Transformation from NaA to MCM-49 Zeolite and Its Catalytic Alkylation Performance

The transformation from NaA (LTA) to MCM-49 (MWW) zeolite was achieved in the synergism of hexamethyleneimine (HMI), NaOH, and SiO₂, in spite of no common composite build units between LTA (lta, sod, and d4r) and MWW (mel and d6r) structure. NaA (SiO₂/Al₂O₃ = 2.0) was employed as the parent zeolite. The samples prepared at different crystallization stages were characterized by XRD, SEM, ²⁹Si/²⁷Al/¹³C MAS NMR, and STEM-EDS to investigate the intermediates during the transformation from NaA to MCM-49. As shown in SEM and STEM-EDS images, MCM-49 was proposed to be transformed gradually from the exterior to the interior of NaA, which was clearly observed by the core (LTA, low SiO₂/Al₂O₃)–shell (MWW, high SiO₂/Al₂O₃) coexisting zeolites as intermediates. With high relative crystallinity and the uniform sizes of crystals, the final MCM-49 was featured by Si enrichment on the external surface, which was proved by the shell (SiO₂/Al₂O₃ = 45.4) wrapping around the core (SiO₂/Al₂O₃ = 22.0). For transformed H-MCM-49 zeolite, the uniform sizes of crystals and the increase of total acid sites contributed to better accessibility of active centers, which achieved simultaneous improvement in ethylene conversion and ethylbenzene selectivity in the liquid-phase alkylation of benzene with ethylene.