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.

No abstract

Tungstate domains supported on ZrO 2 , Al 2 O 3 , TiO 2 , and activated carbon drastically influence the hydronium-ion-catalyzed aqueous-phase dehydration of alcohols. For all catalysts, the rate of cyclohexanol dehydration normalized to the concentration of Brønsted acid sites (turnover frequencies, TOFs) was lower for monotungstates than for polytungstates and larger crystallites of WO 3 . TOFs were constant when reaching or exceeding the monolayer coverage of tungstate, irrespective of the specific nature of surface structures that continuously evolve with the surface W loading. However, the TOFs with polytungstates and large WO 3 crystallites depend strongly on the underlying support (e.g., WO x /C catalysts are 10−50-fold more active than WO x /Al 2 O 3 catalysts). The electrical double layer (EDL) surrounding the negatively charged WO x domains contains hydrated hydronium ions, whose local concentrations change with the support. This varying concentration of interfacial hydronium ions ("local ionic strength") impacts the excess chemical potential of the reacting alcohols and induces the marked differences in the TOFs. Primary H/D kinetic isotope effects (∼3), together with the substantially positive entropy of activation (111−195 J mol −1 K −1 ), indicate that C−H(D) bond cleavage is involved in the kinetically relevant step of an E1-type mechanistic sequence, regardless of the support identity. The remarkable support dependence of the catalytic activity observed here for the aqueous-phase dehydration of cycloalkanols likely applies to a broad set of hydronium-ion-catalyzed organic reactions sensitive to ionic strength.

The mechanistic roles of free hydronium ions, surface hydrides, and interfacial protons during guaiacol hydrodeoxygenation (HDO) on ruthenium nanoparticles have been established. As guaiacol adsorbs on Ru, it loses its strong aromaticity and undergoes a rapid H-shift from its hydroxyl to meta carbons (in relation to its hydroxyl group), causing adsorbed enol and keto surface isomers to exist in chemical equilibrium. HDO occurs via a hydridic H-adatom (H*) attack on the enol, followed by a kinetically relevant C–O bond rupture step, during which water shuttles the hydroxyl proton, enabling its intramolecular attack on the methoxy, evolving to a highly charged [Ru(s)–(C6H5O–)···(H+)···OCH3]† transition state. The competing hydrogenation (HYD) begins with a rapid H* attack on the keto form, before a second, kinetically relevant H* attack without proton involvement. Water, despite shifting the thermodynamics toward the more polar surface keto, promotes HDO to a much greater extent than HYD, because of its dual catalytic roles in reducing the activation free energies(i) it mobilizes the hydroxyl proton of partially saturated guaiacol (Brønsted acid) and functions cooperatively with the Ru metal surface (base) in rupturing the C–O bond and stabilizing the resulting cationic carbon-ring fragment and (ii) water layers solvate the charged [Ru(s)–(C6H5O–)···(H+)···OCH3]† transition state. Free hydronium ions do catalyze a separate homogeneous enol–keto isomerization, but this reaction is kinetically unrelated to HDO catalysis. This mechanistic picture explains the strong effects of a polar protic solvent in hydrodeoxygenation, highlighting the requirements of surface hydrides and interfacial protons acting in tandem to complete a HDO turnover and the cooperative role of the protic solvent and the metal surface in breaking the aromaticity and preferentially stabilizing charged transition states.

The intracrystalline ionic environment in microporous zeolite can remarkably modify the excess chemical potential of adsorbed reactants and transition states, thereby influencing the catalytic turnover rates. However, a limit of the rate enhancement for aqueousphase dehydration of alcohols appears to exist for zeolites with high ionic strength. The origin of such limitation has been hypothesized to be caused by the spatial constraints in the pores via, e.g., size exclusion effects. It is demonstrated here that the increase in turnover rate as well as the formation of a maximum and the rate drop are intrinsic consequences of the increasingly dense ionic environment in zeolite. The molecularly sized confines of zeolite create a unique ionic environment that monotonically favors the formation of alcohol-hydronium ion complexes in the micropores. The zeolite microporous environment determines the kinetics of catalytic steps and tailors the impact of ionic strength on catalytic rates.