Theoretical Investigation of the Effects of the Zeolite Framework on the Stability of Carbenium Ions

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Abstract

Twenty carbenium ions in various zeolite models (8T HZSM-5, 72T HZSM-5, 84T HY, and 80T Hβ) are investigated by theoretical calculation methods in order to reveal the effects of the zeolite framework on the stability of carbenium ion intermediates. Most of the carbenium ions are found to be stable in these zeolite models and exist in the form of an ion pair with the zeolite conjugate bases. The carbenium ions can also be transformed into π complexes and, in some cases, into alkoxy species if the steric constraint around the C−O bond is not pronounced. It is found that zeolite framework inclusion facilitates the formation of ion pairs and concurrently can increase the relative stability of ion pairs compared to π complexes and alkoxy species no matter which effect (electrostatic stabilization or steric constraint destabilization) is predominant. It is also found that bulkier carbenium ions could be accommodated well in the zeolite frameworks with larger channels or cages, and vice versa. This is relevant to the shape selectivity of zeolites. It is shown that the energy difference between the ion pair and the π complex for the carbenium ions in the three zeolite frameworks is related to the proton affinity (PA) of the corresponding neutral hydrocarbons. In general, the larger the PA value, the more negative the energy difference, and thus the higher the stability of the carbenium ion, which is consistent with the experimental observations for many carbenium ions.

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