二氧化矽負載鈮氧化物的製備與優化應用於催化甘油縮醛反應之研究

Abstract

本研究利用含浸法製備二氧化矽負載鈮氧化物，選擇成本低的五氯化鈮為鈮前驅物，但相較於成本高的乙氧基化鈮(V)，存有表面殘餘氯化物影響表面催化特性，我們在製備條件中加入三個優化步驟改善表面催化特性：第一步驟，超音波震盪使鈮分散在二氧化矽表面更加均勻，改善含浸法存在材料均勻度不佳的問題。拉曼光譜顯示1小時的震盪可以使材料均勻度提高；第二步驟水處理，利用90°C水處理去除表面殘餘氯化物，穿透式電子顯微鏡之元素分析檢測發現水處理改善材料表面殘餘氯化物的問題；第三步驟為改變鍛燒條件，用以調整鈮氧化物結構以及酸催化位，UV-Vis吸收光譜和X-ray繞射光譜的結果指出，10 wt%鈮負載之材料當鍛燒溫度超過550°C，表面產生五氧化二鈮結晶，其結晶型態為六方晶系，鍛燒溫度在500°C持溫1小時，材料表面為無晶型且結構扭曲的NbO4有利於酸催化反應。將鈮氧化物負載二氧化矽應用於甘油縮醛反應，甘油加丙酮縮醛化選擇性反應成丙酮縮甘油(Solketal)，提高其經濟利益，在工業上可串起生質柴油的生產鏈。除了探討轉化率和選擇性，運用Operando拉曼即時分析監測反應，觀察分子之間的鍵結以探究反應途徑。 不同鈮負載量之批次實驗結果顯示，隨著鈮負載量升高，反應之甘油轉化率提高，15 wt%的甘油轉換率(45.6%)最高，當鈮負載量大於20 wt%，甘油轉化率也隨之下降。UV-Vis吸收光譜(Ultraviolet–Visible Spectroscopy)指出低鈮負載材料的表面為扭曲且無晶型的鈮氧化物結構，太高的鈮負載量會造成材料表面形成微小Nb2O5結晶。在酸性質分析，氨氣-程序升溫控制脫附儀(NH3- Temperature Programmed Desorption)的結果指向總酸量是造成催化的主要原因。Pyridine吸附-傅立葉轉換紅外光譜(Pyridine adsorption- Fourier Transform Infrared Spectroscopy, FTIR)結果顯示，低鈮負載量的材料只存在路易斯酸，當鈮負載量越大(15 wt%和20 wt%)，材料表面同時存在布忍斯特酸位(Bronsted acid site)以及路易斯酸位(Lewis acid site)。Operando 拉曼光譜分析結果顯示布忍斯特酸位傾向於將半縮酮(Hemiketal)轉為產物。

Silica oxide supported niobium oxide was prepared through the impregnation method. We use niobium(V) chloride as a precursor instead of niobium ethoxide which is a high-cost precursor. However, it presented a problem that strong bonding between niobium and chloride causes residual chloride compound on the surface. In the synthesis process, three optimized steps were adopted to improve surface property. First, the step of ultasonication is to disperse niobium oxide on silica for improving the surface homogeneity which is one of shortcomings in impregnation method. Raman Spectra shows that sonication for 1 hour improves the surface homogeneity. Second, DI water was used to remove surface chloride compound. Energy-dispersive X-ray spectrum shows that DI water removed the residual chloride successfully. Finally, calcined condition was changed to modified the surface niobium species for obtaining more acidity sites. UV-Vis adsorption spectrum and X-ray diffraction spectrum indicated that material calcined at 550°C in 10 wt% niobium loading exhibited the formation of Nb2O5 crystalline phase. Moreover, the material calcined 500°C for 1 hour posscesses distorted amorphous NbO4 which has more acid sites. Silica supported niobium oxide is applied in glycerol acetalization, increasing the economic benefits of glycerol and integrating production chain of biodiesel industry. Glycerol can be acetalized with acetone to produce 5-membered ring 2,2-dimethyl-1,3-dioxolane-4-ylmethanol (solketal) and the 6-membered ring 2,2-dimethyl-1,3-dioxan-5-ol. Despite of the solketal selectivity and glycerol conversion, we use Operando Raman Spectroscopy to monitor the reaction. Operando Raman offers the interaction of molecular to understand the path of reaction. Different loading niobium supported on silica were applied in glycerol acetalization. The conversion results show that higher niobium loading causes high glycerol conversion, and 15 wt% niobium loading possesses the best glycerol conversion (45.6%). However, 30 wt% and 40 wt% have low glycerol conversions relatively. From the characteristic analysis, UV-Vis adsorption spectrum results indicated that low loading niobium materials present distorted amorphous niobium species, however, high niobium loading materials cause small niobium crystallinity on the surface. The acidity analysis includes Pyridine adsorption- Fourier Transform Infrared Spectroscopy (Pyridine adsorption-FTIR) and NH3-Temperature Programmed Desorption (NH3-TPD). The NH3-TPD result shows that the glycerol conversion related to the amount of acid sites. Pyridine adsorption-FTIR indicated that low loading niobium materials only have Lewis acid sites, and more than 15 wt% niobium loading materials posscess Br?nsted acid sites and Lewis acid sites. Operando Raman spectra during reaction indicated that Br?nsted afforded more on converting hemiacetal into production.