Surface-modified silica sols

Manufacturing Methods of Surface-Modified Silica Sols... 

MANUFACTURING METHODS OF SURFACE-MODIFIED SILICA SOLS... 

For manufacturing a surface-modified silica sol, an acidic silica sol [20] from which these Na" " ions have been removed is used in most cases. As shown in... 

Scheme V, silica particles are brought into contact with a cation-exchange resin, and if necessary, with an anion-exchange resin, to obtain an acidic silica sols of pH 2-4. This acidic sol is stable because it is negatively charged even at pH 2-4, according to zeta-potential measurements. Starting from such acidic silica sol, surface-modified silica sols are manufactured.

Positive silica sols and alumina modified silica sols wherein the ultimate silica particles have been modified and/or made electrically positive by partially or completely coating the particle surface with aluminum compounds can also be used in the present invention as a source of amorphous silica. Such sols are described for example by G. B. Alexander and G. H. Bolt in U.S. Pat. No. 3,007,878 and... 

T. T. Hoang, Y. Chen, S. W. May, and R. F. Browner, Analysis of organoselenium compounds using active carbon and chemically modified silica sol-gel by surface-assisted laser desorption/ionization time-of-flight mass spectrometry. Anal. Chem. 76, 2062-2070 (2004). 

Figure 39.5 Impact residual impressions of sol-gel silica coatings reinforced with (a) unmodified silica particles and (b) surface-modified silica particles after impact testing (1 kg from 1 m height). (Reproduced from Ref. [44].)...

An alternative strategy to obtain silica immobilised catalysts, pioneered by Panster [23], is via the polycondensation or co-condensation of ligand functionalised alkoxysilanes. This co-condensation, later also referred to as the sol-gel process [24], appeared to be a very mild technique to immobilise catalysts and is also used for enzyme immobilisation. Several novel functional polymeric materials have been reported that enable transition metal complexation. 3-Chloropropyltrialkoxysilanes were converted into functionalised propyltrialkoxysilanes such as diphenylphosphine propyltrialkoxysilane. These compounds can be used to prepare surface modified inorganic materials. Two different routes towards these functional polymers can be envisioned (Figure 3.4). One can first prepare the metal complex and then proceed with the co-condensation reaction (route I), or one can prepare the metal complex after the... 

In an interesting illustration of the first strategy, Sakaguchi and coworkers covalently attached hemoglobin to an aminopropyl silica particle and then polymerized organoalkoxysilanes on the surface of the hemoglobin-modified silica particle.85 The template was removed via treatment with oxalic acid.85 In more recent work, Zhang and coworkers utilized a similar approach. In their case, the sphere was made from the functionalized biopolymer, chitosan.86 The model template protein, bovine serum albumin, was covalently attached to the chitosan microsphere and then coated with a composite sol prepared from TEOS and an aminosilane.86... 

Recently two heterogeneous TPAP-catalysts were developed, which could be recycled successfully and displayed no leaching In the first example the tetra-alkylammonium perruthenate was tethered to the internal surface of mesopor-ous silica (MCM-41) and was shown [153] to catalyze the selective aerobic oxidation of primary and secondary allylic and benzylic alcohols. Surprisingly, both cyclohexanol and cyclohexenol were unreactive although these substrates can easily be accommodated in the pores of MCM-41. The second example involves straightforward doping of methyl modified silica, denoted as ormosil, with tetra-propylammonium perruthenate via the sol-gel process [154]. A serious disadvantage of this system is the low-turnover frequency (1.0 and 1.8 h-1) observed for primary aliphatic alcohol and allylic alcohol respectively. 

The industrial development of silica sol manufacturing methods is reviewed. Primary attention is focused on the preparation of monodispersed sols from water glass by the ion-exchange method. Details are given for variations of manufacturing process and for the characteristics of both the processes and sols obtained. Furthermore, the following surface modifications of particles are demonstrated silica sols stabilized with ammonia, amine, and quaternary ammonium hydroxide aluminum-modified or cation-coated silica sol and lithium silicate. Finally, future trends in silica sol manufacturing are discussed from the viewpoint of not only raw materials and improvement of the procedures but also the function of the silica sols and their particle shape. 

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