## 2 Results and Discussion
### Catalytic Oxidation of Ethylbenzene
#### 2.1.2 Stability of CeNR in the Liquid-Phase Oxidation Reaction  
In order to get some insight into the stability of the CeNR under the applied oxidizing conditions, the solid was recovered from the reaction of PhEt with _t_-BuOOH, using acetonitrile as solvent, at 70 \({}^{\circ}\)C and used in a second batch run. The solid was separated from the reaction medium by centrifugation, washed consecutively with _n_-hexane, dichloromethane and acetone, and finally dried at room temperature prior to reuse (giving CeNR-PhEt-BuOOH). The kinetic curves for runs 1 and 2 are roughly coincident as well as the dependence of products selectivity on conversion (Figure 4, Table 2). The powder XRD patterns of CeNR and CeNR-PhEt-BuOOH are similar (Figure 1). The TGA analyses of the fresh and used solids gave similar weight losses between 30 \({}^{\circ}\)C and 600 \({}^{\circ}\)C (_ca._ 8 wt. %), under air, and therefore coking is negligible. These results suggest that CeNR is fairly stable under the applied oxidizing conditions. Carrying out the reaction of PhEt in homogeneous phase in the presence of the precursor used for preparing CeNR, e.g., cerium sulphate (using the same mass as CeNR), at 70 \({}^{\circ}\)C, gives 15% conversion after 120 h, and PhEtOOT and acetophenone are formed in 34% and 66% selectivity, respectively (Table 2). The reaction is much faster in the presence of CeNT than that of Ce\({}_{2}\)(SO\({}_{4}\))\({}_{3}.9\)H\({}_{2}\)O, under similar reaction conditions. Commercially available CeO\({}_{2}\) (powder, \(<\) 5 \(\upmu\)m particle size, cubic fluorite-type, irregular morphology and apparently wide particle size distribution, Aldrich) gives 29% conversion after 150 h, at 70 \({}^{\circ}\)C, and PhEtOOT and acetophenone are formed in 63% and 37% selectivity, respectively; under similar conditions, the reaction is faster in the presence of CeNR (64% conversion), and PhEtOOT and acetophenone are formed in 18% and 82% selectivity, respectively. In order to assess the homo/heterogeneous nature of the catalytic reaction in the presence of CeNR, the solid was separated by filtration ([PERSON] 0.2 \(\upmu\)m PVDF w/GMF membrane) after 6 h reaction, at 70 \({}^{\circ}\)C, and the obtained solution was left to react at this temperature until 48 h. Conversion between 6 h and 48 h was approximately a third of that observed in the presence of CeNR (11% _vs._ 32%): possibly, the reaction proceeds in homogeneous phase due to (i) active CeNR, which was not separated by filtration and/or (ii) free radicals formed prior to catalyst filtration. Hypothesis (i) is ruled out by two experiments. The solid was separated from the reaction medium after 120 h by filtration at 70 \({}^{\circ}\)C, and the obtained solution was analysed by ICP-AES for cerium, which was not detected (detection limit _ca._ 5 ppb). Nevertheless, the obtained solution was evaporated to dryness and to the same vessel PhEt, _t_-BuOOH and CH\({}_{3}\)CN were added in the same amounts as those used for the first batch run: after stirring this mixture during 150 h, at 70 \({}^{\circ}\)C, no PhEt reaction was observed, indicating that the'residue' obtained after evaporation did not contain active cerium compounds. Hypothesis (ii) is supported by the fact that hypothesis (i) is ruled out and the reaction stops when a radical scavenger is added, as mentioned in section 2.1.1. Based on these results, it seems clear that CeNR performs as a heterogeneous redox catalyst for generating free radicals, accounting for a radical chain process in the homogeneous phase. When pristine CeNR was heated up to 600 \({}^{\circ}\)C in air no structural modifications were observed for the resulting solid (denoted CeNR600), as ascertained by powder XRD, SEM and FTIR (not shown).