Effects of Local Water Concentrations on Cyclohexanol Dehydration in H-BEA Zeolites

Aqueous-phase acid-catalyzed dehydration of aromatic alcohols over zeolites is one of the major steps in efficient conversion of lignocellulosic biomass to hydrocarbon fuels and value-added chemicals. In this work, the effects of local water concentrations inside the H-BEA zeolite pores on the reaction activity and mechanisms of cyclohexanol dehydration were investigated using ab initio molecular dynamics simulations and density functional theory calculations. Water molecules in H-BEA zeolite pores aggregate and form hydronium water clusters acting as the mobile catalytically active center when more than two water molecules are present at the Brønsted acidic site. Compared with vapor-phase dehydration following the unimolecular E1 mechanism, the presence of a hydronium water cluster inside H-BEA zeolite pores makes the cyclohexanol dehydration to cyclohexane more difficult, where the bimolecular E2 mechanism with the concerted C-O/C-H bond scissions becomes the major dehydration route. At high water concentrations, the protonation of cyclohexanol is a kinetically relevant step.