Trapping hydrogen: Confined catalysis for improved hydrogen borrowing selectivity

Alcohol amination via hydrogen borrowing is an established method for the clean and simple alkylation of amines with alcohols, which are stable and available in bulk; it also 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 need to employ additives to prevent stagnation of the product at the imine stage, which indicates inefficient usage of the borrowed hydrogen atoms. In this work, we designed a catalyst series to demonstrate that confined environments can assist with improved selectivity. 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 3-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 cata-lytic 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.