杂多酸离子液体催化木质素C—O键和苯环氧化裂解

doi:10.11949/0438-1157.20200419

摘要/Abstract

摘要：

木质素是自然界中含量最丰富的可再生芳香聚合物，具备取代或部分替代石油用于生产大宗化学品的潜力。然而，由于木质素复杂的分子结构和化学键连接方式，当前木质素综合利用技术普遍存在目标产物选择性低等问题。因此，新型催化体系的研究对于设计与开发木质素高效转化过程具有重要意义。利用一种新型高效的杂多酸离子液体催化体系，通过选择性氧化裂解木质素的C—O键和苯环，将木质素转化为重要化工产品马来酸二乙酯，并提出了可能的反应机理。研究结果表明，当以含有β-O-4连接的典型木质素模型化合物4-甲氧基-α-[(2-甲氧基苯氧基)甲基]-苯甲醇为反应底物时，在杂多酸离子液体催化剂[BSmim]CuH₂PMo₁₀V₂O₄₀的作用下，180℃下反应5 h后，上述木质素模型化合物转化率可达到98.5%，得到84.3%(质量)和62.2%的马来酸二乙酯收率和选择性。同时，杂多酸离子液体可以通过温度的调节实现与产物的分离，简化了催化剂的回收过程，而且该催化剂具有良好的循环使用性能，重复使用6次后仍表现出较好的催化活性。

关键词: 杂多酸, 离子液体, 木质素, 马来酸二乙酯, 氧化, 选择性

Abstract:

Lignin is the most abundant renewable aromatic polymer in nature and has the potential to replace or partially replace petroleum for the production of bulk chemicals. However, due to the complex molecular structure and chemical bond connection of lignin, the current comprehensive utilization of lignin generally has problems such as low target product selectivity. Therefore, the development of novel catalysts plays a significance role in the utilization of lignin. Herein, a novel and efficient polyoxometalate ionic liquid (POM-IL) catalytic system was provided for the conversion of lignin to diethyl maleate (DEM), a universally used biochemical, through the selective cleavage of the C—O bond and benzene ring of lignin in the ethanol solvent. 4-Methoxy-α-[(2-methoxyphenoxy)methyl]-benzenemethanol, a representative lignin model compound containing b-O-4 linkage, is used as the feedstock. Over [BSmim]CuH₂PMo₁₀V₂O₄₀, 98.5% of the lignin model compound can be converted at 180℃ for 5 h, affording 84.3%(mass) yield and 62.2% selectivity of DEM. Furthermore, this POM-IL could be handily recovered through self-separation with temperature control. Moreover, the catalyst of [BSmim]CuH₂PMo₁₀V₂O₄ shows good recyclability, where a satisfactory DEM yield is demonstrated even after six consecutive runs. Moreover, a plausible pathway for this process was proposed. Therefore, this work provides a new route for the selective conversion of renewable lignin to bulk petrochemicals.

Key words: polyoxometalate, ionic liquid, lignin, dimethyl maleate, oxidation, selectivity

中图分类号:

O 643.3

林泽英,郑歆来,龙金星,刘思洁,李雪辉. 杂多酸离子液体催化木质素C—O键和苯环氧化裂解[J]. 化工学报, 2020, 71(12): 5541-5550.

LIN Zeying,ZHENG Xinlai,LONG Jinxing,LIU Sijie,LI Xuehui. Oxidative cleavage of C—O and benzene ring in lignin catalyzed by polyoxometalate ionic liquids[J]. CIESC Journal, 2020, 71(12): 5541-5550.

图/表 9

表1 杂多酸离子液体催化木质素模型化合物氧化转化

Table 1 Catalytic oxidation of lignin model compound over different POM-ILs

催化剂	Hammett酸值	转化率/%	挥发性产物收率/%(质量)			马来酸二乙酯选择性/%
催化剂	Hammett酸值	转化率/%	马来酸二乙酯	C4酯^①	其他产物^②	马来酸二乙酯选择性/%
no catalyst	—	—	—	—	—	—
[BSmim]H₄PMo₁₀V₂O₄₀	1.023	68.2	48.3	54.0	45.7	48.4
[BSmim]CuH₂PMo₁₀V₂O₄₀	0.813	98.5	84.3	90.2	45.2	62.2
[BSmim]CuHPMo₁₁VO₄₀	0.580	94.2	82.6	88.2	45.4	61.8
[BSmim]CuH₃PMo₉V₃O₄₀	0.841	77.6	61.1	65.1	41.4	57.3
[BSmim]₂CuHPMo₁₀V₂O₄₀	0.437	55.0	34.1	36.9	40.4	44.1
[BSmim]₃CuPMo₁₀V₂O₄₀	0.303	17.0	0.4	0.6	19.6	2.1
CuH₃PMo₁₀V₂O₄₀	—	95.6	81.6	86.7	46.2	61.4
[BSmim]CuH₂PMo₁₀V₂O₄₀^③	0.813	23.7	6.6	8.2	24.8	20.1
[BSmim]CuH₂PMo₁₀V₂O₄₀^④	0.813	17.4	—	—	19.7	—

图1 [BSmim]CuH2PMo10V2O40用量对催化木质素模型化合物氧化转化性能的影响(反应条件为0.5 mmol 4-甲氧基-α-[(2-甲氧基苯氧基)甲基]-苯甲醇，20 ml 乙醇， 180℃， 5 h， 1.00 MPa O2)

Fig.1 Effect of [BSmim]CuH2PMo10V2O40 dosage on the catalytic oxidation of lignin model compound

图2 反应温度对催化木质素模型化合物氧化转化性能的影响(反应条件为0.5 mmol 4-甲氧基-α-[(2-甲氧基苯氧基)甲基]-苯甲醇，0.4 mmol [BSmim]CuH2PMo10V2O40， 20 ml乙醇， 5 h， 1.00 MPa O2)

Fig.2 Effect of reaction temperature on the catalytic oxidation of lignin model compound

图3 反应时间对催化木质素模型化合物氧化转化性能的影响(反应条件为0.5 mmol 4-甲氧基-α-[(2-甲氧基苯氧基)甲基]-苯甲醇，0.4 mmol [BSmim]CuH2PMo10V2O40， 20 ml乙醇， 180℃， 1.00 MPa O2)

Fig.3 Effect of reaction time on the catalytic oxidation of lignin model compound

图4 氧气压力对催化木质素模型化合物氧化转化的影响(反应条件为0.5 mmol 4-甲氧基-α-[(2-甲氧基苯氧基)甲基]-苯甲醇， 0.4 mmol [BSmim]CuH2PMo10V2O40， 20 ml乙醇，180℃， 5 h)

Fig.4 Effect of oxygen pressure on the catalytic oxidation of lignin model compound

图5 杂多酸离子液体的循环使用性能(反应条件为0.5 mmol 4-甲氧基-α-[(2-甲氧基苯氧基)甲基]-苯甲醇， 0.4 mmol [BSmim]CuH2PMo10V2O40，20 ml乙醇，180℃， 5 h， 1.00 MPa O2)

Fig.5 Recycle performance of [BSmim]CuH2PMo10V2O40

表2 杂多酸离子液体催化剂对不同木质素模型化合物的催化性能

Table 2 Catalytic activity of POM-IL on different lignin model compounds

木质素模型化合物	转化率/%	挥发性产物收率/%(质量)			马来酸二乙酯选择性/%
木质素模型化合物	转化率/%	马来酸二乙酯	C4酯	其他产物	马来酸二乙酯选择性/%
	98.5	84.3	90.2	45.2	62.2
	100.0	104.0	110.8	86.8	52.6
	62.7	95.3	101.7	13.6	82.6
	30.1	46.0	48.0	6.2	84.9
	42.7	20.5	21.1	34.2	37.1

图6 木质素模型化合物选择性氧化转化产物的GC-MS分析(反应条件为0.5 mmol 4-甲氧基-α-[(2-甲氧基苯氧基)甲基]-苯甲醇， 0.4 mmol [BSmim]CuH2PMo10V2O40，20 ml 乙醇， 180℃， 5 h， 1.00 MPa O2)

Fig.6 GC-MS profile of products from the selective oxidation of lignin model compound

图7 木质素模型化合物转化反应机理

Fig.7 Proposed reaction mechanism for lignin model compound conversion

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