Aldol reactions are important for C-C bond formation in chemical synthesis, with a wide range of applications in basic and fine chemicals, and potential for upgrading of carbonyl compounds in biomass-derived pyrolysis oils to longer carbon chain compounds. Commercially applied catalysts for aldol condensations are typically soluble acids and bases, and a high level of selectivity control has been achieved using homogeneous catalysis and strategies such as pre-enolization. However, heterogeneous catalysis would facilitate separation and make aldol processes more sustainable. The objective of this investigation was develop selective solid catalysts. Several families of catalysts characterized by different types of acid and base sites were synthesized including (i) propyl sulfonic-acid (PrSO3H) functionalized SBA-15 with various acid loadings and with or without hydrophobization by octyl groups (Oc), (ii) isomorphously substituted BEA zeotypes with Sn, Ti, Zr and Hf, (iii) and sulfated zirconias with various sulfate loadings. Molecular catalysts and resins with appropriate functional groups (-SO3H, -NH2) served as benchmarks. The aldol condensation of benzaldehyde and 2-butanone provided opportunity for regioselectivity through preferential activation at the methylene or the methyl group and for chemoselectivity through preferential fission (to an olefin and a carboxylic acid) or dehydration of the aldol (to the condensation product). Sulfonic acid-functionalized catalysts preferentially formed branched condensation product and metal-substituted BEA zeotypes selectively formed linear condensation product, whereas sulfated zirconia catalysts were not selective. This behavior can be rationalized by the more facile dehydration of the linear aldol, which determines the selectivity if dehydration is rate-limiting in the absence of strong acid or base sites, whereas the favored branched aldol determines selectivity if dehydration is catalyzed by a strong acid. The behavior of sulfated zirconia can be explained by its amphoteric character. Strong acid sites and transport limitation were found to promote the fission of the branched addition product to β-methyl styrene and acetic acid. This reaction was suppressed by hydrophobizing the surface via octyl co-functionalization. The results demonstrate that selectivity can be controlled, but requires solids with well-tuned chemical and morphological properties.

Regioselectivity and chemoselectivity in heterogeneously catalyzed cross-aldol condensations with unsymmetrical ketones

255th National Meeting (2018)

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