The conversion of lignin into valuable chemical products
is important
for the shift away from the petrochemical industry toward a more sustainable
system of biorefineries. However, the recalcitrance and heterogeneity
of lignin have made its selective depolymerization a difficult task.
Photochemical methods of lignin conversion are being investigated
because of the potential to operate photoreactors at milder temperatures
and pressures than thermal methods and to achieve efficient reaction
pathways. Furthermore, light-driven reactions facilitate reaction
pathways that cannot be accessed by conventional/thermal methods.
Most of the current research focuses on photocatalytic methods, which
are interesting due to their potentially high selectivity, but come
with the disadvantage of catalyst costs and separation requirements.
In this work, we continue our investigation into the use of ultraviolet
light-emitting diodes, which aims to utilize the advantages of photochemistry,
while avoiding the use of expensive catalysts. Photosensitizers can
participate in energy transfer, electron transfer, and hydrogen abstraction
in photochemical reactions. Here, we investigated the effects of a
common photosensitizer, benzophenone, on the photochemical conversion
of lignin, and 2-(benzyloxy)phenol (2BP), a compound with an ether
bond between two aromatic units. We monitored the conversion reactions
using complementary techniques of 1H nuclear magnetic resonance
(NMR), diffusion NMR, and in situ Fourier transform infrared (FTIR)
spectroscopy. For 2BP, the reactions with benzophenone progressed
slower and without a difference in the final product formation. However,
several differences were observed in photoreactions utilizing Kraft
lignin and benzophenone compared to those without benzophenone. For
example, a faster decay of the 1H NMR peak corresponding
to aromatic/phenolic protons and different changes in the shape of
methoxy peaks were observed, indicating the formation of different
products. This work demonstrates that benzophenone participates in
the photoreactions of Kraft lignin and that the photoreactions of
Kraft lignin and 2BP are different. Depolymerization of lignin into
smaller fragments was confirmed with diffusion NMR, both with and
without the photosensitizer.