The team set out to investigate the stability of concentrated citrate-based Co–Mo and Ni–Mo impregnation solutions, focusing on the composition of precipitates that form unintentionally when citric acid concentrations are insufficient. This is a practical problem of considerable industrial weight. In catalyst preparation, concentrated impregnation solutions are prized because they allow high metal loadings in a single impregnation step, saving time and energy. But when the solution chemistry is not properly balanced, solids can drop out of solution, and until now there has been genuine uncertainty about what those solids actually are and whether they help or harm the catalyst that eventually results.

To answer these questions, the researchers prepared a series of solutions and precipitates and subjected them to a battery of characterization techniques: X-ray diffraction to identify crystalline phases, infrared spectroscopy to probe molecular structures, scanning electron microscopy paired with energy-dispersive X-ray spectroscopy to examine morphology and elemental composition, and X-ray photoelectron spectroscopy to determine surface chemical states. This multi-technique approach allowed them to pin down the identity of the offending solids with unusual confidence. The verdict was striking: the dominant compounds in the precipitates are Anderson-type heteropoly compounds, a distinctive family of polyoxometalates in which a central heteroatom is encased by a ring of six molybdenum oxide octahedra.

The chemistry turned out to differ sharply depending on the metal system involved. In nickel–molybdenum citrate solutions, the precipitates consist predominantly of Anderson-type ammonium 6-molybdonickelate, a compound in which nickel sits at the center of the molybdenum oxide cage. The cobalt–molybdenum system proved more nuanced. In the absence of alumina, cobalt–molybdenum citrate solutions form sparingly soluble polymolybdates rather than well-defined heteropoly compounds. However, when the solution contacts γ-alumina, the standard support material for hydrotreating catalysts, Anderson-type heteropoly compounds emerge, specifically ammonium 6-molybdocobaltate and ammonium 6-molybdoaluminate. The appearance of the aluminum-containing species is particularly telling, because it signals that the acidic citrate solution is dissolving aluminum from the support itself, a ligand-promoted dissolution process that has been documented in molybdate-on-alumina systems but whose consequences for citrate-based impregnation had not been fully mapped.

Why does citric acid concentration matter so much? The answer lies in competition for metal ions. Citrate is a powerful chelating agent, meaning it wraps around cobalt and nickel ions to form soluble coordination complexes. When enough citric acid is present, essentially all of the promoter metal is held in solution as citrate complexes, and the molybdate species remain dispersed as well. When citric acid falls short, free nickel or cobalt ions become available to react with molybdate anions, nucleating the Anderson-type heteropolyanions that then crystallize as ammonium salts. In effect, citric acid acts as a chemical referee that keeps the players separated; remove the referee, and the metals combine into new compounds on their own schedule, not the manufacturer’s.

The crucial next question was what these precipitates do to catalyst performance. The researchers evaluated the resulting catalysts in the hydrotreating of straight-run gas oil, the sulfur-rich fraction that feeds directly into diesel production. The results upended the intuitive assumption that precipitated solids are simply a defect to be avoided. During sulfidation, the activation step in which the oxide precursors are converted into the active molybdenum sulfide phases, the precipitated heteropoly compounds are transformed into sulfide species and may contribute to the overall hydrodesulfurization performance. In other words, the solids are not inert contaminants; they are convertible precursors that join the active phase population.

This finding reframes the entire problem. Catalysts prepared from solutions in which precipitate formation occurred showed higher activity, and the authors attribute this advantage mainly to increased actual metal loading. When solids form, they still end up deposited on the support during impregnation, carrying their full complement of molybdenum and promoter metal with them. The net effect is that more of the intended metal charge is retained in the final catalyst rather than being lost or unevenly distributed. The heteropoly compounds, once sulfided, deliver their metals into the Co–Mo–S or Ni–Mo–S active phases that perform the actual desulfurization chemistry.

At the same time, the study does not suggest that precipitation should be embraced as a manufacturing strategy. Solution stability remains a quality-control benchmark for reproducible catalyst production, because uncontrolled precipitation can vary batch to batch and complicate the uniform distribution of metals across support pellets. What the work provides instead is mechanistic clarity: the higher activity of precipitate-containing catalysts stems from metal loading rather than from any intrinsic superiority of the heteropoly-derived sulfide phases. That distinction matters for process engineers deciding whether to tune citric acid concentration upward to keep solutions clear or to accept and manage the solids that form.

The broader significance of this research extends across the catalyst preparation community. Citric acid is one of several chelating agents, alongside EDTA and nitrilotriacetic acid, used to enhance hydrotreating catalysts, and the literature stretching back decades documents its promotional effects on the active NiMoS and CoMoS phases. By identifying the specific Anderson-type heteropoly compounds that appear when citrate is insufficient, the Novosibirsk team has given researchers a concrete chemical handle for monitoring and diagnosing solution instability. Techniques such as Raman spectroscopy and X-ray diffraction can now be aimed at known targets, ammonium 6-molybdonickelate in Ni–Mo systems and ammonium 6-molybdocobaltate or 6-molybdoaluminate in Co–Mo systems contacting alumina, rather than treating precipitates as an undifferentiated nuisance.

For an industry under pressure to produce ultra-low-sulfur diesel with ever-greater efficiency, the message is one of controlled chemistry rather than luck. The boundary between a clear, stable impregnation solution and a precipitating one is a citric acid stoichiometry boundary, and on either side of that boundary the resulting catalysts differ in measurable, predictable ways. The Boreskov Institute team’s careful coupling of solution chemistry, solid-state characterization, and realistic hydrotreating tests demonstrates that even the mistakes in catalyst preparation have molecular identities worth understanding. In the quest for cleaner fuels, it turns out that the solids you did not plan for can still pull their weight, provided you know exactly what they are.

Subject of Research: Effect of citric acid concentration on the stability of citrate-based Co-Mo and Ni-Mo impregnation solutions and the performance of hydrotreating catalysts

Article Title: Citrate Impregnation Solutions for the Preparation of Co-Mo/Ni-Mo Hydrotreating Catalysts: How Does Citric Acid Concentration Affect Solution Stability and Catalyst Performance

Article References: Budukva, S. V., Parfenov, M. V., Revyakin, M. E., Suprun, E. A., Bykova, E. S., Uvarkina, D. D., & Kazakov, M. O. (2026). Citrate Impregnation Solutions for the Preparation of Co-Mo/Ni-Mo Hydrotreating Catalysts: How Does Citric Acid Concentration Affect Solution Stability and Catalyst Performance. Catalysis Letters, 156(10), Article 291. https://doi.org/10.1007/s10562-026-05535-w

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05535-w

Keywords: hydrotreating catalysts, citric acid, heteropoly compounds, hydrodesulfurization, impregnation solution, Co-Mo catalysts, Ni-Mo catalysts, Anderson-type polyoxometalates, alumina support, sulfidation, catalyst preparation, straight-run gas oil