Selective depolymerization of lignin to methyl p-coumarate catalyzed by metal oxides
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摘要: 木质素是自然界中含量最丰富的芳香族可再生碳资源,具有极高的利用价值。针对当前木质素解聚技术存在反应条件苛刻、产物选择性低等难题,构建了一种廉价的金属氧化物催化剂体系,研究了草本木质素选择性解聚制备对香豆酸甲酯的性能。考察了不同金属氧化物、反应温度、反应时间以及溶剂等因素对于对香豆酸酯收率和选择性的影响。研究结果表明,金属氧化物ZnO对于草本木质素选择性解聚制备对香豆酸甲酯的过程具有最佳的催化活性。在优化的反应条件下,可获得9.8%的对香豆酸甲酯收率和61.6%的选择性;通过对木质素解聚产物的分析并结合反应前后木质素的傅里叶红外光谱(FT-IR)和二维核磁(2D HSQC NMR)表征结果发现,木质素分子中H结构单元的选择性断裂是其催化解聚过程中对香豆酸甲酯收率和选择性高的主要原因。Abstract: As the most abundant and renewable aromatic source on the earth, lignin is a good alternative to fossil fuel on producing versatile petrochemicals and biofuel. However, current techniques for lignin conversion generally suffer from some key problems of harsh reaction condition and low selectivity of products. In this study, an efficient process was provided for selective depolymerization of herbaceous lignin to a fine chemical of methyl p-coumarate (MPC) by using cost-effective catalysts of metal oxides. The influences of different metal oxides, reaction temperature, time and solvent on the yield and selectivity of MPC were systematically investigated. The results showed that ZnO exhibited the best catalytic activity, where the yield and selectivity of MPC reached 9.80% and 61.6%, respectively, at the optimized reaction conditions. Furthermore, the results of products distribution and comparative investigation on the raw and unreacted lignin using FT-IR and 2D HSQC NMR spectra demonstrated that the efficient cleavage of the ester linkage in lignin was responsible for this good MPC yield and selectivity. Therefore, this work provides a new insight on producing fine chemicals from the renewable lignin.
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Key words:
- lignin depolymerization /
- metal oxide /
- methyl p-coumarate
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表 1 不同金属氧化物催化木质素选择性转化[a]
Table 1 Selective conversion of lignin over different metal oxide catalysts
Catalyst Conversion /% Yield of volatile products w/% Selectivity of MPC /% total MPC 4-hydroxystyrene other None 35.7 7.36 0.00 1.19 6.17 0 Nb2O5 42.5 8.64 0.00 4.37 4.28 0 ZrO2 43.2 8.43 0.00 3.10 5.33 0 CuO 45.1 7.98 1.38 4.33 2.27 17.3 TiO2 47.8 9.00 1.56 2.70 4.73 17.3 Fe2O3 48.3 9.13 1.77 2.89 4.48 19.4 CeO2 49.1 10.26 3.27 1.64 5.35 31.9 Mn2O3 48.9 10.77 4.95 2.33 3.49 46 CaO 51.2 10.84 8.01 0.84 1.99 73.9 ZnO 53.3 15.90 9.80 0.57 5.53 61.6 ZnO[b] 50.3 11.94 5.87 0.51 5.56 49.2 [a] reaction conditions: 0.1 g lignin, 0.05 g catalyst, 15 mL CH3OH, 1.0 MPa N2, 165℃, 8 h
[b] commercial ZnO表 2 不同制备方法获得的ZnO催化剂的比表面积、孔径和孔容
Table 2 Surface area, pore diameter and pore volume of different ZnO
Sample SBET /(m2·g−1) dp[a] /nm vp[b] /(cm3·g−1) As-prepared ZnO 16.5 4.93 0.04 Commercial ZnO 13.2 4.67 0.03 [a] average pore diameter determined by the BJH analysis
[b] total pore volume determined by the BJH analysis表 3 溶剂效应对木质素解聚的影响
Table 3 Solvent effect for selective lignin depolymerization
Solvent Product Yield of volatile products w/% Selectivity of p-HCE /% total p-HCE other Methanol methyl p-coumarate 15.9 9.8 6.1 61.6 Ethanol ethyl p-coumarate 12.4 4.0 8.4 32.6 n-propanol propyl p-coumarate 11.4 3.9 7.5 34.0 n-butanol butyl p-coumarate 8.3 2.3 6 28.0 Reaction conditions: 0.1 g lignin, 0.05 g ZnO, 15 mL solvent, 1.0 MPa N2, 165 ℃, 8 h; p-HCE represented the ester p-coumarate 表 4 木质素各类结构在2D HSQC NMR图谱中的归属
Table 4 Assignment of main lignin 13C-1H cross-signals in the 2D HSQC NMR spectra
Lable δC/δH Assignments Methoxyl 55.6/3.7 C–H in methoxyls Aα 72.3/4.9 Cα–Hα in β–O–4 units (A) Aβ(H/G) 80.3/4.5 Cβ–Hβ in β–O–4 substructures linked to H/G units (A) Aβ(S) 85.5/4.2 Cβ–Hβ in β–O–4 substructures linked to S units (A) Aγ 60.5/3.6 Cγ–Hγ in β–O–4 substructures (A) A’(γ-pCA) 63.2/4.4 Cγ–Hγ in γ-pCA of β–O–4 (A) Bα 87.4/5.5 Cα–Hα in phenylcoumaran substructures (B) Bβ 51.2/3.9 Cβ–Hβ in phenylcoumaran substructures (B) Bγ 62.3/3.8 Cγ–Hγ in phenylcoumaran substructures (B) Cα 83.6/5.0 Cα–Hα in resinol substructures (C) Cβ 53.5/3.5 Cβ–Hβ in resinol substructures (C) Cγ 70.3/4.1 Cγ–Hγ in resinol substructures (C) H2/6 128.4/7.2 C2,6–H2,6 in H units (H) G2 111.6/7.0 C2–H2 in guaiacyl units (G) G5 114.9/6.6 C5–H5 in guaiacyl units (G) G6 120.2/6.8 C6–H6 in guaiacyl units (G) S2,6 104.2/6.7 C2,6–H2,6 in syringyl units (S) S’2,6 106.9/7.4 C2,6–H2,6 in oxidized S units (S′) pCA2/6 130.7/7.5 C2,6–H2,6 in p-coumarate (pCA) pCA3/5 116.2/6.9 C3,5–H3,5 in p-coumarate (pCA) pCA7 145.3/7.5 C7–H7 in p-coumarate (pCA) pCA8 114.3/6.3 C8–H8 in p-coumarate (pCA) FA2 111.6/7.4 C2–H2 in ferulate (FA) FA6 123.2/7.2 C6–H6 in ferulate (FA) FA7 145.0/7.4 C7–H7 in ferulate (FA) -
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