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. Herein, we report a transfer
hydrogenolysis protocol using a heterogeneous ZnIn2S4 catalyst driven by visible light. In this process, alcoholic
groups (CαH–OH) of lignin act as hydrogen
donors. Proliferation of phenolic products to dark substances is suppressed
under visible light illumination at low temperature (below 50 °C);
formation of a light and transparent reaction solution allows visible
light to be absorbed by the catalyst. With this strategy, 71–91%
yields of phenols in the conversion of lignin β-O-4 models and
a 10% yield of p-hydroxyl acetophenone derivatives
from organosolv lignin are achieved. Mechanistic studies reveal that
CαH–OH groups of lignin β-O-4 linkage
are initially dehydrogenated on ZnIn2S4 to form
a “hydrogen pool”, and the adjacent Cβ–O bond is subsequently hydrogenolytically cleaved to two
monomers by the “hydrogen pool”. Thus, the dehydrogenation
and hydrogenolysis reaction are integrated in one-pot with lignin
itself as a hydrogen donor. This study shows a promising way of supplying
phenolic compounds by taking advantages of both renewable biomass
feedstocks and photoenergy.
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