Characterization of NiZnFe2O4 catalyst
The characterization of the prepared NiZnFe2O4 catalyst was investigated by using different analytical methods such as DSC-TGA, FTIR, XRD, EDX, BET, and SEM.
Dsc-tga Analysis
The thermal behavior of the NiZnFe2O4 catalyst (Fig. 3) was obtained by using thermo gravimetric analysis (TGA). The TGA analysis of the NiZnFe2O4 catalyst shows the temperatures at which it is decomposed when heated in a controlled environment under N2.
It may be noted that the final phase is accompanied by several steps of weight loss and was observed at temperatures ranging from 0 to 1000°C. The results reveal that magnetically separable Nickel-Zinc ferrite (NiZnFe2O4) showed the observed weight change below 100 oC. In the first step, minor weight loss at a temperature below 261.64°C occurred at 3.11% due to the loss of water molecules from the surface. Then, in the second step, another weight loss occurred between 261.64°C and 456.14°C, which was 3.920% due to the total dehydration of hydroxide complexes. In the third step, it is evident that higher temperatures are needed and weight loss is 1.789% between temperature ranges of 456.14°C and 566.74°C. In the fourth step, the conversion process starts at 566.74°C and finally gets converted into the magnetic ferrite at a temperature of 673.53°C. This gives confirmation that the ferrite formation has been completed. However, after a further increase in the temperature at 673.53°C, another peak is seen and a slight increment in the weight of the sample, which continues to grow.
Ftir Analysis
Fourier transform infrared (FTIR) spectrum analysis was used to verify the active functional groups in the NiZnFe2O4 catalyst analysis. In the FTIR spectrum of the NiZnFe2O4 catalyst (Fig. 4), the molecular vibrations of ions are usually observed at the absorption peak of Fe-O at 2382 cm− 1, 2311 cm− 1, 1516 cm− 1, 1028 cm− 1, and 619 cm− 1.
Due to a change in crystalline field effect, the peak position and intensity vary with nickel and zinc concentration. In addition to these, the absorption bands observed at 3624 cm− 1 and 1693 cm− 1 prove that on the surface of ferrite catalyst, water is present.
Xrd Analysis
The phase composition and purity of the synthesized NiZnFe2O4catalyst were investigated by X-ray diffraction and are presented in Fig. 5. After thermal treatment of the NiZnFe2O4catalyst, it can be seen that the diffraction peaks appeared at 2θ = 29.99°, 30.09°, 35.41°, 43.05°, 53.96°, 56.82°, and 62.57°, which supports the confirmation of the crystalline nature of the NiZnFe2O4catalyst.
Edx Analysis
The elements in the NiZnFe2O4 catalyst were investigated using EDX, as shown in Fig. 6. The report reveals that this NiZnFe2O4 catalyst contains Ni, Zn, Fe, and O with an atomic percent distribution of 5.48, 6.80, 30.03, and 57.69, respectively. Thus, no other impurities were found, which confirmed the successful synthesis of catalyst. In the synthesized magnetically separable NiZnFe2O4 catalyst, the weight and atomic fractions of each elementary constituent were determined by EDX and are shown in Fig. 6.
Bet Analysis
N2 adsorption-desorption isotherms as well as the corresponding pore size distribution curve for the prepared NiZnFe2O4 catalyst were shown in Fig. 7.
The surface area of the NiZnFe2O4 catalyst was found to be 52.733 m2 g− 1, while the pore volume and average pore radius were 0.1531 cm3/g− 1 and 11.614 nm, respectively. The pore size distribution curve indicates mesopores of very uniform sizes and smooth surfaces, which give the high surface area for catalyzing the reaction. The isotherms of the prepared NiZnFe2O4catalyst were type III isotherms with a H4-type hysteresis loop.
Sem Analysis
The synthesized magnetically separable Nickel-Zinc ferrite (NiZnFe2O4) catalyst was analyzed by employing scanning electron microscopy (SEM) to determine the surface morphology and particle shape as represented in Fig. 8. These SEM images clearly illustrate the NiZnFe2O4 catalyst particles were observed with irregular sizes. They also have a porous nature due to their smooth and soft active surface area, which offers a higher catalytic activity for the transformation of reactant into product.
Catalytic Activity
In order to demonstrate the catalytic activity of the NiZnFe2O4 as a heterogeneous catalyst, first, a one-pot three-component facile synthesis of 2-amino-4H-chromene from the reaction of aromatic salicylaldehyde 1a (1 mmol), and malononitrile 2a (a mmol), with nitromethane 3 (1 mmol) was chosen as a model reaction. As a result, in order to identify the optimal reaction conditions for 2-amino-4H-chromene synthesis, the catalytic efficiency of NiZnFe2O4 on model reaction was investigated under a variety of conditions, including different solvents, amounts of synthesized NiZnFe2O4 catalyst, and temperatures. The results are summarized in Table 1.
Table 1
Optimization of reaction conditionsa
|
Entry
|
NiZnFe2O4catalyst
(mol %)
|
Solvent
|
Temp. (oC)
|
Time (min)
|
Yield (%)b
|
1
|
-
|
-
|
RT
|
60
|
NR
|
2
|
-
|
-
|
Reflux
|
50
|
Trace
|
3
|
-
|
H2O
|
RT
|
40
|
20
|
4
|
-
|
EtOH
|
RT
|
40
|
35
|
5
|
5
|
-
|
RT
|
40
|
42
|
6
|
5
|
-
|
Reflux
|
35
|
42
|
7
|
5
|
Toluene
|
RT
|
25
|
52
|
8
|
5
|
THF
|
RT
|
22
|
50
|
9
|
5
|
CH3CN
|
RT
|
24
|
62
|
10
|
5
|
H2O
|
RT
|
24
|
64
|
11
|
5
|
EtOH
|
RT
|
15
|
85
|
12
|
5
|
EtOH
|
Reflux
|
15
|
85
|
13
|
10
|
EtOH
|
RT
|
10
|
98
|
14
|
12
|
EtOH
|
RT
|
10
|
98
|
15
|
15
|
EtOH
|
RT
|
10
|
98
|
aReaction conditions: Salicylaldehyde 1a (1 mmol), malononitrile 2a (1 mmol), nitromethane 3 (1 mmol) various amount of NiZnFe2O4 catalyst with ethanol solvent (5 mL); bIsolated yields.
|
Initially, the results were investigated by considering several different parameters, including temperature, conventional various solvents such as toluene, EtOH, CH3CN, CH2Cl2, and H2O, and the amount of catalyst were examined in the model reaction.
According to Table 1, initially, the model reaction was tested in the absence of catalyst and solvent, no product was formed in this case (Table 1, entry 1). Also, even at reflux conditions, a trace amount of product 4a was obtained (Table 1, entry 2). However, when the model reaction was repeated without the NiZnFe2O4 catalyst but with the solvent’s ethanol and water, only a trace amount of the desired product 4a was obtained at room temperature (Table 1, entries 3–4). The results indicate that a catalyst is necessary for this conversion. Furthermore, performing the model reaction at room temperature (Table 1, entry 5) and even at reflux conditions (Table 1, entry 6), the yield of desired product 4a in the absence of solvents, is improved due to the addition of the 5 mol % NiZnFe2O4 catalytic amount, but there is no further improvement in the yield of desired product after adding the catalytic amount. The results show that a solvent is essential to improve the yield of the intended product. Furthermore, to find the optimal reaction conditions, we also investigated the effects of a variety of conventional solvents like toluene, THF, CH3CN, and EtOH, along with water in the presence of a 5 mol % magnetically separable catalyst on the model reaction.
When toluene, THF, CH3CN, and H2O were used as solvents, they did not yield a desirable outcome (Table 1, entries 7–10). We notice that, in the presence of EtOH solvent, the yield of desired product 4a effectively increased up to 85%, respectively (Table 1, entry 11). In contrast, with increasing the temperature, there were no improved results in the yield of desired product 4a (Table 1, entry 12). Afterwards, the model reaction was run by adding an increased amount of the NiZnFe2O4 catalyst in the presence of EtOH solvent (Table 1, entries 13–15) at room temperature. It was observed that, when using 10 mol % catalyst, excellent yield of desired product 4a was achieved (Table 1, Entry 13) and no improvement in yield of the desired product was obtained after increasing catalytic amount (Table 1, entries 14–15). As shown in Table 1, clearly observed, the best results of the desired product (98%) were achieved by carrying the model reaction under EtOH solvent condition at room temperature in the presence of 10 mol % of the magnetic synthesized catalyst (Table 1, Entry 13). After finding the optimized reaction conditions, to assess the scope of catalytic activity of NiZnFe2O4 to prepare 2-amino-4H-chromene with respect to various salicylaldehydes 1a-i, and malononitrile 2a, with nitromethane/nitroethane 3, were investigated under optimum conditions. The results are summarized in Table 2 (entries 1–11). Further, to extend the scope of the present methodology, malononitrile was replaced with ethyl cyanoacetate. It was done by the reaction of substituted salicylaldehyde 1a-l, ethyl cyanoacetate 2b, and nitromethane 3a in the presence of a NiZnFe2O4 catalyst. The results are summarized in Table 3 (entries 1–10).
Interestingly, there was no noticeable difference in the yield of the desired product irrespective of whether electron-withdrawing or electron-releasing groups were present on the aromatic aldehyde ring. These studies show that the synthesized NiZnFe2O4 catalyst has greater catalytic activity.
Generally, this reaction is straightforward and precipitates out the desired product from the reaction mixture. Without any laborious step for evaporation of organic solvent or isolation of the catalyst, the reaction mixture affords pure products using recrystallization. All the desired products of this reaction within short reaction times give good to excellent yields under the optimized reaction conditions. The structural assignments of the isolated pure product were characterized by melting point, FT-IR, 1H NMR, and 13C NMR spectral data.
A proposed reaction mechanism for the synthesis of 2-amino-4H-chromene derivatives in the presence of a NiZnFe2O4 catalyst is presented in Scheme 2. Firstly, salicylaldehyde and malononitrile were synergistically activated in the presence of a NiZnFe2O4 catalyst and undergo Knoevenagel condensation, which afforded arylidinemalononitrile as an intermediate (I). Then, this intermediate (I) reacts with nitroalkane via Michael addition, forming an intermediate (II) with a new C-C bond. Finally, an intramolecular heterocyclization of the intermediate (III) and then undergoing tautomerization afforded the formation of the desired 2-amino-4H-chromene derivative (4).
Reusability Of Catalyst
The recyclability of heterogeneous NiZnFe2O4 catalysts with high product yield efficiency is greatly desired in terms of green chemistry. After the reaction was completed, the catalyst was removed from the mixture using an external magnet. It was then cleaned with acetone before being dried at room temperature. For this purpose, the recovered NiZnFe2O4 catalyst was employed for the same model reaction under the optimum reaction conditions for each next run. As shown in Fig. 9, the catalyst can be reused over seven successive runs without significant loss of its catalytic performance. In addition, the weight of the recovered catalyst in each run and the catalyst that was used in the first run has no difference. The heterogeneity of recovered NiZnFe2O4 catalyst was determined by using flame AAS through a hot filtration test. Results reveals similar composition of detectable metals with EDX (Fig. 6) having O (26.82%), Fe (48.80%), Ni (8.85%), and Zn (13.02%).
Table 4
Comparative synthesis of 4a using the previously reported methods versus the present method.
Entry
|
Catalyst
|
Reaction conditions
|
Time (min)
|
Yield (%)
|
Ref.
|
1
|
NaOAc
|
0.41 gm/ 60 oC
|
3 hr
|
82
|
22
|
2
|
Tertiary amine-thiourea
|
10 mol%/ CH2Cl2 (1.5 mL)/RT
|
30 hr
|
88
|
23
|
3
|
Thiophosphonodiamides
|
10 mol%/CH2Cl2(2 mL)/ 20 oC
|
40 min.
|
87
|
28
|
4
|
Cinchonine-squaramide
|
1 mol%/ DCM (2 mL)/ RT
|
24 hr
|
95
|
29
|
5
|
BFE
|
3 mL/ EtOH (5 mL)/ RT
|
60 min
|
89
|
30
|
6
|
Baker’s yeast
|
2 g/ EtOH (20 mL)/ RT
|
30 hr
|
93
|
31
|
7
|
NiZnFe2O4
|
NiZnFe2O4 (10 mol %)/EtOH (5 mL)/RT
|
10
|
98
|
[ * ]
|
*Present work
|
Finally, to compare the efficiency and capability of the present catalyst for the synthesis of the desired product of model reaction 4a with some previous reported catalysts (Table 4, entries 1 to 6). Each of the reported methodologies has its own merits, but some suffer from drawbacks like longer reaction times, low yields, and the use of expensive catalysts. The results summarized in Table 4 clearly show that the current methodology for using NiZnFe2O4 catalysts is superior due to the use of a green chemical approach, rapid procedure, straightforward and easy work-up procedure, and high product yield.