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Highly porous and lightweight aerogels of cellulose nanofibers (CNFs) have emerged as a promising class of material. This study delves into the impact of the composition (lignocellulose nanofibers–LCNFs and CNFs) and the drying methods (supercritical drying and freeze-drying) on the morphology and the properties of nanocellulose-based aerogels. The investigation evaluates the concentrations of nanofibers and the influence of lignin, a constituent of LCNFs recognized for enhancing the rigidity of plant cell walls, on the aerogel's properties. The shrinkage rates, density, pore structure, and mechanical properties of the obtained aerogels are comprehensively compared. Supercritical drying proves advantageous for aerogel formation, resulting in materials with lower density and higher surface area than their freeze-dried counterparts at each concentration level. The use of acetone for supercritical drying contributes to reduce the shrinkage rates compared to ethanol. This decrease is attributed to the formation of a more rigid hydrogel during solvent exchange. Freeze-drying exhibits the lowest shrinkage rates and relatively higher porosity. The presence of lignin in the nanofibers influences the microstructure, yielding smoother and thicker pore walls. This study contributes to the comprehensive understanding of the intricate factors shaping nanocellulose aerogel properties, paving the way for the development of innovative and environmentally-friendly materials. [ABSTRACT FROM AUTHOR]

In this work, nanostructured and transparent cellulose aerogels are synthesized via a purely salt induced approach from non‐modified microcrystalline cellulose type II. The production process requires in contrast to state of the art methods no pretreatment of cellulose or use of expensive cellulose‐solvents: it consists of hydrogel formation via cross‐linking of cellulose with calcium ions, a solvent exchange and a supercritical drying step. A systematic multiparameter study reveals that a high level of structural control is achievable: ratios of macro‐ to mesoporosity and the size of mesopores can be tailored by adjustment of the calcium ion content, while keeping a high overall porosity in the range of 92% – 96 %. The build‐up of homogeneous, fine pore structures results in a significant increase of the specific surface area as compared to conventional calcium‐free aerogels (684 vs. 300 m2 g−1). Remarkably, the Ca2+‐cross‐linking renders aerogels transparent, with Rayleigh scattering being the dominant scattering mechanism. Additional ion exchange to Ca2+ in the hydrogel‐state leads to further reduction of the pore size and to products with optimized optical properties, e.g., light transmission of 91% at an incidents light wavelength of 800 nm and a substrate thickness of 1.5 mm. [ABSTRACT FROM AUTHOR]

An ever-increasing number of applications of oxide aerogels places a high demand on wettability-tuning techniques. This work explores the possibility to cheaply prepare GeO2 aerogels with controlled wettability by an ambient pressure drying (APD) method. GeO2 aerogels are prepared via two synthetic routes. Surface modification is carried out by soaking the gels in a silylating agent solution; type and concentration of the modifier are optimized to achieve a large surface area. The aerogels have been characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, nitrogen adsorption and contact angle measurements. The effect of surface modification on the phase composition and particle size of the aerogels is described. In summary, the work provides a new cheap production method for the preparation of both hydrophobic and hydrophilic GeO2 aerogels with contact angle varying from 30° to 141° and with surface area of 90–140 m2/g, which facilitates the expansion of their diverse applications. GeO2 aerogel synthesis by APD is reported for the first time. [ABSTRACT FROM AUTHOR]

Hydrophilicity is one important drawback of bio‐based aerogels. To overcome this issue, a novel approach for the preparation of mesoporous, water repellent aerogels is introduced, which combines synthesis of cross‐linked bio‐based copolymers from methacrylate copolymerizations, followed by solvent exchange and supercritical drying steps. The influence of monomers with different nonpolar ester groups (methyl, vanillin, tetrahydrofurfuryl) on textural properties and water contact angles of the dry products is assessed. Final aerogels show generally high overall porosities (≈96%), low densities (0.07–0.11 g cm−3) as well as fine, mainly mesoporous networks, and specific surface areas in the range of 120–240 m2 g−1. Hereby, choice of the methacrylate ester groups results in differences of the resulting pore‐size distributions. Water repellency tests show stable static water contact angles in the hydrophobic range (≈100°) achieved for the substrate containing the vanillin ester group. On the contrary the other substrates absorb water quickly, which indicates a decisive role of the ester group. The presented approach opens up a new pathway to bio‐based aerogels with intrinsic hydrophobicity. It is suggested that the properties are tailored by the choice of the monomer structure, hence enabling further adaption and optimization of the products. [ABSTRACT FROM AUTHOR]

Supercritical fluid drying (SCFD) and ambient pressure drying (APD) are two common drying methods in the synthesis of silica aerogels (SA). The SCFD efficiently produces high-quality SA with good reproducibility, however, the produced SA is hydrophilic and has poor moisture tolerance. Meanwhile, the APD is less energy-intensive process but requires an extra amount of organic solvents and hydrophobic silanes to effectively control drying shrinkage. In this study, a new route for synthesizing hydrophobic SA is proposed, using rice husk-derived sodium silicate as a cheap precursor and without the need for high amounts of chemical reagent. First, hydrophilic SA particles (100 µm to 1.0 mm) with a large specific surface area (700 m2/g) were produced using a sol–gel method, ethanol solvent exchange, and SCFD drying. The dried hydrophilic SA were then surface hydrophobized by simple immersion in a silane reagent containing 2%, 5%, and 10% trimethylchlorosilane (TMCS) in n-hexane. These concentration of TMCS was at least ten times lower than that required by the standard APD approach, reducing chemical waste. The TMCS-modified SA shrank by 9.0–17% compared to the original SA, which slightly increasing their final density and thermal conductivity. Hydrophobization was most effective at 5% TMCS, as indicated by the largest surface area (664 m2/g), lowest thermal conductivity (0.038 W/mK), and highest water contact angle (θ = 140°). The hydrophobic SA is more thermally stable than the hydrophilic SA, which can resist temperatures as high as 350 °C. [ABSTRACT FROM AUTHOR]

Bone tissue engineering (BTE) is the most promising strategy to repair bones injuries and defects. It relies on the utilization of a temporary support to host the cells and promote nutrient exchange (i.e., the scaffold). Supercritical CO2 assisted drying can preserve scaffold nanostructure, crucial for cell attachment and proliferation. In this work, agarose aerogels, loaded with hydroxyapatite were produced in view of BTE applications. Different combinations of agarose concentration and hydroxyapatite loadings were tested. FESEM and EDX analyses showed that scaffold structure suffered from partial closure when increasing filler concentration; hydroxyapatite distribution was homogenous, and Young's modulus improved. Looking at BTE applications, the optimal combination of agarose and hydroxyapatite resulted to be 1% w/w and 10% w/v, respectively. Mechanical properties showed that the produced composites could be eligible as starting scaffold for BTE, with a Young's Modulus larger than 100 kPa for every blend. [ABSTRACT FROM AUTHOR]

Polypropylene-based aerogels with high surface area have been developed for the first time. By chemical crosslinking of polypropylene with oligomeric capped-end amino compounds, followed by dissolution, thermally induced phase separation, and the supercritical CO2 drying process or freeze-drying method, the aerogels exhibit high specific surface areas up to 200 m2/g. Moreover, the silica-cage multi-amino compound was utilized in a similar vein for forming hybrid polypropylene aerogels. According to the SEM, the developed polypropylene-based aerogels exhibit highly porous morphology with micro-nanoscale structural features that can be controlled by processing conditions. Our simple and inexpensive synthetic strategy results in a low-cost, chemically resistant, and highly porous material that can be tailored according to end-use applications. [ABSTRACT FROM AUTHOR]

Aerogel composites were synthesized on the basis of the sodium alginate-chitosan interpolymer complex with the inclusion of calcium carbonate microparticles and supercritical drying. It is shown that the textural characteristics of materials did not depend on the morphology of calcium carbonate particles: the specific surface area of aerogels was almost the same for all materials, 380–400 m2/g. The developed porous structure of composites along with the polyelectrolyte nature determined their high water absorption (up to 110 g/g). To impart antimicrobial properties, the materials were impregnated with atranorin isolated from the lichen Hypogymnia physodes, which has a pronounced inhibitory effect on the bacterium P. mirabilis, the main causative agent of wound infections. The minimum suppressive concentration of atranorine was 1 mg/mL. The release of the main amount of atranorine aerosol material included in the matrix occurred within 4 hours, 50%. [ABSTRACT FROM AUTHOR]

Cellulose nanofibers (CNFs) play a crucial role in enhancing the properties of aerogels, making them indispensable in the pharmaceutical sector. This study successfully produced acai bagasse-based CNFs/sodium alginate (SA) aerogels by extracting CNFs from acai biomass through ammonium persulfate oxidation. Cylindrical hydrogels were formed from a CNF/SA solution with varying CNFs content and dried using supercritical CO2. The aerogels underwent physicochemical analyses, and the swelling behaviors of the hydrogels were examined both experimentally and theoretically, considering fluid motion and elastic deformation. The swelling ratio's theoretical forecasts aligned closely with the actual experimental results. It showed a rapid rise at the beginning of the swelling process, followed by a consistent upward pattern with a diminishing rate until eventually reaching a stable state. The aerogels incorporating CNFs in the range of 0.0 to 0.3% exhibited low swelling ratios (ranging from 105 to 120%), along with high Young's modulus (above 10 MPa) and storage modulus (7 to 50 MPa). These findings indicate the promising potential of these aerogels for biomedical applications, specifically in wound closure and the repair of load-bearing tissues. [ABSTRACT FROM AUTHOR]

Tragacanth, an anionic polysaccharide, is a natural material widely investigated for the synthesis of aerogels as drug delivery vehicles. Its biocompatibility, biodegradability, and affordability are all key features for its use in pharmaceutical applications. In this study, tragacanth and tragacanth alginate composite aerogels were prepared using the sol-gel technique followed by supercritical drying. Paracetamol was selected as a model drug for drug loading and release studies owing to its high solubility in ethanol and low solubility in supercritical carbon dioxide. The paracetamol loading into the aerogel pores was confirmed by infrared spectroscopy (IR) and x-ray diffraction (XRD) spectra of the resulting samples. Scanning electron microscopy (SEM) images showed that all aerogels were porous with a macroporous-mesoporous network. Due to the high porosity of the prepared aerogels, a loading of 99 wt% (mg drug/mg aerogel) for tragacanth and 114 wt% (mg drug/mg aerogel) for composite aerogels was achieved. Moreover, the release rate of the drug could be modified by manipulating the aerogel composition. Tragacanth aerogels had a faster release rate, while the addition of alginate prolonged the release rate of the model drug. Various empirical release models were investigated and the release rate was found to follow the Korsmeyer-Peppas (Power Law) model suggesting a diffusion-based release kinetics. Based on the results, the feasibility of utilizing tragacanth for the preparation of drug-loaded aerogels was shown. Highlights: Tragacanth is a suitable polysaccharide for the synthesis of aerogels. Macropores/mesopores of all synthesized aerogels are suitable for drug loading. The presence of alginate retards the release of the model drug up to 20% at each time point. The release kinetics of the synthesized aerogels follow the Korsmeyer-Peppas model. [ABSTRACT FROM AUTHOR]

The inherent characteristics of cellulose-derived aerogels, such as their natural abundance and environmental friendliness, make them highly interesting. However, its significant shrinkage before and after the supercritical drying procedure and low mechanical strength limit its potential application. Here, we propose a strategy to prepare cellulose diacetate aerogels (CDAAs) with low drying shrinkage, exceptional thermal insulation, and superior mechanical strength. The low drying shrinkage (radial drying shrinkage of 1.4%) of CDAAs is attributed to their relative strong networking skeletons, which are greatly formed by tert-butanol solvent exchange in exerting the interaction of reducing the surface tension force. In this case, CDAAs are eventually endowed with the low bulk density of 0.069 g cm−3 as well. Additionally, as-prepared CDAAs possess an abundant three-dimensional networking structure whose pore size is concentrated in the diameter range of ~50 nm, and the result above is beneficial for improving the thermal insulation performance (thermal conductivity of 0.021 W m−1 K−1 at ambient environmental and pressure conditions). On the other hand, the optimal compressive stresses of CDAAs at 3% and 5% strain are 0.22 and 0.27 MPa respectively, indicating a mechanically well robustness. The above evidence demonstrates indeed the exceptional thermal insulation and superior compressive properties of CDAAs. This work may provide a new solution for developing a kind of high-performance cellulose-derived aerogel in the future. [ABSTRACT FROM AUTHOR]

An ever-increasing number of applications of oxide aerogels places a high demand on wettability-tuning techniques. This work explores the possibility to cheaply prepare GeO2 aerogels with controlled wettability by an ambient pressure drying (APD) method. GeO2 aerogels are prepared via two synthetic routes. Surface modification is carried out by soaking the gels in a silylating agent solution; type and concentration of the modifier are optimized to achieve a large surface area. The aerogels have been characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, nitrogen adsorption and contact angle measurements. The effect of surface modification on the phase composition and particle size of the aerogels is described. In summary, the work provides a new cheap production method for the preparation of both hydrophobic and hydrophilic GeO2 aerogels with contact angle varying from 30° to 141° and with surface area of 90–140 m2/g, which facilitates the expansion of their diverse applications. GeO2 aerogel synthesis by APD is reported for the first time.

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Lignin is considered a valuable renewable resource for building new chemicals and materials, particularly resins and polymers. The aromatic nature of lignin suggests a synthetic route for synthesizing organic aerogels (AGs) similar to the aqueous polycondensation of resorcinol with formaldehyde (FA). The structure and reactivity of lignin largely depend on the severity of the isolation method used, which challenges the development of new organic and carbon materials. Resorcinol aerogels are considered a source of porous carbon material, while lignin-based aerogels also possess great potential for the development of carbon materials, having a high carbon yield with a high specific surface area and microporosity. In the present study, the birch hydrolysis lignin and organosolv lignin extracted from pine were used to prepare AGs with formaldehyde, with the addition of 5-methylresorcinol in the range of 75%-25%, yielding monolithic mesoporous aerogels with a relatively high specific surface area of up to 343.4 m2/g. The obtained lignin-based AGs were further used as raw materials for the preparation of porous carbon aerogels (CAs) under well-controlled pyrolysis conditions with the morphology, especially porosity and the specific surface area, being dependent on the origin of lignin and its content in the starting material. [ABSTRACT FROM AUTHOR]

A novel method for fabricating transparent porous γ‐alumina 3D structures by printing at high resolution is presented. The process is based on combining digital light processing 3D printing (DLP) and sol–gel reactions, all performed in transparent solutions, resulting in ceramic monolithic porous structures. The aqueous printing solution contains mainly aluminum chloride, propylene oxide (PO), ethanol, and acrylic acid (AA). The printed polymerized structures are aged, followed by supercritical drying (SCD), and sintered at high temperatures. During aging and sintering, the printed objects shrink, thus enabling a final printing resolution far beyond the nominal value of the used printer. The resulting structures are crystalline γ‐Al2O3, with a very high surface area, above 1800 m2 g−1, and optical transmission above 80% at 600 nm. In addition, SCD enables precise control of pores' dimensions and total surface area, which are essential for applications including thermal insulation and catalyst support and for obtaining heat‐resistant transparent optical devices. [ABSTRACT FROM AUTHOR]

Protein (eggwhite) and chitosan aerogel microparticles characterized by low density, developed morphology, high specific surface area, high porosity, large volume, and small pore diameter, have been obtained. Particle size distribution curves were obtained by laser diffraction, from which the number average and aerodynamic diameters were calculated. The aerodynamic diameter of all of the obtained biopolymer aerogels lies in the range required for targeted delivery of active pharmaceutical ingredients (APIs) to the olfactory region of the nasal cavity. Experimental studies were performed to introduce the "melatonin" API in the pores of biopolymer aerogels at the solvent replacement and supercritical adsorption steps. The mass fractions of APIs in the resulting biopolymer aerogel–API pharmaceutical composites were determined by high-performance liquid chromatography. An X-ray powder diffraction analysis showed that API was present in pharmaceutical compositions mainly in the amorphous state. The results of in vivo experimental studies showed that in the case of intranasal administration of both biopolymer aerogel–API composites, the maximum concentration of API in brain tissues is reached already after 30 min. This proves that the obtained biopolymer aerogels based on protein (eggwhite) and chitosan are promising for use as API carrier matrices in the development of nasal drug delivery systems. [ABSTRACT FROM AUTHOR]

This article presents the results of modeling the hydrodynamics and mass transfer of the supercritical drying process in apparatuses of various volumes. Calculations of hydrodynamics and mass transfer were carried out using the Ansys Fluent software package, exemplified with devices of laboratory and industrial volumes—2 and 70 L, respectively. The modeling aimed to predict the time of supercritical drying and the feasibility of scaling the process. Continuum mechanics principles were employed to model hydrodynamics and mass transfer, treating the multicomponent system as a viscous compressible fluid. The calculation took place in two areas: within a porous body and in the free volume of an apparatus of a certain scale. Additionally, a study was conducted on the hydrodynamics and mass transfer of the process in the presence of a divider, necessary to minimize the formation of stagnant zones in the apparatus. For optimization purposes, the divider calculation was performed separately from the apparatus. The data obtained were then used in subsequent calculations of the hydrodynamics and mass transfer of the process, employing a user-defined function (UDF) written in the C programming language. A preliminary study of the kinetics of supercritical drying in a 2-L apparatus was carried out to assess the possibility of using the proposed mathematical model for an industrial-level apparatus. The simulation demonstrated that the proposed model is capable of describing the process of supercritical drying in devices of various volumes. Furthermore, calculated curves of the kinetics of supercritical drying, profiles of the velocity distribution of supercritical carbon dioxide, and the distribution of the concentration of isopropyl alcohol at various points in time across the cross-section of the apparatus were obtained. [ABSTRACT FROM AUTHOR]

The paper explores a method for the direct synthesis of functional compounds within the bulk of inorganic aerogels, coupled with the supercritical drying of composite materials. The processes for obtaining these materials are delineated into three main stages. The initial stage involves the synthesis of functional compounds on the gel surfaces. The subsequent stage entails the displacement of the solvent from the free volume of the apparatus by flowing supercritical carbon dioxide through the autoclave. The final stage is the supercritical drying process, leading to the removal of solvent from the material pores. The work provides detailed parameters for conducting both synthesis and supercritical drying processes. Additionally, a technological scheme of an industrial installation with a volume of 70 liters, employed for conducting supercritical processes and situated in Niagara LLC, Shchelkovo, is presented and described. To model these processes, a mathematical model was developed using the CHEMCAD software package, facilitating the determination of material and heat balances for individual devices and the overall technological scheme. The paper also calculates the impact of the amount of isopropyl alcohol on carbon dioxide consumption. An analysis of energy costs for the synthesis of functional compounds within aerogel volumes is conducted based on the acquired data. The mathematical model enables the identification of an energy- and resource-efficient method for the technological design of synthesis and supercritical drying processes. Notably, a reduction in energy costs within the technological scheme is achieved through the utilization of heat after compression. [ABSTRACT FROM AUTHOR]

Because of the pronounced degradation of the environment, there has been an escalated demand for the fabrication of eco-friendly and highly efficient products derived from renewable sources. Cellulose aerogels have attracted significant interest attributable to their structural characteristics coupled with biodegradability and biocompatibility. The features of the molecular structure of cellulose allow for the use of various methods in the production of gels. For instance, the presence of hydroxyl groups on the cellulose surface allows for chemical crosslinking via etherification reactions. On the other hand, cellulose gel can be procured by modulating the solvent power of the solvent. In this study, we investigate the impact of the gelation methodology on the structural attributes of aerogels. We present methodologies for aerogel synthesis employing three distinct gelation techniques: chemical crosslinking, cryotropic gelation, and CO2-induced gelation. The outcomes encompass data derived from helium pycnometry, Fourier-transform infrared spectroscopy, nitrogen porosimetry, and scanning electron microscopy. The resultant specimens exhibited a mesoporous fibrous structure. It was discerned that specimens generated through cryotropic gelation and CO2-induced gelation manifested higher porosity (93–95%) and specific surface areas (199–413 m2/g) in contrast to those produced via chemical crosslinking (porosity 72–95% and specific surface area 25–133 m2/g). Hence, this research underscores the feasibility of producing cellulose-based aerogels with enhanced characteristics, circumventing the necessity of employing toxic cross-linking agents. The process of gel formation through chemical crosslinking enables the creation of gels with enhanced mechanical properties and a more resilient structure. Two alternative methodologies prove particularly advantageous in applications necessitating biocompatibility and high porosity. Notably, CO2-induced gelation has not been hitherto addressed in the literature as a means to produce cellulose gels. The distinctive feature of this approach resides in the ability to combine the stages of obtaining an aerogel in one apparatus. [ABSTRACT FROM AUTHOR]

Lignin is considered a valuable renewable resource for building new chemicals and materials, particularly resins and polymers. The aromatic nature of lignin suggests a synthetic route for synthesizing organic aerogels (AGs) similar to the aqueous polycondensation of resorcinol with formaldehyde (FA). The structure and reactivity of lignin largely depend on the severity of the isolation method used, which challenges the development of new organic and carbon materials. Resorcinol aerogels are considered a source of porous carbon material, while lignin-based aerogels also possess great potential for the development of carbon materials, having a high carbon yield with a high specific surface area and microporosity. In the present study, the birch hydrolysis lignin and organosolv lignin extracted from pine were used to prepare AGs with formaldehyde, with the addition of 5-methylresorcinol in the range of 75%–25%, yielding monolithic mesoporous aerogels with a relatively high specific surface area of up to 343.4 m2/g. The obtained lignin-based AGs were further used as raw materials for the preparation of porous carbon aerogels (CAs) under well-controlled pyrolysis conditions with the morphology, especially porosity and the specific surface area, being dependent on the origin of lignin and its content in the starting material.

Aerogels containing rare earth elements are promising compounds for designing various functional materials, since they combine the properties of aerogels - high surface area and porosity, and the luminescent and catalytic properties of rare earth elements. A modified sol-gel method was developed to produce mixed rare earth oxide aerogels (Eu2O3/Gd2O3, Eu2O3/Yb2O3, and Gd2O3/Yb2O3) with high metal to metal molar ratio (1:1) and individual (Eu2O3, Gd2O3, and Yb2O3) aerogels. Rare earth nitrates, propylene oxide, and citric acid were used for the synthesis of monolithic halogen-free rare earth oxide aerogels. The aerogels obtained by supercritical drying in CO2 possess mesoporous structure and high surface area (180–350 m2/g). Uniform distribution of elements in binary oxides was confirmed by EDX. Calcination at 600–800 °С causes crystallization of the amorphous aerogels. XRD patterns of the binary oxides after calcination corresponded to single phase cubic ( I a 3 ¯ ) structure of rare earth M2O3 oxide, which indicated the formation of solid solutions. The Eu2O3/Gd2O3 and Eu2O3 aerogels demonstrate strong luminescence in visible region at near UV excitation, which was also observed after calcination of the aerogels at 800 °C. Highlights: Binary (Eu2O3/Gd2O3, Eu2O3/Yb2O3, and Gd2O3/Yb2O3) and individual (Eu2O3, Gd2O3, and Yb2O3) aerogels with high surface areas were synthesized by supercritical drying in CO2. Influence of propylene oxide and citric acid concentrations on sol-gel process in RE nitrate solutions was studied. After calcination at 600–800 °С the amorphous aerogels get crystalline structure, the binary oxides demonstrate single phase. The binary and individual oxides were characterized by XRD, IR, nitrogen absorption, SEM methods. The as-prepared and calcinated europium-containing aerogels demonstrate luminescence in visible region at near UV excitation. [ABSTRACT FROM AUTHOR]

Bone tissue engineering (BTE) is the most promising strategy to repair bones injuries and defects. It relies on the utilization of a temporary support to host the cells and promote nutrient exchange (i.e., the scaffold). Supercritical CO2 assisted drying can preserve scaffold nanostructure, crucial for cell attachment and proliferation. In this work, agarose aerogels, loaded with hydroxyapatite were produced in view of BTE applications. Different combinations of agarose concentration and hydroxyapatite loadings were tested. FESEM and EDX analyses showed that scaffold structure suffered from partial closure when increasing filler concentration; hydroxyapatite distribution was homogenous, and Young’s modulus improved. Looking at BTE applications, the optimal combination of agarose and hydroxyapatite resulted to be 1% w/w and 10% w/v, respectively. Mechanical properties showed that the produced composites could be eligible as starting scaffold for BTE, with a Young’s Modulus larger than 100 kPa for every blend.

Polypropylene-based aerogels with high surface area have been developed for the first time. By chemical crosslinking of polypropylene with oligomeric capped-end amino compounds, followed by dissolution, thermally induced phase separation, and the supercritical CO2 drying process or freeze-drying method, the aerogels exhibit high specific surface areas up to 200 m2/g. Moreover, the silica-cage multi-amino compound was utilized in a similar vein for forming hybrid polypropylene aerogels. According to the SEM, the developed polypropylene-based aerogels exhibit highly porous morphology with micro-nanoscale structural features that can be controlled by processing conditions. Our simple and inexpensive synthetic strategy results in a low-cost, chemically resistant, and highly porous material that can be tailored according to end-use applications.

Cloud seeding is a weather modification technique aimed at enhanced precipitation from clouds. Silver iodide, the material used in cloud seeding, is toxic to aquatic life. Therefore, precipitation from seeded clouds is harmful to the environment. In response to the concern, we report a novel approach of developing AgI loaded/encapsulated silica seed material for possible cloud seeding process. The traditional sol–gel alkoxide route is adopted to develop AgI/Silica composite. The obtained composite fulfils the requirement of seed material with high surface area (> 1000 m2/g), meso-porous structure, and low density (~ 0.2 g/cm3), which are the main requirements for cloud seeding programme. [ABSTRACT FROM AUTHOR]

A new para-aramid aerogel based on a polymer made by the reaction of terephthaloyl dichloride with 2-(4-aminophenyl)-1H-benzimidazol-5-amine (PABI) is introduced. The aerogel readily bound Pd (+2) ions and was used as a hydrogenation catalyst in some industrially actual reactions. The new material, which did not contain p-phenylenediamine moieties, was prepared in two form factors: bulk samples and spherical pellets of 700–900 μm in diameter. Aerogels were synthesized from 1% or 5% solutions of PABI in N,N-dimethylacetamide via gelation with acetone or isopropanol and had a density of 0.057 or 0.375 g/cm3 depending on the concentration of the starting PABI solution. The specific surface area of the obtained samples was 470 or 320 m2/g. Spherical pellets containing Pd were prepared from a solution of PdCl2 in PABI and were used as heterogeneous catalysts for the gas-phase hydrogenation of unsaturated organic compounds presenting the main types of industrially important substrates: olefins, acetylenes, aromatics, carbonyls, and nitriles. Catalytic hydrogenation of gaseous hexene-1, hexyne-3, cyclohexene, and acrylonitrile C=C bond proceeded with a 99% conversion at ambient pressure, but the catalyst failed to reduce acetone at 150 °C and benzene and ethyl acetate even at 200 °C. The only product of acrylonitrile hydrogenation was propionitrile. The prepared catalysts showed high selectivity, which is important for the chemistry of complex organic compounds. [ABSTRACT FROM AUTHOR]

This paper reports on the synthesis and characterization of La2O3:Eu3+ luminescent aerogels fabricated by the sol–gel method and the supercritical drying technique. The % mol concentration of the Eu3+ ion was varied to study the effects on the luminescent properties of the aerogels. XRD and TEM analysis showed that the La2O3:Eu3+ aerogels exhibited a semi-crystalline behavior regardless of whether the concentration of europium was increased. SEM micrographs revealed a porous structure in the aerogels, which were composed of quasi-spherical nanoparticles that were interconnected and formed coral-shaped agglomerates. Photoluminescence spectroscopy characterization showed that the aerogels had an infrared emission located at λ = 793 nm, and the maximum photoluminescence emission intensity was observed for the aerogel with 50% Eu3+. The results demonstrate that, without heat treatment, it is possible to manufacture luminescent aerogels of rare-earth oxides that can be used in opto-electronic devices. [ABSTRACT FROM AUTHOR]

In this study, aerogels based on graphene oxide decorated with iron oxide nanoparticles are obtained by drying in supercritical isopropanol. For the synthesized samples with the calculated initial iron contents of 9, 18 and 36 wt %, the morphology and structure of the graphene matrix and iron-containing nanoparticles are studied using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods. Comparative investigations are conducted to analyze the carbon and hydrogen composition within the synthesized aerogels structure, followed by an assessment of their magnetic properties at ambient temperature. Sorption experiments are carried out for the extraction of heavy and rare earth elements from multicomponent aqueous solutions of a complex composition. [ABSTRACT FROM AUTHOR]

For the present work, resorcinol-formaldehyde (R–F) aerogels were synthesized through both acid (nitric acid) and base (sodium carbonate) catalyzed sol-gel process followed by supercritical drying (SCD) as well as ambient pressure drying (APD). RF aerogels were carbonized at high temperatures to form carbon aerogels. The effect of the catalyst type and drying methods on the final characteristics were studied in detail. Microstructural and textural characteristics were evaluated through XRD, SEM, BET, RAMAN, FTIR, and TEM techniques. It was found that the combination of base catalysis and SCD delivered a higher micropore density and more uniform pore distribution. The CAs prepared under such conditions displayed a hydrogen uptake of 0.80 wt% at room temperature and 2.85 wt% at liquid nitrogen temperature (77 K). [Display omitted] • The effect of the catalyst on CAs formation has been studied in detail. • Acid and base-catalyzed carbon aerogels abbreviated as ACCA and BCCA. • We have dried CA by supercritical drying (SCD) and ambient pressure drying (APD). • The optimum CA has hydrogen uptake of 0.80% at RT and 2.85% at 77 K. [ABSTRACT FROM AUTHOR]

Silica aerogels have drawn considerable attention due to their low density (almost 95% of the total volume is composed of air), hydrophobicity, optical transparency, low conductivity of heat, and large surface to volume ratio. Sol–gel processing is used to prepare aerogels from molecular precursors. To replace the pore fluid with air while retaining the solid network, a supercritical drying process (the most frequent approach) is used. However, recent technologies use atmospheric pressure to allow for liquid removal followed by chemical alteration of the gel's internal layer, which leaves only a silica network with a porous structure filled with air. This study discusses the sol–gel method for preparing silica aerogels and their various drying processes. Furthermore, various areas of applications of silica aerogels, including electronics, construction, aerospace, purification of water and air, sensing, catalyst, biomedical, absorbent, food packing, textile, etc., are also discussed. Lastly, this review provides a perception of the recent scientific progress along with the futuristic development of silica aerogel. [ABSTRACT FROM AUTHOR]

The production of aerogels for different applications has been widely known, but the use of polysaccharide-based aerogels for pharmaceutical applications, specifically as drug carriers for wound healing, is being recently explored. The main focus of this work is the production and characterization of drug-loaded aerogel capsules through prilling in tandem with supercritical extraction. In particular, drug-loaded particles were produced by a recently developed inverse gelation method through prilling in a coaxial configuration. Particles were loaded with ketoprofen lysinate, which was used as a model drug. The core-shell particles manufactured by prilling were subjected to a supercritical drying process with CO2 that led to capsules formed by a wide hollow cavity and a tunable thin aerogel layer (40 μm) made of alginate, which presented good textural properties in terms of porosity (89.9% and 95.3%) and a surface area up to 417.0 m2/g. Such properties allowed the hollow aerogel particles to absorb a high amount of wound fluid moving very quickly (less than 30 s) into a conformable hydrogel in the wound cavity, prolonging drug release (till 72 h) due to the in situ formed hydrogel that acted as a barrier to drug diffusion. [ABSTRACT FROM AUTHOR]

Promising active pharmaceutical ingredients (APIs) often exhibit poor aqueous solubility and thus a low bioavailability that substantially limits their pharmaceutical application. Hence, efficient formulations are required for an effective translation into highly efficient drug products. One strategy is the preservation of an amorphous state of the API within a carrier matrix, which leads to enhanced dissolution. In this work, mesoporous silica aerogels (SA) were utilized as a carrier matrix for the amorphization of the poorly water-soluble model drug ibuprofen. Loading of tailored SA was performed post-synthetically and solvent-free, either by co-milling or via the melting method. Thorough analyses of these processes demonstrated the influence of macrostructural changes during the drying and grinding process on the microstructural properties of the SA. Furthermore, interfacial SA-drug interaction properties were selectively tuned by attaching terminal hydrophilic amino- or hydrophobic methyl groups to the surface of the gel. We demonstrate that not only the chemical surface properties of the SA, but also formulation-related parameters, such as the carrier-to-drug ratio, as well as process-related parameters, such as the drug loading method, decisively influence the ibuprofen adsorption efficiency. In addition, the drug-loaded SA formulations exhibited a remarkable physical stability over a period of 6 months. Furthermore, the release behavior is shown to change considerably with different surface properties of the SA matrix. Hence, the reported results demonstrate that utilizing specifically processed and modified SA offers a compelling technique for enhancement of the bioavailability of poorly-water soluble APIs and a versatile adjustment of their release profile.

Aerogels are three-dimensional nanostructures of non-fluid colloids connected to porous networks made of loosely packed bonded particles. They are often manufactured utilizing the sol-gel technique following a drying procedure like supercritical, freeze, or ambient pressure drying. It is the lightest solid material and has several unique qualities, including excellent insulation. Intrinsic brittleness and porous nature make their processing and handling complex, which restrict applicability in several real-world dynamic situations. An effective strategy to strengthen the silica aerogel structure is manufacturing composites with an incorporated fibrous material, which expands their uses considerably. This study covers the scientific synthesis, characterization, and applications of silica aerogel. It encourages silica aerogel composites/blankets that are strengthened by additives and fibrous material made from a wide variety of fibers and fabrics, as well as their manufacturing processes and properties. The effect of fibrous material (fiber and fabric) embedment on the final properties of composites has been extensively discussed, considering the amount of loading in the matrix and their unique characteristics, such as density, shrinkage, mechanical, thermal, and acoustic properties. Fiber-reinforced silica aerogel composites'/blankets applications are briefly discussed, indicating advancements in aerogel functions such as thermal sensors, acoustic insulators, and technical textiles such as protective clothing, medical textiles, and insulation blankets. [ABSTRACT FROM AUTHOR]

Graphene-based aerogels (GA) have a high potential in the biomedical engineering field due to high mechanical strength, biocompatibility, high porosity, and adsorption capacity. Thanks to this, they can be used as scaffolds in bone tissue engineering, wound healing, drug delivery and nerve tissue engineering. In this review, a current state of knowledge of graphene (Gn) and graphene oxide (GO) aerogels and their composites used in biomedical application is described in detail. A special focus is paid first on the methods of obtaining highly porous materials by visualizing the precursors and describing main methods of Gn and GO aerogel synthesis. The impact of synthesis parameters onto aerogel microstructure and porosity is discussed according to current knowledge. Subsequent sections deal with aerogels intended to address specific therapeutic demands. Here we discuss the recent methods used to improve Gn and GO aerogels biocompatibility. We explore the various types of GA reported to date and how their architecture impacts their ultimate ability to mimic natural tissue environment. On this basis, we summarized the research status of graphene-based aerogels and put forward the challenges and outlook of graphene-based aerogels dedicated to biomedical usage especially by formation of joints with biocompatible metals. [ABSTRACT FROM AUTHOR]

Activated alumina (AA) with metastable crystal phase and porous structure has shown great potential for fluoride ion removal. Conventional AA is obtained by direct calcination, which limits its fluoride adsorption ability due to the low surface area and limited active sites. In this work, a highly porous AA was synthesized by a sol–gel method, followed with supercritical drying (including ethanol supercritical drying and carbon dioxide supercritical drying) and calcination. The fluoride ion adsorption properties of AA were studied at different concentrations of fluoride in solution. The Langmuir model, Freundlich model, pseudo-first-order model and pseudo-second-order model were used to describe the adsorption process. The results show that the resulting AAs possess a high porosity and a high specific surface area (up to 660 m2/g). The fluoride adsorption capacity of AA increases with the increase of specific surface area and porosity (up to 119.2 mg/g), which is much higher than that of AA prepared by the traditional method (direct calcination). Moreover, all the fitted correlation coefficients of the four models exceed 90%, indicating both chemical and physical adsorption in AA. Highlights: Activated alumina (AA) was prepared by sol–gel method, followed with supercritical drying. The highest fluoride adsorption capacity of porous AA is 119.2 mg/g. The number of active sites of AA increases due to the high porosity and specific surface area. [ABSTRACT FROM AUTHOR]

This work investigated the synthesis and characterization of alginate/starch porous materials and their application as copper ions adsorbents from aqueous media. Initially, pregel aqueous solutions with different biopolymer concentrations (1, 3, and 5% w/w) and alginate contents (25, 50, and 75% w/w) were prepared. Hydrogel formation was performed by internal and external gelation methods. Finally, the drying step was done via CO2sc leading to aerogels and via freeze-drying leading to cryogels. Process parameters influence on the final properties of materials was evaluated by BET isotherms, SEM, EDS, and TGA. Regardless the gelation method applied, interesting materials with meso- and macro-pore structure were prepared from pregel mixtures with 3% w/w biopolymer concentration and an alginate content of only 25% w/w. Low alginate content reduces the final cost of the materials. Concerning copper removal, the adsorption data were well fitted to the pseudo-second order kinetic model for aerogels and cryogels, showing aerogels the highest adsorption capacity (40 mg/g) and removal efficiency (∼ 92%). Materials demonstrated excellent reusability throughout five consecutive adsorption/desorption cycles. Hence, environmentally friendly materials with a high practical value as low-cost bioadsorbents were synthesized, having great performance in the removal of copper ions from aqueous solution. [ABSTRACT FROM AUTHOR]

In this study, we present a detailed comparison between a conventional supercritical drying process and an evaporative drying technique for hierarchically organized porous silica gel monoliths. These gels are based on a model system synthesized by the aqueous sol–gel processing of an ethylene-glycol-modified silane, resulting in a cellular, macroporous, strut-based network comprising anisotropic, periodically arranged mesopores formed by microporous amorphous silica. The effect of the two drying procedures on the pore properties (specific surface area, pore volume, and pore widths) and on the shrinkage of the monolith is evaluated through a comprehensive characterization by using nitrogen physisorption, electron microscopy, and small-angle X-ray scattering. It can clearly be demonstrated that for the hierarchically organized porous solids, the evaporative drying procedure can compete without the need for surface modification with the commonly applied supercritical drying in terms of the material and textural properties, such as specific surface area and pore volume. The thus obtained materials deliver a high specific surface area and exhibit overall comparable or even improved pore characteristics to monoliths prepared by supercritical drying. Additionally, the pore properties can be tailored to some extent by adjusting the drying conditions, such as temperature. [ABSTRACT FROM AUTHOR]

Promising active pharmaceutical ingredients (APIs) often exhibit poor aqueous solubility and thus a low bioavailability that substantially limits their pharmaceutical application. Hence, efficient formulations are required for an effective translation into highly efficient drug products. One strategy is the preservation of an amorphous state of the API within a carrier matrix, which leads to enhanced dissolution. In this work, mesoporous silica aerogels (SA) were utilized as a carrier matrix for the amorphization of the poorly water-soluble model drug ibuprofen. Loading of tailored SA was performed post-synthetically and solvent-free, either by co-milling or via the melting method. Thorough analyses of these processes demonstrated the influence of macrostructural changes during the drying and grinding process on the microstructural properties of the SA. Furthermore, interfacial SA-drug interaction properties were selectively tuned by attaching terminal hydrophilic amino-or hydrophobic methyl groups to the surface of the gel. We demonstrate that not only the chemical surface properties of the SA, but also formulation-related parameters, such as the carrier-to-drug ratio, as well as process-related parameters, such as the drug loading method, decisively influence the ibuprofen adsorption efficiency. In addition, the drug-loaded SA formulations exhibited a remarkable physical stability over a period of 6 months. Furthermore, the release behavior is shown to change considerably with different surface properties of the SA matrix. Hence, the reported results demonstrate that utilizing specifically processed and modified SA offers a compelling technique for enhancement of the bioavailability of poorly-water soluble APIs and a versatile adjustment of their release profile. [ABSTRACT FROM AUTHOR]

Because of the pronounced degradation of the environment, there has been an escalated demand for the fabrication of eco-friendly and highly efficient products derived from renewable sources. Cellulose aerogels have attracted significant interest attributable to their structural characteristics coupled with biodegradability and biocompatibility. The features of the molecular structure of cellulose allow for the use of various methods in the production of gels. For instance, the presence of hydroxyl groups on the cellulose surface allows for chemical crosslinking via etherification reactions. On the other hand, cellulose gel can be procured by modulating the solvent power of the solvent. In this study, we investigate the impact of the gelation methodology on the structural attributes of aerogels. We present methodologies for aerogel synthesis employing three distinct gelation techniques: chemical crosslinking, cryotropic gelation, and CO2-induced gelation. The outcomes encompass data derived from helium pycnometry, Fourier-transform infrared spectroscopy, nitrogen porosimetry, and scanning electron microscopy. The resultant specimens exhibited a mesoporous fibrous structure. It was discerned that specimens generated through cryotropic gelation and CO2-induced gelation manifested higher porosity (93–95%) and specific surface areas (199–413 m2/g) in contrast to those produced via chemical crosslinking (porosity 72–95% and specific surface area 25–133 m2/g). Hence, this research underscores the feasibility of producing cellulose-based aerogels with enhanced characteristics, circumventing the necessity of employing toxic cross-linking agents. The process of gel formation through chemical crosslinking enables the creation of gels with enhanced mechanical properties and a more resilient structure. Two alternative methodologies prove particularly advantageous in applications necessitating biocompatibility and high porosity. Notably, CO2-induced gelation has not been hitherto addressed in the literature as a means to produce cellulose gels. The distinctive feature of this approach resides in the ability to combine the stages of obtaining an aerogel in one apparatus.

The aim of the study is to investigate the influence of the solvents applied both in sol–gel process and for supercritical drying (SCD) on NiO aerogels' properties. NiO aerogels were synthesized using methanol and 2-methoxy-ethanol (MeGl) as sol solvents. SCD was performed using iso-propanol, methanol and tert-butyl-methyl ether as supercritical fluids. The obtained samples were characterized using low-temperature nitrogen adsorption, X-ray diffraction analysis, mass-spectra analysis and STEM and TEM methods. It was found that specific surface area and the phase and chemical composition strongly depend on the synthesis conditions. We revealed that Ni2+ cations were reduced into Ni0 when 2-methoxy-ethanol was applied as a sol solvent. The mechanism of the Ni2+ cations reduction is proposed. We consider that at the stage of sol preparation, the Ni2+–MeGl chelate was formed. This chelate decomposes at the SCD stage with the release of MeGl, which, in turn, eliminates methanol and leads to the formation of aldehyde. The latter is responsible for the nickel reduction. The proposed mechanism was confirmed experimentally. [ABSTRACT FROM AUTHOR]

The objective of this work is to examine the effect of calcium oxide (CaO) nanoparticles on the CO 2 capacity during calcium looping (CaL) in a thermogravimetric analyzer (TGA). This work provides highly reactive nanosized CaO particles synthesized by sol-gel and subsequent supercritical drying techniques resulting in a high surface area of 190 m2/g. Also, stabilized sorbents comprised of two mixed metal oxides of calcium oxide and zirconium oxide (zirconia) nanoparticles are disclosed. The preparation methods for these materials and their applications as sorbents for the calcium looping (CaL) process are presented in this study. The sorbents' performances were conducted using zirconia/ CaO nanoparticles according to our novel patented method. The TGA results exhibited that the zirconia stabilized sorbent had the highest activity with a maximum CO 2 capacity of 15 mol/kg sorbent at a temperature range of 650–700 °C. The sorbents' characterization was performed applying various methods, such as BET surface area and BJH pore size analyses, thermogravimetry analyzer (TGA), and particle size analysis. The experimental results demonstrate that the appropriate amount of zirconia may stabilize CaO nanoparticles structure, increase the CO 2 capacity of sorbents at higher temperatures and intensify the CaO resistance to the sintering during multi-cyclic operations. [Display omitted] • CO 2 capacity and stability of CaO nanoparticles are investigated. • CaO nanoparticles were synthesized using our patented aerogel synthesis. • Zirconia-stabilized CaO sorbent enhanced stability of cyclic operation. • Calcium zirconate CaZrO 3 detected in XRD stabilized sorbents. • Sorbents showed 15 mol CO2/Kg capacity and 10% activity loss after 20 cycles. [ABSTRACT FROM AUTHOR]

A simple and easily operated supercritical CO 2 dryer was designed and manufactured with the aim of producing high-surface-area mesoporous α-Fe 2 O 3 (hematite) and NiO aerogels. The gels were synthesized by a sol−gel method with the aid of propylene oxide (PO), as the gelation agent, and then dried and calcined at different conditions. The effects of drying and calcination conditions on the physicochemical properties of the final aerogels were investigated using X-ray diffraction (XRD), N 2 adsorption-desorption, Fourier-transformed infrared spectroscopy (FT-IR), and field emission scanning electron microscopy (FE-SEM) analyses. It was demonstrated that α-Fe 2 O 3 and NiO aerogels with high surface areas and mesoporosities could be successfully synthesized using the home-made supercritical CO 2 dryer. Supercritical drying of the gels resulted in α-Fe 2 O 3 (186 m2/g) and NiO (178 m2/g) aerogels with 186% and 34% higher surface areas, respectively, than xerogels obtained via simple drying at 80°C using a laboratory oven. In addition, the results showed that supercritical CO 2 drying could enhance preservation of the porous network of the oxide nanostructures at high calcination temperatures via suppression of sintering phenomenon. Calcination of α-Fe 2 O 3 and NiO aerogels at 600°C yielded 225% and 53% higher surface areas than the corresponding xerogel, confirming the significance of drying step in the sol − gel method. Asphaltene adsorption from a model oil with asphaltene concentration of 3000 ppm indicated that the aerogels possessed higher adsorption capacities for the bulky asphaltene molecules than xerogels calcined at the same temperature of 600°C, which was due to their enhanced textural properties. [ABSTRACT FROM AUTHOR]

Abstract—The influence of the conditions of drying elastic polymethylsilsesquioxane aerogels (at atmospheric pressure, during supercritical drying in CO2 or methanol) on their structure (including porosity and specific surface area), hydrophobicity, and mechanical compressive properties is investigated. The structure and physicomechanical properties of the aerogels prepared under different conditions are found to differ insignificantly, which demonstrates the possibility of producing elastic highly porous superhydrophobic materials without the use of expensive equipment for supercritical drying. The polymethylsilsesquioxane aerogels synthesized by supercritical drying in methanol have the best mechanical characteristics under compression (ultimate strength, relative deformation). [ABSTRACT FROM AUTHOR]