1 Introduction

Hydrazones and Schiff bases are versatile class of organic compounds. It contains azomethines (R–CH=NR’) functionality, which is the common facture for both hydrazone and Schiff bases. These azomethine derivatives are interesting compounds because of their remarkable array of biological activity [1, 2]. The literature reviews have shown that Schiff’s base possess a broad range of physiological activities such as anti-microbial, antidyslipidemic, anthelmintic, anti-tuberculosis, anti-inflammatory, anti-convulsant, antitumor, anti-oxidant, anti-viral, antihypertensive and anti-diabetic activities [3]. Vicini et al. [4] have synthesized Benzo[d]isothiazole derivatives by the reaction of benzo[d]isothiazol-3-ylamine with substituted benzaldehydes in anhydrous benzene and the synthesized compounds were studied for biological evaluation. Meanwhile, owing to the unique R, R′-C=N–N-R″-R‴, structure (hydrazone) show a variety of antimicrobial activities, such as herbicide [5], anticonvulsant [6], antimicrobial [7], miticidal [8], antioxidant [9], and anticancer activities [10]. Recently, Rajarajan et al. [11] have synthesized phenylhydrazone derivatives and studied its antimicrobial activities.

Many green acid catalysts like BiCl3-K10 [12], MgSO4–PPTL [13], SiO2–NaHSO4 [14], PSSA [15], K-10 montmorillonite [16], Tandam catalyst [17], MgSO4–PPTS [18], Ti(OR)4 [19] and P2O5–SiO2 [20] have been utilized for Schiff base and hydrazone compounds synthesis. However, there is a noticeable development required in catalyst structures. Typically, in the heterogeneous catalysts based organic transformation reactions, catalysts are revealing its good acidity, high surface area, stable and recyclability. Besides, various metal ions (Mx+) and metal oxides (MxOy) modified mesoporous materials (MCM-41, SBA-15, KIT-6, and TUD-1) have also been employed as a solid acid catalyst for many industrial important organic conversions. Notably, isolated tungsten (W) or oxides of tungsten-based mesoporous catalyst have received much attention, due to its acidity and redox properties [21,22,23,24]. Kundu et al. [25] reported that the effective utilization of tungstic acid modified SBA-15 acid catalyst for one-pot condensation reaction of three components. Recently, W-TUD-1 [26] and WO4/TiTUD-1 [27] type mesoporous catalysts were tested as a solid acid catalyst for industrially relevant Prins cyclisation and esterification reactions. Hence, we have taken efforts for the synthesis of new WO3 modified AlTUD-1 catalyst and examined its activity for thiazole based aryl imines (1-substituted benzylidene 4-methylbenzo[d]thizole-2-amines) and substituted phenylhydrazone compounds preparation. These synthesized catalyst and products are characterized by different spectral techniques.

2 Experimental

2.1 Catalyst synthesis

AlTUD-1 with Si/Al molar ratio of 25 was synthesized by sol–gel, hydrothermal method based on our previously reported procedure [28]. In a typical catalyst preparation, 21 g of tetraethyl orthosilicate (TEOS, Sigma Aldrich) and aluminium isopropoxide (Al (isop)3, Sigma Aldrich) were stirred for 10 min. A mixture of 14 g of triethanolamine (TEA, SRL) and 5 g of water was then added to the above mixture and stirred vigorously for 1 h. Finally, 19.8 g of tetraethylammonium hydroxide (TEAOH, 35%, Sigma Aldrich) was slowly added to the prior solution under continuous stirring for another 2 h. The above-obtained gel was aged at 30 °C for 24 h and then dried at 100 °C for 24 h. The final solid was kept for hydrothermal treatment at (180 °C) for 8 h and successively calcined at 600 °C for 10 h to eliminate the organic molecules and water. 10 wt% of tungsten oxide supported on the amorphous AlTUD-1 catalyst (preheated 250 °C for 2 h) is prepared by the conventional impregnation method using tungstic acid (Aldrich) solution. The mixture was stirred for 12 h and then dried at 100 °C for 6 h. Finally, the sample was calcined at 550 °C for 6 h in an air atmosphere. The obtained material was mentioned as WO3/AlTUD-1.

2.2 Catalyst characterizations

Powder XRD patterns were obtained high angle region employed Rigaku diffractometer using Cu Kα ray (λ = 1.5418 Å). FT-IR spectrum is measured on a Bruker spectrometer (Tensor) in DRIFT resolution of 4 cm−1. BET the specific surface area and BJH pore size distribution were determined from nitrogen sorption using Quantachrome (QuadrasorbSI) porosimeter equipment (77 K) at liquid nitrogen temperature. The morphology was examined by scanning electron micrographs (SEM) imaging using ESEM Quanta 200 with a resolution of 10 kV. Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 fitted with a CCD camera. Diffuse reflectance ultraviolet–visible (DR UV–Vis) spectrum of the sample was recorded on a Thermoscientific spectrometer (Evolution 600) with a diffuse reflectance attachment, using BaSO4 as the reference. FT-Raman spectra of the samples were attained on a Bruker system 1000R using 1064 nm (Nd:YAG laser source) excitation source.

2.3 Synthesis of 1-substituted benzylidene-4-methylbenzo[d]thizole-2-amine over WO3/AlTUD-1

In a typical reaction, equimolar quantities of substituted benzaldehyde (0.01 mol), 4-methylbenzo[d]thiazol-2-amine (0.01 mol) and 20 mL of absolute ethanol with 0.1 g of WO3/AlTUD-1 catalyst were added to 50 mL round bottom (RB) flask fitted with temperature controlled oil bath and magnetic stirrer (Scheme 1). This solution was refluxed for 4 h and the reaction progress is monitored by TLC. After completion, the reaction mixture was cooled and washed with water. The product was recrystallized by ethanol to obtain as pale yellow solid. The uniformity of the final products was monitored by ascending thin layer chromatography (TLC) on silica gel-G. All the solvents used were analytical reagent grade. FT-IR spectra of the products were recorded on a Shimadzu-FT-IR spectrometer (range 4000–400 cm−1) in KBr pellets. The UV Visible absorption spectra under investigation were recorded on Shimadzu-1650 in spectral grade methanol (λmax in cm−1). 1H NMR and 13C NMR spectra for analytical purpose were recorded in CDCl3 on a Bruker instrument at 400 MHz.

Scheme 1
scheme 1

Synthesis of aryl imines over WO3/AlTUD-1 catalyst

The products characterizations are shown below:

  • (E)-N-benzylidene-4-methylbenzo[d]thiazol-2-amine (3a): C15H12N2S; M.pt: 161–162 °C; UV Vis: 345, 222; FTIR (KBr, υ/cm−1): (Ar–CH) 3024, (Ali–CH) 2922, (C=Nthiazole) 1562, (CH=N) 1602, (C–S–C) 754; 1H (NMR) (400 MHz, CDCl3, TMS): δ 7.07–8.19 (8H, m, Ar–H), 2.60 (3H, s, CH3), 8.11 (1H, s, N=CH); 13C NMR (400 MHz, CDCl3): δ 18.1 (CH3), 169.4 (C=Nthiazole), 146.1 (C=N), 145.6–118.9 (aromatic carbons).

  • (E)-N-(4-chlorobenzylidene)-4-methylbenzo[d]thiazol-2-amine (3b): C15H11N2SCl. M.pt: 130–131 °C; UV Vis: 323, 271; FTIR (KBr, υ/cm−1): (Ar–CH) 3065, (Ali–CH) 2922, (C=Nthiazole) 1551, (CH=N) 1613, (C–S–C) 745; 1H NMR (400 MHz, CDCl3, TMS): δ 7.06–7.65 (8H, m, Ar–H), 2.59 (3H, s, CH3), 7.88 (1H, s, N=CH); 13C NMR (400 MHz, CDCl3): δ 18.03 (CH3), 167.4 (C=Nthiazole), 150.6 (C=N), 142.8–118.7 (aromatic carbons).

  • (E)-4-methyl-N-(4-methylbenzylidene)benzo[d]thiazol-2-amine (3c): C16H14N2S. M.pt.:107–108 °C; UV Vis: 340, 225; FTIR (KBr, υ/cm−1): (Ar–CH) 3064, (Ali–CH) 2922, (C=Nthiazole) 1551, (CH=N) 1612, (C–S–C) 744; 1H NMR (400 MHz, CDCl3, TMS): δ 7.06–7.65 (8H, m, Ar–H), 2.55 (3H, s, CH3), 2.39 (3H, s, CH3), 7.87 (1H, s, N=CH); 13C NMR (400 MHz, CDCl3): δ 18.1 (CH3), 167.4 (C=Nthiazole), 150.6 (C=N), 142.8–118.7 (aromatic carbons).

  • (E)-N-(4-methoxybenzylidene)-4-methylbenzo[d]thiazol-2-amine (3d): C16H14N2OS. M.pt.: 149–150 °C; UV Vis: 338, 227; FTIR (KBr, υ/cm−1): (Ar–CH) 3061, (Ali–CH) 2924, (C=Nthiazole) 1508, (CH=N) 1601, (C–S–C) 741; 1H NMR (400 MHz, CDCl3, TMS): δ 6.92–7.63 (8H, m, Ar–H), 2.54 (3H, s, CH3), 3.85 (3H, s, OCH3), 7.83 (1H, s, N=CH); 13C NMR (400 MHz, CDCl3): δ 18.1 (CH3), 55.4 (OCH3), 168.1 (C=Nthiazole), 161.0 (C=N), 148.6–114.2 (aromatic carbons).

  • (E)-4-methyl-N-(3-nitrobenzylidene)benzo[d]thiazol-2-amine (3e): C15H11N3O2S. M.pt.: 189–190 °C; UV Vis: 340, 226; FTIR (KBr, υ/cm−1): (Ar–CH) 3088, (Ali–CH) 2976, (C=Nthiazole) 1533, (CH=N) 1599, (C–S–C) 723; 1H NMR (400 MHz, CDCl3, TMS): δ 7.07–8.22 (8H, m, Ar–H), 2.52 (3H, s, CH3), 8.48 (1H, s, N=CH); 13C NMR (400 MHz, CDCl3): δ 17.9 (CH3), 167.4 (C=Nthiazole), 148.7 (C=N), 141.9–119.1 (aromatic carbons).

2.4 Synthesis of hydrazones over WO3/AlTUD-1

In a typical reaction, equimolar quantities of 3-fluoro benzaldehyde (0.01 mol), substituted phenylhydrazine (0.01 mol) and 20 mL of ethanol with 0.1 g of WO3/AlTUD-1 catalyst were added to 50 mL round bottom (RB) flask fitted with magnetic stirrer (Scheme 2). This solution was refluxed for 4 h and the reaction progress is monitored by TLC. After completion, the reaction mixture was cooled and washed with water. The product was recrystallized by ethanol to obtain as glittering solids. The uniformity of the final products was monitored by ascending TLC on silica gel-G.

Scheme 2
scheme 2

Synthesis of phenylhydrazones over WO3/AlTUD-1 catalyst

The obtained products characterizations are shown below:

  • (E)-4-(2-(3-fluorobenzylidene)hydrazinyl) benzonitrile (6a): C14H10FN3; Mol.Wt. 239; M.pt.: 190–191 °C; UV Vis: 346, 288; FTIR (KBr, υ/cm−1): 3045 (Ar–CH), 3263 (Ar–NH), 1595 (CH=N); 1H NMR (400 MHz, CDCl3, δ, ppm): 7.24–7.92 (m, 8H, Ar–H), 7.82 (S, 1H, NH) 7.98 (S, 1H, –N=CH–); 13C NMR (100 MHz, CDCl3, δ, ppm): 148.3 (CH=N), 144.5–106.3 (aromatic carbons).

  • (E)-1-(4-bromophenyl)-2-(3-fluorobenzylidene) hydrazine (6b): C13H10BrFN2; Mol.Wt. 292; M.pt.: 100–101 °C; UV Vis: 338, 248; FTIR (KBr, υ/cm−1): 3078 (Ar–CH), 3255 (Ar–NH), 1647 (CH=N); 1H NMR (400 MHz, CDCl3, δ, ppm): 7.14–7.85 (m, 8H, Ar–H), 7.86 (S, 1H, NH) 8.03 (S, 1H, –N=CH–); 13C NMR (100 MHz, CDCl3, δ, ppm): 143.2 (CH=N), 137.1–112.1 (aromatic carbons).

  • (E)-1-(3-fluorobenzylidene)-2-(4-tolyl) hydrazine (6c): C14H13FN2, Mol.Wt. 228; M.pt.: 122–123 °C; UV Vis: 338, 248; FTIR (KBr, υ/cm−1): 3055 (Ar–CH), 3294 (Ar–NH), 1618 (CH=N); 1H NMR (400 MHz, CDCl3, δ, ppm): 7.57–7.10 (m, 8H, Ar–H), 7.74 (S, 1H, NH) 7.91 (S, 1H, –N=CH–); 2.28 (S, 3H, –CH3); 13C NMR (100 MHz, CDCl3, δ, ppm): 147.9 (CH=N), 140.5–101.7 (aromatic carbons), 28.7 (CH3).

  • (E)-1-(4-chlorophenyl)-2-(3-fluorobenzylidene) hydrazine (6d): C13H10ClFN2, Mol.Wt. 248; M.pt.: 112–113 °C; UV Vis: 351, 310; FTIR (KBr, υ/cm−1): 3053 (Ar–CH), 3315 (Ar–NH), 1591 (CH=N); 1H NMR (400 MHz, CDCl3, δ, ppm): 7.00–7.90 (m, 8H, Ar–H), 7.62 (S, 1H, NH) 7.92 (S, 1H, –N=CH–); 13C NMR (100 MHz, CDCl3, δ, ppm): 142.3 (CH=N), 141.4–114.3 (aromatic carbons).

3 Results and discussion

3.1 Catalyst characterization

The high angle (2θ = 5°–80°) XRD patterns of AlTUD-1 and typical WO3/AlTUD-1 catalysts are depicted in Fig. 1. AlTUD-1 and WO3/AlTUD-1 exhibits a broad diffraction peak in the 2θ range of 10°–30°, due to the presence of amorphous silica (SiO2). An absence of alumina peaks in XRD patterns of AlTUD-1, confirmed the complete incorporation of Al3+ into TUD-1 framework [28, 29]. Besides, the XRD patterns of the WO3/AlTUD-1 catalyst displayed the minimal diffraction peaks at 23.6°, 33.6° and 54.5° related to WO3 crystalline species [23, 26]. This clearly indicated that the WO3 species are finely dispersed on internal porous walls and an external surface of TUD-1.

Fig. 1
figure 1

Wide angle XRD of AlTUD-1 and WO3/AlTUD-1

The N2 adsorption desorption isotherms and pore size distribution of typical WO3/TUD-1 catalyst are represented in Fig. 2. According to the IUPAC classification, type IV isotherm with H2 hysteresis loop is exhibited, indicating the characteristic of TUD-1 type wormhole pore architectures [23, 26, 29]. The steep desorption and sloping adsorption clearly indicate that the presence of interconnected porous networks [26]. Moreover, the isotherm showed a sharp variation at a relative pressure between 0.6 and 0.9 representing the typical capillary condensation within pores. It is found that catalyst exhibited a surface area of 510 m2 g−1. The pore size distribution (Fig. 2b) measured from the desorption branch using the BJH model with a peak of around 6.4 nm. Also, the pore size distribution of the catalyst had narrow pore size distribution and the sharpness indicated the uniformity of mesopore distribution.

Fig. 2
figure 2

a N2 adsorption–desorption isotherms and b BJH pore size distribution of WO3/AlTUD-1

The FT-IR spectra of WO3/AlTUD-1 and AlTUD-1 catalyst are shown in Fig. 3. The catalysts exhibited the bands at 3450, 1640, 1223, 1100 and 803 cm−1. The bands at 3450 cm−1 and 1640 cm−1 are mainly caused by the O–H stretching vibration and bending vibration mode of the adsorbed water molecules or surface silanol groups (Si–O–H). Further, the peaks observed at 1223, 1100 and 803 cm−1 are due (Si–O–Si) asymmetric and symmetric stretching vibrations. The similar type of vibration bands were reported for metal (Co, Sn, Ti, Zr) containing TUD-1 silicates [29,30,31,32]. Noticeably, the vibration bands corresponding for WO3 in the WO3/AlTUD-1catalyst are shielded by the bands of AlTUD-1.

Fig. 3
figure 3

FTIR spectra of AlTUD-1 and WO3/AlTUD-1 sample

The FT-Raman spectra of WO3/AlTUD-1, AlTUD-1, and WO3 are presented in Fig. 4. In comparison with WO3/AlTUD-1 and bulk WO3 four peaks were noticed at 807, 718, 326 and 273 cm−1 which confirms the presence of crystalline WO3 species [23]. These peaks are assigned to the deformation mode of W–O–W, symmetric stretching mode of W–O and bending mode of W–O, respectively [26, 27]. Apart from WO3 peaks, an intense band at 976 cm−1 was observed, which can be assigned to poly tungstate species or terminal bond W=O of the tetrahedrally coordinated WO3 over the AlTUD-1 surface [27, 33]. Bhuiyan et al. [34] had assigned 970 cm−1 peak as the terminal W=O (symmetric stretching) mode of tungsten oxide (tetrahedrally coordinated) species, which are active sites for metathesis reactions.

Fig. 4
figure 4

FT-Raman spectra AlTUD-1, WO3/AlTUD-1 and bulk WO3

SEM and TEM micrographs of WO3/AlTUD-1 are shown in Fig. 5. From, the SEM image (Fig. 5a), the sample showed the disordered shapes with micrometer size of SiO2 particles with dispersed WO3 particles. The sponge- or wormhole-like three-dimensionally connected mesopore network which is typical for TUD-1 materials was further confirmed by TEM [31, 32] (Fig. 5b). Furthermore, due to low WO3 loading and contrast, it was difficult to identify the metal oxide phases in TEM image. The presence of Al, W and Si are confirmed by the ICP-OES and its quantities are similar to synthesis composition.

Fig. 5
figure 5

a SEM and b TEM images of WO3/AlTUD-1

The UV–vis diffuse reflectance absorption spectrum of WO3/AlTUD-1 is displayed in Fig. 6. A broad peak around 200–450 nm in the catalyst is due to the overlapping bands of isolated tetrahedral [WO4]2− species (240 nm), low oligomeric tungsten oxide species (290 nm) and crystalline WO3 (380 and 450 nm) [21, 26, 27]. However, the absorbance peak values are varied based on the W species (incorporated W or WOx) and the architecture of mesoporous materials. The FT-IR spectrum of pyridine adsorbed on WO3/AlTUD-1 catalyst in the region of 1650–1400 cm−1 is depicted in Fig. 7. The bands at 1595 and 1447 cm−1 are due to the pyridine bonded to the Lewis acid sites [30, 32]. On the other hand, the band at 1492 cm−1 is due to the combinations of Lewis and Bronsted acid sites [26]. Besides, AlTUD-1 leads to an increase in the Lewis acidity.

Fig. 6
figure 6

DR UV Vis of WO3/AlTUD-1

Fig. 7
figure 7

FT-IR spectrum of pyridine adsorbed WO3/AlTUD-1

3.2 Catalyst reactions

The reaction conditions such as time, solvent, catalyst amount and temperature were optimized by screening as given in Schemes 1 and 2. Maximum yield was obtained at 80 °C temperature, 20 mL of the solvent (ethanol), 0.1 g of catalyst and 4 h refluxing time. Further, we extended these optimized reaction conditions to differently substituted benzaldehydes with 4-methyl substituted benzo[d]thiazol-2-amine and the obtained results are presented (Table 1). Notably, 83% of the maximum yield was obtained using the WO3/AlTUD-1 as a catalyst. Substituted benzaldehyde with groups –Cl, –NO2, –OCH3 and –CH3 (electron donating and withdrawing) groups in para position produce the corresponding products in good yields (70–80%). Similarly, 3-fluro aldehyde reacts with different electron withdrawing and electron donating substituted hydrazine produces good yields (75–80%) as shown in Table 2. The yield of products clearly indicates the good dispersion of active WO3 and incorporated Al3+ species on the large porous TUD-1 support materials and interactions taking place between WO3 species and the AlTUD-1 support [27].

Table 1 Synthesis of different group substituted (E)-N-benzylidene-4-methylbenzo[d]thiazol-2-amines compounds
Table 2 Synthesis of different phenylhydrazone compounds

The generally accepted mechanism of Schiff base reaction was proposed by many researchers [35, 36]. By following the mechanism reported by Kumar et al. [37] acid catalysed reaction mechanism is proposed in Scheme 3. The mechanism involves the protonation of both Schiff base and aldehyde with acidic WO3/AlTUD-1 catalyst to option intermediate (1) and (2). This protonated Schiff base intermediate (1) attacked the carbonyl carbon of the intermediate (2) which gives intermediate (3) followed by proton transfer from nitrogen to oxygen to give an intermediate (4). This on further dehydration and deprotonation to produces imines. The solid acid catalyst (WO3/AlTUD-1) promotes dehydration and deprotonation.

Scheme 3
scheme 3

The proposed mechanism for the formation Schiff base over WO3/AlTUD-1 catalyst

4 Conclusions

In summary, finely dispersed tungsten oxide (WO3) introduced mesoporous AlTUD-1 support was successfully synthesized by a modest synthesis procedure. The characterization studies confirmed amorphous, mesoporous and wormhole nature of the catalyst. This WO3/AlTUD-1 catalyst was presented as an efficient catalyst for synthesis of 1-substituted benzylidene-4-methylbenzo[d]thizole-2-amines (thiazo aryl imines) and phenylhydrazones ~ 70–80% of the yield in the optimum reaction conditions. The solid acid catalyst (WO3/AlTUD-1) promotes dehydration and deprotonation owing to the presence of acidic sites (B + L). The catalytic activity related to framework incorporated Al3+, dispersed WO3 and the good accessibility of these active sites to the reactants. Hence, we developed an efficient and simple alternative for the preparation of substituted benzo[d]thiazol-2-amines and phenylhydrazones via three-dimensional mesoporous acid catalyst under feasible reaction method. Further the studies will extend against other microbial species. Also, this catalyst can be exploited for the study of other heterogeneous acid catalysed organic transformations.

5 Supplementary information (SI)

The UV visible, FTIR and NMR (1H and 13C) spectral data of selective compounds are available in supplementary information (Figures S1–S12).