Bottom line. Yes—solvent-free hydrodeoxygenation exists, but the clearly documented examples are mainly continuous processing or whole lignin/bio-oil hydrotreatment, not a controlled, solvent-free batch-autoclave comparison on an isolated lignin phenolic model compound. In the focused primary-source screen for this report, no matched autoclave study was verified that tests the same phenolic model compound, catalyst, H₂ charge, temperature and time both with and without added solvent. Thus “solvent-free is better/worse” cannot be concluded from cross-paper conversion comparisons. The reviewed papers below are a relevant verified set, not an exhaustive census of every paper ever published.
Why use—or omit—a solvent?
A solvent is not merely a diluent in a stirred pressure vessel. It changes the reaction environment, the apparent rate, and sometimes the chemistry. The impact depends on the particular phenol, catalyst, solvent, and reactor fill.
Feed handling & solubilityGuaiacol and substituted phenols can be viscous, low-melting, or poorly miscible with water. A compatible liquid can make a reproducible pumpable charge and maintain contact with catalyst. Water may be an especially poor choice for hydrophobic substrates, while nonpolar hydrocarbons generally dissolve them better.
Mass transfer & mixingIn a batch autoclave, solvent affects viscosity, catalyst suspension, gas–liquid interfacial area, and H₂ transfer. Without it, neat feed may improve volumetric productivity but can become mass-transfer-limited or difficult to stir. Report impeller, rpm, fill fraction, and whether feed is fully molten.
Adsorption & pathwaySolvent molecules compete for catalyst sites and can alter adsorption, tautomer populations, acidity, and selectivity. A direct study of p-cresol/guaiacol/mequinol found different behavior in hydrocarbon versus water media; hydrocarbon media favored aromatic product formation in the reported comparisons [1].
Hydrogen donor vs spectatorAlcohols and tetralin-like compounds may donate hydrogen or undergo transformations; they are not necessarily inert solvents. Product carbon balance must distinguish feed-derived products from solvent-derived products. Solvent blanks and labeled/internal standards may be needed.
Heat, pressure & safetySolvent changes heat capacity, vapor pressure and autogenous pressure. Initial cold H₂ pressure is not hot operating pressure; headspace and solvent vapor matter. Solvent-free does not mean safer: concentrated phenolics, high H₂ pressure, and exotherms still require proper pressure-vessel limits.
Downstream separation & economicsAdded solvent dilutes product and creates recovery/separation duty, but can ease temperature control and catalyst/product separation. Compare productivity per reactor volume and solvent recovery energy—not just conversion.
These are mechanistic and engineering considerations, not claims that any solvent is categorically inert or optimal. The solvent role should be established for the specific catalyst–substrate pair.
What does “solvent-free” mean here?
No added reaction medium means the phenolic feed (or lignin/oil feed) is charged without a separate liquid used primarily as carrier. It does not exclude water generated during HDO, water in wet feed, hydrogen donor reagents, or liquid products formed in situ. Nor is a gas-phase/continuous vapor-feed reactor equivalent to a neat liquid batch charge. In whole-lignin processing, the feed itself may form a liquid phase at temperature; papers sometimes call that solvent-free even though the hot reaction mixture is not a solid.
Verified paper matrix: what solvent and what ratio?
Ratios are given on the reported basis. Where the paper gives feed concentration rather than a weighed solvent mass, that concentration is retained rather than converted into an assumed ratio. The table separates strict model-compound autoclave evidence from adjacent examples.
| Study / evidence class | Substrate & reactor context | Solvent and reported feed:solvent amount | What it supports—and does not |
|---|---|---|---|
| Maglinao et al. (2024), “Role of solvent in selective hydrodeoxygenation of monomeric phenols,” Biomass and Bioenergy 189, 107342. DOI: 10.1016/j.biombioe.2024.107342 Direct solvent comparison | p-Cresol, guaiacol and mequinol; Pd/C and Ru/C; batch runs in a 20 mL 316 stainless-steel pressure reactor. The paper also includes semi-batch experiments. | Compares water with hydrocarbon media including hexadecane; toluene is reported among products/solvent conditions. Exact substrate-to-solvent charge ratio is not verified from the accessible article text in this screen; do not infer it from plot yields or nominal reactor size. | Most useful direct evidence that solvent polarity/media can change conversion and pathway. For p-cresol, the highest toluene mass yield reported was 13.46 ± 5.80% at 250 °C and initial H₂ 4.2 MPa in a hexadecane run; hydrocarbon media gave higher conversion than water in reported comparisons. Not a demonstrated solvent-free arm. Results are not a universal ranking of solvents. [1] |
| Duan et al. (2020), “Effects of Crystallite Sizes of Pt/HZSM-5 Zeolite Catalysts on the Hydrodeoxygenation of Guaiacol,” Nanomaterials 10, 2235. DOI: 10.3390/nano10112235 Batch, solvent used | Guaiacol in a 100 mL stainless batch autoclave; Pt/HZSM-5 catalysts. | 0.5 g guaiacol + 25 mL n-dodecane; feed:solvent = 1:50 g:mL. Mass ratio can be estimated only if the actual solvent density/temperature basis is specified; no need to pretend the volume ratio is a mass ratio. | Clear solvent-based autoclave benchmark. The solvent is not shown by this design to be inert under every condition; it is a medium. No matched neat-guaiacol control identified. [2] |
| “Catalytic Hydrodeoxygenation of Bio-oil Model Compounds over Pt/HY Catalyst” (2016), Catalysts 6, 108. DOI: 10.3390/catal6060108 Batch, solvent used | Guaiacol and other model compounds; batch reactor; reduced Pt/HY. | 0.4 g catalyst + 40 mL of 7.5 wt% guaiacol solution in decane. This corresponds to 3.0 g guaiacol in 40 mL decane (solution mass fraction reported; exact solvent mass not supplied in that formulation excerpt). Feed concentration = 7.5 wt%. | Supports liquid-phase, diluted autoclave operation and reports most decane remained unchanged in the tested experiment. That does not prove zero solvent participation for other catalysts/conditions. No solvent-free paired run identified. [3] |
| “Selective Hydrodeoxygenation of Guaiacol to Cyclohexane over Ru-Catalysts Based on MFI Nanosheets” (2023), Processes 11, 1704. DOI: 10.3390/pr11061704 Batch, solvent used | Steel autoclave, magnetic stirring; Ru catalysts; H₂ charged to 50 bar. | 2.00 g substrate solution at 10 wt% guaiacol in n-dodecane (about 0.20 g guaiacol + 1.80 g solvent; feed:solvent ≈ 1:9 w/w), 0.050 g catalyst. | Useful explicit mass-basis ratio for guaiacol. It is solvent-based, not a neat-feed comparator. [4] |
| “Catalytic Hydrodeoxygenation of Guaiacol on Rh-Based and Sulfided CoMo and NiMo Catalysts” (2011), Energy & Fuels 25, 890–896. DOI: 10.1021/ef101521z Full method ratio not verified here | Guaiacol model-compound HDO; Rh-based and sulfided CoMo/NiMo catalysts. | Solvent-based batch studies are reported in this literature, but exact charge ratio was not verified from accessible primary text in this screen. | Relevant candidate for a broader bibliography, but not included as a quantitative ratio claim absent verified methods. [5] |
| “Hydrodeoxygenation of Guaiacol as a Bio-Oil Model Compound over Pillared Clay-Supported Nickel–Molybdenum Catalysts” (2019), J. Phys. Chem. C. DOI: 10.1021/acs.jpcc.9b01890 Full method ratio not verified here | Guaiacol; autoclave reaction on Ni–Mo clay catalysts. | Exact substrate-to-solvent quantity not verified from accessible method excerpt. | Include in further full-text screening, not as a ratio-bearing record until the methods/supporting information is checked. [6] |
| Wen et al. (2025), β-O-4 dimer/PPE study, Molecular Catalysis 580, 115134. DOI: 10.1016/j.mcat.2025.115134 Adjacent dimer-model evidence | 2-Phenylethyl phenyl ether (PPE), a β-O-4 model, in batch; Ru/Nb₂O₅. | 0.5 g PPE + 50 g decalin = 1:100 w/w; catalyst 0.5 g. Conditions reported as 5 MPa H₂, 250 °C, 2 h. | Strong quantitative solvent-use example for a lignin linkage model, but not a monomeric phenolic substrate and no paired solvent-free PPE control was verified. [7] |
| “Solventless hydrodeoxygenation of dihydroeugenol …” (2026), Reaction Kinetics, Mechanisms and Catalysis. DOI: 10.1007/s11144-026-03188-9 Continuous, not autoclave batch | Dihydroeugenol over Ni/Fe catalysts, continuous reactor, 275–300 °C, 30 bar H₂. | No added solvent (solventless feed). Feed-to-solvent ratio: not applicable. | Confirms solventless HDO of a lignin-derived phenolic molecule is possible in continuous flow. It does not answer whether a neat liquid batch-autoclave run is feasible or superior. [8] |
Excluded from strict “thermal H₂ autoclave HDO” counts
- Wu et al. (2016), “Decomposition of β-O-4 Linked Lignin Model Compound in Anhydrous Ethanol without any Added Catalyst,” BioResources 11, 10349–10377, DOI 10.15376/biores.11.4.10349-10377. 0.100 g dimer + 50 mL ethanol (approximately 1:395 w/v, using reported volume; density-based mass conversion not necessary). This is a solvent/thermal decomposition study without externally charged H₂ and without added catalyst—not a direct solvent-free catalytic HDO comparison. Ethanol can act as a hydrogen donor/reactant; classify separately.
- Ren et al. (2023), water-mediated “hydrogen-free” guaiacol HDO, Green Chemistry, DOI 10.1039/D2GC04340H. Water is the reaction medium and a reactant in the proposed chemo-splitting route; “hydrogen-free” means no externally supplied molecular hydrogen under that process, not solvent-free.
- Solvent-free treatment of whole lignin or pyrolysis oil, and solvent-free HDO of triglycerides/fatty acids, are chemically and operationally adjacent but do not establish solvent-free batch HDO for isolated guaiacol/phenol. Do not pool them with monomer model-compound data.
When is solvent important in an autoclave?
Solvent is usually useful when
- the phenolic feed will not form a stirrable, uniform phase at loading temperature;
- you need controlled low substrate concentration to limit condensation/coke or compare intrinsic kinetics;
- heat removal, catalyst suspension, or reproducibility is a concern;
- you need to model liquid-phase upgrading or co-processing with an oil stream.
Solvent may be unnecessary—or counterproductive—when
- the substrate is liquid/miscible at charge and reaction temperature, and stirring/gas transfer can be demonstrated adequate;
- volumetric productivity and simpler product recovery matter;
- the solvent competes for catalyst sites, changes selectivity, reacts, or burdens product separation.
Do not infer neat-feed behavior from a dilute solution result. Changing from 5–10 wt% solution to neat feed also changes substrate activity, catalyst/feed ratio, gas–liquid transfer, heat capacity, headspace composition, and potentially H₂ partial pressure. Any observed difference is not attributable to “solvent” alone unless those are controlled.
Recommended paired autoclave experiment
- Use a single well-characterized catalyst batch and identical feed mass, catalyst mass, vessel, impeller, temperature, time and initial H₂ charge.
- Compare neat feed with a selected solvent at at least two defined concentrations (e.g., 10 wt% and 30 wt% feed in solvent; these are proposed screening points, not literature optima). Also consider solvent-only/catalyst and solvent-only/no-catalyst blanks.
- Record cold H₂ pressure, hot/autogenous pressure if measurable, headspace, liquid fill fraction, heating/cooling profiles, agitation, and whether the substrate is fully dissolved or molten.
- Report conversion, carbon balance, oxygen removal/deoxygenation, product selectivity and yield separately; quantify gas, aqueous/organic products and coke. Include solvent-derived products in the carbon accounting.
- For polar vs nonpolar media, select solvents compatible with materials and downstream recovery; treat alcohols, water, and hydrogen donors as potentially reactive rather than automatically inert.
The suggested 10/30 wt% points are a practical screen, not a published optimum or universal recommendation.
Scope, confidence and what “all papers” can mean
This report is a focused evidence screen of relevant, identifiable batch/autoclave model-compound work plus one newer continuous solventless example. It is not a systematic review with exhaustive database exports, full-text retrieval of every candidate, or supplementary-information audit for every historical paper. “All possible papers” is not a defensible completeness claim without defining databases, dates, search strings, and inclusion rules.
In this screen, the key negative finding is narrowly worded: no verified matched solvent-free batch-autoclave control for an isolated lignin phenolic model compound was located. This does not prove none exists. Several studies have exact ratios available from open methods; where full text was not accessible, the table explicitly marks the ratio unverified rather than guessing. Ratio bases (w/w, w/v, wt%, g:mL) are deliberately not conflated.
For a defensible exhaustive inventory, search databases such as OpenAlex/Crossref plus publisher indexes and theses using guaiacol, phenol, cresol, anisole, catechol, syringol, eugenol, model compounds, HDO, hydrodeoxygenation, batch, and autoclave; screen references and supporting information; and log every inclusion/exclusion. This webpage is a strong starting matrix, not a claim that the entire corpus is settled.
Sources & references
- Maglinao, R. L. et al. “Role of solvent in selective hydrodeoxygenation of monomeric phenols.” Biomass and Bioenergy 189 (2024), 107342. https://doi.org/10.1016/j.biombioe.2024.107342. Publisher article/abstract and accessible text used for scope, solvent comparison, reactor and reported yield.
- Duan, H. et al. “Effects of Crystallite Sizes of Pt/HZSM-5 Zeolite Catalysts on the Hydrodeoxygenation of Guaiacol.” Nanomaterials 10 (2020), 2235. https://doi.org/10.3390/nano10112235. Open full text; charge 0.5 g guaiacol + 25 mL n-dodecane.
- “Catalytic Hydrodeoxygenation of Bio-oil Model Compounds over Pt/HY Catalyst.” Catalysts 6 (2016), 108. https://doi.org/10.3390/catal6060108. Open full text; 7.5 wt% guaiacol in decane formulation.
- “Selective Hydrodeoxygenation of Guaiacol to Cyclohexane over Ru-Catalysts Based on MFI Nanosheets.” Processes 11 (2023), 1704. https://doi.org/10.3390/pr11061704. Open full text; 10 wt% guaiacol/n-dodecane solution, 2.00 g charge.
- Lin, Y.-C. et al. “Catalytic Hydrodeoxygenation of Guaiacol on Rh-Based and Sulfided CoMo and NiMo Catalysts.” Energy & Fuels 25 (2011), 890–896. https://doi.org/10.1021/ef101521z. Bibliographic record verified; exact ratio not included because primary method ratio was not verified in this screen.
- “Hydrodeoxygenation of Guaiacol as a Bio-Oil Model Compound over Pillared Clay-Supported Nickel–Molybdenum Catalysts.” J. Phys. Chem. C (2019). https://doi.org/10.1021/acs.jpcc.9b01890. Bibliographic record and batch context verified; ratio not asserted.
- Wen, et al. “Exploring the hydrodeoxygenation of lignin β-O-4 dimer model compound and bio-oil by DFT and experimental studies.” Molecular Catalysis 580 (2025), 115134. https://doi.org/10.1016/j.mcat.2025.115134. Ratio as reported in accessible primary text: PPE:decalin 0.5:50 g.
- “Solventless hydrodeoxygenation of dihydroeugenol in a continuous reactor over Ni catalysts modified by Fe and Ce.” Reaction Kinetics, Mechanisms and Catalysis (2026). https://doi.org/10.1007/s11144-026-03188-9. Search-accessible publisher text; continuous, solventless, not batch.
- Wu, Y. et al. “Decomposition of β-O-4 Linked Lignin Model Compound in Anhydrous Ethanol without any Added Catalyst.” BioResources 11 (2016), 10349–10377. https://doi.org/10.15376/biores.11.4.10349-10377. Open full text; adjacent ethanol decomposition, not externally H₂-fed catalytic HDO.
- Ren, X. et al. “Hydrodeoxygenation of guaiacol to phenol using endogenous hydrogen induced by chemo-splitting of water over a versatile nano-porous Ni catalyst.” Green Chemistry (2023). https://doi.org/10.1039/D2GC04340H. Publisher record; water-mediated route, not solvent-free.