The sample used in this study originates from the Kotchari phosphate deposit, located 70 km northeast of Arly and 40 km south of Diapaga, in eastern Burkina Faso. Kotchari lies between 11˚30' and 11˚55' north latitude and between 1˚25' and 2˚00' east longitude, within the rural commune of Tansarga, in the southern part of Tapoa Province (Figure 1). The phosphate rocks are mainly composed of carbonate-fluorapatite (francolite), (Ca₅(PO₄)_2.5(CO₃)_0.5F); hydroxylapatite, a pentacalcium tris(phosphate) hydroxide, (Ca₅(PO₄, CO₃)₃(OH)); as well as alpha-quartz (α-SiO₂) [24].

Figure 1. Location of the sample collection site.

The collected natural phosphate was subjected to mechanical pretreatments structured in several stages. First, a preliminary crushing step was carried out to reduce the initial particle size of the material from the centimeter scale to the millimeter scale. This size reduction was performed using a jaw crusher to ensure fragment homogeneity. The resulting material was then ground using a ROC IMPACT ball mill to reach a micrometric particle size, thereby increasing the specific surface area of the particles, which is essential for catalytic applications.

The particle size distribution was determined by differential sieving using a column of standardized sieves (AFNOR X11-501). The selected fraction, ranging from 63 to 125 μm, complies with international standards for heterogeneous catalysts (ISO 3310-1) and was isolated for further studies. This size range optimizes surface properties while minimizing mass transfer effects during catalyzed reactions.

Subsequently, the phosphatic material was subjected to calcination at 900˚C for two hours, followed by a washing step aimed at removing a large portion of organic matter as well as residual carbonates. A water leaching step was then performed to extract lime and magnesia formed during calcination. The treated sample was dried in an oven for six hours to eliminate any residual moisture.

A portion of the pretreated natural phosphate was then subjected to a second calcination at 900˚C for two hours in a furnace to activate the material, hereafter referred to as the catalyst. This treatment ensures the dehydration and decarbonation of the precursors and promotes the formation of stable crystalline phases. After cooling, the material was ground and sieved (<80 µm), yielding a catalyst with optimal structural and textural properties for catalytic applications.

The synthesis of chalcone was carried out by reacting acetophenone (2 mL, 1.7 × 10⁻² mol) with benzaldehyde (1.8 mL, 1.7 × 10⁻² mol) in 5 mL of distilled water. The experiments were performed at room temperature in the presence of 1 g of catalyst (Scheme 1).

At the end of the reaction, the mixture was heated to approximately 60˚C to ensure complete dissolution of the product. The heterogeneous catalyst was removed by filtration. The filtrate containing the chalcone was then cooled in an ice bath under stirring for one hour, promoting precipitation. The precipitate was collected by vacuum filtration, washed with ice-cold water and then with cold ethanol (5 mL) to remove impurities, and air-dried.

Finally, the crude product was purified by recrystallization in 5 mL of ethanol.

Scheme 1. General method for the synthesis of chalcone.

Scanning electron microscopy(SEM)

The morphology of the different powdered samples was examined using a Microspec-WDX 600/OXFORD microscope. The operating voltage was set at 20 kV, allowing magnifications of up to 30,000×. The samples, previously ground to a particle size below 100 µm, were mounted on double-sided adhesive tape fixed onto the sample holder. The assembly was then coated with a thin layer of gold (Au/Ag) to render it conductive before observation.

Specific surface area measurement

To measure the specific surface area, the sample was pretreated. This pretreatment consisted of degassing and dehydrating the phosphate at 250˚C for 24 hours to remove all previously adsorbed gases. Nitrogen was used as the probe gas in this study. An N₂ adsorption-desorption isotherm was obtained. The measurements were carried out at 77 K, corresponding to the boiling temperature of nitrogen.

The specific surface area and total pore volume of the different materials were estimated from the N₂ adsorption-desorption isotherms at the boiling temperature of liquid nitrogen (77 K). The surface area of the phosphate was determined using a Bel Sorp-max instrument controlled by Bel Japan Inc. software.

X-ray photoelectron spectroscopy(XPS)

The XPS spectra of our samples were recorded using an ESCALAB 250 spectrometer (Thermo Electron), equipped with a monochromatic Al Kα radiation source (1486.6 eV), operating at a power of 150 W (15 kV, 10 mA). Measurements were carried out directly on powdered samples deposited on a sample holder, with an analysis area of approximately 500 μm in diameter, in an ultra-high vacuum chamber (pressure ≤ 2 × 10⁻⁹ Torr). The analysis depth ranges between 3 and 10 nm.

Infrared spectroscopy(IR)

Fourier transform infrared (FT-IR) spectroscopy analysis was carried out using a Thermo Fisher Scientific spectrometer, equipped with attenuated total reflectance (ATR), in the frequency range of 400 - 4000 cm⁻¹.

Proton nuclear magnetic resonance(¹H NMR)

Nuclear magnetic resonance (NMR) experiments were carried out using a Bruker Avance 600 MHz spectrometer for ¹H NMR. Chemical shift values were expressed in parts per million (ppm) on the delta (δ) scale, while coupling constants (J) were measured in hertz (Hz).

X-ray diffraction(XRD)

Single-crystal X-ray diffraction is a technique that provides precise information on atomic arrangement, based on the interaction of X-rays with the crystal lattice. A suitable single crystal was carefully selected and analysed using a Bruker D8 VENTURE diffractometer (Mo-Kα, λ = 0.71073 Å) equipped with a PHOTON II CPAD detector, with an angular scan range in θ from 2.4˚ to 30.5˚.

A total of 30,161 reflections were measured, of which 3276 were identified as independent, with a reproducibility factor R(int) of 0.070. Refinement was carried out on F² using the SHELXL-2018/3 program, applying an empirical weighting scheme [25]. Non-hydrogen atomic parameters were refined freely with anisotropic displacement parameters, while hydrogen atoms were positioned using a geometric model and refined under constraints.

The R₁ factor was 4.3% for significant reflections (I > 2σ(I)), and the weighted factor wR₂ was 10.8%, with a maximum residual electron density of 0.26 eÅ⁻³. The absolute structure was determined from 949 Bijvoet reflection pairs, with a Flack parameter x = 0.1 (8), confirming the assigned configuration [26].

Details of the crystal data and refinement are presented in Table 1.

Table 1. Crystalline data and structure refinement.

The image of raw natural phosphate (Figure 2) shows a heterogeneous structure, consisting of an assembly of irregular agglomerates and fine particles, characteristic of a material rich in mineral impurities and organic matter [27].

In contrast, after calcination, the calcined phosphate exhibits a more homogeneous morphology with more well-defined grains. This results from the decomposition of carbonates, the removal of impurities during thermal treatment, and the stabilization of crystalline phases [28].

Figure 2. SEM image (500×) before (1) and after (2) catalyst preparation.

Table 2 shows a significant decrease in calcium (Ca) content after calcination (48.2% → 39.66%). Calcination may reduce the surface concentration of certain calcium-containing impurities following post-calcination washing [29]. As the main element of the phosphate phase, variations in calcium content directly reflect structural modifications and material density, particularly those induced by calcination, which can affect the compactness of the crystal lattice and the exposure of active surfaces.

The phosphorus (P) content decreases slightly after calcination (22.21% → 19.23%), indicating good stability of phosphorus during the treatments [29][30]. The silicon (Si) content increases significantly after calcination (21.68% → 25.57%), which may result from better exposure of silicate phases at the surface or from element redistribution induced by thermal treatment.

Moreover, iron (Fe) and aluminum (Al) contents also increase after calcination, rising from 2.74% to 4.99% and from 5.04% to 7.77%, respectively. These changes may be explained by a relative concentration effect following the mass loss of other elements or by improved detection due to structural reorganization.

Table 2. Surface composition of raw phosphate and catalyst in atomic percent (at.%).

The specific surface area and pore volume of the raw phosphate (Table 3) are 167.33 m²/g and 0.14 cm³/g, respectively, indicating a porous texture favourable for catalytic activity [31]. However, after calcination at 900˚C (catalyst), a noticeable decrease in these values was observed, with the specific surface area reduced to 140.99 m²/g and the pore volume to 0.12 cm³/g.

The decrease in specific surface area can be interpreted as a consequence of progressive particle fusion. Under the effect of heat, the mobility of solid species increases, promoting particle agglomeration and grain sintering, which leads to crystal growth. This compaction process results in a reduction in the number of accessible pores and, consequently, a significant loss of specific surface area [32].

From a thermodynamic perspective, calcination provides the energy required to overcome kinetic barriers that maintain amorphous or mesoporous structures. The system then tends to minimize its free energy by favouring crystal growth and structural reorganization, leading to matrix densification and the gradual disappearance of micropores. This phenomenon may cause pore closure or reorganization, thereby limiting the accessibility of adsorption sites.

Moreover, certain amorphous phases, characterized by a high specific surface area, may transform into denser crystalline phases, further contributing to the decrease in surface area. This drastic reduction is generally attributed to the progressive disappearance of micropores, structural reorganization, and crystal growth induced by temperature, all of which lead to matrix densification.

This transformation, well documented by H. H. Lim et al. (2001), A. K. Özer et al. (2006), and T. F. Al-Fariss et al., highlights the decisive influence of calcination conditions on the evolution of porous texture and, consequently, on the adsorption properties and potential catalytic activity of these materials [33]-[35].

It is important to note that although this reduction may appear unfavourable, it can also lead to improved thermomechanical stability of the catalyst. Indeed, larger grain size enhances resistance to erosion and degradation under reaction conditions. However, it remains essential to strike a balance to preserve optimal catalytic activity.

Table 3. Measurement of specific surface area, pore volume, and pore diameter.

The detailed analysis of the XPS spectrum of the catalyst (Figure 3) reveals the presence of several characteristic chemical species likely to play a key role in the physicochemical properties of the material. The predominant peak located at 532 eV (O 1s) indicates a high concentration of oxygen-containing groups, mainly associated with hydroxyl (-OH) and carbonyl (C=O) functionalities.

This high oxygen content is of great importance, as it governs surface chemical interactions and can significantly influence the reactivity of the material in heterogeneous catalysis. Oxygen plays a crucial role in the adsorption and desorption processes of reactants, directly affecting the efficiency and kinetics of catalytic reactions. An oxygen-enriched surface promotes the formation of active bonds with reactive species, which can not only accelerate chemical transformations but also improve product selectivity [36][37]. This behavior is particularly important in catalytic systems where surface reactivity is a key factor in optimizing overall catalyst performance.

Furthermore, the presence of a fluorine signal (F 1s, ~685 eV) suggests structural incorporation of fluorine into the matrix, supporting the hypothesis of carbonate-fluorapatite (Ca₅(PO₄)_2.5(CO₃)_0.5F) formation. The detection of iron (Fe 2p, 710 eV), although weak, indicates the presence of metallic traces, likely in the form of iron oxide (Fe₂O₃). Aluminum (Al 2p, 75 eV) is most likely present as alumina (Al₂O₃). Even at low concentrations, these impurities may influence the material’s properties, particularly in terms of chemical reactivity and thermal stability.

The presence of phosphorus (P 2s, 190 eV) indicates the incorporation of phosphate groups ( PO 4 3 − ) and/or P-O bonds, which are key elements in modulating surface properties and chemical interactions of the material [38]. These phosphate groups also open promising prospects in heterogeneous catalysis and biomedical applications due to their ability to interact with metal ions or biomolecules. Indeed, phosphate groups exhibit a strong interaction capacity with metal ions and biomolecules, which can enhance catalytic efficiency and support the development of biocompatible materials. This versatility highlights their key role in the design of functional materials adapted to complex environments [39].

The Si 2p peak at 155 eV suggests the presence of silicon in a specific chemical state, potentially associated with phosphate groups or other structures within the catalyst matrix. The identification of these chemical species within the catalyst underscores its potential for advanced applications, particularly in heterogeneous catalysis and multifunctional systems.

Figure 3. XPS spectrum of the catalyst.

The synthesized product obtained after recrystallization appears as yellow crystals (Figure 4). This morphological characteristic generally indicates a homogeneous chemical composition and a high degree of purity. Furthermore, the melting point of 54˚C, which is in line with the expected values for this type of compound, confirms the quality of the product obtained and the effectiveness of the purification process used.

A single-crystal X-ray diffraction study was carried out to analyze the structure of this compound.

Figure 4. Synthesized product

Figure 5 shows the FT-IR spectrum of the synthesized product, highlighting a characteristic band at 1662 cm⁻¹ corresponding to the stretching vibration of the carbonyl group (C=O) [40][41]. The band observed at 1604 cm⁻¹ confirms the presence of an olefinic carbon-carbon double bond (C=C), while the band at 1500 cm⁻¹ is attributed to an aromatic C=C bond [40].

In addition, the spectrum reveals two weak absorption bands at 3240 and 3195 cm⁻¹, associated with Csp²-H bonds [42]. The FT-IR analysis confirms that the obtained product is a chalcone, based on the characteristic bands related to its functional groups (C=O, C=C, and Csp²-H).

The low multiplicity and moderate intensity of the bands corresponding to the stretching vibrations of aliphatic C-H bonds, observed at frequencies above 3000 cm⁻¹, may be attributed to a decrease in electron density on the carbon atoms [43][44]. This decrease is accompanied by a reduction in the dipole moment associated with these bonds, resulting in weaker infrared absorption.

The carbonyl group (C=O) is distinguished by its strong spectroscopic sensitivity, due to charge transfer between electron donors and acceptors within this group [45]. This results in an intense absorption band in the infrared spectrum. This behaviour is also related to its high dipole moment, characteristic of a highly polar bond, reinforced by multiple bonding and possible π-π interactions between carbon and oxygen orbitals.

Furthermore, the band observed in the region between 1620 and 1500 cm⁻¹, attributed to the C=C group, is consistent with theoretical values [44]. It indicates effective conjugation between the C=C double bond and the carbonyl group (C=O), promoting electron delocalization within the conjugated system [46][47].

The presence of conjugated π-systems in the structure of chalcones plays a fundamental role in their activity, facilitating electron delocalization required for the absorption of reactive oxygen species and interaction with bacterial targets.

In contrast, the peaks below 1000 cm⁻¹ correspond to out-of-plane bending vibrations, also known as wagging vibrations [13]. These vibrations occur when bond angles change without significant variation in interatomic distances.

Figure 5. FT-IR spectrum of the synthesized product.

The ¹H NMR spectrum (500 MHz, DMSO-d₆) of the synthesized product (Figure 6) shows seven signals corresponding to seven distinct protons. This spectrum reveals the presence of aromatic protons, with characteristic multiplets and doublets in the range of 7.48 to 8.17 ppm.

Figure 6. ¹H NMR spectrum (600 MHz) of the synthesized product.

A multiplet at 7.48 ppm corresponds to three aromatic protons (H3, H4, H5), indicating meta and para couplings. A doublet of doublets at 7.59 ppm (J = 8.41 and 7.01 Hz) is assigned to protons H3’ and H5’, typical of meta positions on an aromatic ring. An isolated multiplet at 7.68 ppm corresponds to proton H4’, located at the para position.

Two well-resolved vinylic doublets are observed: one at 7.76 ppm (J = 15.61 Hz), attributed to Hα, and the other at 7.95 ppm (J = 15.68 Hz), attributed to Hβ. In addition, multiplets at 7.90 ppm (2H) correspond to protons H2 and H6, while those at 8.17 ppm (2H) are assigned to protons H2’ and H6’, located at meta and ortho positions, respectively (Figure 7).

The ¹H NMR spectrum of chalcone clearly shows two characteristic doublets corresponding to the vinylic protons Hα and Hβ. The large coupling constant (~15 Hz) suggests a trans configuration of the C=C double bond between Hα and Hβ, thus confirming the presence of a symmetric aromatic system as well as a trans α,β-unsaturated double bond [48].

This trans configuration is particularly favourable for the synthesis of (E)-chalcone derivatives, which are recognized for their significant potential as anticancer, antibacterial, and antifungal agents [49]-[51]. In general, cis isomers are less stable than trans isomers [52], due to increased steric repulsion, conformational differences, higher potential energy, and distinct physical properties.

This difference in stability is of particular importance in organic chemistry, as it influences both the reactivity of compounds and their potential applications. These properties make such compounds promising candidates for further investigations in pharmaceutical research.

Figure 7. Structural representation of (E)-chalcone.

Figure 8. Molecular structure of a chalcone monomer.

The analysis of the molecular structure reveals the presence of various characteristic types of bonds, including aromatic rings, single and double bonds, as well as interactions involving hydrogen atoms (Figure 8).

The molecular structure is characterized by the presence of two nearly parallel benzene rings, where the intermolecular hydrogen bond of the type C9-H9···O1 is represented by a dashed line. Figure 9 illustrates the molecular arrangement in the (a, b) plane, while bond lengths and bond angles are given in Table 4 and Table 5, respectively.

Figure 9. Molecular packing in the (a, b) plane.

Table 4. Bond lengths (Å).

Table 5. Bond angles (˚).

The combined analysis of these results provides a comprehensive understanding of the molecular structure, which is essential for predicting its chemical behaviour and interactions in various contexts.

The synthesized compound crystallizes in the orthorhombic system, belonging to the space group Pca2₁. This crystalline configuration, determined from diffraction data, provides the basis for analyzing molecular interactions and the overall conformation of the compound in the solid state. The study of internal geometric parameters thus allows a better understanding of the factors contributing to the stability and organization of the crystal lattice [53].

The aromatic rings adopt a nearly parallel conformation, forming an angle of 11.70˚ between their mean planes, indicating a slight inclination within the crystal packing. This deviation from coplanarity suggests a certain structural flexibility, which may promote intermolecular interactions such as π-π stacking or C-H···π interactions.

The α,β-unsaturated keto group adopts a syn-periplanar (-sp) conformation, as evidenced by the torsion angle of 18.4 (3)˚ measured for atoms C9-C8-C7-O1. This nearly coplanar spatial arrangement facilitates electron delocalization between the double bond and the carbonyl group. Moreover, the olefinic double bond C9=C8, with a length of 1.336 (3) Å, exhibits an E configuration. This geometry is consistent with sp² hybridization of the involved carbon atoms, as confirmed by proton nuclear magnetic resonance (¹H NMR) spectroscopic data, which show a coupling constant characteristic of the -CH=CH- motif.

The C-H···π interactions observed in the crystal lattice confirm the involvement of aromatic rings in structural cohesion. Two significant contacts are identified: C12-H12···Cg2 (where Cg2 denotes the centroid of the benzene ring formed by atoms C10 to C15), with an H···Cg distance of 2.71 Å and a C-H···Cg angle of 133˚, and C15-H15···Cg2, characterized by an H···Cg distance of 2.70 Å and an angle of 144˚.

C-H···π interactions represent a class of non-covalent interactions that may be dominated either by electrostatic forces or dispersion interactions, depending on the acidity of the C-H group. They contribute to the stabilization of the crystal lattice by ensuring lateral anchoring between molecules through their aromatic rings [54]-[56].

The bond lengths measured within the two benzene rings-C1-C2 (1.397 Å), C2-C3 (1.385 Å), C3-C4 (1.391 Å), C4-C5 (1.385 Å), C5-C6 (1.387 Å), C6-C1 (1.393 Å), as well as C10-C11 (1.398 Å), C11-C12 (1.389 Å), C12-C13 (1.388 Å), C13-C14 (1.390 Å), C14-C15 (1.388 Å), and C15-C10 (1.402 Å)-all fall within a narrow range between 1.385 and 1.402 Å. These values are characteristic of aromatic C-C bonds, intermediate between single and double bonds, confirming π-electron delocalization throughout the ring [57][58].

This near-equality of bond lengths attests to well-established aromaticity, conferring notable electronic stability to the molecule and playing a key role in the intermolecular interactions observed within the crystal lattice.

These remarkable properties of the synthesized product make it a particularly attractive scaffold for the design of therapeutic agents targeting specific biomolecules. Their ability to interact selectively with major biological targets makes them promising candidates for the development of new pharmacological entities [59]-[61]. Several chalcone derivatives have recently achieved pharmacological validation, illustrating their growing therapeutic potential. Among them, butein, sappanchalcone, and okanin stand out for their strong antioxidant activities [62]. Similarly, certain methylated chalcones have demonstrated notable efficacy in anticancer activity models [63]. Furthermore, dimethylamino-chalcones have shown significant activity as scavengers of the superoxide anion generated by stimulated human neutrophils, highlighting their potential role in controlling oxidative stress and inflammation [63].

The synthesised compound corresponds to the BZYACO structure previously reported in the literature [64]. This structural identity, confirmed by comparison of crystallographic data with those in the CSD database, fully validates our synthetic approach and highlights the remarkable efficacy of natural phosphate as an organic catalyst for the formation of this molecular entity.

The evaluation of the antibacterial and anticancer activities of this compound will be the subject of further studies.