Amorphous silica function

Figure 2 Functional groups present on the surface of porous, amorphous silica...

Recently, Muha (83) has found that the concentration of cation radicals is a rather complex function of the half-wave potential the concentration goes through a maximum at a half-wave potential of about 0.7 V. The results were obtained for an amorphous silica-alumina catalyst where the steric problem would not be significant. To explain the observed dependence, the presence of dipositive ions and carbonium ions along with a distribution in the oxidizing strengths of the surface electrophilic sites must be taken into account. The interaction between the different species present is explained by assuming that a chemical equilibrium exists on the surface. 

Thin films of functionalized amorphous silica for immunosensors application. Journal of Sol-Gel Science and Technology, 2, 823-826. 

Control over the material s shape at the nanoscale enables further control over reactants access to the dopant, and ultimately affords a potent means of controlling function which is analogous to that parsimoniously employed by Nature to synthesize materials with myriad function with a surprisingly low number of material s building blocks. A nice illustration is offered by the extrusion catalytic polymerization of ethylene within the hexagonal channels of MCM-41 mesoporous silica doped with catalyst titanocene.36 The structure is made of amorphous silica walls spatially arranged into periodic arrays with high surface area (up to 1400 m2g 1) and mesopore volume >0.7 mLg-1. In this case, restricted conformation dictates polymerization the pore diameter... 

Similarly, Hg(n) binding to thiol-functionalized mesoporous silica for which effective access to all the binding sites (100% of SH groups com-plexed with Hg(n) was achieved in micelle-templated mesostructures with pore diameters larger than 2.0 nm, whereas incomplete filling was always observed with corresponding amorphous silica-based adsorbents.37... 

Two more recent applications for amorphous silicas are expected to grow to large volumes. Precipitated silicas are used in the manufacture of separator sheets placed between cells in automotive batteries. Their function is to provide a controlled path for the migration of conductive ions as a result of the porosity of the silica particles. Additionally, both precipitated silicas and aerogels are being developed for use in low temperature insulation, where the low thermal conductivity of the dry silica powders makes them useful in consumer products such as refrigerators (83). 

In the presence of hydrogen the isomerization of paraffins of five or more carbon atoms over dual function catalysts, such as amorphous silica-alumina supported platinum, can be described by the following scheme ... 

Historically, the isomerization catalysts have included amorphous silica—aluminas, zeolites, and metal-loaded oxides. All of the catalysts contain acidity, which isomerizes the xylenes and if strong enough can also crack the EB and xylenes to benzene and toluene. Dual functional catalysts additionally contain a metal that is capable of converting EB to xylenes. 

Chemical dissolution techniques indicate kaolinite from Cornwall contains 3.1-4.9% of easily soluble Si02 and 1.5-5.9% of easily soluble A1203 (Follett et al., 1965). Most of this material is presumably present as amorphous material. Experiments (by the senior author) with Georgia kaolinite indicate the amount of amorphous material varies as a function of particle size and preparation (Table LX). Amorphous silica and alumina is a common constituent of kaolinite and considerable care must be taken in determining and interpreting the significance of the Si/Al ratio of kaolinites. 

The analysis of Knowles and Turan (2000) of 7 -BN-amorphous silica-3C SiC interfaces showed that Eq. (17.3)could be used to calculate values of the Hamaker constant as a function of the orientation of / -BN with respect to a planar interface containing a thin amorphous silica film, provided that the effective values of static dielectric constant and refractive index for / -BN, /,., and / bx respectively, were taken to be... 

In 1993, L.T. Zhuravlev35 published a review article of work performed in the former USSR on the surface characterization of amorphous silica. This review article is a very important document in the study of the silanol distribution. Also, the energetical aspects of the dehydration and dehydroxylation processes are discussed in detail. The determination of the silanol number as a function of treatment temperature has already been discussed in chapter 4. 

Although micro- and not mesoporous but worth mentioning in any case is a new class of materials in this evolution series of porous silica-based hybrid materials, the so-called ZOLs (zeolites with organic group as lattice). As we will see, many studies in the past focused on the organic functionalization of amorphous silica porous... 

The previous example took inspiration from earlier work of Lin et al.18 In their seminal work, Lin and coworkers functionalized the inner pores of MCM-41 with o-phthalic hemithioacetal moieties that are able to react with amines to produce a highly fluorescent isoindole derivative. In order to enhance selectivity in the sense we have discussed above, the solids were also hydrophobized with different groups such as propyl, phenyl, and pentafluorophenyl in a second step. Interestingly, some of these solids displayed a remarkably selective and differentiable response to dopamine versus the less lipophilic glucosamine. The authors also demonstrated that this selectivity was not observed when amorphous (nonporous) silica functionalized with the same organic groups was used, stressing the importance of the 3D... 

Amorphous Ti/SiCL oxides and crystalline Ti zeolites are two classes of well-studied solid Ti catalysts (11-14). In both classes, a Lewis-acidic Ti atom is anchored to the surrounding siliceous matrix by Si-O-Ti bonds. The oxidant of choice for Ti zeolites such as titanium silicalite 1 (TS-1) and 11-/1 is H2O2, whereas the amorphous, silica-based materials function optimally with organic peroxides such as /-butyl hydroperoxide (/-BuOOH) or ethyl benzene hydroperoxide. However, there are strictly no homogeneous analogues of these materials, and they therefore do not fit within the context of anchoring of homogeneous catalysts. 

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