📄 chemistry

Formation of insoluble Anderson-type heteropoly molybdates in citrate Ni–Mo and Co–Mo impregnation solutions used to prepare HDS catalysts: Causes and effects on catalytic activity

This study reveals that the formation of insoluble Anderson-type heteropoly molybdates in citrate-based Ni–Mo and Co–Mo impregnation solutions leads to increased actual metal loading, which, after transformation into sulfide species during sulfidation, enhances the hydrodesulfurization activity of the resulting catalysts.

Original authors: Sergey V. Budukva, Mikhail V. Parfenov, Mikhail E. Revyakin, Evgenii A. Suprun, Elizaveta S. Bykova, Daria D. Uvarkina, Maxim O. Kazakov

Published 2026-08-03

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CC BY 4.0

Original authors: Sergey V. Budukva, Mikhail V. Parfenov, Mikhail E. Revyakin, Evgenii A. Suprun, Elizaveta S. Bykova, Daria D. Uvarkina, Maxim O. Kazakov

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the world of oil refineries as a massive, high-stakes kitchen where dirty fuel is the main ingredient. Before this fuel can power our cars or heat our homes, it needs a serious detox. The problem? It's full of sulfur, a nasty element that turns into smog and acid rain when burned. To fix this, chemists use a process called hydrodesulfurization (HDS), which is like a molecular scrubbing session. They mix the fuel with hydrogen and pass it over special catalysts—tiny, porous rocks coated with metals like molybdenum, nickel, or cobalt. These metals act as the "chefs" that grab the sulfur and toss it out.

But here's the tricky part: getting those metal chefs to stick to the rocks isn't easy. If you just dump them in, they might clump together or get stuck in the wrong spots, making the catalyst weak. To solve this, scientists use a "glue" called a chelating agent—think of it as a molecular leash. Citric acid, the same stuff that makes lemons sour, is a popular choice. It holds the metal atoms in a neat, dissolved solution so they can spread out evenly on the catalyst rocks. The goal is to create a perfect, uniform coating. But what happens if the solution gets a little too crowded or the recipe is slightly off? Sometimes, instead of a smooth coating, the metals decide to throw a party and form solid clumps, or precipitates, right in the liquid. For a long time, factory workers worried that these clumps were bad news—clogging pipes, wasting expensive metals, and ruining the catalyst.

This paper dives into that exact mystery. The researchers at the Boreskov Institute of Catalysis wanted to know: What exactly are these clumps made of, and do they actually ruin the catalyst, or is there a twist? They looked at solutions containing molybdenum mixed with either nickel or cobalt, using citric acid as the stabilizer. They found that when the amount of citric acid is too low, the metals stop behaving like a smooth solution and start forming specific, crystal-like structures called "Anderson-type heteropoly compounds." It's like the metals suddenly decided to build a castle instead of swimming in the pool.

The big surprise? These "clumps" aren't the villains everyone thought they were. When the researchers tested the catalysts, they found that the ones made with these precipitates were actually more active at removing sulfur than the ones made without them. Why? Because the clumps were loaded with extra metal. When the catalyst goes through the heating and chemical treatment (sulfidation) to become active, these solid clumps break down and turn into the very same super-efficient metal chefs needed for the job. So, the "waste" was actually just a hidden bonus of extra ingredients. However, the authors are careful to note that this doesn't mean factories should try to make clumps. The uneven distribution of these crystals can still cause problems, and the higher activity is simply because there's more metal on the rock, not because the clumps are magically better. The real lesson is that while the clumps aren't a disaster, keeping the solution stable with enough citric acid is still the best way to ensure a consistent, high-quality product.

Technical Summary: Formation of Insoluble Anderson-type Heteropoly Molybdates in Citrate Ni–Mo and Co–Mo Impregnation Solutions

Problem Statement
The preparation of commercial hydrodesulfurization (HDS) catalysts often utilizes impregnation solutions containing organic chelating agents, particularly citric acid (CA), to stabilize active metals (Co or Ni) and molybdenum. While this method enhances catalyst activity by promoting the formation of the Co-Mo-S type II phase, the authors observed the unexpected formation of light-colored precipitates in citrate-based Co-Mo and Ni-Mo solutions during the impregnation of $\gamma$ -Al $_2$ O $_3$ supports. In industrial settings, where large volumes (5–10 m $^3$ ) of solution are processed, such precipitates pose significant operational risks, including filter clogging, pump damage, and the loss of active metals. While precipitate formation in non-citrate systems has been documented, the composition and causes of precipitates in citrate-containing Co-Mo and Ni-Mo solutions remained uncharacterized in the scientific literature. Furthermore, the impact of these precipitates on the final catalytic activity of sulfided catalysts was unknown.

Methodology
The study employed a systematic approach to characterize the precipitates and evaluate their effect on catalyst performance:

Solution Preparation: Impregnation solutions were prepared by dissolving citric acid, cobalt or nickel hydroxides, and ammonium heptamolybdate (AHM) in water. To investigate the cause of precipitation, solutions were prepared with varying Citric Acid to Promoter Metal (CA/Co or CA/Ni) molar ratios. Stable solutions required CA/Co $\ge$ 1.5 and CA/Ni $\ge$ 1.8.
Precipitate Isolation: Precipitates were isolated from two sources: (1) directly from the impregnation solution without support contact (designated S2), and (2) from the solution after contact with $\gamma$ -Al $_2$ O $_3$ granules (designated S1).
Catalyst Preparation: Catalysts were prepared via vacuum impregnation using both stable solutions (no precipitate) and unstable solutions (with precipitate formation). The resulting catalysts were dried, sulfided, and tested.
Characterization: The precipitates and catalysts were analyzed using X-ray Diffraction (XRD), Raman spectroscopy, Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy, Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray (EDX) analysis, and X-ray Photoelectron Spectroscopy (XPS).
Catalytic Testing: Sulfided catalysts were evaluated in the hydrodesulfurization (HDS) of straight-run gas oil (SRGO) in a fixed-bed reactor at 4.0 MPa and temperatures ranging from 340 to 360 °C. Activity was quantified using apparent rate constants normalized by molybdenum content.

Key Contributions and Results

Identification of Precipitate Composition:
- Ni-Mo Systems: In Ni-Mo solutions, the dominant precipitate phase is the Anderson-type heteropoly compound (HPC), ammonium 6-molybdonickelate, $(NH_4)_4[NiMo_6O_{24}H_6]\cdot nH_2O$ . This phase forms both in the absence of the support and upon contact with $\gamma$ -Al $_2$ O $_3$ . When in contact with the support, a minor amount of ammonium 6-molybdoaluminate, $(NH_4)_3[AlMo_6O_{24}H_6]$ , is also detected due to the partial dissolution of the support.
- Co-Mo Systems: The behavior differs significantly. In the absence of the support, Co-Mo solutions primarily form sparingly soluble polymolybdates (e.g., $(NH_4)_8Mo_{10}O_{34}$ and $(NH_4)_2Mo_2O_7$ ). However, upon contact with $\gamma$ -Al $_2$ O $_3$ , the system shifts to form Anderson-type HPCs, specifically ammonium 6-molybdocobaltate and ammonium 6-molybdoaluminate, with the latter predominating.
Mechanism of Formation:
The study proposes that precipitate formation is driven by a deficiency in the chelating agent (citric acid). When the CA/Metal ratio is insufficient, the formation of stable citrate complexes is compromised. Thermodynamically, the formation of Anderson-type HPCs (involving the cooperative binding of six MoO $_6$ octahedra) is more favorable than citrate coordination. Consequently, free molybdate anions react with Co $^{2+}$ , Ni $^{2+}$ , or Al $^{3+}$ cations to form insoluble ammonium salts of heteropoly anions.
Impact on Catalyst Composition and Activity:
- Metal Loading: Catalysts prepared with precipitate formation (CoMo-1, NiMo-1) exhibited higher total metal loadings (Mo, Co, Ni) compared to those prepared from stable solutions (CoMo-2, NiMo-2). This is attributed to the physical deposition of HPC particles onto the support granules during impregnation.
- Phase Transformation: XPS and SEM-EDX analyses of sulfided catalysts revealed that the precipitated HPCs are converted into active sulfide species (Co-Mo-S and Ni-Mo-S phases) during sulfidation. The surface composition of the sulfided catalysts was found to be similar regardless of precipitate formation, indicating that the HPCs successfully transform into the active phase.
- Catalytic Performance:
  - Co-Mo: The CoMo-1 catalyst (with precipitate) showed higher HDS activity than CoMo-2, particularly at 350 and 360 °C. The authors attribute this increase to the higher actual metal loading rather than an intrinsic enhancement of the active phase.
  - Ni-Mo: NiMo-1 and NiMo-2 showed comparable activity. The authors suggest that while NiMo-1 had higher metal loading, the non-uniform deposition of the precipitate may have introduced diffusion or morphological limitations that offset the benefits of the extra metal.

Significance and Claims
The paper concludes that the formation of Anderson-type heteropoly compound precipitates in citrate impregnation solutions is a direct consequence of insufficient citric acid concentration relative to the promoter metal. While these precipitates are often viewed as a defect in industrial processes, the study demonstrates that they are not catalytically inert. Instead, they serve as an additional source of active metals that are successfully converted into active sulfide phases upon sulfidation.

The authors modestly claim that the observed higher activity in Co-Mo catalysts with precipitates is primarily due to increased metal loading rather than a superior intrinsic activity of the precipitate-derived phase. Consequently, while the uncontrolled formation of precipitates is undesirable for process stability and reproducibility, the presence of such precipitates does not necessarily result in a catalyst that fails performance specifications. The primary recommendation remains the stabilization of impregnation solutions (maintaining CA/Co $\ge$ 1.5 and CA/Ni $\ge$ 1.8) to ensure uniform metal distribution and consistent catalyst properties, rather than to avoid the loss of catalytic activity per se.

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