Reaction mechanisms in microwave-assisted lignin depolymerisation in hydrogen-donating solvents

The effective exploitation of lignin, the world's largest renewable source of aromatics, is alluring for the sustainable production of chemicals. Microwave-assisted depolymerisation (MAD) of lignin using hydrogen-donating solvents (HDS) is a promising technique owing to its effective volumetric heating pattern and so-called "non-thermal effects." However, lignin is a structurally complex bio-polymer, and its degradation produces a myriad of products; consequently, MAD reaction mechanisms are generally complex and poorly understood. This review aims to provide a perspective of current research into MAD reaction mechanisms involving HDS, with the goal to give researchers an overall understanding of MAD mechanisms and hopefully inspire innovation into more advanced methods with better yields and selectivity of desired aromatics. Most reaction mechanisms were determined using characterisation methods such as GC-MS, MALDI-TOF, 2D-NMR, GPC, and FT-IR, supported by computational studies in some instances. Most mechanisms generally revolved around the cleavage of the beta-O-4 linkage, while others delved into the cleavage of alpha-O-4, 4-O-5 and even C-C bonds. The reactions occurred as uncatalysed HDS reactions or in combination with precious metal catalysts such as Pt/C, Pd/C and Ru/C, although transition metal salts were also successfully used. Typical MAD products were phenolic, including syringol, syringaldehyde, vanillin and guaiacol.