Reactivity theory of zeolitic Broensted acidic sites
- Eindhoven Univ. of Technology (Netherlands)
- Shell Research b.v., Amsterdam (Netherlands). Koninklijike/Shell-Laboratorium
The unique environment of the Broensted acidic protons in the micropores of zeolites controls the overall catalytic behavior of zeolites to a significant extent. The authors will extensively discuss the use of classical molecular mechanics calculations on extended systems using force fields deduced from ab initio quantum-chemical cluster calculations. This enables a detailed description of local distortions in zeolites around the protonic site. They will try to show the usefulness of this approach, which is based on the prediction of physical properties and structures of a set of zeolitic SiO{sub 2} polymorphs. The reactivity of the zeolitic Broensted acidic site will be discussed in the three following sections. An approach will be followed in which theoretical and experimental data are compared. A discussion on the problem of proton transfer follows. For acid catalysis an understanding of the nature and energetics of carbonium and carbenium ion formation is essential. The authors discuss computed transition states for a few proton transfer reactions and show that in zeolites carbonium ions have to be considered as transition states and not intermediates. The authors will show that the ionicity of the transition state and its geometry determine whether acidity relates to the difference in proton affinity of the two oxygen atoms involved in protonation, or that the value of the strong oxygen-proton bond controls protonation. They also comment on the importance of differences in the proton bond strength to overall catalytic performance. Some additional factors that cause differences in catalytic performance when different zeolites are compared, will also be discussed. 114 refs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 69977
- Journal Information:
- Chemical Reviews, Journal Name: Chemical Reviews Journal Issue: 3 Vol. 95; ISSN CHREAY; ISSN 0009-2665
- Country of Publication:
- United States
- Language:
- English
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