3.1. Synthesis and Characterization of [Cu(II)Br2-BTP@GO] catalyst
3.1.1. FT-IR spectroscopy
The FT-IR spectra of GO, GO-CP, GO-CP-BTP, [Cu(II)Br2-BTP@GO] and reused catalyst are presented in Fig. 1 and Fig. 2. The FT-IR spectrum of GO (Fig. 1), exhibited a strong band in the region of 1726 cm− 1 is revealed to carboxylic acid (C = O) group, and the C–O (epoxy), C–OH and C = C groups were shown at 1051, 1223 and 1618 cm− 1 respectively [50, 51]. Also, a broad band in the region of 3429 cm− 1 is attributed to O–H stretching absorption. Figure 2a exhibited characteristic bands at 1108 and 1025 cm− 1 for Si–O–Si and Si–O–C bonds, respectively. Aliphatic C–H bonds presented at 2890 and 2937 cm− 1 due to the GO modification. The FT-IR spectra of GO-CP-BTP exhibited a peak at 1648 cm− 1 which is attributed to stretching vibration of imine group. This band (imine group) in Fig. 2c at 1670 cm− 1 shifted to lower frequency (1514 cm− 1) in [Cu(II)Br2-BTP@GO] catalyst, which confirmed that coordination of copper was successfully completed onto the surface of GO-CP-BTP. Additionally, the FT-IR spectrum of recovered catalyst was shown in Fig. 2d. As can be seen, the structure of the catalyst does not change after the 5th reaction run (Fig. 2d).
3.1.2. X-ray Diffraction Analysis (Xrd)
The XRD patterns of GO, prepared catalyst and reused catalyst are shown in Fig. 3. For graphene oxide (Fig. 3a) a sharp peak at 2𝜃= 10.86° resultant to (001), which is due to the oxidation of graphite powder [51]. In Fig. 3b a new broad peak represented at around 2𝜃= 23–28°, which is related to the major oxygen functional groups of grapheme oxide have been effectively functionalized. Figure 3c was presented the structure of reused catalyst is retained.
3.1.2. Thermogravimetric Analysis (Tga)
TGA curve of grapheme oxide and copper catalyst were examined in the range of 25–600°C temperature. For the catalyst two degradation phases were displayed (Fig. 4b). Between 25–235°C first loss (about 17%) was related to water adsorbed on graphene oxide and unreacted supporting material. Between 235°C and 414°C the second loss (about 24%) belonged to the decomposition of organic groups.
3.1.3. Fe-sem, Edx And Mapping Analysis
The surface morphology of the copper catalyst was investigated by field emission scanning electron microscopy (FE-SEM) (Fig. 5). SEM images showed rod-like shapes with uniform structures. The FE-SEM images approve the presence of copper nanoparticles. The energy dispersive X-ray (EDX) analysis of catalyst confirmed the presence of copper and other atoms on the surface of obtained catalyst (Fig. 6). Additionally, the elemental mapping showed all of the atoms have been distributed regularly in the surface of the catalyst (Fig. 7). For measured of the amount of copper on the surface of the catalyst used from Atomic Absorption Spectrophotometer (AAS) analyzer and the amount of copper was 0.087 mmol.g− 1. Moreover,CHNS analysis determined the nitrogen content and the amount of ligand on the grapheme oxide is 1.03 mmol g− 1.
3.2. Catalytic Studies
In this work, The [Cu(II)Br2-BTP@GO] catalyst's performance was examined by oxidation of benzyl alcohol to benzaldehyde. The best conditions for the amount of catalyst, time of reaction, type of oxidant and solvent were measured. The effect of the catalyst amount [Cu(II)Br2-BTP@GO] were investigated on the oxidation procedure of benzyl alcohol, after than the room temperature was selected as best temperature. In the absence of the catalyst the yield was negligible and when the amount of the catalyst was increased from 0.02 to 0.03 g the yield did not change. The best result was attained with 0.17 mol% (0.02 gr) of the catalyst (Table 1, entry 5). In this reaction the effect of time was studied and the best time was 3 hour. For investigation of the effect of solvent, different solvents were examined such as acetonitrile, acetonitrile/H2O and H2O, amongst them the water was be found an appropriate solvent for achieving the best reaction condition. Various oxidants such as Na2CO3, tert-BuOOH, H2O2, K2CO3, Et3N and NaIO4 were investigated for the oxidation of benzyl alcohol and tert-BuOOH was showed the best results.
Finally, under optimized reaction conditions, oxidation of different alcohol was evaluated with tert-BuOOH as oxidant at 25°C in water and with the use of Cu(II)Br2–BTP@MNPs catalyst, and results show that the [Cu(II)Br2-BTP@GO] catalyst was better than Cu(II)Br2–BTP@MNPs catalyst. Various alcohols reacted to their corresponding aldehydes and obtained results showed that the nature of substituent on the phenyl ring has no obvious effect on the yield of the product. And linear and cyclic alcohols were successfully oxidized to their corresponding carbonyl compounds (Table 2).
Table 1. Optimization of conditions in the oxidation of benzyl alcohol catalyzed with
Cu(II)Br2–BTP@MNPs
a Reaction conditions: benzyl alcohol (0.25 mmol), tert-BuOOH (0.5 mmol), catalyst, solvent (3 ml)
a GC yield based on the starting benzyl alcohol
Table 2
Result of alcohol oxidation with tert-BuOOH catalyzed by [Cu(II)Br2-BTP@GO]a
Entry
|
Alcohol
|
Carbonyl compound
|
Yield (%)b
|
TOF (h− 1)
|
1
|
Benzyl alcohol
|
Benzaldehyde
|
88
|
172.5
|
2
|
4-Methylbenzyl alcohol
|
4-Methylbenzaldehyde
|
90
|
176.4
|
3
|
4-Chlorobenzyl alcohol
|
4-Chlorobenzaldehyde
|
90
|
176.4
|
4
|
2-Chlorobenzyl alcohol
|
2-Chlorobenzaldehyde
|
86
|
168.6
|
5
|
3-Nitrobenzyl alcohol
|
3-Nitrobenzaldehyde
|
85
|
166.6
|
6
|
4-Nitrobenzyl alcohol
|
4-Nitrobenzaldehyde
|
86
|
168.6
|
7
|
1-Octanol
|
1-Octanal
|
70
|
137.2
|
8
|
Cinnamyl alcohol
|
Cinnamaldehyde
|
79
|
154.9
|
aReaction conditions: alcohol (0.25 mmol), TBHP (0.5 mmol), catalyst (0.17 mol%, 0.02 g), H2O at room temperature after 3 h. |
bGC yield based on starting alcohol. |
Table 3
Comparison of this work with other published works
Entry
|
Catalytic system
|
Time (h)
|
Yield (%)
|
1
|
(L 3= (2-C5H4N)CH2N)Cu(OAc)
|
2
|
90 [52]
|
2
|
CuII ( 8-hydroxyquinoline-imine) complexe
|
8
|
90 [53]
|
3
|
N-heterocyclic carbene complex (Cu- NHC@Pyrm-OMS)
|
8
|
97 [54]
|
4
|
Cu(I)-iodide-2,2′ -dipyridylamine (dpa) catalyst
|
24
|
99[55]
|
5
|
CuII (4-bromobenzoate/2,2-dipyridylamine) complex
|
24
|
100[56]
|
3.2.1. Reusability Of Catalyst
Recyclability and reusability are two main advantage for heterogeneous catalyst. These parameters are important from environmental, economic and industrial points of view. Therefore, the reusability of the [Cu(II)Br2-BTP@GO] catalyst was investigated in oxidation of benzyl alcohol with tert-BuOOH under the optimized reaction conditions. After each catalytic run, the catalyst was separated by centrifugation and then washing with H2O and Et2O, and drying in an oven at 60˚C, and then used in the next run. The results are shown in Fig. 8, exhibited that the catalyst could be reused several times in catalytic reactions without noticeable loss of activity. The copper content of the catalyst after fourth run was measured by Atomic Absorption Spectrophotometer (AAS), which showed a value of about 0.075 mmol/g (about 95% of the initial Cu content), for the catalyst was used in the oxidation of benzyl alcohol.