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Comparative Catalytic Evaluation of Nano-Zr[O.sub.x] Promoted Manganese Catalysts: Kinetic Study and the Effect of Dopant on the Aerobic Oxidation of Secondary Alcohols.

1. Introduction

The catalytic oxidation of alcohols to carbonyl compounds is one of the most valuable and significant organic transformations in synthetic chemistry from the scientific and manufacturing perspective [1-5]. The oxidation products are significant intermediates in perfumes, confectionary, flame-retardants, dyestuffs, cosmetics, agrochemical, and pharmacological industries [6-9]. Conventionally, the oxidation of alcohols into their respective carbonyl compounds is achieved by adding stoichiometric quantities of chromate, hypochlorite, or permanganate as oxidants, which are not environmentally friendly as they are toxic and corrosive in nature [10,11]. Moreover, the desired organic transformation requires harsh conditions such as high temperature and pressure. This process also has some drawbacks, it generates a huge quantity of pollutant and toxic by-products [12, 13]. In comparison, water is the only by-product, by using ecofriendly and low cost oxidants, such as molecular [O.sub.2] to produce carbonyl compounds, so this approach has gained significant attention from the economic and environmental prospective [14, 15]. Furthermore, there are many oxidation catalysts prepared by employing noble metals, such as gold [16-21], palladium [22-25], platinum [26, 27], rhodium [28, 29], and ruthenium [30, 31], which have been extensively utilized for the aerial oxidation of alcohols with high catalytic performances. Consequently, a significant effort has been made in order to explore eco-friendly and low cost catalysts such as nonnoble metals like copper [32-34], cobalt [35-37], nickel [38-40], iron [41, 42], vanadium [43], silver [44], chromium [45,46], molybdenum [47,48], rhenium [49], and zinc [50-52] for aerobic oxidation of alcohols. In addition, it has been extensively reported that the catalytic activity of mixed metal oxide nanoparticles catalysts enhanced remarkably upon doping with other metals probably due to the extremely high surface area of metal nanoparticles [53, 54].

Furthermore, manganese carbonate and various mixed manganese oxide and noble metal doped/supported "Mn" oxides were extensively employed for the oxidation of numerous organic compounds, for instance, oxidation of naphthalene [55], carbon monoxide [56, 57], toluene [58], olefins [59], ethylene and propylene [60], cyclohexane [61], benzene [62], alkyl aromatics [63], nitrogen monoxide [64], and formaldehyde [65].

We have earlier reported mixed metal oxides [19, 44, 53, 54] and metal oxides doped with other transition metals nanoparticles as catalyst such as Ag NPs doped manganese dioxide [44]. With the continued interest in our studies to find new and improved catalysts we carried out a comparative study of Zr[O.sub.x]-MnC[O.sub.3] or Zr[O.sub.x]-[Mn.sub.2][O.sub.3] for the oxidation of primary alcohols [66] and it was found that the Zr[O.sub.x]-MnC[O.sub.3] was an excellent catalyst for the oxidation of primary aromatic alcohols to corresponding aldehydes with molecular [O.sub.2]. In the present report we extend the study further with respect to the oxidation of secondary alcohols. Herein, we report the synthesis of X% Zr[O.sub.x]-MnC[O.sub.3] (where X = 0, 1, 3, 5, and 7), followed by calcination at elevated temperatures, which yielded 1% Zr[O.sub.x]-Mn[O.sub.2] and 1% Zr[O.sub.x]-[Mn.sub.2][O.sub.3]. A comparative study of the catalysts towards the oxidation of secondary benzylic alcohols to the corresponding ketones was carried out employing molecular [O.sub.2] as green oxidizing agent and the results obtained were also compared with the results of the oxidation of primary alcohols and based on this detailed investigation some inferences have been drawn. The oxidation of 1-phenylethanol to acetophenone was selected as a model reaction for optimization of the process. The present procedure is simple, straightforward, mild, and environment-friendly and water is the only by-product in this reaction. It was found that all alcohols used in this study were completely oxidized to corresponding aldehydes and ketones without using any additives or base. Furthermore, the synthesized catalysts have characterized by several types of techniques such as SEM, EDX, TEM, XRD, TGA, and BET.

2. Materials and Methods

2.1. Materials. Manganese(II) nitrate-tetrahydrate (97%), zirconium nitrate (99%), sodium bicarbonate (99%), toluene (98%), benzyl alcohol (99.5%), biphenyl-4-methanol (98%), 2-phenylethanol (98%), furfuryl alcohol (98%), cinnamyl alcohol (98%), diphenylmethanol (99%), 4-chlorobenzhydrol (98%), 1-phenylethanol (98%), 1-(4-chlorophenyl)ethanol (98%), 1-phenyl-2-propanol (98%), 4-phenyl-2-butanol (97%), cyclohexanemethanol (99%), 1-octanol (99%), 5hexen-1-ol (98%), [beta]-citronellol (98%), cyclohexanol (99%), 3-buten-2-ol (97%), and 2-octanol (99%) were purchased from Sigma Aldrich, St. Louis, MO 63118, USA.

2.2. Catalyst Preparation. Zr[O.sub.x] nanoparticles doped MnC[O.sub.3] catalysts of the type X% Zr[O.sub.x]-MnC[O.sub.3] (where X = 0, 1, 3, 5, and 7) were prepared via coprecipitation method where X% denotes w/w%. Stoichiometric amounts of manganese (II) nitrate-tetrahydrate (Mn[(N[O.sub.3]).sub.2]x4[H.sub.2]O) and zirconium nitrate (Zr[(N[O.sub.3]).sub.2]) were dissolved in distilled water. About 100 mL of the stoichiometric mixture of solutions was taken in a round bottomed flask. The solution was heated to 100[degrees]C, while stirring was carried on a mechanical stirrer and 0.5 M solution of sodium hydrogen carbonate (NaHC[O.sub.3]) was added dropwise until the solution attained pH = 9. The solution was continuously stirred at the same temperature for about 3 hours and then left on stirring overnight at room temperature. The solution was filtered using a Buchner funnel under vacuum and the product obtained was dried at 70[degrees]C overnight and calcined at different temperatures (Scheme 1).

2.3. Catalyst Characterization. The morphology of the assynthesized nanocomposite was examined by SEM (Jeol SEM model JSM 6360A (Japan)). Quantitative analysis of the nanocomposite was performed by energy-dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) was carried out using Jeol TEM model JEM-1101 (Japan), which was castoff to identify the size and shape of nanoparticles. Powder X-ray diffraction studies were carried out using Altima IV [Make: Regaku] X-ray diffractometer. BET surface area was investigated on NOVA 4200e surface area and pore size analyzer. The thermal stabilities of the nanoparticles were characterized by thermogravimetric analysis (TGA), using a Pyris 1 TGA instrument (PerkinElmer, USA), with a heating rate of 10[degrees]C/min under a nitrogen gas flow at 20 mL/min. The temperature range was maintained from room temperature to 800[degrees]C using a ceramic pan.

2.4. General Procedure of Oxidation of Alcohols. The protocol followed for the oxidation of alcohols is as previously reported [66].

3. Results and Discussion

3.1. Characterization of the Catalysts

3.1.1. Morphology and Phase Structure. The prepared catalyst by coprecipitation technique was calcined at 300[degrees]C, 400[degrees]C, and 500[degrees]C. The scanning electron microscopy (SEM) micrographs of the as-synthesized catalyst 1% Zr[O.sub.x]-MnC[O.sub.3] and the product 1% Zr[O.sub.x]-MnC[O.sub.3] calcined at temperature 300[degrees]C, 1% Zr[O.sub.x]-Mn[O.sub.2] at 400[degrees]C, and 1% Zr[O.sub.x]- [Mn.sub.2][O.sub.3] at 500[degrees]C are displayed in Figure 1. The SEM micrographs exhibit particles with a well-defined cuboidal morphology. The particle size distribution graph was obtained by using Image J software program (Figures 1(g), 1(h), and 1(i)) and exhibits merely small differences in the particle sizes with changes in calcination temperature. The elemental composition of the catalyst is examined using energy-dispersive X-ray spectroscopy (EDX) and stays within experimental error to the theoretical composition.

3.1.2. Energy-Dispersive X-Ray Spectroscopy (EDX) Analysis. Moreover, the elemental composition of the zirconia/manganese carbonate nanocomposite was also investigated by energy-dispersive X-ray spectroscopy (EDX), which discloses the elemental composition summary of the catalyst prepared as displayed in Figure 2. The intense signal at 5.5-6 keV strongly indicates that "Mn" was the major element, which has an optical absorption in this range due to the surface plasmon resonance (SPR). The mass % ratio of Mn found to be 98.41% which are almost close to theoretical value 99% as shown in Figure 2. A signal at 2keV strongly corresponds to the presence of "Zr" element. It was also eminent that the other signals were also found in the range 0.0-0.5 keV, which signifies the typical absorption of carbon and oxygen.

3.1.3. XRD Analysis. XRD analysis was used to determine the crystal structure of the nanosized Zr[O.sub.x] doped MnC[O.sub.3] catalyst uncalcined (1% Zr[O.sub.x]-MnC[O.sub.3]) and calcined at 300, 400, and 500[degrees]C. Figure 3 displays the existence of rhodochrosite and syn manganese carbonate (JCPDS number 00-007-0268) with space group R-3c (167) which upon calcination at 300[degrees]C transformed to rhodochrosite manganese carbonate oxides (JCPDS number 00-001-0981) (space group R-3c (167)) (Figure 3). Calcination at 400[degrees]C leads to the formation of Mn[O.sub.2] (JCPDS-ICDD number 00-44-0141). In case of calcination at 400[degrees]C as shown in Figure 3 the X-ray diffraction pattern exhibits an amorphous form. Calcination at 500[degrees]C leads to the formation of bixbyite [Mn.sub.2][O.sub.3] (JCPDS number 00-002-0909) (Figure 3). The reflections marked with asterisk (*) could be due to the presence of Zr[O.sub.x]. The synthesized nanocomposite catalysts have been compared with known compounds stated in the literature.

3.1.4. High-Resolution Transmission Electron Microscopy (HRTEM) Analysis. The HRTEM images of 1% Zr[O.sub.x]-MnC[O.sub.3] nanocomposite obtained after calcination at 300[degrees]C (Figure 4) exhibit polycrystalline particles with clear lattice fringes. The interplanar distance calculated from the HRTEM image of the sample calcined at 300[degrees]C (Figure 4) revealed d-spacing 0.29 nm and 0.23 nm corresponding to the (104) and the (110) planes of rhombohedral MnC[O.sub.3] and the similar type of polycrystalline particles with clear lattice fringes structures was noticed from the samples 1% Zr[O.sub.x]-Mn[O.sub.2] calcined at 400[degrees]C and 1% Zr[O.sub.x]-[Mn.sub.2][O.sub.3] at 500[degrees]C.

3.1.5. Thermogravimetric Analysis. Thermogravimetric analysis is a sensitive method that measures mass changes in a sample as it is heated. This method is particularly advantageous for determining degradation of samples at different temperatures. The thermogravimetric analysis was accompanied to notice the change in the weight of the synthesized doped and Zr[O.sub.x]/MnC[O.sub.3] nanocomposite catalysts. Figure 5 displays that a gradual weight loss occurred from room temperature to 800[degrees]C. Figure 5 shows typical TGA thermogram of the synthesized 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst in nitrogen environment. The thermal stability of the 1% Zr[O.sub.x]-MnC[O.sub.3] calcined at 300[degrees]C, 1% Zr[O.sub.x]-Mn[O.sub.2] calcined at 400[degrees]C, and 1% Zr[O.sub.x]-[Mn.sub.2][O.sub.3] attained after calcining at 500[degrees]C catalysts was determined using TGA (Figure 5). The catalyst calcined at 300[degrees]C was stable up to 410[degrees]C with a slight weight loss of <8% ascribed to loss of physisorbed moisture. Increasing the temperature leads to a further weight loss of 15% in the temperature range between 405 and 605[degrees]C, which appears to be due to the loss of C[O.sub.2] of MnC[O.sub.3] to form Mn[O.sub.2] and further oxidation of Mn[O.sub.2] to [Mn.sub.2][O.sub.3] as stated from the literature by Zhu et al. [81]. These findings are in agreement with the results (5% weight loss) obtained for heating the catalysts calcined at 500[degrees]C.

3.1.6. BET Analysis. The surface area of the synthesized catalysts was investigated using BET sorption measurements, in order to determine the surface area and deduce the relationship between surface area and catalytic activity of the as-synthesized catalyst for the present study of oxidation of secondary alcohols. Table 1 exhibited that the specific surface area of the synthesized catalyst calcined at different temperatures such as 300[degrees]C, 400[degrees]C, and 500[degrees]C, that is, 1% Zr[O.sub.x]-MnC[O.sub.3], 1% Zr[O.sub.x]-Mn[O.sub.2], and 1% Zr[O.sub.x]- [Mn.sub.2][O.sub.3], respectively, was about 133.58,53.19, and 17.48 [m.sup.2] x g[.sup.-1], respectively. It can be observed that the 1% Zr[O.sub.x]-MnC[O.sub.3] (i.e., the synthesized material calcined at 300[degrees]C temperature) have higher specific surface area when compared to the catalysts obtained by calcining the prepared material at higher temperatures, that is, 400[degrees]C and 500[degrees]C. The results obtained suggest that at higher calcinations temperatures there is a decrease in the surface area due to sintering. Thus, this may partially be responsible for high catalytic efficiency in case of the catalyst calcined at 300[degrees] C, whereas, in case of 400 and 500[degrees]C calcination temperature, there is a considerable decrease in the specific surface area, possibly due to the agglomeration of Zr[O.sub.x] NPs, which has adverse effect on the catalyst functioning, which leads to poor alcohol conversion. From the above findings, it can be said that calcination treatment of the catalysts plays a significant role in changing the surface area of the as-prepared catalysts, which in turn affects the functioning of the catalyst.

3.2. Catalytic Performances. In order to evaluate the catalytic performance of the prepared material, the aerobic oxidation of secondary alcohol using 1-phenylethanol was used as the model reactant (Scheme 2). This particular reaction was also used as an ideal reaction for optimizing the reaction conditions for other reactions. The effects of several parameters such as effect of % loading of promoter Zr[O.sub.x], reaction time, calcination temperature of the catalyst, catalyst dosage employed, and reaction temperature were studied in detail and the results are presented in Tables 1-4.

3.2.1. Effect of Calcination Temperature on the Catalytic Performance. The catalytic activity of the prepared catalyst calcined at various temperatures 300[degrees]C, 400[degrees]C, and 500[degrees]C, that is, 1% Zr[O.sub.x]-MnC[O.sub.3],1% Zr[O.sub.x]-Mn[O.sub.2], and Zr[O.sub.x]- [Mn.sub.2][O.sub.3], respectively, was studied. It was found that the catalysts studied exhibited a variation in catalytic performance indicating the effect of calcination temperature [13]; nevertheless all the catalysts displayed high selectivity towards acetophenone (>99%). For instance, the catalyst calcined at 300[degrees]C, that is, 1% Zr[O.sub.x]-MnC[O.sub.3], exhibited the highest catalytic conversion yielding 100% conversion of 1-phenylethanol within 20 min and the specific activity calculated was found to be about 20.0 mmol x [g.sup.-1] x [h.sup.-1] (Table 1, entry 1). Further, similar studies employing the other catalysts, obtained by calcining the as-prepared material at different temperatures such as 400[degrees]C and 500[degrees]C, that is, 1% Zr[O.sub.x]-Mn[O.sub.2] and 1% Zr[O.sub.x]- [Mn.sub.2][O.sub.3], were carried out; it was found that the catalyst calcined at 400[degrees]C, that is, 1% Zr[O.sub.x]-Mn[O.sub.2], yielded ~70% alcohol conversion, while the catalyst calcined at 500[degrees]C, that is, 1% Zr[O.sub.x]-[Mn.sub.2][O.sub.3], yielded ~61% conversion. Moreover, as the calcination temperature has evident effect on the surface area of the catalyst, this in turn has an effect on the catalytic performance of the catalyst. Hence, in order to draw a correlation between the calcination temperature and surface area, the surface area of the prepared catalysts was determined employing BET; the results are tabulated in Table 1. It was observed that the specific surface area of the prepared catalyst calcined at various temperatures, that is, 1% Zr[O.sub.x]-MnC[O.sub.3], 1% Zr[O.sub.x]-Mn[O.sub.2], and Zr[O.sub.x]-[Mn.sub.2][O.sub.3], was 133.58, 53.19, and 17.48 [m.sup.2] x [g.sup.-1], respectively. The catalyst 1% Zr[O.sub.x]- MnC[O.sub.3] calcined at 300[degrees]C was found to possess the highest surface area among all other catalysts obtained after calcination at different temperatures. Interestingly, the catalyst calcined at 300[degrees]C, that is, 1% Zr[O.sub.x]-MnC[O.sub.3], possesses highest surface area and gave a 100% conversion product, while the catalyst calcined at 400 and 500[degrees]C, that is, 1% Zr[O.sub.x]-Mn[O.sub.2] and Zr[O.sub.x]-[Mn.sub.2][O.sub.3], was found to possess lower surface area also showing lower 1-phenylethanol conversion. Hence, it can be concluded that the catalytic performance was strongly affected by calcination treatments of the catalyst. Therefore, we chose to use 300[degrees]C as the best calcination temperature to optimize other parameters. The results including alcohol conversion, surface area, specific activity, and acetophenone selectivity over the catalyst was listed in Table 1 and plotted in Figure 6.

3.2.2. Effect of % of Zr[O.sub.x] on the Catalytic Performance. As implied by the literature, the presence of a promoter in a catalytic system enhances the catalytic performance of the catalyst many folds. In the present study in order to study the effect of the presence of Zr[O.sub.x] as promoter and to assess the optimum % of Zr[O.sub.x] doping on MnC[O.sub.3] for best catalytic activity as catalyst for secondary alcohol oxidation, we examined the effect of % Zr[O.sub.x] in the catalytic system by varying load of Zr[O.sub.x] on MnC[O.sub.3] supports from 0 to 7% in the catalyst MnC[O.sub.3], that is, calcined at 300[degrees]C and the obtained catalysts were tested for their catalytic activity. It was found that undoped manganese carbonate catalyst (0% Zr[O.sub.x]-MnC[O.sub.3]) gave about 80.36% conversion of 1-phenylethanol within 20 min of reaction time, and the calculated specific activity was found to be 16.07 mmolx[g.sup.-1]x[h.sup.-1] (Table 2, entry 1). However, after doping Zr[O.sub.x] nanoparticles on MnC[O.sub.3], the catalytic performance remarkably improved, and the resultant catalyst yielded a 100% conversion of 1-phenylethanol within 20 min with a specific activityof 20.0 mmolx[g.sup.-1]x[h.sup.-1] (Table 2, entry 2), while, the other catalysts with higher % of Zr[O.sub.x] nanoparticles doping, that is, 3% Zr[O.sub.x]-MnC[O.sub.3], 5% Zr[O.sub.x]-MnC[O.sub.3], and 7% Zr[O.sub.x]-MnC[O.sub.3], yielded lower percentage of alcohol conversion with 95.41, 63.97, and 42.46%, respectively, resulting in the decrease in the specific activity of the catalyst from 19.08 to 8.49 mmolx[g.sup.-1]x[h.sup.-1] (Table 2, entries 3-5). In addition, the selectivity towards acetophenone remains almost constant (<99) throughout all experiments. From the results obtained it can be concluded that Zr[O.sub.x] nanoparticles play a crucial role in enhancing the catalytic efficiency for the aerobic oxidation of 1-phenylethanol into acetophenone, however as the % of Zr[O.sub.x] increases beyond 1%, a negative impact on the catalytic performance of the catalyst was observed, which may be due to the agglomeration of Zr[O.sub.x] nanoparticles. Therefore, the 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst was the best catalyst among all catalysts synthesized. Consequently, we choose to use 1% Zr[O.sub.x]-MnC[O.sub.3] for further optimization studies. Graphical representation of the results is plotted in Figure 7 and is listed in Table 2.

3.2.3. Effect of Reaction Temperature on the Catalytic Performance. Further studies towards evaluating the influence of reaction temperature on the selective oxidation of 1-phenylethanol was carried out in presence the 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst. The temperature of the reaction was altered from 20[degrees]C to 100[degrees]C and the results include alcohol conversion, specific activity, and selectivity to acetophenone which was monitored. The results obtained by carrying out the reaction at temperatures such as 20,40, 60, 80, and 100[degrees]C are summarized in Table 3 and plotted in Figure 8. It was found that the rate of oxidation reaction of 1-phenylethanol to acetophenone is very much dependent on the reaction temperature. From the results obtained it was found that the optimum temperature for the complete conversion of 1-phenylethanol to acetophenone is 100[degrees]C with the specific activity of 20.0 mmolx[g.sup.-1]x[h.sup.-1]. At lower reaction temperatures it was found that the same catalyst yields a conversion of 1-phenylethanol ranging from 44.82% obtained at 20[degrees]C (Table 3, entry 1) to 87.96% obtained at 80[degrees]C (Table 3, entry 4). However, the selectivity towards acetophenone remained unchanged with >99% while the reaction temperature was varied. Therefore, it was realized that the optimum reaction temperature for this conversion is 100[degrees]C, which was used for all the further studies carried out.

3.2.4. Effect of Catalyst Amount on the Catalytic Performance. The catalyst quantity also has significant effect on the catalytic performance for any conversion reaction; hence a study was carried out to optimize the amount of catalyst required for the oxidation of 1-phenylethanol. The oxidation process was carried out using 100, 200, 300, 400, and 500 mg of catalyst calcined at 300[degrees]C, that is, 1% Zr[O.sub.x]-MnC[O.sub.3] under the conditions optimized from the earlier study. The selectivity towards acetophenone was almost constant throughout all oxidation experiments (<99%), whereas the 1-phenylethanol conversion increases with catalyst amount increasing. The results revealed that, in presence of a low catalyst concentration (100 mg), a low conversion of 41.81% was obtained, which may be owing to the occurrence of fewer catalytic active sites (Table 4, entry 1). As expected, by increasing the catalyst amount to 200 mg, the alcohol conversion also increases to 57.66% (Table 4, entry 2). When the catalyst amount was increased to 500 mg the complete conversion product was obtained within a short reaction time (6 min), with the specific activity of 40.0 mmol x [g.sup.-1] x [h.sup.-1] (Table 4, entry 5). From this study, it can be said that the complete oxidation of 1-phenylethanol into acetophenone can be attained within 6 min with 500 mg of the catalyst. A linear relationship was found between the catalyst amount and alcohol conversion as shown in Figure 9. Under the optimum conditions, a blank reaction was also examined in the absence of the catalyst. No formation of acetophenone was observed in this case which indicates that the prepared catalyst plays a fundamental role in the aerial oxidation of 1-phenylethanol.

In order to recognize the performance of the catalyst in comparison to the previously reported studies, a list of reported catalysts has been compiled as shown in Table 5, which demonstrates the results of 1-phenylethanol oxidation by molecular [O.sub.2] in the presence of various catalysts [3, 8, 9, 15, 36, 52, 67-80, 82]. It was found that the prepared 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst is the utmost effective catalyst among all the mentioned catalysts. In present work, the prepared 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst is used for the selective oxidation of 1-phenylethanol to acetophenone and exhibited a complete conversion and good selectivity within extremely short reaction time of 6 min at 100[degrees]C and highest specific activity (40.0 mmolx[g.sup.-1]x[h.sup.-1]) when compared to the other catalysts for the same oxidation reaction, since the other catalysts reported require a longer reaction time to complete oxidation of 1-phenylethanol, higher reaction temperature, or lower specific activity. For example, Du et al. [8] reported liquid phase selective oxidation of 1-phenylethanol to acetophenone using VOP[O.sub.4] catalyst in combination with TEMPO with molecular [O.sub.2] using water as a solvent. The VOP[O.sub.4] catalyst exhibits relatively low alcohol conversion of 38.5%, selectivity to acetophenone about 89%, and the specific activity of this conversion around 8.02 mmolx[g.sup.-1]x[h.sup.-1] within long reaction time of 6 h at 80[degrees]C. In another example, Farhadi and Zaidi [68] have synthesized a polyoxometalate-zirconia (POM/Zr[O.sub.2]) nanocomposite by sol-gel technique used for aerobic oxidation of 1-phenylethanol to acetophenone. The POM/Zr[O.sub.2] nanocomposite exhibited 88% alcohol conversion and more than 99% acetophenone selectivity along with 11.3 mmolx[g.sup.-1]x[h.sup.-1] specific activity after relatively long reaction time 3 h at room temperature. From the above findings, it can be deduced that the synthesized 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst was found to be the best choice for this oxidation reaction.

3.3. Catalyst Recovery. The catalyst reusability has significant importance from both economic and academic point of view; hence the recyclability of 1% Zr[O.sub.x]-MnC[O.sub.3] for the selective oxidation of 1-phenylethanol was evaluated under optimal circumstances and the results are shown in Figure 10. During this study after the completion of oxidation reaction, the solvent toluene was evaporated, and to the recovered catalyst toluene was added, and the mixture was filtered by simple filtration. The filtered catalyst is washed with toluene again to assure that the remnant from the previous reaction is completely washed off; then the recovered catalyst was dried at 100[degrees]C for 4 h. This process was repeated for five cycles and it was found that apparently there is no appreciable decrease in the activity of the catalyst. During the five recycling reactions, the alcohol conversion decreased from 100% to 91.4%, probably due to the catalyst loss during the filtration method [83]. Moreover, the selectivity of the catalyst towards acetophenone is intact even after subsequent reuse. Thus, the results indicate that the catalyst, that is, 1% Zr[O.sub.x]-MnC[O.sub.3], has a perfect recyclability and stability.

3.4. Oxidation of a Variety of Alcohols with Molecular Oxygen Catalyzed by 1% Zr[O.sub.x]-MnC[O.sub.3] Catalyst. Using the optimized conditions including 2 mmol of alcohol in 10 mL toluene with 20mL[min.sup.-1] oxygen flow rate and 100[degrees]C reaction temperature in presence of 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst (0.5 g) calcined at 300[degrees]C, the conversion of a wide range of alcohols including secondary, primary, benzylic, aliphatic, heteroatomic, and allylic alcohols was carried out to understand the catalytic performance of the catalyst against various substrates. It was observed that the alcohols were oxidized into their corresponding carbonyl derivatives in different reaction times (Table 6, entries 1-18). According to Table 6, all secondary benzylic alcohols have completely converted into their corresponding ketones in extremely short reaction times (Table 6, entries 1-6). An excellent selectivity towards ketones (<99) has been achieved in most of oxidation reactions and no by-products were detected in the reaction mixture. Benzhydrol was the most reactive among all secondary benzylic alcohol and gave 100% conversion within only 5 min (Table 6, entry 3). It can be noted that the 4-chlorobenzhydrol required longer reaction time than benzhydrol to complete the conversion, possibly due to the presence of electron-withdrawing chloro group, that deactivates the aromatic ring by decreasing the electron density (Table 6, entry 4). Moreover, 1-phenylethanol and its derivatives also exhibited complete conversion and more than 99% selectivity in relatively short reaction times (Table 6, entries 1 and 2). Commonly, the oxidation of aromatic alcohols is much easier than aliphatic counterparts [84-86]. Besides, compared to secondary aromatic alcohols the oxidation of secondary aliphatic alcohols to their corresponding ketones exhibited relatively low reactivity towards oxidation process (Table 6, entries 7-9). For instance, the secondary aliphatic alcohols such as cyclohexanol (Table 6, entry 7) require longer reaction times than that of the secondary aromatic alcohols as reported in the case of various other catalysts; hence it can be said that the catalyst is selective towards aromatic alcohols. As expected, it was necessary to increase reaction period, owing to the fact that oxidation of aliphatic alcohols is more difficult than that of secondary aromatic alcohols.

When the other primary, benzylic, allylic, heteroaromatic, and aliphatic alcohols were subjected to oxidation using the similar catalyst, the corresponding aldehydes were formed with varying reaction times under optimum conditions (Table 6, entries 10-18). Thus, the catalyst 1% Zr[O.sub.x]-MnC[O.sub.3] demonstrates excellent catalytic activity against aromatic alcohols (primary and secondary aromatic, allylic, and heteroatomic) when compared to the aliphatic alcohols. Although, in case of aliphatic alcohols, 100% conversion was obtained, the reactions require a longer time. Clearly, the studied catalyst demonstrates the selectivity towards aromatic alcohols. Furthermore, it can be concluded that the catalytic performance is affected by the electronic and steric factors.

4. Conclusions

In conclusion, Zr[O.sub.x] nanoparticles doped MnC[O.sub.3] was introduced as an efficient, cheap, and recyclable catalyst for the selective oxidation of secondary alcohol into their corresponding carbonyl compounds with [O.sub.2] as a green oxidant under base-free condition. Interestingly, the catalytic performance MnC[O.sub.3] as oxidation catalyst was enhanced remarkably after doping Zr[O.sub.x] NPs on MnC[O.sub.3] and it exhibited superior catalytic efficiency in the aerial oxidation of 1-phenylethanol to acetophenone compared to the results reported earlier. An extremely high specific activity of 40.0 mmolx[g.sup.-1]x[h.sup.-1] and complete alcohol conversion with more than 99% selectivity towards acetophenone has been achieved in short reaction time (6 min). In addition, wide range of benzylic, aliphatic, and allylic alcohols was also studied for selective oxidation into their corresponding carbonyl compounds, which yielded complete convertibility within short reaction times under mild reaction conditions. This catalytic system was found to possess several advantages such as 100% conversion within very short reaction times with very high specific activities and excellent selectivities. Therefore, the catalyst developed and the optimized reactions can be applicable for aerobic oxidation of other alcohols.

https://doi.org/10.1155/2017/3958319

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

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.

References

[1] C. Brookes, M. Bowker, and P. P. Wells, "Catalysts for the selective oxidation of methanol," Catalysts, vol. 6, no. 7, article 92, 2016.

[2] M. M. Dell'Anna, M. Mali, P. Mastrorilli, P. Cotugno, and A. Monopoli, "Oxidation of benzyl alcohols to aldehydes and ketones under air in water using a polymer supported palladium catalyst," Journal of Molecular Catalysis A: Chemical, vol. 386, pp. 114-119, 2014.

[3] M. C. Reis, S. D. T. Barros, E. R. Lachter et al., "Synthesis, characterization and catalytic activity of meso-niobium phosphate in the oxidation of benzyl alcohols," Catalysis Today, vol. 192, no. 1, pp. 117-122, 2012.

[4] D. Romano, R. Villa, and F. Molinari, "Preparative Biotransformations: oxidation of Alcohols," ChemCatChem, vol. 4, no. 6, pp. 739-749, 2012.

[5] J. C. Calderon, M. Rios Rafales, M. J. Nieto-Monge, J. I. Pardo, R. Moliner, and M. J. Lazaro, "Oxidation of CO and Methanol on Pd-Ni catalysts supported on different chemically-treated carbon nanofibers," Nanomaterials, vol. 6, no. 10, p. 187, 2016.

[6] R. A. Sheldon, I. W. C. E. Arends, and A. Dijksman, "New developments in catalytic alcohol oxidations for fine chemicals synthesis," Catalysis Today, vol. 57, no. 1-2, pp. 157-166, 2000.

[7] Y. Yu, B. Lu, X. Wang, J. Zhao, X. Wang, and Q. Cai, "Highly selective oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide by biphasic catalysis," Chemical Engineering Journal, vol. 162, no. 2, pp. 738-742, 2010.

[8] Z. Du, J. Ma, H. Ma, J. Gao, and J. Xu, "Synergistic effect of vanadium-phosphorus promoted oxidation of benzylic alcohols with molecular oxygen in water," Green Chemistry, vol. 12, no. 4, pp. 590-592, 2010.

[9] M. L. Kantam, R. S. Reddy, U. Pal et al., "Ruthenium/magnesium-lanthanum mixed oxide: An efficient reusable catalyst for oxidation of alcohols by using molecular oxygen," Journal of Molecular Catalysis A: Chemical, vol. 359, pp. 1-7, 2012.

[10] D. I. Enache, J. K. Edwards, P. Landon et al., "Solvent-free oxidation of primary alcohols to aldehydes using Au-Pd/Ti[O.sub.2] catalyst," Science, vol. 311, no. 5759, pp. 362-365, 2006.

[11] G.-J. ten Brink, I. W. C. E. Arends, and R. A. Sheldon, "Green, catalytic oxidation of alcohols in water," Science, vol. 287, no. 5458, pp. 1636-1639, 2000.

[12] M. B. Smith and J. March, March's advanced organic chemistry: reactions, mechanisms, and structure, John Wiley & Sons, and structure, 2007.

[13] G. Zhan, Y. Hong, V T. Mbah et al., "Bimetallic Au-Pd/MgO as efficient catalysts for aerobic oxidation of benzyl alcohol: a green bio-reducing preparation method," Applied Catalysis A: General, vol. 439-440, pp. 179-186, 2012.

[14] T. Mallat and A. Baiker, "Oxidation of alcohols with molecular oxygen on solid catalysts," Chemical Reviews, vol. 104, no. 6, pp. 3037-3058, 2004.

[15] G. Wu, X. Wang, N. Guan, and L. Li, "Palladium on graphene as efficient catalyst for solvent-free aerobic oxidation of aromatic alcohols: Role of graphene support," Applied Catalysis B: Environmental, vol. 136-137, pp. 177-185, 2013.

[16] C. Shang and Z.-P. Liu, "Origin and activity of gold nanoparticles as aerobic oxidation catalysts in aqueous solution," Journal of the American Chemical Society, vol. 133, no. 25, pp. 9938-9947, 2011.

[17] D. Tsukamoto, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka, and T. Hirai, "Gold nanoparticles located at the interface of anatase/rutile TiO 2 particles as active plasmonic photocatalysts for aerobic oxidation," Journal of the American Chemical Society, vol. 134, no. 14, pp. 6309-6315, 2012.

[18] J. Yu, J. Li, H. Wei, J. Zheng, H. Su, and X. Wang, "Hydrotalcite-supported gold catalysts for a selective aerobic oxidation of benzyl alcohol driven by visible light," Journal of Molecular Catalysis A: Chemical, vol. 395, pp. 128-136, 2014.

[19] S. Alabbad, S. F. Adil, M. E. Assal, M. Khan, A. Alwarthan, and M. R. H. Siddiqui, "Gold & silver nanoparticles supported on manganese oxide: Synthesis, characterization and catalytic studies for selective oxidation of benzyl alcohol," Arabian Journal of Chemistry, vol. 7, no. 6, pp. 1192-1198, 2014.

[20] D. Zhu, D. Duan, Y. Han et al., "Noble metal-free ceria-zirconia solid solutions templated by tobacco materials for catalytic oxidation of CO," Catalysts, vol. 6, no. 9, p. 135, 2016.

[21] D. Guo, Y. Wang, P. Zhao et al., "Selective aerobic oxidation of benzyl alcohol driven by visible light on gold nanoparticles supported on hydrotalcite modified by nickel ion," Catalysts, vol. 6, no. 5, article no. 64, 2016.

[22] K. Layek, H. Maheswaran, R. Arundhathi, M. L. Kantam, and S. K. Bhargava, "Nanocrystalline magnesium oxide stabilized palladium (0): an efficient reusable catalyst for room temperature selective aerobic oxidation of alcohols," Advanced Synthesis and Catalysis, vol. 353, no. 4, pp. 606-616, 2011.

[23] Y. Chen, Z. Guo, T. Chen, and Y. Yang, "Surface-functionalized TUD-1 mesoporous molecular sieve supported palladium for solvent-free aerobic oxidation of benzyl alcohol," Journal of Catalysis, vol. 275, no. 1, pp. 11-24, 2010.

[24] Y. Yan, X. Jia, and Y. Yang, "Palladium nanoparticles supported on CNT functionalized by rare-earth oxides for solvent-free aerobic oxidation of benzyl alcohol," Catalysis Today, vol. 259, pp. 292-302, 2014.

[25] S. Gil, J. M. Garcia-Vargas, L. F. Liotta et al., "Catalytic oxidation of propene over Pd catalysts supported on Ce[O.sub.2], Ti[O.sub.2], Al2O3 and M/Al2O3oxides (M = Ce, Ti, Fe, Mn)," Catalysts, vol. 5, no. 2, pp. 671-689, 2015.

[26] A. Frassoldati, C. Pinel, and M. Besson, "Promoting effect of water for aliphatic primary and secondary alcohol oxidation over platinum catalysts in dioxane/aqueous solution media," Catalysis Today, vol. 173, no. 1, pp. 81-88, 2011.

[27] J. Lu, J. Zang, Y. Wang, Y. Xu, and X. Xu, "Preparation and characterization of zirconia-coated nanodiamonds as a Pt catalyst support for methanol electro-oxidation," Nanomaterials, vol. 6, no. 12, p. 234, 2016.

[28] T. Zweifel, J.-V. Naubron, and H. Grutzmacher, "Catalyzed dehydrogenative coupling of primary alcohols with water, methanol, or amines," Angewandte Chemie--International Edition, vol. 48, no. 3, pp. 559-563, 2009.

[29] L. Liu, M. Yu, B. B. Wayland, and X. Fu, "Aerobic oxidation of alcohols catalyzed by rhodium(iii) porphyrin complexes in water: Reactivity and mechanistic studies," Chemical Communications, vol. 46, no. 34, pp. 6353-6355, 2010.

[30] N. Komiya, T. Nakae, H. Sato, and T. Naota, "Water-soluble diruthenium complexes bearing acetate and carbonate bridges: highly efficient catalysts for aerobic oxidation of alcohols in water," Chemical Communications, no. 46, pp. 4829-4831,2006.

[31] A. Zsigmond, F. Notheisz, G. Csjernyik, and J.-E. Backvall, "Ruthenium-catalyzed aerobic oxidation of alcohols on zeoliteen-capsulated cobalt salophen catalyst," Topics in Catalysis, vol. 19, no. 1, pp. 119-124, 2002.

[32] Z. Hu and F. M. Kerton, "Room temperature aerobic oxidation of alcohols using CuBr 2 with TEMPO and a tetradentate polymer based pyridyl-imine ligand," Applied Catalysis A: General, vol. 413-414, pp. 332-339, 2012.

[33] P. Cruz, Y. Perez, I. Del Hierro, and M. Fajardo, "Copper, copper oxide nanoparticles and copper complexes supported on mesoporous SBA-15 as catalysts in the selective oxidation of benzyl alcohol in aqueous phase," Microporous and Mesoporous Materials, vol. 220, pp. 136-147, 2016.

[34] J. E. Steves and S. S. Stahl, "Copper(I)/ABNO-catalyzed aerobic alcohol oxidation: Alleviating steric and electronic constraints of Cu/TEMPO catalyst systems," Journal of the American Chemical Society, vol. 135, no. 42, pp. 15742-15745, 2013.

[35] S. M. Seyedi, R. Sandaroos, and G. H. Zohuri, "Novel cobalt(II) complexes of amino acids-Schiff bases catalyzed aerobic oxidation of various alcohols to ketones and aldehyde," Chinese Chemical Letters, vol. 21, no. 11, pp. 1303-1306, 2010.

[36] C. Ragupathi, J. Judith Vijaya, S. Narayanan, S. K. Jesudoss, and L. John Kennedy, "Highly selective oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide by cobalt aluminate catalysis: a comparison of conventional and microwave methods," Ceramics International, vol. 41, no. 2, pp. 2069-2080,2015.

[37] V. Mahdavi and H. R. Hasheminasab, "Vanadium phosphorus oxide catalyst promoted by cobalt doping for mild oxidation of benzyl alcohol to benzaldehyde in the liquid phase," Applied Catalysis A: General, vol. 482, pp. 189-197, 2014.

[38] S. R. Ali, P. Chandra, M. Latwal, S. K. Jain, V. K. Bansal, and S. P. Singh, "Synthesis of nickel hexacyanoferrate nanoparticles and their potential as heterogeneous catalysts for the solventfree oxidation of benzyl alcohol," Chinese Journal of Catalysis, vol. 32, no. 11-12, pp. 1844-1849, 2011.

[39] A. R. Hajipour, H. Karimi, and A. Koohi, "Selective oxidation of alcohols over nickel zirconium phosphate," Cuihua Xuebao/Chinese Journal of Catalysis, vol. 36, no. 7, pp. 1109-1116, 2015.

[40] T. Kawabata, Y. Shinozuka, Y. Ohishi, T. Shishido, K. Takaki, and K. Takehira, "Nickel containing Mg-Al hydrotalcite-type anionic clay catalyst for the oxidation of alcohols with molecular oxygen," Journal ofMolecularCatalysisA: Chemical, vol. 236, no. 1-2, pp. 206-215, 2005.

[41] R. Naik, A. Nizam, A. Siddekha, and M. A. Pasha, "An efficient sonochemical oxidation of benzyl alcohols into benzaldehydes by FeCl3/HNO3 in acetone," Ultrasonics Sonochemistry, vol. 18, no. 5, pp. 1124-1127, 2011.

[42] R. Cang, B. Lu, X. Li, R. Niu, J. Zhao, and Q. Cai, "Iron-chloride ionic liquid immobilized on SBA-15 for solvent-free oxidation of benzyl alcohol to benzaldehyde with H2[O.sub.2]," Chemical Engineering Science, vol. 137, pp. 268-275, 2015.

[43] C. Wei, F. Xue, C. Miao et al., "Dehydrogenation of Isobutane with Carbon Dioxide over SBA-15-supported vanadium oxide catalysts," Catalysts, vol. 6, no. 11, p. 171, 2016.

[44] S. F. Adil, M. E. Assal, M. Khan, A. Al-Warthan, and M. H. Rafiq Siddiqui, "Nano silver-doped manganese oxide as catalyst for oxidation of benzyl alcohol and its derivatives: synthesis, characterisation, thermal study and evaluation of catalytic properties," Oxidation Communications, vol. 36, no. 3, pp. 778-791, 2013.

[45] W. Amer, K. Abdelouahdi, H. R. Ramananarivo et al., "Oxidation of benzylic alcohols into aldehydes under solvent-free microwave irradiation using new catalyst-support system," Current Organic Chemistry, vol. 17, no. 1, pp. 72-78, 2013.

[46] O. F. Ozturk, B. Zumreoglu-Karan, and S. Karabulut, "Solvent-free oxidation of benzyl alcohol over chromium orthoborate," Catalysis Communications, vol. 9, no. 7, pp. 1644-1648, 2008.

[47] P. S. N. Rao, K. T. V. Rao, P. S. Sai Prasad, and N. Lingaiah, "The role of vanadia for the selective oxidation of benzyl alcohol Over heteropolymolybdate supported on alumina," CuihuaXuebao/Chinese Journal of Catalysis, vol. 32, no. 11, pp. 1719-1726, 2011.

[48] A. V. Biradar, M. K. Dongare, and S. B. Umbarkar, "Selective oxidation of aromatic primary alcohols to aldehydes using molybdenum acetylide oxo-peroxo complex as catalyst," Tetrahedron Letters, vol. 50, no. 24, pp. 2885-2888, 2009.

[49] S. C. A. Sousa, J. R. Bernardo, P. R. Florindo, and A. C. Fernandes, "Efficient and selective oxidation of alcohols catalyzed by oxo-rhenium complexes," Catalysis Communications, vol. 40, pp. 134-138, 2013.

[50] M. Forouzani, H. R. Mardani, M. Ziari, A. Malekzadeh, and P. Biparva, "Comparative study of oxidation of benzyl alcohol: influence of Cu-doped metal cation on nano ZnO catalytic activity," Chemical Engineering Journal, vol. 275, no. 9, pp. 220226, 2015.

[51] M. Hosseini-Sarvari, T. Ataee-Kachouei, and F. Moeini, "A novel and active catalyst Ag/ZnO for oxidant-free dehydrogenation of alcohols," Materials Research Bulletin, vol. 72, pp. 98-105, 2015.

[52] R. Borthakur, M. Asthana, A. Kumar, and R. A. Lal, "Cooperative catalysis by polymetallic copper-zinc complexes in the efficient oxidation of alcohols under solvent free condition," Inorganic Chemistry Communications, vol. 46, pp. 198-201,2014.

[53] M. R. H. Siddiqui, I. Warad, S. F. Adil, R. M. Mahfouz, and A. Al-Arifi, "Nano-gold supported nickel manganese oxide: synthesis, characterisation and evaluation as oxidation catalyst," Oxidation Communications, vol. 35, no. 2, pp. 476-481, 2012.

[54] S. F. Adil, S. Alabbad, M. Kuniyil et al., "Vanadia supported on nickel manganese oxide nanocatalysts for the catalytic oxidation of aromatic alcohols," Nanoscale Research Letters, vol. 10, no. 1, pp. 1-9, 2015.

[55] T. J. Clarke, S. A. Kondrat, and S. H. Taylor, "Total oxidation of naphthalene using copper manganese oxide catalysts," Catalysis Today, vol. 258, pp. 610-615, 2015.

[56] X. Guo, J. Li, and R. Zhou, "Catalytic performance of manganese doped CuO-Ce[O.sub.2] catalysts for selective oxidation of CO in hydrogen-rich gas," Fuel, vol. 163, pp. 56-64, 2016.

[57] M. Tepluchin, S. Kureti, M. Casapu, E. Ogel, S. Mangold, and J.D. Grunwaldt, "Study on the hydrothermal and S[O.sub.2] stability of Al2O3-supported manganese and iron oxide catalysts for lean CO oxidation," Catalysis Today, vol. 258, pp. 498-506, 2015.

[58] S. C. Kim, Y.-K. Park, and J. W. Nah, "Property of a highly active bimetallic catalyst based on a supported manganese oxide for the complete oxidation of toluene," Powder Technology, vol. 266, pp. 292-298, 2014.

[59] F. Ashouri, M. Zare, and M. Bagherzade, "Manganese and cobalt-terephthalate metal-organic frameworks as a precursor for synthesis of [Mn.sub.2][O.sub.3], [Mn.sub.3][O.sub.4] and [Co.sub.3][O.sub.4] nanoparticles: active catalysts for olefin heterogeneous oxidation," Inorganic Chemistry Communications, vol. 61, pp. 73-76, 2015.

[60] M. Piumetti, D. Fino, and N. Russo, "Mesoporous manganese oxides prepared by solution combustion synthesis as catalysts for the total oxidation of VOCs," Applied Catalysis B: Environmental, vol. 163, pp. 277-287, 2015.

[61] Z. Feng, Y. Xie, F. Hao, P. Liu, and H. Luo, "Catalytic oxidation of cyclohexane to KA oil by zinc oxide supported manganese 5,10,15,20-tetrakis(4-nitrophenyl)porphyrin," Journal of Molecular Catalysis A: Chemical, vol. 410, pp. 221-225, 2015.

[62] H. Einaga, S. Yamamoto, N. Maeda, and Y. Teraoka, "Structural analysis of manganese oxides supported on Si[O.sub.2] for benzene oxidation with ozone," Catalysis Today, vol. 242, pp. 287-293, 2015.

[63] A. S. Burange, S. R. Kale, and R. V. Jayaram, "Oxidation of alkyl aromatics to ketones by tert-butyl hydroperoxide on manganese dioxide catalyst," Tetrahedron Letters, vol. 53, no. 24, pp. 29892992, 2012.

[64] G. Qi and W. Li, "NO oxidation to N[O.sub.2] over manganese-cerium mixed oxides," Catalysis Today, vol. 258, no. 1, pp. 205-213,2015.

[65] J. Pei, X. Han, and Y. Lu, "Performance and kinetics of catalytic oxidation of formaldehyde over copper manganese oxide catalyst," Building and Environment, vol. 84, pp. 134-141, 2015.

[66] M. E. Assal, M. Kuniyil, M. Khan et al., "Synthesis and Comparative Catalytic Study of Zirconia-MnC[O.sub.3] or-[Mn.sub.2][O.sub.3] for the oxidation of benzylic alcohols," ChemistryOpen, vol. 6, no. 1, pp. 112-120, 2017.

[67] L. Qi, X. Qi, J. Wang, and L. Zheng, "A synergistic effect in the combination of H2[O.sub.2], FeAPO-5 and NaBr for selective oxidation of benzyl alcohols," Catalysis Communications, vol. 16, no. 1, pp. 225-228, 2011.

[68] S. Farhadi and M. Zaidi, "Polyoxometalate-zirconia (POM/ Zr[O.sub.2]) nanocomposite prepared by sol-gel process: A green and recyclable photocatalyst for efficient and selective aerobic oxidation of alcohols into aldehydes and ketones," Applied Catalysis A: General, vol. 354, no. 1-2, pp. 119-126, 2009.

[69] H. Zhao, W. Sun, C. Miao, and Q. Zhao, "Aerobic oxidation of secondary alcohols using NHPI and iron salt as catalysts at room temperature," Journal of Molecular Catalysis A: Chemical, vol. 393, pp. 62-67, 2014.

[70] T. Yasu-eda, S. Kitamura, N.-O. Ikenaga, T. Miyake, and T. Suzuki, "Selective oxidation of alcohols with molecular oxygen over Ru/CaO-Zr[O.sub.2] catalyst," Journal of Molecular Catalysis A: Chemical, vol. 323, no. 1-2, pp. 7-15, 2010.

[71] H. P. Mungse, S. Verma, N. Kumar, B. Sain, and O. P. Khatri, "Grafting of oxo-vanadium Schiff base on graphene nanosheets and its catalytic activity for the oxidation of alcohols," Journal of Materials Chemistry, vol. 22, no. 12, pp. 5427-5433, 2012.

[72] M. Shaikh, M. Satanami, and K. V. S. Ranganath, "Efficient aerobic oxidation of alcohols using magnetically recoverable catalysts," Catalysis Communications, vol. 54, pp. 91-93, 2014.

[73] J. Hou, Y. Luan, J. Tang, A. M. Wensley, M. Yang, and Y. Lu, "Synthesis of UiO-66-NH2 derived heterogeneous copper (II) catalyst and study of its application in the selective aerobic oxidation of alcohols," Journal of Molecular Catalysis A: Chemical, vol. 407, pp. 53-59, 2015.

[74] J. He, T. Wu, T. Jiang, X. Zhou, B. Hu, and B. Han, "Aerobic oxidation of secondary alcohols to ketones catalyzed by cobalt(II)/ZnO in poly(ethylene glycol)/C[O.sub.2] system," Catalysis Communications, vol. 9, no. 13, pp. 2239-2243, 2008.

[75] A. S. Burange, R. V. Jayaram, R. Shukla, and A. K. Tyagi, "Oxidation of benzylic alcohols to carbonyls using tert-butyl hydroperoxide over pure phase nanocrystalline CeCrO3," Catalysis Communications, vol. 40, pp. 27-31, 2013.

[76] S. R. Cicco, M. Latronico, P. Mastrorilli, G. P. Suranna, and C. F. Nobile, "Homogeneous and heterogeneous catalytic oxidation of benzylic and secondary alcohols with a metal dioxygenato complex in the presence of 2-methylpropanal and dioxygen," Journal of Molecular Catalysis A: Chemical, vol. 165, no. 1-2, pp. 135-140, 2001.

[77] L.-H. Zhou, X.-Q. Yu, and L. Pu, "Na-promoted aerobic oxidation of alcohols to ketones," Tetrahedron Letters, vol. 51, no. 3, pp. 475-477, 2010.

[78] T. Mitsudome, A. Noujima, T. Mizugaki, K. Jitsukawa, and K. Kaneda, "Efficient aerobic oxidation of alcohols using a hydrotalcite-supported gold nanoparticle catalyst," Advanced Synthesis and Catalysis, vol. 351, no. 11-12, pp. 1890-1896, 2009.

[79] A. Abad, P. Concepcion, A. Corma, and H. Garcia, "A collaborative effect between gold and a support induces the selective oxidation of alcohols," Angewandte Chemie International Edition, vol. 44, no. 26, pp. 4066-4069, 2005.

[80] K. Mori, T. Hara, T. Mizugaki, K. Ebitani, and K. Kaneda, "Hydroxyapatite-supported palladium nanoclusters: a highly active heterogeneous catalyst for selective oxidation of alcohols by use of molecular oxygen," Journal of the American Chemical Society, vol. 126, no. 34, pp. 10657-10666, 2004.

[81] C. Zhu, G. Saito, and T. Akiyama, "A new CaC[O.sub.3]-template method to synthesize nanoporous manganese oxide hollow structures and their transformation to high-performance Li[Mn.sub.2][O.sub.4] cathodes for lithium-ion batteries," Journal of Materials Chemistry A, vol. 1, no. 24, pp. 7077-7082, 2013.

[82] J. Long, X. Xie, J. Xu, Q. Gu, L. Chen, and X. Wang, "Nitrogen-doped graphene nanosheets as metal-free catalysts for aerobic selective oxidation of benzylic alcohols," ACS Catalysis, vol. 2, no. 4, pp. 622-631, 2012.

[83] X. Yu, Y. Huo, J. Yang, S. Chang, Y. Ma, and W. Huang, "Reduced graphene oxide supported Au nanoparticles as an efficient catalyst for aerobic oxidation of benzyl alcohol," Applied Surface Science, vol. 280, pp. 450-455, 2013.

[84] E. Assady, B. Yadollahi, M. Riahi Farsani, and M. Moghadam, "Zinc polyoxometalate on activated carbon: An efficient catalyst for selective oxidation of alcohols with hydrogen peroxide," Applied Organometallic Chemistry, vol. 29, no. 8, pp. 561-565, 2015.

[85] S. Hasannia and B. Yadollahi, "Zn-Al LDH nanostructures pillared by Fe substituted Keggin type polyoxometalate: Synthesis, characterization and catalytic effect in green oxidation of alcohols," Polyhedron, vol. 99, pp. 260-265, 2015.

[86] J. Albadi, A. Alihoseinzadeh, and A. Razeghi, "Novel metal oxide nanocomposite of Au/CuO-ZnO for recyclable catalytic aerobic oxidation of alcohols in water," Catalysis Communications, vol. 49, pp. 1-5, 2014.

Mohamed E. Assal, (1) Mufsir Kuniyil, (1,2) Mujeeb Khan, (1) Mohammed Rafi Shaik, (1) Abdulrahman Al-Warthan, (1) Mohammed Rafiq H. Siddiqui, (1) Joselito P. Labis, (3) and Syed Farooq Adil (1)

(1) Department of Chemistry, College of Science, KingSaud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia

(2) Department of Chemistry, KL University, Guntur, Andhra Pradesh 522502, India

(3) King Abdullah Institute for Nanotechnology, KingSaud University, Riyadh 11451, Saudi Arabia

Correspondence should be addressed to Syed Farooq Adil; sfadil@ksu.edu.sa

Received 15 March 2017; Revised 2 May 2017; Accepted 8 May 2017; Published 31 May 2017

Academic Editor: Mikhael Bechelany

Caption: Scheme 1: Graphical representation of the preparation of Zr[O.sub.x]-Mn carbonate and oxides.

Caption: Scheme 2: Aerobic oxidation of 1-phenylethanol to acetophenone.

Caption: Figure 1: SEM analysis of the as-synthesized catalysts calcined at (a- b) 300[degrees]C; (c-d) 400[degrees]C; (e-f) 500[degrees]C. (a-b) Overview image for assynthesized 1% Zr[O.sub.x]-MnC[O.sub.3]; (c-d) overview image of 1% Zr[O.sub.x]-Mn[O.sub.2]; (e-f) overview image of 1% Zr[O.sub.x]-[Mn.sub.2][O.sub.3]; (g) particle size distribution of 1% Zr[O.sub.x]-MnC[O.sub.3]; (h) particle size distribution of 1% Zr[O.sub.x]-Mn[O.sub.2]; (i) particle size distribution of 1% Zr[O.sub.x]- [Mn.sub.2][O.sub.3].

Caption: Figure 2: Elemental composition from the EDX analysis of the assynthesized catalysts calcined 1% Zr[O.sub.x]-MnC[O.sub.3].

Caption: Figure 3: XRD pattern of the catalyst at different temperatures 1% Zr[O.sub.x]-MnC[O.sub.3] uncalcined; 1% Zr[O.sub.x]-MnC[O.sub.3];1% Zr[O.sub.x]- Mn[O.sub.2]; and 1% Zr[O.sub.x]-[Mn.sub.2][O.sub.3].

Figure 4: The HRTEM images of1% Zr[O.sub.x]-MnC[O.sub.3] nanocomposite.

Figure 5: Thermogravimetric analysis of catalyst calcined at different temperatures 300[degrees]C, 400[degrees]C, and 500[degrees]C; 1% Zr[O.sub.x]- MnC[O.sub.3]; 1% Zr[O.sub.x]-Mn[O.sub.2]; and1% Zr[O.sub.x]-[Mn.sub.2][O.sub.3].

Caption: Figure 6: Graphical representation of 1-phenylethanol oxidation using catalyst calcined at different calcination temperatures.

Caption: Figure 7: The effect of different weight % Zr[O.sub.x] on the oxidation of 1-phenylethanol.

Caption: Figure 8: Catalytic performance of 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst as a function of reaction temperature.

Caption: Figure 9: Catalytic activity of 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst as a function of catalyst amount.

Caption: Figure 10: The reusability of 1% Zr[O.sub.x]-MnC[O.sub.3] catalyst in the oxidation of 1-phenylethanol. (Reaction conditions: 2 mmol of 1-phenylethanol, calcination temperature at 300[degrees]C, oxygen with rate 20 mL [min.sup.-1], 0.5 g of catalyst, reaction temperature at 100[degrees]C, 10 mL of toluene, and 6 min of reaction time.)
Table 1: Effect of the calcination temperature on the catalytic
activities of 1% Zr[O.sub.x]-MnC[O.sub.3] for the selective
oxidation of 1-phenylethanol.[a]

Entry            Catalyst             T([degrees]C)   SA ([m.sup.2]
                                                       [g.sup.-1])

1       1% Zr[O.sub.x]-MnC[O.sub.3]        300           133.58
2       1% Zr[O.sub.x]-Mn[O.sub.2]         400            53.19
3       1% Zr[O.sub.x]-Mn2[O.sub.3]        500            17.48

Entry   Conv.       Sp. activity (mmol x      Sel.
          (%)     [g.sup.-1] x [h.sup.-1])    (%)

1       100.00              20.0              >99
2        70.13              14.03             >99
3        61.34              12.27             >99

[a] Reaction conditions: 2mmol of 1-phenylethanol, 300 mg of
catalyst, oxygen with rate 20 mL [min.sup.-1], reaction
temperature at 100[degrees] C, 10 mL of toluene, and 20 min
of reaction time.

Table 2: Influence of different weight % of Zr[O.sub.x] on the
Oxidation of 1-phenylethanol. [a]

Entry            Catalyst             Conversion
                                         (%)

1       0% Zr[O.sub.x]-MnC[O.sub.3]     80.36
2       1% Zr[O.sub.x]-MnC[O.sub.3]     100.00
3       3% Zr[O.sub.x]-MnC[O.sub.3]     95.41
4       5% Zr[O.sub.x]-MnC[O.sub.3]     63.97
5       7% Zr[O.sub.x]-MnC[O.sub.3]     42.46

Entry    Sp. activity (mmol x      Sel.
        [g.sup.-1] x [h.sup.-1])   (%)

1                16.07             >99
2                20.00             >99
3                19.08             >99
4                12.80             >99
5                 8.49             >99

[a] Reaction conditions: 2 mmol of 1-phenylethanol, 300 mg of
catalyst, calcination temperature at 300[degrees] C, Oxygen with rate
20 mL [min.sup.-1], reaction temperature at 100[degrees]C, 10 mL of
toluene, and 20 min of reaction time.

Table 3: Effect of reaction temperature on the catalytic
performance. [a]

Entry   Reaction temp.    Conv.      Sp. activity(mmol x      Sel.
         ([degrees]C)      (%)     [g.sup.-1] x [h.sup.-1])    (%)

1             20          44.82              8.96              >99
2             40          60.05             12.01              >99
3             60          74.53             14.91              >99
4             80          87.96             17.59              >99
5             100          100               20.0              >99

[a] Reaction conditions: 2 mmol of 1-phenylethanol, 300 mg of
catalyst, calcination temperature at 300[degrees] C, oxygen with rate
20 mL [min.sup.-1], 10 mL of toluene, and 20 min of reaction time.

Table 4: Effect of catalyst amount in oxidation of
1-phenylethanol. [a]

Entry    Catalyst      Conv.     Sp. activity (mmol x      Sel.
        amount (mg)     (%)     [g.sup.1] x [h.sup.-1])     (%)

1           100        41.81             83.62              >99
2           200        57.66             57.66              >99
3           300        72.43             48.29              >99
4           400        86.75             43.38              >99
5           500         100               40.0              >99

[a] Reaction conditions: 2 mmol of 1-phenylethanol, calcination
temperature at 300[degrees]C, oxygen with rate 20 mL [min.sup.-1]
reaction temperature at 100[degrees]C, 10 mL of toluene, and
6 min of reaction time.

Table 5: Comparison between our work and earlier publications for
The selective oxidation of 1-phenylethanol into acetophenone.

Catalyst                  Conv.     Sel.         T         Time
                           (%)      (%)     ([degrees]C)    (h)

1% ZrOx-MnC[O.sub.3]       100      >99         100         0.1
NbP-S2                      72      100          90         24
VOP[O.sub.4]               38.5      89          80          6
Pd/GC                      18.9    >99.9        110          6
Ru/Mg-LaO                   96      >99          80          4
FeAPO                      56.2     100          70          6
Co[Al.sub.2][O.sub.4]     63.45    83.25         80          8
POM/Zr[O.sub.2]             88      >99          RT          3
Fe(N[O.sub.3])3 x           92      >99          RT         48
  9[H.sub.2]O
Ru/CaO-Zr[O.sub.2]         >99      >98          40          6
CuZnO                      >99      >99          RT          3
NG900                      1.8      >98          70          3
Oxo-V SB/G                  89      >99          65        0.33
Cu(II)-Zn(II)               78      >99         100         14
[Fe.sub.3][O.sub.4]         76      >99          80         18
UiO-66-Sal-Cu[Cl.sub.2]     45       99          60         24
Co(II)/ZnO                  95      100          70         13
CeCr[O.sub.3]              100      100          90          6
Co[(acac).sub.2]            94      100          40          7
Na/Co[C.sub.l]3             99      100          RT         46
Au/HT                       99       99          RT          6
Au/Ce[O.sub.2]             >99      >99         100          5
Pd/HAP                     >99      >99          90          1

Catalyst                   Sp. activity (mmol x         Ref.
                          [g.sup.-1] x [h.sup.-1])

1% ZrOx-MnC[O.sub.3]                40.0             This work
NbP-S2                              2.74                [3]
VOP[O.sub.4]                        8.02                [8]
Pd/GC                               0.50                [15]
Ru/Mg-LaO                           2.4                 [9]
FeAPO                               7.45                [67]
Co[Al.sub.2][O.sub.4]               0.79                [36]
POM/Zr[O.sub.2]                    11.73                [68]
Fe(N[O.sub.3])3 x                   1.92                [69]
  9[H.sub.2]O
Ru/CaO-Zr[O.sub.2]                  1.67                [70]
CuZnO                              33.33                [50]
NG900                               0.02                [51]
Oxo-V SB/G                         13.35                [71]
Cu(II)-Zn(II)                      16.39                [52]
[Fe.sub.3][O.sub.4]                 1.4                 [72]
UiO-66-Sal-Cu[Cl.sub.2]             4.34                [73]
Co(II)/ZnO                          1.45                [74]
CeCr[O.sub.3]                      15.15                [75]
Co[(acac).sub.2]                    6.1                 [76]
Na/Co[C.sub.l]3                     0.43                [77]
Au/HT                               416                 [78]
Au/Ce[O.sub.2]                      173                 [79]
Pd/HAP                              208                 [80]

Table 6: Oxidation of a variety of alcohols catalyzed by
1% Zr[O.sub.x]-MnC[O.sub.3]. [a]

Entry     Substrate        Product      Time     Conv.    Sel.
                                        (min)     (%)      (%)

1       [formula not reproducible]        6       100      >99
2       [formula not reproducible]        9       100      >99
3       [formula not reproducible]        5       100      >99
4       [formula not reproducible]        7       100      >99
5       [formula not reproducible]        11      100      >99
6       [formula not reproducible]        15      100      >99
7       [formula not reproducible]        30      100      >99
8       [formula not reproducible]        75      100      >99
9       [formula not reproducible]        90      100      >99
10      [formula not reproducible]        5       100      >99
11      [formula not reproducible]        9       100      >99
12      [formula not reproducible]        20      100      >99
13      [formula not reproducible]        8       100      >99
14      [formula not reproducible]        18      100      >99
15      [formula not reproducible]        25      100      >99
16      [formula not reproducible]        75      100      >99
17      [formula not reproducible]        80      100      >99
18      [formula not reproducible]        90      100      >99

[a] Reaction conditions: 2 mmol of alcohol, 0.5 g of catalyst,
calcination temperature at 300[degrees] C, oxygen with rate 20 mL
[min.sup.-1], 10 mL of toluene, and reaction temperature at
100[degrees]C.
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Title Annotation:Research Article
Author:Assal, Mohamed E.; Kuniyil, Mufsir; Khan, Mujeeb; Shaik, Mohammed Rafi; Warthan, Abdulrahman Al-; Si
Publication:Advances in Materials Science and Engineering
Date:Jan 1, 2017
Words:9877
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