Closely Packed Zeolite Nanocrystals Obtained via Transformation of Porous Amorphous Silica

Abstract

Amorphous silica grains were subjected to a hydrothermal treatment to be transformed into closely packed ZSM-5 zeolite nanocrystalline bodies. Three synthesis approaches have been developed: (I) direct hydrothermal treatment of the amorphous silica grains in a ZSM-5 precursor solution; (II) impregnation of charge-reversed amorphous silica grains with 2−10 nm preorganized units followed by a hydrothermal treatment in a silica-free precursor synthesis solution; and (III) electrostatic adsorption of 50-nm sized ZSM-5 seeds on the amorphous silica grains followed by a hydrothermal treatment with a ZSM-5 precursor solution. The synthesized solids were characterized by X-ray diffraction, Raman spectroscopy, TG analysis, N2 adsorption measurements, and scanning electron microscopy. The resulted zeolitic bodies are built of uniform closely packed nanocrystals and retain the size and morphological features of the initial amorphous silica grains. The crystallinity, the average size of crystallites, and the mechanical properties of the nanozeolite bodies depend strongly on the synthesis procedure. It was found that procedure II provides mechanically stable bodies built of closely packed nanocrystallites with a size of about 40 nm, which are very promising for the production of self-bonded nanozeolite structures.