Unveiling the phenol direct carboxylation reaction mechanism at ZrO2 surface

In the present context of environmental concerns, sustainable solutions must be proposed to dispose of waste CO₂, a well-known greenhouse gas. Among the various emerging projects, upgrading CO₂ molecule into high-value added chemicals appears to be very promising. More particularly, the carboxylation of aromatic compounds to (di-) acid aromatic monomers is of great interest for the high performance polymer industry. Focusing on the direct phenol carboxylation to para-hydroxybenzoic acid as a model reaction, the reactivity of ZrO₂ was investigated in this paper, this material being recently reported in various experimental works for its catalytic efficiency. For the first time, we established the phenol carboxylation mechanism at the surface of a metal oxide material, showing that the reaction can only proceed through an Eley-Rideal mechanism. In this mechanism, CO₂ is strongly chemisorbed at the surface, whereas phenol is physisorbed close to the CO₂ adsorbate. Besides, while the monoclinic and the tetragonal phases often coexist in ZrO₂ particles, we demonstrated that only the monoclinic geometry exhibits a substantial activity. However, the selectivity remains a major challenge, the ortho- isomer being the most abundant product, as in the original Kolbe-Schmitt method. While most of the processes generally reported in literature for the direct carboxylation of phenol are achieved in liquid media, a very few theoretical knowledge is available to describe such a process at solid surfaces. Therefore, we expect the present manuscript to be a pioneer work, aiming at providing a better understanding of metal oxide surface reactivity, paving the road to the rational design of efficient solid catalysts for aromatics carboxylation reactions.