Zeolites

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Inorganic Crystalline Materials".

Deadline for manuscript submissions: closed (20 December 2020) | Viewed by 62856

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Guest Editor
Faculty of Materials Science and Ceramics, AGH University of Science and Technology, 30-059 Krakow, al. Mickiewicza 30, Poland
Interests: zeolites; natural aluminosilicates; sorption; structural studies; FT-IR spectroscopy

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Guest Editor
Faculty of Materials Science and Ceramics, AGH University of Science and Technology, 30-059 Krakow, al. Mickiewicza 30, Poland
Interests: zeolites; geopolymers; structural studies; heavy metals immobilization

Special Issue Information

Dear Colleagues,

Zeolites and zeolite-based materials are characterized by unique physical and chemical properties which result in numerous important applications. These properties arise directly from their specific structure—the system of channels and chambers characterized by well-defined molecular dimensions. If thermal stability, chemical resistance, possibility of generating active sites as well as sorption and ion-exchange properties are taken into account, it is not surprising that zeolites play an extremely important role in various applications of chemical technology, and their importance increases continuously.

Zeolites can be successfully obtained in laboratory conditions by heating the aluminosilicate materials in the presence of alkaline solution. Various silica carriers, including natural raw materials (such as kaolinite, volcanic glasses, diatomite) and industrial wastes (among which fly ash is the most popular) are used in such processes. Current trends provide a green and economic alternative for the synthesis of zeolites using natural or waste clays instead of chemical raw materials.

This Special Issue aims to attract original contributions in topics related to zeolites, covering aspects ranging from the preparation of such materials (especially using natural or waste materials), their characterization, and application in different areas. In particular, reports on the unique synthesis methods of zeolites or improvement of existing ones, as well as structural characterization of resulting materials and their potential use in various fields are welcome.

I believe that this collection will summarize the current state-of-the-art in the field of zeolitic materials and will become a source of new ideas resulting in the development of this group of minerals."

Dr. Magdalena Król
Ms. Paulina Florek
Guest Editors

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Keywords

  • Zeolites
  • Hydrothermal synthesis
  • Alkali activation
  • Zeolite properties
  • Zeolite characterization
  • Zeolite applications

Published Papers (14 papers)

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Editorial

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8 pages, 2103 KiB  
Editorial
Natural vs. Synthetic Zeolites
by Magdalena Król
Crystals 2020, 10(7), 622; https://doi.org/10.3390/cryst10070622 - 17 Jul 2020
Cited by 109 | Viewed by 20161
Abstract
This brief review article describes the structure, properties and applications of natural and synthetic zeolites, with particular emphasis on zeolites obtained from natural or waste materials. Certainly, such short work does not exhaust the complexity of the problem, but it sheds light on [...] Read more.
This brief review article describes the structure, properties and applications of natural and synthetic zeolites, with particular emphasis on zeolites obtained from natural or waste materials. Certainly, such short work does not exhaust the complexity of the problem, but it sheds light on some outstanding issues on this subject. Full article
(This article belongs to the Special Issue Zeolites)
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Research

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16 pages, 4897 KiB  
Article
First Report on the Geologic Occurrence of Natural Na–A Zeolite and Associated Minerals in Cretaceous Mudstones of the Paja Formation of Vélez (Santander), Colombia
by Carlos Alberto Ríos-Reyes, German Alfonso Reyes-Mendoza, José Antonio Henao-Martínez, Craig Williams and Alan Dyer
Crystals 2021, 11(2), 218; https://doi.org/10.3390/cryst11020218 - 22 Feb 2021
Cited by 4 | Viewed by 3322
Abstract
This study reports for the first time the geologic occurrence of natural zeolite A and associated minerals in mudstones from the Cretaceous Paja Formation in the urban area of the municipality of Vélez (Santander), Colombia. These rocks are mainly composed of quartz, muscovite, [...] Read more.
This study reports for the first time the geologic occurrence of natural zeolite A and associated minerals in mudstones from the Cretaceous Paja Formation in the urban area of the municipality of Vélez (Santander), Colombia. These rocks are mainly composed of quartz, muscovite, pyrophyllite, kaolinite and chlorite group minerals, framboidal and cubic pyrite, as well as marcasite, with minor feldspar, sulphates, and phosphates. Total organic carbon (TOC), total sulfur (TS), and millimeter fragments of algae are high, whereas few centimeters and not biodiverse small ammonite fossils, and other allochemical components are subordinated. Na–A zeolite and associated mineral phases as sodalite occur just beside the interparticle micropores (honeycomb from framboidal, cube molds, and amorphous cavities). It is facilitated by petrophysical properties alterations, due to processes of high diagenesis, temperatures up to 80–100 °C, with weathering contributions, which increase the porosity and permeability, as well as the transmissivity (fluid flow), allowing the geochemistry remobilization and/or recrystallization of pre-existing silica, muscovite, kaolinite minerals group, salts, carbonates, oxides and peroxides. X-ray diffraction analyses reveal the mineral composition of the mudstones and scanning electron micrographs show the typical cubic morphology of Na–A zeolite of approximately 0.45 mμ in particle size. Our data show that the sequence of the transformation of phases is: Poorly crystalline aluminosilicate → sodalite → Na–A zeolite. A literature review shows that this is an unusual example of the occurrence of natural zeolites in sedimentary marine rocks recognized around the world. Full article
(This article belongs to the Special Issue Zeolites)
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17 pages, 4194 KiB  
Article
Thermophysical Properties of Kaolin–Zeolite Blends up to 1100 °C
by Ján Ondruška, Tomáš Húlan, Ivana Sunitrová, Štefan Csáki, Grzegorz Łagód, Alena Struhárová and Anton Trník
Crystals 2021, 11(2), 165; https://doi.org/10.3390/cryst11020165 - 7 Feb 2021
Cited by 6 | Viewed by 2736
Abstract
In this study, the thermophysical properties such as the thermal expansion, thermal diffusivity and conductivity, and specific heat capacity of ceramic samples made from kaolin and natural zeolite are investigated up to 1100 °C. The samples were prepared from Sedlec kaolin (Czech Republic) [...] Read more.
In this study, the thermophysical properties such as the thermal expansion, thermal diffusivity and conductivity, and specific heat capacity of ceramic samples made from kaolin and natural zeolite are investigated up to 1100 °C. The samples were prepared from Sedlec kaolin (Czech Republic) and natural zeolite (Nižný Hrabovec, Slovakia). Kaolin was partially replaced with a natural zeolite in the amounts of 10, 20, 30, 40, and 50 mass%. The measurements were performed on cylindrical samples using thermogravimetric analysis, a horizontal pushrod dilatometer, and laser flash apparatus. The results show that zeolite in the samples decreases the values of all studied properties (except thermal expansion), which is positive for bulk density, porosity, thermal diffusivity, and conductivity. It has a negative effect for thermal expansion because shrinkage increases with the zeolite content. Therefore, the optimal amount of zeolite in the sample (according to the studied properties) is 30 mass%. Full article
(This article belongs to the Special Issue Zeolites)
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10 pages, 1085 KiB  
Article
First Occurrence of Willhendersonite in the Lessini Mounts, Northern Italy
by Michele Mattioli and Marco Cenni
Crystals 2021, 11(2), 109; https://doi.org/10.3390/cryst11020109 - 26 Jan 2021
Cited by 1 | Viewed by 1699
Abstract
Willhendersonite is a rare zeolite, with very few occurrences reported globally (Terni Province, Italy; the Eifel Region, Germany; Styria, Austria). Moreover, the data available from these sites are very limited and do not allow a detailed picture of this zeolite’s mineralogical and chemical [...] Read more.
Willhendersonite is a rare zeolite, with very few occurrences reported globally (Terni Province, Italy; the Eifel Region, Germany; Styria, Austria). Moreover, the data available from these sites are very limited and do not allow a detailed picture of this zeolite’s mineralogical and chemical characteristics. In this work, a new willhendersonite occurrence is reported from the Tertiary volcanic rocks of the Lessini Mounts, northern Italy. Morphology, mineralogy and chemical composition of selected crystals were studied by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray Diffraction (XRD), and electron probe microanalyser (EPMA). Willhendersonite occurs within basanitic rocks as isolated, colorless, transparent crystals with prismatic to flattened morphologies. Individual crystals often grow together to form small elongated clusters and trellis-like aggregates. The diffraction pattern exhibits 33 well-resolved diffraction peaks, all of which can be indexed to a triclinic cell with unit cell parameters a = 9.239(2) Å; b = 9.221(2) Å; and c = 9.496(2) Å, α = 92.324(2)°, β = 92.677(2)°, γ = 89.992° (Space Group P1¯). The chemical data point to significant variability from Ca-rich willhendersonite (K0.23Na0.03)Σ=0,26Ca1.24 (Si3.06Al3,00Fe3+0.01)Σ=6,07 O12·5H2O) to Ca-K terms (K0.94Na0.01)Σ=0,95Ca0.99 (Si3.07Al2.93Fe3+0.00)Σ=6,00O12·5H2O). Willhendersonite from the Lessini Mounts highlights the existence of an isomorphous series between the Ca-pure crystals and Ca-K compositions, possibly extended up to a potassic end-member. Full article
(This article belongs to the Special Issue Zeolites)
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16 pages, 4891 KiB  
Article
Quasi Natural Approach for Crystallization of Zeolites from Different Fly Ashes and Their Application as Adsorbent Media for Malachite Green Removal from Polluted Waters
by Denitza Zgureva, Valeria Stoyanova, Annie Shoumkova, Silviya Boycheva and Georgi Avdeev
Crystals 2020, 10(11), 1064; https://doi.org/10.3390/cryst10111064 - 23 Nov 2020
Cited by 7 | Viewed by 2465
Abstract
Worldwide disposal of multi-tonnage solid waste from coal-burning thermal power plants (TPPs) creates serious environmental and economic problems, which necessitate the recovery of industrial waste in large quantities and at commercial prices. Fly ashes (FAs) and slag from seven Bulgarian TPPs have been [...] Read more.
Worldwide disposal of multi-tonnage solid waste from coal-burning thermal power plants (TPPs) creates serious environmental and economic problems, which necessitate the recovery of industrial waste in large quantities and at commercial prices. Fly ashes (FAs) and slag from seven Bulgarian TPPs have been successfully converted into valuable zeolite-like composites with various applications, including as adsorbents for capturing CO2 from gases and for removal of contaminants from water. The starting materials generated from different types of coal are characterized by a wide range of SiO2/Al2O3 ratio, heterogeneous structure and a complex chemical composition. The applied synthesis procedure resembles the formation of natural zeolites, as the raw FAs undergo long-term self-crystallization in an alkaline aqueous solution at ambient temperature. The phase and chemical composition, morphology and N2 adsorption of the final zeolite products were studied by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-Ray Diffraction (XRD) and Brunauer–Emmett–Teller (BET) analyses. The growth of faujasite (FAU) crystals as the main zeolite phase was established in all samples after 7 and 14 months of alkaline treatment. Phillipsite (PHI) crystals were also observed in several samples as an accompanying phase. The final products possess specific surface area over 400 m2/g. The relationships between the surface properties of the investigated samples and the characteristics of the raw FAs were discussed. All of the obtained zeolite-like composites were able to remove the highly toxic dye (malachite green, MG) from water solutions with efficiency over 96%. The experimental data were fitted with high correlation to the second-order kinetics. Full article
(This article belongs to the Special Issue Zeolites)
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11 pages, 4486 KiB  
Article
Synthesis of Pure Phase NaP2 Zeolite from the Gel of NaY by Conventional and Microwave-Assisted Hydrothermal Methods
by Pimwipa Tayraukham, Nawee Jantarit, Nattawut Osakoo and Jatuporn Wittayakun
Crystals 2020, 10(10), 951; https://doi.org/10.3390/cryst10100951 - 19 Oct 2020
Cited by 7 | Viewed by 3482
Abstract
The gel of zeolite NaY has potential as a precursor of other zeolites. The particular interest in this work is to convert the gel of NaY to NaP2. We found that the pure phase NaP2 can be produced simply by the conventional hydrothermal [...] Read more.
The gel of zeolite NaY has potential as a precursor of other zeolites. The particular interest in this work is to convert the gel of NaY to NaP2. We found that the pure phase NaP2 can be produced simply by the conventional hydrothermal (CH) method at 150 °C for 24 h. This NaP2 sample, named CH150, has an average particle size of 10.3 µm and an Si/Al ratio of 1.82. In the case of single crystallization via microwave-assisted hydrothermal (MH) method, various parameters were studied, including the crystallization temperature (90, 150, 175 °C) and time (15, 30, 45, 60 min). The samples were analyzed by X-ray diffraction and scanning electron microscopy. However, mixed phases of P1 and P2 or ANA were obtained from all samples. Another attempt was made by a double crystallization via MH method as followed: at 90 °C for 1 h, quickly cooled down to room temperature in the microwave chamber and aged for 23 h, and finally at 150 °C for 1 h. The sample, named MH90A150, has an average crystal size of 16.45 µm and an Si/Al ratio of 1.85. The high Al content of NaP2 in both samples (CH150 and MH90A150) could lead to interesting applications. Full article
(This article belongs to the Special Issue Zeolites)
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15 pages, 5080 KiB  
Article
Formation Process of Columnar Grown (101)-Oriented Silicalite-1 Membrane and Its Separation Property for Xylene Isomer
by Motomu Sakai, Takuya Kaneko, Yukichi Sasaki, Miyuki Sekigawa and Masahiko Matsukata
Crystals 2020, 10(10), 949; https://doi.org/10.3390/cryst10100949 - 17 Oct 2020
Cited by 10 | Viewed by 2846
Abstract
Silicalite-1 membrane was prepared on an outer surface of a tubular α-alumina support by a secondary growth method. Changes of defect amount and crystallinity during secondary growth were carefully observed. The defect-less well-crystallized silicalite-1 membrane was obtained after 7-days crystallization at 373 K. [...] Read more.
Silicalite-1 membrane was prepared on an outer surface of a tubular α-alumina support by a secondary growth method. Changes of defect amount and crystallinity during secondary growth were carefully observed. The defect-less well-crystallized silicalite-1 membrane was obtained after 7-days crystallization at 373 K. The silicalite-1 membrane became (h0l)-orientation with increasing membrane thickness, possibly because the orientation was attributable to “evolutionally selection”. The (h0l)-oriented silicalite-1 membrane showed high p-xylene separation performance for a xylene isomer mixture (o-/m-/p-xylene = 0.4/0.4/0.4 kPa). The p-xylene permeance through the membrane was 6.52 × 10−8 mol m−2 s−1 Pa−1 with separation factors αp/o, αp/m of more than 100 at 373 K. As a result of microscopic analysis, it was suggested that a very thin part in the vicinity of surface played as effective separation layer and could contribute to high permeation performance. Full article
(This article belongs to the Special Issue Zeolites)
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16 pages, 1475 KiB  
Article
Assessing the Sustainability and Acceptance Rate of Cost-Effective Household Water Treatment Systems in Rural Communities of Makwane Village, South Africa
by Resoketswe Charlotte Moropeng and Maggy Ndombo Benteke Momba
Crystals 2020, 10(10), 872; https://doi.org/10.3390/cryst10100872 - 26 Sep 2020
Cited by 5 | Viewed by 3096
Abstract
The current study investigated the acceptance rate and long-term effectiveness of cost-effective household water treatment systems deployed in Makwane Village. A structured questionnaire was used prior to implementation to collect information such as level of education, level of employment, and knowledge about point-of-use [...] Read more.
The current study investigated the acceptance rate and long-term effectiveness of cost-effective household water treatment systems deployed in Makwane Village. A structured questionnaire was used prior to implementation to collect information such as level of education, level of employment, and knowledge about point-of-use water treatment systems in the target area. The long-term effectiveness was determined by factors such as the Escherichia coli removal efficiency, turbidity reduction, silver leached, and flow rate of the household water treatment devices. The results of the survey prior to deployment revealed that only 4.3% of the community had a tertiary qualification. Moreover, 54.3% of the community were unemployed. The results further revealed that 65.9% of the community were knowledgeable about other point-of-use water treatment methods. The acceptance rate, which was found to be initially higher (100%), reduced after three months of implantation (biosand filter with zeolite-silver clay granular—82.9%; silver-impregnated porous pot filters—97.1%). Moreover, the long-term effectiveness was determined, taking into consideration the adoption rate, and it was found that silver-impregnated porous pot filters have a long life compared to biosand filter with zeolite-silver clay granular. Although household water treatment systems can effectively reduce the burden of waterborne diseases in impoverished communities, the success of adoption is dependent on the targeted group. This study highlights the significance of involving community members when making the decision to scale up household water treatment devices in rural areas for successful adoption. Full article
(This article belongs to the Special Issue Zeolites)
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15 pages, 3652 KiB  
Article
Guided Crystallization of Zeolite Beads Composed of ZSM-12 Nanosponges
by Kassem Moukahhal, Ludovic Josien, Habiba Nouali, Joumana Toufaily, Tayssir Hamieh, T. Jean Daou and Bénédicte Lebeau
Crystals 2020, 10(9), 828; https://doi.org/10.3390/cryst10090828 - 17 Sep 2020
Cited by 1 | Viewed by 3564
Abstract
The direct route using a bifunctional amphiphilic structuring agent for the synthesis of hierarchical nanozeolites coupled with pseudomorphic transformation was used for the crystallization of hierarchized zeolite beads/hollow spheres composed of ZSM-12 (MTW structural-type) with nanosponge morphology. These beads/hollow spheres have the same [...] Read more.
The direct route using a bifunctional amphiphilic structuring agent for the synthesis of hierarchical nanozeolites coupled with pseudomorphic transformation was used for the crystallization of hierarchized zeolite beads/hollow spheres composed of ZSM-12 (MTW structural-type) with nanosponge morphology. These beads/hollow spheres have the same average diameter of 20 µm as their counterpart amorphous mesoporous silica beads used as precursor in the starting synthesis mixture. The effects of synthesis parameters, such as stirring and treatment time at 140 °C, on the morphology, structure, and texture of the materials have been investigated using X-ray diffraction (XRD), N2 sorption, scanning electronic microscopy (SEM), and transmission electronic microscopy (TEM) techniques. Static conditions were found necessary to maintain the morphology of the starting amorphous silica beads. An Ostwald ripening phenomenon was observed with the increase in hydrothermal treatment time leading to the dissolution of the interior of some beads to form core shell beads or hollow spheres with larger crystals on the outer surface. These ZSM-12 beads/hollow spheres possess higher porous volume than conventional ZSM-12 zeolite powder and can be used directly for industrial applications. Full article
(This article belongs to the Special Issue Zeolites)
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9 pages, 2469 KiB  
Communication
Designer Synthesis of Ultra-Fine Fe-LTL Zeolite Nanocrystals
by Fen Zhang, Yunhong Luo, Lei Chen, Wei Chen, Yin Hu, Guihua Chen, Shengyong You and Weiguo Song
Crystals 2020, 10(9), 813; https://doi.org/10.3390/cryst10090813 - 15 Sep 2020
Cited by 1 | Viewed by 2326
Abstract
Nanosized zeolites with larger external surface area and decreased diffusion pathway provide many potential opportunities in adsorption, diffusion, and catalytic applications. Herein, we report a designer synthesis of ultra-fine Fe-LTL zeolite nanocrystals under very mild synthesis conditions. We prepared Fe-LTL zeolite nanocrystals synthesized [...] Read more.
Nanosized zeolites with larger external surface area and decreased diffusion pathway provide many potential opportunities in adsorption, diffusion, and catalytic applications. Herein, we report a designer synthesis of ultra-fine Fe-LTL zeolite nanocrystals under very mild synthesis conditions. We prepared Fe-LTL zeolite nanocrystals synthesized using L precursor. The precursor is aging at room temperature to obtain zeolite L nuclei. In order to investigate more details of Fe-LTL zeolite nanocrystals, various characterizations including X-ray diffraction (XRD), inductively coupled plasma (ICP), diffuse reflectance ultraviolet-visible (UV-Vis) spectroscopy, confirm the tetrahedral Fe3+ species in the zeolite framework. Besides, scanning electron microscope (SEM), Fourier transform infrared spectrometer (FT-IR), dynamic light scattering (DLS) indicate that the average particle size of Fe-LTL zeolite crystals is approximately 30 nm. Thus, ultra-fine Fe-LTL zeolite with large external surface area and shorter diffusion pathway to the active sites might have great potential in the near future. Full article
(This article belongs to the Special Issue Zeolites)
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9 pages, 1823 KiB  
Communication
Size Effects of the Crystallite of ZSM-5 Zeolites on the Direct Catalytic Conversion of L-Lactic Acid to L, L-Lactide
by Qintong Huang, Rui Li, Guangying Fu and Jiuxing Jiang
Crystals 2020, 10(9), 781; https://doi.org/10.3390/cryst10090781 - 3 Sep 2020
Cited by 10 | Viewed by 2858
Abstract
ZSM-5 zeolites are commonly used as a heterogeneous catalyst for reactions. Four ZSM-5 catalysts (with various crystallite sizes and a similar ratio of Si/Al) and their ball-milling/surface-poisoning derivates were used to convert L-lactic acid to L, L-lactide. The reaction products were analyzed by [...] Read more.
ZSM-5 zeolites are commonly used as a heterogeneous catalyst for reactions. Four ZSM-5 catalysts (with various crystallite sizes and a similar ratio of Si/Al) and their ball-milling/surface-poisoning derivates were used to convert L-lactic acid to L, L-lactide. The reaction products were analyzed by three independent analytical methods (i.e., Proton nuclear magnetic resonance (1H NMR), high-pressure liquid chromatography (HPLC), and chiral gas chromatography (GC)) for determining the L, L-lactide yield and L-lactic acid conversion. A clear size effect, i.e., smaller catalysts providing better performance, was observed. Further ball-milling/surface-poisoning experiments suggested that the size effect of the ZSM-5 catalysts originated from the diffusion-controlled nature of the reaction under the investigated conditions. Full article
(This article belongs to the Special Issue Zeolites)
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15 pages, 7205 KiB  
Article
Characterization of Fibrous Mordenite: A First Step for the Evaluation of Its Potential Toxicity
by Dario Di Giuseppe
Crystals 2020, 10(9), 769; https://doi.org/10.3390/cryst10090769 - 31 Aug 2020
Cited by 16 | Viewed by 4163
Abstract
In nature, a huge number of unregulated minerals fibers share the same characteristics as asbestos and therefore have potential adverse health effects. However, in addition to asbestos minerals, only fluoro-edenite and erionite are currently classified as toxic/pathogenic agents by the International Agency for [...] Read more.
In nature, a huge number of unregulated minerals fibers share the same characteristics as asbestos and therefore have potential adverse health effects. However, in addition to asbestos minerals, only fluoro-edenite and erionite are currently classified as toxic/pathogenic agents by the International Agency for Research on Cancer (IARC). Mordenite is one of the most abundant zeolites in nature and commonly occurs with a fibrous crystalline habit. The goal of this paper is to highlight how fibrous mordenite shares several common features with the well-known carcinogenic fibrous erionite. In particular, this study has shown that the morphology, biodurability, and surface characteristics of mordenite fibers are similar to those of erionite and asbestos. These properties make fibrous mordenite potentially toxic and exposure to its fibers can be associated with deadly diseases such as those associated with regulated mineral fibers. Since the presence of fibrous mordenite concerns widespread geological formations, this mineral fiber should be considered dangerous for health and the precautionary approach should be applied when this material is handled. Future in vitro and in vivo tests are necessary to provide further experimental confirmation of the outcome of this work. Full article
(This article belongs to the Special Issue Zeolites)
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16 pages, 2999 KiB  
Article
Use of a Copper- and Zinc-Modified Natural Zeolite to Improve Ethylene Removal and Postharvest Quality of Tomato Fruit
by Johannes de Bruijn, Ambar Gómez, Cristina Loyola, Pedro Melín, Víctor Solar, Norberto Abreu, Federico Azzolina-Jury and Héctor Valdés
Crystals 2020, 10(6), 471; https://doi.org/10.3390/cryst10060471 - 3 Jun 2020
Cited by 25 | Viewed by 4445
Abstract
Ethylene stimulates ripening and senescence by promoting chlorophyll loss, red pigment synthesis, and softening of tomatoes and diminishes their shelf-life. The aim of this work was to study the performance of a novel copper- and zinc-based ethylene scavenger supported by ion-exchange on a [...] Read more.
Ethylene stimulates ripening and senescence by promoting chlorophyll loss, red pigment synthesis, and softening of tomatoes and diminishes their shelf-life. The aim of this work was to study the performance of a novel copper- and zinc-based ethylene scavenger supported by ion-exchange on a naturally occurring zeolite by analyzing its ethylene adsorption capacity and the influence of ethylene scavenging on quality attributes during the postharvest life of tomatoes. The influence of copper- and zinc-modified zeolites on ethylene and carbon dioxide concentrations and postharvest quality of tomatoes was compared with unmodified zeolite. Interactions among ethylene molecules and zeolite surface were studied by diffuse reflectance infrared Fourier transform spectroscopy in operando mode. The percentage of ethylene removal after eight days of storage was 57% and 37% for the modified zeolite and pristine zeolite, respectively. The major ethylene increase appeared at 9.5 days for the modified zeolite treatment. Additionally, modified zeolite delayed carbon dioxide formation by six days. Zeolite modified with copper and zinc cations favors ethylene removal and delays tomato fruit ripening. However, the single use of unmodified zeolite should be reconsidered due to its ripening promoting effects in tomatoes at high moisture storage conditions, as water molecules block active sites for ethylene adsorption. Full article
(This article belongs to the Special Issue Zeolites)
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10 pages, 4280 KiB  
Article
Alkaline Activation of Kaolin Group Minerals
by Oliwia Biel, Piotr Rożek, Paulina Florek, Włodzimierz Mozgawa and Magdalena Król
Crystals 2020, 10(4), 268; https://doi.org/10.3390/cryst10040268 - 2 Apr 2020
Cited by 21 | Viewed by 4119
Abstract
Zeolites can be obtained in the process of the alkali-activation of aluminosilicate precursors. Such zeolite–geopolymer hybrid bulk materials merge the advantageous properties of both zeolites and geopolymers. In the present study, the effect of the type and concentration of an activator on the [...] Read more.
Zeolites can be obtained in the process of the alkali-activation of aluminosilicate precursors. Such zeolite–geopolymer hybrid bulk materials merge the advantageous properties of both zeolites and geopolymers. In the present study, the effect of the type and concentration of an activator on the structure and properties of alkali-activated metakaolin, and metahalloysite was assessed. These two different kaolinite clays were obtained by the calcination of kaolin and halloysite, and then activated with sodium hydroxide and water glass. The phase compositions were assessed by X-ray diffraction, the microstructure was observed via scanning electron microscope, and the structural studies were conducted on the basis of the infrared spectra. The structure and properties of the obtained alkali-activated materials depend on both the type of a precursor and the type of an activator. The formation of zeolite phases was observed when the activation was carried out with sodium hydroxide alone, or with a small addition of water glass, regardless of the starting material used. The higher proportion of silicon in the activator solution does not give crystalline phases, but only an amorphous phase. Geopolymers based on metahalloysite have better compressive strength as the result of the better reactivity of metahalloysite compared to metakaolin. Full article
(This article belongs to the Special Issue Zeolites)
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