Abstract
This paper discusses synthesizing green, recyclable, heterogeneous nickel–chromium oxide (NiCr2O4) catalyst and its application in solvent-free, room-temperature Knoevenagel condensation reaction. Nickel–chromium oxides (Ni–Cr oxides) were prepared using the coprecipitation method in various proportions, such as 2:1, 1:1, and 1:2 ratios. The synthesized catalysts were characterized using X-ray diffraction, SEM-EDX, and BET-surface area analysis. The synthesized catalysts were employed as heterogeneous catalysts in the Knoevenagel condensation model reaction of 4-chlorobenzaldehyde and malononitrile under room temperature, solvent-free grinding reaction conditions, and the results were compared. This paper will discuss the most suitable catalyst and its possible mechanism.
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1 Introduction
The Knoevenagel condensation provides an important alkene precursor with wide applications in important organic transformations, polymers, cosmetics, anticancer agents, and medicinal chemistry.1,2,3,4,5 Developing green and sustainable chemical methodologies using mixed metal oxide catalysts is also becoming an important pillar in current sciences.6,7,8,9,10 Due to the Knoevenagel condensation importance, various homogeneous11,12 and heterogeneous catalysts13,14,15,16,17,18,19,20,21,22,23 are employed for the Knoevenagel condensation reaction. The previously reported methods utilize acid and base catalytic systems in homogeneous and heterogeneous forms to catalyze the Knoevenagel condensation reaction. However, homogeneous catalytic reactions operate at elevated temperatures and require water washing with various downstream steps to remove the catalyst, which becomes the major limitation of reported reactions. On the other hand, solvent-based Knoevenagel condensation is also reported in the literature,20 while MgO/ZrO2-based Knoevenagel condensation reaction requires elevated temperature under solvent-free conditions.18 The grinding technique based on the Knoevenagel condensation reaction is eco-friendly; however, reported methods use MgO and CaO catalysts in stoichiometrically excess amounts and require mineral acid wash to neutralize the base catalyst.21,22 Expensive Pd systems is also reported as a heterogeneous catalyst for the Knoevenagel condensation reaction.23
Considering these facts and constraints in reported methods, we have developed a room-temperature, solvent-free Knoevenagel condensation reaction by using an efficient, recyclable nickel–chromium (Ni–Cr) based heterogeneous catalytic system (Scheme 1) synthesized using the coprecipitation method. This method requires mixed metal oxide in catalytic amounts and operates at room temperature with solvent-free grinding conditions, which gave excellent Knoevenagel condensation yields.
Knoevenagel condensation of benzaldehyde and malononitrile was done using the Ni–Cr oxide catalyst.
A preliminary study synthesized the Ni–Cr oxide catalyst using the coprecipitation method with different Ni and Cr ratios followed by high-temperature calcination. The synthesized catalysts were characterized using XRD, SEM-EDX, and BET surface area. At the initial stage, the synthesized catalysts were employed in the Knoevenagel condensation model reaction of 4-chlorobenzaldehyde and malononitrile (Scheme 2) operated at room temperature and solvent-free grinding conditions.
Knoevenagel condensation of 4-chlorobenzaldehyde and malononitrile was done using a NiCr2O4 catalyst.
The reaction was studied for synthesized catalysts with different Ni–Cr oxide ratios and catalyst amounts. After optimizing the reaction parameters, the catalyst that gave the highest yield was selected for further substrate scope study using various substituted benzaldehydes and for catalyst recycling studies. The Knoevenagel condensation reaction products were confirmed through 1H NMR and FT-IR analysis.
2 Experimental
2.1 General experimental conditions
All the chemicals used in the Knoevenagel condensation reaction of substituted benzaldehydes and malononitrile were made by Sigma Aldrich or SRL. The metal precursors, viz., nickel (II) chloride, chromium (III) chloride (purity 98%), and sodium hydroxide, were purchased from SRL. Ethyl acetate and acetone were used for catalyst washing and recycling study. All reactants and solvents were used without purification. XRD analysis of synthesized catalysts was performed using an Ultima IV X-ray diffractometer Cu radiation (40 kV) with continuous scanning mode. The XRD patterns were analyzed using standard ICDD (International Center for Diffraction data) files. Surface morphology was measured using a FEI Nova NanoSEM 450. An energy-dispersive X-ray detector (EDX) mounted on the microscope was used for the elemental analysis of the synthesized catalysts. Surface morphology was determined using a JEOL JSM 6360A scanning electron microscope. The pH of the 5% catalyst solution was measured using a pH meter under (200 rpm) constant stirring. The BET measurements of the catalysts were carried out on a Surface Area Analyzer Instrument (Micromeritics TriStar II 3020 Version 3.02). The molecular structure of all reaction products was confirmed through 1H NMR analysis at 500 MHz (Bruker Advance III HD NMR) in CDCl3 solvent. FT-IR analyses of reaction products were done on an FT-IR spectrometer (SHIMADZU IRSpirit-X Series), having an IR scan range from 400 to 4000 cm−1. Pyridine FT-IR of catalysts was recorded on BRUKER Tensor II, single reflection diamond ATR at room temperature.
2.1.1 General procedure for the synthesis of Ni–Cr oxide catalyst
Catalysts were synthesized through the co-precipitation method by using sodium hydroxide solution with different Ni and Cr mole ratios.24 In a typical procedure for synthesizing Ni–Cr oxide (1:2) catalyst, 100 ml of 1 M NiCl2·6H2O and 100 ml of 2 M CrCl3·6H2O solution were mixed in a glass vessel. In a separate glass vessel, an excess of 8 M sodium hydroxide solution was taken, and a mixture of Ni–Cr metal chloride solution was added into it dropwise at room temperature (25 ± 2°C) with constant stirring. The obtained precipitate was filtered and washed thoroughly with distilled water till the pH of the filtrate solution became neutral. The washed precipitate was dried in an oven at 120°C for 4 h. The dried precipitate was further calcined at 800°C in a programmed muffle furnace with a rise of 10°C min−1 for 8 h. The calcined catalyst was characterized by using different analysis techniques such as XRD, SEM-EDX, and BET surface area.
2.1.2 General procedure for Knoevenagel condensation of benzaldehyde and malononitrile by using synthesized Ni–Cr oxide (1:2) catalyst
Benzaldehyde (3a-j) (1 mmol), malononitrile (1.1 mmol), and synthesized Ni–Cr oxide (1:2) catalyst (NiCr2O4: 0.3 equivalents) were taken in a mortar pestle. The mixture was grounded at room temperature, and conversion was monitored on TLC. The resulting product and catalyst mixture were taken in ethyl acetate and filtered. The catalyst was washed with ethyl acetate and the combined filtrate was concentrated to get the benzylidinemalononitrile. The washed catalyst was dried at 120°C for 2 h and reused for the next batch. The reaction product was recrystallized in an n-hexane-dichloromethane solution to get a high-purity Knoevenagel condensation product. The product was confirmed using their physical constants, FT-IR, and proton NMR analysis.
2.1.3 Characterization of synthesized Knoevenagel condensation products of benzaldehyde and malononitrile by using synthesized Ni–Cr oxide (1:2) catalyst
3a. 2-(4-chlorobenzylidene)malononitrile: Appearance: solid, color: white, physical constant: 162–163°C, FT-IR (cm−1): 2224, 1H NMR (500 MHz, CDCl3) δ 7.86 (d, J \(=\) 8.6 Hz, 2H), 7.73 (s, 1H), 7.52 (d, J \(=\) 8.6 Hz, 2H).
3b. 2-benzylidenemalononitrile: Appearance: solid, color: cream, physical constant: 83–84°C, FT-IR (cm−1): 2221, 1H NMR (500 MHz, CDCl3) δ 7.91 (d, J \(=\) 7.6 Hz, 2H), 7.78 (s, 1H), 7.64 (t, J \(=\) 7.5 Hz, 1H), 7.55 (t, J \(=\) 7.7 Hz, 2H).
3c. 2-(4-hydroxybenzylidene)malononitrile: Appearance: solid, color: yellow, physical constant: 183°C, FT-IR (cm−1): 2231, 1H NMR (500 MHz, CDCl3) δ 7.88 (d, J \(=\) 8.3 Hz, 1H), 7.82 (d, J \(=\) 8.3 Hz, 1H), 7.65 (s, 1H), 5.94 (bs, 1H).
3d. 2-(3,4-dihydroxybenzylidene)malononitrile: Appearance: solid, color: yellow, physical constant: 222–224°C, FT-IR (cm−1): 3275, 2221, 1H NMR (500 MHz, CDCl3) δ 7.42 (dd, J \(=\) 20.8, 13.2 Hz, 2H), 7.02 (dd, J \(=\) 17.2, 9.1 Hz, 2H), 5.85 (s, 1H), 5.39 (s, 1H).
3e. 2-(3-methoxy-4-hydroxybenzylidene)malononitrile: Appearance: solid, color: faint brown, physical constant: 138–139°C, FT-IR (cm−1): 3350–3300, 2230–2200, 1570, 1450–1550, 1270, 1240, 1H NMR (500 MHz, CDCl3) δ 7.68 (d, J \(=\) 3.2 Hz, 1H), 7.43 (s, 2H), 7.03 (s, 1H), 6.17 (bs, 1H), 3.98 (s, 3H).
3f. 2-(4-methoxybenzylidene)malononitrile: Appearance: solid, color: fluorescent yellow, physical constant: 113–114°C, FT-IR (cm−1): 2221, 1H NMR (500 MHz, CDCl3) δ 7.97–7.82 (m, 2H), 7.65 (s, 1H), 7.04–7.00 (m, 2H), 3.92 (s, 3H).
3g. 2-(2,3-dimethoxybenzylidene)malononitrile: Appearance: solid, color: light green, physical constant: 112–113°C, FT-IR (cm−1): 2350, 1550–1600, 1270, 1230, 1120, 1H NMR (500 MHz, CDCl3) δ 8.26 (s, 1H), 7.82 (dd, J \(=\) 7.0, 2.3 Hz, 1H), 7.18 (d, J \(=\) 7.2 Hz, 1H), 3.95 (s, 3H), 3.91 (s, 3H).
3h. 2-(3,4,5-trimethoxybenzylidene)malononitrile: Appearance: solid, color: fluorescent yellow, physical constant: 139–140°C, FT-IR (cm−1): 2217, 1H NMR (500 MHz, CDCl3) δ 7.65 (s, 1H), 7.19 (s, 2H), 3.98 (s, 3H), 3.91 (s, 6H).
3i. 2-(4-nitrobenzylidene)malononitrile: Appearance: solid, color: pale yellow, physical constant: 160–161°C, FT-IR (cm−1): 2239, 1H NMR (500 MHz, CDCl3) δ 8.39 (d, J \(=\) 8.7 Hz, 1H), 8.08 (d, J \(=\) 8.7 Hz, 1H), 7.88 (s, 1H).
3j. 2-(3-nitrobenzylidene)malononitrile: Appearance: solid, color: pale yellow, physical constant: 101–102°C, FT-IR (cm−1): 2231, 1H NMR (500 MHz, CDCl3) δ 8.66 (t, J \(=\) 1.9 Hz, 1H), 8.48 (ddd, J \(=\) 8.2, 2.1, 0.8 Hz, 1H), 8.33 (d, J \(=\) 7.9 Hz, 1H), 7.89 (s, 1H), 7.80 (t, J \(=\) 8.1 Hz, 1H).
3 Results and discussion
3.1 Catalyst characterization
XRD analysis of synthesized Ni–Cr oxide catalysts is shown in Figure 1. The phase formation of the catalyst was investigated by using an X-ray diffractometer. The XRD patterns of Ni–Cr (1:1) show major peaks at 2θ \(=\) 30.8°, 35.8°, 43.3°, 57.5°, and 63.1°. The diffractogram of Ni–Cr (1:2) depicts more crystallinity, as observed from the sharp peaks at 2θ \(=\) 18.4°, 30.3°, 35.8°, 43.5°, 54.0°, 57.5°, 63.2°, and 75.0°. This matches exactly with the JCPDS pattern of NiCr2O4 numbered (89-6615).25,26 In the case of Ni–Cr (2:1), the values were observed at 2θ \(=\) 30.3°, 35.7°, 53.9°, 57.6°, and 74.8° in which most of the values indicate the presence of an excess of NiO (JCPDS no. \(=\) 04-0835).27,28,29 Thus, from the XRD analysis, it can be concluded that the spinel structure is more predominant in NiCr2O4 oxide (1:2), which results in good yields of the reactions when used as a catalyst. Ni–Cr (2:1) and Ni–Cr (1:1) do not depict exact spinel structure as observed from 2θ values.
XRD analysis of Ni–Cr oxide catalysts with different Ni–Cr oxide mole ratios: (a) Ni–Cr oxide 1:1, (b) Ni–Cr oxide 1:2 (NiCr2O4), and (c) Ni–Cr oxide 2:1.
The morphology of synthesized catalysts was analyzed through SEM analysis (Figure 2). It showed that the particle size of synthesized catalysts ranges in nanometers. The particle size of Ni–Cr oxides was found in the range of 17–150 nm. The Ni and Cr mole percent composition of synthesized Ni–Cr oxide catalysts was confirmed through SEM-EDX analysis (Figure 3) and summarized in Table 1. This analysis confirms the synthesized catalysts have desired Ni–Cr oxide mole ratios. Also, considering SEM-EDX analysis (Figure 3) and pH analysis (Table 1) of all synthesized catalyst, it can be concluded that the sodium hydroxide impurity has been successfully removed from the catalyst. The BET surface analysis of catalysts (Table 1) shows that the Ni–Cr oxide (1:2) has a higher surface area among synthesized catalysts.
SEM images of Ni–Cr oxide catalysts with Ni–Cr oxides: (a) Ni–Cr oxide 1:1, (b) Ni–Cr oxide 1:2 (NiCr2O4), and (c) Ni–Cr oxide 2:1.
SEM-EDX elemental analysis of Ni–Cr oxide catalysts with different Ni–Cr oxide: (a) Ni–Cr oxide 1:1, (b) Ni–Cr oxide 1:2 (NiCr2O4), and (c) Ni–Cr oxide 2:1.
The following table represents the percent compositions of the nickel and chromium in the synthesized catalysts. For the confirmation of the composition of nickel and chromium in the NiCr2O4 spinel structure, the SEM-EDS experimental analysis was carried out (Figure 3). The analysis showed that in Ni–Cr oxide (1:2), the percentage of nickel and chromium was found to be 33.15% and 66.85%, respectively.
3.2 Synthesis of Knoevenagel condensation product by using synthesized Ni–Cr oxide catalyst
Synthesized Ni–Cr oxide catalyst was employed as a heterogeneous catalyst in condensation of 4-chlorobenzaldehyde and malononitrile model reaction to evaluate its catalytic performance. In the preliminary study, the synthesized catalyst and their individual metal oxides were tested on a model reaction between 4-chlorobenzaldehyde (1 mmol), and malononitrile (1.1 mmol) to obtain benzylidinemalonitrile derivative (3a) under grinding conditions at room temperature. The comparative study is summarized in Table 2 to understand the efficacy of the synthesized catalyst in the Knoevenagel condensation reaction. This study observed that Ni–Cr oxide 1:2 (NiCr2O4) was more effective than the other tested catalyst. Product (3a) was not observed under the same reaction conditions without a catalyst (Figure 4).
XRD analysis of Ni–Cr oxide 1:2 (NiCr2O4) catalyst at different calcination temperatures: (a) Uncalcined, (b) 200°C, (c) 400°C, (d) 600°C, and (e) 800°C.
Effect of catalyst amount on % yield (3a) of Knoevenagel condensation reaction of 4-chlorobenzaldehyde and malononitrile using NiCr2O4 catalyst. Reaction conditions: 4-chlorobenzaldehyde (1 mmol), malononitrile (1.2 mmol), grinding at room temperature for 30 min.
At optimized reaction conditions, 0.3 equivalent of NiCr2O4 catalyst was required for the maximum conversion of reactant to get the product (3a) (Figure 5). Using these reaction conditions, the NiCr2O4 catalyst gave the highest yield (92%) of the product (3a) under solvent-free grinding conditions at room temperature. Table 2 shows that NiCr2O4 was a better catalyst than other catalysts due to the synergistic effect of nickel and chromium in the spinel structure, enhancing the yield. In the case of Ni:Cr (2:1), due to excess of NiO, complete phase formation into spinel was not observed, which led to declining of the yield.
The calcination temperature of the catalyst is also one of the important parameters for getting a higher yield of the Knoevenagel condensation product. Figure 6 shows an illustrative study of the effect of the calcination temperature of NiCr2O4 catalyst on the % yield of the Knoevenagel condensation reaction. This study shows that as the calcination temperature of NiCr2O4 increases from 400°C to 800°C, the % yield of Knoevenagel condensation product (3a) also increases. This can be attributed to the transformation of metal hydroxides to the spinel phase with increasing temperature (Figure 4). This also results in the subsequent increase in activity. At 800°C, complete phase formation (spinel) results in yield enhancement.
Effect of catalyst calcination temperature of NiCr2O4 catalyst on % yield of 3a. Reaction conditions: 4-chlorobenzaldehyde (1 mmol), malononitrile (1.2 mmol), catalyst (0.3 eq.), grinding at room temperature for 30 min.
Our study shows that the Ni–Cr oxide (1:2) catalyst, i.e., NiCr2O4, gave the highest conversion among tested catalysts: NiCr2O4 was further taken for catalyst recycling study. It was observed (Figure 7) that the catalyst activity remained intact and gave a good yield for three cycles.
NiCr2O4 catalyst recycling study for Knoevenagel condensation reaction of 4-chlorobenzaldehyde and malononitrile (3a). Reaction Conditions: 4-chlorobenzaldehyde (1 mmol), malononitrile (1.2 mmol), catalyst (0.3 eq.), grinding at room temperature for 30 min.
With these reaction conditions in hand, a substrate scope study was carried out (Table 3). An array of substituted aldehydes was employed in the reaction to get the desired products. To obtain maximum product conversion of tested substrates, the reaction griding time was optimized separately for each substrate (Table 3). It was observed that the presence of strong electron-withdrawing substituents such as –NO2 or electron-donating substituents such as –OH or –OMe did not affect the yield of the reaction much. Unlike the usual trend observed, electron-donating sterically crowded substituents 2,3-methoxy and 3,4,5-trimethoxy gave good yields, requiring longer reaction grinding time. The average yield of Knoevenagel condensation products was 80%, and the average reaction grinding time was 40 min.
Figure 8 shows a possible reaction pathway for NiCr2O4 spinel-catalyzed Knoevenagel condensation reaction of benzaldehyde and malononitrile. Mixed metal oxides are proposed as powerful heterogenous catalysts when present in the spinel phase due to the possibility of fine-tuning metals in the spinel framework, which can alter the structural properties of metal oxides such as acidity, basicity, and physical properties such as surface area.30 The possible reaction mechanism is proposed based on the synergistic effect of Lewis and Bronsted acidity of Nickel and Chromium metal oxides observed in the spinel structure of NiCr2O4 as per the observations based on Pyridine FT-IR and reported reaction mechanism.31 Nickel oxide imparts Lewis acidity towards carbonyl oxygen of aldehyde, which makes carbonyl carbon more electron deficient. At the same time, the oxygen framework of the chromium oxide network interacts with the acidic proton of malononitrile and generates carbanion. As the reaction proceeds, the nucleophilic attack of carbanion occurs on electron-deficient aldehyde to give Knoevenagel condensation product at room temperature with water as a by-product.
Possible reaction pathway for NiCr2O4 catalyzed Knoevenagel condensation reaction of benzaldehyde and malononitrile.
To support these observations, synthesized catalysts were subjected to pyridine FT-IR analysis. From pyridine FT-IR analysis (Figure 9), it was observed that Ni–Cr oxide 1:2 shows peaks at 1441 cm−1 and a strong band from 1581–1609 cm−1 corresponding to hydrogen-bonded pyridine. Peaks at 1441 cm−1, 1460 cm−1, 1486 cm−1, 1492 cm−1 and a strong peak at 1544 cm−1, 1580 cm−1 show Lewis and Bronsted acidity of the catalyst. Considering reported reaction mechanisms for Ni–Cr oxide based catalyst28 and pyridine FT-IR analysis,32,33 it can be concluded that NiCr2O4 spinel catalyst possesses superior catalytic activity at room temperature due to its synergetic effect of Nickel and chromium oxide framework in Knoevenagel condensation reaction.
Pyridine FT-IR analysis of synthesized NiCr2O4 catalyst: (a) frequency range 400–4500 cm−1 and (b) frequency range 1400–1700 cm−1.
4 Conclusion
Various benzylidinemalononitrile derivatives of substituted aromatic aldehydes and malononitrile were successfully synthesized by using Ni–Cr oxide as a green heterogeneous recyclable catalyst under solvent-free, room-temperature conditions. This study successfully established a sustainable synthetic procedure for benzylidinemalononitrile through a simple grinding technique that provides excellent to moderate product yields. The simple co-precipitation method using various concentrations of Ni and Cr in synthesized catalysts provides a very efficient and eco-friendly catalytic system. The results of the comparative study of the different Ni–Cr catalysts showed NiCr2O4 spinel catalyst (Ni–Cr oxide 1:2) as the best possible choice for this reaction. Further, a plausible reaction mechanism is also described based on the results of pyridine FT-IR of catalysts. Through this study, our efforts are to explore the utility of synthesized nickel–chromium oxide catalyst as a green and sustainable heterogeneous catalyst for organic transformations.
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Acknowledgments
The authors are thankful to the Central Instrumentation Facility (CIF) of Modern College of ASC, Ganeshkhind, Pune, for FT-IR characterization, CIF of Modern College of ASC, Shivajinagar, Pune, for pyridine FT-IR characterization, CIF Savitribai Phule Pune University, Pune, for NMR, XRD and FESEM.
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Nagarkar, R.R., Purandare, R.R., Gupte, M.S. et al. Green heterogeneous nickel–chromium oxide catalyst for solvent-free, room-temperature Knoevenagel condensation reaction. J Chem Sci 136, 58 (2024). https://doi.org/10.1007/s12039-024-02292-4
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DOI: https://doi.org/10.1007/s12039-024-02292-4