Executive synthesis
The most convincing conventional hydrotreating evidence retrieved is for oxalic-acid-assisted hydrothermal deposition of W/NiW on γ-Al₂O₃ for DBT hydrodesulfurization (HDS), not direct HDO. Direct HDO precedents include oxalate-functionalized Co/ZSM-5 and Ni-oxalate/Al₂O₃ used for oleic acid, but these chiefly test a metal-precursor effect. The clearest oxalic-acid-leached support study (phonolite/NiW) also uses oxalic acid as a hydrogen donor during reaction, so it does not isolate a support-only effect. Evidence for “oxalic-acid-modified support improves HDO” is therefore much thinner than the broad chelator literature might suggest.
First separate the four roles
| Role | What the acid does | What evidence can establish |
|---|---|---|
| 1. Metal-precursor ligand | Oxalate coordinates Co/Ni or other metal ions; may form sparingly soluble metal oxalate complexes before deposition. | Changes precursor charge, solubility, adsorption and thermal decomposition; potentially changes particle size, reducibility and metal-support interaction. Not automatically equivalent to “chelating Ni and Mo together.” |
| 2. Impregnation/deposition additive | Acidifies solution, can influence molybdate/tungstate condensation and support surface charge; in hydrothermal deposition it can act as dispersant/complexant. | Sequence and pH are decisive. An additive introduced after oxide precursor formation may not prevent aggregation during formation. |
| 3. Support pretreatment / leach | Dissolves selected support elements and surface species; can open pores and change hydroxyl density/acidity. | Requires controls separating leaching, texture, acidity and changed metal loading. “More acidic” is not a standalone mechanism. |
| 4. Reaction reagent / hydrogen donor | Can participate in reaction chemistry (e.g., oxalic acid as a hydrogen donor in a co-processing study). | Cannot be cited as evidence that oxalic acid modified the catalyst during preparation; feed chemistry is a separate variable. |
Oxalic acid in the precursor liquor or deposition step
Conventional sulfided hydrotreating catalysts
In supported CoMo/NiMo hydrotreating, the oxidic precursor must ultimately form promoted MoS₂/WS₂ edge sites. Organic chelators can delay promoter sulfidation and change metal-support bonding, thereby affecting Type I/Type II sulfide formation. Oxalic acid should not be assumed to behave identically to citrate, EDTA or NTA: ligand stability, metal stoichiometry, pH, drying and calcination differ. The study on CoMo/TiO₂–Al₂O₃ that compared several chelators found the oxalic-acid formulation performed worse than the stronger-performing citrate/glycol-containing options; the authors attributed this in part to insoluble cobalt oxalate formation. This is a useful warning against the simple “chelation always helps” rule.
W/Al₂O₃: evidence for deposition engineering
Wang et al. introduced oxalic acid during hydrothermal deposition (OHD), not merely into a routine incipient-wetness solution. In the reported route, alumina is contacted with sodium tungstate; HCl and oxalic acid are added, then the mixture is hydrothermally treated, dried and calcined. Their interpretation is that carboxylate interaction with alumina hydroxyls/unsaturated Al sites weakens W–support interaction, while oxalic acid interaction with tungsten-containing particles and lower capillary forces during drying suppress aggregation. Improved WO₃ dispersion and weaker support bonding favor sulfidation and WS₂ formation. The resulting sulfides were reported to have shorter and more stacked WS₂ slabs and higher HDS activity than conventionally impregnated materials.
The follow-up microwave-hydrothermal study shows why addition order matters: microwave-driven deposition can itself improve dispersion; oxalic acid added after tungstic acid formation had limited influence on WO₃ dispersion, whereas split addition (support pretreatment plus aggregation control) was more effective. This is direct evidence of process-path dependence, not a universal molecular-promoter effect.
Oxalate-derived HDO metals
Ayodele prepared cobalt oxalate separately from cobalt nitrate and oxalic acid, then incorporated it into ZSM-5; a non-oxalate Co/ZSM-5 comparator was made without the ligand. After inert calcination and H₂ reduction, the oxalate-derived sample showed smaller Co particles, higher dispersion and greater reducibility. In oleic-acid HDO, reported conversion was 92% versus 59%. The evidence supports precursor-functionalization effects on Co dispersion/reduction; it does not isolate support modification or establish behavior for sulfided NiMo/CoMo.
A related Ni-oxalate/alumina study prepared nickel oxalate before supporting it on alumina and tested oleic-acid conversion to normal and iso-octadecane. It is an HDO precursor precedent, but comparisons should distinguish oxygen-removal pathway, C₁₇/C₁₈ retention and isomerization from a single overall conversion number.
Oxalic acid as support modification
Support treatment is chemically distinct from putting oxalic acid in a metal solution. Acid leaching can remove exchangeable or framework-associated cations, expose/alter hydroxyl groups, change pore volume and redistribute acid sites. It may create better access or an altered metal-support interface; it can also remove useful components or weaken the support. Because leaching changes several properties together, observed activity is not attributable to acidity alone unless the study includes matched controls.
Phonolite/NiW case: informative, but confounded
In the 2020 phonolite study, 20 g of phonolite was dried, leached at 80 °C for 4 h with 1 M oxalic acid at a 1:10 solid/liquid ratio, filtered, washed, dried and calcined. Ni and W were then added by impregnation (nominal 5 and 10 wt%) and calcined. Oxalic-acid leaching changed elemental composition and textural properties; the oxalic-acid-treated support had greater total pore volume than the HCl-treated comparison, while HCl treatment produced greater mesopore volume. NH₃-TPD acidity also differed. But the catalyst tested with oxalic-acid-treated phonolite additionally used oxalic acid as a hydrogen donor in the hydrotreatment reaction. The HDO/deoxygenation result—about 15 wt% paraffins with phonolite catalysts versus 65 wt% for commercial NiW/silica-alumina—therefore cannot be interpreted as an isolated support-treatment benefit; moreover, paraffin yield and carbon-number pathway matter.
What support-only claims remain unproven
The studies located do not establish a general rule that oxalic-acid pretreatment of alumina or silica improves HDO. The strongest HDS support-interface result comes from hydrothermal W deposition on alumina; that mechanism is a rational hypothesis to test for HDO catalysts, but transfer to different metals, supports, active phases and substrates needs experimental validation.
How activity can change—and how to interpret it
| Measured catalyst change | Plausible activity consequence | Key caveat / measurement |
|---|---|---|
| More dispersed oxide/metal; smaller crystallites | More accessible hydrogenation or sulfide edge sites per gram; often higher rate. | Normalize by catalyst mass and, where possible, exposed metal/active-site count. Confirm by TEM, chemisorption, XRD and spectroscopy. |
| Weaker metal-support interaction | May improve sulfidability/reducibility, increasing active phase fraction. | Too-weak anchoring can promote sintering or migration; quantify sulfidation/reduction and post-reaction stability. |
| Changed precursor coordination/decomposition | Can delay reduction/sulfidation or improve metal placement; may favor promoted sulfide edges. | Oxalate may precipitate cobalt/nickel oxalate, causing nonuniform deposition or blocked pores. Examine solution speciation and thermal decomposition. |
| Changed support pore structure | Can improve diffusion of bulky feeds or reduce accessibility constraints. | Acid leaching can collapse/alter pores; include BET/pore-size distributions and uptake/loading measurements. |
| Changed support acidity | Can promote dehydration, C–O cleavage, skeletal isomerization or cracking depending on catalyst/feed. | More acidity can increase coke/cracking and reduce liquid yield. Separate Brønsted/Lewis sites and track carbon balance. |
| Different product pathway | Higher paraffin yield may reflect HDO, hydrodecarbonylation, decarboxylation or their mixture. | Measure H₂O, CO, CO₂ and carbon-number retention; conversion ≠ deoxygenation degree ≠ desired-product yield. |
Thus, the likely direction is conditional: oxalate may raise activity when it increases active-site dispersion/accessibility and yields an optimally reducible/sulfidable state; it may lower activity when it creates insoluble promoter oxalate, shifts the oxide precursor distribution unfavorably, blocks pores, removes useful support chemistry or drives unselective acidity. Always compare rate/selectivity at matched metal loading and activation protocol.
Preparation routes and a controlled comparison
Reported routes (not interchangeable)
- Oxalate-functionalized Co precursor: dissolve cobalt nitrate hexahydrate in water; add anhydrous oxalic acid and mix at 60 °C for 30 min to form cobalt oxalate; add the precursor to aqueous ZSM-5 dispersion and stir at 70 °C for 6 h; dry at 100 °C for 12 h; calcine under N₂ at 450 °C for 4 h; reduce under H₂ before oleic-acid HDO. The paper reports a 30 wt% Co target. These are the reported Co/ZSM-5 conditions, not a suggested NiMo recipe.
- Oxalic-acid-assisted W hydrothermal deposition: suspend γ-Al₂O₃ in sodium tungstate solution; add HCl and oxalic acid in a controlled sequence; hydrothermally treat in a lined autoclave (the reported paper uses 150 °C/12 h); wash, dry and calcine; prepare the matched conventional-impregnation reference; sulfide both under identical conditions before DBT HDS.
- Oxalic-acid support leaching: dry phonolite; leach with 1 M oxalic acid at 80 °C for 4 h and 1:10 solid/liquid mass-to-volume ratio; filter/wash/dry/calcine; then load Ni/W. Keep an HCl-leached support and untreated support as separate controls. This was combined with oxalic acid as reaction hydrogen donor in its source study.
Suggested experiment to separate the causes (proposed, not a published optimum)
- Use one support batch and fixed nominal elemental loading; verify actual composition by ICP/XRF.
- Run a 2×2 design: untreated vs oxalic-acid-pretreated support × no oxalate vs oxalate in metal solution. Add a pH-matched mineral-acid control and an oxalate-free ionic-strength control.
- For NiMo/CoMo, hold impregnation order, aging, drying, calcination (or deliberate non-calcination), sulfiding/reduction ramp, gas composition, temperature and time constant. Report acid:metal molar ratio and solution pH rather than “a little oxalic acid.” Screen ratios only after verifying precursor solubility; oxalate precipitation is a failure mode, not a target by default.
- Measure support and precursor properties before activity: BET/pore distribution, NH₃-TPD or pyridine-IR, XRD, Raman/UV-vis where appropriate, TPR/TPS, ICP, and microscopy. Characterize sulfided/reduced state, not only calcined precursor.
- Report reaction conversion, deoxygenation degree, product selectivity/yield, carbon balance, gas products, coke and time-on-stream. For fatty acids, distinguish C₁₈-retaining HDO from C₁₇-producing decarboxylation/decarbonylation.
Recommended interpretation: call an effect “precursor-complexation” only if the oxalate-in-solution factor changes performance at fixed support treatment. Call it “support modification” only if pretreatment changes performance at fixed precursor chemistry. If there is an interaction between those factors, report the interaction rather than assigning a single mechanism.
References and evidence notes
DOI links point to publisher records. Primary studies are marked direct or adjacent according to the actual reaction and oxalic-acid role; thesis/review records are not counted as experimental primary journal articles.
Bottom line
For a new hydrotreating/HDO formulation, choose the oxalic-acid role explicitly, then test it with matched controls. Precursor addition is a speciation/deposition intervention; support leaching is a support-composition intervention. The literature supports plausible benefits in dispersion and active-phase formation in specific W/alumina HDS and oxalate-derived Co HDO systems—not a universal activity increase. The most important next evidence is a controlled support-treatment × precursor-oxalate comparison with identical activation and pathway-resolved product balances.