We have applied DFT calculations to investigate the acid-zeolite-catalyzed mixed aldol condensation of acetone with various aldehydes in delta clusters of HZSM-5. We investigate the convergence of activation barriers for two reactions relevant to biomass conversion. In both reaction systems, a δ = 4 Å cutoff is sufficient to converge activation barriers to within chemical accuracy (±1 kcal/mol) in delta clusters of HZSM-5. We have modeled the aldol condensation between the enol and biomass-derived aldehydes, furfural and hydroxymethylfurfural, in δ = 4 Å (~34T) cluster models of HZSM-5. We have discovered that unlike the case of homogeneous acid catalysis, the mechanism for the condensation step in HZSM-5 is concerted. We find that in HZSM-5 protonation of the aldehyde by the zeolite Brønsted acid site and nucleophilic attack on the aldehyde by the enol occur simultaneously. We report barriers for condensation between the enol and furfural or hydroxymethylfurfural as 4.6 kcal/mol and 2.6 kcal/mol, respectively. We conclude that the keto/enol tautomerization of acetone remains rate-determining in HZSM-5 even in the case of condensation with furfural or hydroxymethylfurfural.

Density functional theory study of aldol condensation reactions between acetone and biomass-derived aldehydes in delta clusters of HZSM-5

251st National Meeting (2016)

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