Food Waste and Catering Waste; Focus on Valorisation of Used Cooking Oil and Recovered Triglycerides - The Economic Utilisation of Food Co-Products - page 117

6.6.6.1 Selectivity and Mechanism of Formation of Acrolein and
Acetol
However important for process optimisation, research into catalyst selection
should be coupled with a fundamental understanding of the reaction
mechanism. Theoretical studies found 3-hydroxypropanal to be the most
accessible product via (Bro¨ nsted) acid catalysed dehydration of glycerol
(E a ¼ 22.7 kcal mol -1 compared to acetol E a ¼ 24.9 kcal mol -1 ). 3-Hydroxy-
propanal, is very unstable at high temperatures and undergoes a second dehy-
dration to acrolein. 143 Furthermore, the investigation of acetol stability in the
presence of catalysts with high numbers of Lewis sites demonstrated little
formation of oligomers or coke under the same conditions that acrolein would
do. In addition, the use of the same catalyst using glycerol as a feed demon-
strated an increase of acetol formation with respect to the production of
acrolein. 144 Therefore, there seems to be a correlation between the acid site type
and the products generated, where the presence of Lewis acid sites in the catalyst
will promote the dehydration of glycerol to acetol and the presence of Bro¨ nsted
acid sites would favour acrolein formation, as shown in Schemes 6.8 and 6.9. 145
The understanding of this mechanism might also be relevant to the production
of other chemicals, where selectivity could be tuned to the production of glycerol
(2,3-Epoxypropan-1-ol) 146 or allyl alcohol (2-propenol), for instance. The latter
has been obtained with good selectivity and acceptable yields by using Fe 2 O 3
nanoparticles. 147 The presence of hydroxyl groups on the surface of the catalyst
and its topology seem to play a fundamental role in both the dehydration of
glycerol to acrolein and subsequent hydrogen transfer with glycerol as a donor to
the carbonyl functionality of acrolein reaching yields of 20-25%.

d n 9 r 3 n g | 0

d y y f n n 3 .

(1)

OH

HO

OH

OH HO OH

HO OH

OH

HO

H

+

+

H

HO

O

O

O

HO

HO

M M M M

M M

M M

(2)

OH O

OH OH

ACETOL

(3)

HO

HO O

+H 2 O

M M

M M

Scheme 6.8 Dehydration of glycerol in the presence of Lewis-acid catalyst.