Solvent-Free Synthesis of Azo Dyes Based on β-Naphthol Using Nano-γ-Al2O3/Ti(IV) by Grinding Method

INTRODUCTION
Azo compounds are attractive targets for organic synthesis methodology due to their widespread applications in many fields of technology and medicine [1-5]. Prior to the mid- nineteenth century, materials with coloring properties were extracted mainly from natural sources, mainly from animals or vegetables. However, natural colors were almost completely replaced by the synthetic colors at the beginning of the twentieth century. Today, almost, all dyes and pigments on the market, with the exception of some inorganic pigments, are synthetic materials. Hundreds of new color combinations enter the market every year and become a series of different application [6, 7]. 
According to the classification of dyes, 60-70% of them is azo compounds, in addition to common uses in science and technology, is used as optical materials in the processing of molecular data, ion sensors, and biological applications [8, 9]. The azo dye molecule contains one or more π systems that can produce different colors by changing the electron donor or electron acceptor functional groups. Traditionally, primary aromatic amines are diazotized in concentrated sulfuric acid, and the subsequent coupling process can only continue under acidic conditions. Previously, some acidic catalysts such as Nano-Fe3O4 encapsulated-silica supported boron trifluorid [10], silica sulfuric acid [11], Nano-CuFe2O4-supported sulfonic acid [12], Nano BF3·SiO2 [13], kaolin-SO3H [14], Nano silica chromic acid [15], has been used to synthesize azo dyes.
Supported Ti(V) catalyst is a powerful Lewis acid that is used to promote organic reactions [16-20]. In this research, we have prepared Nano-γ-Al2O3/Ti (IV) as a solid acid, characterize its structure, and use it in the synthesis of azo dyes. A schematic diagram of the proposed method is shown in Fig. 1.

MATERIALS AND METHODS
All chemical reagents were purchased from Fluka, Merck and Aldrich chemical companies and were used without any further purification. All of the products are known compounds which were characterized by comparison of their spectral (FTIR and 1H NMR), and physical data with authentic samples. 1H NMR spectra were recorded on a Bruker DRX-400 Avance spectrometer in CDCl3 and DMSO-d6 as solvents and chemical shift are expressed in δ ppm relative to tetramethyl silane. IR spectra were determined on a Nicolet Magna series FT-IR 550 spectrometer using KBr pellets. Thin layer chromatography (TLC) on commercial aluminium-backed plates of silica gel 60 F254 was used to monitor the progress of the reactions. Melting points were obtained with a micro melting point apparatus (Electrothermal, Mk3). The XRD patterns were collected on a Philips Xpert MPD diffractometer equipped with a Cu Kα anode (λ=1.54 Å) in the 2θ range from 10 to 80˚. Elemental composition was investigated by XRF BRUKER S4 EXPLORER. Average size of Nano-γ-Al2O3/Ti(IV) was analyzed by FESEM and TEM using a Mira 3-XMU and Philips CM120 with a LaB6 cathode and accelerating voltage of 120 kv, respectively. Brunauer–Emmett–Teller (BET) surface area analysis of nano-γ-Al2O3/Ti(IV) was done with Micromeritics, Tristar II 3020 analyzer. Quantitative elemental information (EDS) of Nano-γ-Al2O3/Ti(IV) alumina was measured by EDS instrument, Phenom pro X.

Preparation of Nano-γ-Al2O3 and Nano-γ-Al2O3/Ti(IV) 
NaOH (600 ml, 1 M), was added drop-wise to a slurry containing Al2(SO4)3.18H2O (66 g). The mixture was stirred at room temperature. The resulted suspension was filtered to obtain the white solid Al(OH)3. The solid were washed with distilled water until no more sulfate ions were detected in the washings. After that NaOH (100 ml, 1 M) was added to a beaker containing Al(OH)3 (20 g) to produce NaAl(OH)4. Then PEG 4000 (0.3%) was added to solution and it was neutralized with HCl (0.1 M), to pH 8 until Al(OH)3 produced again. The obtained precipitate filtered and washed with distilled water. The dried solid was calcined in the furnace at 800 ºC for 3 hours through atmospheric air to produce nano-γ-Al2O3 powder.
Follow the procedure below to prepare Nano-γ-Al2O3/Ti(IV). To a mixture of nano-γ-Al2O3 (1 g) and CH2Cl2 (10 ml), TiCl4 (0.5 ml) was added dropwise. The resulting suspension was stirred for 1 hour at room temperature, filtered, washed with chloroform, and dried at room temperature.

Typical procedure for the diazotization and azo coupling reactions 
Aromatic amines (2 mmol), Nano-γ-Al2O3/Ti(IV) (0.5 g) and sodium nitrite (3 mmol) were ground in a mortar with a pestle for a few minutes. In this step, aryl diazonium salt as a yellow solid was formed. Then, 2 mmol of 2-naphthol was added to obtained diazonium salt with vigorous grinding. The progress of the reaction was monitored by TLC (Ethyl acetate‒n-Hexan 7: 3). The crude product was resolved in acetone and the solid acid catalyst was separated by simple filtration. By adding water to filtrate, azo dyes as a pure orange solid were obtained. All the products were identified by a comparison of their melting points, FT-IR and 1H NMR spectra with published data.

1-(3-Nitrophenylazo)-2-hydroxy naphthalene (Table 2, Entry 1)
FT-IR (KBr)/ ῡ (cm-1): 3438, 1616, 1528, 1494, 1446, 1347, 1252, 1201, 834, 758, 734. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.2 (s, NH), 8.52 (t, 3J=8.4 Hz, 2H), 8.09 (d, 3J=7.2 Hz, 1H), 7.93 (s, 1H), 7.74 (d, 3J=9.2 Hz, 1H), 7.61 (m, 3J=7.2 Hz, 3H), 7.45 (d, 3J=6.8 Hz, 1H), 6.81 (d, 3J=9.2 Hz, 1H).

1-(2-Nitrophenylazo)-2-hydroxy naphthalene (Table 2, Entry 2)
FT-IR (KBr)/ ῡ (cm-1): 3426, 1606, 1568, 1480, 1448, 1313, 1192, 1127, 842, 744. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.6 (s, NH), 8.43 (br. s, 1H), 8.31 (br. s, 1H), 7.74 (d, 3J=6.0 Hz, 2H), 7.65 (br. s, 1H), 7.51 (d, 3J=8.0 Hz, 2H), 7.42 (s, 2H), 6.70 (br. s, 1H).

1-(4-Nitrophenylazo)-2-hydroxy naphthalene (Table 2, Entry 3)
FT-IR (KBr)/ ῡ (cm-1): 3436, 1594, 1501, 1452, 1331, 1201, 1152, 1104, 837, 748. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.2 (s, NH), 8.42 (d, 3J=7.6 Hz, 1H), 8.33 (d, 3J=8.4 Hz, 1H), 7.61 (d, 3J=8.8 Hz, 4H), 7.56 (d, 3J=7.2 Hz, 2H), 7.44 (d, 3J=6.8 Hz, 1H), 6.70 (d, 3J=9.2 Hz, 1H).

1-(2-Chlorophenylazo)-2-hydroxy naphthalene (Table 2, Entry 4)
FT-IR (KBr)/ ῡ (cm-1): 3439, 1617, 1554, 1499, 1447, 1258, 1207, 1142, 840, 752. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.5 (s, NH), 8.52 (s, 1H), 8.13 (s, 1H), 7.70 (d, 3J=8.8 Hz, 1H), 7.56 (s, 2H), 7.44 (dd, 3J=8 Hz, 3H), 7.19 (d, 3J=7.6 Hz, 1H), 6.83 (s, 1H).

1-(4-Chlorophenylazo)-2-hydroxy naphthalene (Table 2, Entry 5)
FT-IR (KBr)/ ῡ (cm-1): 3436, 1620, 1561, 1489, 1446, 1252, 1209, 1090, 821, 749, 497. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.1 (s, NH), 8.56 (d, 3J=8.0 Hz, 1H), 7.75 (d, 3J=9.2 Hz, 1H), 7.70 (d, 3J=8.8 Hz, 2H), 7.63 (d, 3J=8.0 Hz, 1H). 7.57 (t, 3J=8.4 Hz, 1H), 7.46 (d, 3J=8.4 Hz, 1H), 7.41 (d, 3J=8.0 Hz, 1H), 7.35(s, 1H), 6.90 (d, 3J=9.6 Hz, 1H). λmax (nm)= 484 (π-π*, N=N).

1-(2-Methoxyphenylazo)-2-hydroxy naphthalene (Table 2, Entry 6)
FT-IR (KBr)/ ῡ (cm-1): 3434, 1615, 1551, 1485, 1444, 1391, 1249, 1204, 1148, 1104, 1020, 837, 746. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.65 (s, NH), 8.52 (s, 1H), 8.07 (s, 1H), 7.67 (d, 3J=4.8 Hz, 1H), 7.54 (d, 3J=4.4 Hz, 3H), 7.37 (d, 3J=4.8 Hz, 1H), 7.10 (d, 3J=4.8 Hz, 1H), 7.01 (s, 1H), 6.80 (dd, 3J=4.8 Hz, 1H), 4.03 (s, 3H, Ar-OCH3).

1-(2-Methylphenylazo)-2-hydroxy naphthalene (Table 2, Entry 7)
FT-IR (KBr)/ ῡ (cm-1): 3435, 1618, 1554, 1504, 1450, 1255, 1205, 1151, 839, 753. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.7 (s, NH), 8.6 (br. s, 1H), 8.1 (br. s, 1H), 7.75 (br. s, 1H), 7.61 (br. s, 3H), 7.40 (br. s, 3H), 6.9 (br. s, 1H), 2.56 (s, 3H, Ar-CH3). 

1-(2, 4-Dimethylphenylazo)-2-hydroxy naphthalene (Table 2, Entry 8)
FT-IR (KBr)/ ῡ (cm-1): 3435, 1612, 1554, 1497, 1299, 1206, 1148, 815, 749. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.7 (s, NH), 8.6 (br. s, 1H), 7.9 (br. s, 1H), 7.84 (br. s, 1H), 7.63 (br. s, 3H), 7.11 (br. s, 3H), 6.94 (br. s, 1H), 2.53 (s, 3H, Ar-CH3), 2.38 (s, 3H, Ar-CH3).

1-(3-Hydroxyphenylazo)-2-hydroxy naphthalene (Table 2, Entry 9)
FT-IR (KBr)/ ῡ (cm-1): 3385, 1599, 1540, 1449, 1268, 864, 489. 1H NMR (400 MHz, DMSO, ppm) δ: 8.15 (d, 3J=8.1 Hz, 1H), 7.7 (d, 3J=7.7 Hz, 1H), 7.3 (m, 2H), 7.1 (m, 2H), 6.8 (d, 3J=6.8 Hz, 1H), 6.7 (t, 3J=6.7 Hz, 1H), 5.97 (t, 3J=5.9 Hz, 2H), 5.9 (d, 3J=5.9 Hz, 1H), 4.85 (s,1H, NH), 4 (s, 1H, OH). 

1-(4-Hydroxyphenylazo)-2-hydroxy naphthalene (Table 2, Entry 10)
FT-IR (KBr)/ ῡ (cm-1): 3426, 1595, 1445, 1264, 836. 1H NMR (400 MHz, DMSO, ppm) δ: 8.45 (d, 3J=8.4 Hz, 1H), 8.1 (d, 3J=8.1 Hz, 1H), 7.15 (t, 3J=7.14 Hz, 1H), 7.05 (d, 3J=7.06 Hz, 1H), 6.75 (t, 3J=6.7 Hz, 1H), 6.7 (d, 3J=6.7 Hz, 1H), 6.45 (d, 3J=6.4 Hz, 2H), 6.4 (d, 3J=6.3 Hz, 2H), 4.5 (s,1H, NH), 3.5 (s, 1H, OH). 

1-(4-Methylphenylazo)-2-hydroxy naphthalene (Table 2, Entry 11)
FT-IR (KBr)/ ῡ (cm-1): 3435, 1616, 1556, 1500, 1446, 1265, 1207, 1140, 815, 748, 498. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.2 (s, NH), 8. 4 (d, 3J=8.0 Hz, 1H), 7.74 (d, 3J=9.4 Hz, 1H), 7.69 (d, 3J=8.0 Hz, 2H), 7.21 (d, 3J=8.4 Hz, 1H), 7.57 (t, 3J=7.6 Hz, 1H), 7.40 (t, 3J=7.6 Hz, 1H), 7.30 (d, 3J=8.0 Hz, 2H), 6.94 (d, 3J=9.2 Hz, 1H), 2.39 (s, 3H, Ar-CH3). 

1-(Phenylazo)-2-hydroxy naphthalene (Table 2, Entry 12)
FT-IR (KBr)/ ῡ (cm-1): 3439, 1617, 1554, 1499, 1447, 1258, 1207, 1142, 840, 752. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.3 (s, NH), 8.58 (d, 3J=8.0 Hz, 1H), 7.74 (t, 3J=8.0 Hz, 3H), 7.61 (d, 3J=7.6 Hz, 1H), 7.57 (t, 3J=7.6 Hz, 1H), 7.49 (t, 3J=7.2 Hz, 2H), 7.41 (t, 3J=7.2 Hz, 1H), 7.33 (t, 3J=6.8 Hz, 1H), 6.88(d, 3J=8.8 Hz, 1H). 

1-(1-Naphthylazo)-2-hydroxy naphthalene (Table 2, Entry 13)
FT-IR (KBr)/ ῡ (cm-1): 3442, 3336, 3031, 2889, 1625, 1513, 1431, 1384, 1270, 1176, 813, 768, 568. 1H NMR (400 MHz, DMSO, ppm) δ: 8.1 (d, 3J=8.05 Hz, 1H), 7.7 (m, 1H), 7.6 (d, 3J=7.6 Hz, 1H), 7.3 (m, 6H), 7.15 (br. t, 1H), 7.05 (d, 3J=7 Hz, 1H), 6.65 (d, 3J=6.6 Hz, 1H), 5.7 (s, 1H, NH).

1-(4-Ethylphenylazo)-2-hydroxy naphthalene (Table 2, Entry 14)
FT-IR (KBr)/ ῡ (cm-1): 3436, 2956, 1618, 1558, 1504, 1446, 1307, 1253, 1206, 1141, 827, 752, 517. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.2 (s, NH), 8.62 (d, 3J=8.4 Hz, 1H), 7.74 (d, 3J=8.8 Hz, 1H), 7.71 (d, 3J=8.8 Hz, 1H), 7.64 (d, 3J=8.4 Hz, 1H), 7.57 (br. t, 1H), 7.40 (br. t, 1H), 7.33 (d, 3J=6.4 Hz, 3H), 6.94 (d, 3J=8.8 Hz, 1H), 2.72 (q, 3J=7.6 Hz, 2H, Ar-CH2CH3), 1.29 (t, 3J=7.6 Hz, 3H, Ar-CH2CH3).

1-(4-Bromophenylazo) -2-hydroxy naphthalene (Table 2, Entry 15)
FT-IR (KBr)/ ῡ (cm-1): 3435, 1619, 1560, 1489, 1251, 1208, 1145, 1069, 819, 750, 495. 1H NMR (400 MHz, CDCl3, ppm) δ: 16.1 (s, NH), 8.54 (br. s, 1H), 7.73 (d, 3J=8.4 Hz, 1H), 7.60 (br. s, 6H), 7.41 (d, 3J=9.2 Hz, 1H), 6.87(br. s, 1H).

RESULTS AND DISCUSSION
In continuation of our research on the applications of solid acids in organic synthesis, we have synthesized, Nano-γ-Al2O3/Ti(IV) as a new heterogeneous acidic reagent and investigated its efficiency in azo dyes preparation at room temperature under grinding conditions.
For identification of the structure of, Nano-γ-Al2O3/Ti(IV), we have studied FT-IR (ATR) spectra of TiCl4 (aq), nano-γ-Al2O3 and , Nano-γ-Al2O3/Ti(IV) (Fig. 2). In TiCl4 (aq) spectrum, a broad band at 3298 (H2O), a middle band at 1619 (Ti-Cl) and a strong band at 875 cm-1 (Ti-O) were observed (Fig 2a). In nano-γ-Al2O3 FT-IR spectrum, a very strong band at 600-1000cm-1 (Al-O) was observed (Figure 2b), Nano-γ-Al2O3/Ti(IV), in addition to γ-Al2O3 signal, two additional bands at 1619 and 3298 show binding of TiCl4 to γ-Al2O3 (Fig 2 c).
The FE-SEM images of the, Nano-γ-Al2O3/Ti(IV) and Nano-γ-Al2O3 Nanoparticles are displayed in Fig 3.
They exhibit irregular spherical shape for Nano particles below 50 nm. Energy-Dispersive X-ray Spectroscopy (EDS) of , Nano-γ-Al2O3/Ti(IV) was measured by EDS instrument provided the presence of the expected elements in the structure of this catalyst and confirmed supporting of TiCl4 on Nano-γ-Al2O3 (Fig 4). The elemental compositions of Nano-γ-Al2O3/Ti(IV) were found to be 58.5, 29.9 and 6.5% for O, Al and Ti, respectively. 
The X-ray diffraction (XRD) pattern of, Nano-γ-Al2O3/Ti(IV) is shown in Fig 5. According to XRD pattern of catalyst, the values of 2θ and FWHM are shown in Table 1. The signals at 2θ equal to 37 (b), 45 (c) and 67 (d) are shown Nano-γ-Al2O3 structure. According to XRD pattern, the two additional signals at 2θ equal to 32 (a) and 75 (e) with FWHM equal to 0.236 and 1.152 respectively, are shown the presence of bonded Ti to Nano-γ-Al2O3 (Fig 5).
The specific surface area of catalyst was measured by Brunauer–Emmett–Teller (BET) theory. Single point surface area at P/Po = 0.184317546 is 73.9645 m²/g and BET surface area is 75.592 m²/g. The N2 adsorption isotherm of catalyst is depicted in Fig 6.
Thermal gravimetric analysis (TG-DTA) pattern of, Nano-γ-Al2O3/Ti(IV) was detected by heating from 50°C to 400 °C and then cooling until 165°C (Fig 7).
Nano-γ-Al2O3/Ti(IV) is stable until 392 °C and only 10.5 % of its weight was reduced due to the removal of catalyst moisture. The char yield of the catalyst in 392°C is 89.5%. According to the TG-DTA diagram of, Nano-γ-Al2O3/Ti(IV) and our study, it was revealed that this reagent is suitable for organic reactions until 400 °C. 
In our opinion, Nano-γ-Al2O3/Ti(IV) can catalyze many organic reactions. This work, we wish to report a simple technique for the synthesis azo dyes based on β-naphthol using Nano-γ-Al2O3/Ti(IV) as a heterogeneous reagent in a green approach and solvent free by grinding method (Fig. 8). Our methodology has advantages such as short reaction time, no time consuming workup, no hazardous solvent and no column chromatography purification. The reaction gave quantitative yields and products formed smoothly under green reaction condition. The results of the synthesis of azo dyes by Nano-γ-Al2O3/Ti(IV) are summarized in Table 2.
The probable reaction mechanism of synthesis of azo dyes is outlined in Fig. 9. In this electrophilic aromatic substitution reaction, the primary aromatic amines (1) convert into its aryldiazonium salt. Aryldiazonium cation (2) is the electrophile and the activated β-naphthol (3) is a nucleophile. Diazonium salts are important synthetic intermediates that can undergo coupling reactions to form azo dyes (4).

CONCLUSION
In summary, we have synthesized azo dyes with using a solid acid reagent, nano-γ-Al2O3/Ti(IV), under grinding and solvent free condition. Short reaction times, high conversions, clean reaction profiles, simple work-up, non-hazardous and excellent yields are some advantages of this protocol. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.