La(OH)3 nanoparticles immobilized on Fe3O4@chitosan composites as novel magnetic nanocatalysts for sonochemical oxidation of benzyl alcohol to benzaldehyde

This work introduces an eco-friendly method for immobilization of La(OH)3 nanoparticles on modified Fe3O4 nanoparticles. The structural and morphological characteristics of the nanocatalyst were determined by various analytical techniques including, FT-IR, EDS, FESEM, VSM and XRD. The catalytic efficiency of the Fe3O4@Cs/La(OH)3 composite as a heterogeneous nanocatalyst was evaluated by selective oxidation of benzylic alcohols to aldehydes. The optimum reaction conditions including time, temperature, nanocatalyst dosage, and solvent were investigated for ultrasound-assisted oxidation processes. Furthermore, the magnetic nanocatalyst was recovered up to seven times without considerable activity loss. Furthermore, the proposed nanocomposite had a remarkable effect on reducing the reaction time and enhancing the yield.


Introduction
Oxidation transformations have attracted much interest due to their potential applications and functionality in the chemical and materials industries. Among different oxidation reactions, oxidation of benzyl alcohols into the corresponding benzaldehydes is a prominent chemical transformation in organic chemistry. [1][2][3][4][5] Aldehydes, which have various applications in different elds such as pharmaceuticals, dyes, perfumes, agriculture, food, beverages, agribusiness industries, and chemicals, are used as valuable oxygen-containing intermediates and raw materials in organic chemistry. In the past, despite numerous available methods for selective oxidation processes, most of them were not without drawbacks, generating a lot of by-products and pollutants. These processes require toxic, expensive, or hazardous chemicals (such as pyridinium chlorochromate (PCC), permanganate (MnO 4 À ), dichromate (Cr 2 O 7 2À ), chromium trioxide (CrO 3 )), as oxidants that lead to safety and ecological problems. 6,7 Thus, the development of a new method for the construction of heterogeneous (nano) catalysts is a matter of increasing attention in the catalysis eld.
In recent years, biopolymer derived nanocatalysts have been considered as heterogeneous catalysts with excellent catalytic activity for chemical transformations, particularly, in oxidation reactions.
Among these, ecofriendly polysaccharides are used as efficient supports in the functionalization of metal nanoparticles. 5 Chitosan (CS) is the second most abundant biopolymer (aer cellulose) on the earth which is applied in many heterogeneous catalytic systems. Utilization of chitosan as catalyst support has attracted profound attention due to its signicant properties such as low cost, resource abundance, hydrophilicity, chemical stability, eco-friendliness, biodegradability, non-toxicity, signicant thermal stability, and antioxidant properties. [8][9][10][11] In addition, the presence of NH 2 and OH functional groups produces appropriate arrangements such as chelating ligands to coordinate various metal ions 9 (Scheme 1).
On the other hand, effective recycling and easy separation are important factors in developing heterogeneous catalysts. 5 In the past few decades, increased use of Fe 3 O 4 nanoparticles (NPs) in heterogeneous catalysts have captured intense attention owing to their unique catalytic properties such as super-paramagnetism, non-toxicity, easy preparation, chemical stability, easy and excellent recyclability, and reusability. 12,13 Scheme 1 Chitosan structure.
The great properties of magnetic chitosan (Fe 3 O 4 @CS) have led to its use in different elds such as drug delivery systems, oxidation and sulfoxidation process, removal of heavy metals, etc. 5 On the other hand, ultrasonic-engineered reactions are more effective than traditional approaches (conventional heating conditions). 5 Ultrasound (US) irradiation can make changes in reactivity, increase modications by improving surrender, reduce reaction time, and nally replace dangerous reagents with safe ones. 14,15 Therefore, selective oxidation reactions using the nanomaterials in conjunction with US irradiation, can be highly efficient. 16 In 1794, lanthanum oxide was discovered by Johann Gadolin. 17 Among the rare earth oxides, lanthanum oxide has been considered as catalyst in various reactions due to its unique properties (good paramagnetic sensitivity, saturated magnetization, magnetostrictive properties, the large bandgap, etc.). 18 Therefore, lanthanum(III) oxide can be a good candidate for improvement of catalytic activity. 18,19 With this background, we designed, prepared and characterized Fe 3 O 4 @CS/La(OH) 3 nanocomposites as a novel heterogeneous catalyst for ultrasound-assisted oxidation reaction. Some of the strange and unique attributes of applied oxidation protocol are short reaction time, great yield, green condition, simple recovery of nano catalysts, and easy workup.

Results and discussion
FT-IR spectroscopy is one of the most important techniques for identifying organic functional groups. The FT-IR spectra of Fe 3 O 4 @CS/La(OH) 3 , Fe 3 O 4 @CS and pure CS were shown in Fig. 1. As shown in Fig. 1a-c, the broad absorption band at 3364, 3358 and 3375 cm À1 belong to the amino and hydroxyl groups of chitosan. The bands at 1649, 1632 and 1637 cm À1 are related to the C]O stretching vibration of the amide group. The bending vibration of the amino group appeared at 1590, 1560 and 1550 cm À1 . Also respectively, 1059, 1018, and 1078 cm À1 represented the C-O stretching vibration of C-OH of chitosan in Fig. 1a-c. As shown in Fig. 1b and c, the absorption band at 559 cm À1 (or 565 cm À1 ) belongs to the Fe-O stretching vibrations. 20 The medium absorption band at 650 cm À1 was because of La-O stretching vibration (Fig. 1c). 17 In the EDS spectrum ( Fig. 2), the presence of all elements including C, N, O, Fe, and La, is determined according to the energy, which indicates the conrmation of product purity.
XRD analysis determines a direct method for the structure of matter and fuzzy composition. This method can be used to determine lattice geometry, unknown materials, crystal size and     JCPDS card no. 01-075-0449) in a good agreement with literature. 21 The broad diffraction peaks that appeared around 2q ¼ 19 for Fe 3 O 4 @CS/La(OH) 3 sample are related to chitosan (Fig. 3c). 22 In addition, the XRD diffraction peaks are observed at 27.2912 , 28.3643 , 39.6213 , and 48.1237 are related to La 2 O 3 which correspond to (222), (300), (400), and (622) respectively (La(OH) 3 ; JCPDS card no. 04-0856). 17,18,23 The observed peaks show that the structure of Fe 3 O 4 and La(OH) 3 have not changed during the composition process.
The particle size, surface properties, and shape of prepared nanocatalyst were observed using FESEM with various magni-cations. The FESEM images of Fe 3 O 4 @CS/La(OH) 3 nanocomposites show a uniform spherical shape with the average particle size about 28 nm (Fig. 4).
Magnetic properties of Fe 3 O 4 NPs and Fe 3 O 4 @CS/La(OH) 3 composites were measured by VSM analysis (Fig. 5). The hysteresis loops of pure   According to the obtained results, a wide range of benzyl alcohols bearing either electron-donating or electron-withdrawing groups were successfully converted to benzaldehyde in short reaction times using Fe 3 O 4 @CS/La(OH) 3 (Table  2). Corresponding products of both groups were achieved without any over-oxidation (

Catalyst reutilization
In addition to the catalytic activity, the stability plays a vital role in catalysis eld. In this work, the reusability of the catalyst was tested under optimum reaction conditions. The results show that the Fe 3 O 4 @CS/La(OH) 3 successfully recovered up to 7th cycle (Fig. 6).
The morphology of the Fe 3 O 4 @CS/La(OH) 3 nanocatalyst aer 7 reuse periods is shown in Fig. 7b. The spherical morphology of Fe 3 O 4 @CS/La(OH) 3 is preserved, indicating that the nanocatalyst was well stable.
XRD of the Fe 3 O 4 @CS/La(OH) 3 nanocatalyst aer 7 reuse periods is shown in Fig. 8b. Characteristic peaks for Fe 3 O 4 @CS/ La(OH) 3 are preserved, indicating that the nanocatalyst was well stable and pure.

Chemicals and apparatus
In this project, all the chemicals, including alcohol and solvents required for the tests, were purchased from Merck and Aldrich. FT-IR samples were collected by KBr pellets and their spectra were detected by PerkinElmer 1600 FTIR spectrometer. The morphology and size of the samples were determined by scanning electron microscopy (SEM) and the crystals were formed by X-ray diffraction (XRD) and scattered X-ray energy spectroscopy (EDX) and vibrating sample magnetometer (VSM). The oxidation products were examined by gas chromatographic spectrometry (GC).

Preparation of Fe 3 O 4 nanoparticles
Fe 3 O 4 magnetic nanoparticles (MNPs) were constructed by the chemical co-precipitation method. 34 Approximately 1.7 g of Fe(II) and 4.75 g of Fe(III) salts were dissolved in deionized water (200 ml). The mixture was stirred at 60 C under N 2 atmosphere, then 7.5 ml of NH 3 solution was added. Then the mixture of reaction was allowed to occur for 1 h at 60 C. Finally, the dark solid was magnetically separated, washed with ionized water, and dried at 60 C overnight. In particular, to avoid the conversion of Fe 3 O 4 to Fe 2 O 3 in air, all of the synthetic procedure was conducted under N 2 atmosphere.

Preparation of Fe 3 O 4 @CS
First, 0.01 g of chitosan was dissolved in 10 ml of ethanoic acid. Subsequently, about 0.25 g of Fe 3 O 4 was added to the chitosan solution and dispersed for half an hour. The resulting solution was mechanically stirred at 60 C. Next, solution (prepared by dissolving 0.02 g of STPP (sodium tripolyphosphate) in 50 ml of deionized water) was dropwise added at a rate of 4.5 ml h À1 . At this stage, the ionic gelation of chitosan was created on the Fe 3 O 4 MNPs surface. Aer ltering, the product was dried for 36 hours at À20 C. Then, the core-shell product of Fe 3 O 4 @CS nanoparticles was obtained. 34

Procedure for the preparation of Fe 3 O 4 @CS/La(OH) 3
Fe 3 O 4 @CS/La(OH) 3 was generated by dispersing 0.1 g of Fe 3 O 4 @CS in deionized water (50 ml) for 1 hour. Next, 0.05 g of LaCl 3 $7H 2 O was added. The whole mixture was stirred about 2 hours under reux condition. The synthesized nanocomposites were collected by an external magnet and were washed with distilled water.

General procedure for oxidation of benzyl alcohols
Benzyl alcohol oxidation and synthesized catalyst were investigated. Benzyl alcohol (1 mmol), nanocatalyst Fe 3 O 4 @CS/ La(OH) 3 (50 mg), and H 2 O 2 (1 ml) were sonicated at 25 C. Aer completion of the oxidation process, the catalyst was separated using a magnet. Then, the organic phase was extracted with EtOAc, and the products were investigated through GC analysis. 35

Conclusions
In summary, we designed and fabricated a novel magnetic nanostructure of Fe 3 O 4 @CS/La(OH) 3 for the oxidation of different types of benzyl alcohols to benzaldehyde under green conditions for the rst time. Accordingly, the utilization of Fe 3 O 4 @CS/La(OH) 3 as a nanocatalyst can not only decrease the reaction time, but also increase the selectivity and yields. The presence of Fe 3 O 4 @CS/La(OH) 3 showed outstanding catalytic  performance with high to excellent conversions for different substituted benzylic alcohols and selectivity for benzyl alcohol at room temperature under US conditions, short reaction time, inexpensive and excellent conversion yields according to the green chemistry principles. Ultrasound irradiation process oxidation of benzyl alcohols with high selectivity is a more effective manner than the conventional heating method due to the synergistic effects between the ultrasound radiation, H 2 O 2 , and the Fe 3 O 4 @CS/La(OH) 3 nanocatalyst. The morphology of the Fe 3 O 4 @CS/La(OH) 3 nanocatalyst conrmed that aer 7 reuse periods, nanocatalyst was well stable and did not reveal a signicant difference.

Conflicts of interest
The authors stated that they had no nancial or personal interest in preparing the material reported in this article.