Chemistry Reference
In-Depth Information
glycerol. This makes any accurate economic evaluation of chemical
processes designed for its transformation rather dicult. Nevertheless, the
transformation of glycerol into more added-value products still remains a
key issue for developing a new bio-based chemical industry.
108-111
Of the several options focused on glycerol conversion, most involve li-
quid-phase transformations, but some gas-phase oxidative approaches
have also been attracting the interest of academic researchers and industry,
i.e., oxidation to acrylic acid
112-121
and ammoxidation to acrylo-
nitrile.
122-124
Concerning both reactions, processes proposed include ei-
ther the one-pot approach (with a single but bifunctional catalyst) or two
in-series catalytic beds. In fact, both reactions include a first step of acid-
catalyzed dehydration of glycerol to acrolein and a second step in which
acrolein is either oxidized to acrylic acid or ammoxidised into acrylonitrile.
Another option recently considered is the ammoxidation of an alternative
alcohol also derived from glycerol, allyl alcohol.
124
As regards the one-pot oxidehydration of acrolein to acrylic acid, the
catalysts with the best performance are those based on hexagonal-structure
tungsten oxide bronzes (HTBs) doped with V and Nb (Table 8.3). The key
property of the catalyst is that of fostering a fast consecutive oxidation step,
in order to avoid the formation of undesired heavy compounds generated by
both the ketal formation and the oligomerization reactions that take place
from the intermediately formed unsaturated aldehyde. The W-V-Nb mixed
oxide is characterized by a relatively large surface area and high concen-
tration of stronger acid sites: two properties which are also important for an
ecient dehydration of glycerol to acrolein. Compared with the bicompo-
nent W-V HTBs, the presence of Nb leads to a remarkable increase in surface
acidity, and also to the development of a mixed oxide characterized by a
larger and more stable surface area.
112
On the other hand, the presence of V
confers on the catalyst the redox properties needed for the ecient oxidation
of acrolein to acrylic acid.
Excellent performance was also obtained with a two-bed reactor
configuration, where the first catalyst showed acidic features (for example,
zeolite ZSM5) and the second catalyst oxidizing properties for acrolein
transformation, such as a V-Mo mixed oxide.
119,120
A detailed study of the reaction mechanism occurring in the oxidation
of glycerol to acrylic acid was carried out by Ueda and co-workers
116
(Scheme
d
n
4
r
4
n
g
|
3
.
8.6). The
first
loss
of water
from glycerol
leads
to
Table 8.3
Summary of catalytic behavior for the oxidehydration of glycerol to acrylic
acid.
Inlet feed: glycerol:
oxygen : water (mol%)
Yield of acrolein,
acrylic acid (%)
Catalyst
T (1C)
Ref.
W-V-O
318
2 : 4 : 54
29, 18
114
W-V-Nb-O
265
6 : 12 : 40
3.5, 50.5
113
Mo-V-Nb-Te-O
N.R.
3 : 6 : 19
1.7, 28.4
116
Fe-V-O/Fe-O
300
1.5 : 1.7 : 30.3
28, 14
117
Search WWH ::
Custom Search