Hydrated Cs⁺-Exchanged MFI Zeolites:  Location of Extraframework Species in Cs_xH_yMFI·zH₂O Phases from X-ray Powder Diffraction and Differential Molar Adsorption Calorimetry

The locations and populations of extraframework species in zeolitic Cs-exchanged H−MFI materials have recently been reported both in several fully dehydrated phases (J. Phys. Chem. B 2006, 110, 97−106) and in hydrated Cs_6.6H_0.3MFI·zH₂O (1.5 < z < 28) samples (J. Phys. Chem. B 2006, 110, 13741−13752). The present paper extends our study to more general compositions, i.e., Cs_xH_yMFI·zH₂O phases spanning the range 0.7 < x < 7.7, 0.3 < y < 5.3 and 4 < z < 32. Structural information is obtained from Rietveld refinements of laboratory X-ray powder diffraction data collected at room temperature (RT, 22 or 26 °C). We discovered that flushing fully hydrated Cs_xH_yMFI·zH₂O samples with a stream of dinitrogen (N₂) gas corresponds to a rather strong isothermal water desorption effect. This effect is intimately correlated with the origin (synthesis method, postsynthesis treatments, and defect concentration) of the Cs-exchanged sample. Special attention is focused on the Cs_2.3H_1.1MFI·24H₂O sample, for which high precision differential molar adsorption calorimetry data corresponding to its complete hydration process exist. In order to be able to interpret the observed water adsorption heats, we had to take several correlations into account:  those existing between the Si/Al ratio and the (H,Li,Na,Cs)/uc cationic contents versus water loadings, in addition to the specific interactions between the cations or protons with aromatics as benzene or p-xylene. It is shown that in the 0−4.5 Al/uc range the maximum water/uc content, in all of the investigated H−MFI samples, is strictly proportional to the Al/uc values (about 8.5 water molecules per Al/uc). In most cation exchanged samples, this value is significantly different:  it is either lower (e.g., Na and Cs) or higher (e.g., Li). In the case of p-xylene sorbed in fully dehydrated H−MFI materials (0 < H/uc < 4.5), all samples adsorb about eight molecules/uc, whereas the presence of extraframework cations lowers dramatically this sorption capacity. It is shown that the crystallinity of a ZSM-5 zeolite might be conveniently estimated by simply inspecting the XRPD profile corresponding to the p-xylene saturated parent H−MFI phase. In this work, it is shown how calorimetric data can be exploited to complement crystal structure results and detect subtle sorbent/sorbate interactions on the molecular level, which cannot be revealed by Rietveld-type powder diffraction profile structure refinements alone. The influence of structural defects (essentially −OH silanol groups attached to Si/Al framework atoms) on the observed adsorption heats is particularly pronounced. The host/guest interactions, observed at very low water loadings for the Cs_2.3H_1.1MFI/water system, are estimated by computer simulations (molecular mechanics (MM) calculations).