An efficient, practical and cost-effective polymer-supported catalyst for the transesterification of methyl methacrylate by 1-butanol

Article abstract of DOI:10.1039/b820704f

A straightforward and cost-effective synthesis of a polymer-supported titanium alkoxide catalyst is reported. The described synthesis involves cheap, readily available and relatively non toxic chemicals, solvents and workup procedures. Efficiency and stability of the catalyst were tested in a laboratory-scale continuous flow reactor under equilibrium conditions over a month period. The average daily titanium leaching was estimated to less than 1% of the total amount of titanium engaged. To our knowledge, this result is the best obtained, to date, concerning the use of heterogeneous titanate for the catalysis of transesterification reactions.

Full text of DOI:10.1039/b820704f

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An efficient, practical and cost-effective polymer-supported catalyst for the
transesterification of methyl methacrylate by 1-butanol
Gae¨lle Baquey,a Marie-He´le`ne Alve`s,a Magalie Graullier,a Alain Riondel,b Jean-Michel Paul,b Marc Birota
and Herve´ Deleuze*a
Received 19th November 2008, Accepted 12th March 2009
First published as an Advance Article on the web 30th March 2009
DOI: 10.1039/b820704f
A straightforward and cost-effective synthesis of a polymer-supported titanium alkoxide catalyst
is reported. The described synthesis involves cheap, readily available and relatively non toxic
chemicals, solvents and workup procedures. Efficiency and stability of the catalyst were tested in a
laboratory-scale continuous flow reactor under equilibrium conditions over a month period. The
average daily titanium leaching was estimated to less than 1% of the total amount of titanium
engaged. To our knowledge, this result is the best obtained, to date, concerning the use of
heterogeneous titanate for the catalysis of transesterification reactions.
polymer-supported titanium catalyst,10 seemed to confirm this
Introduction
behaviour: the grafting of a trialkoxytitanium moiety onto a
Transesterification is an important industrial reaction for the
polymeric network through a titanium–phenoxy bond led to
preparation of high boiling point methacrylic esters from methyl
the synthesis of an active catalyst in transesterification and
methacrylate, the low boiling by-product alcohol (i.e. methanol)
being removed by continuous azeotropic distillation with the
methacrylates.1 This reaction requires a catalyst and, in the
industrial processes currently in use, these catalysts are based
on homogeneous tin or titanium derivatives.2 Recently, bulky
diarylammonium arenesulfonates have been proposed as alter-
native transesterification organocatalysts.3 Although being very
efficient, their high cost preclude, so far, their use in industrial
process, especially in their polymer-supported version.4 Due to
cost and environmental concerns, titanium alkoxides tend to be
preferred when possible.5 The main drawback of such processes
concerns the efficient removal of the catalyst from the final
product. Low-boiling point methacrylic esters can be distilled
off. However, in case of non-distillable compounds, titanium
alkoxide has to be hydrolyzed in situ and separated, in some
cases, a significant part of the waste remains in the commercial
product. In these conditions, the catalyst recycling is difficult.
Therefore, the development of a heterogeneous alternative would
be a significant improvement. Several attempts have been made
in this direction.6,7 However, although the heterogeneous cata-
lysts prepared by direct anchoring of Ti alkoxides on supports
such as silica or zeolites have been claimed to be recyclable, a
loss of activity with use has always been reported attributed to
leaching of the metal in the homogeneous phase. This fact is
mainly due to the usual high sensitivity of Ti–O bonds toward
hydrolysis and alcoholysis, causing the leaching of the metal.8
An early patent reported that titanium phenoxide catalysts were
much more resistant toward hydrolysis and alcohol exchange
than alkyl titanates.9 Our previous studies on the synthesis of a
epoxidation reactions exhibiting a low metal leaching.11 We
present now an application of this observation to a reaction
presenting an industrial interest: The continuous production of
butyl methacrylate by transesterification of methyl methacrylate
with butanol.
Results and discussion
Synthesis of supported catalyst
The general route followed for the synthesis of the polymer-
supported catalyst is represented in Scheme 1. The catalyst
support was a p-acetoxystyrene-co-styrene-co-divinylbenzene
(20/30/50 mol%) copolymer prepared in bead shape by sus-
pension polymerisation.
The optimisation of the experimental parameters of the
support synthesis was previously reported.12 A good site iso-
lation of the grafted catalytic moieties was insured by the
preparation of highly crosslinked macroporous beads with a
low loading level in order to avoid titanate condensation. A
precipitant porogen (2-ethylhexanol, 40 wt%) was used to obtain
a specific surface area value in the medium range.13 The average
characteristics of 16 successive 30 g-scale batches are reported in
Table 1.
Grafting of the tributoxytitanium catalytic species on the
support requires the conversion of acetoxyle functional groups
into phenol groups. In our previous work, this step was
perfor◦med by hydrazinolysis with hydrazine hydrate in dioxane
at 80 C.10 This route was efficient, but the use of these toxic
and potentially dangerous conditions is detrimental in the case
of future industrial developments. In the present work, we
preferred to access the polymer-supported phenol by using a
based-catalysed hydrolysis reaction,14 followed by a treatment
with tetrabutyl titanate in chloroform at room temperature
(Fig. 1).
aUniversite´ de Bordeaux, Institut des Sciences Mole´culaires, UMR 5255
CNRS, 351 Cours de la Libe´ration, F-33405, Talence, France.
E-mail: h.deleuze@ism.u-bordeaux1.fr; Fax: +33 (0)540006994;
Tel: +33 (0)540006444
bArkema, Centre de Recherches et De´veloppements de l’Est, BP 61005
F-57501, Saint-Avold, France
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Table 1 Characteristics of the support beads
Specific surface area
Average particle Standard
Standard deviation s diameter/mm deviation s % C, H, O
Elemental analysis
(BET)/m2 g-1
Standard deviation s Pores size (BJH)/nm
35.0 11.1
208.0
0.7
380.0
12.0
86.80, 7.58, 5.42
Scheme 1 Synthesis of the supported catalyst.
Fig. 1 Daily BMA conversion and Ti leaching.
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Catalytic experiments
˚
containing 3 A molecular sieves, followed by a similar column
filled with the supported catalyst, both columns being placed in
a furnace heated at 115 ◦C 0.5 ◦C. The entry flow was adjusted
in order to allow an average resident time of the reagents of 8 h.
The reaction studied was the transesterification of methyl
methacrylate (MMA) with butanol (BuOH), giving butyl
methacrylate (BMA) and methanol (MeOH) (Scheme 2) in the
bulk (no solvent) with a molar ratio MMA : BuOH (2 : 1)
at 115 ◦C. In these conditions, the experimental equilibrium
constant value is about 0.8, giving a BuOH conversion at
equilibrium around 60%.15
◦
The exit flow was cooled to -5 C to avoid any loss of volatile
compounds. Fractionation of the flow allows an average daily
BMA conversion to be conducted by GC. The collected data
over a 29 days period is reported on Fig. 1. We must highlight
that this experiment can be seen as the equivalent of about 85
batches recycling!
An average BMA conversion value above 50% was reached
after the second day of reaction (the first day was necessary to
establish stable temperature and flow conditions in the column)
and remained at this level for at least ten days. Afterwards, a
slow decrease in conversion was observed with time, but even
after two weeks, the yield in BMA was still around 40%. Later,
the conversion continued to decrease steadily, when the reactor
was stopped, after 29 days, the conversion was still around
25%.
Scheme 2 Transesterification reaction.
In the industrial process, the equilibrium is displaced by
continuous removal of the low boiling alcohol (methanol)
using azeotropic distillation, thus leading to high BMA con-
versions. In this study, our main objective was to evaluate the
lifetime of the heterogeneous catalyst. Therefore, we chose to
work under equilibrium conditions (i.e. in a thermodynam-
ically closed system) in order to obtain more reproducible
results.
Analysis of titanium leaching
In order to evaluate the catalyst leaching during the reaction,
daily reactor eluents were analyzed for titanium traces. The
results are reported on Fig. 1.
Some preliminary experiments were conducted in a 100 mL,
mechanically stirred batch glass reactor (not shown). A BMA
conversion of 50–55%, (85–90% of the equilibrium value) was
obtained reproducibly by heating the mixture to 115 ◦C for 8 h,
in the presence of ca. 0.4 mol% of supported catalyst. However,
recycling catalyst beads using this apparatus appeared tedious
and poorly reproducible mainly due to an almost unavoidable
air intake during the manipulation. Therefore, we developed a
continuous flow reactor with a fixed catalytic bed allowing a
drastic reduction in manipulations. The diagram of the reactor
system is depicted in Scheme 3. We are aware that the reaction
conditions established for a batch stirred reactor are not directly
transposable to a continuous tubular reactor, we chose initially
to maintain the established values as much as possible. The
reagent feedstocks were pumped through a tubular column
Leaching during the first two days was important and can be
attributed to non-grafted catalyst incompletely washed from the
support. Afterward, the daily leaching gradually lowered from
150 to ~ 20 ppm. On average, the daily titanium leaching could
be estimated around 2.5 mg, which corresponds to about 1% of
the total amount of titanium grafted on the support engaged in
the reaction.
Analysis of used catalyst
After washing with chloroform, the residual catalytic activity of
some used beads was investigated in a batch experiment with
a fresh mixture of reagents. In conditions similar to the pre-
liminary experiments described previously, a BMA conversion
of 40–45% was observed, which indicates that the supported
Scheme 3 Continuous flow reactor.
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catalyst was still active after almost a month of use in the
continuous reactor.
we observed previously with silica supports), seems unavoidable
on the long term.
Catalytic activity of the effluents
Conclusion
In order to control the possible catalytic activity of the leached
species, the collected fraction presenting the highest conversion
(n◦ 5, see Fig. 1) was reacted for 72 h with a fresh mixture of
reagents in batch conditions. No increase in conversion could
be detected. The level of titanium alkoxide needed to detect
a catalytic activity in these conditions is well below the Ti
amount leached on this fraction.16 Therefore, it can reasonably
be concluded that the titanium species leached into solution has
little, or no, catalytic activity.
We have prepared a polymer-supported titanium catalyst for
applications in a transesterification reaction of industrial interest
with the idea to replace a homogeneous process.19 We tried
to develop a straightforward and cost-effective route for the
catalyst synthesis using cheap, readily available and relatively
non toxic chemicals, solvents and workup procedures. In order
to assess the efficiency and stability of our catalyst we tested it in
a laboratory-scale continuous flow reactor under equilibrium
conditions. The results obtained indicate a good stability to
metal leaching with deactivation being attributed to the slow
alcoholysis of the phenoxy–titanium bond in the long term.
To our knowledge, this result is the best obtained, to date,
concerning the heterogenisation of a titanate for the catalysis
in transesterification reactions.
Analysis of water content of the reagents flow
Titanium alkoxides are sensitive toward hydrolysis giving inac-
tive species. The water content of the reagents feedstock was
estimated to about 700 ppm using the Karl-Fisher method. This
˚
rather high value led us to insert a tubular column containing 3 A
Experimental
molecular sieve (8 g) into the reactor device before the reactor
column, both◦ columns being placed in the furnace heated at
Instruments
◦
115 C 0.5 C. With this improvement, the water content of
The specific surface area was determined by N2 adsorption
measurements performed on a Micromeritics ASAP 2010. The
resulting data were subjected to the Brunauer, Emmett and
Teller (BET) and the Barrett, Joyner and Halenda (BJH)
treatments.20,21
the flow was reduced to 30 ppm, a value estimated acceptable
for the catalyst stability.
Discussion
Chromatography experiments were performed using a VAR-
IAN 3300 and the Star chromatography software. The capil-
lary column was a DB5-like (5% phenyl groups). The BMA
conversion was calculated from the peak area ratio of BMA
relative to undecane as obtained from GC analysis using a
calibration curve. Elemental analyses on solids were performed
by the Service Central d’Analyses, Vernaison, France. Results
are expressed in weight percent (wt%). Titanium content in
effluents was estimated using ICP-MS technique by Antellis
Company, Toulouse, France. Results are expressed in mg Ti kg-1
(ppm).
Loss of activity of the supported titanium catalyst with time can
be attributed to three main causes: I) hydrolysis of the titanium
alkoxide groups, leading to inactive species, ii) poisoning of the
catalytic active sites and iii) leaching of the metal in solution due
than an alcoholysis of the Ti grafting bond.
We estimate the hydrolytic contribution to catalyst deactiva-
tion as marginal considering the low water level observed in
the reagents flow after addition of a molecular sieves column.
Concerning the second point, it is well known in the industry
that the reaction conditions used for the titanium alkoxide
catalysed transesterification of methacrylic esters involves some
polymerisation of those monomers.17 In order to limit this very
important problem, several radical inhibitors such as phenols
derivatives, thiazines or nitroxyl compounds are added to the
reagents mixture and, in addition, the reaction vessel is always
aerated by a stream of oxygen.18 Even with the added inhibitors,
oxygen aeration was not technically possible in our device, which
strongly limited their efficiency. However, we have not observed
significant polymerisation of methacrylates in our device (which
is usually evidenced by a rapid increase of the fluid pressure in the
column). The total leaching of titanium over the total run length
can be estimated at nearly 30% of the initial amount of titanium
engaged in the reaction (i.e. grafted on the beads placed in the
column). This loss can be correlated with the evolution of BMA
conversion along the run: it reached 55% on day 4 and ended
at 25% on day 29. Therefore, the most accurate explanation for
the significant catalytic activity loss during a long continuous
run seems to be the breaking of the phenoxy–titanium bond
by alkoxy exchange with butanol. This reaction, even if slowed
down with the use of a polystyrenic support (compared to what
Chemicals
All the monomers (styrene (S, Aldrich), divinylbenzene (DVB,
80%, mixture of isomers, Aldrich), p-acetoxystyrene (AS, a gift
from TOSH Corp, Japan) and other components involved in
the polymerisation (azobisisobutyronitrile (AIBN, Acros), 2-
ethylhexanol (Aldrich) and acacia gum (Aldrich) were used
as received. Tetrabutoxy titanium (Aldrich) was also used
without further purification. 1-Butanol and methyl methacrylate
(Aldrich) were distilled under reduced pressure from CaH2. All
solvents were dried according to published procedures.
Synthesis of macroporous poly(4-acetoxystyrene-co-styrene-co-
divinylbenzene) beads (ACP)
The suspension polymerisation was performed in a 500 mL
parallel-sided glass reactor already described.7 In a typical
experiment, the organic phase was composed of acetoxystyrene
(8.6 g, 53 mmol), styrene (8.33 g, 80 mmol), divinylbenzene
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(17.32 g, 133 mmol), AIBN (0.34 g) as the initiator and 2-
ethylhexanol (22.6 g) as the porogen. The mixture was suspended
in water (350 mL) containing a suspension stabiliser (acacia
gum, 7 g) and stirred at 220 rpm. The polymerisation was
performed at 80 ◦C for 15 h, after which time the resulting
spherical particles were washed with water, then continuously
extracted in a Soxhlet apparatus with ethanol (24 h), and finally
dried under vacuum to give 33.5 g of white beads with a diameter
between 200–500 mm. (Y = 98%).
methyl methacrylate (1292 g, 12.91 mol), 4-methoxyphenol
(31 g, 0.249 mol) and phenothiazine (64 g, 0.310 mol) as
polymerisation inhibitors, and undecane (100.8 g, 0.645 mol) as
internal standard for GC analysis. This mixture (water content
700 ppm, KF determination) was pumped through a tubular
˚
column containing 3 A molecular sieve (8 g) followed by the
reactor column, bo◦th columns being placed in the furnace heated
at 115 ◦C
0.5 C. The entry flow (0.025 mL min-1) was
adjusted in order to allow an average residential time of the
reagents of 8 h. After cooling at -5 ◦C in order to avoid any
loss of volatile products, the exit flow was fractionated each 8 h
and the BMA conversion estimated by GC using an internal
standard.
Conversion of ACP to poly(4-hydroxystyrene-co-styrene-co-
divinylbenzene) beads (HP)
The resin beads (31 g) were suspended in methanol (100 mL),
and a sodium hydroxide aqueous solution (200 mL, 1 mol L-1)
was then added and the reaction medium mechanically stirred
for 2 days at RT. Next, the beads were filtered and copiously
washed with successively water and methanol, then continuously
extracted in a Soxhlet apparatus with water (24 h), followed by
THF (24 h), and finally chloroform (24 h). Drying under vacuum
at 50 ◦C gave 29 g of white beads, Y = 100%.
Acknowledgements
This research was strongly supported by the ARKEMA Com-
pany through Aid-in-Grant fundings (to G.B., M.G. and M-
H.A). Me´lanie Bousquet (US2B, Talence, France) is acknowl-
edged for her gracious Karl-Fisher measurements.
Synthesis of catalyst TiP
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