Trapping Hydrogen: Confined Catalysis for Improved Alcohol Amination Selectivity

Abstract

Confined chemistry is a powerful tool in catalysis. In this study, we report hierarchical structures with controlled morphology able to trap labile intermediates and improve a catalytic cascade reaction. We used alcohol amination via hydrogen borrowing as model, a process that gives substituted amines from alcohols and does not require the addition of hydrogen to reduce the imines or the use of coupling agents. A common problem however in those systems is the loss of the borrowed hydrogen atoms, leading to stagnation of the product at the imine stage. To this end, we encapsulated Al₂O₃/Ru(OH)_x nanocatalysts inside mesoporous silica in a yolk-shell architecture and were able to trap the hydrogens to increase the amine yield from 12% to 82%, with a three-fold increase in selectivity without the need of any additive. We found the presence of mesopores in the silica shells to be essential to enable access to the catalytic sites and the yolk-shell gap size to be the key parameter influencing the reactivity of the catalytic system. To the best of our knowledge, this is the first report of a confined hydrogen borrowing reaction, an approach that can be extended to the other types of cascade reactions that produce labile intermediates.