Molecular modelling in zeolite catalysis

Quantum-chemical studies of the alkylation of toluene by methanol in Mordenite illustrate the fundamentals of transition state selectivity. It appears that, as in the enzymes, the reorientation energy of the reactant molecular so as to make the reaction channel to a particular product accessible, is the controlling parameter. This implies that the lock and key selectivity model of shape selective zeolite catalysis is based on the relative stability of pre transition state intermediates (1). These energies are due to the dispersive Van der Waals interaction, not currently require the use of force fields to be simulated correctly. In a second part of the talk we will concentrate on Lewis acidic properties of Zn2+ and the corresponding (ZnOZn)2+ oxycations (2). It will appear that their reactivity sensitively depends on ion-exchange cation positions. Size match with cavity as well as the negative zeolite lattice charge distribution is important. Quantitative prediction of the relative energies is only possible when full consideration of lattice relation is important. Quantitative prediction of the activation is studied. Hydrogen recombination is rate limiting in the ZnOZn2+ cluster, whereas CH activation is rate limiting when catalyzed by Zn2+ (3). Finally the importance of diffusional effects will be discussed for the isomerization of hexane catalyzed by Mordenite. Single-file diffusion occurring in one-dimensional pores has a dramatic effect on the concentration dependence even of monomolecular reactions that are found to proceed through a maximum. Using theoretically and experimentally determined elementary rate parameters on quantitative prediction has been made of crystallite size dependence (4)

Quantum-chemistry of zeolite acidity

Theoretical chemistry applied to zeolite acid catalysis is becoming an important tool in the understanding of the adsorption and interaction of guest molecules with the zeolitic lattice. Especially the understanding of the mechanisms by which zeolite catalyzed chemical reactions proceed becomes possible. It is shown here that the old interpretation of carbonium and carbenium ions as intermediates for zeolite catalyzed reactions has to be replaced by a new approach in terms of positively charged transition states that are strongly stabilized by the zeolitic lattice. The large deprotonation energy of the acidic zeolite is overcome by stabilization of the intermediate or transition state positive charge by the negative charge left in the lattice. The zeolitic sites responsible for the adsorption and/or reaction of guest molecules are the Bronsted-acid and Lewis-base sites. We also show that different transition states are responsible for different kinds of reactions, such as cracking, dehydrogenation, et

Theoretical aspects of heterogeneous catalysis

Quantum chem. aspects of chem. bonding to metal surfaces are discussed with 58 refs. A Frontier Orbital Theory of chemisorption can be developed in which the group orbital local d. of states at the Fermi level replaces the HOMO and LUMO interactions familiar in org. or organometallic chem. Dissocn. of the H2 mol. and H atom recombination are discusse

Theory of chemisorption of hydrogen on transition metals and their alloys

A theor. discussion is given of the changes in the heat of chemisorption of a H atom upon alloying a group-VIII metal with a group-IB metal. The emphasis is on 2 concepts: the ensemble effect and the ligand effect. The ensemble effect ascribes the changes in the heat of chemisorption primarily to changes in the geometry of the adsorption complex, whereas the ligand effect also considers changes in bond strength due to different intrinsic activity of the bonding metal atom

Screening in chemisorption theory

A theory is presented that makes it possible to discuss the effect of plasmon screening on the chemisorption energy as a function of the bond strength/plasmon energy ratio. The correlation effects are shown to disappear for strong bonding and to cancel out in the limiting case of weak bonding. Significant effects are found in the intermediate bonding range, when the tunneling frequency of the adsorbate electrons is of the same order of magnitude as the plasmon frequency. For values representative of chemisorption of H to a transition-metal surface the contribution to the bond energy may be of the order of 20%. The quantum theor. anal. leads to effective potentials, which can be readily incorporated into the Newns-Anderson-type chemisorption Hamiltonian. Explicit expressions for the screened exchange and the Coulomb hole contributions are derived. [on SciFinder (R)

Disguises of complexity

\u3cp\u3eComplementary to the previous chapter a tutorial introduction to complexity science is presented. The chapter focuses on the interrelation of complexity science concepts that vary from mathematics to physics and biology to the social sciences. An interesting aspect of complexity science is that its language as well as tools are of particular use to study problems that require a multidisciplinary approach.\u3c/p\u3