Article pubs.acs.org/IECR
A Highly Efficient and Stable Palladium Catalyst Entrapped within the Cross-Linked Chitosan Membrane for Heck Reactions Minfeng Zeng,*,† Chenze Qi,† Jing Yang,‡ Baoyi Wang,‡ and Xian-Man Zhang† †
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University, Shaoxing 312000, China Institute of High Energy Physics, The Chinese Academy of Science, Beijing 100049, China
‡
S Supporting Information *
ABSTRACT: In this study, chitosan directly cross-linked by PdII cation membranes (Pd-cr-CSM) with good mechanical strength and thermal stabilities have been prepared. Although the prepared Pd-cr-CSM has neither open porous structure nor high specific surface areas, it has similar good catalytic activity and much higher stability as compared with typical prepared chitosan-stabilized palladium heterogeneous catalysts for Heck reactions. It is highly active for the Heck reactions of aryl iodides and bromides with a strong electron-withdrawing group at a palladium catalyst loading of 0.15 mol %. It can be recycled 12 times in dimethyl sulfoxide (DMSO) solution or 7 times in aqueous solution. The high activity and extreme stability of the Pd-cr-CSM catalyst are mainly attributed to the well-entrapped palladium nanoparticles inside the chitosan matrix, which might catalyze the coupling reactions in the free volume holes (open spaces) of the swollen cross-linked chitosan gel networks.
1. INTRODUCTION Palladium catalysis has been widely used in industrial and academic synthetic chemistry, especially for the many carbon− carbon bond transformations, but it is commonly employed in a homogeneous system because of its excellent activity and selectivity.1 However, homogeneous catalysis suffers from a number of drawbacks such as difficult separation, recovery, and reuse of palladium metal.2 It is intricate and costly to completely remove the transition metals and ligands from the reaction mixture to reduce pollution and minimize environmental impacts. Moreover, the transition metal contamination in the final products is of major concern for application of the homogeneous catalytic system in the pharmaceutical, cosmetic, and food industries because the transition metal residue is frequently reached at an unacceptable level in the final products.3 Immobilization of the catalytic system on a solid matrix will not only avoid or at least mitigate these drawbacks of homogeneous catalysts but also offer many additional attractive features including ease of handling, product isolation, and straightforward recovery/recycling of the expensive transition metal catalyst by simple filtration, leaving the final products virtually free of transition metal contamination.4,5 Recently, environmentally friendly and low-cost natural polymers have attracted more and more attention for immobilization of palladium catalysts.6−12 Chitosan is one of the low-cost and nontoxic natural biopolymers, and it can be readily prepared by N-deacetylation of chitin, the most abundant natural biopolymer second only to cellulose. Interestingly, chitosan is soluble in acidic medium, but insoluble in neutral/basic and organic medium, allowing the generation of a wide range of shapes including microspheres, flakes, nanofibers, gel beads, and membranes.6−12 In addition, the abundant amino and hydroxyl groups of the chitosan molecule present unique affinity toward most transition metal species, making it an ideal solid support to immobilize Pd catalyst for applications in organic, aqueous, and ionic liquid media.6,7,13−15 © 2014 American Chemical Society
Palladium leaching is a common problem due to the relatively weaker chelation of the zero-valent palladium species with the surface molecules of the solid supports including chitosan,16−19 indicating it is challenging to prepare a “permanently” stable and active palladium heterogeneous catalyst. The chitosan base-supported palladium catalysts were generally prepared by absorption of palladium species on the solid matrix surface.6 The main advantage of absorption method is it is easier to process when higher Pd amounts are required, whereas the drawback is that the bound strength is often not powerful enough. For example, for the chitosan nanofiber mat-supported Pd catalysts recently prepared by the Iyer group,20 post treatments of thorough washing with Milli-Q water were needed to remove any weakly bound Pd species. Meanwhile, the solid matrix was often prepared with open porous structures to improve the surface specific area and permeability for high activity of the heterogeneous catalysts.6,21,22 However, the open porous structure could reduce the mechanical strength and stability of the polymer supports, resulting in decreased resistance to high reaction temperature and continuous processing. Indeed, most of the reported polymer-stabilized Pd catalysts used in coupling reactions could be recycled about 5−10 times and even fewer times in our recent works.20,23−35 Recently, an excellent palladium heterogeneous catalyst36 was prepared by confining the Pd nanoparticles inside a swollen interpenetrating polymer networks (IPNs) of poly(vinyl alcohol) (PVA) and polyacrylamide (PAA) with no open structure, which could be reused more than 20 times in DMF and 10 times in water for the Heck coupling reaction. The extreme stability was attributed to the nanoscale open spaces Received: Revised: Accepted: Published: 10041
March 29, 2014 May 22, 2014 May 27, 2014 May 27, 2014 dx.doi.org/10.1021/ie501315a | Ind. Eng. Chem. Res. 2014, 53, 10041−10050
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mm/min at 25 °C. Five specimens were tested for each group sample. The thermal stability of the Pd-cr-CSM samples was tested with a TG/DTA 6300 Seiko instrument (Seiko Instruments Inc., Japan). The testing condition was: from 50 to 700 °C, at a heat rate of 20 °C/min, atmosphere of air. The swelling properties of the Pd-cr-CSM samples in DMSO and water were determined according to the following procedure. The preweighed Pd-cr-CSM dry membrane samples were immersed in DMSO (110 °C) or water (90 °C), and then the membrane samples were taken out from the solvent at different interval times (t). After removal of the surface excess solvent, the samples were weighed again and the ratio of solvent absorption (Sa) at the interval time t was calculated with eq 1
(cross-linking mesh) of IPNs, which could allow the catalyzed Heck coupling reaction occurrence. According to the theory of free volume in polymers, such nanoscale open spaces of a dense polymeric material are so-called free volume holes, which are the unoccupied spaces between macromolecular chains. From this work, we think that similarly confining the Pd particles inside a swollen cross-linked chitosan network would be also a good solution to the preparation of a more stable heterogeneous catalyst than usual if the reactions can occur inside the open spaces (free volume holes) of the swollen crosslinked chitosan networks. It was found that the viscosity of the chitosan acidic aqueous solution will significantly increase with addition of a small amount of the PdII cation, making it difficult to prepare suitable heterogeneous catalysts.37−39 Therefore, such a metal ion direct coordination method is much less explored for the preparation of chitosan-stabilized Pd heterogeneous catalysts, although it allows the uniform and strong chelation of Pd species with surrounding chitosan macromolecules.6 However, such technical problems in catalyst preparation should be resolved efficiently by using a lowconcentration chitosan solution as starting solution without additional chemical cross-linking treatment. In the present paper, we have prepared the chitosan directly cross-linked by PdII cation membrane (Pd-cr-CSM) with no open porous structure and found it was also a highly active and stable heterogeneous palladium catalyst. Heck reactions have been employed as the model reaction to evaluate its catalytic activity, stability, and reusability in both dimethyl sulfoxide (DMSO) and aqueous solutions. Meanwhile, positron annihilation lifetime spectroscopy (PALS) has been used to assess the free volume hole size and the possibility of the occurrence of the coupling reaction inside open spaces (free volume holes) of the swollen cross-linked chitosan gel networks.
Sa = (Wt − W0)/W0 × 100%
(1)
where Wt and W0 are the membrane sample weights at the interval time t and t0, respectively. Five specimens were tested for each group sample. X-ray photoelectron spectroscopic (XPS) analysis was performed for the Pd-cr-CSM catalysts using angle-resolved photoemission spectroscopy (ADES-400, VG Co. UK). C1s of 284.6 eV was used as the internal reference for calibration. The morphology of the surface and cross section of the Pdcr-CSM catalysts was observed with a JEM-6360 scanning electron microscope (SEM). The element analysis was tested with an energy dispersive X-ray spectroscopy (Oxford EDX System). The dispersion of the Pd nanoparticles in Pd-cr-CSM catalysts was observed with a JEM-2100F (Japan) highresolution transmission electron microscope (HR-TEM). The samples for the HR-TEM testing were prepared by sonication dispersion of the grounded Pd-cr-CSM catalyst membrane in ethanol solution, followed by spraying on a copper net to form a thin film. The freshly prepared as well as the recycled Pd-cr-CSM heterogeneous catalysts were first completely dissolved in a 2 mL mixture of fuming HNO3 and concentrated HCl v/v 1:3. Then the solution was diluted to 50 mL. The palladium content in the diluted solution was determined by inductively coupled plasma−atomic emission spectroscopy (ICP, Leemann ICPAES Prodigy XP, USA). Positron annihilation lifetime spectroscopic (PALS) measurements were done using a fast−fast lifetime spectrometer (EG&G ORTEC Co., USA) at the ambient temperature. The time resolution was 190 ps at a 60Co prompt peak of 1.18 MeV and 1.33 MeV γ rays. A 6.0 × 105 Bq positron source (22Na) was deposited between two 3 μm thick Kapton films, and they were sandwiched within two identical Pd-cr-CSM samples. Each spectrum contained about 1.0 × 106 counts, and it was consistently modeled using a three-component fitting LT 9.0 program.40 2.4. Catalysis. For Heck coupling reactions in DMSO solvent, a mixture of aromatic halide (1 mmol), alkenes (2 mmol), Pd-cr-CSM (17.6 mg, 1.5 μmol Pd), and CH3CO2K (3 mmol) and 3 mL of DMSO and 0.2 mL of glycol were added together into a 20 mL round-bottom flask. The mixture was heated in an oil bath at 110 °C under magnetic stirring for 5 h. For Heck coupling reactions in aqueous solution, a mixture of aromatic halide (1 mmol), alkenes (1.2 mmol), Pd-cr-CSM (88 mg, 7.5 μmol Pd), CH3CO2K (3 mmol), N-cetyltrimethylammonium bromide (CTAB, 0.025 mmol) in 5 mL of H2O, and 0.2 mL of glycol were added together into a 20 mL round-
2. EXPERIMENTAL SECTION 2.1. Materials. Chitosan used in this study was supplied by Zhejiang Aoxing Biotechnology Co., Ltd., China. Its deacetylated degree was 95%, and its molecular weight was 1.2 × 105. PdCl2 used in this study was supplied by Zhejiang Metallurgical Research Institute Co., Ltd., China. All of the chemical reagents and solvents used in this study were of analytical grade. They were used without further purification. 2.2. Preparation. Chitosan (1.33 g) was dissolved in 100 mL of 2 wt % acetic acid solution at room temperature under magnetic stirring for about 2 h. Then, certain amounts of Na2PdCl4 solution were dropped into the chitosan solution under magnetic stirring. The dropped Na2PdCl4 solution was prepared by dissolution of 0.15 g of PdCl2 and 1 g of NaCl together in 50 mL of deionized water. The mixed solution turned into a fixed gel when 30 mL of chitosan solution was added to 2.0 mL (critical gel adding amount) of such a Na2PdCl4 solution. The resultant gel solution was cast on a Petri dish and naturally dried to form membranes. The removed membranes were then neutralized with 2.0% NaOH solution and then washed to pH 7.0 with deionized water. Finally, the membranes were naturally dried to obtain the Pdcr-CSM catalyst. The prepared membrane (thickness = 70−80 μm) is slightly yellowish and transparent. 2.3. Characterization. The tensile strength of the Pd-crCSM samples was tested with a SANS universal materials testing instrument (Shenzhen SANS Testing Machine Co. Ltd., China). The testing condition was a crosshead speed of 100 10042
dx.doi.org/10.1021/ie501315a | Ind. Eng. Chem. Res. 2014, 53, 10041−10050
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bottom flask. The mixture was heated in an oil bath at 90 °C under magnetic stirring for 12 h. The reaction progress was monitored by TLC and/or GC-MS analysis. The workup and product characterization were followed by the same procedures as in the literature.23 For recycling, the Pd-cr-CSM heterogeneous catalyst was recycled by simple filtration, and then it was rinsed with 2.0 mL of ethanol. Before the next application, it was dried at 110 °C for 20 min to remove the residue ethanol.
3. RESULTS AND DISCUSSION 3.1. Characterization of the Pd-cr-CSM Heterogeneous Catalyst. Addition of the Na 2 PdCl 4 solution
Figure 3. Thermal analysis results of the pure chitosan membrane (CSM) and Pd-cr-CSM.
Figure 1. Formation of the cross-linked chitosan gel membrane.
Figure 4. Swelling ratios of the Pd-cr-CSM membranes in water and DMSO solutions.
× 10−5 mol PdII/g chitosan, whereas the ICP-AES analysis showed that the palladium content is about 1.5 μmol for Pd-crCSM membrane (17.6 mg). The tensile strength was also determined to assess the palladium cross-linking effects on the mechanical properties of the resulting chitosan membranes. The tensile strength of the pure chitosan membrane is 10.6 MPa, indicating that there is about a 4 times increase of the tensile strength for the Pd-crCSM heterogeneous catalyst (37.6 MPa) as shown in Figure 2. Heterogeneous catalysts are generally employed at relatively high reaction temperatures; thus, the thermal stabilities are critical for their practical applications. Weight loss occurs at three major steps, those being 80−120, 175−400, and 450−550 °C, for the thermal degradation of the pure CSM as well as the Pd-cr-CSM membrane as shown in Figure 3. The predominant weight loss of the second and third steps may be associated with destruction of the intermolecular interactions such as hydrogen bonding, cross-linking, thermal oxidation, and carbonization. An inspection of Figure 3 clearly shows that the cross-linked chitosan membrane is more stable than the corresponding non-cross-linked chitosan membrane. The solvent-swelling properties of the prepared Pd-cr-CSM heterogeneous catalysts were determined in both aqueous and DMSO solutions, which are the solvents commonly used for the palladium-catalyzed coupling reactions. Examination of
Figure 2. Dependence of the tensile strength of Pd-cr-CSM on the PdII cation content.
dramatically increased the viscosity of the chitosan solution, and it could even be turned into a fixed gel as shown in Figure 1. The viscosity augmentation is clearly associated with the cross-linking of the chitosan polymer chains through the PdII cation chelation with the surrounding amino, carbonyl, and/or hydroxyl functional groups either in linear or in threedimensional networks. This sol−gel transition was similar to PdII cross-linked crown ether supramolecular gel.41 The critical gel concentration of PdII to chitosan was determined to be 8.5 10043
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Figure 5. SEM-EDX observation of the Pd-cr-CSM: (A) surface; (B) cross-section.
Figure 6. HR-TEM observation of the fresh Pd-cr-CSM.
Figure 4 shows that the solvent absorption could reach equilibrium within 1 h. The maximum swelling ratios of the prepared Pd-cr-CSM heterogeneous catalysts were determined to be about 60 and 90% in DMSO and aqueous solutions, respectively. The slightly higher swelling ratio in the aqueous solution may be associated with the relatively higher polarity of H2O than of DMSO.
Figure 7. X-ray photoelectron spectra (XPS): (A) fresh Pd-cr-CSM membrane; (B) used Pd-cr-CSM membrane.
The scanning electron microscopic (SEM) images indicate that the palladium species is uniformly dispersed on the surface as well as in the cross-section for Pd-cr-CSM. This conclusion is
Table 1. Solvent Effects on the Pd-cr-CSM Catalyzed Heck Cross-Coupling of Iodobenzene with n-Butyl Acrylatea
entry
solvent
yieldb (%)
1 2 3 4 5 6
DMSO (3 mL) xylene (3 mL) dioxane (3 mL) DMSO + glycol (3 mL + 0.2 mL) xylene + glycol (3 mL + 0.2 mL) dioxane + glycol (3 mL + 0.2 mL)
87 12 11 98 89 86
Reaction conditions: 1 mmol of iodobenzene, 2 mmol of n-butyl acrylate, 1.5 μmol of Pd-cr-CSM, 3 mmol of CH3CO2K base in ∼3 mL of solvent at 110 °C for 5 h unless otherwise indicated. bGC-MS yield. a
10044
dx.doi.org/10.1021/ie501315a | Ind. Eng. Chem. Res. 2014, 53, 10041−10050
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Table 2. Pd-cr-CSM Catalyzed Heck Cross-Coupling of Various Aromatic Halides with Different Alkenes in DMSOa
entry
aromatic halide
alkene substrate
yieldb (%) 98 96 99 89 90 95 93 93 95 92 98 97 93 88 33 (trans) 46 (isomer) 34 (trans) 40 (isomer) 98 98 95 66c 73c 72c 54c 65c 33c 51c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
C6H5I 2-CH3C6H4I 4-CH3C6H4I 2-CH3OC6H4I 3-CH3OC6H4I 3-FC6H4I 4-FC6H4I 2-ClC6H4I 3-ClC6H4I 4-BrC6H4I C6H5I 4-CH3C6H4I 4-FC6H4I 4-NO2C6H4I C6H5I
CH2CHCO2(n-C4H9) CH2CHCO2(n-C4H9) CH2CHCO2(n-C4H9) CH2CHCO2(n-C4H9) CH2CHCO2(n-C4H9) CH2CHCO2(n-C4H9) CH2CHCO2(n-C4H9) CH2CHCO2(n-C4H9) CH2CHCO2(n-C4H9) CH2CHCO2(n-C4H9) CH2CHCO2CH3 CH2CHCO2CH3 CH2CHCO2CH3 CH2CHCO2CH3 CH2C(CH3)CO2CH3
16
4-CH3C6H4I
CH2C(CH3)CO2CH3
17 18 19 20 21 22 23 24 25 26
C6H5I 4-CH3OC6H4I 4-BrC6H4I 4-NO2C6H4Br 4-NO2C6H4Br 4-NO2C6H4Br 4-CH3COC6H4Br 4-CH3COC6H4Br 3-CH3COC6H4Br 3-CH3COC6H4Br
CH2CHC6H5 CH2CHC6H5 CH2CHC6H5 CH2CHCO2(n-C4H9) CH2CHC6H5 CH2CHCO2CH3 CH2CHCO2(n-C4H9) CH2CHC6H5 CH2CHCO2(n-C4H9) CH2CHC6H5
Table 3. Pd-cr-CSM Catalyzed Heck Cross-Coupling of Various Aromatic Halides with Different Alkenes in the Aqueous Solutiona
entry
aromatic halide
alkene substrate
yieldb (%) 92 91 29 (trans) 41 (isomer) 87 75 83 81 84 18 31 16 27
1 2 3
C6H5I C6H5I C6H5I
CH2CHCO2(n-C4H9) CH2CHCO2CH3 CH2C(CH3)CO2CH3
4 5 6 7 8 9 10 11 12
2-CH3OC6H4I 4-NO2C6H4I C6H5I 4-CH3OC6H4I 4-BrC6H4I 4-NO2C6H4Br 4-NO2C6H4Br 4-CH3COC6H4Br 4-CH3COC6H4Br
CH2CHCO2(n-C4H9) CH2CHCO2CH3 CH2CHC6H5 CH2CHC6H5 CH2CHC6H5 CH2CHCO2(n-C4H9) CH2CHC6H5 CH2CHCO2(n-C4H9) CH2CHC6H5
a
Reaction conditions: 1 mmol of aromatic halide and 2 of mmol alkenes, 7.5 μmol of Pd-cr-CSM, 3 mmol of CH3CO2K base, 0.025 mmol of CTAB in 5 mL of H2O, and 0.2 mL of glycol at 90 °C for 12 h unless otherwise indicated. bGC-MS yield.
a
Reaction conditions: 1 mmol of aromatic halide, 2 mmol of alkene, 1.5 μmol of Pd-cr-CSM, 3 mmol of CH3CO2K base in a solution of 3 mL of DMSO and 0.2 mL of glycol at 110 °C for 5 h unless otherwise indicated. bGC-MS yield. cReaction time was 10 h.
Scheme 1. Mixture of trans/Regioisomer Coupling Products Obtained for the Heck Reaction of Aromatic Iodides with Methyl Methacrylate
Figure 8. Dependence of the cross-coupling yield on the recycling times for the Pd-cr-CSM catalyst in DMSO and aqueous solution.
No individual separated Pd black spots were found for the fresh Pd-cr-CSM membrane, suggesting that PdII cations dispersed well in the nanometer range in the chitosan network. 3.2. Pd-cr-CSM Catalyzed Heck Coupling of Aromatic Halides with Alkenes. The Heck cross-coupling reaction of aromatic halide with alkene is clearly one of the most studied reactions for carbon−carbon formation.1,42 Thus, we would like to use it as a model reaction to assess the catalytic activity and stability of the prepared Pd-cr-CSM catalyst. The initial reaction conditions were adapted from our previous studies,43−45 and the optimized conditions for the current catalytic systems were as follows: 1.0 mmol of iodobenzene, 2.0 mmol of n-butyl acrylate, 1.5 μmol of Pd-cr-CSM, and 3.0 mmol of KOAc in 3.0 mL of DMSO. Examination of entry 1 of Table 1 shows that an
consistent with the essentially same palladium content level on the surface and the cross-section (Figure 5) as obtained by EDX analysis. The uniform dispersion of the palladium species within the chitosan polymeric network system has been further confirmed by the HR-TEM observation as shown in Figure 6. 10045
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Figure 11. Lifetime spectra of the Pd-cr-CSM and pure CSM.
Figure 9. Dependence of Pd remaining percentage of the Pd-cr-CSM catalyst on the recycling times.
Table 4. Diameters of the Free Volume Hole in the CSM Membrane and Pd-cr-CSM Catalyst
excellent cross-coupling yield (87%) was obtained for the crosscoupling reaction of iodobenzene with n-butyl acrylate in DMSO even at the palladium catalyst loading of 0.15 mol %. A characteristic color change from pale yellow to black was observed for the Pd-cr-CSM catalyst during the coupling reactions, suggesting that the divalent PdII cations of the Pd-crCSM catalyst were in situ reduced to the reductive palladium species (Pd0) by DMSO solvent.43−45 No color change was observed for the Pd-cr-CSM catalyst when the reactions were performed in the nonreductive solvents such as xylene (entry 2) or dioxane (entry 3), and the related coupling products were formed in very low yields. However, the cross-coupling yields could be improved by the addition of a small amount of glycol (entries 5 and 6), accompanied by a similar color change of the Pd-cr-CSM catalyst as observed in DMSO. Interestingly, the addition of a small amount of glycol can also promote the reaction in DMSO (entry 4), resulting in a much quicker color change and a higher coupling yield. The reduction of the divalent palladium cations was further confirmed by XPS analysis of the recycled Pd-cr-CSM catalyst. Figure 7 shows that the palladium 3d5/2 electron binding energy of the recycled Pdcr-CSM catalyst is about 1.7 eV lower than that of the corresponding fresh Pd-cr-CSM catalyst, confirming the reduction of the divalent into the corresponding zero-valent palladium species.46,47 As shown in Table 2, the Pd-cr-CSM catalytic system works well for the Heck cross-coupling of various aromatic iodides having an electron-donating group (entries 2−5) or an electron-withdrawing group (entries 6−10) with n-butyl acrylate. High cross-coupling yield also occurred in the cases
sample
τ3 (ps)
D (Å)
Ds (Å)
pure CSM Pd-cr-CSM
953−2694 953−3248
3.153−6.827 3.153−7.582
5.045−12.131
Table 5. Molecular Size for Some Typical Substrates Used in This Study
Figure 10. HR-TEM images of the Pd-cr-CSM membrane after reuse for first run (A) and fifth run (B). 10046
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Scheme 2. Plausible Illustration of the Catalytic Reactions Process
neous catalyst in the aqueous solution may be associated with the unique amphiphilic chitosan molecular structure as the supporting solid matrix.50 3.3. Stability of the Pd-cr-CSM Heterogeneous Catalyst for the Heck Reactions. Heterogeneous catalyst has the advantage of easy separation and recycling from the reaction mixture, but the cost effectiveness and environmental impact from the heterogenization of homogeneous catalysts are highly dependent upon the catalytic activity and stability of the prepared heterogeneous catalysts. We again employed the Heck reaction of iodobenzene with n-butyl acrylate as the model reaction to assess the stability and reusability of the Pd-cr-CSM catalyst in both DMSO and aqueous solutions. No significant decrease of the coupling yields was observed for the recycled Pd-cr-CSM catalyzed Heck reaction after 12 times in DMSO solution or 7 times in aqueous solution (Figure 8). Clearly, with similar palladium catalyst loading amounts, the prepared Pd-crCSM heterogeneous catalyst can be recycled many more times without loss of the catalytic activity than many other polymerstabilized palladium heterogeneous catalysts,20,23−35 such as Pd supported on chitosan nanofiber mats (7 times),20 porous supports (6 times),23−28 and membranes (7 times).35 Because palladium leaching is usually the main reason for the loss of catalytic activity, we have determined the palladium contents of the freshly prepared as well as the recycled Pd-crCSM heterogeneous catalysts by means of ICP for further evidence of the high stability. Examination of Figure 9 shows that 99.3, 89.8, 76.7, and 63.4% of the palladium content remained after recycling for 1, 5, 9, and 12 times, respectively. There results clearly indicate that the palladium leaching is not significant for the Pd-cr-CSM catalysts during the catalytic reaction as well as the following workup processes. The remarkable stability of the Pd-cr-CSM heterogeneous catalyst can be mainly attributed to the strong chelation of the entrapped palladium species with the abundant carbonyl, amino, and hydroxyl functional groups of the surrounding chitosan matrix molecules. In addition, the high mechanical strength and thermal stability of the Pd-cr-CSM catalysts with no open porous structure favor the improvement of the resistance to harsh reaction conditions. These conclusions are consistent with the HR-TEM images of the Pd-cr-CSM catalysts shown in Figure 10. Examination of Figure 10 clearly shows that the size of the in situ generated black Pd0 nanoparticles is similar, that is, about 50 nm, for the first and fifth recycled Pd-cr-CSM catalysts. The essentially identical size and density of the palladium species for first and fifth recycled Pd-cr-CSM catalysts are consistent with the entrapped model for the Pd0 nanoparticles in the chitosan matrix molecules. It is
of aromatic halides with other alkenes such as methyl acrylate (entries 11−14), methyl methacrylate (entries 15 and 16), and styrene (entries 17−19). All of the cross-coupling reactions resulted in the trans cross-coupling products except in the cases of methyl methacrylate (entries 15 and 16) (as shown in Scheme1), which yielded a mixture of trans/regioisomer coupling products. The formation of regioisomers was attributed to the Pd−H elimination occurring on the methyl group with the formation of high yield of the regioisomers with the terminal alkene. The Pd-cr-CSM catalytic system has low activity for bromobenzene with alkenes, due to the greater strength of the carbon−bromine bond than of the carbon− iodine bond. Nevertheless, decent cross-coupling products were obtained for aryl bromides with strong electron-withdrawing group such as p-NO2 (entries 20−22), p-COCH3 (entries 23 and 24), and m-COCH3 (entries 25 and 26). Similar lower catalytic activities for the Heck coupling reaction of aryl bromides with alkenes were also found in many other polymerstabilized palladium heterogeneous catalysts,20,23−29 such as Pd supported on PVA nanofiber mats (
