Enzymatic hydrolysis lignin dissolution and low-temperature solvolysis in ethylene glycol

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Sang, Yushuai; Ma, Yuhan; Li, Gen; Cui, Kai; Yang, Mingze; Chen, Hong; Li, Yongdan Enzymatic hydrolysis lignin dissolution and low-temperature solvolysis in ethylene glycol

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Chemical Engineering Journal

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10.1016/j.cej.2023.142256 E-pub ahead of print: 01/05/2023

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Sang, Y., Ma, Y., Li, G., Cui, K., Yang, M., Chen, H., & Li, Y. (2023). Enzymatic hydrolysis lignin dissolution and low-temperature solvolysis in ethylene glycol. Chemical Engineering Journal, 463, [142256].

https://doi.org/10.1016/j.cej.2023.142256

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Chemical Engineering Journal 463 (2023) 142256

Available online 10 March 2023

Enzymatic hydrolysis lignin dissolution and low-temperature solvolysis in ethylene glycol

Yushuai Sang

^a^,^b

, Yuhan Ma

^d^,^e

, Gen Li

^a

, Kai Cui

^a^,¹

, Mingze Yang

^a

, Hong Chen

^c

, Yongdan Li

^a^,^*

aDepartment of Chemical and Metallurgical Engineering, School of Chemical Engineering, Aalto University, Kemistintie 1, Espoo, P.O. Box 16100, FI-00076, Finland

bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Laboratory of Applied Catalysis Science and Technology, State Key Laboratory of Chemical Engineering (Tianjin University), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

cSchool of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China

dJoint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207, China

eDepartment of Chemical and Biomolecular Engineering, National University of Singapore, Singapore

A R T I C L E I N F O Keywords:

Catalysis Lignin Ethylene glycol Dissolution Solvolysis

A B S T R A C T

The dissolution and solvolysis processes of enzymatic hydrolysis lignin (EHL) in ethylene glycol are investigated.

Ethylene glycol exhibits high EHL solubility and achieves complete EHL dissolution at room temperature.

Gaussian simulation reveals that van de Waals interactions between ethylene glycol and EHL, including C-H⋯O and lone pair⋯π interactions, break the π-π stacking in EHL, achieving complete EHL dissolution. EHL is partly depolymerized in ethylene glycol at 200 ^◦C even without a catalyst due to the strong van de Waals interactions.

When NaOH and Ni are used as co-catalysts, EHL is efficiently depolymerized at 200 ^◦C, and the overall monomer yield reaches 18.8 wt%. Fourier transform infrared spectroscopy (FT-IR) and molecular dynamics simulation results indicate that the adsorption of ethylene glycol over Ni surface hinders the adsorption of lignin fragments and monomers. Hence, EHL catalytic solvolysis in ethylene glycol occurs in the liquid phase, where OH⁻of NaOH promotes the EHL linkage breakage and active hydrogen atoms formed on Ni surface stabilize the active monomers.

1. Introduction

Lignocellulose, the natural composite of cellulose, hemicellulose and lignin, is the most abundant form of biomass [1]. Different from cellulose and hemicellulose, lignin is composed of aromatic units, such as sinapyl (S), coniferyl (G) and p-coumaryl (H) alcohols as well as ferulic (FA) and p-coumaric (pCA) acids [1]. Nowadays, cellulose and hemicellulose are fully utilized in pulping and the second-generation (2G) bioethanol industries. The 2G biofuel industry produces enzymatic hydrolysis lignin (EHL) as a low-value and large volume byproduct. As a renewable resource of aromatic molecules, EHL is an ideal feedstock for the sustainable production of commercial aromatic chemicals and fuels.

Catalytic lignin solvolysis (CLS) is a promising route to directly produce aromatic chemicals at relatively mild reaction conditions [1]. In CLS reaction, lignin is firstly dissolved and depolymerized in a solvent before contact with a catalyst [2]. However, due to its complex and highly cross-linked structure, lignin cannot be efficiently dissolved in

most of the solvents at ambient temperature, and a high reaction temperature (~300 ^◦C) is often needed for CLS to achieve complete lignin conversion. Nevertheless, high reaction temperature also promotes the repolymerization/condensation of active monomers and self-conversion of solvent, resulting in the formation of char and complex products [3–7]. Yan and his co-workers have examined CLS at relatively low temperature (100 ~ 200 ^◦C) in water with noble metal catalysts, but, due to the low lignin solubility in water, the monomer yields are only around 7 ~ 8 wt% [8,9].

Ethylene glycol is a green solvent that can be produced from the conversion of cellulose and has been widely used for the production of chemicals and fuels [10]. Recently, ethylene glycol was used as a solvent for lignin depolymerization and fractionation, due to its high lignin solubility. For example, Song et al. [11] depolymerized lignosulfonate with a Ni/C catalyst in ethylene glycol at 200 ^◦C under 5 MPa H2, and achieved 68 wt% conversion of lignosulfonate, with 4-propyl guaiacol and 4-ethyl guaiacol as the main monomer products. Ren et al. [12] used

* Corresponding author.

E-mail address: yongdan.li@aalto.fi (Y. Li).

1 Present address: Institute of Engineering Technology, Sinopec Catalyst Co., Ltd., Beijing 101100, China.

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Chemical Engineering Journal

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https://doi.org/10.1016/j.cej.2023.142256

Received 18 November 2022; Received in revised form 1 February 2023; Accepted 1 March 2023

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ethylene glycol as a solvent for fractionation of lignin in poplar sawdust with Ru/C and H₂SO₄as co-catalysts, and obtained 24.1 wt% phenolic monomers at 185 ^◦C for 6 h. Nevertheless, the mechanism of lignin dissolution in ethylene glycol is still not clear and the steps of lignin depolymerization at low temperatures also need to be further elucidated.

Herein, EHL dissolution at room temperature and solvolysis at 200 ^◦C in different solvents are examined. Ethylene glycol achieves complete EHL dissolution and gives the highest monomer yield among the solvents examined. The interaction between ethylene glycol and lignin molecules is investigated with ¹H and ¹³C NMR and Gaussian simulation. The roles of Ni and NaOH catalysts in EHL solvolysis are discussed based on the GPC and HSQC-NMR results as well as molecular dynamics simulation. Based on these results, the mechanisms of EHL dissolution and solvolysis in ethylene glycol are proposed.

2. Materials and methods 2.1. Materials

The EHL was provided by Shandong Long Live biological technology Co., Ltd. which was obtained from the microbial enzymatic hydrolysis of corncob to produce ethanol. The composition of EHL, 91.2 wt% lignin, 0.12 wt% residual carbohydrate and 1.42 wt% ash, has been reported in our previous work [13]. The solvents (AR), including cyclohexane, ethyl acetate, isopropanol, ethanol, methanol and ethylene glycol, were purchased from VWR Chemicals. NiCl2⋅6H2O (>99.9%), NaOH (>99.9%) and NaBH4 (>98%) MgO (>99.9%), ZrO2 (>99.9%) and Al2O3

(>99.9%) were purchased from Sigma Aldrich. Ferulic acid (>99.9%) and coniferyl alcohol (>98%) were also purchased from Sigma Aldrich.

Anisole (99%) was supplied by Acros Organics.

2.2. Methods

2.2.1. Catalyst preparation

The Ni catalyst was prepared via the reduction of NiCl2⋅6H2O with NaBH4. NaOH (0.5 g) and NaBH4 (1 g) were dissolved in 30 mL deionized water and the solution formed was dropped into the solution of NiCl2 (4.05 g NiCl2⋅6H2O in 50 mL deionized water) with magnetic stirring at room temperature. The black precipitate, i.e., the Ni catalyst, was washed with 100 mL deionized water for 4 times and preserved in deionized water before use.

Solid bases, including NaOH/MgO, NaOH/ZrO2, NaOH/Al2O3, were prepared through incipient wetness impregnation technique with pre- scribed 20 wt% NaOH loading. After drying at 100 ^◦C overnight, the simples were calcined at 450 ^◦C for 4 h.

2.2.2. EHL dissolution

The mixture of EHL (1 g) and solvent (20 mL) was treated with ul- trasound for 30 min at room temperature and then left to stand at room temperature for 72 h. The EHL solution and insoluble EHL were separated with filtration, and the insoluble EHL was dried at 80 ^◦C for 24 h.

The amount of dissolved EHL was calculated with Eq. (1).

31P NMR spectra of methanol-soluble (M-soluble) and methanol- insoluble (M-insoluble) EHL were measured according to a method in literature [14]. The sample (40 mg) was dissolved in the mixture of pyridine and deuterated chloroform (1.6:1 v/v, 0.4 mL). Cholesterol (19

mg/ml, 0.2 mL) was added as an internal standard, while chromium-III- acetylacetonate (19 mg/ml, 0.05 mL) was applied as a relaxation reagent. 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (0.1 mL) was employed as a phosphitylation reagent. After phosphitylation for 2 h, the sample was moved into an NMR tube and measured with a Bruker AVANCE III HD 400 MHz instrument. The heteronuclear single quantum coherence nuclear magnetic resonance (HSQC-NMR) spectra of M-soluble and M-insoluble EHL were recorded with the same instrument. For HSQC-NMR, the sample (50 mg) was dissolved in DMSO‑d6 (0.6 mL) as the deuterated NMR solvent.

The ¹H NMR and ¹³C NMR spectra of EHL, the mixture of EHL and methanol (EHL-MET) and the mixture of EHL and ethylene glycol (EHL- EG) were acquired with the same instrument. 50 and 100 mg EHL were dissolved in 0.6 mL DMSO‑d6 for ¹H NMR and ¹³C NMR spectra, respectively. For EHL-MET and EHL-EG, methanol (0.1 mL) and ethylene glycol (0.1 mL) were added into EHL and the DMSO‑d6 mixtures, respectively.

2.2.3. EHL solvolysis

EHL solvolysis was carried out in a 50 mL batch reactor (Parr 4597, Hastelloy C-276) equipped with a temperature controller and a pressure sensor. In a typical test, EHL (1 g), Ni (1 g), NaOH (0.5 g) and ethylene glycol (25 mL) were added into the reactor. The reactor was sealed and purged with nitrogen for six times, and then purged with hydrogen for three times, and finally pressurized to 3 MPa H2 at room temperature.

The reactor was then heated to 200 ^◦C and remained for 6 h with a fixed stirring rate of 600 rpm.

After reaction, the mixture was filtrated to separate the solid catalyst and liquid product. NaOH in the liquid product is neutralized with HCl, and then the liquid product was extracted with deionized water (60 mL) and dichloromethane (30 mL). Floccule formed at the interface of two phases during extraction and was separated with a filtration technique.

The monomer products were extracted into the dichloromethane phase and were analyzed qualitatively with Shimadzu GC–MS (QP2010SE with Optic 4) and quantitatively with an Agilent 7890 GC equipped with an FID. For both GCs, the working conditions were the same. The oven temperature program was set from 45 to 250 ^◦C at 10 ^◦C/min and then held at 250 ^◦C for 7 min. The solvent delay was set as 2 min for the MS detector. HP-5 MS capillary column (30 m ×0.25 mm ×0.25 µm) and a split ratio of 50 were used. The mass detector was set to scan the m/z range from 10 to 500. Anisole was used as an internal standard to quantify the products. The total monomer yield was calculated with Eq.

(2):

The total monomer yield= The weight of total monomers

the weight of EHL added into the reaction (2) The average molecular weight of the EHL and floccule were determined with an Agilent HPLC system with Phenogel (5 μm–5 nm and 100 nm) columns and a UV detector at 280 nm⋅THF was used as an eluent at a rate of 1.0 mL min⁻¹and the analysis was carried out at room temperature. Calibration was performed using polystyrene standards ranging from 30300 g mol⁻¹to 208 g mol⁻¹. The EHL and floccule samples were acetylated before analysis to make them soluble in THF [15]. The HSQC-NMR spectra of EHL and floccule were recorded with

the same Bruker instrument. The sample (50 mg) was dissolved in DMSO‑d6 (0.6 mL) as the deuterated NMR solvent.

The amount of dissolved EHL(%) =The weight of added EHL−The weight of insoluble EHL

The weight of added EHL ×100 (1)

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2.2.4. Adsorption of ethylene glycol and lignin monomers over Ni catalyst The infrared spectra of liquid ethylene glycol as well as adsorbed ethylene glycol and ethylene glycol-lignin monomers mixture were obtained with attenuated total reflectance-Fourier transform infrared spectrometer (ATR-FTIR, PerkinElmer Co.) The scan number was 200 and the spectral resolution was set as 4 cm⁻¹. Adsorbed samples were prepared through heating Ni catalyst (0.5 g) in pure ethylene glycol (10 mL) or ethylene glycol-lignin monomer (0.1 g ferulic acid or coniferyl alcohol in 10 mL ethylene glycol) at 200 ^◦C for 1 h. After cooling, these samples were washed with acetone and dried at 60 ^◦C.

2.2.5. Simulation details

The Gaussian simulation was carried out with the Gaussian16 package using M062X simulation method in conjunction with the 6–31 g (d) basis set [16]. The interaction energies between solvent and phenol or benzene are calculated according to Eq. (3), where H is the enthalpy and BSSE is the Basis Set Superposition Error acronym.

Hint=HA+B− (HA+HB) +EBSSE (3) The non-covalent interaction (NCI) analysis is carried out with software Multiwfn and VMD [17]. Before NCI analysis, structures are firstly optimized with Gaussian simulation.

The Forcite module in Material Studio was used for molecular dynamics simulation of the competitive adsorption of lignin dimer (1-(4- hydroxyphenyl)-2-phenoxypropane-1,3-diol (Fig. 4 (b)) and ethylene glycol over Ni surface. The Ni (111) facet with a (10 ×14 ×4) supercell was chosen as the Ni surface model, above which are 400 ethylene glycol and 2 lignin dimer molecules built with amorphous cell module in Material Studio. Before the simulation, the unit cell and atomic position of the model are firstly optimized. In order to quickly obtained the optimized model, it was quenched for 5 cycles from 26.85 to 226.85 ^◦C, and then underwent an isobaric (NPT) molecular dynamics simulation for one picosecond (ps). After that, this model underwent NPT simulation at 200 ^◦C for 500 ps, and then underwent isothermal molecular dynamics (NVT) simulation at 200 ^◦C for 1000 ps. In all of these simu- lations, Nose and Berendsen were used as temperature and pressure control algorithm, respectively.

3. Results

3.1. EHL dissolution and solvolysis 3.1.1. EHL dissolution

The amount of EHL dissolved in 20 mL solvent was examined at room temperature, and the results are shown in Fig. 1(a). In cyclohexane, ethyl acetate, isopropanol, ethanol and methanol, the amount of dissolved EHL shows a positive linear relationship with the solvent polarity (δ_H). Nevertheless, this relationship does not fit EHL dissolution in ethylene glycol. The solvent polarity of ethylene glycol is close to that of methanol, but ethylene glycol achieves the complete dissolution of EHL, while methanol only dissolves 47.9% EHL. When the EHL-ethylene glycol solution is diluted with 60 mL ethanol, no EHL is precipitated.

Increasing the dosage of EHL to 3 g with keeping the volume of ethylene glycol unchanged (20 mL), the residue remaining on filter paper is not the original EHL solid particles, but a viscous liquid (Fig. S1).

The methanol insoluble EHL cannot be dissolved with fresh methanol, indicating that the soluble and insoluble parts have different structures. Therefore, the contents of hydroxyls in M-soluble and M- insoluble EHL were determined with ³¹P NMR, and the results are depicted in Fig. 1 (b). The contents of aliphatic-OH, aromatic-OH and carboxylic-OH in M-soluble EHL are 1.00, 1.37 and 0.68, respectively, much higher than that of M-insoluble EHL, which are 0.65, 0.74 and 0.35, respectively. The linkages in M-soluble and M-insoluble EHL were determined with HSQC-NMR, and the spectra are illustrated in Fig. 1 (c).

In the spectrum of M-insoluble EHL, the intensity of the signal of β-O-4 linked structures (Aγ) obviously decreases, and more intense signals of C-C linkages are detected, compared to those in the spectrum of M-soluble EHL, indicating that M-insoluble EHL has more C-C linkages and less β-O-4 linkages than M-soluble EHL.

3.1.2. EHL solvolysis

Non-catalytic EHL solvolysis reactions in different solvents were examined at 200 ^◦C under 3 MPa H2 for 6 h. As depicted in Fig. 2 (a), the monomer yields obtained show a positive correlation with the amount of dissolved EHL in these solvents, and ethylene glycol gives the highest total monomer yield, i.e., 5.5 wt%. After that, catalytic EHL solvolysis Fig. 1. (a) The relationship between the δ_Hof solvent and the amount of dissolved EHL, (b) the hydroxyl content of M-soluble and M-insoluble EHL, (c) the HSQC- NMR spectra of M-soluble and M-insoluble EHL.

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reactions in ethylene glycol were examined with Ni catalyst, with NaOH, and with both Ni and NaOH as co-catalysts under the same reaction conditions. The Ni catalyst was prepared via the reduction of NiCl2 with NaBH4, which is a classic catalyst that has been widely employed in many hydrogenation reactions (its characterizations are shown in Fig. S2) [18,19]. Fig. 2 (b) and (c) show the total monomer yields and monomer structures obtained, and Scheme S1 gives the yields of indi- vidual monomers obtained. With Ni catalyst, the total monomer yield is 8.2 wt%, lower than that obtained with NaOH, which is 14.6 wt%. When Ni and NaOH catalysts co-existed, the total monomer yield is up to 18.8 wt%. Without a catalyst, most of the monomers obtained have carbon- –carbon double bonds in their side chains, and para-alkyl phenols (para- ethyl phenol, para-ethyl guaiacol and para-propyl guaiacol) and phenols without para side chains (phenol, guaiacol and syringol) are also detected. With Ni catalyst, carbon–carbon double bonds are

hydrogenated, and para-propanol syringol appears. With NaOH as a catalyst, C2-ketone and C3-ketone substituted syringol appear, which are typical products formed in soluble-base catalyzed lignin conversion reactions [20]. When Ni and NaOH catalysts co-existed, the product distribution is similar to that obtained with only NaOH.

The effects of NaOH, with keeping 1 g Ni catalyst unchanged, and Ni, at 0.5 g dosage of NaOH, amounts on the total monomer yield are plotted in Fig. 2 (d). The total monomer yield is only 12.2 wt% when 0.25 g NaOH is added and increases to 18.8 wt% with 0.5 g NaOH.

Further increasing the amount of NaOH results in the decrease of the total monomer yield. With the increase of the amount of Ni catalyst from 0.25 to 0.75 g, the total monomer yield increases from 16.4 to 18.9 wt%, and is not obviously changed when the Ni catalyst amount further increases to 1 g.

The recyclability of the Ni catalyst is shown in Fig. S3 (a). The Ni Fig. 2.(a) The relationship between the amount of dissolved EHL and the total monomer yields obtained from non-catalyzed EHL solvolysis in different solvents, (b) the total monomer yields and (c) monomer structures obtained from EHL depolymerization in ethylene glycol without and with different catalysts, (d) the effects of the amounts of NaOH and Ni catalyst on the total monomer yields (e) the total monomer yields obtained with Ni catalyst combined with different solid bases. (f) the weight average molecular weight (Mw) and the relative intensity of β-O-4 linkage signals of EHL and the floccule obtained from EHL solvolysis. (Reaction conditions:

1 g EHL, 25 mL solvent, 200 ^◦C, 3 MPa H2, 6 h).

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Fig. 3.The (a) ¹H NMR and (b) ¹³C NMR spectra of EHL, the mixture of EHL and methanol (EHL-MET) and the mixture of EHL and ethylene glycol (EHL-EG).

Fig. 4.(a) The stable structures and interaction energies of P-MET, P-EG, B-MET and B-EG, (b) the stable structures and interaction energies of hydrogen bonds in D- EG and D-ME, (c) NCI analysis of lignin dimer, D-EG and D-MET.

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catalyst (1 g) was separated from the liquid products with filtration and then directly reused with fresh NaOH (0.5 g). During 4 runs of the Ni catalyst, the total monomer yield slightly decreases from 18.8 to 17.0 wt

%. Nevertheless, the XRD pattern of the used Ni catalyst indicates that the Ni catalyst has transformed from an amorphous phase (Fig. S2(a)) to a crystalline phase (Fig. S3 (b)) after the first time run. Hence, the phase transition of the Ni catalyst does not obviously affect its activity on EHL solvolysis.

The activities of solid bases, including MgO and NaOH supported on different metal oxides (NaOH/MgO, NaOH/ZrO2, and NaOH/Al2O3), are also examined (Fig. 2 (e)). When 0.5 g solid bases are added with 1 g Ni as co-catalysts, the total monomer yields obtained are around 10 wt%, much lower than that obtained with 1 g Ni and 0.5 g NaOH as co- catalysts, indicating that all the solid bases examined show much lower activities than NaOH. Increasing the amount of MgO and NaOH/

MgO from 0.5 g to 1 g with keeping 1 g Ni catalyst unchanged, total monomer yields are not obviously improved, slightly increasing from 9.1 to 10.7 wt% and from 11.2 to 12.4 wt%, respectively. Hence, these solid bases cannot efficiently catalyze EHL depolymerization at a low reaction temperature (200 ^◦C).

Although EHL was completely dissolved in ethylene glycol before and after the reaction with or without a catalyst, a floccule appeared between the water and CH2Cl2 phases during product extraction. The floccule is composed of lignin fragments that cannot be dissolved in water and CH2Cl2. The weight average molecular weights (Mw) and β-O- 4 linkage contents of EHL and the floccule samples were analyzed with GPC (Fig. S4) and HSQC-NMR (Fig. S5), respectively, and the results are summarized in Fig. 2 (f). The Mw of EHL is 4333 g/mol. The Mw of the floccule obtained without a catalyst is much lower than that of EHL, which is 2216 g/mol. When a catalyst is added, the Mw decreases in an order: Ni (1768 g/mol) >NaOH (642 g/mol) >both Ni and NaOH (464 g/mol). The intensities of the peaks of β-O-4 linkages in HSQC-NMR spectra were normalized with the peak of DMSO. For EHL, the relative intensity of β-O-4 linkage signal is 0.22. These values of floccule obtained without catalyst and with Ni are similar, which are 0.15 and 0.13 respectively. Nevertheless, this value of floccule obtained with NaOH is only 0.04. When Ni catalyst and NaOH co-exist, the signal of β-O-4 linkage disappears and this value turns to 0.

3.2. Interaction between EHL and solvent

EHL, EHL-MET and EHL-EG samples were analyzed with ¹H NMR and ¹³C NMR to reveal the interactions between EHL and solvent, and signal assignment is based on the published works [21–27]. Fig. 3 (a) plots the ¹H NMR spectra obtained. In the spectra of EHL-MET and EHL- EG, the peaks of H in phenolic hydroxyls (10–8 ppm) shift to higher field (lower δ value), compared to those in the spectrum of EHL. This is due to the cleavage of original intramolecular hydrogen bonds in lignin and the formation of new hydrogen bonds between solvent and phenolic hydroxyls [28]. Nevertheless, in the spectra of EHL-MET and EHL-EG, the positions of the peaks of H in phenolic hydroxyls are similar, indicating that the phenolic O-H⋯O hydrogen bonds in EHL-MET and EHL-EG have similar strengths. In addition, the strong peaks of H in aromatic ring (7.5–6.3 ppm) and aliphatic chain –CH3/–CH2 (1.4–0.6 ppm) also shift to higher field in the spectrum of EHL-MET, and further shift to the same direction in the spectrum of EHL-EG, compared to those in the spectrum of EHL. In the ¹³C NMR spectra (Fig. 3(b)), the peaks ascribed to C4 in G unit (G4, 145.7 ppm), C2/C6 in pCA and H units (pCA2/6 and H2/6, 130.5 ppm), C3/C5 in pCA and H units (pCA3/5 and H3/5, 116.0 ppm) as well as C in CH2 in the aliphatic side chain (29.6 ppm) shift to lower field (higher δ value) in the spectrum of EHL-MET, and further shift to the same direction in the spectrum of EHL-EG, compared to those in the spectrum of EHL. The shifting of these ¹H and ¹³C NMR peaks in the spectra of EHL-MET and EHL-EG suggests the existence of interaction between aromatic and aliphatic C-H in EHL and O in the solvent.

Gaussian simulation was employed to verify the existence and strength of phenolic O-H⋯O and aromatic C-H⋯O interactions in EHL- MET and EHL-EG. Phenol and benzene were used to represent the phenolic OH and benzene ring in EHL, respectively, to exclude the influence of other functional groups. Fig. 4 (a) illustrates the stable structures and interaction energies of phenol-methanol (P-MET), phenol-ethylene glycol (P-EG), benzene-methanol (B-MET) and benzene-ethylene glycol (B-EG) complexes. For P-MET and P-EG, the interaction energies are similar, i.e., −0.44 eV and −0.59 eV, respectively. Hoverer, the interaction energy of B-EG is −0.36 eV, much lower than that of B-MET, which is only −0.06 eV. Therefore, the strengths of phenolic O-H⋯O interaction formed in P-MET and P-EG are similar, but the aromatic C-H⋯O interaction in B-EG is much stronger than that in B- Fig. 5.(a) The structure of lignin model molecule, (b-e) the optimized structure of lignin model molecule and its NCI analysis without ethylene glycol molecules and with seven ethylene glycol molecules.

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MET.

The strength of O-H⋯O hydrogen bonds between aliphatic OH in a lignin dimer (1-(4-hydroxyphenyl)-2-phenoxypropane-1,3-diol) and OH in ethylene glycol and methanol are also calculated. As shown in Fig. 4 (b), the interaction energies of Cγ-OH⋯O and Cα-OH⋯O in lignin dimer- ethylene glycol (D-EG) are −0.55 and −0.69 eV, respectively, slightly lower than these value in lignin dimer-methanol (D-MET), which are

−0.37 and −0.41 eV, respectively. This indicates that the aliphatic O- H⋯O hydrogen bonds in D-EG are slightly stronger than those in D-MET.

The NCI of this lignin dimer, lignin dimer-ethylene glycol (D-EG) and lignin dimer-methanol (D-MET) are further analyzed, and the results are shown in Fig. 4 (c). Blue and green clouds indicate the hydrogen bond and van der Waals interactions, respectively. In the lignin dimer, the van der Waals interaction between the two benzene rings in lignin dimer is ascribed to π-π stacking interaction, which results in the overlapping of two benzene rings [29]. In D-EG, overlapping benzene rings are opened due to the van der Waals interaction between the lignin dimer and ethylene glycol, which are ascribed to the lone pair⋯π interaction between the lone pair of O in ethylene glycol and π electrons in benzene rings [30]. Nevertheless, in D-MET, methanol prefers to form a hydrogen bond with phenolic hydroxyl in lignin dimer, and the benzene rings are still stacked.

The interaction between ethylene glycol and a lignin model molecule (Fig. 5 (a)) consisting of five benzene rings and C-O and C-C linkages, i.

e., α-O-4, β-O-4, 5-O-4, and β-1, was further examined. As shown in Fig. 5 (b) and (c), without ethylene glycol molecule, the lignin model molecule is aggregated due to the intramolecular hydrogen bond and π-π stacking interactions. When seven ethylene glycol molecules are added, the aggregated lignin model molecule is stretched (Fig. 5 (d)), and both hydrogen bond interaction and van der Waals interactions, including C- Fig. 6. The ATR-FTIR spectra of liquid ethylene glycol (Liquid EG), ethylene

glycol adsorbed over Ni catalyst (Adsorbed EG), the mixture of ferulic acid and ethylene glycol adsorbed over Ni catalyst (Adsorbed 0.1 g FA-10 mL EG), and the mixture of coniferyl alcohol and ethylene glycol adsorbed over Ni catalyst (Adsorbed 0.1 g CA-10 mL EG).

Fig. 7.The molecular dynamics simulation of the competitive adsorption of the lignin dimer and ethylene glycol over Ni surface in solvolysis reaction, (a) the model before simulation, (b) the model after simulation for 1000 ps at 200 ^◦C. (c) the distribution of ethylene glycol molecules along the z-axis. (d) The trajectory of the mass center of lignin dimers.

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H⋯O and lone pair⋯π, form between ethylene glycol and the lignin model molecule (Fig. 5 (e)).

3.3. Adsorption of ethylene glycol and lignin monomer/dimer over Ni catalyst

The adsorption of ethylene glycol and lignin monomers, i.e., ferulic acid (FA) and coniferyl alcohol (CA), over Ni catalyst at 200 ^◦C was examined with ATR-FTIR, and the spectra are shown in Fig. 6. In the spectrum of liquid ethylene glycol, the broad band at 3290 cm⁻¹is ascribed to the stretching vibration of –OH, and the bands at 2936 and 2869 cm⁻¹are ascribed to the stretching vibration of –CH2–, whose bending vibrational bands appear in the range of 1455–1200 cm⁻¹, and the bands at 1029 and 866 cm⁻¹are ascribed to the stretching and bending vibration of –C–O–, respectively. In the spectrum of ethylene glycol adsorbed over Ni catalyst, the bond of –OH shifts to 3664 cm⁻¹ and its intensity is significantly weakened compared to that of liquid ethylene glycol, and the stretching vibrational bonds of –CH2– and –C–O– also shift to higher wavenumber, appeared at 2985, 2894 and 1054 cm⁻¹, respectively. The shift of these bonds results from the transformation of electrons from ethylene glycol to Ni atoms. When the mixtures of 10 mL ethylene glycol and 0.1 g FA or CA are adsorbed over Ni catalyst, the spectra obtained are the same as that of pure ethylene glycol adsorbed over Ni catalyst, indicating that these lignin monomers in ethylene glycol can not be adsorbed over Ni catalyst.

The competitive adsorption of the mentioned lignin dimer (1-(4- hydroxyphenyl)-2-phenoxypropane-1,3-diol) and ethylene glycol molecules over the Ni surface was investigated with molecular dynamics simulation. Fig. 7 (a) is the initial state of the model, in which one lignin dimer (Dimer A) is surrounded with ethylene glycol molecules and another one (Dimer B) is over the Ni surface. After 1000 ps simulation (Fig. 7 (b)), Dimer A is still in the liquid phase, while Dimer B remains adsorbed over the Ni surface. The distribution of ethylene glycol molecules along the z-axis (Fig. 7 (c)) indicates that ethylene glycol molecules are enriched over the Ni surface. The trajectory of the mass center of Dimer A and B during 1000 ps (Fig. 7 (d)) shows that Dimer A cannot go through the ethylene glycol molecular layer to adsorb over the Ni surface, while Dimer B steadily adsorbs over the Ni surface.

4. Discussion

4.1. The role of ethylene glycol

EHL dissolution is the first step of EHL solvolysis. We found that ethylene glycol achieves complete EHL dissolution at room temperature, while other solvents, such as methanol, only dissolve part of EHL with high content of hydroxyls and β-O-4 linkages. Previous articles thought that lignin dissolution is mainly attributed to the O-H^…O hydrogen bond between solvent and lignin [28,31–33]. Nevertheless, for lignin

molecules with low content of hydroxyls, the main obstacle to its dissolution is the π-π stacking interaction [34–37]. Gaussian simulation indicates that ethylene glycol forms stronger C-H⋯O and lone pair⋯π interactions with benzene rings of EHL than methanol dose, because one ethylene glycol molecule contains two O atoms. These strong van Der Waals forces break original π-π stacking in EHL and achieve complete EHL dissolution.

The monomer yields obtained from non-catalytic EHL solvolysis show a positive correlation with the EHL solubility of solvents, and the highest monomer yield is obtained in ethylene glycol. As revealed with the GPC and HSQC-NMR results, linkages in EHL are already partly broken in ethylene glycol even at 200 ^◦C without a catalyst, forming monomers and lignin fragments. We speculate that the strong van Der Waals forces between ethylene glycol and EHL may result in the shift of electrons in the benzene ring of EHL, reducing the bond energy of β-O-4 linkages in EHL, as shown in Scheme 1(a).

4.2. The roles of Ni and NaOH 4.2.1. The role of Ni

It has been generally accepted that a large lignin molecule cannot be directly adsorbed on the surface of the solid catalyst due to its large three-dimensional structure [1,2,4]. Our results of molecular dynamics simulation further reveal that the adsorption of ethylene glycol hinders the adsorption of lignin dimer from the liquid phase to the surface of the Ni catalyst. Hence, catalytic EHL hydrogenolysis is not the main reaction pathway, and lignin depolymerization mainly occurs through solvolysis reaction.

The comparison of the results of blank reaction without catalyst and catalytic reaction with Ni catalyst reveals that the Ni catalyst does play a role in the hydrogenation of carbon–carbon double bonds in the side chains of lignin monomers. Nevertheless, the adsorption of lignin monomers is also hindered by the adsorption of ethylene glycol. Hence, the hydrogenation reaction may occur in the liquid phase. we guess that O atoms in ethylene glycol may attract adsorbed H atoms over the Ni surface due to their strong electronegativity, forming a hydrogen- ethylene glycol complex, which may desorb from the Ni surface and involve in the hydrogenation reaction in the liquid phase (Shame 1(b)).

As reported, even at around 200 ^◦C, lignin depolymerization and product repolymerization occur simultaneously in non-catalytic lignin alcoholysis, and intermediates with carbon–carbon double bonds in their side chains more readily undergo repolymerization reactions [38–42]. The Ni catalyst stabilizes active monomers through hydrogenation reactions, suppressing repolymerization reactions. Hence, the addition of the Ni catalyst improves monomer yield and reduces Mw of floccule.

As discussed above, Ni catalyst just plays a role in the hydrogenation reaction of carbon–carbon double bonds in EHL solvolysis reaction. The hydrogenation of carbon–carbon double bonds is relatively easy Scheme 1. (a) the weakening of the β-O-4 linkages in EHL by ethylene glycol. (b) the attracting of adsorbed H by O in ethylene glycol.

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[43,44], and does not require a Ni catalyst with high hydrogenation activity. Hence, the activity of the Ni catalyst is insensitive to the phase transition of Ni from the amorphous phase to the crystalline phase.

4.2.2. The role of NaOH

NaOH is soluble in ethylene glycol and serves as a homogeneous catalyst that directly promotes the breakage of C-O linkages in lignin [45]. Hence, NaOH is more efficient than solid base catalysts for cata- lyzing lignin depolymerization. When NaOH was used as the catalyst, the content of β-O-4 linkage and M_wof floccule were significantly reduced. When Ni and NaOH were used as co-catalysts, the product distribution obtained is similar to that obtained with only NaOH as a catalyst, indicating that the reaction mainly follows the soluble-base catalyzed route. Nevertheless, NaOH also promotes repolymerization/

condensation of active monomers and intermediates, and hence too high NaOH amount results in the decrease of total monomer yield [45,46]. As mentioned above, the Ni catalyst stabilizes active monomers and intermediates, suppressing repolymerization reaction, and hence the combination of Ni and NaOH obtains a higher monomer yield than a single catalyst [47–49].

4.3. Pathways of EHL solvolysis in ethylene glycol

Based on the presented results, the pathways of EHL solvolysis in ethylene glycol at 200 ^◦C with Ni and NaOH as co-catalysts are proposed and presented in Scheme 2. Agglomerated lignin molecule is firstly dissolved and partly depolymerized in ethylene glycol, exposing more functional groups, e.g., –OCH3 and –OH. NaOH depolymerizes dissolved lignin fragments through attacking these functional groups, forming active monomers and intermediates with carbon–carbon double bonds

[45,50]. Active hydrogens are transformed from the Ni surface to the liquid phase with ethylene glycol as a porter, and involve into the hydrogenation of carbon–carbon double bonds. After several cycling of base-catalyzed dehydroxylation and hydrogenation reactions, stable monomers are produced.

5. Conclusion

Ethylene glycol shows high EHL solubility and achieves complete EHL dissolution at room temperature, while methanol only dissolves part of EHL with high content of hydroxyls and β-O-4 linkages. The Gaussian simulation results indicate that ethylene glycol forms strong Van Der Waals interactions with EHL, including C-H⋯O and lone pair⋯π interactions, and these interactions break original π-π stacking in EHL, achieving complete EHL dissolution.

The total monomer yields obtained from non-catalytic EHL solvolysis at 200 ^◦C under 3 H2 for 6 h show a positive correlation with EHL solubility, and ethylene glycol gives the highest total monomer yield, i.e., 5.5 wt%, among the solvents examined. With Ni and NaOH as co- catalysts, the total monomer yield in ethylene glycol reaches 18.8 wt%

under the same reaction condition.

EHL is partly depolymerized at 200 ^◦C without a catalyst due to the strong interactions between EHL and ethylene glycol. NaOH as the homogeneous catalyst directly attracts C-O bonds in EHL and depolymerizes EHL into active monomers and intermediates. The adsorption of lignin fragments over Ni catalyst via C-O bond is hindered with ethylene glycol, and Ni catalyst mainly plays a role in supplying active hydrogen atom to stabilize the active intermediates, suppressing repolymerization side reactions.

Scheme 2. The pathways of EHL depolymerization in ethylene glycol at 200 ^◦C with Ni and NaOH as co-catalysts.

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Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgments

This work has received funding from the European Union’s Horizon 2020 research and innovation program, (BUILDING A LOW-CARBON, CLIMATE RESILIENT FUTURE: SECURE, CLEAN AND EFFICIENT EN- ERGY) under Grant Agreement No 101006744. The content presented in this document represents the views of the authors, and the European Commission has no liability in respect of the content. Y.S. Sang and G. Li would like to express their gratitude to both the China Scholarship Council (202006250156, 202208320030) and the EU-101006744 project.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.

org/10.1016/j.cej.2023.142256.

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