The Catalyst Review December 2024 - 7

SPECIAL FEATURE
3. Common Catalyst Types for Catalytic Fast Pyrolysis (CFP) of Lignin
Metal-Modified Zeolites
Metal-modified zeolites, particularly HZSM-5, have emerged as highly effective catalysts for the catalytic fast pyrolysis
(CFP) of biomass, enabling the production of valuable bio-aromatics such as benzene, toluene, and xylene (BTX).
The strong acidity, high surface area, and microporous structure of HZSM-5 make it a preferred choice for selective
aromatization and deoxygenation of lignin-derived intermediates. However, its narrow pore size can hinder molecular
diffusion, increasing coke formation and limiting efficiency. Modifications with metals like gallium (Ga), zinc (Zn), nickel
(Ni), cobalt (Co), and molybdenum (Mo) address these challenges by altering acidity, creating new active sites, and
suppressing side reactions, ultimately improving catalyst performance and stability.
Gallium-modified HZSM-5 (Ga-ZSM-5) has
demonstrated remarkable efficiency in CFP,
particularly for lignin-derived feedstocks.
Gallium increases pore size, enhances Brønsted
acidity, and promotes decarbonylation and
olefin aromatization. These modifications lead
to significantly higher BTX yields and improved
selectivity, especially for p-xylene. Studies have
shown that Ga-ZSM-5 performs exceptionally
well with a variety of biomass types, including
eucalyptus black liquor, pinewood, and Jatropha
residues, at optimal pyrolysis temperatures
of 550-600°C. Its selectivity typically follows
the order: xylenes > toluene > benzene,
making it a preferred catalyst for sustainable
bio-aromatic production. Additionally, GaZSM-5
outperforms unmodified and other
metal-modified zeolites by minimizing coke
formation, thereby extending catalyst lifespan
and process efficiency (Zheng et al. 2022).
Figure 2. Catalyst types used in catalytic pyrolysis for bio-aromatic based
on the common literature. Source: Zheng et al., 2022
Similarly, zinc-modified HZSM-5 (Zn/HZSM-5) has proven to be a highly effective catalyst for improving BTX yields and
selectivity in CFP. Zinc modifies the acid site distribution by converting Brønsted acid sites to Lewis acid sites, which
enhances hydrogen transfer and decarbonylation reactions. These changes not only reduce coke formation but also
increase BTX yields by up to 19.2% and selectivity by 23.3%, with maximum yields reaching 65.02% for lignin-based
feedstocks. While Ga-modified HZSM-5 is particularly efficient for toluene and xylenes, Zn-modified HZSM-5 excels in
benzene production (Che et al. 2019).
Other metals, such as nickel (Ni), cobalt (Co), and molybdenum (Mo), further expand the capabilities of HZSM-5 in CFP
applications. Ni-modified HZSM-5 enhances hydrogen transfer and deoxygenation reactions, improving bio-oil quality.
However, its high reactivity can increase coke formation, potentially reducing catalyst stability. Co-modified HZSM-5
supports both deoxygenation and hydrogenation, converting Brønsted acid sites to Lewis acid sites, which minimizes
coke buildup and extends catalyst life. Mo-modified HZSM-5 is particularly effective in promoting dehydrogenation
and cracking reactions, achieving BTX yields as high as 78.9%. These metals offer tailored advantages depending on the
desired product profile and operating conditions, enabling the customization of catalysts for specific biomass types.
Comparative studies underscore the advantages of gallium and zinc modifications over other metals for BTX production.
The sequence of effectiveness for aromatic yields is reported as Ga > Zn > Ni > Co > Mg > Cu > unmodified HZSM5.
The incorporation of these metals alters acid site distribution, increasing the ratio of Lewis to Brønsted acid sites
without affecting the total acid amount. This adjustment reduces coke deposition and enhances overall catalytic stability.
The versatility and scalability of metal-modified HZSM-5 underline its critical role in advancing renewable chemical
manufacturing and contributing to global sustainability initiatives.
The Catalyst Review
December 2024
7

The Catalyst Review December 2024

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