Page 110 - Advances in bioenergy (2016)

development/optimization of advanced porous materials as efficient monofunctional and
bifunctional catalysts for the production of transportation fuels from biomass. In this review,
the recent developments on catalysts and processes will be discussed. 4,5

REVIEW ON CATALYSTS FOR BIOMASS CATALYTIC

PYROLYSIS

A plethora of catalytic materials such as zeolites, mesoporous materials with uniform pore size
distribution (MCM-41, MSU, and SBA-15), microporous/mesoporous hybrid materials doped
with noble and transition metals, and base catalysts have been investigated as candidate
catalysts for biomass pyrolysis. These catalysts should be able to selectively favor the
decarboxylation reactions, producing high-quality bio-oil with low amounts of oxygen and
H O. They should also inhibit formation of undesirable oxygenated compounds, such as
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ketones, acids, and carbonyl compounds, which are known to be detrimental for the direct use
or further coprocessing of bio-oil. The hydrothermal stability of the catalysts must also be
improved; thus, resistance to deactivation and catalyst behavior upon regeneration must be
investigated to optimize new catalysts. In general, catalyst development comprises of
controlled formation of appropriate catalyst particles and tailoring the porosity, acidity,
basicity, and metal–support interactions of the candidate catalytic materials.

Microporous Acidic Catalysts

Microporous acidic catalysts are widely used in oil refineries and are known to catalyze the
scission of carbon–carbon bonds of heavier oil fractions. Biomass pyrolysis requires a similar
mechanism for the conversion of heavier oxygenates to lighter ones. In this respect, several
groups have studied the catalytic pyrolysis of biomass over acidic zeolite catalysts, whereas

ZSM-5 has been the main zeolite studied for the upgrading of biomass pyrolysis vapors. 6-11 In
a series of studies, Horne and Williams investigated the effect of the ZSM-5 zeolite on the
pyrolysis of biomass. They found that after catalysis, the oxygenated species in the bio-oil
were markedly reduced, whereas aromatic species increased, producing a premium grade
gasoline-type fuel. Detailed analysis of the upgraded oils showed that there were high
concentrations of economically valuable chemicals as well, but also revealed hazardous,
biologically active polycyclic aromatic species that increased in concentration with increasing
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catalyst bed temperature. They reported oxygen removal from the pyrolysis vapors mainly as
H O at lower catalyst bed temperatures and as CO and CO at higher bed temperatures. They
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observed a shift toward lower molecular weight species with increasing catalyst bed
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temperature as well. Detailed analysis of the aromatic hydrocarbons of the bio-oil derived
from catalytic pyrolysis of biomass with HZSM-5 catalyst showed that the single aromatic ring
species were mainly benzene, toluene, and alkylated benzenes. The polycyclic aromatic
hydrocarbons (PAHs) were mainly naphthalene, phenanthrene, fluorene, and their alkylated

homologues. 3
The complex nature of bio-oil components means that there is only limited understanding of the