Synthesis, characterization, and catalytic properties of zeolites IM-5 and NU-88

摘要

The synthesis, characterization, and catalytic properties of two high-silica zeolites IM-5 and NU-88, whose structures still remain unresolved, are presented. When the 1,5-bis(N-methylpyrrolidinium)prntane and 1,6-bis(N-methylpyrrolidinium)hexane cations are used as organic structure-directing agents, respectively, crystallization of pure IM-5 and NU-88 was possible only from synthesis mixtures with a narrow range of SiO_2/Al_2O_3 and NaOH/SiO_2 ratios. The overall characterization results of this study strongly suggest that IM-5 is a new multidimensional large-pore zeolite, whereas NU-88 is a nanocrystalline material that could be an intergrowth of several hypothetical polymorphs in the beta family of zeolites. Despite its nanocrystalline nature, however, no detectable extraframework Al species were found to exist in H-NU-88, revealing excellent thermal stability. H-IM-5 and H-NU-88 both exhibit a very high l-butene conversion compared to H-ZSM-35 over the period of time studied here, whereas the opposite holds for the formation of isobutene This reveals that their pore topologies are large enough to allow undesird side reactions such as l-butene dimerization followed by cracking to light olefins. They also show the initialn-octane cracking activity comparable to that on H-ZSM-5. However, a notable decrease in n-octane conversion on these two zeolites with increasing time on stream is observed. When the isomerization and cracking activities of all zeolites employed in this study are corrleated with their coke-forming propensities, it can be concluded that both materials present a shape-selective character falling within the category of multidimensional,large-pore zeolites.