Synthesis of Bio-Based Methylcyclopentadiene from 2,5-Hexanedione: A Sustainable Route to High Energy Density Jet Fuels

Article abstract of DOI:10.1002/cssc.202002209

The sustainable, bio-based, platform chemical, 2,5-hexanedione [HD (1)], was efficiently converted to methylcyclopentadiene [MCPD (4)] through a three-step process consisting of intramolecular aldol condensation, catalytic chemoselective hydrogenation, and dehydration. Base-catalyzed aldol condensation of 1 resulted in the formation of 3-methyl-2-cyclopenten-1-one [MCO (2)], which was then converted to 3-methyl-2-cyclopenten-1-ol [MCP (3)] by chemoselective reduction with a ternary Ru catalyst system [RuCl₂(PPh₃)₃/NH₂(CH₂)₂NH₂/KOH]. The hydrogenation proceeded with 96 % chemoselectivity. 3 was then dehydrated over AlPO₄/MgSO₄ at 70 °C under reduced pressure to yield 4, which can undergo an ambient temperature [4+2]-Diels-Alder cyclization to generate dimethyldicyclopentadiene (DMDCPD), a commodity chemical useful for the preparation of high-performance fuels and polymers. Through this approach, advanced jet fuels and materials can be conveniently produced from sustainable cellulosic feedstocks.

Full text of DOI:10.1002/cssc.202002209

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Synthesis of Bio-Based Methylcyclopentadiene from 2,5-
Hexanedione: A Sustainable Route to High Energy Density Jet
Fuels
Authors: Josanne-Dee Woodroffe and Benjamin G. Harvey
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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To be cited as: ChemSusChem 10.1002/cssc.202002209
Link to VoR: https://doi.org/10.1002/cssc.202002209
01/2020

10.1002/cssc.202002209
ChemSusChem
Synthesis of Bio-Based Methylcyclopentadiene from 2,5-Hexanedione: A Sustainable Route to High
Energy Density Jet Fuels
Dr. Josanne-Dee Woodroffe and Dr. Benjamin G. Harvey*
US NAVY, NAWCWD, Research Department, Chemistry Division, China Lake, California 93555
*Corresponding author (email: benjamin.g.harvey@navy.mil)
Keywords: Sustainable Aviation Fuel; Catalysis; Methylcyclopentadiene; Chemoselective Hydrogenation;
Cellulose
ABSTRACT
The sustainable, bio-based, platform chemical, 2,5-hexanedione [HD (1)], was efficiently converted to
methylcyclopentadiene [MCPD (4)] through a three-step process consisting of intramolecular aldol
condensation, catalytic chemoselective hydrogenation, and dehydration. Base-catalyzed aldol condensation
of 1 resulted in the formation of 3-methyl-2-cyclopenten-1-one [MCO (2)], which was then converted to 3-
methyl-2-cyclopenten-1-ol [MCP (3)] by chemoselective reduction with a ternary Ru catalyst system
[RuCl2(PPh3)3/NH2(CH2)2NH2/KOH]. The hydrogenation proceeded with 96% chemoselectivity. 3 was
then dehydrated over AlPO4/MgSO4 at 70 °C under reduced pressure to yield 4, which can undergo an
ambient temperature [4+2]-Diels-Alder cyclization to generate dimethyldicyclopentadiene (DMDCPD), a
commodity chemical useful for the preparation of high-performance fuels and polymers. Through this
approach, advanced jet fuels and materials can be conveniently produced from sustainable cellulosic
feedstocks.
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INTRODUCTION
Over the last several decades, remarkable progress has been made on the development of alternative
transportation fuels from biomass sources.[1-3] This research has been driven by the need to reduce CO2
emissions and help mitigate the impacts of global warming and ocean acidification.[4] Although ground
vehicles, including cars and light trucks, can be easily powered by renewable electricity or hydrogen, the
immense power requirements of commercial and military jet aircraft are currently dependent on
hydrocarbon fuels. Petroleum-derived jet fuels are composed of n-paraffins, isoparaffins, cycloalkanes, and
aromatic compounds. These complex mixtures contain up to 25% aromatic compounds, [5,6] which have low
gravimetric heats of combustion and can increase particulate emissions. To address these issues, a
significant amount of research has been focused on the production of a diverse portfolio of full-performance
bio-based jet fuels with reduced aromatic content. For example, hydrotreatment of lipid/fatty acid
feedstocks or Fischer-Tropsch upgrading of bio-based synthesis gas can readily generate synthetic
paraffinic kerosenes (SPKs),[7-13] which are currently approved for use in commercial aircraft at blend ratios
of up to 50% with traditional jet fuel.[14,15] Most SPKs are composed of C9 ‒ C15 acyclic n-paraffinic and
isoparaffinic hydrocarbons, with no aromatic compounds, and low sulfur content.[16] The lack of aromatics
results in higher gravimetric net heats of combustion (NHOCs), ~43.7 – 44.0 MJ/kg,[17] compared to
conventional jet fuel (≥ 42.8 MJ/kg), but these fuels are limited by their lower densities and volumetric
NHOCs. In addition, these mixtures of acyclic alkanes do not adequately swell nitrile rubber engine seals,
and without engine system modifications, may cause elastomer seals to shrink and malfunction.[18,19] In
contrast, recent studies have shown that 30% cycloalkane blends have the same swelling capability as low
(8%) aromatic fuel blends.[20-23] Cycloalkanes also have higher densities due to their constrained geometries,
and significantly higher volumetric NHOCs than SPKs and traditional jet fuels.[17]
Due to growing interest in the development of high-performance bio-based jet fuels that can out-
perform conventional jet fuels, while reducing net CO2 emissions, a number of new technologies for the
conversion of biomass feedstocks to sustainable cycloalkane fuels have been explored.[24-36] These studies
have even extended to high-density, bicyclic compounds with applications as missile fuels. For example,
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our
group
demonstrated
the
conversion
of
the
acyclic
terpenoid
linalool
to
dimethyltetrahydrodicyclopentadiene (RJ-4) in 2011.[37] RJ-4 has a gravimetric and volumetric NHOC of
42.21 MJ/kg and 39.04 MJ/L (up to 17.7% higher than Jet-A)[17], respectively, with a density of 0.925 g/mL.
Zero-aromatic, high-performance jet fuel surrogates prepared from RJ-4 and various SPKs have recently
been studied with RJ-4 concentrations as high as 50%.[17] Due to its high density, pure RJ-4 has applications
as a high energy density fuel for cruise missile propulsion, [38-40] while its low volatility and high flashpoint
are well suited for ramjet engines.[41] More recently, researchers have shown that RJ-4 can be efficiently
converted to 1,3-dimethyladamantane, a thermally stable jet fuel of interest for high speed applications.[42]
To develop a route to RJ-4 from lignocellulosic feedstocks, it was of interest to explore 2,5-
hexanedione [HD (1)] as a platform chemical for the synthesis of methylcyclopentadiene [MCPD (4),
Scheme 1]. Fructose can be readily converted to 5-hydroxymethylfurfural (HMF) through isomerization
and acid-catalyzed dehydration, followed by selective hydrogenolysis, to generate 2,5-dimethylfuran
(DMF).[43] Alternatively, Mascal has demonstrated the efficient conversion of cellulose to 5-
(chloromethyl)furfural (CMF) followed by a two-stage acetalization/hydrogenation reaction to generate
DMF.[44,45] Hydrolysis of DMF then yields 1 (Scheme 1), which can be readily converted to 3-methyl-2-
cyclopenten-1-one [MCO (2)] by a base-catalyzed intramolecular aldol condensation.[46,47] More recently,
Zhang
demonstrated
the
conversion
of
CMF
to
1
via
a
one-pot
hydrodechlorination/hydrogenolysis/hydrolysis step. 2 was then produced via cyclization with an MgO
catalyst at 453 K.[48] 2 can be converted to 3-methyl-2-cyclopenten-1-ol [MCP (3)] via chemoselective
hydrogenation of the carbonyl group. Although the Luche reduction (NaBH4/CeCl3•7H2O) is
chemoselective and has previously been used for the conversion of 2 to 3,[49] it requires a stoichiometric
amount of NaBH4, which limits the sustainability of this approach for the production of high-performance
jet fuels. In contrast, Ru-based Noyori catalysts[50] exhibit high selectivity for the hydrogenation of carbonyl
groups in the presence of alkenes and in some cases are capable of promoting TONs > 1,000,000, using
cheap and readily available hydrogen gas as the reagent.
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Leveraging the potential of 1 as a readily available bio-based substrate, this work describes the
synthesis of sustainable methylcyclopentadiene (4) and dimethyldicyclopentadiene [DMDCPD (5)] via a
three-step process consisting of aldol condensation, catalytic chemoselective hydrogenation, and
dehydration. Through this approach, we demonstrate the ability to generate both a valuable chemical
precursor and high-performance jet fuel blendstock from ubiquitous biomass feedstocks.
Scheme 1. Synthesis of methylcyclopentadiene (4), dimethyldicyclopentadiene (5), and RJ-4 from biomass
sugars
EXPERIMENTAL SECTION
General. 2,5-hexanedione (≥98%) and tris(triphenylphosphine)ruthenium(II) dichloride (97%), were
obtained from Millipore Sigma and used as received. Ethylenediamine was freshly distilled (62 – 65 °C,
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100 mmHg) prior to use. 1H NMR spectra were recorded on a Bruker Avance III 500 spectrometer at 25
°C, operating at 500.46 MHz. Proton-decoupled 13C{1H} NMR spectra were recorded with the same
instrument at 25 °C, operating at 125.86 MHz. Chemical shifts are reported in parts per million (ppm)
downfield from tetramethylsilane, and are referenced relative to the NMR solvent (CDCl3), according to
the literature values – δ(1H) = 7.26, δ(13C) = 77.23.
Synthesis of 3-methylcyclopent-2-en-1-one [MCO (2)]. 2,5-Hexanedione (41.76 g, 0.37 mol) was added
over 10 min to a stirred solution of KOH (5.25g, 0.094 mol g) in H2O (250 mL) at reflux. The reaction
mixture was refluxed for an additional 30 min, cooled rapidly to 0 °C, saturated with sodium chloride (45.0
g), and filtered through Celite. The solution was then extracted with CH2Cl2 (3 × 200 mL), and the combined
organic extracts were dried (MgSO4) and concentrated under reduced pressure. The crude product was
purified by distillation (50 – 55 °C, 3.6 mmHg) to afford 3-methyl-2-cyclopent-1-one (22.63 g, 64%) as a
colorless oil. 1H NMR (500 MHz, CDCl3) δ 5.92 (s, 1H), 2.56 - 2.55 (m, 2H), 2.40 - 2.39 (m, 2H), 2.12 (s,
3H). 13C NMR (125 MHz, CDCl3) δ 209.8, 178.8, 130.3, 35.5, 32.8, 19.2. MS for C6H8O: 96.06 m/z.
Synthesis of 3-methylcyclopent-2-en-1-ol [MCP (3)] 7.22 mL of a solution containing ethylene diamine
(0.1 M, 0.722 mmol) and KOH (0.2 M, 1.44 mmol) in anhydrous 2-propanol were added to a 6:1 (v/v)
mixture of anhydrous 2-propanol:benzene (140 mL). The mixture was thoroughly degassed under reduced
pressure. RuCl2[P(C6H5)3]3 (0.693 g, 0.722 mmol) was then added to the solution under flowing N2. The
resultant mixture was sonicated for 90 min to obtain a yellow homogenous solution. A [0.5M] solution of
3-methyl-2-cyclopent-1-one (6.94 g, 72.2 mmol) in 2-propanol was degassed and added to the catalyst
solution, which was then transferred to a 600 mL Parr bomb under nitrogen. The bomb was evacuated and
refilled with H2 three times, pressurized to 500 psi, and then vigorously stirred for 3.5 h at ambient
temperature. Upon completion, the reaction mixture was filtered through Celite and concentrated under
reduced pressure. The crude product was combined with the product from a previous reaction (6.12 g) and
°
purified by fractional distillation (39 – 45 C, 2.4 mmHg) to yield 3-methyl-2-cyclopenten-1-ol (10.01 g,
75%) as a colorless oil. GC analysis showed an MCP purity of 94% with 4% methylcyclopentanol and 2%
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starting material (Figure S4). On this basis, the conversion was 98% with 96% chemoselectivity to 3. 1H
NMR (500 MHz, CDCl3) δ 5.41 (d, 1H, J = 1.4 Hz), 4.76 (bs, 1H), 2.27 - 2.15 (m, 1H), 2.144 - 2.139 (m,
1H), 2.137 - 2.131 (m, 1H), 2.05 (bs, 1H), 1.76 (s, 3H). 1.74 - 1.68 (m, 1H). 13C NMR (125 MHz, CDCl3)
δ 145.8, 127.6, 77.9, 35.3, 34.4, 16.8. MS for C6H10O: 98.07, 71.05 m/z.
Synthesis of methylcyclopentadiene [MCPD (4)] and dimethyldicyclopentadiene [DMCPD (5)].
AlPO4, synthesized as previously described,[37,51] (0.894 g, 7.33 mmol) and MgSO4 (1.10 g, 9.17 mmol)
were added to 3 (9.00 g, 0.092 mol) in a 50 mL round bottom flask fitted with a small distillation head. The
receiving flask was lowered into a dry ice bath and the reaction was placed under reduced pressure (40
°
mmHg). The reaction was heated to an oil bath temperature of 70 C for 4 h. The organic phase of the
distillate (5.07 g) was separated from the aqueous layer, placed in a sealed vial over NaCO3 and allowed to
warm to ambient temperature for 24 h. GC-FID analysis revealed a mixture of 7% methylcyclopentadiene,
12% methylcyclopentenols, and 78% dimethyldicyclopentadiene isomers. The residue in the reaction flask
was extracted with dichloromethane (30 mL), filtered through Celite, and concentrated under reduced
pressure to yield a light orange oil (2.04 g). The oil consisted of 8% unreacted 3-methyl-2-cyclopenten-1-
ol, 73% dimethyldicyclopentadiene isomers (DMDCPD), and 9% trimers. The overall yield of MCPD and
DMDCPD was 79%. 1H NMR (500 MHz, CDCl3) δ 6.43 (m), 6.25 – 6.15 (m), 6.02 – 5.97 (m), 5.55 (bs),
5.49 - 5.44 (m), 5.15 (bs), 5.11 (bs), 4.88 (bs), 4.79 (bs), 3.02 – 2.55 (m), 2.54-2.04 (m), 1.69 – 1.57 (m),
1.30 – 1.22 (m). 13C NMR of the major isomer (125 MHz, CDCl3) δ 145.7, 140.4, 126.0, 125.5, 57.8, 50.8,
49.7, 46.3, 42.6, 34.5, 17.2, 16.2. MS for C12H16: 160.1, 80.1 m/z.
RESULTS AND DISCUSSION
The first step in the conversion of 1 to 4 required the intramolecular aldol condensation of 1 to
generate 2. We initially attempted the conversion through a biphasic approach, recently described by Sacia
and co-workers, which utilized an aqueous K3PO4 solution with a toluene overlay at 180 °C.47 Although
Sacia obtained yields of up to 94%, in our hands we were only able to achieve yields of 25% due to the
generation of oligomerization products. This unsatisfactory result was likely due to the much larger (~358-
fold) scale used in the current work. The reaction is very time and temperature sensitive, and our inability
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Scheme 2: Formation of dimers during the base-catalyzed self-aldol condensation of 2.
to rapidly heat and cool our much larger reactor resulted in the generation of heavier reaction products. To
overcome this issue, we cyclized 1 under basic aqueous conditions at reflux temperatures.46 As observed in
the biphasic reaction, carefully controlling the reaction time was critical to limiting the formation of dimers
and trimers. The reaction was monitored by GC-MS at 10-minute increments and showed >99% conversion
to the cyclic enone after 30 min with the formation of 34% dimers. The mass spectra of the dimer peaks
were consistent with molecules formed by the self-aldol condensation of 2 followed by dehydration
(Scheme 2). After 40 min, dimers and trimers represented 58% of the product distribution. To optimize the
yield of 2 we limited the reaction time to 30 min and then immediately cooled the sample in an ice bath to
prevent further oligomerization. This allowed for the preparative scale synthesis of MCO in 64% yield,
similar to the results obtained by Paterson.46 Further improvements in the yield are likely achievable through
more careful control of the temperature and reaction time through advanced synthetic techniques such as
flow chemistry.52 Alternatively, the dimers and trimers can be converted to C12 and C18 cycloalkane-based
fuels by hydrodeoxygenation.48
The chemoselective reduction of 2 was performed under basic conditions using the classic
RuCl2[P(C6H5)3]3-NH2(CH2)2NH2-KOH ternary catalyst system. As previously described by Noyori, the
use of two equivalents of ethylene diamine and one equivalent of KOH in combination with the Ru-
precatalyst is crucial to enhancing the chemoselectivity.[50,53] A modest catalyst loading (1 mol%) and
moderate hydrogen pressure (500 psi) were sufficient to achieve complete conversion of the substrate at
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ambient temperature, with 96% chemoselectivity to the enol. The presence of the methyl group plays a
critical role in enhancing the chemoselectivity. For example, the ruthenium catalyzed hydrogenation of
cyclohex-2-en-1-one has been reported to proceed with 70% chemoselectivity, whereas the hydrogenation
of 3-methylcyclohex-2-en-1-one proceeded with 99.9% chemoselectivity.[53] As a comparison, the
reduction was carried out with NaBH4 and equimolar amounts of CeCl3•7H2O in anhydrous methanol (see
the ESI for experimental details).49 Under these conditions we achieved >99.9% chemoselectivity to 3
(Figure S12). Despite the higher selectivity obtained through this method, the use of stoichiometric NaBH4
and CeCl3•7H2O would be impractical for the synthesis of fuels at commercial scale. Moreover, from a
green chemistry perspective, this approach would result in the production of an onerous waste stream rich
in boron and cerium salts.
Dehydration of secondary alcohols under mild acidic conditions is well known in the literature.[54]
During initial work-ups of 3, dilute HCl solutions were used to extract the product, followed by
concentration of the solution under reduced pressure, and at elevated temperatures. Under these conditions,
NMR studies confirmed the dehydration of the alcohol. Therefore, in an attempt to develop a sustainable
dehydration process, we explored the use of dilute HCl solutions to catalyze the reaction (the results are
summarized in Table S1). Temperatures above 60 °C promoted heavier oligomer formation, while lower
temperatures resulted in a complicated product distribution due to the formation of ethers (Scheme 3),
which were difficult to separate from 5 due to their similar molecular weights and boiling points. Even if
the DMCPD isomers could be efficiently separated from the ethers by distillation, small amounts of ethers
present in the product mixture would lower the gravimetric net heat of combustion of the finished fuels.
Scheme 3. Ether formation during the dehydration of 3 with 1M HCl at temperatures below 60 °C.
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To avoid these issues, we explored the use of a heterogeneous system based on our previous
work.[37,51] 3 was dehydrated over AlPO4/MgSO4 at 70 °C, under reduced pressure (40 mmHg). These
reaction conditions were selected to maximize the conversion of 3 to products, while reducing the
propensity for dimerization and oligomerization of 4. The distillate was allowed to phase separate, and the
organic phase was warmed to room temperature and maintained at that temperature for 48 h, allowing for
Diels-Alder cyclodimerization to generate 5. Small amounts of residual 4 and unreacted starting material
were present in the product mixture (Figure 1). The distillation pot contained primarily 5, along with a small
amount of trimer and residual starting material. The overall yield of product (calculated as the sum of 4 and
5) from the dehydration reaction was 79%, while higher yields could be obtained by recycling unreacted 3.
The dimethyldicyclopentadiene fraction consisted of seven resolvable isomers, similar to
commercial material.[55] However, the relative amounts of each isomer differed substantially compared to
the conventional product distribution. The two most prominent isomers were 3,9-dimethyl-endo-
tricyclo[5.2.1.02,6] deca-3,8-diene (5a), and 4,9-dimethyl-endo-tricyclo[5.2.1.02,6] deca-3,8-diene (5b)
(Scheme 4). The identity of these two isomers was confirmed by comparing the retention time and mass
spectra of these components to our previous results[37] and a literature report.[Error! Bookmark not defined.] The
prevalence of isomer 5a suggests that dehydration of the alcohol to form 1- methylcyclopenta-1,3-diene
(4a) is followed by isomerization to 2-methylcyclopenta-1,3-diene (4b). Diels-Alder cyclodimerization of
4b yields 5a, while a cross-cycloaddition between 4a and 4b yields 5b. It is not clear what impact the
isomer distribution will have on the final fuel properties of derivative RJ-4 mixtures; this topic will be
studied in future work.
The pathway described above resulted in a 38% overall yield for the conversion of 1 to 4/5.
However, other researchers have reported that nearly quantitative yields of 2 can be obtained through the
use of a biphasic reaction system[47] or a solid base catalyst.[48] In addition, although we achieved a 75%
isolated yield of 3 after the chemoselective hydrogenation step, we obtained a 98% conversion of 2 to 3.
This suggests that a more efficient distillation of 3, enabled by running the reaction at large scale, would
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allow for a much higher isolated yield. Finally, the dehydration reaction did not go to completion, and
unreacted 3 could be recycled for subsequent conversion to 4. Therefore, it may be feasible to improve the
Figure 1. Gas chromatogram of crude dimethyldicyclopentadiene (DMDCPD) synthesized from 1.
Scheme 4. Dehydration of 3, followed by Diels-Alder cycloaddition, to generate DMCPD isomers 5a and
5b.
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yield of 4/5 to greater than 80%. Recent work by Zhang et al.[48] has demonstrated a 71.2% yield of 1 from
cellulose. Combining the two approaches at commercial scale may allow for dimethyldicyclopentadiene
yields of greater than 55% from cellulose.
CONCLUSIONS
The development of efficient synthetic routes to commodity chemicals conventionally sourced from
petroleum is a key pillar of sustainable development. This work has demonstrated that 2,5-hexanedione, a
bio-based platform chemical readily generated from lignocellulosic biomass, can be converted to
methylcyclopentadiene (MCPD), an important precursor to high performance polymers and fuels. This
transformation was accomplished by the utilization of a three-step method combining intramolecular aldol
condensation, Ru-catalyzed chemoselective hydrogenation, and dehydration. MCPD can subsequently be
converted to tetrahydrodimethyldicyclopentadiene (RJ-4), a high-density fuel that can increase the density
and volumetric net heat of combustion of jet fuel blends with acyclic synthetic paraffinic kerosenes. The
use of sustainable RJ-4 in jet fuel mixtures may provide an opportunity to reduce particulate emissions and
net greenhouse gas emissions from commercial and military aircraft.
ACKNOWLEDGEMENTS
The authors would like to thank Ms. Roxanne Quintana and Ms. Alicia Hughes for conducting GC-MS and
GC-FID studies. We would also like to thank Dr. Stephen Ayrton (SciPix, The Art of Science) for preparing
the TOC graphic. This work was funded by the Naval Air Warfare Center, Weapons Division PL-219
program.
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TOC Graphic
Ruthenium is the key: The production of jet fuels from abundant biomass sources is critical for sustainable
aviation. This work describes the conversion of 2,5-hexanedione, a bio-based platform chemical readily
derived from cellulose, to methylcyclopentadiene, a commodity chemical useful for the preparation of high-
performance jet fuel. A key step in the process is the chemoselective hydrogenation of 3-methyl-2-
cyclopenten-1-one catalyzed by RuCl2(PPh3)2(H2N(CH2)2NH2).
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