Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 4

Table 1.2 Grafting sites of some supports which can be used in surface
organometallic chemistry.

Support Grafting sites

Alumina Hydroxyl groups (at least five different types); Al-O-Al bridges

(where the bond is not covalent); Lewis acid sites

d n 9 r 4 n g | 1

Silica-Alumina Silanol groups; Si-O(H)-Al bridges

Magnesia Hydroxyl groups; lacunar magnesium sites

Zeolites Protons with different locations which can be more or less

accessible; extra-framework aluminium sites; silanol groups
on the external surface of the crystallites

Carbon All chemical functions can be found: alcohols, amines, ethers,

thiols, ketones, aldehydes, carboxylic acid, etc.

Clays Depending on the distance between the layers the possible

grafting (or exchange) sites can be more or less accessible

Polymers All non-inert chemical functions can be anchorage points for

organometallic complexes

hydrogen bonds. Upon heating at higher temperature, condensation be-
tween two neighboring hydroxyl groups occurs, leading to the evolution of
water molecules and formation of Si-O-Si bridges. As a consequence, the
intensity of the broad infrared band decreases and a new sharp band, at-
tributed to isolated silanol groups, appears at ca. 3750 cm 1 . After heating at
500 1C only isolated silanols are present. Their amount can be determined by
chemical methods (such as by their reactivity with CH 3 Li) or physical tech-
niques (such as quantitative solid-state 1 H MAS NMR). The two values can be
different as the first method will only quantify the hydroxyl groups accessible
to the reagents while the second one will give an estimation of the total
number of hydroxyl groups. In the case of microporous solids the difference
can be very important. In the case of flame silica, which is non-porous, the
two methods lead to an OH density of ca. 1.4 OH nm 2 . 41,42 Upon heating
under vacuum at 700 1C the OH density decreases to ca. 0.7 OH nm 2 . For
such low values, one can reasonably suppose that the hydroxyl groups are far
away from each other and so well-defined grafted organometallic isolated
species will be expected upon reaction with these hydroxyl groups. This is
the key point of surface organometallic chemistry. However, this view is not
fully realistic even if it is sucient in most cases; sometimes a more precise
description of the silica support is needed for explaining the experimental
data. First of all, there are not only monohydroxyl Si-OH groups on the
surface but also dihydroxyl ones ¼ Si(OH) 2 , as evidenced by solid-state 29 Si
MAS NMR. These ¼ Si(OH) 2 groups give infrared bands at the same position
as the Si-OH ones and so they cannot be distinguished by this method. On
the samples heated at high temperature the 29 Si MAS NMR spectra become
broader, preventing the separation of the different Si(OSi) 4 x (OH) x (x ¼ 0-2)
groups and so the presence of some ¼ Si(OH) 2 species cannot be excluded
even if their amount is probably very low. Studies by 1 H double quanta MAS
NMR are more informative. This method allows the observation of protons
pairs which are separated by less than ca. 5 Å. As a consequence, not only

.