The depolymerization of lignin, a significant part of biomass feedstock containing various aryl C-O bonds, is the key step in the catalytic conversion processes of lignocellulose to transportation fuels and valued-added chemicals. In the present work, catalytic C-O bond cleavage of 2-phenyletheyl phenyl ether (PPE) as the model compound of β-O-4 linkage in lignin was investigated over four carbon supported transition metal catalysts (Pd, Pt, Ru, Rh) in aqueous phase using first principles based density functional theory (DFT) calculations. Different major reaction pathways in the PPE conversion including selective C-O bond cleavage (hydrogenolysis and/or hydrolysis) and hydrogenation were identified on four metal surfaces. Hydrogenolysis is the major reaction pathway on Ru(0001) while hydrolysis is the dominant pathway leading to the C6 products (phenol, cyclohexanone and cyclohexanol) and the C8 products (2-cyclohexyl ethanol and 2-phenyl ethanol) on Pd(111) and Pt(111). On Rh(111), hydrolysis and hydrogenolysis are competing pathways for the C-O bond cleavage. The partially hydrogenated PPE products may undergo subsequent C-O bond cleavage via hydrolysis or hydrogenolysis on Pd/C, Rh/C, and Ru/C catalysts while the fully hydrogenated PPE product readily desorbs in aqueous phase. DFT calculations show the Caliphatic-O bond scission of PPE are kinetically preferred over the Caryl-O bond scission over Pd(111), Pt(111), and Ru(111) while both C-O bond scissions of PPE are competitive on Rh(111). Upon the Caliphatic-O bond cleavage, the formation of 2-phenyl ethanol is more favorable than the formation of ethyl benzene, leading to hydrolysis instead of hydrogenolysis on Pd(111) and Pt(111). On the other hand, ethyl benzene and 2-phenyl ethanol formations are comparable on Rh(111), suggesting hydrogenolysis and hydrolysis are competitive reaction pathways. The fundamental understanding of reaction pathways from DFT calculations provides an important roadmap for the efficient conversion of lignocellulose to alternative fuels and useful chemicals selectively.

Fundamental insights into catalytic conversion of lignin-derived model ether compound over supported transition metal catalysts

251st National Meeting (2016)

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