Evidence for the Pairwise Disposition of Grafting Sites on Highly Dehydroxylated Silicas via Their Reactions with Ga(CH3)3

Molecules encountering silica interfaces interact primarily with the hydroxyl groups that terminate the bulk structure. When the nominal surface density is very low, these silanols are presumed to be isolated. Nevertheless, silicas that are highly dehydroxylated by pretreatment at 800 degrees C react with Ga(CH3)(3) at room temperature to give primarily disilanolate-bridged digallium sites, [(CH3)(2)Ga(mu-OSi )](2). The EXAFS at the Ga K-edge shows a prominent Ga-Ga scattering path, regardless of whether an excess or a limiting amount of Ga(CH3)(3) is used. Some dimers are formed by the concerted reaction of Ga(CH3)(3) with an isolated silanol and an adjacent siloxane bond. These grafting sites are proposed to be hydroxyl-substituted 2-rings, formed by condensation within a vicinal Q(2)-Q(3) pair. Other dimers are formed by reaction of Ga(CH3)(3) with vicinal Q(3)-Q(3) pairs which have not condensed, even at 800 degrees C. In a computational model for the dimer sites, the O-O distance is < 2.6 angstrom, which is far shorter than the calculated mean interhydroxyl separation for the thermally treated silicas (12.2 angstrom). This highly nonrandom distribution of surface silanols, in combination with the coupled reaction of isolated silanols and strained siloxane bonds, accounts for the preferential formation of grafted site pairs rather than isolated grafted sites when silica surfaces are chemically modified.