Palladium supported on a novel ordered mesoporous polypyrrole/carbon nanocomposite as a powerful heterogeneous catalyst for the aerobic oxidation of alcohols to carboxylic acids and ketones on water

Article abstract of DOI:10.1039/c9ra10941b

Preparation of an ordered mesoporous polypyrrole/carbon (PPy/OMC) composite has been described through a two-step nanocasting process using KIT-6 as a template. Characterization of the PPy/OMC nanocomposite by various analysis methods such as TEM, XRD, TGA, SEM and N₂ sorption confirmed the preparation of a material with ordered mesoporous structure, uniform pore size distribution, high surface area and high stability. This nanocomposite was then used for the immobilization of palladium nanoparticles. The nanoparticles were almost uniformly distributed on the support with a narrow particle size of 20-25 nm, confirmed by various analysis methods. Performance of the Pd?PPy/OMC catalyst was evaluated in the aerobic oxidation of various primary and secondary alcohols on water as a green solvent, giving the corresponding carboxylic acids and ketones in high yields and excellent selectivity. The catalyst could also be reused for at least 10 reaction runs without losing its catalytic activity and selectivity. High catalytic efficiency of the catalyst can be attributed to a strong synergism between the PPy/OMC and that of supported Pd nanoparticles.

Full text of DOI:10.1039/c9ra10941b

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PAPER
Palladium supported on a novel ordered
mesoporous polypyrrole/carbon nanocomposite as
a powerful heterogeneous catalyst for the aerobic
oxidation of alcohols to carboxylic acids and
ketones on water†
Cite this: RSC Adv., 2020, 10, 13616
Nasim Ganji,a Babak Karimi,
ab Sepideh Najafvand-Derikvandia and Hojatollah Valic
*
Preparation of an ordered mesoporous polypyrrole/carbon (PPy/OMC) composite has been described
through a two-step nanocasting process using KIT-6 as a template. Characterization of the PPy/OMC
nanocomposite by various analysis methods such as TEM, XRD, TGA, SEM and N2 sorption confirmed the
preparation of a material with ordered mesoporous structure, uniform pore size distribution, high surface
area and high stability. This nanocomposite was then used for the immobilization of palladium
nanoparticles. The nanoparticles were almost uniformly distributed on the support with a narrow particle
size of 20–25 nm, confirmed by various analysis methods. Performance of the Pd@PPy/OMC catalyst
was evaluated in the aerobic oxidation of various primary and secondary alcohols on water as a green
solvent, giving the corresponding carboxylic acids and ketones in high yields and excellent selectivity.
The catalyst could also be reused for at least 10 reaction runs without losing its catalytic activity and
selectivity. High catalytic efficiency of the catalyst can be attributed to a strong synergism between the
PPy/OMC and that of supported Pd nanoparticles.
Received 26th December 2019
Accepted 20th March 2020
DOI: 10.1039/c9ra10941b
rsc.li/rsc-advances
chlorite in halogenated solvents, resulting in the generation of
the signicant amounts of waste materials. Therefore, the
Introduction
development of mild, safe and environmentally friendly cata-
lytic approaches based on the use of green reagents and
solvents, and especially using molecular oxygen as a clean, atom
economic and sustainable oxidizing reagent would provide
noteworthy alternatives to the traditional oxidation
methods.11–21 Additionally, a remarkable number of catalysts
especially metal-based systems have successfully been devel-
oped for the aerobic oxidation of alcohols to corresponding
carbonyl compounds, owing to the importance of the use of
catalyst from the perspective of the principles of green
chemistry.
One of the most signicant metals for alcohol oxidation is
palladium. Over the years, various homogeneous Pd catalysts,
have extensively been examined in the aerobic oxidation of
alcohols.22–29 It is worth noting that most of the efficient
homogeneous catalysts in this transformation have containing
nitrogen groups. Earlier mechanistic studies revealed that Pd(0)
intermediates are stabilized through the coordination with the
nitrogen-containing ligands, prior to their aerobic reoxidation
to Pd(II) particles. While, Pd catalysts without the ligands were
extensively deactivated as a result of the Pd black formation.30,31
Moreover, scientists have focused on the development of effi-
cient heterogeneous Pd catalysts for the aerobic oxidation of
alcohols, because of disadvantages of homogeneous
Oxidation of alcohols to carbonyl compounds is one of the most
fundamental chemical reactions in organic synthesis both in
academic and industrial areas,1–5 because the corresponding
products have wide applications in synthetic chemistry as
important precursors and intermediates. Among all the
oxidized products, carboxylic acids are used in large scale for
the preparation of valuable compounds such as pharmaceuti-
cals, agrochemicals, polymers and commercial products.6,7
Global production of aryl carboxylic acids is estimated to be
more than 620 kilotons by 2023.8 However, large scale synthesis
of these carbonyl compounds still relies on the oxidation of
alcohols with stoichiometric amounts of hazardous and
expensive oxidants,9,10 such as permanganate, chromate and
aDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
PO-Box 45195-1159, Gava-zang, Zanjan 45137-6731, Iran. E-mail: karimi@iasbs.ac.
ir; bkarimi48@gmail.com
bResearch Center for Basic Sciences & Modern Technologies (RBST), Institute for
Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
cDepartment of Anatomy and Cell Biology and Facility for Electron Microscopy
Research McGill University, Montreal, Quebec, H3A 2A7, Canada
† Electronic supplementary information (ESI) available: Experimental sections
and additional data such as IR, TG, SEM, TEM, XPS, N2 adsorption–desorption
isotherms, BJH pore size distribution curves and gas chromatograms is
available. See DOI: 10.1039/c9ra10941b
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catalysts,32–42 including the necessity to high catalyst loadings
Polypyrrole (PPy) is one of the most promising polymers for
and excess of ligands or bases as well as difficulties in removing the preparation of polymer–carbon nanocomposites. This
and recycling expensive catalysts or ligands from the reaction polymer has attracted more and more attention during the
mixture. Although, during the recent years a lot of progress has recent years because of its extraordinary properties such as the
been achieved in the eld of heterogeneous Pd catalysts. facile synthesis through chemical and electrochemical
However, the majority of these reports have concentrated on the processes, excellent environmental stability, high conductivity,
conversion of alcohols to aldehydes and ketones and only a few biocompatibility and low price.61,62 This polymer can function-
heterogeneous Pd catalysts have been presented for the oxida- alize the surface structure of carbon materials and also can lead
tion of alcohols to carboxylic acids, for example, Pd/carbon,37 to the higher affinity of materials for metal species by its high
Pd–Bi–Te/activated carbon39–41 and Pd/DNA-MMT.42 Unfortu- coordinating capability, derived from nitrogen atoms and p
nately, most of these heterogeneous Pd systems suffer from the electrons of pyrrole units. In this regard, some research groups
necessity to promoters, low catalytic activity, limited substrate have recently focused on the development of mesoporous PPy–
scope, high reaction temperature and/or wasteful organic carbon composites. For example, Pinnavaia and co-workers re-
solvents. In fact, the preparation of carboxylic acids through the ported the synthesis of a series of polypyrrole-ordered meso-
direct oxidation of alcohols using heterogeneous Pd catalysts is porous carbon composites (PPy–OMCs) by incorporating PPy
still a live challenge. Therefore, the development of an ideal into a mesostructured carbon.63 In another report by Choi et al.,
system based on a heterogeneous Pd catalyst along with the use the surface of an ordered mesoporous carbon was selectively
of molecular oxygen as a green oxidant and also water as a green modied with the PPy.64 Moreover, in 2008 a strategy was pre-
solvent would be very valuable.
sented for the preparation of a number of PPy–OMC composites
Evidently, the nature of solid support is one of the most by the introduction of a PPy layer into the pore structure of
important factors in achieving highly efficient heterogeneous a mesoporous carbon through the vapor inltration of pyrrole
metal-based catalysts, because the support can play different and its subsequent chemical polymerization.65 However, if PPy
roles in the catalytic process and also can inuence the electron chains occupy the spaces of mesopores, the open and ordered
conguration of metal species.43 Among the wide range of solid pore structures that are highly desirable for catalytic applica-
supports, polymers and porous materials are of the most tions will be destroyed to a great extent. Therefore, the prepa-
commonly reported solid supports.44–47 During the years, ration of PPy–carbon composites with controlled open
various polymers, especially polymers having nitrogen-based mesostructure that also benet from electronic and chemical
functional groups in their framework were applied for the properties of both of the involved materials, in an easy and cost-
immobilization of metal nanoparticles, taking the advantage of effective way is still a fundamental challenge.
the high affinity of nitrogen functional groups to metal
particles.48
In this line, herein, we wish to represent a synthetic strategy for
the preparation of an ordered mesoporous PPy–carbon
Another extensively employed supports are porous materials, composite, so that, its mesopore structure remains intact and is
especially ordered mesoporous materials that owing to their kept from being blocked up by PPy chains during the polymeri-
attractive features including high surface areas, suitable and zation. In this approach, rst the mesopores of an ordered mes-
tunable pore sizes, narrow pore size distributions and acces- oporous silica template (KIT-6) are partially lled with sucrose as
sible porous frameworks (which simplify the diffusion of reac- a carbon precursor and then carbonized at 800 ꢀC. In following,
tant and product molecules) have been used as support in pores of the OMC/KIT-6 composite generated during the
different chemical transformations.49–52 Mesoporous carbon carbonization process are lled with a solution containing pyrrole
materials are an interesting category of these porous materials monomers and then the monomers are polymerized using an
that have been developed and applied in various elds because oxidant. Finally, the silica template is removed using a solution of
of their excellent chemical and thermal stability.53–56 However, hydrouoric acid (HF) to obtain the ordered mesoporous PPy/
from the view of application for the preparation of heteroge- carbon nanocomposite, as outlined in Scheme 1. We selected
neous metal-based catalysts, introducing functional groups PPy in this work because of its excellent electronic properties and
having high affinity for metal particles to the surface of porous this fact that pyrrole monomers can easily be adsorbed on carbon
carbons is necessary in most cases, because the interaction
between the metal nanoparticles and the unmodied carbon
surface is typically weak. This weak interaction can give rise to
leaching and aggregation of deposited metal particles during
the catalytic processes. In this connection, the modication of
porous carbon materials has attracted the attention of
researchers.57,58 One of the best functionalization approaches
presented in this area is the polymer-based modication
because resultant polymer-carbon composites could have the
extraordinary and combined properties of the individual
components and also benet from the synergistic effects
between them.59,60
Scheme 1 A schematic illustration of the synthetic procedure for the
preparation of the Pd@PPy/OMC catalyst.
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surfaces due to the molecular similarity between the pyrrole and connection, 5.5 g of P123 and 11.0 g of hydrochloric acid
carbon structure, as well as this point that PPy layers able to stick (37 wt%) were added to 201 g of deionized water and the
on solid surfaces strongly.64
resulting mixture was stirred for 6 h at 35 ꢀC to obtain
Moreover, with the aim of proting advantages of the meso- a homogeneous solution. Then, 5.5 g of n-butanol was poured
porous structure and also phenomenal features of the PPy/carbon into the solution and aer 1 h, 11.44 g of TEOS was also added
ꢀ
composite as a support, palladium species are immobilized on to it. Stir of the mixture was continued at 35 C for 24 h. Next,
this material and subsequently examined in the aerobic oxidation the mixture was transferred into a teon-lined autoclave and
of alcohols in an aqueous medium. To the best of our knowledge, placed under static condition at 130 ꢀC for 36 h. Content of the
there is no report for the use of ordered mesoporous PPy/carbon autoclave was ltered and washed with plenty of deionized
composites in chemical transformations.
water and ethanol. Finally, KIT-6 was obtained as a white solid
by removing the P123 template through the calcination at
550 C for 5 h under the air atmosphere.
ꢀ
Experimental
Characterization
N2 sorption analysis was performed by a Belsorp analyzer
(BELMAX, Japan) at 77 K. Samples were degassed at 373 K for
10 h before the measurements. Specic surface area of the
materials was obtained from the relative pressure range 0.05–
0.15 and their pore size distribution (PSD) was estimated by the
use of the Barrett–Joyner–Halenda (BJH) method from the
adsorption branch. Additionally, total pore volumes were
assigned using the adsorbed volume at P/P0 z 0.995. Pore
structure of the samples was studied by Philips CM-200 and
Titan Krios TEM instruments. Surface morphology of the
materials was investigated using high resolution scanning
electron microscopy (HRSEM) images taken by a TeScan-Mira
III ultrahigh resolution cold eld emission scanning electron
microscope. X-ray powder diffraction (XRD) analysis was per-
formed using a XPERT-PRO MPD diffractometer (CuKa radia-
tion) in the range of 0.8 to 10ꢀ. XPS spectra of the materials were
recorded on a Kratos Analytical X-ray photoelectron spectrom-
eter. Thermogravimetric analysis (TGA) was obtained using
a NETZSCH STA 409 PC/PG instrument (Germany) at scan rates
of 10 K minꢁ1, with typically 5 mg sample from 25 to 800 ꢀC
under both of the N2 and O2 atmosphere. Nitrogen content of
the materials was determined by elemental analysis (C, H, N)
using the vario-EL CHNS instrument. Fourier transform
infrared (FTIR) spectra of the materials were attained using
a Bruker vector 22 instrument in the range of 400 and
4000 cmꢁ1. Yield of oxidation reactions was determined using
a Varian CP-3800 gas chromatograph instrument (GC) equipped
with a capillary column and a ame-ionization detector (FID)
using an internal standard method.
Synthesis of the OMC/KIT-6 composite
Inspired by the Hao's work,68 an OMC/KIT-6 composite was
obtained as follows. The synthesized KIT-6 template was dried
at 100 ꢀC for 12 h and was also degassed under vacuum for 1 h
to remove moisture. 1.25 g sucrose and 0.14 g H2SO4 (98 wt%)
were added to 5 mL deionized water and the resulting solution
was poured into a ask containing 1 gr degassed KIT-6 and the
mixture was then sonicated for 45 min. The mixture was placed
in an oven at 100 ꢀC, and aer 6 h, temperature of the oven
increased to 160 ꢀC and the material was heated in this
temperature for 6 h. The brown solid was then carbonized at
800 ꢀC for 1 h under the argon atmosphere to obtain the OMC/
KIT-6 composite.
Preparation of the PPy/OMC composite
Channels of the OMC/KIT-6 composite generated during the
carbonization process could be used for deposition of PPy
chains through the primary impregnation of Py monomers into
the channels and then their in situ polymerization. For this
purpose, the OMC/KIT-6 composite was degassed under
vacuum for 30 min. The composite was then impregnated with
an H2O : EtOH (1 : 1) solution containing an appropriate
amount of Py monomers, calculated according to the pore
volume of the OMC/KIT-6 composite. Flask of the reaction was
placed into an ultrasonic bath for 45 min. The mixture was
subsequently stirred for 24 h, with the aim of further penetra-
tion of Py monomers into the pores of the OMC/KIT-6 material.
Extra solvents of the mixture were then removed under vacuum
ꢀ
at 45 C. Resulting solid was cooled in an ice bath and then,
Chemicals
200 mL HCl solution (1 mol Lꢁ1) containing ammonium per-
Pluronic P123 (EO20PO70EO20, EO ¼ ethylene oxide, PO ¼
propylene oxide) was obtained from Aldrich. Pd(OAc)2 was
purchased from Acros Organics. Tetraethyl orthosilicate (TEOS),
hydrochloric acid (37%), sulfuric acid (98%) and also solvents
were obtained from Merck Company and were utilized without
further purication. Pyrrole was purchased from Aldrich and
distilled under vacuum for more purication.
sulfate (APS) as an oxidizing agent (molar ratio APS : Py 1.2 : 1)
was added to it. The mixture was stirred at 0–5 C for 24 h to
ꢀ
complete the polymerization process. Finally, the black product
was ltered and washed with plenty of deionized water and
ethanol. Silica template of the as-synthesized PPy/OMC/KIT-6
composite was removed using a 10 wt% HF aqueous solution.
Finally, the PPy/OMC composite was washed with deionized
ꢀ
water and ethanol, and dried at 80 C for 12 h.
Preparation of an ordered mesoporous silica template (KIT-6)
In addition, an OMC material was obtained by removing the
KIT-6, an ordered mesoporous silica template, was prepared KIT-6 template from the initial OMC/KIT-6 composite to
according to the procedures reported in literature.66,67 In this compare with the PPy/OMC composite.
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necked ask. A condenser containing a balloon lled with
pure oxygen was placed on the reaction ask. Aer stirring at
90 ꢀC for an appropriate time, the reaction mixture was ltered
and the solution was extracted with ethyl acetate and H2O.
Finally, the organic layer containing the product was dried over
anhydrous Na2SO4. Yield of the reaction was directly obtained
by GC, without further purication of the product.
Preparation of the Pd@PPy/OMC catalyst
100 mg of the PPy/OMC composite was dispersed in 7 mL THF
and sonicated for 45 min. Subsequently, 4.5 mg Pd(OAc)2 (0.02
mmol) dissolved in 3 mL THF was dropwisely added to the
suspension and the mixture was stirred at room temperature for
12 h. Finally, the obtained Pd@PPy/OMC catalyst was ltered and
washed with THF several times and was dried at 80 ꢀC for 12 h.
Oxidation of primary alcohols using the Pd@PPy/OMC
catalyst
Results and discussion
N2 sorption isotherms and pore size distributions (PSDs) of the
materials (KIT-6, OMC/KIT-6, PPy/OMC/KIT-6, PPy/OMC and
OMC (a mesoporous carbon prepared by the removal of KIT-6
from the OMC/KIT-6 composite for comparing its features with
the PPy/OMC composite)) have been demonstrated in Fig. 1. All
the isotherms display a hysteresis loop at the relative pressure of
0.4–0.8, showing a concentrated distribution of mesopores in
these materials. Also, all of them possess characteristics of a type
IV isotherm. Surface area and textural parameters of these mate-
rials have been summarized in Table 1. Surface areas were
calculated according to the BET equation, and PSDs were evalu-
ated through the BJH method using adsorption branches of the
isotherms. Sharp capillary condensation in the isotherm of KIT-6
Primary alcohol (0.5 mmol), Pd@PPy/OMC (0.2 mol% of Pd),
NaOH (30 mg, 0.75 equiv.) and H2O (0.5 mL) were added into
a two-necked ask. A condenser containing a balloon lled with
pure oxygen was placed on the reaction ask. Aer stirring at
ꢀ
80 C for an appropriate time, the mixture was quenched with
some drop of HCl and then ltered. The solution was extracted
with H2O and ethyl acetate, and its organic layer was dried using
anhydrous Na2SO4. Yield of the reaction was directly obtained
by GC, without further purication of the product.
Oxidation of secondary alcohols using the Pd@PPy/OMC
catalyst
Secondary alcohol (0.5 mmol), Pd@PPy/OMC (1 mol%), NaOH revealed the uniform distribution of mesopores. BET surface area
(90 mg, 2.25 equiv.) and H2O (0.5 mL) were mixed in a two- and total pore volume of KIT-6 were 550 m2 gꢁ1 and 1.46 cm3 gꢁ1
,
Fig. 1 (a) and (b) N2 adsorption–desorption isotherms of the materials; (c) and (d) pore size distributions (PSDs) of the materials evaluated using
the BJH method.
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Table 1 Textural parameters of the materials
isotherms and PSDs of the OMC and PPy/OMC samples demon-
strated that the isotherm of the OMC has almost been maintained
aer the PPy addition, except for a slight decrease in the nitrogen
uptake. Pore sizes of both of them were similar, so that their
distribution curves showed a peak at 1.64 nm (Fig. 1b and d).
Moreover, the acceptable distribution of mesopores for the PPy/
OMC sample conrmed the open mesopore structure of the
PPy/OMC composite and therefore the absence of pore blocking
with PPy chains in the framework. This matter was ascribed to the
presence of the temporarily silica template during the in situ
polymerization of pyrrole inside the mesopores. BET surface areas
and total pore volumes of the OMC and PPy/OMC samples were as
follows: 1000 m2 gꢁ1 and 1.75 cm3 gꢁ1 for the OMC material and
822 m2 gꢁ1 and 1.39 cm3 gꢁ1 for the PPy/OMC composite,
respectively (Table 1).
Low-angle X-ray diffraction (XRD) was used to conrm the
ordered mesoporous structure of the synthesized composite
(Fig. 2). Well-resolved diffraction peaks observed for the KIT-6
template were characteristics of a highly ordered bicontin-
uous cubic Ia3d symmetry structure.69 The PPy/OMC displayed
the XRD pattern of a cubic structure, which was similar to the
KIT-6 pattern,70 conrming that mesostructure of the composite
is an inverse replica of the KIT-6 pore structure. However, the
PPy/OMC replica revealed some decrease in the intensity of its
peaks as compared with the KIT-6, illustrating a partial loss of
the long-range order of structure during the nanocasting
process.71 In addition, the slight shi of the low-angle XRD peak
of 211 toward higher angles for the PPy/OMC composite may be
related to the contraction of the structure aer replication; the
phenomenon that has been reported in some previous
articles.70,72
Sample
aSBET (m2 gꢁ1
)
bVt (m3 gꢁ1
)
cDBJH (nm)
KIT-6
550
288
188
1000
822
628
613
1.46
0.38
0.32
1.75
1.39
1.18
1.17
10.6
7.1
7.1
3.3
3.3
3.3
3.3
OMC/KIT-6
PPy/OMC/KIT-6
OMC
PPy/OMC
Pd@PPy/OMC
Re–Pd@PPy/OMC
a
SBET ¼ specic surface area calculated from the linear part of the BET
plot, (P/P0 z 0.05–0.15). b Vt ¼ total pore volume determined according
c
to N2 adsorbed at P/P0 z 0.995. DBJH ¼ average pore diameter
calculated using BJH method.
respectively (Fig. 1a). As can be seen in the N2 isotherm and PSD of
the OMC/KIT-6 composite, the one-time impregnation of sucrose
into the pores of KIT-6 and its carbonization can create a space
between KIT-6 and OMC, which is then available for the impreg-
nation of pyrrole monomers. Decreasing both of the surface area
and total pore volume from 288 m2 gꢁ1 and 0.37 cm3 gꢁ1 for the
OMC/KIT-6 composite to 188 m2 gꢁ1 and 0.32 cm3 gꢁ1 for the PPy/
OMC/KIT-6 composite conrmed the successful impregnation
and polymerization of pyrrole monomers into the pores generated
during the carbonization process (Fig. 1a and c). Comparing the
Pore structures of the KIT-6, OMC and PPy/OMC materials
were also investigated by using the HRTEM, as shown in Fig. 3.
The well-ordered arrangement of pores and the uniform size of
channels in the HRTEM image of KIT-6 conrmed that the
template has been well synthesized (Fig. 3a). Additionally, the
sufficiency of the one-time impregnation method for preparing
a mesoporous carbon with stable ordered mesostructure was
veried by checking the HRTEM image of the OMC sample
(Fig. 3b). Moreover, the comparison between the HRTEM
images of the OMC and PPy/OMC materials revealed that the
order of the OMC mesostructure was not destroyed by the
Fig. 2 Low-angle X-ray diffraction patterns of KIT-6, PPy/OMC and
Pd@PPy/OMC.
Fig. 3 TEM images of (a) KIT-6, (b) OMC and (c) PPy/OMC.
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Fig. 4 Typical FESEM images of OMC (a: 200 nm, b: 500 nm, c: 1 mm and d: 2 mm) and PPy/OMC (e: 200 nm, f: 500 nm, g: 1 mm and h: 2 mm).
addition of PPy, the result which is in good agreement with the PPy chains. Therefore, the weight loss at this temperature range
XRD and N2 sorption data (Fig. 3c).
was used for estimation the loading amount of the polymer on
Field emission scanning electron microscopy (FESEM) was the OMC that was approximated to be around 14.6% (Fig. 5b).
used to investigate the morphology and surface state of the Chemical composition of the PPy/OMC nanocomposite was also
OMC and PPy/OMC materials (Fig. 4). SEM Images of the OMC determined by the elemental analysis that showed the weight
and PPy/OMC samples, both indicate a cauliower-like struc- percentage of 0.62% for nitrogen.
ture consisted of agglomerated particles (Fig. 4a and e), and
Functional groups of the PPy, PPy/OMC and OMC materials
a nanosheet-like morphology (Fig. 4b–d and f–h). This similarity were characterized by the Fourier-transform infrared
proves the structural stability of the material during the poly-
merization process and removal of the template. However, the
presence of PPy on the surface of the OMC has made the surface
rougher than that of the OMC, as shown in Fig. 4c and g.
Thermogravimetric analysis (TGA) was used in order to
evaluate the thermal stability of the composite and also the
loading amount of PPy on the OMC. This analysis was per-
formed in both of the nitrogen and oxygen atmosphere, in the
temperature range of 25–800 ꢀC with heating rate of
10 ꢀC minꢁ1 (Fig. 5). Weight loss appeared in all samples at T <
150 ꢀC was related to the removal of water and other volatile
compounds from the materials. Broad weight loss observed for
the PPy/OMC and PPy samples in the whole temperature range
could be related to the wide molecular weight distribution of
the PPy chains (Fig. 5a). However, the PPy/OMC nanocomposite
showed a much better thermal stability in comparison with PPy,
as it is clearly observable from Fig. 5a. It should be noted that
when the analysis is carried out in the inert atmosphere, PPy
chains convert to a nitrogen-containing carbon material. Due to
the stability of the OMC at T < 480 ꢀC under the O2 atmosphere,
the weight loss in the PPy/OMC curve at the temperature range
Fig. 5 Thermogravimetric analysis curves under (a) nitrogen and (b)
of 150–480 ꢀC can be attributed to the thermal decomposition of
oxygen atmospheres.
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Fig. 6 Infrared spectra of PPy, OMC and PPy/OMC powders.
spectroscopy (Fig. 6). Spectrum of PPy showed main bands
similar to the PPy samples reported earlier.73 Bands at 1553 and
1474 cmꢁ1 were related to C]C and C–N stretching vibration
modes, respectively. In addition, the band at 1185 cmꢁ1 was
assigned to the C–C breathing vibration mode. Moreover, the
bands at 1303, 1095 and 1042 cmꢁ1 were attributed to the ]C–
H in-plane vibration modes of the pyrrole ring and the bands at
790 and 913 cmꢁ1 were corresponding to the ]C–H out-of-
plane vibration modes.74 There were not many visible peaks in
the FTIR spectrum of the OMC because of its high carbon
content and strong C–C bonds.75 However, the small peaks at
2921 and 2852 cmꢁ1 and the peaks at 1715, 1571 and 1213 cmꢁ1
could be related to the stretching vibrations of C–H, C]O, C]
C, and C–O bonds, respectively. Spectrum of PPy/OMC is similar
to the spectrum of PPy, only with lower intensities of the peaks,
as can be seen in Fig. 6. Characteristic vibration bands at 1521,
1444, 1067 and 805 cmꢁ1 veried the presence of PPy in the
composite very well. The slight shi observed in the position of
some peaks may be related to interactions between the OMC
and PPy layers.
Next, we turned our attention into the investigation of the
performance of the PPy/OMC composite as support for immo-
bilizing palladium species and the application of the resulting
catalyst in organic transformations. For this purpose, the
Pd@PPy/OMC catalyst was prepared by the slow addition of an
appropriate concentration of Pd(OAc)2 to a uniform mixture of
the PPy/OMC composite dispersed in THF. A catalyst with well-
dispersed Pd particles on the surface was expected to be ach-
ieved due to the presence of nitrogen in the support structure.
This material was thoroughly characterized by the XPS, TEM,
XRD, AAS and N2 sorption analyses. Nitrogen adsorption–
desorption isotherm of the Pd@PPy/OMC catalyst provided
a strong evidence for the successful immobilization of the Pd
species on the PPy/OMC material, by showing an appreciable
decrease in the BET surface area and total pore volume from 822
m2 gꢁ1 and 1.39 cm3 gꢁ1 for the PPy/OMC composite to 628 m2
gꢁ1 and 1.18 cm3 gꢁ1 for the Pd@PPy/OMC catalyst, respectively
(Table 1). N2 sorption analysis also demonstrated that the open
mesostructure and the structural order of the parent composite
can be preserved during the deposition of Pd species (Fig. 7).
Therefore, it is expected that enough spaces would still be
Fig. 7 (a) N2 adsorption isotherms of the samples, and (b) pore size
distributions (PSDs) of samples evaluated by the BJH method.
existed in the catalyst nanospaces for the diffusion of
substrates.
Pd@PPy/OMC catalyst also showed a low angle XRD pattern
similar to the pattern observed for the PPy/OMC support, con-
rming the preservation of the ordered cubic structure of the
composite during deposition of the palladium species (Fig. 2).
However, the relatively weaker intensity of the peaks for the
supported palladium catalyst compared to the mesoporous
composite can be attributed to the penetration of the palladium
species into the pores of the PPy/OMC composite. Moreover, the
TEM technique revealed the presence of Pd nanoparticles, and
at the same time maintaining the order of the mesostructure
(Fig. 8a and b). TEM images of the catalyst and the pattern of
particle-size distribution, shown in the inset of Fig. 8a,
demonstrated that the Pd species are almost uniformly
distributed on the PPy/OMC material, with a narrow range of
20–25 nm. Pd content in the catalyst was quantitatively deter-
mined using the atomic absorption spectroscopy (AAS) of the
acid-washed Pd@PPy/OMC sample and showed the loading
amount of 0.2 mmol gꢁ1
.
According to the TGA data, the Pd@PPy/OMC catalyst
demonstrated thermal stability close to thermal stability of the
PPy/OMC composite. This matter conrmed the stability of the
composite aer deposition of Pd particles (Fig. 5). However,
a slightly higher thermal stability observed for the PPy/OMC
composite compared to the Pd@PPy/OMC catalyst under the
O2 atmosphere (the PPy/OMC was decomposed at 480 ꢀC,
whereas the Pd@PPy/OMC was decomposed at 465 ꢀC) was
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Fig. 8 TEM images of Pd@PPy/OMC (a and b) and Re–Pd@PPy/OMC (c and d) samples including the particle-size distribution (insets).
Fig. 9 (a) XPS survey spectrum of the Pd@PPy/OMC; high-resolution XPS spectrum of (b) palladium; (c) carbon and (d) nitrogen.
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Table 2 Optimization of the aerobic oxidation reaction of benzyl alcohol using the Pd@PPy/OMC catalyst
Catalyst
Base
(equiv.)
Solvent
(mL)
T
(ꢀC)
Time
(h)
Yield of
Yield of
Entry
(mol% Pd)b
benzaldehydea (%)
benzoic acida (%)
TONc
1
2
3
4
5
6
7
8
Pd@PPy/OMC (0.4)
Pd@PPy/OMC (0.4)
Pd@PPy/OMC (0.4)
Pd@PPy/OMC (0.4)
Pd@PPy/OMC (0.4)
Pd@PPy/OMC (0.4)
Pd@PPy/OMC (0.4)
Pd@PPy/OMC (0.2)
Pd@PPy/OMC (0.1)
Pd@PPy/OMC (0.2)
Pd@PPy/OMC (0.2)
Pd@PPy/OMC (0.2)
Pd@PPy/OMC (0.2)
Pd@PPy/OMC (0.2)
Pd@PPy/OMC (0.2)
Pd@PPy/OMC (0.2)
PPy/OMC
—
—
H2O (4)
H2O (4)
H2O (4)
H2O (4)
H2O (4)
H2O (2)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
H2O (1)
80
80
80
80
80
80
80
80
80
80
80
70
90
80
80
80
80
80
80
80
80
80
2
0
2
0
0
—
—
12
12
12
12
12
12
12
12
12
12
12
12
6
2
1
2
1.5
2
12
2
2
Na2CO3 (2)
K2CO3 (2)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (2)
NaOH (1)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
NaOH (1.5)
30
25
18
10
0
0
31
1
35
39
0
0
1
15
8
1
18
32
57
27
21
35
73
90
100
100
54
99
48
14
100
100
99
73
0
105
175
365
450
499
1000
1080
990
480
139
1000
1000
990
730
—
9
10
11
12
13
14
15
16
17
18
19
20
21d
22e
Pd@PPy/OMC (0.6)
Pd(OAc)2 (0.6)
Pd(OAc)2(0.6)
Pd@PPy/OMC (0.2)
Pd@PPy/OMC (0.2)
99
1
15
33
67
330
—
50
330
670
a
b
GC yield using internal standard method. The catalyst was prepared by the impregnation of 0.02 mmol Pd(II) in 100 mg PPy/OMC
2 ꢂ Yield acidð%Þ
c
nanocomposite. TON ¼
, although, due to some limitations, the reactivity differences have been compared in all data at the
Catðmol%Þ
d
high end conversions, a more realistic comparison could be achieved at lower initial conversion. The catalyst was prepared by the
impregnation of 0.04 mmol Pd(II) in 100 mg PPy/OMC nanocomposite. The catalyst was prepared by the impregnation of 0.06 mmol Pd(II) in
e
100 mg PPy/OMC nanocomposite.
attributed to the fact that the presence of metal nanoparticles was deconvoluted into ve peaks. The main peak at 284.62 eV
can effectively catalyze decomposition of materials.76,77
was originated from the C–C/C]C carbon groups, while the
X-ray photoelectron spectroscopy (XPS), one of the most other weak peaks at 285.95, 287.22, 288.93 and 291.37 eV were
effective analysis methods for the identication of various attributed to the carbon atoms bound to the oxygen and
species on heterogeneous surfaces, was also employed to obtain nitrogen atoms in the composite (C–N, C–O–C, C]O and
additional insights about the nature of the Pd@PPy/OMC –COOH groups).81,82 Moreover, the binding energy at 400.1 eV in
material at atomic scale (Fig. 9). The signals located at around the N 1s spectrum was corresponding to the nitrogen atom in
200, 284, 340, 400, and 532 eV in the XPS survey spectrum were the PPy part (Fig. 9d).83 The presence of the Cl 2p peak in the
assigned to Cl, C, Pd, N and O, respectively (Fig. 9a). High survey spectrum might be related to the existence of Clꢁ
resolution Pd 3d XPS spectrum displayed three groups of dopants.
signals. The rst pair of the peaks at lower binding energies
Aer the successful preparation and characterization of the
336.13 and 341.39 eV were attributed to the Pd 3d5/2 and Pd@PPy/OMC catalyst, the performance of this material was
Pd 3d3/2 of Pd(0). The second pair of Pd signals, i.e. the 337.99 evaluated in the aerobic oxidation of alcohols in the presence of
and 343.25 eV peaks were assigned to the Pd 3d5/2 and Pd 3d3/2 molecular oxygen, with highlighting the possibility of using
of Pd(II) with lower charge density.78,79 Surprisingly, another pair water as an ideal green solvent. In order to optimize reaction
of signals can be seen at higher binding energies (the 339.36 conditions, the effect of diverse variables such as catalyst
and 344.62 eV peaks). These peaks might be related to the Pd amount, type and quantity of base and amount of water as well
particles that directly interact with existing electron decient as the reaction temperature was examined on the aerobic
groups on the surface of the mesoporous carbon (Fig. 9b).80 oxidation of benzyl alcohol as a test substrate, as shown in
However, the Pd 3d XPS spectrum revealed that dominant Table 2. We started our investigations with the oxidation reac-
component is Pd(II) (69.14%). As shown in Fig. 9c, the C 1s peak tion of benzyl alcohol (1 mmol) with the Pd@PPy/OMC catalyst
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(0.4 mol% [Pd]) on 4 mL H2O at 80 ꢀC temperature. Under these and electron decient derivatives, displayed an admirable
conditions, no progress was observed even aer stirring for 12 h reactivity and afforded the corresponding carboxylic acids in
(Table 2, entries 1 and 2). Therefore, we studied the inuence of excellent yields and selectivities (>90%) (Table 3, entries 1–16).
adding different bases such as K2CO3, Na2CO3 and NaOH to the It is noteworthy that this catalyst system also showed excellent
reaction vessel. NaOH (1.5 equiv.) showed the best result among performance in the oxidation of 4-(methylthio)benzyl alcohol
these various bases by giving 73% benzoic acid as the main that is a highly challenging substrate in metal-catalyzed aerobic
product (Table 2, entries 3–5). It should be noted that except oxidation systems, because this substrate typically deactivates
NaOH, the other bases led to very poor yields and selectivity metal catalysts by its strong coordination to metal centers
under these conditions. Another important point is the forma- (Table 3, entry 5).84 Another remarkable feature of this protocol
tion of the carboxylic acid in almost all of the examined was its capability to oxidize benzylic alcohols bearing sterically
conditions, most likely resulted from the high catalytic activity demanding substituents such as 2-methyl-, 2-chloro- and 2-
of the catalyst. This high catalytic activity may be attributed to metoxy groups, although some slight modications were
the binding of Pd species to strong withdrawing groups onto the required to achieve high efficiency and excellent yields (Table 3,
surface of the PPy/OMC composite. Remarkably, we found that entries 13–16). Cinnamyl alcohol as a complex substrate that
decreasing the amount of water to less than 1 mL results in can undergo several side reactions, because of its active C]C
signicantly higher yields and can improve the repeatability of bond, was mainly oxidized to benzoic acid (Table 3, entry 17).
the reaction (Table 2, entries 5–7). This diminution is also The catalytic system also demonstrated outstanding activity in
noteworthy from the environmental point of view. In the next the oxidation of less reactive primary aliphatic alcohols to the
step, we investigated the impact of different amounts of the corresponding carboxylic acids in good to excellent yields,
catalyst on the reaction process, as shown in Table 2, entries 7– though it was necessary to increase the temperature and
9. Yields and selectivity of the oxidation reaction did not amount of the catalyst to ensure their complete conversions
changed by reducing the amount of the catalyst from 0.4 to (Table 3, entries 18–21). To the best of our knowledge, such
0.2 mol% of [Pd] (Table 1, entries 7 and 8). However, further efficient aerobic oxidation of different primary alcohols to
decreasing of the amount of catalyst (0.1 mol% [Pd]) resulted in carboxylic acids, including challenging substrates, using
a lower yields (entry 9). Screening the quantity of the base and a heterogeneous Pd catalyst possessing a low loading of metal
reaction temperature showed that their decrease can result in and under such green reaction conditions has not been re-
a noticeable reduction in the yields (Table 2, entries 10–13). In ported to date. Despite the low reactivity of secondary alcohols,
addition, the reaction time was optimized and two hours was we discovered that the presented system was also applicable for
chosen as the optimum time of the reaction by giving the the oxidation of secondary benzylic and cyclic aliphatic alcohols
quantitative yield of benzoic acid (Table 2, entries 14–16). Aer on pure water, giving the ketones in excellent yields, just by
these screening experiments, the reaction condition in accor- some modications in the amount of catalyst and NaOH (Table
dance with entry 15 was selected as the optimal reaction 2, entries 22–26). Unfortunately, 2-adamantanol and open-
conditions. We also showed that the bare PPy/OMC support do chain secondary alcohols proved to be very sluggish
not show any activity for the aerobic oxidation of benzyl alcohol substrates, obtaining the corresponding ketones in poor yields,
under the optimal experimental conditions (Table 2, entry 17). even when the catalyst with 2 mol% [Pd] was employed (Table 3,
It is worth mentioning higher increasing in the catalyst amount entries 27–29).
the to 0.6 mol%, did not benecial in improving the catalyst
Recyclability is considered as a crucial factor in developing
performances (Table 2, entry 18). To further prove the impor- novel heterogeneous catalysts from the viewpoint of reducing
tant role of PPy/OMC support of our catalytic system, we also the cost of chemical processes and minimizing environmental
performed an oxidation reactions in the presence of a homoge- destructive effects. To investigate the recyclability of our cata-
neous Pd catalyst (Pd(OAc)2). Interestingly, the reaction using lyst, several runs were performed using benzyl alcohol under
Pd(OAc)2 (0.6 mol% [Pd]) showed a weak performance in the the optimized reaction conditions. In each run of the reaction,
oxidation of benzyl alcohol, most probably because of the Pd aer 2 hours, the Pd@PPy/OMC catalyst was simply recovered
black formation (Table 2, entries 19 and 20).
by ltration and then was washed with plenty of ethyl acetate
Finally, we also investigated a series of Pd@PPy/OMC cata- and water to take away any remaining NaOH and organic
lysts having different initial loadings of Pd in the oxidation contents. The recovered catalyst aer drying was used for the
reaction under the reaction condition similar to entry 15 (Table next run of the oxidation reaction. As shown in Fig. 10, the
2, entries 21 and 22). As can be seen in Table 2, entry 23, by catalyst was successfully utilized in ten reaction cycles and the
increasing the initial loading of Pd to 0.6 mmol gꢁ1 the yields of yields of all runs were almost the same. These series of experi-
reaction dropped remarkably. Hence, 0.2 mmol gꢁ1 was ments conrmed that the catalyst could be reused without loss
selected as the best loading for preparation of catalyst.
of its activity and selectivity. To check the possibility of the
With the optimized reaction conditions in hand, we studied leaching of Pd particles into the solution, a hot ltration test
the possibility of using the Pd@PPy/OMC catalyst for the was performed. In this way, the Pd@PPy/OMC catalyst was
aerobic oxidation of diverse types of primary and secondary separated from the reaction vessel, aer proceeding the oxida-
alcohols to their corresponding carbonyl compounds by testing tion reaction for 30 min (conversion 47%). Solution of the
a wide range of substrates (Table 3). As shown in Table 3, reaction was then transferred to another ask and allowed to
diverse types of primary benzylic alcohols, both electron rich stir under the optimum conditions for 12 h (oxygen atmosphere
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Table 3 Substrate scope for the aerobic alcohol oxidation with the Pd@PPy/OMC catalyst
Entry
1
Substrate
Product
Time (h)
2
Yield (%)
99
TON
990
2
8
98
92
97
83
94
95
98
980
920
970
830
940
950
980
3
10
8
4
5
14
12
13
7
6
7
8a
9
10
15
97
90
970
900
10
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Table 3 (Contd.)
Entry
11
Substrate
Product
Time (h)
15
Yield (%)
TON
830
83
96
12
8
960
13b
14b
15b
16b
17
15
14
18
16
24
95
96
93
87
86
475
480
465
435
860
18c
19c
13
18
94
188
200
100
20c
21c
30
21
100
65
200
130
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Table 3 (Contd.)
Entry
22d
Substrate
Product
Time (h)
20
Yield (%)
99
TON
99
23d
24d
18
15
12
94
94
100
90
100
90
25d
26d
12
24
100
10
100
27d
28d
—
24
24
32
25
—
—
29d
a
0.5 mL isopropanol was added. b Catalyst (0.4 mol% [Pd]) and 90 ꢀC. c Catalyst (1 mol% [Pd]) and 90 ꢀC. d Catalyst (1 mol% [Pd]), 2.25 equiv. NaOH
2 ꢂ Yield acid ð%Þ
Yield ketone ð%Þ
and 90 ꢀC. TONAcid
¼
; TONKetone
¼
:
Cat ðmol%Þ
Cat ðmol%Þ
and 80 ꢀC). Analysis of this catalyst-free reaction demonstrated
<4% progress in the conversion of the reaction. Moreover, the
Pd content of the recovered catalyst determined by AAS was
almost the same as that of the fresh catalyst, conrming the
strong interaction between the support and Pd species. N2
sorption analysis of the recovered catalyst showed an isotherm
similar to that of the fresh catalyst, with a negligible change in
the BET surface area and total pore volume (613 m2 gꢁ1 and 1.17
cm3 gꢁ1 for the SBET and Vt, respectively) (Fig. 7). This obser-
vation along with the TEM results strongly conrmed that the
catalyst has mostly preserved its initial porosity and meso-
structure during the catalysis and recycling processes. TEM
Fig. 10 Recyclability chart of the aerobic oxidation of benzyl alcohol
using the Pd@PPy/OMC catalyst.
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analysis of the recovered catalyst also revealed that the palla-
dium particles are almost uniformly distributed on the surface
of the catalyst, within the range of 20–25 nm (Fig. 8c and d).
However, the TGA analysis of the Re–Pd@PPy/OMC sample
demonstrated a decomposition temperature lower than that of
the Pd@PPy/OMC sample under the O2 atmosphere. Consid-
ering this claim that there is a correlation between the size of
metal nanoparticles and their catalytic effect on the thermal
decomposition of materials,85 we think that a slight change
might have occurred on the size of the Pd nanoparticles during
the oxidation reaction (Fig. 5).
To highlight the crucial role of the new PPy/OMC support in
obtaining the observed excellent reactivity, the performance of
the Pd@PPy/OMC catalyst was compared with a number of Pd-
supported catalysts such as Pd@PPy, Pd@C and Pd@OMC.
These comparative reactions were performed in the presence of
4-bromobenzyl alcohol as a test substrate and the Pd catalysts
with the amount of 0.2 mol% Pd. For more consideration, the
amount of Pd leached into the solutions during the oxidation
reactions was also studied by calculating the difference between
the loading of Pd on the catalysts and the loading of Pd on the
recycled catalysts, shown in Table 4. As can be seen in Table 4,
the Pd@PPy/OMC showed the highest activity and more
importantly the best selectivity in comparison with others.
What is appealing in this study is that in spite of using water as
solvent of the reaction, the amount of 4-bromobenzoic acid
remained below 50% by using of Pd@OMC as the catalyst, even
Scheme 2 A plausible mechanism for the aerobic oxidation reaction
of alcohols in the presence of the Pd@PPy/OMC catalyst.
the uniform and stable distribution of extremely surface active
palladium particles, and the strong support/Pd interaction.
Although, the precise mechanism for this reaction is not
clear at this stage, according to the results obtained in this work
and also study of some mechanisms reported by other research
groups about the aerobic oxidation of alcohols by Pd-based
catalysts,28,37,86 we here proposed a plausible mechanism for
the oxidation reaction of alcohols to carboxylic acids using the
Pd@PPy/OMC catalyst (Scheme 2). The results showed that the
presence of the Pd catalyst and also NaOH is essential for the
progress of the reaction (Table 2). In addition, XPS spectrum of
our catalyst system clearly shows that both Pd(II) and Pd(0) are
simultaneously existed in the supported Pd nanoclusters
(Fig. 9b).
ꢀ
aer running the reaction for 24 h at 80 C and under pure O2
atmosphere (Table 4, entry 2). The Pd@C and Pd@PPy catalysts
showed even inferior activity under the identical conditions.
These catalysts also demonstrated higher amount of the Pd
leaching during the reaction (Table 4, entries 3 and 4). Higher
yields of the products in the case of the Pd@OMC rather than
the Pd@PPy and Pd@C catalysts obviously conrmed the
benecial effect of ordered mesochannels on the progress of the
described oxidation reaction. Therefore, the superior perfor-
mance and stability of the Pd@PPy/OMC catalyst may not only
be attributed to the ordered mesoporous structure of its
support, but it can imply to a great extent on the possible
synergistic effects between its components and also its elec-
tronic properties. These extraordinary features could be led to
Therefore, it seems reasonable to speculate that the catalytic
cycle initiates with binding of alcohol substrate to the Pd(II)
Table 4 Comparing the activity of the Pd@PPy/OMC catalyst with other Pd-supported catalysts in the oxidation of 4-bromobenzyl alcohol
Entry
Catalyst
Time (h)
Aldehyde (%)
Acid (%)
TON
Pd leached (%)
1
2
3
4
Pd@PPy/OMC
Pd@OMC
Pd@C
13
24
24
24
4
95
47
2
950
470
20
<1
5
11
14
18
21
7
Pd@PPy
0
0
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species in the nanoclusters, forming Pd-bound alcohol.
Deprotonation of the Pd-bound alcohol by NaOH, followed by b-
hydride elimination results in the formation of aldehyde. In the
next step, a geminal diol is generated by the nucleophilic
addition of hydroxide anion to aldehyde in the presence of the
hydroxide base in aqueous solution. Eventually, dehydrogena-
tion of geminal diol on the Pd catalyst gives rise to the forma-
tion of carboxylic acid. The Pd(II) catalyst is re-formed by the
oxygenation of Pd(0) species, generated via the reductive elim-
ination of palladium hydride species, in the presence of
molecular oxygen and subsequent hydrolysis.
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Outlook, Application Development, Price Trend, Competitive
Market
Share
&
Forecast,
2019-2025,
https://
Conclusions
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21 H. Yu, S. Ru, G. Dai, Y. Zhai, H. Lin, S. Han and Y. Wei,
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Conflicts of interest
22 T. Mallat and A. Baiker, Catal. Today, 1994, 19, 247–283.
23 K. Mori, K. Yamaguchi, T. Hara, T. Mizugaki, K. Ebitani and
K. Kaneda, J. Am. Chem. Soc., 2002, 124, 11572–11573.
24 S. S. Stahl, Angew. Chem., Int. Ed., 2004, 43, 3400–3420.
25 Y. H. Ng, S. Ikeda, T. Harada, Y. Morita and M. Matsumura,
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There are no conicts to declare.
Acknowledgements
The authors thank the Institute for Advanced Studies in Basic
Science (IASBS) Research Council through
G2019IASBS31100 for supporting this work.
a
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