13 research outputs found
Fabrication of COF-MOF Composite Membranes and Their Highly Selective Separation of H<sub>2</sub>/CO<sub>2</sub>
The search for new types of membrane
materials has been of continuous
interest in both academia and industry, given their importance in
a plethora of applications, particularly for energy-efficient separation
technology. In this contribution, we demonstrate for the first time
that a metalâorganic framework (MOF) can be grown on the covalent-organic
framework (COF) membrane to fabricate COF-MOF composite membranes.
The resultant COF-MOF composite membranes demonstrate higher separation
selectivity of H<sub>2</sub>/CO<sub>2</sub> gas mixtures than the
individual COF and MOF membranes. A sound proof for the synergy between
two porous materials is the fact that the COF-MOF composite membranes
surpass the Robeson upper bound of polymer membranes for mixture separation
of a H<sub>2</sub>/CO<sub>2</sub> gas pair and are among the best
gas separation MOF membranes reported thus far
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)
Detection of Harmful Gases by Copper-Containing MetalâOrganic Framework Films
The stabilization of copper clusters in nanosized metalâorganic
framework crystals, Cu-YÂ(BTC), was achieved by a solvent-exchange
approach, followed by hydrogen reduction. The formation of copper
clusters in the YÂ(BTC) nanocrystals generated during the hydrogen
reduction process was followed by UVâvis spectroscopy. The
Cu-YÂ(BTC) nanocrystals were further assembled in thin films with a
thickness of 250 nm. The distribution and size of the copper clusters
in the films were studied by CO chemisorption, followed by FT-IR spectroscopy
combined with transmission electron microscopy. It was shown that
the copper clusters with a mean diameter of 6 nm were homogeneously
distributed and stabilized in the Cu-YÂ(BTC) films. Further, the Cu-YÂ(BTC)
films were utilized for detection of single harmful gases, such as
CO, chloroform, and 2-ethylthiophene, or mixtures of two compounds.
The high sensitivity, selectivity, and reversibility of the Cu-YÂ(BTC)
films toward single CO, chloroform, and 2-ethylthiophene were demonstrated.
Noteworthy, the Cu-YÂ(BTC) films exhibited a fast response toward CO,
even in the presence of chloroform and 2-ethylthiophene, which was
due to the high activity and accessibility of copper clusters. The
response of Cu-YÂ(BTC) toward 2-ethylthiophene was slower in comparison
with chloroform, which was attributed to the bigger size and higher
viscosity of 2-ethylthiophene
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
Boosting the Catalytic Activity and Stability of Ru Metal Clusters in Hydrodeoxygenation of Guaiacol through MWW Zeolite Pore Constraints
Liquid-phase hydrodeoxygenation (HDO), catalyzed by metal
or metal-acid
sites, provides an effective catalytic strategy to remove oxygen-containing
functionalities of lignin-derived phenolic compounds on the route
to fuels and chemicals. Developing the catalyst with high activity
and stability is crucial for such a chemical process but still remains
a significant challenge. In this contribution, highly dispersed subnanometric
Ru metal clusters (<1.5 nm) encapsulated in the cavities of MWW
zeolites, including HMCM-22 and its siliceous analog ITQ-1, have been
developed for the HDO of guaiacol, an important lignin-derived phenolic
monomer, in an apolar liquid phase under mild conditions (160 °C,
3 MPa H2). We validate the effective encapsulation of Ru
metal clusters in ITQ-1 and HMCM-22 zeolite cavities via complementary
characterization methods. The detailed reaction pathways of the HDO
of guaiacol are depicted by using guaiacol, phenol, and anisole as
reactants. The subnanometric Ru metal clusters confined in MWW zeolite
thin layers (20â30 nm in thickness) show remarkable enhancement
in HDO activity compared to the large metal particles. The close proximity
between Ru metal clusters and BrĂžnsted acid sites (BAS) confined
in zeolite constraints delivers a synergistic effect, leading to an
additional enhancement in catalytic activity as well as product selectivity.
The super stability of the ultrafine Ru metal clusters against sintering
and leaching after successive catalytic runs is achieved. The well-defined
mono- or bifunctional Ru-containing MWW zeolite catalysts enable the
fundamental understanding of HDO of lignin-derived phenolic compounds
in the apolar liquid phase and also provide a prototype for the design
of superior catalysts for other energy-related transformations
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
Three-Dimensional Covalent Organic Frameworks with Dual Linkages for Bifunctional Cascade Catalysis
Covalent organic frameworks (COFs)
are an emerging class of porous
crystalline polymers with broad potential applications. So far, the
availability of three-dimensional (3D) COFs is limited and more importantly
only one type of covalent bond has been successful used for 3D COF
materials. Here, we report a new synthetic strategy based on dual
linkages that leads to 3D COFs. The obtained 3D COFs show high specific
surface areas and large gas uptake capacities, which makes them the
top COF material for gas uptake. Furthermore, we demonstrate that
the new 3D COFs comprise both acidic and basic sites, and act as excellent
bifunctional catalysts for one-pot cascade reactions. The new synthetic
strategy provides not only a general and versatile approach to synthesize
3D COFs with sophisticated structures but also expands the potential
applications of this promising class of porous materials
Crystal Growth Kinetics as a Tool for Controlling the Catalytic Performance of a FAU-Type Basic Catalyst
This study reports on the catalytic
performance of nanosized zeolite
X crystals and their precursors in the reaction of benzaldehyde with
ethyl cyanoacetate. Crystal growth kinetics of FAU-type zeolite is
studied at low temperature (35 °C) in order to discriminate different
crystallization stages. First X-ray crystalline material is detected
after 6 days of hydrothermal treatment. The formation of the crystalline
phase is preceded by changes in the ring structure of an aluminosilicate
precursor as revealed by the combined RamanâHEXRDâsolid-state
NMR analyses. The set of experimental data shows that these changes
are related to the reorganization of the gel structure and the formation
of zeolite units. Prior to the appearance of crystalline material,
the apparently amorphous solid exhibits chemical composition and short-range
order organization similar to that of a crystalline FAU-type zeolite.
Knoevenagel condensation was used to test the catalytic activity of
a series of zeolite intermediates and nanosized zeolite crystals.
The amorphous precursor obtained after 5 days of hydrothermal treatment
showed the highest yield of ethyl α-cyanocinnamate. Superior
catalytic performance of this material was attributed to the combination
of strong basic sites and less restricted and more accessible structure
of the semicrystalline zeolite units. Thus, the crystal growth kinetics
of FAU-type zeolite can be used as a tool to tune the properties of
a catalyst used in Knoevenagel condensation
Fast, Ambient Temperature and Pressure Ionothermal Synthesis of Three-Dimensional Covalent Organic Frameworks
Covalent
organic frameworks (COFs) are an emerging class of porous
crystalline polymers with wide range of potential applications. However,
the availability of three-dimensional (3D) COFs is still limited,
and their synthesis is confined to the high-temperature solvothermal
method. Here, we report for the first time a general and simple strategy
to produce a series of 3D ionic liquid (IL)-containing COFs (3D-IL-COFs)
by using IL as a green solvent. The syntheses are carried out at ambient
temperature and pressure accompanied by a high reaction speed (e.g.,
only three mins for 3D-IL-COF-1), and the IL can be reused without
activity loss. Furthermore, the 3D-IL-COFs show impressive performance
in the separation of CO<sub>2</sub>/N<sub>2</sub> and CO<sub>2</sub>/CH<sub>4</sub>. This research thus presents a potential pathway
to green large-scale industrial production of COFs
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