XPS and structural studies of Fe3O4-PTMS-NAS@Cu as a novel magnetic natural asphalt base network and recoverable nanocatalyst for the synthesis of biaryl compounds

In this research, natural asphalt as a mineral carbonuous material was converted to sodium natural asphalt sulfonate (Na-NAS) and, then, was linked to Fe3O4 MNPs in order to synthesize the magnetic nanocatalyst. Afterwards, Cupper (I) and Cu (II) was grafted on Fe3O4-PTMS-NAS. Moreover, it is worth mentioning that the synthesized the novel magnetic nanocatalyst (Fe3O4-PTMS-NAS@Cu) was successfully used in Suzuki and Stille coupling reactions. The Fe3O4-PTMS-NAS@Cu MNPs were characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), inductively coupled plasma (ICP), BET and X-ray photoelectron spectroscopy (XPS) analysis. Besides, sulfonation of natural asphalt, magnetization of catalyst, grafting of Cu (I) and Cu (II) to NAS and catalyst formation were investigated and proved carefully. This nanocatalyst can be comfortably separated from the reaction medium through an external magnetic field and can also be recovered and reused, while maintaining its catalytic activity.

. The symmetric and asymmetric stretching vibrations at 610 cm −1 and 1128 cm −1 are related to the S=O in the SO 2 bands, which confirm the successful immobilization and connection of the natural asphalt sulfonate on the surface of Fe 3 O 4 -CPTMS (Fig. 1c). The existence of Cu in the structure of the catalyst was approved through stretching vibration of S=O bands that appeared at 1111 cm −1 , as this band shifts to lower frequency due to the grafting of cu on Fe 3 O 4 -PTMS-NASMNPs (Fig. 1d). EDX analysis. One of the suitable analysis to determine elements, present in the structure of nanoparticles, is the energy-dispersive X-ray spectroscopy (EDS). The EDX spectrum of the Fe 3 O 4 -PTMS-NAS@Cu MNPs is demonstrated in Fig. 4, which confirms the presence of Fe, O, S and Cu elements in the structure of the catalyst and proved that the magnetic nanoparticle has been successfully synthesized. Moreover, the EDX mapping analysis (Fig. 5) Fig. 7 indicates the two step weight loss, that the first weight loss (5.81%) was observed at below 260 °C which can be related to the removal of the physically adsorbed solvents and OH groups on the surface of the catalyst. Besides, the second weight loss (10%) occurred between 260 and 520 °C which is related to the decomposition of some organic groups on the surface of the catalyst such as polycyclic rings, R-SO 3 , etc. ICP analysis. In order to determine the amount of Cu on the surface of catalyst, the ICP atomic emission spectroscopy technique was used which indicated that the exact the amount of Cu, stabilized on surface of Fe 3 O 4 -PTMS-NAS MNPs, is found to be 1.70 mmol g −1 .  XPS analysis. In order to evaluate the oxidation states and elemental composition, the X-ray photoelectron (XPS) measurements were used. Figure 9 shows the XPS survey spectrum of Fe 3 O 4 -PTMS-NAS@Cu which clearly demonstrated the existence of C, O, Si, S, Cu, and Fe elements, as we excepted for the final product. The    22 . These results confirmed that the natural asphalt has been sulfonation, successfully. The deconvoluted XPS spectrum of Cu 2p (Fig. 9f) shows two main peaks between 930 and 937 eV, related to Cu 2p 3/2 , and 950.0 and 962.0 eV related to the Cu 2p 1/2 . Eeach of these peaks consists of two main sub-peaks, corresponding to the monovalent Cu (I) and divalent Cu (II). As a results, the XPS spectrum data for Cu 2p confirmed that both Cu oxidation states (I and II) exist in After obtaining the optimum conditions, some derivatives of Suzuki and Stille coupling using different aryl halides were synthesized whose results are demonstrated in Tables 2 and 3.
The suggested mechanism for the Suzuki coupling reaction using Fe 3 O 4 -PTMS-NAS@Cu is demonstrated in Scheme 3. According to the mechanism reported previously 9,24 the first step is the oxidative addition of copper to the aryl halide that the organocopper intermediate (II) is produced. Next, by transmetallation of (II), intermediate (III) is formed. Reductive elimination of the intermediate (III) led to the formation of product and regeneration the copper catalyst (I) which can be continue the catalytic cycle.
Reusability of the catalyst. One of the most significant features of magnetic nanoparticles is the capability of being recovered and reused several times. In this regard, after completion of the reaction, the catalyst was separated from the reaction mixture using an external magnet, washed with EtOAc, dried at room temperature and, then, prepared for the next run. We have studied the activity of Fe 3 O 4 -PTMS-NAS@Cu MNPs in the synthesis of compounds (2a) and (3a) after recovery. The results illustrated that this magnetic nanocatalyst can be recovered and reused for six runs, while maintaining its catalytic activity (Fig. 10).

Leaching study of the catalyst. In order to consider the heterogeneous nature of Fe 3 O 4 -PTMS-NAS@
Cu MNPs in the Stille coupling reaction conditions, hot filtration experiment was performed in the coupling of 4-iodotoluene with triphenyltin chloride. In this study, in half-time of the reaction (the reaction time is 190 min), 56% of product was obtained. Moreover, this reaction was repeated and in half-time of the reaction (after 95 min), the catalyst was separated and, then, the filtrated solution was permitted to continue the reaction without the catalyst for a further 95 min. Thereupon, only 58% of 4-Methyl-1,1′-biphenyl as a product was obtained. These experiments confirm that there is no detectable increase in the product concentration, which might be an evidence for a heterogeneous mechanism during the recycling process.

Characterization of recycled Fe 3 O 4 -PTMS-NASGRONAS@Cu MNPs. Fe 3 O 4 -PTMS-NAS@
Cu MNPs was recycled up to six runs and, then, characterized using SEM and XRD analysis. The SEM image (Fig. 11) after recovery proved the structure of Fe 3 O 4 -PTMS-NAS@Cu MNPs. In addition, XRD patterns of the fresh Fe 3 O 4 -PTMS-NAS@Cu after recovery are shown in Fig. 12, confirming the structure of the catalyst after recovery.
Comparison of the catalyst. The efficiency of Fe 3 O 4 -PTMSNAS@Cu was investigated by comparing our results with the previously reported methods in the coupling of iodobenzene with phenylboronic acid and triphenyltin chloride in the synthesis of 2a and 3a products. As shown in     phenylboronic acid (95%), triphenyltin chloride (95%), aryl halides (98%) and natural asphalt was bought from the Kimia Bitumen Zagros Cooperative, Iran. The reactions were monitored with TLC on silica-gel Polygram SILG/UV254 plates. Fourier-transform infrared spectroscopy (FTIR) was performed using FTIR-8300 spectrometer made by Shimadzu. Proton nuclear magnetic resonance ( 1 H NMR) spectroscopy was also performed on Bruker AVANCE DPX-400 and DPX-500 spectrometers. Chemical shifts were reported in ppm relative to TMS as the internal standard. The morphology of the catalyst was investigated by scanning electron microscopy (SEM) using Mira 3-XMU. TEM was performed with Philips CM300 to measure the size of the particles. The elemental composition was determined using EDS and Mira 3-XMU. The exact value of Cu in the catalyst was estimated applying Inductively Coupled Plasma (ICP) (VISTA-PRO, Australia). X-ray diffraction (XRD) was investigated using a Holland Philips X, the thermogravimetric analysis (TGA) curve was recorded using a PL-STA 1500 device manufactured by Thermal Sciences, superparamagnetic properties of the catalyst were measured using VSM (MDKFD) and the analysis of the surface chemical composition of Fe 3 O 4 -PTMS-NAS@Cu MNPs was conducted using the X-ray photoelectron spectroscopy (XPS) (Thermo Scientific, ESCALAB 250Xi Mg X-ray resource) see Supplementary Information.

Synthesis of Fe 3 O 4 -PTMS-NAS@Cu. Synthesis of natural asphalt sulfonic acid (NASA).
Initially, natural asphalt (1 g) was mixed with (5 mL) of the concentrated sulfuric acid and, then, the mixture was stirred at 220 °C for 2 h. Next, the reaction mixture was cooled to room temperature and, then, it was slowly poured into 20 mL of distilled ice water. Finally, the product (NAS) was extracted using filtration, washed with distilled water for the several runs and, then, dried at 100 °C in oven 9 .
Synthesis of sodium natural asphalt sulfonate (Na-NAS). In the next step, (1 g) of the previous stage product (NASA) was added to (20 mL) of NaOH solution (10%) and, then, the reaction mixture was stirred for 1 h at room temperature. After evaporation of the solvent, the product (Na-NAS) was dried at 100 °C in oven 9 .     (2 mL) was stirred at reflux condition. Progress of the reaction was monitored by TLC. After completion of the reaction, the catalyst was separated from the reaction mixture using an external magnetic field and, then, the product was extracted with ethyl acetate (3 × 10 ml). The solvent was evaporated and, finally, pure biphenyl derivatives were obtained in good to excellent yields.

Conclusion
In conclusion, a new type of magnetically recoverable nanocatalyst (Fe 3 O 4 -PTMS-NAS@Cu MNPs) was synthesized. The efficiency and activity of Fe 3 O 4 -PTMS-NAS@Cu MNPs as an excellent and highly reusable catalyst were investigated in the Suzuki and Stille coupling reactions. In order to characterize this nanocatalyst, various techniques; including, FT-IR, SEM, TEM, EDX, XRD, TGA, VSM, BET, ICP and XPS analysis were used. The XPS analysis confirmed that the natural asphalt has been successfully functionalized. The results obtained from XPS analysis confirmed the presence of sulfonic group, Fe 2+ , Fe 3+ ,Si, Cu (I) and Cu (II) in the structure of the catalyst and are in good agreement with the proposed structure of the catalyst. Short reaction times and also good to excellent yields of the products proved the high catalytic activity of Fe 3 O 4 -PTMS-NAS@Cu MNPs. Moreover, this nanocatalyst has low toxicity, can be extracted from the reaction mixture using an external magnet and reused for the several runs while maintaining its catalytic activity (Supplementary Information).