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, etc