Research Project – Emiel Hensen/Chong Liu
Zeolites are crystalline microporous solids built up from corner-sharing, tetrahedral silicate units. The isomorphic substitutions of framework Si atoms by Al atoms results in the formation of Brønsted acid sites (BAS) in their micropores. Zeolites are widely used as solid acid catalysts in petrochemistry and oil refining processes. Their catalytic activity depends both on the chemical composition and on the structural properties. Although the variation in Brønsted acidity of zeolites is usually only described in terms of the local chemical and topological properties of the zeolite framework, there are other factors that can very strongly influence zeolite acidity and therefore the catalytic activity. One of the most important and not understood yet factors of this kind is the synergetic effect of extraframework metal-containing cations (EFcation) and Brønsted acid sites inside the zeolite micropores. For example, Faujasite-type zeolites modified by extrafamework Al show increased acidity and enhanced alkane cracking activity.1 Alternatively, the introduction of La into low-silica zeolites not only improves their stability, but also enhances reactivity for alkane activation.2 The existence of extraframework cations is believed to have positive influence on the catalytic reactions of zeolites. However, the role of extraframework species on zeolite acidity and catalytic activity is complicated and has not been fully rationalized yet.
In this project, we will carry out a complete theoretical study on the mechanism of acidity enhancement and activity improvement by extraframework species in zeolites. The aim is to answer the question how the synergy between different components of zeolite catalysts gives rise to their unique catalytic performance. The main focus will be on the effect of extraframework Al and La species on the reactivity in such industrial processes as alkylation, cracking and dehydrogenation of hydrocarbons.
1. Almutairi, S. M. T.; Mezari, B.; Filonenko, G. A.; Magusin, P. C. M. M.; Rigutto, M. S.; Pidko, E. A.; Hensen, E. J. M.: Influence of Extraframework Aluminum on the Brønsted Acidity and Catalytic Reactivity of Faujasite Zeolite. ChemCatChem 2013, 5, 452-466.
2. Schüßler, F.; Pidko, E. A.; Kolvenbach, R.; Sievers, C.; Hensen, E. J. M.; van Santen, R. A.; Lercher, J. A.: Nature and Location of Cationic Lanthanum Species in High Alumina Containing Faujasite Type Zeolites. J. Phys. Chem. C 2011, 115, 21763-21776.