Fereshteh Javidfara,
Manoochehr Fadaeian
*a and
Javad Safaei Ghomi
ab
aDepartment of Chemistry, Qom Branch, Islamic Azad University, Post Box: 37491-13191, Qom, Iran. E-mail: fadaeian_m@yahoo.com; Fax: +98 9128236206; Tel: +98 2537780045
bDepartment of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran
First published on 15th December 2022
Ultrasonic (US) irradiation (100 W, 40 kHz) reactions, used as a safe and green technique, are often more efficient than traditional protocols. This leads us to introduce, for the first time, an efficient nanocatalyst and immobilization of La2O3 nanoparticles on Co3O4 nanoparticles for the selective oxidation of benzyl alcohol to the corresponding benzaldehyde at room temperature. The structural and morphological characteristics of the nanocatalysts were determined by FT-IR, XRD, FE-SEM, EDX, and VSM. The catalytic performance of the Co3O4@Cs/La2O3 composites used as heterogeneous nanocatalysts was investigated in the selective oxidation of benzylic alcohols to their corresponding benzaldehydes. Also, the performance of the oxidation parameters, including H2O2 concentration, time, effect of various solvents, and nanocatalyst dosage was checked. Significant benefits of this method can be named by using a non-toxic solvent, easy product isolation, excellent recoverability, low time of reaction, high yield, and ultrasound irradiation. Finally, a possible mechanism was proposed to show the nanocatalytic process.
In terms of quantity used, BZD (with a smell of bitter almonds) is the second most important aromatic molecule (after vanillin). In this field, the catalytic processes that lead to excellent-quality BZD are particularly appealing for use in the perfume and food industries, pharmaceuticals, dyes, beverages, agribusiness industries, and chemicals.3–7
The most plentiful polysaccharide, other than cellulose, is chitin, which has many properties such as biodegradability and biocompatibility.8 Chitosan (CS) is the basic derivative of chitin. Recently, CS has attracted the attention of chemists due to its abundant properties such as inexpensiveness, it is an abundant resource, hydrophilicity, environmentally friendliness, nontoxicity, excellent chemical and thermal stability, and its antifungal, antibacterial, and antioxidant properties. Also, the NH2 and OH groups in the CS structure provide diverse coordination sites for chemical modifications such as chelating ligands to coordinate different metal ions to synthesize magnetic composites,7–12 (Scheme 1).
Recently, cobalt oxide (Co3O4) nanoparticles (NPs) have become an important, magnetic, and well-known p-type semiconductor that has increasingly attracted attention because it is inexpensive and ecofriendly, is simple to prepare and has high chemical and physical stability. The Co3O4 has been studied because it is a promising substance with a great number of applications in lithium-ion batteries, gas sensing, information storage, heterogeneous catalysis, optical devices, magnetic materials, and electrochromic devices.13–15 Co3O4 with valence of more than 3 is unstable in the normal environment. However, other cobalt oxide forms such as Co3O4 and CoO, are more stable and have more advantages in industrial applications.16
Lanthanum oxide (La2O3) one of the rare earth oxides, has been investigated as a catalyst in different reactions because of its unique properties (excellent paramagnetic sensitivity, saturated magnetism, constriction magnetic properties, large bandgap, and so on). It is also a good candidate for studies because of its significant applications in electronics, sensors, insulators, antimicrobial agents, biomedicine, and biocatalysts. Therefore, La2O3 is an excellent choice for research on improving its catalytic activity.1,2,6,7,17–19
In addition, the ultrasound (US) irradiation method is an ecofriendly, safe, and cheap technology which is used as a versatile tool in a vast range of scientific and technological applications.20–22 In contrast to normal thermal heating, US irradiation has many important benefits: improved efficiency, a significant reduction in reaction time, increased selectivity, it is inexpensive and easy to handle and use in various procedures, and the compounds show excellent purity. Additionally, in many cases, reactions under US irradiation could be considered as being ecofriendly procedures and use a small number of solvents.23,24 Therefore, US irradiation causes the least number of side reactions.25
In this regard, a fast and green method for the synthesis of Co3O4@Cs/La2O3 nanocomposites for use as novel heterogeneous nanocatalysts is proposed and its efficiency in the oxidation of benzyl alcohol to BZD is also explored. We developed the oxidation of benzyl alcohol to BZD at room temperature and under US conditions. As understood from this proposed procedure, it greatly complements the goals of green chemistry, and it can be used effectively in the production of BZD, with the use of the Co3O4@Cs/La2O3 nanocomposites as a nanocatalyst. It also shows massive advantages for both use in industry and academia.
In the energy-dispersive X-ray spectroscopy (EDX) spectrum shown in Fig. 2, the presence of the elements: C, Co, La, N, and O, was determined according to their energy, which indicated the confirmation of the product purity.
Data obtained from the X-ray diffraction (XRD) analysis, which was used in analysis of the Co3O4@Cs/La2O3 catalyst synthesized in this project, is shown in Fig. 3. The XRD samples for Co3O4, CS, La2O3 NPs, and the Co3O4@Cs/La2O3 nanocomposite are shown in Fig. 3a–d. The developed peaks indicate the dimensions of the nano sized particles. The characteristic peaks for Co3O4 are found in the region at 2θ of 31.3°, 36.89°, 44.83°, 55.61°, 59.43°, and 65.42°, which corresponded to (220), (311), (400), (422), (511), and (440), respectively. The network parameters agreed well with those of the JCPDS card number 01-078-1969 (standard cubic Co3O4).32 The broad diffraction peaks of CS show 2θ = 10.60 and 19.9° and it was a good match with the results given in past reports.6,33 Also, the La2O3 NPs had peaks that are shown at 27.2912°, 28.3643°, 39.6213°, and 46.4585° and are related to La2O3, which correspond to (222), (300), (400), and (622), respectively, (La2O3, JCPDS card no. 04-0856).34 Characteristic peaks for Co3O4 and La2O3 indicated that the structure of them both had not changed during the combination process.
Scanning electron microscopy (SEM) is a powerful technique for investigating surface morphology. The morphology, size, and form of the catalyst were surveyed by SEM at different magnifications. The result was that the FE-SEM images showed a magnetic Co3O4@Cs/La2O3 nanocomposite consisting of nearly spherical particles with a size of about 32–35 nm (Fig. 4).
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Fig. 4 The FE-SEM images, at different magnifications, of the Co3O4@Cs/La2O3 catalyst, (a) 1 μm, (b) 200 μm, and (c) 2000 nm. |
The magnetic behaviors of Co3O4 and Co3O4@Cs/La2O3 were determined using a vibrating sample magnetometer (VSM) at room temperature (Fig. 5). There was no hysteresis, coercivity, or remanence in bare Co3O4 and Co3O4@Cs/La2O3 samples at room temperature, which showed their typical superparamagnetic behavior. The Co3O4 NPs and Co3O4@Cs/La2O3 had superparamagnetic behavior, which facilitated the magnetic separation. The saturation magnetization values (Ms) of the Co3O4NPs and Co3O4@Cs/La2O3 nanocomposites were 46.5 and 25.8 emu g−1, respectively. However, the addition of the CS layer and the La2O3 NPs on the Co3O4 surface results decreased the magnetic behavior, and the Co3O4@Cs/La2O3 nanocomposite saturation magnetization was sufficient for a rapid magnetic separation by an external magnet.7,35
Entry | CatalystRef | Catalyst (mg) | Time (min) | Temperature (°C) | Oxidant | Solvent | Yieldb (%) |
---|---|---|---|---|---|---|---|
a Reaction conditions: benzyl alcohols (1 mmol), H2O2 (1 ml), Co3O4@CS/La2O3 (0.05 g).b Isolated yield. | |||||||
1 | Co3O4@Cs/La2O3 | 50 | 15 | r.t./US | H2O2 | m-Xylene | 58 |
2 | Co3O4@Cs/La2O3 | 50 | 15 | r.t./US | H2O2 | Toluene | 63 |
3 | Co3O4@Cs/La2O3 | 50 | 15 | r.t./US | H2O2 | CH3CN | 80 |
4 | Co3O4@Cs/La2O3 | 50 | 15 | r.t./US | H2O2 | Ethanol | 85 |
5 | Co3O4@Cs/La2O3 | 50 | 8 | r.t./US | H2O2 | Free solvent | 97 |
6 | Co3O4@Cs | 50 | 8 | r.t./US | H2O2 | Free solvent | 84 |
7 | Co3O4 | 50 | 8 | r.t./US | H2O2 | Free solvent | 73 |
8 | Null | — | 15 | r.t./US | H2O2 | Free solvent | 50 |
9 | Co3O4@Cs/La2O3 | 75 | 10 | r.t./US | H2O2 | Free solvent | 86 |
10 | Co3O4@Cs/La2O3 | 50 | 180 | 130 | H2O2 | m-Xylene | 87 |
11 | Co3O4@Cs/La2O3 | 50 | 160 | 130 | H2O2 | Ethanol | 88 |
12 | Co3O4@Cs/La2O3 | 50 | 300 | 130 | H2O2 | CH3CN | 87 |
13 | Null | — | 240 | 130 | H2O2 | m-Xylene | 91 |
14 | ZPCu37 | 0.005 | 60 | 90 | H2O2 | Free solvent | 90 |
15 | Au-Pd/C39 | 2 | 240 | 80 | H2O2 | Free solvent | 11.32 |
16 | ZnBr2 (ref. 40) | 0.02 | 90 | Reflux | Chloramine-T | CH3CN | 96 |
17 | Au/Al2O3 (ref. 41) | 48 | 15 | 100 | O2 | Toluene | 86 |
18 | (TEAH)H2PW12O40 (ref. 42) | 0.04 | 180 | 100 | H2O2 | H2O | 99.6 |
19 | FSPC43 | 50 | 15 | r.t. | H2O2 | CH3CN | 70 |
20 | WO4@PMO-IL44 | 0.0015 | 720 | 90 | H2O2 | CH3CN![]() ![]() |
75 |
Table 2 summarizes the results from the Co3O4@Cs/La2O3 nanocatalyst oxidation reactions. A wide range of aromatic alcohols has been change to benzaldehyde derivatives with great yields within a short reaction time at room temperature (Table 2). Both electron-donating and electron-withdrawing groups accelerated the oxidation reaction, but the presence of any steric hindrance around the hydroxyl group of the benzyl alcohol substituent at the ortho or meta position caused decreased yields and a longer reaction time (Table 2, entries 1, 4, 6, 8, 9 and 11). However, when compared with the para position, because of the absence of steric hindrance, there was an increase in the yield and a decrease in time.6,36,37
Entry | X | Time (min) | Yieldb (%) | Selectivity (%) |
---|---|---|---|---|
a Reaction conditions: benzyl alcohols (1 mmol), H2O2 (1 ml), Co3O4@CS/La2O3 (0.05 g).b Isolated yield. | ||||
1 | 3-Hydroxy | 10 | 87 | >99 |
2 | 4-Hydroxy | 8 | 97 | >99 |
3 | 4-Chloro | 8 | 97 | >99 |
4 | 2-Chloro | 10 | 83 | >99 |
5 | 4-Methoxy | 8 | 96 | >99 |
6 | 3-Methoxy | 10 | 90 | >99 |
7 | 4-Methyl | 8 | 97 | >99 |
8 | 3-Methyl | 10 | 92 | >99 |
9 | 3-iPr | 12 | 87 | >99 |
10 | 4-Nitro | 8 | 96 | >99 |
11 | 3-Nitro | 10 | 87 | >99 |
12 | 4-Fluoro | 8 | 95 | >99 |
13 | 4-Bromo | 8 | 96 | >99 |
14 | 3-Bromo | 10 | 83 | >99 |
15 | Benzyl alcohol | 8 | 97 | >99 |
According to all this information and the investigation, an acceptable mechanism for this oxidation was proposed. The results demonstrated that La3+ (on the surface of Co3O4@Cs/La2O3) acts as a Lewis acid site in the oxidation of benzyl alcohols. The La3+ was coordinated to the oxygen of the peroxide hydrogen and formed an La–OOH intermediate A as the first step. In the second step, intermediate A reacted with benzyl alcohols to form intermediate B, and the deletion of an H2O molecule from intermediate B, resulted in intermediate C, and at the end, it resulted in the elimination of the secondary H2O molecule and created the corresponding BZD (Scheme 2).6,36–38
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Fig. 7 Comparison of the XRD spectra of the prepared Co3O4@Cs/La2O3 (a) before, and (b) after five runs. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ra05154k |
This journal is © The Royal Society of Chemistry 2022 |