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 (FTIR) and gel permeation chromatography (GPC) to determine the 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 1202 and 1520 g·mol-1, 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 aqueous ionic liquid in presence of the oxidant and transition metal catalysts and the oxidative degradation reaction sequence of lignin towards product formation.
Highlights
Lignin is the second most abundant terrestrial biopolymer on earth and is one of the three main building blocks of lignocellulosic biomass (Das et al, 2012)
Cobalt (Co)-based catalysts have been used for oxidation of lignin model compounds and alcohols in 1-ethyl-3-methylimidazolium diethylphosphate ([C2C1Im][DEP]) (Zakzeski et al, 2010b, 2011)
Apart from acting as a strong Lewis acid, niobium oxide has the capability to stabilize oxidation cation species, which could assist in the electrophilic attack of H2O2 on lignin (Fielicke et al, 2003; Nakajima et al, 2011)
Summary
Lignin is the second most abundant terrestrial biopolymer on earth and is one of the three main building blocks of lignocellulosic biomass (Das et al, 2012). As projected by the 2009 Renewable Fuel Standard (RFS2) on basis of the 2007 Energy Independence and Security Act, the US alone will generate approximately 60 million dry tons of lignin annually from the capacity of cellulosic biorefineries by year 2022 (Tilman et al, 2009; Somerville et al, 2010; Brown and Brown, 2013). This quantity will add on top of the existing ~100 million dry tons of lignin from paper and pulping industry (Chakar and Ragauskas, 2004; Ragauskas et al, 2014). It is critical to convert lignin waste streams to high value-added chemicals to enable cost-competitive biofuels and chemicals production in a biorefinery (Ragauskas et al, 2014; Beckham et al, 2016; Mottiar et al, 2016)
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