Titanium silicalite-1 (TS-1) has
been shown to be a heterogeneous
catalyst with remarkable efficiency and selectivity; however, the
nature of the active Ti site in the MFI framework remains elusive.
Here we report combined experimental and theoretical research on Ti
distribution in the 12 crystallographically distinct T sites of the
MFI framework in high-Ti-loaded TS-1 (2.7 wt % in TiO2).
Using a multishell fit to extended X-ray absorption fine structure,
we show that T4 is the most populated site, in marked contrast to
the preferential substitution sites and the definitely excluded sites
assumed hitherto by diffraction studies. The identification is supported
by a good agreement between calculated and experimental X-ray absorption
near-edge structure studies and by full periodic density functional
theory (DFT) computation. In spite of having the identical most favored
site, the preference order for the remaining sites predicted by DFT
does not fully match the experimental results. This suggests that
Ti distribution in the resulting TS-1 framework is positively correlated
with the thermodynamic stability of pure material but can be affected
by other factors such as interdefects. These new insights may facilitate
the bottom-up design of new zeolites with tailored catalytic performance
and studies on mechanisms of various oxidation reactions.
Studies
of stable catalysts for non-oxidative dehydrogenation of
ethane (NDE) have been challenged by coke deposition from side reactions
and thermal sintering of active species. Herein, we report a catalyst
mechanism that overcomes both challenges to enable highly stable,
active, and selective NDE. The catalyst is made of subnanometric platinum
(Pt) species (i.e., single atoms and clusters) habituated on a two-dimensional
(2D) multilamellar titanium silicalite-1 (M-TS-1) zeolite nanosheet
support (i.e., Pt/M-TS-1). The ultrathin (∼3 nm) M-TS-1 nanosheets
provide high external surface areas, high terminal silanol/titanol
(−OH) groups, and weak Lewis acid sites. The first two characteristics
enhance molecular transport and Pt dispersion in the catalyst support.
The third characteristic prohibits side reactions to avoid coking
and create strong Pt–support interaction, leading to well-dispersed
Pt species against thermal sintering. The M-TS-1 nanosheet-supported
Pt catalyst exhibited durable catalytic activity and high ethylene
selectivity in the NDE.
Simple programmed microfluidic processes show the precise morphology and crystal structure controlled synthesis of nanohybrids using Sn–SnO2 nanohybrids as models.
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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