Monomolecular mechanisms of isobutanol conversion to butenes catalyzed by acidic zeolites: Alcohol isomerization as a key to the production of linear butenes

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

“…At 400 K, the latter offers the lowest apparent free energy barrier, but the lowest intrinsic barrier is provided by the anti-E2 pathway, which renders well the experimental activation energy measured in ZSM-5 . More recent studies taking into account systematically the effect of temperature on the stability of intermediates and transition states have discarded alkoxide-based mechanisms in favor of synchronous E2 mechanisms, starting from isobutanol, tert -butanol, and 2‑butanol …”

“…Isobutanol dehydration was also addressed by the same research group in ZSM-5, but the formation of isobutene only was considered . Recently, new monomolecular dehydration routes of butanols were found by periodic DFT calculations performed on the model CHA framework that are likely to explain the formation of linear alkenes from isobutanol without relying on a posteriori isomerization of alkenes . The most likely ones consist either in first, isomerization of isobutanol into butan-2-ol, then dehydration of the latter into linear but-1-ene and but-2-enes (Figure a).…”

“…At the center, the structure of the transition state is depicted. Reproduced with permission from ref . Copyright 2022 Elsevier.…”

“…696 Recently, new monomolecular dehydration routes of butanols were found by periodic DFT calculations performed on the model CHA framework that are likely to explain the formation of linear alkenes from isobutanol without relying on a posteriori isomerization of alkenes. 667 The most likely ones consist either in first, isomerization of isobutanol into butan-2-ol, then dehydration of the latter into linear but-1-ene and but-2-enes (Figure 53a). Following the energy span concept, this mechanism is more likely than alkoxide-mediated routes, whereas butene isomerization is shown to have a much larger activation barrier than the two latter options.…”

“…The mechanisms, starting from isobutanol, tert-butanol, and 2-butanol. 667 Considering bridging OH groups at the external surface sites of ZSM-5 for 2-propanol dehydration, Prestianni et al concluded to a similar preferred reaction pathway with respect to framework sites in ZSM-5 and faujasite, namely a E2 synchronous dehydration mechanism. 191 The computed activation barriers are significantly higher at the external surface sites with respect to internal bridging OH groups.…”