The most frequent catalysts used in lignin oxidation with O2 in alkaline medium are transition metal salts, such as CuO, CuSO4, FeCl3, and Fe2O3, which have high oxidation potential and easily would allow electron transference from the aromatic rings of lignin; at the same time this high oxidation potential turns the regeneration of the metal salt in the catalytic cycle more difficult. The oxidation with catalyst has been extensively tested on model compounds of lignin, most of them mono­mers. A recent and comprehensive review was recently published by Zakzeski et al. [90] about oxidative catalysis and other perspectives of the catalytic valo­rization of lignin.

Oxidations experiments were performed by Mathias and Rodrigues [118] using CuSO4 (4% of lignin weight) and comparing the reaction rate and yields to the non-catalyzed reaction of kraft lignin at the same conditions. The yield on vanillin was similar, as well as the time to maximum. However, a low degradation rate was found in the case of catalyzed reaction. The same salt was tested in the oxidation of lignosulfonates used at 20% of lignin weight [113]. The authors reported an increment of 1.3 and 1.4 of the yields of vanillin and syringaldehyde produced in the non-catalyzed reaction.

Tarabanko et al., reported 12-13%wt (lignin basis) of vanillin in batch oxida­tion of lignosulfonates using about 16 g/l of Cu(OH)2 [139]. The yield of non-catalyzed reaction was 5.5 wt. % on vanillin at 40 min. The lignosulfonate liquor of the same origin was the raw material for further experiences in contin­uous process [159]. In this case, the authors reported also the syringaldehyde yield. Copper wire and cupric oxide wire were tested as catalyst and simultaneously as reactor packing. In this work, the conclusions about the catalyst effect are some­what difficult due to the simultaneous variation of parameters as, for example, the oxygen rate in continuous process. However, for lignin from other origin, the comparison with non-catalyzed reaction leads us to notice an increase of aldehydes yield (from 1.2-1.8 wt% to 1.5-3.0 wt% on lignin bases) [159].

Other combinations of catalysts were tested in the oxidation of an hardwood lignin: the mixture of CuSO4 with FeCl3 and CuO with Fe2O3 [110]. At reaction temperature of 433 K the yields were incremented by CuSO4/FeCl3 (4.5% for vanillin and 7.4% for syringaldehyde) in comparison with the non-catalyzed reaction at similar reaction conditions (2.5% for vanillin and 4.5% for syringal — dehyde). Based on data reported for 443 K, the additional effect of FeCl3 was evidenced: 4.7 and 9.5% of vanillin and syringaldehyde, respectively, against 3.5 and 6.5% for the reaction at similar conditions using CuSO4 alone.

Besides the Cu(II) catalysis, Bj0rsvik and Minisci [155] also tested salts of Co(II) and Ce(IV) salts in lignin oxidation. The efficiency of the Cu(II) and Co(II) catalysts was similar: 5.9 and 5.8%, respectively; however, in the reaction cata­lyzed by Cu(II), the maximum yield of vanillin was reached faster (70 min) than with Co(II) (90 min). The salt of Ce(IV) shows a lower efficiency in the oxidation of lignosulfonate, which was ascribed to the higher difficulty to reoxidation of the

Ce(III). Besides Cu and Co salts, two commercial platinum-alumina catalysts were studied by Villar et al. [123]. The copper salts produced better results; however, with similar yield of that obtained without catalyst. Moreover, cobalt and plati­num-alumina catalysts showed a negative effect on lignin conversion.

Sales et al. [111, 112] used palladium catalyst supported on y-alumina for oxidation of sugarcane bagasse lignin (batch and continuous). The catalyst revealed effective on increasing the rate of formation of aldehydes, 10-20 times higher compared to the correspondent non-catalytic reactions.

A heterogeneous catalyst was recently reported in literature [124] as effective in the production of aldehydes: the perovskite-type oxide LaFe1_xCuxO3 (x = 0, 0.1, 0.2). The maximum yield of p-hydroxybenzaldehyde, vanillin, and syringaldehyde was significantly improved with the catalyst LaFe0.8Cu0.2O3: 2.49% (at 120 min), 4.56% (at 60 min) and 11.51% (at 30 min), respectively. These values represent 1.66-, 1.42-, and 2.51-fold increase compared to non-catalytic process, respec­tively. Besides this good performance, the catalyst maintained the effectiveness after successive recycling being a promising candidate for research applications in oxidation of lignins from other sources.