Page 113 - Advances in bioenergy (2016)

overall formation of hydrocarbons from pyrolysis oil due to its low hydrogen content. 23

Several groups have studied the catalytic upgrading of lignin pyrolysis vapors over a ZSM-5
zeolite. Lignin is a main constituent component of lignocellulosic biomass and is also a by-
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product of paper pulp mills. Thring et al. studied the catalytic pyrolysis of lignin solubilized
in acetone with HZSM-5 catalyst in a fixed bed reactor and observed high yields of gasoline

range hydrocarbons such as benzene, toluene, and xylene (BTX). Mullen and Boateng 24
observed an increased production of aromatic hydrocarbons as well during the pyrolysis of
lignin in the presence of HZSM-5 catalyst in a Py-GC–MS system. The increased aromatic
hydrocarbon production was likely to be due to the enhanced depolymerization efficiency of
the catalyst that released and converted the aliphatic linkers of lignins to olefins and then to
aromatic compounds by aromatization. They also observed a catalyst deactivation by coke
deposition due to the release of simple phenols from the decomposition of lignin, which
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degraded to coke. Jackson et al. also studied the pyrolysis of lignin over five catalysts and
concluded that HZSM-5 was the best catalyst for producing a deoxygenated liquid fraction.
In an effort to further improve the catalytic effect of the ZSM-5 zeolite on the upgrading of
pyrolysis vapors, some groups studied the incorporation of metals in the zeolite framework.
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According to French and Czernik, the presence of transition metals would affect the mode of
oxygen rejection by producing more carbon oxides and less H O, making in that way more
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hydrogen available for incorporation into hydrocarbons. Zhu et al. performed pyrolysis of
xylan in a Py-GC–MS system over HZSM-5 catalyst and metal-impregnated Fe/HZSM-5 and
Zn/HZSM-5 catalysts and found that the metal-impregnated HZSM-5 materials were more
effective in reducing oxygenated compounds and hence produced higher contents of
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hydrocarbons. French and Czernik evaluated a set of commercial and laboratory synthesized
catalysts for their hydrocarbon production performance and concluded that the best performing
catalysts belonged to the ZSM-5 group, whereas the highest hydrocarbon yields were achieved
with nickel, cobalt, iron, and gallium substituted ZSM-5. Zeolites with larger pores exhibited
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less deoxygenation activity. Park et al. compared the catalytic effects of HZSM-5,
Ga/HZSM-5, and H-Y zeolite and concluded that Ga/HZSM-5 produced more bio-oil than
HZSM-5 and had better selectivity to aromatic hydrocarbons. H-Y was less efficient in bio-oil
upgrading than HZSM-5.

Besides ZSM-5, other microporous zeolites and acidic materials have been studied in the
literature as well. During the conversion of bio-oil over HZSM-5 zeolite, H-Y zeolite, H-
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mordenite zeolite, silicalite, and silica–alumina catalysts, Adjaye and Bakhshi observed that
the acidic zeolites were more effective in converting the bio-oil to hydrocarbons than the less
acidic silica–alumina and nonacidic silicalite, and the highest yield of hydrocarbons was
obtained with the HZSM-5 zeolite. HZSM-5 and H-mordenite zeolites produced more
aromatic than aliphatic hydrocarbons, whereas H-Y, silicalite, and silica–alumina produced
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more aliphatic than aromatic hydrocarbons. 20,29 Aho et al. pyrolyzed pinewood in a fluidized
bed reactor using proton forms of beta, Y, ZSM-5, and mordenite zeolites and found that the
chemical composition of the bio-oil depended on the structure of the zeolite used. The ZSM-5
zeolite gave the highest liquid product yield and exhibited lower selectivity to acids and