Pd-catalyzed Heck coupling reaction is one of the significant synthetic methodologies used to form C–C bonds between olefins and aryl halides and has thus been widely used in the fabrication of various compounds such as natural products, pharmaceuticals, and bioactive materials1,2,3,4. Generally, homogeneous phosphine-based catalysts have played an important role in the Heck coupling reaction3,5,6. However, there are some problems such as the cost of most phosphine ligands, their toxicity, and the incapability of the catalyst to be readily recycled from the media, which limits their large-scale applications7,8. Therefore, efforts to substitute phosphine-based catalysts with heterogeneous solid-supported Pd catalysts have been gaining much interest recently. Different organic/inorganic compounds such as zeolites, carbon materials, metal oxides, MOFs, silica, and polymers have been utilized as carriers to prepare Pd catalysts with high activity and recyclability9,10,11,12,13,14. However, many developed heterogeneous catalysts have poor recyclability and chemical stability, which restrict their application in industrial or academic fields15. The selection or design of suitable supports can help overcome these problems. Moreover, the development of environmentally friendly, economic, highly stable, and recyclable catalytic systems is needed for the Heck coupling reaction from the green chemistry aspect.

Removal of toxic inorganic/organic contaminants from wastewater and soil is a serious and imperative challenge for human life16,17,18,19. Nitrophenols with high risks to the humans and environment are one of the most significant major organic pollutants in agricultural and industrial wastewaters due to their highly toxic nature, stability, and solubility in water20. In particular, 4-NP is an infamous organic pollutant commonly found in industrial wastewaters21. When it is discharged into the environment, it remains for a long time due to its high stability, thus negatively affecting human health, the environment, and aquatic life22. Therefore, 4-NP removal from water/wastewater is of great importance. Various conventional methods such as adsorption, ion exchange, oxidation, reverse osmosis, and membrane separation have previously been applied for the removal of this contaminant23,24,25. However, the treatment or purification of water/wastewater by these methods is not generally sufficient due to some weaknesses such as low adaptability, extraordinary cost, low efficiency, and generation of contamination26,27. Among these processes, catalytic reduction is highly effective due to its simplicity, inexpensiveness, formation of non-toxic compounds, speed, and simplicity of operation28,29. Additionally, reduction products, which are obtained as a result of the catalytic reduction, are valuable compounds for industrial applications. For example, 4-NP reduction product, 4-aminophenol (4-AP), has a significant role in the fabrication of analgesic and antipyretic drugs30,31,32. Therefore, catalytic reduction is a good strategy and the development of highly active catalysts is important.

Recently, the preparation of nanosized metal oxides has gained growing interest from researchers due to their optical, electrical, biological, and catalytic prowess33,34. Therefore, different metal oxides have been designed and used in various fields including water treatment, solar cells, biomedicine, and catalysis35,36. Among metal oxide NPs, ZnO is one of the most effective compounds widely used in various applications due to its reasonable cost, simple fabrication, biocompatibility, high availability, and non-toxicity37,38. Additionally, ZnO particle surfaces can be easily modified with different functionalities. For example, (3-aminopropyl)triethoxysilane (APTES), which is a member of the organosilanes, is a good agent to provide free –NH2 groups on the surface of ZnO39,40. According to the literature, Schiff base ligands are able to coordinate many different metals and metal oxides and can act as catalysts for various organic transformations and different applications41,42,43. In this regard, reactive NH2 groups on the ZnO surface can be further modified by reacting with aldehydes or ketones to form Schiff bases, which strongly interact with metal ions. Schiff base-modified ZnO particles can act as good catalyst supports for the synthesis of different catalysts and the resulting catalysts can be used in different industrial or academic applications.

In this work, Schiff base-modified ZnO particles have been designed as novel catalyst supports for the first time. Pd nanocatalysts have then been synthesized via the stabilization of Pd on the developed support (Scheme 1). The structure of Pd–ZnO–Scb nanocatalyst has been characterized by different spectroscopic techniques such as FT-IR, FE-SEM, TEM, elemental mapping, EDS, and XRD. Characterization studies proved that Pd–ZnO–Scb was spherically shaped with an average particle size of 14 nm. The catalytic activity of Pd–ZnO–Scb was then evaluated in both the Heck coupling reaction and 4-NP reduction. Pd–ZnO–Scb catalyst successfully converted aryl halides to the corresponding Heck coupling products in yields of up to 98% in short reaction times. Additionally, Pd–ZnO–Scb catalyst displayed good performance by reduction of 4-NP within 135 s. Moreover, Pd–ZnO–Scb was easily isolated and reused for up to seven successive runs without changing its morphology and shape.

Scheme 1
scheme 1

Schematic representation of the synthesis of Pd–ZnO–Scb nanocatalyst.


Apparatuses and chemicals

All chemical compounds were purchased from Sigma-Aldrich Chemical Co. FT-IR spectra of ZnO, ZnO–NH2, and ZnO–Scb were recorded using a Perkin Elmer 100 FT-IR spectrophotometer. XRD spectra of ZnO and Pd–ZnO–Scb were recorded using a Rikagu smart lab system. FE-SEM/EDS images of ZnO, ZnO–NH2, ZnO–Scb, and Pd–ZnO–Scb were obtained using a QUANTA–FEG 250ESEM/EDAX–Metek instrument. TEM analysis of Pd–ZnO–Scb nanocatalyst was carried out using a Zeiss Sigma 300 instrument. NMR analyses were performed by a Bruker Avance III 400 MHz spectrometer. The 4-NP reduction was followed by UV–Vis spectroscopy (Genesys 10 S UV–Vis spectrophotometer).

Synthesis of ZnO particles

Zn(NO3)2·6H2O (5.94 g) and (NH4)2CO3 (3.61 g) were separately dissolved in 20 mL of water to form solutions 1 and 2. Solutions 1 and 2 were next mixed and vigorously stirred at room temperature for 3 h to yield ZnO precursors. Finally, the resulting white precipitates were collected by filtration, rinsed with water, dried at 100 °C for 6 h and then calcinated at 550 °C for 4 h.

Synthesis of APTES-functionalized ZnO (ZnO–NH2)

APTES-functionalized ZnO was prepared by heating ZnO (2 g) and APTES (8 mL) in 40 mL of anhydrous toluene for 48 h at 100 °C. Finally, ZnO–NH2 was filtered, washed with EtOH, and dried at 70 °C.

Synthesis of Schiff base-modified ZnO (ZnO–Scb)

ZnO–NH2 (1.5 g) was dispersed in EtOH by sonication for 1 h, followed by the addition of 5 mL of furfural and refluxing the resulting mixture for 72 h. After the reaction was completed, the product denoted as ZnO–Scb, was collected by filtration, rinsed with hot EtOH, and dried at 60 °C.

Decoration of Pd NPs on ZnO–Scb surface (Pd–ZnO–Scb)

1 g of ZnO–Scb and 0.2 g of PdCl2 were added to EtOH (30 mL) and the resulting mixture was heated at 70 °C. After 1 h, the reaction solution turned black, confirming the formation of Pd NPs on the support surface. After another 1 h, the dark-colored Pd–ZnO–Scb catalyst was isolated by filtration, washed with water and EtOH, and then dried at 60 °C.

Pd–ZnO–Scb-catalyzed Heck coupling reaction

A mixture of aryl halide (1 mmol), styrene (1.5 mmol), Na2CO3 (2 mmol), and Pd–ZnO–Scb nanocatalyst (15 mg) in 4 mL of DMF was heated at 120 °C for the required time. At the end of the reaction (monitored by TLC), Pd–ZnO–Scb was recovered from the media and the mixture was extracted with CH2Cl2:H2O (1:1) three times. The organic phase bearing the Heck products was dried on MgSO4 and the products were obtained by evaporating the solvent. For further purification, column chromatography was carried out.

Pd–ZnO–Scb-catalyzed 4-NP reduction

The mixture of an aqueous solution of 4-NP (2 mL, 1.5 × 10–4 M) and NaBH4 (0.5 mL, 0.05 M) was stirred for 2 min at room temperature. 10 mg of Pd–ZnO–Scb were then transferred into the media to start the catalytic reduction of 4-NP and the resulting mixture was stirred for the desired time. The progress of 4-NP reduction was followed by UV–Vis spectroscopy.

NMR data of Heck coupling reaction products


1H NMR (400 MHz, CDCl3): δH = 7.51 (d, 4H), 7.34 (t, 4H), 7.27–7.24 (m, 2H), 7.11 (s, 2H); 13C NMR (100 MHz, CDCl3): δC = 137.40, 128.76, 128.70, 127.64, 126.54.


1H NMR (400 MHz, CDCl3): δH = 7.49–7.43 (m, 4H), 7.33 (t, 2H), 7.24–7.20 (m, 1H), 7.06 (d, 1H), 6.96 (d, 1H), 6.88 (d, 2H), 3.80 (s, 3H); 13C NMR (100 MHz, CDCl3): δC = 159.40, 137.74, 130.24, 128.68, 128.30, 127.77, 127.25, 126.70, 126.31, 114.21, 55.35.


1H NMR (400 MHz, CDCl3): δH = 7.50 (d, 2H), 7.41 (d, 2H), 7.34 (t, 2H), 7.26–7.22 (m, 1H), 7.16 (d, 2H), 7.09 (d, 1H), 7.05 (d, 1H), 2.35 (s, 3H); 13C NMR (100 MHz, CDCl3): δC = 137.58, 134.62, 129.41, 128.66, 128.66, 127.77, 127.41, 126.45, 126.42, 21.25.


1H NMR (400 MHz, CDCl3): δH = 8.21 (d, 2H), 7.62 (d, 2H), 7.55 (d, 2H), 7.42–7.31 (m, 3H), 7.26 (d, 1H), 7.13 (d, 1H); 13C NMR (100 MHz, CDCl3): δC = 146.85, 143.88, 136.24, 133.36, 128.92, 128.86, 127.05, 126.88, 126.33, 124.15.


1H NMR (400 MHz, CDCl3): δH = 7.64–7.57 (q, 4H), 7.53 (d, 2H), 7.39 (t, 2H), 7.33–7.30 (m, 1H), 7.21 (d, 1H), 7.08 (d, 1H); 13C NMR (100 MHz, CDCl3): δC = 141.89, 136.34, 132.50, 132.47, 128.87, 128.66, 126.94, 126.89, 126.77, 119.00, 110.66.


1H NMR (400 MHz, CDCl3): δH = 8.36 (s, 1H), 8.09 (d, 1H), 7.79 (d, 1H), 7.55–7.49 (m, 3H), 7.39 (t, 2H), 7.32 (t, 1H), 7.23 (d, 1H), 7.12 (d, 1H); 13C NMR (100 MHz, CDCl3): δC = 148.81, 139.22, 136.32, 132.23, 131.83, 129.56, 128.87, 128.54, 126.86, 126.14, 122.03, 120.91.

Supplementary data

Contains information about the NMR spectra of Heck coupling reaction products, ninhydrin color test of ZnO–NH2, FE-SEM images, elemental mapping and EDS spectra of samples, and also TEM image and XRD pattern of Pd–ZnO–Scb after seven cycles.

Results and discussion

Pd–ZnO–Scb characterization

The formation of ZnO, ZnO–NH2, and ZnO–Scb were confirmed by FT–IR analysis and the corresponding spectra are given in Fig. 1. Generally, ZnO displays an absorption band below 600 cm−1 related to the Zn–O stretching vibration45. However, this stretching band could not be detected due to the range of ATR/FT-IR. Therefore, XRD, EDS, and FE-SEM analyses were performed to study the fabrication of ZnO. Figure 1b shows the spectrum of ZnO–NH2. The strong peaks located at 2926 and 2867 cm−1 were assigned to the aliphatic C–H stretching vibrations of APTES molecules. Additionally, the peaks at 1575 and 1012 cm−1 were attributed to the NH2 deformation and Si–O stretching vibrations. These important and characteristic peaks confirmed the successful attachment of APTES to the surface of ZnO46. Additionally, a ninhydrin color test was performed to confirm the presence of APTES molecules on ZnO surface. For this purpose, 5 mg of ZnO–NH2 and 5 mg of ninhydrin were refluxed in 10 mL of ethanol47. After 3 min, the color of the suspension turned to purple, resulting in an absorbance peak of 580 nm in the UV–Vis spectrum (Figure S1). Based on the color test and UV–Vis analysis, APTES molecules are attached to the surface of ZnO. The peak at 1645 cm−1 in the FT-IR spectrum of ZnO–Scb indicated the presence of imine stretching vibration, confirming that the condensation reaction of ZnO–NH2 with furfural had been successfully performed.

Figure 1
figure 1

FT-IR spectra of ZnO (a), ZnO–NH2 (b), and ZnO–Scb (c).

The XRD patterns of ZnO and Pd–ZnO–Scb are shown in Fig. 2. In the XRD spectrum of ZnO, the peaks at 2θ values of 31.89°, 34.54°, 36.38°, 47.62°, 56.68°, 62.91°, 66.40°, 68.05°, and 69.18° were attributed to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1) planes of ZnO hexagonal zincite phase, respectively48. These results indicated the high purity of the prepared ZnO. Following the preparation of Pd NPs, ZnO peaks did not change while new peaks were observed at 40.16°, 46.60°, and 82.23°, which were associated with the (111), (200), and (220) planes of Pd, respectively, confirming the stabilization of Pd NPs on the support49.

Figure 2
figure 2

XRD patterns of ZnO (a) and Pd–ZnO–Scb (b).

To examine the morphological properties of synthesized ZnO, ZnO–NH2, ZnO–Scb, and Pd–ZnO–Scb, FE-SEM analysis was performed and the images are illustrated in Figure S2. As it can be observed in Figures S2a-c, ZnO NPs were aggregated and spherical. After the fabrication of ZnO–NH2 and ZnO–Scb, it was observed that ZnO surface was covered with particles, confirming the successful chemical modification of ZnO. The FE-SEM images of Pd–ZnO–Scb indicated the deposition of Pd NPs on ZnO–Scb surface with a nearly spherical shape. EDS analyses were performed to determine the presence of elements in ZnO, ZnO-NH2, and Pd-ZnO-Scb. Additionally, elemental mapping analysis was conducted to investigate the distribution of elements on the surface of the Pd-ZnO-Scb complex. The corresponding spectra are displayed in Figures S3 and S4, respectively. As observed in Figure S3, the EDS spectrum of ZnO showed peaks corresponding to Zn and O elements. The EDS spectrum of ZnO–NH2 displayed the presence of C, N, and Si peaks, related to APTES molecules, which confirmed that APTES molecules were attached to the surface of ZnO. As for Pd–ZnO–Scb spectrum, Pd peaks were observed in addition to the expected elements such as C, N, O, Si, and Zn. Additionally, the presence of Pd was determined using elemental mapping (Figure S4); which indicated the uniform dispersion of Pd on the ZnO–Scb surface.

To further examine the shape and size of Pd–ZnO–Scb, TEM analysis was carried out (Fig. 3). The TEM images of Pd–ZnO–Scb clearly indicated the homogeneous dispersion of the Pd NPs, observed as spherical black spots, on its support. The average diameter of Pd NPs was about 14 nm (Fig. 4).

Figure 3
figure 3

TEM images of Pd–ZnO–Scb.

Figure 4
figure 4

Particle size distribution of Pd–ZnO–Scb.

Investigation of catalytic activity of Pd–ZnO–Scb

After complete characterization of Pd–ZnO–Scb, its catalytic prowess was evaluated in the Heck coupling reaction. In the early step, the reaction between styrene and 1-bromo-4-nitrobenzene was selected as a model reaction and the effects of time, solvent, base, and catalyst loading were then studied to determine the optimal conditions. As observed in Fig. 5, the optimal reaction conditions were 15 mg of Pd–ZnO–Scb, temperature of 120 °C, Na2CO3 as the base, and DMF as the solvent. Afterward, the generality and substrate tolerance of Pd–ZnO–Scb catalytic system was tested in the Heck coupling reaction of different substituted aryl halides under optimal reaction conditions and the results are shown in Table 1. The reaction of styrene and aryl iodides bearing different groups such as –OMe, –Me, and –NO2 was successfully performed with good reaction yields. For example, 4-iodoanisole was converted to the target product with 95% yield within 1.5 h. 1-Iodo-3-nitrobenzene was coupled with styrene with 92% yield. The catalytic potential of Pd–ZnO–Scb was also tested using different substituted aryl bromides and it was found that Pd–ZnO–Scb successfully catalyzed these reactions by providing good isolated yields. For example, 4-bromotoluene was reacted with styrene and the corresponding Heck product reached 91% yield within 3 h. Bromobenzene substrate formed the corresponding Heck product with 94% yield. The reaction of 4-bromobenzonitrile gave the desired product in 96% isolated yield in 1.5 h. The results obtained clearly showed that Pd–ZnO–Scb played a crucial role in the Heck coupling reactions. On the other hand, the catalytic performance of some previously reported catalysts in the Heck coupling reaction between 4-iodoanisole and styrene have been summarized to compare with that of our catalyst (Table 2). It is evident that Pd–ZnO–Scb performed better than the other catalysts in terms of reaction yield and time.

Figure 5
figure 5

Determination of optimal reaction conditions for the reaction of 1-bromo-4-nitrobenzene and styrene in the presence of Pd–ZnO–Scb.

Table 1 Substrate scope for Pd–ZnO–Scb-catalyzed Heck coupling reaction.a
Table 2 Comparison of the catalytic activity of Pd-ZnO-Scb with reported catalysts in Heck coupling reaction between 4-iodoanisole and styrene.

Inspired by the performance of our catalyst in the Heck coupling reaction, it was also used in the catalytic reduction of 4-NP to 4-AP by NaBH4. The reduction of 4-NP in the presence of a catalyst is simple compared to other methods due to the formation of merely one product (4-AP). In addition, the reaction progress can be easily followed by UV–Vis spectroscopy at 400 nm60,61. As Fig. 6a displays, 4-NP, which has a pale-yellow color, showed a maximum absorption at 317 nm in water solvent. Upon the addition of freshly prepared NaBH4 solution into 4-NP solution, the color of the solution changed to deep yellow and the absorption peak at 317 nm shifted to about 400 nm, indicating the formation of 4-nitrophenolate anion. Therefore, 4-NP reduction is carried out on 4-nitrophenolate anion (4-NPT). It was observed that this solution was very stable and the absorption peak at 400 nm did not change even after 5 h without Pd–ZnO–Scb. This indicated that Pd–ZnO–Scb was required for 4-NP reduction. Upon the addition of Pd–ZnO–Scb into 4-NP + NaBH4 mixture, the absorption at 400 nm gradually decreased and disappeared within 135 s. Additionally, a new peak appeared at about 300 nm simultaneously with the reduction reaction, confirming the formation of 4-AP. Moreover, the yellow color of the reaction solution turned colorless at the end of the catalytic reduction. All these findings confirmed the successful conversion of 4-NP to 4-AP by Pd–ZnO–Scb within 135 s without any side products, in concordance with previous studies. Figure 6b illustrates a linear correlation between ln (c/c0) and reaction time (t) for Pd–ZnO–Scb catalyzed 4-NP reduction. The rate constant was found as 0.007 s−1 for 4-NP reduction using the following equation. The catalytic system followed pseudo-first-order kinetics due to NaBH4 concentration being higher than 4-NP.

$${\text{ln }}\left( {{\text{c}}/{\text{c}}_{0} } \right) \, = \, - {\text{kt}}$$

where c0 and c are the initial and final concentrations of 4-NP at tested reaction time (t), respectively, and k (s−1) is the reaction rate.

Figure 6
figure 6

Reduction of 4-NP (a) and linear dependence graph between ln (c/c0) and time for 4-NP (b).

Table 3 summarizes the comparison of the catalytic prowess of Pd–ZnO–Scb with those of various other catalysts in 4-NP reduction. The results showed that Pd–ZnO–Scb was the most active among the catalysts.

Table 3 Comparison of the catalytic activity of Pd-ZnO-Scb with those of other catalysts in 4-NP reduction.

Recyclability potential of Pd–ZnO–Scb

It is known that the recyclability of a catalyst is a crucial factor for both industrial and academic applications in terms of economy, cost-effectiveness, labor, and sustainability. Therefore, the recycling potential of Pd–ZnO–Scb nanocatalyst was investigated in the model Heck coupling reaction under optimized conditions. After each cycle, Pd–ZnO–Scb was isolated by filtration, rinsed with water and ethanol, and dried. The recovered Pd–ZnO–Scb was then directly used in the next reactions. The results revealed that Pd–ZnO–Scb could be reused and recycled up to 7 successive runs giving a product yield of 87%. To check the stability of Pd–ZnO–Scb, its surface was examined by TEM and XRD analyses following the seventh run and the corresponding images are given in Figures S5 and S6. It was found that the particle size, shape, and morphology of the recycled Pd–ZnO–Scb were almost identical to that of the fresh catalyst, indicating the structural stability of Pd–ZnO–Scb.

A hot filtration test was performed on 1-bromo-4-nitrobenzene under the optimal reaction conditions determined in the Heck coupling reaction. The Heck coupling reaction was carried out for 45 min in the presence of Pd–ZnO–Scb nanocatalyst. The catalyst was then recovered from the reaction medium and allowed to react for an additional 45 min under the same conditions to complete the reaction time with the filtrate. It was observed that the Heck coupling reaction did not progress further, indicating that Pd–ZnO–Scb did not leach. Inductively Coupled Plasma (ICP) analysis revealed that the content of Pd NPs loaded on the ZnO–Scb surface was about 9.6%. To check the heterogeneity of catalyst, which is an important factor, the phenomenon of leaching was studied by ICP analysis of the resulting reaction mixture. According to the ICP analysis, the Pd content of the used catalyst was determined as 9.2%.


In summary, Pd–ZnO–Scb has been designed as a retrievable/recyclable heterogeneous nanocatalyst by stabilizing Pd NPs on the prepared Schiff base functionalized ZnO support. The nano-structured Pd–ZnO–Scb was successfully characterized by FT-IR, FE-SEM, TEM, XRD, elemental mapping, and EDS analyses. The catalytic prowess of Pd–ZnO–Scb was then evaluated in the Heck coupling reaction and 4-NP reduction. The results indicated that Pd–ZnO–Scb successfully coupled aryl chlorides, bromides and iodides with styrene, giving 63–98% isolated yields. 4-NP reduction was also efficiently catalyzed by Pd–ZnO–Scb in a short reaction time (135 s). Furthermore, Pd–ZnO–Scb was utilized for seven successive runs with 87% reaction yield and the protection of the nanostructure following the recycling tests was confirmed by TEM analysis. Due to its low cost, simplicity of separation, high yield, stability, and high recoverability/reusability, Pd–ZnO–Scb has a high potential for catalytic transformations and therefore, its catalytic prowess can be evaluated in other applications in the future.