Sulfonated Hyper-cross-linked Porous Polyacetylene Networks as Versatile Heterogeneous Acid Catalysts

Article abstract of DOI:10.1002/cctc.201901815

Two highly sulfonated micro/mesoporous polymers, P(1,3-DEB)-SO₃H and P(1,4-DEB)-SO₃H, with permanent porosity, the specific surface area about 550 m² ? g^?1 and the content of SO₃H groups of 2.7 mmol ? g^?1 were prepared as new acid Porous Polymer Catalysts, PPCs. The PPCs were achieved by easy sulfonation of parent hyper-cross-linked micro/mesoporous polyacetylene-type networks resulting from a chain-growth homopolymerization of 1,3- and 1,4-diethynylbenzenes. New PPCs are reported as highly active and reusable heterogeneous catalysts of esterification of fatty acids with methanol and ethanol, Prins cyclization of aldehydes with isoprenol and intramolecular Prins cyclization of citronellal to isopulegol. The catalytic activity of the micro/mesoporous PPCs (TON values up to 522 mol ? mol^?1) was higher than that of commercial polymer-based heterogeneous catalyst Amberlyst 15 possessing gel texture without permanent pores and that of p-toluenesulfonic acid applied as a homogeneous catalyst.

Full text of DOI:10.1002/cctc.201901815

Accepted Article
Title: Sulfonated hyper-cross-linked porous polyacetylene networks as
versatile heterogeneous acid catalysts
Authors: Lada Sekerová, Pavlína Březinová, Thuy Tran Do, Eliška
Vyskočilová, Jiří Krupka, Libor Červený, Lucie Havelková,
Bogdana Bashta, and Jan Sedláček
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10.1002/cctc.201901815
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Sulfonated hyper-cross-linked porous polyacetylene networks as
versatile heterogeneous acid catalysts
Lada Sekerová[a]*, Pavlína Březinová[a], Thuy Tran Do[a], Eliška Vyskočilová[a]*, Jiří Krupka[a], Libor
Červený[a], Lucie Havelková[b], Bogdana Bashta[b], Jan Sedláček[b]
The preparation of PPCs utilizes organic and polymer synthesis
techniques, which makes possible to prepare PPCs with a wide
spectrum of covalent structures distinguishing thus PPCs from
compositionally less variable heterogeneous catalysts based on
inorganic porous materials.[13–15]
Abstract: Two highly sulfonated micro/mesoporous polymers,
P(1,3-DEB)-SO3H and P(1,4-DEB)-SO3H, with permanent porosity,
the specific surface area about 550 m2·g-1 and the content of SO3H
groups of 2.7 mmol·g-1 were prepared as new acid Porous Polymer
Catalysts, PPCs. The PPCs were achieved by easy sulfonation of
parent hyper-cross-linked micro/mesoporous polyacetylene-type
networks resulting from a chain-growth homopolymerization of
1,3- and 1,4-diethynylbenzenes. New PPCs are reported as highly
active and reusable heterogeneous catalysts of esterification of fatty
acids with methanol and ethanol, Prins cyclization of aldehydes with
isoprenol and intramolecular Prins cyclization of citronellal to
isopulegol. The catalytic activity of the micro/mesoporous PPCs (TON
values up to 522 mol·mol-1) was higher than that of commercial
polymer-based heterogeneous catalyst Amberlyst 15 possessing gel
texture without permanent pores and that of p-toluenesulfonic acid
applied as a homogeneous catalyst.
The diversity of synthetic paths for the preparation of PPCs has been
well documented in publications dealing with the preparation of
organometallic PPCs containing covalently bonded well-defined and
catalytically active organometallic complexes. For example, Ru and Ir
complexes with ethynylated 2,2´-bipyridine ligands were directly
copolycyclotrimerized with tetrakis(4-ethynylphenyl)methane into
organometallic PPCs of the Brunauer-Emmett-Teller surface area,
SBET up 1500 m2·g-1, which were active for aza-Henry reactions and
oxyamination of aldehydes.[16] The PPCs with bipyridine-containing
organometallic segments were also obtained by direct step-growth
condensation polymerization of metallated 2,2´-bipyridine-5,5´-
dicarbaldehyde with melamine cross-linker (SBET up 450 m2·g-1).[17] Xie
et al. reported preparation of organometallic PPCs (SBET = 965 m2·g-1)
from 1,3,5-triethynylbenzene and dibrominated pro-ligand of Salen-
type [Salen = N,N´-bis(salicylidene)ethylenediamine] through the
direct step-growth polymerization performed by means of
Sonogashira coupling and followed by metallation of the parent
networks with Co2+ ions.[18] The same synthetic approaches were
used for the preparation of metal-free organocatalysts of PPC-type.
The direct copolycyclotrimerization was used for preparing PPCs
(SBET up to 731 m2·g-1) with N-heterocyclic carbenes serving as
catalytically active centres of reductive N-formylation of amines with
CO2.[19] The poly(aryleneethynylene)-type PPCs functionalized with
Introduction
The development of heterogeneous catalysts with permanent and
well-defined porosity is one of the main goals of research in the field
of catalysis. The Porous Polymer Catalysts, PPCs, represent a new,
interesting and promising subclass of such heterogeneous
catalysts.[1–8] The PPCs are insoluble and non-swellable polymers
containing permanent pores in the size range of micropores and in
some cases also mesopores and the catalytically active centres as
the part of the micro/mesoporous surface. In terms of polymer
architecture, the PPCs are highly aromatic polymer networks the
permanent porosity of which results from the rigidity of the network
segments and extensive cross-linking. Due to the permanent porosity,
the PPCs are fundamentally different from conventional polymer
supported catalysts based on solvent-swellable polymers.[9–12]
Tröger’s base (SBET
=
750 m2·g-1) were prepared from
1,3,5-triethynylbenzene and diiodinated Tröger’s base monomers
through direct step-growth polymerization performed by means of
Sonogashira coupling.[20] The direct copolymerization through
[a]
[b]
MSc. L. Sekerová; MSc. P. Březinová, T.T Do, Prof. E. Vyskočilová;
Prof. J. Krupka; Prof. L. Červený
Department of Organic Technology
University of Chemistry and Technology Prague
technicka 5, Prague 6, 166 28, Czech Republic
E-mail: lada.sekerova@vscht.cz; Eliska.vyskocilova@vscht.cz
MSc. L. Havelková; Dr. Bogdana Bashta; Prof. J. Sedláček
Department of Physical and Macromolecular Chemistry, Faculty of
Science
Suzuki–Miyura
coupling
and
tetrakis(4-bromophenyl)ethane,
2,7-dibromo-9-fluorenone, and 1,4-benzene diboronic acid
comonomers were used to prepare donor-acceptor PPCs highly
efficient towards hydrogen evolution.[21] In the case of the above-
mentioned PPCs prepared by direct polymerizations, the
organometallic or heteroatomic segments of the PPCs served not only
as catalytically active centres, but simultaneously participated (as the
rigid network struts) in the formation of the porous texture of the PPCs.
This positively influenced the specific surface area of PPCs.
Charles University in Prague
Hlavova 2030, Prague, 128 43, Czech Republic
Supporting information for this article is given via a link at the end of
the document.
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Nevertheless, the PPCs with catalytically active centres in the
pendant groups were also prepared by direct polymerization,
however, their specific surfaces were lower. For example, the
(SBET = 903 m2·g-1)
prepared
by
radical
chain-growth
copolymerization of divinylbenzene and triallylamine.[29] Du et al.
studied the influence of pore size distribution of SO3H-containing
PPCs on their activity in the fructose conversion to
5-hydroxymethylfurfural.[30] The authors revealed the PPCs with a
hierarchical texture containing both micro- and mesopores to be more
efficient than PPCs containing only micropores. Most probably the
mesopores of PPCs facilitated the accessibility of catalytic centres for
the substrate molecules. To introduce mesopores into the networks,
Du. et al. applied an aerogel template approach.[31]
copolymerization
of
tetrakis(4-ethynylphenyl)methane
and
1,3-dibromobenzene substituted in position 5 by a pyrrolidine
derivative performed through Sonogashira coupling provided PPC of
SBET = 228 m2·g-1, which was active in the asymmetric Michael
addition.[22]
The second method for the preparation of PPCs is a two-step
synthesis in which a porous polymer network is first synthesized and
functionalized in the second step by catalytically active groups. The
postpolymerization modification must be properly optimized so that
the polymer network, after modification, retains a satisfactory porous
texture, in particular, a sufficient specific surface area. Wang et al.
described the synthesis of microporous poly(aryleneethynylene)
networks (SBET from 110 to 529 m2·g-1) subsequently modified into
PPCs by radical thiol-yne reaction with aminothiols accompanied by
Recently we reported the chain-growth polymerization of
diethynylarenes as a powerful tool for template-free preparation of
texturally
hierarchized
micro/mesoporous
hyper-cross-linked
networks.[8,32,33] In this article we report (i) the networks of this type
prepared from commercially available 1,3-diethynylbenzene
(1,3-DEB) and 1,4-diethynylbenzene (1,4-DEB) monomers, (ii) the
smooth postpolymerization sulfonation of parent networks into acid
PPCs proceeding under preservation of micro/mesoporous texture
and (iii) the catalytic activity of sulfonated PPCs in esterification of
fatty acids and Prins cyclization reactions.
a negligible change in SBET
micro/mesoporous polyacetylene network decorated with aldehyde
groups by means of chain-growth polymerization of
.
[23] Recently, we reported the synthesis of
3,5-diethynylbenzaldehyde (SBET = 916 m2·g-1). The parent network
was transformed by condensation reaction with ethylenediamine into
NH2 groups bearing PPC organocatalyst (SBET = 649 m2·g-1) active in
aldol condensation.[24] Several articles described the preparation of
acid PPCs by means of sulfonation of aromatic segments of parent
porous networks. The step-growth knitting polymerization of various
arenes (carbazole,[25] phenol, bisphenol A and 2-naphthol[26,27] and
truxene[28]) was used to prepare parent microporous networks, which
were subsequently sulfonated by chlorosulfonic acid to PPCs
decorated with SO3H groups in the amount of from 0.85 to
3.7 mmol·g-1. The SBET values of sulfonated PPCs (from 180 to
719 m2·g-1) were 50 to 60 % lower than those of the parent networks.
The PPCs were active in esterification or acylation reactions. The
SO3H groups-containing PPC [SBET = 406 m2·g-1, 2.3 mmol(SO3H)·g-1]
was also prepared by sulfonation of the microporous network
Results and Discussion
Preparation and characterization of sulfonated PPCs
Two phenylene-rich hyper-cross-linked microporous polymer
networks,
poly(1,4-diethynylbenzene),
P(1,4-DEB)
and
poly(1,3-diethynylbenzene), P(1,3-DEB) were prepared as parent
materials for the modification by sulfonation (Scheme 1). The
networks were synthesized by the chain-growth homopolymerizations
of respective monomers, 1,4-DEB and 1,3-DEB as described
previously.[32,34] Prepared P(1,4-DEB) and P(1,3-DEB) consisted of
the polyene (polyacetylene) chains hyper-cross-linked by either
1,4-phenylene or 1,3-phenylene links, as shown in Scheme 1.
Scheme 1. Synthesis of parent networks P(1,4-DEB) and P(1,3-DEB) and their sulfonation into P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H
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The parent P(1,4-DEB) and P(1,3-DEB) were sulfonated by using
chlorosufuric acid in DCM (see Experimental part) under conditions
applied by Bhaumik et al. for the sulfonation of the microporous
networks containing carbazole and benzene building blocks.[25] The
sulfonated networks were labelled as P(1,4-DEB)-SO3H and
P(1,3-DEB)-SO3H. Table 1 shows the amount of SO3H groups
introduced into P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H as a function
of the amount of chlorosulfuric acid supplied for the sulfonation of
P(1,4-DEB) and P(1,3-DEB). It is obvious that even less than half
(150 mmol·g-1
) of chlorosulfonic acid applied by Bhaumik
network
(375 mmol·g-1network) may be used for sulfonation of P(1,4-DEB) and
P(1,3-DEB) to achieve the same concentration of SO3H groups in the
sulfonated PPCs (2.7 mmol·g-1PPC). The difference between the
concentration of SO3H groups in P(1,4-DEB)-SO3H and
P(1,3-DEB)-SO3H was insignificant. The concentration of SO3H
groups of 2.7 mmol·g-1
achieved in both P(1,4-DEB)-SO3H and
PPC
P(1,3-DEB)-SO3H means that 43 % of monomeric units in these PPCs
were sulfonated. The smoothness of sulfonation was apparently
supported by the presence of mesopores in P(1,4-DEB) and
P(1,3-DEB) facilitating the transport of the sulfonating agent.
For detail characterization and catalytic testing, the samples
P(1,4-DEB)-SO3H No. 1 (Table 1) and P(1,3-DEB)-SO3H No. 5
(Table 1) were chosen.
Figure 1. FTIR spectra of parent and sulfonated networks
Table 1. The amount of SO3H groups in sulfonated PPCs as a function of
the amount of chlorosulfonic acid used for sulfonation of P(1,4-DEB) and
P(1,3-DEB). Reaction time 3 days, room temperature
The nitrogen adsorption/desorption isotherms (77 K) shown in
Figure 2 confirmed the porous texture of P(1,4-DEB) and P(1,3-DEB)
and indicated that P(1,4-DEB) and P(1,3-DEB) contained both
micropores and mesopores. The presence of micropores was evident
from the steep increase in the adsorbed amount in the initial
adsorption stage. The presence of mesopores was indicated both by
the increase of adsorbed amount at p/p0 > 0.8 and the hystereses of
adsorption-desorption isotherms. The specific surface area, SBET, of
P(1,4-DEB) and P(1,3-DEB) was about 1200 m2·g-1, the values of
micropore volume, Vmi and total pore volume, Vtot are in Table 2.
The nitrogen adsorption/desorption isotherms on P(1,4-DEB)-SO3H
and P(1,3-DEB)-SO3H showed that both networks retained
permanent porosity even after extensive sulfonation (Figure 2).
However, the SBET values of the sulfonated networks (~ 550 m2·g-1)
were about 50 % lower than the SBET values of the parent P(1,4-DEB)
and P(1,3-DEB). The values of Vmi and Vtot decreased to a similar
extent due to the sulfonation (Table 2). Nevertheless, the shapes of
the nitrogen adsorption/desorption isotherms of the parent and
sulfonated networks were very similar (Figure 2) and the Vmi/Vtot ratios
of the parent and sulfonated networks were virtually identical (Table 2).
Thus, the total pore volume of the sulfonated networks was similarly
contributed by mesopores as in the case of their parent counterparts.
Figure S1 (in Supplementary material) shows the comparison of
Network
No
Amount of ClSO3H
used for sulfonation
Amount of SO3H
functional groups
[a]
mmol·g-1
mmol·g-1
network
PPC
1
2
3
4
5
6
7
8
375
150
75
2.7
2.8
2.2
1.6
2.7
2.7
2.4
1.1
P(1,4-DEB)-SO3H
P(1,3-DEB)-SO3H
3
375
150
75
3
[a] based on the content of sulfur determined by elemental analysis
Figure 1 shows the FTIR spectra of the parent and sulfonated
networks. Spectra of both P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H
contained strong bands at about 1040 cm−1 and 1220 cm−1
corresponding to C-S and O=S=O stretching vibrations of SO3H
groups, respectively.[35] The broad bands covering the range of about
2800
-
3700 cm-1 in the spectra of P(1,4-DEB)-SO3H and
P(1,3-DEB)-SO3H were due to OH groups from both non-hydrated
and hydrated SO3H substituents. The FTIR spectra of
P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H exhibited the same bands
as ring-sulfonated linear poly(diarylacetylene)s prepared through
postpolymerization sulfonation by Sakaguchi et al.[36,37]
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micropore size distribution of parent and sulfonated networks. The
sulfonation resulted in a slight shift of the maxima of distributions
towards higher micropore diameters most likely due to the partial
disappearance of the smallest micropores caused by their occupation
with SO3H groups.
spectrometry revealed that water and sulfur dioxide were the
dominant gaseous compounds evolved in the course of
P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H heating (see Figure 4).
Figure 3. SEM images of parent and sulfonated networks
The water was released in the temperature range from 100 to 300 °C
in two steps manifested by two maxima on the respective curves in
Figure 4.[27] At the lower temperature (about 100 °C), desorption of the
water trapped by the physisorption proceeded. At higher temperature,
most probably the water molecules, which hydrated the SO3H groups
were released. The observed release of the water confirmed the
expected hygroscopicity of P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H
originating from the high content of SO3H groups in these materials.
The release of SO2 from P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H
proceeded in a temperature range of 150 °C to 450 °C (see Figure 4).
The SO2 release indicated the decomposition of P(1,4-DEB)-SO3H
and P(1,3-DEB)-SO3H, limiting their use to temperatures ≤ 150 °C.
The temperature programmed desorption (TPD) of pyridine was used
to determine the surface acidity of P(1,4-DEB)-SO3H and
P(1,3-DEB)-SO3H. Considering the results of TGA measurements,
P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H were thermally pre-treated
at 160 °C (for 1 h). The saturation by pyridine was performed at the
temperature of 150 °C.[38] The pyridine desorption provided the
following concentrations of the acid centres: 1.30 mmol·g-1 for
P(1,4 DEB)-SO3H and 1.23 mmol·g-1 for P(1,3-DEB)-SO3H. As can be
seen, the acid centre concentrations were virtually identical for both
sulfonated networks. A comparison of the concentration of the acid
centres determined by the TPD method and the concentration of
SO3H groups in P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H
(2.7 mmol·g-1 by elemental analysis, see Table 2) showed that about
45% of the SO3H groups of the sulfonated networks were accessible
Figure 2. N2 adsorption (full points) and desorption (empty points) isotherms
(77ꢀK) on parent and sulfonated networks.
Table 2. Specific surface area, SBET, micropore volume Vmi and total pore
volume Vtot of parent and sulfonated networks
SBET
Vmi
Vtot
Vmi/Vtot
Network
m2·g-1
cm3·g-1
cm3·g-1
P(1,4-DEB)
P(1,4-DEB)-SO3H
P(1,3-DEB)
1204
552
1181
564
0.43
0.21
0.42
0.21
2.43
1.09
1.69
0.82
0.18
0.19
0.25
0.26
P(1,3-DEB)-SO3H
The sulfonation did not modify the morphology of the networks at least
not to the extent, which could be detected by Scanning Electron
Microscopy, SEM: Figure 3 shows the SEM images of parent and
sulfonated networks, which all consisted of aggregated particles with
diameter from 20 to 50 nm.
Figure S2 (in Supplementary material) shows the Thermogravimetric
Analysis (TGA) thermograms of parent and sulfonated networks.
Evidently, the sulfonated networks were thermally less stable (weight
loss of about 40 % at 500 °C, Figure 4, A and B) than the parent ones
(weight loss up to 10 % at 500 °C). The TGA coupled with mass
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for pyridine molecules. It should, however, be noted that
exchange capacity > 1.7 meq·ml-1];[41] and p-toluenesulfonic acid
(p-TSA) applied as homogeneous catalyst (pKa = -5.4 (measured in
H2O; [42]).
P(1,4 DEB)-SO3H and P(1,3-DEB)-SO3H exhibited
a
higher
concentration of acid centres than classical heterogeneous catalysts
of the zeolite-type.[38–40] Due to the reduced thermal stability of
P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H the desorption of pyridine
(proceeding at temperature > 150 °C) was accompanied by the
decomposition of the sulfonated networks (vide supra). In view of this,
it was not possible to determine the strength of the acid centres of
P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H.
Esterification of higher fatty acid by low-chain alcohols
(Scheme 2) can be used for the production of biofuels (e.g. methyl
esters of colza oil). The homogeneous catalysts such as sulfuric or
p-toluenesulfonic acids, which are known to be corrosive and
environmentally unfriendly, are commonly used to catalyse this
reaction. The use of heterogeneous catalysts can be beneficial owing
to the easy separation of the catalyst from the reaction mixture as well
as the possible reuse of the catalyst. Due to this fact large variety of
heterogeneous catalysts was studied in esterification od various
carboxylic acids with various alcohols including recently published
[
e.g.43,44] sulfonated copolymers of styrene and divinylbenzene.
Scheme 2. Reaction scheme of esterification
The results we achieved in the study of esterification reactions are
summarized in (Fig 5 and 6, Table 3). Respective esters (methyl-,
ethyl- or isopropyl-) of the higher fatty acids (lauric, myristic, oleic,
palmitic or stearic) were the only products detected in the reaction
mixtures.
The esterification of lauric acid by methanol was used as the model
reaction. The results showed that both P(1,4-DEB)-SO3H and
P(1,3-DEB)-SO3H were more active in this reaction than the
commercial heterogeneous catalyst Amberlyst 15 applied at the same
weight concentration (see the conversion curves in Figure 5, A). It was
despite the fact that the concentration of acid centres was higher in
Amberlyst 15 (2.4 mmol·g-1) than in P(1,4-DEB)-SO3H and
P(1,3-DEB)-SO3H (~1.3 mmol·g-1). We believe that the permanent
micro/mesoporous texture of PPC-type P(1,4-DEB)-SO3H and
P(1,3-DEB)-SO3H allowed the reactants to access the catalytic
centres more efficiently than they did with the swollen Amberlyst 15
possessing an unstable gel texture without permanent pores.
P(1,4 DEB)-SO3H and P(1,3-DEB)-SO3H showed very similar
catalytic activity in the esterification of lauric acid by methanol (see
Figure 5, A). This is consistent with the fact that both newly prepared
PPCs exhibited (i) a practically equal concentration of acid centres
and (ii) very similar porous texture parameters (Table 2). The
esterification of lauric acid by methanol proceeded with the highest
initial reaction rate if it was catalysed with p-TSA applied in the same
weight concentration as the above discussed heterogeneous
catalysts (see conversion curves in Figure 5, A). However, this mainly
reflected the high concentration of acid centres in p-TSA
(5.3 mmol·g-1). When comparing the Turn Over Number (TON) values
(Table 3), it is apparent that p-TSA exhibited lower catalytic activity in
Figure 4. Temperature profiles of H2O and SO2 releasing from
P(1,4-DEB)-SO3H (A) and P(1,3-DEB)-SO3H (B) in the course of their heating.
The temperature profiles were determined by TGA coupled with mass
spectrometry.
Catalytic activity
The prepared sulfonated networks [P(1,4-DEB)-SO3H and
P(1,3-DEB)-SO3H] were tested as PPC-type heterogeneous catalysts
in three types of typically acid catalysed reactions: esterification of
higher fatty acids by low-chain alcohols, Prins cyclization of various
aldehydes with isoprenol and cyclization of citronellal. The catalytic
activity of P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H was compared
with that of Amberlyst 15 [sulfonated poly(styrene-co-divinylbenzene),
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the studied esterification (TON = 86 mol·mol-1) than P(1,4-DEB)-SO3H
and P(1,3-DEB)-SO3H (TON ~ 400 mol·mol-1). The higher activity of
prepared sulfonated micro/mesoporous networks was not so
surprising, because less acidic sulfonated mesoporous MCM-41 was
previously described as a more active heterogeneous catalyst for
esterification of fatty acids in comparison to homogeneously applied
p-TSA.[45]
the total conversion was achieved within 24 hours. The reaction
courses did not exhibit any systematic dependence on the chain
length and nature (saturated/unsaturated) of the fatty acids. It should
be noted that the total conversion of all mentioned fatty acids was
achieved within 1 h if the concentration of P(1,3-DEB)-SO3H catalyst
of 10 wt.% was used.
Table 3. Turn over numbers of esterification of various higher fatty acids by low-
chain alcohols using various catalysts; 0.5 mmol of acid, 1 wt.% of catalyst, 3 ml
of alcohol, reflux
Conc.of
acid
centres
mmol·g-1
5.81
TON 24 h
Catalyst
Acid
Alcohol
mol·mol-1
p-TSA
86
Amberlyst 15
P(1,4-DEB)-SO₃H
2.40
1.30
212
393
406
358
37
Methanol
Lauric
Ethanol
Isopropylalcohol
Myristic
Palmitic
Stearic
Oleic
1.23
390
378
393
393
P(1,3-DEB)-SO3H
Methanol
Figure 6. Time course of the conversion of acid in the esterification of various
acids (lauric, myristic, palmitic, stearic, oleic) by methanol using
P(1,3-DEB)-SO3H as a catalyst; 0.5 mmol of acid, 1 wt.% of catalyst, 3 ml of
methanol, reflux
Figure 5. The time course of lauric acid conversion in the esterification of lauric
acid by methanol using various catalysts; 100 mg of acid, 1 wt.% of catalyst,
3 ml of methanol, reflux (A) and esterification of lauric acid by various alcohols
(methanol, ethanol, isopropyl alcohol) using P(1,3-DEB)-SO3H as a catalyst;
100 mg of acid, 1 wt.% of catalyst, 3 ml of alcohol, reflux (B)
Prins cyclization is an important reaction in the production of
heterocyclic compounds (containing atoms of oxygen, sulfur or
nitrogen in the ring), which are very often used as intermediates in
syntheses of e.g. pharmaceuticals or fragrant compounds. Prins
Various short-chain alcohols (methanol, ethanol, isopropyl alcohol)
were used for esterification of lauric acid with P(1,3-DEB)-SO3H
catalyst (Figure 5, B). Both the achieved conversions at 1 h
(Figure 5, B) and the TON values (Table 3) decreased in the order
methanol > ethanol >> isopropyl alcohol, which was in good
agreement with literary data for the esterifications catalysed with other
catalysts.[46] Various higher fatty acids (C12 – C18) were used as
substrates for esterification by methanol catalysed with
P(1,3-DEB)-SO3H (see Figure 6 and Table 3). For all the acids tested,
cyclization
can
be
catalysed
by
both
Lewis
acids
(e.g. methyltrioxorhenium or FeCl3)[47,48] and Brønsted acids
(e.g. p-toluenesufonic acid or heteropoly acids)[49,50] The Prins
cyclization of benzaldehyde with isoprenol was chosen as a model
reaction (Scheme 3 A) for the evaluation of catalytic activity of
P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H PPCs (Table 4). Both newly
prepared PPCs were significantly more active in this reaction
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10.1002/cctc.201901815
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FULL PAPER
compared to commercial Amberlyst 15 and p-TSA catalysts (Table 4).
4.0 mol·l-1·min-1·g-1 cat, benzaldehyde 16.3 mol·l-1·min-1·g-1 cat). But the
main reason why the total conversion of the latter mentioned
aldehydes to the desired products was not achieved was the
consumption of isoprenol for the formation of acetals with these
aldehydes. This is in good agreement with our previous findings.[52]
The almost total conversion of aliphatic aldehydes was achieved
within 1 h while using anisaldehyde and cinnamaldehyde as the
substrates only 80% conversion of these compounds was achieved
after 4 h of the reaction. The selectivity to DHP isomers seemed to be
independent on the aldehyde structure.
Scheme 3. Reaction scheme of Prins cyclization of aldehyde with isoprenol
forming substituted tetrahydropyranol (THPol) or dihydropyranes (DHP) as main
products (A) and structures of various aldehydes used as substrates for Prins
cyclization with isoprenol (B)
The conversion of benzaldehyde of 80% was achieved within 30 min
with P(1,4-DEB)-SO3H or P(1,3-DEB)-SO3H, while the time to achieve
this conversion was significantly longer using both Amberlyst 15
(24 h) and p-TSA applied as homogeneous catalyst (4 h). The
cyclization under study provided two expected products: substituted
tetrahydropyranol (THPol) and substituted dihydropyranes (DHP)
(see Scheme 3). The DHP/THPol mole ratios were 7.3 and 3.8 with
Amberlyst 15 and p-TSA, respectively, whereas both
P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H provided the DHP/THPol
ratio of about 1.4 (data corresponding to the conversion of
benzaldehyde of 80 % are reported). The preference of formation of
DHP can be caused by the acidity of used catalysts,[51] which induced
the dehydration reaction. The DHP isomers were preferred using all
mentioned catalysts, the selectivity to DHP isomers was the highest
using Amberlyst 15 (87%) followed by p-TSA (72%) compared to
p(1,4-DEB)-SO3H (53%) and p(1,3-DEB)-SO3H (54%). These results
are in agreement with the fact that DHP isomers are the
thermodynamic products of the reaction and THPol the kinetic one.[49]
Figure 7. Time course of conversion of aldehyde in Prins cyclization of various
aldehydes (benzaldehyde, cinnamaldehyde, anisaldehyde, butyraldehyde and
valeraldehyde) with isoprenol using P(1,3-DEB)-SO3H as a catalyst; 1.2 mmol
of aldehyde and 1.2 mmol of isoprenol, 10 wt.% of P(1,3-DEB)-SO3H, 3 ml
toluene, 70 °C
Table 5. Selectivity to THPol and DHP isomers at 80% conversion of
aldehyde using P(1,3-DEB)-SO3H as a catalyst; 1.2 mmol of aldehyde and
isoprenol, 10 wt.% of P(1,3-DEB)-SO3H, 3 ml toluene, 70 °C
Selectivity to products at 80%
Table 4. Prins cyclization of benzaldehyde with isoprenol using various
catalysts; 1.2 mmol of isoprenol, 1.2 mmol of benzaldehyde; 10 wt.% of
catalyst, 3 ml of toluene, 70 °C
TON 4 h
conversion of aldehyde
Aldehyde
mol·mol-1
%
80% Conversion of benzaldehyde
TON at 24 h
THPol
37
DHP
54
Benzaldehyde
Cinnamaldehyde
Anisaldehyde
84
75
74
93
93
Catalyst
Time to
achieve
h
Product selectivity
%
mol·mol-1
56
49
44
46
33
42
42
45
THPol
DHP
P(1,4-DEB)-SO3H
P(1,3-DEB)-SO3H
Amberlyst 15
p-TSA
0.5
38
53
79
87
39
19
Butyraldehyde
Valeraldehyde
0.5
24
4
37
12
19
54
87
72
Various aldehydes (Figure 7) were used as the substrates for Prins
cyclization with isoprenol (Figure 7, Table 5) with P(1,3-DEB)-SO3H
catalyst. The initial reaction rates were mostly higher when aliphatic
The acid-catalysed intramolecular Prins cyclization of citronellal
is an interesting reaction providing isopulegol as an important fragrant
compound and intermediate for the synthesis of another fragrant
compound, menthol (Scheme 4). Both Lewis acids (e.g. ZnCl2 or ZrO2)
[53,54] and Brønsted acids (e.g. heteropolyacids)[55] can be used as the
catalysts for this reaction.
aldehydes (butyraldehyde 21.6 mol·l-1·min-1·g-1
,
cat
valeraldehyde
16.0 mol·l-1·min-1·g-1 cat) were used as the substrates compared to the
initial reaction rates achieved with benzene ring-containing aldehydes
(cinnamaldehyde
5.9 mol·l1·min-1·g-1
,
anisaldehyde
cat
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10.1002/cctc.201901815
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FULL PAPER
this model reaction three times without significant loss of activity. The
small decrease (about 5%) in obtained yield of isopulegol at the third
use may be caused by partial weight loss of the catalyst between the
uses. The FTIR spectrum of P(1,3 DEB)-SO3H catalyst which was
used in the citronellal cyclization was identical to that of fresh
P(1,3-DEB)-SO3H (see Figure S3 in Supplementary material). This
indicated that the covalent structure of P(1,3-DEB)-SO3H remained
unchanged after its catalytic application. However, a small decrease
of –SO3H functional groups content (from 2.6 mmol·g-1 to
2.0 mmol·g-1) was observed after using the catalyst in the reaction,
which can be induced by the presence of some reactants traces and
is in consistence with the results obtained from N2
adsorption/desorption measurement. The SBET value of the used
P(1,3-DEB)-SO3H (262 m2g-1) was lower than the SBET value of fresh
P(1,3-DEB)-SO3H (comparison of texture characteristics and N2
adsorption/desorption isotherms of fresh and spent P(1,3-DEB)-SO3H
is available in Supplementary material, Table S1 and Figure S4). The
decrease in SBET most likely reflected the collapse or clogging of some
of the pores by the reactants during catalytic application. Although
some textural characteristics changed after the use of the catalyst in
the reaction, it is important to note that P(1,3-DEB)-SO3H can be
repeatedly used in the studied reaction without significant loss of
neither catalytic activity nor selectivity.
Scheme 4. Reaction scheme of cyclization of citronellal to isopulegol
Figure
8
and Table
6
show that P(1,4-DEB)-SO3H and
P(1,3-DEB)-SO3H were highly active not only in esterification and
Prins cyclization with isoprenol but also in the intramolecular
cyclization of citronellal. Nearly quantitative conversion of citronellal
and TON values over 500 mol·mol-1 were achieved with these
catalysts after 4 h of the reaction, whereas the TON values resulting
with Amberlyst 15 and p-TSA were only 131 and 52, respectively
(Table 6). The selectivity to the desired product, isopulegol
(determined at the citronellal conversion of 50 %) was about 90 % with
the P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H catalysts. This was
slightly lower than the selectivity to isopulegol achieved with p-TSA
(100%). The decrease of selectivity to isopulegol was caused by the
formation of the product of aldol condensation of two citronellal
molecules.
Table 7. Reuse of P(1,3-DEB)-SO3H in intramolecular Prins cyclization of
citronellal; 2 g of citronellal, 20 mg of P(1,3-DEB)-SO3H, 30 ml of toluene,
40 °C, 25 min
Yield of isopulegol
Selectivity to isopulegol
Use
%
81
82
76
%
88
91
94
1.
2.
3.
Conclusions
Figure 8 The time course of the conversion of citronellal through the cyclization
catalysed with various catalysts; 200 mg citronellal, 1 wt.% of catalyst, 3 ml
toluene, 40°C
The hyper-cross-linked micro/mesoporous polyacetylene-type
networks with SBET values about 1200 m2·g-1 were prepared by
chain-growth homopolymerization of commercially available
1,3-diethynylbenzene and 1,4-diethynylbenzene monomers. The
permanent porosity of these networks allowed their easy sulfonation
with chlorosulfonic acid into highly sulfonated acid PPCs with
preserved micro/mesoporous texture (SBET about 550 m2·g-1). The
overall concentration of SO3H groups in the sulfonated PPCs was
2.7 mmol·g-1, the concentration of SO3H groups accessible for
pyridine probe molecules (thus for reactants) was about 1.3 mmol·g-1.
We have shown the newly prepared sulfonated PPCs to be highly
active heterogeneous catalysts of esterification of fatty acids with
methanol and ethanol (TON up to 400 mol·mol-1), Prins cyclization of
Table 6. Selectivity to isopulegol at 50% conversion of citronellal
Selectivity to isopulegol
TON 4 h
mol·mol-1
508
Catalyst
%
91
P(1,4-DEB)-SO3H
P(1,3-DEB)-SO3H
Amberlyst 15
p-TSA
91
522
88
131
100
52
As the general advantage of heterogeneous catalysts is that they can
be easily separated from the reaction mixture and reused, the
possibility of the multiple use of P(1,3-DEB)-SO3H was tested in the
cyclization of citronellal (Table 7). The catalyst was twice washed with
2 ml of toluene between each use. P(1,3-DEB)-SO3H can be used in
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10.1002/cctc.201901815
ChemCatChem
FULL PAPER
aldehydes with isoprenol (TON up to 90 mol·mol-1) and intramolecular
Prins cyclization of citronellal to isopulegol (TON up to 500 mol·mol-1).
Using the cyclization of citronellal the reusability of sulfonated PPCs
was confirmed (used three times without significant loss of activity nor
selectivity). The TON values achieved with sulfonated PPCs were
higher than those resulting with commercial polymer-based
heterogeneous catalyst Amberlyst 15 possessing gel texture without
permanent pores. We believe, the high activity of the newly prepared
sulfonated PPCs was apparently due to the high concentration of acid
active centres and the combination of micropores and mesopores in
these catalysts that facilitated the transport of the reactants to the
active centres.
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10.1002/cctc.201901815
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Catalytic test
Experimental Section
P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H were tested as acid
heterogeneous catalysts in three model typically acid catalysed
reactions: Prins cyclization of benzaldehyde with isoprenol,
intramolecular Prins cyclization of citronellal and esterification of
higher fatty acids by short-chain alcohols. The catalytic properties of
P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H were compared with
commercial SO3H groups containing Amberlyst 15 wet and with
p-toluenesulfonic acid.
Materials
All chemicals were used as obtained except for benzaldehyde (p.a.,
Penta, CZ) and dichloromethane (for HPLC, Sigma-Aldrich, CZ),
which were distilled prior to the use. Citronellal (98 %) and
cinnamaldehyde (98 %) were kindly provided by Aroma a.s. (CZ).
1,3-Diethynylbenzene (1,3-DEB) (>96 %), 1,4-diethynylbenzene
(1,4-DEB) (>98 %) and (acetylacetonato)(norbornadiene)rhodium(I)
([Rh(nbd)acac]) were purchased from TCI Chemicals (CZ);
butyraldehyde (>98 %) from Fluka (CZ); ethanol (96 %), isopropyl
alcohol (p.a.), methanol (p.a.), oleic acid (86 %), palmitic acid (p.a.)
and toluene (p.a.) from Penta (CZ); Amberlyst 15 (wet), chlorosulfonic
acid (99 %), isoprenol (97 %), lauric acid (p.a.), myristic acid (>98 %)
and stearic acid (95 %) from Sigma-Aldrich (CZ); valeraldehyde
(97 %) from Acros Organics (CZ) and p-toluenesulfonic acid (p-TSA;
monohydrate, p.a.) from Lachema (CZ). Anisaldehyde (98 %) was
taken from UCT sources and demineralized water was produced by
reverse osmosis at UCT Prague (<1 μS·mL-1).
All reactions were performed in glass flask (25 ml) equipped with
Liebig condenser and placed at the magnetic stirrer (Ares, Velp
Scientifica; 600 rpm) with a heating block (Heidolph; 5 place
monoblock) and temperature regulator (VTF EVO, Velp Scientifica).
Esterification of higher fatty acids (lauric, myristic, oleic, palmitic,
stearic) by short-chain alcohols (methanol, ethanol, isopropyl
alcohol): appropriate amount of catalyst (1 wt.%, 1 mg), 3 ml of alcohol
and 100 mg of acid were placed into the flask. All reactions were
performed at the reflux of used alcohol.
Prins cyclization: appropriate amount of the catalyst (10 wt.%; 10 mg)
was placed into the flask, isoprenol (100 mg, 1.2 mmol), aldehyde
(benzaldehyde, butyraldehyde, valeraldehyde, anisaldehyde or
cinnamaldehyde; 1.2 mmol), and toluene (3 ml) were added. The
reactions were performed at 70 °C.
Preparation of P(1,4-DEB)-SO3H and P(1,3-DEB)-SO3H
Hyper-cross-linked
microporous
polymer
networks,
and
poly(1,4-diethynylbenzene),
P(1,4-DEB)
poly(1,3-diethynylbenzene), P(1,3-DEB) were prepared according to
ref.[34]. The homopolymerizations of 1,4-DEB and 1,3-DEB monomers
(initial concentration 0.6 mol·dm-3) were performed in
dichloromethane at 75 °C [Rh(nbd)acac] complex (concentration
Cyclization of citronellal: appropriate amount of catalyst (1 wt.%,
2 mg) was placed to the flask and 3 ml of toluene and citronellal
(200 mg) were added. The reaction temperature was 40 °C.
18 mol·dm-3) was used as
a
polymerization catalyst. The
Techniques
polymerizations gave quantitative yields of P(1,4-DEB) and
P(1,3-DEB).
Fourier transform IR spectra (FTIR) were measured on a Nicolet
Magna IR 760 using the diffuse reflection mode. Samples were diluted
by KBr before the measurement.
The sulfonation of P(1,4-DEB) and P(1,3-DEB) with chlorosulfonic
acid was carried out through a modified method based on the
procedure described in ref.[25]. The method was optimized with the aim
to minimize the amount of chlorosulfonic acid submitted for
sulfonation. A parent polymer network, P(1,4-DEB) or P(1,3-DEB),
(200 mg) was dispersed in anhydrous DCM (50 ml or less) and kept
under stirring (600 rpm) at 0 °C for 30 min. Then chlorosulfonic acid
(5 mL or less) was added dropwise. The dispersion was continuously
stirred for 3 days at room temperature. The obtained black powder
[P(1,4-DEB)-SO3H or P(1,3-DEB)-SO3H] was filtered and repeatedly
washed with 150 ml of demineralized water until the neutral filtrate
was obtained. After that, the powder was transferred into the round
bottom flask equipped with Dimroth condenser, demineralized water
(150 ml) was added and kept under stirring at reflux for 1 h. This
procedure was repeated two times. Finally, the product was dried
under reduced pressure (100 Pa) at 80 °C.
Thermogravimetric analysis (TGA) was performed on the Setsys
Evolution apparatus (Setaram) under nitrogen atmosphere using
heating rate 10 °C·min-1 in the temperature range 30 and 550 °C.
Thermogravimetric analysis coupled with a mass spectrometer
(TGA-MS) was performed on TG-DTA Setsys Evolution (Setaram,
France) connected with quadrupole spectrometer OmniStarTM
(Pffeifer Vacuum, Germany) under nitrogen atmosphere using
heating rate 10 °C·min-1 in the temperature range 30 and 550 °C.
The nitrogen adsorption/desorption isotherms on the polymers were
measured at 77ꢀK using a Triflex V4.02 apparatus (Micromeritics). The
Brunauer-Emmett-Teller specific surface area, SBET, total pore volume,
Vtot and micropore volume Vmi are reported. The Vmi values were
determined on the base of N2 amount trapped at p/p0 = 0.1. Prior to
the sorption measurements, all samples were degassed on a
Micromeritics SmartVacPrep instrument as follows: starting from the
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10.1002/cctc.201901815
ChemCatChem
FULL PAPER
room temperature the sample was degassed at 353 K (heating ramp
of 0.5 °C·min-1). After 1 h delay at 353 K, the temperature was further
increased up to 383 K with the same heating ramp and the sample
was degassed for 5 h. After that tube with the sample was filled with
N2 and cooled down to room temperature.
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GC analyses of reaction mixture samples were performed on (i)
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detector and non-polar column ZB-5 (Zebron) (for Prins cyclization
and esterification); (ii) Shimadzu GC-17A chromatograph equipped
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Acknowledgements
We acknowledge grant project GACR 17-03474S and GAUK 210119.
This work was realized within the Operational Programme Prague –
Competitiveness (CZ.2.16/3.1.00/24501) and “National Program of
Sustainability” ((NPU I LO1613) MSMT-43760/2015), and we also
acknowledge Specific University Research project (MSMT No 21-
SVV/2019). This work has also been supported by Charles University
Research Centre program No. UNCE/SCI/014.
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Keywords: catalysis•cyclization•sulfonation•porous polymer
catalysts•hyper-cross-linked
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This article is protected by copyright. All rights reserved.

10.1002/cctc.201901815
ChemCatChem
FULL PAPER
FULL PAPER
Highly sulfonated hyper-cross-linked
polyacetylene networks with
permanent porosity and specific
surface area about 550 m2·g-1 were
prepared and successfully used in
esterification and Prins cyclization as
reusable acid heterogeneous
catalysts.
Lada Sekerová*, Pavlína Březinová,
Thuy Tran Do, Eliška Vyskočilová*, Jiří
Krupka, Libor Červený, Lucie
Havelková, Bogdana Bashta, Jan
Sedláček
Page 1-12
Sulfonated hyper-cross-linked porous
polyacetylene networks as versatile
heterogeneous acid catalysts
This article is protected by copyright. All rights reserved.