Page 114 - Advances in bioenergy (2016)

alcohols and higher selectivity to ketones, whereas the mordenite zeolite was effective for
30
31
minimizing the formation of PAHs. Uzun and Sarioglu also studied different zeolite
structures (ZSM-5, H-Y, and USY) for the catalytic pyrolysis of corn stalks. They observed the
highest oil yield with the ZSM-5 zeolite and the lowest with the USY. They also observed the
highest amount of aromatics with the USY catalyst. The H-Y zeolite caused an increase of the
31
aliphatic hydrocarbons. Carlson et al. 32,33 studied ZSM-5, beta, and Y zeolites for the
conversion of biomass compounds to aromatics in a Py-GC–MS system and found that high
catalyst to feed ratios favor aromatic compounds production over coke formation by avoiding
undesirable thermal decomposition reactions in the homogeneous phase. At lower catalyst to
feed ratios, volatile oxygenates were formed including furan-type compounds, acetic acid, and
hydroxyacetaldehyde. ZSM-5 had the highest aromatic yields and the least amount of coke. 32,33
34
Lu et al. also studied HZSM-5 and H-Y zeolites for the upgrading of biomass fast pyrolysis
vapors, along with three other mesoporous materials. The two microporous zeolites proved to
be effective for the deoxygenation of the pyrolysis vapors and formed abundant aromatic
35
34
hydrocarbons and some PAHs after catalysis. Qi et al. performed catalytic pyrolysis of
bamboo in a fixed bed reactor in the presence of NaY zeolite and observed that the bio-oil
yield increased after catalysis and consisted mainly of carboxylic and carbonylic compounds.
Acetic acid was the main component of the catalytic bio-oil and its content was higher than in
the thermal pyrolysis oil, whereas the content of carbonylic compounds was markedly lower. 35
Aho et al. studied the influence of beta, Y, and ferrierite zeolites and their Fe-modified
counterparts on the upgrading of pinewood pyrolysis vapors. They concluded that beta zeolite
was the most active in the deoxygenation reactions, followed by Y and ferrierite. The amount

of methyl-substituted phenols in the bio-oil increased, whereas methoxy-substituted phenols
36
decreased in the presence of the Fe-modified zeolites. In an earlier study, Aho et al. also
investigated the influence of the beta zeolite acidity on the catalytic pyrolysis of biomass. They
observed that zeolites with stronger acidity tend to form less organic oil and more H O and
2
polyaromatic hydrocarbons than less acidic zeolites. 6

Mesoporous Acidic Catalysts

The significant increase in the production of H O and gas at the expense of the organics yield,
2
the formation of undesirable PAHs, and the rapid catalyst deactivation by coke deposition are
serious drawbacks of the zeolitic materials. To overcome these problems, catalysts with a
larger pore size than that of zeolites have recently attracted much attention, as they are
expected to allow the larger molecules of the pyrolysis products, particularly lignin-derived
compounds, to enter, reformulate, and exit the catalyst matrix with less chance of coke

deposition and blocking of pores. 22

Among the mesoporous acidic materials, the MCM-41 catalyst has received the most focus in
the literature. The MCM-41 is the main representative of the mesoporous molecular sieve of
M41S family, which was discovered in 1992, and possesses a hexagonal array of uniform
mesopores whose dimensions can vary from 1.4 nm to greater that 10 nm in size and exhibits
2
high surface areas (>1000 m /g). SBA-15 catalysts have been studied as well. The SBA-15