Tandem isomerization-decarboxylation for converting alkenoic fatty acids into alkenes

Article abstract of DOI:10.1021/cs501019t

We report a facile Ru-catalyzed route to alkenes from unsaturated fatty acids (alkenoic fatty acids) via readily accessible catalyst precursors, [Ru(CO)2RCO2]_n and Ru₃(CO)₁₂. The catalyst apparently functions in a tandem mode by dynamically isomerizing the positions of double bonds in an aliphatic chain and, subsequently, decarboxylating specific isomers with lower activation barriers. Substrates capable of tandem isomerization-decarboxylation processes (oleic acid, undecylenic acid) are readily converted to mixtures of alkenes. A catalytic cycle is proposed that relies on isomerization positioning double bonds proximate to the acid function to enable facile decarboxylation. To elucidate the proposed mechanistic pathway, substrates that do not undergo decarboxylation under these catalytic conditions (methyl oleate) are compared with those that cannot isomerize the position of unsaturation (cinnamic acid). Both were shown to be operational under these catalytic reaction conditions. Another illustrative comparison shows that the saturated octadecanoic acid is 28 times less reactive than the unsaturated counterpart when reacted using this precatalyst.

Full text of DOI:10.1021/cs501019t

Letter
pubs.acs.org/acscatalysis
Tandem Isomerization-Decarboxylation for Converting Alkenoic
Fatty Acids into Alkenes
Rex E. Murray,†,‡ Erin L. Walter,† and Kenneth M. Doll*,†
†Bio-Oils Research Unit, United States Department of Agriculture (USDA), Agricultural Research Service (ARS), National Center for
Agricultural Utilization Research, 1815 North University Street, Peoria, Illinois 61604, United States
‡Department of Natural and Social Sciences, Wayne State College, 1111 Main Street, Wayne, Nebraska 68787, United States
ABSTRACT: We report a facile Ru-catalyzed route to alkenes from unsaturated fatty acids
(alkenoic fatty acids) via readily accessible catalyst precursors, [Ru(CO)2RCO2]n and
Ru3(CO)12. The catalyst apparently functions in a tandem mode by dynamically isomerizing
the positions of double bonds in an aliphatic chain and, subsequently, decarboxylating
specific isomers with lower activation barriers. Substrates capable of tandem isomerization-
decarboxylation processes (oleic acid, undecylenic acid) are readily converted to mixtures of
alkenes. A catalytic cycle is proposed that relies on isomerization positioning double bonds
proximate to the acid function to enable facile decarboxylation. To elucidate the proposed
mechanistic pathway, substrates that do not undergo decarboxylation under these catalytic
conditions (methyl oleate) are compared with those that cannot isomerize the position of
unsaturation (cinnamic acid). Both were shown to be operational under these catalytic
reaction conditions. Another illustrative comparison shows that the saturated octadecanoic
acid is 28 times less reactive than the unsaturated counterpart when reacted using this
precatalyst.
KEYWORDS: unsaturated fatty acid, decarboxylation, isomerization, octadecenoic acid, ruthenium carbonyl carboxylate,
triruthenuium dodecacarbonyl
eactions that convert fatty acids to hydrocarbons have
these pathways have been extensively studied, and both are
Rbeen a topic of investigation for decades. The conversion thermodynamically favorable at high temperature.4−6 However,
1
there are important differences in reaction rate, reagent and
catalyst requirements, and overall mechanism.
of biobased fatty acids into hydrocarbons useful for industrial
applications is a key to the futuristic biorefinery platform which
would enable biobased hydrocarbons, such as olefins, to
efficiently drop into the already extensive infrastructure of the
petrochemical industry. A significant thrust has been focused on
catalytic reactions that accomplish efficient removal of oxygen
species from fatty acid derivatives. There are two main
deoxygenation pathways involving the elimination of carbon
oxides from fatty acids (Scheme 1) and their derivatives. These
pathways are commonly referred to as decarboxylation, carbon
dioxide elimination (Scheme 1, top) and decarbonylation,
carbon monoxide elimination (Scheme 1, bottom). Some
recent reviews discuss these important reactions.2,3 Both of
Although ruthenium and rhodium metals abstract carbonyls
from carboxylic acids and aldehydes,7 catalytic decarbonylation
was first employed in palladium systems8 where much of the
literature has focused on decarbonylation of saturated fatty acid
substrates to form alkenes.9−18 One highly interesting study
shows that the activity of palladium in deoxygenation is affected
by the polarity of the catalyst support.19 In each of these
catalytic systems of varied metals, it is necessary to have either
in situ formation of an anhydride20 or an added one as a
coreactant. The stoichiometric amount of anhydride (Scheme
2) is converted into two moles of carboxylic acid coproduct. A
simpler technology would enhance the viability of a fatty acid to
alkenes biorefinery.
Catalytic oxidative decarboxylation is a different approach in
which Ag1+ is oxidized in situ to Ag2+ by half of an equivalent of
an oxidant, peroxydisulfate, which generates a sulfate
coproduct. The Ag2+ interacts with the fatty acid to give an
oxygen-centered radical, regenerating Ag1+ and a proton which
is trapped by the sulfate formed in the initial step. The oxygen-
centered radical then spontaneously decarboxylates to a carbon-
Scheme 1. Two Distinct Pathways To Remove Oxygen-
a
Containing Species from a Fatty Acid
a
Decarboxylation (top) and decarbonylation (bottom) both yield a
Received: July 16, 2014
Revised: September 3, 2014
Published: September 4, 2014
hydrocarbon, but the product’s level of unsaturation and the co-
products are different in each case.
© 2014 American Chemical Society
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Letter
propanoic acid (99.5%, Sigma-Aldrich, St. Louis, MO) was
added to the Schlenk flask through the side arm while
maintaining an inert (argon) atmosphere. Heating to reflux
gave an orange solution within 1 h. After 7 h of reaction, excess
propionic acid was removed by vacuum while ensuring that the
reaction was not exposed to oxygen. Finally, an orange product
was collected, washed with ethyl ether (99.5%, Sigma-Aldrich),
and dried under vacuum again to afford 0.1215 g (51% of
theoretical) of catalyst. Its identity was confirmed by FT-IR
spectroscopy. In all of the examples, conversion was
determined using an Agilent (Santa Clara, CA) model 7890A
GC-FID with a DB35-MS (30 m × 320 um, 0.25 μm film)
column. Relative response factors for alkanes, methyl esters,
and carboxylic acids were calculated by comparison of authentic
samples. Identities of products were also verified by injection
into a similar GC that was equipped with an MS detector.
The efficacy of the catalyst complex in isomerization
reactions was demonstrated on 1-octadecene (90%, Sigma-
Aldrich) and 7-trans-tetradecene (98%, Sigma-Aldrich). Addi-
tionally, because methyl esters are not decarboxylated under
these catalytic conditions, methyl 10-undecenoate (96%, Sigma-
Aldrich) and methyl 9-cis-octadecenoate (methyl oleate; >99%,
Nu-Check Prep, Elysyian, MN) were also used. As an example,
methyl 10-undecenoate (2.0521 g) was isomerized using 0.0021
g of [Ru(CO)2(EtCO2)]n at 250 °C for 4 h in a 16 × 150 mm
culture tube that was sealed with a septa-capped lid. The
reaction was prepared in an inert-atmosphere glovebox and was
connected to a Schlenk line through a 22 gauge needle. In each
of these systems, distribution of positional isomers was
obtained (Table 1). In another example, 2-methyl-1-decene
(90%, Acros) was isomerized solely to 2-methyl-2-decene (14%
of theoretical yield).
Scheme 2. Balanced Reaction for Decarbonylation of a Fatty
Acid Facilitated by an Anhydride Coreactant
a
a
This reaction produces an alkene, carbon monoxide, and 2 equiv of
carboxylic acid.
centered radical. Abstraction of a hydrogen atom from a solvent
gives the final deoxygenated alkane product. In an interesting
twist, copper can also be added, which will catalyze an oxidative
elimination to give a terminal alkene instead of an alkane.21−23
While this chemistry is interesting, it does have a lot of
requirements making it difficult to implement in a biorefinery
scenario. First, stoichiometric peroxydisulfate is consumed and
a sulfate is produced. Next, a solvent with an easily abstractable
hydrogen is required to produce an alkane, which is
transformed to yet another coproduct. If an alkene is desired,
additional reagents and metal catalysts are needed. Finally,
unproductive reactions may occur on both the oxygen- and
carbon-centered radicals, thus generating disproportionation,
addition, and rearrangement side products. Direct decarbox-
ylation without all of these drawbacks is possible at temper-
atures over 300 °C using palladium catalysts supported on
carbon2 or alumina.24,25 However, a modified approach was
needed to achieve higher conversion at lower temperatures.
The rate of thermal decarboxylation of unsaturated
carboxylic acids is significantly affected by the position of
unsaturation relative to the carboxylic acid moiety. Alpha-beta
or beta-gamma unsaturated acids react the fastest.26−28 There
are many examples of isomerization catalysts in the
literature,29−33 some of which describe the use of metal
carbonyls.34−37 Specifically, ruthenium carbonyl in the presence
of a carboxylic acid forms a complex38 that will readily
isomerize alkenes in the presence39 or absence40 of a phosphine
ligand. The approach used herein (Scheme 3) is to take
There was also no observable fragmentation of the
isomerized materials. Additionally, NMR spectroscopy shows
that the isomerized material does not have a change in the
integration values of the alkane/alkene protons. For example, in
7-trans-tetradecene, the starting material value was 13.7,
whereas it was essentially the same, 13.9, in the isomerized
tetradecene.
Scheme 3. Tandem Isomerization-Decarboxylation Pathway
for the Conversion of Unsaturated Fatty Acids to Alkenes,
Using 9-cis-Octadecenoic Acid As an Example
The efficacy of decarboxylation using the same precatalyst
(Scheme 4) was demonstrated on a proximate isomer, trans-
cinnamic acid (99%, Sigma-Aldrich). Positional isomerization
was not possible because of the relative positions of the ring
and the unsaturation in this material. In an example reaction,
0.0023 g of Ru3(CO)12 and 2.0734 g of trans-cinnamic acid
were prepared as in the prior example, and the reaction was run
at 200 °C for 4 h. GC analysis revealed that ∼29% of the
starting material was converted to styrene, which increased to
>70% when the catalyst amount was increased to 0.2105 g.
Under these same conditions, but without Ru3(CO)12, less than
1% conversion to product was observed, thus clearly
demonstrating the dramatic acceleration of the reaction by
the precatalyst (Table 1). From these results the demonstrated
total turnovers and turnover frequency can be calculated. In the
4 h experiment, 0.0041 mol of substrate was converted by only
3.6 × 10−6 mols of the catalyst which is a demonstration of
1130 total turnovers (377 Ru−1) and a turnover frequency of
280 h−1 (93 h−1 Ru−1).
advantage of this phenomenon by using a catalyst that is
capable of first isomerizing the substrate and then also
facilitating decarboxylation. This tandem approach does not
require additional coreagents, works at temperatures consid-
erably below 300 °C, and has been demonstrated to work on
unsaturated fatty acids of 11 to 18 carbons in length.
The isomerization-decarboxylation catalyst precursor com-
plex, [Ru(CO)2(EtCO2)]n, was synthesized according to a
modified literature method.38 An inert atmosphere glovebox
(nitrogen) was used for manipulation of reagents, where 0.2384
g of Ru3(CO)12 (99%, Acros, Pittsburgh, PA) was placed in a
50 mL Schlenk flask that was connected to a Vigreux column
capped with a vacuum adapter. The apparatus was removed
from the glovebox, and an argon flow from a Schlenk line was
connected to the vacuum adapter. At this time, 20 mL of
The efficacy of the decarboxylation, as measured by turnover
frequency (Table 1), shows that trans-cinnamic acid is readily
converted. Much higher loading of catalyst increased the
observed conversion. However, lower observed turnover
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Table 1. Yield and Products from the Decarboxylation-Isomerization of Unsaturated Substrates and Isomerization of Ester and
a
Alkene Substrates Which Do Not Undergo Decarboxylation under These Conditions
conversion to alkene
(GC %)
turnover frequency
substrate
conditions
(h−1 Ru−1
)
observed product
trans-cinnamic acid
no precatalyst 200 °C, 4 h
0.11 wt % Ru3(CO)12 200 °C, 4 h
10.0 wt % Ru3(CO)12 200 °C, 4 h
1
NA
93
3
styrene
29
74
9-cis-octadecenoic acid no precatalyst 250 °C, 24 h
0.10 wt % Ru3(CO)12 250 °C, 4 h
0
NA
39
12
20
NA
heptadecene, multiple isomers
20
0.10 wt % Ru3(CO)12 250 °C, 24 h
36
10-undecenoic acid
0.89 wt % Ru3(CO)12 250 °C, 4 h
60 (GC-MS)
NA
decene, multiple isomers
methyl 10-
undecenoate
0.10 wt % [Ru(CO)2(EtCO2)]n 250
°C, 4 h
isomerized ester (apparent steady state of
isomers)
0.14 wt % [Ru(CO)2(EtCO2)]n 150
NA
NA
NA
NA
NA
NA
NA
NA
starting material (minor amount of
isomerization)
°C, 4 h
methyl 9-cis-
octadecenoate
0.13 wt % [Ru(CO)2(EtCO2)]n 250
°C, 4 h
0.12 wt % [Ru(CO)2(EtCO2)]n 150
°C, 4 h
0.015 wt % [Ru(CO)2(EtCO2)]n 150
°C, 4 h
isomerized ester (apparent steady state of
isomers)
methyl 9-cis-
octadecenoate
starting material (minor amount of
isomerization)
7-trans-tetradecene
isomerized alkenes (apparent steady state of
isomers)
a
Under reaction conditions where a carboxylic acid substrate is used, the catalyst forms in situ. When non-carboxylic acid substrates are utilized, a
precatalyst with carboxylate ligands must be used to give efficacy.
Aldrich), a saturated analogue of oleic acid. When the
decarboxylation was conducted at 250 °C, only a small amount
of hydrocarbon was produced, ∼7 times less than (Figure 1)
Scheme 4. Catalyzed Decarboxylation of trans-Cinnamic
Acid
frequency is evident, caused by the lower number of possible
turnovers in the experiment.
As with both of the steps in the process, the tandem reaction
was greatly accelerated in the presence of the precatalyst. Both
a large carboxylic acid, 9-cis-octadecenoic acid (oleic acid), and
a smaller terminal acid, 10-undecenoic acid (98%, Sigma-
Aldrich), demonstrated significant conversion. Using the same
methods as described above, 0.0021 g of [Ru(CO)2(EtCO2)]n
and 2.0055 g of oleic acid (>99%, Nu-Check Prep) reacted at
250 °C for 4 h afforded nearly 20% conversion to a mixture of
alkene isomers. The same results were achieved using
Ru3(CO)12 as well, thus indicating similar behavior between
the catalyst precursors (Table 1). In the 9-cis-octadecenoic acid
decarboxylation, using equal catalyst loading, the 4 h experi-
ment has lower conversion but average higher turnover
frequency than the 24 h experiment. This is probably due to
the reaction slowing down as conversion increases due to less
substrate availability, along with inhibition of the decarbox-
ylation reaction caused by the buildup of product. In each of
the systems, both the product and the starting material showed
an apparent steady state of isomers, with no observed
fragmentation. Control experiments demonstrated that, even
at higher temperatures, catalyst-free systems did not undergo
decarboxylation. Experiments which were run for longer times
showed that the amount of decarboxylation continues to
increase with time, albeit at a slower rate. The time required for
complete conversions were studied and compared and are
expected to be highly dependent on substrate, temperature, and
catalyst concentration.
Figure 1. A comparison of the decarboxylation of 9-cis-octadecenoic
□
●
acid ( ) with the saturated octadecanoic acid ( ). The reactions both
contained 0.11 wt % of Ru3(CO)12 (4 h point) or [Ru(CO)EtCO2)]n
(shorter time points) and were run at 250 °C. In the shorter time
experiments, the catalyst which already contained carboxylate groups
was used to avoid an induction period which would have convoluted
the resulting conversions. In the longer experiment, the induction
period is not a substantial factor.
that from oleic acid. This effect was even more pronounced
when the reactions were performed at 225 °C. The conversions
in 24 h under those conditions were 11.1% for the unsaturated
substrate and 0.4% for the saturated species, yielding a ratio of
28:1.
Finally, a reaction system was assembled, where the product
could be removed as the reaction progressed. The substrate
used was 10-undecenoic acid, which when decarboxylated by
this system yields isomers of decene as the product. Because
decene boils at ∼170 °C, an appropriate collector and
condenser can be used to obtain nearly pure product straight
Next, the necessity of the unsaturation in the substrate was
evaluated by the reaction of octadecanoic acid (99%, Sigma-
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without the addition of more catalyst, showing further activity.
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isomerization-decarboxylation using 0.273 g of Ru3(CO)12,
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The most important practical aspect of this work is that it can
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industrial products similar to those produced via conventional
petrochemical means. The hydrocarbons obtained from this
process can be used in fuels, polymers, lubricants, or other
applications where a biobased source of large alkenes is
desirable. The new approach gives industries direct drop-in
replacements for petroleum-based products and also benefits
the agricultural industry.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 309-681-6103. Fax: 309-681-6524. E-mail: Kenneth.
Doll@ars.usda.gov.
Notes
■
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Disclosure: Mention of trade names or commercial products in
this publication is solely for the purpose of providing specific
information and does not imply recommendation or endorse-
ment by the U.S. Department of Agriculture. USDA is an equal
opportunity provider and employer.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors thank Dr. Bryan R. Moser for many helpful
comments during the preparation of this manuscript. This work
was a part of the in-house research of the Agricultural Research
Service of the United States Department of Agriculture.
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