Condensed lignin depolymerization via C–C bond cleavage with a disordered crystalline mesoporous zeolite

Abstract

The depolymerization of condensed lignin—a byproduct of industrial pulping and biorefining—is crucial for sustainable biorefineries. However, it remains a formidable challenge due to the formation of recalcitrant C–C bonds during lignin isolation, restricting the reactivity and accessibility of the lignin macromolecule. Here, we demonstrate that a disordered crystalline mesoporous zeolite (Meso-Z) synergistically integrates strong acidity with enhanced mass transfer efficiency to process this bulk feedstock. Compared to conventional microporous zeolite, Meso-Z achieves 3.7–7.9 times higher yields (32.0–45.6 wt%) of aromatic monomers and dimers (MDs) from condensed lignin oligomers by C–C bond cleavage, thus enabling the total yields to exceed the theoretical maximum defined by C–O bond cleavage by 250–759%. Mechanistic studies reveal that this exceptional performance arises from Brønsted and Lewis acid-assisted hydrolysis and hydrogenolysis of C_α–C_aryl bonds, facilitated by the zeolite’s mesoporosity. Meso-Z also shows robust recyclability, maintaining stable catalytic activity over ten consecutive cycles. This work provides an efficient strategy for depolymerizing condensed lignin, paving the way for the valorization of industrial lignin streams.

Turning industrial lignin waste into useful chemicals is vital for sustainable biorefineries, but processing creates stubborn carbon bonds. A disordered mesoporous zeolite can break these bonds efficiently, releasing many more aromatic products while remaining stable.

Introduction

Lignin depolymerization stands as a cornerstone of lignin valorization, enabling the transformation of complex lignin macromolecules into smaller aromatic intermediates for the production of fuels, chemicals, and functional materials^1–4. A critical challenge, however, arises from condensation reactions during lignin fractionation, particularly under alkaline or acidic pulping conditions^5,6. While these processes cleave labile β–O–4′ linkages (the most abundant aryl ether bonds in native lignin) via proton-mediated mechanisms⁷, they concurrently drive undesirable condensation, forming recalcitrant C_α–C_aryl bonds⁸. The bond dissociation energies (BDEs) of these C–C bonds (75–118 kcal·mol⁻¹) exceed those of β–O–4′ bonds by 20–62 kcal·mol⁻¹, rendering them thermodynamically and kinetically resistant to cleavage—a fundamental bottleneck in condensed lignin utilization⁹.

To circumvent condensation, the ‵lignin-first′ approach has gained traction, employing protective stabilization of reactive intermediates (e.g., benzylic alcohols) through hydrogenation, oxidation, or functionalization^10–14. Such strategies suppress condensation pathways of lignin, enabling high-yield production of monophenols and bisphenols from lignin depolymerization that serve as versatile building blocks for sustainable polymers and drop-in fuels. However, this approach is inherently incompatible with technical lignins (e.g., Kraft, soda, acidic, hydrolysis, and biorefinery lignin), which are already condensed during the chemical cooking processes (Fig. 1). With more than 100 million tons of technical lignin generated annually by pulping/paper and biorefining industries¹⁵, direct C–C bond cleavage—rather than the upstream prevention—emerges as the only viable route for their valorization.

Fig. 1. Schematic illustration of the strategy for the catalytic depolymerization of condensed lignin.

Current strategies for condensed lignin depolymerization produce valuable monomers and dimers with yields lower than 18%, while the presented strategy breaks this limit and raises the yield to over 47% via efficient C–C bond cleavage.

State-of-the-art approaches for C–C bond cleavage in lignin—whether inherent or formed by condensation—have primarily focused on homogeneous oxidative catalysis^16,17 and heterogeneous catalytic cracking^18–23. Samec et al. reported the oxidative cleavage of β–β′ and β–1′ bonds in lignin dimers and oligomers obtained from the reductive catalytic fractionation (RCF) of birch wood using Bobbitt’s salt as an oxidant, which gave 2,6-dimethoxybenzoquinone (DMBQ) as a single product in an isolated yield of 18.0 wt%¹⁷. Though, the economic viability of this approach may be constrained by the oxidant’s cost and challenges in its recovery. Notably, zeolites have been found to offer a compelling alternative combining Brønsted acid-catalyzed C–C bond scission with inherent recyclability. Hensen et al. demonstrated the hydrocracking of various lignin C–C bonds, particularly the most stubborn 5–5′ bond, using a conventional mordenite zeolite embedded Pt catalyst (Pt/H-MOR), which produced 17.1 wt% monocyclic hydrocarbons from Kraft lignin¹⁸. Han et al. reported the catalytic cracking of C_sp2–C_sp3 bonds adjacent to the aromatic rings in native lignin using conventional HY zeolite in water²⁰. By combining proton-assisted C_sp2–C_sp3 bond cracking and C_β–O bond hydrolysis, benzene was produced in a yield of 10.9 wt% from poplar lignin. Our previous studies demonstrated that conventional Hβ zeolite could catalyze the selective hydrolysis of methylene linkages in condensed lignins, achieving aromatic monomer yields up to 10.0 wt%²². However, the inherent microporosity of conventional zeolites, while instrumental for shape-selective catalysis, often imposes severe mass-transfer limitations for the conversion of bulky molecules, such as lignin or lignin-derived oligomers. To overcome this challenge, hierarchical zeolites incorporating a secondary network of mesopores have been developed^24,25. Among them, a distinct category known as disordered crystalline mesoporous zeolites has garnered significant interest²⁶. This term refers to materials that retain the long-range crystalline framework of their microporous counterparts, as evidenced by sharp X-ray diffraction patterns, but are interpenetrated by an interconnected, non-periodic network of mesopores. This disordered mesostructure, in contrast to the ordered channels in templated mesoporous silicates (e.g., MCM-41 and SBA-15), often arises from bottom-up assembly or post-synthetic treatments and creates efficient diffusion pathways. The synergy between the crystalline microporous walls, which host the active sites, and the disordered mesoporous network, which facilitates substrate access, is particularly advantageous for processing complex macromolecules.

Herein, we report a disordered crystalline mesoporous zeolite (Meso-Z) that features strong Brønsted acidity comparable to Hβ zeolite, alongside a markedly increased external surface area and larger pores. This unique property combination unlocks the efficient depolymerization of high-molecular-weight condensed lignin into low-molecular-weight monomers and dimers (MDs). The depolymerization mechanism, encompassing both reaction pathways and mass transfer process, is elucidated through a combination of model reactions, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations.

Results

Catalyst synthesis and characterizations

Meso-Z was synthesized using a gemini surfactant template (C_18-6-6Br₂) as the structure-directing agent. SEM imaging (Fig. 2a) reveals the unique coral-like morphology of the zeolite, featuring a rough surface texture with interconnected mesoporous channels. Characterizations by TEM (Fig. 2b) and XRD (Fig. 2c) confirms the preservation of MFI-like frameworks with broader diffraction peaks (7.9°, 8.7°, 23.0°, and 23.8° 2θ) and well-defined lattice fringes, indicating the coexistence of long-range disorder with local crystallinity. The porosity characteristics, as determined by N₂-physisorption (Fig. 2d), show that the material maintains a high surface area (575 m²·g^-1) comparable to conventional Hβ zeolite (621 m²·g^-1) (Table 1), while exhibiting substantially improved mesoporous features. Mesopores with pore sizes distributed in the range from 7 nm to 50 nm (calculated by NLDFT model) were observed, which were constructed by intra-crystalline stacking of the microporous MFI frameworks²⁶. Correspondingly, the external surface area (313 m²·g⁻¹) is more than three times that of Hβ (100 m²·g⁻¹), accompanied by increased pore volume (1.90 cm³·g⁻¹ vs 0.41 cm³·g⁻¹). Acidic property evaluation with IR-pyridine and NH₃-TPD analyses (Fig. 2e–f, Table 1) show that Meso-Z possesses lower density of total acid sites (456 μmol·g⁻¹, measurable at 150 ^oC) with respect to the conventional acidic zeolites such as Hβ (740 μmol·g^-1) and HZSM-5 (1270 μmol·g⁻¹). However, the density of medium-to-strong Brønsted acid sites (BAS) over Meso-Z, which are catalytically important for C–C bond scission¹⁸, approaches that of Hβ (113 μmol·g⁻¹ vs 140 μmol·g⁻¹). Meanwhile, the density of medium-to-strong Lewis acid sites (LAS), which are crucial for hydrogen-transfer reactions and stabilization of carbonated intermediates²⁷, is larger for Meso-Z (97 μmol·g⁻¹) with respect to Hβ (70 μmol·g⁻¹). This unique combination of mesoporosity and acidity of Meso-Z challenges the limitations of the conventional zeolites, suggesting strong potential for applications in reactions involving large molecules, such as the conversion of lignin.

Fig. 2. Characterizations of disordered crystalline mesoporous zeolite (Meso-Z).

a SEM image. b TEM image. c XRD pattern. d N₂-physisorption isotherm. e IR-pyridine curve. f NH₃-TPD profile. Conventional microporous zeolites are used for comparison.

Table 1.

Acidity and structural properties of zeolites

Zeolite	Density of acid site (μmol NH3·g-1·catalyst) a					SBET (m2·g−1) b	Sext (m2·g−1) c	V (cm3·g−1) d
	Total		Medium-to-strong
	BAS	LAS	BAS		LAS
Meso-Z	332	124	113		97	575	313	1.90
Hβ	350	390	140		70	621	100	0.41
HZSM-5	430	840	270		350	343	86	0.19
USY	170	550	71		209	498	65	0.41
SAPO-11	77	363	37		153	136	42	0.28
MCM-41	75	115	5		5	877	837	0.69

^aCalculated with the amount of acid sites determined by NH₃-TPD and the relative amount of BAS and LAS determined by IR-pyridine;

^bBrunauer-Emmett-Teller (BET) surface area;

^ct-Plot external surface area;

^dSingle point adsorption total pore volume.

Catalytic cleavage of C–C bond in lignin model compound

Meso-Z demonstrated efficient cleavage of C_α–C_aryl bonds in the lignin model compound bisphenol F (BPF, 1), a representative structure mimic for condensed lignin (Fig. 3a). Under hydrothermal conditions (300 °C, 1 h, H₂O), BPF depolymerization yielded 40.1 mol% aromatic monomers, dominated by phenol (96.3% selectivity) (Fig. 3b, Supplementary Table 1). The detection of 2,4′-dihydroxyl diphenyl methane (19.0 mol%) and trace of methyl phenols implicated a Brønsted acid-catalyzed hydrolysis mechanism, where water nucleophilically attacked the—CH₂– group, releasing formaldehyde²⁰. In ethanol (EtOH), monomer yield increased to 46.9 mol%, with 4-methyl phenol and its ethylated derivatives (23.7 mol% in total) constituting half the products. The comparable yields of the phenol derivatives (e.g., phenetole) and the 4-methyl phenol derivatives suggested a potential competing catalytic transfer hydrogenolysis (CTH) pathway mediated by LAS²⁷, where EtOH played the roles as hydrogen donor and capping agent, suppressing repolymerization of the phenols via esterification/ethylation²⁸.

Fig. 3. Catalytic cleavage of C_α–C_aryl bond in bisphenol F (BPF).

a Overview of catalytic depolymerization of BPF via C_α–C_aryl bond cleavage. Yields from BPF depolymerization upon different (b) solvents (9/1 v_organic/v_H2O, 300 °C, 1 h), (c) reaction times (9/1 v_EtOH/v_H2O, 250 °C), (d) reaction temperatures (9/1 v_EtOH/v_H2O, 1 h), (e) EtOH/H₂O ratios (250 °C, 1 h), and (f) zeolites (9/1 v_EtOH/v_H2O, 300 °C, 1 h). g Proposed reaction pathways for BPF depolymerization. General conditions: 0.25 mmol BPF, 20 mg zeolite, 3 mL solvent, atmospheric N₂. Error bars represent standard deviations. Each reaction was run in duplicate/triplicate.

The EtOH/H₂O mixed solvent (9/1 v/v) synergistically enhanced the depolymerization efficiency, achieving a monomer yield of 96.1 mol% (Fig. 3b, Supplementary Table 1)—nearing the theoretical maximum. Product distribution analysis revealed near-equal contributions from the catalytic hydrolysis (46.6 mol%) and hydrogenolysis (49.5 mol%), demonstrating the dual solvent’s unique capacity to integrate both pathways. The improved BPF solubility in the EtOH-dominant system facilitated its liquid-phase mass transfer, while EtOH concurrently acted as a formaldehyde scavenger, suppressing repolymerization by trapping the reactive intermediates²⁹. This dual functionality was critical that substituting EtOH with methanol (MeOH) or isopropanol (iPrOH) reduced monomer yields to 49.8 mol% and 47.2 mol% respectively, whereas the polar aprotic solvents including acetone (ACE), tetrahydrofuran (THF), and dimethylformamide (DMF) triggered deleterious side reactions (< 35 mol% yield) through acid-catalyzed arylation, ring-opening, or hydrolysis (Supplementary Appendix).

Systematic screening identified 300 °C, 1 h, and 9/1 EtOH/H₂O ratio as the optimal reaction conditions for BPF conversion (Fig. 3c–e, Supplementary Tables 2–4). Increasing reaction temperature (Fig. 3c) or duration (Fig. 3d) facilitated BPF conversion since the bond scission was endothermic. Remarkably, a nonlinear influence of the EtOH/H₂O solvent ratio on the depolymerization efficiency of BPF was observed (Fig. 3e). The maximum depolymerization yield was achieved at a 9:1 EtOH/H₂O ratio, while a significant decrease in yield was observed at 7:3 even though hydrogenolysis remained the dominant pathway. As the water content increased further to a 1:9 ratio, the yield gradually recovered with a corresponding enhancement in hydrolysis selectivity. This anomalous trend underscores a sophisticated coupling between mass transfer effects and pathway selection governed by solvent composition. We propose that the optimal 9:1 ratio represents a synergistic balance between hydrogenolysis efficiency and optimal substrate mass transfer. At this specific composition, ethanol serves as an effective hydrogen donor for hydrogenolysis while the solvent mixture maintains ideal solubilization of hydrophobic substrates. The yield reduction at 7:3 is attributed to compromised mass transfer efficiency in this particular solvent microenvironment, which limits overall conversion despite maintaining the intrinsic selectivity of the hydrogenolysis pathway. The subsequent yield recovery in the water-dominated regime (1:9 to 3:7) is primarily driven by the increased concentration of water as a reactant, which shifts the reaction equilibrium to favor the hydrolytic pathway and enhances its kinetic rate, thereby compensating for the diminished mass transfer of the organic substrates. This mechanistic understanding deepens our insight into solvent-mediated depolymerization, demonstrating that solvent composition not only determines reaction pathway selection but ultimately governs pathway efficacy through its crucial role in regulating substrate mass transfer.

Under the optimal conditions, both Meso-Z and conventional Hβ zeolite delivered exceptional performance (> 96% yield, Fig. 3f, Supplementary Table 5), attributable to their appropriate acidities and porosities (Table 1). Although, HZSM-5 possesses higher densities of medium-to-strong BAS (270 μmol·g⁻¹) and LAS (350 μmol·g⁻¹), the overwhelm acidity favors side reactions of the solvent including gasification and C–C coupling, leading to instability of the catalytic system and then low monomer yield²³. The comparable activity of microporous Hβ (97.7 mol% yield) suggested that mesoporosity, while beneficial for mass transfer of the substrates, was not the sole determinant of the catalytic efficiency—a finding that prompted further investigations with real lignin feedstocks.

Mechanistic study regarding reaction pathways

The mechanism for C–C bond cleavage was systematically investigated using structurally diverse lignin models (2–11) with Meso-Z in EtOH/H₂O (9/1 v/v) (Fig. 4). Models 2–4, designed with extended alkyl side chains on the methylene groups to better simulate real lignin structures, exhibited complete conversion with monomer yields of 82.7–90.6 mol% (Supplementary Tables 6–8), confirming the catalyst’s versatility for methylene bond cleavage. The near-quantitative yield (98.3 mol%) from the phenolic-blocked model 5 (Supplementary Table 9) excluded the influence of the EtOH-mediated esterification during the bond cleavage.

Fig. 4. Catalytic cleavage of C_α–C_aryl bonds in different lignin model compounds.

General conditions: 0.25 mmol model compound, 20 mg Meso-Z, 3 mL EtOH/H₂O (9/1 v/v), atmospheric N₂. Each reaction was run in parallel. ^aUsing EtOH as solvent. ^bUsing H₂O as solvent.

For challenging C–C linkages, β–1′ (model 6) and β–β′ (model 7) bonds were cleaved with 64.0 mol% and 27.5 mol% monomer yields, respectively (Supplementary Tables 10 and 11). Control experiments in neat EtOH yielded comparable results (62.9 mol% and 25.9 mol%) while no reaction occurred in H₂O, indicating the dominance of the hydrogenolysis pathway. The reduced bond cleaving efficiencies versus BPF correlated with higher bond dissociation energies (BDEs) of β–1′ and β–β′⁷. In stark contrast, the 5–5′ bonded model 8 resisted depolymerization, forming dimeric products via etherification/alkylation instead, highlighting the essential role of C_α in carbocation stabilization¹⁸. Phenolic group dependency was unequivocally demonstrated that models 9–11, which were free of phenolic –OH, failed to generate monomers, implicating these oxygen atoms as critical protonation sites for both hydrolysis and hydrogenolysis.

Mechanistic studies combining DFT calculations and the formal experimental data revealed a bifunctional pathway for the methylene bond cleavage in BPF (Fig. 5, Supplementary Figs. 2 and 3)^30–38. The reaction initiates through the Brønsted acid-mediated protonation of the phenolic group (ΔE = 55.8 kJ·mol⁻¹, TS1) to form an oxonium intermediate S2, followed by proton migration (ΔE = 86.4 kJ·mol⁻¹, TS2) generating carbocation S3. Subsequent β-scission (ΔE = 55.2 kJ·mol⁻¹, TS3) yields phenol and benzylic carbocation S4³⁹, which partitions between two competitive pathways. Path 1 (ethanol-mediated hydrogenolysis) details the direct hydrogen transfer from ethanol to S4. The process begins with the co-adsorption of ethanol and S4 near a LAS and an adjacent basic site (S5). The hydroxyl hydrogen of ethanol is first transferred to the α-carbon of S4 (ΔE = 154.3 kJ·mol⁻¹, TS4), forming a new carbocation S6 and an ethoxy group (–OCH₂CH₃). Subsequently, a hydrogen atom from the α-carbon of the –OCH₂CH₃ group is transferred to the oxygen of the basic site (ΔE = 46.1 kJ·mol⁻¹, TS5), simultaneously releasing acetaldehyde (S7). The hydrogen atom on the basic site is then transferred back to the carbon framework (ΔE = 121.7 kJ·mol⁻¹, TS6), generating a new carbocation S8. Finally, S8 restores a proton to the Brønsted acid site (ΔE = 144.6 kJ·mol⁻¹, TS7), yielding the stable hydrogenolysis product, 4-methylphenol (S9). On the other hand, Path 2 (hydrolysis) involves the nucleophilic attack of a water molecule on S4. The water molecule directly attacks the carbocation (ΔE = 169.8 kJ·mol⁻¹, TS4′,), leading to the scission of the methylene linkage and the formation of phenol and a hydroxymethyl carbocation intermediate S6′. This is followed by the deprotonation of S6′ back to the BAS (ΔE = 52.0 kJ·mol⁻¹, TS5′), resulting in the formation of formaldehyde (S7′). This intricate mechanism underscores a remarkable synergy between the BAS, LAS, and basic site over Meso-Z. The BAS is primarily responsible for activating the substrate through protonation, generating the pivotal carbocation intermediates. The LAS plays a crucial role in stabilizing these carbocations and polarizing the ethanol molecule, facilitating the key hydride transfer step. Concurrently, the adjacent basic site acts as a hydrogen shuttle, accepting and donating protons throughout the ethanol-to-acetaldehyde conversion cycle. This tripartite cooperation creates an efficient catalytic pocket for the coupled deconstruction of the lignin model and the dehydrogenation of the solvent, which would be unattainable with any single type of site alone.

Fig. 5. DFT calculations for the mechanism of C–C bond cleavage.

Potential energy diagram for proposed reaction pathways of C–C bond cleavage in BPF over Mezo-Z.

Catalytic depolymerization of real condensed lignin

We then applied the optimized catalytic system to the conversion of five representative condensed lignin with high structural complexity, including Kraft lignin (KL), soda lignin (SL), acidic lignin (AL), hydrolysis lignin (HL), and biorefinery lignin (BL) (Fig. 6). Initial treatment by the classical Pd/C-catalyzed hydrogenolysis selectively cleaved the residual C–O bonds (e.g., β–O–4′ and 4–O–5′ linkages) in the lignins (Fig. 6a), as confirmed by complete disappearance of C–O linkage correlations in the HSQC NMR spectra (Supplementary Figs. 4–13, Supplementary Table 12). This step was introduced to eliminate potential interference from C–O bond cleavage during the depolymerization while providing a reference for evaluating the catalytic efficiency of the proposed strategy relative to conventional approaches.

Fig. 6. Catalytic depolymerization of condensed lignin.

a Protocol for the catalytic depolymerization of condensed lignin to aromatic monomers and dimers (MDs). b Qualification of MDs from oligomers depolymerization with Meso-Z by GC-MS. c Molecular weight evolution of Kraft lignin (KL). d Isolated yields of MDs from the depolymerization of five lignin oligomers with Meso-Z and Hβ zeolites. Reaction conditions for Pd/C-catalyzed hydrogenolysis: 500 mg condensed lignin, 100 mg 5%Pd/C, 15 mL EtOH, atmospheric N₂, 250 °C, 4 h. Reaction conditions for oligomer depolymerization with zeolites: 200 mg oligomers, 100 mg zeolite, 15 mL EtOH/H₂O (9/1 v/v), atmospheric N₂, 300 °C, 5 h. Error bars represent standard deviations. Each reaction was run in duplicate/triplicate.

Subsequent fractionation of the hydrogenolysis oil yielded oligomers that only contained C–C bonds (Supplementary Table 13)¹², which were subjected to further depolymerization with Meso-Z and Hβ zeolites. As a result, both zeolites successfully produced a wide range of aromatic MDs from the oligomers (Fig. 6b, Supplementary Appendix), with HSQC NMR revealing concurrent esterification/ethylation events (new ethoxyl, aliphatic hydroxyl, and alkyl groups correlations in Supplementary Figs. 14–18). The emergent alkyl signatures may be originated from the liberation of –CH groups during C–C bond hydrogenolysis, suggesting partial defunctionalization alongside depolymerization. GPC analysis revealed distinct molecular weight (M_w) progression of condensed lignin during the depolymerization (Fig. 6c, Supplementary Figs. 19–23). For instance, initial KL (β–O–4′ content: 3.5%, M_w = 1375 Da) showed limited M_w reduction after hydrogenolysis (1317 Da), yielding three characteristic peaks corresponding to monomers (21.2 min), dimers (20.3 min), and oligomers (19.7 min). Subsequent conversion of the fractionated oligomers (M_w = 1584 Da) over Meso-Z progressively reduced M_w to 988 Da with new peaks of monomer (21.2 min) and dimer (20.4 min), confirming further bond cleavage. The superior efficiency of Meso-Z was quantified by MD yields of 32.0–45.6 wt%, representing a 3.7–7.9-time enhancement over Hβ (4.8–8.6 wt%) (Fig. 6d). These yields far exceeded the theoretical limits for C–O bond cleavage alone (4.4–17.0 wt%), showing remarkable relative maximum yields (RMY) of 250–759%, which demonstrated the unique capacity of Meso-Z to attack C–C backbones in condensed lignin (Fig. 7, Supplementary Tables 13 and 14). This lag underscores how the optimized mass transfer efficiency of Meso-Z enables efficient depolymerization of the high-molecular-weight lignin fractions that conventional zeolites cannot effectively access.

Fig. 7. Mass balance for condensed lignin depolymerization to monomers and dimers (MDs).

The relative maximum yield (RMY) = (MD yield from C-O bond cleavage + MD yield from C-C bond cleavage) / MD yield from C-O bond cleavage × 100%.

Notably, the MD yields obtained from the depolymerization of condensed lignin represents a net mass output resulting from competing reaction pathways. It is well-established that in alcohol-mediated lignin depolymerization, concurrent processes significantly influence the final mass balance¹. On one hand, solvolytic processes, such as the hydrolytic cleavage of methylene linkages, lead to the loss of carbon mass as volatile fragments (e.g., formaldehyde). On the other hand, the alcohol solvent can act as an alkylating agent, incorporating its carbon into the products and thereby increasing the apparent yield^28,29,40,41. This interplay of mass-losing and mass-gaining reactions makes the precise attribution of carbon solely to the native lignin polymer exceptionally challenging. Therefore, while the MD yields serve as a valuable practical metric for process efficiency, the primary evidence for the successful cleavage of recalcitrant C–C bonds—the central focus of this work—is derived from the identification of specific monomeric and dimeric products that serve as unambiguous molecular fingerprints for these specific bond-breaking events.

To validate practical applicability, our system successfully depolymerized gram-scale raw KL, achieving a 33.7 wt% MD yield with a profile analogous to model compounds (Supplementary Figs. 24a–b). The process efficacy was confirmed by the near-complete cleavage of β–O–4′ linkages (Supplementary Fig. 24c) and a significant molecular weight reduction from 1375 Da to 1040 Da (Supplementary Fig. 24d), evidencing substantial macromolecular breakdown. Meso-Z also demonstrated robust recyclability, maintaining a stable MD yield of 28.4 ± 2.2 wt% over ten consecutive cycles (Supplementary Fig. 25a) with consistent production of guaiacol and syringol (Supplementary Fig. 25b). Although gradual deactivation occurred, recalcination fully restored its activity and the regenerated catalyst retained its structural and acidic properties (Supplementary Fig. 25c–f), and exhibited superior anti-coking capability compared to microporous Hβ, as revealed by TGA (Supplementary Fig. 25g), underscoring the durability imparted by the hierarchical porosity⁴².

To demonstrate the utility of the depolymerization products, we attempted to upgrade the isolated aromatic MDs into jet fuel range cycloalkanes via catalytic hydrodeoxygenation (HDO) (Supplementary Fig. 26), achieving a mass efficiency of 54.0 wt%. The resulting blend exhibited a high heating value (34.5 MJ/kg) and H/C_eff ratio (1.58) comparable to conventional high-density jet fuel (e.g., JP-10), underscoring their potential as drop-in biofuels.

Mechanistic study regarding molecular mass transfer

To quantitatively elucidate the mass transfer advantages of the mesoporous structure, we performed molecular dynamics (MD) simulations tracking a lignin oligomer (M_w = 1279 Da, Supplementary Fig. 27) within 0.6 nm (microporous) and 10 nm (mesoporous) zeolite channels (Fig. 8a, b). The structural evolution, analyzed through root mean square deviation (RMSD), revealed a profound confinement effect (Fig. 8c, d). The oligomer in the 0.6 nm pore exhibited minimal structural fluctuation (final RMSD = 1.12 nm), indicating a highly constrained and rigid conformation (Fig. 8c). In stark contrast, the molecule in the 10 nm pore underwent significant structural adaptation (final RMSD = 4.32 nm), demonstrating the greater conformational freedom available in the mesoporous environment (Fig. 8d). This difference in confinement directly translated to dramatic disparities in diffusivity. The mean square displacement (MSD) analysis yielded a diffusion coefficient of 0.0249 × 10⁻⁵cm²·s⁻¹ in the 10 nm pore, which is ~83 times higher than that in the 0.6 nm pore (0.0003 × 10⁻⁵cm²·s⁻¹) (Fig. 8e, f). This unequivocally confirms that the mesopores act as efficient mass transfer highways, enabling the rapid diffusion of bulky lignin fragments that are essentially immobile in microporous networks.

Fig. 8. Molecular dynamics (MD) simulations unveiling the mass-transfer advantage of mesopores.

a, b Representative snapshots of the lignin oligomer conformation within the microporous and mesoporous channels. c, d Root mean square deviation (RMSD) and (e, f) mean square displacement (MSD) analyses quantify the structural flexibility and diffusivity of the oligomer, respectively. g, h Interaction energy profiles (including Lennard-Jones short-range, LJ-SR, and Coulombic short-range, Coulomb-SR, components) between the oligomer and the zeolite frameworks.

Furthermore, interaction energy analyses provided insight into the adsorption strength. The total interaction energy between the oligomer and the 0.6 nm pore wall was significantly stronger (LJ-SR: −1323.65 kJ/mol; Coulomb-SR: −357.21 kJ/mol on average) than in the 10 nm pore (LJ-SR: −818.57 kJ/mol; Coulomb-SR: −286.32 kJ/mol) (Fig. 8g, h). The stronger van der Waals (LJ-SR) attraction in the micropore suggests the oligomer is tightly locked onto the surface, while the weaker interactions in the mesopore allow for smoother surface diffusion⁴³. These simulations provide a multiscale rationale for the superior catalytic performance that the mesoporous architecture not only facilitates rapid molecular transport but also mitigates strong, diffusion-limiting adsorption, thereby ensuring efficient access of lignin oligomers to the active acidic sites located within the microporous domains.

Discussion

In summary, this work provides a robust and efficient route for depolymerizing recalcitrant condensed lignin—a critical bottleneck in lignin valorization. By integrating enhanced mass transfer with synergistic Brønsted-Lewis acid catalysis, we have achieved selective cleavage of the challenging C–C bonds. This study thus moves beyond conventional catalyst design by establishing a structure-transfer-reactivity paradigm specifically tailored for complex biopolymers. The significance of our findings lies in providing a practical and mechanistic framework for upgrading industrial lignin streams into a pool of valuable monomeric and dimeric aromatics. This product stream serves as a versatile platform for the sustainable production of bio-derived aromatics or renewable fuels and materials, potentially displacing petroleum-derived counterparts. Looking forward, the principles elucidated here open avenues for designing next-generation catalysts tailored for native technical lignins. Integrating this catalytic depolymerization process with downstream separation and upgrading steps will be crucial for constructing a closed-loop biorefinery. We envision that this work will inspire further innovation aimed at unlocking the full potential of lignin, transforming it from an industrial waste stream into a central pillar of carbon-efficient chemical manufacturing.

Methods

Chemicals and materials

1-Bromooctadecane (C₁₈H₃₇Br, ≥97.0%, Sigma-Aldrich), 1-bromohexane (C₆H₁₃Br, 98%, Sigma-Aldrich), and N,N,N′,N′-tetramethyl-1,6-diaminohexane (C₁₀H₂₄N₂, 99%, Sigma-Aldrich) were used for the synthesis of organic surfactant. Fumed silica (SiO₂, 0.2–0.3 μm, Sigma-Aldrich) and sodium aluminate (NaAlO₂, ≥98.0%, Aladdin) were used as silicon and aluminum sources for zeolite synthesis. Bisphenol F (1, BPF, C₁₃H₁₂O₂, HPLC, ≥98%, Sigma-Aldrich), bisphenol E (2, BPE, C₁₄H₁₄O₂, HPLC, ≥98%, Sigma-Aldrich), bisphenol A (3, BPA, C₁₅H₁₆O₂, HPLC, 97%, Sigma-Aldrich), bisphenol B (4, BPB, C₁₆H₁₈O₂, HPLC, ≥98%, Sigma-Aldrich), dimethyl bisphenol A (5, C₁₇H₂₀O₂, HPLC, ≥99%, Aladdin), 4,4′-dihydroxylbibenzyl (6, C₁₄H₁₄O₂, 99%, Standardpharm), 4,4′-(tetramethylene) diphenol (7, C₁₆H₁₈O₂, 99%, Standardpharm), 4,4′-dihydroxydiphenyl (8, C₁₂H₁₀O₂, HPLC, >99%, Aladdin), diphenylmethane (9, C₁₃H₁₂, HPLC, >99%, Aladdin), 1,2′-diphenylethane (10, C₁₄H₁₄, HPLC, 99%, Aladdin), 1,4′-diphenylbutane (11, C₁₆H₁₈, 97%, Bidepharm) were used as lignin model compounds. Kraft lignin (KL) and soda lignin (SL) isolated from the black liquor of pulping process and hydrolysis lignin (HL) isolated from the wastewater generated from the hydrothermal treatment process before eucalyptus pulping were obtained from Shandong Sun Paper Industry Co., Ltd (Jining, China). Biorefining lignin (BL) was obtained from the waste stream of xylo-oligosaccharides and bio-ethanol production from corncob from Shandong Changsheng Bio-Technology Co., Ltd (Dezhou, China). Acidic lignin (AL) was prepared with one-year-old birch wood harvested from the forest in Zhangjiakou, China through a reported extracting method⁴⁴. Methanol (MeOH, HPLC, >99.9%, Sigma-Aldrich), ethanol (EtOH, HPLC, >99.9%, Sigma-Aldrich), isopropanol (iPrOH, HPLC, >99.9%, Sigma-Aldrich), acetone (ACE, HPLC, >99.9%, Sigma-Aldrich), acetonitrile (ACN, HPLC, >99.9%, Sigma-Aldrich), tetrahydrofuran (THF, HPLC, >99.9%, Sigma-Aldrich), N,N-dimethylformamide (DMF, HPLC, >99.9%, Sigma-Aldrich), N,N-dimethylacetamide (DMAC, GC, >99.8%, Sigma-Aldrich), dimethylsulfoxide (DMSO, HPLC, >99.7%, Sigma-Aldrich), toluene (HPLC, >99.9%, Sigma-Aldrich), diethyl ether (HPLC, >99.9%, Sigma-Aldrich), heptane (HPLC, ≥99.9%, Sigma-Aldrich), and deionized water were used as solvents. n-Dodecane (C₁₂H₂₆, standard for GC, ≥99.9%, Aladdin), phenol (C₆H₆O, standard for GC, >99.5%, Aladdin), guaiacol (C₇H₈O₂, standard for GC, >99.5%, Aladdin), syringol (C₈H₁₀O₃, standard for GC, >99.5, Aladdin), 4,4’-biphenol (C₁₂H₁₀O₂, HPLC, 97%, Sigma-Aldrich) were used as standard compounds for GC calibration. Sulfuric acid (H₂SO₄, AR, 95.0-98.0%, Sigma-Aldrich) was used for lignin extraction. Hydrochloric acid (HCl, AR, 26%, Fisher), sodium hydroxide (NaOH, AR, >99.0%, Macklin), were used for lignin purification. Sodium sulfate (Na₂SO₄, ACS reagent, >99.0%, Sigma-Aldrich) was used for drying. Pyridine (anhydrous, > 99.8%, Aladdin) and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA, ≥98.0%, Aladdin) were used for the derivation. All chemicals were used as received. Conventional bulk zeolites including Hβ (SiO₂/Al₂O₃ = 30), HZSM-5 (SiO₂/Al₂O₃ = 25), USY (SiO₂/Al₂O₃ = 25), SAPO (SiO₂/Al₂O₃ = 25), and MCM-41 (SiO₂/Al₂O₃ = 25) were purchased from the Catalyst Plant of Nankai University (Tianjin, China). The zeolites were calcined in a muffle furnace at 500 ^oC for 4 h before usage. Commercial Pd/C (5 wt% Pd) and Ni/SiO₂-Al₂O₃ catalysts were purchased from Aladdin Biochemical Technology Co., Ltd (Shanghai, China). H₂ ( > 99.999%) and N₂ ( > 99.999%) were purchased from Shang Yuan Gas Factory (Nanjing, China).

Synthesis of structure directing surfactant

C_18-6-6Br₂ surfactant was synthesized in two steps. First, N,N,N′,N′-tetramethyl-1,6-diaminohexane (0.10 mol) and 1-bromooctadecane (0.01 mol) were dissolved in 100 mL acetonitrile/toluene mixture (1/1 v/v) and heated at 70 °C for 10 h. After cooling to room temperature and solvent evaporation, the solid product with the formula of C₁₈H₃₇–N⁺(CH₃)₂–C₆H₁₂–N(CH₃)₂(Br^-) was precipitated. The product was further filtered, washed with diethyl ether, and dried in a vacuum over at 50 °C. Second, the product (0.01 mol) and 1-bromohexane (0.02 mol) were dissolved in 30 mL acetonitrile and refluxed for 10 h. After cooling to room temperature, the final product with the formula of C₁₈H₃₇–N⁺(CH₃)₂–C₆H₁₂–N⁺(CH₃)₂–C₆H₁₂(Br^-)₂ as denoted C_18-6-6Br₂ surfactant was precipitated. The product was then filtered, washed with diethyl ether, and dried in a vacuum over at 50 °C.

Synthesis of disordered crystalline mesoporous zeolite (Meso-Z)

Meso-Z (Si/Al = 15) was synthesized as follows: fumed SiO₂, NaAlO₂, NaOH, C_18-6-6Br₂, and deionized water were mixed to obtain a gel composition of 30 Na₂O: 1 Al₂O₃: 30 SiO₂: 10 C_18-6-6Br₂: 4000 H₂O, and aged at 60 ^oC for 6 h while stirring. The resulted gel was transferred to a Teflon-coated stainless-steel autoclave, and heated statically at 150 °C for 9 days. After crystallization, the resultant was filtered, washed with deionized water, and dried at 120 °C in an oven. The product was calcined at 550 °C for 6 h under flowing air to remove the organic surfactant. Before reaction, the zeolite was ion-exchanged in 1 M NH₄Cl solution (30 mL/g) at 70 °C for 2 h three times, and washed with deionized water. The zeolite was converted to H⁺ form by calcination at 550 °C for 4 h in a muffle furnace.

Catalytic depolymerization of lignin model compounds

The model reactions were performed in 6 mL batch reactors assembled with tube fittings and bleed valves. Typically, the reactor was loaded with substrate (0.25 mmol), catalyst (20 mg), and solvent (3 mL) with n-dodecane (20 mg) as the internal standard. The reactor was sealed and purged with N₂ for 5 times to dislodge the air without charging. Then, the reactor was buried into a pre-heated sand bath and heated to the required temperatures for specific durations. After completion, the reactor was quenched in ice water to stop the reaction and depressurized. Then, the mixture (liquid and solid residues) was poured into a 25 mL centrifuge tube. The reactor was washed with 5 mL of EtOH and the obtained mixture was poured into the tube as well. The liquid-phase products were collected with a 3.8 cm syringe, filtered with a 0.22 μm PTFE filter, and analyzed with GC-MS/FID.

Lignin purification

Approximately 20 g of lignin was dissolved in 500 mL of NaOH solution (1 mol/L), filtered, and acidolyzed by slowly addition of dilute HCl (36 ~ 40 wt%) while stirring to adjust the pH to 2. Upon completion, the lignin was precipitated, filtered, and washed with deionized water to obtain the purified lignin. Finally, the lignin was freeze-dried for 48 h and stored in a Duran bottle for further usage.

Catalytic hydrogenolysis of condensed lignin to crude oil

The reactions were performed in 100 mL autoclaves. Typically, the autoclave was loaded with lignin (500 mg), Pd/C catalyst (100 mg, 5 wt% Pd), and EtOH (15 mL). The reactor was purged five times with H₂ to dislodge air and charged with 4.0 MPa of H₂. Then, the autoclave was heated to required temperatures for specific durations. Upon completion, the autoclave was quenched in ice water to stop the reaction and depressurized. About 0.5 mL of the liquid products were collected with a 3.8 cm syringe, filtered with a 0.22 μm PTFE filter, and analyzed by GC-MS. The major liquid products were separated and collected by filtration with a sand core funnel, dried by evaporation at 50 °C to obtain the crude lignin oil comprised of aromatic monomers, dimers, and oligomers. The isolated solid residues containing catalyst and insoluble organic materials were washed with additional small portions of EtOH, filtered, and dried in vacuum.

Separation of aromatic monomers and dimers (MDs) and oligomers

The lignin oil from lignin depolymerization was fractionated via two approaches according to the literature². Lignin oil was first dropped on strips of filter papers in a cotton extraction thimble and subjected to a reflux extraction with n-hexane in a Soxhlet extractor for 24 h. Phenolic MDs were dissolved in hot hexane and collected in the flask. Phenolic oligomers, owing to their poor solubility in hot hexane, remained on the filter paper and were washed out using EtOAc, dried at 50 °C in vacuum, and injected to the catalytic reactions.

Catalytic depolymerization of lignin oligomers and raw lignin

The reactions were performed in 100 mL autoclaves. Typically, the autoclave was loaded with lignin oligomers or raw lignin (200 mg), catalyst (100 mg), and EtOH/H₂O (13.5 mL/1.5 mL). The reactor was purged five times with N₂ to dislodge air without charging. Then, the autoclave was heated to required temperatures for specific durations. Upon completion, the autoclave was quenched in ice water to stop the reaction and depressurized. About 0.5 mL of the liquid products were collected with a 3.8 cm syringe, filtered with a 0.22 μm PTFE filter, and analyzed by GC-MS. The major liquid products were separated and collected by filtration with a sand core funnel, dried by evaporation at 50 °C to obtain the crude lignin oil comprised of additional MDs and residual oligomers. The isolated solid residues containing catalyst and insoluble organic materials were washed with additional small portions of EtOH, filtered, and dried in vacuum. In the scale-up experiment, 1 g raw lignin, 200 mg catalyst, and 27 mL/3 mL EtOH/H₂O were loaded.

Catalyst recycling

The recyclability of Meso-Z was tested by subjecting it to ten consecutive cycles of catalytic depolymerization of KL at 300 °C for 5 h. After each cycle of reaction, the spent catalyst mixed with the insoluble organic materials was directly used for the next run without further treatment. The spent catalysts were characterized by thermogravimetric analysis (TGA) for coke determination. The catalyst after ten cycles was regenerated by calcination at 500 °C for 2 h in flowing air for coke removal, and then characterized with XRD, N₂-physisorption, NH₃-TPD, and IR-pyridine to evaluate the changes in the catalyst properties.

Hydrodeoxygenation of dimeric model compounds and lignin-derived MDs

The reactions were performed in 100 mL autoclaves. Typically, the autoclave was loaded with model compounds or lignin-derived MDs (100 mg), Ni/SiO₂-Al₂O₃ catalyst (50 mg), heptane (15 mL), and n-dodecane (20 mg) as the internal standard. The reactor was purged five times with H₂ to dislodge air and charged with 4.0 MPa of H₂. Then, the autoclave was heated to required temperatures for specific durations. Upon completion, the autoclave was quenched in ice water to stop the reaction and depressurized. The liquid products were collected and moved shifted into a 20 mL vial. About 0.5 mL of the liquid products were collected with a 3.8 cm syringe, filtered with a 0.22 μm PTFE filter, and analyzed by GC-MS/FID. The major liquid products were separated and collected by filtration with a sand core funnel. The solid residues were washed with small portions of EtOH, filtered, and dried in vacuum.

Author contributions

X.K., C.L., and R.X. conceived the depolymerization strategy. X.K. designed and participated in all the experiments. L.B. conducted experiments related to the catalyst synthesis and the catalytic depolymerization of lignin model compounds and condensed lignin. X.W. characterized the catalysts and performed the NMR analysis. X.K. and R.X. wrote the paper with input from C.L., W.L., R.B., B.X., T.L., H.Z., and K.B. All authors discussed the results and contributed to revising the paper. X.K. and L.B. contributed equally to this work.

Peer review

Peer review information

Nature Communications thanks Fang Huang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

Source data for Figs. 2c-f, 3b-f, 5, 6b-d, and 8c-h are provided with this paper. Other data supporting the findings of this study are available within the paper and its supplementary information files, or from the corresponding author upon request. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-70103-0.