Unlocking the Potential of NiSO 4 · 6H 2 O/NaOCl/NaOH Catalytic System: Insights into Nickel Peroxide as an Intermediate for Benzonitrile Synthesis in Water

SEM), energy dispersive spectrometer (EDS), X-ray difraction (XRD), and FTIR spectra. Te aqueous basic catalytic system NiSO 4 · 6H 2 O/NaOCl/NaOH (pH � 14) was investigated for the catalytic dehydrogenation of benzylamine and parasubstituents to their corresponding nitriles at room temperature. Te obtained results confrmed the formation of NiO 2 nanocrystalline particles with a size of 20nm. Benzylamine with electron-donating groups showed higher yields of nitriles compared to electron-withdrawing groups. Te mechanism involved in the in situ generated NiO 2 nanoparticles dehydrogenating benzylamine to benzonitrile, with the produced NiO converting back to NiO 2 nanoparticles through the excess of NaOCl.


Introduction
Te green synthesis of organic compounds is of great importance for the protection of the environment from the dangerous climate changes [1].Nitriles are very important functional group because of its presence in many pharmaceutical materials [2,3].Many drugs contain nitrile groups in their structures.Nitriles can enhance the biological activity of drugs by improving their stability, lipophilicity, and metabolic properties.For example, nitrile-containing drugs are used to treat hypertension, cancer, and diabetes [4].Diferent strategies have been developed for the synthesis of nitriles such as the Sandmeyer reaction [5], Rosenmund-von Braun reaction [6], dehydration of amides and aldoximes [7], nucleophilic substitution of alkyl and aryl halides [8], and oxidation of amines [9].However, these methods often involve toxic reagents, harsh conditions, and produce chemical waste.Terefore, there is a strong need for an ecofriendly protocol.A number of transition metals such as ruthenium [10,11], copper [12,13], and nickel [14,15] have been used for the catalytic hydrogenation of benzylamine to nitrile.Grifth and coworkers reported the catalytic dehydrogenation of benzyl amine by trans-[RuO 3 (OH) 2 ] 2− /S 2 O 8 2− in aqueous alkaline medium [10].Taube and coworkers reported the catalytic dehydrogenation of coordinated benzylamine to coordinated benzonitrile [11].Te simple copper-salt catalyst was used in the selective aerobic oxidation of amines to nitriles [12].A number of aliphatic and aromatic amines were oxidized to nitriles by the CuCl 2 / O 2 catalytic system in toluene as a solvent and at 80 °C [13].Nickel peroxide was used for the oxidation of many organic compounds [14].Some alkylamines were dehydrogenated to their corresponding nitriles by NiSO 4 /K 2 S 2 O 8 in good yields [15].Pd@CuO, used as an anodic electrode, resulted in the electro-oxidative coupling of benzyl alcohol and ammonia, producing 83.2% of benzonitrile.Tis method was also successful in converting various primary alcohols to nitriles [16].Xiao et al. [17] developed an efcient chemoenzymatic strategy to prepare nitriles from benzylamine by combining selective oxidation and dehydration.
Metal nanoparticles are of great interest to researchers due to their ease of synthesis, unique functional groups, and exceptional properties.Tey are signifcantly smaller than conventional materials and exhibit enhanced catalytic activity and increased thermal stability [18][19][20][21].Several methods are used for metal nanoparticles preparation, e.g., sol-gel, hydrothermal, chemical vapor deposition, laser vaporization, plane pyrolysis, precipitation, and biosynthesis [21][22][23].
Nickel peroxide (NiO 2 ) nanoparticles have been investigated for their potential use in various applications, including catalysis, energy storage, and biomedical applications.Tey exhibit high stability, high thermal conductivity, and excellent electrochemical properties [24].Te synthesis of nickel oxide nanoparticles can be achieved using various methods, including sol-gel, hydrothermal, and thermal decomposition techniques [25,26].Sodium hypochlorite (bleaching house) is cheap, available, and environmentally acceptable chemical [27].Te synthetic applications of NaOCl in organic synthesis have been reported as a part of researchers' interest in the use of transition metals as a catalysts in organic synthesis [28][29][30][31].NiO 2 serves as both a stoichiometric oxidant and a catalyst for the oxidation of benzyl alcohol.Tis implies that simultaneous stoichiometric and catalytic reactions can potentially take place in diferent reactions [31,32].
According to Ji et al. [32], nanosized NiO 2 powder can be readily obtained by oxidizing Ni(II) salt with a hypochlorite solution, followed by a wet chemical method and calcination.
In this study, NiO 2 nanoparticles will be prepared at room temperature without using any capping agent compared to recently reported [26,33,34].In addition, the optimum conditions and the mechanism of the catalytic dehydrogenation of benzylamine and some parasubstituents to their corresponding nitriles by the catalytic system, NiSO 4 •6H 2 O/NaOCl, in an aqueous basic medium were investigated.Furthermore, the emphasis is placed on investigating several factors to optimize the reaction conditions.

NiO 2 Nanoparticles
Preparation.NiO 2 nanoparticles were synthesized following a procedure described in the literature [31] but with a substitution of NaOCl and NaOH with K 2 S 2 O8 and KOH.In a typical method, 10 mL of 1.0 M NaOH was added to a solution containing NiSO 4 •6H 2 O (0.052 g, 0.2 mM) in 5 mL of H 2 O. Subsequently, commercial NaOCl (10 mL, 14 mM, 5.25%, 0.7 M) was added dropwise with stirring, resulting in the formation of fne black particles.Te precipitate was then fltered, washed with distilled water to remove NaCl, air-dried, and fnally packed:   [36].Te broadening of these peaks can be attributed to the formation of nanoparticles with a nanostructure, resulting in a smaller particle size [37].Under the current experimental conditions, the broadening observed in certain difraction peaks of NiO 2 nanoparticles XRD patterns could be attributed to the existence of minuscule particle-like structures.Furthermore, the XRD pattern exhibited "saw-tooth" refections that are typical of two-dimensional turbostratic phases with layers that lack orientation [38].Te Debye-Scherer's equation was utilized to determine the average crystallite size of NPNP as follows [39]:

Results and Discussion
where D is the average crystallite size, λ = 1.54056Å is the wavelength of CuKα radiation, β is the full width at half-maximum (FWHM) intensity of the peak in radian, θ is Bragg's difraction angle, and K is a constant usually equal to 0.9.Table 1 exposes that all XRD data of the sample and the estimated crystallite size (by using the Scherrer relation) found to vary between 11 nm and 29 nm (average 20 nm) for various identifed difraction peaks confrm the formation of the nanocrystalline structure which is in accordance with the TEM analyses.

SEM, TEM, and EDS. Te SEM micrographs in
Figure 2(a) exhibit the synthesized NiO 2 nanoparticles, indicating that the particles are spherical in shape and have formed nanoclusters due to the accumulation process.Te nanosized crystallites may experience agglomeration because of their small size, which results in a large surface energy.Tis causes the nanocrystals to aggregate during crystal growth, thereby reducing their surface energy.Te SEM images reveal that the average size of the observed NPNP is 24 nm.Te evaluation of crystallinity was performed by comparing the crystallite size determined through SEM analysis.Te resulting crystallinity index is presented in the following: Te crystallinity index, denoted as I cry , was determined using the following parameters: D p � 24 nm for the crystallite size (obtained from SEM image) and D cry � 20 nm for the crystallite size (calculated from the Debye-Scherrer equation).Since the value of I cry � 1.2 is higher than one, it is inferred that the crystallite size corresponds to a polycrystalline structure [40].
Te transmission electron microscopy (TEM) analysis of the NiO 2 nanoparticles, as illustrated in Figure 2(b), revealed their uniform size characterized by spherical shapes and smooth surfaces, exhibiting even distribution.Nevertheless, the small size and high surface energy of Journal of Nanotechnology some particles led to their aggregation into secondary particles [41].In addition, the crystals consisted of mainly spherical particles with the size of 15-20 nm from the TEM observation, which is in good agreement with the result from the XRD patterns.
Te EDS elemental analysis in Figure 2(c) confrms that the NiO 2 nanoparticles consist solely of Ni and O elements.However, the results obtained from EDS analysis (Table 2) reveal some deviations between theoretical and experimental values, which can be attributed to the presence of water attached to the NPNP [42].

FTIR Spectrum NiO 2 Nanoparticles.
Te FTIR spectrum of the NiO 2 nanoparticles sample exhibited typical features of NPNP.Te observed spectrum exhibited a signifcant and wide peak at 3390 cm -1 , indicating the stretching vibration of O-H bonds in interlayer water molecules and hydrogen-bonded OH groups.However, the distinct peak associated with the stretching mode of independent Ni-OH groups was not present due to the hydrogen bonding occurring between hydrogen atoms and intercalated anions or water molecules within the layers.In addition, the peak at 1622 cm -1 was assigned to the bending

4
Journal of Nanotechnology vibration of water.Te bands due to carbonate ion are too weak to suggesting a low carbonate ion content.Te band at 1370 cm -1 was characteristic of interlayer SO 4 2− stretching vibration resulted as a contaminant during the preparation process; the vibration at approximately 660 cm -1 was associated with Ni-O-H bonds, whereas a minor and faint vibration centered around 570 cm -1 was attributed to Ni-O stretching [43][44][45].Te FTIR spectrum of NiO 2 nanoparticles in Figure 3 displays signifcant absorption bands.Te absorption band in the 500-700 cm⁻ 1 range indicates nanocrystals, and the small sample size caused the IR band associated with Ni-O stretching vibration to shift towards the blue region.Tis shift is due to quantum size efect and spherical nanostructures of NPNP [41].

Catalytic Dehydrogenation of Benzyl Amine.
Yamazaki [46] reported the stoichiometric dehydrogenation of benzylamine to benzonitrile by NaOCl in ethanol; this encouraged us to investigate the catalytic dehydrogenation of benzylamine and some parasubstituents containing electron-donating and electron-withdrawing groups by the catalytic system NiSO 4 •6H 2 O/NaOCl/NaOH (pH � 14) to their corresponding nitriles, as shown in Scheme 1.
Benzylamine was selected as a model substrate for the optimization of the reaction conditions.Te results are given in Table 3.
It was found that benzylamine (10 mM) was smoothly dehydrogenated to benzonitrile in 95% yield within two hours.
On the other hand, a control experiment was performed in the absence of NiSO 4 •6H 2 O; the yield of benzonitrile was 20% (entry 2, Table 3).Tis result is in agreement with the reported stoichiometric dehydrogenation of benzylamine [46].
Te efect of the amount of NiSO 4 •6H 2 O on the yield of benzonotrile was studied by performing two reactions; it was found that the yield of benzonitrile was not improved.Tat is because the experiments with 0.1 mM and 0.3 mM of NiSO 4 •6H 2 O gave 51% and 94% of benzonitrile, respectively (entries 3 and 4, Table 3).
Te efect of the amount of the co-oxidant was investigated by conducting one experiment with 5 mL of NaOCl (5.25%) and another one with 10 mL of NaOCl (5.25%).Te yields of benzonitrile were 50% and 60%, respectively (entries 5 and 6, Table 3); these results were probably because the amount of the produced NiO 2 was not enough to dehydrogenate the whole amount of benzylamine and this consequently led to the formation of some other biproducts such as benzaldehyde and Nbenzylidenebenzylamine.
To study the efect of the reaction time, two experiments were conducted under the same conditions of entry 1 but with diferent reaction times, 0.5 h and 1 h, and it was found that the yields of benzonitrile were 35% and 44%, respectively (entries 7 and 8, Table 3).Tese low yields were probably because these reaction times did not allow the catalytic dehydrogenation to go to completion in addition to the formation of some other side products like Nbenzylidenebenzylamine.

Te Produced NiO. Te obtained NiO (equation (1))
powder was fltered and washed with deionized water and recycled by treatment with NaOCl and NaOH to produce NiO 2 nanoparticles that were used for the dehydrogenation of 10 mM of benzylamine.Te yield obtained was 80% of benzonitrile (entries 9, Table 3); this recycling process was repeated two times and gave 60% and 55% of benzonitrile, respectively (entry 10 and 11, Table 3).Te observed low yields in the recycling process can be attributed to the catalyst's loss during the reaction workup, as well as the loss of its active sites.

Reaction Conditions.
All reactions were conducted at room temperature using 20 ml of NaOCl (5.25%), 10 mL of NaOH (1.0 M), 0.2 mM NiSO 4 •6H 2 O, and 10 mM of benzylamine.Entries 2, 3, and 4 are as follows: the amounts of NiSO 4 •6H 2 O were 0.1, 0.3, and 0 mM, respectively.Entries 5 and 6 are as follows: the amounts of NaOCl (5%) were 5 mL and 10 mL, respectively.Entries 7 and 8 are as follows: the reaction times were 0.5 h and 1 h, respectively.Entries 9, 10, and 11 are the recycling of the produced NiO powder after  the workup.Y � yield (%) � number moles of produced nitrile × 100/number moles of benzylamine; TO � turn over � number of moles of product/number of moles of catalyst; TOF (h −1 ) � turn over frequency � number of moles of product/number of moles of catalyst per hour.
Te efect of the parasubstituents on the yield of the nitrile was studied via the dehydrogenation of four substituents containing electron-donating groups (p-CH 3 , p-CH 3 O, OH, and NH 2 ) and four substituents containing electron-withdrawing groups (p-CHO, p-CN, p-NO 2 , and p-CF 3 ).
It was noticed that the yields that were found with the electron-withdrawing group (entries 16, 17, 18, and 19, Table 4) were lower than those with the electron-donating substituents (entries 12, 13, 14, and 15, Table 4).
Te reason behind these observations could be attributed to the activation of the ring by electron-donating groups, which leads to an increase in the dehydrogenation of benzylamine into the corresponding nitrile.Conversely, electron-withdrawing groups deactivate the phenyl ring, causing a slowdown in the catalytic dehydrogenation process.
Te reaction exhibited self-indication, as evidenced by the color change of the reaction mixture from green to black upon the addition of NaOCl, indicating the formation of NPNP.Over time, this color gradually disappeared with the formation of NiO.However, this catalytic dehydrogenation reaction is considered to be selective and catalytic (benzylamine is converted mainly to benzonitrile) and inexpensive.In addition, dehydrogenation was performed at room temperature.However, the obtained results were analogous with some recently reported protocols for the catalytic dehydrogenation of benzylamine but it is appeared to be superior, simpler, and more practical than the most previously reported approaches [47][48][49].Dutta et al. reported the dehydrogenation of benzylamine to benzonitrile by a ruthenium(II) complex bearing a naphthyridinefunctionalized pyrazole ligand in 84% yield in toluene and at 70 °C [50] (our protocol did use any organic additive or organic solvent).Benzonitrile was obtained from benzylamine in 80% yield after 24 hours in dichloroethane as a solvent and at 110 °C (higher temperature and longer reaction time than those applied in this catalytic system) [51].Recently, the complex [RuCl 2 (p-cymene)] 2 catalyzed the conversion of benzylamine to nitrile in dichlorobenzene and in the presence of Me 4 NCl at 150 °C [52] (here, water was used as an ecofriendly solvent).Benzonitrile was produced from the photocatalytic oxidation of benzylamine by zirconium trisulfde (ZrS 3 ) but the workup is complicated and involved many steps [53].To conclude, this protocol was highly selective, producing only nitrile as the product, and water as the sole byproduct.Moreover, it was free from the use of any toxic solvent or chemical.

Mechanism of Catalysis.
Te time-dependent profle illustrated the mechanism of dehydrogenation of benzylamine to benzonitrile through tracing the amount of benzylamine (% BzNH 2 ) and the produced amount of benzonitrile (% BzCN) with time.As expected, the amount of benzylamine was decreased and the amount of benzonitrile was increased, as shown in Figure 4.
Te in situ-generated NiO 2 nanoparticles that were produced from the reaction of NiSO       Journal of Nanotechnology to liberate the corresponding imine (equation ( 6)).Tis imine reacted with one another molecule of NiO 2 to form the unstable PhCHNHNiO 2 (equation ( 7)) that underwent dehydrogenation to liberate benzonitrile and water (equation ( 8)).
NiO Tis catalytic cycle was repeated until the amount of benzylamine was completely consumed, as shown in Scheme 2.

Conclusion
NiSO 4 •6H 2 O and NaOCl in the presence of aqueous 1.0 molar KOH were used to generate NiO 2 nanoparticles.Te particles were characterized and found to be spherical in with an average size of 20 nm, confrming their nanocrystalline structure.Te catalytic system showed good performance in the dehydrogenation of benzylamine and some parasubstituents to benzonitriles.Optimum reaction conditions were determined, and the yields, turnover, and turnover frequency were calculated.Te reaction mechanism was analyzed, and it was determined that the in situgenerated NPNP was the active species accountable for the dehydrogenation process.Tis promising reaction has several advantages as follows: the chemicals are inexpensive, water is used as a solvent at ambient temperature, the yields and turnover are good, and the produced NiO can be recycled for further catalytic reactions.

Figure 4 :Scheme 2 :
Figure 4: Te time-dependent profle: the red column is the percentage yield of benzylamie (BzNH 2 ) and the green column is the percentage yield of benzonitrile (BzCN) with diferent time intervals.

Table 2 :
Te EDS data obtained from the elemental analysis of the NiO 2 nanoparticles. 4 [54]2 O with NaOCl and in the presence of NaOH (equation (1)) were attacked by benzylamine forming the unstable intermediate PhCH 2 NH 2 NiO 2 (equation (5)), which is further dehydrogenated through the hydride abstraction mechanism[54]

Table 3 :
Optimization of the reaction conditions for the catalytic dehydrogenation of benzylamine to benzonitrile by NPNP.

Table 4 :
Te scope of the catalytic dehydrogenation of benzylamines to nitriles NPNP.