Ni/Al₂O₃ versus Ru/Al₂O₃

Hydrodeoxygenation activity, product selectivity and catalyst stability.

Primary scope: reduced, monometallic catalysts on alumina for lignin-derived phenolics, especially phenol and guaiacol. Mixed oxides, acid-modified supports, bimetallic catalysts and real bio-oil are identified separately. This is a critical narrative comparison, not a systematic review or a pooled kinetic ranking.

1. Main findings

There is no defensible universal “Ru > Ni” ranking. Ru/Al₂O₃ can give strong hydrogenation and selective oxygen removal under mild conditions, but a high conversion to cyclohexanol is not complete HDO to hydrocarbons; catalyst structure, water, acidity and feed identity change the comparison.[1][2][3]

Selectivity is conditional, not a fixed property of the metal. Both can produce saturated cyclic products, but atmospheric-pressure Ni/alumina also forms benzene and phenol.[1][2][8] The decisive distinction is oxygenated products versus fully deoxygenated hydrocarbons—not simply substrate disappearance.[1][2][8]

The long-term stability ranking remains unresolved. Ni/alumina shows deactivation during a two-hour gas-phase guaiacol test and structural/deposit changes in an aqueous reforming-coupled study.[2][8][9] Ru has a reported five-cycle case, but these are not matched durability experiments.[2][8][9]

2. What the performance metrics mean

• Conversion, X: disappearance of the starting substrate; it includes hydrogenation without oxygen removal.
• Selectivity, S: proportion of converted feed assigned to a product on a specified molar or carbon basis.
• Yield, Y: product formed relative to feed charged; not interchangeable with selectivity.
• Degree of deoxygenation, DOD: the paper-specific oxygen-removal metric; it must not be substituted for conversion or hydrocarbon yield.
• Activity: preferably an initial rate normalized to catalyst, metal mass or exposed metal sites. A six-hour batch endpoint is not an intrinsic turnover frequency.
• Stability: retention of rate and selectivity with time or reuse, supported by catalyst and liquid-phase analyses. High first-run conversion is not a durability test.

These definitions are used as the comparison framework. Where the original paper's normalization cannot be recovered, its value is explicitly kept on the reported basis rather than recalculated.

3. The clearest accessible same-laboratory comparison

Han, 2014 — phenol, γ-Al₂O₃, dry versus aqueous operation

Evidence: full thesis, not a peer-reviewed journal article. The aqueous test compares 20 wt% Ni with 5 wt% Ru; it is matched in reactor conditions, not metal loading or surface-site inventory.[1]

Testing conditions: 300 mL stirred batch reactor; 300 °C; 5 g phenol; 0.20 g catalyst; 15 g water for aqueous tests; 6 h; 500 rpm.[1] H₂ charged before heating, reported as 3 MPa and also “400 psi”—an internal inconsistency; neither is a verified hot H₂ partial pressure.[1] Calcination: Ni 450 °C, Ru 350 °C, 4 h in air.[1] Reduction: Ni 400 °C, Ru 300 °C, 4 h, H₂ 60 mL min⁻¹.[1]

Interpretation: Ru performs better in this aqueous endpoint test; Ni performs much better without added water.[1] The results do not isolate irreversible deactivation.[1] At lower Ru catalyst dose, performance also falls; that alone does not prove sintering.[1] Read Chapter 2, methods and Tables 2.3–2.6, printed pp. 48–70.[1]

4. Ru/Al₂O₃: selective partial HDO is a different objective

Song et al., 2025 — particle-size control in aqueous guaiacol conversion

Evidence: inspected publisher-indexed main-text passages; direct full-text download blocked; SI not independently inspected. About 1.5 wt% Ru was supported on γ-Al₂O₃ with Ru sizes of 0.6–7.5 nm.[2] The 1.5 nm material gave approximately 99% conversion and approximately 95% cyclohexanol yield; the article uses rounded yield/selectivity language, so these are not treated as identical exact metrics.[2]

Conditions: 20 mL batch reactor; guaiacol 0.3 mmol; catalyst 20 mg; water 3 mL; 190 °C; 5 bar H₂ charged before heating; 6 h; 400 rpm.[2] Activation: 200 °C for 2 h in 10% H₂, 40 mL min⁻¹.[2]

Selective aromatic ether-bond cleavage followed by ring hydrogenation favored cyclohexanol; premature ring saturation could retain the methoxy group as 2-methoxycyclohexanol.[2] Cyclohexanol still contains oxygen.[2] This result therefore demonstrates selective partial HDO, not an approximately 95% hydrocarbon yield.[2]

The main text reports five-cycle stability, referring to SI Fig.[2] S5/Table S2.[2] Recovery procedure, metal loss and numerical cycle-by-cycle data were not verified here; no industrial-lifetime claim is made.[2]

Singh and Dhepe, 2020 — acidic alumina and solvent effects

Evidence: institutional-repository abstract only; article issued in 2020, online in 2019. A 0.5 wt% Ru/acidic-Al₂O₃ catalyst produced an 82% cyclohexanol yield from guaiacol at 225 °C and reported 1 MPa H₂ using cyclohexane as solvent.[3] Time, reactor charge, stirring, pressure basis and recycling protocol were not available in the inspected abstract and are not inferred.[3]

The authors associate cyclohexanol formation with a lower reduction temperature and partially higher Ru oxidation state, and report that product adsorption influences further reactions.[3] This supports preparation- and solvent-sensitive selectivity; it does not establish that this catalyst outperforms Ni/Al₂O₃ at matched conditions.[3]

Valdés-Martínez et al., 2021 — why the support cannot be treated as passive

Evidence: publisher abstract only. For 0.6 wt% Ru catalysts prepared with Ru(NO)(NO₃)₃, phenol, cyclohexanol and ethanol probe reactions indicated cooperative metal–support behavior.[5] Ru/TiO₂ and Ru/ZrO₂ were more active than Ru/SiO₂ and Ru/Al₂O₃ for the studied HDO reactions.[5] Full reaction-specific conditions and stability data were not accessible, so no numerical ranking is reconstructed.[5]

This is contextual evidence that high hydrogenation capability alone does not guarantee efficient C–O cleavage on alumina.[5]

5. Ni/Al₂O₃: hydrogenation is not its only possible pathway

Nunes et al., 2023 — unmodified Ni/alumina control in gas-phase guaiacol HDO

Evidence: full primary article inspected through Europe PMC XML; SI not inspected. The study includes an unmodified 28 wt% Ni/γ-Al₂O₃ control as well as heteropolyacid-modified materials; the latter are not treated as plain Ni/Al₂O₃.[8]

Conditions: fixed-bed glass reactor; 300 °C; atmospheric total pressure; 100 mg catalyst; 5% v/v guaiacol in n-heptane at 3.0 mL h⁻¹; H₂/guaiacol molar ratio 50; 120 min on stream; particles 63–125 µm.[8] Ni/alumina preparation included treatment at 450 °C under N₂ then N₂/H₂ reduction for 4 h; a further H₂ pretreatment was applied for 1 h at reaction conditions.[8] The authors discuss coexisting oxidized and metallic Ni during testing.[8]

The control formed benzene prominently at 10 min; with time, conversion and benzene selectivity decreased while phenol selectivity increased.[8] This contradicts a universal description of Ni/alumina as exclusively ring-hydrogenating.[8] Spent-catalyst XRD showed no detected structural loss; IR indicated retained guaiacol.[8] Deposits/adsorption were discussed, but the observations do not uniquely identify the deactivation mechanism.[8] Conversion/selectivity used chromatographic-area formulas, not a demonstrated complete carbon balance.[8] See sections 2.2/3.3 and Figs. 10–11.[8]

Mortensen et al., 2013 — a useful Ni benchmark, not an alumina-matched Ru comparison

Evidence: inspected abstract. The phenol screen covered 23 catalysts at 275 °C and reported 100 bar H₂ in a batch reactor.[4] A two-step hydrogenation-to-cyclohexanol/deoxygenation-to-cyclohexane model described reduced-metal performance.[4] Ni/Al₂O₃ was active, but the Ru catalyst in the reported activity ranking was Ru/C, not Ru/Al₂O₃; transferring that ordering to an alumina-supported Ni–Ru comparison would confound the metal and the support.[4]

Chen et al., 2020 — aqueous guaiacol with hydrogen generated in situ

Adjacent hydrogen-supply mode; full primary methods/results inspected. The monometallic control was nominally 20 wt% Ni loading on an oxide-form basis, on γ-Al₂O₃.[9] Guaiacol conversion was 95%, with reported selectivities of 45% cyclohexanol and 25% cyclohexanone; cyclohexane was not observed.[9]

Conditions: stirred batch; 1.5 g guaiacol; 75 mL aqueous 1 wt% glycerol; 1 g catalyst; 250 °C; 2 h; 400 rpm; initial 10 bar after helium purging.[9] This is not 10 bar H₂: hydrogen arose from glycerol aqueous-phase reforming.[9] Calcination: 550 °C, 3 h in air.[9] Reduction: 650 °C, 2 h in H₂ at 30 mL min⁻¹.[9]

Spent material showed boehmite formation and sharper Ni diffraction, interpreted as possible sintering.[9] Reported coke yield was 7.7%; Table 6 labels it wt%, while Eq. 5 specifies carbon formed relative to feed carbon.[9] It is not a measured 7.7 wt% deposit on catalyst.[9] No extended time-on-stream or reuse test was established.[9] See methods §2.4, results §3.1/3.2 and Tables 4/6.[9]

6. Similarities and differences: what the literature actually supports

Shared behavior

Hydrogenation and oxygen removal must be separated. Both catalyst families can consume phenolic substrates while leaving oxygenated products.[1][2][4] For phenol, ring hydrogenation can form cyclohexanone/cyclohexanol before further conversion to cyclohexane.[1][2][4] For guaiacol, methoxy-group chemistry introduces additional branches and oxygenated products.[1][2][4]

The alumina formulation is part of the catalyst. Acidity, accessible metal sites and activation history can alter the relative rates of hydrogenation and oxygen removal; “on Al₂O₃” is not sufficient to make two catalysts equivalent.[2][3][5]

Differences, with their limits

The table is a synthesis of the cited studies, not a claim that every row was tested in one common experiment.[1][2][3] The guaiacol Ni entries come from the unmodified control experiments, not the promoted formulations.[8][9]

Aromatics versus cycloalkanes

Do not assume that switching from Ni to Ru on alumina preserves the aromatic ring.[1][2][8] The direct phenol comparison favors cyclohexane, while the mild Ru/guaiacol study targets cyclohexanol.[1][2][8] In contrast, low-pressure Ni/alumina guaiacol conversion can retain aromaticity as benzene or phenol; the latter still contains oxygen.[1][2][8] Aromatic hydrocarbon selectivity must therefore be measured separately at matched conversion.[1][2][8]

7. Catalyst stability: evidence, mechanisms and uncertainties

Observed loss of performance is not automatically a mechanism. Lower aqueous conversion could reflect adsorption competition, changed phase behavior, changes in active-site accessibility or irreversible changes.[1] Assigning leaching, sintering, coke or support transformation requires discriminating measurements rather than an endpoint alone.[1]

Recycling is useful but limited. For Ru/Al₂O₃, five cycles are reported in the 2025 main text, but the uninspected SI limits verification of the recovery and regeneration protocol.[2] This is narrower evidence than a continuous time-on-stream test with metal balance and spent-catalyst characterization.[2]

There is direct Ni deactivation evidence, but not a matched Ni–Ru durability test. Nunes et al. observed declining conversion and benzene selectivity over 120 min, with retained adsorbate indicated by IR.[8][9] Chen et al. observed support hydration, possible Ni sintering and carbon deposits after aqueous reforming-coupled operation.[8][9] Different media, catalyst states and hydrogen supplies prevent transferring those observations into a universal Ru stability advantage.[8][9]

Proposed verification package—not published comparative results: follow initial rate and hydrocarbon selectivity over time; determine Ni/Ru and Al in filtrates by ICP; use fresh/spent TEM or STEM for particle growth; quantify carbon with appropriate blank-corrected TGA/TPO; check support structure and porosity; compare washed-only and re-reduced catalysts. Distinguish reversible adsorption, irreversible structural loss and handling loss. Track the catalyst mass actually recovered after each cycle.

Practical conclusion: choose neither catalyst on a presumed lifetime advantage.[1][2] Establish durability in the intended water content and feed, at comparable conversion and severity.[1][2] The present evidence supports formulation-specific observations, not “Ru never deactivates” or “Ni is inherently unstable.”[1][2]

8. Consolidated testing-conditions register

The study sections give the accessible charges, activation and outcomes. The register below prevents unreported variables from becoming implied experimental facts.

Source mapping follows the row labels; no bar-to-MPa conversions or hot-pressure estimates have been inserted.[1][2][3] The Ni gas-phase, Ni reforming-coupled and Ru-support entries are supported by their respective studies.[8][9][5]

9. How to make a fair Ni–Ru decision

Proposed comparison, not a literature-derived optimum: use one alumina lot and characterize phase, acidity and porosity. Test phenol first to separate ring hydrogenation from hydroxyl removal; then guaiacol to introduce methoxy chemistry, followed by a representative real-feed mixture. Run dry and water-containing feeds with otherwise identical conditions.

Compare catalysts on three bases: catalyst mass, metal mass and exposed-metal sites. Establish a verified reduced state for each catalyst rather than forcing the same activation temperature. Report initial rates and selectivity at matched conversion, complete carbon and oxygen balances where feasible, gas products, recovered catalyst mass and replicate uncertainty. Check mixing and particle-size effects before claiming kinetic control.

For cyclohexanol, selective partial HDO on structured Ru/Al₂O₃ is directly relevant.[1][2][3] For fully deoxygenated cycloalkanes, rank hydrocarbon yield and oxygen removal rather than total conversion.[1][2][3] For aromatic products, measure aromatic retention explicitly instead of extrapolating from ring-saturating tests.[1][2][3]

10. Search scope and access limitations

Searched 10 November 2026 through scholarly-index records, Crossref, publisher discovery and institutional repositories, using hydrodeoxygenation/hydrogenolysis combined with Ni or Ru, alumina/γ-Al₂O₃, phenol/guaiacol and stability/recycling/water terms. The configured OpenAlex connector lacked its API key; direct OpenAlex REST records were used where accessible. No paid Consensus synthesis was consumed.

This is relevance-selected, not exhaustive or bibliometrically ranked. Main-text downloads for several Ru studies were blocked; evidence levels are stated at the point of use. Missing fields mean not accessible in the inspected evidence, not necessarily absent from the original article. The scope does not establish performance for fatty-acid deoxygenation, contaminants, whole lignin or commercial real-bio-oil operation.

Important primary studies not used for unverified numerical claims

• Valdés-Martínez et al., 2017: the study includes Ni, Ru and NiRu on alumina, titania and zirconia. Its original monometallic numerical tables and full testing protocol were not recovered. Consequently, numerical Ni-versus-Ru values repeated in later papers were not adopted as verified measurements.[6]
• Gao et al., 2025: Low-temperature demethoxylation pathways and solvent-driven hydrodeoxygenation for guaiacol upgrading catalyzed by anaerobically synthesized Ru/Al₂O₃, DOI 10.1016/j.jece.2025.120069. Metadata verified; insufficient accessible primary text for inclusion in the quantitative comparison.[7]

The accessible same-condition comparison is therefore thesis-level evidence, while the journal evidence is largely cross-study. This limitation prevents a publication-ready universal activity or stability ranking.

References & access notes

9 distinct source records are cited. Evidence-only leads are identified separately in the text; a verified DOI is not verification of an experimental result.

1. [1]
   Yinglei Han (2014).

   Upgrading of Pyrolysis Bio-oil to Renewable Fuels by Hydrodeoxygenation

   MSc thesis, University of Idaho · No DOI · thesis

   Full thesis: Chapter 2 methods and results; Tables 2.3–2.6, printed pp. 48–70 inspected. Not a peer-reviewed journal article.

   Relevance: Direct same-laboratory monometallic alumina comparison; different loadings and activation treatments, not intrinsic-rate matched.

2. [2]
   Chuqiao Song; Wei Cheng; Xiaojie Wu; Shufang Zhao; Ying Tang; Xin Tang; Yao Xu; Lili Lin; Siyu Yao (2025).

   Optimized Ru catalysts for the selective cleavage of CAr–OCH3 bonds in guaiacol under mild conditions

   Catalysis Science & Technology 15, 1839–1849 · DOI: 10.1039/D4CY01260G

   Publisher-indexed main-text passages, including sections 2.2, 2.4 and results inspected; direct HTML/PDF blocked. SI figures/tables not independently inspected.

   Relevance: Direct Ru/γ-Al2O3 particle-size series; partial main-text access, no Ni comparator.

3. [3]
   Dheerendra Singh; Paresh L. Dhepe (2020).

   Understanding the influence of alumina supported ruthenium catalysts synthesis and reaction parameters on the hydrodeoxygenation of lignin derived monomers

   Molecular Catalysis 480, 110525 · DOI: 10.1016/j.mcat.2019.110525

   Institutional-repository abstract inspected via indexed record; full article and SI not accessed. Issue year 2020, online 2019.

   Relevance: Direct 0.5 wt% Ru/acidic alumina result; only abstract-supported conditions/outcomes included.

4. [4]
   Peter M. Mortensen; Jan-Dierk Grunwaldt; Peter A. Jensen; Anker D. Jensen (2013).

   Screening of Catalysts for Hydrodeoxygenation of Phenol as a Model Compound for Bio-oil

   ACS Catalysis 3, 1774–1785 · DOI: 10.1021/cs400266e

   Institutional/OpenAlex abstract inspected; full text not accessed by parent.

   Relevance: Ni/Al2O3 included; Ru comparator is Ru/C, not Ru/Al2O3, so not a same-support Ni–Ru comparison.

5. [5]
   Omar U. Valdés-Martínez; Jorge Noé Díaz de León; Carlos E. Santolalla; Alfonso Talavera-López; Hugo Avila-Paredes; José Antonio de los Reyes (2021).

   Fundamental Study of Catalytic Functionalities Involved in Effective C–O Cleavage over Ru-Supported Catalysts

   Industrial & Engineering Chemistry Research 60(51), 18880-18890 · DOI: 10.1021/acs.iecr.1c03058

   Publisher abstract inspected through indexed primary record; full text/SI not accessed.

   Relevance: 0.6 wt% Ru supports compared using phenol, cyclohexanol and ethanol; mechanistic context only.

6. [6]
   O.U. Valdés-Martínez; V.A. Suárez-Toriello; J.A. de los Reyes; B. Pawelec; J.L.G. Fierro (2017).

   Support effect and metals interactions for NiRu/Al2O3, TiO2 and ZrO2 catalysts in the hydrodeoxygenation of phenol

   Catalysis Today 296, 219–227 · DOI: 10.1016/j.cattod.2017.04.007

   Crossref/OpenAlex metadata and author-upload abstract inspected; quantitative original tables and testing methods inaccessible.

   Relevance: Important primary lead containing Ni, Ru and NiRu controls; no unverified numerical comparisons used.

7. [7]
   Peng Gao; Min Zheng; Mengni Zhou; Yiwei Ma; Guirong Bao; Chengyan Wang (2025).

   Low-temperature demethoxylation pathways and solvent-driven hydrodeoxygenation for guaiacol upgrading catalyzed by anaerobically synthesized Ru/Al2O3

   Journal of Environmental Chemical Engineering 13(6), 120069 · DOI: 10.1016/j.jece.2025.120069

   Metadata verified using Crossref and Elsevier API. Full article and complete abstract unavailable.

   Relevance: Recent relevant lead, excluded from quantitative synthesis; not used as experimental evidence.

8. [8]
   Rita F. Nunes; Daniel Costa; Ana M. Ferraria; Ana M. Botelho do Rego; Filipa Ribeiro; Ângela Martins; Auguste Fernandes (2023).

   Heterogenization of Heteropolyacid with Metal-Based Alumina Supports for the Guaiacol Gas-Phase Hydrodeoxygenation

   Molecules 28(5), 2245 · DOI: 10.3390/molecules28052245

   Full primary article XML inspected, especially results §2.2, methods §§3.1/3.3, Figs. 10–11 captions and Tables 1–3. Exact plotted values not digitized; SI not inspected.

   Relevance: Unmodified 28 wt% Ni/alumina control is direct evidence; heteropolyacid-modified formulations are not plain Ni/Al2O3.

9. [9]
   Ziyin Chen; Roman G. Kukushkin; Petr M. Yeletsky; Andrey A. Saraev; Olga A. Bulavchenko; Marcos Millan (2020).

   Coupling Hydrogenation of Guaiacol with In Situ Hydrogen Production by Glycerol Aqueous Reforming over Ni/Al2O3 and Ni-X/Al2O3 (X = Cu, Mo, P) Catalysts

   Nanomaterials 10(7), 1420 · DOI: 10.3390/nano10071420

   Full primary article methods and results inspected through repository PDF and Europe PMC XML; Tables 4/6 and Eq. 5 checked. No long-duration cycling evidence.

   Relevance: Monometallic Ni/alumina control; adjacent hydrogen-supply mode (glycerol reforming, no external H2 charge).

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