Cation Siting and Dynamical Properties of Zeolite Offretite from First-Principles Molecular Dynamics

Abstract

We have used density functional theory, both within the local density (LDA) and generalized gradient (GGA) approximations, to study the structure, energetics, and vibrational properties of zeolite offretite in the presence of different monovalent cations (H+, Na+, K+, and Cu+). We find that the spatial locations of the most favorable cation-binding sites are similar for the different cations, being related to the minima of the electrostatic potential. However, the relative stability of the sites does depend on the nature of the counterion, as well as on the Al/Si ratio and on the mutual interactions between cations. At low Al/Si ratios, the preferred site for H+ is in the channel, where it is accessible for reaction with incoming molecules. For both Na+ and Cu+, the most stable site is within the 6-fold ring of the gmelinite cage, but for Na+, two other sites are present within a few tenths of a kilocalorie/mole from the lowest site (small site selectivity). For K+, two sites, one inside the cancrinite cage and the other near the 8-fold ring of the gmelinite cage, are very close in energy, consistent with the X-ray experiments on natural hydrated and dehydrated offretites. Dynamical simulations have been carried out for H− and Na−offretite. The vibrational spectrum of the framework agrees well with the available experiment. OH stretching frequencies calculated for a number of different H+ locations show that more “open” positions, e.g., in the channel, have higher frequencies, in agreement with experiment.

The effect of Si/Al ratios on the catalytic activity of CuY zeolites for DMC synthesis by oxidative carbonylation of methanol: a theoretical study

“…As a result, in this section we investigate the structure of CuY zeolites, including the locations of the Al atoms and Cu + . Then, the most stable configurations of each Si/Al ratio are obtained on the basis of the substitution energy [78][79][80] and binding energy, 81 which are defined as follows:…”

Bro/nsted acid sites in gmelinite

“…26,27 Fast progress in both computer technology and the development of more efficient computer codes is pushing applications in two directions: ͑i͒ toward a still larger number of atoms in the unit cell, and ͑ii͒ toward time consuming applications of molecular dynamics. Typical extensions to large unit cells include the structural study of offretite ͑54 atoms/cell͒, 28,29 mordenite ͑144 atoms/cell͒, 30,31 and faujasite ͑576 atoms/cell͒. 32 Dynamical studies formerly focused to smallsize unit cells such as chabazite, 17 SAPO, 19,33 and sodalite 20,21,23 are now becoming feasible for medium sized cells, such as offretite.…”

“…32 Dynamical studies formerly focused to smallsize unit cells such as chabazite, 17 SAPO, 19,33 and sodalite 20,21,23 are now becoming feasible for medium sized cells, such as offretite. 29 The microscopic mechanism of chemical reactions in zeolites is still unclear. A necessary prerequisite to the modeling of molecular reactions is the detailed knowledge of the local structure and bonding within the active centers of the zeolite ͑e.g., the initial adsorption of hydrocarbons at Bro "nsted acid sites͒.…”

The role of density functional theory in chemistry: Some historical landmarks and applications to zeolites

“…[89][90][91][92][93][94]). In the mid-1990s, she investigated the structural parameters and the energetic of offretite [89][90][91] using the local density approximation within the framework of Car-Parrinello method [65]. In the meantime, the substitution of silicon by aluminum at different tetrahedral sites of zeolite-b and the influence of the counter ion were performed for the first time at a correlated level of theory [92].…”