Light-driven site-selective O-H activation in lignin by triplet excited alkylanthraquinone at simulated natural conditions

doi:10.1016/S1872-2067(25)64764-4

Abstract

Abstract:

Harnessing photocatalyzed hydrogen atom transfer (HAT) for the precise activation of C-H/O-H bonds is a pivotal yet challenging strategy to selectively drive oxidative C-C bond scission in renewable lignin, yielding value-added chemicals with exceptional selectivity. Herein, we present a metal-free photochemical strategy that enables selective C-C bond scission in lignin via a unique synergistic HAT pathway driven by triplet-excited 2-ethylanthraquinone (EAQ*) and hydroxyl radicals (^•OH) generated in situ from EAQH₂ and O₂. Under simulated natural conditions, this process achieves a benzaldehyde yield of 146.6 mol% from a lignin-derived phenolic dimer. Mechanistic investigations reveal that preferential activation of the C_α-OH in lignin facilitates a tandem HAT process, forming alkoxy radical intermediates that undergo β-scission to produce benzaldehyde, as corroborated by extensive control reactions and density functional theory calculations. Furthermore, this straightforward protocol efficiently cleaves the C-C bonds of technical kraft lignins, providing a rapid, scalable, and metal-free protocol for lignin valorization under mild conditions.

Key words: Lignin, Oxidative degradation, Photoredox organocatalyst, 2-Ethylanthraquinone, Benzaldehyde

Lixia Li, Xiuqi Li, Feiyue Li, Xiang Zhen, Mingdong Dong, Jinxing Long, Xiaobing Wang, Zhiyong Jiang. Light-driven site-selective O-H activation in lignin by triplet excited alkylanthraquinone at simulated natural conditions[J]. Chinese Journal of Catalysis, 2025, 76: 65-80.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64764-4

https://www.cjcatal.com/EN/Y2025/V76/I9/65

Figures/Tables 9

Fig. 1. Selective C-C bond cleavage and functionalization via photoredox catalysis.

Table 1 Depolymerization of a model β-O-4 lignin compound over various catalysts a.

Entry	Catalyst	Conv. (%)	Yield (mol%)					Sel. of 2 (%)	Carbon balance (%)
Entry	Catalyst	Conv. (%)	2	3	4	5	Total	Sel. of 2 (%)	Carbon balance (%)
1	No catalyst	<1	—	—	—	—	—	—	—
2		87.7	112.2	27.8	17.3	3.9	161.2	69.6	92.7
3		20.2	—	—	—	18.6	18.6	—	—
4		13.5	—	—	—	—	—	—	—
5		46.7	44.0	10.7	13.7	1.9	70.3	62.6	75.2
6		81.6	97.6	22.5	15.5	3.3	138.9	70.3	85.8
7		94.4	109.9	29.3	12.2	3.7	155.1	70.9	83.2
8		92.7	130.6	18.5	26.5	2.5	168.1	71.7	95.4
9		100	127.2	28.9	16.7	2.1	174.9	72.7	87.3
10		88.5	129.9	14.3	21.3	2.5	168.0	77.3	94.6
11		71.2	98.4	12.4	21.0	2.9	134.7	73.1	94.5
12		93.8	130.4	17.7	34.5	1.9	184.5	70.7	96.7
13		<1	—	—	—	—	—	—	—
14		<1	—	—	—	—	—	—	—
15		15.3	7.6	3.5	—	—	—	—	36.3
16		9.2	1.2	—	—	—	1.2	100	6.5
17^b		<1	—	—	—	—	—	—	—

Fig. 2. (A) Correlation between catalyst dipole moments and yield of product 2. (B) Effects of solvents on catalytic performance (conditions: 0.1 mmol substrate 1, 10 mol% EAQ, hv, open air, 25 °C, 2 h). (C) Catalytic reaction under air or Ar over EAQ (conditions: 0.1 mmol substrate 1, hv, 20 mol% EAQ, 25 °C, 2 h). (D) Different wavelengths. UV-vis spectra (E) of substrate 1, EAQ and reaction liquid at various reaction times and magnification UV-vis spectra (F) of reaction solution at various reaction times.

Fig. 3. (A) LUMO (top) and HOMO (bottom) of AQ, AO, EAQ, AC, and EAC in the electronic ground state and triplet excited state. (B) Minimum energy structures of EAQ in the electronic ground state (left) and triplet excited state (right). (C) Relationship between HOMO energy level of the donor and the conversion of 1. HOMO energy levels of the donor were calculated at the B3LYP/6-31 + G* level.

Fig. 4. (A) Scatter plot of products 2, 3, 4 and 5 at each time point in the degradation of substrate 1. Control experiments (B) and Kinetic isotope effects (C) with deuterium labelled compounds. Standard conditions: substrate (0.1 mmol), EAQ (20 mol%), CH3CN (2 mL), hv, open air, 25 °C, 2 h.

Table 2 Photocatalytic conversion of substrate 1, 1-D and 1’-D in CH3CN and CD3CN.

Entry	Substrate	Solvent	Conv. (%)	Yield (mol%)
Entry	Substrate	Solvent	Conv. (%)	2 + (2-D)	3 + (3-D)	4 + (4-D)	5 + (5-D)	Total
1		CH₃CN	100	146.6	33.2	16.0	1.6	197.4
2		CD₃CN	100	121.2	26.1	13.1	—	160.4
3		CH₃CN	100	137.6	22.1	23.2	—	182.9
4		CD₃CN	100	132.1	21.6	20.0	—	173.7
5		CH₃CN	100	120.0	16.8	15.4	—	152.2
6		CD₃CN	100	118.0	17.9	12.3	—	148.2

Fig. 5. (A) Effect of radical scavenger on the photocatalytic performance of substrate 1. EPR spin trap spectra to detect the reaction intermediates during photoconversion using the trapping agents DMPO to trap O2-•, R• and RO• radicals in CH3OH (B), DMPO to trap •OH in H2O (C), and TEMP to trap 1O2 in H2O (D), under simulated environmental conditions.

Fig. 6. (A) Free energy profile of EAQ-promoted photocatalytic Cα-Cβ bond cleavage of lignin. (B) Reactions with a hydroxyl radical. (C) Proposed reaction pathway.

Fig. 7. 2D HSQC NMR spectra of original bagasse lignin (A,B), recovered bagasse lignin (C,D), original kraft lignin (E,F) and recovered kraft lignin (G,H). Reaction conditions: 50 mg lignin, 2.3 mg mL-1 EAQ, 10 mL CH3CN, 25 °C, open air, 10 h.

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