Production of low-density sodium silicate-based hydrophobic silica aerogel beads by a novel fast gelation process and ambient pressure drying process

We report a method to synthesize low-density transparent mesoporous silica aerogel beads by ambient pressure drying (APD). The beads were prepared by acid–base sol–gel polymerization of sodium silicate in aqueous ammonia solution via the ball dropping method (BDM). To minimize shrinkage during drying, wet silica beads were initially prepared; their surfaces were then modified using trimethylchlorosilane (TMCS) via simultaneous solvent exchange and surface modification. The effects of the volume percentage (%V) of TMCS on the physical and textural properties of the beads were investigated. The specific surface area and cumulative pore volume of the silica aerogel beads increased with an increase in the %V of TMCS. Silica aerogel beads with low packing bed density (0.081 g/cm³), high surface area (917 m²/g), and large cumulative pore volume (2.8 cm³/g) was obtained when 10%V TMCS was used. Properties of the final product were examined by FE-SEM, TEM, BET, and TG–DT analyses. Surface chemical modifications were confirmed by FTIR spectroscopy. The hydrophobic silica aerogel beads were thermally stable up to 411 °C. We discuss our results and compare our findings for modified versus unmodified silica beads.     

Silylation and surface properties of chemically grafted hydrophobic silica

A commercial mesoporous silica (Grace Davison) was chemically grafted with trimethylsilyl chloride (TMSCl) and hexamethyldisilanaze (HMDS). The silylation process brought about some reduction in the specific BET area, the pore volume, and the pore sizes of the samples. Thermogravimetric studies of the silylated samples revealed that the grafting process is kinetically controlled at short reaction times. In the kinetic regime, increasing concentrations of the silylant agent up to 2 wt% in the solvent led to an increase of the extent of the silylated surface, although this limitation disappeared at higher concentrations. Silylation was confirmed by diffuse reflectance infrared Fourier transform (DRIFTS), (29)Si CP-MAS NMR, and photoelectron (XPS) spectroscopic techniques. Solid-state (29)Si MAS-NMR spectra of the silylated samples revealed the presence of -SiCH(3) groups (9.5 ppm) together with two resonances, Q3 (approximately equal to -104 ppm) and Q4 (approximately equal to -114 ppm), coming from siloxane [Qn approximately Si(OSi)n(OH)(4-n), n approximately 2-4] groups, the Q3 signal decreasing upon silylation. The DRIFT spectra of the silylated samples exhibited two well defined bands at 2970 and 2907 cm(-1), due to stretching vibration modes of the C-H bonds in surface -CH(3) groups formed during the silylation process, and also the disappearance of the band at 3740 cm(-1). This observation indicates the complete removal of terminal and geminal hydroxyl groups by grafting with the silylating agent. Similarly, high-resolution photoelectron spectra of the Si2p core levels showed a high binding-energy component (103.5 eV) in all the samples, coming from the Si coordinated with oxide anions in SiO(2), together with a second component at 102.1 eV, which is the fingerprint of Si coordinated by oxide anions and an organic group. Finally, the samples were ranked according to their hydrophobicity, as determined from the temperature-programmed desorption profiles of adsorbed water and 2-methylbutane.     

Mesoporous and biocompatible surface active silica aerogel synthesis using choline formate ionic liquid

In this paper, we report the preparation and characterization of mesoporous and biocompatible transparent silica aerogel by the sol-gel polymerization of tetraethyl orthosilicate using ionic liquid. Choline cation based ionic liquid allows the silica framework to form in a non collapsing environment and controls the pore size of the gel. FT-IR spectra reveal the interaction of ionic liquid with surface -OH of the gel. DSC thermogram giving the evidence of confinement of ionic liquid within the silica matrix, which helps to avoid the shrinkage of the gel during the aging process. Nitrogen sorption measurements of gel prepared with ionic liquid exhibit a low surface area of 100.53 m2/g and high average pore size of 3.74 nm. MTT assay proves the biocompatibility and cell viability of the prepared gels. This new nanoporous silica material can be applied to immobilize biological molecules, which may retain their stability over a longer period.     

Facile method for surface etching of silica aerogel monoliths

Degradation of three different endocrine disruptors (EDs) was thoroughly studied on prepared durable thin layers of titanium dioxide with an anatase crystalline structure. Specially constructed laboratory reactors bringing information on all individual processes (photolysis, photocatalysis, sorption) involved in decomposition of the studied EDs (17α-ethynylestradiol, bisphenol A and 4-nonylphenol) were applied. It was found that photolytic removal of EDs is the fastest degradation process; nevertheless, this method may be less effective regarding all indicators including toxicity. It was verified that individual degradation processes (photolysis and photocatalysis) showed a significantly different influence on toxicity of resulting solutions. During the photolytic process, EDs degradation caused increasing toxicity contrary to the photocatalytic process. Obtained results were corroborated by a mathematical model, which showed that a limitation step for photocatalysis is a sorption and for photolysis a toxicity of resulting products.

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Synthesis of silver nanoparticles supported on silica aerogel using gamma radiolysis

Silver clusters on SiO₂ support have been synthesized using ⁶⁰Co gamma radiation. The irradiation of Ag⁺ in aqueous suspension of SiO₂ in the presence of 0.2 mol dm⁻³ isopropanol resulted in the formation of yellow suspension. The absorption spectrum showed a band at 408 nm corresponding to typical characteristic surface plasmon resonance of Ag nanoparticles. The effect of Ag⁺ concentration on the formation of Ag cluster indicated that the size of Ag clusters vary with Ag⁺ concentration, which was varied from 4×10⁻⁴ to 5×10⁻³ mol dm⁻³. The results showed that Ag clusters are stable in the pH range of 2–9 and start agglomerating in the alkaline region at pH above 9. The effect of radiation dose rate and ratio of Ag⁺/SiO₂ on the formation of Ag clusters have also been investigated. The prepared clusters have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), which showed the particle size of Ag clusters to be in the range of 10–20 nm.     

纳米二氧化硅的表面改性研究

以γ-缩水甘油醚丙基三甲氧基硅烷(GPTMS)对酸催化水解正硅酸乙酯(TEOS)聚合得到的纳米二氧化硅胶粒表面进行接枝改性,用激光粒径仪测定二氧化硅颗粒的粒径,并用透射电子显微镜(TEM)观察了改性前后二氧化硅胶粒的分散状况,采用傅立叶红外(FTIR)光谱法对改性前后的二氧化硅粉体进行了分析,通过热失重分析(TGA)法对GPTMS接枝改性二氧化硅胶粒表面的接枝度进行分析计算,同时对颗粒溶胶的ζ电位进行了测试,结果表明:改性后二氧化硅胶粒分散性大大提高,硅烷偶联剂浓度对接枝度有显著影响,当GPTMS的浓度为1mL/S iO2(g)时,接枝度达到最大,且颗粒表面的物理化学性能发生显著变化。     

NIR studies of the surface modification in silica fillers

Methods of estimating the degree of condensation of the surface silanol groups of silica due to its modification by silane coupling agents are reported.Also, a procedure for estimating the surface silanol groups for the pre- and post-modified silicas for the NIR 7326 cm^–1 band is given.Using electron microscope studies and heats of immersion of silica surfaces, the silane effect on agglomeration of silica particles and, thus, on the physicochemical properties of its surface has been demonstrated.     

Selective complexometric determination of mercury using sodium nitrite as masking agent

A complexo-titrimetric method for the determination of mercury(II) in the presence of other metal ions is described based on the selective masking ability of sodium nitrite. Mercury and other ions are initially complexed with an excess of EDTA and the surplus EDTA is titrated with Pb(NO₃)₂ solution at pH 5.0–6.0 using xylenol orange as indicator. An excess of solid NaNO₂ is then added to decompose the Hg(II)-EDTA complex and the released EDTA is titrated with standard Pb(NO₃)₂ solution. Accurate results were obtained for 10–65 mg of mercury with relative errors <0.3% and standard deviations < 0.03 mg. Sn(IV) and Pd(II) interfere but can be easily masked. The method is applied for the determination of Hg(II) in its alloy compositions and complexes.     

Sodium and potassium silicate-based catalysts prepared using sand silica concerning biodiesel production from waste oil

Heterogeneous catalysts, named SPS (sodium potassium silicates), were synthesized with an alternative silica (MPI silica) obtained from beach sand. In this work, the MPI was modified with NaOH and KOH producing silicate-based catalyst for biodiesel synthesis from waste cooking oil (WCO). The obtained catalyst was characterized by XRD, CO₂-TPD, the Hammett basicity test, XRF, FESEM, EDX, FTIR and TG/DTG. The results confirmed the presence of K₂O/Na₂O oxides and their silicates, the main active sites responsible for the catalytic action. CO₂-TPD and the Hammett basicity data suggested the presence of weak, medium and strong basic sites. Biodiesel yield was about 92% and the SPS catalyst was reused for five cycles. The biodiesel conversion by NMR ¹H was about 93.89%. The DTG deconvolution revealed the decomposition of four typical biodiesel compounds (R² = 0.9987). The method applied for the WCO biodiesel production using SPS catalyst represents an environmentally friendly process, based on low-cost material and reuse of waste biomass.     

A superhydrophobic silica aerogel with high surface area for needle trap microextraction of chlorobenzenes

Microchimica Acta - The authors have synthesized a superhydrophobic silica aerogel by using a sol-gel technique. The material is shown to be an efficient sorbent for needle trap microextraction of...     

Synthesis of rod-like mesoporous silica with hexagonal appearance using sodium silicate as precursor

Using sodium silicate as precursor, rod-like mesoporous silica with hexagonal appearance was synthesized by controlling the pH value of a mixed micelles solution of cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) during hydrolysis of ethyl acetate. The resulting mesoporous silica was characterized by small angle X-ray diffraction, nitrogen gas adsorption-desorption measurement and scanning electron microscopy. Results showed that the regular rod-like shapes with hexagonal appearance were obtained at a 9:1 molar ratio of CTAB to CTAC, and that the amounts and lengths of the rod-like mesoporous silica particles decreased with decreasing CTAB to CTAC molar ratio. There existed a type IV adsorption isotherm and an H1 hysteresis loop in N₂ gas adsorption-desorption curves.     

The influence of using sodium dithionite as SARA agent in miniemulsion ATRP

The supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) of butyl acrylate (nBA) and styrene (Sty) using miniemulsion is reported for the first time. The polymerization was carried out in the presence of Cu(II)Br₂‐based catalytic systems using EHA₆TREN or BPMODA* and sodium dithionite (Na₂S₂O₄) as SARA agent. Kinetic data revealed a controlled polymerization for both monomers, with a very stringent control over the molecular weight distribution (Ð ≤ 1.2) but low monomer conversion. Reaction conditions were optimized in an attempt to understand the kinetics of polymerization, aiming to increase the final monomer conversion while maintaining the control over the polymerization. Self‐chain extension reactions revealed low chain end fidelity, which corroborate the impossibility of increasing monomer conversion even after the judicious variation of the main polymerization parameters (monomers, surfactant, deactivator, and Na₂S₂O₄ concentrations, and its method of addition). The data presented suggest a particular feature of Na₂S₂O₄ in these polymerization systems involving the significant formation of dead chains ends, which has never been observed for any other reported system using this SARA agent. In contrast, control ARGET miniemulsion experiments under the same reaction conditions and using ascorbic acid as reducing agent allowed to achieve high monomer conversions. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 879–888     

Synthesis and surface modification of mesoporous mcm-41 silica materials

Surface modification offers a great opportunity to adjust both the pore diameter and surface properties of MCM-41 type organic–inorganic hybrid materials which result in materials of improved hydrothermal and mechanical stability. Therefore, MCM-41 silica, surface modified with organic ligands, are promising systems with engineered properties and attractive for advanced applications. In the present study, after optimization of the reaction conditions highly ordered MCM-41 silica spheres with uniform mesopores were prepared by the pseudomorphic transformation route. The effect of functionality and alkyl chain length of the alkyl ligands during surface modification was probed by using butyl and octylsilanes with two different functionalities. Due to steric hindrance, the longer chains are assumed to bind only on the outer silica surface and near the entrance of the pores, while the shorter chains are also able to bind to the interior mesopore walls. The resulting materials were comprehensively characterized before and after surface modification using nitrogen sorption techniques, XRD, SEM, solid-state NMR spectroscopy and FTIR spectroscopy. From chromatographic test measurements it was found that the separation power primarily depends on surface coverage and alkyl chain length. On the basis of the present data, surface modified mesoporous silica of MCM-41 type are very promising candidates for future chromatographic applications.     

Salt effect on surface modification of silica by interpolyelectrolyte complexes

The effect of a low-molecular-mass salt on the properties of interpolyelectrolyte complexes formed as a result of interactions between poly(diallyldimethylammonium chloride) and copolymers of maleic acid with propylene or α-methylstyrene in their salt containing non-stoichiometric mixtures has been studied. Properties of such interpolyelectrolyte complexes were found to be determined by the chemical nature of the polyelectrolytes and by the salt concentration. The effect of salt on the surface modification of silica particles via their interactions with interpolyelectrolyte complexes has been examined. Two different ways of the surface modification of silica particles were used: (i) silica particles were contacted with previously prepared interpolyelectrolyte complexes and (ii) silica particles were contacted with cationic polyelectrolyte at first and then anionic polyelectrolyte was added. The efficiency of the surface modification was shown to be also dependent on the salt concentration and the chemical nature of polyelectrolytes. Turbidimetry, quasi-elastic light scattering, laser microelectrophoresis, and polyelectrolyte titration were used to characterize studied systems.     

Maghemite nanocrystal impregnation by hydrophobic surface modification of mesoporous silica

Here, we report the design of a hybrid inorganic/organic mesoporous material through simultaneous pore engineering and hydrophobic surface modification of the intramesochannels to improve the uptake of superparamagnetic maghemite nanocrystals via impregnation techniques. The mesoporous material of the SBA-15 type was functionalized in situ with thiol organo-siloxane groups. Restricting the addition of the thiol organo-siloxane to 2 mol % yielded an inorganic/organic hybrid material characterized by large pores and a well-ordered hexagonal p6mm mesophase. The hydrophobic surface modification promoted the incorporation of 7.5 nm maghemite (gamma-Fe2O3) nanocrystals, prepared through temperature-controlled decomposition of iron pentacarbonyl in organic solvents. The hydrophobic, oleic acid capped superparamagnetic maghemite nanocrystals were incorporated into the porous network via wet impregnation from organic suspensions. Combining diffraction, microscopy, and adsorption data confirmed the uptake of the nanocrystals within the intramesochannels of the silica host. Magnetization dependencies on magnetic field at different temperatures show a constriction in the loop around the origin, which indicates immobilization of maghemite nanocrystals inside the thiol-functionalized silica host.     

Ketoimino groups as silica surface modifiers

The presented work is devoted to Porasil C silica, with organic compounds bonded to its surface and capable of electron-donor-acceptor (EDA) interactions. These packings are a good base for studying interactions among stationary phases and the adsorbate molecules showing electron-donor properties. The presented work concentrates on the phases containing ketoimino groups at their surface. Copper and chromium chlorides were bonded through these to the surface. Physicochemical characteristics of the obtained packings were determined by the use of elemental analysis, differential scanning calorimetry (DSC) and inverse gas chromatography. We examined the influence of the surface modification on the retention parameters of the nucleophilic compounds.     

Study on mono-dispersed nano-size silica by surface modification for underfill applications

In order to improve the rheological behavior of the nanosilica composite no-flow underfill, filler surface treatment using silane coupling agents was investigated to reduce the filler-filler interaction and to achieve the mono-dispersity of the nanosilica in the underfill resin. The experimental conditions of the surface treatment were investigated in a design of experiment (DOE) in terms of the pre-treatment methods, coupling agent types, concentrations, and treatment durations. The particle dispersion after treatment was evaluated by the laser particle analyzer and the transmission electron microscopy (TEM). A mono-dispersed nanosilica solution in the polar medium was achieved using optimal experimental condition. The surface chemistry of the nanosilica was studied using Fourier transformed infrared spectroscopy (FTIR). The wettability of underfill resin and water on the silane treated glass slides was studied using a goniometer. Based on the investigations, the silane-treated nanosilica fillers were incorporated into an underfill resin to formulate a nanocomposite no-flow underfill. It was found that the proper filler treatment could significantly reduce the viscosity of the nanocomposite.     

Synthesis of nanoporous silica aerogel by ambient pressure drying

A crack-free silica aerogel monolith was fabricated from a cheap water glass derived silicic acid solution by adding glycerol, which served as a drying control chemical additive (DCCA). The OH surfaces of the wet gel with glycerol were modified using a TMCS/n-hexane mixture followed by solvent exchange from water to n-hexane. The obtained surface modified wet gel was dried at 75 °C under ambient pressure. The addition of glycerol appears to give the wet gel a more homogeneous microstructure (larger pore size and uniform size distribution) as well as enhanced stiffness. However, glycerol also retards surface modification and solvent exchange. The aerogel synthesized with glycerol added to the silica sol maintained a relatively low bulk density compared with the aerogels aged in a mixed ethanol (EtOH)/TEOS solution. The reproducibility of aerogel production was further improved in the aerogel synthesized with glycerol added to the silica sol and aged in a 70%EtOH/30%TEOS solution.