Synthesis of the molecular sieve ZSM-5 without the aid of templates

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 SiO2Al2O3 ≥ 60, the contribution from α-quartz increases coexisting with ZSM-5 and mordenite. Some hydrous silicon(IV) oxide also is present as an impurity phase. Almost 100% pure α-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 SiO2Al2O3 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.