Eventhough zeolites have been extensively employed as very efficient catalysts over the last decades, their crystallization is still not well understood. In the paper described bellow crystallization of zeolite Beta from viscous gel and incorporation of titanium into its framework were discussed. Investigation by NMR and XAS showed that during first 20 hours of the hydrothermal treatment oligomers aggregated into nanoparticles with the diameter of about 2 nm. After approximately 24 hours the primary nanoparticles began to agglomerate into secondary nanoparticles with the diameter between 5 and 10 nm. These nanoparticles already possessed a zeolitic character. Ti atoms were incorporated into the growing silicate framework at a very early stage of the crystallization and were thus also present in the secondary nanoparticles. Because of very large external surfaces, these secondary nanoparticles could make extremely efficient catalysts.
COBISS.SI-ID: 1795579
Based on the results of carefully designed experiments upgraded with appropriate theoretical modeling, we present clear evidence that the release curves from mesoporous materials are significantly affected by drug-matrix interactions. In experimental curves, these interactions are manifested as a non-convergence at long times and an inverse dependence of release kinetics onpore size. Neither of these phenomena is expected in non-interacting systems. Although both phenomena have, rather sporadically, been observed in previous research, they have not been explained in terms of a general and consistent theoretical model. The concept is demonstrated on a model drug indomethacin embedded into SBA-15 and MCM-41 porous silicates. The experimental release curves agree exceptionally well with theoretical predictions in the case of significant drug-wall attractions. The latter are described using a 2D Fokker-Planck equation. One could say that the interactions affect the relative cross-section of pores where the local flux has a non-vanishing axial component and in turn control the effective transferof drug into bulk solution. Finally, we identify the critical parameters determining the pore size dependence of release kinetics and construct a dynamic phase diagram of the various resulting transport regimes.
COBISS.SI-ID: 4727578
Finely ground calcium carbonate represents an active component during the hydration of Portland cement, which gives rise to the formation of calcium monocarboaluminate and calcium hemicarboaluminate. So far, however, there has been no experimental data on the hydration-time-dependent formation of these two phases, neither on their interrelations, and their amount in a hardened cement paste. Our paper describes the time-related formation of hemicarboaluminate and monocarboaluminate and reports for the first time the conversion of hemicarboaluminate into monocarboaluminate on the basis of X-ray diffraction by following the content of the latter using the Rietveld method. Hemicarboaluminate appears at early hydration times in calcite-containing Portland cement, even in the presence of large amounts of calcium carbonate. As the hydration progresses, a gradual conversion of hemicarboaluminate into monocarboaluminate occurs.
COBISS.SI-ID: 34764037