Selective depolymerization β−O−4 linkage of lignin over Pd/C and NaOH
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摘要: β−O−4醚键是木质素结构中含量最丰富的单元间连接键型,研究高效断裂β−O−4的催化体系对木质素解聚制备单酚具有重要意义。本研究以β−O−4型二聚体模型化合物为原料,结合GC-MS、GC-FID、HSQC NMR表征手段,考察炭负载金属催化剂、反应温度、时间、氢气初始压力等因素对二聚体β−O−4键的断键活性以及单体收率的影响。结果表明,NaOH与炭负载金属催化剂存在协同作用,可以增强β−O−4断键活性。其中,NaOH与Pd/C协同效果最佳,二聚体解聚单体产物从44.1%提高至83.4%。机理研究表明,NaOH协同Pd/C能有效抑制二聚体发生Cα羟基的脱除,显著提升二聚体β−O−4的断键选择性,从而提高了单体产物的收率。NaOH协同Pd/C催化体系对其他醚键(α−O−4)同样存在优异的断键能力。因此,在所做实验的最佳条件下,NaOH协同Pd/C催化体系能高效解聚碱木质素制备单酚化合物,单体产物收率高达37.5%,苯甲醇类选择性高达48.8%。Abstract: β–O–4 is the most abundant linkage in the lignin structure. It is of great significance to convert lignin into monophenols by breaking the β−O−4 linkage. Therefore, β−O−4 dimer model compound was used as raw materials in this paper. The effects of metal catalysts, temperature, time and hydrogen pressure on the conversion of dimer and yield of monophenols were investigated by GC-MS, GC-FID, HSQC NMR characterization methods. The results show that NaOH and carbon-supported metal catalysts have a synergistic effect which can enhance the breakage of β−O−4 linkage, and the best promotion effect is obtained over Pd/C and NaOH with the monomer yield increases from 44.1% to 83.4%. NaOH and Pd/C can inhibit the removal of Cα−OH of the dimer and enhance the breakage of β−O−4 linkage effectively, leading to the increment in monomers. The NaOH and Pd/C catalytic system also shows excellent performance to the breakage of α−O−4. Under the best conditions, alkali lignin is converted into monophenols over NaOH and Pd/C with the yield of 37.5%, and the selectivity of benzyl alcohol is as high as 48%.
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Key words:
- synergistic effect /
- alkali lignin /
- Pd/C /
- NaOH /
- β−O−4
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表 1 催化剂对β–O–4二聚体模型化合物的影响
Table 1 Effect of catalyst on the depolymerization of β–O–4 dimer
Col Catalyst Conv./% Selectivity/% Aromatics/% Mass balance 2 3 4 5 6 7 8 1 Ru/C 99.3 5.2 25.5 3.3 4.6 nd 15.4 51.2 33.7 93.3 2 Pd/C 85.4 nd 38.9 5.0 nd nd 29.9 26.2 44.1 89.4 3 Co/C 69.5 nd 15.8 12.5 nd nd nd 71.7 11.4 95.8 4 Ni/C 63.5 nd 41.7 26.8 nd nd 17.6 14.0 41.0 90.2 5 Pt/C 28.4 nd 37.2 30.3 nd nd 14.2 18.3 14.9 92.8 6 NaOH 87.8 nd 52.2 29.7 18.1 nd nd nd 58.6 70.8 7 Ni/C-NaOH 69.3 5.9 58.1 14.5 15.0 nd 12.4 nd 58.3 89.0 8 Ru/C-NaOH 68.4 6.3 56.4 27.4 7.8 8.4 nd nd 60.9 92.5 9 Pd/C-NaOH 96.5 nd 49.6 40.0 0.9 nd 9.5 nd 83.4 86.9 10 Pt/C-NaOH 86.9 5.1 58.2 26.8 9.6 5.5 nd nd 77.1 90.2 11 Co/C-NaOH 87.4 5.0 51.4 9.8 8.2 nd 26.5 4.1 69.1 86.9 note 1: reaction condition (0.1 g β−O−4 dimer,30 mL EtOH,0.1 g M/C,0.1 g NaOH,150 ℃,3 h,2 MPa H2);
note 2: nd (NOT DETECT);
note 3: aromatics yield was calculated from products 2−7;
note 4: product 2-8 are α-(ethoxymethyl) benzyl alcohol, guaiacol, α-phenethyl alcohol, β-phenethyl alcohol, styrene, phenethyl alkane, 1-methoxy-2-phenethoxybenzene, respectively表 2 不同催化剂解聚产物随温度变化的影响
Table 2 Effect of temperature on depolymerization products over different catalysts
Temperature/℃ Conv./% Selectivity/% Aromatics/% Mass balance 2 3 4 5 6 7 8 Pd/C 120 63.8 1.8 32.9 34.8 nd 1.4 0.3 28.8 31.2 90.7 130 68.8 2.1 33.9 29.0 nd 1.1 7.2 26.7 36.7 92.7 140 77.9 1.1 33.5 25.0 nd 0.8 10.8 28.7 39.5 91.7 150 85.4 nd 38.9 5.0 nd nd 29.9 26.2 44.1 89.4 NaOH 120 43.7 nd 51.1 31.8 17.1 nd nd nd 26.4 82.6 130 50.2 nd 48.5 28.2 23.2 nd nd nd 29.9 79.7 140 78.4 nd 50.6 29.6 19.9 nd nd nd 53.5 75.1 150 87.8 nd 51.3 29.2 19.5 nd nd nd 59.6 70.8 Pd/C-NaOH 120 58.8 1.7 47.4 17.9 19.3 5.1 8.6 nd 48.9 90.1 130 68.2 5.1 47.7 20.6 2.3 16.4 7.9 nd 58.0 89.7 140 87.6 2.7 49.5 30.8 1.5 7.0 8.5 nd 77.2 89.5 150 96.5 nd 49.6 40.0 0.9 nd 9.5 nd 83.4 86.9 160 92.4 nd 46.4 39.8 0.8 nd 13.0 nd 79.5 87.1 120−24 h① 93.2 nd 47.9 40.3 nd nd 11.8 nd 81.2 88.0 note 1: 0.1 g β–O–4 dimer,30 mL EtOH,0.1 g Pd/C,0.1 g NaOH,3 h,2 MPa H2;
note 2: nd: NOT DETECT; ①: reaction condition(120 ℃,24 h,2 MPa H2)表 3 初始氢气压力对β−O−4模型化合物解聚的影响
Table 3 Effect of hydrogen pressure on the depolymerization of dimer
Pressure/MPa Conv./% Selectivity/% Aromatics/% Mass balance 2 3 4 5 6 7 8 0 46.3 5.9 46.0 28.3 1.1 16.4 2.4 nd 31.5 85.2 1 84.7 1.8 52.2 42.2 1.0 0.2 2.6 nd 71.5 86.8 2 96.5 nd 49.6 40.0 0.9 nd 9.5 nd 83.4 86.9 3 94.3 nd 50.5 23.2 1.3 nd 25.1 nd 82.3 88.0 note 1: 0.1 g β−O−4 dimer,30 mL EtOH,0.1 g Pd/C,0.1 g NaOH,150 ℃,3 h; note 2: nd (NOT DETECT) 表 4 NaOH协同Pd/C解聚碱木质素单体及收率
Table 4 Products of alkali lignin depolymerization over Pd/C and NaOH (%)
Product Pd/C NaOH Pd/C-NaOH 3-methyl-phenol 2.3 0.7 0.7 2,3-dimethyl-phenol 1.0 0.2 0.1 2,3,4,6-tetramethyl-phenol 0.5 0.9 0.8 4-ethyl-guaiacol 2.1 1.6 1.9 4-methyl-benzyl alcohol 1.0 6.9 10.3 3-methyl-benzyl alcohol 0.5 1.3 6.3 2,4,6-trimethylbenzyl alcohol 3.3 1.3 1.7 2,4-dimethyl-ethylbenzene 1.9 0.3 1.7 1,2,6-trimethyl-cyclopentane 0.1 0.4 2.0 2-ethyl-5-methyl-cyclohexane 3.5 1.1 0.8 1-methyl-2-ethyl-cyclopropane 0.5 0.3 0.7 1-phenylpropane-1,2-diol 0.4 1.2 0.9 2-methylbenzyl formate 0.7 1.2 0.8 3-phenylbut-3-en-1-ol 1.0 1.6 4.2 2-phenyl-butanol 0.3 0.5 4.7 Total yield 19.1 19.4 37.5 note 1: 0.5 g alkali lignin,0.2 g Pd/C,0.125 mol/L NaOH,24 mL EtOH,16 mL H2O,260 ℃,4 h,2 MPa H2 -
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