DIRECT HYDROTHERMAL CRYSTALLIZATION OF HIGH-SILICA LARGE-PORT MORDENITE

Hydrothermal crystallization of mordenite has been carried out in the temperature range 393-453 K following the oxide mole composition of the starting gel as a Na2O : A12O3 : b SiO2: d (TEA)2O : c H2O, where a = 3.9-12.5, b = 30-40, c = 150-1500, and d = 0 or 1.5. Silica sol, water glass, and microcrystalline silicon dioxide were used as sources of silica, whereas pseudoboehmite, sodium aluminate, and aluminum sulfate.16 H2O were used as sources for alumina. A product silica-to-alumina ratio (SAR) ranging between 12 and 35 was found influenced by the starting gel composition, nature of the sources of silica and alumina, dilution of the gel, pH, temperature of crystallization, and the use of template (tetraethylammonium bromide). Synthesis efficiency (% yield of crystalline solid product based on silica and alumina in the starting gel) was also found to depend upon various parameters during synthesis. Coexistence of other crystalline phases depended on the Na2O content in the starting gel and also on the temperature of the crystallization. Crystallization kinetics fitted in the first-order kinetics equation and apparent activation energies for the processes of nucleation (E(n)) and crystallization (E(c)) were calculated by applying the Arrhenius equation. Large-port mordenite was obtained in the presence or absence of the template with enhanced synthesis efficiency (up to 75%), with the product SAR as high as 35.