Experimental Determination of the Molar Absorption Coefficient of n-Hexane Adsorbed on High-Silica Zeolites

Determination of the molar absorption coefficients of the CH₃ bending mode at (Formula presented.) =1380 cm⁻¹ (ϵ₁₃₈₀) of n-hexane adsorbed from the gas phase on two different dealuminated zeolites is derived by a combination of IR spectroscopy and microgravimetric analysis. High-silica zeolite Y (HSZ-Y) and zeolite ZSM-5 (with SiO₂/Al₂O₃ ratios of 200 and 280, respectively) with different textural and surface features are selected to evaluate the effect of the pore structure and architecture on the value of ϵ₁₃₈₀ of the adsorbed n-hexane. Experimental data indicate that the molecule experiences a different adsorption environment inside zeolites; thus resulting in a significant change of the dipole moment and very different ϵ₁₃₈₀ values: (0.278±0.018) cm μmol⁻¹ for HSZ-Y and (0.491±0.032) cm μmol⁻¹ for ZSM-5. Experimental data are also supported by computational modeling, which confirms the effect of different matrices on the IR absorption intensity. This study reveals that the use of probe molecules for quantitative measurements of surface sites has to be judiciously adopted, especially if adsorption occurs in the restricted spaces of microporous materials.