Catalytic conversion of hydrocarbons over zeolites from first principles

Conversion of hydrocarbons over zeolites is an important industrial process. In the presence of zeolites, hydrocarbons are converted either to smaller molecules (cracking), or they recombine to branched compounds (isomerization). The microscopic steps of the conversion, however, are still not fully understood. This work brings together our previously published as well as new data into a coherent picture of hydrocarbon catalysis. We perform static and molecular dynamics DFT calculations on gmelinite zeolite with adsorbed linear saturated and unsaturated hydrocarbon molecules. Our simulations indicate that the conversion of olefins can proceed through the chemisorption inside the zeolite because species chemisorbed at specific sites are more stable than physisorbed molecules. Desorption at increased temperatures (400-700 K) produces unstable protonated molecules. These are found to be relatively long-lived, being stabilized in the zeolite surroundings. The collapse of protonated molecules then produces branched isomers. Protonation of saturated molecules leads to cracking. The calculated activation energies of both isomerization and cracking are in reasonable agreement with experimental data. (C) 2002 Elsevier Science B.V. All rights reserved.