Full text of DOI:10.1002/1521-4095(200006)12:13<989::AID-ADMA989>3.0.CO;2-K

Adjusting the Porosity of a Silica-Based
Hybrid Material
By Bruno Boury and Robert J. P. Corriu*
Precise control of the porosity of silica-based hybrid materials is the topic discussed
here. One approach in particularÐthe copolycondensation of two precursors fol-
lowed by selective removal of one of the organic spacers (see Figure)Ðis highlighted.
Using this technique, microporous hybrid xerogels are produced that have a high spe-
cific surface area and a highly hydrophobic surface.
New solid materials with attractive properties must com-
bine a specific reactivity with a precise porosity. From this
point of view, the hybrid materials that have been developed
in the last 20 years fit these two requirements.[1] Among the
numerous and various types of hybrid materials, the silica-
based nanostructured hybrid materials have been know since
the pioneering works of Schmidt,[2] Mark,[3] and Wilkes.[4]
They can be prepared by hydrolysis of organoalkoxysilane,
leading to ORMOSIL,[5] silsesquioxanes,[6] or more recently
bridged polysilsesquioxanes[7] (Scheme 1) [R(SiO1.5)n] (n >
2).[8] In such materials the chemical and thermal stability of
the siloxane network is combined with the reactivity and
physical properties of an organic group. Due to the stability of
non-porous resin.[9] In other cases, parameters governing the
kinetic of hydrolysis and polycondensation were found to
±
±
modify the porosity: hybrid xerogels such as O1.5Si C º C
±
±
C6H4 C º C SiO1.5 can be porous or non-porous depending
on the solvent of gelation;[10] and the porosity of hybrid xero-
±
±
gels O1.5Si C6H4 SiO1.5 prepared in tetrahydrofuran depends
on the temperature of gelation.[11] However, a broad distribu-
tion of micro- (<20 Š) and mesopores (>20 Š) is always ob-
tained and limits the usefulness of these tools to adjust the
size and number of pores.
From a general point of view, the design of a precise porosity
has two goals, firstly to generate a porosity that allows access
of the reagents to the reactive centers; this can be achieved by
a high specific surface area and mesoporosity. The second goal
is to define the accessibility to the specific reactive center by
the size of the pore and in that case microporosity is required.
The resulting sieving effect can permit the reaction to be con-
trolled by the size of either the reagent, the products, or the
transition state. Such design of a precise porosity cannot be ad-
dressed in the same terms as for the well-known zeolites[12] or
the molecular network of crystallized hybrid organic/inorganic
solids.[13] In these cases, the microporous architecture is
achieved by a thermodynamic control either through a tem-
plating effect as in the case of the zeolites or it is the result of
supramolecular crystal engineering as in the case of crystal-
lized molecular solids. This is difficult to achieve for the amor-
phous hybrid materials prepared by the sol±gel process be-
cause, as for other materials prepared by such a process, the
structure is mainly determined by the relative rates of hydroly-
sis and condensation (some of them being irreversible).[14]
Efforts to solve these problems have focused on technolog-
ical approaches and, for example, control of the porosity of
these materials was achieved either by preparing thin films[15]
±
the Si C link and the mild conditions used for this process the
introduction of very different reactive R groups is possible,
the latter being homogeneously distributed and covalently
linked to the three-dimensional siloxane framework. There-
fore these materials are potentially useful as coatings (UV,
thermal, and chemical protection), ceramic binders or ceramic
precursors, catalyst or chemical and biochemical sensors, and
luminescent materials.
As in the case of any other materials, the control of the po-
rosity in silica-based hybrid materials is a necessity because,
depending on its future application, either a precise and high
porosity will be necessary or must be avoided. In the case of
the nanostructured hybrid material with the general formula
[R(SiO1.5)n] (n ³ 2) it has been demonstrated that the nature
of the R core can in some cases impose a specific type of
porosity. For example, the hydrolysis of precursors
[R(Si(OMe)3)2] where R is a long alkyl group always gives a
or by using supercritical carbon dioxide as a solvent.[16]
A
[*] Prof. R. J. P. Corriu, Dr. B. Boury
Laboratoire de Chimie MolØculaire et Organisation du Solide
UMR 5637, UniversitØ Montpellier II
Place E. Bataillon, F-34095 Montpellier Cedex 5 (France)
CMC (critical micellar concentration) approach can also be
used and recently [R(Si(OMe)3)n] molecules were used; how-
Adv. Mater. 2000, 12, No. 13, July 5
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989

B. Boury, J. P. Corriu/Adjusting the Porosity
as in the preparation of microporous
oxide, is limited by the homogeneous
distribution of the template in the hy-
brid material.[18] The copolycondensa-
± ±
tion of (RO)3Si R Si(OR)3 with a tet-
raalkoxysilane is also conceivable but
the difference of reactivity between
very different precursors and the evolu-
tion of the structure upon aging and
drying is difficult to control
(Scheme 2b).
Recently, Shea et al. reported the
generation of porosity in hybrid xero-
gels by a chemical route, with material
of the general formula [R(SiO1.5)n]
where the organic group is a dialkylene
carbonate group. The elimination of
Scheme 1. Schematic depiction of the preparation of silica-based hybrid materials by the sol-gel process
with organic-bridged silsesquioxane precursors.
CO2 from each of the organic moieties
ever, they lead to the fabrication of mesoporous hybrid mate-
rials [R(SiO1.5)n] (n ³ 2) and the accessibility to the organic
group is not yet clear.[17]
The generation of microporosity in these materials by the
most direct method would be to use an organic templating
agent at the hydrolysis/polycondensation step and subse-
quently to remove it by washing (Scheme 2a). This approach,
by a post-processing thermal treatment leads to modification
of the material's reactivity, meanwhile generating a porosity
that is not precisely described.[19] We have also considered
such a chemical route to control the porosity of hybrid materi-
als [R(SiO1.5)n] (n > 2). Our approach is based on the copoly-
condensation of two precursors followed by the selective re-
moval of one of the organic spacers as shown in Scheme 2c. It
is close to the new concept successfully used to introduce mi-
croporosity in a crystallized hybrid material such as phospho-
nate. We have demonstrated the interest and the limitation of
such an approach in the case of silica-based hybrid materials,
by studying a model where elimination of an organic group,
the 1,4- bisethynylbenzene, from hybrid xerogels of the gener-
±
±
±
±
±
±
al formula {[O1.5Si C º C C6H4 CºC SiO1.5]x[O1.5Si C6H4
±
C6H4 SiO1.5 1±x
]
}
was achieved by chemical treatment,
Scheme 3.[20] The starting hybrid xerogels were obtained by
copolycondensation in various ratios of two precursors: the
1,4-bis(trimethoxysilylethynyl)benzene (A) with a labile 1,4-
bisethynylbenzene moiety and the 4,4¢-bis(trimethoxysilyl)bi-
phenyl (B) with a biphenyl group that will act as pillars. These
precursors were chosen for their similar structure (size and ge-
ometry) and functionality. A random copolycondensation
seems to be achieved despite the higher reactivity of A. By
comparison with previous studies, this allows us to modulate
the ratio of pillar and labile organic groups.
We used a chemical treatment in order to remove quantita-
tively the 1,4-bisethynylbenzene, whose efficiency was pre-
viously demonstrated in the case of pure hybrid xerogels such
[21]
±
± ±
±
as O1.5Si C º C R C º C SiO1.5
.
From an experimental
point of view this is performed by reacting a suspension of the
hybrid xerogels in an excess of methanol, water, and a catalyt-
ic amount of fluoride anion for 4 days at reflux. The thermal
oxidation that is widely used to remove organic groups from a
hybrid xerogel must be avoided in the present case as it can-
not be selective.
The resulting materials are hybrid xerogels with a high speci-
fic surface area and where the biphenyl groups act as pillars
linking silica network. As a consequence, the surface of the ma-
Scheme 2. General routes for the introduction of porosity in a silica-based hy-
brid material prepared by a sol-gel process.
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Adv. Mater. 2000, 12, No. 13, July 5

B. Boury, J. P. Corriu/Adjusting the Porosity
sopores (Æ > 20 Š) and it is important to fo-
cus on the major parameters that can influ-
ence their size and distribution.
1) The molar ratio of each of the precur-
sors and their distribution in the starting
material are certainly the most important
parameters since they determine the pro-
portion of the voids and the pillars in the
resulting materials, and moreover their vi-
cinity. Thus, a high ratio of the 4,4¢-bis(tri-
methoxysilyl)biphenyl precursor (B) limits
the proximity of voids created by the elimi-
nation of the organic group and avoids the
collapse of the materials. In addition, a ran-
dom copolycondensation of the precursors
limits the formation of microdomains
where several labile organic groups are
close to each other and which leads to the
formation of mesopores.
Scheme 3. Preparation and chemical treatment of the hybrid material prepared by co-polycondensa-
tion of two precursors.
2) By comparison with a crystallized ma-
terial, the amorphous character of the silica-based hybrid xe-
rogels complicates the attainment of very well-defined porosi-
ty. In this case, voids initially present between the organic
groups are finally added to the void created and modify the
expected pore volume.
terial is highly hydrophobic. In all cases a high level of micro-
pores are formed, their size (4 < Æ < 20 Š) being mainly very
close to the size of the organic group that has been removed
(Si±Si distance 11.5 Š) (Fig. 1). However, there are some me-
3) Chemical treatment in the presence of F± anion and
water induces reorganization of the silica network.[21] How-
ever, such reorganization is apparently limited by increasing
the proportion of pillars, this might result from the hydropho-
bic character of the biphenyl groups, which prevent the access
of the polar reagents.
±
4) The hydrolysis of the Si C(sp) bond leads to the forma-
tion of silanols, whose volume reduces the size of each pore
created by the elimination of the organic group. In addition
some of them can polycondense with each other.
In conclusion, we have presented the possibility of creating
microporosity in an amorphous hybrid material by its chemi-
cal modification. The model that we have developed illus-
trates the high reactivity of the organic group in these materi-
als and the possibility of doing chemistry within the material.
It also allows us to point out the key parameters of this ap-
proach that could be extended to other materials. Further de-
velopment will have to take into account the organization in
such materials and the functionality created inside the hybrid
material.
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Fig. 1. Pore size disribution vs. pore diameter for the hybrid materials obtained
after elimination of the bisethynylbenzene group by chemical treatment.
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992
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Adv. Mater. 2000, 12, No. 13, July 5