Optimization of mesoporous titanosilicate catalysts for cyclohexene epoxidation via statistically guided synthesis.

An efficient approach to improve the catalytic activity of titanosilicates is introduced. The Doehlert matrix (DM) statistical model was utilized to probe the synthetic parameters of mesoporous titanosilicate microspheres (MTSM), in order to increase their catalytic activity with a minimal number of experiments. Synthesis optimization was carried out by varying two parameters simultaneously: homogenizing temperature and surfactant weight. Thirteen different MTSM samples were synthesized in two sequential 'matrices' according to Doehlert conditions and were used to catalyse the epoxidation of cyclohexene with tert-butyl hydroperoxide. The samples (and the corresponding synthesis conditions) with superior catalytic activity in terms of product yield and selectivity were identified. In addition, this approach revealed the limiting values of each synthesis parameter, beyond which the material becomes catalytically ineffective. This study demonstrates that the DM approach can be broadly used as a powerful and time-efficient tool for investigating the optimal synthesis conditions of heterogeneous catalysts.

Kinetics and mechanism of the sphere-to-rod transition of triblock copolymer micelles in aqueous solutions.

The kinetics of the sphere-to-rod transition of micelles composed of triblock copolymers of ethylene oxide and propylene oxide (EO(20)PO(70)EO(20)) have been investigated using dynamic light scattering (DLS) and cryogenic electron transmission microscopy (Cryo-EM). Sphere-to-rod transition is induced by a solvent jump, initiated by adding KCl and ethanol to an aqueous micellar solution. The growth process of the wormlike micelles depends on the experimental conditions and has two distinct regions that can be described as initiation period and actual growth to equilibrium. All growth curves exhibit a single relaxation time that represents the lifetime of the micelles. The growth curves collapse into a master curve, when shifted by the relaxation time, indicating that the actual growth process of the micelles in all samples occurs through the same mechanism. The relaxation time decreases with increasing surfactant concentration. Additionally, some of the formed micelles exhibit a caterpillarlike shape in which some of the original spherical species can still be detected. These facts suggest that the micelles grow longer predominantly by random coagulation/fragmentation reactions involving micellar species of different sizes. However, the appearance of a unimer peak is detected with DLS during the growth stage. This implies that unimer exchange may also contribute to the elongation of the micelles.

Effects of salts and ethanol on the population and morphology of triblock copolymer micelles in solution.

The morphological changes of micelles composed of triblock copolymer of ethylene oxide and propylene oxide (EO20PO70EO20) in the presence of different inorganic salts and ethanol have been investigated using dynamic light scattering (DLS), rheometry, and cryogenic transmission electron microscopy (cryo-EM). The following salts were studied: KF, KCl, KI, LiCl, and CsCl. In the presence of KF, KCl, and CsCl, spherical and wormlike micelles coexist. LiCl and KI have little influence on the morphology of the micelles, whereas KF has the most pronounced effect. In agreement with the well-known Hoffmeister anion salt series, F- has the strongest effect of the three anions studied (F-, Cl-, I-). In contrast, the effectiveness of the cation type does not follow the original Hoffmeister cation series. The addition of ethanol to the KCl micellar solutions leads to the formation of more or longer wormlike micelles, which start to interact at certain copolymer concentrations depending on the volume fraction of ethanol added. Both the dilute and the semidilute regimes of the wormlike micelles were studied. The length of the micelles reaches a maximum value at around 8-10 vol % ethanol, after which it decreases again. At higher ethanol concentrations (18 vol %), spherical micelles are formed. Conclusions from this study enhance our understanding of the role played by ethanol and salts in the formation of micelle-templated mesoporous materials, such as SBA-15.

Power-law distribution of pressure fluctuations in multiphase flow.

Bubbling fluidized beds are granular systems, in which a deep layer of particles is set in motion by a vertical gas stream, with the excess gas rising as bubbles through the bed. We show that pressure fluctuations in such a system have non-Gaussian statistics. The probability density function has a power-law drop-off and is very well represented by a Tsallis distribution. Its shape is explained through the folding of the Gaussian distribution of pressure fluctuations produced by a monodisperse set of bubbles, onto the actual distribution of bubble sizes in the bed, assuming that bubbles coalesce via a Smoluchowski-type aggregation process. Therefore, the Tsallis statistics arise as a result of bubble polydispersity, rather than system nonextensivity.

Effects of surface roughness on self- and transport diffusion in porous media in the Knudsen regime.

The effect of surface roughness on Knudsen diffusion in nanoporous media is investigated by means of dynamic Monte Carlo simulations in three-dimensional rough fractal pores. These simulations yield new insight and explain a number of apparent inconsistencies by revealing a striking difference between the roughness dependence of transport diffusion and gradientless (self- or tracer) diffusion. Both analytical and simulation results show a significant roughness dependence of self-diffusion in the Knudsen regime. Transport diffusion, on the other hand, is roughness independent, as the fluxes do not depend on the detailed residence time and molecular trajectories.