Etherification of 5-hydroxymethyl-2-furfural (HMF) with ethanol to biodiesel components using mesoporous solid acidic catalysts

The etherification of 5-hydroxymethyl-2-furfural (HMF) with ethanol is studied over a series of mesoporous silica catalysts (Al-MCM-41 materials with different Si/Al ratio, and zirconia or sulfated zirconia supported over SBA-15) and compared with the behavior of H2SO4 and Amberlyst®15. The observed reaction products were 5-(ethoxymethyl)furan-2-carbaldehyde (EMF), 1,1-dietoxy ethane (DE) and ethyl 4-oxopentanoate (EOP). The selectivity to EMF and EOP is closely related to the presence of Lewis and/or Brønsted acidity on the catalyst, while the formation of DE is probably related to defect sites. The latter, being less reactive, catalyze the side reaction to DE only when strong Lewis and/or Brønsted acid sites are absent. Catalysts with only a strong Brønsted acidity react selectively to form EOP. When strong Lewis acid sites are present in the catalyst, e.g. by introducing ZrO2 in SBA-15 or when extra-framework isolated Al3+ sites are present in the mesoporous channels, a high selectivity to EMF was observed. The results indicate that EMF, DE or EOP can be obtained selectively by direct reaction of HMF with bioethanol by tuning the acidity of the catalyst. EMF is a value biodiesel component, but the results also evidence the possibility to obtain selectively EOP in a one-step reaction, opening interesting perspectives to produce valeric biofuels by subsequent selective hydrogenation.