A protocol for the preparation of iron oxide nanocrystals of two different (nanorods and octahedrons) morphologies has been developed and the synthesized nanocrystals were well characterized by TEM and XRD. These two nanocrystals have been applied for the selectivie oxidation of aryl-methanol and vinyl-arene. Moreover, the magnetic catalysts have easily separated from reaction mixture by a magnet and are reused without appreciable loss of catalytic activity. The oxidation processes avoid the use of toxic catalysts and volatile and hazardous organic solvents.
Selective oxidations of aryl-methanol and vinyl-arene to aryl-aldehyde are synthetically important because of the wide applications of these products in organic reactions and also it is difficult to control the further oxidation to acid. Traditionally, oxidation of benzyl alcohol to benzaldehyde is performed with many oxidizing agents and was used in stoichiometric amount [
At first, we have synthesized both the iron oxide nanorods and octahedrons using hydrogen peroxide by solvothermal technique. In a simple experimental procedure, a mixture of FeCl3 and FeCl2 was used in equimolar ratios as the precursor. Ethylenediamine and water were mixed in equal volume ratios before being used as the solvent. Appropriate amounts of the precursor were dissolved in the selected amount of the solvent in such a way that the mixture filled up to 80% of a Teflon lined stainless steel autoclave used for the solvothermal process. Hydrogen peroxide was added to the chamber before being closed and placed in a preheated oven at 175°C for 8 h. 150 and 300
The powder products were characterized by X-ray diffraction (XRD) study to identify the products as well as their crystal structure. Both the products exhibited identical XRD patterns and literature survey (JCPDS) shows the formation of magnetite (Fe3O4) phase of iron oxide. Figure
XRD pattern representing both the nanorods and octahedrons of iron oxide.
TEM images of the Fe3O4 (a) nanorods and (b) octahedrons.
Further high-resolution TEM (HRTEM) (not shown here) reveals that these nanorods were single crystalline in nature. Figure
The synthesized iron oxide nanorods and octahedrons were employed as catalysts for the selective oxidations benzylic alcohols by H2O2 (Scheme
Nanoiron oxide catalyzed oxidation of benzyl alcohol and styrene derivative by H2O2.
In a trial reaction, we have applied both iron oxide nanomaterials (i.e., nanorod and octahedron) for the oxidation of benzyl alcohol. We have observed a comparable reactivity and selectivity of the two nanomaterials with slightly higher reactivity for nanorods (Table
The comparison of reactivity of nanorods and octahedron iron oxide nanomaterials for selective oxidation of benzyl alcohol with hydrogen peroxide.
Catalyst | Temperature | Time (h) | Yield (%) | Selectivity (%) |
---|---|---|---|---|
Nanorods | 70°C | 12 | 78 | 97 |
Octahedron | 80°C | 12 | 70 | 96 |
None | 100°C | 24 | 15 | 80 |
The product was extracted with ethyl acetate. The details of experimental procedure were given in the experimental section. The selectivity of all the oxidized products was determined from 1H NMR spectroscopic analysis of crude product.
A variety of substituted benzyl alcohol and styrene derivative were oxidized by H2O2 catalyzed by nanoiron oxide. The results are summarized in Table
Iron oxide nanorod catalyzed oxidation of benzylic alcohol by H2O2.
| ||||
---|---|---|---|---|
Entry | R | Time (h) | Yield (%)a | Selectivity (%)b |
1 | H | 12 | 78 | 97 |
2 | 4-Me | 10 | 81 | 99 |
3 | 4-Cl | 12 | 80 | 98 |
4 | 4-OMe | 11 | 82 (80)c | 99 (98)c |
5 | 4-NO2 | 12 | 75 (65)c | 98 (95)c |
Iron oxide nanorod catalyzed oxidation of styrene by H2O2.
| ||||
---|---|---|---|---|
Entry | R | Time (h) | Yield (%)a | Selectivity (%)b |
1 | H | 12 | 72 | 98 |
2 | 4-Me | 12 | 80 (70)c | 99 (97)c |
3 | 4-OMe | 12 | 80 | 99 |
5 | 4-NO2 | 12 | 75 | 98 |
The improved activity of the catalyst most probably originates from the nanometer size of nano-iron oxide. In general, nanoscale heterogeneous catalysts should offer higher surface areas and low-coordinated sites, which are responsible for the higher catalytic activity. Importantly, note that the ferromagnetic property of the catalyst made the isolation and reuse of this catalyst very easy. In the presence of a magnetic stirrer bar, nano-iron oxide moved onto the stirrer bar steadily and the reaction mixture became clear within 10 s (Figure
Separation of the IONPs by using external magnet.
After washing with acetone and drying in air, the nano-iron oxide can be directly reused for five runs without significant loss of catalytic activity for selective oxidation of 4-methoxy benzyl alcohol to 4-methoxybenzaldehyde (Table
Iron oxide catalyzed oxidation of 4-methoxybenzaldehyde.
| |||
---|---|---|---|
Runs | Time (h) | Yield (%)a | Selectivity (%)b |
1 | 12 | 82 | 99 |
2 | 12 | 82 | 99 |
3 | 12 | 81 | 98 |
4 | 12 | 80 | 98 |
5 | 12 | 80 | 98 |
A plausible mechanism for the iron oxide nanorod catalyzed oxidation of alcohol by H2O2 has been presented in Scheme
Plausible mechanism for the oxidation of alcohol.
In general, all these, oxidation reactions are simple, moderate to good yielding, and highly selective. This protocol did not require any solvent; thus, this procedure avoided volatile and toxic organic solvents concerning green chemistry. Moreover, here we have used environment-friendly and easily accessible nano-iron oxide as catalyst. The oxidizing reagent (H2O2) is also a very cheap, mild, and an environment friendly reagent, which produced water as only by-product. The catalyst (nano-iron oxide) and reagent (H2O2) used in this protocol thus fulfilled the criteria for “green chemistry.”
In conclusion, a protocol has been developed for the preparation of iron oxide nanoparticles of two different (nanorods and octahedrons) morphologies and well characterized by TEM and XRD. These two nanomaterials showed comparable selectivity on the oxidation of aryl-methanol and vinyl-arene. Moreover, the magnetic catalysts have easily separated from reaction mixture by a magnet and are reused without appreciable loss of catalytic activity. The oxidation processes avoid use of toxic catalysts and volatile and hazardous organic solvents. Certainly, this observation provides great promise towards more practical applications.
4-Methoxy benzyl alcohol (1 mmol) was heated with 30% (v/v) H2O2 (1 mmol) at 70°C in presence of catalytic amount of nano-iron oxide (1 mol%) and stirred at that temperature for 12 h (TLC). The reaction was cooled to room temperature and the product was extracted with ethyl acetate (
Styrene (104 mg, 1 mmol) was heated with 30% (v/v) H2O2 (2 mmol) at 70°C in presence of catalytic amount of nano-iron oxide (1 mol%) and stirred at that temperature for 12 h (TLC). The reaction was cooled to room temperature and the product was extracted with ethyl acetate (
All the products in Tables