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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 the molecular environment around catalytically active site allows to enhance catalytic reactivity via a hitherto unexplored pathway. In zeolites, the presence of water creates an ionic environment via formation of hydrated hydronium ions and the negatively charged framework Al tetrahedra. The high density of cation-anion pairs determined by the aluminum concentration of a zeolite induces a high local ionic strength that increases the excess chemical potential of sorbed and uncharged organic reactants. Charged transition states (carbocations for example) are stabilized, reducing the energy barrier and leading to higher reaction rates. Using the intramolecular dehydration of cyclohexanol on H-MFI in water, we show quantitatively the enhancement of the reaction rate by the presence of high ionic strength as well as potential limitations of this strategy.

Acid catalysis by hydronium ions is ubiquitous in aqueous-phase organic reactions. Here we show that hydronium ion catalysis, exemplified by intramolecular dehydration of cyclohexanol, is markedly influenced by steric constraints, yielding turnover rates that increase by up to two orders of magnitude in tight confines relative to an aqueous solution of a Brønsted acid. The higher activities in zeolites BEA and FAU than in water are caused by more positive activation entropies that more than offset higher activation enthalpies. The higher activity in zeolite MFI with pores smaller than BEA and FAU is caused by a lower activation enthalpy in the tighter confines that more than offsets a less positive activation entropy. Molecularly sized pores significantly enhance the association between hydronium ions and alcohols in a steric environment resembling the constraints in pockets of enzymes stabilizing active sites.

Strategies to understand and mitigate the corrosive interactions of zeolites in aqueous phase under reaction conditions have been explored using zeolite BEA as an example. The states of Si and Al atoms after chemical modification and during gradual degradation were followed by crosspolarization enhanced 29 Si MAS NMR and 27 Al MAS NMR as well as IR spectroscopy. The key to stabilizing a zeolite for aqueous phase catalysis is to reduce the pore concentration of water in the presence of reacting substrates. The concentration of tetrahedral aluminum, which is charge balanced by hydrated hydronium ions, is the most important parameter determining the concentration of water in the zeolite pores. Lower intraporous water concentrations, largely independent of ubiquitous defects, led to longer zeolite lifetimes during cyclohexanol dehydration. The concentration of intraporous water was directly related to the rate of hydrolysis of Si 4+ from the zeolite lattice and its removal from the crystal. Dissolution of Si 4+ led eventually to a loss of confinement of the catalytically active hydronium ions and decreased the catalytic activity. At low Brønsted acid site concentrations, water bound to lattice defects begins to exert a measurable influence on the stability under reaction conditions.

Hydronium ions in the pores of zeolite H-ZSM5 show high catalytic activity in the elimination of water from cyclohexanol in aqueous phase. Substitution induces subtle changes in rates and reaction pathways, which are concluded to be related to steric effects. Exploring the reaction pathways of 2-, 3-, and 4-methylcyclohexanol (2-McyOH, 3-McyOH, and 4-McyOH), 2-and 4-ethylcyclohexanol (2-EcyOH and 4-EcyOH), 2-n-propylcyclohexanol (2-PcyOH), and cyclohexanol (CyOH) it is shown that the E2 character increases with closer positioning of the alkyl and hydroxyl groups. Thus, 4-McyOH dehydration proceeds via an E1-type elimination, while cis-2-McyOH preferentially reacts via an E2 pathway. The entropy of activation decreased with increasing alkyl chain length (ca. 20 J mol −1 K −1 per CH 2 unit) for 2-substituted alcohols, which is concluded to result from constraints influencing the configurational entropy of the transition states.

No abstract

In the presence of sufficient concentrations of water, stable,hydrated hydronium ions are formed in the pores and at the surface of solid acids such as zeolites.F or am edium-pore zeolite,s uch as zeolite MFI, hydrated hydronium ions consist of eight water molecules and have an effective volume of 0.24 nm 3 .I ntheir presence,larger organic molecules can only adsorb in the portions of the pore that are not occupied by hydronium ions.Asaconsequence,the available pore volume decreases proportionally to the concentration of the hydronium ions.T he higher charge density (the increasing ionic strength) that accompanies an increasing concentration of hydronium ions leads to an increase in the activity coefficients of the adsorbed substrates,t hus,w eakening the interactions between the organic part of the molecules and the zeolite and favoring the interactions with polar groups.T he quantitative understanding of these interactions makes it possible to link ac ollective property such as hydrophilicity and hydrophobicity of zeolites to specific interactions on molecular level. Understandingandcontrollingtheinteractionsofmoleculesin confined space of varying polarity is central to the action of enzymes and to properties that link zeolite catalysis with synthetic mimics of enzyme functions. [1] Thee nvironment interacts with the sorbed molecules,either via directed bonds that include hydrogen bonding and electron pair donoracceptor interactions or via nondirected dispersion forces. [2] Therelative dominance of the two types of interactions in an aqueous environment makes specific regions of the materials hydrophilic or hydrophobic, which is,i nt urn, manifested in colligative properties such as the specific surface tension, wetting, and the enrichment of polar or nonpolar components in complex mixtures.T his classification of interactions is widely used conceptually,but the classifications are empirical ("hydrophobicity scale") [3] and are difficult to extend and transfer between systems.Thes imultaneous presence of polar and nonpolar domains in proximity in catalyst/enzyme environments and its importance for catalytic properties make it imperative to explore the heterogeneity and its impact on interactions with substrates at an atomistic and molecular level. Zeolites,which have aw ell-defined three-dimensional pore structure with nonpolar channel walls and polar Brønsted acidic OH groups (Brønsted acid sites,B AS), are ideal systems to explore this complexity.The combination of active sites and pore structure offers au nique way to influence both ground and transition states in catalyzed reaction pathways.Understanding the influence of such an environment on reacting molecules becomes even more complex, when zeolites are used in the presence of water. Then, the Brønsted acid sites transform into charged hydrated hydronium ions (H 3 O + hydr. ). [4] It has been observed that the hydronium ions strongly affect adsorption properties in the micropore confines. [5] On am acroscopic level, zeolites with very low concentration...