Effects of Lignin Structure on Hydrodeoxygenation Reactivity of Pine Wood Lignin to Valuable Chemicals

Abstract

Hydrodeoxygenation (HDO) of two dilute acid flowthrough pretreated softwood lignin samples, including residual lignin in pretreated solid residues (ReL) and recovered insoluble lignin in pretreated liquid (RISL), with apparent different physical and chemical structures, was comprehensively studied. A combination of catalysts (HY zeolite and Ru/Al2O3) was employed to investigate the effects of lignin structures, especially condensed structures, on the HDO upgrading process. Results indicated that the condensed structure and short side chains in lignin hindered its HDO conversion under different reaction conditions, including catalyst loading and composition, hydrogen pressure, and reaction time. In addition to lignin structure, HY zeolite was found crucial for lignin depolymerization, while Ru/Al2O3 and relatively high hydrogen pressure (4 MPa) were necessary for upgrading unstable oxy-compounds to cyclohexanes at high selectivity (>95 wt %). Since the lignin structure essentially affects its reactivity during HDO conversion, the yield and selectivity of HDO products can be predicted by detailed characterization of the lignin structure. The insights gained from this study in the fundamental reaction mechanisms based on the lignin structure will facilitate upgrading of lignin to high-value products for applications in the production of both fuels and chemicals.

Rice straw as a feedstock for biofuels: Availability, recalcitrance, and chemical properties

“…Its characterization is essential to evaluate its utility, strength, and integrity for subsequent downstream processing and applications. Nuclear magnetic resonance (NMR) spectroscopy emerged as a powerful tool for detailed structural elucidation of lignin . One dimensional (1D) proton ( 1 H), carbon ( 13 C) and phosphorus ( 31 P) NMR and two‐dimensional (2D) heteronuclear single quantum coherence (HSQC) techniques have significantly contributed to deciphering the structural and functional properties of lignin .…”

“…Lignin is primarily composed of three types of monolignols/hydroxycinnamyl alcohols ( p ‐ coumaryl, coniferyl, and sinapyl alcohols) linked with each other by different types of ether and carbon‐carbon bonds like β‐O‐4, 4‐O‐5, β‐β, β‐1 and β‐5 to make phenylpropanoid units such as p‐hydroxyphenyl (H), guaiacyl (G) and syringyl (S). Among these, the β‐O‐4 linkage is the most predominant ether bond (about 40–60%) in rice straw lignin . Lignin is proposed to be attached to carbohydrates by benzyl esters, benzyl ethers and phenyl glycosides.…”

Structural Characterization of Lignin and Its Degradation Products with Spectroscopic Methods

“…Furthermore, the lignin-carbohydrate complex (LCC) increases the difficulty of structural analysis and isolation of lignin from LCBM [15]. The value-added utilization of lignin and its degradation products are one of the ultimate goals especially for biorefineries; therefore, the comprehensive understanding of the structure of lignin is crucial necessary, since it can provide theoretical direction on constructing and optimizing degradation processes, generating of valuable aromatic chemicals Journal of Spectroscopy to act as low-molecular-mass feedstocks [16], estimating the economic viability, and so forth. Traditionally, there are two ways to isolate lignin from other components in LCBM, so-called degradation processes: one is to extract cellulose and hemicellulose leaving most of the lignin as solid residue, and the other one is to extract lignin by using fractionation methods leaving the other components.…”

Promising Techniques for Depolymerization of Lignin into Value‐added Chemicals

“…Hydrocarbons are generally formed with the yield of 10–50 % in a transparent form ,,,. The carbon numbers of the hydrocarbon compounds range mainly from 6–9 and 12–17 which contain one or two cyclohexyl (or aryl) units, respectively, and which cover the scope of gasoline and diesel, respectively ,,,,,. And it is easy to separate the product into gasoline or diesel fractions by distillation as with the crude oil.…”