Potentialities of silane-modified silicas to regulate palladium nanoparticles sizes

A promising approach to control palladium nanoparticle sizes by application of silane modified silicas was suggested. The combination of reductive properties of silicon hydride groups and hydrophobic properties of alkylsilyl groups which act as agglomeration limiters for metal nanoparticles gives an opportunity to synthesize uniformly distributed particles with a specified size. Silicas modified with triethoxysilane (TES) and diisopropylchlorosilane (DIPCS), as well as, the combination of hexamethyldisilazane (HMDS) and triethoxysilane were applied for formation of such bifunctional matrices. Properties of the silane-modified silica samples and changes occurred during the formation of palladium nanoparticles were studied by IR spectroscopy. Thermal stability of surface chemical compounds was investigated by thermogravimetric analysis (TGA); low-temperature nitrogen adsorption was used to study structural properties of the applied materials. With the use of transmission electron microscopy (TEM) the dependence of palladium nanoparticle size on the nature of support surface layer was found.     

Polymerization and polymers of silicic acid

Linear relationships among molecular weight, polymerization time, and gelation for the condensation of any monomer, including those of unknown size and functionality, are developed and applied to data on soluble silicic acids generated from tetraethyl silicate and from sodium silicate. The results suggest the formation of monomers containing ca. 12–15 ? OSi units with functionality of ca. 2.05 that condense with a rate constant of ca. 10^?4 liter/mole sec and an activation energy of 40–70 kJ/mole. One model compatible with these characteristics and the stoichiometry involved is a ladder polymer ca. 3 ? SiO units wide. Polymer isolation was achieved by replacing residual ? OH with (CH₃)₃Sio? , as well as by freezing of of aqueous solutions, which yielded fibers under special conditions. Solutions of the uncapped and capped polymers and melts of the latter had low viscosity even for fractions with M?_n ～100,000. This implies a coiled or globular nature for the polymers, which is supported by their limited propensity for film and fiber formation. Attemps to improve this situation by copolymerization, the use of other capping agents, and by the polymerization of precapped monomers were unsuccessful.     

Preparation of polysiloxanes from silicic acid. IX. Partially silylated silicic acid and its spinnability

The reaction in different molar ratios of trimethylchlorosilane to silicic acid offered partially silylated silicic acids PSSX (X = OH) with various degrees of silylation. Esterification and acetylation of PSSX (X = OH) with 1-butanol or acetylchloride gave polysiloxanes PSSX (X = OBuⁿ and OCOCH₃). Some of them showed an excellent spinnability and provided a fine fiber with a length of about 250 cm. A fiber drawing test from concentrated PSSX solutions of various solvents revealed that spinnability markedly depended on the DS, functional group X of PSSX, and solvent. Spinnability may also be correlated to solubility parameter of PSSX and solvent.     

Thermal condensation of phthalimide with malonic acid

When phthalimide is reacted with malonic acid in the presence of zinc acetate, 1-hydroxy-1-methyl-1H-3-(1-oxoisoindolin-3-ylidenemethyl)isoindole, the zinc complex of 1-hydroxy-8,1322,27-diimino-1,615,20-dinitrilotetrabenzo[b,d,l,q]eicosin, and zinc tetrabenzoporphin are formed depending on the temperature. The compounds have been characterized by their electronic absorption spectra, IR and mass spectra, and also by their x-ray photoelectron spectra. A possible scheme for the synthesis of zinc tetrabenzoporphin has been proposed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 480–484, April, 1988.     

Synthesis and structure of grafted layer of silicas modified with alkanesulfonic acid

A convenient method for preparation of silicas modified with alkanesulfonic acid through the reaction of NaHSO₃ with the C₂H₃ groups grafted on the SiO₂ surface was proposed. The influence of the carrier and the structure of the grafted layer on the modification process was studied by DRIFT, MAS NMR, and temperature-programmed desorption mass spectrometry (TPD-MS). It was found that at most 40% of the C₂H₃ groups transform to the acid sites, which mostly have the structure of 2-Si-ethanesulfonic acid. The mechanism of the thermal decomposition of the grafted groups, which are stable up to 240 °C, was proposed. The monomeric structure of the grafted layer leads to partial olygomerization of the C₂H₃ groups through the N=N bonds.     

Modeling three-dimensional network formation with an atomic lattice model: application to silicic acid polymerization

We present an atomic lattice model for studying the polymerization of silicic acid in sol-gel and related processes for synthesizing silica materials. Our model is based on Si and O atoms occupying the sites of a body-centered-cubic lattice, with all atoms arranged in SiO(4) tetrahedra. This is the simplest model that allows for variation in the Si-O-Si angle, which is largely responsible for the versatility in silica polymorphs. The model describes the assembly of polymerized silica structures starting from a solution of silicic acid in water at a given concentration and pH. This model can simulate related materials-chalcogenides and clays-by assigning energy penalties to particular ring geometries in the polymerized structures. The simplicity of this approach makes it possible to study the polymerization process to higher degrees of polymerization and larger system sizes than has been possible with previous atomistic models. We have performed Monte Carlo simulations of the model at two concentrations: a low density state similar to that used in the clear solution synthesis of silicalite-1, and a high density state relevant to experiments on silica gel synthesis. For the high concentration system where there are NMR data on the temporal evolution of the Q(n) distribution, we find that the model gives good agreement with the experimental data. The model captures the basic mechanism of silica polymerization and provides quantitative structural predictions on ring-size distributions in good agreement with x-ray and neutron diffraction data.     

Condensation and structure of silicic acid in tetrahydrofuran

Silicic acid (SA) was extracted with THF from aqueous sodium metasilicate (SMS) solutions neutralized with hydrochloric acid, followed by silylation to give silylated SA from which the condensation and structure of SA in tetrahydrofuran (THF) were investigated. The degree of extraction markedly depends on SA and HCl concentrations. The condensation of 0.86 and 3.5 mol/L SA-THF solutions was followed by measuring the viscosity of the solutions at 0 and 20°C. The silylated SA was isolated as a distillate (LS) and a residue (HS) with low and high M?_ns (ca. 1300 and 3800) by vacuum distillation. The ratio of LS against HS decreased as a SA concentration in THF increased. In an aqueous solution, SA exists as lower molecular weight SAs compared with those in THF. SAs such as monomer, dimer, cyclic tri- and tetramer were the main components in a 0.1 mol/L aqueous solution. On the extraction with THF from an aqueous solution, SA was found to undergo condensations to form more polymerized SAs. From the THF solutions of SA concentration above 0.5 mol/L, the HS was obtained as a main component (73%) which was expected to have ladder-like structures. © 1992 John Wiley & Sons, Inc.     

Features of condensation of thiobarbituric acid with salicylaldehyde

Russian Journal of General Chemistry -     

Preparation of polysiloxanes from silicic acid. X. Preparation and properties of allyldimethylsilylated silicic acids

In order to obtain polysiloxanes capable of forming a film, the preparation and properties of allyldimethylsilylated silicic acid was investigated. The reaction in variable molar ratios of allyldimethylchlorosilane to silicic acid gave partially silylated silicic acids with different degrees of silylation. They provided not only fibrous polysiloxanes but also films from concentrated polymer solutions. Spinnability and film formation depended on the solvent, degree of silylation, molecular weight, and allyl group.     

Comparison between phenylboronic acid and iron loaded silicas to selectively preconcentrate catecholamines on-line coupled with HPLC

Summary The use of the on-line preconcentration of catecholamines on small precolumns packed with iron loaded and phenylboronic acid silicas was investigated. For both preconcentration materials, parameters which can influence the retention of catecholamines on the support such as pH, ionic strength, polarity and flow rate of the sample solution are described to predict the mechanism of retention on these silicas. On-line coupling with an ion-pair chromatography column is presented. Combined with amperometric detection, these methods have been applied to the quantitative determination of catecholamines in urine samples, using 1,2-dihydroxybenzylamine as an internal standard.     

Influence of water molecules on dimerization of silicic acid

By means of the quantum-chemical MO LCAO SCF method in the valence approximation of MNDO, it has been shown that effective interaction of two molecules of Si(OH)₄ is possible with the participation of a water molecule that plays the role of a bridge in forming a six-center complex.Institute of Surface Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 4, pp. 485–487, July–August, 1991. Original article submitted February 14, 1991.     

Tributylstannylated silicic acid as precursor of organofunctionalized silica gel

Tributylstannylated silicic acid (TBSA), which was regarded as a protected polymeric silanol against self-condensation to give silica gel, was newly prepared from the reaction of water glass (WG) and bis(tributyltin) oxide (TBO). The ratios of Si/Sn contained in TBSA were determined by gravimetric analysis to be in the range of 2–3. The gelation of TBSA in usual organic solvents such as hexane, benzene, and dichloromethane was not observed over 3 weeks. In addition, TBSA was shown to be a convenient precursor for the preparations of silica gel modified with organofunctional groups. From the reaction of TBSA with trimethoxysilanes and aromatic alcohols, the silica gels havingorganofunctional groups were obtained with the elimination of a tributyltin group. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 655–663, 1998     

Interaction of silicic acid with poly(1‐vinylimidazole)

Poly(1‐vinylimidazole) reacts with silicic acid and poly(silicic acid), giving rise to water‐soluble complexes and insoluble composites because of hydrogen bonding. The composition, structure, and morphology of the obtained products have been studied with elemental analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy. The main direction of the reaction depends not only on the initial ratio of the components, concentration, and pH but also on the sequence of the reagent mixing: the presence of poly(1‐vinylimidazole) macromolecules during the formation of silicic acid stabilizes soluble complexes, which precipitate with an excess of H₄SiO₄ only. These soluble complexes may serve as a pattern of particles responsible for the transport of silicic acid in diatom algae and other organisms that assimilate silicon from the environment. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 820–827, 2006