Hemicelluloses supported palladium/xylan nanocomposites containing N and O ligands: Highly-performance heterogeneous catalysts for Suzuki reaction

Article abstract of DOI:10.1016/j.carbpol.2019.03.005

Two kinds of xylan/palladium catalysts (PACMX-Pd and EDAX-Pd) were simply fabricated by integrating two N bidentate ligands into the skeleton of oxygen-rich xylan and followed with the in-situ reduction of palladium. The Pd loading in PACMX-Pd and EDAX-Pd nanocomposites was 0.353 mmol/g and 0.302 mmol/g, respectively. The catalysts exhibited an excellent catalytic activity for the Suzuki reaction with a yield of high up to 100% as well as a superior turnover frequency (TOF: 9626 h^?1). Moreover, the catalysts showed outstanding stability and could be reused for at least 5 times without significant decrease in activity. Compared with other polymers and non-polymers supported palladium catalysts, the as-prepared biomass-based catalysts possess potential applications in a wide range of organic catalysis.

Full text of DOI:10.1016/j.carbpol.2019.03.005

Accepted Manuscript
Title: Hemicelluloses supported palladium/xylan
nanocomposites containing N and O ligands:
highly-performance heterogeneous catalysts for Suzuki
reaction
Author: Fan Du Lei Zhang Jiliang Ma Xinwen Peng
PII:
S0144-8617(19)30271-1
DOI:
Reference:
https://doi.org/doi:10.1016/j.carbpol.2019.03.005
CARP 14674
To appear in:
Received date:
Revised date:
Accepted date:
19 November 2018
26 February 2019
3 March 2019
Please cite this article as: Du, F., Zhang, L., Ma, J., and Peng, X.,Hemicelluloses
supported palladium/xylan nanocomposites containing N and O ligands: highly-
performance heterogeneous catalysts for Suzuki reaction, Carbohydrate Polymers
(2019), https://doi.org/10.1016/j.carbpol.2019.03.005
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Graphical Abstract
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Highlights:
⚫
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Palladium/xylan heterogeneous catalysts containing N and O
ligands were synthesized.
The prepared catalysts showed excellent catalytic activity with
yield up to 100%.
The activity of catalysts exhibited no significant loss after
recycle for 5 times.
The reaction conditions are milder, compared with other
palladium-based catalysts.
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Hemicelluloses
supported
palladium/xylan
nanocomposites
containing N and O ligands: highly-performance heterogeneous
catalysts for Suzuki reaction
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Fan Du, Lei Zhang, Jiliang Ma, Xinwen Peng*
State Key Laboratory of Pulp and Paper Engineering, South China University of
Technology, 381 Wushan Road, Guangzhou 510640, China
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*Corresponding authors’ E-mail: fexwpeng@scut.edu.cn (Xinwen Peng),
Tel.: +86-020-87111860; Fax: +86-020-871118 60.
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Abstract
Two kinds of xylan/palladium catalysts (PACMX-Pd and EDAX-Pd) were simply
fabricated by integrating two N bidentate ligands into the skeleton of oxygen-rich
xylan and followed with the in-situ reduction of palladium. The Pd loading in
PACMX-Pd and EDAX-Pd nanocomposites was 0.353 mmol/g and 0.302 mmol/g,
respectively. The catalysts exhibited an excellent catalytic activity for the Suzuki
reaction with a yield of high up to 100% as well as a superior turnover frequency
-1
(TOF: 9626 h ). Moreover, the catalysts showed outstanding stability and could be
reused for at least 5 times without significant decrease in activity. Compared with
other polymers and non-polymers supported palladium catalysts, the as-prepared
biomass-based catalysts possess potential applications in a wide range of organic
catalysis.
Keywords: Biomass, Xylan, Palladium, Nanocomposites, Suzuki reaction
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1. Introduction
Recently, carbon-carbon cross-coupling reactions catalyzed by palladium have
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attracted enormous interest in synthetic organic chemistry (Hekmati, Bonyasi,
Javaheri, & Hemmati, 2017; Chen, Engle, Wang, & Yu, 2009). Coupling reactions
such as Suzuki (Putta, Sharavath, Sarkar, & Ghosh, 2015), Heck (Ma et al., 2008),
and Sonogashira (Gogoi, Dewan, & Bora, 2015) provide a one-step method for
synthesizing complex compounds, including pharmaceuticals, herbicides and natural
products, as well as engineering materials such as conducting polymers, molecular
wires, and liquid crystals (Chen et al., 2009). Palladium catalysts, in the form of Pd
(Ⅱ)/Pd (0) complexes or Pd nanoparticles, have been proven to be efficient and
irreplaceable catalysts for C-C bond formation reactions (Fortea-Pérez et al., 2015;
Narayanan & El-Sayed, 2003). In order to maintain the stability of Pd (Ⅱ) or Pd (0)
and obtain satisfactory yield, homogeneous systems are commonly used. However,
palladium is difficult to be separated from the catalysts, leading to undesired residue
metals in final products, which is a serious problem in pharmaceutical industry due to
the closely regulated level of heavy metals such as Pd in active pharmaceutical
intermediates (Zhang et al., 2013; Zhong et al., 2014).
Substituting homogeneous catalysts with heterogeneous catalysts is an effective
strategy to solve this problem due to the residues can be easily separated by filtration
or centrifugation and the catalysts can be reused for several times (Veerakumar,
Thanasekaran, Lu, Liu, & Rajagopal, 2017; Crudden, Sateesh, & Lewis, 2005).
Supporting homogeneous catalysts on inorganic matters or organic polymers is a
typical method to prepare heterogeneous catalysts. Various materials, including
sporopollenin (Baran, Sargin, Kaya, Mentes, & Ceter, 2017), ionic liquids
(Nikoorazm, Ghorbani-Choghamarani, Noori, & Tahmasbi, 2016), zeolite
(Baghbanian, Yadollahy, Tajbakhsh, Farhang, & Biparva, 2014), carbon (Zhong et al.,
2015), silica (Sharma & Gupta, 2015), metal oxides (Amoroso, Colussi, Del Zotto,
Llorca, & Trovarelli, 2013; Dumbre, Yadav, Bhargava, & Choudhary, 2013), and
polymers (Rangel, Maya, Sanchez, de la Campa, & Iglesias, 2015; Xu, Song, Li, &
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Tan, 2015) have been employed to support heterogeneous palladium catalysts.
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However, the heterogeneous Pd catalysts show lower activity and selectivity than
their homogeneous counterparts (Choudhary, Nishimura, & Ebitani, 2014), which is
due to the Pd leaching from the support and its poor accessibility of the active sites to
the reactant (Hekmati, Bonyasi, Javaheri, & Hemmati, 2017). Therefore, designing a
highly active and stable palladium heterogeneous catalyst is urgent.
Tuning the specific composition, morphology and size of catalyst is crucial for the
catalytic activity of Pd complex (Julkapli & Bagheri, 2015). In addition, previous
studies demonstrated that supports with functional groups containing, oxygen,
nitrogen, or sulfur ligands can enhance the stability of Pd nanoparticles, because these
heteroatoms have a high affinity towards Pd (Ⅱ) or Pd (0), thus improving the
catalytic activity of their transition metal complexes (Kardanpour et al, 2014; Oliveira,
He, Klein Gebbink, & de Jong, 2015; Kostas, Tenchiu, Arbez-Gindre, Psycharis, &
Raptopoulou, 2014). Biopolymers, which are a class of organic compounds
containing large amounts of O or N or S heteroatom, are considered to be good
catalytic materials for their inherent features in chemical composition and chain
structure. Chitosan-based (containing O and N) palladium (0) catalysts have been
confirmed to have excellent catalysis performance in Suzuki coupling reaction
(Yilmaz, Baran, & Mentes, 2018; Baran & Menteş, 2017). Cellulose (containing O)
was also utilized as a support for palladium (Li et al., 2017). Pd immobilized on lignin
(containing O and S) as a catalyst for cross-coupling reactions in water was reported
by Coccia etal (Coccia, Tonucci, d’Alessandro, D’Ambrosio, & Bressan, 2013). In
addition, alginate (Jouannin et al., 2012), gelatin (Cacchi et al., 2012; Khazaei,
Khazaei, & Rahmati, 2015), starch (Khalafi-Nezha & Panahi, 2011), cyclodextrin
(Senra et al., 2009) and their derivatives were also employed as supports in catalysis.
However, hemicelluloses have been rarely applied in Pd catalytic system and the
prepared xylan-palladium nanocomposites can catalyze the Suzuki reactions with
equivalent yields in a relatively mild reaction condition.
Xylan-type hemicellulose has abundant hydroxyl, carbonyl, and aldehyde
carboxylic acid groups along the backbone and side chains, and thus it has great
potential to stabilize metal particles. Our group successfully prepared Pd/xylan
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catalysts by a simple in-situ reduction method for organic synthesis reaction (Chen,
Zhong, Peng, Lin, & Sun, 2014; Chen et al., 2014). 1, 10-phenanthrolin-5-amine and
ethylenediamine have been widely used in transition-metal-catalyzed reactions
(Mandegani et al., 2015; Yang, Li, & Wang, 2011) because they possess high
electron-donating ability, short metal-ligand bond lengths, strong metal-ligand bond
energies, and greater bond dissociation energies (Xu et al., 2015), which are suitable
ligands for palladium particles stabilization.
Herein, novel xylan supported palladium complexes were prepared by grafting N
ligands in xylan-type hemicelluloses and then incorporated palladium via in-situ
reduction and showed a superior catalytic performance in the Suzuki cross-coupling
-1
reactions under moderate condition with a TOF as high as 9626 h . Furthermore, both
catalysts have excellent recovery and recycle ability. The as-prepared catalysts have a
great potential in green chemical catalysis.
2. Materials and Methods
2.1. Materials
Palladium acetate (Pd(Ac)
2
), 1, 10-phenanthrolin-5-amine, ethylenediamine,
hydrochloride (EDC·HCl),
1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
N-hydroxysuccinimide (NHS), epichlorohydrin, 1, 8-diazabicyclo [5.4.0] undec-7-ene
(DBU), aryl halides, arylboronic acids were purchased from Aladdin Reagent Co., Ltd
and were used without further purification. Solvents used for coupling reactions,
ethanol (EtOH), methanol (MeOH), acetonitrile, 1,4-dioxane, hexane, tetrahydrofuran
(THF) and toluene were purchased from National Medicine Group Chemical Reagent
Co., Ltd. Sodium hydroxide (NaOH), potassium hydroxide (KOH), potassium
carbonate (K
2
CO
3
), sodium carbonate (Na
2
CO ), triethylamine (TEA) and other
3
reagents were also commercially available from National Medicine Group Chemical
Reagent Co., Ltd. Xylan-type hemicelluloses were prepared according to literature
(Peng, Ren, Zhong, & Sun, 2012). The sugar analysis showed sugar composition
(relative weight percent): 89.4% xylose, 5.8% arabinose, 1.9% glucose, 0.7%
galactose, 1.8% glucuronic acid and 0.6% galactose acid.
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2.2. Synthesis of 1, 10-phenanthrolin-5-amine functionalized carboxymethyl xylan
(PACMX)
The typical procedure for the preparation of PACMX was designed as followed.
Carboxymethyl xylan was firstly prepared by a similar method as Peng et al (See
Supplementary data). Subsequently, carboxymethyl xylan (0.8 mmol) and EDC·HCl
(4.0 mmol) were suspended in 12 mL deionized water, the pH was previously
o
adjusted to 5.5. After stirring under low temperature (＜20 C) for 15 min, 4.0 mmol
NHS was added to the above solution and the activation stage was maintained for 2 h,
followed by the addition of 0.4 mmol 1, 10-phenanthrolin-5-amine which had been
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dissolved in 8 mL EtOH and the reaction mixture was stirred for 24 h below 20 C.
The obtained solid residual was filtered, thoroughly washed with EtOH and vacuum
dried to give PACMX. The typical schematic outline of the preparation of PACMX
was depicted in Fig. S1a.
2.3. Synthesis of ethylenediamine functionalized xylan (EDAX)
The representative synthetic procedure for EDAX was illustrated in Fig. S1b. 0.66
g xylan-type hemicelluloses powder (approximately 5 mmol of xylose) was
o
homo-dispersed evenly in 5.5 mL water and heated to 85 C with stirring (500 rpm)
for 10 min. As the dispersion being cooled down to room temperature, sodium
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hydroxide (2 mL, 200 g L ) was added dropwise (20 drops per minute) into the
o
mixture and swollen for 30 min at 85 C, obtaining alkaline xylan solution. In addition,
0.02 mmol ethylenediamine and 0.01 mmol epichlorohydrin were mixed together at
room temperature and the reaction progressed violently. After 10 min the alkaline
xylan solution was added into the above mixture and stirred at room temperature for 6
h. The obtained yellow solution was precipitated with EtOH (100 mL) and filtered,
o
then the residue was washed with EtOH and vacuum dried at 60 C to obtain EDAX.
2.4. Synthesis of the palladium grafted PACMX and EDAX (PACMX-Pd and
EDAX-Pd)
0.02 g palladium acetate and 0.2 g PACMX/EDAX were suspended in 10 mL EtOH.
o
The solution was stirred for 12 h at 60 C. During the reaction, the Pd (Ⅱ) was
reduced gradually, and the yellowish solution turned to be deep brown, signifying the
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conversion of Pd (Ⅱ) to Pd (0). After the reaction, the brown colored mixture was
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filtrated, washed with EtOH and then vacuum dried at 60 C to give PACMX-Pd and
EDAX-Pd catalysts. The local structures of palladium/xylan nanocomposites
(PACMX-Pd/EDAX-Pd) were proposed as shown in Fig. S2, and the corresponding
ball and stick model structures were also presented.
2.5. Catalytic performance for Suzuki coupling reaction
In a typical procedure, aryl halide (0.5 mmol), arylboronic acid (0.6 mmol), base
(1.0 mmol) and catalyst (23 mg PACMX-Pd, containing 1.5 mol% Pd or 4.3 mg
EDAX-Pd, containing 0.26 mol% Pd) were added into a 35 mL pressure tube. The
solvent (2 mL) was added to disperse the reactants well and the resulting mixture was
stirred under air atmosphere. Upon the completion of the reaction, the dispersion was
filtered and concentrated under vacuum. The products were purified by flash column
chromatography on a silica gel and identified by H NMR and 13C NMR
spectroscopy.
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2.6. Recycling tests for PACMX-Pd and EDAX-Pd catalysts
Recycling tests were performed to check the stability of PACMX-Pd and EDAX-Pd
catalysts during the reaction. After the Suzuki reaction of 4-iodoanisole with
phenylboronic acid, the catalysts were separated from the reaction mixture by
centrifugation. The supernatants were stored and the Pd leaching of catalysts were
analysed. The residual catalysts were washed with EtOH, water and vacuum dried
overnight. Fresh 4-iodoanisole, phenylboronic acid, K
2
CO and reused catalyst were
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added, and then the same reaction was performed under room temperature for 4 h.
Each cycle followed the general procedure for Suzuki cross coupling reactions
mentioned above.
2.7. Heterogeneity test
23 mg PACMX-Pd (containing 1.5 mol% Pd) or 4.3 mg EDAX-Pd (containing 0.26
mol% Pd) was added in a 35 mL pressure tube, which was followed by K
2
CO (1.0
3
mmol), 4-iodoanisole (0.5 mmol), and phenylboronic acid (0.6 mmol). Then EtOH (2
mL) was added to disperse the reactants and the reaction mixture was performed
under room temperature. After 1 h of reaction, the catalysts were separated by
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filtration. Subsequently, an additional amount of fresh K
2
CO (1.0 mmol) was added
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into the filtrate and the system was reacted under the same condition for another 3 h.
2.8. Characterization
FT-IR spectra of the samples were recorded on a Vector 33 infrared spectrum
instrument (Bruker Corporation, Germany). The structures were analyzed by XRD
measurements with a Bruker-D8 Advance diffractometer using Cu Kα radiation
(λ=0.154 nm). The electronic states of Pd and N were determined by XPS and
performed on Thermo Scientific ESCALAB 250 spectrometer system using Al Kα
radiation (hν=1486.6 eV) with contaminated C as an internal standard (C 1s=284.6
eV). The thermal stabilities of PACMX, EDAX, PACMX-Pd and EDAX-Pd catalysts
were confirmed by TGA (TGA Q500, TA). The morphologies of samples were
obtained on a JEM-2100 (HR) transmission electron microscopy working at 200 kV.
The as-prepared samples were also observed using a scanning electron microscope
(Zeiss Merlin, Germany). The Pd content was determined by Z-2000 (Japan) atomic
1
13
absorption spectrometer (AAS). All H NMR and C NMR spectra were conducted
on a 600 MHz Bruker (AVANCE III HD 600) spectrophotometer using CDCl as
3
solvent and tetramethylsilane (TMS) as internal standard. All the spectroscopic data of
the products were similar with those reported in the former literature and
commercially available.
3. Results and discussion
3.1. Characterization of PACMX-Pd and EDAX-Pd catalysts
PACMX-Pd and EDAX-Pd were prepared by a simple two steps. The synthesis routes
of functionalized xylan (PACMX/EDAX) were illustrated in Figure 1. PACMX was
obtained by the reaction of carboxymethyl xylan and 1, 10-phenanthrolin-5-amine in
the presence of EDC·HCl and NHS. EDAX was synthesized by modifying xylan with
ethylenediamine and epichlorohydrin under NaOH condition. Subsequently,
palladium nanoparticles were supported onto PACMX/EDAX via in-situ reduction of
Pd(Ac) at room temperature, the solution turned from yellowish to deep brown ,
2
indicating the successful reduction of Pd (Ⅱ) into Pd (0) and simultaneously the
coordination of Pd to PACMX/EDAX. As Fig. S2 shown, the palladium was anchored
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by four nitrogen atoms at the functionalized xylans.
The FT-IR spectra of PACMX, EDAX, PACMX-Pd and EDAX-Pd are shown in
Fig. 1. The peak assigned to the C=N stretching vibration of PACMX (Fig. 1c) can be
-1
clearly observed at 1645 cm (Govindaiah, Lee, Jung, Lee, & Kim, 2009). After
coordination with palladium particles (Fig. 1d), the stretching vibration of C=N group
-1
-1
(1645 cm ) is shifted to 1642 cm due to the nitrogen atom in the benzene ring of
PACMX actively participated to give its nonbonding electron pair to the metal
particles. As shown in Fig. 1a and 1b, after Pd NPs being immobilized onto the
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surface of EDAX, a spectral shift for N-H bond in the amino group from 3420 cm to
-1
-1
3397 cm is obviously observed. The band at 1640 cm is ascribed to N-H in-plane
bending vibration of amide group (Yang, Tan, Wang, & Wang, 2013). After being
-1
hydrided with palladium, the peak of N-H in EDAX shifts to 1614 cm . All these
results indicate that strong ligand bond formed between EDAX and palladium
particles.
EDAX
EDAX-Pd
(d)
(c)
(b)
PACMX
PACMX-Pd
1
642
1
645
3397
1
640
614
(a)
1
3420
4
000 3500 3000 2500 2000 1500 1000
500
-
1
Wavelength (cm )
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Fig. 1. FT-IR spectra of EDAX (a). EDAX-Pd (b). PACMX (c) and PACMX-Pd (d).
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To further illustrate the interaction between palladium nanoparticles and the carrier,
the oxidation states of the surface elements of PACMX-Pd and EDAX-Pd catalysts
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were determined by X-ray photoelectron spectroscopy (XPS) (Fig. 2). The XPS
narrow scan of palladium on the surface of PACMX-Pd (Fig. 2c) showed two peaks.
The binding energy of the doublet peaks at 335.0 eV (Pd 3d5/2) and 340.3 eV (Pd 3d3/2
are attributed to Pd (0) state. The peak spliting at 338.1 eV and 342.6 eV corresponds
to the characteristic state of Pd (Ⅱ) (Yang et al., 2013). The difference of N1s binding
energies between PACMX and PACMX-Pd was 0.1 eV (Fig. 2d). These results
suggest that a coordination bond between palladium and nitrogen was formed in
PACMX-Pd. For EDAX-Pd (Fig. 2g), the binding energies at 335.0 eV and 340.3 eV
indicate the presence of Pd (0), and Pd (Ⅱ) split at 338.1 and 343.4 eV (Chen, Zhong,
Peng, Lin, & Sun, 2014). The difference of N1s binding energies between EDAX and
EDAX-Pd was 1.0 eV (Fig. 2h), which indicates that the strong ligand formed
between EDAX and palladium particles. These results are in good agreement with
FT-IR spectra.
)
399.56
335.04
(
c)
PACMX
PACMX-Pd
(d)
340.28
399.47
342.56
338.08
O 1s
396
398
400
402
404
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344
342
340
338
336
334
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330
Binding Energy (eV)
Binding Energy (eV)
Pd 3d
(
b)
C 1s
C 1s
N 1s
N 1s
O 1s
(
a)
1
200
1000
800
600
400
200
0
Binding Energy (eV)
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338.08
399.98
(g)
(h)
EDAX
EDAX-Pd
335.04
343.36
340.28
O 1s
398.98
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342
340
338
336
334
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330
404
402
400
Binding Energy (eV)
398
396
Binding Energy (eV)
(f)
Pd 3d C 1s
N 1s
O 1s
C 1s
(e)
N 1s
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1000
800
600
400
200
0
Binding Energy (eV)
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2
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94
Fig. 2. XPS wide scan of PACMX (a) and PACMX-Pd (b). XPS narrow scan of palladium on
the surface of PACMX (c) and nitrogen in PACMX and PACMX-Pd (d). XPS wide scan of
EDAX (e) and EDAX-Pd (f). XPS narrow scan of palladium on the surface of EDAX (g) and
nitrogen in EDAX and EDAX-Pd (h).
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02
03
The formation of palladium nanoparticles was also confirmed from the wide angle
powder XRD patterns of PACMX-Pd and EDAX-Pd hybrids (Fig. 3). The XRD
spectra suggest that PACMX and EDAX are amorphous since no characteristic peak
was observed, while a phase in PACMX-Pd and EDAX-Pd was emerged in XRD
patterns at 2θ = 40.0°, 46.1°, and 67.5° , corresponding to (111), (200), and (220)
planes of Pd crystal (PDF#46-1043), respectively (Cao, Wu, Su, & Cao, 2015).
12
Page 13 of 33

（111）
EDAX
EDAX-Pd
PACMX
PACMX-Pd
(d)
（200）
(
c)
（220）
(b)
(a)
PDF#46-1043
20 30
10
40
50
60
70
80

(degree)
3
3
04
05
Fig. 3. XRD patterns of PACMX (a). PACMX-Pd (b). EDAX (c) and EDAX-Pd (d).
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
The morphologies of PACMX-Pd and EDAX-Pd at a higher resolution TEM were
recorded. It was shown that the spherical palladium particles with uniform size were
homogeneously dispersed on PACMX (Fig. 4a) and EDAX (Fig. 4e). Average sizes
for the two catalysts were about 5 nm and 4 nm, respectively. HR-TEM images of
individual nanoparticle showed clear lattice fringes with an interplanar distance of
approximately 0.22 nm, corresponding to Pd (111) planes (Fig. 4c and 4g) (Cao et al.,
2015; Maity & Maitra, 2014). The selected area electron diffraction pattern (Fig. 4d
and 4h) displayed five diffused rings, which are assigned to the (111), (200), (220),
(222), and (311) reflections of face-centered cubic Pd nanoparticles (Maity et al.,
2014), which well agrees with the XRD results. As shown in Fig. 4b and 4f, a slight
aggregation was observed in the recycled PACMX-Pd and EDAX-Pd, which could
result in the reduced catalytic activity of the Pd nanocatalysts. The SEM images in
Supplementary data Fig. S4 showed that both catalysts had coralloid porous as
compared to PACMX and EDAX, which is very beneficial for the catalytic reactions.
The EDX spectra of PACMX-Pd and EDAX-Pd indicate the presence of metallic Pd
signals.
13
Page 14 of 33

(
a) 35
(b)
(c)
3
2
0
5
2
1
1
0
5
0
(d)
5
0
3
4
5
6
7
8
9
10
Particle Size (nm)
(e)
(f)
(g)
2
1
0
0
(h)
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
Particle Size (nm)
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Fig. 4. TEM images and histogram of the freshly prepared (a) and reused (for five times, b)
PACMX-Pd particles. HR-TEM images and diffraction patterns of the as-prepared PACMX-Pd
particles (c-d). TEM images and histogram of the as-prepared (e) and recycled (for five times, f)
EDAX-Pd particles. HR-TEM images and diffraction patterns of the as-prepared EDAX-Pd
particles (g-h).
Thermal stability of catalysts is also a key factor for its catalytic activity. The
thermal behaviors of catalysts were depicted in Fig. S5 by TGA curves. PACMX,
o
EDAX, PACMX-Pd, and EDAX-Pd started to decompose at about 185 C, indicates
the loaded palladium particles didn’t significantly affect the initial decomposition
temperature. Besides, the remaining weights of PACMX-Pd and EDAX-Pd were
higher than those of PACMX and EDAX. These results reveal that the thermal
stabilities of PACMX-Pd and EDAX-Pd were suitable for Suzuki catalytic
application.
3.2. Catalytic activities of PACMX-Pd and EDAX-Pd for the Suzuki coupling
reactions
Suzuki cross coupling reactions were thoroughly performed for testing the catalytic
activities of PACMX-Pd and EDAX-Pd. In the screening experiments, the reaction
conditions, including solvent, base and catalyst loading in PACMX-Pd and EDAX-Pd
14
Page 15 of 33

3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
were optimized. Firstly, various solvents were examined in the Suzuki coupling
reaction of 4-iodoanisole with phenylboronic acid using PACMX-Pd and EDAX-Pd
as catalysts and K
2
CO as a base. For PACMX-Pd (Table 1, entries 1-8), EtOH,
3
MeOH and CH CN gave higher yields of 4-methoxybiphenyl (100%, 97.8%, and
3
100%, respectively), while the reactions in less polar solvents, for example, 1,
4-dioxane, hexane, THF and toluene, were less effective (Chen, Zhong, Peng, Lin, &
Sun, 2014). PACMX-Pd was not stable in H O and therefore the catalytic reaction did
2
not proceed smoothly in aqueous media. For the sake of safety and environmental
friendliness, EtOH was chosen as a desirable solvent for the reaction catalyzed by
PACMX-Pd, and a remarkable yield of 70% could be achieved even if the reaction
just proceeded for 1 h (Table 1, entry 9). EDAX-Pd also showed a good catalytic
activity in EtOH and MeOH with yields of high up to 100% (Table 1, entries 10 and
11). Compared with PACMX-Pd, EDAX-Pd showed a moderate catalytic activity in
2
H O with a yield of 80.3% (Table 1, entry 8 and 17) due to the hydrophilcity of
EDAX-Pd. Among various solvents (Table 1, entries 10-17), EtOH was also selected
as the optimal solvent for EDAX-Pd catalyzed Suzuki coupling reactions, and giving
an impressive yield of 59.5% within 1 h (Table 1, entry 18).
Table1. Effect of solvent on the Suzuki coupling reaction of 4-iodoanisole with phenylboronic
acida.
b
Entry
Catalyst
Solvent
Yield /%
1
2
3
4
5
6
7
8
PACMX-Pd
PACMX-Pd
PACMX-Pd
PACMX-Pd
PACMX-Pd
PACMX-Pd
PACMX-Pd
PACMX-Pd
EtOH
100
97.8
100
63
MeOH
CH CN
3
1,4-dioxane
hexane
THF
49.5
47.2
43.5
16.3
toluene
2
H O
15
Page 16 of 33

9
PACMX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EtOHc
EtOH
70
10
100
100
65.3
15.5
84.5
37.8
87
1
1
2
3
4
5
6
7
8
MeOH
1
1
1
1
1
1
1
CH CN
3
1,4-dioxane
hexane
THF
toluene
H
2
O
80.3
59.5
EtOHd
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
aReaction conditions: 4-iodoanisole (0.5 mmol), phenylboronic acid (0.6 mmol), K
2
CO (1 mmol),
3
PACMX-Pd (23 mg, containing 1.5 mol% Pd), EDAX-Pd (4.3 mg, containing 0.26 mol% Pd),
b
solvent (2 mL), at room temperature, 4 h. Isolated yields (each experiment was repeated for 3
times, and the final isolated yield was the average of 3 similar yields). c, d1 h.
For screening desirable bases, the Suzuki reactions with various bases were
evaluated in EtOH at room temperature. As shown in Table 2, both PACMX-Pd and
EDAX-Pd didn’t proceed well with organic bases such as TEA and DBU (Table 2,
entry 4-5 and 9-10). However, the inorganic bases (Table 2, entries 1-3 and 6-8)
generally exhibited high efficiencies in the Suzuki coupling reaction of 4-iodoanisole
with phenylboronic acid. Among the bases, K
Consequently, the following reactions were carried out using EtOH as a solvent and
CO as a base to examine the effect of the Pd loading on the product yield (Table 3).
2
CO
3
showed the highest yield of 100%.
K
2
3
As for PACMX-Pd, the yield was enhanced from 60% to 100% as the Pd content
increased from 0.2 mol% to 1.5 mol%, while it remained almost unchanged after
improved the dosage of Pd loading (Table 3, entries 1-6). In terms of EDAX-Pd, the
yield was up to 100% when the Pd content reached to 0.26 mol% and being constant
with further increasing the Pd loading (Table 3, entries 8-13). These results imply that
PACMX-Pd and EDAX-Pd with less palladium exhibited excellent catalytic activity
towards Suzuki reaction and the optimal Pd loading for PACMX-Pd and EDAX-Pd
16
Page 17 of 33

3
3
3
3
3
3
79
80
81
82
83
84
was 1.5 mol% and 0.26 mol%, respectively. Control experiments indicated that there
was no product acquired in the absence of Pd (Table 3, entries 7 and 14), suggesting
that PACMX-Pd and EDAX-Pd could act as highly effective catalysts. In addition, the
pure palladium acetate was also used for the reaction (Table 3, entry 15), and it was
found that Pd(OAc) was not as effective as PACMX-Pd and EDAX-Pd.
2
17
Page 18 of 33

3
85
Table 2. Effect of base on the Suzuki coupling reaction of 4-iodoanisole with phenylboronic acida.
b
Entry
Catalyst
Base
Yield /%
1
2
PACMX-Pd
PACMX-Pd
K
2
CO
3
100
Na
2
CO
3
50.1
3
4
5
6
7
PACMX-Pd
PACMX-Pd
PACMX-Pd
EDAX-Pd
KOH
TEA
DBU
80
26
28
2
K CO
3
100
67.5
EDAX-Pd
Na
2
CO
3
8
9
EDAX-Pd
EDAX-Pd
EDAX-Pd
KOH
TEA
DBU
52.8
47.3
56.8
10
3
3
3
3
3
86
87
88
89
90
aReaction conditions: 4-iodoanisole (0.5 mmol), phenylboronic acid (0.6 mmol), various bases (1
mmol), PACMX-Pd (23 mg, containing 1.5 mol% Pd), EDAX-Pd (4.3 mg, containing 0.26 mol%
b
Pd), EtOH (2 mL), at room temperature, 4 h. Isolated yields (each experiment was repeated for 3
times, and the final isolated yield was the average of 3 similar yields).
18
Page 19 of 33

3
3
91
92
Table 3. Effect of Pd loading on the Suzuki coupling reaction of 4-iodoanisole with phenylboronic
acida.
b
Entry
Catalyst
Pd/mol%
Yield /%
1
2
PACMX-Pd
PACMX-Pd
0.2
60
69
0.26
3
4
5
6
7
PACMX-Pd
PACMX-Pd
PACMX-Pd
PACMX-Pd
PACMX-Pd
0.50
1.0
1.5
2.0
0
62
69
100
100
0
8
9
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
EDAX-Pd
0.2
0.26
0.50
1.0
1.5
2.0
0
84
100
100
100
100
100
0
1
1
1
1
1
1
0
1
2
3
4
5
Pd(OAc)
2
3.38
90.1
3
3
3
3
3
3
3
93
94
95
96
97
98
99
aReaction conditions: 4-iodoanisole (0.5 mmol), phenylboronic acid (0.6 mmol), K
2
CO (1 mmol).
3
b
EtOH (2 mL), at room temperature, 4 h. Isolated yields (each experiment was repeated for 3 times,
and the final isolated yield was the average of 3 similar yields).
The substrate scope of this Suzuki reaction between various aryl halides and
arylboronic acids was investigated under the optimized reaction condition, and the
results were summarized in Tables 4 and 5. For the both catalysts, electron-deficient
aryl halides and electron-rich phenylboronic acid afforded higher coupling yields up
19
Page 20 of 33

4
4
4
4
4
4
4
4
4
4
4
4
4
4
00
01
02
03
04
05
06
07
08
09
10
11
12
13
to 100%, especially aryl iodides containing methoxyl, nitro and ether group could
smoothly reacted with phenylboronic acid (Table 4 and 5, entries 6, 8, and 11).
However, deactivated aryl chlorides did not give good yields in the coupling reactions
with phenylboronic acid (Tables 4 and 5, entries 5 and 10). Moreover, arylboronic
acids with either electro-withdrawing or electro-donating groups on the phenyl ring
was reacted with iodobenzene (Table 4 and 5, entries 1, 13 and 14), the yields of the
products were good to excellent. It was reported that electron-rich ligands promoted
the oxidative addition that is a crucial step for enhancing the catalytic activity of
palladium catalyst in Suzuki coupling reaction (Cho et al., 2014). As a consequence,
the high catalytic activities of PACMX-Pd and EDAX-Pd can be attributed to their
electron-rich N bidentate ligand. Moreover, the TOF values were recorded up to 1540
-1
-1
h for PACMX-Pd, and the corresponding values were observed as high as 9626 h
for EDAX-Pd (Luconi et al., 2018; Dong, Wu, Chen, & Wei, 2017).
20
Page 21 of 33

4
4
14
15
Table 4. The Suzuki coupling reactions of aryl halides and arylboronic acids catalyzed by
PACMX-Pda.
.
PACMX Pd, K CO (1 eq.)
2
3
R1
X
(OH)2B
R2
R1
R2
+
EtOH, Room temperature, Air
aryl halides
.5 eq.
boronic acid
0.6 eq.
biphenyls
0
R1
R2
b
TOF (h-1)
713
Entry
X
I
Time/h
Yield /%
92.6
49.2
60.5
16.2
trace
100
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
H
8
12
8
Br
I
252
CH
CH
CH
3
3
3
466
Br
Cl
I
12
24
4
83
trace
1540
376
OCH
OCH
3
Br
I
3
8
48.9
100
O=CCH
O=CCH
O=CCH
3
3
3
4
1540
1540
189
Br
Cl
I
4
100
1
0
1
2
3
4
8
24.5
96.3
82.2
91.4
71.1
1
1
1
1
NO
NO
H
2
2
4
1483
1266
1407
547
Br
I
H
4
OCH3
4
O=CCH3
I
H
8
4
4
4
4
16
aReaction conditions: aryl halide (0.5 mmol), arylboronic acid (0.6 mmol), K
2
CO (1 mmol),
3
b
17
18
19
PACMX-Pd (23 mg, containing 1.5 mol% Pd), EtOH (2 mL), at room temperature. Isolated
yields (each experiment was repeated for 3 times, and the final isolated yield was the average of 3
similar yields). TOF: turnover frequency ((yield of product / per mol of Pd)/time of reaction)
21
Page 22 of 33

4
4
20
21
Table 5. The Suzuki coupling reactions of aryl halides and arylboronic acids catalyzed with
EDAX-Pda.
.
EDAX Pd, K CO (1 eq.)
2
3
R1
X
(OH)2B
R2
R1
R2
+
EtOH, Room temperature, Air
aryl halides
.5 eq.
boronic acid
0.6 eq.
biphenyls
0
R1
R2
b
TOF (h-1)
3384
2269
1110
1100
trace
9626
2743
9626
9626
trace
9626
7768
7835
3658
Entry
X
I
Time/h
Yield /%
70.3
70.7
69.2
68.6
trace
100
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
H
8
12
24
24
24
4
Br
I
CH
CH
CH
3
3
3
Br
Cl
I
OCH
OCH
3
Br
I
3
8
57
O=CCH
O=CCH
O=CCH
3
3
3
4
100
Br
Cl
I
4
100
1
0
1
2
3
4
12
4
trace
100
1
1
1
1
NO
NO
H
2
2
Br
I
H
4
80.7
81.4
76
OCH3
4
O=CCH3
I
H
8
4
4
4
4
4
22
23
24
25
26
aReaction conditions: aryl halide (0.5 mmol), arylboronic acid (0.6 mmol), K
2
CO (1 mmol),
3
b
EDAX-Pd (4.3 mg, containing 0.26 mol% Pd), EtOH (2 mL), at room temperature. Isolated yield
(each experiment was repeated for 3 times, and the final isolated yield was the average of 3 similar
yields). TOF: turnover frequency ((yield of product / per mol of Pd)/time of reaction)
We compared the results achieved in this work with those catalyzed by other
22
Page 23 of 33

4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
catalysts reported recently for the Suzuki coupling reactions, and the results were
listed in Supplementary data Table S1. Our catalysts showed superiority to some
degree, such as lower reaction temperature, shorter reaction time, and higher yield.
3.3. Recyclability of PACMX-Pd and EDAX-Pd
The recyclability of the catalysts was examined for the Suzuki reaction between
4-iodoanisole and phenylboronic acid under standard condition with 23 mg
PACMX-Pd (containing 1.5 mol% Pd) and 4.3 mg EDAX-Pd (containing 0.26 mol%
Pd), and the results were shown in Fig. 8. After the completion of the first cycle
(100% yield), the catalysts were recovered by centrifugation and then vacuum dried.
The recovered catalysts were directly used for the second run under the same reaction
conditions. The yield of the product was decreased to 87.9% for PACMX-Pd catalysis
in the fifth run, indicating that PACMX stabilized Pd NPs was highly stable and
recyclable. However, it yielded only 57.4% in the fifth run for EDAX-Pd. The results
indicate that the rigid ligand modified xylan (PACMX) is superior to the flexible
ligand modified one (EDAX). Furthermore, the AAS measurement revealed that the
loss concentration of Pd was less than 1 ppm after being used for five times,
demonstrating PACMX and EDAX could act as good substrates to stabilize Pd NPs
and the Pd leaching was negligible. According to the TEM analysis, the Pd
th
nanocatalysts appeared a slight aggregation in the 5 cycle (especially the EDAX-Pd).
Hence, the reduced catalytic activity in recycling can ascribed to the agglomeration of
Pd nanocatalysts.
23
Page 24 of 33

PACMX-Pd
EDAX-Pd
1
00 100
1
00
96
9
4.7
8
7.8
87.9
8
6
4
2
0
0
0
0
0
7
3.5
7
2.6
72
5
7.4
1
2
3
4
5
Runs
4
4
48
49
Fig. 8. Recyclability of PACMX-Pd and EDAX-Pd.
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
3.4. Heterogeneity of PACMX-Pd and EDAX-Pd
In order to explore the heterogeneity of the catalysts, the following experiments
were conducted. After the typical Suzuki reactions proceeding for 1 h, the catalyst
was quickly filtered off and the filtrate along with the fresh base was stirred for
another 3 h at room temperature. Within 1 h of the reaction, the product yield was
70% and 59.5% for PACMX-Pd and EDAX-Pd (Table 1, entries 9 and 18),
respectively. However, the yield increased by only 1% and 3% for PACMX-Pd and
EDAX-Pd in the following 3 h, indicating the reaction was inefficient in the absence
of catalysts. Therefore, we can conclude that the catalysts were heterogeneous and
efficient for the Suzuki reactions.
4. Conclusions
In summary, we synthesized N bidentate ligand functionalized xylan supported
palladium composites and applied as heterogeneous catalysts for catalyzing Suzuki
cross-coupling reaction. The palladium/xylan heterogeneous catalysts showed unique
features, such as the good usability under air atmosphere, ease of separation,
24
Page 25 of 33

4
4
4
4
4
4
4
4
4
4
66
67
68
69
70
71
72
73
74
75
providing high yield of products with a small amount of catalysts and no Pd leaching
was occurred during the catalytic reaction. A series of aryl halides were effectively
coupled with arylboronic acid under room temperature by the catalysis of PACMX-Pd
-1
and EDAX-Pd, and giving excellent TOF values up to 9626 h . Additionally, the
catalysts can be reused for five times without considerable deactivation and the high
stability of PACMX or EDAX indicating their potential for grafting various metal
species as catalytic active centers for a wide range of metal-catalyzed reactions. The
xylan-based complex may also open the pathway for other organic catalysis reactions
in which Pd NPs are involved.
25
Page 26 of 33

4
4
4
4
4
4
4
4
4
4
4
4
4
4
76
77
78
79
80
81
82
83
84
85
86
87
88
89
Abbreviations
PACMX, 1, 10-phenanthrolin-5-amine functionalized carboxymethyl xylan;
PACMX-Pd, palladium grafted PACMX; EDAX, ethylenediamine functionalized
xylan; EDAX-Pd, palladium grafted EDAX; Pd NPs, palladium nanoparticles;
EDC·HCl, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride; NHS,
N-hydroxysuccinimide; DBU, 1, 8-diazabicyclo [5.4.0] undec-7-ene; EtOH, ethanol;
MeOH, methanol; THF, tetrahydrofuran; NaOH, sodium hydroxide; KOH, potassium
hydroxide; K
2
CO
3
, potassium carbonate; Na
2
CO , sodium carbonate; TEA,
3
triethylamine; FT-IR, fourier transform infrared spectra; XRD, X-ray powder
diffraction; XPS, X-ray photoelectron spectroscopy; TGA, thermal gravimetric
analysis; TEM, transmission electron microscopy; HR-TEM, high resolution
transmission electron microscopy; AAS, atomic absorption spectrometer; NMR,
nuclear magnetic resonance spectra; SEM, scanning electron microscope; EDX,
energy dispersive X-ray spectroscopy.
4
90
Acknowledgements
4
4
4
4
4
4
4
91
92
93
94
95
96
97
We wish to thank for the National Natural Science Foundation of China (31430092,
21736003), Guangdong Natural Science Funds for Distinguished Young Scholar
(2016A030306027, 2017A030306029), National Natural Science Foundation of
Guangdong (2017A030313130), Tip-top Scientific and Technical Innovative Youth
Talents of Guangdong Special Support Program (2015TQ01C488), State Key
Laboratory of Pulp and Paper Engineering and Fundamental Research Funds for the
Central Universities.
26
Page 27 of 33

4
4
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
98
99
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
References
Amoroso, F., Colussi, S., Del Zotto, A., Llorca, J., & Trovarelli, A. (2013).
Room-temperature Suzuki-Miyaura reaction catalyzed by Pd supported on rare earth
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