Carbon dioxide anion radical as a tool to enhance lignin valorization

Lignin is particularly recalcitrant for valorization via the existing pretreatment methods due to its complex cross-linking polymeric network. The aim of this study is to develop a novel integrated pretreatment strategy to exploit the potential of lignocellulosic biomass as resource for production of biofuels and aromatic chemicals. In this regard, a novel UV/TiO₂/HCOOH reaction was proposed to systematically generate hydroxyl radical (OH) and carbon dioxide radical anion (CO₂^-) to depolymerize lignin. Usage of 2,3-dihydrobenzofuran as a simple probe molecule showed cleavage β-O-4 linkage occurred via H abstraction mechanism. The addition of methyl viologen as CO₂^- scavengers proved the presence of CO₂^- in this UV/TiO₂/HCOOH reaction. Lignin and wheat straw were used to investigate the effect of different parameters, including formic acid concentration and TiO₂ dosage, on the efficiency of the reaction. At optimized conditions, the highest phenolic concentrations attained were 173.431 and 66.802 mg/g lignin and wheat straw, respectively. A cycle test was designed with the aim to favor the complete consumption of formic acid through more pretreatment cycles for producing the highest possible Total Phenolic Compounds (TPC) in the liquid phase. After the third consecutive cycle, 103.651 ± 5.964 mg-TPC/g, was obtained. Meanwhile it was found the remaining wheat straw solid fibers used for biogas production, showed 11.0% increase biogas production and increased degradation rate compared to the untreated wheat straw.