(434d) Silica-Titania Mixed Oxide Mesoporous Thin Films: Incorporation Of Ti By Surfactant Complexation

Tetrahedrally
coordinated titanium atoms incorporated into a silica network are believed to
be the active oxidation sites required for heterogeneous silica-supported
titania epoxidation catalysts.  Sol-gel chemistry provides an opportunity to
produce well mixed metal oxides, but the high reactivity of titanium precursors
compare to silicon precursors impedes the molecular level incorporation of
Ti(IV) in silica matrix by Si-O-Ti bridges.  The difference in reactivities
often generates titanium in unwanted states- either isolated titanium with
coordination numbers greater than four or aggregated bulk titania nanoparticles
silica.  In this work, homogeneous distribution of titanium at the pore surface
is promoted by a ligand-assisted templating method with n-dodecyl β-D-maltoside (β-C₁₂G₂).  Preferred
interactions or possible complexation between the titanium atoms of a Ti
precursor and maltose headgroups of maltoside surfactant should have two favorable
effects. First, complexation inhibits fast
hydrolysis and condensation, thus promoting homogeneous incorporation of
titania.  Second, complexation with the surfactant fixes titanium at the
micelle surface, allowing it to be incorporated selectively only at the
micelle/silica interface. To promote well-ordered materials and to allow
control over titania site density, a mixed surfactant system of β-C₁₂G₂
and the cationic surfactant cetyltrimethylammonium bromide (C₁₆TAB)
is used for pore templating.  Ordered hexagonal mesoporous silica-titania mixed
oxide thin films are synthesized by a solvent evaporation-driven self-assembly
method.  The final mesostructure of the mixed oxide films are predicted from a
ternary phase diagram of C₁₆TAB/ β-C₁₂G₂ / water (developed
earlier) by replacing the volume of water with an equivalent volume of silica
and titania. A series of films with predictable mesostructures are synthesized
with different amount titanium loading by employing a 1:1 molar ratio of β-C₁₂G₂
to titanium precursor.  The degrees of homogeneity (indicated by
tetracoordinated Ti) in these films are compared with that of films synthesized
with same loading of titanium but without any β-C₁₂G₂ surfactant.  The
titanium in films synthesized with utilization of transition metal-carbohydrate
complexation is highly dispersed, while site isolated titanium atoms with
octahedral coordination and/or uncontrolled segregation of titania in silica
support occurs without sugar-based surfactant. Silica-titania mixed oxide
mesoporous films are characterized here by UV-vis spectroscopy, Fourier
transform infrared (FTIR) spectroscopy, powder X-ray diffractometry (XRD), and
transmission electron microscopy (TEM).