Fabrication of Fe3O4@PVA-Cu Nanocomposite and Its Application for Facile and Selective Oxidation of Alcohols

Fe3O4@PVA-Cu nanocomposite was introduced as an affordable catalyst for selective oxidation of alcohols into various aldehydes and ketones. The synthesized nanocomposite was characterized by applying essential analyses. The peaks that are appeared in FT-IR spectroscopy confirmed the production of the Fe3O4@PVA-Cu nanocomposite. In addition, EDX analysis proved the presence of oxygen, carbon, iron, and copper elements in the catalyst. Further, TGA analysis showed high thermal stability of the nanocomposite. VSM technique was applied to examine the magnetic property of the nanocomposite. The results demonstrated a high magnetic property in the catalyst, which enables easy separation of it from the reaction solution. TEM and SEM imaging showed the nanoscale size of the particles (~20 nm) in the catalyst. Additionally, XRD data was compatible with that of Fe3O4 nanoparticles. The application of the nanocomposite has been studied in the selective oxidation of alcohols in the presence of acetonitrile as solvent, and hydrogen peroxide as a supplementary oxidizing agent. This technique is remarkably facile and inexpensive. Further, the products showed high yields. In addition, the calculated TON and TOF values indicated the phenomenal efficiency of the nanocomposite in preparation of targeted products.


INTRODUCTION
In recent decades, introducing nanoscale materials has led to a remarkable change almost in all areas of science. Accordingly, specific and unique features of nanomaterials in the chemical reactions have attracted the attention of scientists in the field of chemistry. Nanomaterials are a class of compounds with at least one dimension in size ranging from 1 to 100 nm (Kreyling et al., 2010). Unlike bulk materials, nanoscale materials show distinct traits just because of their small size (Valenti et al., 2016). In general, nanoparticles can be categorized into two subclasses: Simple nanoparticles and nanocomposites or core/shells. The latter plays an important role in nanoscience. Core/shell nanoparticles are composed of a core that is placed in the inner part of shell material (Chaudhuri and Paria, 2012).
Some nanoparticles, like iron, nickel, and cobalt and their oxides such as Fe 3 O 4 (Maleki et al., 2016), NiO (Tadic et al., 2015), and Co 3 O 4 (Moro et al., 2013), have wide application in chemical, material, medicinal, and electronic sciences (Alivisatos, 2004;Kopelman et al., 2005 Graphical Abstract | Synthesis of Fe 3 O 4 @PVA-Cu NC and it's application for selective oxidation. Maleki et al., 2019). But they are extremely unstable, as their large surface area makes them highly active (Hutten et al., 2004). However, nanoparticles covered with a protection material show higher stability. Also, the protective shell can improve and develop the application of nanomaterials (Salama et al., 2018;Cheng et al., 2019). There are different types of materials that can be used as a shell to improve the stability of nanoparticles (including inorganic metals and polymers) (Zhu and Diao, 2011). Among various types of nanocomposites, researchers choose the shell material most suitable for their target reaction process.
Polyvinyl alcohol (PVA) ([CH 2 CHOH] n ) is one of the most useful polymers that can be used in preparation of nanocomposites. PVA is produced from based-catalyzed hydrolysis of acetate groups in polyvinyl acetate (Rahimi et al., 2020b). It has special characteristics that enable it to act as a good substrate in the synthesis of nanocomposites. These characteristics include high solubility in water, resistance to chemicals, environmentally friendly, cheap, full of hydroxyl groups, etc. (Song et al., 2018). Despite of such precious characteristics, the rich hydroxyl groups make it unstable in aqueous solutions. One of the best ways to prevail over this limitation is to protect the -OH groups with a cover material such as hydrogen sulfate (Maleki et al., 2019b), clay (Kokabia et al., 2007), zinc acetate (Yang et al., 2004), TiO 2 (Yang, 2007), etc. Copper is an element that can be used as a useful protecting cover for improving and modifying PVA attributes (Khan et al., 2019). Copper element has singular properties, including propounded electrical conductivity and antibacterial activities (Lu et al., 2004;Kong et al., 2019). The copper chelated PVA can easily overcome the instability of polyvinyl alcohol by functionalizing hydroxyl groups (Hojo et al., 1978;Maleki et al., 2019a).
Selective oxidation of alcohols, which leads to the formation of aldehydes and ketones, considered among one of the most important processes in industry and scientific research (Su et al., 2010;Pereira et al., 2019;Rahimi et al., 2020a). Aldehydes and ketones are among the most important precursors for the synthesis of various compounds with precious properties (Fey et al., 2001). In past decades, numerous synthetic routes have been reported for the oxidation of alcohols. Most utilize hazardous and expensive oxidants, while others have strict conditions such as high pressure and temperature (Ghalavand et al., 2019;Lopes et al., 2019). Therefore, it is necessary to introduce a green and low-cost strategy based on an  (Jiang et al., 2016), and air atmosphere (Noyori et al., 2003). This is to produce aldehydes and ketones through the oxidation reaction of alcohols (Ghalavand et al., 2019). Additionally, an appropriate catalyst in the oxidation reaction can significantly reduce the need for using detrimental oxidants (Dijksman et al., 2001;Rautiainen et al., 2014). During the past years, different strategies have been developed for the synthesis of aldehydes and ketones through the oxidation of alcohols with metal catalysts, especially Pd (Verma et al., 2017), Ru (Ray et al., 2016;Sarmah et al., 2018), Ag (Mir et al., 2019), Au (Zhan et al., 2018), and copper-based (Sasano et al., 2018) catalysts, which are limited in scope, have long reaction time, and in most cases low conversion rate (Table 1).
Herein, based on unique properties of nanocomposites and our previous research on the synthesis of nanomagnetic composites and their catalytic application in chemical reactions, we introduced the synthesis of the heterogeneous Fe 3 O 4 @PVA-Cu nanocomposite (Maleki and Rahimi, 2018;Maleki et al., 2019c). Moreover, we investigated the application of this nanocomposite for the selective oxidation of alcohols into aldehydes and ketones (Scheme 1).

Delineation of Fe 3 O 4 @PVA-Cu Nanoparticles
Fourier-Transform Infrared Spectroscopy (FT-IR) Figure 1 illustrates the FT-IR spectra of the synthesized catalyst, Fe 3 O 4 , PVA, and Fe 3 O 4 @PVA. The FT-IR spectrum of the recovered Fe 3 O 4 @PVA-Cu is also shown. According to Figure 1b, the observed intense peaks at wavelengths of ∼3,436 and ∼2,931 are related to OH groups and vibrational mode of C-H bonds of PVA, respectively. The FT-IR data indicated that, the intensity of hydroxyl groups signal at the wavelength of ∼3,430, which are corresponded to PVA, has been decreased (Figure 1d). This is due to the chelation of copper with oxygen atoms of PVA in the catalyst. The signal at ∼588 cm −1 in Figure 1d demonstrates the formation of the covalent bond between Fe and O. The signals at SCHEME 1 | Synthesis of the Fe 3 O 4 @PVA-Cu nanocomposite and its application in selective oxidation of alcohols. ∼1,433 and ∼2,900 cm −1 are related to vibrations of C-H bonds associated to PVA. As shown in Figure 1e, all signals of the recycled catalyst are compatible with the primary Fe 3 O 4 @PVA-Cu nanoparticles. In both spectra (Figures 1d,e), the signal corresponding to the Cu-O bond was occurred at ∼600 cm −1 (Hajipour et al., 2015).

Energy-Dispersive X-Ray Spectroscopy (EDX)
For identifying the related elements in the catalyst, we employed the EDX spectroscopy technique. EDX analyses of Fe 3 O 4 @PVA (a), Fe 3 O 4 @PVA-Cu (b), and also EDX spectrum of recovered Fe 3 O 4 @PVA-Cu (c) were provided. As shown in Figure 2A, presence of carbon, oxygen, and iron elements are, respectively, confirmed by the recorded peaks at 0.29, 0.53, 6.44, and 7.1 KeV. Moreover, the signals that are located at 0.8, 8.1, and 9.0 KeV in Figures 2B,C, demonstrate the presence of copper in the nanocomposite. Therefore, the copper remained unchanged after recyclization of the catalyst. In addition, quantitative measurements indicated the chelation of copper elements with oxygen atoms of PVA ( Table 2). The results showed that, 1.53 percent of the final product is associated with the chelated copper elements.  Thermogravimetric Analysis (TGA) We conducted the TGA analysis to evaluate the weight loss of the product in different thermal ranges. Figure 3A shows TGA analysis of Fe 3 O 4 @PVA-Cu in temperature ranged from 20 to 800 • C in the surrounding atmosphere. TGA analyses of Fe 3 O 4 and Fe 3 O 4 @PVA were provided for comparison in Figure 1b in the same conditions. The increase of weight observed at the beginning of the graph is due to adsorption of surrounding moisture. It was depleted by increasing the temperature ranged between 75 and 200 • C. The approximate loss of 5% in weight was observed from 210 to 300 • C. This corresponds to dehydration of H 2 O groups that were trapped in the structure of the product. As indicated in Figure 3A, ∼8% loss of weight occurred at the temperatures ranged between ∼300 to ∼700 • C. Separation of copper and removal of hydroxyl groups in the form of water molecules from PVA is propounded in this thermal range. Eventually, the product was fully broke down at the elevated temperature, which is estimated at ∼700 • C. The results demonstrate the higher thermal stability of the nanocomposite compared with unfunctionalized PVA (Abdul Kareem and Anu kaliani, 2011).

Vibrating Sample Magnetometer (VSM)
We used the VSM analysis to assess the magnetic property of the nanocomposite. The graph illustrates that, Fe 3 O 4 @PVA and Fe 3 O 4 @PVA-Cu nanoparticles show magnetization of ∼29 and ∼39 emu/g, respectively (Figure 4). Comparison of the curves in Figure 4, shows ∼10 emu/g increase in the magnetic property of the Fe 3 O 4 @PVA-Cu nanoparticles. This is due to chelation of Cu 2+ with oxygen atoms of PVA. The high magnetic property of Fe 3 O 4 @PVA-Cu nanocomposite causes the catalyst to be smoothly isolated from the reaction solution by using an external magnet field.

Electron Microscopy Techniques (SEM and TEM)
We used microscopy-imaging techniques for identification of morphology and structure of the catalyst. Figure 5 shows SEM (Figure 5a) and TEM (Figure 5b) images of the final product. In addition, size distribution of nanoparticles is provided in Figure 5c. The spherical morphology of the Fe 3 O 4 @PVA-Cu nanoparticles are clearly shown with an average diameter of ∼20 nm in SEM image. The results in Figure 5c demonstrate that most of the particles have size ranged from 20 to 25 nm. Moreover, the TEM analysis confirmed the particles size. As illustrated in TEM image of the catalyst (Figure 5b), dark areas on the gray matrix clearly indicate the dispersion of nanoparticles on PVA.

X-Ray Crystallography (XRD)
XRD analysis was conducted to identify the crystalline structure of the catalyst. Figure 6 indicates XRD patterns of copper Fe 3 O 4 @PVA catalyst (a) and Fe 3 O 4 nanoparticles (b). The signal around 20 • confirms the presence of PVA in the product (Li et al., 2015). Furthermore, comparison of the appeared signals at 18.5 • , 30.5 • , 35 • , 38.5 • , 43 • , 44.5 • , 58 • , 63 • , and 78 • (see Figure 6A) with the standard data of Fe 3 O 4 (JCPDS: 19-0629) demonstrates the production of the nanocomposite (Todaka et al., 2003). Moreover, the Debye-Scherrer equation determined the mean size of ∼20 nm for particles, which is compatible with measurements in the electron microscopy analysis.

Optimization of Reaction Variables for the Selective Oxidation of Alcohols Using Fe 3 O 4 @PVA-Cu Nanoparticles
We used the reaction of benzyl alcohol (0.5 mmol) in order to optimize the different variables in the selective oxidation reaction of alcohols in the presence of copper-functionalized Fe 3 O 4 @PVA. This reaction was implemented in the presence of several solvents with the different amount of the catalyst. In addition, we tested the oxidation effect of H 2 O 2 on the model reaction. As presented in Table 3 (entries 1-9), the best performance occurred using acetonitrile as solvent in the trial of different solvents for the reaction ( Table 3, entry 5). To obtain the best amount of Fe 3 O 4 @PVA-Cu for the reaction, we applied a diverse amount of the catalyst to the sample reaction. Further, the reaction was performed in the absence of the catalyst. As shown in Table 3 (entries 10-15), the best result was obtained using 20 mg of Fe 3 O 4 @PVA-Cu for the reaction.
To evaluate the optimal amount and oxidation effect of H 2 O 2 in the model reaction, we implemented the reaction in the different amounts of hydrogen peroxide. As mentioned in   Frontiers in Chemistry | www.frontiersin.org 6 July 2020 | Volume 8 | Article 615 aldehydes and ketones was studied ( Table 3, entries 20-22). The results have shown the ineffectiveness of these compounds to catalyze the selective oxidation reaction of alcohols.

Extending Catalytic Application of Fe 3 O 4 @PVA-Cu Nanoparticles to Selective Oxidation of Alcohols
In the interest of exerting catalytic application of Fe 3 O 4 @PVA-Cu in the selective oxidation of alcohols, we extended the optimized reaction conditions to various alcohols ( Table 4). In this research, the oxidation reaction of alcohols was carried out in acetonitrile, as a solvent, in the presence of Fe 3 O 4 @PVA-Cu, as catalyst and oxidant. Additionally, hydrogen peroxide was used as the auxiliary oxidizing agent. The reaction solution was stirred under reflux condition until completion of the reaction. High values of calculated TONs and TOFs (see Table 4) show excellent efficiency of the catalyst for selective conversion of alcohols into desired products. According to Table 4 (entries 4-8, 12), maximum yields of products are shown by the alcohols with electron donation groups on ortho or meta position of the phenyl group. As electron donating groups in ortho and meta positions increase the negative charge on benzylic carbon, the removal of its hydrogen from intermediate complex would be easier.

Evaluation of Catalyst Reusability
Recyclability of the catalyst is a significant factor for the green synthesis of organic compounds. This feature also has a great impact on reducing the costs of the reaction. For this purpose, we tested the reusability of the Fe 3 O 4 @PVA-Cu nanoparticles for the selective oxidation reaction of alcohols. Figure 7 shows infinitesimal loss of catalytic activity after reusing the catalyst for five times in the oxidation reaction of benzyl alcohol as the model reaction under optimized conditions. In each trial, we separated the nanomagnetic catalyst by using an external magnetic field. Then, it was washed with acetone and deionized water for several times. The results demonstrated a high stability of Fe 3 O 4 @PVA-Cu magnetic nanocomposites in the reaction.

Suggested Mechanism for Selective Oxidation of Alcohols in the Presence of Fe 3 O 4 @PVA-Cu Nanocomposite
Scheme 2 shows a plausible mechanism for selective oxidation of alcohols in the presence of Fe 3 O 4 @PVA-Cu nanocomposite and hydrogen peroxide. The results implied that Cu(II) of nanocomposite acts as the active site in the selective oxidation reaction of alcohols in the presence of H 2 O 2 . In the first step, the chelated copper coordinated to the oxygen of hydrogen peroxide and creates intermediate complex A (Hojo et al., 1978). Then, the complex A reacted with corresponding alcohols to give the intermediate B. Further, the elimination of a water molecule from the complex resulted the intermediate complex C. In the next step, the removal of another water molecule provided the desired products (2a-2n), and regeneration of the catalyst completed the cycle (Narisawa et al., 1989;Yu et al., 2010).

Synthetic Routes
In-situ Synthesis of Fe 3 O 4 @PVA Nanoparticles In the first step, PVA 72000 MW (2.0 g) was added to a 250 mL round flask containing distilled water (40 mL) and was mixed on a magnetic stirrer at 80 • C under the protection of inert gas (N 2 ). After the creation of a homogeneous and transparent mixture, ammonia (12 mL) was added to the mixture dropwise. We continued adding ammonia to the mixture until pH = 12 was acquired. Then, the prepared aqueous solutions of FeCl 3 .6H 2 O (2.5 g in 10 mL deionized water) and FeCl 2 .4H 2 O (1.0 g in 10 mL deionized water) were added to the mixture gradually. When the addition was finished, we left the flask to continue stirring for 2 consecutive hours at ambient temperature. After completion of the reaction, we separated the produced black magnetic nanoparticles by using an external magnetic field. Then we washed the nanoparticles with excessive amount of deionized water and acetone. In the next step, the nanoparticles were  dried in a vacuum oven at 70 • C. Then, the black powder of Fe 3 O 4 @PVA nanoparticles was provided by using a ball mill.

Preparation of Fe 3 O 4 @PVA-Cu Nanoparticles
Fe 3 O 4 @PVA nanoparticles (1.0 g) was added to a flask, which was containing distilled water (10 mL). The mixture was dispersed for about 10 min. Then, Cu(CH 3 COO) 2 (aq.) (5% wt.) was added to the dispersed aqueous mixture of Fe 3 O 4 @PVA nanoparticles dropwise. After 2 h stirring the mixture at room temperature, the produced mixture was filtered off and completely washed with water (50 mL) and ethanol (50 mL) for three times. Eventually, the yielded product was dried at room temperature.

Synthesis of Aldehydes and Ketones (2a-2n) From the Selective Oxidation of Alcohols Using
Fe 3 O 4 @PVA-Cu Nanoparticles Alcohol (0.5 mmol), of Fe 3 O 4 @PVA-Cu (20 mg), and hydrogen peroxide (3 mol %) were added to a flask, which was containing acetonitrile (2 mL) as solvent. The reaction solution was stirred until completion of the reaction. The reaction process was tracking with Thin Layer Chromatography (TLC) technique. After producing the target product, which was detected by TLC, the catalyst was removed from the container by using an external magnetic field. In the end, we purified the products with GC-MS technique. SCHEME 2 | The propounded mechanism for the selective oxidation of alcohols in the presence of Fe 3 O 4 @PVA-Cu.

CONCLUSION
In this study, we produced Fe 3 O 4 @PVA-Cu nanocomposite, which was characterized by employing TEM, SEM, EDX, XRD, TGA, VSM, and FT-IR techniques. Moreover, the application of the prepared nanocomposite was thoroughly examined for the selective oxidation of alcohols. Various aldehydes and ketones were synthesized in the presence of Fe 3 O 4 @PVA-Cu and hydrogen peroxide as the supplementary oxidant. Using the prepared heterogeneous nanocatalyst provided an environmentally friendly, economical, highly selective, and straightforward strategy for selective oxidation of alcohols. Moreover, simple isolation of the catalyst from the reaction solution with negligible loss of catalytic activity in further reactions is another advantage of this method.

DATA AVAILABILITY STATEMENT
All datasets presented in this study are included in the article/supplementary material.

AUTHOR CONTRIBUTIONS
JR, and MN as the students of AM's research group carried out the analyses, characterization, and participated in discussing the results, prepared the draft of the manuscript, and revised the manuscript. AM designed the study, edited and revised the manuscript, and managed all steps of the project. All authors read and approved the final manuscript.