Catalytic depolymerization of lignin via transfer hydrogenation strategy over skeletal CuZnAl catalyst

Selective cleavage of the aromatic CO bonds in lignin is a crucial step for lignin valorization to produce value-added chemicals, however, the development of efficient catalysts for mild conversions is still challenging. Herein, non-precious skeletal CuZnAl catalysts were firstly reported for the depolymerisation of lignin dimers and real organosolv lignin via the catalytic transfer hydrogenation (CTH) method. Typical lignin dimers, including α-O-4, β-O-4 and 4-O-5 types, were all effectively converted to the corresponding aromatics and alcohols in isopropanol under mild reaction condition. Furthermore, birch organosolv lignin was also well depolymerized into monophenols in 56.1 wt% yield. Experiments revealed that the water brought by the wet catalyst effectively promoted the CO bond cleavage, but slightly decreased the catalyst reactivity toward hydrogenation in isopropanol. Reaction evolution profiles and control experiments suggested that three reaction pathways were included, with the CO ether bond cleavage and subsequent hydrogenolysis taking precedence over the dehydrogenation or hydrodeoxygenation routes. This work provides an economical and environmentally-friendly method for the selective cleavage of lignin and lignin model compounds into value-added chemicals, which holds great promise in industrial application.