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

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.