Gold nanoparticles onto cerium oxycarbonate as highly efficient catalyst for aerobic allyl alcohol oxidation

Article abstract of DOI:10.1016/j.catcom.2020.105989

Au nanoparticles, generated by the metal vapor synthesis technique, were supported onto cerium oxycarbonate monohydrate (Ce₂O(CO₃)₂·H₂O) giving Au@Ce₂O(CO₃)₂·H₂O. The obtained heterogeneous catalyst was used in the aerobic allyl alcohol oxidation reaction performed in toluene, showing a notably higher catalytic substrate conversion and isomerization activity compared to Au onto ceria, which is the reference catalyst for this type of catalysis. Results originating from catalytic recycling experiments and PXRD, HRTEM and XPS measurements carried out on recovered Au@Ce₂O(CO₃)₂·H₂O, confirmed the stability of the catalyst under aerobic oxidation reaction conditions and hence its recyclability, without the need of a regeneration step.

Full text of DOI:10.1016/j.catcom.2020.105989

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Gold nanoparticles onto cerium oxycarbonate as highly efficient
catalyst for aerobic allyl alcohol oxidation
Werner Oberhauser, Claudio Evangelisti, Marcello Marelli,
Vladimiro Dal Santo, Cinzia Cepek, Marco Bellini
PII:
S1566-7367(20)30065-0
DOI:
https://doi.org/10.1016/j.catcom.2020.105989
CATCOM 105989
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
11 February 2020
18 March 2020
19 March 2020
Please cite this article as: W. Oberhauser, C. Evangelisti, M. Marelli, et al., Gold
nanoparticles onto cerium oxycarbonate as highly efficient catalyst for aerobic allyl
alcohol oxidation, Catalysis Communications (2019), https://doi.org/10.1016/
j.catcom.2020.105989
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Gold nanoparticles onto cerium oxycarbonate as highly
efficient catalyst for aerobic allyl alcohol oxidation
Werner Oberhausera,*, Claudio Evangelistib,*, Marcello Marellic, Vladimiro Dal
Santod, Cinzia Cepeke, Marco Bellinia
aCNR-Istituto di Chimica dei Composti Organometallici, Via Madonna del Piano 10, 50019 Sesto
Fiorentino, Italy. E-mail: werner.oberhauser@iccom.cnr.it
bCNR-Istituto di Chimica dei Composti Organometallici, Via Moruzzi 1, 56124 Pisa, Italy. E-mail:
claudio.evangelisti@cnr.it
cCNR-Istituto di Scienze e Tecnologie Chimiche “Giulio Natta” (SCITEC), Via G. Fantoli 16/15,
20133 Milano, Italy
dCNR-Istituto di Scienze e Tecnologie Chimiche “Giulio Natta” (SCITEC), Via C. Golgi 19, 20133
Milano, Italy
eCNR-IOM Laboratorio TASC, Area Science Park, s.s. 14 km 163.5, 34012 Basovizza, Trieste, Italy
ABSTRACT
Au nanoparticles, generated by the metal vapor synthesis technique, were supported onto cerium
oxycarbonate monohydrate (Ce2O(CO3)2·H2O) giving Au@Ce2O(CO3)2·H2O. The obtained
heterogeneous catalyst was used in the aerobic allyl alcohol oxidation reaction performed in
toluene, showing a notably higher catalytic substrate conversion and isomerization activity
compared to Au onto ceria, which is the reference catalyst for this type of catalysis. Results
originating from catalytic recycling experiments and PXRD, HRTEM and XPS measurements
carried out on recovered Au@Ce2O(CO3)2·H2O, confirmed the stability of the catalyst under
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aerobic oxidation reaction conditions and hence its recyclability, without the need of a regeneration
step.
Keywords: Au nanoparticles, cerium oxycarbonate, allyl alcohol, oxidation, air
1. Introduction
Allylic alcohols are challenging substrates in oxidation reactions, since carbon-carbon double
bond isomerization, hydrogenation as well as epoxidation reactions might simultaneously occur,
notably decreasing thus the chemoselectivity for the desired α,β-unsaturated carbonyl compound.
Homogeneous Mn- [1], Cu- [2,3], or Fe-based [4] catalysts have been used to bring about high
selectivity in allyl alcohol oxidation reactions towards the carbonyl functionality. Au nanoparticles
(NPs), supported onto ceria, have found to be highly promising catalysts for the aerobic allyl
alcohol oxidation conducted in apolar solvents or under solventless reaction conditions [5,6].
Brønsted bases, such as inorganic carbonates [7], acetates, borates [8] and mixed metal oxide
supports [9,10] notably accelerate in combination with Au the aerobic alcohol oxidation reaction.
Ceria is a well-established inorganic support for a number of metal NPs-based reactions ranging
from photocatalytic and water-gas shift reactions to selective alcohol oxidation reactions [11]. In
particular, the lattice defects in ceria, leading to the simultaneous presence of Ce3+ and Ce4+ centers
[12,13] on the support surface, make this support not innocent towards redox-reactions. In fact, Ce
4+
oxidizes in part Au0 atoms to Au+ on the NPs’ surface, decreasing hence the amount of
catalytically active Au0 centers [5].
Herein we describe the synthesis and application of Au NPs supported onto cerium
oxycarbonate monohydrate (Ce2O(CO3)2·H2O) [14-16] for the aerobic allyl alcohol oxidation
reaction conducted in toluene.
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2. Results and discussion
The hydrothermal reaction of Ce(NO3)3·6H2O with urea at 90 °C, gave Ce2O(CO3)2·H2O
(Scheme 1) as off white powder. The high crystallinity of the latter material and its conversion to
CeO2 at temperatures > 200 °C is shown by PXRD spectra (Fig. S1) [14-16]. BET measurements
carried out on the latter material indicated a very low surface area of 1.123 m2/g (Fig. S2, Table S1)
and the absence of a significant microporosity. CO2 adsorption was not observed, which is
indicating that the bridging oxide groups located on the support surface are not the active Brønsted
bases sites [17]. Hence the carbonate oxygen atoms are thus the real Brønsted base sites.
Au NPs were generated by the MVS technique [18-20] and supported onto Ce2O(CO3)2·H2O
giving Au@Ce2O(CO3)2·H2O (1) (Scheme 1). This approach has the huge advantage of generating
acetone-stabilized Au NPs, which are featured by the metallic state of the surface atoms. The
obtained NPs which can be easily dispersed onto solid supports by impregnation at room
temperature [18]. 1 was isolated without post-annealing treatments and ICP-OES analysis of it
revealed an Au-loading of 0.5 wt%.
Scheme 1. Synthesis of Ce2O(CO3)2·H2O and 1.
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IR spectra acquired for Ce2O(CO3)2·H2O and 1 (KBr, transmission mode) (Fig. S3) showed
-1
strong absorption bands in the wave number region between 1490 and 1419 cm , which were
-1
assigned to the carbonate stretching modes [21]. The broad adsorption band at 3440 cm was
indicating the presence of coordinated water molecules [22] and ethanol, originating from the
support synthesis.
HAADF-STEM micrographs of 1 showed the presence of Ce2O(CO3)2·H2O crystallites of non-
regular shape in the size range from 0.5 to 2 µm (Fig. S4), while HRTEM micrographs (Fig. 1)
proved the presence of Au NPs’ on the support surface characterized by an average size of 4.0 ± 1.5
nm.
Fig. 1. Representative HRTEM micrograph and histogram for 1.
The XPS spectra acquired for as-synthesized 1 showed for Au 4f the presence of only Au0 on the
NPs’ surface (i.e. binding energy (BE) (Au 4f7/2) = 83.6 eV (Au0)) (Fig. 2, upper trace) [23], while
the pattern observed for the Ce 3d BEs (Fig. S5, upper trace) is characteristic for the presence of
only Ce3+ [24,25].
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Fig. 2. Au 4f XPS spectra of as-synthesized 1 (upper trace) and of recovered 1 (lower trace)
acquired with an energy resolution of ~0.8 eV.
The O 1s XPS spectra of 1 and of Ce2O(CO3)2·H2O (Fig. S6) exhibited a BE of 530.2 eV, which
was assigned to bridging oxide units, while the BE of 531.6 eV was ascribed to the presence of
carbonate oxygen atoms [26]. The O 1s XPS peak characterized by a BE of 532.9 eV was assigned
to the C-O group of adsorbed ethanol [27], which originated from support synthesis.
Catalyst 1 (100 mg), characterized by a Au content of 0.5wt%, contained 2.538 μmol of Au
atoms of which 0.989 μmol were surface Au atoms (i.e. Au dispersion of 39%). This latter catalyst
was tested in the aerobic oxidation of selected allyl alcohols (i.e. cinnamyl alcohol (A) and 1-octen-
3-ol (B)) (Scheme 2), applying a substrate to catalyst molar ratio of 5792. Results obtained from
catalytic reactions in terms of surface atom-related TOF (h-1) and chemoselectivity are compiled in
Table 1 and the reaction products obtained are shown in Scheme 2.
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Scheme 2. Aerobic allyl alcohol oxidation reactions catalyzed by 1 in toluene.
A blank reaction carried out with Ce2O(CO3)2·H2O in the presence of substrate A (Table 1,
entry 1), confirmed a negligible substrate conversion of ca. 2%, which indicates the need of Au to
catalyze the oxidation reaction.
Table 1, here
Catalyst 1 converted substrate A into the α,β-unsaturated carbonyl compound A1 with a
chemoselectivity of 100% (Table 1).
Under comparable catalytic conditions (i.e. cinnamyl alcohol, toluene, 120 °C), Au@CeO2,
which is the most promising heterogeneous catalysts for selective aerobic allyl alcohol oxidation
reactions in organic solvents, showed a chemoselectivity for cinnamylaldehyde of 99% but a lower
substrate conversion compared to catalyst 1 (i.e. 538 h-1 vs 698 h-1) which might be due to the fact
that Au@CeO2 exhibits a notable fraction of oxidized Au atoms (i.e. Au+ is not the active form of
the catalyst) on the NPs’ surface [6]. Pd@apatite showed a low chemoselectivity of only 63% for
cinnamaldehyde along with a catalyst activity (i.e. TOF of 402 h-1) that is inferior to those obtained
for 1 and Au@CeO2 [6]. Au NPs in the size range 6-9 nm and located onto the ternary mixed-oxide
support Cu5Mg1Al2Ox were shown to be suitable for 1-phenylethanol oxidation, whereas its activity
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for the cinnamyl alcohol oxidation was low (TOF of 243 h-1) while the chemoselectivity for
cinnamyl alcohol was of 99%, which is typical for Au-based catalysts [9]. Au onto mixed Ga-Al
oxide has been tested only at 80 °C applying a substrate to catalyst molar ratio of 200. As a result, a
TOF value of only 76 h-1 was obtained [10].
Catalyst 1 showed for substrate B a higher conversion compared to A, at the expense of
chemoselectivity for the unsaturated carbonyl compound B1. In fact, a notable percentage of the
saturated carbonyl compound B2 was obtained (Table 1, entries 7-10). A blank reaction conducted
with Ce2O(CO3)2·H2O in the presence of B gave B1 with 3% yield as the only organic product
(Table 1, entry 6). This experimental result confirmed the need of Au to catalyze the allyl alcohol
isomerization reaction. The selectivity for B2 was further increased to 93% by conducting the
catalytic reaction in the absence of air (i.e. under nitrogen atmosphere) (Table 1, entry 11). Under
the latter experimental condition octan-3-ol (3%) was obtained which indicates that B was acting to
some extend as hydrogen acceptor under anaerobic experimental conditions.
Recycling experiments with 1 in the presence of substrates A and B were carried out, showing
only a small activity drop in terms of surface-related TOF values (Table 1). Most importantly, the
chemoselectivity of the catalytic reactions remained almost identical (Table 1, entries 4/5 vs 3 and
8/9 vs 7). In addition, recycling experiments were conducted without the need of a catalyst
reactivation step (i.e. washing the recovered catalyst with base), which is indicating that substrate
over-oxidation to carboxylates, which strongly coordinate surface Au atoms and contribute hence to
catalyst deactivation, did not occur with 1 [28,29]. 1 was recovered by centrifugation and
decantation of the catalytic solution, followed by washing the solid catalyst with toluene and drying
at 50 °C in vacuum. In order to estimate the amount of leached Au in solution, the reaction
suspension was filtered at 80 °C and the obtained clear solution analyzed by ICP-OES. As a result,
a Au content of 25 ppb was found in solution which corresponds to a Au leaching of 0.25%.
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PXRD (Fig S7), Ce 3d (Fig. S5, lower trace) and O 1s-XPS (Fig. S6, lower trace) spectra of
recovered 1 proved the stability of the support material under the chosen catalytic reaction
conditions oxidation. In addition, an intense peak in the O 1s spectrum centered at a BE of 532.9
eV, was assigned to the alcohol functional group of adsorbed cinnamyl alcohol, proving hence the
strong interaction between the substrate and surface carbonate groups [30].
The Au 4f XPS spectrum of recovered 1 (Fig. 2, lower trace) confirmed the presence of only
Au0 on the NPs’ surface after the aerobic oxidation reaction, while HRTEM (Fig. S8) and
representative HAADF-STEM micrographs (Fig. S9) confirmed a slight increase of the mean NPs’
size to 4.1 ± 1.3 nm and to 4.7 ± 1.5 nm after a 5 and 20 h reaction time, respectively. As a
consequence, the amount of surface Au atoms of recovered 1 was found to be almost identical to
that of as-synthesized 1, after a reaction time of 5 h or slightly decreased to 0.889 μmol, which
corresponds to a Au dispersion of 35%, after a reaction time of 20 h. Although, a drop of the
amount of surface Au atoms (10%) was observed for long reaction times (20 h), the catalytic
activity, expressed in surface-related TOF, was almost unaltered for the first recycling experiment
(Table 1, entry 4 vs 3).
We propose the following catalytic cycles operative for the 1-catalyzed allyl alcohol oxidation
reaction (Scheme 3) after combining experimental results obtained from catalytic batch reactions
and physical measurements carried out on the recovered catalyst.
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Scheme 3. Proposed catalytic cycles operative for the 1-catalyzed aerobic allyl alcohol oxidation in
toluene.
The alcohol functional group of the substrate interacts with the surface carbonate units of the
catalyst’s support (Scheme 3, A). Then deprotonation of the alcohol functional group by carbonate
(Brønsted base sites) gave the alcoholate which is coordinated to the Au NPs’ surface (Scheme 3,
B). Indeed, DFT calculations showed that the alcoholate formation in aerobic alcohol oxidation
reactions is significantly favored in the presence of a Brønsted base [31,32]. The rate-determining
step (i.e. α-hydrogen abstraction from coordinated alcoholate) leads to the formation of Au hydride
species (Scheme 3, C) [5,33,34] and the concomitant release of the corresponding unsaturated
carbonyl compound.
Species C reacts either with oxygen giving water [28] and the regenerated catalyst 1, or
undergoes an insertion reaction with the C-C double bond (Scheme 3, E) of a sterically less
hindered allyl alcohol, such as substrate B, followed by β-hydride elimination, which gives species
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C and the enolate [35]. The enolate tautomerizes in solution giving B2. The isomerization activity of
the Au hydride species increases in the absence of oxygen, due to a higher concentration of Au
hydride species on the NPs’ surface compared to the aerobic experimental reaction condition.
3. Conclusions
Au nanoparticles, generated by the metal vapor synthesis technique, were supported onto
Ce2O(CO3)2·H2O, a crystalline, Ce3+-based inorganic material. Au XPs measurements confirmed
the presence of only Au atoms in the metallic oxidation state on the NPs’ surface. Aerobic allyl
alcohol oxidation reactions conducted with the obtained heterogeneous catalyst in toluene showed:
(i) A catalytic conversion of allyl alcohols to the unsaturated carbonyl compound (major
compound) which is superior to that of Au@CeO2, known as reference system for this kind of
chemistry. (ii) A partial allyl alcohol isomerization activity to the corresponding saturated carbonyl
compound, when the allyl group of the substrate is sterically not hindered, which contrasts results
obtained with Au@CeO2 [6]. The selectivity for the saturated carbonyl compound can be increased
to 93% under anaerobic reaction conditions. Recycling experiments, conducted without the need of
a catalyst regeneration step, confirmed the stability of the support material, as well as of the
catalytically active Au NPs in terms of size (Au dispersion) and oxidation state of the surface atoms
even after a prolonged reaction time.
Conflict of interest
There are no conflicts to declare.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://...
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Credit author statement
Werner Oberhauser: writing of the draft and PXRD measuremnts; Claudio Evangelisti: catalyst
preparation, Marcello Marelli: TEM measurements; Vladimiro Dal Santo: CO2-chemisorption
experiments, Cinzia Cepek: XPS study; Marco Bellini: physisorption measurements
Declaration of interests
The authors declare thatthey have no known competing financial interests orpersonal relationships that
could have appeared to influence the work reported in this paper.
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Table 1. Aerobic allyl alcohol oxidation in toluene by catalyst 1.a
Entry
t(h)
Substrate
Conv. (%)/[TOF (h-1)]b
Selectivity (%)c
B1
A1
B2
1d
2
5
5
20
20
20
5
5
5
5
20
20
A
A
A
A
A
B
B
B
B
B
B
2 [n.d.]
100
100
100
100
100
23 [698]
85 [627]
75 [617]
70 [n.d.]
3 [n.d.]
47 [1380]
45 [1338]
43 [n.d.]
100 [n.d.]
100 [n.d.]
3
4e
5f
6d
7
100
82
81
80
79
4
18
19
20
21
93
8e
9f
10
11g
a
Catalytic condition: 1 (100 mg, Au (2.54 μmol)), substrate (14.7 mmol), toluene (50 mL), T (120
°C), p(air) = (20 bar at 120 °C).
b
Surface atom-related TOF (h-1) defined as mmol (converted substrate) / [mmol (Au NPs’ surface
atoms) × t(h)]
c Selectivity (%) defined as mmol(product) / Σmmol(products) × 100.
d Ce2O(CO3)2·H2O (100 mg) instead of 1.
e First recycling.
f Third recycling.
g Under N2-atmosphere. Octan-3-ol (3%) was obtained.
Graphical abstract:
Highlights:




Au NPs onto Ce2O(CO3)2·H2O were synthesized by metal vaporsynthesis technique.
High catalyticactivity in the aerobicallyl alcohol oxidation was observed in toluene.
High allyl alcohol isomerization occurred with sterically less hindered allyl alcohols.
The catalyticperformance was maintained withoutthe need of aregeneration step.
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