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

A special issue of Molecules (ISSN 1420-3049).

Deadline for manuscript submissions: closed (31 October 2017)

Special Issue Editors

Guest Editor
Prof. Dr. Roger Gläser

Institute of Chemical Technology, Universität Leipzig, Linnéstr. 3, 04103 Leipzig, Germany
Website | E-Mail
Interests: Chemical technology/Industrial chemistry; Heterogeneous catalysis; Redox molecular sieves; Zeolites; Porous materials with defined and hierarchical micro-/meso-/macroporosity; Supported noble metal catalysts; Alternative solvent systems as media for heterogeneous catalysis (supercritical fluids, gas-expanded liquids); Emission reduction (DeNOx); Utilization of renewable resources; Energy storage and conversion; Mass transfer effects in heterogeneous catalysis
Assistant Guest Editor
Dr. Nicole Wilde

Institute of Chemical Technology, Universität Leipzig, Linnéstr. 3, 04103 Leipzig, Germany
Website | E-Mail
Interests: heterogeneous catalysis; utilization of renewables; micro-/mesoporous zeolites; hierarchically structured materials; textural characterization; aqueous-phase processing

Special Issue Information

Dear Colleagues,

In recent years, the class of zeolites, molecular sieves and related nanoporous materials, has been greatly expanded. These materials are of particular interest, both in industry and in academia, due to their large variety of properties and, thus, the areas of their applications, spanning from adsorption and separation to heterogeneous catalysis, and, nowadays, also in green chemistry, are growing continuously. Considerable progress in this dynamic field of research has been made, notably, in the synthesis and characterization of novel materials. With an emphasis on understanding the interplay of physico-chemical properties, such as structure, composition, texture, and morphology, with their function and behavior in sorption and catalysis, perspectives for new applications can be based on a rational material design.

In view of this, this Special Issue is aimed at featuring the most recent progress in research and development on the synthesis, characterization, and applications of zeolites and related nanoporous materials in the fields of catalysis and green chemistry. Submissions are welcome across the following themes, but are not limited to this list:

  • Materials and their preparation
    • zeolites and related microporous materials (AlPOs, SAPOS), metal-organic frameworks, covalent organic frameworks and related organic coordination polymers,
    • micro-/meso-/macroporous and hierarchically structured materials, layered materials, ordered mesoporous materials,
    • novel synthesis strategies (sol-gel, solvothermal/ionothermal synthesis, solvent-free synthesis, dry gel synthesis, clear gels), novel structure-directing agents, bio-based templates, template-free synthesis.
  • Characterization of materials and modes of operation
    • characterization of nanoporous materials with respect to structure, composition, porosity, and morphology, including spectroscopic and diffraction methods,
    • understanding modes of operation by in-situ/operando spectroscopy, application-oriented characterization, modelling and simulation approaches.
  • Applications in catalysis and green chemistry
    • catalysis in refining and petrochemistry including light olefins production and heavy-feedstocks upgrading,
    • utilization of greenhouse gases (CO2 and CH4) including reduction of CO2, reforming, and aromatzation and (partial) oxidation of methane,
    • energy conversion (generation and conversion of hydrogen, photocatalysis),
    • valorization of biomass and biomass-derived feedstocks,
    • environmental aplications including off-gas (automotive) and waste water treatment,
    • fine chemical synthesis via novel routes based on zeolites and related catalysts.

We encourage you to submit full papers, short communications, and reviews to this Special Issue highlighting the challenges and opportunities in the field of zeolites and related nanoporous materials.

Prof. Dr. Roger Gläser
Dr. Nicole Wilde
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • zeolites
  • metal-organic frameworks
  • organic coordination polymers
  • ordered mesoporous materials
  • hierarchically structured materials
  • synthesis
  • bio-based templates
  • spectroscopic and textural characterization including in-situ and operando methods
  • refining and petrochemistry
  • utilization of bio-based resources
  • CO2 and CH4 utilization
  • green chemistry
  • energy conversion
  • environmental protection
  • fine chemicals synthesis

Published Papers (18 papers)

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Research

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Open AccessArticle Zeta Potential of Beta Zeolites: Influence of Structure, Acidity, pH, Temperature and Concentration
Molecules 2018, 23(4), 946; https://doi.org/10.3390/molecules23040946
Received: 11 March 2018 / Revised: 11 April 2018 / Accepted: 13 April 2018 / Published: 18 April 2018
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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
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Open AccessArticle Continuous Separation of Light Olefin/Paraffin Mixtures on ZIF-4 by Pressure Swing Adsorption and Membrane Permeation
Molecules 2018, 23(4), 889; https://doi.org/10.3390/molecules23040889
Received: 23 February 2018 / Revised: 29 March 2018 / Accepted: 5 April 2018 / Published: 11 April 2018
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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
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Open AccessFeature PaperArticle Anomaly in the Chain Length Dependence of n-Alkane Diffusion in ZIF-4 Metal-Organic Frameworks
Molecules 2018, 23(3), 668; https://doi.org/10.3390/molecules23030668
Received: 7 February 2018 / Revised: 8 March 2018 / Accepted: 13 March 2018 / Published: 15 March 2018
Cited by 1 | PDF Full-text (4085 KB) | HTML Full-text | XML Full-text
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
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Open AccessArticle Enhanced Hydrolysis of Cellulose in Ionic Liquid Using Mesoporous ZSM-5
Molecules 2018, 23(3), 529; https://doi.org/10.3390/molecules23030529
Received: 31 October 2017 / Revised: 25 January 2018 / Accepted: 30 January 2018 / Published: 27 February 2018
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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
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Open AccessArticle A Facile Route toward the Increase of Oxygen Content in Nanosized Zeolite by Insertion of Cerium and Fluorinated Compounds
Received: 24 November 2017 / Revised: 15 January 2018 / Accepted: 19 January 2018 / Published: 24 January 2018
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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
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Open AccessArticle Supported Zeolite Beta Layers via an Organic Template-Free Preparation Route
Molecules 2018, 23(1), 220; https://doi.org/10.3390/molecules23010220
Received: 7 November 2017 / Revised: 11 January 2018 / Accepted: 15 January 2018 / Published: 21 January 2018
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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
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Open AccessArticle Comparative Study between Direct and Pseudomorphic Transformation of Rice Husk Ash into MFI-Type Zeolite
Received: 3 November 2017 / Revised: 13 December 2017 / Accepted: 16 December 2017 / Published: 21 December 2017
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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
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Open AccessArticle Are Diatoms “Green” Aluminosilicate Synthesis Microreactors for Future Catalyst Production?
Molecules 2017, 22(12), 2232; https://doi.org/10.3390/molecules22122232
Received: 11 November 2017 / Revised: 6 December 2017 / Accepted: 12 December 2017 / Published: 16 December 2017
Cited by 2 | PDF Full-text (2327 KB) | HTML Full-text | XML Full-text | Supplementary Files
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
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Open AccessFeature PaperArticle Nb-Based Zeolites: Efficient bi-Functional Catalysts for the One-Pot Synthesis of Succinic Acid from Glucose
Molecules 2017, 22(12), 2218; https://doi.org/10.3390/molecules22122218
Received: 14 November 2017 / Revised: 8 December 2017 / Accepted: 12 December 2017 / Published: 14 December 2017
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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
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Open AccessArticle Effect of Steam Deactivation Severity of ZSM-5 Additives on LPG Olefins Production in the FCC Process
Molecules 2017, 22(10), 1784; https://doi.org/10.3390/molecules22101784
Received: 11 October 2017 / Revised: 18 October 2017 / Accepted: 18 October 2017 / Published: 21 October 2017
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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
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Review

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Open AccessFeature PaperReview Metal-Incorporated Mesoporous Silicates: Tunable Catalytic Properties and Applications
Molecules 2018, 23(2), 263; https://doi.org/10.3390/molecules23020263
Received: 1 December 2017 / Revised: 22 January 2018 / Accepted: 24 January 2018 / Published: 29 January 2018
Cited by 1 | PDF Full-text (5056 KB) | HTML Full-text | XML Full-text
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
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Open AccessReview Zeolites as Catalysts for Fuels Refining after Indirect Liquefaction Processes
Molecules 2018, 23(1), 115; https://doi.org/10.3390/molecules23010115
Received: 25 November 2017 / Revised: 30 December 2017 / Accepted: 1 January 2018 / Published: 6 January 2018
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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
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Open AccessReview CO2 Recycling to Dimethyl Ether: State-of-the-Art and Perspectives
Received: 13 November 2017 / Revised: 20 December 2017 / Accepted: 22 December 2017 / Published: 24 December 2017
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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
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Open AccessFeature PaperReview Nanoparticle/Metal–Organic Framework Composites for Catalytic Applications: Current Status and Perspective
Molecules 2017, 22(12), 2103; https://doi.org/10.3390/molecules22122103
Received: 31 October 2017 / Revised: 26 November 2017 / Accepted: 27 November 2017 / Published: 30 November 2017
Cited by 1 | PDF Full-text (7399 KB) | HTML Full-text | XML Full-text
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
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Open AccessReview Probing Gas Adsorption in Zeolites by Variable-Temperature IR Spectroscopy: An Overview of Current Research
Molecules 2017, 22(9), 1557; https://doi.org/10.3390/molecules22091557
Received: 5 July 2017 / Revised: 6 September 2017 / Accepted: 11 September 2017 / Published: 15 September 2017
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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
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Open AccessReview An Introduction to Zeolite Synthesis Using Imidazolium-Based Cations as Organic Structure-Directing Agents
Molecules 2017, 22(8), 1307; https://doi.org/10.3390/molecules22081307
Received: 15 June 2017 / Revised: 1 August 2017 / Accepted: 4 August 2017 / Published: 6 August 2017
Cited by 2 | PDF Full-text (20360 KB) | HTML Full-text | XML Full-text
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
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Open AccessReview Synthesis and Modification of Clinoptilolite
Molecules 2017, 22(7), 1107; https://doi.org/10.3390/molecules22071107
Received: 10 May 2017 / Accepted: 26 June 2017 / Published: 4 July 2017
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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
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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
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Open AccessFeature PaperPerspective The Molecular Design of Active Sites in Nanoporous Materials for Sustainable Catalysis
Molecules 2017, 22(12), 2127; https://doi.org/10.3390/molecules22122127
Received: 2 November 2017 / Revised: 23 November 2017 / Accepted: 28 November 2017 / Published: 2 December 2017
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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
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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
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