Enhancing the catalytic activity of hydronium ions through constrained environments

Abstract

The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Here we demonstrate that hydronium ions confined in the nanopores of zeolite HBEA catalyse aqueous phase dehydration of cyclohexanol at a rate significantly higher than hydronium ions in water. This rate enhancement is not related to a shift in mechanism; for both cases, the dehydration of cyclohexanol occurs via an E1 mechanism with the cleavage of Cβ–H bond being rate determining. The higher activity of hydronium ions in zeolites is caused by the enhanced association between the hydronium ion and the alcohol, as well as a higher intrinsic rate constant in the constrained environments compared with water. The higher rate constant is caused by a greater entropy of activation rather than a lower enthalpy of activation. These insights should allow us to understand and predict similar processes in confined spaces.

Tailoring nanoscopic confines to maximize catalytic activity of hydronium ions

“…We note that the free energy profile illustrated in Fig. 3 , purely based on kinetic and isotope measurements, is remarkably consistent with DFT calculations shown for aqueous phase (mimicked by hydronium ion and 20 water molecules inside the pore) cyclohexanol dehydration on zeolite HBEA 34 and 1-propanol dehydration on zeolite HZSM-5 (ref. 46 ) via E1-type pathways.…”

“…2 ). In the present case, only 15–19% of the hydronium ions were on time average associated with cyclohexanol in a solution containing 0.02 M H 3 PO 4 and 0.32 M cyclohexanol at reaction conditions 34 . Full alcohol–hydronium ion association in aqueous solutions would require alcohol concentrations of more than 5 M cyclohexanol.…”

“…Here, [C 6 H 11 OH] a is the concentration of the association complex formed between cyclohexanol and hydronium ion that depends on the alcohol–hydronium ion association equilibrium constant ( K L,a ) and the initial concentration of cyclohexanol. For dilute homogeneous acids ([H 3 O + aq ]=3–4 × 10 −3 M), K L,a was determined to be ∼40 and weakly temperature dependent (433–473 K) 34 . Consequently, at low alcohol concentrations (for example, 0.1 M for H/D isotope experiments), the concentration of the association complex is proportional to the total alcohol concentration.…”

Report of the Basic Energy Sciences Roundtable on Liquid Solar Fuels

“…However, the understanding of confined processes for solution-phase reactions is still at an early stage. Probing and controlling the effects of nanostructuring, confinement 103 , and surface interactions within pores to create microenvironments represent a significant challenge in solar fuels and related electrochemistry. A recent example of a confinement effect 104 shows how confining CO at the buried interface between a Pt electrode and a semi-permeable SiO 2 layer significantly alters the peak potential associated with CO oxidation (Figure 12).…”

Confinement Effects and Acid Strength in Zeolites

“…The catalytic activity of such hydronium ions also bene ts from the constrained environment, with rates approximately 1-2 orders of magnitude higher than those in an open aqueous environment [46][47][48] . Organic molecules appeared to adsorb in such environments only in the void left by hydrated hydronium ion clusters.…”