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Article

Chitosan-Modified SBA-15 as a Support for Transition Metal Catalysts in Cyclohexane Oxidation and Photocatalytic Hydrogen Evolution

by
Assemgul S. Auyezkhanova
1,*,
Alima K. Zharmagambetova
1,
Eldar T. Talgatov
1,
Aigul I. Jumekeyeva
1,
Sandugash N. Akhmetova
1,
Zhannur K. Myltykbayeva
2,
Imge Kalkan
3,
Atıf Koca
3,
Akzhol A. Naizabayev
1 and
Aigul T. Zamanbekova
1
1
D.V. Sokolsky Institute of Fuel, Catalysis, and Electrochemistry, Kunaev, Almaty 050010, Kazakhstan
2
Research Institute of New Chemical Technologies and Materials, Al-Farabi Kazakh National University, Almaty 050040, Kazakhstan
3
Department of Chemical Engineering, Faculty of Engineering, Marmara University, Istanbul 34854, Turkey
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(7), 650; https://doi.org/10.3390/catal15070650
Submission received: 5 June 2025 / Revised: 27 June 2025 / Accepted: 30 June 2025 / Published: 3 July 2025
(This article belongs to the Special Issue Homogeneous and Heterogeneous Catalytic Oxidation and Reduction)
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Abstract

This work aims to study the catalytic properties of Fe, Cr, and Cu catalysts deposited on chitosan–silica (SBA-15) composites in liquid phase oxidation of cyclohexane (CH) with H2O2 and photocatalytic hydrogen evolution reaction. The catalysts were obtained by consecutive adsorption of chitosan (CS) and metal ions (Fe3+, Cr3+, Cu2+) on SBA-15 at ambient conditions. Characterization of the catalysts by XRD, IR spectroscopy, XPS, TEM, SEM, etc., showed the CS and metal ion adsorption on the solid support. Modification with CS provided better immobilization of the metal ions on SBA-15. The synthesized catalysts demonstrated different performance in liquid phase oxidation of cyclohexane with H2O2 under mild conditions at 40 °C and atmospheric pressure. Cyclohexane conversion on Fe–CS/SBA-15 (18.5%) and Cr–CS/SBA-15 (21.6%) was higher than on Cu–CS/SBA-15 (9.3%). The influence of different conditions of the reaction such as time, temperature, catalyst dosage, substrate and oxidant ratio on cyclohexane conversion in the presence of the most efficient Cr–CS/SBA-15 catalyst was also studied. The optimal reaction conditions were found to be the following: duration of reaction—4 h, temperature of reaction—50 °C, mcat—0.03 g, a substrate/H2O2 ratio of 1:3. In addition, Cr–CS/SBA-15 and Fe–CS/SBA-15 catalysts were studied in a photocatalytic H2 evolution reaction. The Fe-containing catalyst demonstrated superior efficiency in photocatalytic H2 evolution. The total volume of hydrogen produced within 3 h was 103 mL/g. Thus, this study demonstrates that chitosan possesses promising potential in the design of the supported catalysts for cyclohexane oxidation and photocatalytic hydrogen evolution reactions.

Graphical Abstract

1. Introduction

Catalytic oxidation of cyclohexane to a cyclohexanone and cyclohexanol mixture, known in the literature as KA-oil, is one of the most important industrial processes. KA-oil is used to obtain valuable compounds such as caprolactam and adipic acid [1,2,3,4,5,6]. This reaction is considered challenging because of the high stability of the H–C bond. Selective oxidation of cyclohexane under industrial conditions is carried out in the liquid phase at 150–160 °C and an O2 or air pressure of 10–20 bar employing salts of Co or Mn as homogeneous catalysts [7,8]. One of the disadvantages of the process is the high contamination of the homogeneous catalyst and its difficult and expensive separation from the reaction mixture. The growing demand for KA-oil, along with the adverse environmental effects of the current process, necessitates the replacement of conventional homogeneous catalysts with more efficient and eco-friendly heterogeneous alternatives [1,5,9,10].
Heterogeneous catalysts showed promising catalytic properties to be used in the oxygenation of cyclohexane [9,10,11,12,13]. Heterogeneous catalysts based on Pd [2,5,13], Ru [4,14], Au [8,15,16,17], and Ag [18,19] are characterized by high efficiency in the selective oxidation of cycloalkanes. However, their high cost and toxicity necessitate their replacement by more available metals such as Co, Cu, and Fe [3,20,21,22,23,24,25,26].
Kirar J.S. et al. [3] synthesized Cu nanoparticles deposited on layered double hydroxide (LDH) using the chemical reduction method. The activity, selectivity, and stability of the developed heterogeneous nanocatalysts were studied in cyclohexane oxidation at 353 K. Tert-butyl hydroperoxide (TBHP) was taken as an oxidant. The conversion of cyclohexane and KA-oil selectivity were 52.3% and 97.2%, respectively. The authors in their other work developed Co and Fe nanoparticles fixed on LDH [20]. The cobalt systems showed higher activity than iron catalysts. The results of structural analysis indicate the strong fixation of transition metal nanoparticles on the layered double hydroxide, which promotes the dispersibility of metal nanoparticles. The highest cyclohexane conversion and KA-oil selectivity in the presence of the CoNPs@LDH nanocatalyst were 45.7% and 95.8%, respectively. Xie S. et al. [21] have reported that cyclohexane conversion on carbon-based Cu2O/BGC catalyst was 77.1% with a cyclohexanone selectivity of 68.5%. Jian J. et al. [22] developed highly dispersed Co nanocatalysts fixed on SBA-15, in the presence of which the yield of products and selectivity of KA-oil were 9.6% and 81.1%, respectively. In the study in [23], Cu-MgAlO catalytic systems were developed and tested in cyclohexane oxidation. It was shown that copper can activate the C-H bond and improve the decomposition of cyclohexyl hydroperoxide to KA-oil. The highest product yield (8.3%) and KA-oil selectivity (82.9%) were observed on 9%Cu-MgAlO catalysts at 150 °C and an O2 pressure of 0.6 MPa for 2 h. In [24], mesostructured MCoS cobalt silicates were described. The degree of substrate conversion on the prepared catalysts increases with increasing molar ratio of cyclohexane/TBHP from 1:1 to 1:2. At the same time, the selectivity for cyclohexanol decreases due to the following oxidation of cyclohexanol to cyclohexanone. Wu M. et al. [11] presented a cobalt catalyst deposited on magnesium oxide (Co/MgO) for the cyclohexane oxidation with O2. The presence of 0.2%Co/MgO catalyst at 140 °C and 0.5 MPa O2 for 4 h led to a substrate conversion of 12.5% and KA-oil selectivity of 74.7%. In [25], a Cu catalyst on TiO2 was prepared and tested for cyclohexane oxidation to a cyclohexanol and cyclohexanone mixture under mild conditions. The product yield and selectivity were 16.8% and 98.0%, respectively. In [26], cobalt-molybdenum oxide fixed on mesoporous silica supports such as SBA-15, KIT-6 and FDU-12 was used for liquid phase cyclohexane oxidation with molecular oxygen. All investigated catalysts demonstrated 7–8% substrate conversion with a selectivity of 85% to KA-oil. The possibility of catalyst regeneration by repeated calcination was shown.
Thus, according to the literature, available transition metals (Co, Cu, Fe) can successfully activate the C-H bond, and a new approach is needed to develop efficient catalysts for such reactions.
Photocatalytic evolution of hydrogen has attracted considerable attention for its sustainability and high-energy conversion efficiency [27,28,29,30,31,32,33]. The design of new effective catalysts to increase the reaction rate and hydrogen yield is an important area of research in this field [32,33]. Various transition metal oxides, hybrid materials, carbon-based nanomaterials, etc., are used as photocatalysts [27,28,29,30,31,32,33]. Among them, the photocatalysts containing transition metals (Fe, Ni, Cu and Co) are promising [34,35].
Polymer–metal complexes (PMCs) supported on inorganic materials have a great potential for use as catalysts in different reactions. Immobilization of PMCs on a solid support can provide such benefits as good dispersion and stabilization of active metal particles and facile separation of the catalyst from reaction mixture. However, to our knowledge, there are only a few reports on the study of polymer–metal complexes immobilized on solid supports in the oxidation of cyclohexane and no reports for their application in photocatalytic hydrogen evolution.
Recent studies have shown the promising application of natural polysaccharides as renewable, non-toxic and biodegradable materials in the design of catalysts [36,37,38]. A large number of works are devoted to chitosan (CS)-based catalytic systems for various chemical processes, owing in part to the presence of functional groups in the structure of polysaccharide that can interact with and stabilize active metal particles [27,28,29,30,31,32,33,34,35,36,37]. Various silicate materials (SBA-15, MCM-41, ZSM-5, etc.) due to their characteristics such as large surface area and pores and high thermal stability are potential candidates as supports for the fixation of active metal particles [9,12,22,39].
Therefore, in this work, Cr, Fe and Cu catalysts fixed on mesoporous silica modified with chitosan were characterized using physicochemical methods and tested in cyclohexane oxidation to KA-oil with H2O2 as a “green” oxidant. The photocatalytic properties of the prepared catalysts were also studied in the H2 evolution reaction.

2. Results and Discussion

2.1. Characterization of Catalysts

The catalysts were prepared by sequential adsorption of chitosan (CS) and metal ions (Fe3+, Cr3+, Cu2+) on SBA-15. The elemental contents of the catalysts identified by EDX measurements are shown in Table 1. For comparison, the supported catalysts without polymers were prepared. The content of the active phase (metal) in Cr–CS/SBA-15 and Fe–CS/SBA-15 was close to 3% wt., while in the Cu–CS/SBA-15 the metal content was lower. For the catalysts without polymer modification, only a small amount of metal was detected on the SBA-15 surface. Thus, the polymer promotes the fixation of the active phase on the solid support.
Figure 1 presents the SEM and EDX elemental mapping images of Cr, Fe, Si and O from the Cr–CS/SBA-15 and Fe–CS/SBA-15. All elements (Cr, Si, O for Cr–CS/SBA-15 and Fe, Si, O for Fe-CS/SBA-15) are uniformly distributed, which is consistent with the SEM images and suggests that Cr and Fe are homogeneously fixed on CS/SBA-15.
Figure 2 demonstrates the IR spectra of initial components and Cr–CS/SBA-15 catalyst. The spectrum of SBA-15 shows a broad absorption peak at 3467 cm−1, which belongs to hydroxyl (O–H and Si–OH) stretching bands (Figure 2a) [40]. The absorption peak at 1104 cm−1 was ascribed to the antisymmetric stretching vibrations of Si-O-Si [40,41,42]. IR spectra of chitosan displayed absorption peaks at 3442 cm−1 (νO–H), 2929 and 2888 cm−1 (νC–H), 1644 cm−1 (C=O stretching in NH2-C=O), 1562 cm−1 (δN–H), and 1080 cm−1 (νC–O) (absorption frequencies of β-d-pyranoside in chitosan) (Figure 2b) [40,41,43,44]. When chitosan is dissolved in dilute HNO3 solution, protonation of NH-groups occurs to form a positively charged water-soluble cationic polyelectrolyte [43]. This was confirmed by shifting of the absorption bands of -NH-groups from 1562 cm−1 to 1532 cm−1 (Figure 2c) [45,46].
IR spectra of CS/SBA-15 and Cr–CS/SBA-15 showed a presence of functional groups of both chitosan and SBA-15. Moreover, a comparison of IR spectra of CS/SBA-15 (Figure 2d) with those of the protonated form of chitosan (chitosanium nitrate, Figure 2c) and Cr–CS/SBA-15 (Figure 2e) showed that chitosan interacted with both SBA-15 surface and Cr ions through its NH-groups. For instance, formation of CS/SBA-15 composite was accompanied by the absorption band of the -NH-groups shifting from 1532 cm−1 to 1522 cm−1. Further, after addition of Cr ions to the CS/SBA-15 composite, the absorption band is shifted to 1518 cm−1.
Figure 3 shows XRD patterns of CS, SBA-15 and chromium catalysts based on them. A broad peak at 10–30° presented in the XRD pattern of SBA-15 is characteristic of amorphous silica (Figure 3a) [47,48,49,50]. Modification of SBA-15 with Cr and CS was not accompanied by the appearance of new diffraction peaks (Figure 3c,d). No diffraction peaks associated with chromium species can be explained by low metal loading in both Cr/SBA-15 and Cr–CS/SBA-15 catalysts [50]. In the case of Cr–CS/SBA-15, the diffraction peak of chitosan is overlapped by the broad peak of SBA-15 (Figure 3a,b) [43,50].
According to BET measurements, the specific surface area of Cr–CS/SBA-15 and Fe–CS/SBA-15 catalysts was almost two times lower than that of initial SBA-15, which can indicate the filling of micropores of silica with iron and chromium particles stabilized with chitosan (Table 2).
The initial SBA-15 and the synthesized Cr- and Fe- catalysts supported on CS-modified silica were studied by SEM (Figure 4). In the initial silica, larger aggregates of SBA-15 with length of 50–80 μm and width of 10–12 μm, as well as smaller anisometric aggregates with size of 10–30 μm, were observed (Figure 4a). After modification with chitosan, no large aggregates were detected. Only small aggregates of 10–30 μm size remained visible (Figure 4b,c). This change is likely due to the addition of the polymer, which may have caused particles to adhere around the larger aggregates, rendering them no longer visible. The appearance of brighter particles in the SEM images of the catalysts can be attributed to the presence of heavier elements, such as iron and chromium [51].
The TEM images of the catalysts clearly demonstrate the well-ordered, parallel mesoporous channels characteristic of SBA-15 structures (Figure 5a,c) [52,53]. Chromium and iron particles are poorly traceable, indicating its fine dispersion in the SBA-15 porous structure. It is rather difficult to determine the size of metal particles due to insufficiently clear contrast between the metal and chitosan-modified support (Figure 5b,d).
The lines of silicon, oxygen, carbon, nitrogen and chromium were detected in the survey X-ray photoelectron spectrum (XPS) of the Cr–CS/SBA-15 catalyst (Figure 6a), confirming the presence of both chitosan and chromium on the silica surface. Similar results were observed in the case of CS-containing iron catalyst. The survey XPS spectrum of the Fe–CS/SBA15 showed the presence of silicon, oxygen, carbon, nitrogen and iron peaks (Figure 6b).
The Cr 2p3/2 region of the XPS spectrum of the Cr–CS/SBA15 (Figure 6c) is characterized by a symmetric single peak at 578.0 eV, which is attributed to Cr(III) species [54,55]. The Fe 2p3/2 region of the XPS spectrum of the Fe–CS/SBA15 (Figure 6d) could be deconvoluted into three peaks at 710.6, 712.6 and 717 eV, corresponding to Fe2O3, FeOOH [56] and Fe3+ satellite peak [57], respectively. The deconvoluted N 1s signals in the XPS spectrum of both the CS-modified catalysts (Figure 6e,f) showed that nitrogen atoms are present on their surface in two different chemical states. According to the literature [58], the peaks at ~400 and ~402 eV can be assigned to –NH2 and –NH3+ groups of chitosan, respectively. Chitosan was added to SBA15 in the form of a polyelectrolyte, containing protonated amino groups (–NH3+). The presence of deprotonated amino groups (–NH2) in the surface of the catalysts was probably due to the fact that the catalysts were washed with water after their preparation. The N 1s peaks were positively shifted to compare them with the reference values reported for –NH2 (339.4 eV) and –NH3+ (401.0 eV) groups of chitosan. These shifts can be explained by the interaction of the polysaccharide with both the metal species (Fe or Cr) and SBA-15 surface.

2.2. Oxidation of Cyclohexane

Cyclohexane (CH) is oxidized to cyclohexanol (CHOL) and cyclohexanone (CHON) via the formation of cyclohexyl hydroperoxide as an intermediate product [59,60] (Scheme 1). To confirm this mechanism, the reaction products were recognized using gas chromatography (GC) before and after the adding of triphenylphosphine (PPh3), which can react with cyclohexyl hydroperoxide (CyOOH) to form CHOL [59,60]. For all analyzed systems, the addition of PPh3 led to an increase in the yield of the alcohol in the reaction products, confirming the presence of CyOOH as a reaction intermediate. It should be noted that only CHOL and CHON were detected among the reaction products, indicating that all catalysts were highly selective toward KA-oil.
The proposed route for metal-catalyzed cyclohexane oxidation using hydrogen peroxide as an oxidant is outlined by the following main steps [6,61,62]:
Mn+ + H2O2 → HO• + M(n+1)+ + HO−
M(n+1)+ + H2O2 → HOO• + H+ + Mn+
HO• + C6H12 → C6H11• + H2O
C6H11• + HOO• → C6H12OOH
C6H11OOH→C6H11OH + 1/2O2
C6H11OOH→C6H10O + H2O
Equations (1) and (2) illustrate the formation of oxygen-centered radicals, HOO• and HO•. The hydroxyl radical (HO•), generated through the reduction of H2O2 in the presence of the reduced form of the metal catalyst (Equation (1)), reacts with cyclohexane to produce an alkyl radical R• (Equation (3)). The interaction of the alkyl radical R• with the hydroperoxyl radical HOO• leads to the formation of alkyl hydroperoxide (C6H12OOH) (Equation (4)). In the presence of a catalyst, the alkyl hydroperoxide can subsequently decompose into cyclohexanol, cyclohexanone, and oxygen (Equations (5) and (6)).
A comparison of the activity and selectivity of the CS-containing catalysts in the oxidation of cyclohexane (CH) is shown in Table 3. In the presence of chromium and copper catalysts, the reaction predominantly yielded cyclohexanol (CHOL). The selectivity toward CHOL over Cr–CS/SBA-15 and Cu–CS/SBA-15 catalysts was 77.3% and 62.4%, respectively. Cyclohexane conversion values reached 21.6% and 18.5% for Cr–CS/SBA-15 and Fe–CS/SBA-15 catalysts, respectively. The copper catalyst exhibited the lowest activity among the studied systems, with cyclohexane conversion not exceeding 9.3%.
Thus, Cr–CS/SBA-15 and Fe–CS/SBA-15 catalysts were more efficient for CH oxidation to KA-oil yields compared to a similar Cu(II)-based catalyst, which is consistent with previously reported data [60,63].
To study the effect of operational parameters on cyclohexane conversion, a series of experiments were carried out in the presence of the Cr–CS/SBA-15 catalyst. The reaction conditions such as catalyst dosage (10–50 mg), reaction duration (2–6 h), temperature (30–60 °C) and amount of H2O2 added (0.3–0.9 mL) were varied (Table 4, Table 5, Table 6 and Table 7).
An increase in reaction time from 2 to 4 h resulted in a twofold increase in cyclohexane conversion. However, further increasing the reaction time to 6 h did not lead to any major change in the conversion (Table 4). Therefore, subsequent experiments were performed at a reaction time of 4 h. The observed change in the CHOL/CHON ratio over time is due to the sequential oxidation of cyclohexanol to cyclohexanone. Initially, cyclohexanol accumulates as the primary product, but as the reaction proceeds, it undergoes further oxidation to form cyclohexanone, leading to a shift in the ratio. This behavior is consistent with the proposed reaction mechanism, where the conversion of cyclohexyl hydroperoxide to cyclohexanol occurs more rapidly than its conversion to cyclohexanone [7,64].
Performing the reaction at different temperatures (30–60 °C) showed that the conversion of cyclohexane increased with increasing the temperature to 50 °C. However, further increasing the temperature to 60 °C was accompanied with insignificant decrease in the conversion, which can be attributed to self-decomposition of H2O2 at higher temperatures [65]. In addition, increasing the temperature to 60 °C led to a significant increase in selectivity to CHON (Table 5). This is consistent with data reported in [65], according to which the temperature affects the product distribution in the oxidation process. Thus, a reaction temperature of 50 °C was found to be effective for cyclohexane oxidation under studied conditions.
The effect of the hydrogen peroxide amount was varied in the range of 0.3–0.9 mL corresponding to a concentration of the oxidant of 0.5–1.4 mol/L. Taking into account that the initial concentration of cyclohexane (CH) in the reaction mixture was 0.45–0.50 mol/L, the [CH]:[H2O2] molar ratios were approximately 1:1, 1:2 and 1:3. At the [CH]:[H2O2] molar ratio = 1:1, the conversion is minimal (3.7%), and the reaction favors the formation of cyclohexanol (CHOL) with a selectivity of 97.3%. Increase in the amount of hydrogen peroxide added resulted in an increase in the conversion and selectivity to CHON. The maximum yield of products was observed at the [CH]:[H2O2] ratio of 1:3 (Table 6).
Increasing the catalyst dosage from 0.01 to 0.03 g resulted in an increase in the conversion of cyclohexane from 12.8% to 21.6%, and, in all cases, cyclohexanol was the main product. However, further increase in the catalyst amount was accompanied with insignificant decrease in the conversion, while the selectivity to CHOL remained almost unchanged (Table 7). This was probably due to non-productive decomposition of H2O2 in excess of the catalyst [64]. Thus, the maximum KA-oil yield was achieved under the following reaction parameters: temperature—50 °C, reaction duration—4 h, catalyst dosage—0.03 g, and [CH]:[H2O2] ratio = 1:3.
Stability is a key catalytic characteristic of heterogeneous catalysts. Therefore, recycling studies were conducted using the optimal Cr–CS/SBA-15 catalyst. After each reaction cycle, the catalyst was filtered and reused. Cr–CS/SBA-15 continued its activity during the oxidation of the first two substrate portions. Upon further cycles, a slight decrease in cyclohexane conversion was observed (Figure 7). Elemental analysis of the spent catalyst indicated no chromium leaching (Table 1). The turnover number (TON) was calculated to be 147.
A comparison of the catalytic performances of Cr–CS/SBA-15 and Fe–CS/SBA-15 under the optimal reaction conditions with other known catalysts for cyclohexane oxidation is presented in Table 8. Despite the fact that the reaction was performed at a relatively lower temperature (50 °C), the conversion achieved with both the catalysts is comparable to that reported for several other heterogeneous catalysts.
Thus, the study of the catalytic performance of chitosan-modified catalysts showed the promising potential of using chitosan in the development of heterogeneous catalysts for selective oxidation of cyclohexane to KA-oil under mild conditions.

2.3. Photocatalytic H2 Production

The photocatalytic properties of the most effective Cr–CS/SBA-15 and Fe–CS/SBA-15 catalysts were tested in photocatalytic hydrogen evolution.
Before conducting photocatalytic H2 evolution experiments, the optical properties of Cr–CS/SBA-15 and Fe–CS/SBA-15 were evaluated using UV-vis diffuse reflectance spectroscopy method (Figure 8).
UV-vis absorption spectra presented in Figure 8a show that the Cr–CS/SBA-15 and Fe–CS/SBA-15 samples possess slightly different optical properties, affecting their absorption of light.
Kubelka–Munk function of the Tauc plot method was used to calculate the bandgaps of the photocatalysts:
(hυF(Rα)^(1/n) = A × (hυ − Eg)
where hυ is photon energy (h is Planck constant; υ is vibration frequency); α is absorption coefficient; Eg is direct bandgap; A is a constant; n = ½; R is reflectance; and F(R) is proportional to the absorption coefficient (α) [30].
Figure 8b displays the Kubelka–Munk function plots with (f(R)hν)2 on the y-axis versus energy (hν) on the x-axis. The bandgap was determined by extending the tangent line until it intersects the hν axis. The bandgap values were found to be 2.21 eV for Cr–CS/SBA-15 and 2.23 eV for Fe–CS/SBA-15.
UV-Vis spectroscopy data confirmed the difference in the optical properties of the catalysts: there is a slight shift in absorption spectra, which is associated with a change in the band structure due to modification by chitosan and metals. The calculation of the bandgap using the Kubelka–Munck method showed values of 2.21 eV for Cr–CS/SBA-15 and 2.23 eV for Fe–CS/SBA-15, indicating their ability to absorb visible light efficiently.
The total volume of hydrogen produced within 3 h was 23.4 mL/g for Cr–CS/SBA-15 and 103 mL/g for Fe–CS/SBA-15.
The photocatalytic activities of Cr–CS/SBA-15 or Fe–CS/SBA-15 were investigated using the colloidal dispersions of Cr–CS/SBA-15 or Fe–CS/SBA-15 particles in a 0.35 mol dm−3 Na2S/0.25 mol dm−3 Na2SO3 aqueous mixture that acted as a sacrificial electrolyte (pH = 13.0) throughout photocatalytic HER analyses in a homemade photocatalytic reactor. The dispersion was illuminated with a solar simulator with 1000 Wm−2 (1 sun) light intensity, and the evolved hydrogen gas was recorded with respect to time (Figure 9). The photocatalytic HER mechanism in the Na2S/Na2SO3 sacrificial electrolyte is proposed in Equations (8)–(12) [68,69,70]:
PC + hν → PC (e−(CB) + h+ (VB))
H2O + PC (e−(CB)) → ½ H2 + OH− + PC
SO32− + 2OH− + PC (h+ (VB)) → SO42− + H2O +PC
2S2− + 2PC (h+ (VB)) → S22− + 2PC
S22− + 2SO32− → 2S2O32−
The catalysts exhibited varying efficiencies in the photocatalytic hydrogen evolution reaction, with Fe–CS/SBA-15 demonstrating significantly higher activity (103 mL H2/g) compared to Cr–CS/SBA-15 (23.4 mL H2/g). This enhanced performance can be attributed to the superior charge transfer capacity of iron and its favorable interaction with photogenerated electron–hole pairs. These properties are supported by differences in the bandgap energy and light absorption efficiency of the materials. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of functional chitosan groups on the catalyst surfaces, which play a crucial role in stabilizing metal particles and modulating their electronic states. The interaction of Cr(III) with the amino groups of chitosan enhances metal dispersion, while the presence of Fe contributes to the formation of catalytically active sites. This, in turn, facilitates efficient charge separation and transfer, which are critical for photocatalytic activity.
Thus, the photocatalytic performance of Cr–CS/SBA-15 and Fe–CS/SBA-15 is governed by their physicochemical properties, including surface morphology, metal distribution, bandgap structure, and the chemical states of active sites. The superior activity of Fe–CS/SBA-15 is likely due to its optimized band structure and enhanced charge carrier dynamics, making it a promising candidate for photocatalytic hydrogen production.

3. Materials and Methods

3.1. Materials

Chitosan (CS, Mw 250,000), chromium (III) nitrate nonahydrate (Cr(NO3)3 9H2O, 99%), ferric (III) chloride hexahydrate (FeCl3 · 6H2O, 98%), copper chloride dihydrate (CuCl2 · 2H2O, 99%), mesostructured silica SBA-15, cyclohexane (99.8%), hydrogen peroxide (35 wt% aqueous), acetonitrile (99%), and triphenylephosphine were purchased from Sigma-Aldrich, St. Louis, MO, USA.

3.2. Preparation of Cr-, Fe-, Cu-Containing CS-Stabilized Catalysts Fixed on SBA-15

The catalysts were synthesized by successive adsorption of chitosan and then metal ions from aqueous solutions onto mesostructured silica SBA-15 as described in [43]. Chitosan (1 g) was solubilized in 100 mL of 1% solution of nitric acid at 60 °C and stirred for 4 h to make 1% chitosan solution [43]. Then, a 5 mL of this solution was introduced dropwise to the aqueous suspension of silica (1 g in 15 mL of water) at ambient temperature and constant stirring [43]. Furthermore, an aqueous solution of a metal salt (chromium (III) nitrate, ferric (III) chloride hexahydrate, or copper chloride dihydrate) was dropwise added, and the mixture was stirred for 3 h. The amount of the metal salts added was taken from the calculation to obtain catalysts with 3%wt. metal (Cr, Fe or Cu) content. The resulting mother liquor was kept for 24 h, after which the precipitate was washed with distilled water and air-dried at room temperature. For comparison, similar Cr/ZnO, Fe/ZnO and Cu/ZnO catalysts were synthesized using the same methodology except for the polysaccharide addition step.

3.3. Characterization of the Catalysts by Physicochemical Methods

IR spectra were recorded on a Nicolet iS5 from Thermo Scientific, Waltham, MA, USA (resolution of 3 cm−1 in the 4000–400 cm−1 region). Pellets IR analysis were prepared by grinding a mixture of 1 mg sample with 100 mg dry potassium bromide (KBr) and then pressing the mixture into a mold [43].
X-ray diffractograms of the samples were registered on a DRON-4-0.7 diffractometer (Bourevestnik, Saint Petersburg, Russia) using a Co-Kα radiation at a wavelength of 0.179 nm [43].
An Accusorb analyzer (Micromeritics, Norcross, GA, USA) and an energy dispersive detector (EDX Oxford Instruments, Oxford, UK) built into the microscope were used to measurement the specific surface area of the studied samples by the Brunauer–Emmett–Teller (BET) method. Preliminary degassing of the studied samples was carried out in vacuum at 60 °C for 4 h, and then the adsorption isotherm was taken. Nitrogen was used as a gas adsorbent [43].
JSM-6610 LV scanning electron microscope (JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 15–20 kV was operated for obtaining scanning electron microscopy (SEM) images. An energy dispersive detector (EDX Oxford Instruments, Oxford, UK) built into the microscope was used to perform elemental analysis [43].
Ultraviolet–visible diffuse reflectance spectra were recorded using T92+ UV-Vis spectrophotometer (PG instruments, Wibtoft, UK). The spectra were measured against BaSO4 at a wavelength range of 240–700 nm [30]. The obtained diffuse reflectance data were converted to absorbance spectra using the Kubelka–Munk function. Bandgap energies (Eg) were evaluated by Tauc plots of [F(R∞) E]1/2 versus photon energy E, where R∞ = RSample/RBaSO4 [30].
Transmission electron microscopy (TEM) micrographs were acquired on a Zeiss Libra 200FE transmission electron microscope (Carl Zeiss, Oberkochen, Germany) with an accelerating voltage of 100 kV [43].
X-ray photoelectron spectra (XPS) of the catalysts were taken on a ESCALAB 250Xi X-ray and Ultraviolet Photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) [43].

3.4. Photocatalytic Measurements

A photocatalytic hydrogen evolution reaction (PCHER) was performed in a 250 mL handmade quartz reactor. A HAL-320 Compact Xenon Light Source Solar Simulator was used as a light source, and light intensity was determined with a light meter (HD 2302.0). The light intensity was focused as one-sun (1000 Wm−2). For each reaction, 70 mg of photocatalyst was dispersed in 50 mL of an aqueous solution containing 0.35 M Na2S/0.25 M Na2SO3 mixture as sacrificial reagent by using an ultrasonic bath. Then, the mixture was transferred to the quartz reactor and the sacrificial agent was added. The generated hydrogen was tested using a gas chromatograph (Agilent 6890 GC, Santa Clara, CA, USA) with pure argon as the carrier gas [30].

3.5. Methodology of Oxidation

Cyclohexane oxidation in acetonitrile medium was performed in a thermostated glass reactor at ambient pressure with stirring. The following experiment parameters were chosen: acetonitrile (5 mL), catalyst (0.01–0.05 g), cyclohexane (0.3 mL), oxidant (0.3–0.9 mL), temperature of the reaction (30–60 °C), reaction duration (2–8 h).
The oxidation products were analyzed using gas chromatography on a Chromos GC1000 chromatograph (Chromos Engineering, Dzerzhinsk, Russia) with a flame ionization detector using a BP21 capillary column (FFAP) with a polar phase (PEG modified with nitroterephthalate) of 50 m length and 0.32 mm inner diameter [43]. Selectivity was estimated as the fraction of the target product present in the reaction products at a given degree of substrate conversion [43].

4. Conclusions

In this study, a natural polysaccharide chitosan was used to prepare Cr, Fe and Cu catalysts supported on SBA-15. It was shown that modification of SBA-15 with chitosan led to improved sorption properties of the silica towards the metal ions (Cr3+, Fe3+ and Cu2+). Comparison of the IR spectra of Cr–CS/SBA-15 with those of a protonated form of chitosan and CS/SBA-15 composite showed that the polysaccharide interacted with both the SBA-15 surface and Cr ions through its NH-groups. SEM study of SBA-15 and SBA-15-based catalysts demonstrated that modification of the silica with chitosan and metal ions (Cr3+ and Fe3+) led to changes in its morphology. This modification also resulted in a decrease in the SBA-15 specific surface area, which can be attributed to the filling of micropores of the silica with iron and chromium particles stabilized with chitosan. XPS analyses showed that the suggested adsorption sites for metal ions (Cr, Fe) are amine functional groups of chitosan supported on SBA-15. Thus, the formation of polymer–metal complexes supported on SBA-15 was confirmed by a complex of physical and chemical methods.
The obtained catalysts were tested in the oxidation of cyclohexane with H2O2 as an oxidant. The Cr–CS/SBA-15 and Fe–CS/SBA-15 showed higher catalytic activity compared to the Cu–CS/SBA-15 catalyst. The conversion of cyclohexane reached 21.6% for Cr–CS/SBA-15 and 18.5% for Fe–CS/SBA-15, which are comparable to the performance of other catalysts from the literature. The cyclohexane conversion can also be affected by reaction conditions such as time of the reaction, temperature, catalyst loading, and substrate to oxidant molar ratio. The optimal reaction parameters determined for the most active Cr–CS/SBA-15 catalyst were as follows: temperature—50 °C, reaction time—4 h, catalyst dosage—0.03 g, and [cyclohexane]:[H2O2] ratio = 1:3.
The prepared catalysts were also active in a photocatalytic hydrogen (H2) evolution reaction. The Fe–CS/SBA-15 catalyst exhibited superior activity, achieving 103 mL H2/g compared to 23.4 mL H2/g for Cr–CS/SBA-15. This enhanced performance is attributed to its optimized band structure and improved charge carrier dynamics, making Fe–CS/SBA-15 a promising candidate for photocatalytic hydrogen evolution.
Thus, this work demonstrates chitosan–silica composites can be used as promising supports for the design of heterogeneous metal catalysts. Depending on the nature of the active metal, CS-containing catalysts exhibit notable activity in both the selective oxidation of cyclohexane to KA-oil and photocatalytic hydrogen evolution.

Author Contributions

Conceptualization, A.S.A.; methodology, E.T.T., A.K.Z., Z.K.M. and A.K.; software, A.A.N., S.N.A. and A.I.J.; validation, A.I.J., S.N.A., A.T.Z. and I.K.; formal analysis, A.S.A., A.K.Z. and E.T.T.; investigation, A.I.J., S.N.A., A.A.N., A.T.Z., Z.K.M. and I.K.; resources, A.S.A., E.T.T. and Z.K.M.; data curation, A.I.J., S.N.A., A.A.N. and A.T.Z.; writing—original draft preparation, A.S.A., A.K.Z. and E.T.T.; writing—review and editing, A.S.A., A.K.Z., Z.K.M. and A.K.; visualization, A.K.Z. and A.K., I.K.; supervision, A.S.A.; project administration, A.S.A.; funding acquisition, A.S.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Committee of Science of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP19679984).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The XPS and TEM studies were performed on the equipment of the Resource Centers “Physical methods of surface investigation” and “Nanotechnology” of the Scientific Park of St. Petersburg University. The XRD, IRS, BET, and SEM studies were carried out on the equipment of the Laboratory of Physical Research Methods of D.V. Sokolsky Institute of Fuel, Catalysis and Electrochemistry.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM/EDX elemental mapping images of Cr–CS/SBA-15 (a) and Fe–CS/SBA-15 (b) catalysts.
Figure 1. SEM/EDX elemental mapping images of Cr–CS/SBA-15 (a) and Fe–CS/SBA-15 (b) catalysts.
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Figure 2. IR spectra of the SBA-15 (a), chitosan (b), chitosanium nitrate (c), CS/SBA-15 (d), and Cr–CS/SBA-15 (e).
Figure 2. IR spectra of the SBA-15 (a), chitosan (b), chitosanium nitrate (c), CS/SBA-15 (d), and Cr–CS/SBA-15 (e).
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Figure 3. Diffractograms of SBA-15 (a), chitosan (b), Cr/SBA-15 (c), and Cr–CS/SBA-15 (d).
Figure 3. Diffractograms of SBA-15 (a), chitosan (b), Cr/SBA-15 (c), and Cr–CS/SBA-15 (d).
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Catalysts 15 00650 g004
Figure 4. SEM images of SBA-15 (a), Cr–CS/SBA-15 (b), and Fe–CS/SBA-15 (c).
Figure 4. SEM images of SBA-15 (a), Cr–CS/SBA-15 (b), and Fe–CS/SBA-15 (c).
Catalysts 15 00650 g004
Catalysts 15 00650 g005
Figure 5. TEM images of the Cr–CS/SBA-15 (a,b) and Fe–CS/SBA-15 (c,d) catalysts at different magnifications.
Figure 5. TEM images of the Cr–CS/SBA-15 (a,b) and Fe–CS/SBA-15 (c,d) catalysts at different magnifications.
Catalysts 15 00650 g005
Catalysts 15 00650 g006aCatalysts 15 00650 g006b
Figure 6. Survey XPS spectra of Cr–CS/SBA-15 (a) and Fe–CS/SBA-15 (b), and high-resolution XPS spectra of Cr2p (c), Fe2p (d), and N1s (e,f) of Cr–CS/SBA-15 (c,e) and Fe–CS/SBA-15 (d,f).
Figure 6. Survey XPS spectra of Cr–CS/SBA-15 (a) and Fe–CS/SBA-15 (b), and high-resolution XPS spectra of Cr2p (c), Fe2p (d), and N1s (e,f) of Cr–CS/SBA-15 (c,e) and Fe–CS/SBA-15 (d,f).
Catalysts 15 00650 g006aCatalysts 15 00650 g006b
Catalysts 15 00650 sch001
Scheme 1. Oxidation of cyclohexane with hydrogen peroxide.
Scheme 1. Oxidation of cyclohexane with hydrogen peroxide.
Catalysts 15 00650 sch001
Catalysts 15 00650 g007
Figure 7. Consecutive oxidation of cyclohexane portions on Cr–CS/SBA-15. Experimental conditions: Vsub—0.3 mL; VH2O2 = 0.9 mL; acetonitrile (5 mL); T—40 °C; P—1 atm.
Figure 7. Consecutive oxidation of cyclohexane portions on Cr–CS/SBA-15. Experimental conditions: Vsub—0.3 mL; VH2O2 = 0.9 mL; acetonitrile (5 mL); T—40 °C; P—1 atm.
Catalysts 15 00650 g007
Catalysts 15 00650 g008
Figure 8. UV-vis absorption spectra (a) and Tauc plots (b) for Cr–CS/SBA-15 and Fe–CS/SBA-15.
Figure 8. UV-vis absorption spectra (a) and Tauc plots (b) for Cr–CS/SBA-15 and Fe–CS/SBA-15.
Catalysts 15 00650 g008
Catalysts 15 00650 g009
Figure 9. Photocatalytic H2 production efficiency of Cr–CS/SBA-15 and Fe–CS/SBA-15.
Figure 9. Photocatalytic H2 production efficiency of Cr–CS/SBA-15 and Fe–CS/SBA-15.
Catalysts 15 00650 g009
Table 1. The EDX elemental analysis of the studied samples.
Table 1. The EDX elemental analysis of the studied samples.
SampleMass., %
OSiClKCaMgAlVZnMe
Cr–CS/SBA-1553.243.30.1-0.3-- 3.1
Cr–CS/SBA-15spent55.139.1-1.10.2--0.21.23.1
Fe–CS/SBA-1552.543.31.3-0.2----2.7
Fe–CS/SBA-15spent56.539.91.30.20.3----1.8
Cu–CS/SBA-1552.643.21.8--0.10.1--2.2
Cr/SBA-1555.043.90.1------1.0
Fe/SBA-1555.143.80.2------0.9
Cu/SBA-1555.244.20.3------0.3
Table 2. Specific surface area of SBA-15, Cr–CS/SBA-15, Fe–CS/SBA-15.
Table 2. Specific surface area of SBA-15, Cr–CS/SBA-15, Fe–CS/SBA-15.
SampleSpecific Surface Area, m2/g
SBA-15418.5
Cr–CS/SBA-15282.9
Fe–CS/SBA-15263.6
Table 3. Catalytic properties of Me–CS/SBA-15 catalysts in oxidation of cyclohexane *.
Table 3. Catalytic properties of Me–CS/SBA-15 catalysts in oxidation of cyclohexane *.
MetalSelectivity, % Conversion, %
CHONCHOL
Cr22.777.321.6
Fe53.546.518.5
Cu37.662.49.3
* Experimental conditions: mcat—0.03 g; Vsub—0.3 mL; VH2O2 = 0.9 mL; solvent acetonitrile (5 mL); t—4 h; T—50 °C; P—1 atm.
Table 4. The influence of reaction time on cyclohexane oxidation *.
Table 4. The influence of reaction time on cyclohexane oxidation *.
Time, hSelectivity, % Conversion, %
CHONCHOL
229.870.210.4
422.777.321.6
631.368.719.8
* Experimental conditions: mcat—0.03 g; Vsub—0.3 mL; VH2O2 = 0.9 mL; solvent acetonitrile (5 mL); t—2–6 h; T—50 °C; P—1 atm.
Table 5. Effect of temperature on cyclohexane oxidation *.
Table 5. Effect of temperature on cyclohexane oxidation *.
Temperature, °CSelectivity, % Conversion, %
CHONCHOL
3035.164.95.7
4034.765.316.2
5022.777.321.6
6066.633.318.9
* Experimental conditions: mcat—0.03 g; Vsub—0.3 mL; VH2O2 = 0.9 mL; solvent acetonitrile (5 mL); t—4 h; T—30–60 °C; P—1 atm.
Table 6. Effect of [CH]:[H2O2] molar ratio on cyclohexane oxidation *.
Table 6. Effect of [CH]:[H2O2] molar ratio on cyclohexane oxidation *.
[CH]:[H2O2] RatioSelectivity, % Conversion, %
CHONCHOL
1:12.797.33.7
1:240.759.311.8
1:322.777.321.6
* Experimental conditions: mcat—0.03 g; Vsub—0.3–0.9 mL; VH2O2 = 0.3 mL; solvent acetonitrile (5 mL); t—4 h; T—50 °C; P—1 atm.
Table 7. Effect of catalyst dosage on cyclohexane oxidation *.
Table 7. Effect of catalyst dosage on cyclohexane oxidation *.
Catalyst Loading, gSelectivity, % Conversion, %
CHONCHOL
0.0127.872.212.8
0.0227.572.517.3
0.0322.777.321.6
0.0423.476.619.2
0.0521.178.919.4
* Experimental conditions: mcat—0.01–0.05 g; Vsub—0.3 mL; VH2O2 = 0.9 mL; solvent acetonitrile (5 mL); t—4 h; T—50 °C; P—1 atm.
Table 8. A comparison of catalytic performances of Cr–CS/SBA-15 with other known supported catalysts in cyclohexane oxidation with hydrogen peroxide.
Table 8. A comparison of catalytic performances of Cr–CS/SBA-15 with other known supported catalysts in cyclohexane oxidation with hydrogen peroxide.
CatalystT, °CSelectivity, % Conversion, %Ref.
CHONCHOL
TBA-CuPOM/MCM8073.027.020.0[12]
[Ru(VFCH)2·2H2O]+-Y8051.848.229.3[14]
40FeOx/ZSM-57097.12.93.6[66]
CrCoAPO-5(sc)9055.244.815.9[67]
Cr–CS/SBA-155022.777.321.6This work
Fe–CS/SBA-155053.546.518.5This work
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Auyezkhanova, A.S.; Zharmagambetova, A.K.; Talgatov, E.T.; Jumekeyeva, A.I.; Akhmetova, S.N.; Myltykbayeva, Z.K.; Kalkan, I.; Koca, A.; Naizabayev, A.A.; Zamanbekova, A.T. Chitosan-Modified SBA-15 as a Support for Transition Metal Catalysts in Cyclohexane Oxidation and Photocatalytic Hydrogen Evolution. Catalysts 2025, 15, 650. https://doi.org/10.3390/catal15070650

AMA Style

Auyezkhanova AS, Zharmagambetova AK, Talgatov ET, Jumekeyeva AI, Akhmetova SN, Myltykbayeva ZK, Kalkan I, Koca A, Naizabayev AA, Zamanbekova AT. Chitosan-Modified SBA-15 as a Support for Transition Metal Catalysts in Cyclohexane Oxidation and Photocatalytic Hydrogen Evolution. Catalysts. 2025; 15(7):650. https://doi.org/10.3390/catal15070650

Chicago/Turabian Style

Auyezkhanova, Assemgul S., Alima K. Zharmagambetova, Eldar T. Talgatov, Aigul I. Jumekeyeva, Sandugash N. Akhmetova, Zhannur K. Myltykbayeva, Imge Kalkan, Atıf Koca, Akzhol A. Naizabayev, and Aigul T. Zamanbekova. 2025. "Chitosan-Modified SBA-15 as a Support for Transition Metal Catalysts in Cyclohexane Oxidation and Photocatalytic Hydrogen Evolution" Catalysts 15, no. 7: 650. https://doi.org/10.3390/catal15070650

APA Style

Auyezkhanova, A. S., Zharmagambetova, A. K., Talgatov, E. T., Jumekeyeva, A. I., Akhmetova, S. N., Myltykbayeva, Z. K., Kalkan, I., Koca, A., Naizabayev, A. A., & Zamanbekova, A. T. (2025). Chitosan-Modified SBA-15 as a Support for Transition Metal Catalysts in Cyclohexane Oxidation and Photocatalytic Hydrogen Evolution. Catalysts, 15(7), 650. https://doi.org/10.3390/catal15070650

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