Catalytic performance of Pd-doped α-MnO2 for oxidation of benzyl alcohol under solvent-free conditions
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摘要: 通過共沉淀和原位煅燒轉化方法, 將Pd摻雜δ-MnO2前驅體煅燒后制備得到Pd摻雜α-MnO2納米棒催化材料.通過氮氣物理吸附、X射線衍射、透射電子顯微鏡、掃描電子顯微鏡、熱重分析、X射線光電子能譜等技術對催化材料進行了表征.掃描電鏡和透射電鏡結果顯示, α-MnO2納米棒表面沒有明顯的Pd納米顆粒, 表明Pd可能摻雜到α-MnO2晶格中.純α-MnO2的還原溫度在390℃左右, 但Pd摻雜可以極大地促進α-MnO2還原, 還原溫度可低至約200℃左右.研究了所制備催化劑在無溶劑條件下對于以分子氧為氧化劑選擇性催化氧化苯甲醇為苯甲醛的催化性能.結果表明: 在無溶劑及用純氧氣為氧化劑條件下, Pd摻雜α-MnO2納米棒對苯甲醇氧化顯示出增強的催化活性; 所摻雜的氧化態Pd物質可增強催化材料中的氧遷移率; 在這些Pd摻雜α-MnO2催化材料中, 當以Pd (3%, 質量分數) -MnO2為催化劑時, 在110℃反應4 h后, 苯甲醇的轉化率為39%, 遠高于同條件下以純α-MnO2為催化劑時18. 3%的苯甲醇轉化率.Abstract: Liquid-phase selective oxidation of benzyl alcohol to benzaldehyde is one of the most important processes in both laboratory and chemical industry processes due to the remarkable values of benzaldehyde in the production of flavours, fragrances, and biologically active compounds. In the traditional processes for selective oxidation of benzyl alcohol using a stoichiometric or excessive amount of toxic and expensive inorganic oxidants, such as ammonium permanganate in aqueous acidic medium, a large amount of toxic waste is produced. A few studies on the benzyl alcohol-to-benzaldehyde oxidation by environmentally clean oxidants (O2 or H2O2) in the presence of organic solvents (e. g., toluene, p-xylene, and trifuorotoluene) have been reported; however, the usage of organic solvent is neither economical nor environmental friendly. Even though the solvent-free oxidation of benzyl alcohol to benzaldehyde using tert-butylhydroperoxide (TBHP) as oxidant has been reported, the co-product of tert-butanol from the consumption of TBHP will be left in the reaction solution, necessitating further separation. Therefore, various heterogeneous catalysts have been developed for solvent-free selective oxidation of benzyl alcohol using flowing air or oxygen; however, in most of these systems, the reaction temperature is still high (> 130 ℃) and/or conversion/selectivity is still low. Thus, the development of efficient heterogeneous catalysts for the solvent-free se-lective oxidation of benzyl alcohol with high selectivity and yield using molecular oxygen from air as the oxidant at low temperature is needed. In this study, Pd-doped α-MnO2 nanorods were prepared from Pd-doped δ-MnO2 precursors via a co-precipitation and in situ calcination transformation method. These catalysts were extensively characterized by various techniques, such as N2 adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The SEM and TEM results indicate that there are no obvious Pd nanoparticles on the surface of α-MnO2 nanorods, signifying the possible doping of Pd into the lattice of α-MnO2. The reduction temperature of pureα-MnO2 is around 390 ℃, while the doped Pd could greatly promote α-MnO2 reduction to lower temperatures at around 200 ℃. The applications of Pd-doped α-MnO2 nanorods as catalysts for selective aerobic oxidation of benzyl alcohol to benzaldehyde under solventfree conditions with molecular oxygen were investigated. As compared with pure α-MnO2, the Pd-doped α-MnO2 nanorods show enhanced catalytic activity for selective oxidation of benzyl alcohol under solvent-free conditions with O2, which can be attributed to the beneficial presence of oxidized palladium species and enhanced oxygen mobility resulting from the doping Pd species. In these Pddoped α-MnO2 nanorods, when Pd (3%) -MnO2 was used as catalyst, a 39% conversion of benzyl alcohol was achieved. It is much higher than the 18. 3% conversion when pure α-MnO2 used as catalyst at 110 ℃ and reaction time of 4 h.
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Key words:
- nanorods /
- α-MnO2 /
- Pd-doping /
- solvent-free oxidation /
- sol-gel preparation
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圖 1 X射線衍射圖. (a) α-MnO2; (b) Pd (1%) -MnO2; (c) Pd (2%) -MnO2; (d) Pd (3%) -MnO2
Figure 1. XRD patterns: (a) α-MnO2; (b) Pd (1%) -MnO2; (c) Pd (2%) -MnO2; (d) Pd (3%) -MnO2
圖 2 掃描電鏡照片. (a) α-MnO2; (b) Pd (1%) -MnO2; (c) Pd (2%) -MnO2; (d) Pd (3%) -MnO2
Figure 2. SEM images: (a) α-MnO2; (b) Pd (1%) -MnO2; (c) Pd (2%) -MnO2; (d) Pd (3%) -MnO2
圖 3 透射電鏡照片. (a, b) α-MnO2; (c, d) Pd (3%) -MnO2
Figure 3. TEM images: (a, b) α-MnO2; (c, d) Pd (3%) -MnO2
圖 4 所制備催化材料在5%H2/Ar條件下的熱重曲線(升溫速率: 10℃·min-1)
Figure 4. TGA curves of catalysts under 5%H2/Ar with increasing rate of 10℃·min-1
圖 5 Pd 3d X射線光電子能譜譜圖. (a) Pd (1%) -MnO2; (b) Pd (2%) -MnO2; (c) Pd (3%) -Mn O2
Figure 5. Pd 3d XPS spectra: (a) Pd (1%) -MnO2; (b) Pd (2%) -MnO2; (c) Pd (3%) -MnO2
圖 6 Mn 2p X射線光電子能譜譜圖. (a) α-MnO2; (b) Pd (1%) -MnO2; (c) Pd (2%) -MnO2; (d) Pd (3%) -Mn O2
Figure 6. Mn 2p XPS spectra: (a) α-MnO2; (b) Pd (1%) -MnO2; (c) Pd (2%) -MnO2; (d) Pd (3%) -MnO2
圖 7 O 1s X射線光電子能譜譜圖. (a) α-MnO2; (b) Pd (1%) -MnO2; (c) Pd (2%) -MnO2; (d) Pd (3%) -MnO2
Figure 7. O 1s XPS spectra: (a) α-MnO2; (b) Pd (1%) -Mn O2; (c) Pd (2%) -MnO2; (d) Pd (3%) -MnO2
圖 8 用Pd摻雜α-MnO2納米棒作為催化劑時苯甲醇氧化的反應路徑
Figure 8. Reaction processes of benzyl alcohol oxidation using Pd-dopedα-Mn O2 nanorods
圖 9 苯甲醇的轉化率和苯甲醛的選擇性隨著反應溫度及反應時間的變化曲線
Figure 9. Conversion of benzyl alcohol and selectivity of benzaldehyde dependent on reaction temperature and time
表 1 樣品的元素含量結果
Table 1. Element content results of as-prepared samples
樣品名稱 元素質量分數/% O Mn K Pd α-MnO2 39.70 51.71 8.59 0 Pd(1%)-MnO2 34.72 55.04 9.45 0.79 Pd(2%)-MnO2 32.28 56.32 9.71 1.69 Pd(3%)-MnO2 30.03 57.90 10.32 1.75 
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表 2 所制備催化材料在不同條件下的催化性能
Table 2. Catalytic performance of as-prepared catalysts under different conditions
序號 催化劑 溫度/℃ 氣氛 轉化率/% 選擇性/% 產率/% 1 無催化劑 110 氧氣 ＜0.1 >99.9 ＜0.1 2 α-MnO2 110 氧氣 18.3 >99.9 18.3 3 Pd(1%)-MnO2 110 氧氣 26.0 87.8 22.8 4 Pd(2%)-MnO2 110 氧氣 34.3 80.3 27.5 5 Pd(3%)-MnO2 110 氧氣 39.0 85.7 33.4 6 Pd(3%)-MnO2 110 空氣 24.9 96.6 24.1 7 Pd(3%)-MnO2 110 氮氣 6.5 >99.9 6.5 8 Pd(3%)-MnO2 100 氧氣 16.5 96.6 15.9 9 Pd(3%)-MnO2 120 氧氣 56.6 62.5 35.4 注: 催化反應條件為10 mg催化劑, 20 mmol苯甲醇, 101.325 kPa, 4 h.
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