Molecules

Research

14 pages, 4579 KiB

Open AccessArticle

Zeta Potential of Beta Zeolites: Influence of Structure, Acidity, pH, Temperature and Concentration

by Xuan Liu, Päivi Mäki-Arvela, Atte Aho, Zuzana Vajglova, Vladimir M. Gun’ko, Ivo Heinmaa, Narendra Kumar, Kari Eränen, Tapio Salmi and Dmitry Yu. Murzin

Molecules 2018, 23(4), 946; https://doi.org/10.3390/molecules23040946 - 18 Apr 2018

Cited by 60 | Viewed by 8905

Abstract

Measurements of the zeta potential of solid heterogeneous supports are important for preparation of metal supported catalysts and for shaping zeolites into extrudates. In the current work, different types of heterogeneous support materials such as SiO₂, Al₂O₃, [...] Read more.

Measurements of the zeta potential of solid heterogeneous supports are important for preparation of metal supported catalysts and for shaping zeolites into extrudates. In the current work, different types of heterogeneous support materials such as SiO₂, Al₂O₃, and a range of beta zeolites of different silica- to-alumina ratio were analysed. It was observed that parameters such as temperature, pH and acidity significantly affect the zeta potential. In several instances, depending on the materials’ acidity and microstructure, maxima in zeta potential were observed. The solid materials were thoroughly characterized using XRD, SEM, EDX, TEM, nitrogen physisorption, Al-NMR and FTIR with pyridine before zeta potential measurements. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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13 pages, 3614 KiB

Open AccessArticle

Continuous Separation of Light Olefin/Paraffin Mixtures on ZIF-4 by Pressure Swing Adsorption and Membrane Permeation

by Maximilian Hovestadt, Sebastian Friebe, Lailah Helmich, Marcus Lange, Jens Möllmer, Roger Gläser, Alexander Mundstock and Martin Hartmann

Molecules 2018, 23(4), 889; https://doi.org/10.3390/molecules23040889 - 11 Apr 2018

Cited by 21 | Viewed by 7398

Abstract

In this study, two zeolitic imidazolate frameworks (ZIFs) called ZIF-4 and ZIF-zni (zni is the network topology) were characterized by sorption studies regarding their paraffin/olefin separation potential. In particular, equilibrated pure and mixed gas adsorption isotherms of ethane and ethene were recorded at [...] Read more.

In this study, two zeolitic imidazolate frameworks (ZIFs) called ZIF-4 and ZIF-zni (zni is the network topology) were characterized by sorption studies regarding their paraffin/olefin separation potential. In particular, equilibrated pure and mixed gas adsorption isotherms of ethane and ethene were recorded at 293 K up to 3 MPa. ZIF-4 exhibits selectivities for ethane in the range of 1.5–3, which is promising for continuous pressure swing adsorption (PSA). ZIF-4 shows high cycle stability with promising separation potential regarding ethane, which results in purification of the more industrial desired olefin. Furthermore, both ZIF materials were implemented in Matrimid to prepare a mixed matrix membrane (MMM) and were used in the continuous separation of a propane/propene mixture. The separation performance of the neat polymer is drastically increased after embedding porous ZIF-4 crystals in the Matrimid matrix, especially at higher feed pressures (3–5 barg). Due to the smaller kinetic diameter of the olefin, the permeability is higher compared to the paraffin. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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16 pages, 4085 KiB

Open AccessFeature PaperArticle

Anomaly in the Chain Length Dependence of n-Alkane Diffusion in ZIF-4 Metal-Organic Frameworks

by Seungtaik Hwang, Arun Gopalan, Maximilian Hovestadt, Frank Piepenbreier, Christian Chmelik, Martin Hartmann, Randall Q. Snurr and Jörg Kärger

Molecules 2018, 23(3), 668; https://doi.org/10.3390/molecules23030668 - 15 Mar 2018

Cited by 17 | Viewed by 5676

Abstract

Molecular diffusion is commonly found to slow down with increasing molecular size. Deviations from this pattern occur in some host materials with pore sizes approaching the diameters of the guest molecules. A variety of theoretical models have been suggested to explain deviations from [...] Read more.

Molecular diffusion is commonly found to slow down with increasing molecular size. Deviations from this pattern occur in some host materials with pore sizes approaching the diameters of the guest molecules. A variety of theoretical models have been suggested to explain deviations from this pattern, but robust experimental data are scarcely available. Here, we present such data, obtained by monitoring the chain length dependence of the uptake of n-alkanes in the zeolitic imidazolate framework ZIF-4. A monotonic decrease in diffusivity from ethane to n-butane was observed, followed by an increase for n-pentane, and another decrease for n-hexane. This observation was confirmed by uptake measurements with n-butane/n-pentane mixtures, which yield faster uptake of n-pentane. Further evidence is provided by the observation of overshooting effects, i.e., by transient n-pentane concentrations exceeding the (eventually attained) equilibrium value. Accompanying grand canonical Monte Carlo simulations reveal, for the larger n-alkanes, significant differences between the adsorbed and gas phase molecular configurations, indicating strong confinement effects within ZIF-4, which, with increasing chain length, may be expected to give rise to configurational shifts facilitating molecular propagation at particular chain lengths. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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10 pages, 3076 KiB

Open AccessArticle

Enhanced Hydrolysis of Cellulose in Ionic Liquid Using Mesoporous ZSM-5

by Tianlu Chen, Chunrong Xiong and Yousheng Tao

Molecules 2018, 23(3), 529; https://doi.org/10.3390/molecules23030529 - 27 Feb 2018

Cited by 19 | Viewed by 4144

Abstract

Mesoporous ZSM-5 prepared by alkaline treatment was demonstrated as an efficient catalyst for the cellulose hydrolysis in ionic liquid (IL), affording a high yield of reducing sugar. It was demonstrated that mesoporous ZSM-5 (SiO₂/Al₂O₃ = 38) had 76.2% [...] Read more.

Mesoporous ZSM-5 prepared by alkaline treatment was demonstrated as an efficient catalyst for the cellulose hydrolysis in ionic liquid (IL), affording a high yield of reducing sugar. It was demonstrated that mesoporous ZSM-5 (SiO₂/Al₂O₃ = 38) had 76.2% cellulose conversion and 49.6% yield of total reducing sugar (TRS). In comparison, the conventional ZSM-5 had a mere 41.3% cellulose conversion with 33.2% yield of TRS. The results indicated that the important role of mesopores in zeolites in elevating the TRS yield may be due to the diffusional alleviation of cellulose macromolecules. The effects of reaction time, temperature, and the ratio of catalyst to cellulose were investigated for optimal reaction conditions. It was found that IL could enter the inner channel of mesoporous ZSM-5 to promote the generation of H⁺ from Brönsted acid sites, which facilitated hydrolysis. Moreover, the mesoporous ZSM-5 showed excellent reusability for catalytic cycles by means of calcination of the used one, promising for its practical applications in the hydrolysis of cellulose. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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12 pages, 4117 KiB

Open AccessArticle

A Facile Route toward the Increase of Oxygen Content in Nanosized Zeolite by Insertion of Cerium and Fluorinated Compounds

by Sarah Komaty, Clément Anfray, Moussa Zaarour, Hussein Awala, Valérie Ruaux, Samuel Valable and Svetlana Mintova

Molecules 2018, 23(2), 37; https://doi.org/10.3390/molecules23020037 - 24 Jan 2018

Cited by 13 | Viewed by 5678

Abstract

Enriching oxygen content within nanosized zeolite X (as synthesized Na-X) by insertion of cerium (ion exchanged Ce-X) and functionalization with bromoperfluoro-n-octane (fluorinated F-X) is reported. The materials were fully characterized by powder X-ray diffraction (XRD), dynamic light scattering (DLS), zeta potential, [...] Read more.

Enriching oxygen content within nanosized zeolite X (as synthesized Na-X) by insertion of cerium (ion exchanged Ce-X) and functionalization with bromoperfluoro-n-octane (fluorinated F-X) is reported. The materials were fully characterized by powder X-ray diffraction (XRD), dynamic light scattering (DLS), zeta potential, thermogravimetric analysis (TGA), nitrogen adsorption, and nuclear magnetic resonance (¹⁹F NMR). The O₂ adsorption in the zeolite samples at various concentrations (0 to 165 Torr) at −196 °C was studied by in situ FTIR. The modification of nanosized zeolites did not alter their colloidal stability, crystallinity, porosity, and particle size distribution. The inclusion of cerium and bromoperfluoro-n-octane considerably increase the oxygen capacity by 33% for samples Ce-X and F-X in comparison to the as-synthesized Na-X zeolite. Further, toxicity tests revealed that these materials are safe, which opens the door for their implementation in medical applications, where controlled delivery of oxygen is highly desirable. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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11 pages, 2146 KiB

Open AccessArticle

Supported Zeolite Beta Layers via an Organic Template-Free Preparation Route

by Stephanie Reuss, Dirk Sanwald, Marion Schülein, Wilhelm Schwieger, Shaeel A. Al-Thabaiti, Mohamed Mokhtar and Sulaiman N. Basahel

Molecules 2018, 23(1), 220; https://doi.org/10.3390/molecules23010220 - 21 Jan 2018

Cited by 5 | Viewed by 5167

Abstract

Layers of high silica zeolites, synthesized with an organic structure directing agent (OSDA) and grown onto porous support structures, frequently suffer from the thermal stress during the removal of OSDA via the calcination process. The different thermal expansion coefficients of the zeolite and [...] Read more.

Layers of high silica zeolites, synthesized with an organic structure directing agent (OSDA) and grown onto porous support structures, frequently suffer from the thermal stress during the removal of OSDA via the calcination process. The different thermal expansion coefficients of the zeolite and the support material, especially when stainless steel is used as a support, causes enormous tension resulting in defect formation in the zeolite layer. However, the calcination is an easy procedure to decompose the OSDA in the pore system of the zeolite. Recently, methods to synthesize zeolite beta without the use of an organic structure directing agent have been described. In the present study, a seed-directed synthesis is used to prepare OSDA-free zeolite beta layers on stainless steel supports via an in situ preparation route. For the application as membrane, a porous stainless steel support has been chosen. The beta/stainless steel composites are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). To prove its possible application as a membrane, the beta/stainless steel composites were also tested by single gas permeances of H₂, He, CO₂, N₂, and CH₄. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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959 KiB

Open AccessArticle

Comparative Study between Direct and Pseudomorphic Transformation of Rice Husk Ash into MFI-Type Zeolite

by Hallah Ahmad Alyosef, Hans Roggendorf, Denise Schneider, Alexandra Inayat, Julia Welscher, Wilhelm Schwieger, Tom Münster, Gert Kloess, Suzan Ibrahim and Dirk Enke

Molecules 2018, 23(1), 1; https://doi.org/10.3390/molecules23010001 - 21 Dec 2017

Cited by 10 | Viewed by 7051

Abstract

Pre-shaped mesoporous amorphous rice husk ash (RHA) and MCM-41 derived from RHA as a silica source were transformed into MFI-type zeolites using two different structure-directing agents. Tetrapropylammonium hydroxide (TPAOH) was utilized as an alkali source for silica dissolution and structure control during the [...] Read more.

Pre-shaped mesoporous amorphous rice husk ash (RHA) and MCM-41 derived from RHA as a silica source were transformed into MFI-type zeolites using two different structure-directing agents. Tetrapropylammonium hydroxide (TPAOH) was utilized as an alkali source for silica dissolution and structure control during the direct transformation of RHA into zeolite. A monopropylamine (PA)-containing alkaline solution (NaOH) was used for the pseudomorphic transformation of RHA or MCM-41 into zeolite. The hydrothermal conversion of RHA or MCM-41 into MFI-type zeolites was investigated as a function of reaction time at 175 °C. With PA as template, the crystallization took place inside and on the outer surface of RHA or MCM-41 without losing the original shape of the initial silica sources, while TPAOH led to the formation of conventional MFI-type zeolite crystals due to the complete dissolution of RHA. The final products were characterized by X-ray diffraction, nitrogen adsorption, scanning electron microscopy, and optical emission spectroscopy. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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2327 KiB

Open AccessArticle

Are Diatoms “Green” Aluminosilicate Synthesis Microreactors for Future Catalyst Production?

by Lydia Köhler, Susanne Machill, Anja Werner, Carolin Selzer, Stefan Kaskel and Eike Brunner

Molecules 2017, 22(12), 2232; https://doi.org/10.3390/molecules22122232 - 16 Dec 2017

Cited by 22 | Viewed by 5816

Abstract

Diatom biosilica may offer an interesting perspective in the search for sustainable solutions meeting the high demand for heterogeneous catalysts. Diatomaceous earth (diatomite), i.e., fossilized diatoms, is already used as adsorbent and carrier material. While diatomite is abundant and inexpensive, freshly harvested and [...] Read more.

Diatom biosilica may offer an interesting perspective in the search for sustainable solutions meeting the high demand for heterogeneous catalysts. Diatomaceous earth (diatomite), i.e., fossilized diatoms, is already used as adsorbent and carrier material. While diatomite is abundant and inexpensive, freshly harvested and cleaned diatom cell walls have other advantages, with respect to purity and uniformity. The present paper demonstrates an approach to modify diatoms both in vivo and in vitro to produce a porous aluminosilicate that is serving as a potential source for sustainable catalyst production. The obtained material was characterized at various processing stages with respect to morphology, elemental composition, surface area, and acidity. The cell walls appeared normal without morphological changes, while their aluminum content was raised from the molar ratio n(Al):n(Si) 1:600 up to 1:50. A specific surface area of 55 m²/g was measured. The acidity of the material increased from 149 to 320 µmol NH₃/g by ion exchange, as determined by NH₃ TPD. Finally, the biosilica was examined by an acid catalyzed test reaction, the alkylation of benzene. While the cleaned cell walls did not catalyze the reaction at all, and the ion exchanged material was catalytically active. This demonstrates that modified biosilica does indeed has potential as a basis for future catalytically active materials. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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3853 KiB

Open AccessFeature PaperArticle

Nb-Based Zeolites: Efficient bi-Functional Catalysts for the One-Pot Synthesis of Succinic Acid from Glucose

by Magdi El Fergani, Natalia Candu, Simona M. Coman and Vasile I. Parvulescu

Molecules 2017, 22(12), 2218; https://doi.org/10.3390/molecules22122218 - 14 Dec 2017

Cited by 21 | Viewed by 6869

Abstract

The one-pot production of succinic acid from glucose was investigated in pure hot water as solvent using Nb (0.02 and 0.05 moles%)-Beta zeolites obtained by a post-synthesis methodology. Structurally, they are comprised of residual framework Al-acid sites, extra-framework isolated Nb (V) and Nb [...] Read more.

The one-pot production of succinic acid from glucose was investigated in pure hot water as solvent using Nb (0.02 and 0.05 moles%)-Beta zeolites obtained by a post-synthesis methodology. Structurally, they are comprised of residual framework Al-acid sites, extra-framework isolated Nb (V) and Nb₂O₅ pore-encapsulated clusters. The Nb-modified Beta-zeolites acted as bi-functional catalysts in which glucose is dehydrated to levulinic acid (LA) which, further, suffers an oxidation process to succinic acid (SA). After the optimization of the reaction conditions, that is, at 180 °C, 18 bar O₂, and 12 h reaction time, the oxidation of glucose occurred with a selectivity to succinic acid as high as 84% for a total conversion. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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4540 KiB

Open AccessArticle

Effect of Steam Deactivation Severity of ZSM-5 Additives on LPG Olefins Production in the FCC Process

by Andrey A. Gusev, Antonios C. Psarras, Konstantinos S. Triantafyllidis, Angelos A. Lappas and Paul A. Diddams

Molecules 2017, 22(10), 1784; https://doi.org/10.3390/molecules22101784 - 21 Oct 2017

Cited by 23 | Viewed by 8550

Abstract

ZSM-5-containing catalytic additives are widely used in oil refineries to boost light olefin production and improve gasoline octanes in the Fluid Catalytic Cracking (FCC) process. Under the hydrothermal conditions present in the FCC regenerator (typically >700 °C and >8% steam), FCC catalysts and [...] Read more.

ZSM-5-containing catalytic additives are widely used in oil refineries to boost light olefin production and improve gasoline octanes in the Fluid Catalytic Cracking (FCC) process. Under the hydrothermal conditions present in the FCC regenerator (typically >700 °C and >8% steam), FCC catalysts and additives are subject to deactivation. Zeolites (e.g., Rare Earth USY in the base catalyst and ZSM-5 in Olefins boosting additives) are prone to dealumination and partial structural collapse, thereby losing activity, micropore surface area, and undergoing changes in selectivity. Fresh catalyst and additives are added at appropriate respective levels to the FCC unit on a daily basis to maintain overall targeted steady-state (equilibrated) activity and selectivity. To mimic this process under accelerated laboratory conditions, a commercial P/ZSM-5 additive was hydrothermally equilibrated via a steaming process at two temperatures: 788 °C and 815 °C to simulate moderate and more severe equilibration industrial conditions, respectively. n-Dodecane was used as probe molecule and feed for micro-activity cracking testing at 560 °C to determine the activity and product selectivity of fresh and equilibrated P-doped ZSM-5 additives. The fresh/calcined P/ZSM-5 additive was very active in C₁₂ cracking while steaming limited its activity, i.e., at catalyst-to-feed (C/F) ratio of 1, about 70% and 30% conversion was obtained with the fresh and steamed additives, respectively. A greater activity drop was observed upon increasing the hydrothermal deactivation severity due to gradual decrease of total acidity and microporosity of the additives. However, this change in severity did not result in any selectivity changes for the LPG (liquefied petroleum gas) olefins as the nature (Brønsted-to-Lewis ratio) of the acid/active sites was not significantly altered upon steaming. Steam deactivation of ZSM-5 had also no significant effect on aromatics formation which was enhanced at higher conversion levels. Coke remained low with both fresh and steam-deactivated P/ZSM-5 additives. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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Review

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13 pages, 5056 KiB

Open AccessFeature PaperReview

Metal-Incorporated Mesoporous Silicates: Tunable Catalytic Properties and Applications

by Anand Ramanathan and Bala Subramaniam

Molecules 2018, 23(2), 263; https://doi.org/10.3390/molecules23020263 - 29 Jan 2018

Cited by 18 | Viewed by 5176

Abstract

A relatively new class of three-dimensional ordered mesoporous silicates, KIT-6, incorporated with Earth-abundant metals such as Zr, Nb, and W (termed as M-KIT-6), show remarkable tunability of acidity and metal dispersion depending on the metal content, type, and synthetic method. The metal-incorporation is [...] Read more.

A relatively new class of three-dimensional ordered mesoporous silicates, KIT-6, incorporated with Earth-abundant metals such as Zr, Nb, and W (termed as M-KIT-6), show remarkable tunability of acidity and metal dispersion depending on the metal content, type, and synthetic method. The metal-incorporation is carried out using one-pot synthesis procedures that are amenable to easy scale-up. By such tuning, M-KIT-6 catalysts are shown to provide remarkable activity and selectivity in industrially-significant reactions, such as alcohol dehydration, ethylene epoxidation, and metathesis of 2-butene and ethylene. We review how the catalytic properties of M-KIT-6 materials may be tailored depending on the application to optimize performance. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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17 pages, 1686 KiB

Open AccessReview

Zeolites as Catalysts for Fuels Refining after Indirect Liquefaction Processes

by Arno De Klerk

Molecules 2018, 23(1), 115; https://doi.org/10.3390/molecules23010115 - 6 Jan 2018

Cited by 23 | Viewed by 6410

Abstract

The use of zeolite catalysts for the refining of products from methanol synthesis and Fisher–Tropsch synthesis was reviewed. The focus was on fuels refining processes and differences in the application to indirect liquefaction products was compared to petroleum, which is often a case [...] Read more.

The use of zeolite catalysts for the refining of products from methanol synthesis and Fisher–Tropsch synthesis was reviewed. The focus was on fuels refining processes and differences in the application to indirect liquefaction products was compared to petroleum, which is often a case of managing different molecules. Processes covered were skeletal isomerisation of n-butenes, hydroisomerisation of n-butane, aliphatic alkylation, alkene oligomerisation, methanol to hydrocarbons, ethanol and heavier alcohols to hydrocarbons, carbonyls to hydrocarbons, etherification of alkenes with alcohols, light naphtha hydroisomerisation, catalytic naphtha reforming, hydroisomerisation of distillate, hydrocracking and fluid catalytic cracking. The zeolite types that are already industrially used were pointed out, as well as zeolite types that have future promise for specific conversion processes. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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4190 KiB

Open AccessReview

CO₂ Recycling to Dimethyl Ether: State-of-the-Art and Perspectives

by Enrico Catizzone, Giuseppe Bonura, Massimo Migliori, Francesco Frusteri and Girolamo Giordano

Molecules 2018, 23(1), 31; https://doi.org/10.3390/molecules23010031 - 24 Dec 2017

Cited by 156 | Viewed by 16292

Abstract

This review reports recent achievements in dimethyl ether (DME) synthesis via CO₂ hydrogenation. This gas-phase process could be considered as a promising alternative for carbon dioxide recycling toward a (bio)fuel as DME. In this view, the production of DME from catalytic hydrogenation [...] Read more.

This review reports recent achievements in dimethyl ether (DME) synthesis via CO₂ hydrogenation. This gas-phase process could be considered as a promising alternative for carbon dioxide recycling toward a (bio)fuel as DME. In this view, the production of DME from catalytic hydrogenation of CO₂ appears as a technology able to face also the ever-increasing demand for alternative, environmentally-friendly fuels and energy carriers. Basic considerations on thermodynamic aspects controlling DME production from CO₂ are presented along with a survey of the most innovative catalytic systems developed in this field. During the last years, special attention has been paid to the role of zeolite-based catalysts, either in the methanol-to-DME dehydration step or in the one-pot CO₂-to-DME hydrogenation. Overall, the productivity of DME was shown to be dependent on several catalyst features, related not only to the metal-oxide phase—responsible for CO₂ activation/hydrogenation—but also to specific properties of the zeolites (i.e., topology, porosity, specific surface area, acidity, interaction with active metals, distributions of metal particles, …) influencing activity and stability of hybridized bifunctional heterogeneous catalysts. All these aspects are discussed in details, summarizing recent achievements in this research field. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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7399 KiB

Open AccessFeature PaperReview

Nanoparticle/Metal–Organic Framework Composites for Catalytic Applications: Current Status and Perspective

by Wenlong Xiang, Yueping Zhang, Hongfei Lin and Chang-jun Liu

Molecules 2017, 22(12), 2103; https://doi.org/10.3390/molecules22122103 - 30 Nov 2017

Cited by 132 | Viewed by 14791

Abstract

Nanoparticle/metal–organic frameworks (MOF) based composites have recently attracted significant attention as a new class of catalysts. Such composites possess the unique features of MOFs (including clearly defined crystal structure, high surface area, single site catalyst, special confined nanopore, tunable, and uniform pore structure), [...] Read more.

Nanoparticle/metal–organic frameworks (MOF) based composites have recently attracted significant attention as a new class of catalysts. Such composites possess the unique features of MOFs (including clearly defined crystal structure, high surface area, single site catalyst, special confined nanopore, tunable, and uniform pore structure), but avoid some intrinsic weaknesses (like limited electrical conductivity and lack in the “conventional” catalytically active sites). This review summarizes the developed strategies for the fabrication of nanoparticle/MOF composites for catalyst uses, including the strategy using MOFs as host materials to hold and stabilize the guest nanoparticles, the strategy with subsequent MOF growth/assembly around pre-synthesized nanoparticles and the strategy mixing the precursors of NPs and MOFs together, followed by self-assembly process or post-treatment or post-modification. The applications of nanoparticle/MOF composites for CO oxidation, CO₂ conversion, hydrogen production, organic transformations, and degradation of pollutants have been discussed. Superior catalytic performances in these reactions have been demonstrated. Challenges and future developments are finally addressed. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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4027 KiB

Open AccessReview

Probing Gas Adsorption in Zeolites by Variable-Temperature IR Spectroscopy: An Overview of Current Research

by Edoardo Garrone, Montserrat R. Delgado, Barbara Bonelli and Carlos O. Arean

Molecules 2017, 22(9), 1557; https://doi.org/10.3390/molecules22091557 - 15 Sep 2017

Cited by 9 | Viewed by 6641

Abstract

The current state of the art in the application of variable-temperature IR (VTIR) spectroscopy to the study of (i) adsorption sites in zeolites, including dual cation sites; (ii) the structure of adsorption complexes and (iii) gas-solid interaction energy is reviewed. The main focus [...] Read more.

The current state of the art in the application of variable-temperature IR (VTIR) spectroscopy to the study of (i) adsorption sites in zeolites, including dual cation sites; (ii) the structure of adsorption complexes and (iii) gas-solid interaction energy is reviewed. The main focus is placed on the potential use of zeolites for gas separation, purification and transport, but possible extension to the field of heterogeneous catalysis is also envisaged. A critical comparison with classical IR spectroscopy and adsorption calorimetry shows that the main merits of VTIR spectroscopy are (i) its ability to provide simultaneously the spectroscopic signature of the adsorption complex and the standard enthalpy change involved in the adsorption process; and (ii) the enhanced potential of VTIR to be site specific in favorable cases. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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20360 KiB

Open AccessReview

An Introduction to Zeolite Synthesis Using Imidazolium-Based Cations as Organic Structure-Directing Agents

by Paloma Vinaches, Katia Bernardo-Gusmão and Sibele B. C. Pergher

Molecules 2017, 22(8), 1307; https://doi.org/10.3390/molecules22081307 - 6 Aug 2017

Cited by 31 | Viewed by 10610

Abstract

Zeolite synthesis is a wide area of study with increasing popularity. Several general reviews have already been published, but they did not summarize the study of imidazolium species in zeolite synthesis. Imidazolium derivatives are promising compounds in the search for new zeolites and [...] Read more.

Zeolite synthesis is a wide area of study with increasing popularity. Several general reviews have already been published, but they did not summarize the study of imidazolium species in zeolite synthesis. Imidazolium derivatives are promising compounds in the search for new zeolites and can be used to help understand the structure-directing role. Nearly 50 different imidazolium cations have already been used, resulting in a variety of zeolitic types, but there are still many derivatives to be studied. In this context, the purpose of this short review is to help researchers starting in this area by summarizing the most important concepts related to imidazolium-based zeolite studies and by presenting a table of recent imidazolium derivatives that have been recently studied to facilitate filling in the knowledge gaps. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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603 KiB

Open AccessReview

Synthesis and Modification of Clinoptilolite

by Pavlina Ambrozova, Jindrich Kynicky, Tomas Urubek and Vinh Dinh Nguyen

Molecules 2017, 22(7), 1107; https://doi.org/10.3390/molecules22071107 - 4 Jul 2017

Cited by 59 | Viewed by 9305

Abstract

Clinoptilolite is a natural mineral with exceptional physical characteristics resulting from its special crystal structure, mainstreamed into a large zeolite group called heulandites. An overall view of the research related to the synthesis, modification and application of synthetic clinoptilolite is presented. A single [...] Read more.

Clinoptilolite is a natural mineral with exceptional physical characteristics resulting from its special crystal structure, mainstreamed into a large zeolite group called heulandites. An overall view of the research related to the synthesis, modification and application of synthetic clinoptilolite is presented. A single phase of clinoptilolite can be hydrothermally synthesized for 1–10 days in an autoclave from various silica, alumina, and alkali sources with initial Si/Al ratio from 3.0 to 5.0 at a temperature range from 120 to 195 °C. Crystallization rate and crystallinity of clinoptilolite can be improved by seeding. The modification of clinoptilolite has received noticeable attention from the research community, since modified forms have specific properties and therefore their area of application has been broadening. This paper provides a review of the use of organic compounds such as quarter alkyl ammonium, polymer, amine and inorganic species used in the modification process, discusses the processes and mechanisms of clinoptilolite modification, and identifies research gaps and new perspectives. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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Other

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4875 KiB

Open AccessFeature PaperPerspective

The Molecular Design of Active Sites in Nanoporous Materials for Sustainable Catalysis

by Stephanie Chapman, Matthew E. Potter and Robert Raja

Molecules 2017, 22(12), 2127; https://doi.org/10.3390/molecules22122127 - 2 Dec 2017

Cited by 13 | Viewed by 6869

Abstract

At the forefront of global development, the chemical industry is being confronted by a growing demand for products and services, but also the need to provide these in a manner that is sustainable in the long-term. In facing this challenge, the industry is [...] Read more.

At the forefront of global development, the chemical industry is being confronted by a growing demand for products and services, but also the need to provide these in a manner that is sustainable in the long-term. In facing this challenge, the industry is being revolutionised by advances in catalysis that allow chemical transformations to be performed in a more efficient and economical manner. To this end, molecular design, facilitated by detailed theoretical and empirical studies, has played a pivotal role in creating highly-active and selective heterogeneous catalysts. In this review, the industrially-relevant Beckmann rearrangement is presented as an exemplar of how judicious characterisation and ab initio experiments can be used to understand and optimise nanoporous materials for sustainable catalysis. Full article

(This article belongs to the Special Issue Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry)

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Zeolites and Related Nanoporous Materials: Synthesis, Characterization and Applications in Catalysis and Green Chemistry

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