NOTES
1460
Similar results were obserrcd for isomerization a t 471°, but a t 527" t'he existeiice of a maximum was not clearly demonstrated. However, the data a t 527' are not sufficient to exclude the existence of a maximum between 5.8 and 20 atm. of hydrogen, or a t a still higher pressure. Discussion The role of hydrogen in the kinetics of dehydrocyclization izj interpreted as follotvs: The dehydrocyclization :reaction involves a preliminary dehydrogenation step prior to ring closure. Increasing hydrogen pressure might then be expected to suppress the initial dehydrogenation step and hence decrease the rate of dehydrocyclization, as observed for hydrogen pressures above 5.8 atmospheres in the present study. However, a t lower hydrogen pressures, background reactions involving extensive dehydrogenation and polymerization become important. These reactions lead to extensive coverage of the platinum by hydrogen deficient hydrocarbon residues. The limiting factor in the reaction is then the rate of removal of the residues by reaction with hydrogen, thus accounting for the beneficial effect of hydrogen on the rate. Studies on the dissociative chemisorptioii of hydrocarbons over a variety of met:& supply direct evidence for the formation of such hydrogen deficient residues, and for their removal by reaction with hydrogen.6 Thc ability of hydrogen to clean up a platinum catalyst cxposed to benzene a t high temperature' also has been demonstrated. Similar considerations on the rob. of hydrogen also apply to the isomcrization reaction, although the maxima in rates may occur a t different hydrogen pressures. The temperature coefficients of the rc:iction rates varg markedly with hydrogen pressure. At low hydrogen pressurc the temperature cocfficients are !jmall and can even be negative, as observed for i sonieri zation . Apparently the amoiin t of surface not covered by residues decreases appreciably wit,h temperature. At higher hydrogen prcssiires, where coverage of the platinum by hydrocarhon residues is presumably not a major limitation, the temperature coefficients are much higher. The effects, of hydrogcn reported here are quitc similar t o those found in the isomerization-dch~drois~meri:zatio~i of methylcyclopentane ovcr a similar catalyst,*and would appear to be charxctmistic of reactions over bifunctional cat#alysts such a,splatillurn-on-alumina. (fi)
A . I
