Acid-base Catalyzed Sol-gel Process Research Articles

Synthesis and Characterization of Erbium-Doped Silica Films Obtained by an Acid-Base-Catalyzed Sol-Gel Process.

Erbium-doped silica films were synthesized using a two-step sol-gel methodology that involved acid and base catalysts, with erbium concentration ranging from 0.2% to 6% and annealing temperatures varying from 500 °C to 900 °C. The photoluminescence spectra showed that the samples exhibiting efficient emission were annealed at 800 °C and 900 °C and doped with 3% and 6% erbium. The X-ray diffraction analysis revealed that the internal structure of the films was influenced by the different annealing temperatures and the doping concentrations. Samples with dominant 4f transitions were modelled. The results suggest that the proposed method is a promising approach for the synthesis of erbium-doped silica films with potential applications in optical devices.

Long-Term Performance and Stability of Interlayer-Free Mesoporous Silica Membranes for Wetland Saline Water Pervaporation.

Wetland water is an alternative water resource around wetland areas. However, it is typically saline due to seawater intrusion and contains high natural organic matter (NOM) that is challenging to treat. This study evaluated the stability of interlayer-free mesoporous silica matrix membranes employing a dual acid–base catalyzed sol–gel process for treatment of saline wetland water. The silica sols were prepared under a low silanol concentration, dip-coated in 4 layers, and calcined using the rapid thermal processing method. The membrane performance was initially evaluated through pervaporation under various temperatures (25–60 °C) using various feeds. Next, the long-term stability (up to 400 h) of wetland saline water desalination was evaluated. Results show that the water flux increased at higher temperatures up to 6.9 and 6.5 kg·m−2·h−1 at the highest temperature of 60 °C for the seawater and the wetland saline water feeds, respectively. The long-term stability demonstrated a stable performance without flux and rejection decline up to 170 h operation, beyond which slow declines in water flux and rejection were observed due to fouling by NOM and membrane wetting. The overall findings suggest that an interlayer-free mesoporous silica membrane offers excellent performance and high salt rejection (80–99%) for wetland saline water treatments.

Silica Aerogel as Super Thermal and Acoustic Insulation Materials

Silica aerogels are light weight, nanostructured, and highly porous materials with an open pore structure. Due to their excellent characteristics, such as extremely low thermal conductivity, low density and high porosity, the silica aerogels become promising potential adsorbents, catalysts, thermal insulation, and acoustic absorption materials for environmental purposes. This paper presents the synthesis of a highly flexible polymer modified silica aerogel with the use of a cellulose-methyltriethoxysilane (MTES) precursor in a two-step acid-base catalyzed sol-gel process. The physical properties of the resulting aerogels were characterized by thermogravimetry, scanning electron microscopy, nitrogen adsorption-desorption, contact angle, thermal conductivity measurements, compression testing and Fourier transform infrared spectroscopy. The fabricated aerogel exhibited high flexibility with a Young’s modulus of compression of 0.33 MPa and the density of 0.132 g/cm3. They were hydrophobic in nature and had low thermal conductivity. Preparation of aerogel with solid waste (fly ash/bottom ash) is also discussed. The preliminary results showed that the materials have great potential for environmental application, i.e. enhancement of solid waste recycling rate by converting waste to high value-added materials, super thermal and acoustic insulation materials in green building and removal of oil spilled into surface drainage.

Sol-gel derived flexible silica aerogel as selective adsorbent for water decontamination from crude oil

Oil spills are the most important threat to the sea ecosystem. The present study is an attempt to investigate the effects of sol-gel parameters on seawater decontamination from crude oil by use of flexible silica aerogel. To this goal, methyltrimethoxysilane (MTMS) based silica aerogels were prepared by two-step acid-base catalyzed sol-gel process, involving ambient pressure drying (APD) method. To investigate the effects of sol-gel parameters, the aerogels were prepared under two different acidic and basic pH values (i.e. 4 and 8) and varied ethanol/MTMS molar ratios from 5 to 15. The adsorption capacity of the prepared aerogels was evaluated for two heavy and light commercial crude oils under multiple adsorption-desorption cycles. To reduce process time, desorption cycles were carried out by using roll milling for the first time. At optimum condition, silica aerogels are able to uptake heavy and light crude oils with the order of 16.7 and 13.7, respectively.

Development of hydrophobic and optically transparent monolithic silica aerogels for window panel applications

In the present paper, highly transparent, monolithic and hydrophobic silica aerogel monoliths were prepared by using tetraethoxysilane (TEOS) as precursor and methyltriethoxysilane (MTES) as co-precursor with different MTES/TEOS molar ratios (M) by two-step acid–base catalyzed sol–gel process followed by supercritical alcohol drying. The molar ratio of TEOS, ethanol (EtOH), water (0.001 M oxalic acid catalyst) and ammonium hydroxide (1 M NH4OH) was kept constant at 1:5:3.5:3.5 respectively, while the molar ratio of MTES/TEOS (M) was varied from 0 to 0.75. It has been observed that as the M value increases, the gelation time increases. It has been found that lower (0.25) M values resulted in highly transparent (optical transmission >90 % for a 10 mm thick sample at 700 nm wavelength) and negligible volume shrinkage ( 10 % volume shrinkage but excellent hydrophobicity. A good compromise of acceptable optical transmittance (~87 % optical transmission at 700 nm wavelength for a 10 mm thick sample) with negligible volume shrinkage (6 %) were obtained at M = 0.50. The hydrophobicity of the aerogels was tested by measuring the contact angle between a water droplet and silica aerogel surface. The hydrophobicity was confirmed by Fourier transform infra-red spectroscopy and contact angle measurements. the aerogels have been characterized by percentage of volume shrinkage, optical transmittance, scanning electron microscopy, thermal conductivity measurements and thermogravimetric analysis and differential thermal analysis.

Preparation and characterization of hydrophobic silica aerogel sphere products by co-precursor method

In the present paper, silica aerogel balls were prepared using methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) co-precursor with different MTES/TEOS molar ratio (I) by two-step acid-base catalyzed sol–gel process and molding technology followed by the carbon dioxide supercritical drying. The physical properties of various silica aerogels whose I varied from 0 to 1.0 were studied by BET and SEM. Approving aerogel ball was obtained by using acetone as exchanging solvent at I of 0.8. Better properties are less crack, little shrinking percentage (17%), low apparent density (0.103 g/cm3), large surface area (996.35 m2/g) and high pore volume (3.32 cm3/g). Characterized by contact angle measurements and thermal stability, the characterizations of aerogel spheres were strongly affected by the MTES/TEOS molar ratio. Hydrophobic property increased with the increase in I value, and silica aerogels at I = 0.8 and 1.0 have a superhydrophobic characterization with the highest contact angle (152°). The TG-DTA analysis shows that the silica aerogel spheres transform hydrophobic to hydrophilic at around 450 °C, which is due to oxidation of Si–CH3 to Si–OH groups. The surface chemical modification was confirmed by FT-IR spectrums, which demonstrated that Si–CH3 groups be beneficial to molding of silica aerogels.

Preparation and characterization of silica composite xerogels with broadband absorption in mid- and far-infrared windows

The silica composite xerogels with broadband absorption in mid- and far-infrared windows have been synthesized by a two-step acid-base catalyzed sol–gel process and freeze drying technique using water glass, triethylamine hydrochloride and 10nm γ-Al2O3 as the silica source, mid-infrared absorbent and far-infrared absorbent respectively. The structure and property of the composite xerogels were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 physisorption (at 77K) and Fourier-transform infrared spectroscopy (FT-IR). Results show that the triethylamine hydrochloride crystallized adhering to the whole three-dimensional network of the silica xerogel substrate, while the γ-Al2O3 agglomerated into spheres or particles. The specific surface area, total pore volume and average pore diameter of the silica xerogel substrate are about 158.2m2/g, 0.7030cm3/g and 17.78nm respectively. The specific surface area, total pore volume and average pore diameter of the composite xerogels are about 28.15–274.6m2/g, 0.1146–0.6571cm3/g and 9.572–17.08nm respectively. The composite xerogels have the characteristic of broadband absorption in mid- and far-infrared windows. The relative absorption intensity in mid- or far-infrared windows is enhanced in proportion with the increase of the triethylamine hydrochloride and γ-Al2O3 respectively.

Substituent effects on the glass transition phenomena of polyorganosilsesquioxane particles prepared by two-step acid-base catalyzed sol-gel process

Polyorganosilsesquioxanes (RSiO3/2) (R: organic group) particles with different organic groups such as alkyl and aryl groups were prepared using a two-step acid–base catalyzed sol–gel process. Spherical particles were formed at room temperature using the two-step acid–base catalyzed sol–gel process in which trifunctional organoalkoxysilanes with R = methyl, n-propyl, n-butyl, phenyl, and benzyl were used as starting materials. Polyorganosilsesquioxanes with longer alkyl groups or phenethyl group prepared by the two-step acid–base catalyzed sol–gel process were oily material at room temperature. Polyorganosilsesquioxanes with shorter alkyl or aryl groups exhibited the glass transition. In contrast, polyorganosilsesquioxanes with longer alkyl groups showed no glass transition behavior. The occurrence of glass transition phenomena of polyorganosilsesquioxanes was concluded to be strongly dependent on the chain length of organic substituents.

Powerful adsorption of silver(I) onto thiol-functionalized polysilsesquioxane microspheres

The strong adsorbability of Ag(I) ions onto poly(3-mercaptopropylsilsesquioxane) (PMPSQ) microspheres synthesized through a two-step acid–base catalyzed sol–gel process method was systematically examined. The effect of adsorption time, initial Ag(I) concentration, and solution pH was studied to optimize the Ag(I) adsorbability of PMPSQ microspheres. The PMPSQ microspheres demonstrate a powerful Ag(I) adsorbability with an adsorptivity above 99.99% when the initial Ag(I) concentration is lower than 10 mM and the highest Ag(I) adsorbance of 1140 mg/g at an initial Ag(I) concentration of 150 mM. Adsorption phenomena appeared to follow Langmuir isotherm. The kinetic studies indicated that the adsorption process well fits the pseudo-second-order kinetics with a very rapid initial adsorption rate of 15.28 mg g −1 min −1. The appropriate solution pH for Ag(I) adsorption is around 2.0–5.4. The PMPSQ microspheres demonstrate a promising application in the removal of Ag(I) ions from aqueous solutions.

Studies on rheological properties of methyltriethoxysilane (MTES) based flexible superhydrophobic silica aerogels

The flexible and superhydrophobic properties of silica aerogels are extremely important requirements for their long-term applications. Therefore, the present paper deals with the synthesis and characterization of flexible and superhydrophobic silica aerogels utilizing a two-step acid–base catalyzed sol–gel process with a novel precursor, methyltriethoxysilane (MTES), followed by supercritical drying. Solvent and catalysts used for the synthesis were methanol (MeOH), oxalic acid (C 2H 2O 4) and ammonium hydroxide (NH 4OH), respectively. Silica alcosol was prepared by varying the MeOH/MTES molar ratio (S) from 6.45 to 19.35 by keeping the each of oxalic acid (0.001 M)/MTES and maintaining a constant ammonium hydroxide (10 M)/MTES molar ratio at 4. It was observed that the Young’s modulus ( Y) decreased from 15.03 × 10 4 to 3.95 × 10 4 N/m 2 with an increase in S value from 6.45 to 19.35. Utilizing constant molar ratio of MeOH/MTES at 19.35, experiments produced monolithic, less shrinkage and flexible aerogels. The effect of various sol–gel parameters such as acid and base catalysts concentration and gel-aging on the flexibility and hydrophobicity of the aerogels were investigated. The aerogels have been characterized by bulk density, percentage of volume shrinkage, Young’s modulus, elastic limit measurements, strain energy, Poisson’s ratio and contact angle measurements. The microstructural studies were carried out using Transmission Electron Microscopy. The hydrophobicity was confirmed by Fourier Transform Infrared (FTIR) spectroscopy.

The fabrication of microstructure surface of super-hydrophobic coating by surface gelation technology

The microstructured surface of materials were fabricated by a two-step acid-base catalyzed sol-gel process. In fluorinated polymer with PTFE doping, the well-proportioned composite sols were prepared using sol-gel processing under the hydrochloric acid and deficiency of water conditions. After the substrate was coated by composite sols, and the gelation treatment on the surface of composite coating, the micrometer-scale and nanometer-scale hierarchical structures were formed in surface layer of material. XPS and TEM technologies were employed to identify that the gelation occurs just on the surface of composite coating. The morphology of coating surface was observed by SEM and AFM technologies. The microstructured surface of material can be fabricated using this inexpensive and easily controlled method on low surface energy resin materials, the super-hydrophobic coatings materials can be prepared.

Methyltriethoxysilane: New precursor for synthesizing silica aerogels

Traditionally, silica aerogels are synthesized using three silicon alkoxides, namely, tetraethoxysilane (TEOS), tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) for various applications in science as well as in technology. Among all these precursors, only MTMS-based silica aerogels are inherently superhydrophobic with so far reported, the highest contact angle of 173°. In the present paper, we reported a new precursor, namely, methyltriethoxysilane (MTES) for synthesizing the silica aerogels having the novel properties as that of MTMS-based aerogels. The aerogels have been prepared using the MTES by two-stage acid–base catalyzed sol–gel process followed by supercritical drying. The solvent and catalysts used for the synthesis were methanol (MeOH), oxalic acid (C 2 H 2 O 4 ) and ammonium hydroxide (NH 4 OH), respectively. The aerogels of different densities were obtained by varying the molar ratio of MeOH/MTES ( S ) from 6.45 to 19.35. In order to get good quality aerogels in terms of low density, high contact angle and less volume shrinkage, the oxalic acid ( A ) and NH 4 OH ( B ) concentrations were varied from 0 to 1 and from 2 to 13.36 M, respectively. Monolithic aerogels have been obtained for the values of A = 0.01 M and B = 13.36 M. Simultaneously, the aerogels are superhydrophobic with contact angle as high as 163°. Furthermore, the effects of molar ratio of H 2 O/MTES ( W 1), i.e. acidic water and H 2 O/MTES ( W 2), i.e. basic water on the physical properties of the aerogels have also been studied. The molar ratio of MTES:MeOH:acidic water:basic water was optimized at 1:19.35:3.57:3.57, respectively. The aerogel thermal stability was studied by TGA–DTA while the hydrophobicity was quantified in terms of the contact angle measurements and FTIR studies. The as-prepared aerogels have been characterized by bulk density, porosity, volume shrinkage, thermal conductivity, contact angle measurements, transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. The physical properties of the aerogels have been explained by taking into account of sol–gel reactions and the gel network formation.

Glass transition and thermal softening of poly(phenylsilsesquioxane) particles prepared using two-step acid–base catalyzed sol–gel process

Spherical poly(phenylsilsesquioxane) (PhSiO3/2) particles prepared using a two-step acid–base catalyzed sol–gel process had a ‘core-shell’ structure: their inside and outside parts were composed respectively of PhSiO3/2 species with low and high molecular weights. The PhSiO3/2 particles prepared using the two-step acid–base catalyzed sol–gel process exhibited two glass transition temperatures (Tg). The PhSiO3/2 species in the inside of the particles showed glass transition behavior at a lower temperature than those in the outside of the particles, indicating that Tg is mainly dependent on the materials’ respective molecular weights.

Structure of Polyphenylsilsesquioxane Particles Prepared by Two-Step Acid-Base Catalyzed Sol–Gel Process and Formation of Hollow Particles

Spherical polyphenylsilsesquioxane (PhSiO(3/2)) particles, one of the inorganic-organic hybrid materials, were synthesized by a two-step acid-base catalyzed sol-gel process, and hollow particles were successfully prepared by washing the as-prepared particles with organic solvents. It was found that the inside and outside parts of the as-prepared particles were composed of PhSi03/2 species with relatively low and high molecular weight, respectively, i.e., the PhSiO(3/2) particles had a kind of "core-shell" structure. Because the core portion in the as-prepared particles was soluble in ethanol and tetrahydrofuran, hollow particles were obtained through washing the as-prepared PhSiO(3/2) particles with ethanol or tetrahydrofuran. Furthermore, the molecular weight of the as-prepared particles was varied by the concentration of phenyltriethoxysilane used as a starting alkoxide. As a result of the variation of the molecular weight, the hollow PhSiO(3/2) particles with different stabilities against organic solvents were formed.

Preparation and Characterization of Polyaminophenylsilsesquioxane Particles by Two-step Acid–Base-catalyzed Sol–Gel Process

Abstract Spherical polyaminophenylsilsesquioxane (H2NC6H4SiO3/2) particles were prepared by a two-step acid–base-catalyzed sol–gel process, and we found that the particles showed the glass transition behavior. Amino groups in the H2NC6H4SiO3/2 particles were partially changed to amido ones by acetylation using acetic anhydride.

Thermoplastic and thermosetting properties of polyphenylsilsesquioxane particles prepared by two-step acid-base catalyzed sol-gel process

Polyphenylsilsesquioxane (PhSiO3/2) particles were synthesized by a two-step acid-base catalyzed sol-gel process, in which phenyltriethoxysilane used as a starting alkoxide was hydrolyzed and condensed in the first step under two different acid-catalyzed conditions. The PhSiO3/2 particles prepared under an acid-catalyzed process without using a solvent were thermally hardened after the first heating up to 200°C. In contrast, the PhSiO3/2 particles prepared under an acid-catalyzed process with ethanol as a solvent were thermally softened with the repeated heating processes up to 200°C. From the gel permeation chromatography measurements, it was found that the average molecular weight of the particles prepared with the use of ethanol was decreased in comparison with that of the particles prepared without using ethanol. The addition of ethanol in the sols during hydrolysis and condensation reaction under the acid-catalyzed process caused the decrease in the average molecular weight, which should result in the changes of thermal property of PhSiO3/2 particles from thermosetting to thermoplastic.

Effects of Phenyltriethoxysilane Concentration in Starting Solutions on Thermal Properties of Polyphenylsilsesquioxane Particles Prepared by a Two-Step Acid-Base Catalyzed Sol-Gel Process

Spherical polyphenylsilsesquioxane particles were prepared by a two-step acid-base catalyzed sol-gel process, in which various amounts of ethanol were used as a solvent to vary the concentration of phenyltriethoxysilane (PhSi(OEt)3) used as a starting alkoxide. The decrease in the PhSi(OEt)3 concentration led to the decrease in the average molecular weight and glass transition temperature of the polyphenylsilsesquioxanes. Thus, the change of the PhSi(OEt)3 concentration in the preparation of polyphenylsilsesquioxane particles by the two-step acid-base catalyzed sol-gel process is an effective means to control the glass transition temperature of the particles.

Preparation and characterization of poly(phenylsilsesquioxane) particles by two-step acid-base catalyzed sol-gel process

Preparation and characterization of poly(phenylsilsesquioxane) particles by two-step acid-base catalyzed sol-gel process

Synthesis of Highly Porous Organic/Inorganic Hybrids by Ambient Pressure Sol-Gel Processing

This paper reports the synthesis of highly porous organic/inorganic hybrids by a two-step acid-base catalyzed sol-gel process and ambient pressure drying. In the method organic and inorganic precursors are copolymerized so as to incorporate organic ligands into the solid network. The two-step acid-base catalyzed process was used to prevent phase segregation during the hydrolysis and co-condensation of organic and inorganic precursors. The organic ligands incorporated into the solid gel network modify the surface chemistry. Thus, the wetting angle is significantly increased so that the collapse of the gel network is greatly reduced upon the removal of pore fluid during drying. Organic/inorganic hybrids with BET surface areas above 1250 m2/g, porosities above 75% and pore sizes of ∼8 nm have been synthesized.