Two new magnetic nanocomposites of graphene and 12-tungestophosphoric acid: characterization and comparison of the catalytic properties in the green synthesis of 1,8-dioxo-octahydroxanthenes

Abstract

Two well dispersed H₃PW₁₂O₄₀ immobilized to magnetite graphene oxide (Fe₃O₄/GO/PW) and magnetite graphene aerogel (Fe₃O₄/GA/PW) nanocomposites were synthesized, via coprecipitation and coprecipitation-solvothermal methods, respectively. The catalytic activity, leaching and recyclability of the two prepared catalysts were compared in the synthesis of 1,8-dioxo-octahydroxanthenes (DOX) in water. Fe₃O₄/GO/PW showed higher activity, and lower leaching followed by a higher reusing number compared with Fe₃O₄/GA/PW. The as-prepared Fe₃O₄/GO/PW nanocomposite was fully characterized by transmission electron microscopy, scanning electron microscopy, a vibrating sample magnetometer, energy dispersive X-ray spectroscopy, X-ray diffraction, laser particle size analyzer, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller (BET) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The excellent conversions show that the catalyst has been used as a highly effective catalyst for the synthesis of DOX. The novelty of this catalyst is the low leaching of PW in aqueous media and reusability of the catalyst for at least four times with an appreciable loss of its catalytic activity.

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Introduction

It is generally approved that there is a growing need for more environmentally acceptable processes in the chemical industry. This trend towards what has become known as ‘Green Chemistry’^1,2 or ‘Sustainable Technology’ necessitates a paradigm shift from traditional concepts of process efficiency to one that assigns economic value to eliminating waste at source and avoiding the use of toxic or hazardous substances. The attention to the process parameters in the synthesis of stable magnetic nanoparticles (MNPs), to get a controlled product, is of particular importance in view of the utilization of their properties^3–5 in various fields. They can be used in catalytic, environmental, biological, biomedical and electronical applications. Much attention has been addressed to the preparation of MNPs.^3–5 Moreover different strategies have been developed to preserve their stability against agglomeration or precipitation and oxidation phenomena including polymers,^6–8 silica,^9,10 precious metals^11,12 and carbon by coating of some compounds.^13–21 The carbon covering has many advantages over other coatings due to its feasibility to be easily functionalized. The porous carbons, comprised of one-dimensional (1D) carbon nanotubes (CNTs) or two-dimensional (2D) graphenes have been used in a wide variety of fields, including energy storage, sensors, catalysis and environmental science and engineering.^22–33 Furthermore, carbon-based catalysts have been proposed as low cost renewable “green catalysts” able to be prepared from either biomass or from household waste. Compared with a CNT monolith, graphene-based metamaterials, one atom-thick two dimensional layers of sp² bonded carbon, possesses several unique properties, including highly thermal conductivity, fracture strength, large specific area and extraordinary electronic transport properties.^34,35 Recently, the combination of various heteropoly acids (HPAs) with inorganic supports as nanocatalysts have been successfully developed and evaluated in a number of important industrial reactions.^36–40 Supported HPA catalysts are important for various applications because of environmental and economic considerations. They also have excellent activity and present a greater number of surface acid sites than their bulk components.^41,42 Moreover they have been pointed lately as versatile green catalysts for a variety of organic reactions.^43,44 Immobilization of HPAs in the magnetically recoverable nanoparticle supports will provide facile production process and offer an opportunity for their applications in the chemical or pharmaceutical industries. In this study, two different kind of materials, namely magnetite graphene oxide (Fe₃O₄/GO) and magnetite graphene aerogel (Fe₃O₄/GA) were firstly prepared as supports for the immobilization of HPAs. An easy controlled one-step chemical coprecipitation method was used to introduce 2D Fe₃O₄/GO, meanwhile 3D Fe₃O₄/GA was prepared by combination of a modified hydrothermal reduction process and chemical coprecipitation methods. Afterwards, PW was immobilized on magnetic supports, Fe₃O₄/GO and Fe₃O₄/GA, through chemical coprecipitation method to produce Fe₃O₄/GO/PW and Fe₃O₄/GA/PW. These are novel heterogeneous catalytic systems that possesses both a high separation efficiency and a relatively high surface area to produce maximize catalyst loading and activity. The effects of these two supports on the catalytic activity, leaching, and recyclability were investigated. Based on the obtained results, Fe₃O₄/GO/PW was used for the syntheses of biologically useful building blocks, namely 1,8-dioxo-octahydroxanthenes (DOX) in water.^45–53

Experimental

Materials and methods

FeCl₃·6H₂O (99%) and FeSO₄·7H₂O (99%), NaOH (98%), HCl (37%), graphite, PW (>99), other reagents and solvents used in this work were obtained from Merck, Aldrich or Fluka without further purification. Transmission electron microscopy (TEM) was obtained using a TEM microscope (Jeol JEM-2100 with an accelerating voltage of 200 kV). Scanning electron microscopy (SEM) has been performed using an AIS2300C microscope with scanning range from 0 to 20 keV. Energy-dispersive X-ray (EDX) measurements were made with an IXRF model 550i attached to SEM. SEM/EDX samples were prepared by coating of solid particles into a conductive layer. The size distribution of the samples was obtained using a laser particle size analyzer (HPPS5001, Malvern, UK). X-ray diffraction (XRD) patterns were obtained on an Inel French, EQUINOX 3000 model X-ray diffractometer using Cu-Kα radiation. Fourier transform infrared (FTIR) spectra were recorded with KBr pellets using a FTIR spectrometer ALPHA. Brunauer–Emmett–Teller (BET) surface areas and pore volumes were measured on a sorptometer kelvin 1042 using nitrogen adsorption at 77 K. Magnetic properties were measured using a BHV-55, Riken, Japan vibrating sample magnetometer (VSM). The inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Spectro Ciros CCD spectrometer were used to leaching measurements. NMR spectra were recorded on a Bruker Avance 200 MHz NMR spectrometer with CDCl₃ as solvent and TMS as internal standard. Thin layer chromatography on precoated silica gel fluorescent 254 nm (0.2 mm) on aluminum plates was used for monitoring the reactions.

Preparation of the catalysts

a. Preparation of GO. Graphite oxide was synthesized from graphite powder according to the modified Hummer's method.⁵⁴ Briefly, 23 mL of 98% sulfuric acid was added in a three-neck flask, which was placed in an ice bath with continuous stirring. Subsequently, graphite powder (4 g), NaNO₃ (2 g) and KMnO₄ (12 g) were separately added into this flask with vigorous agitation. Then, ice bath could be removed, and the suspension was stirred overnight until its color turned into taupe brown. The ice bath was replaced, and 120 mL of H₂O was gradually added into the mixture and diluted mixture was heated to 98 °C for 15 min. After that, the temperature was reduced to 60 °C and 30% hydrogen peroxide was added slowly until visible bubbles were observed. The above mixture was centrifuged to remove the supernatant, and the GO was washed several times by deionized water. Afterwards, the obtained material was dried in oven at 60 °C for 6 h.

b. Preparation of Fe₃O₄/GO. Fe₃O₄/GO was synthesized by chemical coprecipitation method,⁵⁵ conducted as follows: the synthesized GO was dispersed in deionized water (1 mg mL⁻¹) and ultrasonicated for 3 h to get a clear dispersion of GO. Ferric chloride (160 mg) and ferrous sulfate (60 mg) were gradually added to 50 mL of 1 mg mL⁻¹ GO at 50 °C under N₂ atmosphere. Approximately 25 mL of ammonia solution (25%) was added slowly to precipitate Fe²⁺ and Fe³⁺ ions for preparation of magnetite particles. The solution was cooled to room temperature. Then, the solution was applied by external magnetic field to separate the Fe₃O₄/GO. Finally, the Fe₃O₄/GO was washed with high purity water several times and dried in oven at 60 °C.

c. Preparation of Fe₃O₄/GA. The magnetic Fe₃O₄ nanoparticles were prepared by a modified chemical co-precipitation method.⁵⁶ Then, 0.1 mmol Fe₃O₄ nanoparticles were added to 10 mL of as-fabricated GO suspension (2 mg mL⁻¹) and ultrasonicated for 30 min and 30 μL ethylenediamine was added to the suspension as assistant reducing agent. Then the mixture was transferred to an ice bath and was ultrasonically dispersed for 1 h. 2.5 mL of the mixture was placed in a 25 mL Teflon-lined autoclave and maintained at 120 °C for 2 h. After that, the autoclave was cooled to room temperature and the as-formed hydrogel was washed with a solvent composed of distilled water and ethanol with the volume ratio of 5 : 1 several times. Finally, the obtained Fe₃O₄/GA was vacuum-dried overnight at room temperature.

d. Preparation of Fe₃O₄/GO/PW and Fe₃O₄/GA/PW. For the immobilization of PW on as-prepared supports (Fe₃O₄/GO or Fe₃O₄/GA particles), solution of PW (0.7 g in 5 mL of dry methanol), was added dropwise to a suspension of 1.0 g of as-prepared support in methanol (50 mL). The mixture was stirred 24 h at room temperature to obtain Fe₃O₄/GO/PW or Fe₃O₄/GA/PW catalysts. Finally, the prepared catalysts were collected by a permanent magnet and dried in a vacuum at 40 °C.

General procedure for the synthesis of DOX

For the synthesis of DOX, to a mixture of an aldehyde (1.0 mmol), dimedone (2.0 mmol), water (4 mL) was added the catalyst (Fe₃O₄/GO/PW or Fe₃O₄/GA/PW, 0.02 g) in reflux condition. The progress of the reaction indicated by thin-layer chromatography (TLC). After completion of the reaction the mixture was cooled to room temperature and the solid (containing catalyst and product) was filtered and washed with water (10 mL). The product was dissolved in acetonitrile, and the catalyst was recovered from the product using an external magnet. The solvent was evaporated in a vacuum to obtain the crude product, which was purified by recrystallization from ethanol. The remaining catalyst was washed with diethyl ether, dried under vacuum and reused in a subsequent reaction.

Large scale synthesis

Reaction of dimedone and benzaldehyde was selected for large-scale synthesis. The reaction of dimedone (20.0 mmol) with benzaldehyde (10.0 mmol), in the presence of Fe₃O₄/GO/PW (0.2 g) was done at 70 °C. Purification of the product and recovery of the catalyst are similar to above procedure.

Leaching as well as heterogeneity test

To check the leaching of Fe₃O₄/GO/PW and Fe₃O₄/GA/PW, after completion of the reaction between dimedone and benzaldehyde, in the presence of catalyst in 4 mL water at 70 °C, the content of PW filtrates was evaluated quantitatively by ICP-AES. For rigorous proof of heterogeneity, a test was carried out by hot filtration of the catalyst from the reaction mixture after obtaining a conversion of about 50% was done. Then, the reaction was continued with the residual solution.

Results and discussion

The synthetic process of Fe₃O₄/GO/PW and Fe₃O₄/GA/PW are indicated in Scheme 1 in two different ways. Graphite oxide was prepared by oxidation of graphite powder under harsh oxidizing conditions by Hummer's method and was converted to graphene oxide by ultrasonication. In pathway a, the Fe₃O₄/GO was prepared by chemical coprecipitation method of iron ions. Fe³⁺ and Fe²⁺ ions were captured by carboxylate anions on the surface of GO sheets. Then ammonia solution was gradually added to precipitate Fe₃O₄. Immobilization of Fe₃O₄/GO with a methanol solution of PW occurs via formation of hydrogen bonding between hydroxyl group on the surface of GO and PW molecule. Fe₃O₄/GA/PW was prepared by pathway b. Magnetic Fe₃O₄ NPs were prepared according to the literature using ferric chloride hexahydrate and ferrous chloride tetrahydrate upon addition of NH₄OH through the chemical co-precipitation method. GO aqueous suspension was first mixed with reducing agent ethylenediamine and then freshly synthesized Fe₃O₄ nanoparticles. After hydrothermal treatment, GO was gradually self-assembled into a hydrogel with simultaneous deposition of Fe₃O₄ nanoparticles. The soft hydrogels, fabricated by the modified hydrothermal method, showed a very small volume shrinkage. Fe₃O₄/GA aerogel samples after drying were obtained. Afterwards, Fe₃O₄/GA was used as support for immobilization of PW.

Scheme 1 Schematic illustration for preparation of Fe₃O₄/GO/PW and Fe₃O₄/GA/PW nanocatalysts.

At the beginning, PW content of two different magnetic catalysts were compared with previously synthesized catalyst with carbon base, PW/SMNs (starch magnetic nanoparticles supported PW)⁵⁷ with EDX spectrums. The EDX spectrums of the Fe₃O₄/GO/PW, Fe₃O₄/GA/PW and PW/SMNs are presented in Fig. 1a, b and c, respectively.

Fig. 1 EDX spectrum of (a) Fe₃O₄/GO/PW (b) Fe₃O₄/GA/PW and (c) PW/SMNs.

In Table 1 the molar ratio of carbon, oxygen, phosphor, iron and tungstate of the catalysts are illustrated. From these results, PW content of three different catalyst are similar. So, comparison of the catalytic activity and other factors for these catalysts are quite logical. Moreover, the EDX elemental mapping images spectrum of the Fe₃O₄/GO/PW, Fe₃O₄/GA/PW and PW/SMNs show uniform distribution of P and W (from PW) and C, O, Fe (from support).

Table 1 Content of different elements in Fe₃O₄/GO/PW, Fe₃O₄/GA/PW and PW/SMNs

Element	Fe3O4/GO/PW	Fe3O4/GA/PW	PW/SMNs
C (%)	11.145	10.321	11.895
O (%)	11.594	14.801	12.325
P (%)	2.730	2.671	2.92
Fe (%)	25.178	23.193	22.98
W (%)	49.354	49.014	49.88

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The FTIR spectra of GO, Fe₃O₄ nanoparticles, Fe₃O₄/GO, PW and Fe₃O₄/GO/PW are presented in Fig. 2a–e. FTIR spectrum of Fe₃O₄/GO⁵⁸ (Fig. 2c) shown the presence of various functional groups like OH (3429 cm⁻¹), COOH (1223 cm⁻¹), C–O (824 cm⁻¹) and C–O (1133 cm⁻¹) related to graphene oxide. Also, Fe–O stretching vibration peak in the Fe₃O₄/GO was observed at 610 cm⁻¹, in which for Fe₃O₄ (Fig. 2b) nanoparticles appear at 586 cm⁻¹. PW Keggin structure is well known and comprised of a PO₄ tetrahedron surrounded by four W₃O₁₃ groups designed by edge-sharing octahedra. These groups are linked to each other by corner-sharing oxygens.⁵⁹ The described structure gives rise to four kindes of oxygen, being responsible for the fingerprint bands of the Keggin ion between 700 and 1200 cm⁻¹. PW presents typical bands for absorptions at 1080 (P–O), 984 (W=O), 896 and 814 (W–O–W) cm⁻¹ (Fig. 2d). In Fe₃O₄/GO/PW, the characteristic bands are at the same wavenumbers with Fe₃O₄/GO with a slight shift confirming the interaction to the support (Fig. 2e).

Fig. 2 FTIR spectra of the (a) GO (b) Fe₃O₄ (c) Fe₃O₄/GO (d) PW and (e) Fe₃O₄/GO/PW.

The SEM and TEM images of the synthesized Fe₃O₄/GO/PW are shown in Fig. 3a and b. From Fig. 3a it can be seen clearly that the iron nanoparticles and PW were successfully combined with the graphene sheets. Furthermore, it understand that GO nanosheets have rough surface which provide a large surface area to combination with Fe₃O₄ nanoparticles and PW coating. As shown in Fig. 3b, Fe₃O₄/GO/PW had well-defined composite structure composed of the graphene oxide sheets, Fe₃O₄ and PW nanoparticles. Closer examination reveals that the particles had an average diameter of about 15–20 nm.

Fig. 3 (a) SEM and (b) TEM images of Fe₃O₄/GO/PW nanocatalyst.

Size distribution of the Fe₃O₄/GO and Fe₃O₄/GO/PW derived from a laser particle size analyzer, illustrated in Fig. 4a and b. Indicated that the mean diameter of them is 640 and 856 nm, respectively. As shown in the TEM image (Fig. 3b), aggregation of individual nano particles was occurred. Such aggregations cause the difference between particle size resultant from TEM analysis and the measurements made using the laser particle size analyzer.

Fig. 4 Grain size distribution of (a) Fe₃O₄/GO and (b) Fe₃O₄/GO/PW.

The powder X-ray diffraction (XRD) patterns of Fe₃O₄/GO and Fe₃O₄/GO/PW are shown in Fig. 5. The diffraction peak in XRD at 10.1 was attributed to the planar structure of GO. Likewise, all the characteristic diffraction peaks of the pure crystal of Fe₃O₄ at 2θ of 30.35, 35.69, 43.38, 53.78 and 62.91 were present in the Fe₃O₄/GO which further indicated the successful synthesis of GO and Fe₃O₄/GO.⁶⁰

Fig. 5 XRD patterns of (a) Fe₃O₄/GO and (b) Fe₃O₄/GO/PW.

When PW is impregnated on Fe₃O₄/GO, characteristic peaks assigned to PW are comparable to those for the Fe₃O₄/GO, which implies maintenance of their crystalline character. The presence of the peaks illustrates that there is no significant variation in the structure of the PW during the preparation which confirmed by FTIR spectroscopy.⁶¹

In order to investigate the porous structure and surface area of Fe₃O₄/GO/PW the N₂ adsorption–desorption isotherm were conducted and shown in Fig. 6. The isotherm curve of Fe₃O₄/GO/PW is close to type IV with a weak hysteresis loop in the relative pressure, which indicated the presence of porous structure of Fe₃O₄/GO/PW composite. The measured BET surface area of the Fe₃O₄/GO/PW is 168.124 m² g⁻¹. In addition, the average pore diameter of the catalyst is 33.87 Å, calculated from desorption branch of the nitrogen isotherm with the BJH method. The corresponding BJH desorption cumulative pore volumes is 0.181 cm³ g⁻¹.

Fig. 6 N₂ adsorption–desorption isotherm and of Fe₃O₄/GO/PW.

The magnetic property of Fe₃O₄/GO and Fe₃O₄/GO/PW were measured using VSM (Fig. 7). The magnetic measurement (Ms) of Fe₃O₄/GO and Fe₃O₄/GO/PW are about 60 and 30 emu g⁻¹, respectively. There is no hysteresis, proposing that Fe₃O₄/GO/PW is superparamagnetic.

Fig. 7 Magnetization measurements for (a) Fe₃O₄/GO and (b) Fe₃O₄/GO/PW.

In the following, the catalytic activities, leaching and reusability of three different magnetic catalysts were compared using the reaction of benzaldehyde and dimedone as a model system in water (Scheme 2).

Scheme 2 Model reaction.

Results show that higher catalytic activity in short reaction time was found in the presence of the Fe₃O₄/GO/PW (Fig. 8).

Fig. 8 Comparison of catalytic activity of three different catalysts in the model reaction. (Reaction conditions; benzaldehyde (1 mmol), dimedone (2 mmol), water (4 mL), catalyst (0.02 g), T = 75 °C.)

The reusability and leaching of three different magnetic catalysts were studied by employing recovered catalysts in the model reaction (Fig. 9a and b). For Fe₃O₄/GO/PW, the interacting species were well dispersed on the support surface and leaching was low during the reaction which confirmed by ICP-AES measurement as well. So, the Fe₃O₄/GO/PW catalyst could be recovered and reused several times; only 4.9% of the initial PW content totally leached into the reaction mixture during four successive runs. The yield of the product was not substantially changed (10% difference) after the fourth run. Fe₃O₄/GA/PW and PW/SMNs could be reused at least four times, albeit with slightly decrease in activities compared to Fe₃O₄/GO/PW. This behavior may indicate that some leaching of the active species should occur during the subsequent reaction cycles which confirmed by the ICP-AES measurement. As a result, it seems that 11.6% and 26.2% of the PW were leached out from the Fe₃O₄/GA/PW and PW/SMNs surface during four runs (Fig. 9b).

Fig. 9 Comparison of reusability and leaching of three different catalysts in the model reaction. (Reaction conditions; benzaldehyde (1 mmol), dimedone (2 mmol), water (4 mL), catalyst (0.02 g), T = 75 °C.)

Another test was performed to check if this active catalyst, Fe₃O₄/GO/PW, is indeed heterogeneous (Fig. 10). After the hot filtration no further conversion was observed in the model reaction as a proof that no homogeneous catalyst was present in the reaction mixture.

Fig. 10 Reaction progress before and after filtration of the catalysts.

According to the results obtained above, future experiments were carried out using Fe₃O₄/GO/PW as the best catalyst. The effect of the catalyst loading and reaction temperature on the efficiency was evaluated in the model reaction (Table 2). It was observed that the substrates did not react at room temperature. Whereas by increasing the temperature up to 70 °C, a significant improvement was observed and yield of the product was improved to 95% it may be because of the low solubility of dimedone in the reaction mixture at room temperatures (Table 2, entries 1 and 2). The effect of the catalyst loading was also studied. Decreasing in the catalyst quantity from 0.04 to 0.02 g had no effect on product yield or reaction time. Further decrease from 0.02 to 0.01 g in the catalyst quantity led to increasing in time of the reaction (Table 2, entries 2–4). It should be noted that the yield of product was 22% after 10 min when the reaction was performed in the presence of Fe₃O₄/GO only. In addition, the reaction did not proceed in the absence of the catalyst.

Table 2 Optimization of the reaction conditions in model reaction in the presence of Fe₃O₄/GO/PW catalyst

Entry	Catalyst	Temp. (°C)	Time (min)	Yield (%)
1	Fe3O4/GO/PW (0.04 g)	25	30	—
2	Fe3O4/GO/PW (0.04 g)	70	10	95
3	Fe3O4/GO/PW (0.02 g)	70	10	95
4	Fe3O4/GO/PW (0.01 g)	70	10	60
5	Fe3O4/GO (0.02 g)	70	10	22
6	—	70	30	—

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Encouraged by these results; we used the Fe₃O₄/GO/PW catalyst in the reaction of dimedone with various aromatic aldehydes to synthesize structurally diverse DOX; the results are summarized in Table 3.

Table 3 Synthesis of DOX in the presence of Fe₃O₄/GO/PW

Entry	R	Product^a	Time (min)	Yield^b (%)
a All products were identified by comparing their spectral data with those of the authentic samples.^62b Isolated yields.c Refer to large scale syntheses.
1	H		10	95
2	H		30	90
3^c	4-Cl		15	96
4	4-Br		15	96
5	4-OH		20	84
6	4-NO2		10	98
7	3-NO2		15	92
8	2-NO2		20	93
9	4-OCH3		15	90
10	3,4-OCH3		15	85
11	2,5-OCH3		20	84
12	4-NMe2C6H4		12	95

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According to the literature⁶³ and results obtained in our experiments, a mechanism is proposed for the synthesis of DOXs as shown in Scheme 3. Initially, the acid catalyst was protonated the carbonyl group of the aldehyde and converted the aldehyde into a convenient electrophile. Then intermediate I was formed via condensation of dimedone with the aromatic aldehyde. Subsequently, intermediate II was produced through combination of a second activated dimedone with intermediate I. Finally, removal of one water molecule of intermediate II led to cyclization and followed by final formation of xanthene products.

Scheme 3 The proposed mechanism for the synthesis of DOX.

Conclusions

Two novel magnetic nanocomposite, Fe₃O₄/GO/PW and Fe₃O₄/GA/PW was successfully synthesized through one-step chemical coprecipitation and coprecipitation-solvothermal methods, respectively. Evaluation of these two new synthesized catalysts in term of synthetic methods, activities, leaching and recyclability, showed that Fe₃O₄/GO/PW has better property than Fe₃O₄/GA/PW for more investigations. Fe₃O₄/GO/PW was characterized by FTIR, XRD, VSM, TEM, SEM, EDX, BET and laser particle size analyzer. Fe₃O₄/GO/PW could be separated rapidly under external magnet; this is an important advantage of the use of a magnetically separable catalyst. The Fe₃O₄/GO/PW catalyst was used in aqueous synthesis of DOX which are biologically interesting compounds. The method is easily operated, cost-effective, environmentally friendly with high-purity products in excellent yields. The detecting results could indicated our protocol make the reaction suitable for scale-up and commercialization.

Acknowledgements

The authors thank the Razi University Research Council for support of this work.

Notes and references

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