Preparation, characterization and catalytic activity of MoO₃/CeO₂–ZrO₂ solid heterogeneous catalyst for the synthesis of β-enaminones

2.1 Catalyst Preparation

The CeO₂–ZrO₂ (1:1) binary oxides were prepared by a homogeneous co-precipitation method. An aqueous solution containing the requisite quantities of zirconium oxychloride and ammonium ceric nitrate was prepared separately by using deionized water and mixed together with constant stirring followed by the addition of 20 ml 5% polyethylene glycol (PEG-400) as structure directing agent. This solution was hydrolyzed with 1:1 aqueous ammonia with constant stirring until the pH of solution reached to 9. A yellowish precipitate was formed and the precipitate was allowed to settle down in an electric oven at 60 °C for 24 h. The resulting precipitate was filtered and washed with deionized water to remove the chloride and dried at 120 °C for 12 h.

The MoO₃/CeO₂–ZrO₂ materials, containing 8–20 wt% MoO₃ were prepared by impregnation method. To incorporate the molybdenum oxide, the requisite quantities of ammonium heptamolybdate were dissolved in excess water and the finely powdered oven-dried hydrous ceria–zirconia support was added to this solution, each mixture was stirred at 80 °C for 6 h. The excess water was air dried completely and the resulting sample was dried at 120 °C for 12 h. Finally the dried powders were calcined at 500 °C for 5 h in air atmosphere.

2.2 Catalyst characterization

The X-ray powder diffraction patterns of catalyst were recorded on Bruker 8D advance X-ray diffractometer using Cu Kα radiation of wavelength = 1.54060 Å, with the scanning rate of 2°/min was used for all samples. The FT-IR spectrums were recorded on JASCO-FTIR/ 4100 Japan, using dry KBr as standard reference in the range of 4000–500 cm⁻¹. In order to understand the surface morphology and to assess the surface dispersion of MoO₃ supported on CeO₂–ZrO₂, the SEM analysis were carried out with JEOL; JSM-6330 LA operated at 20.0 kV and 1.0000 nA. The presence of metal and their elemental composition were recorded by using energy dispersive spectroscopy.

2.3 General procedure for the synthesis of β-enaminones

Substituted aniline (1 mmol) and dimedone (1 mmol) were taken in 50 ml beaker and 0.2 g catalyst was added and well mixed with the help of glass rod. The resulting mixture was subjected to microwave irradiation at 450 W for an appropriate time (Table 3). After completion of the reaction (as monitored by TLC), 10 ml ethyl acetate was added to the reaction mixture to separate the catalyst by filtration. The solvent was evaporated and crude product was obtained. The products were characterized by ¹H NMR, IR and Mass spectroscopic techniques.

Spectral data for some of the representative compounds:

(3a): ¹H NMR (400 MHz, CDCl₃): δ 7.7 (s, 1H), 7.3 (d, 2H), 7.1 (d, J = 7.1 Hz, 3H), 5.5 (s, 1H), 2.5 (s, 2H), 2.3 (s, 2H),1.0 ppm (s, 6H); FTIR (KBr): 3429, 3070, 1490, 1379,1280, 1606 cm⁻¹ ; m/z: 215.

(3b): ¹H NMR (400 MHz, CDCl₃): δ 7.5 (s, 1H), 7.2 (d, J = 7.3 Hz, 2H), 7.1 (d, J = 7.1 Hz, 2H), 5.5 (s, 1H), 2.3 (s, J = 2.3 Hz, 2H), 2.2 (s, 2H), 1.1 ppm (s, 6H). FTIR (KBr): 3429, 3074, 1493, 1379, 1280,1606, 605 cm⁻¹; m/z: 250.

3.1 XRD analysis

The powder X-ray diffraction pattern of (a) pure CeO₂–ZrO₂ and (b–f) 8–20 wt% MoO₃ supported CeO₂–ZrO₂ catalysts calcined at 500 °C in the presence of air for 5 h are shown in Fig. 1. The unsupported CeO₂–ZrO₂ sample exhibits broad diffraction patterns due to the poor crystallinity. As can be noted from Fig. 1(a), that the diffraction patterns of unsupported CeO₂–ZrO₂ and MoO₃ supported CeO₂–ZrO₂ samples differ very much indicating a strong influence of the impregnated molybdenum oxide on the surface of CeO₂–ZrO₂. From the close inspection of XRD patterns pertaining to the unsupported CeO₂–ZrO₂ samples, an interesting observation to be noted is that the pure CeO₂–ZrO₂ sample exhibits diffraction patterns due to the formation of cubic solid solution, the highly intense and sharp peaks were obtained at 2θ = 28.90°, 33.56°, 48.23°, 57.23° and 78.00° corresponding to the planes (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (3 3 1), respectively, indicate the cubic structure of CeO₂–ZrO₂ [JCPDS card no. 280271]. While the orthorhombic phase could be identified in the case of 8–20 wt% MoO₃/CeO₂–ZrO₂, the XRD patterns are displayed in Fig. 1(b–f). In each case highly intense and sharp peaks were obtained at 2θ = 22.51°, 25.70°, 29.17°, 41.86°, 45.32°, 48.93°, 52.87°, 57.12°, 62.65°, 68.63° corresponding to the planes (5 0 1), (4 1 1), (8 0 0), (7 2 0), (4 2 2), (1 1 0 2), (1 2 3), (0 1 4), (8 0 4), (0 4 0), respectively. It has been seen that, X-ray powder diffraction patterns of MoO₃ promoted CeO₂–ZrO₂ samples are crystalline in nature, indicating a strong interaction between the MoO₃ and CeO₂–ZrO₂ support.

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Figure 1

XRD patterns of series of (a) unsupported CeO₂–ZrO₂, (b) 8%, (c) 10%, (d) 12%, (e) 15%, (f) 20 wt% MoO₃/CeO₂–ZrO₂ calcined at 500 °C.

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3.2 FT-IR analysis

The FT-IR spectrums of series of MoO₃/CeO₂–ZrO₂, using dry KBr as standard reference in the range of 4000–500 cm⁻¹ are shown in Fig. 2. From the IR spectrum it was observed that in each sample (a–f) the peak in the range 3200–3500 cm⁻¹ appeared due to the hydroxyl group adsorbed on the surface of each catalyst and which helps to generate the Bronsted acidic sites. Similarly in all the cases the peak is in the range of 1500–1600 and 1410–1446 cm⁻¹ due to the deformation of surface hydroxyl group (Lagashetty et al., 2007). As can be noted from the spectrum of each sample, the peak at in the range of 1010–1079 cm⁻¹ is assigned for the M–O–M bonding (M = Mo, Ce, Zr). The bands in the range 500–991 cm⁻¹ were assigned to the Mo⚌O, Mo–O–Mo stretching modes of vibration for crystalline MoO₃ (Sohn and Chun, 2003). The appearance of these bands indicates that, the MoO₃ is strongly dispersed on the surface of CeO₂–ZrO₂ catalyst.

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Figure 2

FT-IR spectrum of (A) unsupported CeO₂–ZrO₂, (B) (a) 8%, (b) 10%, (c) 12%, (d) 15%, (e) 20 wt% MoO₃/CeO₂–ZrO₂ calcined at 500 °C.

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3.3 SEM analysis

Typical SEM images of series of MoO₃/CeO₂–ZrO₂ catalytic materials are presented in Fig. 3. As can be seen from Fig. 3(a) which shows the SEM image of pure CeO₂–ZrO₂ sample, it is clear that particles are homogeneously aggregated and which are irregular in shape. Also some porosity is generated and that may be due to the addition of 5% PEG as a structure directing agent. Fig. 3(b–f) shows SEM image of 8–20 wt% loaded MoO₃, it was observed that, MoO₃ is strongly interacted and highly dispersed on the surface of the CeO₂–ZrO₂ catalyst.

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Figure 3

SEM micrograph of (a) unsupported CeO₂–ZrO₂, (b) 8%, (c) 10%, (d) 12%, (e) 15%, (f) 20 wt% MoO₃/CeO₂–ZrO₂ calcined at 500 °C.

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3.4 EDS analysis

The presence of metals and elemental compositions of the 20 wt% MoO₃/CeO₂–ZrO₂ catalyst are shown in Fig. 4, represents the elemental distribution of at Mo, Ce, Zr, and O as (0.53 at.%), (29.18 at.%), (12.67 at.%), and (57.62 at.%), respectively.

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Figure 4

EDS Spectrum of 20 wt% MoO₃/CeO₂–ZrO₂ calcined at 500 °C.

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3.5 Crystallite size calculation

The particle sizes of different wt% MoO₃ supported CeO₂–ZrO₂ samples estimated using Debye–Scherrer equation: $$T=0.94\lambda /\beta \cos \theta \text{,}$$ where T = particle size, λ = wavelength, θ = diffraction angles, β = FWHM (full width half maximum).

From Table 1, it was observed that the particle size and crystallinity (from XRD) increases with increase in Mo wt% loading on CeO₂–ZrO₂.

3.6 Catalytic activity results

In order to evaluate the catalytic activities of 0, 8, 10, 12, 15 and 20 wt% MoO₃ supported on CeO₂–ZrO₂ catalysts are investigated using Scheme 1. The results in terms of reaction time and product yields obtained are summarized in Table 2. Catalytic activity results reveal that, the pure CeO₂–ZrO₂ material exhibited negligible activity in comparison with the other 8–20 wt% MoO₃ loaded CeO₂–ZrO₂ catalysts, which may be due to the poor crystallinity of the CeO₂–ZrO₂ material (Fig. 1(a)). From the X-ray diffraction patterns (Fig. 1(b–f)) it was observed that, the crystallinity increases with increase in Mo wt% loading, which indicates that, molybdenum oxide is highly crystalline in nature. Also, Table 2 shows, the 20 wt% MoO₃/CeO₂–ZrO₂ catalyst exhibits good catalytic activity for the synthesis of β-enaminones probably that might be due to the small particle size and possess better acidic sites as the acidic nature of molybdenum oxide (Williams et al., 1991; Carbucicchio and Trifiro, 1980; Bruckman et al., 1993; Ono et al., 1987; Ma et al., 2004). Therefore, it was further used as a catalyst to prepare the various derivatives of β-enaminones (Table 3). A variety of different substituted anilines possessing an electron donating (CH₃, OCH₃, OH) and electron withdrawing groups (NO₂) offered good yield (82–95%) and reactions were completed in a very short time. We have also examined some heteroaromatic anilines, which gave reasonably good yields but demanded little more reaction time after the crystallization oily drops are formed. From the results obtained it is concluded that, the catalytic activity increases with increase in Mo wt% loading (8–20%).

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Scheme 1

The synthesis of β-enaminones catalyzed by 20 wt % MoO₃/CeO₂-ZrO₂.

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Table 3 Synthesis of β-enaminones derivatives catalyzed by 12 wt% MoO₃/CeO₂–ZrO₂ catalyst in microwave.

Compound	Ar–NH2	Time (min)	Yield (%)a	MP (°C)
3a		3	95(95, 95, 94)b	183–184
3b		3	92	190–192
3c		3	95	195–196
3d		2	85	124–126
3e		3	92	194–196
3f		2	93	193–195
3g		8	86	(Viscous oil)
3h		8	82	(Viscous oil)

a Isolated yields.

b After consecutive run.

Furthermore, we have investigated the reusability and recycling of 20 wt% MoO₃/CeO₂–ZrO₂ catalyst. At the end of reaction, the catalyst was recovered by simple filtration from the reaction mixture and washed with n-hexane and subjected to a second run of the reaction process. To assure that the catalyst was not dissolved in n-hexane, the catalyst was dried at 60 °C and activated at 100 °C for 1 h in (HTMF), after filtration and before using and reusing for the next reaction. The result shows that these catalysts are not soluble in n-hexane. The comparison of efficiency of 20 wt% MoO₃/CeO₂–ZrO₂ in the synthesis of 3a after three times is reported (Table 3, entry 3a). There were no significant decreases in efficiency of the recovered catalyst compared to the fresh material. The catalyst was reusable at least up to three times.