While C–O bond cleavage is pivotal in the depolymerization/valorization
of lignin, it is still challenging to control the reaction selectivity
under high activity due to the higher dissociation energy of aromatic
C–O bonds relative to other reactions such as direct ring hydrogenation.
Herein, we report the activation of Al2O3-supported
earth-abundant MnO with embedded Ru to enhance the selective hydrogenolysis
of aromatic C–O bonds in both a model compound and real lignin.
Complementary characterizations demonstrate that the embedment of
Ru into the MnO phase generates vacancy-enriched MnO under a hydrogen
atmosphere, and such abundant active sites enable about threefold
enhancement of the specific reaction rate for C–O bond hydrogenolysis.
Moreover, the defective MnO overlayer on Ru nanoparticles has a stronger
interaction with the O in diphenyl ether with preferential vertical
adsorption, which inhibits the activation and hydrogenation of the
aromatic ring, leading to higher selectivity for direct C–O
bond cleavage. In the depolymerization of real lignin, the bimetallic
Ru–MnO shows significantly higher (fivefold) activity than
monometallic Ru under the tested condition. This work provides a general
framework for the rational design of highly efficient catalysts for
selective C–O bond cleavage.
Aromatics are desirable products from the depolymerization/valorization
of lignin; however, it is still challenging to achieve selective hydrogenolysis
of the C–O bond with the preservation of aromatic rings. In
this work, the electronic Ru-Al2O3 interaction
was tailored by controlling the sizes of supported Ru nanoparticles
to regulate the profiles of energy barriers for selective hydrogenolysis
of diphenyl ether (DPE, modeling compound of lignin). Complementary
characterizations and kinetic studies demonstrate that a stronger
electronic metal–support interaction (EMSI) occurs between
smaller Ru nanoparticles and Al2O3, leading
to a more electron-deficient Ru domain. This tailored electronic structure
dramatically increases the barrier of an undesired secondary reaction
(i.e., ring hydrogenation), outstripping that of DPE hydrogenolysis
for the production of aromatics. In addition, although the latter
barrier also increases over smaller and more electron-deficient Ru,
the much more abundant active site compensates for the increased barrier
and enhanced the apparent reactivity. The catalyst bearing the smallest
Ru particle displays the highest activity and selectivity under the
tested conditions. This work provides an approach to control selectivity
in hydrotreatment by regulating the energy barriers along the reaction
pathway.
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