Effects of Framework Flexibility on the Adsorption and Diffusion of Aromatics in MFI-Type Zeolites

Abstract

We systematically study how the degree of framework flexibility affects the adsorption and diffusion of aromatics in MFI-type zeolites as computed by Monte Carlo simulations. It is observed that as the framework is more flexible, the zeolite structure is inherently changed. We have found that framework flexibility has a significant effect on the adsorption of aromatics in MFI-type zeolites, especially at high pressure. Framework flexibility allows the zeolite framework to accommodate to the presence of guest aromatic molecules. For very flexible zeolite frameworks, loadings up to two times larger than that in a rigid zeolite framework are obtained at a given pressure. We assessed the “flexible snapshot” method, which captures framework flexibility using independent snapshots of the framework. We have found that this method only works well when the loadings are low. This suggests that the effect of the guest molecules on the zeolite framework is important. Framework flexibility lowers the free-energy barriers between low energy states, increasing the rate of diffusion of aromatics in the straight channel of MFI-type zeolites for many orders of magnitude compared to a rigid zeolite framework. The simulations show that framework flexibility should not be neglected and that it significantly affects the diffusion and adsorption properties of aromatics in an MFI-type zeolite.

In situ imaging of the sorption-induced subcell topological flexibility of a rigid zeolite framework

“…Furthermore, averaged cell-scale results cannot indicate the local structure of deformed opening pores caused by successive twisted tetrahedrons. Crystal dynamics simulation techniques have been applied to trace the motion and vibration of specific atoms within zeolite frameworks, and some progress toward describing framework flexibility has been made, but supporting experimental evidence has been lacking (2,9,(24)(25)(26). Thus, new experimental technology with ultrahigh spatial resolution is needed to reveal the subcell local structural evolution of the zeolite framework when guest molecules break through the pore size limit, especially in practical applications.…”

Competitive Adsorption of Xylenes at Chemical Equilibrium in Zeolites

“…The insight from this work can be considered for the design and/or improvement of applications of zeolites for the processing of xylenes. However, it is known that the predictions of adsorption of aromatics in zeolites are sensitive to the flexibility of the framework, 124 , 155 to the flexibility of aromatic molecules, 156 and to the partial charges used for the models. 62 These factors are not considered in this work.…”

“… 80 For very flexible zeolite frameworks, loadings up to two times larger than in a rigid zeolite framework are obtained at a given pressure. 124 However, models for framework flexibility should not be blindly applied to zeolites. 80 The intra-framework interactions in flexible framework models induce small but important changes in the atom positions of the zeolite and hence in the adsorption isotherms.…”

Identification of an efficient adsorbent for ethanol sensing at room temperature using quartz crystal microbalance

“…Another point is that the presence of silanol groups, anionic charges, and cations in the external surface of zeolites help the formation of local electrostatic fields that can contribute to the adsorption of ethanol. [50][51][52]. The presence of the intense LA band which corresponds to the bridging of extra-framework aluminum species in BEA zeolite can allow the volumetric adsorption of ethanol by the formation of alcohol-water dimer, alcohol-water-hydronium ion clusters, or extended hydrogen-bonded water network [53].…”