The Catalyst Review December 2024 - 8

SPECIAL FEATURE
Mesoporous Catalysts and Hierarchical
Zeolites
Microporous zeolites like HZSM-5 have been
instrumental in converting biomass into valuable
bio-aromatics. Their high catalytic
activity and
shape-selectivity stem from their strong acidity and
well-defined micropores. However, their narrow
pore size (<1 nm) poses a significant limitation,
restricting the diffusion of larger oxygenated
intermediates, such as syringyl aromatics and
levoglucosan, found in biomass-derived feeds.
This restriction often leads to repolymerization
on the catalyst surface, increasing coke formation
and resulting in rapid catalyst deactivation.
Recycling and regeneration provide only limited
improvements, as the catalyst's efficiency declines
over successive cycles, and oxygen content in the
bio-oil increases.
Figure 3. Product yields of pine catalytic pyrolysis with different metalloaded
catalysts. Source: Zheng et al. 2017
To overcome these challenges, hierarchical zeolites
that integrate mesopores (2-50 nm) with micropores
have been developed. These structures improve
the diffusion of bulky molecules, reduce coke
formation, and enhance overall catalytic efficiency. Hierarchical HZSM-5 catalysts are synthesized through methods like
dealumination, desilication, or templating. For instance, alkali treatments using NaOH or TPAOH selectively remove
parts of the framework to create mesopores. Studies show that these modifications significantly improve the stability and
selectivity of the catalyst, particularly for producing single aromatic compounds. For example, Zhang et al. demonstrated
that alkali treatment could reduce coke formation by up to 10.4%, depending on the Si/Al ratio, underscoring the
importance of precise material design (Yan et al. 2021).
Despite their advantages, some hierarchical catalysts face challenges. The use of secondary mesoporous templates
can lead to structural irregularities, affecting the uniformity of acidity and catalytic performance. Addressing these
inconsistencies requires careful optimization of synthesis methods and materials.
Mesoporous molecular sieves like MCM-41 and SBA-15 have emerged as alternatives to purely microporous catalysts.
Their larger pores and higher surface areas enhance diffusion, making them suitable for catalytic biomass pyrolysis.
For example, Al-MCM-41 and Cu-MCM-41 have been shown to improve the production of aromatic hydrocarbons and
phenolic compounds. Copper increases aromatic yields, while aluminum enhances phenolic compound production.
Similarly, Zn-Al-modified MCM-41 reduces oxygenates and achieves aromatic yields as high as 24.31% (Sun et al. 2021).
However, mesoporous catalysts often exhibit weaker selectivity for aromatics compared to their microporous counterparts.
To address this, micro - mesoporous composite systems have been developed, combining the advantages of both
structures. For instance, ZSM-5/MCM-41 core-shell catalysts have achieved aromatic yields of 50.31% and monocyclic
aromatic selectivity of 42.83% (Wang et al., 2021). These hybrid systems improve product yields and stability by enhancing
diffusion while maintaining strong shape-selectivity.
Recent innovations have pushed the boundaries of catalyst design. Dai et al. created a core-shell HZSM-5@MCM-41
catalyst that improved aromatic yields from lignin pyrolysis. Kim et al. demonstrated that Al-SBA-15's larger pore size
enables better conversion of bulky phenolic compounds into aromatics. Additionally, palladium-supported SBA-15
catalysts have efficiently converted lignin-derived oligomers into phenolic monomers, reducing unwanted byproducts
like aldehydes and ketones (Dai et al. 2024).
In conclusion, hierarchical and mesoporous catalysts improve diffusion properties, tailoring acidity, and enhancing
stability and enable more efficient and sustainable biomass conversion.
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The Catalyst Review
December 2024

The Catalyst Review December 2024

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