Mesoporous titanosilicate molecular sieves prepared at ambient temperature by electrostatic (S+I-, S(+)X(-)I(+)) and neutral (S degrees I degrees) assembly pathways: A comparison of physical properties and catalytic activity for peroxide oxidations

Hexagonal mesoporous titanosilicates with distinguishable framework charges and textural mesoporosity, namely, Ti-MCM-41 and Ti-HMS, were prepared at ambient temperature by electrostatic and neutral assembly processes, respectively. Titanium incorporation at the 2 mol % level for both materials was accompanied by increases in lattice parameters and wall thicknesses, but the framework pore sizes remained unaffected. Cross-linking of the anionic framework of as-synthesized Ti-substituted MCM-41 prepared by electrostatic S+I- and S(+)X(-)I(+) assembly pathways (where S+ is a quaternary ammonium surfactant and I- and I+ are ionic silicon precursors) was enhanced significantly by Ti substitution, as judged by Si-29 MAS NMR. The neutral framework of as-synthesized Ti-HMS formed by S degrees I degrees assembly (where S degrees is a primary amine and I degrees is a neutral silicon precursor) exhibited the same high degree of cross-linking as the unsubstituted silica analog. UV-vis and XANES spectra for the calcined farms of Ti-MCM-41 and Ti-HMS indicated (i) the presence of site-isolated Ti species in the framework, (ii) predominantly tetrahedral coordination for Ti, along with some rehydrated five- and six-coordinated sites, and (iii) Ti siting that was virtually independent of the framework assembly pathway. All mesoporous molecular sieves exhibited catalytic activities superior to that of titanium silicalite for the liquid phase peroxide oxidations of methyl methacrylate, styrene, and 2, 6-di-tert-butylphenol. The exceptional catalytic activity in the case of Ti-HMS, especially toward larger substrates, was attributable to the small crystallite size and complementary textural mesoporosity that facilitates substrate access to framework Ti sites.