nanoparticles: an efficient and reusable catalyst in

Full paper Received: 2 February 2016

Revised: 11 April 2016

Accepted: 14 April 2016

Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/aoc.3509

Copper(II) nanoparticles: an efficient and reusable catalyst in green oxidation of benzyl alcohols to benzaldehydes in water‡ Razieh Mirsafaeia, Majid M. Heravib*, Tayebeh Hosseinnejadb and Shervin Ahmadic Copper(II) nanoparticles immobilized onto 3-aminopropyltriethoxysilane (APTES) supported on mesoporous silica KIT-5 nanocomposite (AK) was prepared. The APTES group on KIT-5 was manipulated as a suitable coordinating agent for copper(II) and fully characterized using FT-IR, SEM, EDX, ICP, TGA, XRD and UV–visible methods. Moreover, a quantitative description for metal–ligand interactions in the APTES-KIT-5 mesoporous silica-supported copper(II) acetate complex was assessed via quantum chemistry computations. This novel transition metal supported heterogeneous surface was used as an effective catalyst in low loading for selective oxidation of differently substituted benzylic alcohols in water, using H2O2 as oxidant to give the corresponding carbonyl compounds in high to excellent yields. This heterogeneous nanocatalyst can be recovered and reused several times without any significant loss of activity. Copyright © 2016 John Wiley & Sons, Ltd. Keywords: copper(II) supported catalysts; oxidation in water; nanocatalyst; density functional theory; quantum theory of atoms in molecules

Introduction The most common technique applied for functionalization of microporous and mesoporous materials is the covalent binding of an appropriate metal complex to a solid support. This technique can be applied using two strategies: (1) grafting and (2) tethering, in which the spacer (‘tether’) is placed between the wall of the material and the metal complex.[1,2] Mesoporous organosilica and silica are frequently decorated with certain organic functional groups for the adsorption of metal ions such as Pb2+, Cu2+ and Hg2+ in solution. Furthermore, mesoporous carbon and polymer materials decorated with N-containing functional groups are often employed to adsorb CO2.[3] In comparison with calcium carbonates, zeolites, metal–organic frameworks and activated carbon, N-included mesoporous carbon shows a longer lifetime and high-temperature adsorption performance. Mesoporous materials are also found to be suitable for adsorption of molecules, chiefly due to their large pore size and high surface area.[4] They are also currently utilized as either heterogeneous catalysts or supports. In heterogeneous catalysis, the quality of the surface area of the catalyst is important since it determines the availability of catalytic sites for reactants.[5,6] Thus, mesoporous silica-supported catalysts are well known as being highly effective, due to their large area, and are more effective in organic heterogeneous catalysed reactions.[7,8] Among them, the nanocage silica KIT-5 with unique properties discovered by Ryoo and co-workers has been proven as a highly ordered cage-type mesoporous material with cubic Fm3m close-packed symmetry, high surface area, large pores and a high specific pore volume.[9] Recently, the development of efficient catalytic systems for oxidations in water or aqueous media has attracted much attention.[10–12] To achieve selective oxidations, various Appl. Organometal. Chem. (2016)

homogeneous and heterogeneous catalysts have been reported in the chemical literature,[13–15] the latter of particular interest due to the well-known and well-established inherent advantages of heterogeneous catalysis over homogeneous catalysis.[16,17] However, the selective oxidation of primary alcohols to the corresponding aldehydes remains very difficult, because the oxidation often proceeds further to afford the corresponding carboxylic acids.[18] In this respect, various catalysts and methods are known for selective oxidation of primary alcohols. Transition metals and their compounds and complexes have been known and established for their homogeneous and heterogeneous catalytic activity in oxidation. This activity is ascribed to their ability to adopt multiple oxidation states and to form complexes.[19–26] The utilization of transition metals such as V,[19] Cu,[20,21] Mo,[22,23] W,[23] Ru,[24] Pd[25] and Mn[26] along with an appropriate oxidizing agent has recently attracted much attention and is frequently employed as an alternative to conventional methodologies which require stoichiometric amounts of non-ecofriendly reagents and harsh reaction conditions. However, most of the protocols involving the utilization of transition metals suffer from requiring harsh reaction conditions such as hazardous or toxic solvents, high temperature and one or more equivalents of environmentally unfriendly oxidizing agents.[27,28] * Correspondence to: Majid M. Heravi, Department of Chemistry, Alzahra University, Vanak, Tehran, Iran. E-mail: [email protected] ‡ Dedicated to my son, OMID, on the occasion of his 35th birthday. a Department of Chemistry, Yazd Branch, Islamic Azad University, Yazd, Iran b Department of Chemistry, Alzahra University, Vanak, Tehran, Iran c Iran Polymer and Petrochemical Institute, Tehran, Iran

Copyright © 2016 John Wiley & Sons, Ltd.

Mirsafaei R., Heravi M. M., Hosseinnejad T. and Ahmadi S. Thus, an economic and environmental heterogeneous catalyst working under mild reaction conditions, in green solvents and using inexpensive and non-toxic oxidative reagents is still in much demand.[29] Undoubtedly oxidations in water are superior from known and established points of view.[10–12] Hydrogen peroxide is an attractive, atom-economic and environmentally friendly oxidant as it is inexpensive, non-toxic and commercially available and produces only water as a by-product.[30,31] We are interested in oxidation reactions[32–35] and immobilization of nanoparticles on suitable supports[36–40] for taking advantage of the superiority of heterogeneous catalysis.[41–45] Recently we have reported the preparation of Cu(I) nanoparticles on modified KIT-5 as an efficient recyclable catalyst and its applications in click reactions in water.[46] In continuation of our activities towards green chemistry using nanoparticles, in this work we report the preparation, computational studies[45,47,48] and application of a novel system of Cu(II) nanoparticles immobilized on nanocage-like mesoporous KIT-5 as a support and a 3-aminopropyltriethoxysilane (APTES) group as a coordinating agent for Cu(II). Through modification of the KIT-5 surface with moderate polarity (APTES), suitable for adsorption of Cu(OAc)2 and reduction, Cu(II) particles of a few nanometres in size were successfully confined inside the nanocages of modified KIT-5, and employed as an active catalyst in the successful selective oxidation of primary and secondary alcohols in water. Furthermore, we have performed computational assessments of the chelating behaviour in APTES-KIT-5 mesoporous silica-supported copper(II) acetate complex to obtain a quantitative description for metal–ligand interactions which also confirmed the experimental characterization of the copper(II) nanocatalyst. It should be stated that a computational investigation of the catalytic role of APTES-KIT-5 silica-supported copper(II) nanocatalyst in selective oxidation of substituted benzylic alcohols is underway and another comprehensive computational report is planned.

Experimental Materials and solvents All starting chemicals and reagents were commercially available and were used without further purification. The reagents and solvents were purchased from Merck, Fluka or Aldrich and were used as received. APTES and copper acetate were purchased from Merck and used without further purification. For drying tetrahydrofuran, it was distilled off over sodium in the presence of benzophenone until its colour changed to a deep blue. Silica gel KIT-5 (particle size, 10.38 nm; surface area, 1090 m2 g 1; pore diameter, 2.62 nm) and APTES/KIT-5 (AK) were prepared in accordance with our previously published procedure.[43]

Alpha spectrometer. Scanning electron microscopy (SEM; Philips, XL30, SE detector) was used to observe the morphology of the modified and unmodified mesoporous silicas. Energy-dispersive X-ray spectroscopy (EDX; Genesis, with an SUTW detector equipped with SEM equipment) was used in order to confirm the presence of silica. Copper content was measured using inductively coupled plasma (ICP) analysis with a Varian Vistapro analyser.

In Situ nanocopper(ii) preparation Preparation of cubic mesoporous KIT-5

Three-dimensional large cage-type face-centred cubic Fm3m mesoporous KIT-5 nanocages were synthesized successfully. The large pore cage-type mesoporous silica, denoted KIT-5, was prepared using Pluronic F127 (EO106PO70EO106) template as a structure-directing agent and tetraethylorthosilicate (TEOS) as the silica precursor. In a typical synthesis, F127 (2.5 g, 0.198 mmol) was dissolved in 120 g of distilled water and concentrated hydrochloric acid (5.25 g, 0.05 mol, 35 wt% HCl). To this mixture, TEOS (12 g, 0.057 mol) was added. The mixture was stirred for 24 h at 45°C to produce the mesostructured product. Then, the reaction mixture was heated at 95°C for 24 h under static conditions for hydrothermal treatment. The solid product was then filtered, washed with deionized water and dried at 100°C. Finally, the samples were calcined at 550°C for 6 h to remove the template. Preparation of amine-functionalized mesoporous KIT-5

The AK nanocomposites were synthesized following the procedure given in our previous work.[43] Briefly, a suspension of APTES (1.00 g) and calcined KIT-5 (1.00 g) in toluene (20 ml) was refluxed with continuous stirring under inert atmosphere for 24 h. The resultant mixture was filtered and washed with a 1:1 mixture of acetone and chloroform to remove the un-reacted APTES, and then dried under vacuum at 80°C. The un-reacted APTES was removed by Soxhlet extraction using dry dichloromethane for 20 h and dried at 60°C. The sample was denoted as AK ligand (shown in Scheme 1). Coordination of Cu(II) to AK ligand

In a round-bottom flask equipped with a condenser, Cu(OAc)2 (1.00 g) was added to the modified support AK ligand (1.00 g) in methanol (3 ml). The reaction mixture was refluxed for 6 h. The solid was filtered and extracted in a Soxhlet extractor with methanol for 10 h, then it was dried at 60°C for 1 h to give Cu(II) supported on AK (Cu(II)/AK composite) as shown in Scheme 2. ICP analysis demonstrated that the copper content in the heterogeneous catalyst is 15.60% (w/w). Therefore, each gram of heterogeneous catalyst contains 1.717 mmol of copper.

Techniques Melting points were determined with an X6 digital microscope melting point apparatus. X-ray diffraction (XRD) patterns of the materials were recorded with a Bruker D8 advanced powder X-ray diffractometer using Cu Kα (λ = 1.54 Å) as the radiation source in the range 2θ = 2–90°. Fourier transform infrared (FT-IR) spectra were recorded in KBr discs with a Bruker FT-IR spectrophotometer. Thermal gravimetric analysis (TGA) data were obtained with a SetaramLabsys TG (STA) in the temperature range 10–600°C and heating rate of 5°C min 1 under nitrogen atmosphere. UV–visible solution spectra were recorded using a thermo-spectronic Helios

wileyonlinelibrary.com/journal/aoc

Scheme 1. Preparation of AK ligand.


Appl. Organometal. Chem. (2016)

Green catalytic oxidation of benzyl alcohols to benzaldehydes In the case of copper(II), LANL2DZ effective core potentials were used together with the accompanying basis sets to describe the valence electron density.[52] The stationary points were determined as minima through verifying the presence of all real frequencies. All DFT computations were performed using the GAMESS suite of programs.[53]

Results and discussion Scheme 2. Coordination of Cu(II) to AK ligand.

Catalytic benzyl alcohol oxidation in water Benzyl alcohol was selected as a model compound for the optimization of the oxidation process. A typical experimental procedure was as follows. In a round-bottom flask, to a mixture of catalyst (0.05 g) and benzyl alcohol (1 mmol), 2 ml of water plus hydrogen peroxide (30%) aqueous solution (containing 3.0 mmol H2O2) was added dropwise. This mixture was stirred at reflux condition for a period of time. The progress of the reaction was monitored by TLC (silica gel 60 F254) and/or GC (CP WAX 52, Varian CP 3800). Evaporation of solvent gave the corresponding aldehyde. Upon the completion of the reaction, the solid catalyst was recovered by centrifugation, and washed repeatedly with ether and ethanol. The recovered catalyst was dried under vacuum and was directly used for the next reaction.

Computational details Recently, we have assessed computationally metal–ligand interactions in modified poly(styrene-co-maleic anhydride) palladium nanocatalyst,[45] copper (I)-aminated KIT-5[46] and Nsulfamic-aminated KIT-5 nanocatalysts[47] via quantum chemistry approaches. Armed with these experiences, we have concentrated on presenting a quantitative description for metal–ligand interactions in the APTES-KIT-5 mesoporous silica-supported copper(II) acetate complex by performing density functional theory (DFT)[48] and quantum theory of atoms in molecules (QTAIM) computations.[49,50] In this respect, we have designed an effective model for AK ligand 1 and Cu(II)/AK complex 2 (as illustrated in Scheme 3) considering that this model is a credible comprise between accuracy and time-saving efficiency of computational procedure. We first determined the optimized structure of compounds 1 and 2 at M06/6-31G* level of theory.[51] It should be mentioned that M06 functional has been introduced recently as a hybrid meta-GGA (generalized gradient approximation) exchange-correlation functional that was parametrized including both transition metals and non-metals and was recommended for application in organometallic and inorganometallic thermochemistry, kinetic studies and noncovalent interactions.[51]

Scheme 3. Simple model for APTES-KIT-5 mesoporous silica-supported copper(II) acetate complex.


KIT-5 mesoporous silica surfaces were modified with APTES as a linking agent via a one-step chemical reaction. Ethoxy group in the APTES structure was chemically bound with surface hydroxyl groups of KIT-5 mesoporous silica (Scheme 1). APTES contains one free amine moiety and was applied as a coordinating agent for copper(II) in methanol. In order to introduce copper(II) into the nanocages of KIT-5, APTES was employed to modify the surface of KIT-5. Owing to the strong coordination capacity of amino groups, Cu(OAc)2, even with a high loading, can be easily adsorbed by KIT-5 modified with amino groups.

FT-IR Spectroscopic analysis Figure 1 shows the FT-IR spectra before and after the reaction of AK ligand with Cu(OAc)2. The major peaks for silica including the broad antisymmetric Si–O–Si stretching (1300 to 1000 cm 1) and the symmetric stretching (820–740 cm 1) are observed. In AK ligand spectrum, the 2935 cm 1 peak can be assigned to C–H bond stretching, while NH2 group shows two bands for primary amines: N–H stretching (3390 cm 1) and N–H bending (1650 cm 1). After the reaction of AK ligand with Cu(II), the C–N stretching band shifts to lower frequency (1444 cm 1), while N–H absorption shifts to higher frequency (1604 cm 1), which confirm the chemical interactions of nitrogen lone pair electrons with vacant copper(II) orbitals (Fig. 1).

UV–visible spectroscopic analysis Figure 2 presents the solid-state UV–visible spectra of AK ligand and copper(II) nanocatalyst. The absorptions around 223, 255 and 323 nm are attributed to n–π* and π–π* electron transfers for the silica and amine groups. In the UV–visible spectrum of Cu(II)/AK complex composite, a new absorption around 706 nm appears which can be assigned to d–d transitions after loading of copper(II).

Figure 1. FT-IR spectra of AK ligand and copper(II) nanocatalyst.



Mirsafaei R., Heravi M. M., Hosseinnejad T. and Ahmadi S.

Figure 2. Solid-state UV–visible spectra of (a) AK ligand and (b) copper(II) nanocatalyst.

SEM Characterization The morphology and the location of metallic species on the surface of the catalyst were analysed using an SEM instrument equipped with EDX capability. Figure 3 shows SEM images of AK support before and after immobilization of copper(II). The porosity of AK, shown in Fig. 3(b), has an average size of 21–49 nm. After introduction of copper, clear changes in the morphologies of the catalyst are observed. The SEM images of the Cu(II)/AK catalyst (Figs 3(c) and (d)) clearly show the Cu(II) nanoparticles are homogeneously immobilized on the modified surface. As shown in Fig. 3(d), the copper(II) catalyst is nanometric, being 32–40 nm in size. Comparisons of EDX analysis of pure KIT-5 with that of Cu(II)/AK complex (after extracting with methanol in a Soxhlet extractor) confirm the addition of copper(II) to the catalyst, which suggests the formation of metal complexes with the anchored ligand (Fig. 4).

Figure 3. SEM images of (a) pure KIT-5 and (b) AK and (c, d) copper(II) immobilized on AK.


Figure 4. EDX analysis of pure KIT-5 (top) and Cu(II)/AK complex (bottom).

XRD Results Figure 5 displays the wide-angle XRD pattern of Cu(II)/AK nanocatalyst (2θ = 2–90°). The crystallite size of particles was determined using Bragg’s equation (nλ = 2dsin θ), found to be approx. 45 nm for Cu(II)/AK catalyst (2θ ≈ 11°). Figure 5 shows that all reflection peaks could be indexed to the pure cubic crystal phase of nanocrystalline Cu(II). Sharp peaks of copper (2θ = 11.0°, 12.0°, 14.8° and 23.7°) indicate the crystalline character of the product. No specific peaks are observed for impurities. This pattern is in reliable agreement with the reported pattern for copper acetate (Ref. 00-027-1126) and also confirms the formation of Cu(II) nanoparticles on APTES/KIT-5. This behaviour can be mainly attributed to the fact that symmetry has been destroyed by the hybridization in the ordered mesoporous silica loaded with guest matter. To determine the copper content, the supported copper was treated with concentrated HCl and HNO3 (1:1) followed by ICP-OES analysis. The copper content was evaluated to be 15.60% (w/w).

Figure 5. XRD pattern of copper(II) nanocatalyst.



Green catalytic oxidation of benzyl alcohols to benzaldehydes Thermal properties The thermal stability of pure AK and Cu(II)/AK catalyst was initially investigated using TGA (under nitrogen flow), confirming the incorporation of copper acetate groups in AK. As illustrated in Fig. 6, the catalyst shows a slight loss of mass (about 5%) below 110°C, probably due to the loss of a small quantity of adsorbed water. If the decomposition is performed at 127°C, the yield of decomposition is only 4%. At higher temperatures, the decomposition yield increases continuously. Decomposition of copper acetate on AK support starts with a yield of 6.58% at 200°C and 30% at 280°C. With increasing temperature, the decomposition of the aminopropyl group in AK occurs at about 350°C.[47] The TGA curve of the copper(II) catalyst at higher temperatures demonstrates a more marked mass loss in comparison to that observed for AK. This can be chiefly due to the gradual desorption and thermal decomposition of the copper groups. It should be stated that the copper groups decompose at lower temperatures in comparison with AK. The total percentage of the mass loss of the catalyst is determined to be around 38%, and 7% for AK. The mass loss for the copper(II) catalyst also attests to the presence of copper groups inside the pores. This catalyst is stable up to 200°C and can be remarked as a catalyst of choice in various organic reactions in the temperature range 80–140°C. However, AK is thermally stable even up to 400°C, and therefore it is frequently applied as a thermally stable catalyst support in several organic transformations.

Computational assessment On the basis of our obtained DFT and QTAIM computational results, we can present a quantitative description for metal–ligand interactions and also interpret some experimental elucidations in the characterization of the APTES-KIT-5 mesoporous silicasupported copper(II) acetate complex from structural and electronic viewpoints. In Fig. 7, we presented the optimized geometries of AK ligand 1 and Cu(II)/AK complex 2 calculated at M06/6-31G* level of theory with the atomic numbering. In Fig. 7, we also present M06/6311G** calculated C–N bond length (and also bond order in parentheses) in each of AK ligand 1 and Cu(II)/AK complex 2. Comparative survey of the optimized structures of AK ligand 1 and Cu(II)/AK complex 2 demonstrates a geometrical deformation

Figure 7. Atomic numbering and optimized structures obtained at M06/631G* level of theory for AK ligand 1 and Cu(II)/AK complex 2. C–N bond length (and bond order) calculated at 6-311G** level of theory.

in AK ligand through complexation. In the following, the structural stability of Cu(II)/AK complex 2 is analysed based on the various electronic indicators. The bond orders of some key bonds in AK ligand 1 with their corresponding bonds in Cu(II)/AK complex 2 are listed in Table 1 to characterize the variation of bond orders via complexation. The data in Table 1 demonstrate that the bond order of C–N bond decreases from 1.02 in AK ligand to 0.98 in Cu(II)/AK complex (while there occurs an increase in bond order of N–H bonds) due to the donation of shared electrons from this chemical bond to copper(II) cation through complexation, which is in agreement with our FT-IR spectroscopic observations. In the next step, we focused on topological analysis of electron density via the QTAIM method[49,50] to interpret the nature of

Table 1. Calculated values of some selected bond orders in AK ligand and Cu(II)/AK complex obtained at M06/6-311G** level of theory (note that numbering of atoms is in accordance with Fig. 7) AK ligand N1–C1 N1–H1 N1–H2 Cu–N1

1.023 0.946 0.943

Cu(II)/AK complex 0.897 0.984 0.991 0.419

Figure 6. TGA curves of AK and Cu(II) catalyst.




Mirsafaei R., Heravi M. M., Hosseinnejad T. and Ahmadi S. metal–ligand interactions in Cu(II)/AK complex 2. In this respect, we analysed M06/6-311G** calculated wave function of electron density using the AIM2000 program package.[54] QTAIM molecular graphs of AK ligand 1 and Cu(II)/AK complex 2 including all bond and ring critical points and their associated bond paths are displayed in Fig. 8. Table 2 gives QTAIM calculated values of electron density, ρb, its Laplacian, ∇2ρb, electronic kinetic energy density, Gb, electronic potential energy density, Vb, total electronic energy density, Hb, and |Vb|/Gb ratio at some selected bond critical points (BCPs) for AK ligand 1 and Cu(II)/AK complex 2. It is important to mention that

the electron density at BCPs usually corresponds to the strength of the bond between two atoms. Values of ρb < 0.1 au are indicative of an electrostatic interaction; it is usually correlated with a relatively small and positive value of ∇2ρb.[50,55] By contrast, for a covalent interaction, usually ρb > 0.1 au and ∇2ρb is usually negative with the same order as ρb.[50,55] Moreover, a good authentic indicator for classifying interatomic interactions is the total electronic energy density that is defined as Hb = Gb + Vb at BCPs. For electrostatic interactions, Hb has a positive value and for covalent interactions it is negative. On the basis of the results reported in Table 2, the following can be stated: (i) through complexation, calculated values of ρb decrease at C–N BCP and increase at N–H BCP, which confirms the donation of shared electrons to the metal centre and is in agreement with variations in the stretching frequency of the FT-IR spectrum; (ii) the large values of electron density together with the negative values of ∇2ρb and Hb at C–N and N–H BCPs indicate the covalent character of C–N and N–H chemical bonds in AK ligand 1 and Cu(II)/AK complex 2; and (iii) the small values of electron density with the positive values of ∇2ρb and Hb at Cu–N BCP demonstrate that the metal–ligand interactions do not have covalent character. For Cu–N BCP, calculated value of 1 < |Vb|/Gb2 confirms the presence of partially covalent–electrostatic interaction. Furthermore, a more stringent analysis of QTAIM molecular graphs demonstrates some new ring critical points which can be attributed to the interaction of copper(II) and acetate groups with oxygen and hydrogen atoms of AK ligand and leads to an electronic stabilization effect in the complexation procedure. Investigation of catalytic activity for oxidation of alcohols

Figure 8. Complete molecular graphs (MGs) of AK ligand 1 and Cu(II)/AK complex 2 obtained using QTAIM analysis of M06/6-311G** electron density functions. Bond critical points: red circles; ring critical points: yellow circles; bond paths: pink lines.

Table 2. Mathematical properties of some selected BCPs in AK ligand 1 and Cu(II)/AK complex 2. The properties have been obtained via QTAIM analysis of the M06/6-311G** calculated wave function of electron density (note that numbering of atoms is in accordance with Fig. 8) ρb AK ligand BCP(N1–C1) 0.2664 BCP(N1–H1) 0.3357 BCP(N1–H2) 0.3343 Cu(II)/AK complex BCP(N1–C1) 0.2507 BCP(N1–H1) 0.3433 BCP(N1–H2) 0.3361 BCP(Cu–N1) 0.0784

2

∇ ρb

Gb

Vb

Hb

|Vb|/Gb

0.6944 0.1193 1.4768 0.0632 1.4592 0.0630

0.4123 0.4957 0.4910

0.2930 3.4559 0.4325 7.8435 0.4280 7.7936

0.5928 1.5612 1.6016 0.2732

0.3457 0.5175 0.5163 0.1377

0.2470 0.4539 0.4584 0.0347

0.0987 0.0636 0.0579 0.1030


3.5025 8.1367 8.9170 1.3368

In order to investigate the effect of solvent on the oxidation reaction, various solvents such as acetonitrile, toluene, acetone, n-hexane and water and solvent-free condition were chosen for oxidation of benzyl alcohol (Table 3). The results show that the best solvent for this reaction is water. For solvent-free condition, the yield of benzaldehyde is low. The effect of catalyst amount in the oxidation of benzyl alcohol was investigated (Table 4). The results show that with increasing amount of catalyst to 0.05 g, in fixed time, the yield of reaction increases. Increasing in the amount of catalyst to 0.20 g does not have any considerable effect on the reactivity. After the investigation of the effects of different parameters, the best conditions were chosen and various alcohols were oxidized in the presence of a catalytic amount of Cu(II)/AK complex and H2O2 as an oxidant in refluxing water. However, this reaction was

Table 3. Effect of solvent in oxidation of benzyl alcohola Solvent Acetonitrile Solvent free Toluene Acetone n-Hexane Water

B.p. (°C lit.)

Time (min)b

Yield (%)

81–82 — 110–111 56 69 100

65 40 85 240 35 20

55 42 70 73 80 90

a

All reactions carried out in reflux condition in the presence of benzyl alcohol (1 mmol), 0.05 g of copper nanocatalyst and 3 mmol of H2O2 and 5 ml of solvent. b Time of maximum conversion.



Green catalytic oxidation of benzyl alcohols to benzaldehydes Table 4. Effect of catalyst amount in oxidation of benzyl alcohola

Table 6. Catalyst reusability of Cu(II)/AK complexa

Entry 1 2 3 4 5

Catalyst amount (g)

Yield (%)

Selectivity (%)

Entry

Time (min)

Yield (%)

Selectivity (%)

— 0.03 0.05 0.10 0.20

5 70 90 91 90

100 100 100 100 100

1 2 3 4 5 6

21 21 21 21 21 21

90 90 85 77 75 70

100 100 100 100 100 100

a

Reaction conditions: benzyl alcohol =1 mmol, solvent = water, amount of H2O2 = 3 mmol, time = 21 min at reflux condition.

performed with H2O2 without catalyst in water and during 24 h produces a trace of aldehydes and so does not work.[56] Table 5 summarizes the results of the oxidation of various benzylic alcohols using Cu(II)/AK complex as catalyst. The reaction yields are considerably dependent on substituents (–Cl, –OCH3, –OH, –NO2 and –NH2) and on their positions. o-Hydroxy and p-hydroxy and o-amino electron-donating substituents are converted to the corresponding aldehydes in excellent yields (entries 3–6). On the other hand, the greatest slowing effect is detected with strong electron-withdrawing nitro substituents (entries 8–10), whereas, halogen substituents cause modest yields (entry 2). Similarly, the oxidation of secondary benzylic alcohol also leads to the corresponding ketone in satisfactory yield (entry 12). All of the corresponding aldehydes are obtained with excellent selectivity, and over-oxidation into carboxylic acids is not observed.

a

Reaction conditions: alcohols =1 mmol, amount of catalyst =0.05 g, H2O2 = 3 mmol, solvent = water. All reactions were carried out in reflux condition.

Table 7. Comparison of efficiency of various catalysts with that of Cu (II)/AK complex in the oxidation of benzyl alcohol to benzaldehyde No.

Catalyst

1 5 mol% TEMPO 2 0.25 mol% Cu(OAc)2 3 1 mol% Pd on SBA-15 4 H14[NaP5W30 O110]/SiO2 5 Caffeinilium chlorochromate 6 Cu(II)/AK complex (0.05 g)

Conditions Time (min) Yield (%)a

Ref.

H2O/60°C H2O/60°C

720 720

98 55

56 56

H2O/reflux

480

>99

57

Acetic acid/reflux CH2Cl2/reflux

13

99

34

45

95

58

H2O/reflux

21

90

This work

a

Yield refers to isolated and pure product.

Table 5. Cu(OAc)2 supported on AK as catalyst in oxidation of alcohols to corresponding aldehydesa

Entry 1 2 3b 4 5 6 7 8 9 10 11 12

,

R

Time(min)c

Yield (%)d e

H o-Cl m-OH o-OH p-OH o-NH2 m-OMe o-NO2 p-NO2 m-NO2 Hydroquinone Diphenylmethanol

21 23 50 5 5 7 22 23 35 22 5 60

90 75 85 97 98 97 83 48 61 56 96 70

a

Reaction conditions: alcohols =1 mmol, amount of catalyst =0.05 g, H2O2 = 3 mmol, solvent = water. All reactions carried out in reflux condition, except entries 5, 9, 11, which were performed at room temperature. b Acetone was used as a solvent. c Time of maximum conversion (determined by TLC and/or GC). d Isolated yield. e All reaction give corresponding aldehyde in 100% selectivity.


Catalyst recycling One of the major reasons for supporting a homogeneous metal complex on silica is to improve the activity and reusability of the catalyst. To examine the reusability of the silica-gel-based complex catalyst, the catalyst was separated by filtration after the reaction, washed, dried under vacuum and then reused in a second run. The yield and structure of the final products were determined and compared with that of being used in the original run. As evident from Table 6, this catalyst could be used for the oxidation of alcohols to aldehyde up to six times without loss of selectivity. To show the advantages of the Cu(II)/AK complex as a heterogeneous nanocatalyst in this reaction, our results and reaction conditions for the oxidation of benzyl alcohol to benzaldehyde (Table 5, entry 1) were compared with previously reported homogeneous and heterogeneous catalysis of this reaction (Table 7)[34,56–58]. The results show that our catalyst is quite comparable with other catalysts regarding the yields and reaction times.

Conclusions Via modification of the mesoporous cage-like material KIT-5 followed by adsorption of Cu(OAc)2 and reduction with methanol, copper(II) nanoparticles with a uniform size distribution were successfully confined in the nanocages of KIT-5, leading to a new solid catalyst for the selective oxidation of alcohols. The solid catalyst was characterized using FT-IR spectra, SEM–EDX and solid-state UV–visible spectra. FT-IR and UV–visible specrta show



Mirsafaei R., Heravi M. M., Hosseinnejad T. and Ahmadi S. the structure of Cu(II)/AK complex, confirming that the ligand is attached to the silica surface. UV–visible and EDX analysis also demonstrate the existence of copper(II) on the resulting catalyst. Moreover, we have employed quantum chemistry computational methods to interpret and confirm the experimental characterization of the nanocatalyst from computational viewpoints. This new nanocatalyst was applied as a highly efficient catalyst for the selective and environmentally friendly oxidation of benzylic alcohols to corresponding aldehydes with hydrogen peroxide in green media. The main advantages of this catalytic system are ease of preparation, needing mild reaction conditions, easy work-up procedure, low cost of the catalyst and finally it can be recycled and reused several times without loss of activity. This catalyst showed a high activity for the selective oxidation alcohols to the corresponding aldehydes and ketones. Acknowledgements The MMH is grateful for partial financial support from the Iran National Science Foundation (INSF). The authors are thankful to the Alzahra University, Iran Polymer and Petrochemical Institute for providing the SEM-EDX facility and Islamic Azad Yazd University for providing FT-IR analyses.

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