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Synthesis of palladium-coated magnetic nanoparticle and its application in Heck reaction
Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 116–121
Synthesis of palladium-coated magnetic nanoparticle and its application in Heck reaction Zhifei Wang a , Pengfen Xiao a , Bin Shen b , Nongyue He a,∗ a
State Key Laboratory of Bioelectronics, Southeast University, 2 Sipailou, Nanjing 210096, PR China b Department of Chemistry, Southeast University, 2 Sipailou, Nanjing 210096, PR China Received 11 March 2005; received in revised form 14 October 2005; accepted 25 October 2005 Available online 1 December 2005
Abstract In this paper, we synthesized the new palladium catalyst based on magnetic nanoparticles by the “bottom-up” approach, and then its catalytic behavior in the cross coupling of acrylic acid with iodobenzene was investigated. When compared with the traditional palladium catalysts supported on carbon or the others, such catalyst has a good performance in the first run (i.e. TOF of 3749 h−1 for NaOAc base). However, the activity in re-use is poor and especially influenced by the base in the reaction. The results of palladium leaching analysis and TEM image indicate that one of the causes for the above activity drop is the leaching of Pd nanoparticle (or palladium ion) from the surface of magnetic nanoparticle into the solution. Finally, the reaction of styrene with iodobenzene (or bromobenzene) was also employed to investigate its catalytic behavior. © 2005 Elsevier B.V. All rights reserved. Keywords: Palladium catalyst; Magnetic body; Nanoparticle; Heck reaction; Bottom-up
1. Introduction In recent years, there are growing interests on the catalytic properties of transition metal nanoparticles because of their large surface area and a great ratio of atoms remaining at the surface [1–4]. Although many researches have been done, there is still the paramount challenge for the wide application of transition metal nanoparticle as catalysts in the industry, i.e. how to separate and recycle them completely from the products. At present, the main idea presented for recyclable systems may be built in liquid–liquid and solid–liquid modes [1]. For liquid–liquid system, due to the high interfacial tension between water and low-polar organic liquids, the area of the interface is small even with vigorous stirring. Therefore, separation and catalytic efficiency come to a contradiction. Another approach to separate and recycle metal nanoparticles is to immobilize them on solid supports such as organic polymeric (resin) [5], inorganic microsphere (Pd/C, Pd/SiO2 , Pd/Al2 O3 , etc.) [1,6,7], making it really easy and simple to separate the catalyst from the reaction products mixture. However, with solid catalysts suspended within liquids the transport of reactants within the liquid to the catalyst
∗
Corresponding author. Tel.: +86 25 83792245; fax: +86 25 83619983. E-mail address: [email protected] (N. He).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.028
bodies as well as the transport of them within porous catalyst bodies can be rate limiting because the transport rate to the surface of the catalyst bodies is proportional to 1/D (D is the diameter of catalyst body) [8]. To raise the rate of the reaction, it is therefore attractive to utilize small catalyst bodies. Recently, there are some significant advances in this field, such as the application of dendritic catalyst [3,4], but the main problem related to this approach is the separation and leaching of heterogeneous dendritic catalyst by the nanofiltration membrane, which does not meet the large-scale application in the industry. Thus, it is necessary to develop new catalysts with the magnetic cores and their shell consisting of the catalytic species for the ease separation by the external magnetic field. So for, the reports on the magnetic nanoparticles as catalyst supports are mainly focused on nickel–iron alloy nanoparticle with its surface coated by carbon layer [9,10], and other paramagnetic nanoparticles are less studied. In comparison with other superparamagnetic nanoparticle, Fe3 O4 nanoparticle could be easily produced by the co-deposition of ferrite with the basic solution. In addition, many investigations have been done on the surface modification due to its application in biological technology [11–13], which laid good foundation on further research as the catalyst support. Herein, we synthesized the new palladium catalyst based on magnetic body of silica coated Fe3 O4 by “bottom-up” approach,
Z. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 116–121
and its catalytic behavior in Heck reaction was also investigated. Considering the fact that Fe3 O4 is fairly reactive, especially with acid encountered in liquid-phase catalytic reaction frequently, the silica was employed to shield the inner magnetic core from the external environment. In comparison with the other shells such as carbon layer, the silica layer around magnetite provides the surface silanol groups, which can react with silane coupling agents to produce stable dispersions in non-aqueous solvent without the risk of aggregation. Also, its isoelectric point is lowed to pH ∼3 [13]. So, the utilization of the silica shell can improve the stability of the catalyst in both aqueous solvent and non-aqueous solvent. 2. Experimental 2.1. Material All chemicals used were of analytical grade from Shanghai Chemical Reagent Corporation except iodobenzene, which is chemical grade. Water used in the experiments was deionized (DI), doubly distilled and deoxygenated prior to use. 2.2. Preparation of Fe3 O4 nanoparticle Fe3 O4 nanoparticles were produced by the chemical coprecipitation. Typically, a solution of mixture of 0.85 ml of 12 mol/l HCl, 25 ml of deoxygenated DI water, 5.2 g of FeCl3 and 2.0 g of FeCl2 was prepared under N2 protection. Then the resulting solution was added drop-wise into 250 ml of 1.5 mol/l NaOH solution under vigorous stirring. The obtained precipitate was diluted to 7 g/l by tetramethylammonium hydroxide (TMA) and DI water after the washing by DI water for three times. 2.3. Preparation of silica coated Fe3 O4 nanoparticle (SiO2 /Fe3 O4 )
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42 ml of H2 O, 28 ml of ethanol and 0.067 g of PVP was refluxed for 3 h. After isolating APTS coated SiO2 /Fe3 O4 nanoparticle from the reactant, the as-synthesized colloid solution of Pd nanoparticles was added and the resulting solution was stirred for 3 h. Finally, the catalyst was separated from the reaction medium under the magnetic field, and then washed with DI water for three times. 2.5. General procedure for the catalytic tests The cross coupling of acrylic acid with iodobenzene was performed under different reaction conditions. Typically, 1 equiv (4 mmol) of iodobenzene, 2 equiv (8 mmol) of acrylic acid, 15 mg of catalyst, 20 ml of a CH3 CN solution and 2.5 equiv (10 mmol) of NaOAc were added to 60 ml of water, and the mixture was heated at reflux for 12 h. After cooling to room temperature, the catalyst was separated from the reaction medium by the magnetic field. Then, H2 O and 5% HCl were added to the residual mixture, from which white solid was formed. Finally, the solid was filtered, washed several times with fresh water, and then recrystallized by water/ethanol (3:1) solution. For the reaction of iodobenzene and styrene, the residue was extracted with ether and chromatographed over silica gel after CH3 CN was removed by rotary evaporation. 2.6. Characterization The particle size and morphology of the samples were determined by transmission electronic microscopy (TEM) with JEM200CX operating at 200 kV. Magnetization measurements of both Fe3 O4 nanoparticle and SiO2 coated Fe3 O4 nanoparticle were performed at room temperature using vibration sample magnetometer (VSM). 3. Results and discussion
A 31 ml of the silicate solution (the silicate solution was obtained by 0.58 wt% sodium silicate stock solution pass an acid exchange resin column) was mixed with 169 ml of ferrofluid diluted with DI water to the concentration of 1.4 g/l. After that, pH was adjusted to 10 by slow titration with 0.5 mol/l HCl. Then, the obtained solution was stirred for 2 h and allowed to stand for 2 days. Further silica growth on the obtained particles was performed according to St¨ober method. To 500 ml of the sol in 1:4 water/ethanol containing the obtained particle was added 0.3 ml of TEOS and 0.6 ml of ammonia. The solution was allowed to stand for 12 h under mild magnetic stirring. The surface modification of SiO2 coated Fe3 O4 nanoparticle with 3-aminopropyl triethoxysilane (APTS) was performed by the addition of 0.4 ml of APTS solution and rapid stirring for 8 h.
Catalyst Pd/(SiO2 /Fe3 O4 ) is prepared by ligand-mediated immobilization of Pd0 nanoparticle on functionalized oxide surfaces as shown in Scheme 1. It can be found that several Fe3 O4
2.4. Incorporation of Pd nanoparticle into SiO2 coated Fe3 O4 nanoparticle Pd nanoparticles were prepared according to the literature [14]. Typically, a mixture of 30 ml of a 2 mM H2 PdCl4 solution,
Scheme 1. Procedure of the preparation of catalyst Pd/(SiO2 /Fe3 O4 ). (A) Fe3 O4 nanoparticle; (B) SiO2 /Fe3 O4 ; (C) APTS coated SiO2 /Fe3 O4 nanoparticle; (D) Pd/(SiO2 /Fe3 O4 ).
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Z. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 116–121
Fig. 1. TEM image of the nanoparticles. (A) Fe3 O4 nanoparticle; (B) SiO2 coated Fe3 O4 nanoparticle prepared at 0.08 g/l Fe4 O3 nanoparticle, 1:4 H2 O/ethanol, 50 l of TEOS per 500 ml of reaction mixture, 0.2 ml of NH3 OH; (C) SiO2 coated Fe3 O4 nanoparticle prepared at 0.17 g/l Fe4 O3 nanoparticle, 1:4 H2 O/ethanol, 0.5 ml of TEOS per 500 ml of reaction mixture, 1 ml of NH3 OH; (D) SiO2 /Fe3 O4 coated with Pd nanoparticle.
nanoparticles were firstly embedded in silica coating by the St¨ober method, and then the surfaces of the obtained nanoparticles were modified with APTS through the reaction between –OH on the surface and APTS. Finally, the as-synthesized Pd colloid was bound to the catalyst carrier through the pendent amine group. So far, there have been several reports on the preparation of SiO2 coated Fe3 O4 nanoparticle [11–13]. Herein, the thin silica layer was firstly deposited on the magnetic particle by mixing the silicate solution with aqueous ferrofluid, and the further silica growth on the above-obtained nanoparticle was performed by the St¨ober method [15]. The representative TEM image of Fe3 O4 nanoparticle prepared by the chemical co-precipitation is shown in Fig. 1A, from which it can be seen that most of the particles are quasi-spherical with an average diameter of 8 nm. By adjusting experimental conditions such as the amount of TEOS and concentration of Fe3 O4 nanoparticle, we were able to vary the shell thickness and control the number of Fe3 O4 nanoparticle in core. In this work, two kinds of typical SiO2 coated Fe3 O4 nanoparticles were prepared. Fig. 1B and C give the TEM images of those nanoparticles containing one and several embedded magnetite particles, respectively. Considering the requirement for SiO2 coated Fe3 O4 nanoparticle as catalyst support, it must have relatively high saturation and appropriate surface area. SiO2 coated Fe3 O4 nanoparticles with some embedded iron oxide were chosen as the catalyst support.
From Fig. 1B, it can be found that its average diameter is about 100 nm. After the functionalization of SiO2 coated Fe3 O4 nanoparticles by APTS, the as-synthesized Pd colloid, which size is about 3 nm according to the literature [14], was bound to functionalized oxide surfaces by the strong coordination of –NH2 group with the palladium surface. In order to assure that the surface of SiO2 coated Fe3 O4 nanoparticles was covered by palladium nanoparticles, the concentration ratio of Pd colloid to SiO2 coated Fe3 O4 nanoparticles was kept at 104 :1. As can be seen from the TEM image (Fig. 1D), palladium particles are nearly uniformly distributed on the surface of the SiO2 coated Fe3 O4 nanoparticle and its coverage is high, which means that more surface palladium atoms are available for catalysis as compared to using the bulk palladium as catalyst, resulting in high activity per gram of supported catalysis. The element analysis of catalyst by the flame atomic absorption spectroscopy (AAS) indicates that the content of palladium is about 0.8 wt%. Fig. 2 shows the magnetization curves of the samples. It can be seen that after the reaction, the saturation of Fe3 O4 particle decreases from 31.4 to 9.8 emu/g for the SiO2 coated Fe3 O4 nanoparticle. There is no hysteresis, and both remanence and coercivity are zero, suggesting that such nanospheres are superparamagnetic. In order to investigate its catalytic behavior, Heck reaction of the cross coupling of acrylic acid with iodobenzene was studied.
Z. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 116–121
Fig. 2. Magnetization curves obtained by VSM at room temperature for Fe3 O4 nanoparticle (a) and SiO2 coated Fe3 O4 nanoparticle (b).
Considering the solubility requirement of the catalyst, the reactant and product, the mixture solvent (CH3 CN/H2 O, 1:3, v/v) was used for the reaction. The base was used to neutralize the acid (HX) ensuing from the formal exchange of a hydrogen atom with an aryl group. At the end of each reaction, the catalyst was separate from the reaction medium under the external magnetic field of 1.4 T. During the reaction, no obvious agglomeration of the catalyst was observed. Fig. 3 gives the effect of the reaction time on the product yield. It can be seen that after the reaction for 10 h, the product yield changes little with longer time and it reaches the maximum 67% when the reaction time is 12 h. So, 12 h has been chosen as the reaction time for each of the reaction investigated in this paper. The change of product yield as a function of times of cycle under the different reaction conditions is shown in Fig. 4. It can be seen that with the increase of the times of cycle, the catalyst activity decreases greatly. For example, for the reaction using NaOAc as the base, the product yield decreases from 67 to 37% after the sixth run. It can also be seen that the extent of the drop in catalyst activity varies with the different base. For NEt3 base, the catalyst activity is nearly zero after six cycles,
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Fig. 4. Product yield as a function of the times of cycle under the difference reaction condition.
indicating that the active species attached to the surface of the catalyst is lost. If we consider the possible mechanism occurred in Heck reaction, this phenomenon can be explained. During the reaction, the catalysis often performs by the way of the oxidative addition of the halide to the surface atoms of Pd particles, which leads to the release of palladium in the +2 oxidation state [16]. So, the good coordinating agents towards palladium (+2) should enhance the extent of metal leaching. Palladium leaching after the first cycle was measured to further confirm the above assumption (Table 1). It can be seen that after the first run palladium leaching in the reaction using NEt3 is the maximum 151 ppm as compared with other bases, which is consistent with the coordination ability between the base and palladium ion. If palladium leaching is only by way of the palladium ion, another question would arise, that is, the catalyst by using NEt3 as the base should be more active than that by using other base. So, we deduce that pre-absorbed Pd nanoparticle is lost from the surface of the magnetic support during the reaction besides its leaching by the ion. Fig. 5 presents the TEM image of the used catalyst by using NEt3 as the base after six cycles. It can be seen that the catalyst surface looks smooth as compared with the rough surface before the reaction, which indicates that most of nanoparticles are lost during the reaction. Several bigger palladium nanoparticles directed by the arrow in the image are also found on the surface of the catalyst, indicating that a Table 1 Effects of bases on the reaction of idodebenzne and acrylic acid and Pd leaching of catalysts Entry
Base
Conversion (%)
TOF (h−1 )
Pd leached into solvents (ppm)
1 2 3 4 5a
Na2 CO3 NEt3 Na3 PO4 NaOAc NEt3
58 52 63 67 78
3247 2911 3526 3749 1337
21 151 35 23 Blank
a
Fig. 3. Effect of the reaction time on product yield.
Reaction condition: catalyst: Pd-Y; catalyst loading: 30 mg; reaction temperature: 353 K; reactant: 10 mmol iodobenzene; 15 mmol butylacrylate, 12 mmol NEt3 , 10 ml solvent dimethylformamide (DMF) or tolunen.
Z. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 116–121
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Table 2 Product yields for other Heck reactions in CH3 CN/H2 O (temperature, reflux; time 12 h; base, 2.5 equiv of NaOAc) Entry
ArX
Alkene
Products
Conversion (%)
1
71
2
58
3
63
4
51
few Pd nanoparticles have grown into bigger nanoparticle by the redeposition of Pd ion leached from the support during the reaction onto the surface of the support after completion of the reaction. Table 1 also gives the turnover frequency (TOF) values of the catalyst by using different bases. TOF were calculated as (moles product) × (moles of catalyst)−1 × (reaction time (h))−1 . For the sake of simplicity in the calculations, we consider that the size of the catalyst support is 100 nm and the coverage of palladium nanoparticle on the surface of catalyst is uniform. It can be seen that the catalyst shows good performance with TOF of 3749 h−1 (for NaOAc base) at the first run as compared with other palladium catalysts supported on silica, alumina and carbon [1,6,7]. In order to further examine the catalyst activity on other Heck reaction, bromo-substrate rather than iodo-substrate and styrene instead of acrylic acid were used (Table 2). It can be seen that the catalyst has good performance for the cross coupling of styrene with iodobenzene (NaOAc is used as the base). In addition, as is usually the case, iodoaromatic is found to be more active substrates than bromoaromtic.
4. Conclusion In summary, we successfully synthesized the silica coated Fe3 O4 nanoparticle as the catalyst support in this work, and then palladium catalyst was obtained by the coordination of 3 nm palladium nanoparticle with –NH2 group on the support surface. TEM image shows that palladium particles are nearly uniformly dispersed on the SiO2 coated Fe3 O4 particle with high coverage. Comparing with the traditionally supported catalyst, the activity measurement shows this kind of catalyst has good activity for Heck reaction in the first run. However, the activity in re-use is poor and especially influenced by the base in the reaction. The results of palladium leaching analysis and TEM image indicate that the drop of activity in re-use is due to the leaching of palladium ion (or palladium nanoparticle) into the solution. Considering the fact that Pd leaching and redeposition may depend on the state of Pd dispersion (particle size and the interactions of metal/support) and the surface properties of support, we suggest that the bases with the strong coordination with Pd should be avoided in future research. In a word, as the fundamental research, this paper provides the attempt to solve the recycle problem of metal nanoparticle as the catalyst in reaction by synthesizing Pd coated magnetic nanoparticle catalyst and giving its preliminary activity results in Heck reaction. Acknowledgements This research was funded by the National Natural Science Foundation of China 60571032, 20505020 and the Doctoral Program of Higher Education 20050286014. Thanks to Z.C. Wang (State Key Lab. Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, CAS) for help using transmission electron microscope. References
Fig. 5. TEM image of the used catalyst after the sixth run by using Et3 N as the base.
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Synthesis of palladium-coated magnetic nanoparticle and its application in Heck reaction Zhifei Wang a , Pengfen Xiao a , Bin Shen b , Nongyue He a,∗ a
State Key Laboratory of Bioelectronics, Southeast University, 2 Sipailou, Nanjing 210096, PR China b Department of Chemistry, Southeast University, 2 Sipailou, Nanjing 210096, PR China Received 11 March 2005; received in revised form 14 October 2005; accepted 25 October 2005 Available online 1 December 2005
Abstract In this paper, we synthesized the new palladium catalyst based on magnetic nanoparticles by the “bottom-up” approach, and then its catalytic behavior in the cross coupling of acrylic acid with iodobenzene was investigated. When compared with the traditional palladium catalysts supported on carbon or the others, such catalyst has a good performance in the first run (i.e. TOF of 3749 h−1 for NaOAc base). However, the activity in re-use is poor and especially influenced by the base in the reaction. The results of palladium leaching analysis and TEM image indicate that one of the causes for the above activity drop is the leaching of Pd nanoparticle (or palladium ion) from the surface of magnetic nanoparticle into the solution. Finally, the reaction of styrene with iodobenzene (or bromobenzene) was also employed to investigate its catalytic behavior. © 2005 Elsevier B.V. All rights reserved. Keywords: Palladium catalyst; Magnetic body; Nanoparticle; Heck reaction; Bottom-up
1. Introduction In recent years, there are growing interests on the catalytic properties of transition metal nanoparticles because of their large surface area and a great ratio of atoms remaining at the surface [1–4]. Although many researches have been done, there is still the paramount challenge for the wide application of transition metal nanoparticle as catalysts in the industry, i.e. how to separate and recycle them completely from the products. At present, the main idea presented for recyclable systems may be built in liquid–liquid and solid–liquid modes [1]. For liquid–liquid system, due to the high interfacial tension between water and low-polar organic liquids, the area of the interface is small even with vigorous stirring. Therefore, separation and catalytic efficiency come to a contradiction. Another approach to separate and recycle metal nanoparticles is to immobilize them on solid supports such as organic polymeric (resin) [5], inorganic microsphere (Pd/C, Pd/SiO2 , Pd/Al2 O3 , etc.) [1,6,7], making it really easy and simple to separate the catalyst from the reaction products mixture. However, with solid catalysts suspended within liquids the transport of reactants within the liquid to the catalyst
∗
Corresponding author. Tel.: +86 25 83792245; fax: +86 25 83619983. E-mail address: [email protected] (N. He).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.028
bodies as well as the transport of them within porous catalyst bodies can be rate limiting because the transport rate to the surface of the catalyst bodies is proportional to 1/D (D is the diameter of catalyst body) [8]. To raise the rate of the reaction, it is therefore attractive to utilize small catalyst bodies. Recently, there are some significant advances in this field, such as the application of dendritic catalyst [3,4], but the main problem related to this approach is the separation and leaching of heterogeneous dendritic catalyst by the nanofiltration membrane, which does not meet the large-scale application in the industry. Thus, it is necessary to develop new catalysts with the magnetic cores and their shell consisting of the catalytic species for the ease separation by the external magnetic field. So for, the reports on the magnetic nanoparticles as catalyst supports are mainly focused on nickel–iron alloy nanoparticle with its surface coated by carbon layer [9,10], and other paramagnetic nanoparticles are less studied. In comparison with other superparamagnetic nanoparticle, Fe3 O4 nanoparticle could be easily produced by the co-deposition of ferrite with the basic solution. In addition, many investigations have been done on the surface modification due to its application in biological technology [11–13], which laid good foundation on further research as the catalyst support. Herein, we synthesized the new palladium catalyst based on magnetic body of silica coated Fe3 O4 by “bottom-up” approach,
Z. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 116–121
and its catalytic behavior in Heck reaction was also investigated. Considering the fact that Fe3 O4 is fairly reactive, especially with acid encountered in liquid-phase catalytic reaction frequently, the silica was employed to shield the inner magnetic core from the external environment. In comparison with the other shells such as carbon layer, the silica layer around magnetite provides the surface silanol groups, which can react with silane coupling agents to produce stable dispersions in non-aqueous solvent without the risk of aggregation. Also, its isoelectric point is lowed to pH ∼3 [13]. So, the utilization of the silica shell can improve the stability of the catalyst in both aqueous solvent and non-aqueous solvent. 2. Experimental 2.1. Material All chemicals used were of analytical grade from Shanghai Chemical Reagent Corporation except iodobenzene, which is chemical grade. Water used in the experiments was deionized (DI), doubly distilled and deoxygenated prior to use. 2.2. Preparation of Fe3 O4 nanoparticle Fe3 O4 nanoparticles were produced by the chemical coprecipitation. Typically, a solution of mixture of 0.85 ml of 12 mol/l HCl, 25 ml of deoxygenated DI water, 5.2 g of FeCl3 and 2.0 g of FeCl2 was prepared under N2 protection. Then the resulting solution was added drop-wise into 250 ml of 1.5 mol/l NaOH solution under vigorous stirring. The obtained precipitate was diluted to 7 g/l by tetramethylammonium hydroxide (TMA) and DI water after the washing by DI water for three times. 2.3. Preparation of silica coated Fe3 O4 nanoparticle (SiO2 /Fe3 O4 )
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42 ml of H2 O, 28 ml of ethanol and 0.067 g of PVP was refluxed for 3 h. After isolating APTS coated SiO2 /Fe3 O4 nanoparticle from the reactant, the as-synthesized colloid solution of Pd nanoparticles was added and the resulting solution was stirred for 3 h. Finally, the catalyst was separated from the reaction medium under the magnetic field, and then washed with DI water for three times. 2.5. General procedure for the catalytic tests The cross coupling of acrylic acid with iodobenzene was performed under different reaction conditions. Typically, 1 equiv (4 mmol) of iodobenzene, 2 equiv (8 mmol) of acrylic acid, 15 mg of catalyst, 20 ml of a CH3 CN solution and 2.5 equiv (10 mmol) of NaOAc were added to 60 ml of water, and the mixture was heated at reflux for 12 h. After cooling to room temperature, the catalyst was separated from the reaction medium by the magnetic field. Then, H2 O and 5% HCl were added to the residual mixture, from which white solid was formed. Finally, the solid was filtered, washed several times with fresh water, and then recrystallized by water/ethanol (3:1) solution. For the reaction of iodobenzene and styrene, the residue was extracted with ether and chromatographed over silica gel after CH3 CN was removed by rotary evaporation. 2.6. Characterization The particle size and morphology of the samples were determined by transmission electronic microscopy (TEM) with JEM200CX operating at 200 kV. Magnetization measurements of both Fe3 O4 nanoparticle and SiO2 coated Fe3 O4 nanoparticle were performed at room temperature using vibration sample magnetometer (VSM). 3. Results and discussion
A 31 ml of the silicate solution (the silicate solution was obtained by 0.58 wt% sodium silicate stock solution pass an acid exchange resin column) was mixed with 169 ml of ferrofluid diluted with DI water to the concentration of 1.4 g/l. After that, pH was adjusted to 10 by slow titration with 0.5 mol/l HCl. Then, the obtained solution was stirred for 2 h and allowed to stand for 2 days. Further silica growth on the obtained particles was performed according to St¨ober method. To 500 ml of the sol in 1:4 water/ethanol containing the obtained particle was added 0.3 ml of TEOS and 0.6 ml of ammonia. The solution was allowed to stand for 12 h under mild magnetic stirring. The surface modification of SiO2 coated Fe3 O4 nanoparticle with 3-aminopropyl triethoxysilane (APTS) was performed by the addition of 0.4 ml of APTS solution and rapid stirring for 8 h.
Catalyst Pd/(SiO2 /Fe3 O4 ) is prepared by ligand-mediated immobilization of Pd0 nanoparticle on functionalized oxide surfaces as shown in Scheme 1. It can be found that several Fe3 O4
2.4. Incorporation of Pd nanoparticle into SiO2 coated Fe3 O4 nanoparticle Pd nanoparticles were prepared according to the literature [14]. Typically, a mixture of 30 ml of a 2 mM H2 PdCl4 solution,
Scheme 1. Procedure of the preparation of catalyst Pd/(SiO2 /Fe3 O4 ). (A) Fe3 O4 nanoparticle; (B) SiO2 /Fe3 O4 ; (C) APTS coated SiO2 /Fe3 O4 nanoparticle; (D) Pd/(SiO2 /Fe3 O4 ).
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Z. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 116–121
Fig. 1. TEM image of the nanoparticles. (A) Fe3 O4 nanoparticle; (B) SiO2 coated Fe3 O4 nanoparticle prepared at 0.08 g/l Fe4 O3 nanoparticle, 1:4 H2 O/ethanol, 50 l of TEOS per 500 ml of reaction mixture, 0.2 ml of NH3 OH; (C) SiO2 coated Fe3 O4 nanoparticle prepared at 0.17 g/l Fe4 O3 nanoparticle, 1:4 H2 O/ethanol, 0.5 ml of TEOS per 500 ml of reaction mixture, 1 ml of NH3 OH; (D) SiO2 /Fe3 O4 coated with Pd nanoparticle.
nanoparticles were firstly embedded in silica coating by the St¨ober method, and then the surfaces of the obtained nanoparticles were modified with APTS through the reaction between –OH on the surface and APTS. Finally, the as-synthesized Pd colloid was bound to the catalyst carrier through the pendent amine group. So far, there have been several reports on the preparation of SiO2 coated Fe3 O4 nanoparticle [11–13]. Herein, the thin silica layer was firstly deposited on the magnetic particle by mixing the silicate solution with aqueous ferrofluid, and the further silica growth on the above-obtained nanoparticle was performed by the St¨ober method [15]. The representative TEM image of Fe3 O4 nanoparticle prepared by the chemical co-precipitation is shown in Fig. 1A, from which it can be seen that most of the particles are quasi-spherical with an average diameter of 8 nm. By adjusting experimental conditions such as the amount of TEOS and concentration of Fe3 O4 nanoparticle, we were able to vary the shell thickness and control the number of Fe3 O4 nanoparticle in core. In this work, two kinds of typical SiO2 coated Fe3 O4 nanoparticles were prepared. Fig. 1B and C give the TEM images of those nanoparticles containing one and several embedded magnetite particles, respectively. Considering the requirement for SiO2 coated Fe3 O4 nanoparticle as catalyst support, it must have relatively high saturation and appropriate surface area. SiO2 coated Fe3 O4 nanoparticles with some embedded iron oxide were chosen as the catalyst support.
From Fig. 1B, it can be found that its average diameter is about 100 nm. After the functionalization of SiO2 coated Fe3 O4 nanoparticles by APTS, the as-synthesized Pd colloid, which size is about 3 nm according to the literature [14], was bound to functionalized oxide surfaces by the strong coordination of –NH2 group with the palladium surface. In order to assure that the surface of SiO2 coated Fe3 O4 nanoparticles was covered by palladium nanoparticles, the concentration ratio of Pd colloid to SiO2 coated Fe3 O4 nanoparticles was kept at 104 :1. As can be seen from the TEM image (Fig. 1D), palladium particles are nearly uniformly distributed on the surface of the SiO2 coated Fe3 O4 nanoparticle and its coverage is high, which means that more surface palladium atoms are available for catalysis as compared to using the bulk palladium as catalyst, resulting in high activity per gram of supported catalysis. The element analysis of catalyst by the flame atomic absorption spectroscopy (AAS) indicates that the content of palladium is about 0.8 wt%. Fig. 2 shows the magnetization curves of the samples. It can be seen that after the reaction, the saturation of Fe3 O4 particle decreases from 31.4 to 9.8 emu/g for the SiO2 coated Fe3 O4 nanoparticle. There is no hysteresis, and both remanence and coercivity are zero, suggesting that such nanospheres are superparamagnetic. In order to investigate its catalytic behavior, Heck reaction of the cross coupling of acrylic acid with iodobenzene was studied.
Z. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 116–121
Fig. 2. Magnetization curves obtained by VSM at room temperature for Fe3 O4 nanoparticle (a) and SiO2 coated Fe3 O4 nanoparticle (b).
Considering the solubility requirement of the catalyst, the reactant and product, the mixture solvent (CH3 CN/H2 O, 1:3, v/v) was used for the reaction. The base was used to neutralize the acid (HX) ensuing from the formal exchange of a hydrogen atom with an aryl group. At the end of each reaction, the catalyst was separate from the reaction medium under the external magnetic field of 1.4 T. During the reaction, no obvious agglomeration of the catalyst was observed. Fig. 3 gives the effect of the reaction time on the product yield. It can be seen that after the reaction for 10 h, the product yield changes little with longer time and it reaches the maximum 67% when the reaction time is 12 h. So, 12 h has been chosen as the reaction time for each of the reaction investigated in this paper. The change of product yield as a function of times of cycle under the different reaction conditions is shown in Fig. 4. It can be seen that with the increase of the times of cycle, the catalyst activity decreases greatly. For example, for the reaction using NaOAc as the base, the product yield decreases from 67 to 37% after the sixth run. It can also be seen that the extent of the drop in catalyst activity varies with the different base. For NEt3 base, the catalyst activity is nearly zero after six cycles,
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Fig. 4. Product yield as a function of the times of cycle under the difference reaction condition.
indicating that the active species attached to the surface of the catalyst is lost. If we consider the possible mechanism occurred in Heck reaction, this phenomenon can be explained. During the reaction, the catalysis often performs by the way of the oxidative addition of the halide to the surface atoms of Pd particles, which leads to the release of palladium in the +2 oxidation state [16]. So, the good coordinating agents towards palladium (+2) should enhance the extent of metal leaching. Palladium leaching after the first cycle was measured to further confirm the above assumption (Table 1). It can be seen that after the first run palladium leaching in the reaction using NEt3 is the maximum 151 ppm as compared with other bases, which is consistent with the coordination ability between the base and palladium ion. If palladium leaching is only by way of the palladium ion, another question would arise, that is, the catalyst by using NEt3 as the base should be more active than that by using other base. So, we deduce that pre-absorbed Pd nanoparticle is lost from the surface of the magnetic support during the reaction besides its leaching by the ion. Fig. 5 presents the TEM image of the used catalyst by using NEt3 as the base after six cycles. It can be seen that the catalyst surface looks smooth as compared with the rough surface before the reaction, which indicates that most of nanoparticles are lost during the reaction. Several bigger palladium nanoparticles directed by the arrow in the image are also found on the surface of the catalyst, indicating that a Table 1 Effects of bases on the reaction of idodebenzne and acrylic acid and Pd leaching of catalysts Entry
Base
Conversion (%)
TOF (h−1 )
Pd leached into solvents (ppm)
1 2 3 4 5a
Na2 CO3 NEt3 Na3 PO4 NaOAc NEt3
58 52 63 67 78
3247 2911 3526 3749 1337
21 151 35 23 Blank
a
Fig. 3. Effect of the reaction time on product yield.
Reaction condition: catalyst: Pd-Y; catalyst loading: 30 mg; reaction temperature: 353 K; reactant: 10 mmol iodobenzene; 15 mmol butylacrylate, 12 mmol NEt3 , 10 ml solvent dimethylformamide (DMF) or tolunen.
Z. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 116–121
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Table 2 Product yields for other Heck reactions in CH3 CN/H2 O (temperature, reflux; time 12 h; base, 2.5 equiv of NaOAc) Entry
ArX
Alkene
Products
Conversion (%)
1
71
2
58
3
63
4
51
few Pd nanoparticles have grown into bigger nanoparticle by the redeposition of Pd ion leached from the support during the reaction onto the surface of the support after completion of the reaction. Table 1 also gives the turnover frequency (TOF) values of the catalyst by using different bases. TOF were calculated as (moles product) × (moles of catalyst)−1 × (reaction time (h))−1 . For the sake of simplicity in the calculations, we consider that the size of the catalyst support is 100 nm and the coverage of palladium nanoparticle on the surface of catalyst is uniform. It can be seen that the catalyst shows good performance with TOF of 3749 h−1 (for NaOAc base) at the first run as compared with other palladium catalysts supported on silica, alumina and carbon [1,6,7]. In order to further examine the catalyst activity on other Heck reaction, bromo-substrate rather than iodo-substrate and styrene instead of acrylic acid were used (Table 2). It can be seen that the catalyst has good performance for the cross coupling of styrene with iodobenzene (NaOAc is used as the base). In addition, as is usually the case, iodoaromatic is found to be more active substrates than bromoaromtic.
4. Conclusion In summary, we successfully synthesized the silica coated Fe3 O4 nanoparticle as the catalyst support in this work, and then palladium catalyst was obtained by the coordination of 3 nm palladium nanoparticle with –NH2 group on the support surface. TEM image shows that palladium particles are nearly uniformly dispersed on the SiO2 coated Fe3 O4 particle with high coverage. Comparing with the traditionally supported catalyst, the activity measurement shows this kind of catalyst has good activity for Heck reaction in the first run. However, the activity in re-use is poor and especially influenced by the base in the reaction. The results of palladium leaching analysis and TEM image indicate that the drop of activity in re-use is due to the leaching of palladium ion (or palladium nanoparticle) into the solution. Considering the fact that Pd leaching and redeposition may depend on the state of Pd dispersion (particle size and the interactions of metal/support) and the surface properties of support, we suggest that the bases with the strong coordination with Pd should be avoided in future research. In a word, as the fundamental research, this paper provides the attempt to solve the recycle problem of metal nanoparticle as the catalyst in reaction by synthesizing Pd coated magnetic nanoparticle catalyst and giving its preliminary activity results in Heck reaction. Acknowledgements This research was funded by the National Natural Science Foundation of China 60571032, 20505020 and the Doctoral Program of Higher Education 20050286014. Thanks to Z.C. Wang (State Key Lab. Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, CAS) for help using transmission electron microscope. References
Fig. 5. TEM image of the used catalyst after the sixth run by using Et3 N as the base.
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