Green synthesis of palladium nanoparticles via branched polymers: a bio-based nanocomposite for C–C coupling reactions

Abstract

Catalytic process is the key process for many chemical industries. In this study, a novel heterogeneous Pd (CMH–Pd(0)) has been prepared by the deposition of palladium nanoparticles (Pd NPs) onto the surface of carboxymethyl functionalized hemicelluloses using ethanol as solvent and in situ reducing agent. The as prepared catalyst was characterized by TEM, HR-TEM, XRD, FT-IR, TGA and XPS. The loading level of Pd in the CMH–Pd(0) catalyst was 0.38 mmol g⁻¹. The catalyst showed high catalytic activity and versatility towards Heck coupling reactions under aerobic conditions and could be readily recovered and reused in at least five successive cycles without obvious loss in activity. The catalyst is promising for its renewability, environmental benefits, efficient catalytic activity, mild reaction conditions, simple product work-up and easy catalyst recovery.

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Introduction

The awarding of Nobel prizes to the winners who developed cross-coupling catalysis reactions indicates the significance of these pioneering works as a landmark in organic chemistry and their great influence on material science.¹ Since the discovery of these reactions, they have been widely utilized in organic synthesis, especially, palladium occupies a special position of crucial importance in the field of catalysis for different organic transformations.^2–4 Recently, palladium nanoparticle catalysts have been used widely in various C–C coupling reactions.^5,6 Especially, palladium-catalyzed Heck cross-coupling reaction of aryl halides with olefins is found to be one of the most powerful synthetic methods for the formation of C–C bonds between alkenes and aryl or alkyl halides. These coupling products find good applications as intermediates in the preparation of materials, natural products, and bioactive compounds.⁷ Among the cross-coupling processes, the Heck reaction is especially widely used, because of its attractive attributes such as commercial availability, air and water stability and functional-group compatibility, and thus has been widely used in many industrial processes, especially in pharmaceuticals,⁸ fine chemicals,⁹ natural product synthesis,¹⁰ material science,¹¹ bioorganic chemistry¹² and conducting polymers.¹³

Traditionally, homogeneous catalyst system such as Pd(OAc)₂ or PdCl₂ was used for Heck reaction. Although, conventional catalytic systems have a series of advantages, the utilization of green, efficient chemistry approaches and reusable catalysts are of great importance in the light of contemporary design of synthetic processes. During Heck reaction, Pd(0) acts as active site to initiate the catalytic cycle, and the catalytic activity depends on the capacity of Pd(0) species to activate the carbon–halogen bond by oxidative addition, which can be further adjusted by the steric and electronic properties of the ligands attached to the nanoparticles. In this regard, phosphine ligands have been extensively utilized because of its effectiveness in stabilizing the Pd(0).¹⁴ However, despite general utility of the phosphine ligands, they are susceptible to oxidation during the reaction, which results in phosphine oxides and “palladium black”, and thus decreasing the catalytic activity. In addition, several other shortcomings such as the pollution of products, tedious work-up, high cost, and difficulty in separation and recovery of the catalyst from the reaction mixture always puzzled researchers.^2,10,15

In order to overcome these drawbacks, many researchers have made great efforts to develop environmentally benign Pd heterogeneous catalysts. Most of the reported supports for catalytic applications are based on various inorganic and organic materials, such as metal oxides,¹⁶ porous silicate,¹⁷ silica materials,¹⁸ clay,¹⁹ zeolites,²⁰ ionic liquid,¹⁵ carbon materials²¹ and synthetic polymers.²² However, the synthesis of these supports involves either high temperature calcinations or polymerization of petrochemical and expensive feed stocks. Taking into consideration of all these issues as well as the increasing environmental emphasis on materials and processes, tremendous efforts have been made to develop natural polymers supported catalysts.²³ Biopolymers are the most important environment-friendly resources with several interesting features, for example, renewability, high sorption capacity, stability of metal anions, and facile functionalization. Recently, Marzieh et al.²⁴ reported cellulose supported palladium(0) for Heck coupling reactions. Nicola and co-workers²⁵ utilized lignin as support for Heck, Suzuki and Sonogashira cross-coupling reactions in water with satisfactory product yields and selectivity. In addition, the nanopalladium-entrapped several other biopolymers, such as chitosan,¹ alginate,²⁶ gelatin,²⁷ starch,²⁸ cyclodextrin²⁹ and DNA³⁰ were fabricated as first successful achievements of such strategy and were further evaluated as the ideal supports for various catalytic applications.

Among various biopolymers, hemicelluloses are heteropolysaccharides with various branches. The units of main chain include xylose, glucose, uronic acid, etc.³¹ Hemicelluloses possess many hydroxyl groups, and therefore hemicelluloses can bind and stabilize heavy metal ions.³² In addition, as compared to cellulose, chitosan and other high crystalline biopolymers, hemicelluloses show unique features, for example, controlled surface chemistry, solubility in various solvents. Thus hemicelluloses have great potential to be modified and utilized. Our interest focuses on developing hemicelluloses based functional materials and especially, developing hemicelluloses or their derivatives as the natural ligands for palladium nanoparticle preparation to respond to the current call for environment friendly and sustainable green chemistry.³³ Carboxymethyl hemicelluloses (CMH) which have both carboxyl groups and hydroxyl groups are synthesized by hemicelluloses with monochloroacetic acid (MCA), and the reaction scheme is shown in Fig. 1. It has been reported that chelating ligands containing O atom result in more stable and efficient Pd catalysts.¹³ Thus, CMH can be used as excellent natural ligands for catalysts preparation. In this study, we focused on the preparation of hemicelluloses-based heterogeneous catalyst by the anchoring of palladium on the surface of CMH with O–O ligand interaction, where the oxygen on both hydroxyl groups and carboxy groups acted as ligand and stabilizer for Pd NPs, and its application in Heck cross-coupling reaction. The superiority of physical and chemical structure, convenience for preparation, natural polydentate ligand and minimization of catalysts cost made our ideal catalytic CMH-based nanocomposite exhibit relatively high catalytic efficiency, selectivity and stability in the C–C coupling reactions.

Fig. 1 Schematic illustration for the preparation of CMH.

Experimental section

Chemicals

All of reagents used including halides, olefins, palladium acetate, MCA and p-toluenesulfonic acid were purchased from Aladdin Reagent Co., Ltd., and were used without further purification. N,N-Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol (EtOH), acetonitrile, triethylamine and sodium hydroxide were purchased from National Medicine Group Chemical Reagent Co., Ltd. Hemicelluloses were prepared according to literature.³⁴ The sugar analysis showed the proportion of sugar composition (relative weight percent): 89.4% xylose, 5.80% arabinose, 1.90% glucose, 0.70% galactose, 1.80% glucuronic acid, and 0.60% galactose acid.

Preparation of CMH

The carboxymethylation of hemicelluloses was shown in Fig. 1. The typical procedure for the preparation of CMH was according to the literature,³⁵ with slight modifications. The details were shown in ESI.† And six samples were prepared by changing the reaction conditions (Table S1 in ESI†).

Preparation of CMH–Pd(0)

CMH powder (1.0 g) and palladium acetate (0.1 g) were suspended in 50 mL EtOH, and then Pd(II) in the slurry was reduced by EtOH at 60 °C for 12 h. After the reaction, the black slurry was filtered and washed for several times with ethanol and finally with diethyl ether and dried at 50 °C under vacuum to give the catalyst CMH–Pd(0) with a Pd loading of 0.38 mmol g⁻¹ calculated by inductively coupled plasma atomic emission spectroscopy (ICP-MS).

Characterizations

Fourier transform infrared spectra (FT-IR) were measured through a Bruker Tensor 27 FT-IR spectrometer. The thermal stabilities of CMH and CMH–Pd(0) catalyst were determined by TA Q500 thermogravimetric analyzer (TGA). The morphologies of the samples were recorded by a JEM-2100 (HR) transmission electron microscopy (TEM) working at 200 kV. The content of palladium in catalyst was determined by TEM-energy dispersive spectrometer (EDS) and ICP-MS using Agilent 7700 equipment. X-ray powder diffraction (XRD) patterns were collected using a Bruker-D8 Advance diffractometer with Cu Kα radiation (λ = 0.154 nm). X-ray photoelectron spectroscopy/ESCA (XPS) was recorded on Axis Ultra DLD instrument using Al Kα radiation (hν = 1486.6 eV) with contaminated C as an internal standard (C_1s = 284.6 eV). ¹H-NMR (600 Hz) and ¹³C-NMR (150 Hz) spectra were accumulated on a Bruker AVANCE III HD 600 spectrophotometer system.

General procedure for Heck coupling reaction

A mixture of aryl halide (1.0 mmol), olefin (1.2 mmol), triethylamine (2.0 mmol) and CMH–Pd(0) catalyst (50 mg, 1.9 mol% Pd) were added into a 35 mL pressure tube, 3 mL DMF was then added. The mixture was heated and stirred at 120 °C for 6 h. After the reaction was completed, the catalyst was filtrated and washed with ethyl actate for several times. Before extraction by ethyl actate for three times, the filtrate was diluted with water and the organic phase was dried over anhydrous Na₂SO₄ and then evaporated under vacuum to obtain crude product. The isolated product was collected, purified by flash column chromatography and characterized by ¹H-NMR and ¹³C-NMR.

General procedure for catalyst recovery

The catalytic activity of CMH–Pd(0) catalyst was examined after reaction. In a reaction vessel, iodobenzene (1.0 mmol), ethyl acrylate (1.2 mmol), triethylamine (2.0 mmol) and CMH–Pd(0) catalyst (50 mg, 1.9 mol% Pd) were mixed in DMF. The mixture was heated and stirred at 120 °C for 6 h. The heterogeneous catalyst was separated by filtration and washed with ethyl actate for three times, which followed by drying at 70 °C. The catalyst was directly used for the second run, and the same process was repeated for the next run.

Heterogeneity test

A mixture of iodobenzene (1.0 mmol), ethyl acrylate (1.2 mmol), triethylamine (2.0 mmol) and CMH–Pd(0) catalyst (50 mg, 1.9 mol% Pd) were added into a 35 mL pressure tube, 3 mL DMF was then added. The mixture was heated and stirred at 120 °C. After 2 h of reaction progress, the catalysts were separated by hot filtration. The filtrate was further reacted under the same conditions for another 4 h. The following process after the Heck reaction was performed by the same procedure mentioned in the general procedure for Heck coupling reaction.

Results and discussion

Characterization of CMH–Pd(0) catalyst

Fig. S1a† shows the typical absorption frequencies of hemicelluloses at 3431, 2920, 1629, 1465, 1250, 1169, 986, and 897 cm⁻¹.³⁵ Compared to the spectra of hemicelluloses, the peak at 1735 cm⁻¹ in Fig. S1b† is ascribed to the carbonyl vibration absorption, and the new bond at 1605 cm⁻¹ is attributed to –COO⁻ stretching vibration. The band at around 1419 cm⁻¹ is assigned to –CH₂ scissoring vibration.³⁵ Appearance of these bands suggests that the reaction between hemicelluloses and MCA occurs successfully. Interestingly, the FT-IR spectra of CMH–Pd(0) is similar to that of CMH scaffold in terms of the characteristic peaks, which indicates that the physical bonding exists between Pd NPs and CMH.³⁶

The ¹H-NMR spectrums of hemicelluloses and CMH are shown in Fig. S2 in ESI.† The major signals at 4.39, 4.00, 3.71, 3.47, 3.29, and 3.21 ppm in Fig. S2a† are corresponded to the H-1, H-5eq, H-4, H-3, H-5ax, H-2 of non-substituted β-D-Xyl, respectively.³⁷ In Fig. S2b,† the new band at 4.52 ppm is ascribed to the substituted carboxymethyl –CH₂–. These observations confirm the occurrence of carboxymethylation of hemicelluloses with MCA. Fig. S2c and S2d† show the representative ¹³C-NMR spectra of native hemicelluloses and CMH. The signals at 101.69, 76.37, 73.68, 72.72, and 62.98 ppm in Fig. S2c† are assigned to C-1, C-4, C-3, C-2, and C-5 of the β-D-xylpyranosyl units of hemicelluloses, respectively. Acetyl –CH₃ in xylan gives a signal at 23.28 ppm. The ¹³C-NMR spectrum of CMH shows the new signal at 176.12 ppm, which is assigned to the carboxyl groups. The signal at 69.94 ppm is ascribed to the methylene carbon atoms of carboxymethyl groups.³⁵ In conclusion, the results further indicated the reaction between hemicelluloses and MCA occurred.

The structure of CMH–Pd(0) was further studied by means of powder X-ray diffraction (XRD), as shown in Fig. 2. The broad peaks of CMH–Pd(0) were observed at 2θ = 40.0°, 46.1° and 67.5°, corresponding to (111), (200) and (220) crystalline planes of the face-centered cubic (fcc) lattice (PDF # 46-1043).³⁸

Fig. 2 XRD diffraction patterns of (a) CMH and (b) CMH–Pd(0) catalyst.

Fig. 3 displays the XPS spectra of CMH and CMH–Pd(0). Besides O and C elements in CMH, CMH–Pd(0) contains Pd element. The Pd 3d spectrum shows two typical peaks at 334.3 eV and 339.6 eV for 3d_5/2 and 3d_3/2, respectively, which correspond to Pd(0).³⁸ These results well agree with XRD results.

Fig. 3 XPS spectra of (a) CMH and (b) CMH–Pd(0) catalyst.

Fig. 4 showed the TEM images of CMH–Pd(0) before and after catalysis. A typical TEM image of CMH–Pd(0) was shown in Fig. 4a, which clearly showed the formation of metallic Pd nanoparticles with size range of 11–19 nm and the average particle size was about 15 and 16 nm. The freshly prepared CMH–Pd(0) particles were homogeneous on the CMH surface. High resolution transmission electron microscopy (HR-TEM) image of an individual Pd nanoparticle showed clear lattice fringes with an interplanar distance of approximately 0.22 nm, corresponding to (111) planes of Pd (Fig. 4d). There were clearly numerous crystal forms on the surface of the as-prepared CMH–Pd(0), as indicated by the corresponding electron diffraction pattern form (Fig. 4c).³⁹ The element Pd presented in the product was shown in Fig. 4e. All of these results clearly confirmed that the palladium complex was immobilized onto xylan molecular chains.

Fig. 4 (a) TEM images and histogram of the as-prepared fresh CMH–Pd(0) catalyst, (b) TEM images of CMH–Pd(0) after five cycles of Heck reaction, (c) electron diffraction pattern from the metallic Pd nanoparticles, (d) HRTEM image of CMH–Pd(0) catalyst and (e) EDS spectrum of CMH–Pd(0) catalyst.

Generally, heating is necessary in the Heck cross-coupling reaction. Hence, the thermal stability of CMH–Pd(0) catalyst was studied by TGA. As illustrated in Fig. 5, compared to CMH, the catalyst exhibited relatively good thermal stability up to 210 °C. Weight loss along with exothermic peak occurred when temperature was higher than 210 °C. With the increase of temperature, the decrease of catalyst weight was more obvious. The decomposition of CMH was complete when temperature was raised to 360 °C and the oxide of palladium was remained.²⁴

Fig. 5 TGA traces of (a) CMH and (b) CMH–Pd(0) catalyst recorded at heating rate of 15.00 °C min⁻¹ under nitrogen flow.

CMH–Pd(0) catalyzed Heck reaction

The Heck coupling reaction of iodobenzene with ethyl acrylate or styrene was utilized as a model reaction to investigate the catalysis performance of CMH–Pd(0), and the results were listed in Tables 1 and 2. The results indicated that solvent, catalyst loading, reaction temperature, time and DS of CMH were the key factors that significantly affected the catalytic efficiency of Pd NPs for the formation of C–C bonds. Subsequently, the influence of solvent on the reaction was investigated (Table 1, entries 1–5). Solvent screening gave a maximum yield of 74% in DMF (Table 1, entry 1). A relatively high yield of 84% for the cross-coupled direct arylation product could be obtained within 6 h (entry 16 in Table 1). However, H₂O and CH₃CN resulted in trace amount of product (entries 4 and 5). The moderate yields (53% and 48%) (entries 2 and 3) indicate that reactions of iodobenzene with ethyl acrylate in EtOH and DMSO were also not effective compared to DMF. The reasons may be explained as follows. On the one hand, CMH could dissolve in H₂O and DMSO, so the structure of CMH–Pd(0) was not stable when the reaction was carried out in H₂O or DMSO. On the other hand, without reflux system, 120 °C was over the boiling points of CH₃CN, EtOH and H₂O, which may not facilitate the reaction. Therefore, DMF was the optimum solvent.

Table 1 Optimization of the Heck reaction conditions^a

Entry	Catalyst (mg)/DS = 1.77	Solvent	Temp (°C)	Time (h)	Yield^b (%)
a Reaction conditions: 1.0 mmol iodobenzene, 1.2 mmol ethyl acrylate, 2.0 mmol triethylamine, 3 mL solvent. b Isolated yield was based on the iodobenzene.
1	50	DMF	120	8	74
2	50	EtOH	120	8	53
3	50	DMSO	120	8	48
4	20	CH3CN	120	8	8
5	50	H2O	120	8	4
6	0	DMF	120	8	Trace
7	12.5	DMF	120	8	60
8	25	DMF	120	8	67
9	75	DMF	120	8	73
10	100	DMF	120	8	74
11	50	DMF	60	8	16
12	50	DMF	80	8	65
13	50	DMF	100	8	67
14	50	DMF	140	8	64
15	50	DMF	120	4	83
16	50	DMF	120	6	84
17	50	DMF	120	10	75
18	50	DMF	120	12	71

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Table 2 Optimization of the DS of CMH for Heck reaction^a

Entry	DS	M w	Temp (°C)	Time (h)	Yield^b (%)
a Reaction conditions: 1.0 mmol iodobenzene, 1.2 mmol styrene, 2.0 mmol triethylamine, 3 mL DMF. b Isolated yield was based on the iodobenzene.
1	0	283 410	120	6	84
2	0.055	177 080	120	6	56
3	0.082	148 100	120	6	59
4	0.0534	183 250	120	6	54
5	0.109	113 650	120	6	62
6	0.177	82 727	120	6	77
7	0.24	51 383	120	6	90

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In addition, the effect of the catalyst loading on product yield was examined (Table 1, entries 6–10 and 1). When the amount of catalyst was reduced to 12.5 mg (0.48 mol%), a good yield 60% (Table 1, entry 7) was retained, indicating that CMH–Pd(0) was a relatively active catalyst for the direct coupling reactions of aryl halides with olefins. Controlling experiment demonstrated that no reaction could occur in the absence of the CMH–Pd(0) (entry 6 in Table 1), suggesting that the as-prepared CMH–Pd(0) catalyst actually acted as the real catalyst. The results showed that the optimal catalyst dosage was 1.9 mol%.

The optimal reaction temperature was established by monitoring the coupling reaction of iodobenzene with ethyl acrylate as a function of temperature from 60 °C to 140 °C (Table 1, entries 11–14 and 1). The reaction processed successfully and the yield exceeded 60% even at low temperature (80 °C, Table 1, entry 12). The yield of product increased when the reaction temperature was increased from 60 to 120 °C. Due to the side reactions the yield of product decreased slightly when the temperature was raised to 140 °C,²⁴ for further exploration, 120 °C was selected as the optimum temperature.

As shown in Table 2, the DS and M_w of CMH had significant influence on the catalytic activity of the palladium nanocomposites. Hemicelluloses was a kind of ideal support with the yield up to 84%, however, a good yield 90% was obtained when the DS of CMH was up to 0.24. With the increase of DS of CMH, the product yield of Heck reaction was increased, which indicated that carboxy groups have better ability for Pd NPs coordination and stabilization. However, the increase of DS of CMH resulted in the decrease of M_w which might further have great influence on the service life of the CMH–Pd(0).³⁵ Because of the decrease of M_w, the molecular chain and side chains of CMH were broken and shortened, the twisting capacity of CMH molecular chains for Pd NPs stabilization was impaired. Therefore, for further study, DS = 0.24 was chose as the typical DS.

The reaction scope was also investigated using a range of functionalized aryl halides and olefins under the optimized reaction condition, and the results were summarized in Table 3. In general, all the aryl iodides afforded the corresponding products in good to excellent yields ranging from 90% to 99%. As for different vinyl substrates, such as styrene, acrylic acid, methyl acrylate, ethyl acrylate and n-butyl acrylate resulted in high yields (Table 3, entries 4–8). For aryl bromides, the catalyst showed lower activity (Table 3, entries 3, 9, 11, 13, 15). Meanwhile, aryl bromides with electron-withdrawing groups afforded the corresponding products in better yield up to 87% and 94% (Table 3, entries 13 and 15). The coupling reactions of aryl chlorides with ethyl acrylate required extended reaction time and the yields of corresponding coupling products were unsatisfactory, because the oxidative-addition of C–Cl bond to catalyst species was usually difficult. Functional groups including methoxyl, methyl, carbonyl and nitryl are also tolerated. ¹H-NMR and ¹³C-NMR spectra of all products were confirmed. The results indicated that CMH–Pd(0) as a novel catalyst was a relatively good choice for Heck coupling reactions.

Table 3 Heck reactions of aryl halides with olefins^a

Entry	R1	X	R2	Yield^b/%	Product
a Reaction conditions: 1.0 mmol aryl halide, 1.2 mmol olefin, 2.0 mmol triethylamine, 50 mg CMH–Pd(0) (1.9 mol% Pd), 3 mL DMF, 120 °C, 6 h. b Isolated yield. c At 120 °C for 24 h.
1	H	I		97	3a
2	H	I		90	3b
3	H	Br		32	3a
4	p-CH3O	I		91	3c
5	p-CH3O	I		95	3d
6	p-CH3O	I		96	3e
7	p-CH3O	I		96	3f
8	p-CH3O	I		92	3g
9	p-CH3O	Br		34	3c
10	p-CH3	I		96	3h
11	p-CH3	Br		31	3h
12	p-CH3CO	I		94	3i
13	p-CH3CO	Br		87	3i
14	p-NO2	I		99	3j
15	p-NO2	Br		94	3j
16^c	p-NO2	Cl		63	3i
17^c	p-CH3CO	Cl		55	3j
18^c	p-CH3O	Cl		12	3c
19^c	p-CH3	Cl		Trace	3h

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Reusability of CMH–Pd(0) catalyst

The recyclability of the as-prepared CMH–Pd(0) catalyst was studied by using the reaction between iodobenzene and ethyl acrylate as a model reaction. The catalyst could be recovered by simple filtration. After being washed adequately by EtOH and dried in air, the catalyst could be reused for the next reaction, and the results are listed in Table 4. The yield of the product was decreased from 97% to 89% in the fifth run, and retained nearly 92% of its original reactivity, indicating that CMH stabilized Pd NPs catalyst was highly stable and recyclable. The decreasing activity of the catalyst upon recurrent usage can be explained by the fact that the aggregation of the Pd NPs (Fig. 4b). As we all known, Pd NPs are not stable and prone to aggregate because of their large surface cohesion energy. The formation of Pd NPs by reduction of Pd(II) is composed of two steps, that is, the nucleation and the growth of nuclei.⁴⁰ When the initial Pd nuclei is formed, the hydroxyl-groups and carboxyl of CMH participate in controlling growth of Pd nuclei. The strong bonding interaction between CMH and Pd atom prevents individual particle from aggregation, and thus giving well dispersed Pd NPs with small size. After five reaction cycles, the stability of CMH is decreased, thus the interaction between Pd NPs and CMH reduced. In this work, excellent activity and recyclability overcame the reused and environmental problems for homogeneous catalyst. Therefore, the wildly available and renewable raw materials, good catalytic properties and reuse performance can significantly decrease the overall cost, and improve the efficiency of the synthetic process for practical applications.

Table 4 Successive Heck reaction using recovered catalysts^a

Entry	Catalyst	Solvent	Temp (°C)	Yield^b (%)
a Reaction conditions: 1.0 mmol iodobenzene, 1.2 mmol ethyl acrylate, 2.0 mmol triethylamine, 3 mL DMF at 120 °C for 6 h. b Isolated yield was based on the iodobenzene.
1	H–Pd (50 mg)	DMF	120	97
2	H–Pd 1st reuse	DMF	120	95
3	H–Pd 2nd reuse	DMF	120	95
4	H–Pd 3rd reuse	DMF	120	90
5	H–Pd 4th reuse	DMF	120	87
6	CMH–Pd (50 mg)	DMF	120	97
7	CMH–Pd 1st reuse	DMF	120	96
8	CMH–Pd 2nd reuse	DMF	120	96
9	CMH–Pd 3rd reuse	DMF	120	94
10	CMH–Pd 4th reuse	DMF	120	89

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Heterogeneity of the CMH–Pd(0) during the Heck coupling reaction

In order to prove the heterogeneous nature of the CMH–Pd(0) and the absence of Pd leaching, the following reaction conditions were conducted. First, the standard reaction was processed at 120 °C for 2 h (the product yield was 75% checked by column purification of the product), and then CMH–Pd(0) was hot filtrated, the reaction mixture was left stirring at 120 °C for another 4 h. The product yield was only 80%. In another test, after full conversion (6 h, 97% yield), the filtrate was analyzed by ICP-MS, the results showed that the amount of palladium was 0.2 ppm. This indicated that the leakage of Pd NPs during catalytic experiments was negligible and the nature of reaction was heterogeneous.

Taking iodobenzene reacting with styrene as an example, we compared the results achieved in this work with other biopolymer-based catalysts supported catalysts for the Heck coupling reaction, and the results are listed in Table 5. As shown in Table 5, CMH–Pd(0) catalyst showed some extensive improvement in reaction conditions, such as reaction time, catalyst dosage and yield. For example, CELL–Pd(0), PdNPs@XH afforded a much longer reaction time and higher dosage of palladium catalysts, although high yield was obtained. In addition, the procedure for the preparation of PNP–SSS is complex and not green, in which thionyl chloride and chloroform were employed during the synthesis reaction, despite the yield was higher than CMH–Pd(0) achieved. Catalysis for the Heck reaction by utilizing renewable CMH–Pd(0) catalyst based on biomass presents an ideal chemical process. The CMH–Pd(0) catalyst is promising for its renewability, environment benefit, efficiently catalytic activity, thus has great potential to be applied into the industry processes.

Table 5 Catalytic performance of different Pd-based catalysts in the Heck reaction

Entry	Catalyst	Solvent	T (°C)	t (h)	Catalyst dosage (mol%)	Yield (%)	Ref.
a Palladium on calcium carbonate combined to 2-hydroxypropyla/b-cyclodextrins. b Cellulose supported Pd(0). c Pd nanoparticles supported on silica–starch substrate. d Xylan-type hemicellulose supported Pd(0). e Bacteria cellulose nanofibers supported Pd(0).
1	HPCD/Pd/CaCO3^a	H2O/DMF	110	24	1	80	2
2	CELL–Pd(0)^b	DMF	120	12	2.3	100	23
3	PNP–SSS^c	H2O	Reflux	1.5	1.2	95	28
4	PdNPs@XH^d	CH3CN	90	8	2	92	35
5	Pd/BC^e	DMF	120	8	0.1	87	37
6	CMC–Pd(II)	DMF	120	6	0.9	69	This work
7	H–Pd(0)	DMF	120	6	1.7	84	This work
8	CMH–Pd(0)	DMF	120	6	1.9	90	This work

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Conclusion

In summary, CMH–Pd(0) catalyst is a very active and stable bio-based nanocomposite catalyst for the Heck coupling reaction. Furthermore, the catalyst could be easily recovered and reused by simple filtration, which was perfect in terms of cost and environmental sustainability. The catalyst could be reused at least five times without considerable deactivation. Driven by all these advantages, CMH–Pd(0) catalyst will have great potential to be applied in pharmaceutical industry and other environment benign chemical industries.

Conflict of interest

The authors declare no competing financial interest.

Abbreviations

CMH	Carboxymethyl hemicelluloses
Pd NPs	Palladium nanoparticles
MCA	Monochloroacetic acid
DMF	N,N-Dimethylformamide
DMSO	Dimethyl sulfoxide
EtOH	Ethanol
CMH–Pd(0)	Carboxymethyl hemicelluloses with Pd(0) catalyst
FT-IR	Fourier transform infrared spectra
TGA	Thermal gravimetric analysis
TEM	Transmission electron microscopy
HR-TEM	High resolution transmission electron microscopy
XRD	X-ray powder diffraction
EDS	TEM-energy dispersive spectrometer
XPS	X-ray photoelectron spectroscopy/ESCA
ICP-MS	Inductively coupled plasma mass spectrometry

Acknowledgements

The project is supported by the National Natural Science Foundation of China (21404043, 31430092 and 21336002), Pearl River S&T Nova Program of Guangzhou (2014J2200063), Special Funds for Public Welfare Research and Capacity Building in Guangdong Province (2015A010105005), Research Fund for the Doctoral Program of Higher Education (201301721200240), Fundamental Research Funds for the Central Universities.

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Footnote

† Electronic supplementary information (ESI) available: Details for CMH preparation, FT-IR and ¹H NMR spectra of CMH, spectral data and copies of ¹H and ¹³C-NMR spectra for the synthesized compounds. See DOI: 10.1039/c6ra02242a

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