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Absolute Reactivity of the 4-Methoxycumyl Cation in Non-Acid Zeolites
journal contribution
posted on 2000-06-08, 00:00 authored by Melanie A. O'Neill, Frances L. Cozens, Norman P. ScheppThe reactivity of the 4-methoxycumyl cation in a series of alkali metal cation-exchanged zeolites
(LiY, NaY, KY, RbY CsY, NaX, NaMor, and Naβ) in the absence and presence of coadsorbed alcohols and
water is examined using nanosecond laser flash photolysis. In dry zeolites, the absolute reactivity of the
carbocation is found to be strongly dependent on the nature of the alkali counterion, the Si/Al ratio, and the
framework morphology, with the lifetime of the carbocation in Naβ being almost 10000-fold longer than in
CsY. The results suggest a mechanism for carbocation decay involving direct participation of the zeolite
framework as a nucleophile, leading to the generation of a framework-bound alkoxy species. Intrazeolite addition
reactions of alcohols and water to the 4-methoxycumyl cation can be described in terms of both dynamic and
static quenching involving molecular diffusion through the heterogeneous topology and rapid coupling between
the alcohol and the carbocation encapsulated within the same cavity. The dynamics of the quenching reactions
are different from similar reactions in homogeneous solution due to both the passive and active influences of
the zeolite environment. In a passive sense, the zeolite decreases the reactivity of the nucleophilic quencher by
hindering molecular diffusion. However, the zeolite actively promotes the efficiency of intracavity coupling
by enhancing the deprotonation of the oxonium ion intermediate, allowing the reaction to go to completion.
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framework morphologyzeolite frameworkNa βAbsolute Reactivityalkali counterionzeolite environmentquenching reactionszeolite decreasesIntrazeolite addition reactionsoxonium ioncarbocation encapsulatednucleophilic quencherKYnanosecond laser flash photolysiscoadsorbed alcoholscarbocation decayreactivityRbY CsY
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