2 research outputs found
Investigation on Titanium Silicalite‑1 Zeolite Synthesis Employing ATPAOH as an Organic Structure Directing Agent
Tetrapropylammonium
hydroxide (TPAOH) as an organic structure directing
agent (OSDA) is of great importance for the preparation of titanium
silicalite-1 (TS-1) zeolite. In this paper, we employed a new OSDA,
allyltripropylammonium hydroxide (ATPAOH), in the synthesis process
and successfully synthesized ATS-1 zeolite (MFI type). Compared with
traditional OSDA TPAOH, one of TPAOH’s propyl groups is substituted
by an allyl group, which endows ATPAOH with unique properties. On
the one hand, ATPAOH accelerates the crystallization rate of titanium
silicalite zeolite remarkably due to the strong interaction between
Ti species and ATPAOH during the crystallization period. On the other
hand, ATPAOH is beneficial for the formation of isolated 6-coordinated
Ti species, thus leading to the generation of lower amount of anatase.
Owing to its abundant active Ti species, ATS-1 prepared by ATPAOH
as OSDA exhibits a much better catalytic performance for the cyclohexanone
ammoximation reaction than TS-1 prepared by TPAOH as OSDA
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 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