Development of a Method for the Synthesis of Ammoniacal Niobium Oxalate Trihydrate for Use as a Precursor in Chemical Reactions

In this project, a method was developed to synthesize ammoniacal niobium oxalate (NH₄)(NbO(C₂O₄)(H₂O)ᵧ)·XH₂O, aimed at its application in the preparation of niobium-incorporated materials. This transition metal exhibits excellent properties and is widely available in Brazil. Its versatility makes it essential for metal alloys and electronic components. The synthesis was based on physical and chemical processes, including fusion, decantation, filtration, and complexation, using niobium pentoxide and potassium bisulfate. Rigorous procedures were implemented to ensure precursor quality. Material characterization was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and thermogravimetric analysis (TGA), which collectively confirmed the material's morphology, structure, and thermal stability. The results confirmed the efficient synthesis of high-purity niobium oxalate, with the developed method demonstrating technical feasibility by preserving the material's essential physicochemical properties, as verified by the characterization techniques. Key innovations included replacing acid leaching with a more efficient method, implementing neutral-pH decantation instead of traditional acidic processes, and the strategic use of an aqueous acetic acid solution in the washing step. This combination of advancements enabled a 75% reduction in water consumption during the washing stage, thus establishing a significantly more sustainable synthesis protocol for producing this precursor without compromising final product quality.

Keywords:
Synthesis; Ammonium Niobium Oxalate Trihydrate; Niobium; Precursor

Visual Abstract

1. Introduction

In recent decades, the significance of niobium-based materials in both economic and geopolitical contexts has increased due to their strategic applications. These include superconducting magnets used in nuclear reactor manufacturing and high-temperature-resistant alloys employed in the aerospace industry¹,².

Niobium (Nb) is a transition metal with high melting (2477 °C) and boiling points (4744 °C), a density of 8.57 g/cm³, and an electronic configuration of [Kr]4d⁴5s¹. It exhibits superconductivity below 9.3 K and has an electrical resistivity of 15.2 μΩ·cm. Niobium's most stable oxidation state is +5, found mainly in niobium pentoxide (Nb₂O₅), which is used in catalysts and electronic devices. Brazil holds over 95% of global niobium reserves, sourced mainly from Columbite-Tantalite and Pyrochlore minerals³-⁹.

Recent research has focused on improving the physicochemical properties of niobium precursors. Niobium extraction and processing are challenging due to the environmental risks associated with fluoride-containing complexing agents in traditional methods. Ammonium niobium oxalate (ANO), derived from niobium pentoxide, is gaining attention for its high reactivity, stability, low cost, solubility in oxalate solutions, and low toxicity. Studies aim to optimize the synthesis of such precursors for enhanced properties and broader applications¹⁰-¹⁹.

The synthesis of niobium oxalates typically involves dissolving hydrated niobium oxide in an aqueous oxalic acid solution with the addition of an appropriate salt²⁰. According to Cotton et al.¹⁷, oxalic acid acts as a chelating agent, while oxalate ions serve as bidentate ligands, facilitating the formation of stable niobium complexes. The coordination number of oxalate ligands around the niobium ion defines the classification of these complexes, which are historically categorized as mono-, bis-, and tris-(oxalate)niobates¹⁷-²⁰.

The selection of suitable precursors is primarily determined by both the desired final product and the specific synthetic methodology to be employed. In the case of ammonium niobium oxalate, this compound has been established as an ideal precursor for aqueous-phase synthesis protocols. This designation stems from its well-documented physicochemical properties, including high aqueous solubility, low toxicity, and exceptional stability under ambient conditions. Collectively, these characteristics enable its widespread application in wet-chemical synthesis routes while simultaneously minimizing environmental and safety concerns¹⁰-¹²,¹⁵.

Mathern et al.¹⁸ characterized the structure of ammonium tris(oxalato)oxiniobate monohydrate, revealing a pentagonal bipyramidal geometry where three oxalate groups coordinate the niobate core. This structural determination was key to understanding the chemical interactions and properties of niobium oxalate complexes²¹.

Various studies have confirmed the effectiveness of oxalates as niobium precursors. For example, Fontes et al.²¹ synthesized niobium carbide (NbC) from oxalate and Nb₂O₅, showing that oxalate-derived NbC had smaller crystallite sizes than commercial NbC. Medeiros et al.²⁰ achieved NbC synthesis at lower temperatures, and Prado et al.²² developed a reusable cellulose acetate/Nb₂O₅ catalyst. Additionally, Souto et al.² synthesized CuNb₂O₆, and Feliczak and Nowak²³ pioneered the use of tris(oxalate) ammonium niobium(V) complexes as precursors for reactive mesoporous molecular sieves²¹-²⁵.

In this study, ammoniacal niobium oxalate (NH₄)(NbO(C₂O₄)(H₂O)Y)·XH₂O was synthesized from industrial niobium pentoxide, replacing the conventional acid leaching step of columbite-tantalite ore - a methodology that requires high consumption of corrosive reagents and generates contaminating waste. The developed approach represents a significant innovation by eliminating the need for direct mineral processing, using a pre-purified compound (commercial Nb₂O₅) as raw material. Additionally, the washing step was optimized using acetic acid, reducing the required water volume by 75% compared to previous protocols. These modifications not only simplified the process flowchart but also minimized environmental impacts, aligning with the principles of green chemistry. The obtained precursor was characterized by advanced techniques, confirming its suitability for industrial applications and functional materials synthesis.

2. Materials and Methods

Ammoniacal Niobium Oxalate is synthesized from Niobium Pentoxide (Nb₂O₅) through controlled physical and chemical processes that optimize its reactivity for oxalate synthesis²,¹⁸,²⁰-²²,²⁶. The resulting niobium complex is characterized using techniques such as X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and Thermogravimetric Analysis (TGA) to confirm its crystalline structure, particle morphology, elemental composition, and thermal stability²⁷.

The niobium precursor is synthesized from niobium pentoxide (Nb₂O₅) via controlled physical and chemical processes to produce ammoniacal niobium oxalate. Due to the high melting point of Nb₂O₅, potassium bisulfate (KHSO₄) is added as a fluxing agent to lower the fusion temperature. The solid mixture is homogenized in an agate mortar, heated in a platinum crucible using a Bunsen burner until fully melted, then cooled and ground into a fine powder — a critical step for reducing particle size and optimizing subsequent purification and complexation stages²⁰,²⁶,²⁸.

The fused powder undergoes aqueous purification through agitation and heating to separate components by density differences.

Next, the material is washed to remove impurities and residual ions using a diluted acetic acid solution and deionized water at controlled temperatures, with a fume hood and vacuum filtration system for efficiency²⁰,²⁶.

After washing, the powder undergoes a complexation process with oxalic acid and ammonium oxalate in precise stoichiometric ratios to form the desired niobium precursor. The solution is heated and stirred constantly until complete homogenization, and the pre-washed powder is gradually introduced, promoting crystallization of the product²⁰,²⁶. The crystallized solid is removed, ground, weighed, and stored in a sealed container, ensuring its integrity for scientific and industrial applications.

The Ammoniacal Niobium Oxalate was characterized using X-ray diffraction (Bruker D2 Phaser with Cu Kα radiation, λ=1.54Å, Ni filter, 10 mA current, 30 kV voltage, and Lynxeye detector), scanning electron microscopy (ZEISS Auriga 40 FEG-SEM equipped with a Bruker E-Flash detector for morphological and chemical analysis), and thermogravimetric analysis (TG 209 F3 Tarsus - NETZSCH) to evaluate functional group decomposition under temperature variation.

The methodological innovations in this work focus on three key advances: (i) replacement of the conventional niobium ore acid leaching step with direct fusion of niobium pentoxide (Nb₂O₅) as the initial stage; (ii) implementation of a neutral-pH (non-acidic) decantation process post-fusion, reducing reagent consumption; and (iii) optimization of the fused material washing protocol, achieving a 75% reduction in water usage compared to traditional methodologies. These adaptations delivered significant gains in process efficiency and environmental sustainability while maintaining final product quality.

3. Results and Discussion

3.1. Crystal structure of ammonium niobium oxalate

Based on the reference data from the ICSD (card number 200049) for the trihydrated oxalate, the characteristic diffraction peaks of this compound were observed at 13°, 15°, 17°, 27°, 29°, and 30°¹⁸,²⁰-²²,²⁶ in Figure 1. These values align closely with the diffractograms available in the database, confirming the structural identification of the analyzed material, successful formation of the desired crystalline phase, and high purity and crystallinity of the product. The accurate reproduction of the reference structure validates the effectiveness of the synthesis method, particularly regarding reaction conditions and reagent stoichiometry. Furthermore, this structural correspondence is crucial for ensuring the material's physicochemical properties in specific applications.

Figure 1
X-ray Diffractogram of the Synthesized Ammonium Niobium Oxalate.

Furthermore, the diffraction pattern analysis reveals the presence of an amorphous fraction within the complex, attributed to its organic matrix, specifically the oxalate groups (C₂O₄). This characteristic highlights the hybrid nature of the material, where the amorphous phase is directly associated with the incorporation of the organic structure into the niobium complex¹⁹.

3.2. Morphology of ammonium niobium oxalate

Medeiros et al.²⁵ states that the chemical properties of niobium oxalate remain unchanged regardless of the synthesis method. The micrographic analysis conducted by Souto²⁶ shows that the niobium precursor consists of agglomerated particles with varying shapes and sizes, some exhibiting porosity. These observations highlight how synthesis conditions influence the material's morphology without altering its chemical composition. Electron microscopy further confirms the irregularity and size distribution of the particles, with dimensions in the micrometer range (10⁻⁶ m), supporting the study's findings, as illustrated in Figure 2²,¹⁸,²¹-²³,²⁵,²⁶.

Figure 2
Micrograph of Niobium Oxalate magnified at A) 500x, B) 2000x, and C) 6000x.

At 10,000x magnification (Figure 3a), the pores are clearly visible, with larger particles showing pores around 1 µm and smaller ones less than 0.5 µm. Higher magnification (15,000x - Figure 3b) reveals two distinct morphologies, with varying particle sizes, irregular shapes, and small pores under 1 µm.

Figure 3
Micrograph of Niobium Oxalate magnified at A) 10000x and B) 15000x.

3.3. Elemental analysis of ammonium niobium oxalate

The analysis by Energy Dispersive X-ray Spectroscopy (EDS) - with quantitative and spectral data detailed in Figure 4 and Table 1, respectively - confirmed that the studied material exhibits an elemental composition matching the theoretical formulation of niobium oxalate. Quantitative detection revealed only the constituent elements (Nb, O, N, and C) in compatible stoichiometric proportions²,¹⁸,²⁰-²²,²⁶. This analysis verifies the successful synthesis of the target compound, free from detectable metallic or organic contaminants using the employed technique.

Figure 4
Chemical microanalysis of Ammonium Niobium Oxalate.

Table 1
Atomic percentage of niobium oxalate.

Ammonium Niobium Oxalate - (NH4​)(NbO(C2​O4​)2​(H2​O)Y) . X H2​O
at.% O	at.% Nb	at.% N	at.% C
45.84	11.82	23.86	18.48

3.4. Thermal decomposition of ammonium niobium oxalate

Given that the material is a complex derived from an inorganic oxide and contains an organic fraction, thermal analysis was crucial to study its degradation. A thermogravimetric analysis was performed to examine the mass loss as the temperature increased. The analysis, shown in Figure 5, revealed five distinct stages of decomposition, starting at 72 °C and ending at 318 °C, resulting in a 69% mass loss. The initial mass for the analysis was 10.3 mg (0.103 g), and after the process, the remaining mass was 3.19 mg.

Figure 5
TGA of Niobium Oxalate.

In Stage I (72 °C - 138 °C), a 4.4% mass loss (0.453 mg) occurred due to dehydration, with the removal of hydration water ¹⁷,²⁰.

Reaction 1

$\left(N{H}_4\text{}\right)(NbO\left(C_2{O}_4\text{}\right)\text{}\left(H_{2}O{)}_Y\right)\cdot X{H}_2\text{}O\to \left(N{H}_4\text{}\right)(NbO{(C_2{O}_4\text{})}_2\left(H_2\text{}O{)}_Y\text{}\right)+X{H}_2\text{}O\left(g\right)$

Stage 2 (138 °C - 188 °C) shows a mass loss of 10.4% (1.07 mg), attributed to the decomposition of coordination waters, which have stronger bonds to the complex and require more energy to be released²⁰,²⁶,²⁹.

Reaction 2

$\left(N{H}_4\text{}\right)(NbO{(C_2\text{}{O}_4\text{})}_2\text{}\left(H_2\text{}O{)}_Y\right)\to \left(N{H}_4\text{}\right)(NbO\left(C_2\text{}{O}_4\text{}{)}_2\text{}\right)+Y{H}_2\text{}O\left(g\right)$

Stage 3 (188 °C - 207 °C) shows a mass loss of 8.2% (0.846 mg) due to the decomposition of the ammonium ion (NH₄)⁺, releasing ammonia (NH₃), a typical temperature range for ammonium cation deprotonation²⁰,²⁶,²⁹.

Reaction 3

$\left(N{H}_4\text{}\right)(NbO\left(C_2\text{}{O}_4\text{}{)}_2\text{}\right)\to NbO{(C_2\text{}{O}_4\text{})}_2\text{}+N{H}_3\text{}\left(g\right)$

Stage 4 involves the most significant decomposition, with a mass loss of 46% (4.738 mg). This is due to the thermal decomposition of the oxalate ligands, which break down into carbon dioxide (CO₂) and carbon monoxide (CO) ²⁰,²⁶.

Reaction 4

$NbO{(C_2\text{}{O}_4\text{})}_2\text{}\to N{b}_2\text{}{O}_5\text{}+C{O}_2\text{}\left(g\right)+CO\left(g\right)$

Finally, Stage 5, which begins at 318 °C, shows no further decomposition, with the mass remaining constant. After the elimination of water molecules, ammonia, and oxalate, niobium pentoxide (Nb₂O₅) remains as the residual product ²⁰,²⁶. This compound was used as the primary reagent for the preparation of the oxalate precursor.

Table 2 summarizes the decomposition that occurred during the thermogravimetric analysis (TG).

Table 2
TG results for ammoniacal niobium oxalate.

STAGE	T0 (°C)	Tf (°C)	% Mass	Equivalent molecule
I	72	138	-4.40	X . H2O (Hydration)
II	138	188	-10.4	Y . H2O (Coordination)
III	188	207	-8.20	NH3
IV	207	318	-46.0	C2O4 → (CO2 e CO)
V	318	-	= 31	Nb2O5

The thermal events observed during the analysis can be corroborated through the DTG curve. This curve represents the derivative of mass loss, allowing the generation of a graph that indicates the rate of mass variation of the analyte as the temperature changes. The DTG enables confirmation of mass losses during the process, as, with each decomposition, an alteration in the curve can be observed, demonstrating that a transformation is occurring. The fact that the peak presented in the DTG curve is negative, i.e., directed downward, indicates that mass loss is occurring²⁷.

4. Conclusions

This study successfully synthesized a niobium precursor for specific chemical reactions, demonstrating significant potential for both industrial and academic applications. The methodological innovations of this research focus on three key advances: (i) replacement of the conventional acid leaching step for niobium ore with direct fusion of niobium pentoxide (Nb₂O₅) as the initial stage; (ii) implementation of a neutral-pH (non-acidic) decantation process post-fusion, reducing reagent consumption; and (iii) optimization of the fused material washing protocol, achieving a 75% reduction in water usage compared to traditional methodologies. These adaptations delivered substantial gains in process efficiency and environmental sustainability while maintaining final product quality. The precursor was obtained through this innovative approach and characterized by XRD (X-ray diffraction), SEM (scanning electron microscopy), and EDS (energy-dispersive spectroscopy), confirming its purity and structure. This research not only validates the developed synthesis method but also lays the foundation for future studies to further optimize the process and explore the precursor’s properties in areas such as catalysis and advanced materials development. The results highlight the potential of this approach to advance niobium chemistry by combining operational efficiency with environmentally sustainable practices.

• Data Availability
  The entire dataset supporting the results of this study was published in the article itself.

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Edited by

• Associate Editor:
  Eliana Muccillo.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

The entire dataset supporting the results of this study was published in the article itself.

Publication Dates

• Publication in this collection
  22 Aug 2025
• Date of issue
  2025

History

• Received
  29 Jan 2025
• Reviewed
  25 Apr 2025
• Accepted
  01 June 2025