Catalysis of hydrocarbon conversion over zeolite-supported platinum-rhenium bimetallic clusters

The existence of mixed PtRe clusters in Y-zeolites is manifested from the high hydrogenolysis activity leading to methane exhibited by bimetallic catalysts compared to monometallic catalysts when converting n-heptane or cyclopentane. For instance, the turnover frequency of methane formation from cyclopentane hydrogenolysis over a PtRe/NaY catalyst at 240{dollar}spcirc{dollar}C and 1 atm, is 187 times higher than the average value of those over Pt/NaY and Re/NaY.; PtRe/Y-zeolite catalysts prepared from Re{dollar}sb2{dollar}(CO){dollar}sb{lcub}10{rcub}{dollar}, NaReO{dollar}sb4{dollar}, or (Re(NH{dollar}sb2{dollar}C{dollar}sb2{dollar}H{dollar}sb4{dollar}NH{dollar}sb2)sb2{dollar}O{dollar}sb2{dollar}) Cl, all contain significant amounts of bimetallic clusters that can be detected by hydrocarbon hydrogenolysis reactions. Although ion exchange is an attractive method for preparing metal-containing zeolites, the low efficiency of exchanging (Re(NH{dollar}sb2{dollar}C{dollar}sb2{dollar}H{dollar}sb4{dollar}NH{dollar}sb2)sb2{dollar}O{dollar}sb2rbracksp{lcub}+{rcub}{dollar} ion into NaY depreciates this precursor. As expected, PtRe/NaY catalysts prepared from NaReO{dollar}sb4{dollar} impregnation do not exhibit an extent of alloying as high as those from Re{dollar}sb2{dollar}(CO){dollar}sb{lcub}10{rcub}{dollar} deposition, because the anionic nature of ReO{dollar}sb4sp{lcub}-{rcub}{dollar} decreases its chance of staying inside zeolite particles. Thus, deposition of Re{dollar}sb2{dollar}(CO){dollar}sb{lcub}10{rcub}{dollar} onto Pt/NaY is a more promising way to prepare bimetallic zeolites than the other two methods used.; The mobility of Re species is indispensable for the formation of mixed PtRe clusters on catalyst supports. Migration of Re oxide species in calcined samples is not confined within a zeolite particle. Interparticle migration of Re oxide can be inferred from the observation that bimetallic clusters are detected via the probe reactions in a physical mixture of Pt/NaHY and Re/NaHY.; Presulfided PtRe/NaY exhibits superior activity maintenance to presulfided Pt/NaY, which is reminiscent of alumina-supported catalysts used in naphtha reforming. This result agrees with a model which assumes that the sulfur atoms strongly adsorbed on the Re atoms of mixed PtRe ensembles impose geometrical hindrance on the growth of harmful graphitic entities.