metals Article
Ag2O Nanoparticles-Doped Manganese Immobilized on Graphene Nanocomposites for Aerial Oxidation of Secondary Alcohols Mohamed E. Assal 1 , Mohammed Rafi Shaik 1 ID , Mufsir Kuniyil 1,2 ID , Mujeeb Khan 1 , Abdulrahman Al-Warthan 1 , Mohammed Rafiq H. Siddiqui 1, * ID and Syed Farooq Adil 1, * 1
2
*
ID
Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia;
[email protected] (M.E.A.);
[email protected] (M.R.S.);
[email protected] (M.K.);
[email protected] (M.K.);
[email protected] (A.A.-W.) Department of Chemistry, Koneru Lakshmaiah Education Foundation, Vaddeswaram, Guntur 522502, Andhra Pradesh, India Correspondence:
[email protected] (M.R.H.S.);
[email protected] (S.F.A.); Tel.: +966-11-467-0439 (S.F.A.)
Received: 23 May 2018; Accepted: 17 June 2018; Published: 19 June 2018
Abstract: Ag2 O nanoparticles-doped MnO2 decorated on different percentages of highly reduced graphene oxide (HRG) nanocomposites, i.e., (X%)HRG/MnO2 –(1%)Ag2 O (where X = 0–7), were fabricated through straight-forward precipitation procedure, and 400 ◦ C calcination, while upon calcination at 300 ◦ C and 500 ◦ C temperatures, it yielded MnCO3 and manganic trioxide (Mn2 O3 ) composites, i.e., [(X%)HRG/MnCO3 –(1%)Ag2 O] and [(X%)HRG/Mn2 O3 –(1%)Ag2 O], respectively. These nanocomposites have been found to be efficient and very effective heterogeneous catalysts for selective oxidation of secondary alcohols into their respective ketones using O2 as a sole oxidant without adding surfactants or nitrogenous bases. Moreover, a comparative catalytic study was carried out to investigate the catalytic efficiency of the synthesized nanocomposites for the aerobic oxidation of 1-phenylethanol to acetophenone as a substrate reaction. Effects of several factors were systematically studied. The as-prepared nanocomposites were characterized by TGA, XRD, SEM, EDX, HRTEM, BET, Raman, and FTIR. The catalyst with structure (5%)HRG/MnO2 –(1%)Ag2 O showed outstanding specific activity (16.0 mmol/g·h) with complete conversion of 1-phenylethanol and >99% acetophenone selectivity within short period (25 min). It is found that the effectiveness of the catalyst has been greatly improved after using graphene support. A broad range of alcohols have selectively transformed to desired products with 100% convertibility and no over-oxidation products have been detected. The recycling test of (5%)HRG/MnO2 –(1%)Ag2 O catalyst for oxidation of 1-phenylethanol suggested no obvious decrease in its performance and selectivity even after five subsequent runs. Keywords: silver oxide nanoparticles; secondary alcohols; catalyst; graphene; oxidation
1. Introduction Catalytic oxidation of alcohols into their respective carbonyls such as aldehydes and ketones is of crucial importance in synthetic chemistry [1]. Aldehyde and ketone derivatives are employed as high value intermediates and precursors in several industries, such as perfumery, vitamins, cosmetics, confectionary, flavors, insecticides, aniline dyes, flame-retardants, and agro-processing [2,3]. Conventionally, oxidation of alcohols was performed by using expensive, polluting, and hazardous stoichiometric amount of oxidizing reagents (e.g., CrO3 , SeO2 , NaClO, KMnO4 , PCC, Na2 Cr2 O7 , RuO4 , and Br2 ) [4,5]. However, there are disadvantages to using these oxidants, such as high cost and
Metals 2018, 8, 468; doi:10.3390/met8060468
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the production of large amounts of toxic waste, which largely reduce their industrial applications. Recently, huge efforts have been devoted to developing more green catalytic protocols to reduce the disadvantages of conventional oxidation approaches [6]. Therefore, the use of environmentally friendly oxidants like molecular O2 has received growing interest for the alcohol oxidation from the viewpoint of green chemistry, because O2 is cheap, natural, abundant, and generates innocuous by-products (H2 O and H2 O2 ) [7]. Several catalytic protocols for alcohol oxidation using O2 as a clean oxidizing agent have developed and exhibited high performance [8]. In the last several years, a plethora of studies have used noble metals like palladium [9], platinum [10], gold [11], and ruthenium [12] as efficient catalysts for selective alcohol oxidation. In addition to their high costs, these noble metals also suffer from some drawbacks due to difficulties in synthesis, and the rarity of these precious metals make these catalysts impractical for industry [13]. Consequently, considerable efforts have been devoted to replacing these precious catalysts with inexpensive and plentiful non-noble metals or transition metals such as V [7], Cu [14], Ni [15], Co [16], Fe [17], Zr [18], and Zn [19]. Additionally, the metal or metal oxide NPs-based catalysts were reported as highly effective for selective alcohol oxidation. Furthermore, it has been broadly reported that the efficiency of metal NPs significantly improved upon doping with other metals owing to the metallic NPs have huge surface area [20]. Among different types of metal oxides, MnO2 has been found to be the most stable metal oxide which has superior properties [21], such as high activity, highly specific capacitance and is an eco-friendly material [22]. Hence, Mn-containing catalysts have been extensively applied as highly effective catalysts for the catalytic oxidation of alcohols [23]. Graphene, a unique crystal of an individual layer of sp2 hybridized carbon atoms, was densely packed into a 2D honeycomb lattice [24]. Single graphene sheet is known as the strongest material ever measured [25]. In 2004, the individual sheet of graphene was experimentally isolated for the first time via mechanical exfoliation of graphite; a plethora of studies have been reported on graphene due to the extraordinary properties and numerous applications since that date [26]. Graphene is an amazing material, which received widespread interest owing to the fact it has huge surface area (2630 m2 /g), superior corrosion resistance, exceptional thermal conductivity (5000 W·m−1 ·K−1 ), high electron mobility (250,000 cm2 /Vs), high chemical stability, ultrathin thickness, and highly electrically conductive (6000 S·cm−1 ) [27]. However, the preparation of single-crystalline, defect-free, single-layer graphene is still a challenge. The exceptional optical, catalytic, electronic, and magnetic properties of graphene based metal nanocomposites have been exploited in many industrial applications such as supercapacitor, touch screens, batteries, sensors, solar cells, and catalysis [27]. Therefore, huge efforts have devoted to combine uniformly different types of nanomaterials, including metallic NPs with graphene or its derivatives, which remarkably improved the properties of these materials; this is probably due to the synergistic effect between the graphene and NPs [28]. Besides enhancing their properties, the metal NPs also behave as a stabilizing agent against the agglomeration of single graphene layers [29]. Moreover, the combination of metal NPs with graphene also assisted the exfoliation of single graphene sheets [30]. Graphene and its derivatives are widely utilized as a promising supporting material for metallic NPs due to their superior features, e.g., huge surface area, superior chemical stability, and simple reusability of metals from spent catalysts [31]. To exploit these fascinating properties of graphene in catalysis, it is necessary to produce large quantity of the graphene by using graphene oxide (GO) and HRG [32]. GO consists of myriad oxygenic functionalities on the basal plane such as hydroxyl, carboxyl, epoxy, and carbonyl groups [33], to promote the oxidation reaction [31]. GO can reduced to highly reduced graphene (HRG) via chemical or thermal reduction reactions [34]; most of oxygenated functionalities in GO layer can be reduced and left behind as numerous topological defects and carbon vacancies [35]. Recently, HRG has received growing attention as a promising material for catalyst supports in the oxidation of several organic substrates [36], maybe due to the fact that the HRG surface behaved as the anchoring site for the aromatic alcohols and oxygen on the surface by π–π stacking near the metal NPs [36]. This advantage, together with its extremely specific surface area, offers HRG excellent
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catalyst support in the selective oxidation process [30]. In general, catalytic active metallic NPs with smaller particle size usually showed higher catalytic activity owing to their extremely high surface area active exposed sites [37]. However, owing to their small size, metal NPs are not Metalsand 2018,increased 8, x FOR PEER REVIEW 3 of 22 stable and readily agglomerated because of their high surface energy, which reduces their catalytic performance stability [38].this In order to solve this problem, found catalytically that immobilizing stability [38]. and In order to solve problem, researchers found researchers that immobilizing active catalytically active sites onto be a[37]. suitable solution in [37]. expected, in the the catalyst current sites onto supports could be asupports suitablecould solution As expected, theAs current study study the catalyst with exhibited graphene higher support exhibited higher than thatsupport. withoutGraphenegraphene with graphene support reactivity than that reactivity without graphene support. Graphene-based metal NPstohave been found to be efficient catalysts for numerous organic based metal NPs have been found be efficient catalysts for numerous organic transformations, transformations, such as Suzuki coupling of [39], reduction alcohols [40], aniline [41], such as Suzuki coupling [39], reduction alcohols [40],ofpreparation ofpreparation aniline [41],ofoxidation of oxidation of ammonia borane [42], and oxidation of methanol [43]. Especially, graphene-supported ammonia borane [42], and oxidation of methanol [43]. Especially, graphene-supported metal NPs metal NPs were broadly as a heterogeneous for theoxidation selective oxidation alcohols were broadly applied as applied a heterogeneous catalyst forcatalyst the selective of alcoholsofinto their into their corresponding carbonyl compounds, [44],[45], MnO /GO [45], Au/RGO [46], corresponding carbonyl compounds, e.g., Pd/GCe.g., [44],Pd/GC MnO2/GO Au/RGO [46], Pd/HRG [47], 2 Pd/HRG [47], and Au/GQDs/Fe O [48]. and Au/GQDs/Fe3O4 [48]. 3 4 In connection connection with our efforts in the utilization of several metallic oxide NPs as effective catalysts for the liquid-phase oxidation of alcohols using oxygen [22,49,50], we compare between the catalytic efficiency of Ag without using using graphene graphene support (HRG) and the catalytic Ag22O NPs-doped MnO22 without performance after afterusing usingHRG HRGasas a catalyst support toward the oxidation of 1-phenylethanol in a catalyst support toward the oxidation of 1-phenylethanol in order order to understand the support effect (Scheme 1). The prepared nanocomposites were fabricated, to understand the support effect (Scheme 1). The prepared nanocomposites were fabricated, and their and their performances catalytic performances examined aerial oxidation of alcohols via green ideal green methodology, catalytic examined in aerialinoxidation of alcohols via ideal methodology, i.e., i.e., using a greenoxidizing oxidizingagent agentwhile whilebeing beingfree freeofof any any base base or or surfactants. surfactants. The as-obtained using a green as-obtained nanocomposites have characterized by by XRD, XRD, TGA, TGA, SEM, SEM, EDX, EDX, HRTEM, HRTEM, BET, BET,Raman, Raman,and andFT-IR. FT-IR.
Scheme 1. 1. Schematic depiction for forthe thefabrication fabricationof of(X%)HRG/MnCO (X%)HRG/MnCO33–(1%)Ag –(1%)Ag22O, O, (X%)HRG/MnO (X%)HRG/MnO22–– Scheme Schematic depiction (1%)Ag22O, 3–(1%)Ag2O nanocomposites. (1%)Ag O, and and (X%)HRG/Mn (X%)HRG/Mn2O 2 O3 –(1%)Ag2 O nanocomposites.
2. Materials Methods 2. Materials and and Methods 2.1. Preparation of HRG HRG 2.1. Preparation of In the the beginning, beginning, graphene graphene oxide oxide (GO) (GO) was was prepared prepared from from pristine pristine graphite graphite by by the the Hummers Hummers In procedure [51]. Then, GO has been reduced to HRG using hydrazine hydrate by following our earlier procedure [51]. Then, GO has been reduced to HRG using hydrazine hydrate by following our publication [52]. [52]. earlier publication 2.2. Preparation of (X%)HRG/MnO2–(1%)Ag2O Catalyst (X%)HRG/MnO2–(1%)Ag2O nanocomposite was prepared via precipitation procedure (where X = 0, 1, 3, 5 and 7). In a typical preparation, stoichiometric amount of Mn(NO3)2·4H2O, AgNO3, and HRG were mixed in a round bottomed flask and subjected to ultrasonication for 0.5 h. After that, the solution was heated to 80 °C, while vigorously stirring using a mechanical stirrer and 0.5 M solution
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2.2. Preparation of (X%)HRG/MnO2 –(1%)Ag2 O Catalyst (X%)HRG/MnO2–(1%)Ag2O nanocomposite was prepared via precipitation procedure (where X = 0, 1, 3, 5 and 7). In a typical preparation, stoichiometric amount of Mn(NO3)2 ·4H2O, AgNO3, and HRG were mixed in a round bottomed flask and subjected to ultrasonication for 0.5 h. After that, the solution was heated to 80 ◦ C, while vigorously stirring using a mechanical stirrer and 0.5 M solution of NaHCO3, and was added drop wise until the pH of solution reached 9 for 3 h; then the stirring continued over night at room temperature. The solution was filtered and dried, followed by calcination in muffle furnace at 300, 400, and 500 ◦ C. 2.3. Catalyst Characterization The as-obtained nanocomposites were characterized by various techniques such as XRD, TGA, SEM, EDX, HRTEM, BET, Raman, and FT-IR [52]. 2.4. General Procedure for Oxidation of Alcohols In a typical experiment, a mixture of the 1-phenylethanol (2 mmol), toluene (10 mL), and the catalyst (300 mg) was transferred in a glass three-necked round-bottomed flask (100 mL); the resulting mixture was then heated to desired temperature with vigorous stirring. The oxidation experiment was started by bubbling oxygen gas at a flow rate of 20 mL·min−1 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. (Santa Clara, CA, USA), equipped with a flame ionization detector (FID) and a 19019S-001 HP-PONA column. 3. Results and Discussion 3.1. Catalyst Characterization 3.1.1. X-Ray Diffraction Analysis (XRD) XRD analysis was carried out to investigate the crystallographic structure of the as-synthesized nanocomposite catalyst. Figure 1 displays characteristic XRD patterns of the graphite, GO, HRG, un-supported MnO2 –(1%)Ag2 O, and (5%)HRG/MnO2 –(1%)Ag2 O nanocomposite. The graphite’s XRD pattern displayed a sharp diffraction signal at 2θ = 26.5◦ , with a d-spacing of 3.37 Å as exhibited in Figure 1a. The GO crystalline degree is moderately small with a wide-ranging typical diffraction band at approximately 2θ = 11.8◦ , which corresponds to the characteristic structure of GO as displayed in Figure 1b. The graphite peak at 2θ = 26.5◦ vanishing and the presence of a new band at 2θ = 11.8◦ designate the development of GO through graphite oxidation. This change principals to a rise in an inter-planar distance from 3.37 to 4.83 Å for graphite and GO, respectively, which might be ascribed due to the presence of several O-containing functional groups. The XRD diffraction of HRG nanosheets is noticed at around 2θ = 24.6◦ with (002) plane, which is a finger-print peak of HRG and diffraction peak at 2θ = 11.8 vanishing evidently, designating that GO is entirely reduced to HRG as exhibited in Figure 1c. The MnO2 –(1%)Ag2 O XRD pattern highly matches with the stated data of pyrolusite MnO2 (JCPDS file No. 24-0735), while the nanocomposite (5%)HRG/MnO2 –(1%)Ag2 O shows a related XRD pattern as MnO2 –(1%)Ag2 O with HRG characteristic band at 2θ = 24.6◦ , specifying the existence of HRG in the as-synthesized nanocomposite (Figure 1e).
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Figure 1. XRD analysis of (a) graphite, (b) GO, (c) HRG, (d) MnO2–(1%)Ag2O, and (e)
Figure 1.1.XRD XRD analysis (a) graphite, GO, HRG, (d) 2 O, MnO 2O, and (e) Figure analysis of (a) of graphite, (b) GO, (c)(b) HRG, (d)(c) MnO and2–(1%)Ag (e) (5%)HRG/MnO 2 –(1%)Ag 2– (5%)HRG/MnO2–(1%)Ag2O. (5%)HRG/MnO 2 –(1%)Ag 2 O. (1%)Ag2 O. 3.1.2. Fourier Transforms Infrared Spectroscopy (FT-IR)
3.1.2. Fourier Fourier Transforms Transforms Infrared Spectroscopy (FT-IR) The FT-IR spectral analysis of GO, HRG,(FT-IR) and (5%)HRG/MnO2–(1%)Ag2O nanocomposite is 3.1.2. Infrared Spectroscopy displayed in Figure 2. In the FT-IR spectrum of GO (Figure 2a), the absorption peak at 3440 cm−1 can
The FT-IR FT-IR spectral spectral analysis analysis of of GO,HRG, HRG,and and(5%)HRG/MnO (5%)HRG/MnO2–(1%)Ag –(1%)Ag2O nanocomposite is is The nanocomposite 2 2 O molecules, be attributed to hydroxyl groupsGO, (–OH) stretching modes of (–COOH) groups and water −1 −1 can displayed in in Figure Figure 2. In Inatthe the FT-IR spectrum ofstretching GO(Figure (Figure 2a),the theand absorption peak 3440cm cm can the intense peak 1740FT-IR cm−1 resembles (C=O) of (−COOH), the absorption strong peak displayed 2. spectrum of GO 2a), absorption peak atat3440 −1 is allocated at around 1630 cm to the stretching vibration of carbon backbone (C=C/C−C) from be attributed to hydroxyl groups (–OH) stretching modes of (–COOH) groups and water molecules, be attributed to hydroxyl groups (–OH) stretching modes of (–COOH) groups and water molecules, unoxidized graphite lattice. Furthermore, thestretching three peaks of situated at 1395, and 1225, and absorption 1060 cm−1 can strong be −1 1 resembles the intense peak 1740 cm (C=O) peak the intenseascribed peak at at 1740 cm−resembles (C=O) stretching(−COOH), of (−COOH), the and the absorption strong to −1 the stretching modes of (C−OH), (C−O−C), and (C−O), correspondingly. FT-IR spectrum at around 16301630 cm cmis−1allocated to the stretching vibration of backbone(C=C/C (C=C/C−C) from peak at around allocated to the vibration of carbon carbon backbone of HRG (Figure 2b)is displayed that the stretching broad intense peak at 1215 cm−1 associated with (C−OH) −C) from −−1 1 can unoxidized graphite lattice. Furthermore, the three peaks situated at 1395, 1225, and 1060 cm can be be −1 weak absorption at approximately 1636atm1395, is ascribed (C=C) group, unoxidizedstretching graphitevibration lattice.and Furthermore, thepeak three peaks situated 1225, toand 1060 cm the skeletalmodes aromatic vibration; additional peaks matching to oxygen comprising functional ascribed to todue thetostretching stretching modesofof (C−O−C), (C−O), correspondingly. FT-IR ascribed the (C(C−OH), −OH), (C −O− C),and and (C−O), correspondingly. FT-IR spectrum spectrum groups vanished when associated with GO. The (5%)HRG/MnO 2–(1%)Ag2O nanocomposite −1 associated FT-IR − 1 of HRG (Figure 2b) displayed that the broad intense peak at 1215 cm with (C−OH) of HRG (Figure 2b) displayed that the broad intense peak at 1215 cm associated with (C −OH) spectra evidently shows the whole reduction of maximum of oxygenated functional groups on the −1 − 1 stretching vibration and weak absorption peak at approximately 1636 m is ascribed to (C=C) group, stretching vibration and weak absorption peak at approximately 1636 m is ascribed to (C=C) group, surface of GO (Figure 2c). The intense bands attributed to (C=O), (C−O−C), and (C−O) stretching due to to the thevibrations skeletal aromatic aromatic vibration; additional peaks matching towith oxygen comprising functional at 1740, 1225, and 1060 additional cm−1 were not observed associated GO, comprising and the oxygenfunctional due skeletal vibration; peaks matching to oxygen possessing functionalities on the GO plane are efficiently reduced. Moreover, a wide-ranging peak at groups vanished when associated with GO. The (5%)HRG/MnO 2–(1%)Ag 2O nanocomposite FT-IR groups vanished when associated with GO. The (5%)HRG/MnO2 –(1%)Ag2 O nanocomposite FT-IR 1634 cm−1shows resultant to whole (C=C) stretching mode accredited toof theoxygenated skeletal aromatic vibrationgroups was spectra evidently the reduction of maximum functional on the spectra evidently shows the whole reduction of maximum of oxygenated functional groups on the witnessed (Figure 2c). Meanwhile, a sharp, intense absorption peak situated at 580 cm−1 is allocated surface of of GO GO(Figure (Figure2c). 2c).The Theintense intensebands bandsattributed attributedtoto (C=O),(C(C−O−C), and(C (C−O) stretching surface (C=O), −O−C), and −O) stretching to Mn−O vibrations in MnO2. −11 were not observed associated with GO, and the oxygen − vibrations at 1740, 1225, and 1060 cm vibrations at 1740, 1225, and 1060 cm were not observed associated with GO, and the oxygen possessing functionalities a wide-ranging peak at possessing functionalities on on the theGO GOplane planeare areefficiently efficientlyreduced. reduced.Moreover, Moreover, a wide-ranging peak −1 − 1 1634 cm resultant to (C=C) stretching mode accredited to the skeletal aromatic vibration was at 1634 cm resultant to (C=C) stretching mode accredited to the skeletal aromatic vibration was witnessed (Figure (Figure 2c). 2c). Meanwhile, Meanwhile, aa sharp, sharp,intense intenseabsorption absorptionpeak peaksituated situatedatat580 580cm cm−−11 is is allocated allocated witnessed to Mn Mn−O to −Ovibrations vibrationsin inMnO MnO2. . 2
Figure 2. FT–IR analysis of (a) GO, (b) HRG, and (c) (5%)HRG/MnO2–(1%)Ag2O.
Figure2.2.FT–IR FT–IRanalysis analysisof of(a) (a)GO, GO,(b) (b)HRG, HRG,and and(c) (c)(5%)HRG/MnO (5%)HRG/MnO2–(1%)Ag 2O. Figure 2 –(1%)Ag2 O.
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3.1.3. Thermogravimetric Analysis (TGA) 3.1.3. Thermogravimetric Analysis (TGA) TGA analysis was conducted to study the thermal stability of graphite, GO, HRG, MnO2– TGA analysis was conducted to study the thermal stability of graphite, GO, HRG, MnO2 – (1%)Ag2O, and (5%)HRG/MnO2–(1%)Ag2O as shown in Figure 3. The TGA curves endorses the whole (1%)Ag2 O, and (5%)HRG/MnO2 –(1%)Ag2 O as shown in Figure 3. The TGA curves endorses the whole reduction of GO to HRG using hydrazine monohydrate as reducing agent. Figure 3a,b showed that reduction of GO to HRG using hydrazine monohydrate as reducing agent. Figure 3a,b showed that the the thermal stability of GO is much inferior to graphite. The pristine graphite TGA curve shows an thermal stability of GO is much inferior to graphite. The pristine graphite TGA curve shows an entire entire weight loss of approximately 1% (Figure 3a). Moreover, GO displays ~6% weight loss nearby weight loss of approximately 1% (Figure 3a). Moreover, GO displays ~6% weight loss nearby 100 ◦ C, 100 °C, evidently due to the absorbed H2O molecules loss and volatile compounds, followed by the evidently due to the absorbed H2 O molecules loss and volatile compounds, followed by the highest highest weight loss of approximately 43% owing to the pyrolysis of the oxygenated carrying weight loss of approximately 43% owing to the pyrolysis of the oxygenated carrying functional groups functional groups in the temperature range of 200–370 °C, and, finally, a weight loss of around 11% in the temperature range of 200–370 ◦ C, and, finally, a weight loss of around 11% owing to the pyrolysis owing to the pyrolysis of the carbon skeleton, which is perceived in the temperature range of 370– of the carbon skeleton, which is perceived in the temperature range of 370–800 ◦ C. HRG thermogram 800 °C. HRG thermogram (Figure 3c) displays an entire weight loss of less than 19% in the same (Figure 3c) displays an entire weight loss of less than 19% in the same temperature range, owing to temperature range, owing to the reduction of most of the oxygenated carrying functionalities. The the reduction of most of the oxygenated carrying functionalities. The (5%)HRG/MnO2 –(1%)Ag2 O (5%)HRG/MnO2–(1%)Ag2O displayed entire weight loss of nearly 17% in the identical temperature displayed entire weight loss of nearly 17% in the identical temperature range (Figure 3e), which is range (Figure 3e), which is marginally more than the weight loss apparent from the MnO2–(1%)Ag2O marginally more than the weight loss apparent from the MnO2 –(1%)Ag2 O thermal degradation thermal degradation pattern, indicating efficient reduction of GO. As a consequence, it is established pattern, indicating efficient reduction of GO. As a consequence, it is established that the as-synthesized that the as-synthesized (5%)HRG/MnO2–(1%)Ag2O catalyst is stable up to 520 °C. (5%)HRG/MnO2 –(1%)Ag2 O catalyst is stable up to 520 ◦ C.
Figure 3. TGA graph of (a) graphite, (b) GO, (c) HRG, (d) MnO2–(1%)Ag2O, and (e) (5%)HRG/MnO2– Figure 3. TGA graph of (a) graphite, (b) GO, (c) HRG, (d) MnO2 –(1%)Ag2 O, and (e) (5%)HRG/MnO2 – (1%)Ag2O. (1%)Ag2 O.
3.1.4. SEM and EDX 3.1.4. SEM and EDX The size and morphology of the as-obtained (5%)HRG/MnO2–(1%)Ag2O nanocomposite was The size and morphology of the as-obtained (5%)HRG/MnO –(1%)Ag2 O nanocomposite was analyzed by SEM technique and associated with MnO2–(1%)Ag2O2 as displayed in (Figure 4). The analyzed by SEM technique and associated with MnO2 –(1%)Ag2 O as displayed in (Figure 4). MnO2–(1%)Ag2O catalyst displayed micro size but precise cuboidal shape particles; nevertheless, the The MnO2 –(1%)Ag2 O catalyst displayed micro size but precise cuboidal shape particles; nevertheless, (5%)HRG/MnO2–(1%)Ag2O nanocomposite (Figure 4b), remarkably, displays aggregation of rather the (5%)HRG/MnO2 –(1%)Ag2 O nanocomposite (Figure 4b), remarkably, displays aggregation of rather smaller size crystals, which appear on the surface through proper surface nucleation/growth process. smaller size crystals, which appear on the surface through proper surface nucleation/growth process. Furthermore, the elemental composition analysis of the as-prepared (5%)HRG/MnO2–(1%)Ag2O Furthermore, the elemental composition analysis of the as-prepared (5%)HRG/MnO2 –(1%)Ag2 O nanocomposite is also investigated through EDX analysis, as noticed in Figure 5. The presence of Ag, nanocomposite is also investigated through EDX analysis, as noticed in Figure 5. The presence of Ag, Mn, C, and O2 is clearly specified in the EDX spectrum, and the percentage of composition is within Mn, C, and O2 is clearly specified in the EDX spectrum, and the percentage of composition is within the theoretical range. the theoretical range.
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–(1%)Ag222O and (b) (5%)HRG/MnO Figure 4. 4. SEM SEM images images of of (a) (a) MnO MnO222–(1%)Ag Figure (5%)HRG/MnO222–(1%)Ag –(1%)Ag22O nanocomposite. 2 Onanocomposite.
Figure 5. 5. EDX EDX spectra spectra of of the the (5%)HRG/MnO (5%)HRG/MnO222–(1%)Ag Figure –(1%)Ag222OOnanocomposite. nanocomposite.
3.1.5. (HRTEM) 3.1.5. High High Resolution Resolution Transmission Transmission Electron Electron Microscope Microscope (HRTEM) 22O nanocomposite morphology and size HRG, HRG, MnO MnO222–(1%)Ag –(1%)Ag222O, O, and and (5%)HRG/MnO (5%)HRG/MnO22–(1%)Ag 2 –(1%)Ag2 O nanocomposite morphology and size were examined using HRTEM. Figure 6a shows extremely were examined using HRTEM. Figure 6a shows extremely exfoliated exfoliated HRG HRG nanolayers, nanolayers, which which are are 2–(1%)Ag22O thin, flake-like, silky, and transparent, whilst the HRTEM image of the (5%)HRG/MnO 2 thin, flake-like, silky, and transparent, whilst the HRTEM image of the (5%)HRG/MnO2 –(1%)Ag2 O nanocomposite nanosize of the Ag22O the catalyst with 0.95 ± 0.14 nm as average nanocompositeclearly clearlyshows showsthe the nanosize of the Agin 2 O in the catalyst with 0.95 ± 0.14 nm as diameter with spherical shape, and is homogenously distributed on the crumpled nanosheets, average diameter with spherical shape, and is homogenously distributed on theHRG crumpled HRG as illustrated in Figure 6d–f. Additionally, the HRTEM image of the MnO 2 –(1%)Ag 2 22O catalyst nanosheets, as illustrated in Figure 6d–f. Additionally, the HRTEM image of the MnO2 –(1%)Ag 2O 2O NPs of 2.16±0.23 nm, which is larger than the Ag22O NPs (without graphene) displays the Ag 2 catalyst (without graphene) displays the Ag2 O NPs of 2.16±0.23 nm, which is larger than the obtained byobtained using graphene This can be This possibly attributed the graphene Ag2 O NPs by usingsupport. graphene support. can be possiblytoattributed to thenanolayers graphene reducing the aggregation of Ag22O NPs 6b,c). Notably, the size of Ag 22O NPs in the nanolayers reducing the aggregation of Ag(Figure O NPs (Figure 6b,c). Notably, the size of Ag 2 2 O NPs in (5%)HRG/MnO 22–(1%)Ag22O nanocomposite is found to be smaller than that of the Ag22O NPs in the the (5%)HRG/MnO2 –(1%)Ag2 O nanocomposite is found to be smaller than that of the Ag2 O NPs in 2–(1%)Ag22O (Figure 6c,f), that is, this may be the reason catalyst without graphene support, i.e., MnO the catalyst without graphene support, i.e., 2MnO 2 –(1%)Ag2 O (Figure 6c,f), that is, this may be the why (5%)HRG/MnO 22–(1%)Ag22O nanocomposite showed enhanced efficiency compared to MnO22– reason why (5%)HRG/MnO2 –(1%)Ag2 O nanocomposite showed enhanced efficiency compared to 22O catalyst. (1%)Ag MnO –(1%)Ag O catalyst.
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Figure Figure 6. 6.HRTEM HRTEMimages imagesof of(a) (a)HRG, HRG,(b) (b)MnO MnO22–(1%)Ag –(1%)Ag22O, O,(d,e) (d,e)(5%)HRG/MnO (5%)HRG/MnO22–(1%)Ag –(1%)Ag22O, O,and and(c,f) (c,f) Figure 6. distributions HRTEM imagesof of (a) HRG, (b) MnO2O 2–(1%)Ag2O, (d,e) (5%)HRG/MnO2–(1%)Ag2O, and (c,f) particle size MnO 2–(1%)Ag and (5%)HRG/MnO 2 –(1%)Ag 2 O nanocomposite, particle size distributions of MnO2–(1%)Ag O and (5%)HRG/MnO –(1%)Ag O nanocomposite, respectively. 2 2 2 particle size distributions of MnO2–(1%)Ag2O and (5%)HRG/MnO2–(1%)Ag2O nanocomposite, respectively. respectively.
3.1.6. Raman Spectroscopy 3.1.6. Raman Spectroscopy 3.1.6. Raman Spectroscopy Figure 7 exhibits the Raman spectrum of GO, MnO2–(1%)Ag2O, and (5%)HRG/MnO2–(1%)Ag2O FigureFigure 7 exhibits the Raman spectrum ofofGO, 22–(1%)Ag O,and and(5%)HRG/MnO (5%)HRG/MnO 2–(1%)Ag 7 exhibits the Raman spectrum GO,MnO –(1%)Ag 22O, –(1%)Ag 2O 2O nanocomposite. Raman spectra of the MnO2–(1%)Ag O and (5%)HRG/MnO 2MnO 2 –(1%)Ag2 O 2nanocomposite nanocomposite. Raman spectra the MnO O−1and (5%)HRG/MnO 2–(1%)Ag Raman spectraof ofpeak thesituated MnO2–(1%)Ag 2–(1%)Ag 22O (5%)HRG/MnO 2–(1%)Ag O 2O2 (Figurenanocomposite. 7b,c) demonstrate a characteristic at 642 cm ,and confirming the existence of 2MnO −1, −1 nanocomposite (Figure 7b,c) demonstrate a characteristic peak situated at 642 cm confirming the nanocomposite (Figure 7b,c) demonstrate a characteristic peak situated at 642 cm , confirming the in both MnO2–(1%)Ag2O and (5%)HRG/MnO2–(1%)Ag2O nanocomposite. Additionally, the presence existence of MnO in both MnO 2–(1%)Ag 2O and and (5%)HRG/MnO (5%)HRG/MnO2–(1%)Ag 2O2O nanocomposite. existence of MnO 2 in2 both MnO 2–(1%)Ag 2O 2–(1%)Ag nanocomposite. of HRG support in the (5%)HRG/MnO2–(1%)Ag2O nanocomposite is confirmed by the existence of Additionally, the presence of HRG support 1inthe the (5%)HRG/MnO (5%)HRG/MnO2–(1%)Ag 2O nanocomposite is Additionally, the peaks presence of HRG 2–(1%)Ag2O nanocomposite is two characteristic at ~1591 and support 1337 cm−in correspondingly, commonly denoted as D-band and −1 correspondingly, confirmed by the existence of two characteristic peaks at ~1591 and 1337 cm −1 correspondingly, −1 (Figure 7a), confirmed byGthe existence of in two characteristic peaks and at ~1591 and 1337 cm1346 G-bands. The and the D bands GO spectrum are shifted located at 1605 and cm commonly denoted as D-band and G-bands. The G and the D bands in GO spectrum are shifted and 2 commonly denoted as D-band and G-bands. The Gby and D bands GO spectrum are shifted and whichlocated is due to 1605 the destruction the sp 7a), character thethe ofin graphite and existence −1 (Figure at and 1346 cmof which is due tooxidation the destruction of the to sp2GO character by the of −1 (Figure 7a), which is due to the destruction of the sp2 character by the located at 1605 and 1346 cm oxygen-possessing functional groups on the GO plane. The G peak in (5%)HRG/MnO –(1%)Ag oxidation of graphite to GO and existence of oxygen-possessing functional groups on the2GO plane.2 O is 1 from 1 , oxygen-possessing oxidation of graphite to GO andtoexistence of functional groups on thefrom GO plane. −1 from shiftedThe byG ~14 cmin−(5%)HRG/MnO 1605 1591 cm2−O slight shift is noticed thecm D−1peak 1346 to peak 2–(1%)Ag iswhile shifteda by ~14 cm 1605 toin 1591 , while a slight −1 −1 − 1 The G peak in (5%)HRG/MnO 2–(1%)Ag 2Oto is shifted by ~14 cm from 1605 to 1591 cm , while a slight −1 shift is noticed in the D peak from 1346 1337 cm , indicating an efficient reduction of GO to HRG. 1337 cm , indicating an efficient reduction of GO to HRG. Interestingly, the obtained observations are very −1, indicating an efficient reduction of GO to HRG. shift in the D peak from 1346 to 1337 cmmuch the obtained observations are very in accordance with the XRD results. muchisInterestingly, innoticed accordance with the XRD results. Interestingly, the obtained observations are very much in accordance with the XRD results.
7. Raman analysis of (a) GO, (b) MnO2–(1%)Ag2O, and (c) (5%)HRG/MnO2–(1%)Ag2O FigureFigure 7. Raman analysis of (a) GO, (b) MnO2 –(1%)Ag2 O, and (c) (5%)HRG/MnO2 –(1%)Ag2 O nanocomposites. nanocomposites.
Figure 7. Raman analysis of (a) GO, (b) MnO2–(1%)Ag2O, and (c) (5%)HRG/MnO2–(1%)Ag2O nanocomposites.
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3.1.7. BET Surface Area BET surface area analysis of the as-obtained catalysts was determined, to compare the changes 8, xto FOR PEER REVIEW at various temperatures and the existence of HRG in 9 ofthe 22 catalytic in surfaceMetals area2018, due calcination protocol and understand 3.1.7.to BET Surface Area the correlation between the surface areas of the synthesized materials and the effectiveness of the catalytic protocol for alcohol oxidation. Table 2 displayed that the BET surface area analysis of the as-obtained catalysts was determined, to compare the changes surface areas of thearea fabricated catalystsat(without graphene), i.e., 3 –(1%)Ag 2 O, 2 –(1%)Ag2 O, in surface due to calcination various temperatures and theMnCO existence of HRG in theMnO catalytic 2 and Mn2 Oprotocol O,understand are aboutthe 52,correlation 84, and between 42 m /g, the surface areas of the and 2to therespectively, surface areas ofwhereas the synthesized materials 3 –(1%)Ag and the effectiveness of the it catalytic forvarious alcohol oxidation. Table 2 displayed the◦surface prepared catalyst after doping with protocol HRG at temperatures (300 ◦ C,that 400 C, and 500 ◦ C), areas of the fabricated catalysts (without graphene), i.e., MnCO3–(1%)Ag2O, MnO2–(1%)Ag2O, and i.e., (5%)HRG/MnCO3 –(1%)Ag2 O, (5%)HRG/MnO 2 –(1%)Ag2 O, and (5%)HRG/Mn2 O3 –(1%)Ag2 O, Mn2O3–(1%)Ag2O, are about 52, 84, and 42 m2/g, respectively, whereas the surface areas of the 2 increased prepared to 107,catalyst 149, after anddoping 99 mit with /g, HRG respectively. As anticipated, catalytic at various temperatures (300 °C, 400the °C, and 500 °C), performance i.e., after loading HRG on the catalyst, (5%)HRG/MnCO –(1%)Ag O, (5%)HRG/MnO –(1%)Ag2 O, (5%)HRG/MnCO 3–(1%)Ag 2O, i.e., (5%)HRG/MnO 2–(1%)Ag2O, and (5%)HRG/Mn 2 O 3 –(1%)Ag22O, 3 2 2/g, respectively. As anticipated, the catalytic performance after increased to 107, 149, and 99 m and (5%)HRG/Mn2 O3 –(1%)Ag2 O, was also increased. loading HRG on the catalyst, i.e., (5%)HRG/MnCO3–(1%)Ag2O, (5%)HRG/MnO2–(1%)Ag2O, and
(5%)HRG/Mn2O3–(1%)Ag2O, was also increased. 3.2. Catalytic Studies 3.2. Catalytic Studies efficiency of the synthesized nanocomposites, oxidation of 1-phenylethanol To explore the catalytic To explore catalytic efficiency the synthesized of (Scheme 1using molecular O2 as the a clean oxidant wasof employed as ananocomposites, representativeoxidation example 2). phenylethanol using molecular O 2 as a clean oxidant was employed as a representative example Different catalysts were synthesized by altering the wt. % of HRG in the catalyst and calcination (Scheme 2). Different catalysts were synthesized by altering the wt. % of HRG in the catalyst and treatment.calcination Additionally, the effect of catalyst quantity, temperature, and reaction time on the treatment. Additionally, the effect of catalyst quantity, temperature, and reaction time on effectiveness of the synthesized catalysts is studied in detail. the effectiveness of the synthesized catalysts is studied in detail.
Scheme 2. Schematic depiction of oxidation of 1-phenylethanol into acetophenone catalyzed by the
Scheme 2.as-fabricated Schematiccatalyst. depiction of oxidation of 1-phenylethanol into acetophenone catalyzed by the as-fabricated catalyst. 3.2.1. Impact of HRG Support
3.2.1. Impact of HRG Support The performance of the oxidation catalyst can be improved using graphene or its derivatives as a catalyst support [42,43,53]. From our previously reported study, it was found that Ag2O NPs was
The performance of the oxidation be improved using graphene or its derivatives as found to be an excellent promoter tocatalyst the MnO2can catalyst, and the MnO 2–Ag2O(1%) catalyst exhibited a catalyst highest support [42,43,53]. From previously reported study,2Oit was wasselected found and that further Ag2 O NPs was catalytic activity [54]. our Herein, the catalyst MnO2–(1%)Ag developed by addition of different percentages of HRG in order to fine-tune its performance. Firstly, found to be an excellent promoter to the MnO2 catalyst, and the MnO2 –Ag2 O(1%) catalyst exhibited we have studied the catalytic efficiency of pure, highly reduced graphene oxide for the aerobic highest catalytic activity [54]. Herein, the catalyst MnO2 –(1%)Ag2 O was selected and further developed oxidation of 1-phenylethanol using O2 as an oxidizing agent at 100 °C. It was found that HRG alone by addition of different percentages of HRG in 1, order its performance. is not active for this oxidation process (Table entryto 1).fine-tune Several catalysts with changing Firstly, weight we have studied the catalyticofefficiency of pure, highly reduced graphene oxideand forexamined the aerobic oxidation of percentages HRG, i.e., (0–7%) HRG in the nanocomposite, were synthesized in terms of theirusing activities The results revealed that the catalysts 1-phenylethanol O2 asforan 1-phenylethanol oxidizing agentoxidation. at 100 ◦ C. It was found that HRG alone is not active for (1%)HRG/MnO2–(1%)Ag2O and (3%)HRG/MnO2–(1%)Ag2O display alcohol conversion of 69.81% this oxidation process (Table 1, entry 1). Several catalysts with changing weight percentages of HRG, and 91.34%, respectively, within 25 min of reaction time at 100 °C (Table 1, entries 3, 4). By raising i.e., (0–7%)theHRG synthesized and examined terms of their wt.% in of the HRGnanocomposite, in the catalyst, thewere conversion of 1-phenylethanol was alsoin increased, and the activities for 1-phenylethanol oxidation. The results revealed that the catalysts (1%)HRG/MnO2 –(1%)Ag2 O and (3%)HRG/MnO2 –(1%)Ag2 O display alcohol conversion of 69.81% and 91.34%, respectively, within 25 min of reaction time at 100 ◦ C (Table 1, entries 3, 4). By raising the wt.% of HRG in the catalyst, the conversion of 1-phenylethanol was also increased, and the (5%)HRG/MnO2 –(1%)Ag2 O yielded 100% conversion along with 16.0 mmol/g·h specific activity, which is highest performance
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among all other HRG percentages (Table 1, entry 5). Further increase of the weight percentage of HRG led to decrease in the performance of the catalyst; this is may be due to high percentage of HRG support, which led to the blocking of the active sites of the catalyst (Table 1, entry 6). Interestingly, the MnO2 –(1%)Ag Ox FOR catalyst without HRG support gave only 60.39% conversion 10 under identical Metals 2018,28, PEER REVIEW of 22 circumstances (Table 1, entry 2). (5%)HRG/MnO2–(1%)Ag2O yielded 100% conversion along with 16.0 mmol/g·h specific activity, which is highest performance among all other HRG percentages (Table 1, entry 5). Further increase Table 1. The catalytic properties of various catalysts for the aerial oxidation of 1-phenylethanol. of the weight percentage of HRG led to decrease in the performance of the catalyst; this is may be due to high percentage of HRG support, which led to the blocking of the active sites of the catalyst Conversion Specific Activity Selectivity (Table 1, entry 6). Interestingly, the MnO2–(1%)Ag 2O catalyst without HRG support gave only 60.39% Entry Catalyst (%) (mmol/g·h) (%) conversion under identical circumstances (Table 1, entry 2).
1 HRG 3.94 0.63 >99 MnOproperties O various catalysts 60.39 for the aerial9.66 >99 2 –(1%)Ag2of Table21. The catalytic oxidation of 1-phenylethanol. (1%)HRG/MnO2 –(1%)Ag2 O 69.81 11.17 >99 3 Entry Catalyst Conversion (%) Specific Activity Selectivity (%) 4 (3%)HRG/MnO 91.34 14.61 (mmol/g·h) >99 2 –(1%)Ag 2O 5 (5%)HRG/MnO 16.0 >99 1 HRG >99 3.94 100.0 0.63 2 –(1%)Ag2 O 6 MnO (7%)HRG/MnO 13.51 >99 2 –(1%)Ag2 O 2 2–(1%)Ag2O 60.39 84.45 9.66 >99 /MnO 2–(1%)Ag2O 3 11.17 >99and 20 mL/min Note: Conditions: 2(1%)HRG mmol of 1-phenylethanol, 30069.81 mg catalyst, 400 ◦ C calcination temperature, MnO2–(1%)Ag2O 91.34 14.61 >99 oxygen rate at4100 ◦(3%)HRG C for 25 /min. 5 >99 (5%)HRG/MnO2–(1%)Ag2O 100.0 16.0 84.45 13.51 >99 6 (7%)HRG/MnO2–(1%)Ag2O
Therefore, itNote: could be deduced the support played a fundamental role in enhancement Conditions: 2 mmol ofthat 1-phenylethanol, 300 (HRG) mg catalyst, 400 °C calcination temperature, and 25 min. The effectiveness of the (5%)HRG/MnO2 –(1%)Ag2 O oxygen rate at 100 °C for of the efficiency 20 ofmL/min the present catalytic protocol. nanocomposite is much higher than that of MnO2 –(1%)Ag O nanocatalyst, possibly due to the increase Therefore, it could be deduced that the support2 (HRG) played a fundamental role in in the adsorption of theofreactant aromatic alcohols HRG surface π–π of interaction near enhancement the efficiency of the present onto catalytic protocol. Thethrough effectiveness the (5%)HRG/MnO 2–(1%)Ag 2O nanocomposite is muchto higher than support that of having MnO2–(1%)Ag the Ag2 O NPs attached on the HRG layers, in addition graphene huge2Osurface area nanocatalyst, possibly due to the increase in the adsorption of the reactant aromatic alcohols onto that can homogenously disperse the active sites (Ag O–MnO2 NPs). Moreover, the existence of carbon HRG surface through π–π interaction near the Ag22O NPs attached on the HRG layers, in addition to defects and graphene oxygen support carrying functionalities onthat HRG anchored NPs and prevented 2 Osites having huge surface area can surface homogenously dispersethe the Ag active (Ag 2O– MnO 2 NPs). Moreover, the existence of carbon defects and oxygen carrying functionalities on HRG the aggregation of graphene sheets. The product selectivities toward acetophenone stayed motionless surface anchored the Ag2O(Table NPs and prevented1–6). the aggregation of graphene sheets. The product (>99%) throughout all processes 1, entries The obtained data are collected in Table 1 and selectivities toward acetophenone stayed motionless (>99%) throughout all processes (Table 1, entries are depicted1–6). in Figure 8. Thus, it can be deduced that the (5%)HRG/MnO –(1%)Ag O nanocomposite 2 it can be2deduced The obtained data are collected in Table 1 and are depicted in Figure 8. Thus, exhibited highest among the all-prepared nanocomposites, therefore is selected in the that theefficiency (5%)HRG/MnO 2–(1%)Ag 2O nanocomposite exhibited highest and efficiency amongitthe allnanocomposites, and therefore it is selected in the later studies to optimize other factors. later studiesprepared to optimize other factors.
Figure 8. Graphical representation of 1-phenylethanol oxidation using catalyst (a) MnO2–(1%)Ag2O,
Figure 8. Graphical representation of 1-phenylethanol oxidation using catalyst (a) MnO2 –(1%)Ag2 O, 2–(1%)Ag2O, (c) (3%)HRG/MnO2–(1%)Ag2O, (d) (5%)HRG/MnO2–(1%)Ag2O, and (b) (1%)HRG/MnO (b) (1%)HRG/MnO –(1%)Ag O, (c) (3%)HRG/MnO2 –(1%)Ag2 O, (d) (5%)HRG/MnO2 –(1%)Ag2 O, 2 –(1%)Ag 2 O. (e) (7%)HRG/MnO 2 2 and (e) (7%)HRG/MnO2 –(1%)Ag2 O.
3.2.2. Impact of Calcination Calcination process has a strong impact on the structure of the catalyst, including changes in the oxide composition, morphology, and particle size. In addition, the calcination process has profound influence on the catalytic activity of the prepared material. Subsequently, as-prepared nanocomposites are subjected to calcination treatment at 300, 400, and 500 ◦ C. The resulting materials are investigated
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for their catalytic efficiency as oxidation catalysts. The results revealed that the catalysts showed high selectivity for acetophenone throughout all oxidation reactions (>99%). Nevertheless, the conversion of 1-phenylethanol was strongly effected by calcination temperature. The catalyst calcined at 300 ◦ C, i.e., (5%)HRG/MnCO3 –(1%)Ag2 O, exhibited lower catalytic activity, which gave 89.11% of acetophenone from the oxidation of 1-phenylethanol within 25 min (Table 2, entry 4), while in case of 500 ◦ C calcination temperature, i.e., (5%)HRG/Mn2 O3 –(1%)Ag2 O, the alcohol conversion was reduced to 59.74%, which was the least alcohol conversion detected among the catalysts synthesized (Table 2, entry 6). For 400 ◦ C calcination, (5%)HRG/MnO2 –(1%)Ag2 O exhibits 100% conversion of 1-phenylethanol along with superior specific activity concerning 16 mmol/g·h under similar conditions (Table 2, entry 5). Table 2. Catalytic oxidation of 1-phenylethanol under different calcination temperatures. Entry
Catalyst
T. (◦ C)
SA (m2 /g)
Conv. (%)
Specific Activity (mmol/g·h)
Sel. (%)
1 2 3 4 5 6
MnCO3 –(1%)Ag2 O MnO2 –(1%)Ag2 O Mn2 O3 –(1%)Ag2 O (5%)HRG/MnCO3 –(1%)Ag2 O (5%)HRG/MnO2 –(1%)Ag2 O (5%)HRG/Mn2 O3 –(1%)Ag2 O
300 400 500 300 400 500
51.7 84.3 41.8 107.1 149.1 98.7
58.47 60.39 39.81 89.11 100.0 59.74
9.36 9.66 6.37 14.26 16.0 9.56
>99 >99 >99 >99 >99 >99
Note: Conditions: 2 mmol of 1-phenylethanol, 300 mg catalyst, and 20 mL/min oxygen rate at 100 ◦ C for 25 min.
When the attained results are compared to the catalyst without graphene support, i.e., MnO2 – (1%)Ag2 O, it was found that under identical circumstances, the alcohol conversion obtained was only 60.39%. The calculated specific activity of the catalyst, i.e., MnO2 –(1%)Ag2 O, was found to be 9.66 mmol/g·h, which was lower than specific activity of the catalyst containing HRG (Table 2, entry 2). Based on these findings, it is evident that using graphene support improves the efficiency of catalyst significantly. Notably, the obtained catalytic results are consistent with BET surface area data of the synthesized catalyst as well. The catalyst (5%)HRG/MnO2 –(1%)Ag2 O calcined at 400 ◦ C have the highest surface area among the other temperatures and gave full conversion of 1-phenylethanol. The catalyst heated at 300 and 500 ◦ C, i.e., (5%)HRG/MnCO3 –(1%)Ag2 O and (5%)HRG/Mn2 O3 –(1%)Ag2 O, respectively, displayed lower 1-phenylethanol conversion and lower surface area. Thus, it can be concluded that the catalytic performance was substantially affected by calcination process. However, acetophenone selectivity was almost independent of surface area of the prepared catalysts. 2018, obtained 8, x FOR PEERwere REVIEW 12 of 22 The catalyticMetals results summarized in Table 2 and presented in Figure 9.
Figure 9. Graphical illustration of aerial oxidation of 1-phenylethanol catalyzed by (a) MnCO3–
Figure 9. Graphical illustration of aerial oxidation of 1-phenylethanol catalyzed by (a) MnCO3 – (1%)Ag2O, (b) MnO2–(1%)Ag2O, (c) Mn2O3–(1%)Ag2O, (d) (5%)HRG/MnCO3–(1%)Ag2O, (e) (1%)Ag2 O, (b) Mn(f)2 O O, (d) (5%)HRG/MnCO3 –(1%)Ag2 O, (e) Ag2 O(1%)– 2 –(1%)Ag 2 O, (c)and 3 –(1%)Ag22O O(1%)–MnO 2/(5%)HRG, (5%)HRG/Mn 3–(1%)Ag2O. Ag2MnO MnO2 /(5%)HRG, and (f) (5%)HRG/Mn2 O3 –(1%)Ag2 O. 3.2.3. Impact of Reaction Temperature
Usually, the temperature had a pronounced effect on catalytic oxidation of alcohols. Therefore, the influence of temperature during the oxidation of 1-phenylethanol was also assessed by altering the temperature from 20 °C to 100 °C for the aerial oxidation of 1-phenylethanol in presence of (5%)HRG/MnO2–(1%)Ag2O catalyst. The results are compiled in Table 3 and plotted in Figure 10. The attained results reveal that the effectiveness of the present nanocomposite is directly proportional to the reaction temperature, whereas the selectivities toward acetophenone were all as high as >99%.
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3.2.3. Impact of Reaction Temperature Usually, the temperature had a pronounced effect on catalytic oxidation of alcohols. Therefore, the influence of temperature during the oxidation of 1-phenylethanol was also assessed by altering the temperature from 20 ◦ C to 100 ◦ C for the aerial oxidation of 1-phenylethanol in presence of (5%)HRG/MnO2 –(1%)Ag2 O catalyst. The results are compiled in Table 3 and plotted in Figure 10. The attained results reveal that the effectiveness of the present nanocomposite is directly proportional to the reaction temperature, whereas the selectivities toward acetophenone were all as high as >99%. For instance, at low temperature 20 ◦ C, a relatively low alcohol conversion of 33.14% was observed (Table 3, entry 1). As anticipated, high reaction temperature contributed to higher catalytic activity and led to the considerable enhancement of the performance of the prepared catalyst. At high temperature (100 ◦ C), a complete conversion was accomplished under same circumstances (Table 3, entry 5). Thus, the temperature of 100 ◦ C has been chosen for further optimization studies. Table 3. Influence of temperature on the performance of the synthesized nanocomposite. Entry
Temperature (◦ C)
Conv. (%)
Specific Activity (mmol/g·h)
Sel. (%)
1 2 3 4 5
20 40 60 80 100
33.14 50.20 66.41 82.93 100.0
5.30 8.03 10.63 13.27 16.0
>99 >99 >99 >99 >99
Metals 2018,Conditions: 8, x FOR PEER Note: 300REVIEW mg catalyst (5%)HRG/MnO2 –(1%)Ag2 O calcined at 400 ◦ C, 2 mmol of 1-phenylethanol,13 of 22 and 20 mL/min oxygen rate for 25 min.
Figure10. 10.Dependence Dependenceof of1-phenylethanol 1-phenylethanolconversion conversionon onreaction reactiontemperature. temperature. Figure
3.2.4.Effect Effectof ofAmount Amountof ofCatalyst Catalyst 3.2.4. Therole roleof ofcatalyst catalystquantity quantityduring duringthe theoxidation oxidationprocess processwas wasalso alsoexamined examinedwhile whilekeeping keeping The other optimized parameters constant, and the attained results are depicted in Figure 11 and compiled other optimized parameters constant, and the attained results are depicted in Figure 11 and compiled inTable Table4.4. ItIt isis clear clear that, that, by byraising raising the thecatalyst catalyst quantity, quantity,the thealcohol alcoholconversion conversionwas wasalso alsoraised raised in gradually, but the selectivity for acetophenone remained unchanged (above 99%). According to Table gradually, but the selectivity for acetophenone remained unchanged (above 99%). According to Table 4, 4, using a low catalyst amount (50 mg), a poor conversion of 23.06% was obtained, which could be using a low catalyst amount (50 mg), a poor conversion of 23.06% was obtained, which could be dueto tofewer feweractive activesites sites(Table (Table4,4,entry entry1). 1).As Asanticipated, anticipated,by byraising raisingthe thecatalyst catalystquantity quantityto to100 100mg, mg, due the 1-phenylethanol conversion also increased to 39.85% (Table 4, entry 2). Further increase in catalyst the 1-phenylethanol conversion also increased to 39.85% (Table 4, entry 2). Further increase in catalyst to300 300mgmg resulted in catalyst the catalyst being afforded which 100% produced 100% to resulted in the being afforded excellentexcellent efficiency,efficiency, which produced conversion conversion with 16.0 mmol/g·h specific activity within 25 min of the reaction (Table 4, entry 5), whilst the rest yielded lower than 100% conversion at similar circumstances. The current study shows that only 3.0 g of the catalyst was required to achieve complete conversion of 1-phenylethanol to acetophenone within 25 min. As seen in Figure 11, the conversion of 1-phenylethanol is directly proportional to the catalyst amount. To ensure the catalytic performance, blank test without the asfabricated catalyst was conducted under the optimum conditions. No formation of acetophenone was
other optimized parameters constant, and the attained results are depicted in Figure 11 and compiled in Table 4. It is clear that, by raising the catalyst quantity, the alcohol conversion was also raised gradually, but the selectivity for acetophenone remained unchanged (above 99%). According to Table 4, using a low catalyst amount (50 mg), a poor conversion of 23.06% was obtained, which could be due to fewer active sites (Table 4, entry 1). As anticipated, by raising the catalyst quantity to 100 mg, Metals 2018, 8, 468 13 of 22 the 1-phenylethanol conversion also increased to 39.85% (Table 4, entry 2). Further increase in catalyst to 300 mg resulted in the catalyst being afforded excellent efficiency, which produced 100% conversion with 16.0 mmol/g·h specific activity of the(Table reaction (Table5), 4, entry whilst with 16.0 mmol/g ·h specific activity within 25 within min of 25 themin reaction 4, entry whilst5), the rest the rest lower yielded lower than 100% conversion at similar circumstances. The current that yielded than 100% conversion at similar circumstances. The current study study showsshows that only only 3.0 g of the catalyst was required to achieve complete conversion of 1-phenylethanol to 3.0 g of the catalyst was required to achieve complete conversion of 1-phenylethanol to acetophenone acetophenone within 25 min. As seen in Figure 11, the conversion of 1-phenylethanol is directly within 25 min. As seen in Figure 11, the conversion of 1-phenylethanol is directly proportional to the proportional to the amount. To ensure the catalytic blank test withoutcatalyst the ascatalyst amount. Tocatalyst ensure the catalytic performance, blankperformance, test without the as-fabricated fabricated catalyst was conducted under the optimum conditions. No formation of acetophenone was was conducted under the optimum conditions. No formation of acetophenone was detected by GC, detected by GC, indicating that the prepared catalyst is necessary for this oxidation process. indicating that the prepared catalyst is necessary for this oxidation process.
Figure Conversionofof 1-phenylethanol as a function of (5%)HRG/MnO catalyst (5%)HRG/MnO 2–(1%)Ag Figure 11. 11. Conversion 1-phenylethanol as a function of catalyst amount.2O 2 –(1%)Ag2 O amount. Table 4. Influence of variation of amount of catalyst (5%)HRG/MnO2 –(1%)Ag2 O. Entry
Catalyst Dosage (mg)
Conv. (%)
Specific Activity (mmol/g·h)
Sel. (%)
1 2 3 4 5 6
50 100 150 200 250 300
23.06 39.85 55.12 70.81 84.70 100.0
22.14 19.13 17.64 16.99 16.26 16.00
>99 >99 >99 >99 >99 >99
Note: Conditions: catalyst (5%)HRG/MnO2 –(1%)Ag2 O calcined at 400 and 20 mL/min oxygen rate at 100 ◦ C for 25 min.
◦ C,
2 mmol of 1-phenylethanol,
3.3. Reusability of Catalyst The recyclability and stability of the catalyst gained growing interest owing to the fact it is greatly important from both environmental and economic viewpoints. In order to assess the durability and reusability of the (5%)HRG/MnO2–(1%)Ag2O catalyst, oxidation of 1-phenylethanol was conducted five consecutive times under optimal conditions, as described in Figure 12. After the complete first oxidation experiment, the catalyst can be easily filtered by simple filtration or centrifugation and washed sequentially with toluene and reused for up to five catalytic reactions. The results, which are illustrated in Figure 12, elucidated no obvious decrease in the performance after five runs. The conversion 1-phenylethanol decreases gradually from 100% to 13.94% after five cycles, presumably due to weight loss during filtration. In the fifth cycle, the selectivity toward acetophenone remained unchanged above 99%. Therefore, the results indicated that the synthesized catalyst possesses excellent reproducibility and stability.
centrifugation and washed sequentially with toluene and reused for up to five catalytic reactions. The results, which are illustrated in Figure 12, elucidated no obvious decrease in the performance after five runs. The conversion 1-phenylethanol decreases gradually from 100% to 13.94% after five cycles, presumably due to weight loss during filtration. In the fifth cycle, the selectivity toward remained unchanged above 99%. Therefore, the results indicated that the synthesized Metals acetophenone 2018, 8, 468 14 of 22 catalyst possesses excellent reproducibility and stability.
Figure 12. Recycling results of the (5%)HRG/MnO2–(1%)Ag2O catalyst. Conditions: (150 mol %)
Figure 12. Recycling results of the (5%)HRG/MnO2 –(1%)Ag2 O catalyst. Conditions: (150 mol %) (5%)HRG/MnO2–(1%)Ag2O calcined at 400 °C, 2 mmol of 1-phenylethanol, and 20 mL/min oxygen (5%)HRG/MnO2 –(1%)Ag2 O calcined at 400 ◦ C, 2 mmol of 1-phenylethanol, and 20 mL/min oxygen rate at 100 °C for 25 min. rate at 100 ◦ C for 25 min.
The efficiency of our catalytic system was compared with other reported catalysts for 1phenylethanol as illustrated in Table In the present oxidation of 1- for The efficiencyoxidation, of our catalytic system was 5.compared with study, other the reported catalysts phenylethanol to acetophenone is completed in relatively short reaction time (25 min) with more than of 1-phenylethanol oxidation, as illustrated in Table 5. In the present study, the oxidation 99% acetophenone selectivity. Additionally, other catalysts listed take a longer period to complete 1-phenylethanol to acetophenone is completed in relatively short reaction time (25 min) with more oxidation of 1-phenylethanol. For example, Karami et al. [55] have prepared palladium NPs than 99% acetophenone selectivity. Additionally, other catalysts listed take a longer period to dispersed on the surface of polystyrene (Pd NPs/PS) and employed as an oxidation catalyst. The Pd complete oxidation of 1-phenylethanol. For example, Karami et al. [55] have prepared palladium NPs NPs/PS catalyst exhibited complete 1-phenylethanol conversion and more than 99% selectivity for
dispersed on the surface of polystyrene (Pd NPs/PS) and employed as an oxidation catalyst. The Pd NPs/PS catalyst exhibited complete 1-phenylethanol conversion and more than 99% selectivity for acetophenone after 15 h was obtained with 1.80 mmol/g·h specific activity, which is lower compared to the specific activity (16 mmol/g·h) with 100% conversion and >99 acetophenone selectivity after 25 min in this work. In another example, Reis et al. [56] reported the use of mesoporous niobium phosphate as an oxidation catalyst using H2 O2 as an oxidizing agent, but it required very long period 24 h to yield 72% 1-phenylethanol conversion along with lower specific activity (2.74 mmol/g·h) by comparison with the as-prepared (5%)HRG/MnO2 –(1%)Ag2 O catalyst. Table 5. A comparison between the efficiency of our catalyst and earlier reported catalysts. Catalyst
Conv. (%)
(5%)HRG/MnO2 –(1%)Ag2 O 100 Pd NPs/PS 100 NbP-S2 72 84.0 MoVI O2 (bp-bhz)(MeOH) 15% w/w Ag-OMS-2 78.9 AuCNT 80 VO(ephedrine)2 @MNPs 96 CoAl2 O4 78.54 POM/ZrO2 88 Ru/CaO–ZrO2 95 Pd-pol 98 Ru/Mg–LaO 96
Sel. (%)
T (◦ C)
Time
Specific Activity (mmol/g·h)
Ref.
>99 >99 100 >99 85.43 94 >99 >99
100 85 90 80 75 RT 80 80 RT 90 100 80
25 min 15 h 24 h 0.5 h 4h 24 h 3h 5h 3h 2h 16 h 4h
16.0 1.80 2.74 13.77 0.16 9.60 1.57 11.73 7.92 2.65 2.40
This study [55] [56] [57] [58] [59] [60] [16] [61] [62] [63] [64]
Metals 2018, 8, 468
15 of 22
Table 5. Cont. Catalyst FeAPO-5/NaBr 2 wt% Ir/TiO2 Au/LDH Ru/MnOx /CeO2 Au/NiAlO PW11 -DMA16 /CMPS CoPc@Cell Co(II)/ZnO Fe(NO3 )3 ·9H2 O/NHPI Na/CoCl3 Au/TiO2 Fe3 O4 FeCl3 -imine@SiO2
Conv. (%)
Sel. (%)
T (◦ C)
Time
Specific Activity (mmol/g·h)
Ref.
56.2 >99 >99 99 96 92 91 95 92 99 100 76 90
100 >99 >99.5 99 >99 98.7 >99 100 >99 100 >99 >99 >99
70 80 80 27 80 90 RT 70 RT RT 110 80 80
8h 3h 2h 7h 1h 6h 5.5 h 13 h 48 h 46 h 24 h 18 h 6h
7.45 2.67 9.28 1.41 8.0 2.56 3.31 1.45 1.92 0.43 0.83 1.40 3.26
[65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77]
3.4. Oxidation of a Various Alcohols over (5%)HRG/MnO2 –(1%)Ag2 O Catalyst After optimizing the reaction circumstance, we expanded the oxidation process to the oxidation of a broad range of secondary benzylic and aliphatic alcohols, which also underwent oxidation to give the corresponding ketones; the results are tabulated in Table 6. According to this table, all secondary alcohols were smoothly transformed into the respective ketones with 100% convertibility (Table 6, entries 1–6). Moreover, selectivity towards ketones of more than 99% was achieved. It is worth mentioning that diphenylmethanol was the most reactive among all secondary aromatic alcohol and gave 100% conversion within only 20 min, while 4-chlorobenzhydrol gave 100% conversion after prolonged reaction time, which may be due to 4-chlorobenzhydrol bearing electron-withdrawing group that deactivated the phenyl ring by decreasing the electron density (Table 6, entries 3 and 4). Additionally, 1-phenylethanol and its derivatives also gave 100% conversion in short periods (Table 6, entries 1 and 2). Comparatively, aromatic alcohols are more reactive than the aliphatic ones. According to the Table 6, the oxidation of secondary aliphatic alcohols exhibited relatively lower reactivity compared to benzylic ones. For instance, the oxidation of cyclohexanol, 1-buten-3-ol, and 2-octanol to corresponding aliphatic ketones occurred with relatively longer reaction times compared to aromatic ones (Table 6, entries 7–9). Encouraged by these excellent results, we tried to expand the oxidation process to a broad range of alcohols like primary benzylic, allylic, heterocyclic, and aliphatic alcohols. Fortunately, these alcohols are selectively oxidized to their respective aldehydes employing optimum conditions. It is clear that the catalytic system was found to be also effective for the selective oxidation of primary aromatic alcohols (Table 6, entries 10–22). In addition, selectivity toward aldehydes is more than 99%, because there no over-oxidation to acids occurred, except that the aldehyde was observed. Usually, the oxidation rate of aromatic alcohols with electron-releasing substituents was relatively higher than that of alcohols carrying electron-withdrawing groups. For example, oxidation of an aromatic alcohol-bearing, electron-donating group such 4-methoxylbenzyl alcohol gave 100% conversion within 30 min (Table 6, entry 12), while oxidation of alcohol-containing, electron-withdrawing group such as complete oxidation of 4-nitrobenzyl alcohol occurred within longer period (55 min) (Table 6, entry 13). Furthermore, para-substituted alcohols gave full oxidation after shorter periods compared to ortho- and meta-positions, which may be attributed to the fact that para-position has lower steric resistance relative to other positions. For example, para-methylbenzyl alcohol was fully transformed to its para-methylbenzaldehyde within just 30 min (Table 6, entry 11), wheras metaand ortho-methylbenzyl alcohol were fully converted to the respective aldehydes after longer times of 40 and 45 min, respectively, in comparison to the para-position (Table 6, entries 14 and 15). Additionally, the prepared catalyst was found to be also highly effective towards oxidation of allylic alcohols in the respective aldehydes. For example, cinnamyl alcohol was rapidly transformed to cinnamaldehyde with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl alcohol as example of heteroaromatic
to orthoand meta-positions, which be attributed to the fact that para-position has lower steric to orthoand meta-positions, which maymay be attributed to the fact thatafter para-position has lower steric entry 13). Furthermore, para-substituted alcohols gave full oxidation after shorter periods compared entry 13). Furthermore, para-substituted alcohols gavegave full oxidation shorter periods compared to and meta-positions, which may be to fact that para-position has lower steric to13). orthoand meta-positions, which may be attributed to the the fact that para-position has lower steric entry 13). Furthermore, para-substituted alcohols full oxidation after shorter periods compared entry Furthermore, alcohols gave full oxidation shorter periods compared resistance relative topara-substituted other positions. For example, para-methylbenzyl alcohol was fully transformed resistance relative to other other positions. Formay example, para-methylbenzyl alcohol was fully transformed to orthoorthoand meta-positions, which may be attributed attributed to the the fact thatafter para-position has lower steric to orthoand meta-positions, which be attributed to fact that para-position has lower steric entry entry 13). 13). Furthermore, Furthermore, para-substituted para-substituted alcohols alcohols gave gave full full oxidation oxidation after after shorter shorter periods periods compared compared resistance relative to other positions. For example, para-methylbenzyl alcohol was fully transformed resistance relative to positions. For example, para-methylbenzyl alcohol was fully transformed to orthoand meta-positions, which may be attributed to the fact that para-position has lower steric to orthoand meta-positions, which may be attributed to the fact that para-position has lower steric resistance relative to other positions. For example, para-methylbenzyl alcohol was fully transformed resistance relative to other positions. For example, para-methylbenzyl alcohol was fully transformed to orthoand meta-positions, which may be attributed to fact theentry fact that para-position has lower steric to orthometa-positions, which may be attributed to that para-position has lower steric to para-methylbenzaldehyde itsand para-methylbenzaldehyde within just 30para-methylbenzyl min (Table 6, entry 11), wheras metaand orthoto its para-methylbenzaldehyde within just 30attributed min (Table 6, 11),alcohol wheras metaand orthoresistance relative to other positions. Formay example, alcohol was fully transformed resistance relative to other positions. For example, para-methylbenzyl was fully transformed orthoorthoand and meta-positions, meta-positions, which which may be attributed be to the the to the fact fact that that para-position para-position has has lower lower steric steric to its para-methylbenzaldehyde within just 30 min (Table 6, entry 11), wheras metaand orthoto its within just 30 min (Table 6, entry 11), wheras metaand orthoresistance relative to other other positions. For example, para-methylbenzyl alcohol was fully transformed resistance relative to other other positions. For example, para-methylbenzyl alcohol was fully transformed to its para-methylbenzaldehyde within just 30 min (Table 6, entry 11), wheras metaand orthoto its para-methylbenzaldehyde within just 30 min (Table 6, entry 11), wheras metaand orthoresistance relative to positions. For example, para-methylbenzyl alcohol was fully transformed resistance relative to positions. For example, para-methylbenzyl alcohol was fully transformed methylbenzyl alcohol were fully converted tomin the respective aldehydes after longer times of 40orthoand methylbenzyl alcohol were fully converted tojust the respective aldehydes after longer times of 40 and to its para-methylbenzaldehyde within just 30 (Table 6, entry 11), wheras metaand orthoto its para-methylbenzaldehyde within 30 min (Table 6, entry 11), wheras metaand resistance resistance relative relative to other to other positions. positions. For For example, example, para-methylbenzyl para-methylbenzyl alcohol alcohol was was fully fully transformed transformed methylbenzyl alcohol were fully converted to the respective aldehydes after longer times of 40 and methylbenzyl alcohol were fully converted to the respective aldehydes after longer times of 40 and to its para-methylbenzaldehyde para-methylbenzaldehyde within just 30 min (Table 6, entry 11), wheras metaand orthoto its para-methylbenzaldehyde within just 30 min (Table 6, entry 11), wheras metaand orthomethylbenzyl alcohol were fully converted to the respective aldehydes after longer times of 40 methylbenzyl alcohol were fully converted tomin themin respective aldehydes after longer times ofand 40 and to its within just 30 (Table 6, entry 11), wheras metaand orthoto its para-methylbenzaldehyde within just 30 (Table 6, entry 11), wheras metaand ortho45 min, respectively, in comparison to the para-position (Table 6, entries 14 and 15). Additionally, the 45 min, respectively, in comparison to the para-position (Table 6, entries 14 and 15). Additionally, the methylbenzyl alcohol were fully converted to the respective aldehydes after longer times of 40 and methylbenzyl alcohol were fully converted to the respective aldehydes after longer times of 40 and to min, its to para-methylbenzaldehyde its para-methylbenzaldehyde within within just 30 min 30 min (Table (Table 6,6, entry 6,6, entry 11), 11), wheras wheras metametaand and orthoortho45 min, respectively, inwere comparison tojust the para-position (Table entries 14 and 15). Additionally, the 45 respectively, inwere comparison toconverted the para-position (Table entries 14 and 15). Additionally, the methylbenzyl alcohol fully to the respective aldehydes after longer times of 40 and methylbenzyl alcohol fully converted to the respective aldehydes after longer times of 40 and 45 min, respectively, in comparison to the para-position (Table 6, entries 14 and 15). Additionally, the 45 min, respectively, in comparison to the para-position (Table 6, entries 14 and 15). Additionally, the methylbenzyl alcohol were fully converted to the respective aldehydes after longer times of 40 and alcohol were fully converted to the respective aldehydes after longer times of 40 and Metalsmethylbenzyl 2018, 8, 468 16 of 22 prepared catalyst was found to be also highly effective towards oxidation of allylic alcohols in the prepared catalyst was found to be also highly effective towards oxidation of allylic alcohols in the 45 min, respectively, inwere comparison to the para-position (Table 6, entries 14 and 15). Additionally, 45 min, respectively, infound comparison to the para-position (Table 6, entries 14 and 15). Additionally, methylbenzyl methylbenzyl alcohol alcohol fully converted converted to the to respective the respective aldehydes aldehydes after after longer times times of 40 ofand 40 prepared catalyst toalso be also highly effective towards oxidation oflonger allylic alcohols in and the prepared catalyst was found to fully be highly effective towards oxidation of allylic alcohols in 45 min, respectively, inwere comparison to the para-position (Table 6, entries entries 14 and 15). Additionally, 45 min, respectively, inwas comparison to the para-position (Table 6, entries entries 14 and 15). Additionally, the prepared catalyst was found to be also highly effective towards oxidation of allylic alcohols in prepared catalyst was found to be also highly effective towards oxidation of allylic alcohols in 45 min, respectively, in comparison to the para-position (Table 6, 14 and 15). Additionally, the 45 min, respectively, in comparison to the para-position (Table 6, 14 and 15). Additionally, the respective aldehydes. For example, cinnamyl alcohol was rapidly transformed toalcohols cinnamaldehyde respective aldehydes. For example, cinnamyl alcohol was rapidly transformed cinnamaldehyde prepared catalyst was found to be highly effective towards oxidation of allylic in the prepared catalyst was found to also be also highly effective towards oxidation oftoallylic alcohols in 45 min, 45 min, respectively, respectively, inwas comparison inFor comparison to the to para-position the para-position (Table (Table 6,rapidly entries 6,transformed entries 14 and 14 15). 15). Additionally, Additionally, respective aldehydes. example, cinnamyl alcohol was transformed to cinnamaldehyde respective aldehydes. For example, cinnamyl alcohol was rapidly cinnamaldehyde prepared catalyst found toalso be also highly effective towards oxidation ofto allylic alcohols in the the prepared catalyst was found to beto also highly effective towards oxidation of and allylic alcohols in the the respective aldehydes. For example, cinnamyl alcohol was rapidly transformed to cinnamaldehyde respective aldehydes. For example, cinnamyl alcohol was rapidly transformed to cinnamaldehyde prepared catalyst was found be also highly effective towards oxidation of allylic alcohols in the prepared catalyst was found to be highly effective towards oxidation of allylic alcohols in with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl respective aldehydes. For example, cinnamyl alcohol was rapidly transformed to cinnamaldehyde respective aldehydes. For example, cinnamyl alcohol was rapidly transformed to cinnamaldehyde prepared prepared catalyst catalyst was was found found to be to also be also highly highly effective effective towards towards oxidation oxidation of allylic of allylic alcohols alcohols in the in the with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl respective aldehydes. For example, cinnamyl alcohol was rapidly transformed to cinnamaldehyde cinnamaldehyde respective aldehydes. For example, cinnamyl alcohol was rapidly transformed toFurthermore, cinnamaldehyde alcohols was also completely oxidized to furfural, and more than 99% selectivity towards furfural was with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl with complete conversion and selectivity within 60 min (Table 6, entry 21). furfuryl respective aldehydes. For example, cinnamyl alcohol was rapidly transformed to respective aldehydes. example, cinnamyl alcohol was rapidly alcohol as example example of For heteroaromatic alcohols was also completely oxidized tocinnamaldehyde furfural, and more alcohol ascomplete example of For heteroaromatic alcohols was also completely oxidized to to furfural, and more withalcohol complete conversion andexample, selectivity within 60 min (Table 6, transformed entry 21). 21). Furthermore, furfuryl with conversion and selectivity within 60 min (Table 6, entry Furthermore, furfuryl respective respective aldehydes. aldehydes. For example, cinnamyl cinnamyl alcohol alcohol was was rapidly rapidly transformed transformed to cinnamaldehyde to cinnamaldehyde as of heteroaromatic alcohols was also completely oxidized to furfural, and more alcohol as example of heteroaromatic alcohols was also completely oxidized to furfural, and more with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl obtained in 120 min (Table 6, entry 22). In addition, it is found that the full oxidation of aliphatic alcohols alcohol as example of heteroaromatic alcohols was also completely oxidized to furfural, and more alcohol as example of heteroaromatic alcohols was also completely oxidized to furfural, and more with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl with complete conversion and selectivity within 60 min (Table 6, entry 21). Furthermore, furfuryl than 99% selectivity towards furfural was obtained in 120 min (Table 6, entry 22). In addition, it is is than 99% selectivity towards furfural was obtained in 120 min (Table 6, entry 22). In addition, it is alcohol as example of heteroaromatic alcohols was also completely oxidized to furfural, and more alcohol as example of heteroaromatic alcohols was also completely oxidized to furfural, and more with with complete complete conversion conversion and and selectivity selectivity within within 60 min 60 min (Table (Table 6, entry 6, entry 21). 21). Furthermore, Furthermore, furfuryl furfuryl than 99% selectivity towards furfural was obtained in 120 min (Table 6, entry 22). In and addition, it than 99% selectivity towards furfural was obtained in 120 min (Table 6, oxidized entry 22). Infurfural, addition, it is alcohol as example of heteroaromatic alcohols was also completely to and more alcohol as example of heteroaromatic alcohols was also completely oxidized to furfural, more is more difficult than benzylic ones. In this regard, the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, than 99% selectivity towards furfural was obtained in 120 min (Table 6, entry 22). In addition, it is than 99% selectivity towards furfural was obtained in 120 min (Table 6, entry 22). In addition, it is alcohol as example of heteroaromatic alcohols was also completely oxidized to furfural, and more alcohol as example of heteroaromatic alcohols was also completely oxidized to furfural, and more found that the full oxidation of aliphatic alcohols is more difficult than benzylic ones. In this regard, found that the full oxidation of aliphatic alcohols is more difficult than benzylic ones. In this regard, than 99% selectivity towards furfural was obtained in 120 min (Table 6, entry 22). In addition, it is than 99% selectivity towards furfural was obtained in 120 minthan (Table 6, entry 22). In addition, it is alcohol alcohol as99% example asselectivity example of heteroaromatic of heteroaromatic alcohols alcohols was also also completely completely oxidized oxidized to22). furfural, to22). furfural, and and more more found that the full oxidation offurfural aliphatic alcohols is6, more difficult than benzylic ones. In this regard, found that the full oxidation of aliphatic alcohols is was more difficult benzylic ones. In this regard, than towards was obtained in 120 min (Table 6, entry In addition, it is than 99% selectivity towards furfural was obtained in 120 min (Table 6, entry In addition, it is octan-1-ol, and citronellol occurs in slightly longer periods (Table entries 23–26). Based on the above results, found that the full oxidation of aliphatic alcohols is than benzylic ones. In regard, found that thecyclohexanemethanol, full oxidation of aliphatic alcohols isoctan-1-ol, more difficult than benzylic ones. In this regard, than 99% towards furfural was obtained indifficult 120 min (Table 6, entry 22). Inthis addition, it is is than 99% selectivity towards furfural was obtained in 120 min (Table 6, entry 22). In addition, is the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, and citronellol occurs in it slightly the oxidation of 5-Hexen-1-ol, and citronellol occurs in slightly found that theselectivity full oxidation of aliphatic alcohols is more more difficult than benzylic ones. In this regard, found that the full oxidation of aliphatic alcohols is more difficult than benzylic ones. In this regard, than than 99% 99% selectivity towards towards furfural was was obtained obtained inis 120 in 120 min min (Table (Table 6, entry 6, entry 22). 22). In addition, In addition, it is it the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, and citronellol occurs in slightly the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, and citronellol occurs in slightly found that the full oxidation of aliphatic alcohols more difficult than benzylic ones. In this regard, found that theselectivity full oxidation offurfural aliphatic alcohols isofmore more difficult than benzylic ones. In this regard, it could be deduced that the oxidation different types alcohols using (5%)HRG/MnO –(1%)Ag O 2ones. 2slightly the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, and citronellol occurs in the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, and citronellol occurs in slightly found that the full oxidation of aliphatic alcohols is more difficult than benzylic In this regard, found that the full oxidation of aliphatic alcohols is difficult than benzylic ones. In this regard, longer periods (Table 6, of entries 23–26). Based onis the the above results, itbenzylic could be deduced the longer periods (Table 6, entries entries 23–26). Based onis the the above results, itbenzylic could be deduced that the the the longer oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, andand citronellol occurs in that slightly the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, citronellol occurs in that slightly found found that that the full the full oxidation oxidation aliphatic of aliphatic alcohols alcohols more more difficult difficult than ones. ones. In this In this regard, regard, periods (Table 6, entries 23–26). Based on above results, it could be deduced longer periods (Table 6, 23–26). Based on above results, it could be deduced the the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, and citronellol occurs in that slightly the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, andthan citronellol occurs in slightly catalyst isthe strongly affected by steric hindrance and electronic density. longer periods (Table 6, entries 23–26). Based on the above results, it could be deduced that the longer periods (Table 6, entries 23–26). Based on the above results, it could be deduced that the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, and citronellol occurs in slightly the oxidation of cyclohexanemethanol, 5-Hexen-1-ol, octan-1-ol, and citronellol occurs in slightly oxidation of different types of alcohols using (5%)HRG/MnO 2 –(1%)Ag 2 O catalyst is strongly affected oxidation of different types of alcohols using (5%)HRG/MnO 2 –(1%)Ag 2 O catalyst is strongly affected longer periods (Table 6, types entries 23–26). Based on the above results, it could beisoccurs deduced that the the longer periods (Table 6,ofentries 23–26). Based on octan-1-ol, the above results, it could be deduced that the the oxidation of(Table cyclohexanemethanol, of(Table cyclohexanemethanol, 5-Hexen-1-ol, 5-Hexen-1-ol, octan-1-ol, and2and citronellol citronellol in that slightly in affected slightly oxidation of different of23–26). alcohols using (5%)HRG/MnO 2–(1%)Ag 2it O catalyst isoccurs strongly oxidation ofperiods different types alcohols using (5%)HRG/MnO 2–(1%)Ag O catalyst strongly affected longer 6, entries 23–26). Based on the above results, could be deduced that the longer periods 6, entries Based on the above results, it could be deduced the oxidation of different types alcohols using (5%)HRG/MnO 2–(1%)Ag 2it O catalyst is strongly affected oxidation of(Table different types of23–26). alcohols using (5%)HRG/MnO 2–(1%)Ag 2it O could catalyst isdeduced strongly affected longer periods (Table 6,of entries 23–26). Based on the the above results, be that the longer periods 6, entries Based on the above results, be that the by steric hindrance and electronic density. by steric hindrance and electronic density. oxidation of different types of alcohols using (5%)HRG/MnO 2–(1%)Ag 2it Ocould catalyst isdeduced strongly affected oxidation of different types of alcohols using (5%)HRG/MnO 2–(1%)Ag 2it O catalyst is strongly affected longer longer periods (Table 6, entries 6, entries 23–26). 23–26). Based Based on the on above above results, results, could could beis deduced be that that the the by steric hindrance and electronic density. by steric hindrance and electronic density. Table 6.periods Catalytic oxidation of various alcohols over (5%)HRG/MnO catalyst. oxidation of(Table different types of alcohols using (5%)HRG/MnO 2–(1%)Ag 2O catalyst isdeduced strongly affected oxidation of different types of alcohols using (5%)HRG/MnO 2–(1%)Ag 2O catalyst strongly affected 2 –(1%)Ag 2O by steric hindrance and electronic density. by steric hindrance and electronic density. oxidation of different types of alcohols using (5%)HRG/MnO 2–(1%)Ag 2O catalyst is strongly affected oxidation of different types of alcohols using (5%)HRG/MnO 2–(1%)Ag 2O catalyst is strongly affected by steric hindrance and electronic density. by steric hindrance and electronic density. oxidation oxidation of different of different types types of alcohols of alcohols using using (5%)HRG/MnO (5%)HRG/MnO 2–(1%)Ag 2–(1%)Ag 2O catalyst 2O catalyst is strongly is strongly affected affected by steric steric hindrance and electronic density. by steric hindrance and electronic density. by hindrance and electronic density. by steric hindrance and electronic density. Table 6. Catalytic oxidation of various alcohols (5%)HRG/MnO 2–(1%)Ag 2O catalyst. Table 6. Catalytic oxidation of various alcohols overover (5%)HRG/MnO 2–(1%)Ag 2O catalyst. by steric by steric hindrance hindrance andand electronic electronic density. density. Table 6. Catalytic oxidation of various alcohols (5%)HRG/MnO 2–(1%)Ag 2O catalyst. Table 6. Catalytic oxidation of various alcohols overover (5%)HRG/MnO 2OSel. catalyst. Time 2–(1%)Ag Conv. 6. oxidation of alcohols over (5%)HRG/MnO 2–(1%)Ag 2O catalyst. Table 6. Catalytic oxidation of various alcohols over (5%)HRG/MnO 2–(1%)Ag 2O catalyst. EntryTable Product Table 6. Catalytic Catalytic oxidation of various various alcohols overover (5%)HRG/MnO 2–(1%)Ag 2O(%) catalyst. Table 6.Substrate Catalytic oxidation of various alcohols (5%)HRG/MnO 2–(1%)Ag 2O catalyst. (min) (%) Entry Substrate Product Time (min) Conv. Sel. (%) Entry Product Time (min) Conv. Sel. Sel. (%) (%) Table 6.Substrate Catalytic oxidation of various alcohols over (5%)HRG/MnO 2–(1%)Ag 2(%) O catalyst. Table 6. Catalytic Catalytic oxidation of various various alcohols overover (5%)HRG/MnO 2–(1%)Ag 2(%) O catalyst. catalyst. Entry Substrate Product Time (min) Conv. Entry Product Time (min) Conv. Sel. (%) Table 6.Substrate Catalytic oxidation of various alcohols (5%)HRG/MnO 2–(1%)Ag 2(%) O catalyst. Table 6. oxidation of alcohols over (5%)HRG/MnO 2–(1%)Ag 2(%) O Table Table 6. Catalytic 6.Substrate Catalytic oxidation oxidation of various of various alcohols alcohols overover (5%)HRG/MnO (5%)HRG/MnO 2–(1%)Ag 2–(1%)Ag 2(%) O catalyst. 2O catalyst. Entry Product Time (min) Conv. Sel. (%) Entry Substrate Product Time (min) Conv. (%) Sel. (%) Entry Substrate Product Time (min) Conv. (%) Sel. (%) Entry Substrate Product Time (min) Conv. (%) Sel. (%) Entry Substrate Product Time (min)Conv. Conv. (%) Sel. Sel. (%) Entry Substrate Product Time (min) (%) (%) Entry Substrate Product (min)Conv. Conv. Entry Substrate Product TimeTime (min) (%) (%) Sel. Sel. (%) (%) Entry Substrate Substrate Product Product TimeTime Conv. Conv. (%) Sel. (%) (%) 25(min)100 100 >99 25(min) 100 >99 111 Entry 25 >99(%) Sel. 11 25 100 >99 25 100 >99 11 1 25 100 >99 25 100 >99 25 25 100 100 >99 >99 1 1 25 100 >99 25 25 100 100 >99 >99 11 1 25 100 >99 1 1 25 25 100 100 >99 >99 2 222 2 22 2
22 2 2 2 2 2
35 35 35 35 3535 35 35 35 35 35 35 35 35 35
100 100 100 100100 >99 100 100 100 100 100 100 100 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
33 333 33 3
33 3 3 3 3 3
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
100 100 100 100 100 100 100 >99 100 100 100 100 100 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
44 4 44 44 4
44 4 4 4 4 4
25 25 25 25 25 25 25 2525 25 25 25 25 25 25
100 100 100 100 100 100 100 100 100 100 >99 100 100 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
55 55 555 5
55 5 5 5 5 5
45 45 45 45 45 45 45 45 45 45 4545 45 45 45
100 100 100 100 100 100 100 100 100 100 100 100100 >99 100 100
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
66 66 6 666
66 6 6 6 6 6
60 60 60 60 60 60 60 60 60 60 60 6060 60 60
100 100 100 100 100 100 100 100 100 100 100 100100 >99 100 100
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
77 77 77 77
77 7 7 7 7 7
95 95 95 95 95 95 95 95 95 95 95 95 95 9595
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 >99
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW
17 of17 22of 22 17 of17 22of 22 17 of17 22of 22
888 8
8 8 8
180 180 180180 180 180 180
100 100100 >99 100 100 100 100
>99 >99 >99 >99 >99 >99
9 999
9 9 9
210 210 210210 210 210 210
100 100 100 100
100 100 >99 100
>99 >99 >99 >99 >99 >99
10 10 10 10 10 10 10
35 35 35 35 35 3535
100 100 100 100 100 100 100 >99
>99 >99 >99 >99 >99 >99
11 11 11 11 11 11
30 30 30 30 30 30
100 100 100 100 100 100
>99 >99 >99 >99 >99 >99
12 12 12 12
30 30 30 30
100 100 100 100
>99 >99 >99 >99
Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW 8 2018, Metals 2018, 8, 8x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW 8 2018, Metals 2018, 8, 8x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW 8 8 Metals 2018, 8, 468 8 8
180 180 180 180 180 180 180 180 180 180 180 210 210 180 180 180 210 210 180 210 210 180 210 210 210 210 210 210 210 210 210 210 35 35Time 35 35 35 35(min) 35 35 35 35 35 35 35 35 35 35
100 100 100 100 100 100 100 100 100 100 100 100 100 100 Conv. 100 100 (%) 100 100 100 100 100
100 100 100 100 100 100 100 100 100 100 100 100 100 100 Sel. 100 100 (%) 100 100 100 100 100
8 89 8 9 89 9 9 9 9 9 10 10 Entry 10 10 10 10 10 10
8 89 8998 9 9 9 99 10 10 10 10 10 10 10 10
11 11 11 11 11 11 11 11 11
11 11 11 11 11 11 11 11
30 30 30 30 30 30 30 30
3030 30 30 30 30 30 30 30
100 100 100 100 100 100 100 100 100
12 12 12 12 12 12 12 12 12
12 12 12 12 12 12 12 12
30 30 30 30 30 30 30 30
30 3030 30 30 30 30 30 30
13 13 13 13 13 13 13 13 13
13 13 13 13 13 13 13 13
55 55 55 55 55 55 55 55
14 14 14 14 14 14 14 14 14
14 14 14 14 14 14 14 14
15 15 15 15 15 15 15 15 15
17 of17 22of 22 17 of17 22of 22 17 of17 22of 22 17 of17 22of 22 17 of17 22of 22 17 of 17 22 of 22 >99of 17 >99 17 22 of 22 >99of17 >99 17 22of 22 >99 >99 17>99 of 22 >99
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
100 >99 100 100 100 100 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
100 100 100 100 100 100 100 100 100
100 100 >99 100 100 100 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
55 55 5555 55 55 55 55 55
100 100 100 100 100 100 100 100 100
100 100 100 >99 100 100 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
40 40 40 40 40 40 40 40
40 40 40 4040 40 40 40 40
100 100 100 100 100 100 100 100 100
100 100 100 100 >99 100 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
15 15 15 15 15 15 15 15
45 45 45 45 45 45 45 45
45 45 45 45 4545 45 45 45
100 100 100 100 100 100 100 100 100
100 100 100 100 100 >99 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
16 16 16 16 16 16 16 16 16
16 16 16 16 16 16 16 16
55 55 55 55 55 55 55 55
55 55 55 55 55 5555 55 55
100 100 100 100 100 100100 100 100
100 100 100 100 100 100 >99 100 100
>99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
17 17 17 17 17 17 17 17 17
17 17 17 17 17 17 17 17
65 65 65 65 65 65 65 65
65 65 65 65 65 65 65 65 65
100 100 100 100 100 100 100 100100
100 100 100 100 100 100 100 >99 100
>99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18
80 80 80 80 80 80 80 80
80 80 80 80 80 80 80 80 80
100 100 100 100 100 100 100 100
100 100 100 100 100 100 100 100 >99
>99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
Table 6. Cont. Substrate
Product
100
Metals 2018, 8, 468
18 of 22
Table 6. Cont. Metals 2018,2018, 8, x FOR PEER REVIEW Metals 8, x FOR PEER REVIEW Entry Substrate Metals Metals 2018,2018, 8, x FOR 8, x FOR PEER PEER REVIEW REVIEW Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 2018, 8, x FOR PEER REVIEW Metals 2018, 8, x FOR PEER REVIEW Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 2018,2018, 8, x FOR PEERPEER REVIEW Metals 8, x FOR REVIEW
Product
Time (min)
Conv. (%)
Sel. (%)
18 of18 22 of 22 18 of18 22of 22 18 of18 22of 22 18 of18 22of 22 18 18 of18 22 of 22 18 of 22of 22 18 of18 22of 22 18 of 18 22 of 22
19 19 19 19 19 19 19 19 19
19 19 19 19 19 19 19 19
100 100 100100 100 100 100 100 100 100 100 100 100 100 100 100 100
100 100 100 100 100 100 100 100 100
100 >99 100 100 100 100 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99
20 20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20
140 140 140140 140 140 140 140 140 140 140 140 140 140 140 140 140
100 100 100 100 100 100 100 100 100
100 100 >99 100 100 100 100 100 100
>99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
21 21 21 21 21 21 21 21 21 22 22 22 22 22 22 22 22 22 23 23 23 23 23 23 23 23 23 24 24 24 24 24 24 24 25 24 25 24 25 25 25 25 25 25 25 26 26 26 26 26 26 26 26 26
21 21 21 21 21 21 21 21 22 22 22 22 22 22 22 22 23 23 23 23 23 23 23 23 24 24 24 24 24 24 25 24 25 24 25 25 25 25 25 25 26 26 26 26 26 26 26 26
60 60 60 60 60 6060 60 60 60 60 60 60 60 60 60 60 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 90 90 90 90 90 90 90 90 90 90 90 90 9090 90 90 90 200 200 200 200 200 200 200 200 200 200 200 200200 220 200 200 220 220 220 200 200 220 220 220 220 220 220 220 220 220 220220 220 220 130 130 130 130 130 130 130 130 130 130 130 130 130 130 130130 130 Note: Conditions: as Figure 12. Note: Conditions: in Note: Note: Conditions: Conditions: as in inas as Figure in Figure Figure 12. 12. 12. Note: Conditions: in Figure Note: Conditions: as inas Figure 12. 12. Note: Conditions: as in 12. Note: Conditions: as in Figure 12. Note: Conditions: as in Figure Figure 12. Note: Conditions: as in Figure 12. Note: Conditions: as in Figure 12. Note: Conditions: as in Figure 12. Note: Conditions: in Figure Note: Conditions: as inasFigure 12. 12. Note: Conditions: as in Figure 12. Note: Conditions: as in Figure 12. Note: Conditions: as in Figure 12.
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 1100 00 100 1100 100 00 100 100 100 100 1100 00 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
100 100 100 >99 100 100 100 100 100 100 100 100 100 >99 100 100 100 100 100 100 100 100 100 >99 100 100 100 1100 00 100 1100 00 1>99 00 100 1100 00 100 100 100 100 100 100 >99 100 100 100 100 100 100 100 100 >99 100
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99
>99 >99 >99 >99 >99 >99 >99 >99
4. 4. Conclusions 4. Conclusions Conclusions 4. Conclusions Conclusions 4. 4. Conclusions 4. Conclusions 4. Conclusions 4.InConclusions Conclusions 4. we here an cheap, recyclable, and very mild protocol for In summary, we present here an effective, cheap, recyclable, and very mild protocol for the 4. 4. Conclusions Conclusions In summary, summary, In summary, summary, we present present we present present herehere here an effective, effective, an effective, effective, cheap, cheap, recyclable, recyclable, andand and veryvery very mildmild mild protocol protocol for the the for the the 4. 4. Conclusions In we an cheap, recyclable, protocol for InConclusions summary, we present here an effective, cheap, recyclable, and very mild protocol for the oxidation of alcohols with 100% convertibility and selectivity using Ag 2 O NPs-doped MnO 2 oxidation of alcohols with 100% convertibility and selectivity using Ag 2 O NPs-doped MnO 2 4. Conclusions 4. Conclusions In summary, we present here an effective, cheap, recyclable, and very mild protocol for the In summary, we present here an effective, cheap, recyclable, and very mild protocol for the oxidation oxidation of alcohols of alcohols with with 100% 100% convertibility convertibility and and selectivity selectivity using using Ag Ag 2 O NPs-doped 2 O NPs-doped MnO MnO 2 Inof summary, we present here an effective, effective, cheap, recyclable, and very mild protocol for the22 In summary, we present here an effective, cheap, recyclable, and very mild protocol for the In summary, we present here an cheap, recyclable, and very mild protocol for oxidation of alcohols with 100% convertibility and selectivity using Ag 2O NPs-doped MnO In summary, we present here an effective, cheap, recyclable, and very mild protocol for the oxidation alcohols with 100% convertibility and selectivity using Ag 2O NPs-doped MnO 2 the immobilized on HRG support as the oxidation catalyst using oxygen as an oxidizing immobilized on support as oxidation catalyst using oxygen as an oxidizing In of summary, we present here an effective, cheap, recyclable, and mild protocol for 4. Conclusions oxidation of alcohols with convertibility and selectivity Ag 2eco-friendly O NPs-doped MnO 2 In summary, weHRG present here anthe effective, cheap, recyclable, andusing very mild protocol for the oxidation alcohols with 100% convertibility and selectivity using Ag 2eco-friendly O MnO 2 the immobilized immobilized on HRG on HRG support support as100% the as the oxidation oxidation catalyst catalyst using using oxygen oxygen as an asvery an oxidizing oxidizing oxidation of HRG alcohols with 100% convertibility and selectivity using AgNPs-doped 2eco-friendly O NPs-doped MnO 2 oxidation alcohols with 100% convertibility and selectivity using Ag 2eco-friendly O NPs-doped MnO 2 the In summary, weHRG present here an effective, cheap, recyclable, andas very mild protocol for the Inof summary, we present here an effective, cheap, recyclable, and very mild protocol for oxidation of alcohols with 100% convertibility and selectivity using Ag 2eco-friendly O NPs-doped MnO 2 immobilized on support as the oxidation catalyst using oxygen as an oxidizing oxidation of alcohols with 100% convertibility and selectivity using Ag 2eco-friendly O NPs-doped MnO 2 immobilized on support asoxidation the oxidation catalyst using oxygen an oxidizing agent. For 1-phenylethanol as a substrate model, (5%)HRG/MnO 2–(1%)Ag 2MnO O agent. For 1-phenylethanol oxidation as a substrate model, (5%)HRG/MnO 2–(1%)Ag oxidation of alcohols with 100% convertibility and selectivity using Ag 2eco-friendly O NPs-doped 2 immobilized on HRG support as the oxidation catalyst using oxygen as an oxidizing oxidation of alcohols with 100% convertibility and selectivity using Ag 2eco-friendly O NPs-doped MnO 2 2O immobilized on HRG support as the oxidation catalyst using oxygen as an oxidizing agent. agent. For For 1-phenylethanol 1-phenylethanol oxidation oxidation as as a substrate a substrate model, model, (5%)HRG/MnO (5%)HRG/MnO 2 –(1%)Ag 2 –(1%)Ag 2 O 2 O immobilized on HRG support as the oxidation catalyst using oxygen as an eco-friendly oxidizing immobilized on HRG support as the oxidation catalyst using oxygen as an eco-friendly oxidizing oxidation alcohols with 100% convertibility and selectivity using Ag 2O MnO oxidation of HRG alcohols with 100% convertibility and selectivity using AgNPs-doped 2eco-friendly O NPs-doped In summary, we present here an effective, cheap, recyclable, and very mild protocol immobilized on HRG support as the oxidation catalyst using oxygen as an oxidizing agent. 1-phenylethanol oxidation as aacatalyst substrate model, (5%)HRG/MnO 2for –(1%)Ag immobilized on support as the oxidation using oxygen as an eco-friendly oxidizing agent. ForofFor 1-phenylethanol oxidation asconversion) acatalyst substrate model, (5%)HRG/MnO 2–(1%)Ag 2MnO O2 2O2 nanocomposite gave higher conversion (100% than the MnO 2–(1%)Ag 2eco-friendly O catalyst without nanocomposite gave higher conversion (100% conversion) than the MnO 2–(1%)Ag 2O catalyst without immobilized on HRG support as the oxidation using oxygen as an oxidizing agent. For 1-phenylethanol oxidation as substrate model, (5%)HRG/MnO 2–(1%)Ag immobilized on HRG support as the oxidation using oxygen as an eco-friendly oxidizing agent. For 1-phenylethanol oxidation as aacatalyst substrate model, (5%)HRG/MnO 2–(1%)Ag 2O 2O nanocomposite nanocomposite gave gave higher higher conversion conversion (100% (100% conversion) conversion) than than the MnO the MnO 2–(1%)Ag 2–(1%)Ag 2eco-friendly O catalyst 2O catalyst without without agent. For 1-phenylethanol oxidation as a substrate model, (5%)HRG/MnO 2–(1%)Ag agent. For 1-phenylethanol oxidation as substrate model, (5%)HRG/MnO 2MnO –(1%)Ag 2O 2O immobilized on HRG support as the oxidation catalyst using oxygen as an eco-friendly oxidizing immobilized on HRG support as the oxidation catalyst using oxygen as an oxidizing the oxidation of alcohols with 100% convertibility and selectivity using Ag O NPs-doped 2 2–(1%)Ag 2 without agent. For 1-phenylethanol oxidation as substrate model, (5%)HRG/MnO 2–(1%)Ag nanocomposite gave higher conversion (100% than the MnO 2O catalyst nanocomposite gave higher conversion (100% than the MnO 2–(1%)Ag 2O catalyst without agent. For 1-phenylethanol oxidation asconversion) aconversion) substrate model, (5%)HRG/MnO 2–(1%)Ag 2O 2O using graphene support. A full conversion with >99% acetophenone selectivity accomplished using graphene support. A full conversion with acetophenone selectivity accomplished agent. For 1-phenylethanol oxidation as aa>99% substrate model, (5%)HRG/MnO 2–(1%)Ag nanocomposite gave higher conversion (100% than the MnO 2–(1%)Ag 2was O catalyst without nanocomposite gave higher conversion (100% conversion) than the MnO 2–(1%)Ag 2was O catalyst without agent. For 1-phenylethanol oxidation as aconversion) substrate model, (5%)HRG/MnO 2–(1%)Ag 2O 2O using using graphene graphene support. support. Athe full Aoxidation full conversion conversion with with >99% >99% acetophenone acetophenone selectivity selectivity accomplished accomplished nanocomposite gave higher conversion (100% conversion) than the MnO 2–(1%)Ag 2was O catalyst without nanocomposite gave higher conversion (100% conversion) than the 2–(1%)Ag 2was O catalyst without agent. For 1-phenylethanol oxidation as aconversion) substrate model, (5%)HRG/MnO 2–(1%)Ag 2O 2O agent. For 1-phenylethanol oxidation as a>99% substrate model, (5%)HRG/MnO 2–(1%)Ag immobilized on HRG support as catalyst using oxygen as anMnO eco-friendly oxidizing agent. nanocomposite gave higher conversion (100% than the MnO 2–(1%)Ag 2was O catalyst without using graphene support. A full conversion with acetophenone selectivity accomplished nanocomposite gave higher conversion (100% conversion) than the MnO 2–(1%)Ag 2was O catalyst without using graphene support. A full conversion with >99% acetophenone selectivity accomplished after a short period. The obtained specific activity (16 mmol/g·h) is much higher than that found in after a short period. The obtained specific activity (16 mmol/g·h) is much higher than that found in nanocomposite gave higher conversion (100% conversion) than the MnO 2 –(1%)Ag 2 O catalyst without using graphene support. A full conversion with >99% acetophenone selectivity was accomplished nanocomposite gave higher conversion (100% conversion) than theisMnO 2–(1%)Ag2was O catalyst without using support. A full conversion with >99% acetophenone selectivity accomplished after after agraphene short agraphene short period. period. The The obtained obtained specific specific activity activity (16>99% mmol/g·h) (16 mmol/g·h) much isMnO much higher higher than than that that found found in in in using graphene support. A conversion full conversion with >99% acetophenone selectivity was accomplished using graphene support. A conversion with >99% acetophenone selectivity accomplished nanocomposite gave higher conversion (100% conversion) than MnO 22 –(1%)Ag 2was O catalyst without nanocomposite gave higher (100% conversion) than the 2–(1%)Ag 2than O catalyst without For 1-phenylethanol oxidation as afull substrate model, (5%)HRG/MnO –(1%)Ag Ohigher nanocomposite gave 2the using support. A full conversion with acetophenone selectivity was accomplished after a short period. The obtained specific activity (16 mmol/g·h) is much higher that found using graphene support. A full conversion with >99% acetophenone selectivity was accomplished after a short period. The obtained specific activity (16 mmol/g·h) is much than that found in earlier publications. Using this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic earlier publications. Using this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic using graphene support. A full conversion with >99% acetophenone selectivity was accomplished after a short period. The obtained specific activity (16 mmol/g·h) is much higher than that found in using graphene support. A full conversion with >99% acetophenone selectivity was accomplished after a short period. The obtained specific activity (16 mmol/g·h) is much higher than that found in earlier earlier publications. publications. Using Using this this catalytic catalytic system, system, various various aromatic, aromatic, allylic, allylic, heterocyclic, heterocyclic, andaccomplished and aliphatic aliphatic after agraphene short period. The obtained specific activity (16 mmol/g·h) iswithout much higher than that found in after aagraphene short period. The obtained specific activity (16 mmol/g·h) is much higher than that found in using support. Athis full conversion with >99% acetophenone selectivity was accomplished using support. Athis full conversion with >99% acetophenone selectivity was higher conversion (100% conversion) than the MnO –(1%)Ag O catalyst using graphene 2various 2mmol/g·h) after short period. The obtained specific activity (16 is much higher than that found in earlier Using catalytic system, various aromatic, allylic, heterocyclic, and aliphatic after short period. The obtained specific activity (16 mmol/g·h) isallylic, much higher than that found in earlier publications. Using catalytic system, aromatic, heterocyclic, and aliphatic alcohols can be fully transformed to respective aldehydes and ketones with excellent yields and short alcohols can be fully transformed to respective aldehydes and ketones with excellent yields and short after aa publications. short period. The obtained specific activity (16 mmol/g·h) is much higher than that found in publications. this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic afterearlier a publications. short period. TheUsing obtained specific activity (16 mmol/g·h) isallylic, much higher than that found in earlier Using this catalytic system, various aromatic, heterocyclic, and aliphatic alcohols alcohols can can be fully be fully transformed transformed to respective to respective aldehydes aldehydes and and ketones ketones with with excellent excellent yields yields and and short short earlier publications. Using this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic earlier publications. Using this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic after aAshort period. Thewith obtained activity (16 mmol/g·h) isallylic, much higher than that found in after a publications. short period. The obtained specific activity (16 mmol/g·h) is much higher than that found in support. full conversion >99% acetophenone selectivity wasketones accomplished after ayields short earlier Using this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic alcohols can be fully transformed to respective aldehydes and with excellent and short earlier publications. Using this catalytic system, various aromatic, heterocyclic, and aliphatic alcohols can be fully transformed tospecific respective aldehydes and ketones with excellent yields and short reaction times. The current catalytic protocol is highly selective, yielding only desired aldehydes or reaction times. The current catalytic protocol is highly selective, yielding only desired aldehydes or earlier publications. Using this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic alcohols can be fully transformed to respective aldehydes and ketones with excellent yields and short earlier publications. Using this system, various aromatic, allylic, heterocyclic, and aliphatic alcohols can be fully transformed to respective aldehydes and ketones with excellent yields and short reaction reaction times. times. The The current current catalytic catalytic protocol protocol is highly is highly selective, selective, yielding yielding only only desired desired aldehydes aldehydes or or alcohols can be fully transformed to respective aldehydes and ketones with excellent yields and short alcohols can be fully transformed to respective aldehydes and ketones with excellent yields and short earlier publications. Using this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic earlier publications. Using this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic period. The obtained specific activity (16 mmol/g · h) is much higher than that found in earlier alcohols can be fully transformed totorespective respective aldehydes and ketones with excellent yields and short reaction times. The current catalytic protocol is highly selective, yielding only desired aldehydes or alcohols canwithout be The fully transformed totorespective and ketones with excellent yields and short reaction times. current catalytic protocol isaldehydes highly selective, yielding only desired aldehydes or ketones without further oxidation acids. The significant advantages of catalytic system are as ketones further oxidation acids. The significant advantages of this catalytic system are as alcohols can be fully transformed to and ketones with excellent yields and short reaction times. The current catalytic protocol is highly selective, yielding only desired aldehydes or alcohols canwithout be The fully transformed totorespective aldehydes and ketones with excellent yields and short reaction times. current catalytic protocol is highly selective, yielding only desired aldehydes or ketones ketones without further further oxidation oxidation acids. torespective acids. The The significant significant advantages advantages of this this of this catalytic catalytic system system are as are as reaction times. The current catalytic protocol isaldehydes highly selective, yielding only desired aldehydes or reaction times. The current catalytic protocol is highly selective, yielding only desired aldehydes or alcohols can be fully transformed toto respective aldehydes and ketones with excellent yields and short alcohols can be fully transformed toto aldehydes and ketones with excellent yields and short publications. Using this catalytic system, various aromatic, allylic, heterocyclic, and aliphatic reaction times. The current catalytic protocol is highly selective, yielding only desired aldehydes or ketones without further oxidation acids. The significant advantages of this catalytic system are as reaction times. The current catalytic protocol is highly selective, yielding only desired aldehydes or ketones without further oxidation acids. The significant advantages of this catalytic system are as follows: (a) straightforward and easy to handle procedure; (b) clean oxidant; (c) not using any follows: (a) straightforward and easy to handle procedure; (b) clean oxidant; (c) not using any reaction times. The current catalytic protocol is highly selective, yielding only desired aldehydes or ketones without further oxidation to acids. The significant advantages of this catalytic system are as reaction times. The current catalytic protocol is highly selective, yielding only desired aldehydes or ketones without further oxidation to acids. The significant advantages of this catalytic system are as follows: follows: (a) straightforward (a) straightforward and and easy easy to handle to handle procedure; procedure; (b) clean (b) clean oxidant; oxidant; (c) not (c) not using using any any ketones without further oxidation to acids. The significant advantages of this catalytic system are as ketones without further oxidation to acids. The significant advantages of this catalytic system are as reaction times. The current catalytic ishandle highly selective, yielding only desired aldehydes or reaction times. The current protocol isprocedure; highly selective, yielding only desired aldehydes or alcohols can without be fully transformed tocatalytic respective aldehydes andadvantages ketones with excellent yields and ketones without further oxidation tobases; acids. The significant advantages ofoxidant; this catalytic system areany as follows: (a) straightforward and easy to procedure; (b) clean (c) not using follows: (a)surfactants straightforward and easy to handle (b) clean (c) not using any ketones further oxidation toprotocol acids. The significant ofoxidant; this catalytic system are as additional and nitrogenous (d) mild circumstances; (e) cheap oxidant and catalyst; additional surfactants and nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and catalyst; ketones without further oxidation to acids. The significant advantages of this catalytic system are as follows: (a) straightforward and easy to handle procedure; (b) clean oxidant; (c) not using any follows: (a) straightforward and easy to handle procedure; (b) clean oxidant; (c) not using any ketones without further oxidation to acids. The significant advantages of this catalytic system are as additional additional surfactants surfactants and and nitrogenous nitrogenous bases; bases; (d) mild (d) mild circumstances; circumstances; (e) cheap (e) cheap oxidant oxidant and and catalyst; catalyst; follows: (a)surfactants straightforward and easy tois(d) handle procedure; (b) (e) clean oxidant; (c) not using any follows: (a) straightforward and easy to handle procedure; (b) clean oxidant; (c) not using any without further oxidation toprotocol acids. The significant advantages ofonly this catalytic system are as ketones without further oxidation to The significant advantages ofoxidant; this catalytic system areany as shortketones reaction times. The current catalytic highly selective, yielding desired aldehydes follows: (a) straightforward and easy to handle procedure; (b) clean (c) not using additional and nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and catalyst; follows: (a) straightforward and easy toacids. handle procedure; (b) clean oxidant; (c) not using any additional surfactants and nitrogenous bases; mild circumstances; cheap oxidant and catalyst; (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous catalyst; (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous follows: (a) straightforward and easy to handle procedure; (b) clean oxidant; (c) not using additional surfactants and nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and catalyst; follows: (a)surfactants straightforward and easy to handle procedure; (b) clean oxidant; (c) not using any any additional and nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and (f) full (f) full convertibility; convertibility; (g) complete (g) complete selectivity; selectivity; (h) fast (h) fast reaction; reaction; (i) reusable (i) reusable heterogeneous heterogeneous catalyst; additional surfactants and nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and additional surfactants and nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and catalyst; follows: (a) straightforward and easy to handle procedure; (b) clean oxidant; (c) not using any follows: (a) straightforward and easy to handle procedure; (b) clean oxidant; (c) not using any or ketones without further oxidation to acids. The significant advantages of(e) this system are catalyst; additional surfactants and nitrogenous bases; (d) mild circumstances; (e)catalytic cheap oxidant and (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous additional surfactants andcomplete nitrogenous bases; (d) mild circumstances; cheap oxidant and (f) full convertibility; (g) selectivity; (h) fast reaction; (i) reusable heterogeneous catalyst; and (j) being applicable to aato various types of alcohols. All these advantages will cause this and (j) being applicable aa selectivity; various types of alcohols. All these advantages will cause this additional surfactants and nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous catalyst; additional surfactants andcomplete nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and (f) full convertibility; (g) (h) fast reaction; (i) reusable heterogeneous catalyst; and and (j) being (j) being applicable applicable to to various various types types of alcohols. of alcohols. All All these these advantages advantages will will cause cause this this this (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous additional surfactants and nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and catalyst; additional surfactants and nitrogenous bases; (d) mild circumstances; (e) cheap oxidant and catalyst; as follows: (a) straightforward and easy to handle procedure; (b) clean oxidant; (c) not using any (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous catalyst; and (j) being applicable to a various types of alcohols. All these advantages will cause (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous catalyst; and (j) being applicable to a various types of alcohols. All these advantages will cause this methodology to be very beneficial and applicable to the industrial synthesis of carbonyl compounds. methodology to be very beneficial and applicable to the industrial synthesis of carbonyl compounds. (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous catalyst; and (j) being applicable to a various types of alcohols. All these advantages will cause this (f) full convertibility; (g) complete selectivity; (h) fast reaction; (i) reusable heterogeneous catalyst; and (j) being applicable to a various types of alcohols. All these advantages will cause this methodology methodology to be toapplicable very benitrogenous very beneficial beneficial and and applicable applicable to the tofast industrial thereaction; industrial synthesis synthesis of carbonyl of carbonyl compounds. compounds. and (j) convertibility; being applicable tovarious aand various types of alcohols. All these advantages will cause this and being to aato types of alcohols. All these advantages will cause this (f) convertibility; (g) complete selectivity; (h) fast reaction; reusable heterogeneous catalyst; (f)(j) full (g) complete selectivity; (h) (i)these reusable heterogeneous catalyst; additional surfactants and bases; (d) mild circumstances; (e) cheap oxidant and catalyst; and (j) being various types of alcohols. All advantages will cause this methodology to be very and applicable to the industrial synthesis of carbonyl compounds. andfull (j) being applicable tobeneficial various types ofto alcohols. All(i) these advantages will cause this this methodology toapplicable be very beneficial applicable the industrial synthesis of carbonyl compounds. and (j) being applicable aaand various types of alcohols. these advantages will cause to be very and applicable to the industrial synthesis of carbonyl compounds. andmethodology (j) being applicable tobeneficial atovarious types ofto alcohols. All All these advantages willcompounds. cause this methodology to be very beneficial applicable the industrial synthesis of carbonyl methodology to be very beneficial and applicable to the industrial synthesis of carbonyl compounds. methodology to be very beneficial and applicable to the industrial synthesis of carbonyl compounds. and (j) being tobeneficial aselectivity; types ofto alcohols. All these advantages will cause this this and (j) being tovarious aand various types of All these advantages will cause (f) full convertibility; (g) complete (h) fast reaction; reusable heterogeneous catalyst; methodology toapplicable be very very and applicable toalcohols. the (i) industrial synthesis of carbonyl carbonyl compounds. methodology toapplicable be very beneficial applicable the industrial synthesis of carbonyl compounds. methodology be beneficial applicable to the industrial synthesis of compounds. methodology to betovery beneficial andand applicable to the industrial synthesis of carbonyl compounds. methodology to betovery beneficial and and applicable to the synthesis of carbonyl compounds. methodology be very beneficial applicable to industrial the industrial synthesis of carbonyl compounds.
Metals 2018, 8, 468
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and (j) being applicable to a various types of alcohols. All these advantages will cause this methodology to be very beneficial and applicable to the industrial synthesis of carbonyl compounds. Author Contributions: S.F.A. and M.E.A. gave the idea of the work; M.E.A., S.F.A., M.K. (Mujeeb Khan), and M.R.S. helped to write the project; M.E.A., M.K. (Mufsir Kuniyil), and M.R.S. performed the experimental section and characterization; and A.A.-W. and M.R.H.S. provided scientific guidance for successful completion of the study and also assisted to write the paper. All authors read and approved the final paper. Funding: This work is funded by the research group project No. RG-1436-032. 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. Conflicts of Interest: The authors declare no conflict of interest.
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