Chemisorption Of Dimethylchlorosilane On The Silica Surface
CHEMISORPTION OF DIMETHYLCHLOROSILANE ON THE SILICA SURFACE
A. M. Varvarin, L. A. Belyakova
Institute of Surface Chemistry of National Academy of Sciences,
Kiev, Ukraine
Hydride silicas possess unique sorption and oxidation-reduction properties. Hydride silicas are recently used more often as intermediates for the synthesis of heterogeneous catalysts. The most often application of hydride silicas is the implementation of solid-phase hydrosilylation reactions of olefins and high temperature condensation of different organic chlorine-substituted compounds. The stable chemical bonds Si‑C between the silica surface and grafted organic reagent are formed in both cases.
The chemisorption of dimethylchlorosilane (CH3)2Si(Cl)H (DMCS) on silica is almost not studied. Therefore the interaction of DMCS with the surface of highly dispersed amorphous silica (Aerosil A‑300), which is hydroxylated in varying degrees, has been investigated.
It was established that the interaction of dimethylchlorosilane with Aerosil surface, which is preliminary evacuated at different temperatures (from 25 to 600oC) was performed according to the mechanism of electrophilic substitution of proton in the free silanol groups (SEi). The bonded by mutual hydrogen linkage.
OH-groups (pretreatment at 200 and 400oC) as well as siloxane bridges of silica surface do not react with DMCS.
Taking into consideration the obtained results it is possible to come to the following conclusions. If the temperature of pretreatment of Aerosil increases, the reaction ability of isolated hydroxyl groups on the silica surface towards DMCS decreases as:
=Si–OH 200оC > =Si–OH 400oC > =Si–OH 600oC .
We consider that the efficiency of dimethylchlorosilane chemisorption on the silica surface is depended on the nucleophility of oxygen atom of the
attacked silanol group. The decrease of electron density of oxygen atom in the OH-group along with the increase of the temperature of silicon dioxide pretreatment is associated with formation of new siloxane bridges in the process of dehydroxylation and, therefore, with the increase of delocalization degree of 2p-electrones of oxygen atom (with the increase of (p-d)p-conjugation in siloxane chain).
The thermal stability of surface compounds, formed as a result of chemisorption of dimethylchlorosilane on highly dispersed silica has been investigated. It was established that grafted hydride dimethylsilyl groups are stable up to 250oC in air and up to 500oC in vacuum.
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