Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 17

1.6 Applications in Catalysis
We will give some examples of reactions catalyzed by the above complexes
with the aim to show the advantages of the synthesis of catalysts by surface
organometallic chemistry.

d n 9 r 4 n g | 1

1.6.1 Hydrogenolysis of Alkanes
As shown above the hydrides are prepared by treatment under hydrogen of
the surface alkyl complexes. However, instead of the expected neopentane,
only methane (and ethane in the case of group 4 elements) is observed in the
gaseous phase. This reaction is also observed when an alkane is contacted
with the hydride in presence of hydrogen. The key step is a b-alkyl transfer,
quite similar to the b-hydrogen transfer but which occurs more rarely
(Scheme 1.10). The driving force for the reaction is the hydrogenation of the
olefin which will shift the equilibrium to the right. Indeed, in absence of
hydrogen no reaction proceeds and the opposite reaction can be performed:
the catalyst polymerizes ethylene via the classical mechanism of insertion in
the Zr-C bond.
This reaction is of no interest for small alkanes but can become important
when applied to polyolefins, which can be considered as alkanes with a very
long chain. For example, it can be applied to the hydrogenolysis of waxes.
Indeed Fischer-Tropsch synthesis leads to the formation of such side
products which cannot be directly valorized industrially. Degradation by use
of a group 4 hydride allows their direct transformation into diesel fuel and
gasoline. 73,74
Another interesting example is the transformation of polymers into more
valuable compounds. For example, polystyrene can react with the zirconium
hydride in presence of hydrogen. Three reactions are observed: (i) breaking
of a carbon-carbon bond of the chain, (ii) breaking of a carbon-phenyl bond
leading to the evolution of benzene, and (iii) hydrogenation of the phenyl
group of the polymer. Depending on the conditions the last two reactions
can be prominent, leading to the formation of a new styrene/ethylene/vinyl
cyclohexane terpolymer with interesting physical properties. 75

.

Zr : b - Alkyl transfer

H 2

Zr IV Zr IV

R R C 2 H 6 +RH+Zr-H

Ta : a - Alkyl transfer

R

H 2

Ta III Ta V

R

CH 4 +RCH 3 +Ta-H

Scheme 1.10 Elementary reactions involved in the hydrogenolysis of alkanes by
supported hydrides of groups 4 and 5.