Performance of cobalt titanate towards H₂O₂ based catalytic oxidation of lignin model compound

Abstract

Mixed metal cobalt titanium oxide (CoTiO₃) prepared by solution phase method has been evaluated for the liquid phase catalytic oxidation of vainlly alcohol to vanillin using H₂O₂ as an oxygen source. The morphology, phase composition and crystal structure of the freshly prepared and reused CoTiO₃ catalyst was studied by SEM, EDX, XRD, XPS and Raman spectroscopy. Vanillyl alcohol conversion was influenced by various experimental conditions such as reaction time, temperature, molar ratio of reactants, catalyst loading, nature of solvent and reaction medium. The design a heterobimetallic oxide catalyst which can efficiently perform the high conversion and selective oxidation of vanillyl alcohol into fine chemicals, such as vanillin and vanillic acid. It has been found that during the 5 h reaction in NaOH, the CoTiO₃ exhibits remarkable conversion of 99% and excellent selectivity of 99.8% to vanillin was achieved in acetic acid and isopropanol solvents, respectively. The oxidation reaction mechanism over the catalyst was postulated based on the observation product from the HPLC analysis. CoTiO₃ catalyst can retain its performance without significant change in the catalytic activity after four consecutive cycles.

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1. Introduction

Lignin, one of the three subcomponents of lignocellulosic biomass (along with cellulose and hemicellulose), represents more than 20% of the total mass of the Earth’s biosphere.^1,2 However, essentially due to its complex structure, this renewable polymer derived from biomass is mainly burned as a source of energy in the pulp and paper industry.^1,2 In the current context of sustainable development and green chemistry,³ lignin valorization represents an important challenge in the biorefinery area in order to diminish the reliance on oil.^4,5 Today, the valorization of lignin into the production of chemical feedstocks represents a real challenge in terms of both sustainability and environmental protection. Because of the complexity of lignin, most of the lignin oxidation studies were performed from model compounds and are very well documented in the literature.^6–10 In the past few years, some novel strategies for oxidation of lignin and lignin compounds have been investigated, such as electrochemistry,^11,12 use of mesoporous materials,⁸ photocatalysis¹³ and use of vanadium-based catalysis.¹⁴

These studies reveal that some issues need to be resolved, such as the lack of selectivity, decomposition of the catalyst, low amounts of lignin required and difficult separation of products after the reactions are completed. These problems can be overcome by designing suitable heterogeneous catalysts because they offer many advantages over their homogenous counterparts such as easy handling, separation and reusability. Several noble metal-based heterogeneous catalysts such as Au/CeO₂, Pt/C, Ru/TiO₂ etc. have been explored for this purpose and such reactions are mediated by the use of alkali.^15–17 Although noble metal catalysts exhibit high activity for oxidation reactions, they can be expensive, minimizing their potential for commercial applications.¹⁸ Oxides of other transition metals, such as titanium, chromium, manganese, cobalt and copper are also known to be effective catalysts for oxidation reactions. Among these metals, titanium and cobalt are the most attractive because of their strong capability for dioxygen activation (through metal–O₂ adduct formation) and because these are more cost effective compared to noble metals. Recently, a mixed metal oxide catalyst of manganese and cobalt oxide (CoMn₂O₄) was found to be an active species for oxidation reactions and catalyzed oxidation of vanillyl alcohol to vanillin.¹⁹

In light of this study, we were attracted and motivated towards the mixed-metal cobalt titanium oxide system as a focus of our present study. We wish to report the preparation, characterization and detailed catalytic performance of CoTiO₃ for liquid phase oxidation of vanillyl alcohol mediated with H₂O₂. The catalytic reaction parameters such as time, mass loading of catalyst, mole ratio and reusability of the CoTiO₃ catalyst have also been systematically carried out during current study. It was observed that the change of acidic to basic medium significantly enhances the selectivity and conversion of vanillyl alcohol to vanillin. Furthermore, the reusability of catalyst was tested up to four times and no significant change in the catalytic activity was observed.

2. Experimental

2.1. Materials and methods

All chemicals were purchased from various commercial sources such as: cobalt(II) acetate tetra hydrate (Merck 99%), titanium(IV) iso-propoxide (Sigma-Aldrich 98%), vanillyl alcohol (Sigma-Aldrich 98%), hydrogen peroxide (Sigma-Aldrich solution 30% in water), and acetic acid (Merck 96%). The chemicals were used without further purification.

2.1.1. Synthesis of catalyst. In a typical synthesis, 1.0 g (4.0 mmol) cobalt(II) acetate tetrahydrate was dissolved in 25 ml methanol in a 100 ml Schlenk tube fitted with an inert gas/vacuum line adapter and magnetic stirrer. 1.2 ml titanium(IV) iso-propoxide (4.0 mmol) was added drop by drop via syringe to the solution. The contents were stirred for 4 h to obtain a clear purple solution. The reaction mixture was evaporated to dryness under vacuum to produce a deep purple solid.

The as synthesized solid was ground to fine powder and calcined in air at the rate of 2 °C min⁻¹ until 500 °C and was kept at this temperature for 6 h.

2.2. Characterization

The XRD pattern of the catalysts was recorded on a D8 Advance X-ray diffraction-Bruker AXS using Cu Kα radiation (d = 1.54 Å) at a voltage of 40 kV and current of 40 mA at ambient temperature. The surface morphology and chemical composition of the catalyst were analysed by a scanning electron microscope (FEI Quanta 400) equipped with an energy dispersive X-ray spectrometer EDX (INCA Energy 200, Oxford Inst.) operated at an accelerating voltage of 20 kV and a working distance of 9 mm. FT-Raman spectra were recorded with a Renishaw inVia Raman microscope, using the 514 nm excitation source of Ar⁺¹ laser with a 0.01 mw output power. X-ray photoelectron spectroscopy was studied using an ULVAC-PHI Quantera II with a 32-channel Spherical Capacitor Energy Analyzer under vacuum (1 × 10⁻⁶ Pa) using monochromated Al Kα radiation (1486.8 eV) and natural energy width of 680 meV. The carbonaceous C 1s line (284.6 eV) was used as a reference to calibrate the binding energies.

2.3. Typical method for the oxidation of vanillyl alcohol

The oxidation reaction was carried out in a 50 ml two-necked round bottom flask fitted with a water-cooled reflux condenser. In a typical oxidation reaction, the calculated quantities of vanillyl alcohol (0.15 g, 1.0 mmol) and acetonitrile (25 ml) were added to a round bottom flask and kept on a magnetic stirrer with oil bath at 358 K. After the desired temperature was attained, the amount of catalyst was added while stirring (850 rpm), followed by H₂O₂ (0.2 ml, 2.0 mmol). About 0.5 cm³ of the reaction mixture was withdrawn periodically by means of a syringe and filtered. The progress of the reaction was monitored by analyzing the filtrate with an Agilent HPLC 1100 series liquid chromatograph equipped with an ultraviolet detector. HPLC analysis was performed on a 25 cm RP-18 column. The products and reactant were detected using a UV detector at λ_max = 270 nm. Aqueous acetonitrile (15%) with (1%) acetic acid was used as the mobile phase at a column temperature of 28 °C and a flow rate of 1.0 ml min⁻¹.

3. Results and discussion

3.1. Structural and morphological characterization

The phase composition and chemical formula of the as-synthesized Co–Ti oxide catalyst was determined from XRD and the resultant X-ray pattern is presented in Fig. 1 (blue line). A careful matching of the fresh Co–Ti oxide XRD pattern with the standard inorganic crystal structure database available in “PANalytical X’Pert HighScore Plus” software identifies the formation of CoTiO₃ as the catalyst product. The cobalt titanium oxide (CoTiO₃) crystallizes in the rhombohedral crystal system with cell parameters a = b = 5.0440, c = 13.9610 Å; α = β = 90° and γ = 120°and is identified by its peaks at 2θ = 24.0, 32.9, 35.4, 40.6, 49.2, 53.5, 60.3, 63.7, 71.4, 75.3, 84.1 and 87.9 produced by their Miller indices (012), (104), (110), (113), (024), (116), (214), (300), (119), (220), (134) and (226). All peaks in the XRD pattern show excellent matching with standard pattern (ICSD = 00-001-1040) in terms of their peak position, 2θ value and d-spacing. No possible crystalline impurities such as Co₂O₃, Co₃O₄ and TiO₂ were detected from the XRD pattern. This clearly indicates the formation of pure crystalline CoTiO₃ at 500 °C.

Fig. 1 X-ray diffractogram of freshly prepared CoTiO₃ (blue line) catalyst prepared at 500 °C. Green line represents the X-ray diffractogram of reused catalyst after performing the oxidation reaction.

After performing the catalytic oxidation reaction, the reused catalyst was again characterized by XRD (Fig. 1, green line) in order to observe any change in its phase composition and chemical identity. It is worth noticing that no significant change in the composition and phase has been detected and finger prints of reused catalyst are similar to the fresh CoTiO₃ in terms of its peak position and 2θ values. Both the XRD patterns are dominated by the peak at 2θ = 32.9° also reveals that CoTiO₃ product is well crystallized. However the crystallinity of the reused catalyst has been reduced as compared to the freshly prepared CoTiO₃.

Recent literature implies that attempts to synthesize pure CoTiO₃ generally require higher temperatures exceeding 600 °C and often produce TiO₂ and Co₂TiO₄ as impurity phases.^20–22 Furthermore, it needs prolonged annealing at higher temperature of 700 °C to enhance the crystallinity of the material. On the contrary, in the present case, a solution method is used at a relatively low temperature of 500 °C to furnish a high crystalline CoTiO₃ product free from all other titania and cobalt oxide phases.

The cobalt titanate phase that was identified from the XRD pattern was further confirmed by Raman spectroscopy as is shown in Fig. 2.

Fig. 2 Raman spectrum of CoTiO₃ catalyst.

The Raman scattering vibration modes detected at 193, 208, 237, 267, 335, 383, 480, 521 and 689 cm⁻¹ are characteristic of ilmenite CoTiO₃. The Raman spectroscopy results are in good agreement with our XRD pattern and also matched well with the previously reported Raman data for the CoTiO₃ materials.²⁰

The change in surface morphology of the as-synthesized CoTiO₃ catalyst before and after the vanillyl alcohol oxidation reaction has been studied by scanning electron microscopy and images are shown in Fig. 3. Before carrying out the catalytic reaction, the CoTiO₃ catalyst is comprised of large size crystallites (Fig. 3a) and a high resolution image reveals the rough and stony surface of the catalyst (Fig. 3b). It is generally believed that the rough surface of the catalyst can better perform the catalytic action because of its large surface area, as compared to a smooth and plane surface which has a smaller surface area. After execution of the oxidation reaction the morphology of CoTiO₃ changes to oval shaped crystallites which exhibit a smooth and plane surface (Fig. 3c and d).

Fig. 3 Surface topographic images of CoTiO₃ catalyst (a and b) before the vanillin oxidation reaction; (c and d) after the vanillin oxidation reaction.

The stoichiometric composition of the catalyst before and after the catalytic oxidation reaction was established from energy dispersive X-ray analysis and EDX spectra are shown in Fig. 4. It has been shown that the elemental composition of the catalyst before and after catalytic oxidation reaction remains unchanged and EDX spectra show the expected stoichiometric ratio indicating that the composition of the catalyst before and after oxidation reaction is in fact CoTiO₃. The molar ratio of Co/Ti obtained from different peak areas of the EDX spectra is thus close to the expected 1 : 1 ratio of CoTiO₃.

Fig. 4 EDX spectra revealing the stoichiometric composition of CoTiO₃ catalyst (a–c) before and (d–f) after oxidation reactions.

Fig. 5 indicates the EDX mapping of the CoTiO₃ catalyst before and after the catalytic oxidation reaction and reveals that Co, Ti and O atoms are homogeneously distributed throughout the catalyst’s surface.

Fig. 5 EDX mapping revealing the distribution of Co, Ti and O atoms on the surface of catalyst before and after oxidation reactions.

The XPS survey scan plot (Fig. 6a) indicates the presence of the major cobalt, titanium, and oxygen components on the surface of the catalyst. Fig. 6b–d shows the high resolution XPS spectra in the Co 2p and Ti 2p and O 1s region for the catalyst powder. Fig. 4 shows that the binding energies of Co 2p3/2 and Co 2p1/2 are 780.0–780.8 eV, and 796.2–796.4 eV, respectively. These values are comparable to the 780.7 (2p3/2) and 796.3 eV (2p) that was found in Co₃O₄.²³ The binding energies of Ti 2p3/2 (457.2–458.6 eV) and Ti 2p (463.0–464.2 eV) are agreeable with that of the TiO powder (458.5 eV for Ti 2p3/2)²⁴ and the binding energy of O 1s is between 529.2–530.2 eV. Overall the XPS spectra are in good agreement with the previously reported CoTiO₃ prepared by a modified sol–gel method.²⁵

Fig. 6 XPS spectra of CoTiO₃ powder prepared at 500 °C (a) survey scan plot. (b) Co 2p (c) Ti 2p (d) O 1s.

3.2. Liquid phase oxidation of vanillyl alcohol

To determine the optimum reaction condition of vanillyl alcohol oxidation various reaction parameters were studies, such as effect of time, catalyst amount, molar ratio of vanillyl alcohol : H₂O₂, reaction temperature, solvent type and the effect of alkaline medium on the reaction. Under the optimum reaction conditions, the oxidation of vanillyl alcohol without catalyst did not show any activity and the reusability analyses were also investigated. Moreover, the reactions also were performed under optimum reaction conditions in alkaline medium. The idea behind using base is to deprotonate the phenolic-OH to form the phenoxy anion, which is more reactive, favours coordination to CoTiO₃ and reacts with more selectivity to vanillin.²⁶ The equation for the oxidation of vanillyl alcohol is shown in Scheme 1.

Scheme 1 Liquid phase oxidation of vanillyl alcohol products in CoTiO₃ catalyst.

3.2.1. Influence of reaction time. The effect of reaction time on vanillyl alcohol conversion and selectivity to vanillic acid and vanillin were studied for CoTiO₃ and the results are shown in Fig. 7. The conversion increased slowly from 28% to 67% when the time increased from 30 min to 5 h. Further extending the reaction time to 6 h did not improve the oxidation. Based on the highest conversion, the 5 h reaction time was selected as an optimum time for this catalytic system. However, the selectivity of vanillic acid was found to be decreased from 87% to 77% within time increase, this decrease is consistent with the increased selectivity of vanillin from 10% to 19% in reaction time 30 min to 5 h.

Fig. 7 Effect of time on conversion and selectivity. Reaction conditions: amount of CoTiO₃ (100 mg); molar ratio of vanillyl alcohol : H₂O₂ (1 : 2); reaction temperature (358 K); acetonitrile as solvent (25 ml).

3.2.2. Catalytic activity with different mass of catalyst. The prepared catalyst was tested in the liquid phased oxidation of vanillyl alcohol. As illustrated in Fig. 8, as the mass of the catalyst increased the conversion of vanillyl alcohol to the corresponding product was found to be increased sharply from 24% to 67% when the catalyst mass used was increased from 25 to 100 mg. The selectivity of acid formatting was found to slightly decrease as the catalyst mass was increased. However, further increase in the catalyst mass to 125 mg resulted in a decrease in vanillyl alcohol conversion to 59%. The reason behind that could be due to the rapid decomposition of H₂O₂ over the large surface area available with a higher mass catalyst.^27,28 As a result, the selectivity of vanillin was observed slightly increased associated with decreased formation of vanillic acid. On the other hand, the absence of the catalyst leads to a maximum value of 3% vanillyl alcohol conversion to vanillic acid.

Fig. 8 Effect of catalyst loading on conversion and selectivity. Reaction conditions: reaction time (5 h); molar ratio of vanillyl alcohol : H₂O₂ (1 : 2); reaction temperature (358 K); acetonitrile as solvent (25 ml).

3.2.3. Influence of temperature. The reaction temperature was found to have a great influence on the conversion of the reaction. Thus, in the oxidation of vanillyl alcohol the effect of temperature on the reaction was studied in the range of 338–363 K (Fig. 9). The reaction was conducted at a fixed parameter (vanillyl alcohol : H₂O₂ molar ratio 1 : 2 and a catalyst amount of 100 mg in 25 ml acetonitrile). The conversion was found to increase from 51% at 338 K to 72% at 363 K. A plausible explanation for this observation could be the enhanced decomposition of H₂O₂ at high temperature.²⁷ However, there is no significant change in the selectivity of vanillic acid and vanillin at different temperature. Therefore, 358 K was preferred as suitable reaction temperature for the subsequent optimization studies, since there are moderate conversion and rapid decomposition of H₂O₂.

Fig. 9 Effect of reaction temperature on conversion and selectivity. Reaction conditions: reaction time (5 h); molar ratio of vanillyl alcohol : H₂O₂ (1 : 2); amount of CoTiO₃ (100 mg); acetonitrile as solvent (25 ml).

3.2.4. Influence of H₂O₂ mole ratio. To determine the optimum ratio of H₂O₂ needed for this oxidation reaction of vanillyl alcohol, four different ratios were studied. The results obtained from the reactions are shown graphically in Fig. 10. The conversion was ranging from 51–67% for 1 to 4 H₂O₂ molar ratio. However, the results clearly show that when H₂O₂ is taken in more than equimolar ratio (vanillyl alcohol : H₂O₂: 1 : 2, 1 : 3) the conversion significant increased (67%). Generally, the decomposed of H₂O₂ produced water as by-product,²⁹ and that might poison the catalyst by adsorbing strongly to the catalyst CoTiO₃ and blocking the active site, thus inhibiting the conversion of vanillyl alcohol to further products. Furthermore, decomposition of H₂O₂ increased the formation of vanillic acid, however, the formation of guaiacol decreased with increasing of H₂O₂ ratio, while the formed of vanillin did not significantly change. Moreover, it can be seen from Fig. 10, which indicates that an increase in H₂O₂ more than 2 equimolar ratio no significant change in the performance of the catalyst.

Fig. 10 Effect of molar ratio on conversion and selectivity. Reaction conditions: reaction time (5 h); amount of CoTiO₃ (100 mg); reaction temperature (358 K); acetonitrile as solvent (25 ml).

3.2.5. Reusability of the catalyst. In order to study the stability of the CoTiO₃ heterogeneous catalyst a regenerating test was carried out. After observing the completion of the reaction over a fresh catalyst, which was carried out as mentioned in typical reaction for the oxidation of vanillyl alcohol, the liquid contents were removed from the flask and CoTiO₃ catalyst washed several times with acetone. Then, dried under vacuum at 100 °C for 2 h. This dried catalyst was reused in a second cycle for the oxidation of vanillyl alcohol and a similar procedure was followed for four subsequent oxidation cycles, the results of which are shown in Fig. 11. It can be observed that the vanillyl alcohol conversion decreased ca. 15% and 20% after the third and fourth reuse, respectively. However, the selectivity of vanillic acid and vanillin was found to change slightly after the third recycle. To further confirm the catalyst nature of CoTiO₃ after the fifth reuse, the XRD, SEM and Raman spectra were analysed and are described in Section 3.1.

Fig. 11 Effect of re-used catalyst (reusability) on conversion and selectivity. Reaction conditions: reaction time (5 h); amount of CoTiO₃ (100 mg); molar ratio of vanillyl alcohol : H₂O₂ (1 : 2); reaction temperature (358 K); acetonitrile as solvent (25 ml).

3.2.6. Influence of solvent. Since the catalytic activity is strongly dependent on the basic character of the solvent, the influence of the nature of the solvent on the oxidation of vanillyl alcohol and vanillin selectivity was investigated over six different solvent systems with 100 mg catalyst mass and 1 : 2 vanillyl alcohol : H₂O₂ molar ratio, and the results are shown in Fig. 12. Based on the experiment, the highest conversion was achieved in acetic acid (91%), followed by ethyl alcohol (70%), acetonitrile (67%), N,N-dimethyl formaldehyde (35%), tetrahydrofuran (20%) and acetone (1%). Keeping the above fact in mind, it is understood that the increasing conversion by using acetic acid as a solvent is directly proportional to the formation of peracetic acid, which is in situ generation by reacted acetic acid with H₂O₂ in the presence of an acid catalyst.³⁰ And this in turn accelerates and acts as an oxidant source for the oxidation reaction, as shown in Scheme 2. However, the selectivity of vanillin was significant increased when acetic acid was used as a solvent compared to other solvents,^31,32 and this could be due to the possibility of formation of O₂ in higher quantities than radical hydroxide. Nonetheless, acetic acid was chosen as optimal solvent medium due to formation of peracetic acid and this led to higher conversion and increases the selectivity to vanillin. So under these reaction conditions, in the oxidation of vanillyl alcohol to vanillin, a major role is played by peracetic acid, as opposed to oxidation of vanillyl alcohol to vanillic acid where a major role is played by H₂O₂.

Fig. 12 Effect of solvent on conversion and selectivity. Reaction conditions: reaction time (5 h); amount of CoTiO₃ (100 mg); molar ratio of vanillyl alcohol : H₂O₂ (1 : 2).

Scheme 2 Role of formation peracetic acid in oxidation of vanillyl alcohol.

3.2.7. Influence of basic medium. The effect of NaOH on the conversion and selectivity of aldehyde product (vanillin) in the oxidation of vanillyl alcohol is displayed in Fig. 13A. The amount of base (NaOH) added was equivalent to the amount of H₂O₂ (2.0 mmol). The role of alkaline is to deprotonate the phenol to form the phenoxy anion, and in addition it seems that the generation of sodium peroxide occurs immediately after addition of H₂O₂ to the alkaline oxidation reaction mixture (eqn (1)). The latter, can be used as an oxidizing agent as well as oxygen source and that enhances and accelerates the reaction activity and the selectivity of aldehyde. However, according to lack solubility of NaOH in acetonitrile the acetic acid and isopropanol were chosen as a solvent to examine the effect of alkaline on the oxidation of the vanillyl alcohol reaction (see Fig. 13B). It has been established that conversion and selectivity of vanillin was significantly influenced by change of solvent. When the catalytic reactions were studied in acetic acid, a conversion of 99% and 68% selectivity of vanillin product was obtained. However, using isopropanol as solvent, slightly reduces the conversion to 91% but enhances the selectivity vanillin to 99.8%. A comparable oxidation reaction in the presen[small chi, Greek, tilde]e of isopropanol as solvent and without a base led to much lower conversion (3%) of vanillyl alcohol. As a result, this hypothesis indicates that the base plays an important part in the selective oxidation of the vanillyl alcohol process.

Fig. 13 Effect of basic medium on conversion and selectivity. Reaction conditions: amount of CoTiO₃ (100 mg); amount of NaOH (2.0 mmol); molar ratio of vanillyl alcohol : H₂O₂ (1 : 2); reaction temperature (358 K); solvent (A): acetic acid (25 ml), (B): isopropanol (25 ml).

Eqn (1): reactivity of H₂O₂ in alkaline medium to form oxygen in oxidation reaction.

3.2.8. The proposed reaction mechanism. Detection of oxidation products from the HPLC analysis suggests a free radical mechanism may be operating for the vanillyl alcohol oxidation under the studied reaction conditions. Scheme 3 illustrates the proposed mechanism for the oxidation of vanillyl alcohol in presence of H₂O₂. Initially the catalyst reacts with H₂O₂ to form a hydroperoxyl radial which is acting as a source of highly reactive oxidant and, this in turn attacks the catalyst forming an intermediate. The hydroperoxyl radial is a conjugate acid of superoxide anion (⁻O₂), which is a very important intermediate in combustion chemistry and known to play a major role in oxidation reactions.^33,34 Moreover, this chemical is more stable in the atmosphere than superoxide. Additionally, its well known that the formation of hydroperoxyl radial can be through direct photolysis of H₂O₂ as well as through the Fenton reaction.³⁵ However, as vanillyl alcohol is activated on the catalyst surface the electron-rich hydroperoxyl attacks the partially positive carbon and the hydrogen of the hydroxyl group by oxygen to produce an activated oxygen species, which helps to remove the hydroxyl from the hydroperoxyl to produce water as a by-product. Meanwhile, the vanilloxy cation releases a proton which is attacked by electron-rich oxygen of the hydroperoxyl group to form the final acid product. Another possibility is that the electron-rich oxygen of the hydroperoxyl group attacks the proton on the activated oxygen species (on catalyst) and forms the final vanilla product. It was found that when the reaction medium is more basic the formation of aldehyde product increased and that could be due to the formation of superoxide rather that hydroperoxyl radical which is going to prefer attacking the hydrogen on activated oxygen species (on catalyst) over the proton of vanilloxy carbon.

Scheme 3 The suggested mechanism for the formation of main oxidation product (vanillin and vanillic acid) from vanillyl alcohol in presence of CoTiO₃.

4. Conclusion

The CoTiO₃ catalyst was prepared by mixing equimolar Co(OAc)₂ and Ti(ⁱPro)₄ in methanol, followed by calcination at 500 °C. However, the applicability of this process demands a heterogeneous catalyst which can offer potential advantages of easy handling, separation, reusability and cost effectiveness. The as synthesized CoTiO₃ material was characterized by XRD, SEM, EDX and XPS analyses and employed for the catalytic oxidation of the vanillyl model compound with the help of H₂O₂ in acid and base medium. The presence of titanium species was responsible for their enhanced catalytic activity. In the oxidation of vanillyl alcohol, the vanillin product was obtained as the major component in basic conditions (99.8%) while the highest conversion was obtained in acid medium (99%). No products were obtained without the presence of the catalyst. In the present study, we identified that CoTiO₃ possesses these catalytic properties and can be used for practical applications. Through a simple regeneration technique, the prepared catalyst was found to be stable and could be reused several times with minor loss of catalytic activity.

Acknowledgements

The authors are grateful for the financial support from High Impact Research HIR F-000032 to complete this work.

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