Selective oxidation of kraft lignin over zeolite-encapsulated Co(II) [H4]salen and [H2]salen complexes

Abstract

The Co([H4]salen) and the corresponding Co([H2]salen) complexes were encapsulated in NaY by the impregnation (IM) and flexible ligand ship-in-a-botttle (SB) method for oxidation of kraft lignin to obtain chemical compounds. The neat and encapsulated complexes were characterized by XRD, FTIR, DR UV-Vis spectroscopy and BET, which showed the observed changes in the molecular structure to be correlated to the enhanced activity of the complexes. GC-MS confirmed that the catalytic reactions of kraft lignin in the presence of peracetic acid produced major products including 2-methoxy phenol, 2-hydroxy benzaldehyde, 4-hydroxy-3,5-dimethoxyphenyl ethanone. The encapsulated complexes exhibited improved reactivity compared to the corresponding neat complexes. Additionally, there was a greater improvement in TOF value for complexes upon IM-immobilization as compared to SB-immobilization. The improved salen, that is, [H4]salen, was beneficial to the enhancement of catalytic activity as compared to [H2]salen, as revealed by the high TOF values due to the incorporation of a modified coordination environment of the central cobalt cation by C=N hydrogenation. Actually, Co([H4]salen)/IM was proved to be most active. Lignin conversion and TOF were high over the neat cobalt complexes when using 100% H2O as the solvent, and the values were high over the corresponding encapsulated analogues when using 80% H2O + 20% CH3OH as the solvent. It can be found that encapsulation and hydrogenation of the complexes can improve the selectivity to 2-methoxy phenol in the oxidation of kraft lignin. Copyright © 2014 Wiley Periodicals, Inc.