Introduction

Dyes and pigments are widely used in cosmetics, textiles, paper, printing and many other industries. It is thus not surprising that organic dyes such as methyl orange (MO) and Rhodamine B(RhB) are one of the most common anthropogenic water pollutants in industrial effluent. Most of these compounds are acutely toxic, mutagenic and either known or suspected carcinogens1, 2. Many technologies have been designed for the treatment of wastewater containing organic dyes, including adsorption3, coagulation3, 4 and reverse osmosis5. However, the mentioned methods are not sufficient and effective for dye catalytic reduction to nonhazardous products6. As a consequence, searching for an effective and suitable approach for the efficient removal of dyes is extremely essential. Chemical reduction method in the presence of metal nanoparticles and NaBH4 is considered to be a feasible and potential approach for dye decolonization due to its advantages of low-cost, high-efficiency, and easy operation7, 8.

Noble metal based nanomaterials have recently gained significant attention, since they have outstanding physicochemical properties and great potentials in various fields such as optical, catalytic, biomedical and environmental application9,10,11,12. Among them, palladium nanoparticles (Pd NPs) are the most promising nanoparticles served in several industries and academic synthetic chemistry laboratories as effective catalysts for many organic reactions13. However, in the practical application, noble metal NPs can agglomerate easily because of their large surface area, which subsequently results in poor catalytic activity and durability13,14,15.

To solve this problem, a large number of supporting materials, such as polymers12, 16, metal oxides17,18,19,20, clays21 and carbon based materials22,23,24 have been used to support and stabilize NPs. Among many types of carbon based support, graphitic carbon nitride (g-C3N4) has been considered as an ideal support for various metal nanoparticles25,26,27,28,29. However, the pristine g-C3N4, still suffers from unsatisfactory adsorption performance owing to its insufficient active sites and limited specific surface area. An important strategy for improving the adsorption capacity of g-C3N4 is enhancing the active sites by doping heteroatoms (e.g. S, O, B, P)30,31,32,33. In addition, the element doping approach can increase some defects of bulk g-C3N434, 35, thus providing more active sites for binding target ions. Among doped g-C3N4, S-g-C3N4 (SGSN) showed improved electron transfer and catalytic performance36, 37. For example, Li et al.33 showed that doping g-C3N4 with S can facilitate the adsorption ability of Pb(II) since soft S ligands serve as Pb(II) scavenger. Thus, use of SGCN can improve the performance of graphitic carbon as a support.

Magnetic nanoparticles offer significant promise due to their magnetic properties, allowing for easy and fast recovery with a conventional magnet38, 39. Therefore, the design and development of new magnetic catalysts that can be easily separated from the solution is of great importance.

One of the major problems associated with the immobilized metallic heterogeneous catalysts is the low catalyst loading and high catalyst leaching. In the conventional immobilization of metal NPs on a solid support, only one layer of the support surface is available and consequently the metal loading is expected to be low. This problem can be addressed by coating of solid surfaces by functional polymers17, 40, 41. Among various functional polymers, poly (1-vinylimidazole) (PVI) has been intensively studied as a compound for anchoring metal ions in solution. In some cases, PVI has been employed due to its complex formation capability42, whereas other studies focused on the utility of PVI for the preparation of polymer-grafted nanoparticles41. Vinylimidazole (VI) has also been successfully used for the synthesis an ion-imprinted silica supported organic–inorganic hybrid for heavy metal ions removal43 and carrying metal-chelated beads for reversible use in yeast invertase adsorption44.

Ionic liquids, ILs, are a class of very applicable organic salts that can be applied as catalysts, carbon precursor and solvents45,46,47. These organic salts can also be successfully used for the immobilization of catalytic species on the supports48.

In the pursuit of our research on the design of novel hybrid catalytic systems based on g-C3N449,50,51 and IL52,53,54, herein, we report the synthesis of a novel magnetic heterogeneous hybrid catalyst. In this catalytic system, magnetic SGSN was functionalized with vinyl IL and then polymerized with vinyl imidazole to form PVI. The resulting hybrid was then applied as a support for Pd immobilization, (Fig. 1). The prepared SGCN/Fe3O4/PVIs/Pd nanocomposite was then used as a magnetic catalyst for the catalytic reduction of MO and RhB in the presence of NaBH4. In addition, the kinetic and the effects of the reaction temperature, the catalyst amount and the reaction time on the removal of MO and RhB were investigated. Moreover, the recyclability of SGCN/Fe3O4/PVIs/Pd was studied.

Figure 1
figure 1

The schematic procedure of the synthesis of the SGCN/Fe3O4/PVIs/Pd catalyst.

Result and discussion

Catalyst characterizations

The X-ray diffraction (XRD) was applied to monitor the crystal phase of SGCN (Fig. S1) and SGCN/Fe3O4/PVIs/Pd (Fig. 2). Typically, the strongest peak observed for SGCN at 2θ = 27.6° can be representative of interlayer stacking of aromatic system (002). A small diffraction peak at 2θ =  ~ 13.1° can be indexed to the (100) plane and assigned to the in-plane aromatic structural packing33, 55. Regarding SGCN/Fe3O4/PVIs/Pd nanocomposite XRD pattern, the peak at 2θ = 27.6° had a considerably reduced intensity and became broader, while the peak at 13.1° vanished, owing to the introduction of Fe3O4, Pd NPs or the interaction of the Pd NPs and SGCN in SGCN/Fe3O4/PVIs/Pd nanocomposite33, 38. Eleven characteristic diffraction peaks of Fe3O4 are found in XRD pattern of SGCN/Fe3O4/PVIs/Pd nanocomposite (denoted as black circles)56, suggesting that Fe3O4 has been successfully immobilized on S-g-C3N4. The indexed (111) and (200) diffraction peaks at 39.66°, 46.46° and 82.08° are assigned to the Pd NPs (JCPDS No. 46–1,043), corresponding to the face centered cubic (fcc) Pd lattices57.

Figure 2
figure 2

XRD pattern of SGCN/Fe3O4/PVIs/Pd nanocomposite.

The FTIR spectra of SGCN, SGCN/Fe3O4 and SGCN/Fe3O4/PVIs/Pd nanocomposite are presented in Fig. 3. FTIR spectra of all the above mentioned materials presented similar absorption bands at 800 and 1,200–1,600 cm−1, which are attributed to triazine units, aromatic –C=C/–C=N/–C–N bonds, as well as the band at 3,100–3,500 cm−1 that can be assigned to –NH and –OH groups30, 33. The presence of S–C bond at 701 cm-1 in the FTIR spectrum of SGCN, implied the successful incorporation of sulfur into g-C3N4 structure33, 37. As for SGCN/Fe3O4 and SGCN/Fe3O4/PVIs/Pd, the absorption band at 568 cm−1 can be due to Fe–O bond38.

Figure 3
figure 3

FT-IR spectra of SGCN, SGCN/Fe3O4 and SGCN/Fe3O4/PVIs/Pd nanocomposite.

The morphologies of SGCN and SGCN/Fe3O4/PVIs/Pd samples were examined with TEM as shown in Fig. 4a–c. Figure 4a shows the film-like morphology with a layered structure of the SGCN (a unique folded graphene like structure composed of spatially interconnected nanosheets). Figure 4b,c corroborated that Fe3O4 and Pd nanoparticles are highly dispersed on the surface of the support. Furthermore, the EDS analysis of SGCN/Fe3O4/PVIs/Pd nanocomposite (Fig. S2) showed the presence of Fe and Pd atoms, affirming successful incorporation of metallic nanoparticles on the hybrid support. Moreover, the presence of S, C and N is indicative of SGCN. Notably, the absence of Br atom can be ascribed to its low content.

Figure 4
figure 4

TEM images of (a) SGCN and (b, c) SGCN/Fe3O4/PVIs/Pd nanocomposite.

FESM image and elemental mapping analysis of SGCN/Fe3O4/PVIs/Pd nanocomposite were also recorded, Fig. S3. It was found that both Pd and magnetic nanoparticles were dispersed homogeneously on the composite.

Magnetic properties of the Fe3O4 and SGCN/Fe3O4/PVIs/Pd samples were investigated at room temperature, Fig. S4. It was confirmed that the magnetization saturation of SGCN/Fe3O4/PVIs/Pd was 38.42 emu/g, lower than that of Fe3O4 (51.3 emu/g). This result can be justified by considering the fact that Fe3O4 nanoparticles were embedded in the support that is a non-magnetic compound58.

Fig. S5 showed the thermogravimetric analysis (TGA) results of SGCN and SGCN/Fe3O4/PVIs/Pd. As for SGCN, upon increase of temperature up to 550 °C, the sublimation or decomposition of SGCN initiated. This process is completed at 650 °C. For SGCN/Fe3O4/PVIs/Pd nanocomposite, however, the stability of the nanocomposites greatly decreased (the decomposition temperature is shifted to 449.1 °C, which is lower than that of SGCN). This is assigned to the oxidation and decomposition of PVIs.

Kinetic and thermodynamic studies of the reduction reaction of dyes in the presence of SGCN/Fe3O4/PVIs/Pd catalyst

The catalytic activity of the SGCN/Fe3O4/PVIs/Pd nanocomposite was evaluated in the reduction reaction of MO and RhB dyes with NaBH4 as the reducing agent and the progress of reaction monitored with the help of ultraviolet–visible (UV–Vis) absorption spectroscopy. The initial experiments established that in the absence of the catalyst, no reaction progress was apperceived, indicating that the catalyst play an important role in the reduction process. In the next step, the influence of SGCN/Fe3O4/PVIs/Pd loading on MO and RhB catalytic reduction was assessed. In this regard, the catalytic performances of different amounts of catalyst (1, 2, 3, 4 and 5 mg) were evaluated under similar operating condition (performing the reaction at room temperature, in water as solvent). Experimental results affirmed that the conversion of the reactions increased by the increment of the content of SGCN/Fe3O4/PVIs/Pd up to an optimum level (2 mg for MO and 4 mg for RhB) and further increase of SGCN/Fe3O4/PVIs/Pd loading had no remarkable effect on the reaction conversion, Table S1.

The reduction progress for both dyes over time was monitored by measuring the temporal evolution of UV–Vis absorption spectra of the reaction mixtures under SGCN/Fe3O4/PVIs/Pd catalysis (Fig. 5). As shown, the absorption peaks of the dyes (λmax = 465 nm for MO and λmax = 550 nm for RhB) decreased gradually as the reaction elapsed. This implied high efficiency of SGCN/Fe3O4/PVIs/Pd for dye decolorization in a short time of the reaction (40 s for MO and 50 s for RhB).

Figure 5
figure 5

Time-dependent UV–visible spectra for the catalytic reduction of (a) MO and (b) RhB dyes by NaBH4 in the presence of optimum amount of SGCN/Fe3O4/PVIs/Pd.

The MO and RhB catalytic reduction processes followed the pseudo-first-order kinetic, which can be described by the following equation59, 60:

$$\ln {\raise0.7ex\hbox{${C_{0} }$} \!\mathord{\left/ {\vphantom {{C_{0} } C}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$C$}} = kt$$
(1)

In that equation, the values of C0 (dye concentration at the start of the reaction) and C (dye concentration at time t) can be obtained from the absorbance at t = 0 and t (A0 and At) respectively. Hence, the values of the rate constant (k) for the reduction of dyes can be calculated from the slope of ln (C0/C) vs. time (Fig. S6).

The k values for the reduction of both MO and RhB at four different reaction temperatures (293, 298, 303 and 308 K) were similarly measured, reported in Table 1. As tabulated, k value of the reaction increased with the increment of the reaction temperature (Table 1). k values at different temperatures can be helpful for estimating the activation energies (Ea). More exactly, having the Arrhenius equation in hand, Eq. (2), and R and k values, Ea can be measured from the plot of ln k vs. 1/T as shown in Fig. S7 and Table 1.

$$\ln k = \ln A - \left( {{\raise0.7ex\hbox{${E_{a} }$} \!\mathord{\left/ {\vphantom {{E_{a} } {RT}}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${RT}$}}} \right)$$
(2)
Table 1 The values of thermodynamic and kinetic parameters of reduction reaction of MO and RhB dyes in the presence of the SGCN/Fe3O4/PVIs/Pd catalyst.

For the calculation of activation thermodynamics parameters, i.e. activation entropy (ΔS#) and the activation enthalpy (ΔH#), Eyring equation (Eq. 3) and Eyring plot (ln (k/T) vs. 1/T), Fig. S8, were exploited.

$$\ln \left( {{\raise0.7ex\hbox{$k$} \!\mathord{\left/ {\vphantom {k T}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$T$}}} \right) = \ln \left( {{\raise0.7ex\hbox{${k_{B} }$} \!\mathord{\left/ {\vphantom {{k_{B} } h}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$h$}}} \right) + {\raise0.7ex\hbox{${\Delta S^{\# } }$} \!\mathord{\left/ {\vphantom {{\Delta S^{\# } } R}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$R$}} - {\raise0.7ex\hbox{${\Delta H^{\# } }$} \!\mathord{\left/ {\vphantom {{\Delta H^{\# } } R}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$R$}}\left( {{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 T}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$T$}}} \right)$$
(3)

In Eq. (3), kB and h are constant and known values. Moreover, the study of the kinetic parameters provided the value of ln (k/T). Hence, ΔS# and ΔH# can be evaluated from the intercept and slop of Eyring plot respectively. The measured ΔS# values for the reduction reactions of MO and RhB were assessed as -33.678 and −45.626 J/mol K, respectively. ΔH# values of the reduction reactions were measured as 68.397 and 65.929 kJ/mol for MO and RhB dyes, respectively (Table 1).

Mechanism

According to the literature61, the plausible mechanism for the reduction of MO and RhB in the presence of SGCN/Fe3O4/PVIs/Pd can be defined as follow: First, borohydride ions are generated through dissociation of sodium borohydride. Secondly, the as-generated BH4 ions are adsorbed on Pd nanoparticles that are the main catalytic species for the reduction reaction. On the other hand, the organic dyes that possess aromatic moieties in their structures can be adsorbed onto SGCN/Fe3O4/PVIs/Pd through π-π stacking interactions. Thirdly, the adsorbed dyes were reduced by the generated hydride ions, Fig. 6. Finally, the reduced dye will be released from SGCN/Fe3O4/PVIs/Pd.

Figure 6
figure 6

The plausible mechanism for dye reduction.

Recyclability

Considering the importance of the reuse of the heterogeneous catalysts in the practical application, the recyclability of SGCN/Fe3O4/PVIs/Pd for the reduction reaction of both dyes was examined. To accomplish this purpose, SGCN/Fe3O4/PVIs/Pd was separated by applying an external magnetic from the reaction mixture and then employed for the next reaction run under the same reaction condition. This cycle was repeated up to eight consecutive reaction runs and the obtained yields of each runs for both dyes were measured and compared (Fig. 7). As shown in Fig. 7, SGCN/Fe3O4/PVIs/Pd could be recycled for 8 reaction runs with only slight loss of the catalytic activity. Furthermore, the Pd leaching of SGCN/Fe3O4/PVIs/Pd was also investigated for the catalyst reused after eight runs. It was gratifyingly found out that Pd leaching was insignificant (0.01 wt% of initial Pd loading), showing the efficiency and stability of SGCN/Fe3O4/PVIs for Pd anchoring.

Figure 7
figure 7

The results of the recyclability of the catalyst for reduction of MO and RhB.

Next, the stability of the recycled SGCN/Fe3O4/PVIs/Pd was evaluated by recording FTIR spectrum of the recycled SGCN/Fe3O4/PVIs/Pd after eight runs for the reduction of MO and RhB (Fig. S9a). It was found that the spectra of the recycled SGCN/Fe3O4/PVIs/Pd for both reactions are similar to that of fresh SGCN/Fe3O4/PVIs/Pd and no absorbance band has been disappeared upon recycling. Moreover, the TEM analysis of the recycled catalyst after eight cycles did not show major morphological changes (Fig. S9b). These implied that the structure and morphology of SGCN/Fe3O4/PVIs/Pd were not destroyed after recycling.

Experimental

The detail of used materials and apparatus is elaborated in SI. Herein, the syntheses of the catalyst and dye reduction are explained.

Synthesis of the catalyst

Synthesis of S-g-C3N4 nanosheet (SGCN)

Sulphur-doped graphitic carbon nitride (S-g-C3N4) nanosheets were synthesized by carbonization of thiourea in a muffled furnace30. In brief, 10 g of thiourea was placed in crucibles with a cover and calcined at 530 °C in a muffle furnace for 2 h. After calcination, the obtained yellow powder marked as SGCN was ground into fine powder and collected for further usage.

Synthesis of the SGCN/Fe3O4

SGCN/Fe3O4 nanocomposites was synthesized by precipitation method6. Briefly, SGCN powder (0.5 g) was added to 120 mL of distilled water and ultrasonicated (60 W) for 20 min at room temperature. Then, FeCl3.6H2O (1.37 g) and FeCl2·4H2O (0.5 g) were dissolved into 12 mL distilled water and the resultant solutions were added to the SGCN suspension under stirring. Stirring was continued for 60 min at 80 °C, and then 10 mL of ammonia solution was added and the mixture was continuously stirred for 60 min, after which the suspension was allowed to cool naturally. Then, the as prepared precipitate was collected by an external magnet, washed with water and ethanol, and dried at 50 °C overnight. The obtained product (named as SGCN/Fe3O4) was used in the next step.

Synthesis of SGCN/Fe3O4/PVIs Hybrids

SGCN/Fe3O4 (1.0 g) was dispersed in 45 mL toluene in a clean round-bottom flask. Subsequently, 1,4-dibromobutane (1 mL) was added to the stirring mixture. The resulting mixture was heated, refluxed and kept at 110 °C for 24 h under vigorous agitation. At the end of the reaction, the product was subjected to magnetic separation and washed sequentially with EtOH to thoroughly remove unreacted 1,4-dibromobutane from the surface of SGCN/Fe3O4. The final product, denoted as SGCN/Fe3O4 -(CH2)4Br, was then dried under vacuum at 60 °C overnight to remove the residual solvent. To introduce IL, a mixture containing SGCN/Fe3O4-(CH2)4Br (1.0 g) and 1-vinylimidazole (1.5 mL) in 35.0 mL EtOH was stirred for 15 min and then refluxed at 60 °C for 3 h under vigorous agitation to form SGCN/Fe3O4-(CH2)4IL. Growth of poly (1-vinylimidazole) (PVI) was achieved via free radical polymerization of 1-vinylimidazole and SGCN/Fe3O4-(CH2)4IL in the presence of AIBN (50 mg) as initiator. Polymerization was continued at 80 °C for 24 h under Ar atmosphere. Upon completion of the polymerization reaction, the product was separated magnetically and then washed with ethanol several times. The produced SGCN/Fe3O4/ PVIs was dried under vacuum at 60 °C to remove the residual solvent.

Preparations of SGCN/Fe3O4/PVIs/Pd

Stabilization of Pd nanoparticles was realized through wet-impregnation procedure15. 1.2 g of the SGCN/Fe3O4/PVIs was agitated in 50 mL toluene. Afterwards, a solution of 0.1 mmol of Pd(OAc)2 in 20 mL of toluene was added gradually. After agitation at room temperature for 2 h, a solution sodium borohydride in H2O (10 mL, 0.2 N), was added to provide Pd(0) nanoparticles. At the end, SGCN/Fe3O4/PVIs/Pd was collected, washed with MeOH /EtOH, and dried under vacuum for 13 h. Figure 1 present a schematic illustration of preparation of SGCN/Fe3O4/PVIs/Pd. Using ICP method, Pd loading was measured as 0.07 mmolg-1.

Catalytic reduction of dye

To decolorize the dyes, MO or RhB (2 mL), scan content of SGCN/Fe3O4/PVIs/Pd and sodium borohydride (2 mL, 0.01 M) were mixed in water and stirred. The progress of de-colorization was traced by using time-dependent UV–vis spectroscopy61. At the end of the reaction, SGCN/Fe3O4/PVIs/Pd was collected, rinsed repeatedly with EtOH: H2O (1:1) and dried. This experiment was repeated at four temperatures (20, 25, 30 and 35 °C).

Conclusion

In summary, growth of PVI on IL decorated magnetic SGCN has been reported to furnish an efficient support for stabilization of Pd NPs. The resulting catalyst, SGCN/Fe3O4/PVIs/Pd, was characterized and applied for the catalytic reduction of MO and RhB in aqueous media at room temperature. The results confirmed high efficiency of the catalyst for reduction of both dyes in almost 1 min, probably because of the chelation properties. The study of the reaction temperature confirmed that the higher the reaction temperature, the faster the reaction proceeded. Moreover, the effect of the catalyst loading was studied to find out the optimum catalyst loading for both reactions. The rate constants of both reactions were calculated at four different temperatures and using some conventional calculation, Ea, ΔS# and ΔH# values for MO were found to be 68.35 kJ/mol, − 33.67 J/mol K and 68.39 kJ/mol respectively. These values for RhB were 72.63 kJ/mol, − 45.62 J/mol K and 65.92 kJ/mol. Moreover, the recycling of SGCN/Fe3O4/PVIs/Pd confirmed facile recovery of the catalyst and its excellent recyclability up to eight runs. This catalyst has good potential to real-life applications because of easy handling and separation, and long-term stability. In fact, in this study the chemistry of graphitic carbon nitride was modified by incorporation of heteroatom and introduction of PVI to furnish a potential support for Pd immobilization and developing a catalyst for removal of dyes.