Chemistry Reference
In-Depth Information
OH
2
OH
(1)
OH
O
HO
OH
+
HO
OH
+
+
d
n
9
r
3
n
g
|
0
OH
2
(2)
HO
OH
H
3
O
+
HO
OH
+
(3)
-H
2
O
O
O
HO
OH
HO
d
y
y
f
n
n
3
.
HYDROXYPROPANAL
ACROLEIN
(4)
O
OH
H
3
O
+
H
2
O
+
+
Scheme 6.9
Dehydration of glycerol with Bro¨ nsted-acid catalyst.
6.6.7 Oligoglycerol and Ethers
Oligomers, and alkyl ethers of glycerol might find applications in defined
markets within food industries, as emulsifiers, additive stabilisers and fuel
additives (Scheme 6.10).
148
Diglycerol and other oligoglycerols such as trig-
lycerol and glycerol alkyl ethers are compounds with high commercial
potential. There are a number of applications where they are employed as
precursors of
renewable-based lubricants, antifogging and antifoaming
agents,
149
components of sunscreen formulations,
150
corrosion and cold-
property improvers in mineral fuels and biofuels.
151
Oligomerisation of glycerol, is promoted by strongly alkaline conditions and
high temperatures (above 200 1C, generally 220-240 1C), using either homo-
geneous reagents with different solubility (Na
2
CO
3
, NaOH, CaO, MgO)
152,153
or functionalised silicas and mesoporous catalysts (Cs MCM-41).
154
The use of
sodium or potassium carbonate is favoured over the equivalent hydroxides for
the activation of glycerol. This is considered to be due to better solubility. The
selectivity, however, is poor, yielding a complex reaction mixture leading to a
wasteful work-up (Scheme 6.11). Nevertheless, examples of selective synthesis
to linear oligomers have been shown with the use of high surface area activated
CaO and MgO as catalysts with high conversion and selectivity to linear di- and
triglycerol as shown in Scheme 6.13.
155
Other examples relevant to this work are the production of methyl-glycerols,
mono- and diethers. Methyl glycerols have reportedly been used as a cryo-
preservation agent
156
and as an intermediate for the synthesis of dioxolanes,
157
which are well-known fuel additives.
158
In addition the selective methylation of
glycerol with dimethyl sulfate (DMST) in the presence NaOH has been studied
by a number of authors.
159,160
Methyl-O-glycerols have found applications as
diesel additives and as a cryogenation agent (Scheme 6.12).
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