abstracts - Институт катализа им. Г.К. Борескова

OP-III-32where δ - characteristic size of inter-particle gas gaps;d - mean value of catalyst particle diameter;D - diffusion coefficient (about 0.5 cm 2 /s. for propyl radicals at 1 atm and 700-800 K).Calculations show that if catalyst grain size is about 0.05 cm, characteristic times formono-molecular reactions of propyl radicals will exceed the τ D values. This means that intergraindiffusion offers the possibility of secondary heterogeneous transformations for propylradicals formed on external surface of catalyst particles or in inter-grain gas phase. As to bimolecularreactions with oxygen, they can increase lifetime of propyls in the gas phase andalso modify their reactivity in heterogeneous processes. This provides with additionalopportunities for the tuning of combined catalytic systems.A more profound analysis could be made based on the detailed modeling of reactionkinetics taking into account heterogeneous activation of alkane, diffusion in the gas phaseaccompanied with homogeneous reactions and, finally, alkyl radical reaction on the secondsurface responsible for the formation of desired product(s). Unfortunately, due to the scarce ofinformation about mechanism and kinetics of free radical reactions, especially heterogeneous,such modeling can be performed only for methane. The obtained results confirm thathomogeneous reactions become important for the overall process when the characteristic sizeof gas gaps exceeds 10 -3 -10 -2 cm.The above micro-kinetic and mass-transfer estimates are illustrated by severalexperimental examples, which demonstrate a significant increase of efficiency in combinedcatalytic systems, including:- oxidative coupling of methane over mechanical mixtures of catalytic and "inert" materials;- oxidative conversion of methane to synthesis gas in combined oxide-metal catalytic layer;- oxidation of C 3 -C 4 alkanes into unsaturated oxygenates.393