The knowledge of host–guest interactions occurring in confined space between porous solids and embedded molecules of different origin is an important task to improve adsorption properties of materials, thus extending their application fields. In this work, the interactions of toluene and n-hexane molecules (selected as models of organic pollutants coming from industrial waste of oil refineries and gas stations) on different dealuminated high silica zeolites were studied by means of both experimental and computational approaches. Zeolites with different textural and surface features were selected as adsorbents and the effect of their physicochemical properties (i.e., pore size architecture and type and amount of surface OH sites) on sorption capacity were studied. High silica Y and ZSM-5 zeolites (with a SiO2/Al2O3 ratio of 200 and 280, respectively) were selected as model sorbents. FTIR and SS-NMR spectroscopy were used to study the type and strength of the host−guest interactions between the molecules and the zeolite surface. Gravimetric analysis allowed the determination of the sorption capacity of both zeolites and their affinity to pollutants. The interactions between the silica surfaces and pollutants molecules computed at the DFT level, and supplemented by empirical formulation of dispersion energies, led to estimate the intensity of hydrogen bonding and cooperative van der Waals interactions.

An experimental study of the interactions of an equimolar binary gaseous mixture of toluene and n-hexane, model molecules of aromatic and aliphatic fuel-based pollutants, with two dealuminated high silica zeolites is here presented for the first time. Zeolites Y and ZSM-5 with diverse textural and surface properties were chosen as adsorbents, and the effects of their physicochemical features (predominantly the pore size architecture and silanol content) on sorption capacity were studied. The host−guest (i.e., sorbent-molecules) interactions were studied by FTIR and SS-NMR spectroscopies. IR optical adsorption isotherms of both toluene and n-hexane coadsorbed on the zeolites allowed the determination of the concentration of the adsorbed molecules. Variable temperature SS-NMR spectroscopy allowed the description of the mobility of the pollutant molecules when coadsorbed at the interface of the zeolites micropores. Finally, we describe how the proposed innovative approach can be of general use to determine the selectivity of adsorbent materials for a mixture of hydrocarbons.

Determination of the molar absorption coefficients of the CH bending mode at ν˜ =1380 cm (ϵ ) 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 SiO /Al O 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 ϵ 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 ϵ values: (0.278±0.018) cm μmol for HSZ-Y and (0.491±0.032) cm μmol 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.

Two amorphous mesoporous silica (AMS) samples with different textural properties were prepared, characterized by a multi-technique approach and tested as adsorbents for the purification of natural gas from aromatic hydrocarbons.