Preparation and Application of a Hydrochar-Based Palladium Nanocatalyst for the Reduction of Nitroarenes

In the present study, a novel heterogeneous catalyst was successfully fabricated through the decoration of palladium nanoparticles on the surface of designed Fe3O4-coffee waste composite (Pd-Fe3O4-CWH) for the catalytic reduction of nitroarenes. Various characterization techniques such as XRD, FE-SEM and EDS were used to establish its nano-sized chemical structure. It was determined that Pd-Fe3O4-CWH is a useful nanocatalyst, which can efficiently reduce various nitroarenes, including 4-nitrobenzoic acid (4-NBA), 4-nitroaniline (4-NA), 4-nitro-o-phenylenediamine (4-NPD), 2-nitroaniline (2-NA) and 3-nitroanisole (3-NAS), using NaBH4 in aqueous media and ambient conditions. Catalytic reactions were monitored with the help of high-performance liquid chromatography. Additionally, Pd-Fe3O4-CWH was proved to be a reusable catalyst by maintaining its catalytic activity through six successive runs. Moreover, the nanocatalyst displayed a superior catalytic performance compared to other catalysts by providing a shorter reaction time to complete the reduction in nitroarenes.


Introduction
Due to the increasing demand for sustainable recovery and the exploitation of biowaste, the conversion of biowaste into valuable products has attracted a lot of attention. At the same time, the atmospheric emissions from the use of fossil fuels cause problems such as global warming, climate change and environmental pollution. Furthermore, fossil fuel reserves are rapidly depleting, while energy consumption is increasing dramatically [1,2]. Until now, the most sustainable exploitation pathway for biowaste has been its conversion to biofuels; however, new applications are emerging. Coffee is one of the most widely consumed beverages in the world and is created from roasted beans of the plant genus Coffea and family Rubiaceae. On average, around 1 kg of soluble coffee can produce two kilograms of wet ground coffee [3]. As a result, large quantities of used coffee grounds from coffee shops are disposed of in landfills. Therefore, the disposal of coffee grounds accelerates the time required for the landfill to reach its capacity. Furthermore, this contributes to a global problem of food loss and waste, now estimated to be 2.1 billion tons of food wasted and a lost economic value of USD1.5 trillion globally by 2030 [4].
Hydrochar is a carbon-based material that is prepared by the hydrothermal carbonization of high moisture biomass waste, such as sewage sludge, algae or grass, in an aqueous environment at temperatures in the range of 180-260 • C [5]. Biochar is the solid product of biomass pyrolysis at temperatures in the range of 300-800 • C. The main advantage of hydrothermal carbonization over conventional pyrolysis is the potential to use wet biomasses as feedstock. Such carbonaceous materials prepared from spent coffee grounds have received much attention recently for their economic value and promising applications in environmental treatment technology. Although biochars exhibit higher surface areas and more extended porosities compared to hydrochars, the latter commonly have a higher number of oxygen-containing surface groups. Depending on the requirements, all these attributes are highly desirable for the development of functional materials such as catalysts or adsorbents. Various biochars and hydrochars have been used as substrates to disperse and stabilize nanoparticles (NPs) to enhance their reactivity for catalytic reactions [6][7][8].
Some examples of pollutants of concern today include heavy metals, herbicides, oil spills, pharmaceuticals and fertilizers. Compounds containing nitro groups have been determined in aqueous environments [9]. Due to the mutagenic and carcinogenic properties of nitro compounds, it is necessary to investigate their environmental fate as part of a strategy to prevent the contamination of receiving bodies. So far, various methods have been developed to remove nitro compounds from wastewater, including photochemical degradation, adsorption, microbial degradation, membrane distillation and electrocoagulation. However, these methods often have practical limitations, such as a low removal efficiency, cost inefficiency and the formation of harmful by-products. The catalytic reduction in nitro compounds to amino derivatives is an alternative and emerging process for the elimination of toxic nitro compounds from the environment. As a result of their unique and distinctive properties, nanomaterials have attracted great interest in recent years. In particular, transition metal/metal oxide nanocatalysts, with their unique physical and chemical properties, have attracted significant attention for their application in various fields [10][11][12]. The design and preparation of such catalysts has attracted a lot of attention for industrial processes, since they can be magnetically recovered after use, washed and reapplied [13].
In the framework of circular bio-economy, the rationale behind this work is to develop a novel pathway for the utilization of coffee waste and the production of a high added-value material. Therefore, spent coffee grounds were converted to hydrochar through hydrothermal carbonization. This process helps to increase the structural and chemical stability of the coffee grounds. The resultant hydrochar was then used as a substrate for the deposition of Fe 3 O 4 particles, followed by the dispersion of Pd nanoparticles on the magnetic substrate surface. The chemical structure and composition of the nanocatalyst (referred to as Pd-Fe 3 O 4 -CWH thereof) were determined by various imaging and spectroscopic methods. Pd-Fe 3 O 4 -CWH was then applied as heterogeneous nanocatalyst for the reduction in 4-nitrobenzoic acid (4-NBA), 4-nitroaniline (4-NA), 4-nitro-o-phenylenediamine (4-NPD), 2-nitroaniline (2-NA) and 3-nitroanisole (3-NAS), using NaBH 4 as a reducing reagent. The respective aniline products were determined by high performance liquid chromatography. A detailed investigation of the mechanism of reduction in the nitro groups was beyond the scope of this study. Finally, the reusability of the nanocatalyst was investigated by applying it in six successive catalytic runs.

Preparation and Characterization of Pd-Fe 3 O 4 -CWH Nanocatalyst
Hydrochar was prepared through hydrothermal carbonization at 200 • C and 2 h treatment time.
Fe 3 O 4 -CWH was obtained by the following procedure, discussed in detail in our previous study [5]. First, FeSO 4 ·7H 2 O (4.2 g) and FeCl 3 ·6H 2 O (6.1 g) were dissolved in 100 mL distilled water and heated to 90 • C. Ammonium hydroxide (10 mL-26%) and a suspension of 1 g of CWH in 200 mL of water were mixed, the mixture was stirred at 90 • C for 40 min and, finally, cooled to 25 • C. Fe 3 O 4 -CWH was collected as a black precipitate by filtering, being repeatedly washed with distilled water until a neutral pH was reached, dried at 70 • C for 18 h and stored. The next procedure was applied to load the Pd nanoparticles onto Fe 3 O 4 -CWH. A total of 0.25 g of Fe 3 O 4 -CWH was suspended in 30 mL water and a specific quantity of Na 2 PdCl 4 (as the Pd precursor) was added, representative of a 5% Pd loading. After 40 min of stirring at 25 • C, an ascorbic acid solution (n ascorbicacid :n Pd 1 4 2:1) was added and allowed to react for 130 min. After filtration, the solid catalyst was rinsed repeatedly with distilled water. Pd-Fe 3 O 4 -CWH was recovered with extremal magnet after drying at 80 • C for 12 h. The preparation of Pd-Fe 3 O 4 -CWH nanocatalyst is presented in Figure 1. Pd loading on Pd-Fe3O4-CWH was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo Scientific iCAP 6500, Manchester, UK).

Preparation and Characterization of Pd-Fe3O4-CWH nanocatalyst
Hydrochar was prepared through hydrothermal carbonization at 200 °C and 2 h treatment time.
Fe3O4-CWH was obtained by the following procedure, discussed in detail in our previous study [5]. First, FeSO4·7H2O (4.2 g) and FeCl3·6H2O (6.1 g) were dissolved in 100 mL distilled water and heated to 90 °C. Ammonium hydroxide (10 mL-26%) and a suspension of 1 g of CWH in 200 mL of water were mixed, the mixture was stirred at 90 °C for 40 min and, finally, cooled to 25 °C. Fe3O4-CWH was collected as a black precipitate by filtering, being repeatedly washed with distilled water until a neutral pH was reached, dried at 70 °C for 18 h and stored. The next procedure was applied to load the Pd nanoparticles onto Fe3O4-CWH. A total of 0.25 g of Fe3O4-CWH was suspended in 30 mL water and a specific quantity of Na2PdCl4 (as the Pd precursor) was added, representative of a 5% Pd loading. After 40 min of stirring at 25 °C, an ascorbic acid solution (nascorbicacid:nPd ¼ 2:1) was added and allowed to react for 130 min. After filtration, the solid catalyst was rinsed repeatedly with distilled water. Pd-Fe3O4-CWH was recovered with extremal magnet after drying at 80 °C for 12 h. The preparation of Pd-Fe3O4-CWH nanocatalyst is presented in Figure 1.

Reduction in Nitro Compounds to Anilines
For the reduction in the nitro compounds to the respective amino derivatives, 20 mg of Pd-Fe 3 O 4 -CWH was transferred into 1 mL of nitro compound (3 × 10 −4 M), followed by stirring for 1 min at room temperature. Freshly prepared NaBH 4 (0.08 M, 0.4 mL) was then added to the reaction medium and the nitro compound reduction was followed by HPLC. Finally, the nanocatalyst was removed from the reaction media by a magnetic bar and reactivated by washing with water before using it for subsequent runs. Kinetic studies were performed at 25 • C by using 4-NBA as the model substrate and an excess concentration of NaBH 4 .

HPLC Analysis
The analysis of the reduced nitro-aromatic compounds was performed by using a PerkinElmer Flexar Series HPLC system (Waltham, MA, USA). Separation was achieved on a ZORBAX SB Phenyl column (150 mm × 4.6 mm, 5 µm, Agilent Technologies, Santa Clara, CA, USA) maintained at 25 • C. The mobile phase used was 20/79 v/v acetonitrile/water, to which 1% acetic acid was added. The flow rate was set at 1.0 mL·min -1 and the injection volume at 10 µL. UV detection was set at 270 nm.

Figure 2 depicts FE-SEM images and associated EDS data of Pd-Fe 3 O 4 -CWH. FE-SEM
images of CWH showed an irregular but porous surface morphology (Figure 2a,b). Hydrochars are typically amorphous materials with a low degree of crystallinity [14,15]. Following the deposition of Fe 3 O 4 , it was observed that CWH's surface morphology was not affected, but only covered by Fe 3 O 4 particles (Figure 2c,d). The Fe 3 O 4 cluster sizes varied widely, ranging from tenths to hundreds of nanometers, confirming earlier studies [16].  The XPS analysis ( Figure 5) also confirmed the fabrication of the Pd-Fe 3 O 4 -CWH nanocatalyst by the display of Fe 2p (709.4 eV (2p 3/2 ), 723.8 eV (2p 1/2 )) and Pd 3d (335.3 eV (3d 5/2 ), 341.6 eV (3d 3/2 )) of metallic Pd(0), respectively, in the spectrum [21,22]. The two characteristic peaks of Pd(II), commonly seen at 338.0 and 343.6 eV in published works, were not observed [23]. This indicated the dominance of the Pd(0) speciation on our nanocatalyst.    (Figure 4b). Additionally, the two diffraction peaks at 40.19° and 46.65°, assigned to the (111) and (200) planes of metallic Pd [19,20] were easily observed in the XRD pattern of the nanocatalyst ( Figure  4b). These peaks further confirmed the presence of Pd NPs on the Fe3O4-CWH surface. For a more detailed surface morphology, a TEM analysis of the Pd-Fe3O4-CWH nanocatalyst was performed and the corresponding images are displayed in Figure 5. The images highlighted the presence of Pd NPs as black dots, nearly homogenously grafted on Fe3O4-CWH.  metallic Pd [19,20] were easily observed in the XRD pattern of the nanocatalyst ( Figure  4b). These peaks further confirmed the presence of Pd NPs on the Fe3O4-CWH surface. For a more detailed surface morphology, a TEM analysis of the Pd-Fe3O4-CWH nanocatalyst was performed and the corresponding images are displayed in Figure 5. The images highlighted the presence of Pd NPs as black dots, nearly homogenously grafted on Fe3O4-CWH.  The XPS analysis ( Figure 6) also confirmed the fabrication of the Pd-Fe3O4-CWH nanocatalyst by the display of Fe 2p (709.4 eV (2p3/2), 723.8 eV (2p1/2)) and Pd 3d (335.3 eV (3d5/2), 341.6 eV (3d3/2)) of metallic Pd(0), respectively, in the spectrum [21,22]. The two characteristic peaks of Pd(II), commonly seen at 338.0 and 343.6 eV in published works, were not observed [23]. This indicated the dominance of the Pd(0) speciation on our nanocatalyst.

Catalytic Activity Test
The performance of the nanocatalyst was explored by employing it in the catalytic reduction in various nitro compounds in the presence of NaBH4 as a hydrogen source. To test the catalytic performance of Pd-Fe3O4-CWH and establish the optimal reaction conditions (amount of catalyst, concentration of NaBH4 and nitro compound) in the reduction in nitro compounds, 4-NA was selected as a representative substrate and control studies were performed ( Table 1). The progress of catalytic reactions was followed by high performance liquid chromatography by monitoring the retention times of both substrates and reduced products ( Figure 7). As seen in Table 1, the desired product was obtained in less time in the presence of 20 mg of catalyst, 3 × 10 −4 M of nitro compound and 0.4 mL of NaBH4 (0.08 M). Subsequently, the catalytic behavior of Pd-Fe3O4-CWH was examined in the reduction in 2-NA, 4-NBA, 4-NPD and 3-NAS reductions with the detected optimal conditions and the obtained findings shown in Table 1. As seen in Table 2, the nitro compounds were successfully reduced to the corresponding aniline derivatives in very short times. For example, the Pd-Fe3O4-CWH nanocatalyst provided a complete reduction in 4-NBA within 60 s. Pd-Fe3O4-CWH catalyzed the 4-NA reduction within 82 sec. Furthermore, the reduction in 2-NA proceeded quickly and was completed within 90 sec. In a similar manner, the catalytic reductions in 4-NPD and 3-NAS were completed within 168 and 428 sec, respectively. On the other hand, the reduction in 4-NA was performed without the Pd-Fe3O4-CWH catalyst, and no product formation was observed after 1 h. This result revealed the importance of the Fe3O4-CWH catalyst against the reduction in nitroarenes. Additionally, based on previous studies, the possible reaction mechanism for the Pd-Fe3O4-CWH catalyzed reduction in nitroarenes was given in Scheme 1 [24,25].

Catalytic Activity Test
The performance of the nanocatalyst was explored by employing it in the catalytic reduction in various nitro compounds in the presence of NaBH 4 as a hydrogen source. To test the catalytic performance of Pd-Fe 3 O 4 -CWH and establish the optimal reaction conditions (amount of catalyst, concentration of NaBH 4 and nitro compound) in the reduction in nitro compounds, 4-NA was selected as a representative substrate and control studies were performed ( Table 1). The progress of catalytic reactions was followed by high performance liquid chromatography by monitoring the retention times of both substrates and reduced products ( Figure 6). As seen in Table 1, the desired product was obtained in less time in the presence of 20 mg of catalyst, 3 × 10 −4 M of nitro compound and 0.4 mL of NaBH4 (0.08 M). Subsequently, the catalytic behavior of Pd-Fe 3 O 4 -CWH was examined in the reduction in 2-NA, 4-NBA, 4-NPD and 3-NAS reductions with the detected optimal conditions and the obtained findings shown in Table 1. As seen in Table 2, the nitro compounds were successfully reduced to the corresponding aniline derivatives in very short times. For example, the Pd-Fe 3 O 4 -CWH nanocatalyst provided a complete reduction in 4-NBA within 60 s. Pd-Fe 3 O 4 -CWH catalyzed the 4-NA reduction within 82 s. Furthermore, the reduction in 2-NA proceeded quickly and was completed within 90 s. In a similar manner, the catalytic reductions in 4-NPD and 3-NAS were completed within 168 and 428 s, respectively. On the other hand, the reduction in 4-NA was performed without the Pd-Fe 3 O 4 -CWH catalyst, and no product formation was observed after 1 h. This result revealed the importance of the Fe 3 O 4 -CWH catalyst against the reduction in nitroarenes. Additionally, based on previous studies, the possible reaction mechanism for the Pd-Fe 3 O 4 -CWH catalyzed reduction in nitroarenes was given in Scheme 1 [24,25].        To indicate the superiority of Pd-Fe3O4-CWH over other reported catalysts, its catalytic performance for the reduction in 2-NA was compared to that of other catalysts reported in the literature. As seen in Table 3, our nanocatalyst compared favorably with respect to the reaction time for the complete reduction in 2-NA. Furthermore, some of the Scheme 1. Proposed reaction mechanism for Pd-Fe 3 O 4 -CWH catalyzed reduction in nitroarenes.

4-NA (M) NaBH4 (M) Pd-Fe3O4-CWH (mg) Reaction Time (s)
To indicate the superiority of Pd-Fe 3 O 4 -CWH over other reported catalysts, its catalytic performance for the reduction in 2-NA was compared to that of other catalysts reported in the literature. As seen in Table 3, our nanocatalyst compared favorably with respect to the reaction time for the complete reduction in 2-NA. Furthermore, some of the catalysts reported in the table required multiple, complex preparation steps, and used substrates from non-renewable sources. For the kinetic study, a high excess of NaBH 4 meant that the rate constant could be assumed to be independent of the NaBH 4 concentration and a pseudo-first-order kinetics model could be applied to the reduction in 4-NBA [35]. The pseudo-first-order rate constant (k) value was calculated from the slope of the following equation: where 4-NBA t and 4-NBA 0 are the 4-NBA concentration at time t and initial concentration, respectively. As indicated by the regression coefficient (R 2 = 0.9829), the reduction data fitted very well to the pseudo-first-order model ( Figure S1). This observation agreed well with earlier studies, which examined the reduction of nitroarenes under the influence of various catalysts [36,37]. The rate constant was determined as 0.1479 min −1 , indicating a kinetically unhindered process with no induction period, in contrast to some earlier studies that showed that Pd catalysts have an initial stage of reduced performance during the reduction in nitro compounds [38][39][40].

Recyclability of Pd-Fe 3 O 4 -CWH
One of the most important properties of heterogeneous catalysts is their reusability, a valuable attribute for industrial applications. Therefore, the recycling potential of Pd-Fe 3 O 4 -CWH was evaluated on 4-NBA reduction. The nanocatalyst was readily separated from reaction media magnetically after the 4-NBA reduction and rinsed with water to use in the further reactions ( Figure 7). Reusability/recoverability studies showed that Pd-Fe 3 O 4 -CWH was reused up to a number six runs without any significant loss of catalytic performance (Figure 8). Pd loading on Pd-Fe3O4-CWH was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo Scientific iCAP 6500, Manchester, UK).

Preparation and Characterization of Pd-Fe3O4-CWH nanocatalyst
Hydrochar was prepared through hydrothermal carbonization at 200 °C and 2 h treatment time.
Fe3O4-CWH was obtained by the following procedure, discussed in detail in our previous study [5]. First, FeSO4·7H2O (4.2 g) and FeCl3·6H2O (6.1 g) were dissolved in 100 mL distilled water and heated to 90 °C. Ammonium hydroxide (10 mL-26%) and a suspension of 1 g of CWH in 200 mL of water were mixed, the mixture was stirred at 90 °C for 40 min and, finally, cooled to 25 °C. Fe3O4-CWH was collected as a black precipitate by filtering, being repeatedly washed with distilled water until a neutral pH was reached, dried at 70 °C for 18 h and stored. The next procedure was applied to load the Pd nanoparticles onto Fe3O4-CWH. A total of 0.25 g of Fe3O4-CWH was suspended in 30 mL water and a specific quantity of Na2PdCl4 (as the Pd precursor) was added, representative of a 5% Pd loading. After 40 min of stirring at 25 °C, an ascorbic acid solution (nascorbicacid:nPd ¼ 2:1) was added and allowed to react for 130 min. After filtration, the solid catalyst was rinsed repeatedly with distilled water. Pd-Fe3O4-CWH was recovered with extremal magnet after drying at 80 °C for 12 h. The preparation of Pd-Fe3O4-CWH nanocatalyst is presented in Figure 1.   One of the most important properties of heterogeneous catalysts is their reusability, a valuable attribute for industrial applications. Therefore, the recycling potential of Pd-Fe3O4-CWH was evaluated on 4-NBA reduction. The nanocatalyst was readily separated from reaction media magnetically after the 4-NBA reduction and rinsed with water to use in the further reactions ( Figure 2). Reusability/recoverability studies showed that Pd-Fe3O4-CWH was reused up to a number six runs without any significant loss of catalytic performance (Figure 8).

Conclusions
It was demonstrated that coffee waste hydrochar can be used as a substrate for the development of an efficient nanocatalyst. Overall, our approach combined concepts of waste management, circular bio-economy and wastewater treatment. Therefore, biomassbased materials can provide sustainable building blocks for the production of catalysts for environmental remediation. Fe 3 O 4 and Pd nanoparticles were deposited on the hydrochar surface through a simple process and the resultant catalyst exhibited a positive performance in the reduction in nitroarenes. Kinetically, the nanocatalyst showed no induction period, achieving complete reductions for a wide range of nitro compounds in very short times. Furthermore, the stability and reusability of the catalyst was established by collecting it through the use of a magnet and applying it in successive reaction runs. Further work should focus on testing the performance of the catalyst in the reduction in or oxidation reactions of other organic contaminants and investigating the economics of the process in detail, identifying necessary steps before scaling-up application.
Supplementary Materials: The following are available online, Figure S1: Fitting of the 4-NBA reduction data to the pseudo first-order kinetic model.