Quantum-chemical studies of alkene chemisorption in chabazite: A comparison of cluster and embedded-cluster models

Abstract

Quantum-chemical studies of ethene, propene and isobutene chemisorption at an aluminosilicate BrÔnstedsite in the zeolite chabazite are reported. Comparison of the results using di †erent cluster models and a acid qm/mm (quantum mechanical/molecular mechanical) embedded cluster approach are compared and contrasted. As in previous studies, the activation barriers for the chemisorption process leading to a surface alkoxide are found to follow a carbenium ion trend, i.e. ethene [ propene [ isobutene. In contrast to previous studies, however, results indicate that the stability of the alkoxide is also very sensitive to a number of factors, the dominant one being steric interactions with the acid site, i.e. the stability order is ethene [ propene [ isobutene. This steric e †ect and other, less dominant, contributions are only observed when host environment e †ects are included in the model, in the present case via constraints on the cluster boundaries and via the qm/mm embedded-cluster approach. The possible formation of stable carbenium ions in the pores of acidic zeolites is discussed.

Revisiting carbenium chemistry on amorphous silica-alumina: Unraveling their milder acidity as compared to zeolites

“…The newly formed CÀO surf bond length is almost constant from one site to another on ASA (1.48 ± 0.04 Å). We consider here that the alkoxide intermediates are anchored either on the O surf atom of the BAS itself or on the neighboring O surf atom as reported in the literature devoted to zeolitic materials [58,59,65,68,70,77,95,97,[100][101][102][103]. The various possible kinds of intermediates and their energetics are exemplified in Fig.…”

Alkylation of benzene with ethylene over faujasite zeolite investigated by the ONIOM method

“…With such limited models, the effect of the framework which can significantly change the structure and energetics of the system is not taken into account. Neglecting the extended framework effect has been shown to lead to discrepancies between the cluster results and the actual zeolite behaviors [21,22]. The recent development of hybrid methods, such as embedded cluster or combined quantum mechanics/molecular mechanics (QM/MM) [14,15,[21][22][23][24][25] methods, as well as the more general ONIOM (Our-own-N-layer Integrated molecular Orbital + molecular Mechanics) method [26,27] has brought a larger system within reach of obtaining accurate results.…”

QM/MM: what have we learned, where are we, and where do we go from here?

“…A shifted-charge scheme [67] has been developed to work with force fields where the neutral-groups feature is not available. (Of course, the scheme can also be used for force fields having the neutral-groups feature).…”