PNNL

Abstract

The kinetics of the acid-catalyzed dehydration of cyclohexanol is investigated using a mineral acid, H3PO4, and two HBEA zeolites (Si/Al = 71 and 75) with different Al T-site distributions, aiming to derive structure-property relations for liquid-phase dehydration on solid acids. Aqueous phase adsorption and calorimetric measurements show that adsorption of cyclohexanol molecules in the zeolites is favorable at reaction temperatures of 160-200 °C. With respect to cyclohexanol in the solution state, the dehydration in HBEA proceeds with an enthalpy of activation (136 kJ mol-1) that is less than that for aqueous H3PO4, but with an entropy of activation that is essentially the same (~60 J mol-1 K-1). The dehydration of neat liquid cyclohexanol over HBEA occurs with a significantly lower activation barrier (by ~40 kJ mol-1) and less entropy gain (by ~60 J mol-1 K-1) than in aqueous phase. The effect is attributed to differences in solvation environments caused by changes in the intrazeolite concentrations of cyclohexanol and water that are ultimately determined by the bulk activities of cyclohexanol and water, together with their relative adsorption strengths on HBEA. Accordingly, solvent-free conditions significantly reduce the number of intraporous water molecules that solvate the zeolitic proton. In consequence, proton transfer from the H3O+(H2O)n cluster to cyclohexanol becomes increasingly favorable as fewer water molecules are associated with the proton. DFT calculations predict that, in the absence of water, the protonated cyclohexanol dimer is the dominant surface species, which undergoes C–O bond cleavage via a concerted pericyclic pathway.