Sustainable production of benzene from lignin

The study addresses the challenge of producing benzene, an essential commodity chemical traditionally derived from fossil resources, via a sustainable route using lignin. The research question is how to efficiently and selectively abstract benzene rings from lignin by transforming the robust Csp2-Csp3 and Csp2-O bonds into Csp2-H bonds without hydrogenating the aromatic ring or relying on harsh conditions and exogenous hydrogen. The context highlights growing benzene demand, environmental drawbacks of fossil-based production routes, and lignin’s abundance as a renewable aromatic source. Prior lignin valorization methods typically yield complex mixtures with only trace benzene and often require severe conditions leading to undesired side reactions, particularly ring hydrogenation. The purpose is to design a practical, mild, and selective catalytic strategy that couples Brønsted acid-catalyzed Csp2-Csp3 bond deconstruction with hydrogenolysis of Csp2-O bonds using hydrogen abstracted from lignin itself, thereby enabling exclusive benzene production in water under nitrogen.

Recent advances in lignin valorization include aldehyde-stabilized depolymerization (Luterbacher et al.) to guaiacyl/syringyl monomers via hydrogenolysis or oxidation; oxidative depolymerization to syringaldehyde and vanillin (Stahl); direct hydrodeoxygenation to alkyl-substituted arenes and hydrocarbons (Wang); integrated processes to phenol (Sels; Wang); and catalytic pyrolysis, hydrodeoxygenation, and combined catalytic processing producing complex aromatic mixtures where benzene is low-yield. These approaches face limitations: harsh conditions, low benzene selectivity, and competing hydrogenation of the aromatic ring when exogenous H2 is used. Prior work by the authors introduced self-supported hydrogenolysis (SSH) of aromatic Csp2-O(CH3) bonds to Csp2-H without external hydrogen, preventing ring saturation. Other catalysts (e.g., MoO3) can hydrodeoxygenate lignin-derived compounds under H2 to arenes but risk hydrogenating the ring. Thus, a method integrating selective Csp2-Csp3 cleavage and Csp2-O hydrogenolysis without external H2 remains unmet.

Catalyst design and preparation: A multifunctional catalyst RuW/HY30 (RuW alloy nanoparticles supported on high-silica HY zeolite, Si/Al=30) was prepared by wet impregnation of ammonium metatungstate and Ru precursors onto pre-calcined zeolite, followed by H2/Ar reduction up to 900 °C and passivation. Controls included Ru/HY30, W/HY30, RuW on SiO2 and Al2O3, and other bimetallic MW/HY30 (M=Ni, Co, Fe, Mo, Cu), as well as RuW on other zeolites (Beta, HZSM-5, MCM-41). Reaction media and conditions: Water served as the solvent; reactions conducted under N2 (0.1 MPa) at 180–240 °C depending on substrate, with 800 rpm stirring. Model reaction optimization: 1-(4-methoxyphenyl)-1-propanol (1a) was used to mimic lignin phenylpropanol units containing one methoxy (Csp2-OCH3) and one 1-hydroxypropyl (Csp2-Csp3(OH)) substituent. Systematically varied catalyst composition/support, temperature, time, and presence/absence of exogenous H2. Analytical methods: GC-FID with internal standard for organic-phase quantification; GC-MS and 1H NMR for identification; solid-state NMR (13C MAS, 2D 13C{1H} HETCOR), 27Al NMR and MQMAS to probe intermediates and acid site environments; XRD with Rietveld refinement, XPS, XAFS (EXAFS/XANES) for catalyst structure and alloying; N2 physisorption for textural properties; TEM/HRTEM for nanoparticle morphology; ICP for metal loadings. Mechanistic studies: DFT calculations modeled Brønsted acid sites as reaction centers for Csp2-Csp3 transformation, exploring protonation, dehydration to oxonium/carbonium ions, γ-methyl shift, subsequent protonation at aromatic Csp2, and β-scission to form Csp2-H (benzene). The SSH pathway for Csp2-O cleavage on RuW used hydrogen abstracted from methoxy groups, generating formaldehyde that can further serve as hydrogen source for phenolic Csp2-O cleavage. Scope evaluation: Tested a portfolio of G- and S-type phenylpropanol monomers and dimers with various substituents (methoxy, hydroxyl, isopropoxy; ether linkages) under graded conditions (190–210 °C, catalyst load 0.25–0.35 g), tracking formation of benzene as sole liquid product at full conversion. Lignin extraction and conversion: Lignin was extracted from diverse woods and bamboo using acetone/water in an autoclave, recovered and characterized by 1H/13C 2D HSQC and quantitative 13C NMR. Catalytic conversion of isolated lignins was performed in water at 240 °C for 12 h under N2 with RuW/HY30. Scale-up: 50 g lignin with 50 g RuW/HY30 and 650 mL water in a 1-L autoclave at 240 °C to isolate benzene. Mass balance and product handling: Ethyl acetate capture of volatiles, phase separation, recovery of catalyst and residual lignin oil; mass balance reported. Catalyst recycling was assessed in model reactions via recovery, washing, and reuse.

• Developed a water-based, exogenous-H2-free in situ refining strategy using RuW/HY30 that exclusively converts lignin-derived phenylpropanol structures to benzene by coupling Brønsted acid-catalyzed Csp2-Csp3 deconstruction (HY30) with RuW-catalyzed hydrogenolysis of Csp2-O via SSH using hydrogen abstracted from lignin’s methoxy groups.
• Model compound performance: For 1-(4-methoxyphenyl)-1-propanol (1a), RuW/HY30 in water at 180 °C under N2 achieved 99.9% conversion with 97.3% selectivity to benzene (trace anisole 2.5% selectivity) and no cyclohexane; full conversion to benzene in 6 h without any ring-saturation products. In contrast, with exogenous H2, significant hydrogenation/byproducts formed: benzene (43.3%), cyclohexane (16.9%), n-propylbenzene (35.8%), n-propylcyclohexane (3.9%).
• Catalyst roles and necessity: HY30 alone cleaves Csp2-Csp3 to anisole but not Csp2-O; RuW is necessary for SSH of Csp2-O to benzene without ring hydrogenation. Ru/HY30 or W/HY30 behave like HY30 (no Csp2-O cleavage). Other bimetallic MW/HY30 (NiW, CoW, FeW, MoW, CuW) fail to perform SSH under N2 and show lower benzene selectivity even under H2. RuW on SiO2 or Al2O3 yields n-propylbenzene, demonstrating the unique requirement of HY zeolitic Brønsted acidity and mesoporosity.
• Structure–function insights: XRD/Rietveld verified RuW alloy on HY; EXAFS showed Ru–W coordination (Ru–W CN ~3.1; W–Ru CN ~0.9), and XPS indicated electron transfer from W to Ru. HY30 exhibits strong Brønsted acidity and substantial mesoporosity, facilitating fast Csp2-Csp3 deconstruction. HY with lower Si/Al (3–15) or other zeolites (Beta, HZSM-5) reduced benzene selectivity and increased n-propylbenzene due to micropore diffusion limits and different acid site distributions. MCM-41 (pure silica) lacked activity for Csp2-Csp3 cleavage.
• Mechanism: DFT supports a pathway involving protonation of the aliphatic OH, dehydration to carbonium ion, γ-methyl shift to form a tertiary alcohol, protonation at aromatic Csp2, and β-scission yielding benzene (Csp2-H). Solid-state 13C and 2D 13C{1H} HETCOR NMR detected key intermediates (e.g., shifts for Cα, Cβ, Cγ/Cβ3) and bonding of Cβ3 to the Brønsted acid center; 27Al MQMAS/NMR identified a new framework Al(IV) environment consistent with [Al–O(Cβ3)–Si] during γ-methyl shift. SSH on RuW hydrogenolyzes Csp2-O(CH3), producing formaldehyde which supplies active hydrogen for subsequent phenolic Csp2-O cleavage; formaldehyde-dependent benzene formation was confirmed with 4-(1-hydroxypropyl)phenol.
• Substrate scope: A broad set of G- and S-type phenylpropanol monomers and dimers, including methoxy-, hydroxy-, and isopropoxy-substituted derivatives and ether-linked dimers, were converted with >99% benzene yield at full conversion under tailored conditions (190–210 °C; Ru 3.0–3.5 wt%, W 17–20 wt% on HY30).
• Lignin conversion: Isolated lignins from pine, cedrela, poplar, willow, eucalyptus, peach, apple-wood, cedar, and bamboo were refined to benzene with yields all >10 wt% on lignin; pine lignin achieved up to 18.8% benzene yield under 240 °C, 12 h, N2. Intermediates (2,6-dimethoxyphenol, 2-methoxyphenol, phenol) were observed transiently and fully converted to benzene.
• Composition dependence: Benzene yield correlates with lignin unit composition; S-unit-rich lignins gave lower mass yields than G/H-rich lignins at equivalent phenylpropanol content. Methoxy group availability supports SSH and phenolic C–O cleavage.
• Scale-up: 8.5 g of pure benzene obtained from 50.0 g pine lignin with no saturation byproducts; slight yield decrease attributed to handling losses. Mass balance for lignin transformations: 92 ± 5%.
• Catalyst stability: RuW/HY30 retained crystallinity and nanoparticle size distribution after reaction; reusable over cycles in model reactions.

The results directly address the challenge of selectively abstracting benzene rings from lignin by demonstrating that Brønsted acid sites in mesoporous HY30 can deconstruct Csp2-Csp3 bonds in phenylpropanol motifs while RuW alloy centers perform SSH of Csp2-O bonds using hydrogen sourced from methoxy groups within lignin. This synergy eliminates the need for exogenous hydrogen and prevents detrimental aromatic ring hydrogenation, a common issue in hydroprocessing. Mechanistic DFT and operando solid-state NMR provide a consistent picture of the γ-methyl shift and β-scission sequence on HY30 and confirm SSH-driven hydrogen supply for both methoxy and phenolic C–O cleavage on RuW. The broad substrate scope, successful conversion across diverse lignins with >10 wt% benzene yields (up to 18.8%), and clean product slate underscore the strategy’s generality and selectivity. The process leverages water as a green medium and yields benzene as the sole liquid product, simplifying separation and improving process economics. The scalable demonstration and acceptable mass balance suggest feasibility for practical application, while composition-dependent yields highlight the importance of feedstock characterization and optimization.

This work introduces an in situ refining route for sustainable benzene production from lignin using a RuW/HY30 catalyst in water under nitrogen. The catalyst’s bifunctionality enables ordered deconstruction of Csp2-Csp3 and Csp2-O bonds to Csp2-H without ring saturation, yielding benzene exclusively. Mechanistic elucidation via DFT and solid-state NMR substantiates the Brønsted acid-mediated γ-methyl shift/β-scission pathway and RuW-enabled SSH hydrogenolysis. The method achieves up to 18.8 wt% benzene yield from lignin and scales to 8.5 g benzene from 50 g lignin, with broad applicability across lignin types. This strategy opens a viable, renewable pathway for benzene production and highlights the potential for catalyst and process design targeting specific lignin structures. Future research could focus on increasing benzene yields via catalyst optimization (acid site distribution/mesoporosity, RuW composition), tailoring to lignin composition (enrichment of G/H units), continuous-flow reactor development, catalyst longevity and regeneration studies, and techno-economic and life-cycle assessments for industrial deployment.

• Benzene yields, while exclusive, are moderate (maximum 18.8 wt% on lignin) and vary with lignin composition; S-unit-rich lignins produce lower mass yields than G/H-rich lignins at equivalent phenylpropanol content.
• Lignin conversions require elevated temperatures (typically 240 °C for 12 h) which may impact energy efficiency and scalability.
• The approach relies on methoxy-derived hydrogen for SSH; lignins with low methoxy content may limit phenolic C–O cleavage efficiency without additional hydrogen sources.
• Slight yield loss upon scale-up was observed due to product handling and separation; process engineering is needed to minimize losses.
• The mass balance for lignin transformations is 92 ± 5%, indicating some unaccounted fractions (e.g., gaseous/light products or analytical loss).