lFour isomeric, unsaturated nitriles were hydrogenated, separately and as mixtures, over a Pd/alumina catalyst in a three-phase system at various pressures and temperatures. The nitriles were allylcyanide, cis- and trans-crotonitrile and methacrylonitrile and all have the general formula C4H5N. By varying hydrogenation conditions (temperatures, hydrogen pressure and substrate concentrations) activation energies and reaction orders were determined. Allylcyanide had the lowest activation energy because of the stability of its structure (non-conjugated) compared to the other substrates (conjugated). Reaction orders were -1, 0.5 and 1 for allylcyanide, crotonitrile and methacrylonitrile respectively. These findings indicate a difference in the strength of adsorption of each reactant, where allylcyanide is strongly adsorbed to the catalyst surface and methacrylonitrile weakly adsorbed. Isomerisation, which usually accompanies double bond hydrogenation in alkenes, was not observed. This lack of isomerisation may be due to the presence of more than one unsaturated group and the lack of an appropriate acid/base site on the support.
A range of different solvents were used to identify solvent effects for every isomer. A full kinetic analysis involving activation energies and reaction orders was undertaken for IPA and 1-propanol. An effect on reaction orders was observed and was believed to be related to competitive adsorption by the solvent. The activity of the system was sensitive to solvent and reactant. No single solvent parameter could be shown to trend with the variations observed. Variation of conversion in different solvents was related to substrate structure.
Competitive hydrogenation revealed subtle differences between the isomers. Allylcyanide did not show a significant change in reactivity when reacting in the presence of other isomers possibly because of it being more strongly adsorbed or because it adsorbed on corner or edge sites. Both crotonitrile and methacrylonitrile had their reactivity reduced in the presence of allylcyanide and when competing against each other. The behaviour was in keeping with the understanding developed for alkene hydrogenation over palladium
