Reactivity Of Hydrochlorosilanes Clnsi(ch3)3-nh (n=1-3) Relative To Silica Surface
REACTIVITY OF HYDROCHLOROSILANES ClnSi(CH3)3-nH (n=1-3) RELATIVE TO SILICA SURFACE
A.M. Varvarin, L.A. Belyakova, L.N. Kravchuk
Institute of Surface Chemistry, Ukrainian National Academy of Sciences, General Naumov st. 17, 03164 Kiev, Ukraine
Silicas modified by silicon-hydride groups (hydride silicas) are used as specific adsorbents, chromatographic supporters as well as for preparation of catalysts. The most simple method of introduction of ºSi-H groups into surface layer of silica is interaction of hydrosilanes with silicon dioxide. In order to clarify the peculiarities and the regularities of chemisorption of organosilicon compounds with Si-H bond on silicon dioxide surface the interaction of silica with hydrosilanes ClnSi(CH3)3-nH (n=1-3) was studied. These hydrosilanes being reactive series are perspective for fulfillment of directional syntheses in the surface layer of SiO2.
The investigation of chemical interaction of hydrochlorosilanes with silica surface pretreated at 600, 400 or 200° C was performed using IR spectroscopy method as well as gravimetric and volumometric analysis.
It has been established that the only isolated silanol groups are chemisorption centers of silica surface pretreated under different temperatures as a result of the interaction with hydrosilanes: the concentration of grafted silicon-hydride groups is equal to that for free OH groups on the surface of starting silicon dioxide. The siloxane bridges and OH groups interconnected by hydrogen bonds (the temperature of pretreatment of silica is 200° C) do not participate in reaction.
It has been shown that the efficiency of fixation of silicon-hydride groups on the surface of silicon dioxide increases with decreasing of silica pretreatment temperature. In other words, with increasing of temperature of surface dehydroxylation the reactivity of isolated silanol groups of silicon dioxide relative to hydrochlorosilanes decreases:
ºSi-OH200 > ºSi-OH400 > ºSi-OH600
We consider that this fact is due to decrease of electronic density on the oxygen atoms of OH groups during silica dehydroxylation as a result of the
formation of new siloxane bridges and an increase in the extent of delocalization of 2p-electrons of oxygen atoms ((p-d)p conjugation in Si-O-Si chain). With increasing of temperature of silica pretreatment along with decrease of nucleophilicity of oxygen atoms of OH groups the increase of proton-donor ability (or acidity) of free OH groups take place.
Really, we have show (by IR spectroscopy method and by nonaqueous titration of silicas different hydroxylation degrees) that as temperature of silica pretreatment increases, the acidity of isolated OH groups enhances: an increase of shift of absorption band for OH groups under adsorption of benzene from 113 cm-1 to 130 cm-1 for 400° C-surface and a decrease of pKa of silanol groups from 8.68 to 8.00 are observed. Thus, the activity of hydrochlorosilanes in reactions with isolated hydroxyl groups of silica pretreated at 200-600° C changes symbately to nucleophilicity of oxygen atoms of silanol groups.
The series of the change of hydrochlorosilanes reactivity relative to isolated OH groups is established also:
Cl3SiH > Cl2Si(CH3)H > ClSi(CH3)2H.
It has been shown that the activity of hydrochlorosilanes in reaction with OH groups depends on a value of effective positive charge on silicon atoms of attacked molecules which is due to total inductive effect of substituents at silicon atoms.
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