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

Quantum-chemical studies of ethene, propene and isobutene chemisorption at an aluminosilicate Brønsted-acid site in the zeolite chabazite are reported. Comparison of the results using different cluster models and a 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 effect and other, less dominant, contributions are only observed when host environment effects 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.