Page 112 - Advances in bioenergy (2016)

acid sites, mainly the stronger ones. 18

The catalytic upgrading of the pyrolysis vapors is known to reduce the bio-oil yield and
increase the gas, coke, and H O yields. Some groups have also reported a reduction in the
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biomass residue when the catalyst came in contact with the biomass feed. Atutxa et al. studied
the effect of an HZSM-5 zeolite on the in situ pyrolysis of sawdust at 400°C. The gas yields
increased with increasing catalyst mass, whereas the liquid yield decreased notably. They also
observed a slight reduction in char. The proportion of CO over CO increased with increasing
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catalyst mass. Atutxa et al. mainly related the reduction in the total liquid product with the
transformation of the heavy liquid fraction into light liquid fraction and gases. The light
fraction was more severely deoxygenated than the heavy fraction, which is evidence of the
higher global reactivity of the compounds of the lighter fraction, especially of alcohols and
acetic acid. This observation was in good agreement with the work with bio-oil model
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compounds of Horne and Williams and Gayubo et al. 15,16 The catalytic pyrolysis oil was less
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oxygenated, less viscous, less corrosive, and more stable. Wang et al. studied the pyrolysis
of three kinds of biomass impregnated with HZSM-5 zeolite and similarly found that the
presence of HZSM-5 in the biomass increased the maximum weight loss rate. Zhang et al. 20
studied the pyrolysis of corncob in a fluidized bed with HZSM-5 catalyst and observed that the
catalyst caused a marked decrease of heavy oil fraction and an increase of H O, coke, and
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noncondensable gas yields. The use of HZSM-5 also led to a remarkable increase of aromatic
hydrocarbons in the oil fraction and to a decrease of all the other types of compounds. The

oxygen content of the catalytic oil decreased to 14.69% from 40.28% in the noncatalytic oil. 20
Other groups have demonstrated the selectivity of ZSM-5 toward aromatics and its ability to
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reduce oxygenates as well. Samolada et al. studied the conversion of representative bio-oil
model compounds over different catalytic materials and concluded that the HZSM-5 zeolite
completely converted undesirable carbonyls to hydrocarbons with a simultaneous loss of the
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organic liquid fraction and a dramatic increase of H O. Pattiya et al. investigated the
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upgrading of the vapors from the pyrolysis of cassava rhizome in a pyrolysis-gas
chromatography–mass spectrometry (Py-GC–MS) system using various catalysts. They
concluded that the ZSM-5 catalyst was the most active catalyst and increased the formation of
aromatic hydrocarbons, phenols, and acetic acid. It also decreased oxygenated lignin-derived
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compounds and carbonyls. Lastly, Adjaye et al. studied the effect of mixtures of HZSM-5
and silica alumina on the product distribution during the conversion of maple wood to bio-oil
and observed that the organic liquid that was produced with silica–alumina consisted mainly
of aliphatic hydrocarbons, whereas the organic liquid product that HZSM-5 produced
consisted mostly of aromatic hydrocarbons. The addition of HZSM-5 to silica–alumina
reduced coke formation and increased organic liquid product and gas yields. The gradual
increase of HZSM-5 in the mixture changed the hydrocarbon products from aliphatic to
aromatic. These observations suggest that the HZSM-5 is a more effective hydrogen transfer

catalyst than the less acidic silica–alumina and the aliphatics formed during upgrading are
converted into the thermodynamically favored aromatics. As aromatics have a lower H–C ratio
than aliphatic hydrocarbons, more hydrogen has to be put into the aliphatics, which reduces the