HYDROGENOLYSIS OF n-HEXANE
491
marked effect (Table 3). Increasing temperatures should (6% Pt/SiO2) were higher at 6-fold hydrogen excess, but
have decreased the hydrogen coverage at constant H2 pres- the Mf values were as low as 2–3 as opposed to Mf � 8 at
sure. The hydrogen coverage should have increased at a 48-fold hydrogen excess (16). Some authors regarded the
higher p(H2) values. However, both increasing temperature formation of “Ir–C” ensembles less likely (5, 6, 12). How-
and hydrogen pressure increased hydrogenolysis activity. ever, S a´ rk a´ ny attributed the isomerization ability of Ir to
A closer inspection of hydrogenolysis characteristics seems some adsorbed hydrocarbons also influencing the method
to resolve this apparent disagreement. The results indicate of reactive adsorption of the alkane feed (33). Ponec (2, 12,
two more or less independent fragmentation processes su- 34) reported higher hydrogenolysis selectivity at the low-
perimposed on each other: single and multiple hydrogenol- and the high-temperature ends of the temperature range.
ysis. The former is favored at higher hydrogen coverages: Such a marked temperature effect was not observed here.
at lower temperatures (Fig. 1, Table 2) and higher hydro-
To sum up, dispersion and Cl content strongly influence
gen pressures (Table 4). As the amount of available surface the activity of Ir/Al2O3 catalysts but the selectivity changes
hydrogen decreased, more and more molecules suffered a are not profound. The overwhelming hydrogenolysis arises
total disruption in addition to the less and less important by superposition of a “single” and a “multiple” rupture,
single hydrogenolysis (Tables 2 and 4). The competition of higher temperatures and lower hydrogen pressures favor-
the two processes is seen as a function of hydrogen pressure ing the latter process.
at 513 K, whereas single splitting is no more important at
6
03 K (Table 4). The continuous decrease of Mf values with
ACKNOWLEDGMENTS
the temperature indicates a “real” multiple rupture, i.e., the
disruption of the reactant into several fragments during one
A.M. thanks the French Ministry of Foreign Affairs for supporting her
sojourn on the surface rather than “slicing off” end methyl practice semester in Budapest during which the experiments were done.
S.B. thanks the French Space Agency, CNES (Centre National d’Etudes
Spatiales) for financial help. Z.P. is grateful to the University of Poitiers
for inviting him as a guest professor, rendering the completion of the
manuscript possible during this stay.
groups step by step. The value of Mf decreased from 17
to 8 from 450 to 550 K on Ir/Al2O3 (32). These values are
somewhat higher than those reported here. With the contin-
uous decrease of Mf from � 8 to below 1 (Table 2), its value
necessarily will reach and cross unity at a certain temper-
ature. This, however, may be an intermediate stage rather
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6
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8
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1
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1
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(
1
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