CATALYTIC CONVERSION OF LIGNIN BY IMMOBILIZED Cu[H-4]SALEN AND [H-2]SALEN COMPLEXES UNDER HYDROTHERMAL CONDITIONS

A comparative investigation was made of the effect of catalysis by immobilized Cu([H-4]salen) on the hydrothermal conversion of organosolv lignin. The immobilization and hydrogenation of the complexes led to increased yield of liquid products compared with the corresponding unimmobilized and unhydrogenated complexes in catalytic conversion of lignin. In addition, the method of immobilization of the complexes affected the catalytic performance in lignin conversion, in which a higher yield of liquid products was obtained with the SB-immobilized complexes (ship-in-bottle method) than with the NI-immobilized complexes (impregnation method). The yield of liquid products obtained with Cu([H-2]salen) at 250 degrees C and 4 MPa O-2 in water was 6.00%, compared with 16.80% and 22.08% respectively for Cu([H-4]salen)/IM and Cu([H-4]salen)/SB. Additionally, the addition of organic solvent had a marked effect on the catalytic performance of Cu([H-4]salen), and the highest yield of liquid products (27.06%) was observed with Cu([H-4]salen)/SB in water/methanol (80/20, v/v). The total yield of liquid products varied depending on temperature and oxygen pressure, and reached 46.01% under reaction conditions of 280 degrees C and 6 MPa O-2 in water/methanol (80/20, v/v). GC-MS analysis showed the main compounds in the liquid products to be phenols, with a yield of 32.16%. Other compounds included alcohols, ketones, aromatics, olefins, cycloalkanes, and alkanes. A mechanism for their formation was proposed based on the oxidation of a lignin model compound under hydrothermal conditions.