Poly(vinylsulfonic acid)-grafted solid catalysts: New materials for acid-catalysed organic synthetic reactions

Article abstract of DOI:10.1039/c0gc00241k

The synthesis, characterisation and application of novel high-density poly(vinylsulfonic acid)-grafted solid acid catalysts are described. A graft, radical polymerization procedure was employed, allowing the immobilisation of the acid form of vinylsulfonic acid monomer onto various carrier materials, such as polystyrene, silica or polysaccharide-based gels. The highest acid-exchange capacity (as determined by acid-base titration methods) achieved with these new materials was 5.2 mmol H⁺ g^-1. The properties of these PVS-grafted materials as solid state acid catalysts have been examined from several perspectives, including their fundamental properties as materials with extremely high acid dissociation characteristics, their structural features as revealed from IR and solid-state NMR measurements, their thermal stability properties, and their surface morphologies, humidity dependencies and functionality. Compared to many other types of acid catalysts, these high-density poly(vinylsulfonic acid)-grafted materials demonstrated superior catalytic performance in esterification, Friedel-Crafts acylation, and condensation reactions. Moreover, these novel materials show high stability, significant anticorrosion capability and can be easily recycled. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0gc00241k

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PAPER
www.rsc.org/greenchem | Green Chemistry
Poly(vinylsulfonic acid)-grafted solid catalysts: new materials for
acid-catalysed organic synthetic reactions
a
b
a
b
Teruyuki Okayasu, Kei Saito, Hiroyuki Nishide* and Milton T. W. Hearn*
Received 21st June 2010, Accepted 2nd August 2010
DOI: 10.1039/c0gc00241k
The synthesis, characterisation and application of novel high-density poly(vinylsulfonic
acid)-grafted solid acid catalysts are described. A graft, radical polymerization procedure was
employed, allowing the immobilisation of the acid form of vinylsulfonic acid monomer onto
various carrier materials, such as polystyrene, silica or polysaccharide-based gels. The highest
acid-exchange capacity (as determined by acid–base titration methods) achieved with these new
+
-1
materials was 5.2 mmol H g . The properties of these PVS-grafted materials as solid state acid
catalysts have been examined from several perspectives, including their fundamental properties as
materials with extremely high acid dissociation characteristics, their structural features as revealed
from IR and solid-state NMR measurements, their thermal stability properties, and their surface
morphologies, humidity dependencies and functionality. Compared to many other types of acid
catalysts, these high-density poly(vinylsulfonic acid)-grafted materials demonstrated superior
catalytic performance in esterification, Friedel–Crafts acylation, and condensation reactions.
Moreover, these novel materials show high stability, significant anticorrosion capability and
can be easily recycled.
Introduction
yield and selectivity can be obtained with such catalysts, the use
of corrosive mineral acids however necessitates a neutralization
step after the reaction is completed to separate the product from
the hazardous acids, generating considerable waste. Similarly,
metal-containing Lewis-acid catalysts also require careful re-
moval after the reaction by adsorption onto, e.g. Fuller’s earth,
which also produces a large amount of waste. For example, a
minimum of four moles of hydrochloric acid and one mole of
hydrated aluminium salt waste are generated for every mole of
product for the majority of industrial Friedel–Crafts processes
The use of recyclable catalysis for chemical reactions is a key
green chemistry goal. Acid catalysed reactions is one area where
1
the use of recyclable catalysts would have great scope. For ex-
ample, acid-catalyzed synthetic reactions such as esterification,
Friedel–Crafts acylation and other condensation reactions have
found numerous important applications, producing many value-
2
added organic intermediates and other products. Preparation
of organic esters and aromatic ketones using acid-catalyzed
esterification and ketonization reactions is of particular im-
portance in the industrial manufacture of compounds used as
food preservatives, pharmaceuticals, cosmetics, perfumes, plas-
ticisers, solvents and other chemical auxiliaries. For example,
bisfurylalkanes obtained by condensation reactions are relevant
intermediates in several fields of macromolecular chemistry, with
their derivatives used in many applications, e.g. as foundry cores
and moulds, corrosion-resistant materials, and as precursors for
graphitic composites and adhesives.
4
involving the reaction of phenyl ethers with acyl chlorides.
Therefore, to achieve minimal waste production, easier product
recovery and greater atom efficiencies, it is imperative that
“greener” catalytic processes are developed.
Within an industrial setting, the use of heterogeneous catalysis
is often favoured for many synthetic reactions. The deployment
of heterogeneous methods with an active acid species can result
in improvement in the overall economical efficiency of the
process and is usually preferable from an environmental point
In conventional practice, strong mineral acids catalysts, such
as sulfuric acid or hydrogen fluoride, are deployed as single
use reagents in homogeneous systems due to their miscibility
in the reaction medium. Lewis-acid catalysts like anhydrous
aluminium trichloride, trifluoroborane or organotin chloride,
5
of view. Many heterogeneous catalysts have been previously
developed and reported for use in acid-catalyzed organic reac-
6
tions, including heteropolyacids, zeolites, and titania. In the
absence of co-additives, these types of heterogeneous catalysts
suffer from the disadvantages of relatively low catalytic activity
under equimolar conditions, acidities that are difficult to adjust,
and a lack of stability.
2,3
have been similarly used for such reactions. Although high
The use of sulfonic acid immobilized inorganic materials has
also attracted attention. Such acid immobilized inorganic mate-
rials characteristically combine extremely high surface areas and
larger pore diameters than conventional inorganic acids such
as zeolites and can be used at lower temperatures. However,
these materials also suffer from the same limitations as many
a
7
Department of Applied Chemistry, Waseda University, Tokyo,
1
69-8555, Japan. E-mail: nishide@waseda.jp; Fax: +81-3-3209-5522;
Tel: +81-3-3200-2669
b
Centre for Green Chemistry, Monash University, Clayton, Victoria,
3
800, Australia. E-mail: Milton.Hearn@sci.monash.edu.au;
Fax: +61-3-9905-8501; Tel: +61-3-9905-4547
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Green Chem., 2010, 12, 1981–1989 | 1981

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other currently used heterogeneous acid catalysts, requiring
special conditions, such as stoichiometric imbalance reactions
to achieve high catalytic activities. For example, when such
heterogeneous acid catalysts are used in esterification reactions,
carboxylic acids are often reacted with large molar excesses
of the alcohol to bias the chemical equilibrium. Nevertheless,
when compared to the mineral or Lewis acid type of acid
catalysts, such solid acid catalysts have a number of distinct
advantages, including their potential for improved ease of
handling and re-use in batch or continuous (flow) liquid phase
reactions, coupled with the possibility for lower toxicity and
corrosiveness.
form of the VSA, thus overcoming previous constraints due
to difficulties in the purification of the acid form of the VSA
monomer by cation exchange from the sodium salt of VSA.
Moreover, these studies document the general versatility of
the immobilised PVS as a solid acid catalyst and demonstrate
the catalytic activity of the fully characterized PVS-grafted
polystyrene (PSt) and related materials in typical acid-catalyzed
reactions such as esterification, acylation and condensation
reactions.
Experimental
Recently, in a preliminary report we described the use of a
poly(vinylsulfonic acid) (PVS)-grafted polystyrene acid catalyst
as an organic solid state acid catalyst. The preparation of
Chemicals
8
Vinylsulfonic acid (VSA) was supplied from Asahi Kasei
Finechem Co., Ltd. and purified as described previously. The
purified VSA did not contain any polymerization inhibitor. The
monomer conversion was 98.1% (0.7% water), and its metal
content was 3 ppb Fe, < 20 ppb Ca, < 10 ppb Na, < 1 ppb Cr,
8
this catalyst involved the macromolecular introduction via a
graft polymerization procedure of multiple sulfonic acid groups
as part of the polymer coating onto the carrier surface. The
catalytic properties of this new solid acid catalyst were thus not
dependent on the surface area of the carrier material per se
but rather involved a readily accessible surface cladding with
many sulfonic acid groups juxtapositioned within a polymeric
network. We have now extended this earlier study and describe
below the applications and synthesis of the highest acid density
sulfonic acid polymer yet prepared as a grafted material
polymer generated by radical polymerization of the acid form of
1
< 2 ppb Cu. The VSA purity was also monitored by H NMR
1
3
1
and C NMR measurements. H NMR (500 MHz, D
d = 6.22 (1H, dd, J = 16.6, 10.2 Hz, CH CH), 5.58 (1H, d, J =
16.6 Hz, CHH CH), 5.33 (1H, d, J = 10.2 Hz, CHH CH); C
NMR (125 MHz, D O; ppm) d = 138.6, 123.4. The radical ini-
2
O; ppm)
2
1
3
2
tiator, 4,4¢-azo-bis(4-cyanovaleric acid) (ACV, Fluka) was used
after drying at 25 C in vacuo. N-Ethoxycarbonyl-2-ethoxy-1,2-
dihydroquinoline (EEDQ), aminomethylated polystyrene beads
(4.0 mmol g NH
◦
9
vinylsulfonic acid (VSA) resulting in an immobilised poly(vinyl-
-
1
-1
sulfonic acid) (PVS, ion-exchange capacity: IEC = 5.2 meq g ),
as the solid state analogue of sulfuric acid. In this manner,
a high density solid acid catalyst was generated via polymer
synthesis with the favourable attributes of reusability. Moreover,
with the immobilised grafted PVS, it can be noted that the
sulfonic acid groups exist in a grafted ‘tentacular’ environment
whereby the diffusional paths and access of the reagents to
the catalytic sites do not have the same constraints as found
with porous silica materials bearing immobilised sulfonic acids
as polymeric monolayers. With such monolayer silica-based
materials, higher catalytic efficiencies can often be achieved
when the acid density is lower rather than higher. In contrast,
with high density immobilised grafted PVS, the catalytic site
accessibility and dispersivity will be different when porous silica
is employed rather than other types of materials because of
the higher order architecture of silica, and these features will
differentiate the catalytic activities. The immobilized PVS’s were
thus expected to have broad application uses, such as a strong
cation exchange for use in water retention, as an anti-static agent,
as a capture material for amines and other basic compounds, and
as heterogeneous acid catalysts allowing high substrate turn-over
numbers due to their high acidity and selectivity, coupled with
their capability to be reused.
2
loading; NH -PSt beads) and 3-aminopropyl-
2
-
1
functionalized silica (1.0 mmol g NH
2
loading; NH –Si) were
2
ꢀ
R
purchased from Aldrich. Sepharose 6 FF was purchased from
GE Healthcare. Other chemicals were of analytical reagent
grade.
Synthesis procedure for ACV-attached PSt
The ACV-attached PSt was synthesized as follows. ACV
(32 mmol) and EEDQ (64 mmol) were dissolved in DMF
(400 mL) in a three-necked flask. The solution was bubbled with
Ar gas for 30 min. NH -PSt beads (4 g) were then immersed in
the solution, and the mixture degassed again for 30 min before
commencement of the reaction, which was carried out at 25 C
for 8 h under an Ar atmosphere. The ACV-attached PSt beads
were washed consecutively with DMF and ethanol and dried
in vacuo at 30 C for 1 day. The ACV-attached PSt beads (9 g)
were obtained as a faint yellow powder. Elemental analysis; C:
70.6%. H: 7.2%, N: 12.3%.
2
◦
◦
Synthesis procedure for the PVS-grafted PSt solid acid catalyst
The poly(vinylsulfonic acid) (PVS)-grafted PSt was synthesized
as follows. VSA (40 mmol) and the ACV-attached PSt beads
(0.5 g) were added in a round-bottom flask. The reaction was
In this paper, we firstly document the extremely high acidity
of VSA in its acid form in both aqueous and water-free state.
Secondly, we describe in detail the synthetic approach employed
for the immobilization of VSA onto various carriers such as
polystyrene, silica and polysaccharide materials under various
experimental conditions to achieve high protonic ion exchange
capacities. Based on this approach, we have obtained PVS
densely-grafted onto different carrier surfaces using a very high
acid purity (>98.1 wt%) and metal-free (<20 ppb) state acid
◦
carried out at 65 C for 2 days under an Ar atmosphere. The
PVS-grafted PSt beads were filtered and extensively washed
three times with water, methanol, and acetone by decantation to
remove any non-immobilized PVS and monomers. The product
was then dried in vacuo at 90 C for 2 days. The PVS-grafted
PSt beads (0.8 g) were obtained as a pale brownish powder.
Elemental analysis; C: 40.7%. H: 6.0%, N: 2.0%, S: 17.6%.
◦
1
982 | Green Chem., 2010, 12, 1981–1989
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Synthesis procedure for PVS-grafted SepharoseꢀR
DTA curves of the samples were measured using a Rigaku
TG8120. T was measured using a TA Q200 differential scanning
g
ꢀ
R
First, epoxy-activated Sepharose was synthesized as follows.
colorimeter. Water uptake of a hydrated sample was measured
ꢀ
R
Suction-dried Sepharose 6 FF (250 g), thoroughly washed with
water (5 mL ¥ 300 mL), was placed in a round bottom flask and
mixed with 2 M NaOH (250 mL) and NaBH (12.4 mmol) at
◦
by immersing the sample in water at 25 C for 48 h, the sample
was then removed and the recovered samples quickly weighed
on a microbalance.
Water uptake was calculated from:
4
room temperature for 2 h by means of an overhead mechanical
stirrer. Epichlorohydrin (150 mL) was then added and the
suspension stirred for a further 21 h. The resulting epoxy-
activated gel was collected by vacuum filtration and washed
with water (5 ¥ 300 mL), 20% ethanol aqueous solution (5 ¥
00 mL), and once more with water (5 ¥ 300 mL). Secondly, the
aminated Sepharose was prepared as follows. An aliquot of
the wet cake of the epoxy-activated gel (240 mL) was suspended
Water uptake = (W - W )/W ¥ 100 [wt%]
s
d
d
(1)
where W
respectively.
The number-average molecular weight (M
molecular weight (M ) and polydispersity index (M
s
and W
d
are the weights of wet and dried samples,
3
ꢀ
R
n
), weight-average
/M ) of
w
w
n
the ungrafted free polymer sample were determined by gel
in 2 M NH
4
OH (240 mL) and gently shaken for 21 h at
◦
◦
ꢀR
permeation chromatography (GPC) at 40 C using a system
28
C. The aminated Sepharose gel was then washed with
equipped with a Tosoh guard column (TSKguardcolumn PWXL
.0 mm ¥ 4 cm) and two TSKgel 10 mm columns (G3000PWXL
and G4000PWXL) with a differential refractive index detector
Shimadzu, RID-10A). Water containing 20% acetonitrile and
.2 M NaNO was used as an eluent at a flow rate of 1.0 mL
min . The sample was calibrated with PSS standard samples
,
1
0 volumes of distilled water. The process employed for the
6
graft polymerization with VSA after preparation of the aminated
carrier was similar to the procedures used for the preparation of
the ACV-attached PSt and the PVS-grafted PSt.
(
0
2
-
1
Synthesis procedure for PVS-grafted silica
-
1
with M value in the range of 1,000,000–910 g mol .
n
PVS-grafted silica was prepared using NH
similar to the ACV-attached PSt and PVS-grafted PSt. The PVS-
grafted silica was obtained as a pale brownish powder.
2
–Si in a manner
1
H solid-state MAS NMR spectra were recorded on a JEOL
CMX-400 spectrometer using a double-resonance MAS probe
supporting rotors of 2.5 mm o.d. The resonance frequency of
1
H is 399.8 MHz. The dried samples were spun at 24 kHz. The
Synthesis procedure for PSS-grafted PSt
p/2 pulse lengths were set to 2 ms. A recycle delay of 3 s was
1
3
used. C Solid-state CP/MAS NMR spectra were recorded on
a JEOL CMX-400 spectrometer using a double-resonance MAS
probe supporting rotors of 5 mm o.d. The resonance frequency
Poly(styrenesulfonic acid) (PSS)-grafted PSt was synthesized
using NH -PSt (4 g) in a manner similar to the ACV-attached
PSt and the PVS-grafted PSt. The grafting monomer, 4-
styrenesulfonic acid was prepared by cation-exchange of sodium
-styrenesulfonate. The PSS-grafted PSt was obtained as a pale
2
1
3
of C was 100.5 MHz. The dried samples were spun at 10 kHz.
1
3
A contact time of 5 ms and a recycle delay of 5 s were used.
C
4
chemical shift was referenced to hexamethylbenzene. XPS data
were obtained using a JEOL JPS-9010TR spectrometer with an
Mg-Ka line source. All spectra were calibrated with respect to
the position of the carbon 1 s peak at 284.6 eV.
yellowish powder.
Material characterisation
UV spectra were obtained with a JASCO V-670 UV-vis-NIR
spectrometer. The effective range was from 190 nm to 2700 nm.
Catalytic reactions
+
The calculated H
0
values were based on the value at [H ] =
-
1
1
mol L . The water content of the sulfonic acid derivatives
was determined by Karl Fischer coulometry using a Metrohm
31 KF apparatus. Anolyte and catholyte were coulomat AG
The catalytic performance of the new catalysts was tested for
esterification, acylation and condensation reactions. Catalytic
reactions were carried out in a three-necked round-bottom
flask fitted with a reflux condenser to prevent the escape of
volatile liquid components. The flask with its content was
heated at a constant temperature in a heating oil bath and
stirred continuously. The mixture was heated to the desired
reaction temperatures, and then the dry catalyst was added
with continuous stirring. For the esterification reactions, ten
recycling cycles were examined without loss of catalytic activity,
whilst the recycling tests for the catalytic condensation reaction
of 2-methylfuran with acetone were carried out five times. The
used catalyst was filtrated from the reaction mixture and reused
in a new reaction cycle without any treatment. In order to
monitor the progress of the reaction, reaction products were
8
and AK. The water content of all sulfonic acid derivatives was
less than 0.6 wt%. The acid capacities of the solid acid catalysts
were determined by acid–base titration with 0.02 M NaOH using
a Metrohm 808 Titrando autotitrator & 801 stirrer. The titration
experiments were performed in duplicate and the average num-
ber for the acid capacities reported. The particle diameters were
measured by a Nikon SMZ-2B stereoscopic microscope and a
Keyence VHX-900 digital stereoscopic microscope. SEM images
were recorded on a Hitachi S4500S field-emission scanning
electron microscope. The organic content of the synthesized
materials was quantified by elemental analysis performed on
a Carlo Erba Elemental Analyser EA CHNS analyzer. Fourier
transform infrared (FT-IR) spectra of samples were recorded on
JASCO FT/IR-6100 with KBr pellet technique. The effective
collected at different time intervals, separated from solid catalyst
particles by means of a syringe filter, and analyzed by
1
H
-
1
-1
1
range was from 400 cm to 4000 cm . Thermal stabilities were
evaluated by thermogravimetry-differential thermal analysis
NMR in CDCl
3
using internal TMS as standard. H NMR
spectra were recorded using a JEOL-Lambda 500 and ECX 500
spectrometers. Commercial Amberlyst 15 catalyst (Aldrich)
ꢀ
R
(
TG-DTA) and differential scanning calorimeter (DSC). TG-
This journal is © The Royal Society of Chemistry 2010
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View Article Online
ꢀ
R
and Nafion silica composite catalyst (SAC-13, Aldrich) were
also used for the same reaction as the heterogeneous catalyst
control of all reactions. Sulfuric acid, vinylsulfonic acid, and
benzene sulfonic acid were used as the homogeneous catalyst
controls in the esterification reactions.
of VSA, individual water molecules behave as a base with a more
pronounced solvating behaviour towards ionised protons rather
than maintaining the bulk hydrogen bonded network of water
as a solvent, the absorbance of the unprotonated form of the
indicator increased, and the H value in an aqueous solution
0
was higher than observed for a non-aqueous solution.
Results and discussion
Preparation of the PVS-grafted acid catalysts and their
characterisation
Acid dissociation ability of the high-purity and metal-free acid
form monomer of VSA
As part of these investigations, PVS has been grafted onto
a variety of carrier surfaces, such as polystyrene, silica and
cross-linked agarose, by radical polymerization. Scheme 1 shows
an example of the procedure employed with an immobilized
initiator and PVS. In this case, the preparation of the grafted
PVS catalyst involved two steps: (a) the introduction of the
radical initiator onto the carrier surface, and (b) radical poly-
merization of the acid form of monomeric VSA. Firstly, the
aminated carrier surface was allowed to react with 4,4¢-azobis(4-
The remarkably high acid dissociation ability of the high-
purity and metal-free acid form of VSA was elucidated from
a combination of spectroscopic and Hammett methods. Thus,
the Brønsted acidic scale of various sulfonic acid derivatives was
determined using UV-visible spectroscopy with a basic indicator
10
(
p-nitroaniline; pK(I)aq = 0.99). The Brønsted acidity was
evaluated from the Hammett acidity functions. The properties
of protons depend on both the nature of the solvent and the
nature and concentration of the acid. In a given dissociating
11
cyanovaleric acid) (ACV), a functional free radical initiator.
solvent (s), the Hammett function (H
= pK(I)aq + log([I]
where pK(I)aq is the pK
0
) is defined as
N-Ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was
then used as a condensation agent. Previously we have shown
that EEDQ, which is available at low cost, allows the coupling of
acylamino acids with amino acid esters in high yield in a single
+
H
0
s
/[IH ]
s
)
(2)
a
value of the indicator referenced
+
12
to an aqueous solution, and [IH ]
s
and [I]
s
are the molar
one-pot operation. Then, the ACV-attached PSt was mixed
concentrations of the protonated and unprotonated forms of
the indicator in the solvent(s), respectively. Due to the high
acidity of the VSA sample, the absorbance at 378 nm of the
unprotonated form of the basic indicator decreased (Fig. 1).
with the acid form of VSA and the mixture heated to initiate the
polymerization via radicals anchored at the carrier surface.
+
Hence, the [I]/[IH ] ratio was determined from the measured
absorbance differences after the addition of the acid sample,
and the acidity order and the H values of VSA determined.
0
Fig. 1 Absorption spectra of p-nitroaniline for various concentrations
of VSA (a) in water and (b) in acetonitrile.
From these results, a linear correlation was established
between the H
showing a relatively low acid function value (H
pared to other organic sulfonic acids, such as methanesulfonic
acid (MSA; H = 1.7) or benzenesulfonic acid (BSA; H = 2.3)
in water. Moreover, VSA showed surprisingly strong acidity
= -1.4) in non-aqueous solutions. This result indicates that
VSA exhibits a very high acid dissociation propensity compared
to other sulfonic acid derivatives both in aqueous and non-
aqueous solutions. This tendency is derived from the strongly
electron withdrawing nature of the vinyl group and concomitant
stabilization of the sulfonate anion in VSA. Since in the presence
0
values and the proton concentration, with VSA
0
= 0.74) com-
Scheme 1 (a) Polymerization of vinylsulfonic acid; (b) preparation for
grafting a carrier surface with poly(vinylsulfonic acid).
0
0
(
H
0
For aminomethylated PSt beads (NH -PSt) as a starting
2
material, the conditions for the graft polymerization of VSA are
given in Table 1. The number of sulfonic acid groups introduced
onto the carrier surface by the PVS graft polymerization was
measured by a neutralization titration method. The introduction
of the sulfonic acid groups was similarly confirmed by elemental
1
984 | Green Chem., 2010, 12, 1981–1989
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a
b
Table 1 Graft polymerization conditions of vinylsulfonic acid onto a carrier surface
c
Immobilized acid groups
(mmol H g )
Entry
Carrier
VSA/mmol
Solvent
[Solvent]/mol L-
1
[Surfactant ]/mmol L
-1
Reaction time/h
+
-1
1
2
3
4
5
6
7
8
9
PSt
PSt
PSt
PSt
PSt
PSt
PSt
PSt
Si
40
40
20
20
20
20
20
20
40
20
20
20
Bulk
Bulk
Water
Water
Water
—
—
4
4
4
4
10
4
—
4
4
—
—
2
20
48
48
48
20
20
20
20
20
20
20
20
3.8
5.2
0.8
1.0
0.7
2.2
3.0
2.2
1.8
0.7
1.0
Dissolution
20
100
—
—
—
—
20
—
—
d
TEG
DMF
DMF
Bulk
Water
DMF
DMF
1
1
1
0
1
2
Si
Si
ꢀ
R
Sepharose
4
a
ꢀR
◦
b
c
ACV-attached PSt = 0.5 g, ACV-attached Si = 0.2 g, ACV-attached Sepharose = 0.2 g, reaction T = 65 C. Tween 85. Calculated by titration
d
experiment. TEG: Tetra(ethyleneglycol).
+
analysis. VSA was successfully used as the acid form monomer
in this direct graft polymerization onto the carrier surface. The
graft polymerization under the VSA monomer bulk condition
shown in Table 1, entries 1–2 gave extremely high sulfonic acid
the PSS-grafted PSt (3.9 mmol H g-1), which was obtained
from the radical polymerization of 4-styrenesulfonic acid in
a manner similar to the preparation of the PVS-grafted PSt.
However, when water/surfactant or DMF solution conditions
were used, the introduced levels were not as high as obtained
for the PVS immobilized samples prepared with the monomer
+
-1
densities (5.2 mmol H g as a maximum loading). The graft
ratio of the PVS-grafted PSt was 128% as calculated from the
neutralization titration of the PVS-grafted PSt. On the other
hand, uniformly lower acid densities, approximately 1.0 mmol
ꢀ
R
bulk conditions. When Sepharose was used as a carrier, the
carrier was degraded by the high acidity of the VSA. This
outcome did not change even when various other conditions
were explored, such as changes in solvent composition or
temperature. Accordingly, we selected the PVS-grafted PSt as a
particularly promising material for use as the solid acid catalyst.
+
-1
H g (Table 1, entries 3–5), were obtained with reactions in
aqueous solution, regardless of the amount of added surfactant.
When DMF or tetra(ethyleneglycol) (TEG) was used as solvent,
the PVS-grafted PSt was obtained with comparatively high acid
+
-1
introduction levels (2.2–3.0 mmol H g ). These acid levels of the
PVS-grafted PSt were much higher than those found for strong
Brønsted acid sites in conventional sulfonic acid immobilized
Thermal properties of the PVS-grafted PSt acid catalyst
Along with acid–base titration experiments and elemental
analysis, the introduction of sulfonic acid polymer onto the
carrier surface was confirmed by IR-spectra and H solid state
MAS NMR spectra based on procedures reported previously.
Thermal gravimetric analysis (TG/DTA) was conducted over a
+
-1
7
solid acid catalysts (in the range of 0.5–1.5 mmol H g ).
+
-1
The value of 5.2 mmol H g as a maximum loading of the
PVS-grafted PSt was also higher than the density of sulfonic
acid groups with commercially available solid acid catalysts,
1
8
ꢀ
R
+
-1
ꢀR
such as Amberlyst 15 (4.7 mmol H g ) and Nafion SAC-13
(
◦
◦
temperature range from 25 to 450 C at a heating rate of 10 C
+
-1 13
0.8 mmol H g ).
-
1
min under N
with PVS and compared to the parent support material sample,
NH -PSt. The TG/DTA curves presented in Fig. 2 revealed
that the weight residue between the PVS-grafted PSt and the
2
atmosphere for the materials after modification
In order to determine the molecular weight of the grafted
PVS polymer, the molecular weight of the free PVS polymer was
first determined. The M of the PVS polymer grafted onto the
carrier surface was then correlated to that of the ungrafted free
2
n
NH
2
-PSt were quite different because of the presence of the
14
polymer produced under similar reaction conditions. Since the
molecular weight of the ungrafted free PVS polymer was M
n
=
4
4
1.0 ¥ 10 , M
w
= 2.4 ¥ 10 , the value of M
w
/M = 2.3 at a maximum
n
(
Table 1, entry 2), whilst for the PSS-grafted PSt, the molecular
3
weight of the ungrafted free PSS polymer was M = 4.3 ¥ 10 ,
M
n
4
w
= 1.3 ¥ 10 , with M
w
/M
n
= 3.0.
Furthermore, the graft polymerization of PVS on silica
ꢀ
R
and Sepharose carriers was also investigated, utilising the
same synthetic approach. In the case of the silica carrier, the
introduction levels of VSA were similar to that achieved with
+
the PVS-grafted PSt, i.e. the highest acid density (1.8 mmol H
-
1
g ) was obtained under the VSA monomer standard conditions.
Given the number of the reactive amino groups present in
-
1
the NH
2
-functionalized silica (1.0 mmol g NH
2
loading), the
◦
Fig. 2 TG-DTA curves of the PVS-grafted PSt from 25 to 450 C at a
heating rate of 10 C min under N atmosphere; inset: TG-DTA curves
2
◦
-1
acid density of the PVS-grafted Si was comparable to the high
levels of the PVS-grafted PSt. This tendency did not change for
of the NH -PSt.
2
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high-density grafting organic sulfonic acid polymer. A weight
loss below 110 C was observed for the PVS-grafted PSt, which
the high-density graft polymerization of PVS onto the carrier
surface.
◦
is due to the removal of the physisorbed water from the surface
of the grafted polymer.
The humidity dependence of water uptake was measured for
the PVS-grafted PSt at 25 C under relative humidity close
◦
This physisorbed water is due to the high water absorbing
property of sulfonic acid moieties in spite of the strictly reduced-
pressure drying. The decomposition of the PVS-grafted PSt was
characterised by two general steps, one in the region of 200–
to 75%. The water uptake of the PVS-grafted PSt (174 wt%)
was 10 times higher than that of the NH
2
-PSt (17 wt%). This
result was expected, since water should have little effect on the
NH
2
-PSt beads. Furthermore, the PVS-grafted PSt possessed
◦
◦
ꢀR
300 C and the other over 400 C. The former transitional weight
an extremely high water affinity compared to Amberlyst 15
ꢀ
R
loss involves the oxidation and degradation of sulfonic acid
(114 wt%) or Nafion SAC-13 (95 wt%). From a standpoint
of a swelling property, the PVS-grafted PSt was expanded by
1.2 times from its initial particle diameter as assessed by optical
microscopy.
◦
groups which probably begin near to 200 C. The decomposition
◦
of cross-linked polystyrene begins near to 400 C, a tendency that
was consistent with that of the NH -PSt. Thermal gravimetric
2
analysis results revealed that the PVS-grafted PSt is thermally-
◦
stable up to 200 C in a nitrogen flow atmosphere.
Catalytic activity of the PVS-grafted PSt
The T
g
(glass-transition temperature) of the PVS-grafted PSt
◦
The catalytic performance of the PVS-grafted PSt was eval-
uated in several acid catalyzed reactions, such as esterification,
acylation and condensation reactions. The efficiency of the PVS-
grafted PSt was then compared to other known acid catalysts
was observed at -24 C measured by DSC over a temperature
range from -80 C to 200 C at a heating rate of 10 C min and
at a cooling rate of 30 C min under an Ar atmosphere, and was
almost the same as found for the PVS homopolymer (-27 C).
The slight difference in T values between the PVS-grafted PSt
andPVS homopolymer can be attributed to immobilization onto
the carrier surface.
◦
◦
◦
-1
◦
-1
◦
(
homogeneous catalyst: sulfuric acid, vinylsulfonic acid, and
g
ꢀ
R
benzenesulfonic acid; and heterogeneous catalysts: Amberlyst
1
ꢀ
R
5 and Nafion SAC-13). The experimental conditions were
chosen to enable comparative assessment of the best perfor-
mance of these homogeneous/heterogeneous catalytic materials
in these reactions. The schematic for these catalytic reactions is
presented in Scheme 2.
Surface morphology and humidity behavior of the PVS-grafted
PSt acid catalyst
The particle diameter distribution of the PVS-grafted PS t was
80–100 mm, similar to the starting material as measured by a
stereoscopic microscope, though the particle shape of the NH
2
-
PSt was unequal in size and distinctly heterogeneous.
The surface morphology of the PVS-grafted PSt was mon-
itored by field-emission scanning electron microscopy (FE-
SEM). Typical SEM images are shown in Fig. 3. It is apparent
from these SEM images that the surface morphology of the
PVS-grafted PSt is quite different from the NH
grafted PSt had a more planar surface than the NH
2
-PSt. The PVS-
-PSt due to
2
Scheme 2 Various acid-catalyzed reactions: (a) esterification of car-
boxylic acids with alcohols, (b) acylation of anisole with acetic anhy-
dride, and (c) condensation of 2-methylfuran with acetone.
Esterification is an industrially important reaction, producing
2
organic esters at the level of multi-tonnes per year. Previously,
we have described in a preliminary report the catalytic activities
of a PVS-grafted PSt for esterification reactions using acetic acid
8
with alcohols using equimolar amounts of reactants. Here, we
extend this reaction to various other carboxylic acids. The reac-
tion rate constants for these esterification reactions are shown in
Table 2. All these reactions showed much higher conversion
levels in the presence of the catalyst than in the absence of
catalyst, and the same held for the reaction rate constants.
Fig. 3 Field-emission scanning electron micrographs of the surface
observation of (a) PVS-grafted PSt and (b) NH -PSt.
2
1
986 | Green Chem., 2010, 12, 1981–1989
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a
Table 2 Reaction rate constant of the esterification of carboxylic acids with ethanol catalyzed by various catalysts
(¥10 L mol-1 s )
-
4
-1
Rate constant k
1
b
Catalyst
Acetic acid
Butyric acid
Caproic acid
Lauric acid
Benzoic acid
PVS-grafted PSt
11
2.9
1.5
0.50
12
2.7
1.2
0.42
—
0.87
0.55
0.21
—
0.21
0.022
0.018
—
ꢀ
R
Amberlyst 15
5.6
2.0
18
ꢀ
R
Nafion SAC-13
VSA
BSA
Sulfuric acid
No catalyst
13
18
0.44
9.1
12
0.11
—
—
0.10
—
—
0.076
—
—
0.0011
a
Reaction conditions: carboxylic acid = 0.1 mol, ethanol = 0.1 mol, homogeneous acid catalysts = 0.45 mmol, heterogeneous acid catalysts = 0.1 g,
◦
b
T = 65 C. VSA: vinylsulfonic acid, BSA: benzenesulfonic acid.
The reaction rate constants generally tended to decrease with
increasing alkyl chain length of the carboxylic acid species as the
reactant. Moreover, the acid-catalyzed esterification reactions
were affected more by the nature of the carboxylic acids than
that of alcohols.
Table 3 Product conversion and selectivity of the acylation of anisole
a
with acetic anhydride catalyzed by various solid catalysts
Acid capacity
(mmol H g ) Conversion (%) Selectivity (%)
+
-1
Catalyst
c
PVS-grafted PSt 4.5
50
54
25
62
0
98
97
96
97
0
Even though the reaction rate constants of the PVS-grafted
PSt were lower than that found for the homogeneous catalysts,
they were 2–5 times higher compared to the other solid catalysts.
Significantly, the rate constants of the PVS-grafted PSt were
ꢀ
R
Amberlyst 15
4.7
ꢀR
Nafion SAC-13 0.8
VSA
No catalyst
9.2
—
ꢀ
R
also higher than that of Amberlyst 15, which has almost
equal acid densities, in these esterification reactions. Moreover,
comparison of the PVS-grafted PSt with the Nafion SAC-13 also
confirmed that on an equivalent acid basis the PVS-grafted PSt
was more catalytically active, i.e. with an equivalent acid amount
a
Reaction conditions: anisole = 10 mmol, acetic anhydride = 10 mmol,
◦
b
catalyst = 0.2 g, T = 90 C, reaction time = 1 h. Selectivity toward
c
p-methoxyacetophenone. The other product was the o-isomer. The
sample had the same acid capacity used for the esterification reactions.
◦
of 0.45 mmol at a temperature of 65 C the reaction conversion at
conventional solid acid catalysts in the same molar proportions
(Table 3). In terms of conversion, the catalytic activity of the
2
h with the PVS-grafted PSt was nearly twice that of the Nafion
ꢀ
R
SAC-13. The PVS-grafted PSt structure, due to the nature of
the grafted polymer chains containing numerous sulfonic acid
groups, is presumed to undergo a significant expansion in the
solvent during the esterification reactions. From NMR studies, it
was obvious that the reaction sample contained only the reactant
and the ester product. Thus, there was no by-product formation
or leakage of the PVS from the carrier surface. The PVS-grafted
PSt showed high catalytic activity in the case of 1-butanol. The
reaction rate of the esterification of ethanol or 1-butanol with
PVS-grafted PSt was higher than that of Nafion SAC-13, but
ꢀ
R
lower than that of Amberlyst 15. The outcome was partially
due to the strong adsorption of the p-methoxyacetophenone,
one of the main products obtained in this reaction, on the acid
sites of the PVS-grafted PSt catalyst, and thereby preventing its
further catalytic activity. However, the PVS-grafted PSt showed
a very high p-selectivity, and compared favorably with the other
conventional solid acid catalysts.
The catalysis by the PVS-grafted PSt in the condensation of
2-methylfuran and acetone to 2,2-bis(5¢-methylfuryl)-propane
-
3
-1
acetic acid acid-catalyzed by PVS-grafted PSS (1.1 ¥ 10 L mol
-
1
-4
-1 -1
◦
s
and 7.1 ¥ 10 L mol s , respectively) was much higher than
(bisfuran) at 50 C (molar ratio = 1 : 2.5 : 0.037) were also
-
4
-1 -1
those acid-catalyzed by PSS-grafted PSt (4.4 ¥ 10 L mol
s
compared with those of several other solid acid catalysts. Table 4
shows the results for these various solid acid catalytic reactions.
The PVS-grafted PSt showed excellent catalytic activity. The
catalytic activities of the other solid acid catalysts, such as
-
4
-1 -1
and 2.2 ¥ 10 L mol s , respectively). This result indicates that
the grafted polymer, PVS-PSt, has a extremely high capability as
an acid catalyst compared to other sulfonic acid polymers such
as PSS. This property of the new immobilised PVS-PSt material
thus reflects an important advantage of this type of grafted
polymeric solid catalyst with its high acidity, which translates to
comparatively higher catalytic efficiency and conversion results.
Moreover, the results confirm that the PVS-grafted PSt can be
reused repeatedly without any significant loss of activity.
ꢀ
R
ꢀR
Amberlyst 15 and Nafion SAC-13 under the same catalytic
conditions were much lower than that shown by the PVS-grafted
PSt. The faster reaction progress in the case of the PVS-grafted
PSt compared to the other solid acid catalysts is also shown in
Fig. 4.
From the results of various catalytic reactions, the PVS-
grafted PSt has been found to be highly active even with water-
generating reactions. This behaviour is significantly different to
Similarly, the use of the PVS-grafted PSt led to efficient
Friedel–Crafts acylation of anisole with acetic anhydride. Acetic
anhydride was chosen as an acylating agent instead of acetyl
chloride, in order to simulate avoidance of formation of the
corrosive HCl by-product, which could arise in an actual large
scale process. Anisole was reacted with acetic anhydride with the
ꢀ
R
that found with Amberlyst 15, which is known to be deacti-
vated by water solvation. This property reflects a remarkable
advantage of the PVS-grafted PSt due to the characteristic high
water retentivity of PVS, which favours the equilibrium moving
towards the formation of the targeted product. In order to
establish the reusability of the catalyst for this condensation
◦
PVS-grafted PSt (molar ratio = 1 : 1 : 0.09) at 90 C for 1 h in the
absence of solvent. The same reaction was repeated with other
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Table 4 Product conversion and selectivity of the synthesis of 2,2-bis(5-
steel was hardly observed by XPS in the case of the PVS-grafted
PSt compared with the case of sulfuric acid.
methylfuryl)propane using the PVS-grafted PSt in comparison with
a
commercial solid acid catalysts and literature precedents
b
Acid capacity
(mmol H g ) Time/h (%)
Conversion Selectivity
+
-1
Conclusions
Catalyst
(%)
c
In conclusion, high-density PVS-grafted PSt has been synthe-
sized by graft polymerization of PVS onto the carrier surface
and applied as a new class of solid acid catalysis material.
With the synthetic conditions described, an acid density of
PVS-grafted PSt 4.5
2
2
2
82
59
18
99
94
96
ꢀ
R
Amberlyst 15
4.7
0.8
ꢀ
R
Nafion
SAC-13
HMS-proSO
MCM41-SO
VSA
d
e
3
H
H
1.0e
24
24
2
73
85
99
0
98
96
98
0
+
-1
d
5.2 mmol H
g
for the PVS-grafted PSt could be obtained
3
1.0
9.2
—
as a maximum loading. The PVS-grafted PSt has been found
to be very effective as a heterogeneous acid catalyst in various
synthetic esterification, acylation, and condensation reactions
with very high selectivity. The high catalytic activity of the PVS-
grafted PSt was due to the high density and strong acidity of
the sulfonic acid groups on the carrier surface. As such, this
study introduces a new avenue to carry out such solid phase acid
catalysed synthetic reactions in a more benign, green chemical
manner, whilst minimizing pollution/contamination risks due
to reactor corrosion and allowing the same catalyst to be used
repeatedly in a recycling mode of operation.
No catalyst
—
a
Reaction conditions: 2-methylfuran = 22 mmol, acetone = 55 mmol, cat-
◦
b
alyst = 0.18 g, T = 50 C. Selectivity for 2,2-bis(5-methylfuryl)propane,
c
based on 2-methylfuran. The sample had the same acid capacity used
d
e
for the esterification reactions. Ref. 7(g). Based on the lowest value in
the range quoted in reference.
Acknowledgements
This work was partially supported by the Global COE pro-
gram “Practical Chemical Wisdom” at Waseda University
from MEXT, Japan and the Australian Research Council. The
authors thank Dr T. Shibue for the assistance with the solid-state
NMR measurements, Mr S. Enomoto for the assistance with the
FE-SEM and XPS measurements, Mr P. Maharjan and Mr T.
Hibino for the experimental help. The authors are grateful to
Prof. R. Jackson and Mrs E. Campi for fruitful discussions. We
also thank Asahi Kasei Finechem Co., Ltd. for supplying the
high-purity, metal-free acid form of the VSA monomer.
Fig. 4 Time-conversion curves for condensation reaction of 2-
methylfuran with acetone at 50 C (᭹: the PVS-grafted PSt, ᭺: the
◦
5
th time of recycling PVS-grafted PSt, ꢀ: AmberlystꢀR 15, ꢁ: NafionꢀR
SAC-13).
reaction, the catalyst was recovered by filtration and reused in
subsequent cycles. The catalyst was found to retain its activity
for at least five cycles of reuse. Although the product yield
was slightly decreased from 82 to 74% after the fifth cycle, this
difference was due to the small handling losses of the catalyst
upon recovery. The selectivity remained almost constant over
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Green Chem., 2010, 12, 1981–1989 | 1989