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O
HO
(1)
(2)
R
SO
3
H
SO
3
HO
OH
H
2
O
OH
d
n
9
r
3
n
g
|
0
n
OH
OH
Si
Si
H
H
H
H
H
H
O O O
O
O
O
O O O
O
O
O
O
O
(3)
(4)
H
2
O
d
y
y
f
n
n
3
.
HO
O
R
R
OH
OH
SO
3
SO
3
H
n
OH
OH
n
Si
Si
H
H
Diffusion
H
H
H
H
O
O O
O
O
O
O O O
O
O
O
Scheme 6.16
Mechanism of acylation of glycerol over functionalised silicas.
mechanism described is highly surface dependent. Silicas without functional-
isation have a strong anity for glycerol, however, the introduction of orga-
nosulfonate acid groups dramatically changes the adsorption/desorption and
diffusion dynamics of reagents and products. The surface nature of the catalyst
allows the single reaction of a moiety from glycerol (Scheme 6.16), with the
selectivities increasing as the chain length of the FFA increases to 12 to 18C.
When the product is formed, the a
nity of the compound for the catalyst
changes, diffusing more easily from the active site, leaving it free for an addi-
tional glycerol moiety to react. The selectivity would be expected to decrease in
the same conditions if acetic acid is employed instead of the free fatty acids, due
to the reduction of lipophilicity of the acid.
Barrault also justifies the introduction of ionic liquids in the silica matrix to
increase the stability and catalyst life, which is especially relevant when using
biodiesel glycerine as the feed for the acetylation process.
181
6.6.9 Hydrogenolysis
Hydrogenolysis of glycerol is selectively catalysed by heterogeneous catalysts
such as CuCrO
3
and Pd/C system. Others systems described in literature lead to
variable yields and selectivity (Pt/C, Cu/alumina, Raney Ni, Raney Cu,
Ru/alumina, etc.).
182
The mechanism proposed by Suppes and coworkers
comprises the conversion of glycerol to acetol as a first step and the hydroge-
nation of the ketonic compound.
183
Acetol formation in contrast, is generally
favoured by the presence of Lewis sites (Scheme 6.7), so it is produced as the
main product under low hydrogen pressure and in the presence of catalysts
such as CuCrO
3
, Pd/C, CuO-ZnO at temperatures above 180 1C.
182
This
mechanism would rule out previously proposed reaction paths such as the
formation of glyceraldehyde as an intermediate.
184
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