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polarisation on Pt/␥-Al2O3 catalysts. These sites then interact
strongly with the intermediate products, as observed in TPR anal-
electron-deficient metal particles in the vicinity of the metal-
support (Al2O3) created Lewis and Bronsted acid sites [70,71].
conversion of MCP was Ir/␥-Al2O3. This catalyst showed the abil-
ity to open MCP with an atom economy (SRO = 100%) at 180 ◦C.
Opening occurred at the secondary–secondary position following
the dicarbene mechanism. The Pt/␥-Al2O3 and Pt-Ir/␥-Al2O3 cat-
alysts became active at 220 ◦C to provide 100% selectivity toward
ring opening (atom economy).
•
3.2.4. Cracking selectivity
The order of reactivity in the ring opening of MCP was Pt-Ir/␥-
Al2O3 (100%, ˛ = 29%) = Pt/␥-Al2O3 (100%, ˛ = 5%) > Ir/␥-Al2O3
(88%, ˛ = 65%). Ir/␥-Al2O3 opened the ring of MCP via a dicarbene
mechanism that split C–C bonds at the secondary–secondary
positions. The Pt/␥-Al2O3 and Pt-Ir/␥-Al2O3 opened the ring via a
multiplet mechanism with C–C rupture at the secondary–tertiary
positions. The dicarbene pathway leads to the formation of 2-
MP and 3-MP as dominant products, whilst the multiplet path
favoured the formation of 2-MP, 3-MP and n-H. Pt-Ir/␥-Al2O3
behaved like its counterpart Pt/␥-Al2O3, whilst ring opening
possibly occurring only at Pt sites under our conditions. In the
Pt-Ir/␥-Al2O3 catalyst, a synergistic effect was observed but the
separate metal entities seemed to exist within the catalyst.
The ring opening products decreased with increasing tempera-
ture, whilst the cracking products increased. This observation was
ascribed to the possible formation of entities with a high number
of coordination sites on the catalyst surface. The most selective
catalysts for cracking were Ir/␥-Al2O3 and the Pt-Ir/␥-Al2O3. The
Pt-Ir/␥-Al2O3 catalyst behaved like its Ir/␥-Al2O3 counterpart in
the cracking reaction.
As observed in Table 2, the more selective catalyst for cracking
seemed to be Ir/␥-Al2O3 (11.7% at 220 ◦C), whilst the other cat-
alysts were inactive under the same conditions. In this case, one
part of the primary products formed by opening of MCP at the
secondary–secondary positions could not therefore be desorbed as
2-MP or 3-MP. These products remained adsorbed on the surface
of the catalyst and underwent multiple C–C breaking before des-
orption. Amongst the cracking products, C2 was the most common.
For the other catalysts, the cracking products started to form at
260 ◦C, where the cracking selectivity decreased in the order: 78.1%
Ir/␥-Al2O3 > 52% Pt-Ir/␥-Al2O3 > 5.4% Pt/␥-Al2O3. Again, Ir/␥-Al2O3
was more reactive toward multiple C–C bonds breaking com-
pared to Pt/␥-Al2O3 (about 15-fold) and Pt-Ir/␥-Al2O3 (1.5-fold).
Although the cracking product distribution was different amongst
the three catalysts, similarities were observed between Ir/␥-Al2O3
and Pt-Ir/␥-Al2O3, which desorbed C2 as the dominant product.
In this respect, the obtained results showed the hard reactivity of
Ir/␥-Al2O3 for cracking reactions and the Ir-like behaviour of the
Pt-Ir/␥-Al2O3 bimetallic catalysts. Increasing the temperature to
300 ◦C, selectivities for cracking reached 100% for Ir/␥-Al2O3 and
Pt-Ir/␥-Al2O3, with C2 as the major product, and 27.6% for the Pt/␥-
Al2O3 catalyst. The high selectivity in the cracking reaction for the
tions have already been reported for other catalysts [11,72–74].
We observed that for all the catalysts, the cracking selectivities
increased with temperature and were accompanied by simultane-
ous decrease in ring opening selectivity (Table 2). It could thus be
remarked that the most pronounced difference between the cat-
alysts appeared at low temperature. At high temperature, C1 was
the dominant product regardless of the catalyst, indicating that the
high temperature favoured methane formation. This behaviour was
attributed to the possible formation of metallic structures with low
•
only on Pt/␥-Al2O3. This behaviour was explained by the forma-
tion of carbonaceous entities on the Pt/␥-Al2O3 surface.
Acknowledgments
The authors thank REALISE (REseau Alsace de Laboratoires en
Ingénierie et Sciences pour l’Environnement) for facilitating this
study. I. Fechete gratefully acknowledges CNRS France for financial
support.
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A comparative study was performed between Pt-Ir/␥-Al2O3,
Pt/␥-Al2O3, and Ir/␥-Al2O3 catalysts in the conversion of MCP as
a function of temperature on an isodispersed metallic phase. The
results of this study suggested that:
•
The selectivity for ring opening varied with temperature and
nature of the metal employed. The most active catalyst in the
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