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 1 , Luísa Margarida Dias Ribeiro de Sousa Martins 1,2, *,
Sónia Alexandra Correia Carabineiro 3, *, José Luís Figueiredo 3 and
Armando José Latourrette Pombeiro 1
1
2
3
*
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049-001 Lisboa, Portugal; [email protected] (A.P.C.R.); [email protected] (A.J.L.P.)
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; [email protected]
Correspondence: [email protected] (L.M.D.R.S.M.); [email protected] (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 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.
Keywords: gold nanoparticles; C-H activation; hydrocarboxylation; cyclohexane; catalyst recycling;
water; green metric
1. Introduction
The single-pot carboxylation of Cn alkanes to Cn+1 carboxylic acids by CO is a particularly
attractive alkane functionalization procedure [1–17], in view of the increasing industrial demand for
carboxylic acids and of the drawbacks of their current synthetic methods [18–21]. However, catalytic
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
80 ◦ 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
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and water (72% yield), in the presence of peroxodisulfate oxidant, in water/acetonitrile medium at
◦ C and in the presence of a tetracopper(II) catalyst has been achieved [10]. In this improved
circa
50 50
at circa
°C and in the presence of a tetracopper(II) catalyst has been achieved [10]. In this improved
system,
water plays
playsthe
theroles
rolesofofboth
bothreactant
reactant
and
solvent
[10].
In contrast
to carboxylation
the carboxylation
in
system, water
and
solvent
[10].
In contrast
to the
in TFA
TFA
[4,6–8],
the
carboxylation
of
cyclohexane
by
CO
in
the
H
O/MeCN/K
S
O
system
proceeds
2
2 2 8 proceeds to some
[4,6–8], the carboxylation of cyclohexane by CO in the H2O/MeCN/K
2S2O8 system
to
someinextent
in the absence
any metal
catalyst,
leading
to the
formation
(uptoto12%
12% yield)
yield) of
extent
the absence
of anyofmetal
catalyst,
leading
to the
formation
(up
of
cyclohexanecarboxylic
more
efficiently
in the
presence
of aof
metal
(V, Mn,
cyclohexanecarboxylicacid.
acid.However,
However,ititcan
canproceed
proceed
more
efficiently
in the
presence
a metal
(V,
Fe
orFe
Cu)
[10,11,22],
leading
to higher
yieldsyields
of carboxylic
acid. acid.
Mn,
orpromotor
Cu) promotor
[10,11,22],
leading
to higher
of carboxylic
In
farfar
any
tested
homogeneous
catalytic
systems
[22][22]
havehave
the
In spite
spite of
ofthe
theabove
aboveachievements,
achievements,soso
any
tested
homogeneous
catalytic
systems
drawback
of
not
being
re-usable,
thus
the
search
for
a
more
efficient
and
eco-friendly
heterogeneous
the drawback of not being re-usable, thus the search for a more efficient and eco-friendly
processes
for the
synthesis
such
industrially
important
commodities
continues.
Goldcontinues.
catalysts
heterogeneous
processes
forofthe
synthesis
of such
industrially
important
commodities
are
currently
“hot
topic” of
research,
showasapplication
in many reactions
industrial
Gold
catalystsaare
currently
a “hot
topic”as
ofthey
research,
they show application
in manyofreactions
of
and
environmental
importance
[23–28].
Several
variables
have
been
considered
as
important
factors
industrial and environmental importance [23–28]. Several variables have been considered as
influencing
the structure,
reactivity,
and catalytic
activity.and
Among
them activity.
are the method
preparation,
important factors
influencing
the structure,
reactivity,
catalytic
Amongofthem
are the
the
nature
the support,
particularly,
the gold
nanoparticle
size
method
of of
preparation,
theand
nature
of the support,
and
particularly,
the[23–28].
gold nanoparticle size [23–28].
Herein,
report
the use
of gold
as promotors ofascyclohexane
Herein, wewe
report
the
use nanoparticles
of gold nanoparticles
promotorshydrocarboxylation.
of cyclohexane
We
have chosen the We
above-mentioned
protocol
[10] and theprotocol
use of gold
as athe
metal
hydrocarboxylation.
have chosen the
above-mentioned
[10] and
use promotor
of gold asina
n Bu N][AuCl ], Au
n
view
of
the
ability
of
[
C-scorpionate
gold
complexes,
and
Au
nanoparticles
4
4
metal promotor in view of the ability of [ Bu4N][AuCl4], Au C-scorpionate gold complexes, and Au
to
catalyze thetoperoxidative
of cyclohexane
to cyclohexane
KA oil (cyclohexanol
cyclohexanone
nanoparticles
catalyze theoxidation
peroxidative
oxidation of
to KA oiland
(cyclohexanol
and
mixture)
[29,30].
Moreover,
gold
nanoparticles
are supported
carbon xerogels
different
cyclohexanone
mixture)
[29,30].
Moreover,
gold nanoparticles
areon
supported
on carbonwith
xerogels
with
treatments,
in order toinprovide
recyclable
forcatalysts
the one-pot
hydrocarboxylation
of cyclohexane
different treatments,
order to
provide catalysts
recyclable
for the
one-pot hydrocarboxylation
of
to
cyclohexanecarboxylic
acid (Schemeacid
1). (Scheme 1).
cyclohexane
to cyclohexanecarboxylic
Scheme
Hydrocarboxylation of
of cyclohexane
cyclohexane to cyclohexanecarboxylic
Scheme 1.
1. Hydrocarboxylation
cyclohexanecarboxylic acid catalysed by gold
gold
nanoparticles
nanoparticles supported
supported on
on carbon
carbon xerogels.
xerogels.
To the
the best
best of
of our
our knowledge,
knowledge, this
this is
is the
the first
first report
report dealing
dealing with
with hydrocarboxylation
hydrocarboxylation of
of alkanes
alkanes
To
using
gold
nanoparticles
as
catalysts.
In
fact,
the
only
reports
found
in
literature
so
far,
dealing
with
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
hydrocarboxylation of hydrocarbons using gold catalysts, refer to hydrocarboxylation of alkynes and
to gold
gold complexes
complexes (not
(not gold
gold nanoparticles)
nanoparticles) [31,32].
[31,32]. Moreover,
the only
only report
report for
for hydrocarboxylation
hydrocarboxylation
to
Moreover, the
using
carbon
materials
deals
with
1,3-butadiene
using
a
Rh(I)
complex
immobilized
on activated
activated
using carbon materials deals with 1,3-butadiene using a Rh(I) complex immobilized on
carbon
as
the
catalyst
[33].
Therefore,
our
work
is
also
the
first
report
of
such
reaction
carried
out
carbon as the catalyst [33]. Therefore, our work is also the first report of such reaction carried out using
using carbon
based catalysts.
carbon
xerogelxerogel
based catalysts.
2. Results and Discussion
2.1. Characterisation of Xerogel Supports
Supports
The carbon
carbon xerogel
xerogelwas
wasused
usedasas
a support
in original
its original
as prepared
oxidised
a support
in its
formform
as prepared
(CX),(CX),
oxidised
(-ox),
(-ox),
and oxidised
with nitric
acid subsequently
and subsequently
treated
sodium
hydroxide
(-ox-Na).
and oxidised
with nitric
acid and
treated
with with
sodium
hydroxide
(-ox-Na).
The
The
characterization
details
of these
samples
found
Table1 1and
andFigure
Figure1,1, which
which include the
characterization
details
of these
samples
cancan
be be
found
in in
Table
textural
textural and
and surface
surface characterisation.
characterisation.
Table
pore size,
size, as
as expected.
expected [29,30,34–37].
Table 11 shows
shows that CX is mainly mesoporous and has a large pore
By comparing the parameters of the oxidized (CX-ox) samples with those of the parent material (CX),
it
liquid
phase
activation
slightly
decreased
the surface
area andarea
poreand
volume,
it isisobserved
observedthat
that
liquid
phase
activation
slightly
decreased
the surface
pore probably
volume,
due
to some
wallpore
collapse
to the or
presence
of numerous
oxygen-containing
surface groups,
probably
duepore
to some
wall or
collapse
to the presence
of numerous
oxygen-containing
surface
groups, which might partially block the access of N2 molecules to the smaller pores [37]. Nitric acid
consumes large amounts of carbon atoms and changes the structure of pores, merging some of them
together.
Molecules 2017, 22, 603
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which might partially block the access of N2 molecules to the smaller pores [37]. Nitric acid consumes
large amounts of carbon atoms and changes the structure of pores, merging some of them together.
Molecules 2017, 22, 603
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Table 1. Description and characterisation of carbon xerogel samples: surface area (SBET ), total pore
Table(V
1.pDescription
and characterisation
of carbonvolume
xerogel(V
samples:
surfacearea
area(S(Sexternal
BET), total
pore
volume
), average mesopore
width (L), micropore
), obtained
micro ), external
◦
volume
(V
p
),
average
mesopore
width
(L),
micropore
volume
(V
micro
),
external
area
(S
external
),
obtained
by adsorption of N2 at −196 C, and amounts of CO and CO2 desorbed, as determined by temperature
by adsorption
of N2 at −196
°C, and amounts of CO and CO2 desorbed, as determined by temperature
programmed
desorption
(TPD).
programmed desorption (TPD).
SBET
SBET
(m2 /g)
(m2/g)
CX
CX
604604
CX-ox
570
CX-ox
CX-ox-Na 570560
CX-ox-Na
560
Sample
Sample
V
Vp 3p
(cm
/g)
3
(cm /g)
0.91
0.91
0.80
0.80
0.75
0.75
LL
(nm)
(nm)
13.7
13.7
18.8
18.8
17.6
17.6
Vmicro
Vmicro
(cm3 /g)
(cm3/g)
~0
~0
0.038
0.038
0.036
0.036
Sexternal
Sexternal
(m2 /g)
(m2/g)
604
604
512
512
496
496
CO
CO
(µmol/g)
(µmol/g)
492
492
4609
4609
3720
3720
CO2
CO2
(µmol/g)
(µmol/g)
135
135
3774
3774
3793
3793
Figure
1 1shows
desorbingin
indifferent
differenttemperature
temperature
ranges,
Figure
showsthe
theidentification
identificationof
of types
types of
of groups
groups desorbing
ranges,
according
to
what
is
already
established
in
the
literature
[37–41].
Upon
oxidation
treatment,
the
amounts
according to what is already established in the literature [37–41]. Upon oxidation treatment, the
of amounts
CO and of
COCO
(Table 1 and
Figure
1). Figure
Figure 1).
1a Figure
shows 1a
theshows
CO desorption
2 increase
and COenormously
2 increase enormously
(Table
1 and
the CO
profiles.
The
effect
of
oxidation
treatments
is
also
seen
in
these
data.
The
largest
CO
evolution
desorption profiles. The effect of oxidation treatments is also seen in these data. The largest CO
of evolution
the -ox materials
starts
at around
350at ◦ C,
whereas
-ox-Naforsamples,
the temperature
of the -ox
materials
starts
around
350 for
°C, the
whereas
the -ox-Na
samples, the is
slightly
higher.isThat
can be
due to
thecan
destruction
carboxylic
anhydrides
(that anhydrides
desorb as CO
and
temperature
slightly
higher.
That
be due toofthe
destruction
of carboxylic
(that
CO
in
that
temperature
range
[37–41])
also
contributing
to
the
increase
of
the
carboxylate
groups,
desorb
as
CO
and
CO
2
in
that
temperature
range
[37–41])
also
contributing
to
the
increase
of
the
2
groups,
as proposed
in literature
[42]. Moreover,
theisprofile
-ox-Na is
a little
sharper,
ascarboxylate
proposed in
literature
[42]. Moreover,
the profile
of -ox-Na
a littleofsharper,
more
intense,
and
more
intense, andathas
its maximum
at a higher
temperature
of theThis
-ox sample.
This
suggests
has
its maximum
a higher
temperature
than
that of thethan
-ox that
sample.
suggests
that
phenol
that phenol
groupsas(that
COconverted
[37–41]) are
converted
intowhich
phenolates,
which
are[42].
moreThe
stable
groups
(that desorb
CO desorb
[37–41])asare
into
phenolates,
are more
stable
CO2
[42].
The
CO
2
desorption
profiles
(Figure
1b)
of
the
-ox
and
-ox-Na
materials
also
show
a
considerable
desorption profiles (Figure 1b) of the -ox and -ox-Na materials also show a considerable increase
◦ C,
in theofamount
of carboxylic
acid(which
groups decompose
(which decompose
in the temperature
200–
in increase
the amount
carboxylic
acid groups
in the temperature
range range
200–350
°C, as described
in the literature
[37–41])
when compared
to the original
materials.
as 350
described
in the literature
[37–41]) when
compared
to the original
materials.
Phenols
Carbonyls/
Quinones
Anhydrides
Carboxylic acids
Anhydrides
CX-ox-Na
CX-ox
Lactones
CX-ox
CX-ox-Na
CX
(a)
CX
(b)
Figure1. 1. Temperature
Temperature programed
programed desorption
desorption (TPD)
Figure
(TPD) profiles
profiles for
for the
thecarbon
carbonxerogel
xerogelmaterials.
materials.
Desorption of CO (a) and CO2 (b) is shown, with identification of types of groups desorbing in
Desorption of CO (a) and CO2 (b) is shown, with identification of types of groups desorbing in
different temperature ranges (the different colour bars are only indicative of the temperature ranges
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
expected for the desorption of different groups, and do not provide any information on their amounts).
amounts).
2.2.
Characterisation
2.2.
CharacterisationofofGold
GoldCatalysts
Catalysts
The
nominal
gold
loading
Table 2,
The
nominal
gold
loadingwas
was3%
3%wt.
wt.(see
(seeMaterials
Materialsand
andMethods).
Methods). However,
However, as
as shown
shown in
in Table
only
CX
showed
an
actual
loading
near
that
value
(2.8%).
Regarding
the
functionalized
samples,
it has
2, only CX showed an actual loading near that value (2.8%). Regarding the functionalized samples,
it
been
reported
that
surface
oxygen
groups
can
act
as
anchoring
sites
for
metallic
precursors
[43,44].
has been reported that surface oxygen groups can act as anchoring sites for metallic precursors
However,
in the particular
case of Au
on xerogels
by theby
colloidal
method
usedused
in this
work
[43,44]. However,
in the particular
caseloaded
of Au loaded
on xerogels
the colloidal
method
in this
(see
Materials
and Methods),
it seemsit that
thethat
presence
of oxygenated
groupsgroups
is detrimental
for Au
work
(see Materials
and Methods),
seems
the presence
of oxygenated
is detrimental
for Au 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.
Molecules 2017, 22, 603
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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.
Molecules 2017, 22, 603
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Table 2. Average gold nanoparticle size and dispersion (calculated from TEM measurements) and gold
Table(calculated
2. Average by
gold
nanoparticle
sizespectroscopy)
and dispersionon(calculated
from
TEM xerogel
measurements)
and
loading
atomic
absorption
the different
carbon
materials.
gold loading (calculated by atomic absorption spectroscopy) on the different carbon xerogel materials.
Sample
AuAverage
AverageSize
Size(nm)
(nm)
Metal
(%)
Gold
Sample
Au
Metal Dispersion
Dispersion (%)
GoldLoading
Loading(%)
(%)
Au/CX
16.6
6.9 **
2.8
Au/CX
16.6* *
6.9
2.8
Au/CX-ox
14.2
8.1
1.4
Au/CX-ox
14.2
8.1
1.4
Au/CX-ox-Na
13.7
8.4
0.5
Au/CX-ox-Na
13.7
0.5
*—calculated
thespherical
sphericalnanoparticles,
nanoparticles,
nanorods.
*—calculatedtaking
takinginto
intoaccount
account only
only the
notnot
thethe
nanorods.
Figure
2 showssome
someselected
selected transmission
transmission electron
(TEM)
images
of the
It
Figure
2 shows
electronmicroscopy
microscopy
(TEM)
images
of samples.
the samples.
can be seen that gold nanoparticles are deposited mostly in the form of nanorods on CX (Figure 2a,b).
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],
Although spherical nanoparticles are usually obtained with the colloidal method [30,45–47], nanorods
nanorods are also often reported in literature [48,49]. They are usually formed through a seedare also often reported in literature [48,49]. They are usually formed through a seed-mediated method,
mediated method, which includes the formation of “seed” nanoparticles and the growth of such
which includes the formation of “seed” nanoparticles and the growth of such seeds into rods [49].
seeds into rods [49]. Also other agglomerates of particles are seen (Figure 2b). Au on CX-ox and CXAlso other agglomerates of particles are seen (Figure 2b). Au on CX-ox and CX-ox-Na (Figure 2c,d,
ox-Na (Figure 2c,d, respectively) show more regular sphere-like particles, which are larger on CX-ox
respectively)
(Figure 2c).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:
CXCX-ox
(a,b),(c)CX-ox
(c) and CX-ox-Na
(d). Gold
Figure
2. TEM
images
carbon
xerogel
samples:
CX (a,b),
and CX-ox-Na
(d). Gold nanoparticles
arespots,
seen as
darker
spots,(a,b),
withor
rod-like
(a,b), or spherical/obliquous
arenanoparticles
seen as darker
with
rod-like
spherical/obliquous
(c,d) shapes. (c,d) shapes.
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
Molecules 2017, 22, 603
5 of 12
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 ·3H2 O used in homogeneous
medium, i.e., in aqueous solution (Figure 3, Table 3).
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.
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
a
Total TON
375
311
111
245
23
Yield/% c
CyCOOH Cy-H =O
CyOH Cy-H (OH)2
b
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
Reaction conditions: cyclohexane (1.00 mmol), p(CO) = 2–20 atm, K2 S2 O8 (1.50 mmol), catalyst (2–20 µmol), H2 O
(3.0 mL)/MeCN (3.0 mL), 30–80 ◦ C, 6 h in an autoclave (13.0 mL capacity); b Turnover number = moles of products
per mole of catalyst; c Moles of product per 100 mol of cyclohexane; Cy = C6 H11 .
Molecules 2017, 22, 603
6 of 12
60
60
6060
60 60
50
50
5050
50 50
20
Products yield /%
Products
Productsyi
yiel
eldd /%
/%
Products yield /%
10
10
1010
10 10
0
0
40
30
20
Products yield /%
Products yield /%
Moreover, the Au/CX-ox-Na/CO/K2 S2 O7 /H2 O/MeCN system exhibits its maximum performance
at the mild temperature of 50 ◦ C (compare entries 1, 3 and 4, Table 3) requiring a significantly
(up
to2017,
16603
times)
lower
systems
Molecules
2017,
22,
603 amount of metal promoter than in the previously reported
6 12
of
12
Molecules
2017,
22,
603
Molecules
2017,
22,
603
6of
of
124).
Molecules
22,
603
6 (Table
of612
Molecules
2017,
22,
Molecules
2017,
22,
603
6 of
12 6 of 12
In fact, considering the cyclohexanecarboxylic acid yield per amount of metal promotor (Table 4) the
ox-Na
system
exhibits
performance
on
cyclohexane
hydrocarboxylation
relative
ox-Na
system
exhibits
significantly
better
performance
cyclohexane
hydrocarboxylation
relative
ox-Na
system
exhibits
significantly
better
onon
cyclohexane
hydrocarboxylation
relative
ox-Na
system
exhibits
significantly
better
performance
on cyclohexane
hydrocarboxylation
relative
system
exhibits
significantly
better
performance
onperformance
cyclohexane
hydrocarboxylation
relative
ox-Naox-Na
system
exhibits
significantly
better
performance
on
cyclohexane
hydrocarboxylation
relative
Au/CX-ox-Na
system
exhibits
significantly
better
on
cyclohexane
hydrocarboxylation
to
the
formerly
tested
promotors
[10]
(cyclohexanecarboxylic
acid
yield
is
1.5
times
higher
than
theformerly
tested
promotors
[10](cyclohexanecarboxylic
(cyclohexanecarboxylic
acid
yield
istimes
1.5
higher
than
that
tototested
the
tested
acid
isyield
1.5
times
higher
than
that
torelative
the
formerly
promotors
[10]
(cyclohexanecarboxylic
yield
istimes
1.5
higher
than
thatthat
the
formerly
tested
promotors
[10]promotors
(cyclohexanecarboxylic
acidacid
yield
is yield
1.5
higher
than
that
to the to
formerly
promotors
[10]
(cyclohexanecarboxylic
acid yield
is
1.5
times
higher
than
toformerly
thetested
formerly
tested
[10] (cyclohexanecarboxylic
acid
istimes
1.5that
times
higher
than
ofofthe
the
literature
34}]O)
(BOH)
4entries
224,,]2entries
4
and
16,
respectively,
of
the
literature
best
catalyst
[OCu
43{N(CH
2(BOH)
CH
O)
}
4(BOH)
,
entries
4
and
16,
respectively,
of
literature
best
catalyst
42{N(CH
][BF
4]
]
entries
4
and
16,
respectively,
ofof
ofthat
the
literature
best
catalyst
[OCu
4[OCu
{N(CH
22CH
24O)
32}O)
42(BOH)
4][BF
44]][BF
2, and
4
and
16,
respectively,
of
the
literature
best
catalyst
4{N(CH
CH
O)32}CH
44432][BF
4(BOH)
]
2,44][BF
entries
4
and
16,
respectively,
of
of theof
literature
best
catalyst
[OCu
4[OCu
{N(CH
2[OCu
CH
2O)
}44{N(CH
(BOH)
4][BF
,
entries
4
16,
respectively,
of
of
the
literature
best
catalyst
CH
}
][BF
]
,
entries
4
and
16,
respectively,
2
2
3 4
4
4 2
Table
4).
Table
4).
Table
4).
4).
4).
Table Table
4). Table
of Table 4).
40
30
4040
40 40
3030
30 30
2020
20 20
0
00 0
cyclohexanol
cyclohexanone
cyclohexane-1,2-diol
cyclohexanecarboxilic
cyclohexanol
cyclohexanone
cyclohexane-1,2-diol
cyclohexanecarboxilic
acid
cyclohexanol
cyclohexanone
cyclohexane-1,2-diol
cyclohexanecarboxilic
acid
cyclohexanol
cyclohexanone
cyclohexane-1,2-diol
cyclohexanecarboxilic
cyclohexanol
cyclohexanone
cyclohexane-1,2-diol
cyclohexanecarboxilic
acid acid acid
cyclohexanol
cyclohexanone
cyclohexane-1,2-diol
cyclohexanecarboxilic
acid
no
rom
oter
llC44
uu
XX AX A
XXu-ox
XXN-oxNN aaN a
u-oxXN/C-ox
u /C
Xa-oxno
rom
/C
/C
-ox
/C
-oxno poter
u /C
XuuA-ox
u /CAANXuuA-oxp rom
u oter
H CA lu4 H/CAACXuuAl4AHHu uCCH /C
A /C
u /C
-ox
A/C
a/C
no p rom no
oter
Arom
uno
Hppoter
Cp lrom
4A oter
AAlXu4u/C/CXAAX-ox
uAA/C
X/C
aAX -ox-
Figure
3.3.Products
Products
yields
from
cyclohexane
hydrocarboxylation:
metal-free
(),
Figure
yields
from
cyclohexane
hydrocarboxylation:
metal-free
( ),promoted
promoted
Figure
Products
yields
from
cyclohexane
hydrocarboxylation:
metal-free
(),
promoted
Figure
3.
Products
yields
cyclohexane
hydrocarboxylation:
metal-free
(),
promoted
byby
3. 3.
Products
yields
from
cyclohexane
hydrocarboxylation:
metal-free
promoted
3.
Products
yields
from
cyclohexane
hydrocarboxylation:
metal-free
(), (),
promoted
by by by
FigureFigure
3. Figure
Products
yields
from
cyclohexane
hydrocarboxylation:
metal-free
(),
promoted
by
HAuCl
4.3H2O in homogeneous conditions () and by Au NPs deposited on different carbon xerogels:
HAuCl
4
.3H
2
O
in
homogeneous
conditions
()
and
by
Au
NPs
deposited
on
different
carbon
xerogels:
HAuCl
.3H
O
in
homogeneous
conditions
(
)
and
by
NPs
deposited
on
different
carbon
HAuCl
4
.3H
2
O
in
homogeneous
conditions
()
and
Au
NPs
deposited
on
different
carbon
xerogels:
HAuCl
4
.3H
2
O
in
homogeneous
conditions
()
and
by
Au
NPs
deposited
on
different
carbon
xerogels:
HAuCl
4
.3H
2
O
in
homogeneous
conditions
()
and
by
Au
NPs
deposited
on
different
carbon
xerogels:
HAuCl4.3H2O in homogeneous
conditions () and by Au NPs deposited on different carbon xerogels:
4
2
as
prepared
(CX)
(),
treated
with
nitric
acid
(CX-ox)
oxidized
with
nitric
acid
asprepared
prepared
(CX)
(),
treated
with
nitric
acid
()and
and
oxidized
with
nitric
acidand
and
as
(CX)
(),
treated
with
nitric
acid
(CX-ox)
()
and
oxidized
with
nitric
acid
and
prepared
(CX)
(treated
),with
treated
with
nitric
acid
(CX-ox)
((CX-ox)
) oxidized
and
oxidized
with
nitric
acid
and
subsequently
as as
prepared
(),
treated
with
nitric
acid
(CX-ox)
()
and
oxidized
with
nitric
acid
and
as prepared
(CX)(CX)
(),
with
nitric
acid
(CX-ox)
()
and()
oxidized
with
nitric
acid
and
as prepared
(CX)
(),
treated
nitric
acid
(CX-ox)
()
and
with
nitric
acid
and
subsequently
treated
with
sodium
hydroxide
(CX-ox-Na)
().
subsequently
treated
with
sodium
hydroxide
(CX-ox-Na)
().
subsequently
treated
with
sodium
hydroxide
(CX-ox-Na)
subsequently
treated
with
sodium
hydroxide
(CX-ox-Na)
subsequently
treated
with
sodium
hydroxide
(CX-ox-Na)
(). ().().
treated
with
sodium
hydroxide
(CX-ox-Na)
( ).
subsequently
treated
with
sodium
(CX-ox-Na)
().
a for the cyclohexane hydrocarboxylation to
Table
4.promotors
Metal
promotors
performance
comparison
afor
a the
a for
a for
Table
4.Metal
Metal
promotors
performance
comparison
forthe
the
cyclohexane
hydrocarboxylation
to
4.
promotors
performance
comparison
cyclohexane
hydrocarboxylation
Table
4. 4.Metal
promotors
performance
comparison
foraa cyclohexane
the
cyclohexane
hydrocarboxylation
4.Table
Metal
performance
comparison
hydrocarboxylation
to to toto
Table Table
4. Metal
promotors
performance
comparison
the
cyclohexane
hydrocarboxylation
to
Metal
promotors
performance
comparison
for
the
cyclohexane
hydrocarboxylation
Table
cyclohexanecarboxylic
acid.
cyclohexanecarboxylic
acid.
cyclohexanecarboxylic
acid.
cyclohexanecarboxylic
cyclohexanecarboxylic
cyclohexanecarboxylic
acid. acid.acid.
cyclohexanecarboxylic
acid.
CyCOOH
Yield
CyCOOH
Yield
CyCOOH
Yield
CyCOOH
CyCOOH
YieldYield
CyCOOH
Yield
Carbon
Atom
b/μmol
Atom
Carbon
Atom
Carbon
Atom
Carbon
Carbon
Atom Atom
Carbon
CyCOOH
Yield
Entry
Metal
Promoter
(%)
of
M
b
b
b
b
b
c
d
Entry
Metal
Promoter(%) /μmol
(%)
/μmol
of
Entry
Metal
Promoter
(%)
of
MM c Efficiency/%
Entry
Metal
Promoter
/μmol
of M
Promoter
(%) (%)
/μmol
of/μmol
M
Entry Entry
Metal Metal
Promoter
of M
Carbon
Atom
Economy/%
b
c
c
d d
c
d
c
d
d Economy/%
Economy/%
Efficiency/%
Entry
Metal Promoter
(%)
/µmolEfficiency/%
of MEfficiency/%
Efficiency/%
Economy/%
Economy/%
Economy/%
Efficiency/%
Promotor
c
d
Efficiency/%
Economy/%
Promotor
Promotor
Promotor
Promotor
Promotor
Promotor
HAuCl
.3H
O2Oee e
e 22O
e 442.3H
eHAuCl
HAuCl
4.3H
5.2 7.7
7.7
5.2
7.7
.3H
O
1 1 11 1 HAuClHAuCl
24O
5.2 5.2 5.2
7.7 7.7 7.7
1
4.3HHAuCl
2O4.3H
5.2
e 2O e
1
HAuCl
·
3H
5.2
7.7
2
Au/CX
6.9
9.3
4
e
e
e
e
e
2
Au/CX
6.9
9.3
Au/CX
6.9
9.3
Au/CX
6.9
9.3
2 2 22
6.9
9.3
2
Au/CXAu/CX
6.9
9.3
e
33.5
Au/CX e
6.9
9.3
33.5
33.5 33.5 33.533.5
33
Au/CX-ox
12.7
18.4
e
e
e
e
e
33.5
e
Au/CX-ox
12.7
18.4
3
Au/CX-ox
12.7
18.4
Au/CX-ox
3 3
Au/CX-ox
3
Au/CX-ox
12.7 12.7 12.7 12.718.4 18.4 18.4 18.4
3
Au/CX-ox e
4
Au/CX-ox-Na
27.3
37.9
e ee
e
e
eAu/CX-ox-Na
27.337.9 37.9 37.937.9
37.9
Au/CX-ox-Na
Au/CX-ox-Na
4 4 444 Au/CX-ox-Na
Au/CX-ox-Na
27.3 27.3 27.327.3
4
Au/CX-ox-Na
27.3
37.9
5
Cr(OH)3·2.5H2O [10]
0.4
0.6
3
·2.5H
2
O
[10]
0.4
0.6
5
Cr(OH)
3
·2.5H
2
O
[10]
0.4
0.6
5
Cr(OH)
3·2.5H
2O [10]
Cr(OH)
·2.5H
2O [10]
0.4 0.4 0.4 0.6
0.6 0.6 0.6
5 5
Cr(OH)
3·2.5H
23O
[10]
0.4
5
Cr(OH)
Cr(OH)
·2.5H
2 O [10]
65
K2Cr23O
7 [10]
1.0
3.9
6
K
2
Cr
2
O
7
[10]
1.0
3.9
6
K
2
Cr
2
O
7
[10]
1.0
3.9
6
K
2
Cr
2
O
7
[10]
1.0
3.9
6
K
2
Cr
2
O
7
[10]
1.0
3.9
6
K2Cr2O7 [10]
1.0
6
K2 Cr2 O7 [10]
1.0 3.9
3.9
7
MoO
3 [10]
0.00.0
0.0
0.0
3
[10]
0.0
0.0
MoO
0.0
0.0
3MoO
[10]3 [10]
0.0
0.0
7 7 777
MoOMoO
3 [10]
0.0
0.0
7
MoO3 [10]
0.0
0.0
3
8
HH
4[PMo
11VO
40]·34H
2O O
[10]
0.40.4
1.1
[PMo
VO
]O
·34H
[10] 0.4
1.1
4411
11
40
[PMo
11
VO
402]·34H
22O
[10]
0.4 1.1
1.1
8 H11
H
4[PMo
1140
VO
40]·34H
2O
[10]
HVO
]·34H
[10]
H440[PMo
4[PMo
11
40
]·34H
2O
[10]
0.4 0.4 0.4
1.1 1.1 1.1
8 8 H848[PMo
VO
]·34H
2VO
O
[10]
8
99
MnO
2 [10]
0.5
1.4
MnO
[10]
0.5
1.4
22 [10]
0.5 1.4
1.4
MnO
2MnO
[10]2 [10]
2 [10]
0.5 0.5 0.5
1.4 1.4 1.4
9 9 99
[10] MnO
0.5
9
MnO2 MnO
10
Fe(OH)
[10]
1.6
10
Fe(OH)
3·0.5H
2O
[10]
1.01.0
1.6
3 ·0.5H
2O
33.5
10
Fe(OH)
32·0.5H
2O
[10]
1.0
1.6
Fe(OH)
·0.5H
2O
[10]
1.0
1.6
Fe(OH)
3·0.5H
O
[10]
1.0
1.6
10 10 10
Fe(OH)
·0.5H
2O3[10]
1.0
1.6
10
Fe(OH)
3·0.5H
23O
[10]
1.0
1.6
33.5
11
Co(acac)
[10]
0.9
33.5
33.5 33.5 33.533.5
11
Co(acac)
33
[10]
0.60.6
0.9
3
[10]
0.6
0.9
11
Co(acac)
3
[10]
0.6
0.9
Co(acac)
3
[10]
0.6
0.9
Co(acac)
3
[10]
0.6
0.9
11 11 11
Co(acac)
3
[10]
0.6
0.9
11
Co(acac)
12
Zn(NO
)2 [10]
0.8
12
Zn(NO
3)32 [10]
0.50.5
0.8
2 [10]
0.5 0.8
0.8
12 Zn(NOZn(NO
Zn(NO
23)[10]
3)2 )[10]
Zn(NO
3)Zn(NO
2 [10]
0.5 0.5 0.5
0.8 0.8 0.8
12 12 12
3)2 [10]
0.5
12
13
Cu(NO
1.0
3.3
23·)2.5H
2 O [10]
13
Cu(NO3)32·2.5H
2O [10]
1.0
3.3
14
[Cu(H
tea)(N
[10]
2.0
3.3
13
Cu(NO
)22·2.5H
O[10]
[10]
1.0 3.3
3.3
3)23[10]
·2.5H
2O
Cu(NO
3)2·2.5H
O [10]
13 13 13
Cu(NO
3)2Cu(NO
2O
·2.5H
2O
1.0 1.0 1.0
3.3 3.3 3.3
32)]
13
Cu(NO
3)2·2.5H
[10]
1.0
14
[Cu(H
2tea)(N3)] [10]
2.04.9
3.3
15
[Cu
(tpa)]
2nH
3.8
2 (H
2 tea)
22tea)(N
2 O [10]2.0
[10]
2.0 3.3
3.3
14
[Cu(H
2tea)(N
3n
)]3·)]
[10]
2tea)(N
3)] [10]
[Cu(H
23tea)(N
3)]
[10]
2.0 2.0 2.0
3.3 3.3 3.3
14 14 14
tea)(N
)][Cu(H
[10]
14
[Cu(H2[Cu(H
15
[Cu
2(H2tea)
2(tpa)]
2O ][BF
[10]4 ]2 [10]
4.9
3.8
[OCu
{N(CH
CH
O)3 }n4·2nH
(BOH)
18.1
7.1
16
4
2
2
4
15[Cu
[Cu
2(H
tea)
2n
(tpa)]
n2·2nH
2O
[10]4.9
4.9 3.8
3.8
2(tpa)]
(H
22tea)
2[10]
(tpa)]
·2nH
2O
[10]
[Cu
2[Cu
(H
tea)
(tpa)]
·2nH
O [10]
15 15
2(H
2tea)
n
·2nH
2O n[10]
4.9 4.9 4.9
3.8 3.8 3.8
15
[Cu15
2(H
2tea)
2(tpa)]
n2·2nH
2O
[OCu
4{N(CH2CH2O)3}4(BOH)4][BF4]2 [10]
18.1
7.1p(CO) = 20 atm, K S O
a 16
Typical
(unless
otherwise
stated)
cyclohexane
2 2 8
{N(CH
2CH
}4(BOH)
418.1
][10]
2 [10] 18.1 18.1
18.17.1 (1.00
7.1
16[OCu
43(BOH)
4][BF
4]2conditions:
18.1
16
[OCu
4[OCu
CH
O)
3}24O)
(BOH)
44][BF
4]42][BF
[10]
7.1 7.1
164{N(CH
4{N(CH
22O)
3}224CH
]2reaction
[10]
7.1mmol),
16
[OCu
2CH
2{N(CH
O)2CH
34}{N(CH
44(BOH)
4(BOH)
][BF
42]O)
234}][BF
[10]
16
[OCu
◦
a
(1.50
mmol),(unless
H2 O (3.0
mL)/MeCN
(3.0
mL), 50conditions:
C, 6 h incyclohexane
an autoclave(1.00
(13.0mmol),
mL capacity);
Moles
Typical
otherwise
stated)
reaction
p(CO) =b 20
atm, of
a Typical
a (unless
a Typical
a Typical
a Typical
(unless
otherwise
stated)
reaction
conditions:
cyclohexane
(1.00
mmol),
p(CO)
=
20
c
Typical
(unless
otherwise
stated)
reaction
conditions:
cyclohexane
(1.00
mmol),
p(CO)
=
20
atm,
(unless
otherwise
stated)
reaction
conditions:
cyclohexane
(1.00
mmol),
p(CO)
=
20
atm,
otherwise
stated)
reaction
conditions:
cyclohexane
(1.00
mmol),
p(CO)
=
20
atm,
(unless
otherwise
stated)
reaction
conditions:
cyclohexane
(1.00
mmol),
p(CO)
=
20
atm,
cyclohexanecarboxylic
100mL)/MeCN
mol of cyclohexane;
C6 H
of carbon
in CyCOOH
per atm,
total
b
K2S2O8 (1.50 mmol),acid
H2Oper
(3.0
(3.0 mL),Cy
50=°C,
611h; inAmount
an autoclave
(13.0
mL capacity);
d Molecular weight of desired product per combined molecular bweightb of starting
b b
b
carbon
in
reactants
×
100%;
2
S
2
O
8
(1.50
mmol),
H
2
O
(3.0
mL)/MeCN
(3.0
mL),
50
°C,
6
h
in
an
autoclave
(13.0
mL
capacity);
K
2
S
2
O
8
(1.50
mmol),
H
2
O
(3.0
mL)/MeCN
(3.0
mL),
50
°C,
6
h
in
an
autoclave
(13.0
mL
capacity);
K
2
S
2
O
8
(1.50
mmol),
H
2
O
(3.0
mL)/MeCN
(3.0
mL),
50
°C,
6
h
in
an
autoclave
(13.0
mL
capacity);
K
2
S
2
O
8
(1.50
mmol),
H
2
O
(3.0
mL)/MeCN
(3.0
mL),
50
°C,
6
h
in
an
autoclave
(13.0
mL
capacity);
K
K2S2O8 (1.50 mmol), H2O (3.0 mL)/MeCN (3.0 mL), 50 °C, 6 h in an autoclave (13.0 mL capacity);
c
Moles of
cyclohexanecarboxylic
e p(CO) = 2 atm. acid per 100 mol of cyclohexane; Cy = C6H11; Amount of carbon in
materials
×
100%;
c
c
c
c
c
Moles
cyclohexanecarboxylic
acid
per100
ofcyclohexane;
cyclohexane;
Cy
=C
carbon
ofofcyclohexanecarboxylic
acid
per
mol
ofCy
11carbon
;11; Amount
ofof
Moles
of cyclohexanecarboxylic
acid
per
100
mol
ofmol
cyclohexane;
=Cy
C
116C
;H
Amount
of carbon
in inin
ofMoles
cyclohexanecarboxylic
acid
per
100
of100
cyclohexane;
=;Cy
CAmount
6H
11
;6=HAmount
ofAmount
carbon
incarbon
Moles Moles
of cyclohexanecarboxylic
acid per
100
mol
ofmol
cyclohexane;
= CCy
6H11
of6H
in
d Molecular weight of desired product per combined
CyCOOH per total carbon in reactants d× 100%;
d Molecular
d Molecular
d weight
d Molecular
weight
of
desired
product
per
combined
CyCOOH
pertotal
total
carbon
×100%;
100%;
weight
of
desired
product
per
combined
CyCOOH
per
ininreactants
× Molecular
Molecular
weight
of
desired
product
per
combined
CyCOOH
per total
carbon
in× reactants
× 100%;
weight
of
desired
product
per
combined
CyCOOH
percarbon
total
carbon
incarbon
reactants
×reactants
100%;
of
desired
product
per
combined
CyCOOH
per
total
in
reactants
100%;
molecular weight of starting materials × 100%; ee p(CO)
= 2 atm.
eatm.
e×p(CO)
e100%;
atm.
molecular
weight
starting
= =2 2atm.
molecular
weight
ofof
starting
materials
p(CO)
= 2 atm.
molecular
weight
of starting
× 100%;
= p(CO)
2e p(CO)
atm.
molecular
weight
of
starting
materials
×materials
p(CO)
=×100%;
2100%;
molecular
weight
of starting
materials
×materials
100%;
In addition, the present catalytic system was evaluated by green metrics such as atom economy
addition,
thepresent
present
catalytic
system
was
evaluated
green
metrics
such
asatom
atom
economy
InInaddition,
the
catalytic
system
was
byby
green
metrics
as
economy
In the
addition,
the
present
catalytic
system
was
evaluated
by green
metrics
such
as atom
economy
In addition,
the present
catalytic
system
was
evaluated
by green
metrics
such
as such
atom
economy
In addition,
present
catalytic
system
was
evaluated
by evaluated
green
metrics
such
as
atom
economy
(molecular weight of desired product per combined molecular weight of starting materials) or carbon
(molecular
weight
desired
product
percombined
combined
molecular
weight
starting
materials)
carbon
(molecular
weight
ofofdesired
product
molecular
weight
ofofstarting
materials)
ororcarbon
(molecular
of
desired
product
per per
combined
molecular
of
starting
materials)
or carbon
(molecular
weight
of
desired
product
per
combined
molecular
weight
of
starting
materials)
or carbon
(molecular
weight
of weight
desired
product
per
combined
molecular
weight
of weight
starting
materials)
or carbon
efficiency (amount of carbon in CyCOOH per total carbon in reactants). Such metrics were not
efficiency
(amount
ofcarbon
inCyCOOH
CyCOOH
per
total
inreactants).
reactants).
Such
metrics
were
not
efficiency
(amount
of
in
per
total
carbon
inSuch
Such
metrics
were
efficiency
(amount
ofincarbon
in CyCOOH
total
carbon
in reactants).
Such
metrics
were
not not
efficiency
(amount
of carbon
incarbon
CyCOOH
per per
total
carbon
incarbon
reactants).
Such
metrics
were
not
efficiency
(amount
of
carbon
CyCOOH
per
total
carbon
in
reactants).
metrics
were
not
included in the previously reported systems. Thus, to compare our system with the previous ones
included
the
previously
reported
systems.
Thus,
compare
ourthe
system
with
theprevious
previous
ones
included
ininthe
previously
reported
systems.
totocompare
our
system
included
in previously
the
previously
reported
systems.
Thus,
to our
compare
system
with
the the
previous
included
in
the
reported
systems.
Thus,
toThus,
compare
our our
system
with
thewith
previous
onesonesones
included
in
the
previously
reported
systems.
Thus,
to compare
system
with
previous
ones
Molecules 2017, 22, 603
7 of 12
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
Molecules efficiency
2017, 22, 603(amount of carbon in CyCOOH per total carbon in reactants). Such metrics 7were
of 12
carbon
not included in the previously reported systems. Thus, to compare our system with the previous
(reported
in [10]),
carbon
efficiency
and the and
atomthe
economy
were determined
(Table 4). Although
ones
(reported
in the
[10]),
the carbon
efficiency
atom economy
were determined
(Table 4).
presenting
the
same
atom
economy
value,
our
gold
systems
exhibit
markedly
higher
Although presenting the same atom economy value, our gold systems exhibit markedly higher carbon
carbon
efficiency values
values (Table
(Table 4),
4), which
which is
is aa significant
significant improvement
improvement in
in terms
terms of
of the
the sustainability
sustainability of
of the
the
efficiency
hydrocarboxylation reaction.
reaction.
hydrocarboxylation
Another
important
the
possibility
of of
being
recycled
andand
reAnother important advantage
advantageofofthe
thepresent
presentsystems
systemsis is
the
possibility
being
recycled
used.
Recycling
of
the
best
catalyst,
Au/CX-ox-Na,
was
tested
on
up
to
seven
consecutive
cycles.
On
re-used. Recycling of the best catalyst, Au/CX-ox-Na, was tested on up to seven consecutive cycles.
completion
of each
stage,
thethe
products
were
analyzed
and
On
completion
of each
stage,
products
were
analyzed
andthe
thecatalyst
catalystwas
wasrecovered
recoveredby
by filtration,
filtration,
thoroughly
washed,
and
then
reused
for
a
new
set
of
cyclohexane
hydrocarboxylation
experiments.
thoroughly washed, and then reused for a new set of cyclohexane hydrocarboxylation experiments.
The filtrate
filtratewas
wastested
tested
a new
reaction
(by addition
of reagents),
fresh reagents),
no oxidation
was
The
in in
a new
reaction
(by addition
of fresh
and no and
oxidation
was detected.
detected.
Figurethe
4 shows
the excellent
recyclability
of theAu/CX-ox-Na:
system Au/CX-ox-Na:
in the second,
third,
Figure
4 shows
excellent
recyclability
of the system
in the second,
third, fourth,
fourth,
fifth,
sixth,
and
seventh
run,
the
observed
activity
was
99.8%,
99.7%,
98.4%,
98.3%,
97.7%
and
fifth, sixth, and seventh run, the observed activity was 99.8%, 99.7%, 98.4%, 98.3%, 97.7% and 97.5%
of
97.5%
of
the
initial
one,
being
the
selectivity
maintained.
the initial one, being the selectivity maintained.
4. Effect
Figure 4.
Effect of
of the
the catalyst
catalyst recycling
recycling on the yield of cyclohexanecarboxylic acid obtained by
hydrocarboxylation of cyclohexane
cyclohexane catalyzed
catalyzed by
byAu/CX-ox-Na.
Au/CX-ox-Na. Reaction conditions:
conditions: cyclohexane
cyclohexane
8 8(1.50
2O2 O
(3.0
mL)/MeCN
(3.0(3.0
mL),
50
(1.00 mmol), p(CO)=
p(CO)= 22 atm,
atm, K
K22SS2O
(1.50mmol),
mmol),catalyst
catalyst(2(2μmol),
µmol),HH
(3.0
mL)/MeCN
mL),
2O
50
C, and
hours
in an
autoclave
(13.0
capacity).
°C,◦and
six six
hours
in an
autoclave
(13.0
mLmL
capacity).
3. Materials and Methods
3.1. Reagents
All the reagents and solvents were purchased from commercial
commercial sources
sources and
and used
used as
as received.
received.
The water used for
for all
all reactions
reactions and
and analyses
analyses was
was double
double distilled
distilled and
and deionised.
deionised.
3.2.
Carbon Materials
Materials Preparation
3.2. Carbon
Preparation
Carbon
a
Carbon xerogel
xerogel (CX)
(CX) was
wasprepared
preparedby
bypolycondensation
polycondensationofofresorcinol
resorcinoland
andformaldehyde,
formaldehyde,using
using
pH
ofof6,6,according
a pH
accordingtotoa aprocedure
proceduredescribed
describedelsewhere
elsewhere[29,30,34–37].
[29,30,34–37].ItItwas
wasused
usedin
in its
its original
original form
form
(CX),
oxidized
(-ox),
and
oxidized
with
nitric
acid
and
subsequently
treated
with
sodium
(CX), oxidized (-ox), and oxidized with nitric acid and subsequently treated with sodium hydroxide
hydroxide
(-ox-Na).
CX-ox was
was obtained
obtained by
by refluxing
refluxing CX
CX with
with 75
75 mL
mL of
of aa 55 M
(-ox-Na). CX-ox
M nitric
nitric acid
acid solution,
solution, per
per gram
gram of
of
carbon
material,
for
3
h,
then
separated
by
filtration
and
washed
with
deionized
water
until
neutral
carbon material, for 3 h, then separated by filtration and washed with deionized water until neutral
pH, similarly to what was reported earlier [29,35–37]. CX-ox-Na was obtained by treating CX-ox with
75 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.
Molecules 2017, 22, 603
8 of 12
pH, similarly to what was reported earlier [29,35–37]. CX-ox-Na was obtained by treating CX-ox with
75 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 N2 adsorption at 77 K in a Quantachrome Nova
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 ·3H2 O (Alfa Aesar, Karlsruhe, Germany),
in water, adding polyvinyl alcohol (Aldrich, Darmstadt, Germany) and NaBH4 (Aldrich), resulting
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
N2 flow for 3 h at 350 ◦ C (shown by elemental analysis to be efficient for this purpose), and then, the
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 DM = (6ns M)/(ρNdp ), where ns is the number of atoms at the
surface per unit area (1.15 × 1019 m−2 for Au), M is the molar mass of gold (196.97 g mol−1 ), ρ is the
density of gold (19.5 g cm−3 ), N is Avogadro’s number (6.023 × 1023 mol−1 ) and dp is the average
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 HNO3 and H2 SO4 mixture. The resulting solution was
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
9 of 12
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
600 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 (H2 O or NCMe) were also performed, to assess if the carboxylation reactions proceeded
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 H2 O/MeCN, under mild conditions (50 ◦ C, 2 atm of CO).
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-01-0145FEDER-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.
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Sample Availability: Samples of the compounds are not available from the authors.
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