Incorporation of isolated Ti sites into mesoporous silica thin films by sugar surfactant complexation

One of the challenges in transition metal heterogeneous catalysis is the generation of supports with well-defined porosity containing transition metal sites of uniform coordination, complexity, and spacing. For instance, tetrahedrally coordinated titanium atoms incorporated into a silica network are believed to be the active oxidation sites for heterogeneous epoxidation catalysis. Sol-gel chemistry provides an opportunity to produce well mixed metal oxides, but the high reactivity of titanium precursors compared to silicon precursors impedes the uniform incorporation of isolated tetracoordinated Ti(IV) sites. Here, a homogeneous distribution of titanium at the pore surface is promoted by a ligand-assisted templating method with n-dodecyl β-d-maltoside (C₁₂G₂) surfactant. Preferential interactions or complexation between the titanium and maltose headgroups of this surfactant are hypothesized to promote homogeneous incorporation of titania only at the micelle/silica interface. To generate well-ordered materials with controlled titania site density, a mixed surfactant system of C ₁₂G₂ and cetyltrimethylammonium bromide (C ₁₆TAB) is used for pore templating. A series of Si-Ti mixed oxide thin films with different levels of titanium loading (from 1.15 to 11.1 atom% of cations) is synthesized by a solvent evaporation-driven self-assembly method with staged precursor addition to facilitate complexation between C ₁₂G₂ and the titanium precursor. The degrees of homogeneity (indicated by tetracoordinated Ti) in these films are compared with those of films synthesized with the same loading of titanium but without C ₁₂G₂ surfactant or without pre-complexation. The titanium in films synthesized using maltoside complexation is highly dispersed, while some fraction of octahedrally coordinated titanium atoms is generated without C₁₂G₂ surfactant and without pre-complexation.