Gold nanoparticles deposited on surface modified carbon materials as reusable catalysts for hydrocarboxylation of cyclohexane

Article abstract of DOI:10.1016/j.apcata.2017.08.028

Gold nanoparticles were deposited on different carbon materials and used as catalysts for the alkane hydrocarboxylation reaction. Cyclohexane hydrocarboxylation to cyclohexanecarboxylic acid was carried out in the presence of CO and water, peroxodisulfate, in water/acetonitrile medium, at ca. 50 °C, with gold nanoparticles deposited by a colloidal method on carbon nanotubes and activated carbon with three different surface chemistries: in their original forms (CNT or AC, respectively), oxidized with HNO₃ (-ox) or oxidized with HNO₃ and subsequently treated with NaOH (-ox-Na). Au/CNT-ox-Na was the best catalyst, yielding cyclohexanecarboxylic acid up to 88.2% yield, with excellent recyclability (97.5% of the initial activity was maintained after five consecutive catalytic cycles).

Full text of DOI:10.1016/j.apcata.2017.08.028

Accepted Manuscript
Title: Gold nanoparticles deposited on surface modified
carbon materials as reusable catalysts for hydrocarboxylation
of cyclohexane
Authors: A.P.C. Ribeiro, L.M.D.R.S. Martins, S.A.C.
Carabineiro, J.L. Figueiredo, A.J.L. Pombeiro
PII:
S0926-860X(17)30408-8
DOI:
Reference:
http://dx.doi.org/10.1016/j.apcata.2017.08.028
APCATA 16383
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
27-4-2017
1-8-2017
19-8-2017
Please cite this article as: A.P.C.Ribeiro, L.M.D.R.S.Martins, S.A.C.Carabineiro,
J.L.Figueiredo, A.J.L.Pombeiro, Gold nanoparticles deposited on surface modified
carbon materials as reusable catalysts for hydrocarboxylation of cyclohexane, Applied
Catalysis A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.08.028
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Highlights
Gold nanoparticles deposited on functionalized activated carbon (AC) and carbon
nanotubes (CNT) were used as catalysts for hydrocarboxylation of cyclohexane.
Carbon materials with different surface chemistries were tested: in original forms (CNT
or AC, respectively), oxidized with HNO
3
(-ox) or oxidized with HNO and subsequently
3
treated with NaOH (-ox-Na).
The hydrocarboxylation reaction was carried out in the presence of CO, water and
peroxodisulfate in water/acetonitrile medium and mild conditions, at ca. 50 ºC.
Au/CNT-ox-Na was the best catalyst, yielding cyclohexanecarboxylic acid up to 88.2%
yield, with excellent recyclability, maintaining 97.5% of the initial activity after 5
consecutive catalytic cycles.
Gold nanoparticles deposited on surface modified carbon materials as reusable
catalysts for hydrocarboxylation of cyclohexane
a
,a,b
,c
c
A.P.C. Ribeiro, L.M.D.R.S. Martins,* S.A.C. Carabineiro,* J.L. Figueiredo, A.J.L.
Pombeiroa
a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049-001 Lisboa, Portugal.
b
Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa,
Instituto Politécnico de Lisboa, Rua Conselheiro Emídio Navarro, 1959-007 Lisboa,
Portugal. E-mail: lmartins@deq.isel.ipl.pt
c
Laboratório de Catálise e Materiais, Laboratório Associado LSRE-LCM, Faculdade de
Engenharia, Universidade do Porto, Rua Dr. Roberto Frias 4200-465 Porto, Portugal.
Email: sonia.carabineiro@fe.up.pt

Abstract
Gold nanoparticles were deposited on different carbon materials and used as catalysts for
the alkane hydrocarboxylation reaction. Cyclohexane hydrocarboxylation to
cyclohexanecarboxylic acid was carried out in the presence of CO and water,
peroxodisulfate, in water/acetonitrile medium, at ca. 50 ºC, with gold nanoparticles
deposited by a colloidal method on carbon nanotubes and activated carbon with three
different surface chemistries: in their original forms (CNT or AC, respectively), oxidized
with HNO
3
(-ox) or oxidized with HNO and subsequently treated with NaOH (-ox-Na).
3
Au/CNT-ox-Na was the best catalyst, yielding cyclohexanecarboxylic acid up to 88.2%
yield, with excellent recyclability (97.5% of the initial activity was maintained after five
consecutive catalytic cycles).
Keywords: gold nanoparticles; hydrocarboxylation; C-H activation; cyclohexane;
catalyst recycling

1
. Introduction
The carboxylation of C
n
alkanes to Cn+1 carboxylic acids, using CO, is an attractive
process for alkane functionalization [1-17] that might have important applications in
industry and surpass the disadvantages of the present synthesis procedures [2, 5, 7, 15].
However, this reaction is very challenging from the chemical point of view, due to the
need of C-H activation, that is very difficult to achieve for the least reactive lower alkanes
(
with 1 to 6 carbon atoms). Some authors [1, 3, 4] reported on the carboxylation of
cyclohexane to cyclohexanecarboxylic acid (achieving 4.3% yield relative to the
substrate) using CO and peroxodisulfate in trifluoroacetic acid (TFA), at 80 °C, using a
Pd(II) Cu(II) system as catalyst. A strongly acidic medium is needed, since the alkane is
/
rather inert.
More recently, many works dealt with the improvement of alkane carboxylation towards
the sustainable production of carboxylic acid, using greener and safer solvents [12-14, 16,
1
7]. Other authors reported on the hydrocarboxylation of cyclohexane to
cyclohexanecarboxylic acid (with a 72 % yield), using CO and water, in the presence of
peroxodisulfate oxidant, in water/acetonitrile medium, at ca. 50 °C, on a tetracopper(II)
catalyst, where water acts both as reagent and solvent [12]. Additionally, and contrarily
to the reaction carried out in TFA [4, 6, 9, 11], the carboxylation of cyclohexane by CO
in the H
2
O/MeCN/K
2
S
2
8
O system proceeds without catalyst up to a certain point,
producing cyclohexanecarboxylic acid (up to 12% yield).
Naturally, the reaction proceeds more effectively in the presence of a catalyst (V, Mn, Fe
or Cu) [12-14], with higher yields of carboxylic acid being achieved. Nevertheless, the
homogeneous catalytic systems studied so far [13] cannot be re-used. Therefore, the quest
for more efficient and eco-friendly heterogeneous methods that would allow catalyst
recycling is still a challenge.
Supported gold catalysts have found application in different reactions of environmental
and industrial importance [8, 10, 18-23]. Their catalytic activity is mainly influenced by
the preparation method, the nature of the support and, in particular, the gold nanoparticle
size [8, 10, 18-21]. However, the hydrocarboxylation of alkanes, using gold nanoparticles
(
supported on carbon nanotubes or activated carbon) as catalysts, has not been previously
reported. Only alkyne hydrocarboxylation using gold complexes (not gold nanoparticles)
24, 25], and the hydrocarboxylation of 1,3-butadiene catalysed by a Rh(I) complex
[
immobilized on activated carbon [26] can be found in the literature. The
hydrocarboxylation of cyclohexane using gold nanoparticles on carbon xerogels was very

recently reported by us [27]. However, the results found in the present work, for gold on
activated carbon and carbon nanotubes, using the above-mentioned protocol [12]
(
Scheme 1), are far superior than those previously published for xerogels.
Scheme 1. Hydrocarboxylation of cyclohexane to cyclohexanecarboxylic acid catalyzed
by gold nanoparticles supported on carbon materials (AC = activated carbon; CNT =
carbon nanotubes).
2
. Experimental
2
.1 Materials preparation
All the reagents and solvents were purchased from commercial sources and used as
received. The water used for all reactions and analyses was double distilled and deionised.
2
.1.1. Carbon materials (supports)
Activated carbon (AC) from Sigma-Aldrich Norit® RO 0.8, and multi-walled carbon
nanotubes (CNT) from Nanocyl™ NC3100, were used in their original form, oxidized (-
ox), and oxidized with nitric acid and subsequently treated with sodium hydroxide (-ox-
Na). The –ox materials were obtained by refluxing 5 g of AC or CNT with 375 mL of a
5
M nitric acid solution, for 3 h, then separated by filtration and washed with deionized
water until neutral pH, similarly to what was reported earlier [23, 28-30]. The –ox-Na
samples were obtained by treating 2g of the -ox ones with 150 mL of a 20 mM NaOH
aqueous solution, in reflux for 1 h, as reported in the literature [23, 29, 30]. These
materials were also separated by filtration and washed until neutral pH.
2
.1.2. Gold catalysts
Gold (3% nominal wt.) was loaded on the carbon supports by a colloidal method [22, 31-
3]. This procedure consisted in dissolving 0.053 g of the gold precursor, HAuCl ·3H
Alfa Aesar), in 350 ml of water, adding 0.05 g of polyvinyl alcohol (Aldrich) and 0.023
g of NaBH (Aldrich), previously dissolved into 5 ml of water, resulting in a ruby red
3
4
2
O
(
4
solution to which the carbon support was added under stirring. The solution of the carbon
material started to lose colour, as Au was deposited on the support. The stirring was

maintained until total loss of colour (2-3 days in the case of carbon nanotubes and around
week for activated carbon samples). The colourless solution was filtered, the catalyst
1
washed thoroughly with distilled water until the filtrate was free of chloride (around 1.5
L of water were used) and dried at 110 °C overnight. The organic scaffold was removed
from the supports by a heat treatment under N
2
flow for 3 h at 350 °C, and then, the
catalyst was activated by further treatment under hydrogen flow for 3 h, also at 350 °C.
2
2
.2. Materials characterization
.2.1. Carbon materials (supports)
The carbon materials were characterised by N adsorption at -196 ºC in a Quantachrome
2
Nova 4200e apparatus, using the Brunauer-Emmett-Teller (BET) method for total surface
area determination, Barrett-Joyner-Halenda (BJH) for pore size distribution and Boer's t-
method for micropore volume and external surface area.
The determination of oxygenated surface functional groups was performed by
temperature programmed desorption–mass spectrometry (TPD–MS), according to
procedures already established in previous publications [32-35]. CO and CO
2
TPD
profiles were obtained with a fully automated Altamira AMI-300 equipment. The sample
(
(
100 mg) was placed in a U-shaped quartz tube inside an electrical furnace. A flow of He
3
25 cm /min) was passed through the sample for 1 h at room temperature. Afterwards, the
material was heated up to 1100 ºC, at 5 ºC/min linear temperature increase also under He
3
flow (25 cm /min). A quadrupole mass spectrometer (Ametek Dycor DyMaxion) was
used to monitor CO and CO
2
signals, with temperature increase. For quantification of the
CO and CO released, calibration of these gases was carried out at the end of the analysis.
2
2
.2.2. Gold catalysts
The Au/CNT and Au/AC samples were imaged by transmission electron microscopy
TEM). The analyses were performed on a JEOL 2010F Field Emission Gun
Transmission Electron Microscope (FEG-TEM), working in both High Resolution TEM
HR-TEM) and Scanning-Transmission (STEM) modes at 200 kV accelerating voltage.
(
(
The average gold particle size was determined from measurements made on up to 100
particles (depending on sample). Gold size distribution histograms were also obtained.
The dispersion was calculated from the formula [36]:
6
Mns

1.17


,
D


Nd d nm

where M represents the molar mass of Au (197 g/mol), n the number of atoms of Au per
s
1
9
-2
3
unit area (1.15×10 m ), ρ the density of Au (19.3 g/cm ), N is Avogadro’s number, and
d (nm) the average particle size, determined by TEM, considering the spherical gold
nanoparticles.
X-ray photoelectron spectroscopy (XPS) analyses of the Au 4f region were carried out
for selected samples in order to confirm the expected reduced state of gold. The
equipment used was a VG Scientific ESCALAB 200A spectrometer using Al Kα
radiation (1486.6 eV). The charge effect was corrected taking the C 1s peak as a reference
(
binding energy of 285 eV). CASAXPS software was used for data analysis.
SEM analyses were carried out on a scanning electron microscope JEOL 7001F with
Oxford light elements EDS detector and EBSD detector.
In order to determine the loading of gold, samples were incinerated at 600 °C and the
resulting ashes were dissolved in a concentrated HNO
3
and H
2
SO mixture. The resulting
4
solution was diluted and analysed by atomic absorption spectroscopy (AAS) using a
Unicam 939 atomic absorption spectrometer and a hollow cathode lamp Heraeus 3UNX
Au. The amount of Na on selected samples was determined using a similar procedure.
2
.3. Catalytic tests
The single-pot reactions were undertaken in stainless steel autoclaves, by reacting, at
typical temperatures of 30-80 °C and in a water/acetonitrile medium with cyclohexane,
carbon monoxide (pressures from 2 to 20 atm), gold catalyst (2-20μmol) and potassium
peroxodisulfate. The reaction mixture was stirred for 3–6 h (typically 6 h) using a
magnetic stirrer and an oil bath, whereupon it was cooled in an ice bath, degassed, opened
and the contents transferred to a Schlenk flask. Diethyl ether (9.0–11.0 mL) and 90 mL
of cycloheptanone (GC internal standard) were added. The obtained mixture was
vigorously stirred for 10 min, and the organic layer was analysed typically by GC.
Chromatographic analyses were undertaken by using a Fisons Instruments GC 8000
series gas chromatograph with a DB-624 (J&W) capillary column (flame ionization
detector) and the Jasco-Borwin v.1.50 software. GC-MS analyses were performed in a
gas chromatograph Perkin Elmer Clarus 600 C instrument GC-MS, using He as the carrier
gas, equipped with a 30 m × 0.22 mm × 25 μm BPX5 (SGE) capillary column. The
internal standard method was used to quantify the organic products.
Blank tests i) without any catalyst, ii) only with CM, CM-ox and CM-ox-Na (CM = CNT
or AC), and iii) using only one of the solvents (H O or NCMe) were also performed, to
2

assess if the carboxylation reactions proceeded in the absence of metal catalyst and the
importance of each support and solvent. Moreover, aqueous solutions of the gold
precursor were also tested for comparison (homogenous medium).
Each catalytic essay was repeated at least three times in order to ensure reproducibility of
the results. Material balances were also verified according to the degrees of precision of
the data.
3
. Results and Discussion
3
.1. Characterisation of the supports
Activated carbon (AC) and carbon nanotubes (CNT) were used as supports in their
original forms, oxidised (-ox), and oxidised with nitric acid and subsequently treated with
sodium hydroxide (-ox-Na). The textural and surface characterization details of these
samples can be found in Table 1 and Figure 1.
Table 1. Description and characterisation of carbon samples: surface area (SBET), total
pore volume (V
external), obtained by adsorption of N
as determined by temperature programmed desorption (TPD).
p
), average mesopore width (L), micropore volume (Vmicro), external area
(
S
2
at -196 °C, and amounts of CO and CO desorbed,
2
S
BET
2
V
p
V
micro
S
external
2
CO
CO
2
Sample
L (nm)
3
b
3
(m /g)a
123
123
78
(
m /g)
(cm /g)
0.65
(cm /g)
0.50
0.32
0.24
~0
(µmol/g) (µmol/g)
AC
1274
829
607
292
457
345
4.7
498
354
AC-ox
AC-ox-Na
CNT
0.46
4.3
5016
5602
384
3189
3290
130
0.33
4.5
1.81
36.6c
13.2 c
13.7 c
292
457
345
CNT-ox
1.78
~0
1705
1176
829
CNT-ox-Na
1.34
~0
787
a
-
-
-
considered as mesoporous surface area in mesoporous materials.
only the micro- and mesopore volumes.
b
c
values refer to spaces between the agglomerated nanotubes.
Table 1 shows that the -ox treatment increased the BET surface area of CNTs. These
materials are not porous and have a cylindrical configuration, so the “porosity” measured
is related to the free spaces found in the bundles. The treatment with acid causes opening
of the end tips, which explains the increase in the surface area observed for the CNT-ox

sample. The spaces between the nanotubes (that was calculated in a way similar to the
mesopore diameter for the AC samples) was lower for CNT-ox, possibly due to
aggregation of the nanotubes caused by oxidation [37, 38]. The treatment with NaOH
does not change that (as shown by the CNT-ox-Na data).
The acid treatment slightly decreased the surface area and micropore volume for AC,
most likely due to collapse of some of the pore-walls, or to the oxygenated surface groups
present, that can possibly partially block the micropores and prevent the access of N
2
molecules [28]. The acid treatment destroys several atoms of carbon and modifies the
pore structure, merging some of them. The treatment with NaOH decreases the surface
area for all samples. Table 1 and Figure 1 show that the acid treatment increases the
presence of oxygenated surface groups, which desorb in TPD as CO and CO
Na procedure greatly modifies the CO desorption, its amount decreasing for CNT but
increasing for AC. As for CO desorbing groups, an increase was found for AC, but for
CNT a small decrease was seen.
2
. The -ox-
2
The TPD results are shown in Figure 1, along with the identification of the several surface
groups that characteristically desorb at diverse temperature ranges, as described in
literature [28, 34, 35, 39, 40]. The CO desorption profiles (Figure 1, right) of -ox and -
2
ox-Na materials show an increase in the amount of carboxylic acids (that decompose
around 250 °C [28, 34, 35, 39, 40], compared to pristine samples). However, the profiles
for -ox-Na materials are broader, and their centre is a little shifted to higher temperatures,
possibly as carboxylic acids are converted to (sodium) carboxylates, that are more stable,
as shown with carbon xerogels [41].
Figure 1 (left) shows the CO desorption profiles. The -ox and -ox-Na treatments increased
the quantity of surface groups that desorbed as CO. For the CNT-ox (left, bottom), the
CO evolution started around 300 °C, whereas for CNT-ox-Na, the temperature was
approximately 450 °C. This increase in temperature can be due to the destruction of
carboxylic anhydrides (that desorb in the form of CO in this temperature range [28, 34,
3
5, 39, 40]), which are originated by the treatment with NaOH, that contributes to the
increase of the amount of carboxylate groups, as referred in literature [41]. Also, the peaks
of CNT-ox-Na are sharper and more intense and have their maxima at temperatures higher
than those of CNT-ox. It is possible that phenol groups (desorbing as CO in this
temperature interval [28, 34, 35, 39, 40]) were converted into phenolates, known to be
more stable [41]. Comparable results were obtained for AC samples (left, top).

Figure 1. TPD profiles for the carbon materials: activated carbon (top) and carbon
nanotubes (bottom). Desorption of CO (left) and CO (right) is shown, with identification
2
of types of groups desorbing in different temperature ranges (the different colour bars are
only indicative of the temperature ranges expected for desorption of different groups, and
do not provide any information on their amounts).
3
.2. Characterisation of gold catalysts
The nominal gold loading was ~3% wt. However, as shown in Table 2, only CNT-ox and
CNT-ox-Na showed an actual loading around that value. Regarding the functionalized
samples, it has been reported in literature that surface oxygen groups can serve as anchors
to the metallic precursors [42, 43]. That is true for functionalized CNT materials,
compared with the pristine sample. However, and in spite of the larger amount of surface
groups present in AC, not much change is seen after functionalization, AC-ox showing
lower loading than AC, and AC-ox-Na having a similar loading to the pristine sample
(
Table 2). It is possible that the microporous nature of AC materials (Table 1) is
detrimental for gold deposition, as hypothesized in a previous publication [22].

Table 2. Size ranges, average sizes and dispersion of gold nanoparticles (obtained from
TEM measurements of the spherical gold nanoparticles), and gold loading (obtained by
atomic absorption spectroscopy) on the different carbon materials.
Au size
range (nm)
8-30
Au average Dispersion Gold loading
Sample
size (nm)
17.4
9.0
(%)
6.7
(%)
2.2
1.9
2.2
2.2
3.1
3.1
AC
AC-ox
4-16
13.0
14.6
10.4
11.4
16.7
AC-ox-Na
CNT
4-13
8.0
2-23
11.3
10.3
7.0
CNT-ox
CNT-ox-Na
1-20
1-15
Figure 2 shows selected HR-TEM and HAADF images of the gold on carbon samples.
Not many nanoparticles were found, and most of them were usually isolated. It can be
seen that gold is sometimes deposited in the form of elongated nanoparticles (Figures
2
a,d). Although spherical nanoparticles are usually obtained with the colloidal method
[
22, 31-33], other forms, just like nanorods, are also reported often in literature [44, 45].
They are usually formed through a seed-mediated method, that consists in the formation
of “seed” nanoparticles and their growth into rods [45].
Table 2 shows the size ranges and average sizes of gold nanoparticles. The corresponding
size histograms can be found in Figure S1 of Supporting Information. The average size is
larger for the pristine materials (17.4 nm for AC and 11.3 nm for CNT – Figures 2a and
2
d, respectively), than for the -ox materials (9 nm for AC-ox and 10.3 nm for CNT-ox,
Figures 2b and 2e, respectively) and the ox-Na materials, the latter showing the smallest
values (8 nm for AC-ox-Na and 7 nm for CNT-ox-Na, Figures 2c and 2f, respectively).
Also the functionalized CNT samples showed smaller gold nanoparticles (down to 1 nm,
Table 2 and Figure S1e and S1f), compared to AC samples, in which the lowest size
measured was 4 nm (Table 1 and Figures S1b and S1c). In all cases, there was
agglomeration of gold nanoparticles (one example is shown in Figure 2d). Comparing the
same support treatment (pristine, -ox or -ox-Na), the gold sizes are smaller on CNT than
on AC (Table 2). As mentioned above, it is possible that the microporous nature of AC
materials (Table 1) is detrimental for gold deposition [22].
The dispersion (%) is the number of gold atoms exposed on the external surface, out of
the total gold atoms present on the solid [46-48]. This value is calculated based on the

average gold nanoparticle size (see Experimental) and thus has an inverse trend (Table
). The smaller the gold size, the higher the dispersion.
2
In a previous work of ours dealing with gold nanoparticles on several carbon materials,
including AC and CNT [22], for 1% Au loading, spherical nanoparticles of ca. 5.1 nm
and 6.8 nm were obtained for CNT and AC, respectively. It is possible that the larger
loading used in this work may have led to agglomeration. Although without much
influence on the gold loading (at least for AC), the presence of surface oxygenated groups
seems beneficial for the creation of smaller, less agglomerated nanoparticles. As said
above, surface oxygen groups can act as anchors for the metallic precursors [42, 43]
(
especially those of basic nature, like carbonyls and quinones, that have an enhancing
effect [42]), leading to the formation of smaller and better dispersed nanoparticles
compared to the non-functionalized materials). Alegre et al. [42] also referred that the
(
presence of more acidic functionalities, like carboxylic acids and anhydrides, may result
in a negatively charged carbon surface, making the interaction with the also negatively
charged metal precursor more difficult). In fact, it can be seen that the amount of
carbonyls and quinones (desorbed as CO above 700ºC, Figure 1, left) increased with the
NaOH treatment. A decrease in the anhydrides is also seen in the CO profile (especially
for CNT-ox-Na sample), which might explain the higher Au loading and smaller gold
nanoparticle size of this sample.
Since samples prepared by the colloidal method are directly reduced with hydrogen (as
stated in the Experimental part), Au 4f XPS was carried out only in the Au/AC-ox-Na
and Au/CNT-ox-Na samples for confirmation of the reduced (metallic) state. Results are
shown in Figure S2 of Supporting Information.

Figure 2. HRTEM (a-d, f) and HAADF (e) images of gold nanoparticles on AC (a), AC-
ox (b) and AC-ox-Na (c), CNT (d), CNT-ox (e) and CNT-ox-Na (f). Au nanoparticles are
seen as darker spots in HRTEM images (a-d, f) and as white spots in HAADF image (e).
3
.3. Catalytic Activity
Gold nanoparticles supported on different CNT or AC samples exhibit different catalytic
activities (Figure 3). The desired cyclohexanecarboxylic acid was achieved up to 88.2%
yield with Au/ CNT-ox-Na (entry 1, Table 3). However, KA oil (cyclohexanol and
cyclohexanone mixture) and cyclohexane-1,2-diol were also obtained, although in much
lower (<9% total) yields. The conversion of cyclohexane to cyclohexanecarboxylic acid
follows the order CM-ox-Na > CM-ox > CM (CM = carbon material) (Figure 3). This
can be related to the smaller gold nanoparticle size, found on -ox-Na and -ox (compared

to the pristine materials, Table 2), for a long time known to be linked with catalytic
activity [8, 10, 18-21].
Figure 3. Product yields from cyclohexane hydrocarboxylation: metal-free (),
promoted by HAuCl
on different carbon nanotubes (---) or activated carbons (—): as prepared (CNT or AC)
), treated with nitric acid (CNT-ox or AC-ox) () and oxidized with nitric acid and
4
.3H
2
O in homogeneous conditions (), and by Au NPs deposited
(
subsequently treated with sodium hydroxide (CNT-ox-Na or AC-ox-Na) ().
It is also well known that the presence of alkali metals enhances the activity of gold
catalysts [49-51]. Thus, we could think that the presence of sodium carboxylate and
phenolate groups might also be beneficial to the catalytic activity (although, the supports
alone, without gold, revealed no catalytic activity). However, it was found that Na is
leached during the reaction; nevertheless, the catalysts remain active for several cycles,
as will be seen ahead. Higher activities (for the same Au amount and reaction conditions)
were found for gold nanoparticles deposited on carbon samples, when compared to
HAuCl
4
.3H O used in homogeneous medium, i.e., in aqueous solution (Figure 3, Table
2
3
).

a
Table 3. Selected data for cyclohexane hydrocarboxylation promoted by Au/CM (CM =
AC or CNT).
Entry
Catalyst
Au /
P(CO) /
atm
T/º
C
Total
TON/S
Yield /% c
Conv./
%
b
d

mol
CyCOOH
88.2
Cy-H=O
CyOH
2.2
Cy-H(OH)
1.6
2
3
1
2
2
2
2
20
2
2
2
2
2
2
20
2
2
0
2
20
2
2
2
2
2
2
20
2
2
2
2
2
2
2
50
2.93×10
5.3
97.3
3
2
3
3
3
2
3
4
5
6
7
8
9
50
30
80
50
50
50
50
50
30
80
50
50
50
50
50
2.27×10
7.78×10
1.50×10
2.03×10
3.35×10
2.90×10
3.28×10
1.78×10
9.57×10
1.71×10
2.19×10
2.23×10
2.23×10
-
63.0
15.9
44.4
42.2
63.0
39.9
79.4
32.5
12.7
34.2
31.4
42.5
21.9
5.7
5.9
7.5
6.3
16.3
5.9
9.7
7.3
7.5
6.1
7.5
2.4
4.4
7.9
7.5
8.1
6.4
12.3
8.2
0.0
0.0
0.9
2.3
0.0
2.8
2.3
0.5
0.1
0.3
3.9
0.4
0.2
0.0
0.3
76.4
25.8
50.0
68.7
76.4
60.5
95.4
52.8
27.1
50.7
63.5
58.5
30.1
6.8
Au/CNT-ox-Na
Au/CNT-ox
Au/CNT
3
3
3
2
Au/AC-ox-Na
10
11
12
13
14
15
16
3
3
3
3
8.3
18.0
7.9
7.9
10.2
8.1
Au/AC-ox
Au/AC

3.9
4.3
0.8
0.3
AuHCl
4
2
-
10.3
1.7
2.9
15.2
a
Reaction conditions: cyclohexane (1.00 mmol), p(CO)= 2-20 atm, K
2
S
2
O
8
(1.50 mmol),
catalyst (2-20 μmol), H
2
O (3.0 mL)/MeCN (3.0 mL), 30-80 °C, 6 h in an autoclave (13.0
b
mL capacity). TON = Turnover number = moles of products per mole of catalyst. TON/S
=
moles of product per mole of surface gold atoms (S), calculated by dividing the TON
value by the dispersion value, D (see Supporting Information for further details) [46-48,
c
11. d Conv. = conversion
5
2-54]. Moles of product per 100 mol of cyclohexane; Cy = C
6
H
=
moles of product per moles of reagent.
The fact that CNT-ox-Na shows better performance than AC-ox-Na, that is, slightly
higher catalytic activity (97.3% conversion for the former and 95.4% for the latter,
respectively), and also higher cyclohexane carboxylic acid yield (88.2% and 79.4%
respectively, Figure 3 and entries 1 and 8 of Table 3) might also be linked with the smaller
gold nanoparticle size of the former (7 nm average) compared to the later (8 nm average),
as shown in Table 2. However, comparing CNT-ox with AC-ox, the former has a slightly
better performance than the latter (Figure 3 and entries 6 and 13 of Table 3), and that is
not related with the gold nanoparticle size, which is larger for CNT-ox (10.3 nm average)
than for AC-ox (9 nm, average), as shown in Table 2. This might be caused by a textural
effect, that is, the active centres are more accessible for the reactants (as CNT are
nonporous), or by an electronic effect (as CNT have a more graphitic structure), which
might favour the interactions with both the reactants or the intermediate radicals [23].
These effects might also explain why CNT materials have better performances than AC
counterparts. Apparently the larger surface area and larger amount of surface groups of

AC, compared to CNT, do not play a determining role in this reaction, as also found for
the peroxidative oxidation of cyclohexane [22]. Nevertheless, the presence of surface
groups on –ox and –ox-Na materials, formed by the surface treatments, has beneficial
effects, since the functionalized samples showed gold nanoparticles of smaller size
compared to the pristine materials (Table 2), as already mentioned. In fact, as stated
earlier, the surface oxygen groups can act as anchorage sites to the metallic precursors,
especially those of basic nature [42], as stated above, which increase upon the treatment
with NaOH.
In terms of TON-S values, Table 3 shows that the best results were obtained for Au/CNT-
3
3
ox and Au/AC-ox-Na (3.35×10 and 3.28×10 , entries 8 and 6, respectively), followed by
3
3
Au/CNT-ox-Na and Au/CNT (2.93×10 and 2.90×10 , entries 1 and 7, respectively),
3
being the worse results obtained for AC-ox and AC (2.23×10 , entries 13 and 14,
respectively). Although the tendency is not the exactly same as the trend found for
conversion, it can still be seen that the surface groups are beneficial and that CNT is in
general better than AC, as stated above.
The catalytic activity of gold nanoparticles on AC- or CNT-based materials is also
dependent on the reaction conditions. It was found that higher pressures of CO do not
enhance the production of carboxylic acid, the best being the 1:1 molar ratio of CO
relative to cyclohexane (comparing entries 1 and 2 or 7 and 8 of Table 3). In all
hydrocarboxylation systems known to date, a 10:1 molar excess of CO, relative to
substrate, is required (see conditions of Table S1 of Supporting Information) [13]. This is
a very important advantage of our systems, in terms of environmental protection and
process safety.
Moreover, the Au/CM-ox-Na/CO/K
performance at the mild temperature of 50 °C (comparing e.g., entries 1, 3 and 4 of Table
) requiring a significant (up to 16 times) less amount of metal promoter than the
2
S
2
O
7
/H O/MeCN systems exhibit their maximum
2
3
previously reported systems (Table S1 of Supporting Information). In fact, considering
the cyclohexanecarboxylic acid yield per amount of metal promotor (Table S1 of
Supporting Information), the Au/CNT-ox-Na system exhibits significantly better
performance on cyclohexane hydrocarboxylation compared to the formerly tested
promotors [12] (cyclohexanecarboxylic acid yield is 1.5 times higher than that of the best
catalyst reported in the literature [OCu
6, respectively, of Table S1 of Supporting Information).
In addition, the present catalytic system was evaluated by green metrics, namely atom
4
{N(CH
2
CH
2
O)
3
}
4
(BOH)
4
][BF
4
]
2
, entries 4 and
1

economy (molecular weight of desired product per combined molecular weight of starting
materials) or carbon efficiency (amount of carbon in CyCOOH per total carbon in
reactants). Such metrics were not included in the previously reported systems. Thus, to
compare our system with those reported in [12], the carbon efficiency and the atom
economy were determined (Table S1 of Supporting Information). Although presenting
the same atom economy value, our gold catalysts exhibit markedly higher carbon
efficiency values (Table S1 of Supporting Information), which is a significant
improvement in terms of the sustainability of the hydrocarboxylation reaction.
1
00
88.2
88
87.9
87.6
87.2
86.9
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
Cycle
Figure 4. Effect of the catalyst recycling on the yield of cyclohexanecarboxylic acid
obtained by hydrocarboxylation of cyclohexane catalyzed by Au/CNT-ox-Na. Reaction
conditions: cyclohexane (1.00 mmol), p(CO)= 2 atm, K
2
S
2
8
O (1.50 mmol), catalyst (2
μmol), H O (3.0 mL)/MeCN (3.0 mL), 50 °C, 6 h in an autoclave (13.0 mL capacity).
2
Another important advantage of the present systems is the possibility of being recycled
and re-used. Recycling of the best catalyst, Au/CNT-ox-Na, was tested up to six
consecutive cycles. After each stage was completed, an analysis of products was carried
out and the catalyst was filtered, thoroughly washed with water and acetonitrile, and used
again for a new set of hydrocarboxylation experiments. The filtered (liquid) material was
tested in a new reaction (adding fresh reagents), with no carboxylation being detected.
Moreover, an additional test was performed to check if leaching of the active species
would occur: after 1 hour, the reaction was terminated, the gold catalyst was removed by
filtration and the resulting filtrate was heated at 50 ºC in the presence of CO for another
hour. No increase of the cyclohexane conversion was observed. Figure 4 shows the

excellent recyclability of the Au/CNT-ox-Na material: in the second, third, fourth, fifth
and sixth run, the activity was 99.8%, 99.7%, 99.3%, 98.7% and 98.5% of the initial
value, while the selectivity remained the same. Moreover, it was also found that the
oxidation state of gold remains unchanged (metallic) after the reaction both for Au/CNT-
ox-Na and Au/AC-ox-Na (Figure S2 of Supporting Information). Also no significant
changes were observed on the shape and size of gold nanoparticles of the used Au/CNT-
ox-Na material (Figure S3). The fact that Na is leached shows that the catalysis is
performed by gold nanoparticles (clearly detected by SEM in the samples after reaction,
as seen in Figure S3).
The previously proposed mechanism for carboxylation of cyclohexane [12] involves the
•
free cyclohexyl radical (Cy ) generated by H-atom abstraction from C
6
H12 by the sulphate
•−
radical SO4 derived from thermolysis of K
2
S
2
O
8
. Then, the cyclohexyl radical reacts
•
with carbon monoxide resulting in the acyl radical C
6
H
11CO that is then oxidized by
2
−
11C(O)OSO −
S
2
O
8
generating the acyl sulfate C
6
H
3
. This species is hydrolysed by water
forming the cyclohexanecarboxylic acid. The gold-promoted route involves the Au /Au+
0
redox couple, as depicted in Scheme 2. However, other n/n+1 pair of oxidation states are
+
O
2−.
possible, since Au might be further oxidized by the strong oxidant S
2
8
The cyclohexanone (CyO), cyclohexanol (CyOH) and dihydroxycyclohexane (Cy(OH)
2
)
by-products are formed by a side reaction: oxidation of cyclohexane instead of
2
-
hydrocarboxylation. The oxidizing agents are S
2
O
8
and O , and the reaction is expected
2
to occur via a radical mechanism, as reported in other cyclohexane oxidation reactions
catalyzed by homogeneous copper [55, 56], iron [57] or vanadium [58, 59] complexes,
•
and heterogenized homogenous catalysts [23, 29]. The cyclohexyl radical (Cy ), formed
upon dehydrogenation of CyH by sulfate radical, derived from thermolysis of
peroxydisulfate, undergoes fast reaction with O
2
to form the peroxyl radical CyOO. This
is obtained
follows usual reaction pathways as indicated in those references. Cy(OH)
2
upon further oxidation of CyOH, occurring similarly, but, as expected, at an activated C
atom adjacent to that already hydroxylated at CyOH.

H2O
2
-
-
S2O
8
HSO4
4-

SO
2
-
-
SO4
-
½
S2O
HSO4
8

CO
Au+
Au0
S2O82
-
-

SO42
- + SO4
H+
H O
2
Scheme 2. Proposed simplified radical mechanism for the carboxylation of cyclohexane.
4
. Conclusions
In summary, gold nanoparticles were successfully deposited on carbon materials as
prepared and with different surface treatments: oxidized with HNO ; and oxidized with
HNO and subsequently treated with NaOH. The catalytic activity of those materials was
assessed for the single-pot hydrocarboxylation of cyclohexane, in H O/MeCN, under
3
3
2
mild conditions (50 °C, 2 atm of CO). Au/CNT-ox-Na exhibited the best performance,
yielding cyclohexanecarboxylic acid up to 88.2% yield, and excellent recyclability,
maintaining 97.5% of the initial activity after five consecutive catalytic cycles. These
results are relevant towards the sustainable production of carboxylic acids.
Acknowledgements: This work has been partially supported by the Fundação para a
Ciência e a Tecnologia (FCT), Portugal, and its projects PTDC/QEQ-ERQ/1648/2014,
PTDC/QEQ-QIN/3967/2014 and UID/QUI/00100/2013. This work is a result of project
“
AIProcMat@N2020 - Advanced Industrial Processes and Materials for a Sustainable
Northern Region of Portugal 2020”, with the reference NORTE-01-0145-FEDER-
00006, supported by Norte Portugal Regional Operational Programme (NORTE 2020),
0
under the Portugal 2020 Partnership Agreement, through the European Regional
Development Fund (ERDF) and of Project POCI-01-0145-FEDER-006984 – Associate
Laboratory LSRE-LCM funded by ERDF through COMPETE2020 – POCI – and by
national funds through FCT. SACC acknowledges Investigador FCT program
(
IF/01381/2013/CP1160/CT0007), with financing from the European Social Fund and the
Human Potential Operational Program. The authors acknowledge Dr. Alessandro

Benedetti (University of Vigo) and Dr. Carlos Sá (CEMUP) for assistance with TEM and
XPS measurements, respectively.
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