INTRODUCTION

A₂B₂O₇ pyrochlore type materials (where A is a medium-large cation and B is an octahedrally coordinated, high-charge cation) with space group of can be described as a fluorite superstructure with O anions ordered around the A and B cations. These materials have been widely studied as ferroelectric and/or magnetic materials, ionic conductors, catalysts and radiation resistant materials [1-6]. Pyrochlore materials have attracted great interest due to their ability to form substituted and defective structures, permitting interesting physical properties [7-9]. Rare earth (RE) compounds have been extensively studied as potential laser host materials, oxygen ion conductors, and fluorescent lamp phosphors due to their attractive optical, electric, and magnetic properties, which are attributed to the electronic transitions of rare earth ions between the 4f energy levels [10–13]. In the RE family, Sm³⁺ is one of the most important active ions and can show intense line-like absorption bands for special infrared light because of its closely lying energy level structure [14]. In recent years, considerable efforts have been focused on the fabrication of ceramic materials and RE oxides [12–20]. Doping is expected to introduce defect and change in the lattice energy of the crystals. La₂O₃ has also the greatest effect among the various additives in changing the lattice energy of crystals [21].

The Biginelli reaction was originally reported by Biginelli in 1891 [23]. It is a methodology for the synthesis of 3,4-dihydropyrimidin-2-(1H)-one derivatives (DHPMs) in a one-step procedure. DHPMs have shown several applications [24]. Several metal oxides have been reported as nanocatalysts for the Biginelli reactions including Bi₂V₂O₇ [6], alumina supported Mo catalysts [25], nano ZnO as a structure base catalyst [26], MoO₃–ZrO₂ nanocomposite [27], MnO₂–MWCNT nanocomposites [28], TiO₂ nanoparticles [29], Mg–Al–CO₃ and Ca–Al–CO₃ hydrotalcite [30], Bi₂O₃/ZrO₂ nanocomposite [31], ZrO₂–Al₂O₃–Fe₃O₄ [32], imidazole functionalized Fe₃O₄@SiO₂ [33], Alumina supported MoO₃ [34], ZrO₂-pillared clay [35], ZnO nanoparticle [36], Fe₃O₄-CNT [37], TiO₂-MWCNT [38], Fe₃O₄@mesoporous SBA-15 [39], Bi₂Mn₂O₇ [40] and RuO₂ [41].

Recently we have reported the synthesis of the nanostructured Bi₂Mn₂O₇ and Bi₂V₂O₇. The cooperative catalytic performance between the metal ions in the above mentioned materials have also been reported in Biginelli reactions [6, 40]. In continuation of our research on the effect of different factors on the cooperative catalysis of the Biginelli reaction, herein we report the synthesis of La³⁺ and Sm³⁺ - doped Bi₂Mn₂O₇ with general formula Bi_2-xLa_xMn₂O₇ and Bi_2-xSm_xMn₂O₇ nanomaterials. To the best of our knowledge, there is no report on the synthesis of the La³⁺ and Sm³⁺-doped Bi₂Mn₂O₇ nanomaterials. Besides, there is no report on the catalytic application of the obtained materials in the Biginelli reaction and the investigation of the correlation of the dopant metal ions with the catalytic application. So, the doped nanomaterials were used as nanocatalyst in Biginelli reaction for the synthesis of various DHPMs and the expected cooperation was seen again. Besides, it was rationalized that the hard/soft natures of the metal ions would play important role in the catalytic activity of such catalysts.

MATERIALS AND METHODS

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 elemental analyses of the obtained materials were examined with a Philips XL30 scanning electron microscope (Philips, Amsterdam, Netherlands) equipped with an energy-dispersive X-ray (EDX) spectrometer. The morphologies of the obtained materials were examined with a field emission scanning electron microscope (Hitachi FE-SEM model S-4160). The UV-Vis diffuse reflectance spectra of the samples were recorded using an Avantes Spectrometer model Avaspec-2048-TEC. Cell parameter refinement was reported by celref software version 3 (Laboratoire des Materiaux et du Génie Physique de l’Ecole Supérieure de Physique de Grenoble). The purity of the DHPMs 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.

Hydrothermal synthesis of Bi_2-xLa_xMn₂O₇

In typical synthetic experiment, 0.01, 0.02 or 0.03 mmol La(NO₃)₃ (Mw=324.92 g mol^-1) was mixed respectively with 2.05, 2.04 or 2.03 mmol Bi(NO₃)₃.5H₂O (Mw=485.07 g mol^-1) in 70 mL of hot aqueous solution of 1M NaOH. 2.06 mmol MnO₂ (Mw=86.94 g mol^-1) was then added and the resulting mixture were transformed into 100 mL Teflon – lined stainless steel autoclaves. The autoclaves were sealed and heated at 180 °C for 48h. When the reactions were completed, the autoclaves were cooled down to room temperature by water immediately. The prepared powders were collected, washed with distilled water and dried at 110 °C for 20 min and the black powders of the target materials were obtained.

Hydrothermal synthesis of Bi_2-xSm_xMn₂O₇

The nanomaterials were synthesized following a similar procedure as described for Bi_2-xLa_xMn₂O₇ except 0.01, 0.02, 0.03 or 0.04 mmol Sm₂O₃ (Mw= 348.8 g mol^-1) were mixed respectively with 2.05, 2.04, 2.03 or 2.02 mmol Bi(NO₃)₃.5H₂O.

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 0.014 g of La³⁺ or Sm³⁺ - doped Bi₂Mn₂O₇ were placed in a round-bottom flask under solvent free conditions. The suspension was stirred at 104 °C. The progress of 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.

RESULTS AND DISCUSSIONS

PXRD analysis

The crystal phases of the synthesized materials were examined by powder X-ray diffraction technique. Fig. 1 and 2 show the PXRD patterns of the La³⁺ and Sm³⁺ - doped Bi₂Mn₂O₇ nanomaterials. The measured powder XRD data are similar to those of the corresponding un-doped Bi₂Mn₂O₇ nanomaterial [22]. Fig. 1 shows the PXRD patterns of La³⁺ - doped Bi₂Mn₂O₇. It was found that with increasing the dopant amount to 0.03 mmol (S₃), there were some impurity peaks corresponding to the La₂O₃. So, the doping limitation was 0 to 0.02 mmol of La³⁺ and S₂ was used for EDX analysis. Excess mole percent concentration of the dopant agent in the reaction mixture, resulted in impurity peaks in the XRD pattern in Fig. 1c [42, 43]. The diffraction lines at 2θ ≈ 39, 46, 55, 60 and 64° are assigned by their peak positions to the excess La₂O₃ [42, 43]. The X-ray diffraction data for La₂O₃ were identified using the Joint Committee on Powder Diffraction Standards (JCPDS) file: 73-2141.

Fig. 2 shows the PXRD patterns of Sm³⁺ - doped Bi₂Mn₂O₇. It was clear that with increasing the dopant amount to 0.04 mmol, some impurity peaks correspond to Sm₂O₃ were detected. So, the doping limitation was 0 to 0.03 mmol of Sm³⁺ and S₅ was used for EDX analysis. The excess amount of the dopant was appeared as impurity peaks in the XRD pattern of Fig. 1d [44, 45]. The diffraction lines at 2θ ≈ 35, 36, 47, and 63° are assigned by their peak positions to Sm₂O₃ [44, 45].

Table 1 shows the interplanar spacing (d) data calculated from Bragg’s equation for pure and La³⁺ and Sm³⁺ - doped Bi₂Mn₂O₇ nanomaterials. It was found that the d values were increased with increasing the dopant amount of La³⁺ and Sm³⁺ into Bi₂Mn₂O₇. Besides, table 1 shows the cell parameter refinement data of the as-synthesized nanomaterials with standard deviation (SD) data for each sample. It was found that the cell parameters values were increased with increasing the dopant amounts. However, when doping the ions into the crystal system, the d and a values for S₂ and S₃ were decreased.

Table 2 shows the crystal sizes of the as-synthesized nanomaterials in different dopant concentrations calculated by Scherrer equation:

 (1)

In this equation, t is the entire thickness of the crystalline sample, λ is the X-ray diffraction wavelength (0.154 nm), and k is the Scherrer constant (0.9), B_1/2 of FWHM is the full width at half its maximum intensity and H is the half diffraction angle at which the peak is located. The data mentioned in table 2 shows that with doping La³⁺ into Bi₂Mn₂O₇, the crystal sizes were increased compared to pure Bi₂Mn₂O₇ nanomaterial. It was found that with doping Sm³⁺ into Bi₂Mn₂O₇, the calculated crystal sizes were decreased to nearly the value for pure Bi₂Mn₂O₇.

Elemental analysis

The EDX spectra of the obtained nanomaterials as a function of La₂O₃ and Sm₂O₃ concentrations are presented in Fig. 3 a and b. The spectra indicated the X-rays emitted from various elements. The peaks corresponding to La, Sm, Bi, Mn, and O atoms present in the sample are labeled. The respective energy positions and the specific X-ray lines from various elements are also indicated. The Fig.s illustrated the EDX analyses for the sample doped with 0.02 and 0.03 mmol of La₂O₃ and Sm₂O₃, respectively, which verify the doping and the compositional analysis of La³⁺ and Sm³⁺ in Bi₂Mn₂O₇ nanomaterial. The normalized elemental analyses of the doped materials were 1.79 wt. % for La³⁺ and 2.84 wt. % for Sm³⁺.

Microstructure analysis

Typical FESEM images recorded from La³⁺-doped Bi₂Mn₂O₇ nanomaterials are shown in Fig. 4 and 5. From the FESEM images in Fig. 4a–b, with the magnification of 30,000×, and 60,000×, respectively, it was obvious that the morphologies of the obtained materials were mainly rod-like, and some particles were present on the surface of the rods. At higher magnification of 60,000× in Fig. 4b, it was clear that the thickness sizes of the rods were about 100 nm and the particles diameter sizes were about 50 nm.

Fig. 5 a and b with the magnifications of 30,000×, and 100,000× show that with increasing the dopant amount, the morphologies of the doped nanomaterials did not change considerably but the thickness size of the rods was decreased to about 60 nm and the particles’ diameter sizes were about 30 – 60 nm.

FESEM images of Sm³⁺-doped Bi₂Mn₂O₇ nanomaterials are shown in Fig. 6, Fig. 7 and Fig.8. Fig. 6 a – b with the magnifications of 30,000×, 60,000×, and 90,000× showed that the obtained nanomaterials had wire and rod-like morphologies. Fig. 6 b shows that the thickness sizes of the rod structures were about 70 nm and the length size was about 250 nm. It shows that the thickness sizes of the wires were about 70 nm and the length sizes were about 1.5 µm.

Fig. 7 a – d with the magnifications of 4,000×, 6,000×, 6,000× and 20,000× show that the morphology of the S₄ was a mixture of prism and flower-like structures. It was clear that with increasing the dopant amount, the morphology was changed from wire and rod to flower structures. It seemed that the flower had been composed of some prisms that joined together and formed a flower structure. It was clear that the flowers thickness sizes were in micrometer ranges and were about 2µm. Fig. 7 a and d show that there are two kinds of prisms in view of their sizes. The length sizes of the larger prisms were about 15 µm and for the smaller ones were about 1.5 µm.

Fig. 8 a – d with the magnifications of 2,000×, 10,000×, 15,000×, and 60,000× show that the morphologies of the obtained nanomaterials were a mixture of prisms, wires and rod-like structures. It was found that there were some prisms that had been grown on the surface of the larger ones. It was also clear that there were some particles with rod-like morphologies on the surface of the prisms. Fig. 8d shows the wire and rod structures in one view. This figure shows that the wires thickness sizes were about 60 nm and the length sizes were about 1.5 µm. Also, the rod thickness sizes were about 200 nm and the length sizes were about 1 µm.

Optical property

The direct optical band gaps for pure Bi₂Mn₂O₇ and La³⁺ and Sm³⁺ doped Bi₂Mn₂O₇ nanomaterials are shown in Fig. 9 a-c. To calculate the optical band gaps, we changed transmittance data to absorption ones using A = - log (T/100) formula; where A is absorption and T is transmittance. According to the results of Pascual et al. [46], the relation between the absorption coefficient and incident photon energy can be written as (αhν)² = A(hν - Eg), where A and Eg are constant and direct band gap energy, respectively. Band gap energies were evaluated by extrapolating the linear part of the curve to the energy axis. It was found that by doping La³⁺ and Sm³⁺ into Bi₂Mn₂O₇, the band gap was increased; i.e., 3.8 and 4.30 eV for S₂; and 3.9 and 4.3 eV for S₅.

Catalytic studies

Biginelli reaction for the synthesis of DHPMs

The one-pot condensation between ketoesters, aldehydes and urea, in the presence of either Lewis or mineral acids, results in the synthesis of DHPMs. In this study, DHPMs were prepared from the condensation of aromatic aldehydes, ethyl acetoacetate in presence of 0.014 g of doped Bi₂Mn₂O₇ (S₂ and S₅) at 104 °C for 66 min under solvent free conditions (Fig. 10). The optimum conditions were used as for pure Bi₂Mn₂O₇ [40]. The results are collected in table 3.

Table 4 shows the catalytic efficiency of the synthesized doped Bi₂Mn₂O₇ nanomaterials compared to that of pure Bi₂Mn₂O₇ [40]. The optimized conditions from the previous work were used [40]. In two earlier works, we have reported the catalytic efficiency of Bi₂Mn₂O₇ and Bi₂V₂O₇ nanomaterials in the Biginelli reactions [6, 40]. In those works, we concluded that the band gap had an important effect on the catalytic performance of the studied nanomaterials. Bi₂V₂O₇ with the lower band gap was the more efficient catalyst. Another conclusion which was drawn was that the hard and soft nature of the metal ions was also important. Bi₂V₂O₇ with the slightly softer V⁴⁺ compared to Mn⁴⁺ in Bi₂Mn₂O₇ had shown better catalytic performance. To further confirm this idea we prepared La³⁺ and Sm³⁺ doped Bi₂Mn₂O₇. Our results collected in table 4 shows that by doping the M³⁺ metal ions into Bi₂Mn₂O₇, the band gap was increased. The increased band gap has resulted in the decreased catalytic performance of the synthesized nanomaterials in Biginelli reactions. It could also be concluded that since the harder M³⁺ ions have replaced the softer Bi³⁺ metal ion, the catalytic performance was also decreased.

To show the merit of the present work, we have compared the doped Bi₂Mn₂O₇ nanocatalysts results with some of the previously reported catalysts in the synthesis of DHPMs (table 5). It is clear that the doped Bi₂Mn₂O₇ nanocatalysts showed greater activity than some other heterogeneous catalysts.

CONCLUSION

In this work, La³⁺ and Sm³⁺ - doped Bi₂Mn₂O₇ nanomaterials were synthesized via a mild condition hydrothermal method. The catalytic application of the synthesized nanomaterials was investigated in Biginelli reaction in solvent free conditions. It was found that the doped-Bi₂Mn₂O₇ nanocatalysts had considerable efficiency in the synthesis of DHPMs. A correlation between the band gap results and the hard/soft nature of the metal ions was concluded.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.