A neutral templating route to mesoporous molecular sieves and their catalytic application for peroxide oxidation of large aromatic molecules

Because of their uniform pore size and ability to "sieve" molecules on the basis of their size and shape microporous molecular sieves and zeolites (with pores of <2.0 nm) are widely used in a number of adsorption, ion-exchange and catalytic processes. The recently discovered hexagonal mesoporous molecular sieves MCM41 (with uniform pore size in the range of 2.0--10.0 nm) by the scientists at Mobil provide an unique opportunity for a logical extension of these applications toward processing and transforming of much larger molecules. The preparation of MCM-41 was originally accomplished by a templating mechanism involving electrostatic interactions between assemblies of positively charged quaternary ammonium cation surfactants (S+) and anionic inorganic species (I--) as framework precursors. Recently, Stucky, and his co-workers extended this electrostatic approach by providing three complementary templating pathways, namely, the charge-reversed S--I + and the counterion (X-- or M+)-mediated S+X--1+ or, S-- M+I-- routes. However, these electrostatic templating pathways afford mesoporous molecular sieves with low degree of framework cross-linking, limited framework wall thickness, large crystallite size and little or no textural mesoporosity which does not contribute to improving thermal stability and to accessing the framework-confined mesopores. In addition, due to the strong electrostatic interactions and charge matching the cationic template is strongly bound to the framework and difficult to recover by non-destructive methods.; Our efforts to circumvent these limitations have materialized into a neutral templating approach (S° I°) which allows for the preparation of hexagonal mesoporous molecular sieves (denoted HMS) with: (i) more completely cross linked framework; (ii) thicker pore walls; (iii) superior thermal stability; (iv) small crystallite size and complementary textural mesoporosity for better access of the framework-confined mesopores. Due to the lack of electrostatic interactions the method allows for the effective and environmentally benign recovery and recycling of the neutral template by simple solvent extraction.; A new I° S°-S° I° biomimetic templating approach to the synthesis of lamellar silicas with high degree of framework cross-linking, exceptional thermal stability, sorption properties typical of a pillared lamellar material, and specific surface area and pore volume similar to that of the MCM-41 and HMS is demonstrated. The simultaneous biomimetic templating of silica layers and intragallery pillars occurs in the intralayer regions of multilamellar vesicles of neutral diamine surfactant. This biomimetic method also provides for the efficient and environmentally benign recycling of the neutral template by simple solvent extraction.; The new S° I° templating approach has been used to prepare transition metal-substituted HMS derivatives. These new mesoporous metallosilicates exhibit exceptional catalytic activity for peroxide oxidation of bulky aromatics with kinetic diameters that are too large (larger than 0.6 nm) to access the pore structure of the conventional microporous transition metal-substituted molecular sieves such as titano- and vanadosilicates. Due to its complementary textural mesoporosity Ti-HMS exhibits superior catalytic activity for peroxide oxidation of the bulky 2,6-di-tert-butyl-phenol relative to the electrostatic Ti-elative to the electrostatic Ti-MCM-41 counterparts.