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Fast-pyrolysis bio-oil that is currently produced from lignocellulosic biomass in demonstration and semicommercial plants requires significant modification to become an acceptable transportation fuel. The high acidity and chemical instability of bio-oils render them incompatible with existing petroleum refinery processes that produce gasoline and diesel fuels. To facilitate the use of bio-oil as a feedstock in a traditional refinery infrastructure, there is considerable interest in upgrading bio-oils through chemical pathways that include converting the carboxylic acids and reactive carbonyl compounds into esters and acetals using low-cost alcohols. In this article, we discuss our observations with different approaches to esterification and etherification chemistry using a crude bio-oil derived from mixed hardwoods. The high water content in crude bio-oils (ca. 20−30%) creates equilibrium limitations in the condensation reactions that hamper the upgrading process in that the neutralization and stabilization steps cannot easily be driven to completion. The lowest acid number that we were able to obtain without causing serious degradation of the flow properties of the bio-oil had a total acid number of about 20, a value that is still too high for use in a traditional petroleum refinery.
Moëns et al. (Mon,) studied this question.