大的潛力應用於工業催化以及能源開發上。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