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

Photocatalysis has high potential in the cleavage of Cβ–O bond in lignin into high-value aromatic monomers; however, the inefficient Cα–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 Cβ–O bond can be improved by the generation of the Cα radical intermediate through Cα–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 Cα–H bond in PP-ol from 2.38 to 1.87 eV, therefore improving the Cα–H bond activation. The isotopic experiments of H2O/D2O indicate that the efficiency of *OH generation is an important step in Cα–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.


Photoelectrochemical (PEC) measurements
The photoelectrochemical (PEC) measurements were performed in a standard three-electrode system using an electrochemical workstation (CHI660E, Chenhua, shanghai).The indium tin oxide (ITO) glass with photocatalysts served as the working electrode, with a platinum foil as the counter electrode and an Ag/AgCl electrode as the reference electrode.The electrolyte consisted of 30 mg PP-ol and 0.2 M Na 2 ClO 4 in a mixture of 20 mL CH 3 CN and 30 mL H 2 O.
A xenon arc lamp (manufactured by Perfect Light Company) equipped with a PE300BF type light bulb and a 420 nm UV filter was used as light source.For the working electrode, 5 mg of photocatalysts was dispersed in 40 μL ethanol with 5 μL Nafion.The obtained slurry was then evenly spread onto a 3.0 × 1.0 cm 2 conducting ITO glass substrate with an active area of about 1.0 cm 2 and then dried in air.

Alkylation and Regeneration Experiments
The alkylation and regeneration of thiol groups experiments were modified from the method reported in the literature 1 .For the inhibition of thiol groups on the 3% MoS 2 /ZIS-300 photocatalyst surface, 50 mg of 3% MoS 2 /ZIS-300 and 20 μL of BPTMOS were added into 10 mL of cyclohexane and then stirred at 80 °C for 6 h.After the reaction, the BPTMOS treated 3% MoS 2 /ZIS-300 was collected by centrifugation and rinsed with cyclohexane several times before being dried under vacuum at 60 °C for 4 h.As for the regeneration, the inhibition of -SH groups on the surface could be removed by an aqueous solution of NaSH.Specifically, 30 mg of BPTMOS treated 3% MoS 2 /ZIS-300 photocatalyst and 100 mg of NaSH were added into 10 mL of deionized water, and then the mixture was stirred at 60 °C for 2 h.After the regeneration, the obtained 3% MoS 2 /ZIS-300 was collected by centrifugation.The sample was washed with water several times and then dried in vacuum oven at 60 °C for 4 h.

Coumarin (Cou)
PL with Cou was used as a molecular probe to evaluate the relative concentration of *OH radical intermediates in the reaction system 2 .Specifically, 0.1 mM Cou was added to the reaction system with varying ratios of water from 0 to 0.8.3% MoS 2 /ZnIn 2 S 4 photocatalysts

S3
were dispersed by magnetic stirring, and the reactor was sealed tightly after 30 min of argon purge (10 mL min -1 ).The sealed reactor with 200 rpm of magnetic stirring was illuminated under 1 h of visible light irradiation.The solution was collected by centrifugation and transferred into a cuvette.Photoluminescence (PL) measurements (RF-6000, Shimadzu) were conducted at an excitation wavelength of 335 nm to detect the relative concentration of *OH radical intermediates in different ratios of water.

PL Emission Spectra of 3% MoS 2 /ZIS-300 with and without PP-ol
In the photocatalytic reaction system, the interaction between photogenerated charge carriers and PP-ol is a crucial step in the cleavage of the C β -O bond in PP-ol to aromatic monomers.Therefore, PL measurements were conducted with and without PP-ol to evaluate this interaction.For the experiment, 10 mg 3% MoS 2 /ZIS-300 photocatalysts were added to the reaction solvent (CH 3 CN/H 2 O, v/v = 2/3) either with or without 10 mg of PP-ol.The mixture was then transferred to a cuvette.Photoluminescence (PL) measurements (RF-6000, Shimadzu) were performed at an excitation wavelength of 420 nm to detect the emission intensity of the prepared solution, both with and without PP-ol.

Scavengers Controlled Experiments
In the PP-ol conversion process, the photogenerated holes (h + ) and electrons (e -), hydroxyl radicals from water oxidation and the formation of C α radical intermediates through activation of C α -H bond in PP-ol play critical roles in improving conversion rate to desirable aromatic monomers.To investigate these roles, different scavengers were used to capture C α radicals, h + , and e -.These scavengers include radical scavengers (30 mg of DMPO), C α radical scavengers (30 mg of TEMPO), hole scavengers (20 mg of Na 2 S and 10 mg of Na 2 SO 3 ), hydroxyl radical scavengers (0.1 mM of Cou) and electron scavengers (30 mg of Na 2 S 2 O 8 ).In detail, 10 mg of PP-ol, 10 mg of 3% MoS 2 /ZIS-300, and the specific amount of respective scavengers were added to the reaction solvent (CH 3 CN/H 2 O, v/v = 2/3).After 1 h of visible light irradiation, the obtained solution was analyzed by GC to calculate the conversion rate of PP-ol and the yields of various products.

Lignin Extraction from Ground Birch Sawdust
Lignin was extracted from ground birch sawdust using a Soxhlet extractor 3,4 .In detail, 10 g of ground birch sawdust was mixed with 160 mL of ethanol, 40 mL of water, and 4 mL of aqueous HCl solution (37%) in a Soxhlet extractor.The extraction process was performed at a temperature of 80 °C.After 20 h of extraction, the extracted lignin was collected by rotary evaporation.The obtained lignin was then washed with water several times and vacuum dried at 60 ℃ for 2 h.

Theoretical Calculation Details:
Vienna ab-initio simulation package (VASP) was used to conduct the density functional theory (DFT) calculations.The projector augmented wave (PAW) potential and Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) for exchange-correlation function were applied to describe the ionic cores [5][6][7][8] .A plane wave basis set with cutoff energy was conducted at 500 eV.The convergence criterion for total energy was set at 10 −5 eV and the residual force was 0.02 eV Å −1 .Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method with a width of 0.10 eV.Electronic energy was considered self-consistent when the energy change of the whole simulated system was smaller than 10 −7 eV.Geometry optimization was considered convergent when the energy change was smaller than 10 −6 eV.Grimme's DFT-D3 methodology was used to describe the dispersion interactions among all the atoms.Bulk ZnIn 2 S 4 and MoS 2 were calculated using their primitive cells, incorporating k-point meshes of 5 × 5 × 5 and 16 × 16 × 16, respectively, to ensure accurate Brillouin zone sampling.To simulate the ZnIn 2 S 4 monolayer accurately and avoid interactions from periodic images, a vacuum region of 15 Å was introduced.For the heterojunction model comprising a half-unit-cell MoS 2 /ZnIn 2 S 4 monolayer, both components were simulated with a 3 × 3 × 3 kpoint mesh and separated by a 15 Å vacuum region, ensuring minimal interaction across the periodic boundary in the two-dimensional monolayer system.The quantitative analysis of PP-ol and products was determined using the internal standard method, which helps minimize errors during the measurement of reaction solutions.
Specifically, various weights of substrates were added to 5 mL of reaction solution along with 8 mg of methylparaben, used as the internal standard (ISTD), to prepare standard samples.These samples were then tested using GC equipment (Figure S1a).The following equation based on the calibration curve was used to determine the unknown weight of compounds: Where A is the peak area of a compound, A ISTD is the peak area of internal standard, m chemcials is the weight of a compound.a is the slope compensation factor and b is the constant compensation factor, which is determined from the calibration process.
Calibration equations were obtained for different chemical compounds as shown in Figure S1b and the inset table in Figure S1b.In these equations, y represents A/A ISTD , and x represents m chemcial .The errors (R²) of the fitting equations are all above 0.999.Based on the calculated weights of different chemical compounds, the conversion rate of PP-ol and selectivity/yield of the products can be accurately determined.GC-MS analysis was conducted to qualify the generated products from photocatalytic conversion of PP-ol.The results were then compared with the standard mass spectra of these products in the library.As shown in Figure S10, both the standard and detected mass spectra of phenol, acetophenone, PP-ol, and DB-one from GC-MS analysis were presented to S17 confirm the generated products were phenol, acetophenone, and DB-one.For PP-one, the standard mass spectra were not available in the GC-MS library, so we compared it with previously published results 9 and confirmed the generation of PP-one is one of byproducts in the photocatalytic conversion of PP-ol.

Figure S1 .
Figure S1.(a) GC spectra of various weights of chemicals (0.5 mg, 1 mg, 2 mg, 3 mg, 5 mg, 7 mg, 8 mg, 10 mg) for calibration to calculate the conversion rate of PP-ol and the selectivity/yield of all generated products.The chemicals include phenol, acetophenone, PPol, PP-one, and DB-one.(b) Calibration fitting curves for these chemicals based on the GC results.

Figure S11 .
Figure S11.The 1 H and 13 C NMR spectra for standard chemicals and the reaction solution.For NMR analysis of standard chemicals, chemical is 10 mg, solvent (CH 3 CN/H 2 O (v/v = 2/3)) is 2.5 mL, D 2 O is 2.5 mL; for NMR analysis of reaction solution, 2.5 mL reaction solution is mixed with 2.5 mL D 2 O.Typical reaction condition: lignin model compound PPol is 10 mg, photocatalyst is 10 mg, solvent (CH 3 CN/H 2 O (v/v = 2/3)) is 5 mL, Ar is at 1 atm, visible light power is 0.35 W cm -2 , 1h.

Table S1 .
The results of elemental analysis by ICP-OES.

Table S2 .
Photocatalytic performance of heterogenous catalysts for PP-ol conversion in literature.