Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 20

( SiO)Ta V Cl 2 Me 2 precatalyst. During the initiation step ethylene reduces this
complex, allowing then the generation of the key metallacycle intermediates
and thus the selective production of the requested 1-hexene. DFT calcula-
tions have explained this unexpected reactivity and why other tantalum
compounds are not active. 80

d n 9 r 4 n g | 1

1.6.6 Epoxidation and Deperoxidation Reactions
Such reactions involve oxygen or oxygenated molecules and so cannot be
catalyzed by alkyl or hydride complexes such as those depicted above.
However, by reaction of these species with alcohols it is possible to obtain
new well-defined surface complexes which can be used for these reactions.
Mono-, di- and tri-grafted titanium complexes can then be obtained which
are active systems for the epoxidation of oct-1-ene. A study of these different
systems and of their transformation during the catalytic reaction has
allowed an understanding of the behavior of the industrial epoxidation
catalyst developed by Shell, which deactivates rapidly at the beginning before
becoming stable upon time. 81
Another example is the asymmetric epoxidation of allyl alcohol. In solu-
tion this reaction is usually made by use of the Sharpless catalyst which is
based on a titanium complex with a tartrate ligand. A system has been de-
signed on the surface by taking into account that the coordination sphere of
the metal must contain one bond with the surface, two bonds for coordin-
ation of the tartrate ligand and it must also accommodate the two reagents,
allyl alcohol and the peroxide. As a consequence five bonds are needed
around the metal and so a group 5 metal complex should be expected, such
as tantalum. The coordination sphere around the metal can be built by (i)
grafting reaction of the Ta[CH 2 -C(CH 3 ) 3 ] 3 [ ¼ CH-C(CH 3 ) 3 ] complex on silica
dehydroxylated at high temperature (formation of the surface-metal bond);
(ii) reaction of the grafted complex with ethanol in order to replace the alkyl
and alkylidene ligands by ethoxy ones; (iii) addition of the asymmetric lig-
and, ( þ )-diisopropyl tartrate (Scheme 1.13). The resulting system is active for
the asymmetric epoxidation of allyl alcohol with results quite comparable to
those achieved in homogeneous catalysis. 69
A third example is the deperoxidation of cyclohexyl hydroperoxide. An
important way of synthesis of adipic acid developed by Rhodia passes
through the oxidation of cyclohexane in three steps. In a first step cyclo-
hexane is oxidized, by a classical Fenton mechanism, into a mixture of
cyclohexyl peroxide, cyclohexanol and cyclohexanone. In a second step, this
mixture is transformed into cyclohexanol and cyclohexanone by oxidation of
the peroxide. Finally, the two oxygenated compounds are oxidized into
adipic acid by nitric acid. Surface organometallic chemistry was used in
order to find a heterogeneous system for the second step. Indeed this re-
action is usually made in solution in presence of a chromium salt which is
highly toxic. The aim of this study was mainly to obtain a purely hetero-
geneous system without any leaching even if

.

it was not very active.