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Original article
12 (
1
); 54-68
doi:
10.1016/j.arabjc.2018.03.021

Ag2O nanoparticles/MnCO3, –MnO2 or –Mn2O3/highly reduced graphene oxide composites as an efficient and recyclable oxidation catalyst

, , , , , , , ,
a Department of Chemistry, College of Science, King Saud University, P.O. 2455, Riyadh 11451, Saudi Arabia
b Department of Chemistry, K L University, Guntur 522502, Andhra Pradesh, India
c Department of Chemistry, College of Science, Prince Sattam Bin Abdulaziz University, Al Kharj 16278, Saudi Arabia
d Department of Chemistry, RGUKT-Basar, Mudhole, Adilabad 504107, Telangana, India

⁎Corresponding authors. rafiqs@ksu.edu.sa (Mohammed Rafiq H. Siddiqui), sfadil@ksu.edu.sa (Syed Farooq Adil)

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Silver oxide nanoparticles doped manganese (IV) oxide along with varying percentages of highly reduced graphene oxide (HRG) [Ag2O(1%)–MnO2/(X%)HRG] nanocomposites were fabricated through a simple co-precipitation method followed by calcination at 400 °C. The as-prepared nanocomposite upon calcination at 300 °C and 500 °C temperatures, yields the manganese carbonate (MnCO3) and manganese (III) oxide (Mn2O3) composites i.e. Ag2O(1%)–MnCO3/(X%)HRG and Ag2O(1%)–Mn2O3/(X%)HRG, correspondingly. The structural composition of the prepared nanocomposites has confirmed by several characterization techniques. The nanocomposites have successfully utilized as a catalyst for liquid-phase oxidation of aromatic alcohols in presence of O2 as a green oxidant under alkali-free conditions. In addition, a comparative study was performed to assess the activity of the manganese carbonates and manganese oxides for aerial oxidation of benzyl alcohol into benzaldehyde as a model reaction. Effects of various parameters have thoroughly examined in detail and the Ag2O(1%)–MnO2/(5%)HRG catalyst exhibited the highest activity in the aerial oxidation of benzyl alcohol to benzaldehyde with a 100% conversion and >99% selectivity in a remarkably short reaction time (35 min) than the undoped precursor i.e. Ag2O(1%)–MnO2. The presence of HRG dopant greatly enhanced the catalytic performance of Ag2O–MnO2 nanocatalysts could be attributed to the presence of carbon vacancies and topological defects as well as oxygen carrying functionalities on the HRG surface and increase in the surface area. The as-prepared catalyst could be efficiently recycled and reused up to five times without a discernible drop in its catalytic performance and the product selectivity remained unchanged. The prepared catalyst i.e. Ag2O(1%)–MnO2/(5%)HRG was employed as oxidation catalyst for a series of various substituted benzylic and aliphatic alcohols into their respective aldehydes and yielded complete conversion with excellent product selectivity with no further oxidation to acids.

Keywords

Ag2O NPs
Graphene
MnO2
Alcohol oxidation
Catalyst
1

1 Introduction

Among the various organic transformations carried out using heterogeneous catalysis, selective oxidation of organic substrates has widely applied in synthetic chemistry industrially and academically (Tanaka et al., 2012). Amongst the numerous catalytic oxidation reactions, the oxidation of alcohols has gained extensive attention due to its use in the manufacturing of carbonyl compounds (Della Pina et al., 2012), especially benzaldehyde which is considered as the second most significant aromatic compound after vanillin (Kochi and Sheldon, 1981). Generally, aldehydes have extensively used as intermediates or precursors in numerous industries such as food processing, perfumery, confectionary, vitamins, flame-retardants, dyestuffs, insecticides, and drugs (Luo et al., 2014). The traditional methods employed for oxidation of alcohols into corresponding carbonyl compounds not only include employing expensive, toxic and corrosive oxidants (such as chromate, hypochlorite or permanganate etc.), but also consumes huge amounts energy to supply the high temperature and pressure required by the reactions (Palmisano et al., 2007). Over the past decades, enormous efforts have been devoted to developing more environmentally benign green approaches to overcome the drawbacks of traditional oxidation methods (Yurdakal et al., 2008) which was the replacement of the expensive, toxic and corrosive oxidants with green and cheap oxidants such as molecular oxygen (Luo et al., 2014). Recently, selective oxidation of alcohols was performed employing molecular oxygen as an ideal green and sustainable oxidant (Shi et al., 2012). Apart from the issue of finding a replacement of hazardous oxidant with green oxidant, the other issue is to find a better catalyst as mostly employed catalysts are based on noble metals such as gold (Wang et al., 2015), palladium (Wang et al., 2012), platinum (Hong et al., 2014), ruthenium (Yang et al., 2015) and rhodium (Wusiman and Lu, 2015) or combinations of them (Chen et al., 2012). Beside their costs, the noble metals employed for the oxidation of alcohols these precious metals also possess serious toxicity issues, and the rareness of these precious metals makes these catalysts unpractical for commercial applications (Xie et al., 2014). Therefore, tremendous effort has been devoted to exchange these precious metals with low cost and abundant non-noble metals such as copper (Cruz et al., 2016), cobalt (Ragupathi et al., 2015), nickel (Hajipour et al., 2015), iron (Cang et al., 2015), vanadium (Behera and Parida, 2012), chromium (Noshiranzadeh et al., 2014), molybdenum (Rao et al., 2011), cerium (Mandal et al., 2013), niobium (Furukawa et al., 2011), zinc (Forouzani et al., 2015) and zirconium (Goh et al., 2015) have been employed for selective oxidation of alcohols.

Among the several transition metal oxides, manganese dioxide was found to be most stable and inexpensive metal oxide with high catalytic performance (Bhaumik et al., 2016). Wherein it was utilized as a catalyst for catalytic oxidation of alcohols to the respective carbonyls (Hu et al., 2016). In addition, various Mn-based catalysts also have been extensively applied as catalysts for the oxidation of various organic substrates. For instance, oxidation of naphthalene (Garcia et al., 2013), carbon monoxide (Gao et al., 2016), toluene (Liu et al., 2017), formaldehyde (Bai et al., 2016), benzene (Huang et al., 2015), 4-tert-butyltoluene (Hua Yu et al., 2012), and alkyl aromatics (Burange et al., 2012).

Other than the various supports used for the preparation of oxidation catalysts, carbon-based material such as activated carbon, graphene, graphene oxide (GO), highly reduced graphene (HRG) has also been employed as support. Graphene, a carbon allotrope, has gained prominence as catalyst support as it remarkably improves the performance of the composites (Stankovich et al., 2006; Yang et al., 2018). Recently, HRG has attracted intense attention as an efficient material for catalyst supports or dopant for the preparation of catalysts for oxidation reactions due to presence of oxygen-carrying functional groups as well as carbon vacancies and topological defects present on the HRG surface, the possibility of sorption and intercalation of ions and molecules increases (Scheuermann et al., 2009) which in turn improves catalytic performance. The presence of HRG in the catalytic system has dual advantages (1) the HRG surface which behaves as the anchoring site for the reactant alcohols and oxygen on the surface by π–π stacking near the metal nanoparticles (Shakir et al., 2016), (2) the metallic nanoparticles usually are not stable and easy to agglomerate because of high surface energy, which decreases their catalytic efficiency and stability (Qu et al., 2014), which can be prevented due to the presence of HRG.

Incorporation of metal and/or metal oxide nanoparticles with carbon supports to produce graphene based metal and metal oxide nanocomposites have been reported to be effective catalysts to catalyze various chemical reactions, e.g. Suzuki cross-coupling reactions (Mondal et al., 2015), reduction of aromatic alcohols (Mirza-Aghayan et al., 2015), dehydrogenation of ammonia borane (Zhao et al., 2016), oxidation of CO (Esrafili et al., 2016), oxidation of glycerol (Xu et al., 2017), oxidation of formic acid (Xu et al., 2018) and synthesis of aniline (Qusti et al., 2014). Particularly, graphene based metal and metal oxide nanocomposites has extensively utilized as a promising catalyst for alcohol oxidation to their respective aldehydes and ketones, e.g. Pd NPs/GC (Wu et al., 2013), MnO2/GO (Hu et al., 2016), Ag NPs/GOSH (Zahed and Hosseini-Monfared, 2015), Au NPs/HRG (Yu et al., 2013), Pd-Co/RGO (Wang et al., 2014), Au/GQDs/Fe3O4 (Wu et al., 2016), and Fe3O4–Pt/HRG (Wu et al., 2012).

Continuing our efforts on the utilize of different mixed metal oxide NPs as a highly effective heterogeneous catalyst for aerial oxidation of alcohols (Adil et al., 2017; Assal et al., 2016; Assal et al., 2017a). Herein, we have used HRG as a dopant to enhance the catalytic activity of Ag2O doped MnO2 nanocatalysts and its catalytic performance was compared with that of Ag2O NPs–MnO2 (without HRG) in order to study the improvement of catalytic efficiency upon the use of HRG as a dopant in the catalytic system towards the aerobic oxidation of alcohols (Scheme 1). The catalytic activities of the synthesized nanocomposites with varying percentage of HRG have been examined for the selective oxidation of alcohols in liquid-phase employing an eco-friendly approach, i.e. using molecular O2 as an environmentally benign oxidant and in the absence of any bases or additives. To the best of our knowledge, this is the first study employing Ag2O NPs doped MnO2 along with HRG co-dopant as a catalyst for aerobic oxidation of alcohols. Additionally, the prepared materials have analyzed by various characterization techniques like XRD, SEM, EDX, HRTEM, TGA, Raman, FT–IR, and BET.

Schematic diagram for the synthesis of Ag2O(1%)–MnCO3/(X%)HRG, Ag2O(1%)–MnO2/(X%)HRG, and Ag2O(1%)–Mn2O3/(X%)HRG catalysts.
Scheme 1 Schematic diagram for the synthesis of Ag2O(1%)–MnCO3/(X%)HRG, Ag2O(1%)–MnO2/(X%)HRG, and Ag2O(1%)–Mn2O3/(X%)HRG catalysts.

2

2 Experimental

2.1

2.1 Synthesis of HRG

Initially, Graphene oxide (GO) has prepared from graphite by the Hummers method (Hummers and Offeman, 1958). Then, GO reduced by hydrazine hydrate to HRG and the entire method was provided in the supplementary file.

2.2

2.2 Preparation of Ag2O(1%)–MnO2/(X%)HRG catalysts

Ag2O(1%)–MnO2/(X%)HRG nanocomposites have prepared via co-precipitation procedure where (X = 0–7) and the entire procedure is provided in the supplementary file.

2.3

2.3 Characterization

The as-synthesized catalysts have characterized by several techniques and all details are provided in the supplementary file.

2.4

2.4 Activity testing

The general procedure for benzyl alcohol oxidation is provided in the supplementary file.

3

3 Results and discussion

3.1

3.1 Catalyst characterization

3.1.1

3.1.1 XRD analysis

The crystallographic structure of the synthesized nanocomposites has been studied by X-ray diffraction technique (XRD). Fig. 1 depicts typical XRD patterns of the pristine graphite, GO, HRG, Ag2O(1%)–MnO2, and Ag2O(1%)–MnO2/(5%)HRG samples. The XRD pattern for graphite exhibited a high crystalline degree with a strong and sharp diffraction line at 2θ = 26.5° which related to diffraction peak at (0 0 2) plane with a d-spacing of 3.37 Å as shown in Fig. 1a (Wang and Hu, 2011). On the contrary, the crystalline degree of GO is relatively low with a broad characteristic band at 2θ = 11.8°, which is a characteristic structure of GO (Fig. 1b). The vanishing of the graphite peak at 2θ = 26.5° and appearance of a new peak at 2θ = 11.8° with the (0 0 2) crystal plane, indicates the formation of GO via graphite oxidation (Mirza-Aghayan et al., 2012). This shift leads to an increase in an interplanar distance from 3.37 to 4.83 Å for graphite and GO, correspondingly, could be due to the introduction of oxygen carrying functionalities and the trapped water molecules between the graphite sheets upon oxidation process (Mirza-Aghayan et al., 2015). XRD diffraction for HRG nanosheets, has observed reflection at 2θ = 24.6° with (0 0 2) plane which is a characteristic peak of HRG and disappearance of the reflection at 2θ = 11.8°, indicates that GO has completely reduced to HRG by hydrazine monohydrate as shown in Fig. 1c (Zhou et al., 2014). The XRD pattern of Ag2O(1%)–MnO2 matches well with the reported data of pyrolusite manganese dioxide (JCPDS file No. 24-0735) while the Ag2O(1%)–MnO2/(5%)HRG nanocomposite displays a similar XRD pattern as Ag2O(1%)–MnO2 along with characteristic diffraction peak of HRG at 2θ = 24.6° indicating the presence of HRG in the synthesized nanocomposite (Fig. 1e).

XRD patterns of (a) pristine graphite, (b) GO, (c) HRG, (d) Ag2O(1%)–MnO2, and (e) Ag2O(1%)–MnO2/(5%)HRG.
Fig. 1 XRD patterns of (a) pristine graphite, (b) GO, (c) HRG, (d) Ag2O(1%)–MnO2, and (e) Ag2O(1%)–MnO2/(5%)HRG.

3.1.2

3.1.2 FT−IR spectroscopy

FT-IR analysis of GO, HRG, and Ag2O(1%)–MnO2/(5%)HRG samples were depicted in Fig. 2. In the FT-IR spectra of GO (Fig. 2a), the absorption bands at 3440 cm−1 can be ascribed to the (O—H) stretching vibrations of (—COOH) groups and H2O molecules, (Wang et al., 2013) the intense peak at 1740 cm−1 corresponds to (C⚌O) stretching of (—COOH) on the surface (Hou et al., 2010), and the sharp absorption peak situated at 1630 cm−1 has assigned to the stretching vibration of carbon backbone (C⚌C/C—C) from unoxidized graphite lattice (Sitko et al., 2013). Additionally, the three absorptions existing at 1397, 1225, 1060 cm−1 can be attributed to the stretching vibrations of (C—OH), (C—O—C) and (C—O), respectively (Sitko et al., 2013). As illustrated in Fig. 2b the HRG spectrum showed the broad band at 1214 cm−1 corresponding to (C—OH) stretching vibration and a weak band at nearly 1636 cm−1 is for (C⚌C) group, due to the skeletal aromatic vibration, other peaks related to oxygen carrying functionalities vanished (Shinde et al., 2015). Compared with GO, the FT-IR spectra of Ag2O(1%)–MnO2/(5%)HRG nanocomposite obviously displays the full reduction of most of the oxygenated functionalities on GO surface (Fig. 2c). The sharp peaks associated with (C⚌O), (C—O—C), and (C—O) vibrations at 1740, 1225, and 1060 cm−1 were not noticed compared with GO, suggesting that the oxygen functionalities on GO surface are have successfully reduced. Further, a broad-ranging band located at 1634 cm−1 belongs to (C⚌C) stretching mode is owing to the skeletal aromatic vibration was noticed (Fig. 2c) (Gao et al., 2013). Meanwhile, intense absorption at around 580 cm−1 is observed which is associated with Mn—O vibrations in MnO2 (Yang et al., 2011).

FT–IR spectra of (a) GO, (b) HRG, and (c) and Ag2O(1%)–MnO2/(5%)HRG.
Fig. 2 FT–IR spectra of (a) GO, (b) HRG, and (c) and Ag2O(1%)–MnO2/(5%)HRG.

3.1.3

3.1.3 TGA analysis

TGA has carried out to study the thermal stability of the Ag2O(1%)–MnO2/(5%)HRG nanocomposite and is compared to the thermal stability of the precursors such as pristine graphite, GO, HRG and Ag2O(1%)–MnO2. TGA patterns confirm that complete reduction of graphene oxide (GO) to highly reduced graphene oxide (HRG) by using hydrazine hydrate as a reductant. TGA curves obtained for pristine graphite, GO, HRG, Ag2O(1%)–MnO2, and Ag2O(1%)–MnO2/(5%)HRG nanocomposite samples has shown in Fig. 3. Fig. 3a, b displayed that the thermal stability of GO is much lower than pristine graphite sample. TGA curve of graphite exhibits an overall weight loss of nearly 1% in the range of (25–800 °C) as described in Fig. 3a (Sharma et al., 2013). Conversely, GO displays ∼6% weight loss at approximately 100 °C, clearly owing to the loss of physisorbed H2O and volatile impurities which may be held in the material, followed by a fundamental weight loss of ∼43%, which is attributed to decomposition of the oxygen functionalities such as carbonyl, hydroxyl, carboxyl, and epoxy groups in (200–370 °C) (Mungse et al., 2012). Eventually, a weight loss of about 11%, due to the thermal decomposition of the carbon skeleton is noticed in the temperature range of 370–800 °C (Metin et al., 2012), the results obtained are in good agreement with earlier publication (Geng et al., 2014). The thermogram of HRG (Fig. 3c) shows a total weight loss of less than 19%, which attributed to the reduction of the most of oxygenic carrying functionalities under identical temperatures. The Ag2O(1%)–MnO2/(5%)HRG exhibited total weight loss around 17% in the same temperature range (Fig. 3e), which is slightly more than the weight loss, apparent from the degradation graph of Ag2O(1%)–MnO2, which indicated that effective reduction of GO to HRG by removing most of the oxygenated functional groups. As a result, it is concluded that the Ag2O(1%)–MnO2/(5%)HRG nanocomposite is thermally stable up to a temperature of 520 °C.

TGA analysis of (a) graphite, (b) GO, (c) HRG, (d) Ag2O(1%)–MnO2, and (e) Ag2O(1%)–MnO2/(5%)HRG.
Fig. 3 TGA analysis of (a) graphite, (b) GO, (c) HRG, (d) Ag2O(1%)–MnO2, and (e) Ag2O(1%)–MnO2/(5%)HRG.

3.1.4

3.1.4 Scanning electron microscope (SEM) and elemental analysis (EDX)

The morphology of the synthesized catalyst Ag2O(1%)–MnO2/(5%)HRG obtained via co-precipitation is monitored using scanning electron microscopy (SEM) and compared with Ag2O(1%)–MnO2 (Fig. 4) wherein the Ag2O(1%)–MnO2 (Fig. 4a) and the Ag2O(1%)–MnO2/(5%)HRG nanocomposite (Fig. 4b). The Ag2O(1%)–MnO2 catalyst exhibits micro size but well-defined cuboidal morphology, however, the Ag2O(1%)–MnO2/(5%)HRG (Fig. 4b), interestingly exhibits aggregation of rather smaller size crystals which looks like grown on the surface through proper surface nucleation/growth process. Moreover, elemental analysis of the prepared Ag2O(1%)–MnO2/(5%)HRG nanocomposite has also determined via energy-dispersive X-ray spectroscopy (EDX), as observed in Fig. 5. The existence of silver, manganese, carbon, and oxygen is evidently shown in EDX analysis, and the composition percentage is within the theoretical range.

SEM images of (a) Ag2O(1%)–MnO2 and (b) Ag2O(1%)–MnO2/(5%)HRG.
Fig. 4 SEM images of (a) Ag2O(1%)–MnO2 and (b) Ag2O(1%)–MnO2/(5%)HRG.
EDX spectra of the Ag2O(1%)–MnO2/(5%)HRG nanocomposite.
Fig. 5 EDX spectra of the Ag2O(1%)–MnO2/(5%)HRG nanocomposite.

3.1.5

3.1.5 High-resolution transmission electron microscope (HRTEM)

The morphology and size of the HRG, Ag2O(1%)–MnO2, and Ag2O(1%)–MnO2/(5%)HRG nanocomposite were determined by using high-resolution transmission electron microscope (HRTEM). Fig. 6a displays highly exfoliated HRG nanosheets, which possesses a thin layer like silky transparent shape. Whereas, HRTEM micrograph of the as-prepared Ag2O(1%)–MnO2/(5%)HRG nanocomposite clearly reveals the nanosize of the Ag2O in the catalyst with 0.95 ± 0.14 nm as average diameter with spherical morphology and are well distributed on the crumpled HRG nanosheets as displayed in Fig. 6(d–f). It is noteworthy to mention that the HRTEM image of unsupported Ag2O(1%)–MnO2 catalyst exhibits the Ag2O nanoparticles of 2.16 ± 0.23 nm average diameter, which is larger than the Ag2O nanoparticles obtained by using graphene support. This might be due to graphene nanosheets inhibiting the agglomeration of the Ag2O nanoparticles (Fig. 6b, c) (Khan et al., 2015). Interestingly, the average size of Ag2O nanoparticles in Ag2O(1%)–MnO2/(5%)HRG is found to be smaller than that of the Ag2O nanoparticles in unsupported Ag2O(1%)–MnO2 catalyst (Fig. 6c and f), that is probably the reason as to why Ag2O(1%)–MnO2/(5%)HRG nanocomposite exhibits higher catalytic performance compared to Ag2O(1%)–MnO2 catalyst. It is important to note that the obtained results from HRTEM analysis are in good agreement with BET surface area measurements and the catalytic results, which the Ag2O(1%)–MnO2/(5%)HRG catalyst possesses much higher surface area and catalytic performance compared to Ag2O(1%)–MnO2 catalyst without graphene support.

HRTEM micrographs of (a) HRG, (b) Ag2O(1%)–MnO2, (d, e) Ag2O(1%)–MnO2/(5%)HRG, (c, f) particle size distributions of Ag2O(1%)–MnO2 and Ag2O(1%)–MnO2/(5%)HRG nanocomposite respectively.
Fig. 6 HRTEM micrographs of (a) HRG, (b) Ag2O(1%)–MnO2, (d, e) Ag2O(1%)–MnO2/(5%)HRG, (c, f) particle size distributions of Ag2O(1%)–MnO2 and Ag2O(1%)–MnO2/(5%)HRG nanocomposite respectively.

3.1.6

3.1.6 Raman spectroscopy

Raman spectra is a highly informative characterization technique for analyzing the structure of graphene based metal nanocomposites. Fig. 7 shows the Raman spectra of GO, Ag2O(1%)–MnO2, and Ag2O(1%)–MnO2/(5%)HRG nanocomposite. The Raman spectrum of the Ag2O(1%)–MnO2 and Ag2O(1%)–MnO2/(5%)HRG nanocomposite (Fig. 7b,c), exhibit a characteristic band appearing at 642 cm−1, which is related to the symmetric lattice vibration (Mn–O), indicating the presence of MnO2 in both Ag2O(1%)–MnO2 as well as Ag2O(1%)–MnO2/(5%)HRG nanocomposite (Han et al., 2014). In addition, the presence of HRG in the Ag2O(1%)–MnO2/(5%)HRG catalyst was confirmed by the existence of two characteristic peaks at ∼1591 and 1337 cm−1 correspondingly, commonly denoted as D-band and G-band (Fan et al., 2013). The D-band belongs to the disordered carbon structure induced by lattice defects and the G-band is corresponds to well-ordered structure (Qi et al., 2013). Fig. 7a displays GO spectrum. The G and the D peaks have shifted and appear at 1605 and 1346 cm−1, correspondingly, which is attributed to the demolition of the sp2 structure during oxidation of graphite to graphene oxide and occurrence of oxygen functionalities on the GO surface (Liu et al., 2012). The G characteristic band in Ag2O(1%)–MnO2/(5%)HRG shifted by ∼14 cm−1 from 1605 to 1591 cm−1, whilst a small shift is observed in the D-band from 1346 to 1337 cm−1, implying a complete reduction of graphene oxide to highly reduced graphene oxide (Allen et al., 2010). Notably, the Raman results obtained are in agreement with the XRD results.

Raman spectra of (a) GO, (b) Ag2O(1%)–MnO2 and Ag2O(1%)–MnO2/(5%)HRG nanocomposites.
Fig. 7 Raman spectra of (a) GO, (b) Ag2O(1%)–MnO2 and Ag2O(1%)–MnO2/(5%)HRG nanocomposites.

3.1.7

3.1.7 Surface area (BET) measurements

The Brunauer–Emmett–Teller (BET) surface area of the synthesized materials were evaluated, to compare the changes in surface area due to calcination at different temperatures and the presence of HRG in the catalytic system and to comprehend the connection between the specific surface areas of the fabricated catalysts and the efficiency of the catalytic system for oxidation of benzylic alcohols. Table 2 revealed that the surface areas of the as-prepared catalysts (without HRG support) i.e. Ag2O(1%)–MnCO3, Ag2O(1%)–MnO2, and Ag2O(1%)–Mn2O3 is approximately 52, 84, and 42 m2 g−1 correspondingly. Whilst, the surface areas of the synthesized nanocomposite after doping it with HRG at different calcination temperatures 300 °C, 400 °C, and 500 °C i.e. Ag2O(1%)–MnCO3/(5%)HRG, Ag2O(1%)–MnO2/(5%)HRG, and Ag2O(1%)–Mn2O3/(5%)HRG respectively, increased to 107,149, and 99 m2 g−1 correspondingly. As expected, the catalytic efficiency after doping the catalyst with HRG i.e. Ag2O(1%)–MnCO3/(5%)HRG, Ag2O(1%)–MnO2/(5%)HRG, and Ag2O(1%)–Mn2O3/(5%)HRG also increased.

3.2

3.2 Catalytic application

The main objective of the present work is to employ the as-synthesized catalyst in selective oxidation, for the oxidation of benzyl alcohol into benzaldehyde utilizing O2 as a green oxidizing agent, with high conversion and selectivity in short reaction time. In addition, a comparison was carried out between the various manganese salts i.e. manganese carbonate and manganese (IV, III) oxides obtained when the as-synthesized material is calcined at different temperatures and the changes in catalytic activity due to the incorporation of HRG as a dopant. Several variations including the percentage of the HRG, calcination temperature, catalyst concentration, reaction time, and temperature are carried out and its influence on the catalytic efficiency was evaluated. The oxidation of benzyl alcohol using molecular O2 was selected as a model reaction as described in Scheme 2.

Schematic illustration of aerial oxidation of benzyl alcohol using the as-obtained catalyst.
Scheme 2 Schematic illustration of aerial oxidation of benzyl alcohol using the as-obtained catalyst.

3.2.1

3.2.1 Effect of weight percentage of HRG

The efficiency of any catalyst can be fine-tuned by utilizing graphene support (Hu et al., 2016; Wang et al., 2014; Wu et al., 2013). From our previously reported study, it was found that Ag2O NPs are a superior promoter to the MnO2 catalyst and Ag2O(1%)–MnO2 catalyst was found to be the best catalyst among all other catalysts for the benzyl alcohol oxidation (Assal et al., 2017b). In the present study, the catalyst Ag2O(1%)–MnO2 was chosen and further modified by the addition of HRG, in order to study the changes in the catalytic activity due to the presence of HRG in the catalytic system. In the present work, the catalytic activity of graphene (HRG) i.e. without Ag2O NPs/MnO2, did not yield any oxidation product with benzyl alcohol (Table 1, entry 1). The catalytic performances of the different Ag2O(1%)–MnO2/(X%)HRG nanocomposites (while, X = 0, 1, 3, 5, and 7) where the percentage of HRG was varied from 1 to 7 (w/w%) and its catalytic performance was evaluated for oxidation of benzyl alcohol. The results demonstrated that, the nanocomposites Ag2O(1%)–MnO2/(1%)HRG and Ag2O(1%)–MnO2/(3%)HRG provide alcohol conversion of 70.6% and 84.3%, respectively, under same circumstances (Table 1, entries 3 and 4). By increasing the percentage of HRG in the nanocomposite i.e. Ag2O(1%)–MnO2/(5%)HRG, the catalytic activity significantly increased and yielded a complete conversion within 35 min with high specific activity 11.4 mmol g−1 h−1 (Table 1, entry 5). Further increase in the wt% of HRG led to slight reduction in the catalytic efficiency of the catalyst, which can be attributed to the blocking the active sites of the catalyst due to the high percentage of HRG (Table 1, entry 6). Notably, unsupported Ag2O(1%)–MnO2 catalyst gave only 67% conversion of benzyl alcohol (Table 1, entry 2) in the same reaction time. Consequently, it was concluded that the graphene doping plays a vital role in improving the performance of the synthesized catalyst and the superior activity of Ag2O(1%)–MnO2/(5%)HRG is attributed to the adsorption of the aromatic alcohols on the catalyst surface which is enhanced further by the presence of HRG on the surface by π–π interaction along with Ag2O NPs. Furthermore, the existence of carbon defects and oxygen carrying functional groups on HRG plane which anchored the Ag2O NPs. The product selectivity remains (>99) unchanged during all oxidation experiments (Table 1, entries 1–6). The obtained results have tabulated in Table 1 and depicted in Fig. 8. Therefore, from the obtained data it can be deduced that the Ag2O(1%)–MnO2/(5%)HRG nanocomposite is the best among the other catalysts prepared and will be used to optimize other parameters.

Table 1 The catalytic activities of various catalysts for benzyl alcohol oxidation.
Entry Catalyst Conv. (%) Sp. activity (mmol g−1 h−1) Sel. (%)
1 HRG 3.8 0.4 >99
2 Ag2O(1%)–MnO2 67 7.6 >99
3 Ag2O(1%)–MnO2/(1%)HRG 70.6 8.1 >99
4 Ag2O(1%)–MnO2/(3%)HRG 84 9.6 >99
5 Ag2O(1%)–MnO2/(5%)HRG 100.0 11.4 >99
6 Ag2O(1%)–MnO2/(7%)HRG 96 11.0 >99

Conditions: 2 mmol benzyl alcohol, 300 mg catalyst, 400 °C calcination temperature, 20 mL min–1 oxygen flow rate, 100 °C temperature, 10 mL toluene, and 35 min reaction time.

Graphical illustration of benzyl alcohol oxidation using the catalysts (a) Ag2O(1%)–MnO2, (b) Ag2O(1%)–MnO2/(1%)HRG, (c) Ag2O(1%)–MnO2/(3%)HRG, (d) Ag2O(1%)–MnO2/(5%)HRG, and (e) Ag2O(1%)–MnO2/(7%)HRG.
Fig. 8 Graphical illustration of benzyl alcohol oxidation using the catalysts (a) Ag2O(1%)–MnO2, (b) Ag2O(1%)–MnO2/(1%)HRG, (c) Ag2O(1%)–MnO2/(3%)HRG, (d) Ag2O(1%)–MnO2/(5%)HRG, and (e) Ag2O(1%)–MnO2/(7%)HRG.

3.2.2

3.2.2 Role of calcination treatment

Calcination treatment has a significant impact on the composition of the synthesized material, which leads to changes in the oxide composition, morphology, particle size and surface area, which in-turn effects the catalytic activity of the prepared catalysts (Singh et al., 2017). Hence, the influence of the calcination temperature on the catalytic efficiency of the as-prepared catalyst was explored to get the optimum calcination treatment required for the best effectiveness of this catalytic system, the obtained results have been tabulated in Table 2 and plotted Fig. 9. The as-prepared catalyst calcined at 300 °C, 400 °C, and 500 °C yields i.e. Ag2O(1%)–MnCO3/(5%)HRG, Ag2O(1%)–MnO2/(5%)HRG, and Ag2O(1%)–Mn2O3/(5%)HRG respectively. In case of catalyst heated at 300 °C i.e. Ag2O(1%)–MnCO3/(5%)HRG, a 95% conversion of benzyl alcohol has obtained within 35 min of reaction time (Table 2, entry 4). Whilst, the catalyst obtained upon calcination at 500 °C i.e. Ag2O(1%)–Mn2O3/(5%)HRG, the conversion of benzyl alcohol reduced to 43.82% which is lower than the conversion obtained by employing the catalyst calcined at 300 °C (Table 2, entry 6). In case of catalyst heated at 400 °C, i.e. Ag2O(1%)–MnO2/(5%)HRG a complete alcohol conversion to aldehyde has obtained and the specific activity was found to be ∼11.4 mmol g−1 h−1 (Table 2, entry 5). Additionally, the catalytic activity of the catalysts without graphene (HRG) i.e. Ag2O(1%)–MnCO3, Ag2O(1%)–MnO2, and Ag2O(1%)–Mn2O3 it was found that 62%, 67%, and 40% conversion of benzyl alcohol to benzaldehyde (Table 2, entries 1–3). Accordingly, it was evident that the presence of HRG in the catalytic system enhances the performance of catalyst considerably.

Table 2 Effects of calcination treatment on the catalytic activities.
Entry Catalyst T (°C) SA (m2 g−1) Conv. (%) Sp. activity (mmol g−1 h−1) Sel. (%)
1 Ag2O(1%)–MnCO3 300 52 62 7.1 >99
2 Ag2O(1%)–MnO2 400 84 67 7.6 >99
3 Ag2O(1%)–Mn2O3 500 42 40 4.6 >99
4 Ag2O(1%)–MnCO3/(5%)HRG 300 107 95 10.8 >99
5 Ag2O(1%)–MnO2/(5%)HRG 400 149 100 11.4 >99
6 Ag2O(1%)–Mn2O3/(5%)HRG 500 99. 44 5.0 >99

Conditions: 2 mmol benzyl alcohol, 300 mg catalyst, 100 °C temperature, 20 mL min−1 oxygen flow rate, 10 mL toluene, and 35 min reaction time.

Graphical representation of aerial oxidation of benzyl alcohol using the catalysts (a) Ag2O(1%)–MnCO3, (b) Ag2O(1%)–MnO2, (c) Ag2O(1%)–Mn2O3, (d) Ag2O(1%)–MnCO3/(5%)HRG, (e) Ag2O(1%)–MnO2/(5%)HRG, and (f) Ag2O(1%)–Mn2O3/(5%)HRG.
Fig. 9 Graphical representation of aerial oxidation of benzyl alcohol using the catalysts (a) Ag2O(1%)–MnCO3, (b) Ag2O(1%)–MnO2, (c) Ag2O(1%)–Mn2O3, (d) Ag2O(1%)–MnCO3/(5%)HRG, (e) Ag2O(1%)–MnO2/(5%)HRG, and (f) Ag2O(1%)–Mn2O3/(5%)HRG.

As reported in the literature that surface area of the catalyst had a marked impact on the efficacy of the catalyst and graphene-based nanocomposites possess higher surface area, interestingly, similar trends are observed in this study as well. The HRG nanocomposites prepared in the study have found to possess surface area twice as much as their precursors, which has improved the catalytic performance of the catalysts prepared. The catalyst Ag2O(1%)–MnO2/(5%)HRG calcined at 400 °C possess a surface area of 149 m2 g−1 while the catalyst without HRG i.e. Ag2O(1%)–MnO2 possess a surface area of 84.3 m2 g−1. Furthermore, in case of the catalyst calcined at 300 and 500 °C i.e. Ag2O(1%)–MnCO3/(5%)HRG and Ag2O(1%)–Mn2O3/(5%)HRG showed lower catalytic activity and also have found to possess lower surface area. Hence, it can be said that the catalytic activity has strongly influenced by calcination temperature and surface area of the catalyst and the catalyst with the highest surface area among the catalyst prepared yields a 100% conversion. Therefore, we chose to use 400 °C as the best calcination temperature in further studies to optimize other factors.

3.2.3

3.2.3 Effect of temperature

Reaction temperature had a pronounced impact on the performance of the catalyst, which in turn accelerates the rate of oxidation of alcohols. The influence of reaction temperature on the conversion of benzyl alcohol and the benzaldehyde selectivity has studied by considering five different temperatures in the range of (20–100 °C), like 20, 40, 60, 80 and 100 °C for the liquid phase oxidation of benzyl alcohol in presence of Ag2O(1%)–MnO2/(5%)HRG while other optimized parameters are unchanged. The results obtained have listed in Table 3 and graphically depicted in Fig. S1. It can be observed that the selectivity of benzaldehyde remained unchanged with >99%, while the benzyl alcohol conversion has affected and appears to be directly proportional to the reaction temperature. Only 40% conversion of benzyl alcohol has obtained when the reaction was carried out at low temperature (20 °C) (Table 3, entry 1). As predicted, high temperature yielded a higher oxidation rate and led to the noticeable improvement of catalytic efficiency of the catalyst. At elevated temperature i.e., 100 °C, a 100% conversion was achieved along with high specific activity of 11.4 mmol g−1 h−1 under similar conditions (Table 3, entry 5). Apparently, best results were achieved by performing the reaction at 100 °C temperature, which can be attributed as an optimum temperature, for the optimum performance of the Ag2O(1%)–MnO2/(5%)HRG catalyst. Therefore, further optimization of the reaction conditions is carried out at 100 °C.

Table 3 The impact of temperature on the catalytic activity of Ag2O(1%)–MnO2/(5%)HRG catalyst.
Entry Temperature (°C) Conv. (%) Sp. activity (mmol g−1 h−1) Sel. (%)
1 20 40 4.6 >99
2 40 56 6.4 >99
3 60 71 8.1 >99
4 80 86 9.8 >99
5 100 100 11.4 >99

Conditions: 2 mmol benzyl alcohol, 400 °C calcination temperature, 300 mg catalyst, 20 mL·min−1 oxygen flow rate, 10 mL toluene, and 35 min reaction time.

3.2.4

3.2.4 Effect of catalyst concentration

In order to ascertain the optimum amount of catalyst, the oxidation of benzyl alcohol was performed using different amounts of the Ag2O(1%)–MnO2/(5%)HRG catalyst, while other optimized parameters have maintained as confirmed earlier. The obtained results have collected in Table 4 and presented in Fig. S2. The results displayed that the conversion of alcohol increased with the rising catalyst dosage. Meanwhile, the benzaldehyde selectivity is almost unchanged during all oxidation reactions (above 99%). When the dosage of the catalyst increased from 50 mg to 300 mg, the alcohol conversion increased significantly from 26% to 100% within 35 min at 100 °C (Table 4, entries 1–6). The present study shows that only 300 mg of the catalyst is sufficient for full transformation of benzyl alcohol after a relatively short period.

Table 4 Effect of the quantity of catalyst Ag2O(1%)–MnO2/(5%)HRG on the catalytic properties.
Entry Catalyst amount (mg) Conv. (%) Sp. activity (mmol g−1 h−1) Sel. (%)
1 50 26 18.1 >99
2 100 44 14.9 >99
3 150 59 13.4 >99
4 200 74 12.7 >99
5 250 88 12.0 >99
6 300 100.0 11.4 >99

Conditions: 2 mmol benzyl alcohol, 400 °C calcination temperature, 100 °C temperature, 20 mL min−1 oxygen flow rate, 10 mL toluene, and 35 min reaction time.

To ascertain that the oxidation product i.e. benzaldehyde, is indeed produced from the catalytic oxidation of benzyl alcohol and toluene (solvent) oxidation is not affecting the results obtained, a similar experiment was carried out in the absence of the benzyl alcohol, under optimal reaction circumstances. It was found that no benzaldehyde formation is observed, which indicates that the benzaldehyde produced, is due to the aerial oxidation of benzyl alcohol and not because of the oxidation of toluene. Similarity, a blank run i.e. the reaction performed without the prepared catalyst using identical conditions to confirm that the desired benzaldehyde is being formed because of the catalytic activity of the as-prepared catalyst and not because of the self-oxidation of benzyl alcohol in the presence of oxygen and no oxidation product is obtained. The results show that the as-made catalyst plays a crucial role in the aerial oxidation of benzyl alcohol.

3.3

3.3 Catalyst recycling test

The recyclability and stability of catalyst is a very important theme for cost reduction in commercial applications. Therefore, a five recycling experiments are performed to test the stability of the Ag2O(1%)–MnO2/(5%)HRG by selecting the oxidation of benzyl alcohol as a model reaction under optimal reaction conditions. The obtained results have illustrated in Fig. 10. After every single catalytic cycle, the catalyst can be easily isolated by simple filtration. The filtered catalyst was washed many times with solvent and then dried in oven at 100 °C for 4 h. The filtered Ag2O(1%)–MnO2/(5%)HRG catalyst is then reused for benzyl alcohol oxidation under same circumstances. The results reveal that, no noticeable loss in the catalytic activity after each recycling experiment. During the five catalytic runs investigation, the benzyl alcohol conversion decreases from 100% to 93%. This is might be related to the inevitable mass loss through the filtration step due to attrition (Yu et al., 2013). Meanwhile, the selectivity towards benzaldehyde is unchanged during all recycling reactions. These results indicate that the as-prepared catalyst is a superior stability and reusability.

The recyclability of Ag2O(1%)–MnO2/(5%)HRG catalyst.
Fig. 10 The recyclability of Ag2O(1%)–MnO2/(5%)HRG catalyst.

Moreover, in order to further elucidate the superior catalytic activity of the Ag2O(1%)–MnO2/(5%)HRG nanocomposite, the tested catalyst was compared with other graphene-based catalysts found in the literature as outlined in Table 5. The obtained results indicate that, among all listed catalysts Ag2O(1%)–MnO2/(5%)HRG seems to be the most efficient catalyst for oxidation of benzylic alcohols in terms of reaction time, conversion, and specific activity. From the comparison made it was found that the prepared catalyst afforded a complete conversion and selectivity within relatively short reaction time 35 min at 100 °C and highest specific activity (11.4 mmol g−1 h−1), when compared to all other, reported similar catalysts. Interestingly, all listed catalysts take a relatively long reaction time to achieve a complete oxidation of benzyl alcohol and showed lower specific activity. For instance, Hu et al. (Hu et al., 2016) fabricated the MnO2/graphene oxide (GO) composites by an in-situ growth method and used it as a catalyst for the transformation of benzyl alcohol. MnO2/GO nanocomposite afforded 96.8% alcohol conversion and 100% selectivity toward benzaldehyde with lower specific activity (1.6 mmol g−1 h−1) within 3 h, which is a longer reaction time compared to the reaction time required by our catalytic system reported in this work. Another example, Zahed et al. (Zahed and Hosseini-Monfared, 2015) reported the use of silver nanoparticles (Ag NPs) incorporated on thiolated partially reduced graphene oxide (Ag NPs/GOSH) as catalyst for the oxidation of benzyl alcohol, but it required extremely long reaction time (24 h) to provide 61% benzyl alcohol conversion and 58% benzaldehyde selectivity along with low specific activity of 5.1 mmol g−1 h−1. Hence, it can be stated that our catalyst is the best catalyst for the oxidation of alcohols among similar catalysts.

Table 5 Comparison between the performance of Ag2O(1%)–MnO2/(5%)HRG catalyst and earlier reported graphene-based catalysts for the benzyl alcohol oxidation.
Catalyst Conv. (%) Sel. (%) T. (°C) t. (h) Sp. activity (mmol g−1 h−1) Ref.
Ag2O(1%)–MnO2/(5%)HRG 100 >99 100 0.6 11.4 This study
MnO2/GO (10/100) 97 100 110 3 1.6 Hu et al. (2016)
Ag NPs/GOSH 61 58 80 24 5.1 Zahed and Hosseini-Monfared (2015)
Ag NPs/rGO 12 8 80 24 1.0 Zahed and Hosseini-Monfared (2015)
iGO-T 46 – RT 20 0.9 Shakir et al. (2016)
γ-MnO2/GO 91 – 50 5 1.8 Kadam et al. (2016)
Au/NG 67 40 70 6 0.4 Xie et al. (2012)
NG-900 15 100 80 10 0.1 Long et al. (2012)
Pd/GO 36 34.1 110 6 1.0 Wu et al. (2013)
Au/RGO 65 93 100 8 5.4 Yu et al. (2013)
Ag NPs/GO 33 55 80 24 2.8 Zahed and Hosseini-Monfared (2015)
Pd(II)-AAPTMS@GO 96 99 60 3 2.1 Rana et al. (2015)
GO–N–PW 76 99 100 6 10.6 Liu et al. (2014)
GO-100 100 100 80 5 1.1 Geng et al. (2014)

3.4

3.4 Application of Ag2O(1%)–MnO2/(5%)HRG in the aerobic oxidation of a wide range of alcohols

To explore the present catalyst further, oxidation of a variety of benzylic alcohols and citronellol as example of aliphatic alcohols with molecular O2 s carried out and Ag2O(1%)–MnO2/(5%)HRG was employed as catalyst while all the optimized conditions were maintained, such as various benzylic alcohols (2 mmol) is taken in 10 mL toluene with 20 mL min−1 oxygen flow rate, at reaction temperature 100 °C in presence of the 0.3 g of prepared catalyst calcined at 400 °C. A wide range of derivatives of benzyl alcohol have been completely transformed into respective aldehydes at different reaction times (Table 6, entries 1–20). A >99% selectivity of aldehydes have achieved in all experiments without further oxidation to acids have been found. The attained results obviously illustrate that electronic effects have an important effect on the oxidation rate. It was found that the electron-rich aromatic derivatives carrying electron-releasing substituents possess higher reactivity and show shorter periods, whilst reaction times for the aromatic alcohols with electron-deficient substituent are longer (Dell’Anna et al., 2014). However, the oxidation of aromatic alcohols with electron-donating groups like (—OCH3, —CH3, —OH, and —C(CH3)3) into their respective aldehydes with complete conversions in relatively short reaction times. Whilst, the oxidation rate is slower when the aromatic alcohol has an electron-deficient substituent like (—F, —Cl, —NO2, and —CF3). For an example, the aromatic alcohol contains a para-electron-releasing substituent like 4-methoxylbenzyl alcohol, was fully oxidized to 4-methoxybenzaldehyde within only 30 min (Table 6, entry 2). While, 4-(Trifluoromethyl)benzyl alcohol that possessing a para-electron-withdrawing substituent, need longer period (55 min) (Table 6, entry 14). Moreover, it was noticed that the presence of substituted group on the para-position of benzyl alcohol is easily oxidized compared to benzyl alcohol with substituents at the ortho- and meta-position, could be because of the minimum steric resistance experienced at the para-position compared to other positions (Kadam et al., 2016). In this context, para-methylbenzyl alcohol has fully transformed into para-methyl benzaldehyde after 30 min (Table 6, entry 3), whereas meta- and ortho-methylbenzyl alcohol take longer time 40 and 45 min for complete conversion, respectively (Table 6, entries 5 and 7). The effect of steric hindrance with bulky groups like (4-C(CH3)3, 4-CF3, 2,3,4-TriOMe, 2,4-DiCl, and 2,3,4,5,6-PentaF) connected to the phenyl ring is noticeable, as there is a decrease in the percentage of conversion product in such alcohols with bulky groups, which can be attributed to the steric resistance that impede the interaction of the alcohol with the catalyst surface, in turn effecting the oxidation of the bulky alcohols (Table 6, entries 13, 14, 17, 18, 20) (Borthakur et al., 2014). Therefore, it can be concluded that the catalytic performance of the catalystic system is significantly influenced by two parameters, electronic properties and steric hindrance.

Table 6 Aerobic oxidation of various alcohols catalyzed by Ag2O(1%)–MnO2/(5%)HRG.
Entry Reactant Product t. (min) Conv. (%) Sel. (%)
1 35 100 >99
2 30 100 >99
3 30 100 >99
4 35 100 >99
5 40 100 >99
6 40 100 >99
7 45 100 >99
8 45 100 >99
9 45 100 >99
10 50 100 >99
11 50 100 >99
12 50 100 >99
13 55 100 >99
14 55 100 >99
15 55 100 >99
16 65 100 >99
17 65 100 >99
18 75 100 >99
19 80 100 >99
20 80 100 >99
21 130 100 >99

Conditions: 2 mmol alcohol, 0.3 g catalyst, 400 °C calcination temperature, 10 mL toluene, 20 mL min−1 oxygen flow rate at 100 °C.

On the contrary, the oxidation of benzylic alcohols has more easier than that of aliphatic alcohols and required longer time for full oxidation (Assady et al., 2015). For an example, the oxidation of citronellol into citronellal takes place in longer times, might be attributed to absence of conjugation in the β-position of the hydroxyl group (Table 6, entry 21). Hence, it could be deduced that as-obtained catalyst is selective towards oxidation of aromatic alcohols than aliphatic alcohols.

4

4 Conclusion

In this work, highly reduced graphene composites of Ag2O nanoparticles doped MnO2 is presented as an efficient, cheap, environment-friendly and recyclable catalyst for the selective aerobic oxidation of alcohols. Comparison between the catalyst without graphene i.e. Ag2O(1%)–MnO2 and Ag2O(1%)–MnO2/(5%)HRG reveal that inclusion of graphene in the catalytic system enhances the catalytic performance for the oxidation of benzylic alcohols to their respective aldehydes. A 100% conversion of benzyl alcohol with >99% product selectivity has accomplished within 35 min of reaction time and with 11.4 mmol g−1 h−1 specific activity, which is found to be much higher than the specific activity of various graphene-based catalysts previously reported. Furthermore, a variety of aromatic and aliphatic alcohols have evaluated for aerobic oxidation to the respective aldehydes with complete conversion and selectivity within short reaction times. The oxidation of substituted benzyl alcohols containing electron-releasing substituents required shorter periods than those bearing electron-withdrawing or bulky groups to respective aldehydes. In addition, the oxidation of citronellol as an example of aliphatic alcohols is found to be more difficult than aromatic alcohols, might be because of the good affinity between the π-electrons on the basal surfaces of graphene support and electrons on the phenyl ring in aromatic alcohols. Meanwhile, the as-fabricated catalyst can be recovered and reused five times without manifest loss of its catalytic efficiency and the selectivity remained almost motionless (>99%). Notable superiorities of the current catalytic strategy are: (a) simple work-up methodology; (b) green oxidizing agent; (c) no external additives or nitrogenous bases; (d) low-cost oxidant and catalyst; (e) complete conversion and selectivity; (f) extremely rapid reaction; (g) mild reaction conditions; (h) excellent recyclability and (i) applicable to a variety of alcohols. All these features will be very much useful and can be industrially applicable for the oxidation of alcohols.

Acknowledgment

The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the research group project No. RG-1436-032.

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Appendix A

Supplementary material

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2018.03.021.

Appendix A

Supplementary material

Supplementary data 1

Supplementary data 1


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