Theory of zeolites and related materials

The aim of this project is the investigation of the structure and chemical reactivity of zeolites (i.e. of microporous aluminoslicates) and related materials (AlPO`s, clays), and of chemical processes catalyzed by these materials, using ab-initio density-functional-theory (DFT) and post-DFT approaches. It is now generally accepted that quantum mechanics is an indispensable tool for the investigation of the structure and chemical reactivity of microporous solid acids like protonated or metal-exchanged zeolites and silico-aluminophosphates (SAPO`s). These techniques also develop an increasing impact on the study of chemically related complex materials such as clays. Quantum-mechanical simulations allow to assign experimental signatures to structural models and to predict the stability and reactivity of different configurations. Quantum-mechanical simulations also permit to explore the reaction mechanisms by calculating the energy and free-energy profiles for molecular reactions promoted by the catalyst. The results of the first phase this project have deepened our understanding of zeolites, but also opened several interesting fields for further research. The first concerns the structure and reactivity of the active sites in metal- exchanged zeolites and SAPO`s. For transition-metal ions (TMI) in particular, the cation-framework interaction is determined by covalent bonds formed by d-orbitals of the cation and p-orbitals of the framework oxygen atoms. The properties of these bonds, and hence the location and chemical reactivity of the cation depend on the state of activation of the oxygen atoms bonding to the TMI, and hence on distribution of Si/Al substitution sites over the framework. For the second phase of this project we plan a comprehensive investigation of the structure and reactivity of the active sites in TMI-exchanged zeolites and SAPO`s. A network of international cooperation will contribute to the breadth of these investigations. A second point where considerable progress has been achieved during the first phase of this project is the determination of free-energy barriers for catalytic reactions. We have developed and implemented a range of algorithms for dynamical transition-state searches and performed first applications to proton-exchange reactions of alkanes. In future we plan to apply these techniques to model a range of hydrocarbon reactions in zeolites: monomolecular cracking of alkanes and bifunctional-bimolecular transformations (cracking, isomerization) of alkanes (de-hydrogenation at a Lewis-site, followed by formation of carbenium ions at a Br$\o$nsted site and either isomerization or cracking by $\beta$-scisssion). Particular attention will be paid to the influence of the structure of the zeolitic framework on the selectivity of the reaction. In addition we plan to perform studies of the environmentally important de-NO$_x$ reaction by the selective catalytic reduction of methane by NO. These proposed investigations will be complemented by studies of molecular adsorption in clays (comparing the microporous with the layered aluminosilicates). In addition we plan exploratory studies of the application of hybrid-functionals (i.e. of functionals mixing DFT and exact (Hartree-Fock) exchange) to small-cell zeolites and SAPO`s. The intention of these studies is to shed some light on possible shortcomings of the current DFT functionals in describing sorbent-zeolite interactions.

The aim of this project was the investigation of the structure and chemical reactivity of microporous solid acid catalysts (mostly zeolites, but also of related materials such as SAPO`s, clays, and ionic liquids) and the simulation of chemical processes catalyzed by these materials, using ab-initio density functional techniques. Solid acid catalysts play an important role in many industrially and environmentally important processes, ranging from hydrocarbon conversions in refining to de-NOx reactions in automotive catalytic converters. The focus of the project is on the fundamental aspects of acid-based reactions in zeolites, concentrating on: (1) The investigation of the active sites in protonated and metal-exchanged zeolites-probing their Brønsted and Lewis acidity via molecular adsorption and vibrational spectroscopy. (2) The investigation of the structure and reactivity of the active sites in metal-exchanged silico-aluminophosphates (SAPOS), in close analogy to the work on zeolites. (3) In these investigations particular attention should be given to the importance of the choice of the exchange- correlation functional, exploring the influence of post-DFT corrections such as the admixture of exact exchange (in hybrid functionals) and of dispersion forces. (4) The development of techniques allowing to determine the reaction path and the free energy reaction barrier, using an approach beyond harmonic transition-state theory. (5) The simulation of catalytic reactions, concentrating on (a) hydrocarbon conversions, including proton-exchange, de-hydrogenation, cracking and isomerization (branching) of alkanes and alkenes, and (b) de-NOx reactions such a N2 O decomposition and the selective catalytic reduction of NO. (6) The investigation of the formation of metallic clusters in the cavities of zeolites, concentrating on the important topic of clusters-support interaction. This part of the project is realized in cooperation with the FWF Project on "Transition-metal clusters" (P19712-N16). (7) The investigation of materials related to zeolites by their chemical composition and/or properties, in particular layered aluminosilicates (clays) and transition-metal oxides with layered structures. (8) The comparative investigation of other acid catalysts, in particular ionic liquids. Important progress has been realized in all project areas, as documented by the publications and conference contributions listed below. As particular highlights we mention (i) the first investigations of periodic models of zeolites and SAPO`s using hybrid functionals, (ii) the first investigation of van der Waals interactions on the molecular adsorption in zeolites using post-DFT ab-initio methods, (iii) the implementation and application of first-principles free-energy methods to acid-based reactions in zeolites, and (iv) the elucidation of the nature of cluster-support interactions.