J. Hafner

Periodic trends in hydrodesulfurization: in support of the Sabatier principle

Abstract Periodic effects form the basis for progress in understanding the role of structure/function relationships in hydrodesulfurization (HDS). Theoretical results of Toulhoat, and Raybaud et al. based on ab initio calculations using density functional theory calculations as well as experimental results of Berhault et al. have recently confirmed the initial proposal of Pecoraro and Chianelli about the Sabatier principle interpretation for HDS activity either for transition metal binary bulk sulfides or for cobalt promotion of MoS2-based catalysts. An optimum HDS activity corresponds to a moderate active site-organic sulfur-containing reactant interaction. An update is presented here with a special attention to the electronic foundations of the Sabatier principle as applied to HDS catalysis.

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.

Acid-based Catalysis in Zeolites Investigated by Density-Functional Methods

Zeolites are a unique class of microporous aluminosilicates with multiple applications as molecular sieves, detergents, desiccants and acid catalysts. Their catalytic activity is determined by Brønsted and Lewis acid sites created by protonation or activation by metallic cations. The reactivity of the acid sites is strongly influenced by the geometry and the flexibility of the zeolitic framework. Recent investigations of the reactivity of zeolites and simulations of catalytic reactions based on periodic-density-functional calculations are reviewed.

Ab initio study of structure and interconversion of native cellulose phases.

Dispersion-interaction corrected DFT simulations are performed to study the structure of two allomorphs of native cellulose I. Good agreement between theoretical and experimental data is achieved. Two H-bond patterns, previously identified experimentally, are examined for both allomorphs. The transition mechanism for the conversion between the phase I(α) and I(β) is studied by means of constrained relaxations. New metastable intermediate phase occurring on the I(α) → I(β) route is identified, and the corresponding structural data are reported.

Ab initio density functional investigation of the (001) surface of mordenite

Structural and acidic properties of the (001) surface of mordenite have been studied using density functional theory including generalized gradient corrections to the exchange-correlation functional. Our calculations, in agreement with experimental results, show that the surface structure of mordenite differs only moderately from the bulk structure, no reconstruction of the surface has been found. Part of the terminal silanol groups form weak hydrogen bonds with other framework oxygen sites, the lengths of hydrogen bonds vary in the range 1.9–2.6 A depending on the position of the Al site. The OH-stretching frequencies for various acid sites in the mordenite framework have been calculated. In agreement with experiment, calculated OH-stretching frequencies of terminal silanol groups are shifted by about 100 cm−1 with respect to the Bronsted acid sites. Those silanol groups whose OH-stretching frequencies are modified by the presence of hydrogen bonds are shown to absorb in the region typical for the Bronst...

Adsorption and vibrational spectroscopy of ammonia at mordenite: Ab initio study

The adsorption of ammonia at various active centers at the outer and inner surfaces of mordenite, involving Brønsted acid (BA) sites, terminal silanol groups, and Lewis sites has been investigated using periodic ab initio density-functional theory. It is shown that ammonia forms an ammonium ion when adsorbed at strong BA sites. The calculated adsorption energies for different BA sites vary in the interval from 111.5 to 174.7 kJ/mol depending on the local environment of the adduct. The lowest adsorption energy is found for a monodentate complex in the main channel, the highest for a tetradentate configuration in the side pocket. At weak BA sites such as terminal silanol groups or a defect with a BA site in a two-membered ring ammonia is H bonded via the N atom. Additional weak H bonds are formed between H atoms of ammonia and O atoms of neighboring terminal silanol groups. The calculated adsorption energies for such adducts range between 61.7 and 70.9 kJ/mol. The interaction of ammonia with different Lewis sites is shown to range between weak (DeltaE(ads)=17.8 kJ/mol) and very strong (DeltaE(ads)=161.7 kJ/mol), the strongest Lewis site being a tricoordinated Al atom at the outer surface. Our results are in very good agreement with the distribution of desorption energies estimated from temperature-programmed desorption (TPD) and microcalorimetry experiments, the multipeaked structure of the TPD spectra is shown to arise from strong and weak Brønsted and Lewis sites. The vibrational properties of the adsorption complexes are investigated using a force-constant approach. The stretching and bending modes of NH(4) (+) adsorbed to the zeolite are strongly influenced by the local environment. The strongest redshift is calculated for the asymmetric stretching mode involving the NH group hydrogen bonded to the bridging O atom of the BA site, the shift is largest for a monodentate and smallest for a tetradentate adsorption complex. The reduced symmetry of the adsorbate also leads to a substantial splitting of the stretching and bending modes. In agreement with experiment we show that the main vibrational feature which differentiates coordinatively bonded ammonia from a hydrogen-bonded ammonium ion is the absence of bending modes above 1630 cm(-1) and in the region between 1260 and 1600 cm(-1), and a low-frequency bending band in the range from 1130 to 1260 cm(-1). The calculated distribution of vibrational frequencies agrees very well with the measured infrared adsorption spectra. From the comparison of the adsorption data and the vibrational spectra we conclude that due to the complex adsorption geometry the redshift of the asymmetric stretching is a better measure of the acidity of an active sites than the adsorption energy.

Entropy effects in hydrocarbon conversion reactions: free-energy integrations and transition-path sampling.

The standard approach to ab initio simulations of activated chemical processes is based on the harmonic-oscillator/rigid-rotor approximation to transition state theory. However, there is increasing evidence that these approximations fail for reactions involving loosely bound reactant and/or transitions states where entropy makes a significant contribution to the free-energy reaction barrier. Examples are provided by the conversion (proton exchange, dehydrogenation, monomolecular cracking) of short alkanes over acidic zeolites. For proton exchange and monomolecular cracking the reaction path may be described reasonably well by simple vectorial reaction coordinates and the free energy of activation may be derived by free-energy integration schemes such as the Blue-Moon ensemble technique in combination with constrained ab initio molecular dynamics simulations. For alkane dehydrogenation, however, several reaction scenarios are in competition and techniques such as transition-path sampling must be used to determine the dominant reaction mechanism. In our paper we describe the fundamental aspects of these techniques and discuss their application to compute free-energy barriers for proton exchange between isobutane and acidic chabazite and for monomolecular cracking of propane. Dehydrogenation of propane has been studied using transition-path sampling. In this case the static approach based on harmonic transition state theory not only fails in producing accurate reaction barriers but even leads to incorrect predictions of reaction intermediates and products.

Crystal structure prediction of LiBeH3 using ab initio total-energy calculations and evolutionary simulations

The stable crystal structure of LiBeH(3) is predicted on the basis of ab initio total-energy calculations using density-functional theory and an extended database of candidate structures and using global optimizations based on an evolutionary algorithm. At the level of density-functional theory, a CaSiO(3)_1-type structure with space group P2(1)/c, containing BeH(4) tetrahedra linked in chains, is the ground-state structure of LiBeH(3) (alpha-LiBeH(3)). It is found to be lower in energy than the structures proposed in previous studies. The analysis of the electronic structure shows that alpha-LiBeH(3) is an insulator with a band gap of about 4.84 eV and exhibits strong covalent bonding in the BeH(4) tetrahedral complexes. Calculations at finite temperatures and high pressures suggest that at T=408 K and ambient pressure a structural transition from alpha-LiBeH(3) (CaSiO(3)-type) to a YBO(3)-type structure with space group Cmcm occurs and that at a pressure of 7.1 GPa alpha-LiBeH(3) undergoes a pressure-induced structural transition from the alpha-phase to a MgSiO(3)-type structure with space group C2/c. The calculated enthalpies of formation (-45.36 and -30.12 kJ/mol H(2) without and with zero-point energy corrections) are in good agreement with the experimental result, indicating that LiBeH(3) is a potential hydrogen storage material with low activation barriers for hydrogen desorption.

Ab-initio energy profiles for thiophene HDS on the MoS2 (1010) edge-surface

Abstract We establish the energy profiles for thiophene hydrodesulfurization on the edge-(1010) surface for relevant model mechanisms. Two kinds of adsorption configurations ( η 1 and η 5 ) for the thiophene and its hydrogenated derivatives are taken into account. We have considered a supply of hydrogen either from Mo-SH surface groups or from Mo-H groups. The η 5 configuration, which is likely to occur at low surface coverages, is clearly the most favorable one for the hydrogenation steps. In the η 1 configuration, the role of co-adsorbed -SH groups becomes crucial for the hydrogen transfer during the hydrogenation steps. At this stage of the study, the sulfur removal step appears as the most unfavorable step for a non-promoted catalyst.

On the structure and dynamics of secondary n-alkyl cations

A variety of computational studies was undertaken to examine and establish the relative importance of open versus closed structures for unbranched secondary n-alkyl cations. First, the PW91 level of density functional theory was used to optimize over 20 minimum-energy structures of sec-pentyl, sec-hexyl, and sec-heptyl ions, demonstrating that closed structures are more stable than open ones on the potential energy surface (PES). Second, PW91 was used with a theoretical Andersen thermostat to perform a molecular dynamics simulation (150 ps) of C9H19+ at a typical catalytic temperature of 800 K, demonstrating that the structure preference is inverted on the free-energy surface. Third, both quantum (rigid-rotor/harmonic oscillator) and classical partition functions were used to demonstrate that the simulated structure-opening at catalytic temperatures is due to the floppiness of the open forms, which improves its free energy by both lowering its zero-point vibrational energy and increasing its molecular ent...