The synthesis and cracking behavior of zeolite-amorphous silica-alumina composites prepared using gels with high alumina and low organic template content

Composites of ZSM-5 and amorphous silica-alumina have been prepared from starting gels with SiO₂/Al₂O₃ molar ratios of 10 and below and tetrapropyl ammonium (TPA+)/Al₂O₃ molar ratios below 0.5. The syntheses have been followed by periodic removal of aliquots from the gel with subsequent analysis. At SiO₂/Al₂O₃ = 10 and TPA⁺/Al₂O₃ = 0.44 mordenite formed. When the SiO₂/AI₂O₃ ratio was reduced to 8.3, but the ratio TPA⁺/Al₂O₃ was kept at 0.45, crystallization progressed from the initial formation of P_c to a mixture of Pc plus ZSM-5 to finally only ZSM-5 at the end of the run. At the lower SiO₂/Al₂O₃ ratio of 6 the P_c phase persisted, in a mixture with ZSM-5, until the end of the run. At the lowest SiO₂/Al₂O₃ ratio employed, 3.5, only P_c was detected for a TPA⁺/Al₂O₃ ratio of 0.4. In all cases an amorphous silica-alumina coexisted with the zeolites. Samples removed periodically from gels having initial molar ratios of SiO₂/Al₂O₃ = 8.3 and TPA⁺/Al₂O₃ = 0.45 were calcined and subsequently converted to the H+ form of ZSM-5 and evaluated in the cracking of n-hexane. These catalysts, which contained between 10 and 42 wt% HAIZSM-5 were more active than either pure HAIZSM-5 or mechanical mixtures prepared by the addition of preformed ZSM-5 powder (from 10 to 42 wt%) to an amorphous silica-alumina gel. All catalysts examined did show comparable selectivity to C₆-C₁₀ aromatics when compared at equal conversions. Both types of catalysts, namely those crystallized in situ and those prepared mechanically, had comparable numbers of strong acid sites (relative to the amount of HAIZSM-5) as measured by the temperature-programmed desorption of ammonia. The better connectivity between the two phases is suggested to be responsible for the greater activity of the composites in which the ZSM-5 crystallizes in situ, compared to the mechanical mixtures.