The influence of solvent composition on the coordination environment of the Co/Mn/Br based para-xylene oxidation catalyst as revealed by EPR and ESEEM spectroscopy

Abstract

The industrially important para-xylene oxidation reaction, based on a Co/Mn/Br catalyst, operates in a water/acetic acid (H₂O/AcOH) solvent system. The correct H₂O/AcOH ratio of the solvent is crucial in controlling the reaction yields and selectivities. However, the influence of this variable solvent system on the catalyst structure and coordination environment is not well understood. Using UV-vis spectroscopy, we observed the formation of tetrahedral Co²⁺ species when the solvent composition was below 10 wt% H₂O. These were considered to be tetrahedral Co²⁺ species with either 2 or 3 coordinating Br⁻ ligands. The pronounced CW EPR linewidth changes observed in the Mn²⁺ signals revealed a strong correlation on the solvent H₂O content. Detailed analysis revealed that these variations in the linewidth were attributed to the changing coordination sphere around the Mn²⁺ centres, with a maximum linewidth occurring at 8–10 wt% H₂O. The narrow linewidths below 8 wt% H₂O were found to result from substitution of H₂O/AcOH ligands by Br, whereas above 8 wt% H₂O a further narrowing of the linewidth was actually caused by greater amounts of H₂O coordination. To confirm this, 3-pulse ESEEM measurements on the Mn²⁺ were conducted in the solvent compositions corresponding to 3, 8, 13.7 and 20 wt% H₂O. The results showed a marked change in the number (n) of coordinated H₂O molecules (ranging from n = 0, 0, 1.0 to 4.0 respectively for the 3–20 wt% H₂O content). For the first time, these findings provide a crucial insight into the relationship between solvent composition and catalyst structure in this industrially important catalytic reaction.