reduced graphene oxide microspheres supported Pd nanoparticles catalysts for Heck coupling reactions

Journal Pre-proof Preparation of porous chitosan/reduced graphene oxide microspheres supported Pd nanoparticles catalysts for Heck coupling reactions Xiu Zheng, Jing Zhao, Mengdie Xu, Minfeng Zeng

PII:

S0144-8617(19)31251-2

DOI:

https://doi.org/10.1016/j.carbpol.2019.115583

Reference:

CARP 115583

To appear in:

Carbohydrate Polymers

Received Date:

14 August 2019

Revised Date:

3 November 2019

Accepted Date:

6 November 2019

Please cite this article as: Zheng X, Zhao J, Xu M, Zeng M, Preparation of porous chitosan/reduced graphene oxide microspheres supported Pd nanoparticles catalysts for Heck coupling reactions, Carbohydrate Polymers (2019), doi: https://doi.org/10.1016/j.carbpol.2019.115583

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Preparation of porous chitosan/reduced graphene oxide microspheres supported Pd nanoparticles catalysts for Heck coupling reactions

Xiu Zheng, Jing Zhao, Mengdie Xu, and Minfeng Zeng*

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Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, College of

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Chemistry and Chemical Engineering, Shaoxing University, Shaoxing 312000, China

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Corresponding author, Email: [email protected], Fax: 8657588345682 1

Highlights 1. Novel porous Pd@CS/RGO microsphere catalyst was prepared. 2. Pd nanoparticles and layered RGO nanosheets dispersed well in CS matrix. 3. The catalyst showed remarkable catalytic performance in Heck reactions.

Abstract

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Novel porous chitosan/reduced graphene oxide microspheres supported Pd nanoparticles catalysts

(Pd@CS/RGO) were prepared by a combination of silica nanoparticles etching and freeze-drying treatments of CS/RGO/silica/PdCl2 composite microspheres. The microstructure of the

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Pd@CS/RGO microspheres catalysts have been investigated by X-ray photo electron spectroscopy

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(XPS), Raman spectroscopy, high resolution transmission electron microscopy (HR-TEM),

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thermo-gravimetric analysis (TGA), and X-ray diffraction (XRD), etc. The results revealed that: the novel catalysts showed open porous structure; CS had good miscibility with RGO nanosheets;

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Pd nanoparticles were well incorporated within CS/RGO matrix; the thermal stabilities of the catalysts were improved significantly over CS. Meanwhile, the Pd@CS/RGO catalysts have been

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demonstrated as highly active and easily recyclable catalysts for Heck reactions. The preparation process is simple, and the structure and performance of the catalytic material can be governed by

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changing the mass ratios of CS/RGO/silica/PdCl2 and the pore-forming process conditions.

Keywords: chitosan, reduced graphene oxide, palladium, catalysis

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Introduction The typical Heck coupling reaction refers to the coupling reaction between alkenes with organic moieties bearing suitable leaving groups such as halides, triflates, and diazonium functional groups (Mizoroki et al., 1971; Heck & Nolley, 1972). For the past decades, Heck coupling reactions have been frequently utilized in C-C bond formation to generate natural products, biologically active molecules, and fine chemical products, and so on (Ghosh et al., 2019;

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McGlacken & Bateman，2009; Balanta et al., 2011; Jagtap 2017). Generally, homogeneous Pd catalysts are required for the proceeding of the reactions. However, homogeneous Pd catalysis

systems have several disadvantages, such as necessary of addition of various ligands, difficulties

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in the recycling of the expensive Pd catalyst, Pd contamination in the products, etc. Therefore, for green, economy, and safety considerations, it is desirable to develop facile and efficient methods

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for Heck reactions using heterogeneous Pd catalysts (Nasrollahzadeh et al., 2017; Roy & Uozumi,

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2018; Liu & Astruc, 2018). The heterogeneous Pd catalysts are usually prepared via immobilization of Pd nanoparticles on suitable solid supports, and they can facilitate easy

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separation and recycling of both the catalyst and product from the reaction mixture. Among various supports, for the containing of amounts of polar functional groups (such as

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amino groups and hydroxyl groups) on the molecular backbone, natural polysaccharide of

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chitosan (CS) is considered as one of the most excellent candidates due to its strong complexation capability with transition metals (Molnár, 2019; Berillo & Cundy 2018; Levy-Ontman, et al., 2018; El Kadib, 2015; Guibal, 2005). Moreover, CS is easy to process into different forms, such as films, microspheres, and fibers, and so on. Therefore, Pd supported on CS-based materials heterogeneous catalysts have attracted more and more attentions both in academic and industrial fields. However, the mechanical strength and stability of CS is limited. Under actual harsh 3

reaction conditions and long-time recycling process, such as high temperature, continuous stirring, solvent swelling, etc., the catalysts can be only recycled for limited times (Baran et al., 2018; Zeng et al., 2014; Bradshaw et al., 2011). For these reasons, it is necessary to explore modified CS supports with much improved comprehensive properties. Recently, graphene family nanomaterials, such as graphene (GR), graphene oxide (GO), and reduced graphene oxide (RGO), are considered as the novel nanoscale reinforcing modifiers for

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construction of the next generation of nanocomposite materials (Papageorgiou et al., 2017; Wang et al., 2017; Das & Prusty, 2013; Wan et al., 2016). GR is an exciting and advanced carbon

nanomaterial that consists of sp2 carbon atoms covalently bonded in a honeycomb crystal lattice,

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having excellent optical, electrical and mechanical properties, etc. GO is usually prepared by the

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oxidation of graphite to graphite oxide and followed by the subsequent chemical, mechanical or

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thermal exfoliation of graphite oxide to graphene oxide sheets. Besides the high specific surface area, it is rich in oxygen-containing functional groups such as hydroxyl groups, carboxyl groups,

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epoxy groups and carbonyl groups, and easily complexes with transition metals, organic molecules, polymers, and so on. Due to the destroying of the conjugated structure, the electrical

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conductivity and mechanical properties of GO are obviously lower than GR. And GO is often considered as a surface-functionalized of GR. GO can be readily converted into reduced RGO via

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chemical, thermal, and light reductions. Usually, RGO is a partially reduced product of GO, having higher electrical conductivity and mechanical property than GO. Functional groups bound to the surface of GO/RGO may improve the interfacial interactions between GO/RGO and CS matrix (Zhang et al., 2018; An et al., 2018; Sivashankari et al., 2018; Han et al., 2011). In addition, GO and RGO themselves have been proven as excellent supporting materials for many metallic 4

nanocatalysts, like Pd, Au, Fe3O4 and etc (Navalon et al., 2016; Nasrollahzadeh et al., 2018). It is mainly attributed to their unique structures, good chelation capabilities, high surface areas, high thermal stabilities, excellent solvent-resistant capabilities, feasible loading capacity of metallic nanocatalysts, and so on. Therefore, it will be interesting to prepare novel CS/graphene family nanomaterials (GR, GO, and RGO) for immobilization of Pd nanoparticles with synergistically improved properties. To date, some studies have been reported on the CS/graphene family

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nanomaterials (GR, GO, and RGO) composites as supports for Pd nanoparticles used as effective biosensors ( Wang et al., 2018; Sun et al., 2012; Jin et al., 2011; Wang et al., 2013). However, few studies are available on application of them in organic coupling reactions.

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In this study, one of the graphene family nanomaterials, RGO was selected as the modifier

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for CS to construction a novel Pd@CS/RGO heterogeneous catalyst with excellent comprehensive

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properties for Heck coupling reactions. Effective immobilization of Pd nanoparticles on CS/RGO supports will result in the advantages of high dispersion degree of metal particles, high activity,

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and low loss of metal species during recycling process. CS-based supports can be easily fabricated into various forms such as membranes, fibers, and microspheres (Guibal, 2005). The heterogenous

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Pd supported on CS-based supports catalysts is often preferentially designed in the form of microsphere for its core characteristics (Wang, et al., 2015; Molnár, 2019), including simple

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design, easy recovery from reaction mixture, high activity, excellent stability, and easy scale up, etc. Porogen etching is one of the convenient and most-used methods to prepare CS-based microspheres with porous structure. Inorganic silica particles (etchable in NaOH or HF solution) and water soluble polymers (like polyethylene glycol, polyvinyl pyrrolidone, polyvinyl alcohols etc.) are often used as porogens (Zeng & Ruckenstein, 1996, 1998, 1999; Zeng, et al., 2012a, 5

2012b). However, for the remarkable chain shrinkages of the CS macromolecules during drying, the induced open pores would then even change to closed (Zeng, et al., 2012b). Recently, without using traditional porogen etching method, novel CS/GO porous microspheres have been successfully prepared for functional materials applications. For example, using supercritical CO2 drying method, El Kadib et al. fabricated CS-GO high macroporous microspheres with improved hydrothermal stability and chemical stability (Frindy et al., 2017). Yu et al. prepared GO/CS

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aerogel microspheres with honeycomb-cobweb and radically oriented microchannel structures by

electrospraying and freeze-casting (Yu et al., 2017). However, the above literatures approaches often require special conditions, such as low temperature, high pressure, and complex

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instrumentation, etc. In our opinion, a combination of two or more simple pores-forming

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techniques might be another effective way in developing CS/RGO porous microspheres with low

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instrumentation requirements and suitable for large-scale production. Freeze-drying is another frequently-used and convenient pores-forming method for polysaccharide hydrogels. Herein, a

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combination of silica-porogen etching and freeze-drying methods has been adopted to prepare Pd@CS/RGO composite microspheres with porous structure. The microstructure of the prepared

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Pd@CS/RGO catalysts were investigated with several techniques, such as scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), X-ray

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diffraction (XRD), X-ray photo electron spectroscopy (XPS), Raman spectroscopy, and thermo-gravimetric analysis (TGA), etc. The catalytic performances of the prepared Pd@CS/RGO catalysts applied in Heck coupling reactions were investigated.

2. Experimental 6

2.1 Material CS (pharmaceutical grade) was supplied by Zhejiang Aoxing Biotechnology Co., Ltd., China. The deacetylation degree of CS was 95% and its molecular weight was 1.2 × 105. RGO nanosheets (SE1430 type, with specific surface area of 206.3 m2/g and C/O mass ratio of 4.7/1) were supplied by the Sixth Element (Changzhou) Materials Technology Co. Ltd., China. Colloidal silica solution (Ludox AS-40, 40 wt %) was supplied by Shanghai Sigma-Aldrich trade Co., Ltd.,

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China. PdCl2 (analytical purity grade) was supplied by Zhejiang Metallurgical Research Institute

Co., Ltd., China. The reaction substrates used in the Heck coupling reactions were supplied by Energy Chemical, Sun Chemical Technology (Shanghai) Co., Ltd. China. The other used

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chemicals and solvents were all in analytical purity grade, and they were supplied by Sinopharm

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Chemical Reagent Co., Ltd. China.

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2.2 Preparation of the Pd@CS/RGO microsphere catalysts

The preparation process can be illustrated in Scheme S1. And the detail procedure is as

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follows. Firstly, 2 g of CS was dissolved in 100 mL of 2 wt% acetic acid to form homogeneous solution under magnetic stirring. Then, certain amounts of RGO (RGO content: 0%, 5%, and 10%)

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was mixed with the CS solution under ultrasonic dispersion for 1 h. Afterwards, certain amount of colloidal silica solution (mass ratio of CS/silica: 1/0, 1/3, and 1/6) was added drop-wise to the

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CS/RGO mixture. After magnetic stirring for 2 h, 2.5 mL of the PdCl2 solution was drop wisely added to the CS/RGO/silica mixture to obtain homogeneous CS/RGO/silica/PdCl2 blend suspension. The PdCl2 solution was prepared by dissolving of PdCl2 (0.3 g) and NaCl (about 2 g) in 100 mL deionized water. The resulting CS/RGO/silica/PdCl2 blend suspension was drop wisely added to a 50 mL NH3·H2O (25 wt%) solution through a 0.7 mm syringe needle, forming 7

composite microspheres due to the precipitation of CS in basic bath. The obtained microspheres were washed with deionized water and dried at 60 °C. The dried CS/RGO/silica/PdCl2 microspheres was treated with NaOH (5 wt%) bath at 80 °C for about 8 h for etching off the silica-porogens. After washing with distilled water until neutral, the etched microspheres were cooled at a controlled temperature of -20 °C in a refrigerator for 24 h. Finally, the frozen microspheres were freeze-dried with a FD-1A-50 vacuum freeze dryer (Boyikang Experimental

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Instruments Co. Ltd., Beijing, China) for 24 h. After freeze drying, the novel porous Pd@CS/RGO microsphere catalysts were reduced by ethanol and air-dried. As shown in Table 1, the prepared porous Pd@CS/RGO microsphere catalysts were labeled according to the mass ratio of

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CS/RGO/silica. Meanwhile, the content of Pd in the resulting Pd@CS/RGO microspheres

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catalysts as determined by ICP-AES was found to be 0.27-0.34 wt% (as shown in Table 1). As the

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RGO and silica percentage increase, the relative content of Pd in the catalysts shows a slight decrease.

Table 1. Each prepared Pd@CS/RGO microsphere catalyst was labeled according to the mass ratio. Mass ratio of CS/silica

Pd content a

Pd@CS/RGO-5%-0

5%

1/0

0.34

Pd@CS/RGO-5%-3

5%

1/3

0.32

Pd@CS/RGO-5%-6

5%

1/6

0.29

Pd@CS/RGO-10%-0

10%

1/0

0.34

Pd@CS/RGO-10%-3

10%

1/3

0.31

Pd@CS/RGO-10%-6

10%

1/6

0.27

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RGO loading percentage

Catalysts

a

from ICP-AES determination

2.3 Characterizations of the Pd@CS/RGO microsphere catalysts XRD patterns of the Pd@CS/RGO microsphere catalysts were recorded with a PANalytical 8

Empyrean X-ray diffraction system (Netherlands). The XRD testing conditions were set as follows: scanning rate of 2°/min, 2θ range from 5° to 80°. Raman spectra of the Pd@CS/RGO microsphere catalysts samples were recorded with a Renishaw’s inVia Raman Microscope (UK). XPS analysis of the surface microstructure information of the Pd@CS/RGO microsphere catalysts samples were performed with a thermo Scientific ESCALAB 250Xi X-ray photo-electron spectrometer (US). The morphology and elements analysis of the Pd@CS/RGO microsphere catalysts samples were

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examined by means of a JEM-6360 (Japan) scanning electron microscope (SEM) equipped with

an energy dispersive X-ray spectroscopy (EDS, Oxford EDX System). Before SEM observation, the samples were coated with thin layer of Pt to improve the conductivity of the samples. The

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morphology of the Pd@CS/RGO microsphere catalysts samples were further observed with a

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JEM-2100F (Japan) high resolution transmission electron microscope (HR-TEM). Before

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HR-TEM observation, the Pd@CS/RGO microspheres were firstly grounded to powders and

dispersed sonically in 5 mL ethanol. Then, the suspension was dropped on a copper net and dried.

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N2 adsorption-desorption isotherms (at 77 K) of the Pd@CS/RGO microsphere catalysts samples were determined with a Micromeritics TriStar II series surface area and porosity analyzer (US).

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The thermo gravimetric analysis of the Pd@CS/RGO microsphere catalysts samples were performed with a Mettler Toledo TGA/DSC 2 STAR system (Zurich, Switzerland). The TGA

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testing conditions were set as follows: scanning rate of 20 °C/min, from 30 to 700 °C, air atmosphere. The Pd content within the Pd@CS/RGO microsphere catalysts were determined by a Leemann ICP-AES Prodigy XP inductively coupled plasma-atomic emission spectrometer (US). 2.4 Catalytic application of the Pd@CS/RGO microsphere catalysts in Heck coupling reactions

In a 50 mL reaction tube, a mixture of 1 mmol of aromatic halide, 2 mmol of n-butyl acrylate, 9

0.08g of Pd@CS/RGO microspheres catalysts (containing about 2 μmol of Pd), 3 mmol of potassium acetate, solvents (3 mL of dimethyl sulfoxide (DMSO) + 0.2 ml ethylene glycol) was stirred at 110 °C (heated in oil bath) for 5 h. The progress of the Heck coupling reactions was monitored by thin layer chromatography (TLC) method. The chemical structure and yield of the Heck coupling reaction products were confirmed with H1NMR (Bruker Avance III 400 NMR analyzer, Bruker Inc., Switzerland) and gas chromatography-mass spectrometry (GC/MS) analysis

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(Agilent 6890N/5975 MSD GC/MS, Palo Alto, CA, USA) equipped with an Agilent HP-5

capillary column (30m×0.25mm×0.25μm). Proton NMR spectra of the coupling products were recorded in CDCl3 on the NMR analyzer (400M Hz). The oven-temperature program in GC/MS

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spectroscopy analysis was initially set as 80 °C and ramped to 260 °C at a rate of 10 °C/min, and

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maintained for 2 min at every step. The GC/MS yield is based on the 1 mmol of aromatic halides

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consumption using peaks area normalization treatment of the aromatic halides and the corresponding coupling products. The characterization results of the coupling products are shown

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in the supporting information file, and which is consistent with our recent work (Zeng et al., 2014, 2016). In the recycling experiments, the Pd@CS/RGO microsphere catalysts were separated from

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the reaction mixture via simple filtration. Then the microspheres were rinsed with ethanol for 3

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times and dried. Finally, they were reused in a next run of Heck coupling reaction.

3.

Results and discussions

3.1 Preparation and characterization of Pd@CS/RGO microspheres catalysts The porous structure of the prepared Pd@CS/RGO microspheres catalysts were powerfully confirmed with the SEM observation. It is well known that both silica porogen etching and 10

freeze-drying are effective methods to induce porous structure for polymeric materials. However, as shown in Fig.S1, after etching with NaOH, the air-dried Pd@CS/RGO microspheres show dense structure with almost no pores. It means that the open pores of the microspheres formed by silica etching undergo closing because of the shrinkage of the materials during air-drying. However, freeze-drying method can greatly reduce the closure of the open pores due to the little shrinkage of the polymeric materials. At the same time, besides the open pores formed by etching

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of silica porogens, numerous new pores can be induced by the sublimation of the frozen H2O molecules in the microspheres. As shown in Fig. 1, regardless of the addition of silica porogens, all the samples show open porous structure both on the outer surface and cross section of the

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microspheres. The pore size on the outer surface (5-10 μm) is much bigger than that of cross

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section (below 1 μm). Unfortunately, all the prepared Pd@CS/RGO microspheres catalysts

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showed low BET specific surface area value (<10 m2/g) with N2 adsorption-desorption isotherms analysis. This phenomenon is similar with the porous CS-based microspheres prepared by other

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processes, such as thermally induced phase separation (about 30 m2/g) (Li, et al., 2017), polymers porogen etching (< 20 m2/g) (Zeng, et al., 2012a, 2012b), freeze-drying (also <10 m2/g) (Ma, et al.,

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2017). This may be due to the large size (above nano level) and insufficient interconnectivity of the induced pores. An EDX-scanning result of the Pd@CS/RGO-5%-3 (Fig. 1G) shows that the

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microspheres are mainly composed of C (from CS and RGO), N (from CS), O (from CS and RGO), Na (from added Na+ during Pd immobilization), Si (from residue silica particles after effectively etching), and Pd (from successful Pd immobilization). Clearly, the Pd content from EDX scanning is consistent with the ICP-AES determination results.

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Fig. 1. SEM images of the Pd@CS/RGO microsphere catalysts: A. outer surface Pd@CS/RGO-5%-0; A’. cross section of Pd@CS/RGO-5%-0; B. outer surface Pd@CS/RGO-5%-3; B’. cross section of Pd@CS/RGO-5%-3; C. outer surface Pd@CS/RGO-5%-6; C’. cross section of Pd@CS/RGO-5%-6; D. outer surface Pd@CS/RGO-10%-0; D’. cross section of Pd@CS/RGO-10%-0; E. outer surface Pd@CS/RGO-10%-3; E’ cross section of Pd@CS/RGO-10%-3; F. outer surface Pd@CS/RGO-10%-6; F’. cross section of Pd@CS/RGO-10%-6; G. EDX scanning results of cross section of Pd@CS/RGO-5%-3.

of of of of of of the

Fig.2 shows the XRD pattern of pure CS, RGO, and Pd@CS/RGO prepared with different 12

addition of RGO and silica amounts. For the pure RGO sample, a broad reflection peak centered at 2θ=23.2° appears, indicating the RGO nanosheets are stacked in several layers. According to the Bragg equation, the interlayer distance of the RGO nanosheets can be estimated as 0.38 nm. For the pure CS samples, two broad reflection peaks at 2θ=19.9°, 10.5° are attributed to the crystallization of CS. For all the Pd@CS/RGO samples, a combination of the reflection peaks of CS and RGO is found. And obviously, the character peak of RGO shifts to lower 2θ value of 21.7°.

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The interlayer distance of RGO in the Pd@CS/RGO samples can be estimated as 0.41 nm, which is a bit larger than that of pure RGO. It suggests that a number of polar groups (-OH and/or NH2

groups) of CS molecules form strong interactions (including H-bonding) with oxygen-containing

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functional groups of RGO. Meanwhile, it is found that the reflection peak at around 19.9°

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(attributed to the crystalline of CS) of Pd@CS/RGO-5%-3 (Fig. 2C), Pd@CS/RGO-5%-6 (Fig.

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2D) are much stronger than that of Pd@CS/RGO-5%-0 (Fig. 2B). Similarly, the reflection peak at around 19.9° of Pd@CS/RGO-10%-3 (Fig. 2F), Pd@CS/RGO-10%-6 (Fig. 2G) are much

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stronger than that of Pd@CS/RGO-10%-0 (Fig. 2E). These phenomenons suggest that the regularity of the CS macromolecules stacking (i.e. crystallinity) increases after the silica porogens

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etching. During the silica etching by NaOH at 80 °C for 8h, a number of CS macromolecules might undergo regular rearrangements due to the enough spaces left by the etched silica porogens.

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However, in the cases of Pd@CS/RGO-5%-0 and Pd@CS/RGO-10%-0 with no porogens etching, it is difficult for CS macromolecules to rearrange due to the density structure.

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Fig.2 XRD patterns of the Pd@CS/RGO microsphere catalysts: A. RGO; B. Pd@CS/RGO-5%-0; C. Pd@CS/RGO-5%-3; D. Pd@CS/RGO-5%-6; E. Pd@CS/RGO-10%-0; F. Pd@CS/RGO-10%-3; G. Pd@CS/RGO-10%-6; H. CS.

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Fig. 3 shows TGA curves of Pd@CS/RGO-5% (A) and Pd@CS/RGO-10% (B) series microsphere catalysts with different silica porogen loading. For all the samples, weight loss

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around 100 °C is due to the bonded water evaporation. And the ending mass ratio of all the Pd@CS/RGO microsphere samples are close to 0%, confirming that the added silica porogens are

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almost totally etched off. The beginning decomposition temperature of CS is found at around

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234 °C. As a kind of carbon material, RGO has much better thermal stability with a beginning decomposition temperature at around 487 °C. In the Pd@CS/RGO-5% series, the TGA curve of Pd@CS/RGO-5%-3 almost overlaps with that of Pd@CS/RGO-5%-6. Both samples show improved beginning decomposition temperature of 274 °C (about 40 °C higher than CS). Meanwhile, it is found that the TGA curve of Pd@CS/RGO-5%-0 is almost overlapped with pure CS. Similar phenomenon is found in the series of Pd@CS/RGO-10% with different adding 14

amounts of silica porogens. The reason might be explained in the view of different crystallinity as confirmed in XRD characterization. Higher crystallinity is advantageous for better thermal stability.

Considering

the

former

characterization

results,

Pd@CS/RGO-10%-3

and

Pd@CS/RGO-10%-6 with better comprehensive physical properties are selected as the candidate

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catalysts for the further application assessments in Heck reactions.

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Fig. 3 TGA curves of the Pd@CS/RGO microsphere catalysts: (A) Pd@CS/RGO-5%; (B) Pd@CS/RGO-10%.

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The HR-TEM characterization can demonstrate the morphology and internal microstructure of the Pd nanoparticles and RGO nanosheets within Pd@CS/RGO composite in detail. Fig. 4

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shows the HR-TEM observation results of Pd@CS/RGO-10%-3 microsphere catalyst with different magnifications. As seen from Fig. 4A, many Pd0 nanoparticles (sized 5-10 nm) and RGO

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nanosheets disperse well in the CS matrix. The lattice fringes of Pd0 nanoparticle can be clearly

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seen from Fig. 4B, further confirming the successfully immobilization of Pd species within the CS/RGO composite microspheres. Meanwhile, multilayers (10-15 layers) of RGO nanosheets with wrinkles are clearly visualized in Fig. 4C. The thickness of about 12 layers of RGO nanosheets is around 0.00502 μm, indicating that the average interlayer distance of RGO nanosheets is about 0.418 nm, which is well consistent with the XRD characterization results.

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ro of -p re lP na ur Jo Fig. 4 HR-TEM images of the Pd@CS/RGO-10%-3 microsphere catalyst with different magnification: A. 100,000×; B. 800,000×; C. 400,000×. Fig. 5 shows the Raman spectra of the RGO and Pd@CS/RGO microsphere catalysts samples. RGO sample shows D band at 1346 cm-1 and G band at 1589 cm-1, respectively. The D 16

band is attributed to the sp2-derived carbons in the imperfect aromatic structure, indicating the presence of defects on the surface of RGO nanosheets due to disrupting the C=C groups by oxygen containing groups. The G band is attributed to the in-plane vibrations of graphite carbons. And the ID/IG ratio implies the disordered degree of the carbon materials. The ID/IG ratio of RGO is found as 2.1, indicating a disordered carbon-rich microstructure. This ratio of the prepared Pd@CS/RGO-10%-3 and Pd@CS/RGO-10%-6 microsphere catalysts is found a bit decrease to

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1.8 and 1.9, respectively. This phenomenon might be related to the formation of strong interaction

between the oxygen containing groups of RGO and polar groups (-OH and –NH2 groups) of CS

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matrix.

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Fig. 5 Raman spectra of the Pd@CS/RGO-10%-3 microsphere catalysts. Fig. 6 shows the C1s, O1s, N1s, and Pd3d XPS spectra of CS, RGO, and Pd@CS/RGO-10%-3

samples. In Fig. 6(A), the C1s peaks of CS are observed at 284.6 eV (attributed to C-C), 286.0 eV (attributed to C-N, C-O), and 287.6 eV (attributed to C=O), respectively. As for RGO, the C1s peaks of C-C, C-O, and O-C=O are observed at 284.6 eV (almost same with that of CS), 286.0 eV (almost same with that of CS), and 288.5 eV, respectively. The C1s peaks of Pd@CS/RGO-10%-3 17

samples are almost a merger of the peaks of CS and RGO except a distinct shift of C1s peak of O-C=O to 289.0 eV. It suggests the formation strong interactions of the O-C=O groups on the surface of RGO nanosheets with polar groups of CS molecules. In Fig. 6(B), the O1s peaks of C-O and C=O in CS are observed at 531.9 eV and 532.6 eV, respectively. As for RGO, two split peaks at 531.6 eV and 533.0 eV are attributed O1s peaks of C-O and O-C=O, respectively. The O1s peaks of Pd@CS/RGO-10%-3 are observed at 531.4 eV, and 532.3 eV, respectively, which should be

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attributed to the combination of the O1s peaks of CS and RGO. The N1s peak at 398.8 eV is

attributed to amine or amide groups of CS molecules. The Pd3d peaks of Pd@CS/RGO-10%-3 sample are observed at 334.9 eV (attributed Pd0) and 336.3 eV (attributed to Pd2+), respectively

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(Wagner et al., 1979). The ratio of Pd0 and Pd2+ mixture is roughly 0.42，indicating the presence of

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29.5% of Pd0 and 70.5 of Pd2+ in the fresh (original) Pd@CS/RGO-10%-3 catalyst.

Fig. 6 The C1s, N1s, O1s, Pd3d XPS spectra of Pd@CS/RGO-10%-3 microsphere catalyst. 18

3.2 Catalytic performances of Pd@CS/RGO microspheres catalysts in Heck reactions The Heck coupling reaction between iodo benzene and n-butyl acrylate was used as a model reaction to evaluate the catalytic performances of the prepared Pd@CS/RGO microspheres catalysts. Pd supported on air-dried Pd@CS/RGO-10%-3 with no pore structure was used for comparison. As shown in Fig. 7A, both Pd@CS/RGO-10%-3 and Pd@CS/RGO-10%-6 exhibit much higher catalytic activities for the reaction than that of air-dried Pd@CS/RGO-10%-3,

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indicating that porous structure is advantageous for better catalytic performance. Although the amount of silica porogen added in the case of Pd@CS/RGO-10%-6 is twice that of

Pd@CS/RGO-10%-3, their catalytic activities are almost the same (99% yield). And the

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microsphere catalysts are convenient in separation out the reaction mixture for use in next reaction

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run. Fig. 7B shows the recyclability of such two prepared catalysts. Obviously, the recycling

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capability of the Pd@CS/RGO-10%-3 catalyst (can recycle 13 times) is better than that of the Pd@CS/RGO-10%-6 (can recycle 9 times). Theoretically, the more the silica porogens is added,

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the more the micro-defects are induced. Micro-defects are usually disadvantageous for good mechanical properties. As confirmed in Fig. S2, the loss of the Pd@CS/RGO-10%-6 catalyst is

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faster than Pd@CS/RGO-10%-3 during recycling process. In the case of Pd@CS/RGO-10%-6, the poorer recyclability should be much related with its poorer mechanical stability due to more

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defects induced. In other words, in the case of Pd@CS/RGO-10%-3, the more excellent catalytic performance both in activity and recyclability might be attributed to a better balance between the porous structure and mechanical stabilities. The recycled Pd@CS/RGO-10%-3 microsphere catalyst (see Fig. S3) shows little changes in morphology compared with the fresh Pd@CS/RGO-10%-3 microsphere catalyst, confirming the good stability of the catalyst. ICP-AES 19

determination of Pd content of the 13 times recycled Pd@CS/RGO-10%-3 microsphere catalyst is about 0.19 wt% (about 60% of Pd retained), indicating the slow leaching of Pd species during recycling. Meanwhile, it is found that the mixture ratio of Pd0 to Pd2+ changes to 0.13 (i.e. 11.5% of Pd0 and 88.5% of Pd2+) for the recycled Pd@CS/RGO-10%-3 microsphere catalyst (see Fig. S4). It indicates that the Pd0 loss might be the main causes of the progressive loss of catalytic efficiency during recycling. On the one hand, both CS and RGO have good chelation abilities with

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re

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significantly improved after modification of RGO nanosheets.

ro of

Pd species. On the other hand, the mechanical and thermal stabilities of CS matrix are

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Fig. 7 Heck coupling yield of the model reaction between iodo benzene and n-butyl acrylate vs reaction time (A) and recycling times (B) catalyzed with Pd@CS/RGO microsphere catalyst. Reaction conditions: 1 mmol of iodo benzene, 2.0 mmol of n-butyl acrylate, 3 mmol of CH3CO2K base, 0.002 mmol of Pd microspheres catalysts in a solution of 3.0 ml DMSO and 0.2 ml glycol at 110 °C for 5h. It is well known that the comprehensive properties of a heterogeneous catalyst can be well

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illustrated in terms of turn over number (TON), turn over frequency (TOF), and number of recycles (Molnár & Papp, 2017). Because of different experimental procedures, it should be difficult to compare the activities and recycling capabilities of different catalysts. Nevertheless, as shown in Table 2, the Pd@CS/RGO-10%-3 microsphere catalyst was compared with previous reported catalysts for similar Heck reaction (iodobenzene coupling with acrylate or acrylic acid). 20

Clearly, the values of TON, TOF and number of recycles of the Pd@CS/RGO-10%-3 microsphere catalyst (entry 6) are larger than our recent prepared CS-based porous microsphere or membrane catalysts (entry 1 and 2), indicating improved comprehensive properties. Due to higher specific surface area and mechanical stability, the previous Pd@MMT/CS catalyst (entry 3) shows reasonably larger values of TOF and number of recycles than Pd@CS/RGO-10%-3 microsphere catalyst. It is found that the Pd@CS/RGO-10%-3 microsphere catalyst can be recycled 6 more

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runs than Pd@CS mat catalyst (entry 4), indicating better recycling capabilities. However, it seems that the Pd@CS/RGO-10%-3 microsphere catalyst has far below recycling capabilities than

a magnetic Fe3O4-CS-Pd complex catalyst prepared by Yang et al. (entry 5), which can maintain

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high activity in incredible 60 recycling runs. In the recycling experiment, the magnetic separated

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catalyst was directly reused in next run with no generally-required solvent rinsing to remove the

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retained products and high temperature drying for further use in next run. Reasonably, a bit loss of catalyst in these steps is difficult to completely avoid. It should be one of the reasons for the much smaller recyclable numbers of the catalyst of this work as compared with Yang’s work.

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Table 2. A comparison of Pd@CS/RGO catalyst with the previous reported catalysts in Heck coupling reactions. Metal (mol%)

TON

TOF (h-1)

Number of cycles

Pd@PCSM microspheres

1%

95

19

13

Pd@CS membranes

0.5%

245

49

8

3

Pd@MMT/CS hybrids

0.2%

480

120

30

4

Pd@CS mats

0.17%

576

36

7

5

Fe3O4-CS-Pd complex

0.13%

307.5

205

66

6

Pd@CS/RGO microsphere

0.2%

495

99

13

1

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2

Catalyst

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Entry

21

Ref. Zeng, et al., 2012a Zeng, et al., 2018 Zeng, et al., 2016 Bradshaw, et al., 2011 Yang, et al., 2008 this work

Table 3. Catalytic performances of Pd@CS/GO-10%-3 microspheres catalysts applied in Heck

Entry

Aromatic halides

Alkenes

Cross-coupling Yield a

1

C6H5I

CH2=CHCO2(n-C4H9)

99% (trans)

2

2-ClC6H4I

CH2=CHCO2(n-C4H9)

79% (trans)

3

3-FC6H4I

CH2=CHCO2(n-C4H9)

4

4-FC6H4I

CH2=CHCO2(n-C4H9)

5

2-CH3C6H5I

CH2=CHCO2(n-C4H9)

6

3-CH3OC6H4I

CH2=CHCO2(n-C4H9)

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coupling reactions between aromatic halides and alkenes.

7

4-CH3OC6H5I

CH2=CHCO2(n-C4H9)

90% (trans)

8

C6H5I

CH2=CHCO2(t-C4H9)

98% (trans)

9

4-FC6H4I

CH2=CHCO2(t-C4H9)

97% (trans)

10

4-CH3OC6H5I

11

C6H5I

12

95% (trans)

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re

-p

80% (trans) 89% (trans)

95% (trans)

CH2=CHC6H5

95% (trans)

4-FC6H4I

CH2=CHC6H5

94% (trans)

13

4-CH3OC6H5I

CH2=CHC6H5

92% (trans)

14

C6H5I

CH2=CHCN

95% (cis/trans: 0.40)

4-FC6H4I

CH2=CHCN

96% (cis/trans: 0.47)

16

4-CH3OC6H5I

CH2=CHCN

88% (cis/trans: (0.41))

17

C6H5Br

CH2=CHCO2(n-C4H9)

trace b

18

3-CH3COC6H4Br

CH2=CHCO2(n-C4H9)

58% b (trans)

19

4-CH3COC6H4Br

CH2=CHCO2(n-C4H9)

67% b (trans)

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ur

na

CH2=CHCO2(t-C4H9)

15

a

98% (trans)

GC/MS yield, b 10h. As shown in Table 3, the Pd@CS/RGO-10%-3 microsphere catalyst can be successfully 22

extended to the Heck coupling reactions between other aromatic halides and n-butyl acrylates. It works well for the aromatic iodides substituted with either an electron-withdrawing group such as o–Cl (entry 2, 79% yield), m-F (entry 3, 98% yield), and p-F (entry 4, 95% yield ) or an electron-donating group such as o-CH3 (entry 5, 80% yield), and m-COCH3 (entry 6, 89% yield), and m-COCH3 (entry 7, 90% yield). Also, coupling reactions between aromatic iodides with other alkenes, such t-butyl acrylates (entries 8-10), styrene (entries 11-13), and acrylonitrile (entries

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14-16), can be well catalyzed by the catalyst, indicating the high general applicability of the

catalyst. However, the Heck-coupling reaction between bromo-benzene and n-butyl acrylates is poorly catalyzed by this catalytic system with trace yield (entry 17). It is due to the much more

-p

difficulties in breaking of C-Br bonding than C-I bonding. Nevertheless, C-Br bonding can be

re

activated via strong electron-withdraw group’s substitution. As seen, moderate yields is achieved

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in the entries 18-19 for Heck reactions between strong electron-withdraw group’s substituted aromatic bromides, such as m-COCH3 (entry 18, 58% yield), and p-COCH3 (entry 19, 67% yield).

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Overall, the prepared Pd@CS/RGO microsphere catalysts show comparable catalytic activities to recently reported heterogeneous Pd catalysts supported on graphene family nanomaterials and CS,

ur

etc (Siamaki et al., 2011; Saito et al., 2014; Srivastava, 2015; Zeng et al., 2014, 2016, 2018).

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4. Conclusions

In this study, novel Pd@CS/RGO microspheres catalysts have been successfully prepared by a combination

of

etching

off

silica

nanoparticles

and

freeze-drying

treatments

of

CS/RGO/silica/PdCl2 microspheres. Silica nanoparticles act as effective porogens for the porous microsphere catalysts and freeze-drying treatment can guarantee the pores opening during the 23

drying process after silica porogens etching. It has been demonstrated that the well-dispersed RGO nanosheets with 10-15 layers forms strong interactions with CS molecules and notable improvement on the material stability can be achieved. Pd nanoparticles sized in 5-10 nm were also well dispersed in the porous CS/RGO matrix. A good balance between porous structure and high stabilities was achieved in the case of Pd@CS/RGO-10%-3. It showed excellent catalytic activities for the Heck reactions between aromatic halides and alkenes. And it can be reused for 13

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times without significant decrease in coupling yields. Moreover, the staring material of both

natural polysaccharide CS and RGO nanosheets are cheap and easy to get. Consequently, the novel Pd@CS/RGO microsphere catalyst has a great organic synthesis application potential both

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in academic and industrial fields.

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Acknowledgments: This work is supported by the National Natural Science Foundation of China

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(Grant No. 11875193 and 11475114), Zhejiang Provincial Fundamental Public Welfare Research Project of China (Grant No. LGG18E030004).

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Conflicts of Interest: The authors declare no conflict of interest.

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