Transformation
from NaA to MCM-49 Zeolite and Its
Catalytic Alkylation Performance
- Publication date
- Publisher
Abstract
The transformation from NaA (LTA)
to MCM-49 (MWW) zeolite was achieved
in the synergism of hexamethyleneimine (HMI), NaOH, and SiO<sub>2</sub>, in spite of no common composite build units between LTA (<i>lta</i>, <i>sod</i>, and <i>d4r</i>) and
MWW (<i>mel</i> and <i>d6r</i>) structure. NaA
(SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> = 2.0) was employed as
the parent zeolite. The samples prepared at different crystallization
stages were characterized by XRD, SEM, <sup>29</sup>Si/<sup>27</sup>Al/<sup>13</sup>C 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 SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub>)–shell
(MWW, high SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub>) 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 (SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> = 45.4) wrapping around the core (SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> = 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
