DFT study of selective hydrogenation of non-conjugated dienes on a Pd surface: the case of 2,5-norbornadiene

In this work a DFT modeling of various routes of hydrogenation of a non-conjugated bicyclic diene-norbornadiene (NBD)-was carried out using the all-electron scalar-relativistic approximation. According to calculations, the determining role in selective hydrogenation of NBD to norbornene (NBE) belongs to the type of palladium surface and adsorption configuration (exo or endo) of NBD and NBE molecules. Pd(111) surface shows selectivity in NBD adsorption, as well as activity in selective hydrogenation of NBD to NBE. Gibbs activation energies of hydrogenation of NBD to NBE and NBE to NB on Pd(111) differ by 4.3 kcal/mol in favor of the former route, which determines the selectivity of NBD hydrogenation. Formation of byproduct nortricyclane (NTC) involves overcoming a large Gibbs activation energy (29.2 kcal/mol), explaining its experimental trace amounts. Despite the high selectivity of Pd(100) surface in NBD adsorption, it is almost inactive in its hydrogenation. Notably, Gibbs activation energy of hydrogenation route of NBD to NBE on Pd(100) is 5.2 kcal/mol higher than that of NBE to NB on Pd(100) and 9.2 kcal/mol higher than that of NBD to NBE on Pd(111). Despite the significant increase in the adsorption energies of NBD and NBE, that was calculated with Grimme's D3 dispersion corrections, most of this difference is compensated when calculating activation energy.

Main DFT calculations were carried out using the scalar-relativistic approximation. The PBE exchange-correlation functional and all-electron basis sets L2 for C and H atoms and L11 for Pd atoms were used. The Pd(111) and Pd(100) surfaces were modeled as 3-layered flat clusters Pd₇₀ and Pd₇₃, respectively. Geometry optimization for key intermediates and TSs using the PBE-D3(BJ) functional, def2-TZVP basis sets, and def2-ECP pseudopotentials was performed in ORCA 6.1.1 software. For the most energetically favorable adsorption configurations of NBD and NBE molecules, DFT-PBE and PBE-D3(BJ) calculations were performed under periodic boundary conditions in CP2K software.