Synthesis, characterization and application of a nano-manganese-catalyst as an efficient solid catalyst for solvent free selective oxidation of ethylbenzene, cyclohexene, and benzylalcohol

Applied Surface Science 276 (2013) 487–496

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Synthesis, characterization and application of a nano-manganese-catalyst as an efficient solid catalyst for solvent free selective oxidation of ethylbenzene, cyclohexene, and benzylalcohol Davood Habibi ∗ , Ali Reza Faraji ∗ Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan 6517838683, Iran

a r t i c l e

i n f o

Article history: Received 27 November 2012 Received in revised form 16 March 2013 Accepted 19 March 2013 Available online 1 April 2013 Keywords: Mn nano-catalyst Oxidation Ethylbenzene Cyclohexene Benzylalcohol Acetophenone 2-Cyclohexene-1-one Benzaldehyde

a b s t r a c t The object of this study is to synthesize the heterogeneous Mn-nano-catalyst (MNC) which has been covalently anchored on a modified nanoscaleSiO2 /Al2 O3 , and characterized by FT-IR, UV-Vis, CHN elemental analysis, EDS, TEM, and EDX. The method is efficient for the highly selective oxidation of ethylbenzene, cyclohexene, and benzylalcohol without the need to any solvents, using tert-butyl hydroperoxide (TBHP) as an oxidant. Oxidation of ethylbenzene, cyclohexene, and benzylalcohol gave acetophenone, 2-cyclohexene-1-one and benzaldehyde, respectively, as major products. Reaction conditions have been optimized by considering the effect of various factors such as reaction time, amounts of substrates and oxidant, Mn-nano-catalyst and application of various solvents. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The oxidation of organic substrates into useful organic compounds is a fundamental reaction in the organic chemistry both for basic researches and chemical industries [1,2]. Recently, significant amounts of transition metal complexes accompanied by Schiff base as ligand have been used as heterogeneous catalysts, due to their high activity, eco-friendly and selectivity [3–5]. Production of benzylic and allylic ketons were carried out before by oxidizing the C H bond using stoichiometric amount of KMnO4 [6], H2 O2 [7,8], CrO3 SiO2 [9], and tert-butyl hydroperoxide (TBHP) [10–16] as oxidizing agents. Transition metal complexes mainly used as catalyst to promote the oxidation reaction as the manganese complexes have considerable advantages [17–20]. Considerable researches have been devoted to find efficient catalysts for the selective side chain oxidation of alkyl aromatics and allylic substrates. The following catalytic systems were used for the oxidation of ethylbenzene(EB) to acetophenone (AP) such as, metal acetylacetone [21], metalloporphyrins [22,23], macrocyclic [24] and metal complexes supported

on alumina/silica [25], salen-Mn (III)/MCM-41 [26], Si/Al-Pr-NHet-N = methyl-2-Pyridylketone-Mn [15], SF-ATPS-Mn(III) TMCPP [27], Mn(salen)-POM [28], M-APO-11 (M = Co, Mn and V) [29], Zeolite encapsulated metal complexes [30], Co tetraphenylporphyrins [31] and the PS-PAR-Co [32]. The allylic oxidation of olefin to ␣,␤-unsaturated ketone is an important transformation in natural product synthesis [33]. For the article reported in oxidation of cyclohexen to 2-cyclohexen-1-one, [Mn(H4 C6 N6 S2 )] [34], [Mn(Sal-1.3-phen)]-NaY [35], [Mn(bpy)2 ]Cl2 -Al2 O3 [36], manganese porphyrin [37] and salophen Mn(III) complexes [26] can be mentioned. In addition, to study the variation of the Mn catalyst, we report a simple and general synthesis of manganese ions immobilized onto the factionalizedSiO2 -Al2 O3 mixed oxide with the Schiff base ligand, and a methodology planned for the selective and ecofriendly oxidation of ethylbenzene, cyclohexene and benzylalcohol with TBHP. 2. Experimental 2.1. Materials and instruments

∗ Corresponding authors. Tel.: +98 811 8282807; fax: +98 811 8380709. E-mail addresses: [email protected] (D. Habibi), alireza [email protected] (A.R. Faraji). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.121

All reagents were purchased from the Merck and Fluka chemical companies. Reagents were used without extra purification, but solvents were purified with standard methods. Inductively coupled

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Scheme 1. Typical preparation procedure of Si/Al-APTMS-BPK-Mn catalyst.

plasma (ICP) measurements for Mn content evaluation were performed using a Perkin-Elmer ICP/6500. Infrared was collected on KBr pellets using a JASCO FT/IR (680 plus) spectrometer and the position of an infrared band is given in reciprocal centimeters (cm−1 ). Diffuse reflectance spectra were registered on a JASCO550 UV–vis spectrophotometer which was equipped with a diffuse reflectance attachment in which BaSO4 was used as the reference. Type and quantity of the resulting products from oxidation were determined by a HP 6890/5973 GC/MS instrument and analyzed by a Shimadzu GC-16A gas chromatograph(GL-16A gas chromatograph with a 5 m × 3 mm OV-17 column, 60–220 ◦ C (10 ◦ C/min), Inj. 230 ◦ C, Det. 240 ◦ C). For elemental analysis a CHN-Rapid Heraeus elemental analyzer (Wellesley MA) was used. Before carrying out the Nitrogen (99.999%) adsorption experiments, the sample was outgassed at 393 K for 14 h, then the experiment was carried out at 76 K using a volumetric apparatus (Quantachrome NOVA automated gas sorption analyzer). The specific surface areas were calculated, using the BET (Stephen Brunauer, Paul Hugh Emmett, and Edward Teller) method. The images of scanning electron micrograph (SEM) and transmission electron microscopy (TEM) were taken using a Philips 501 microscope and a Tecnai F30TEM operating at 300 kV, respectively. In addition, energy dispersive X-ray analysis was conducted on each sample. In order to count nanoparticles in reverse microemulsion, size distribution was measured by Zetasizer Nano-ZS-90 (ZEN 3600, MALVERN instruments). 2.2. Preparation of Organometallic nanosized SiO2 /Al2 O3 SiO2 /Al2 O3 nanosized was used as the support prepared by the sol–gel method [15]. At first, 3.5 g of nanosized SiO2 /Al2 O3 was activated at 500 ◦ C for 5 h under air and then refluxed with 4.3 mL of trimethoxysilylpropylamine (3-APTMS) in dry toluene (50 mL) for 24 h. The solid achieved during this process was filtered and washed off with dry methanol at 100 ◦ C under vacuum for 5 h. Then bipyridylketone (BPK) was added to the suspended solution of SiO2 /Al2 O3 -APTMS in dry methanol. To synthesize the SiO2 /Al2 O3 APTMS-BPK-Mn (Scheme 1), 2.0 g of SiO2 /Al2 O3 -APTMS-BPK was suspended in 50 ml of ethanol in a round bottom flask followed by adding of 3.0 mmol Mn(OAc)2 ·4H2 O. The mixture was refluxed during 24 h under magnetically stirring.

with a refluxed condenser. The mixture was stirred at desired temperature. After filtering and washing with solvent, the filtrate was monitored by GC analysis. The products were identified by GC–MS techniques. The conversion and selectivity were calculated with GC area normalization. Finally, comparative experiments were performed under different conditions. 3. Results and discussion 3.1. Characterization Using BPK and Mn(OAc)2 ·4H2 O through the covalently immobilization, caused heterogeneous Mn catalyst synthesized onto the SiO2 /Al2 O3 mixed oxides, as illustrated in Scheme 1. The formation of Mn catalyst onto the SiO2 /Al2 O3 was verified using CHN, FT-IR, UV–vis, SEM, TEM and EDX. The surface area, pore size and volume of the modified support were significantly reduced compared to the parent SiO2 /Al2 O3 (this decrease indicates the decrease in interaction between adsorbate, N2 molecules, and the nano-sized SiO2 /Al2 O3 surface after modification with organic chains). The loading of manganese in the heterogeneous manganese catalyst was characterized by elemental analyses. The final metal content was around 0.35 mmol/g, indicating that 74.4% of the immobilized ligands were complexed with manganese ions (Table 1). 3.1.1. The FT-IR spectra In Figure 1, the FT-IR spectra of SiO2 /Al2 O3 -APTMS, SiO2 /Al2 O3 APTMS-BPK, and SiO2 /Al2 O3 -APTMS-BPK-Mn are shown. The strong absorption bands related to Si O Si stretching vibrations

2.3. General procedure for the oxidation of ethylbenzene, cyclohexene and benzyl alcohol In this procedure the heterogeneous catalyst (5.0 mg), the substrate (9.0 mmol) and an oxidant (9.0 mmol, 80% aqueous solution TBHP) were added in three necked round bottom flask equipped

Fig. 1. FTIR spectra of: (A) SiO2 /Al2 O3 -APTMS; (B) SiO2 /Al2 O3 -APTMS-BPK; (C) SiO2 /Al2 O3 -APTMS-BPK-Mn.

489

0.045 0.031 0.026 0.018 498 378 320 274

36 25 20 17

Pore volume (cm3 /g) Surface area (m2 /g)

Structural parameterse

Pore diameter (Ao )

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e

c

d

Molar ratio of SiO2 /Al2 O3 was 60:40, determined from EDX analysis. Nitrogen was estimated from the elemental analyses. Mn content determined from EDX analysis. Determined from the N-contents. Determined from the Mn-content, assume that cobalt ions coordinated with Schiff base ligands. The pore size calculated using the BJH method. a

– – – 74.4 – – – 0.35 – 2.67 3.51 1.46 – – – 1.94 – 3.75 4.92 2.04 – 8.13 10.9 9.49

Mn C

N

SiO2 /Al2 O3 nanosizeda SiO2 /Al2 O3 -APTMS SiO2 /Al2 O3 -APTMS-BPK SiO2 /Al2 O3 -APTMS-BPK-Mn

b

%Coordinated Schiff base groups to Mn ions Immobilized -Mn Schiff base-complex (mmol/g mixed oxide)d Organic functional group (mmol/g mixed oxide)c Elemental analysis (wt.%)b Sample

Table 1 Chemical composition and physicochemical properties of the immobilized Mn-nano-catalyst on theSiO2 /Al2 O3 mixed oxide.

Fig. 2. DR UV–vis spectra of: (A) SiO2 /Al2 O3 ; (B) SiO2 /Al2 O3 -APTMS; (C) SiO2 /Al2 O3 APTMS-BPK; (D) SiO2 /Al2 O3 -APTMS-BPK-Mn.

was observed in the spectrum of SiO2 /Al2 O3 at 1010–1290 and 798 cm−1 . The FT-IR spectrum of SiO2 /Al2 O3 -APTMS showed several signals originating from amino propyl- groups, which are related to C H stretching modes of the propyl groups, appeared in the area of 1450–1560 cm−1 and 2935–2860 cm−1 . By observing the spectrum mentioned, it was concluded that the SiO2 /Al2 O3 was modified by amino-spacer groups successfully. The N–H deformation peak at 1540–1560 cm−1 confirmed the successful functionalization of the SiO2 /Al2 O3 with 3-APTMS. In the IR spectrum (Fig. 1, sec. B) the C N imine vibration signal was observed at 1630 cm−1 , which showed the condensation reaction between BPK with organo-functionalized SiO2 /Al2 O3 . The peaks in the 3066–3020 cm−1 range and the peaks at 1437–1434 cm−1 were attributed to the C H stretching vibrations of pyridine groups and C C stretching vibration of pyridine groups, respectively. After complexing of Mn with immobilized BPK over modified SiO2 /Al2 O3 , the weak absorption peak at 413 cm−1 was appeared which is attributed to the Mn–N bands. 3.1.2. The UV–vis spectra By comparing the UV–vis spectrum of the SiO2 /Al2 O3 with the SiO2 /Al2 O3 -APTMS-BPK (Fig. 2), was observed that the UV–vis spectrum of the SiO2 /Al2 O3 had the side-band adsorption near 249 nm but, in the SiO2 /Al2 O3 -APTMS-BPK transitions ␲ → ␲* and n → ␲* of the ligands caused strong adsorption in the 255–320 nm. A characteristic feature of immobilized Mn(II) species after the reaction with Mn(OAc)2 ·4H2 O is changing the color of SiO2 /Al2 O3 -APTMSBPK from yellow to deep brown. Consequently, UV–visible spectra indicated the appearance of several new metal d–d transition bands around 490 nm upon complexation. 3.1.3. SEM, TEM and EDS Selected images showing the scanning electron microscope (SEM) and the transmission electron microscope (TEM) of catalyst (Figs. 3 and 4) and also its size distribution (Fig. 5) are presented. The average particle size in reverse microemulsion solution was around 31 nm. According to the small nanoparticle size and ligand capping as an obstacle in agglomeration, the Mn-nanocatalyst could be used as a suitable catalyst for oxidation of different substrates such as ethylbenzene, cyclohexene and benzyl alcohol. The energy dispersive spectrum (EDS) of Mn-nano-catalyst is shown in Fig. 6. In the EDS spectrum of the nano-manganese catalyst, signals related to Si, Al, O and Mn were observed. The existence of Mn signal in the spectrum resulted from the Mn complexes with active sites of the organic functional groups ( C N) that increased the catalytic

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Fig. 5. Particle size distribution on the SiO2 /Al2 O3 -APTMS-BPK-Mn.

activity of the synthesized catalyst in comparison to the unmodified support. 3.2. Oxidation of ethylbenzene Fig. 3. The SEM image of SiO2 /Al2 O3 -APTMS-BPK-Mn nano-catalyst.

Fig. 4. The TEM image of SiO2 /Al2 O3 -APTMS-BPK-Mn catalyst.

To optimize the oxidation condition of ethylbenzene (EB) by TBHP in the presence of nano manganese catalyst, various parameters such as the amount of the substrate to oxidant, the temperature, and the nature of the solvent, on the performance of the heterogeneous catalyst were investigated. In this reaction major products are acetophenone (1) benzaldehyde (2) and benzoic acid (3) (Scheme 2). The GC analysis did not show any oxidation products of the aromatic ring in effluents. In order to study the effects of solvents on the oxidation of EB, acetonitrile, ethanol, water, benzene and dichloromethane were examined at 100 ◦ C (Fig. 7). The reaction had low conversion at the presence of coordinating solvents such as ethanol, while using water as a solvent made no reaction. It seems that the donor electrons of these solvents had more ability to occupy the vacant space around the existed metal in catalyst, so this prevented coordinating from oxidant molecules [44]. The major product of oxidation of EB was acetophenone. The maximum percentage of EB conversion decreased on different solvents in the following order: 49.0% (benzene) > 34.0% (acetonitrile) > 32.0% (dichloromethane) > 25.0% (ethanol) > 0.0% (water). The use of protic-polar solvents favored the production selectivity of benzaldehyde and benzoic acid, while aprotic-polar solvents led to acetophenone [45]. It seemed that the acetophenone selectivity decreased with a protic solvent (acetonitrile, 51.0% > benzene, 45.0% > dichloromethane, 37.0% > ethanol, 31.0% > water, 0.0%). Thus, dipole moments of the solvents probably played an essential role in oxidation of EB. More investigations

Fig. 6. The EDS spectrum of Mn- nano-catalyst.

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491

Scheme 2. Possible products that can be obtained from the oxidation of ethylbenzene.

showed that the solventless reaction had high selectivity and more catalytic activity (Fig. 7). This shows that the EB and solvents were competing for the active sites of metal center on catalyst. In order to evaluate the effect of substrate/oxidant mole ratio on the catalytic activity and selectivity in solventless condition, the reactions were carried out at different mole ratios of EB:TBHP (1:1, 1:3 and 1:5) at 100 ◦ C after 24 h (Table 2). It was indicated that the different molar ratios of EB:TBHP had strong effects on catalytic activity and selectivity. In most of the experiments acetophenone was the major product, and the best catalytic active was obtained at the EB:TBHP molar ratio of 1:5. In all EB oxidation condition, as the reaction time increased, the conversion and selectivity to acetophenone increased as well (Table 2). Increasing the temperature for accelerating the reaction was apparently effective. Consequently, the temperature effects (50 ◦ C, 80 ◦ C and 100 ◦ C) were also monitored on the oxidation of EB at the EB:TBHP molar ratio of 1:1 after 24 h in the presence of NMC (Fig. 8). Generally, by increasing the reaction temperature and time, the conversions grew higher (67%) and also the reaction selectivity to acetophenone increased (96%) while the selectivity to benzaldehyde and benzoic acid decreased. Thus raising temperature was effective for the oxidation reaction since the energy was not sufficient for the activation of oxygen molecules or the catalytic circulation at low temperature. To estimate the reusability of catalyst, after the first run, the catalyst was filtrated and washed with ether and dried at 80 ◦ C under vacuum and then reused for the next run under the same conditions (Fig. 9). A few losses of activity and selectivity were revealed, indicating that the catalyst had high stability during the catalytic processes. Finally another experiment was carried out in order to prove the important role of the catalyst in oxidation of EB. After removal of the catalyst, the extra EB was added to the filtrate which no product was obtained. Then, to prove that whether metals were coordinated to the Schiff base ligand or with Si–OH and Al–OH groups on the

surface of SiO2 /Al2 O3 , some Mn acetate ions were added to the blank SiO2 /Al2 O3 and the reaction took place in the same condition, but the conversion of ethylbenzene was almost traced. Oxidation mechanism of ethylbenzene with TBHP in presence of the SiO2 /Al2 O3 -APTMS-BPK-Mn catalyst took place in several steps [15,47]. In first step, metal center of catalyst coordinated with oxygen present in TBHP, then oxidant became active as oxidation reaction took place, as it has been shown in Scheme 3. Ethyl benzene converted to (1-hydroproxyethyl) benzene by activated oxidant via the radical mechanism. (1-hydroproxyethyl)benzene could convert to acetophenone with dehydration or convert to benzaldehyde with loosing methanol. The rate of acetophenone production was greater than benzaldehyde production since dehydration is easier than the elimination of methanol. So the amount of benzoic acid production was trace (Scheme 3). In Table 3 our catalyst is being compared with the literate catalysts. The catalyst and applied method have advantages in terms of heterogeneous nature, simple preparation, reusability, high conversions and selectivity.

Fig. 7. The influence of solvent on the ethylbenzene oxidation by the SiO2 /Al2 O3 APTMS-BPK-Mn catalyst. Conditions: ethylbenzene, 2 mmol; Cat., 50 mg; TBHP, 2 mmol; 24 h; T, 100 ◦ C.

Fig. 8. The influence of reaction temperature on the ethylbenzene oxidation and selectivity to acetophenone by the SiO2 /Al2 O3 -APTMS-BPK-Mn catalyst. Conditions: ethylbenzene, 2 mL; Cat., 50 mg; TBHP, 10 mmol; solventless.

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Table 2 The influence of different oxidant ratio on the ethylbenzene oxidation by the SiO2 /Al2 O3 -APTMS-BPK-Mn catalyst. Entry

Condition

Time (h)

Ethylbenzene conversion (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1:1 (TBHP 80%)

2 6 12 18 24 2 6 12 18 24 2 6 12 18 24

15 22 42 46 52 24 32 49 56 67 44 51 60 73 87

1:3 (TBHP 80%)

1:5 (TBHP 80%)

Product selectivity (%) Acetophenone

Benzaldehyde

Benzoic acid

42 46 50 65 68 64 69 75 83 87 69 75 87 90 96

17 16 13 14 15 11 13 10 9 5 10 10 6 6 3

41 38 37 21 17 25 18 15 8 8 21 15 7 4 1

Conditions: ethylbenzene, 2 mmol; Cat., 50 mg; T, 100 ◦ C; solventless.

Scheme 3. Proposed mechanism for the oxidation of ethylbenzene catalyzed by SiO2 /Al2 O3 -APT-BPK-Mn in the presence of TBHP.

Scheme 4. Catalytic oxidation of cyclohexene with SiO2/Al2O3-APTMS-BPK-Mn.

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493

Table 3 Comparison of literature catalysts and our catalyst system for oxidation of ethylbenzene. Entry

1 2 3 4 5 6 7 8 9 10 a b

Catalytic system

Kumar et al. [38] Vetrivel and Pandurangan [48] Bhoware and Singh [49] Jothiramalingam et al. [50] Cavalerio and co-workers [39] Caveleiro and co-workers [39] George and Sugunan [51] Murugesan and co-workers [40] Parida and Dash [16] Present work

Reaction conditions

Ethyl benzene conversion (%)

0.2 g Mn-SBA-15, TBHP, solvent-free, 80 ◦ C, 8 h 0.3 g Mn-MCM-41, TBHP, solvent-free, 80 ◦ C, 24 h 0.05 g Co-MCM-41(100), TBHP, solvent-free, 80 ◦ C, 24 h 0.1 g Zr-k-OMS, TBHP, CH3 CN, 65 ◦ C, 10 h 3 ␮mol Mn (TDCPP) Cl, H2 O2 , r.t., 6.5 h 3 ␮mol Mn (␮- NO2 TDCPP) Cl, H2 O2 , r.t., 5.5 h 0.1 g CNCr-2, TBHP, CH3 CN, 70 ◦ C, 8 h 0.2 g Mn-SBA-15, TBHP, solvent-free, 80 ◦ C, 8 h 0.05 g 6/Mn-MCM-41, TBHP, CH3 CN, 80 ◦ C, 6 h 0.05 g SiO2 /Al2 O3 -APTMS-BPK-Mn, solvent-free, 100 ◦ C, 24 h

25 60 26 62 64 66 45 9.8 57.7 67

Selectivity APa

BZb

Other

37 39 85 98 75 66 69 53.4 82 84

10 9.1 21 20 13.17 18 8

62 50 5.9 4 10 30 33 6

AP: acetophenone. BZ: benaldehyde.

3.3. Oxidation of cyclohexene To examine the catalytic activity of the NMC, oxidation of cyclohexene was carried out under solventless conditions at 100 ◦ C by applying TBHP as oxidant (Scheme 4). Products of this reaction were2-cyclohexene-1-one (1), cyclohexene oxide,(2),and 1.2-cyclohexandiol(3) which were identified by GC-MS and quantified by GC analysis. Fig. 10 shows that at same temperature 100 ◦ C, but with a different molar ratios of CH:TBHP (1:1, 1:3 and 1:5) cyclohexene

Fig. 9. Reusability of the Mn-nano-catalyst on the oxidation of ethylbenzene and selectivity to acetophenone. Conditions: ethylbenzene, 2 mmol; Cat., 50 mg; TBHP, 10 mmol; T, 100 ◦ C; solventless.

conversion and the selectivity of the products have been changed. Changing the molar ratios of CH:TBHP from 1:1 to 1:5 within 24 h caused enhancement of the cyclohexene conversion from 40 to 84 and the selectivity to 2-cyclohexene-1-one from 65.6 to 95%. Thus,

Fig. 10. The influence of different oxidant ratio on the cyclohexene oxidation by the SiO2 /Al2 O3 -APTMS-BPK-Mn catalyst. Conditions: Cyclohexene, 2 mmol; Cat., 50 mg; T, 100 ◦ C; solventless.

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Scheme 5. Plausible reaction mechanism for the oxidation of cyclohexene catalyzed by the SiO2 /Al2 O3 -APT-BPK-Mn in the presence of TBHP.

it is obvious that under the mentioned reaction conditions, the C C double bond is less reactive than allylic hydrogen [46]. In order to examine the reusability of the immobilized manganese catalyst onto the SiO2 /Al2 O3 , several successive experiments were carried out by application of the used catalyst. According to Fig. 11, there is no significant loss of activity and selectivity even after fifth run. Therefore, the results clearly proved that the NMC catalyze conversion of cyclohexene (84%) with ca. 95.0% selectivity to 2-cyclohexene-1-one under solventless condition. After the coordination reaction between oxygen in TBHP with manganese, oxidant activated and conversion of cyclohexene to hydroperoxycyclohexene was carried out.

Hydroperoxycyclo- hexane, converted to 2-cyclohexene-1-one with dehydration (step 2). Also, hydroperoxy- cyclohexane could be converted as well to cyclohexene oxide in reaction with cyclohexene and finally to cyclohexanediol with subsequent reaction with water. Since the reaction will take place at 100 ◦ C, so elimination of water is easier than epoxidation of cyclohexene, so it is obvious that the rate of step 2 is greater than step 3 which states the reason for the great amount of 2-cyclohexene-1-one in comparison to cyclohexanediol (Scheme 5) [47,52]. In Table 4 our catalyst is being compared with the literate catalysts. 3.4. Oxidation of benzyl alcohol The liquid-phase oxidation of benzyl alcohol was performed by TBHP at 25, 40, 60, 80 and 100 ◦ C over immobilized NMC under different conditions without the use of any solvent (Scheme 6). Table 5 confirms the highness of catalytic activity and excellent selectivity to benzaldehyde (100%) in all experimental condition. GC analysis showed that benzaldehyde was the sole product (confirmed by performing three replicate experiments). By increasing the temperature from 25 to 100 ◦ C, the benzyl alcohol conversion increased. However, by reusing the catalyst, the percentage of

Fig. 11. Reusability of the Mn-nano-catalyst on the oxidation of cyclohexene and selectivity to 2-cyclohexene-1-one. Conditions: Cyclohexene, 2 mmol; Cat., 50 mg; TBHP, 10 mmol; 24 h; T, 100 ◦ C; solventless.

Scheme 6. Oxidation of benzyl alcohol catalyzed by the SiO2/Al2O3-APTMS-BPKMn.

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495

Table 4 Comparison of oxidation of cyclohexen with other studies in literature. Entry

1 2 3 4 5 6 7 8 9 10 11 a b

Catalytic system

Xia and co-workers [26] Salavati-Niasari [34] Clark and co-workers [53] Lei et al. [54] Farzaneh et al. [36] Ghiaci et al. [55] Selvam and co-workers [56] Chang and co-workers [57] Sehlotho and Nykong [58] Park and co-workers [59] Present work

Reaction conditions

Cyclohexen conversion (%)

2.4 mg salophen Mn (ш) complex, O2 , CH2 Cl2 , 70 ◦ C, 12 h 10 × 10−50 mol [Mn (H4 C6 S2 )]-NaY, TBHP, CH2 Cl2 , r.t., 8 h 0.2 Co-salen-SBA 15, H2 O2 , MW 300 W, 90 ◦ C, 0.33 h 8 mg PAMAMSA (3.0) Mn, O2 , Ethanol, 70 ◦ C, 6 h 0.5 g [Mn (sal-1,3–phen)]-NaY, TBHP, CH2 Cl2 , r.t., 8 h 5 mg (Ru/Co/Ce) (THNO), TBHP, CH2 Cl2 (7.6 wt.%) (Cr) MCM-41, TBHP, chloro benzene, 120 ◦ C, 12 h 0.35 g VSB-5, H2 O2 , CH3 CN, 60 ◦ C, 8 h (1.7 mg/ml) CoPc, TBHP, DMF/dichloromethane, r.t., 8 h 100 mg Co (III) SBA-15, H2 O2 , CH3 CN, 40 ◦ C, 12 h 0.05 g SiO2 /Al2 O3 -APTMS-BPK-Mn, solvent-free, 100 ◦ C, 24 h

82.3 90.3 >99 45 76.5 73.6 51.1 84.1 92 34.8 84

Selectivity Ketonea

Otherb

59.3 87.5 89 71 73.8 80.3 76.3 3.8 60.8 10.2 95.0

40 12 11 20 26 19.7 20 96 39.2 89.8 5.0

2-Cyclohexene 1-one. Cyclohexene-1-ole and cyclohexene epoxide.

Table 5 Oxidation of benzyl alcohol with heterogeneous Mn-nano-catalyst. Catalyst

T (◦ C)

Conversion (mol%)

Selectivity (mol%) (benzaldehyde)

Nano-catalyst Nano-catalyst Nano-catalyst Nano-catalyst Nano-catalyst Nano-catalysta Nano-catalystb Nano-catalystc Nano-catalystd

25 40 60 80 100 60 60 60 60

26.8 39.2 55.8 76.1 82.6 69.8 81.0 55.0 54.3

100 100 100 100 100 100 100 100 100

Reaction conditions: 1 mmol benzyl alcohol; 1 mmol TBHP; 0.03 g catalyst; 12 h; without solvent. a Benzyl alcohol: TBHP (1:2). b Benzyl alcohol: TBHP (1:3). c Second run. d Third run. Table 6 Comparison of oxidation of benzyl alcohol. Entry

1 2 3 4 5 6 a b

Catalytic system

Yang and co-workers [41] Song and co-workers [42] Yang and co-workers [43] Wang et al. [60] Huang and co-workers [61] Present work

Reaction condition

Conversion (%)

◦

0.2 g k (2) Mn(1)-C, O2 , Solvent-free, 100 C, 6 h 10 wt.% of Cu Mn, 10 mol% TEMPO, O2 , CH2 Cl2 , 120 ◦ C, 6 h 200 mg Mn2Ni8, O2 , toluene, 100 ◦ C, 1 h 0.1 g Si W11 Zn, H2 O2 , water, 90 ◦ C, 9 h (1 mmol) benzyl alcohol, 10 mol% Mn3 O4 , O2 , DMF, 80 ◦ C, 4 h 0.03 g SiO2 /Al2 O3 -APTMS-BPK-Mn, Solvent-free, 80 ◦ C, 12 h

>99 62.6 62 99 96 76.1

Selectivity BZa

Otherb

99 97.5 98 100 96 100

1 2.5 2 – – –

BZ: benzaldehyde. Other: benzoic acid.

benzyl alcohol conversion and benzaldehyde selectivity were constant after three times. The nature of the recovered catalyst after three times reusing had been followed by FTIR spectrum, and no significant change was observed. This indicated that the heterogeneous manganese catalyst with high catalytic performance and stability for the selective oxidation of benzyl alcohol was developed by covalently anchored on the SiO2 /Al2 O3 nanosized. In Table 6 our catalyst is being compared with the literature catalyst which shows the high conversion and excellent selectivity.

Thanks are due to the Iranian Nanotechnology Initiative and Chemistry Department of Bu-Ali Sina University for supporting of this work.

4. Conclusion

References

In this research, a simple and efficient catalysis system of SiO2 /Al2 O3 nanosized anchored manganese complexes with a number of bidentate ligands of N, N atoms in the oxidation of ethylbenzene, cyclohexene, and benzyl alcohol had been applied. As the result oxidation of alkyl aromatic, allylic site and double bond with the oxidant of TBHP without the use of any solvent were occurred. The high percentage yield and excellent selectivity of reactions

typically in the oxidation of benzyl alcohol were proved. Other extension of the method such as applying different catalysts with different oxidant is currently under investigation. Acknowledgment

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