Synthesis, characterization of Co1-xNixAl2O4 Spinel system and catalytic role in the synthesis of dihydropyrimidinone

In the present investigation, Spinel systems with chemical composition Co 1 − x Ni x Al 2 O 4 (x = 0.0, 0.25, 0.5, and 0.75) have been successfully synthesized by the co-precipitation citrate precursor technique. The phase formation, crystal structure, and impurity check were conrmed by X-ray powdered diffraction (XRD) and Fourier transforms infrared (FTIR) spectroscopy technique. The particle size estimation was done using a transmission electron microscope (TEM). Investigation of magnetic behavior and parameters such as saturation magnetization (M S ), coercivity (H R ), and retentivity (M R ) was done using a vibrating sample magnetometer (VSM). The catalytic activity of prepared spinel systems was explored for the one-pot synthesis of dihydropyrimidinone derivatives. The catalytic product was identied by comparison of melting point and the spectral data (FTIR).


Introduction
The nanostructured cobalt aluminate spinels have been the center of attraction due to their unique electrical, magnetic, optical, and catalytic properties. These materials nd their application in the eld of high-density magnetic recording, microwave devices, applications such as plastics coloring and glass coatings, etc. [1][2][3][4][5]6,7]. The unusual magnetic behavior of FCC structure resulting from geometrical frustration makes it a system of great importance as it is an exemplary system possessing degenerate frustrated ground states which are the result of competition between various exchange interactions and further lead to spin-liquid behavior [8][9][10]. The spinel composition CoAl 2 O 4 is also known for its catalytic properties arising from high surface area and optimum basicity of the spinel structure and hence have been the subject of an investigation to explore their potential catalytic applications for organic synthesis [11][12][13][14]. Studies have shown that these spinel oxides are better catalysts on account of their activity and stability. The phenomenon of synergism in two or three metal-based catalysts is a fascinating aspect of catalytic research [15].
Researchers have synthesized cobalt aluminates using several methods such as microwave combustion method, low-temperature combustion method, sol-gel process, hydrothermal method, reverse microemulsion process, sonochemical process, polymeric-aerosol pyrolysis, freeze-drying, ultrasonic-  3 were dissolved in 100mL of double-distilled water in a beaker labeled as 'A'. In another beaker labeled as 'B, an equimolar amount of citric acid was dissolved separately in double-distilled water. The citrate solution from beaker B was added dropwise to the solution in beaker A and continuously stirred on a magnetic stirrer for 2 hours with the temperature maintained at 60°C. The temperature was raised further to initiate slow evaporation and reduce the quantity to half, leading to the setting of the desirable gel. The gel thus obtained was subjected to an overnight and slow decomposition process at an elevated temperature of 200°C. The uffy mass obtained at the end of decomposition was then subjected to heat treatment at 300°C to eliminate the carbonaceous matter. The dried powder was ball milled for 4 hours to attain uniformity in particle size. The processed powders were further heated at nally 800°C in the air for a total time of 10 hours, and were furnace cooled, and stored in airtight containers for characterization. The process of material preparation is as shown in Figure 1.

Characterization of as-synthesized materials:
The prepared materials were characterized by 3 Results And Discussion 3.1 X-ray diffraction analysis The prepared compositions were characterized by X-ray powder diffraction. The X-ray diffraction patterns were recorded at room temperature with a 2θ scanning angle ranging from 20° to 70°. Figure 2 shows the Rietveld re nement of the XRD pattern for the Co indicating the formation of pure spinel phase belonging to Fd3m space group without any impurity [19,20]. The goodness of re nement quality is evident from the re nement parameters (c2, R wp , R exp , RBragg, and RF) listed in table 1. The variation of lattice constant 'a', x-ray density, and cell volume as a function of Ni +2 concentration are shown in Figure 3 (a,b) The lattice constant was seen to decrease initially for x=0.0 to x=0.25 and further showed a marginal rise with increasing Ni +2 at the tetrahedral site. The initial decrease in lattice constant can be attributed to smaller ionic radii of Ni +2 ions (0.83Å) replacing the larger Co +2 ions with ionic radii of 0.88Å [21]. For the compositions beyond x=0.25 i.e. for x=0.50 and 0.75, the increase in lattice constant could be due to electrostatic shielding caused by 3d electrons of Ni +2 ions diluting the nuclear dominance and causing marginal expansion in the tetrahedral environment [22]. A trend similar to that of lattice constant variation was observed in the values of cell volume while x-ray density was seen to vary inversely ( Figure 2b).  [23][24][25][26][27]. No other impurity or organic matter absorption bands were observed indicating the purity of the materials, in agreement with the results obtained by XRD. FTIR measurements were also helpful to illustrate the formation of spinel from the precursor.

VSM data analysis
The magnetic hysteresis loops obtained at room temperature for Co 1-x Ni x Al 2 O 4 nano-powders are shown in Figure 5 The samples were seen to exhibit paramagnetic behavior [1]. The values of saturation magnetization (M S ), coercive eld (H C ), and remnant magnetization (M R ) are listed in Table 3. The M S values were seen to vary with Ni +2 content and showed a maximum value of 0.72emu/g for x=0.5 and a minimum of 0.51emu/g for x=0.25 while the remnant magnetization was seen to remain almost constant with values ranging between 0.03emu/g to 0.05emu/g. The squareness values calculated using the following equation (1) were found to be very low con rming the paramagnetic behavior of the nanopowders. However, these values were much higher than the reported squareness limit for superparamagnetic materials [28][29][30]. The magnetic moment m calculated for all the samples using equation (2) was seen to remain in the range of 0.016 to 0.022 Bohr magneton with increasing Ni concentration in the spinel lattice [31][32][33][34][35].
Where Mx is the molecular weight of the composition.
The anisotropy constant was calculated using equation 3 given below showed a trend similar to that of HC. The plot of variation in anisotropy constant 'K' with increasing Ni concentration is shown in gure 6.

TEM image analysis
The transmission electron micrographs obtained on CoAl 2 O 4 and Co 0.25 Ni 0.75 Al2O 4 nanocrystalline powders along with particle size distribution histograms are shown in Figure 7. The observed particle size of these nanocrystalline spinel aluminates was in the range of 20 nm to 47 nm.

The Bignelli reaction as a model catalytic application
The as-synthesized spinel Co-Ni aluminate compositions were explored for catalytic e ciency of synthesizing dihydropyrimidinone derivative as a model test reaction as shown in Figure 8. A solution of Benzaldehyde (10 mmol, 1.06 g), ethylaceto acetate (13mmol, 1.69 g), and urea (15 mmol, 0.90g) was re uxed at 85-90°C in ethanol in the presence of materials (0.2g) under investigation, for 3 hours. On completion of the reaction, the catalyst was ltered off from the mixture, and the ltrate was collected in crushed ice. The product obtained was recrystallized using ethyl acetate. The synthesized product was identi ed by comparison of melting point (mp) and the spectral data (FTIR).
The catalytic product was con rmed by melting point (mp) and FTIR spectroscopy.  [35]. The e ciency nanopowders of Co 1-X Ni X Al 2 O 4 can be enhanced to produce higher yield by increasing the Ni concentration in the composition as these nanopowders possess higher surface area in comparison to that of heteropoly acids.  Figure 9 shows the FTIR spectra of the product and the characteristic absorbance peaks which are summarised in Table.5, con rming the formation and purity of dihydropyrimidinone The melting point of the puri ed product was found to be ranging between 205 o C to 207 O C which again indicated the purity of the reaction product [36][37][38][39][40]. solution. Samples were seen to exhibit pure spinel phase and particle size ranging between 25 nm to 40 nm. Structural parameters such as lattice constant, cell volume, and X-ray density were seen to change with varying Ni +2 concentrations.
The FTIR spectra con rmed the existence of characteristic spinel bands con rming the phase purity. All the samples exhibit paramagnetic behavior with saturation magnetization varying between 0.7 to 0.5 emu/g. The application of Co 1-x Ni x Al 2 O 4 nanopowders as a catalyst in the synthesis of dihydropyrimidinone was investigated. The reaction showed an enhancement of 15 percent in percentage yield with pure CoAl 2 O 4 employed as catalyst. The percentage yield was seen to increase further by 21 percent with increasing Ni content in the spinel structure. The e ciency of Co 1-x Ni x Al 2 O 4 can be improved further by increasing Ni concentration and can be used as more e cient catalyst due to its larger surface area.

Declarations
We the authors of the manuscript entitled "Synthesis, characterization of Co 1-x Ni x Al 2 O 4 Spinel system and catalytic applications in the synthesis of dihydropyrimidinone" declare that 1. The article is original.
2. The article has been written by the stated authors who are all aware of its content and approve its submission.
3. The article has not been published previously 4. The article is not under consideration for publication elsewhere 5. No con ict of interest exists, or if such con ict exists, the exact nature must be declared.
If accepted, the article will not be published elsewhere in the same form, in any language, without the written content of the publisher. The report also includes the investigation of the catalytic e ciency effect of Ni +2 substituted cobalt aluminate in synthesizing dihydropyrimidinone via Bignelli reaction.

Figure 6
Variation of anisotropy constant 'K' with Ni concentration Figure 7 Transmission electron micrographs and particle size distribution histograms of CoAl2O4 and Co0.25Ni0.75Al2O4

Figure 8
Bignelli reaction as a model catalytic application FTIR spectra of dihydropyrimidinone using Co1-XNiXAl2O4 nanocatalysts.

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