Naked Au nanoparticles monodispersed onto multifunctional cellulose nanocrystal-graphene hybrid sheets: Towards efficient and sustainable heterogeneous catalysts

Article abstract of DOI:10.1039/c7nj03798h

A simple, clean, and efficient synthesis method has been developed for deposition of ～3 nm monodisperse "naked" gold nanoparticles (Au NPs) onto cellulose nanocrystal-graphene (CNC-G) hybrid sheets without the use of additional reducing and capping agents. The as-prepared catalysts (Au@CNC-G) were examined using FT-IR spectroscopy, thermogravimetric analysis (TGA), atomic force microscopy, electron microscopy, wide-angle X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). After some optimization, the Au@CNC-G catalyst exhibited robust catalytic activity, impressive stability, and good flexibility for the "one-pot" reaction of an alkyne, an amine, and an aldehyde (A³-coupling) in water. The catalytic activity of the Au@CNC-G catalyst could be attributed to the synergistic effect derived from the coexistence of CNC and graphene. The Au³⁺ ions could be in situ reduced to create fine Au NPs and simultaneously anchored on the surface of CNC-G hybrid sheets through coordination, leading to the uniform dispersion of the Au NPs. Moreover, the Au@CNC-G catalyst could be conveniently recycled and reused many times, thus lowering the cost and minimizing the environmental pollution caused by organic solvents and heavy metal ions.

Full text of DOI:10.1039/c7nj03798h

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Naked Au nanoparticles monodispersed onto multifunctional
cellulose nanocrystals-graphene hybrid Sheets：towards
efficient and sustainable heterogeneous catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
‖
Yameng Wang,a,‖ Huinian Zhang,b, Xiangjun Lin,a Shuai Chen,b Zhongjie Jiang,a Junzhong
www.rsc.org/
Wang,*b Jianlin Huang*a, Fang Zhangc, and Hexing Lic
A simple, clean, and efficient synthesis method has been developed for deposition of ∼3 nm monodisperse “naked” gold
nanoparticles (Au NPs) onto cellulose nanocrystals-graphene (CNC-G) hybrid sheets without the use of additional reducing
and capping agents. The as-prepared catalysts (Au@CNC-G) were examined using FT-IR spectroscopy, thermogravimetric
analysis (TGA), atomic force microscopy, electron microscopy, wide-angle X-ray diffraction, and X-ray photoelectron
spectroscopy (XPS). After some optimization, the Au@CNC-G catalyst exhibited robust catalytic activity, impressive
stability, and good flexibility for “one-pot” reaction of an alkyne, an amine, and an aldehyde (A3-coupling) in water. The
catalytic activity of the Au@CNC-G catalyst could be attributed to a synergestic effect derived from the coexistence of CNC
and graphene. The Au3+ ions could be in situ reduced to create fine Au NPs and simultaneously anchored on the surface of
CNC-G hybrid sheets through coordination, leading to uniform dispersion of Au NPs. Moreover, the Au@CNC-G catalyst
could be conveniently recycled and reused for many times, thus lowering the cost and minimizing the environmental
pollutions
caused
by
organic
solvents
and
heavy
metallic
ions.
particles to the supports.5,6 Gold nanoparticles (Au NPs) are
one of the most important catalysts for transformation of
various chemicals due to their outstanding catalytic
Introduction
properties.7
Notably,
the
size,
morphology,
and
Metal nanoparticles deposited onto the surface of various
supporting materials to form supported metal catalysts have
been widely studied in academic and industrial field.1,2
However, the synthesis of most supported metal catalysts
usually uses reducing agents, dispersing or capping agents, and
organic solvents, which not only are undesirable for the
environment but also increase the cost. Moreover, it is still a
huge challenge for stability and recyclability of these
supported metal catalysts because of the self-aggregation of
tiny metal particles3,4 and the insufficient bonding of the metal
monodispersity of Au NPs have a decisive effect on the
catalytic activity.8 However, very small Au NPs have a general
tendency of dissolution and self-aggregation under service
conditions. To overcome these problems, great efforts have
been devoted to rational design of supported Au NPs
catalysts.9 Thus, many kinds of materials have been used as
supports for Au NPS, including polymers,10 mesoporous
carbons,11 oxides,12 hybrid silica,13 and graphene14. However,
the preparation of supported Au NPs catalysts by these
methods usually use some reducing agents such as hydrazine
hydrate, sodium borohydride, hydrogen, or thiol-containing
silane based on the reduction of Au3+. Besides, these methods
generally require two or more steps to synthesize supported
Au NPs catalysts. Thus,
a facile, efficient, and greener
approach to synthesis of supported Au NPs catalysts is highly
desirable. Recently, Zhang et al. report that the Au NPs
deposited on cellulose nanocrystals (CNs) under hydrothermal
conditions using CNs as a reducing agent and stabilizing
template.15 Tang et al. present a novel and general strategy to
grow ultrafine Au clusters and other metal (Pt, Pd) clusters on
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reduced graphene oxide (rGO) sheets without any additional reduced graphene oxide.20 The multifunctional Au@CNC-G
protecting molecule or reductant.16
hybrid sheets were synthesized according to a co-assemble
strategy. Typically, a CNCs aqueous solution (10 ml, 19 mg mL-1)
and a graphene aqueous dispersion (40 ml, 4.8 mg mL-1) were
mixed by sonication for 10 min and further stirred at room
temperature for 30 min to get a homogeneous colloidal
solution. After that, a certain amount of 0.12 M HAuCl4 ethanol
solution was added into above colloidal solution and kept
under stirring at room temperature for 30 min. Then, the novel
Au@CNC-G nanosheets with different Au-loading was
obtained via a simple vacuum-filtration, washing and drying
procedure.
Scheme 1 Schematic synthesis of Au@CNC-G catalysts
Very recently, Shi et al. report a facile, one-pot, and green
The Au@CNC catalyst was prepared according to the
synthesis of cellulose nanocrystal (CNC)-supported Au NPs, modified procedure33. In a typical experiment, 40 mL of CNCs
avoiding toxic chemicals, extreme conditions, and complicated suspension (1.9 mg mL-1) was mixed with 40 mL of HAuCl4
procedures.17 Moores et al. reported the monodisperse and solution (0.4 mM). The mixture was stirred under magnetic
ultra-small gold nanoparticles, without external reducing stirring for 5 min to homogenize the suspension. Then, the
agents or bulk solvents.18 Despite the great progress in mixture was placed in a hydrothermal synthesis reaction kettle
designing facile and green synthesis of supported Au NPs at 120 °C for 10 h. Finally, the Au@CNC catalyst was obtained
catalysts, developing chemical synthesis to be conducted with via a simple vacuum-filtration, washing and drying procedure.
environmentally friendly and sustainable resources and
processes remains urgently needed for green chemistry.
For comparison, the Au@G and Au@HS-CNC hybrids were
also prepared by the similar process using graphene and HS-
In this study, we developed a simple, facile, and greener CNC as a support, respectively.
method for deposition of monodispersed gold nanoparticles Catalyst characterizations
(Au NPs) on multifunctional cellulose nanocrystals-graphene
The gold loadings were determined on an inductively
(CNC-G) hybrid sheets to obtain Au@CNC-G catalysts, which coupled plasma optical emission spectrometer (ICP-OES,
exhibit excellent catalytic activity, stability, and flexibility. Both Varian VISTAMPX). The samples electronic states were
the CNC and the graphene in the hybrid structure play an analyzed by X-ray photoelectron spectroscopy (XPS
importent role in the reduction, growth, and immobilization of measurements were performed using a Thermo ESCALAB 250
Au NPs. Experimental results suggest that the synergic effect spectrometer, employing an Al-KR X-ray source with a 500 mm
was derived from the abundant HO-groups on the surface of electron beam spot). All the binding energy values were
CNC and a strong interfacial coupling of graphene with small calibrated by using C 1s = 284.6 eV as a reference. X-ray
oxygen containing functional groups. The optimized Au@CNC- powder diffraction (XRD) was recorded at a scan rate of
G catalyst exhibited much higher catalytic activity and stability 0.02 °/s using the Cu Ka (λ = 1.540600 Å) line. Transmission
than that supported only by CNC, graphene, or other supporting electron microscopy (TEM) images were performed with a
mayterials in the A3-coupling reactions. Such a facile, efficient, JEM-2100F microscope at an acceleration voltage of 200 kV.
and greener strategy may be applicable to the preparation of The samples were supported on ultrathin carbon film-
supported metal nanoparticles catalysts for other applications. supported Cu TEM grids. The FTIR spectra characterization were
carried out with Nicolet Magna 550. The thermogravimetric (TG)
Experimental section
analyses were carried out on a STA 449C under a stream of N2 at a
heating rate of 5 °C min−1
.
Materials
Catalytic activity tests
In a typical run of the water-medium A3-coupling reaction,
Reagents: All chemicals were analytical grade reagents and
used as received without further purification. Graphite (99.95% 0.60 mmol phenylacetylene, 0.4 mmol benzaldehyde, 0.48
purity, Qingdao Huarun Graphite Co., Ltd.), Cotton from mmol piperidine, 2.0 ml H2O and 0.57 mol% Au@CNC-G
Whatman cellulose filter aid was used as the cellulose source. catalyst were mixed in a 25 ml oven dried reaction tube. After
Deionized water (18.2 MΩ cm, Millipore Milli-Q Purification reacting in nitrogen atmosphere at 80 °C for 12 h, the reaction
System) was used throughout. Sulfuric acid, HAuCl4, solvents mixture was filtrated and then extracted with acetone,
and other reagents were purchased from Sigma-Aldrich followed by analyzing on a gas chromatograph (GC-MS, Agilent
chemical company.
5977E) equipped with a JWDB-5, 95% dimethyl 1-(5%)-
diphenylpolysiloxane column and a FID detector. The column
Catalyst preparation
Au@CNC-G catalyst: Firstly, suspensions of cellulose temperature was programmed from 50 to 320°C at a ramp
nanocrystals (CNCs) were prepared as previously described by speed of 5°C/min N2 was used as carrier gas. For product NMR
hydrolysis using 64% w/w sulfuric acid at 45 °C for 45 minutes analysis, after reaction mixture was diluted with 0.5 mL deuterated
under constant stirring.19 A large size of 10
defective graphene sheets that were synthesized by
modified electrochemical exfoliation of graphite to replace MgSO4, then filtered and analyzed by 1H NMR. 1H NMR spectra
∼40 μm and weakly chloroform (CDCl3), filtered and eluted through a short silica gel
a
plug with more deuterated chloroform (2 x 0.5 mL), and dried over
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Figure 1. (a, b) Electrochemically exfoliated graphene: (a) an
SEM image, (b) a Raman spectrum, and a typical selected area
electron diffraction pattern (SEAD) of a graphene flake. (c)
TEM image and (d) AFM of Cellulsoe NanoCrystals.
were recorded on a Varian 400 MHz spectrometer and the chemical
shifts were reported in parts per million (δ) relative to internal
standard TMS (0 ppm) and all NMR spectra were recorded at room
temperature. The reaction conversions were calculated based on
benzaldehyde, and the selectivity is 100%. The reproducibility was
checked by repeating the experiments at least three times. In order
to determine the recyclability of the Au@CNC-G catalyst, it
was allowed to settle after each run of the reaction, and the
clear supernatant liquid was decanted slowly. The flexible film
catalyst was washed thoroughly with acetone and re-used with
a fresh charge of reactants for subsequent recycling under the
same reaction conditions. The amount of gold leached from
the heterogeneous catalyst was also calculated by ICP analysis.
(a)
(d)
(c)
(f)
(b)
(e)
CNC
Results and discussion
Figure 2. (a-c) TEM images of self-assembled CNC-G hybrid.
(d,e) TEM images of monodisperse Au NPs on the surface of
CNC-G hybrid, (f) HRTEM image of a representative gold
nanoparticle in (e).
The preparation of the novel Au@CNC-G catalysts was briefly
illustrated in Scheme 1 (see catalyst preparation for details).
First, the cellulose nanocrystals and graphene sheets are
synthesized respectively according to pervious works.19,20
Graphene flakes have large lateral sizes of ∼30 μm (Figure 1a), and
single-crystalline like (Figure 1b inset). The Raman spectrum (Figure
1b) presents that relatively strong intensity of D at 1334 cm-1 could
be assigned to significant defects, which benefits good dispersion in
22
ethanol or water.21,
As shown in Figure 1c, the rod-like
morphology of the CNC has been distinctly seen, and their
dimensions are about ∼20 nm wide by ∼200 nm long, which could
be further confirmed by the AFM pictures (Figure 1d). The
multifunctional CNC-G hybrid sheets are prepared by a co-assemble
strategy. Shortly, a CNCs aqueous solution (10 ml, 19 mg mL-1) and a
graphene aqueous dispersion (40 ml, 4.8 mg mL-1) were mixed by
sonication for 10 min and further stirred at room temperature for
30 min to get a homogeneous colloidal solution. Their morphology
are analysed by TEM (Figure 2a-c, S1), which confirmed the
CNC were homogeneously coated well on the surface of
graphene sheets via a hydrogen bonding interaction and the
rod-like morphology and size of the CNC in hybrid sheets could
been retained well after reaction. The TEM image (Figure 2d)
also revealed that a large number of Au NPs were formed and
uniformly distributed on the surface of CNC-G hybrid sheets.
Figure
3. The wide-angle XRD patterns for CNC-G and
Au@CNC-G (a). XPS spectra of Au4f in Au@CNC-G and Au@G
(b), C1s and O1s in CNC-G and Au@CNC-G (c) and (d).
The high-resolution TEM (HR-TEM) images (Figure 2e, f) show
that the characteristic lattice of Au NPs can be clearly seen and
the lattice space and size are estimated to be 0.23 and
∼3 nm
respectively.23 As shown in Figure 3a, in comparison with CNC-
G hybrid sheets, the wide-angle XRD diffraction patterns of
Au@CNC-G shows additional four peaks derived to the 311,
220, 200 and 111 diffractions of face-centred cubic Au NPs.24
In addition to the four peaks, the XRD pattern of the both
samples also exhibits a clear cellulose Iβ polymorph with the
200 diffraction centred about 23° and a peak signal around 16°
due to 110 plane,25 and the two peaks intensity obviously
decrease after deposited Au NPs. A small peak was also
observed around 11° distributed to electrochemically
exfoliated graphene with some oxygen groups.26 The XRD
patterns confirm the existence of Au NPs in the Au@CNC-G,
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and the deposition of Au NPs on multifunctional CNC-G hybrid groups on the surface of CNC could play a dominant role in
sheets did not change the crystallographic property of CNC. dispersion and stability of Au NPs, while the strong interface of
The in situ reduction and growth of Au NPs on the CNC-G graphene sheets might mainly induce reduction in Au3+ to Au0
hybrid sheets ensures that the Au NPs contact with the CNC-G during the redox process, which can be further confirmed by
hybrid sheets
interaction/coupling of the CNC-G hybrid sheets exhibited an groups (e.g., C-H, C-O, C=C, COOH) disappeared for the Au@CNC-G
excellent ability to anchor Au NPs, leading to induce and catalyst due to the redox reaction between the Au3+ and graphene
confine the growth of Au NPs.27 Therefore, the size of Au NPs The similar reaction mechanism have also been reported by
in CNC-G hybrid sheets was able to be controlled of
3 nm and Wu et al.31 Moreover, the graphene could also promote the
well, and
the strong interfacial FTIR (Figure 4) that in comparison with CNC-G, most functional
.
∼
the Au NPs are homogeneously dispersed on the surface of nucleation and growth of Au NPs. We further investigated the
CNC-G hybrid sheets (Figure 2d). The Au@CNC-G catalyst was effects of Au NPs’ growth process under different conditions
further characterized by X-ray photoelectron spectroscopy (Figure S3). The TEM images indicated that prolonging the
(XPS) to investigate the surface nature of Au NPs and CNC-G reaction time leaded to the gradually growth of Au NPs but did
hybrid sheets. The XPS spectra (Figure 3b, c, d) showed that not result in an obviously increase, indicating that an
the binding energy of the Au species in the Au@CNC-G was instantaneous nucleation process occurred and that no
84.71 and 88.43 eV corresponding to Au4f5/2 and Au4f7/2 in the marked self-aggregation happened following nucleation. This
metallic state,9, 28 which demonstrated that the Au3+ ions have phenomenon also implies a strong anchoring effect between
been reduced to the Au0 states. The binding energy of Au0 in the Au nuclei and the surfaces of CNC-G hybrid sheets.
Au@CNC-G shifted negatively by 0.18 eV in comparison with However, when the reaction temperature was over 40
℃, self-
the corresponding Au0 in Au@G, suggesting that the Au0 aggregation obviously occurred, leading to rapidly increase the
accepted more electrons from the CNC-G hybrid sheets than particle size, which might be attributed to the weaker
that from only graphene sheets. This could be attributed to the anchoring ability of the surface of CNC-G hybrid sheets to Au
more electrons-donation derived from abundant HO-groups nuclei,32 and the CNC could also boost the Au3+ reduction
on the surface of CNC, leading to the higher electron density leading to easier self-aggregation of Au NPs at higher
on the surface of CNC-G hybrid sheets. The Au oxidation temperature.33 More interesting, the morphologies and sizes
states29, 30 are absent in the XPS spectrum of the Au@G and of Au NPs, as well as the flexibility of as-prepared Au@CNC-G
Au@CNC-G catalysts, which can further confirm the completed catalyst were almost intact after dispersion in aqueous
reduction of Au3+ to Au0. Meanwhile, in comparison with CNC- solution for three weeks (Figure 5), which is very important for
G hybrid sheets, the binding energy of C1s and O1s in organic reactions in aqueous media.34
Au@CNC-G shifted positively by 0.29 and 0.39 eV respectively,
which further confirmed the transformation from the Au3+ to
Au0, and the Au NPs evenly dispersed on the surface of CNC-G
hybrid sheets by coordination model. Furthermore, it is worth
mentioning that the size of Au NPs increased to several
100
80
60
COOH,
C=C
C-O
alkoxy
-CH
-OH
Figure 5. Photographs and TEM pictures of Au@CNC-G catalyst
was dispersed before (a), (b) and after (c), (d) in water for 3
weeks respectively.
40
20
0
Au@CNC-G
CNC-G
C-O
(expoxy)
Furthermore, thermogravimetric analysis (Figure S7) showed
that the deposition of Au NPs onto CNC-G significantly
improved the thermal stability of the Au@CNC-G catalyst,
which might be due to a stronger chemical coordination
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
Figure 4. FT-IR spectra of the CNC-G, and Au@CNC-G catalyst between the Au NPs and supports.
respectively.
In order to explore the catalytic application for these novel
hundred nanometers at the same conditions without addition
of the CNC (Figure S2). While, there is no Au NPs observed
without addition of the graphene at present conditions. These
results indicated that the CNC and graphene play a synergic
promoting effect in the formation and growth process of Au
NPs. Therefore, it can be deduced that the abundant HO-
Au@CNC-G catalysts, the water-medium one-pot reaction of
phenylacetylene, benzaldehyde, and piperidine (A3-coupling)
was selected as the probe reaction to examine the catalytic
performances. First, the catalytic performances of the
Au@CNC-G catalysts with different synthetic conditions were
investigated (Figure S4). The optimum Au@CNC-G catalyst
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provided excellent yield (95%) of propargylamines (Table 1, aliphatic and aromatic aldehydes. Even unreactive aldehydes
entry1). With the optimum catalyst on the hand, the catalytic
such as 1-Naphthaldehyde can be transformed by the
Au@CNC-G catalyst into the corresponding propargylamine
product with high yield (84%). For the long chain aldehydes
with ten-carbon chains had a lower activity, giving lower yields
Table
1:
Three-component coupling of benzaldehyde, (39%). This can be attributed to the lower reactivity of the
Table 2. Three-component coupling of aldehyde, amine, and
phenylacetylene catalyzed by Au@CNC-G catalyst in water
medium.a
piperidine, and phenylacetylene catalyzed by Au-based
catalysts.a
Entry
Aldehyde
Amine
Product
Yield(%)b
1
2
3
4
5
6
7
formaldehyde
piperidine
piperidine
piperidine
piperidine
piperidine
piperidine
piperidine
98
95
94
86
84
83
82
benzaldehyde
Propargylamine Yield
Entry
Catalysts
(%)b
95
31
80
74
73
98
0
ethylbutyraldehyde
isobutyraldehyde
1-Naphthaldehyde
2-Methylbutyraldehyde
Cyclohexanecarboxaldehyde
1
2
3
4
5
6
7
Au@CNC-G
Au@G
Au@GOc
Au@CNCd
Au@HS-CNCe
HAuCl4
CNC-G
8
9
Hydrocinnamaldehyde
decanal
piperidine
piperidine
55
39
aAll reactions were carried out with benzaldehyde (0.4 mmol),
piperidine (0.48 mmol), phenylacetylene (0.6 mmol), 2 mL H2O
and catalyst containing Au (0.57 mol %) in a sealed well tube, at
80 °C for 12 h. bYield were determined by GC-MS and 1H NMR of
the crude reaction mixture. c,d,eCatalysts were prepared by
using a modified procedure reported as Ref. 32, Ref. 33 and
Ref. 25 respectively ).
10
11
Valeraldehyde
benzaldhyde
piperidine
47
66
morpholine
activation of different catalysts were also studied that the pure
CNC-G hybrid sheets (Table 1, entry 7) showed inactivity,
indicating Au NPs is active sites, as well as the Au@G (Table 1,
entry 2) gave very poor performances may be attributed to the
formation of large aggregates of Au NPs (Figure S2). While the
Au@GO and Au@CNC catalysts also gave the moderate yields
(80 and 74 %) under present conditions, respectively.
Meanwhile, the optimum catalyst Au@CNC-G exhibited a
slight performances increase compared with Au@HS-CNC
catalyst, which gave 93% yield (Table 1, entry 5), and
comparable to that of the homogeneous catalyst (Table 1,
entry 6). Notably, the optimum Au@CNC-G catalyst
demonstrated the higher activity compared to the most of
other supported Au NPs catalysts recently reported (Tables
S1). These results further suggest that the multifunctional
CNC-G hybrid sheets show the synergic promoting effect
attributed to HO-groups of CNC resulted in the uniform
dispersion of Au NPs and strongly anchored to the surface of
CNC-G hybrid sheets, meanwhile the more exposed active specific
surface area was provided derive from large “naked” Au NPs
12
13
14
15
16
17
18
benzaldhyde
pyrrolidine
azepane
43
34
97
87
64
60
54
benzaldhyde
formaldehyde
formaldehyde
formaldehyde
ethylbutyraldehyde
ethylbutyraldehyde
morpholine
pyrrolidine
azepane
morpholine
pyrrolidine
19
ethylbutyraldehyde
azepane
27
aAll reactions were carried out with aldehyde (0.4 mmol),
amine (0.48 mmol), phenylacetylene (0.6 mmol), 2.0 mL H2O
and catalyst containing Au (0.57 mol %) in a sealed well tube,
at 80 °C (oil bath) for 12 h. Yields were determined by GC-MS
and 1H NMR of the crude reaction mixture.
b
terminal aldehyde with ten-carbon chains. We also found
moderate to good yields when the cyclic dialkylamines such as
pyrrolidine, morpholine and azepane were used (Table 2,
entries 11-19). These results furtherly indicated the generality
of the strategy for the efficient synthesis of propargylamine
products from various aldehydes and amines.
thereby benefited fast diffusion of reactants and products
.
Furthermore, we examined the substrate scope with various
substrates including amines and aldehydes under the optimized
reaction conditions (Table 2). The water-medium A3-coupling
reactions proceeded with good to excellent products for
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The most important superiority of the supported catalysts amount of Au in the reaction mixtures, which strongly
was that they could be easily recycled and used repetitively. As suggested that this catalytic reaction was a heterogeneous
shown in Figure 6, the optimum Au@CNC-G catalyst displays
process in nature.9
Conclusions
In summary, we have developed a simple and clean process for
deposition of
∼3 nm naked Au nanoparticles onto
multifunctional CNC-G hybrid sheets without using any
reducing and capping agent. The “naked” Au NPs bonded
strongly to the surface of CNC-G hybrid sheets. The Au@CNC-G
hybrid catalyst displayed superior catalytic activity for A3-
coupling reaction in a water-medium. Experimental results
suggest that a strong synergetic effect among the three
components of the hybrid catalyst system play a decisive role
on the structure and properties of the tightly hybridized
material. The HO-groups on the surface of the CNC
coordinated and coupled with graphene could in-situ reduce
Au3+ ions to create narrow nanosized Au NPs anchored on the
surface of CNC-G hybrid sheets. Moreover, this synergetic
effect also increased the catalyst durability due to the
enhancement of metal-support interaction and the high
hydrothermal stability of Au@CNC-G catalysts. It is also
demonstrated that the catalyst can be recycled ten times
without significant deactivation. These findings pave the way
for the greener synthesis of nanocellulose/graphene-
supported noble catalysts for applications in catalysis,
environment and energy storage.
Figure 6. The recycling test of Au@CNC-G for the three-
component coupling of formaldehyde, piperidine, and
phenylacetylene (A3-coupling) in water-medium. The reaction
conditions were given in Table 1.
impressive durability in water-medium A3-coupling of
formaldehyde,
piperidine,
and
phenylacetylene.
The optimized Au@CNC-G catalyst could be reused for more
than 10 times with negligible activity loss. On one hand, the
“naked” Au NPs strong covalently bonded to the surface of
CNC-G hybrid sheets could effectively inhibit the self-
aggregation and leaching of Au NPs active sites. On the other
hand, the highly homogenous monodispersitity of Au NPs on
the surface of CNC-G hybrid sheets could devote the maximum
exposed active specific surface area. This synergistic promoting
effect could be explained by the strong interfacial interaction
between the Au NPs and CNC-G hybrid sheets by means of
coordination, additionally, the good dispersion of CNC in the
graphene sheets resulting in a highly monodispersitity of Au
NPs in Au@CNC-G. Besides the high recycling efficiency of the
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
This work was supported by three grants form National Natural
Science Foundation of China (21403274, 21503259, 11474101,
21373255), the Hundred Talent Program of Chinese Academy of
Sciences (2015YCR001), and the Fundamental Research Funds
catalysts with negligible loss of Au active sites (ICP analysis: the for the Central Universities (2015ZM151), the Guangdong
Innovative and Entrepreneurial Research Team Program (No.
2014ZT05N200).
optimum Au@CNC-G catalyst has Au loading of 0.558 mol%
(initial loading of 0.57 mol%) after being reused 10 times,
which indicated there is neglected free Au species leaching),
the identical, FTIR spectra (Fig. S5), XPS spectra (Fig. S6), TG
curves (Fig. S7), and TEM pictures (Fig. S8) clearly
demonstrated the preservation of the catalyst structure,
morphology and the chemical composites after being reused
10 times, which could sufficiently account for the excellent
durability of the catalysts upon recycling. For heterogeneous
catalysts operating in liquid phase systems, the key point is
that the active components from heterogeneous catalyst can
leach into the reaction mixture, leading to formation of an
active homogeneous catalyst.35 In order to testify: the
heterogeneous catalyst was removed by hot filtering when the
conversion was up to 50% in A3-coupling reactions, and then
the solution reaction was continued under the same
conditions. The conversion of the aldehyde remained
unchanged, and ICP-AES analysis confirmed an undetectable
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