Thomas Demuth

Catalytic isomerization of 2-pentene in H-ZSM-22—A DFT investigation

The skeletal isomerization of a 2-pentene molecule catalyzed by acidic ZSM-22 was investigated by ab initio DFT studies. Two different scenarios proposed in the literature were tested. First a reaction including an alkyl shift was considered: a methyl or ethyl group is detached from the carbenium ion chain and reattached at another site in the residual hydrocarbon chain. However, this mechanism is rather unlikely, since the alkyl ion is a high-energy species, so its detachment from the carbenium ion induces a high activation energy. We find that the more likely pathway for skeletal isomerization inside the channels of ZSM-22 involves the rearrangement of the carbenium ion into a protonated dimethylcyclopropane and implies the formation of relatively stable secondary carbenium ions as transient intermediates.

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. ...

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.