Catalytic oxidation of cyclohexene by supported gold nanoclusters synthesized in a two-liquid phases system containing eucalyptus essential oil

Article abstract of DOI:10.1016/j.mcat.2020.110922

Gold nanoclusters (d < 1 nm) have been synthesized in a two-liquid phases system, the aqueous phase being formed by dissolving gold in a solution of ammonium chloride in concentrated nitric acid (1: 4, w/w), while eucalyptus essential oil is the organic phase. The mesoporous SBA-15 material functionalized with mercaptopropyl groups (after surfactant removal: d_p = 6.3 nm, 3.5 wt% S) was contacted with aliquots of the Au-containing essential oil phase taken at 1, 3, 6 and 8 days of contact time between both phases. In this way, gold was immobilized on the support, ranging from 0.4 wt% for the 1-day sample, to 2.7 wt% in the 3-days material. UV–vis spectra show the presence of gold nanoclusters in these samples, but the surface plasmon resonance at 520 cm^?1, characteristic of Au nanoparticles, was not detected save for the 3-days sample. ¹³C MAS NMR and TG evidence that the thiol groups of the support remain mostly unaltered for the 1-day sample, but oxidation to sulfonic acid groups becomes apparent for contact time > 3 days, and reaches nearly 60 % of the total sulphur species after 8 days of contact time as estimated from XPS analysis. The Au-SH-bearing catalyst is inactive for cyclohexene oxidation with molecular oxygen in liquid phase, but those having sulfonic groups are active and selective for its allylic oxidation. It has been found for the 8-day catalyst that the gold nanoclusters partially evolve spontaneously in the reaction medium to form gold nanoparticles, and this agglomeration process parallels the increase in catalyst activity.

Full text of DOI:10.1016/j.mcat.2020.110922

Molecular Catalysis 488 (2020) 110922
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Catalytic oxidation of cyclohexene by supported gold nanoclusters
synthesized in a two-liquid phases system containing eucalyptus essential oil
J. Agúndez, C. Ares, C. Márquez-Álvarez, J. Pérez-Pariente*
Instituto de Catálisis y Petroleoquímica (ICP-CSIC), C/Marie Curie 2, 28049 Cantoblanco, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords:
Gold nanoclusters (d < 1 nm) have been synthesized in a two-liquid phases system, the aqueous phase being
formed by dissolving gold in a solution of ammonium chloride in concentrated nitric acid (1: 4, w/w), while
eucalyptus essential oil is the organic phase. The mesoporous SBA-15 material functionalized with mercapto-
Gold nanoclusters
Oxidation
Cyclohexene
Eucalyptus oil
SBA-15
propyl groups (after surfactant removal: d = 6.3 nm, 3.5 wt% S) was contacted with aliquots of the Au-con-
p
taining essential oil phase taken at 1, 3, 6 and 8 days of contact time between both phases. In this way, gold was
immobilized on the support, ranging from 0.4 wt% for the 1-day sample, to 2.7 wt% in the 3-days material.
UV–vis spectra show the presence of gold nanoclusters in these samples, but the surface plasmon resonance at
−
1
13
5
20 cm , characteristic of Au nanoparticles, was not detected save for the 3-days sample. C MAS NMR and TG
evidence that the thiol groups of the support remain mostly unaltered for the 1-day sample, but oxidation to
sulfonic acid groups becomes apparent for contact time > 3 days, and reaches nearly 60 % of the total sulphur
species after 8 days of contact time as estimated from XPS analysis. The Au-SH-bearing catalyst is inactive for
cyclohexene oxidation with molecular oxygen in liquid phase, but those having sulfonic groups are active and
selective for its allylic oxidation. It has been found for the 8-day catalyst that the gold nanoclusters partially
evolve spontaneously in the reaction medium to form gold nanoparticles, and this agglomeration process par-
allels the increase in catalyst activity.
1
. Introduction
majority of these methods, aqueous extracts of plants are mixed with
diluted aqueous solutions of HAuCl , in such a way that the formation
4
Metal nanoparticles and clusters are attracting an increasing interest
of the AuNPs takes place in the aqueous solution that contains a variety
of complex soluble biomolecules derived from the plant extract. These
biomolecules are responsible for the reduction of Au(III) to Au(0) and
the stabilization of the resulting Au NPs. In very few reports essential
oils are also used [10–12], but even in these cases the oils are mixed
with ethanol/water mixtures [10,11] or acetone [12] and then a small
for catalytic applications for their unique properties in important che-
mical and energy-related processes (see references [1–3] for very recent
developments in this area). More specifically, gold nanoparticles
(
AuNPs) are able to catalyze a large number of chemical reactions,
among them the selective oxidation of organic molecules [4,5]. The
catalytic performance of the AuNPs in these reactions is related, among
other factors, to their size and morphology, which in turn are depen-
dent upon their synthesis pathway. Hence, this has prompted intensive
research in designing new synthesis methodologies that aimed not only
at the control of these two properties, but also to cope with sustain-
ability aspects of the synthesis process. On this regard, increasing at-
tention is being devoted to environmental benign routes, and in parti-
cular to the use of substances derived from plants that can play both the
role of reduction of the gold precursors and capping agents of the
nascent AuNPs. Following this approach, a large number of plant ex-
tracts have been used, rendering AuNPs of different sizes and
morphologies, usually in the 10−50 nm range [6–9]. In the vast
portion of the resulting solution is added to the aqueous HAuCl solu-
4
tion, in such a way that a water-rich, single liquid phase system is again
formed. Typically, the concentration of the essential oil in these solu-
tions is 1 % (v/v) [11].
Far from these one-liquid phase methods, a new two-liquid phases
approach has been developed based on the use of water-insoluble es-
sential oils as organic phase, and a gold water solution formed by dis-
solving gold lumps in an ammonium chloride solution in concentrated
nitric acid (this solution is a variety of the well-known aqua regia,
usually prepared by mixing nitric and hydrochloric acids, 1:3 mol ratio)
as aqueous phase [13,14]. The development of this procedure has been
inspired by historical eighteenth century recipes to prepare the so-
⁎
Corresponding author.
E-mail address: jperez@icp.csic.es (J. Pérez-Pariente).
https://doi.org/10.1016/j.mcat.2020.110922
Received 16 December 2019; Received in revised form 6 March 2020; Accepted 23 March 2020
Available online 13 April 2020
2468-8231/ © 2020 Elsevier B.V. All rights reserved.

J. Agúndez, et al.
Molecular Catalysis 488 (2020) 110922
called potable gold, then a reputed medicine with a longstanding tra-
dition that was nevertheless forgotten in nineteenth century [15,16].
Following this procedure and by using rosemary (Rosmarinus officinalis)
essential oil, a mixture of gold nanoparticles (1−8 nm) and small na-
noclusters are formed in the oil phase at high concentration of gold in
the aqueous phase [13]. However, if the concentration is reduced, only
gold nanoclusters are obtained [14]. Moreover, these gold nanoclusters
can be conveniently immobilized on ordered mesoporous materials
2.2. Synthesis of the gold nanoclusters
Gold nanoclusters were prepared from a two-liquid phases system
according to a procedure previously described [13,14]. A gold lump
(0.1507 g, Johnson-Matthey, 99.99 %) was dissolved under gentle
stirring in 48.2 g of aqua regia, prepared by mixing (4:1, w/w) nitric
acid (Panreac, 65 wt%) and ammonium chloride (Sigma-Aldrich, > 98
wt%), while heating at 40 °C in a sand bath. This solution has a 1:320
gold-to-aqua regia weight ratio. After cooling, the resulting golden
yellow solution was distributed in two 50 mL decanting funnels, and
then 12 g of eucalyptus essential oil were gently added to each of them,
where it remains as a top layer over the gold solution. The system is
then left at room temperature undisturbed, i.e., the two liquid phases
are not mixed, and aliquots of the organic layer are taken at selected
time intervals to prepare the Au-SBA-15 materials, as it is described
below. The area of the contact surface between both liquid phases was
(
SBA-15) functionalized with thiol groups, and the resulting solids be-
have as selective catalysts for the allylic oxidation of cyclohexene with
molecular oxygen in mild, liquid phase conditions [14].
The reduction of metal and the subsequent stabilization of the
nascent gold entities carried out by crude plant derivatives is a complex
chemical phenomenon that involves several biomolecules therein pre-
sent [17]. On this regard, nearly thirty different individual molecules
have been identified in rosemary essential oil, being 1,8-cineol (24.9 wt
2
%
), camphor (20.9 wt%) and alfa-pinene (21.9 wt%) the three more
10.7 cm . The eucalyptus essential oil (Eucalyptus globulus) was supplied
abundant compounds [13]. Hence, it could be reasoned that the specific
chemical composition of the essential oil would have an impact on the
resulting gold entities. Based on this hypothesis, we have explored in
this work the synthesis of gold entities by using the two-liquid phase
system described above by replacing the rosemary essential oil by eu-
calyptus (Eucalyptus globulus) essential oil. The qualitative composition
of this oil is closely related to that of rosemary, but the content of cineol
is much higher (55 wt%), as well as that of alfa-pinene, but to a lesser
extent (22 wt%), while no camphor is detected and the concentration of
the remaining components is < 4 wt% (see below). As it has been dis-
cussed above, the use of rosemary oil can lead to AuNPs or gold na-
noclusters depending on the synthesis conditions. Therefore, a suitable
support for their immobilization should have the appropriated textural
properties (large surface area and pore volume) and in particular pores
with diameter large enough to accommodate these gold entities. These
requirements are satisfied by ordered mesoporous materials [18], and
in particular by SBA-15 functionalized with thiol groups, which have
been successfully used previously to immobilize gold nanoclusters
prepared by using rosemary oil [14]. In the present work, the metal
clusters have also been immobilized on the mesoporous SBA-15 mate-
rial functionalized with mercaptopropyl moieties and the resulting so-
lids used as catalysts for the liquid phase oxidation of cyclohexene with
oxygen at atmospheric pressure.
by the Spanish company El Granero Integral (The Integral Barn) and has
the following chemical composition (wt. %) as determined by GC–MS
employing a gas chromatograph (Agilent 6890) coupled with a mass
spectrometer (Agilent 5973 N) using a capillary column made of me-
thylpolysiloxane (30 m x0.25 mm x0.25 μm), heating from 70 to 290 °C
at 6 °C /min: 55.1 % 1,8-cineole, 22.3 % alpha-pinene, 3.8 % p-men-
thenyl acetate, 2.2 % allo-aromadendrene, 2.2 % alpha-terpineol, 2.2 %
trans-pinocarveol, 2.1 % p-cymene, 0.6 % phellandrene, 0.6 % beta-
pinene, 0.5 % 4-terpineol, 0.3 % borneol, 0.3 % myrcene, 0.1 % cam-
phene (see Fig. S1 of Supplementary Information for the molecular
structure of the three most abundant compounds).
2.3. Immobilization of the gold nanoclusters on SBA-15
Aliquots of 3.75 mL of the organic layer were taken after 1, 3, 6 and
8 days of the addition of eucalyptus oil and each of them was mixed
with 18.75 mL of ethanol. To this solution 0.500 g of the extracted SBA-
15 material were added and the mixture was stirred at room tempera-
ture for 3 h. After that, the solid was separated by centrifugation and
washed with four portions of 40 mL each of ethanol. The corresponding
samples were denoted as Ex, x = 1, 3, 6 and 8, as shown in Scheme 1.
2.4. Characterization techniques
Powder X-ray diffraction was carried out using a PANalytical X’pert
Pro instrument (Cu Kα radiation). Gold content of the solid was de-
termined by inductively coupled plasma (ICP-OES) spectrometry with
an ICP Winlab Optima 3300 DV Perkin-Elmer spectrometer.
Thermogravimetric analyses were performed in a Perkin-Elmer TGA7
instrument, in air (40 mL/min) with a 20 °C/min heating ramp from 25
to 900 °C. CHNS elemental analyses were done in a LECO CHNS-932
analyser provided with an AD-4 Perkin-Elmer scale. Nitrogen adsorp-
tion-desorption isotherms where measured in a Micromeritics ASAP
2420 apparatus at the temperature of liquid nitrogen (-196 °C). The
samples were degassed in situ at 70 °C in vacuum for 16 h prior to
analysis. Surface area was determined using the BET method. The pore
volume and the average pore diameter were calculated by applying the
BJH protocol to the adsorption branch of the isotherm. Diffuse re-
flectance UV–vis spectra were recorded on a Cary 5000 Varian spec-
trophotometer equipped with an integrating sphere with the synthetic
polymer Spectralon as reference. The data were expressed according to
the Kubelka-Munk function. MAS NMR spectra were recorded with a
2
. Experimental
2.1. Synthesis of thiol-containing SBA-15
Propyl-thiol mesoporous SBA-15 was prepared from a gel with
molar composition: 1.0 TEOS:0.111 MPTMS:0.0186 P123:6.42 HCl:180
H
2
O, where TEOS stands for tetraethyl orthosilicate (Sigma-
Aldrich, > 99 %); MPTMS for (3-mercaptopropyl)trimethoxysysilane
(
Sigma-Aldrich, 95 %); P123 for Pluronic 123, the triblock co-polymer
PEO20PPO70PEO20, m.w. ∼5800 (Sigma-Aldrich); and HCl for hydro-
chloric acid (Panreac, 37 wt%), following the procedure described in ref
[
14]: 125 mL of 1.9 M HCl were placed in a 500 mL plastic bottle pro-
vided with a cover having a hole for the insertion of a PTFE (Poly-
tetrafluoroethylene) stirrer blade, and 4 g of P123 were added. Then,
the bottle was heated at 40 °C in a silicone oil bath, and 8.2 mL of TEOS
were added. After 45 min, 764 μL of MPTMS were added, and the
mixture was stirred for 22 h. After that, it was poured into a stainless-
steel autoclave provided with a Teflon liner, and heated statically at
1
13
Bruker AV 400 WB spectrometer. H to C cross-polarization (CP)
1
1
00 °C for 24 h. The autoclave was then cooled and its content filtered,
spectra were recorded using a π/2 rad pulses of 4.5 μs for H, a contact
1
3
washed with ethanol and dried at room temperature overnight. The
dried sample was treated with ethanol (200 mL of ethanol per g of
sample) under stirring in a 1 L round-bottom flask at 90 °C for 24 h in
order to remove the surfactant. The resulting sample is denoted as E-
ext.
time of 5 ms and a recycle delay of 3 s. For the acquisition of the
C
2
9
spectra, the samples were span at a rate of 5–5.5 kHz. Si MAS NMR
spectra were recorded in a 4 mm probe at 79.5 MHz.
Transmission electron microscopy studies were carried out using a
JEOL 2100 F electron microscope operating at 200 kV in conventional
2

J. Agúndez, et al.
Molecular Catalysis 488 (2020) 110922
Scheme 1. Synthesis of SBA-15 silica-supported gold catalysts.
and STEM/HAADF mode. The catalysts were dispersed in ethanol and
dropped onto a holey carbon copper grid for the observations. X-ray
photoelectron spectra (XPS) have been collected using a SPECS in-
dimethylpolysiloxane capillary column of 15 m of length and 0.25 mm
of i. d. Octane was used as internal standard. Five reaction products
were identified; two of them resulting from the addition of oxygen to
the cyclohexene double bond, namely cyclohexene epoxide and cyclo-
hexanediol, and three coming from the allylic oxidation of the cyclo-
hexene ring: 2-cyclohexen-1-ol, 2-cyclohexen-1-one and 2-cyclohexenyl
hydroperoxide. Cyclohexene conversion was calculated from the yields
of these five products.
−
10
strument with UHV system (pressure in the range of 10
mbar),
equipped with a PHOIBOS 150 9MCD energy analyzer. A non-mono-
chromatic Mg K (1253.6 eV) X-ray source was used, working at a
α
power of 200 W, with an acceleration voltage of 12 kV. High-resolution
regions were recorded with a pass energy of 20 eV. For analysis, the
powder samples were pressed into self-supporting wafers and stuck on
the sample holder with double-sided adhesive conductive carbon tape.
Spectra were referenced against the O 1s emission line (binding energy
set to 532.9 eV) to correct for charging effect. Decomposition of ex-
perimental peaks into components (70 % Gaussian, 30 % Lorentzian)
was done using a non-linear, least squares fitting algorithm and a
Shirley baseline. Relative atom ratios were calculated from the sum
area of the core-level components of Si, S and Au, using the relative
sensitivity factors provided by Casa XPS software.
3
. Results and discussion
3.1. Extracted SH-SBA-15 sample
The synthesis procedure of the SH-SBA-15 material has been de-
scribed in the experimental section. Both the as-made and the extracted
(
the surfactant-free product recovered after ethanol extraction) samples
show the X-ray diffraction pattern characteristic of well-ordered SBA-
5 mesoporous materials, with 100 = 9.5 nm and unit cell size
= 11.0 nm for the extracted one (Fig. S2 and Table 1). The nitrogen
1
d
2.5. Catalytic tests
a
0
adsorption/desorption isotherm (Fig. S3) is also characteristic of SBA-
The oxidation of cyclohexene with molecular oxygen was carried
2
1
5 materials, with surface area and pore volume of about 600 m /g and
out by the procedure described in [14], which was developed from that
reported in [19]. The catalytic reaction was carried out in a 50 mL four-
neck round-bottom glass flask, provided with a condenser through
which water at 5 °C was circulated to minimize the evaporation of re-
agents and products. The flask was immersed in a silicone oil bath to
keep the reaction temperature at 65 °C, measured inside the reaction
mixture by a thermometer inserted in one of the necks. The experi-
mental set-up was not protected against environmental light. The re-
action mixture was made of 0.049 mol of cyclohexene (4.055 g, Sigma-
Aldrich, 99 %), 0.4055 g of octane (10 wt% referred to cyclohexene;
Sigma-Aldrich, > 99 %), 0.2027 g of a ter-butyl hydroperoxide solution
3
0
.67 cm /g, respectively. The pore diameter is 6.3 nm, close to that
reported for mercaptopropyl-functionalized SBA-15 materials [20], but
clearly smaller than that of pure SBA-15 materials, which is usually
around 7 nm [21]. This reduction of pore size can be attributed to the
presence of organosilane propylthiol groups attached to the surface of
the SBA-15 channels. Moreover, the chemical analysis reveals the pre-
sence of both sulphur and carbon in the sample (Table 2), with a Si/S
ratio of 10.6, i.e., nearly 10 % of the Si atoms in the structure are at-
tached to one mercaptopropyl moiety. A more direct evidence of the
actual occurrence of the co-condensation process of MPTMS and TEOS
and the consequent presence of propylthiol groups in the solid is
(
TBHP, 5 wt% referred to cyclohexene; ∼ 5.5 M in decane, Sigma-Al-
drich), 3.041 g of toluene (75 wt% of the cyclohexene; Panreac, > 99.5
), and 0.050 g of catalyst. O (1.8 mL/min) was bubbled through the
Table 1
%
2
Structural and textural properties of SH-SBA-15 support and catalysts.
stirred reaction mixture. The catalysts were previously heated at 100 °C
for 1 h in the reaction flask equipped with a tube containing molecular
sieve 5A in order to remove the traces of water coming from the cat-
alysts, cooled down to the reaction temperature of 65 °C, and then the
reagents were added. The trap was removed and replaced by the con-
denser before the reaction starts. Aliquots of 0.20 mL of the reaction
mixture were taken at given time intervals and analysed by GC in a
Varian CP-300 instrument, by using a FactorFour™ (Varian VF-1 ms)
2
Sample Unit cell parameter a
0
S
BET (m /
Pore volume
Pore size
3
(
nm)
g)
(cm /g)
(nm)
E-ext
E1
11.0
10.9
11.0
10.9
10.9
603
381
395
318
296
0.67
0.48
0.47
0.43
0.42
6.3
5.6
5.5
5.6
5.6
E3
E6
E8
3

J. Agúndez, et al.
Molecular Catalysis 488 (2020) 110922
Table 2
Chemical composition (wt%) and TG results of the SH-SBA-15 support (E-ext)
and catalysts.
Sample a
C
H
S
Au
C/S b S/Au b %Weight loss T > 140 °C
E-ext
E1
13.9 3.2 3.5
–
10.5
–
24.1
27.9
20.6
22.2
26.3
16.9 3.4 3.3 0.4 13.6
12.2 3.0 3.3 2.7 9.9
13.2 3.0 3.3 2.6 10.7
15.1 3.2 3.2 2.1 12.6
52
7.5
7.8
9.3
E3
E6
E8
a
Traces of N are found in the samples ranging in increasing order from
.1 wt% for E1 to 0.4 wt% for E8.
0
b
Atomic ratio.
100
80
60
40
20
0
-20
13
δ
C (ppm)
Fig. 2. 13C{ H}CP/MAS NMR spectra of extracted E-ext (top) and Au-con-
taining E8 (bottom) samples. Spectral deconvolution is shown to highlight the
location of relevant resonance peaks of the anchored functional groups as dis-
cussed in the text.
1
1.5
1.0
-20
-40
-60
-80
-100
-120
-140
-160
29
δ
Si (ppm)
2
9
Fig. 1. Si MAS NMR spectra of the extracted E-ext (top) and Au-containing E8
E6
m
0.5
0.0
(
bottom) samples. Deconvolution of spectra into five Qn and T components is
also shown.
E3
E1
E ext.
Table 3
Chemical shift and peak area percentage of Q and Tm silicon species de-
n
termined by deconvolution of 29Si MAS-NMR spectra.
2
00
400
600
800
Sample Chemical shift (ppm)
Peak area (%)
Wavelength (nm)
Q2
Q3
Q4
T2
T3
Q2
Q3
Q4
T2
T3
1
1
0
0
.5
E-ext
E8
−92 −102 −111 −58 −67 4.3 31.7 49.5 5.4 9.1
−93 −101 −111 −59 −66 3.9 30.8 55.0 3.5 6.8
provided by 29Si MAS NMR of sample E-ext (Fig. 1). Two different sets
of signals are observed in the spectrum. Three resonances are found at
.0
.5
.0
2
3
4
−
92 (weak), −102 and −111 ppm, assigned to Q , Q and Q species,
n
respectively, where Q refers to [Si(OSi)
n
(OH)4-n] environments, with
E8 after reaction
n = 0, 1, 2, 3 and 4. The high intensity of the signal at −102 ppm,
n
which accounts for 37 % of total area of Q signals (Table 3), evidences
the high concentration of silanol Si−OH groups present in the sample.
It should be noted, nevertheless, that an undetermined fraction of these
silanol species would actually correspond to Si-ethoxy moieties, as
E8
E ext.
1
3
proven by C CP/MAS NMR (see below). A second group of signals
appears at −67 and −58 ppm, being the former more intense than the
latter. These signals, which account for 14.5 % of total Si species, are
200
400
600
800
3
2
m
Wavelength (nm)
attributed respectively to T and T sites, where T corresponds to si-
lane species (those having C-Si bonds) in environments described as
Fig. 3. UV–vis spectra of the Au-containing samples.
[
RSi(OSi) (OH)3-m], m = 0, 1, 2 and 3 [22], confirming in this way that
m
the MPTMS is indeed forming part of the silica framework located at the
channel surface, in agreement with the observed reduction of the
average pore size reported above. Further confirmation of the presence
of mercaptopropyl moieties in the sample is provided by the
13
1
C{ H}CP/MAS NMR spectra shown in Fig. 2. The intense resonance at
7 ppm is contributed by the α carbon, adjacent to the SH group, as
well as by the carbon atom in β position, while that at 11 ppm arises
2
4

J. Agúndez, et al.
Molecular Catalysis 488 (2020) 110922
1
1
1
00
with the eucalyptus essential oil/ethanol solution is quite similar to that
of the extracted one (Table 1). Surface area and total pore volume of the
Au-containing materials are smaller than those of the SH precursor,
while pore size is slightly reduced (Table 1).
0
.0
9
8
7
6
0
0
0
0
The amount of gold present in the samples raises from 0.4 wt% for
the material prepared from the eucalyptus oil phase that was in contact
with the gold solution for one day (sample E1), to 2.7 wt% for the 3-
days sample (E3), and then decreases slightly for longer contact time
-
0.2
0.4
(
Table 2). This decrease can be attributed to a precipitation of part of
-
the gold from the eucalyptus oil solution, because gold crystals are
found on the inner wall of the decanting funnel around the interphase
between both liquid phases as the contact time is prolonged beyond 3
days. The amount of gold in the 1-day sample is twice that found for a
similar preparation procedure and contact time previously reported by
using rosemary essential oil [14], which is 0.19, evidencing the higher
efficiency of the eucalyptus oil for transferring gold from the aqua regia
to the oil phase. This behaviour should arise from differences in the
respective chemical composition of the two oils, and in particular to the
concentration of 1,8-cineole present in the eucalyptus oil, which is
nearly twice that of rosemary oil. On this regard, the efficiency of black
cardamom aqueous extracts to reduce gold when added to tetra-
chloroauric solutions has been attributed to the high content (65 wt%)
of 1,8-cineole of this plant [25]. In the same way, Eucalyptus oleosa leaf
aqueous extracts, also rich in 1,8-cineole, have been reported to reduce
00
0
.0
9
8
7
6
0
0
0
0
-
0.2
0.4
-
3
+
Au
from tetrachloroauric solutions [26]. However, in these two
cases, gold nanoparticles were obtained, in contrast to the gold na-
noclusters formed following the procedure reported here, as it will be
discussed below.
00
0
.0
The amount of S in the Au-containing samples is close to that of the
extracted gold-free support, although the Au deposition procedure leads
to a slight (< 10 %) loss of sulphur (Table 2). Partial removal of mer-
captopropyl groups attached to the silica support upon contact with the
9
8
7
6
0
0
0
0
-
0.1
0.2
2
9
aqua regia solution of gold is also confirmed by Si MAS NMR. As
m
shown in Table 3, the amount of T species (silane groups) determined
2
9
from the Si MAS NMR spectrum of sample E8 (Fig. 1), accounts for
10.3 % of total silicon species. This result corresponds to a decrease of
silane groups of 29 % respect to sample E-ext, indicating that silane
removal might be higher than expected according to the decrease of
bulk sulphur content determined by chemical analysis. Chemical ana-
lyses show that the S/Au ratios attained are in the range 7−52. The
carbon content increases for the E1 and E8 samples, particularly for the
former, which suggests that some organic material from the oil phase is
incorporated into the solids.
-
-0.3
2
00
400
600
800
Temperature (ºC)
Fig. 4. TG/DTG curves of the extracted support E-ext (top) and Au-containing
samples E1 (middle) and E8 (bottom).
The UV–vis spectra of AuNPs present a characteristic surface
−1
plasmon resonance at ∼520 cm
[27]. Moreover, UV–vis bands of
from the γ carbon, which is attached to the Si atom in the silane
gold clusters stabilized with thiol groups and having a metal core size of
0.8 nm have been reported in the range 300−450 nm [28], while UV
bands at ∼ 250 nm have been attributed to gold clusters having a
partial positive charge [29]. The UV–vis spectra of the Au-containing
samples prepared in this work are shown in Fig. 3. Bands of weak in-
tensity are observed at ∼ 370 nm and ∼ 250 nm, but the band of the
[
23,24]. The two signals at 16 and 59 ppm are attributed respectively to
the methyl and methylene carbon atoms of the ethoxy groups
–O−CH −CH ), which are formed by reaction of silanol groups pre-
sent in the material with hot ethanol during the extraction process
(
2
3
[
23,24]. Finally, the signal at 70 ppm would correspond to the me-
−1
thylene carbon atoms adjacent to ether oxygen atoms of residual
P123 surfactant that remains in the material after the extraction process
plasmon resonance at ∼ 520 cm
characteristic of AuNPs is not de-
tected. An exception to this behaviour is sample E3, the one with the
−
1
[
23]. According to this assignment, the surfactant methyl groups would
highest Au content, where a band of low intensity at ∼550 cm with a
−1
also contribute to the intensity of the peak at 16 ppm. The presence in
the sample of these carbon-bearing chemical species in addition to
mercaptopropyl silane explains that the C/S atom ratio recorded in
Table 2 is higher than that expected for the S-bearing organic moiety.
weak shoulder at 515 cm
is observed, and a new intense band at
350 nm is detected while that at 370 nm is absent. This result is in
strong contrast with the formation of gold nanoparticles of ∼ 28 nm of
average size when Eucalyptus oleosa leaf aqueous extracts are used to
3
+
reduce Au
from tetrachloroauric solutions [26]. This evidences the
efficiency of the two-liquid phases method based on the use of essential
oils to render small gold nanoclusters, in contrast with the AuNPs
produced from plant extracts using a single aqueous phase, as it has
been discussed above.
3.2. Gold-containing samples
Au entities present in the eucalyptus oil phase were immobilized on
the SH-SBA-15 extracted material by following the procedure described
in the experimental section, subsection 2.3. The unit cell size of the
materials recovered after contacting the extracted SH-SBA-15 sample
Some noticeable differences have been observed between the ex-
tracted and the Au-containing samples concerning their thermogravi-
metric analysis, which have been plotted in Fig. 4. The TG curve of the
5

J. Agúndez, et al.
Molecular Catalysis 488 (2020) 110922
S(ox)
Au(red) 4f7/2
2p1/2
2p3/2
Au(red) 4f5/2
S(red)
p3/2
2
2p1/2
Au(ox)
Au(ox)
4f7/2
4
f5/2
172 170 168 166 164 162
90
88
86
84
Binding energy (eV)
Binding energy (eV)
Fig. 5. Deconvoluted S 2p and Au 4f core-level XP spectra of samples E1 (bottom) and E8 (top). Labels indicate the assignment of doublets to reduced sulphur (S(red)
)
and gold (Au(red)) and oxidized sulphur (S(ox)) and gold (Au(ox)) species.
Table 4
XPS characterization data of selected gold-containing samples.
Sample
BE (eV)
Atomic ratio
S/Si
Si 2p
S 2p3/2
163.6
S 2p1/2
Au 4f7/2
85.1
Au 4f5/2
88.8
Au/Si
0.007
0.014
Au/S
0.103
0.212
S
ox/S a
E1
E8
103.5
103.5
164.8
170.1
164.8
169.7
0.068
0.24
0.58
1
68.8
163.6
68.4
84.9
86.3
88.5
89.7
0.065
1
a
Fraction of oxidized sulphur species.
extracted sample shows a weight loss below 100 °C due to water des-
orption. A sharp and intense weight loss is observed centred at
progressive change for samples prepared with gold solutions obtained
after 3 and 6 days of contact time of the oil with the acid solution
(samples E3 and E6, respectively), with no further changes found for
longer contact time (sample E8). As shown in Fig. 4 for sample E8, these
profiles exhibit a weaker and smoother weight loss at ca. 320 °C com-
pared to those of samples E-ext and E1, which leads to a broader peak in
the first derivative plot, slightly shifted towards higher temperature (ca.
335 °C). Furthermore, an additional weight loss step appears above
450 °C. The TGA patterns of samples E3, E6 and E8 are quite similar to
those previously reported for analogous materials synthesized from
rosemary oil [14] and also for mesoporous materials bearing oxidized
mercaptopropyl groups [24]. Indeed, it has been shown that the de-
composition of alkyl-sulfonic acid groups anchored on SBA-15 by direct
synthesis takes place at 450 °C [20]. Therefore, our TGA results suggest
that part of the mercaptopropyl groups are oxidized during the gold
deposition process if the contact time between the eucalyptus oil and
the gold-containing aqua regia solution is prolonged for at least 3 days.
A more direct evidence of the actual oxidation of thiol to sulfonic
∼
320 °C, which is followed by a continuous and smooth weight loss at
T > ∼380 °C. This two last thermal events are due basically to the
desorption/combustion of the organic material present in the sample, in
particular the mercaptopropyl moieties [20,24] and, to some extent,
also the remnant surfactant and ethoxy groups detected by NMR. The
water desorbed by the condensation of the silanol groups detected by
NMR will also contribute to the total weight loss at high temperature.
The TGA of the 1-day sample is quite similar to the extracted one,
save for the presence of an additional thermal feature centred at 130 °C,
which can be ascribed to the removal of some volatile organic material
incorporated from the essential oil phase during the gold deposition
process, in agreement with the higher amount of carbon found in this
sample compared to the support E-ext (Table 2). The similarity of the
TG patterns of the extracted and 1-day samples suggests that the gold
deposition process does not modify essentially the nature of the func-
tional groups therein present when the support was in contact with the
oil solution obtained after only 1 day of reaction with aqua regia.
However, the weight loss profile at temperatures above 250 °C, at
which the removal of the attached silane groups occur, shows a
1
3
groups is given by the C CP/MAS NMR spectrum of the 8-days sample
(Fig. 2), which shows resonances at 11, 18 and 54 ppm that can be
assigned, respectively, to methylene carbons attached to Si, and those
6

J. Agúndez, et al.
Molecular Catalysis 488 (2020) 110922
5
4
3
2
1
0
0
0
0
0
0
with aqua regia from 1 to 8 days. Nonetheless, this enhancement of
oxidation capacity does not increase significantly the leaching of sul-
phur. Indeed, according to the S/Si atomic ratios calculated from XPS
analysis (Table 4), the decrease of total S concentration of sample E8
respect to sample E1 is below 5%, in agreement with bulk chemical
analyses (Table 2).
The oxidation of thiol groups that takes place during the gold de-
position treatment can be attributed to the presence of strong oxidant
compounds that would be formed in the essential oil in contact with
aqua regia. This oxidant activity of the essential oil, which becomes
apparent specially for contact times of 3 days or longer, should be
linked to the changes that we have observed in its colour in that period
of time. No changes in colour are detected for the 1-day sample, but the
oil starts to acquire a light brown-orange coloration after 3 days, which
intensity increases for longer contact times (Fig. S4). These changes
strongly suggest that the terpenic compounds initially present in the oil
react with the strongly acidic and oxidant solution of gold in aqua regia,
and the compounds resulting from this reaction (which have not yet
been identified) should be able to oxidize thiol to sulfonic groups.
Supported gold species present on the catalysts have been char-
acterized by X-ray photoelectron spectroscopy. The Au 4f core-level
spectrum of sample E1 (Fig. 5) exhibits two broad peaks than can be
fitted with a single doublet (3:4 intensity ratio) with BE for the Au 4f7/2
component of 85.1 eV (FWHM =1.8 eV) and a spin-orbit splitting of
3.7 eV (Table 4). This doublet shows a positive shift of 1.3 eV and a
broadening of ca. 0.6 eV respect to spectra reported for Au° nano-
particles within the size range 1−3 nm bonded to thiol groups [32,33].
This result is in agreement with the UV–vis spectrum that indicates the
lack of Au nanoparticles in this sample, which would contain smaller
gold entities. Indeed, it has been shown for supported gold nanoclusters
that BE increases respect to bulk 4f core-level as cluster size decreases
[34,35]. Furthermore, significant broadening has been observed with
decreasing cluster size for clusters of less than 20 atoms. Thus, spectrum
of sample E1 suggests that gold would be present as very small clusters
in this sample. In is also worth noting that the Au/S atomic ratio cal-
culated for sample E1 from the XPS spectrum (Table 4) is around 5
times higher than the bulk ratio (Table 2), which indicates a noticeable
enrichment of gold in the outermost region of the support. Compared to
sample E1, the Au 4f spectrum of sample E8 (Fig. 5) shows higher in-
tensity, in agreement with its higher gold content, with a Au/S ratio of
ca. 0.2, twice that of sample E1 (Table 4). However, the Au/S ratio of
sample E8, is much closer to the bulk ratio, which indicates lower gold
surface enrichment in this sample respect to sample E1. Also, the XPS
spectrum of sample E8 shows the Au 4f doublet characteristic of Au°
nanoclusters slightly shifted to lower BE (84.9 eV for the 4f7/2 compo-
nent) and narrower (FWHM =1.3 eV) in comparison to sample E1. This
result indicates that sample E8 would contain larger gold entities, al-
though the high BE supports the absence of nanoparticles. However, it
can be clearly seen in Fig. 5 that the Au 4f peaks show some tailing
towards high BE and an additional doublet is required for a correct
fitting of the spectrum, which accounts for 11 % of total Au 4f peak
area. The BE (86.3 eV for the 4f7/2 component) and spin-orbit splitting
(3.4 eV) of this species agree with values reported for Au(III) complexes
[36,37] and oxide phases [38–41], indicating that a small fraction of
the gold content of sample E8 is present as oxidized Au(III).
0
10
20
30
40
50
Time (h)
Fig. 6. Conversion of cyclohexene as a function of time for the gold-free support
E-ext (black squares) and gold catalysts E1 (green circles), E3 (brown up tri-
angles) and E8 (blue down triangles). Solid lines have been drawn as visual aid
only.
in β and α positions respect to the –SO H moiety in propylsulfonic
3
groups [22]. Other signals found at 70 and 15 ppm, that were present
also in sample E-ext, are assigned to residual P123 species [23], which
do not seem to be affected by the gold impregnation treatment. How-
ever, this treatment removes the ethoxy groups present in the extracted
material, as the corresponding NMR signals have vanished. It can be
noticed that the signal detected at 27 ppm, corresponding to carbon
atoms attached to the –SH moiety, exhibits much lower intensity
compared to sample E-ext, and, on the other hand, no signals of dis-
ulphide bridges arising from the eventual condensation of thiol groups
(
signals in the 20−50 ppm range) are detected, nor that at 64 ppm
attributed to carbon atoms attached to S(O ) chemical species [23].
2
Therefore, it can be concluded that, in the case of sample E8 (as well as
E6 and, in a lesser extent, sample E3) a large fraction of thiol groups
present in the extracted sample are oxidized to sulfonic acid groups
during the gold deposition process. This conclusion is further supported
by XPS analysis, which provides direct evidence of the chemical state of
sulphur. As shown in Fig. 5, the S 2p core-level spectrum of sample E8
exhibits two asymmetric peaks that can be deconvolved into doublets
with a characteristic spin-orbit splitting in the range 1.2–1.3 eV and a
2
:1 intensity ratio. The low BE doublet, which 2p3/2 component appears
at 163.6 eV (Table 4), can be assigned to thiol while the high BE one
168.4 eV) corresponds to high oxidation state sulphur species such as
(
sulfonate groups [24,30,31]. The spectrum, however, do not show a
peak at ca. 162 eV that would be expected for thiol groups bound to
gold [30,32], which could be due to the low amount of thiol groups that
are interacting with Au because of the low Au loading of the samples.
According to the relative areas of both high and low BE peaks, it can be
estimated that nearly 60 % of the thiol species were oxidized to sulfonic
acid during the gold deposition treatment (Table 4), which is in close
agreement with the results of bulk techniques discussed above. It is
interesting to notice that the XPS spectrum of sample E1 (Fig. 5) also
exhibits both doublets in the S 2p region, although with lower intensity
of the high BE peaks, revealing that in this sample oxidation extends to
3.3. Cyclohexene oxidation
2
4 % of thiol groups. This shows that even the oil obtained after only 1
The E1, E3 and E8 samples were tested in the cyclohexene oxidation
with molecular oxygen at atmospheric pressure, in the conditions de-
scribed in the experimental section, and the conversion as a function of
the reaction time is plotted in Fig. 6. It can be seen that only for the 3-d
and 8-d catalysts the conversion is significantly higher than that of
gold-free thiol-containing sample, approaching 40 %–45 % at 48 h of
reaction, while the presence of gold in the 1-day sample doesn’t en-
hance the catalytic activity as compared with that of the Au-free
day of contact with aqua regia has a certain thiol oxidation capacity.
Nevertheless, the oxidation would in this case affect only to thiol groups
located on the external surface of the support, for sulfonic acid species
are observed when probing the outermost layers of the sample by XPS
but they are not detected when using bulk techniques such as TGA and
1
3
C NMR, as discussed above. The results clearly show that the thiol
oxidation capacity of the essential oil increases with its contact time
7

J. Agúndez, et al.
Molecular Catalysis 488 (2020) 110922
Fig. 7. TEM (top) and STEM (bottom) images of the E8 catalyst after reaction.
material. A similar result has been reported for the 1-day sample pre-
pared by the same method but using rosemary oil [14], and it can be
taken as strong evidence that gold nanoclusters attached to thiol groups
are essentially inactive for cyclohexene oxidation in the reaction con-
ditions here described, probably due to their high stability. This can be
taken as a general conclusion derived from this study at least for the
materials and conditions here described. The manifestation of activity
by the gold entities present in the E3 and E8 catalysts is therefore linked
to the transformation of thiol groups into sulfonic acid groups that
ranging from clusters to nanoparticles. In the TEM images of this
sample (Fig. 7) gold nanoparticles with a maximum size of 20−25 nm
are clearly identified, but the STEM mode allows the detection of
abundant much smaller gold entities, some of them with sizes below
1−2 nm, about the detection limit of the instrument. The occurrence of
this yet incomplete growing process of the gold clusters toward nano-
particles that takes place in the reaction medium has also been observed
for catalysts prepared from rosemary oil by following the same proce-
dure here reported [14]. Therefore, there seems to be a positive cor-
relation between catalytic activity and the coalescence of the gold na-
noclusters to yield nanoparticles. A similar conclusion has been
reported for the same reaction by using silica-supported triphenylpho-
1
3
occurs during the gold deposition process, as shown by C MAS NMR
discussed above (Fig. 2). Moreover, it has been observed that the colour
of the catalysts changed as the reaction proceeds, from pale yellowish in
the pristine state, to pink and then purple. This behaviour suggests that
at least part of the gold nanoclusters initially present in the catalysts
gradually evolve in the reaction medium to form gold nanoparticles.
Indeed, in the UV–vis spectrum of the E8 catalyst after reaction the
sphine-stabilized Au clusters, which are inactive for the cyclohexene
9
oxidation, much in the same way reported here for the SH-Au clusters
[42]. In that case, the catalytic activity is triggered by the formation of
Au nanoparticles more than 2 nm in size. In our case, the broad size
distribution of gold clusters/nanoparticles present in the catalysts under
actual working conditions and their evolution in the reaction medium
preclude to establish a clear correlation between the size of gold entities
and the catalytic activity. It can be also noticed that the activity of these
catalysts is quite close to that prepared from rosemary oil reported in
−
1
characteristic plasmon resonance band at ∼ 525 cm
is clearly ob-
served (Fig. 3). However, the two bands associated to the presence of
clusters are still detected, although with lower intensity, and a red-shift
−
1
−1
of the 250 cm band to 280 cm is observed. This result points to the
coexistence in the used catalyst of an ample variety of gold entities,
8

J. Agúndez, et al.
Molecular Catalysis 488 (2020) 110922
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
evidences that the eucalyptus oil behaves as a very efficient phase-
transfer, reductant and capping agent of the gold entities present in the
strongly acidic and oxidant aqueous phase. Upon contacting the gold-
rich oil phase with the functionalized support, most of the thiol groups
are oxidized to sulfonic acid groups if the contact time between the oil
and the aqueous phase is sufficiently long. Gold is present in these
materials as very small clusters, and minor amount of Au(III) is present
at the longest contact time (8 days). It has been found that as far as the
thiol groups remain unaltered, the catalyst is inactive for the oxidation
of cyclohexene, which has been attributed to the presence of gold na-
noclusters strongly interacting with thiol groups forming probably very
stable thiolate configurations. However, the reaction is triggered if the
thiol groups are oxidized to sulfonic groups. In this latter case, it has
been observed a partial agglomeration of the gold nanoclusters toward
gold nanoparticles during the reaction, which seems to be the main
responsible for the catalytic activity. The allylic oxidation of cyclo-
hexene is the major catalytic route, where 2-cyclohexenyl hydroper-
oxide is an intermediate product which evolves to form the stable 2
0
10
20
30
40
50
Conversion(%)
-
cyclohexen-1-one and 2-cyclohexen-1-ol, with enone/enol ratio of ∼ 2
Fig. 8. Selectivity to the reaction products, 2-cyclohexen-1-ol (red squares), 2-
cyclohexen-1-one (green up triangles), cyclohexenyl hydroperoxide (violet
stars), cyclohexene epoxide (cyan diamonds) and cyclohexanediol (black down
triangles), as a function of conversion for catalysts E1 (full symbols), E3 (open
symbols) and E8 (crossed open symbols). Solid lines have been drawn as visual
aid only.
at high conversion.
CRediT authorship contribution statement
J. Agúndez: Conceptualization, Resources, Investigation,
Visualization, Writing - review & editing. C. Ares: Investigation. C.
Márquez-Álvarez: Investigation, Visualization, Writing - review &
editing. J. Pérez-Pariente: Conceptualization, Methodology,
Supervision, Writing - original draft.
[
14] with a similar Au content (2.57 wt%). This suggests that the in-
trinsic activity of the gold entities is basically not affected by the nature
of the essential oil used in the preparation of the gold nanoclusters.
The selectivity to the several reaction products is plotted in Fig. 8
for all the catalysts as a function of conversion. It can be seen that the
three products resulting from the allylic oxidation of the cyclohexene
Declaration of Competing Interest
(
2-cyclohexen-1-one, 2-cyclohexen-1-ol and 2-cyclohexenyl hydroper-
The authors declare that there are no conflicts of interest.
oxide) account for nearly 90 % of the total reaction products, and hence
the allylic oxidation is a preferred reaction route over the epoxidation
of the double bond. Concerning the distribution of products coming
from this major route, it can be observed in the figure that the se-
lectivity to 2-cyclohexenyl hydroperoxide is higher than ∼50 % for
conversion lower than ∼20 %, but it decreases rapidly as the conver-
sion increases, and the enone becomes the major product for the Au-
containing catalysts as the conversion raises beyond ∼30 %. This
evolution of the product selectivity with the conversion suggests that
the hydroperoxide is transformed into enone and enol as the reaction
proceeds. Moreover, it can be observed at conversion < 10 % that both
the enol and the hydroperoxide are primary products, but the se-
lectivity to the former decreases rapidly and reaches a minimum at ∼
Acknowledgements
The authors acknowledge the current Spanish Ministry of Science,
Innovation and Universities (formerly the Ministry of Economy and
Competitiveness) for the funding of this work through the project
MAT2016-77496-R.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.110922.
1
5 % conversion, and raises slightly beyond that point. On the contrary,
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