Ag2O–MnO2/Graphene Oxide Nanocomposite

Catalytic oxidation of alcohol to their analogous carbonyls is one of the key organic reactions in both scientific and industrial applications, with universal production of 10,000 million tons/year of carbonyls in the 20th century . Such as, aldehyde and ketone derivatives are extensively employed as precursors in insecticide, flame-retardant, cosmetic, confectionery, flavoring, pharmaceutical, and beverage industries . Traditionally, the current oxidation process depends on the utilization of strong stoichiometric oxidizing agents, which are usually expensive and toxic (e.g., permanganate, hypochlorite, or dichromate, etc.) . Moreover, the strong nature of these conventional oxidizing agents leads to an over-oxidation of alcohols to CO or carboxylic acids, thus decreasing the selectivity towards aldehydes . Alternatively, due to the growing environmental concerns, the applications of eco-friendly oxidants, such as molecular O and aqueous H O , have been preferred, which only produces water as the byproduct . Created by: Farooq Syed , Mujeeb Khan , Mohammed Rafi Shaik , Mufsir Kuniyil , M Rafiq Siddiqui , Abdulrahman Alwarthan

Moreover, graphene-based materials have also attracted increasing attention in the field of catalysis due to their extraordinary optical, electronic, and catalytic properties . The unique 2D structure of graphene allows introducing a variety of functionalities on its surface through covalent and non-covalent interactions . This property can be successfully exploited for the development of graphene-based sustainable catalysts . In this regard, the precursors of graphene, i.e., graphene oxide (GRO)-based materials, have generated significant excitements in the field of catalysis due to their remarkable physicochemical and surface properties . Particularly, the presence of a variety of oxygenated groups, a large number of defects, and a unique 2D structure have rendered graphene oxide as perfect material in catalysis . In addition, the negatively charged surface of GRO can be easily exploited for the dispersion of other catalytically active materials, including metal and metal oxide nanoparticles (NPs), on its surface to enhance their resulting properties . Several metal and/or metal oxide NPs like Pd , Ag , Co O , ZrO , and Fe O have incorporated on GRO nanolayers and achieved superior performance and selectivity. Recently, GRO-based metal nanocomposites have been explored and exhibited considerable potential in the oxidation of various organic moieties, including alcohols , cyclohexane , amines , benzene , aryl benzene and alkenes . It has found that the GRO in these nanocomposites has a positive impact in enhancing the catalytic efficiency and selectivity.
The methods for the binding of nanomaterials on the surface of GRO are broadly classified in two different categories i.e., post immobilization (ex-situ hybridization) and in situ binding (in-situ crystallization). In the post immobilization method, separately prepared dispersions of graphene or graphene oxide and pre-synthesized NPs are mixed to obtain nanocomposite, whereas in-situ binding is performed by the simultaneous reduction of graphene oxide and metal precursors . So far, several methods have been reported for the synthesis of graphene-based nanocomposites, including chemical, electrochemical, microwave based synthesis etc. Among these methods, chemical method is widely used for the large-scale synthesis of graphene-based nanocomposites. However, this method typically involves hazardous chemicals and functionalization ligands, which often have adverse effects on the environment.
This gave the impetus to researchers to find alternative eco-friendly methods for the synthesis of graphene based nanocomposites . Recently, the environmental friendly mechanochemical preparation methods have emerged as suitable alternative to the commonly applied chemical approaches for the preparation of nanomaterials with outstanding properties for advanced applications. Nowadays, these methods are also gaining significant popularity for the preparation of graphene-based nanocomposites XRD analysis is an imperative technique to study the crystallinity of the synthesized samples. Figure 1 illustrates     The morphology of the as-prepared (1%)Ag O-MnO and (1%)Ag O-MnO /(5 wt.%)GRO nanocomposite is also examined by SEM as displayed in Figure 5. The SEM micrographs of the catalyst without GRO i.e.
(1%)Ag O-MnO showed well-defined cuboidal shaped particles with micro size. Whereas, the nanocomposite displayed much smaller cuboidal shaped microparticles due to ball mill process. The nanoparticles after ball milling causes a smaller but wide size distribution. This will lead to increase the total surface area of the catalyst.
Furthermore, the BET analysis has revealed the surface area of the samples, the surface area of (1%)Ag O-MnO (without GRO) is approximately 84 m .g as demonstrated in Table 2. Whereas, the nanocomposite after doping with GRO demonstrated an enhanced surface area of 158 m .g . These results are in good agreement with the results of SEM and TEM analyses, which revealed the presence of small-sized nanoparticles in the composite. Hence, it is assume that the preparation of (1%)Ag O-MnO /(5 wt.%)GRO by the induction of GRO through an eco-friendly mechanochemical process leads to the formation of a stable and efficient nanocatalyst.

Influence of wt% GRO
The efficacy of the catalytic performance could be enhanced using graphene or its derivatives as a supporting material and an efficient promoter . Previously, we have found that the silver oxide nanoparticles was found to be an efficacious promoter to the manganese dioxide, and the ( i.e. (1%)Ag O-MnO /(X wt.%)GRO (X = 1 -7), it is found to have a strong impact on the catalytic efficacy.
The obtained results from the catalytic tests were summed up in Table 1.
As seen in Table 1

Role of various graphene supports
Based on our former reported publications, we have also compared the efficiency of Ag ONPs/MnO immobilized on various graphene supports such as GRO and highly reduced graphene oxide (HRG) for oxidation of BnOH to understand the role of graphene. The obtained data were compiled in Table 2 and depicted in Figure 6. As displayed in  the GRO support owing to the existence of superabundant oxygen-carrying groups, which also behaves as nucleation sites. Nevertheless, with respect to HRG, the nucleation sites are less in number that results in more aggregated growth of active catalyst. In addition, the specific surface area of GRO based nanocomposite higher than that of HRG-nanocomposite. Therefore, it can be stated that utilizing GRO for synthesizing the metallic oxide catalysts could be useful for homogeneous growth of catalyst on the surface. Figure 6. Impact of time on catalytic performances of fabricated catalysts.

Impact of temperature
In general, the temperature played an important role in the catalytic system and had an obvious impact on catalytic efficacy of the catalyst. Therefore, a series of reactions were carried out at different Whilst, the selectivities to BnH were achieved <99% for all catalysts.   7. Impact of temperature on the BnOH conversion using various catalysts.

Impact of catalyst dosage
To other factors constant and the attained catalytic data were plotted in Figure 8. Figure 8 clearly shows that BnOH conversion increased linearly as the catalyst doses raised from 50 mg to 300 mg. However, the selectivity to aldehyde was almost motionless throughout oxidation processes (<99% the BnH is only produced from the catalytic oxidation of BnOH and not from toluene oxidation. Additionally, a blank test (no catalyst) has also been carried out at the optimized circumstances to ascertain that the produced BnH is being obtained owing to the catalytic efficacy of the fabricated catalyst. It was noticed that no BnH has been noticed, proving that the synthesized catalyst is indispensable for this transformation. Furthermore, to illustrate the importance of the oxidizing agent (O ), the experiment was performed over (1%)Ag O-MnO /(5 wt.%)GRO catalyst using air without bubbling O . At the optimum circumstances, the obtained data displayed that only 31.8% conversion of BnOH has been detected, which is much lesser than that of 100% convertibility achieved when the process is conducted using gaseous O .
Recyclability and stability of the catalyst is a significant aspect in enhancement of heterogeneous catalysis systems for the industrial applications. The reusability of the (1%)Ag O-MnO /(5 wt.%)GRO nanocomposite was investigated for the catalytic oxidation of BnOH at the optimal circumstances. The catalytic activities of the (1%)Ag O-MnO /(5 wt.%)GRO catalyst at different cycles for BnOH oxidation was illustrated in Figure 9. The reusability data disclosed that we are able to reuse the fabricated catalyst for six runs with no manifest loss in its performance after every cycle. CuNPs@rGO composite as an effective catalyst for the BnOH oxidation, but it needs a relatively longer reaction time (16 h) to give <99% conversion and 98.6% selectivity towards BnH alongside lower specific performance of 8 mmol.g .h . Moreover, Ag O-MnO /(5 wt.%)GRO has found to be an efficient and selective for the aerial oxidation of various kinds of alcohols, an indication of its considerable versatility. According to Table 4, all primary benzylic alcohols were easily oxidized to their respective aldehyde derivatives with 100% conversions within relatively short periods (Table 4, entries 1-11). Besides, perfect selectivities (<99%) toward respective aldehydes were accomplished for all alcohols that used in this study without further oxidation to carboxylic acids. It can be observed that the nature of substituents (electron-releasing or electronwithdrawing) on the benzylic alcohols has an explicit impact on the oxidation rate. Benzylic alcohols bearing electron-releasing groups were found to be more active and were oxidized to the respective aldehydes in shorter reaction periods. The rate of oxidation process was slower when the benzyl alcohol contained an electron-withdrawing group. That might ascribed to the decrease of electronic cloud density on aromatic ring caused by -ve induction effect . Additionally, it was observed that the p-substituted benzyl alcohols is easily oxidized by comparing with the ortho-and meta-substituted alcohols, presumably due to the para-derivative has lower steric hindrance by comparing with other derivatives . In this regard, p-methylbenzyl alcohol was wholly converted to p-methylbenzaldehyde within only 35 minutes (Table 4, entry 3), whereas o-and m-methylbenzyl alcohol were fully oxidized within longer times of 45 and 50 minutes, correspondingly (Table 4, entries 4 and 5). It was also found that the steric hindrance had a significant effect on the oxidation rate, the bulk group (e.g., trifluoromethyl, trimethoxy and pentaflouro) connected to benzyl alcohol reduce the efficiency of the oxidation process, might be owing to the steric hindrance that prohibits oxidation of the alcohols possessing bulk groups (Table 4, entries 9-11) . It is noteworthy to mention that cinnamyl alcohol as an example of allylic alcohol achieved 100% conversion and <99% selectivity of cinnamaldehyde within 50 minutes (Table 4, entry 12). Regarding furfuryl alcohol as an example of hetero-aromatic alcohol has also wholly transformed to furfural after 110 minutes (Table 4, entry 13). Moreover, the current catalytic methodology has also applicable to the oxidation of secondary benzylic alcohols, for example, α-phenyl-ethanol was selectively transformed to acetophenone with complete conversion within only 25 min ( Indeed, the oxygenation of aliphatic alcohols is significantly more complicated with respect to the aromatic counterparts , excellent results are obtained by oxidation of primary aliphatic alcohols using the present catalytic strategy. In this regard, the oxidation of citronellol into the citronellal occurs in relatively longer reaction times (Table 4, entries 15). Compared to secondary benzylic alcohols, the oxidation of secondary aliphatic alcohols showed a lower reactivity towards this oxidation transformation.
Obviously, it was indispensable to elongate the time, due to the oxidation of benzylic secondary alcohols is easier than that of aliphatic ones. As estimated, full oxidation of α-phenyl-ethanol occurs within only 25 minutes, while the entire oxidation of 2-octanol occurred after longer time of 240 minutes (Table 4, entries 14 and 16). Accordingly, the present catalytic methodology has affected by dual factors, electronic and steric impacts. As a conclusion, in general, the current catalytic oxidation protocol has found to be efficacious for oxygenation of various kinds of alcohols include benzylic, aliphatic, allylic, heterocyclic, primary and secondary alcohols, indicating the versatility of Ag O-MnO /(5 wt.%)GRO catalyst for aerial selective alcohol oxidation. The Ag O-MnO was prepared separately through a co-precipitation procedure. In brief, stoichiometric amounts of AgNO and Mn(NO ) were dissolved in distilled water (100 ml) followed by the dropwise addition of 0.5 M NaHCO solution at 100 °C for 3 hrs (till the pH of the resultant solution reaches to 9 maintain the temperature inside the container, the milling process was paused at regular intervals).
The prepared materials were characterized using several instruments and all experimental details are described in the supplementary file.
Oxidation of benzyl alcohol was performed in a glass flask equipped with a magnetic stirrer, reflux condenser, and thermometer. A mixture of benzyl alcohol (2 mmol), toluene (10 mL), and the catalyst (0.3 g) was transferred in a glass three-necked round-bottomed flask; the resulting mixture was then heated to the desired temperature with vigorous stirring. The oxidation experiment was started by bubbling O gas at a flow rate of 20 mL.min into the reaction mixture. After the reaction, the solid catalyst was filtered off by centrifugation and the liquid products were analyzed by gas chromatography to determine the conversion of the alcohol and product selectivity by (GC, 7890A) Agilent Technologies Inc, equipped with a flame ionization detector (FID) and a 19019S-001 HP-PONA column.
After the completion of first oxidation process, the used catalyst was separated by centrifuge, then washed many times with toluene and dried at 95 °C for 5 hrs for the next run. The dried catalyst was used for next run under similar aforementioned conditions.
Herein we report a cost-effective and eco-friendly mechanochemical approach for the preparation of GRO time. The obtained specific performance (13.3 mmol.g .h ) was much better than that presented in previous literature. Our catalytic strategy is highly selective, producing only desired products with no over-oxygenation to carboxylic acids. The merits of our catalytic methodology are: (a) facile process, (b) inexpensive and clean oxidant, (c) no surfactants or nitrogenous bases are required, (d) mild catalytic conditions, (e) cost-effective recoverable catalyst, (f) complete convertibility, (g) full selectivity, (h) rapid process and (i) applicable to virtually all types of alcohols. So, these highlights make this catalytic strategy to be highly applicable in the industrial applications for manufacturing of carbonyls.