Preparation of Porous Acidic Oxides from Group 13 Element-containing Oligosilsesquioxanes

Abstract

The controlled calcination of newly-synthesized group 13 element-containing silsesquioxanes, PhB[(Me₃SiO) (c-C₅H₉)₇Si₇O₁₁] 1 and [PPN]⁺{Ga[(Me₃SiO)(c-C₅H₉)₇Si₇O₁₁]₂}^-3b, as well as [HNEt₃]⁺{Al[(Me₃SiO)(c-C₅H₉)₇Si₇O₁₁]₂}^-2, [HNEt₃]⁺{Ga[(Me₃SiO)(c-C₅H₉)₇Si₇O₁₁]₂}^-3a, and [C₁₄H₁₈N₂H]⁺[(c-C₅H₉)₇Si₇O₁₂GaCl]^-4 at around 823K produces acidic oxides with high BET surface areas of 330-520m²g^-1 and uniformly-controlled micropores of 5.1Å diameter. Detailed investigation of these oxides revealed that the pore structures are not greatly influenced by the presence of the group 13 elements and counter cations. However, the acidic properties are greatly affected by the structure of the silsesquioxane precursors, and partly controlled by the group 13 elements and counter cations. In particular, oxides from 2, 3a and 3b were found to have high BET surface areas and large numbers of acidic sites. The oxides from 3a have relatively strong acidic sites, whereas oxides from 2 and 3b have weak and moderate strength acidic sites. The oxides from 4 do not show significant surface acidity. Differences in the dispersion of the oxides of group 13 elements in the silica matrixes, which are closely related to the structure of precursors, are considered partly account for the differences in acidic character.