## 2 Results and Discussion
### Stability of Copolymer Catalysts during Aerobic Oxidation of PMT
Thus, the presence of primary NH\({}_{2}\) groups and carboxyl groups formed in the copolymer structure as a result of the decomposition of NHPI groups results in the adsorption of Co(II) or Co(III) cations. XPS measurements were performed for the spent catalysts containing 60 mol% of the functional monomer with different cross-linker contents (3, 5, or 7 mol% of DVB), which has been previously analyzed in the fresh form (cf. Figure 4). The results collected for the materials after 6 h of the catalytic run, isolation, washing with PMT and effective drying are shown in Figure 9. In the XPS N 1s spectrum of the 60%NHPI_7% DVB material with the highest cross-linking degree, beside the peak characteristic for N-OH functionalities, an additional peak appears at 399.2 eV. This can be attributed to the presence of amine groups in hydroxylamine trapped in the copolymer structure [61]. Interestingly, in the case of this sample, significant amounts of Co are also found on the surface. The chemical nature of these species can be determined on the basis of the XPS Co 2p spectrum (Figure 9). The parent Co 2p\({}_{3/2}\) and 2p\({}_{1/2}\) peaks at 781.9 eV and 797.5 eV (spin-orbit splitting \(\Delta\) = 15.6 eV), respectively, with very intense satellite structure (at 786.0 eV and 802.6 eV), suggest that high-spin Co(II) complexes were formed with NH\({}_{2}\)OH species [62; 63]. The intensities of the peaks attributed to photoemission from Co atoms in the less cross-linked samples are significantly lower. It is also worth noting that the content of NH\({}_{2}\)OH and, thus, the formed Co(II) complexes is strongly correlated with the catalytic activity in the aerobic PMT oxidation. The high cross-linking most likely causes that the hydroxylamine released as a result of the decomposition of NHPI groups remains bound inside the polymer network, while attaching Co(II) cations. Consequently, this leads to a fast loss of activity of the material. Fortunately, there is a relatively simple way to regenerate the spent catalyst, which was confirmed for one of the most active materials--60%NHPI_5%DVB. After the first catalytic cycle, in which after 6 h the PMT conversion of 23.3% was achieved, the catalyst was separated, washed with PMT and introduced into the fresh reaction system. After the repeated 6 h test, the obtained PMT conversion significantly decreased 6.6% due  
Figure 9: XPS N 1s and Co 2p spectra collected for the 60 NHPI_3%DVB, 60 NHPI_5%DVB and 60 NHPI_7%DVB copolymer catalysts after the aerobic PMT oxidation at 80 \({}^{\circ}\)C (0.1 g copolymer catalyst, 5.0 mg AIBN, 5.0 mg Co(OAc)\({}_{2}\)-4H\({}_{2}\)O, 2.0 mL PMT, 1 atm O\({}_{2}\), 6 h). to the limited number of available active sites. The initial nitrogen content of 5.3 wt% corresponding to 3.78 mmol N-OH/g reduced after the first test to 2.4 wt% (1.71 mmol N-OH/g). The used catalyst was activated by repeating the last two steps of NHPI group formation, i.e., the reaction leading to the generation of phthalic anhydride structure, and then its transformation into the active forms of NHPI. In this way, the N content was restored to the level of 4.8 wt% (3.43 mmol N-OH/g), which allowed to restore high catalytic activity of 21.3% PMT conversion.