Silica-Supported zirconium complexes and their polyoligosilsesquioxane analogues in the transesterification of acrylates: Part 2. activity, recycling and regeneration

Article abstract of DOI:10.1002/adsc.200900274

The catalytic activity of both supported and soluble molecular zirconium complexes was studied in the transesterification reaction of ethyl acrylate by butanol. Two series of catalysts were employed: three well defined silica-supported acetylacetonate and n-butoxy zirconium(IV) complexes linked to the surface by one or three siloxane bonds, (=SiO)Zr(acac)₃ (1) (=3iO)₃Zr(acac) (2) and (=SiO)₃Zr(0-n-Bu) (3), and their soluble polyoligosilsesquioxy analogues (c-C₅H₉) ₇Si₈O₁₂(CH₃)₂Zr(acac) ₃ (I'), (c-C₅Hc,)₇Si₇O ₁₂Zr(acac) (Z'), and (cC₅Hg)₇Si ₇O₁₂Zr(O-W-Bu) (3'). The reactivity of these complexes were compared to relevant molecular catalysts [zirconium tetraacetylacetonate, Zr(acac)₄ and zirconium tetra-n-butoxide, Zr(O-n-Bu)₄]. Strong activity relationships between the silica-supported complexes and their polyoligosilsesquioxane analogues were established. Acetylacetonate complexes were found to be far superior to alkoxide complexes. The monopodal complexes 1 and V were found to be the most active in their respective series. Studies on the recycling of the heterogeneous catalysts showed sig-nificant degradation of activity for the acetylacetonate complexes (1 and 2) but not for the less active tripodal alkoxide catalyst, 3. Two factors are thought to contribute to the deactivation of catalyst: the lixivation of zirconium by cleavage of surface siloxide bonds and exchange reactions between acetylacetonate ligands and alcohols in the substrate/product solution. It was shown that the addition of acetylacetone to the low activity catalyst Zr(O-M-Bu)₄produced a system that was as active as Zr(acac)₄. The applicability of ligand addition to heterogeneous systems was then studied. The addition of acetylacetone to the low activity solid catalyst 3 produced a highly active catalyst and the addition of a stoichiometric quantity of acetylacetone at each successive batch catalytic run greatly reduced catalyst deactivation for the highly active catalyst 1.

Full text of DOI:10.1002/adsc.200900274

FULL PAPERS
DOI: 10.1002/adsc.200900274
Silica-Supported Zirconium Complexes and their Polyoligosilses-
quioxane Analogues in the Transesterification of Acrylates:
Part 2. Activity, Recycling and Regeneration
Valꢀrie Salinier,a Gerald P. Niccolai,a,c Vꢀronique Dufaud,a,b,*
and Jean-Marie Basseta,*
a
Laboratoire de Chimie, Catalyse, Polymꢁres et Procꢀdꢀs UMR 5265 CNRS-CPE Lyon, 43 bd du 11 Novembre 1918,
69622 Villeurbanne cedex, France
Fax : (+33)-4-7243-1795; e-mail: basset@cpe.fr
Present address: Laboratoire de Chimie, UMR 5182 ENS/CNRS, Ecole Normale Supꢀrieure de Lyon, 46 allꢀe d’Italie,
b
F-69364 Lyon Cedex 07, France
Fax : (+33)-4-7272-8860; e-mail: vdufaud@ens-lyon.fr
Present address: ICAR, UMR 5191 ENS LSH, 15 parvis Renꢀ-Descartes, 69342 Lyon cedex 07, France
c
Received: April 18, 2009; Revised: July 6, 2009; Published online: September 10, 2009
Abstract: The catalytic activity of both supported nificant degradation of activity for the acetylaceto-
and soluble molecular zirconium complexes was nate complexes (1 and 2) but not for the less active
studied in the transesterification reaction of ethyl ac- tripodal alkoxide catalyst, 3. Two factors are thought
rylate by butanol. Two series of catalysts were em- to contribute to the deactivation of catalyst: the lixi-
ployed: three well defined silica-supported acetyl- vation of zirconium by cleavage of surface siloxide
ACHTUNGTRENNUNGacetonate and n-butoxy zirconium(IV) complexes bonds and exchange reactions between acetylaceto-
linked to the surface by one or three siloxane nate ligands and alcohols in the substrate/product so-
ꢀ
ꢀ
bonds, ( SiO)Zr
(acac)3 (1) ( SiO)3Zr
R
ꢀ
( SiO)3Zr
N
ACHTUNGTRENNUNG
silsesquioxy analogues (c-C5H9)7Si8O
U
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
tivity relationships between the silica-supported com- catalytic run greatly reduced catalyst deactivation for
plexes and their polyoligosilsesquioxane analogues the highly active catalyst 1.
were established. Acetylacetonate complexes were
found to be far superior to alkoxide complexes. The Keywords: acetylacetonate and alkoxy ligands ex-
monopodal complexes 1 and 1’ were found to be the change; catalyst regeneration; recycling; silica-sup-
most active in their respective series. Studies on the ported zirconium complexes; transesterification of
recycling of the heterogeneous catalysts showed sig- (meth)acrylates
Introduction
dustry. The acid-[2] and base[3]-catalyzed transesterifi-
cation reactions were largely investigated during the
Transesterification reactions represent one of the clas- 1950s and 1960s. They are now replaced by metal cat-
sic reactions in organic chemistry. They are of great alysts (Al, Pb, Sn, etc.)[4] and particularly transition
practical industrial interest because they are often re- metal catalysts (Ti, Zr, etc.)[5] which render the trans-
alized under mild conditions and thus can be a con- esterification more chemo-, stereo- and regioselective.
venient means to prepare esters not accessible by These latter suffer, nevertheless, from the problems of
direct esterification reactions.[1] Transesterification homogeneous catalysis concerning the recovery of the
products are used not only in organic synthesis but catalyst from the reaction mixture at the end of the
also in polymerization reactions and can find large ap- reaction and eventually its recycling. The develop-
plications in industry particularly in the coatings in- ment of selective, ecofriendly and reusable solid cata-
2168
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 2168 – 2177

Silica-Supported Zirconium Complexes and their Polyoligosilsesquioxane Analogues Part 2
FULL PAPERS
lyst for organic transformations is a very active re- and alcoholysis. Note that in the case of transesterifi-
search area and in the particular case of transesterifi- cation reaction this lack of stability represents a sig-
cation reactions a large variety of heterogeneous cata- nificant limitation since the alcohol is used both as re-
lysts has been reported so far including Brønsted acid actant and solvent. On the other hand, it was deter-
solids,[6] organic nitrogen bases-modified mesoporous mined that zirconium(IV) acetylacetonate complexes
silica materials[7] and titanates immobilized on differ- such as Zr
ent supports such as alumina, silica and polymers.
ACHTUNGTRENNUNG
The high activity and selectivity of titanates as ho- the transesterification of (meth)acrylates and more
mogeneous catalysts in the transesterifications of stable than their titanate counterparts upon recy-
acrylic esters which are useful substrates for a variety cling.[9b]
of synthetic transformations[5] has prompted consider-
With this in mind, our initial objective in this study
able effort to find efficient immobilization strategies was thus to produce an active supported analogue of
not only from an environmental and economical point the homogeneous Zr(acac)4 using a silica carrier,
AHCTUNGTRENNUNG
of view but also to favour the formation on the solid which would simplify the overall process. This was ac-
surface of monomeric titanium species thought to be complished via surface organometallic chemistry by
the true active catalytic centers. Supported transes- extending established work from our laboratory on
terification titanium alkoxides-based catalysts were supported zirconium complexes.[12] We have described
synthesized by Blandy and co-workers[8] by reacting in part 1 of this series of publications[13] the synthesis
XTi(OR)3 (X=CH3, Cl, O-i-Pr) with alumina and and the characterization of three relatively well de-
silica surfaces. These catalysts were able to catalyze fined silica-supported acetylacetonate and butoxy zir-
the transformation of ethyl propionate by 1-dodeca- conium(IV) complexes linked to the surface by one
ꢀ
nol to produce 1-dodecyl propionate. Although recy- or three siloxane bonds, ( SiO)ZrACTHUNGTRENNNUG
ꢀ
ꢀ
cling of the solid catalysts was reported, the activity
ACHTUNGTREN(GNUN SiO)3ZrACHTUNRTGEN(NGUN acac) (2), and ( SiO)3ZrACTHUNGTRENNUNG
C
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
tion of methyl methacrylate (MMA) with 2-ethylhex- catalytic behaviour of these supported and molecular
anol or 1-dodecanol but here again moderate activity complexes in the transesterification of acrylates, the
was achieved.[9a] An improvement of the activity was recycling of the surface species, easier to recover after
obtained by anchoring alkoxytitanates, TiACTHUNTRGNE(UNG O-i-Pr)4, on the catalysis than their soluble analogues, and explore
resins with increasing levels of cross-linking.[9b] How- the question of catalyst ageing and regeneration.
ever, the rate of the transesterification decreased
slowly upon recycling due to some plugging of the
pores by poly(methacrylate). Recently, in order to im-
À
prove the stability of the SiO Ti bond in silica-grafted Results and Discussion
titanium catalysts a passivation step was introduced in
the catalyst synthesis protocol by reacting the residual Choice of Catalytic Testing Conditions
silanol groups with 1-(trimethylsilyl)imidazole. This
synthetic route did not lead to any further improve- With the objective of developing a general use heter-
ment since titanium leaching was always detected in ogeneous transesterification catalyst and studying its
the heterogeneous transesterification of methyl meth- behaviour under laboratory and micropilot reaction
acrylate with butanol in the liquid phase.[10]
conditions, one must choose a relevant model system
At the industrial level, Rohm and Haas has de- and reaction protocol capable of producing and repro-
scribed, in two patents,[11] the preparation of titanium ducing measurable results. The approach taken in this
and zirconium alkoxides or acetylacetonate catalysts study was to choose a simple general homogeneous
supported on organic or inorganic supports and catalyst [ZrACTHNUTRGEN(UNG acac)4] and a model reaction (see below),
having a polymerizable function which can be poly- and then proceed through a series of experimental
merized before or after the grafting. These catalysts and analytical protocols to determine which would
were reported as being able to transesterify an impor- most simply provide an adequate measure of activity.
tant number of esters among them acrylates, not With regards to the catalytic reaction, we chose to
always easy to transform, under mild conditions.
study the transformation of ethyl acrylate (AE) by n-
Thus, although attempts to recycle heterogeneous butanol to give n-butyl acrylate (ABu) and ethanol
titanates have been reported, a loss of activity due to (Scheme 1). As was mentioned in the introduction,
the leaching of the metal in solution was often ob- (meth)acrylates are useful synthons for the synthesis
served. This is not surprising since titanium-based cat- of relevant fine chemicals, polymers and important in-
alysts are known to be sensitive towards hydrolysis termediates for oil and coatings industry, as evidenced
Adv. Synth. Catal. 2009, 351, 2168 – 2177
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2169

Valꢀrie Salinier et al.
FULL PAPERS
Figure 1. Monopodal and tripodal acetylacetonate and butoxy zirconium surface complexes and their soluble polysilsesquiox-
ane models.
Scheme 1.
by the significant number of mentions in the patent (or, if equilibrium is reached more quickly, we note
literature.[14]
the time at which the extent of reaction passes 95%).
Several options were considered with respect to the The selectivity of the reaction, that is the number of
reaction protocol.[15] In our case, we chose to work in moles of butyl acrylate formed divided by the number
a batch reactor and evaluate the activity in terms of of moles of n-BuOH which had reacted, was found to
the kinetic approach to the reaction equilibrium. Cat- be unity within experimental error in all cases report-
alysis was therefore realized very simply in a two- ed in this paper.[16]
necked flask under argon at atmospheric pressure, at
708C, without any solvent, with an equimolar mixture
of ethyl acrylate and n-butanol and a molar ratio n-
BuOH/Zr of 1000.
The advancement of each test is represented by Evaluation of the Catalytic Activity of the Surface
plot of “extent of reaction” versus time. Extent of re- Species and of their Molecular Analogues
action, a, is defined as the percentage of n-BuOH
converted to n-butyl acrylate divided by the equilibri- Recall that, in Part 1 of this series of reports,[13]
a
um constant at the temperature of the reaction (0.51 series of silica-supported zirconium acetylacetonate
at 708C) [Eq. (1)] and is expressed as a percentage. and alkoxide complexes and their polyoligosilses-
Thus, the plot begins at a=0 and if equilibrium is ob- quioxane analogues were synthesized and fully char-
tained, the curve levels off at a=100%. In tabular acterized by a series of spectroscopic and chemical
form, activity is expressed as initial rate of butyl acry- techniques. These complexes were submitted to the
late formation (determined graphically, expressed as standard catalytic reaction protocol described above,
mMminÀ1) and as the extent of the reaction at 6 h together with the homogeneous benchmark catalysts,
2170
asc.wiley-vch.de
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 2168 – 2177

Silica-Supported Zirconium Complexes and their Polyoligosilsesquioxane Analogues Part 2
FULL PAPERS
Figure 2. Catalytic activity of the surface species 1, 2 and 3
in the transesterification of ethyl acrylate by n-butanol.
Figure 3. Catalytic activity of the soluble analogues 1’, 2’ and
3’ in the transesterification of ethyl acrylate by n-butanol.
zirconium tetraacetylacetonate, Zr
nium tetra-n-butoxide, Zr(O-n-Bu)4.
Before testing the reactivity of the surface zirconi-
ACHTUNGTERN(NGNU acac)4, and zirco-
AHCTUNGTRENNUNG
um species, a blank run was performed on the silica the ligand systems have a strong effect on catalytic ac-
support, previously dehydroxylated at 5008C. Under tivity. The tripodal acetylacetonate complex 2 was far
the standard conditions, no n-butyl acrylate product less active (8.6 mMminÀ1), and the monopodal alkox-
was observed after six hours of reaction in the reac- ide complex 3 was nearly inactive (0.65 mMminÀ1).
tion medium. Thus, the support exhibits no measura- This result is not pre-ordained, as one can note that
ble activity toward transesterification.
for titanium homogeneous catalysts the alkoxides are
A first series of tests was performed comparing among the most active catalysts for transesterification
three heterogeneous catalysts, the monopodal tris- and esterification reactions but that was not the case
acetylacetonate complex 1, the tripodal mono-acetyla- here.
cetonate complex, 2, and the tripodal monobutoxide
This catalytic activity trend was mirrored for the
complex 3, to the homogeneous analogue, ZrACTHNUTRGNE(NUG acac)4, molecular catalysts 1’, 2’, and 3’ (Table 1, Figure 3).
(Figure 2).[17] The supported catalysts had similar zir- The monopodal silsesquioxane catalyst 1’ was twice as
conium loadings (2.64–2.68%wt Zr). Note especially active as its heterogeneous analogue 1, (52 mMminÀ1
that in this series of experiments, the number of moles vs. 26 mMminÀ1), indeed nearly as active as Zr
ACHTUNGTRENNUNG
constant.
the tripodal acetylacetonate complex, 2’ drops to
The three heterogeneous catalysts all exhibit cata- 2.2 mMminÀ1, and again the alkoxide catalyst 3’ was
lytic activity, although inferior to the activity of the nearly inactive (0.65 mMminÀ1).
homogeneous Zr
G
Thus, one observes, in terms of initial catalytic ac-
erogeneous catalysts, the monopodal tris-acetylaceto- tivity, that acetylacetonate complexes are superior to
nate complex 1, was only half as active as ZrACTHNUTRGNE(NUG acac)4 alkoxide complexes in this series, and that structure-
(60 mMminÀ1 vs. 26 mMminÀ1, Table 1). Furthermore, activity trends exhibited by the supported complexes
are mirrored for the molecular analogues.
Table 1. Transesterification of ethyl acrylate by n-butanol
catalyzed by silica-supported and molecular zirconium com-
plexes.
Recycling of the Surface Zirconium Species 1, 2 and 3
Catalyst Zr loading for solid
catalyst [%wt]
a at 6 h
Initial rate
[mMminÀ1]
We considered that catalyst stability and regeneration
were of primal interest. Thus, a preliminary study of
the performance of each heterogeneous catalyst over
several batch reaction cycles was investigated. The
protocol was as follows. Fresh catalyst was employed
under the conditions detailed above and allowed to
react for six hours. The catalyst was then allowed to
settle overnight and the solution was withdrawn via
syringe. Fresh substrate solution was then added and
the reactor was again heated to 708C. After six hours
of reaction, the same recycling/reaction protocol was
repeated, thus producing three successive six-hour
Zr
N
(eq. at
175 min)
60
1
2
2.64
2.68
85
33
6
92
27
5
26
8.6
0.65
52
2.2
3
2.67
1’
2’
3’
Zr
Homogeneous
Homogeneous
Homogeneous
0.65
0.80
(O-n- Homogeneous
5
Bu)4
Adv. Synth. Catal. 2009, 351, 2168 – 2177
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2171

Valꢀrie Salinier et al.
FULL PAPERS
catalytic runs (performed over three days) for each
catalyst.
The results for monopodal surface catalyst 1 are
shown in Figure 4. One observes a sharp decrease in
activity. In terms of initial activity, the second cycle
was 3.5 times less active than the fresh catalyst (cycle
1, 26 mMminÀ1; cycle 2, 7.5 mMminÀ1) and the third
cycle further deactivated by a factor of three (cycle 3,
2.5 mMminÀ1, an overall activity loss of 90% from the
initial run).
One might envisage that this drop in activity could
be due to lixivation. To investigate this hypothesis, a
number of different methods were employed. The zir-
conium microanalysis of used catalyst at the end of
the third catalytic run (after two recyclings) showed
that approximately 25% of the grafted zirconium had
been lost (1.92%wt Zr vs. 2.64%wt Zr). Otherwise
stated, during the third run, more than 75% of the
grafted zirconium is present but catalytic activity has
dropped off by 90%. This suggests that lixivation
cannot be the sole cause for catalyst deactivation.[18]
Given that the catalyst is in powdered form, one
might postulate also that some finely divided catalyst
is lost at the decantation step of the recycling proto-
col, that is, a finely divided powder was suspended in
the supernatant. As a control, a catalyst analogous to
ꢀ
Figure 4. Recycling of ( SiO)Zr
N
1
was prepared using high specific surface
(380 m2·gÀ1) silica beads. The activity of this catalyst
mirrored, at a very slightly inferior activity level, that
of 1 over three cycles.
ꢀ
Figure 5. Recycling of ( SiO)3Zr
G
Acetylacetone was detected (but not quantified)[19]
in solution after each catalytic run. This observation
lead to a study of ligand exchange as a possible origin
of the sharp catalytic activity losses observed, below.
Similar recycling behaviour was observed for the
tripodal monoacetylacetonate complex 2 (Figure 5).
Initial activity dropped sharply from 8.6 mMminÀ1 for
fresh 2 to 3.2 mMminÀ1 for the second cycle (63% ac-
tivity loss) and 0.65 mMminÀ1 for the third cycle
(92% overall activity loss). In this case, however, zir-
conium microanalysis of used catalyst showed no dif-
ference with that of fresh catalyst. Significantly, acety-
lacetone was once again detected (but not quanti-
fied)[19] in the solution.
Finally, the relatively inactive tripodal butoxide zir-
conium catalyst 3 was submitted to the recycling pro-
tocol (Figure 6). This relatively poor catalyst exhibit-
ed no significant activity loss over the three cycles (in-
ꢀ
Figure 6. Recycling of ( SiO)3ZrO-n-Bu (3).
itial activity for each cycle was 0.65 mMminÀ1). As Regeneration of Catalytic Activity
shall be seen below, the fact that the initial activity of
the tripodal alkoxide catalyst 3 and that of the used Given that the loss of activity of supported acetylacet-
tripodal acetylacetonate catalyst 2 were identical may onate complexes was accompanied by the presence of
not be accidental. One should also note that zirconi- acetylacetone in the product solution and that the tri-
um microanalysis showed no significant change be- podal butoxide zirconium complex did not lose activi-
tween fresh catalyst and used catalyst.
ty on recycling, it seemed clear that part of deactiva-
tion could be due to the replacement of acetylaceto-
nate ligands by butoxide ligands. The exchange be-
2172
asc.wiley-vch.de
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 2168 – 2177

Silica-Supported Zirconium Complexes and their Polyoligosilsesquioxane Analogues Part 2
FULL PAPERS
ZrACTHUNTRGNE(UGN O-n-Bu)4, as the catalyst. Recall that the activity
of this catalyst is very close to that shown by 3 and 3’
(Table 1). In Figure 7, we have represented two ex-
periments – in the first one, following the standard
catalytic protocol, the mixture of butanol and ethyl
acrylate were allowed to react in the presence of
ZrACTHNUTRGNEU(GN O-n-Bu)4 at 708C for six hours, resulting in a low
initial rate of reaction (0.8 mMminÀ1), which does not
vary significantly given the very low conversions. In a
second experiment, an identical protocol was followed
for the first three hours of reaction, at which point
four equivalents of acetylacetone were introduced.
The catalytic activity of the reaction immediately in-
creases to a level of the same order as that of
ZrACTHUNTRGENNG(U acac)4 catalyst: if one measures “initial activity”
Figure 7. Transesterification of ethyl acrylate by butanol cat-
alyzed by Zr(O-n-Bu)4 with and without added acetylace-
tone after 3 h.
from the point where the acetylacetone was added,
ACHTUNGTRENNUNG
one observes 23 mMminÀ1, which can be compared to
the 60 mMminÀ1 observed for Zr
ACTHNUTRGNEU(NG acac)4. Clearly, one
can regenerate at least partially catalytically active
species from inactive catalysts in this manner.
tween alkoxide ligands and b-diketone chelating li-
This protocol was used to determine the optimal
gands at zirconium and titanium has been thoroughly ligand to zirconium ratio. That is, in each experiment
studied[20] and it has been established that the ex- the standard protocol was followed for three hours
change is facile, that mixed alcoholate/chelate com- with ZrACTHNUTRGNEG(NU O-n-Bu)4, at which point different quantities
plexes are often stable and less susceptible to hydro- of acetylacetone were added to the reactor (Figure 8).
lytic degradation than the simple alkoxides.[21] The Important increases in activity were observed in all
formation of zirconium alkoxides from acetylaceto- cases, even when less than one equivalent of acacH
nate complexes seemed likely due to the high concen- was employed. The best activity was obtained by the
tration of butanol (initially one thousand times that of addition of three equivalents of acetylacetone
the zirconium) and a reaction temperature (708C) (60 mMminÀ1), equivalent to that observed for
sufficient to facilitate ligand dissociation. Further- ZrACTHNUTRGNENUG(acac)4. Higher concentrations of acetylacetone
more, we recognized that we might be able to regen- appear to inhibit catalytic activity (Figure 9). One
erate the deactivated catalysts by addition of acetyl- might suppose that the catalysis involves coordination
acetone.
of substrate to zirconium, and that excess acetylace-
The hypothesis was first explored in a homogene- tone competes favourably with substrate for coordina-
ous system, using the inactive tetrabutoxyzirconium, tion. It would follow that the most active species
Figure 8. Influence of the quantity of added acetylacetone on the transesterification reaction catalyzed by ZrACTHNUGTRENUNG(O-n-Bu)4.
Adv. Synth. Catal. 2009, 351, 2168 – 2177
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2173

Valꢀrie Salinier et al.
FULL PAPERS
As a preliminary step to our studies on the use of
ligand addition to maintain activity under industrial
micropilot conditions,[22] a study was performed in
order to check if the addition of acetylacetone to the
batch reactor feed would have an effect on catalyst
stability. The monopodal trisacetylacetonate zirconi-
um catalyst 1, which exhibited the highest activity but
deactivated very quickly, was chosen for this study.
Fresh catalyst was employed as usual for the recycling
series of experiments, that is, reacted with the sub-
strate mixture at 708C for six hours, left to stand at
room temperature overnight, and separated by de-
cantation. In the second and third cycles, a stoichio-
metric quantity of acetylacetone (1 equivalent per zir-
conium, 11 mg) was added to the substrate mixture
(acacH is thus ~0.04%wt of the reactor feed). As can
be seen in Figure 11, very little change in activity was
observed for the first recycled run (indeed, it is slight-
ly more active), and in subsequent runs the activity
Figure 9. Influence of the quantity of added acetylacetone
on the transesterification reaction catalyzed by Zr
ACHTUNGTNER(NUNG O-n-Bu)4
at 270 min.
would be a trisacetylacetonate species, but verifica- losses are far less significant than those observed
tion of this hypothesis has not been pursued.
when pure substrate is employed (see Figure 4). It
may be that this loss of activity is due to lixivation,
given the evidence for this process mentioned above
(see also ref.[18]). Nevertheless, the study provides
very encouraging indications as to the direction to
Ligands Exchange in Heterogeneous Catalysis
Recall that the overall objective of this project was follow in the development of stable and robust heter-
the development of more active and stable heteroge- ogeneous transesterification catalysts.
neous catalysts for industrial application. Given the
ligand exchange hypothesis, and in the context of
multicycle industrial application, we consider that the Conclusions
two tripodal catalysts, 2 and 3, are functionally equiv-
alent and thus we have two types of systems, monopo- In this study, a series of homogeneous and heteroge-
dal and tripodal. Thus, the in situ generation of an neous zirconium catalysts was evaluated for the trans-
active catalyst from an inactive catalyst was attempted esterification of ethyl acrylate by butanol under vary-
by adding acetylacetone (6 equivalents) to a test of ing catalytic protocols. It was shown that the relation-
the tripodal butoxide catalyst, 3 (Figure 10). The re- ship between the structure of the catalyst and its ac-
sultant system exhibited an activity similar to that of tivity and stability for silica-supported zirconium com-
the tripodal zirconium acetylacetonate material 2.
plexes was mirrored by their polyoligosilsequioxane
ꢀ
Figure 10. Effect of added acetylacetone on the transesterification reaction catalyzed by ( SiO)3Zr
N
2174
asc.wiley-vch.de
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 2168 – 2177

Silica-Supported Zirconium Complexes and their Polyoligosilsesquioxane Analogues Part 2
FULL PAPERS
silica surface in catalyst degradation and on industrial
micropilot studies of the transesterification reaction.
Experimental Section
All manipulations, when needed, were conducted under
strict inert atmosphere or vacuum conditions using Schlenk
techniques. The solvents were dried using standard methods
and stored over activated 4 ꢃ molecular sieves. Zr
ACHTUNGTRENNUNG(acac)4
and Zr(O-n-Bu)4 in solution in n-butanol were purchased
AHCTUNGTRENNUNG
from Aldrich Chemical and used without further purifica-
tion. Ethyl acrylate was provided by Elf-Atochem. Before
reaction with molecular zirconium complexes, silica (Aerosil
from Degussa, 200 m2 gÀ1) was first calcined at 5008C under
a stream of O2 for 15 h followed by dehydroxylation under
vacuum (10À5 mm Hg) at the same temperature. Three
silica-supported acetylacetonate and butoxy Zr(IV) com-
plexes with different coordination spheres and links with the
silica support have been prepared and fully characterized in
Part 1 of this series of papers (for more details see ref.[13]).
ꢀ
Figure 11. Attempt to regenerate monopodal surface species
ꢀ
( SiO)Zr
N
( SiO)ZrACTHNURTGENNG(U acac)3 (acac=acetylacetonate ligand) (1) was ob-
tained by direct reaction of ZrACTHUNGTRNEUNG(acac)4 with silica(500) whereas
analogues. Acetylacetonate complexes were found to
be far superior to alkoxide complexes. Little differ-
ence in terms of absolute initial activity was observed
between the heterogeneous catalysts (1, 2 and 3) and
homogenous catalysts (1’, 2’ and 3’). The monopodal
complexes 1 and 1’ were found to be the most active
in their respective series. Studies on the recycling of
the heterogeneous catalysts showed significant degra-
dation of activity for the acetylacetonate complexes
(1, 2) but not for the less active tripodal alkoxide cat-
alyst 3. Two main factors contribute to the deactiva-
tion of catalyst. The lixivation of zirconium by cleav-
age of surface siloxide bonds, presumably by alcohol,
was observed with the monopodal catalyst 1, but to
no significant extent for 2 and 3. Ligand exchange re-
actions between alcohols in the substrate/product so-
ꢀ
ꢀ
( SiO)3ZrACTHNUGTRENNGU(acac) (2) and ( SiO)3ZrACHUTNGTRENN(GUN O-n-Bu) (n-Bu=butyl
ꢀ
À
ligand) (3) were synthesized by reaction of ( SiO)3Zr H
with, respectively, acetylacetone and n-butanol at room tem-
perature. Polyoligosilsesquioxane analogues for each of the
supported species have also been prepared from the trisila-
nol, (c-C5H9)7Si7O9(OH)3, to produce tripodal molecular
models and from its derivative blocked on two silanol
groups, (c-C5H9)7Si8O11ACHTNUTRGNE(UNG CH3)2(OH), to provide monopodal
models.[13]
Catalytic Tests
Protocol: The liquid phase transesterification of ethyl acry-
late by n-butanol was chosen as a model reaction to evalu-
lution and the catalyst leading progressively to the ate the performance of the solid catalysts as well as that of
their molecular analogues. The reaction was followed to
equilibrium at a temperature of around 708C with an equi-
molar amount of ethyl acrylate and n-butanol. The catalytic
reaction was carried out under argon in the absence of sol-
vent, in a two-necked flask equipped with a condenser. The
reaction mixture was composed of ethyl acrylate (4 g, 0.04
mol), n-butanol (3 g, 0.04 mol), hydroquinone methyl ether
(EMHQ) stabilizer (200 ppm) and the reaction scale was
fixed at 8 mL. The amount of catalyst was adjusted to give 4
10À5 mol based on zirconium with a BuOH/Zr molar ratio of
less active alkoxide analogues was also invoked and
further studied. The facility of ligand exchange under
catalytic conditions was demonstrated: the low activi-
ty homogeneous catalyst ZrACHTNUGTRENUNG(O-n-Bu)4 was rendered
much more active by the addition of actetylacetone.
The addition of an optimal amount of acetylacetone
(3 equivalents) produced a system that was as active
as Zr
ACHTUNGTRENNUNG(acac)4. The applicability of ligand addition to
heterogeneous systems was then studied. The addition
of six equivalents of acetylacetone to the low activity 1000. In the case of the silica blank, 500 mg of silica previ-
ously dehydroxylated at 5008C were used.
solid catalyst 3 was shown to produce a highly active
catalyst. Initial studies on producing steady catalytic
activity were invoked, and it was shown that the addi-
tion of a stoichiometric quantity of acetylacetone
(0.04%wt with respect to zirconium) to each portion
of the batch catalytic reactor feed greatly reduced cat-
alyst deactivation of the highly active, easily degraded
catalyst 1.
Analysis: The reaction was monitored by taking aliquots
(0.1 g) periodically, which were analyzed on a DELSI DI
200 gas chromatograph equipped with a flame ionization de-
tector and a column packed with Chromosorb 101 (1.50 m,
60–80 mesh). Nitrogen was used as gas carrier. Conversion
and yield were determined by GC based on relative area of
the GC signals referred to an internal standard (octane)
calibrated to the corresponding pure compounds (Drel
=
Future publications in this series will concentrate
Æ5%). GC rate program: 2 min at 1008C, heating 8 de-
on ligand effects on the reaction, on the role of the grees· minÀ1 up to 2508C, 2 min at 2508C.
Adv. Synth. Catal. 2009, 351, 2168 – 2177
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2175

Valꢀrie Salinier et al.
FULL PAPERS
[5] D. Seebach, E. Hungerbꢄhler, R. Naef, D. Schnurren-
berger, B. Weidmann, M. Zꢄger, Synthesis 1982, 138.
[6] B. S. Balaji, M. Sasidharan, R. Kumar, B. Chanda,
Chem. Commun. 1996, 707.
[7] Y. V. S. Rao, D. E. De Vos and P. A. Jacobs, Angew.
Chem. 1997, 109, 2776; Angew. Chem. Int. Ed. Engl.
1997, 36, 2661.
[8] a) C. Blandy, J. L. Pellegatta, B. Gilot, J. Catal. 1994,
150, 150; b) C. Blandy, J. L. Pellegatta, P. Cassoux,
Catal. Lett. 1997, 43, 139.
Recycling Procedure
In the recycling studies, fresh catalyst was used as for a stan-
dard catalytic run and allowed to react for 6 h. Stirring and
heating were removed and the reactor was let stand over-
night. The limpid and colourless supernatant was then care-
fully withdrawn with a syringe and the damp catalyst
reused.
Ligands Exchange in Homogeneous Catalysis
[9] a) H. Deleuze, X. Schultze, D. C. Sherrington, Polymer
1998, 39, 6109; b) H. Deleuze, X. Schultze, D. C. Sher-
rington, J. Mol. Catal. A: Chem. 2000, 159, 257.
[10] M. C. Gaudino, R. Valentin, D. Brunel, F. Fajula, F.
Quignard, A. Riondel, Appl. Catal. A: General 2005,
280, 157–164.
[11] a) Rohm and Haas Co. , US Patent 5,183,930, 1993;
b) Rohm and Haas Co. , European Patent 0557131-B1,
1998.
Ethyl acrylate (50 g, 0.5 mol), n-butanol (37 g, 0.5 mol) and
200 ppm of EMHQ stabilizer were introduced in a Schlenk
flask (250 mL) under argon topped with a condenser. The
solution was then warmed up at 708C before the introduc-
tion of ZrACHTUNGTRENNUNG
(O-n-Bu)4 (5 10À4 mol). After 3 h of reaction a
known quantity of ligand was added. The reaction was then
followed for another 4 h 30.
[12] a) F. Quignard, A. Choplin, J.-M. Basset, J. Chem. Soc.
Chem. Commun. 1991, 1589; b) S. A. King, J. Schwartz,
Inorg. Chem. 1991, 30, 3771; c) F. Quignard, C. Lꢀcuy-
er, C. Bougault, F. Lefebvre, A. Choplin, D. Olivier, J.-
M. Basset, Inorg. Chem. 1992, 31, 928; d) J. Corker, F.
Lefebvre, C. Lꢀcuyer, V. Dufaud, F. Quignard, A. Cho-
plin, J. Evans, J.-M. Basset, Science 1996, 271, 966; e) F.
Quignard, C. Lꢀcuyer, A. Choplin, D. Olivier, J.-M.
Basset, J. Mol. Catal. 1992, 74, 353; f) F. Quignard, C.
Lꢀcuyer, A. Choplin, J.-M. Basset, J. Chem. Soc.
Dalton Trans. 1994, 1153.
Acknowledgements
The authors acknowledge Elf-Atochem for financial support
and N. Ferret for fruitful discussions.
References
[1] a) J. Otera, Chem. Rev. 1993, 93, 1449; b) H. E. Hoy-
donckx, D. E. De Vos, S. A. Chavan, P. A. Jacobs, Top.
Catal. 2004, 27, 83–96.
[13] See Part 1 of the series: V. Salinier, J. M. Corker, F. Le-
febvre, F. Bayard, V. Dufaud, J.-M. Basset, Adv. Synth.
Catal. 2009, 351, 2155–2167.
[2] a) J. Colonge, E. Le Sech, R. Marey, Bull. Soc. Chim.
Fr. 1957, 776; b) C. E. Rehberg, W. A. Faucette, C. H.
Fisher, J. Am. Chem. Soc. 1944, 66, 1723; c) J. K.
Haken, J. Appl. Chem. 1963, 13, 168; d) H. G. Hagen-
meyer Jr. , D. C. Hull, Ind. Eng. Chem. 1949, 41, 2920;
e) E. R. Alexander, H. M. Busch, J. Am. Chem. Soc.
1952, 74, 554; f) S. Fujisaki, Y. Ishii, Y. Yano, A. Nishi-
da, J. Org. Chem. 1994, 59, 1191.
[3] a) R. W. Taft, Jr., M. S. Newman, F. H. Verhoek, J. Am.
Chem. Soc. 1950, 72, 4511; b) S. Alleman, J.-L. Rey-
mond, P. Vogel, Helv. Chim. Acta 1990, 73, 73; c) J.
Barry, G. Bram, A. Petit, Tetrahedron Lett. 1988, 29,
4567; d) Y. Ali, M. A. Hanna, Bioresour. Technol. 1994,
50, 153; e) B. Freeman, E. H. Pryde, T. L. Mounts, J.
Am. Oil. Chem. Soc. 1984, 64, 1638; f) B. Freeman,
R. O. Butterfield, E. H. Pryde, J. Am. Oil Chem.Soc.
1986, 63, 1375; g) D. S. Wulfman, B. McGiboney, B. W.
Peace, Synthesis 1972, 49; h) F. Perron, T. C. Gahman,
K. F. Albizati, Tetrahedron Lett. 1988, 29, 2023; i) D. F.
Taber, J. C. Amedio, Y. K. Patel, J. Org. Chem. 1985,
50, 3618; j) S. Hatakeyama, K. Satoh, K. Sakurai, S.
Takano, Tetrahedron Lett. 1987, 28, 2717; k) D. J. Bru-
nelle, Tetrahedron Lett. 1982, 23, 1739.
[14] a) G . Protzmann, H. Trautwein, J. Knebel, T. Schuetz,
G. Koelbl, T. Kehr, G. Lauster, German Patent DE
102007031470A1, 2009; b) A. Riondel, G. Baquey, H.
Deleuze, M. Birot, WO Patent 2008125787A1, 2008;
c) N. Murata, M. Naoe, Y. Tsuji, K. Kitamura, Japanese
Patent JP 2008106019A, 2008; d) A. Benderly, M. R.
Ryan, D. R. Weyler, U.S. Patent 2007287841A1, 2007;
e) J. Kamei, K. Hayashi, A. Kobayashi, Japanese
Patent JP 2007314502A, 2007.
[15] Many variations on protocol and analysis were ex-
plored in the pre-study. Final choices were based on a
minimal condition of demonstrated reproducibility
(less than 2% variance in yield at any point over three
independent runs) with the objective of minimizing cat-
alyst use and apparatus complexity. Among the factors
studied were 1) comparison of batch results to proto-
cols involving the continuous removal of the ethyl acry-
late/ethanol azeotrope; 2) scale of the reaction between
8 mL and 100 mL scale; 3) the effect of the addition of
molecular sieves (no effect); 4) the effect of added sol-
vent (dodecane, no significant advantage); and 5) sub-
strate to catalyst ratio (between 100 and 1000).
[16] Neither polymerization nor Michael addition products
were observed, perhaps due to the low temperature
and the presence of a radical trap (hydroquinone stabil-
izer).
[4] a) J. Otera, S. Ioka, H. Nozaki, J. Org. Chem. 1989, 54,
4013; b) A. Orita, A. Mitsutome, J. Otera, J. Org.
Chem. 1998, 63, 2420; c) A. Orita, K. Sakamoto, Y.
Hamada, A. Mitsutome, J. Otera, Tetrahedron 1999, 55,
2899; d) J. Otera, T. Yano, A. Kawabata, H. Nokzaki,
Tetrahedron Lett. 1986, 27, 2383; e) J. Otera, T. Yano,
R. Okawara, Organometallics 1986, 5, 1167.
[17] For simplicity, the relatively inactive ZrACHTUNGRTNEUNG(O-n-Bu)4 is
not represented in Figure 3 and Figure 4. The activity is
nearly equivalent to that of the heterogeneous catalysts
2176
asc.wiley-vch.de
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 2168 – 2177

Silica-Supported Zirconium Complexes and their Polyoligosilsesquioxane Analogues Part 2
FULL PAPERS
3 and 3’. The allure of the activity of Zr
(O-n-Bu)4 is
vious possibility of chemisorption and physisorption on
the silica surface (which itself is undergoing modifica-
tion during the reaction). The added value of this quan-
tification was not deemed sufficient for the effort re-
quired.
represented together with a discussion of its signifi-
cance in the catalyst regeneration section (see Figure 7
and Figure 8).
[18] Several different exploratory studies of lixivation were
performed. These incomplete studies were all equiva-
lent with the hypothesis that some lixivation was pres-
ent during catalysis with the monopodal catalyst 1 but
very little lixivation was observable for 2 and 3. For ex-
ample, catalysts 1 and 2 (powders) were tested in the
presence of silica beads. Zirconium microanalysis of
the beads after reaction showed a similar degree of
transfer of zirconium in the case of 1 (~20% transfer),
but very little in the case of 2. In another study, catalyst
1 was continuously extracted (Soxhlet) with butanol for
three days (the extraction bed temperature was not
measured but was clearly superior to the 708C catalytic
test temperature): zirconium microanalysis of the solid
showed loss of 28% of the initial zirconium.
[20] a) H. Nabavi, S. Doeuff, C. Sanchez, J. Livage, J. Non
Cryst. Solids 1990, 121, 31; b) J. Livage, C. Sanchez, J.
Non Cryst. Solids 1992, 145, 11; c) C. Sanchez, M. In, P.
Toledano, P. Griesmar, Mt. Res. Soc., Symp. Proc. 1992,
271; d) C. Sanchez, J. Livage, M. Henry, F. Babonneau,
J. Non Cryst. Solids 1987, 100, 65; e) A. Leaustic, F. Ba-
bonneau, J. Livage, Chem. Mater. 1989, 1, 248.
[21] a) J. Livage, C. Sanchez, M. Henry, S. Doeuff, Solid
State Ionics 1989, 32–33, 633; b) F. Babonneau, S.
Doeuff, A. Leaustic, C; Sanchez, C. Cartier, M. Verda-
guer, Inorg. Chem. 1988, 27, 3166; c) S. Doeuff, Y.
Dromzee, F. Daulelle, C. Sanchez, Inorg. Chem. 1989,
28, 4439.
[22] a) Atochem Elf SA, French Patent FR 2747675, 1997;
b) Atochem Elf SA, French Patent FR 2747596, 1997.
A note more fully describing this study is under prepa-
ration for publication.
[19] Accurate quantification of the acetylacetonate would
have required very careful study, given the relatively
low concentration in the solution coupled with the ob-
Adv. Synth. Catal. 2009, 351, 2168 – 2177
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2177