Hyper-Cross-Linked Polyacetylene-Type Microporous Networks Decorated with Terminal Ethynyl Groups as Heterogeneous Acid Catalysts for Acetalization and Esterification Reactions

Article abstract of DOI:10.1002/chem.201802432

Heterogeneous catalysts based on materials with permanent porosity are of great interest owing to their high specific surface area, easy separation, recovery, and recycling ability. Additionally, porous polymer catalysts (PPCs) allow us to tune catalytic activity by introducing various functional centres. This study reports the preparation of PPCs with a permanent micro/mesoporous texture and a specific surface area S_BET of up to 1000 m² g^?1 active in acid-catalyzed reactions, namely aldehyde and ketone acetalization and carboxylic acid esterification. These PPC-type conjugated hyper-cross-linked polyarylacetylene networks were prepared by chain-growth homopolymerization of 1,4-diethynylbenzene, 1,3,5-triethynylbenzene and tetrakis(4-ethynylphenyl)methane. However, only some ethynyl groups of the monomers (from 58 to 80 %) were polymerized into the polyacetylene network segments while the other ethynyl groups remained unreacted. Depending on the number of ethynyl groups per monomer molecule and the covalent structure of the monomer, PPCs were decorated with unreacted ethynyl groups from 3.2 to 6.7 mmol g^?1. The hydrogen atoms of the unreacted ethynyl groups served as acid catalytic centres of the aforementioned organic reactions. To the best of our knowledge, this is first study describing the high activity of hydrogen atoms of ethynyl groups in acid-catalyzed reactions.

Full text of DOI:10.1002/chem.201802432

A Journal of
Accepted Article
Title: Hyper-cross-linked polyacetylene type microporous networks
decorated with terminal ethynyl groups as heterogeneous acid
catalysts for acetalization and esterification reactions
Authors: Lada Sekerová, Miloslav Lhotka, Eliška Vyskočilová, Tomáš
Faukner, Eva Slováková, Jiří Brus, Libor Červený, and Jan
Sedláček
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To be cited as: Chem. Eur. J. 10.1002/chem.201802432
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Hyper-cross-linked polyacetylene type microporous networks
decorated with terminal ethynyl groups as heterogeneous acid
catalysts for acetalization and esterification reactions
Lada Sekerová[a], Miloslav Lhotka[b], Eliška Vyskočilová[a]*, Tomáš Faukner[c], Eva Slováková[c], Jiří
Brus[d], Libor Červený[a] and Jan Sedláček[c]*
Abstract: Heterogeneous catalysts based on materials with
permanent porosity are of great interest owing to high specific
surface area and easy separation, recovery and recycling.
Additionally, Porous Polymer Catalysts (PPCs) allow us to tune
catalytic activity by introducing various functional centres. This study
Introduction
Porous Polymer Catalysts (PPCs) are a promising new subclass
of heterogeneous catalysts based on materials with permanent
porosity.[1-8] In contrast to conventional polymer-supported
catalysts based on solvent-swellable polymers,[9-12] PPCs show
permanent porosity with sizes ranging from micropores to
mesopores and with high specific surface area. Most PPCs
possess the architecture of polymer networks whose porosity
results from the rigidity of the segments forming the networks
and from extensive cross-linking. PPCs can be prepared to have
a wide spectrum of chemical functionalities that differ them from
most heterogeneous catalysts based on inorganic porous
supports.
Synthesis of PPCs using both step-growth and chain-growth
polymerization has been described in the literature. Step-growth
polymerization using Sonogashira coupling was efficient for
copolymerization of 1,3,5-triethynylbenzene with Tröger's base
derivative containing two iodo substituents into porous
organocatalysts, with a Brunauer-Emmett-Teller surface area
SBET of up to 750 m2 g-1, which were active in benzaldehyde
alkylation with diethylzinc.[13] The same synthetic path was used
to prepare a PPC organocatalyst with boron-dipyrromethene
units active in selective thioanisole oxidation.[14] Organometallic
PPCs active in sulphide oxidation with O2 (SBET ~ 1300 m2 g-1)
were prepared by Suzuki-Miyaura coupling of 1,4-
reports the preparation of PPCs with
a
permanent
micro/mesoporous texture and a specific surface area SBET of up to
1000 m2 g-1 active in acid catalysed reactions, namely aldehyde and
ketone acetalization and carboxylic acids esterification. These PPC-
type conjugated hyper-cross-linked polyarylacetylene networks were
prepared
by
chain-growth
homopolymerization
and
of
1,4-
diethynylbenzene,
1,3,5-triethynylbenzene
tetrakis(4-
ethynylphenyl)methane. However, only some ethynyl groups of the
monomers (from 58 to 80%) were polymerized into the
polyacetylene network segments while the other ethynyl groups
remained unreacted. Depending on the number of ethynyl groups
per monomer molecule and covalent structure of the monomer,
PPCs were decorated with unreacted ethynyl groups from 3.2 to 6.7
mmol g-1. The hydrogen atoms of the unreacted ethynyl groups
served as acid catalytic centres of the aforementioned organic
reactions. To our best knowledge, this is first study describing the
high activity of hydrogen atoms of ethynyl groups in acid catalysed
reactions.
phenyldiboronic
acid
and
[tetrakis(4´-
bromophenyl)porphyrin](FeIII).[15] Ru and Ir complexes with
ethynylated 2,2´-bipyridine ligands were copolycyclotrimerized
with tetrakis(4-ethynylphenyl)methane into organometallic
photocatalysts (SBET up to 1500 m2 g-1) active in aza-Henry
[a]
L. Sekerová, Dr. E. Vyskočilová. Prof. L. Červený
Department of Organic Technology
University of Chemistry and Technology Prague
Prague 6, 166 28, Czech Republic
email: eliska.vyskocilova@vscht.cz
Dr. M. Lhotka
Department of Inorganic Technology
University of Chemistry and Technology Prague
Prague 6, 166 28, Czech Republic
Dr. T. Faukner, Dr. E. Slováková, Prof. J. Sedláček
Department of Physical and Macromolecular Chemistry
Faculty of Science, Charles University
Prague 2, 128 40, Czech Republic
email: jan.sedlacek@natur.cuni.cz
reactions
and in aldehyde oxyamination.[16]
Knitting
polymerization of 2,4,6-tris(carbazolo)-1,3,5-triazine was used to
prepare base PPC organocatalysts (SBET ~ 840 m2 g-1) for the
Knoevenagel reaction.[17] The same synthetic path provided
organometallic PPCs with Ru active centres complexed with
2,2´-bis(diphenylphosphino)-1,1´-binaphthyl segments active in
asymmetric hydrogenation.[18] Knitting polymerization of
carbazole with 1,4-bis(bromomethyl)benzene followed by
sulfonation of the parent network provided PPC efficient in
esterification and transesterification reactions.[19]
Chain-growth copolymerization of divinylbenzene (DVB) with
sodium p-styrene sulfonate followed by ion exchange with
sulfuric acid provided acid PPCs of the hyper-cross-linked
polymer network type (SBET up to 535 m2 g-1) active in acetic acid
esterification.[20] The organometallic PPC active in Huisgen 1,3-
dipolar cycloadition and in Glaser homocoupling of alkynes was
[b]
[c]
[d]
Dr. J. Brus
Institute of Macromolecular Chemistry
Academy of Sciences of the Czech Republic v.v.i.
Prague 6, 162 06, Czech Republic
Supporting information for this article is given via a link at the end of
the document
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10.1002/chem.201802432
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prepared using the same type of copolymerization of DVB with
N-(1,10-phenanthroline-5-yl)acrylamide followed by metalation
of the 1,10-phenanthroline segments of the porous networks
with Cu2+.[21] The hyper-cross-linked PPCs prepared by chain-
growth homopolymerization (referred to as self-polymerization)
have also been described in a review.[7] In these reactions, the
monomers containing both (i) catalytic active centres or their
precursors and (ii) vinyl groups were radically polymerized into
PPCs. For example, bis-1,2-(diphenylphospino)ethan with one
vinyl group on each phenyl ring was polymerized into a hyper-
cross-linked porous polymer of SBET ~ 950 m2 g-1, which was
subsequently metalated into an organometallic PPC active in
alkene hydroformylation.[22] Tetrastyrylporphyrin was another
multi-vinylic monomer efficiently polymerized into a hyper-cross-
linked polymer network. Poly(tetrastyrylporphyrin) metalation
with Co provided an organometallic PPC active in the
transformation of epoxides to carbonates in reaction with CO2.[23]
The above survey shows that PPCs are mostly prepared by
polymerization of (co)monomers already decorated with
catalytically active centres or their precursors. Preparing
the main chains, polyacetylene networks achieve microporous
texture, even when cross-linking is less extent.[27] Therefore, we
hypothesised that chain-growth polymerization of ethynylarenes
into polyacetylene networks could be used to prepare hyper-
cross-linked PPCs because fewer ethynyl groups are required
for polymerization, thereby allowing the introduction of a higher
number of catalytically active groups (centres) into PPCs.
Accordingly, this study reports the preparation of acid PPC
organocatalysts via chain-growth homopolymerization of
ethynylarenes with two, three or four ethynyl groups per
molecule as the only type of functional group. Only some ethynyl
groups of the monomers were consumed in the polymerization
while the other ethynyl groups remained unreacted and served
as the acid catalytic centres of acetalization and esterification
reactions. To our best knowledge, this is first study describing
the high activity of hydrogen atoms of ethynyl groups in acid
catalysed reactions.
Results and Discussion
extensively
cross-linked
PPCs
with
permanent
micro/mesoporous texture requires using (co)monomers with
catalytically active centres and with a sufficient number of
polymerizable groups per monomer molecule. When preparing
PPCs by step-growth polymerization, (co)monomers with an
average number of polymerizable groups per molecule ≥ 2.5
were used, and the conditions were optimized to achieve a
nearly quantitative conversion of these groups. When preparing
hyper-cross-linked PPCs by chain-growth polymerization, fewer
polymerized groups per monomer molecule are required for
polymer network formation. For example, the aforementioned
chain-growth copolymerization of DVB with functionalized
styrenes provided PPCs, although the average number of
polymerized (vinyl) groups per (co)monomer molecule was only
1.5.[7] However, even when synthesising PPCs by chain-growth
polymerization of vinylic monomers, polymerization feeds with a
higher number of vinyl groups per monomer molecule were often
used to obtain more extensively cross-linked products. Cross-
linking was extended to offset the partly flexible saturated main
chains of polyvinylic networks that do not sufficiently contribute
to the formation of a permanent microporous texture, which is a
key drawback of polyvinylic PPCs.
Preparation of microporous polymer networks decorated
with terminal ethynyl groups
To prepare hyper-cross-linked microporous polymer networks
extensively decorated with terminal ethynyl groups, we used
chain-growth polymerization catalysed by the [Rh(nbd)acac]
complex and monomers with two, three and four terminal ethynyl
groups,
namely
1,4-diethynylbenzene
(DEB),
1,3,5-
triethynylbenzene (TEB) and tetrakis(4-ethynylphenyl)methane
(TEPM) (Scheme 1). Chain-growth polymerization of alkynes
with one ethynyl group per monomer molecule catalysed by Rh-,
W- or Mo-based catalysts yielding the linear substituted
polyacetylenes has been used for several decades.[28,29] The
ethynyl groups of the monomer molecules are transformed into
polyene (polyacetylene)-type conjugated main chains in this
polymerization [see Scheme 1 depicting the polymerization of
phenylacetylene to linear poly(phenylacetylene)]. Recently, we
found that Rh(I) catalysts also polymerized diethynylarenes
(diethynylbenzenes and 4,4´-diethynylbiphenyl),[24-26] and
mixtures of diethynylarenes with monoethynylarenes.[27] In this
case, the polymerization provided non-swellable polyacetylene
networks with permanent microporosity in which the polyene
chains were cross-linked with arylene links.
We have recently published the preparation of highly rigid hyper-
cross-linked porous polymers by chain-growth (coordination)
polymerization
of
diethynylarenes
(specifically
Chain-growth polymerization of DEB, TEB and TEPM (see
Scheme 1) was performed under conditions given in ref.[30] (see
Experimental part). Quantitative yields of the respective polymer
networks, P(DEB), P(TEB) and P(TEPM), were assessed in all
polymerizations. The prepared polymer networks were totally
insoluble in the solvents tested (DCM, methanol, THF and
benzene). The dark red colour of P(DEB), P(TEB) and P(TEPM)
indicated their conjugated character (see Figure S1 for the DR
UV/vis spectra in ESI). Thermogravimetric analysis (TGA)
confirmed the high thermal stability of the polymer networks
prepared (see Figure S2 for the TGA curves in ESI). A mass
loss of 5 wt.% was observed by TGA (t95%) at the following
values of temperature: 419°C [P(DEB)], 529°C [P(TEB)] and
diethynylbenzenes)[24-26] or by copolymerization of these
monomers with monoethynylarenes.[27] This polymerization
provides micro/mesoporous polymer networks (SBET up to 1400
m2 g-1) with rigid conjugated main chains of the polyacetylene
type in which double and single bonds alternate between carbon
atoms. These chains are cross-linked by arylene (most
commonly phenylene) links in the same way as in
poly(divinylbenzene), P(DVB) networks. Thus, the architecture
of polyacetylene networks resembles that of cross-linked
P(DVB), although the conjugated unsaturated main chains of
polyacetylene networks are significantly more rigid than the
saturated main chains of P(DVB). Due to the higher rigidity of
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Scheme 1. Structure of the monomers PhA, DEB, TEB and TEPM. A to D represent possible types of monomeric units in polymer networks P(DEB), P(TEB) and
P(TEPM).
505°C [P(TEPM)]. Figure 1 shows 13C CP/MAS spectra of
P(DEB), P(TEB) and P(TEPM). The 13C CP/MAS NMR spectra
of all polymer networks contained (i) a broad, partly resolved
signal in the region from 115 to 150 ppm that corresponds to the
resonance of aromatic carbons and carbons of the polyene
main-chains and (ii) signals around δ = 82 ppm and δ = 75 ppm
that correspond to carbons of non-transformed (unreacted)
terminal ethynyl groups. The central sp3 carbon of the TEPM
monomeric units was manifested in the 13C CP/MAS NMR
spectrum of P(TEPM) by a signal at 64 ppm. The presence of
unreacted terminal ethynyl groups in all polymer networks
prepared was also confirmed by IR spectroscopy [see bands at
2110 cm-1 (νC≡C) and 3300 cm-1 (ν≡C-H) in Figure S3 in ESI].
The quantification of 13C CP/MAS spectra of P(DEB), P(TEB)
and P(TEPM) provided values of the number of terminal ethynyl
groups (per monomeric unit) that were transformed during the
formation of the polymer networks from individual monomers
(nTE) and the values of the respective degree of conversion of
ethynyl groups (ξE) (see Table 1). The values of nUE in Table 1
express the number of terminal ethynyl groups (per monomeric
unit) remaining in the polymer networks. Because the
monomeric units of P(DEB), P(TEB) and P(TEPM) differ in molar
mass, we also report the content of unreacted ethynyl groups in
the polymer network in mmol g-1 (cE) (Table 1). The values of nTE
of the polymer networks increased with the number of ethynyl
groups in the parent monomers (nE0). However, this increase
was not proportional to nE0. While the DEB monomer with two
ethynyl groups provided P(DEB) in which 1.6 ethynyl groups per
monomeric units were converted, the monomer TEPM with four
ethynyl groups provided P(TEPM) with only partly enhanced nTE
= 2.3 (see Table 1). Therefore, the value of ξE decreased in the
series P(DEB) > P(TEB) > P(TEPM), ranging from 0.80 in the
case of P(DEB) to only 0.58 in the case of P(TEPM). The
decrease in ξE with increasing nE0 most likely resulted from steric
hindrance in transforming a higher number of ethynyl groups per
monomeric unit. All polymer networks contained a significant
amount of unreacted ethynyl groups [nUE from 0.4 for P(DEB) to
Figure 1. 13CP/MAS NMR spectra of polymer networks P(DEB), P(TEB) and
P(TEPM).
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Table 1. Covalent and textural characteristics of the polyacetylene polymer networks: number of ethynyl groups per molecule of monomer, nE0, number of
ethynyl groups (per monomeric unit) transformed in polymerization, nTE, degree of conversion of ethynyl groups in polymerization, ξE, number of unreacted
ethynyl groups in the polymer networks (per monomeric unit), nUE, content of ethynyl groups in the polymer networks in mmol g-1, cE, overall specific surface
area, SBET, specific surface area of micropores, Smicro, volume of micropores, Vmicro
.
cE
SBET
Smicro
Vmicro
Polymer
network
nE0
nTE
ξE
nUE
(mmol g-1)
(m2 g-1)
(m2 g-1)
(cm3 g-1)
P(DEB)
P(TEB)
2
3
4
1.6
2.0
2.3
0.80
0.67
0.58
0.4
1.0
1.7
3.2
6.7
4.1
1007
914
832
741
699
0.4
0.2
0.3
P(TEPM)
711
1.7 for P(TEPM)].The values of cE ranged from 3.2 to 6.7 mmol
g-1 and increased in the series P(DEB) < P(TEPM) < P(TEB).
The results of 13C CP/MAS NMR spectroscopy partly allowed us
to specify the covalent structure of the polymer networks.
P(DEB) most likely consists of two types of monomeric units: (i)
linear units (40 mol % content) in which only one ethynyl group
of DEB was transformed into a segment of the polyacetylene
main chain while the second ethynyl remained unreacted
(Scheme 1, units AP(DEB)) and (ii) cross-linking units (60 mol %
content) in which both ethynyl groups of DEB were transformed
into the segments of the polyacetylene main chains and
interconnected two main chains via 1,4-phenylene links
(Scheme 1, units BP(DEB)). P(TEB) may contain linear units with
only one transformed ethynyl group (Scheme 1, units AP(TEB))
and two types of cross-linking monomeric units in which either
two or three ethynyl groups were transformed (Scheme 1, units
BP(TEB) and CP(TEB), respectively). Four types of monomeric units
can be contained in P(TEPM): a linear monomeric unit (Scheme
1, units AP(TEPM)) and three types of cross-linking units with two,
three or four transformed ethynyl groups (Scheme 1, units
BP(TEPM), CP(TEPM) and DP(TEPM), respectively). Unfortunately, the
content of the individual unit types in P(TEB) and P(TEPM)
could not be determined from 13C CP/MAS NMR spectra.
Nitrogen adsorption isotherms (77 K) on P(DEB), P(TEB) and
P(TEPM) (see Figure 2) showed a steep increase in the quantity
of N2 adsorbed in the relative pressure (p/p0) region from 0 to
0.1 (p0
= 101325 Pa), thus confirming the presence of
micropores in the polymer networks. P(TEB) and P(DEB) also
contained mesopores (in addition to micropores) as shown by
the steep increase in adsorbed N2 at p/p0 > 0.6 and by significant
hystereses on adsorption/desorption isotherms. The Brunauer–
Emmett–Teller specific surface area of the polymer networks
(SBET) ranged from 711 to 1007 m2 g-1, slightly decreasing in the
order P(DEB) > P(TEB) > P(TEPM), although the extent of
cross-linking of the polymer networks (roughly proportional to
nTE) increased in the same order. In polymer networks with a
higher extent of cross-linking, the tight packing of some links and
chain segments may have restricted the formation of micropores
in the polymer networks. Nevertheless, the difference in SBET of
the P(DEB), P(TEB) and P(TEPM) may also be affected by the
geometry of the DEB, TEB and TEPM monomers used to
prepare the polymer networks. The evaluation of the desorption
branches of N2 isotherms by the Barrett-Joyner-Halenda (BJH)
method confirmed the presence of mesopores in P(DEB) and
P(TEB). The mesopore diameter distributions of both networks
were broad (see Figure S4 in ESI) with maxima at about 9 nm
[P(DEB)] and 6 nm [P(TEB)]. The micropore diameter of P(DEB),
P(TEB) and P(TEPM) was around 0.7 nm. Figure S5 in ESI
shows Scanning Electron Microscopy (SEM) images of P(DEB),
P(TEB) and P(TEPM). The networks consisted of approximately
spherical particles 20 – 100 nm in diameter.
Figure 2. N2 adsorption (full symbols) and desorption (open symbols)
isotherms at 77 K on the polymer networks P(DEB), P(TEB) and P(TEPM).
DEB, TEB and TEPM and other aromatics with ethynyl groups
have often been described as the starting materials for the
synthesis of structurally diverse microporous polymers by
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various step-growth polymerizations. Based on the pioneering
studies by Cooper et al.,[31] step-growth polymerization through
Sonogashira coupling yielding conjugated microporous
poly(aryleneethynylene)s is arguably the most commonly used
technique in this field. The poly(aryleneethynylene)s prepared
from DEB and TEB have a SBET of up to 1000 m2 g-1,[31,32] and
poly(aryleneethynylene)s prepared from TEPM have a value of
SBET = 1917 m2 g-1.[33] However, a nearly quantitative conversion
of ethynyl groups of the monomers by step-growth
phenylacetylene has been described as a cocatalyst in the
transformation of some transition metal complexes into
homogenous catalysts,[34] no systematic study on the use of
terminal alkynes as acid catalysts has been published in the
literature thus far. This may be due to difficulties in separating
the product from the alkyne that would be used as
homogeneous catalyst.
a
The acidity of acetylenic hydrogen should be not only
maintained in the terminal ethynyl groups of P(DEB), P(TEB)
and P(TEPM), but also increased upon conjugation of triple
bonds of ethynyl groups with conjugated segments of the
polymer networks. We confirmed the Brønsted acidity of P(DEB),
P(TEB) and P(TEPM) by an indirect method based on
measuring the decrease of basicity of dimethyl sulfoxide
(DMSO) caused by the presence of dispersed acid solid.[35] Due
to the presence of dispersed terminal ethynyl groups containing
P(DEB), P(TEB) or P(TEPM), pH of DMSO dropped from 13 to 9
(see Experimental part for details). Conversely, pH of DMSO
decreased only marginally (from 13 to 12) due to adding
reference linear poly(phenylacetylene) P(PhA), which did not
contain the terminal ethynyl groups (see Scheme 1). It should be
noted that the sensitivity of the method used was not sufficient to
determine any differences in acidity between P(DEB), P(TEB)
and P(TEPM). Based on the above finding, we decided to study
the activity of P(DEB), P(TEB) and P(TEPM) as heterogeneous
acid organocatalysts. Two typical acid-catalysed reactions have
been chosen for this purpose: acetalization of carbonyl
compounds and esterification of carboxylic acids by methanol. In
the initial stage of the catalytic study, we confirmed
polymerization
is
required
to
prepare
microporous
poly(aryleneethynylene)s with high SBET values. In this respect,
the chain-growth technique used by us for the polymerization of
DEB, TEB and TEPM differs: for obtaining microporosity and
satisfactory SBET values of polyacetylene-type P(DEB), P(TEB)
and P(TEPM) partial conversion of ethynyl groups of the
monomers was sufficient that allowed preparing conjugated
microporous polymer networks extensively decorated with
(unreacted) terminal ethynyl groups (Table 1). Moreover, the
content of the unreacted terminal ethynyl groups of polymer
networks can be partly controlled by selecting monomer
molecules with a different number of terminal ethynyl groups.
Catalytic activity of polymer networks P(DEB), P(TEB) and
P(TEPM)
The acetylenic hydrogen of terminal ethynyl groups is known to
have Brønsted acid character. This property is used in chemical
reactions with terminal alkynes as substrates. However, the acid
character of the acetylenic hydrogen may also make it possible
to use terminal alkynes as acid organocatalysts. Although
Table 2. Yield of acetals achieved in the acetalization of various aldehydes or ketones with methanol catalysed with the polymer networks P(DEB), P(TEB) and
P(TEPM) (100 mg aldehyde or ketone, 10 mg polymer network, 3 mL methanol, 60°C, reaction time 0.25 h and 7 h).
P(DEB)
P(TEB)
P(TEPM)
Yield (%) at
reaction time:
Aldehyde
or Ketone
Yield (%) at
reaction time:
Yield (%) at
reaction time:
0.25 h
7 h
0.25 h
15
7 h
0.25 h
7 h
Pentanal
Hexanal
1.8
1.4
3.3
0.5
1.3
3.5
4.8
49
38
22
18
32
31
14
(18)a
(12)a
(6)a
80
98
91
50
88
36
13
(14)a
(15)a
(12)a
(10)a
(15)a
(9)a
11
13
84 (24)a
89 (22)a
81 (17)a
28 (9)a
76 (22)a
40 (17)a
15 (5)a
12
Heptanal
Isobutanal
Isopentanal
Acetone
19
8.6
1.5
10
(8)a
5.4
14
(12)a
(17)a
(6)a
16
12
Butanone
12
(3)a
9.8
a Turn over number (TON) in mol of converted substrate per mol of ethynyl groups of the catalyst.
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Table 3. Yield of methyl esters achieved in the esterification of various carboxylic acid with methanol catalysed with either the polymer network P(TEB) or
phenylacetylene applied as a homogeneous catalyst (100 mg carboxylic acid, 10 mg catalyst, 5 mL methanol, 65°C).
P(TEB)
Phenylacetylene
Yield (%) at
Yield (%) at
reaction time:
Carboxylic
Acid
reaction time:
1 h
2.4
1.9
5.5
7 h
13
14
21
24 h
1 h
0.9
0.5
1.0
7 h
5.2
5.8
4.6
24 h
Acetic
Propionic
39 (10)a
30 (6)a
31 (4)a
12 (2)a
8.2 (1)a
12 (1)a
cis-Hex-3-enoic
a Turn over number (TON) in mol of converted substrate per mol of ethynyl groups of the catalyst.
the positive catalytic effect of P(DEB) on the isopentanal
acetalization by methanol (see Experimental part for conditions):
32% conversion of isopentanal to isopentanal dimethyl acetal
was achieved with P(DEB) within 7 h while only 3% conversion
was assessed in the same reaction performed without the
catalyst. Because P(DEB) may contain residua of the
[Rh(nbd)acac] complex (applied as polymerization initiator), we
tested [Rh(nbd)acac] (dissolved in methanol) for its eventual
activity in isopentanal acetalization. The result was negative,
thus confirming that acetalization was not catalysed by
[Rh(nbd)acac]. Isopentanal acetalization by methanol was also
performed with dispersed linear P(PhA) polymer (Scheme 1),
which contained polyene main chains substituted with phenyl
groups similarly to P(DEB), albeit without terminal ethynyl
groups. The result was also negative: poly(phenylacetylene) did
not catalyse isopentanal acetalization. The above finding
strongly indicates that the catalytic activity of P(DEB) in
acetalization resulted from the acidity of acetylenic hydrogens of
the terminal ethynyl groups present in P(DEB), which
corroborates the certain ability of phenylacetylene to catalyse
acetalization: 8% conversion of isopentanal was achieved when
applying phenylacetylene as a homogeneous catalyst. However,
the catalytic activity of phenylacetylene was lower than that of
P(DEB), which may reflect the lower acidity of acetylenic
hydrogen of phenylacetylene than that of acetylenic hydrogens
of the more conjugated P(DEB).Table 2 summarizes the results
of the P(DEB)-, P(TEB)- and P(TEPM)-catalysed acetalization of
various carbonyl compounds by methanol. The only products
formed in these reactions were the expected dimethyl acetals
(no hemiacetals or any side products were detected). The
catalytic activity of the networks in aldehyde acetalization
(expressed by aldehyde conversion) increased approximately in
the same order in which the content of terminal ethynyl groups in
the networks (cE) increased, that is, [P(DEB) < P(TEPM) ≤
P(TEB)] (see Table 1 and 2). This finding further confirms that
the terminal ethynyl groups of the networks served as the acid
catalytic centres. Based on the comparison of TON values listed
in Table 2, which quantify the ability of ethynyl groups of the
individual networks to catalyze acetalization of aldehydes,
P(TEPM) appeared to be slightly more efficient than P(TEB) and
P(DEB). This may be related to the fact that ethynyl groups in
P(TEPM) are relatively distant from knots of the networks and
can therefore be better sterically accessible for substrate
molecules (see Scheme 1). The catalytic activity of the networks
in acetalization depended on the character of the substrates.
Pentanal, hexanal and heptanal acetalization generated the
expected acetals in yields ≥ 80% when P(TEPM) and P(TEB)
were used as the catalyst. Conversely, isobutanal acetalization
was less efficient: with P(TEB) catalyst, an acetal yield of only
50% was achieved, most likely because the branching of the
butane chain near the HC=O group sterically complicated the
isobutanal acetalization. In turn, an acetal yield of 88% was
achieved in the P(TEB)-catalysed acetalization of isopentanal in
which the branching of substrate alkane chain apparently
caused no sterical hindrance because it was farther from the
aldehyde group.
P(DEB), P(TEB) and P(TEPM) also catalysed the acetalization
of acetone and butanone with methanol to acetone dimethyl
acetale and butanone dimethyl acetale, respectively. The yield
achieved after 0.25 h was similar to that assessed after the
acetalization of most aldehydes in the same period. However,
the final yield of acetals (after 7 h of reaction) was lower than
that of aldehyde acetalization. Lengthening the reaction time
failed to increase the yield. Notwithstanding, we believe that
further optimizing the reaction conditions may increase the
efficiency of P(DEB), P(TEB) and P(TEPM) in ketone
acetalization. It should be noted that no traces of product were
detected (by GC) in the acetalization of acetone and butanone
performed under the same conditions without adding P(DEB),
P(TEB) or P(TEPM). No products were detected when acetone
and butanone acetalization was attempted in the presence of
phenylacetylene.[36]
The most catalytically active network, P(TEB), was tested for
reusability in isopentanal acetalization [200 mg of isopentanal,
20 mg of P(TEB), 4 mL of methanol, 60°C, 4 h]. After each
reaction cycle, P(TEB) was separated, washed four times with
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DCM and dried (always at room temperature) prior to repeated
use. Isopentanal conversion was as follows, in consecutive
cycles: 95% (1-st cycle), 84% (2-nd cycle), 68% (3-rd cycle),
68% (4-th cycle). Thus, even at the third and fourth use, P(TEB)
showed satisfactory catalytic activity. The decrease in catalytic
activity between the first and third reaction cycles may reflect
deterioration of the access to the smallest pores of P(TEB) as a
result of possible reaction components trapping in these pores.
P(TEB) network was also active in esterification reactions,
namely the esterification of acetic, propionic and cis-hex-3-enoic
acids with methanol at 65°C (see Table 3). The yields of the
respective methyl esters achieved after 24 h were, however,
only moderate (ranging from 30 to 39%). Nevertheless, P(TEB)
was significantly more active in the studied esterifications than
phenylacetylene applied as a homogeneous catalyst in the same
reactions (see Table 3). The studied esterifications provided no
esters when performed without adding a catalytically active
component. The P(TEB) network is a less efficient esterification
catalyst than conventional mineral acids. However, easy
separation of P(TEB) from the liquid part of the reaction system
is a clear advantage of this catalyst.
analogue to the networks. The acidity of the acetylenic hydrogen
atoms may be increased in the networks prepared due to the
conjugation of ethynyl groups with conjugated network segments.
To our best knowledge, this is first study describing the high
activity of hydrogen atoms of ethynyl groups in acid catalysed
reactions.
Experimental Section
Materials
1,4-Diethynylbenzene (DEB), 1,3,5-triethynylbenzene (TEB), tetrakis(4-
ethynylphenyl)methane
(TEPM),
acetylacetonato(norborna-2,5-
diene)rhodium(I), [Rh(nbd)acac] (all by TCI Europe), acetic acid,
butanone, propionic acid (all by Lach-Ner, Czech Republic), acetone,
methanol (both by Penta, Czech Republic), heptanal, hexanal, cis-hex-3-
enoic acid, isobutanal, isopentanal, pentanal, propanal, phenylacetylene
and tetrahydrofuran (THF) (all by Sigma-Aldrich) were used as obtained.
Dichloromethane (DCM) (Lach-Ner, Czech Republic) was distilled from
P2O5. Linear poly(phenylacetylene) was prepared by polymerization of
phenylacetylene with [Rh(nbd)acac] polymerization catalyst in DCM
according to ref.[37]
DEB, TEB and TEPM polymerization
Conclusions
All polymerizations were performed in DCM at 70°C in sealed thick-wall
ampoules under argon atmosphere. The complex [Rh(nbd)acac] was
used as a polymerization catalyst. The reaction time was 72 h. The initial
concentrations in the reaction mixtures were: [[Rh(nbd)acac]]0 = 18 mmol
dm-3, [monomer]0 = 0.6 mol dm-3 (in the case of DEB and TEB
monomers), [monomer]0 = 0.25 mol dm-3 (a lower concentration was
used due to worse solubility of TEPM in DCM). Polymerizations provided
quantitative yields of the respective polymer networks, P(DEB), P(TEB)
and P(TEPM). Typical example of polymerization reaction
(polymerization of DEB): 1 g (7.9 mmol) of DEB was placed into an
ampoule, dissolved in 10 mL of DCM and subsequently mixed (under
stirring with a magnetic stirrer bar) with a solution of 70 mg (0.24 mmol)
of [Rh(nbd)acac] in 3 mL of DCM. Within two to three minutes, the
gelation point was reached, preventing further stirring. The ampoule was
then sealed and placed in the oven preheated to 70°C. After 3 days of
reaction at 70°C, the solid polymer network was separated and washed
repeatedly with DCM until the colourless filtrate was obtained. Polymer
network was subsequently dried in vacuum at room temperature for 7
days, and the yield of the polymer network was determined
gravimetrically.
Hyper-cross-linked conjugated polymer networks of the
polyacetylene-type extensively decorated with terminal ethynyl
groups were prepared by chain-growth homopolymerization of
1,4-diethynylbenzene, 1,3,5-triethynylbenzene and tetrakis(4-
ethynylphenyl)methane. In the polymerization, only some
ethynyl groups of the monomers (from 58 to 80%) were
converted into polyacetylene-type segments of the main chains
of the networks while the other ethynyl groups remained
unreacted in the prepared networks. Depending on the number
of ethynyl groups per monomer molecule and covalent structure
of the monomer, the prepared networks were decorated with
unreacted ethynyl groups from 3.2 to 6.7 mmol g-1. Conversely,
the networks prepared showed permanent micro or
micro/mesoporous texture (SBET from 700 to 1000 m2 g-1)
although the polymerizations proceeded only under partial
transformation of the polymerizable (ethynyl) groups of the
monomers.
The prepared networks were active as heterogeneous
organocatalysts in reactions known to require acid catalysis,
such as aldehyde and ketone acetalization by methanol. The
respective dimethyl acetals were the only products from the
acetalizations catalysed by these networks. The catalytic activity
of the networks in acetalization correlated well with the content
of the ethynyl groups in the networks. The network prepared
from 1,3,5-triethynylbenzene was shown to also catalyse the
esterification of carboxylic acid with methanol although the yields
of the respective methyl esters were only moderate. We
attributed the catalytic activity of the networks to the acidity of
the acetylenic hydrogen atoms of the ethynyl groups abundantly
present in the networks. Surprisingly, the networks prepared
showed higher catalytic activity than the phenylacetylene that we
tested as a homogeneous ethynyl-group containing catalytic
Application of the polymer networks P(DEB), P(TEB) and P(TEPM)
as heterogeneous catalysts
The prepared polymer networks were applied as heterogenous catalysts
of (i) acetalization of aldehydes and ketones with methanol (reaction
temperature 60°C) and (ii) esterification of carboxylic acids with methanol
(reaction temperature – boiling point of methanol, ~ 65°C, oil bath 90°C).
Acetalization and esterification were performed in a stirred batch reactor
(400 rpm). Most often, 10 mg of the polymer network was dispersed in
methanol (from 3 to 5 ml). The dispersion was stirred for 20 min to
solvate the surface of the polymer network. Then, 100 mg of substrate
was added under stirring, and the course of the reaction was monitored
by Gas Chromatography (GC). The samples were centrifuged using
rotation centrifuge (Hettich Zentrifuge EBA 20), and the liquid part was
analyzed by GC.
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10.1002/chem.201802432
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Techniques
[4]
[5]
J. Jiang, C. Wang, A. Laybourn, T. Hasell, R. Clowes, Y. Z. Khimyak,
J. Xiao, S. J. Higgins, D. J. Adams, A. I. Cooper, Angew. Chemie Int.
Ed. 2011, 50, 1072.
All 13C CP/MAS NMR spectra were measured at 11.7 T using a Bruker
Avance 500 WB/US NMR spectrometer in a double-resonance 3.2-mm
probehead at 20 kHz spinning frequency. The 90° (1H) pulse length was
2.5 µs, the strength of the spin-locking field B1(1H, 13C) expressed in
frequency units ω1/2 π = γB1 was 64 kHz, the recycle delay was 6 s, and
the cross-polarization contact time to reach quantitative results was 3 ms.
Active cooling was used to offset the frictional heating of rotating
samples.[38] Diffuse reflectance (DR) UV/vis spectra of polymer networks
were recorded using a PerkinElmer Lambda 950 spectrometer. Samples
were diluted with BaSO4. Fourier transform IR spectra were measured on
a Nicolet Magna IR 760 using the diffuse reflection mode. Samples were
diluted with KBr. Thermogravimetric analysis (TGA) was performed on a
TA Q500 apparatus under nitrogen atmosphere with a heating rate of
10°C min-1 from 40 to 800°C. The samples were treated at 100°C for 60
min under nitrogen flow prior to analysis to remove trapped moisture.
Nitrogen adsorption/desorption isotherms on the polymer network
samples were measured at 77 K using a static volumetric adsorption
system TriFlex analyzer, Micromeritics, Norcross, USA. The samples
were degassed at 373 K (12 h) prior to N2 adsorption analysis. The
adsorption isotherms were fitted using the (i) Brunauer–Emmett–Teller
(BET) method, to assess the specific surface area (SBET), and the (ii) t-
plot method to determine the external specific surface area (Sext). The
specific surface area of the micropores (Smicro) was assessed using the
following equation: Smicro = SBET - Sext. Horvath–Kawazoe method was
used to estimate the diameter of micropores. To determine the mesopore
diameter distribution the desorption branches of the N2 isotherms were
evaluated by the Barrett-Joyner-Halenda (BJH) method. GC analysis of
H. Bohra, P. Li, C. Yang, Y. Zhao, M. Wang, Polym. Chem. 2018, 9,
1972.
[6]
[7]
G. Deng, Z. Wang, Macromol. Rapid Commun. 2018, 39, 1700618.
K. Dong, Q. Sun, X. Meng, F.-S. Xiao, Catal. Sci. Technol. 2017, 7,
1028.
[8]
Y.-B. Zhou, Z.-P. Zhan, Chem. - An Asian J. 2018, 13, 9.
S. Schlick, E. Bortel, K. Dyrek, Acta Polym. 1996, 47, 1.
B. M. Antunes, A. E. Rodrigues, Z. Lin, I. Portugal, C. M. Silva, Fuel
Process. Technol. 2015, 138, 86.
[9]
[10]
[11]
J. Sedláček, M. Pacovská, D. Rédrová, H. Balcar, A. Biffis, B.
Corain, J. Vohlídal, Chem. - A Eur. J. 2002, 8, 366.
P. Drabina, J. Svoboda, M. Sedlák, Molecules 2017, 22, 865.
X. Du, Y. Sun, B. Tan, Q. Teng, X. Yao, C. Su, W. Wang, Chem.
Commun. 2010, 46, 970.
[12]
[13]
[14]
[15]
[16]
M. Liras, M. Iglesias, F. Sánchez, Macromolecules 2016, 49, 1666.
L. Chen, Y. Yang, D. Jiang, J. Am. Chem. Soc. 2010, 132, 9138.
Z. Xie, C. Wang, K. E. DeKrafft, W. Lin, J. Am. Chem. Soc. 2011,
133, 2056.
[17]
[18]
[19]
Y. Zhang, S. A, Y. Zou, X. Luo, Z. Li, H. Xia, X. Liu, Y. Mu, J. Mater.
Chem. A 2014, 2, 13422.
T. Wang, Y. Lyu, X. Chen, C. Li, M. Jiang, X. Song, Y. Ding, RSC
Adv. 2016, 6, 28447.
reaction mixtures was performed on (i)
a
Shimadzu GC-17A
S. Bhunia, B. Banerjee, A. Bhaumik, Chem. Commun. 2015, 51,
5020.
chromatograph equipped with a flame ionisation detector and a non-polar
ZB-5 column and on (ii) a Shimadzu GC-2010 Plus chromatograph
equipped with a mass spectrometer Shimadzu GC-MS QP 2010 Ultra
and a non-polar ZB-1 column. Scanning Electron Microscopy (SEM) was
performed using Tescan Lyra3 apparatus (TESCAN Brno, ltd., Czech
Republic) at accelerating voltage of 10 kV. The values of DMSO pH were
determined using automatic pH meter (Hanna Instruments HI 3220
pH/ORP Meter). The dispersions of polymer (10 mg in 15 mL of DMSO)
were prepared and placed to the magnetic stirrer (450 rpm). The values
of pH were measured for a period of 30 s at 23 °C.
[20]
[21]
F. Liu, W. Kong, C. Qi, L. Zhu, F.-S. Xiao, ACS Catal. 2012, 2, 565.
Q. Sun, Z. Lv, Y. Du, Q. Wu, L. Wang, L. Zhu, X. Meng, W. Chen,
F.-S. Xiao, Chem. - An Asian J. 2013, 8, 2822.
[22]
[23]
[24]
Q. Sun, Z. Dai, X. Liu, N. Sheng, F. Deng, X. Meng, F.-S. Xiao, J.
Am. Chem. Soc. 2015, 137, 5204.
Z. Dai, Q. Sun, X. Liu, C. Bian, Q. Wu, S. Pan, L. Wang, X. Meng, F.
Deng, F.-S. Xiao, J. Catal. 2016, 338, 202.
V. Hanková, E. Slováková, J. Zedník, J. Vohlídal, R. Sivkova, H.
Balcar, A. Zukal, J. Brus, J. Sedláček, Macromol. Rapid Commun.
2012, 33, 158.
Acknowledgements
[25]
[26]
E. Slováková, M. Ješelnik, E. Žagar, J. Zedník, J. Sedláček, S.
Kovačič, Macromolecules 2014, 47, 4864.
Acknowledgements: Funding from the Czech Science
Foundation (Project No.17-03474S) is gratefully acknowledged
together with financial support from specific university research
(MSMT No.21-SVV/2018). This work was realized within the
D. Bondarev, R. Sivkova, P. Šuly, M. Polášková, O. Krejčí, R.
Křikavová, Z. Trávníček, A. Zukal, M. Kubů, J. Sedláček, Eur. Polym.
J. 2017, 92, 213.
Operational
Programme
Prague
–
Competitiveness
[27]
S. Stahlová, E. Slováková, P. Vaňkátová, A. Zukal, M. Kubů, J. Brus,
D. Bondarev, R. Moučka, J. Sedláček, Eur. Polym. J. 2015, 67, 252.
T. Masuda, Polym. Rev. 2017, 57, 1.
(CZ.2.16/3.1.00/24501) and “National Program of Sustainability”
((NPU I LO1613) MSMT-43760/2015). We also thank Dr. Carlos
V. Melo for editing the manuscript.
[28]
[29]
[30]
J. Sedláček, H. Balcar, Polym. Rev. 2017, 57, 31.
E. Slováková, A. Zukal, J. Brus, H. Balcar, L. Brabec, D. Bondarev,
J. Sedláček, Macromol. Chem. Phys. 2014, 215, 1855.
J.-X. Jiang, F. Su, A. Trewin, C. D. Wood, H. Niu, J. T. A. Jones, Y.
Z. Khimyak, A. I. Cooper, J. Am. Chem. Soc. 2008, 130, 7710.
Q. Chen, M. Luo, T. Wang, J. X. Wang, D. Zhou, Y. Han, C. S.
Zhang, C. G. Yan, B. H. Han, Macromolecules 2011, 44, 5573.
J. R. Holst, E. Stöckel, D. J. Adams, A. I. Cooper, Macromolecules
2010, 43, 8531.
Keywords:
Hyper-cross-linked
polymers;
microporous
polymers; acetalization; polyacetylenes; porous polymer
catalysts.
[31]
[32]
[33]
[1]
[2]
[3]
Q. Sun, Z. Dai, X. Meng, F. Xiao, Chem. Soc. Rev. 2015, 44, 6018.
P. Kaur, J. T. Hupp, S. T. Nguyen, ACS Catal. 2011, 1, 819.
S. K. Kundu, A. Bhaumik, ACS Sustain. Chem. Eng. 2015, 3, 1715.
This article is protected by copyright. All rights reserved.

10.1002/chem.201802432
Chemistry - A European Journal
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[34]
[35]
[36]
K. J. Ivin, J. C. Mol, Olefin Metathesis and Metathesis
Mikulov, April, 2017, n.d.
Polymerization, Academic Press, 1997.
[37]
[38]
O. Trhlíková, J. Zedník, H. Balcar, J. Brus, J. Sedláček, J. Mol.
Catal. A Chem. 2013, 378, 57.
S. Gronet, R. A. Moss, M. S. Platz, M. J. Jr, Carbanions, Reactive
Intermediate Chemistry, John Wiley And Sons, New Jersey, 2004.
L. Sekerová, M. Frýbová, E. Vyskočilová, L. Červený, Presented In
Part At The 5th International Conference On Chemical Technology,
J. Brus, Solid State Nucl. Magn. Reson. 2000, 16, 151.
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Entry for the Table of Contents
FULL PAPER
Arenes substituted only with ethynyl
groups were used for polymerization
construction of new hyper-cross-linked
Porous Polymer Catalysts. In the
polymerization, only some ethynyl
groups of the monomers were
Lada Sekerová, Miloslav Lhotka, Eliška
Vyskočilová*, Tomáš Faukner, Eva
Slováková, Jiří Brus, Libor Červený and
Jan Sedláček*
Page No. – Page No.
converted into polymer segments
while the other ethynyl groups
remained unreacted and served as
acid catalytic centres of acetalization
and esterification reactions.
Hyper-cross-linked polyacetylene
type microporous networks
decorated with terminal ethynyl
groups as heterogeneous acid
catalysts for acetalization and
esterification reactions
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