SYNTHESIS OF THE MOLECULAR-SIEVE ZSM-5 WITHOUT THE AID OF TEMPLATES
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abstract
Attempts were made to investigate compositional constraints in the synthesis of pentasil molecular sieves, including aluminosilicate zeolites using no organic templates, from the gel composition aSiO2:Al2O3:bNa2O:1500 H2O, in the temperature range 423-463 K, where a = 20 - , b = 4.5 - 10. When a < 30, the system crystallizes into mordenite as a major phase along with traces of ZSM-5. At SiO2 Al2O3 60, the contribution from -quartz increases coexisting with ZSM-5 and mordenite. Some hydrous silicon(IV) oxide also is present as an impurity phase. Almost 100% pure -quartz is obtained when the gel contains no added alumina. At a composition of a = 40 and b = 4.5-6.0, pure ZSM-5 crystallizes with the occlusion of Na+ in excess of charge compensation on the zeolite framework. As the Na2O content is increased in the gel, the ZSM-5 product is found to be lower in framework silica-alumina ratio, probably due to the greater solubility of silica. The ZSM-5 product so obtained exhibited sorption characteristics (BET surface area, uptake of n-hexane, water at 298 K, p/P0 = 0.8) slightly different from those exhibited by ZSM-5 with the same SiO2 Al2O3 ratio synthesized from a system containing an organic template. The structural stability of the zeolite ZSM-5 synthesized using no template was also found to be lower as compared with that for the ZSM-5 synthesized using a template. These differences may result from the occluded excess sodium. 1989.
