Tautomerisation plays a key role in the Pd catalysed β-O-4′ bond cleavage in lignin derivatives. In this work, dispersion-corrected density functional theory (DFT-D3), has been used to investigate important species of the reaction. More specifically, a keto structure, namely 2-phenoxy-1-phenylethanone, and its enol form have been used as lignin derivative models. To investigate how the adsorption energies are affected by adsorbate coverage, we have used two different Pd(100) super cells; one smaller p(6 × 4) and one larger p(6 × 6). In the gas phase, the isolated keto tautomer is more stable than its enol form by 5.1 kcal/mol. However, the molecules have different affinities to the catalyst surface. At higher coverage, both the keto and enol models prefer to adsorb in an atop configuration on the surface. Furthermore, we find that both the keto and the enol models binds strongly to the surface through their phenyl rings. Previously, we studied the adsorption of similar molecular models on the Pd(100) surface, but with a methyl group instead of the phenyl ring. This resulted in much lower adsorption energies, about 10 % of the ones calculated this study. Despite the strong binding of the phenyl groups, the enol still adsorbs onto the surface through the cleavage of the C=C bond, leading to two types of di-sigma complexes depending on the position of the newly formed Pd-C sigma bonds. Our simulations show that there is a intermolecular repulsion between adsorbates on the surface, and consequently, the molecules are found to bind more strongly to the surface at low coverages (by 8-14 kcal/mol comparing the small and large super cells). The most stable adsorption complex found in this work corresponds to a low-coverage enolate structure. These results are important for experimental design purposes, as previous experiments have shown that the enol form is key for an efficient β-O-4′ bond cleavage.

Lignin intermediates on the Pd surface: Effect of coverage

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.