Computational approaches to the determination of active site structures and reaction mechanisms in heterogeneous catalysts

A high-performance selective oxidation system for the facile production of fine chemicals

Mn(III)AlPO-5 and Cr(VI)AlPO-5 redox (microporous) catalysts are effective, in the presence of dissolved acetylperoxyborate (APB) under mild conditions (333-373 K), and much superior to the titanosilicate, TS-1 (also a single-site heterogeneous catalyst), in the selective oxidation of primary, secondary, benzylic and other unsaturated alcohols, p-cymene, methyl cyclohexene and other speciality organics which are of value in the fine-chemical and pharmaceutical industries.

Point defects in ZnO

We have investigated intrinsic point defects in ZnO and extended this study to Li, Cu and Al impurity centres. Atomic and electronic structures as well as defect energies have been obtained for the main oxidation states of all defects using our embedded cluster hybrid quantum mechanical/molecular mechanical approach to the treatment of localised states in ionic solids. With these calculations we were able to explain the nature of a number of experimentally observed phenomena. We show that in zinc excess materials the energetics of zinc interstitial are very similar to those for oxygen vacancy formation. Our results also suggest assignments for a number of bands observed in photoluminescence and other spectroscopic studies of the material.

New all-atom force field for molecular dynamics simulation of an AlPO4-34 molecular sieve

A force field of the triclinic framework of AlPO(4)-34, important in methanol-hydrocarbon conversion reactions, was developed using an empirical potential function. Molecular dynamics simulation of an AlPO(4)-34 triclinic framework segment of 1216 atoms, containing the template molecules isopropylamine and water, was performed with explicit consideration of atomic charges. The average RMS difference between instantaneous positions of the framework atoms during 1 ns simulation and their positions in the structure determined from single crystal X-ray diffraction was calculated, and the average structure of the flexible framework was determined. The computed Debye-Waller factors and simulated FTIR spectra are in good agreement with the experimental data. The new force field permits detailed molecular dynamics simulations of flexible, charged aluminophosphate molecular sieves which should lead to a better understanding of the catalytic processes and the crucial role played by templating molecules.

Activation energy for hydrogen abstraction from methane over Li-doped MgO: A density functional theory study

Hydrogen abstraction from methane over Li-doped MgO is studied by means of density functional theory. The generalized synchronous transit method is applied to determine the transition state of the reaction. This method allows a transition state search that is more comprehensive compared with previous studies. The convergence of the calculated activation barrier with respect to cutoff energy, k-point mesh, vacuum layer thickness, and number of ionic layers in the crystal slab is examined. The activation barrier is calculated to be 0.745+/-0.01 eV (71.9+/-1.0 kJ/mol).

Single-Site Heterogeneous Catalysts

Intellectually, the advantages that flow from the availability of single-site heterogeneous catalysts (SSHC) are many. They facilitate the determination of the kinetics and mechanism of catalytic turnover-both experimentally and computationally-and make accessible the energetics of various intermediates (including short-lived transition states). These facts in turn offer a rational strategic principle for the design of new catalysts and the improvement of existing ones. It is generally possible to prepare soluble molecular fragments that circumscribe the single-site, thus enabling a direct comparison to be made, experimentally, between the catalytic performance of the same active site when functioning as a heterogeneous (continuous solid) as well as a homogeneous (dispersed molecular) catalyst. This approach also makes it possible to modify the immediate atomic environment as well as the central atomic structure of the active site. From the practical standpoint, SSHC exhibit very high selectivities leading to the production of sharply defined molecular products, just as do their homogeneous analogues. Given that mesoporous silicas with very large internal surface areas are ideal supports for SSHC, and that more than a quarter of the elements of the Periodic Table may be grafted as active sites onto such silicas, there is abundant scope for creating new catalytic opportunities.

Catalysis: principles, progress, prospects

In this introductory paper, we endeavour to bridge the gaps that currently exist between the three main subdivisions of catalysis: enzymatic, homogeneous and heterogeneous. Hitherto, there has been a tendency for each of these three divisions to grow separately using their own concepts, phrases and techniques. However, there is much that unites them, not least the notion of the catalytically active site and, in particular, its often unusual (constrained) state of electronic or atomic environmental disposition. We identify many points of similarity between, for example, the mode of action of, metalloenzymes on the one hand and the recent generation of transition metal ions embedded within nanoporous (usually siliceous) solids on the other. Useful unifying principles emerge from considerations of free-energy/reaction-coordinate plots. We present a number of tabulations and comparisons designed to facilitate the understanding of the mode of operation of existing, and the performance of new, catalysts. In doing so, we have drawn on our own work as well as that of others, including contributions that are to be found in this volume, with the intention of covering the great variety of catalytic phenomena.