Adsorption sites extra-framework cations

The CO-TPD technique together with DFT calculations were previously successfully used to characterize monovalent copper positions in Cu-ZSM-5 and Cu-Na-FER catalysts[4, 5]. Recently it was observed that the CO molecule can also form adsorption complexes, where the CO molecule is bonded between two extra-framework cations [6]. It is likely that the formation of similar species between the Cu+ and K+ ions can also occur. The presence of adsorption complexes on such heterogeneous dual cation site was evidenced by the FTIR experiments [7]. The formation of CO complexes on dual cation sites was not considered in our previous TPD models where three types of Cu+ sites were taken into account. 

The various interactions contributing towards total energy of physical adsorption include dispersion, polarization, field-quadrupole and close range repulsion mteractions. Framework oxygens and extra framework cations are the principal sites for interactions with... 

A1r Separation Properties. Self-bound LSX adsorbents have an enhanced ability to selectively adsorb nitrogen from air. For thermodynamically driven adsorption processes, the quantity of a gas adsorbed by a zeolite at a given pressure and temperature Is a function of Its the affinity for the cationic adsorption sites as well as the quantity of sites available for Interaction. Electronic charge balance dictates that the LSX will have the maximum number of cationic sites available for direct Interaction with weakly Interacting adsorbates. The electric field within the zeolite cavity 1s dependent on both structure and the charge density of the extra-framework cation. Small polyvalent cations 1n the dehydrated/dehydroxylated state, especially calcium, show high selectivity for N2 from a1r.(l2)... 

Chlorobenzenes are well known as important precursors of PCDD/Fs and are suitable model compounds for the complete oxidation of chlorinated POPs. Noble metal-based catalysts such as Pt/Al203 show high efficiencies but promote the formation of polychlorinated benzenes. Zeolites have unique properties for the deep oxidation of chlorinated compounds thanks to their well-defined framework and the presence of acid sites or transition metal cations. As stated by Corma, zeohtes present interesting properties of reactant/product partitioning and of molecules preactivated by the molecular confinement effect. Moreover, their adsorptive properties can be modulated by modifying the nature of the extra framework cations and the Si/Al ratio. 

The evolutions of the differential enthalpies of adsorption as. a ftmction of the coverage for the two different Faujasite forms are reported in Fig.2. We observe that the adsorption of CH4 on DAY gives within the experimental error, almost constant differential enthalpy values before slightly decreasing. This type of evolution suggests a relatively heterogeneous adsorbate/adsorbent interaction. The heterogeneity of the DAY sample comes from the residual 1.9 Na extra-framework cations or from a low concentration of textural defect sites... 

The interaction of CO and acetonitrile with extra-framework metal-cation sites in zeolites was investigated at the periodic DFT level and using IR spectroscopy. The stability and IR spectra of adsorption complexes formed in M+-zcolitcs can be understood in detail only when both, (i) the interaction of the adsorbed molecule with the metal cation and (ii) the interaction of the opposite end of the molecule (the hydrocarbon part of acetonitrile or the oxygen atom of CO) with the zeolite are considered. These effects, which can be classified as the effect from the bottom and the effect from the top, respectively, are critically analyzed and discussed. 

Multinuclear solid state nuclear magnetic resonance (NMR) has been applied to study the interaction of pyrrole with extra framework compensating cations in zeolites LiNaY and LiNaX. Upon adsorption over zeolite LiNaY, Na and Li cations migrate towards accessible positions in the supercage to interact with one molecule of pyrrole. The adsorption over zeolite LiNaX decreases the mobility of SIIT Na cations, while pyrrole molecules do not interact with Li" cations. At lower loading, pyrrole adsorbs over more basic sites, which are associated with Na cations in zeolite LiNaY. 

Multinuclear solid state chemical shift NMR of C, N, H and Xe nuclei has been widely used to investigate the environments experienced by adsorbates, and NMR of protons and metal cations (such as aluminium) in framework and extra-framework positions reveals changes in environment of these sites in the solid upon adsorption of molecules. The specific appHcation of NMR to the study of structure in adsorption is outlined below, whereas appHcations in diffusion are described in Section 7.4. The adsorption process can be followed either by observing changes at the adsorption sites or within the adsorbates. NMR is inherently a less-sensitive technique than infrared spectroscopy, particularly in the study of dilute spins such as and data collection times on adsorbed hydrocarbons can reach hours. 

The differential heats of adsorption on Bronsted sites and Lewis sites (cationic species) are not easily comparable [53]. For the former it is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of ammonia, while for Lewis sites the differential heat of adsorption represents the energy associated with the transfer of electron density towards an electron deficient, coordinatively imsaturated site, and probably an energy term related to a relaxation of the strained smface. Micro calorimetric studies of several zeohtes (H-mordenite, USY, H-ZSM-5) treated in such a way as to contain a noticeable amount of extra-framework aluminum have shown that the distribution of the sites with respect to the differential heats of NH3 adsorption is exponential for the Lewis sites (Freund-lich isotherm) and hnear for the Brbnsted sites (Temkin isotherm) [53]. 

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