Silica aerogel-carbon nanotube composites: Mechanistic insights into condensation reactions
Pedro Maximiano, Pedro Simões
Abstract
The mechanisms and energetics of condensation reactions between carboxyl or hydroxyl groups of oxidized carbon nanotubes and silanol groups of a silica aerogel were unveiled in several catalytic conditions via density functional theory calculations. The silanol–carboxyl condensation follows one-step SN2@Si mechanisms, which involve a simultaneous nucleophilic attack by an O atom of the carboxyl group on the Si atom and transfer of a proton to a leaving silanol group, via a pentacoordinated-Si transition state. AAC2 nucleophilic acyl substitution routes are very unlikely given the rather high activation free energies predicted. Different possible protonation sites and proton transfer pathways were explored for the cationic SN2@Si mechanism. The silanol-hydroxyl reaction occurs via a single-step SN2@Si mechanism in neutral media and via a two-step mechanism in basic conditions. In both cases, the nucleophilic hydroxyl O atom attacks the Si atom and an H is transferred to a silanol group that leaves as water. The condensation reactions in neutral media (no catalyst) have high activation free energies and thus require high temperatures, with the silanol–carboxyl reaction being more favoured under these conditions. One-pot synthesis of silica aerogel-CNT composites in strongly acidic media promotes the condensation between the silanol and carboxyl moieties, although the process is a chemical equilibrium. This reaction becomes unfeasible in basic conditions, which preferentially catalyze silanol–hydroxyl reactions.