Microalgae lipids as a feedstock for the production of benzene

Article abstract of DOI:10.1039/c8gc00423d

A two-step one-pot synthesis of benzene from the five-fold unsaturated fatty acid eicosapentaenoic acid (EPA), a component of microalgae oils, is presented. By a sequence of olefin metathesis and the catalytic dehydrogenation of the resulting 1,4-cyclohexadiene, two equivalents of benzene are effectively formed per EPA substrate molecule. As the only major by-products, 5-octenoic acid and 5-decenedioic acid are formed. Performing the dehydrogenation step under hydrogen pressure results in the formation of their saturated analogues, sebacic acid and octanoic acid, both desirable products, while the simultaneous dehydrogenation step to benzene is not hampered.

Full text of DOI:10.1039/c8gc00423d

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Microalgae lipids as a feedstock for the production
of benzene†
Cite this: DOI: 10.1039/c8gc00423d
Dennis Pingen,
Julia Zimmerer,
Nele Klinkenberg and Stefan Mecking
*
A two-step one-pot synthesis of benzene from the five-fold unsaturated fatty acid eicosapentaenoic acid
(EPA), a component of microalgae oils, is presented. By a sequence of olefin metathesis and the catalytic
dehydrogenation of the resulting 1,4-cyclohexadiene, two equivalents of benzene are effectively formed
per EPA substrate molecule. As the only major by-products, 5-octenoic acid and 5-decenedioic acid are
formed. Performing the dehydrogenation step under hydrogen pressure results in the formation of their
saturated analogues, sebacic acid and octanoic acid, both desirable products, while the simultaneous
dehydrogenation step to benzene is not hampered.
Received 6th February 2018,
Accepted 13th March 2018
DOI: 10.1039/c8gc00423d
rsc.li/greenchem
Introduction
Results and discussion
Aromatics are essential key compounds of the chemical indus-
try. In particular, benzene is the starting material for the pro-
duction of numerous agrochemical and pharmaceutical
actives, dyes and other chemicals. In view of the finite nature
of petrochemical sources and the environmental impact of
fossil fuel recovery, schemes that provide access to aromatics
from renewable sources are desirable in the long term.
In principle, aromatics occur in large amounts in lignin.
However, lignin is a complex mixture containing numerous
oxygenated substituted benzene motifs linked to a high molar
mass biopolymer. Efforts to produce single clean aromatic
compounds by breaking down lignin efficiently have not been
successful to date.1,2 Thus, lignin is used as a low value fuel
for the very largest part.3 Possible schemes for the efficient
generation of benzene utilizing suitable renewable molecules
are lacking.
We now report on the catalytic generation of benzene from
eicosapentaenoic acid (EPA). EPA is contained in substantial
amounts in various microalgae or can also be produced by
yeasts.4–6 While the limited achievable cell densities are cur-
rently a bottleneck for their production, microalgae offer the
benefit of not competing with food production for arable land
and fresh water, and they reproduce very rapidly to yield high
oil content biomass.7–13
Towards this aim, we pursued ring closure by olefin metathesis
with subsequent dehydrogenation to yield benzene
(Scheme 1). For this concept, it is notable that olefin meta-
thesis by ruthenium alkylidenes has been shown to be compa-
tible with fatty acid substrates, plant oils and other renewable
feedstocks.14–18 Also, the catalytic dehydrogenation of six-
membered cyclic olefins to benzene, driven by the stability of
the aromatic systems, is a proven reaction.19,20
Olefin metathesis usually converts the substrates into an
equilibrium mixture of all possible products accessible by the
formal cleavage of the double bonds and recombination of the
methylidene fragments. For multiple unsaturated substrates
like EPA, a large range of linear and cyclic products are concei-
vable. Advantageously, 1,4-cyclohexadiene is a thermodynamic
sink.21–23 This not only favours the formation of this desired
product, but ideally also strongly reduces the number of
further products formed, namely 5-octeneoic acid, 5-decene-
dioic acid and 3-hexene which are the result of net EPA chain-
end homo-metathesis (Scheme 1).
Towards the aim of an efficient self-metathesis reaction of
eicosapentaenoic acid, we screened a range of commercial
metathesis catalysts (Fig. 1).
The metathesis reactions were run until no change in the
composition of the reaction mixture occurred anymore, as
observed by GC. These times appeared to vary from 4 hours,
which the Grubbs 2nd generation catalyst required (Table 1,
entry 2), to 15 minutes, when Hoveyda–Grubbs 2 achieved its
maximum conversion (Table 1, entry 5). In all cases, the pro-
ductivity was high; the lowest conversion achieved reached
93% with the Grubbs 1st generation catalyst (Table 1, entry 1).
The selectivity towards 1,4-cyclohexadiene was however, not
always complete. In most cases this appeared to be around 70
Chair of Chemical Materials Science, Universitätsstrasse 10, University of Konstanz,
Konstanz, Germany. E-mail: Stefan.Mecking@uni-konstanz.de
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc00423d
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Scheme 1 Self-metathesis of eicosapentaenoic acid, and a possible subsequent homo-metathesis of the produced 5-octenoate to produce
3-hexene and 5-decenedioate, and two equivalents of benzene after dehydrogenation.
dioic acid. The remaining 3-hexene was not observed due to
the overlap with the solvent signal (Fig. S2, ESI†). In only a few
cases (entries 1, 2 and 4, Table 1) the metathesis was incom-
plete and multi-unsaturated linear products were observed. All
in all, the Hoveyda–Grubbs 2nd generation catalyst appears to
be the most suitable catalyst for the self-metathesis of EPA,
achieving a maximum conversion and selectivity for 1,4-
cyclohexadiene.
For the dehydrogenation, different solid-supported Pd cata-
lysts were studied, initially employing neat 1,4-cyclohexadiene
as a substrate. This showed the presence of a solvent to be ben-
eficial to perform the dehydrogenation efficiently. A hydrogen
acceptor however, was not required although hydrogen is
expelled from 1,4-cyclohexadiene to form benzene (ESI,† vide
infra). Complete conversion to benzene was observed in these
experiments under suitable reaction conditions.
Based on these, in view of a straightforward one-pot pro-
cedure, olefin metathesis was carried out in the presence of a
solvent, and without any further purification, a solid dehydro-
genation catalyst was added and the reaction mixture was
heated to optimum dehydrogenation conditions (Table 2). It
Fig. 1 Metathesis catalysts employed in the screening of eicosapentae-
noic acid metathesis.
to 80%, where catMETium was much lower (55%, Table 1, could be expected that the non-cyclic mono-unsaturated mole-
entry 6) and Hoveyda–Grubbs 2 (100%, Table 1, entry 5) pro- cules (5-octenoate, or 5-decenedioate) present in the post-
vided the highest selectivity. Overall, 5-octenoate and 5-decene- metathesis mixture of eicosapentaenoic acid might serve as a
dioic acid were the main remaining additional products. The hydrogen acceptor.19,20,24,25 This was however, not observed to
5-decenedioic acid results from 5-octenoic self-metathesis or a large extent; only a small portion of octanoate and decane-
direct EPA chain metathesis. These products are observed as a dioic acid was found at most, whereas the major part of the
mixture of both in their thermodynamic equilibrium from the unsaturated linear molecules was unaffected (apart from some
metathesis, about 50% of 5-octenoic acid and 25% 5-decene- double bond isomerization; Fig. S6, ESI†).
Table 1 Productivity of several metathesis catalysts in the self-metathesis of EPA
Entry
Metathesis cat. (Fig. 1)
Time (min)
Conv.a (%)
1,4-CHD select.a (%)
Main (by) producta
1
2
3
4
5
6
7
8
9
10
G1
G2
G3
HG1
60
240
15
60
15
75
105
75
75
105
93
100
100
100
100
99
70
93
78
94
100
55
78
72
91
84
Undecadienoateb
Multiple unsat.b
5-Octenoate
Undecadienoateb
5-Octenoate
HG2
CatMETium RF1
Umicore M2
Umicore M31
Umicore M73 SIPr
Umicore M73 SIMes
5-Octenoate
5-Octenoate
5-Octenoate
5-Octenoate
99
100
100
99
5-Octenoate
Conditions: Eicosapentaenoic acid (5 mmol), metathesis catalyst (0.005 mmol, 0.1 mol%), neat, 45 °C. a Determined via GC analysis. Conversion
is determined on the EPA consumption. The selectivity was determined with respect to the maximum theoretical amount (2 equiv. of 1,4-cyclo-
hexadiene per EPA starting material). b Incomplete self-metathesis product.
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Table 2 Results of one-pot metathesis and the dehydrogenation of eicosapentaenoic acid to benzene
Dehydrogenation catalyst
(loading)
Metathesis conv. and
select. for 1,4-CHDa
Overall benzene
select.a,b (%)
Entry
Conv. of 1,4-CHDa (%)
1b,c
2b
3c
4
Pd/C (1 mol%)
Pd/C (0.1 mol%)
Pd/C (1 mol%)
100
100
99
99
99
99
38
100
99
99
99
99
99
38
75
73
87
76
67
66
28
Pd/C (1 mol%)
5
Pd(OAc)2 (1 mol%)
Pd/Al2O3 (0.1 mol%)
Pd/C (1 mol%)
6
7c,d
Conditions: HG2 (0.005 mmol, 0.1 mol%), 10 mL toluene solvent, 15 min at 45 °C, then addition of the supported Pd catalyst and 110 °C for
20 h. a Analysed by GC, the conversion of the metathesis was determined with respect to the EPA consumption. The selectivity of 1,4-cyclohexa-
diene and benzene was determined with respect to the maximum theoretical amount of 2 equiv. 1,4-cyclohexadiene from EPA. b 0.2 equiv. AMS
(anthraquinone-2-sulfonic acid sodium salt) added as a hydrogen acceptor. c Chlorobenzene instead of toluene as a solvent. d Reaction performed
in a single step: HG 2 and Pd catalysts were placed in a Schlenk tube under an inert atmosphere, solvent and then EPA were added, and the
mixture was heated to 110 °C and stirred for 20 h.
The metathesis was not adversely influenced by the presence could be affected in parallel to the formation of benzene by
of
a
solvent;
full
and
selective
conversion
to hydrogenation.
In order to identify suitable reaction conditions, post-meta-
1,4-cyclohexadiene was retained (ESI†). The presence of a
residual ruthenium catalyst is not detrimental to the catalytic thesis/post-dehydrogenation mixtures were subjected to hydrogen-
dehydrogenation, though the conversion to benzene appeared to ation under H2 pressure (Table 3, entries 1 and 2). Even at only 5
be slightly affected in the case of using the post-metathesis reac- bar hydrogen pressure, the unsaturated 5-octenoic acid and
tion mixture. This was also found when employing the other pre- 5-decenedioic acid were fully hydrogenated. With this insight, the
viously tested Pd precursors (Table 2, entries 5 and 6).
metathesis/dehydrogenation reactions were performed according
As the metathesis is the faster reaction, we investigated to the aforementioned protocol, but with hydrogen present
whether both reactions could be combined into a single step during the dehydrogenation step (Table 3, entries 3 to 5).
(Table 2, entry 7), that is combining the substrate, solvent and
Indeed, benzene formation still occurs at these hydrogen
both catalysts in the initial reaction mixture and gradually pressures due to the high driving force of aromatization, while
heating to the dehydrogenation temperature of 110 °C. at the same time the linear olefinic compounds are hydrogen-
Unfortunately, the metathesis step was now hampered and did ated effectively (Scheme 2). Note that the composition of the
not reach full conversion (38%). The subsequent dehydrogena- (saturated) acids is shifted in favour of sebacic acid beyond the
tion did proceed relatively well; all in all a total selectivity 2 : 1 molar ratio expected from a metathesis equilibrium.
towards benzene of 28% based on EPA was achieved.
Possibly, the diacid is hydrogenated more rapidly, and shifts
Although the unsaturated linear C10 diacid and C8 mono- the ongoing metathesis by this removal of the metathesis
acid are useful compounds, their saturated analogues are product. The hydrogen released in the dehydrogenation to
often preferred. In particular, sebacic acid is in strong benzene is exactly the amount required to hydrogenate the
demand. As the dehydrogenation catalyst employed can also remaining non-cyclic unsaturated molecules; the partial
affect hydrogenation according to the concept of microscopic pressure of the hydrogen evolved apparently is not sufficient to
reversibility, we probed whether this hydrogenation, however actually perform the olefin hydrogenation.
Table 3 Hydrogenation of mixtures containing benzene formed in the 2-step metathesis and dehydrogenation reactions of 1,4-cyclohexadiene
formed in the EPA metathesis reactions
Composition of saturated linear acid portion (mol%)
Conv. of (found)
Hydrogenation selectivity
Entry H2 (bar) 1,4-CHDa (%)
C6H6 select.a (%) of linear olefins (%)
8 : 0 Acida (%) 1,10-Diacida (%) Other productsb (%)
Hydrogenation on the post-metathesis, post-dehydrogenation mixture:
1
2
5
10
100
100
99
100
100
100
56
44
39
38
5
18
Simultaneous post-metathesis dehydrogenation/hydrogenation:
3
4
5
5
10
1
100
100
100
96
100
98
100
100
89
25
26
75
75
67
25
0
7
0
Conditions: EPA (5 mmol), HG2 (0.005 mmol, 1 mol%), 45 °C, 30 min, then toluene (10 mL), Pd/C (10% w/w, 1 mol%), 110 °C, 20 h.
a Determined via GC analysis. Selectivity was determined with respect to the maximum theoretical amount based on EPA. b Other products are
the result of incomplete metathesis. These consist mainly of undecadienoates.
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as received. EPA was distilled before use. Pd/C (10%), Pd/Al2O3
(5%) and Pd(OAc)2 were obtained from ABCR. The metathesis
catalyst precursors, Grubbs 1st generation, Grubbs 2nd gene-
ration, Grubbs 3rd generation and Hoveyda–Grubbs 1st gene-
ration, were purchased from Sigma-Aldrich and used as
received. CatMETetium was purchased from ABCR. Umicore
metathesis catalyst precursors were kindly donated by
Umicore. Hydrogen (N5.0, 99.999%) was obtained from Air
Liquide. Gas chromatography was performed on a PerkinElmer
GC Clarus 500 system equipped with an elite-5 column
(length = 30 m, inner diameter = 0.25 mm, thickness = 25 µm)
and a FID detector via the following programs: 1 min at 90 °C,
30 °C min−1 to 280 °C, 280 °C for 8 minutes (method 1), or
3 min at 50 °C, 20 °C min−1 to 280 °C, 280 °C for 5 minutes
(method 2), both with an injector temperature of 300 °C and a
detector temperature of 280 °C. GC-MS measurements were
conducted on an Agilent GC7890A system equipped with an
inert MSD 5975C triple-axis detector. The column in the GC-MS
was an HP-5 ms. NMR spectra were recorded on a Bruker
Scheme 2 Self-metathesis and the subsequent dehydrogenation of
EPA resulting in benzene, 5-decenedioic acid and 5-octenoic acid in the
absence of hydrogen. In the presence of even 1 atmosphere hydrogen,
benzene and the saturated compound are formed.
1
Avance III 400 MHz spectrometer. H NMR spectra were refer-
enced to the residual protiated solvent (CDCl3).
Conclusions
Catalysis
The one-pot procedure of catalytic metathesis and dehydro-
genation enables an efficient production of benzene from eico-
sapentaenoic acid. The latter is a component of microalgae oil
feedstocks, amongst others. This avenue overcomes the
current lack of routes to desirable individual aromatic com-
pounds from renewable sources, rather than complex mixtures
of oxygenated aromatics which are also not separable. Benzene
is essential for the production of many actives, dyes and other
chemicals. As the only major by-products, decenedioic acid
and octenoic acid are formed. Remarkably, these can be hydro-
genated simultaneously to the dehydrogenation step to form
their saturated analogues. In particular, sebacic acid is in
strong demand for high-performance polyamides, corrosion
protection and other applications.
Procedure for the metathesis of eicosapentaenoic acid. In
the glovebox, the appropriate metathesis catalyst precursor
(0.005 mmol, 0.1 mol%) was weighed into a Schlenk tube. The
Schlenk tube was closed with a septum and transferred out of
the glovebox. Under an inert atmosphere, eicosapentaenoic
acid (5 mmol) was added via a syringe. At time intervals of
0, 15, 30, 45, 75, 105, 180 and 240 minutes, samples were
taken via a syringe from the reaction mixture. The sample was
quenched with EVE and filtered over silica to remove all metal
residues. The sample was subsequently analysed by GC.
Procedure for the dehydrogenation of 1,4-cyclohexadiene.
The appropriate Pd source (Pd/C (10% w/w) 5–0.1 mol%,
Pd(OAc)2 1 mol%, Pd/Al2O3 (5% w/w) 1–0.1 mol%) was loaded
into a Schlenk tube. In case AMS or dimethylfumarate was
applied, these were added (0.2 mmol, 62.1 mg or 1 equiv.,
1.44 g, 0.25 equiv., 360 mg, respectively). The Schlenk tube
was closed with a septum and evacuated and purged with N2
3 times. Using a syringe, the solvent (chlorobenzene or toluene)
was added, followed by the addition of 1,4-cyclohexadiene
(10 mmol), also via a syringe. The reaction mixture was
then heated to the stated temperature and stirred for the
appropriate time. After this time, the reaction mixture was
While the production of benzene from fatty acids is cer-
tainly not economically viable today, this scheme does provide
a perspective to access this important building block from
non-fossil sources efficiently.
Experimental
General considerations
All reactions were performed under an inert gas atmosphere cooled to room temperature and a sample was taken from the
and mixed using a magnetic stirring bar, unless stated other- mixture and filtered over silica to remove any catalyst. The
wise. Chlorobenzene was degassed and stored over molecular sample was analysed by GC.
sieves under an inert atmosphere, toluene was distilled over
Procedure for the single-step one-pot metathesis and
Na and stored under an inert atmosphere, and methanol was dehydrogenation
distilled over Mg and stored over molecular sieves under an
One-step procedure. In a Schlenk tube, Pd/C 10% w/w
inert atmosphere. Ethyl vinyl ether (EVE), 1,4-cyclohexadiene (0.1 mmol, 1 mol%) was weighed, and the tube was evacuated
and anthraquinone-2-sulfonic acid sodium salt (AMS) were and purged with an inert gas. To this, Hoveyda–Grubbs 2nd
obtained from Sigma-Aldrich and used as received. generation catalyst (0.005 mmol, 0.1 mol%) was added in the
Eicosapentaenoic acid (EPA) and the Hoveyda–Grubbs 2nd glovebox. The Schlenk tube was closed with a septum and
generation catalyst were purchased from Carbosynth and used transferred out of the glovebox. Using a syringe, chlorobenzene
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or toluene (10 mL) was added, followed by the addition of eicosa-
pentaenoic acid (5 mmol). The mixture was heated to 110 °C over
the course of 25 minutes and then stirred at this temperature for
20 h. After the reaction mixture was cooled to room temperature,
a sample was filtered over silica and analysed by GC.
Procedure for the two-step one-pot metathesis and dehydro-
genation. In a Schlenk tube, the Hoveyda–Grubbs 2nd gene-
ration catalyst (0.005 mmol, 0.1 mol%) was weighed in the glo-
vebox. The Schlenk tube was closed with a septum and trans-
ferred out of the glovebox. Using a syringe, chlorobenzene or
toluene (10 mL) was added. The mixture was heated to 45 °C
and eicosapentaenoic acid (5 mmol) was added. The mixture
was stirred for 15 min. After this, Pd/C 10% w/w (0.1 mmol,
1 mol%) was added and the entire mixture was further heated
to 110 °C and stirred at this temperature for 20 h. After the
reaction mixture was cooled to room temperature, a sample
was filtered over silica and analysed by GC.
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Conflicts of interest
The authors declare no conflict of interests.
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