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.
Highlights
The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water
Thermochemical and kinetic measurements are used in conjunction with density functional theory (DFT) and isotope labelling, to elucidate quantitatively the reaction pathway in the aqueous-phase dehydration of alcohols in constrained environment and analyse the benefits of such a sterically tailored environment based on transition state theory
It is noteworthy that turnover frequencies (TOFs) were obtained by normalizing the rates to the concentration of total Brønsted acidic sites (BAS) in HBEA150 (Supplementary Table 2), as we have shown earlier that all the BAS are present in the form of solvated hydronium ions[4], which are active in aqueous-phase dehydration[11]
Summary
The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Despite the seemingly ubiquitous use in organic conversion sequences, the dehydration of alcohols by hydronium ions in aqueous phase is surprisingly challenging, requiring reaction temperatures above 100 °C to occur at industrially acceptable rates[1] The reasons for this lie in significant enthalpic and entropic barriers for the formation of carbocationic intermediates and for their decomposition to form the olefin and water. In contrast, are able to catalyse dehydration of alcohols with high rates at temperatures close to ambient[2], which is attributed to the unique microenvironment of the catalytically active centers in the three-dimensional enzyme structures and the nearly concerted acid base interactions In translating this concept to inorganic catalysts we have shown in recent preliminary experiments that zeolite pores are able to substantially increase the rate at which hydronium ions catalyse reactions[3]. Was B158 kJ mol À 1 at two alcohol concentrations (0.32 and 0.90 M; see Table 1 and Supplementary Fig. 4)
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