Stabilities of C3–C5 alkoxide species inside H-FER zeolite: a hybrid QM/MM study

Nieminen, Ville; Sierka, Marek; Murzin, Dmitry Yu.; Sauer, Joachim

doi:10.1016/j.jcat.2005.01.035

Cited by 96 publications

(163 citation statements)

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“…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.…”

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confidence: 90%

“…This results in too low energies of adsorption, but may also change the relative energies of chemisorbed species compared to adsorption p complexes and carbenium ions. [15] Future refinements of the present results by explicit inclusion of dispersion interactions may be obtained by an MP2/DFT hybrid technique [23] or by adding a damped dispersion term. [22] The results obtained with the PBE/PW91 family of functionals require less correction than those with functionals including Becke exchange.…”

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confidence: 92%

“…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 obtained.…”

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

2005

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“…While the Brønsted acid site locations can be similar in different 10T ring zeolites, they can display considerable selectivity differences, including the production of isobutene from linear butanes [1]. This is because the framework topologies can alter the catalytic reactions through (a) moderation of the pKa of the Brønsted acid site, (b) hinder/allow the formation of key intermediates due to the local pore volume, (c) stabilize stationary points on the potential energy surface through long range interactions as a result of high Si/Al ratios or the presence of heavy metal additives or (d) impose diffusional limitations as a result of coking/time on stream.Quantum mechanical (QM) calculations [3][4][5][6][7] are often used to model the reactivity and selectivity of zeolites since through a consideration of the relative energies and structures of the zeolite-substrate complexes, transition states (TS) and products, one can often get insights into chemical processes not possible by experimental techniques. A common feature of zeolite simulations is their consideration of acidity, focusing either on the non-bonded 'physisorption' of various small molecules to the Brønsted acid site [8], or more complex reactions that occur at it.…”

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“…Note the secondary 1-butene TS is also an observed outlier in the 3T, and 3T:46T correlation suggesting it is an artifact of the MM treatment. Here, the Spearman's correlations coefficient of 0.80 is observed.It should be noted that Niemenen et al [3] showed that DFT models typically give low adsorption energies due to the underestimation of the van der Waals interactions. Additionally, Boronat et al [32] report that QM:MM ONIOM calculations do not treat the interaction between the layers correctly but performing full single point energies on the optimized structures is more reliable.…”

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Application of QM simulations and multivariate analysis in the study of alkene reactivity in the zeolite H‐ZSM5

Gleeson

2008

Journal of Chemometrics

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Application of QM simulations and multivariate analysis in the study of alkene reactivity in the zeolite H-ZSM5 Duangkamol Gleeson a *Reported herein are the results of an investigation into the effect of the extended framework of the zeolite ZSM-5 on the reaction energetics and structures of (a) the physisorbed complex formed between the zeolite and six alkenes, (b) the corresponding chemisorbed alkoxide intermediate and (c) the transition states (TS) connecting the two. For this, quantum mechanical (QM) simulations of ZSM-5 in the presence and absence of the zeolite framework have been employed. A 46T density functional theory (DFT) cluster model and a 3T:46T DFT:UFF ONIOM model are used to represent the former scenario and a simple 3T DFT cluster model for the latter. The structural implications of neglecting the zeolite framework have been rigorously compared using the multivariate statistical method principal components analysis (PCA). This method allows one to assess the correlated nature of the changes in structure along the reaction coordinate, for multiple different alkenes, in a facile, reliable way. . ZSM-5 is a well-studied medium pore-size zeolite from the MFI family. It is characterized by a three-dimensional pore system consisting of sinusoidal 10T ring with intersecting 10T ring channels. While the Brønsted acid site locations can be similar in different 10T ring zeolites, they can display considerable selectivity differences, including the production of isobutene from linear butanes [1]. This is because the framework topologies can alter the catalytic reactions through (a) moderation of the pKa of the Brønsted acid site, (b) hinder/allow the formation of key intermediates due to the local pore volume, (c) stabilize stationary points on the potential energy surface through long range interactions as a result of high Si/Al ratios or the presence of heavy metal additives or (d) impose diffusional limitations as a result of coking/time on stream.Quantum mechanical (QM) calculations [3][4][5][6][7] are often used to model the reactivity and selectivity of zeolites since through a consideration of the relative energies and structures of the zeolite-substrate complexes, transition states (TS) and products, one can often get insights into chemical processes not possible by experimental techniques. A common feature of zeolite simulations is their consideration of acidity, focusing either on the non-bonded 'physisorption' of various small molecules to the Brønsted acid site [8], or more complex reactions that occur at it. Alkoxide intermediate formation involves the transfer of the Brønsted proton on the zeolite O1 atom to the C1 of the alkene with the subsequent formation of a bond between the C2 and O2 atoms of the substrate and zeolite, respectively (Figure 1). Theoretical studies on alkoxide intermediates have often been restricted to relatively small cluster models [9][10][11] Studies comparing gas phase cluster models to those that encode information about the active site are generally restricted to ...

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