The Heck reaction catalyzed by the noble metal Pd is one of the most important coupling reactions in organic syntheses [1, 2]. Homogeneous and heterogeneous Pd catalysts are often used in the Heck reaction. However,homogeneous catalysts are used in less than one fifth of industrial applications due to the facts that the homogeneous catalysts are easily lost after reaction,difficult to separate from the reaction system and certainly costly [3]. Generally,Pd catalysts are initially synthesized as heterogeneous catalysts followed by loading onto active carbon [4],metal oxides [5],zeolites [6, 7],polymers [8],or clay [9].
Magnetic catalysts have the incomparable advantages over the traditional catalysts because of their high activity,magnetic recyclability,and reusability. Research on the magnetic nanocatalysts for the Heck reaction had been reported earlier [10, 11]. However,due to their high specific surface areas and magnetic properties,magnetic nanoparticles are often easily self-agglomerated [12]. Therefore,SiO2,C,and metal oxides,polymers are usually used to modify the magnetic nanoparticles and to obtain magnetic nanoparticles with special surface properties that still keep their dispersiveness [13, 14, 15, 16]. g-Al2O3 has been used as supporter,adsorbent,and catalyst,owing to its low cost,good chemical stability,high surface area,acidic sites,and controllable synthetic process [17, 18, 19, 20]. In the catalytic industry,g-Al2O3 is an ideal supporter. Particularly,the catalysts that are formed by loading the noble metal on the g-Al2O3 were used in organic reactions [20, 21, 22, 23]. It is still difficult,however,to separate the catalyst from the liquid phase after the reaction. In our previous research,we found an easy way to synthesize monodispersed Fe3O4 [24],which had uniformed diameter. Thus we hypothesized that if we could coat the Fe3O4 with g-Al2O3 to obtain core-shell structures,and then load Pd onto the Fe3O4@g-Al2O3 nanospheres, the resulting catalysts would combine their advantages of high activity,excellent mesoporous structure,magnetic recyclability, and reusability.
In this paper,we illustrated the construction of magnetically recyclable Pd/Fe3O4@g-Al2O3 nanocomposites possessed the core- shell structure. To evaluate the activity and the stability of the Pd/ Fe3O4@g-Al2O3 nanocomposites,the Heck coupling reactions were chosen as the model reaction. Results showed that the catalyst could be easily separated from the reaction system by employing an external magnetic field,because of the superparamagnetic behavior of Fe3O4. Furthermore,it can be reused for several cycles with sustained selectivity and activity. 2. Experimental 2.1. Synthesis of Fe3O4@g-Al2O3
Fe3O4 was synthesized according to a slightly modified solvothermal method [24]. Briefly: FeCl3·6H2O (10.8 g),NaAc (28.8 g) and cetyltrimethyl ammonium bromide (CTAB,0.014 g) were dissolved in 400 mL of glycol under stirring. The obtained homogeneous yellow solution was transferred into a Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at 200 ℃ under 400 rpm stirring speed. After heating for 12 h,the autoclave was cooled naturally to room temperature. The obtained black magnetic particles were separated with a permanent magnet,washed with ethanol six times,and dried in vacuum at 60 ℃ for 24 h.
The obtained Fe3O4 particles (0.1 g) were dispersed in an aluminumisopropoxide (AIP) ethanol solution (60 mL,0.016 mol/L) under ultrasonication. After30 min,the solutionwas transferredto a three-neck flask (250mL) and stirred for 12 h at 45 ℃ to obtain the Fe3O4 particleswhose surfacewas saturated with AIP. Subsequently, 50 mL of ethanol/water (5/1,v/v) was added into the solution,and the mixture was allowed to stir for another 1 h to complete the hydrolysis of AIP. Then the mixture was transferred to a Teflon-sealed autoclave and heated at 80 ℃ for 20 h. The obtained particles were separated with a permanent magnet,washed several times with deionized water and ethanol,and then dried in vacuum at 50 ℃ for 12 h. After that the products were put into a tube furnace and the system was purged with N2. Then the tube furnace was heated from room temperature to 500 ℃ (1℃/min) under the N2 (5 mL/min) ambience,and kept at 500 ℃ for 4 h. After cooling to room temperature naturally,the Fe3O4@g-Al2O3 was collected. 2.2. Preparation of Pd/Fe3O4@g-Al2O3
The obtained Fe3O4@g-Al2O3 particles (0.1 g) were dispersed in a 2.5 mL PdCl2 aqueous solution (0.6 mg/mL) under ultrasonication. After 30 min,the solution was diluted to 100 mL and transferred to a three-neck flask,stirred for 12 h under 25 ℃, separated by a permanent magnet,and finally dried in vacuum at 100 ℃ for 12 h. The obtained products were put into a tube furnace,which was purged with H2. Then the tube furnace was heated from room temperature to 200 ℃ (1 ℃/min) under the H2 (5 mL/min) ambience,and kept at 200 ℃ for 3 h. After cooling to room temperature,Pd/Fe3O4@g-Al2O3 (1.5 wt%) was collected. By changing the volume of PdCl2 added,we synthesized a series of Pd/ Fe3O4@g-Al2O3 catalysts with different Pd loadings in 1.5%,3.0%, 4.5%,6.0%,respectively. They were marked as Pd-1,Pd-2,Pd-3,Pd- 4,respectively. 2.3. Catalytic reaction
For the Heck reactions,10 mg of Pd/Fe3O4@g-Al2O3 catalyst, arylhalide,olefin,dodecane (as an internal standard substance), and base were added to the solvent. The reaction was carried out under reflux condition. The effects of Pd content,solvents, substrates,and time were investigated individually. The catalyst was collected by an external permanent magnet,and the product was analyzed by gas chromatography (GC). For recycling,the collected catalyst was washed with tetrahydrofuran and separated by an external permanent magnet,then dried under vacuum at 60 ℃ for 12 h. Then the catalysts were utilized for another run of catalytic testing. Each time,catalyst loss was measured by precision electronic balance. 2.4. Characterizations
The X-ray diffraction (XRD) pattern was collected on a D/Max 2500 VB 2+/PC diffractometer (Rigaku,Japan) with Cu-Ka irradiation (λ = 1.5418Å ,200 kV,50 mA) with 2u values between 10° and 80°. Transmission electron microscopy (TEM) was performed with a JEOL (JEM-2100) transmission electron microscope (JEOL,Japan) operated at 200 kV accelerating voltage. The N2 adsorption-desorption analysis was conducted with an ASAP 2020 M automatic specific surface area and aperture analyzer (MICROMERITICS,USA). Magnetic properties of the samples were measured using a vibrating sample magnetometer (VSM; Lake Shore Model 7400,USA) under magnetic fields up to 18 kOe. The Pd loading amount was determined by inductively coupled plasma mass spectrometry (ICP-MS,SPECTRO ARCOS EOP; SPECTRO Analytical Instruments GmbH,Germany). 3. Results and discussion 3.1. The structure of the catalyst
Fig. 1 shows the crystallinity and phase composition of the samples by X-ray powder diffraction (XRD). The peaks of all samples could be indexed to face center cubic magnetite phase (Fe3O4; JCPDS No. 19-0629). The extra weak diffraction peaks at 45.9°,66.7° could be indexed to the characteristic diffraction peaks of g-Al2O3 (2u = 37.68,45.78,66.68; PDF No. 10-425),which indicates that both Fe3O4 and g-Al2O3 had been obtained [25]. Since the peak at 2u = 37.68 was so close to the Fe3O4 characteristic diffraction peak that they could not be distinguished by the XRD. Fig. 1 shows that the Pd/Fe3O4@g-Al2O3 nanocomposites displayed the characteristic diffraction peaks indexed to the Pd (0),which suggested the presence of Pd (0) on Pd/Fe3O4@g-Al2O3 composites. The intensity became stronger as the concentration of Pd increased.
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| Fig. 1.Wide-angle XRD patterns of Pd/Fe3O4@g-2O3 with different Pd loading. | |
Fig. 2 shows typical SEM and TEM images of Fe3O4@g-Al2O3 and Pd/Fe3O4@g-Al2O3. It was easily observed,as in Fig. 2a-d,that the diameters of the as-synthesized Fe3O4 nanoparticles were about 300 nm with a narrow size distribution,and that the thickness of the g-Al2O3 shell was about 40 nm. If observed carefully we could find some porous structures,which resulted from the stacking of smaller g-Al2O3 particles on the outer space of the g-Al2O3 shell. Fig. 2e-f illustrated the TEM images of different loading of Pd/ Fe3O4@g-Al2O3 (Pd-1,Pd-2,Pd-3,Pd-4). One could see from the figure that there were some small Pd clusters,which stacked by smaller Pd nano-particles with the average diameter of 3-4 nm on the surface of Fe3O4@g-Al2O3. As the Pd loading increases,the clusters tend to increase. However,excess Pd loading may lead to Pd cluster agglomerating.
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| Fig. 2.SEM images of Fe3O4 (a) and Fe3O4@g-2O3 (b); TEM images of Fe3O4@g-2O3 (c,d); TEM images of Pd-1 (e),Pd-2 (f),Pd-3 (g),Pd-4 (h). | |
Nitrogen adsorption-desorption isotherms of all the samples are illustrated in Fig. 3,and the textural and structural characteristics of Pd/Fe3O4@g-Al2O3 with different Pd loading were shown in Table 1. The isotherms of all the samples are type III (definition by IUPAC). The appearance of type 3-H hysteresis loops in isotherms was indicated because there was no presence of orderly mesoporous structures in Fe3O4@g-Al2O3 magnetic particles. After Pd was loaded,the BET surface area decreased compared with Fe3O4@g-Al2O3 magnetic particles. As the Pd loading increased,the decrease of BET surface area became more significant. However,the pore volume remained constant, indicating that the Pd clusters were loaded on the surface of the Fe3O4@g-Al2O3 [26],which was consistent with the TEM results.
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Table 1 Textural and structural characteristics of Pd/Fe3O4@g-2O3 with different Pd loading. |
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| Fig. 3.N2 adsorption-desorption isotherms of Pd/Fe3O4@g-2O3 with different Pd loading. | |
Magnetic properties of the samples were investigated by a vibrating sample magnetometer (VSM) at room temperature with the applied magnetic field ranging from -18,000 to 18,000 Oe. As illustrated in Fig. 4,all samples were superparamagnetic nanoparticles. Compared with Fe3O4,the saturation magnetization (MS) intensity of the Fe3O4@ g-Al2O3 and Pd-1,Pd-2,Pd-3,Pd-4 successively decreased. Even so,the MS of the Pd/g-Al2O3@Fe3O4 was adequately high to allow its easy separation from the reaction system.
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| Fig. 4.Magnetization curves of Pd/Fe3O4@g-2O3 with different Pd loading. (a) Fe3O4,(b) Fe3O4@g-2O3,(c) Pd-1,(d) Pd-2,(e) Pd-3,(f) Pd-4. | |
The Heck reaction of bromobenzene with styrene was carried out as a model reaction to evaluate the catalytic ability of the Pd/Fe3O4@g-Al2O3. We systematically investigated the effects of Pd content,temperature,base,and solvent on the reaction. Timedependent Heck reaction activity of the Pd/Fe3O4@g-Al2O3 catalysts with different Pd loadings is shown in Fig. 5,with reaction conditions: 10 mg of Pd/Fe3O4@g-Al2O3 catalyst,bromobenzene (20 mmol),styrene (30 mmol),dodecane (10 mmol,as internal standard substance),Na23 (24 mmol),NMP (N-methyl- 2-pyrrolidone,40 mL),and reaction temperature (120 ℃). It can be observed from Fig. 5 that the conversion of bromobenzene increased to as much as 100% given sufficient reaction time. The difference is that with the increase of Pd content,the conversion rate of bromobenzene became faster. The conversion of bromobenzene increased slowly with a low Pd/bromobenzene molar ratio (Fig. 5a,0.007%) and needed 24 h to finish the reaction, whereas a higher Pd/bromobenzene molar ratio (Fig. 5c,0.021%) needed 14 h to finish the reaction. Furthermore,an increase in the Pd/bromobenzene molar ratio could not help to decrease the reaction time (Fig. 5d). For the Pd atom utilization,the conversion of Pd-1-Pd-4 was 60.5%,81.7%,96.8%,99.3% in 14 h,respectively. The conversion of bromobenzene rose to about 18% when the Pd content increased 1.5 wt%,and it decreased with the increased Pd content,which means that the dominant factor was changed from the number of active sites to the mass transfer rate of the substrates [27]. In addition,increasing the Pd content led to serious Pd clusters agglomerating (Pd-4) judged by the TEM image,which suggests that the a Pd content of Pd-3 (4.5 wt%) was adequate to catalyze the reaction.
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| Fig. 5.Time-dependent Heck activities of Pd/Fe3O4@g-2O3 catalysts with different Pd loading. | |
Table 2 show the Heck activity of bromobenzene and styrene catalyzed by Pd-3 catalysts under different reaction conditions. As we can see from entry 1 to entry 4,temperature was a key factor for the Heck reaction. As the temperature rose,the conversion of bromobenzene increased; when the temperature reached to 120 ℃, a 96.8% conversion of bromobenzenewas achieved with a high yield of 91.2%. High temperature enabled more molecule activated,thus increasing the reaction rate. For the purposes of energy saving and prolonging the life of the catalyst,lower reaction temperatures are more desirable,thus 120 ℃ was the best reaction temperature for the Pd-3 catalyst. It was also found that inorganic base was more effective than organic base,and Na23 was the ideal base among Na23,NaOAc,and Et3N tested (entry 4 to entry 6)
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Table 2 Heck reaction activities of Pd-3 catalysts under different reaction conditions a |
Table 3 shows a comparison of the activity and reaction conditions of supported Pd catalysts in the Heck reaction of bromobenzene with styrene that had been published in the literature. As we can see from the table,the diameter of Pd was the key factor. Decreasing the diameter of the catalyst means more active sites can be exposed to the reaction system. Comparison with the TOF value of different catalyst revealed that the Pd-3 catalyst had a higher TOF value at a lower reaction temperature (120 ℃),shorter reaction times (14 h),and lower Pd molar content (0.021 mol%) as compared to other supported Pd-based catalysts such as Pd loaded on mesoporous silica or TiO2 [5]. The activity of Pd on carbon nanofiber [28] was higher than that of the Pd-3 catalyst,but carbon nanofiber was not as effective as Fe3O4@g- Al2O3 in terms of the separation from the reaction systems, stability,and recyclability.
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Table 3 Comparison of the activity and reaction conditions of supported Pd catalysts in the Heck reaction of bromobenzene with styrene published in the literature. |
The condition of Heck reaction was rigorous,requiring an argon atmosphere to avoid the Pd leaching while maintaining the high activity in an anhydrous environment,and phosphine ligands to stabilize the catalyst,but the phosphine ligands are toxic and should be avoided. The solvents were often DMF (N,N-dimethylformamide),DMAc (dimethylacetamide),NMP (Nmethyl- 2-pyrrolidone),DMSO (dimethyl sulfoxide),etc. In the coupling reaction,the organic groups of the substrates varied from hydrophilic groups to hydrophobic groups or from electronwithdrawing groups to electron donating groups. So the nature of the solvents and substrates can influence the catalytic performance of the catalyst. Commonly,a catalyst had a high activity only in a specific solvent [5, 31]. In contrast,our Pd-3 catalyst was suitable for most of the common solvents with a high conversion and yield,as shown in Table 4.
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Table 4 Heck reaction activities of Pd-3 catalysts under different reaction conditions.a |
The effects of substituted groups in substrates were also examined. As illustrated in Table 5,the conversion of substrates with electron withdrawing groups in the para-position,at entry 1 to entry 2,was much higher than the substrates with electron donating groups,at entry 4 to entry 5. This result was the same as the other supported Pd catalysts result,which could be explained by the fact that the electron-withdrawing groups facilitate the oxidative addition during the Heck reaction [32]. When the olefin changed from styrene to straight-chain olefins,such as butyl acrylate or methyl acrylate,the conversion of bromobenzene decreased under the same reaction conditions,as shown at entry 6 to entry 7.
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Table 5 Heck reaction of various aryl bromides with different olefins catalyzed by Pd-3.a |
For practical application in the Heck reaction,the shelf life of the heterogeneous catalysts and their reusability are very important factors. Here our Pd-3 catalyst was reused for six times, each time after reaction the catalyst was separated from the reaction system by a permanent magnet,and the collected catalyst was washed with tetrahydrofuran and dried under vacuum at 60 ℃ for 12 h; catalyst loss was measured by precision electronic balance,then the catalyst was used in the next cycling under the same reaction conditions and giving 94.2%,89.4%,89.1%,84.7% and 78.6% isolated yield for the second,third,fourth,fifth and sixth runs,respectively,as shown in Fig. 6.
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| Fig. 6.Recycling of the Pd-3 catalyst in Heck reaction. | |
Fig. 6 also shows that the weight loss of the catalyst was only 2% or less in each regeneration process,and the total weight loss of the catalyst was only 10% after six cycles. A baseline loss may be unavoidable in the regenerate process and the Pd leaching,which is the reason that the catalyst activity dropped each cycle. The conversion of bromobenzene decreased as the catalyst weight decreased,but remained above 80% with selectivity remaining at 100% after being used for six times. After use,the catalyst was dissolved in concentrated nitric acid and analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to determine the content of Pd (0.301 mmol/g). After being used six times,the core- shell structure and the g-Al2O3 were retained. These were characterized by TEM as shown in Fig. 7. Parts of the Pd clusters were aggregated,maybe that was one of the reasons that the catalytic activity decreased compared to the fresh catalyst [28, 33]. However,during the six runs of reusing process the Pd-3 catalyst retained the high activity and magnetic reusability.
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| Fig. 7.TEM images of the Pd-3 catalyst recycled for six times in Heck reaction. | |
In summary,we had synthesized magnetic Fe3O4@g-Al2O3 microspheres with core-shell structures by a coating and calcining process on inorganic magnetic core (Fe3O4) followed by loading Pd active component on the surface of the Fe3O4@g-Al2O3 microspheres by reducing PdCl2. The catalyst of Pd was highly dispersed on the surface of the Fe3O4@g-Al2O3 microspheres with a diameter of 3-4 nm. Using the Pd/Fe3O4@g-Al2O3 (4.5 wt%) catalyst,the conversion of bromobenzene and the yield of product of about 96.8% and 91.2%,respectively,were obtained at 120 ℃ in 14h. After being recycled for six times,the catalyst gave a conversion of bromobenzene of above 80% and the recovery of the catalyst was above 90%. The nano-Pd particles were kept well dispersed in the used Pd/Fe3O4@g-Al2O3 catalyst,which show a good applicability in industry. Acknowledgment
Financial support from the National Natural Science Foundation of China (No. 21173018) is gratefully acknowledged.
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