Catalyst-free synthesis of biodiesel precursors from biomass-based furfuryl alcohols in the presence of H2O and air

Article abstract of DOI:10.1039/c9gc03179k

Production of biodiesel from biomass resources usually requires elongation of carbon numbers from typical C₅ and C₆ platform molecules through C-C coupling reactions, which were catalyzed by acid, base or metal catalysts traditionally. Herein, a catalyst-free method was developed to produce bis(furan-2-yl)methane derivatives (BFMs) from furfuryl alcohol derivatives (FAs) in the presence of H₂O and air without any other additional catalysts. An 81% yield of bis(5-methylfuran-2-yl)methane (BMFM) can be obtained from 5-methylfurfuryl alcohol (5-MFA) and a 59% total yield of C₁₁ biodiesel was obtained from 5-methylfurfural (5-MF). In addition, a H₂O and air mediated free radical decarboxylation mechanism was proposed based on the detailed mechanistic studies. This strategy offers a green, low-cost and environmentally friendly approach to synthesize biodiesel precursors from biomass based platform molecules.

Full text of DOI:10.1039/c9gc03179k

Green Chemistry
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Catalyst-free synthesis of biodiesel precursors
from biomass-based furfuryl alcohols in the
presence of H2O and air†
Cite this: Green Chem., 2019, 21,
6326
Shengxiang Qin,‡b Teng Li,‡a Man Zhang,a Hongyu Liu,a Xin Yang,a Nianxin Rong,a
a,c
Jun Jiang,a Yalin Wang,a Hua Zhang *b and Weiran Yang
*
Production of biodiesel from biomass resources usually requires elongation of carbon numbers from
typical C5 and C6 platform molecules through C–C coupling reactions, which were catalyzed by acid,
base or metal catalysts traditionally. Herein, a catalyst-free method was developed to produce bis(furan-
2-yl)methane derivatives (BFMs) from furfuryl alcohol derivatives (FAs) in the presence of H2O and air
without any other additional catalysts. An 81% yield of bis(5-methylfuran-2-yl)methane (BMFM) can be
obtained from 5-methylfurfuryl alcohol (5-MFA) and a 59% total yield of C11 biodiesel was obtained from
5-methylfurfural (5-MF). In addition, a H2O and air mediated free radical decarboxylation mechanism was
proposed based on the detailed mechanistic studies. This strategy offers a green, low-cost and environ-
mentally friendly approach to synthesize biodiesel precursors from biomass based platform molecules.
Received 10th September 2019,
Accepted 19th October 2019
DOI: 10.1039/c9gc03179k
rsc.li/greenchem
genation of furan derivatives can only provide C5–C6 biogasoline
Introduction
candidates.21–25 Biodiesel production from biomass is another
Transportation fuels and chemicals are mainly resourced from important research area. Biodiesel, a common fuel with a high
non-renewable fossil resources, such as crude oil, coal and heat value, containing long linear, branched, and cyclic C9–C20
natural gas, which may be unavailable in the future due to alkanes, can be produced from the furan derivatives by the
their unsustainability.1–3 Therefore, it is urgent to explore sus- carbon chain elongation reaction.26 Lately, varieties of
tainable resources as replacements. As a low-cost, abundant methods have been developed to elongate the carbon chains
and renewable resource, biomass is an ideal alternative to of furan derivatives to prepare biodiesel precursors, such as
fossil resources, which has received widespread attention in aldol condensation,27–32 hydroxyalkylation/alkylation26,33–37
recent years.4–6 Many furan derivatives can be efficiently and C–C coupling.38 For example, the Ouyang group28 used a
obtained from cellulose and hemicellulose,7 the major com- basic ionic liquid as a base catalyst for aldol condensation of
ponents of biomass, such as 5-hydroxymethylfuran (5-HMF),8,9 furfural with acetoin, and 78% yield of the condensation
furfural,10–12 and 5-methylfurfural (5-MF),13–17 which are product was obtained (Scheme 1a). Pioneering studies by
platform molecules for the production of other value-added Corma,33,39 Zhang,35,36 and Rode37 have achieved excellent
chemicals to be used in the polymer, agricultural, and results with high dimer product yields by reacting aryl or
pharmaceutical industries.18–20
alkyl aldehyde compounds with 2-methylfuran using an acid
One of the most important applications of the furan deriva- catalyst (Scheme 1b). Recently, a new method to increase the
tives is as a rich carbon source for the production of biofuels chain length was reported by the Wang group,38 which can
via further hydrodeoxygenation reaction. Direct hydrodeoxy- coproduce biodiesel precursors and H2 from lignocellulose-
derived 2-methylfuran and 2,5-dimethylfuran via acceptorless
dehydrogenative C–C coupling driven by visible light over a
Ru-doped ZnIn2S4 catalyst (Scheme 1c). However, most of
aSchool of Resource Environmental and Chemical Engineering, Nanchang University,
No. 999 Xuefu Avenue, Jiangxi 330031, PR China. E-mail: wyang16@ncu.edu.cn
bSchool of Chemistry, Nanchang University, No. 999 Xuefu Avenue, Jiangxi 330031,
PR China. E-mail: huazhang@ncu.edu.cn
cKey Laboratory of Poyang Lake Environment and Resource Utilization
(Nanchang University), Ministry of Education, No. 999 Xuefu Avenue,
Jiangxi 330031, PR China
these methods use acid, base or noble metals as the catalyst,
which leads to the corrosion and pollution problems and thus
high production cost.
Here, we developed an efficient catalyst-free method to syn-
thesize bis(furan-2-yl)methane derivatives (BFMs) as biodiesel
precursors from furfuryl alcohol derivatives (FAs), which can
be obtained in large quantities from biomass-lignocellulose40
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc03179k
‡These authors contributed equally to this work.
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Scheme 2 Synthesis of BHCMs from heteroaryl compounds.
Scheme 1 Biodiesel precursors from biomass-based derivatives.
yield of bis(furan-2-yl)methane (BFM) with the corresponding
amount of formaldehyde over the Rh(PPh3)3Cl catalyst.
Ramasastry and co-workers50 developed a method for the self-
condensation of FAs followed by elimination of an aldehyde to
generate BFMs by using the Yb(OTf)3 catalyst (Scheme 2b).
Other bis(heterocyclyl)methane derivatives (BHCMs) were syn-
thesized by the hydroxyalkylation/alkylation of heteroaryl com-
pounds with aldehydes or ketones over Lewis or Brønsted
acids, or by the electrophilic substitution between heteroaryl
methanol derivatives and another heteroaryl compound with
an acid catalyst51 (Scheme 2c). Thus, our attempt to use only
H2O and air is a big advancement to the current methods for
the production of BFMs.
Experimental section
Materials
Fig. 1 Pathway for preparing biodiesel precursors from lignocellulose.
Unless otherwise noted, all reactants or reagents were obtained
from commercial suppliers and used as received without
(Fig. 1). The reaction conditions are mild and green without further treatment. 5-Methylfurfuryl alcohol was obtained from
additional catalysts or reagents. Only H2O and air were proved Energy Chemical (Shanghai, China), 9-Dingchem (Shanghai,
to be necessary in this process (Scheme 1d). In addition, a China), and Ark (Chicago, USA). 2-Methylfuran, 5-methyl-
radical decarboxylation mechanism was proposed by further furancarboxylic acid, furfuryl alcohol, 2-furanoic acid NaBH4,
mechanistic study.
Pd/C (5.0 wt%), undecane, 1,2-dichloroethane, toluene and
Besides being used as biodiesel precursors, BFMs can be ethyl acetate were obtained from Aladdin (Shanghai, China).
directly used as a valuable odorant and monomer for the Isopropylmagnesium chloride, 5-methylfurfural, SOCl2,
preparation of sensors for ion-selective chromatography,41
a
CD3OD, DMSO-D6, CDCl3 and D2O were obtained from Inno-
dielectric material by furan/imide Diels–Alder polymers,42 and Chem (99.9%, Beijing, China). H-beta zeolite was obtained
a useful intermediate in organic synthesis.43,44 There are from Nankai University Catalyst Co., Ltd. Cyclohexane was
several methods reported for the production of BFMs. BFMs obtained from Inno-Chem (99.5%, Beijing, China), Xilong
can be obtained by the self-condensation of FAs followed by Scientific (Shantou, China), and Tokyo Chemical Industry
elimination of an aldehyde catalyzed by an acid.45–49 For (Shanghai, China). Milli-Q H2O was used in the experiments.
example, the Balaban group45 reported that high-yield The CHEMNNS-10-C276 high pressure stainless steel reactor,
(80–82%) BFMs were produced from FAs with polyphosphoric temperature controller and IKA magnetic stirring apparatus
acid (PPA) in aqueous acetone (Scheme 2a). The Dzhemilev were obtained from Anhui Kemi Machinery Technology
group41 found that furfuryl alcohol (FA) can provide an 80% Co., Ltd.
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Experimental procedure
The EPR data were obtained using a Bruker A300 spectrometer
(Bruker, Germany). NMR spectra of the compounds were
recorded on Bruker Advance III (400 MHz) spectrometers with
tetramethylsilane as an internal standard. 5-MFA conversion
and BMFM yield were calculated using the following formulae:
All experiments were performed in a Schlenk tube or a high
pressure reactor with a magnetic stirrer. Typically, reactants
(5-methylfurfuryl alcohol, 5-methylfurancarboxylic acid, fur-
furyl alcohol etc.) and solvents were added in the vessel, and
then the reaction was performed at the desired reaction tem-
perature under an air atmosphere (or filled with N2, O2, or
their mixtures). Upon completion of the reaction, the reaction
mixture was cooled down to room temperature, and the reac-
tion mixture was analyzed by GC, GC-MS, UPLC and other
necessary analytical methods. Standard solutions were used to
obtain the calibration curves, and the concentrations of the
compounds were calculated by the external standard method.
5‐MFA conversion ¼
Moles of initial 5‐MFA ꢀ Moles of final 5‐MFA
ꢁ 100%
Moles of initial 5‐MFA
Moles of final BMFM
Moles of initial 5‐MFA
BMFM yield ¼
ꢁ 100%
2
Calculation formula for reaction conversion and yield
Analysis of the reaction mixture
The reaction mixture was collected and filtered through a
0.22 µm syringe filter. The organic phase was analyzed by
GC-MS (Thermo Scientific TRACE 1310) using a chromato-
graphic column (HP-5 capillary column: 30 m × 320 μm ×
Results and discussion
Catalyst-free process development
0.25 μm) and GC (Agilent 7820A gas) using a chromatographic Initially, we envisioned to develop a catalytic process to
column (HP-5 capillary column: 30 m × 320 μm × 0.25 μm). produce bis(5-methylfuran-2-yl)methane (BMFM) as a bio-
The aqueous phase was measured by UPLC (Waters, Acquity diesel precursor by oxidative coupling of 5-methylfurfuryl
H-class) using an organic acid column (Alltech OA1000- alcohol (5-MFA), which can be easily obtained from biomass-
organic acid column: 6.5 mm ID × 300 mm). The gas phase based 5-methylfurfual (5-MF)13–17 (Scheme S-1†). To our sur-
was analyzed with a top air phase model (PANNA-A91PLUS). prise, it was found that this process can be achieved without
The trace metals were measured with a Thermo Scientific 7000 additional metals or acid catalysts. After extensive screening,
series inductively coupled plasma-atomic emission spectro- 79% GC yield (74% isolated yield) of BMFM was obtained from
meter (ICP-AES). Free radical adduct product was detected by 5-MFA (0.2 mmol) in a biphasic reaction system with H2O
ultra performance liquid chromatography quadrupole time- (1.0 mL) and cyclohexane (1.0 mL) under an air atmosphere in
of-flight mass spectrometry (UPLC-X500RQTOF), using an a sealed tube at 100 °C for 2 h (Table 1, entry 1). The effects of
ACQUITY UPLC@BEH C18 1.7 μm (2.1 mm × 50 mm) column. deviations to the standard reaction conditions are listed in
Table 1 Exploration and optimization of the reaction conditionsa
Solvent (mL)
H2O/cyclohexane
Entry
Atmosphere
Temp. (°C)
Time (h)
Conv.b (%)
Yieldb (%)
1
2
3
4
5
6
7
8
1.0/1.0
0.0/1.0
0.018/1.0
0.5/1.0
2.0/1.0
1.0/0.0
1.0/1.0d
1.0/1.0e
1.0/1.0f
1.0/1.0
1.0/1.0
1.0/1.0
1.0/1.0
1.0/1.0
1.0/1.0
1.0/1.0
1.0/1.0
50.0/50.0
Air
Air
Air
Air
Air
Air
Air
Air
Air
N2
100
100
100
100
100
100
100
100
100
100
100
25
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
6
2
2
90
26
35
94
85
95
80
85
84
21
94
74
85
94
90
91
91
96
79(74)c
0
6
71
70
49
60
69
44
4
9
10
11
12
13
14
15
16
17g
18h
O2
43
0
Air
Air
Air
Air
Air
Air
Air
60
36
78
68
76
78
(81)c
140
100
100
100
100
a 0.2 mmol 5-MFA. b The conversion and yield were determined by UPLC and GC. c Isolated yield in parentheses. d 1,2-Dichloroethane. e Toluene.
f Ethyl acetate. g The tube was covered with aluminum foil. h 40.0 mmol 5-MFA.
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Table 1. In the absence of H2O, no BMFM was detected any other unexpected influences,53 and a similar result was
(Table 1, entry 2). Thus, H2O is crucial for this transformation. also obtained. In conclusion, this catalytic system is indeed a
To further study the function of H2O to be a catalyst or a catalyst-free process.
solvent, five equivalents of H2O (18 μL) instead of 1.0 mL H2O
Mechanism investigation experiments
were used for the reaction. However, only 6% yield of BMFM
was produced (Table 1, entry 3). When the amount of H2O was
increased to 0.5 mL and 2.0 mL, the results were comparable
to those obtained under the standard conditions (Table 1,
entries 4 and 5), which proves that H2O is required as an indis-
pensable solvent. With only H2O as the solvent but not as the
organic solvent, the yield of BMFM is 49% (Table 1, entry 6).
Thus, the addition of an organic solvent is beneficial for the
reaction, probably due to the stabilization of the reaction inter-
mediates.52 When 1,2-dichloroethane, toluene or ethyl acetate
was used instead of cyclohexane, 60%, 69% and 44% yield of
BMFM was obtained, respectively (Table 1, entries 7–9), so
cyclohexane was selected as the appropriate organic solvent.
When N2 was applied to replace air, only a very small amount
of BMFM (4% yield) was detected (Table 1, entry 10). When O2
was used rather than air in this reaction, the yield of BMFM
decreased to 43% (Table 1, entry 11). Therefore, a moderate
concentration of O2 can promote the reaction, which will be
discussed in more detail in the mechanistic study section.
The effect of reaction temperature and reaction time was
also studied. At 25 °C, no BMFM was detected (Table 1, entry
12). The yield of BMFM increased to 36% at 60 °C, and 78% at
140 °C in 2 h (Table 1, entries 13 and 14). In 1 h, 68% yield of
BMFM could be produced, which is slightly lower than that in
2 h (Table 1, entry 15). In addition, the BMFM yield only
improved a little as the reaction time was extended to 6 h
(Table 1, entry 16). Thus, the conversion of 5-MFA to BMFM is
fast in this system.
To study the reaction mechanism, the product distribution vs.
reaction time was performed (Fig. S-1†), which showed that
both the conversion and the yield are more than 50% in just
10 min at 100 °C. As a result, the reaction happened too
quickly to capture any useful intermediates.
It was reported that the dimerization of furfuryl alcohol
derivatives could go through two different mechanisms.41,46,50
One is that a carbocation (compound A) is formed from a fur-
furyl alcohol derivative catalyzed by a Brønsted acid, and then
the electrophilic substitution between compound A and the
furfuryl alcohol derivative produces a dimer and an aldehyde
(Scheme 3a). Another mechanism is that the furfuryl alcohol
derivative undergoes etherification and then intramolecular
Friedel–Crafts type reaction catalyzed by
a Lewis acid
(Scheme 3b), which also generates a molecule of aldehyde.
However, no HCHO was detected in the reaction liquid. We
speculated that a gaseous product (CO or CO2) could be
generated. Indeed, CO2 was detected after the reaction.
Furthermore, using HCHO as the reactant, no CO2 was
detected under the same conditions. Therefore, CO2 is directly
produced from the 5-MFA dimerization process (Scheme S-2†).
Thus, this reaction does not follow either carbocation or ether
mechanism as reported before.
Air has been proved necessary for this reaction in the pre-
vious section, which is better than pure O2. To further study
the effect of O2, the experiments were carried out with
different proportions of N2 and O2 (Fig. 2). The BMFM yield
To study the influence of light, the reaction without light
(the tube was covered with aluminum foil) was carried out.
The result was similar to that with light, which ruled out a
photocatalytic mechanism (Table 1, entry 17).
The reaction was also performed on a large scale. 4.49 g
(40.0 mmol) 5-MFA, 50.0 mL cyclohexane, and 50.0 mL H2O
were added into a 500 mL round bottom flask, and then
refluxed for 2 h under an air atmosphere. An 81% (16.3 mmol,
2.86 g) BMFM isolated yield was obtained (Table 1, entry 18).
Confirmation of the catalyst-free process
To further verify that this reaction is a catalyst-free process, a
series of control experiments were performed. The trace
metals were measured by inductively coupled plasma-atomic
emission spectrometry (ICP-AES) analysis on the reaction
mixture after the reaction. The concentrations of metal ions in
the reaction solution were all below the detection limit of the
ICP-AES analysis. Thus, the catalytic effect of trace metal ions
can be negligible (Table S-1†). Besides, cyclohexane, H2O and
5-MFA from different sources (including purified 5-MFA) were
used, respectively, to exclude the influence of impurities in the
raw materials, and similar results were obtained (Tables S-2
and S-3†). Furthermore, the experiment was repeated with a
Scheme 3 Proposed mechanisms in the literature studies.41,46,50
brand new reactor, a new stirrer and ultrapure H2O to exclude Fig. 2 Effect of O2 partial pressure on this reaction.
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Scheme 4 Free radical trapping experiments.
increased first and then decreased slowly with the increase of
O2 partial pressure, and the highest yield of BMFM was
obtained (50%) as the ratio of O2 and N2 was around 21%. Low
O2 partial pressure had a negative effect on the BMFM yield,
implying that the oxidation reaction happened during this
reaction. High O2 partial pressure may cause overoxidation of
5-MFA, leading to a decreased BMFM yield.
Knowing that this reaction requires O2 and generates CO2,
it is most probably an oxidative decarboxylation process. As
per literature search, decarboxylation under mild conditions
usually follows a free radical mechanism,54,55 so the possibility
of a radical mechanism was explored. First, the radical trap-
ping experiments were carried out. When three equivalents of
Scheme 5 Condensation pathway investigation experiments.
To make the condensation pathway clear, experiments with
different reactants were carried out (Scheme 5). No BMFM was
formed from pure 5-methylfurancarboxylic acid (MFCA)
(Scheme 5a), which indicated that BMFM was not produced by
the dimerization of MFCA. According to some references,56–58
the process of converting an alcohol into an acid generally
goes through an aldehyde. Thus, 5-methylfurfural (5-MF) as a
substrate was also investigated under the standard conditions.
No BMFM was detected either (Scheme 5a). Based on these
results and considering that air is better than pure O2, we
speculated that it is probably a partial oxidation process that
leads to the reaction between a molecule of alcohol and a
molecule of acid.59–61 Consequently, reactions of 5-methyl-
furfuryl alcohol (5-MFA) with one equivalent of 5-MF or MFCA,
respectively, under standard conditions were implemented.
With 5-MF, the BMFM was obtained with only 35% yield
(Scheme 5b). Repeating the same sequence with MFCA
resulted in the formation of 62% BMFM (Scheme 5c). Then
the standard conditions were adjusted by using N2 instead of
air considering that the oxidation of alcohol is not needed
anymore. A 74% yield of BMFM was obtained with an equal
amount of 5-MFA and MFCA, similar to that with 5-MFA alone
under the standard conditions (Scheme 5d). According to
these results, it is safe to conclude that the process is a partial
oxidation reaction and the condensation is between 5-MFA
and MFCA. Based on the literature studies,62,63 the dimeriza-
tion of 5-MFA and MFCA may undergo esterification and
then decarboxylation. However, no BMFM was detected
from (5-methylfuran-2-yl)methyl 5-methylfuran-2-carboxylate
(MMMC) (Scheme 5e), which excluded the ester pathway.
Thus, MFCA probably undergoes decarboxylation first and
then dimerization reaction happens between 5-MFA and the
decarboxylation product.
the
radical
inhibitor
2,2,6,6-tetramethylpiperidinooxy
(TEMPO) were added into the reaction under the standard
conditions, the yield of BMFM dropped sharply to trace,
which indicated that this transformation could undergo a
free radical pathway. Besides, an addition product of the free
radical intermediate (5-methylfurancarboxylic acid radical,
MFCA•) and TEMPO was detected by Ultra Performance
Liquid Chromatography Quadrupole Time-of-flight Mass
Spectrometry (UPLC-QTOF-MS) (Scheme 4 and Fig. S-2†).
Therefore, 5-methylfurancarboxylic acid (MFCA) is a possible
intermediate, which can be formed by 5-MFA oxidation by O2.
The He group has also reported that benzyl alcohols can be
oxidized to aromatic carboxylic acids in bis(methoxypropyl)
ether with atmospheric O2 as the oxidant.56 In order to further
probe the free radical species formed during the reaction, an
electron paramagnetic resonance (EPR) study was performed
(Fig. 3). Free radical signals were observed in both H2O and
cyclohexane. Therefore, the reaction has been subjected to a
free radical process.
To probe the decarboxylation process of MFCA, MFCA was
used as a reactant under the standard conditions, and 10% of
Fig. 3 EPR free radical signal detection experiments.
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MFCA reacts to generate MFCA• and 2-methylfuran. At the
same time, 5-MFA+ is generated from 5-MFA by eliminating a
molecule of H2O with H+ coordination. Finally, 5-MFA+ and
2-methylfuran produce BMFM through electrophilic substi-
tution reaction.
Mechanism verification experiments
To further confirm the formation of a carbocation, 2-methyl-1-
(5-methylfuran-2-yl)propan-1-ol (MFPL) was used as the start-
ing material. No dimer was detected under the standard con-
ditions as expected, since MFPL is unable to form an acid
(Scheme 8a). However, when equivalent amount of MFPL
and 5-MFA were used as the starting materials, the dimer
5,5′-(2-methylpropane-1,1-diyl)bis(2-methylfuran) (MDBF) was
detected as the major product by GC-MS (Scheme 8b). MDBF
is formed as the major product instead of BMFM most prob-
ably due to the fact that MFPL can form a carbocation more
easily. In addition, when FA was used instead of 5-MFA, the
dimer product (5-(2-methyl-1-(5-methylfuran-2-yl)propyl)furan-
2-yl)methanol (MPFM) was detected (Scheme 8c). It must be
that MFPL can form the carbocation (MFPL+) readily, and then
MFPL+ attacks the active hydrogen of the furan ring to form
the dimerization product by an intermolecular electrophilic
substitution reaction.
Scheme 6 MFCA decarboxylation and 2-methylfuran intermediate
experiments.
2-methylfuran was detected. When three equivalents of
TEMPO were added to the reaction under the same conditions,
no 2-methylfuran was detected (Scheme 6a). Thus, decarboxyl-
ation of MFCA undergoes a radical pathway to form 2-methyl-
furan. A sulfonic acid anion, which is similar to MFCA−, has
been reported to form the corresponding free radical by oxi-
dation with O2.64–66 Therefore, MFCA probably forms 5-methyl-
furan-2-carboxylate (MFCA−) first by ionization in H2O, which
further converts to MFCA• by oxidation and then goes through
decarboxylation to form 2-methylfuran, and the dimerization
process could happen between 5-MFA and 2-methylfuran. To
confirm this hypothesis, equal amounts of 5-MFA and
2-methylfuran were reacted under the standard conditions,
and the detected BMFM yield was higher than that obtained
with 5-MFA alone under the same conditions (Scheme 6b).
These pieces of evidence indicate that MFCA can form
2-methylfuran by free radical decarboxylation and 2-methyl-
furan is involved in the dimerization reaction.
To understand a clearer role of H+ in the mechanism, the
effect of acidity or alkalinity on the reaction was studied.
When 0.1 mL CH3COOH was added to the reaction system, no
effect on the reaction result was observed, while no product
was obtained after adding 0.1 mL Et3N (Scheme S-3†). It is
possible that the MFCA formed during the reaction provides
H+, which plays a crucial role in the carbocation production.
This cannot occur in an alkaline environment.
The effect of H2O on the reaction was also studied in depth.
It was reported that the decarboxylation process of some 2-fur-
ancarboxylic acid derivatives also requires H2O.67 Therefore,
experiments were designed with and without H2O by using
MFCA as the substrate under the otherwise standard con-
ditions. The decarboxylation product 2-methylfuran was
detected with 10% yield in the presence of H2O, and a trace
amount of 2-methylfuran was detected in the absence of H2O
(Scheme S-3†). Therefore, besides being a solvent, H2O also
plays a vital role in the decarboxylation process.
Therefore, the reaction mechanism is proposed in
Scheme 7 for the catalyst-free conversion of 5-MFA to BMFM.
First, 5-MFA is oxidized to MFCA under O2. MFCA further con-
verts to MFCA•, which splits into CO2 and 5-methylfuran-2-free
radical (2-MF•) by decarboxylation. Afterwards, 2-MF• and
Scheme 7 Possible mechanism for the synthesis of BMFM.
Scheme 8 Electrophilic substitution reaction studies.
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ditions, which has great potential for the synthesis of
biodiesel.
Conclusions
In summary, a novel approach was reported to synthesize
BFMs as biodiesel precursors from biomass-based furfuryl
alcohols in the presence of only H2O and air without catalysts.
Up to 81% yield of bis(5-methylfuran-2-yl)methane (BMFM)
can be obtained from 5-methylfurfuryl alcohol (5-MFA) at
100 °C for 2 h in a biphasic system of H2O and cyclohexane.
A free radical decarboxylation mechanism for this reaction was
proposed based on control experiments and intermediate
trapping exploration. Part of 5-MFA is oxidized to 5-methyl-
furancarboxylic acid (MFCA) first. Then 2-methylfuran is
formed by the radical decarboxylation of MFCA. Finally, the
dimerization happens between 5-MFA+ and 2-methylfuran by
electrophilic substitution. H2O is indispensable as a solvent
and for the decarboxylation of MFCA. Air is better than O2 for
this partial oxidation reaction. Based on this method, 59%
total yield of C11 biodiesel was synthesized from 5-MF by
hydrogenation, dimerization and hydrodeoxygenation. Overall,
this synthetic method has great potential in practical appli-
cations for the production of biodiesel precursors, with fea-
tures such as being catalyst-free, requiring mild conditions
and short reaction time, and being environmentally benign.
This synthetic method is an important addition in regard to
industrial applications for energy development needs.
Scheme 9 Substrate scope.
Substrate scope
Besides 5-MFA, furfuryl alcohol (FA) was also used as the start-
ing material. Few BFM was obtained from FA under the stan-
dard conditions. By increasing the reaction temperature and
prolonging the reaction time (150 °C, 8 h), only 12% of BFM
was obtained. We believe the reason for the high reactivity of
5-MFA is that the methyl group at the 5-position can stabilize
the free radical and the carbocation intermediate, so that the
5-MFA reactivity is promoted. The instability of FA in a high
temperature aqueous solution is another reason for the low
BFM yield. FA can react with 2-furanoic acid (FCA) to form
BFM in 17% yield under N2 (Scheme 9a). Additionally, in
asymmetric synthesis experiments, a 38% yield of 2-(furan-2-
ylmethyl)-5-methylfuran (FMMF) was synthesized via FA and
MFCA (Scheme 9b).
Conflicts of interest
Production of C11 biodiesel from 5-MF
Based on this work, a synthetic route was developed to syn-
thesize C11 biodiesel from 5-MF (Scheme S-4†). Different from
lipid-based biodiesel (C17–C18, freezing point 22–29 °C), C11
biodiesel is a kind of light diesel oil with a −26 °C freezing
point, which can be used at low temperature. First, a 98% yield
of 5-MFA can be obtained from 5-MF by hydrogenation, then
BMFM was synthesized under the standard conditions
described above with 79% yield, and finally, a 76% yield of C11
biodiesel was synthesized from BMFM via hydrodeoxygenation
over Pd/C and H-beta zeolite (Scheme 10). A 59% total yield of
C11 biodiesel was obtained from 5-MF based on this method.
This synthetic method is a simple operation under mild con-
There are no conflicts to declare.
Acknowledgements
This work was supported by the funds from financial support
of the National Natural Science Foundation of China (Grant
No. 21808100), the “Thousand Young Talents” Program of
China, and the “Thousand Talents” Entrepreneurship Program
of Jiangxi Province.
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