Evidence of a Structural Change in Pure-Silica MEL upon the Adsorption of Argon

Abstract

In this work, we investigate the structure of argon adsorbed on pure-silica MEL at liquid nitrogen temperature. Our goal is to provide a microscopic interpretation for the appearance of a substep in the adsorption isotherm at intermediate loadings before saturation. For that purpose, we first perform time-of-flight neutron diffraction experiments of the loaded zeolite, before and after the substep. The measured spectra reveal that, after the substep, a considerable ordering of the adsorbate builds up, but there is also evidence of a zeolite structural change. These experimental data were then used in conjunction with Reverse Monte Carlo simulations to obtain theoretical structural models of the adsorbate/adsorbent system. Interestingly, the spatial distribution of the adsorbate predicted by Reverse Monte Carlo is considerably different from that obtained from grand canonical Monte Carlo simulations, even at half loading. We ascribe these discrepancies to deficiencies in the argon–zeolite interatomic potential. Besides, at high loading, we observe a different distribution of atoms along the channels parallel to the x-axis and those parallel to the y-axis. This might be attributed to a zeolite structural change distorting the tetragonal symmetry of MEL.

Adsorption of n-Alkanes in MFI and MEL: Quasi-Equilibrated Thermodesorption Combined with Molecular Simulations

“…Other reasons of these discrepancies may be higher flexibility of the MEL framework or its phase transition upon adsorption. Such transitions were reported for adsorption of toluene and argon. , Therefore, the force field used for molecular simulations could not fully reflect adsorption phenomena for this particular zeolite. It is worth noticing that the level of agreement between the calculated and experimental isotherms obtained by Marguta et al is very close to that found for adsorption isobars of C6–C8 in MEL in this work.…”

Experimental and Simulation Studies of the Stepped Adsorption of Toluene on Pure-Silica MEL Zeolite

“…To finalize, we would like to stress the usefulness of the N -RMC method to extract structural models compatible with experimentally measured diffraction patterns of adsorbed fluids on zeolites. In a previous work, we have applied this method to the adsorption of a simple gas, such as argon, on pure silica MEL . In this work, we show that the method is also applicable for more complex molecules, such as toluene.…”

“…The N -RMC method is a variant of RMC modeling specifically designed to deal with tightly confined media. As the method has already been detailed elsewhere, , only a brief description is provided here. In short, the N -RMC method consists in performing, in addition to particle/molecule displacement and rotation attempts, insertion and deletion trials for adsorbate molecules.…”

“…At half load, we have used the rigid zeolite approximation, whereas at high load, we have performed also simulations that incorporate the zeolite flexibility. In order to avoid large deformations of the zeolite structure, restraints over the Si–O bond distance and Si–O–Si bending angles were introduced . The initial simulation box was built by replicating the experimental unit cell in the three dimensions of space to obtain a 4 × 4 × 6 supercell.…”

“…These changes have not yet been so well characterized as those of ZSM-5. There is, however, experimental evidence of their occurrence upon increasing the temperature − and upon the adsorption of argon at liquid nitrogen temperature …”