(81e) Preparation of Mesoporous Materials: Approaching to Industrial Viability

Since the discovery of the ordered mesoporous
materials [1-2] many
papers have been published about preparation of these kind of materials, but
most of the proposed methods are very far of being applicable at industrial
scale due to cost of precursors, catalysts, structure directing surfactants,
and complex processes which are proposed.

In most of the processes alcoxides of the
corresponding metal are proposed as precursors, acid or basic catalysts are
used, and surfactants used as structure directing agents are not normally
recovered.

In this work, meso and
meso-macroporous materials are prepared using metasilicate as precursor,
experiments using acid resins as acidifying agent are presented, and
experiments using recovered surfactant are also presented. Influence of
concentration of precursor and surfactant on properties of obtained materials is
presented.

In case of using acid resin the
preparation method is the following. First, water and surfactant are and kept under
stirring at 50ºC to ease the melt of the surfactant until a clear solution was
obtained. Then ion exchange resin is added to the water and surfactant solution.
Later on, a sodium silicate solution is added drop by drop. The resulting slurry
is stirred, so the Na⁺ of the sodium silicate solution substituted the
H⁺ on the exchange sites of cation resin. The filtered liquid, which
did not have condensed silica yet, is placed in an oven at 100 ºC during 24 h
in order to allow the silicate condensation producing a white precipitate that
corresponds to the ordered mesoporous material. In case of using macroemulsions
as templates for macroporous materials, HCl is used as catalysts.

Surfactant
is extracted from material by ethanol in a soxhlet, and recovered by
evaporating the ethanol in a rotavapor.

The
material is finally dried and calcined in order to eliminate the residual
surfactant

Materials
are characterized by Small-angle X-ray diffraction scattering (SAXS), Scanning electron microscopy (SEM), Transmission electron
microscopy (TEM),  Hg intrusion porosimetry and N₂-sorption analysis.

In figure 1, examples of macromesoporous
material and the ordered mesoporous structure are presented.

Figure 1. SEM
and TEM  images showing macroporous and mesoporous structures obtained

Mesostructure was confirmed by SAXS.
High specific surfaces were obtained depending on the values of variables
studied, the less silicate or surfactant concentration the more specific
surface. No significant differences in specific surface were observed when
recovered surfactant is used.

As main conclusion of this work, remark
that very good materials with high specific surfaces and ordered mesostructure
can be obtained using inorganic precursor, acid resin as catalyst, and
recovered surfactant, which can make much less expensive the fabrication of
this kind of materials.

 [1] C.T.
Kresge, M.E. Leonowicz, W.J. Roth, J.S. Beck. Ordered
mesoporous molecular sieves synthetized by a liquid-crystal template mechanism.
Nature 359, 1992, 710

[2] J.S.
Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge,, K.D Schmitt, C.T.W
Chu, D.H. Olson, E.W. Sheppard, S.B. McCulle, J.B. Higgins, J.L. Schlender.
Anew family of mesoporous molecular sieves prepared with liquid crystal
templates, J.Am. Chem. Soc. 114, 1992, 10834

Keywords: Silica,
emulsion, meso-macroporous, ordered mesopores, cooperative templating mechanism