Aldol condensations are versatile C-C bond formation reactions that are widely used in chemical synthesis. An emerging application is upgrading of biomass-derived pyrolysis oils to fuels, whereby short chain carbonyl compounds, which make up a large fraction of such oils, are converted to molecules with longer carbon chains while oxygen is removed in the form of water. Commercially applied catalysts for aldol condensations are often soluble acids and bases with the known separation problems. The development of solid catalysts faces a number of challenges, including poisoning of basic sites by carboxylic acids present in bio-oils, leaching or even dissolution of the solid, diffusion limitations in pores, and vastly different reactivities of aldehydes and ketones resulting in unwanted condensations or side reactions.
Efforts in the literature in recent years have focused on catalyst activity and stability, and inorganic solids such as metal oxides and metal-substituted zeolites were evaluated. The seminar will focus on steering selectivity in aldol reactions by varying catalyst properties and reaction conditions. Regioselectivity is needed to preferentially obtain branched or straight carbon chains in aldol condensations involving asymmetric ket-2-ones, and chemoselectivity is needed to shift from production of condensation products to production of olefins. Catalyst families investigated include resins, functionalized ordered mesoporous silicas, sulfated zirconia, zeolites and metal-substituted zeotypes. It will be shown that selectivity strongly depends on the nature of the surface sites, catalyst porosity and the solvent. Reaction pathways will be associated with particular sites, and it will be shown how diffusion limitations affect selectivity.