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Article

Study of the Oxidation of Phenol in the Presence of a Magnetic Composite Catalyst CoFe2O4/Polyvinylpyrrolidone

by
Tatyana V. Shakiyeva
1,
Binara T. Dossumova
1,
Larissa R. Sassykova
1,2,*,
Madina S. Ilmuratova
1,2,
Ulzhan N. Dzhatkambayeva
1 and
Tleutai S. Abildin
1,2
1
Center of Physical-Chemical Methods of Research and Analysis, Al-Farabi Kazakh National University, Almaty 050012, Kazakhstan
2
Faculty of Chemistry and Chemical Technology, Al-Farabi Kazakh National University, Almaty 050040, Kazakhstan
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(19), 8907; https://doi.org/10.3390/app14198907
Submission received: 30 July 2024 / Revised: 15 September 2024 / Accepted: 25 September 2024 / Published: 3 October 2024
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Abstract

:
The development of new catalytic systems based on cobalt and iron compounds for the production of oxygen-containing compounds is an urgent task of chemical technology. The purpose of this work is the synthesis of CoFe2O4 stabilized with polyvinylpyrrolidone (PVP), the study of the catalyst by physico-chemical research methods, and the determination of the effectiveness of the CoFe2O4/PVP catalyst in the phenol oxidation reaction. In this work, magnetic composites CoFe2O4 and CoFe2O4 stabilized with polyvinylpyrrolidone were synthesized by co-deposition. A comparison of the characteristics of the properties of the synthesized cobalt (II) ferrite (CoFe2O4) and the composite material CoFe2O4/PVP based on it is carried out. The obtained samples were examined using X-ray phase analysis (XRD), the Debye–Scherrer method, scanning electron microscopy (SEM), Mossbauer and IR Fourier spectroscopy, as well as thermogravimetric analysis (TGA). The textural properties were determined based on the analysis of nitrogen isotherms. The catalytic properties of the synthesized materials in the process of phenol oxidation in the presence of hydrogen peroxide are considered. The analysis of the reaction mixtures by HPLC obtained by the oxidation of phenol in the presence of a CoFe2O4/PVP catalyst showed a decrease in the concentration of phenol in the first 15 min of the process (by 55–60%), and then within 30 min, the concentration of phenol decreased to 21.83%. After 2 h of the process, the conversion of phenol was already more than 95%. The final sample after the reaction contained 28% hydroquinone and 50% benzoquinone. It was found that the synthesized magnetic composites exhibit catalytic activity in this process.

1. Introduction

Researchers are intensively studying the possibilities of synthesizing nanoscale materials, and this is due to the advantages in the structure and properties of nanoscale composites compared to conventional polycrystalline materials [1,2,3,4,5,6,7,8,9]. The valuable properties of nanomaterials have proved to be extremely in demand in adsorption and catalytic processes and in biomedicine [10,11,12,13,14,15,16,17,18,19,20,21,22,23]. Catalytic reactions in the presence of catalysts based on nanomaterials are widely used to purify water and air from various impurities [1,3,5,7,9,15,18].
Among the iron oxides with magnetic properties, ferrites stand out due to their high surface area, chemical and temperature stability over a wide pH range, and high resistance to oxidation [1,2,13,20,22,23]. Ferrites of transition metals with micro- and nanocrystalline structures are widely used to produce magnetic liquids, magnetic media, protective shields against electromagnetic and ionizing radiation, as contrast agents in magnetic resonance studies, anode materials in lithium ion batteries, as well as catalysts for obtaining valuable organic compounds [24,25,26,27,28,29,30,31,32,33,34,35,36,37].
Cobalt ferrite (CoFe2O4) is a reverse spinel-type and can be synthesized by the hydrothermal method, ball grinding, chemical reduction, sol-gel, microwave oven, and co-precipitation methods [9,22,24,25,27]. The co-deposition method is a simple, less toxic synthesis option with a high yield and ease in separating the target substance from the liquid after the process. Cobalt (II) ferrite is a promising object that has received close attention from researchers, since it has very important characteristics that can be applied for technical purposes. Being a magnetic material, it can be used as sorbents of various toxic compounds, for example, organic dyes. It is known that cobalt (II) ferrite is also used for the synthesis of catalytic composites, sorbents, and inorganic compositions [1,2,9,33,37,38,39,40,41,42,43,44,45].
It is well known that nanostructured materials tend to decrease their surface energy, and this, of course, is the cause of thermodynamic instability. Unmodified nanoparticles can be stabilized either by the sorption of molecules from the environment or by reducing the surface area using coagulation or agglomeration. Various materials can be used to stabilize metal nanoparticles, for example, water-soluble polymers [1,2,3,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64]. Small organic molecules serve as stabilizers, and organic molecules can bind to the surface of metal particles in the form of metal ligands. Stabilizer polymers promote the presence of a strong bond at various sites and can be bonded to a metal particle in different places [1,2,3,9,65,66,67,68,69,70,71,72,73,74,75,76,77,78]. The literature describes a number of studies related to the production of ferrites stabilized by polymers [79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94]. All these studies were mainly focused only on the preparation of the material and its characteristics, without describing the effectiveness of use for a particular process. In these works, different polymers (polyaniline, chitosan, polystyrene, polyacrylic acid, polyethyleneimin) are used for stabilization, as well as catalysts based on other metals (Pd, Pt, Ag, Fe), and not cobalt ferrite.
Poly(n-vinyl-2-pyrrolidone) (PVP) is one of the most well-known polymers used to stabilize nanoparticles, since it meets the necessary requirements of both ligands and steric sites [1,2,3]. For catalysts based on PVP-stabilized nanomaterials, the size of nanoparticles and the stability of the nanomaterial were analyzed under the conditions of a catalytic reaction. It was found that the use of PVP stabilization for the nanocatalysts led to a reduction in the size of Pd nanoparticles to 3 nm in the Suzuki reaction and improved the catalytic activity of the catalyst [93].
The purpose of this study is to synthesize a magnetically controlled magnetic composite, CoFe2O4 (known as cobalt ferrite), stabilized with polyvinylpyrollidone (PVP); to study the catalyst by physico-chemical research methods; and to determine the effectiveness of the CoFe2O4/PVP catalyst in the phenol oxidation reaction.

2. Materials and Methods

2.1. Materials

For the experiment, chemicals of analytical purity were used without additional purification. Polyvinylpyrrolidone (PVP (Mw = 10,000)) was purchased from AppliChem (Ottoweg 4, 64291 Darmstad, Germany). The following were used: iron chloride (FeCl3∙6H2O) and iron sulfate (FeSO4∙7H2O) of chemically pure grade, manufactured in Russia; Co(NO3)2∙6H2O of chemically pure grade, manufactured in Russia; and ammonium hydroxide (25%) of chemically pure grade, manufactured in Russia. Iron (III) chloride hexahydrate (FeСl3 6H2O, of “reagent grade”), cobalt (II) nitrate hexahydrate (Сo(NO3)2∙6H2O, of “reagent grade”), and ammonium hydroxide (25% NH4OH, of “reagent grade”) were used to obtain spinel ferrites by coprecipitation. In order to ensure intensive mixing during the synthesis of magnetic composites, a 250 mL glass mini-reactor equipped with a stirrer with a speed controller up to 1500 rpm was used.

2.2. Synthesis of CoFe2O4 and CoFe2O4/Polyvinylpyrrolidone Magnetic Composite Catalyst

Nanosized magnetic composites stabilized with PVP (Mw = 10,000) based on cobalt ferrite were obtained by chemical precipitation. Nanocrystals of the magnetic composite stabilized with PVP were obtained by the chemical coprecipitation of the corresponding salts: divalent cobalt and trivalent iron ions in an alkaline solution.
For the synthesis of CoFe2O4/PVP, an aqueous solution of iron (III) Fe(NO3)3×6H2O nitrate and cobalt (II) Co(NO3)2×6H2O nitrate and PVP were mixed for three minutes in a vortex mode in an electromagnetic homogenizer; then, in a mini reactor in an inert medium (argon) with intensive stirring (600 rpm), a 25% ammonia solution (pH-10) was added drop by drop to the resulting suspension at a temperature of 40 °C. The resulting black precipitate of the composite was washed by decanting with bidistilled water and separated using a neodymium magnet and dried at a temperature of 90–100°. A detailed description of the methodology for obtaining catalysts and conducting experiments is described in our previous articles [37,47,58].
The composition and structure of the synthesized catalysts were determined by SEM, X-ray diffraction, TGA, and IR Fourier spectroscopy. The reaction products were analyzed by high-performance liquid chromatography (HPLC).

2.3. Determination of Structural Characteristics of the Obtained Magnetic Composites and Process Products

Diffraction patterns were measured in the Bragg–Brentano geometry in the angle range of 2θ = 15–100°. All results were processed under atmospheric pressure of 101,325 Pa = 760 mm Hg and Tair = 20 °C. The Profex 5.0.2 program was used for the qualitative and quantitative analyses of the diffraction patterns. Quantitative analysis was performed in the program using the Rietveld method with the criterion of the minimum value of the root-mean-square deviation χ2. Qualitative analysis was performed based on the results of the energy-dispersive analysis of SEM. The average diameter of the synthesized composites was calculated from the width of the X-ray line using the Debye–Scherrer equation.
The IR spectra of magnetic composites were recorded on a Vertex 70v IR Fourier spectrometer (Bruker, 76275 Ettlingen, Germany) with a resolution of 4 cm−1, with a computer system for recording and processing spectra in the wavenumber scanning range of 4000–500 cm−1 using a PIKE MIRacle ATR single attenuated total reflection (ATR) attachment with a germanium crystal. The results were processed using the OPUS 7.2.139.1294 software.
The thermogravimetry method was used to assess the crystallization and sorbed water content of the samples. The thermal analysis of the samples was performed on a DTG 60H derivatograph (Shimadzu, Kyoto, Japan). TGA curves were recorded at a heating rate of 5 °C/min in air; the sample weight was 20–30 mg.
Textural properties were studied by analyzing N2 adsorption–desorption isotherms at −196.15 °C obtained on a Quantachrome NOVATOUCH LX4 adsorption analyzer equipped with long cells with a flask and an outer diameter of 9 mm. Before the analysis, the samples were degassed for 16 h at a temperature of 120 °C in accordance with IUPAC recommendations. The total pore volume (VT) and BET were calculated using the Quantachrome TouchWinTM software, version 1.22, in accordance with the procedure used in the works [34,95].
Mössbauer spectroscopy was performed using the Express Mössbauer spectrometer MS1104Em (Southern Federal University, Rostov-on-Don, Russian Federation). α-Fe calibration was performed for all samples. The source of gamma quanta of Co57. A set of Mössbauer spectra was produced at room temperature. The following designations are noted in the text: Is is an isomeric shift, mm/c; Qs is a quadrupole splitting, mm/c; H is the magnetic field on the Fe nuclei, mm/c; S is the area of the component; and G is the line width, mm/c.
The reaction of phenol with oxygen in the presence of H2O2 on PVP-stabilized CoFe2O4 nanocomposites was carried out using the technique described in our previous articles [37,58,65] in a duck-type glass thermostatic reactor with intensive stirring (400 rpm), temperature heating, in an oxidizing oxygen atmosphere. To perform the experiments, 10 mL of phenol solution with an initial substrate concentration of 10 mmol/L was loaded into the reactor. Then, 0.02 g of the nanocomposite catalyst was added. The reactor was closed and purged with oxygen 3 times. Then, a given atmosphere (oxygen) was created in the reactor. The reactor was shaken until a constant volume of the gas phase was established within the experimental error, after which the remaining components of the system were quickly pressed through a glass tap. This moment was taken as the beginning of the reaction. The liquid volume in all experiments was 10 mL, and the gas volume was 180 mL. The temperature was maintained using a U-10 thermostat. The results were processed in coordinates W = f (Q), where W is the oxygen absorption rate (mol/L min) and Q is the amount of absorbed oxygen (mol/L). During the reaction, samples of the reaction mixture were periodically taken from the reactor (at the reaction moments of 0, 15, 30, 60, 90, and 120 min). The reaction mixture was analyzed by HPLC. HPLC was calibrated for the identification and quantification of many of the expected oxidized phenol intermediates (hydroquinone, p-nitrophenol, resorcinol, p-benzoquinone, pyrocatechol, trans-muconic acid, and phenol) [92].
The concentration of phenol and its expected oxidized products (p-nitrocatechin, hydroquinone, p-nitrophenol, resorcinol, p-benzoquinone, pyrocatechin, trans-muconic acid, and phenol) was controlled by analyzing liquid aliquots taken during the reaction in the Jasco HPLC system at a wavelength of 246 nm (UV-2075 plus detector, Tokyo 193-0835, Japan). For this purpose, a biphenyl column and 0.3 mL∙min−1 (PU-2089 Plus, Tokyo 193-0835, Japan) mixture of acetonitrile (A) and acidified ultrapure water (B) H3PO4 were used.
The conversion of phenol (XPhOH) was calculated using Formula (1):
XPhOH (%) = (C0 PhOH − C PhOH)/C0 PhOH
where C0 is the initial concentration of the substrate in the solution and C is the current concentration of the substrate.
The concentration of H2O2 in the solution was determined at a wavelength of 405 nm using a T70 spectrophotometer from PG instruments Ltd. (Lutterworth, UK).

3. Results

3.1. Studying the Properties of Nanocomposites Using Physico-Chemical Research Methods

The phase state of the magnetic composite CoFe2O4/PVP was studied. Diffractograms were measured in the Bragg–Brentano geometry in the angle range of 2θ = 15–100°.
The results of X-ray diffraction analysis of the obtained composites are presented in Table 1.
According to the results of the X-ray phase analysis of unstabilized CoFe2O4, in addition to the main phase CoFe2O4 (spinel), Fe2O3 (hematite) was found (Figure 1a), and the only crystalline phase present in the CoFe2O4/PVP sample was cobalt (II) ferrite with a cubic spinel structure with the space group Fd-3m (Figure 1b), PDF Number 010-74-6403. According to the results of the X-ray phase analysis, the unit cell parameters of CoFe2O4 and CoFe2O4/PVP were 0.83880 and 0.83939 nm, respectively. The X-ray spectra of CoFe2O4 nanoparticles show clearly defined diffraction peaks at 2θ from 30.134, 35.494, 37.129, 43.383, 53.519, 57.052, and 62.651°, corresponding to (220), (311), (222), (400), (422), (511), and (440); the planes and the results are in good agreement with JCPDS card No. 22-1086. The sharp and clear peaks confirm that the CoFe2O4 nanoparticles have a pure crystalline phase with a cubic spinel structure.
The calculation of the crystal lattice parameters showed that, when cobalt is replaced with iron, the crystal lattice parameter increases, as a result of the fact that the radius of Co2+ is greater than the radius of Fe2+, and this is consistent with Vegard’s law, which states that there is a linear relationship between the crystal lattice parameter and the percentage of the element. It is calibrated with respect to Fe (KC: α-iron).
The structure and phase composition of a magnetic composite CoFe2O4/PVP were studied using Mössbauer spectroscopy (Figure 2; Table 2 and Table 3). Figure 2 shows the Mössbauer spectra of the magnetic composites CoFe2O4 and CoFe2O4/PVP.
For CoFe2O4:
Absolute area of the experimental spectrum, w/s: 1.30657. The absolute area of the calculated spectrum, w/s: 1.30745 (100.07 % of exp.). Maximum of the exp. spectrum (effect), %: 4.527 +/− 0.113 (2.50 % of max.). χ2 = 1.343.
For CoFe2O4/PVP:
Absolute area of the experimental spectrum, w/s: 2.70104. The absolute area of the calculated spectrum, w/s: 2.50534 (92.75% of the exp.). Maximum of the exp. spectrum (effect), %: 5.243 +/− 0.112 (2.13 % of max.). χ2 = 2.704.
The Mössbauer spectra of the obtained magnetic composite СoFe2O4/PVP (Figure 2b, Table 3) showed the contents of trivalent iron ions in tetrahedral and octahedral environments, and also have a doublet spectrum. The parameters of this doublet included isomeric Is shift and quadrupole splitting. The decrease in effective fields can be explained by a decrease in the particle size of cobalt ferrite stabilized by PVP (particle diameter is less than 16 nm). Surface stabilization leads to a decrease in the size of ferrite nanocrystallites in nanocomposites.
IR spectra were obtained in the range from 500 to 4000 cm−1. Figure 3 shows the IR spectra of the magnetic CoFe2O4, CoFe2O4/PVP composite, and pure PVP for comparison.
The detection of hydrocarbons by the method of the elemental analysis of composites [58] based on iron and cobalt nanoparticles stabilized by PVP shows that there is an interaction between nanoparticles and polymer, and suggests the effective formation of nanocomposites. Based on the results of the SEM analysis [65], it was shown that PVP forms composites together with metal nanocrystals. PVP can be adsorbed on iron nanoparticles due to a weak coordination bond that stabilizes it [75,88,89,90,91]. For nanocomposites, the bands in the region of 600–800 cm−1 are caused by stretching vibrations of the Fe-O bond in oxides. The absorption bands at 735, 663, 649, and 626 cm−1 are natural vibrations of composite nanoparticles embedded in the PVP matrix. PVP-specific bands were found in the polymer matrix at 1657 cm−1 (Raman amide band), 1498 cm−1, 1461 cm−1, 1423 cm−1 and 1372 cm−1 (deformation vibrations of CH2 groups in the pyrrolidone cycle), and 1287 cm−1 (bending vibrations of III–C-H amide), with small shifts compared to pure PVP.
This may indicate that PVP forms a composite together with a ferrite nanocrystal. The inclusion of CoFe2O4 nanoparticles in the polymer matrix leads to a slight displacement of some bands in nanocomposites. The characteristic peaks for PVP are consistent with the literature data [88,89,90,91]. The analytical wavenumbers for determining PVP are 1638 cm−1 (amide combination band); 1443 cm−1, 1425 cm−1, and 1374 cm−1 (deformation vibrations of CH2 groups in the pyrrolidone cycle); and 1294 cm−1 (amide III –C–H deformation vibrations) [90,91].
The N2 adsorption isotherms for CoFe2O4 and CoFe2O4/PVP are shown in Figure 4, and the obtained textural properties are shown in Table 4. As can be seen from Table 4, the most developed surface has CoFe2O4/PVP. The formation of cobalt ferrite stabilized with polyvinylpyrrolidone makes it possible to obtain a material with a significant increase in surface area (Table 4); at the same time, a slight decrease in the unit cell parameter was noted, as evidenced by calculations of the lattice parameter. A decrease in the size of crystallites may be associated with the formation of cobalt (II) ferrite clusters on the surface of polyvinylpyrrolidone, which prevents their agglomeration into larger particles.
After the process, the composite materials were removed from the reaction system using a neodymium magnet.
It is evident from Figure 4 that the magnetic composites exhibit typical type IV isotherms with H2-type hysteresis, according to the classification established by IUPAC. This type of isotherm is specified by mesoporous materials, which are shown in Figure 4 in images of samples of (a) CoFe2O4 (a) and CoFe2O4/PVP (b).
Figure 5 shows the DTA/TGA curves for CoFe2O4 (b) and CoFe2O4/PVP (c) nanocomposites in comparison with free PVP (a). The DTA/TGA results show that CoFe2O4 nanoparticles can improve the thermal stability of these nanocomposites. The measurements were carried out in a nitrogen atmosphere at a heating rate of 10 S/min. The TGA curve of pure PVP (Figure 5a) shows that, when the sample was heated from room temperature to 150 °C, 4.23% of the mass was lost, which is explained by the evaporation of the dissolved water molecules. This is confirmed by the endothermic effect characteristic of such a process on the DTA curve with a peak at 105.6 °C. The next mass loss of 10.77% with a small endoeffect at 325 °C is due to the evaporation of residual solvents and small molecules from the polymer structure. Weight loss of up to 80.12% is due to the decomposition or thermal degradation of the polymer itself. The DTA curve shows that this process begins at 373.6 °C and complete decomposition is achieved at temperatures above 500 °C, which is in good agreement with the literature data [96].
The TGA data obtained for the CoFe2O4 nanocomposites are characterized by a mass loss of 7.09% due to the removal of adsorbed physical and chemical water. At the same time, a clear endoeffect for weakly bound absorbed water at 97.2 °C and a wide diffuse peak of the gradual dehydration of crystallization water can be distinguished on the DTA curve. At the same time, the total mass loss of the CoFe2O4/PVP composite increased to 23.75%, which is explained by the evaporation of water molecules in the polymer matrix, and is also caused by the decomposition of PVP from the magnetic composition. This is confirmed by the endoeffect on the DTA curve in the range of about 52–340 °C.
Thus, the mass loss of the nanocomposites is significantly less than that of pure PVP. The thermal stability of the CoFe2O4/PVP nanocomposites is higher than that of pure PVP. This may be due to the interaction of CoFe2O4 nanoparticles with the PVP molecule [97].

3.2. Testing of Synthesized CoFe2O4/PVP Nanocomposite during Phenol Oxidation

The reactions were carried out under optimum conditions determined in the course of studies on the transformation of organic substrate in the presence of catalysts. The tests were carried out in a reactor at 80 °C and at atmospheric pressure.
Figure 6 presents the chromatograms of the starting substances: phenol (a), hydroquinone (b), and benzoquinone (c).
The proposed scheme for the oxidation of phenol with oxygen in the presence of an aqueous solution of hydrogen peroxide is shown in Figure 7.
  • The kinetic regularities of the oxidation reaction of phenol with hydrogen peroxide in the presence of CoFe2O4/PVP were studied at different pH values and concentrations of reagents (catalyst, hydrogen peroxide, and phenol).
  • The results were processed in coordinates W = f (Q) and φ = f (Q),
  • where
  • W is the rate of oxygen absorption (mol/L∙min);
  • Q is the amount of oxygen absorbed (mol/L).
  • The reaction rate was calculated using the following formula:
W = f(Ci∙t) = kI∙Cni
  • where f = (Plab∙T0)/(P0 (T0 + Texp,)),
  • where Texp is the temperature of the experiment, °C.
  • T0 is the standard temperature of 273.2 °C.
  • P0 is the standard pressure of 101.3 Pa.
  • Plab is the pressure in the laboratory, kPa.
  • Then, W = ∆V/∆T∙f/(22.4∙60∙10),
  • where 22.4 is the volume of 1 mole of gas, L/mol.
  • ∆V/∆T is the volume of absorbed oxygen (mL) per unit time (min).
  • 60 is 60 s in 1 min,
  • 10 is the volume of the starting substance, mL.
  • The amount of absorbed oxygen was calculated using the following formula:
Q = Vt − V0∙f/(22.4∙10),
  • where Vt is a volume of absorbed oxygen (mL) at a certain point in time (min).
  • V0 is a volume of absorbed oxygen (mL) at the beginning of the experiment (min).
  • The calculation of the activation energy (Ea) of the reaction at different temperatures (T1 and T2) and at the rate constants for these reactions k1 and k2, respectively, was carried out using the following formula:
Ea = 2.3∙(lg k2 − lg k1)∙8.314∙T1∙T2/T2 − T1,
  • where k1 and k2 are the rate constants at temperatures T1 and T2;
  • 8.314 is the universal gas constant, J/mol.
When varying the pH, it was found that the maximum rate of phenol oxidation is achieved at pH = 3.5, which corresponds to a low rate of hydrogen peroxide decomposition reaction. When studying the effect of hydrogen peroxide concentration on the rate of phenol oxidation (Table 5), it was found that, with an increase in the concentration of H2O2 from 362 mg/L to 1448 mg/L, the reaction rate increases. A further increase leads to a slight increase in the reaction rate, which is explained by an increase in the rate of decomposition of H2O2 as a result of side reactions. Carrying out the reaction at a concentration of H2O2 of 362 mg/L leads to the incomplete conversion of phenol (56–57%) due to an insufficient amount of hydrogen peroxide.
Figure 8a shows the conversion curves of phenol oxidation on various catalyst suspensions from 0.01 to 0.1 g. The reaction rate increases in direct proportion with an increase in the amount of catalyst.
The directly proportional dependence of the initial oxidation rate on the amount of catalyst (Figure 8b) indicates the course of the reaction in the external kinetic region. With an increase in the amount of catalyst from 0.01 to 0.1 g, the volume of absorbed oxygen increases from the calculated amount. At low catalyst concentrations (0.01 g), there is insufficient oxygen absorption (i.e., below the calculated amount). The analysis of the final sample after the reaction showed that the selective oxidation of phenol occurs, and undesirable by-products are formed at high concentrations (0.08–0.1 g). The overall selectivity and yield of the products are maximal, with a catalyst content of 0.02 g.
With the increase in the catalyst content, the proportion of hydroquinone in the reaction mass decreases, and the proportion of 1,4 benzoquinone increases. The observed dependence is associated with an increase in the number of active sites on which hydroquinone is reoxidized as it accumulates in the reaction mass.
The effect of phenol concentration on the rate and selectivity of the process was studied at a temperature of 70 °C and an oxygen pressure of 1.0 MPa. Figure 9 shows the experimental results demonstrating the effect of the initial phenol concentration on the rate of its oxidation with oxygen in the presence of H2O2 on CoFe2O4/PVP. An increase in the phenol concentration from 0.05 mol/L to 0.3 mol/L increases the initial reaction rate; the optimal phenol concentration was 0.1 mol/L, since the maximum yield of the product (benzoquinone) is achieved at this concentration. This may be due to the fact that the catalytic reaction can be inhibited by a high concentration of the substrate (phenol).
With the increase in the concentration of phenol, the oxidation rate increases and the amount of absorbed oxygen increases proportionally in accordance with the stoichiometry of reaction (2):
C6H5OH + O2 → O = C6H4 = O + H2O
The effect of temperature on the rate and selectivity of phenol oxidation was studied at 1.0 MPa in the range of 40–80 °C. The experiments were carried out with an initial reaction volume of 10 mL, a catalyst sample of 0.02 g, a phenol concentration of 0.01 mol/L, pH 3.5, and H2O2 of 120 μL. The conversion curves of phenol oxidation at different temperatures are shown in Figure 10. With the increase in the reaction temperature from 40 °C to 70 °C, the initial rate of formation of products increases, and the growth of the initial rate of formation of 1,4-benzoquinone exceeds the growth of the initial rate of formation of hydroquinone, which indicates a greater acceleration of the secondary oxidation reaction with an increase in temperature, as well as the acceleration of side condensation reactions. The maximum yield of benzoquinone, 48–52%, is observed at temperatures of 70–80 °C, which is confirmed by the HPLC data. In the studied temperature range, a linear dependence of lg k on 1/T is observed (Figure 10b). The apparent activation energy determined from this dependence and calculated by the Arrhenius equation in this temperature range is 37.8 kJ/mol. Intermediate products were detected, such as catechin, hydroquinone, and formic acid, as oxidation products.
Hydroquinone, 1,4-benzoquinone, and condensation products were found in the reaction mass after phenol oxidation. Figure 11 shows a chromatogram of phenol oxidation products after 120 min; the main product is benzoquinone (50%).
In the presence of CoFe2O4/PVP, the content of the oxidized intermediate product detected by HPLC increases sharply in the first 30 min of the reaction; then, for 2 h of the reaction, the content of the main products (hydroquinone, benzoquinone) does not change significantly at a phenol conversion of 95%. The HPLC analysis of the reaction mixtures obtained in the presence of the CoFe2O4/PVP catalyst revealed a decrease in the phenol concentration already at the beginning of 15 min (55–60%), and for 30 min, the phenol concentration decreased to 21.83%. After 2 h, the phenol conversion is already more than 95%. Totals of 28% hydroquinone and 50% benzoquinone were detected in the product.
For the control experiments, we conducted preliminary studies of the process of oxidation of phenol with oxygen on CoFe2O4 in order to assess the concentration of phenol in the sample and to perform a primary qualitative assessment of the oxidation of phenol. The content of phenol and benzoquinone was analyzed by UV and IR spectroscopy. In the IR spectra of phenols, the characteristic absorption bands of valence vibrations of the OH group lie in the frequency range of 3390–3600 cm−1, and valence vibrations of the C–O group are observed at 1230 cm−1. In the UV spectra, the absorption bands of phenol are as follows: 210 (ɛ = 6200 L/mol·cm) and 270 nm (ɛ = 1450 L/mol·cm).
Figure 12 shows the absorption spectra of the initial aqueous solution of phenol (a) and from the oxidation time ((b, 1 h) and (c, 2.0 h)).
In the UV spectrum of the initial solution of phenol, absorption bands are observed in the region of 193, 210.8, and 270 nm (Figure 12a), characteristic of phenol. Depending on the oxidation time, a shoulder in the region of 207 nm (Figure 12b) and a plateau in the region of 275 nm are similarly observed in the UV spectra.

4. Conclusions

In this study, magnetic composites CoFe2O4 and CoFe2O4 stabilized with polyvinylpyrrolidone (PVP) were synthesized by co-deposition. Rietveld’s refinement of CoFe2O4/PVP confirms that these composites have a pure spinel cubic structure without any additional phases with the space group Fd-3m.
The obtained samples were characterized by various physico-chemical methods. The X-ray image of the CoFe2O4/PVP nanocomposite shows that CoFe2O4 is a cubic spinel structure with a lattice constant of a = 0.83880 nm and has a size in the range from 10 to 15 nm. According to the results of X-ray phase analysis, the unit cell parameters of CoFe2O4 and CoFe2O4/PVP are 0.83880 and 0.83939 nm, respectively. The stabilization of CoFe2O4 using PVP can minimize the agglomeration of CoFe2O4 nanoparticles and forms a composite together with a ferrite nanocrystal. The decrease in effective fields can be explained by a decrease in the particle size of cobalt ferrite stabilized by PVP (particle diameter is less than 16 nm). Surface stabilization leads to a decrease in the size of ferrite nanocrystallites in nanocomposites. It can be noted that the magnetic composites CoFe2O4 stabilized with polyvinylpyrrolidone CoFe2O4 prepared in this work are characterized by a sufficiently developed specific surface area, the value of which was in the range of 126–156 m2/g.
Magnetic nanoparticles of CoFe2O4/PVP can improve the thermal stability of the nanocomposite due to the interaction between the CoFe2O4 nanoparticle and PVP. CoFe2O4/PVP nanocomposites exhibit ferromagnetic behavior.
Thus, a comparative study of the properties of cobalt ferrite and CoFe2O4/PVP was carried out. The effectiveness of using the synthesized magnetic composite CoFe2O4/PVP for the oxidation of phenol in the presence of hydrogen peroxide in order to obtain oxygen-containing compounds is shown.

Author Contributions

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

Funding

This research was funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan, grant No. AP14870308 “Development of technology for catalytic petrochemical synthesis of oxygen-containing compounds from aromatic hydrocarbons in the presence of nanoscale magnetic composites”, Center of Physicochemical Methods of Research and Analysis.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 14 08907 g001aApplsci 14 08907 g001b
Figure 1. Results of the X-ray diffraction of CoFe2O4 (a) and CoFe2O4/PVP (b).
Figure 1. Results of the X-ray diffraction of CoFe2O4 (a) and CoFe2O4/PVP (b).
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Figure 2. Mössbauer spectra of the magnetic composites: (a) CoFe2O4; (b) CoFe2O4/PVP.
Figure 2. Mössbauer spectra of the magnetic composites: (a) CoFe2O4; (b) CoFe2O4/PVP.
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Figure 3. IR spectra of CoFe2O4, CoFe2O4/PVP, and PVP.
Figure 3. IR spectra of CoFe2O4, CoFe2O4/PVP, and PVP.
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Figure 4. Adsorption (1) and desorption (2) isotherms of N2 for CoFe2O4 (a) and CoFe2O4/PVP (b).
Figure 4. Adsorption (1) and desorption (2) isotherms of N2 for CoFe2O4 (a) and CoFe2O4/PVP (b).
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Figure 5. TGA of PVP (a), CoFe2O4 (b), and CoFe2O4/PVP (c) nanocomposites. Here the designation [1] in red is the curve number.
Figure 5. TGA of PVP (a), CoFe2O4 (b), and CoFe2O4/PVP (c) nanocomposites. Here the designation [1] in red is the curve number.
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Figure 6. Typical chromatograms of phenol (a), hydroquinone (b), and benzoquinone (c). Red color indicates peak numbering, green triangles indicate the beginning and end of peaks (for calculating peak area).
Figure 6. Typical chromatograms of phenol (a), hydroquinone (b), and benzoquinone (c). Red color indicates peak numbering, green triangles indicate the beginning and end of peaks (for calculating peak area).
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Figure 7. The proposed mechanism of the phenol oxidation process.
Figure 7. The proposed mechanism of the phenol oxidation process.
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Figure 8. Conversion curves of phenol oxidation on different amounts of catalyst on CoFe2O4/PVP at T-353K, PO2-1.0 MPa. Dependence of the oxygen absorption rate, WO2, on the amount of absorbed oxygen, QO2 (a). Designation of curves: 1—0.01 g, 2—0.02 g, 3—0.06 g, 4—0.1 g. The directly proportional dependence of the initial oxidation rate on the amount of catalyst (b).
Figure 8. Conversion curves of phenol oxidation on different amounts of catalyst on CoFe2O4/PVP at T-353K, PO2-1.0 MPa. Dependence of the oxygen absorption rate, WO2, on the amount of absorbed oxygen, QO2 (a). Designation of curves: 1—0.01 g, 2—0.02 g, 3—0.06 g, 4—0.1 g. The directly proportional dependence of the initial oxidation rate on the amount of catalyst (b).
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Figure 9. Oxidation of phenol with oxygen in the presence of H2O2 on CoFe2O4/PVP. Dependence of the oxygen absorption rate, WO2, on the amount of absorbed oxygen, QO2 (a). Dependence of the oxygen absorption rate, WO2, on the concentration of phenol, СС6Н5ОН (b). Process conditions: T = 0.02 g CoFe2O4/PVP catalyst, 120 μL H2O2, PO2 = 1 MPa, CC2H5OH∙10, mol/L: 1—0.5; 2—1.0; 3—2.0; 4—3.0.
Figure 9. Oxidation of phenol with oxygen in the presence of H2O2 on CoFe2O4/PVP. Dependence of the oxygen absorption rate, WO2, on the amount of absorbed oxygen, QO2 (a). Dependence of the oxygen absorption rate, WO2, on the concentration of phenol, СС6Н5ОН (b). Process conditions: T = 0.02 g CoFe2O4/PVP catalyst, 120 μL H2O2, PO2 = 1 MPa, CC2H5OH∙10, mol/L: 1—0.5; 2—1.0; 3—2.0; 4—3.0.
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Figure 10. Phenol oxidation over CoFe2O4/PVP catalyst at different temperatures: (a) conversion curves of phenol oxidation as a function of temperature. Curve designations: 1—T = 40 °C; 2—T = 60 °C; 3—T = 70 °C; 4—T = 80 °C. (b) Dependence of lg k on 1/T∙103.
Figure 10. Phenol oxidation over CoFe2O4/PVP catalyst at different temperatures: (a) conversion curves of phenol oxidation as a function of temperature. Curve designations: 1—T = 40 °C; 2—T = 60 °C; 3—T = 70 °C; 4—T = 80 °C. (b) Dependence of lg k on 1/T∙103.
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Figure 11. Chromatogram of the products of the phenol oxidation reaction on CoFe2O4/PVP after 120 min of the process. Reaction conditions: 0.02 g CoFe2O4/PVP catalyst, 120 μL H2O2, Cphenol—0.01 mol/L, PO2 = 1 MPa, T = 80 °C. Red color indicates peak numbering, green triangles indicate the beginning and end of peaks (for calculating peak area). Here, the peaks of interest to us are the following compounds: phenol (8), hydroquinone (3), and benzoquinone (6).
Figure 11. Chromatogram of the products of the phenol oxidation reaction on CoFe2O4/PVP after 120 min of the process. Reaction conditions: 0.02 g CoFe2O4/PVP catalyst, 120 μL H2O2, Cphenol—0.01 mol/L, PO2 = 1 MPa, T = 80 °C. Red color indicates peak numbering, green triangles indicate the beginning and end of peaks (for calculating peak area). Here, the peaks of interest to us are the following compounds: phenol (8), hydroquinone (3), and benzoquinone (6).
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Figure 12. Absorption spectra of the initial aqueous solution of phenol and from the oxidation time: (a) initial phenol, (b) oxidation time of 1 h, (c) oxidation time of 2.0 h.
Figure 12. Absorption spectra of the initial aqueous solution of phenol and from the oxidation time: (a) initial phenol, (b) oxidation time of 1 h, (c) oxidation time of 2.0 h.
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Table 1. Results of the X-ray phase analysis of CoFe2O4 and CoFe2O4/PVP.
Table 1. Results of the X-ray phase analysis of CoFe2O4 and CoFe2O4/PVP.
CompositesPhasea, nmc, nmConcentration, %X-ray Density, g/cm3Space Group
CoFe2O4Fe2O3 (hematite)0.503671.3751436.805.266R-3 c
CoFe2O4 (spinel)0.83880-63.205.281Fd-3m
CoFe2O4/PVPCoFe2O4 (spinel)0.83939-1005.270Fd-3m
Table 2. Parameters of the Mössbauer spectrum of CoFe2O4.
Table 2. Parameters of the Mössbauer spectrum of CoFe2O4.
NoNameIs, mm/сQs, mm/sH, keVSrelative, %G, mm/s
1-CFe3+_octahedron0.3680−0.2091514.3850.880.3216
2-CFe3+_octahedron0.43690.3681510.928.120.2215
3-CFe3+_tetrahedron0.2844−0.0071482.7522.600.5139
4-CFe3+_octahedron0.3052−0.1597495.3418.410.2637
Note: Is is the isomer shift, mm/s; Qs is the quadrupole splitting, mm/c; H is the magnetic field on the Fe nuclei, mm/c; S is the area of the component; and G is the width of the line, mm/s.
Table 3. Parameters of sextets and doublets of CoFe2O4/PVP.
Table 3. Parameters of sextets and doublets of CoFe2O4/PVP.
NoNameIs, mm/сQs, mm/sH, keVSrelative, %G, mm/s
1-CFe3+_octahedron0.4299−0.0044408.9311.980.7759
2-CFe3+_octahedron0.39090.0293446.4623.320.5719
3-CFe3+_tetrahedron0.28700.0280486.8221.450.5049
4-DFe3+_octahedron0.3555−0.0396475.1538.750.6014
5-DDoublet_10.09050.5371-4.500.7759
Note: Is is the isomer shift, mm/s; Qs is the quadrupole splitting, mm/c; H is the magnetic field on the Fe nuclei, mm/c; S is the area of the component; and G is the width of the line, mm/s.
Table 4. Textural and structural parameters of the magnetic composites CoFe2O4 and CoFe2O4/PVP.
Table 4. Textural and structural parameters of the magnetic composites CoFe2O4 and CoFe2O4/PVP.
SampleParameters
Sspecific, m2/gVpore, cm3/gAverage Crystallite Size According to Scherrer, nm (311)Lattice Parameter, nm
CoFe2O4126.000.2395232.439650.83880
CoFe2O4/PVP156.000.21109715.509870.83939
Table 5. Phenol conversion in the presence of CoFe2O4/PVP.
Table 5. Phenol conversion in the presence of CoFe2O4/PVP.
NoIndicatorsResearch Results
1Amount of H2O2, µL60120240480
2Reaction duration, h.12121212
3Hydroquinone yield, %0.60.319.828.220.52118.817.0
4Benzoquinone yield, %0.30.239.249.638.734017.817.7
5Phenol conversion, %57.056.078.095.089.089.097.499.0
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Shakiyeva, T.V.; Dossumova, B.T.; Sassykova, L.R.; Ilmuratova, M.S.; Dzhatkambayeva, U.N.; Abildin, T.S. Study of the Oxidation of Phenol in the Presence of a Magnetic Composite Catalyst CoFe2O4/Polyvinylpyrrolidone. Appl. Sci. 2024, 14, 8907. https://doi.org/10.3390/app14198907

AMA Style

Shakiyeva TV, Dossumova BT, Sassykova LR, Ilmuratova MS, Dzhatkambayeva UN, Abildin TS. Study of the Oxidation of Phenol in the Presence of a Magnetic Composite Catalyst CoFe2O4/Polyvinylpyrrolidone. Applied Sciences. 2024; 14(19):8907. https://doi.org/10.3390/app14198907

Chicago/Turabian Style

Shakiyeva, Tatyana V., Binara T. Dossumova, Larissa R. Sassykova, Madina S. Ilmuratova, Ulzhan N. Dzhatkambayeva, and Tleutai S. Abildin. 2024. "Study of the Oxidation of Phenol in the Presence of a Magnetic Composite Catalyst CoFe2O4/Polyvinylpyrrolidone" Applied Sciences 14, no. 19: 8907. https://doi.org/10.3390/app14198907

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