# Selective Allylic Oxidation of Terpenic Olefins Using Co-Ag Supported on SiO2 as a Novel, Efficient, and Recyclable Catalyst

Authors: Abderrazak Aberkouks; Ayoub Abdelkader Mekkaoui; Mustapha Ait Ali; Larbi El Firdoussi; Soufiane El Houssame
Journal: Journal of Chemistry
 (2020)

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## Abstract

Co-Ag supported on the SiO2 catalyst was synthesized by the sol-gel method and characterized using XRD, FT-IR, TG-DTG, BET, CV, and SEM/EDX analysis. The catalytic performance of the resulting catalyst was examined by the oxidation of mono and sesquiterpenic olefins using hydrogen peroxide and tert-butyl peroxide as oxidant agents. Various parameters such as catalyst amount, temperature, and solvents have been studied. The Co-Ag supported on the SiO2 catalyst showed a high activity, selectivity, and recyclability for the selected oxidation reaction.


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## Body

## 1. Introduction

Solid catalysts are used for converting a vast array of chemicals and fuels, and contribute significantly to economic progress [1, 2]. Recently, the development of solid materials in catalytic oxidation has attracted an economic and ecologic interest because of their easy recovery and the possibility of regeneration which results in a reduction of the environmental impact [3–5]. Moreover, glasses doped with transition metals formed one important class of materials in catalysis field [6]. Among the methods of preparation, sol-gel affords solid materials with high surface area and homogeneity [7]. A high-level incorporation of active metals in the ceramic matrix can be obtained using the sol-gel method than conventional impregnation methods [8].The allylic oxidation of mono and sesquiterpenic natural olefins, cheap constituents of essential oil extracted from plants [9], constitutes a great route for the synthesis of a wide range of oxygenated products with high added values such as aldehydes, alcohols, ketones, esters, ethers, and phenols [10]. The oxygenated compounds are used in perfumery, aromatherapy, flavoring industry, fine chemical synthesis, food industry, and therapeutic drug [11, 12]. In addition, verbenone is a precursor for the synthesis of taxol used in the treatment of ovarian cancer [13]. The nootkatone, a much popular product replacing the orange aroma [14], was synthesized from β-pinene [15].The allylic oxidation of terpenic olefins catalyzed by palladium complexes was the subject to various studies [16, 17], and the development of eco-friendly process using eco-compatible reagents and/or metal catalysts in green solvents remained a strong challenge [18]. However, various catalytic systems using molecular complexes [19, 20] or heterogeneous catalysts [21–23] were limited by the use of toxic organic solvents, high amount of oxidants, long reaction time, and difficult catalyst’s recovery.However, in recent years, bimetallic nanoparticles as catalysts instead of monometallic ones are of a great interest from both scientific and technological point of view. In bimetallic catalysts, the electronic effect plays an important role which describes the charge transfer and alloying of the constituting elements can result in the structural changes of the bimetallic nanoparticles. Therefore, bimetallic nanoparticles may be more effective, more stable, and more selective heterogeneous catalysts due to the synergistic effects occurring between the two different metals [24–27].Moreover, recently, we have developed Co-Ag codoped ZnO as heterogeneous coordination catalysts for selective oxidation of styrene. Among Zn1−x−yAgxCoyO prepared nanocatalysts, the monometallic ones (Ag-doped ZnO and Co-doped ZnO) are much less efficient [28].As part of our ongoing research on the development of new heterogeneous nanocatalytic systems [29] as well as the allylic functionalization of natural terpenes [28, 30], herein we report the preparation of 1 mol% Co-1 mol% Ag supported on SiO2 by the sol-gel method as a novel catalyst in the oxidation reaction. The physiochemical properties of the prepared catalyst were examined by XRD, FT-IR, TG-DTG, BET, and SEM/EDX analysis. It showed a good activity and recyclability for the oxidation of terpenic olefins to their oxygenated derivatives using hydrogen peroxide and tert-butyl peroxide as oxidant agents.


## 2. Experimental Details

### 2.1. Materials

Tetraethyl orthosilicate (TEOS 99.99%), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), silver nitrate (Ag(NO3)), nitric acid (HNO3 67%), (+)−α-pinene (≥99%), (+)-β-pinene (98%), (R)-(+)-limonene (97%), (+)-valencene (≥70%), hydrogen peroxide (30 wt%), tert-butyl hydroperoxide (70%), acetonitrile, hexane, ethyl acetate, acetone, chloroform, ethanol, and methanol were purchased from Sigma-Aldrich (of chemical reagent grade) and used as received.


### 2.2. Characterization

Thermogravimetric analysis (TGA) was recorded on a TA Instrument Q500 apparatus in the flowing air at a heating rate of 10°C min−1. BET-specific surface areas and average pore diameter of the prepared catalyst were measured by N2 adsorption-desorption technique using the Micromeritics analyser P/N 05098-2.0 Rev A. The stretching vibration frequencies of the catalyst were recorded by FT-IR spectroscopy in the range of 400–4000 cm–1 using a Bruker vertex70 DTGS. Spectrometer XRD measurements were performed on a XPERT-MPD Philips diffractometer using Cu-K α radiation as the X-ray source in the 2θ range of 20°–80°. The size, morphology, and elemental mapping of the microstructures were carried out on VEGA3 TESCAN microscope equipped with an energy dispersive X-ray spectrometer (EDAX TEAM). The cyclic voltammetry (CV) study was performed between 1500 mV/−500 mV and 300 mV/−200 mV with a carbon paste electrode modified by our prepared catalyst in a 0.1 M HCl electrolyte at a scan rate of 100 mV·s−1 at room temperature.


### 2.3. Catalyst Preparation

A detailed preparation of SiO2 by the sol-gel method is described in the literature [31–33]. The preparation was carried out as follows: a mixture of 10.4 g of tetraethyl orthosilicate (TEOS), 4.6 g of ethanol, and 4.6 g of 0.15 M nitric acid was stirred for 1 hour at room temperature. The resulting clear solution was combined with needed amount of Co(NO3)2·6H2O and Ag(NO3) dissolved in 4.6 g of ethanol, stirred for 1 h at room temperature, and dried at 80°C for 24 hours. The dried gel was ground into fine powders and calcined at 600°C for 5 h under the air to obtain nanopowders of 1 mol% Co-1 mol% Ag supported on SiO2.


### 2.4. Catalytic Activity

A 50 mL rotaflow tube equipped with magnetic stirring and immersed in bath oil at 80°C was used to mix 0.5 mL of the substrate (terpenic olefin), 0.1 g of nanocatalyst (Co-Ag supported SiO2), and 5 mL of acetonitrile. Then, 1.3 mL of H2O2 (30% wt) was added, and the resulting solution was stirred at 80°C. The reaction process was monitored using gas chromatography (GC) equipped with a FID using an Rtx-5 capillary column. Dodecane was used as an internal standard for the quantitative analysis of the reaction products. After completion of the reaction, 5 mL of distilled water was added to the residue and extracted with ethyl acetate (3 × 10 ml). The combined organic phases were dried over MgSO4 and evaporated under reduced pressure. The catalyst was separated from the reaction mixture, washed, dried, and then reused for further use.


## 2.1. Materials

Tetraethyl orthosilicate (TEOS 99.99%), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), silver nitrate (Ag(NO3)), nitric acid (HNO3 67%), (+)−α-pinene (≥99%), (+)-β-pinene (98%), (R)-(+)-limonene (97%), (+)-valencene (≥70%), hydrogen peroxide (30 wt%), tert-butyl hydroperoxide (70%), acetonitrile, hexane, ethyl acetate, acetone, chloroform, ethanol, and methanol were purchased from Sigma-Aldrich (of chemical reagent grade) and used as received.


## 2.2. Characterization

Thermogravimetric analysis (TGA) was recorded on a TA Instrument Q500 apparatus in the flowing air at a heating rate of 10°C min−1. BET-specific surface areas and average pore diameter of the prepared catalyst were measured by N2 adsorption-desorption technique using the Micromeritics analyser P/N 05098-2.0 Rev A. The stretching vibration frequencies of the catalyst were recorded by FT-IR spectroscopy in the range of 400–4000 cm–1 using a Bruker vertex70 DTGS. Spectrometer XRD measurements were performed on a XPERT-MPD Philips diffractometer using Cu-K α radiation as the X-ray source in the 2θ range of 20°–80°. The size, morphology, and elemental mapping of the microstructures were carried out on VEGA3 TESCAN microscope equipped with an energy dispersive X-ray spectrometer (EDAX TEAM). The cyclic voltammetry (CV) study was performed between 1500 mV/−500 mV and 300 mV/−200 mV with a carbon paste electrode modified by our prepared catalyst in a 0.1 M HCl electrolyte at a scan rate of 100 mV·s−1 at room temperature.


## 2.3. Catalyst Preparation

A detailed preparation of SiO2 by the sol-gel method is described in the literature [31–33]. The preparation was carried out as follows: a mixture of 10.4 g of tetraethyl orthosilicate (TEOS), 4.6 g of ethanol, and 4.6 g of 0.15 M nitric acid was stirred for 1 hour at room temperature. The resulting clear solution was combined with needed amount of Co(NO3)2·6H2O and Ag(NO3) dissolved in 4.6 g of ethanol, stirred for 1 h at room temperature, and dried at 80°C for 24 hours. The dried gel was ground into fine powders and calcined at 600°C for 5 h under the air to obtain nanopowders of 1 mol% Co-1 mol% Ag supported on SiO2.


## 2.4. Catalytic Activity

A 50 mL rotaflow tube equipped with magnetic stirring and immersed in bath oil at 80°C was used to mix 0.5 mL of the substrate (terpenic olefin), 0.1 g of nanocatalyst (Co-Ag supported SiO2), and 5 mL of acetonitrile. Then, 1.3 mL of H2O2 (30% wt) was added, and the resulting solution was stirred at 80°C. The reaction process was monitored using gas chromatography (GC) equipped with a FID using an Rtx-5 capillary column. Dodecane was used as an internal standard for the quantitative analysis of the reaction products. After completion of the reaction, 5 mL of distilled water was added to the residue and extracted with ethyl acetate (3 × 10 ml). The combined organic phases were dried over MgSO4 and evaporated under reduced pressure. The catalyst was separated from the reaction mixture, washed, dried, and then reused for further use.


## 3. Results and Discussion

### 3.1. Catalyst Characterization

The XRD pattern generated by 1 mol% Co-1 mol% Ag supported SiO2 is shown in Figure 1. The wide peaks between 18° and 40° associated with the crystalline phase are not revealed as it was intended for an amorphous structure [34–36]. Based on the XRD study, the amount of the doped metal does not affect the amorphous structure of silica.Figure 1
XRD patterns of fresh and recycled 1 mol% Co-1 mol% Ag supported SiO2.Thermogravimetric analysis of the Co-Ag/SiO2 nanocatalyst studied the loss of mass as a function of temperature. The TG-DTG curves of the precursor obtained by the sol-gel method revealed a thermal event (Figure 2). The peak located at 150°C corresponds to a loss of crystal water from the catalyst (3.5%).Figure 2
TG-DTG curve of 1 mol% Co-1 mol% Ag supported SiO2.The BET (Brunauer-Emmett-Teller) analysis of 1 mol% Co-1 mol% Ag supported on the SiO2 catalyst was performed. The specific surface area of the catalyst was determined to be 306.18 m2/g, and the calculated BJH pore size was 7.87 nm. The nitrogen adsorption/desorption isotherm diagram and the pore size distribution diagram are shown in Figures 3 and 4. The sample corresponds to a type IV isotherm and hysteresis type H1 in the Brunauer classification. This type of diagram has been attributed to the predominance of mesopores, and the pore sizes are in the range between 2 and 14 nm.Figure 3
N adsorption/desorption isotherms of 1 mol% Co-1 mol% Ag supported SiO2.Figure 4
Distribution of the pore diameter of 1 mol% Co-1 mol% Ag supported SiO2.Structural characterization of the nanocatalyst 1 mol% Co-1 mol% Ag supported on SiO2 was also performed using FT-IR data, based on the spectrum shown in Figure 5. The band at 460 cm−1 corresponds to the bending mode of the Si-O-Si bonds. The peaks at 806 cm−1 and 1080 cm−1 bands are, respectively, attributed to the symmetric and asymmetric stretching modes of these links [37, 38]. The bands at 960 cm−1 and 1650 cm−1 are assigned to the Si-OH groups and the physisorbed water in the sample [39], while the broad and intense at 3450 cm−1 are attributed to the presence of OH groups in the silica framework [40].Figure 5
FT-IR spectrum of 1 mol% Co-1 mol% Ag supported SiO2.The scanning electron microscopy (SEM) of 1 mol% Co-1 mol% Ag supported on the SiO2 catalyst and pure SiO2 are shown in Figure 6. The amount of silver and cobalt codoped in SiO2 do not change the morphology compared to its support. EDX results confirm that codoped and undoped SiO2 nanopowders consist of elements of silicon, cobalt, and silver (Figure 6) in good agreement with the literature [41, 42] and providing further evidence for the successful synthesis of our nanocatalyst.Figure 6
SEM images of (a) pure SiO2 and (b) 1 mol% Co-1 mol% Ag supported SiO2.
(a)
(b)Element distribution in the hybrid has been carried out for 1 mol% Co-1 mol% Ag supported SiO2 (Figure 7). All the elements representing our prepared catalyst were studied and showed a good distribution. For cobalt and silver, the studied area shows a good dispersion and distribution in a nanoscale range.Figure 7
EDX elemental mapping analysis of 1 mol% Co-1 mol% Ag supported SiO2: (a) the studied area and the distribution of (b) all elements, (c) oxygen, (d) silicon, (e) silver, and (f) cobalt.
(a)
(b)
(c)
(d)
(e)
(f)In order to study the metal predominant oxidation state of supported catalyst, cyclic voltammetry (CV) experiments represent a good alternative [29]. CV was performed for both the 1 mol% Co-1 mol% Ag supported SiO2 prepared nanocatalyst and the monometallic supported silica Co/SiO2 and Ag/SiO2 (Figure 8). The study of cobalt and silver electrodes presents two specific zones of surface oxide formation and reduction. The monometallic Ag/SiO2 redox voltammetric profile presents an anodic peak at 0.13 V (Ag0-->Ag+) and a cathodic peak at −0.04 V (Ag+-->Ag0), while the monometallic Co/SiO2 shows the characteristic redox peaks of Co3O4 [43]. Moreover, the prepared 1 mol% Co-1 mol% Ag supported SiO2 (Co-Ag/SiO2) shows, in addition to the characteristic redox voltammetric profile of Co3O4, an anodic peak at 0.13 V with no obvious cathodic peak which confirms the presence of metallic silver (Ag0). Therefore, the CV study indicates the presence of cobalt and silver under the Co3O4 and Ag0 oxidation state, respectively.Figure 8
CV curves of carbon paste electrode modified with Co/SiO2, Co-Ag/SiO2 (performed between 1500 mV and 500 mV) and Ag/SiO2 (performed between 300 mV/−200 mV) at a scan rate of 100 mV·s−1.


### 3.2. Catalytic Application

The catalytic performance of the Co-Ag supported SiO2 was examined by assessing the oxidation of α-pinene 1 chosen as a model substrate (Scheme 1). The reaction was optimized by varying the catalyst amount, reaction time, effect of temperature, and the nature of solvent.Scheme 1
Catalytic oxidation ofα-pinene 1 using Co-Ag supported SiO2.


#### 3.2.1. Effect of Catalyst Amount

The catalytic oxidation ofα-pinene 1 using hydrogen peroxide was performed by varying the amount (0.02 g to 0.15 g) of 1 mol% Co-1 mol% Ag supported SiO2 (Table 1). The conversion of α-pinene 1 resulted in the increase of the catalyst amount. The best result was obtained with the 0.1 g nanocatalyst, and a conversion of 82% and a yield of 55% for both verbenol and verbenone are reached. In the absence of the catalyst, no reaction took place even after stirring for a long reaction time.Table 1
Effect of catalyst amount.
Amount of catalyst (g)
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
0.02
16
Trace
Trace
5.7
9
Trace
0.05
78.5
Trace
36.6
27.4
Trace
Trace
0.1
82
Trace
27
28
10
Trace
0.15
98
Trace
13
41
17
11
Reaction conditions: 0.5 mLα-pinene and 1.3 mL H2O2; 60°C; acetonitrile (5 ml); 24 h. Yields are determined by GC using dodecane as internal standard.


#### 3.2.2. Effect of the Reaction Time

In order to shed more light on this point, a kinetic study was carried out with 1 mol% Co-1 mol% Ag supported SiO2 (Table 2). Based on these results, the evolution of α-pinene 1 versus time shows that verbenone was formed as a major product. A total conversion of α-pinene 1 was reached after 24 hours with 50% of verbenone and 20% of myrtenal yields, respectively.Table 2
Effect of reaction time.
Time (h)
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
6
42
7
11
24
0
0
12
48
12
11
25
0
0
20
78
14
11
42
4
2
24
100
2
12
50
7
20
Reaction conditions: 0.5 mLα-pinene 1, 1.3 mL H2O2; 0.1 g catalyst; 80°C; acetonitrile (5 ml); 24 h. Yields are determined by GC using dodecane as internal standard.


#### 3.2.3. Effect of Reaction Temperature

Table3 shows the influence of the temperature on the catalytic oxidation of α-pinene 1. The increase of the temperature to 120°C resulted in the increase of the conversion and the selectivity to verbenone decreases due to the formation of isomerization products and the opening of the cyclobutane ring. This confirms that the cleavage of the C=C bond is greater at lower temperatures and the opening of the cyclobutane ring and isomerization reactions is more favorable than the C=C cleavage at high temperatures [16].Table 3
Effect of reaction temperature.
Temperature (°C)
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
R.T
0
0
0
0
0
0
60
80
4
20
29
3
4
80
100
2
12
50
7
20
100
95
4
7
33
6
8
120
96
4
12
44
2
5
Reaction conditions: 0.5 mLα-pinene, 1.3 mL H2O2; catalyst 0.1 g; 24 h; acetonitrile (5 ml). Yields are determined by GC using dodecane as internal standard.


#### 3.2.4. Effect of the Solvent

In order to evaluate the catalytic performance of Co-Ag supported SiO2, an examination of the effect of various protic and aprotic solvents in the presence of hydrogen peroxide (30 wt%) has been carried out (Table 4).Table 4
Effect of solvent.
Solvent
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
Free solvent
100
7
16
30
6
Trace
Ethanol
100
5
31
37
6
2
Methanol
94
15
59
13
Trace
2
Chloroforme
40
11
25
5
Trace
Trace
Acetone
98
7
6
46
3
14
Acetonitrile
100
2
12
50
7
20
Reaction conditions: 0.5 mLα-pinene, 1.3 mL H2O2; catalyst 0.1 g; 80°C; 24 h. Yields are determined by GC using dodecane as internal standard.According to Table4, polar and aprotic solvents (acetonitrile and acetone) are more active and selective towards verbenone. In the presence of polar and protic solvents (methanol and ethanol), the selectivity increases towards verbenol while in the absence of solvent, the α-pinene 1 is completely converted, and a low yield of verbenone (30%) was obtained with the formation of isomerization products and the opening cyclobutane ring. Among the solvents used, acetonitrile seems to be the best solvent.


#### 3.2.5. Co-Ag/SiO2 Catalytic Performance

Under the optimized conditions, catalytic oxidation of various terpenic olefins has been carried out (Table5). As shown in Table 5, limonene was converted to the corresponding allylic ketone, carvone as the major compound (entry 1). However, β-pinene leads to the formation of myrtenal as a major product (entry 2). Performing the reaction with valencene resulted in a total conversion and gave, mainly, nootkatone in the 80% yield (entry 3).Table 5
Catalytic oxidation of terpenic olefins.
Entry
Substrate
Conv. (%)
Yield (%)
1
56
2
95
3
100
Reaction conditions: 0.5 mL substrate, 1.5 mL H2O2; catalyst 0.1 g; 80°C; acetonitrile; 24 h. Yields are determined by GC using dodecane as internal standard.


#### 3.2.6. Effect of TBHP

The catalytic oxidation of various terpenic olefins usingtert-butyl hydroperoxide (TBHP 70%), a nonpolluting and economic oxidant agent [5, 18], has been carried out (Table 6). All the terpenic olefins show good activities, and the corresponding ketones were obtained in a moderate-to-good yield. As shown in Table 6, it appeared that limonene and β-pinene (entries 5 and 6) were less reactive compared to α-pinene and valencene (entries 4 and 7).Table 6
Effect of TBHP.
Entry
Substrate
Conv. (%)
Yield (%)
4
98
5
72
6
72
7
100
Reaction conditions: 0.5 mL substrate, 1.5 mL TBHP; catalyst 0.1 g; 80°C; acetonitrile (5 ml); 24 h. Yields are determined by GC using dodecane as internal standard.


#### 3.2.7. Recycling of Catalyst

To investigate the reusability of the 1 mol% Co-1 mol% Ag supported SiO2 nanocatalyst, the catalytic performance was evaluated in four consecutive cycles (Figure 9). After each cycle, the catalyst was separated from the reaction mixture by simple filtration, washed with water, and dried at 200°C for 2 hours while being reused. According to Figure 9, the catalyst was stable after four cycles. Investigation of XRD analysis shows that the identity of the recovered Co-Ag/SiO2 remains similar to the fresh nanocatalyst (Figure 1). However, SEM morphology of the nanocatalyst after four cycles showed some agglomerated particles compared to the fresh one (Figure 10). Moreover, EDX elemental mapping of recycled Co-Ag/SiO2 indicates no changes of dispersion and distribution of all elements compared to the fresh nanocatalyst (Figure 11).Figure 9
Oxidation ofα-pinene during recyclability of 1 mol% Co-1 mol% Ag supported SiO2.Figure 10
SEM images of (a) fresh and (b) recycled 1 mol% Co-1 mol% Ag supported SiO2.
(a)
(b)Figure 11
EDX elemental mapping analysis of recycled 1 mol% Co-1 mol% Ag supported SiO2 after four cycles: (a) the studied area and the distribution of (b) all elements, (c) oxygen, (d) silicon, (e) silver, and (f) cobalt.
(a)
(b)
(c)
(d)
(e)
(f)


## 3.1. Catalyst Characterization

The XRD pattern generated by 1 mol% Co-1 mol% Ag supported SiO2 is shown in Figure 1. The wide peaks between 18° and 40° associated with the crystalline phase are not revealed as it was intended for an amorphous structure [34–36]. Based on the XRD study, the amount of the doped metal does not affect the amorphous structure of silica.Figure 1
XRD patterns of fresh and recycled 1 mol% Co-1 mol% Ag supported SiO2.Thermogravimetric analysis of the Co-Ag/SiO2 nanocatalyst studied the loss of mass as a function of temperature. The TG-DTG curves of the precursor obtained by the sol-gel method revealed a thermal event (Figure 2). The peak located at 150°C corresponds to a loss of crystal water from the catalyst (3.5%).Figure 2
TG-DTG curve of 1 mol% Co-1 mol% Ag supported SiO2.The BET (Brunauer-Emmett-Teller) analysis of 1 mol% Co-1 mol% Ag supported on the SiO2 catalyst was performed. The specific surface area of the catalyst was determined to be 306.18 m2/g, and the calculated BJH pore size was 7.87 nm. The nitrogen adsorption/desorption isotherm diagram and the pore size distribution diagram are shown in Figures 3 and 4. The sample corresponds to a type IV isotherm and hysteresis type H1 in the Brunauer classification. This type of diagram has been attributed to the predominance of mesopores, and the pore sizes are in the range between 2 and 14 nm.Figure 3
N adsorption/desorption isotherms of 1 mol% Co-1 mol% Ag supported SiO2.Figure 4
Distribution of the pore diameter of 1 mol% Co-1 mol% Ag supported SiO2.Structural characterization of the nanocatalyst 1 mol% Co-1 mol% Ag supported on SiO2 was also performed using FT-IR data, based on the spectrum shown in Figure 5. The band at 460 cm−1 corresponds to the bending mode of the Si-O-Si bonds. The peaks at 806 cm−1 and 1080 cm−1 bands are, respectively, attributed to the symmetric and asymmetric stretching modes of these links [37, 38]. The bands at 960 cm−1 and 1650 cm−1 are assigned to the Si-OH groups and the physisorbed water in the sample [39], while the broad and intense at 3450 cm−1 are attributed to the presence of OH groups in the silica framework [40].Figure 5
FT-IR spectrum of 1 mol% Co-1 mol% Ag supported SiO2.The scanning electron microscopy (SEM) of 1 mol% Co-1 mol% Ag supported on the SiO2 catalyst and pure SiO2 are shown in Figure 6. The amount of silver and cobalt codoped in SiO2 do not change the morphology compared to its support. EDX results confirm that codoped and undoped SiO2 nanopowders consist of elements of silicon, cobalt, and silver (Figure 6) in good agreement with the literature [41, 42] and providing further evidence for the successful synthesis of our nanocatalyst.Figure 6
SEM images of (a) pure SiO2 and (b) 1 mol% Co-1 mol% Ag supported SiO2.
(a)
(b)Element distribution in the hybrid has been carried out for 1 mol% Co-1 mol% Ag supported SiO2 (Figure 7). All the elements representing our prepared catalyst were studied and showed a good distribution. For cobalt and silver, the studied area shows a good dispersion and distribution in a nanoscale range.Figure 7
EDX elemental mapping analysis of 1 mol% Co-1 mol% Ag supported SiO2: (a) the studied area and the distribution of (b) all elements, (c) oxygen, (d) silicon, (e) silver, and (f) cobalt.
(a)
(b)
(c)
(d)
(e)
(f)In order to study the metal predominant oxidation state of supported catalyst, cyclic voltammetry (CV) experiments represent a good alternative [29]. CV was performed for both the 1 mol% Co-1 mol% Ag supported SiO2 prepared nanocatalyst and the monometallic supported silica Co/SiO2 and Ag/SiO2 (Figure 8). The study of cobalt and silver electrodes presents two specific zones of surface oxide formation and reduction. The monometallic Ag/SiO2 redox voltammetric profile presents an anodic peak at 0.13 V (Ag0-->Ag+) and a cathodic peak at −0.04 V (Ag+-->Ag0), while the monometallic Co/SiO2 shows the characteristic redox peaks of Co3O4 [43]. Moreover, the prepared 1 mol% Co-1 mol% Ag supported SiO2 (Co-Ag/SiO2) shows, in addition to the characteristic redox voltammetric profile of Co3O4, an anodic peak at 0.13 V with no obvious cathodic peak which confirms the presence of metallic silver (Ag0). Therefore, the CV study indicates the presence of cobalt and silver under the Co3O4 and Ag0 oxidation state, respectively.Figure 8
CV curves of carbon paste electrode modified with Co/SiO2, Co-Ag/SiO2 (performed between 1500 mV and 500 mV) and Ag/SiO2 (performed between 300 mV/−200 mV) at a scan rate of 100 mV·s−1.


## 3.2. Catalytic Application

The catalytic performance of the Co-Ag supported SiO2 was examined by assessing the oxidation of α-pinene 1 chosen as a model substrate (Scheme 1). The reaction was optimized by varying the catalyst amount, reaction time, effect of temperature, and the nature of solvent.Scheme 1
Catalytic oxidation ofα-pinene 1 using Co-Ag supported SiO2.


### 3.2.1. Effect of Catalyst Amount

The catalytic oxidation ofα-pinene 1 using hydrogen peroxide was performed by varying the amount (0.02 g to 0.15 g) of 1 mol% Co-1 mol% Ag supported SiO2 (Table 1). The conversion of α-pinene 1 resulted in the increase of the catalyst amount. The best result was obtained with the 0.1 g nanocatalyst, and a conversion of 82% and a yield of 55% for both verbenol and verbenone are reached. In the absence of the catalyst, no reaction took place even after stirring for a long reaction time.Table 1
Effect of catalyst amount.
Amount of catalyst (g)
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
0.02
16
Trace
Trace
5.7
9
Trace
0.05
78.5
Trace
36.6
27.4
Trace
Trace
0.1
82
Trace
27
28
10
Trace
0.15
98
Trace
13
41
17
11
Reaction conditions: 0.5 mLα-pinene and 1.3 mL H2O2; 60°C; acetonitrile (5 ml); 24 h. Yields are determined by GC using dodecane as internal standard.


### 3.2.2. Effect of the Reaction Time

In order to shed more light on this point, a kinetic study was carried out with 1 mol% Co-1 mol% Ag supported SiO2 (Table 2). Based on these results, the evolution of α-pinene 1 versus time shows that verbenone was formed as a major product. A total conversion of α-pinene 1 was reached after 24 hours with 50% of verbenone and 20% of myrtenal yields, respectively.Table 2
Effect of reaction time.
Time (h)
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
6
42
7
11
24
0
0
12
48
12
11
25
0
0
20
78
14
11
42
4
2
24
100
2
12
50
7
20
Reaction conditions: 0.5 mLα-pinene 1, 1.3 mL H2O2; 0.1 g catalyst; 80°C; acetonitrile (5 ml); 24 h. Yields are determined by GC using dodecane as internal standard.


### 3.2.3. Effect of Reaction Temperature

Table3 shows the influence of the temperature on the catalytic oxidation of α-pinene 1. The increase of the temperature to 120°C resulted in the increase of the conversion and the selectivity to verbenone decreases due to the formation of isomerization products and the opening of the cyclobutane ring. This confirms that the cleavage of the C=C bond is greater at lower temperatures and the opening of the cyclobutane ring and isomerization reactions is more favorable than the C=C cleavage at high temperatures [16].Table 3
Effect of reaction temperature.
Temperature (°C)
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
R.T
0
0
0
0
0
0
60
80
4
20
29
3
4
80
100
2
12
50
7
20
100
95
4
7
33
6
8
120
96
4
12
44
2
5
Reaction conditions: 0.5 mLα-pinene, 1.3 mL H2O2; catalyst 0.1 g; 24 h; acetonitrile (5 ml). Yields are determined by GC using dodecane as internal standard.


### 3.2.4. Effect of the Solvent

In order to evaluate the catalytic performance of Co-Ag supported SiO2, an examination of the effect of various protic and aprotic solvents in the presence of hydrogen peroxide (30 wt%) has been carried out (Table 4).Table 4
Effect of solvent.
Solvent
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
Free solvent
100
7
16
30
6
Trace
Ethanol
100
5
31
37
6
2
Methanol
94
15
59
13
Trace
2
Chloroforme
40
11
25
5
Trace
Trace
Acetone
98
7
6
46
3
14
Acetonitrile
100
2
12
50
7
20
Reaction conditions: 0.5 mLα-pinene, 1.3 mL H2O2; catalyst 0.1 g; 80°C; 24 h. Yields are determined by GC using dodecane as internal standard.According to Table4, polar and aprotic solvents (acetonitrile and acetone) are more active and selective towards verbenone. In the presence of polar and protic solvents (methanol and ethanol), the selectivity increases towards verbenol while in the absence of solvent, the α-pinene 1 is completely converted, and a low yield of verbenone (30%) was obtained with the formation of isomerization products and the opening cyclobutane ring. Among the solvents used, acetonitrile seems to be the best solvent.


### 3.2.5. Co-Ag/SiO2 Catalytic Performance

Under the optimized conditions, catalytic oxidation of various terpenic olefins has been carried out (Table5). As shown in Table 5, limonene was converted to the corresponding allylic ketone, carvone as the major compound (entry 1). However, β-pinene leads to the formation of myrtenal as a major product (entry 2). Performing the reaction with valencene resulted in a total conversion and gave, mainly, nootkatone in the 80% yield (entry 3).Table 5
Catalytic oxidation of terpenic olefins.
Entry
Substrate
Conv. (%)
Yield (%)
1
56
2
95
3
100
Reaction conditions: 0.5 mL substrate, 1.5 mL H2O2; catalyst 0.1 g; 80°C; acetonitrile; 24 h. Yields are determined by GC using dodecane as internal standard.


### 3.2.6. Effect of TBHP

The catalytic oxidation of various terpenic olefins usingtert-butyl hydroperoxide (TBHP 70%), a nonpolluting and economic oxidant agent [5, 18], has been carried out (Table 6). All the terpenic olefins show good activities, and the corresponding ketones were obtained in a moderate-to-good yield. As shown in Table 6, it appeared that limonene and β-pinene (entries 5 and 6) were less reactive compared to α-pinene and valencene (entries 4 and 7).Table 6
Effect of TBHP.
Entry
Substrate
Conv. (%)
Yield (%)
4
98
5
72
6
72
7
100
Reaction conditions: 0.5 mL substrate, 1.5 mL TBHP; catalyst 0.1 g; 80°C; acetonitrile (5 ml); 24 h. Yields are determined by GC using dodecane as internal standard.


### 3.2.7. Recycling of Catalyst

To investigate the reusability of the 1 mol% Co-1 mol% Ag supported SiO2 nanocatalyst, the catalytic performance was evaluated in four consecutive cycles (Figure 9). After each cycle, the catalyst was separated from the reaction mixture by simple filtration, washed with water, and dried at 200°C for 2 hours while being reused. According to Figure 9, the catalyst was stable after four cycles. Investigation of XRD analysis shows that the identity of the recovered Co-Ag/SiO2 remains similar to the fresh nanocatalyst (Figure 1). However, SEM morphology of the nanocatalyst after four cycles showed some agglomerated particles compared to the fresh one (Figure 10). Moreover, EDX elemental mapping of recycled Co-Ag/SiO2 indicates no changes of dispersion and distribution of all elements compared to the fresh nanocatalyst (Figure 11).Figure 9
Oxidation ofα-pinene during recyclability of 1 mol% Co-1 mol% Ag supported SiO2.Figure 10
SEM images of (a) fresh and (b) recycled 1 mol% Co-1 mol% Ag supported SiO2.
(a)
(b)Figure 11
EDX elemental mapping analysis of recycled 1 mol% Co-1 mol% Ag supported SiO2 after four cycles: (a) the studied area and the distribution of (b) all elements, (c) oxygen, (d) silicon, (e) silver, and (f) cobalt.
(a)
(b)
(c)
(d)
(e)
(f)


## 3.2.1. Effect of Catalyst Amount

The catalytic oxidation ofα-pinene 1 using hydrogen peroxide was performed by varying the amount (0.02 g to 0.15 g) of 1 mol% Co-1 mol% Ag supported SiO2 (Table 1). The conversion of α-pinene 1 resulted in the increase of the catalyst amount. The best result was obtained with the 0.1 g nanocatalyst, and a conversion of 82% and a yield of 55% for both verbenol and verbenone are reached. In the absence of the catalyst, no reaction took place even after stirring for a long reaction time.Table 1
Effect of catalyst amount.
Amount of catalyst (g)
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
0.02
16
Trace
Trace
5.7
9
Trace
0.05
78.5
Trace
36.6
27.4
Trace
Trace
0.1
82
Trace
27
28
10
Trace
0.15
98
Trace
13
41
17
11
Reaction conditions: 0.5 mLα-pinene and 1.3 mL H2O2; 60°C; acetonitrile (5 ml); 24 h. Yields are determined by GC using dodecane as internal standard.


## 3.2.2. Effect of the Reaction Time

In order to shed more light on this point, a kinetic study was carried out with 1 mol% Co-1 mol% Ag supported SiO2 (Table 2). Based on these results, the evolution of α-pinene 1 versus time shows that verbenone was formed as a major product. A total conversion of α-pinene 1 was reached after 24 hours with 50% of verbenone and 20% of myrtenal yields, respectively.Table 2
Effect of reaction time.
Time (h)
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
6
42
7
11
24
0
0
12
48
12
11
25
0
0
20
78
14
11
42
4
2
24
100
2
12
50
7
20
Reaction conditions: 0.5 mLα-pinene 1, 1.3 mL H2O2; 0.1 g catalyst; 80°C; acetonitrile (5 ml); 24 h. Yields are determined by GC using dodecane as internal standard.


## 3.2.3. Effect of Reaction Temperature

Table3 shows the influence of the temperature on the catalytic oxidation of α-pinene 1. The increase of the temperature to 120°C resulted in the increase of the conversion and the selectivity to verbenone decreases due to the formation of isomerization products and the opening of the cyclobutane ring. This confirms that the cleavage of the C=C bond is greater at lower temperatures and the opening of the cyclobutane ring and isomerization reactions is more favorable than the C=C cleavage at high temperatures [16].Table 3
Effect of reaction temperature.
Temperature (°C)
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
R.T
0
0
0
0
0
0
60
80
4
20
29
3
4
80
100
2
12
50
7
20
100
95
4
7
33
6
8
120
96
4
12
44
2
5
Reaction conditions: 0.5 mLα-pinene, 1.3 mL H2O2; catalyst 0.1 g; 24 h; acetonitrile (5 ml). Yields are determined by GC using dodecane as internal standard.


## 3.2.4. Effect of the Solvent

In order to evaluate the catalytic performance of Co-Ag supported SiO2, an examination of the effect of various protic and aprotic solvents in the presence of hydrogen peroxide (30 wt%) has been carried out (Table 4).Table 4
Effect of solvent.
Solvent
Conversion (%)
Yield (%)
Epoxide2
Verbenol3
Verbenone4
Myrtenol5
Myrtenal6
Free solvent
100
7
16
30
6
Trace
Ethanol
100
5
31
37
6
2
Methanol
94
15
59
13
Trace
2
Chloroforme
40
11
25
5
Trace
Trace
Acetone
98
7
6
46
3
14
Acetonitrile
100
2
12
50
7
20
Reaction conditions: 0.5 mLα-pinene, 1.3 mL H2O2; catalyst 0.1 g; 80°C; 24 h. Yields are determined by GC using dodecane as internal standard.According to Table4, polar and aprotic solvents (acetonitrile and acetone) are more active and selective towards verbenone. In the presence of polar and protic solvents (methanol and ethanol), the selectivity increases towards verbenol while in the absence of solvent, the α-pinene 1 is completely converted, and a low yield of verbenone (30%) was obtained with the formation of isomerization products and the opening cyclobutane ring. Among the solvents used, acetonitrile seems to be the best solvent.


## 3.2.5. Co-Ag/SiO2 Catalytic Performance

Under the optimized conditions, catalytic oxidation of various terpenic olefins has been carried out (Table5). As shown in Table 5, limonene was converted to the corresponding allylic ketone, carvone as the major compound (entry 1). However, β-pinene leads to the formation of myrtenal as a major product (entry 2). Performing the reaction with valencene resulted in a total conversion and gave, mainly, nootkatone in the 80% yield (entry 3).Table 5
Catalytic oxidation of terpenic olefins.
Entry
Substrate
Conv. (%)
Yield (%)
1
56
2
95
3
100
Reaction conditions: 0.5 mL substrate, 1.5 mL H2O2; catalyst 0.1 g; 80°C; acetonitrile; 24 h. Yields are determined by GC using dodecane as internal standard.


## 3.2.6. Effect of TBHP

The catalytic oxidation of various terpenic olefins usingtert-butyl hydroperoxide (TBHP 70%), a nonpolluting and economic oxidant agent [5, 18], has been carried out (Table 6). All the terpenic olefins show good activities, and the corresponding ketones were obtained in a moderate-to-good yield. As shown in Table 6, it appeared that limonene and β-pinene (entries 5 and 6) were less reactive compared to α-pinene and valencene (entries 4 and 7).Table 6
Effect of TBHP.
Entry
Substrate
Conv. (%)
Yield (%)
4
98
5
72
6
72
7
100
Reaction conditions: 0.5 mL substrate, 1.5 mL TBHP; catalyst 0.1 g; 80°C; acetonitrile (5 ml); 24 h. Yields are determined by GC using dodecane as internal standard.


## 3.2.7. Recycling of Catalyst

To investigate the reusability of the 1 mol% Co-1 mol% Ag supported SiO2 nanocatalyst, the catalytic performance was evaluated in four consecutive cycles (Figure 9). After each cycle, the catalyst was separated from the reaction mixture by simple filtration, washed with water, and dried at 200°C for 2 hours while being reused. According to Figure 9, the catalyst was stable after four cycles. Investigation of XRD analysis shows that the identity of the recovered Co-Ag/SiO2 remains similar to the fresh nanocatalyst (Figure 1). However, SEM morphology of the nanocatalyst after four cycles showed some agglomerated particles compared to the fresh one (Figure 10). Moreover, EDX elemental mapping of recycled Co-Ag/SiO2 indicates no changes of dispersion and distribution of all elements compared to the fresh nanocatalyst (Figure 11).Figure 9
Oxidation ofα-pinene during recyclability of 1 mol% Co-1 mol% Ag supported SiO2.Figure 10
SEM images of (a) fresh and (b) recycled 1 mol% Co-1 mol% Ag supported SiO2.
(a)
(b)Figure 11
EDX elemental mapping analysis of recycled 1 mol% Co-1 mol% Ag supported SiO2 after four cycles: (a) the studied area and the distribution of (b) all elements, (c) oxygen, (d) silicon, (e) silver, and (f) cobalt.
(a)
(b)
(c)
(d)
(e)
(f)


## 4. Conclusion

The 1 mol% Co-1 mol% Ag supported SiO2 catalyst prepared by the sol-gel method is an efficient and recyclable catalyst. The prepared catalyst shows a significant activity, a good reusability, and stability towards the oxidation of terpenic olefins using hydrogen peroxide and tert-butyl peroxide as green oxidizing agents. In the presence of H2O2, the prepared catalyst showed a high selectivity towards ketones and aldehydes in good yields. However, in the presence of TBHP, the reaction leads to the formation of ketones and aldehydes with a loss of selectivity and yields. The investigation of the recycled catalyst indicates that the recovered Co-Ag/SiO2 remains similar to the fresh one in terms of catalytic yields, morphology, dispersion, and distribution over four consecutive cycles.



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*Source: 1241952-2020-01-24.xml*