Gold nanoparticles deposited on surface modified carbon xerogels as reusable catalysts for cyclohexane C-H activation in the presence of CO and water

Article abstract of DOI:10.3390/molecules22040603

The use of gold as a promotor of alkane hydrocarboxylation is reported for the first time. Cyclohexane hydrocarboxylation to cyclohexanecarboxylic acid (up to 55% yield) with CO, water, and peroxodisulfate in a water/acetonitrile medium at circa 50 °C has been achieved in the presence of gold nanoparticles deposited by a colloidal method on a carbon xerogel in its original form (CX), after oxidation with HNO₃ (-ox), or after oxidation with HNO₃ and subsequent treatment with NaOH (-ox-Na). Au/CX-ox-Na behaves as re-usable catalyst maintaining its initial activity and selectivity for at least seven consecutive cycles. Green metric values of atom economy or carbon efficiency also attest to the improvement brought by this novel catalytic system to the hydrocarboxylation of cyclohexane.

Full text of DOI:10.3390/molecules22040603

molecules
Article
Gold Nanoparticles Deposited on Surface Modified
Carbon Xerogels as Reusable Catalysts for
Cyclohexane C-H Activation in the Presence of CO
and Water
Ana Paula da Costa Ribeiro , Luísa Margarida Dias Ribeiro de Sousa Martins 1,2,*,
1
3
,
3
Sónia Alexandra Correia Carabineiro *, José Luís Figueiredo and
Armando José Latourrette Pombeiro 1
1
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1
049-001 Lisboa, Portugal; apribeiro@tecnico.ulisboa.pt (A.P.C.R.); pombeiro@tecnico.ulisboa.pt (A.J.L.P.)
2
3
Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de
Lisboa, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal
Laboratório de Catálise e Materiais, Laboratório Associado LSRE-LCM, Faculdade de Engenharia,
Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal; jlfig@fe.up.pt
Correspondence: lmartins@deq.isel.ipl.pt (L.M.D.R.S.M.); sonia.carabineiro@fe.up.pt (S.A.C.C.);
Tel.: +351-21-831-7226 (L.M.D.R.S.M.); +351-22-041-4907 (S.A.C.C.)
*
Academic Editor: Georgiy B. Shul’pin
Received: 12 February 2017; Accepted: 3 April 2017; Published: 9 April 2017
Abstract: The use of gold as a promotor of alkane hydrocarboxylation is reported for the first time.
Cyclohexane hydrocarboxylation to cyclohexanecarboxylic acid (up to 55% yield) with CO, water,
◦
and peroxodisulfate in a water/acetonitrile medium at circa 50 C has been achieved in the presence
of gold nanoparticles deposited by a colloidal method on a carbon xerogel in its original form (CX),
after oxidation with HNO (-ox), or after oxidation with HNO and subsequent treatment with NaOH
3
3
(-ox-Na). Au/CX-ox-Na behaves as re-usable catalyst maintaining its initial activity and selectivity
for at least seven consecutive cycles. Green metric values of atom economy or carbon efficiency
also attest to the improvement brought by this novel catalytic system to the hydrocarboxylation
of cyclohexane.
Keywords: gold nanoparticles; C-H activation; hydrocarboxylation; cyclohexane; catalyst recycling;
water; green metric
1
. Introduction
The single-pot carboxylation of C
attractive alkane functionalization procedure [
carboxylic acids and of the drawbacks of their current synthetic methods [18
n
alkanes to Cn+1 carboxylic acids by CO is a particularly
17], in view of the increasing industrial demand for
21]. However, catalytic
1
–
–
carboxylation of saturated hydrocarbons, such as alkanes, requiring C-H activation, is a considerable
chemical challenge, in particular for the least reactive lower alkanes (1 to 6 carbon atoms).
Fujuwara et al. [1,2,4] found that a cyclohexane undergoes carboxylation to cyclohexanecarboxylic
acid (4.3% yield relative to the substrate) with CO and peroxodisulfate in trifluoroacetic acid (TFA) at
◦
8
0 C, catalysed by a Pd(II)/Cu(II) system. The strongly acidic medium is required due to the inertness
of the alkane.
In recent years, intensive research has been focused on the improvement of alkane carboxylation
towards future sustainable carboxylic acid production [10 11 15 17 22], namely regarding the use of
greener and safer solvents. Hydrocarboxylation of cyclohexane to cyclohexanecarboxylic acid with CO
,
, , ,
Molecules 2017, 22, 603; doi:10.3390/molecules22040603
www.mdpi.com/journal/molecules

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and water (72% yield), in the presence of peroxodisulfate oxidant, in water/acetonitrile medium at
◦
circa 50 C and in the presence of a tetracopper(II) catalyst has been achieved [10]. In this improved
system, water plays the roles of both reactant and solvent [10]. In contrast to the carboxylation in
TFA [4,6–8], the carboxylation of cyclohexane by CO in the H O/MeCN/K S O system proceeds
2 2 2 8
to some extent in the absence of any metal catalyst, leading to the formation (up to 12% yield) of
cyclohexanecarboxylic acid. However, it can proceed more efficiently in the presence of a metal (V, Mn,
Fe or Cu) promotor [10,11,22], leading to higher yields of carboxylic acid.
In spite of the above achievements, so far any tested homogeneous catalytic systems [22] have the
drawback of not being re-usable, thus the search for a more efficient and eco-friendly heterogeneous
processes for the synthesis of such industrially important commodities continues. Gold catalysts
are currently a “hot topic” of research, as they show application in many reactions of industrial
and environmental importance [23–28]. Several variables have been considered as important factors
influencing the structure, reactivity, and catalytic activity. Among them are the method of preparation,
the nature of the support, and particularly, the gold nanoparticle size [23–28].
Herein, we report the use of gold nanoparticles as promotors of cyclohexane hydrocarboxylation.
We have chosen the above-mentioned protocol [10] and the use of gold as a metal promotor in
n
view of the ability of [ Bu N][AuCl ], Au C-scorpionate gold complexes, and Au nanoparticles
4
4
to catalyze the peroxidative oxidation of cyclohexane to KA oil (cyclohexanol and cyclohexanone
mixture) [29 30]. Moreover, gold nanoparticles are supported on carbon xerogels with different
,
treatments, in order to provide recyclable catalysts for the one-pot hydrocarboxylation of cyclohexane
to cyclohexanecarboxylic acid (Scheme 1).
Scheme 1. Hydrocarboxylation of cyclohexane to cyclohexanecarboxylic acid catalysed by gold
nanoparticles supported on carbon xerogels.
To the best of our knowledge, this is the first report dealing with hydrocarboxylation of alkanes
using gold nanoparticles as catalysts. In fact, the only reports found in literature so far, dealing with
hydrocarboxylation of hydrocarbons using gold catalysts, refer to hydrocarboxylation of alkynes and
to gold complexes (not gold nanoparticles) [31,32]. Moreover, the only report for hydrocarboxylation
using carbon materials deals with 1,3-butadiene using a Rh(I) complex immobilized on activated
carbon as the catalyst [33]. Therefore, our work is also the first report of such reaction carried out using
carbon xerogel based catalysts.
2
. Results and Discussion
2
.1. Characterisation of Xerogel Supports
The carbon xerogel was used as a support in its original form as prepared (CX), oxidised
(-ox), and oxidised with nitric acid and subsequently treated with sodium hydroxide (-ox-Na).
The characterization details of these samples can be found in Table 1 and Figure 1, which include the
textural and surface characterisation.
Table 1 shows that CX is mainly mesoporous and has a large pore size, as expected [29,30,34–37].
By comparing the parameters of the oxidized (CX-ox) samples with those of the parent material (CX),
it is observed that liquid phase activation slightly decreased the surface area and pore volume, probably
due to some pore wall collapse or to the presence of numerous oxygen-containing surface groups,

Molecules 2017, 22, 603
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which might partially block the access of N molecules to the smaller pores [37]. Nitric acid consumes
2
large amounts of carbon atoms and changes the structure of pores, merging some of them together.
Table 1. Description and characterisation of carbon xerogel samples: surface area (SBET), total pore
volume (V
by adsorption of N at
programmed desorption (TPD).
p
), average mesopore width (L), micropore volume (Vmicro), external area (Sexternal), obtained
◦
−
196 C, and amounts of CO and CO desorbed, as determined by temperature
2
2
S
Vp
(cm /g)
L
(nm)
V
S
CO
(µmol/g)
CO2
(µmol/g)
BET
2
micro
3
external
2
Sample
3
(
m /g)
(cm /g)
(m /g)
CX
CX-ox
CX-ox-Na
604
570
560
0.91
0.80
0.75
13.7
18.8
17.6
~0
0.038
0.036
604
512
496
492
4609
3720
135
3774
3793
Figure 1 shows the identification of types of groups desorbing in different temperature ranges,
according to what is already established in the literature [37 41]. Upon oxidation treatment, the amounts
–
of CO and CO increase enormously (Table 1 and Figure 1). Figure 1a shows the CO desorption
2
profiles. The effect of oxidation treatments is also seen in these data. The largest CO evolution
◦
of the -ox materials starts at around 350 C, whereas for the -ox-Na samples, the temperature is
slightly higher. That can be due to the destruction of carboxylic anhydrides (that desorb as CO and
CO in that temperature range [37–41]) also contributing to the increase of the carboxylate groups,
2
as proposed in literature [42]. Moreover, the profile of -ox-Na is a little sharper, more intense, and
has its maximum at a higher temperature than that of the -ox sample. This suggests that phenol
groups (that desorb as CO [37–41]) are converted into phenolates, which are more stable [42]. The CO2
desorption profiles (Figure 1b) of the -ox and -ox-Na materials also show a considerable increase
◦
in the amount of carboxylic acid groups (which decompose in the temperature range 200–350 C,
as described in the literature [37–41]) when compared to the original materials.
Phenols
Carbonyls/
Quinones
Carboxylic acids
Anhydrides
Anhydrides
CX-ox
CX-ox-Na
CX-ox
Lactones
CX-ox-Na
CX
CX
(
a)
(b)
Figure 1. Temperature programed desorption (TPD) profiles for the carbon xerogel materials.
Desorption of CO ( ) and CO ( ) is shown, with identification of types of groups desorbing in
a
b
2
different temperature ranges (the different colour bars are only indicative of the temperature ranges
expected for the desorption of different groups, and do not provide any information on their amounts).
2
.2. Characterisation of Gold Catalysts
The nominal gold loading was 3% wt. (see Materials and Methods). However, as shown in Table 2,
only CX showed an actual loading near that value (2.8%). Regarding the functionalized samples, it has
been reported that surface oxygen groups can act as anchoring sites for metallic precursors [43 44].
However, in the particular case of Au loaded on xerogels by the colloidal method used in this work
see Materials and Methods), it seems that the presence of oxygenated groups is detrimental for Au
,
(

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loading, as much less gold was loaded on CX-ox (1.4%) and even less on CX-ox-Na (0.5%), as depicted
in Table 2.
Table 2. Average gold nanoparticle size and dispersion (calculated from TEM measurements) and gold
loading (calculated by atomic absorption spectroscopy) on the different carbon xerogel materials.
Sample
Au Average Size (nm)
Metal Dispersion (%)
Gold Loading (%)
Au/CX
Au/CX-ox
Au/CX-ox-Na
16.6 *
14.2
13.7
6.9 *
8.1
8.4
2.8
1.4
0.5
*
—calculated taking into account only the spherical nanoparticles, not the nanorods.
Figure 2 shows some selected transmission electron microscopy (TEM) images of the samples.
It can be seen that gold nanoparticles are deposited mostly in the form of nanorods on CX (Figure 2a,b).
Although spherical nanoparticles are usually obtained with the colloidal method [30
,45–47], nanorods
are also often reported in literature [48 49]. They are usually formed through a seed-mediated method,
,
which includes the formation of “seed” nanoparticles and the growth of such seeds into rods [49].
Also other agglomerates of particles are seen (Figure 2b). Au on CX-ox and CX-ox-Na (Figure 2c,d,
respectively) show more regular sphere-like particles, which are larger on CX-ox (Figure 2c).
(
a)
(b)
(
c)
(d)
Figure 2. TEM images carbon xerogel samples: CX (
a
,
b
), CX-ox (c) and CX-ox-Na (d). Gold nanoparticles
are seen as darker spots, with rod-like (a,b), or spherical/obliquous (c,d) shapes.

Molecules 2017, 22, 603
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Table 2 shows a summary of the values of the average gold nanoparticle size and dispersion.
It can be seen that the average size is larger for CX (16.6 nm, calculated only for spherical nanoparticles,
as nanorods larger than 100 nm are also observed—Figure 2a,b). 14.2 nm was found for CX-ox and
13.7 nm for CX-ox-Na. In both cases, there was agglomeration of gold nanoparticles (one example is
shown in Figure 2c). Consequently, dispersion is smaller on CX and larger on CX-ox-Na, although the
value of 8.4% can still be considered low.
In a previous work of ours, dealing with gold nanoparticles on several carbon materials, including
xerogels [30], for 1% Au loading, spherical nanoparticles of ca. 4.4 nm were obtained with a metal
dispersion of 26.2%. Most likely, the larger loading used in this work promoted agglomeration as
well as nanorods formation in the case of CX. Although apparently detrimental for gold loading,
the presence of surface oxygenated groups seems beneficial for the formation of spherical, less
agglomerated nanoparticles. As stated above, it was previously reported that surface oxygen groups
can act as anchors for the metallic precursors [43
,44] and that can result in smaller and better dispersed
nanoparticles (at least compared with the unfunctionalized support).
2
.3. Catalytic Results
Gold nanoparticles supported on different carbon xerogel samples exhibited different catalytic
activities (Figure 3). The desired cyclohexanecarboxylic acid was achieved with up to 54.5% yield
with Au on CX-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 (<10%) yields. The conversion
of cyclohexane to cyclohexanecarboxylic acid (and also to the other oxidation products) follows the
order CX-ox-Na > CX-ox > CX (Figure 3). This can be related with the smaller gold nanoparticle size
found on CX-ox-Na and CX-ox, compared to that of CX (Table 2), which is expected to affect catalytic
activity [23–28].
It is also well known that the presence of alkali metals enhances the activity of gold catalysts [50–52]
Thus, 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). Higher
activities (for the same Au amount and reaction conditions) were found for gold nanoparticles
.
deposited on carbon xerogel samples, when compared to HAuCl4
medium, i.e., in aqueous solution (Figure 3, Table 3).
·
3H O used in homogeneous
2
The catalytic activity of gold nanoparticles on xerogels is also dependent on the reaction conditions.
It was found that high pressures of CO do not enhance the production of carboxylic acid, the best
being the 1:1 molar ratio of CO relative to cyclohexane (compare entries 1 and 2 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 4) [22]. This is a very important advantage for our system, in terms of the
environment and in process safety.
Table 3. Selected data a for cyclohexane hydrocarboxylation promoted by Au/CX-ox-Na.
Yield/% c
CyCOOH Cy-H=O
CyOH Cy-H(OH)2
◦
Total TON b
Entry
Au/µmol
P(CO)/atm Temperature/ C
1
2
3
4
5
2
2
2
2
20
2
20
2
2
2
50
50
30
80
50
375
311
111
245
23
54.5
28.3
12.0
24.1
19.9
9.1
19.4
5.3
14.4
12.3
7.1
14.5
4.9
9.5
12.5
4.3
3.9
0.8
0.9
0.5
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
◦
b
(
3.0 mL)/MeCN (3.0 mL), 30–80 C, 6 h in an autoclave (13.0 mL capacity); Turnover number = moles of products
per mole of catalyst; Moles of product per 100 mol of cyclohexane; Cy = C6H11.
c

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Moreover, the Au/CX-ox-Na/CO/K S O /H O/MeCN system exhibits its maximum performance
2
2
7
2
◦
at the mild temperature of 50 C (compare entries 1, 3 and 4, Table 3) requiring a significantly
up to 16 times) lower amount of metal promoter than in the previously reported systems (Table 4).
(
In fact, considering the cyclohexanecarboxylic acid yield per amount of metal promotor (Table 4) the
Au/CX-ox-Na system exhibits significantly better performance on cyclohexane hydrocarboxylation
relative to the formerly tested promotors [10] (cyclohexanecarboxylic acid yield is 1.5 times higher than
that of the literature best catalyst [OCu {N(CH CH O) } (BOH) ][BF ] , entries 4 and 16, respectively,
4
2
2
3 4
4
4 2
of Table 4).
60
50
40
30
20
10
0
cyclohexanol
cyclohexanone
cyclohexane-1,2-diol
cyclohexanecarboxilicacid
noprom oter
A uH Cl4
A u/CX
A u/CX-ox A u/CX-ox-N a
Figure 3. Products yields from cyclohexane hydrocarboxylation: metal-free (
HAuCl .3H O in homogeneous conditions ( ) and by Au NPs deposited on different carbon xerogels:
), treated with nitric acid (CX-ox) ( ) and oxidized with nitric acid and subsequently
treated with sodium hydroxide (CX-ox-Na) ().

), promoted by

4
2
as prepared (CX) (


Table 4. Metal promotors performance comparison a for the cyclohexane hydrocarboxylation to
cyclohexanecarboxylic acid.
CyCOOH Yield
(%) /µmol of M
Carbon
Atom
Economy/%
b
Entry
Metal Promoter
Efficiency/% c
d
Promotor
e
1
2
3
4
HAuCl ·3H O
5.2
6.9
12.7
27.3
7.7
9.3
18.4
37.9
4
2
e
Au/CX
e
33.5
Au/CX-ox
Au/CX-ox-Na
e
5
6
7
8
9
Cr(OH) ·2.5H O [10]
0.4
1.0
0.0
0.4
0.5
1.0
0.6
0.5
1.0
2.0
4.9
18.1
0.6
3.9
0.0
1.1
1.4
1.6
0.9
0.8
3.3
3.3
3.8
7.1
3
2
K Cr O [10]
2
2
7
MoO [10]
3
H [PMo11VO40]·34H O [10]
4
2
MnO [10]
2
10
11
12
13
14
15
16
Fe(OH) ·0.5H O [10]
3
2
33.5
Co(acac) [10]
3
Zn(NO ) [10]
3
2
Cu(NO ) ·2.5H O [10]
3
2
2
[Cu(H tea)(N )] [10]
2
3
[Cu (H tea) (tpa)]n·2nH O [10]
2
2
2
2
[OCu {N(CH CH O) } (BOH) ][BF ] [10
]
4
2
2
3 4
4
4 2
a
Typical (unless otherwise stated) reaction conditions: cyclohexane (1.00 mmol), p(CO) = 20 atm, K
2
S
2
O
8
◦
b
(
1.50 mmol), H
2
O (3.0 mL)/MeCN (3.0 mL), 50 C, 6 h in an autoclave (13.0 mL capacity);
Moles of
c
cyclohexanecarboxylic acid per 100 mol of cyclohexane; Cy = C
6
H
11
;
Amount of carbon in CyCOOH per total
d
carbon in reactants
materials × 100%; p(CO) = 2 atm.
×
100%; Molecular weight of desired product per combined molecular weight of starting
e

Molecules 2017, 22, 603
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In addition, the present catalytic system was evaluated by green metrics such as atom 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 the previous
ones (reported in [10]), the carbon efficiency and the atom economy were determined (Table 4).
Although presenting the same atom economy value, our gold systems exhibit markedly higher carbon
efficiency values (Table 4), which is a significant improvement in terms of the sustainability of the
hydrocarboxylation reaction.
Another important advantage of the present systems is the possibility of being recycled and
re-used. Recycling of the best catalyst, Au/CX-ox-Na, was tested on up to seven consecutive cycles.
On completion of each stage, the products were analyzed and the catalyst was recovered by filtration,
thoroughly washed, and then reused for a new set of cyclohexane hydrocarboxylation experiments.
The filtrate was tested in a new reaction (by addition of fresh reagents), and no oxidation was detected.
Figure 4 shows the excellent recyclability of the system Au/CX-ox-Na: in the second, third, fourth,
fifth, sixth, and seventh run, the observed activity was 99.8%, 99.7%, 98.4%, 98.3%, 97.7% and 97.5% of
the initial one, being the selectivity maintained.
Figure 4. Effect of the catalyst recycling on the yield of cyclohexanecarboxylic acid obtained by
hydrocarboxylation of cyclohexane catalyzed by Au/CX-ox-Na. Reaction conditions: cyclohexane
(
5
1.00 mmol), p(CO)= 2 atm, K S O (1.50 mmol), catalyst (2 µmol), H O (3.0 mL)/MeCN (3.0 mL),
2 2 8 2
◦
0 C, and six hours in an autoclave (13.0 mL capacity).
3
. Materials and Methods
3
.1. Reagents
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.
3
.2. Carbon Materials Preparation
Carbon xerogel (CX) was prepared by polycondensation of resorcinol and formaldehyde, using a
pH of 6, according to a procedure described elsewhere [29
(
(
, ,34–37]. It was used in its original form
30
CX), oxidized (-ox), and oxidized with nitric acid and subsequently treated with sodium hydroxide
-ox-Na). CX-ox was obtained by refluxing CX with 75 mL of a 5 M nitric acid solution, per gram of
carbon material, for 3 h, then separated by filtration and washed with deionized water until neutral

Molecules 2017, 22, 603
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pH, similarly to what was reported earlier [29
,35
–
37]. CX-ox-Na was obtained by treating CX-ox with
7
5 mL of a 20 mM NaOH aqueous solution, per gram of carbon material, in reflux for 1 h, as reported
in the literature [29 35 36]. This material was also separated by filtration and washed until neutral pH.
,
,
3
.3. Carbon Materials Characterisation
The carbon materials were characterised by N adsorption at 77 K in a Quantachrome Nova
2
4200e apparatus (Boynton Beach, FL, USA), using the Brunauer-Emmett-Teller (BET) theory 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. Their surface chemistry was characterised
by temperature programed desorption (TPD) using an Altamira AMI-300 apparatus (Pittsburgh, PA,
USA), with a coupled Ametek Dycor DyMaxion quadrupole mass spectrometer (Pittsburgh, PA, USA).
3
.4. Gold Loading
Gold (nominal 3% wt) was loaded on the xerogel supports by the colloidal method [30,45–47],
which consists of dissolving the gold precursor, HAuCl4 3H O (Alfa Aesar, Karlsruhe, Germany),
·
2
in water, adding polyvinyl alcohol (Aldrich, Darmstadt, Germany) and NaBH (Aldrich), resulting
4
in a ruby red solution to which the xerogel support was added under stirring. After a few days,
the solution of CX started to lose colour, as Au was deposited on the support. The colourless solution
was filtered, the catalyst washed thoroughly with distilled water until the filtrate was free of chloride
◦
and dried at 110 C overnight. Solutions of CX-ox and CX-ox-Na took more time and even so the
deposition was not complete (as found out later when the amount of gold present was determined),
as these solutions never turned colourless. However, the same filtering and washing procedures were
followed as with CX. The organic scaffold was removed from the supports by heat treatment under
◦
N flow for 3 h at 350 C (shown by elemental analysis to be efficient for this purpose), and then, the
2
◦
catalyst was activated by further treatment under hydrogen flow for 3 h also at 350 C.
3
.5. Gold Catalysts Characterisation
The Au/xerogel samples were imaged by transmission electron microscopy (TEM). The analyses
were performed on a Leo 906E apparatus (Austin, TX, USA), at 120 kV. Samples were prepared by
ultrasonic dispersion in hexane and a 400 mesh formvar/carbon copper grid (Agar Scientific, Essex,
UK) was dipped into the solution for TEM analysis.
The average gold particle size was determined from measurements made on about 300 particles.
The metal dispersion was calculated by D = (6n
s
M)/(
ρ
Nd
p
), where n
s
is the number of atoms at the
M
19
−2
−1
surface per unit area (1.15
×
10
m
for Au), M is the molar mass of gold (196.97 g mol ),
ρ
is the
−
3
23
−1
density of gold (19.5 g cm ), N is Avogadro’s number (6.023
particle size (determined by TEM, assuming that particles are spherical).
In order to determine the loading of gold, samples were incinerated at 600 C and the resulting
ashes were dissolved in a concentrated HNO and H SO mixture. The resulting solution was
×
10 mol ) and d is the average
p
◦
3
2
4
diluted and analysed by atomic absorption spectroscopy (AAS) using a Unicam 939 atomic absorption
spectrometer (Kent, UK) and a hollow cathode lamp Heraeus 3UNX Au.
3
.6. Catalytic Tests
The single-pot reactions were carried out 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 (2–20 µmol) 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 gas chromatograph
GC). A Fisons Instruments GC 8000 series gas chromatograph (Agilent Technologies, Santa Clara, CA,
(

Molecules 2017, 22, 603
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USA) with a DB-624 (J&W) capillary column (flame ionization detector) and the Jasco-Borwin v.1.50
software (Jasco, Tokyo, Japan) were used. GC-MS analyses were performed in a Perkin Elmer Clarus
6
00 C GC-MS instrument (Shelton, Connecticut, USA) equipped with a 30 m × 0.22 mm × 25 µm
BPX5 (SGE) capillary column, using He as the carrier gas. The internal standard method was used to
quantify the organic products, since the desired cyclohexanecarboxylic acid was not isolated from the
reaction mixture.
Blank tests (i) without any catalyst; (ii) only with CX, CX-ox and CX-ox-Na; and (iii) using only one
of the solvents (H O or NCMe) were also performed, to assess if the carboxylation reactions proceeded
2
in the absence of the metal catalyst, and the importance of each support and solvent. Moreover,
aqueous solutions of the gold precursor were also tested for comparison (homogenous medium).
4
. Conclusions
Gold nanoparticles were successfully deposited on carbon xerogel, as prepared and with
different treatments: with nitric acid; and oxidized with nitric acid and subsequently treated with
sodium hydroxide. The catalytic activity of the said materials was assessed for the single-pot
◦
hydrocarboxylation of cyclohexane, in H O/MeCN, under mild conditions (50 C, 2 atm of CO).
2
Au/CX-ox-Na exhibited the best performance, yielding cyclohexanecarboxylic acid up to 54.5% yield,
and excellent recyclability, maintaining 97.5% of the initial activity after seven consecutive catalytic
cycles. Green metric values of carbon efficiency also confirmed the improvement brought by this novel
catalytic system to the hydrocarboxylation of cyclohexane.
These results have an important implication on the design of gold catalysts and are of potential
significance for the sustainable production of carboxylic acids.
Acknowledgments: 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-000006,
supported by Norte Portugal Regional Operational Programme (NORTE 2020), 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—Programa
Operacional Competitividade e Internacionalização (POCI)—and by national funds through FCT—Fundação
para a Ciência e a Tecnologia. SACC acknowledges Investigador FCT program (IF/01381/2013/CP1160/CT0007),
with financing from the European Social Fund and the Human Potential Operational Program. Authors thank
Pedro Tavares and Lisete Fernandes (UME/CQVR/UTAD) for assistance with the TEM analyses.
Author Contributions: S.A.C.C. prepared and characterized the catalysts. A.P.C.R. and L.M.D.R.S.M. conceived
and designed the experiments; A.P.C.R. performed the experiments; A.P.C.R., L.M.D.R.S.M. and S.A.C.C. analysed
the data; L.M.D.R.S.M. and S.A.C.C. wrote the paper; L.M.D.R.S.M., J.L.F. and A.J.L.P. provided the means needed
for the realization of this work. All authors read and approved the manuscript.
Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
References
1
.
Nishiguchi, T.; Nakata, K.; Takaki, K.; Fujiwara, Y. Transition Metal Catalyzed Acetic Acid Synthesis from
Methane and CO. Chem. Lett. 1992, 21, 1141–1142. [CrossRef]
2
.
Nakata, K.; Yamaoka, Y.; Miyata, T.; Taniguchi, Y.; Takaki, K.; Fujiwara, Y. Palladium(II) and/or
copper(II)-catalyzed carboxylation of small alkanes such as methane and ethane with carbon monoxide.
J. Organomet. Chem. 1994, 473, 329–334. [CrossRef]
3.
4.
5.
Derouane, E.D.; Haber, J.; Lemos, F.; Ramôa Ribeiro, F.; Guinet, M.E. Catalytic Activation and Functionalisation
of Light Alkanes; NATO ASI Series; Kluwer Academic Publ: Dordrecht, The Netherlands, 1998; Volume 44.
Jia, C.; Kitamura, T.; Fujiwara, Y. Catalytic Functionalization of Arenes and Alkanes via C-H Bond Activation.
Acc. Chem. Res. 2001, 34, 633–639. [CrossRef] [PubMed]
Pombeiro, A.J.L.; Fraústo da Silva, J.J.R.; Fujiwara, Y.; Silva, J.A.L.; Reis, P.M.; Palavra, A.F. Catalysts and
Process for Direct Conversion of Methane into Acetic Acid. WO 2004/037416 A3, 6 May 2004.

Molecules 2017, 22, 603
10 of 12
6
.
Reis, P.M.; Silva, J.A.L.; Palavra, A.F.; Fraústo da Silva, J.J.R.; Kitamura, T.; Fujiwara, Y.; Pombeiro, A.J.L.
Single-Pot Conversion of Methane into Acetic Acid in the Absence of CO and with Vanadium Catalysts Such
as Amavadine. Angew. Chem. Int. Ed. 2003, 42, 821–823. [CrossRef] [PubMed]
7
.
.
Reis, P.M.; Silva, J.A.L.; Palavra, A.F.; Fraústo da Silva, J.J.R.; Pombeiro, A.J.L. Vanadium-catalyzed
carboxylation of linear and cyclic C5 and C6 alkanes. J. Catal. 2005, 235, 333–340. [CrossRef]
Kirillova, M.V.; Kuznetsov, M.L.; Reis, P.M.; da Silva, J.A.L.; Fraústo da Silva, J.J.R.; Pombeiro, A.J.L.
Direct and Remarkably Efficient Conversion of Methane into Acetic Acid Catalyzed by Amavadine and
8
Related Vanadium Complexes. A Synthetic and a Theoretical DFT Mechanistic Study. J. Am. Chem. Soc. 2007
,
1
29, 10531–10545. [CrossRef] [PubMed]
9
1
1
1
.
Kirillova, M.V.; Kuznetsov, M.L.; da Silva, J.A.L.; Guedes da Silva, M.F.C.; Fraústo da Silva, J.J.R.;
Pombeiro, A.J.L. Amavadin and Other Vanadium Complexes as Remarkably Efficient Catalysts for One-Pot
Conversion of Ethane to Propionic and Acetic Acids. Chem. Eur. J. 2008, 14, 1828–1842. [CrossRef] [PubMed]
0. Kirillova, M.V.; Kirillov, A.M.; Kuznetsov, M.L.; Silva, J.A.L.; Frausto da Silva, J.J.R.; Pombeiro, A.J.L.
Alkanes to carboxylic acids in aqueous medium: Metal-free and metal-promoted highly efficient and mild
conversions. Chem. Commun. 2009, 17, 2353–2355. [CrossRef] [PubMed]
1. Kirillova, M.V.; Kirillov, A.M.; Pombeiro, A.J.L. Mild, Single-Pot Hydrocarboxylation of Gaseous Alkanes to
Carboxylic Acids in Metal-Free and Copper-Promoted Aqueous Systems. Chem. Eur. J. 2010, 16, 9485–9493.
[CrossRef] [PubMed]
2. Silva, T.F.S.; Luzyanin, K.V.; Kirillova, M.V.; da Silva, M.F.G.; Martins, L.M.D.R.S.; Pombeiro, A.J.L. Novel
Scorpionate and Pyrazole Dioxovanadium Complexes, Catalysts for Carboxylation and Peroxidative
Oxidation of Alkanes. Adv. Synth. Catal. 2010, 352, 171–187. [CrossRef]
13. Phan, A.; Czaja, A.U.; Gándara, F.; Knobler, C.B.; Yaghi, O.M. Metal–Organic Frameworks of Vanadium as
Catalysts for Conversion of Methane to Acetic Acid. Inorg. Chem. 2011, 50, 7388–7390. [CrossRef] [PubMed]
14. Martins, L.M.D.R.S.; Pombeiro, A.J.L. Tris(pyrazol-1-yl)methane metal complexes for catalytic mild oxidative
functionalizations of alkanes, alkenes and ketones. Coord. Chem. Rev. 2014, 265, 74–88. [CrossRef]
15. Pombeiro, A.J.L. Toward Functionalization of Alkanes under Environmentally Benign Conditions.
In Advances in Organometallic Chemistry and Catalysis, The Silver/Gold Jubilee ICOMC Celebratory Book;
Pombeiro, A.J.L., Ed.; J. Wiley & Sons: New York, NY, USA, 2014; pp. 15–25.
1
1
6. Martins, L.M.D.R.S.; Pombeiro, A.J.L. Water-Soluble C-Scorpionate Complexes—Catalytic and Biological
Applications. Eur. J. Inorg. Chem. 2016, 2016, 2236–2252. [CrossRef]
7. Shul’pin, G. New Trends in Oxidative Functionalization of Carbon-Hydrogen Bonds: A Review. Catalysts
2
016, 6, 50. [CrossRef]
18. Weissermel, W.; Horpe, H.J. Industrial Organic Chemistry, 2nd ed.; VCH Press: Weinheim, Germany, 1993.
19. Seidel, A.; Bickford, M. Encyclopedia of Chemical Technology, 5th ed.; Wiley: New York, NY, USA, 2004.
20. Fritz, U. Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Wiley-VCH: Weinheim, Germany, 2002.
21. Seidel, A.B.M. (Ed.) Kirk-Othmer Encyclopedia of Chemical Technology; J. Wiley & Sons: New York, NY,
USA, 2014.
2
2. Kirillova, M.V.; Kirillov, A.M.; Pombeiro, A.J.L. Metal-Free and Copper-Promoted Single-Pot
Hydrocarboxylation of Cycloalkanes to Carboxylic Acids in Aqueous Medium. Adv. Synth. Catal. 2009, 351,
2
936–2948. [CrossRef]
3. Hutchings, G.J.; Haruta, M. A golden age of catalysis: A perspective. Appl. Catal. A Gen. 2005, 291, 2–5.
CrossRef]
2
[
2
2
4. Bond, G.C.; Louis, C.; Thompson, D.T. Catalysis by Gold; Imperial College Press: London, UK, 2006; Volume 6.
5. Carrettin, S.; Blanco, M.C.; Corma, A.; Hashmi, A.S.K. Heterogeneous Gold-Catalysed Synthesis of Phenols.
Adv. Synth. Catal. 2006, 348, 1283–1288. [CrossRef]
2
2
2
6. Carabineiro, S.A.C.; Thompson, D. Catalytic Applications for Gold Nanotechnology. In Nanocatalysis;
Heiz, U., Landman, U., Eds.; Springer: Berlin/Heidelberg, Germany, 2007; pp. 377–489.
7. Carabineiro, S.A.C.; Thompson, D. Gold Catalysis. In Gold: Science and Applications; Corti, C., Holliday, R., Eds.;
CRC Press, Taylor & Francis Group: Boca Raton, FL, USA; London, UK; New York, NY, USA, 2010; pp. 89–122.
8. Wu, P.; Loh, K.P.; Zhao, X.S. Supported Gold Catalysts for Selective Oxidation of Organics. Sci. Adv. Mater.
2
011, 3, 970–983. [CrossRef]

Molecules 2017, 22, 603
11 of 12
2
3
3
9. De Almeida, M.P.; Martins, L.M.D.R.S.; Carabineiro, S.A.C.; Lauterbach, T.; Rominger, F.; Hashmi, A.S.K.;
Pombeiro, A.J.L.; Figueiredo, J.L. Homogeneous and heterogenised new gold C-scorpionate complexes as
catalysts for cyclohexane oxidation. Catal. Sci. Technol. 2013, 3, 3056–3069. [CrossRef]
0. Carabineiro, S.A.C.; Martins, L.M.D.R.S.; Avalos-Borja, M.; Buijnsters, J.G.; Pombeiro, A.J.L.; Figueiredo, J.L.
Gold nanoparticles supported on carbon materials for cyclohexane oxidation with hydrogen peroxide.
Appl. Catal. A Gen. 2013, 467, 279–290. [CrossRef]
1. Patil, N.T.; Konala, A. Mechanistic Dichotomy with Alkynes in the Formal Hydrohydrazination/Fischer
Indolization Tandem Reaction Catalyzed by a Ph3PAuNTf2/pTSA Binary System. Eur. J. Org. Chem. 2010
010, 6831–6839. [CrossRef]
,
2
3
3
3
2. Dupuy, S.; Gasperini, D.; Nolan, S.P. Highly Efficient Gold(I)-Catalyzed Regio- and Stereoselective
Hydrocarboxylation of Internal Alkynes. ACS Catal. 2015, 5, 6918–6921. [CrossRef]
3. Protzmann, G.; Luft, G. A new immobilized Rh(I)-carbonylation catalyst. Appl. Catal. A Gen. 1998, 172,
1
59–163. [CrossRef]
4. Carabineiro, S.A.C.; Thavorn-amornsri, T.; Pereira, M.F.R.; Serp, P.; Figueiredo, J.L. Comparison between
activated carbon, carbon xerogel and carbon nanotubes for the adsorption of the antibiotic ciprofloxacin.
Catal. Today 2012, 186, 29–34. [CrossRef]
3
5. Martins, L.M.D.R.S.; Peixoto de Almeida, M.; Carabineiro, S.A.C.; Figueiredo, J.L.; Pombeiro, A.J.L.
Heterogenisation of a C-Scorpionate Fe-II Complex on Carbon Materials for Cyclohexane Oxidation with
Hydrogen Peroxide. ChemCatChem 2013, 5, 3847–3856. [CrossRef]
3
6. Sutradhar, M.; Martins, L.M.D.R.S.; Carabineiro, S.A.C.; Guedes da Silva, M.F.C.; Buijnsters, J.G.;
Figueiredo, J.L.; Pombeiro, A.J.L. Oxidovanadium(V) Complexes Anchored on Carbon Materials as Catalysts
for the Oxidation of 1-Phenylethanol. ChemCatChem 2016, 8, 2254–2266. [CrossRef]
3
3
3
4
4
7. Carabineiro, S.A.C.; Thavorn-Amornsri, T.; Pereira, M.F.R.; Figueiredo, J.L. Adsorption of ciprofloxacin on
surface-modified carbon materials. Water Res. 2011, 45, 4583–4591. [CrossRef] [PubMed]
8. Figueiredo, J.L.; Pereira, M.F.R.; Freitas, M.M.A.; Órfão, J.J.M. Modification of the surface chemistry of
activated carbons. Carbon 1999, 37, 1379–1389. [CrossRef]
9. Figueiredo, J.L.; Pereira, M.F.R.; Freitas, M.M.A.; Órfão, J.J.M. Characterization of Active Sites on Carbon
Catalysts. Ind. Eng. Chem. Res. 2007, 46, 4110–4115. [CrossRef]
0. Figueiredo, J.L.; Pereira, M.F.R. The role of surface chemistry in catalysis with carbons. Catal. Today 2010, 150,
2
–7. [CrossRef]
1. Carabineiro, S.A.C.; Pereira, M.F.R.; Órfão, J.J.M.; Figueiredo, J.L. Surface Chemistry of Activated Carbons.
In Activated Carbon: Classifications, Properties and Applications; Kwiatkowski, J.F., Ed.; Nova Science Pub Inc.:
New York, NY, USA, 2011; pp. 125–168.
4
4
4
2. Maia, F.; Mahata, N.; Jarrais, B.; Silva, A.R.; Pereira, M.F.R.; Freire, C.; Figueiredo, J.L. Jacobsen catalyst
anchored onto modified carbon xerogel as enantioselective heterogeneous catalyst for alkene epoxidation.
J. Mol. Catal. A Chem. 2009, 305, 135–141. [CrossRef]
3. Alegre, C.; Gálvez, M.E.; Baquedano, E.; Moliner, R.; Pastor, E.; Lázaro, M.J. Oxygen-Functionalized Highly
Mesoporous Carbon Xerogel Based Catalysts for Direct Methanol Fuel Cell Anodes. J. Phys. Chem. C 2013
17, 13045–13058. [CrossRef]
,
1
4. Alegre, C.; Gálvez, M.E.; Baquedano, E.; Pastor, E.; Moliner, R.; Lázaro, M.J. Influence of support’s oxygen
functionalization on the activity of Pt/carbon xerogels catalysts for methanol electro-oxidation. Int. J.
Hydrogen Energy 2012, 37, 7180–7191. [CrossRef]
4
5. Önal, Y.; Schimpf, S.; Claus, P. Structure sensitivity and kinetics of d-glucose oxidation to d-gluconic acid
over carbon-supported gold catalysts. J. Catal. 2004, 223, 122–133. [CrossRef]
4
6. Rodrigues, E.G.; Carabineiro, S.A.C.; Chen, X.; Delgado, J.J.; Figueiredo, J.L.; Pereira, M.F.R.; Órfão, J.J.M.
Selective Oxidation of Glycerol Catalyzed by Rh/Activated Carbon: Importance of Support Surface
Chemistry. Catal. Lett. 2011, 141, 420–431. [CrossRef]
4
7. Rodrigues, E.G.; Carabineiro, S.A.C.; Delgado, J.J.; Chen, X.; Pereira, M.F.R.; Órfão, J.J.M. Gold supported on
carbon nanotubes for the selective oxidation of glycerol. J. Catal. 2012, 285, 83–91. [CrossRef]
8. Afrooz, A.R.M.N.; Sivalapalan, S.T.; Murphy, C.J.; Hussain, S.M.; Schlager, J.J.; Saleh, N.B. Spheres vs. rods:
The shape of gold nanoparticles influences aggregation and deposition behavior. Chemosphere 2013, 91, 93–98.
4
[CrossRef] [PubMed]

Molecules 2017, 22, 603
12 of 12
4
9. Gao, J.; Xu, M. Metal Nanoparticles of Various Shapes. Advantages of Metal Nanoparticles, ECE-580 Mid-Term
Paper 2007. Available online: http://www.slideserve.com/nero/metal-nanoparticles-of-various-shapes
accessed on 7 April 2017).
0. Huang, J.; Wang, Y.; Zheng, J.; Dai, W.-L.; Fan, K. Influence of support surface basicity and gold particle size
on catalytic activity of Au/ -AlOOH and Au/ -Al O catalyst in aerobic oxidation of -diols to lactones.
Appl. Catal. B Environ. 2011, 103, 343–350. [CrossRef]
(
5
γ
γ
α,ω
2
3
5
1. Nepak, D.; Srinivas, D. Effect of alkali and alkaline earth metal ions on benzyl alcohol oxidation activity of
titanate nanotube-supported Au catalysts. RSC Adv. 2015, 5, 47740–47748. [CrossRef]
5
2. Shigenobu, H.; Mitsuyuki, S.; Tamotsu, K.; Takaharu, O.; Kenzi, T. Catalytic Activity of Silver and Gold
Metals Doped with Alkali Metals. Bull. Chem. Soc. Jpn. 1977, 50, 842–846.
Sample Availability: Samples of the compounds are not available from the authors.
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2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).