Iron-Oxide-Supported Ultrasmall ZnO Nanoparticles: Applications for

Research Article pubs.acs.org/journal/ascecg

Iron-Oxide-Supported Ultrasmall ZnO Nanoparticles: Applications for Transesterification, Amidation, and O‑Acylation Reactions Vilas B. Gade,†,§ Anuj K. Rathi,‡,§ Sujit B. Bhalekar,† Jiri Tucek,‡ Ondrej Tomanec,‡ Rajender S. Varma,‡ Radek Zboril,*,‡ Sharad N. Shelke,*,† and Manoj B. Gawande*,‡ †

P. G. & Research Center, Department of Chemistry, S.S.G.M. College, Kopargaon, Dist.Ahmednagar (MS) 423601, India Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University, Šlechtitelů 27, 783 71 Olomouc, Czech Republic

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‡

S Supporting Information *

ABSTRACT: An efficient maghemite−ZnO nanocatalyst has been synthesized via a simple coprecipitation method, where ZnO nanoparticles are uniformly decorated on the maghemite core and characterized by XRD, SEMEDS, ICP-AES, XPS, TEM, HRTEM, and Mö ssbauer spectroscopy; maghemite nanoparticles are in the typical size range 10−30 nm with ultrasmall (3−5 nm) ZnO nanoparticles. A competent and benign protocol is reported for various organic transformations, namely, transesterification, amidation, and O-acylation reaction in good to excellent yields (75−97%) using magnetically separable and reusable maghemite−ZnO nanocatalyst.

KEYWORDS: Maghemite−ZnO nanoparticles, Magnetic cleansing, Transesterification, Acylation, Sustainable protocol

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without any need for catalyst filtration and centrifugation. Magnetic nanoparticles (magnetite and maghemite) have emerged as flexible and unique supports for immobilization as they are obtainable from earth-abundant inexpensive precursors, and a variety of metals, organoligands, and metal NHC can be immobilized and find applications in numerous sustainable organic processes.21−23 Transesterification is a vital reaction which has broad applications,24,25 and this transformation is often catalyzed by a selection of protic and Lewis acids, and an assortment of bases, enzymes, and antibodies.26,27 However, a more proficient and commonly applicable transesterification procedure involving nonhazardous catalyst is still needed. Amides are significant functional groups in natural and synthetic molecules, and not surprisingly, amide bond formation has been comprehensively explored in organic transformations,28,29 often necessitating a sequence of hydrolysis (for esters), activation, and final reaction with amines. Thus, a search for an atom economical catalytic amide bond formation is highly desirable pursuit. The acylation of hydroxyl groups is one of the most important reactions in organic synthesis, and it can be accomplished with acylating agents, namely, acid halides and acid anhydrides in the presence of protic or Lewis acid

INTRODUCTION Nanomaterials in the form of solid support have been widely deployed as sustainable nanocatalysts to confront various sustainable issues in catalysis.1−3 Such nanocatalytic systems possess unique and precisely controlled active sites like homogeneous catalysts and therefore combine the ideal characteristics of both homogeneous and heterogeneous catalysts.4−8 In view of the dissimilar properties of materials on the nanoscale compared to their bulk counterparts, research on nanoparticles has seen unprecedented growth, and numerous applications are projected. To impart certain desirable traits, it may be a requisite step to modify the surface of such materials.9 In this context, magnetic nanoparticlesupported catalytic systems have acquired tremendous attention owing to their extraordinary properties including ease of availability, chemical and thermal stability inertness, and great surface area to volume ratio.10−14 Magnetic nanocomposites are of generous interest to researchers in various areas including magnetic fluids,15 catalysis,16 biotechnology,17 and environmental remediation.18 An array of suitable protocols have been established for the preparation of magnetic nanoparticles of various compositions, with the ideal size range of 10−20 nm. Lately, the applications of magnetic nanoparticle-supported catalysts have garnered attention due to their exceptional properties such as high surface area, and superparamagnetic behavior.19,20 The apparent superiority of magnetic-supported catalysts is in that they can be reused and recycled by a magnet © 2017 American Chemical Society

Received: December 26, 2016 Revised: February 17, 2017 Published: February 23, 2017 3314

DOI: 10.1021/acssuschemeng.6b03167 ACS Sustainable Chem. Eng. 2017, 5, 3314−3320

Research Article

ACS Sustainable Chemistry & Engineering catalyst30 or pyridine derivatives;31 many procedures require a longer reaction time, harsher reaction conditions, or tedious workup, or entail the use of expensive catalysts. The development of simple, cost-effective, versatile, and eco-friendly procedure is nevertheless actively sought. Zinc oxide (ZnO) nanoparticles are a relatively economical and nontoxic entity with some environmental benefits32 as they are utilized for wastewater treatment to remove chemical and biological pollutants.33 In recent years, the potential utility of ZnO-NPs has been explored as anticancer34 and antibacterial agents. Also, ZnO nanoparticles/catalysts were successfully employed in various catalytic processes including Ugi-reaction,35 synthesis of 3-substituted indoles in water,36 domino Knoevenagel-heteroDiels−Alder reaction,37 and one pot multicomponent reactions,38 among others.39 Due to its amphoteric nature, and being a relatively eco-friendly catalyst, there is demand for ZnO catalyzed organic reactions. In perpetuation of our research activities on applications of advanced nanomaterials and sustainable protocols,40−46 herein we report maghemite-supported-ZnO nanocatalyst for various aforementioned organic transformations such as transesterification, amide synthesis, and acylation of hydroxyl groups (Scheme 1).

General Procedure for the Preparation of Maghemite-ZnO NPs. Maghemite (3 g) was added to a solution of zinc chloride (210 mg) in deionized water (100 mL). The ensuing mixture was stirred for 1 h, and the suspension was adjusted to pH 12−13 by addition of sodium hydroxide (1.0 M) and further stirred for 20 h. The aqueous layer was removed with the help of an external magnet, and the obtained material was washed with deionized water (5 × 50 mL) under sonication, further washed by ethanol, and dried under vacuum at 60 °C for 12 h to afford maghemite-ZnO. General Procedure for the Transesterification Reaction. A mixture of methyl benzoate (7.3 mmol), alcohol (14.6 mmol), and maghemite-ZnO (100 mg) was stirred in a sealed tube at 150 °C for the appropriate time (Tables 1 and 2). Upon completion of the

Table 1. Optimization of the Transesterification Reaction with Methyl Benzoate and 1-Propanol Catalyzed by Maghemite-ZnOa

no. 1 2 3 4 5 6 7 8 9 10 11

Scheme 1. Maghemite-ZnO Catalyzed Organic Transformations

catalyst

maghemite-ZnO maghemite-ZnO maghemite-ZnO maghemite-ZnO maghemite-ZnO maghemite maghemite-ZnO maghemite-ZnO

solvent

time (h)

THF THF THF THF toluene toluene

16 16 16 16 16 14 12 12 12 12 12

temp RT 100 RT 100 100 150 150 150 150 150 150

°C °C °C °C °C °C °C °C °C

yieldb (3a) NR NR trace 20 35 52 83c 30c 42c 44d 62e

a

Reaction conditions: methyl benzoate (7.3 mmol), solvent (10 mL), 1-propanol (14.6 mmol), maghemite-ZnO (100 mg), sealed tube. NR: no reaction. RT: room temperature. bIsolated yield. cReaction carried out without solvent. d25 mg catalyst used. e50 mg catalyst used.

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reaction (monitored by TLC), the catalyst was seized with the help of an external magnet, and the obtained crude material, after concentration under reduced pressure, was purified via column chromatography (silica 100−200; n-hexane/ethyl acetate mixture) to afford the desired product. General Procedure for the Amidation Reaction. A mixture of methyl benzoate (7.3 mmol), amine (22 mmol), toluene (5 mL, 3:1, v/v), and maghemite-ZnO (100 mg) was stirred in a sealed tube at 150 °C for the appropriate time (Table 3). Once the reaction was deemed completed (monitored by TLC), the toluene was removed under reduced pressure, and the obtained material was diluted with water and extracted with ethyl acetate (3 × 50 mL). The concentrated crude product was purified by column chromatography (silica 230− 400; n-hexane/ethyl acetate mixture) to afford the desired product. General Procedure for the Acetylation of Alcohol. To a stirred mixture of phenol (6.9 mmol) and acetic anhydride (7.6 mmol) was added maghemite-ZnO (100 mg), and the ensuing mixture was stirred at room temperature for 3 h (Table 4). After completion of the reaction, the catalyst was removed by external magnet, and the reaction mixture was poured in crushed ice; crude product was collected by filtration which was subjected to column chromatography (silica 100−200; n-hexane/ethyl acetate mixture) to afford the pure desired product.

EXPERIMENTAL SECTION

Materials and Methods. Zinc chloride (99.9%), ammonium hydroxide, sodium hydroxide, sodium sulfate (99%), and other solvents were purchased from Sigma-Aldrich and were used as such. For TLC, Merck Kieselgel 60 F254 precoated aluminum sheets were used. TLC spots were visualized using UV light and iodine. The IR spectra were scanned on PerkinElmer spectrum version 10.4.2. The NMR spectra were recorded on Bruker Avance II 400 and 200 MHz (PMR) and 100 MHz (CMR) instrument using CDCl3 as a solvent and TMS as an internal standard. General Procedure for the Synthesis of Maghemite (γFe2O3) NPs. In a typical coprecipitation synthesis of maghemite, FeSO4·7H2O (6.06 g, 21.79 mmol) and FeCl3·6H2O (11.75 g, 45.08 mmol) were dissolved in 120 mL of deionized water under N2 atmosphere. The obtained mixture was stirred for 15 min and heated at 60 °C under vigorous stirring. After the desired temperature was achieved at 60 °C, aqueous ammonia (30 mL, 25−28% w/w) was added dropwise, and heating was continued for 2 h under N2 atmosphere. The ensuing blackish precipitate was separated by a magnet and washed thoroughly with water until the supernatant liquor reached neutrality and dried in an oven at 100 °C for 12 h.

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RESULTS AND DISCUSSION Characterization of the Catalyst. The maghemite-ZnO nanocatalyst, prepared by the coprecipitation method, was exhaustively characterized by several characterization techni3315


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ACS Sustainable Chemistry & Engineering Table 2. Maghemite-ZnO Catalyzed Transesterification Reactions with Different Alcoholsa

Table 3. Maghemite-ZnO Catalyzed Amidation Reactions with Different Aminesa

a

Reaction conditions: methyl benzoate (7.3 mmol), amine (22 mmol), toluene (5 mL), maghemite-ZnO (100 mg), sealed tube, 150 °C. b Isolated yield. cReaction performed in aqueous ammonia. a

Reaction conditions: methyl benzoate (7.3 mmol), alcohol (14.6 mmol), maghemite-ZnO (100 mg), sealed tube, time (12 h). bIsolated yield.

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Fe Mössbauer spectroscopy was employed to identify the chemical nature of iron oxide phase and assess its magnetic properties; the recorded 57Fe Mössbauer spectra are shown in Figure 3, and values of the Mössbauer hyperfine parameters derived from spectra fitting are summarized in Table S1 in Supporting Information. At room temperature, the profile of the 57Fe Mössbauer spectrum of the Fe2O3-zinc sample can be well-fit with only one broad sextet (see Figure 3a); the derived values of the Mössbauer hyperfine parameters (see Table S1) fall into the range frequently observed for γ-Fe2O3. As the Mössbauer resonant lines are of non-Lorentzian nature, the roomtemperature 57Fe Mössbauer spectrum was correctly fitted with a distribution of the hyperfine magnetic field (Bhf). The curving of the resonant lines to the center of the spectrum is frequently reported for a system of magnetic nanoparticles whose superspins undergo collective magnetic excitations.49,50 As there is no sign of a doublet component, with regard to the characteristic time of the Mössbauer technique (∼10−8 s), the nanoparticle assembly still stays in the magnetically blocked state at room temperature. The blocked spectral feature indirectly indicates that the average size of γ-Fe2O3 nanoparticles present in the system exceeds ∼15 nm. If the temperature is lowered to 5 K and an external magnetic field of 5 T is applied, the Mössbauer resonant lines further split up with clear emergence of two sextets (see Figure 3b). The sextet showing a lower isomer shift (δ) and higher effective hyperfine magnetic field (Beff) value belongs to the tetrahedral (T) cation

ques as described below in detail. The structure of maghemiteZnO was studied by X-ray diffraction (XRD) measurements, which reveal typical diffraction from the (110), (210), and (211) planes when compared with parent maghemite sample, confirming the presence of maghemite with partially ordered vacancies (PDF card 01-089-5892). Further, Rietveld analysis was performed on both XRD patterns to calculate the lattice parameter of maghemite (a = 3.355 Å) and estimate the size of the crystalline domains (around 15 nm). The XRD pattern of maghemite-ZnO shows the diffraction lines 100, 002, 101, 102, 110, 103, 112, and 201 for crystalline Wurtzite phase of ZnO (Figure 1), and particle size is further confirmed by TEM and HRTEM analysis. The exact content of ZnO was confirmed by ICP-MS, which was found to be 7 wt % in the sample. The oxidation states of Zn and Fe species in the final nanocatalyst were ultimately confirmed by X-ray photoelectron spectroscopy (XPS) (Figure 2). Surface composition of the maghemite-ZnO powder was established from the characteristic XPS peak intensities of Fe and ZnO (i.e., Fe 2p3/2 and ZnO 2p3/2, respectively). In the XPS profile of maghemite-ZnO, the Fe 2p3/2 line shape corresponds to Fe2O3.47 The high resolution spectrum of the ZnO 2p3/2 line at 1021.42 eV corresponds to Zn (2+) in ZnO.48 3316


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ACS Sustainable Chemistry & Engineering Table 4. Maghemite-ZnO Catalyzed Solvent-Free OAcylation Reactions with Different Phenolsa

Figure 2. (a) Fe 2p3/2 line spectrum, and (b) XPS spectrum of maghemite-ZnO. The position of the ZnO is denoted by blue color.

sites of the γ-Fe2O3 inverse spinel crystal structure while the sextet with a higher δ and lower Beff value is ascribed to the γFe2O3 octahedral (O) cation sites (see Table 1).51 The analysis further confirms that the spectral T:O ratio is very close to 3:5 (see Table S1), which is observed for perfectly stoichiometric γFe2O3 where Fe3+ ions fill all the T cation sites and 5/3 of O cation sites and 1/3 of O cation sites are unoccupied to establish the neutral charge over the γ-Fe2O3 crystal structure.51 The isomer shift values of both sextets lie in the interval characteristic for a high spin (S = 5/2) Fe3+ valence state.51 The sample is of single-phase character as there are no signs of other spectral components originating from other phases of iron oxide origin (e.g., α-Fe2O3, Fe3O4). Concerning the magnetic properties of the nanoparticle system, nearly zero intensity of the second and fifth Mössbauer resonant lines for both sextets indicates that the atomic magnetic moments inside a nanoparticle are practically perfectly aligned in an externally applied magnetic field showing a ferrimagnetic arrangement.51 However, a spin canting phenomenon cannot be fully excluded as a complete vanishing of the second and fifth Mössbauer resonant lines is not observed as long as the difference between Beff of T sites and O sites does not reach 10 T (almost 8 T, see Table S1). Most probably, it may originate from the surface atomic moments with reluctance to incline completely in the direction of the external magnetic field due to enhanced surface

a

Reaction conditions: phenol (6.9 mmol), acetic anhydride (7.6 mmol), maghemite-ZnO (100 mg). bIsolated yield. c2 equiv of acetic anhydride used.

Figure 1. XRD patterns of maghemite support and maghemite-ZnO. The Miller indices corresponding to maghemite and ZnO are shown in black and blue numbers, respectively. 3317


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ACS Sustainable Chemistry & Engineering

over the spherical maghemite surface (Figure 4c−f). The perfect core−shell nature of the catalyst is investigated by a single line spectrum, where ZnO nanoparticles are clearly shown on the surface of maghemite core (Figure S3b, Supporting Information). Catalytic Applications. The catalytic efficiency of synthesized catalyst was assessed for the transesterification reaction of methyl benzoate with different alcohols, amidation reaction of methyl benzoate with various amines, and acylation reaction of phenolic reactants with acetic anhydride. Initially, the reaction condition was optimized for transesterification reaction using methyl benzoate as model substrate with 1propanol at 150 °C in the presence of maghemite-ZnO catalyst. The effects of solvent, time, temperature, and amount of the catalyst were investigated (Table 1). Notably, reaction smoothly proceeds without any solvent to afford 83% of corresponding product while in the presence of tetrahydrofuran (THF), and toluene which afforded 20% and 52% yields, respectively (Table 1, entries 3−7). A parallel experiment without catalyst and with bare maghemite was also performed; 30% and 42% product formation was observed, respectively (Table 1, entries 8 and 9). Notably, with low catalysts loadings such as 25 and 50 mg, comparatively lesser yields were obtained (Table 1, entries 10 and 11). After optimization of the reaction conditions, a variety of aliphatic alcohols including propan-1-ol, prop-2-yn-1-ol, propan-2-ol, 3-methylbutan-1-ol, and 2-methylpropan-2-ol were subjected to this reaction, and good yields (70−84%) of corresponding products were obtained; phenyl methanol gave 84% yield of benzyl benzoate (Table 2). Further, the catalytic activity was explored for the amidation reaction between methyl benzoate and aliphatic amine and benzyl amine as coupling partners in toluene at 150 °C. Initially, reaction was performed between methyl benzoate and benzyl amine as model substrate using 100 mg of maghemiteZnO catalyst, and results revealed that N-benzylbenzamide was obtained in excellent yield (92%) while without catalyst only 25% product was formed (Table 3, entry 4, and Table S2, Supporting Information). After successful optimization, substrate scope was examined using propan-1-amine, butan-1amine, piperidine, and benzyl amine as substrates; good to excellent yields were obtained (Table 3). Finally, the catalytic competence of maghemite-ZnO catalyst was analyzed for the acylation of 2-naphthol as model substrate with acetic anhydride. The reaction is completed within 3 h using 100 mg of maghemite-Zn catalyst, while without catalyst only 15% of product formed (see Table S3, Supporting Information). Various types of electron withdrawing group containing derivatives such as 2,5-dichlorophenol and 4nitrophenol displayed 85% and 96% yield, respectively (Table 4, entries 4 and 5). Additionally, this catalytic system is effective for the acylation of naphthalen-2-ol, 7-hydroxy-4-methyl-2Hchromen-2-one, and benzyl alcohol which affords 92%, 96%, and 94% yields, respectively (Table 4, entries 1, 5, and 6). All studied reactions including transesterification, amidation, and O-acylation reactions are comparable/better than some previously reported protocols (Tables S4−S6, Supporting Information). The reusability of the catalytic system is a prerequisite for the development of a cost-effective and sustainable catalyst. The reusability of the catalyst was performed for O-acylation reaction of hydroquinone with acetic anhydride in the presence of maghemite-ZnO catalyst under optimized conditions. The

Figure 3. 57Fe transmission Mössbauer spectra of the γ-Fe2O3-zinc oxide sample collected (a) at room temperature and zero external magnetic field and (b) at a temperature of 5 K and in an external magnetic field of 5 T. The inset in panel a shows the distribution of the hyperfine magnetic field (Bhf).

anisotropy and/or anisotropy induced by interparticle magnetic interactions. Nevertheless, the degree of spin canting is very low, thus not degrading the magnetic features of the studied nanoparticle system. TEM and HRTEM analysis of maghemite-ZnO (Supporting Information, Figures S1 and S2, and Figure 4b) discloses that the overall diameter of the spherical maghemite-ZnO NPs is in the range between 10 and 30 nm, which is in accord with the XRD data. The elemental (ZnO, Fe, and O) scanning confirms uniform distribution of ultrasmall ZnO nanoparticles (3−5 nm)

Figure 4. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of maghemite-ZnO: (a) HAADF image of maghemite-ZnO at 10 nm scale; (b) HRTEM image of maghemiteZnO at 10 nm scale; (c−e) HAADF images showing Fe, O, and ZnO mapping at 9 nm scale, respectively; (f) HAADF image showing ZnO, Fe, and O together. 3318


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also grateful to the Principle Dr. K. P. Kakade and Dr. A. B. Nikumbh (HOD), S. S. G. M. College, Kopargaon, Ahmednagar (MH), for providing necessary research facilities and constant encouragement. The author gratefully acknowledges the support by BCUD, Savitribai Phule Pune University, Pune for providing research stipend. The authors thank Ms. J. Straska for TEM, Mr. M. Petr for XPS, and Dr. C. Aparicio for XRD analysis. The authors also gratefully acknowledge the assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project LM2015073, and support from the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LO1305.

recycling experiments indicate the excellent conversion after 5 cycles without major loss of activity (Figure 5). During the recycling process, the catalyst was magnetically separated and washed with ethanol and dried at 60 °C under vacuum.

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Figure 5. Maghemite-ZnO catalyzed O-acylation reaction of hydroquinone with acetic anhydride.

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CONCLUSIONS In summary, an active and versatile maghemite-ZnO catalyst was prepared from low-cost precursors by simple coprecipitation technique. Maghemite decorated ultrasmall ZnO nanoparticles (3−5 nm) served as expedient catalysts for common organic transformations, namely, transesterification, amidation, and O-acylation reactions. Maghemite-ZnO nanocatalyst was stable and held its initial catalytic performance even after five runs. The simple procedure, mild reaction conditions, economic feasibility, and proficient to excellent yields of products render this an attractive sustainable alternative.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03167. Detailed information about instrument and characterization techniques, relevant images, and optimization tables of reactions and additional table of data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jiri Tucek: 0000-0003-2037-4950 Rajender S. Varma: 0000-0001-9731-6228 Radek Zboril: 0000-0002-3147-2196 Manoj B. Gawande: 0000-0003-1575-094X Author Contributions §

V.B.G. and A.K.R. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the Director of SAIF, Panjab University (Chandigarh, India), for the spectral analysis. We are 3319


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