Experimental and theoretical studies about the adsorption of toluene on ZSM5 and mordenite zeolites modified with Cs

In this work, we studied the adsorption capacity and thermal stability of toluene on ZSM5 and mordenite zeolites with different exchange cations (H, Na and Cs). The interaction of the hydrocarbon molecules with the adsorption sites was also addressed with both experimental and theoretical techniques. Flow-adsorption measurements were carried out in order to obtain breakthrough curves as well as TPD experiments. Molecular simulations were performed using the Density Function Theory (DFT). The results show that, in general, mordenite samples have a better adsorption capacity than ZSM5 samples and that the thermal stability of the adsorbed toluene is more influenced by the exchanged cations than by the zeolite structure. The overall basicity of the samples depends on the exchanged cations, following the Cs > Na > H order. The main interaction of toluene takes place between the π electrons of the aromatic ring and the Lewis sites generated, and another interaction takes place between C–H groups and the zeolite network oxygen.The increase of the Cs loading decreases the surface area and pore volume of the zeolites, thus decreasing the available sites for the overall toluene adsorption at 100 °C. Through TPD experiments both the Φ parameter, which represents the fraction of toluene retained at temperatures higher than 100 °C, and the temperature at which the toluene desorbs were measured. It was observed that in the Cs exchanged samples, Φ is higher but the thermal stability (temperature of the maximum desorption rate on TPD) follows the order NaMOR = NaZSM5 > CsNaZSM5 > CsNaMOR > CsHMOR = CsHZSM5 > HMOR > HZSM5. The molecular calculations for the NaMOR structure determined that the interaction energies were higher for NaMOR when compared with the HMOR structure and increased even more when Cs was exchanged. These facts are in line with the experimental results.

The figure shows a zeolite cluster of 12 members, formed by tetrahedral T–O–T (T = Si or Al) that interact with a molecule of toluene.Figure optionsDownload as PowerPoint slideHighlights
► The Cs exchange increases the zeolitic oxygen charge which determines the basicity.
► The structure of 1-D MOR appears to be more convenient than the 3-D of ZSM5.
► The simulation results show that CsNaMOR have high interaction energy with toluene.