Hierarchically porous zirconia through precursor-directed large-scale synthesis

Abstract

Two new precursors, produced by modification of zirconium t-butoxide with 1-dimethylamino-propanol-2 (HDMAP), solid Zr 2 (DMAP) 3 (O t Bu) 5 (1), and liquid Zr 2 (DMAP) 4 (O t Bu) 4 (2), were obtained by reaction of 1.5 and 2 equivalents of HDMAP, respectively, in toluene on Zr(O t Bu) 4 . The produced compounds were characterized by Fourier-transform infrared spectroscopy, 1 H and 13 C nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA) to estimate their stability and volatility. Action of traces of water in solvents or contact with humid air transforms 1 and 2 into less soluble crystalline Zr 2 (DMAP) 3 (O t Bu) 4 (OH) (3). Molecular structures of compounds 1 and 3 were established using single-crystal X-ray studies and for 2, they were elucidated by applying 2D 1 H– 13 C-correlated NMR spectra. The crystals of 1 were subjected to hydrolysis via either storage in ambient atmosphere or immersion into boiling water and the resulting products were characterized by X-ray powder diffraction, TGA, scanning electron microscopy, and atomic force microscopy techniques. The product of hydrolysis in air, ZrO 2 -1, is essentially nonporous, while hydrolysis in boiling water results in ZrO 2 -2 with hierarchical macro-, meso-, and microporosity. Both materials are essentially X-ray amorphous with diffraction patterns appearing as oblique curves, resembling unresolved profiles of the monoclinic baddeleyite structure of ZrO 2 . Heat treatment at 200 and 400 °C does not affect essentially the morphology or porosity and leaves the phase composition unchanged, while that at 600 °C converts both samples into a tetragonal ZrO 2 phase. The ZrO 2 -2 material is via this treatment losing microporosity and becoming macro–mesoporous with a well-defined pore size of about 3 nm. Heat treatment at 900 °C results in collapse of pores and transformation into a well-defined monoclinic baddeleyite structure for both materials.[Figure not available: see fulltext.].