Hierarchically porous rutile titania: Harnessing spontaneous compositional change in mixed-metal oxides

In numerous applications involving fluid flow through a porous structure, performance improvements, such as better flow characteristics and increased fluid-solid contact, are obtained when multiple length scales of porosity are present in the porous medium. To this end, we have developed a route to hierarchically porous rutile titania that obviates the need for preformed or preassembled structure-directing agents. The preparation involves the successive leaching of phases and components from a dense composite of wurtzite ZnO and inverse spinel Zn2TiO4. The resulting monoliths are composed of grains of highly crystalline rutile titania and possess a hierarchy of interconnected pores with length scales of 5 mu m and 100 nm. Ex situ solid-state Zn-67 and Ti-47,Ti-49 NMR has helped probe local environments around these elements in both the mixed and pure crystalline materials that are formed during the leaching processes.