G. Seisenbaeva, O. Dudarko, V. Kessler
Oct 2, 2018
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Journal
Journal of Sol-Gel Science and Technology
Abstract
Two new precursors, produced by modification of zirconium t-butoxide with 1-dimethylamino-propanol-2 (HDMAP), solid Zr2(DMAP)3(OtBu)5 (1), and liquid Zr2(DMAP)4(OtBu)4 (2), were obtained by reaction of 1.5 and 2 equivalents of HDMAP, respectively, in toluene on Zr(OtBu)4. The produced compounds were characterized by Fourier-transform infrared spectroscopy, 1H and 13C 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 Zr2(DMAP)3(OtBu)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 1H–13C-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, ZrO2-1, is essentially nonporous, while hydrolysis in boiling water results in ZrO2-2 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 ZrO2. 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 ZrO2 phase. The ZrO2-2 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.HighlightsHeteroleptic zirconium alkoxide–aminoalkoxide precursors were synthesized and characterized.Precursor hydrolysis in boiling water offered hierarchically macro–meso–microporous zirconia resulting from ligand-driven reaction mechanisms discussed in detail.Heat treatment at 600 °C results in disappearing of microporosity and transformation into a crystalline tetragonal ZrO2.Heating to 900 °C leads to collapse of pores and transformation into a monoclinic ZrO2.