Hydration of α-pinene catalyzed by oxalic acid/polyethylene glycol deep eutectic solvents
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摘要: 以天然有机二元羧酸草酸(OA)作为氢键供体,各种聚合度的聚乙二醇(PEG)作为氢键受体,构建出羧酸功能化低温共熔体(DES),将其用于催化α-蒎烯水合制备α-松油醇的反应中。傅里叶变换红外光谱(FT-IR)、核磁共振氢谱(1H NMR)、热重分析(TGA)等表征证实了OA与PEG之间氢键的形成。DES中PEG组分的存在对其酸强度影响较小,但随PEG分子量和用量的增加,DES总酸量减小,从而降低其催化活性。研究表明,分子量最小的PEG200与OA制备的OA/0.6PEG200 DES具有较佳的催化 α-蒎烯水合反应性能,在DES催化剂用量0.03 mol(以OA计),α-蒎烯用量0.06 mol,水用量0.3 mol,反应温度75 ℃,反应时间8 h的优化条件下,获得81.5%的α-蒎烯转化率及51.2%的α-松油醇选择性。催化剂相反应结束后静置冷藏过夜即可分层分离,且循环使用性能良好。该OA/0.6PEG200 低温共熔体制备简单,原子经济性高,为一步法催化α-松油醇的清洁制备开辟了新路线。Abstract: A series of carboxylic acid-functionalized deep eutectic solvents (DES) were constructed by a natural organic dicarboxylic acid, oxalic acid (OA), as the hydrogen bond donor and the polyethylene glycol (PEG) with different polymerization degrees as the hydrogen bond acceptors, which are used in the hydration of α-pinene to produce α-terpineol. Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance spectroscopy (1H NMR), and thermogravimetric analysis (TGA) were used to prove the hydrogen bonding between OA and PEG. It is found that the presence of PEG has a less impact on the acid strength of DES. However, an increase in both molecular weight and dosage of PEG results in a decrease in total acidity and catalytic activity. Among them, OA/0.6PEG200, a DES catalyst prepared by PEG with the smallest molecular weight, exhibits a favorable catalytic performance. Under an optimal reaction condition with 0.03 mol of DES (based on OA), 0.06 mol of α-pinene, 0.3 mol of water, at 75 °C for 8 h, an α-pinene conversion of 81.5% and an α-terpineol selectivity of 51.2% are obtained. The catalyst phase can be separated by refrigerating overnight after reaction and reused directly with relatively stable catalytic performance. Thus, OA/0.6PEG200, as a DES catalyst prepared by a simple and highly atom economical process, will offer a clean catalytic route for the one-step production of α-terpineol.
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Key words:
- α-pinene /
- α-terpineol /
- hydration /
- deep eutectic solvents
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表 1 聚乙二醇聚合度对DES催化性能的影响
Table 1 The effect of the degree of polymerization of polyethylene glycol on the catalysis of DES
HBD HBA Conversion of
α-pinene/ %Selectivity of
α-terpineol/%Distribution of reaction products/ % isomerization products e f a b c d OA − 51.5 26.7 4.5 11.6 24.2 26.5 30.4 2.8 OA PEG200 91.7 45.0 4.6 16.4 18.8 6.4 51.0 2.8 OA PEG400 58.6 50.8 7.5 15.1 15.8 5.4 52.3 3.9 OA PEG600 61.4 49.1 7.4 16.1 16.2 4.1 51.1 5.1 OA PEG2000 53.0 50.0 7.8 15.1 16.1 4.2 53.2 3.6 reaction conditions: 0.06 mol of α-pinene, 0.3 mol of water, 0.03 mol (based on OA) of catalyst, 80 ℃, 8 h; products: a (camphene),
b (limonene), c (terpinolene), d (other terpenes), e (total hydration products), f (other by-products)表 2 DES催化剂的酸量
Table 2 Acidity of DES catalyst
Catalyst Acidity/ (mmol H+·g−1) OA 24.07 OA/ 0.6PEG200 6.62 OA/ 0.6PEG400 4.49 OA/ 0.6PEG600 3.71 OA/ 0.6PEG2000 1.09 OA/ 0.8PEG200 5.22 OA/ 1.0PEG200 4.32 OA/ 1.2PEG200 3.67 OA/ 2.0PEG200 1.83 表 3 DES催化剂的密度
Table 3 Density of DES catalysts
DES ρ/(g·cm−3) OA/ 0.6PEG200 1.303 OA/ 0.8PEG200 1.256 OA/ 1.0PEG200 1.213 OA/ 1.2PEG200 1.190 OA/ 2.0PEG200 1.148 表 4 柠檬烯、异松油烯、α-松油醇的相互转化
Table 4 Mutual conversion of limonene, terpinolene, and α-terpineol
Reactants Components of the reaction mixture/ % isomers ofterpene e f g a b c d Limonene − 87.7 1.3 4.0 5.4 0.5 1.1 Terpinolene − 4.0 76.0 18.0 0.9 0.4 0.7 α-terpineol − 11.0 11.0 9.0 56.5 11.9 0.6 reaction conditions: 0.06 mol of reactants, 0.3 mol of water, 0.03 mol(based on OA) of OA/0.6PEG200, 75 ℃, 8 h; products: a (camphene), b (limonene), c (terpinolene), d (other terpenes), e (α-terpineol), f (other hydration products), g (by-products) -
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