Hybrid materials for molecular sieves

Hybrid microporous organosilica membranes for molecular separations made by acid-catalyzed solgel synthesis from bridged silsesquioxane precursors have demonstrated good performance in terms of flux and selectivity and remarkable hydrothermal stability in various pervaporation and gas separation processes. The availability of wide range of α,ω-bis(trialkoxysilyl)alkane and 1,4-bis (triethoxysilyl)benzene precursors allows tuning of membrane properties such as pore size and chemistry. This chapter presents an overview of the synthesis and application of hybrid organosilica microporous membranes in liquid and gas separation processes. After a concise discussion of the history of solgel-derived microporous ceramic membranes for molecular separations, the solgel chemistry of bridged silsesquioxanes and all relevant processing steps needed to obtain a supported microporous films suitable for molecular separations are discussed. The performance of these membranes is correlated with the membrane compositional properties, such as nature, stiffness and length of the bridging group, and details of the solgel process

Stabilization and destabilization of zirconium propoxide precursors by acetylacetone

The stabilizing and destabilizing mechanism in the action of acetylacetone on zirconium propoxide precursors is revealed; the nature of heteroleptic intermediates provides an explanatio

Nona-coordinated MO6N3 centers M = Zr, Hf as a stable building block for the construction of heterometallic alkoxide precursors

The modification of zirconium or hafnium alkoxides with diethanolamine, H2dea, leads to the formation of unique nona-coordinated M{μ-η3-NH(C2H4O)2}3 cores. The mechanism is used to develop a self-assembly approach to the first thermodynamically stable zirconium–titanium and hafnium–titanium precursors, Zr{μ-η3-NH(C2H4O)2}3[Ti(OiPr)3]2 (1) and Hf{μ-η3-NH(C2H4O)2}3[Ti(OiPr)3]2 (2). Mass spectrometric characterization of these compounds demonstrates their volatility. In addition to the solution stability of these compounds the volatility makes them attractive single source precursors for MOCVD and ALD applications. These precursors are also interesting candidates for application in sol–gel synthesis of microporous materials as the stability of the core prevents self-assembly of ligands on the outer surface of the primary particles formed during the hydrolysis. A n-propoxide analog of 1 can be prepared from zirconium n-propoxide but does not yield any crystalline material. It is demonstrated that 1 can be prepared from [Zr(OnPr)(OiPr)3(iPrOH)]2, however, with a lower yield compared to the use of zirconium isopropoxide. The single crystals obtained from systems containing zirconium isopropoxide, titanium isopropoxide and triethanolamine H3tea turned out to be Ti2(OiPr)2({μ-η4-NH(C2H4O)3}2)2 (4). Theoretical calculations indicate that the octacoordinate M{μ-η4-N(C2H4O)3}2 core, anticipated in reaction with H3tea, will have metal–nitrogen bonds that are too long for its stabilization. This explains why the formation of 4 is thermodynamically favored over the formation of heterometallic species

A modified thermo-mechanical modeling approach for shape memory alloy behaviour13;

The Brinson-Lammering constitutive model is modified to account for the nonlinear shape memory alloy behaviour. The model is divided into three modules by keeping each of the three parameters of stress, strain and temperature constant. Experiments were conducted with a NiTiCu material to obtain the model constants. The constants were then incorporated in the model and the behaviour predicted. A good correlation is obtained between the theory and experiments.13

The molecular composition of non-modified and acac-modified propoxide and butoxide precursors of zirconium and hafnium dioxides

Long-term storage at 0 °C of a paraffin-sealed flask with commercial 70 wt% solution of zirconium n-propoxide in n-propanol resulted in crystallization of an individual oxoalkoxide complex Zr4O(OnPr)14(nPrOH)2 in over 20% yield. The structure of this molecule can be described as a triangular Zr3(μ3-O)(OR)10(ROH) core of 3 edge-sharing octahedrons with an additional Zr(OR)4(ROH) unit attached through a pair of (μ-OR) bridges. Mass spectrometric and 1H NMR investigation of the commercial samples of the most broadly applied zirconium and hafnium n-propoxides and n-butoxides indicate the presence of analogous species in the commercial alkoxide precursors. The content of oxo-alkoxide species in the commercial precursors has been estimated to be ~20% for n-propoxide and ~35% for zirconium n-butoxide. A new route has been presented for synthesis of the individual crystalline mixed ligand precursor [Zr(OnPr)(OiPr)3(iPrOH)]2, from zirconium n-propoxide. A high yield has been observed (~90%), indicative of an almost complete precursor transformation. Mass spectrometry has shown that the synthesized mixed ligand precursor is dimeric, which makes it an attractive alternative to zirconium n-propoxide. Addition of 1 eq of Acetylacetone to zirconium or hafnium alkoxide precursors results in formation of dimeric [M(OR)3(acac)]2 in high yields. These species have limited stability (much higher for Hf than for Zr) and transform in solution into hydrolysis-insensitive M(acac)4 through very unstable M(acac)3(OR) intermediates containing 7-coordinated metal centers. This transformation can be followed kinetically in hydrocarbon solvents by 1H NMR and is noticeably accelerated by addition of parent alcohols. The obtained results clearly reveal limited applicability of EXAFS and XANES techniques for the study of such systems, especially in the context of structure prediction.\ud Electronic supplementary material The online version of this article (doi:10.1007/s10971-009-1988-0) contains supplementary material, which is available to authorized users

Microporous Zirconia–Titania Composite Membranes Derived from Diethanolamine-Modified Precursors

Microporous zirconia-titania composite membranes have been fabricated by sol-gel processing using diethanolamine-modified precursor solutions. Microporous materials made from powders calcined at 400 °C show type I nitrogen sorption behavior. Supported ~0.1 µm thick membranes (see figure) exhibit molecular-sieving properties and are expected to have great potential for separation and reaction applications under harsh conditions