Hydrothermal Liquefaction of α‐O‐4 Aryl Ether Linkages in Lignin

Abstract

By using lignin model compounds with relevant key characteristic structural features, the reaction pathways of a-O-4 aryl ether linkages under hydrothermal conditions are elucidated. Experimental resultsa nd computational modelings uggest that the a-O-4 linkages in lignin undergo catalyzed hydrolysis and elimination to give phenolic and alkenylbenzene derivatives as major products in subcritical water.T he decreased relative permittivity of water at these high temperatures and pressures facilitates the elimination reactions. The alkyl group on the a-carbon andt he methoxy groups on the phenyl rings both have positive effects on the rate of conversion of a-O-4 linkages in native lignin.Hydrothermal liquefaction (HTL) is becomingacommon methodt oc onvert biomass into fuel and chemicals, with several companies worldwide exploring the possibilities to commercialize such processes (e.g.,L icella/Canfor,G enifuel). In a typical HTL process, feedstocks are heatedi nw ater under subcritical conditions (250-374 8C, 4-22 MPa). [1] As the temperature increases above 374 8C, more hydrothermalg asification processes occur,e specially for biomass. [1] HTL is superiort o many other biomass conversion processes,e specially in terms of the relativelyl ow oxygen content and the stabilityo ft he resulting liquids. Biocrudes produced from the HTLp rocessa re of significantly higher energy density than pyrolysis oils. [2] In addition, unlike gasification and pyrolysis processes, pre-drying of feedstock for HTL is, by definition, unnecessary.Biomass is primarily made up of macromolecules such as cellulose, hemicellulose, lignin, and proteins.L ignin, which constitutes 25-35 %o ft he organic matrix of lignocellulosic biomass, [3,4] is the most readily availables ource of renewable aromatics; hence its valorization has become av igorous area of research. [5][6][7][8] The complex structure of lignin contains aromatic units connected by linkages such as b-O-4, a-O-4, 4-O-5, 5-5, b-5, and b-b crosslinks. Functional groups such as benzylic alcohols, [9] aldehydes [10] and methoxy groups [11] are also present in the structure of lignin.The distinct chemistry of lignin under hydrothermalc onditions is less well studied [12][13][14][15] than that under pyrolysis conditions, under which linkages are broken down primarily through free-radical chemistry. [16][17][18][19]

Hydrothermal Treatment of Biomass Feedstocks for Sustainable Production of Chemicals, Fuels, and Materials: Progress and Perspectives

“…However, there was no cleavage at the stable 4-O-5 linkages . The reaction pathways of α-O-4 linkages underwent catalyzed hydrolysis and elimination to give phenolic and alkenylbenzene derivatives . In the research study conducted by Atta-Obeng et al, the major structural changes occurred at 300 °C of HTC with production of droplets of bio-oil .…”

“…176 The reaction pathways of α-O-4 linkages underwent catalyzed hydrolysis and elimination to give phenolic and alkenylbenzene derivatives. 177 In the research study conducted by Atta-Obeng et al, the major structural changes occurred at 300 °C of HTC with production of droplets of bio-oil. 175 NMR results confirmed the structural changes at 350 °C, in which aromatic carbon was condensed via aromatization process.…”

The future of aviation soars with HTL-based SAFs: exploring potential and overcoming challenges using organic wet feedstocks

“…78 Additionally, HTL bio-crude has less oxygen and moisture, which increases its calorific value and product stability 93 and has a clearly higher energy density than pyrolysis oil. 94 Despite the pilot-scale research in continuous systems over the past decade, most studies on HTL have only been conducted on a small scale, in a batch reactor. The HTL reaction and the separation of the extracted products are the two main components of the experiment.…”

Bio-based platform chemicals synthesized from lignin biorefinery

“…184 Additionally, the methoxy of the aromatic ring and the alkyl groups on the α-carbon are conducive to decreasing the required time and increasing the conversion rate of ether bonds, such as α-O-4 and β-O-4. 185,186 Notably, the HTL reaction pathway of lignin is a competitive reaction between bond cleavage and condensation reactions, which results in a poor yield of aromatical chemicals. Fortunately, solvent liquefaction provides a promising option to avoid condensation reactions because it usually utilizes hydrogen-donating solvent or atmosphere (e.g., methanol and ethanol) as the substitute for water to stabilize the active intermediates, 14 .…”