Bio-assisted synthesized Pd nanoparticles supported on ionic liquid decorated magnetic halloysite: an efficient catalyst for degradation of dyes

Using natural materials, i.e. halloysite nanoclay that is a biocompatible naturally occurring clay and Heracleum persicum extract that can serve as a green reducing agent, a novel magnetic catalyst, Fe3O4/Hal-Mel-TEA(IL)-Pd, has been designed and fabricated. To prepare the catalyst, halloysite was first magnetized (magnetic particles with mean diameter of 13.06 ± 3.1 nm) and then surface functionalized with melamine, 1,4 dibromobutane and triethanolamine to provide ionic liquid on the halloysite surface (5 wt%). The latter was then used as a support to immobilize Pd nanoparticles that were reduced by Heracleum persicum extract. The characterization of the catalyst established that the loading of Pd in Fe3O4/Hal-Mel-TEA(IL)-Pd was very low (0.93 wt%) and its specific surface area was 63 m2g−1. Moreover, the catalyst showed magnetic property (Ms = 19.75 emu g−1) and could be magnetically separated from the reaction. The catalytic performance of the magnetic catalyst for reductive degradation of methyl orange and rhodamine B in the presence of NaBH4 in aqueous media was investigated. The activation energy, enthalpy, and entropy for the reduction of methyl orange were estimated as 42.02 kJ mol−1, 39.40 kJ mol−1, and −139.06 J mol−1 K−1, respectively. These values for rhodamine B were calculated as 39.97 kJ mol−1, 34.33 kJ mol−1, and −155.18 Jmol−1K−1, respectively. Notably, Fe3O4/Hal-Mel-TEA(IL)-Pd could be reused for eight reaction runs with negligible loss of the catalytic activity (~3%) and Pd leaching (0.01 wt% of the initial loading).

In the continuation of our research on disclosing the catalytic utility of Hal 23,24 , recently we have reported facile recovery and high reusability of magnetic Hal 25,26 . On the other hand, our research on the catalytic activity of IL-Hal hybrid 27,28 showed that the presence of ILs on the Hal can efficiently improve anchoring of nanoparticles and suppressing their leaching. In this work we design and synthesize a novel catalyst through multi-step procedure, Fig. 1, in which Hal was first magnetized and then reacted with melamine and 1, 4 di bromobutane. The resulting compound then tolerated reaction with triethanolamine to form IL. In the final step, Pd nanoparticles were supported on the IL-decorated magnetic Hal with the aid of Heracleum persicum extract as a biological reducing agent. To evaluate the catalytic activity of the resulting catalyst, the reduction of two dyes MO and RhB was studied in aqueous media. The reusability of the catalyst was also investigated. Moreover, the kinetic parameters, including activation energy, entropy and anthalpy of reduction of each dye have been calculated.
Results and discussions characterization. As depicted in Fig. 2, EDS analysis of the catalyst confirmed the presence of Al, Si and O atoms that can be attributed to the Hal structure. Notably, Si, O and C atoms can also represent the conjugation of CPTES. The presence of Fe atom indicated the successful magnetization of Hal. Observation of Pd atom also confirmed that Pd paricles were incorporated to the structure of the catalyst. The presence of C, N and Br atoms can be assigned to the organic moieties (melamine and IL). Figure S1 showed the elemental mapping analysis of Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd. It was found that magnetic and Pd nanoparticles were well dispersed on the support.
In Fig. 3A the TEM image of Fe 3 O 4 /Hal is presented. As depicted, upon magnetization, Hal preserved its cylindrical morphology. Moreover, it can be seen that magnetic nanoparticles were located both on Hal exterior and interior surfaces. The measurement of the magnetic nanoparticle average size, Fig. 3A, showed that the synthetic procedure led to the formation of relatively small Fe 3 O 4 nanoparticles (mean diameter of 13.06 nm with standard deviation of 3.1). The TEM analysis of the catalyst, Fig. 3B, also showed the cylinders of Hal, indicating that functionalization with organic moieties did not lead to the collapse of Hal structure. The dark black spots on the TEM image of the catalyst can be assigned to the magnetic and Pd nanoparticles.
In the next step, the structure of the catalyst was studied by recording its XRD pattern and comparing it with that of Fe 3 O 4 /Hal. As depicted in Fig. 4   To study the magnetic feature of Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd its magnetization curve was recorded and compared with that of bare magnetic nanoparticles, Fig. S2. The results showed that the magnetic saturation (Ms) value of Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd (19.75 emu g −1 ) was far lower than that of bare Fe 3 O 4 (47.1 emu g −1 ). It is worth noting that although the magnetic saturation of the catalyst was low, Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd was magnetic enough to be readily separated from the reaction mixture by using an external magnet.
Next, the thermal stability of    Table S1, showed that in the case of MO, the conversion of the reduction process increased with the increase of the catalyst loading from 1 to 2 mg. However, further increase of the catalyst amount had no effect on the reaction conversion. Regarding RhB, increase of the catalyst amount up to 4 mg led to the increase of the conversion of the reduction process, while further increase of this value to 5 mg did not result in any increase in the reaction conversion, Table S1. Considering these results the optimum catalyst amounts for MO and RhB were found to be 2 and 4 mg respectively.
The temporal UV-vis spectral changes of MO and RhB dyes in the course of the catalytic reduction in the presence of optimum amounts of the catalyst and NaBH 4 are illustrated in Fig. 6. The decrease of the absorbance at λ max = 465 nm (MO) and λ max = 550 nm (RhB) demonstrated efficient decolorization of dyes in a very short reaction time (50 s for MO and 60 s for RhB). To elucidate whether bare magnetic nanoparticles were catalytically active for the reduction of MO and RhB, the reduction reaction of both dyes in the presence of NaBH 4 was performed by using 4 mg bare magnetic nanoparticles as catalyst. Monitoring the progress of the reaction certified that within 1 min, MNPs did not promote the reduction of MO and RhB. This result confirmed that in 1 min that is the reaction time, in which the reduction reaction completed in the presence of Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd, MNPs had no role in the reduction reaction, but facilitated the separation of the catalyst from the reaction media. www.nature.com/scientificreports www.nature.com/scientificreports/ According to the literature, catalytic reduction of dye by metallic nanoparticles in the presence of excess amount of NaBH 4 is of pseudo-first-order kinetics 31 . The concentration of dye (C t ) in the course of the reduction can be calculated from the absorbance of the dye at its λ max . Accordingly, the rate of the reduction reaction can be measured by considering the decrease of the absorbance of dyes at its λ max versus time. As the ratio of the absorbance of dye at reduction time t = A t to initial time, t = A 0 is equal to the concentration ratio C t /C 0 , the apparent rate constant (k app ) can be defined by the following equations: The values of k app for the dyes can be calculated from the slope of ln (A t /A 0 ) versus t (s) plot. The value of k app obtained at various temperature (298, 303, 308 and 313 K) are reported in Table 1.
It was found that the kinetic energy of the reactants increased upon increasing of the reaction temperature. This observation can be attributed to the increase of the intensity of the collision of the reactants at elevated temperature. Based on Arrhenius equation (Eq. 3), the activation energies (E a ) of the reduction reactions of two dyes were estimated by plotting lnk vs. 1/T, Fig. S5 and Table 1. In those plots the R 2 value for MO and RhB were 0.935 and 0.864 respectively.
In this equation E a is activation energy, A stands for Arrhenius factor, T is temperature and R represents ideal gas constant = 8.314 JK −1 mol −1 . In the following, the thermodynamic parameters, i.e. the activation entropy (ΔS # ) and the activation enthalpy (ΔH # ) were calculated by using the Eyring equation (Eq. 4).  Figure S6 shows the plots of ln(k/T) vs. 1/T for MO and RhB. In those plots the R 2 value for MO and RhB were 0.931 and 0.838 respectively. The entropy values of the reduction reaction of MO and RhB were calculated to be −139.06 and −155.18 Jmol.K −1 respectively. Enthalpies of the reduction reactions were measured to be 39.40 and 34.33 kJmol −1 for MO and RhB dyes, respectively (Table 1).
To confirm the merit of Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd for the reduction of organic dyes, its performance for the reduction of MO was compared with some of the other catalysts that have been used for the reduction of this dye in the presence of NaBH 4 , Table 2   www.nature.com/scientificreports www.nature.com/scientificreports/ The plausible mechanism for the reduction of MO and RhB in the presence of Fe 3 O 4 /Hal-Mel-TEA(IL) is presented in Fig. 7. As shown, sodium borohydride dissociates at the first step of the reaction to produce borohydride ions. Subsequently, BH 4 − ions are adsorbed on the surface of the Pd nanoparticles. Meanwhile, dyes are adsorbed onto the catalyst through π-π stacking interactions. Upon sorption of dyes on Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd, the generated hydride ions transferred to them and dyes will be reduced. In the final step, the reduced dye will be detached from Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd and allows the catalytic cycle to be repeated.

Reusability
The reusability of Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd for the reduction of both MO and RhB was investigated. To this purpose, the catalyst was separated from the reaction mixture by using an external magnet and then reused for the next run of the reaction. This cycle was repeated for eight consecutive reaction runs and the obtained yields were measured and compared, Fig. 8. As shown in Fig. 8 experimental Preparation of magnetic Hal. In the first step of the preparation of the catalyst, Hal was magnetized. For this purpose, 2.5 g Hal was dispersed in 120 mL deionized water and the amounts of 1.37 g FeCl 3 . 6H 2 O and 0.5 g FeCl 2 .4H 2 O were added to the Hal suspension. Then, the reaction mixture was stirred at 60 °C. In the following, 10 mL NH 3 was introduced to the reaction mixture. This mixture was then stirred vigorously for 1 h. At the end of the reaction, the precipitate was collected by an external magnet, washed several times with deionized water and dried at ambient temperature.

Functionalization of magnetic Hal with CPTES (Fe 3 o 4 /Hal-cl).
In the next step, magnetic Hal was functionalized with CPTES. More precisely, 1 g magnetic Hal was dispersed in 60 mL dried toluene and then 2.5 mL CPTES was added in a drop wise manner. Subsequently, the mixture was refluxed at 110 °C overnight. Upon completion of the reaction, the product was separated by an external magnet and washed with ethanol and dried at room temperature for 24 h.   2 (1.5 mM) was introduced and the resulting mixture was stirred vigorously at room temperature for 4 h. After that, 10 mL of Heracleum persicum extract was added and the mixture heated at 60 °C overnight. At the end of the reaction, the final product (Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd NPs) was collected by an external magnet, washed with ethanol/water and dried at room temperature overnight. The schematic procedure for the synthesis of the catalyst is illustrated in Fig. 1. According to ICP result, the Pd content of Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd was estimated as 0.93 wt%. Moreover, the content of magnetic nanoparticles was estimated to be 17.6 wt%.
Catalytic reduction of dye. To reduce the dyes, dye (2 mL) was dissolved in water and then proper amounts of conclusion Using Hal and Heracleum persicum extract as natural materials, Fe 3 O 4 /Hal-Mel-TEA(IL)-Pd was prepared through magnetization of Hal followed by surface functionalization and palladation. The composite was then applied as an efficient and magnetically separable catalyst for the reductive degradation of MO and RhB in aqueous media. The catalyst showed excellent catalytic activity for the reduction of both dyes in aqueous media. The activation energy, enthalpy, and entropy for the reduction of methyl orange were estimated as 42.02 kJ mol −1 ,