Monolithic Xerogels Research Articles

Macroporous high‐entropy spinel oxide monoliths as efficient oxygen evolution electrocatalyst

AbstractIn this paper, macroporous high‐entropy spinel oxide (HESO) (Fe0.2Ni0.2Co0.2Mn0.2Zn0.2)3O4 monoliths were successfully fabricated via a sol–gel method followed by calcination. Appropriate polyacrylic acid and propylene oxide contents allow the formation of three‐dimensional co‐continuous xerogel monoliths, and the water/glycerol ratio controls the macropore size of monoliths. Subsequent calcination achieves the precipitation of HESO (Fe0.2Ni0.2Co0.2Mn0.2Zn0.2)3O4 with a singular phase and exceptional structural stability. The macroporous HESO (Fe0.2Ni0.2Co0.2Mn0.2Zn0.2)3O4 demonstrates remarkable performance in the oxygen evolution reaction (OER) with an overpotential of 333 mV at 100 mA cm−2 and a Tafel slope of 43.2 mV dec–1, surpassing that of RuO2 (391 mV) under identical conditions. Furthermore, the catalytic stability of the HESO catalyst remains superior even after 24 h of testing. This process offers a promising avenue for the development of macroporous high‐entropy oxide OER catalysts for overall water splitting.

The Thermophysical Aspects of the Transformation of Porous Structures in Versatile Nanostructured Materials

The technology of obtaining porous nanostructures is based on ecological organosilicon materials and their uses in some spheres of human life, for example, for medical preparations, for thermal insulation of building structures and industrial equipment, and for cleaning. The purpose of this study was to establish correlations between various experimental parameters (shear stress, speed pulsations, temperature, viscosity, and processing time) and the rheological characteristics of suspensions obtained by the method of liquid-phase dispersion; it was a study of hydrodynamic effects and the processes of heat and mass exchange in liquid systems during the liquid-phase dispersion of hydrogel monoliths by means of discrete-pulse activation in a special rotary apparatus. The dehydration of hydrogels was carried out by two methods: convective drying in a layer and spraying in the coolant flow. Experiments have shown that the key parameters for obtaining stable homogeneous suspensions are a synergistic combination of concentration factors and processing time. To obtain adsorbents in the form of pastes with specified adsorption properties and a monolith size of up to 300 μm, the optimal parameters were a hydrogel concentration of 70% and a processing time in the double-recirculation mode. Xerogels obtained by convective drying are a polydisperse mixture of strong monoliths and fragile aggregates. In contrast, xerogel monoliths obtained by spray drying show great homogeneity in terms of dispersion and strength characteristics. The rheological parameters of the hydrogel dispersions, which depend on the concentration and hydrodynamic treatment modes, are the dominant factors affecting the moisture extraction during drying. This study marks the first investigation into the resilience of porous organosilicon structures against the influence of intense turbulence fields and mechanical stresses experienced within the rotor apparatus during suspension production.

“Ship-in-a-bottle” entrapment of biomolecules in MOF-based xerogel monoliths for high-performance electrochemical hydrogen evolution

The “ship-in-a-bottle” entrapment of bioactive molecules in metal–organic framework (MOF)-based xerogel monoliths based on a controlled mesopore architecture was reported.

The Development of Fe3O4-Monolithic Resorcinol-Formaldehyde Carbon Xerogels Using Ultrasonic-Assisted Synthesis for Arsenic Removal of Drinking Water.

Inorganic arsenic in drinking water from groundwater sources is one of the potential causes of arsenic-contaminated environments, and it is highly toxic to human health even at low concentrations. The purpose of this study was to develop a magnetic adsorbent capable of removing arsenic from water. Fe3O4-monolithic resorcinol-formaldehyde carbon xerogels are a type of porous material that forms when resorcinol and formaldehyde (RF) react to form a polymer network, which is then cross-linked with magnetite. Sonication-assisted direct and indirect methods were investigated for loading Fe3O4 and achieving optimal mixing and dispersion of Fe3O4 in the RF solution. Variations of the molar ratios of the catalyst (R/C = 50, 100, 150, and 200), water (R/W = 0.04 and 0.05), and Fe3O4 (M/R = 0.01, 0.03, 0.05, 0.1, 0.15, and 0.2), and thermal treatment were applied to evaluate their textural properties and adsorption capacities. Magnetic carbon xerogel monoliths (MXRF600) using indirect sonication were pyrolyzed at 600 °C for 6 h with a nitrogen gas flow in the tube furnace. Nanoporous carbon xerogels with a high surface area (292 m2/g) and magnetic properties were obtained. The maximum monolayer adsorption capacity of As(III) and As(V) was 694.3 µg/g and 1720.3 µg/g, respectively. The incorporation of magnetite in the xerogel structure was physical, without participation in the polycondensation reaction, as confirmed by XRD, FTIR, and SEM analysis. Therefore, Fe3O4-monolithic resorcinol-formaldehyde carbon xerogels were developed as a potential adsorbent for the effective removal of arsenic with low and high ranges of As(III) and As(V) concentrations from groundwater.

Poly(methylphenylsiloxane)-modified resorcinol-terephthalaldehyde phenolic xerogel monoliths

Fabrication of polymer porous xerogel with environment-friendly and simplified procedure is important target for reducing pollution and production cost. In this research, a sustainable method to prepare thermal insulator of polysiloxane-phenolic composite porous xerogel monoliths was studied by replacing hazardous and volatile formaldehyde with terephthalaldehyde in phenolic component and applying facile ambient drying process. The homogeneous dispersion of poly(methylphenylsiloxane) and resorcinol-terephthalaldehyde in xerogels was achieved by adjusting catalysis condition, water-ethanol content and prepolymer component. Typical poly(methylphenylsiloxane) and resorcinol-terephthalaldehyde composite xerogel monolith had density of about 0.3 g/cm3, 65.4% mass retention rate at 800 °C under nitrogen atmosphere, and thermal conductivity 0.063 W/(m·K) at room temperature. According to Thermogravity analysis coupled with Fourier Transform Infrared Spectra (TG-FTIR), pyrolysis mechanism and structure transformation were revealed. The typical pyrolyzed derivatives at 600 °C and 800 °C well preserved the integrated structure, low density, thermal insulation property, and thermal oxidation stability.

Effects of NO2 aging on bismuth nanoparticles and bismuth-loaded silica xerogels for iodine capture

The capture of long-lived radioactive iodine (129I) from oxidizing off-gasses produced from reprocessing used nuclear fuel is paramount to human health and environmental safety. Bismuth has been investigated as a viable iodine getter but the phase stability of bismuth-based sorbents in an oxidizing environment have not yet been researched. In the current work, bismuth nanoparticle-based sorbents, as free particles (Bi-NPs) and embedded within silica xerogel monoliths made with a porogen (TEO-5), were exposed to I2(g) before and after aging in 1 v/v% NO2 at 150 °C. For unaged sorbents, BiI3 was the dominant phase after iodine capture with 8–30 mass% BiOI present due to native Bi2O3 on the surface of the unaged nanoparticles. After 3 h of aging, 82 mass% of the Bi-NPs was converted to Bi2O3 with only a small amount of iodine captured as BiOI (18 mass%). After aging TEO-5 for 3 h, iodine was captured as both BiI3 (26 %) and BiOI (74 %) and no Bi2O3 was detected.”. Additionally, bismuth lining the micrometer-scale pores in the TEO-5 led to enhanced iodine capture. In a subsequent exposure of the sorbents to NO2 (secondary aging), all BiI3 converted to BiOI. Thus, direct capture of iodine as BiOI is desired (over BiI3) to minimize loss of iodine after capture.

Synthesis and properties of Cerium-doped organic/silica xerogels: A potential UV filter for photovoltaic panels

The UV-induced polymeric encapsulant degradation in the photovoltaic panel is a challenge in extending the operating lifetime of such devices. To overcome that, an anti-reflective coating material that blocks the UV incidence can be an alternative. In this sense, this work presents high optical quality GPTS/TEOS monolithic xerogels doped with cerium prepared by the sol–gel process. X-ray diffraction, thermal analysis, and infrared spectroscopy results reveal an amorphous hybrid structure stable with the dopant concentration, up to 1 wt%. The undoped matrix shows a wide transmission window, and the cerium addition results in a redshift of the cut-off edge, from ∼300 to 410 nm, and a slight reduction in reflectance. Moreover, all samples present a refractive index well matched with the ethylene-vinyl acetate (EVA) encapsulant, which minimizes reflectance losses. These results highlight the potential of the cerium-doped GPTS/TEOS system as a candidate for UV filter coating on photovoltaic panels.

Elastic and plastic mechanical properties of nanoparticle-based silica aerogels and xerogels

This paper reports and analyzes, for the first time, the effective Young's modulus and hardness of nanoparticle-based silica aerogels and xerogels with porosity ranging from 46 to 81%. Pure silica aerogel and xerogel monoliths were synthesized by (i) gelation of aqueous suspensions of silica nanoparticles on omniphobic substrates (PTFE or perfluorocarbon liquids), (ii) aging, (iii) drying at ambient temperature and pressure, and (iv) calcination. The aging and calcination conditions were varied to elucidate their effect on the mechanical properties of the monoliths. Both the effective Young's modulus and the hardness of the mesoporous slabs were measured by nanoindentation and were found to obey a power law as a function of the effective density. No effect of the synthetic conditions or structural parameters other than porosity were observed. Interestingly, the effective hardness was linearly proportional to the effective Young's modulus. The elastic properties of the present nanoparticle-based materials were compared with those of the molecular precursor-based silica aerogels and xerogels reported in the literature. A single relationship was proposed that can be used to estimate the effective Young's modulus of nanoparticle-based and molecular precursor-based silica aerogels and xerogels with porosity between 0 and 98% and Young's modulus between ∼10−4 and 70 GPa. Deposition of an alumina coating was also demonstrated as a way to increase the hardness of these mesoporous monoliths by a factor 2–13 for porosity of 40–73%. The experimental results and the accompanying analysis broaden the understanding of structure-property relationships in mesoporous silica and will help in the design and fabrication of mesoporous silica components.

Enhanced Adsorption and Mass Transfer of Hierarchically Porous Zr-MOF Nanoarchitectures toward Toxic Chemical Removal.

Zirconium-based metal-organic frameworks (Zr-MOFs) have shown tremendous prospects as highly efficient adsorbents against toxic chemicals under ambient conditions. Here, we report for the first time the enhanced toxic chemical adsorption and mass transfer properties of hierarchically porous Zr-MOF nanoarchitectures. A general and scalable sol-gel-based strategy combined with facile ambient pressure drying (APD) was utilized to construct MOF-808, MOF-808-NH2, and UiO-66-NH2 xerogel monoliths, denoted as G808, G808-NH2, and G66-NH2, respectively. The resulting Zr-MOF xerogels demonstrated 3D porous networks assembled by nanocrystal aggregates, with substantially higher mesoporosities than the precipitate analogues. Microbreakthrough tests on powders and tube breakthrough experiments on engineered granules were conducted at different relative humidities to comprehensively evaluate the NO2 adsorption capabilities. The Zr-MOF xerogels showed considerably better NO2 removal abilities than the precipitates, whether intrinsically or under simulated respirator canister/protection filter environment conditions. Multiple physicochemical characterizations were conducted to illuminate the NO2 filtration mechanisms. Analysis on adsorption kinetics and mass transfer patterns in Zr-MOF xerogels was further performed to visualize the underlying structure-activity relationship using the gravimetric uptake and zero length column methods with cyclohexane and acetaldehyde as probes. The results revealed that the synergy of hierarchical porosities and nanosized crystals could effectively expedite the intracrystalline diffusion for the G66-NH2 xerogel as well as alleviate the surface resistance for the G808-NH2 xerogel, which led to accelerated overall adsorption uptake and thus enhanced performance toward toxic chemical removal.

Preparation of monolithic transparent mullite-based glass-ceramics by the sol–gel method

Monolithic transparent mullite-based glass ceramics were prepared in the system SiO2–Al2O3–B2O3–ZnO–K2O by the sol–gel method. Heat treatment of the resulted monolithic xerogels at 950 °C for 6 h, in air atmosphere resulted dense, homogeneous and transparent glass-ceramics. The activation energy for crystallization, the Avrami constants and the mechanism of crystallization were determined by the Marotta, Augis-Bennet and the modified Kisinger methods, using the differential thermal analysis. X-ray diffraction and transmission electron microscope were also used to determine the crystallinity of the specimens, and the morphology of the precipitated crystalline phase. Also, transparency of the prepared sol and the synthesized glass and glass ceramics were evaluated by UV–vis spectrophotometer. In addition, the volume percent of the precipitated mullite, which was calculated by the internal standard method, was 34%. The transmission of the samples in the range of visible wavelengths was about 79%. According to the results, the Avrami constant for crystallization was 1.24 that means mullite was precipitated in the glassy phase through the surface crystallization mechanism.

Synthesis and Characterization of Magnetic Xerogel Monolith as an Adsorbent for As(V) Removal from Groundwater

Arsenic contamination of groundwater is still a global problem due to the toxicity at low dose on human health confirmed by epidemiological studies. Magnetic xerogel monoliths (MXs) were synthesized by the sol-gel polymerization using resorcinol, formaldehyde, alkaline catalyst and magnetite. The varying molar ratios of magnetite and resorcinol (M/R) in the gel were evaluated for As(V) removal from groundwater. The surface chemistry, structure and morphology of MXs related to arsenic adsorption were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and point of zero charge. Batch adsorption experiments were carried out to investigate the effects of Fe contents, initial pH and adsorbent dose on As(V) removal performance. The MXs with molar ratio of M/R at 0.15 gave the maximum As(V) adsorption capacity and removal with values of 62.8 µg/g and 86.7%, respectively. The adsorption data were well described by the Elovich equation of the kinetic model and the Freundlich isotherm. The thermodynamic studies demonstrated that the adsorption process was endothermic and spontaneous in nature. MXs showed to be a good alternative for As(V) removal from groundwater and achieving the efficient desorption, and thus fulfilled the Mexican standard for drinking water.

Prediction of the pH effect on arsenic (V) removal by varying catalyst of magnetic xerogel monoliths based on FREN model

Abstract Magnetic xerogels monoliths (MCs) were simultaneously prepared and formed by the cross-linking polymerization of resorcinol and formaldehyde using the alkaline catalyst and magnetite. The varying of molar ratio of resorcinol and catalyst (R/C) was studied and characterized by isoelectric point (IEP), point of zero charge (pHpzc), scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), N2 adsorption and Fourier transform infrared spectroscopy (FTIR). The result of XRD and EDX confirmed the presence of magnetite into the gel at 1.19% with low molar ratio of magnetite and resorcinol ratio at 0.01. The surface morphology and textural properties of MCs affect directly with SBET, total pore volume and volume of mesopore increase when molar of R/C increases. The behavior of arsenic (As(V)) adsorption by using MCs, was studied in groundwater into the ranges of pH from 2.0 to 7.0. MC50 shows the maximum As(V) uptake and removal were 72 μg/g and 73.5% at pH 5, respectively, while MC100 gave the best performance within the application range of pH both of acidic and neutral region. Furthermore, the prediction technique based on an adaptive fuzzy rules emulated network was utilized for evaluation of the arsenic removal performance.

Hydrophobic and heat-resistant poly(methylphenylsiloxane)-modified resorcinol–formaldehyde composite xerogel monoliths and the carbonized derivatives

Resorcinol–formaldehyde (RF) aerogels are widely used as thermal insulation materials. The present investigation of RF aerogels emphases on improving hydrophobic property and high-temperature residual yields. In this study, polysiloxane-modified RF composite xerogel monoliths were prepared by gelation of poly(methylphenylsiloxane) prepolymer with RF in cosolvent and ambient pressure drying. The influence of phenyl content on structure and thermal property of xerogels was investigated. The as-prepared RF composite monoliths were lightweight and hydrophobic with density lower than 0.300 g/cm3 and water contact angles approaching 130°. Morphology analyses indicated that polysiloxane grew and formed a continuous layer on RF porous skeleton with controlled reaction parameter. Thermogravimetry analyses (TGA) certified that the controlled incorporation of polysiloxane in composite xerogels enhanced the thermal degradation temperature at which the weight loss of RF xerogels reaches 5% (T5%) and increased the residual yields at 1000 °C. The carbonized derivatives were obtained for further enhancing the thermal stability by removal of small molecules during carbonization. Xerogel monoliths preserved hydrophobic property till 500 °C carbonization due to the thermal stability of poly(methylphenylsiloxane). The heat-resistant composite xerogels remained monolith structure after 800 °C treatment under nitrogen condition.

Sol–gel derived silica/polyethylene glycol hybrids as potential oligonucleotide vectors

Abstract

Quantum yield measurements by thermal lens in highly absorbing samples: The case of highly doped rhodamine B organic/silica xerogels

In this work, rhodamine B-doped GPTS/TEOS-derived organic/silica monolithic xerogels were prepared by the predoping sol-gel process. Thermal lens method was applied to determine thermo-optical properties of the samples. Since the samples present a high optical absorption coefficient, the high absorption theoretical TL model was introduced. Thermal diffusivities, optical absorption coefficients, and fluorescence quantum yield values were determined for samples with different rhodamine B (RhB) concentrations, from ${10}^{\ensuremath{-}5}$ to ${10}^{\ensuremath{-}2}$ M. The dye-doped organic/silica xerogels present a high fluorescence quantum yield (up to 90%), maintaining the value over 50% until a concentration of $\ensuremath{\sim}1.6$ mM. To the best of our knowledge, RhB-doped transparent monoliths with a high fluorescence quantum yield with the highest RhB concentration reported in a solid hybrid matrix were obtained.

A facile preparation of transparent methyltriethoxysilane based silica xerogel monoliths at ambient pressure drying

Abstract The highly transparent and hydrophobic methyltriethoxysilane (MTES) based silica xerogel monoliths with excellent integrity and low thermal conductivity were facilely synthesized by a novel method without complex steps of surface modification and solvent exchange via ambient pressure drying. The effects of water to MTES molar ratios, namely, n (Water)/n (MTES) = 8:1, 9:1, 10:1, 11:1, 12:1 and 13:1, on the pore structure, surface morphology, water contact angle, optical transmittance and thermal conductivity were systematically investigated. It was found that the average transparency and hydrophobicity of the silica xerogels decreased and increased linearly with the increase of n (Water)/n (MTES) value, respectively. Furthermore, the thermal conductivity was also strongly dependent upon the n (Water)/n (MTES) value which directly influenced the completion degree of MTES hydrolysis. A simplified pore model is suggested based on the experimental results to correlate the formation of pore structures with the property of silica xerogels. The average transparency of 65–69% in the visible light range of 300–800 nm with the thermal conductivities of 0.048–0.052 W/(m·K) and water contact angles of 105°∼110° could be simultaneously achieved with the silica xerogel monoliths by controlling the n (Water)/n (MTES) values of 10:1 and 11:1.

Preparation and characteristic of three-dimensional NiCo alloy/carbon composite monoliths with well-defined macropores and mesostructured skeletons

Binary NiCo alloy/carbon composite monoliths with well-defined macropores and mesostructured skeletons have been facilely fabricated by a solgel process followed by heat treatment at Ar atmosphere, and the morphology control, heat treatment and pore structures of the composite monoliths were investigated in detail. Synergic control of phase separation and solgel transition allows the formation of three-dimensional nickel–cobalt hydroxide-based xerogel monoliths with interconnected macropores and co-continuous solid skeletons. Heat treatment at 270–340 °C promotes the precipitation of nickel–cobalt oxides and the pyrolysis of PAA into carbon, and at and above 400 °C, some in situ formed carbon can carbothermally reduce nickel–cobalt oxides to generate the NiCo alloy and form NiCo alloy/carbon composites, without any deterioration of the macrostructure of monoliths. The as-prepared NiCo alloy/carbon composite monolith possesses hierarchical pore structure with a macropore size of 0.5 μm, a mesopore size of 2 nm, a specific surface area up to 167 m2 g−1 and a porosity as high as 80%. This three-dimensional hierarchically porous binary NiCo alloy/carbon monoliths are promisingly used in wide applications such as catalysis, filtration, separation, sensor.

Reassessment of the Electronic Structure of Cr(VI) Sites Supported on Amorphous Silica and Implications for Cr Coordination Number

The electronic structure of isolated Cr(VI) sites supported on silica was reinvestigated using multiple, complementary electronic spectroscopies applied to transparent xerogel monoliths. The absorption spectrum exhibits three previously reported peaks, at 22 800, 29 100, and 41 500 cm–1, as well as a previously unresolved band at ca. 36 900 cm–1. The emission is a long-lived red luminescence with λmax = 13 600 cm–1, emanating from the lowest excited state. Assignment of the excited states was facilitated using time-dependent density functional theory (TD-DFT) calculations performed on cluster models. All of the observed electronic transitions and their energies are accounted for by dioxoCr(VI) sites. The lowest energy observed excitation at 22 800 cm–1 populates a singlet excited state, while the emitting state is the corresponding triplet state, accessed by intersystem crossing from the singlet state. Spectroscopic bands observed at 29 100, 36 900, and 41 500 cm–1 were assigned, based on the TD-DFT calcu...

Synthesis, Reduction, and Electrical Properties of Macroporous Monolithic Mayenite Electrides with High Porosity.

Room-temperature stable macroporous mayenite electride (C12A7:e–) has been successfully prepared via a sol–gel method accompanied by phase separation, followed by heat-treatment and reduction processes. The obtained xerogel monoliths possess controllable macrostructure and a porosity of more than 60%, depending on adjusting the amount of poly(ethylene oxide) as a phase separation inducer. Heat-treatment allows the formation of multicrystals Ca12Al14O32Cl2 and Ca12Al14O33 (C12A7), and the porosity increases to 78.67% after being heat-treated at 1100 °C. Further reduction promotes the transformation from Ca12Al14O32Cl2 or C12A7 to C12A7:e– as well as the conversion from an insulator to a semiconductive electride. The carrier concentration of the electride reaches 3.029 × 1018 cm–3 after being reduced at 1100 °C under Ar atmosphere, and the porosity still remains 66%. The macrostructure of the resultant mayenite electride before and after heat-treatment and reduction is perfectly preserved, indicating that the obtained macroporous monolithic mayenite electride could be utilized in the electronic components.

Facile preparation of mesoporous silica monoliths by an inverse micelle mechanism

A well-known drawback of sol–gel materials is their tendency to crack because of the high capillary pressure supported during drying. We have pioneered a facile and low-cost route to obtain monolithic xerogels, from a silica precursor and a surfactant, mixed under ultrasonic agitation. This route presents a clear interest for practical application at industrial scale. In this paper, a model to explain the formation of silica monoliths in the presence of the surfactant is presented. It is demonstrated that a stable microemulsion of water in the silica oligomer media is produced due to the combined effect of surfactant, producing inverse micelles, and ultrasonic agitation. The model proposed, suggests that the water is encapsulated in the surfactant micelles that act as nanoreactors, producing silica primary particles. The growth of these silica seeds continues outside the micelles until the formation of the constituent particles of the xerogel. Next, the particles are packed, and mesopores are produced from the interparticle spaces. This mesoporosity prevents xerogel cracking because it reduces capillary pressure during gel drying. An in-depth investigation of the structure of the xerogels revealed that they are effectively composed of silica nanoparticles of nearly uniform size values that could match with the size of the surfactant inverse micelles. Finally, it was demonstrated that surfactant and water content present a significant effect on the final structure of the xerogels. An increase of surfactant content produces a reduction in particle size, whereas an increase of water produces an opposite effect.