Activity of continuous flow synthesized Pd-based nanocatalysts in the flow hydroconversion of furfural

Article abstract of DOI:10.1016/j.tet.2017.02.056

A number of synthesized supported Pd nanomaterials on aluminosilicate supports were investigated in the continuous flow hydroconversion of furfural to a range of furanic derivatives as compared to a commercial Pd/C catalyst. Excellent furfural conversions (>60%) and varying selectivities to products at mild temperatures (90 °C) could be obtained for most catalytic systems, with interesting selectivities to furfuryl alcohol (FOL), 2-methylfuran (MF) and tetrahydrofurfuryl alcohol (THFOL) depending on type of Pd catalyst. A significant deactivation with time on stream was observed for continuous flow synthesized nanomaterials with respect to commercial 5%Pd/C.

Full text of DOI:10.1016/j.tet.2017.02.056

Accepted Manuscript
Activity of continuous flow synthesized Pd-based nanocatalysts in the flow
hydroconversion of furfural
Antonio J. Garcia-Olmo, Alfonso Yepez, Alina M. Balu, Pepijn Prinsen, Araceli Garcia,
Audrey Maziere, Christophe Len, Rafael Luque
PII:
S0040-4020(17)30204-1
10.1016/j.tet.2017.02.056
TET 28497
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 19 December 2016
Revised Date: 22 February 2017
Accepted Date: 24 February 2017
Please cite this article as: Garcia-Olmo AJ, Yepez A, Balu AM, Prinsen P, Garcia A, Maziere A, Len C,
Luque R, Activity of continuous flow synthesized Pd-based nanocatalysts in the flow hydroconversion of
furfural, Tetrahedron (2017), doi: 10.1016/j.tet.2017.02.056.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Activity of continuous flow synthesized Pd-based nanocatalysts in the flow
hydroconversion of furfural
Antonio J. Garcia-Olmoa, Alfonso Yepeza, Alina M. Balua, Pepijn Prinsena, Araceli
Garciaa, Audrey Maziereb, Christophe Lenb, Rafael Luquea, b*
aDepartamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales,
Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 306, E14014, Cordoba (Spain), e-mail:
q62alsor@uco.es
bSorbonne Universités, Université de Technologie de Compiègne (UTC), EA 4297 UTC-
ESCOM, CS 60319, 60203 Compiègne Cedex, France
Abstract
A number of synthesized supported Pd nanomaterials on aluminosilicate supports were
investigated in the continuous flow hydroconversion of furfural to a range of furanic
derivatives as compared to a commercial Pd/C catalyst. Excellent furfural conversions
(>60%) and varying selectivities to products at mild temperatures (90ºC) could be
obtained for most catalytic systems, with interesting selectivities to furfuryl alcohol
(FOL), 2-methylfuran (MF) and tetrahydrofurfuryl alcohol (THFOL) depending on type
of Pd catalyst. A significant deactivation with time on stream was observed for
continuous flow synthesized nanomaterials with respect to commercial 5%Pd/C.
Keywords: furfural conversion; flow chemistry; Pd; nanoparticles; hydrogenation;
heterogeneous catalysis
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Introduction
The catalytic conversion of biomass-derived feedstocks to useful chemical,
materials and fuels became a highly relevant topic in recent years due to the possibilities
to switch towards a more sustainable bio-based economy different from the
petroleum-oriented approach [1]. Furfural is one of such top value-added biomass
derived chemicals from lignocellulosic biorefineries, generally produced from sugars
via dehydration [2]. From the chemical point of view, the advantage of furfural as
platform chemical relates to the possibility of its wide range of transformations into
chemicals, fuels and materials that include several valuable compounds such as biofuel
precursors (gamma-valerolactone, methyl tetrahydrofuran) to chemicals and materials
(furfural alcohol, FOL) as recently highlighted in some excellent reports [3, 4-10].
The hydroconversion of furfural into different high added value chemicals and
fuel precursors attracted a particular number of research efforts in recent years [2-13]. In
these reports, a number of homogeneous and heterogeneous catalytic systems were
disclosed for the production of FOL, 2-methylfuran (MF) and related products under
different reaction conditions (mostly batch type of chemistries, or vapor phase flow
processes at relatively high temperatures) [12-18].
Liquid phase continuous flow processes have been reported to provide several
relevant advantages in studies related to the conversion of biomass-platform molecules
to high added value chemicals (e.g. levulinic acid to gamma-valerolactone and 2-
MTHF) [19]. These included the possibility to follow reaction pathways and even fine-
tuning reactivity for a number of transition and noble metal heterogeneous catalysts to
direct the conversion of levulinic acid to pentanoic acid or alternatively to 2-MTHF
[19]. Most importantly, the stability of the catalysts under typical biomass conversion
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conditions (i.e. aqueous solution, moderate to high temperatures, presence of acid, etc.)
could be tested as well as catalyst deactivation into the reaction mixture, to propose
alternatives for the preparation increasingly active and stable heterogeneous systems
[20-23]. Further to this, the preparation and design of heterogeneous catalysts under
continuous flow conditions has been of particular relevance in our group in recent times
[24, 25]. In this regard, we have designed different catalytically active supported
nanoparticles on porous materials for a number of redox and acid-base chemistries
under microwave assisted irradiation and conventional heating (batch). However, the
stability of such nanocatalysts was never investigated to date in any catalytic processes
under continuous flow conditions.
Based on these premises, this contribution was aimed to investigate the activity
and stability of continuous flow synthesized Pd-containing aluminosilicates in the liquid
phase flow hydroconversion of furfural at low temperatures (80-90oC). The activity of
synthesized Pd-containing materials was compared with respect to that of a commercial
catalytic system (5% Pd/C). Different reaction conditions were screened and optimised
for which times of reaction, selectivity to products and catalyst stability were tested in
view of a potential translation of optimized results to the conversion of furanic waste
streams.
Experimental
Continuous flow synthesis of catalysts
A simple approach for the design of flow synthesis of the nanomaterials
employed in this work has been previously reported [25]. Firstly, a preformed parent
support (Al-SBA-15 or Al/Zn-SBA-15) was synthesized using a modification of the
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protocol previously reported by our group [26]. In the case of AlZn-SBA-15, the
synthesis of the support was also previously reported [27]. The final supports were then
filtered off, dried overnight at RT, and calcined in a N2 atmosphere at 600ºC for 2 h,
then in air for an additional 6 h. From the support, the continuous flow preparation of
the different catalyst was conducted in an innovative continuous flow setup [25] as
follows:
A stainless-steel reactor (1.2 mL volume) was packed with the SBA support for
all catalysts preparation (typically 0.5 g) set between two plugs of quartz wool to
prevent the solid support from moving in the reactor upon pumping in the flow of the
solution of the metal precursor. Separately, different solutions were prepared by using
Pd acetate (0.83 mL in 50 mL ethyl acetate) as Pd precursor to achieve a theoretical 1
wt.% Pd loading in the final material (with the exception of a 2 wt.% Pd/AlSBA). The
solution was filtered off prior to flowing it through the system to prevent the presence of
any undissolved metal precursor. The system started with pumping a solution of pure
ethanol (0.5 mL min-1, 5 min) through the catalyst bed to wet the catalyst. The
temperature of the reactor was set to 100ºC (optimum temperature [25]) and the system
was then changed to the Pd solution, which was pumped through at an optimum flow
rate of 0.3 mL min-1. At the end of the continuous flow process, the system was
switched back to ethanol, which was pumped through the Pd-incorporated catalyst at
100ºC to remove physisorbed and unreacted Pd species on the catalyst.
Pd-containing systems were denoted as Pd/AlSBA, Pd/AlZnSBA, PdFe/AlSBA
and Pd/AlSBAred. Pd/AlSBA stands for a 0.9%Pd/Al-SBA-15 material prepared under
flow conditions, while similarly Pd/AlZnSBA corresponds to 0.5%Pd/AlZnSBA-15
containing 0.3 wt.%Zn. Pd/AlSBAred corresponded to a higher loaded material (1.9
wt.% Pd) on Al-SBA-15 that underwent a reduction under hydrogen atmosphere (50 mL
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min-1, 400ºC, 2 h) prior to the reaction. Last, a similarly loaded unreduced PdFe/AlSBA
material contains 0.8 wt% Pd as well as 2.5 wt.% Fe on the parent Al-SBA-15. In this
way, four different catalytic systems were prepared to investigate and compare the
effect of 1) Pd loading; 2) presence of additional metals (i.e. Zn, Fe) and 3) catalyst
conditioning (e.g. reduction pre-reaction, for the activity of Pd2+ vs Pd0 species). All
these synthesized nanomaterials were characterized using various techniques and
compared to a commercial 5%Pd/C catalyst from Alfa Aesar, containing 4.93 wt% Pd
with a metal area of 20 m2 g-1.
Materials characterization
Materials were characterized by using powder N2 physisorption, X-ray
diffraction, and ICP-MS analyses.
N2 adsorption measurements were performed at 77 K by using a Micromeritics
ASAP 2000 volumetric adsorption analyzer. The samples were outgassed for 24 h at
1008C under vacuum (P0=10-2 Pa) and subsequently analyzed. The linear part of the
BET equation (relative pressure 0.05–0.30) was used for the determination of the
specific surface area. Mean pore size diameter and pore volumes were obtained from
porosimetry data by using the BJH method.
Wide-angle X-ray diffraction experiments were recorded on
a
Pan-Analytic/Philips X’pert MRD diffractometer (40 kV, 30 mA) with CuKa
(l=0.15418 nm) radiation. Scans were performed over a 2q=10–80º at step size of
0.0188 with a counting time per step of 5 s.
XPS measurements were performed in an ultra high vacuum (UHV)
multipurpose surface analysis system (Specst model, Berlin, Germany) operating at
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pressures <10−10 mbar using a conventional X-ray source (XR-50, Specs, Mg-Ka,
1253.6 eV) in a “stopand-go” mode to reduce potential damage due to simple
irradiation. The survey and detailed Fe and Cu high-resolution spectra (pass energy 25
and 10 eV, step size 1 and 0.1 eV, respectively) were recorded at room temperature
using a Phoibos 150-MCD energy analyser (Phoibos, Berlin, Germany). Powdered
samples were deposited on a sample holder using double-sided adhesive tape and
subsequently evacuated under vacuum (<10−6 Torr) overnight. Eventually, the sample
holder containing the degassed sample was transferred to the analysis chamber for XPS
studies. Binding energies were referenced to the C1s line at 284.6 eV from adventitious
carbon. Deconvolution curves for the XPS spectra were obtained using the Casa XPS
software.
The metal content in the materials was determined by using ICP in a Philips PU
70000 sequential spectrometer equipped with an Echelle monochromator (0.0075 nm
resolution) and coupled to a mass spectrometer. Samples were digested in HNO3 and
subsequently analyzed by using ICP-MS at the SCAI of Universidad de Cordoba.
Continuous flow hydroconversion of furfural
Reactions were conducted in an H-Cube mini flow hydrogenation instrument
from ThalesNano (Figure 1). A 0.2 M furfural solution in ethyl acetate was prepared
using furfural (0.83 mL furfural in 50 mL ethyl acetate) as starting material. The
solution is filtered off prior to flowing it through the system to avoid the presence of
any undissolved species. The system starts with pumping a solution of pure methanol as
washing solvent (0.3 mL min-1, 15 min) through the system. Subsequently, methanol is
changed to the solvent used in the reaction, in this case ethyl acetate (0.3 mL min-1,
10 min) and the system is set to the desired reaction temperature (90 ºC) and hydrogen
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pressure (50 bar), as optimum reaction conditions. When the desired parameters are
achieved, a 0.2 M furfural solution is pumped through the system in order to carry out
the continuous flow hydroconversion of furfural. As the reaction is running, several
samples were taken at different times for subsequent analysis and check the catalyst
deactivation in time under the investigated reaction conditions. The furfural solution
(brownish) gradually turns into colorless in the course of the reaction.
At the end of the continuous flow process, the system is again switched to ethyl
acetate (0.3 mL min-1, 10 min) in order to remove unreacted furfural species or different
products of reaction and the reaction parameters (temperature and pressure) are
switched off. Eventually, the system is washed again with methanol (0.3 mL min-1,
20 min). Based on the dead volume of the reactor, 20 min of reaction is equivalent to
approximately 60 seconds of residence time of the furfural solution in the catalyst.
Figure 1. Flow hydroconversion of furfural using the H-Cube mini system. Reproduced
by permission of the Royal Society of Chemistry.
The continuous flow hydroconversion of furfural was conducted by passing
hydrogen generated in-situ in the H-Cube equipment from water electrolysis and the
flow solution through a packed column containing different solid catalysts (CatCart®)
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as illustrated in Figure 1. All catalysts were packed in our group (weight of material
packed ca. 0.2 g per cartdrige) in 30 mm long catcarts, compatible with the H-Cube
mini
system,
using
the
catcart
packer
from
Thalesnano
(http://thalesnano.com/products/CatCarts_and_compatible_systems/catcart-packer).
Collected samples were analyzed by GC and GC MS using an Agilent 5890N
fitted with a SUPELCO EQUITY TM-1 fused silica capillary column (60 m × 0.25 mm
× 0.25 µm) and a flame ionisation detector (FID). The identity of the products was
confirmed by GC MS at the SCAI of Universidad de Cordoba. The experimental error
in all data provided is within typical GC errors (ca. 5-10%) and carbon balances of all
reactions conducted in this work are > 95%.
Results and discussion
Materials were characterized by a number of techniques including XRD (low
and wide angle), N2 physisorption ICP-MS (for metal content) and SEM/TEM. Figure 1
depicts representative low/wide angle XRD patterns of Pd/AlSBA materials which
exhibited the typical diffraction lines of the SBA-15 structure, in good agreement with
previous reports from the group [26, 28]. For the particular example of unreduced Pd
systems (Figure 1, right plot), a good match with a palladinite (PdO, COD 1009031)
phase was observed in the materials as compared to the clear presence of Pd0 for
Pd/AlSBAred samples (Figure 2, fresh and used). In any case, all samples exhibited
similar XRD patterns, with only minor differences in terms of line intensity.
Both Al-SBA-15 and AlZn-SBA-15 supports exhibited a high surface area
(>900 m2 g-1), which was significantly reduced upon Pd incorporation as shown in
Table 1. This phenomenon is clearly related to the incorporation of Pd on the surface of
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the mesoporous aluminosilicate as well as most importantly the flow synthetic protocol
that seems to deteriorate the external surface of the materials, hence reducing their
surface areas [24]. Nevertheless, despite such decrease in surface area, the structural
quality of the materials was preserved as illustrated for AlZnSBA materials (Figure 3)
as well as even after their use in the catalytic reaction, despite a reduction of over 300
m2 g-1 in surface area after Pd incorporation.
1
2
3
4
5
20
40
60
80
2θ
2θ
Figure 1. Low angle (left image) and wide angle (right image) XRD patterns of
Pd/AlSBA material (0.9 wt.% Pd). Wide angle patterns show a good correspondence
with a PdO phase (Palladinite, COD 1009031).
1%Pd/Al-SBA reduced used
1%Pd/Al-SBA reduced fresh
Pd palladium
20
40
60
80
2 θ
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Figure 2. Wide angle XRD pattern of Pd/AlSBAred materials (fresh and reused), clearly
showing the presence of zerovalent Pd in the materials.
1000
800
600
400
200
0
1000
800
600
400
200
0
AlZnSBA
Pd/AlSBA fresh
Pd/AlSBA used
Pd/AlZnSBA
0,8
0,0
0,2
0,4
0,6
0,8
1,0
0,0
0,2
0,4
0,6
1,0
Relative Pressure (P/P
)
0
Relative pressure (P/Po)
Figure 3. Representative N2 physisorption isotherms of parent AlZn-SBA-15 as
compared to Pd/AlZnSBA (left) and Pd/AlSBA materials (fresh and used).
Table 1. Textural properties of synthesized Pd-containing materials
Material
Surface area Pore size Pore Volume Pd
content, Other metals
(m2/g-1)
756
912
370
642
430
363
20
(nm)
8.9
9.2
8.0
9.0
8.6
8.1
-
(mL g-1)
0.72
1.25
0.48
0.94
0.62
0.24
-
wt.%, ICP-MS Wt.%, ICP-MS
Al-SBA-15
AlZn-SBA-15
Pd/AlSBA
-
1.34 (Al)
-
0.18 (Zn)
0.9
0.5
0.8
1.0
4.9
-
Pd/AlZnSBA
PdFe/AlSBA
Pd/AlSBAred
5%Pd/C
0.15 (Zn)
2.5 (Fe)
-
-
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Average mesopore diameters were estimated from the adsorption branch of the nitrogen isotherm using
the BJH method. BET surface area was estimated by the multipoint BET method using the adsorption
data. Mesopore volume from the isotherms at a relative pressure of 0.95.
Importantly, pore sizes were not significantly affected by the metal incorporation
under the flow approach which obviously relate to the low metal content but also to the
inherent surface nature of the flow protocol (with minor migration of Pd particles to the
pores of the materials) [25]. Metal loadings measured by ICP-MS were also included on
Table 1, illustrating a general good match between theoretical aimed Pd loading (1 wt.%
for all materials, except 2 wt.% for Pd/AlSBAred) for comparative purposes, with the
exception of Pd/AlZnSBA for which a reduced Pd loading (0.5 wt.%) was observed.
We have previously reported a similar behavior for Fe-containing catalysts in which the
presence of Zn in the materials could have an effect on metal incorporation in flow [27],
although this result was not completely understood.
After characterization, Pd-containing materials were tested in the continuous
flow hydroconversion of furfural under optimised reaction conditions. Table 2 shows
moderate to good catalytic activities for most Pd-systems achieved at short residence
times (typically 35 seconds, 20 minutes reaction) achieved at 90oC and 50 bar hydrogen
pressure. A maximum conversion of ca. 90% was achieved for Pd/AlSBAred
(1.9 wt.% Pd) as compared to a conversion of 68% achieved for Pd/AlSBA (0.9 wt.%
Pd) after 20 minutes of reaction in flow. Comparatively, lower conversion was observed
for Pd/AlZnSBA (43%, associated to a lower Pd content) and surprisingly for the
PdFe/AlSBA material, for which Fe content did seem to have a detrimental effect on
activity. An almost quantitative conversion was obtained for the commercial 5%Pd/C
catalyst containing 5-10 times higher Pd loading as compared to the continuous flow
synthesized systems.
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Interestingly, the selectivity to products was also rather different between
catalysts, with two main products FOL and THFOL observed for all continuous flow
synthesized materials as compared to FOL and other products (2-methylfuran and
pentenols) observed for commercial 5%Pd/C.
The presence of additional metals in the materials did seem to have a significant
effect both on activity and selectivity to products. Pd/AlSBA provided a major
selectivity (63%) to THFOL after complete hydrogenation of both the carbonyl group
and the furan ring, promoted by the presence of Pd(0) in the catalyst [14]. However, the
presence of Zn in Pd/AlZnSBA (even at low loadings and lower conversion in the
systems) seemed to favor a more selective reduction of the pendant carbonyl to generate
FOL as major product (67% selectivity). The observed selectivity switch may be due to
the more metallic character of Pd in Pd/AlZnSBA via partial interaction with Zn in the
materials. Interestingly, the co-addition of Fe with Pd in the continuous flow synthesis
seemed to have a remarkable detrimental effect in the final PdFe/AlSBA system, for
which a poor 20% conversion was obtained (76% FOL selectivity).
However, all continuous flow synthesized systems, previously reported to be
stable under batch reaction conditions for the case of similar Fe-containing materials
[24], were not particularly stable under the investigated flow conditions, with a
complete deactivation after 1 h on stream (ca. 100 seconds residence time of the feed in
the catalyst). Comparatively, the commercial 5%Pd/C experienced a moderate
deactivation (>99 to ca. 52% conversion at longer reaction times) under otherwise
identical reaction conditions. Importantly, TOF values calculated for the different
catalytic systems seem to follow a similar trend, despite being quite low, and
remarkably inproved in flow synthesized materials as compared to the commercial Pd/C
system under the investigated conditions. An interesting behavior was also observed for
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5%Pd/C with a remarkable change in selectivity from 2-MF as main product to FA
(Table 2).
Table 2. Catalytic activity of various Pd-containing systems in the continuous flow
hydroconversion of furfural
Selectivity (mol %)
FOL THFOL
TOFb
min-1
Time Conversion
Material
(min)
(mol %)
Pd/AlSBA
20
60
68
<10
<5
43
<5
-
37
63
2.4
<0.1
-
>99
>99
67
120
20
-
33
-
2.7
-
Pd/AlZnSBA
60
>99
-
120
20
-
-
FePd/AlSBA
20
91
89
<10
99
77
80
52
76
24
>99
>95
-
0.7
3.2
1.0
-
Pd/Al-SBAred
20
-
60
<5
>99
120
20
a
a
5% Pd/C
(commercial)
-
-
0.4
0.1
<0.1
<0.1
a
60
30
80
-
a
120
720
-
>95
-
Reaction conditions: 0.2 M furfural in ethyl acetate, 0.3mL/min, 90ºC, 50 bar,
Hydrogen ON. 20 minutes of reaction time equals to ca. 35 seconds of residence time of
the feed in the catalyst.a a high selectivity to other products was observed including
2-methyl furan (MF) and pentenols (ring opening products).bTOF values calculated
as (# molecules reacted)/(# active sites-Pd-)(unit time, min), where unit time stands for
the residence time of the furfural solution in the catalyst.
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Characterisation of the catalysts was performed after the continuous flow
catalytic reaction to ascertain potential causes of the observed deactivation in the
systems. XPS data clearly indicated a significant reduction in the areas of the Pd signal
possibly due to leaching observed in the materials in the course of the reaction, in good
agreement with leaching studies which show most Pd was lost in the materials (>70% of
total Pd content in all cases leached into solution). We believe the significant leaching
observed for these nanocatalysts is related to the synthetic protocol (flow preparation
[24]) and the inherent external surface deposition of Pd which is washed away under the
investigated continuous flow conditions. Further to these, Nitrogen physisorption
measurements, XRD and related characterization techniques (Figure 2, Figure 3, right)
indicated a good preservation of the mesoporous aluminosilicates after reaction, which
in good agreement with ICP-MS, point to leaching as the main reason for deactivation
in the continuous flow synthesized nanomaterials.
Pd3d
2
x 10
190
180
170
160
150
140
130
120
110
100
Pd (II)
Pd/AlSBA (used)
Pd (II)
Pd3d5/2
Pd (II)
Pd3d3/2
Pd (II)
Pd/AlSBA (fresh)
Pd (IV)
Pd (IV)
340
348
344
336
332
Binding Energy (eV)
Figure 4. XPS of Pd/AlSBA (fresh) and used catalyst in the continuous flow reaction.
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According to XRD data, the catalysts seem to be partially (or completely)
reduced after reaction, something that could not be fully supported by XPS results from
used catalysts (Figure 4, also results for other catalysts) due to the very weak bands
obtained for Pd in the used nanocatalysts (after reaction). Comparatively, the used
5%Pd/C system showed low leaching under the investigated short reaction conditions
(although deactivation of ca. 50% takes place at longer times of reaction) and a clear
reduction of Pd species from Pd (II) to Pd (0), see Figure 5.
Pd3d
Pd (II)
Pd (II)
Pd (0)
Pd (0)
Pd (0)
Pd (0)
Pd (II)
Pd (II)
360
355
350
345
340
335
330
325
Binding Energy (eV)
Figure 5. XPS data of 5%Pd/C fresh (before reaction, top spectra) vs 5%Pd/C used
(after 2 h on stream, bottom spectra). Clear differences in Pd (II) vs Pd (0) species can
be observed for both catalysts.
Pd (0) species favoured the hydrogenation of the carbonyl group to FA at low
temperatures, supporting the reduction in the hydrogenolytic capacity (FA to 2-MF)
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with time on stream of the system due to a reduction of Pd (II) to Pd (0) under the
investigated flow conditions.
Conclusions
A study of the catalytic activity and deactivation of continuous flow synthesized
Pd-containing aluminosilicates proved that catalysts were nicely active (TOF values) in
comparison to a commercial 5% Pd/C system containing over 5-10 times higher Pd
content. However, synthesized Pd systems exhibited a poor stability under the
investigated conditions, combining leaching and sintering, with respect to a highly
active and stable commercial 5%Pd/C which deactivates moderately under the
investigated reaction conditions. This deactivation was found to be more severe at
longer times of reaction, with 5%Pd/C losing up to ca. 50% of its initial activity.
Based on these findings, continuous flow designed nanomaterials do not seem
optimum for the proposed flow hydroconversion reaction, for which alternative systems
currently under development in our laboratories will be designed and reported in the
future.
Acknowledgments
R.L. gratefully acknowledges support from Consejeria de Ciencia e Innovacion, Junta
de Andalucia under project P10-FQM-6711. A.J. G-O gratefully acknowledges COST
Action FP1306 for financial support under an STSM (COST-STSM-FP1306-33358) at
the group of C.L. in UTC in 2016. R.L. gratefully acknowledges support from Sorbonne
Universites for a visiting Professorship at UTC Compiegne in 2016.
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