Reversing aggregation: direct synthesis of nanocatalysts from bulk metal. Cellulose nanocrystals as active support to access efficient hydrogenation silver nanocatalysts

Article abstract of DOI:10.1039/c5gc01281c

A highly atom-economical synthetic method to access nanocatalysts from bulk metal is described. A water suspension of cellulose nanocrystals was exposed to an Ag wire, under air and light exposure. In 2 weeks, Ag nanoparticles of size 1.3 nm ± 0.3 nm were deposited onto the biopolymer. These species were active for the hydrogenation of aldehydes, 4-nitrophenol, alkenes and alkynes.

Full text of DOI:10.1039/c5gc01281c

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Reversing aggregation: direct synthesis of
nanocatalysts from bulk metal. Cellulose
nanocrystals as active support to access
efficient hydrogenation silver nanocatalysts†
Cite this: DOI: 10.1039/c5gc01281c
Received 11th June 2015,
Accepted 25th August 2015
DOI: 10.1039/c5gc01281c
Madhu Kaushik, Alain You Li, Reuben Hudson, Mitra Masnadi, Chao-Jun Li and
Audrey Moores*
www.rsc.org/greenchem
A highly atom-economical synthetic method to access nanocata- cing agents or energy were needed to turn bulk into
lysts from bulk metal is described. A water suspension of cellulose nanomaterials.
nanocrystals was exposed to an Ag wire, under air and light
In this article, we present the novel synthetic method to
exposure. In 2 weeks, Ag nanoparticles of size 1.3 nm ± 0.3 nm access metal nanoparticles based on this principle, thus
were deposited onto the biopolymer. These species were active for bypassing two industrial steps necessary to access such nano-
the hydrogenation of aldehydes, 4-nitrophenol, alkenes and materials: (1) the need to form metal halide salts produced
alkynes.
industrially via reacting pure metals with halohydric acids and
2) the reduction of the resulting halogenide salts, or other
(
Many different synthetic methods have been developed to
access metal nanomaterials with well controlled size, shape,
aspect ratio and thus properties. However, because metal
nanoparticles (NPs) are kinetically stabilized materials, their
synthesis often relies on intensive use of solvents, reagents,
reducing and capping agents. The concept of atom economy,
derivatives originating from these salts. For this study, we used
cellulose nanocrystals (CNCs) as a water suspendable, and
mildly reducing support. Ag NPs were produced in days at
room temperature, in the presence of light, in water, from a
simple Ag wire and without using any other additive than
CNCs. Characterization by transmission electron microscopy
1
developed by Trost and adopted within the 12 principles of
(
TEM) and X-ray photoelectron spectroscopy (XPS), including
2
green chemistry for organic synthesis, remains to be systema-
Auger analysis, was performed to establish that the resulting
material was a hybrid composed of CNCs onto which small Ag
NPs were deposited (AgNPs@CNC). In this work, we used the
resulting Ag NPs in the catalytic hydrogenation of aldehydes to
3
tically applied to nanosynthesis. In recent years, efforts have
been made to develop more sustainable synthetic methods to
3
access these high-value materials, including solvent-free
4
3d,5
methods, biomass-based approaches
and greener reducers
9
afford primary alcohols in water, at 100 °C and 40 bars H2.
6
development. An interesting avenue relies on by-passing the
need to reduce metal salts and use a reduced metal form as
starting material. While metal (0) organometallic species have
been developed, they often require some synthetic effort and
careful handling. Bulk metal grinding is also a well-known
approach, suffering from poor NP size and shape control.
We (Li) had recently demonstrated the homogeneous version
10
of this reaction using Ag salts with ligands. AgNPs@CNC
were also effective in the hydrogenation of 4-nitrophenol,
alkenes and alkynes. The aqueous suspension of AgNPs@CNC
served as a biphasically separable catalytic system, recyclable
up to 4 times, with negligible leaching.
7
4
b
A
distinct and potentially milder approach relies on a result
CNCs are a class of functional nanomaterial readily
8
recently delineated by Hutchison and his group. Bulk Cu and
11
obtained from cellulose via sulfuric acid hydrolysis. CNCs
Ag materials were shown to generate nanoparticles in their
surface vicinity, as the results of exposure to moisture, light
and air. In this interesting work, no external oxidizing, redu-
have well-defined size and morphology, high specific surface
area, high aspect ratio, high crystalline order, chirality, high
mechanical strength, and controllable surface chemistry. In
addition, CNCs are inexpensive, renewable, biodegradable,
non-toxic, and accessible industrially in large scale, from a
1
2
local plant in Quebec. These properties have made CNCs
useful in a range of applications, including the production of
iridescent and birefringent films, reinforcing fillers in plastics
and polymers, chiral materials, hydrogels, aerogels and super-
Centre for Green Chemistry and Catalysis, Department of Chemistry, McGill
University, 801 Sherbrooke St West, Montreal, QC H3A 0B8, Canada.
E-mail: audrey.moores@mcgill.ca; Fax: +1 (514) 398 3797; Tel: +1 (514) 398 4654
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
1
1b,12b,13
c5gc01281c
capacitors.
They have also been shown to act as active
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1
4
15
and water suspendable support to stabilize NPs of Pd, Pt,
1
6
16a,17
16a
18
Au, Ag,
Cu
and Se. Specifically, AgNPs@CNCs were
, using
or CNCs themselves as a reducing
Ag NPs constitute interesting materials, used for
previously synthesized from Ag salts, such as AgNO
either NaBH ,
4
agent.
antibacterial applications,
oxidation, and alcohols cross-coupling reactions.
3
1
6a,17a–c
1
7d,17e
1
7a,d,e,19
19a,20
4-nitrophenol reductions,
21
22
In an initial set of experiments, silver wires were immersed
in CNC suspensions with concentrations of either 0.5%, 1% or
1
.5% (w/w) in pure water. Some vials were exposed to light,
while the others were kept in the dark (by wrapping with alu-
minium foil). All samples were air-exposed. After 45 days, the
Ag content in the samples was measured using inductively
coupled plasma-mass spectroscopy (ICP-MS) (Table 1).
Fig. 1 Ag content over 20 days by ICP-MS measurements. The values
observed are an average of five data points for each type of experiment.
The results were bimodal. High, and essentially similar,
concentrations of Ag were observed when CNCs were present
in the suspension and when the suspensions were exposed to
light (Table 1, entries 1–3). Any other circumstance led to the
presence of a baseline quantity of Ag (Table 1, entries 4–8).
Light proved essential to silver oxidation in accordance with
pended material was undergone. A sample of 0.5% CNC sus-
pension, stirred at room temperature for 20 days while
exposed to Ag wire, light and air was dried onto a Cu TEM
grid. On TEM images, CNCs were observed as well-dispersed,
low contrast whiskers and the Ag NPs appeared as darker
spots, mostly onto the CNCs, with an average size of 1.3 nm ±
8
the work of Hutchison. But CNCs were also essential, proving
their key role in the silver wire depletion.
0.3 nm (Fig. 2, S2–4†). The presence of Ag was confirmed by
The kinetics of this process was further explored with a
EDS (Fig. S4†). A careful selection of focusing conditions
allowed to overcome the usual difficulties associated with
0
.5% CNC suspension exposed to the Ag wire at room tempera-
ture (Fig. S1†). Its Ag content was monitored by ICP-MS upon
sampling on days 3, 7, 10, 15 and 20. Two sets of experiment
were carried out side by side, one in the presence, one in
absence of light. Fig. 1 shows a steady increase of the Ag
content in the CNC suspension for the first 10 days with light,
2
3
CNCs/metal hybrid composites imaging. Reduction of Ag
under TEM conditions was excluded based on time analysis,
Ag NPs size and specific location onto CNC and XPS results
2
4
(
vide infra). These TEM results were in agreement to the pre-
1
2b,23,25
−
1
viously reported CNC images,
showing that the CNCs
after which it reached a plateau around 160 μg L
.
This value
did not aggregate in suspension and retained both their high
surface area and rod-like form. Next, XPS experiments were
performed to gain insight into the Ag oxidation state. Precise
quantification of Ag(I) and Ag(0) species is difficult in the
photoelectron region, because both species have binding ener-
gies which are close. In the Auger region, Ag(I) and Ag(0) each
produce an individual peak, at 355.7 eV and 357.21 eV respect-
ively. They were both observed in AgNPS@CNCs samples
is comparable to results obtained at 45 days, indicating the
achievement of an equilibrium state. Again, absence of CNC or
light led to no significant increase in Ag concentration (Fig. 1).
The role of air was probed by immersing a Ag wire in a
degassed CNC suspension, in the presence of light. No signifi-
cant amount of Ag was observed in this case either.
The ICP-MS measurements were performed after proper Ag
digestion and were thus a means to quantify Ag in suspensions
in all its possible forms, i.e. Ag salts and suspended Ag col-
loids. In order to gain more insight into the role of the CNC in
the observed phenomenon as well as the nature of the ICP-MS
measured Ag, a complete characterization of the resulting sus-
(
Fig. S5a†). XPS being a surface technique, sputtering was
performed on the samples for 15 seconds and led to Ag(I)
peak disappearance, leaving the Ag(0) peak alone (Fig. S5b†).
Standards for this method were obtained using Ag metal
(
Fig. S6†).
Table 1 Ag content after 45 days in CNC suspensions exposed to Ag
wire, measured by ICP-MS. All samples were air exposed
CNC concentration
(in w/w)
Light
exposure
Ag content
1
Entry
(in μg L−
)
1
2
3
4
5
6
7
8
0.5
1.0
1.5
No CNC
0.5
1.0
1.5
No CNC
Yes
Yes
Yes
Yes
No
No
No
No
163.1
166.3
144.4
5.7
4.1
4.5
4.4
5.1
Fig. 2 TEM images of Ag NPs supported on CNCs.
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a
This study thus proved that: (1) Ag(0) NPs (with partially oxi- Table 2 Hydrogenation of unsaturated compounds using AgNPs@CNCs
dized surface) were produced at the surface of CNCs upon
b
Entry
1
Substrate
Product
Yield
immersing a Ag wire in a CNC suspension; (2) both CNCs and
light were necessary for this reaction to proceed. A proposed
mechanism is depicted in Fig. 3. According to the study of
Hutchison and coll., light, moisture and air are able to oxidize
96%
8
the Ag wire into Ag(I) salts. In absence of CNCs, the reaction
stops at this stage and the Ag concentration in suspension
remains marginal. In fact, in absence of potential support,
2
72%
2
6
AgNPs tend to dissolve in pure water. When CNCs are in sus-
pension, the Ag salts are consumed towards the growth of Ag
NPs at the CNC surface, causing more Ag depleting from the
wire and a sharp Ag concentration increase measured by ICP.
CNCs are nanowhiskers composed of cellulose strings packed
in a crystalline fashion. Cellulose itself is a polymer of cello-
biose, a dimer of glucose. Therefore, the surface of CNCs is
lined with hydroxyl groups, which have been shown before to
3
4
78%
83%
2
7
enable the reduction of Ag(I) back into Ag(0). Cellulose was
also shown to allow this reduction under microwave con-
2
8
ditions. In the CNC synthesis process, by sulfuric acid hydro-
lysis, some hydroxyl groups are turned into sulfate esters
5
6
43%
89%
(sulfur content = 3%). These sulfate groups have been proven
to favour stabilisation and nucleation of Ag NPs in a recent
1
7b
research.
A recent review on the evaluation of Ag NPs syn-
thesis according to sustainable principles may be used to
3
d
assess this novel method. Over the 8 evaluation criteria used,
this method ranks very high on 7 of them (reducing agent,
capping agent, solvent, local resources used, reaction tempera-
ture, equipment and size range).
7
8
98%
91%
Beyond its key role in the Ag NPs synthesis, the CNC sus-
pension also acted as a stabilizing medium for the nanocata-
lysts, which were stable as non-aggregated, small NPs, and a
readily usable and separable catalytic system. The
AgNPs@CNCs suspension obtained was tested as catalysts for
the hydrogenation of unsaturated compounds, including alde-
hydes, alkenes, alkynes and nitro, in water (Table 2). To start
with, benzaldehyde was chosen as the model substrate for
optimisation of temperature and pressure conditions
9
100%
0%
10
N/A
2
(Table S1†). At 100 °C and 40 bars H , benzaldehyde was suc-
a
b
Reaction conditions: 100 °C, 40 bars H
Determined by GC-MS.
2
, 24 h, water as solvent.
cessfully hydrogenated to benzyl alcohol with a yield of 96%
Table 2, entry 1). The homogeneous counterpart of this reac-
(
tion was recently demonstrated by us (Li) under comparable
conditions and AgNPs@CNCs offer an appealing means of het-
1
0
erogenizing this process in an atom-economical fashion.
Further screening was performed on substituted aromatic
aldehydes (Table 1, entries 2–5). Methyl and methoxy substi-
tution in para position, as well as hydroxyl in ortho, afforded
good yields. Trifluoromethyl however gave poor results, poss-
ibly due to solubility issues. Vinyl aldehydes also worked well,
with complete reduction of both CvO and CvC bonds (entry
Fig. 3 Pictorial representation of the mechanism for the generation of
Ag NPs from Ag wire in aqueous CNC suspensions.
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). Complete reduction of C–C double and triple bonds was ence (CREATE) in Green Chemistry and McGill University for
also demonstrated on styrene and phenylacetylene (entries 7 their financial support. We thank FPInnovations for providing
and 8). We were also pleased to see that a nitroarenes (entry 9) the CNC starting material and Kevin Wilkinson and Madjid
could be turned into an aniline derivative, in a process that is Hadioui from Université de Montreal for ICP-MS analysis. We
more atom economical than the classic NaBH , silver-catalysed are grateful to Hojatollah Vali and Kelly Sears for useful con-
4
reduction. To the best of our knowledge, this is the first versations on TEM analysis and Rajender Varma for interesting
example of this reaction being performed with Ag NPs and H
2
insight into silver nanoparticle synthesis. Elisabeth François-
as a reducer. Ketones however proved unreactive under these Moores is thanked for contributing to the graphical abstract
conditions (entry 10). The catalyst could also be easily recycled material.
by addition of an organic solvent followed by phase separation,
and the aqueous AgNPs@CNCs suspension was reused in a sub-
sequent reaction. The yields were quantitative in the first four
cycles, after which the activity reduced drastically (Fig. S7†).
Notes and references
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a drop in catalytic activity because of diffusion limitation. The
leaching of Ag was quantified from the product of benz-
aldehyde hydrogenation. It was determined that from the
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