Concerted Reaction Mechanism in Deuteration and H/D Exchange of Nitriles over Transition Metals

Abstract

In acetonitrile hydrogenation the selectivity to primary amine decreases in the order of Ru, Ni, Rh, Pd, Pt. Ruthenium is highly selective for primary amine and platinum for tertiary amine. The selectivity of the metals is related to their propensity for multiple bond formation. The metal known for the highest propensity to form multiple bonds has the highest selectivity to primary amine. In the deuteration of nitriles both D addition and H/D exchange take place. Most of the primary and secondary amine molecules that are formed when acetonitrile and D2mixture is passed over the catalysts have fewer D atoms than predicted by stoichiometry. D atoms are added to the C atom of the CN group, but H atoms are preferentially added to the N atom. For instance, for diethylamine the preferred isotopomers are (CH3CD2)2NH, (CH3CD2)–NH–(CD2CD3), and (CD3CD2)2NH. Many of these H atoms originate from isotope exchange of D2with the methyl group. A concerted mechanism is proposed in which dissociation of C–H bonds of some CH3CN molecules results in H transfer to other CH3CN molecules that are adsorbed with their C≡N bond parallel to the surface. Particularly with Ru it is also conceivable that this H transfer is instrumental in the amine desorption step when strong C–Ru bonds have to be broken. A second source of H atoms is a surface overlayer that is formed over all metals and will, initially, have a H/D ratio different from the overall ratio in the feed.