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

Abstract

ABSTRACTThe 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. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40809.