2.1. Materials and Equipments
All the reagents and chemicals that were employed in this study are research standard and they obtained from Merck and Sigma-Aldrich company. Magnesium nitrate hexahydrate (Mg(NO3)2.6H2O) extrapure AR, 99%, Aluminium nitrate nonahydrate (Al(NO3)3.9H2O) extrapure, 98%, 2-aminophenol, aromatic aldehydes, ethanol, de-ionized water and carbol fuchsin dye were purchased and used without further purification .
PANalytical x’pert PRO Powder X-ray diffractometer with 15 KVA UPS support was used for the examination of crystalline nature and purity of spinel material. FT-IR were recorded with PERKIN ELMER TWO within the range 4000-400 cm-1. Morphology of the nanomaterial was analysed by FE-SEM and EDAX done by TESCAN OXFORD VEGA 3 INCA. HR-TEM analysis was done by JEOL Ltd, Tokyo, Japan. UV measurement was done by LAB INDIA-UV3092. XPS high-resolution survey spectrum was obtained from Thermo Scientific make (Model: K-Alpha – KAN9954133). The surface area was identified using BET surface analyser Quantachrome, Nova- 1000.
All the heterocyclic products were characterized by 1H NMR, 13C NMR data analysed by using 500 MHz FT-NMR Spectrometers BRUKER AVIII 500. and MASS spectral data recorded by LC-QTOF-HRMS Waters ACQITY H-CLASS + UPLC/XevoG2 XS QTOF.
2.2. Preparation of MgAl2O4 nanospinel
De-ionized water was used as the extracting medium to create the green extract of lemongrass and MgAl2O4 nanomaterial. To get rid of any dust particles from the selected leaves of lemongrass, it was thoroughly cleaned with distilled water. For four days, the biomaterial was placed shaded region of sunlight in an environment which is free of dust for drying the leaves. The dried leaves were ground mechanically to get fine powder, 10 g of which was placed in a beaker containing 200 mL of deionized water. The suspension was stirred for 30 minutes at 80℃ and filtered using Whatman No.1 filter paper in order to remove the solid residue.
For the synthesis of nanocrystalline MgAl2O4, 0.1 M Mg(NO3)2.6H2O and 0.1 M Al(NO3)3.9H2O were prepared. About 25 mL of 0.1 M Mg(NO3)2.6H2O and 50 mL of 0.1 M Al(NO3)3.9H2O were mixed (1:2 ratio) and to this mixture 25 mL of lemongrass extract slowly added with constant stirring (rpm 800) and temperature was kept constant to 80℃. After continuous stirring of the mixture for 2 hours, the volume of the mixture was reduced to 30 mL by keeping in a hot air oven and calcined at 800℃ in a muffle furnace for 2 hours. The white solid powder of MgAl2O4 nanospinels were obtained and the formation of spinel is confirmed by spectroscopic analysis [21].
2.3. Photocatalytic removal of Carbol Fuchsin dye
Carbol Fuchsin (CF) was photocatalytically decomposed under sunshine irradiation in order to analyse the photocatalytic efficiency of the MgAl2O4 nanocatalyst. First, dye solutions containing catalyst were stirred in the dark for the first 30 minutes to establish the adsorption-desorption equilibrium. After 30 minutes, the contents were exposed to sunlight. An appropriate quantity of aliquots was taken at periodically and centrifuged for 5 min for the separation of suspended particles. The resultant solution was then examined using a UV-visible double beam spectrophotometer at the maximum absorbance wavelength of 547 nm. The procedure is reproduced to different catalyst concentrations, different solution pH, and various CF concentrations. The following calculation has been used to compute the percentage of degradation of dye [1].
% of degradation = (C0 – C/C0) ×100 (1)
Applying pseudo-first order reaction, the kinetics of the reaction rate was examined.
ln C0/C = Kapp t (2)
Where Co and C are concentration of CF dye at zero time and time t respectively and Kapp is the pseudo-first order rate constant.
2.4. General procedure for the synthesis of 2-Aryl substituted benzoxazole derivatives
The mixture of 2-aminophenol (1 mmol), aromatic aldehyde (1 mmol), and MgAl2O4 nanoparticle (0.02 g) were added to 10 mL of ethanol and 10 mL of water. The suspension was stirred at 60 ℃ and the progress of the reaction was monitored by using TLC (hexane: ethyl acetate 6:4). After complete conversion, the mixture was centrifuged and the catalyst was separated. The product was extracted into EtOAc, the organic phase was washed with water and dried with sodium sulphate. The solvent was evaporated to get the crude, which was recrystallized from ethanol [14].
Scheme for the preparation of benzoxazole
Spectral data of compounds
2-(4-nitrophenyl)-1,3 benzoxazole (1)
FT-IR (KBr): 3450, 1592, 1508, 1341, 1194, 845 cm-1, 1H NMR d 6.6-8.7 (m, 8H) ppm; 13C NMR d156.64, 155.47, 141.55, 130.56, 129.08, 124.09, 123.86, 123.03, 115.91 ppm; Mass m/z: 243 (100%)
2-(2-nitrophenyl )1,3 benzoxazole (2)
FT-IR (KBr): 3415, 1613, 1530, 1348, 1230, 734 cm-1, 1H NMR: d 6.8-9.7 (m, 8H) ppm; 13C NMR: d 152.69, 133.53, 131.24, 130.34, 129.20, 124.39, 122.88, 115.82 ppm; Mass m/z: 243(100%)
2-(2-hydroxyphenyl)1,3 benzoxazole (3)
FT-IR (KBr): 3408, 3248, 1620, 1516, 1180, 740 cm-1, 1H NMR: d 6.5-8.8 (m, 8H) ppm; 13C NMR: d 160.05, 159.93, 156.34, 138.97, 132.78, 132.14, 122.52, 119.23, 116.41 ppm; Mass m/z: 214(100%)
2-(4-hydroxyphenyl)1,3 benzoxazole (4)
FT-IR (KBr): 3415, 3240, 1641, 1578, 1160, 831 cm-1, 1H NMR: d 6.76-9.78 (m, 8H) ppm; 13CNMR:d 190.88, 160.06, 156.86, 143.17, 132.08, 130.12, 127.85, 122.08, 115.86ppm; Mass m/z:214(100%)
2-Phenyl-1,3 benzoxazole (5)
FT-IR (KBr): 3429, 1620, 1508, 1180, 1096, 831 cm-1, d 6.7-8.6 (m, 8H) ppm; 13CNMR:d 157.6, 156.7, 143.1, 136.9, 131.3, 129.2, 128.7, 122.9, 116.1 ppm; Mass m/z:198(100%)
2-(1-naphthyl)-1,3 benzoxazole (6)
FT-IR (KBr): 3338, 1613, 1502, 1170, 775 cm-1, 1H NMR: d 6.43--9.23 (m,9H) ppm; 13C NMR:d 158.90, 156.31, 131.21, 129.43, 128.60, 127.29, 126.20, 125.45, 122.61, 115.72, 115.50, 115.24 ppm; Mass m/z:248 (100%)
2-(4-fluorophenyl)-1,3 benzoxazole (7)
FT-IR (KBr): 3429, 1606, 1502, 1226, 1152, 838 cm-1, 1H NMR: d 6.5-9.9(m,8H) ppm; 13C NMR:d 192.34, 164.95, 156.99, 148.49, 142.27, 132.46, 130.62, 122.44, 116.40 ppm; Maass m/z: 216(100%)
2-(4-methoxyphenyl)-1,3 benzoxazole (8)
FT-IR (KBr): 3059, 1613, 1516, 1446, 1257, 1160, 834 cm-1, 1H NMR: d 6.7-8.5 (m, 8H) ppm; 13C NMR:d 161.9, 157.0, 156.3, 143.4, 130.4, 129.8, 122.7, 116.1, 114.6, 55.8 ppm; m/z: 228 (100%)