Estimation of spacing between 3-bromopropyl functions grafted on mesoporous silica surfaces by a substitution reaction using diamine probe molecules

Abstract

We present here the results of the substitution of 3-bromopropyltriethoxysilane-grafted mesoporous SBA15 (pore size 7.6 nm) with a series of diamines, CH₃NH(CH₂)_nNHCH₃ (C2DA, C3DA and C6DA for n = 2, 3 and 6, respectively) or NH₂(CH₂)_nNH₂ (C4DA and C5DA for n = 4 and 5, respectively). The outcome of the reactions was closely related to the spacing of the surface bromopropyl groups. Amount of bromine that remained after the reaction decreased linearly with the amount of C2DA, and it disappeared completely when C2DA : Br = 1 : 1 in the initial reactant mixture. This result indicates the complete conversion of C2DA and Br by a one-to-one substitution (i.e. formation of a linear species). By contrast, the rate of decrease of Br was twice as fast during substitutions with amines other than C2DA when diamine/Br < 0.5 (in the initial mixture), suggesting a one-to-two substitution and the formation of a bridge species. We show that the C/N ratios in the elemental analysis after the complete substitution of Br are a simple and reliable indicator of the distributions of the pair spacings of the bromopropyl groups. The results are compared by a geometric calculation in which the configurations and the minimum limits of neighbouring distances are ignored. The formation of linear and bridge forms were confirmed by ¹³C-NMR analysis. Reactions using MCM41 (pore size 2.4 nm) and fumed silica were also explored for comparison. We found that the order of the pair spacings of the bromopropyl groups grafted onto these silicas was MCM41 < SBA15 ≪ fumed silica. This result disagrees with the outcome that was expected from the number of bromopropyl groups per unit nm², 0.8, 1.1 and 1.2 for MCM41, SBA15 and fumed silica, respectively, which was derived from the BET specific surface area and bulk elemental analyses. All of these results are consistent with a mode of grafting where the reaction with silanols competes with diffusion into the pore channels.