A MORE GENERAL-APPROACH TO DISTINGUISHING HOMOGENEOUS FROM HETEROGENEOUS CATALYSIS - DISCOVERY OF POLYOXOANION-STABILIZED AND BU(4)N(-STABILIZED, ISOLABLE AND REDISSOLVABLE, HIGH-REACTIVITY IR-APPROXIMATE-TO-190-450 NANOCLUSTER CATALYSTS())

A more general approach to distinguishing between so-called homogeneou s vs heterogeneous catalysts has been developed and intrinsically: tes ted in answering the question ''what is the true catalyst in the activ e hydrogenation system which evolves from cyclohexene, hydrogen, and t he discrete, polyoxoanion-supported Ir(I) catalyst precursor (Bu(4)N)( 5)Na-3[(1,5-COD)Ir.P2W15Nb3O62]?''. The approach developed and utilize d consists of four categories of experiments: (i) catalyst isolation a nd characterization studies, with an emphasis initially,on TEM (transm ission electron microscopy); (ii) initial kinetic studies, emphasizing whether or not the isolated catalyst can account for the observed kin etics, especially any induction period seen, and whether pr not the re action exhibits a +/-10% reproducible rate; (iii) quantitative phenome nological catalyst poisoning and recovery experiments;(iv) additional kinetic and mechanistic studies and chemical tests, all interpreted wi th strict adherence to the principle that the correct description of t he catalyst (i.e., the correct mechanism) will explain all of the data . The present approach has identified a previously unknown type of hyb rid homogeneous-heterogeneous, Ir-similar to 190-45.polyoxoanion/BU4N catalyst of average composition [Ir(0)(similar to 300) (P4W30Nb6O1231 6-)(similar to 33)](BU4N)(similar to 300)Na-similar to 233. The discov ery of, and ability to distinguish, even an unprecedented hybrid homog eneous-heterogeneous catalyst strongly suggests that the present appro ach will be more generally applicable to the difficult and often unsol ved mechanistic problem of distinguishing catalysis by a discrete, hom ogeneous metal complex vs that by a soluble metal nanocluster or collo id ''heterogeneous'' catalyst. Some false starts and incorrect leads i n the early stages of this work are discussed, research which illustra tes some of the pitfalls to be avoided in attempts to distinguish homo geneous from heterogeneous catalysts. A minimum mechanistic scheme for the catalyst's evolution, consisting of the autocatalytic generation of the Ir-similar to 190-450 nanoclusters, is shown to account for all of the observed results, including the findings of the rate-enhancing effects of H+, H2O, and acetone impurities that were puzzling in the earlier stages of this work, before the Ir-similar to.190-450 nanoclus ter catalysts were identified.