Elucidating the Reaction Pathways of Veratrylglycero-β-guaiacyl Ether Degradation Over Metal-Free Solid Acid Catalysts with Hydrogen

Abstract

Efficient conversion of lignin to high-yield aromatic compounds for the potential production of fuels and chemicals is vital to the economics of the modern biorefinery industry. Herein, we report a novel catalytic process involving the cleavage of β-O-4 bonds, which are the major interunit linkages in native lignin. This work is distinct in that a detailed mechanistic analysis of the reaction pathways of veratrylglycero-β-guaiacyl ether (VGE) catalyzed by transition-metal-free solid acid zeolite under aqueous conditions at high hydrogen pressure has been performed. VGE degradation produced high yields of monomers (~ 87%), including guaiacol (48.2%), 1-(3,4-dimethoxyphenyl)ethanol (10.3%), 1-(3,4-dimethoxyphenyl)-2-propanol (6.1%), 3,4-dimethoxyphenylpropanol (4.7%), 3,4-dimethoxycinnamyl alcohol (4.1%), and 1,2-dimethoxy-4-propylbenzene (2%), which were identified and confirmed by both the in-situ solid-state magic angle spinning (MAS) 13C NMR under real-time conditions and two-dimensional gas chromatography analysis. The variety of products reveal the crucial roles of hydrogen, water, and acid sites for heterolytic cleavage of the β-O-4 bond in VGE. Decarbonylation, hydrogenolysis, hydrogenation, and dehydration reaction pathways are proposed and further validated using first-principles calculations.