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maximum acrylic acid yield (28%) at low glycerol concentra-
and feed molar ratios open important questions. Despite the
fact that all of the studied materials efficiently dehydrated
glycerol into acrolein and that they all had V as the only active
element to perform the partial oxidation, their catalytic behav-
ior is sometimes opposite. This highlights that close proximity
of acid and redox sites (i.e., V ions) at the atomic level is man-
datory to efficiently perform the oxidehydration reaction of
glycerol on a single catalyst.
tions (feed composition glycerol/oxygen/water/nitrogen=
1
:2:40:57 mol%).
At higher pressures of the reactants, both partial and total
oxidation were hampered, which provoked an increase in the
intermediate product (acrolein) and finally the formation of
heavy compounds.
At this stage, it is worth mentioning that V-P oxides were
[
23]
previously studied by others for this reaction. Among the V-
P-O materials prepared, it was found that the one treated at
In the pursuit of the intimate relations that link the catalyst
structure and the catalytic results, in situ FTIR spectroscopy
analyses were performed by studying the oxidation of the in-
termediate product, that is, acrolein, on the different multi-
functional catalysts. Attempts were also made to adsorb glyc-
erol on the surfaces of the catalysts, but its high boiling point
prevented it from being transferred into the IR cell in a control-
8008C was the best performing phase, with acrolein selectivity
up to 64% but only trace amounts of acrylic acid. Moreover,
regardless of the catalyst thermal treatment, acrylic acid was
always produced in trace amounts. Although the different re-
sults obtained in this work upon using the industrial VPP cata-
lyst could be explained by taking into account the fact that
the physicochemical properties (e.g., surface area, heat treat-
[46]
lable fashion, as previously reported. This made it impossible
to control the stoichiometry of the two reactants, glycerol and
oxygen, inside the IR cell. In 2014, C. Sievers and co-workers re-
ported an ex situ method that could be used to overcome this
[
22]
ments, preparation method, etc.) of the latter catalyst are
different than those of the previously reported V-P oxides, it is
safe to say that the major role is actually played by the reac-
tion conditions. Indeed, upon testing the industrial catalyst
under the same reaction conditions as those reported in
Ref. [23], the catalytic performance was almost identical for
both catalysts (see Figure S3).
[43]
issue with some catalysts. Indeed, the catalyst could be slur-
red for 24 h in an aqueous solution of glycerol, and water was
finally removed. However, V-based systems are known to leach
vanadium species if dispersed in aqueous solutions, which un-
fortunately made the application of this ex situ method unreli-
able, if not impossible, in the case of the bifunctional catalytic
systems reported herein.
To further explore the structure–reactivity correlations of
multifunctional catalysts for the oxidehydration of glycerol, the
catalytic behavior of the WV sample was compared to that of
the ion-exchanged catalyst, VO-WO (Figure S4b). Despite the
x
In situ FTIR spectroscopy study with acrolein
formation of acrolein on VO-WO , acrylic acid was formed in
x
just minor amounts (3%), whereas CO was formed in remark-
In the first step of the one-pot oxidehydration of glycerol, that
is, dehydration of glycerol to acrolein, acid sites of specific acid
strength and type are required to favor the desired dehydra-
x
able amounts. This may be related to the lower redox behavior
of the V ions, which decreases the oxidation rate of the acrole-
in intermediate and favors secondary reactions (finally leading
to heavy compounds). Despite the fact that the content of va-
nadium in the ion-exchanged sample was lower than that in
[42]
tion reaction to acrolein. The presence of Lewis acid sites
and, particularly, Brønsted acid sites is fundamental to direct
the dehydration of the secondary hydroxy group selectively to
[43]
WV (VO-WO 0.15 vs. 0.21, see Table S1), the distinct catalytic
form acrolein. In the second step, that is, the oxidation of
acrolein to acrylic acid, both Lewis acid and redox sites are in-
x
behavior cannot be attributed merely to the different composi-
tions; indeed, previously reported V-containing HTBs with in-
framework V species and a similar V/W ratio to that in VO-WOx
showed acrylic acid selectivity >20% under identical reaction
conditions (see Ref. [14] and Figure S4A). Moreover, differently
[47]
volved. Depending on the surface properties of the catalyst
(acid–base character, nature of oxygen species, and V surface
sites), the adsorbed intermediate species evolve into different
products such as acrylic acid, acetaldehyde, acetic acid, and
[48]
from WV, VO-WO did not show any significant change in cata-
COx.
x
lytic performance upon exposure to higher partial pressures of
the reactants (see Figure S4B). These results seem to point out
that the presence of V species in the framework positions en-
hances the oxidation properties of the transition element.
However, as discussed in the in situ FTIR spectroscopy studies
Preliminary evaluation of the interaction of acrolein with the
surfaces of the catalysts was performed under anaerobic condi-
tions at room temperature (Figure 6). For the WV, VO-WOx,
VCoAPO, and VPP catalysts, rather complex IR spectra can be
observed after acrolein adsorption. The band at approximately
À1
(see below), the picture at the molecular level is actually oppo-
n˜ =1695 cm can be assigned to the stretching vibration of
site, in which the strong acid sites in VO-WO play a major role
the C=O moiety interacting with the OH groups (i.e., Brønsted
x
[48]
in the formation of consecutive products owing to strong ad-
sorption of the reaction intermediates. Overall, although the
absolute values of the product selectivities might also depend
on the real accessibility of the V sites on the different samples
acid sites) on the catalyst surface, whereas bands at approxi-
À1
mately n˜ =1680, 1666, and 1650 cm are associated to the
same carbonyl group interacting with the Lewis acid sites,
5
+
4+
mainly V /V ions, in different oxidation states and/or coor-
dination environments (e.g., tetrahedral, octahedral, polyoxo
species, dimers, etc.). Owing to the different crystal structures
of VCoAPO and VPP, a different nature of the vanadium ions is
expected; in fact, this is observed in the IR spectra (Figure 6).
(
e.g., acrolein and other C molecules can enter the micropores
3
[
42]
of zeolites, whereas HTBs have only external surface area
[
29]
available for catalysis), the remarkable differences displayed
by the catalysts reported herein as a function of temperature
&
ChemSusChem 2016, 9, 1 – 12
6
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Chieregato, Alessandro
