Engineered Half-Unit-Cell MoS2/ZnIn2S4 Monolayer Photocatalysts and Adsorbed Hydroxyl Radicals-Assisted Activation of Cα-H Bond for Efficient Cβ-O Bond Cleavage in Lignin to Aromatic Monomers

Abstract

Photocatalysis has high potential in the cleavage of C_β-O bond in lignin into high-value aromatic monomers; however, the inefficient C_α-H bond activation in lignin and a low hydrogen transfer efficiency on the photocatalyst's surfaces have limited its application in photocatalytic lignin conversion. This study indicates that the cleavage of the C_β-O bond can be improved by the generation of the C_α radical intermediate through C_α-H bond activation, and the formation of desirable aromatic products can be significantly improved by the enhanced hydrogen transfer efficiency from photocatalyst surfaces to aromatic monomeric radicals. We elaborately designed the half-unit-cell MoS₂/ZnIn₂S₄ monolayer with a thickness of ∼1.7 nm to promote the hydrogen transfer efficiency on the photocatalyst surfaces. The ultrathin structure can shorten the diffusion distance of charge carriers from the interior to the surfaces and tight interface between MoS₂ and ZnIn₂S₄ to facilitate the migration of photogenerated electrons from ZnIn₂S₄ to MoS₂, therefore improving the selectivity of desirable products. The adsorbed hydroxyl radical (*OH) on the surfaces of MoS₂/ZnIn₂S₄ from water oxidation can significantly reduce the bond dissociation energy (BDE) of C_α-H bond in PP-ol from 2.38 to 1.87 eV, therefore improving the C_α-H bond activation. The isotopic experiments of H₂O/D₂O indicate that the efficiency of *OH generation is an important step in C_α-H bond activation for PP-ol conversion to aromatic monomers. In summary, PP-ol can completely convert to 86.6% phenol and 82.3% acetophenone after 1 h of visible light irradiation by using 3% MoS₂/ZnIn₂S₄ and the assistance of *OH, which shows the highest conversion rate compared to previous works.

Keywords: Cβ−O bond cleavage; activation of Cα−H bond; half-unit-cell MoS2/ZnIn2S4 monolayer; hydrogen transfer efficiency; lignin valorization.

Scheme 1. C_α Radical Intermediate Mechanism of C_β–O Bond Fragmentation in PP-ol over Reported Photocatalysts

Reproduced from refs (3,13), Copyright [2017, 2019, and 2020] American Chemical Society.

Figure 1

(a) Schematic diagram for the preparation of a half-unit-cell MoS₂/ZnIn₂S₄ monolayer. AFM images and the corresponding height profile of ZIS-300 (b, c) and 3% MoS₂/ZIS-300 (d, e). The inset graphs in (b) and (d) correspond to particle thickness distributions. TEM and HRTEM images, and element mappings of ZIS-300 (f, g, j) and 3% MoS₂/ZIS-300 (h, i, k).

Figure 2

(a) UV–vis diffuse reflectance spectra of ZIS-0, ZIS-300, and x% MoS₂/ZIS-300. (b) Band structure diagram of ZIS-300 and x% MoS₂/ZIS-300. PEC properties of x% MoS₂/ZIS-300: (c) transient photocurrent responses and (d) EIS spectra. (e) DOS of half-unit-cell ZIS-300 and half-unit-cell MoS₂/ZIS-300. (f) Charge density distribution of half-unit-cell MoS₂/ZIS-300. The blue and red parts indicate the accumulation and depletion of electrons, respectively.

Scheme 2. Photocatalytic Conversion of Lignin Model (PP-ol) to Phenol, Acetophenone, PP-One, and DB-One

Figure 3

Conversion rate of PP-ol and the yields of products with (a) x% MoS₂/ZIS-300 and (b) BPTMOS-treated and NaSH-regenerated 3% MoS₂/ZIS-300. Reaction condition: lignin model compound PP-ol is 10 mg, photocatalyst is 10 mg, solvent (CH₃CN/H₂O (v/v = 2/3)) is 5 mL, Ar is at 1 atm, and visible light power is 0.35 W cm^–2, 1 h.

Figure 4

DFT calculation for E_PP-ol, BDE of C_α–H bond, and L_Cα–H on three conditions: (1) MoS₂/ZIS-300 surfaces without *H and *OH, (2) MoS₂/ZIS-300 surfaces with *H, and (3) MoS₂/ZIS-300 surfaces with *OH.

Figure 5

(a) Photocatalytic conversion of PP-ol in H₂O–CH₃CN and D₂O–CH₃CN solvent conditions via 1 h of visible light irradiation. (b) Effect of water amount on the conversion rate of PP-ol and the yields of products. (c) Fluorescence spectra of coumarin solution with different ratios of H₂O/CH₃CN. Conversion rate of PP-ol and the yields of products with (d) effect of reaction time on conversion rate under 3% MoS₂/ZIS-300 in H₂O condition and (e) effect of reaction time on conversion rate under 3% MoS₂/ZIS-300 in D₂O condition. (f) The calculated KIE under H₂O and D₂O conditions. Reaction condition: lignin model compound PP-ol is 10 mg, photocatalyst is 10 mg, solvent (CH₃CN/H₂O (v/v = 2/3)) is 5 mL, Ar is at 1 atm, visible light power is 0.35 W cm^–2, 1 h.

Figure 6

(a) Controlled experiments by 3% MoS₂/ZIS-300 with different scavengers. Reaction conditions: lignin model compound PP-ol is 10 mg, 3% MoS₂/ZIS-300 is 10 mg, solvent (CH₃CN/H₂O (v/v = 2/3)) is 5 mL, Ar is at 1 atm, visible light power is 0.35 W cm^–2, 1 h. Hole scavengers: 20 mg of Na₂S and 10 mg of Na₂SO₃; electron scavengers: 30 mg of Na₂S₂O₈; radical scavengers: 30 mg of DMPO; C-centered radical scavengers: 30 mg of TEMPO; hydroxyl radical scavengers: 0.1 mM Cou. (b) PL emission spectra of 3% MoS₂/ZIS-300 with and without PP-ol under visible light irradiation (excitation wavelength at 420 nm). (c) Proposed mechanism of C_β–O bond fragmentation in the photocatalytic conversion of PP-ol over a 3% MoS₂/ZIS-300 photocatalyst.

Scheme 3. Conversion of Different Lignin Models to Desirable Aromatic Monomers after 2 h of Visible Light Irradiation

(a) MP-ol;(b) PPP-ol;(c) DMP-ol;(d) PEB. Reaction conditions: lignin model compounds are 10 mg, 3% MoS₂/ZIS-300 is 10 mg, solvent (CH₃CN/H₂O (v/v = 2/3)) is 5 mL, Ar is at 1 atm, and visible light power is 0.35 W cm^–2, 2 h.

Figure 7

(a) GC-MS spectra for photocatalytically converted products from wood extraction lignin solution. Inset: Solution color before and after the photocatalytic reaction. Reaction conditions: wood extraction powder is 80 mg, CdS-150 is 20 mg, H₂O and CH₃CN mixed solution (CH₃CN/H₂O (v/v = 2/3)) is 7 mL, Ar is at 1 atm, and visible light power is 0.35 W cm^–2, 10 h. (b) Conversion rate of PP-ol and yields of products with recycled 3% MoS₂/ZIS-300 under 1 h of visible light irradiation.