engleski

A comparison of the structural characteristics of the hydrogels used for the synthesis of LTA and MFI zeolites

Solid phase (gel) separated from freshly prepared hydrogel having the batch molar composition: 2.5Na2O-8TPABr-60SiO2-800H2O as well as the solids drawn off the reaction mixture, during its hydrothermal treatment at 443 K and solid phase (gel) separated from freshly prepared hydrogel having the batch molar composition: 0.468Na2O×Al2O3×3.382SiO2×55.44H2O as well as the solids drawn off the reaction mixture, during its hydrothermal treatment at 353 K were analyzed by different methods such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning-electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction (ED) and atomic force microscopy (AFM). It has been found that the first mentioned freshly prepared gel represents a “hierarchical structure” in which the largest individual gel aggregates (having the size 200 nm – 1000 nm, or more) are composed of smaller particles having the size in the range 40 nm – 80 nm, which represent aggregates of ≤ 10 nm particles. The second gel is mainly composed of disc-shaped primary particles as well of partially or even fully crystalline entities. Heating of the hydrogels causes structural changes in the gel matrix. 36T units of MFI structure present in the freshly prepared gel gradually transforms into fully crystalline nuclei of silicalite-1 and the “pyramidal-shaped” particles that have crystal structure closer to faujasite than zeolite A transform to zeolite A. The nuclei formed inside both the gel particles and/or small nanocrystals formed by a limited grow of nuclei in the gel matrix, can grow only after their release from the gel dissolved during the crystallization, i.e. when they are in full contact with the liquid phase.

hydrogels; zeolite A; Silicalite

Rad je prihvaćen za publikaciju u Proceedings simpozija a biti će objavljen i u book series “Topics in Chemistry and Material Science”