Modelling the adsorption on the surface of metal catalysts applied in catalytic hydrogenations – running
Compounds containing nitrogen, phosphorous and sulphur, respectively, are usually important and valuable pharmaceutical, plastic or agrochemical intermediates. During the preparation of these substances, heterogeneous catalytic hydrogenation is a frequently applied process. However, the activity of a catalyst (typically supported palladium, platinum, ruthenium or rhodium catalysts) often decreases or ceases completely during the hydrogenation of such type compounds, i.e. the catalyst is poisoned.
It has long been known that the reason of this phenomenon is the formation of a dative bond between the unshared pair of electrons of nitrogen, phosphorous or sulphur atoms and the d-orbits of precious metals. Thus a strong, chemical-like and very specific bond forms between the active component of a catalyst and the poisoning molecules, which inhibits the further catalytic processes.
During our research work we would like to obtain more detailed information about the interactions between catalytic metals and substrates. We will examine how certain factors influence poisoning of the catalysts during the hydrogenation, what kind of relationship is between the structure of substrate and the activity, as well as the structure of the metal catalyst. We will investigate how poisoning of the spent catalyst can be eliminated in an easy way, for instance, by the rehydrogenation of the catalyst, i.e. hydrogen is able to push off the poisoning molecules from surface of the catalyst and to restore the original activity of the catalyst. Furthermore, we try to elucidate the relationships between the structure (nanosized texture) of a catalyst and its activity.
In order to give answers to the above-mentioned questions molecular modelling calculations will also be performed to model the process of adsorption on a metal surface in a realistic way. Using semiempirical methods (e.g. PM3) several conformers of some catalytically unhydrogenable pyrrole derivatives, as well as the energy profiles of their hydrogenation (Gibbs free energy of activation, heat of reaction, heat of formation, etc.) were already described successfully, explaining the failure of these reactions. Significant differences were observed in the hydrogenation of aromatic and aliphatic type nitriles (e.g. benzonitrile, benzyl cyanide), namely lower isolated yield and primary amine selectivity were achieved in the reduction of benzyl cyanide than that of benzonitrile under the same conditions. This was attributed to the dissimilar adsorption abilities of the imine type intermediates, on the basis of density functional (Becke3LYP/6–31G*) calculations. We plan to determine the mechanism of the reactions and provide new thermodynamical data on the hydrogenation of nitriles over various carbon supported precious metal catalysts using molecular modelling, as well. For this, we elaborate on both discrete molecular calculations at the atomic orbital approach by GAMESS software and simulation of periodic systems using plane wave bases by Quantum Espresso, both within the frame of the density functional theory. Considering the complexity of a heterogeneous system at the atomic level, high level calculations and significant storage capacities are sorely needed, like the BME HPC Cluster has.
- Project owner:
- Dr. Hegedűs László (Szerves Kémia és Technológia Tanszék)
- Szerves Kémia és Technológia Tanszék (VBK-SZSKEM)