Bio-derived ZnO nanoflower: a highly efficient catalyst for the synthesis of chalcone derivatives

Abstract

The green, eco-friendly synthesis of ZnO nanoparticles using the peel of Musa balbisiana and their use as nanocatalysts in the synthesis of chalcones derivatives is reported here. Bio-derived ZnO nanoparticles were characterized by XRD, XPS, FTIR, SEM, BET and TEM techniques. The single step condensation of substituted aryl carbonyls is an attractive feature to obtain substituted chalcones in 88–98% yields in less than 2 min under microwave irradiation in solvent free conditions. A short reaction time with excellent yields of chalcones is the main advantage of our study.

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1. Introduction

Among various metal oxides, ZnO has come to the limelight for its semiconducting properties, unique antibacterial, antifungal, wound healing and UV filtering properties, and high catalytic and photochemical activity.¹ Over the past several years, plants and different natural sources have come up as low cost, energy-efficient, eco-friendly and non-toxic sources for the synthesis of nanomaterials.^2,3 The synthesized nanoparticles have the advantage of good polydispersity, dimensions and stability with a negligible synthesizing cost. Moreover, using plant extracts for nanoparticle synthesis can be advantageous over other biological processes because it eliminates the elaborate process of maintaining cell cultures and can also be suitably scaled up for large-scale nanoparticle synthesis.⁴ Many examples are found in the literature for eco-friendly synthesis of ZnO nanoparticles using leaf extracts such as Corriandrum Sativum with Zn(CH₃CO)₂·2H₂O as precursor,⁵ Calotropis procera,⁶ seaweeds such as green Caulerpa peltata, red Hypnea valencia and brown Sargassum myriocystum,⁷ orange juice,⁸ Calotropis procera latex,⁹ aqueous leaf extract of Acalypha indica,¹⁰ and leaf extract of Calotropis Gigantea.¹¹

Musa balbisiana is a medicinal and low-cost plant in North East India. The peel of the plant is a food additive and helps with normalizing digestive disorders of the stomach. It is widely used to produce soaps and detergents for washing clothes and shampooing hair.^12,13

In the present investigation we synthesized ZnO nanoparticles with a green and low cost method using the peel of Musa balbisiana. This procedure is environmentally benign for the production of well characterized nanoparticles without the use of harsh, toxic and expensive chemicals.

Furthermore, this procedure is valuable due to its cost effectiveness. Besides the green synthesis, the catalytic activity of ZnO in the microwave synthesis of chalcones in solvent free conditions is reported.

2. Experimental section

2.1. Materials

Materials used for the synthesis of ZnO nanoparticles are zinc nitrate [Zn(NO₃)₂·2H₂O] (Merck, India) and Musa balbisiana peel extract which was prepared by burning the peel of the plant.

2.2. Synthesis of ZnO nanoparticles

In this method, the peel of Musa balbisiana was dried and then burnt. To 1 g of the resulting ash, 10 ml of distilled water were added and the resulting mixture was filtered. 4 ml 1 M Zn(NO₃)₂·2H₂O solution were added to the filtrate and the resulting mixture was stirred for 20 min. A white precipitate was obtained. The precipitate was then filtered and washed three/four times with distilled water. The precipitate was heated for 2 h at 120 °C to form powder ZnO nanoparticles. This is the first report of an eco-friendly green synthesis of ZnO nanoparticles by using the peel of Musa balbisiana.

2.3. Characterization

Scanning electron microscopy (SEM) characterization was performed on JEOL JSM-6360 at 15 kV. X-ray diffraction (XRD) measurements were carried out by Rigaku X-ray diffractometer (Model: ULTIMA IV, Rigaku, Japan) with Cu-Kα X-ray source (λ = 1.54056 Å) at a voltage of 40 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALAB 300 (Thermo-VG Scientific). The high resolution transmission electron microscopy (HR-TEM) images were recorded by a JEOL Model 2100 EX, Japan operated at a voltage of 200 kV. Specific surface area, pore volume, and average pore diameter were measured with the Autosorb-1 (Quantachrome, USA). The specific surface area of the samples was measured by adsorption of nitrogen gas at 77 K and applying the Brunauer–Emmett–Teller (BET) calculation. The pore size distribution was derived from desorption isotherms using the Barrett–Joyner–Halenda (BJH) method. The ¹H & ¹³C NMR spectra were recorded at room temperature in CDCl₃ on a Bruker DPX-300 spectrometer and chemical shifts were reported relative to SiMe₄.

2.4. Claisen–Schmidt condensation

Acetophenone (10 mmol) and benzaldehyde (10 mmol) were mixed with ZnO nanoparticles (5 mol%). Then the mixture was irradiated in a microwave reactor for 1 min after setting the reaction power at 40% (maximum output 400 W) at 100 °C. After cooling to room temperature, ethyl acetate (15 ml) was added to the reaction mixture and the mixture was filtered through filter paper to separate the solid catalyst. The ZnO catalyst could be reused for five consecutive times for the condensation of acetophenone and benzaldehyde. After washing the filtrate, the separated organic layer was concentrated under reduced pressure and the product was purified by column chromatography using hexane/ethyl acetate as the solvent system in different concentrations to obtain the pure compound.

3. Results and discussion

3.1. Characterization of ZnO nanoparticles

In the present investigation, ZnO nanoparticles are synthesized using the peel of Musa balbisiana. It contains 233.60 g of K⁺, 2.00 g of Na⁺, 161.40 g of CO₃²⁻ and 6.62 g of Cl⁻ when prepared from 1 kg of the ash.^12,13 These ions may be responsible for the synthesis of ZnO nanoparticles [Scheme 1S, ESI†].

The formation of ZnO nanoparticles was confirmed using XRD, XPS, FTIR, SEM and TEM analysis. In XRD analysis, the 2θ values 31.7°, 34.4°, 36.3°, 47.4°, 56.6°, 62.8° and 67.9° assigned to the (100), (002), (101), (102), (110), (103) and (112) plane indicate the wurtzite structure of the ZnO nanoparticles (JCPDS card no. 36-1451). The corresponding ‘d’ spacing values of the ZnO nanoparticles are 2.87, 2.71, 2.49, 1.91, 1.54 and 1.35 (Fig. 1). All diffraction peaks of the sample correspond to the characteristic hexagonal wurtzite structure of the ZnO nanoparticles (a = 0.315 nm and c = 0.529 nm). It is similar to the reported data.¹⁴

Fig. 1 XRD pattern of ZnO nanoparticles.

In the XPS analysis, the instrument was standardized against the C 1s spectral line at 284.6 eV. The binding energy of the Zn 2p_3/2 and Zn 2p_1/2 component is recorded to be 1028.6 eV and 1051.8 eV respectively (Fig. 2A). The binding energy of O 1s is found at 530.16 eV (Fig. 2B). Such results suggested that there is no impurity existing within the sample. The result is supported by the reported data.^15,16 In the FTIR analysis, the peaks observed at 3452.30 and 1119.15 cm⁻¹ may be due to O–H stretching and deformation of atmospheric vapours, respectively. The peaks at 1634.00 and 620.93 cm⁻¹ correspond to Zn–O stretching and deformation vibration, respectively (Fig. 1S, ESI†). The metal–oxygen frequencies observed for the metal oxide are in accordance with the literature values.^17,18

Fig. 2 XPS spectra of (A) Zn 2p and (B) O 1s of ZnO nanoparticles.

The SEM images indicate the formation of a flower like morphology of ZnO. The petal size of the ZnO nano flower is in the range of 100–400 nm (Fig. 3A and B). The flower like morphology consists of petal like small nanosheets. However, it is difficult to examine the surface structure of the nanosheets by SEM images and therefore it was examined by HR-TEM analysis. The results showed the formation of flower like clusters of the ZnO nanostructure. The nanoparticles overlap each other which strongly supports the formation of a flower like nanostructure along with oval shaped nanoparticles (Fig. 4A and B). The size of ZnO nanoparticles was found to be in the range of 5.0 ± 0.2–22.0 ± 1.2 nm. The average size of the particles is 10.5 ± 0.8 nm. The distance between the two atomic layers is 0.26 nm. The formation of a flower like structure of the nanoparticles may be due to synergistic effects of ions like K⁺, CO₃²⁻, Na⁺, Cl⁻ etc. which are available in the extract during the synthesis of the nanoparticles. A similar result was observed in other metal oxides with the same precursors.^12,13

Fig. 3 SEM images (A and B) of ZnO synthesized using Musa balbisiana.

Fig. 4 TEM image (A and B) of ZnO synthesized using Musa balbisiana.

In order to understand the role of ions like K⁺, CO₃²⁻ etc. we have performed an experiment using commercially available reagents. In this experiment ZnO nanoparticles were synthesized using 1 mM K₂CO₃ with 1 mM Zn(NO₃)₂·2H₂O, then we heated the synthesized product at about 120 °C for 1 h. The synthesized nanoparticles were characterized using XRD, TEM and FTIR analysis.

In the XRD analysis, the 2θ values 31.6°, 34.4°, 36.4°, 47.4°, 56.6°, 62.8° and 67.9° assigned to the (100), (002), (101), (102), (110), (103) and (112) plane strongly indicated the formation of ZnO nanoparticles [Fig. 2S, ESI†] The TEM image indicated the formation of pentagonal and hexagonal ZnO nanoparticles. The size of the nanoparticles ranged from 8.0 ± 0.2 to 30.0 ± 1.1 nm [Fig. 3S, ESI†]. The average size of the particles was 18.2 ± 0.2 nm. In the FTIR analysis the peaks at 3378 cm⁻¹, 1620 cm⁻¹, and 1122 cm⁻¹ accounted for H₂O and CO₂ which are usually taken up from the environment [Fig. 4S, ESI†]. The results were supported by reported data.¹⁸ The size and shape of the nanoparticles were similar to those synthesized using the peel extract of Musa balbisiana. So, from this observation it is revealed that ions like K⁺, CO₃²⁻, Na⁺, Cl⁻ etc. may be responsible for the synthesis of ZnO nanoparticles.

3.2. Claisen–Schmidt condensation reaction

As we were emphasizing, a solventless single step synthesis of chalcones without using any protecting group, the reaction between benzaldehyde (A) and acetophenone (B), was studied in detail taking equimolar mixtures of A and B under microwave irradiation in different conditions to optimize the reaction.

We studied the reaction using ZnSO₄, Zn(NO₃)₂, ZnCl₂, commercial ZnO nanopowder (characterized by XRD and TEM analysis, ESI, Fig. 5S & 6S†), the ZnO nanocatalyst and without any catalyst. The results are presented in Table 1S, ESI.† The reaction was carried out under solvent free conditions by mixing benzaldehyde and acetophenone in the microwave at 100 °C with the power at 40% (maximum output 400 W). No product was formed when a catalyst was not used at 80 °C and 100 °C keeping the other reaction conditions unchanged as a control reaction (Table 1S,† entries 1 and 2). From Table 1S,† it was observed that the use of 5 mol% ZnO produced the best result (98%) with 0.8 min microwave irradiation at 100 °C. Thereafter reactions were carried out with different substituted aldehydes and ketones [Table 1]. The electron donating and withdrawing substituents on the aryl ring were well tolerated to give moderate to high yields of the desired chalcones. This result was significant and the yields ranged from 88–98%. The isolated compounds were characterised by ¹H NMR and ¹³C NMR analysis. The data for all products were comparable with the commercial compounds (Scheme 2S, ESI†).

Table 1 Claisen–Schmidt condensation reaction

Entry	Substrate	Product	Temp (°C)	Time^a (min)	Yield^b (%)	TON	TOF (h^−1)
a Reactions performed at 80 °C & 100 °C and monitored using TLC until all the aldehyde and acetophenone were found to be consumed.b Isolated yield after column chromatography with 2% standard deviation.
1			100	0.8	98	1.96	147.3
2			100	1.2	92	1.84	92.6
3			100	1.0	94	1.88	117.4
4			100	0.9	92	1.86	124.0
5			100	1.2	90	1.8	90.0
6			100	1.1	91	1.82	99.4
7			100	1.3	88	1.76	81.4

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We have tested the reusability of the ZnO catalyst in the condensation reaction of benzaldehyde and acetophenone. The ZnO catalyst was recovered by filtration and washed with hot water/ethanol to remove any absorbed products. The catalyst was reused without obvious loss of catalytic activity for up to five cycles and the efficiency remained almost the same (1^st recycle 98%, 2^nd recycle 97%, 3^rd recycle 96%, 4^th recycle 95% and 5^th recycle 94% chalcone was obtained) (Table 2). This was further confirmed by using XRD and TEM analysis after the 5^th recycle [Fig. 7S & 8S†]. TEM and XRD investigations also showed that the activity, morphology and size distribution of the ZnO nanocatalyst remained unchanged after use over 5 times. Further investigations were carried out to obtain precise evidence through adsorption–desorption of nitrogen molecules. The BET surface area and total pore volume of ZnO were found to be 11.402 m² g⁻¹ and 0.1859 m³ g⁻¹ respectively for the fresh catalyst. The specific surface area of the recovered catalysts decreased marginally to 140 (5^th run) compared to 237 m² g⁻¹ of the freshly prepared catalyst [Fig. 9S, ESI†]. The decrease of the surface area of the catalyst after reaction may be due to the partial destruction of the support by the small amount of base used in the reaction. It was observed that the adsorption–desorption hysteresis loop of the catalyst used in the 5^th run ranging between P/P₀ = 0.3 and 0.9 shifted to P/P₀ = 0.6 and 0.9. This may be due to the change in the structure of the pores. The BJH pore size distribution curve of the recovered catalyst shows a slight broadening of the distribution pattern compared to the fresh catalyst [Fig. 10S, ESI†] indicating a breakdown of the pore walls forming larger pores. However, the evidence strongly implies that there is no loss of efficiency of the catalyst after the 5^th recycle.

Table 2 Recycling potential of ZnO nanocatalyst^a

No. of cycle	Run 1	Run 2	Run 3	Run 4	Run 5
a Reaction conditions: 10 mmol benzaldehyde, 10 mmol acetophenone, 5 mol% ZnO nanocatalyst.
Yield (%)	98	97	96	95	94
Time (min)	0.8	0.8	0.8	0.8	0.8
TON	1.96	1.94	1.92	1.90	1.88
TOF (h^−1)	147.3	145.8	144.3	142.8	141.3

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In a plausible mechanism, it has been observed that nano-ZnO activates the aldehyde and reacts with the enol form of the ketone to form the condensed product. One H₂O molecule is eliminated from the condensed product to form 1,3-diphenyl-1-phenylpropenone (chalcone) and its derivatives (Scheme 1).¹⁹

Scheme 1 Plausible mechanism for the condensation reaction.

The catalytic performance of the chalcones was compared with earlier supported catalysts and it was observed that the use of nano-ZnO as catalyst in the reaction between benzaldehyde and acetophenone under microwave irradiation at 100 °C showed the highest yield of chalcones (Table 3). So, this is a suitable, simple, efficient method for the synthesis of chalcones and its derivatives.

Table 3 Comparison of nano-ZnO catalyst for chalcone formation from benzaldehyde and acetophenone with an earlier report

Sl No.	Catalyst	Time (min)	Temperature	Yield (%)	Reference
1	BF3–Et2O/dioxane	15	Room temperature	90	20
2	I2–Al2O3 (neutral)	1.5	M.W. (300 W) 60 °C	95	19
3	NH4Cl/solvent free MW	3	M.W. (480 W)	95	21
4	BiCl3/solvent free	20	140 °C	85	22
5	Phosphonium ionic liquid	150	145 °C	80	23
6	Nano-ZnO	0.8	100 °C	98	Present work

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4. Conclusion

This is the first report of a green, efficient and eco-friendly synthesis of ZnO nanoparticles using the peel of Musa balbisiana. Ions like K⁺, Na⁺, CO₃²⁻ etc. may be responsible for the synthesis of a ZnO micro-flower like nanostructure. We have developed a novel, quick, environmentally safe and clean process for the synthesis of chalcone and its derivatives. The mild reaction conditions, easy work-up and clean reaction profile render this approach an interesting alternative to the existing methods.

Acknowledgements

The authors thank the Director of CSIR-North East Institute of Science & Technology, Jorhat, Assam for providing facilities and valuable advice. The authors also thank SAIF, Shillong for TEM analysis. The authors thank the SEED Division, DST and CSIR, New Delhi for financial support. IS thanks CSIR New Delhi for fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14225j

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