Functionalization of A3B-type porphyrin with Fe3O4 MNPs. Supramolecular assemblies, gas sensor and catalytic applications

Article abstract of DOI:10.1016/j.cattod.2017.01.014

The aim of this study was to synthesize different multifunctional materials containing 5-(4-carboxyphenyl)-5,10,15-tris(4-phenoxyphenyl)-porphyrin and iron oxide magnetic nanoparticles (MNPs). Besides, a new type of polymeric material comprising the same

Full text of DOI:10.1016/j.cattod.2017.01.014

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Functionalization of A3B-type porphyrin with Fe3O4 MNPs.
Supramolecular assemblies, gas sensor and catalytic applications
Carmen A. Maka,∗, Miquel A. Pericasa,b, Eugenia Fagadar-Cosmac
a Institute of Chemical Research of Catalonia, Av. Països Catalans, 16, 43007, Tarragona, Spain
b Department de Química Organica, Universitat de Barcelona, c/Martí i Franques 1-11, 08028, Barcelona, Spain
c Institute of Chemistry Timisoara of Romanian Academy, M. Viteazul Ave. 24, 300223, Timisoara, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
The aim of this study was to synthesize different multifunctional materials containing 5-(4-
carboxyphenyl)-5,10,15-tris(4-phenoxyphenyl)-porphyrin and iron oxide magnetic nanoparticles
(MNPs). Besides, a new type of polymeric material comprising the same porphyrin and a polysaccha-
ride, k-carrageenan, was also developed. All synthesized materials were characterized by FT-IR, TEM,
XRD, 1H NMR, HPLC, UV–vis and fluorescence spectroscopy and tested as catalysts for the Knoevenagel
condensation of aldehydes at room temperature. The best performing porphyrin–based catalyst, which
also contain MNPs and a silica linker, was additionally tested for its capacity to detect CO2. All synthesized
materials tested as catalysts for the above mentioned reaction proved comparable efficiency providing
the desired adducts in short reaction times and with high yields. The novel designed catalytic systems
met the sustainable chemistry requirements because they can be recovered and several times reused.
© 2017 Elsevier B.V. All rights reserved.
Received 14 July 2016
Received in revised form
19 December 2016
Accepted 9 January 2017
Available online xxx
Keywords:
Porphyrin
Magnetic nanoparticles
Optical sensor
Knoevenagel condensation
1. Introduction
ual components can enhance a desired optical, electrical or redox
property of the overall material [17–23].
One of the major demands of sustainable chemistry regarding
catalysis is that the novel designed catalytic systems should be
finally recovered in high purity and reused in the process. For this
reason, magnetic nanoparticles, which present excellent properties
as heterogeneous catalytic supports, have already been used for dif-
ferent porphyrinic compounds immobilization [1]. These types of
organic-inorganic hybrid nanomaterials have been widely used in
recent years, particularly in biomedicine, as carriers being able to
easy penetrate the cell of living tissues [2–4].
Studies on synthetic porphyrins have increased significantly in
recent years due to the many versatile optoelectronic and redox
properties which makes them suitable candidates for a wide range
of technical [5] and medical applications [6] and also for their
supramolecular chemistry [7]. For this reason, they have been
used as photosensitizers or imaging agents in medicinal chemistry
[8–11], as sensitive materials in detection devices [12,13] and also
as catalysts[14–16]. Hybrid materialscomprisingof porphyrinsand
a polymeric or an inorganic nanomaterial matrix have also drawn a
lot of attention because the combined characteristics of the individ-
Herein we designed three different multifunctional hybrid
nanomaterials containing 5-(4-carboxyphenyl)-5,10,15-tris(4-
phenoxyphenyl)-porphyrin (COOH-POPP), that are tested as
catalysts for the Knoevenagel condensation of aldehydes at room
temperature and for their ability to detect and monitor CO2 in
wet environment. Porphyrin based materials have previously been
reported for selective oxidation reactions, as well as for the Kno-
evenagel condensation reaction [24], proving to be very efficient
catalysts for these transformations. However, to the best of our
knowledge, the use of porphyrin functionalized magnetic nanopar-
ticles for such catalytic systems has not been studied so far. Even
more, the materials reported herein possess several advantages
compared to previous reports [14], such as good activity combined
with short reaction times and mild conditions and easy recycling
of the catalyst by simple magnetic decantation. Furthermore, one
of the as-designed hybrid materials was also tested as optical
sensor for CO2 detection, proving to be multifunctional.
The best results were obtained for the material con-
taining the functionalized porphyrin, Fe3O4 MNPs and 1,2-
bis(triethoxysilyl)ethane (BTSE) as silica linker (Fig. 1).
The increased interest in novel materials tailored to capture and
convert CO2 was the determining factor for studying these materi-
als for gas detection as well. Porphyrins are used in metal–organic
hybrids for capturing and CO2 detection because of the presence of
∗
Corresponding author.
E-mail address: cmak@iciq.es (C.A. Mak).
http://dx.doi.org/10.1016/j.cattod.2017.01.014
0920-5861/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: C.A. Mak, et al., Functionalization of A3B-type porphyrin with Fe3O4 MNPs. Supramolecular assemblies,
gas sensor and catalytic applications, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.01.014

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Fig. 1. Functionalization of magnetite nanoparticles (MNPs) with 1,2-bis(triethoxysilyl)ethane (BTSE) and 5-(4-carboxyphenyl)-5,10,15-tris(4-phenoxyphenyl)-porphyrin
(COOH-POPP).
the two basic nitrogen atoms into the internal cavity, which facil-
itate strong interactions with CO2, the same as those reported in
human physiology. [25]
phenoxybenzaldehyde in a particular ratio of 4:3:1, as previously
reported [27]
2.1.2. Synthesis of Fe3O4 MNPs
Iron oxide magnetic nanoparticles were synthesized accord-
ing to a previously reported method, by thermal decomposition of
Fe(acac)3 in the presence of oleic acid and oleylamine as surfactants.
[28]
The syntheses of all conjugated materials: c1, c2, c3 (structures
in Fig. 2) were also done in accordance with the previous reported
methods. [29,30] (see supporting information for detailed proce-
dure).
2. Experimental section
2.1. Reagents
2.1.1.
5-(4-carboxyphenyl)-5,10,15-tris(4-phenoxyphenyl)-porphyrin
(COOH-POPP) synthesis
All synthesized materials were fully characterized by FT-IR, TEM,
AFM, XRD, 1H NMR, HPLC, UV–vis spectroscopy and elemental anal-
ysis (see supporting information for details).
Although the most used method to obtain carboxylic acid por-
phyrins is through hydrolysis of ester type Zn-metalloporphyrins
[26], the synthesis of A3B unsymmetrical mixed substituted 5-
(4-carboxyphenyl)-5,10,15-tris(4-phenoxyphenyl)-porphyrin was
based on Adler -Longo multicomponent reaction [8] consisting
in the condensation reaction of pyrrole and the two different
substituted benzaldehydes 4-carboxylmethylbenzaldehyde and 4-
2.1.3. General procedure for the Knoevenagel reaction
The corresponding aldehyde (0.2 mmol), malonitrile (0.3 mmol),
and catalyst (2 mol% c1/c2/c3) were mixed and stirred at room tem-
Fig. 2. Structures of the synthesized porphyrin-based materials.
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Fig. 3. TEM image of Fe3O4 MNPs (a); of material c1 (b); of material c3 (c); of material c1 at higher magnification (d); of material c2 (e).
perature. All the reactions were done in neat conditions except
those for which the starting materials were both solids. In those
cases, water/ethanol 1/1 was used as solvent. The reaction was
monitored by TLC until the total consumption of the starting
material was observed. Then, the catalyst was removed by mag-
netic decantation and the organic mixture was concentrated and
purified by column chromatography on silica gel, eluting with hex-
anes/EtOAc, to give the desired adduct.
tion time for 5-(4-carboxyphenyl)-5,10,15-tris(4-phenoxyphenyl)-
porphyrin was 2.69 min (the second eluted peak in Supporting
information, Fig. S4).
The 1H NMR spectrum shows a significant diminution of the
current of cycle that causes on one side the shielding of internal
NH protons and on the other side the un- shielding of ˇ pyrrolic
protons. The signal corresponding to NH pyrrolic protons appears
as broad singlet at −2.70 ppm; the signals for the six equivalent
ˇ pyrrolic protons appear as doublet at 8.92-8.94 ppm and for
the other two ˇ pyrrolic protons the signal is a broad singlet at
8.84 ppm. The signals for the eight 2,6H-phenyl protons are to
be seen as two doublets, each corresponding to four protons, at
8.53 ppm and 8.17 and as a multiplet in the range of 7.50–7.59 ppm
for the eight 3,5H-phenyl protons. The protons belonging to the
phenoxy groups are not differentiated, although they do not appear
as equivalent, and their signal is a multiplet at 7.35–7.41 ppm.
For all obtained compounds a detailed characterization was per-
formed. First of all, the samples were sent to elemental analysis,
based on which we could confirm the presence of N in all the ana-
lyzed materials. The materials were also studied by IR, TEM, ESEM
and EDX. (see SI for detailed spectra). Based on TEM images we
could observe that the MNPs remain monodisperse, they do not
change in shape or size in the case of material c1 (Fig. 3(b)), as for
material c3 a significant process of aggregation of the nanoparti-
3. Results and discussion
3.1. Characterization of Fe3O4, (COOH-POPP) porphyrin and
(MNPs) - (BTSE) -(COOH-POPP) nanomaterials
The Fe3O4 MNPs were characterized by IR, TEM and XRD. The
nanoparticles are monodispersed spherical and of approximately
5 nm in diameter (Fig. 3(a)).
The (COOH-POPP) porphyrin was fully characterized as previ-
ously reported [26] and the identity of product was confirmed by
HPLC and 1H NMR (see Supporting information Figs. S3, S4).
The mixture of the porphyrin products was separated by HPLC
on a KROMASIL 100, Sil, (5 ␮m 250 × 4.0 mm) reversed phase ana-
lytical column chromatography in a mixture of solvents: ethyl
acetate:DMSO:propionic acid = 90 mL:10 mL:300 ␮L. The reten-
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Fig. 5. The UV–vis spectrum of (MNPs)- (BTSE) −(COOH-POPP) nanomaterial.
tion bond; the intense band at 1234 cm−1 is attributed to aromatic
C bonds; the band from1483 cm−1 is attributed to valence
C
O
vibrations of C N bonds; broadened 1587 cm−1 band is corre-
sponding to asymmetric stretching of entire porphyrin macrocycle
due to all vibrations of the methyn bridges and of the pyrrole rings
but also of symmetric C C stretching in phenyl; the peak from
1686 cm−1 corresponds to stretching of C O bond; the band at
3032 cm−1 is assigned to asymmetric stretching of C H phenyl
vibrations and the band at 3311 cm−1 belongs to the stretching
vibration of N H bond [26–29].
Fig. 4. Superposed FT-IR spectra of 5-(4-carboxyphenyl)-10,15,20-tris(4-
phenoxyphenyl)-porphyrin (full line) and the nanocomposite c3 material
(dashed line).
The FT-IR spectrum (Fig. 4) of the c3 new nanocomposite
material shows a reduced number of bands that are differently posi-
tioned and broadened in comparison with the FT-IR spectrum of the
bare porphyrin. These novel bands can be attributed to the presence
of both magnetite and silica in the hybrid material. The formation
of this new material is also confirmed by the presence of the intense
cles is noticed. This fact can be due to the bulky presence of three
porphyrin unit associated with the presence of the bis-linker, that
facilitates both the linkage of the MNPs and the side by side (J
type) and sandwich (H-type) aggregates. (Fig. 3(c)). The aggrega-
tion behavior observed for material c3 is somewhat expected. As
mentioned in the experimental part, for the design of this hybrid
material we make use of a bis-triethoxysilane linker. The possible
phenomena that occurs is the iron oxide magnetic nanoparticles
get functionalized to one-another, due to the presence of this lig-
and. The ideal scenario would be that one of the triethoxysilane
units attaches to the surface of the nanoparticles and the other to
the porphyrin. However, it is well known, that the triethoxysilane
group has high affinity towards the surface of the nanoparticles. For
this reason, this tendency of aggregation takes place in the case of
material c3.
Besides, the weak forces, Van der Waals or hydrophobic, hydro-
gen bond and ␲- ␲ stacking got rise to triangular type morphology
of porphyrin based hybrid materials (Fig. 7) with dimensions in the
range of 350–400 nm. Self- organization gave multi lamellar trian-
gles, uniformly in shape, dimensions and orientation, due to J–type
and H-type aggregation specific to functionalized porphyrins, that
balance side- by- side with sandwich- type aggregation. [30]
One very interesting fact, worth mentioning, was observed in
the case of material c1. When studying the TEM images at higher
magnification very small nanoparticles, besides the iron oxide ones,
were observed. (Fig. 3(d)). These nanoparticles are believed to be
porphyrin nanoparticles which are formed due to the presence
of the polysaccharide. This can be confirmed by the fact that the
same small nanoparticles, of an average diameter of 2 nm, were
also observed in the case of material c2 (Fig. 3(e)).
band attributed to stretching vibrations of the Si
also Si
Fe bridges at 900 cm−1 [31,32,33]. The bond formed
between the silica material and the porphyrin molecules is also
confirmed by the presence of the very intense band at 1058 cm−1
O Si bonds and
O
,
attributed to asymmetric stretching mode of Si
O
O
C bond [34]. The
C bond and/or ꢀ
weak band at 1457 cm−1 is attributed to the C
C
N bond and is due to porphyrin [27] The medium- intense signals
at 2871 cm−1 and 2973 cm−1, can be attributed to stretching C
H
vibrations in methylene groups from the linker. The large, medium
intensity band at 3424 cm−1 in the nanocomposite FT-IR spectrum
is related to O H stretching vibration due to the presence of H2O.
3.3. UV–vis characterization of the (MNPs)- (BTSE)
-(COOH-POPP) nanomaterial
The UV–vis spectrum of (MNPs)- (BTSE) -(COOH-POPP) nano-
material (Fig. 5) presents the characteristic bands of bare
porphyrin, namely: the intense Soret band at 419 nm assigned to
*
the transition from a1u (␲) - eg (␲) and the four Q bands of less
intensity with absorption maxima located at 514, 549, 591 and
*
647 nm, that are generated by a2u (␲) - eg (␲) transitions. The
spectrum belongs to etio pattern, meaning that the intensities of
Q bands are decreasing as follows: QIV > QIII > QII > QI.
Moreover, the presence of the combined hybrid materials could
be confirmed by EDX and elemental analysis. Even more, the results
from EDX match the ones obtained from elemental analysis in all
cases (see supporting information for details, Figs. S5–S7).
3.4. UV–vis response of the (MNPs)- (BTSE) -(COOH-POPP)
nanomaterial to CO2 exposure
The CO2 influence (stream gas 2 mL min−1) on absorption prop-
erties of (MNPs)-(BTSE)-(COOH-POPP) nanomaterial in DMF was
monitored by UV–vis spectroscopy.
By increasing the CO2 gas concentration, the intensity of absorp-
tion of the Soret band progressively increases (Fig. 6). In a wide
concentration range of CO2, varying from 30 ␮M to 200 ␮M, the
dependence between absorption intensity measured at 419 nm and
the concentration of CO2 in solution was linear and characterized by
an excellent correlation coefficient (Fig. 6), meaning that the mate-
3.2. FT-IR characterization of the (MNPs)- (BTSE) -(COOH-POPP)
nanomaterial in comparison with bare porphyrin
The FT-IR spectrum (Fig. 4) of the bare porphyrin displayed
bands at 796 cm−1, attributed to the bending of C H pyrrole bond
and to out of plane ␥ N H deformation in free base porphyrin; the
1sharp band at 1161 cm−1 is assigned to in-plane N H deforma-
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Fig. 6. The CO2 influence on absorption properties of (MNPs) − (BTSE) −(COOH-POPP) nanomaterial. The dependence between absorption intensity measured at 419 nm
and the concentration of CO2 in solution.
rial might be used in formulation for sensitive sensor devices able
to detect trace amounts of CO2 before being harmful for human
life. Based on the knowledge that 13 ␮M represents the normal
concentration of CO2 in our atmosphere, (which translates to 400
parts per million or 0.04% concentration) the optical sensor based on
this (MNPs)-(BTSE) -(COOH-POPP) nanomaterial can detect any
increase, before producing toxic response (headaches, respiratory
distress with loss of consciousness) to the staff working on space
orbital stations [35].
restructuration of the surface. Ovoidal aggregates of different sizes,
in the range between 114–299 nm, but also having the same orien-
tation are formed. The material shows large deep pores, probably
generated by the interaction with the gas.
3.6. Knoevenagel condensation of aldehydes at room temperature
One of the most important reactions used in industry to form
C
C bonds is the Knoevenagel condensation [36]. The obtained
The increase of intensity of absorbtion registered by (MNPs)-
compounds are useful candidates for the synthesis of biologically
active intermediates in the pharmaceutical industry [37]. Due to
the presence of the amino group in the porphyrin macrocycles the
synthesized materials were also tested for the Knoevenagel con-
densation of aldehydes with malonitrile.
The active basic N-sites of the porphyrin are the ones responsi-
ble for the condensation reaction (Fig. 8). These internal sites that are
present in the tetrapyrrole ring, help promotes the proton removal
from the methylene group of malonitrile thus generating a carban-
ion, which will then attack the electrophilic aldehydes to yield the
condensation products (Fig. 8).
(BTSE)-(COOH-POPP) nanomaterial after CO2 exposure is
a
completely different behavior in comparison with another
nanomaterial based on porphyrin, 5-(4-pyridyl)-10,15,20-tris(4-
phenoxyphenyl)-porphyrin − polyvinylpyrrolidone hybrid, when
by increasing CO2 concentrations the intensity of absorption was
decreased [12].
3.5. Atomic force microscopy of (COOH-POPP) in comparison with
the (MNPs)- (BTSE) -(COOH-POPP) before and after CO2 exposure
The samples were obtained by evaporation from a THF solution
containing the porphyrin COOH-POPP on pure silica plates. Self-
assembling of porphyrin at the air - THF interface was investigated.
The weak forces, Van der Waals or hydrophobic, hydrogen bond
and ␲-␲ stacking got rise to triangular type morphology (Fig. 7)
with dimensions in the range of 350–400 nm. Self-organization
gave multi lamellar triangles, uniformly in shape, dimensions and
orientation.
(MNPs)- (BTSE)-(COOH-POPP) material is characterized by the
same presence of triangles but only few and although pre-
serving the orientation, randomly dispersed. (MNPs)- (BTSE)
-(COOH-POPP) nanomaterial after CO2 exposure suffer a complete
The reactions proceed well at room temperature, in neat condi-
tions, leading to the desired adducts in high yields.
Therefore, catalysts c1, c2 and c3 (Fig. 2), were applied for the
condensation of 4-methylbenzaldehyde with malonitrile (molar
ratio 1:1,5), all of them proving to be highly efficient for this
transformation (Scheme 1). The cyano- groups in malonitrile can
form undesirable hydrolysis products. For this reason, it is more
appropriate to use an excess of malonitrile compared to the initial
aldehyde used for the catalytic reaction.
The best results were observed with material c3 which gave a
93% yield of the desired product, 3a, compared to 88% and 91%
in the cases of c1, c2 respectively. Moreover, a control reaction
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Fig. 7. a–c. 2D AFM image in contact mode and topography of multilayered deposition of CPOPP porphyrin (a); (MNPs)-(BTSE)-(COOH-POPP) material before (b) and after
CO2 exposure (c).
using just the MNPs was performed in order to verify if there is
any background reaction present. In this case no conversion was
observed, fact which confirms that the active catalyst in the mate-
rials is indeed the porphyrin. After choosing the catalyst we then
test different solvent systems in order to find the best conditions
for the above mentioned reaction (Table 1). The solvent system was
chosen, based on the nature of the aldehyde. Although the model
reaction performs good in neat conditions, and even in water these
conditions could not be applied to all the other substrates studied
herein. Since the other aldehydes were in solid form, the reaction
could not be performed in neat conditions. Even more, the chosen
substrates were not soluble in water, so for this reason the mixture
water:ethanol was chosen for these transformations.
After optimizing the reaction conditions, catalyst c3 was chosen
for the investigation in the Knoevenagel condensation of aldehydes
with malonitrile (Scheme 2). Both aromatic and aliphatic aldehydes
were tested, the results being summarized in Table 2.
In Table 1 the results are mentioned as conversion, due to the
fact that they are based on the NMR of the crude mixture, whilst
for the results listed in Table 2, the values are for the pure products.
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Table 2
7
Knoevenagel condensation of different aldehydes with malonitrile[a]
.
Substrate
Product
Yield (%)[b]
benzaldehyde
88
3b
2-hydroxybenzaldehyde
78
3c
3d
3e
4-bromobenzaldehyde
4-nitrobenzaldehyde
3-nitrobenzaldehyde
butanal
87
92
91
83
Fig. 8. Mechanism for the porphyrin catalyzed Knoevenagel condensation.
Scheme 1. Condensation of 4-methylbenzaldehyde with malonitrile.
3f
However, since the selectivity is 100%, yields and conversions are
identical.
Most of the synthesized compounds were obtained in almost
pure form, being separated from the reaction mixture by simple
magnetic decantation. All reactions were done in neat conditions,
apart for the substrates that are solids (3d, 3e, 3f) in which cases
the reaction was performed in water/ethanol 1/1. However, these
substrates also lead to the desired adducts in very good yields.
The differences observed in yields for the studied substrates are
directly correlated with the different substituents that they pos-
sess. The activity is based on electronic effects, steric interactions
and the position of substitution on the aromatic ring, in agree-
ment with already published results [38–43]. It can be observed
that the aromatic aldehydes bearing electron-withdrawing groups,
such as bromo and nitro (Table 2, entries 3d and 3e) react faster,
3g
[a] - All reactions performed with 0.6 mmol of aldehyde (1), 0.9 mmol of malonitrile
(2), and 2 mol% of c3 in 0,5 mL of solvent at room temperature.
[b] - Isolated yield after column chromatography.
providing the desired adducts with higher yields in comparison
with the aldehydes that have electron-donating groups. Even more,
based on these results, it can be stated that electron-withdrawing
groups favor the addition of the malonitrile to the aldehyde,
whilst the electron-donating groups facilitate the dehydration step
(Scheme 2). For this reason, the rate determining step is consid-
ered to be the first step, which is the slowest [42]. The influence
of the position of substitution on the aromatic ring (Table 2)
reveals that the reaction is accelerated in the following sequence:
ortho < meta < para [39].
The main advantage of such systems is their recyclabil-
ity by the facile recovery of the catalyst from the reaction
media by simple magnetic decantation. Therefore, the recycling
experiment was also performed (Fig. 9) for the condensation of 4-
methylbenzaldehyde with malonitrile. The catalyst remains active
for at least five consecutive runs, without significant loss in reac-
tivity.
Table 1
Optimization of reaction conditions[a]
.
Entry
Solvent
Conversion (%)[b]
1
2
3
4
5
6
water
ethanol
water/ethanol 1/1
acetonitrile
tetrahydrofuran
neat
>99
n.c.
99
98
n.c.
>99
[a] - All reactions performed with 0.2 mmol of aldehyde (1), 0.3 mmol of malonitrile
(2), and 2 mol% of c3 in 0,5 mL of solvent at room temperature.
[b] - Determined by 1H NMR of the reaction crude.
4. Conclusions
Three different multifunctional materials containing 5-(4-
carboxyphenyl)-5,10,15-tris(4-phenoxyphenyl)-porphyrin
and
iron oxide magnetic nanoparticles (MNPs) were obtained and
characterized. A novel polymeric material comprising the same
porphyrin and a polysaccharide, k-carrageenan, was also devel-
oped. All synthesized materials were characterized by FT-IR,
TEM, AFM, XRD, 1H NMR, HPLC, UV–vis spectroscopy and tested
as catalysts for the Knoevenagel condensation of aldehydes at
Scheme 2. General reaction scheme for the Knoevenagel condensation of aldehydes
with malonitrile.
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Acknowledgements
This work was supported by the EU-ITN network Mag(net)icFun
(PITN-GA-2012-290248) and STAR Safeair Project 76/2013. This
work is dedicated to the Anniversary of 150 years since Romanian
Academy was established. The authors thank to Mihaela Birdeanu
for AFM spectra.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2017.01.
014.
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Please cite this article in press as: C.A. Mak, et al., Functionalization of A3B-type porphyrin with Fe3O4 MNPs. Supramolecular assemblies,
gas sensor and catalytic applications, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.01.014