Mechanistic aspects of formation of MgO nanoparticles under microwave irradiation and its catalytic application

Advanced Powder Technology xxx (2017) xxx–xxx

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Original Research Paper

Mechanistic aspects of formation of MgO nanoparticles under microwave irradiation and its catalytic application Aravind L. Gajengi a, Takehiko Sasaki b, Bhalchandra M. Bhanage a,⇑ a b

Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400 019, India Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan

a r t i c l e

i n f o

Article history: Received 8 January 2016 Received in revised form 1 February 2017 Accepted 2 February 2017 Available online xxxx Keywords: MgO NPs Microwave Heterogeneous Formylation Amines

a b s t r a c t This work reports a preparation of Mg(OH)2 and MgO nanoparticles (NPs) using magnesium acetate in benzylamine and mechanistic study of its formation. The benzylamine acts as a solvent, base, promoter and capping agent in this reaction. The structure and morphology of particles were analyzed by X-ray diffraction pattern (XRD), transmission electron microscopy (TEM), high resolution TEM (HRTEM), selected area energy dispersion (SAED), energy-dispersive X-ray spectroscopy (EDAX), thermogravimetric analysis (TGA), FT-IR, CO2–temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) surface area analysis techniques. The application of as prepared MgO NPs was used in catalysis as a catalyst for the formylation of amines with recyclability studies of nanocatalyst. Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Recently, developing novel methods for the synthesis of nanomaterials is gaining considerable attention in material science [1]. Magnesium oxide (MgO) is an important material which is widely used in catalysis [2,3], dye removing agents [4], defluorination [5], sensors [6] and CO2 storage materials [7]. The MgO NPs shows excellent catalytic activity due to small particle size and availability of high surface area. As the particle size decreases, the relative number of surface atoms increases, and thus the activity increases. The materials having a high surface area are of interest in catalysis as a catalyst or support of catalyst. Some of the organic reaction in which MgO or metal supported MgO used as catalyst for Claisen– Schmidt condensation reaction [2], N-methylation of indole and o-methylation of phenol [7], Suzuki–Miyaura cross-coupling reaction [8] etc. There are several methods of synthesis of MgO NPs with different size and morphology such vapour ablation method [9], wet chemical method [10], hydrothermal method [11], sol-gel [12], thermal decomposition [13], ultrasound synthesis [14]. Nowadays, microwave-assisted synthesis of metal and metal oxide NPs has attracted great attention because of several advantages such as it is simple, require less time and energy efficient [15]. Recently we showed the importance of microwave assisted ⇑ Corresponding author. Fax: +91 3361 1020. E-mail addresses: (B.M. Bhanage).

[email protected],

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method for synthesis of various nanoparticles [16–20]. The microwave synthesis have some advantages over conventional methods as it improves the kinetics of reactions by one or two orders of magnitude due to rapid initial heating and generation of the localized high pressure zone at reaction sites. The efficiency of microwave power (P) dissipated per unit volume is given by following equation:

P ¼ cE2 f e00 where c, E, f and e00 are radiation velocity, electric field in the material, radiation frequency and dielectric loss constant respectively. These are most vital parameters which decide the ability of heating material in the microwave field. We herein report a rapid, template/capping agent and additive free method for synthesis of Mg(OH)2 and MgO NPs using benzylamine as solvent. The detailed mechanism for the formation of MgO NPs investigated. This synthesized MgO NPs shows high catalytic activity towards the formylation of amines with formic acid under microwave irradiation method with recyclability of the catalyst. 2. Experimental details 2.1. Materials All commercial reagents, Mg(CH3COO)24H2O and benzylamine were purchased from M/S Merck Chemicals Pvt. Ltd. India and were used directly without further purification.

http://dx.doi.org/10.1016/j.apt.2017.02.004 0921-8831/Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

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2.2. Preparation of MgO NPs In a typical procedure, the mixture of 1.0 g of Mg(CH3COO)24H2O and 10 mL benzylamine was transferred to a 30 mL teflon lined tube and kept in a microwave oven for 2 min at 360 watts (W) using on/off mode with a time interval of 30 s. After microwave heating, the white turbid precipitate was formed which indicates the formation of Mg(OH)2 NPs. These particles were separated by centrifugation and then washed twice with distilled water followed by ethanol. The Mg(OH)2 NPs was calcinationed in oven at 550 °C for 5 h which gave MgO NPs (Scheme 1).

column (Perkin–Elmer, Clarus 400, 30 m  0.32 mm  0.25 m). After completion, the reaction mixture was filtered to separate catalyst. The filtrate containing desired product was quenched in water and were extracted using ethyl acetate (5 ml  3). The organic layer was washed with saturated NaHCO3 to remove excess of formic acid. The combined organic layer was then dried over Na2SO4 and evaporated under vacuum. The obtained crude product was purified by column chromatography using silica gel (100–200 mesh size) with petroleum ether/ethyl acetate (PEEtOAc, 95:05) as eluent to afford the pure product. The products are well known in the literature and were confirmed by GC–MS analysis of the comparison with literature data.

2.3. Characterization of prepared MgO NPs The prepared MgO NPs were characterized by using various analytical techniques such as TEM and SAED (Philips, CM 200, operating voltage of 200 kV model CM 200), HRTEM (JOEL, JEM2100F operating voltage of 200 kV with a probe size under 0.5 nm), TGA (PerkinElmer STA 6000), XPS (PHI 5000 Versa Probe Scanning ESCA Microprobe) and EDAX (Oxford instrument, model 51-ADD0007). The FT-IR spectra were recorded using Perkin Elmer-100 Spectrometer, XRD analysis done by using Shimadzu XRD-6100, BET surface area and CO2-TPD by using TPDRO 1100 thermo scientific model. 2.4. General procedure for N-formylation of amines In a 25 mL sealed tube, amine (1 mmol), formic acid (3 mmol) and MgO NPs (20 mg) were added and kept for microwave irradiation at 480 W for 2 min. The reaction progress was monitored by GC equipped with a flame ionization detector (FID) and a capillary

Scheme 1. Synthesis of MgO NPs by microwave irradiation method.

3. Results and discussion 3.1. Characterization of Mg(OH)2 and MgO NPs The morphology of prepared Mg(OH)2 and MgO NPs was observed by transmission electron microscopy (TEM). The TEM images show Mg(OH)2 are in the nano range and having flake like morphology (Fig. 1a and b) and its SAED pattern as shown in Fig. 1a inset which indicates its crystalline nature. The HRTEM images of Mg(OH)2 are shown in Fig. 1c and d. The images in Fig. 2a and b show flake like morphology of MgO which are lacunal and loose connection of particles and its SAED pattern as shown in Fig. 2a inset which indicates the crystalline nature of MgO NPs. The HRTEM of MgO NPs as shown in Fig. 2c and d. The HRTEM images of Mg(OH)2 (Fig. 1c and d) and MgO NPs (Fig. 2c and d) having a particle size approximately 6 nm and 16 nm which is complying with average crystallite size obtained by XRD using Scherrer’s equation. The energydispersive X-ray spectroscopy of MgO NPs (Fig. 2e), which indicates magnesium and oxygen elements only, suggesting MgO NPs were in pure form and particle size distribution histogram (Fig. 2f) which shows the most of the particles are in the range of 12–22 nm. Phase identification was done by using X-ray diffraction technique with an X-ray wavelength of Cu Ka radiation at k = 1.5405 Å with a scanning rate of 2°/min from 10° to 80°.

Fig. 1. (a and b) TEM with SAED pattern (inset a) and (c and d) HRTEM images of Mg(OH)2 NPs.

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Fig. 2. (a and b) TEM with SAED pattern (inset a) and (c and d) HRTEM images (e) EDAX (f) particle size distribution histogram of MgO NPs.

Fig. 3. XRD pattern (a) Mg(OH)2 and (b) MgO NPs.

Fig. 3 indicates planes (0 0 1), (1 0 0), (1 0 1), (1 0 2), (1 1 0), (1 1 1), (1 0 3), (2 0 1) which can be indexed to the planes of hexagonal Mg(OH)2 (JCPDS 7-239) and (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2)

which can indexed to the planes of cubic MgO (JCPDS 75-0447). The average crystallite size of synthesized Mg(OH)2 and MgO NPs were determined according to Scherrer’s equation: D = kk/bcos h

Please cite this article in press as: A.L. Gajengi et al., Mechanistic aspects of formation of MgO nanoparticles under microwave irradiation and its catalytic application, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.02.004

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Fig. 4. FT-IR of (a) Mg(OH)2 and (b) and MgO NPs.

Fig. 5. CO2 TPD of (a) MgO NPs and (b) Bulk MgO.

(where h is Bragg angle, b is the full width at half maximum in radians) and were found 6 nm and 16 nm respectively. Fig. 4 shows FT-IR spectra of synthesized Mg(OH)2 and MgO NPs. Peak at 3382 cm1, 3699 cm1 and 571 cm1 (Fig. 4a) which are characteristic peak due to OAH stretching band and MgAO stretching in Mg(OH)2. The peaks at 1440–1591 cm1 shows the bending vibration of OAH stretching. Whereas, the peak at 662 cm1 (Fig. 4b) is due to stretching vibrations of the bond between Mg and O in MgO NPs. Peak at 3395 cm1 is due to moisture absorbed by MgO NPs. The basicity of commercially available bulk MgO and prepared MgO NPs was measured by CO2-TPD analysis. Before analysis, the sample was pre-treated with helium gas from 25 °C to 550 °C for

one hour to remove adsorbed water molecules and other impurities. After that sample was cooled to room temperature and then saturated with CO2 at 50 °C. After saturation, TPD was carried out from 25 °C to 1000 °C at temperature ramp of 10 °C/min using helium as inert gas at a flow rate of 20 cm3/min. The amount of CO2 desorbed from MgO NPs and bulk MgO was 516 mmol/g and 246 mmol/g respectively, which reveals MgO NPs are more basic than bulk MgO which is due to nano sized MgO (Fig. 5). Desorption peak for CO2 appeared at 900 °C (Fig. 5a) which is absent in (Fig. 5b) indicated the presence of strong basic sites on MgO NPs than bulk MgO NPs. The more basicity of MgO NPs than bulk MgO due the MgO NPs have more BET specific surface area than the bulk MgO (Fig. 7a and b). However, the spectrum of MgO NPs

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Fig. 6. (a) XPS survey spectrum, (b) O 1s region, (c) Mg 2p region, (d) Mg 2s Region.

Fig. 7. (a and b) N2 adsorption-desorption of Bulk MgO and MgO NPs (c) TGA MgO NPs.

Fig. 8. Proposed reaction mechanism for synthesis of MgO NPs.

shows much difference from bulk MgO’s one. MgO NPs has stronger basic sites than bulk MgO. It’s not only by specific surface area but also the intrinsic difference of the MgO NPs surface.

The typical XPS as shown in Fig. 6, survey spectrum (Fig. 6a) indicates major XPS peak for Mg and O present on the surface of the catalyst. The main Mg KLL Auger emission peaks were found

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Fig. 9. (a) GC of final reaction mixture, (b) mass spectra of N-benzylacetamide, (c) mass spectra of N-bezylidene-1-phenylmethanamine.

in the range 304.4–308.8 eV were attributes of oxidation of polycrystalline Mg and there is no other visible contribution of metallic magnesium [21]. A weak peak around 285 eV corresponds to the C 1s due to slight contamination of the carbon element which generally occurs on the specimen surface [22]. Peaks at 530.7 eV correspond to the O 1s spectrum (Fig. 6b). Fig. 6c and d indicates peaks at 49.51 eV, 88.9 eV, correspond to Mg 2p, Mg 2s, respectively, which is in accordance with the literature [22]. BET surface area of bulk MgO and prepared MgO NPs were measured by nitrogen adsorption-desorption method. Before BET analysis, samples were pre-treated, ranging from 25 °C to 550 °C for 1 h to remove adsorbed water and other impurities using helium gas. The N2 adsorption and desorption was carried out by using liquid nitrogen. Area under the peak corresponds to the surface area of respective material. The obtained specific surface area for bulk MgO (Fig. 7a) was found to be 17 m2/g and 70 m2/g for MgO NPs (Fig. 7b) respectively. The high surface area of MgO NPs than bulk MgO is due to nano range particles of MgO NPs. Thermal analysis of MgO NPs was carried out using TGA measurement in the inert N2 atmosphere from 30 °C to 900 °C with ramp rate 20 °C per min (Fig. 7c). The TGA curve shows the small weight loss at 100 °C which is due to the moisture absorbed by the MgO NPs. The second weight loss observed around 280 °C was possibly associated with leaving lattice hydroxyls [23]. A plausible mechanistic pathway for the formation of MgO NPs has been illustrated in Fig. 8. The proposed mechanism was further supported by GC–MS analysis report of mother liquor and it shows the formation of N-benzylacetamide and N-bezylidene-1phenylmethanamine which are confirmed by GCMS (Fig. 9). Initially, ANH2 group of benzylamine reacts with magnesium acetate to give A, which on rearrangement leads to the formation of Mg

Scheme 2. N-formylation of amines catalysed by MgO NPs under microwave irradiation.

(OH)2 NPs and N-benzylacetamide takes place. A similar type of mechanism has also been reported in the literature [16,24,25]. Nbezylidene-1-phenylmethanamine is formed due to self coupling of benzylamine under microwave.

3.2. Catalytic activities of MgO NPs Nowadays, green and environmentally benign protocols achieved by catalysis which avoid the use of volatile organic solvents, toxic reagents, harsh reaction conditions and timeconsuming processes. The use of nanocatalyst has an advantage over the bulk catalyst because has several advantages such as large surface-to-volume ratio, low catalyst loading. In this aspect, some groups have reported the formylation of amines using MgO and some other catalysts [26–28]. The catalytic activity of prepared MgO NPs was examined for formamide synthesis using amine and formic acid, which shows excellent catalytic activity (Scheme 2). The MgO NPs acts as a base as well as a catalyst in the reaction. The initial reaction of aniline (1 mmol) and formic acid (3 mmol) without MgO NPs was carried out under microwave irradiation at 480 W. However, lower yields of N-formyl aniline (10%) (Table 1,

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A.L. Gajengi et al. / Advanced Powder Technology xxx (2017) xxx–xxx Table 1 Synthesis of formamide using amine and formic acid under microwave irradiation using MgO NPsa. Entry

Substrate

Yield (%)b

Product

1

98, 40c, 10d

2

81

3

61

4

90

5

99

6

99

7

80

entry 1) was observed. Furthermore, we have carried out reaction using MgO NPs and MgO Bulk as a catalyst which gives 98% and 40% yield of the corresponding product within the two minutes (Table 1, entry 1). Next, we have screened various electron donating groups on a amine (Table 1, entries 2–3) as well as an electron withdrawing group (Table 1, entry 4), heterocyclic amines (Table 1, entries 6–8) and aliphatic amines (Table 1, entry 5 and 10) which gives good to excellent yield of the corresponding N-formylation products. The reason for better catalytic activity is due to particle size in nano scale, higher surface area, strong basic sites and lattice planes of MgO NPs. The comparison of the present work with various other reported catalyst as shown in Table 2, these catalyst have one or more disadvantage such as longer reaction time, expensive catalyst, complicated preparation of catalyst and difficult to recovery of the catalyst.

Fig. 10. Recyclability of MgO NPs for amide synthesis (GC yield).

8

87

9

98

10

97

a All reactions were performed using an amine (1 mmol), formic acid (3 mmol) and MgO NPs (20 mg) at 480 W for 2 min. b GC yield, all compound are known and are matched with GC–MS data. c Reaction carried using Bulk MgO. d Without catalyst.

Fig. 11. Comparison of TGA (a) fresh and (b) recycled MgO NPs.

Table 2 Comparison of catalytic activity with other reported catalysts. No

Catalyst

Amount of catalyst

Solvent

Time

Yield

Reference

1 2 3 4 5 6 7

dppe HIL-[Ch-SO3H]3W12PO40 MOF MSA Thiamine hydrochloride Ionic liquid MgO NPs

0.1 mol% 20 mg 20 mg 2 mol% 10 mol% 5 mol% 20 mg

Toluene Neat Neat Ethanol Neat Neat Neat

4h 5 min 20 min 50 min 10 min 1h 2 min

96 99 97 90 96 97 98

[29] [30] [31] [32] [33] [34] This work

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3.3. Recycle study After completion of the reaction, the catalyst was separated by filtration and the obtained catalyst was washed several times with ethanol, dried at 80 °C for 30 min in the oven and used further for the next reaction. The catalyst shows excellent activity for up to four recycle runs without significant change in catalytic activity with respect to fresh catalyst (Fig. 10). To investigate for the moderate decrease in the catalytic activity at fourth recycle, the 4th recycled catalyst was analyzed for TGA and FTIR studies and compared with the fresh catalyst. The 4th recycled catalyst shows slightly decrease in the weight loss as compared to fresh catalyst (Fig. 11). The FT-IR spectra of 4th recycled catalyst shows the shift of peaks 662–589 cm1. The peaks at 3434 cm1 is due to moisture absorbed by MgO NPs (Fig. S1 supporting information). Due to these changes there may be a moderate decrease in the catalytic activity. 4. Conclusion In summary, an efficient synthesis of Mg(OH)2 and MgO NPs using benzylamine and its mechanism of formation has been developed. The reported protocol is an inexpensive and rapid method for the preparation of MgO NPs. The MgO NPs shows the higher BET surface area, stronger basic sites as compared to bulk MgO. Furthermore, we tested the catalytic activity of synthesized MgO NPs for the formylation of amines with formic acid under microwave irradiation. The developed protocol is advantageous due to ease of preparation of MgO NPs, separation by simple filtrations during the work up and shows excellent activity with good recyclability. Acknowledgements The author (Aravind L. Gajengi) thankful to the university grant commission (UGC), India for providing financial support under University Grand Commission Basic Research Programme (UGC-BSR). We are also thankful to Department of Science and Technology (DST), India for DST-Nano Mission Project No. [SR/NM/NS-1097/2011] and Indo-Japan collaborative project No. [DST/INT/JSPS/P-152/2013] for financial assistance. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apt.2017.02.004. References [1] A.H. Wani, M.A. Shah, A unique and profound effect of MgO and ZnO nanoparticles on some plant pathogenic fungi, J. Appl. Pharm. Sci. 2 (2012) 40–44. [2] A.B. Patil, B.M. Bhange, Novel and green approach for the nanocrystalline magnesium oxide synthesis and its catalytic performance in Claisen-Schmidt condensation, Catal. Commun. 36 (2013) 79–83. [3] S.T. Gadge, A. Mishra, A.L. Gajengi, N.V. Shahi, B.M. Bhanage, Magnesium oxide as a heterogeneous and recyclable base for the N-methylation of indole and omethylation of phenol using dimethyl carbonate as a green methylating agent, RSC Adv. 4 (2014) 50271–50276. [4] N.K. Nga, P.T.T. Hong, T.D. Lam, T.Q. Huy, A facile synthesis of nanostructured magnesium oxide particles for enhanced adsorption performance in reactive blue 19 removal, J. Colloid Interface Sci. 398 (2013) 210–216. [5] B. Nagappa, G.T. Chandrappa, Mesoporous nanocrystalline magnesium oxide for environmental remediation, Microporous Mesoporous Mater. 106 (2007) 212–218. [6] N.K. Reddy, Q. Ahsanulhaq, J.H. Kim, Y.B. Hahn, Behavior of n-ZnO nanorods/pSi heterojunction devices at higher temperatures, Appl. Phys. Lett. 92 (2008) 043127.

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