Research Interests:

Zeolites are crystalline microporous aluminosilicates widely used as molecular sieves and catalysts in industrial chemical processes. In the past thirty years, attention was drawn to ZSM-5 and other silicon-rich zeolites (with Si/Al > 8). Their protonic forms are currently used as acid catalysts for hydrocarbon transformations in petrochemistry. Their transition metal exchanged forms have been discovered as exceptional redox catalysts for nitrogen oxide abatement, and a selective oxidation of hydrocarbons by nitrous oxide.
The species assumed to be the active sites in the mentioned reactions, i.e. protons, metal ions and metal-oxo species, are positively charged and compensate the negatively charged alumosilicate framework. Therefore the Al siting in zeolite frameworks governs the location of the active sites as well as their properties. This is important for both the acid catalyzed hydrocarbon syntheses as well as redox reactions. Thus, the Al siting in zeolites is of crucial importance for their catalytic behavior.
A typical feature of many silicon-rich zeolites is a high number of crystallographically distinguishable tetrahedral framework sites (T sites, T = Si or Al) resulting in a high variability of the Al siting.
Diffraction techniques cannot distinguish between Al and Si atoms and thus do not allow direct identification of the Al siting in zeolites.
On the other hand, 27Al solid state NMR spectroscopy was found to be a powerful tool for analyzing the coordination of Al atoms in zeolites. Recent developments of multiple quantum NMR experiments opened new possibilities for studying the structure of AlO4- in zeolite frameworks.
Key to the identification of the Al siting in silicon rich zeolites is assigning the observed 27Al resonances to individual framework T sites.
Developments in computational chemistry have allowed calculating reasonable zeolite structures as well as NMR parameters.
We developed the bare charged zeolite framework model and showed that it represents a realistic approach to calculate the local geometry of AlO4- tetrahedra in fully hydrated counter cation containing silicon rich zeolites. The predictions of the 27Al isotropic chemical shifts corresponding to Al in the individual T sites are based on calculations of the local structure around the Al sites using a quantum mechanics - molecular mechanics hybrid approach (QM-Pot). The subsequent evaluation of the NMR shielding values uses the GIAO method. Both steps are based on density functional theory (DFT). The calculated Al shieldings are converted to Al isotropic chemical shifts employing the experimental and theoretical NMR parameters obtained for the silicon rich chabasite zeolite which serves as a secondary standard.

We are also interested to employ periodic DFT methods to study transition metal exchanged silicon rich zeolites which are important catalysts. We study the N2O decomposition into N2 and O2 on Fe(II) exchanged silicon rich zeolites.