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

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).