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

The determination of molar absorption coefficients of the bending CH3 mode at 1380 cm-1 (e1380) of n-hexane adsorbed from the gas phase on two different dealuminated zeolites is here derived by a combined infrared spectroscopy and microgravimetric analysis. High silica Y (HSZ-Y) and ZSM-5 zeolites (with a SiO2/Al2O3 ratio of 200 and 280, respectively) with different textural and surface features were selected aiming to evaluate the effect of the pore structure and architecture on e1380 of the adsorbed n-hexane. Experimental data indicated that the molecule experienced a different adsorption environment inside of zeolites thus resulting in a significant change of the dipole moment and very different e1380 values: 0.278 ± 0.018 cm µmol-1 for HSZ-Y and 0.491 ± 0.032 cm µmol-1 for ZSM-5, respectively. Experimental data are also supported by computational modeling, which confirms the effect of the different matrices on the IR absorption intensity. This study enlightens that the use of probe molecules for quantitative measurements of surface sites has to be judiciously adopted, especially when the adsorption occurs in restricted spaces of microporous materials.