Adsorption of Toluene and Water over Cationic-Exchanged Y Zeolites: A DFT Exploration

When less is more: does more Na+-cations mean more adsorption sites for toluene in faujasites?

The unique properties of zeolites make them an interesting material to be used in separation processes. The possibility of tailoring some of their characteristics, like the Si/Al ratio, allows optimizing their synthesis for a given task. Concerning the adsorption of toluene by faujasites an understanding of the effect of cations is necessary to foster the elaboration of new materials, which can capture molecules with a high degree of selectivity and sensitivity. Undoubtedly, this knowledge is relevant for a wide range of applications going from the elaboration of technologies for improving the air-quality to diagnostic procedures to prevent health risks. The studies reported here using Grand Canonical Monte Carlo simulations elucidate the role of Na-cations in the adsorption of toluene by faujasites with different Si/Al ratios. They detail how the location of the cations inhibits or enhances the adsorption. The cations located at site II are shown to be those enhancing the adsorption of toluene on faujasites. Interestingly, the cations located at site III generate a hindrance at high loading. This becomes an impediment for the organization of toluene molecules inside faujasites.

Bimetallic Copper-Silver Systems Supported on Natural Clinoptilolite: Long-Term Changes in Nanospecies’ Composition and Stability

Long-term changes in species of copper-silver bimetallic systems on natural clinoptilolite obtained by ion exchange of Cu2+ and Ag+ and then reduced at different temperatures were studied. Even after storage under ambient conditions, XRD and UV-Vis diffuse reflectance spectra indicate the presence of nanospecies and larger particles of reduced copper and silver. Scanning electron microscopy of aged bimetallic samples, reduced at the highest temperature (450 °C) and the pristine sample for their preparation, also aged, showed the presence of silver particles with a size of about 100 nm. They are formed in the initial ion-exchanged sample (without reduction) due to the degradation of Ag+ ions. The particles in the reduced sample are larger; in both samples they are evenly distributed over the surface. The presence of silver affects the stability and the mechanism of decomposition/oxidation of reduced copper species, and this stability is higher in bimetallic systems. The decomposition pattern of recently reduced species includes the formation of smaller nanoparticles and few-atomic clusters. This can occur, preceding the complete oxidation of Cu to ions. Quasicolloidal silver, which is present in fresh bimetallic samples reduced at lower temperatures, transforms after aging into Ag8 clusters, which indicates the stability of these nanospecies on natural clinoptilolite.