Page 111 - Advances in bioenergy (2016)

reactions that take place during zeolite catalysis. Horne and Williams investigated the reaction
of oxygenated biomass pyrolysis model compounds over a ZSM-5 catalyst in an attempt to
simplify the problem. They found that methanol could be catalytically converted to
hydrocarbon products at temperatures 300–350°C, whereas furfural, anisole, and
cyclopentanone required higher temperatures. With the exception of anisole, increasing catalyst
bed temperature reduced coke formation for the upgrading of the oxygenated compounds. The
optimum catalysis temperature was 500–550°C. At that temperature, however, anisole gave
high yields of coke, suggesting that the phenolic compounds are the major coking components
present in biomass-derived pyrolysis vapors. The oxygen bond in anisole appeared to be

refractory to the catalytic upgrading, leading to the formation of large quantities of phenolic
compounds and gave only low yields of aromatic hydrocarbons. 14

Gayubo et al. 15,16 studied the transformation of oxygenated bio-oil model compounds over an
HZSM-5 zeolite as well. They found that alcohols transformed via dehydration from olefins at
lower temperatures to higher olefins at 250°C and to paraffins and aromatic hydrocarbons
above 350°C. Phenol exhibited low reactivity on the HZSM-5 zeolite. 2-Methoxyphenol had
low reactivity as well and furthermore generated thermal coke. These observations for the
phenolic compounds were in good agreement with the observations of Horne and Williams. 14

The rate of deactivation by coke deposition was low for both alcohols and phenols.
Acetaldehyde had a low reactivity to hydrocarbons with noticeable tendency to form coke
depositions, which was attributed to its capacity for oligomerization. The transformation of
ketones and acetic acid occurred through decarboxylation and, to a lesser extent, dehydration.
Above 400°C, this transformation gave olefins and aromatics, similar to the reaction scheme of
alcohols. The generation of coke was more significant than in the corresponding process for
alcohols and increased the formation of olefins, which were the intermediate products of the
reaction scheme. 15,16 In a later study, Gayubo et al. investigated the transformation of mixtures
of bio-oil model compounds on the HZSM-5 zeolite and found that the results agree well with
the results in the case of using pure compounds. However, synergistic effects were observed in
the case of using model compound mixtures instead of pure compounds due to the high
reactivity of some of the primary pyrolysis products. The catalyst underwent severe
deactivation by coke deposition, which was enhanced by the thermal coke from the degradation

of acetaldehyde. The presence of furfural enhanced the tendency of 2-methoxyphenol to
degrade to coke and led to the conclusion that the feasibility of the process of bio-oil
upgrading into hydrocarbons requires the previous separation of certain components of the bio-
oil such as aldehydes, oxyphenols, furfural, and their derivatives in order to reduce the amount
of coke formed. 17

18
Vitolo et al. studied the upgrading of bio-oil over HZSM-5 and tentatively attributed catalytic
activity to its acidic sites that, through a carbonium ion mechanism, promote deoxygenation,
decarboxylation, and decarbonylation of the oil constituents as well as cracking,
oligomerization, alkylation, isomerization, cyclization, and aromatization. They also
investigated the behavior of the zeolite, using it in repeated upgrading-regenerating cycles and
observed that the material undergoes irreversible poisoning after the fifth upgrading-

regenerating cycle. The deactivation was due to the disappearance of a significant amount of