Solid state synthesis, characterization, optical properties and cooperative catalytic performance of bismuth vanadate nanocatalyst for Biginelli reactions

Abstract

Bi₂V₂O₇ nano powders were synthesized via a solid state reaction at 500 °C for 8 h using Bi(NO₃)₃ and VO(acac)₂ at stoichiometric 1 : 1 Bi : V molar ratio as raw materials. The synthesized material was characterized by the powder X-ray diffraction (PXRD) technique and Fourier-transform infrared (FTIR) spectroscopy. Structural analysis was performed by the FullProf program employing profile matching with constant scale factors. The results showed that the pattern had a main Bi₂V₂O₇ structure with a space group of Fd3̄m. Lattice parameters were found as a = b = c = 10.259079 Å, α = γ = β = 90°. FESEM and TEM images showed that the synthesized Bi₂V₂O₇ particles had mono-shaped sphere morphologies. Ultraviolet-visible spectrum analysis showed that the nanostructured Bi₂V₂O₇ powders possessed strong light absorption properties in the ultraviolet light region. Catalytic performance of the synthesized nanomaterial was also investigated in Biginelli reactions which showed excellent efficiency.

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1. Introduction

Among oxides and fluorides with general formula A₂B₂X₇ (where A is a medium-large cation and B is an octahedrally coordinated, high-charge cation), there are many compounds widely studied for their great technological properties.^1,2 The pyrochlore phase is understood as a defect structure of fluorite type with general formula A₂B₂O₆ or A₂B₂O₇. This structure exhibits ccp arrays of cations with anions partially occupying the tetrahedral sites to form a distorted structure containing intrinsic defects. Research on pyrochlore materials have been focused on applications such as catalysis,^3,4 electronics,⁵ optical⁶ and magnetic properties⁷ and solid oxide fuel cell (SOFC) electrode materials.⁸ Bi-containing perovskites have received a lot of attention as lead-free ferroelectric^9,10 and multi-ferroic^11,12 materials. Among them, bismuth vanadate semiconductor materials have also received great interest for their photocatalytic application for splitting of water into hydrogen and oxygen^13–16 and organic wastewater treatment.^17–19 Several compounds of Bi–V–O have been previously synthesized via different methods including BiVO_3.2,²⁰ BiVO₃, BiVO₄, Bi₄V₂O_10.5, Bi_1.62V₈O₁₆,²¹ different phases of Bi₂VO_5.5,²² Bi₄V₂O₁₁ and Bi₄V₂O₁₀,²³ Bi₄V₂O₁₁,²⁴ Bi₂VO_5.5 and so on.²⁵

The Biginelli reaction, which was originally reported by Biginelli in 1891,²⁶ is a methodology for the synthesis of 3,4-dihydropyrimidin-2-(1H)-one derivatives (DHPMs) in a one-step procedure. DHPMs have already shown biological activities.²⁷ Several metal oxides have been reported as nanocatalysts for the Biginelli reactions including alumina supported Mo catalysts,²⁸ nano ZnO as a structure base catalyst,²⁹ MoO₃–ZrO₂ nanocomposite,³⁰ MnO₂–MWCNT nanocomposites,³¹ TiO₂ nanoparticles,³² Mg–Al–CO₃ and Ca–Al–CO₃ hydrotalcite,³³ Bi₂O₃/ZrO₂ nanocomposite,³⁴ ZrO₂–Al₂O₃–Fe₃O₄,³⁵ imidazole functionalized Fe₃O₄@SiO₂,³⁶ alumina supported MoO₃,³⁷ ZrO₂-pillared clay,³⁸ ZnO nanoparticle,³⁹ Fe₃O₄–CNT,⁴⁰ TiO₂–MWCNT,⁴¹ Fe₃O₄@mesoporous SBA-15.⁴² In the present study, a solid state method was explored for the synthesis of nanostructured Bi₂V₂O₇ powders using Bi(NO₃)₃ and VO(acac)₂ as raw materials. To the best of our knowledge, there is no information available in the literature about the solid state synthesis of nanostructured Bi₂V₂O₇ crystallites. The band gap energy of the as-prepared Bi₂V₂O₇ nanomaterial was initially estimated from ultraviolet-visible spectra. Catalytic application of the synthesized Bi₂V₂O₇ was also investigated in Biginelli reactions. It was found that the synthesized Bi₂V₂O₇ nanocatalyst had excellent efficiency in the synthesis of DHPMs.

2. Experimental

2.1. General remarks

All chemicals were of analytical grade, obtained from commercial sources, and used without further purification. Phase identifications were performed on a powder X-ray diffractometer D5000 (Siemens AG, Munich, Germany) using CuKα radiation. The morphology of the obtained materials was examined with a field emission scanning electron microscope (Hitachi FE-SEM model S-4160). Bi₂V₂O₇ particles were dispersed in water and cast onto a copper grid to study the sizes and morphology of the particles by TEM (Transmission Electron Microscopy) using a Philips – CM300 – 150 kV microscope. From the TEM images the average of particle size distribution was carried out using Image software. FTIR spectra were recorded on a Tensor 27 (Bruker Corporation, Germany). Absorption spectra were recorded on an Analytik Jena Specord 40 (Analytik Jena AG Analytical Instrumentation, Jena, Germany). BET surface areas were acquired on a Beckman Coulter SA3100 Surface Area Analyser. The purity of products was checked by thin layer chromatography (TLC) on glass plates coated with silica gel 60 F254 using n-hexane/ethyl acetate mixture as mobile phase.

2.2. Catalyst preparation

In a typical synthetic experiment, 0.50 g (1.03 mmol) of Bi(NO₃)₃ (M_w = 485.08 g mol⁻¹) and 0.27 g (1.03 mmol) of VO(acac)₂ (M_w = 262.14 g mol⁻¹) were mixed in a mortar and ground until a nearly homogeneous powder was obtained. The obtained powder was added into a 25 mL crucible and treated thermally in one step at 500 °C for 8 h. The crucible was then cooled normally in oven to the room temperature. The obtained powder was collected for further analyses. The synthesis yield for Bi₂V₂O₇ (M_w = 631.88 g mol⁻¹) was 0.30 g (92%).

2.3. General procedure for the synthesis of DHPMs

In a typical procedure, a mixture of aldehyde (1 mmol), ethyl acetoacetate (1 mmol), urea (1.2 mmol) and 0.02 g (3.1 × 10⁻² mmol) of Bi₂V₂O₇ as catalyst were placed in a round-bottom flask under solvent free conditions. The suspension was stirred at 90 °C. The reaction was monitored by thin layer chromatography (TLC) [6 : 4 hexane : ethylacetate]. After completion of the reaction, the solid crude product was washed with deionized water to separate the unreacted raw materials. The precipitated solid was then collected and dissolved in ethanol to separate the solid catalyst. The filtrate was left undisturbed at room temperature to afford the crystals of the pure product.

3. Result and discussion

3.1. Characterizations

The phase composition of the Bi₂V₂O₇ nanomaterial was examined by powder X-ray diffraction technique. Fig. 1 shows the PXRD pattern of the obtained material in the 2θ range 4–70°. The results of structural analysis performed by the FullProf program employing profile matching with constant scale factors are also included in Fig. 1. Red bars are the observed intensities while the black ones are the calculated data. The blue one is the difference: Y_obs − Y_calc. The Bragg reflections positions are indicated by green bars. The pattern has a well fitted defect pyrochlore structure profile with face centered cubic structure. Lattice parameters were found as a = b = c = 10.259079 Å and α = γ = β = 90° with a space group of Fd3̄m.²⁰ The results showed that the pattern had a main Bi₂V₂O₇ structure. A small fraction of unreacted V₂O₃ was found in the structure which is identified in Fig. 1.⁴³

Fig. 1 PXRD pattern of the synthesized Bi₂V₂O₇ nanomaterial and the rietveld analysis. ◊ is due to V₂O₃ impurity.

Fig. 2 shows the FESEM images of the synthesized Bi₂V₂O₇ nanomaterial. Fig. 3 also shows the TEM images as well as the particle size distribution profile. As could be seen from Fig. 2 and 3, the materials are consisted of nearly homogeneous narrow size distributed spherical particles. Fig. 3 also indicates that the maximum particle size distribution were in the range of 40–60 nm.

Fig. 2 FESEM images of the synthesized Bi₂V₂O₇ nanomaterial.

Fig. 3 TEM images of the synthesized Bi₂V₂O₇ nanomaterial. (e) Particle size distribution.

UV-vis absorption spectrum of the Bi₂V₂O₇ nanomaterial is shown in Fig. 4a. The optical band gap is also shown in Fig. 4b. The pure Bi₂V₂O₇ nanomaterial displays a typical visible absorption edge at about 720 nm. According to the results of Pascual et al.,⁴⁴ the relation between the absorption coefficient and incident photon energy can be written as (αhν)² = A(hν − E_g), where A and E_g are constant and direct band gap energy, respectively. Band gap energy was evaluated by extrapolating the linear part of the curve to the energy axis. It was found that the band gap was 2.0 eV.^45,46

Fig. 4 Plots of (a) UV-vis spectrum and (b) (ahν)² versus hν for Bi₂V₂O₇ nanomaterial.

Fig. 5 shows the FTIR spectrum of the obtained material. There are some peaks at around 420, 472, 516, 611, 655, 794, 1020, 1097 and 1151 cm⁻¹ that are the characteristic bands for Bi₂V₂O₇. The band at 420 cm⁻¹ is attributed to Bi–O vibration;^47,48 the bands at 516–611 and 655 cm⁻¹ (ref. 24) are attributed to the O–V–O deformation mode vibration. The broad band at about 794–950 cm⁻¹ is assigned to the V–O asymmetric stretching vibration.^25,47–49 The signal at around 1020 cm⁻¹ is also assigned to the V=O stretching vibration. This band is usually observed in vanadium oxide compounds with intermediate oxidation state between V⁵⁺ and V⁴⁺ ions.⁵⁰ The peak at around 601 cm⁻¹ is assigned to the Bi–O stretching vibration and the peak at around 1090 cm⁻¹ is assigned to the Bi–O vibrations caused by the interaction between the Bi–O bonds and their surroundings.^51,52 The surface area, average pore size and average pore volume of the synthesized material was analyzed by BET and BJH textural analyses. The sample was degassed at 150 °C for 120 min in the nitrogen atmosphere prior to N₂-physical adsorption measurements. The specific surface area (S_BET) of the obtained materials was determined with adsorption–desorption isotherms of N₂ at 77 K. The surface area, pore volumes and average pore diameters of the synthesized materials are summarized in Table 1.⁵³

Fig. 5 FTIR spectrum of the synthesized Bi₂V₂O₇ nanomaterial.

Table 1 BET data for Bi₂V₂O₇ showing the textural properties of the obtained materials

SBET (m^2 g^−1)	Pore size (Å)	Pore volume (cm^3 g^−1)
3.0	139.6	0.01

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3.2. Catalytic studies

3.2.1. Biginelli reaction for the synthesis of DHPMs. The direct synthesis involves the condensation between ketoesters, aldehydes and ureas in the presence of either Lewis or mineral acids. DHPMs were prepared by the reaction of aromatic aldehyde, ethyl/methyl acetoacetate and urea with 3.1 × 10⁻² mmol of Bi₂V₂O₇ at 90 °C for 60 min under solvent-free conditions (Scheme 1). Several parameters affecting the catalytic reactions were analysed and optimized.

Scheme 1 Schematic representation of the reaction pathway for the synthesis of DHPMs.

3.2.2. Effect of different parameters on the catalytic reaction. Different parameters such reaction time, temperature and the amount of the nanocatalyst were optimized on the catalytic reaction using benzaldehyde as the aldehyde derivative. Table 2 shows the effect of the different parameters on the reaction efficiency. It was found that the efficiency was increased with increasing the amount of the nanocatalyst however by changing its amount over 0.02 g, the efficiency was nearly constant. So we found that the optimal amount of the nanocatalyst was 0.02 g. The effect of temperature on the reaction was also investigated using the optimized amount of the catalyst. According to the Table 2, it was found that the reaction efficiency was increased with increasing the temperature from 60 to 90 °C but over 90 °C the efficiency maintained almost constant. Hence, the optimal temperature for the reaction was found to be 90 °C. Table 2 also shows that the efficiency of the synthesis procedure was increased with increasing the reaction time from 15 to 60 min and the efficiency maintained constant with increasing the reaction time. So, 0.02 g of the catalyst, 90 °C reaction temperature, and 60 min reaction time were used for the other Biginelli reactions in this work.

Table 2 Optimization of the different conditions for the synthesis of DHPMs^a

Amount of catalyst (g)	Time (min)	Temperature (°C)	Yield (%)
a Benzaldehyde : ethylacetoacetate : urea molar ratios is as follows: 1 : 1 : 1.2.
0.01	60	90	43
0.02	60	90	89
0.03	60	90	89
0.04	60	90	89
0.05	60	90	89
0.02	60	60	58
0.02	60	70	77
0.02	60	80	81
0.02	60	90	89
0.02	60	100	89
0.02	15	90	50
0.02	30	90	66
0.02	45	90	81
0.02	60	90	89
0.02	90	90	89

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The optimized parameters from the previous section were used for the synthesis of other derivatives and the results are collected in Table 3. Scheme 1 shows a summary of the reaction pathway.

Table 3 Biginelli reactions using ethyl/methyl acetoacetate and urea with different benzaldehyde derivatives

R1	R2	mmol of product	Turnover number (TON)
H	OEt	0.89	29
4-Cl	OEt	0.92	30
2-Cl	OEt	0.98	32
4-Br	OEt	0.62	20
4-F	OEt	0.69	22
2-OMe	OEt	0.98	32
4-OMe	OEt	0.72	23
4-Cl	OMe	0.98	32
2-Cl	OMe	0.90	29
4-Br	OMe	0.93	30
4-F	OMe	0.98	32
2-OMe	OMe	0.92	30
4-OMe	OMe	0.62	20

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The turnover numbers (TON, mol of product per mol of catalyst) for the synthesis of DHPMs were calculated by the following equation.⁵⁴

Table 4 shows the catalytic efficiency of the synthesized Bi₂V₂O₇ nanomaterial compared to the starting materials Bi(NO₃)₃ and VO(acac)₂. The optimized conditions from the previous section were used. As could be seen from Table 4, Bi₂V₂O₇ was much more efficient catalyst compared to the two starting materials which mean that the presence of the both metal ions was important and the two ions have acted cooperatively.

Table 4 Comparison study of the catalytic ability of the synthesized Bi₂V₂O₇ with raw materials

Catalyst	Reagents	Time (min)	Yield
Bi2V2O7	Benzaldehyde	60	89
VO(acac)2	Benzaldehyde	60	38
Bi(NO3)3	Benzaldehyde	60	35

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To show the merit of the present work, we have compared Bi₂V₂O₇ nanocatalyst results with some of the previously reported catalysts in the synthesis of DHPMs (Table 5). It is clear that Bi₂V₂O₇ showed greater activity than some other heterogeneous catalysts.

Table 5 Comparison study of the catalytic ability of the synthesized Bi₂V₂O₇ with other catalysts

Catalyst	R1	Catalyst amount	Condition	Yield%	Time (min)	Ref.
Bi2V2O7	H	3.1 × 10^−2 mmol	Solvent-free, 90 °C	89	60	This work
	4-Cl			92
	2-Cl			98
Bi2O3/ZrO2	H	20 mol%	Solvent-free, 80–85 °C	85	120	34
	4-Cl			85	120
	2-Cl			82	165
ZrO2–Al2O3–Fe3O4	H	0.05 g	Ethanol, reflux, 140 °C	82	300	35
	4-Cl			66
	2-Cl			40
Mo/γ-Al2O3	H	0.3 g	Solvent-free conditions at 100 °C	80	60	37
ZnO	H	25 mol%	Solvent-free conditions at 90 °C	92	50	39
	4-Cl			95

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4. Conclusion

In this work, Bi₂V₂O₇ nanomaterial was synthesized via solid state method. PXRD patterns and structural analysis done by the FullProf program employing profile matching showed that the synthesis was successfull. FESEM and TEM images showed the sphere-like structure in the as-synthesized materials. The catalytic application of the synthesized nanomaterial was investigated in Biginelli reaction in solvent free conditions. It was found that Bi₂V₂O₇ nanomaterial had excellent efficiency in the synthesis of DHPMs. Besides, the two metal ions have performed the catalytic reactions cooperatively.

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