Preparation of a novel poly(vinylsulfonic acid)-grafted solid phase acid catalyst and its use in esterification reactions

Article abstract of DOI:10.1039/b823177j

A high-density sulfonic acid polymer, grafted onto a carrier surface, was synthesized from the acid form of vinylsulfonic acid monomer, and its catalytic activity as a new class of heterogeneous acid catalyst demonstrated in esterification reactions. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b823177j

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Preparation of a novel poly(vinylsulfonic acid)-grafted solid phase acid
catalyst and its use in esterification reactionsw
Teruyuki Okayasu,a Kei Saito,b Hiroyuki Nishide*a and Milton T. W. Hearn*b
Received (in Cambridge, UK) 24th December 2008, Accepted 4th June 2009
First published as an Advance Article on the web 23rd June 2009
DOI: 10.1039/b823177j
A high-density sulfonic acid polymer, grafted onto a carrier
surface, was synthesized from the acid form of vinylsulfonic acid
monomer, and its catalytic activity as a new class of hetero-
geneous acid catalyst demonstrated in esterification reactions.
in water, ꢀ4.9 in acetonitrile), and a very high proton
conductivity in dry membrane form (on 10ꢀ3–10ꢀ6 S cmꢀ1 in
the temperature range from 30 to 150 1C under dry condition).
Esterification is an industrially important reaction, producing
organic esters at the level of tonnes per year.2 Conventional
mineral acid catalysts, such as sulfuric acid, are miscible with the
reaction medium, and need to be neutralized after the reaction to
separate the product ester from the toxic acid.3 Metal-containing
Lewis acid catalysts, like organotin chloride, also require careful
removal after the reaction by adsorption onto Fuller’s earth, which
also produces a large amount of waste.4 The use of a hetero-
geneous solid catalyst system is thus preferable from an environ-
mental and economical point of view.5 Distinct advantages of solid
acid catalysts can include low toxicity and low corrosiveness; ease
of handling, recovery and reuse in liquid phase reactions; and the
possibility of their use in a continuous process. Many hetero-
geneous catalysts have been previously reported in the literature to
be active in esterification reactions, including heteropolyacids,
zeolites, titania, and sulfonic acid immobilized inorganic
materials.6,7 However, these types of heterogeneous catalysts suffer
from disadvantages of low catalytic activity under equimolar
conditions in the absence of additives, such as H2O2, acidities that
are difficult to adjust, and typically lack of stability.
Poly(vinylsulfonic acid) (PVS) has a high acid density
(ion-exchange capacity; IEC = 9.2 meq gꢀ1) and is the
solid-state analogue of sulfuric acid. PVS is expected to have
various applications, such as strong cationic exchange, for
water retention, and as an acid catalyst. Hitherto, the
preparation of PVS in very high acid-purity and metal-free
state from vinylsulfonic acid (VSA) has presented constraints
due to difficulties in the purification of the acid form of the
VSA monomer and in the cation exchange of the sodium form
of PVS. We have recently succeeded in polymerizing vinyl-
sulfonic acid (VSA) of very high acid-purity (498.1 wt%) and
metal-free (o20 ppb) in water to yield PVS (Scheme 1a).1 PVS
exhibited very high acid properties in both aqueous and water-
free states (acid dissociation constant of VSA: pKHI+Aꢀ = 0.049
In this communication, we describe the synthesis and use of
PVS grafted onto a polystyrene (PSt) carrier from the acid
form VSA, thus providing a new type of organic solid catalyst.
Because of both the high polymerizability of VSA under the
developed conditions and the high acid density of PVS, the
graft polymerization of PVS resulted in a high density of
catalytically active acid sites on the heterogeneous catalyst.
We demonstrated the catalytic activity of the PVS-grafted PSt
for esterification.
We have grafted PVS onto a PSt carrier by the radical
polymerization of VSA. Scheme 1 shows an example of the
procedure with immobilized initiator and VSA. This synthesis
methodology does not introduce in a one-at-a-time mono-
molecular fashion sulfonic acid groups through individual
covalent anchoring via alkyl chains onto the carrier surface,
like
a a
conventional solid acid catalyst,7 but rather
macromolecular introduction of multiple sulfonic acid groups
as a polymer onto the carrier surface. Firstly, the aminomethyl
groups localized at the carrier surface are allowed to react with
4,40-azobis(4-cyanovaleric acid) (ACV), a functional free
radical initiator.8 Next, the ACV-attached PSt was mixed with
the acid form VSA and heated to achieve polymerization
initiated by the radicals anchored at the carrier surface. VSA
was successfully used as the acid form monomer in this direct
graft polymerization onto the carrier surface. In addition, we
have also studied PVS grafted onto silica and Sepharoses
Scheme 1
a Department of Applied Chemistry, Waseda University,
Tokyo 169-8555, 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 3800, Australia. E-mail: Milton.Hearn@sci.monash.edu.au;
Fax: +61-3-9905-8501; Tel: +61-3-9905-4547
w Electronic supplementary information (ESI) available: Experimental
procedures for the synthesis, characterization and catalytic properties
of the PVS-grafted PSt. See DOI: 10.1039/b823177j
ꢁc
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Table 1 Characterization of the PVS-grafted PSt
Elemental analysis
Immobilized group/mmol gꢀ1
Water uptake (wt%)
C
H
N
S
Entry
Sample
1
2
3
NH2-PSt
ACV-attached PSt
PVS-grafted PSt
85.1
70.6
40.7
9.2
7.2
6.0
5.7
12.3
2.0
—
—
17.6
4
—
5.2a
17
19
174
a
Calculated by titration experiment.
carriers in the same way. However, the carrier was degraded
by the high acidity of VSA when Sepharoses was used.
The PVS-grafted PSt beads and the ungrafted free PVS
polymer were separated by filtration and purified. Since the
molecular weight of the polymer grafted on the carrier can be
correlated with that of the ungrafted free polymer produced in
the reaction solution, the Mn of the graft PVS polymer is
almost equal or at least proportional to that of the ungrafted
free PVS polymer.9 The molecular weight of the ungrafted free
PVS polymer was Mn = 1.0 ꢂ 104, Mw = 2.4 ꢂ 104, Mw/Mn =
2.3. The graft ratio of the PVS-grafted PSt was 128% as
calculated from the neutralization titration of the PVS-grafted
PSt. The introduction of sulfonic acid polymer onto the carrier
surface was confirmed from other several perspectives. The IR
spectrum of the PVS-grafted PSt shows a large band, attrib-
uted to hydroxyl groups coordinated to sulfonic acid cation,
with a maximum at around 3400 cmꢀ1 in the OH stretching
region compared to that of the NH2-PSt. The two bands
observed at 1200 and 1000 cmꢀ1 correspond to the asymmetric
stretching vibration and symmetric stretching vibration bands
Scheme 2
high water affinity compared to Amberlysts 15 (114 wt%)
and Nafions SAC-13 (95 wt%).
The catalytic performance of the PVS-grafted PSt was
determined for the esterification of acetic acid with various
alcohols (molar ratio 1 : 1, Scheme 2) and the efficiency of the
PVS-grafted PSt was compared to other known homogeneous
catalysts (sulfuric acid, vinylsulfonic acid, methanesulfonic
acid, and benzenesulfonic acid) and heterogeneous catalysts
(Amberlysts 15 and Nafions SAC-13). In these experiments,
0.1 g of the PVS-grafted PSt sample corresponds to acid
densities of 0.45 mmol. All the catalysts showed much higher
conversion levels than in the absence of catalyst (Fig. 1). This
tendency was similar for the reaction rate constant (Table 2).
Ester conversion and reaction rate constant generally
depended on the nature of the reactant alcohols. From the
viewpoint of heterogeneous catalysis, the PVS-grafted PSt
gives a reaction rate constant more than 2–10 times higher
than other solid catalyst Amberlysts 15 and Nafions SAC-13.
It is clear that the PVS-grafted PSt has an extremely high
catalytic activity as a heterogeneous catalyst. This high catalytic
activity of the PVS-grafted PSt was due to the high sulfonic
acid density on the carrier surface, the high affinity for water,
and the strong acidity derived from VSA. In comparison, the
1
of the OQSQO, respectively. The H solid-state MAS NMR
spectrum of the PVS-grafted PSt shows a sharp peak at d =
6.1 ppm, which correlates with the presence of a strong
hydrogen bond network due to a high density of sulfonic acid
protons, in the solid phase state.10 The 13C solid-state
CP/MAS NMR spectrum of the PVS-grafted PSt showed a
broad peak of the PVS alkyl chain at d = 10–60 ppm and a
short peak of the amide linker between the carrier surface and
the graft polymers at d = 178.5 ppm. Measurement of the acid
density by neutralization titration methods provided the
number of sulfonic acid groups introduced onto the carrier
by the PVS graft polymerization. A sulfonic acid density of
5.2 mmol H+ gꢀ1 for the PVS-grafted PSt could be obtained
as a maximum loading (Table 1). Sulfonic acid densities for the
polystyrenesulfonic acid-grafted PSt (3.9 mmol H+
was obtained from 4-styrenesulfonic acid in a manner similar
to the PVS-grafted PSt, or Amberlysts 15 (4.7 mmol H+ ꢀ1),
g
ꢀ1) which
g
PVS-grafted silica were up to 1.8 mmol H+ ꢀ1. The intro-
g
did not show the same level of high catalytic abilities as found
duction of the sulfonic acid groups was similarly confirmed
from the elemental analysis. This level was much higher than
those found for strong Brønsted acid sites in conventional
sulfonic acid immobilized solid acid catalysts (in the range
0.5–1.5 mmol H+
g
ꢀ1)6 and was approximately 7 times higher
than the density of sulfonic acid groups (0.8 mmol H+ gꢀ1
)
found for Nafions, a highly active perfluorosulfonated ionomer.11
The humidity dependence of water uptake was measured for
the PVS-grafted PSt at 25 1C under relative humidity close to
75% (Table 1). The water uptake of the PVS-grafted PSt was
10 times higher than that of aminomethylated PSt (NH2-PSt).
This result was expected, since water should have little effect
on the carrier of the PSt beads, with the water uptake
attributed to the water-absorbing properties of the PVS.
Furthermore, the PVS-grafted PSt possessed an extremely
Fig. 1 Esterification of acetic acid with ethanol at 65 1C with
equimolar amounts of the reactants (K: 0.1 g of the PVS-grafted
PSt, J: 0.1 g of Amberlysts 15, ’: 0.1 g of Nafions SAC-13,
&: 0.45 mmol of sulfuric acid, E 0.45 mmol of vinylsulfonic acid, };
no catalyst).
ꢁc
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Table 2 Reaction rate constant of the esterification of acetic acid with alcohols catalysed by various catalystsa
Rate constant (k1/ꢂ10ꢀ4 L molꢀ1 sꢀ1
2-Propanol
)
Catalystb
Methanol
Ethanol
1-Butanol
1-Decanol
PVS-grafted PSt
Amberlysts 15
Nafions SAC-13
PVS
VSA
MSA
BSA
Sulfuric acid
No catalyst
60
14
4.6
—
—
—
—
—
11
2.2
0.56
0.18
—
—
—
—
—
0.068
7.1
2.6
1.6
9.4
16
13
12
17
4.0
2.0
1.3
—
—
—
—
—
0.30
5.6
2.0
—
18
16
13
18
0.44
0.64
0.33
a
Reaction conditions: acetic acid = 0.1 mol, alcohols = 0.1 mol, homogeneous acid catalysts = 0.45 mmol, heterogeneous acid catalysts = 0.1 g,
b
temperature = 65 1C. PVS: poly(vinylsulfonic acid), VSA: vinylsulfonic acid, MSA: methanesulfonic acid, BSA: benzenesulfonic acid.
for the PVS-grafted PSt. No by-product formation, i.e. ether
formation, or desorption of PVS from the carrier surface was
seen, since the NMR spectrum of the reaction sample did not
reveal any components other than the reactants and the ester
product.
loss of activity, and pollution risks like reactor corrosion are
kept to a minimum.
This work was partially supported by the Global COE
program ‘‘Practical Chemical Wisdom’’ at Waseda University
from MEXT, Japan, and the Australian Research Council.
Reusability of a heterogeneous catalyst is key for industrial
production. To examine the reusability of the catalyst, the
PVS-grafted PSt was filtered from the reaction mixture and
reused in a new reaction cycle without any treatment. The
progress of the reaction was monitored by taking small
aliquots of samples at different time intervals. Fig. 2 shows
that every recycled sample demonstrated almost identical
conversion to the ester as the fresh catalyst sample, i.e. about
55% of ethyl acetate, indicating that there is no significant
effect on the activity when the PVS-grafted PSt was reused. It
apparently stays chemically stable and active in the presence of
water and acetic acid. Furthermore, the corrosion of stainless
steel was hardly observed by XPS in the anticorrosion test of
the PVS-grafted PSt compared with the case of sulfuric acid.
In addition, the catalytic performance of the PVS-grafted PSt
was also determined for Friedel–Crafts acylation reaction and
condensation reaction, and it showed high catalytic activity. In
particular, the synthesis of 2,2-bis(5-methylfuryl)propane
using the PVS-grafted PSt highlighted the advantages of this
new solid phase catalyst in terms of conversion compared to
other catalysts.
Notes and references
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In conclusion, the PVS-grafted PSt is one of the ultimate
solid acid materials that has both high acid densities and
intrinsic strong acid strength derived from VSA in a synergistic
manner. The PVS-grafted PSt showed very high catalytic
activity as a heterogeneous acid catalyst for esterification.
The catalyst can be reused repeatedly without any observed
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Fig. 2 Recycling of the PVS-grafted PSt in the synthesis of ethyl
acetate (catalyst = 0.1 g, temperature = 65 1C, reaction time = 3 h).
ꢁc
This journal is The Royal Society of Chemistry 2009
4710 | Chem. Commun., 2009, 4708–4710