We have performed density functional theory calculations to investigate the acid-zeolite-catalyzed aldol condensation of acetone with aldehydes relevant to biomass conversion in cluster models of HZSM-5. These calculations have been benchmarked by kinetics experiments on selected reaction systems. We have investigated the convergence of activation barriers for acetone condensation with furfural and hydroxy-methyl-furfural (HMF) in cluster models of HZSM-5, using the delta-cluster approach to determine minimal cluster sizes for achieving converged barriers, as reported previously by us [ACS Catalysis <b>5</b>, 2859 (2015)]. We have modeled the acid-catalyzed reaction in HZSM-5 in two steps: keto/enol tautomerization of acetone, and bimolecular combination between each aldehyde and the acetone enol. We have found that a delta-cluster cutoff of δ = 4 Å is sufficient to converge activation barriers with respect to cluster size, yielding computationally-tractable HZSM-5 clusters of up to ~34T where T = Si or Al tetrahedral atoms. We have discovered that, unlike the case of homogeneous acid catalysis, the mechanism of bimolecular combination in HZSM-5 is concerted, with zeolite protonation of the aldehyde and nucleophilic attack on the protonated aldehyde occurring simultaneously. Our calculations have yielded bare barriers of 20. kcal/mol for acetone keto/enol tautomerization, and 4.2 kcal/mol / 2.6 kcal/mol for enol bimolecular combination with furfural / HMF, respectively, predicting that the keto/enol step remains rate-limiting despite the bulky nature of these aldehydes. Our kinetics experiments on acetone/furfural condensation in HZSM-5 over the temperature range 80-120<sup>o</sup>C have established that this aldol chemistry is under reaction control by using HZSM-5 with different particle sizes, and have yielded apparent activation energies of 14.5 ± 2.3 kcal/mol (100 nm ZSM-5), 13.9 ± 0.8 kcal/mol (800 nm ZSM-5), and 13.6 ± 1.9 kcal/mol (5 mm ZSM-5). Our DFT-computed enthalpy of keto/enol activation, 18 kcal/mol, is in broad agreement with the experimental activation energies, but also indicates the presence of more active Brønsted acid sites not considered by our calculations.

Aldol condensation of biomass-derived feedstocks in HZSM-5 zeolite: Quantum chemistry and experiments point to keto/enol as the rate-determining step

253rd National Meeting (2017)

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