Silica surface processes, research

Co-containing POMs have been found to be among the most efficient catalysts for homogeneous aerobic oxidation and co-oxidation processes [91-93]. This prompted many researchers to design solid Co-POM-containing materials [78,94-100]. Thus, various Co-POMs have been deposited on cotton cloth [94] and silica [100], datively [95] or electrostatically [96,97] bonded to NH2-modified silica surfaces (vide infra) as well as intercalated in LDHs [78,98,99]. The resulting materials were successfully used for aerobic oxidation of aldehydes, alkenes, alkanes, alcohols and some other organic substrates. 

Especially researchers from the states of the former USSR have performed detailed studies on the reaction mechanism of SOCl2 with the silica surface.32 It was suggested that the anomalously low temperature of chlorination of the silica surface is related to the initial process of electrophilic substitution of a proton of the silanol group, the formation of intermediate compounds and their decomposition, according to reaction scheme (E). 

This work considers the state of the art with reference to the problem of the synthesis of surface chemical compounds with Si-C bonds directly on silica surface with the main accent being given to reactions of solid-phase hydrosilylation. In recent years the major regularities of such processes were the subject of numerous researches and the results obtained allow us to classify the solid-phase hydrosilylation reaction as the basis for promising methods of chemical modification of silica surface. 

Matrix solid-phase dispersion (MSPD) was developed by researchers at Louisiana State University s School of Veterinary Medicine in order to isolate, identify, and quantify veterinary drug residues in livestock (Barker and Hawley, 1992). Compared to traditional methods, MSPD reduces solvent use by 98% and turnaround time by as much as 90%. The method involves the mechanical blending of a sample matrix with bulk C-18 sorbent. The C-18 hydrophobic phase has the ability to incorporate the lipids in meat and other food products into its matrix. Mechanical shearing forces initially disrupt the sample structure and disperse the sample over the surface of the C-18 bonded silica. The process causes the sample and polymer phase to become semidry, which then allow the material to be packed into a column (see Fig. 9.4). 

In much of the definitive IR work on the silica surface researchers have chosen to work with fumed silica. This choice was mainly for experimental reasons (the ease of preparing the self-supporting disk), but also because it minimizes another important issue — the nature of porous silica surface. A major advance in the past decade has been in the controlled synthesis of many sUica polymorphs with variable pore size. Accordingly, the past decade has seen a renewed enthusiasm for the study of porous silicas, their reaction with chemical probes, and H2-D2 exchange reactions. An increasing body of evidence indicates that the basic silica structure is similar in both cases, but that accessibihty and derivatization of the porous silicas can stericaUy alter the process and the kinetics of the reactions. 

This paper is devoted to the sorption of uranyl, which exhibits a complex aqueous and surface chemistry. We review briefly the sorption behaviour of An in the environment, and illustrate the variety of environmental processes using published data of uranyl sorption in the Ban-gombe natural reactor zone. After summarizing the general findings of the mechanisms of An sorption, we then focus particularly on the current knowledge of the mechanisms of uranyl sorption. A major area of research is the influence of the aqueous uranyl speciation on the uranyl surface species. Spectroscopic data of U(VI) sorbed onto silica and alumina minerals are examined and used to discuss the role of aqueous uranyl polynuclear species, U02(0H)2 colloids and uranyl-carbonate complexes. The influence of the mineral surface properties on the mechanisms of sorption is also discussed. 

Switching from the very hydrophilic clays towards other inorganic nanoparticles, e.g., colloidal silica, leads, in the interplay with polymerization in miniemulsions, into a potential structural complexity, which covers the whole range from embedded particles (such as in the case of the calcium carbonate and carbon blacks) to surface bound inorganic layers (such as in the case of the clays). For basic research they are ideal systems to analyze complex structure formation processes in emulsions, since the original droplet shows a structure which is essentially established by molecular forces and local energy considerations, and which is ideally just solidified into a polymer structure. 

Periodic mesoporous silicas were reported for the first time in the literature by researchers at the Mobil Oil Corporation in the early 1990s, although a synthetic process that yields very similar reaction products was patented twenty years prior (MoUer and Bein, 1998). The reactive internal surfaces of these solids have been used to attach functional groups that can act as complexing agents for metal cations. However, as well as complexation and similar uses like ion exchange and sorption, the channels have been used to grow metal clusters and wires. 

---