No abstract

Hierarchically porous TiO(2)/ZrO(2) millimeter-sized beads were synthesized using a sol-gel templating technique, and investigated for suitability as radionuclide sorbents using uranyl as a radionuclide-representative probe. The bead properties were varied by altering either composition (22, 36, and 82 wt % Zr in the Ti/Zr composite) or calcination temperature (500 or 700 °C). Uranyl adsorption was higher for the crystalline beads (surface area: 52-59 m(2) g(-1)) than the amorphous beads (surface area: 95-247 m(2) g(-1)), reaching a maximum of 0.170 mmol g(-1) for the 22 wt % Zr sample. This was attributed to the higher surface hydroxyl density (OH nm(-2)), presence of limited microporosity, and larger mesopores in the crystalline beads. Mass transport properties of the crystalline beads were not compromised by the large bead diameter: sorption rates comparable to those reported for powders were achieved and rates were higher than exclusively mesoporous reported systems, thereby highlighting the importance of pore hierarchy in designing materials with improved kinetics. Chemical stability of the sorbent, an important property for processes involving corrosive effluents (e.g., radioactive waste), was also assessed. Crystalline beads displayed superior resistance against matrix leaching in HNO(3). Stability varied with composition: the 22 wt % Zr sample demonstrated the highest stability.

Macro- and mesoporous hybrid materials have applications in the fields of drug delivery, catalysis, biosensing, and separations. The pore size requirements must be well-understood to maximize the performance (e.g., load capacity and accessibility) of such materials. Hybrid materials were prepared by coating five distinct macroporous commercial membranes with zirconium titanium oxide through sol-gel chemistry. Calcination of these templated materials produced oxide membranes which had a suite of macropore and mesopore architectures, pore volumes, and surface areas. These differences in physical properties were used to conduct a fundamental study on the relationship between the pore size and volume and the polymer incorporation. Metal oxide membranes were postsynthetically modified with poly(ethyleneimine) (PEI) ranging in molecular weight from 1300 to 1 000 000 Da (1.2-11 nm in hydrodynamic diameter). The incorporation of the polymer from a 9 wt % solution at pH 10 was highly dependent on the pore size and pore volume. As the surface area increased, loading capacity decreased, indicating that much of the increased internal surface, due to small pore diameters (< or = 8 nm), was inaccessible to the macromolecules. Exclusion of PEI from small mesopores was apparent even for the lowest molecular weight polymer. A high maximum loading of 1.25 mg m(-2) of 600 000-1 000 000 Da PEI was achieved in the metal oxide with the largest minimum mesopore diameter. Thus, mesopore diameter and pore volume must be considered when designing a mesoporous solid support.

A simple and engineering friendly one-step process has been used to prepare zirconium titanium mixed oxide beads with porosity on multiple length scales. In this facile synthesis, the bead diameter and the macroporosity can be conveniently controlled through minor alterations in the synthesis conditions. The precursor solution consisted of poly(acrylonitrile) dissolved in dimethyl sulfoxide to which was added block copolymer Pluronic F127 and metal alkoxides. The millimeter-sized spheres were fabricated with differing macropore dimensions and morphology through dropwise addition of the precursor solution into a gelation bath consisting of water (H(2)O beads) or liquid nitrogen (LN(2) beads). The inorganic beads obtained after calcination (550 °C in air) had surface areas of 140 and 128 m(2) g(-1), respectively, and had varied pore architectures. The H(2)O-derived beads had much larger macropores (5.7 μm) and smaller mesopores (6.3 nm) compared with the LN(2)-derived beads (0.8 μm and 24 nm, respectively). Pluronic F127 was an important addition to the precursor solution, as it resulted in increased surface area, pore volume, and compressive yield point. From nonambient XRD analysis, it was concluded that the zirconium and titanium were homogeneously mixed within the oxide. The beads were analyzed for surface accessibility and adsorption rate by monitoring the uptake of uranyl species from solution. The macropore diameter and morphology greatly impacted surface accessibility. Beads with larger macropores reached adsorption equilibrium much faster than the beads with a more tortuous macropore network.

Temperature-based processes combined with a sol–gel templating technique were used to prepare hierarchically porous amorphous TiO2/ZrO2 beads with tunable mesopore sizes for improved dye sorption capacities.