Pd on poly(1-vinylimidazole) decorated magnetic S-doped grafitic carbon nitride: an efficient catalyst for catalytic reduction of organic dyes

A novel magnetic catalyst, (SGCN/Fe3O4/PVIs/Pd) was synthesized by growing of poly(1-vinylimidazole) on the surface of ionic liquid decorated magnetic S-doped graphitic carbon nitride, followed by stabilization of palladium nanoparticles. Catalytic activity of the prepared heterogeneous catalyst was explored for the catalytic reduction of hazardous dyes, methyl orange and Rhodamine B, in the presence of NaBH4. Besides, the effects of the reaction variables on the catalytic activity were investigated in detail. The kinetics study established that dye reduction was the first order reaction and the apparent activation energy was calculated to be 72.63 kJ/mol and 68.35 kJ/mol1 for methyl orange and Rhodamine B dyes, respectively. Moreover, ΔS# and ΔH# values for methyl orange were found to be − 33.67 J/mol K and 68.39 kJ/mol respectively. These values for Rhodamine B were − 45.62 J/mol K and 65.92 kJ/mol. The recycling test verified that the catalyst possessed good stability and reusability, thereby making it a good candidate for the catalytic purposes. Furthermore, a possible catalytic mechanism for dye catalytic reduction over SGCN/Fe3O4/PVIs/Pd was proposed.

www.nature.com/scientificreports/ element doping approach can increase some defects of bulk g-C 3 N 4 34, 35 , thus providing more active sites for binding target ions. Among doped g-C 3 N 4 , S-g-C 3 N 4 (SGSN) showed improved electron transfer and catalytic performance 36,37 . For example, Li et al. 33 showed that doping g-C 3 N 4 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 magnet 38,39 . 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 polymers 17,40,41 . 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 capability 42 , whereas other studies focused on the utility of PVI for the preparation of polymer-grafted nanoparticles 41 . Vinylimidazole (VI) has also been successfully used for the synthesis an ion-imprinted silica supported organic-inorganic hybrid for heavy metal ions removal 43 and carrying metal-chelated beads for reversible use in yeast invertase adsorption 44 .
Ionic liquids, ILs, are a class of very applicable organic salts that can be applied as catalysts, carbon precursor and solvents [45][46][47] . These organic salts can also be successfully used for the immobilization of catalytic species on the supports 48 .
In the pursuit of our research on the design of novel hybrid catalytic systems based on g-C 3 N 4 49-51 and IL 52-54 , 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/Fe 3 O 4 /PVIs/ Pd nanocomposite was then used as a magnetic catalyst for the catalytic reduction of MO and RhB in the presence of NaBH 4 . 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/Fe 3 O 4 / PVIs/Pd was studied.

Result and discussion
Catalyst characterizations. The X-ray diffraction (XRD) was applied to monitor the crystal phase of SGCN (Fig. S1) and SGCN/Fe 3 O 4 /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 packing 33,55 . Regarding SGCN/ Fe 3 O 4 /PVIs/Pd nanocomposite XRD pattern, the peak at 2θ = 27.6° had a considerably reduced intensity and became broader, while the peak at 13 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 groups 30,33 . The presence of S-C bond at 701 cm -1   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/Fe 3 O 4 /PVIs/Pd catalysis (Fig. 5). As shown, the absorption peaks of the dyes (λ max = 465 nm for MO and λ max = 550 nm for RhB) decreased gradually as the reaction elapsed. This implied high efficiency of SGCN/Fe 3 O 4 /PVIs/Pd for dye decolorization in a short time of the reaction (40 s for MO and 50 s for RhB).
The MO and RhB catalytic reduction processes followed the pseudo-first-order kinetic, which can be described by the following equation 59, 60 : In that equation, the values of C 0 (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 (A 0 and A t ) respectively. Hence, the values of the rate constant (k) for the reduction of dyes can be calculated from the slope of ln (C 0 /C) vs. time (Fig. S6). www.nature.com/scientificreports/ 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 (E a ). More exactly, having the Arrhenius equation in hand, Eq. (2), and R and k values, E a can be measured from the plot of ln k vs. 1/T as shown in Fig. S7 and Table 1.  (Table 1).
Mechanism. According to the literature 61 , the plausible mechanism for the reduction of MO and RhB in the presence of SGCN/Fe 3 O 4 /PVIs/Pd can be defined as follow: First, borohydride ions are generated through dissociation of sodium borohydride. Secondly, the as-generated BH 4 − 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/Fe 3 O 4 /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/Fe 3 O 4 /PVIs/Pd.

Recyclability.
Considering the importance of the reuse of the heterogeneous catalysts in the practical application, the recyclability of SGCN/Fe 3 O 4 /PVIs/Pd for the reduction reaction of both dyes was examined.  www.nature.com/scientificreports/ To accomplish this purpose, SGCN/Fe 3 O 4 /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/Fe 3 O 4 /PVIs/Pd could be recycled for 8 reaction runs with only slight loss of the catalytic activity. Furthermore, the Pd leaching of SGCN/Fe 3 O 4 /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/Fe 3 O 4 /PVIs for Pd anchoring. Next, the stability of the recycled SGCN/Fe 3 O 4 /PVIs/Pd was evaluated by recording FTIR spectrum of the recycled SGCN/Fe 3 O 4 /PVIs/Pd after eight runs for the reduction of MO and RhB (Fig. S9a). It was found that the spectra of the recycled SGCN/Fe 3 O 4 /PVIs/Pd for both reactions are similar to that of fresh SGCN/Fe 3 O 4 /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/Fe 3 O 4 /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-C 3 N 4 nanosheet (SGCN).
Sulphur-doped graphitic carbon nitride (S-g-C 3 N 4 ) nanosheets were synthesized by carbonization of thiourea in a muffled furnace 30 . 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. 3 O 4 . SGCN/Fe 3 O 4 nanocomposites was synthesized by precipitation method 6 . Briefly, SGCN powder (0.5 g) was added to 120 mL of distilled water and ultrasonicated (60 W) for 20 min at room

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/Fe 3 O 4 /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 www.nature.com/scientificreports/ 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, E a , 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/Fe 3 O 4 /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.