Abstract
Nanocomposites of highly reduced graphene oxide (HRG) and ZnOx nanoparticles doped manganese carbonate containing different percentages of HRG were prepared via a facile co-precipitation method. The prepared sample calcined at 300 °C yielded i.e., ZnOx(1%)–MnCO3/(X%)HRG (where X = 0–7), calcination at 400 °C and 500 °C, yielded different manganese oxides i.e., ZnOx(1%)–MnO2/(X%)HRG and ZnOx(1%)–Mn2O3/(X%)HRG respectively. The prepared catalyst were subjected to catalytic evaluation and a comparative catalytic study between carbonates and oxides for the liquid-phase aerobic oxidation of benzylic alcohols to corresponding aldehydes using molecular oxygen as an eco-friendly oxidant without adding additives or bases. The influence of various parameters such as percentage of HRG, reaction time, catalyst amount, calcination and reaction temperature was systematically examined to optimize reaction conditions using oxidation of benzyl alcohol as a substrate model. It was found that the catalytic performance is remarkably enhanced after using HRG as catalyst co-dopant for the aerobic oxidation of alcohols, possibly owing to the presence of carbon defects and oxygenated functional groups on HRG surface. The as-synthesized catalysts were characterized by SEM, EDX, XRD, Raman, TGA, BET, and FT-IR. Under optimal conditions, the catalyst with composition ZnOx(1%)–MnCO3/(1%)HRG calcined at 300 °C exhibited remarkable specific activity (57.1 mmol·g−1·h−1) with 100% conversion of benzyl alcohol and more than 99% product selectivity within extremely short time (7 min). The as-prepared catalyst was re-used up to five consecutive times without significant decrease in its activity and selectivity. To the best of our knowledge, the achieved specific activity is the highest so far compared to the earlier reported catalysts used for the benzyl alcohol oxidation. A wide range of substituted benzylic and aliphatic alcohols were selectively oxidized into their corresponding aldehydes with complete convertibility and selectivity in short reaction times without over-oxidation to the acids. Due to their significant low cost, superior reproducibility, excellent catalytic efficiency, the ZnOx(1%)–MnCO3/(X%)HRG nanocomposites possess several application prospect in other organic chemistry reactions.
Highlights
Selective oxidation of aromatic and aliphatic alcohols to their corresponding carbonyl compounds is a significant and fundamental organic transformation because the carbonyl compound i.e., aldehyde and ketone derivatives formed are important starting material/intermediate in the confectionaries, perfumery, food, dyestuffs, cosmetics, and pharmaceutical industries [1,2,3,4,5]
In continuation of our efforts on the use of various mixed metal oxide nanoparticles as an efficient catalyst for the aerobic oxidation of benzylic and aliphatic alcohols [15,25,30,52,62], we report here, a simple and straightforward procedure for the synthesis of ZnOx nanoparticles doped manganese carbonates and oxides supported with different percentages of highly reduced graphene and employed for aerial oxidation of alcohols in order to understand the effect of presence of highly reduced graphene oxide (HRG) in the catalytic system (Scheme 1)
The influence of reaction temperature on the aerial oxidation of benzyl alcohol is explored by varying the temperature from 20 ◦ C to 100 ◦ C while using ZnOx (1%)–MnCO3 /(1%)HRG as catalyst
Summary
Selective oxidation of aromatic and aliphatic alcohols to their corresponding carbonyl compounds is a significant and fundamental organic transformation because the carbonyl compound i.e., aldehyde and ketone derivatives formed are important starting material/intermediate in the confectionaries, perfumery, food, dyestuffs, cosmetics, and pharmaceutical industries [1,2,3,4,5]. Oxidation of alcohols into corresponding aldehydes and ketones via conventional methods includes harmful, toxic, corrosive oxidizing agents such as (chromate, hypochlorite or permanganate etc.). Ru [18] as a catalyst for the aerobic oxidation of alcohols with high catalytic activities and selectivities, often these catalysts have disadvantages owing to being expensive, difficulty in preparation, and rarity of these noble metals that limit their applications [19]. It has been extensively reported that the catalytic performance of metal oxide nanoparticles catalysts remarkably improved upon doping with other metals probably due to the synergistic effects between metal and metal oxide nanoparticles as well as the significant augmentation of the surface area [30]
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