Acidic Silica Sol Research Articles

Preparation of High-Temperature Resistant Aerogels by Incorporating Aluminum Sol into Composite Silica Sources Using Ambient Pressure Drying.

To reduce production costs and enhance the high-temperature resistance of SiO2 aerogels, an aluminum-doped silica aerogel (ASA) was successfully prepared using the sol-gel method and atmospheric drying method. The composite silica sources included TEOS and inexpensive acidic silica sol, while the aluminum source was aluminum sol. The study investigated the influence of the molar ratio of acidic silica sol to TEOS, Al/Si, and calcination temperature on the composition, structure, and high-temperature resistance of the ASA. The results indicate that a sample with an acidic silica sol to TEOS molar ratio of 0.8 achieved a specific surface area of 683.204 m2·g-1. The Al/Si molar ratio significantly impacted the high-temperature resistance of the ASA, with the sample having a molar ratio of 0.02 Al/Si displaying the highest specific surface area of 705.956 m2·g-1 at 600 °C. Moreover, this surface area remained at 273.099 m2·g-1 after calcination at 1000 °C, notably higher than the sample without aluminum sol (16.082 m2·g-1). Mechanism analysis indicated that the addition of aluminum sol to the SiO2 aerogel inhibited phase transitions, and both acidic silica sol and aluminum sol particles enhanced the aerogel structure, contributing to a marked improvement in high-temperature resistance.

Fabrication of Flexible Transparent Conductive Films via an Energy-Efficient In Situ Chemical Welding and Reinforcement Strategy.

Recently, flexible transparent conductive films composed of metal nanowires have received significant interest, particularly for flexible electronics. However, the high contact resistance among metal nanowires and the weak bonding effect between metal nanowires and substrates often result in films whose conductivity, adhesion, and flexibility fall short of the stringent requirements demanded by real-world uses. We developed a simple method to fabricate high-performance flexible transparent conductive films via successively spin-coating silver nanowires (AgNWs) and acidic silica sol onto the surface of the substrate. Under the capillary force of ethanol and the etching effect of hydrochloric acid, the adjacent AgNWs are induced to chemically weld in situ to form a stable network. The resulting composite film exhibits a sheet resistance of only 8.5 Ω/sq, marking an impressive 80% decrease compared with the pristine AgNW film. Meanwhile, the silica sol acts as a filler, improving light transmittance while further reinforcing the network structure and firmly bonding it to the substrate. Thus, the delamination of the nanowires under bending motion is effectively inhibited, and the resulting film was endowed with resistance remaining below 15 Ω/sq after 3000 bending and 200 tape peeling. The energy-efficient in situ chemical welding and reinforcement method for nanowires provides an innovative strategy for the batch preparation of flexible transparent conductive films.

Improved Sublimation Resistance of Skutterudites by Silane Modified Ceramic-Based Coating

The organic/inorganic hybrid sol was prepared by hydrolysis polymerization of methyl-triethoxysilane (MTES) with acidic silica sol. Zirconium silicate, zircon sand and refractory fiber were respectively added to the hybrid sol to increase the stability of the coating in high temperature service. P-type skutterudite(SKD) protective coating obtained by multi-layer coating and certain pretreatment. Protective effect was studied by observing the diffusion of antimony from the SKD into the coating. The results show that the coating has the advantages of simple process and high strength and high stability after aging. The sublimation of antimony is suppressed and it is expected to be applied to the protection of SKD in vacuum.

TEAOH‐Templated SAPO‐34 Zeolite with Different Crystallization Processes and Silicon Sources: Crystallization Mechanism and MTO Performance

SAPO‐34 zeolites were synthesized by one‐, two‐, and three‐stage crystallization processes with fumed silica (FS), acidic silica sol (AS), basic silica sol (BS) and tetraethoxysilane (TEOS) as silicon sources. Properties of samples were characterized by XRD, SEM, EDX, N2‐adsorption/desorption, NH3‐TPD and 29Si NMR. In contrast with one‐ and two‐stage processes, the three‐stage one favors the formation of SAPO‐34s with higher purity, higher Si/Al ratio, smaller crystallite/crystal size, better dispersion and more acidic sites. Crystallization mechanism was proposed. It is supposed that three‐stage process favors the formation of high supersaturated SBUs (secondary building units) and CBUs (composite building units) before nucleation. Thus massive nuclei are formed during nucleation and well‐dispersed small size SAPO‐34 crystals are formed in three‐stage process. Three‐stage SAPO‐34s are longer catalytic lifetime and better olefin selectivity in MTO process. TEOS‐3 has the best MTO performance. It was found that acidity of sample and mesopores in 2–4 nm range have little effect on the MTO performance of SAPO‐34. Catalytic lifetime and olefin selectivity are mainly depending on the size of crystallites, the dispersity of crystallites, and the purity of SAPO‐34.

Preparation of spherical aminopropyl-functionalized MCM-41 and its application in removal of Pb(II) ion from aqueous solution

Abstract Mesoporous silica microspheres were prepared by the polymerization-induced colloid aggregation (PICA) and pseudomorphic synthesis methods. The prepared microspheres have high specific surface area and MCM-41 type structure. In the PICA process, acidic silica sol was utilized as silica source and the effect of molar ration (formaldehyde/urea) was investigated. Moreover, the influences of reaction time and temperature were also studied. The specific surface area of porous and mesoporous silica microspheres were 186.4 m2/g and 900.4 m2/g, respectively. The materials were characterized by SAXS, FTIR, SEM, TEM and nitrogen sorption measurements. The prepared silica microspheres were functionalized by (3-Aminopropyl)triethoxysilane and then used to remove the lead from aqueous solution. The result indicates that the grafted silica microspheres have rapid adsorption capacity and good reproducibility. The adsorption data was fitted well with the Langmuir isotherm model, and the maximum adsorption capacities for MCM-41 silica microspheres were 102.7 mg/g.

Research of silica aerogels prepared by acidic silica sol under the condition of atmospheric pressure drying

Acidic silica sol was used as precursor to prepare SiO2 aerogels using the atmospheric pressure drying technology. The influence of pH, water bath temperature and concentrations of silica sol on the structure parameters and morphology were systematically studied. SiO2 aerogel prepared with the sol concentration of 20% possessed uniform structure and the best structure parameters. Water bath temperature exhibited an obvious effect on morphology, specific surface area and porosity. Moreover, the optimal SiO2 aerogel showed a good heat resistance up to 700 °C. A lower thermal conductivity was 0.019 W/m · K at room temperature (25 °C) and 0.044 W/m · K at 600 °C.

Attrition-resistant Ni[sbnd]Mg/SiO2[sbnd]Al2O3 catalysts with different silica sources for fluidized bed syngas methanation

Attrition-resistant NiMg/SiO2Al2O3 catalysts for fluidized bed syngas methanation were prepared by combining co-precipitation and spray granulation using different silica sources including acidic silica sol (SS), sodium silicate (NS) and tetraethyl orthosilicate (TEOS). Air-jet attrition tests showed that the attrition strength of the resulting catalysts followed an order of C-10TEOS > C-33SS > C-10NS > C-10SS, where C-10TEOS, as an example, refers to the catalyst with 10 wt.% silica using TEOS as its silica source. Characterizations show that the porosity and skeletal structure have strong correlation with catalyst attrition strength. Simultaneous hydrolysis of TEOS and co-precipitation caused C-10TEOS to have dense and continuous skeletal structure to have the high strength of its NiMg/SiO2Al2O3 precursor particles and thus the improved attrition resistance of sprayed catalyst. Silica sol filled in the voids among precursor particles resulted in a compact framework in C-33SS but its NiMg/Al2O3 precursor had weak attrition resistance. Atmospheric syngas methanation over these catalysts at an SV of 600 NL g−1 h−1 in a fixed bed reactor clarified an activity order of C-10TEOS > C-10NS ≈ C-33SS. The TEOS also enabled highly dispersed metallic Ni and many surface active sites to facilitate the methanation activity and stability of this catalyst.

Influence of acidic and alkaline silica sols on the performance of Cu/Zn/Al slurry catalysts

A series of Cu/Zn/Al/Si slurry catalysts were prepared using acidic and alkaline silica sols by the complete liquid-phase technology. The samples were characterized with XRD, H2-TPR, FT-IR, BET, NH3-TPD, XPS and TEM techniques. Addition of acidic silica sol to the catalyst gave the highest CO conversion and dimethyl ether (DME) selectivity, being 65.38% and 76.26% respectively. The acidic silica sol weakens the interaction between Cu and other components, resulting in a rapid reduction of Cu species and exposure of more active lattice planes of Cu0. In addition, the acidic/alkaline properties of silica sol also influence the number and the strength of acid sites in the sample. It was found that both the strong and the weak acid sites decreased in the strength. The acidic silica sol significantly increased the weak acid sites, mesopores and specific surface area, consequently promoting the methanol dehydration and increasing the DME selectivity.

Synthesis and characterization of sol–gel derived continuous spinning alumina based fibers with silica nano-powders

Silica nano-powders were used as the Si source to substitute acidic silica sol in the SiO2–Al2O3 sol mixture for benefiting the sintering of the fibers at 1250 °C. With the increase of nano-silica, grain diameter and porosity of the fibers decreased firstly and increased subsequently, a minimum value of 35.86 nm and 0.86% was exhibited at the nano-silica content of 20%. The solid content, linear growth model and homogeneity of the precursor sol were not affected by the presence of nano-silica, although the continuous spinning length became low. NMR Analysis of 27Al indicated that polymerization degree of the sol was enhanced by nano-silica. The nano-particle contributed to the reduced intermolecular distance of gel, so the appropriate content existed for resulting the reduced grain size and compact structure of the fibers.

Surface Modification of Colloidal Silica Nanoparticles: Controlling the size and Grafting Process

Surface modification of colloidal silica nanoparticles without disrupting the electric double layer of nanoparticles is a major challenge. In the work, silane was employed to modify colloidal silica nanoparticles without inducing bridge flocculation obviously. The effect of pH value of the silica sol, the amount of silane in feed, and reaction temperature on the graft amount and the final size of modified particles was investigated. The increased weight loss by TG and the appearance of T 2 and T 3 except for Q 2 and Q 3 signals by CP/MAS 29 Si NMR of the modified samples verified the successful grafting of silane. The graft amount reached 0.57 mmol/ g, which was slightly lower than theory value, and the particle size remained nearly the same as unmodified particles for acidic silica sol at the optimum condition. For alkaline silica sol after modification, aggregates composed of several nanoparticles connected together with silane moleculars as the bridge appeared.

Silica-based superhydrophobic coating by a single-step process

Silica-based superhydrophobic coatings were produced on glass substrate by dip-coating process. Colloidal solutions containing nanoparticles of silica synthesized via seed growth method (single- and dual-sized particles) and dispersed in three different solvents, ethanol, silica sol and acidic silica sol, were used. Coatings with dual-sized particles in ethanol showed superhydrophobicity with a water contact angle of 155°. The coating adhesion was improved with the use of acidic silica sol as solvent. Coatings prepared from colloidal solution containing 30 wt.% of dual-sized silica particles of 600 and 800 nm in acidic silica sol, showed hydrophilicity with water contact angle of 45° before heat treatment due to the presence of –OH groups from acidic sol. After heat treatment at 250°C for 2 h in atmospheric ambience, the –OH groups reacted to form cross-linked Si-O-Si resulting in superhydrophobicity of the coating with water contact angle of 147°. In this work, silica based superhydrophobic coating was fabricated by adjusting the ratios of the dual-sized silica particles without carrying out any secondary step such as chemical modification or fluorination process on the coating.

Poly(L‐lactic acid)/silicone dioxide nanocomposites prepared via in situ melt polycondensation of L‐lactic acid in the presence of acidic silica sol: Dispersion stability of nanoparticles during dehydration/oligomerization

AbstractIn this work, the stability of nanoparticles during the dehydration/oligomerization (D/O) stage in situ melt polycondensation of L‐lactic acid to prepare poly(L‐lactic acid) (PLLA)/SiO2 nanocomposite was studied. The change in the appearance of the reaction mixture was tracked, and the resultant oligo(L‐lactic acid) (OLLA)/SiO2 and PLLA/SiO2 nanocomposites were characterized by transmission electron microscopy, 1H‐NMR, and light transmittance. The electric double layer and the grafted OLLA chains provided static and steric stabilities during the early and late phases, respectively. However, there existed an intermediate transitional phase with weak stability when the static stability was weakened, but sufficient steric stability had not yet been established; this led to soft or hard aggregation, depending on the SiO2 loading and agitation conditions. At low or moderate SiO2 loading (<5–10%), the soft aggregation could be depressed with appropriate agitation conditions and redispersed with the aid of gradually established steric interaction energy. Consequently, well‐dispersed PLLA/SiO2 nanocomposites with SiO2 loadings of 5 and 10% were successfully prepared by in situ melt polycondensation with an arc stirrer at 400 and 600 rpm, respectively, during the D/O stage. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

A comparative study of Fe/SiO2 Fischer–Tropsch synthesis catalysts using tetraethoxysilane and acidic silica sol as silica sources

Fe/SiO2 Fischer–Tropsch synthesis (FTS) catalysts using tetraethoxysilane (TEOS) and acidic silica sol (AcSS) respectively as silica sources were prepared by co-precipitation method. The catalysts were studied through N2 adsorption, X-ray diffraction, Mössbauer effect spectroscopy, and temperature-programmed reduction and desorption. The FTS performances of catalysts were evaluated in a fixed-bed reactor at 280°C, 1.5MPa, 2000h−1, and H2/CO molar ratio of 2. The characterization results indicated that the TEOS derived catalyst displayed a more uniform pore size distribution, easier reduction and higher capability for CO and H2 adsorption than the AcSS derived catalyst. In FTS reactions, the TEOS derived catalyst showed higher activity and C5+ selectivity than the AcSS derived catalyst.

Optimum Parameters for Fining Extract of Garlic Soluble Solids with Acidic Silica and Bentonite

To produce dried garlic soluble solids extract (GSSE), the method of the economical and rapid method of fining fructan extracts solution was investigated. The acidic silica sol and the bentonite as fining reagents at different concentrations were added into the garlic juice at soluble solids (SS) 6%,12% and 15% (W/W) and at pH 4, 5, 6, 7. The results showed that pH values affected fining firstly, and silcan sol did secondly at SS = 12% but bentonited did secondly at SS = 15%. When silica sol and bentonite were added at 0.2% and 0.1% (V/V), respectively, and at pH 4, the clarifying operation not only reached an acceptable yield of protein precipitate and other impurities, but it was also economically viable. It is concluded that acceptable fining parameters waries based on the GSSE and could be obtained according to the test for the producing dried GSSE as the source of fructan.

A general approach for superhydrophobic coating with strong adhesion strength

Superhydrophobic coatings with strong adhesion strength, high chemical stability and good self-cleaning function are fabricated by a new colloid assembly method. This method involves the utilizing of dual-sized particles to achieve a micro/nano structure, and an acidic silica sol to bond the particles with substrates. The resulting coating has a contact angle higher than 160° and a hysteresis angle of 2°, showing excellent superhydrophobic properties. The adhesion strength of the coating is proved to be strong according to the ASTM international standard. The smoothing effect of the bonders can be completely overcome by adjusting the ratios and sizes of the dual-sized silica particles. Because of its flexibility and simplicity, the present colloid assembly method may be suitable for large-scale coating.

Preparation and properties of poly[styrene‐co‐(butyl acrylate)‐co‐(acrylic acid)]/silica nanocomposite latex prepared using an acidic silica sol

AbstractBACKGROUND: Polyacrylate/silica nanocomposite latexes have been fabricated using blending methods with silica nanopowder, in situ polymerization with surface‐functionalized silica nanoparticles or sol–gel processes with silica precursors. But these approaches have the disadvantages of limited silica load, poor emulsion stability or poor film‐forming ability.RESULTS: In this work, poly[styrene‐co‐(butyl acrylate)‐co‐(acrylic acid)] [P(St‐BA‐AA)]/silica nanocomposite latexes and their dried films were prepared by adding an acidic silica sol to the emulsion polymerization stage. Morphological and rheological characterization shows that the silica nanoparticles are not encapsulated within polymer latex particles, but interact partially with polymer latex particles via hydrogen bonds between the silanol groups and the COOH groups at the surface of the polymer particles. The dried nanocomposite films have a better UV‐blocking ability than the pure polymer film, and retain their transparency even with a silica content up to 9.1 wt%. More interestingly, the hardness of the nanocomposite films increases markedly with increasing silica content, and the toughness of the films is not reduced at silica contents up to 33.3 wt%. An unexpected improvement of the solvent resistance of the nanocomposite films is also observed.CONCLUSION: Highly stable P(St‐BA‐AA)/silica nanocomposite latexes can be prepared with a wide range of silica content using an acidic silica sol. The dried nanocomposite films of these latexes exhibit simultaneous improvement of hardness and toughness even at high silica load, and enhanced solvent resistance, presumably resulting from hydrogen bond interactions between polymer chains and silica particles as well as silica aggregate/particle networks. Copyright © 2009 Society of Chemical Industry

Sol–gel synthesis of porous and dense silica microspheres

Abstract An acidic silica sol (35 ± 2 wt% equivalent SiO2) having a gelling time of 9–10 min has been used as an aqueous phase for obtaining a w/o emulsion in CCl4 as oil phase in presence of a surfactant, Tween 80. The silica sol was allowed to form gel at room temperature via polycondensation among the –Si–OH groups forming the porous silica gel microspheres. The surface area of the microspheres heated at 500°, 700° and 900 °C was found to be 227 m2/g, 167 m2/g and 81 m2/g indicating the gradual densification. The decreased surface area and unchanged –Si–O–Si– asymmetric stretching vibration at 1084 cm−1 up to 700 °C probably indicate the formation of extensive cross-linked gel structure in the microsphere. The appearance of the –Si–O–Si– asymmetric stretching vibration at 1104 cm−1 and the absence of porosity while heating at 1000 °C indicate the formation of dense silica glass microspheres.

Poly(L‐lactic acid)/silicon dioxide nanocomposite prepared via the in situ melt polycondensation of L‐lactic acid in the presence of acidic silica sol: Isothermal crystallization and melting behaviors

AbstractIn a previous article, we reported the preparation and characterization of a nanocomposite of poly(L‐lactic acid) (PLLA) and silica via the in situ melt polymerization of L‐lactic acid in the presence of acidic silica sol. In this study, the isothermal crystallization and melting behaviors of a PLLA/silicon dioxide (SiO2) nanocomposite with 5 wt % well‐dispersed SiO2 nanoparticles (PLLASN5) and pure PLLA were comparatively studied with differential scanning calorimetry and polarized optical microscopy. The SiO2 nanoparticles acted as nucleation agents in the PLLA matrix and enhanced its nucleation rate and overall crystallization rate, especially at high crystallization temperatures. However, no deleterious effect on the crystal morphology or crystallinity was observed. The crystals that formed at a low temperature were imperfect; therefore, double melting peaks occurred during the second heating scan because of melt recrystallization. With the crystallization temperature increasing, the crystals became increasingly perfect; as a result, the low melting peak increased and shifted to a higher temperature. The existence of SiO2 nanoparticles had no effect on the equilibrium temperature of the PLLA matrix. Pure PLLA and PLLASN5 have the same equilibrium temperature of 171.5°C. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Poly(l-lactic acid)/SiO2 nanocomposites via in situ melt polycondensation of l-lactic acid in the presence of acidic silica sol: Preparation and characterization

In situ melt polycondensation of l-lactic acid (LLA) in the presence of acidic silica sol (aSS) is proposed for the first time to prepare PLLA/SiO2 nanocomposites. The SiO2 nanoparticles were readily dispersed in LLA monomer, which has similar polarity and hydrophilicity to the silica sol medium. During the polycondensation process, both the matrix and the surface of SiO2 nanoparticles changed from high polarity/hydrophilicity to weak polarity/hydrophobicity due to simultaneous chain growth in the organic phase and chemical grafting on the particle surface. The chemical grafting provided steric stabilization and ensured satisfactory nano-scale dispersion in the final nanocomposites. The introduction of SiO2 nanoparticles resulted in unchanged yield and better color. The molecular weight kept almost constant at low SiO2 content (<8wt%) but decreased at higher SiO2 content. The method is also characterized by commercially available and cheap starting material and environmentally benign process. It appears to be a promising approach for the preparation of PLLA/SiO2 nanocomposites.

Dependences of Young’s modulus of porous silica low dielectric constant films on skeletal structure and porosity

Porous silica films were prepared by spin coating the mixtures of acidic silica sol and nonionic surfactant template. The (a) porosity and (b) skeletal structure of the films were varied by adjusting the (a) template concentration and the (b) annealing temperature, respectively. Fourier transform infrared spectroscopic ellipsometry was employed to evaluate the skeletal silica structure of the films. The analysis was focused on the midinfrared (1000–1300cm−1) spectral structure which is assigned as the asymmetric stretching vibration mode of Si–O–Si bonds [Kamitsos et al., Phys. Rev. B 48, 12499 (1993)]. The spectral structure depended on both porosity and chemical bonding structure. Bruggemann’s effective medium theory was employed to obtain the spectrum of “skeletal” silica from that of “porous” silica. The skeletal silica structure was then discussed in terms of the peak positions of the transverse optical (ωTO) and longitudinal optical (ωLO) vibration modes of Si–O–Si network. It was shown that the Young’s elastic modulus of skeletal silica correlates well with ωLO2∕ωTO2. We have obtained good correlations between ωLO2∕ωTO2 of skeletal silica and elastic modulus E for two series of porous silica films with around 55% and 40% porosity. The experimental results show that the structural change in silica skeleton strongly affects the mechanical properties of porous silica low-k films.