Insights into Oxidative Conversion of Lignin to High-Added-Value Phenolic Aldehydes

Abstract

In this paper, the influence of some lignin features on its oxidative conversion to high-added-value phenolic aldehydes is discussed. Four softwood and three hardwood lignins are considered due to their different origins regarding the wood species, pulping process, and lignin isolation process. The lignins were characterized based on nitrobenzene oxidation (NO), frequency of phenolic hydroxyl (OHph) groups, and molecular weight determination, attempting to evaluate their potential to produce vanillin (V) and/or syringaldehyde (Sy) through the alkaline oxidation with oxygen. The raw-materials, Pinus spp. kraft lignin (LWest), Lignoboost kraft lignin (LBoostS), and Organosolv beech lignin (LOrgs) were selected to perform bench-scale oxidations with O2. These lignins contain lower content of nonlignin material and show higher yields of NO and high OHph groups content. In the oxidation with O2, LWest provide the highest production of V, 4.4% w/w (corresponding to 36% of NO yield). Considering the characteristics of LOrgs, an unexpected low Sy yield (2.5% w/w) was obtained (19% of the NO yield). The high reactivity of syringyl nuclei may be pointed out as the main reason for the faster production and degradation of Sy in oxidation. The concentration profile of products, the variation of pH in the reactive medium, and the variation of temperature with the reaction time of oxidation with O2 are shown and discussed with reference to the investigated lignins features.

Catalytic Oxidation and Depolymerization of Lignin in Aqueous Ionic Liquid

“…Pokutsa et al (2009) reported that elevation of temperature from 80 to 120°C improved the conversion of precipitated hardwood lignin from 84.6 to 97.8% using H2O2 in a NaOH medium. In this study, for both catalysts, increasing temperature led to increases in product concentration, probably due to the formation of active hydroxyl and superoxide ions at higher temperatures as a result of hydrogen peroxide disintegration (Rodrigues Pinto et al, 2010).…”

“…Formation of vanillin from Kraft lignin by different oxidants such as H2O2, O2, and nitrobenzene in NaOH medium has been reported (Xiang and Lee, 2000;Rodrigues Pinto et al, 2010;Pandey and Kim, 2011). It was reported that the IL, [C2C1im] [OAc] possesses dual basic and acidic characteristics as a function of temperature (Varanasi et al, 2012).…”

Lignin Degradation Processes and the Purification of Valuable Products

“…Distribution of principal lignin building block linkages in hardwood and softwood (%), adapted from [13]. Lignin is the most abundant renewable source of aromatic molecules and the second largest renewable source of carbon on earth, after cellulose [3,14].…”

“…Compared to hydrogenolysis, lignin oxidation leads to more complex aromatic compounds with greater functionality [2]. Lignin oxidation plays an important role in the conversion of lignin to aldehydes such as vanillin and syringaldehyde [13,[144][145][146]. However, the spectrum of potential products also includes carboxylic acids, aliphatic alcohols and hydrocarbon gases, with the product ratios depending on the lignin source, type of catalyst and reaction conditions [2,5].…”

Experimental and Theoretical Analysis of the Pyrolysis Mechanism of a Dimeric Lignin Model Compound with α-O-4 Linkage

“…Among these bonds, the content of the β-O-4 linkage is the highest (45% to 60%) (Adler 1977;Chakar and Ragauskas 2004;Rodrigues Pinto et al 2010;Shen et al 2010;Pandey and Kim 2011). Therefore, in previous mechanism studies, dimeric lignin model compounds with β-O-4 linkage were preferred as research objects (Drage et al 2002;Liu et al 2011;Chu et al 2013).…”