Vanadium-Catalyzed Deoxydehydration of Glycerol Without an External Reductant

Article abstract of DOI:10.1002/cctc.201701049

A vanadium-catalysed deoxydehydration (DODH) of neat glycerol has been developed. Cheap and readily available ammonium metavanadate (NH₄VO₃) affords higher yields of allyl alcohol than the well-established catalyst methyltrioxorhenium. A study in which deuterium-labelled glycerol was used was undertaken to further elucidate the dual role of glycerol as both an oxidant and reductant. This study led to the proposal of a metal-catalysed DODH mechanism for the production of allyl alcohol and a deeper understanding of the formation of the byproducts acrolein and propanal.

Full text of DOI:10.1002/cctc.201701049

Accepted Article
Title: Vanadium-catalysed Deoxydehydration of Glycerol Without an
External Reductant
Authors: Allan Robertson Petersen, Lasse Bo Nielsen, Johannes R.
Dethlefsen, and Peter Fristrup
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10.1002/cctc.201701049
ChemCatChem
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Vanadium-catalysed Deoxydehydration of Glycerol Without an
External Reductant
Allan R. Petersen, Lasse B. Nielsen, Johannes R. Dethlefsen and Peter Fristrup*[a]
Abstract: The vanadium-catalysed deoxydehydration (DODH) of
neat glycerol has been developed. Cheap and readily available
ammonium metavanadate (NH4VO3) affords higher yields of allyl
alcohol than the well-established catalyst methyltrioxorhenium
(MTO). A study using deuterium-labelled glycerol was undertaken to
further elucidate glycerols dual role as both oxidant and reductant.
heterogeneous catalyst ReOx–Au/CeO2 converted glycerol to
allyl alcohol in 91% yield with a TON of 300.[11]
OH
Catalyst
R2
R2
+
+
RedO
+
H2O
Red
R1
R1
OH
Scheme 1. General scheme for the deoxydehydration reaction. Red
=
reductant. RedO = oxidised reductant.
This study led to the proposal of
a
metal-catalysed DODH
deeper
mechanism for the production of allyl alcohol and
a
Converting glycerol to allyl alcohol requires the use of a
stoichiometric reductant. Early studies on formic acid-mediated
DODH, pioneered by Kamm and Marvel, have more recently
been expanded and improved upon.[12–14] Yet under continuous-
flow conditions, triethyl orthoformate rivals formic acid as a
reagent for the DODH of glycerol.[15] Comparably, secondary
alcohols have literature precedence as reductants in Mo-
catalysed and Re-catalysed DODH reactions.[8,16–19] Yet their
use on an industrial scale would require recycling of the
secondary alcohols through hydrogenation of the corresponding
ketones. Alternatively, if neat glycerol is used, it undergoes a
disproportionation thus assuming the role of both oxidant and
reductant. The DODH of neat glycerol preconditions that only
half of the glycerol can be reduced to allyl alcohol (see Scheme 2).
The other half of the glycerol, which acts as the reductant, is
oxidised. Hence allyl alcohol, water and an oxidised component
are formed.
understanding of the formation of byproducts acrolein and propanal.
Introduction
The efficient use of glycerol has been of interest for some time.
Deoxydehydration (DODH) could provide the first step towards
sustainable acrylic acid by converting glycerol to allyl alcohol.[1]
DODH is the removal of two vicinal OH groups from a diol or
polyol to yield an alkene (see Scheme 1). It requires
a
stoichiometric reductant and may be viewed as the reverse of a
dihydroxylation. The reaction is typically catalysed by rhenium-
based catalysts, however molybdenum-based and vanadium-
based catalysts have also been developed.[2,3] DODH has the
ability to efficiently reduce the oxygen content of biomass while
retaining useful functionality. Since 2009, DODH and particularly
rhenium-catalysed DODH have received renewed focus due to
their success in using biomass-derived substrates.[4–6]
Nonetheless, the allure of glycerol as a substrate has been
present from the onset. In 1996 Cook and Andrews reported, in
their seminal work on rhenium-catalysed DODH, the conversion
of glycerol and erythritol to allyl alcohol and butadiene
respectively.[7] PPh3 was employed as the reductant and as a
consequence the quest to replace PPh3, with a more atom-
efficient reductant, commenced. Shiramizu and Toste
investigated the use of 3-octanol as the reductant for the Re-
catalysed DODH of glycerol and erythritol.[8] Hydrogen has also
been explored as reductant in DODH reactions. The
heterogeneous catalyst ReOx–Pd/CeO2 has been shown to
convert glycerol to 1-propanol by sequential DODH and
hydrogenation.[9,10] In a subsequent study, hydrogenation was
curtailed by replacing palladium with gold. Hence the
Abu-Omar and co-workers reported the DODH of neat
glycerol at 165 °C using methyl trioxorhenium (MTO) as the
catalyst.[20] Subsequently, ReO3 was shown by Canale et al. to
supersede MTO as a catalyst for the DODH of neat glycerol.[21]
Yields of allyl alcohol as high as 34% were achieved by
conducting the reaction under a hydrogen atmosphere as this
decreased the reaction time and increased the selectivity.
The product of oxidation of glycerol is dependent upon the
metal used. Rhenium is proposed to oxidise glycerol to
dihydroxyacetone and/or glyceraldehyde which under the
reaction conditions polymerises.[20,21] So far, molybdenum is the
only other metal capable of DODH of glycerol.[16,22] Intriguingly,
instead of oxidation, glycerol undergoes oxidative deformylation
during the Mo-catalysed DODH of glycerol (see Scheme 2).[16,22]
Previous work:
Re catalyst
OH
OH
O
O
OH
OH
2
2
+
+
+
2 H2O
HO
OH
OH
HO
HO
OH
+
Mo catalyst
O
+
2 H2O
HO
H
H
H
[a]
Dr. A. R. Petersen, Dr. L.B. Nielsen, Dr. J. R. Dethlefsen and Dr. P.
Fristrup
This work:
O
Department of Chemistry
Technical University of Denmark
Kemitorvet 207
DK-2800 Kgs. Lyngby, Denmark
E-mail: peter.fristrup@gmail.com
HO
HO
OH
O
1 mol% NH4VO3
OH
OH
2
+
or
+
2 H2O
HO
OH
275 °C
OH
Scheme 2. General scheme for the DODH of glycerol.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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The relatively low reaction temperature and high yields can
make Re-catalysed DODH appear advantageous when
compared to DODH catalysed by cheaper metals.[2] Rhenium
catalysts allow temperature sensitive substrates to undergo
DODH without extensive decomposition. Nevertheless, glycerol
is neither sensitive nor expensive. The production of allyl alcohol,
obtained by the DODH of glycerol, does not merit the use of
metals as costly and scarce as rhenium. Despite the need for
higher reaction temperatures, the DODH of glycerol catalysed by
cheaper earth-abundant metals is more appropriate.
Homogeneous catalysts based on vanadium have
demonstrated significant potential in the DODH of vicinal
diols.[23,24] A recent publication on converting 2,3-butanediol to
butane, using heterogeneous vanadium-based catalysts, has
also shown promise.[25] Herein, we present the vanadium-
catalysed DODH of neat glycerol to afford allyl alcohol. To the
best of our knowledge, this is the first example of DODH of
glycerol catalysed by vanadium. Reactions using deuterium-
labelled glycerol were used to elucidate the reaction. The results
were compared to those obtained while using MTO as the
catalyst. These labelling studies also led to the proposal of a
mechanism for the formation of acrolein and other byproducts.
only
a
trace amount of allyl alcohol and acrolein while
Mn(OAc)3·2H2O was found to be nearly unreactive. Although
Nb2O5 gave only trace amounts of allyl alcohol and acrolein,
Nb2OEt10 produced a complex mixture of products of which more
acrolein was produced than allyl alcohol.
Table 1. Test of various transition metal-based catalysts for the
DODH of glycerol in a reactive distillation setup.[a]
Yields [%] of
T
[°C]
Entry Catalyst
t [h]
vol.
cond.
allyl
acrolein[e]
prod.[b] prod.[c] alcohol[d]
1
NH4VO3
NH4VO3
AHM
AHM + 3 eq.
Bu4NOH (1 M
in MeOH)
MTO (MeReO3) 170
Mn(OAc)3
275
275
236
5
5
7
87
80
58
71
68
51
22
22
12
3.7
3.0
1.4
2[f]
3[g]
4[g]
250
4
81
83[i]
11
1.4
5[f]
6
7[f,h]
3
4
5
53
8
49
4
17
6.4
275
Trace
Trace
0.1
·2H2O
(Bu4N)2W6O19
275
30
27
Trace
[a] Reaction conditions: catalyst (1.25 mmol, 1 mol%), glycerol (11.51 g,
0.125 mol). [b] The yield of volatile products (vol. prod.) was calculated as the
mass loss in the reaction flask divided by the mass of glycerol. [c] The yield of
condensable products (cond. prod.) was calculated as the mass of products
collected divided by the mass of glycerol. [d] Yields of allyl alcohol were
determined by GC, using THF as the internal standard. [e] Yields of acrolein
were determined by 1H NMR spectroscopy using 1,4-dioxane as the internal
standard. [f] catalyst (2.5 mmol, 1 mol%), glycerol (23.02 g, 0.25 mol). [g]
0.179 mmol (NH4)6Mo7O24·4H2O used (1 mol% with respect to Mo). [h]
0.417 mmol (Bu4N)2W6O19 used (1 mol% with respect to W). [i] Includes MeOH
from the Bu4NOH additive.
Results and Discussion
Initial Catalyst Screening
A series of high-valent transition metals with terminal oxo
ligands were surveyed to find a cheaper alternative to rhenium
for the DODH of glycerol (see Table 1). The reactions were
carried out in a distillation setup allowing for the continual
separation of allyl alcohol and water from the reaction media.
Among the different catalysts tested, ammonium metavanadate
(NH4VO3) was found to effectively catalyse the DODH of glycerol.
Heating 1 mol% NH4VO3 in neat glycerol to 275 °C yielded up to
22% allyl alcohol as part of the condensed products (for GC and
GC/MS see Figure S18 and S19 in ESI). A small amount of
Catalyst Precursors and Additives
Having determined that NH4VO3 is a suitable catalyst for the
DODH of neat glycerol, we chose to investigate alternative
vanadium-based catalyst precursors and possible additives to
determine their effect on the yield and selectivity of the reaction
(see Table 2). Although there are some distinct differences in
reactivity between the vanadium compounds tested, none of
them were able to surpass an allyl alcohol yield of 22%. NaVO3
acrolein, typically less than 5%, was also obtained. After 5 hours, and V2O4 were found to give lower yields of allyl alcohol than
NH4VO3 (see Table 2, entries 2 and 3). We tentatively propose
that the lower solubility of NaVO3 in glycerol compared to that of
NH4VO3 accounts for the differing reactivity. V2O4 remains as a
black powder throughout the reaction in comparison V2O5 reacts
rather differently and is completely dissolved before the reaction
temperature has been reached. The complexes, V(acac)3 and
VO(acac)2, display reactivity very similar to V2O5 and NH4VO3
(see Table 2, entries 1 and 4-6). The addition of NH4H2PO4,
ZnCO3 and (Bu4N)2W6O19 decreased the yield of allyl alcohol
substantially, while B(OH)3 had less of an effect on the yield.
When 1 mol% of VOSO4·xH2O was employed as the catalyst,
a particularly rapid reaction was observed. After 30 minutes at
230 °C, the stirring in the reaction mixture stopped as the
residue had begun to solidify. Analysis of the condensed
products showed no allyl alcohol, instead large amounts of
acrolein had formed. We attribute this to the presence of
sulphate ions as this reaction is analogous to the known
conversion of glycerol to acrolein by potassium sulfate.[27]
the total amount of volatile products constituted ~80 wt% of the
initial glycerol of which 60 – 70 wt% was condensable. Hence
roughly 10 wt% of the glycerol is converted into gasses that are
too volatile to be collected by the condenser, this is something
that will be discussed in more detail in a later section.
Intriguingly, a higher allyl alcohol yield was obtained when
the reaction was conducted using NH4VO3 as the catalyst
compared to methyltrioxorhenium (MTO) (see Table 1, entries
1,2 and 5). Indeed, the vanadium catalyst requires a higher
reaction temperature. Yet, MTO afforded a 17% yield of allyl
alcohol after 3 hours. This yield did not increase with prolonged
reaction times and is similar to what has been reported by
Canale et al.[21] Of the other metals surveyed, only molybdenum
was found to give an appreciable amount of allyl alcohol. Using
ammonium heptamolybdate (AHM) as the catalyst, a 12% yield
of allyl alcohol was obtained. The addition of Bu4NOH, which
previously has been found to improve the yield of Mo-catalysed
DODH and HDO reactions, did not improve the yield of allyl
alcohol.[16,26] The polyoxotungstanate, (Bu4N)2W6O19, produced
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Table 2. Test of vanadium-based catalyst precursors and additives for
the DODH of glycerol in a reactive distillation setup.[a]
Yields [%] of
t
Entry
Catalyst Additive
[h]
vol.
cond.
allyl
acrolein[e]
prod.[b] prod.[c] alcohol[d]
1
2
3
4
5
6
NH4VO3
NaVO3
V2O4
V2O5
V(acac)3
VO(acac)2
NH4VO3 B(OH)3
NH4VO3 NH4H2PO4
NH4VO3 ZnCO3
NH4VO3 (Bu4N)2W6O19
—
—
—
—
—
—
5
4
5
4
4
4
4
5
5
5
87
55
21
64
68
65
—
59
43
56
71
45
16
54
54
56
48
55
37
50
22
14
6
22
21
20
17
9
3.7
1.0
1.3
5.8
5.8
6.4
3.4
2.8
1.1
2.8
7[f,g]
8[f,g]
9[f,g]
10[f,h]
[i]
10
8
[a] Reaction conditions: catalyst (1.25 mmol, 1 mol%), glycerol (11.51 g,
0.125 mol), 275 °C. [b] The yield of volatile products (vol. prod.) was
calculated as the mass loss in the reaction flask divided by the mass of
glycerol. [c] The yield of condensable products (cond. prod.) was calculated
as the mass of products collected divided by the mass of glycerol. [d] Yields of
allyl alcohol were determined by GC, using THF as the internal standard. [e]
Yields of acrolein were determined by 1H NMR spectroscopy using 1,4-
dioxane as the internal standard. [f] Catalyst (2.5 mmol, 1 mol%), glycerol
Figure 1. Time course of the DODH of glycerol using 1 mol% NH4VO3 as the
catalyst at 275 ºC.
The Residue
When the residue was partitioned between H2O and DCM,
small quantities of glycerol were obtained from the aqueous
phase. Analysis of the DCM phase, where the majority of the
residue resided, was inconclusive. Accordingly, information
about the composition of the residue was gathered by other
means.
(23.02 g, 0.25 mol). [g] Additive (2.5 mmol,
1 mol%) [h] 0.417 mmol
(Bu4N)2W6O19 used (1 mol% with respect to W). [i] Not quantified.
Increasing the catalyst loading from 1 mol% to 2 mol% did
not increase the yield of allyl alcohol; however when 0.1 mol%
NH4VO3 was used only 8% allyl alcohol was obtained after
8 hours at 275 °C. Consequently, 1 mol% NH4VO3 was used in a
series of experiments that were conducted to determine the
product evolution over time (see Figure 1). The reaction times
were varied between one and seven hours. Plotting the yield of
both allyl alcohol and acrolein against reaction time led to a
straight line up to five hours. After five hours, the production of
allyl alcohol ceases and the residue in the reaction flask has
become a black tar. Heating the reaction mixture beyond five
hours resulted in the continued formation of water and acrolein
until the residue solidifies. This indicates that allyl alcohol and
acrolein may be formed independently of each other.
The choice of distillation set-up had a significant impact on
the yield of allyl alcohol. A short-path condenser was found to
produce the highest yields of allyl alcohol. In order to maximise
the yield of allyl alcohol, its efficient and continual removal from
the reaction flask is necessary, which thereby avoids the
degradation of the product under the reaction conditions. Left
behind in the reaction flask is a black residue with the
consistency of tar.
The conversion of glycerol to allyl alcohol is expected to
produce two equivalents of water. A non-volatile oxidation
product is also produced, which is proposed to be the main
component of the residue (see Scheme 2). Similarly, the
conversion of glycerol to acrolein is also expected to produce
two equivalents of water. During the V-catalysed DODH of neat
glycerol, 22% allyl alcohol and 3% acrolein is produced, hence
the amount of water which is expected to form can be calculated.
The sum of the allyl alcohol and acrolein produced, together with
the expected amount of water amounts to less than half the
mass of the condensed products obtained. Consequently, the
amount of water produced far exceeds the two equivalents per
allyl alcohol and acrolein in the condensed products. Rapid
dehydration or polymerisation of the non-volatile oxidation
products, formed when glycerol is oxidised, may produce water
as a byproduct. Furthermore, we propose that any acrolein,
which does not escape the reaction medium, would quickly
polymerise at the reaction temperature. This makes it difficult to
accurately quantify how much glycerol is dehydrated to acrolein
and as consequence, how much water should be produced.
In an attempt to recreate the black residue obtained from the
DODH reaction, 20 mmol of dihydroxyacetone was heated in
glycerol for 4 hours at 275 °C. The distillate collected contained
7% allyl alcohol and 1% acrolein with respect to
dihydroxyacetone. Also, slightly more than 3 equivalents of
water were obtained for every equivalent of dihydroxyacetone.
This indicates that dihydroxyacetone dehydrates and reacts
further under the reactions conditions.
A black residue was also obtained when MTO was used as
the catalyst. This residue has previously been proposed to be
polymerised
dihydroxyacetone.[20]
The
higher
reaction
temperature employed when NH4VO3 is used as the catalyst
leads instead to the dehydration of dihydroxyacetone.
Interestingly, heating the V-catalysed DODH reaction past the
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1) 0.4 eq. NaBD4,
THF/H2O, 5 °C
2) H+/ MeOH, 65 °C
5 hours mark led to further dehydration. After 8 hours of heating,
a dry black solid was left behind in the reaction flask. Elemental
analysis of this solid determined that the material was 70.39% C,
7.13% H and 0.08% N. Thus the material cannot be solely
comprised of dihydroxyacetone as the carbon content is too high.
During the V-catalysed DODH of neat glycerol, roughly
10 wt.% of the volatile products were converted into gasses that
were not condensed by a water condenser. Bubbling the gasses
through a solution of Br2 in DCM and analysing the products
OH
HO
OH
D
O
d1-GLY
O
O
84%
O
1) 0.4 eq. NaBH4,
THF/H2O, 5 °C
O
O
2) H+/ MeOH, 65 °C
OH
Et3N, D2O
O
O
HO
OH
CH3CN, RT
D
D D D
D
D D D
O
O
d4-GLY
87%
91%
Scheme 3. Synthesis of deuterium-labelled glycerol.
demonstrated that only
a trace amount of propene was
produced. When the gas was bubbled through a 0.1M Ba(OH)2
solution, a white precipitate of BaCO3 immediately formed,
indicating that CO2 was the major component of the gas. A
report by Tang et al. highlights the importance of the VV/VIV
redox couple in the oxidative C–C bond cleavage of
glyceraldehyde and 1,3-dihydroxyacetone to afford formic acid
and CO2.[28] Formic acid was not identified in the condensed
products. Nevertheless, as the formic acid-mediated DODH is
well established, the lack of formic acid in the condensed
products is unsurprising.[12–15,29]
Deoxydehydation of Labelled Glycerol
A study using deuterium-labelled glycerol was undertaken.
The two most efficient catalysts from Table 1, NH4VO3 and MTO,
were used in this study. This helped elucidate the mechanisms
by which allyl alcohol and acrolein were produced. The results
from the DODH of 2-d1-glycerol and 1,1,3,3-d4-glycerol are
collected in Table 3. In general, introducing one or more C–D
bonds into glycerol reduced the yield of allyl alcohol produced.
When NH4VO3 was used as the catalyst, the yield of allyl alcohol
obtained from 2-d1-glycerol was 17%; in comparison only 12%
allyl alcohol was obtained from 1,1,3,3-d4-glycerol. Yet the
highest yield was obtained by the DODH of fully protiated
glycerol.
Synthesis of Deuterium-Labelled Glycerol
Since the DODH of neat glycerol can be viewed as a
disproportionation, we hoped to clarify the dual role of glycerol
as both oxidant and reductant. Being able to distinguish between
the primary and secondary C–H bonds in glycerol would aid
determining where glycerol is oxidised. Using deuterium-labelled
glycerol as the substrate allowed for this. Inevitably, this study
required the synthesis of deuterium-labelled glycerol on a gram-
scale. In turn, this required the development of an affordable
synthetic procedure.
The complementary pair of isotopologues, 2-d1-glycerol (d1-
GLY) and 1,1,3,3-d4-glycerol (d4-GLY), were both synthesised
from the same starting material, 1,3-dipropionyloxyacetone (see
Scheme 3). 2-d1-glycerol was prepared by the reduction of 1,3-
dipropionyloxyacetone with sodium borodeuteride. As observed
by Bentley and McCrae, this affords a mixture of the 1,3-
diglyceride and the 1,2-diglyceride.[30] Transesterification of the
intermediate diglycerides with methanol afforded 2-d1-glycerol in
84% yield.
The preparation of 1,1,3,3-d4-dihydroxyacetone, by Fronza
et al., inspired a similar approach for the preparation of 1,3-
dipropionyloxy-1,1,3,3-d4-acetone.[31] 1,3-dipropionyloxyacetone
was found to undergo H/D exchange more readily in acetonitrile
than THF. This allowed for the facile incorporation of deuterium
from D2O (see Figure S10 in ESI). Consequently, 1,1,3,3-d4-
glycerol was prepared by an analogous route to 2-d1-glycerol
(see Scheme 3). 1,3-dipropionyloxy-1,1,3,3-d4-acetone was
reduced with sodium borohydride and the subsequent
transesterification afforded the desired product in 87% yield.
Thereby, the use of LiAlD4, to reduce diethyl 2-acetoxymalonate
to 1,1,3,3-d4-glycerol, was avoided.[32,33]
The 1H NMR spectra in D2O of the condensed products from
the V-catalysed DODH of glycerol, 2-d1-glycerol and 1,1,3,3-d4-
glycerol are displayed in Figure 2. Fully protiated allyl alcohol
displays a characteristic signal of a doublet of doublets of triplets
at 6.01 ppm (see Figure 2a). When 2-d1-glycerol was employed
1
as the substrate, the H NMR spectrum is absent of a signal at
6.01 ppm where the proton in the 2-position of allyl alcohol is
expected (see Figure 2b). On the other hand, when 1,1,3,3-d4-
glycerol was employed as the substrate, the product 1,1,3,3-d4-
allyl alcohol displayed a broad singlet at 6.00 ppm in the 1H
NMR spectrum of the condensed product (see Figure 2c).
Accordingly, no H/D scrambling was detected in the deuterated
allyl alcohol produced. The DODH of 2-d1-glycerol and 1,1,3,3-
d4-glycerol yielded 2-d1-allyl alcohol and 1,1,3,3-d4-allyl alcohol
respectively (for GC/MS see Figure S20 in ESI).
Similarly, when MTO was used as the catalyst, the DODH of
2-d1-glycerol and 1,1,3,3-d4-glycerol afforded 2-d1-allyl alcohol
and 1,1,3,3-d4-allyl alcohol respectively (for 1H NMR and GC/MS
spectra see Figure S11 and Figure S21 respectively, in the ESI).
The yield of allyl alcohol obtained from the two deuterated
glycerol substrates was comparable to each other, yet
noticeably lower than with fully protiated glycerol.
Table 3. DODH of glycerol and deuterium-labelled glycerol in a reactive
distillation setup.[a]
Yields [%] of
T
[°C]
Entry Catalyst Substrate
vol.
cond.
allyl
acrolein[e]
prod.[b] prod.[c] alcohol[d]
1
2
3
NH4VO3 glycerol
NH4VO3 d1-GLY
NH4VO3 d4-GLY
275 56
275 85
275 55
46
70
46
20
17
12
2.9
6.6
1.0
glycerol
+ d1-GLY (1:1)
glycerol
+ d4-GLY (1:1)
4[f]
NH4VO3
275 65
275 59
54
50
21
14
5.4
1.4
5[g]
NH4VO3
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6
7
8
9[f]
MTO
MTO
MTO
glycerol
d1-GLY
d4-GLY
glycerol
+ d1-GLY (1:1)
glycerol
+ d4-GLY (1:1)
170 53
170 46
170 28
44
39
22
17
10
9
6.0
5.2
1.2
#
a)
*
*
MTO
MTO
170 48
170 36
40
28
13
12
7.0
2.0
10[h]
[a] Reaction conditions: catalyst (1.0 mmol, 1 mol%), substrate (0.1 mol), reaction
time 4 h. [b] The yield of volatile products (vol. prod.) was calculated as the mass
loss in the reaction flask divided by the mass of glycerol. [c] The yield of
condensable products (cond. prod.) was calculated as the mass of products
collected divided by the mass of glycerol. [d] Yields of allyl alcohol were
determined by GC, using THF as the internal standard. [e] Yields of acrolein were
determined by 1H NMR spectroscopy using 1,4-dioxane as the internal standard.
[f] Catalyst (1.0 mmol, 1 mol%), glycerol (4.60 g, 0.05 mol), 2-d1-glycerol (4.65 g,
0.05 mol). [g] Catalyst (1.0 mmol, 1 mol%), glycerol (4.60 g, 0.05 mol), 1,1,3,3-d4-
glycerol (4.81 g, 0.05 mol). [h] Catalyst (0.66 mmol, 1 mol%), glycerol (3.04 g,
0.033 mol), 1,1,3,3-d4-glycerol (3.18 g, 0.033 mol).
#
b)
6.1 6.0 5.9 5.8 5.7 5.6 5.5 5.4 5.3 5.2 5.1 5.0 4.9 4.8 4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7
δ (ppm)
Figure 3. 1H NMR spectra in D2O of the condensed products obtained using
1 mol% NH4VO3 for the DODH of equimolar mixtures of: (a) glycerol and 2-d1-
glycerol, (b) glycerol and 1,1,3,3-d4-glycerol. * = H2O and # = 1,4-dioxane
(internal standard).
Competition experiments were also carried out. These
contained equimolar mixtures of fully protiated glycerol and
either 2-d1-glycerol or 1,1,3,3-d4-glycerol. For both catalysts, the
yield of allyl alcohol obtained was higher in the mixtures than for
the corresponding deuterated glycerol on its own. Intriguingly,
when NH4VO3 was employed for the experiment containing fully
protiated glycerol and 2-d1-glycerol, the yield of allyl alcohol was
marginally higher than when glycerol was used on its own.
The 1H NMR spectra in D2O of the condensed products
display clear evidence of the superposition of fully protiated allyl
alcohol with either 2-d1-allyl alcohol or 1,1,3,3-d4-allyl alcohol
(see Figure 3 for NH4VO3 and Figure S12 in the ESI for MTO).
Again, no H/D scrambling was observed for either of the
catalysts.
The condensed products obtained from the competition
experiments employing equimolar mixtures of glycerol and 2-d1-
glycerol were carefully distilled. The 1H NMR spectra of the
fractions containing allyl alcohol are displayed in the ESI as
Figure S15 for NH4VO3 and Figure S17 for MTO. The
relationship between the integrals of the peaks attributed to allyl
alcohol show that a 1:1 ratio of deuterated and non-deuterated
allyl alcohol was obtained. This was the case in all competition
experiments.
The Origins of Acrolein
Irrespective of whether NH4VO3 or MTO is used as the
catalyst, acrolein is always formed as a byproduct during the
DODH of neat glycerol. V-based heterogeneous catalysts show
precedence for the dehydration of glycerol to acrolein.[34–36]
Similarly, it is well-established that rhenium catalysts can
catalytically dehydrate alcohols to alkenes.[6,37,38] Likewise, the
catalytic dehydration of glycerol to acrolein is well
documented.[39,40] It is generally acknowledged that glycerol can
dehydrate via two separate pathways (see Scheme 4).[41,42] The
first pathway produces hydroxyacetone (acetol) via the loss of
one of the two primary alcohol groups in glycerol. The second
pathway leads to acrolein via two successive dehydration steps.
The initial elimination of water, by the loss of the secondary
alcohol group, forms an enol intermediate which isomerises to 3-
hydroxypropanal. This intermediate is believed to be particularly
unstable and is therefore not observed before it dehydrates to
form acrolein.
OH
OH
O
OH
HO
OH
OH
- H2O
H2
#
#
#
a)
b)
c)
*
*
*
H3
OH
H1
- H2O
HO
H3
H1 H1
H3
H2
O
OH
O
HO
- H2O
H
Scheme 4. Dehydration of glycerol to afford hydroxyacetone or acrolein.
If acrolein had formed via the dehydrogenation of allyl
alcohol, it is expected that 2-d1-acrolein and 1,1,3,3-d4-acrolein
would be formed during the DODH of 2-d1-glycerol and 1,1,3,3-
d4-glycerol, respectively. Nonetheless, the DODH of 2-d1-
glycerol, using NH4VO3 as the catalyst, led to the formation of a
mixture of acrolein and 2-d1-acrolein (see Figure 4). The aldehyde
signal for acrolein usually appears as a doublet at 9.50 ppm. In
the 1H NMR spectrum of the condensed products, obtained from
6.1 6.0 5.9 5.8 5.7 5.6 5.5 5.4 5.3 5.2 5.1 5.0 4.9 4.8 4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7
δ (ppm)
Figure 2. 1H NMR spectra in D2O of the condensed products obtained using
1 mol% NH4VO3 for the DODH of: (a) glycerol, (b) 2-d1-glycerol and (c)
1,1,3,3-d4-glycerol. * = H2O and # = 1,4-dioxane (internal standard).
the DODH of 2-d1-glycerol, this signal appears as
a
superposition of a doublet and a singlet attributed to acrolein
and 2-d1-acrolein, respectively (see Figure 4b). In addition, the
integral of the signal of the proton in the 2-position of acrolein,
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10.1002/cctc.201701049
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OH
H/D
which appears at 6.42 ppm is close to half (0.59) the value of the
integrals of the other three protons. The 1H NMR spectrum of the
condensed products from the DODH of 1,1,3,3-d4-glycerol
displayed a broad singlet at 6.44 ppm attributed to 1,3,3-d3-
acrolein (see Figure 4c).
D
H
H
(a)
(b)
HO
HO
OH
OH
O
O
O
HO
HO
- H2O
- H2O/HDO
D
d1-GLY
H
OH
H
D
O
D
H3
- HDO
- H2O
H1
a)
b)
c)
D
H2
D
D D
D
D
D
D
H3
O
d4-GLY
H2
Scheme 5. Formation of acrolein by the dehydration of deuterium-labelled
H3
H1
9.50
§
δ (ppm)
glycerol.
Deuteration of glycerol also affects the yield of acrolein
obtained. When deuterium was introduced in the 1 and 3
positions of glycerol, the yield of acrolein was reduced. The
reduction in the yield of acrolein by the dehydration of 1,1,3,3-d4-
glycerol may come at the cost of an increase in hydroxyacetone
production. Although hydroxyacetone is not observed in the
condensed products, it is plausible that it is produced and reacts
further with the other components in the residue.
9.50
δ (ppm)
§
9.9 9.7 9.5 9.3 9.1 8.9 8.7 8.5 8.3 8.1 7.9 7.7 7.5 7.3 7.1 6.9 6.7 6.5 6.3
δ (ppm)
Propanal
Figure 4. 1H NMR spectra in D2O of the condensed products obtained using
1 mol% NH4VO3 for the DODH of: (a) glycerol, (b) 2-d1-glycerol and (c)
1,1,3,3-d4-glycerol. § = propanal.
As noted by the group of Abu-Omar, when MTO is used as
the catalyst for the DODH of neat glycerol, small amounts of
propanal are also formed.[20] Interestingly, propanal is also
observed when NH4VO3 is used as the catalyst, albeit in
significantly lower quantities. When 2-d1-glycerol was used as
When MTO was used as the catalyst, for the DODH of 2-d1-
glycerol, the ratio of acrolein to 2-d1-acrolein produced increased.
Deuteration of the acrolein component appears to be less
pronounced compared to when NH4VO3 was used as the
catalyst (see Figure S13 in the ESI). The ratio of acrolein to 2-d1-
1
the substrate, the signals in the H NMR spectrum attributed to
propanal were identical to those obtained when fully protiated
glycerol was used as
a substrate. The aforementioned
distillation of the condensed products obtained from
a
1
acrolein was found to be roughly 4:1. Meanwhile, the H NMR
competition experiment employing an equimolar mixture of
glycerol and 2-d1-glycerol allowed for the separation of acrolein
and propanal from remainder of the sample consisting mainly of
spectrum of the condensed product from the DODH of 1,1,3,3-
d4-glycerol, catalysed by MTO, also displayed a broad singlet at
6.44 ppm attributed to 1,3,3-d3-acrolein (see Figure S13 in the
ESI). Due to the overlapping spectra of acrolein and 2-d1-
acrolein, the GC/MS spectra of the acrolein isotopologues were
less clean-cut when compared to the 1H NMR spectra.
Nonetheless, the expected increase in m/z is visible (see Figure
S22 for NH4VO3 and Figure S23 for MTO in the ESI).
To the best of our knowledge, this is the first example of
using deuterium-labelled glycerol to elucidate the dehydration of
glycerol to acrolein. Gratifyingly, the results obtained herein
corroborate the proposed mechanism. We agree that acrolein is
produced by two sequential dehydrations of glycerol (see Scheme
5). For 2-d1-glycerol, the initial dehydration is accompanied by a
tautomerisation that transfers a proton to the central carbon. As
a result, the second dehydration can eliminate either H2O or
HDO producing a mixture of acrolein and 2-d1-acrolein (see
Scheme 5a). In contrast, the initial dehydration of 1,1,3,3-d4-
glycerol leads to an isotopologue of 3-hydroxypropanal that can
only produce 1,3,3-d3-acrolein when it dehydrates (see Scheme
5b).
1
allyl alcohol and water. Judging by the H NMR spectrum of the
propanal and acrolein distillate, fully protiated propanal had been
produced (see Figure S14 for NH4VO3 and Figure S16 for MTO
in the ESI). Unlike the signals for allyl alcohol and acrolein, the
spectra did not show evidence of the superposition of propanal
and 2-d1-propanal. Therefore propanal cannot be produced by
the isomerisation of allyl alcohol. Equally, it cannot be produced
by the hydrogenation of acrolein. Instead, propanal must have
been formed through a pathway whereby the central carbon has
been oxidised such that the deuterium has been lost.
Gratifyingly, the 1H NMR spectrum of the condensed products
obtained from the DODH of 1,1,3,3-d4-glycerol, did not display
any signals that could be assigned to propanal, suggesting that
fully deuterated propanal (d6-propanal) had been produced (see
S11 in the ESI).
Abu-Omar and co-workers note that both acrolein and
propanal are produced independently to allyl alcohol.[20] In
addition, it was proposed that propanal is produced by the
dehydration of 1,2-propanediol.[43] We hypothesise that if 1,2-
propanediol was formed by the reduction of hydroxyacetone, this
would account for the loss of deuterium. Thus, the propanal
formed would be fully protiated regardless of whether the
hydroxyacetone was formed by the dehydration of glycerol or 2-
d1-glycerol.
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10.1002/cctc.201701049
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FULL PAPER
Mechanistic Discussion
the alkene from a metal diolate or orthoester-type intermediate
(see Scheme 6).
During the alkene extrusion step, neither formic acid
mediated-DODH nor metal-catalysed DODH break any of the
Various mechanisms have been reported for the conversion
of glycerol to allyl alcohol. Some of these mechanisms operate
without the addition of an external reductant. Konaka et al.
demonstrated that the catalyst K/ZrO2-FeOx afforded a 27%
yield of allyl alcohol.[44] The production of allyl alcohol was
reported to occur via a hydrogen transfer mechanism. As no
external hydrogen gas was added, the hydrogen atoms were
proposed to come from either formic acid, generated in situ, or
the decomposition of H2O over ZrO2.
In contrast, Lari et al. reported a dehydration/hydrogenation
mechanism for the conversion of glycerol to allyl alcohol by a
bifunctional catalyst (see Scheme 6A).[45] The catalyst is
comprised of silver nanoparticles deposited on a ZSM-5 zeolite.
Here glycerol is dehydrated to acrolein at the Brønsted acid sites
of the zeolite. Subsequently, the silver nanoparticles
hydrogenate acrolein to allyl alcohol by means of molecular
hydrogen added to the system.
existing C–H bonds in glycerol. As
a consequence, the
deuterium-labelling experiments do not help to distinguish
between the two mechanisms. Nonetheless, neither formic acid
nor an oxidant is added to the reaction mixture of the V-
catalysed DODH of glycerol. Furthermore, the aerobic oxidation
of glycerol to formic acid can be ruled out as the V-catalysed
DODH of glycerol also performs well under a stream of N2.
Formic acid could be generated via the oxidative C–C bond
cleavage of glycerol. Yet, the difference in the oxidation states
between glycerol and formic acid necessitates that something
needs to be reduced for formic acid to be generated. Everything
considered, formic acid mediated-DODH is unlikely to be the
standalone mechanism, yet we cannot exclude it from
contributing to the production of allyl alcohol. In any case, the
metal catalyst is still required.
The mechanism of the metal-catalysed DODH reaction is
generally acknowledged to comprise of three steps:
condensation, reduction and alkene extrusion.[3] The sequence
of events remains a topic of discussion and two pathways have
been proposed (see Figure 5). In Pathway A, the condensation
step precedes the reduction step whereas in Pathway B, the
reduction step precedes the condensation step. Adding to the
complexity is the determination of which step has the highest
energy barrier, but this is dependent on the specifics of the
H2
OH
O
OH
(a)
HO
RO
OH
OH
- 2 H2O
O
OH
OH
(b)
(c)
+
+
+
CO2
ROH
O
OH
O
M
O
M
O
O
Scheme 6. Reported mechanisms for the conversion of glycerol to allyl
alcohol. R = HCO.
reaction such as the catalyst, reductant and substrate.
Formic acid mediated-DODH was used as early as 1921 as
Reduction
a
method of converting glycerol to allyl alcohol.[12] Two
Condensation
O
M
Red
HO
OH
O
O
molecules of formic acid condense with glycerol to give an
orthoester-type intermediate which upon heating extrudes allyl
alcohol and releases CO2 (see Scheme 6B).[13] Moreover, formic
acid can also be generated in situ. While using a Nb-Si-V
catalyst in the presence of H2O2, formic acid, produced by
oxidative cleavage of glycerol, reacts with glycerol to afford allyl
alcohol.[46]
Pathway A
R
- H2O
- RedO
R
M
Alkene Extrusion
O
O
M
O
O
-
R
R
The first example of using metal-catalysed DODH for the
conversion of glycerol to allyl alcohol was reported by Cook and
Andrews.[7] Subsequently, this reaction has been investigated by
- H2O
- RedO
Red
Reduction
HO
R
OH
several groups using
a
variety of catalysts and
Pathway B
M
O
reductants.[8,11,20,21] Allyl alcohol is formed by the extrusion of the
alkene from a metal diolate (see Scheme 6C).
Condensation
Due to the study using deuterium-labelled glycerol we can
rule out the dehydration/hydrogenation mechanism. The V-
catalysed DODH of 2-d1-glycerol afforded solely 2-d1-allyl
alcohol, however a mixture of acrolein and 2-d1-acrolein was
obtained. If allyl alcohol had formed via the hydrogenation of
acrolein, it is expected that a mixture of allyl alcohol and 2-d1-
allyl alcohol would be formed. Equally, acrolein cannot have
formed via the dehydrogenation of allyl alcohol. In addition, as
no H/D scrambling was observed in any of the experiments
involving deuterium-labelled glycerol, we propose that allyl
alcohol and acrolein are formed independent of each other.
Acrolein is formed by the dehydration of glycerol and not from
allyl alcohol. Moreover, allyl alcohol is formed by the extrusion of
Figure 5. Proposed pathways for deoxydehydration. Red = reductant. RedO =
oxidised reductant.
Of the three types of steps in the proposed catalytic cycle,
the condensation step would be the least affected by deuteration
on the three-carbon backbone of glycerol. As for the alkene
extrusion step, at most a secondary KIE could be observed. A
primary KIE could be observed if the reduction step involves C–
H bond cleavage. In contrast, a secondary KIE would be
expected if oxidative C–C bond cleavage is preferred during the
reduction step. The reduction step may also be viewed as the
oxidation of glycerol.
Glycerol has also been employed as the reductant in the Mo-
catalysed deoxygenation of sulfoxides.[47] Intriguingly, the
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10.1002/cctc.201701049
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primary oxidation products of glycerol were better reductants
than glycerol itself. As a consequence consecutive oxidation of
glycerol was observed. Thus, six molecules of bis(p-
tolyl)sulfoxide could be reduced by consuming one molecule of
glycerol. The glycerol was oxidised to yield two molecules of
CO2 and one molecule of formic acid. In contrast, the selective
oxidation of glycerol to dihydroxyacetone has been reported by
exchange of one secondary C–H bond for a C–D bond has the
comparable affect on the yield of allyl alcohol to exchanging 4
primary C–H bonds with C–D bonds.
In contrast, attempts at using alcohols as reductants for V-
catalysed DODH has been less successful. When compared to
PPh3 and Na2SO3, 2,4-dimethyl-3-pentanol proved to be an
inefficient reductant for the DODH of 1-phenyl-1,2-ethanediol
n
Waymouth
and
co-workers.[48]
The
catalyst,
catalysed by Bu4NVO3.[23] In addition, benzyl alcohol has been
[Pd(OAc)(neocuproine)]2(OTf)2, shows remarkable selectivity for
the oxidation of vicinal 1,2-diols and polyols to the corresponding
hydroxy ketones.[49]
tested as a reductant for DODH reactions catalysed by the
complex
(nBu4N)[VO2(salhyd)]
(salhyd
=
N-(2-
benzylidene)thiophene-2-carbohydrazide).[24] Yet Na2SO3, H2
and especially CO afforded higher yields of alkenes than when
benzyl alcohol was used as a reductant. Furthermore, DFT
calculation carried out by Fu and co-workers demonstrate that
the use of secondary hydroxyl groups with the vanadium catalyst,
Similarly, it would be of interest to determine whether
NH4VO3 or MTO selectively oxidises glycerol on either the
primary or secondary position. Or whether oxidative C–C bond
cleavage occurs during the reduction step making the
interpretation of the deuterium labelling study taxing.
Concurrently, oxidative C–C bond cleavage also allows for the
production of formic acid which in itself is capable of converting
glycerol to allyl alcohol.
Hence, measuring reliable reaction rates was impeded by
several inherent limitations of the reaction. Additionally, whether
NH4VO3 or MTO was used as the catalyst, both reactions
required a period of time to reach the reaction temperature. A
rate based upon the yield of allyl alcohol obtained in the
condensed products may not reflect the true rate of reaction,
especially as the condensed products are collected dropwise
from the short-path condenser. Instead, under the premise that
deuteration of glycerol has little affect on the condensation and
extrusion steps. Not to mention, deuteration is also expected to
have little affect on oxidative C–C bond cleavage. The yield of
allyl alcohol was used as a measure of the ease of the oxidation
of glycerol. This in turn should mirror the ease of the reduction
step in the catalytic cycle.
In several publications, primary and secondary alcohols
have been used as both solvent and reductant for DODH of
vicinal diols catalysed by MTO. Specifically for the DODH of
glycerol, 1-hexanol, 1-heptanol, benzyl alcohol, cyclohexanol,
2,4-dimethyl-3-pentanol, 2-octanol and 3-octanol have been
employed.[8,20,21,50] The secondary alcohols 2,4-dimethylpentanol
and 3-octanol are reported to give the highest yields of allyl
alcohol. Hence one might postulate that MTO may show some
selectivity towards the oxidation of secondary C–H bonds over
primary C–H.
The DODH of 2-d1-glycerol and 1,1,3,3-d4-glycerol using
MTO as the catalyst gave comparable yields of allyl alcohol,
both of which were lower than when fully protiated glycerol was
employed (see Table 3). If MTO was completely selective for the
oxidation of glycerol at the 2-position, the allyl alcohol yield when
using 1,1,3,3-d4-glycerol would be expected to be close if not
identical to that for fully protiated glycerol. Likewise, if the
oxidation was selective to primary C–H bonds, then 2-d1-glycerol
would give a higher yield of allyl alcohol than 1,1,3,3-d4-glycerol.
Given that in glycerol primary C–H bonds outnumber secondary
C–H bonds 4 to 1, if no selectivity was observed, 1,1,3,3-d4-
glycerol should afford a lower yield of allyl alcohol than 2-d1-
glycerol. Taken together, we reason that MTO displays some
selectivity towards oxidising glycerol in the 2-position. As the
(nBu4N)[VO2(dipic)] (dipic
=
pyridine-2,6-dicarboxylate), is
kinetically disfavoured.[51] The oxidation of 1,2-propanediol to
hydroxyacetone, leading to the concomitant reduction of the
vanadium centre from +V to +III, has an energy barrier of
47.5 kcal/mol. We postulate that the high reaction temperature
needed for the V-catalysed DODH of neat glycerol is, in part,
due to the poor performance of glycerol as a reductant.
NH4VO3 appears to show either selectivity towards the
oxidation of primary C–H bonds or no selectivity at all.
Deuteration in the 2-position has little affect on the yield of allyl
alcohol and this effect was nullified during the competition
experiment. In comparison, the DODH 1,1,3,3-d4-glycerol affords
a significantly lower yield of allyl alcohol.
For all but one of the competition experiments, a reduction in
the yield of allyl alcohol was observed when compared to fully
protiated glycerol. In the competition experiments, one might
expect the catalyst to preferentially oxidise protiated glycerol
such that the allyl alcohol obtained in the condensed products
was enriched in deuterated allyl alcohol. Yet, no KIE was
observed in the product distribution and equimolar amounts of
allyl alcohol and deuterated allyl alcohol is obtained. The lack of
KIE being observed in the competition experiments could be due
to a preceding condensation being irreversible or that the
condensation step is rate determining.
DFT calculations propose that Re-catalysed and Mo-
catalysed DODH reaction employing alcohols as reductants,
proceed via an inner sphere mechanism.[16,52–55] Accordingly, the
alcohol is bound to the metal centre it reduces. When acting as
the reductant, whether glycerol is bound to the metal as a
monodentate or bidentate is unknown. If glycerol was bound as
a monodenate, sterics would favour the binding of one of the
primary alcohols over the more hindered secondary alcohols.
Considering the low selectivity in the oxidation of glycerol, we
reason that glycerol binds in a bidentate fashion. As a bidentate,
glycerol can form either a five-membered chelate by binding
through a primary and a secondary OH group or a six-
membered chelate could be formed if the two primary OH group
bind to the metal centre. Three recent publications, pertaining to
the mechanism of the DODH of vicinal diols catalysed by
(nBu4N)[VO2(dipic)], shed light on the condensation step.[51,56,57]
Except for one case, the energy barrier of the condensation of
the second hydroxyl to give a chelating diolate was lower in
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10.1002/cctc.201701049
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FULL PAPER
energy than the initial condensation of the diol with the
vanadium oxo moiety. The mechanism by Fu and co-workers
proposes that the condensation of (nBu4N)[VO2(dipic)] with 1,2-
propanediol to give an alkoxide is lower in energy than the
subsequent second condensation step which affords the diolate.
Nevertheless, binding in a bidentate fashion has a significant
energy barrier, which may be lowered by invoking a water
molecule in a proton relay mechanism.[56] If the binding of
glycerol is irreversible and/or rate-determining, the lack of H/D
scrambling seen in the allyl alcohol product suggests that the
reduction step, which involves the oxidation of glycerol, also is
irreversible.
Council for Independent Research, Grant 11-105487, and a
grant from the Villum Foundation, Grant 341/300-12301 2.
Keywords: glycerol • vanadium • rhenium • DODH •
deoxydehydration
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We have discovered that NH4VO3 is an effective catalyst for
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The vanadium-catalysed
Allan R. Petersen, Lasse B. Nielsen,
Johannes R. Dethlefsen and Peter
Fristrup*
deoxydehydration (DODH) of neat
glycerol using cheap and readily
available ammonium metavanadate
(NH4VO3) affords higher yields of allyl
alcohol than the well-established
catalyst methyltrioxorhenium (MTO).
Using deuterium-labelled glycerol
helped elucidate the dual role of
glycerol as both oxidant and
Page No. – Page No.
Vanadium-catalysed
Deoxydehydration of Glycerol
Without an External Reductant
reductant, but also understand the
formation of byproducts acrolein and
propanal.
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Author(s), Corresponding Author(s)*
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Title
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