(633b) Crystallization of Mordenite from a Clear Solution

Zeolite mordenite is one of the solid acid materials used in hydrocracking and isomerization processes. It has been widely prepared from hydrogel solutions in which a silica precursor and an alumina precursor are mixed and then taken to high temperature for hydrothermal synthesis. For fundamental studies, clear solutions in which all particles are in the nanometer scale have become the accepted norm. Synthesis of mordenite from a clear solution was first reported by Ueda (American Mineralogist, 65 (1980)). However, the synthesis protocol requires heating the sample to 100 oC to dissolve the gel phase by increasing the silica solubility. This step is not desirable for studying the early stages of the crystallization. We developed a new protocol to prepare mordenite starting from a clear solution at room temperature. Crystallization was studied using small angle x-ray scattering (SAXS), pH and conductivity, X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and nitrogen adsorption. SAXS showed that the starting particles grow rapidly during the first half hour from 7 nm to 25 nm and a precipitated solid is formed within the first hour. The condensation of these particles increases the pH of the system, a fact which can be attributed to the condensation reaction between charged particles and silanol groups. XRD showed that the initial particles are amorphous materials. It also showed that the crystallization process of mordenite is composed of a long induction time (20 hrs) and a rapid crystal growth (1 hr) period. Heterogeneous nucleation is proposed to be the main mechanism of mordenite crystallization in this system as indicated by electron microscopy. The absence of nanoparticles and the continuous change of pH during the crystallization course show that this is possibly a solution mediated mechanism for gel evolution and nuclei formation.