Obtaining high selectivity of aromatic monomers from renewable lignin has been extensively pursued but is still unsuccessful, hampered by the need to efficiently cleave C−O/C−C bonds and inhibit lignin proliferation reactions. Our studies show semiconductor photocatalysts can efficiently catalyze C−O and C−C bonds cleavage of lignin β-O-4 and β-1 models into aromatics. Cleavage of the C−O bond of lignin β-O-4 models is realized by a dual light wavelength switching (DLWS) strategy. Pd/ZnIn2S4 is employed to selectively catalyze the oxidation of α-CH−OH to α-C=O group in ethanol with 455 nm blue LEDs. Afterwards, the wavelength is switched to 365 nm to excite anatase TiO2, which catalyzes the subsequent C−O bond cleavage of lignin β-O-4 ketones through transfer hydrogenolysis. In this way, β-O-4 models are cleaved into two monomers in one pot without the separation of lignin β-O-4 ketones. To avoid wasting of ethanol and efficiently use the hydrogen sources, we employ lignin itself or the residual carbohydrates in extracting lignin as hydrogen donors to achieve C−O bond cleavage. Through a photocatalytic self-transfer hydrogenolysis strategy, the hydrogen in α-CH−OH group is abstracted and finally transfers to ZnIn2S4 photocatalyst to form a “hydrogen pool” via reduction by conduction band electrons. The hydrogen in the “hydrogen pool” then reduces the C−O bond of β-O-4 ketones to offer aromatic monomers. With this strategy, 71% to 91% yields of phenols are obtained. Particularly, p-hydroxyl acetophenone derivatives are also obtained with a 10% mass yield from organosolv poplar lignin. Besides, no darkening of the color of organosolv poplar lignin is observed. To obtain high selectivity of aromatic monomers through C−O bond cleavage, cleavage of C−C bond is also needed. Adopting lignin β-1 models as substrates, CuOx/CeO2/anatase-NTs shows exceptionally high selectivity for C−C bond cleavage. Benzaldehydes with &gt; 96% yields were obtained. Highly dispersed CuOx clusters on ceria domains enhance the Ce3+ concentration and thus increase the catalytic activity; meanwhile, CuOx on the exposed anatase surface suppresses the unwanted oxy-dehydrogenation reaction. We attempt to demonstrate the feasibility of obtaining aromatic monomers from renewable lignin by photocatalysis at low temperatures and to alleviate proliferation of phenolic products.

Photocatalytic cleavage of lignin into aromatics

254th National Meeting (2017)

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