Interactions between poly(ethylene oxide)-based surfactants and transition metal alkoxides: Their role in the templated construction of mesostructured hybrid organic-inorganic composites

Article abstract of DOI:10.1039/b002518f

The interactions between PEO-based templates and metallic centres are key in the synthesis of mesostructured materials. In low-water-content media, strong chelation between metal centres and the non-ionic polar heads leads to polymer unfolding, and thus t

Full text of DOI:10.1039/b002518f

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Interactions between poly(ethylene oxide)-based surfactants and
transition metal alkoxides: their role in the templated construction of
mesostructured hybrid organic–inorganic composites
Galo J. de A. A. Soler–Illia and Clement Sanchez*

L e t t e r
Chimie de la Matie
place Jussieu, T our 54 5e, 75252 Paris, cedex 05, France. E-mail: clems=ccr.jussieu.fr
re Condensee, (CNRS UMR 7574), Universite Pierre et Marie Curie,
`  
4
Received (in Montpellier, France) 29th March 2000, Accepted 9th May 2000
Published on the Web 21st June 2000
The interactions between PEO-based templates and metallic
centres are key in the synthesis of mesostructured materials. In
low-water-content media, strong chelation between metal
centres and the non-ionic polar heads leads to polymer
unfolding, and thus to worm-like phases. Larger quantities of
water and acid added to the reaction bath weaken the coordi-
nation bonds that compose the hybrid interface. These resulting
weak interactions (mainly hydrogen bonding) help the folding of
the templating agents, and yield more ordered mesophases.
Block copolymers PEO ÈPPO ÈPEO have also been
n
m
n
extensively used to create architectures of large pore size
silicas in aqueous or alcoholic media. In these environments,
the liquid crystalline phases self assemble, directing the PEO
blocks towards the outer side of the micelles.22 Several studies
have proved that there is a partition of low-weight silica poly-
mers between the solvent and the solventÈmicellar interface,
prior to the construction of the inorganic framework.23 The
subsequent polymerisation step allows for the formation of
highly controlled periodic structures. It is believed that the
interactions taking place between PEO-based surfactants
The synthesis of mesostructured hybrid materials and meso-
porous metal oxides has experienced an impressive burst in
the last years. Initially developed for silica-based structures,
the introduction of new surfactants as templates allowed the
extension of this approach to most transition metals. Tran-
sition metal alkoxides (or halogenides) are mostly used as pre-
cursors.1h17 The higher Lewis acidity and the unsaturated
coordination of these precursors make them prone to react
strongly with nucleophilic species, leading thus to fast polym-
erisation reactions.18,19 When the reactivity of the transition
metal precursors is not controlled, precipitates comprised of
ill-deÐned transition metal-oxo polymers are obtained. The
inhibited growth of these metal-oxo polymers can produce
sols or bushy structures, which invade the whole volume,
forming gels rather than precipitates. This variability arises
from the many di†erent ways in which oligomers and poly-
mers can be linked and organised when they are dispersed in a
solvent.
The possible tailoring of these materials to build meso-
structured hybrid architectures through templated growth
processes depends on the control of the growth of these oxo
polymers and thus on the control of transition metal alkoxide
precursor reactivity.20 Although some results have been
obtained via the traditional low-weight ionic surfactants,21 the
most successful templating agents for non-silica mesoporous
phases seem to be neutral and able to interact with the metal
centres.2
(PEOÈPPOÈPEO triblock copolymers and diblock C E ) and
i j
the metallic centres are a key feature in obtaining meso-
structured phases, the precursors of mesoporous transition
metal oxides. However, to the best of our knowledge, the
nature of these interactions has not been investigated.
The present work tries to shed some light on the inter-
actions that occur in the Ðrst steps (polymerisation, drying) of
the formation of mesostructured hybrids. These processes are
of paramount importance in the tailoring of the resulting
mesoporous metal oxide phases. Ti(IV), Zr(IV) and V(V) alkox-
ides were used as typical model systems. 51V NMR and UV-
Visible spectroscopies were used to probe the environment
experienced by the metallic centres during the Ðrst steps of
the construction of the mesostructured hybrid material. A
scheme describing the formation of mesostructured hybrids
synthesised in the presence of poly(ethylene oxide)-based
surfactants and transition metal alkoxide precursors is Ðnally
proposed.
Ti(OBun) (TBT), Ti(OPri) (TPT) or Ti(OEt) (TET) were
4
4
4
used as metal sources for the construction of the meso-
structured titania; VO(O-tert-Am) (VOTAM)24,25 was also
3
used as an inorganic precursor for comparison purposes.
Commercially provided surfactants [Pluronics: P123,
(PEO) (PPO) (PEO) , F127, (PEO) (PPO) (PEO)
20
70
20
106
70
106
and P3100, (PEO) (PPO) (PEO) Èplease note that all
.5 17 1.5
PEO surfactants are capped with OH groupsÈand Brij 56,
1
Three main approaches have been developed, depending on
the chemical features of the couple surfactantÈprecursor: (i)
the importance of the matching of the H-bonding between the
growing titaniaÈsilica- or alumina-based polymers and the
templating phase has been postulated by Pinnavaia et al.;5h8
C
H
(PEO) OH] were dried prior to use by heating at
1
6 33
10
70 ¡C under vacuum; all solvents (ethanol, THF, toluene) were
dried and kept over 5 AŽ molecular sieves. Initial solutions
were prepared under Ar in a glove box.
The typical synthesis of mesostructured transition metal
oxides (MTMO)-based hybrid materials in the presence of
poly(ethylene oxide)-based surfactants can be described as
(ii) the Ligand Assisted Templating model stresses the forma-
tion of surfactantÈmetal internal sphere acidÈbase Lewis com-
plexes prior to mesophase formation;2a,9h12 (iii) recently,
Stucky and coworkers3,4 have shown that poly(ethylene
oxide)-based surfactants are very e†ective in the templating of
numerous metal oxides. Moreover, they suggested that non-
hydrolytic reactions take place in their low-water-content
systems, allowing the oxide network to be built in a more con-
trolled way.
follows. The metal source [Ti(OR) ] is Ðrst dissolved in
4
ethanol, or ethanolÈacidic water solutions; the surfactant is
then added to the resulting sol while stirring. In all cases, the
metal-to-surfactant ratios (S) were kept between 1 and 150;
the hydrolysis ratio (H) was set between 1 and 10; the
HCl : Ti ratio varied between 0 \ p \ 4.26
The initial transparent sols give rise to di†erent kinds of
DOI: 10.1039/b002518f
New J. Chem., 2000, 24, 493È499
493
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solids, depending on their treatment. Aging at 50È70 ¡C under
stirring yields white precipitates, while glass-like xerogels are
obtained upon leaving the systems in open recipients, at tem-
peratures ranging from ambient to 60 ¡C. The latter exhibit a
slightly yellow colour, which becomes more intense upon
ethanol evaporation from the precursor gels (this colour is
deeper for vanadium, which turns to orange, than for
titanium- and zirconium-based solutions). As demonstrated
through XRD and TEM experiments (see Fig. 1) pseudo-
hexagonal, lamellar or worm-like hybrid phases show up,
depending on the experimental conditions; the powders
display characteristic ordering distances very close to those
reported by Stucky and coworkers for mesostructured
titania.3,4 On the contrary, systems with very low water con-
tents (i.e., atmospheric moisture as the only water source)
non-polar solvents exhibit a very intense yellow colour, which
is more pronounced for higher S ratios. UV-Visible absorp-
tion experiments performed on these solutions exhibit an
absorption band in the 400 nm region, for each templating
agent (Fig. 2). The position and intensity of this band are
characteristic of a charge transfer band, particularly a
r
] t
LMCT.27 The appearance of this kind of band in
O2p
2gM
the visible region is a typical feature observed in the case of
chelated d0 metallic centers with ligands such as peroxo,
acetylacetonato, acetato, etc.28 As expected, the absorption
intensity correlates with the reduction potential of the metal;
typically, e [ e [ e . In all cases, the absorbance increases
with the polymer concentration, showing a non-linear depen-
dence (Fig. 2, inset). This may be due to the existence of
several superimposed bands belonging to various molecular
complexes. In principle, both ÈOCH CH OÈ and
V
Ti
Zr
display vermicular textures, in the range of 20 to 40 AŽ , inde-
pendently of the size of the templating polymer. Heating (up
to 120 ¡C under nitrogen Ñow, followed by a slow ramp up to
2
2
ÈOCH(CH )CH OÈ fragments (known for their role as metal
3
2
scavengers) can coordinate the metal centres, thus expanding
their coordination number from four to six. All these com-
plexes are clearly identiÐed from 51V NMR experiments
(vide infra).
350È400 ¡C under oxygen atmosphere) leads to mesoporous
TiO in all cases.
2
The deeper coloration intensity of the gel-like materials
observed upon ethanol evaporation suggests that inner sphere
processes take place over the Ti centres, probably an exchange
reaction involving the template, the solvent and water. The
complex ensemble of these processes drastically a†ects the
self-assembly and thus the mesostructuration of the hybrid
phases. In order to investigate these features, and separate the
role of ethanol in the tuning of the surfactantÈprecursor inter-
actions, non-protic solvents such as THF or toluene have
been used. All surfactantÈmetal alkoxide solutions in these
The presence of nucleophilic molecules (such as alcohol or
water) in these systems produces drastic changes in the UV-
visible spectra. The CT band strongly decreases and even dis-
appears upon ethanol addition to any of the di†erent
surfactantÈalkoxide sols [Fig. 2(c)]. The presence of polar
environments destroys the complexing interaction previously
observed under anhydrous conditions. Substitution reactions
occurring between the solvent (ethanol or water) and the
polymer, or transalcoholysis between the Ðxed alkoxo groups
Fig. 1 Upper: TEM micrographs (microtomy) showing typical vermicular and pseudo-hexagonal textures, obtained for titanium alkoxideÈ
EtOHÈPluronic systems, using di†erent synthesis conditions. Lower: XRD (j \ 1.5406 A, graphite-Ðltered) of the corresponding samples. (a)
Ž
TBT : EtOH : P3100 \ 1 : 30 : 0.1, synthesised by solvent removal at ambient temperature; (b) TBT : EtOH : F127 : HCl : H O \
2
1
: 30 : 0.015 : 1: 4, precipitate, aged 8 days at 60 ¡C and calcined at 350 ¡C.
494
New J. Chem., 2000, 24, 493È499

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solutions, also exhibiting an orange colour. Methoxyethanol
can be considered as a model ligand that mimics the polar
heads of the PEO-based surfactants, presenting an alcoholic
end and a b-methoxy group, able to coordinate a metal centre.
VO(O-tert-Am) in THF or toluene solution is character-
3
ised by two 51V NMR resonances (Fig. 3, spectrum a). The
Ðrst one, very intense and located at [684 ppm, corresponds
to VO(O-tert-Am) (I) while a second weak resonance located
3
at [666 ppm corresponds to the Ðrst hydrolysis product
VOOH(O-tert-Am) (IIa).31,32 The latter displays a seven-
2
resonance multiplet, the origin of which is to be found in a
2
J
coupling due to the presence of VÈOÈV species
VvV
(
IIb).32,33 Upon addition of MeOCH CH OH (Fig. 3, spec-
2
2
trum b), the most important new resonance appears at [650
ppm (singlet). This signal lies at a similar value to that
observed for VO(OEt)(O-tert-Am) , [651 ppm; this is in
2
agreement with the similarity of the MeOCH CH OH tips
2
2
and EtOH, and should correspond to the substitution of one
tert-amyloxy group by one methoxyethanol coordinated to
vanadium as a monodentate ligand (III). On the other hand,
the other peaks belonging to the VO(OEt) (O-tert-Am)
series ([625 ppm for x \ 2; [590 ppm for x \ 3) are not
present. Two new resonances show up at [634 (signal A,
composite) and [612 (B, singlet) ppm, becoming increasingly
important upon further addition of the complexing agent. In
principle, the A signal can be derived from any of the reso-
nances at lower d. However, the shift to higher d values is
x
3~x
small
enough
to
rule
out
the
possibility
of
VO(OCH CH OMe) (O-tert-Am). The separation of 33 ppm
2
2
2
from the resonances of species II is somewhat too large to be
ascribed to a hypothetical VO(OH)(OCH CH OMe)(O-tert-
2
2
Am) that should derive from the hydroxo species; moreover,
the eventual parent resonance at [666 ppm remains even
when resonance A is signiÐcant. The separation of 17 ppm
could, though, be ascribed to a derivative of species III, in
which the b-ether function of methoxyethanol acting as a chel-
ator increases the coordination to Ðve, and shifts the reso-
nance to higher Ðelds. Therefore, we propose that the chelated
isomer of species III (species IV) should correspond to this
resonance. Moreover, some of the O atoms belonging to the
chelating MeOCH CH OH can link a minor fraction of vana-
2
2
dium moieties, thus explaining the presence of VÈOÈV coup-
ling. Excess ligand enhances the resonance at [611 ppm,
which we assigned to a disubstituted species, containing two
methoxyethanol ligands linked to vanadium, with both mono-
dentate and bidentate coordination modes (VI). Note that, in
this case, the position of the line is also shifted ]14 ppm with
Fig. 2 Representative UV-Vis spectra of metal alkoxideÈsurfactant
systems. (a) VOTAMÈF127 in toluene; inset: e†ect of surfactant con-
centration on absorbance (410 nm) for this system ([VOTAM] \ 0.1
mol dm~3). (b) TBTÈP3100 in toluene; inset: e†ect of surfactant con-
centration on absorbance ([TBT] \ 0.1 mol dm~3). (c) E†ect of EtOH
addition on the system depicted in (a).
respect to the corresponding VO(OEt) (O-tert-Am).
2
and the terminal OH groups of the surfactant polar head, can
account for the observed behaviour. Such alcoholysis reac-
tions have been reported to occur spontaneously between
small alcohol molecules and transition metal alkoxides (such
as titanium and vanadium) while they take a much longer
time and need to be catalysed for silicon alkoxides.29,30 The
drastic role of ethanol is not surprising, in view of the high
molecular weights of the polymeric surfactants compared to
EtOH. Even small quantities of EtOH in the system suffice to
shift the alcoholysis equilibrium to the right, increasing the
formation of ethoxy-substituted metallic species.
Using VO(O-tert-Am) as a probe, these reactions between
PEO moieties and metallic centres have been carefully investi-
3
Fig. 3 51V NMR spectra (79 MHz) of the VOTAMÈ
MeOCH CH OH
system
in
THF.
(S \ [VOTAM]/
gated through 51V NMR spectrometry. Due to the absence of
concrete references, preliminary experiments were Ðrst per-
formed on methoxyethanol (MeOCH CH OH)ÈVOTAM
2
2
[
MeOCH CH OH]). (a) Pure VOTAM (S \ O); (b) S \ 1; (c)
2
2
S \ 0.5; (d) S \ 0.1; (e) S \ 0.01. The assignments correspond to
Scheme 1.
2
2
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495

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The signal located at [650 ppm is the most intense new
observed, even with variations of the templating agent or the
(S \ [VOTAM]/[Surf]) ratio from 2.5 to 100. Pluronic 3100È
VOTAM systems show some additional resonances to those
already described, at [660 (doublet) [643 and [624 ppm.
The position of the [660 ppm signal corresponds to those
signal at low MeOCH CH OH concentrations, suggesting
2
2
that the Ðrst step of the interactions should be the result of the
following transalcoholysis:
VO(O-tert-Am) ] MeOCH CH OH ]
3
2
2
observed in the case of (PriO)VO(O-tert-Am) ;31 therefore,
2
MeOCH CH OVO(O-tert-Am) ] tert-AmOH
this new series may be ascribed to the presence of some
2
2
2
capping PriO groups, instead of EtO. In fact, less than 20% of
this polymer is in the triblock form, see for example, ref 35.
Interestingly, the ““square peaksÏÏ (multiplets) located at
upon which, and once the bulky alcoxy groups are removed,
chelation is an easy and almost direct process.
The assignments of all MeOCH CH OHÈVOTAM species
[
633 ppm are important, indicating the presence of VÈOÈV
bridging. This fact suggests the possibility that the polymeric
skeleton acts like a template: once the metallic centres are
2
2
are summarised in Scheme 1. Upon ethanol addition, the reso-
nances corresponding to chelated moieties fade, while the
characteristic VO(O-tert-Am) (OEt)
signals start to
““trappedÏÏ by the PEO or PPO fragments, nucleophilic reac-
3
~x
x
appear. This is characteristic of labile V(V) complexes;
although VOTAM should be sterically hindered, once substi-
tution and/or chelation takes place, the nucleophilic substitut-
ion driven by ethanol can easily lead to ethoxy-substituted
moieties.34
tions between the immobilised cationic species can take place,
leading to the formation of local oligomers, or even clusters,
attached to the polymer. These oligomers can act as anchor-
ing points for the growing inorganic phase; the formation of a
hybrid interface should act in this case as a heterogeneous
nucleation site.
It can be concluded that the interaction mechanism is
essentially the same for V and Ti alkoxides: initial trans-
alcoholysis by the alcoholic ether polar head in parallel with
chelation of the metallic centres, to yield hybrid surfactant-
metal species, on the basis of the following arguments.
All VOTAMÈsurfactant systems display NMR behaviour
similar to that found in MeOCH CH OHÈVOTAM solu-
2
2
tions, either in THF or toluene (see, e.g., curves b, c and d in
Fig. 4). Only minor changes from this general pattern were
(i) Transition metal alkoxides (Ti, Zr), initially tetra-
coordinated, may expand their coordination sphere by acco-
modating Lewis base O-donors (solvent, other alkoxides).29 A
well-known example is the alkoxide association in solution (in
oligomers) that proceeds through l -OR bridges, enhancing
2
the coordination number of the metal from four to six and
thus leading to a lowering of the reactivity of the metal
centres. Chelation with neutral species, in this case, the PEO
or PPO moieties, is also feasible, and should proceed by an
analogous mechanism, as suggested by the detected VÈOÈV
bridging.
(
ii) Titanium alkoxides are also prone to very fast trans-
alcoholysis reactions as evidenced through 47,49Ti NMR.30
Moreover, transalcoholysis is well-known preparative
pathway for many transition metal alkoxides.36 Although, to
our knowledge, no thorough NMR studies have been per-
formed on the subject, X-ray structures of titanium complexes
chelated by methoxyalcohols and phenols have recently been
reported.37
a
Fig. 4 51V NMR spectra (79 MHz) of VOTAMÈpolymer systems in
THF. (S \ [VOTAM]/[polymer]). (a) MeOCH CH OH (S \ 0.5); (b)
2
2
P123 (S \ 2.5); (c) F127 (S \ 10); (d) Brij 56 (S \ 0.1). The asterisks
denote a lesser peak attributable to double hydrolysis of VOTAM, see
ref. 31. Inset: shape of the ““square plus lorentzianÏÏ peaks for the
system in (b).
Scheme 1 Assignment of vanadium moieties according to the 51V NMR data.
496
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(
iii) The UV-visible absorption spectra of all the systems
ed in many experiments that yield mesostructured hybrid
materials.
made from PEO-based surfactants and di†erent metal alkox-
ides exhibit the same features. The presence of a charge-
transfer band also indicates an increase of coordination.
Pathway A considers an anhydrous medium, where conden-
sation (either through hydrolytic or nonhydrolytic processes)
is not possible before the formation of the mesotextured
hybrid phase (i.e., a dry solvent system with titanium alkox-
ides as precursors). In the initially solvent-rich systems
(typically, surfactant : Ti : ethanol ratios are on the order of
0.01È0.1 : 1 : 30), chelation by the polar head of the sur-
factants should not be favoured. However, upon solvent
evaporation, which is speeded by the ageing temperature (i.e.,
40È70 ¡C), the bonding of the polar heads to the metallic
centres becomes increasingly predominant, favouring the for-
Hitherto, our results indicate that in a non-protic solvent,
transalcoholysis and chelation are the key processes that take
place between PEO-type polymers and metal centres. These
metallic centres can belong either to transition metal alkox-
ides, transition metal chloroalkoxides or even to already pre-
formed metal-oxo building units. In fact, molecular metal
alkoxide precursors are not the only ones able to bind to the
PEO or PPO groups of the surfactants through fast alco-
holysis or chelation reactions; already-condensed transition
metal-oxo species can also bind. This is clearly shown by the
yellow colouration observed when titanium oxo-alkoxo clus-
mation of Ti(OR)
(OEt) (O-Surf) (0 O x ] y O n) species
n~x~y
x
y
(the transalcoholysis process triggers pathway A). It must be
kept in mind, thus, that in most block copolymers, the binding
to a metallic precursor will modify the relative solubility of the
functional blocks, therefore altering the micellisation behav-
iour.39 In this case, and for low hydrolysis ratios (H @ 1), the
polymer ends will probably be capped with hydrophobic
species that decrease the solubility di†erence, thus promoting
polymer unfolding. Finally, a strong hybrid interface is
formed, generating nucleation sites upon which the inorganic
network is formed. As a consequence, the polar heads of
PEO-based templates should act as inhibitors, avoiding the
massive segregation of a nonporous dense oxidic phase (at this
point, it should be recalled that the chelation of transition
metal alkoxide-based species is a well-known strategy used for
the control of the condensation rates of many transition metal
alkoxides19,40).
ters such as Ti16O16
(OEt) or Ti
O
(OPri) are mixed
32
12 16
16
with pluronic- or dendrimer-type copolymers.26 Such alco-
holysis reactions were also evidenced through 13C NMR
experiments that showed the transformation of the cluster
Ti
O
(OEt) into Ti
O
(OEt) (OPrn) when the former
1
6 16
32
16 16
32
8
is dissolved in n-propanolÈtoluene solutions.38
However, the synthesis conditions of mesostructured tran-
sition metal oxides are indeed more complicated. In the pres-
ence of trace water, hydrolysisÈcondensation reactions
between the metallic centres take place in parallel with chela-
tion or transalcoholysis reactions. At the same time, the coor-
dination of the cationic moieties (Scheme 2) can induce drastic
modiÐcations in the folding behaviour of the templating
agents, leading to their reticulation, thus preventing their ade-
quate conÐguration to act as dense, organic templates. In a
limiting case, they might even promote unfolding, leading to
vermicular bicontinuous phases; this should be favoured in
low-water conditions, such as those attained in ““cleanÏÏ
systems, that is those in which water comes from impurity
sources, for H @ 1. This e†ect should be more pronounced in
PEOÈPPO copolymers, where both fragments are able to
Condensation of the metal moieties can occur if water is
provided through air moisture or residual traces, present in
the templates, and also in some cases via a non-fully dried
solvent.41 These phases are generally obtained as monoliths,
or xerogels, and present more-or-less ordered vermicular
(worm-like) structures; there is no dramatic di†erence in the
mesostructures (by XRD or TEM) for these nonaqueous syn-
thesis methods when the size of the polymer is drastically
changed. The rapid condensation, which permits the nucle-
ation of dense local clusters, and the intrinsic viscous nature of
the growing gels, which hinders rearrangement, should be
responsible for the lack of long-range order. In fact, the bicon-
tinuous worm-like materials present a partial curvature
towards the inorganic region; it seems that, in this case, the
small inorganic clusters, incompletely hydrolysed, are coated
by a PEO or PPO crust. This e†ect is less marked with Brij-
Triton- or Igepal-type (PEOÈalkyl) templates, which are more
versatile in this sense, although their maximum pore range is
chelate the metal centres, than in their C E counterparts.
The presence of polar molecules like ethanol or water
i j
masks the relatively feeble metalÈstructurant interactions,
blurring the coordination-based interface. This hybrid inter-
face is thus dominated by the weaker dipolar or H-bonding
interactions. At the same time, this lack of metalÈpolymer
contact allows for polymerÈpolymer interactions to be
enhanced, therefore permitting an improved hydrophobic
PPOÈPPO contact. This phase segregation is indeed critical
in the formation of the liquid crystal-like templates. The role
of water is twofold, in all cases: from the point of view of
template structuring, its higher polarity accentuates the di†er-
ence between PEO and PPO blocks, promoting domain
separation. Furthermore, its presence leads to inorganic poly-
merisation, which takes place ideally in the polar (aqueous or
alcoholicÈaqueous) phase. It must be kept in mind that
hydrolysisÈpolymerisation processes are orders of magnitude
faster for transition metals than for silica. At this stage,
however, the fates of high- and low-water-containing systems
di†er considerably.
somewhat restrained (up to ca. 60 AŽ ); this should be not sur-
prising, in view of the better di†erentiation that is expected on
the basis of the quite di†erent hydrophobicity of the polymer
blocks.
When TiCl is used as precursor, a large amount of HCl is
4
formed upon dissolution in ethanol, the solution is strongly
acidic and TiCl (OEt) species are formed. Such species can
n~x
x
also condense through nonhydrolytic processes to form
Scheme 3 summarises the proposed reaction pathways,
based on existing models2 and the new data reported in the
present work. Once the reactants are mixed, and the Ðrst fast
exchange processes (i.e., alcoholysis, hydrolysis) are accom-
plished, a complex evolution follows, which is very sensitive to
the external parameters; one of the most crucial is the water-
to-metal ratio, H. The reacting systems can follow two main
paths, depending on H, which we will call aqueous or
Ti (OH) O (OR) oligomers.42 The yield of such transform-
p
y z
q
ations should be important for temperatures higher than
90 ¡C. In a classical example (mesostructures obtained from
TiCl Èethanol systems)4 the synthesis was performed at 60 ¡C
4
and thus it seems quite reasonable that the formation of the
transition metal-oxo polymers (building blocks) are built
through a set of nonhydrolytic reactions coupled with
hydrolysisÈcondensation reactions occurring with the already
discussed water sources. Note that large quantities of an inor-
ganic acid, added or generated in situ, is also a way to control
hydrolysisÈcondensation reactions and avoid precipitation.
Through the B or aqueous route, rapid condensation pro-
cesses lead to the formation of oxo oligomers, clusters or small
polymers prior to the formation of the textured template. This
““anhydrousÏÏ conditions. For the sake of clarity these two
main branches will be described separately, as extremes.
However, it must be kept in mind that an intermediate situ-
ation is always possible. In fact, organisation, structuring,
hydrolysis and condensation are simultaneous processes
under the chemical conditions and synthetic protocols report-
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decreases. In the presence of protons as condensation inhibi-
tors, the size of these oligomeric species has been determined
by SAXS to be on the order of 20 AŽ ;45 the presence of OH
groups make these species hydrophilic. The eventual inter-
action between these subunits and the polymers should be due
to H-bonding-type interactions, rather than covalent coordi-
native bonds, as in the previously presented case. This is
evident from the white colour of precipitates and xerogels
obtained from the aqueous procedures. Although chelation of
the metallic centres belonging to the transition metal-oxo
building blocks by surfactants is favoured upon solvent
evaporation,
Ti O X
(OEt) (O-Surf)
moieties
p z pn~x~y~z@2
x
y
should be rarely formed. In this media, the oxo-metal oligo-
mers are hydrophilic; thus, the capping of the polymers
should enhance, rather than blur, the solubility di†erence of
both blocks, promoting an improved segregation. M-(O-Surf)
chelating bonds could possibly have a role in the nucleation of
structured precipitates. The co-condensation of the hydro-
philic clusters around the micelles allows the formation of the
inorganic network. This stage is partially hindered by the
presence of acid, the Cl~ counteranions can remain at the
interface, as proposed in ref. 4. Complete condensation is diffi-
cult to achieve, and trapped water and EtOH, as well as
incompletely removed EtO groups, are likely to be present in
all obtained solids; as seen by FTIR and TGA analysis.26
Condensation may be increased by resorting to a hydrother-
mal post-synthesis treatment, as described by Bagshaw.46
Water plays a determining role as (co)solvent, in which
better polymer folding (and hence, structuration) is possible.
For typical conditions (H O 10), the water consumed by
Scheme 2 Strong (upper) and weak (lower) metalÈpolymer inter-
actions present in di†erent media. Cf. the A and B paths shown in
Scheme 3.
is the principal problem in dealing with transition metals:
their high reactivity. However, the control of hydrolysis and
condensation, through the addition of complexants9,43 or
acids,26 can overcome the problem. Upon dissolution, Ti(OR)
n
precursors undergo fast exchange reactions with EtOH and
HCl molecules, yielding
a
distribution of species
Ti(OR) (OEt) or TiCl (OEt) , or a mixture of both.
hydrolysis and condensation does not exceed 1.9 n .47 The
n~x
x
n~x
x
Ti
Hydrolysis processes are practically instantaneous under these
conditions (H A 2, p [ 1).44 Subsequently, polycondensation
reactions between metal alkoxides can occur, yielding metal-
oxo condensates of general formula TiX (OH) O
remaining typical, water : surfactant mole ratios are on the
order of 40È500. In the case of Brij 56, this represents an ideal
Ðnal composition of ca. 30È60% wt of water; Ðne tuning of
the water-to-surfactant parameter should in principle place
the systems in the right position to attain the desired meso-
phase. Although the presence of the metal cations changes the
x
y 2~(x`y)@2
(
X \ OR or even Cl). Typically, x B 0.3È0.7 and y B 0È0.2;
while the value of y increases with the water content, x
Scheme 3 Proposed mechanistic scheme for hybrid mesophase formation.
498
New J. Chem., 2000, 24, 493È499

View Article Online
1
1
1
7
8
9
D. Khushalani, O
Chem., 1999, 9, 1491.

. Dag, G. A. Ozin and A. Kuperman, J. Mater.
systemsÏ behaviour radically, ordinary phase diagrams can be
considered a Ðrst-hint approximation.
J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem.,
The presented results show the importance of polymer-to-
metal coordination in the early stages of the formation of
hybrid mesostructured phases, which are the precursors of
mesoporous solids. Adequate control of these processes is
essential to develop well-deÐned interfaces. The development
of a strong or weak hybrid interface is a key feature through-
out in obtaining mesostructured xerogels or precipitates: as
shown, it can direct to the formation of narrow worm-like
tunnels or the development of well-deÐned pores, fully
developing the templating potential of high weight and
volume triblock copolymers. Moreover, it is also important in
the design of the template elimination process. Strongly
bound polymers can only be removed by an energic thermal
treatment; on the contrary, if the interactions at the interfacial
level are sufficiently masked, smoother processes (such as
extraction procedures) can be envisaged.
1
988, 18, 259.
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20 P. Behrens, Angew. Chem., Int. Ed. Engl., 1996, 35, 515.
21 M. Linden, J. Blanchard, S. Schacht, S. A. Schunk and F. Schu
Chem. Mater., 1999, 11, 3002.
G. Wanka, H. Ho†mann and W. Ulbricht, Macromolecules, 1994,
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th,
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7, 4145.
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Fredrickson and B. F. Chmelka, Macromolecules, 1999, 32, 4332.
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Acknowledgements
M. Nabavi, Ph.D. T hesis, Universite
989.
(a) B. Alonso, Ph.D. T hesis, Universite

Pierre et Marie Curie, Paris,
1
The authors would like to thank J. Maquet and M.-N. Rager
Ž

Pierre et Marie Curie,
for their help and skill in NMR measurements, and P.-A.
Albouy for the small angle XRD measurements. A. Bouchara
and A. Louis are gratefully recognised. G. J. A. A. S.-I. wishes
to acknowledge CONICET (Argentine Republic) for support
Paris, 1998; (b) B. Alonso and C. Sanchez, J. Mater. Chem., 2000,
10, 377.
D. C. Crans, H. Chen and R. A. Felty, J. Am. Chem. Soc., 1992,
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D. C. Crans, F. Jiang, J. Chen, O. P. Anderson and M. M. Miller,
(
PIP 4196) and a postdoctoral fellowship, and GabboÏs.
Inorg. Chem., 1997, 36, 1038.
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499