The Catalyst Review December 2024 - 9

SPECIAL FEATURE
Bimetallic Catalysts
Bimetallic catalysts have emerged as a promising solution for converting lignin into bio-aromatics, such as benzene,
toluene, and xylene (BTX). By combining two metals with a single catalytic framework, these systems leverage synergistic
effects that enhance reaction selectivity, increase BTX yields, and mitigate common challenges such as coke formation
and catalyst deactivation.
One notable example is the combination of zinc (Zn) and gallium (Ga) on an HZSM-5 zeolite. Zinc facilitates hydrogen
transfer reactions, stabilizing reactive intermediates during pyrolysis, while gallium enhances the aromatization of olefins
into BTX. Together, Zn and Ga achieve significantly higher yields of bio-aromatics than their monometallic counterparts.
Studies report that Zn-Ga/HZSM-5 achieves up to 69.3% selectivity for bio-aromatics under optimized conditions, making
it a highly effective catalyst for lignin conversion (Zheng et al. 2017).
Another effective pairing is molybdenum (Mo) and cobalt (Co) on HZSM-5. Molybdenum excels at deoxygenation,
removing oxygen as CO or CO2, while cobalt supports hydrogen transfer reactions that stabilize lignin-derived
intermediates and reduce the formation of undesired by-products. This synergy is evident in the performance of Mo-Co/
HZSM-5, which achieved an aromatic yield of 41% when applied to biomass pyrolysis (Ren et al. 2018).
Nickel (Ni) and zinc (Zn) also form an efficient bimetallic system for lignin conversion. Zinc enhances deoxygenation
and hydrogenation, while nickel facilitates cracking reactions, breaking down larger oxygenates into simpler aromatic
compounds. For example, Zn-Ni/ZSM-5 achieved an aromatic yield of 68.5% with enhanced selectivity for benzene and
toluene.
Bimetallic catalysts work through
complementary mechanisms that improve
the
overall conversion
process.
The
deoxygenation activity of one metal can
be paired with the hydrogen transfer or
cracking capabilities of the other, optimizing
the formation of BTX. Additionally, these
catalysts often reduce coke formation by
stabilizing reactive intermediates that
would otherwise polymerize and deactivate
the catalyst. For instance, Zn and Ga work
together to fine-tune acidity and pore
structure in HZSM-5, resulting in a catalyst
that not only produces high yields of BTX
but also remains active for longer periods.
Figure 4. Different catalysts used for CFP of lignin. Source: Socar
Despite these advancements, challenges
persist. Bimetallic catalysts can be complex
to design, requiring precise control over
metal loading, distribution, and ratios to
achieve optimal performance. Additionally, while some systems reduce coke formation, others, such as those involving
nickel, may still experience significant deactivation due to coke deposition. Scaling these catalysts for industrial
applications also poses economic challenges, especially when using rare or expensive metals like gallium.
4. Co-Pyrolysis with Hydrogen Donors
Co-pyrolysis, the simultaneous thermal decomposition of lignin and a hydrogen-rich feedstock, is a promising strategy to
enhance the yield and quality of bio-aromatics. Hydrogen donors stabilize reactive intermediates during lignin pyrolysis,
reducing coke formation and favoring the production of compounds like benzene, toluene, and xylene (BTX). The low
hydrogen-to-carbon ratio of lignin leads to reactive oxygenated intermediates that polymerize into coke. Adding hydrogen
donors redirects reactions towards light aromatics by stabilizing these intermediates (Zheng et al. 2022).
Hydrogen donors also facilitate secondary reactions, such as methylation, enhancing selectivity for specific aromatics like
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December 2024
The Catalyst Review

The Catalyst Review December 2024

Table of Contents for the Digital Edition of The Catalyst Review December 2024