Theory of zeolites and related materials

Zeolites are crystalline microporous aluminosilicates with periodic arrangements of cages and channels that find widespread industrial application as catalysts, sorbents, molecular sieves and ion exchangers. In addition zeolite films are being considered for potential applications as selective membranes, chemical sensors, and components in microelectronic devices. The framework structure surrounding the channels and cavities with diameters corresponding to the size of small molecules is built of SiO4 tetrahedra, with some of the Si atoms replaced by Al. The Al atoms replacing Si induce a negative charge on the lattice which is counterbalanced by positively charged metal ions or protons in the pores. These cations endow the zeolites with their characteristic ion-exchange capacity and with their unique functionality as size- and shape-selective catalysts. Ever since in 1962 zeolites have first been used as catalysts, intense research efforts have been focused on the understanding of their structure and chemical properties and on attempts to modify their chemical behavior by changing the framework structure, the Si/Al ratio or by the addition of acidic, basic or redox-active metal ions. Currently, our knowledge of catalytic processes in zeolites is being revolutionized by the application of computer modeling techniques which are able to predict with growing accuracy and reliability the structural and dynamical properties of materials. The most advanced of these modeling techniques are based on density-functional (DFT) theory. However, the application of these techniques to materials and processes which are as complex as zeolites and the chemical processes they catalyze remains a formidable challenge - only recently, DFT codes capable of handling such tasks have been developed. The Vienna ab-initio simulation package is among the most performant of these codes. It has been used to investigate the structure and chemical reactivity of zeolites as complex as mordenite (150 atoms/cell) and to study reaction scenarios for ion-exchange-prozesses and for a variety of catalytic reactions such as isomerization, alkylation and disproportionation of complex molecules (mostly hydrocarbons). Within the proposed project we intend to address fundamental questions of zeolite research using ab-initio density- functional calculations. These include in particular the structural properties of the active sites in protonated and metal-exchanged zeolites and the characterization of their Brønsted and Lewis acidity. We shall also investigate a new class of hybrid zeolites containing either organic functional groups in the cavities covalently bound to the framework or organic groups incorporated within the framework. We propose to extend the studies of zeolite surfaces started recently and to explore the chemical reactivities of surface silanols and bridging hydroxyl groups at the surface. The investigations of metal groups inside zeolites and of zeolite surfaces are strongly connected to the role of zeolites as support for metallic and oxidic catalysts which will form an important part of the project. The studies of these fundamental structural and chemcial properties of zeolites will be complemented by simulation of chosen catalytic reactions. In particular we shall concentrate on hydrocarbon conversions in acid and bifunctional catalysts and on surface-specific reactions. We also plan to extend the investigations to materials closely related to zeolites (in particular clays).

Zeolites are crystalline microporous aluminosilicates with periodic arrangements of cages and channels that find widespread industrial application as catalysts, sorbents, molecular sieves and ion exchangers. In addition zeolite films are being considered for potential applications as selective membranes, chemical sensors, and components in microelectronic devices. The framework structure surrounding the channels and cavities with diameters corresponding to the size of small molecules is built of SiO4 tetrahedra, with some of the Si atoms replaced by Al. The Al atoms replacing Si induce a negative charge on the lattice which is counterbalanced by positively charged metal ions or protons in the pores. These cations endow the zeolites with their characteristic ion-exchange capacity and with their unique functionality as size- and shape-selective catalysts. Ever since in 1962 zeolites have first been used as catalysts, intense research efforts have been focused on the understanding of their structure and chemical properties and on attempts to modify their chemical behavior by changing the framework structure, the Si/Al ratio or by the addition of acidic, basic or redox-active metal ions. Currently, our knowledge of catalytic processes in zeolites is being revolutionized by the application of computer modeling techniques which are able to predict with growing accuracy and reliability the structural and dynamical properties of materials. The most advanced of these modeling techniques are based on density-functional (DFT) theory. However, the application of these techniques to materials and processes which are as complex as zeolites and the chemical processes they catalyze remains a formidable challenge - only recently, DFT codes capable of handling such tasks have been developed. The Vienna ab-initio simulation package is among the most performant of these codes. It has been used to investigate the structure and chemical reactivity of zeolites as complex as mordenite (150 atoms/cell) and to study reaction scenarios for ion-exchange-prozesses and for a variety of catalytic reactions such as isomerization, alkylation and disproportionation of complex molecules (mostly hydrocarbons). Within the proposed project we intend to address fundamental questions of zeolite research using ab-initio density- functional calculations. These include in particular the structural properties of the active sites in protonated and metal-exchanged zeolites and the characterization of their Brønsted and Lewis acidity. We shall also investigate a new class of hybrid zeolites containing either organic functional groups in the cavities covalently bound to the framework or organic groups incorporated within the framework. We propose to extend the studies of zeolite surfaces started recently and to explore the chemical reactivities of surface silanols and bridging hydroxyl groups at the surface. The investigations of metal groups inside zeolites and of zeolite surfaces are strongly connected to the role of zeolites as support for metallic and oxidic catalysts which will form an important part of the project. The studies of these fundamental structural and chemcial properties of zeolites will be complemented by simulation of chosen catalytic reactions. In particular we shall concentrate on hydrocarbon conversions in acid and bifunctional catalysts and on surface-specific reactions. We also plan to extend the investigations to materials closely related to zeolites (in particular clays).