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Article

Novel Catalysts Based on Synthetic Mesoporous Silicates of the MCM-41 Type and Hydroxyapatite for Desulfurization of Model Fuel

by
Nadezhda O. Donskaya
1,
Margarita A. Goldberg
1,*,
Alexander S. Fomin
1,
Anna O. Koptelova
2,
Polina D. Domashkina
2,
Ekaterina A. Eseva
2,
Olga S. Antonova
1,
Anatoliy A. Konovalov
1,
Alexander V. Leonov
2,
Egor A. Kudryavtsev
3,
Fadis F. Murzakhanov
4,
Marat R. Gafurov
4,
Argam V. Akopyan
2,
Sergey M. Barinov
1 and
Vladimir S. Komlev
1
1
Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskiy Pr. 49, Moscow 119334, Russia
2
Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskie Gory 1, Building 3, Moscow 119991, Russia
3
Joint Research Center of Belgorod State National Research University «Technology and Materials», Belgorod State National Research University, Pobedy Str. 85, Belgorod 308015, Russia
4
Institute of Physics, Kazan Federal University, 18 Kremlevskaya Str., Kazan 420008, Russia
*
Author to whom correspondence should be addressed.
Ceramics 2025, 8(2), 61; https://doi.org/10.3390/ceramics8020061
Submission received: 1 April 2025 / Revised: 7 May 2025 / Accepted: 20 May 2025 / Published: 21 May 2025
Browse Figures
Versions Notes

Abstract

:
Nanopowders of hydroxyapatite (HA) and Fe-substituted hydroxyapatite (HAFe) were synthesized by wet precipitation on either MCM-41 (a synthetic, mesoporous aluminosilicate material) or an aluminum-containing MCM-41 (AlMCM) support. According to X-ray diffraction data, all of the synthesized materials are composite powders consisting of amorphous silicate and an HA phase with low crystallinity. The presence of aluminum and iron in the structure of the powders resulted in further amorphization. The obtained samples showed high specific surface areas (SSAs), ranging from 162.3 to 186.6 m2/g for MCM-41-HA and from 112.6 to 127.2 m2/g for AlMCM-HA. The hysteresis loops were found to be of type H3, indicating the formation of slit-like pores in the intercrystalline space, as confirmed by transmission electron microscopy, which revealed the presence of lamellar and flake-like particles. Catalytic activity tests showed that the conversion of dibenzothiophene depended on the iron concentration in the material and the acidity of the support. To further improve the catalytic activity of the materials, they were impregnated with molybdenum compounds. Active molybdenum peroxo complexes formed under these conditions enabled 100% conversion of dibenzothiophene. To our knowledge, this is the first study on the influence of MCM-41-HA- or AlMCM-HA-based materials on dibenzothiophene conversion via oxidative desulfurization using hydrogen peroxide as an oxidant.

Graphical Abstract

1. Introduction

Today, petroleum is the world’s most important source of energy, corresponding to a third of the world’s energy consumption. The Russian Federation ranks third in the world in terms of reserves of heavy hydrocarbons [1]. The problem of the development of an effective catalyst for the cleansing of oil from sulfur-containing organic substances is important from both applied and basic research points of view. Due to the continuous increase in requirements for environmental safety and the performance of various fuels, regulated permissible concentrations of impurities in fuels are continuously decreased.
There is a clear tendency towards deterioration in the quality of extracted hydrocarbon feed, in particular, the quality of oil, which is associated with both an increase in the proportion of asphalt-resin compounds and an increase in the content of heteroatoms, in particular, sulfur atoms [2]. The presence of sulfur compounds in oil has a significant negative impact on processing, including the poisoning of catalysts, increased wear on processing equipment, and the pollution of the environment with sulfur compounds [3]. On the other hand, there is a tendency towards tightening environmental requirements for petroleum products, in particular, for motor fuels in terms of sulfur content [4]. These problems stimulate both the improvement of existing methods based on hydroprocesses and the development of new alternative methods. Among the various alternative hydrogen-free processes (extraction, adsorption, biodesulfurization, etc.), the most interesting from the point of view of the possibility of practical implementation is the oxidative desulfurization method, which allows for reducing the total sulfur content in various petroleum products down to ultra-low values of less than 10 ppm under relatively mild conditions (atmospheric pressure and temperature up to 80 °C) [5,6]. It is important to note that the process involves the use of industrially available reagents (in particular, hydrogen peroxide as an oxidizer), which is promising for its implementation in small oil refineries, where there is no possibility of introducing hydrodesulfurization.
Hydroxyapatite (HA) is an insoluble calcium phosphate with chemical formula Ca10(PO4)6(OH)2 and has found broad applications in biomedicine as a bone grafting material [7,8,9,10]. The use of HA-based materials as adsorbents, including for water purification, and as catalysts or catalyst supports is also growing in popularity [11,12,13,14].
The structure of HA contains two channels, and HA has both alkaline and acidic centers in its lattice, which enable its activity as an oxidative and reductive catalyst [15,16]. The existence of two different calcium positions is of interest because properties of HA have been successfully altered via specific modifications of these centers [17]. HA can give rise to solid solutions and substituted forms in terms of both the cation and anion; these features make it possible to regulate HA’s properties, including catalytic activity, selectivity, and stability in various reactions [18,19,20,21].
How to increase specific surface area (SSA) is one of the most important questions in the development of modern catalysts, and therefore, these projects often involve such materials as activated graphene [22,23], silica [24,25], alumina [26,27], or aluminosilicates [28,29] with SSAs above 1000 m2/g. For instance, the use of the synthetic aluminosilicate mesoporous material MCM-41 with an SSA up to 1100 m2/g is widespread [30,31]. MCM-41 has been popular due to its hexagonal arrangement of homogeneous mesopores, a simple synthesis method, and unrestricted experimental conditions [31,32]. The general procedure for the preparation of MCM-41 has been to use simple organosilicate or inorganic reagents as silica sources (e.g., tetramethylorthosilicate, tetraethylorthosilicate, or aqueous glass [33,34,35]) and a structure-forming agent. A drawback of this process has been the lack of an inexpensive and nontoxic source of silicon. Transition metals, such as tungsten [36], molybdenum [37], and vanadium [38], and noble metals, such as platinum [39] and palladium [40], are commonly employed as active components that enhance the oxidizing ability of MCM-41 [41,42]. MCM-41 has shown efficiency in the epoxidation of alkenes and cyclooctene and in oxidation with hydrogen peroxide [43,44]. On the other hand, the oxidative desulfurization conversion in the presence of pure MСM-41 does not exceed 8% [38].
The aim of our work was to investigate the effects of HA and of the introduction of iron cations into HA at either 5 or 10 mol.% on the catalytic activity of a composite based on either MCM-41 or aluminum-containing MCM-41 (AlMCM) in oxidative desulfurization reactions.

2. Materials and Methods

2.1. Synthesis of Samples

To obtain composite materials based on HA and a mesoporous silicate material of the MCM-41 type, the synthesis involved precipitation from aqueous solutions. The concentration of the calcium phosphate phase or of the iron-substituted calcium phosphate was selected to be 10 wt.%, and the degree of substitution with iron in HA was either 5 mol.% (HAFe1) or 10 mol.% (HAFe2). Materials were obtained on a support consisting of either MCM-41 or AlMCM. To obtain composite materials, each support was synthesized beforehand by the direct method [38].
At the next stage, 0.5 g of either MCM-41 or AlMCM was dispersed in 20 mL of distilled water. The dispersion was carried out from 30 to 60 min depending on the degree of agglomeration in an ultrasonic bath (Grand home-style ultrasonic cleaner) having a volume of 0.5 L, operating frequency of 35 kHz, power of 55 W, and amplitude modulation of 1000 Hz. After dispersion, a visual inspection was carried out, during which an incomplete breakdown of agglomerates on the AlMCM substrate was registered.
The synthesis of HA was carried out by means of a magnetic stirrer in accordance with Equation (1):
10Ca(NO3)2 + 6(NH4)2HPO4 + 8NH4OH → Са10(PO4)6(OH)2 + 20NH4NO3 + 6H2O
Either MCM-41 or AlMCM pre-dispersed in 25 mL of distilled water was added to a calcium nitrate solution (25 mL) with constant stirring. An ammonium phosphate solution (25 mL) was introduced into the resulting suspension, and 40 mL of a 25% aqueous NH4OH solution was also added during the synthesis to maintain a pH of 11–12.
The synthesis of iron-containing HA powders was performed via a similar approach with either silicate or aluminosilicate pre-dispersed by ultrasonication and mixed with a calcium nitrate solution. A ferric chloride solution served as an iron source (Equation (2)):
(10 − x)Ca(NO3)2 + 6(NH4)2HPO4 + (2x/3)FeCl3 + 8NH4OH → Ca(10−x)Fe2x/3(PO4)6(OH)2 + (20 − 2x)NH4NO3 + 2xNH4Cl + 6H2O,
where x = 0.05 or 0.1.
The ripening of materials in mother liquors lasted for 21 days at 37 °C [45]. The decoding of sample names is presented in Table 1.
Catalysts were also synthesized based on the obtained composite materials via processing with molybdenum compounds. The initial materials were impregnated with a solution of ammonium heptamolybdate with constant stirring under reduced pressure in a rotary evaporator, followed by the drying and calcination of the obtained catalyst at 550 °C. The amount of ammonium heptamolybdate was chosen so that the molybdenum content of the final catalyst would be 10 wt.%.

2.2. Sample Characterization

The powdered materials were characterized by X-ray diffraction (XRD) (Shimadzu XRD-6000, Kyoto, Japan, CuKα radiation, 10–60° [2Θ]), with the identification of phase composition using the ICDD PDF2 database. The indexing of the peaks was carried out with card ICDD No. 09–0432 for HA. Quantitative phase analysis and lattice parameter estimation by the Rietveld method were performed in the PHAN% software [46].
Fourier-transform infrared (FTIR) spectra of the samples were recorded by the KBr method on a Nicolet Avatar 330 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA); the spectra were obtained in a range from 4000 to 400 cm−1 to determine functional chemical groups in the samples.
Particle morphology was investigated via transmission electron microscopy (TEM; JEOL JEM 2100, Tokyo, Japan) performed on carbon-sputtered samples, at an accelerating voltage of 200 kV, with a diffraction dispersive detector (Oxford Instruments, Abingdon, UK) along with the Aztec software.
SSA was determined by the Brunauer–Emmett–Teller (BET) method, and pore-volume and pore-size distributions of the as-synthesized powders were assessed by means of the Barret–Joyner–Halenda analyzer model (BJH) via low-temperature nitrogen adsorption measurements (Micromeritics TriStar Analyzer, Micromeritics Instrument Corporation, Norcross, GA, USA). From the obtained specific surface area data, the average particle sizes were calculated using the following formula:
D = 6000/ρ∙SSA,
where D is the particle diameter, nm, ρ is the density of HA, g/cm3, and SSA is the specific surface area, m2/g.
Continuous wave (CW) electron paramagnetic resonance (EPR) measurements were performed on a Bruker Elexsys E580 spectrometer (Billerica, MA, USA) in an X-band (νmw = 9.6 GHz) frequency range at room temperature (297 K).
Molybdenum-impregnated samples were subjected to examination by Raman spectroscopy (Optosky ATR6600Pro, OptoskyPhotonics Imc., Xiamen, China) with an excitation wavelength of 1064 nm.
The oxidation of model mixtures was performed via the following procedure: 0.009–0.037 g of a catalyst and 0.003–0.034 mL of hydrogen peroxide (50 wt.%) were added to 5 mL of model fuel (a mixture of DBT with dodecane). The oxidation was conducted under the different conditions in the range of 5–60 min at 50–80 °C. The conversion conditions were established based on our previous results [47]. Each experiment was carried out repeatedly to obtain a minimum of three convergent results, which differ from the average value by less than 3%.
The composition of reaction products was analyzed (and the purity of starting substances was controlled) by gas chromatography on a Kristall-2000M chromatograph (Chromatech, Yoshkar-Ola, Russia) equipped with a flame ionization detector (column: Zebron L = 30 m, d = 0.32 mm; liquid phase: ZB-1) with temperature programming from 100 to 250 °C. Concentrations were calculated from a change in the relative peak area of the substrate (DBT) and products (wt.%).

3. Results

Because the components of the investigated composites—HA and structured silicate/aluminosilicate—have the main X-ray reflections in different ranges of 2Θ (in HA from 20° to 60° [47,48], ICDD card 9-432, and in MCM-41 or AlMCM, in the region of ultra-small angles <7° {2θ = 2.86°, 4.72°, or 5.40°} [49], ICDD card 00-049-1712), it was difficult to acquire a single spectrum capturing both ranges of 2Θ angles. In addition, in studies on HA/MCM-41 composites, the disappearance of peaks at low angles has been found to be related to the destruction of the structure during HA formation [50]; this process can also be proven by TEM.
In our XRD data, all diffractograms of the synthesized materials indicated low crystallinity with a halo in the 2θ zone of 15–35° and also showed peaks in the region 30–33° representing the three most intense peaks of the apatite phase at 31.77, 32.19, and 32.90 2θ. The broad peak in the 2Θ 15–30° region centered at 20–23° is characteristic of an amorphous silicate phase [50,51,52] (Figure 1). The introduction of iron cations into the HA lattice decreased the intensity of apatite phase peaks at 30–33°; for compositions with 10 mol.% of Fe, the apatite reflection almost disappeared, by turning into a shoulder of the silicate peak. Thus, the introduction of iron cations into HA leads to an amorphization of the powder material. The introduction of the Al additive into MCM-41 also diminished the degree of the crystallization of powder materials, as evidenced by the reduced intensity of the halo (Figure 1 and Figure S1).
The presented IR spectra (Figure 2) clearly show the main peaks characteristic of vibrations of silicon dioxide mesh in the regions of 460, 805, and 1200–1080 cm−1 [51]. When HA/HAFe was deposited on the MCM-41 matrix, the ν1 (PO4) vibration peak at 960–962 cm−1 as well as a substantial amount of adsorbed CO2 (intense vibration in the region of 1480 cm−1) were evident in the MCM-41 matrix. The main peak of the phosphate group in the 1200–1000 cm−1 region strongly overlaps with an intense peak of O-Si-O vibrations. Another characteristic region for phosphate is 650–550 cm−1, in which the presence of more intense bands should also be seen (and was), compared to the support material without the deposition. Of interest is the hydroxyl peak at 3745 cm−1, which is characteristic of MCM-41 [53]. This band is associated with the silanol group, which is located at the edges of MCM-41 particles. After the deposition of the apatite phase on the surface of the support, the intensity of these vibrations dropped sharply, possibly indicating the presence of chemical bonding between silanol groups and deposited HA/HAFe.
According to BET data, the application of HA or HAFe significantly decreases SSA, porosity, and pore volume as compared to MCM-41 without the deposition (Table 2). Relative to the silicate without the deposition, the change in the average pore diameter was considerable. As the degree of iron substitution went up, SSA increased from 162.3 to 186.6 m2/g and pore volume enlarged by 25.8%. Adsorption–desorption isotherms are of type IV according to IUPAC. Hysteresis loops of all samples were attributed to H3-type behavior, which characterizes split-shape pores.
For the AlMCM series, a smaller SSA was noticeable in comparison with the composite materials based on MCM-41; this phenomenon can be explained by a reduction in the SSA of the original aluminosilicate and by the presence of the AlMCM support’s agglomerates that could not be broken up during the preparation procedure before the application of HA. Nonetheless, the tendency of the total pore volume to enlarge with the increasing iron content was still present. The enlargement of micropore volume from 0.0039 to 0.0065 cm3/g was noted too.
The adsorption–desorption isotherm data (Figure 3a,c) pointed to the mesoporous structure of the obtained materials. Pore size distribution plots (Figure 3b,d) confirmed that most of the pore volume falls within the range of pore sizes from 2 to 50 nm. The materials are also characterized by an H3-type hysteresis loop, implying the formation of materials containing pores of the split-shape type.
According to TEM data, composite powders MCM-HA and MCM-HAFe2 each consist of two fractions: structured silicate and apatite powders (Figure 4). The particle shape of the composite materials did not change with the increasing iron content and was flake-like and lamellar [54]; the particle size diminished somewhat (from 50–40 to 20 nm) with the elevation of iron concentration. Dark zones belonging to MCM-41 are present in micrographs.
Mapping-mode imaging confirmed the emergence of calcium phosphate on the surface of MCM-41 (Figure 5). Silicon, calcium, phosphorus, and iron were found to be uniformly distributed on the whole surface of the examined particles. These findings mean the absence of the formation of separate zones of MCM-41 not covered by calcium phosphate as well as no iron oxide formation at the 10 mol.% substitution during the synthesis of the powders.
An appreciable change in morphology was documented with the increasing aluminum content of the aluminosilicate carrier (Figure 6). The AlMCM-HA composite powders contain agglomerates having a curved cylindrical and toroidal shape (up to 500 nm in length and 100 nm in width) and are covered with small HA particles up to 50 nm in size.
The pure HA sample, just as the mesoporous MCM-41 material, is paramagnetically pure, and no EPR spectrum should be observed. A single broad resonance line at g = 4.3 for Fe3+ ions (100–150 mT) in MCM-HAFe2 and in the 300–400 mT region was detected in the studied samples; this outcome may be related to the presence of free radicals on the surface of the samples (Figure 7). The obtained EPR spectra were explained by isotropic interaction and the presence of strong zero-field splitting for Fe3+ ions owing to the influence of a local crystal field (an electric-field gradient) [47]. The high absorption capacity of the compounds leads to an increased concentration of carbonate radicals on the surface and causes a strong spin–spin interaction and homogeneous broadening of the resonance lines. The asymmetry of the EPR lines in the region g = 2.00 (B0 = 310 mT, the red line in Figure 7) is due to the anisotropy of spectroscopic splitting factor Δg = 0.02, indicating the partial incorporation of radicals into the crystal lattice.
Electron-nuclear double resonance (ENDOR) spectra were obtained using a Mims pulse sequence and a 150 W radio frequency generator. The ENDOR spectrum at room temperature in the X-band contained a weakly structured extremely broad line, which did not provide additional information about the material’s features. This phenomenon is caused by short relaxation times and the scattering of the magnitude of hyperfine structure.
The activity of composite MCM-HA materials and of MCM-HAFe1-2, AlMCM-HA, AlMCM-HAFe1-2, and molybdenum-impregnated catalysts was investigated in hopes of achieving a complete conversion of the model compound.
Subsequent to the impregnation process, the samples were subjected to Raman spectroscopy analysis. As demonstrated in Figure S2, the Raman spectra of both the iron-containing and iron-free samples exhibited bands that corresponded to the vibrations of the MoO4 group [55], specifically the Mo-O-Mo stretching mode at 872 cm−1.
Figure 8 presents the results of the activity assay of the synthesized catalysts deposited on mesoporous silicate MCM-41. It should be noted that the catalysts representing HA deposited on the carrier (MCM-HA) with different iron concentrations manifested almost no catalytic activity. Namely, when these materials were employed, the kinetic curve reached a plateau corresponding to 20% conversion, which may be explained by both the idle adsorption of DBT in the pores of the carrier (which is 7%) and by the oxidation of DBT by hydrogen peroxide at 70 °C. According to the results, when the iron content of HA was increased, the conversion of DBT monotonically decreased. For example, the catalyst 10%Mo/MCM-HA achieved 100% conversion of DBT within 30 min of the reaction, whereas under similar conditions, the catalyst 10%Mo/MCM-HAFe1 helped to oxidize only half of the substrate; in the presence of the catalyst 10%Mo/MCM-HAFe2, the conversion was less than 40%.
When the alumina-containing carrier was utilized, the DBT conversion in the presence of the catalyst 10%Mo/AlMCM-HAFe1 was higher by approximately 10% as compared to the corresponding catalyst based on the MCM-41 carrier (Figure 9). This outcome may be a consequence of the greater acidity of the modified carrier, which enabled a more efficient coordination of sulfur-containing compounds and their oxidation. On the contrary, no such improvements in the kinetic curve of DBT oxidation were observed with the catalyst 10%Mo/AlMCM-HAFe2. A possible reason is that at such a high iron content, most of the hydrogen peroxide decomposes and does not have a chance to enter into the oxidation reaction. Furthermore, without molybdenum, in the presence of HAFe catalysts deposited on AlMCM, the reaction almost failed.

4. Discussion

The present work is focused on the preparation of HA- and HAFe-based mesoporous powders on MCM-41 or AlMCM supports. HA is a promising material as a catalyst [47], a catalyst support [56,57], and a sorbent for water and soil treatment [58,59]. MCM-41 and its more acidic analog AlMCM [60] have a well-developed surface and ordered porous structure, which allow these materials to be used in a wide range of applications. Nonetheless, the production of MCM-41 and AlMCM involves costly reagents (that serve as sources of silicon), namely tetraethoxysilane and tetramethoxysilane [61], along with sodium silicate solutions, colloidal silica, and fumed silica [62,63]. To produce HA, accessible and inexpensive precursors such as calcium nitrate and ammonium phosphate are required, thus contributing to a lower cost of the resultant functional materials. Iron may influence biological [64,65], catalytic [66], and textural properties [48] of such powders, thereby inspiring us to evaluate its effect on the composite materials based on HA and mesoporous silica. In our work, the introduction of iron enlarged the SSA of the composite materials, with the observed values reaching 186.6 m2/g for MCM-HAFe2 and 112.6 m2/g for AlMCM-HAFe2; these are much greater than those of pure mesoporous HA [67,68].
Meso-HA can be obtained when MCM-41 is applied as a template, as demonstrated in [69]. In that study, Ca2+ and H2PO4− ions were deposited on the surface of Si-MCM-41 to create a pH-sensitive platform for the adsorption and release of ciprofloxacin molecules. Their BET data revealed an SSA of 51.17 m2/g, which is three times smaller than that of an analogous powder prepared in our work. The pore diameter and pore volume were measured there and proved to be 2.4 nm and 0.30 cm3/g, respectively.
The composite proposed by us has considerable potential in terms of the largest SSA increase for studies dealing with nanocomposites based on HA/MCM-41 and HA/SBA-15 for applications involving water purification from fluoride [70]. These composites have an SSA of 987 m2/g, with the HA content ranging from 30 to 35 wt.%. The activation of the materials is achieved through heating at 500 °C in N2 flow for 10 h, followed by calcination from 100 to 500 °C at a heating rate of 2 °C/min for 2 h. Unfortunately, data pertaining to catalytic activity and iron incorporation into the lattice are not presented in that article.
The material most similar to ours in terms of composition is the nanocomposite obtained by Hosseini et al. [71]. In that work, superparamagnetic materials based on MCM-41, HA, and Fe3O4 were prepared to realize a drug delivery system. Judging by X-ray phase analysis, their powders are identical to those obtained in our work. XRD patterns of both sets of materials show an amorphous halo with poorly resolved hydroxyapatite peaks. In addition, FTIR spectroscopy data confirmed the presence of phosphate groups in the obtained composites, as evidenced by the presence of a high-intensity zone at 1000 cm−1. In that article, no information is provided about the SSA and catalytic activity of the materials.
Several papers have been dedicated to the subject of HA serving as a carrier of MCM-41 [72]. In the study just cited, mesoporous silicate was deposited on the HA surface by the sol-gel method at room temperature. The effect of ripening time on textural characteristics was investigated, and according to BET data, adsorption–desorption isotherms correspond to type IV according to IUPAC classification. The SSA of the samples without ripening was approximately 36.2 m2/g, and after ripening for 48 h, it increased to 334 m2/g. The article does not present data on the effect of iron and aluminum, nor does it include data on catalytic activity.
To the best of our knowledge, this is the first study on an influence of MCM-41-HA composition on the conversion of DBT in the green process of oxidative desulfurization using hydrogen peroxide as an oxidant. DBT and its derivatives are the main sulfur-containing compounds in fuel oil, especially in diesel fuels. Our results indicate that the process is characterized by mild conditions (a temperature of 70 °C and a release of water as a single coproduct) and is effective [47]. The addition of iron did not help to improve the conversion, which remained below 20%. By contrast, the incorporation of molybdenum oxide into the composite material substantially enhanced the conversion, giving 100% conversion with 10%Mo/MCM-HA or 10%Mo/AlMCM-HA (see Figure 10). This is due to the formation of active molybdenum-based peroxycomplexes in the presence of H2O2 that react with benzothiophene [73,74,75].
To enhance the oxidizing capacity of MCM-41, it has been proposed that the impregnation of the material with various metal groups may be a viable solution. Examples of such groups are rare-earth metals such as praseodymium [76], cerium, and erbium [77] as well as noble metals such as silver [78,79], palladium [39,80], and platinum [39,79]. The molybdenum-containing compounds are economically advantageous and possess high catalytic activity in DBT oxidative desulfurization. A high conversion of DBT has been achieved in catalyst systems based on 14 wt.% MoO3/γ-Al2O3 [81], MoO2/g-C3N4 [82], NiMo on MCM-41 [83], or 3Mo/NKM-5 [84]. Furthermore, a catalyst composed of an MCM-41 support impregnated with molybdenum–vanadate–tungsten oxides has been shown to ensure 100% conversion after 2 h of the process at 1 wt.% of the catalyst, with a H2O2:S ratio of 4:1, when performed at 60 °C [38]. In our work, it was estimated that 10%Mo/MCM-HA and 10%Mo/AlMCM-HA would cause 100% conversion of DBT after 30 min at 70 °C, with a similar H2O2:S ratio, thereby performing highly efficient catalysis. Thus, MCM-HA and AlMCM-HA may serve as efficient catalyst supports. The studies of real oil oxidation desulfurization and the reusability of the catalysts could shed light on the prospects of the developed materials in the industry.

5. Conclusions

Precipitation from aqueous solutions in the presence of mesoporous materials, namely MCM-41 and AlMCM, was found to result in the formation of apatite structures and iron-doped apatite phases. During the synthesis of HA on MCM-41 or AlMCM, the presence of silanol groups on these mesoporous materials was shown to facilitate the chemisorption of apatite structure, irrespective of iron concentration.
According to our BET data, the obtained materials possess mesoporous structure with a broad distribution of pore sizes. The MCM-41–based composite materials are distinguished by larger SSAs as compared to AlMCM-based materials, and an increase in the iron content gave powders with an SSA of 186 m2/g. Furthermore, hysteresis loops were found to be of the H3 type, indicating the emergence of slit-like pores within the space between lamellar crystals, according to ref. [68]. This hypothesis is further substantiated by our TEM data, which revealed the presence of lamellar and flake-shaped particles.
Subsequently, catalytic activity assays were conducted to ascertain the effect of iron concentration on the conversion of DBT. It was observed that as the iron concentration was increased in each formulation, the conversion of DBT diminished. A possible theory to explain this outcome is that iron reacts more vigorously with hydrogen peroxide, thereby inducing the decomposition of the oxidant and its off-target consumption.
A major finding is an apparent enhancement of catalysis as a consequence of the molybdenum impregnation. It can be supposed that the role of HA is to bind molybdenum compounds and prevent their leaching in the presence of an aqueous hydrogen peroxide solution. Accordingly, low-cost composite catalysts were prepared, with the most efficient compositions being 10%Mo/MCM-HA and 10%Mo/AlMCM-HA and leading to 100% DBT conversion within 30 min at 70 °C. To our knowledge, this is the first investigation into the impact of MCM-41-HA composition on the conversion of DBT via oxidative desulfurization by means of hydrogen peroxide as an oxidant.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/ceramics8020061/s1: Figure S1: XRD data on MCM-HA and AlMCM-HA; Figure S2: Raman spectra of MCM-HA and MCM-FeHA after molybdate impregnation.

Author Contributions

Conceptualization, M.A.G.; Methodology, N.O.D. and O.S.A.; Investigation, A.S.F., A.O.K., P.D.D., E.A.E., O.S.A., A.A.K., A.V.L., E.A.K., and F.F.M.; Writing—original draft, N.O.D., M.A.G., and A.S.F.; Writing—review and editing, M.A.G., M.R.G., A.V.A., S.M.B., and V.S.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Russian Science Foundation, grant number 22-79-10293.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD data on HA, HAFe1, and HAFe2 powders deposited on either (a) MCM-41 or (b) AlMCM as well as pure MCM-41 and AlMCM materials, where M denotes silicate, and A denotes apatite phases.
Figure 1. XRD data on HA, HAFe1, and HAFe2 powders deposited on either (a) MCM-41 or (b) AlMCM as well as pure MCM-41 and AlMCM materials, where M denotes silicate, and A denotes apatite phases.
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Figure 2. FTIR spectra of HA and HAFe1–4 powders deposited on either (a) MCM-41 or (b) AlMCM.
Figure 2. FTIR spectra of HA and HAFe1–4 powders deposited on either (a) MCM-41 or (b) AlMCM.
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Figure 3. Adsorption–desorption isotherms and pore size distributions of HA, HAFe1, and HAFe2 powders deposited on either MCM-41 (a,b) or AlMCM (c,d).
Figure 3. Adsorption–desorption isotherms and pore size distributions of HA, HAFe1, and HAFe2 powders deposited on either MCM-41 (a,b) or AlMCM (c,d).
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Figure 4. Morphology of MCM-HA (a,b) and MCM-HAFe2 (c,d).
Figure 4. Morphology of MCM-HA (a,b) and MCM-HAFe2 (c,d).
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Figure 5. Elemental distribution maps of MCM-HAFe2.
Figure 5. Elemental distribution maps of MCM-HAFe2.
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Figure 6. Morphology of AlMCM-HA.
Figure 6. Morphology of AlMCM-HA.
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Figure 7. EPR spectra of samples MCM-41, MCM-HA, and MCM-HAFe2 in X-band pulsed mode at room temperature. The red line indicates the center of gravity of the EPR signal at g = 2.00.
Figure 7. EPR spectra of samples MCM-41, MCM-HA, and MCM-HAFe2 in X-band pulsed mode at room temperature. The red line indicates the center of gravity of the EPR signal at g = 2.00.
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Figure 8. Effects of the catalyst type and reaction time on the conversion of DBT. Catalysts are based on MCM-41. Reaction conditions: 70 °C, w(cat.) = 0.5%, and H2O2:S = 4:1.
Figure 8. Effects of the catalyst type and reaction time on the conversion of DBT. Catalysts are based on MCM-41. Reaction conditions: 70 °C, w(cat.) = 0.5%, and H2O2:S = 4:1.
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Figure 9. Effects of the catalyst type and reaction time on the conversion of DBT. Catalysts are based on AlMCM. Reaction conditions: 70 °C, w(cat.) = 0.5%, and H2O2:S = 4:1.
Figure 9. Effects of the catalyst type and reaction time on the conversion of DBT. Catalysts are based on AlMCM. Reaction conditions: 70 °C, w(cat.) = 0.5%, and H2O2:S = 4:1.
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Figure 10. The scheme of the catalyst structure impregnated on the surface of the MoO4.
Figure 10. The scheme of the catalyst structure impregnated on the surface of the MoO4.
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Table 1. The compositions of the investigated materials.
Table 1. The compositions of the investigated materials.
Sample CodeComposition
Type of Mesoporous SilicaHA Concentration, wt.%Fe Concentration, mol.%
MCM-HAMCM-41100
MCM-HAFe1MCM-41105
MCM-HAFe2MCM-411010
AlMCM-HAaluminum-containing MCM-41100
AlMCM-HAFe1aluminum-containing MCM-41105
AlMCM-HAFe2aluminum-containing MCM-411010
Table 2. Textural parameters of MCM-41- and AlMCM-based materials.
Table 2. Textural parameters of MCM-41- and AlMCM-based materials.
SampleSSA (m2/g)Total Pore Volume (cm3/g)Average Pore Diameter (nm)D (nm)
MCM-41 series
MCM-41925.10.933.22.94
MCM-HA162.30.6214.416.1
MCM-HAFe1173.20.7616.815.1
MCM-HAFe2186.60.7815.914.0
AlMCM series
AlMCM843.91.355.23.2
AlMCM-HA123.30.3913.221.2
AlMCM-HAFe1127.20.3813.420.6
AlMCM-HAFe2112.60.4518.523.2
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Donskaya, N.O.; Goldberg, M.A.; Fomin, A.S.; Koptelova, A.O.; Domashkina, P.D.; Eseva, E.A.; Antonova, O.S.; Konovalov, A.A.; Leonov, A.V.; Kudryavtsev, E.A.; et al. Novel Catalysts Based on Synthetic Mesoporous Silicates of the MCM-41 Type and Hydroxyapatite for Desulfurization of Model Fuel. Ceramics 2025, 8, 61. https://doi.org/10.3390/ceramics8020061

AMA Style

Donskaya NO, Goldberg MA, Fomin AS, Koptelova AO, Domashkina PD, Eseva EA, Antonova OS, Konovalov AA, Leonov AV, Kudryavtsev EA, et al. Novel Catalysts Based on Synthetic Mesoporous Silicates of the MCM-41 Type and Hydroxyapatite for Desulfurization of Model Fuel. Ceramics. 2025; 8(2):61. https://doi.org/10.3390/ceramics8020061

Chicago/Turabian Style

Donskaya, Nadezhda O., Margarita A. Goldberg, Alexander S. Fomin, Anna O. Koptelova, Polina D. Domashkina, Ekaterina A. Eseva, Olga S. Antonova, Anatoliy A. Konovalov, Alexander V. Leonov, Egor A. Kudryavtsev, and et al. 2025. "Novel Catalysts Based on Synthetic Mesoporous Silicates of the MCM-41 Type and Hydroxyapatite for Desulfurization of Model Fuel" Ceramics 8, no. 2: 61. https://doi.org/10.3390/ceramics8020061

APA Style

Donskaya, N. O., Goldberg, M. A., Fomin, A. S., Koptelova, A. O., Domashkina, P. D., Eseva, E. A., Antonova, O. S., Konovalov, A. A., Leonov, A. V., Kudryavtsev, E. A., Murzakhanov, F. F., Gafurov, M. R., Akopyan, A. V., Barinov, S. M., & Komlev, V. S. (2025). Novel Catalysts Based on Synthetic Mesoporous Silicates of the MCM-41 Type and Hydroxyapatite for Desulfurization of Model Fuel. Ceramics, 8(2), 61. https://doi.org/10.3390/ceramics8020061

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