Acid and base catalysed reactions in one pot with site-isolated polyHIPE catalysts

Article abstract of DOI:10.1039/c9ra01053j

The polyHIPE catalysts based on styrene, vinyl benzyl chloride, and divinylbenzene co-polymerisation were functionalised with carboxylic and tertiary amine groups. Catalyst characterisation showed covalent bonding of the graft polymers. The macroporous and highly interconnected structure of polyHIPEs allows isolation of the acid and base functional groups and allows the presence of these otherwise incompatible functionalities on the same catalyst. The functionalised polyHIPE catalysts were shown to perform two reactions; (i) acid-catalysed acetal hydrolysis and (ii) base-catalysed Knoevenagel condensation in one-pot with 97% yield. The yield obtained is substantially higher than that observed with the homogeneous or resin polymer type catalysts due to the compartmentalisation of the active sites and improved mass transfer through the open porous polyHIPE structure.

Full text of DOI:10.1039/c9ra01053j

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Acid and base catalysed reactions in one pot with
site-isolated polyHIPE catalysts†
Cite this: RSC Adv., 2019, 9, 8175
ab
a
a
*
Erdem Yavuz,
Nikolay Cherkasov
and Volkan Degirmenci
The polyHIPE catalysts based on styrene, vinyl benzyl chloride, and divinylbenzene co-polymerisation were
functionalised with carboxylic and tertiary amine groups. Catalyst characterisation showed covalent
bonding of the graft polymers. The macroporous and highly interconnected structure of polyHIPEs
allows isolation of the acid and base functional groups and allows the presence of these otherwise
incompatible functionalities on the same catalyst. The functionalised polyHIPE catalysts were shown to
perform two reactions; (i) acid-catalysed acetal hydrolysis and (ii) base-catalysed Knoevenagel
condensation in one-pot with 97% yield. The yield obtained is substantially higher than that observed
with the homogeneous or resin polymer type catalysts due to the compartmentalisation of the active
sites and improved mass transfer through the open porous polyHIPE structure.
Received 9th February 2019
Accepted 4th March 2019
DOI: 10.1039/c9ra01053j
rsc.li/rsc-advances
polyHIPE polymerisation. It was then followed by the post-
polymerisation reaction with amine molecules (i.e. morpho-
1. Introduction
line, tris(2-aminoethyl)amine). Acid site addition to polyHIPEs
can be accomplished by post-modication of the polyHIPE by
sulfonation18,19 and it was reported for hydration of
cyclohexane.20
High internal phase emulsions (HIPEs) can be formed with
a droplet (internal) phase volume ratio of up to 99%.1 Curing of
the non-droplet phase followed by the drying of the droplet
phase results in highly porous polymers with interconnected
windows known as polyHIPEs.1,2 These macroporous structures
nd numerous applications, such as membranes for separa-
tions,3 adsorbents for water purication,4 drug delivery
carriers,5 and tissue engineering scaffolds,6 and have been the
subject of recent reviews.7,8 However, the literature is relatively
limited on the catalytic applications, because polyHIPEs do not
possess any catalytic active sites, and thus, the surface modi-
cation is essential to introduce catalytic properties to the
polyHIPEs.
One of the most common strategies to functionalise poly-
HIPEs is the surface-initiated atom transfer radical polymeri-
zation (ATRP).9–11 Surface-initiated ATRP is a controlled gra
polymerization technique which can be used with various
functional monomers at mild temperatures (below 100 ꢀC) in
aqueous or organic solvents to tailor the polymer surfaces.12–16
Catalytic sites of metal nanoparticles, organic functional
moieties, and acid–base functionalities have been introduced to
polyHIPEs by post-polymerisation functionalisation following
the ATRP. The addition of basic sites on polyHIPEs can be
achieved by the amine functionalisation.17 The procedure was
initiated with the commonly employed method of GMA-based
In this work, we introduced vinyl benzyl chloride into poly-
HIPE, a molecule that enables further surface functionalisation
of polyHIPEs,21–23 because the chloromethyl groups allow easy
post-polymerisation functionalisation. We prepared polyHIPE
by polymerising the continuous phase of a water/oil HIPE with
styrene, vinyl benzyl chloride, and 2% (w/w) of divinylbenzene
in the oil phase (Scheme 1). It is known that increasing the
electrolyte concentration reduces the tendency towards Ostwald
ripening – a phenomenon in which small droplets diffuse
through continuous phase and form larger droplets which leads
to the evolution of a non-homogenous structure over time and
eventually destabilise the emulsions.24,25 Therefore, we used
CaCl2, as electrolyte to prevent Ostwald ripening and enhance
the stability of the resulting concentrated emulsions. In addi-
tion, vinyl benzyl chloride co-monomer allowed obtaining pol-
yHIPEs with a moderate cell size. The decreased cell size is
caused by co-adsorption of vinyl benzyl chloride at the emul-
sions interface along with the span 80 surfactant.16 Styrene also
acts as a co-monomer and reduces the initiation sites on poly-
mer support which helps to increase the length of the gra
chains and results in exible chains.
In order to obtain polyHIPE with Brønsted acid sites, we
applied the post-polymerisation functionalisation by gra
polymerisation of tert-butyl acrylate through chlorine initiation
sites on the parent polyHIPE. Further hydrolysis resulted in the
formation of carboxylic-functionalised Brønsted acidic poly-
HIPE. Similarly, gra polymerization of GMA and successive
aSchool of Engineering, University of Warwick, Coventry, CV4 7AL, UK. E-mail: v.
degirmenci@warwick.ac.uk
bIstanbul Technical University, Department of Chemistry, Faculty of Science, 34469
Maslak, Istanbul, Turkey
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra01053j
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Scheme 1 Preparations of VBC–St–DVB polyHIPEs.
modication with diethylamine resulted in basic polyHIPE. The procedure is presented in the ESI, S1.† Briey, 4-vinyl benzyl
main advantage of the polyHIPE open-pore structures over chloride, styrene, divinylbenzene, and span 80 were mixed in
other types of polymer supported catalysts (such as macro- a molar ratio of 0.96 : 1.00 : 0.20 : 0.13 in a round-bottom ask
porous beads) is the decreased diffusion limitations. For tted with a PTFE paddle and a mechanical overhead stirrer.
example in the gel-type beads, the catalytic activity is diffusion The mixture was purged with nitrogen and an aqueous solution
limited due to limitations on solvent accessibility to the active containing K2S2O8 and CaCl2 (also purged with nitrogen) was
sites through the beads.26,27 Highly cross-linked macroporous added dropwise over 30 minutes under stirring of 300 rpm.
(pore diameters [ 50 nm) polymers are proposed to overcome Aerwards, stirring continued for 1 h at 300 rpm and 10 min at
this diffusion limitation.28 Unlike beads, the macroporous 50 rpm to release trapped air. The emulsion was placed in to
polymers do not swell by absorption of the solvent but instead a PET mold and cured at 60 ꢀC for 48 h followed by Soxhlet
reactants diffuse through the porous framework. In this case, extraction with water and isopropanol (each for 24 h) and drying
however, the reaction rates remain low because the rates still in vacuum (2 mbar) for 24 h.
depend on the mass transfer of molecules through diffu-
sion.24,29 PolyHIPEs, conversely, could allow mass transport
2.2 Graing the polyHIPEs
through convection due to their large interconnected pores1,30
Scheme 2 shows a simplied graing procedure of the poly-
where interconnected porosity are among the most desired
HIPEs. Firstly, chlorine sites in the polyHIPE were activated and
properties of a heterogeneous catalyst.31–34
graed with either of tert-butyl acrylate or glycidyl methacrylate.
The obtained materials were either hydrolysed to form poly-
HIPE–COOH or modied with diethylamine to form a poly-
HIPE–NR2.
The applications of these polyHIPEs for organic synthesis
reactions provide promising green chemistry alternatives to
traditional synthetic chemistry. The advantages include easy
separation through ltration and thus lower energy requirement,
The synthesis of polyHIPE–COOH included chlorine initiation
the reuse of the catalyst and lower solvent use in one-pot reaction
performed by combining the 1.0 g polyHIPE with 2.00 mmol
cascades.35–37 Moreover, polyHIPEs render new opportunities
CuBr, 4.00 mmol bipyridine, tert-butyl acrylate (84.0 mmol) in
such as ow chemistry in packed bed or wall coated micro-
12 mL toluene in a 3-neck ask under nitrogen atmosphere. The
reaction was carried out at 80 ꢀC for 20 h, the solid material was
ltered, washed with 50 mL of toluene, ethanol, water, and again
reactors.38 PolyHIPEs allow for impossible otherwise one-pot
cascades of reactions,39,40 provide a signicant improvement in
process intensication due to the use of a single solvent and
ethanol. The graed polyHIPE was transferred into a 2.5 wt%
a single purication step to obtain a product that traditionally
ethylenediaminetetraacetic acid (EDTA) solution for 24 h to
needs several individual synthetic steps.41 We have tested the
remove the remaining copper salt. The graed powdered poly-
HIPE was ltered and washed with excess water, then 30 mL
ethanol and dried under vacuum of 2 mbar at 40 ꢀC for 24 h to
catalytic activity of our Brønsted acidic and basic polyHIPE
materials in the one-pot hydrolysis of benzaldehyde dimethyl
acetal into benzaldehyde followed by the Knoevenagel conden-
sation of benzaldehyde into 2-benzyl-malononitrile. As a result,
provide polyHIPE graed with poly(tert-butyl acrylate).
The graed polyHIPE was hydrolysed to obtain polyHIPE–
polyHIPEs are shown to be efficient and reusable catalysts for the
COOH: 1 g of the material was transferred into a round bottom
one-pot acid and base catalysed reaction cascades.
ask containing 10 mL dioxane and 5 mL HCl at room
temperature.
The mixture was le at reux temperature for 6 h and then
cooled down to room temperature. The resulted polymer was
transferred into 20 mL dioxane and washed with 20 mL ethanol,
2. Experimental
2.1 Synthesis of catalysts
The catalyst synthesis procedure consisted of 3 parts: (i) the 50 mL water, and 20 mL ethanol. The polyHIPE–COOH ob-
ꢀ
synthesis of the polyHIPE material as a catalyst support fol- tained was dried under vacuum of 2 mbar at 40 C for 24 h.
lowed by graing the polyHIPE with either (ii) acid or (iii) base
The polyHIPE–NR2 synthesis was performed using the
functional groups. The initial polyHIPE was obtained by poly- method adapted from ref. 42, where we showed that methac-
merisation of 4-vinyl benzyl chloride and styrene, cross-linked rylate can be graed onto a sulphonamide-based polystyrene
with divinylbenzene, in the presence of the span 80 surfactant resin and modied with diethylamine. The procedure included
and CaCl2 electrolyte (Scheme 1). The detailed synthesis the same chlorine initiation process as before, but tert-butyl
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Scheme 2 Grafting procedure of polyHIPEs to obtain acid- and base-functionalised catalysts.
acrylate was replaced with an equimolar amount of glycidyl
methacrylate. The graing reaction was performed at 65 ꢀC for
20 h. PolyHIPE–PGMA obtained was poured into 50 mL dioxane
and washed with 50 mL ethanol, water, and ethanol. PolyHIPE–
NR2 was obtained by polyHIPE–PGMA modication with amine
functionality by mixing 1 g of the material into a round bottom
ask containing 10 mL diethylamine and 10 mL ethanol at 0 ꢀC.
The mixture was stirred at room temperature for 24 h and then
at 50 ꢀC for 24 h. Aerwards, the material was washed with
50 mL water, ethanol, and dried under vacuum of 2 mbar at
2.3 Catalyst characterisation and testing
Content of functional groups was determined by acid–base back
titration. In case of acid groups, 25 mg of polyHIPE–COOH was
le in contact with 100 mM of NaOH for 24 h at room
temperature. 1 mL of the ltrate was titrated with a 10 mM HCl
solution in the presence of phenolphthalein indicator. In case
of base groups, 25 mg of polyHIPE–NR2 was le in contact with
100 mM of HCl for 24 h at room temperature. 1 mL of the ltrate
was titrated with a 10 mM NaOH solution in the presence of
phenolphthalein indicator. The functional groups were also
measured with chemisorption of ammonia and propionic acid
ꢀ
40 C for 24 h.
Fig. 1 SEM images of the polyHIPE obtained along with (a) void and (b) window diameter distributions.
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Table 2 Results of polyHIPE elemental analysis
vapours from the gas phase by measuring the breakthrough
curves through a polyHIPE samples held at the temperature of
100 ꢀC. The details of the procedure are provided in the ESI, S2.†
Scanning electron microscopy (SEM) images were obtained
with a Zeiss SUPRA 55VP Field Emission Scanning Electron
Microscope operating at 3.0 kV. The samples were prepared by
dispersing the powder onto a double-sided adhesive surface.
Nitrogen adsorption isotherms were measured at À196 ꢀC using
a Quantachrome Quadrasorb SI Surface Area and Pore Size
Analyzer. Prior to measurement, the samples were degassed for
12 h at 100 ꢀC. The average pore size was determined using the
Barrett–Joyner–Halenda (BJH) method. The carbon, nitrogen,
chlorine, and hydrogen contents were determined using
a CE440 Elemental Analyser. Fourier transform infrared (FTIR)
spectroscopy was performed on a Perkin-Elmer Spectrum 100
instrument from a pellet made of KBr mixed with polyHIPE.
Catalyst testing was performed in a 10 mL round-bottomed
ask. In a typical run, the polyHIPE catalyst containing 50
mmol acid functional groups was mixed with the catalyst con-
taining 50 mmol base functional groups and added into
a degassed solution of 500 mmol benzaldehyde dimethyl acetal,
1000 mmol ethyl cyanoacetate, 3.0 mL toluene, 50 mL water and
80 mL tetradecane. The mixture was stirred at 80 ꢀC for 3 h and
the liquid phase was analysed with a Shimadzu GC-2010 gas
chromatograph equipped with a 30 m Stabilwax column and an
FID detector. The concentration was calculated using the
internal standard (tetradecane) and the carbon balance for all
the reaction conditions studied was always above 95%. In
recycling experiments, the solid catalyst was recovered, washed
with toluene, ethanol, and water followed by vacuum drying at
Elemental content (wt%)
Material
C
H
N
Cl
PolyHIPE
78.34
74.83
73.32
71.36
68.13
6.76
6.83
9.06
7.95
6.28
—
—
4.45
—
11.32
4.54
1.02
5.03
6.81
PolyHIPE–PGMA
PolyHIPE–NR2
PolyHIPE–PtBA
PolyHIPE–COOH
—
completely isolated. Through this approach incompatible acid
and base catalysts can be synthesized and used in tandem that
enables consecutive reaction cascades.
Table 1 shows the functional group content determined by
different methods. The gravimetric results (the mass increase
on graing) shows an excellent agreement with the acid–base
titration. Gas-phase chemisorption of either ammonia (for
polyHIPE–COOH) or propionic acid (for polyHIPE–NR2) showed
different results. The capacity of acid groups determined by
chemisorption was considerably underestimated lower likely
ꢀ
because an elevated temperature of 100 C (required to mini-
mise physisorption of ammonia) resulted in adsorption only
over stronger acid sites. The base group capacity determined by
chemisorption was, on the contrary, considerably over-
estimated44–46 likely because of partial dimerization of adsorbed
propionic acid. It is worth noting that the original polyHIPEs
showed negligible functional group content based on the
titration and chemisorption methods. Hence, several alterna-
tive methods show that the functionalised polyHIPEs contain
the desired acid and base functional groups.
Elemental analysis was performed to study the intermediate
forms of polyHIPE (Table 2). A signicant chlorine content was
observed in all the polyHIPEs – the nding indicates that atom
transfer radical polymerization used is a living polymerization
method with a signicant number of initiator groups remaining
on polymers aer gra reactions. Chlorine content, as expected,
decreased aer graing due to its substitution with the
ꢀ
50 C for 24 h. The catalyst was reused with a fresh charge of
solvent and reactants.
3. Results and discussion
3.1 Physicochemical properties of catalysts
First, polyHIPE material was studied with SEM. The
morphology of polyHIPE is complex so we used the terminology
introduced by Cameron and Barbetta43 where the large spher-
ical pores are termed as voids and the smaller pores which
interconnects voids are dened as windows. The hierarchical
pore system of polyHIPE can be clearly seen in Fig. 1. The pol-
yHIPE with 80% porosity has voids with a narrow diameter
distribution between 3 and 10 mm. The material also has
signicant number of smaller windows interconnecting voids
with a diameter of 1–4 mm (Fig. 1b). The larger spherical voids
can be imaged as small reactors. Different functional groups
can be attached onto the interior of the voids which are
Table 1 Content of functional groups determined by various methods
Functional group content (mmol gÀ1
)
Analysis method
PolyHIPE–COOH
PolyHIPE–NR2
Gravimetry
Titration
Gas-phase chemisorption
3.70
3.55
1.75
2.80
2.70
4.85
Fig. 2 FTIR spectra of (a) initial and grafted polyHIPEs, (b) polyHIPE–
PtBA, (c) polyHIPE–COOH, (d) polyHIPE–PGMA, (e) polyHIPE–NR2.
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Fig. 3 Photographs of the reaction mixture containing a polyHIPE grafted with tert-butyl acrylate and an ethylenediaminetetraacetic (EDTA) acid
solution. Blue colour of the solution confirms formation of the Cu–EDTA complexes which indicate successful grafting of the chloride groups.
Two polyHIPE materials were studied (a) a blank one without chlorine groups, (b) the one used previously.
additional of polymeric chains. For example, chlorine content accessible to graing. During chlorine activation, CuBr is oxi-
in the polyHIPE almost halved on graing. A clear demonstra- dised into Cu2+ which is extracted with EDTA forming a blue
tion of the polyHIPE–NR2 formation is the observed nitrogen solution.47 Fig. 3 shows that the blank polyHIPE formed a col-
content of 4.5% - the content corresponds to 3.17 mmol gÀ1 ourless solution, while the initial polyHIPE formed a blue
functional groups. This result agrees well with the functional solution. The experiment indicates that atom transfer radical
group content determined gravimetrically and with acid–base polymerization was not taking place in the absence of chloride
titration (Table 2).
surface groups of the blank polyHIPE sample. Therefore, poly-
Fig. 2 shows FTIR spectra of the materials obtained. The HIPE graing was performed by the substitution of chloride
peak at 1485 cmÀ1 observed in all the spectra which is the groups in the polyHIPE structure forming strongly-bound
characteristic of the aromatic ring originated from the polyHIPE graed functional polymers.
structural motif. A strong peak at 1720 cmÀ1 arises from
The effect of graing onto the polyHIPE surface was studied
carbonyl (C]O) stretching vibrations found in polyHIPE– by nitrogen physisorption with pore size distribution shown in
COOH as well as polyHIPE–PtBA and polyHIPE–PGMA mate- Fig. 4. The data shows that the micro and mesoporous struc-
rials. The latter also shows strong peaks at 1250 and 905 cmÀ1 ture was not affected by graing indicating that the functional
corresponding to, respectively, the symmetric and asymmetric polymers were attached mainly onto the outer surface of the
vibrations of the epoxy rings. Hydrolysis of polyHIPE–PtBA polyHIPE voids. The BET surface area changed insignicantly
(which forms polyHIPE–COOH) results in a shi in the C]O on draing decreasing from 9.9 m2 gÀ1 observed for polyHIPE
peak from 1722 cmÀ1 to 1619 cmÀ1 and broadening of the OH to 9.2 m2 gÀ1 and 9.7 m2 gÀ1, respectively, observed for poly-
stretching vibrations between 2500–3300 cmÀ1. Both observa- HIPE-g-PGMA and polyHIPE–PtBA. The modest surface area
tions conrm the formation of the carboxyl groups. Therefore,
the spectroscopy data shows that gra reactions were per-
formed successfully forming the anticipated compounds
(Scheme 2).
Characterisation data shows that the desired functional
groups were successfully applied onto the catalyst. We antici-
pated to perform graing via the chlorine groups evenly
distributed throughout the polymer surface. Their distribution
and the nature of binding, however, might be disputed. On one
hand, the substitution of chlorine groups with gra polymers
was also expected to form strong covalent bonds. On the other
hand, thermal polymerisation of the graed molecules could
have taken place forming a loosely-bound polymers trapped
inside the polyHIPE structure. The latter arrangement is
undesirable because it provides the possibility of leaching the
active groups – the groups would react in a way similar to the
homogeneous reactants.
Hence, we studied experimentally the tert-butyl acrylate
graing procedure onto two polyHIPE materials: the one used
previously and a blank polyHIPE obtained without vinyl chlo-
Fig. 4 N2 Adsorption isotherms of polyHIPE; polyHIPE–PGMA and
robenzene. The blank polyHIPE contained no chlorine groups polyHIPE–PtBA.
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observed agrees with the SEM data on prevailing macro- between benzaldehyde (2) and malononitrile to form benzyli-
porosity (>50 nm) of the polyHIPEs.
denemalononitrile (3), Scheme 3.
Macroporosity of the graed polyHIPEs was studied by the
The combination of polyHIPE–COOH and polyHIPE–NR2
analysis of the SEM pictures as shown in Fig. 5. Graing had catalysts in the one-pot cascade reaction gives the product yield
little effect onto the apparent morphology – the materials (3) of 97% aer 3 h reaction at 80 ꢀC (Table 3, entry 1). The result
observed were macroporous structures formed of large voids proves that no signicant neutralization occurred during the
interconnected with smaller windows. Hence, the graing reaction. The catalytic reaction that took place at a room
procedure did not damage the original polyHIPE structure.
temperature gave only 57% yield (3), which could be attributed
to the reduced reaction rates at a lower temperature (Table 3,
entry 2). When the catalytic reaction was carried out in the
presence of only PolyHIPE–NR2, almost no product (3) forma-
tion was observed (Table 3, entry 3). Interestingly, de-
acetalization reaction proceeded with a 10% yield of benzalde-
hyde (2), which might have been catalysed by water. When we
bypassed the rst reaction stage and combined benzaldehyde
with malononitrile, 100% yield was observed in the presence of
the polyHIPE–NR2 catalyst (Table 3, entry 4). Therefore, the
3.2 Activity tests for one-pot reaction cascades
Solid catalysts with compartmentalised functionalities has the
potential to perform notably different reactions in a single pot.
To study this concept, we used polyHIPE materials with well-
dened porosity and functionality in
a model reaction
cascades of acid-catalysed hydrolysis of benzaldehyde dimethyl
acetal (1) and base-catalysed Knoevenagel condensation
Fig. 5 SEM images of polyHIPE-g-PGMA (a1–a3), polyHIPE-g-PtBA (b1–b3) and catalyst mixture after the reaction (c1–c3). Scale bars are 20 mm.
Scheme 3 One-pot cascade reaction: acid-catalysed hydrolysis of benzaldehyde dimethyl acetal and Knoevenagel condensation between
benzaldehyde and malononitrile.
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Table 3 Catalytic performance of acid and base functional polyHIPEs in one-pot cascade reactions: hydrolysis and Knoevenagel condensation
Catalyst
Conversion (1) (%)
Yield (2) (%)
Yield (3) (%)
a
b
1
2
3
4
5
6
7
8
PolyHIPE–COOH + polyHIPE–NR2
PolyHIPE–COOH + polyHIPE–NR2
100
100
11
100
100
100
54
3
43
10
0
96
69
29
36
97
57
1
100
4
31
25
64
c
PolyHIPE–NR2
PolyHIPE–NR2
d
e
Benzoic acid + polyHIPE–NR2
PolyHIPE–COOH + anilinef
A-21 + CG50 Type 1g
h
PolyHIPE–COOH + polyHIPE–NR2
100
a
Reaction conditions: benzaldehyde dimethyl acetal (0.50 mmol), malononitrile (1.0 mmol), catalyst (polyHIPE–NR2, 10 mol%, + polyHIPE–COOH,
(10 mol%) referred to benzaldehyde dimethyl acetal), anhydrous toluene (3 mL) + H2O (50 mL), tetradecane (50 mL), 80 ꢀC, 3 h. The same as (a)
b
except room reaction temperature. The same as (a) except using only the basic polyHIPE–NR2 catalyst. The reaction with benzaldehyde (0.50
c
d
mmol), malononitrile (1.0 mmol) and polyHIPE–NR2 catalyst at 80 ꢀC, 3 h. The same as (a), but polyHIPE–COOH was replaced with benzoic
e
acid (10 mol%). The same as (a), but polyHIPE–NR2 was replaced with aniline (10 mol%). The same as (a), but with the (Amberlite CG50 –
f
g
Type 1) and basic (Amberlyst A-21) resins instead of polyHIPE. Equimolar functional group substitution. The same as (a), but the catalysts
h
were removed aer 1 h and reaction solution was allowed to react for another 2 h.
polyHIPE catalysts efficiently catalyse the one-pot cascade amine and acids leached out, we performed Sheldon ltration.37
reactions that requires acid and base catalysts present at the The cascade reactions were performed for 1 h achieving 64%
same time in the reaction medium and the second reaction product (3) yield. Aer hot ltration, the reaction mixture was
proceeds to full completion.
allowed to react for another 2 hours, but the yield did not
Additional experiments were carried out to understand increase further (Table 3, entry 8). Hence, the polyHIPEs are
whether the heterogeneous polyHIPE catalyst can be truly heterogeneous catalysts operating via surface-bound
substituted with soluble homogeneous counterparts. Therefore, functional groups which show negligible leaching.
the reaction cascades were carried out with polyHIPE–NR2 and
The recyclability of the polyHIPE catalysts were examined by
benzoic acid (as a soluble acid catalyst to substitute polyHIPE– isolating the porous catalysts from the reaction medium via
COOH). In this case, product (3) was obtained with a minor ltration followed by washing with toluene, ethanol, water and
yield, while benzaldehyde (2) was the main product (Table 3, toluene again respectively to regenerate the active sites (in
entry 5). When the reaction cascades were carried out with 10 mL solvent for 2 h). With this procedure, the catalyst can be
polyHIPE–COOH and soluble amine (aniline to substitute pol- recycled at least 4 times with only a minor loss of activity (Fig. 6).
yHIPE–NR2), the yield of product (3) was only 31% (Table 3, It should be noted that some acidic and basic groups can be
entry 6). The results conrm that compartmentalization is vital neutralized in this process, but the recycling experiment
for the one-pot cascade reaction. Small organic molecules such conrmed that the majority of the sites were le mostly intact.
as benzoic acid and aniline easily penetrate into acidic and The catalysts aer 4 reaction runs were examined by SEM
basic porous polymers and deactivate the corresponding active (Fig. 5c) which showed no change in the morphology of the
sites resulting in minor reaction yields.
polyHIPE structure. Hence, it can be concluded that the poly-
Finally, we compared the polyHIPE with commercial resin- HIPE catalysts were stable over several reaction cycles.
based catalysts. PolyHIPE has a macroporous structure with
functional groups on the catalyst surface, while the resin-based
catalysts have a lower porosity with the functional groups
distributed through the beads. In the experiment,
commercially-available acidic (Amberlite CG50 – Type 1) and
basic (Amberlyst A-21) polymeric resins were used in the one-
pot cascade reaction resulting in 54% conversion and only
25% yield (Table 3, entry 7). The commercial resin catalysts
showed signicantly low conversion and product yields than
polyHIPE catalysts which is likely due to the fact that the reac-
tion rates are limited by the diffusion through the polymer
beads. Hence, polyHIPE catalysts have a signicant advantage
compared to commercial polymeric resins due to their open
pore structure allowing accelerated mass transport.
3.3 Catalyst stability and leaching
In order to prove that the catalytic activity is based on
Fig. 6 Catalyst recycling results of the polyHIPE catalyst in the one-
covalently-bounded amine and acid groups rather than the free pot reaction cascades.
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4. Conclusions
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In this work, we showed that the macroporous polymer mate-
rials, polyHIPEs, can be obtained using simple processes. The
catalysts provide highly hierarchical pore system: spherical
pores are the rst level of porosity which are interconnected by
windows – the second level of porosity. Such interconnected
porous systems are ideal for catalysis applications due to
enhanced mass transfer rates. The polyHIPEs obtained were
graed with either carboxylic groups or tertiary amine groups to
introduce acid and base functionality. The characterisation data
conrmed the nature, quantity and strong bonding of the
functional groups to the polyHIPE structure. Strong binding,
this way, allows performing mutually opposite reactions in one
pot. The incorporation of the both acid and base functionalities
into the same polyHIPE material is challenging due to the
incompatibility of these functional groups, which requires
careful optimisation of synthesis procedures, which will be
studied in future.
An example of such a reaction was studied – acid-catalysed
hydrolysis of acetals followed by base-catalysed Knoevenagel
condensation reaction. A combination of base- and acid-
functionalised polyHIPE catalysts showed yield of 97% of the
nal product with good catalyst recyclability an no damage to
the polyHIPE structure was observed. A series of control
experiments demonstrated that the homogeneous acid, bases
as well as commercial resin-based materials provide a substan-
tially poorer result. The unique advantages of graed polyHIPE
catalysts come from macroporous interconnected structure
allowing for excellent mass-transfer rates and compartmental-
ised environment with active sites that allows the isolation of
otherwise incompatible acid and base functionalities simulta-
neously. Long term catalyst stability tests are needed for scale
up and industrial application, which will be studied in future.
˘
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The authors declare no conict of interest.
Acknowledgements
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EY and VD acknowledge the Scientic and Technological
Research Council of Turkey (TUBITAK) for nancial support
through International Post-Doctoral Research Fellowship
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