Theoretical study of the adsorption of ethylene on alkali‐exchanged zeolites

Abstract

ABSTRACT:The structures of alkali-exchanged faujasite (X-FAU, X ϭ Li ϩ or Na ϩ ion) and ZSM-5 (Li-ZSM-5) zeolites and their interactions with ethylene have been investigated by means of quantum cluster and embedded cluster approaches at the B3LYP/6-31G (d, p) level of theory. Inclusion of the Madelung potential from the zeolite framework has a significant effect on the structure and interaction energies of the adsorption complexes and leads to differentiation of different types of zeolites (ZSM-5 and FAU) that cannot be drawn from a typical quantum cluster model, H 3 SiO(X)Al(OH) 2 OSiH 3 . The Li-ZSM-5 zeolite is predicted to have a higher Lewis acidity and thus higher ethylene adsorption energy than the Li-FAU zeolites (16.4 vs. 14.4 kcal/mol), in good agreement with the known acidity trend of these two zeolites. On the other hand, the cluster models give virtually the same adsorption energies for both zeolite complexes (8.9 vs. 9.1 kcal/mol). For the larger cation-exchanged Na-FAU complex, the adsorption energy (11.6 kcal/mol) is predicted to be lower than that of Li-FAU zeolites, which compares well with the experimental estimate of about 9.6 kcal/ mol for ethylene adsorption on a less acidic Na-X zeolite.

Vapor‐Phase Beckmann Rearrangement of Oxime Molecules over H‐Faujasite Zeolite

“…In this study, the total Madelung potential is represented by 649 explicit charges within 120 pm to the quantum cluster and 1210 surface charges. Previous studies [29][30][31][32][33]64] revealed that the calculated adsorption properties agree well with experimental estimates, suggesting that the SCREEP embedded approach is a sufficiently accurate and practical model for studying reaction mechanisms on zeolites. In this current investigation, only the active site region (H 2 SiOAl(OH) 2 O(H)SiH 2 ) and the adsorbate molecule were allowed to relax, while the remaining atoms were kept fixed at the experimental structure.…”

C2-Hydrocarbon Adsorption in Nano-porous Faujasite: A DFT Study

“…The simple naked M + C 2 H 4 models obviously overestimate as compared to the interaction of C 2 H 4 in real zeolite system due to the large electrostatic field generated by the naked metal cation. [49] This shows that the oxygen in zeolite fragment cluster is strongly coordinated with smaller cation, as the same observation is obtained the Sanderson electronegativity index of monovalent ion exchanged nano-porous Faujasite.…”

“…Figure 1) are calculated based on the same theory. Previously, Ketrat and Limtrakul [49] demonstrated the interactions of ethylene in Faujasite type zeolite with B3LYP/6-31G (d,p) level of theory. In addition to this, various DFT functional such as B3P86 [50], PW91, PBE, etc.…”

Structure and adsorption of a basic probe molecule on H-ZSM-5 nanostructured zeolite: An embedded ONIOM study

“…1) to reproduce the long-range electrostatic field from the extended zeolite framework. Using two sets of point charges in such a way has already been employed in studies of several zeolite systems [24,25,27].…”

“…Furthermore, they often introduce an unrealistic perfect symmetry into the model. Hybrid methods, such as the embedded cluster or combined quantum mechanics/molecular mechanics (QM/MM) methods [23][24][25][26][27][28][29][30][31], as well as the more general ONIOM ('Our-own-N-layer integrated molecular orbital + molecular mechanics') method [32][33][34][35][36][37][38][39][40][41][42] provide a cost effective computational strategy for including the effects of the zeolite framework.…”