Unveiling New Product Formations beyond Conventional Pathways in De-Halogenation of Halo-Acetic Acids Using Ni-Encapsulated Sol-Gel Catalysts ^†

Abstract

The urgency of water remediation and the conversion of toxic pollutants into non-toxic compounds is increasingly crucial in our industrialized world. Heterogeneous catalysts based on metal nanoparticles, which are cost-effective, non-toxic, and readily available, have garnered significant attention in the market due to their unique catalytic properties. This study presents sol–gel-based hybrid silica matrices that encapsulate nickel, designed for the efficient reductive de-halogenation of tri-bromoacetic acid (TBAA), di-bromoacetic acid (DBAA), mono-bromoacetic acid (MBAA), tri-chloroacetic acid (TCAA), mono-chloroacetic acid (MCAA), and Chloroacetanilide (CAA). A detailed study of the product distribution from each halo-acetic acid (HAA) is presented. The study points out that other products are formed from Ni-catalyzed reduction reactions of HAAs, breaking the conventional rules of stepwise reduction mechanisms. The plausible mechanisms of the catalytic processes are discussed.

1. Introduction

Water—the most needed and inevitable part of our lives—comes from different resources around us. Since, in most cases, it is contaminated with various bacteria, viruses, and parasites, chlorine disinfection is considered a popular solution. However, drinking water disinfection by-products (DBPs) are an unintended consequence of this, formed by the reaction of disinfectants with naturally occurring organic matter [1]. Scientists across various disciplines find reducing toxic compounds into non-toxic forms a compelling area of study. Specifically, bromo-organic and chloro-organic compounds, categorized as disinfection by-products (DBPs) and implicated in numerous health hazards, demand attention [2].

In the past, these reactions were studied using Au⁰-NPs [3], Ag-NPs [3] and Fe⁰-NPs [4] encapsulated in organically modified silica (ORMOSIL) matrices as catalysts for this purpose. The results pointed out that the nature of the metal in the M⁰-NPs dramatically affects the nature of the products obtained, i.e., the mechanism of de-halogenation is affected by the nature of the metal, though one uses the same reducing agent, BH₄⁻, and the same procedure [3,4]_. We thought it could be interesting to check the catalytic properties of another first-row transition metal, and thus Ni was chosen as a less toxic, economical, and abundant metal [5]. Two catalysts were studied: Ni(0)@ORMOSIL and Ni(II)@ORMOSIL. The latter was reported to be extremely efficient in the catalytic reduction of nitrobenzene [6]. This study focuses on the de-halogenation of bromo- and chloro-derivatives of acetic acid, employing the sodium borohydride reduction method [7]. Surprisingly, the results point out that fumaric acid (FA) is a major product of the catalytic reduction of tribromoacetic acid (TBAA) and trichloroacetic acid (TCAA), along with maleic acid (MA) in trace amounts. These products were not observed when other M⁰-NPs were used as catalysts. Notably, the results demonstrate a higher efficiency and superior reduction compared to earlier reported outcomes using Au⁽⁰⁾ [3], Ag⁽⁰⁾ [3], and Fe⁽⁰⁾ [4].

The sol–gel technique stands out as a straightforward and effective approach for entrapping large molecules within a matrix [8]. Additionally, sol–gel chemistry provides a heightened level of precision in controlling particle size and morphology [9]. The matrices used in this study are M@ORMOSIL (organically modified silicates), representing a hybrid sol–gel matrix. This type of matrix is prepared by adjusting the molar percentage and alkyl chain length of the organic group. Catalysts enclosed within the inner pores of these matrices exhibit increased efficiency, heightened activity, enhanced selectivity and greater stability within the confines of the organosilica gels compared to catalysts in solutions [10]. This not only facilitates the separation of the solid catalytic material but also streamlines its efficient recycling [10]. Pre-catalyst sol–gel matrices are prepared by adsorbing transition metal cations through an ion-exchange mechanism and reducing it to the zero-valent state during the catalytic test reaction, using sodium borohydride as previously explained [6]. This catalyst preparation method was discovered to be more straightforward than nanoparticle encapsulation, exhibiting stability and allowing the use of various solvent media. Additionally, it proves to be more efficient in catalysis, as evidenced in Meistelman’s study comparing ZVI@ORMOSIL and Ni(II)@ORMOSIL [6].

2. Results and Discussion

2.1. Catalyst Characterization

The characterization of the Ni(0)@ORMOSIL catalyst was performed in this study, the characterization of the Ni(II)@ORMOSIL catalyst was previously reported [6]. The SEM images along with the EDX results for the blank ORMOSIL and nickel-encapsulated ORMOSIL are shown below (Figure 1). The EDX results validate the existence of Ni enclosed within the matrix, alongside Si, O, and C. An analysis of certain specific areas with few white spots visible on the surface indicated that Ni is present only as minimal traces on the surface, while most of it is well encapsulated inside, as is expected for dopped matrices prepared via the sol–gel synthesis route.

When analyzing the XRD results, as anticipated, a broad peak of amorphous SiO₂ was consistently observed at a 2θ angle of 23.8° in the case of ORMOSIL (SiO₂) and Ni-encapsulated ORMOSIL. For the synthesized Ni nanoparticles, specific crystalline syn phases of Ni at 2θ angles of 44°, 51° and 76° corresponding to (1 1 1), (2 0 0) and (2 2 0) planes, respectively, were observed. Along with these peaks, nickel boride peaks were also visible at 2θ angles of 37° 38°,40°, 42°, 46°, 47°, and 49° corresponding to the (2 1 0), (1 2 1), (2 0 1), (2 1 1), (0 3 1), (1 1 2), and (2 2 1) planes, respectively. But in the case of 1% Ni(0)@ORMOSIL, the intensity of these peaks was poorly visible, which was probably due to its lower Ni content. (see Figure 2). From the XRD data of the Ni(0) nanoparticles, we calculated the particle size using the Scherrer equation, which came to around 25 nm [11]. The TEM image taken for Ni nanoparticles is provided in Figure S5.

The N₂ adsorption–desorption isotherms obtained for the catalysts belonged to the type-IV category (see Figure 3), suggesting the mesoporous nature of the sol–gel matrix. The surface area, pore volume, and pore diameter obtained are presented in

Table 1. B.E.T. analysis results.

	Average Pore Radius (Å)	Surface Area (m2/g)	Pore Volume (cm3/g)
1% Ni(0)@ORMOSIL	11.38	539	0.31
1% Ni(II)@ORMOSIL	18 [6]	602 [6]	0.34 [6]
Blank ORMOSIL	11.45	639.9	0.37

:

2.2. Catalytic Activity and Mechanism

2.2.1. MBAA Reduction Reaction

The blank matrices do not catalyze the reduction of MBAA. However, 0.05 g of 0.5% Ni(II)@ORMOSIL converts 100% of 0.01 M of MBAA into AA with an [NaBH₄]/[MBAA] mole ratio of 4. This is considerably more efficient than the earlier reported results on the de-bromination of MBAA by ZVI-entrapped ORMOSIL [4]. This result indicates that Ni(II)@ORMOSIL is a considerably better catalyst for de-bromination; therefore, we decided to study its performance.

2.2.2. TBAA Reduction Reaction

Next, we decided to study the mechanism behind TBAA reduction by determining the product distribution using Ni(0)@ORMOSIL as the catalyst. The effects of varying the ratio between the substrate and the reducing agent (Figure 4) and employing different amounts of catalyst (Figure 5) were studied.

The results point out that the following:

• In all of the experiments we conducted, no traces of TBAA were detected. The findings indicate that sodium borohydride is capable of fully converting TBAA into DBAA. Previous studies have highlighted the rapid hydrolysis of TBAA in the presence of water, leading to the immediate formation of DBAA [12].
• The de-bromination increases with the increase in the catalyst used, at constant concentrations of TBAA and BH₄⁻. This suggests that the competition between de-bromination and hydrogen evolution is dramatically affected by the amount of BH₄⁻/H adsorbed by each Ni(0) nanoparticle. More concisely, the notable reduction in the amount of DBAA as well as MBAA suggests higher de-bromination which eventually leads to less available H atoms on the surface for evolution. Meanwhile, the improved ratio of BFA/FA to AA indicates that the radicals react with each other leading to dimerization, rather than reacting with hydrogen leading to AA.
• The formation of FA and bromo-fumaric acid (BFA) proves that CBr_kH_2−kCO₂⁻ radicals are involved in the process. It is of interest to note that analogous results were reported for Au(0)@ORMOSIL [3,13] but not for Ag(0)@ORMOSIL [3] and Fe(0)@ORMOSIL [4]. Interestingly, in the catalytic de-bromination of TBAA by Au(0)@ORMOSIL, succinic acid is the final product, whereas for Ni(0)@ORMOSIL, the final products are FA, MA, and BFA.

Fumaric acid, maleic acid, and bromo-fumaric acid were conclusively identified through the analysis of both the ¹H and ¹³C NMR data. This confirmation is essential because the quantitative representation of these compounds was comparatively lower in the HPLC results. Detailed spectra showing the peak identification using standards are presented in Figure S1. The ¹H NMR data are presented in Figure 6, and the ¹³C NMR data in Figure S2. ¹³C NMR was crucial since both DBAA and MA show the same chemical shift value in the ¹H NMR data.

A comparison of the properties of the four types of nickel catalysts (Ni(H₂O)₆²⁺, Ni(II)@ORMOSIL, Ni⁰-NPs suspended in water and Ni(0)@ORMOSIL) was performed. The results presented in Figure 7 point out that:

• All these catalysts induce de-bromination, forming the same products but in different ratios. The yields of bromide were measured using ion chromatography. The results that corroborate the organic products are summarized in Table S1.
• The de-bromination with Ni(H₂O)₆²⁺, as measured by the combined amounts of DBAA and MBAA, is somewhat better than that with Ni(II)@ORMOSIL, and Ni⁰-NPs suspended in water surpasses that with Ni(0)@ORMOSIL. This is tentatively attributed to the location of the Ni(0)-NPs in the pores of the ORMOSIL matrices. Thus, BH₄⁻ probably reacts with the Ni(0)-NPs when no substrate is around, increasing the hydrogen evolution reaction (HER) yield [14].
• The yield of FA by the ORMOSIL catalysts is lower than that observed in the aqueous solutions. This is attributed to the lower chance that two TBAA substrates will be present near a Ni(0) entrapped in a pore when it is reduced.
• The ORMOSIL catalysts are still preferred due to the possibility of recycling them.

2.2.3. DBAA Reduction Reaction

Experiments involving the reduction of DBAA using 1% Ni(0)@ORMOSIL in two different ratios, as shown in Figure 8, confirm the expected outcome: higher yields of AA, FA, and BFA compared to the reduction of TBAA (Figure 4). A significant improvement in reduction is observed with a decrease in the [DBAA]:[NaBH₄] ratio, resulting in higher FA yields. Interestingly, minimal amounts of maleic acid (MA) were detected in this scenario, indicating a notable preference for FA formation over MA. This preference is attributed to the dimerization of CHBrCO₂^.− radicals on the Ni⁰-NPs surface, favoring the formation of trans-⁻₂OCCHBrCHBrCO₂⁻.

2.2.4. TCAA, MCAA, and CAA Reduction Reactions

De-chlorination of TCAA and MCAA is more difficult due to the stronger C-Cl bond than the C-Br bond in TBAA and MBAA [15]. Optimization of the reaction conditions using 0.5 g of 1% Ni(II)@ORMOSIL, required ratios of [TCAA]:[NaBH₄] = 1:20 and [MCAA]:[NaBH₄] = 1:15 with a substrate concentration of 0.05 M. The reaction time needed for completion was approximately 5 h. The reduction of TCAA yielded 49% MCAA, 21% AA and the remaining 30% consisted of FA and MA. This is quite surprising as the catalytic reduction of TCAA using Ag⁰-NPs, Au⁰-NPs, and Fe⁰-NPs encapsulated in ORMOSIL catalysts yielded no products stemming from the dimerization of radicals on the surfaces of the M⁰-NPs [3,4].

An attempt to reduce MCAA resulted in its 27% conversion into AA alone. Along with this, we tested another major category of pollutants containing the aniline group, specifically the 2-chloroacetanilide (CAA). Upon induction with NaBH₄ in two different ratios with the CAA in the presence of a Ni catalyst, 2-chloroacetanilide (CAA) underwent reductive dehalogenation, yielding phenylacetamide (PA), Figure 9.

Finally, the best results of the catalytic reduction of different substrates with Ni(II)@ORMOSIL were consolidated and are shown in Figure 10.

2.2.5. Mechanistic Analysis

It is commonly accepted that the M⁰-NP-catalyzed reductive de-halogenations by BH₄⁻ are initiated by reaction (1) that is followed by reaction (2) in aqueous media [13,16].

M⁰-NP + (n/4)BH₄⁻ + (3/4)n H₂O → (M⁰-NP)-H_nⁿ⁻ + (n/4)B(OH)₃ + (3 n/4)H⁺

(1)

(M⁰-NP)-H_nⁿ⁻ + m H₂O ⇄ (M⁰-NP)-H_n+m^(n−m)− + m OH⁻

(2)

The equilibrium constant of reaction (2) depends on the nature of M and upon the pH of the solution, and with the formation of B(OH)₃ and OH⁻, the reaction medium will be slightly alkaline. However, recent DFT studies indicate that reaction (1) involves the transfer of less hydrogen to the M⁰-NPs’ surfaces, at least for Au⁰-NPs and Ag⁰-NPs [17,18,19]. Furthermore, some BH_k(OH)_4−k⁻ remains adsorbed on the M⁰-NPs. No detailed mechanism of the hydrolysis of BH₄⁻ on Ni⁰-NPs by DFT calculation was performed. In the absence of an oxidizing substrate, the next step is the hydrogen evolution reaction either by (3) or (4) or (5).

(M⁰-NP)-H_n+m^(n−m)− → (M⁰-NP)-H_n+m−2^(n−m)− + H₂

(3)

(M⁰-NP)-H_n+m^(n−m)−-H₂O → (M⁰-NP)-H_n+m−1^{(n−m−1)−} + H₂ + OH⁻

(4)

[(M⁰-NP)-H_n+ℓ]-BH_k(OH)_4−k^{(n−ℓ+1)−} → [(M⁰-NP)-H_n+ℓ−1]-BH_k−1(OH)_4−k^(n−ℓ)− + H₂

(5)

In the presence of an oxidizing substrate, reactions (3)–(5) compete with the reduction of the substrates which results in its de-halogenation. This can occur in four possible ways: either by a hydrogen atom transfer (6) or by an electron transfer (7) or by a reaction of the substrate with the adsorbed BH_k(OH)_4−k⁻ (8) or (9) resulting in the formation of a radical.

(M⁰-NP)-H_n+m^(n−m)− + X_kCH_3−kCO₂⁻ →
{(M⁰-NP)-H_n+m−1}^(n−m)− + X_k−1CH_3−kCO₂^∙− + X⁻ + H⁺   (k = 1; 2; 3 X = Br or Cl)

(6)

{(M⁰-NP)-H_n+m}^(n−m)− + X_kCH₃₋kCO2⁻ → {(M⁰-NP)-H_n+m}^{(n−m−1)−} + X⁻ +
X_k−1CH_3−kCO₂^∙−

(7)

{(M⁰-NP)-H_n+m}-BH₄^(n−m+1)− + X_kCH_3−kCO₂⁻ →
{(M⁰-NP)-H_n+m}-BH₃^(n−m+1)− + X_k−1CH_3−kCO₂^∙− + X⁻ + H⁺

(8)

{(M⁰-NP)-H_n+m}-BH₄^(n−m+1)− + ^XkCH3_−kCO₂⁻ → {(M⁰-NP)-H_n+m}-BH₄^(n−m)− +
X_k−1CH_3−kCO₂^∙− + X⁻

(9)

The radicals thus formed are expected to react with the M⁰-NPs to form an M-C bond as the reactions of alkyl radicals with M⁰-NPs are known to be fast [20,21]. Alternatively, the radicals X_k−1CH_3−kCO₂^.− might abstract a hydrogen atom from the hydrogen atoms bound to the M⁰-NPs or from the adsorbed BH_k(OH)_4−k⁻.

{(M⁰-NP)-H_n+m}^{(n−m−1)−} + X_k−1CH_3−kCO₂^∙− → {(M⁰-NP)-H_n+m}-CX_k−1H_3−kCO₂H^(n−m)−

(10)

The transients {(M⁰-NP)-H_n+m}-CX_k−1H_3−kCO₂H^(n−m)− thus formed decompose via one of the following reactions:

{(M⁰-NP)-H_n+m}-CX_k−1H_3−kCO₂H^(n−m)− → {(M⁰-NP)-H_n+m−1}^(n−m)− +
CX_k−1H_4−kCO₂H

(11)

{(M⁰-NP)-H_n+m}-X_k−1CH3_−kCO₂H^(n−m)− + H₂O →
{(M⁰-NP)-H_n+m}^{(n−m−1)−} + CX_k−1H_4−kCO₂H + OH⁻

(12)

[{(M⁰-NP)-H_n+m}-BH₄]-X_k−1CH₃₋kCO2H^(n−m+1)− →
[{(M⁰-NP)-H_n+m}-BH₃]^(n−m+1)− + CX_k−1H_4−kCO₂H

(13)

For transients of the type {(M⁰-NP)-H_n+m}-(X_k−1CH_3−kCO₂H)_l^(n−m)− and (k−1) ≥ 1, the results prove that for M = Ni the following reaction occurs.

{(Ni⁰-NP)-H_n+m}-(X_k−1CH_3−kCO₂H)_l^(n−m)− →
{(Ni⁰-NP)-H_n+m}- (X_k−1CH_3−kCO₂H)_l−2^(n−m)− + (-CX_k−1H_3−kCO₂H)₂

(14)

A reaction analogous to reaction (14) was reported only for X = Br and k−1 = 2 on Au⁰-NPs. This reaction does not occur on Fe⁰-NPs [4] and Ag⁰-NPs [3] and no M⁰-NPs for X = Cl [3,4,13].

The (-CX_k−1H_3−kCO₂H)₂ are further de-halogenated to form the radicals (HO₂CX_k−2H_3−k-CX_k−1H_3−kCO₂H). These radicals then react with the Ni⁰-NPs to form {(Ni⁰-NP)-H_n+m}-(HO₂CX_k−2H_3–k-CX_k−1H_3−kCO₂H)^(n−m)−. The latter transients are expected to decompose via β-elimination⁸ to form HO₂CX_k−2H_3−k = CX_k−2H_3−kCO₂H. When (k−2) > 0, de-halogenation forms a mixture of fumaric acid and bromo-fumaric acid for X = Br. To the best of our knowledge, this is the first system for which the de-halogenation of a chloro-compound is easier than that of a bromo-compound. It is also of interest to note that the results point out that fumaric acid is the major product of the β-elimination process and only traces of maleic acid are formed.

Surprisingly, on Ni⁰-NPs, fumaric acid is the final product, whereas on Au⁰-NPs [3] and Fe⁰-NPs [4] the final product of de-bromination is succinic acid.

3. Materials and Methods

3.1. Materials

NiCl₂∙6H₂O > 98%, trichloroacetic acid (TCAA) 99%, monochloroacetic acid (MCAA) 99%, and dichloroacetic acid (DCAA) 99% were purchased from Alfa Aesar (Heysham, UK). Tri-bromoacetic acid (TBAA) 97%, di-bromoacetic (DBAA) 96%, mono-bromoacetic acid (MBAA) 98+%, tetraethyl-orthosilicate (TEOS) ≥ 99%, methyl-trimethoxy-silane (MTMOS) ≥ 99.0%, and APS (3-Aminopropyl-triethoxysilane) were purchased from Sigma Aldrich (Saint Louis, MO, USA). Acetic acid (AA) (glacial 99.7%), fumaric acid (FA) 99% and NaBH₄ powder (99%) were purchased from Thermo Scientific (Waltham, MA, USA). Hydrochloric acid (HCl, 37%), HPLC grade 85% H₃PO₄, maleic acid (MA) and LCMS-grade ethanol were purchased from Merk (Boston, MA, USA). The 30% NH₃ solution was purchased from Carlo Erba Reagents (Cornaredo, Italy). Acetonitrile (LCMS-grade) was purchased from Bio Lab Ltd. (Jerusalem, Israel). 2-chloroacetanilide (CAA)/2-chloro-N-phenyl acetamide (95%) was purchased from Angene (Nanjing, China). All chemicals were of A.R. grade and were used as received. All aqueous solutions were prepared from deionized water purified by a Millipore Milli-Q set up with a final resistivity of >10 MΩ/cm.

3.2. Instrumentation and Measurements

The Brunauer–Emmett–Teller (BET) isotherm measurements for the determination of specific surface area, pore volume, and pore size distribution were conducted utilizing a Nova (Tokyo, Japan) 3200e Quantachrome analyzer. Before analysis, the samples underwent a 2 h degassing at 120 °C under vacuum. The surface area was derived from the linear segment of the BET plot, while the pore size distribution was assessed employing the Barrett–Joyner–Halenda (BJH) model and the Halsey equation. To understand the surface morphology of the catalysts synthesized, SEM images along with the elemental composition assessment and Energy Dispersive X-ray spectroscopy (EDS) were taken. The analysis was performed on an Ultra-High-Resolution MAIA 3 FE-SEM (Tescan, Brno, Czech Republic) with Microanalysis -EDX Aztec (Oxford) with an X-MAX^N active area of 80 mm² and a resolution of 127 eV. The high-resolution transmission electron microscopy (HRTEM) was done using a JEOL (Tokyo, Japan) 2100, operated at 200 keV. Phase identification and a deeper understanding of the sample purity were achieved through powder X-ray diffraction analysis using the diffractometer Rigaku SmartLab SE (Akishima, Japan) with Cu-Kα radiation (λ = 1.546 Å) using the database from ICDD (Newtown Square, PA, USA) PDF-2 2019 and an X-ray generator at 40 kV. Measurements were made in θ/2θ geometry in the range of 10–90° with a step of 0.03 and a rate of 0.3°/minute. The phase analysis was performed using SmartLab (Singapore) Studio II version 4.2.44.0 with the powder XRD module. Ion-chromatography (IC) was conducted on the Dionex ICS-IC system from Thermo Fisher Scientific (ICS 2100 anions and ICS 1100 cations).

De-halogenation of the halo acetic acids was monitored by using an HPLC system, Dionex Ultimate 3000 (Germering, Germany) with a UV/Visible detector (λ = 210 nm) along with an Agilent HPLC column (Eclipse XDB-C18, 3 μm) (Vilnius, Lithuania), with dimensions of 4.6 × 150 mm. The eluent was H₂O: acetonitrile; 98:2 with 0.2% ortho-phosphoric acid (H₃PO₄), maintaining a pH between 2–3, a flow rate of 1 ml/minute, and a 25 °C column oven temperature. ¹H and ¹³C NMR spectra (400 MHz and 100 MHz) were recorded on a Bruker advanced III HD NMR Spectrometer equipped with a 5 mm BBFO probe and Z-axis gradient coil. The samples were dissolved in a mixture of 90% H₂O/10% D₂O.

3.3. Syntheses

The catalysts were prepared using a two-step acid-base sol-gel synthesis method as previously described [6]. A mixture of 8.9 mL of ethanol (0.152 moles), 6.0 mL of TEOS (0.027 moles), and 1.6 mL of MTMOS (0.011 moles) was thoroughly blended to achieve a uniform solution. In this case, the ratio of TEOS to MTMOS precursors was set at 70:30. To this mixture, 2.8 mL of 0.26 M HCl solution was added drop by drop. In the homogeneous mixture, 100 µL of 3-Aminopropyl-triethoxysilane (APS) was added. After thorough mixing, NiCl₂.6 H₂O was introduced to make up the desired mole percentage (either 0.5% or 1%) of the total silica content, followed by adding 1.5 mL of 2% NH₃ solution to facilitate the conversion of the solution into a gel. The resulting solution was continuously stirred to promote gelation and later kept for aging and drying. Subsequently, the dried gel was finely powdered, thoroughly washed with water, dried again, and used for catalytic experiments.

In this study, catalysts containing Ni(0) incorporated within the sol-gel matrix were studied. To achieve this, nickel nanoparticles were synthesized and encapsulated within the sol-gel structure. The nanoparticles were produced through a chemical reduction method employing sodium borohydride as the reducing agent [22]. In brief, the necessary metal salt (NiCl₂^.6 H₂O) was dissolved in de-ionized water, followed by the gradual addition of NaBH₄ in a 1:6 molar ratio with the metal salt solution. The pH was adjusted using a NaOH solution, and the entire reaction was conducted under a nitrogen atmosphere. After thorough mixing for one hour, the resulting nanoparticles were separated from the solution, washed multiple times with ethanol and de-ionized water, and then vacuum-dried. The synthesized nanoparticles were subjected to XRD analysis. These were incorporated into the ORMOSIL by adding a 1.0 M ethanolic suspension of these nanoparticles in an amount that constituted a 1-mole percentage of the total silica content. This addition was carried out after adjusting the pH using an NH₃ solution during the sol–gel synthesis stage [4,6].

Blank matrices were also prepared following the same procedure without the addition of nickel salt.

3.4. Catalytic Tests

For the catalytic tests, the required substrate solution was combined with a suspension of the catalyst in de-ionized water. The reaction was conducted at room temperature, maintaining a pH range between 1–2. After stirring for a short duration, solid NaBH₄ was gradually added, leading to a change in pH to a range of 8–9. The resulting solution was stirred for a total of 60 min to ensure a complete reaction (the reaction time had been previously optimized). Subsequently, the catalyst was separated by centrifugation, and the filtrate was subjected to analysis. The 2-chloroacetanilide (CAA) reaction was conducted in a solvent mixture of H₂O and methanol in a 7:3 ratio for 60 min. Each experiment was repeated a minimum of three times and the reusability of the catalyst matrices was also confirmed (see Figures S3 and S4).

4. Conclusions

Exploiting the expansive realm of sol–gel chemistry, Ni(II)@ORMOSIL and Ni(0)@ORMOSIL catalysts were synthesized and characterized. These Ni catalysts successfully de-halogenated TBAA, DBAA, MBAA, TCAA, MCAA, and CAA, demonstrating the clear formation of products with the least possible amount of catalyst and time. Additionally, we observed the formation of fumaric acid and traces of maleic acid during the comprehensive de-halogenation of TBAA and TCAA, with Ni exhibiting a distinctive propensity for their production, contrary to what was observed with Ag [3], Au [3], and Fe [4]. The study directs toward the other possibilities of product formation from Ni-catalyzed reduction reactions of HAAs, breaking the conventional rules of stepwise reduction mechanisms. The heterogeneous catalysts demonstrate promising reusability, establishing their efficacy in industrial catalytic applications in an accessible manner.