Comparison of microwave and mechanochemical energy inputs in the catalytic oxidation of cyclohexane

Article abstract of DOI:10.1039/c8dt00866c

The effect of microwave and mechanochemical ball milling energy inputs was studied for the peroxidative oxidation (with aqueous H₂O₂) of cyclohexane to cyclohexanol and cyclohexanone, over CoCl₂ and/or V₂O₅

Full text of DOI:10.1039/c8dt00866c

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DOI: 10.1039/C8DT00866C.
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On the comparison of microwave and mechanochemical energy inputs in catalytic
oxidation of cyclohexane
a
a,b,*
c
a
c
Ana P.C. Ribeiro, Elisabete C.B.A. Alegria,
Maximilian N. Kopylovich, Ana M. Ferraria,
a,*
Ana M. Botelho do Rego, Armando J.L. Pombeiro
a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049ꢀ001 Lisboa, Portugal.
b
Chemical Engineering Departament, Instituto Superior de Engenharia de Lisboa, Instituto
Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1, 1959ꢀ007 Lisboa, Portugal.
c
Centro de QuímicaꢀFísica Molecular, Institute of Nanoscience and Nanotechnology and IBBꢀ
Institute for Bioengineering and Biosciences, Instituto Superior Técnico, Universidade de
Lisboa, 1049ꢀ001 Lisboa, Portugal
Abstract
The effect of microwave and mechanochemical ball milling energy inputs was studied for the
peroxidative oxidation (with aqueous H
cyclohexanone, over CoCl and/or V dispersed (ꢀm scale) catalysts. The maximum total
yield of cyclohexanol and cyclohexanone of 43% after 1 h reaction at 30 °C, in acetonitrile and
under microwave irradiation (5 W), was achieved over the CoCl ꢀV (3:1) catalyst prepared
2 2
O ) of cyclohexane to cyclohexanol and
2
2 5
O
2
2 5
O
by ball milling. Cyclohexanol is the main final product with selectivity up to 93% over
cyclohexanone. Conducting the oxidation reaction under microwave irradiation at the same
conditions but without any mechanochemical treatment of the catalyst prior to use resulted in a
lower total yield of 30% with a lower selectivity (69%) towards cyclohexanol over
cyclohexanone. The sole application of mechanochemical treatment for the catalyst preparation
and the catalytic oxidation of cyclohexane allowed to reach yields of 29% after 1 h of reaction,
at room temperature, without microwave irradiation and any additive and in the absence of any
organic solvent. Ball milling is shown to provide a convenient solventꢀfree method to disperse
those solid catalysts and to promote the above cyclohexane oxidation, although, in the latter
case, not so effectively as microwave irradiation.

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Keywords: Mechanochemistry, ball milling, microwave, solventꢀfree, composites, catalysts,
cyclohexane oxidation.
2

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1. Introduction
Application of alternative energy inputs, such as microwave or ultrasonic irradiation, is one of
the ways to perform effective chemical transformations under mild reaction conditions [1]. As
a result, a number of existing synthetic protocols can be significantly improved in comparison
to those performed under conventional heating. For instance, electromagnetic microwaves
(MW) with wavelengths of 1 mm – 1 m can directly transfer energy to polar intermediates or
catalysts thus improving the efficiency and decreasing the reaction time. However, the
utilization of the MW irradiation is not always advantageous due to its rather complicated
relation to the composition of the reaction mixture, reaction mechanism and a number of other
factors [2].
Reactions can also be promoted by applying mechanical energy, for example by ball milling
(BM). If mechanical grinding is under consideration, the elevated rates of the performed
reactions are related to high concentrations of the starting materials, interaction upon large
areas, formation of point defects and dislocations in the crystal lattices, breaking the order of
the crystalline structure, production of new surfaces and cracks, ionic exchange, etc. [1b]. At
the point of collision of the edges, the solids deform and melt, forming hot points where the
molecules reach high excitation states [1b]. However, the energy accumulated in the defects of
the crystalline structure can also lead to slower chemical transformations. Hence,
mechanochemical processes can significantly differ from those performed under other energy
inputs. For example, conventional heating of solid HgCl and AgCl leads to their melting and
subliming, while their milling results in metal recovery with Cl gas evolution [1b].
2
Although advantageous effects of alternative energy inputs are well recognized and reviewed
during last decade [1], an experimental comparison of microwave and mechanochemical
treatments has not yet been intentionally performed for the same catalytic reaction. Hence, we
decided to fill this blank and compare microwave and mechanochemical energy inputs in
catalytic oxidation of cyclohexane. We choose this reaction due to our longstanding interest in
such a transformation [3] and its importance in industry [4].
3

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2
2
. Experimental
.1. Materials
Vanadium oxide (V
2
5 2
O , Sigma Aldrich), cobalt chloride (CoCl , Sigma Aldrich), multiwalled
carbon nanotubes (MWCNTꢀHP 150, Baeyer Materials), acetonitrile (MeCN, Fisher
Chemical), cyclohexane (C , Sigma Aldrich), 30% aqueous solution of hydrogen peroxide
, Sigma Aldrich) were handled under normal ambient conditions without any purification
prior to use.
6
H
6
2 2
(H O
2.2. Preparation and characterization of dispersed catalysts
V
2
O
5
and/or CoCl with different molar ratios (total mass: 200 mg) were directly placed in a
2
PM100/200 Retsch GmbH planetary ball mill equipped with a 50 mL bowl and mechanically
grinded with 10 stainless steel balls of 10 mm diameter for 1 h similar to the procedures
described before [5]. Multiwalled carbon nanotubes (CNTs) additives (loadings from 0.1 to 5
wt% or ca. 100ꢀ1000 metalꢀperꢀcarbon molar ratios) were also added in some cases to prepare
catalytic composites [5a, 5c]. The milling frequency was 450 rpm, with rotational inversions
every 5 minutes. All the dispersed mixtures were prepared by dry milling. The dispersed
materials were characterized by scanning electron microscopy (SEM) using a JEOL 7001F
2 2 5
equipment. The composition of the (1:1) CoCl ꢀV O ball milled mixture was studied by Xꢀray
Photoelectron Spectroscopy (XPS) using a XSAM800 spectrometer from Kratos. The
characterization of each individual salt was reported previously [5a]. The operating and
recording conditions as well as details on the data treatment were as described in [5a]. The
sensitivity factors used for the mixture analysis were those of the software library when the
radiation source is the aluminum anode.
2.3. Catalytic studies
100 ꢁmol of catalyst (dispersed with or without ball milling treatment), 3 mL MeCN (solvent)
and 5 mmol (540 ꢀL) cyclohexane (substrate) were introduced into a pyrex tube. Subsequently,
0 mmol of a 30% aqueous hydrogen peroxide solution (oxidant) was added. For the
1
experiments under microwave radiation, the tube was sealed and placed in the microwave
reactor and the system was stirred under 5 W at 30 °C for 1 or 2 h. CAUTION: mixtures of
4

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H
2
O
2
with organic compounds are potentially explosive! At the end of the reaction, 10 mL of
diethyl ether (for the substrate and organic products extraction) and 90 ꢁL of cycloheptanone
internal standard) were added. The final mixture was stirred for 10 min and finally a sample (1
l) was taken from the organic phase and analyzed by GC. Following a method developed by
Shul'pin [6], an excess of triphenylphosphine (PPh ) was added prior to the GC analysis, to
reduce the cyclohexyl hydroperoxide formed to the corresponding alcohol and the hydrogen
(
ꢁ
3
peroxide
cyclohexanone/cyclohexanol
cyclohexanone/(cyclohexanol+cyclohexyl peroxide)
experiments were performed using different amounts of H
to
water.
Respectively,
the
reported
and
discussed
to
below
the
(
ONE/OL)
ratio
corresponds
[
ONE/(OL+ROOH)] ratio. Blank
and confirmed that no
2
O
2
cyclohexane oxidation product was obtained in the absence of a metal catalyst.
For performing the oxidation of cyclohexane exclusively under mechanochemical (ballꢀ
milling) treatment, the calculated amount of catalytic material was directly added to the
grinding bowl of the ball mill, whereafter the substrate (cyclohexane) and the oxidant (H O ,
2
2
30% aq. solution) were added and the reaction performed as described above, but without
addition of the NCMe solvent.
Gas chromatographic measurements were carried out using a FISONS Instruments GC 8000
series gas chromatograph with a FID detector and a capillary column (DBꢀWAX, column
length: 30 m; internal diameter: 0.32 mm) (He as the carrier gas) and the JascoꢀBorwin v.1.50
software. The temperature of injection was 240 °C. The initial temperature of the column was
maintained at 100 °C for 1 min, then raised 10 °C/min up to 180 °C and held at this
temperature for 1 min. The attribution of the peaks was made by comparison with
chromatograms of genuine samples.
3
3
. Results and Discussion
.1. Preparation and characterization of the catalysts
2 5 2
The dispersed catalytic materials were prepared by mechanical treatment of V O and/or CoCl
in a planetary ball reactor [7] as indicated above. The choice of those systems was performed
based on our previous studies [3,5] and literature data [2,4]. According to SEM images (Figure
1), the produced materials consist of nonꢀuniform microcrystallites with different shapes and
5

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2 2 5
particle sizes above 1 µm for the ball milled CoCl (Figure 1a), V O (Figure 1b), as well as
*
the CoCl ꢀV O (3:1) mixture (Figure 1c,d). More details on morphology and structure of the
2
2
5
prepared catalysts can be found in our previous papers devoted to the preparation and study of
related dispersed materials [5].
(a)
(b)
(c)
(d)
Fig. 1. SEM images: a) Ballꢀmilled CoCl
2 2 5
, 10 ꢀm scale; b) Ballꢀmilled V O , 100 ꢀm scale; c)
CoCl ꢀV (3:1), 100 µm scale; d) CoCl ꢀV O (3:1), 10 µm scale.
2
2
O
5
2 2 5
XPS shows that after ball milling the composition of each salt is nearly the same as before
mixing.
*
Here and further molar ratios are indicated.
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Cl 2p3/2
Cl 2p
Co 2p3/2
Co2+
3700
O 1s
11500
Cl 2p1/2
11800
V 2p
Satellites
2
700
700
V 2p3/2
9
500
500
6
800
1
V 2p1/2
7
1800
700
7
95 790 785 780 775
536 530 524 518 512
208 205 202 199 196 193
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
(a)
(b)
(c)
2 2 5
Fig. 2. CoCl ꢀV O (1:1) XPS regions (a) Co 2p3/2; (b) O 1s and V 2p; and (c) Cl 2p.
Figure 2 shows the relevant XPS regions of the ball milled mixture CoCl ꢀV O (1:1). In Table
2
2
5
1
, the atomic ratio Co/Cl > 0.5, suggests that the mixture is composed not only of cobalt
chloride, but also of some cobalt hydroxides and/or oxyhydroxides, as in CoCl [5a]. Actually,
the main peaks in Co 2p3/2, centred at 782.2 ± 0.2 eV and 783.9 ± 0.2 eV (Figure 2a), are both
2
2
+
assigned to Co in CoCl
different resistivities [5a]. Such effect, which was also observed for a CoCl
studied previously [5a], is also detected in the Cl 2p region: Cl 2p was fitted with two doublets
the main Cl 2p3/2 component being centred at ~200.3 eV) revealing the presence of some
2
and in Co(OH)
2
but, most probably, included in powder phases with
2
ꢀFe mixture
2 3
O
(
differential charge resulting from mixed phases with different conductivities and/or important
contact potential between powder particles. V 2p, detected next to O 1s, with V 2p3/2 centred at
517.6 eV ± 0.1 eV, confirms that the vanadium oxide is quite stable, keeping the stoichiometry
of the starting material (V O , [5a]) even after ball milling (Table 1). O 1s was fitted with three
2
5
peaks: the main peak, centred at 530.6 eV is assigned to oxygen in V
2 5
O (it is not detected in
CoCl [5a]); and two other peaks, at the high binding energy side, with low relative intensities,
2
centred at 532.4 and 534 eV, which are assigned to oxygen bound to cobalt (as attested by the
quantitative analysis) and to some oxidized carbonaceous contamination [8].
7

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Table 1. XPS quantitative results for the precursor salts (studied in [5a]) and the mixed
material CoCl ꢀV (1:1).
2
2 5
O
Co/Cl
0.8
O530.6/V
CoCl
2
V O5
2.5
2.4
2
CoCl
2
ꢀV
2
O
5
0.9
3
.2. Catalytic studies
To choose the most appropriate catalytic material for further comparative studies, several ballꢀ
milled CoCl ꢀV mixtures with different metal ratios (1:1, 3:1 and 1:3) and starting V and
CoCl samples (also dispersed in the ball mill under the same conditions) were screened for the
peroxidative microwaveꢀassisted oxidation of cyclohexane with H (30% aq. solution)
Table 2). CNTs were also added in several cases since our previous studies [5] demonstrated
2
2
O
5
2 5
O
2
2
O
2
(
that their addition can provide significant promoting effect. In all cases the formation of a
mixture of cyclohexyl hydroperoxide (CyOOH, primary product), cyclohexanol (CyOH) and
cyclohexanone (CyO) (Scheme 1a, Figure S1) was observed. The alcohol and ketone are the
major products upon metalꢀassisted decomposition of CyOOH, the formation of the alcohol
3
being promoted by reduction of CyOOH with triphenylphosphine (PPh ) (Scheme 1b) [6].
2 2
Scheme 1. Microwaveꢀassisted peroxidative (with H O ) oxidation of cyclohexane.
The CoCl ꢀV O 3:1 ballꢀmilled mixture showed remarkable activity under the established
2
2
5
experimental conditions, resulting in a 43% (in relation to the substrate) overall yield of
cyclohexanol and cyclohexanone with total turnover (TON) of 22 (22 moles of products per
mole of metal compound) (entry 2, Table 2) and a selectivity up to 93% for
(cyclohexanol+cyclohexyl peroxide), after 1 h of reaction at 30 °C and under microwave
irradiation (5 W). In the tested catalytic systems, the cyclohexanone/cyclohexanol ratio is in
8

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the range of 0.07ꢀ0.7 (Table 2) whereas in the industrial processes [4,9] the ratio generally falls
in the 0.5ꢀ0.7 range. The common high predominance of cyclohexanol over cyclohexanone in
our systems suggests the presence of cyclohexyl hydroperoxide in the reaction mixture as a
primary oxidation product of cyclohexane which, after treatment with PPh
3
(see experimental
part), gives the corresponding alcohol in a relatively large quantity [6] (Scheme 1b).
The use of a ball milled CoCl
relatively to the use of any starting component, from 1.3% for CoCl
3.3% for CoCl ꢀV (3:1) (entries 11, 10 and 2, respectively, Table 2). The achieved yield
for the CoCl ꢀV (3:1) mixture (43.3% overall yield), with a concomitant high selectivity, is
2
ꢀV
2
O
5
mixture (3:1) resulted in a marked yield increase
2
or 13.5% for V to
2 5
O
4
2
2 5
O
2
2 5
O
much higher than that (ca. 4ꢀ6%) of the industrial process [4,9] in spite of the mild reaction
conditions used and is comparable to those obtained [3h] in the presence of Fe(III)ꢀCr(III)
hydroxides (up to 45%). However, the latter catalytic system requires the use of a strong acid
(
HNO ) [3h]. The combination of such a considerable yield reached in a short reaction time (1
3
h) under mild conditions (30 ºC, 5 W) and without any additive is a rare example which points
out an outstanding activity of the catalytic system based on the CoCl
mixture prepared by ball milling treatment.
2
ꢀV
2
O
5
(3:1) binary
9

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2 2 5
Table 2. MWꢀassisted peroxidative oxidation of cyclohexane, over ball milled CoCl ꢀV O
a
mixtures, V
2
O
5
or CoCl
2
as catalysts.
c
Yield (%)
b
Entry
Catalyst
Ratio
ONE/(OL+ROOH)
d
OL+ROOH
ONE
4.5
Tot
1
2
3
4
5
6
7
1:1
3:1
1:3
ꢀ
6.5
40.3
3.4
0.7
7.0
6.0
8.5
11.0
43.3
4.9
0.7
0.07
0.4
0.4
0.1
0.1
0.7
CoCl
2
ꢀV
2
O
5
3.0
1.5
0.3
0.8
0.6
6.0
CoCl
2
ꢀ0.1% CNTs
1.0
CoCl
CoCl
2
2
ꢀ1% CNTs
ꢀ5% CNTs
ꢀ
7.8
ꢀ
6.6
(CoCl ꢀV O )ꢀ0.1%
3:1
14.5
2
2
5
CNTs
ꢀV )ꢀ1% CNTs
8
9
(CoCl
2
2
O
5
3:1
9.9
23.6
8.9
6.8
2.2
4.6
0.2
1.2
16.7
25.8
13.5
1.3
0.7
0.09
0.5
(CoCl ꢀV O )ꢀ5% CNTs
3:1
2
2
5
1
0
1
2
V
2
O
5
ꢀ
ꢀ
ꢀ
1
1
CoCl
2
1.1
0.2
CNTs
0.4
1.6
3.0
a
Reaction conditions (BM + MW): cyclohexane (5.0 mmol), 100 µmol of catalyst (2 mol% vs.
substrate, after ball mill treatment, BM), NCMe (3 mL), H 30% aq. solution (10 mmol), 1 h, 30 ºC,
microwave (MW) irradiation (5 W); yield and TON determined by gas chromatography upon treatment
2
O
2
b
c
3
with PPh (see text). Molar ratio between the components of the prepared catalyst mixture. Molar
yield (%) based on substrate, i.e. moles of products [cyclohexanol (OL) and cyclohexanone (ONE)] per
d
1
00 moles of cyclohexane. Total yield = moles of products [OL + ONE]/100 moles of C
6
H12. CNTs =
multiwalled carbon nanotubes.
The presence of CNT additives affects differently the catalytic activity of the studied systems
Table 2). Thus, for CoCl a yield improvement is observed when 1 and 5 wt% of CNTs have
(
2
been used, namely from 1.3% in the absence of CNTs (entry 11, Table 2) to 7.8 and 6.6%,
entries 5 and 6, Table 2, respectively). However, for the CoCl ꢀV systems the yield
(
2
2 5
O
decreases in the presence of CNTs (entries 7ꢀ9, Table 2). This hampering effect of the CNTs
contrasts with a few other cases where the presence of such additives can result in a significant
improvement in the catalytic performance [5].
1
0

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In order to compare the influence of MW and mechanochemical BM energy inputs, we have
chosen the most active catalytic material, i.e. the CoCl ꢀV (3:1) mixture. With this catalytic
2
2 5
O
material, some tests were carried out using the following combinations (Scheme 2): i)
preparation of the catalyst by ball milling (1 h) and its subsequent application in the
peroxidative oxidation of cyclohexane under MW irradiation (1h, 30 °C) (BM + MW); (ii)
conducting the oxidation reaction under MW irradiation without any BM treatment of the
catalyst before use (MW) and (iii) conducting the catalyst preparation and the oxidation
reaction by ball milling in one pot without MW irradiation (BM + BM). The results are
presented in Table 3.
Scheme 2. Comparison of microwave and mechanochemical energy inputs
1
1

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2 2 5
Table 3. Peroxidative oxidation of cyclohexane over CoCl ꢀV O (3:1) catalyst using MW and
a
BM treatment.
b
Yield (%)
Entry
Method
Reaction Time (h)
1
c
OL
ONE
TOTAL
d
1
BM + MW
40.3
3.0
43.3
e
2
MW
MW
1
2
20.9
29.1
9.5
30.4
39.9
e
3
10.8
f
4
BM + BM
1
2
1
1
1
22.4
26.6
0.2
6.2
5.5
0.1
0.7
0.4
28.6
32.1
0.3
f
5
BM + BM
g
6
BM
BM
ꢀ
h
7
5.9
6.6
i
8
0.7
1.1
a
Reaction conditions: 5 mmol of substrate, 100 ꢀmol metal composite (2 mol% vs. substrate), NCMe
(
3 mL) (only for the BM + MW and MW methods, entries 1ꢀ3), 10 mmol of H O (aq. 30%), 30 ºC,
2
2
b
microwave irradiation (5 W). Molar yield (%) based on substrate, i.e. moles of product per 100 mol of
c
substrate determined by GC. Total Yield = moles of products [cyclohexanol (OL) + cyclohexanone
d
(
ONE)]/100 mol of C
6
H
12, determined by GC after treatment with PPh
3
. BM+MW = preparation of the
e
catalyst with ball milling treatment (BM) and catalysis under MWꢀirradiation. MW = preparation of
the catalyst without ball mill treatment, by manual grinding, and used directly in the peroxidative
f
oxidation under MWꢀirradiation. BM+BM = catalyst preparation and catalytic reaction exclusively by
g
ball milling treatment, without MWꢀirradiation and without NCMe solvent. Oxidation reaction
h
performed by ball milling without any catalyst. Catalytic reaction performed with ball milled catalyst
i
and without microwave irradiation. Catalytic reaction performed with catalyst mixture used without
previous ball milling treatment (mixed manually) and without microwave irradiation.
The first method (BM + MW), that is the one in which the catalyst is prepared by ball milling
and then used in the peroxidative oxidation of cyclohexane under MW irradiation results in the
highest overall product yield (43%, entry 1, Table 3) with the maximum 93% selectivity
towards cyclohexanol over cyclohexanone. Conducting the oxidation reaction under MW
irradiation under the same conditions but without any ball milling treatment of the catalyst
1
2

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before use (MW method) gives a lower total yield of 30.4% with a 69% selectivity towards
cyclohexanol over cyclohexanone (entry 2, Table 3).
The results obtained in the third procedure (BM + BM) (entries 4 and 5, Table 3) are rather
interesting, since the catalyst preparation and the catalytic reaction were assisted only by ball
milling, without any additive and in the absence of any organic solvent, leading to high overall
yields of 28.6 and 32.1% after 1 or 2 h of reaction, respectively, at room temperature.
Additionally, the oxidation reactions were performed in the absence of microwave irradiation
(
entries 6ꢀ8, Table 3) using catalytic mixtures with (entry 6, Table 3) and without (entry 8,
Table 3) previous ball milling treatment. In both cases, the yields were quite low (6.6 and
.1%, respectively), demonstrating once again the importance of MW irradiation in the
1
catalytic reaction and the advantage of using ball milled catalytic mixtures as catalysts. The
oxidation of cyclohexane was also performed exclusively by ball milling without addition of
metal composites and residual amounts of product were detected (entry 7, Table 3).
Among the reported vanadium or cobalt metal catalysts applied for alkane oxidations with
2 2
H O , the achieved herein product yields are remarkable [3,10]. Besides that, the catalytic
systems under study involve greener reaction conditions, since they do not involve any additive
or organic solvent (in the case of the sole use of mechanochemical treatment, BM + BM
method) and perform well at room temperature in a short reaction time (1 h). To compare, the
binuclear oxidovanadium(V) complex bearing an NO
2
ꢀdonor Schiff base ligand
bis(2ꢀ
2
2
[
{VO(EtO)(EtOH)}
2
(1κ O,κN:2κ O,κNꢀL)]·2H
2
O
(H
4
L
=
hydroxybenzylidene)oxalohydrazonic acid), a homogeneous catalyst towards the oxidation of
gaseous and liquid alkanes by aqueous H under mild conditions, leads to 27.6% yield of
2
O
2
oxygenates (cyclohexanol + cyclohexone) in the presence of the acid coꢀcatalyst 2ꢀ
pyrazinecarboxylic acid (PCA), using H O (50% aqueous solution) as oxidant, in acetonitrile,
2
2
at 50ºC and 4 h of reaction [11].
4
. Conclusions
Cobalt and/or vanadium dispersed catalytic materials were prepared by a simple and solventꢀ
free mechanochemical method, i.e., ballꢀmilling. The prepared heterometallic systems act as
efficient catalysts for the peroxidative oxidation (with aq. H
2 2
O ) of cyclohexane to
cyclohexanol and cyclohexanone, under mild conditions. The microwave and
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mechanochemical energy inputs were compared and it was shown that the most effective
combination comprises the catalytic reaction performed under microwave irradiation and using
the CoCl ꢀV O (3:1) catalyst prepared mechanochemically. However, the catalyst preparation
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and the catalytic reaction can be achieved in oneꢀpot at room temperature and by using
exclusively the mechanochemical BM energy input with considerable yields of 29 and 32%
after 1 and 2 h, respectively. Hence, mechanochemical methods appear to be quite promising in
the field of alkane functionalization and deserve to be further explored for a variety of catalytic
systems and reactions.
Acknowledgments
Financial support from the Fundação para a Ciência e a Tecnologia, Portugal (fellowships
SFRH/BPD/90883/2012 to A.P.C.R., SFRH/BPD/108338/2015 to A.M.F. and the
"
Investigador 2013" contract to M.N.K. respective IF/01270/2013/CP1163/CT0007 project, the
UID/QUI/00100/2013, PTDC/QEQꢀQIN/3967/2014 and UID/NAN/50024/2013 projects) and
from Instituto Politécnico de Lisboa (MechSynCat ProjectꢀConcurso Anual de IDI&CA,
710044/2016) is acknowledged.
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