Protonated Isobutene in Zeolites: tert‐Butyl Cation or Alkoxide?

Abstract

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.[1] However, NMR spectroscopic studies failed to find simple carbenium ions as intermediates, and instead produced evidence for surface alkoxides. [2,3] 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.[4] So far, evidence for persistent carbenium ions has only been produced for cyclic alkenyl or aromatic carbenium ions by NMR, [5] UV/Vis, [6] and IR spectroscopy [7] or computational techniques.[8]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. [9][10][11][12] Only the embedded cluster study by Boronat et al. [12] reports a local minimum on the PES for the tert-butyl cation in mordenite. It is, however, about 26 kJ mol À1 less stable than the adsorption p complex of isobutene with the Brønsted site. Nevertheless, all experimental attempts [13,14] 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 (H-FER, 1) with formation of the p complex 2 (Scheme 1). We applied periodic boundary conditions to a large simulation cell of dimensions 1870 1417 1496 pm 3 . [15] 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 HAlSi 71 O 144 . After aluminum substitution at the crystallographic position T2, [16] the proton is most stable at O7. [15] We used the Perdew-Burke-Ernzerhof (PBE) density functional [17] with a plane wave basis set. [18] 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 ...

Theoretical Chemistry of Zeolite Reactivity

“…Inclusion of zero-point vibrations and finite temperature effects further stabilized the carbenium ions relative to the covalently bound alkoxides. It was concluded that already at 120 K formation of tert-butyl cation in H-ferrierite becomes thermodynamically more favorable than formation of the covalently bound species [37]. However, this theoretical prediction lacks the experimental support, because simple carbenium ions have never been observed by either NMR or infrared spectroscopy upon olefin adsorption to hydrogen forms of zeolites [38].…”

“…Only the π-complex of butane (1) with the Brønsted acid site of the zeolite was found to be more stable than the isolated alkene separated from the zeolite [37]. The stabilization however was rather minor (PBE/PW, FER pbc , Table 11.1).…”

“…On the other hand, when the long-range interactions with the zeolite framework and its structural details are explicitly included in the computation either within the embedded cluster approach with a very large part of the zeolite lattice as a low-level model [35] or by using periodic DFT [36,37] potential energy surface corresponding to the tert-butyl cation can be located for various zeolite topologies.…”

“…Indeed, applying DFT under periodic boundary conditions to the realistic system containing isobutene adsorbed in ferrierite a rather different picture was observed [37] as compared to the situation when a small cluster model was used to mimic the zeolite active site [34]. Only the π-complex of butane (1) with the Brønsted acid site of the zeolite was found to be more stable than the isolated alkene separated from the zeolite [37].…”

Proton Transfer in Zeolites

“…Because of technical limitations vibrational analysis, which could prove that this cation is a local minimum on the potential energy surface, that is a metastable species, have only recently been made. Within a periodic DFT study of isobutene/H-FER a complete vibrational analysis for all atoms in the unit cell was made [48], and as part of a hybrid QM/MNDO study on an embedded cluster model of isobutene/H-MOR a vibrational analysis was made with a limited number of atoms [49]. Both reached the + H + -H 2 -H + (6) (7) Figure 22.9 Carbenium ion, 6, obtained by hydride abstraction from xylene and its deprotonation product, 7.…”