Deoxydehydration of glycerol to allyl alcohol catalyzed by rhenium derivatives

Article abstract of DOI:10.1039/c4cy00631c

The deoxydehydration (DODH) of glycerol is effectively catalyzed by rhenium derivatives, either in neat glycerol or in the presence of solvents (in particular alcohols), in air or under hydrogen bubbling. Methyltrioxorhenium (MTO) and ReO₃were the only rhenium catalysts tested that can selectively catalyze the DODH reaction at very low temperatures (140 °C). The presence of oxygen is not necessary, although under nitrogen the reaction requires higher temperatures to occur. On the other hand, the presence of hydrogen often noticeably increased the selectivity versus allyl alcohol formation, reaching the considerable value of 90% in the case of reaction conducted in 2,4-dimethyl-3-pentanol with ReO₃. The DODH reaction always exhibits a definite induction time that, in the case of MTO, corresponds more or less to the time required for its demethylation. Metal catalysts in both high-likely rhenium(vi)-and low oxidation states are involved. Re-addition of fresh glycerol at the end of the reaction indicates the feasibility of the reuse of the catalysts. This journal is

Full text of DOI:10.1039/c4cy00631c

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Deoxydehydration of glycerol to allyl alcohol
catalyzed by rhenium derivatives
Cite this: DOI: 10.1039/x0xx00000x
a
b
a
*a
Valentino Canale, Lucia Tonucci, Mario Bressan and Nicola d’Alessandro
Received 00th January 2012,
Accepted 00th January 2012
The deoxydehydration (DODH) of glycerol is effectively catalyzed by rhenium derivatives,
either in neat or in presence of solvents (in particular alcohols), in air or under hydrogen
DOI: 10.1039/x0xx00000x
bubbling. Methyltrioxorhenium (MTO), together with ReO , were the only rhenium catalysts
3
www.rsc.org/
tested able to selectively catalyze the DODH reaction at very low temperatures (140 °C). The
presence of oxygen is not necessary, although under nitrogen the reaction requires higher
temperatures to occur. On the other hand, the presence of hydrogen often increased noticeably
the selectivity versus allyl alcohol formation, reaching the considerable value of 90% in the
case of reaction conducted in 2,4-dimethyl-3-pentanol with ReO . The DODH reaction always
3
exhibits a definite induction time that, in the case of MTO, corresponds more or less to the
time required for its demethylation. Metal catalysts in both high - likely rhenium(VI) - and low
oxidation states are involved. Re-addition of fresh glycerol at the end of reaction indicates the
feasibility of the re-use of the catalysts.
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diols to the corresponding alkenes, via a cyclic thionocarbonate
Introduction
intermediate, followed by
triethylphosphite reagents. In 1996 Cook and Andrews applied the
reaction on renewable biomass carbohydrates in presence of rhenium
metal catalyst and triphenylphosphine as reductant. Replacement
of the organophosphorous reagents with greener reducing agents was
then proposed, among them H2, sulfite and alcohols. Recently ,
a
reduction by trimethyl- or
2
4
1
According to the modern concepts of "Green Chemistry", by-
products and waste, obtained in large quantities from several
industrial applications, are opening new scenarios and exciting
2
5
2
possibilities. Glycerol is the by-product formed during the
26
27
28
transesterification of vegetable oils for the production of biodiesel:
for every ton of biodiesel produced, about 10 kg of glycerol are
the Abu-Omar group reported a rhenium catalysed DODH of neat
29
3
glycerol to give, as main reaction product, allyl alcohol. The fact
that glycerol acts, at the same time, as the reducing agent, the
reactant and the solvent, makes the “green” convenience of the
above process; on the other hand, even if glycerol was quantitatively
converted, the yield in allyl alcohol never exceeded 50% mol/mol,
by being the remaining 50 % transformed into dihydroxyacetone,
which, in large part, spontaneously polymerizes in the reaction
medium.
released. Until a few years ago, all the glycerol necessary for its
applications, like food (24%), personal care (23%), oral hygiene
(17%), tobacco (11%), pharmaceutical (7%), polyols (8%) and other
minor applications (10%), was synthesized from propylene, mainly
via epichlorohydrin. Currently, Solvay , by simply reversing the
direction of the process in the industrial plant, produces
epichlorohydrin starting exclusively from bioglycerol (Epicerol
Tavaux, France). In fact, in this regard, Solvay has formed a strong
partnership with Diester, a leader in the production and marketing of
biodiesel in France.
4
TM
TM
;
5
In the present investigation we describe the conversion of
glycerol to allyl alcohol, operating either in absence or in presence
of solvent and in aerobic conditions or under
a hydrogen
In the last decade there have been alternative proposals for the
expansion of the applications of glycerol, i.e. as an additive in
atmosphere. Two distinct classes of solvents were selected, namely
chemically inert and oxidizable ones, the latters also acting as a
sacrificial reducing agents. Initially, we took into consideration only
methyltrioxorhenium (MTO) catalyst, which was widely employed
6
7
cement, as antifreeze agent (both as is or transformed into the
8
9
more appropriate 1,2-propanediol ) or as a sustainable solvent.
However, with the increasing biodiesel production and the
resulting increased supply of glycerol, the market is presently
nearing saturation. This prompts research to investigate for selective
chemical transformations of glycerol, aiming to obtain derivatives
with high added value and wide-ranging uses as new fuels, polymers
2
3a,26,27b,29
in previous investigations.
However, although MTO is
30
reasonably stable in both organic and aqueous solvents, we felt
appropriate to extend the investigation to more stable, and sometime
less expensive rhenium derivatives, like ReO
3
, NH
4
ReO
4
2
, Re (CO)10
1
0
and pentamethylcyclopentadienyltrioxorhenium (Cp*ReO
3
).
and fine chemicals.
Glycerol, like all other natural polyols, is exceedingly oxygen-
rich, when compared with the majority of current commodity
Experimental
11
chemicals and fuels. The result is a poor solubility in organic
solvents, a relative thermal instability and a limited chance for All commercial reagents and solvents were used as received without
selective functionalization. Conversion of glycerol to less further purification. Glycerol, NH ReO cis- and trans-1,2-
oxygenated derivatives might open new scenarios in several fields, cyclohexanediol were purchased from Sigma-Aldrich, while MTO,
,
4
4
1
2
like synthesis of biopolymers (e.g. via 1,3-propanediol),
Re
introduction of new fuels of alcoholic nature (e.g. propanol) and Chemical Inc. Cp*ReO
production of building blocks for industrial applications (e.g. allyl procedure previously reported in the literature.
2
(CO)10, ReO
3
and Cp*Re(CO)
3
were purchased from Strem
1
3
3
was prepared following
a
synthetic
3
1
1
4
alcohol, acrolein, propene). Catalysis and biocatalysis are probably Nuclear magnetic resonance (NMR). NMR spectra were recorded
the more important tools able to drive the selective transformation of on Bruker Avance DRX-300 spectrometers (7.05 Tesla) equipped
any biomass or biomass-derived feedstocks, like glycerol, to the with a high-resolution multinuclear probe that operated in the range
desired added value chemical. Although heterogeneous catalysis was of 30-300 MHz. Spectra were recorded in an NMR tube (5 mm) that
deeply investigated in the past, recently, many research groups are contained a closed co-axial capillary tube that was filled with a 30
trying to develop innovative homogeneous catalytic pathways able mM 3-trimethylsilyl-2,2,3,3-tetradeutero propionic acid (sodium
to competitively obtain good results in term of yields, selectivities salt). Free induction decays were acquired at 22°C using a pulse
1
5
1
6
and sustainability of the entire procedure.
sequence (Bruker-made; zgcppr) that suppresses the water signal at
Several catalytic pathways were proposed to achieve efficient, 4.7 ppm. The spectral width was -1 to 12 ppm (3894.081 Hz). A 90°
cheap and “green” deoxygenation, among them selective thermal excitation pulse and a 1 s relaxation delay were used to collect 64
1
7
15b
18
1
processing, hydrogenolysis,
acid-catalyzed dehydration and scans. Proton NMR ( H NMR) spectra in CDCl
3
solution were
19
bio-mediated reactions. However, there is an alternative and very acquired using tetramethylsilane as reference, and the standard pulse
1
3
competitive methodology, namely deoxydehydration reaction sequence (zg) was used. For the carbon NMR spectra ( C NMR), a
2
0
(
DODH), which to date have been only marginally investigated.
proton decoupling pulse sequence (zgdc) was used. The spectral
DODH can remove two adjacent hydroxyl groups from vicinal diols width was 0-240 ppm (18115.941 Hz), while the 90 excitation pulse
to afford the corresponding unsaturated derivatives, namely, in the and 5 s relaxation delay were used to collect 12,000 scans.
case of glycerol, allyl alcohol, a useful building block commonly GC-MS analyses. They were performed using an ISQ single
employed for several applications like, for example, the synthesis of quadrupole instrument by Thermo Fisher Scientific. The GC column
DAP (diallyl phthalate; a precursor of diallyl phthalate thermoset was a low-polar HP-5MS capillary column (Hewlett & Packard), 30
polymers, mainly used in moldings and coatings for electronic m length, 0.25 mm diameter and 0.25 ꢀm film thickness. High purity
21
devices). Noteworthy, in a recent publication by Shiramizu and helium was used for the carrier gas at a constant flow of 0.7 ml/min.
Toste, was reported an extension of the DODH reaction to not For the water solution, 1 ꢀL was injected while for headspace
vicinal diols allowing alternative potential applications on natural analysis, the sample was thermostated at 45 °C for 3 min, after that
2
2
feedstock. DODH, which always requires the presence of a suitable 100 ꢀL of aerial top layer solution was injected to the GC apparatus
2
3
catalyst, often high-valent oxorhenium complexes, was first by a gas-tight syringe, with a 5:1 split ratio. The temperature
reported by Corey and Winter in 1963, for the conversion of vicinal
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program were: i) solutions; began at 50°C with a hold time of 5 min Fig. 1 Schematic representation of the reactor employed for
and then increased at a rate of 10°C/min to 250 °C with a hold time rhenium catalyzed DODH of glycerol
of 5 min; ii) headspace; isotherm at 50 °C for 30 min.. The injection
inlet temperature was 250 °C and mass spectrometer transfer line Reactions in neat glycerol
were held at 280 °C. The electron impact (EI) ion source was held at
ReO3,
cyclopentadienyl trioxorhenium (Cp*ReO
NH ReO were tested as catalysts. At 140 °C DODH of
glycerol (in air or H ) took place only in presence of MTO and
ReO while, at higher temperature (170 °C), the reaction
methyltrioxorhenium
(MTO),
pentamethyl-
2
00 °C, with a filament bias of -70 V. Mass spectra were collected
3
), Re
2
(CO)10 and
from 31 to 450 m/z at 100 spectra/sec.
GC analyses. For methane detection, the GC analyses were
4
4
2
performed using
a Hewlett and Packard gaschromatograph
3
apparatus, model 6890. The GC column was a polar HP-INNOWax
capillary column (Agilent Technologies), 60 m length, 0.32 mm
diameter and 0.5 ꢀm film thickness. High purity helium was used
for the carrier gas at a pressure on the head of the column of 50 kPa
occurred with all the above catalysts, at various degree of
conversions, yields, selectivities and amounts of by-products
diallyl ether, acrolein and allyl formate) (Table 1).
(
(
1 ml/min, measured at 60 °C). For the water solution, 1 ꢀL was
injected while for headspace analysis, the sample was thermostated a
5 °C for 3 min, after that 100 ꢀL of aerial top layer solution was
Table 1 Allyl alcohol yields with several rhenium catalysts, in
aerobic or H atmosphere and in neat glycerol. In bracket are
reported the % of by-products; a: acrolein; b: diallyl ether; c: allyl
formate.
2
4
†
injected to the GC apparatus by a gas-tight syringe, with a 5:1 split
ratio. The temperature program were: i) solutions; began at 50°C
with a hold time of 5 min and then increased at a rate of 10°C/min to
Catalyst
Air
H
2
2
50°C with a hold time of 5 min; ii) headspace; isotherm at 50 °C
for 30 min.. The injection inlet temperature was 260 °C and 280 °C
for the FID detector.
140 °C
React. timeφ
14; a(1); b(1); c(0)
1300
32; a(2); b(tr); c(0)
500
MTO
Typical reaction procedure. In a 10 mL glass vessel containing a
magnetic stirrer and provided with a rubber septum were initially
placed 5 mmol (0.460 g) of glycerol and 0.1 mmol of metal catalyst.
In the reaction performed in solution, 5 mL of solvents were added.
The vessel was connected to a cold water trap (100 mL, around
170 °C
15; a(3); b(2); c(1)
25; a(3); b(1); c(1)
React. time
180
160
0
°C) by a steel double pointed needle dipping one. The vessel was
1
40 °C
0
0
then continuously purged with air, nitrogen or hydrogen (≈1 bubble
per second), depending on the typology of the experiment,
meanwhile it was heated to the desired temperature by
thermostated oil bath. The cold trap was sampled at the desired
reaction time (250 ML each time), for the NMR spectra and for the
headspace gaschromatographic analyses (GC and GC-MS). Above
all on the reactions conducted in alcoholic solvents, with both
4 4
NH ReO
React. time
170 °C
480
480
a
tr; a(0); b(0); c(0)
5; a(1); b(1); c(0)
React. time
720
600
140 °C
0
0
Cp*ReO
3
3
catalysts (MTO and ReO ), the reaction mixtures underwent the
React. time
480
10; a(2); b(1); c(0)
250
480
same clear-cut and reproducible changes in colour during the DODH
reaction, by turning from yellowish (immediately after the induction-
time) into red-purple, when the formation of allyl alcohol reaches the
maximum rate, and finally black at the end of reaction, i.e. when
glycerol was completely removed.
170 °C
21; a(1); b(tr); c(0)
React. time
200
140 °C
32; a(3); b(2); c(0)
1275
34; a(0); b(0); c(0)
ReO
3
Results and discussion
React. time
1005
Rhenium-catalysed DODH of glycerol was carried out in
aerobic or deoxygenated conditions, in the presence or the
170 °C
30; a(4); b(3); c(1)
210
30; a(tr); b(tr); c(0)
React. time
210
0
absence of H
2
stream, and at temperatures comprised between
130 °C and 170 °C. Recovery of reaction products was
140 °C
0
480
§
achieved by a ice-cooled water trap collecting the gas-flow Re
coming from the reaction vessel, where air (or N or H ) was
2
(CO)10
React. time
480
§
2
2
continuously bubbled (Fig. 1). The reported yields were
generally referred to those measured at the end of the reaction,
regardless of the reaction times required.
170 °C
React. time
1080
1080
†
:
One mole of diallyl ether, to be formed, require two moles of allyl
alcohol, therefore the present yield must be doubled for a correct mass
balance evaluation.
φ
:
Reaction times are expressed in minutes
2
At 170 °C, the reaction occurs, but Re (CO)10 sublimes, making
§
:
difficult the purging of reaction mixture; therefore, yields are scarcely
reproducible
Under aerobic conditions (air bubbling), the best selectivity
in allyl alcohol was achieved at 140 °C with ReO
ca 21 hrs), while with MTO the yield was definitely lower
13%; dotted lines in Fig. 2); in both experiments, a
3
(32 % after
(
1
quantitative conversion of glycerol was observed (by H NMR).
In the experiments at 140 °C, with MTO or ReO we always
observed distinctive induction times in the 40-60 min range
3
(
Figure S1 and S2). Finally, the measured mass balances were
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3
always lower than the expected 50%, the maximum value for a Table 2 Allyl alcohol yields with MTO and ReO catalysts in
DODH reaction conducted in presence of only glycerol, acting various solvents.
also as reducing agent.
Solvent
Air
H
2
5
4
4
3
3
2
2
1
1
0
5
0
5
0
5
0
5
0
5
0
1
0
40°C
170°C
0
140°C
0
170°C
0
MTO
Ethylene
carbonate
ReO
3
0
0
0
0
0
0
0
0
MTO
DMPU
ReO
MTO
ReO
MTO
3
0
0
0
0
0
0
0
7; a(7)
0
200
400
600
800
1000
1200
1400
1600
reaction time (min)
Sulpholane
3
§
§
†
§
§
†
28;
a(2)
70;
a(1)
Fig. 2 Rhenium catalyzed (ReO
neat glycerol at 140 °C. Time course of allyl alcohol (triangles:
ReO ; circles: MTO; continuous line: under H bubbling; dotted
line: under air bubbling)
3
and MTO) DODH of glycerol in
1
-Hexanol
3
2
ReO
3
20;
a(2)
†
†
†
22
†
†
†
At
conversions were observed with both catalysts, while at 170 °C
where the reactions were obviously faster), the reaction
patterns were similar, with only slightly larger amounts of by-
products (e.g. for MTO: diallyl ether, 2%, allyl formate, 1%,
acrolein, 3%), thus indicating that higher temperatures did not
dramatically trigger side-reactions (Table 1): diallyl ether
which requires two mol of glycerol to be formed) and allyl
formate likely arise from the allyl alcohol pathway, as indicated
by a previous study, where rhenium catalysts were proved to
a slightly lower temperature (130°C), negligible
MTO
ReO3
MTO
61;
a(2)
87;
a(1)
(
DMP
64;
a(1)
91;
a(tr)
n.e.i.
13;
a(11)
n.e.i.
22-25;
a(7)
(
2
1
-Octanol
,3-
ReO
3
§
0
§
§
§
3
2
catalyze the dehydration of alcohols to symmetrical ethers.
MTO
4;
a(2)
1;
a(1)
31;
a(26)
In another set of experiments, we conducted the same
reactions by bubbling nitrogen instead of air. At 170 °C yields
in allyl alcohol were similar (14%, MTO) or lower (23%,
Propanediol
ReO
3
0
2;
a(2)
0
31;
a(25)
3
ReO ) to those reported above for the aerobic reactions, while
at 140 °C yields collapsed to less than 5% (Figures S3 and S4).
Finally, by bubbling a moderate flux of hydrogen at 140°C
the yields in allyl alcohol reached the noticeable values of 32-
MTO
0
0
0
0
0
0
1
-Phenyl-
#
3
3
4 % (with both MTO and ReO ; continuous lines in Fig. 2; for
ethanol
a representative GC-MS see Figure S6), even if at higher
temperature (170°C) yields were lower (25-30%) and with
larger amounts of by-products (especially with MTO). The
mass balance was not easily evaluable, because of the feasible
competition between glycerol and hydrogen in the reduction
step: nevertheless, the whole molar glycerol transformation
ReO
3
0
0
n.e.i.: Not easily interpretable.
§
†
3
: The solubility of ReO is very low.
: The temperature of the reaction solution is lower than the boiling
point of the solvent.
#
: Acetophenone was the only product detected inside the water trap.
(
allyl alcohol + 2 allyl ether + acrolein) reached about 34-35 %
at 140 °C and 30 % at 170 °C. The reaction mixtures contained,
as expected, the polymerized form of glyceraldehyde and/or
dihydroxyacetone, while no evidence of intact MTO was found
at the end of the reaction, as indicated by the absence of the
Therefore, further experiments were carried out with a
choice of alcoholic solvents, i.e. 1-hexanol, 2,4-dimethyl-3-
pentanol (DMP), 2-octanol and 1,3-propanediol, with the aim to
replace as reducing agents both glycerol and hydrogen;
experiments were carried out at 140°C and, when possible, at
1
diagnostic H NMR signal of the methyl group.
1
70 °C, under air or by H
At 140 °C the aerobic reactions in DMP led to higher yields
of allyl alcohol (61% - 64%, for MTO and ReO , respectively;
see Fig. 3), while in 1-hexanol yields were definitely lower
29% - 19%, for MTO and ReO , respectively; see Fig. 4). In 2-
2
bubbling.
Reaction in solution
3
We tested some inert, green solvents, like sulpholane, ethylene
carbonate and dimethylpropylurea (DMPU), by both air and H2
bubbling and in presence of the only two catalysts, which had
proven to be effective in neat. Only the reactions performed in
(
3
octanol the solubility of glycerol was too low to allow the
detection of a clear and unambiguous reactivity of the substrate,
whereas in 1,3-propanediol the reactions did not occur. Under
sulpholane at 170 °C, in presence of H
2
lead to allyl alcohol
although in low yields, around 7%), together with acrolein
around 7%); glycerol was recovered unchanged when
(
(
H
2
atmosphere, the yields of allyl alcohol reached the
outstanding values of 87% (MTO) and 91% (ReO ) in DMP,
3
conducting the reactions in DMPU and ethylene carbonate
Table 2).
3
and 70% (MTO) and 22% (ReO ) in 1-hexanol (continuous
1
(
lines in Fig. 3 and Fig. 4; for a representative H NMR
4
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spectrum and headspace-GC-MS chromatogram of the aqueous DMP, we detected (combining the amounts found in the
trap see Figures S7 and S8); in 1,3-propanediol no reaction was reaction vessel and in the water trap) 3.9 mmol of the
observed. Acrolein was never formed, whereas several other corresponding ketone, i.e. 10% of the initial DMP; considering
by-products were detected, all involving transformations of the also that ca. 10% of the initial glycerol (i.e. 0.5 mmol) was not
solvents (i.e. oxidation, acetalization, etherification etc.), like converted to allyl alcohol, we conclude that, in large part (ca.
hexanal, 2,4-dimethyl-3-pentanone, 2-ethenyl-1,3-dioxane, 85%), it is the alcoholic solvent acting as the reducing agent.
dihexyl ether, etc.
Another dedicated experiment was performed to follow the
fate of MTO during the DODH reaction, by recording the H
1
NMR spectra of the crude reaction mixture and focusing on the
signal of the methyl group, which appears at 2.52 ppm. At 170
100
°
C, demethylation was quantitative and almost immediate, but
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
also at the lowest temperature examined (140 °C), MTO
disappeared in a very short time (< 1 hr) and, significantly,
within the observed induction time. Methane was detected by
GC (see Figure S9). This result implies that the catalytically
active species does not necessarily contain the methyl group
and furthermore, that demethylation could occur already in the
above-mentioned induction step. From Fig. 5 (and Figure S10),
it is clear that only when MTO undergoes complete
demethylation, the DODH reaction becomes dominant, leading
3
to allyl alcohol. Significantly, also in the case of Cp*ReO we
detected, at the end of reaction, the presence of the
corresponding hydrocarbon, i.e. pentamethylcyclopentadiene.
The reaction were also carried out in deuterated DMP (3-D-
0
500
1000
1500
2000
2
,4-dimethyl-3-pentanol): although it is not easy to quantify the
reaction time (min)
reaction rate because of the erratic induction time always
present, the reaction with deuterated alcohol is slightly slower
than that with DMP.
Fig. 3 Rhenium catalyzed (ReO
,4-dimethyl-3-pentanol at 140 °C. Time course of allyl alcohol
triangles: ReO ; circles: MTO; continuous line: under
bubbling; dotted line: under air bubbling)
3
and MTO) DODH of glycerol in
2
(
3
H
2
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
500
1000
1500
2000
2500
reaction time (min)
3
Fig. 4 Rhenium catalyzed (ReO and MTO) DODH of glycerol in
1
-hexanol at 140 °C. Time course of allyl alcohol (triangles: ReO ;
3
2
circles: MTO; continuous line: under H bubbling; dotted line:
under air bubbling)
At 170°C only the experiments in 1,3-propanediol and 2-
octanol have been carried out (the other two alcohols possess a
boiling point lower than 170°C). In air no reaction occurred.
Under H
moderate yields (31% and 23%, with MTO and ReO
respectively), together with large amounts of acrolein (26%-
8%); under the same conditions, but in 2-octanol, MTO lead to
a 22-25% yields of allyl alcohol, together with 7% of acrolein
2
and in 1,3-propanediol, allyl alcohol was formed in
3
,
2
and smaller amounts of diallyl ether, while with ReO
of products was obtained, not easily quantifiable.
3
a number
The relatively high reactivity observed in 1-hexanol and
DMP under hydrogen, prompted us to try to quantify the
amounts of the carbonyl derivative deriving from the solvent
itself. A dedicated experiment was designed with MTO catalyst
in the presence of 5 mmol of glycerol: starting from 40 mmol of
1
Fig. 5 H NMR spectra of the crude reaction mixture performed as
follows: in 2,4-dimethyl-3-pentanol solution in presence of glycerol
and MTO at 140 °C under air bubbling (from bottom to top, after 0,
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15, 30, 45, 100 and 190 minutes). The signal indicated with the
arrow is relative to the methyl of MTO.
Finally, it is noteworthy that further amounts of allyl
alcohol were formed (at a comparable rate) by adding fresh
glycerol to the exhaust reaction mixture containing both MTO
3
and ReO .
Discussion
Among the rhenium catalysts tested in our study, only MTO
and ReO
to allyl alcohol. Indeed, at higher temperatures, also other
rhenium catalysts, like NH ReO and Re (CO)10, are active, but
they exhibited much lower selectivities. A temperature of
40°C results as the best compromise in order to achieve good
3
were able to selectively catalyze DODH of glycerol
4
4
2
1
conversions, acceptable reaction times and satisfactory
selectivities to allyl alcohol (Figs. 6 and 7). The nature of the
carrier gas, although significantly influencing the amount of
formed allyl alcohol, does not change the reaction pattern,
2
which is the same under an inert atmosphere (N ), and in
aerobic conditions, thus indicating that the presence of oxygen
is not critical; on the other hand, the presence of hydrogen,
especially with alcoholic solvents, contributes to a significant
increase of conversions and selectivities (Figs. 6 and 7).
A comparison of the two catalysts, i.e. ReO
3
and MTO,
evidenced that ReO leads to comparable yields and
3
selectivities either in air or under hydrogen current, while MTO
is less efficient when in aerobic conditions. The use of
alcoholic solvents maximizes, as expected, the production of
allyl alcohol: however, the two alcohols tested namely 1-
hexanol and DMP, behave differently, the secondary DMP
exhibiting better reactivity.
Fig. 6 Product selectivity at 140 °C and 170 °C of MTO-catalyzed
DODH of glycerol in neat and in presence of hydrogen or air.
To better clarify the role of hydrogen, we decided to delve
into the topic using deuterium gas rather than H , always in the
2
same experimental conditions seen above (DMP as solvent, 140
°
3
C and both MTO and ReO as catalysts). Whereas the reaction
rates were quite similar (Figure S11), we did not observe
deuteration neither of the allyl alcohol nor on any of the
reaction by-products (usually present in tiny amounts). A
previous study, although conducted in different experimental
conditions, demonstrated that H
2
may be involved in the
DODH reaction mechanism. However, our results with D
appear to rule out the participation of H to the reaction
2
6
2
2
pathway, even if contributing to maximize the turnover
numbers of the catalyst.
3
Fig. 7 Product selectivity at 140 °C and 170 °C of ReO -catalyzed
DODH of glycerol in neat and in presence of hydrogen or air.
6
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Another important question is the pathway by which
acrolein is produced. Even if it is feasible that acrolein results
upon the oxidation of allyl alcohol, the produced amount does
not change dramatically when the reactions at 170 °C are
1
2
Green chemistry, theory and practice, (Eds: P. T. Anastas and J. C.
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2
conducted in aerobic or anaerobic (N atmosphere) conditions;
2
c
in addition, by heating a 1-hexanol solution of allyl alcohol and
MTO under aerobic conditions, no acrolein was found neither
inside the reaction mixture nor inside the cold trapping water
d
e
2
9
solution. In agreement with other authors, we believe that
,
acrolein is formed by parallel pathway, completely
a
2
3
f
independent from the allyl alcohol one.
Biofuels, Bioprod. Biorefining, 2009,
3
, 72-89.
Finally, in order to confirm if the reaction involves, as
nd
2
3a,23b,29,33
3
4
The future of glycerol
:
2
edition (Eds: M. Pagliaro and M. Rossi),
widely assumed,
adduct, we performed a typical catalytic reaction (at 140 °C,
with ReO ) in presence of the cis or the trans isomer of 1,2-
the formation of a metal-diolate
Royal Society of Chemistry: Cambridge, 2010.
3
R. Christoph, B. Schmidt, U. Steinberner, W. Dilla and R. Karinen,
cyclohexanediol. Only the cis isomer was found to react, giving
cyclohexene as the main product, while trans-1,2-
cyclohexanediol was recovered unchanged, even after
prolonged reaction times (48 hrs), a clear indication that the
facile formation of the diolate adduct is a key step for the
DODH reactions, and, in addition, that the step itself is highly
stereospecific.
From the experiments performed in deuterated DMP, it is
clear that the coordination of alcohol for the H/D transfer plays
an important role on the rate determining step (slower the
reaction with 3-D-DMP). Even if we have not at hand other
strong mechanistic data, however we are driven to think that
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ReO
presence of MTO proceeds, above all, only after MTO was
unequivocally transformed into demethylated species,
possibly a polyMTO one, akin to the polymeric organometallic
3
and MTO behave analogously: indeed, the reaction in the
9
1
Y. Gu, F. Jerome, Green Chem., 2010, 12, 1127.
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oxides previously characterized by Herrmann in water,
alternatively, a stable MTO-glycerol adduct, akin to those
6
c
3
5
d
previously identified and characterized at lower temperatures.
e
Conclusions
6
Allyl alcohol is obtained in good yields, starting from the
renewable and (presently) inexpensive glycerol in the presence
1
1
4, 1017.
2 V. Rangaswamy, G. Balu, N. Revagade, A. Hiremath and M.
Kannabiren, Patent application. International Publication Number
WO 2010/079500 A2.
of MTO or ReO
presence of H noticeably speeds-up the reaction and increases
allyl alcohol yields up to a significant 90%, when conducted in
3
and at temperatures as low as 140 °C; the
2
DMP. This is an important finding, because, at the moment, it 13 M. E. Thibault, D. V. Di Mondo, M. Jennings, P. V. Abdelnur, M. N.
represents one of the lower reported temperature for an
effective DODH reaction employing high-valent rhenium
Eberlin and M. Schlaf, Green Chem., 2011, 13, 357.
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1
a
2
8,29
catalysts.
Although MTO clearly acts as a pre-catalyst, none
, was
b
)
of the other rhenium derivatives tested, apart from ReO
3
able to catalyze the DODH reaction at the same, low
temperature, thus suggesting a non-trivial structure of the
catalytically active species.
c
,
2
1
d
Acknowledgements
1
1
1
5 (
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(ITQSA) (CIPE fundings 20.12.04; DM 28497) for financial
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