Catalytic C-O bond cleavage of model diaryl ether compounds that are derived from the depolymerization process of lignincellulosic biomass over supported transition metal catalysts is one of the most important catalytic steps in the efficient conversion of lignincellulosic biomass to transportation fuels and valuable chemicals. Previous studies have shown that the C-O bond cleavage activity and the product selectivity of diaryl ethers, which are largely determined by the dominant pathway among three parallel pathways including hydrogenolysis, hydrolysis, and hydrogenation, vary from different transition metal catalysts, solvent environments and substituents.1-3
In the present study, the reaction mechanisms of catalytic C-O bond cleavage of diphenyl ether over supported palladium catalyst has been investigated using density functional theory (DFT) calculations. It is found that hydrolysis, which is leading to the formation of phenol and cyclohexanol, is the dominant reaction pathway for aqueous phase diphenyl ether conversion in the presence of hydrogen. The partial hydrogenation of one of aromatic rings plays the key role in the selective C-O bond cleavage. The fundamental insights from DFT calculation results not only confirm our experimental observation, but also provide a general picture of how the selective C-O bond cleavage reactivity and selectivity over transition metal catalysts in aqueous phase.
(1) He, J.; Zhao, C.; Lercher, J. A. Ni-catalyzed cleavage of aryl ethers in the aqueous phase. J. Am. Chem. Soc. 2012, 134, 20768-20775.
(2) He, J.; Zhao, C.; Mei, D.; Lercher, J. A. Mechanisms of selective cleavage of C–O bonds in di-aryl ethers in aqueous phase. J. Catal. 2014, 309, 280-290.
(3) He, J. Y.; Lu, L.; Zhao, C.; Mei, D. H.; Lercher, J. A. Mechanisms of catalytic cleavage of benzyl phenyl ether in aqueous and apolar phases. J. Catal. 2014, 311, 41-51.
This work is supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multi-program national laboratory operated for DOE by Battelle. Computing time was granted by the scientific theme user project of the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) and by the National Energy Research Scientific Computing Center (NERSC). EMSL is a national scientific user facility located at PNNL and sponsored by DOE Office of Biological and Environmental Research.