Self-supported gold/chitosan nanocatalyst for chemoselective hydrogenation in Π-conjugated C[dbnd]C–C[dbnd]O system

Article abstract of DOI:10.1016/j.catcom.2018.08.003

A self-supported gold/chitosan nanocatalyst was easily obtained by drying a water dispersion followed by powdering it. In this system chitosan acts concomitantly as a stabilizer and support for gold nanoparticles. The self-supported gold/chitosan was applied as catalyst for C[dbnd]C and C[dbnd]O hydrogenation in conjugate and non-conjugate systems. The catalyst showed activity, driving selectivity to saturated alcohols or saturated ketones (selectivities ≥99%) depending on the reducing agent used, NaBH₄ or H₂ respectively. When H₂ was used, the catalyst showed activity only in conjugate C[dbnd]C–C[dbnd]O, forming saturated aldehydes and ketones. For isolated C[dbnd]C, C[dbnd]O or conjugate C[dbnd]C–C[dbnd]C bonds, no activity was observed.

Full text of DOI:10.1016/j.catcom.2018.08.003

Accepted Manuscript
Self-supported gold/chitosan nanocatalyst for chemoselective
hydrogenation in π-conjugated CC-CO system
Anike H. Virgili, Leandro Luza, Jesum A. Fernandes, Tania M.H.
Costa, Eliana W. de Menezes, Edilson V. Benvenutti
PII:
S1566-7367(18)30279-6
doi:10.1016/j.catcom.2018.08.003
CATCOM 5464
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
8 May 2018
19 July 2018
3 August 2018
Please cite this article as: Anike H. Virgili, Leandro Luza, Jesum A. Fernandes, Tania
M.H. Costa, Eliana W. de Menezes, Edilson V. Benvenutti , Self-supported gold/chitosan
nanocatalyst for chemoselective hydrogenation in π-conjugated CC-CO system. Catcom
(2018), doi:10.1016/j.catcom.2018.08.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As
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ACCEPTED MANUSCRIPT
Self-supported gold/chitosan nanocatalyst for chemoselective
hydrogenation in π-conjugated C=C–C=O system
a
a
b
a
Anike H. Virgili , Leandro Luza *, Jesum A. Fernandes , Tania M. H. Costa , Eliana
a
a
W. de Menezes *, Edilson V. Benvenutti *
a
LSS – Laboratory of Solids and Surfaces, Instituto de Química, UFRGS, CP 15003,
9
b
1501-970 Porto Alegre-RS, Brazil.
GSK Carbon Neutral Laboratory for Sustainable Chemistry, University of
Nottingham, NG7 2TU, Nottingham, UK.
*
Corresponding Author: Edilson V. Benvenutti (e-mail: benvenutti@ufrgs.br)

ACCEPTED MANUSCRIPT
Abstract
A self-supported gold/chitosan nanocatalyst was easily obtained by drying a water
dispersion followed by powdering it. In this system chitosan acts concomitantly as a
stabilizer and support for gold nanoparticles. The self-supported gold/chitosan was
applied as catalyst for C=C and C=O hydrogenation in conjugate and non-conjugate
systems. The catalyst showed activity, driving selectivity to saturated alcohols or
saturated ketones (selectivities ≥ 99%) depending on the reducing agent used, NaBH
4
or
H
2
respectively. When H was used, the catalyst showed activity only in conjugate
2
C=C–C=O, forming saturated aldehydes and ketones. For isolated C=C, C=O or
conjugate C=C–C=C bonds, no activity was observed.
Keywords: biopolymer, green catalyst, C=C/C=O hydrogenation, hybrid,
heterogeneous catalysis.

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1
. Introduction
Chitosan is a natural, non-toxic and biodegradable polymer that has been
studied and applied in several technological areas, such as biomedical,
pharmaceutical, environmental and catalysis [1-4]. Due to the presence of amine
groups, chitosan can be solubilized in acidic medium and presents chelating
ability for metal ion complex, ensuring adsorption capacity. This metal-chitosan
affinity allows its use as stabilizing agent for the synthesis of metal nanoparticles,
in which the adsorbed metal ions are reduced to zero-valent metal by solid
chitosan itself [5,6], or by using additional reducing agents [7]. Metal
nanoparticles-chitosan systems have been applied as heterogeneous catalyst in
several reactions [5-8].
Supported gold nanoparticles (AuNP) are one of the most used catalysts
for oxidation of CO (Au/TiO
are also very selective in the hydrogenation of nitro compounds (Au/ZnAl-
hydrotalcite) [11], conjugated dienes (Au/Al ) [12], and alkynes (Au/SiO
Au/Al ) [12]. Additionally, supported AuNP, are extensively employed to
2
nanotube) [9] and alcohols (Au/SiO ) [10]. They
2
2
O
3
2
,
2
O
3
obtain high-added-value allylic alcohols in the hydrogenation of α,β-unsaturated
compounds [13-15]. In these reactions, unsaturated alcohols are obtained by 1,2-
1
2
addition of hydrogen to C=O via η O (end-on) and η CO (π or di-σ) adsorption
modes (Fig. 1, pathways A, B, and C) since the adsorption of π-conjugated C=C–
C=O systems is not favoured on Au surfaces [16].
However, chemoselective hydrogenation to form saturated carbonyl
compounds involving Au catalysts is rarely reported [12] and generally reached
via 3,4- or 1,4-addition steps on group 10 metal surfaces [17,18]. In these cases,
the compound adsorbs on metal surfaces through: i) only ethylenic double bond

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2
via η CC (π or di-σ) adsorption modes (Fig. 1, pathways D and E) or; ii) by
3
2
1
4
2
2
means of the C=C–C=O system via η (η CC+η O) or η (η CC+η CO: di-π or
η4CCCO) (Fig. 1, pathways F, G, and H) [18]. In the second case, the kinetic
factor leads to the hydrogenation of C=C bond (3,4-addition) more easily and
quickly than for C=O bond (1,2-addition). Furthermore, the 1,4-addition of
hydrogen also improves the selectivity for saturated carbonyl products once an
enolic compound is formed and isomerized to the same saturated specie, as the
3,4-addition [17,18]. Thus, it is very important to have knowledge about the
behaviour of the double bonds, both ethylenic and carbonylic, during the reaction
steps.
The source of hydrogen can also play a central role in the adsorption
modes of π-conjugated C=C–C=O systems. Sodium borohydride (NaBH
hydrazine, formic acid, and alcohols are hydrogen donors capable of producing
molecular hydrogen (H ) in the reaction medium and/or transferring hydrogen
4
),
2
directly through its adsorption onto the catalyst surface. In aqueous media, this
characteristic is a clear advantage in terms of reactivity, since they are highly
water-soluble reductants when compared to the gaseous H
2
. However,
and other
comparisons between hydrogen activation mechanisms by using H
2
hydrogen donors are still scarce. These pathways are crucial to control the
activities and selectivities in catalytic hydrogenations, especially in compounds
with different functional groups, such as α,β-unsaturated carbonyl substrates. In
these cases, the metal-hydride/support-proton species, formed via an ionic
mechanism, prefers to hydrogenate the polar C=O (1,2-addition) and the
homolytically cleaved H, created by means of a dissociative chemisorption, than
to hydrogenate the less polar C=C (3,4-addition).

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In this work, a novel self-supported AuNP/Chitosan catalytic system was
developed, using a procedure not yet reported. Chitosan, dissolved in acid
aqueous solution was used to stabilize gold nanoparticles in water dispersion.
Powder AuNP/Chitosan was produced by a simple drying procedure. This system
is easy to obtain and presents interesting morphological and textural properties
that allow its use as heterogeneous catalyst. The hydrogenation reactions of
C=C–C=O and C=C–C=C conjugate bonds, as well as isolated C=C and C=O
bonds, were used in order to study the reactivity of AuNP/Chitosan nanocatalyst.
2
. Experimental
2.1 Synthesis of Self-supported gold/chitosan nanocatalyst (AuNP/Chitosan)
Chitosan solution was prepared by dissolving 150 mg of chitosan powder
(Sigma-Aldrich) in 10 mL of acetic acid under stirring for 24 h, the first 4 h under
−
1
heating at 50 °C. Then water (10 mL), 5 mmol
∙
L HAuCl (1 mL) and 0.02
4
−
1
mol
∙
L NaBH (10 mL) were added under stirring, resulting in AuNP water
4
dispersion. This dispersion, hereafter designated as AuNP/Chitosan, was
evaporated under vacuum at ambient temperature to obtain self-supported sheet-
like material, which was powdered to be used as catalyst. The images of
AuNP/Chitosan, as water dispersion, as sheet-like material and as powder are
shown in Supplementary Material (Fig. S1). The amount of gold was estimated as
0.033 mmol of gold atoms per gram of material, corresponding to 0.65 wt%.
2.2 Characterization of AuNP/Chitosan
AuNP/Chitosan water dispersion and AuNP/Chitosan solid as sheet-like
material were analyzed by transmission ultraviolet-visible spectroscopy (UV-Vis)

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using UV-160 1PC Shimadzu spectrophotometer. Transmission electron
microscopy (TEM) was performed in JEOL JEM-1220 microscope. Quantikov
software was used to determine the AuNP size distribution and standard deviation
.
Upon analysis of several images of water dispersion and powdered
AuNP/Chitosan, it was considered populations of 323 and 212 gold nanoparticles,
respectively. N
2
isotherms were determined at N boiling point, using Tristar II
2
3020 Kr Micromeritics equipment, for samples previously degassed at 120 °C,
under vacuum (12 h). The specific surface area was determined by BET
multipoint technique [19]. X-ray photoelectron spectroscopy (XPS)
measurements were performed using Kratos AXIS Ultra DLD instrument with
−
9
chamber pressure at 5×10 Torr, using C signal at 284.8 eV as reference.
2.3 Catalytic hydrogenation procedures
Powdered AuNP/Chitosan was employed in the catalytic hydrogenation of
α,β-unsaturated carbonyl compounds using NaBH
4
(Aldrich) or H (>99.999%,
2
White-Martins). As general procedure, a first set of reactions, presented on
Tables 1 and 2, AuNP/Chitosan catalyst (0.6 µmol Au), 2-cyclohexen-1-one (1)
(
substrate/Au = 833) and solvent (5 mL of water) were placed in a 50 mL
erlenmeyer at room temperature. NaBH (5 mmol) was added to 5 mL of water.
4
Aliquots (100 μL) were taken during reaction. For a second set of reactions,
AuNP/Chitosan catalyst (0.6 µmol Au), carbonyl compounds (2-cyclohexen-1-
one (1), cyclohexanone (2), 2-cyclohexen-1-ol (3), 2-methyl-2-cyclohexen-1-one
(
5), t-crotonaldehyde (8) and t-cinnamaldehyde (11)), an alkene (cyclohexene
14)), and a diene (1,3-cyclohexadiene (15)) (substrate/Au = 833) and water (10
(
mL) were placed in 50 mL Parr 4590 Multi-reactor. Compound structures are

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depicted on Tables 1 and 2. The reaction vessel was pressurized under 10 or 30
bar of H and heated at 100, 150 or 200 °C. Conversion and selectivity were
2
determined by GC-analysis of 100 μL aliquots extracted using dichloromethane,
in Agilent Technologies GC System 6820 with a DB-17 column, FID detector at
−
1
260 °C. The conditions were: nitrogen as carrier (1 mL
∙min ); column head
pressure of 10 psi; temperature program from 40 °C (10 min) to 250 °C at a
−
1
heating rate of 10 °C∙min . TOF was calculated as mol substrate converted/(mol
Au × minute) from the slope of plots TON vs time at initial rates extrapolated to
zero conversion [20].
3
. Results and discussion
3.1 Characterization of AuNP/Chitosan
The UV-Vis spectra of AuNP/Chitosan water dispersion and solid are
shown in Fig. 2. The bands show maxima at 521 nm for water dispersion and at
39 nm for solid. These bands are due to the collective oscillation of electrons, an
5
occurrence known as localized surface plasmon resonance absorption for
spherical gold nanoparticles [21]. The observed red shift for curve b was
interpreted considering the agglomeration of AuNP as solid [22] and as
consequence of refraction index changes of matrices [23].
Infrared spectrum of AuNP/Chitosan is shown in Supplementary Material
(
Figure S2), along with the spectrum of chitosan submitted to dissolving and
drying process. Both present the typical spectrum of chitosan with a fraction of
chitin dissolved in acetic acid [24,25]. The main bands are the N-H bending
1650 cm ) and N-H deformation of amide (1558 cm ), skeletal vibration of C-O
2
-
1
-1
(
-
1
stretching below 1100 cm . The acetic acid used to dissolve the chitosan is

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-
1
identified by band C=O stretching at 1759 cm and COOH deformation at 1223
-
1
-1
cm . The band at 1381 cm is due to nitrate of aqua regia gold precursor
solution, and it is present only in the AuNP/Chitosan material. This result is clear
evidence that the chitosan structure is maintained in AuNP/Chitosan material.
TEM images of AuNP/Chitosan water dispersion and powder are presented in
Fig. 3. It is shown that the nanoparticles exhibit spherical shape, with an average
diameter of (4.9±1.4) nm for the water dispersion and (5.1±1.0) nm for the solid
powder. Even as powder, the metal nanoparticles were rather uniform in size and shape,
confirming that the red shift observed in the UV-Vis spectrum for the solid, should be
due to agglomeration of AuNP or matrix effects.
Textural analysis reveals that the materials are non-porous. However, the BET
2
-1
surface area of AuNP/Chitosan powder was (5.0±0.5) m g , twice the area of the
2
-1
reference chitosan (2.3±0.5) m g . These different surface area values can be explained
by taking into account that this new AuNP/Chitosan material came from a dispersion
containing gold, strong acids and NaBH , components that were not present in the
4
solution used to obtain the reference chitosan. The presence of these new components,
necessary to obtain the catalyst may produce differences in the textural properties. Even
regarding that AuNP/Chitosan is a polymeric material, the resulting BET surface area
was very interesting considering the possibility of its use as catalyst.
The N 1s and Au 4f XPS spectra of AuNP/Chitosan are shown in Supplementary
Material (Fig. S3). The N 1s spectrum presents two peaks, with maxima at 399.5 and
4
01.7 eV. These peaks are usually observed in chitosan and they are assigned to the
non-protonated and protonated amine groups of chitosan [26]. The presence of
protonate nitrogen is due to the remaining acid from the HAuCl solution and from the
acetic acid used to dissolve the chitosan. The Au 4f XPS spectrum is characteristic of
4

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0
Au species [10,27]. The peak intensity for gold was much lower than that of N 1s
peaks, and this result was interpreted regarding that the catalyst, which is composed
mainly by chitosan, presents high nitrogen content. On the other hand, the gold amount,
planned for this catalyst was 0.65 wt.%. Therefore, no peaks corresponding to N–Au
interaction could be detected.
3.2 Catalytic performance in hydrogenation reactions
The hydrogenation of 2-cyclohexen-1-one (1) was used as a benchmark probe to
check the catalytic performance of powder AuNP/Chitosan. By using NaBH as
4
reducing agent in aqueous medium, it was possible to verify that both double bonds,
C=O and C=C, were hydrogenated, with 18% for 2-cyclohexen-1-ol (3), and 82% for
cyclohexanol (4), with conversion of 97% and TOFs up to 77 min−1 (Table 1, entry 1)
(Supplementary Material Fig. S4a). These results suggest that the substrate 2-
cyclohexen-1-one (1) can adsorb on Au surface by two ways: i) to a lesser extent via
1
2
η O or η CO (Fig. 1: A and B) providing the allylic alcohol by means of 1,2-addition of
3
4
hydrogen and, ii) preferentially via η or η pathways generating the fully hydrogenated
product (Fig. 1: F, G and H). This behavior suggests that the H formed can be activated
by both the ionic and the homolytic mechanisms. Thus, by using NaBH as source of
2
4
hydrogen, the chitosan provides both types of hydrogen for the -conjugated system.
Cyclohexanone (2) was hydrogenated selectively to cyclohexanol (4) with
−
1
conversion of 90% and TOFs up to 8 min , whereas the unsaturated alcohol 2-
cyclohexen-1-ol (3) did not undergo hydrogenation (Table 1, entries 2 and 3). These
1
2
results confirm the occurrence of η O or η CO adsorption modes (1,2-addition) (Fig. 1:
2
A and B) and practically exclude the η CC pathways (3,4-addition) (Fig. 1 D and E) by
using NaBH as reducing agent. It is noteworthy that C=C bonds can be hydrogenated
4

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by AuNP only when conjugated to the C=O group (Table 1, entries 1 and 3). This fact is
in opposition to reports that describe the low adsorption of entire π-conjugated C=C–
C=O systems on Au surfaces [16] and may be related to the presence of chitosan, since
this biopolymer acts concomitantly as stabilizer and as support for AuNP, probably
changing the coordination strength/ability of the substrate and the availability of
activated hydrogen type on AuNP surface, affecting its catalytic performance. The same
reactivity trend was also observed in the hydrogenation of α,β-unsaturated carbonyl
compounds using AuNP supported in γ-Al
amount of NaBH (from 5 to 0.5 mmol) provided lower conversion (53%) and activity
12 min ), but did not display significant changes in the selectivity, 22% for 2-
2
3
O modified with ionic liquid [12]. The
4
−
1
(
cyclohexen-1-ol (3) and 78% for cyclohexanol (4) (Table 1, entry 4).
Changes in activity and especially in selectivity were observed by using H
2
(Table 1, entries 5-8) (Supplementary Material Fig. S4b). At 100 °C, AuNP/Chitosan
catalyst was almost inactive, even at moderate H
2
pressures (10 and 30 bar) (Table 1,
pressure has no significant
entries 5 and 6), indicating that, at this temperature, the H
2
influence on the activity. Conversely, as the temperature was increased to 150 and 200
C, higher conversions (62% and 99%, respectively) and activities (TOFs up to 9 min−1
°
−
1
and 32 min , respectively) were attained with full selectivity for cyclohexanone (2)
(Table 1, entries 7 and 8). This fact evidences that the increase in temperature is an
important factor to reach higher activity, whereas the selectivity remain practically
constant. It is probable that the substrate 2-cyclohexen-1-one (1) adsorbs on AuNP
2
3
4
surface via η CC pathways (3,4-additon) (Fig. 1: D and E) or by η or η modes (3,4- or
,4-addition) (Fig. 1: F, G and H), which suggests that chitosan environment adapts to
the reducing agent (H in these cases) and modifies the selectivity even in the presence
of both homolytic/heterolytic hydrogen.
1
2

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In the search for evidences about the adsorption modes and hydrogen activation,
the AuNP/Chitosan catalyst was evaluated by carrying out the hydrogenation under H
2
of unsaturated products, such as cyclohexanone (2) and 2-cyclohexen-1-ol (3), as well
as α,β-unsaturated carbonyl compounds, such as 2-methyl-2-cyclohexen-1-one (5), t-
crotonaldehyde (8), t-cinnamaldehyde (11), alkene, such as cyclohexene (14), and
conjugated diene, such as 1,3-cyclohexadiene (15) (Table 2 and Supplementary
Material, Fig. S4c).
It is important to point out that both isolated C=O and C=C were not
hydrogenated (Table 2, entries 2 and 3). These results can be an indication that neither
1
2
2
η O and η CO (1,2-additon) (Fig. 1: A and B) nor η CC (3,4-addition) pathways (Figure
1
: D and E) occur in these reaction conditions. Thus, it is plausible assume that only η3
4
or η modes (Fig. 1: F, G and H) are present on AuNP surface using H
2
during the
course of the reaction.
Additionally, the C=C bond is selectively hydrogenated (≥97%) in α,β-
unsaturated carbonyl compounds: 2-methyl-2-cyclohexen-1-one (5), t-crotonaldehyde
(
8) and t-cinnamaldehyde (11), which reveals the preference for the homolytic
activation of hydrogen, but it does not exclude the presence of hydride/proton species
selectivities <3%). The observed activities can be related to the electronic effects
caused by ketones and aldehydes groups, as well as steric effects due to the volume
occupied by different substituents, such as –H, –CH and –Ph. (Table 2, Entries 4-6).
(
3
It is important to highlight that substrates containing isolated C=C or conjugated
C=C–C=C bonds did not undergo hydrogenation, as observed for 2-cyclohexen-1-ol (3),
cyclohexene (14) and 1,3-cyclohexadiene (15) (Table 2, entries 3, 7 and 8). Therefore,
the presence of conjugated C=C–C=O system is extremely important in order to
hydrogenate C=C bonds selectively even in the presence of heterolitically cleaved

ACCEPTED MANUSCRIPT
hydrogen. This fact is related to the self-supported catalyst employed here, where
chitosan acts concomitantly as stabilizing agent and as support for AuNP, which may
favor the adsorption modes that were proposed, driving the selectivity according to on
the reducing agent.
4
. Conclusions
In this study a self-supported AuNP/Chitosan (water dispersion and powder) was
developed, where chitosan acts as stabilizer and support for AuNP. The AuNP/Chitosan
in solid form was easily obtained by drying the water dispersion. It was observed that
AuNP have spherical shape with diameter of about 5 nm, both in water dispersion and
in solid form. The powder of self-supported AuNP/Chitosan was applied as catalyst for
C=C e C=O hydrogenation in conjugated and non-conjugated system. The results show
that AuNP/Chitosan promotes hydrogenation only in π-conjugated C=C–C=O system,
3
4
via η and/or η adsorption modes, driving the selectivity to saturated alcohols or
saturated ketones, depending on the reducing agent used in the hydrogenation, NaBH
or H , respectively.
4
2
Acknowledgements
Authors thank to CNPq, FAPERGS and CAPES for financial support and grants,
CMM-UFRGS for TEM images and Prof. Jairton Dupont for his lab facilities.

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Figure Captions
Fig. 1: Adsorption modes of the α,β-unsaturated carbonyl compounds on metal
nanoparticles surface.
Fig. 2: UV-Vis spectra of AuNP/Chitosan. a) water dispersion; b) solid.
Fig. 3: TEM images of AuNP/Chitosan and size distribution. a) water dispersion
and b) powder.

ACCEPTED MANUSCRIPT
Table 1 Selective hydrogenation of the 2-cyclohexen-1-one by AuNP/Chitosan catalyst.
Reducing
Agent
Conversion/%
[time/h]
Selectivity/%
Entry[a,b]
T/ °C
TOF/ min−1
2
3
4
1
2[b]
3[c]
4[d]
5
NaBH
NaBH
NaBH
NaBH
4
4
4
4
25
25
97 [1.5]
90 [10.0]
―
―
18
―
―
22
0
82
77
8
―
>99
―
25
―
―
12
0.3
0.5
9
25
53 [6.0]
1 [24.0]
2 [24.0]
62 [10.0]
>99 [6.0]
―
78
10 bar H
30 bar H
30 bar H
30 bar H
2
2
2
2
100
100
150
200
>99
>99
>99
>99
<1
<1
<1
<1
6
0
7
0
8
0
32
[a]
Reaction conditions: Au (0.6 µmol), 2-cyclohexen-1-one/Au = 833, H
2
O (10 mL), NaBH
mmol) (entries 1, 2, and 3); Substrate: cyclohexanone; Substrate: 2-cyclohexen-1-ol; [d]0.5
mmol of NaBH
4
(5
[b]
[c]
4
.

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Table 2 Selective hydrogenation by AuNP/Chitosan catalyst, using H as reducing
2
agent.
Conversion/%
Entry[a]
Substrate
Selectivity/%
TOF/ min−1
32
[
time/h]
1
>99 [6.0]
― [24.0]
― [24.0]
2
3
―
―
―
―
4
15 [24.0]
3
5
6
>99 [5.0]
>99 [6.0]
46
27
7
8
― [24.0]
― [24.0]
―
―
―
―
[a]Reaction conditions: Au (0.6 µmol), substrate/Au = 833, H
00 °C.
O (10 mL), H (30 bar),
2
2
2

ACCEPTED MANUSCRIPT
HIGHLIGHTS


Easy way to obtain powdered self-supported gold/chitosan nanocatalyst;
Activity and chemoselectivity only in the hydrogenation of conjugate C=C–
C=O;

No activity in the hydrogenation of isolated C=C, C=O or conjugate C=C–C=C
bonds.

Figure 1

Figure 2

Figure 3