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Isobutene Protonation in H‑FAU, H‑MOR, H‑ZSM-5, and H‑ZSM-22

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journal contribution
posted on 30.08.2012, 00:00 by Cuong M. Nguyen, Bart A. De Moor, Marie-Françoise Reyniers, Guy B. Marin
Kinetics and thermodynamics of isobutene protonation in H-FAU, H-MOR, H-ZSM-5, and H-ZSM-22 have been studied in a temperature range of 300–800 K, combining PW91-D//PW91 periodic density functional theory calculations with statistical thermodynamics. At temperatures relevant for industrial zeolite-catalyzed processes (500–800 K), the tert-butyl carbenium ion is more stable than the tert-butoxy in H-MOR, H-ZSM-5, and H-ZSM-22. Entropy contributions govern the standard Gibbs free energy stability of the chemisorbed intermediates. Due to the absence of a C–O covalent bond, formation of the tert-butyl carbenium ion is accompanied by a lower entropy loss and, consequently, has a higher stability than the tert-butoxy in H-MOR, H-ZSM-5, and H-ZSM-22. At 800 K, the protonation toward tert-butoxy in H-FAU, H-MOR, and H-ZSM-5 and to the tert-butyl carbenium ion in H-ZSM-22 is 5 to 7 orders of magnitude faster than the protonation toward isobutoxy. Among the four zeolites, the lowest activation energy is found in H-ZSM-22.