The Dehydrogenation Reaction of Light Alkanes Catalyzed by Zeolites

Furtado, E. A.; Milas, Ivan; LINS, J; Nascimento, Mac

doi:10.1002/1521-396x(200109)187:1<275::aid-pssa275>3.0.co;2-9

Cited by 25 publications

(21 citation statements)

References 47 publications

(40 reference statements)

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“…The effects of the framework were absent in these studies since they were carried out using T3 and T5 clusters, which may have led to a difference in the mechanism. However, a concerted mechanism has been previously reported for isobutane dehydrogenation on a T5 cluster 45 and for linear alkanes in MFI containing nonframework Ga. 46 In addition to influence of the cluster model and presence of nonframework atoms, the mechanism is also sensitive to pore size. In the case of small-pore zeolites such as chabazite, transition state decay to alkene and H 2 via H-shift has been observed for methyl dehydrogenation of propane using a periodic model.…”

Section: ■ Theoretical Methodsmentioning

confidence: 93%

Insights into the Kinetics of Cracking and Dehydrogenation Reactions of Light Alkanes in H-MFI

et al. 2013

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Monomolecular reactions of alkanes in H-MFI were investigated by means of a dispersion-corrected density functional, ωB97X-D, combined with a hybrid quantum mechanics/molecular mechanics (QM/MM) method applied to a cluster model of the zeolite. The cluster contains 437 tetrahedral (T) atoms, within which a T5 region containing the acid site along with the representative alkane is treated quantum mechanically. The influence of active site location on reaction energetics was examined by studying cracking and dehydrogenation reactions of n-butane at two regions in H-MFI–T12, where the proton is at the intersection of straight and sinusoidal channels, and T10, where the proton is within the sinusoidal channel. Two transition states were observed for cracking: one where the proton attacks the C–C bond and another where it attacks a C atom. Dehydrogenation proceeds via a concerted mechanism, where the transition state indicates simultaneous H2 formation and proton migration to the framework. Intrinsic activation energies can be determined accurately with this method, although heats of adsorption were found to be higher in magnitude relative to experiments, which is most likely mainly caused by the MM dispersion parameters for the zeolite framework atoms. Intrinsic activation energies calculated for reactions at the T10 site are higher than those at T12 owing to differences in interaction of the substrate with the acid site as well as with the zeolite framework, demonstrating that Brønsted acid sites in H-MFI are not equivalent for these reactions. Apparent activation energies, determined from calculated intrinsic activation energies and experimentally measured heats of adsorption taken from the literature, are in excellent agreement with experimental results.

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Section: ■ Theoretical Methodsmentioning

confidence: 93%

Insights into the Kinetics of Cracking and Dehydrogenation Reactions of Light Alkanes in H-MFI

et al. 2013

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“…Numerous studies published since the early 2000 s [122][123][124][125][126][127][128][129][130] have demonstrated that long-range electrostatic interactions are critical not only for accurately capturing experimentally measured activation barriers, but also for providing a complete picture of transition state structures involved in catalysis. One such seminal study on the impact of long-range electrostatics on the calculated activation barrier for alkane cracking in zeolites was conducted by Zygmunt and coworkers [122].…”

Section: Impact Of Long-range Electrostaticsmentioning

confidence: 99%

“…These results provided early evidence that the TS structures for the cracking of alkanes on H-MFI are especially sensitive to long-range electrostatic effects. In later studies [124][125][126] by Nascimento and coworkers, the dehydrogenation and cracking of isobutane over HZSM-5 were both examined at the B3LYP/6-31G(d,p) level. Activation energies predicted for T5 and T20 clusters were found to differ by up to 12 kcal/mol.…”

Section: Impact Of Long-range Electrostaticsmentioning

confidence: 99%

Impact of long-range electrostatic and dispersive interactions on theoretical predictions of adsorption and catalysis in zeolites

et al. 2018

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In this paper, we review the importance of long-range zeolite framework interactions in theoretical predictions for a variety of zeolite-catalyzed processes, and we show why such interactions must be determined accurately in order to reproduce experimentally measured adsorption and activation energies. We begin with an overview of the different strategies that have been used to account for long-range coulombic and dispersive interactions of zeolite framework atoms with species adsorbed at an active site. These methods include full periodic-DFT calculations and multi-layer hybrid techniques. Electrostatic interactions are observed to have a more significant impact than dispersive interactions on the geometries of ion-pair transition states and adsorbed species. Stabilization of the TS relative to reactant complexes is also dictated by electrostatic interactions. Dispersion effects are found to significantly stabilize both transition and reactant states for adsorbed species, especially those which have dimensions that provide good fits within the zeolite pore or cavity. We also show that the relevance of particular active site configurations can be missed, if the effects of long-range interactions are neglected. As a case in point, we demonstrate that a site previously considered inactive for ethane dehydrogenation, [GaH 2 ] + may in fact be more active than previously thought, when the impact of long-range interactions on the predicted activation energy is taken into account. Finally, the use of hybrid quantum mechanics/molecular mechanics approaches on extended, finite zeolite clusters has emerged as an accurate, highly cost-effective, and versatile alternative towards overcoming some of the present-day limitations of periodic calculations.

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“…The zeolite properties characterization by molecular modeling requires the use of large clusters to take into account the confinement effects. − Despite the relative success of cluster calculations − in studies of catalysis by zeolites, they involve high computational costs. Alternatively, hybrid methods, which combine quantum and classical mechanics, can be used to model large systems at moderate computational costs.…”

Section: Introductionmentioning

confidence: 99%

Evaluation of Brønsted Sites Inside the H-MOR Employing NH₃: A Theoretical Study

et al. 2013

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ONIOM and pseudoespectral calculations have been carried out to determine geometries and adsorption energies of NH3 on H-mordenite, H-MOR, with density functional theory. The influence, on the adsorption energy, of a second acid site, close to the one where the adsorption is taking place, was investigated as well as the possibility of adsorbing two NH3 molecules on neighboring sites. The NH3 adsorption energies over MOR with one and two acid sites agree with the temperature-programmed desorption TPD results.

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The Dehydrogenation Reaction of Light Alkanes Catalyzed by Zeolites

Cited by 25 publications

References 47 publications

Insights into the Kinetics of Cracking and Dehydrogenation Reactions of Light Alkanes in H-MFI

Insights into the Kinetics of Cracking and Dehydrogenation Reactions of Light Alkanes in H-MFI

Impact of long-range electrostatic and dispersive interactions on theoretical predictions of adsorption and catalysis in zeolites

Evaluation of Brønsted Sites Inside the H-MOR Employing NH₃: A Theoretical Study

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The Dehydrogenation Reaction of Light Alkanes Catalyzed by Zeolites

Cited by 25 publications

References 47 publications

Insights into the Kinetics of Cracking and Dehydrogenation Reactions of Light Alkanes in H-MFI

Insights into the Kinetics of Cracking and Dehydrogenation Reactions of Light Alkanes in H-MFI

Impact of long-range electrostatic and dispersive interactions on theoretical predictions of adsorption and catalysis in zeolites

Evaluation of Brønsted Sites Inside the H-MOR Employing NH3: A Theoretical Study

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Evaluation of Brønsted Sites Inside the H-MOR Employing NH₃: A Theoretical Study