Catalytic Oxidation and Depolymerization of Lignin in Aqueous Ionic Liquid

Abstract

Lignin is an integral part of the plant cell wall, which provides rigidity to plants, also contributes to the recalcitrance of the lignocellulosic biomass to biochemical and biological deconstruction. Lignin is a promising renewable feedstock for aromatic chemicals; however, an efficient and economic lignin depolymerization method needs to be developed to enable the conversion. In this study, we investigated the depolymerization of alkaline lignin in aqueous 1-ethyl-3-methylimidazolium acetate [C2C1Im][OAc] under oxidizing conditions. Seven different transition metal catalysts were screened in presence of H2O2 as oxidizing agent in a batch reactor. CoCl2 and Nb2O5 proved to be the most effective catalysts in degrading lignin to aromatic compounds. A central composite design was used to optimize the catalyst loading, H2O2 concentration, and temperature for product formation. Results show that lignin was depolymerized, and the major degradation products found in the extracted oil were guaiacol, syringol, vanillin, acetovanillone, and homovanillic acid. Lignin streams were characterized by Fourier transform infrared spectroscopy and gel permeation chromatography to determine effects of the experimental parameters on lignin depolymerization. The weight-average molecular weight (Mw) of liquid stream lignin after oxidation, for CoCl2 and Nb2O5 catalysts were 1,202 and 1,520 g mol, respectively, lower than that of Kraft lignin. Polydispersity index of the liquid stream lignin increased as compared with Kraft lignin, indicating wide span of the molecular weight distribution as a result of lignin depolymerization. Results from this study provide insights into the role of oxidant and transition metal catalysts and the oxidative degradation reaction sequence of lignin toward product formation in presence of aqueous ionic liquid.

Microbial fuel cell‐mediated lignin depolymerization: a sustainable approach

“…On the basis of the lignin depolymerization mechanism and HPLC results, there might be the presence of several lower and similar MW phenolic compounds including catechol (MW 110), guaiacol (MW 124), creosol (MW 138), 2‐Methoxy‐4‐vinylphenol (MW 150), vanillin (MW 152), 4‐ethylguaiacol (MW 152), syringol (MW 154) and vanillyl alcohol (MW 154) . In a previous study, five major products – guaiacol, syringol, vanillin, acetovanillone and homovanillic acid – were reported (with highest concentrate of guaicol) when Kraft lignin was treated with H 2 O 2 in aqueous ionic liquid . Beside its controlled electrochemical depolymerization of lignin and chemical treatment, the present method also may provide an efficient and ecofriendly technique to produce vanillin and related molecules, without using any ionic liquid, external power supply, harsh solvents or toxic chemicals.…”

Protic Ionic Liquids for Lignin Extraction—A Lignin Characterization Study

“…Depending on the biomass source utilized along with the lignin extraction method (pretreatment), various lignins can be liberated that may have different properties [ 7 , 8 ]. Typical pretreatment methods, such as physical (milling and grinding), physicochemical (steam pretreatment/auto hydrolysis, hydrothermolysis, and wet oxidation), chemical (alkali, dilute acid, oxidizing agents, organic solvents, and ionic liquids), biological, electrical, or a combination of these, have been studied for the extraction of lignin from biomass [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. Therefore, there is a need for the selective extraction of high quality lignin (purity and functionality) with physiochemical properties of interest for certain applications.…”

Investigating Lignin-Derived Monomers and Oligomers in Low-Molecular-Weight Fractions Separated from Depolymerized Black Liquor Retentate by Membrane Filtration

“…The complexity of the lignin molecule itself and the condensation reactions during the kraft process increase the complexity and recalcitrance of kraft lignin, making depolymerization more difficult. Numerous studies have been carried out on the depolymerization of kraft lignin during recent decades, including homogeneous base-catalyzed depolymerization (BCD) using NaOH as the catalyst [ 6 , 7 , 8 , 9 , 10 ], acid-catalyzed depolymerization [ 11 , 12 ], ionic liquid-catalyzed depolymerization [ 13 , 14 , 15 , 16 ], oxidative depolymerization using oxidants or an oxidizing protocol [ 17 , 18 , 19 , 20 ], and heterogeneously catalyzed depolymerization using supported transition-metal catalysts (e.g., Ni, Mo, etc.) [ 21 , 22 , 23 ].…”