Desilation reaction

Catalyzing influence on reactions in silicate systems may be excited by an increase of crystal-forming solution alkalinity. This is evidenced by zeolite synthesis experience (17) and by direct experiments of Campbell and Fyfe (3) and Kerr (8). However, an increase of pH may affect not only growth rate, but displacement of desilication reactions towards products poorer in silica (15, 16). These 2 effects must be distinguished when analyzing the influence of alkalinity growth. 

Direct reductive cleavage by catalytic hydrogenolysis of a cyclopropane C-C-bond is only possible in exceptional cases, where the three membered ring is further activated by a phenyl or a vinyl group. With unpoisoned catalyst a subsequent reductive desil-oxylation occurs to afford esters like 124 (Eq. 48), whereas addition of small amounts of triethylamine allows isolation of the desired siloxy compounds (e.g. 125, Eq. 49). Interestingly, both reactions demonstrate that the cleavage of the cyclopropane bond proceeds non-stereoselectively with inversion and retention at C-l and C-2, respectively 79). 

Also, reaction of N-trimethylsilyl-N -alkylcarbodiimides 240 with haloketenes affords -lactams 241, which are easily desilated with absolute methanol to give halogenated 1 -alkyl-4-imino-2-acetidinones 242. ... 

Desilylation reactions are based on the well-known stabilization of a,/3-carbenium centers by silicon. The conversion of allylsilanes into alkenes by use of the BF3. (AcOH)2 complex has been studied in considerable detail. The reaction occurs by protonation, followed by nucleophile-induced desilylation (Eq. 61) [107]. The stereochemical implications have also been considered in detail, and in the deuteration-desilyation sequence shown in Eq. (62) the structure shown is the major product [108]. Protonation-desilation of alkynes to give allenes in yields ranging from 70 to 97 % can evidently be considered to involve a /3-vinyl cation (Eq. 63) [109]. 

ZSM-5 zeolites attracted particular attention due to their ability to catalyze various reactions in petrochemistry, oil refining and fine chemistry [1,2,3]. However, for many useful chemical reactions, size exclusion virtually operates, as the steric requirements of the reactants and products are beyond the pore sizes. There are several methods how to improve accessibility of active sites located inside microporous structure [4,5]. The first possibility how to diminish the role of the molecular transport is a creation of secondary mesopores in zeolite ciystals, which can be done by post-synthesis treatments such as desilication [6] or dealumination [7], however, these treatments cannot provide zeolites having mesopores uniform in size and lattice positions [8]. The second... 

Phosphate ions cause a release of silicic acid from aluminosilicates, thus causing their desilication. In this way insoluble alumino- and ferri-phos-phosilicates are formed, with irreversibly-bound phosphate ions. Insoluble forms of phosphorus are also obtained by the reaction of phosphate ions with hydrated A1 and Fe sesquioxides and hydroxylated aluminium ions or with products of the hydrolysis of clay minerals. 

Figure 1 Variation of silica activity with temperature for several mineral assemblages. Arabic numerals correspond to the reactions listed in Table 1. Roman numerals correspond to the order of desilication steps in the CIPW normative classification. Curves calculated from data tabulated in Nicholls t a3. (1971) and Carmichael et al. (in press).

It is highly probable that lattice vacancies created by removal of silicon may be filled up in the same way and under similar conditions as those remaining after release of aluminum. Thus, recrystallization of the desilicated products to particles with well-ordered crystal structure but traversed by nanopores is obviously effected by water steam present as reaction product of the dehydroxy-lation of hydroxyl nests . After the desihcation process, the zeolite is in the sodium (or potassium) form and this is known to be highly resistant towards hydrothermal effects. Therefore, it is to be expected that steaming represents the most effective method for the stabilization of desilicated zeolites avoiding the risk of concurrent dealumination. 

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