Catalytic Alkaline Oxidation of Lignin and its Model Compounds: a Pathway to Aromatic Biochemicals

Abstract

Catalytic oxidation via the application of molecular oxygen and copper complexes is a useful pathway toward valuable low molecular mass compounds from in situ or waste stream lignins. In this study, two dimeric β-ether model compounds, one β-ether oligomer, and a milled wood lignin sample from Loblolly pine were catalytically oxidized. Yields and stability of the aromatic aldehyde and acid products were measured. Nuclear magnetic resonance spectroscopy and gel permeation chromatography were used to monitor structure/composition and molecular mass changes of the lignin before and after catalytic oxidation to study the degree of depolymerization and structure of the residual lignin. Oxidized units appear to be derived from β-aryl ether, phenylcoumaran, and biphenyl ether components. To date, this method breaks down the lignin polymeric structure reasonably effectively, producing low molecular mass products; this work also highlights some of the issues that need to be overcome to optimize this approach.

Predominant formation of aromatic aldehyde and acid from a dimeric β-O-4-type lignin model compound under hydrogen peroxide bleaching conditions with high pH levels

“…Because the disappearance of compound 1 was still about 22-24% or 15-17% based on its initial amount at an initial pH of 13.3 or 12.8, respectively, at a reaction time of 120 min and the degradation rate of compound 1 was constant throughout the reaction, a prolonged reaction with adding H 2 O 2 stepwise must further afford much larger amounts of veratraldehyde and veratic acid with finally attaining the yield of more than 70% even based on the initial mole amount of compound 1. The above total yields of veratraldehyde and veratic acid (68-78%) are higher than those reported in the previous paper [24], although special catalysts, such as 1,10-phenanthroline and CuSO 4 used in the paper [24], were not employed in the H 2 O 2 system. In the previous report, less than 5% and 65-70% of veratraldehyde and veratric acid, respectively, as well as 3% of residual compound 1 (which is called compound 2 in this previous report) were obtained based on the initial mole amount of compound 1, when compound 1 was treated in an aqueous alkaline solution at an oxygen pressure of about 1.…”

“…A prolonged reaction must afford larger amounts of veratraldehyde and veratric acid, which makes it possible to say that the total yields based not only on the amount of disappearing compound 1 but also on the initial mole amount of compound 1 are no lower than those shown in the previous paper where compound 1 was subjected to an oxidation system employing the special catalysts, 1,10-phenanthroline and CuSO 4 [24]. These high yields result from the reactivity of O⋅, the species responsible for the degradation of compound 1.…”

“…1), which is an analogue of vanillic acid, were obtained as the major degradation products with high yields in the above-described alkaline hydrogen peroxide treatments at high pH levels. It should be emphasized that special catalysts, such as 1,10-phenanthroline and CuSO 4 used in the previous report [24], were not employed in this study, which is significantly different from previous reports [4,24].…”

“…pressure in the presence of catalysts, 1,10-phenanthroline and CuSO 4 [24]. Roughly speaking, around 10% and 50% of vanillin can be obtained from a macromolecular lignin sample and lignin model compounds, respectively, in the presence of special catalysts.…”

Fractionation and Characterization of Three Main Components from Pennisetum sinese Roxb. (P. sinese) by Microwave-assisted H2O2-NaOH Extraction

“…The alkaline oxidative method is one of the most attractive strategies because it is versatile, low cost, nontoxic, and environmentally friendly (Azarpira et al 2014). It effectively and selectively removes and depolymerizes lignin from wood pulp under relatively mild reaction conditions (Villar et al 2001).…”