Influence of Intracrystalline Ionic Strength in MFI Zeolites on Aqueous Phase Dehydration of Methylcyclohexanols

Abstract

The impact of the concentration of hydrated hydronium ions and in turn of the local ionic strength in MFI zeolites has been investigated for the aqueous phase dehydration of 4‐methylcyclohexanol (E1 mechanism) and cis‐2‐methylcyclohexanol (E2 mechanism). The E2 pathway with the latter alcohol led to a 2.5‐fold higher activity. The catalytic activity normalized to the hydronium ions (turnover frequency, TOF) passed through a pronounced maximum, which is attributed to the increasing excess chemical potential of the alcohols in the pores, increasing in parallel with the ionic strength and the additional work caused by repulsive interactions and charge separation induced by the bulky alcohols. While the maximum in rate observed is invariant with the mechanism or substitution, the reaction pathway is influencing the activation parameters differently.

Hydronium ion and water complexes vs. methanol on solid catalyst surfaces: how confinement influences stability and reactivity

“…It was recently shown that the changed ionic strength and acid site density of MFI zeolites is responsible for a volcano-plot like change of the reaction rate of dehydration reactions. 31 Notably, it is stated that a H + (H 2 O) n complex with n = 8 sterically fills the pores of the MFI zeolites and that too close acid sites thus block each other for reactant access, leading to slower dehydration. An important but often ignored surface group is the silanol or Si(OH) group.…”

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

“…The mechanism of the dehydration of both cyclohexanol and methylcyclohexanols in zeolites was studied in-depth by Lercher's group, especially systemically considering the influence of intracrystalline ionic strength, solvent, Si/Al ratio of zeolites, water concentration, etc. 99,166–173 For the unimolecular cyclohexanol dehydration via the E1 mechanism in the gas phase, the cyclohexyl carbenium cation (C 6 H 11 + ) is formed by C–O bond cleavage of protonated cyclohexanol, and the calculated Bader charge of +0.93 | e | and stationary point on the free energy surface confirmed the existence of C 6 H 11 + as an intermediate (Fig. 16a and b).…”

“…Moreover, the density of cation-anion pairs can be determined based on the Si/Al radio of acidic zeolites, which further induces a high local ionic strength and increases the excess chemical potential of related organic reactants simultaneously. 166,169,171,173 The charged transition states are stabilized by cation-anion pairs, reducing the energy barrier and leading to higher reaction rates (Fig. 16d).…”

“…Therefore, the formation of the ethyl carbenium ion (instead of the aromatic carbenium ions) in zeolite from ethanol associated with the hydronium ion in the aqueous phase was considered as the rate-determining step by Eckstein et al 308 In this regard, the solvent effect significantly affects the stability of the carbenium ions in zeolites, and the mechanistic insightful understanding of the solvent effect in zeolite-catalyzed reactions was investigated by Lercher's group. 166,167,169,172,309 The alkylation of phenol with tertbutanol preferentially occurs through a concerted path to directly form the product without the preparation of tert-butyl carbenium ion as an intermediate. 160 Notably, the alkylation of aromatics with large alcohol or alkene is different, and the bulky alcohol and alkene would be not easy to form surface alkoxide because of the steric hinderance, and the carbenium ions of alcohol dehydration or alkene protonation by BAS are more stable intermediates.…”

Dynamic Pt–OH−·H2O–Ag species mediate coupled electron and proton transfer for catalytic hydride reduction of 4-nitrophenol at the confined nanoscale interface

“…4a) was closely related to the electron transfer process between proton of water and hydride of NaBH 4 as an electron donor by surface dark states even without the involvement of d 8 –d 10 group metals. Very recently, in the confined sub-nanopores of microporous zeolites, by 1 H NMR, Lercher's group captured similar H species, 77–85 which was called hydronium ions [H + (H 2 O) n ], but their formation was just simply attributed to the physical confinement of the nanocavity. The capturing of activated intermediate hydrogen species not only answered the origin of non-metal catalysts for hydride catalytic reductivity of 4-NP, 50,51 but also shed new light on the H + activation of water by non-metal catalysts in the electrocatalytic reactions.…”