一種利用金屬附載於中孔洞二氧化矽奈米球來進行選擇性氧化芳香性碳氫化合物的策略

Abstract

鐵與鐵氧化物附載於二氧化矽奈米球一直以來在催化領域的應用上有相當大潛力的催化劑，原因不外乎鐵在催化表現上的高活性以及二氧化矽奈米粒子其高的比表面積、適當的孔徑大小、好的重複使用率和容易分離的特性，且直徑約在一百奈米大小的二氧化矽奈米球，使它能夠很好的分散在溶劑之中，以利於催化上的應用。 在此篇論文中，我們透過一鍋合成法合成出了不同附載量具高度分散性氧化鐵於二氧化矽奈米球上的催化劑，這些催化劑能夠有效率地在溫和的環境下，催化芳香族化合物的碳氫鍵氧化，且具有可調控的選擇性，於穿透式電子顯微鏡和掃描式電子顯微鏡能量分散光譜儀的鑑定中，可以發現到氧化鐵是高度分散於二氧化矽的結構之中，在氮氣吸脫附圖的結果之中，這種材料即便在高的負載率(16.4 w/w%)仍然展現了非常高的比表面積(~800m２g-1)且能夠維持相當於未負載鐵時的均勻孔徑大小(2.3 nm)，此外我們也試著利用同步輻射X光吸收光譜來進行材料的鑑定，發現到鐵原子成功取代了結構中矽原子的位置，並且釐清在催化反應前後鐵於結構之中的氧化態和局域結構。 這些異相催化劑能夠在溫和的條件下，將苯有效率地轉換成酚且只有少量的過度氧化產物，並且對甲苯進行以sp2碳氫鍵活化為主的氧化反應生成對甲酚和鄰甲酚。我們抓住二氧化矽骨架能夠限制住氧化鐵奈米粒子粒徑這一點，透過於合成過程中改變氧化鐵前驅物的量來得到於骨架中不同大小的氧化鐵，進而去調控出具有不同反應性的鐵催化劑，在反應結束後，可以容易地從反應系統中將催化劑分離，並且重複使用數次仍維持相當的活性。 我們嘗試了數個控制實驗去研究了反應的機制，發現到這個催化系統不是典型的費頓反應，第一、當調控在酸性環境下時，氫氧自由基生產速率會下降，會使得目標產物生成速率也隨之下降，但在此系統中沒有觀察到這樣的現象發生，第二、透過動力學同位素實驗的結果，KIE值(0.96)與費頓反應的1.6-1.7有很大的差距，第三、反應系統在加入自由基捕獲劑時，產物的轉換數和產率沒有影響，說明了反應途徑不是透過自由基所主導。 綜合以上，我們開發了有效率的方式去合成負載高分散氧化鐵的二氧化矽奈米球催化劑，將其用於較難進行的氧化芳香性碳氫鍵，更進一步驗證了催化系統機制與費頓機制相異的特別性質，結合這種高活性的碳氫鍵氧化性、具有容易分離性和低廉合成方法……等等特性，我們相信這種材料將會是一個於工業應用上具有高度潛力的催化劑。

Metal oxides incorporated into mesoporous silica nanoparticles (MSNs) have been considered as great potential catalysts for further catalytic application, due to high surface area, adequate pore size, good reusability and easy separation. With a particle size of ~100 nm, the MSNs can also be suspended in solution very well for catalysis application in liquid state. Herein, we reported a series of catalysts that are assembled by incorporation of different amounts of iron oxide species into MSNs with high dispersion by using an one-pot direct synthesis at ambient temperature. These catalysts can perform efficiently catalytic oxidation of aromatic arenes into the corresponding oxidized products with controllable selectivity under mild condition. TEM, SEM-EDX images show that iron species are highly dispersed inside the silica materials. Nitrogen adsorption-desorption isotherms demonstrate that the obtained materials exhibit large surface area (~1000 m2 g−1), and uniform pore size (2.3 nm) even though high concentration of iron species were dispersed in MSNs (16.4%). X-ray absorption spectroscopy is used to clarify the oxidation state (Fe+3) and local structure of the iron species dispersed in MSNs before and after catalytic reaction. The heterogeneous catalysts can efficiently carry out C–H bond activations of benzene to form phenol as the major product, and of toluene to form benzaldehyde and/or benzyl alcohol in sp3 C-H bond oxidation and o-, p-cresol and/or methyl-p-benzoquinone in sp2 C-H bond oxidation under mild temperature. The particle size of iron oxide confined in the silica framework can be controlled by introducing different amount of iron precursor during synthetic process. Thus, the reactions can be facilely tuned and controlled to selectively yield an sp2 or sp3 C–H bond oxidation of toluene. Moreover, these heterogeneous catalysts are robust and reuseable. They are easily separate from the reaction mixture and can be fully recovered and reused for several cycles with only small decay of activity. Several control experiments were performed to show that the catalytic oxidation is not a typical fenton-like reaction mechanism (FeII and H2O2 for hydroxyl radical). First, the formation rate of phenol is not affected at the low pH environment where the hydroxyl radical formation rate is thought to be decrease a lot. Second, we have detected no apparent effect of the radical trapping agent on the TONs or product yields of arene oxidation, indicating that there is no radical formation (such as hydroxyl radicals) in this reaction pathway Third, kinetic isotope effect (KIE) experiments were well done and shown difference to the fenton-like mechanism. Overall, we developed an effective method to synthesize a series of high dispersion Fe-MSN catalysts for difficult aromatic C-H bond oxidation and clarified that our catalytic system is not a fenton-like reaction. With the properties of high activity for C-H bond oxidation, easy separation, cheap synthesis process, we believe that the MSN-supported iron oxide catalysts have high potential for industrial application.