Elisa Rodeghero

Competitive adsorption of VOCs from binary aqueous mixtures on zeolite ZSM-5

Adsorption equilibria of methyl tert-butyl ether (MTBE)/toluene (TOL), and 1,2-dichloroethane (DCE)/MTBE binary mixtures in aqueous solution on ZSM-5 were measured over a wide range of concentrations. In comparison with the single-component data, the loading of all of the three compounds was reduced in the presence of a second component in the mixture. The binary system was described by a competitive dual site Langmuir adsorption isotherm. The model was chosen on the basis of the results obtained from X-ray diffraction studies of the adsorbent material loaded with equimolar binary mixtures. Rietveld structure refinements provide information about the relative position of molecules inside the structure after TOL-MTBE and DCE-MTBE mixture adsorption. The short intermolecular distances between the adsorption sites of MTBE, DCE and TOL inside the zeolite framework clearly prevent the simultaneous occupancy of one site by more than one component, when these compounds are adsorbed from binary mixtures.

Influence of water on the retention of methyl tertiary-butyl ether by high silica ZSM-5 and Y zeolites: a multidisciplinary study on the adsorption from liquid and gas phase

Ambient temperature adsorption isotherms have been determined for methyl tert-butyl ether (MTBE) in aqueous solutions on high silica ZSM-5 and Y zeolites which differ from each other in framework topology and pore window apertures. The comparison of their adsorption properties was performed to evaluate the importance of the competitive interference of water with pore size. The adsorption Henrys constants for MTBE decrease with an increase in the zeolite pore size, indicating that confinement of the adsorbate in the zeolite pores is a driving force for adsorption. On the contrary, the saturation capacity increases with an increase in the adsorbent pore size (i.e. about 95 and 250 mg g−1 for ZSM-5 and Y, respectively). These data were confirmed from Rietveld refinement, that clearly revealed the incorporation of about 8 and 32 MTBE molecules per ZSM-5 and Y unit cell respectively. X-ray diffraction and IR spectroscopy indicate that significant clustering of the guest molecules (i.e. water and MTBE) occurs in the two investigated zeolites during adsorption from both the liquid and gas phase. The clusters originate from hydrogen bonding between MTBE and water molecules and their interaction with zeolite lead to framework flexibility for both zeolites. However, the framework deformation in Y is more evident than in ZSM-5 and occurs with a change of the real symmetry. These findings could have an impact on the adsorption, separation and catalytic use of zeolites for oxygenate compounds.

Formation of Supramolecular Clusters at the Interface of Zeolite X Following the Adsorption of Rare-Earth Cations and Their Impact on the Macroscopic Properties of the Zeolite

The adsorption behavior of neodymium (Nd3+ ) and yttrium (Y3+ ) cations on synthetic FAU zeolite X in its sodium form (NaX) has been investigated by means of macroscopic (adsorption isotherm determination and thermal analysis) and microscopic measurements (including solid-state NMR spectroscopy and X-ray powder diffraction). The multidisciplinary study reveals some unexpected features. Firstly, adsorption constants of cations are not correlated to their ionic radii or hydration enthalpy. The adsorption constant of Y3+ on NaX was indeed about twice that of Nd3+ , which is the opposite of what could be expected based on the size of the cations. In addition, adsorption was accompanied by partial dealumination of the zeolite framework. The extent of dealumination changed depending on the exchanged cations, with the extent being more significant on the Nd-exchanged zeolite than on the Y-exchanged one. The most interesting finding of this study, however, is the presence of supramolecular clusters composed of water, Nd3+ , residual sodium ions, and extra-framework aluminum at the interface of Nd-exchanged zeolite. The hypothesis that these host-guest complexes are responsible of the significantly different behavior exhibited by NaX towards the adsorption/desorption of Nd3+ and Y3+ has been formulated.