Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 19

d n 9 r 4 n g | 1

Scheme 1.12 Catalytic cycle proposed for the transformation of ethylene into pro-
pene on the supported tungsten hydride.

30000

25000

i-Pr

i-Pr

20000

N

t-Bu 15000

W

O

10000

Si

O

5000

O

O

0

0 200 400 600 800 1000 1200 1400

.

Time (min)

Figure 1.4 Propene metathesis (TON as a function of time) for the supported
tungsten imido carbene shown on the left.

1.6.4 Olefin Metathesis
As shown above, olefin metathesis catalysts derived from those developed by
Schrock and colleagues can be supported on silica. 57 The resulting materials
can be very active for olefin metathesis. For example the system shown in
Figure 1.4 can have turnover numbers higher than 20 000 after 20 h.
Recently, more active systems based on tungsten oxo complexes were
developed and patented. 78 Typically these catalysts are based on the grafting
reaction of W( ¼ O)F[CH 2 C(CH 3 ) 3 ] 3 on an oxide support and lead to very high
activities in the metathesis of propene.

1.6.5 Trimerization of Ethylene
Selective trimerization of ethylene to 1-hexene has been one of the most
exciting developments of olefin catalysis in the past decades. A supported
catalyst for the ethylene oligomerization has been designed very recently by a
molecular approach. 79 It operates eciently without any co-catalyst and
delivers 1-hexene in good selectivity. This system has been obtained by
grafting reaction of TaCl 2 Me 3 on dehydroxylated silica, leading to the