Monolithic Silica Research Articles

Efficient reverse saturable absorption of sol-gel hybrid plasmonic glasses

Monolithic silica sol-gel glasses doped with platinum(II) acetylide complexes possessing respectively four or six phenylacetylene units (PE2-CH2OH and PE3-CH2OH) in combination with various concentrations of spherical and bipyramidal gold nanoparticles (AuNPs) known to enhance non-linear optical absorption, were prepared and polished to high optical quality. The non-linear absorption of the glasses was measured and compared to glasses doped solely with AuNPs, a platinum(II) acetylide with shorter delocalized structure, or combinations of both. At 532 nm excitation wavelength the chromophore inhibited the non-linear scattering previously found for glasses only doped with AuNPs. The measured non-linear absorption was attributed to reverse saturable absorption from the chromophore, as previously reported for PE2-CH2OH/AuNP glasses. At 600 nm strong nonlinear absorption was observed for the PE3-CH2OH/AuNPs glasses, also attributed to reverse saturable absorption. But contrary to previous findings for PE2-CH2OH/AuNPs, no distinct enhancement of the non-linear absorption for PE3-CH2OH/AuNPs was observed. A numerical population model for PE3-CH2OH was used to give a qualitative explanation of this difference. A stronger linear absorption in PE3-CH2OH would cause the highly absorbing triplet state to populate quicker during the leading edge of the laser pulse and this would in turn reduce the influence from two-photon absorption enhancement from AuNPs.

Tracer-Size-Dependent Pore Space Accessibility and Long-Time Diffusion Coefficient in Amorphous, Mesoporous Silica

Heterogeneous porous materials are widely used as fixed beds in adsorption, separation, and catalysis. An important aspect of mass transport in these materials is the hindrance to diffusion of molecules with dimensions approaching the pore size. We numerically study hindered diffusion of finite-size, passive (i.e., nonadsorbing, nonreacting) tracer particles in physically reconstructed amorphous, mesoporous silica from a hierarchical, macroporous–mesoporous silica monolith. The long-time, effective diffusion coefficient Deff is determined as a function of λ, the ratio of the tracer diameter to the average size of the mesopores. Results demonstrate a strong reduction of Deff with increasing tracer size. Comparison with theoretical models of hindered diffusion in a single uniform, cylindrical pore reveals that these models significantly overestimate Deff for λ > 0.2. Morphological analysis of the mesopore pore space accessible for finite-size tracers shows that its effective geometrical and topological prop...

Facile synthesis of large-sized monolithic methyltrimethoxysilane-based silica aerogel via ambient pressure drying

In this study, large-sized monolithic methyltrimethoxysilane-based silica aerogels were prepared via a facile sol–gel route using ambient pressure drying. The structural, morphological, and hydrophobic properties of the aerogels were characterized by scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller method, and contact angle. Thermal conductivity, thermal stability, and mechanical properties of the samples were also evaluated. The ambient pressure dried aerogels showed macro-pore structure and low density (as low as 75 kg m−3). The Young’s modulus of the aerogels was observed to increase from 0.043 to 1.102 MPa with an increase in the density of the aerogels from 75 to 141 kg m−3. Simultaneously, the aerogels exhibited superhydrophobicity (>150°), low thermal conductivity (0.036 W m−1 K−1), and good thermal stability in air atmosphere. Both the simple fabricating process and the superior performance of the monolithic silica aerogel make it as a promising candidate for energy-saving sector. Large-sized monolithic methyltrimethoxysilane-based silica aerogel with superhydrophobicity was successfully prepared by a one-step sol–gel method via ambient pressure drying using methyltrimethoxysilane as precursor, distilled water as solvent, and ammonia as alkali catalyst.

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.

Monolithic polydimethylsiloxane-modified silica composites prepared by a low-temperature sol–gel micromolding technique for controlled drug release

Sol–gel derived multi-component silica composites are widely accepted as smart materials in orthopedic surgery as bone fillers and bioactive skeleton drug delivery systems. This paper discusses the effect of hydroxy-terminated polydimethylsiloxane (PDMS) (10, 20, 30 and 40% (w/w)) on the physicochemical properties of low temperature sol–gel processed polydimethylsiloxane/calcium phosphate/silica (PDMS-modified CaP/SiO2) composites. The micromolding technique was employed to obtain PDMS-modified CaP/SiO2 composites–monolithic granule-type formulations. The effectiveness of PDMS-modified CaP/SiO2 granules as potential skeleton drug delivery systems was studied in vitro using Rhodamine B (ROD) as a model for highly water-soluble molecules. Results indicated that the composites with PDMS contents at 20 and 30% (w/w) showed similar mechanical properties to those of human cancellous bones. The content of PDMS had a significant effect on the release of ROD. These results showed that PDMS-modified CaP/SiO2 granules with 20 and 30% (w/w) PDMS could provide the zero-order release profile of highly water-soluble molecules.

Silver metal nano-matrixes as high efficiency and versatile catalytic reactors for environmental remediation

Nano-porous metallic matrixes (NMMs) offer superior surface to volume ratios as well as enhanced optical, photonic, and electronic properties to bulk metallic materials. Such behaviours are correlated to the nano-scale inter-grain metal domains that favour the presence of electronic vacancies. In this work, continuous 3D NMMs were synthesized for the first time through a simple diffusion-reduction process whereby the aerogel matrix was functionalized with (3-Mercaptopropyl)trimethoxysilane. The surface energy of the silica monolith templates was tuned to improve the homogeneity of the reduction process while thiol functionalization facilitated the formation of a high density of seeding points for metal ions to reduce. The diameter of NMMs was between 2 and 1000 nm, corresponding to a silver loading between 1.23 and 41.16 at.%. A rates of catalytic degradation kinetics of these NMMS which is three orders of magnitude higher than those of the non-functionalized silver-silica structures. Furthermore, the enhancement in mechanical stability at nanoscale which was evaluated by Atomic Force Microscopy force measurements, electronic density and chemical inertness was assessed and critically correlated to their catalytic potential. This strategy opens up new avenues for design of complex architectures of either single or multi-metal alloy NMMs with enhanced surface properties for various applications.

Improved mechanical and thermal insulation properties of monolithic attapulgite nanofiber/silica aerogel composites dried at ambient pressure

To overcome the brittleness and the dry shrinkage of silica aerogels, natural attapulgite nanofiber is successfully used as reinforcement for monolithic silica aerogel composites possessing ultralow density and excellent thermal insulation properties. The attapulgite/silica aerogel composites are obtained by dispersing the acid-treated attapulgite fibers in silica sol, followed by aging and ambient pressure drying. The acid treatment of attapulgite fibers obviously improved the adhesion between silica aerogel and attapulgite fibers surface. The microstructure analysis showed that silica particles firmly adhered to the surface of the rod shaped crystal of attapulgite fibers, which played a supporting role as structure skeleton and substantially improved the integrity and mechanical strength of the aerogel composites. No obvious variation of the mesoporous silica structure was observed since the nanosized attapulgite fibers are effectively incorporated into the network, instead of separated into small fragments as pure aerogel shown. As the fiber content increased, the density decreased to 0.163 cm3/g first and then increased, while the thermal conductivity increased slightly with ranging between 0.0198 and 0.0228 W m−1 K−1. The aerogel samples with compression strength varying from 0.8 to 2.5 MPa were obtained. All these promising characteristics indicated that ATP/silica aerogel composites have diverse applications, especially in heat preservation field.

Photoluminescence of silica monoliths prepared from cold sintering of nanometric aerosil precursors under high pressure

Blue photoluminescent silica monoliths were obtained by cold sintering of nanosized precursors under high pressure. The samples were prepared from hydrophobic and hydrophilic fumed silica AEROSIL® precursors which were placed inside a soft capsule of hBN and pressed at 7.7GPa. Practically no photoluminescence (PL) emission was observed for the nanometric powdered precursors while a relatively intense emission was measured for the sintered monoliths, which was dependent on the excitation wavelength (2.47–5.63eV). The emission was observed in the range between 2.0 and 3.5eV for all samples with the peak at ~2.7eV for excitation between 3.26–3.44eV. The blue band is probably related to the defects produced during the cold sintering process related to plastic deformation and internal stress of the monolith. The intensity of PL spectra of the monoliths obtained from the hydrophilic precursor (A200) was higher than the intensity of the spectra observed for the monoliths obtained from hydrophobic precursors (R974, R816 and R812) probably due to the presence of the carbon groups at the surface of the hydrophobic precursors, hindering the cold sintering process.

Functionalization of hierarchically porous silica monoliths with polyethyleneimine (PEI) for CO2 adsorption

A novel CO2 sorbent was prepared by modifying polyethyleneimine (PEI) on hierarchically porous silica (HPS) monoliths with well-defined macro-mesopores through wet impregnation method. HPS monoliths before and after functionalization were characterized by scanning electron microscopy (SEM), differential thermal analysis/thermogravimetry (DTA/TG), fourier transform infrared spectroscopy (FT-IR) and nitrogen adsorption/desorption analysis (BET), and the CO2 adsorption-regeneration properties of as-synthesized sorbents were investigated by thermogravimetric analysis (TG). The effects of loaded PEI amount as well as adsorption temperature and CO2 partial pressure on the CO2 adsorption performance of the as-prepared sorbents were also studied. The results showed that the porous structure of HPS-PEI sorbents was preserved after functionalization, while the specific surface area and pore volume decreased with increasing amount of amines to some extent. The use of PEI had a remarkable improvement on the CO2 adsorption capacity. With the increasing amount of PEI loaded, the CO2 adsorption capacity improved significantly. The highest CO2 adsorption capacity of 2.44 mmol (g sorbent)−1 was obtained at 75 °C in the CO2 partial pressure of 100 kPa for the HPS loaded with 60 wt% PEI. Adsorption/desorption cycles indicated that the PEI-modified HPS sorbents had a good stability and regeneration.

High temperature annealing for structural optimization of silica aerogels in solar thermal applications

Optically transparent, thermally insulating monolithic silica aerogel, with its high solar transmittance and low thermal conductivity, is well-suited for solar thermal applications, particularly concentrated solar power systems. The properties of silica aerogel are directly determined by the structure of the highly porous, interconnected silica network. By using high temperature annealing to control this structure post-synthesis, we were able to optimize the material to increase solar transmittance with minimal effect on the effective thermal conductivity using an easy and scalable method. Samples made from two silica aerogel chemistries and two annealing temperatures (400°C and 600°C) were investigated as a function of annealing time. The annealed samples showed a maximum increase in solar spectral transmittance of over 3% while the effective thermal conductivity (including radiative and conductive contributions) was shown to increase by as much as 40%, indicating a need to optimize the annealing time for maximum performance. The properties of the characterized aerogels were used to demonstrate aerogel annealing optimization using a concentrated solar power receiver model operating at 400°C. The model predicted a maximum receiver efficiency gain of 1% by annealing for 24h at 400°C, representing a significant gain in overall system efficiency.

A Facile Method to Prepare Aligned Porous Magnetic Silica Monoliths from Directional Freeze-Drying

A Facile Method to Prepare Aligned Porous Magnetic Silica Monoliths from Directional Freeze-Drying

Ordered Mesoporous Silica Monoliths as a Versatile Platform for the Study of Magnetic and Photomagnetic Prussian Blue Analogue Nanoparticles

AbstractInvited for the cover of this issue is the group of Anne Bleuzen at Institut de Chimie Moléculaire et des Matériaux d'Orsay, Université Paris‐Saclay, France. The cover image shows the versatility of the chemistry that can be developed in the ordered pores of silica monoliths.

Front Cover: Ordered Mesoporous Silica Monoliths as a Versatile Platform for the Study of Magnetic and Photomagnetic Prussian Blue Analogue Nanoparticles (Eur. J. Inorg. Chem. 10/2017)

Front Cover: Ordered Mesoporous Silica Monoliths as a Versatile Platform for the Study of Magnetic and Photomagnetic Prussian Blue Analogue Nanoparticles (Eur. J. Inorg. Chem. 10/2017)

Variation of surface area of silica monoliths by controlling ionic character/chain length of surfactants and polymers

To achieve maximum pore diameter and surface area of porous silica monoliths different structure directing agents (SDAs) (Cationic/anionic/neutral/mixture) were used. BET study showed alteration of SDA’s chain length, composition or concentration have a cumulative effect on controlling the surface area and pore size. PEG in combination with other SDAs like CTAB, SDS, BTMAC, Triton-X100 etc. controlled internal agglomeration along with particles size. The maximum amount of surface area was observed by using maximum chain length of PEG and CTAB.

Morphological Properties of Methacrylate-Based Polymer Monoliths: From Gel Porosity to Macroscopic Inhomogeneities.

Shaping chemical interfaces of hard and soft matter materials into physical morphologies that guarantee excellent transport properties is of central importance for technologies relying on adsorption, separation, and reaction at the interface. Polymer monoliths with a hierarchically structured pore space, for example, are widely used in flow-driven processes, whose efficiency depends on the morphology of the support material over several length scales. Compared with alternative support structures, particularly silica monoliths, polymer monoliths yield lower efficiency, which suggests a suboptimal morphology. Based on physical reconstruction by serial block-face scanning electron microscopy we evaluate the structural features of a methacrylate-based polymer monolith from the pore scale to the column scale. The morphological data reveal a homogeneous polymer skeleton with a solute-impenetrable core-porous shell architecture and a heterogeneous macropore space that suffers from inhomogeneities at the short-range and the transcolumn scale. Although the morphology of the polymer phase is favorable to efficient mass transport, the performance of the polymer monolith is limited by severe transcolumn gradients in macroporosity and macropore size. We propose to overcome these morphological limitations by pursuing a preparation strategy that involves active rather than passive shaping of the macropore space, for example, by using silica monoliths as templating structures for polymer monolith preparation.

Formulation of Aminosilica Adsorbents into 3D-Printed Monoliths and Evaluation of Their CO2 Capture Performance.

Amine-based materials have represented themselves as a promising class of CO2 adsorbents; however, their large-scale implementation requires their formulation into suitable structures. In this study, we report formulation of aminosilica adsorbents into monolithic structures through a three-dimensional (3D) printing technique. In particular, 3D-printed monoliths were fabricated using presynthesized silica-supported tetraethylenepentamine (TEPA) and poly(ethylenimine) (PEI) adsorbents using three different approaches. In addition, a 3D-printed bare silica monolith was prepared and post-functionalized with 3-aminopropyltrimethoxysilane (APS). Characterization of the obtained monoliths indicated that aminosilica materials retained their characteristics after being extruded into 3D-printed configurations. Adsorptive performance of amine-based structured adsorbents was also investigated in CO2 capture. Our results indicated that aminosilica materials retain their structural, physical, and chemical properties in the monoliths. In addition, the aminosilica monoliths exhibited adsorptive characteristics comparable to their corresponding powders. This work highlights the importance of adsorbent materials formulations into practical contactors such as monoliths, as the scalabale technology platform, that could facilitate rapid deployment of adsorption-based CO2 capture processes on commercial scales.

Mesopore etching under supercritical conditions – A shortcut to hierarchically porous silica monoliths

Hierarchically porous silica monoliths are obtained in the two-step Nakanishi process, where formation of a macro–microporous silica gel is followed by widening micropores to mesopores through surface etching. The latter step is carried out through hydrothermal treatment of the gel in alkaline solution and necessitates a lengthy solvent exchange of the aqueous pore fluid before the ripened gel can be dried and calcined into a mechanically stable macro–mesoporous monolith. We show that using an ethanol–water (95.6/4.4, v/v) azeotrope as supercritical fluid for mesopore etching eliminates the solvent exchange, ripening, and drying steps of the classic route and delivers silica monoliths that can withstand fast heating rates for calcination. The proposed shortcut decreases the overall preparation time from ca. one week to ca. one day. Porosity data show that the alkaline conditions for mesopore etching are crucial to obtain crack-free samples with a narrow mesopore size distribution. Physical reconstruction of selected samples by confocal laser scanning microscopy and subsequent morphological analysis confirms that monoliths prepared via the proposed shortcut possess the high homogeneity of silica skeleton and macropore space that is desirable in adsorbents for flow-through applications.

Enantioselective Transamination in Continuous Flow Mode with Transaminase Immobilized in a Macrocellular Silica Monolith

ω-Transaminases have been immobilized on macrocellular silica monoliths and used as heterogeneous biocatalysts in a continuous flow mode enantioselective transamination reaction. The support was prepared by a sol-gel method based on emulsion templating. The enzyme was immobilized on the structured silica monoliths both by adsorption, and by covalent grafting using amino-functionalized silica monoliths and glutaraldehyde as a coupling agent. A simple reactor set-up based on the use of a heat-shrinkable Teflon tube is presented and successfully used for the continuous flow kinetic resolution of a chiral amine, 4-bromo-α-methylbenzylamine. The porous structure of the supports ensures effective mass transfer and the reactor works in the plug flow regime without preferential flow paths. When immobilized in the monolith and used in the flow reactor, transaminases retain their activity and their enantioselectivity. The solid biocatalyst is also shown to be stable both on stream and during storage. These essential features pave the way to the successful development of an environmentally friendly process for chiral amines production.

Preparation of hierarchically mesoporous silica monolith using two components of poly(ethylene glycol) as cooperative dual-templates

In this work, two components of poly(ethylene glycol) (PEG) are used as dual-templates for facile and cost-effective preparation of hierarchically mesoporous silica monoliths (HMSMs) with controllable mesostructures and co-continuous macrostructures from low-toxicity tetraethoxysilane. The influence of different preparative parameters on the porous structures was studied, including the proportions of PEGs and acidity. The typical HMSM (PEG-1.5/0.5) with skeleton mesopore size of 6.3nm, skeleton thickness of 1.25μm and macropores of 1.87μm can be obtained. The total pore volume and high surface area reach 711m2g−1 and 3.37cm3g−1 (including mesopore volume of 0.93cm3g−1), respectively. The PEG400 and PEG35k are proved to play cooperative roles as dual-templates in harnessing the mesoporous skeletons and co-continuous macrostructures. Interestingly, in such preparation system, the use of PEG400 is found to be play key role in inducing the formation of skeletons with relatively large mesopores, while the amount of PEG35k is a more sensitive factor in shaping the macroporous structures. This work is believed to open a new path to engineering the macroporous and mesoporous structures via the TEOS/PEG/HCl(aq.) synthesis system.

A comparative study of extractant and chromatographic phases for the rapid and sensitive determination of six phthalates in rainwater samples

Six phthalic acid esters were determined in rainwater samples, from which a very low sample volume was collected. This method combines on-line in-tube solid-phase microextraction coupled to high-performance liquid chromatography with a diode-array detector. In order to obtain a short analysis time and to reduce the consumption of organic solvents, two chromatographic phases (C18 monolithic and cyanopropyl silica) are compared. Although three critical pairs are found, faster separation, good resolution and lower pressures are achieved using C18 monolithic column. In order to achieve a simple and sensitive method, two commercial capillaries (a porous polymer with divinylbenzene-4-vinylpyridine and a liquid-phase capillary with 95% poly(dimethylsiloxane)-5% poly(diphenylsiloxane)) are tested for the extraction process. Due to great differences of hydrophobicity among the six phthalates, the selection of a modifier is necessary for a good extraction. The best conditions are achieved using 5 mL of sample containing 40% methanol in a 70 cm-long porous polymer capillary. The procedural blanks are controlled and taken into account in the calculation of the detection limits. Except for dimethylphthalate, the method detection limits are in the range from 0.2 to 0.9 ng mL−1 and the inter-day precision is between 5.3% and 12.5%. The recoveries were within the range of 71%–101%. Rainwater samples are analyzed in order to examine the dilution effect and washout of phthalates in the atmosphere. Dibutyl phthalate is the predominant phthalate found and di-(2-ethylhexyl) phthalate is detected in all analyzed samples.