The influence of solvent composition on the coordination environment of the Co/Mn/Br based -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 (H 2 O/AcOH) solvent system. The correct H 2 O/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 Co 2+ species when the solvent composition was below 10 wt% H 2 O. These were considered to be tetrahedral Co 2+ species with either 2 or 3 coordinating Br − ligands. The pronounced CW EPR linewidth changes observed in the Mn 2+ signals revealed a strong correlation on the solvent H 2 O content. Detailed analysis revealed that these variations in the linewidth were attributed to the changing coordination sphere around the Mn 2+ centres, with a maximum linewidth occurring at 8-10 wt% H 2 O. The narrow linewidths below 8 wt% H 2 O were found to result from substitution of H 2 O/AcOH ligands by Br, whereas above 8 wt% H 2 O a further narrowing of the linewidth was actually caused by greater amounts of H 2 O coordination. To confirm this, 3-pulse ESEEM measurements on the Mn 2+ were conducted in the solvent compositions corresponding to 3, 8, 13.7 and 20 wt% H 2 O. The results showed a marked change in the number ( n ) of coordinated H 2 O molecules (ranging from n = 0, 0, 1.0 to 4.0 respectively for the 3-20 wt% H 2 O 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. The industrially important para -xylene oxidation reaction, based on a Co/Mn/Br catalyst, operates in a water/acetic acid (H 2 O/AcOH) solvent system.