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Article

Safety and Process Intensification of Catalytic Reduction of 4-Nitophenol Using Sodium Borohydride in Flow Microreactor System

by
Ahmed Ibrahim Elhadad
and
Magdalena Luty-Błocho
*
AGH University of Krakow, Faculty of Non-Ferrous Metals, Al. A, Mickiewicza 30, 30-059 Kraków, Poland
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(11), 1038; https://doi.org/10.3390/catal15111038
Submission received: 11 September 2025 / Revised: 3 October 2025 / Accepted: 7 October 2025 / Published: 2 November 2025
(This article belongs to the Special Issue Catalytic Conversion of Small Molecules for Energy and Environmental Sustainability)
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Abstract

In this work, a novel approach for the catalytic reduction of 4-nitrophenol to 4-aminophenol using sodium borohydride is proposed. It was shown that a continuous-flow microreactor system is an optimal tool for PdNP synthesis with dimensions of 3.0 ± 0.5 nm, as well as the performance of catalytic tests with high process efficiency, while keeping a high level of safety. The results obtained from the microreactor system allowed for 100% conversion to 4-aminophenol and were compared to processes carried out in a batch reactor, as well as to a hybrid system which was a combination of a microreactor (synthesis of PdNPs) and batch reactor (catalytic test). These investigations were enhanced by kinetic studies, for which a stopped-flow spectrophotometer was applied due to the extremely high rate of the reaction, i.e., formation of PdNPs (2.1 s), as well as to measure in situ the rate of the heterogeneous catalytic process. To visualize the progress of the heterogeneous reaction more precisely, color coding based on transmittance measurements was employed. Furthermore, to deepen the understanding of the process, a detailed mechanism supported by DFT calculations for the conversion of 4-nitrophenol to 4-aminophenol in the presence of PdNPs was proposed.

Graphical Abstract

1. Introduction

The reaction between 4-nitrophenol (4-NP) and sodium borohydride is used as a model catalytic reaction for testing different metallic catalysts [1,2,3,4]. This is mainly due to the fact that the catalyzed reaction is quite simple to carry out (only two components are required), it does not require complicated equipment (the process is carried out in a batch reactor), and the catalytic course of the reaction itself can be detected even with the naked eye. Of course, for more accurate progress in reaction detection, in most cases a spectrophotometer is a sufficient tool [5,6]. In turn, this device is much cheaper compared to techniques based, for example, on liquid chromatography [7,8], mass and infrared spectrometry [9], etc. Moreover, the use of spectrophotometric techniques does not require expensive training for the person performing the analysis. Thus far, reactions between 4-NP and sodium borohydride have been carried out with the use of noble metal catalysts like silver [10,11], gold [12,13,14], platinum [15,16], and palladium [17], as well as bimetallic structures [18,19]. The nanometric size of the catalyst provides a high surface-to-volume ratio and, along with the chemical properties of the individual metals, ensures high catalytic efficiency. Among the metals mentioned above, palladium seems to be the most efficient catalyst (see Table S1, Supplementary Materials) used for 4-nitrophenol conversion. Thus, for a better insight, Table 1 contains more details related to the applications of nanometric palladium or Pd-containing structures.
The data collected in Table 1 indicate that palladium nanoparticles in the pure form, as well as more complex nanometric structures containing Pd, are efficient catalysts allowing the complete conversion of 4-NP to 4-aminophenol (4-AP) in a short period of time. In Table 1, for more precise comparison of the obtained experimental data, only research containing similar catalyst morphology was mentioned, i.e., size below 5 nm. The observed differences between reported results were attributed to variations in catalyst composition and initial conditions, such as reagent concentrations and temperature. In all cases, the catalyst synthesis and catalytic reaction processes were carried out in a batch reactor.
In this context, there are concerns about the safety of the synthesis process of catalysts, as well as catalytic test performance. It is known that reagents like 4-NP and NaBH4 are very harmful (Supplementary Materials, Table S2). In addition, NaBH4 is mutagenic and reproductively toxic (H360FD). Moreover, sodium borohydride is unstable and decomposes at a rate strongly dependent on the pH medium and temperature [24,25]. Even when employing noble metal nanoparticles, attention should be paid to potential safety concerns [26,27]. The presented arguments strongly recommend conducting research in protective conditions, but not in an open laboratory space. In this case, it is a good idea to use a microreactor system to carry out the whole process, including the synthesis of catalysts as well as their catalytic performance.
Since microreactors appeared in laboratories, the approach to their processes has changed completely—introducing several improvements and benefits. Microreactors have been used in many chemical syntheses, both organic [28,29] and inorganic [30], demonstrating the enormous power of these tools. In certain cases, the use of a microreactor enabled the synthesis of platinum nanoparticles at temperatures exceeding the boiling point of water, made possible by the implementation of a back-pressure regulator [31]. Microreactors were successfully applied for the synthesis of noble metal nanoparticles [32,33], as well as to develop one-step monometallic [34] and bimetallic catalyst synthesis and deposition on a catalyst carrier [35], giving lots of opportunities in process modulation and adjustment.
Taking into account the aspects related to safety mentioned above, i.e., high process efficiency, process intensification, etc., the whole process related to the synthesis of the catalyst and catalytic tests was carried out in a microreactor system in this work. This process was compared to a similar one carried out in a batch reactor, as well as demonstrating a hybrid approach. To our knowledge, this is the first work that shows how safely and efficiently the catalytic process can be carried out using a flow microreactor. Moreover, the presented process related to the synthesis of palladium nanoparticles and the catalytic process was strongly supported by the kinetic study. The obtained results were integrated into process optimization, in both the hybrid and microreactor systems.

2. Results and Discussion

2.1. Set-Up of the Process Carried out in the Batch, Hybrid, and Microreactor Systems

Depending on the approach—batch, hybrid, or microreactor system—the process of PdNP synthesis and performance of catalytic tests were carried out in different systems. In the first approach, a batch system was employed. This system contains a set of batch reactors (glass, Duran), the first of them used for the synthesis of the PdNPs and the second containing a mixture of the catalyzed reaction. The process contains two steps. The first relates to the mixture of the solution containing Pd(II) ions with NaBH4. As a result, the faint yellow color, coming from the metal ions, changes to light brown coming from colloidal PdNPs. Then, 10–1000 µL of the solution was added to the next batch reactor containing the catalyzed mixture, i.e., 4-nitrophenolate (4-NPe), which was formed as a result of 4-NP’s mixture with sodium borohydride.
The process carried out in the hybrid system also contains two stages, as schematically presented in Figure 1.
The first is related to the synthesis of PdNPs, and this process was carried out in the microreactor system. Next, catalytic tests were performed in the batch reactor. For this purpose, freshly prepared catalysts collected from the microreactor system were introduced using an automatic pipette into the set of batch reactors containing the catalyzed reaction.
The last approach was carried out entirely in the microreactor system. This system also contains two steps (Figure 2). In the first one, the solutions containing reagents, i.e., metal ions and the reductant, were introduced to the mixer. Then, a solution with reacting species was directed via a capillary to the next mixer, where the mixing of freshly prepared PdNPs and 4-nitrophenolate took place. After that, components were directed to the capillary loop to make sure that the whole process was completed.
The flow rate of reagents in the hybrid and microreactor systems was set based on kinetic studies (see Section 2.2).

2.2. Kinetic Background

Knowledge about the reaction rate is very important, basically for two main reasons. The first is to better understand the reaction itself. This issue contains the following aspects: to follow the compounds’ “interaction”, sometimes to detect their behavior (the kinetic curve reveals the character of the process, i.e., first-order, autocatalytic reaction, etc.) [36], does not directly help to predict the reaction mechanism. The second reason relates to the proper design of the product and process optimization. This is also basic knowledge, which enables us to transfer the results from the laboratory to the industrial scale [37]. In our case, the kinetic study of both the reduction of Pd(II) ions and the rate of catalytic reaction is crucial for proper process design in a microreactor system. Therefore, to achieve this goal, it is necessary to determine the reaction rates of all processes considered in this study.

2.2.1. Synthesis of PdNPs—Kinetic, Mechanism, and Morphology Analysis

After mixing Pd(II) ions with a solution containing reducing agents in the batch reactor, a color change was observed. This optical property was also recorded spectrophotometrically and is shown in Figure 3. It was confirmed that, following reagent mixing in the batch reactor, the characteristic spectrum of Pd(II) ions, with maxima at 207 and 236 nm (Figure 3), underwent a transformation, resulting in a newly registered spectrum. A similar observation was noted during the synthesis of PdNPs in the microreactor system. In both cases, an increase in spectrum intensity across the entire wavelength range was observed, suggesting the appearance of a metallic phase in the solution. Moreover, the obtained spectra contained two less intense peaks, which may originate either from an unreacted metal precursor or a newly formed Pd(II)-containing compound. Given the substantial excess of sodium borohydride compared to the metal precursor, the presence of unreacted Pd(II) ions was ruled out. The peaks at 207 and 236 nm had been previously detected [38] and were identified as signals associated with PdO. This oxide can spontaneously form in a solution containing dissolved oxygen.
The color change and registered turbidity confirmed that a reduction reaction between Pd(II) ions and sodium borohydride took place. The process can be written as:
Pd II + NaBH 4 → k PdNPs + products
Equation (1) in fact contains more reactions, which include atoms, clusters, and autocatalytic growth leading to final PdNP formation. These reactions demonstrate interaction on the liquid and metallic phase surface, which is important in the context of a preferred small size of the catalyst. Thus, the knowledge about kinetics is crucial for this purpose and also for proper design of the process of palladium nanoparticle synthesis in the microreactor system. The synthesis carried out in the batch reactor under selected reaction conditions indicated that the process of nanoparticle formation is fast. Hence, it was not possible to determine the reaction rate properly using standard spectrophotometry. Thus, in order to register the kinetic curve of this process, stopped-flow spectrophotometry was applied. Due to the limit of the used xenon lamp, we decided to observe the process from the site of the product instead of the Pd(II) ions, and consequently, the kinetic curve was registered at 400 nm, as depicted in Figure 4.
The registered kinetic curve does not have a typical character. Mostly, the appearance of the metallic phase is associated with the registration of a sigmoidal dependence, illustrating slow nucleation and fast autocatalytic growth [39]. Instead, the registered kinetic curve depicted in Figure 4 seems to be composed of three stages (Supplementary Materials, Figure S2), and all are fast. However, the first (stage I) takes about 0.1 s (Supplementary Materials, Figure S2), and its character is very close to sigmoid. The next, i.e., stage II, has a “linear” character, and it suggests that the process can be of zero order. Such a behavior was not previously observed. It may also be inferred that the recorded kinetic curve represents a superposition of individual kinetic profiles arising from parallel processes involving palladium ions and/or generated palladium nuclei, each associated with a distinct reaction pathway. The kinetic curve depicted in Figure 4 exhibits a steady state after 2.1 s, indicating that the process of palladium formation was completed. This value was used for the calculation of the flow rate in the microreactor system. Based on these data, the flow rates of the reagents were established at a value of FR = 3.59 mL/min (see Table 2). It is worth noting that during the entire process, from nanoparticle synthesis to the catalytic test, a microcapillary was additionally introduced. The length of the Teflon capillary used for the catalytic test was 49 cm (1.25 mm inner diameter), and this value was calculated taking into account the total flow rate and the rate of the catalytic reaction.
As already mentioned, due to the distinct nature of the obtained kinetic curve, it was not possible to apply the Watzky–Finke model to determine the kinetic rate constants responsible for nucleation and autocatalytic growth [39,40]. Therefore, PdNPs obtained from the synthesis carried out in the batch and microreactor systems were analyzed using UV–Vis spectrophotometry (Figure 3) and HRSTEM (Figure 5). Comparison of the samples obtained in these two systems showed distinct differences in the color of colloidal palladium. It was darker in the case of the sample produced in the batch reactor. Moreover, the registered UV–Vis spectrum was also more intense compared to the sample obtained in the microreactor system. To provide an explanation, both samples were analyzed using HRSTEM. The obtained results are compared in Figure 5.
The obtained results reveal a significant difference between samples produced using different systems. Please note that the size diagram shown in Figure 5a,c excludes particles exhibiting coalescence, and such particles were not considered in the calculation of this diagram. The mean dimensions are 3.0 ± 0.5 nm and 4.0 ± 0.5 nm for syntheses conducted in the microreactor and batch reactor, respectively. Furthermore, palladium nanoparticles synthesized in the microreactor exhibit a higher proportion of smaller particles compared to those obtained in the batch reactor. Additionally, a greater degree of coalescence was observed in nanoparticles synthesized in the batch reactor—causing polycrystalline growth and aggregation. The appearance of coalescence may negatively affect catalyst efficiency due to the limitation of the active area.

2.2.2. Reaction Between 4-Nitrophenol and Sodium Borohydride—Kinetics and Mechanism

After mixing an aqueous solution containing 4-nitrophenol with sodium borohydride, the conversion to the 4-nitrophenolate anion takes place (Figure 6). This process is a consequence of the substrates’ properties, i.e., sodium borohydride and 4-NP. It is known that in an aqueous solution, NaBH4 undergoes hydrolysis [41]. During this process, gaseous hydrogen (H2) is released from the reacting solution to the environment. This leads to an increasing pH of the medium. Consequently, it further alters the equilibrium between the neutral form of 4-nitrophenol and that of its conjugate base, the 4-nitrophenolate anion (Figure 6).
This transformation shows a significant shift in the UV–Vis absorbance maximum from 319 nm (neutral 4-NP) to 400 nm (4-nitrophenolate anion), and it can also be seen by the naked eye as color changes from light yellow to intensive yellow (see Figure 6d), respectively. Based on the literature, it is known that the reaction between 4-NP and NaBH4 is fast, but nobody has measured its rate so far [7,42]. For this reason, the reaction between 4-NP and sodium borohydride in aqueous solution was tracked using a stopped-flow spectrophotometer. Surprisingly, even using this spectrophotometer, it was not possible to measure the reaction rate due to the limit of the device. Based on the obtained kinetic data, it can be concluded that the reaction time is slower than 1 ms. The fragment of the registered kinetic curve is shown in the Supplementary Materials (see Supplementary Materials, Figure S3).

2.2.3. Reaction Between 4-Nitrophenolate and Sodium Borohydride in the Presence of Palladium Nanocatalyst—Kinetics and Mechanism

The addition of palladium nanoparticles to the mixture containing 4-nitrophenolate (1, Figure 7) and unreacted sodium borohydride, leads to its further conversion to 4-aminophenol [20,43,44] (Figure 6c). The role of PdNPs is quite important in this process since they act as the transfer medium for electrons involved in hydride transfer to the nitro (–NO2) functional group of 4-nitrophenolate as proposed by Malik et al. [45]. This mechanism is depicted in Figure 7a.
A catalyst-controlled process is used to reduce 4-nitrophenolate. In the basic medium, due to the addition of NaBH4, 4-NP exists in its deprotonated form and undergoes a stepwise reduction mechanism on the surface of the PdNP catalyst. The hydrolysis of NaBH4 produces molecular hydrogen (H2) and sodium metaborate (NaBO2). The borohydride ion BH-4 acts as a strong nucleophile, transferring electrons to the PdNPs, leading to an increase in the electron density over them. This electron-rich environment on the PdNPs weakens the H-H bond of the adsorbed molecular hydrogen, resulting in the formation of the palladium hydride bond (Pd-H), which is essential for the reduction process. Subsequently, 4-NP is activated and adsorbs on the PdNP surface according to the Langmuir–Hinshelwood (L-H) mechanism [46]. The hydride attacks the nitrogen atom with the assistance of oxygen protonation, which makes nitrogen more electrophilic, followed by a dehydration process to obtain a nitroso intermediate (Ar-NO, 3, Figure 7a) where Ar represents a para-substituted phenyl ring with a deprotonated hydroxyl group (–O−), shown in Figure 7b. The second step involves the interaction of Ar-NO with another hydride, followed by oxygen protonation after N=O bond breakage to produce the hydroxylamine intermediate (Ar-NHOH, 6, Figure 7a). A third step is the reduction of hydroxylamine via hydride, followed by water elimination, forming an amine that can be further protonated while adsorbed or after desorption from the catalyst surface [47]. Each step of this mechanism is justified by theoretical energy calculations for each probable partial intermediate system (Supplementary Materials, Table S3) and the most frequently confirmed intermediates in the literature.
The summary heterogeneous reaction can be written as
4 − NP + 3 NaBH 4 + 4 H 2 O → PdNPs 4 − AP + 3 NaBO 2 + 9 H 2
This reduction process is commonly monitored using UV–Vis spectroscopy. The disappearance of the 400 nm peak of 4-nitrophenolate and the simultaneous appearance of the 4-aminophenol peak at a wavelength ≈300 confirms the progress and completion of the reaction.

2.2.4. Catalyst Performance in the Batch, Hybrid, and Microreactor System—Kinetic Study

The synthesized PdNPs in the batch reactor in a volume range from 10 to 1000 µL were added to the set of batch reactors containing the catalyzed mixture, i.e., 4-nitrophenolate. A different amount of catalyst was introduced to determine the minimum quantity needed to achieve high process efficiency in a short time in the catalytic mixture while minimizing catalyst usage. Depending on the volume of PdNPs used, 4-aminophenol was obtained at different times. As expected, the full conversion was observed for the sample containing the highest amount of the catalyst (>100 µL). For all samples, the UV–Vis spectra were recorded and are depicted in Figure 8.
Due to the low rate of the process in the sample containing 10 µL (sample A, Figure 8) of the catalyst, the spectra were registered after 1 h, at which time the rest of the samples (B–E, Figure 8) were converted to the final product, i.e., 4-AP. The location of new spectra with maxima at 300 and 233 nm confirmed full 4-nitrophenolate conversion into 4-aminophenol, where the color turned from yellow to either colorless (sample B, Figure 8) or light reddish-brown (samples C–E, Figure 8). The intensity of the color was correlated with the amount of PdNPs introduced to the reacting solution. It was also confirmed by the Tyndall effect, as well as registered turbidity on the UV–Vis spectrum. The original image of the obtained samples (A–E) together with their improvements and conversion is shown in Supplementary Materials, Figure S4a.
Similar tests were performed in the hybrid system. For this purpose, different volumes of freshly prepared catalysts synthesized in the microreactor system were introduced using a pipette to the set of batch reactors containing 4-nitrophenolate. The obtained color change depending on the volume of PdNPs introduced to the catalytic reaction (see also Supplementary Materials, Figure S4b for details), as well as the sample of spectral evolution registered for the catalytic conversion of 4-NPe into 4-AP for a smaller volume of the catalyst, is shown in Figure 9.
The addition of a small portion of the catalyst (sample A) to the reaction mixture results in only about 50% conversion of 4-nitrophenolate to 4-aminophenol within 2 h (Figure 9). Even extending the reaction time further does not lead to complete conversion. This can be attributed to an insufficient amount of catalyst, which is necessary for optimal performance during NaBH4 activity. The results obtained from the batch reactor and hybrid system confirmed that conversion of 4-NPe into 4-AP is more efficient in the case of the catalyst obtained in the microreactor system, and this can be related to the smaller size of the PdNPs.
Due to the low process efficiency (sample A, Figure 9), the volume of the applied catalyst was increased up to 50 µL, and spectral evolution was registered for both systems, i.e., the batch and the hybrid system. For this purpose, freshly synthesized PdNPs in the batch reactor and in the microreactor were introduced directly to a quartz cuvette containing the deprotonated 4-NP (6 mL) solution. Then, 3 mL of the mixture was inserted into the spectrophotometer holder. The obtained spectral evolution coming from both experiments is shown in Figure 10a,b.
The evolution of the obtained spectra confirms the progress of the reaction (Figure 10a,b). The recorded spectra, with maxima at 233 and 300 nm, suggest the formation of 4-aminophenol. To verify the reaction rate, kinetic curves were drawn based on the evolution of the obtained spectra at a selected wavelength of 400 nm (Figure 10c,d). From fitting equations to the obtained data, the values of the rate constant were determined and equal to 0.085 min−1 and 0.172 min−1 for the batch and hybrid system, respectively. The character of the fitted curve has an exponential character, and it suggests the pseudo-first order of the reaction with respect to 4-NPe, when an excess of NaBH4 was used. This also confirmed that particles produced in the microreactor system are more efficient, and it is strictly correlated with their morphology (see Figure 3). Moreover, to register a color change of the catalyzed solution, we used a novel approach, which allowed for color coding using a special tool available in LabSolution UV-Vis software. This approach offers monitoring of accurate color change inside the spectrophotometer without any external light source interference. This approach also eliminates the need for image correction, which is typically required when processing data with software like Adobe Photoshop CC 2019 (see Supplementary Materials, Figure S4a,b) and similar tools. Additionally, this measurement guarantees the reproducibility of the recorded results, regardless of geographical location. The software used can estimate the color coding of the samples based on transmittance measurements. To collect the necessary data, transmittance was measured throughout the reaction progress. These data were then transformed to generate coding according to the Lab* scale, ultimately producing a color scale bar as depicted in Figure 10a,b.

2.2.5. Comparison of the Kinetics of Heterogeneous Catalysis Carried out in the Batch, Hybrid, and Microreactor Systems

A comparison of the data obtained in the batch reactor, hybrid system, and microreactor was possible after process modification. This was related to the modification of the volume of the mixed reagents in the microreactor system, as well as in the stopped flow used for tracking the reaction rate. For these systems, the volumetric ratio of all reagents should be kept at the same level in order to eliminate negative effects that appear in a flow (internal forces, problems with introducing a stream with a lower flow rate to the stream with a much higher flow rate, etc.), as well as for proper stopped-flow operation. Therefore, to ensure comparability of the experiments, the volumetric ratio was set to 1:1, maintaining the reagent concentrations at the same level as in the previous study. For these experimental conditions, kinetic curves were again registered (Figure 11). In the case of the batch reactor system, analysis of the kinetic curve was performed using a standard spectrophotometer. In contrast, a hybrid system was utilized where PdNPs were prepared within the microreactor and their catalytic behavior in the 4-NP reduction process was studied using the batch reactor. Meanwhile, to reflect the course of the catalytic reaction in the flow microreactor, an additional system integrated into a stopped-flow spectrophotometer was used. The use of a pre-mixing set and a delay time of 2.1 s is needed for complete Pd(II) reduction, enabling the formation of palladium nanoparticles before mixing with the catalyzed reaction. After this time, the nanoparticles were automatically injected into the measurement chamber where they were mixed with 4-nitrophenolate. The registered kinetic curves coming from different systems are depicted in Figure 11.
The character of the obtained kinetic curves and fitted equations to experimental data suggests that the process of 4-nitrophenolate reduction in the presence of the catalyst is of first order. Moreover, the determined kinetic rate constants increase in the order kbatch (0.058 s−1) < khybrid (0.095 s−1) < kmicroreactor (1.076 s−1). The obtained results suggest that crucial parts of the catalytic reaction are PdNPs and the environment for reaction performance. It also seems that a microreactor system is a perfect tool for very quick conversion of 4-nitrophenolate to 4-aminophenol, since it assures the highest rate. The process of synthesis of PdNPs is very quick, and it seems that freshly prepared particles are more efficient compared to those from the hybrid and batch reactors. Based on the data obtained from HRSTEM analysis (see Figure 5), it can be concluded that the morphology of palladium nanoparticles synthesized in the batch reactor differs from that of those synthesized in the microreactor. The proportion of particles with a diameter of 2–3 nm is higher in the synthesis conducted in the microreactor, leading to more efficient catalytic performance. The results obtained also indicate that the proposed process for nanoparticle synthesis and catalytic reaction testing in a flow microreactor yields better outcomes compared to those reported in the literature (Table 1). Moreover, such a comprehensive approach eliminates potential environmental exposure to reagent toxicity and contributes to improved environmental sustainability.

3. Materials and Methods

3.1. Chemicals

As a metal precursor, a stock solution of H2PdCl4 (0.093 mol/dm3) was prepared from pure metals (Mennica Panstwowa, 99.99%, Warsaw, Poland). For the process of nanoparticle synthesis, a 0.2 mm solution of H2PdCl4 was obtained by dilution of 21 µL of stock solution with deionized water.
As a reducing agent, sodium borohydride (p.a., POCH, Gliwice, Poland) was used. The stock solution with a concentration of 40 mm was prepared by dissolving 30 mg of powder in deionized water. A fresh solution of sodium borohydride worked as a reducing agent for Pd(II) ions, leading to PdNP formation as well as one component responsible for the reduction of 4-NP to 4-AP. For Pd (II) reduction, 66 µL was taken from the NaBH4 stock solution in 10 mL of deionized water to form a concentration of 0.01 mg/mL. Prepared solution was then added to 10 mL of 0.2 mm H2PdCl4 solution within the batch and microreactor. For 4-NP reduction, 0.75 mL of NaBH4 stock solution was mixed with 1.5 mL of 500 µM 4-NP and 3.75 mL of deionized water to obtain a mixture of 125 µM 4-nitrophenol and 5 mm NaBH4.

3.2. Method of Analysis

3.2.1. Spectra Registration and Kinetic Studies

UV–Vis spectrophotometry (PC 2501, Shimadzu, Tokyo, Japan) working in the wavelength range of 190–900 nm, equipped with a thermostatic and reference cell, was used for characteristic spectra registration of the reagents, catalyst, and products of catalyzed reactions. For this purpose, about 3 mL of the solution containing the mentioned chemicals was introduced to the quartz cell (Hellma, path length = 1 cm), and deionized water was used as a reference solution. The analysis was performed at 20 °C. Moreover, for samples containing catalyzed reactions, the spectral evolution was also registered.
To register selected spectra evolution, as well as to measure the transmittance value required for color coding (LabSolution software), we used the UV–Vis spectrophotometer UV-1900i (Shimadzu, Tokyo, Japan) working in the wavelength range 190–1100 nm and equipped with measurement and reference cells.
To determine the rate of the reactions considered in this study, i.e., reduction of Pd(II) ions using NaBH4; nucleation and autocatalytic growth of palladium nanoparticles; reaction between 4-NP and sodium borohydride; and finally catalyzed reaction between 4-NPe and NaBH4 in the presence of metallic phase, a stopped-flow spectrophotometer (Applied Photophysics, Leatherhead, UK) was applied. This device, contrary to a standard spectrophotometer, allows for very rapid and automatic reagent mixing, analysis, and data collection (limit of the reaction ca. 1 ms). Stopped flow is equipped in a thermostatic quartz cell (path length 1 cm) and works in the wavelength range of 190–700 nm.

3.2.2. Catalyst Analysis

PdNPs coming from synthesis carried out in a batch reactor and a microreactor system were analyzed using UV–Vis spectrophotometry and high-resolution scanning transmission electron microscopy (HRSTEM, Titan3 G2 60–300, Thermo Fisher Scientific, Bleiswijk, The Netherlands). Samples for further analysis were collected immediately after the appearance of the metallic phase in the solution. For this purpose, about 3 mL of the solution in a quartz cuvette was collected and inserted in a thermostatic spectrophotometer cell. In contrast, for HRSTEM analysis, 100 µL of the solution containing PdNPs was dispersed on a copper grid covered with 30 nm carbon film. The excess solution from the sample was removed using filter paper (diameter 3 cm) placed under copper grids. Then, the sample was dried at room temperature and analyzed after two days.

3.2.3. Density Functional Theory Calculation

The molecular structure of the proposed intermediates formed during catalytic reaction within the reaction mechanism was drawn and optimized using Avogadro software (Application Version: 1.2.0) to generate an ORCA input file for quantum calculations. Single-point energy calculations were performed using ORCA (version 6.0.1), where the Restricted Hartree–Fock (RHF) method and the def2-SVP basis set were employed. Total electronic energies in Hartree units (Eh) were calculated to compare the stability of the intermediates; the lower the energy, the more favorable the intermediate.

3.2.4. Color Coding and Data Processing

Photographs of real samples were captured inside a light box using a digital camera, and then the images were processed in Adobe Photoshop using curve adjustments to decrease the effect of ambient and other sources of light. The Eyedropper tool was applied to identify the white color within the picture based on a white background, effectively removing ambient color bias. The results were very similar to those obtained by the Shimadzu spectrophotometer using LabSolution software.
The Shimadzu color analysis add-on software for LabSolution’s UV–Vis supports the CIELAB system, which was used to obtain the color coding where different standard values were selected for lightness index L* and color coordination a*, b*. D65 and 10 degrees were chosen as standard settings for the illuminant and observation fields of view, respectively.

3.3. Batch, Hybrid, and Microreactor Systems

The reaction between Pd(II) ions and sodium borohydride was carried out in a glass batch reactor or in a microreactor chip (Supplementary Materials, Figure S1). In the batch reactor system, the same volume of reducing agent was added to the solution containing the metal precursor (1 mL:1 mL) using an automatic pipette. The small volume of reagents allowed for fast mixing. In contrast, in the case of PdNP synthesis carried out in a hybrid or microreactor system, freshly prepared solutions containing reagents were loaded into regents loops using medical syringes (Hawkmed HK-400, Shenzhen Hawk Medical Instrument Co., Ltd., Shenzhen, China). Then chemicals were pumped into the microreactor chip, in which mixing and reduction reactions took place. After this process, the solution containing PdNPs was carried out via capillary to the sampler (Hybrid system) or directed to the mixer (full microreactor system) in order to connect the product with a stream of the solution containing reduced 4-NP, i.e., 4-nitrophenolate, and then transported the solution via microcapillary loop to the sampler.

4. Conclusions

This work presents better performance and safety of a continuous-flow microreactor system for the synthesis and catalytic application of palladium nanoparticles (PdNPs) in the reduction of 4-nitrophenol. The synthesized PdNPs and their use directly in the microreactor showed a much higher reaction rate constant (k = 64.56 min−1) than for those synthesized in a batch (k = 3.48 min−1) or during synthesis in a flow, but tested in a batch (k = 5.7 min−1). The microreactor system also enabled complete conversion (100%), in just 5 s, far exceeding the reaction time of the hybrid as well as the conversion (45 s, 99.35%) and batch (140 s, 98.62%). The smaller and more homogeneous (3.0 ± 0.5 nm) PdNPs produced in the microreactor, compared to those from batch synthesis, also exhibit improved catalytic efficiency. These results confirm that mixing nanoparticle synthesis with catalytic application inside a microreactor platform not only improves both the reaction kinetics and the product yield, but also reduces hazards related to handling reactive chemicals, thus supporting its potential for safe, effective, and scalable environmental remediation processes. Moreover, a more detailed mechanism for 4-NP to 4-AP conversion was proposed, taking into account existing data, using DFT calculations for energy determination, and selecting a more probable reaction path. The knowledge about possible intermediates can be very important for catalytic tests performed in media other than aqueous. Additionally, to enhance the visualization of the heterogeneous reaction progress, we also utilized color coding based on transmittance measurements. This approach enabled the creation of a color map that remains unaffected by the experimenter’s geographical location.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal15111038/s1, Figure S1: Real photo of the microreactor, where 1—input for Pd(II) ions, 2—NaBH4 used as reducing agent, 3—output which contains PdNP solution; Figure S2: Kinetic curve obtained as result of reduction reaction between 110 µL (0.2 mm) of H2PdCl4 and 110 µL (0.01 mg/mL) of NaBH4 at 20 °C, measured using the stopped-flow method at a wavelength of 400 nm. Absorption recorded for (a) 0.05, (b) 0.1 and (c) 1 s, path length = 1 cm; Figure S3: Kinetic curve obtained for 125 µM 4-NP after addition of 5 mm NaBH4 using a path length of 2 mm; Figure S4: Processing of the digital images using Adobe Photoshop adjustments to illuminate the effect of other light sources was performed, then the color sample was obtained for different samples of the catalytic test via different volumes of the PdNP catalyst synthesized within the batch reactor (a) where the image was taken after 1 h and the microreactor (b) where images were taken every 15 s. Table S1: Selected noble metal catalysts used for 4-NP reduction; Table S2: Hazardous pictograms of 4-NP and NaBH4 reagents; Table S3: Theoretical energy calculations for each probable partial intermediate system. The intermediate number is the number introduced within the mechanism in Figure 7a, where structures having the same intermediate number have the same number of atoms and were compared together to determine which is the most stable. The chosen structure is highlighted in blue and introduced into the mechanism.

Author Contributions

Conceptualization, A.I.E. and M.L.-B.; methodology, A.I.E. and M.L.-B.; formal analysis, A.I.E. and M.L.-B.; investigation, A.I.E.; writing—original draft preparation, A.I.E. and M.L.-B.; visualization, A.I.E. and M.L.-B.; project administration, M.L.-B.; funding acquisition, M.L.-B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Higher Education, Grant No. SKN/SP/603483/2024.

Data Availability Statement

The original contributions presented in this study are included in the article and Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Gu, S.; Wunder, S.; Lu, Y.; Ballauff, M.; Fenger, R.; Rademann, K.; Jaquet, B.; Zaccone, A. Kinetic Analysis of the Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles. J. Phys. Chem. C 2014, 118, 18618–18625. [Google Scholar] [CrossRef]
  2. Li, M.; Chen, G. Revisiting Catalytic Model Reaction P-Nitrophenol/NaBH4 Using Metallic Nanoparticles Coated on Polymeric Spheres. Nanoscale 2013, 5, 11919–11927. [Google Scholar] [CrossRef]
  3. Yelboğa, M.; Akbayrak, M. Efficient Reduction of Highly Toxic 4-Nitrophenol with Ultra-Low Platinum Loading on Cobalt (II, III) Oxide Support: Facile Synthesis and High Turnover Frequency. Int. J. Hydrogen Energy 2025, 102, 800–815. [Google Scholar] [CrossRef]
  4. Pi, S.; Ma, F.; Cui, D.; Feng, L.; Zhou, L.; Li, A. Catalytic Reduction of 4-Nitrophenol by Green Silver Nanocomposites Assembled Using Microbial Extracellular Polymer Substances. Environ. Res. 2021, 197, 111006. [Google Scholar] [CrossRef]
  5. Kabiri, B.; Heidari, H. Synthesis and Catalytic Activity of Silver- Reduced Graphene Oxide and Silver- Magnetite- Reduced Graphene Oxide Nanocomposites in the Reduction of 4-Nitrophenol. Sci. Rep. 2025, 15, 14539. [Google Scholar] [CrossRef]
  6. Ahmed, M.A.; Ahmed, M.A.; Mohamed, A.A. Removal of 4-Nitrophenol and Indigo Carmine Dye from Wastewaters by Magnetic Copper Ferrite Nanoparticles: Kinetic, Thermodynamic and Mechanistic Insights. J. Saudi Chem. Soc. 2023, 27. [Google Scholar] [CrossRef]
  7. Lomonosov, V.; Asselin, J.; Ringe, E. Solvent Effects on the Kinetics of 4-Nitrophenol Reduction by NaBH4 in the Presence of Ag and Au Nanoparticles. React. Chem. Eng. 2022, 7, 1728–1741. [Google Scholar] [CrossRef] [PubMed]
  8. Ribeiro, A.P.C.; Santos, B.M.; Faustino, R.F.C.; Pombeiro, A.J.L.; Martins, L.M.D.R.S. C-Heterogenized Re Nanoparticles as Effective Catalysts for the Reduction of 4-Nitrophenol and Oxidation of 1-Phenylethanol. Catalysts 2022, 12, 285. [Google Scholar]
  9. Ehsani, A.; Nejatbakhsh, S.; Soodmand, A.M.; Farshchi, M.E.; Aghdasinia, H. High-Performance Catalytic Reduction of 4-Nitrophenol to 4-Aminophenol Using M-BDC (M = Ag, Co, Cr, Mn, and Zr) Metal-Organic Frameworks. Environ. Res. 2023, 227, 115736. [Google Scholar] [CrossRef]
  10. Kästner, C.; Thünemann, A.F. Catalytic Reduction of 4-Nitrophenol Using Silver Nanoparticles with Adjustable Activity. Langmuir 2016, 32, 7383–7391. [Google Scholar] [CrossRef] [PubMed]
  11. Geng, Q.; Du, J. Reduction of 4-Nitrophenol Catalyzed by Silver Nanoparticles Supported on Polymer Micelles and Vesicles. RSC Adv. 2014, 4, 16425–16428. [Google Scholar] [CrossRef]
  12. Noël, S.; Bricout, H.; Addad, A.; Sonnendecker, C.; Zimmermann, W.; Monflier, E.; Léger, B. Catalytic Reduction of 4-Nitrophenol with Gold Nanoparticles Stabilized by Large-Ring Cyclodextrins. New J. Chem. 2020, 44, 21007–21011. [Google Scholar] [CrossRef]
  13. Thawarkar, S.R.; Thombare, B.; Munde, B.S.; Khupse, N.D. Kinetic Investigation for the Catalytic Reduction of Nitrophenol Using Ionic Liquid Stabilized Gold Nanoparticles. RSC Adv. 2018, 8, 38384–38390. [Google Scholar] [CrossRef]
  14. Neal, R.D.; Inoue, Y.; Hughes, R.A.; Neretina, S. Catalytic Reduction of 4-Nitrophenol by Gold Catalysts: The Influence of Borohydride Concentration on the Induction Time. J. Phys. Chem. C 2019, 123, 12894–12901. [Google Scholar] [CrossRef]
  15. Ghosh, S.K.; Mandal, M.; Kundu, S.; Nath, S.; Pal, T. Bimetallic Pt–Ni Nanoparticles Can Catalyze Reduction of Aromatic Nitro Compounds by Sodium Borohydride in Aqueous Solution. Appl. Catal. A Gen. 2004, 268, 61–66. [Google Scholar] [CrossRef]
  16. Li, X.; Li, G.; Zang, W.; Wang, L.; Zhang, X. Catalytic Activity of Shaped Platinum Nanoparticles for Hydrogenation: A Kinetic Study. Catal. Sci. Technol. 2014, 4, 3290–3297. [Google Scholar] [CrossRef]
  17. Chatterjee, S.; Bhattacharya, S.K. Size-Dependent Catalytic Activity of PVA-Stabilized Palladium Nanoparticles in p-Nitrophenol Reduction: Using a Thermoresponsive Nanoreactor. ACS Omega 2021, 6, 20746–20757. [Google Scholar] [CrossRef]
  18. Berahim, N.; Basirun, W.J.; Leo, B.F.; Johan, M.R. Synthesis of Bimetallic Gold-Silver (Au-Ag) Nanoparticles for the Catalytic Reduction of 4-Nitrophenol to 4-Aminophenol. Catalysts 2018, 8, 412. [Google Scholar] [CrossRef]
  19. König, R.Y.G.; Schwarze, M.; Schomäcker, R.; Stubenrauch, C. Catalytic Activity of Mono- and Bi-Metallic Nanoparticles Synthesized via Microemulsions. Catalysts 2014, 4, 256–275. [Google Scholar] [CrossRef]
  20. Cui, Y.; Liang, B.; Zhang, J.; Wang, R.; Sun, H.; Wang, L.; Gao, D. Polyethyleneimine-Stabilized Palladium Nanoparticles for Reduction of 4-Nitrophenol. Transit. Met. Chem. 2019, 44, 655–662. [Google Scholar] [CrossRef]
  21. Ashraf, J.; Khan, G.A.; Ain, M.U.; Ghanem, M.A.; Mohammed, K.M.H.; Ahmed, W. Synthesis and Catalytic Activity of Core-Shell Gold-Palladium Nanoworms towards the Catalytic Reduction of 4-Nitrophenol and Methyl Orange. Inorg. Chem. Commun. 2024, 169, 113039. [Google Scholar] [CrossRef]
  22. Zhao, L.; Du, X.; Gao, F.; Hua, Y.; Li, H.; Zhang, X.; Di, L.-B. Oxygen Plasma Synthesis of Pd/GO with Enhanced Co-Adsorption Ability of 4-Nitrophenol and NaBH4 for Boosting 4-Nitrophenol Reduction Activity. Appl. Catal. A Gen. 2025, 696, 120203. [Google Scholar] [CrossRef]
  23. Xu, H.; Zhang, H.; Hou, T.; Wang, D.; Tu, W. Efficient AuPd Catalysts with Layered Material Supporters for the Reduction of 4-Nitrophenol. J. Nanoparticle Res. 2022, 24, 108. [Google Scholar] [CrossRef]
  24. Retnamma, R.; Novais, A.Q.; Rangel, C.M. Kinetics of Hydrolysis of Sodium Borohydride for Hydrogen Production in Fuel Cell Applications: A Review. Int. J. Hydrogen Energy 2011, 36, 9772–9790. [Google Scholar] [CrossRef]
  25. Shin, S.; Kim, Y.; Jin, J.-H.; Jung, J. Heat-Induced Dry Hydrolysis of Sodium Borohydride/Oxalic Acid Dihydrate Composite for Hydrogen Production. ACS Omega 2022, 7, 979–986. [Google Scholar] [CrossRef] [PubMed]
  26. Luty-Błocho, M.; Varon Hoyos, M.; Hessel, V. Implications of the Use of Functional Nanomaterials for the Environment and Health. In Functional Nanomaterials for Sensors; CRC Press: Boca Raton, FL, USA, 2023; pp. 289–319. ISBN 9781003263852. [Google Scholar]
  27. Mejía, Y.R.; Reddy Bogireddy, N.K. Reduction of 4-Nitrophenol Using Green-Fabricated Metal Nanoparticles. RSC Adv. 2022, 12, 18661–18675. [Google Scholar] [CrossRef] [PubMed]
  28. Watts, P.; Haswell, S.J. The Application of Micro Reactors for Organic Synthesis. Chem. Soc. Rev. 2005, 34, 235–246. [Google Scholar] [CrossRef]
  29. Mason, B.P.; Price, K.E.; Steinbacher, J.L.; Bogdan, A.R.; McQuade, D.T. Greener Approaches to Organic Synthesis Using Microreactor Technology. Chem. Rev. 2007, 107, 2300–2318. [Google Scholar] [CrossRef]
  30. Abou-Hassan, A.; Sandre, O.; Cabuil, V. Microfluidics in Inorganic Chemistry. Angew. Chemie Int. Ed. 2010, 49, 6268–6286. [Google Scholar] [CrossRef]
  31. Luty-Błocho, M.; Wojnicki, M.; Grzonka, J.; Kurzydłowski, K.J. The Synthesis of Stable Platinum Nanoparticles in the Microreactor. Arch. Metall. Mater. 2014, 59, 509–512. [Google Scholar] [CrossRef]
  32. Zhao, C.-X.; He, L.; Qiao, S.Z.; Middelberg, A.P.J. Nanoparticle Synthesis in Microreactors. Chem. Eng. Sci. 2011, 66, 1463–1479. [Google Scholar] [CrossRef]
  33. Pach, A.; Szot, A.; Fitzner, K.; Luty-Błocho, M. Opportunities and Challenges in the Synthesis of Noble Metal Nanoparticles via the Chemical Route in Microreactor Systems. Micromachines 2024, 15, 1119. [Google Scholar] [CrossRef]
  34. Luty-Błocho, M.; Wojnicki, M.; Pacławski, K.; Fitzner, K. The Synthesis of Platinum Nanoparticles and Their Deposition on the Active Carbon Fibers in One Microreactor Cycle. Chem. Eng. J. 2013, 226, 46–51. [Google Scholar] [CrossRef]
  35. Pach, A.; Zaryczny, A.; Michałek, T.; Kamiński, H.; Kutyła, D.; Tokarski, T.; Chat-Wilk, K.; Hessel, V.; Luty-Błocho, M. One-Step Synthesis of Pt–Pd@ACF Catalyst in the Microreactor System for the Hydrogen Evolution Reaction. Ind. Eng. Chem. Res. 2024, 63, 7018–7030. [Google Scholar] [CrossRef]
  36. Ašperger, S. Chemical Kinetics and Inorganic Reaction Mechanisms; Springer Science & Business Media: Cham, Switzerland, 2003. [Google Scholar] [CrossRef]
  37. Stevens, W.F. Chemical Engineering Kinetics. Use in the Scale-Up of Chemical Processes. Ind. Eng. Chem. 1958, 50, 591–593. [Google Scholar] [CrossRef]
  38. Luty-Błocho, M.; Wojnicki, M.; Włoch, G.; Fitzner, K. Green Method for Efficient PdNPs Deposition on Carbon Carrier in the Microreactor System. J. Nanoparticle Res. 2018, 20, 239. [Google Scholar] [CrossRef]
  39. Bentea, L.; Watzky, M.; Finke, R. Sigmoidal Nucleation and Growth Curves Across Nature Fit by the Finke–Watzky Model of Slow Continuous Nucleation and Autocatalytic Growth: Explicit Formulas for the Lag and Growth Times Plus Other Key Insights. J. Phys. Chem. C 2017, 121. [Google Scholar] [CrossRef]
  40. Finke, R.G.; Watzky, M.A.; Whitehead, C.B. Response to “Particle Size Is a Primary Determinant for Sigmoidal Kinetics of Nanoparticle Formation: A ‘Disproof’ of the Finke–Watzky (F-W) Nanoparticle Nucleation and Growth Mechanism. Chem. Mater. 2020, 32, 3657–3672. [Google Scholar] [CrossRef]
  41. Chen, W.; Ouyang, L.Z.; Liu, J.W.; Yao, X.D.; Wang, H.; Liu, Z.W.; Zhu, M. Hydrolysis and Regeneration of Sodium Borohydride (NaBH4)—A Combination of Hydrogen Production and Storage. J. Power Sources 2017, 359, 400–407. [Google Scholar] [CrossRef]
  42. Fu, M.; Li, M.; Zhao, Y.; Bai, Y.; Fang, X.; Kang, X.; Yang, M.; Wei, Y.; Xu, X. A Study on the High Efficiency Reduction of P-Nitrophenol (4-NP) by a Fe(OH)3/Fe2O3@Au Composite Catalyst. RSC Adv. 2021, 11, 26502–26508. [Google Scholar] [CrossRef] [PubMed]
  43. Reddy Bogireddy, N.K.; Mejia, Y.R.; Aminabhavi, T.M.; Barba, V.; Becerra, R.H.; Ariza Flores, A.D.; Agarwal, V. The Identification of Byproducts from the Catalytic Reduction Reaction of 4-Nitrophenol to 4-Aminophenol: A Systematic Spectroscopic Study. J. Environ. Manag. 2022, 316, 115292. [Google Scholar] [CrossRef] [PubMed]
  44. Ayad, A.I.; Luart, D.; Dris, A.O.; Guénin, E. Kinetic Analysis of 4-Nitrophenol Reduction by “Water-Soluble” Palladium Nanoparticles. Nanomaterials 2020, 10, 1169. [Google Scholar] [CrossRef] [PubMed]
  45. Malik, M.A.; Alshehri, A.A.; Abomuti, M.A.; Danish, E.Y.; Patel, R. Catalytic Reduction of 4-Nitrophenol to 4-Aminophenol. Toxics 2021, 9, 103. [Google Scholar] [CrossRef]
  46. Wang, J.; Zhang, J.; Song, Y.; Xu, X.; Cai, M.; Li, P.; Yuan, W.; Xiahou, Y. Functionalized Agarose Hydrogel with in Situ Ag Nanoparticles as Highly Recyclable Heterogeneous Catalyst for Aromatic Organic Pollutants. Environ. Sci. Pollut. Res. Int. 2023, 30, 43950–43961. [Google Scholar] [CrossRef]
  47. Tomishige, K.; Honda, M.; Sugimoto, H.; Liu, L.; Yabushita, M.; Nakagawa, Y. Recent Progress on Catalyst Development for Ring-Opening C-O Hydrogenolysis of Cyclic Ethers in the Production of Biomass-Derived Chemicals. Carbon Neutrality 2024, 3, 17. [Google Scholar] [CrossRef]
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Figure 1. The scheme of the process carried out in the hybrid system. Notation: 4-NP: 4-nitrophenol; 4-NPe: 4-nitrophenolate; 4-AP: 4-aminophenol.
Figure 1. The scheme of the process carried out in the hybrid system. Notation: 4-NP: 4-nitrophenol; 4-NPe: 4-nitrophenolate; 4-AP: 4-aminophenol.
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Catalysts 15 01038 g002
Figure 2. The scheme of the process carried out in the microreactor system.
Figure 2. The scheme of the process carried out in the microreactor system.
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Catalysts 15 01038 g003
Figure 3. UV–Vis spectrum of 0.2 mm H2PdCl4 (A) and formed PdNPs as a result of the reaction between 1 mL of Pd(II) ions and 1 mL (0.01 mg/mL) of NaBH4. The process of PdNPs was carried out in the batch reactor (B) and in the microreactor system (C). Conditions: 0.2 mm H2PdCl4, 0.01 mg/mL NaBH4, T = 20 °C, FR = 3.59 mL/min.
Figure 3. UV–Vis spectrum of 0.2 mm H2PdCl4 (A) and formed PdNPs as a result of the reaction between 1 mL of Pd(II) ions and 1 mL (0.01 mg/mL) of NaBH4. The process of PdNPs was carried out in the batch reactor (B) and in the microreactor system (C). Conditions: 0.2 mm H2PdCl4, 0.01 mg/mL NaBH4, T = 20 °C, FR = 3.59 mL/min.
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Catalysts 15 01038 g004
Figure 4. Kinetic curve registered for the reaction between 110 µL (0.2 mm) of H2PdCl4 and 110 µL (0.01 mg/mL) of NaBH4 at 20 °C, measured using the stopped-flow method at a wavelength of 400 nm. Data were recorded for 5 s, path length = 1 cm.
Figure 4. Kinetic curve registered for the reaction between 110 µL (0.2 mm) of H2PdCl4 and 110 µL (0.01 mg/mL) of NaBH4 at 20 °C, measured using the stopped-flow method at a wavelength of 400 nm. Data were recorded for 5 s, path length = 1 cm.
Catalysts 15 01038 g004
Catalysts 15 01038 g005aCatalysts 15 01038 g005b
Figure 5. HRSTEM results of PdNPs synthesized in the batch reactor (a,b) and in the microreactor system (c,d) at different magnifications. Distribution of the PdNP diameter synthesized via batch (a) and microreactor (c). Diameters of the samples are taken from HRSTEM images at a 10 nm scale. Conditions: PdNPs were synthesized using 0.2 mm H2PdCl4 and 0.01 mg/mL NaBH4 (volumetric ratio 1:1).
Figure 5. HRSTEM results of PdNPs synthesized in the batch reactor (a,b) and in the microreactor system (c,d) at different magnifications. Distribution of the PdNP diameter synthesized via batch (a) and microreactor (c). Diameters of the samples are taken from HRSTEM images at a 10 nm scale. Conditions: PdNPs were synthesized using 0.2 mm H2PdCl4 and 0.01 mg/mL NaBH4 (volumetric ratio 1:1).
Catalysts 15 01038 g005aCatalysts 15 01038 g005b
Catalysts 15 01038 g006
Figure 6. The conversion of 4-nitrophenol (a) into 4-nitrophenolate (b) in the presence of NaBH4 and then to 4-aminophenol in the presence of the catalyst (c); real color change of the solution demonstrating conversion of 4-NP via 4-NPe into 4-AP (d).
Figure 6. The conversion of 4-nitrophenol (a) into 4-nitrophenolate (b) in the presence of NaBH4 and then to 4-aminophenol in the presence of the catalyst (c); real color change of the solution demonstrating conversion of 4-NP via 4-NPe into 4-AP (d).
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Catalysts 15 01038 g007
Figure 7. Possible mechanism of 4-nitrophenolate reduction in aqueous medium in the presence of the catalyst (a). All numbers mentioned within the mechanism used as an intermediate reference (Supplementary Materials, Table S3) of the DFT energy calculation; the explanation of “Ar” mentioned in the text, where “X” can be any substitution like NO2, NO, NHOH, and NH2 (b).
Figure 7. Possible mechanism of 4-nitrophenolate reduction in aqueous medium in the presence of the catalyst (a). All numbers mentioned within the mechanism used as an intermediate reference (Supplementary Materials, Table S3) of the DFT energy calculation; the explanation of “Ar” mentioned in the text, where “X” can be any substitution like NO2, NO, NHOH, and NH2 (b).
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Catalysts 15 01038 g008
Figure 8. UV–Vis absorption spectra recorded 1 h later for a catalytic mixture containing 125 µM 4-nitrophenol and 5 mm NaBH4, with the addition of 10 µL (A), 100 µL (B), 200 µL (C), 500 µL (D), and 1 mL (E) of PdNPs synthesized via a batch reactor; inset is the samples’ color obtained using a digital camera and Adobe Photoshop adjustments.
Figure 8. UV–Vis absorption spectra recorded 1 h later for a catalytic mixture containing 125 µM 4-nitrophenol and 5 mm NaBH4, with the addition of 10 µL (A), 100 µL (B), 200 µL (C), 500 µL (D), and 1 mL (E) of PdNPs synthesized via a batch reactor; inset is the samples’ color obtained using a digital camera and Adobe Photoshop adjustments.
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Catalysts 15 01038 g009
Figure 9. UV–Vis spectral evolution registered for 4-nitrophenolate reduction in the presence of PdNPs (synthesized via the microreactor), sample A. Inset: Color change of the catalytic mixture after addition different volume of catalyst A—10, B—100, C—200, D—500, and E—1000 µL PdNPs (synthesized via the microreactor), where the color were recorded using digital camera and Adobe Photoshop adjustments. Conditions: 125 µM 4-NP, 5 mm NaBH4, 10 µL of PdNPs. Volumetric ratio of the catalytic mixture to the PdNPs (6000:10). Spectrum recorded after 15 min (first spectrum), where the time interval is 5 min between each run.
Figure 9. UV–Vis spectral evolution registered for 4-nitrophenolate reduction in the presence of PdNPs (synthesized via the microreactor), sample A. Inset: Color change of the catalytic mixture after addition different volume of catalyst A—10, B—100, C—200, D—500, and E—1000 µL PdNPs (synthesized via the microreactor), where the color were recorded using digital camera and Adobe Photoshop adjustments. Conditions: 125 µM 4-NP, 5 mm NaBH4, 10 µL of PdNPs. Volumetric ratio of the catalytic mixture to the PdNPs (6000:10). Spectrum recorded after 15 min (first spectrum), where the time interval is 5 min between each run.
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Catalysts 15 01038 g010
Figure 10. Spectral evolution registered for the reduction of 4-nitrophenolate in a batch reactor after the addition of 50 µL of PdNPs, and their corresponding kinetic curves registered at 400 nm, for PdNPs synthesized via batch reactor (a,c) and microreactor (b,d). Inset: the color change with time recorded using Shimadzu spectrophotometry. Conditions: 125 µM 4-NP, 5 mm NaBH4, 50 µL of PdNPs. Volumetric ratio of the catalytic mixture to the PdNPs (6000:50).
Figure 10. Spectral evolution registered for the reduction of 4-nitrophenolate in a batch reactor after the addition of 50 µL of PdNPs, and their corresponding kinetic curves registered at 400 nm, for PdNPs synthesized via batch reactor (a,c) and microreactor (b,d). Inset: the color change with time recorded using Shimadzu spectrophotometry. Conditions: 125 µM 4-NP, 5 mm NaBH4, 50 µL of PdNPs. Volumetric ratio of the catalytic mixture to the PdNPs (6000:50).
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Catalysts 15 01038 g011
Figure 11. Kinetic curves registered at 400 nm for catalytic reaction carried out in the batch reactor (a), hybrid (b), and microreactor system (c). Conditions: 125 µM 4-NP, 5 mm NaBH4. PdNPs were synthesized via batch reactor (a) and microreactor (b) or in situ in stopped flow (c). In the case of a heterogeneous catalytic test carried out using stopped flow, 90 µL of PdNPs were synthesized and then added to 90 µL of catalytic mixture.
Figure 11. Kinetic curves registered at 400 nm for catalytic reaction carried out in the batch reactor (a), hybrid (b), and microreactor system (c). Conditions: 125 µM 4-NP, 5 mm NaBH4. PdNPs were synthesized via batch reactor (a) and microreactor (b) or in situ in stopped flow (c). In the case of a heterogeneous catalytic test carried out using stopped flow, 90 µL of PdNPs were synthesized and then added to 90 µL of catalytic mixture.
Catalysts 15 01038 g011
Table 1. Overview of PdNPs or Pd-based bimetallic/hybrid structures used as catalysts for 4-NP reduction.
Table 1. Overview of PdNPs or Pd-based bimetallic/hybrid structures used as catalysts for 4-NP reduction.
Synthesis of Catalyst Nanoparticles
Catalyst
Composition:		 PdNPs Pdn-PEI NPsAu-Pd
Core–Shell (Pd@Au) NWs		Pd/GO-
O2P 		Graphene-
Supported AuPd (1:3)
Precursor
concentration		0.2 mm H2PdCl42 mm Na2PdCl4aqH2PdCl4 (2.0 mm)0.0554 g/mL H2PdCl4 0.8 mm HAuCl4
HAuCl4
(0.1 M)		2.5 mm H2PdCl6
Volume of metal salt solutions10 mL110 µLMolar ratio Na2PdCl4:PEI = 25,
100, 150		80 μL0.185 mL 3 mL
50 μL
Reducing agent of the precursor0.01 mg/mL NaBH4Fivefold molar
excess
of NaBH4		Ascorbic acid (0.1 M)Plasma treatmentNaBH4
(8 × 10−5 mol)
Reducing agent volume (mL)10-0.28 -3
Particle size (nm)4.0 ± 0.53.0 ± 0.54.0 ± 0.53.0 ± 0.5 3.0 ± 0.5 3.64~6.20 (66 ± 22) ×
(13 ± 1)		2.63.03
Catalyst information
Catalyst volume50 µL50 µL1 mL1 mL90 µL25 μL100 µL--
Catalyst mass (mg)1.1 × 10−31.1 × 10−32.1 × 10−22.1 × 10−21.9 × 10−3* 6 × 10−3* 2.7 × 10−8--
Catalytic test conditions
SystemBatch-
reactor		Hybrid Batch
reactor		Hybrid Micro
reactor		Quartz
cuvette 		-Batch-
Temperature20 °CRT--29.9 °C
Time34 min22 min140 s45 s5 s 10 min60 s-
Reagent concentrations and volumes
[4-NP] (mm)0.50.60.10.032
V4-NP (mL)1.51.50.250.250.02250.250.23
[NaBH4] (M)0.040.50.1 0.250.25
VNaBH4 (mL)0.750.750.1250.1250.01111.0-20
VWater (mL)3.753.750.6250.6250.0561---
Catalytic activity
k (min−)0.0850.1723.485.764.560.280.53.4710.5-
Conversion (%)98.6299.35--10090----
Ref.
ReferencesHerein[20][21][22][23]
Year20252019202420252022
* Values in the table are calculated or estimated from graphs and not strictly mentioned within the paper.
Table 2. Flow rate values of reagents used in hybrid and microreactor systems.
Table 2. Flow rate values of reagents used in hybrid and microreactor systems.
ProcessFlow Rate of the Stream Containing Pd(II) IonsFlow Rate of the Stream Containing NaBH4Diameter
mm		Length
cm		Flow Rate of the Stream Containing 4-Nitrophenolate * + PdNPsContact Time
s		System
Synthesis of PdNPs1.795 mL/min1.795 mL/min0.8257.18 mL/min2.1Hybrid and
Microreactor
Catalytic tests performance1.795 mL/min1.795 mL/min1.25495Microreactor
* 4-nitrophenolate is an intermediate product in the catalytic conversion of 4-nitrophenol using sodium borohydride (see, Section 3.1 for more details).
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Elhadad, A.I.; Luty-Błocho, M. Safety and Process Intensification of Catalytic Reduction of 4-Nitophenol Using Sodium Borohydride in Flow Microreactor System. Catalysts 2025, 15, 1038. https://doi.org/10.3390/catal15111038

AMA Style

Elhadad AI, Luty-Błocho M. Safety and Process Intensification of Catalytic Reduction of 4-Nitophenol Using Sodium Borohydride in Flow Microreactor System. Catalysts. 2025; 15(11):1038. https://doi.org/10.3390/catal15111038

Chicago/Turabian Style

Elhadad, Ahmed Ibrahim, and Magdalena Luty-Błocho. 2025. "Safety and Process Intensification of Catalytic Reduction of 4-Nitophenol Using Sodium Borohydride in Flow Microreactor System" Catalysts 15, no. 11: 1038. https://doi.org/10.3390/catal15111038

APA Style

Elhadad, A. I., & Luty-Błocho, M. (2025). Safety and Process Intensification of Catalytic Reduction of 4-Nitophenol Using Sodium Borohydride in Flow Microreactor System. Catalysts, 15(11), 1038. https://doi.org/10.3390/catal15111038

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