Direct synthesis of nanosized-ZSM-5/SBA-15 analog composites from preformed ZSM-5 precursors for improved catalytic performance as cracking catalyst

摘要

Nanosized-ZSM-5/SBA-15 analog composites (ZSC) were successfully synthesized in a two-step process from zeolite seed solutions containing intact ZSM-5 nanocrystals (nano-ZSM-5) by high-temperature synthesis in mild acidic media (200 ℃, pH 3.5). The ZSC solids exhibit domains of nano-ZSM-5 blended homogeneously with highly condensed, well-ordered mesoporous SBA-15 analogs containing zeolite building units. The content of nano-ZSM-5 phase and SBA-15 analog phase can be controlled by a single synthesis parameter, i.e., the precrystallization time of the initially formed zeolite seed solution. Compared to conventional Al-SBA-15, the acidic properties, particularly the surface density of strong Br?nsted acid sites, of the ZSC are significantly improved while its micro-/mesostructures are well maintained even upon steaming at 800 ℃ for 24 h. The catalytic activity of the ZSC solids compared to that of commercial ZSM-5 and Al-SBA-15 samples was evaluated in the gas phase cracking of cumene and 1,3,5-tri-isopropylbenzene (TIPB) as probe reactions. The results show that the ZSC materials are effective for the conversion of both cumene and TIPB due to the increased density of the Br?nsted acid sites with high accessibility provided by well-dispersed nano-ZSM-5 in the mesoporous SBA-15 analog matrix.