Structural characteristics of silicas

V. M. Gun ko, J. Skubiszewska-Zieba, R. Leboda, and V. V. Turov, Impact of Thermal and Hydrothermal Treatments on Structural Characteristics of Silica Gel Si-40 and Carbon/Silica Gel Adsorbents, Colloids Surf. A 235 (2004) 101-111. 

Table 4. Structural characteristics of silica gel Si-100 and carbosils with phosphorus.7...

Table 2. Structural characteristics of silicas before surface modification.

FIGURE 1.267 The H chemical shift as a function of temperature at different amounts of water adsorbed onto initial and hydrothermally treated (HTT) silica gel Si-40. (Adapted from Colloids Surf. A Physicochem. Eng. Aspects., 235., Gun ko, V.M., Skubiszewska- Zi ba, J., Leboda, R., and Turov, V.V., Impact of thermal and hydrothermal treatments on structural characteristics of silica gel Si-40 and carbon/silica gel adsorbents, 101-111, 2004c. Copyright 2004, with permission from Elsevier.)... 

Vollet DR, Ntmes LM, Donatti DA, Ibanez Ruiz A, Maceti H (2008) Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS. J Non-Cryst Solids 354 1467-1474 Livage J, Herrry M, Sanchez C (1988) Sol-Gel Chemistry of Transition Metal Oxides. Prog. Solid State Chem.l8 259-341... 

Table 1. Structural characteristics of initial, heated (T) and hydrothermally treated at 150°C (HTT) silica gel Si-40 and carbosils prepared with acenaphthene pyrolysis under static conditions.15...

Table 1 presents the structural characteristics of adsorbents prepared with Si-40 and acenaphthene.15 Larger changes in the Si-40 structure are caused by hydrothermal treatment, despite a relatively low temperature (150°C), than by heating at 500°C. However in the case of carbon-silica adsorbents, both hydrothermal modification and high-temperature pyrolysis changes the pore structure to a large extent. 

Table 1. Structural characteristics of initial fumed silicas and hybrid pyrocarbon/silica adsorbents prepared from liquid solutions of polymers and subsequent carbonization.5...

Table 3. Structural characteristics of initial silica gel Si-100 and carbosils prepared by pyrolysis of glucose (GL) under static (A) and dynamic (R) conditions.6...

PNP poorly adsorbs onto silica surfaces.6,11 Therefore, it can be used to estimate the structural characteristics of the carbon phase in carbosils. An increase in the Cc value leads to a significant enhancement of PNP adsorption per m2 of carbosil surface (Figure 3A), which is greater for the A series (Table 3). 

N. Vervoort, P. Gzil, G. V. Baron, and G. Desmet, Model column structure for the analysis of the flow and band-broadening characteristics of silica monoliths, /. Chromatogr. 1030 (2004) 177-1S6. 

Mesoporous melamine-formaldehyde and phenolic-formaldehyde resins were synthesized in the process of polymerization in the presence of fumed silica as an inorganic template. The surface and structural characteristics of the obtained sorbents were investigated using XPS technique and sorption from gas phase. The parameters characterizing porous structure of the synthesized resins in a dry state were determined from nitrogen adsorption/desorption isotherms. The sorption processes of benzene and water vapor accompanied by simultaneous swelling of both polymers were also studied. 

The aim of this work was to examine and compare structural characteristics of vanadia-doped silica aerogels with standard liquid nitrogen adsorption and small angle X-ray scattering. 

Astala, Auerbach, and Monson277 used plane wave LDA calculations to examine structural characteristics of five all-silica zeolites SOD, LTA, CHA, MOR, and MFI. The first four of these materials can be examined in calculations with 36-72 atoms in a periodic unit cell, but the MFI structure is considerably larger and requires 288 atoms in a single unit cell. Astala et al. argue that LDA calculations give more accurate results for the structure and energies of silica polymorphs than GGA calculations. Demuth et al. used LDA and GGA calculations to examine the properties of Al substitutions in morde-nite both in a protonated form and in the presence of Na cations.278... 

Graetsch, H. (1994) Structural characteristics of opaline and microcrystalline silica minerals. In Heaney, P.J., Prewitt, C.T. Gibbs, G.V. (Eds) Silica Physical Behaviour, Geochemistry and Materials Applications. Reviews in Mineralogy 29, Washington Mineralogical Society of America, pp. 209-232. 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions. Their fascinating properties, such as ion-exchange, separation, and catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. Traditionally, the term zeolite refers to a crystalline aluminosilicate or silica polymorph based on comer-sharing TO4 (T = Si and Al) tetrahedra forming a three-dimensional four-connected framework with uniformly sized pores of molecular dimensions. Nowadays, a diverse range of zeolite-related microporous materials with novel open-framework stmctures have been discovered. The framework atoms of microporous materials have expanded to cover most of the elements in the periodic table. For the structural chemistry aspect of our discussions, the second key component of the book, we have a chapter (Chapter 2) to introduce the structural characteristics of zeolites and related microporous materials. 

Porous structure characteristics of unsupported silica materials exhibiting an ordered microporosity (N2 adsorption measurements) [68]... 

THE MODIFICATION OF STRUCTURAL AND ADSORPTION CHARACTERISTICS OF SILICA AND ALUMINA BY THE MOLECULAR LAYERING METHOD... 

These conceptions, explaining the fimctions of the chelating agents, were mainly developed for the silica-supported HDS catalysts [12-15]. However, the data presented here indicate their validity for the stevensite-supported HDS catalysts. Nevertheless, taking into account differences in the crystal structure, sur ce chemistry of the smectites, and the corresponding characteristics of silica, additional study is necessary to elucidate the case. 

Information about the secondary structures (a-helices, /5-sheets, random coil) can be useful for understanding conformation changes of proteins upon the immobilization process. More specifically, circular dichroism (CD) [70] and FT-IR spectroscopy [56, 58, 61, 71-73] have been applied to study the structural characteristics of various proteins adsorbed on mineral surfaces. Kondo and coworkers [70] have studied the modification in a-helix content of proteins adsorbed on ultrafine silica particles with CD and found a decrease upon immobilization. Circular dichroism is not usually used because this technique is applicable only for the study of enzymes immobilized on nano-sized mineral particles due to problems arising from light scattering effects. On the other hand, infrared spectroscopy does not suffer from light scattering perturbations and has thus been used for the study of the conformation of proteins when they are immobilized on solid supports [57, 58]. 

The linear viscoelastic properties in the melt state of highly grafted polymers on spherical silica nanoparticles are probed using linear dynamic oscillatory measurements and linear stress relaxation measurements. While the pure silica tethered polymer nanocomposite exhibits solid-like response, the addition of a matched molecular weight free matrix homopolymer chains to this hybrid material, initially lowers the modulus and later changes the viscoelastic response to that of a liquid. These results are consistent with the breakdown of the ordered mesoscale structure, characteristic of the pure hybrid and the high hybrid concentration blends, by the addition of homopolymers with matched molecular weights. 

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