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Article Type: Original Research

Low temperature hydrothermal synthesis, evaluation of band gap energies and catalytic performance for Biginelli reactions of Sr2-xAxNb2O7+δ (A=Eu3+ and Nd3+) (x = 0.01 and 0.05) nanomaterials

shahin khademinia ¹ ( Department of Inorganic Chemistry, Faculty of Chemistry, Semnan University, Semnan 35351-19111, Iran )
mahdi behzad ² ( Department of Inorganic Chemistry, Faculty of Chemistry, Semnan University, Semnan 35351-19111, Iran )

Submited date : 2017-12-29 Accepted date : 2018-03-01

Keywords: Crystal Structure, Hydrothermal, Sr2Nb2O7, Rietveld, Biginelli,

Abstract :

Nano powders Eu3+ and Nd3+ - doped Sr2Nb2O7 were prepared by a low temperature hydrothermal method at 120 ºC for 48 h followed by annealing at 400 ᵒC for 3 h among Sr(NO3)2 and Nb2O5, Eu2O3 and Nd2O3 raw materials at stoichiometric 1:1 Sr:Nb molar ratio. Characterization of the synthesized materials was performed by X-ray powder diffraction (XRPD) technique. FullProf program employing profile matching with constant scale factors was employed for structural analysis. The results showed that the patterns had a main Sr2Nb2O7 orthorhombic crystal structure with space group Cmc21. FESEM images showed that the synthesized nanomaterials had flower morphologies. Ultraviolet-visible spectra analysis showed that the synthesized Eu3+ and Nd3+ - doped Sr2Nb2O7 nanomaterials had light absorption in the ultraviolet light region. The direct optical band gap energies obtained from UV-Vis absorption spectra were 3.45, 3.50 and 3.80 eV for pure Sr2Nb2O7, S2 and S4, respectively. The catalytic activity of the obtained materials in the one-pot synthesis of the heterocyclic compounds 3,4-dihydropyrimidin-2(1H)-ones (DHPMs) in Biginelli reaction is investigated. The optimized 0.03 g of the catalyst, 95 ºC reaction temperature, and 60 min reaction time are used for the other Biginelli reactions in this work.

References:

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Low temperature hydrothermal synthesis, evaluation of band gap energies and catalytic performance for Biginelli reactions of Sr2-xAxNb2O7+δ (A=Eu3+ and Nd3+) (x = 0.01 and 0.05) nanomaterials

Abstract

Nano powders Eu3+ and Nd3+ - doped Sr2Nb2O7 were prepared by a low temperature hydrothermal method at 120 ºC for 48 h followed by annealing at 400 ᵒC for 3 h among Sr(NO3)2 and Nb2O5, Eu2O3 and Nd2O3 raw materials at stoichiometric 1:1 Sr:Nb molar ratio. Characterization of the synthesized materials was performed by X-ray powder diffraction (XRPD) technique. FullProf program employing profile matching with constant scale factors was employed for structural analysis. The results showed that the patterns had a main Sr2Nb2O7 orthorhombic crystal structure with space group . FESEM images showed that the synthesized nanomaterials had flower morphologies. Ultraviolet-visible spectra analysis showed that the synthesized Eu3+ and Nd3+ - doped Sr2Nb2O7 nanomaterials had light absorption in the ultraviolet light region. The direct optical band gap energies obtained from UV-Vis absorption spectra were 3.45, 3.50 and 3.80 eV for pure Sr2Nb2O7, S2 and S4, respectively. The catalytic activity of the obtained materials in the one-pot synthesis of the heterocyclic compounds 3,4-dihydropyrimidin-2(1H)-ones (DHPMs) in Biginelli reaction is investigated. The optimized 0.03 g of the catalyst, 95 ºC reaction temperature, and 60 min reaction time are used for the other Biginelli reactions in this work.

Keywords: Sr2Nb2O7, Rietveld, Hydrothermal, Crystal Structure, Biginelli.

1. Introduction

Rare earth (RE) oxides are extensively studied in recent years because of their unique electronic, optical, and chemical properties, and their potential applications in various fields [1, 2]. The rare earth materials are used in a wide range of advanced technologies. Europium shows emission features and is used as a phosphor activation agent in colour cathode-ray tubes and in liquid-crystal flat displays [3-7]. Among many RE oxides, neodymium oxide is widely used in photonic applications [8], luminescent and thermo luminescent materials [9, 10], protective coatings [11, 12] and thin films [13]. The oxides of rare earths including neodymium are used in important applications, such as high-efficiency phosphors and catalysts. They show good catalytic properties in several reactions, including synthesis of ammonia and oxidative coupling of methane [14-16]. There are several reports about doping metal ions into Sr2Nb2O7 crystal system including N [17], Ba and Ta [18], La [19], Zn [20], Mo [21], Ti [22].

The Biginelli reaction is a methodology for the one-pot synthesis of 3,4-dihydropyrimidin-2-(1H)-one derivatives (DHPMs) [23,24]. DHPMs have shown biological activities [25]. Several metal oxides have been reported as nanocatalyst for the Biginelli reactions including alumina supported Mo catalysts [26], nano ZnO as a structure base catalyst [27], MoO3 – ZrO2 nanocomposite [28], MnO2–MWCNT nanocomposites [29], TiO2 nanoparticles [30], Mg–Al–CO3 and Ca–Al–CO3 hydrotalcite [31], Bi2O3/ZrO2 nanocomposite [32], ZrO2–Al2O3–Fe3O4 [33], imidazole functionalized Fe3O4@SiO2 [34], Alumina supported MoO3 [35], ZrO2-pillared clay [36], ZnO nanoparticle [37], Fe3O4-CNT [38], TiO2-MWCNT [39], Fe3O4@mesoporous SBA-15 [40], Bi2V2O7 [41], Bi2Mn2O7 [42], La3+ and Sm3+ doped Bi2Mn2O7 [43], Mn2Sb2O7 [44], and etc.

In the present work, a facile and low temperature hydrothermal reaction is explored for the synthesis of nanostructured Eu3+ and Nd3+ doped Sr2Nb2O7 powders using Nb2O5, Sr(NO3)2, Eu2O3, Nd2O3 and NaOH in the present study. To the best of our knowledge, there is no report about doping the above mentioned lanthanide ions into Sr2Nb2O7 crystal system. Also, studying the catalytic activity of the nanomaterials in the Biginelli reactions is reported for the first time in the present work. The rietveld analysis and crystal structure investigation is used for the characterization of the obtained targets. The effect of Eu3+ and Nd3+ dopants on the morphology and optical property of the obtained materials are also studied by FESEM and UV-Vis techniques. Catalytic application of the synthesized nanomaterials was also investigated in Biginelli reactions for 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 done on a powder X-ray diffractometer D5000 (Siemens AG, Munich, Germany) using CuKα radiation. Field emission scanning electron microscope (Hitachi FE-SEM model S-4160) was used to examine the morphology of the obtained materials. Philips XL30 scanning electron microscope (Philips, Amsterdam, Netherlands) equipped with energy-dispersive X-ray (EDX) spectrometer was employed for studying the elemental analyses of the obtained materials. Absorption spectra were recorded on a Analytik Jena Specord 40 (Analytik Jena AG Analytical Instrumentation, Jena, Germany). The purity of products was checked by thin layer chromatography (TLC) on glass plates coated with silica gel 60 F254 using a 6:4 volumetric ratio of n-hexane:ethyl acetate mixture as mobile phase and comparison of melting points with authentic samples. Melting points were obtained on a thermoscientific 9100 apparatus.

2.2. Materials preparation

2.2.1. Synthesis of Nd3+ - doped Sr2Nb2O7

The synthesis procedure is according to our previous reported work [45]. For the synthesis of the targets, 0.20 g (0.752 mmol) of Nb2O5 (Mw = 265.82 gmol−1), 0.158 g (0.747 mmol) or 0.156 g (0.737 mmol) of Sr(NO3)2 (Mw = 211.62 gmol−1) and 0.0034 g (0.01 mmol) (S1) or 0.0017 g (0.05 mmol) (S2) of Nd2O3 (Mw= 336.48 gmol-1) were added into 50 mL of hot aqueous solutions of 2 M NaOH under magnetic stirring at 80 ᵒC, respectively. The resulting solution was stirred for 15 min. Then the obtained solution was transferred into a 100-mL Teflon lined stainless steel autoclave. The autoclave was sealed and treated thermally at 120 ˚C for 48 h in an oven. When the synthesis process was completed, the autoclave was cooled to room temperature by quenching in water immediately. The prepared powder was washed with distilled water for several time and dried at 110 ˚C for 20 min under normal atmospheric condition. The prepared powder was then transferred in a crucible and treated thermally again in a one step at 400 ˚C for 3h in a furnace. After the desired time, the crucible was allowed to cool down normally to room temperature.

2.2.2. Synthesis of Eu3+-doped Sr2Nb2O7

In another typical synthetic experiment, 0.20 g (0.752 mmol) of Nb2O5 (Mw = 265.82 gmol−1), 0.158 g (0.747 mmol) or 0.156 g (0.737 mmol) of Sr(NO3)2 (Mw = 211.62 gmol−1) and 0.00352 g (0.01 mmol) (S3) or 0.00176 g (0.05 mmol) (S4) of Eu2O3 (Mw= 351.92 gmol-1) were added to 50 mL of hot aqueous solutions of 2 M NaOH under magnetic stirring at 80 ˚C, respectively. The resulting solution was stirred for 15 min. Then it was transferred to a 100-mL Teflon lined stainless steel autoclave. The autoclave was sealed and heated at 120 ˚C for 48 h in an oven. When the desired reaction was completed, the autoclave was cooled to room temperature by quenching in water, immediately. The prepared powder was washed with distilled water and dried at 110 ˚C for 20 min under normal atmospheric condition. The obtained dried powder was treated thermally in a one step at 400 ˚C for 3h in a furnace. When the desired time was elapsed, the furnace was shut down and allowed to cool down normally to room temperature.

2.2.3 General procedure for the synthesis of DHPMs

In a typical procedure [42-44], a mixture of aldehyde (1 mmol), ethyl acetoacetate (1 mmol), urea (1.2 mmol) and 0.03 g (≈ 8.3 × 10-2 mmol) of Sr2Nb2O7 or Eu3+ and or Nd3+ doped Sr2Nb2O7 (Mw ≈ 361 gmol-1) as catalyst were placed in a round-bottom flask under solvent free conditions. The suspension was stirred at 95 °C. The reaction was monitored by thin layer chromatography (TLC) [6:4 n-hexane:ethyl acetate]. After completion of the reaction, the solid crude product was washed with deionized water to separate the unreacted raw materials. The remaining solid was then dissolved in ethanol to separate the heterogeneous catalyst. The solid catalyst was washed with acetone and dried in oven at 90 ˚C to be used in the next cycles. The ethanolic solution was evaporated to dryness to obtain the target DHPMs.

3. Characterization

Figure 1 shows the XRPD analysis of the doped Sr2Nb2O7 samples obtained in the θ-2θ geometry with Cu-Kα radiation. Structural analysis is done by the FullProf program by employing profile matching with constant scale factor. The blue bars show the Bragg positions of the main phase Sr2Nb2O7. The red bars show the Bragg positions of the impurity phase Nb2O5 (JCPDS no: 00-030-873). The results showed that the patterns had a main Sr2Nb2O7 crystal structure with space group . The data show that doping process has influence on the crystal growth and purity of the obtained materials. It can also been found that the other crystallographic parameters such as interplanar spacing, crystallite size, dislocation density and strain values are effected by changing and increasing the dopant amounts.

$F:\PAPERS\kafi ahmadi\Submitted manuscripts\Sr2Nb2O7 doped\XRD.jpg$

Figure 1. XRPD patterns of the synthesized Nd3+ and Eu3+doped Sr2Nb2O7 nanomaterials and the rietveld analyses. Where (a) is S1, (b) is S2, (c) is S3 and (d) is S4.

Rietveld analyses data of the obtained samples are summarized in table 1. The values of RBragg, RF and χ2 show that the analyses are well. The data show that increasing the crystal phase purity has a considerable effect on the goodness of the refinement. So, the refinement for S1 and S3 was performed better than those for S2 and S4. Besides, the phase purity value of the obtained materials show that the parameter was decreased with increase the dopant amount in the crystal system. This can be due to the difference between the ionic radii of the dopant amount and strontium ion, causes a disorder in the unit cell when a considerable amount of the dopant incorporates in the unit cell cavity. Also, the crystal phase growth was studied by investigating the counts value of the sharpest peak in the XRPD pattern. It was found that the value was decreased with increasing the dopant amount in the crystal system. The disorder, mentioned above, can be responsible for the phenomenon. However, there is another parameter that is the difference charge between dopant and Sr2+. The difference creates another deficiency in the crystal system due to the deviation of the stoichiometry of oxygen in A2B2O7 formula. The experimental observations indicate the more the deviation the less the crystal growth and purity.

Table 1. Quantitative phase analysis for doped - Sr2Nb2O7 nanomaterials.

Sample	Rietveld parameters			Phase purity (%)	Counts
Sample	RBragg	RF	χ2	Phase purity (%)	Counts
S1	2.12	1.05	1.73	100	407
S2	1.36	0.74	1.63	88	366
S3	1.37	0.802	1.74	91	408
S4	0.899	2.11	1.58	66	164

Table 2 shows the lattice parameters of the obtained targets. The data also show the unit cell volume of the samples calculated by formula 1. It shows that the volume of the samples is decreased with increasing the dopant amount in the crystal system. The data show that the decreasing in the unit cell volume for Eu3+ doped Sr2Nb2O7 is larger than that for Nd3+ doped Sr2Nb2O7. This is due to the smaller ionic radii of Eu3+ (1.07 Å) compared to Nd3+ (1.12 Å).

Table 2. Lattice parameters data for pure and doped - Sr2Nb2O7 nanomaterials.

Sample	a (Å)	b (Å)	c (Å)	Volume (Å3)
S1	3.91127	26.72003	26.72003	2792
S2	3.82528	26.55433	26.55433	2697
S3	3.86266	26.63212	26.63212	2739
S4	3.80668	26.60026	26.60026	2693

Interplanar spacing (d) data of the obtained nanomaterials are summarized in table 3. The d parameter data were calculated by Bragg's equation and formula (2). The obtained data are in good consistence with each other's. It was found that the d parameter values were decreased by doping the lanthanide ions into the crystal system. It is due to the substitution of Sr2+ (1.26 Å) by Nd3+ (1.12 Å) and Eu3+ (1.07 Å) with smaller ionic radii. So there is a contraction in the unit cell with doping the lanthanide ions into Sr2Nb2O7 crystal system.

Table 3. Interplanar spacing (d) data for pure and doped - Sr2Nb2O7 nanomaterials.

	Pure Sr2Nb2O7	S1	S2	S3	S4
dBragg (Å)	3.902	3.0473	3.0335	3.0551	3.0522
dcal. (Å)	3.600	3.0717	3.0225	3.0442	3.0152
2θ (°)	22.77	29.28	29.42	29.21	29.24

The crystallographic data of the obtained samples were calculated and compared to the observed data. The unit cell volume can be obtained from the bellow formula:

(1)

Where a, b and c are the lattice parameters and V is the cell volume.

The interplanar spacing is calculated by the following formula:

(2)

With using the peak with maximum intensity at 29.4 ᵒ, with the (h k l) value of (152), the relation is as below:

Table 4 shows the crystallite sizes, dislocation density and strain date of the as-synthesized nanomaterials in different dopant concentrations calculated via Scherrer equation:

(3)

In this equation, D 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), B1/2 of FWHM is the full width at half its maximum intensity and θ is the half diffraction angle at which the peak is located. The crystallite size data mentioned in table 4 show that the values were increased compared to pure Sr2Nb2O7 nanomaterial with doping Eu3+ into Sr2Nb2O7. It was also found that with doping Nd3+ into Sr2Nb2O7, the calculated crystallite sizes were decreased to the value nearly equal to the pure Sr2Nb2O7.

The value of the dislocation density (δ) which is related to the number of defects in the crystal was calculated from the average values of the crystallite size (D) by the relationship given below:

(4)

It was found that the dislocation density was decreased when doping the dopant ions in the crystal system. The behavior is due to the changing the crystallite sizes of the materials with doping the lanthanide ions in Sr2Nb2O7 crystal structure.

The strain (e) values were determined with the use of the following formula:

(5)

The variation in the strain as a function of doping processes is included in table 4. The decreasing in the strain with doping and increasing the dopants amounts is probably due to the enhancement in the degree of crystallite of the obtained target.

Table 4. Scherrer data information for pure and doped - Sr2Nb2O7 nanomaterials.

3.1. Elemental analysis

Figure 3 (a and b) shows the EDX spectra of the obtained nanomaterials as a function of Nd2O3 and Eu2O3 concentrations. The spectra indicated the X-rays emitted from various elements. The peaks corresponded to Nd, Eu, Sr, Nb, and O atoms present in the samples are labeled. The respective energy positions and the specific X-ray lines from various elements are also indicated. The figure illustrates the EDX analyses for the samples doped theoretically with 0.05 mmol of Nd2O3 and Eu2O3 into the crystal system which verify the doping and the compositional analysis of Eu3+ and Nd3+ in Sr2Nb2O7. The normalized elemental analyses of the doped materials showed that the experimental formulas were Sr1.96Nd0.04Nb2O7 and Sr1.963Eu0.037Nb2O7.

$C:\Users\Admin\Desktop\edx SrNbO.jpg$

Figure 2. Elemental analyses data for a) S2 and b) S4.

3.2. Morphology analysis

Figure 3 shows the FESEM images of S1 and S2. Figure 3 a and b shows the FESEM images of S1. It was found that the morphology of the obtained material was flower. However, it indicated that the flower was composed of rod structures which join together in a point and make a flower structure. Figure 3 b shows that the thickness size of the rods is about 80 - 100 nm. Figure 3 c and d shows the FFESEM images of S2. It shows that the morphology is changed when the dopant amount is increased. It shows that there are two types of flower morphology. One type is formed from rod structures. The second type is formed from joining plate structures to each other. It was found that the thickness size of the plates was about 30 - 40 nm.

$G:\شاهین\مقالات نوشته شده در دوره دکتری\doped Sr2Nb2O7\سنتز 45\total.jpg$

$G:\شاهین\مقالات نوشته شده در دوره دکتری\doped Sr2Nb2O7\سنتز 46\total.jpg$

Figure 3. FESEM images of S1 and S2.

Figure 4 a and b shows the FESEM images of S3. Figure 4 a indicates that the size and morphology of the obtained target are homogeneous. Besides, figure 4 b shows that the morphology of the material is flower formed by crossing rods to each others. The thickness size of the rods is about 100 - 120 nm. Figure 4 c and d shows the FESEM images of S4. The figure indicates that the rod-formed flower structure has been changed to the rods with heterogeneous morphology. The thickness size of the rods is about 100 - 120 nm. Figure 4 d shows that the morphology of the target was changed to the plate-formed flower with increasing the dopant amount. It is found that the plate thickness size is about 30 - 40 nm.

$G:\شاهین\مقالات نوشته شده در دوره دکتری\doped Sr2Nb2O7\سنتز 47\total.jpg$

$G:\شاهین\مقالات نوشته شده در دوره دکتری\doped Sr2Nb2O7\سنتز 48\total.jpg$

Figure 4. FESEM images of S3 and S4.

The direct optical band gap energies of the pure and doped Sr2Nb2O7 nanomaterials obtained from UV-Vis absorption spectra are shown in figure 5. 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 a constant and the direct band gap energy, respectively. The band gap energies were evaluated from extrapolating the linear part of the curve to the energy axis. It was found that the direct optical band gaps were 3.45, 3.50 and 3.85 for pure Sr2Nb2O7, S2 and S4, respectively. The data show that increasing the dopant amount increased the direct optical band gaps of the targets. Increasing the optical bannd gap can be due to the decreasing the unit cell volume and lanthannnide contraction effect.

Figure 5. Plots of Plots of (ahν)2 versus hν (eV) for a) Pure Sr2Nb2O7, b) S2 and c)S4.

3.3. Catalytic studies

3.3.1. 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 acetoacetate and urea with 8.3 × 10-2 mmol of each of the obtained samples at 95 °C for 60 min under solvent-free conditions (Scheme 1). Several parameters affecting the catalytic reactions were analysed and optimized.

3.3.2. 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 5 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 from 0.02 to 0.03; however by changing its amount over 0.04 g, the efficiency was decreased. So we found that the optimal amount of the nanocatalyst was 0.03 g. The effect of temperature on the reaction was also investigated using the optimized amount of the catalyst. According to the table 5, it was found that the reaction efficiency was increased with increasing the temperature from 70 to 95 ºC but over the temperature, the efficiency maintained nearly constant. Hence, the optimal temperature for the reaction was found to be 95 ºC. Table 5 also shows that the efficiency of the synthesis procedure was increased with increasing the reaction time from 50 to 60 min and the efficiency maintained constant with increasing the reaction time. So, 0.03 g of the catalyst, 95 ºC reaction temperature, and 60 min reaction time were used for the other Biginelli reactions in this work.

Table 5. Optimization of the different conditions for the synthesis of DHPMs.

Sample	2θ	θ value	B1/2 (°)	B1/2 (rad)	cosθB	D (nm)	δ(lines/m2)	e
Sr2Nb2O7	22.73	11.365	0.3000	0.005200	0.980390	27.18	13.5	1.27
S1	29.28	14.64	0.2558	0.004462	0.96753	32.10	9.7	1.08
S2	29.42	14.71	0.2558	0.004462	0.96722	32.12	9.7	1.08
S3	29.21	14.61	0.2558	0.004462	0.96769	32.10	9.7	1.08
S4	29.24	14.62	0.2558	0.004462	0.96762	32.10	9.7	1.08

Amount of catalyst (g)	Time (min)	Temperature (°C)	Yield (%)
0.02	60	80	64
0.02	60	95	70
0.02	70	95	70
0.03	50	70	62
0.03	60	80	77
0.03	60	95	80
0.03	70	95	82
0.04	50	70	58
0.04	60	80	65
0.04	70	80	68
0.04	60	95	65
0.04	70	95	72

The optimized parameters from the previous section were used for the synthesis of other derivatives and the results are collected in table 6. The data showed that doping the lanthanide ions changed the activity and so the catalytic performance of the obtained materials. It was found that when the doping the cations into the crystal system, the performance was increased for the derivative 2-Cl and 4-Cl benzaldehyde derivatives. The catalytic data indicate that the hard/soft nature of the cations play a more important role in the catalytic performance for the Biginelli reactions when an electron withdrawing substituent agent as the benzaldehyde derivative is used in replace of benzaldehyde agent. Scheme 1 shows a summary of the reaction pathway.

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

Table 6. Biginelli reactions using ethyl acetoacetate and urea with different benzaldehyde derivatives.

Sample	Band gap (eV)	Yield (%)
Sample		H	4- Cl	2- Cl
S1	3.45	80	86	91
S2		80	85	92
S3	3.50	77	68	95
S4		73	98	78
S5	3.85	65	92	81

To show the merit of the present work, we have compared the obtained nanocatalysts results with some of the previously reported catalysts in the synthesis of DHPMs (Table 7). It is clear that the synthesized nanomaterials showed greater activity than some other heterogeneous catalysts. It is clear that the present conditions for the synthesis of DHPMs is more facile and the yields of the obtained materials under the present conditions are comparable with those of the catalysts applied at sever catalytic conditions including at higher reaction temperature, using reflux and reaction solvent.

Table 7. Comparison study of the catalytic ability of the synthesized Bi2Mn2O7 with other catalysts.

Catalyst	R1	Catalyst amount	Reaction Condition	Yield %	Time (min)	Ref.

Sr2Nb2O7	H	0.03 g	solvent-free, 95 °C	80	60	This work
	4-Cl	0.03 g		86
	2-Cl	0.03 g		91

Eu3+-doped Sr2Nb2O7	H	0.03 g	solvent-free, 95 °C	80	60	This work
	4-Cl	0.03 g		85
	2-Cl	0.03 g		92

Nd3+-doped Sr2Nb2O7	H	0.03 g	solvent-free, 95 °C	73	60	This work
	4-Cl	0.03 g		98
	2-Cl	0.03 g		78

Mn2Sb2O7	H	0.04 g	solvent-free, 103 °C	81	62	[37]
	4-Cl			92
	2-Cl			77

La3+-doped Bi2Mn2O7	H	0.014 g	solvent-free, 104 °C	92	66	[36]
	4-Cl			51
	2-Cl			78
Sm3+-doped Bi2Mn2O7	H	0.014 g	solvent-free, 104 °C	82	66	[36]
	4-Cl			37
	2-Cl			71

Bi2Mn2O7	H	2.2 × 10-2 mmol	solvent-free, 104 °C	96	66	[35]
Bi2Mn2O7	4-Cl	2.2 × 10-2 mmol	solvent-free, 104 °C	89	66	[35]
	2-Cl			86

Mo/γ - Al2O3	H	0.3 g	solvent-free, 100 °C	80	60	[28]

Bi2O3/ZrO2	H	20 mol%	solvent-free, 80-85 °C	85	120	[31]
Bi2O3/ZrO2	4-Cl	20 mol%	solvent-free, 80-85 °C	85	120	[31]
	2-Cl			82	165

ZrO2–Al2O3–Fe3O4	H	0.05 g	Ethanol, reflux, 140 °C	82	300	[32]
ZrO2–Al2O3–Fe3O4	4-Cl	0.05 g	Ethanol, reflux, 140 °C	66	300	[32]
	2-Cl			40

Bi2V2O7	H	3.1 × 10-2 mmol	solvent-free, 90 °C	89	60	[34]
	4-Cl			92
	2-Cl			98

ZnO	H	25 mol%	solvent-free, 90 °C	92	50	[38]
ZnO	4-Cl	25 mol%	solvent-free, 90 °C	95	50	[38]

4. Conclusion

In this his work, Nd3+ and Eu3+ doped Sr2Nb2O7 nanomaterials were synthesized via hydrothermal method followed by annealing at 400 ᵒC for 3 h. Structural analyses were performed by FullProf program employing profile matching. The data showed that the syntheses were successful and the patterns had a main Sr2Nb2O7 orthorhombic crystal structure with space group Cmc21. The crystallographic data of the obtained materials such interplanar spacing, crystallite size, crystal phase growth and purity, dislocation density and strain values were investigated and related to the type and amount of the dopant ion. FESEM images showed the flower structure in the as-synthesized materials. Direct band gap energy was evaluated by extrapolating the linear part of the curve to the energy axis. It was found that the band gaps were increased with doping Nd3+ and Eu3+ into Sr2Nb2O7 crystal system. The catalytic data indicated that the hard/soft nature of the cations played a more important role in the catalytic performance for the Biginelli reactions when an electron withdrawing substituent agent as the benzaldehyde derivative was used in replace of benzaldehyde agent.

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