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Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity...
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Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship

A special issue of Gels (ISSN 2310-2861). This special issue belongs to the section "Gel Analysis and Characterization".

Deadline for manuscript submissions: closed (20 September 2023) | Viewed by 16908

Special Issue Editors

Dr. Zoltán Dudás

E-Mail Website
Guest Editor
Neutron Spectroscopy Department, Centre for Energy Research, H-1121 Budapest, Hungary
Interests: sol–gel synthesis; physicochemical characterization; magnetic properties; optical properties; biological activity; medical applications; controlled drug release; structure–activity relationship
Special Issues, Collections and Topics in MDPI journals
Dr. Adél Len

E-Mail Website
Guest Editor
Neutron Spectroscopy Department, Centre for Energy Research, H-1121 Budapest, Hungary
Interests: small-angle scattering; electron microscopy; nanomaterials; aerogels; xerogels; structural characterization; neutron scattering
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

This Special Issue on “Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship” is dedicated to recent developments from fundamental aspects to the synthesis, characterization, and applications of silica xero- and aerogels.

Different synthesis routes can be applied for the synthesis of silica particles. Especially the sol–gel technique offers a flexible and versatile approach to synthesize a wide range of nano-/micro-structures. The present Special Issue has the general aim to present the different synthesis approaches, the relations between the synthesis parameters, and the final product properties.

The increasing number of scientific papers on the relation between silica materials/composites’ structure and activity shows the importance of this topic; therefore, this Special Issue also focuses on silica xero- and aerogels’ chemical composition and textural and morphological characterization at the micro- and nano-scale level. Exhaustive characterization could be achieved using common characterization methods such as: FT-IR or Raman spectroscopy, contact angle measurements, thermal analysis, XRD, electron microscopy (SEM and TEM), and porosimetry measurements. The presented methods can be completed very well with special characterizations methods such as small-angle X-ray or neutron scattering and 18O or 29Si solid-state NMR spectroscopy.

We welcome papers about the application of different silica materials and composites in biology, optics, catalysis, medicine, construction, etc.

Meanwhile, we sincerely hope that this Special Issue will provide some new insights on silica materials’ structure–activity relationship.

Dr. Zoltán Dudás
Dr. Adél Len
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 submissions that pass pre-check are 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. Gels 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 2600 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

  • synthesis of xero- and aerogels
  • sol–gel techniques
  • physicochemical characterizations
  • structure–activity relationship
  • porous structures
  • small-angle scattering
  • electron microscopy
  • applications of silica xerogels and aerogels
  • biological activity
  • catalytic activity
  • magnetic properties
  • optical properties

Related Special Issue

Published Papers (10 papers)

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Research

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19 pages, 7343 KiB  
Article
Application of Silica-Aerogel-Fibre-Based Thermal Renders for Retrofits in Building Walls: A Comparative Assessment with Benchmark Solutions
by Marco Pedroso, José Dinis Silvestre, M. Glória Gomes, Jéssica D. Bersch and Inês Flores-Colen
Gels 2023, 9(11), 861; https://doi.org/10.3390/gels9110861 - 30 Oct 2023
Cited by 1 | Viewed by 1092
Abstract
The current climate change context raises the demand for reducing energy and environmental impacts while keeping an economic balance and building users’ comfort. Thermal insulation solutions are potential allies in ensuring the adequacy of existing buildings for challenging sustainability requirements. In this scenario, [...] Read more.
The current climate change context raises the demand for reducing energy and environmental impacts while keeping an economic balance and building users’ comfort. Thermal insulation solutions are potential allies in ensuring the adequacy of existing buildings for challenging sustainability requirements. In this scenario, silica-aerogel-fibre-based thermal renders are innovative solutions for which integrated approaches still lack information, and they should be compared with benchmark multilayer solutions, such as those based on expanded polystyrene (EPS), extruded polystyrene (XPS), mineral wool (MW), and insulated corkboard (ICB), to evidence their prospective economic, environmental, and energy benefits. This paper quantifies the optimum insulation thicknesses, life cycle savings, payback periods, and environmental impacts of innovative thermal renders compared to conventional thermal insulation materials when applied as a retrofit in existing facade walls. The results show that cost-optimised thermal renders with sisal fibres led to the best overall performance. Higher heating needs led to higher optimum render thicknesses and life cycle savings. With a 0.02 m thickness, aerogel-fibre-based thermal renders outperformed other materials in terms of heating-degree days (HDD) from 1000 °C·day onwards; they can save approximately EUR 60∙m−2, 1000 MJ∙m−2, and 100 kg CO2 eq∙m−2 while presenting a U-value 13% lower throughout their 30-year lifetime when compared with the second-best multilayer solution with XPS. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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13 pages, 3431 KiB  
Article
Preparation of Super-Flexible Silica Aerogel and Its Application in Oil–Water Separation
by Linghan Li, Tianci Xu, Faping Zhang, Chunhua Du and Song He
Gels 2023, 9(9), 739; https://doi.org/10.3390/gels9090739 - 12 Sep 2023
Viewed by 1445
Abstract
Using silica as the precursor, and methyltrimethoxysilane and dimethyldimethoxysilane as the silicon sources, a super-flexible hydrophobic lipophilic gel solid was prepared via hydrolysis, drying, solvent replacement, and atmospheric-pressure drying. The characterization test showed that the sample had good flexibility, hydrophobicity, an amorphous structure, [...] Read more.
Using silica as the precursor, and methyltrimethoxysilane and dimethyldimethoxysilane as the silicon sources, a super-flexible hydrophobic lipophilic gel solid was prepared via hydrolysis, drying, solvent replacement, and atmospheric-pressure drying. The characterization test showed that the sample had good flexibility, hydrophobicity, an amorphous structure, and a hydrophobic contact angle of 137°. Through the adsorption separation experiment, it was concluded that the adsorption separation rate of aerogel to oil substances is related to the viscosity of the oil substances. The hydrophobic and oleophilic properties of flexible silicon aerogel materials can be applied to many aspects, such as crude oil leakage and kitchen waste oil recovery, with broad future development prospects and great research significance. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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17 pages, 3648 KiB  
Article
Tunability of Hybrid Silica Xerogels: Surface Chemistry and Porous Texture Based on the Aromatic Precursor
by Beatriz Rosales-Reina, Guillermo Cruz-Quesada, Nataly Padilla-Postigo, Marian Irigoyen-Razquin, Ester Alonso-Martínez, María Victoria López-Ramón, Maialen Espinal-Viguri and Julián J. Garrido
Gels 2023, 9(5), 382; https://doi.org/10.3390/gels9050382 - 05 May 2023
Viewed by 1192
Abstract
The interest in new materials with specific properties has increased because they are essential for the environmental and technological needs of our society. Among them, silica hybrid xerogels have emerged as promising candidates due to their simple preparation and tunability: when they are [...] Read more.
The interest in new materials with specific properties has increased because they are essential for the environmental and technological needs of our society. Among them, silica hybrid xerogels have emerged as promising candidates due to their simple preparation and tunability: when they are synthesised, depending on the organic precursor and its concentration, their properties can be modulated, and thus, it is possible to prepare materials with à la carte porosity and surface chemistry. This research aims to design two new series of silica hybrid xerogels by co-condensation of tetraethoxysilane (TEOS) with triethoxy(p-tolyl)silane (MPhTEOS) or 1,4-bis(triethoxysilyl)benzene (Ph(TEOS)2 and to determine their chemical and textural properties based on a variety of characterisation techniques (FT-IR, 29Si NMR, X-ray diffraction and N2, CO2 and water vapour adsorption, among others). The information gathered from these techniques reveals that depending on the organic precursor and its molar percentage, materials with different porosity, hydrophilicity and local order are obtained, evidencing the easy modulation of their properties. The ultimate goal of this study is to prepare materials suitable for a variety of applications, such as adsorbents for pollutants, catalysts, films for solar cells or coatings for optic fibre sensors. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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18 pages, 7714 KiB  
Article
The Methyl Functionality of Monolithic Silica Xerogels Synthesized via the Co-Gelation Approach Combined with Surface Silylation
by Selay Sert Çok, Fatoş Koç, Zoltán Dudás and Nilay Gizli
Gels 2023, 9(1), 33; https://doi.org/10.3390/gels9010033 - 30 Dec 2022
Cited by 4 | Viewed by 1497
Abstract
The present research aims to investigate the chemical and morphological properties of the methylated silica xerogels produced via the co-gelation approach combined with surface silylation. In the sol–gel synthesis, methyltrimethoxysilane (MTMS) and tetraethylorthosilicate (TEOS) were utilized as silica precursors and trimethylchlorosilane (TMCS) served [...] Read more.
The present research aims to investigate the chemical and morphological properties of the methylated silica xerogels produced via the co-gelation approach combined with surface silylation. In the sol–gel synthesis, methyltrimethoxysilane (MTMS) and tetraethylorthosilicate (TEOS) were utilized as silica precursors and trimethylchlorosilane (TMCS) served as a silylating agent. Structural changes were observed depending on the MTMS/TEOS molar ratio and on the post-synthesis-performed surface silylation of the xerogels. Post-synthesis silylation plays a critical role in the exchanging of the surface silanols with methyl groups, preserving the monolithic form. The morphological and structural changes were followed by SEM, 29Si-MAS-NMR, FTIR spectroscopy, nitrogen porosimetry, and contact angle measurements. The results have shown significant structural variations depending especially on the MTMS content. With an increasing MTMS content, the morphology of the samples has changed from a micro/mesoporous texture to a meso/macroporous texture. A higher degree of methyl substitution has been achieved for the silylated samples both confirmed by the FTIR and 29Si-NMR results. On the other hand, only the samples with a high MTMS content could preserve their structural integrity after evaporative drying, and all have exhibited a high degree of hydrophobicity with θ > 140°. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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12 pages, 3372 KiB  
Article
Hierarchical Porous SiO2 Cryogel via Sol-Gel Process
by Marius Horváth and Katalin Sinkó
Gels 2022, 8(12), 808; https://doi.org/10.3390/gels8120808 - 09 Dec 2022
Cited by 4 | Viewed by 1511
Abstract
The aim of this research work was to develop a new, low-cost and low-energy-consuming preparation route for highly porous silica systems. The precursor gel systems were synthesized by sol-gel chemistry. The starting materials were TEOS and water glass in the sol-gel syntheses. The [...] Read more.
The aim of this research work was to develop a new, low-cost and low-energy-consuming preparation route for highly porous silica systems. The precursor gel systems were synthesized by sol-gel chemistry. The starting materials were TEOS and water glass in the sol-gel syntheses. The effect of the chemical composition, the catalysis, the pH, and the additives were investigated on the structure and porosity of the cryogels. The gel systems were treated by freeze-drying process to obtain porous cryogel silica products. The cryogel systems possess hierarchical structures. The conditions of the freeze-drying process were also studied to increase the porosity. Small angle X-ray measurements, scanning electron microscope technique, and infrared spectroscopy were applied for the investigation of gel and cryogel systems. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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19 pages, 6368 KiB  
Article
Novel Silica Hybrid Xerogels Prepared by Co-Condensation of TEOS and ClPhTEOS: A Chemical and Morphological Study
by Guillermo Cruz-Quesada, Maialen Espinal-Viguri, María Victoria López-Ramón and Julián J. Garrido
Gels 2022, 8(10), 677; https://doi.org/10.3390/gels8100677 - 20 Oct 2022
Cited by 5 | Viewed by 2199
Abstract
The search for new materials with improved properties for advanced applications is, nowadays, one of the most relevant and booming fields for scientists due to the environmental and technological needs of our society. Within this demand, hybrid siliceous materials, made out of organic [...] Read more.
The search for new materials with improved properties for advanced applications is, nowadays, one of the most relevant and booming fields for scientists due to the environmental and technological needs of our society. Within this demand, hybrid siliceous materials, made out of organic and inorganic species (ORMOSILs), have emerged as an alternative with endless chemical and textural possibilities by incorporating in their structure the properties of inorganic compounds (i.e., mechanical, thermal, and structural stability) in synergy with those of organic compounds (functionality and flexibility), and thus, bestowing the material with unique properties, which allow access to multiple applications. In this work, synthesis using the sol-gel method of a series of new hybrid materials prepared by the co-condensation of tetraethoxysilane (TEOS) and 4-chlorophenyltriethoxysilane (ClPhTEOS) in different molar ratios is described. The aim of the study is not only the preparation of new materials but also their characterization by means of different techniques (FT-IR, 29Si NMR, X-ray Diffraction, and N2/CO2 adsorption, among others) to obtain information on their chemical behavior and porous structure. Understanding how the chemical and textural properties of these materials are modulated with respect to the molar percentage of organic precursor will help to envisage their possible applications: From the most conventional such as catalysis, adsorption, or separation, to the most advanced in nanotechnology such as microelectronics, photoluminescence, non-linear optics, or sensorics. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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28 pages, 7266 KiB  
Article
Enhancement of the Catalytic Performance and Operational Stability of Sol-Gel-Entrapped Cellulase by Tailoring the Matrix Structure and Properties
by Corina Vasilescu, Simona Marc, Iosif Hulka and Cristina Paul
Gels 2022, 8(10), 626; https://doi.org/10.3390/gels8100626 - 01 Oct 2022
Cited by 3 | Viewed by 1274
Abstract
Commercial cellulase Cellic CTec2 was immobilized by the entrapment technique in sol–gel matrices, and sol–gel entrapment with deposition onto magnetic nanoparticles, using binary or ternary systems of silane precursors with alkyl- or aryl-trimethoxysilanes, at different molar ratios. Appropriate tailoring of the sol–gel matrix [...] Read more.
Commercial cellulase Cellic CTec2 was immobilized by the entrapment technique in sol–gel matrices, and sol–gel entrapment with deposition onto magnetic nanoparticles, using binary or ternary systems of silane precursors with alkyl- or aryl-trimethoxysilanes, at different molar ratios. Appropriate tailoring of the sol–gel matrix allowed for the enhancement of the catalytic efficiency of the cellulase biocatalyst, which was then evaluated in the hydrolysis reaction of Avicel microcrystalline cellulose. A correlation between the catalytic activity with the properties of the sol–gel matrix of the nanobiocatalysts was observed using several characterization methods: scanning electron microscopy (SEM), fluorescence microscopy (FM), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA/DTA). The homogeneous distribution of the enzymes in the sol–gel matrix and the mass loss profile as a function of temperature were highlighted. The influence of temperature and pH of the reaction medium on the catalytic performance of the nanobiocatalysts as well as the operational stability under optimized reaction conditions were also investigated; the immobilized biocatalysts proved their superiority in comparison to the native cellulase. The magnetic cellulase biocatalyst with the highest efficiency was reused in seven successive batch hydrolysis cycles of microcrystalline cellulose with remanent activity values that were over 40%, thus we obtained promising results for scaling-up the process. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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14 pages, 3542 KiB  
Article
Imidazolium Ionic Liquids as Designer Solvents Confined in Silica Nanopores
by Ana-Maria Putz, Adél Len, László Trif, Zsolt Endre Horváth and László Almásy
Gels 2022, 8(6), 388; https://doi.org/10.3390/gels8060388 - 19 Jun 2022
Cited by 1 | Viewed by 1910
Abstract
Composite silica xerogels were prepared via acid catalysed sol–gel route using tetraethoxysilan (TEOS) as silica precursor, and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] or 1-butyl-3-methylimidazolium chloride [BMIM][Cl] ionic liquids, used simultaneously as co-solvents, catalysts and pore templates, at various IL-to-silica ratios. Morphology of the [...] Read more.
Composite silica xerogels were prepared via acid catalysed sol–gel route using tetraethoxysilan (TEOS) as silica precursor, and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] or 1-butyl-3-methylimidazolium chloride [BMIM][Cl] ionic liquids, used simultaneously as co-solvents, catalysts and pore templates, at various IL-to-silica ratios. Morphology of the xerogels prepared using the different IL templating agents were investigated using scanning electron microscopy (SEM), nitrogen sorption and small angle neutron scattering (SANS). The thermal behavior of the composites was analyzed by thermal gravimetry, whereas the compositions were checked by infrared spectroscopy and EDX. The differences in the morphology and thermal behavior of the composites due to the different IL additives were revealed. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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25 pages, 9841 KiB  
Article
Molybdate Recovery by Adsorption onto Silica Matrix and Iron Oxide Based Composites
by Florin Matusoiu, Adina Negrea, Mihaela Ciopec, Narcis Duteanu, Petru Negrea, Paula Svera and Catalin Ianasi
Gels 2022, 8(2), 125; https://doi.org/10.3390/gels8020125 - 16 Feb 2022
Cited by 4 | Viewed by 1608
Abstract
Aggressive industrial development over the last century involved different heavy metals being used, including high quantities of molybdenum, which need to be treated before discharge in industrial waters. Molybdenum’s market price and industrial applicability make its recovery a big challenge. In the present [...] Read more.
Aggressive industrial development over the last century involved different heavy metals being used, including high quantities of molybdenum, which need to be treated before discharge in industrial waters. Molybdenum’s market price and industrial applicability make its recovery a big challenge. In the present study the possibility to recover molybdenum ions from aqueous solutions by adsorption on a composite material based on silica matrix and iron oxides—SiO2FexOy—was evaluated. Tests were performed in order to determine the influence of adsorbent material dose, initial solution pH, contact time and temperature over adsorption capacity of synthesized adsorbent material. For better understanding of the adsorption process, the obtained experimental data were modelled using Langmuir, Freundlich and Sips adsorption isotherms. Based on the obtained data, it can proved that the Sips isotherm was describing with better orderliness the studied process, obtaining a maximum adsorption capacity of 10.95 mg MoO42− for each gram of material. By modelling the studied adsorption process, it was proven that the pseudo-second order model is accurately describing the adsorption process. By fitting experimental data with Weber-Morris model, it was proven that MoO42− adsorption is a complex process, occurring in two different steps, one controlled by diffusion and the second one controlled by mass transfer. Further, studies were performed in order to determine the optimum pH value needed to obtain maximum adsorption capacity, but also to determine which are the adsorbed species. From pH and desorption studies, it was proven that molybdate adsorption is a physical process. In order to establish the adsorption mechanism, the thermodynamic parameters (ΔG0, ΔH0 and ΔS0) were determined. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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Review

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13 pages, 1537 KiB  
Review
Compendium of Safety Regulatory for Safe Applications of Aerogels
by Antonella Caterina Boccia, Alfio Pulvirenti, Carlos A. García-González, Fabia Grisi and Monica Neagu
Gels 2023, 9(11), 842; https://doi.org/10.3390/gels9110842 - 24 Oct 2023
Cited by 4 | Viewed by 1787
Abstract
An increasing number of aerogels as nanostructured highly porous materials are entering the market in every day products, with an attractive portfolio of properties for emerging applications ranging from health care and leisure to electronics, cosmetics, energy, agriculture, food and environmental. However, the [...] Read more.
An increasing number of aerogels as nanostructured highly porous materials are entering the market in every day products, with an attractive portfolio of properties for emerging applications ranging from health care and leisure to electronics, cosmetics, energy, agriculture, food and environmental. However, the novelty in properties and forms of aerogels makes the development of a legislative framework particularly challenging for ensuring the safe development and use of nano-enabled products. The presented safety regulatory Compendium intends to share knowledge with the international aerogels community, as well as end-users and stakeholders, on the regulatory and safe handling procedures, as best safety practices, to be followed during the production process, handling, transport and end-use of aerogel-based formulations to mitigate human and environmental risks considering lack of data availability for this purpose in general. Full article
(This article belongs to the Special Issue Advances in Silica Xero- and Aerogels: From Synthesis to Structure–Activity Relationship)
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