François Hutschka

Adsorption of linear hydrocarbons in zeolites: A density-functional investigation

An extensive first-principles periodical study of adsorption properties of linear hydrocarbons in zeolites is presented. The applicability of density-functional theory to weak interactions is inspected within both local-density (LDA) and generalized-gradient (GGA) approaches for C1 to C6 linear hydrocarbons. The LDA adsorption energies are due to the overbinding ∼2.5 times larger than the GGA values. A compact diagram is constructed showing the increase of the adsorption energy with the length of the adsorbed molecule and with the concentration of acid sites in the zeolite support. The flow of the electron density induced by the adsorption indicates that the adsorption on the acid site is realized through the hydrogen bonding between the OH group and the CH3 group. The pattern of the reconstructed bonding, however, is more complex than that of the simple hydrogen bond. The regions of redistributed electron density within the adsorbed molecule are spread over the whole CH3 group and the adjacent C–C bond. ...

Ab initio investigation of the adsorption of benzene in mordenite

The adsorption of benzene in pure-silica and acid mordenite has been investigated using density functional theory, both in the local density approximation and including generalized-gradient corrections. Benzene acts as a test molecule for studying the interaction between an aromatic hydrocarbon and a zeolite. Different adsorption modes have been studied for several acid sites. All configurations have been optimized by minimizing the total energy with respect to all lattice parameters and to the atomic coordinates. The strength of the adsorption is directly correlated to the local structural distortion of the zeolitic framework, especially of the acid site. Only if strong adsorption occurs, the molecule itself is slightly deformed. Furthermore, an analysis of the differential charges has been performed, indicating substantial polarization effects for the acid site as well as for the molecule. Finally, the red-shift of the OH-stretching frequencies due to the adsorption has been calculated.

Spontaneous proton transfer between O-sites in zeolites

Abstract We have conducted a first-principles room-temperature molecular dynamics (MD) simulations of gmelinite. For one water molecule, the MD provides a complete scenario of the reaction pathway of the spontaneous proton transfer (PT). Water adsorbs at the acid site (i). One hydrogen atom of the adsorbed water connects to the framework O atom (ii) constituting a six-membered ring. Within this ring, the water molecule attracts the acidic proton, thus creating an unstable hydronium cation (H3O+) (iii). Another proton then transfers to the framework and completes the intrazeolite PT (iv). The calculated adsorption energy of one water molecule is 107 kJ/mol.

A periodic density functional theory study of thiophenic derivative cracking catalyzed by mordenite

A periodic density functional theory study of thiophenic derivative cracking catalyzed by proton- or lithium-exchanged mordenite has been performed. The same qualitative trends in activation energies as those described employing the cluster approach method have been obtained. However, the zeolite framework appears not to stabilize the transition state complexes. This is explained by the zwitterionic nature of the thiophenic derivatives cracking transition state complexes. The zeolite framework has more subtle effect on reactivity, as shown by the alteration of the ionic nature of the transition state complex in the case of better fit with the zeolite cavity. Notwithstanding, thiophenic derivative cracking catalyzed by zeolites remains difficult. General comments concerning the use of zeolite catalysts in hydrodesulfurization are made. Predictions on zeolite-based catalysts more suitable to achieve hydrodesulfurization are described.

Bro/nsted acid sites in gmelinite

The structural properties of purely siliceous and Al-substituted protonated gmelinite, a zeolite with a medium-sized unit cell, are investigated by means of first-principles local-density functional calculations. For acid sites introduced into an Al-free structure, optimized geometries are compared with experimental data. The substitution of a Si–O fragment by Al–O–H induces a pronounced local deformation of the structure. Four symmetrically distinct O sites (O1–O4) are classified, according to the pattern of the deformation, into two categories showing rather localized (sites 2 and 3), and mostly delocalized (sites 1 and 4) distortions. Relative stabilities of protonated structures are shown to depend on the initial geometry of the site. Larger Si–O–Si angles lead to a higher stability of the acid site. Two approaches, a static and a dynamical one, are used to derive OH stretching frequencies. Both of them prove a rather complex relation between the infrared (IR) frequency, the acidity, and the local env...

Spontaneous proton transfer in Na zeolites

Abstract First-principles room-temperature molecular dynamics (MD) simulations are conducted to investigate proton transfer (PT) in Na zeolites. The MD are performed on the unit cell containing two Al-sites, one of them saturated with H (acid site) and the second one with Na coordinated with three water molecules. The creation of the charged H3O+ cations is suppressed by the Na cation. Spontaneous barrierless PT between the O-sites in the zeolite, however, is possible. The lifetime of the hydronium cation is extremely short (⩽5 fs). The presence of Na+ cations leads to a modified mechanism but does not suppress the proton transfer around the Al-site in zeolites.

Catalyic conversion of hydrocarbons in zeolites from first principles

The application of the density functional techniques to processes of the conversion of hydrocarbons in zeolites has been reviewed. The conversion of hydrocarbons over zeolites is an important industrial process. The microscopic steps of the conversion, however, are still not satisfactorily understood. In order to examine reaction pathways, both static and molecular dynamics density functional theory (DFT) calculations have been performed. Simulated structural and spectral properties compare reasonably with experimental data. Comparison of energies of physisorption and chemisorption indicates possible reaction channel of the conversion through the chemisorption at the specific O-sites of the zeolite.

15-P-12-Ab-initio study of the adsorption and reactions of hydrocarbons in mordenite

Publisher Summary This chapter presents the ab initio study of the adsorption and reactions of hydrocarbons in mordenite. It discusses the interaction of n -alkanes, n -alkenes, and benzene with acid mordenite using density functional theory. The adsorption strength is correlated to the local structural distortion of the zeolite framework, especially of the acid site. Only for strong adsorption, the molecule is slightly deformed as in the case of benzene adsorption. The analysis of the differential indicates pronounced polarization effects on both the acid site and the adsorbed molecule. The H/D exchange reaction of benzene has been analyzed by calculating the reaction-energy diagram.

15-O-03 - A theoretical study of the methylation of toluene by methanol over acid mordenite

Publisher Summary This chapter presents a theoretical study of the alkylation of toluene by methanol catalyzed over acid mordenite. Cluster density functional theory (DFT) as well as periodic structure DFT calculations have been performed to obtain full reaction energy diagrams of the elementary reaction steps leading to the formation of the three xylene isomers. The use of periodic structure calculations allows taking into account the framework electrostatic contributions and steric constraints that are important in zeolite catalysis.

A DFT study of the isomerization reactions of aromatics catalyzed by acidic zeolites.

Publisher Summary This chapter presents a density functional theory (DFT) study of the isomerization reactions of toluene catalyzed by acidic zeolite. Monomolecular isomerization reactions have been considered and analyzed. The different reaction pathways have been discussed. The uses of periodical structure calculations and small cluster approach method calculations allow analyzing the effect of zeolite electrostatic contributions and steric constraints on reactivity.