Does Ionic Strength in Zeolite Pores Impact Brønsted Acid-Catalyzed Reactions?

Ionic strength has been proposed as the main factor affecting the dehydrating activity of cyclohexanol in an aqueous solution. However, the rate of cyclohexanol dehydration varied by a factor up to ∼15 on H-ZSM-5 samples with similar ionic strengths but different particle sizes. Site accessibility limitations were revealed, as the rate exhibited an inverse correlation with the average particle size of H-ZSM-5 samples. We employed uptake measurements and in situ transmission Fourier transform infrared (FTIR) spectroscopy to systematically investigate the adsorption behavior of cyclohexanol in H-ZSM-5: the diffusion of cyclohexanol into the pore of H-ZSM-5 in aqueous solution was facile, whereas cyclohexene demonstrated a tendency for oligomerization on the Brønsted acid sites (BAS) so that only a fraction of BAS participated in the reaction. We developed a procedure to estimate the density of the “effective” BAS in cyclohexanol dehydration. Consistent turnover frequencies (TOFs) of cyclohexanol dehydration (∼3.5 × 10–3 s–1) were observed for all H-ZSM-5 samples in this study when rates were normalized to the density of the “effective” BAS. Our results suggest that ionic strength in zeolite pores does not significantly impact BAS-catalyzed cyclohexanol dehydration reactions.