No abstract

The modification of different zirconium propoxide and hafnium propoxide precursors with 2,2,6,6,-tetramethyl-3,5-heptanedione (Hthd) was investigated by characterization of the isolated modified species. The complexes [Zr(OnPr)3(thd)](2), [Zr(OnPr)(OiPr)2(thd)]2, Zr(OiPr)(thd)3, [Hf(OnPr)3(thd)]2, and Hf(OiPr)(thd)3 were isolated and characterized. The structure of the n-propoxide analogue of Zr(OiPr)(thd)3 could not be refined, but its existence was clearly demonstrated by XRD and 1H NMR. The modification of the propoxide precursors involves mono- and trisubstituted intermediate compounds and does not involve a disubstituted compound; thus, the commercial product that is claimed to be "Zr(OiPr)2(thd)2" and is most commonly used for the MOCVD preparation of ZrO2 does not exist. No evidence was found for the presence of such a compound in either zirconium- or hafnium-based systems. Formation of the dimeric hydroxo-di-thd-substituted complex, [Hf(OH)(OiPr)(thd)2]2, which could be isolated only for hafnium-based systems, occurs on microhydrolysis. All heteroleptic intermediates are eventually transformed to the thermodynamically stable Zr(thd)4 or Hf(thd)4) The compounds obtained from isopropoxide precursors showed a higher stability than those with n-propoxide ligands or a combination of both types. In addition, it is important to note that residual alcohol facilitates the transformation and strongly enhances its rate. The unusually low solubility and volatility of MIV(thd)4 has been shown to be due to close packing and strong van der Waals interactions in the crystal structures of these compounds.

Optical properties of a thin amorphous silica membrane and the supported

Inorganic microporous materials show great potential for applications in industrial catalysis, separation technology, membranes, sensors, and optical devices. The most common amorphous, microporous inorganic material is silica. Due to its small pores and straightforward synthesis, amorphous silica has been considered as a promising material for membrane applications. However, its moderate hydrothermal stability limits its extended application in harsh environments.[1] Of particular interest as alternative materials for stable, high-performance membranes are sol-gel-derived amorphous oxides of transition metals, such as titania and zirconia. These materials show superior stability compared to silica; [2] however, their synthesis is complicated by the high reactivity of the required precursors.[3] To date, a few groups have been able to prepare amorphous titania and zirconia membranes for aqueous nanofiltration.[4]Herein, we describe the preparation and characterization of microporous zirconia-titania composite membranes. Preparation has been carried out via two different synthesis routes. The first route is based upon using the diethanolamine-stabilized heterometallic precursor Zr{l-g 3 -NH(C 2 H 4 O) 2 } 3 [Ti-(O i Pr) 3 ] 2 (1).[5] The second route involves synthesis using 2, a mixture of zirconium n-propoxide, titanium n-propoxide, and diethanolamine in a molar ratio equivalent to that in 1. Figure 1a shows thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) data for uncalcined powders derived from either 1 or 2. The data are similar for both powders. A sharp initial weight loss is observed with an onset at 50°C, corresponding to the removal of adsorbed solvent molecules. A gradual decrease in weight is subsequently observed up to 300°C. This is attributed to the removal of ligands from the outer surface of the material. Mass spectrometry confirmed the presence of nitrogen in the sweep gas, originating from the amine groups of the ligands. In the range ∼ 300-350°C, a sharp decrease in weight is observed, which is accompanied by a large change in the heat flow centered at 350°C. This can be attributed to the removal of ligands located inside the material. Mass spectrometry again confirmed a high concentration of nitrogen in the sweep gas. A further increase of temperature to 450-600°C causes removal of the remaining alkoxide ligands.The DSC data suggest that crystallization occurs at ∼ 750°C, as evidenced by the sharp exothermic peak at this COMMUNICATIONS