Catalyst characterization
First, the morphology of the Pd@h-Fe2O3@C was studied using TEM technique. Clearly, from Fig. 1a, the hollow spherical morphology can be observed. Moreover, it is clearly that the spheres are surrounded with gray coverage, confirming the successful incorporation of glucose and organic layer in the structure of the catalyst 35,36. In addition, tiny dark dots on the magnetic spheres that are indicative palladium NPs can be obvious. Notably, the palladium NPs were distributed on the porous carbon homogeneously. Using TEM technique the average size of palladium NPs was estimated and the size distribution curve of the nanoparticles is shown in Fig. 1b. The Gaussian-fitted curve shows the mean diameter (d) as 9.39 nm.
Next, FTIR spectroscopy was applied to verify the formation of Pd@h-Fe2O3@C and the compounds prepared in the course of synthesis of Pd@h-Fe2O3@C. The FTIR spectra of the Pd@h-Fe2O3@C (a), h-Fe2O3@glu-MFR (b) and pure glucose (c) are depicted in Supplementary, Fig. S1. This spectrum (a) clearly showed the characteristic bands of h-Fe2O3, i.e. the strong absorption bands at 470-590 cm-1, which can be assigned to Fe–O stretching and the strong band at 3463 cm-1, which can be attributed to -OH groups 20. The characteristic bands of glucose (c) can be listed as the bands at 3411, 2943 and 1461 cm-1 that can be assigned to the -OH functionality and -CH2 stretching. The FTIR spectrum of h-Fe2O3@glu-MFR (b) exhibited the characteristic bands at 1604, 2850, 2942 and 3406 cm-1 can be due to the C=N, -CH2 stretching and -OH functionality, which confirms the conjugation of organic layer. Moreover, the FTIR spectrum (b) showed the characteristic bands of magnetic core (h-Fe2O3), implying that the magnetic core preserved its structure upon functionalization processes. The FTIR spectrum (a) of Pd@h-Fe2O3@C is significantly distinguished from others and the intensity of the characteristic bands became very weak. This observation of some of characteristic bands is quite expectable and can be due to the high temperature thermal treatment and carbon generation.
The structure and formation of γ-Fe2O3 hollow sphere as well as palladium NPs was also studied by X-ray diffraction pattern of the catalyst (see Supplementary Fig. S2). As shown, the characteristic peaks of γ-Fe2O3 hollow sphere were appeared at 2θ = 30.6° {220}, 34.9° {311}, 43.5° {400}, 54.1° {422}, 57.5° {511}, 63.5° {440} and 74.2° {533} (labelled as F) can indicate the typical cubic structure hematite (JCPDS card No. 39-1346) 20. In the XRD pattern of Pd@h-Fe2O3@C, the peaks labelled as P are characteristic peaks of palladium NPs at 2θ = 40.0°, 46.6°, 68.5°, 82.3° and 87.1° (JCPDS, No.46-1043) that can be assigned to the {111}, {200}, {220}, {311} and {222} planes 37. In the XRD pattern of Pd@h-Fe2O3@C, three bands was observed that can be assigned to the carbon material. Precisely, the bands at 2θ = 25.3°, 45.0° 81.7° that can be indexed to {002}, {100} and {110} crystal planes respectively 38, belonging to the hexagonal graphite (JCPDS card No. 41-1487) 39. It is worth mentioning that the characteristic bands at 2θ = 81.7° was overlapped with that of Palladium NPs. Furthermore, the average diameter size of Palladium NPs and γ-Fe2O3 were calculated by employing Debyee-Scherrer equation, being about 10.2 and 19.5 nm.
Raman spectroscopy was also applied for the characterization of the catalyst (see Supplementary Fig. S3). The Raman spectrum of Pd@h-Fe2O3@C exhibited two bands at 1357 (D-band) and 1601 cm-1 (G-band), related to graphitic carbon, can confirm the graphitic nature of the catalyst. In greater detail, the D-band is indicative of the sp3 configuration that can be attributed to presence of intrinsic defects and the G-band can be attributed to the graphitic carbon 40-42. In this line, the ID/IG was calculated and measured to be 0.83, can confirm disordered graphitic structures or highly defective 43.
To elucidate whether decoration of the surface of h-Fe2O3 with carbon shell and palladium NPs can alter the magnetic property, Pd@h-Fe2O3@C was studied by room temperature vibrating sample magnetometer (VSM), and its magnetic features compared with that of h-Fe2O3 (see Supplementary Fig. S4). As obvious, the maximum saturation magnetization (Ms) values of h-Fe2O3 and Pd@h-Fe2O3@C were found to be 45.1 and 33.1 emu/g, respectively. Clearly, the Ms value of the hollow Fe2O3 nano sphere (45.1 emu/g) is higher than that of the catalyst which may be due to the incorporation of non-magnetic compounds and immobilization of palladium NPs on the surface of magnetic core. However, the hysteresis loops of the catalyst showed a paramagnetic behaviour without aggregation that can be easily separated from the reaction mixture using an external magnetic force.
In Supplementary Fig. S5, the thermal stability of the (a) h-Fe2O3@glu-MFR, (b) h-Fe2O3 and (c) Pd@h-Fe2O3@C were recorded using TG analysis and were compared. As shown, the h-Fe2O3 possessed high thermal stability and exhibited only the weight loss below 200 °C that is representative of loss of water. The comparison of h-Fe2O3@glu-MFR with that of h-Fe2O3 indicates that apart from the weight losses of magnetic core, an major weight loss at 450 °C can be detected that can be attributed to the degradation of organic layer. More detailed, the weight loss of about 19% can be due to the decomposition of MRF as an organic motif that indicates the successful formation of MRF in the structure of the catalyst. Considering the thermo gram of Pd@h-Fe2O3@C, it can be concluded that this sample exhibited significantly higher thermal stability compared to that of others, confirming the successful carbonization.
In the following, the effect of the carbonization of organic shell on the textural properties of the catalyst was studied. To this purpose, the N2 adsorption-desorption isotherms of Pd@h-Fe2O3@C and h-Fe2O3 were recorded and depicted in Supplementary Fig. S6. As shown, the isotherms of two samples are distinguished. The shape of h-Fe2O3 exhibited type II isotherm, while Pd@h-Fe2O3@C showed type IV with H3 hysteresis loops 38. To further verify this issue, the specific surface area of two samples were calculated and compared. The specific surface area of the catalyst was calculated to be 426 m2g-1 which was higher than that of h-Fe2O3 (53 m2g-1). This can indicate the porous nature of Pd@h-Fe2O3@C. The total pore volume of two samples were also compared (see Supplementary Fig. S6). More precisely, this value for Pd@h-Fe2O3@C was much higher than that of h-Fe2O3, indicating the carbonization of organic layer resulted in the formation of pores. Moreover, the pore size distribution curves of the h-Fe2O3 and Pd@h-Fe2O3@C were obtained by the BJH method using the pore volumes in the measurement of N2 desorption isotherms. As is evident from pore size distribution result of Pd@h-Fe2O3@C, two types of pores with mesoporous (2 nm) and micropores (8.9 and 11 nm) were clear. Nevertheless, compared to the Pd@h-Fe2O3@C, the microporous size uniformity has increased and appeared at 8.9 and 11 nm, presumably due to the carbonization of organic layer.
Finally, ICP-AES was exploited for measuring the content of palladium NPs in the catalyst. To prepare the sample for the analysis, a known quantity of Pd@h-Fe2O3@C was digested in a mixture of concentrated HCl and HNO3 solution. Finally, the obtained extract was analysed and the results confirm that the content of palladium was 3.1 w/w%.
Investigation of the catalytic activity
To assess the catalytic activity of this heterogeneous system, the Pd@h-Fe2O3@C was utilized as a recyclable catalyst in Suzuki coupling reaction. Initially, the reaction of iodobenzene 1 and phenylboric acid 2 was selected as a model substrate. The reaction was conducted in the present of Na2CO3 and Pd@h-Fe2O3@C (0.5 mol%) at 75 ℃ in EtOH condition. After 1 h, the nature of the reaction mixture quickly changed to a dark viscous solid. After purification by column chromatography, the biphenyl 3a was afford in 35% yield. The solvent was then screened in the reaction, such as water, EtOH, water/EtOH, DMF, CH3CN, toluene and THF. As shown in Supplementary Table S1, the outcomes exposed that a mixture of water/EtOH can well support the reaction effectively. After using different solvent, a series of parameters including catalysis, reaction temperature and base were examined to find and secure the optimal reaction conditions. Ultimately, the experimental results established that the biphenyl could achieved with 95% yield when the model reaction was carried out in the present of Pd@h-Fe2O3@C (0.5 mol%) with Na2CO3 at 75 ℃ in water/EtOH.
Further, to determine its scope by applying various aryl halides, a range of reactions was carried out under the optimal reaction conditions. As shown in Table 1, a series of aryl halides with electron-donating and electron-withdrawing group and phenyl boronic acid were used to the reaction under optimized conditions. Eventually, the Pd@h-Fe2O3@C efficiently catalyzed the coupling reaction between aryl halides with phenylboronic acid and biphenyls were attained in high to excellent yields after purification. In detail, the electronic effect of the substituents was generally found have no considerable influence since aryl iodides bearing donor- and acceptor substituents reacted with phenylboronic acid to afford the expected coupled products in excellent yields. Worthy to mention that, when iodobenzene was substituted by bromobenzene, the reactions needed longer reaction times for being completed. Notably, the scope of this methodology was found not to be operative to chlorobenzene.
Furthermore, the performance of Pd@h-Fe2O3@C catalyst was tested for the Sonogashira coupling reaction using iodobenzene 1 and phenylacetylene 4 as a model substrate. As shown in Supplementary Table S2, the optimization reaction conditions were investigated in different solvents, temperatures, bases, and in the presence of various amount of the Pd@h-Fe2O3@C catalyst. Unlike the previous reaction and as the first parameter of this set, the role of Pd@h-Fe2O3@C as a catalyst was studied. Notably, in the absence of any catalyst and only in the presence of Na2CO3 (2 mmol) at 50 ℃, the product was not formed (see Supplementary Table S2, entry 10), while, the use of a small amount (0.175 mol%) of catalyst provided the desired product in moderate to excellent yields (53-98%) (see Supplementary Table S2, entries 1, 6 and 7). It was found that, when the model substrate was applied in the presence of Na2CO3 (2 mmol) at 50 ℃ using Pd@h-Fe2O3@C (0.35 mol%) as catalyst, the yield of product could reach 95% after separation. Using the optimized reaction conditions, the scope and generality of this method were exemplified in the reaction of aryl halides 1 and phenyl/aliphatic acetylenes 4 using Pd@h-Fe2O3@C catalyst, and the outcomes are presented in Table 2. The cross-couplings of phenyl/aliphatic acetylenes with aryl iodides bearing electron donating groups, -OMe, -Me and -COMe gave products in satisfactory yields. Moreover, arylbromides and chlorides are efficiently reacted as substrates in this process, though, the reaction of aryl chlorides needed longer reaction times for being completed. Noticeably, propargylalcohol as an aliphatic acetylene was also fruitfully coupled to arylhalides with satisfactory yields. Nevertheless, the highest yields were obtained for aryl acetylene. Noteworthy, all compounds are known and some were identified by comparing physical properties through FTIR and melting point analyses.
Next, the catalyst structure-catalytic activity relationship was studied. Initially, the result of loading of Pd on the support was studied. In this regard, apart from the catalyst two more samples with different loading of Pd, i.e. Pd@h-Fe2O3@C (Pd 2 and 3 wt%) were provided and their catalytic activities for catalyzing the model Sonogashira reaction was compared with that of the catalyst. The outcomes showed that the loading of Pd meaningfully influence the catalytic activity and use of lower content of Pd is more effective. Then, the influence of each component in the structure of the catalyst to the catalysis was studied. For this purpose, several control catalysts, including, Pd@h-Fe2O3, Pd@h-Fe2O3@glu, Pd@h-Fe2O3@glu-MRF, Pd@h-Fe2O3@glu-RF, Pd@h-Fe2O3@glu-RF-C and Pd@h-Fe2O3@C were prepared (see experimental section). To synthesis Pd@h-Fe2O3, Pd was easily coated on h-Fe2O3 and the prepared product was applied for catalyzing the model Sonogashira reaction (Table 3, entry 1). As shown, Pd@h-Fe2O3 is not an active catalyst and the product was obtained only 35%. Then, it was studied whether introducing of Glu shell is able to improve the catalytic activity. In this line, Pd@h-Fe2O3@glu was provided and its catalytic activity was examined and the product was obtained with 50% yield (Table 3, entry 4). This results approved that Glu can somewhat improve the catalytic activity. For further revealing the key role of Glu in the catalysis, two control catalysts, Pd@h-Fe2O3@MRF and Pd@h-Fe2O3@MRF-C was prepared, in which the Glu was not present in the structure of the catalyst and the resorcinol-formaldehyde-melamine polymer was adjusted on the surface of h-Fe2O3 and subsequently palladated and carbonized (Table 3, entries 2 and 3). Based on the comparison of the results from catalytic activity of the catalyst and these samples the contribution of Glu was confirmed. In details, donor- and acceptor substituted it was found that Glu component not only influenced the catalytic activity, but also improved the separation and reusability of the catalyst. More exactly, the separation of the catalyst was simpler and more efficient than that of Pd@h-Fe2O3@MRF and Pd@h-Fe2O3@MRF-C, while, the ICP examination of these samples approved the higher loading of Pd NPs in the catalyst.
Approving the role of melamine as a nitrogen source, the effect of N-precursor by creating another control samples, Pd@h-Fe2O3@glu-RF, Pd@h-Fe2O3@glu-RF-C in which melamine was omitted in the structure of polymer was clarified and compared with Pd@h-Fe2O3@glu-MRF and catalyst. As tabulated, melamine as an N-rich precursor has the ability to produce product with the highest catalytic activity (Table 3, entries 5-8). To elucidate the effect of polymer's type on the structure of the catalyst and Pd loading, the specific surface area of these samples were compared and showed that this value decreased in the following order: Pd@h-Fe2O3@C (426 m2g-1)> Pd@h-Fe2O3@glu-RF-C (156 m2g-1)> Pd@h-Fe2O3@glu-MRF (42 m2g-1)> Pd@h-Fe2O3@glu-RF (33 m2g-1). By these results the effect of the N-rich carbon precursor on the content and specific area of the carbon coated h-Fe2O3 was confirmed. Further, the effects of composite components on the loading and leaching of Pd NPs in catalyst as well as control samples were measured via ICP analysis and studied, Table 3. As tabulated, the order of loading and leaching of Pd increased in the following order: Pd@h-Fe2O3> Pd@h-Fe2O3@MRF> Pd@h-Fe2O3@glu> Pd@h-Fe2O3@glu-RF> Pd@h-Fe2O3@glu-MRF> Pd@h-Fe2O3@MRF-C> Pd@h-Fe2O3@glu-RF-C> Pd@h-Fe2O3@C. The lowest loading of Pd was observed in Pd@h-Fe2O3 sample which can be due to the absence of strong chemical interaction between Pd NPs and h-Fe2O3 surface. It is worth mentioning that the low catalytic activity of this sample can be allocated to the low Pd loading. The loading of Pd in Pd@h-Fe2O3@MRF, Pd@h-Fe2O3@glu, Pd@h-Fe2O3@glu-RF and Pd@h-Fe2O3@glu-MRF samples compared to that of Pd@h-Fe2O3 can confirm that the presence of carbon shell could increase Pd immobilization. Nevertheless, the Pd leaching of this sample was relatively high. Pd@h-Fe2O3@MRF-C and Pd@h-Fe2O3@glu-RF-C that possessed carbon shell, the Pd anchoring were further enhanced compared to Pd@h-Fe2O3@MRF and Pd@h-Fe2O3@glu-RF, indicating the encouraging effect of carbonization of polymeric shell on immobilization of Pd NPs. Furthermore, the Pd leaching in samples were repressed compared to that of Pd@h-Fe2O3@glu-MRF and Pd@h-Fe2O3@glu-RF. In Pd@h-Fe2O3@C, the loading of Pd was increased compared to that of Pd@h-Fe2O3@glu-MRF, representing that Pd anchoring on carbonized sample was most better that un-carbonized one.
Catalyst recyclability
To explain whether Pd@h-Fe2O3@C can be considered as a reusable catalyst, the recycling of that catalyst was carried out for the Sonogashira model reaction and the results exhibited in Supplementary as Fig. S7. In detail, after the first run had been accomplished, the reaction mixture was cooled to ambient temperature, diluted by ethylacetate and the recovered catalyst was magnetically separated, washed with EtOH and dried and the recycled catalyst re-used for the subsequent run of Sonogashira model reaction under the same conditions. As shown, the Pd@h-Fe2O3@C catalyst was subjected to five successive runs and the catalytic activity of catalyst weakened gradually. Noteworthy, up to the fourth reaction run, only a reasonable decrease was detected, subsequently a more obvious loss of activity after the fifth run and the yield of product reached to 44%. To consider the effects of reusing on the morphology of the catalyst, FTIR spectra of the recycled catalyst was recorded and compared with that of the fresh catalyst, Fig 2. The FTIR spectrum of the recycled Pd@h-Fe2O3@C demonstrated the specific bands of the fresh Pd@h-Fe2O3@C. Nevertheless, some difference between two spectra were observed, that can be due to the disposition of organic substances on the surface of the catalyst. Outstandingly, Pd leaching was insignificant after five reaction runs and can confirm that the catalyst was stable upon recycling and can be reused for the successive reaction runs.
The development and application of the reusable heterogeneous catalysts in the C-C coupling reactions have been the subject of various studies. Under convinced conditions, leaching of the supported Pd particles has also been observed. Information on the active Pd species in solution can be attained by the hot filtration test and it is classically accomplished by disturbing the reaction at a low conversion and then carrying it on after catalyst removal. If no additional conversion is identified in the catalyst-free solution phase, the leaching of Pd may be excluded. Herein, to establish the heterogeneous nature of the Pd@h-Fe2O3@C catalyst, the hot filtration test was investigated using iodobenzene and phenyl acetylene as the model substrates under the optimized reaction condition. After 1 h, the reaction was stopped and the catalyst was separated from the hot reaction mixture and the yield of product was analyzed with GCMS (45% conversion). The reaction was continued using the filtrate in the absence of the solid catalyst for more 3 h and no product was found, indicating that the leaching of Pd NPs during the catalytic cycle not observable. Furthermore, the ICP-AES analysis of the filtrate exhibited Pd content under detection level (i.e. <0.01 ppm) which more enumerates non-leaching of either Pd metal during the reactions, thereby confirming its heterogeneous stability and nature.