Study of Type-A Zeolites. Part 2:  Effect of Dehydration on the Effective Aperture Dimension

Abstract

A new insight into the role played by water molecules in the crystalline framework of type-A zeolites is demonstrated. The effect of dehydration on the effective free aperture dimension, D f, is studied by utilizing temperature-programmed decapsulation of He and Ne. The interplay between the various known mechanisms governing D f is directly sensed by the decapsulation behavior of differently sized inert atoms. The results are qualitatively interpreted using pure dimensional considerations, revealing the occurrence of a strict relation between the content of water molecules and the redistribution of zeolitic cations in determining D f. Water content is shown to have a strong effect on the blocking state of the O8 and O6 zeolitic windows, which in turn, governs gas accessibility to the α and β cages. It is proposed that D f is regulated via a subtle balance between two coupled principal mechanisms. One involves direct lattice adjustments, where two opposite sub-mechanisms seem to play competitive roles. Removal of water molecules by simultaneous heating and pumping enlarges D f, while hydroxyl groups elimination from within the zeolitic channels results in their partial collapse, thus reducing D f. The second mechanism involves the regulation of aperture blocking via relocations of the counterions initiated by increasing vacancies due to removal of water molecules. While dehydration continuously contracts the O6 apertures, a two-stage effect is observed for the wider O8 windows. Upon dehydration at large water contents, O8 apertures are reduced; then, beyond a critical extent of dehydration, the net effect is reversed, widening the O8 windows.

An empirical dependence of frequency in the oscillatory sorption of H2 and D2 in Pd on the first ionization potential of noble gases

“…Thus it has been found that merely the presence of argon during pretreatment of certain types of gold supported catalysts can improve their performance in the process of epoxidation 2 and in the low temperature oxidation of CO. 3 The presence of helium or neon can strongly alter sorption capacity of the type A zeolites for water. 4,5,6 Likewise, both argon and helium were found to change the rate as well as the equilibrium of adsorption of various C 4 alkanes in the zeolites BEA, MFI and FER, with the effects consistently stronger for Ar than for He 7,8,9 . The difference in strength between the respective actions of Ar and He indicates that the kind of noble gas used in those experiments was consequential.…”

Sensing the Physicochemical Nature of He and Ne in Micropores by Adsorption Measurements

“…These observations revealed a strong temperature effect on the effective free aperture dimension of zeolitic windows and constrictions in CMSF, and that was utilized to establish the closure status of constrictions and/or windows versus temperature. 7,8 Figure 1 depicts representative TPD profiles of He decapsulated from dehydrated 3, 4, and 5A zeolites, following sorption during sample cooling between 573 and 77 K followed by long-lasting evacuation of residual gas at 77 K.…”

“…While Table 1 refers to the pore closure status of dehydrated zeolite samples, the situation for hydrated samples is much different since the presence of water molecules enhances pore closure. 8 TPD studies 7,8 show that depending on both pre- evacuation and sample temperatures, pores and apertures may be effectively opened in a controllable fashion to varying degrees, thus providing a powerful straightforward tool for tuning in advance the closure status of samples throughout a fairly wide range of extents. Consequently, comfort and efficient planning of volumetric measurements are feasible.…”

“…After these few examples, only a small number of additional systematic studies addressing this issue can be found in the literature. This work utilizes the findings of recently detailed systematic observations of He and Ne encapsulation in carbon molecular sieve fibers (CMSF) and A zeolites, − in an attempt to explore the physicochemical nature of the gases adsorbed on these porous materials.…”

“…Zeolites may well offer the realization of such encapsulating systems. In a recent detailed study, He and Ne were admitted in a controlled manner into either one or both of the α and β crystalline cages of type A zeolites at atmospheric pressure. − Unambiguous, efficient, reversible, and highly selective encapsulation of these atoms was achieved between 77 K and mildly elevated temperatures. In another related work, encapsulation of He and Ne in an amorphous solid in the form of CMSF was also found to occur between room temperature (RT) and liquid nitrogen temperature (LN 2 ).…”