(66b) Reductive Depolymerization of PAA-Fractionated Lignin Via Using Cupmo Catalyst

Lignin is a key feedstock in biorefinery processes for accomplishing a successful total biomass utilization. It is typically fractionated from carbohydrates such as cellulose and hemicellulose prior to its conversion. However, it is not avoidable to modify its intact structure. Especially, harsh reaction conditions during this fractionation cause unwanted decomposition and condensation, resulting in low end-product yield. Another challenge in lignin valorization processes is the use of expensive catalysts. Various heterogeneous catalysts such as Pt, Pd, and Ru have been applied for effective conversion of lignin; however, these catalysts are costly even though their regeneration is feasible. Therefore, many researchers try to develop earth-abundant metal catalysts to replace current expensive ones.

Cu is one of the earth-abundant metals but is still effective in many thermochemical processes. Successful catalytic hydrodeoxygenation of lignin has been reported in supercritical solvent system such as methanol, using an earth-abundant element Cu-doped porous metal oxide (PMO) as the catalyst, at a relatively mild temperature (300 °C) without addition of external hydrogen. In this single one-pot reaction system, supercritical methanol was catalyzed by the Cu-doped PMO catalyst, releasing H₂ as an in situ liquid syngas precursor. The hydrogen transferred from supercritical solvent to lignin would then result in the hydrogenolysis of phenyl ether bonds coupled with the hydrogenation of aromatic rings.

In this study, peracetic acid (PAA) effectively fractionated lignin preserving relatively high b-O-4 linkage content and minimal condensation due to its mild reaction conditions. CuPMO catalysts were prepared and applied for reductive depolymerization of the fractionated lignins. The fractionated lignins were characterized with two-dimensional heteronuclear single quantum coherence (2D HSQC) nuclear magnetic resonance (NMR), ³¹P NMR, and gel permeation chromatography (GPC) to understand the impact of PAA on lignin characteristics. The applied catalysts were characterized by inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (TPR), and NH₃ temperature-programed desorption (TPD). The depolymerized products were analyzed by GPC and gas chromatography mass spectrometry (GC-MS).