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

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journal contribution
posted on 2015-04-01, 00:00 authored by Enhui Xing, Yanchun Shi, Aiguo Zheng, Jin Zhang, Xiuzhi Gao, Dongyun Liu, Mudi Xin, Wenhua Xie, Fengmei Zhang, Xuhong Mu, Xingtian Shu
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.

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