Kinetic approach of surface acidity of W2N, Mo2N and NbN catalysts using methylbutynol as molecular probe

In order to get more information on the acidic sites of early transition metal nitrides, methylbutynol (2-methylbut-3-yn-2-ol) (MBOH) conversion was performed on bulk W2N, NbN and Mo2N passivated by oxygen after synthesis. The reaction proceeds through two parallel routes leading either to 2-methylbut-1-en-3-yne (Mbyne) by dehydration, or to 3-methylbut-2-enal (prenal) by isomerisation. Deactivation during time on stream occurred for both routes, the selectivity remaining constant. This deactivation can be modelled by a second-order rate law allowing to give accurate initial reaction rates. Zero-order reactions were shown to occur, leading to constant reaction rates along the two routes: r(Mbyne) = 3.71 x 10(-4) mol min(-1) g(-1) and r(prenal) = 1.94 x 10(-4) mol min(-1) g(-1) in the case of W2N. Similar global activation energies were also found for both ways: E-a,E- Mbyne = 24.7 kJ mol(-1) and E-a,E- prenal = 22.1 kJ mol(-1). All these data were interpreted in terms of acidic hydroxyl and metal-axo groups, linked to the same surface transition metal atom. Dehydration occurs on Bronsted acid sites, whereas isomerisation occurs on metal-oxo ones. Kinetic coupling of two catalytic cycles, linked by the same elementary step (MBOH chemisorption), leads to the saturation of all surface active sites by the reactant, with two different turnover rates. This kinetic model permitted a coherent interpretation of all the data. The differences observed between W2N, NbN and Mo2N are interpreted in terms of acid-base properties of the catalytic materials. (C) 2002 Elsevier Science B.V. All rights reserved.