Silica formation/reaction mechanism

Fig. 3.4 Schematic drawings illustrating the nucleation of silica precursor and possible mechanisms of reactions leading to silica formation. (A) The precursor hydrolysis. THEOS nucleates on a polysaccharide macromolecule through hydrogen bonds that are formed with hydroxy groups in the biomacromolecule. Per-...

More recently, dealumination was achieved by fluorination of zeolites at ambient temperature with a dilute fluorine-in-air stream, followed by high-temperature calcination (102). The suggested reaction mechanism involves the formation of different aluminum-fluorine compounds along with zeolites containing hydroxyl and fluorine nests. During the high-temperature calcination, it is assumed that silica insertion occurs, similar to the scheme in Figure IB. 

Over the last years the utilisation of supramolecular arrays of surfactant molecules as structure-directing templates [1] has been applied to the synthesis of numerous mesostructured aluminophosphates [2-11]. In most cases the preparations were carried out in aqueous systems under hydrothermal conditions, but tetraethylene glycol and/or unbranched primary alcohols were also used [2,4]. Several discussions have been made on the reaction mechanisms that are involved in the syntheses of mesostructured materials [1,12-15] and recently a number of in-situ investigations on the formation processes of mesostructured silica phases in aqueous media have been reported these studies employed small angle X-ray diffraction [16-19] as well as 2H, 13C, 29Si, and 8lBr NMR spectroscopy and polarised light optical microscopy [17]. 

Some of the extensive literature surrounding a relatively simple reaction, exchange on silica will be summarized in this section and contrasted with that for magnesium oxide in the next section. This reaction proceeds via O -centers on both catalysts, but the sites, modes of production and the reaction mechanisms themselves are very different. This reaction is very useful for illustrating the possible effects, on mineral catalysts, of processes common in natural systems, i.e., artifacts of mineral formation and weathering and electronic excitation. 

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). 

Surface Superbasic Sites of One-electron Donor Character. - The reaction of alkali metal with anionic vacancies on the oxide surfaces (equation 1) leads to the creation of colour centres of F type. The transfer of one electron from the alkali metal atom to an anionic vacancy is the reason for the formation of these defects. The largest quantities of this type of active centre are obtained by evaporation of the alkali metal onto an oxide surface calcined at about 1023 K, at which temperature the largest quantity of anionic vacancies is formed. Oxide surfaces calcined at such high temperatures contain only a small quantity of OH groups ca. 0.5 OH per 100 for MgO and 0.8 OH per 100 for AI2O3), so their role in the reaction is small and the action of alkali metal leads selectively to the creation of defects of the electron in anionic vacancy type. The evidence for such a reaction mechanism is the occurrence of specific colours in the oxide. Magnesium oxide after deposition by evaporation of sodium, potassium, or a caesium turns blue, alumina after sodium evaporation becomes a navy blue in colour, and silica after sodium evaporation becomes violet-brown in colour. ... 

The chemistry involved in the formation of mesoporous silica thin films is qualitatively well understood. However, specific reaction mechanisms of the individual steps are still debated. In addition, owing to the complexity of the sol-gel reaction pathways and cooperative self-assembly, full kinetic models have not been developed. From the time of mixing, hydrolysis reactions, condensation reactions, protonation and deprotonation, dynamic exchange with solution nucleophiles, complexation with solution ions and surfactants, and self-assembly, all occur in parallel and are discussed here. Although the sol-gel reactions involved may be acid or base catalyzed, mesoporous silica film formation is carried out under acidic conditions, as silica species are metastable and the relative rates of hydrolysis and condensation reactions lead to interconnected structures as opposed to the stable sols produced at higher pH. Silicon alkoxides are the primary silica source (tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, etc.) and are abbreviated TMOS, TEOS, and TPOS, respectively. Starting from the alkoxide, Si(OR)4, in ROH and H2O solution, some of the general reactions are ... 

Systematic studies carried out at C.F.R.I. pertainining to catalytic vapour phase synthesis of pyridine bases, namely 2 4-picolines through cyclodehydrogenation reaction of acetaldehyde and anunonia have been described. Metal oxide modified amorphous silica-aliunina and crystalline alumino-silicate namely ZSM-5 zeolites were found to be active and selective catalysts towards the formation of the lower pyridines bases. The method of preparation, pretreatment vis-a-vis the acidity have been found to affect the catalytic activity and selectivity. It is interesting to observe that metal modified crystalline alumino-silicate ZSM-5 is more selective than amorphous silica-alumina for the formation of 2 4-picoline. A plausible reaction mechanism based on the findings of this study has been proposed. 

Since cation-radical formation in the chemisorption of hydrocarbons has not previously been considered in the catalytic literature, the nature, reactions, and mechanism for formation of such species should be of considerable importance to the elucidation of catalytic reaction mechanisms particularly in view of the fact that Webb (20) has found spectral evidence for the formation of species other than carbonium ions from butene-2 adsorbed on silica-alumina. It is not possible at the present time to define either the role of cation-radicals in acid catalysis or the chemical nature of the electrophilic surface sites involved in their formation. 

Using the concept that silica-alumina catalysts contain intrinsically strong acidic sites, several authors have proposed detailed reaction mechanisms involving the intermediate formation of carbonium ions to... 

The reaction mechanism during gel formation differs from that of precipitated silicas and comprises sol formation followed by the gelation step, where the liquid precursor sol has the same chemical composition as the gel formed from it. This might be described by interaction of separate micelles in the sol first by hydrogen bonding, followed by condensation. More details may be taken... 

Fe-oxide Studies. The oxidation of Co(II)EDTA to Co(III)EDTA by the ferrihydrite-coated silica under dynamic flow conditions exhibited breakthrough characteristics (data not shown) that were consistent with earlier research (6). The reaction quickly reached a steady-state condition that was pH and flow rate dependent. Previous studies by Brooks et al. (6) showed that in the absence of oxygen, the amount of oxidation slowly decreased with time, suggesting that in the absence of oxygen, the amount of oxidation strongly depended on the amount of Fe(III) originally in the flow field. Brooks et al. (6) postulated that Fe(III) served as the oxidant in the ferrihydrite-Co(II)EDTA system with subsequent formation of solid phase Fe(II). XANES spectroscopy was used to determine the speciation of the solid phase as a fimction of reaction time in order to identify the reaction mechanism. 

These observations are not consistent with the occurrence of a looped, nonpolar, hydrophobic layer as implied by the complete reaction of both silyl groups with the silica, an adsorption mechanism proposed earlier. They are not consistent either with the formation of chemical bonds between filler particles. 

More elaborate and ambitious studies on the dissolution reactions of silica were conducted by Xiao and Lasaga (1994, 1996). Their objective was to provide full descriptions of the reaction pathway of quartz dissolution in acidic and basic solutions, from the adsorption of H2O or OH on a site, the formation of possible reaction intermediates and transition states, to the hydrolysis of the Si-O-Si bonds. Also, their aim was to extract kinetic properties such as changes in activation energy, kinetic isotope effects, catalytic and temperature effects, and the overall rate law form. The reaction mechanisms investigated were... 

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