Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 5

pairs of protons involved in hydrogen bonds are seen but also protons at a
slightly higher distance. An intense signal is observed for pairs of isolated
protons even on silica treated at 700 1C (for which the OH density is 0.7 OH
nm 2 ), probing unambiguously that the repartition of the hydroxyl groups is
not fully homogeneous. 43 Additionalexperimentsbytriplequanta 1 HMAS
NMR or spin counting were performed. 43,44 These experiments showed that
some hydroxyl groups are present as nests of three silanols located on a cycle
containing six silicon atoms as those observed on the (111) face of cristobalite.
In that case, the distance between the hydrogen atoms will be ca. 3Å.One,two
or three cycles can be adjacent.
Finally, upon heating at a very high temperature (ca. 1000 1C), highly
strained cycles containing two silicon atoms and two oxygen atoms are
formed by condensation between two adjacent silanols. These cycles are
highly reactive even if their amount is low (0.14 nm 2

d n 9 r 4 n g | 1

on silica dehy-

droxylated at 1000 1C while the amount of residual hydroxyl groups is

0.4 nm 2 ).
Scheme 1.1 summarizes the variety of species which are present on
dehydroxylated silica as deduced from these studies. Depending on the
nature of the silica (non-porous flame silica, mesoporous silica, etc.) the
relative amount of these species will be different, leading then in some cases
to different reaction products.
As shown above, even if silica can be considered as the simplest support,
there is not only one species on its surface. For other oxides such as alumina
the situation is more complicated and more than five different types of hy-
droxyl groups can be observed, with all their combinations, without taking
into account the Lewis acid sites. The most complex support is probably
carbon, as its surface contains a lot of functional groups covering all the
fields of organic chemistry. This complexity of the surface support will have
a consequence on the number of species which will be obtained upon re-
action with organometallic complexes as the strength of the bond between
the metal and the surface will be more or less strong. It is for this reason that
preliminary studies are always made on silica and mainly on silica dehy-
droxylated at relatively high temperature (500 or 700 1C).

.

Scheme 1.1 Hydroxyl groups and siloxane cycles on the surface of silica.