Self-Terminated Cascade Reactions That Produce Methylbenzaldehydes from Ethanol

2- and 4-Methylbenzaldehyde, which are useful precursors for phthalic anhydride and terephthalic acid, form by sequential aldol condensations between acetaldehyde and enals and subsequent dehydrocyclization during ethanol upgrading reactions on hydroxyapatite catalysts. The selectivities for methylbenzaldehydes exceed 30%, as a result of rapid cyclization reactions and steric protection that hinders further growth. Such pathways compete with a set of alternating condensation and hydrogenation steps, known as the Guerbet reaction, which produces broad distributions of n- and iso-alcohols. The selectivities to C-8 aromatic products increase in proportion to the ratio of the acetaldehyde to ethanol concentrations. These reactions provide new pathways for the selective conversion of bioethanol to value-added chemicals.