Nickel nanoparticles supported on graphene as catalysts for aldehyde hydrosilylation

Article abstract of DOI:10.1016/j.molcata.2015.11.011

Nickel nanoparticles (NPs) supported on different undoped or doped with N or B graphenes (Gs) have been tested as catalyst for the hydrosilylation of aldehydes to obtain the corresponding siloxanes with high conversion and good selectivity in short reaction time. The different Gs employed were obtained by pyrolysis under inert atmosphere of alginate or chitosan, modified or not with boric acid. Then the metal NPs obtained by polyol reduction method using ethylene glycol were adsorbed on Gs. The Ni-containing G catalysts were characterized by electron microscopy, XPS and Raman spectroscopy. The scope of the Ni/G catalyst includes aliphatic and aromatic aldehydes as well as a variety of hydrosilanes.

Full text of DOI:10.1016/j.molcata.2015.11.011

Journal of Molecular Catalysis A: Chemical 412 (2016) 13–19
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Nickel nanoparticles supported on graphene as catalysts for aldehyde
hydrosilylation
Juan F. Blandeza, Iván Esteve-Adella, Ana Primoa, Mercedes Alvaroa,
Hermenegildo Garcíaa,b,∗
a Instituto Universitario de Tecnología Química CSIC-UPV and Departamento de Química, Univ. Politécnica de Valencia, Av. de los Naranjos s/n, 46022
Valencia, Spain
b Center of Excellence for Advanced Materials Research, King Abdullaziz University, Jeddah, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel nanoparticles (NPs) supported on different undoped or doped with N or B graphenes (Gs) have
been tested as catalyst for the hydrosilylation of aldehydes to obtain the corresponding siloxanes with
high conversion and good selectivity in short reaction time. The different Gs employed were obtained by
pyrolysis under inert atmosphere of alginate or chitosan, modified or not with boric acid. Then the metal
NPs obtained by polyol reduction method using ethylene glycol were adsorbed on Gs. The Ni-containing
G catalysts were characterized by electron microscopy, XPS and Raman spectroscopy. The scope of the
Ni/G catalyst includes aliphatic and aromatic aldehydes as well as a variety of hydrosilanes.
© 2015 Elsevier B.V. All rights reserved.
Received 18 September 2015
Received in revised form
11 November 2015
Accepted 12 November 2015
Keywords:
Heterogeneous catalysis
Nickel nanoparticles
Graphene as support
Aldehyde hydrosilylation
1. Introduction
higher activity than Cu for the oxidative coupling of silanes and
alcohols, Cu as catalyst has advantages in terms of affordability and
Due to its large surface area, extended ꢀ orbitals, 2D morphol-
ogy and high dispersability, graphene (G) and related materials are
suitable supports of metal nanoparticles (MNPs) exhibiting high
catalytic activity [1–7]. In some cases, it has been found that the
activity of MNPs supported on G is better than the activity of
these MNPs on other supports including different types of car-
bon nanoforms and metal oxides [3,4,7,8]. On one hand, overlap
between the p and d orbitals of transition metal with ꢀ orbitals
of G can modulate the electronic density on the MNPs and also
may result in a strong metal–support interaction necessary to avoid
leaching and minimize MNP size growth. On the other hand, the
adsorption capacity of G bringing substrates and reagents near the
active MNP can contribute to increase the reaction rates.
Recently, we have reported that Cu NPs supported on G is a
highly efficient catalyst to promote the dehydrogenative coupling
of alcohol with hydrosilanes (Eq. (1)) [9]. This reaction has the
advantage over the conventional silylation (Eq. (2)) of overcoming
the use of halosilanes and minimizing the production of corrosive
byproducts (HX), increasing atom efficiency. Although Pd exhibits
sustainability. Similarly, the dehydrogenative coupling of hydrosi-
lanes and amines (Eq. (3)) has also been recently reported using Pd
supported on G as catalyst [10]. For both processes, it was found
that G as support leads to a more efficient catalyst than when the
MNPs are deposited on other materials.
(1)
(2)
(3)
(4)
Continuing with this line of research aimed at exploiting the
potential of Gs as support of MNPs, in the present manuscript we
describe that Ni NPs supported on G is a convenient catalyst for
∗
Corresponding author at: Center of Excellence for Advanced Materials Research,
King Abdullaziz University, Jeddah, Saudi Arabia. Fax: +966 34963877809.
E-mail address: hgarcia@qim.upv.es (H. García).
http://dx.doi.org/10.1016/j.molcata.2015.11.011
1381-1169/© 2015 Elsevier B.V. All rights reserved.

14
J.F. Blandez et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 13–19
Fig. 1. Two TEM images at different magnification taken for Ni/G. The inset shows the size distribution of Ni NPs.
the addition of silanes to aromatic aldehydes. This reaction (Eq.
(4)) is similar to the Strecker addition of halosilanes to aldehydes
catalyzed by diluted acids. Also in this case, the hydrosilylation has
the advantage of avoiding halosilanes as reagents.
are formed by reduction of ethylene glycol and becoming sponta-
neously deposited on G in the same step.
2.2. Catalytic tests
The corresponding catalyst (0.06 mmol% of metal respect to sub-
strate) was introduced in an ampoule equipped with a magnetic
bar. The aldehyde (10 mmol) was added under argon atmosphere
and the ampoule was sonicated for 30 min. Then, hydrosilane
(5 mmol) and dodecane as internal standard were added to the
ampoule and the reactor sealed. The reaction mixture was mag-
netically stirred at 120 ◦C in an oil bath preheated at the reaction
temperature. At the end of the reaction, the reaction mixture was
cooled to room temperature and an aliquot of 0.1 mL was diluted
with anhydrous toluene (0.5 mL), filtered, and injected into GC,
determining the conversion and product yields based on the inter-
nal standard. The rest of the reaction mixture was filtered and the
2. Experimental
2.1. Catalysts synthesis and characterization
As supports of MNPs and based on related precedents for the
dehydrogenative coupling of hydrosilanes and alcohols or amines
[9,10], a G obtained by exfoliation of turbostratic graphitic carbon
obtained by pyrolysis of alginate was used. In addition, B- and N-
doped Gs [(B)G and (N)G, respectively] were obtained pyrolysing
the borate ester of alginate [(B)G] or chitosan [(N)G] at 1000 ◦C
under inert atmosphere, followed by exfoliation of the graphitic
carbon residues by sonication [11–13]. For the sake of comparison
graphene oxide (GO) prepared from graphite by Hummers oxi-
dation and exfoliation was also included as support in the study
[14].
All these Gs are well documented in the literature and had
been previously used as metal-free catalysts or as support of MNPs
[9,10,15–17]. The G samples employed in the present study were
characterized by TEM, AFM, Raman and XPS and their properties
coincide with those reported in the literature for these G samples
[10,18–21].
MNPs were obtained by the polyol method consisting in the
reduction of salts of the corresponding metals in hot ethylene glycol
[22]. Deposition was performed simultaneously to the formation
of the MNPs by suspending the corresponding G in ethylene gly-
col at the same time that the reduction of the metal ions is taking
place [22]. In the present study, MNPs of Cu, Ni and Pd supported
on various Gs were prepared. The Cu/G and Pd/G catalysts corre-
spond to those samples previously reported in the literature [9,10].
G-supported catalysts were analyzed to determine the metal con-
tent that was 5 wt% in all cases. TEM allowed determining the
morphology and average MNP size and XPS was used to establish
the oxidation state of Ni. The single layer morphology of Ni/G was
established by AFM.
The Ni NPs employed as catalysts for the addition reaction
between the aldehyde and the hydrosilane were supported on the
G samples following the synthesis described in the literature [10].
Accordingly, a suspension in ethylene glycol of graphene and the
metal precursor, NiCl2, was sonicated for 1 h. After sonication the
mixture was heated at 120 ◦C for 24 h at reflux temperature. Ni NPs
liquid phase diluted in deuterated chloroform and analysed by 1
H
NMR spectroscopy (Varian Geminis 300 MHz) to determine prod-
uct selectivity. Supporting information contains spectroscopic data
of the hydrosilylation products 3a–h.
3. Results and discussion
As support of MNPs and based on related precedents for the
dehydrogenative coupling of hydrosilanes and alcohols or amines
[9,10], a G obtained by exfoliation of turbostratic graphitic car-
bon resulting from pyrolysis of alginate was initially used. Two
additional doped Gs, (B)G and (N)G, were obtained pyrolysing at
1000 ◦C under inert atmosphere the borate ester of alginate [(B)G]
and chitosan [(N)G], followed by exfoliation of the graphitic car-
bon residues by sonication [11,13]. For the sake of comparison GO
prepared from graphite by Hummers oxidation and exfoliation was
also included as support in the study [14].
All these Gs are well documented in the literature and had been
previously used either as metal-free catalysts or as support of MNPs
[9,10,15–17]. The G samples employed in the present study were
characterized by chemical analysis, TEM, AFM, Raman and XPS and
their properties coincide with those reported in the literature for
these G sample [10,18–21].
MNPs were obtained by the polyol method consisting in the
reduction of the salts of the corresponding metals in hot ethylene
glycol [22]. Deposition was performed simultaneously to the for-
mation of the MNPs by suspending G in ethylene glycol at the same
time that the reduction of the metal ions was taking place [22]. In

J.F. Blandez et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 13–19
15
Fig. 2. XRD and Raman spectrum for the Ni/G sample used as catalyst.
the present study, MNPs of Cu, Ni and Pd supported on various Gs
were prepared. The Cu/G and Pd/G catalysts correspond to those
samples previously reported in the literature [9,10]. G-supported
catalysts were characterized by: (i) chemical analysis to determine
the metal content, that was 3.2 wt%, (ii) TEM allowed to determine
the morphology and average MNP size, and (iii) XPS was used to
establish the oxidation state of Ni. The single layer morphology of
Ni/G was established by AFM.
Fig. 1 shows two representative TEM images recorded for Ni/G
and the histogram of the Ni NP size distribution that gives an
average dimension of 4 nm. The average particle size estimated by
statistical analysis of TEM images coincide with the average particle
size calculated from XRD pattern of Ni/G (Fig. 2) using the width of
the most intense (1 1 1) peak corresponding to Ni metal by applying
the Scherrer equation.
Besides on G, Ni NPs were also supported on (N)G and (B)G. In
the literature there are theoretical calculations as well as experi-
mental data showing that the presence of heteroatoms on G can
increase the interaction of this support with MNPs, providing a
strong anchoring of Ni NPs on G and modulating the electronic den-
sity of the MNPs [23]. In addition, heteroatoms on G, such as B or N,
can act as active centers in the reaction mechanism cooperating to
the catalysis [24–29]. For the sake of comparison, the present study
also includes Cu NPs supported on (N)G and (B)G that correspond
to the samples already reported [9].
3.1. Catalytic activity
In the initial stage of our work, the addition of dimethylphenylsi-
lane (1a) to benzaldehyde (2a) to form benzyloxyphenyldimethyl-
silane (3a) was selected and proceeded to optimization of the
reaction conditions and screening of the catalyst activity of the
various samples. The results obtained are summarized in Table 1.
Cu/G has been found previously a good catalyst for the dehydro-
genative silylation of alcohols [9] and, therefore, it was an obvious
candidate to check as catalyst for aldehyde hydrosilylation. As it
can be seen in Table 1, while under certain conditions Cu/G can
promote a high conversion of silyl ethers (high Cu ratio, 120 ◦C),
the selectivity toward the target compound was always unsatisfac-
tory due to the formation of disiloxane (4a). Under other conditions,
selectivity to the addition product 3a using Cu/G could be high (see
Table 1, entry 2), but, then, conversion was unsatisfactorily low.
The use of N(G) or B(G) as supports instead of G does not appear
to play a crucial role enhancing the performance of Cu. In con-
trast to Cu catalysts, Pd/G was able to achieve complete conversion
with essentially complete selectivity in very short reaction times
(Table 1, entry 18). Besides Pd, we were interested in determining
whether or not affordable, first-row transition metals could also
lead to high yields and selectivity toward 3a. Aimed at this purpose
the activity of Ni/G was screened at temperatures between 80 and
140 ◦C and loadings from 0.012 to 0.06 mol% (Table 1, entries 9–17).
As it can be seen in Table 1, using Ni/G at 120 ◦C and working at
0.06 mol% of Ni, very high conversions with almost complete selec-
tivity were attained already at 3 h reaction time, conversion and
selectivity increasing slightly from 3 to 5 h reaction time (Table 1,
entries 10 and 11). Fig. 4 shows the temporal evolution of 1a conver-
sion and 3a yield using Ni/G as catalyst (0.06 mol%) under optimal
conditions. Decreasing the temperature results not only in lower
conversion for 24 h reaction time, but also in a lower selectivity
to 3a due to the formation of disiloxane (4a). Decreasing Ni to
substrate mol ratio from 0.06 to 0.012 leads to a remarkable high
XPS measurements showed the presence of Ni and C in the Ni/G
sample. Fig. 3 presents the experimental high resolution C 1s and
Ni 2p recorded for Ni/G and the best deconvolution to individual
components. According to this analysis there are three types of C at
binding energy (BE) values of 287.7, 284.5 and 282. 5 eV in a atomic
proportion of 7, 41 and 37%, respectively. While the two compo-
nents with the higher BE can be safely attributed to C atoms bonded
to O and graphenic C, respectively, the component with the low-
est BE value is unusual. We attribute this component to graphenic
C interacting with Ni NPs and having higher electron density than
expected for G. This assignment is in agreement with the observa-
tion in XPS that the BE values for Ni atoms is higher than for metallic
Ni. Thus, the high resolution Ni peak in XPS could be resolved into
two peaks at BE values of 855.7 and 854.0 eV with a relative atomic
proportion of 0.2 and 0.1%, respectively. These BE values are unusu-
ally high for Ni(0) that should appear at about 852 eV. Thus, the
data of XPS suggesting Ni(II) does not seem to fit with those of XRD
indicating Ni(0). One possibility to reconcile these two conflicting
experimental data for the Ni oxidation state is to assume that XPS is
probing the external layers of Ni NPs that have become oxidized by
ambient O2. However this external NiO passivating Ni NPs should
be very thin compared to the size of metallic Ni(0), since NiO is
undetectable in XRD. Therefore, XRD will report on the majority
of Ni atoms and XPS on the thin shell. Alternatively, it could be
that the charge transfer that causes a 2 eV shift to lower BE in one
component of graphenic C (37%) would be also responsible for the
apparent 2 eV higher value of Ni(0). This proposal will imply an
interaction between Ni NPs and G, strong enough to shift the BE
value of certain C and Ni atoms in close contact. Whatever the rea-
son, it appears that the Ni supported on G has electron deficiency
and has positive character.

16
J.F. Blandez et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 13–19
Ni2p
C1s
135000
90000
45000
Ni2p3/2
114000
108000
Ni2p1/2
275
280
285
290
295 856 864 872 880 888
Binding Energy (eV)
Binding Energy (eV)
Fig. 3. High resolution C 1s and Ni 2p XPS peaks of Ni/G and their best deconvolution to three (C) and two (Ni) components.
Table 1
Results of the catalytic addition of dimethylphenylsilane (1a) to benzaldehyde (2a) in the presence of different catalysts with or without MNPs supported on G.
Entry
Catalyst
T (◦C)
t (h)
Load (mol%)
Conversion of 1ab (%)
Selectivity to 3ab (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Cu/G
Cu/G
Cu/G
Cu/G
Cu/G
Cu/G
Cu/(N)G
Cu/(B)G
Ni/G
Ni/G
Ni/G
Ni/G
Ni/G
Ni/G
Ni/G
Ni/(N)G
Ni/(B)G
Pd/G
GO
120
120
50
24
48
24
168
3
3
24
24
24
3
0.06
0.06
0.06
0.06
0.06
0.12
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.012
0.012
0.06
0.06
0.06
–
64
68
48
54
26
94
39
73
80
95
73
72
79
39
56
94
99
99
99
78
78
96
96
68
96
99
0
50
120
120
120
120
120
120
120
100
80
120
120
120
120
120
120
120
120
100
96
100
88
77
84
97
60
86
100
0
5
24
24
24
48
24
24
0.08
24
24
24
(N)G
(B)G
–
–
20
30
47
78
a
Reaction conditions: 1a (5 mmol), 2a (10 mmol), catalyst (0.06 mol%), n-dodecane as internal standard (0.1 mmol), Ar atmosphere.
Conversion and selectivity were determined by GC based on calibration with internal standard.
b
0.0006 mol% As observed for the case of the Cu/G the presence of
N or B heteroatoms on G was detrimental for the catalytic perfor-
mance, being this negative influence particularly notable in the case
of (N)G. It could be that in the present case the presence of N or B
heteroatoms on G influences the electronic density of Ni NPs by
heteroatom–Ni interactions and this result in a less active sites.
The activation energy (Ea) for the addition of hydrosilane 1a to
benzaldehyde (2a) using Cu/G or Ni/G was determined using the
Arrhenius plot from the influence of the temperature on the initial
reaction rate. As an example, Fig. 5 shows the Arrhenius plot for
the reaction between 1a and 2a catalyzed by Ni/G catalyst, while
the time-conversion plots at different temperatures from which the
initial reaction rates were determined are presented as an inset of
this figure. The Ea values obtained for Ni/G and Cu/G were 19.8 and
31.5 kJ/mol, respectively, indicating the higher catalytic activity of
Ni/G compared to Cu/G.
100
50
0
0
2
4
Time (h)
Fig. 4. Time–conversion of 1a (᭿) and time-yield (᭹) of 3a plots for the hydrosilyla-
tion of 2a by 1a. Reaction conditions: 1a (5 mmol), 2a (10 mmol), catalyst (0.1 mol%),
dodecane as internal standard (0.1 mmol), Ar atmosphere, 120 ◦C.
A reusability test was performed by recovering the Ni/G cat-
alyst after the reaction, washing it with hexane and drying it at
room temperature. The used Ni/G catalyst was resuspended by
sonication in a fresh reaction medium. This test shows that the
Ni/G catalyst undergoes strong deactivation during the reaction.
While almost a complete conversion was achieved for the fresh
conversion and selectivity, although much longer reaction times
are required to reach almost complete conversion. The maximum
turnover number achieved for the addition of phenyldimethylsi-
lane (1a) to benzaldehyde (2a) using Ni/G as catalyst in the present
study was 105 measured at 20 days reaction time working at
120 ◦C reaction temperature with a Ni to substrate mol ratio of

J.F. Blandez et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 13–19
17
ICP analysis of the liquid phase after removal of the Ni/G catalyst
indicated that Ni leaching is not the cause of deactivation since the
percentage of the initial Ni that migrated to the solution was only
about 1% of the Ni content of the Ni/G catalyst. TEM images of the
three-times used catalyst show the presence of Ni NPs, supported
on the carbon layers without much apparent increase in the aver-
age particle size as determined by statistical analysis of the average
particle size of images of sufficient magnification. The most likely
reason for the strong deactivation of Ni/G catalyst upon use seems
to be the change in the 2D morphology of the G sheets as conse-
quence of folding and wrapping of the sheet to form a 3D object.
Fig. 7 presents selected TEM images to illustrate these changes in
the morphology of the Ni/G upon its use as catalysts. As it can be
seen in these images, the 2D morphology of G is not clearly observed
after its use as catalyst and most probably G undergoes corrugation
leading to 3D objects by folding and wrapping of the 2D sheet.
The scope of Ni/G as catalyst was screened by performing the
carbonyl addition using various hydrosilanes and aldehydes. The
results obtained are summarized in Table 2. This table shows that
besides hydrosilane 1a, other silanes also give high conversion and
selectivity to the corresponding siloxanes (Table 2, entries 2 and 3)
and only when the bulky triphenylsilane (1d) was used as reagent
the addition product was not formed (Table 2, entry 4), probably
due to the strong steric encumbrance. The only product observed
in the reaction using triphenylsilane (1d) as reagent was the cor-
responding triphenylsilanol. On the other hand, when different
aldehydes were used as substrates with the dimethylphenylsilane
(1a) longer reaction times were necessary to achieve high con-
versions. In agreement with a reaction mechanism involving a
nucleophilic attack to the carbonyl group, when aldehydes have
an electron donor group as substituent on the ring, the conversion
decreased and longer reaction times were necessary to achieve high
conversions (Table 2, entry 7). Nevertheless, the selectivity to the
expected O-benzylsiloxane 3 was not excessively influenced by the
electron density of the ring and moderate selectivity values toward
the addition product were obtained for substituted benzaldehy-
des having both electron donor or acceptor substituents (Table 2,
100
50
2
1
0
0
20 40 60 80
Time / h
24
26
28
1/T (1/K) ·10-4
Fig. 5. Arrhenius plot for the reaction between 1a and 2a catalyzed by Ni/G. The
inset shows the time-conversion plots at 80, 10, 120 and 140 ◦C from which the
initial reaction rates (k) were calculated.
100
50
0
0
2
4
6
8
10
Time / h
Fig. 6. Time–conversion plot for three consecutive uses of the same Ni/G catalyst.
First use ᭿, second use ᭹, and third use ꢀ.
catalyst in 3 h, to achieve complete conversion in the second and
third runs 8 and 168 h were necessary, respectively. Fig. 6 shows
the time–conversion plots for three consecutive uses of the same
Ni/G sample.
Fig. 7. TEM images at different magnifications taken for the Ni/G sample after being used three times as catalyst panels (a) and (b) show that the average Ni nanoparticles
size has not changed with respect to the fresh catalyst. Frame (c) presents a broader view of the sample. Panel (d) shows folding of the graphene sheet.

18
J.F. Blandez et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 13–19
Table 2
Catalytic activity of Ni/G for hydrosilylation of silanes and aldehydes.a
Entry
Silane
Aldehyde
Time
5
Product
Converb (%)
100
Selecc (%)
1
99
(1a)
(2a)
(2a)
(3a)
(3b)
2
5
100
81
(1b)
3
5
100
80
(2a)
(1c)
(3c)
4d
5
100
0
(2a)
(1d)
(3d)
(3e)
(3f)
5
6
24
24
100
98
70
70
(2b)
(2c)
(1a)
(1a)
7
8
48
95
76
(1a)
(2d)
(3 g)
24
24
98
15
65
(1a)
(1a)
(2e)
(2f)
(3 h)
(3i)
9e
0
a
Reaction conditions: silane (5 mmol), aldehyde (10 mmol), catalyst (0.06 mol%), dodecane as internal standard (0.1 mmol), Ar atmosphere, temperature 120 ◦C.
Conversion was determined by GC using internal standard.
Selectivity was determined by 1H NMR spectroscopy.
Only triphenylsilanol was formed.
b
c
d
e
2f (6 mmol).

J.F. Blandez et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 13–19
19
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Among the aldehydes tested, only furfuraldehyde failed to afford
the expected hydrosilylation product (Table 2, entry 9).
4. Conclusions
The present study has shown that Ni NPs (4 nm average particle
size) supported on G are conveniently prepared by reducing Ni2+
salt in ethylene glycol containing G [22]. Ni/G is general catalyst
to promote the hydrosilylation of aldehydes with high selectivity,
reaching TON of 105 and exhibiting relatively low activation energy,
while Cu/G failed to catalyze this reaction. Ni/G promotes the addi-
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aliphatic aldehydes. Although Ni/G becomes deactivated during the
course of the reaction, no Ni leaching or Ni particle size is observed.
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