Elementary steps and reaction pathways in the aqueous phase alkylation of phenol with ethanol

•Hydronium ions confined in H-MFI pores catalyze alkylation of phenol with ethanol in aqueous phase.•Both C-and O-alkylation of phenol proceed via ethyl carbenium ion intermediates.•Apparent alkylation rate per hydronium ion increases with decreasing BAS concentration in H-MFI zeolites.•H-MFI zeolite with lower BAS concentration shows stronger phenol adsorption from aqueous phase.•Intrinsic rate per hydronium ion is constant among H-MFI zeolites with different BAS concentrations.

The hydronium ion normalized reaction rate in aqueous phase alkylation of phenol with ethanol on H-MFI zeolites increases with decreasing concentration of acid sites. Higher rates are caused by higher concentrations of phenol in the zeolite pores, as the concentration of hydronium ions generated by zeolite Brønsted acid sites decreases. Considering the different concentrations of reacting species, it is shown that the intrinsic rate constant for alkylation is independent of the concentration of hydronium ions in the zeolite pores. Alkylation at the aromatic ring of phenol and of toluene as well as O-alkylation of phenol have the same activation energy, 104 ± 5 kJ·mol−1. This is the energetic barrier to form the ethyl carbenium ion from ethanol associated with the hydronium ion. Thus, in both the reaction pathways the catalyst involves a carbenium ion, which forms a bond to a nucleophilic oxygen (ether formation) or carbon (alkylation).

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