abstracts - Институт катализа им. Г.К. Борескова

KS-I-1THE EFFECTS OF ACTIVE SITE COMPOSITION AND STRUCTURE ON THEMECHANISM AND KINETICS OF HYDROCARBON OXIDATIONBell A.T.Department of Chemical Engineering, University of California, California, USAE-mail: alexbell@uclink.berkeley.eduIt is well established empirically that the activity and selectivity of hydrocarbon oxidationcatalysts are affected by the local composition and structure of the active sites. However, theprecise definition of what is meant by an active site is usually difficult or impossible toestablish for catalysts consisting of either bulk or dispersed metal oxides, and as a resultdiscussions of structure-performance relationships are often imprecise and lead to longstandingcontroversy. These difficulties arise from the presence of a multitude of possibleactive sites on bulk and dispersed materials and the lack of experimental methods that canenable the isolation of those sites exhibiting the highest level of activity. In an attempt tosurmount this limitation, there has appeared a growing interest in the experimental andtheoretical investigation of catalysts consisting of isolated-sites, some times referred to also assingle sites, for which all of the active centers are exactly or nearly the same. Examples ofsuch sites are monomeric metal-oxo species supported on an oxide, supported andunsupported heteropolyacids, and dissolved organometallic complexes. This talk willillustrate the level of detail that can be obtained from studies of methane oxidation occurringon isolated molybdate species supported on silica on iron oxo species cation-exchanged intoZSM-5, and the epoxidation of cyclooctene on iron(III) porhpyrin complexes. It will beshown that in the case of isolated molybdate species, experimental evidence, supported byquantum chemical calculations, lead to the conclusion that the oxidation of CH 4 by O 2 toCH 2 O proceeds via a peroxide intermediate. Peroxide species are also thought to be theactive intermediate in the case of CH 4 oxidation by N 2 O over Fe-ZSM-5. Theoreticalcalculations show why in this case CO 2 and H 2 O are formed rather than CH 3 OH or CH 2 O.The epoxidation of cyclooctene and several other olefins by H 2 O 2 oniron(III)tetrakisperfuorophenyl porphyrin catalysts is shown to be sensitive to the compositionof the axial ligand associated with the porphyrin, the solvent in which the reaction is carriedout, and coordination of the olefin to the iron center of the porphyrin. The active intermediatefor expoxidation is found to be an iron(IV) pi-radical cation formed via the heterolyticcleavage of the O-O bond of H 2 O 2 coordinated to the iron center in the porphyrin. The extentof heterolytic versus homolytic cleavage of the O-O bond dictates the selectivity of H 2 O 223