Kinetic modeling of metal-catalyzed reactions of alkanes

Linear alkanes having two to four carbon atoms react with H-2 by hydrogenolysis, giving smaller alkanes, or in the case of n-butane also by isomerization to isobutane. The dependence of the rates on H-2 pressure is described by rate equations derived from mechanisms involving a partially dehydrogenated reactive intermediate; the precise form of the equation depends on the number of H atoms (zero, one, or two) assumed to take part on the slow step. An equation in which one H atom is used has been applied to the results obtained with Ru, Rh, and Pt catalysts and provides values for the rate constant k and for the equilibrium constants for H-2 chemisorption K-H and alkane dehydrogenation K-A. Both k and K-A increase with temperature on Ru and Pt catalysts, leading respectively to values of the true activation energy E-t of similar to 40-80 kJ mol(-1) and of enthalpy changes of 60-90 kJ mol(-1). Deficiencies of such rate equations include failure to allow quantitatively for carbon deposition and the assumption that both reactants utilize the same sites. Possible modifications to meet these criticisms will, however, require more complex rate equations, from which significant numerical values of the constants may not be obtainable.