SYNTHESIS AND CONTROL OF THE SIZE OF LARGE MORDENITE CRYSTALS USING POROUS SILICA SUBSTRATES

A new method for the synthesis of large mordenite crystals from hetero geneous aluminosilicate reaction mixtures is presented. The method inv olves heat treatment of porous silica gels in air at elevated temperat ures prior to their use in mordenite synthesis. Heat treatment lowers the surface area and pore volume (porosity) of X-ray amorphous porous silica gel particles without substantially changing the size of the re maining pores. Thus, a decrease in accessibility of the internal surfa ce area of silica to dissolution is realized. The result is nucleation of fewer mordenite crystals which grow larger. The method employed to grow large mordenite crystals also leads to the formation of other cr ystalline phases (quartz and/or zeolite P-c) coexisting with mordenite , the amounts of which increase with increasing heat treatment tempera ture of silica. For the investigated composition only large-pore (140 and 150 Angstrom) silica gels resulted in the synthesis of large morde nite crystals (up to 175 mu m) when they were heated to 800-900 degree s C prior to use. The use of heat-treated, medium-pore (60 Angstrom) s ilica gel in zeolite synthesis resulted in growth of substantially sma ller mordenite crystals with size up to 80 mu m Small-pore (22 and 25 Angstrom) silica gels could not be used to grow large mordenite crysta ls by heating them before use in syntheses. Upon heating at elevated t emperatures prior to use, these silica gels resulted in synthesis of p redominantly quartz and no significant size increase of mordenite crys tals was observed at any heat-treatment temperature. It is also shown that the combination of heat and mechanical (grinding) treatments of p orous silica allows control of the crystallization and the size of mor denite crystals.