13 research outputs found

    Fabrication of COF-MOF Composite Membranes and Their Highly Selective Separation of H<sub>2</sub>/CO<sub>2</sub>

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    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 metal–organic 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

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    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 (10–20 nm)

    Detection of Harmful Gases by Copper-Containing Metal–Organic Framework Films

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    The stabilization of copper clusters in nanosized metal–organic framework crystals, Cu-Y­(BTC), was achieved by a solvent-exchange approach, followed by hydrogen reduction. The formation of copper clusters in the Y­(BTC) nanocrystals generated during the hydrogen reduction process was followed by UV–vis spectroscopy. The Cu-Y­(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-Y­(BTC) films. Further, the Cu-Y­(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-Y­(BTC) films toward single CO, chloroform, and 2-ethylthiophene were demonstrated. Noteworthy, the Cu-Y­(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-Y­(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

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    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 (5–10 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 6–15 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

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    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 (20–30 nm in thickness) show remarkable enhancement in HDO activity compared to the large metal particles. The close proximity between Ru metal clusters and Brþnsted 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

    Three-Dimensional Covalent Organic Frameworks with Dual Linkages for Bifunctional Cascade Catalysis

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    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

    High-Visible-Light Photoactivity of Plasma-Promoted Vanadium Clusters on Nanozeolites for Partial Photooxidation of Methanol

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    Cold VCl<sub>3</sub>–plasma 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

    Crystal Growth Kinetics as a Tool for Controlling the Catalytic Performance of a FAU-Type Basic Catalyst

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    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 Raman–HEXRD–solid-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

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    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

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    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
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