Kinetic Study ofn-Butane Isomerization over Pt−H-Mordenite

The kinetics of n-butane isomerization over bifunctional Pt−H-mordenite was studied by varying reactant partial pressure and temperature. The main products were isobutane, propane, and pentanes. Yields of other products, especially methane, ethane, ethene, and propene, were very low, which suggests that protolytic cracking and hydrogenolysis were present only to a minor extent. Especially the protolytic cracking was inhibited, since after introduction of Pt into H-mordenite all strong Brønsted acid sites disappeared and the total density of acid sites decreased. The reaction rate showed complex dependence on the reactant partial pressure, suggesting that two reaction mechanisms on the acid sites, i.e., monomolecular or bimolecular, for isobutane formation are present and the prevailing mechanism depends on the reaction conditions. Three kinetic models were developed based on the current understanding of the reaction mechanisms including hydrogenation and dehydrogenation steps on the metal sites, skeletal isomerization on the acid sites, and deactivation due to coke formation. Model A enabled the monomolecular isobutane formation path, model B the bimolecular, and model C both reaction paths. At high temperatures and low reactant dilutions, model B described the isomerization efficiency accurately but was not able to predict the increase in the selectivity to isobutane with increasing reactant dilutions at low temperature, opposite to models A and C. The kinetic modeling supported the conclusion that the prevailing mechanism depends on the reaction conditions.