The crystal structure of the zeolite ferrierite

Vaughan, Philip A.

doi:10.1107/s0365110x66004298

Cited by 297 publications

(200 citation statements)

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“…The ferrierite structure as determined by Vaughan 17 has been used and kept rigid in the simulations. All aluminum atoms were replaced by silicon atoms since only the dispersive interactions between the hydrocarbon and the zeolite framework are taken into account.…”

Section: Simulation Detailsmentioning

confidence: 99%

Chain Length Effects of Linear Alkanes in Zeolite Ferrierite. 2. Molecular Simulations

et al. 1998

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Configurational-bias Monte Carlo technique simulations are performed in the grand canonical ensemble to calculate the adsorption isotherms and the isosteric heats of adsorption of propane, n-butane, n-pentane, and n-hexane in ferrierite. The results are generally in good agreement with experiment results. The locations of the molecules in the ferrierite pore structure obtained from the simulations agree well with the locations obtained by earlier 13 C NMR measurements. It is found that only short n-alkane molecules up to n-pentane can access the entire two-dimensional ferrierite pore structure; longer molecules adsorb only in the 10-ring channels and not in the ferrierite 8-ring cage. Adsorption of n-pentane in this cage is possible but only at relatively high pressures due to the high number of repulsive interactions with framework oxygen ions. The results presented are furthermore used to set limits to the size parameter describing the Lennard-Jones interaction between the n-alkane united atoms and the framework oxygen atoms.

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Section: Simulation Detailsmentioning

confidence: 99%

Chain Length Effects of Linear Alkanes in Zeolite Ferrierite. 2. Molecular Simulations

et al. 1998

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“…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.…”

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confidence: 99%

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

2005

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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 ...

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“…1). 1,2 The pore sizes of the aluminum-containing FER framework with free diameters of d ≤0.48 nm (8-MR) and d ≤0.54 nm (10-MR). 1,3 The main channel is outlined by elliptical 10-MR, while the side channel is formed by 8-MR.…”

Section: Introductionmentioning

confidence: 99%

“…1,2 The pore sizes of the aluminum-containing FER framework with free diameters of d ≤0.48 nm (8-MR) and d ≤0.54 nm (10-MR). 1,3 The main channel is outlined by elliptical 10-MR, while the side channel is formed by 8-MR. It is possible to synthesize low-silica FER without a structure-directing agents (SDA), while the addition of an SDA is essential for the successful synthesis of high-silica (Si/Al ratio >5) FER.…”

Section: Introductionmentioning

confidence: 99%