Protonated isobutene in zeolites: tert-butyl cation or alkoxide?

Acid zeolite catalysts are industrially used for a variety of hydrocarbon transformation processes. Initially it was assumed that these reactions follow mechanisms known from chemistry in superacidic media, and involve carbocations as intermediates formed upon protonation of hydrocarbons by Brønsted acid sites. However, NMR spectroscopic studies failed to find simple carbenium ions as intermediates, and instead produced evidence for surface alkoxides. 3] Around the same time, quantum chemical calculations employing small cluster models showed that alkoxides are minima on the potential energy surface (PES) and, hence, intermediates, while carbenium ions represent saddle points on the PES and are only present as extremely short-lived transition states. So far, evidence for persistent carbenium ions has only been produced for cyclic alkenyl or aromatic carbenium ions by NMR, UV/Vis, and IR spectroscopy or computational techniques. Among the carbenium ions derived from small alkenes, the tert-butyl cation has attracted much interest because it is more stable than primary or secondary carbenium ions; the competing formation of tert-butoxide may be sterically hindered. Previous computational studies have shown that, depending on the framework and the position at the zeolite wall to which they are bound, tert-butoxides may be as unstable as the tert-butyl cation. Only the embedded cluster study by Boronat et al. reports a local minimum on the PES for the tert-butyl cation in mordenite. It is, however, about 26 kJmol 1 less stable than the adsorption p complex of isobutene with the Brønsted site. Nevertheless, all experimental attempts 14] to produce evidence for either the tertbutyl cation or an alkoxide have been unsuccessful so far, and the existence of tert-butyl cations in zeolites remains controversial. Herein we report density functional theory (DFT) calculations for the reaction of isobutene with H-ferrierite (HFER, 1) with formation of the p complex 2 (Scheme 1). We applied periodic boundary conditions to a large simulation cell of dimensions 1870A 1417A 1496 pm. We show that the complex with the tert-butyl cation (4)—one possible structure formed upon proton transfer to isobutene in 2—is a local minimum on the potential energy surface. For the first time we have evaluated entropy contributions to assess the stability of 4 relative to the adsorption p complex 2 and other possible proton transfer products, that is, surface alkoxides (3, 5), at finite temperatures. Our simulation cell of H-FER (1) has the composition HAlSi71O144. After aluminum substitution at the crystallographic position T2, the proton is most stable at O7. We used the Perdew–Burke–Ernzerhof (PBE) density functional with a plane wave basis set. Stationary points obtained by relaxation of the positions of all atoms in the cell are characterized by harmonic frequencies, from which zeropoint vibrational energies, finite temperature energy, and entropy contributions are obtained. All calculations employed the CPMD code. The calculated structures and the corresponding reaction energies are shown in Figure 1 and Table 1. Formation of the p complex of isobutene with the Brønsted acidic site is exothermic (2, DE0 = 6.6 kJmol ). In contrast, chemisorption of isobutene is an endothermic process. Of the two different alkoxides, the isobutoxide (5) is more stable than the tert-butoxide (3). The longer C O bond in tert-butoxide (161 pm) compared to isobutoxide (151 pm) indicates increased steric constraints due to the three methyl groups at the C O carbon atom. Both the adsorbed isobutene and the tert-butyl cation are connected by hydrogen bonds to the zeolite framework. The corresponding bond lengths are 190.2 and 242.5 pm for r(AlOH···C) and r(AlOH···C) in the p complex and 174.0 pm for r(AlO···HC) in the carbenium ion structure. The carbenium ion complex 4 is electronically least favored. It is 46 kJmol 1 less stable than the adsorption p complex, while isobutoxide is 14 kJmol 1 and tert-butoxide is 27 kJmol 1 less stable. Similar relative energies have been found in a previous DFT (PW91) study for chabasite, ZSM-22, and mordenite Scheme 1. Reactions considered for the protonation of isobutene in H-FER (1).