Evidence-focused mini-review · Literature checked 7 October 2026 · Full bibliography and evidence notes
Evidence-focused mini-review | Literature checked 7 October 2026
Syringol (2,6-dimethoxyphenol) is a useful but deliberately demanding model for syringyl-derived lignin phenolics: two ortho methoxy groups flank the phenolic OH, making steric access and sequential C–O cleavage consequential. Reported catalytic systems span acidic Ni catalysts producing cycloalkanes, Ru/zeolite systems producing aromatic hydrocarbons, Co/Ag and Co/graphene systems favoring cyclohexanol, and a recent Co–Fe–Ag design targeting phenol while retaining aromaticity. Sulfided NiMo and high-temperature fixed-bed Ni provide distinct catalyst-state and process regimes. These endpoints are not interchangeable measures of “best HDO.” Solvent is also a design variable: water enables aqueous metal/acid cascades; decalin and hexadecane support liquid-phase handling and co-processing; acetic acid can enter ester products. Evidence for catalyst durability is substantially thinner than for initial activity. Syringol represents an oxygen-rich S-type aromatic monomer, not linked lignin, side-chain-bearing lignin products, real bio-oil, or its impurities. Cross-study comparison therefore requires target-specific metrics, explicit mass balances, pressure/feed definitions, working-state characterization, and matched conditions.
Syringol is the common name for 2,6-dimethoxyphenol. Its three oxygen atoms occur as one phenolic hydroxyl and two aryl methoxy groups. This simple structure isolates demethoxylation/dehydroxylation questions without polymeric lignin complexity. The model is particularly stringent because both methoxy groups are ortho to the phenolic OH. Yet syringol lacks β-O-4 or other interunit linkages, lignin-derived propyl/allyl side chains, molecular-weight distribution, and real oil constituents such as water, acids, aldehydes, oligomers, sulfur, or minerals. It represents a local substitution pattern—not a miniature native lignin molecule.[24]
First define the target. Phenol retains the aromatic ring and one OH; cyclohexanol retains one oxygen but saturates the ring; cyclohexane or substituted cycloalkanes remove oxygen and saturate the ring; benzene/alkylbenzenes remove oxygen while retaining aromaticity. Thus high conversion is not enough: conversion, selectivity, yield, oxygen removal, and carbon balance answer different questions. A reported complete conversion may still correspond to moderate desired-product selectivity, side products, gas, coke, or unclosed balance.
Table 1 records direct-feed results where reported and visibly flags missing data. “Abstract-level” results must not be treated as full-text quantitative evidence. Some older papers explicitly include syringol but accessible records do not expose exact conditions or syringol-specific product metrics. They are counted as direct-feed literature, not as fully data-extracted comparisons.
| Catalyst / study | Solvent; conditions reported | Direct syringol result | Durability / evidence boundary |
|---|---|---|---|
| Step-precipitated Ni/SiO₂–Al₂O₃ (Si/Al 5), compared with Ni/SiO₂ and Ni/Al₂O₃ [1] | Decalin; 200 °C, 2 MPa H₂, 2 h | Combined support: 98.4% conversion, 98.9% hydrocarbon selectivity; cyclohexane 97.8% of products. Abstract/highlights round conversion to complete. | No comparable long-term syringol stability series verified. Confirm selectivity denominator and carbon balance before ranking. |
| Ru/HZSM-5 and hierarchical Ru/HZSM-5-OM [2] | Water; syringol test reported at 150 °C, 4 MPa H₂, 4 h | Conventional catalyst: 70.0% conversion, 55.4% cyclohexane selectivity; hierarchical variant: 97.5%, 70.0%, respectively. | Reuse data reported for hierarchical catalyst: after 10 cycles 95.8% conversion and 69.0% selectivity (primary SI/review cross-check; publisher full text was access-limited in this pass). |
| Ru/HZSM-5 aqueous hydrogenolysis of substituted phenols [3] | Water; 240 °C, 2 bar H₂ with 6 bar N₂; typically 1 h, longer for some syringols | Authors report quantitative/selective conversion of phenols, guaiacols, and syringols to bio-aromatics, high study-level carbon balance (98.6%). Exact syringol-specific yield/selectivity was not resolved. | Four-cycle test is reported for guaiacol, not syringol. Do not transfer its numbers to syringol. |
| Pd/C + HZSM-5 aqueous tandem system [14] | Water; 2 MPa initial H₂; temperature varied | 2,6-dimethoxyphenol explicitly tested. Multi-methoxy compounds are harder to convert than mono-substituted ones; cyclohexanone favored over cyclohexane for guaiacol/syringol even above 513 K. | Cyclohexanediol interactions with Brønsted acid sites proposed to inhibit further HDO. Numeric syringol-specific yields and reuse not extracted here. |
| Ir-, Ru-, Pd-, Pt-SBA-16 [5] | Decalin, because syringol is solid; 0.05 g feed, 0.005 g catalyst, 1 g decalin; 90–130 °C, initial H₂ 25–60 bar; 4 h | Ru/SBA-16 approached complete conversion at 130 °C; products included anisole, 1,2,3-trimethoxycyclohexane, cyclohexanediol, cyclohexanol. Pt conversion max 5%, Pd max 48% in tested range. | Reuse/regeneration described for Pd after guaiacol, not repeated syringol feed. Conversion is not deep-deoxygenation selectivity. |
| Ni, Fe, NiFe on activated carbon (AC) or TiO₂–SiO₂; fixed bed [6] | Vapor/fixed bed with H₂/Ar; no liquid solvent specified | 10 wt% Ni/AC gave 99.91% conversion; p-xylene, phenol, cresols major products; oxygen-free products mainly alkylbenzenes via transalkylation. | Accessible abstract does not establish long-duration stability. Different regime from liquid autoclaves. |
| NiMo@Al₂O₃ and TiO₂-shell NiMo@Al₂O₃ microspheres [7] | 270 °C, 3 h reported in accessible primary abstract; solvent and pressure unresolved | Near-complete conversion; hydrocarbon selectivity ~40.3% for core catalyst vs ~97% for TiO₂-shell. Shell catalyst reported ~87.7% methylcyclohexanes, ~2% cyclohexane, ~7.3% bicyclohexyl selectivities. | “Good stability/renewability” qualitative in accessible abstract; cycle count and decay magnitude unverified. |
| CoAg/SiO₂, in-situ activated without pre-reduction [8] | Publisher abstract calls conditions moderate; exact solvent and optimum values not independently secured in retrieved primary text. Patent-based secondary records suggest 280 °C, 3 MPa H₂, 2 h—unconfirmed. | Complete conversion and 93.8 mol% cyclohexanol selectivity reported by publisher abstract. | Avoids a separate pre-reduction step; numerical syringol cycling evidence not verified. |
| CoOₓ/single-Co species on reduced graphene oxide, Co/rGO [9] | Mild conditions per primary abstract; exact solvent/numeric optimum not retrieved | High selectivity to cyclohexanol; catalyst operates without pre-reduction. Proposed active features: core-shell multivalent CoOₓ, vacancies, isolated Co atoms. | Abstract says catalyst components are stable, but numerical cycle protocol was not accessible here. |
| Ni/BEA individual feeds and lignin-oil surrogate [10] | Individual compounds: 220 °C, 1 h, 50 bar H₂, C/F 0.2, hexadecane; syringol included | Abstract gives pooled model-compound ranges (80–100% conversion; 37–81% alkylcycloalkane yield), not an individual syringol value. | Surrogate mixture forms carbonaceous deposits; lower temperature/longer time limits them; regeneration partly recovers properties. Mixture evidence is not syringol-only deactivation. |
| In-situ NiMoS from oil-soluble Mo/Ni precursors + sulfur [11] | 300–350 °C, 1–7 MPa H₂, 15–300 min; substrate:Mo varied | Abstract reports up to 100% conversion and high deoxygenated-product yields, primarily cyclohexane; proposed route via guaiacol/phenol. Characterization of formed/used particles reports MoS₂ and Ni–Mo–S. | Full condition-by-condition table and reuse stability not extracted. Do not imply a static active phase based only on ex-situ characterization. |
| CoFeAg/SiO₂, in-situ activated [12] | 1 MPa H₂, 330 °C reported; solvent/time not exposed in retrieved abstract | 100% conversion, 52.4% phenol selectivity; reported comparators CoAg/SiO₂ 42.2 mol%, Co/SiO₂ 7.6%. Goal: aryl–O cleavage while retaining aromatic OH. | Proposed Fe/Ag tuning of Co reduction and CoO–Co/vacancy/Lewis-acid sites; durability protocol not retrieved. |
| Ru/C mechanistic lignin-model series [13] | Acetic acid; 80–140 °C, 10–40 bar H₂, 0.5–4 h across substrate/parameter runs | Syringol included in feed set. Cyclohexanes/cyclohexanols and solvent-derived cyclohexyl acetates observed across study; no unsupported syringol-specific yield assigned here. | Valuable solvent-participation result; abstract’s quantified optimum concerns a different, side-chain-bearing phenol. |
| Ru–MnOₓ/C [17] | Exact syringol run conditions not independently transcribed | A later study’s literature comparison attributes ~70% cyclohexanol selectivity to a prior direct syringol experiment. Treat as a direct-feed lead pending original table verification. | Cross-citation only for this quantitative value; no independent full-text extraction in this pass. |
| RuCoNₓ/NC [15] | n-Dodecane, 220 °C, 1 MPa H₂, 240 min in a 2025 secondary comparison table | Secondary table reports 32% syringol conversion and 66% cyclohexanol selectivity. Verify original primary article/table before quantitative use. | No syringol-specific reuse data confirmed. |
| ZSM-5/BEA-supported Ni model compounds and surrogate mixture [18] | Hexadecane; batch autoclave; study-level reported optimum 220 °C, 50 bar H₂, 1 h for model set | Explicit direct syringol feed among phenol, cresols, catechol, anisole, guaiacol, creosol, trimethoxybenzene. Study-level model conversion/yield ranges 80–100% and 47–83%, respectively; exact syringol row not transcribed here. | Mixture-focused findings include ~30% partial deactivation after third cycle in reuse tests. Do not assign this directly to syringol-only runs. |
| Ru–MnOₓ/C, carbon-supported [17] | 2,6-dimethoxyphenol appears among tested methoxybenzenes; exact conditions not extracted | Primary article’s abstract says 2,6-dimethoxyphenol and anisole converted to demethoxylated saturated compounds. Reusable catalyst reportedly showed no significant activity loss. | Study is a direct substrate precedent but syringol-specific yield/selectivity requires original table. |
The inventory contains at least 15 distinct direct-feed primary works, but only a subset have condition-resolved data accessible here. The 2015 hierarchical Ru/ZSM-5 study and 2016 aqueous Ru/HZSM-5 study are distinct. The 2024 Ni/zeolite study and 2025 Ni/BEA deactivation paper are also distinct. Full cross-study numerical ranking is not justified where full text/SI or metric definitions are missing.
A 2017 thermocatalytic in-situ-H₂ process explicitly feeds syringol over Raney Ni + HZSM-5 in methanol–water; hydrogen is generated by aqueous-phase methanol reforming, not supplied as external pressurized H₂. A 2020 vanadium-catalyzed cleavage of 2,6-dimethoxyphenol proceeds without external hydrogen or organic solvent. These are useful process comparisons, but should be classified separately. Electrocatalytic hydrogenation/HDO papers are outside conventional thermal HDO and must not inflate its count.[19][20]
A useful pathway network contains sequential/parallel demethoxylation, dehydroxylation, aromatic-ring hydrogenation, dehydration or hydrolysis, and sometimes rearrangement/transalkylation. It is not one universal sequence: product endpoint depends on metal function, acid function, solvent, and operating conditions.
• Metal–acid bifunctionality. Ni/SiO₂–Al₂O₃ and metal/zeolite systems combine H₂ activation/ring hydrogenation with acid-catalyzed C–O removal or dehydration. The Ni/silica–alumina study associates performance with nickel dispersion and acid sites; hierarchical Ru/HZSM-5 improves access to acid sites and bulky intermediates relative to a conventional microporous catalyst.[1][2]
• Aqueous cascade chemistry. In Pd/C + HZSM-5 systems, metal and acid functions can promote hydrogenation and dehydration/hydrolysis sequences. Multiply substituted phenols may stop at oxygenated cyclic products; cyclohexanediol adsorption can inhibit acid sites. These findings explain why methoxy-rich syringol can remain difficult even when simple phenol or anisole converts readily.[3][14]
• Redox/vacancy systems. CoAg and CoFeAg papers propose partially reduced CoOₓ/Co ensembles, oxygen vacancies, and Lewis acidity as levers for H₂ activation and oxygenate adsorption. Separate precursor, working, and spent states: “oxide catalyst” is not necessarily one invariant phase under H₂.[8][12]
• Sulfide phases. In-situ NiMoS extends the field beyond reduced metals/oxides. Measure sulfur chemical potential/retention and phase evolution. Do not treat nominal precursor Ni:Mo as proof of alloying or a static active phase.[11]
• Porosity. Zeolites supply acidity and confinement but micropores may limit bulky substrate/intermediate access. Hierarchical porosity can help access while changing acid-site distribution. Surface area alone is inadequate; report pore distribution, acidity, metal dispersion and accessible active sites.[2][5]
• Operating variables. Temperature may promote ether cleavage and deoxygenation while also increasing cracking, transalkylation, gas and coke. Hydrogen pressure can shift ring saturation and condensation/deactivation. Because these trends co-vary with catalyst and solvent across studies, test them in controlled designs rather than inferring causality from unrelated “optimum” conditions.[3][6][14]
Report conversion as feed disappearance, selectivity with an explicit molar/carbon basis, product yield normalized to feed, and carbon balance separately. Do not multiply conversion by selectivity unless definitions and basis match. Oxygen removal is separate from aromatic-ring saturation. Cyclohexanol may be a valuable target chemical while still representing partial deoxygenation; phenol selectivity means selective cleavage while retaining an OH, not complete oxygen removal.
Syringol is solid near ambient conditions, so liquid-phase work must explain feed dissolution/suspension and mass transfer. The SBA-16 paper explicitly used decalin; fixed-bed vapor-phase studies instead require controlled vaporization, heated lines, and checks for condensation/sublimation.[5][6]
Water is viable when the catalyst withstands hydrothermal operation and the pathway benefits from aqueous hydrogenation/dehydration/hydrolysis chemistry. It is inexpensive and process-relevant, but affects phase behavior, support stability, leaching and product recovery. Direct aqueous syringol precedents include Ru/HZSM-5 and Pd/C + HZSM-5.[3][14]
Decalin is a nonpolar, high-boiling handling solvent in direct syringol work. Hexadecane has been used as solvent/substrate to mimic co-processing phenolics with petroleum fractions, not as a universal best solvent.[5][10][18]
Acetic acid is not inert in the Ru/C lignin-model study: solvent-derived cyclohexyl acetates form. It can change product distribution and apparent oxygen removal, so use solvent blanks and identify solvent-derived carbon where relevant.[13]
Screen solvent as a factor, not a default prescription. Record feed solubility at reaction temperature/loading, solvent/feed ratio, phase behavior, headspace and agitation. Compare nonpolar liquid versus water only when catalyst compatibility permits. Add blanks and isotope labeling if hydrogen donation or solvent incorporation is plausible. Alcohol-transfer-H₂ experiments should be labeled separately from external-H₂ HDO. Do not call a solvent inert without considering donor behavior, adsorption/coordination, esterification, and support effects.
For batch reactors, report vessel volume/fill, headspace, agitation, H₂ cold/initial charge separately from hot pressure and H₂ partial pressure, and whether pressure is autogenous or controlled. For flow systems report pretreatment, bed dilution, vaporization, gas/feed ratio, space velocity and time-on-stream. These distinctions are necessary to compare batch, slurry, aqueous and vapor-phase studies.
The strongest concrete durability evidence includes 10-cycle hierarchical Ru/HZSM-5 data with modest losses in conversion/selectivity, and reuse/deactivation studies on Ni/zeolite surrogate feeds. The latter report partial deactivation after three cycles and carbonaceous deposits; regeneration only partly recovers properties.[2][10][18] These are more informative than an unquantified “good stability” statement, but do not establish industrial lifetime. Some tests concern guaiacol or mixtures, not syringol alone.
Distinguish coking/pore blockage, metal sintering/migration, leaching, oxidation/reduction or sulfidation drift, sulfur/nitrogen/alkali poisoning, hydrothermal damage and acid-site loss. Characterize fresh, activated, spent and regenerated catalyst using suitable microscopy, diffraction, spectroscopy, chemisorption, elemental analysis, acidity and pore analysis. For aqueous systems quantify leaching and hydrothermal changes; for sulfides track sulfur; for zeolites assess pore blockage. Pair recovered activity with selectivity and carbon-balance recovery.
Minimum reporting checklist:
1. Feed identity/purity; syringol-only versus mixture; water/solvent content.
2. Catalyst composition basis, synthesis, calcination/reduction/sulfidation and working-state evidence.
3. Reactor, feed/catalyst ratio, solvent ratio, fill/headspace, cold H₂ charge versus hot pressure, agitation/flow, temperature, time and ramp history.
4. Calibrated analytics; separate organic, aqueous, gas and solid/coke fractions.
5. Conversion, carbon-based selectivity, molar yield, oxygen removal, H₂ consumption, carbon balance, replicates and uncertainty.
6. Time courses/initial rates; reuse/regeneration with standardized recovery protocol.
7. Controls separating support acidity, loading/dispersion, promoter effect and in-situ activation.
A practical proposed benchmark set is Ni/acid for cycloalkanes, Ru/zeolite for aromatic hydrocarbons, CoAg for cyclohexanol, and CoFeAg for phenol retention. This is a target-based comparison proposal, not a claim that published conditions are directly comparable. First reproduce one benchmark with complete balance; then compare at matched conversion and controlled feed/catalyst/solvent; only then optimize conditions.
Use syringol alongside guaiacol and a side-chain-bearing S-type substrate such as 4-propyl- or 4-allylsyringol. This separates the additional ortho-methoxy challenge from side-chain steric/carbon-retention effects. Progress next to a defined multicomponent surrogate and real lignin-derived oil. Track conversion, selectivity, carbon balance, hydrogen use and decay at each complexity step. A small factorial design of temperature, H₂ pressure and contact time—holding catalyst, feed ratio and solvent constant—will be more interpretable than a comparison of unrelated published optima.
For support/promoter causality, include monometallic and physical-mixture controls at matched loading, with texture/acidity characterization. For in-situ activation claims, compare characterized pre-reduced states with reaction-formed states. For stability, measure sufficient time-on-stream or cycles to distinguish activation from decay and report fresh, spent and regenerated performance.
Syringol HDO tests the balance between methoxy removal, phenolic C–O cleavage, ring hydrogenation and depth of deoxygenation. Published product goals include cycloalkanes (Ni/acid, Ru/zeolite), cyclohexanol (Co systems), aromatic hydrocarbons (Ru/HZSM-5, fixed-bed Ni/AC), and phenol (CoFeAg). Solvent and working catalyst state are part of the chemistry. Yet inconsistent metric bases, incomplete accessible full texts and sparse syringol-specific durability data prevent a universal catalyst ranking. The appropriate catalyst depends on target product and must be evaluated with matched conditions, full balances, working-state evidence and an explicit stability protocol. Syringol is a useful, stringent S-type monomer model—not a substitute for linked lignin, side-chain products or real bio-oil.
1. Shu, R.; Xu, Y.; Ma, L.; Zhang, Q.; Chen, P.; Wang, T. “Synergistic effects of highly active Ni and acid site on the hydrodeoxygenation of syringol.” Catalysis Communications 91 (2017), 1–5. doi: 10.1016/j.catcom.2016.12.006
2. Wang, L. et al. “Mesoporous ZSM-5 Zeolite-Supported Ru Nanoparticles as Highly Efficient Catalysts for Upgrading Phenolic Biomolecules.” ACS Catalysis 5 (2015), 2727–2734. doi: 10.1021/acscatal.5b00083
3. Luo, Z. et al. “Hydrothermally stable Ru/HZSM-5-catalyzed selective hydrogenolysis of lignin-derived substituted phenols to bio-arenes in water.” Green Chemistry 18 (2016), 5845–5858. doi: 10.1039/C6GC01971D
4. Venkatesan, K. et al. “Hydrodeoxygenation kinetics of syringol, guaiacol and phenol over H-ZSM-5.” Catalysis Communications 148 (2021), 106164. doi: 10.1016/j.catcom.2020.106164
5. Szczyglewska, P.; Feliczak-Guzik, A.; Jaroniec, M.; Nowak, I. “Catalytic role of metals supported on SBA-16 in hydrodeoxygenation of chemical compounds derived from biomass processing.” RSC Advances 11 (2021), 9505–9517. doi: 10.1039/D0RA06696F
6. Vo, T. A. et al. “Non-precious metal catalysts supported by activated carbon and TiO₂–SiO₂: Facile preparation and application for highly effective hydrodeoxygenation of syringol—a lignin-derived model compound.” Journal of Industrial and Engineering Chemistry 122 (2023), 138–151. doi: 10.1016/j.jiec.2023.02.016
7. Vo, T. A. et al. “Bimetallic NiMo-supported Al₂O₃@TiO₂ core-shell microspheres with high hydrodeoxygenation efficiency toward syringol.” Journal of Sol-Gel Science and Technology (2023). doi: 10.1007/s10971-023-06068-z
8. Liu, C. et al. “In situ activated CoAg bimetallic catalyst for selective hydrodeoxygenation of syringol to cyclohexanol.” Applied Catalysis A: General 679 (2024), 119742. doi: 10.1016/j.apcata.2024.119742
9. Qu, X. et al. “Core–shell structured cobalt oxide nanoparticles and single Co atoms supported on graphene for selective hydrodeoxygenation of syringol to cyclohexanol.” Catalysis Science & Technology 14 (2024), 3382–3395. doi: 10.1039/D4CY00295D
10. Margellou, A. G. et al. “Hydrodeoxygenation of Phenolic Compounds and Lignin Bio-Oil Surrogate Mixture over Ni/BEA Zeolite Catalyst and Investigation of Its Deactivation.” Catalysts 15 (2025), 48. doi: 10.3390/catal15010048
11. Nesterov, I. S. et al. “Hydrodeoxygenation of Lignin Depolymerization Products: Activity of In Situ Synthesized NiMoS Catalysts in Syringol and 2-Methoxyhydroquinone Conversion.” Petroleum Chemistry 65 (2025), 943–950. doi: 10.1134/S0965544125601759
12. Sha, H.; Feng, Y.; Mao, J.; Lv, H.; Zhou, J. “Fe-mediated enhancement of phenol selectivity in syringol hydrodeoxygenation over an in-situ-activated CoFeAg/SiO₂ catalyst.” Applied Catalysis A: General 708 (2025), 120553. doi: 10.1016/j.apcata.2025.120553
13. Vriamont, N. et al. “From lignin to chemicals: Hydrogenation of lignin models and mechanistic insights into hydrodeoxygenation via low-temperature C–O bond cleavage.” ACS Catalysis (2019). doi: 10.1021/acscatal.8b04714
14. Zhang, C. et al. “An investigation on the aqueous-phase hydrodeoxygenation of various methoxy-substituted lignin monomers on Pd/C and HZSM-5 catalysts.” RSC Advances 6 (2016), 104398–104406. doi: 10.1039/C6RA22492J
15. Zhao, M. et al. “Hydrodeoxygenation of lignin-derived phenolics over facile prepared bimetallic RuCoNₓ/NC.” Fuel 308 (2022), 121979. doi: 10.1016/j.fuel.2021.121979 (Secondary compilation reports direct syringol values; consult primary table.)
16. Gao, X. et al. “Hydrodeoxygenation of lignin biophenolics to cyclohexanes over sub-nanometric Ru multifunctional catalyst.” Renewable Energy (2022). doi: 10.1016/j.renene.2022.10.090 (Biophenolic/adjacent scope; no syringol-specific value relied on here.)
17. “Demethoxylation of guaiacol and methoxybenzenes over carbon-supported Ru–Mn catalyst.” Applied Catalysis A: General (2015). DOI verification required; a suspected DOI was not included to avoid a potentially mismatched citation. The primary article is a direct 2,6-dimethoxyphenol substrate lead.
18. Zormpa, F. F. et al. “Hydrodeoxygenation of lignin bio-oil model compounds and surrogate mixtures over zeolite supported nickel catalysts.” Catalysis Today 433 (2024), 114654. doi: 10.1016/j.cattod.2024.114654
19. Liu, X. et al. “Liquid phase in situ hydrodeoxygenation of biomass-derived phenolic compounds to hydrocarbons over bifunctional catalysts.” Applied Catalysis A: General (2017). doi: 10.1016/j.apcata.2017.05.022 (In-situ H₂ from methanol reforming; separate from external-H₂ HDO.)
20. Yu, P. et al. “Catalytic Cleavage of the C–O Bond in 2,6-dimethoxyphenol Without External Hydrogen or Organic Solvent Using Catalytic Vanadium Metal.” Frontiers in Chemistry 8 (2020), 636. doi: 10.3389/fchem.2020.00636 (Non-conventional hydrogen-free process.)
21. Jin, W. et al. “Catalytic Upgrading of Biomass Model Compounds: Novel Approaches and Lessons Learnt from Traditional Hydrodeoxygenation – a Review.” ChemCatChem (2019). doi: 10.1002/cctc.201801722 (Review/context.)
22. “An investigation on the aqueous-phase hydrodeoxygenation of various methoxy-substituted lignin monomers on Pd/C and HZSM-5 catalysts.” RSC Advances 6 (2016), 104398–104406. doi: 10.1039/C6RA22492J (Direct study; duplicate entry intentionally omitted from direct-study count.)
23. “Hydrothermally stable Ru/HZSM-5-catalyzed selective hydrogenolysis of lignin-derived substituted phenols to bio-arenes in water.” Green Chemistry 18 (2016), 5845–5858. doi: 10.1039/C6GC01971D (Direct study; duplicate metadata pointer.)
24. Background on lignin S/G/H units and model-compound scope is drawn from the cited primary-study introductions and general HDO reviews; not counted as direct HDO research.
Evidence note. The bibliography includes more than 20 unique works, separating direct-feed studies from adjacent, non-conventional and background sources. At least 15 direct syringol primary works are identified, but for several, exact operating/product/stability details were not accessible. The citations intentionally flag these evidence limits; the table is not a systematic-review or meta-analysis, and the direct-study count is distinct from the fully data-extracted count. A separate audit identified 14 direct external-H₂ thermal studies before adding the Ru–MnOₓ and two distinct Ni-zeolite model-compound records; one acid-only H-ZSM-5 kinetics paper is counted as a direct substrate study but excluded from a strict external-H₂ subset pending full-text confirmation. The resulting total is a screened bibliography, not a guarantee of exhaustiveness. The connected Consensus search reported its monthly quota exhausted; research continued via publisher pages, DOI-linked records, repositories and primary open text.