tentatively assigned to a combination of C–C stretching
and CH3 deformation mode of methylacetylene molecules
adsorbed on silanols and chromium sites, respectively.
which instantaneously are able to form an aromatic ring. A
highly coordinatively unsaturation of the active Cr sites is
inferred to explain the reported IR data.
The immediate formation of the aromatic molecule is testi-
fied by the evolution of the spectra reported in Fig. 4(c). Up
to Tboil (dotted and bold spectra), no significant IR features
are observed in this spectral region, we just note a slight back-
ground fluctuation. Upon further increase in T the ring
stretching of the aromatic molecule is observed at 1618,
1608, 1577 and 1507 cmꢁ1. In particular, the presence of the
1618, 1608 cmꢁ1 doublet implies that the formed aromatic
molecules do not posses a centre of symmetry inversion.39
The formation of 1,3,5-trimethylbenzene (mesitylene) is testi-
fied by the shape of summation bands of out-of plane aromatic
C–H bending vibrations shown in the inset of Fig. 4(c).39
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On the perturbation of the silanol modes by acetylene and
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3750 cmꢁ1 is only partial, while it is almost complete upon
interaction with methylacetylene (compare the bold spectra
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T
boil . In this first interval, the dominant thermodynamic para-
meter is the equilibrium pressure. At higher temperatures than
boil , the adducts formed on the unreactive silanols undergo a
T
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mium sites start to react. In both cases, acetylene and methyl-
acetylene result in the immediate formation of benzene and
1,3,5-trimethylbenzene, respectively (Schemes 1 and 2), with-
out the evidence of any measurable intermediate product in
the time scale reachable by these experiments (5 s). No other
by-products are observed. As for the reaction path reported
in Scheme 2, the presence of other isomers is in principle pos-
sible, however, the repulsions between two adjacent CH3
groups in the intermediate precursor would favour the sym-
metric compound.
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Scheme 1
37 S. Bordiga, G. Ricchiardi, G. Spoto, D. Scarano, L. Carnelli,
´
A. Zecchina and C. Otero Arean, J. Chem. Soc., Faraday Trans.,
1993, 89, 1843.
38 S. Bordiga, S. Bertarione, A. Damin, C. Prestipino, G. Spoto,
C. Lamberti and A. Zecchina, J. Mol. Catal. A, 2003, in press.
39 N. B. Colthup, L. H. Daly and S. E. Wiberley, Introduction to Infra-
red and Raman Spectroscopy, Academic Press, New York, 1975.
´
40 C. Paze, S. Bordiga, C. Lamberti, M. Salvalaggio, A. Zecchina
and G. Bellussi, J. Phys. Chem. B, 1997, 101, 4740.
Scheme 2
Phys. Chem. Chem. Phys., 2003, 5, 4414–4417
4417