Characterization
In the present work, we have synthesized a simple, green, biodegradable, and economical catalyst, Pd(0)@MnO2-CF. The schematic representation for the synthesis of catalyst is shown in Scheme 1. In the first step, MnO2 and cotton fabric were ultrasonicated to disperse MnO2 on surface of cotton fabric. Then, MnO2 modified cotton fabric was used for the immobilization of Pd(0) nanoparticles. The chemical composition of synthesized catalyst, Pd(0)@MnO2-CF, is determined using various modern characterization techniques such as FTIR, TGA, FEGSEM, EDX, ICPAES, XRD and XPS.
The surface morphology of the catalyst, Pd(0)@MnO2-CF was studied using FEG-SEM (Fig. 1). The SEM micrographs indicate that MnO2 and Pd nanoparticles are uniformly spread throughout the surface of cotton fabric. It is also inferred from SEM micrographs that MnO2 and Pd nanoparticles have spherical morphology. The spherical shape is not liable to agglomeration and hence accounts for increased surface area. Further, to confirm their uniform distribution on cotton support, elemental mapping was studied, which authenticates that both MnO2 and Pd are uniformly immobilized on to the cotton surface.
The elemental composition in Pd(0)@MnO2-CF was determined by energy dispersive X-ray spectroscopy (Fig. 2). The peaks corresponding to C, O, Mn and Pd are present in EDX spectrum. It confirms that MnO2 and Pd are successfully anchored onto the surface of cotton fabric. Furthermore, absence of any additional peak eliminates the possibility of any contamination. The peak intensities also confirm the proper binding of all the elements in accordance to their quantities used. Further, Inductively coupled plasms atomic emission spectrum indicates that 3.14 and 1.21 w/w% of Pd and Mn are loaded onto Pd(0)@MnO2-CF.
The XRD spectra of CF, MnO2-CF and Pd(0)@MnO2-CF are shown in Fig. 3. The XRD spectrum of cotton fabric (CF) showed four well-defined peaks at 2θ = 15˚, 16.6˚, 22.7˚, and 34.5˚, corresponding to the (1\(\stackrel{-}{1}\)0), (110), (200), and (004) planes of the cellulose type I, respectively (Ali et al. 2018). However, because of amorphous nature of MnO2, no peak corresponding to planes of MnO2 was observed in XRD spectrum of MnO2-CF. The XRD pattern of Pd(0)@MnO2-CF exhibits peaks at 40° and 46.1°, which correspond to face centered cubic lattice planes (111) and (200) of Pd(0) (Seyednejhad et al. 2019).
To ascertain the elemental state of Mn and Pd in the catalyst, XPS analysis of the fresh and reused catalyst was carried out. Figure 4 depicts the XPS spectra of fresh Pd(0)@MnO2-CF. The Pd 3d peaks can be deconvoluted into a doublet, and the peaks at binding energy 335.2 and 340.1 eV are assigned to Pd 3d5/2 and Pd 3d3/2 of Pd(0) (Keshipour et al. 2013). In addition, Mn 2p shows peaks at 641.8 and 653.4 eV, which can be ascribed to the Mn 2p3/2 and Mn 2p1/2 of Mn4+ in MnO2 (Wang et al. 2014). Further, to confirm the stability of catalyst, the XPS of Pd(0)@MnO2-CF has been carried out after five catalytic runs (Fig. 4b and 4d). In the XPS of reused catalyst, the peaks for Pd 3d5/2 and Pd 3d3/2 have been observed at 335 and 340 eV while the peaks for Mn 2p3/2 and Mn 2p1/2 are centred at 641.7 and 653.2 eV respectively. This suggests that the bonding environment around the elements has not altered and the catalyst is stable under the reaction conditions.
The thermogravimetric analysis (TGA) curves of CF, MnO2-CF and Pd(0)@MnO2-CF are represented in Fig. 5. TGA curves indicate initial weight loss of around 2% up to 100°C which can be due to evaporation of solvent and water molecules trapped on the surface. Further, up to 250°C, no considerable weight loss has been noticed in the TGA curves of CF, MnO2-CF and Pd(0)@MnO2-CF. This confirms the stability of support as well the catalyst up to this temperature range. After this, a noticeable and significant weight loss has occurred up to 360°C, which is due to depolymerization and degradation of cellulose polymer (Ali et al. 2017 and Ahmad et al. 2016).
The FTIR spectra of CF, MnO2-CF and Pd(0)@MnO2-CF are shown in Fig. 6. The peak at around 3340 cm− 1 corresponds to O-H stretching vibrations (Baruah et al. 2014), whereas peak at 2891 cm− 1 is due to the C–H stretching vibrations. The peaks at 1427 cm− 1, 1384 cm− 1, 1336 cm− 1, 1336 cm− 1, 1053 cm− 1 and 897 cm− 1 are associated with CH2 scissoring, C-H bending, O-H in plane bending, CH2 wagging, C–O–C stretching vibration and cellulosic β-glycosidic linkages respectively (Baruah et al. 2014). No change in the IR bands at 1427 cm− 1 and 897 cm− 1 in case of MnO2-CF suggests that incorporation of MnO2 onto cotton fabric did not modify the original structure of cellulose (Zhou et al. 2011). A decrease in intensity of peak of O-H stretching vibrations (3340 cm− 1 ) in the FTIR spectra of Pd(0)@MnO2-CF as compared to that of CF indicates the formation of Pd(0)–oxygen linkage (Baruah et al. 2014).
Catalyst Testing For Oxidations
In view of immense importance of carbonyl compounds, Pd(0)@MnO2-CF has been utilized for the oxidation of primary and secondary alcohols. To find out the best reaction conditions for oxidation of alcohols, the reaction conditions have been optimized by using 4-bromobenzyl alcohol as a test substrate. To select the best solvent, the oxidation with test substrate using TBHP as an oxidant was performed in different solvents such as ethanol, toluene, acetonitrile, ethanol:water (1:1) and water. It has been observed that the best results were obtained in case of water (entry 11, Table 1). After selecting the solvent, the optimum temperature for the oxidation of alcohols was selected by carrying out oxidation with test substrate at different temperatures viz. room temperature, 40°C, 60°C, 80°C and 100°C (Table 1). The catalyst was found to be inactive at room temperature, but the yield of the corresponding product increased with increase in temperature up to 80°C and after that no further increase in conversion was observed; therefore, 80°C was selected as the optimum temperature (entry 11, Table 1). Further, the model reaction was also investigated for selecting the best oxidant for oxidation of alcohols. For this, the model reaction was performed in the presence of different oxidants such as air, molecular oxygen (O2), TBHP and H2O2 (Table 1). The results indicated that no reaction occurred in the presence of air, while the best yield in minimum time was obtained in case of TBHP (entry 11, Table 1). Thus, the optimum conditions for oxidation of primary and secondary alcohols are: water as solvent, 80°C as optimum temperature and TBHP as an oxidant. Now, in order to examine the role of MnO2, the model reaction was performed with Pd(0)@CF (without MnO2) (entry 3, Table 2). In this case, the yield of the corresponding product was noticeably reduced (58%). This might be due to the fact that MnO2 stabilizes palladium nanoparticles due to its high surface area and thermal stability. Further, with MnO2-CF, no reaction occurred which confirms the role of palladium as a catalyst during the oxidation of alcohols (entry 4, Table 2)
The substrate scope of catalyst was further explored by subjecting various structurally different aromatic alcohols to oxidation under the selected reaction conditions and the results are formulated in Table 3. The oxidation of both primary and secondary alcohols containing electron-releasing and electron-withdrawing groups proceeded smoothly. It is pertinent to mention that formation of over-oxidized product i.e. carboxylic acid was not observed in case of any substrate. Also, good yield of the products was obtained for both primary and secondary alcohols.
Table 1
Optimization of the solvent, oxidant and temperature for Pd(0)@MnO2-CF catalyzed oxidation of alcohols and oxidative deprotection of oximes
Entry
|
Oxidation of alcoholsa
|
Oxidative deprotection of oximesb
|
Solvent
|
Oxidant
|
Temp(°C)
|
Time (min)
|
Yieldc
(%)
|
Solvent
|
Temp (°C)
|
Time (h)
|
Yieldd
(%)
|
1
2
|
Ethanol
Toluene
|
TBHP
TBHP
|
80
80
|
30
30
|
63
61
|
Ethanol
Water
|
80
80
|
3.5
3.5
|
Traces
65
|
3
|
Acetonitrile
|
TBHP
|
80
|
30
|
82
|
Acetonitrile
|
80
|
3.5
|
Traces
|
4
|
Ethanol:Water (1:1)
|
TBHP
|
80
|
30
|
78
|
Ethanol:Water (1:1)
|
80
|
3.5
|
45
|
5
|
Water
|
Air
|
80
|
30
|
NR
|
Toluene
|
RT
|
3.5
|
NR
|
6
|
Water
|
O2
|
80
|
30
|
80
|
Toluene
|
40
|
3.5
|
56
|
7
|
Water
|
H2O2
|
80
|
30
|
40
|
Toluene
|
60
|
3.5
|
79
|
8
|
Water
|
TBHP
|
RT
|
30
|
NR
|
Toluene
|
80
|
3.5
|
88
|
9
|
Water
|
TBHP
|
40
|
30
|
52
|
Toluene
|
100
|
3.5
|
88
|
10
|
Water
|
TBHP
|
60
|
30
|
75
|
|
|
|
|
11
|
Water
|
TBHP
|
80
|
30
|
85
|
|
|
|
|
12
|
Water
|
TBHP
|
100
|
30
|
85
|
|
|
|
|
aReaction conditions: 4-bromobenzyl alcohol (1 mmol, 0.187 g), TBHP (0.5 mmol, 0.045 g), Pd(0)@MnO2-CF (1 cm×1 cm) and solvent (10 mL).
bReaction conditions: 4-bromobenzaldehyde oxime (1 mmol, 0.2 g), TEMPO (0.25 mmol, 0.078 g), Pd(0)@MnO2-CF (1 cm×1 cm) and solvent (10 mL).
cColumn Chromatography yield.
dIsolated yield.
Table 2
Optimization of the catalyst for the oxidation of alcohols and oxidative deprotection of oximes
Entry
|
Catalyst
|
Oxidation of alcoholsa
|
Oxidative deprotection of oximesb
|
Time (min)
|
Yieldc (%)
|
Time (h)
|
Yieldd (%)
|
1
2
|
No catalyst
Pd(0)@MnO2-CF
|
5
30
|
NR
85
|
6
3.5
|
44
88
|
3
|
Pd(0)@CF
|
30
|
62
|
3.5
|
58
|
4
|
MnO2-CF
|
30
|
NR
|
3.5
|
45
|
aReaction conditions: 4-bromobenzyl alcohol (1 mmol, 0.187 g), TBHP (0.5 mmol, 0.045 g), Pd(0)@MnO2-CF (1 cm×1 cm) in water (10 mL) at 80°C.
bReaction conditions: 4-bromobenzaldehyde oxime (1 mmol, 0.2 g), TEMPO (0.25 mmol, 0.078 g), Pd(0)@MnO2-CF (1 cm×1 cm) in toluene (10 mL) at 80°C.
cColumn chromatography yield.
dIsolated yield.
Proposed Mechanism
To ascertain the radical mechanism of oxidation of alcohols, a radical-trapping experiment was performed using (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as radical quencher. The test reaction (entry 3a, Table 3) was performed until the conversion was up to 50% (15 min), then radical quencher i.e. TEMPO (1 mmol, 0.156 g) was added to the reaction mixture and the reaction was continued for another 30 min. No considerable change was observed, which indicates that oxidation of alcohol proceeded via radical intermediates. In the proposed mechanism, initially, Pd(0)@MnO2-CF on reaction with TBHP forms t-butyl peroxide complex (I) and radical t-BuO· (tert-butyl alkoxo radical). Then t-BuO· reacts with TBHP to give another radical t-BuOO· (tertbutyl peroxo radical) which extracts the benzylic hydrogen from the alcohol to give the radical intermediate (II). Then the t-butyl peroxide complex (I) reacts with the radical intermediate II to produce the desired product (III) and the catalyst is regenerated back (Fig. 7).
Catalyst Testing For Oxidative Deprotection Of Oximes
The optimum reaction conditions for the oxidative deprotection of oximes were selected by carrying out the reaction using 4-bromobenzyldehyde oxime in the presence of TEMPO as the test reaction. Screening of solvent, temperature and catalyst was done by performing similar experiments. Among different solvents, the best results were obtained in case of toluene (entry 8, Table 1). Also, results indicated that good yield of product was obtained at 80°C (entry 8, Table 1). In the absence of TEMPO, a significant reduction in the yield was observed, which is suggestive of the fact that TEMPO has an important role in the oxidative deprotection of oximes. After optimizing the reaction conditions, the scope of this catalytic system was extended to various oximes having different functional groups (Table 4). It was found that all the substrates containing electron-donating as well as electron-withdrawing groups gave good yield of the corresponding aldehydes. However, the reaction proceeded slowly for 4-bromoacetophenone oxime compared to other benzaldehyde oximes, indicating higher selectivity of the catalyst for benzaldehyde oximes.
Proposed mechanism for oxidative deprotection of oximes
A plausible reaction pathway for the oxidative deprotection of oximes to form carbonyl compound is represented in Fig. 8. The mechanism begins with the coordination of Pd(0) with the oxime resulting in the formation of complex I. TEMPO then interacts with complex I and abstracts a hydrogen atom from the O-H bond of the oxime to give TEMPOH, which again re-oxidized to TEMPO by aerial oxygen and also forms hydroperoxyl radical, which attacks the imine group of the oxime producing oxaziridine ring as the reaction intermediate. Finally, rearrangement in the oxaziridine intermediate gives the carbonyl compound and the catalyst is regenerated.
Catalyst Testing For Degradation Of Methyl Orange
The catalytic activity of Pd(0)@MnO2-CF was explored for the degradation of methyl orange (MO) in the presence of sodium borohydride. During the reduction reaction, change in the absorbance value at λmax was monitored for the methyl orange (Fig. 9). It has been observed the peak intensity decreased gradually due to degradation of chromophores on stirring with NaBH4 and Pd(0)@MnO2-CF and after 8 minutes, 100% degradation of dye occurred. In general, the electron transfer from the BH4− to MO dye is responsible for the reduction by converting azo group to non-toxic amine group (Chandrasekaran et al. 2020).
Mechanism of dye degradation
The mechanism of reduction of methyl orange can be explained by electron relay effect (Fig. 10). Both BH4− (donor) and methyl orange (acceptor) diffuse from the aqueous solution and are adsorbed on the surface of the catalyst. Then, there occurs electron transfer in which BH4− donates the electrons to methyl orange via Pd(0)@MnO2-CF. The Pd nanoparticles decompose NaBH4 to produce H2 and the generated reactive hydrogens reduces the azo group of methyl orange to imine and then to the amine stage. The reduced dye molecule is then detached from the surface of Pd(0)@ MnO2-CF and finally diffuses out.
To confirm the recyclability of Pd(0)@MnO2-CF, oxidation of 4-bromobenzyl alcohol (3a, Table 3) and oxidative deprotection of 4-bromobenzaldehyde oxime (4a, Table 4) was performed under the selected optimized conditions. After the completion of the reaction, the catalyst was removed from the reaction mixture using lab tweezers and washed with deionised water (3×5 mL), ethyl acetate (3×5 mL) and ethanol (3×5 mL) and dried for 20 minutes at room temperature. The catalyst was again used to carry out the similar reaction using fresh substrates under similar reaction conditions. A minimal drop in catalytic activity of the catalyst was observed up to 5 catalytic runs in case of both the reactions which confirms the heterogeneity and stability of synthesized Pd(0)@MnO2-CF catalyst (Fig. 11).