8 research outputs found
Control of Na-EMT Zeolite Synthesis by Organic Additives
Organic additives were used to control
the formation of EMT-type
zeolite in a sodium-rich initial system. Triethanolamine was employed
as a nucleation suppression agent able to complex the aluminum during
the synthesis of the EMT-type zeolite. The commonly used tetramethylammonium
(TMA) chloride and tetraethylammonium (TEA) chloride as structure
directing agents in zeolite synthesis were also employed in this study.
The triethanolamine has the most pronounced effect on the crystal
growth process of the EMT-type zeolite. The use of triethanolamine
resulted in the formation of large crystals (100 nm) with the framework
composition different from the counterpart obtained in the organic-free
system. Therefore, the function of triethanolamine is attributed to
the immobilization of Al in the initial gel and thus partial suppression
of zeolite nucleation. The immobilization of a part of Al resulted
in the EMT zeolite crystals with a higher Si/Al ratio (1.4). In contrast,
the TMA and TEA organic additives have a limited impact on the physicochemical
properties of the EMT crystals, the latter being a consequence of
large presence of Na in the system. The bulky TMA and TEA ions prevent
the aluminosilicate precursor species from agglomeration and formation
of dense gels and thus resulting in the crystallization of nanosized
EMT zeolite crystals (10â20 nm)
Nucleation and Crystal Growth Features of EMT-Type Zeolite Synthesized from an Organic-Template-Free System
The process of formation of ultrasmall EMT-type zeolite
from organic-template-free
homogeneous suspensions is presented. The formation of transparent
uniform suspension utilizing sodium aluminate, sodium silicate, and
sodium hydroxide under controlled mixing is found to be of primary
importance to control the nucleation and growth process of EMT-type
crystals. The investigation of zeolite intermediates reveals the formation
of uniformly sized gel particles (5â10 nm in size). The mean
hydrodynamic diameter of the crystalline EMT-type zeolite corresponds
to the size of the amorphous particles formed after preparation of
the clear precursor suspension. Controlled formation of uniform precursor
particles predetermines, to great extent, the following nucleation
and growth steps and, thus, the characteristics
of the ultimate product. The amorphous particles are transformed to
single EMT-type crystals 6â15 nm in size at 303 K within 36
h. Small changes in the initial composition or the preparation procedure
lead to the formation of other sodalite-cage-containing zeolites.
Thus, it is of critical importance to control the nucleation kinetics
in order to obtain the EMT-type material as pure phase. Besides the
EMT zeolite, the crystallization fields of other zeolites upon low-temperature
synthesis conditions are studied. The careful control of gel chemistry,
combined with slow nucleation kinetics at low temperature, can provide
access to important nanoscale zeolites while avoiding the use of expensive
organic templates
Synthesis and Catalytic Behavior of Ferrierite Zeolite Nanoneedles
The proton form of nanosized, needlelike
ferrierite zeolite, which was synthesized using choline and Na<sup>+</sup> cations as structure-directing agents, was found to be much
more efficient for the skeletal isomerization of 1-butene to isobutene
than the corresponding cation form of conventional, submicrometric
ferrierite with a platelike shape, mainly because of the considerably
lower density of strong acid sites, but as well as a result of the
higher density of 10-ring pore mouths
High-Visible-Light Photoactivity of Plasma-Promoted Vanadium Clusters on Nanozeolites for Partial Photooxidation of Methanol
Cold VCl<sub>3</sub>âplasma
is employed for the preparation
of highly dispersed vanadium oxide clusters on nanosized zeolite.
Different types of zeolites, such as EMT, FAU (z.X), and Beta, are
used. The activity of the prepared catalysts is studied in the selective
photooxidation of methanol under polychromatic visible and UV irradiations.
The physicochemical properties and catalytic performance of plasma-treated
zeolite Beta (P-V<sub>2</sub>O<sub>5</sub>@Beta) catalyst is compared
with zeolite Beta (V<sub>2</sub>O<sub>5</sub>@Beta) and amorphous
silica (V<sub>2</sub>O<sub>5</sub>@SiO<sub>2</sub>) impregnated vanadium
oxide catalysts. Pure V<sub>2</sub>O<sub>5</sub> is used as a reference
material. The set of catalytic data shows that plasma-prepared zeolite
Beta based catalyst displays the highest activity. Complementary characterization
techniques including XRD, N<sub>2</sub>-sorption, FTIR, ionic exchange,
pyridine adsorption, Raman, NMR, TPR, and EDX-TEM are used to study
the impact of the preparation approach on the physicochemical properties
and catalytic performance of photocatalysts
Recrystallization on Alkaline Treated Zeolites in the Presence of Pore-Directing Agents
In
previous works aiming at understanding the mesoporous network
after alkaline treatment in the presence of organic additives, conventional
bulk characterization techniques led to the conclusion that the dissolved
zeolite does not undergo any kind of recrystallization [Verboekend, D., Cryst. Growth. Des. 2013, 13, 5025â5035]. Here for the first time, we demonstrate
using the data obtained from <sup>1</sup>H and <sup>129</sup>Xe NMR
spectroscopy that such recrystallization does occur, which leads to
the formation of a very thin coating of the mesopore walls. This demonstration
is done on a beta (BEA) zeolite treated in the presence of TPA<sup>+</sup> in an alkaline solution. The formation of a small amount
of nanosized crystals or embryonic phases of silicalite-1 (MFI) zeolite
is evidenced, as well as their homogeneous dispersion on the mesoporous
surface of the beta zeolite. We think that these results may explain
why a homogeneous mesopore size distribution is obtained, when organic
pore-directing agents are used in the zeolite hierarchization process
performed in an alkaline medium
Opening the Cages of Faujasite-Type Zeolite
Zeolites are widely used in industrial
processes, mostly as catalysts
or adsorbents. Increasing their micropore volume could further improve
their already exceptional catalytic and separation performances. We
report a tunable extraction of zeolite framework cations (Si, Al)
on a faujasite-type zeolite, the archetype of molecular sieves with
cages and the most widely used as a catalyst and sorbent; this results
in ca. 10% higher micropore volume with limited impact on its thermal
stability. This increased micropore volume results from the opening
of some of the small (sodalite) cages, otherwise inaccessible to most
molecules. As more active sites become accessible, the catalytic performances
for these modified zeolites are substantially improved. The method,
based on etching with NH<sub>4</sub>F, is also applicable to other
cage-containing microporous molecular sieves, where some of the most
industrially relevant zeolites are found
Opening the Cages of Faujasite-Type Zeolite
Zeolites are widely used in industrial
processes, mostly as catalysts
or adsorbents. Increasing their micropore volume could further improve
their already exceptional catalytic and separation performances. We
report a tunable extraction of zeolite framework cations (Si, Al)
on a faujasite-type zeolite, the archetype of molecular sieves with
cages and the most widely used as a catalyst and sorbent; this results
in ca. 10% higher micropore volume with limited impact on its thermal
stability. This increased micropore volume results from the opening
of some of the small (sodalite) cages, otherwise inaccessible to most
molecules. As more active sites become accessible, the catalytic performances
for these modified zeolites are substantially improved. The method,
based on etching with NH<sub>4</sub>F, is also applicable to other
cage-containing microporous molecular sieves, where some of the most
industrially relevant zeolites are found
Insight in the Alginate Pd-IonogelsîžApplication to the TsujiâTrost Reaction
A new class of catalytic
ionogels based on the entrapment of an
ionic liquid phase within a sodium alginate matrix was applied to
the Pd-catalyzed allylic substitution reaction. High activity and
promising recyclability were obtained in CâN bond formation.
Scanning electron microscopy, scanning transmission electron microscopy,
and solid-state NMR studies provided a better insight into the catalytic
behavior, and the stability of the catalytic ionogels was studied
through a recycling process