Carbodeoxygenation of biomass: The carbonylation of glycerol and higher polyols to monocarboxylic acids

Article abstract of DOI:10.1002/chem.201203069

Glycerol is converted to a mixture of butyric and isobutyric acid by rhodium- or iridium-catalysed carbonylation using HI as the co-catalyst. The initial reaction of glycerol with HI results in several intermediates that lead to isopropyl iodide, which upon carbonylation forms butyric and isobutyric acid. At low HI concentration, the intermediate allyl iodide undergoes carbonylation to give vinyl acetic acid and crotonic acid. Higher polyols C_nH _n+2(OH)_n are carbonylated to the corresponding C _n+1 mono-carboxylic acids. Copyright

Full text of DOI:10.1002/chem.201203069

DOI: 10.1002/chem.201203069
Carbodeoxygenation of Biomass: The Carbonylation of Glycerol
and Higher Polyols to Monocarboxylic Acids
Timur Coskun, Christopher M. Conifer, Laura C. Stevenson, and
[
a]
George J. P. Britovsek*
Abstract: Glycerol is converted to a mixture of butyric and isobutyric acid by rho-
dium- or iridium-catalysed carbonylation using HI as the co-catalyst. The initial re-
action of glycerol with HI results in several intermediates that lead to isopropyl
iodide, which upon carbonylation forms butyric and isobutyric acid. At low HI
concentration, the intermediate allyl iodide undergoes carbonylation to give vinyl
acetic acid and crotonic acid. Higher polyols C H (OH) are carbonylated to the
Keywords: butyric acid
monoxide · carbonylation · glycerol ·
iridium · rhodium
· carbon
n
n+2
n
corresponding Cn+1 mono-carboxylic acids.
Introduction
tion reactions are widespread in the chemical industry, for
example, in the production of acetic acid from methanol in
the Monsanto and Cativa processes, and the underlying
The development of new catalytic processes for the conver-
sion of biomass into biofuels and biochemicals is currently
mechanisms for the carbonylation of lower alcohols are now
[1–3]
[12–15]
of great interest.
Biodiesel is the second most produced
well understood.
The carbonylation of higher alcohols,
biofuel after bioethanol and accounted for 27.5% of the
and in particular polyols, has received much less attention
[4]
[16–23]
global biofuels output in 2011. A major drawback of bio-
diesel is the production of approximately 1 kg of glycerol as
a by-product for every 9 kg of biodiesel. As a consequence,
research into new conversion processes for glycerol as a C3
platform chemical into value-added products is therefore,
both from an economical and environmental point of view,
so far.
We therefore decided to investigate the applica-
tion of carbonylation for the carbodeoxygenation of bio-
mass-derived polyols, starting with glycerol.
Deoxygenated
Hydrodeoxygenation : Biomass þ H !
þ H O
2
2
Biomass
[
5]
ð1Þ
of considerable importance.
Besides gasification to
[6]
syngas, several conversions of glycerol are already being
explored, for example, hydrogenolysis to 1,2- and 1,3-pro-
panediol or the oxidation to dihydroxyacetone.
Deoxygenated
Biomass
Carbodeoxygenation : Biomass þ CO !
þ CO2
ð2Þ
[7–11]
Other
potential products that are being considered are acrylic acid,
[5]
glycerol carbonate and epichlorohydrin.
The first studies on the carbonylation of glycerol were re-
Biomass-derived compounds such as glycerol are oxygen
rich and de-oxygenation is usually carried out by reaction
with H and the formation of H O, generally termed hydro-
[24]
ported by Nakamura in 1979.
A rhodium complex
[
{RhCl(CO) } ] was used in combination with HI as the co-
2
2
2
2
[10]
catalyst at 1808C and 35 bar CO pressure and glycerol was
converted after 80 min to a mixture of 45 mol% butyric
deoxygenation [Eq. (1)]. An alternative strategy, which is
used here, is carbodeoxygenation: the carbonylation with
(
BA) and 30 mol% isobutyric acid (IBA), according to the
CO and the formation of CO [Eq. (2)]. The cost of CO and
2
overall reaction given in Equation (3). The only other report
in the literature on the carbonylation of glycerol is a recent
high-throughput study by Schunk and co-workers, who used
H are comparable, but the product spectrum from carbo-
2
deoxygenations will be different and complementary to the
products obtained through hydrodeoxygenation. Carbonyla-
[
{RhCl(CO) } ] and CH I as the catalyst system and ob-
2 2 3
served the formation of unsaturated acids such as vinyl
acetic acid (VA) and crotonic acid (CA), in addition to the
saturated acids BA and IBA.
[
a] Dr. T. Coskun, Dr. C. M. Conifer, L. C. Stevenson,
Dr. G. J. P. Britovsek
Department of Chemistry
[22]
Imperial College London, Exhibition Road
South Kensington, London SW7 2AZ (UK)
Fax. (+44)20-75945804
E-mail: g.britovsek@imperial.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203069.
6840
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 6840 – 6844

FULL PAPER
The saturated and unsaturated C acids are not the prod-
1308C and 30 bar CO pressure, a mixture of eight products
was obtained after 5 h, which were identified as the saturat-
ed acids (BA and IBA), the unsaturated acids (VA and
CA), as well as allyl acetate (AA), allyl iodide (AI), iso-
propyl acetate (IPA) and isopropyl iodide (IPI), together
with 38 mol% unreacted T (see run 1 and Figure 1).
4
ucts one would immediately predict from the carbonylation
of glycerol and no explanation has been provided so far for
this surprising transformation. We present here the results
of our extensive study on the carbonylation of glycerol, in-
cluding mechanistic studies and a comparison between the
performance of rhodium versus iridium-based catalysts. This
systematic study demonstrates that both catalyst systems can
give selective conversion of glycerol to a mixture of BA and
In the experiments with CH I as the co-catalyst, the HI
3
required for the CꢀO bond activation is formed in situ by
the carbonylation of CH I and hydrolysis. Schunk and co-
3
IBA. While these C acids are of industrial interest as esters
workers, who also used CH I, noted that the carbonylation
4
3
in solvent applications, they could also be converted to
other valuable chemicals such as butanol/isobutanol, butyr-
aldehyde or methacrylic acid.
of glycerol was affected by the amount of water in the
system: at low water concentration, significant amounts of
the unsaturated C acids VA and CA were obtained, where-
4
as a higher water concentration led to the formation of BA
[22]
and IBA. In our experiments under the same conditions
as in run 1, a significant improvement was observed when
HI was used as the co-catalyst (run 2). Full conversion of tri-
acetin was observed and no unsaturated acids were ob-
tained. Increasing the temperature to 1808C and prolonged
reaction times resulted in the conversion to BA and IBA as
the major reaction products (runs 3 and 4). It is tempting to
propose that the unsaturated
Results and Discussion
Carbonylation studies: Carbonylation experiments were car-
ried out in acetic acid/water at various pressures (20–45 bar)
and temperatures (130–1808C), selected runs are listed in
Table 1. In acetic acid, glycerol is rapidly acetylated to a
acids VA and CA could be in-
termediates in the formation of
[
a]
Table 1. Results of the rhodium- and iridium-catalysed carbonylation reactions
.
Run Sub Co
Cat
T
t
All products [mol%]
8C] [h] IBA BA IPA IPI AA AI PA PI CA VA
Total
[mol%]
BA and IBA by isomerisation
and hydrogenation (through the
water–gas shift reaction). How-
ever, the presence of IPI and
IPA, and n-propyl iodide (PI)
and n-propyl acetate (PA) at
[
T
ACHTUNGTRENNUNG
[
[
[
[
[
b]
b]
b]
b]
b]
1
2
3
4
5
T
T
T
T
G
T
CH
HI
HI
HI
HI
CH
3
I
[Rh] 130
[Rh] 130
[Rh] 180
[Rh] 180 17 41
[Rh] 180
5
5
5
1.3
5.8
26
1.6 22
4.6 57
13 4.9 2.1
0
0
0
0
5.5 8.1 38
96
100
83
100
100
49
33
51
56
33
33
5.9
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
44
59
63
0.1
12
5.2
0
1.0
0
0.6 1.7
0
0
0
0
0
0
5
5
5
5
5
5
35
0.1
14
0.4
6.5
13
1.5
0
5.5
0
30
0
0
4.8
0
[
c]
c]
c]
c]
c]
c]
c]
6
7
8
9
0
1
2
3
3
I
I
[Ir]
[Ir]
[Ir]
[Ir]
[Ir]
[Ir]
[Ir]
[Ir]
[Ir]
130
180
130
150
180
0.3 0.3 14
higher
temperatures
(see
[
[
[
[
[
[
AA CH
1.0
0.5 0.5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
run 3), are not easily explained
by this mechanism.
The iridium-catalysed car-
T
T
T
T
T
T
T
T
HI
HI
HI
HI
CH
HI
HI
HI
0.5 12
8.0 31
6.1 14
18
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
11
2.0
0
0
0
0
0
0
0
0
1
1
1
3
1
15
18
2.9
0
180 17 17
180 17 8.1
180
180
35
40
96
53
bonylation
reaction
with
3
I
6.7 11
44 7.8
9.8 11
14 4.2
12
7.9
16
4.5
0.7 2.3
1.0 4.8
0.9 7.8
[
d]
[NBu ][IrCl (CO) ] in combina-
ACHTUNGTRENNUNG
4
2
2
1
5
5
31
7.5
[
e]
4
tion with CH I as the co-cata-
3
[
f]
1
5
none 180 17 16
0
0
39
lyst, at 1308C and 30 bar CO
[
[
(
[
(
a] Sub=Substrate (T=triacetin, G=glycerol, AA=allyl acetate); Co=Co-catalyst; Cat=Catalyst: [Rh]= pressure, produced mainly AA
RhCl(CO) and [Ir]=[NBu [IrCl (CO) ]. [b] Conditions: [RhCl(CO) (0.05 mmol), CH I (13 mmol) or HI and AI, and small amounts of
57 wt%, 13 mmol), triacetin (27 mmol), acetic acid (6.7 mL), water (1.5 mL), CO (30 bar). [c] Conditions:
NBu [IrCl (CO) ] (0.1 mmol), CH I (13 mmol) or HI (57 wt%, 13 mmol), triacetin (27 mmol), acetic acid
6.7 mL), water (1.5 mL), CO (30 bar). [d] Promotor [Ru (CO)12], (127 mg, 0.2 mmol) was added. [e] Instead
, 3:1, 40 bar pressure). [f] no catalyst, HI (57 wt%, 13 mmol), triacetin
27 mmol), acetic acid (6.7 mL), water (1.5 mL).
2
]
2
4
]
A
H
U
T
E
N
N
2
2
2
]
2
3
the acids (run 6). In a separate
experiment (run 7), in which
AA was used as the substrate, it
was confirmed that AA is
indeed an intermediate in the
4
]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
2
3
3
of CO, syngas was used (CO/H
(
2
mixture of mono-, di- and tria-
cetoxyesters. For convenience,
we therefore used the less vis-
cous triester triacetin (1,2,3-tri-
acetoxypropane, T) as the sub-
strate, which gave very similar
results as glycerol (for example,
see runs 3 vs. 5). In our initial
experiments,
[{RhCl(CO) } ]
2 2
was used in combination with
CH3I as the co-catalyst. At Figure 1. Product distribution from the carbonylation of triacetin (run 1 in Table 1, all values in mol%).
Chem. Eur. J. 2013, 19, 6840 – 6844
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6841

J. P. Britovsek et al.
formation of BA and IBA.
Using HI as the co-catalyst re-
sulted in an increased yield of
BA and IBA, which was further
improved by raising the temper-
ature (runs 8–10) and longer re-
action times (run 11). Under
these conditions, HI is clearly a
superior co-catalyst than CH3I
(
runs 8 vs. 6 and 11 vs. 12).
It should be noted that under
the same conditions as run 11,
but in the absence of a metal
catalyst, carbonylation took
place due to HI by the Koch re-
[
25]
action (run 15).
The results
from run 11 are only marginally
better than this blank experi-
ment without a metal catalyst.
It is well known that iridium-
1
2
Figure 2. H NMR spectra of the reaction of triacetin (T) with ten equivalents of DI (53% in D O) in
based carbonylation catalysts CD COOD (for labels, see Figure 3; *=triacetin hydrolysis products, #=allyl alcohol, + =1,3-diiodo-2-propa-
3
rapidly deactivate due to the nol).
ꢀ
formation of [IrI (CO) ] and
4
2
promotors are needed that can
[
30]
reversibly bind iodide in order to regenerate the deactivated
that were easily identified,
the sequence of transforma-
[
26]
catalyst.
Indeed, orange crystals obtained at the end of
tions shown in Figure 3 is observed. Double nucleophilic
substitution results in the formation of 1,3-diiodo-2-acetyl
propane (M), which is followed by elimination of iodine and
a 2,1-shift of the acyl group to the terminal position to give
allyl acetate (AA). Similar observations and formation of
allyl acetate have been reported from the reaction of 1,3-di-
several runs were identified as [NBu4]
A
H
U
G
R
N
N
[IrI (CO) ] by IR
4
[15]
2
ꢀ
1
spectroscopy (n(CO)=2109 and 2071 cm ). The best re-
sults for iridium were obtained in the presence of the pro-
motor [Ru (CO) ] (run 13). These results are comparable to
[31]
3
12
those obtained with the rhodium-based catalyst under the
same conditions (run 13 vs. 3). Instead of CO, syngas (CO/
H =3:1) was used in run 14 for the carbonylation reaction,
2
[32]
chloro-2-acetyl propane with sodium iodide. Further sub-
stitution results in allyl iodide (AI) and addition of DI fol-
but gave inferior results. Experiments at different pressures
have shown that the BA/IBA ratio increases with increasing
pressure, similar to the observations reported by Dekleva
and Forster for the carbonylation of n-propanol and isopro-
lowed by elimination of I generates propene (P), which un-
2
dergoes addition of DI to give finally isopropyl iodide (IPI),
together with some isopropyl acetate (IPA). Oxidative addi-
tion of IPI at rhodium and iridium results in isomerisation
[16,17]
[33]
panol.
to both isopropyl and n-propyl metal complexes, and as a
result, the carbonylation of IPI results in the formation of
both BA and IBA. It should be noted that at higher temper-
atures (1808C) small amounts of n-propyl iodide are ob-
served in the reaction mixture, which suggests that CO in-
sertion competes with reductive elimination at these temper-
atures.
Mechanistic studies: Despite the presence of three hydroxyl
functionalities in glycerol, carbonylation results exclusively
in monocarboxylic acids, rather than tricarboxylic acids.
Similarly, the carbonylation of 1,2-diols, such as glycol, 1,2-
propane diol and cyclohexane diol, produce only monocar-
[22,23]
boxylic acids.
In order to explain this surprising selectiv-
Several observations support the proposed reaction se-
quence in Figure 3. Whilst the conversion of triacetin in
runs 8–12 was quantitative, the total amount of product re-
covered from the liquid phase at the end of the reaction did
not exceed 56 mol%. Qualitative analysis of the gas phase
ity, we have investigated the reaction of triacetin with HI in
1
acetic acid by H NMR spectroscopy. First investigations on
the reaction of glycerol with HI date back to the 19th centu-
[
27,28]
ry,
and a study in the 1950s revealed that allyl iodide is
1
an intermediate in the reaction and that eventually isoprop-
by GC and H NMR analysis at the end of these runs re-
[29]
yl iodide is formed. We have revisited this reaction under
conditions that are similar to those used for the carbonyla-
tion experiments. The product mixture that is formed upon
vealed the presence of propene, which would explain the ob-
served carbon imbalance in these experiments. Noteworthy,
propene was also observed by Dekleva and Forster in the
[16,17]
reacting triacetin with ten equivalents of DI (53% in D O)
carbonylation of n-propanol and isopropanol.
It can be
2
1
in CD COOD was monitored by H NMR spectroscopy (see
seen from Figure 2 that the reaction of DI with allyl iodide
(AI) and with propene (P) to give IPI are slow and require
extensive heating at 1008C. These reactions will be acceler-
3
Figure 2). Over time and with increasing temperature, be-
sides partial hydrolysis to di- and monoacetylated glycerol
6842
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Chem. Eur. J. 2013, 19, 6840 – 6844

Carbodeoxygenation of Biomass
FULL PAPER
Figure 3. Sequence of intermediates and products from the reaction of triacetin with HI.
ated at higher HI concentrations and increased temperature.
The formation of the unsaturated acids VA and CA are the
products observed at low temperature, short reaction times
formation of pentanoic acid (3.3 mol%) and 2-methylbutyric
acid (5.2 mol%). The lower conversion in this case is be-
lieved to be due to the lower effective iodide concentration,
as seven equivalents of iodide are needed in this case to
reduce erythritol to 2-iodobutane. Furthermore, the reaction
of sorbitol C H (OH) with HI has been reported to gener-
and CH I as the co-catalyst and must therefore originate
3
from the carbonylation of AI, before further conversion of
AI to IPI can take place (see Figure 3). Conversely, the use
of HI as the co-catalyst, higher temperatures and longer re-
action times favour the formation of IPI and result in the
carbonylation to the saturated acids BA and IBA exclusive-
ly.
6
8
6
[39]
ate 2-iodo hexane. Nakamura reported that the carbony-
lation of ethylene glycol resulted in propionic acid and that
sorbitol gave the isomeric C acids heptanoic acid and 2-
7
[24]
methyl hexanoic acid.
Taking all these results into ac-
The overall reaction of glycerol to IPI requires five equi-
valents of iodide and generates two equivalents of iodine
count, the following general reaction can be postulated for
the carbodeoxygenation of polyols as shown in Equation
(4). For methanol (n=1) only one product, acetic acid, is
obtained, which is the industrially applied Monsanto or
Cativa process. Also for ethylene glycol (n=2) only one
product, propionic acid, is formed, whereas for higher poly-
ols for which n>2, a mixture of two isomeric Cn+1 mono-
carboxylic acids is obtained.
(
see Figure 3). Iodide was added to the reactor either in the
form of HI or CH I, but only 0.5 equivalents of iodide was
3
used in all runs relative to triacetin, which implies that I2
ꢀ
must be reduced in situ back to I . The reductant is believed
[34]
to be either H , formed by the water–gas shift reaction,
2
and the reaction of I with H to HI is catalysed by rhodium
2
2
[35,36]
or iridium complexes.
It is also possible that I reacts
2
ꢀ
ꢀ
with [IrI (CO) ] to give [IrI (CO) ] , which can react with
2
2
4
2
ꢀ
CO and water to regenerate the catalyst [IrI (CO) ] togeth-
2
2
[34]
er with CO and two molecules of HI. The use of syngas
2
(
CO/H ) instead of CO (run 14) was also investigated, but
2
did not lead to better conversion. Deactivation of the iridi-
um catalyst is likely to prevent both carbonylation and
[
37]
iodide re-activation. Noteworthy, the reduction of glycerol
Conclusion
with HI to allyl iodide and I is related to the recently revis-
2
ited di-dehydroxylation reaction of glycerol to allyl alcohol
with formic acid, whereby formic acid acts as the stoichio-
metric reductant.
The catalyst systems [{RhCl(CO) } ] in combination with the
2
2
ꢀ
co-catalyst HI, or [IrCl (CO) ] together with the promotor
2
2
[38]
[Ru (CO) ] and HI, can carbonylate glycerol to butyric and
3 12
The sequence of reactions shown here for triacetin in
Figure 3 also applies to other 1,2-diols and higher polyols re-
sulting in the formation of mono-iodo alkanes. H NMR
iso-butyric acid with comparable activities and selectivities.
A key intermediate, allyl acetate, is formed prior to the
actual carbonylation reaction by the reduction of glycerol
with HI. At low HI concentration, allyl acetate is carbony-
lated to vinyl acetic acid and crotonic acid, whereas at
higher HI concentrations allyl acetate is converted to iso-
propyl iodide, which is carbonylated to butyric and isobuty-
ric acid. The carbonylation reaction can be applied to higher
1
analysis of the reaction of ethylene glycol with six equiva-
lents of DI in CD COOD has shown the formation of ethyl-
3
ene as an intermediate and ethyl iodide as the final product
(
see Figure S1 in the Supporting Information). Preliminary
results on the carbonylation of erythritol C H (OH) under
4
6
4
the same conditions as for run 3 in Table 1, resulted in the
polyols C H (OH) to give the corresponding C
n+1
mono-
n
n+2
n
Chem. Eur. J. 2013, 19, 6840 – 6844
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6843

J. P. Britovsek et al.
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products using this carbodeoxygenation strategy.
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Acknowledgements
5
We thank the Department of Chemistry at Imperial College London for
financial support and Johnson Matthey for a generous loan of rhodium
and iridium salts. We thank Dr. Ayako Miyazaki for the translation of
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Received: August 29, 2012
Revised: February 19, 2013
Published online: March 27, 2013
6844
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