Zirconocene-catalysed biodiesel synthesis from vegetable oil with high free fatty acid contents

Article abstract of DOI:10.1016/j.jorganchem.2018.06.021

A highly efficient transformation of vegetable oils into biodiesel catalysed by air-stable and water-tolerant zirconocene perfluorooctanesulfonate Lewis acid has been developed. By combining the direct esterification of free fatty acids (FFAs) and transesterification of triglycerides (TGs), the simultaneous transformation of the two components to biodiesel is achieved in good yields. Furthermore, this catalyst, when used in the synthesis of biodiesel from commercial oleifera, behaves as a reaction-induced self-separation catalyst. During the course of the reaction, it switches from homogeneous to heterogeneous; upon completion of the reaction, the catalyst precipitates as a white solid, which can be easily recycled.

Full text of DOI:10.1016/j.jorganchem.2018.06.021

Accepted Manuscript
Zirconocene-catalysed biodiesel synthesis from vegetable oil with high free fatty acid
contents
Yiqiang Deng, Xiaojian Hu, Lihua Cheng, Hanlu Wang, Linhai Duan, Renhua Qiu
PII:
S0022-328X(18)30517-5
10.1016/j.jorganchem.2018.06.021
JOM 20480
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 27 April 2018
Revised Date: 12 June 2018
Accepted Date: 24 June 2018
Please cite this article as: Y. Deng, X. Hu, L. Cheng, H. Wang, L. Duan, R. Qiu, Zirconocene-catalysed
biodiesel synthesis from vegetable oil with high free fatty acid contents, Journal of Organometallic
Chemistry (2018), doi: 10.1016/j.jorganchem.2018.06.021.
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ACCEPTED MANUSCRIPT
Zirconocene-catalysed biodiesel synthesis from vegetable oil with
high free fatty acid contents
a,
b,c,
a
a
a
Yiqiang Deng *, Xiaojian Hu *, Lihua Cheng , Hanlu Wang , Linhai Duan ,
c,
Renhua Qiu *
a
College of Chemical Engineering, Guangdong University of Petrochemical
Technology, Maoming 525000, Guangdong, China
Department of Chemistry, School of Basic Medicine, Changsha Medical University,
Changsha, 410219, Hunan, China.
b
c
College of Chemistry and Chemical Engineering, Hunan University, Changsha,
4
10082, Hunan, China.
*
Corresponding author:
Y.Q. Deng, Tel/Fax: +86-668-2981080; E-mail: dyq3211@126.com,
X.J. Hu, Tel/Fax: +86-731-88602817; E-mail: 32897069@qq.com,
R.H. Qiu, Tel/Fax: +86-731-88821171; E-mail: renhuaqiu@hnu.edu.cn
1

ACCEPTED MANUSCRIPT
1
. Introduction
The quest for alternative sources of energy has become a main research area in
chemical industries owing to the increasing demand for energy, necessity to lower
the emission of greenhouse gases and environmental pollution, and fast
diminishing supply of fossil fuels [1]. Biodiesel is a diesel-fuel substitute
composed of fatty acid esters obtained from renewable sources (i.e. vegetable oils,
VOs) by catalytic transesterification with primary aliphatic alcohols, regarded as a
promising alternative sources of energy [2]. The alcoholysis of VOs for biodiesel
production has attracted much attention, since VOs are relatively cheap raw
materials widely abundant in nature [3]. However, because of the undesirable
saponification of their high free fatty acid contents, biodiesel synthesis usually
involves direct esterification of free fatty acids (FFAs) and transesterification of
triglycerides (TGs) with alcohols [4]. A recent example of industrial biodiesel
production from VOs using alkaline catalysts still required the pre-esterification of
FFAs before the transesterification of TGs, resulting in energy waste [5]. Instead,
the enzymatic production of biodiesel from VOs successfully combines these two
reactions [6]. However, the requirements for a high-quality final product and
reduction in wastewater production during the separation and purification steps,
together with the high cost of the enzyme, makes the production of biodiesel by
clean and sustainable biocatalysis still doubtful [7].
Our interest in Lewis acid chemistry has greatly contributed to organic synthesis,
especially in the above two key reactions used in biodiesel production, namely
2

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esterification and transesterification [8-11]. Various Lewis acid catalytic systems
have been used in (trans)esterifications, including organotin complexes as well as
alkylphenylsulfates [12-13]. Zirconocene shows high catalytic activity in a variety
of Lewis-acid-catalysed reactions [14], such as the Mannich reaction, allylation of
aldehydes, Friedel–Crafts acylation, as well as esterifications, such as Iglesias et al.
prepared Zirconium-containing SBA-15 materials and used as catalyst in the
production of fatty acid methyl esters from low grade oleaginous feedstock.
Catalytic assays in continuous mode in a fixed bed reactor showed a higher
resistance of Zr-SBA-15/bentonite pellets against catalyst deactivation[15-17].
Herein, we report that the incorporation of a large electron-withdrawing and
hydrophobic counter anion results in an efficient air-stable and water-tolerant
zirconocene catalyst (1) for biodiesel synthesis from vegetable oil with high free
fatty acid contents. The efficiency of the reaction results from a combination of the
direct esterification of free fatty acids (FFAs) and transesterification of
triglycerides (TGs) in one-pot. Furthermore, we found that this catalyst behaves as
a reaction-induced self-separation catalyst in the synthesis of biodiesel from
commercial oleifera. During the course of the reaction, the reaction switches from
homogeneous to heterogeneous. Upon completion of the reaction, the catalyst
precipitates as a white solid, which after a simple separation can be easily recycled
five times.
2
2
. Experimental
.1. General method
3

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All chemicals were purchased from Aldrich. Co. Ltd and used as received unless
otherwise indicated. The preparation of catalyst was carried out under nitrogen
atmosphere with freshly distilled solvents unless otherwise noted. THF and hexane
were distilled from sodium/benzophenone. Acetonitrile was distilled from CaH2.
The NMR spectra were recorded at 25 °C on INOVA-400M (USA) calibrated with
tetramethylsilane (TMS) as an internal reference. Elemental analyses were
performed by VARIO EL III. TG-DSC analysis was performed on a HCT-1
(
HENVEN, Beijing, China) instrument. The acidity was measured by Hammett
indicator method as described previously. Acid strength was expressed in terms of
Hammett acidity function (Ho) as scaled by pKa value of the indicators.
2
.2 Preparation of Cp Zr(H O) (OSO C F ) ·THF (1)
2 2 3 2 8 17 2
To a solution of Cp ZrCl (292 mg, 0.99 mmol) in THF (20 mL) was added a
2
2
solution of AgOSO C F (1.21 g, 2.0 mmol) in THF (10 mL). The mixture was
2
8 17
stirred in the dark at 25 °C for 1 h, and subject to filtration. The filtrate was
combined with dry hexane (40 mL). Then, the solution was stored in the
refrigerator for 24 h to furnish colorless crystals of 1·THF(794 mg, 65%):
1
M.p.133-136 °C. H NMR (400 MHz, Acetone-d ): δ = 6.76 (s, 10 H, CpH), 3.63
6
1
9
(
t, J = 5.6 Hz, 4 H, THF), 1.82 – 1.76 (m, 4 H, THF); F NMR (376 MHz;
Acetone): δ = -81.65 (t, J = 9.8 Hz, 3 F, CF3–), -115.02 (s, 2 F, –CF2–), -121.05
(
-
s, 2 F, –CF2– ), -122.17 – -122.48 (m, 6 F, –(CF2)3–), -123.28 (s, 2 F, –CF2–),
126.75 – -126.82 (m, 2 F, –CF2–). Elemental analysis for Cp Zr(OSO C F )
8 17 2
2
2
after pumping for a week: calcd. (%) for C H F O S Zr: C, 25.60; H, 0.83;
2
6
10 34
6 2
4

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found: C, 25.67; H, 0.82. Elemental analysis for Cp Zr(OSO C F ) ·4H O after
2
2
8
17 2
2
standing in open air for 2 days: calcd (%) for C H F O S Zr: C, 24.18; H, 1.40;
2
6
18 34 10 2
found: C, 24.34; H, 1.33. C H F O S Zr, Prismatic, colorless, Mr = 1345.83,
3
0
24 34 10 2
Dx = 1.905 Mg m−3, Triclinic, P¯ 1, a = 9.9295 (13) Å, b = 11.9551 (16) Å, c =
0.269 (3) Å, α = 82.833 (3)°, β = 79.491 (3)°, γ = 87.397 (3)°, V = 2346.7 (5) Å3,
2
Z = 2, θ = 1.7–25.5°, 0.50 × 0.24 × 0.21 mm,T = 293 (2) K, measured
reflections/independent reflections 12436/8583, Rint = 0.104, h = −12-11,k =
−
14-11, l = −24-22, R1 = 0.095,wR2 = 0.277, S = 0.95.CCDC No. 631296.
2
.3 Typical procedure for direct esterification of alcohol with oleic acid using
catalyst 1.
To a round-bottom flask was added ethanol (46 mg, 1.0 mmol) and 1 equivalent of
oleic acid (282 mg, 1.0 mmol) and catalyst 1 (12.6 mg, 0.01 mmol, 1.0 mol% relative
to ethanol). The mixture was stirred at 100°C for 24 h and monitored by TLC. Then
the mixture was diluted with petroleum ether (10 mL × 3). By means of filtration, the
catalyst was separated and used for the next cycle, and the filtrate was washed twice
with 10 mL of saturated brine, and extracted by petroleum ether (10 mL × 2).
Subsequently the portions of petroleum ether were combined together, dried by
sodium sulfate, and evaporated to obtain the crude ester. Finally, the ester was subject
to short flash column chromatography on silica gel (petroleum ether: ethyl acetate =
2
0:1, Rf = 0.4) to afford the colorless liquid, yield, 89%.
2
.4 Typical procedure for transesterification of alcohol with triolein using
catalyst 1.
To a round-bottom flask was added ethanol (138 mg, 3.0 mmol) and 1 equivalent
of triolein (885 mg, 1.0 mmol) and catalyst 1 (37.8 mg, 0.03 mmol, 1.0 mol% relative
to ethanol). The mixture was stirred at 100 °C for 24 h and monitored by TLC. Then
the mixture was diluted with petroleum ether (10 mL × 3). By means of filtration, the
catalyst was separated and used for the next cycle, and the filtrate was washed twice
5

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with 10 mL of saturated brine, and extracted by petroleum ether (10 mL × 2).
Subsequently the portions of petroleum ether were combined together, dried by
sodium sulfate, and evaporated to obtain the crude ester. Finally, the ester was subject
to short flash column chromatography on silica gel (petroleum ether: ethyl acetate =
2
0:1, Rf = 0.4) to afford the colorless liquid, yield, 85%.
Methyl oleate (5)
1
H NMR (400 MHz, CDCl ) δ 5.35 – 5.29 (m, 2H), 3.63 (s, 3H), 2.29-2.25 (m, 2H),
3
13
2
.05 – 1.98 (m, 4H), 1.61-1.58 (m, 2H), 1.26-1.23 (m, 20H), 0.87-0.85 (m, 3H); C
NMR (101 MHz, CDCl ) δ 174.19, 129.97, 128.02, 51.35, 34.05, 31.91, 31.52, 29.76,
3
2
2
9.67, 29.58, 29.53, 29.46, 29.35, 29.32, 29.26, 29.16, 29.11, 29.08, 27.19, 27.17,
+
5.61, 24.93, 22.68, 22.57, 14.08, 14.04. Ms (EI): m/z = 296.2 [M+H] .
Ethyl oleate (7)
1
H NMR (400 MHz, CDCl ) δ 5.35-5.33 (m, 2H), 4.14-4.09 (m, 2H), 2.34 – 2.21 (m,
3
2
3
3
2
H), 2.05-2.00 (m, 4H), 1.63-1.61 (m, 2H), 1.30-1.23 (m, 23H), 0.88 (t, J = 5.6 Hz,
1
3
H); C NMR (101 MHz, CDCl ) δ 173.89, 130.05, 128.04, 60.15, 34.38, 31.92,
3
1.54, 29.78, 29.70, 29.60, 29.54, 29.33, 29.18, 29.12, 27.21, 25.63, 24.98, 22.70,
+
2.59, 14.26, 14.12, 14.08.Ms(EI) : m/z = 310.2 [M+H] .
3
. Results and discussion
The
synthesis
of
zirconocene
bis(perfluorooctanesulfonate)
[
Cp Zr(H O) (OSO C F ) ] (1) is straightforward and involves the treatment of
2
2
3
2
8 17 2
Cp ZrCl with AgOSO C F in THF in the absence of light for one hour
2
2
2
8 17
according to our reported procedure (Fig. 1) [10].
Complex 1 shows strong Lewis acidity (0.8<H <3.3), high air (stable in open air
o
for one year), and thermal-stability. Moreover, it displays high catalytic activity in
various carbon–carbon and carbon–heteroatom bond formation reactions [10].
Owing to its air-stability, Lewis acid catalyst 1 can be successfully used in both
6

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direct esterification and transesterification reactions (Scheme 1). Thus, the
(
(
trans)esterification of pure oleic acid or triolein with ethanol in a strict 1:1 ratio
ratio of R groups of 2 or 3 to ethanol) in the presence of 1 mol% of 1 at 100 °C for
2
4 h affords product 5 in 89% and 85% yield, respectively. Increasing the ratio
from 1:1 to 1:2 relatively increases the yield of 5 to 93% and 92%, respectively.
Therefore, the two basic reactions for the transformation of VOs into biodiesel are
successfully achieved.
We hence applied catalyst 1 for the transformation of VOs to biodiesel.Various
types of crude VOs containing FFAs and TGs in different molar ratios were
selected as the raw material to test the catalytic efficiency; the results are
summarized in Table 1.
Since oleifera is mainly composed of FFAs and TGs, we first applied our
catalyst to the transformation of a mixture containing the reported ratio of pure
FFAs and TGs for oleifera, thus simulating the composite of the commercial
oleifera. This mixture was efficiently and simultaneously transesterified and
directly esterified using 1 mol% of catalyst 1 and two equivalents of ethanol at
1
00 °C (the loadings of catalyst 1 and ethanol were based on the oleic functional
group in oleifera) to produce biodiesel in 94% yield (Table 1, entry 1); the
conversion of FFAs and TGs was 93% and 97%, respectively (entry 1). The
1
products were analysed by comparing their H NMR spectra with that of biodiesel
obtained via the commercial synthetic method using the same tea seed oil as
starting material. Subsequently, we applied our catalyst to transform mixtures of
7

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pure FFAs and TGs simulating other VOs, such as canola, palm, soybean,
cottonseed and sunflower (entries 2–6); the yield of the desired ester 5 was 78%,
7
7%, 82%, 66% and 84%, respectively. These results suggest that the higher the
concentration of FFAs in the VO mixture, the higher is the yield of biodiesel
produced via our method, and that the TGs are transesterified before FFAs are
transformed to the desired ester, which may be due to the dynamic balance
between the products and TGs and FFAs.
The reaction can be scaled up to 31 mmol (8.83 g of oleifera), affording 5 with a
9
2% yield (Scheme 2). Moreover, the catalyst loading can be reduced to 0.5 mol%,
affording 5 in 82% yield. Similarly, the reaction with methanol gives the
corresponding product (7) in 92% and 80% yields using 1 mol% and 0.5 mol% of
1
, respectively. The reduction of catalyst loading represents a significant reduction
in the amounts of fluorine-containing compounds that might leak into the
environment, which makes the use of this catalyst advantageous in industrial
applications.
Since the mixture simulating oleifera was successfully converted to desired
biodiesel, we then used commercial oleifera (containing 2‒4 wt% H O) as starting
2
material; the result is show in Fig. 2. The reaction using 1 mol% of catalyst 1
afforded 5 in only 60% isolated yield possibly due to the water contained in the
starting material. Interestingly, when the volume of ethanol was increased tenfold
(
40 mL) and catalyst 1 loading was 2.7 mol%, a self-separation of the catalyst
from the reaction mixture occurred after 48 hat 80 °C and product 5 was obtained
8

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with 92% yield (Fig. 2).
At the beginning of the reaction, the reactants are well dissolved, except for
complex 1 (Fig. 2a), which only dissolves partially at a loading of 2.7 mol% (the
loading of catalyst 1 and ethanol are based on the oleic functional group in
oleifera). At the end of the reaction, complex 1 precipitates from the reaction
mixture plausibly due to a polarity change of the reaction system (Fig. 2c). In
homogeneous catalysis (Fig. 2b), all catalytic sites are accessible and fully utilized,
and the chemoselectivity of the reaction can be easily modified. On the other hand,
in heterogeneous catalysis, the catalyst can be easily recovered and recycled.
Hence, our method offers the advantages of both homogeneous and heterogeneous
catalysis.
Since the catalyst can be easily recovered from the transformation of
commercial oleifera into biodiesel, we then evaluated its recyclability. As shown
in Table 2, the catalyst can be easily recycled five times without a significant loss
1
in catalytic efficiency. The recovered catalyst was analysed by H NMR
spectroscopy,which showed that the recovered catalyst was the same as the fresh
catalyst, implying that the catalyst is suitable for recycling, chemically stable, as
its structure is not destroyed by the reaction media and substrates, and thermally
stable as it withstands a temperature of 80 °C for five cycles.
A possible explanation for the reaction-induced self-separation ability of
catalyst 1 in the transformation of commercial oleifera to biodiesel is the special
structure of the catalyst. As shown in Fig. 3, the C F chains of the anion are
8
17
9

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clustered together to produce hydrophobic domains, which generate three separate
areas in the crystal structure of the catalyst. One area is the hydrophobic and
lipophobic C F group, which acts as a shield and enables the Lewis acid centre to
8
17
be water-tolerant and air-stable. A second area is the lipophilic cyclopentadienyl
region, which dissociates from the organic substrate and acts as an organic
directing group; the third area is the zirconium cationic ion Lewis acid centre,
which acts as a hydrophilic area and allows the coordination of water molecules,
and is where the reaction takes place. Upon completion of the reaction, the
lipophilic ester dissociates from the cyclopentadienyl group and the hydrophilic
side products (alcohol or water) dissociate from the coordinated water molecules,
pushing the equilibrium towards the formation of the product. The produced ester
and water molecules leave the catalyst due to the hydrophobic nature of its
interior. Furthermore, the self-separation ability of the desired ester and side
products (alcohol or water) facilitates the precipitation of the catalyst in the final
step.
Based on the current findings and literature reports, we proposed the catalytic
process as follows (Fig. 4): first, the organic substrates approach the C8F17
hydrophobic and lipophobic area, then dissociate from the organic
cyclopentadienyl ligand, and consequently coordinate with the zirconium Lewis
acid centre through exchange of the three coordinated water molecules. This leads
to the activation of the carbonyl group of the starting material, which can then
easily react with the alcohol to form a new C–O bond. After rearrangement of the
1
0

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carbonyl group, the desired product is formed and the catalyst liberated. However,
the real catalytic mechanism is still under investigation in our lab.
4
. Conclusion
In summary, we have developed a highly efficient simultaneous transformation of
the two components (FFAs and TGs) of vegetable oils (VOs) to biodiesel by
combining the direct esterification of free fatty acids (FFAs) and transesterification of
triglycerides (TGs) using the air-stable and water-tolerant zirconocene
perfluorooctanesulfonate Lewis acid. The catalyst, when used for the transformation
of commercial oleifera to biodiesel, behaves as a reaction-induced self-separation
catalyst that facilitates the separation, recovery, and recyclability of the catalyst.
Acknowledgment
This work was financially supported by Science and Technology Planning Project
of Guangdong Province (2014A010105051, 2016B020211002), National Natural
Science Foundation of China (21403038, 21676076), the Scientific Research
Starting Foundation for Doctors of Guangdong University of Petrochemical
Technology (2015rc04) and the Funding for the scientific research project of the
Hunan Provincical Education Department (17C0150). The authors thank Profs.
Akihiro Orita and Junzo Otera (Okayama University of Science) and Profs. Xinhua
Xu and Shuang-Feng Yin (Hunan University) and Prof. Lifen Peng (Hunan
University of Science and Technology) for their helpful discussion.
11

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3

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Figure captions
Fig. 1 Synthesis of zirconocene bis(perfluorooctanesulfonate)
Fig. 2 Photographs of the transformation of commercial oleifera to biodiesel
catalysed by zirconocene bis(perfluorooctanesulfonate).(a, left) At the beginning
of the reaction; (b, middle) homogeneous mixture during the reaction at 80 °C; (c,
right) upon completion, the reaction becomes heterogeneous: the upper layer
contains the product and unconsumed reactants, while the precipitate at the bottom
is catalyst 1 in its hydrated form.
Fig. 3 X-ray structure of catalyst 1.
Fig. 4 A possible catalytic mechanism for the current catalytic reaction.
Scheme 1. Zirconocene(1)-catalysed (trans)esterification(the loadings of catalyst 1
and ratio of 2:4 or 3:4 are based on the oleic functional group R and R’ of 4).
Scheme 2. Application of zirconocene bis(perfluorooctanesulfonate) in biodiesel
synthesis from oleifera (the loadings of catalyst 1 are based on the oleic functional
group in oleifera)
1
4

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a
Table 1 Transformation of various types of VOs to biodiesel catalysed by 1
Molar ratio Conv. of
:3 2 (%)
Conv. of
3 (%)
Yield of
5 (%)
94
Ent. VOs
2
1
2
3
4
5
6
oleifera
1:12.5
1:5
97
98
99
99
92
98
93
canola
palm
66
44
31
-
78
1:2.08
1:1.04
1:0.78
1:0.55
77
soybean
cottonseed
sunflower
82
66
9
84
a
Catalyst 1 (0.01 mmol); loadings of 1 and 4 as well as isolated yield of 5 were
calculated based on R groups in 2 and 3.
Table 2 Recyclability of catalyst 1 in the transformation of commercial oleifera to
a
biodiesel
Cycle
1
2
3
4
5
Yield(%)
92
95
92
94
92
92
90
93
90
94
Recov. Cat.(%)
a
Conditions: EtOH (40 mL), Catalyst 1(1 mol%), commercial olefiera (9.03 g), reflux,
4
8 h.
1
5

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Fig. 1
Fig. 2
OH
1
(2.7 mol%)
o
+
EtOH
RCOOEt + HO
, 92%
OH
commercial oleifera
80 C, 48 hrs
4
6
9.03 g
5
31 mmol ,oleic
40 mL
functional group
R = CH (CH ) CH=CH(CH ) CH -
3 2 7 2 6 2
Fig. 3
1
6

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Fig. 4
Scheme 1.
Scheme 2.
1
7

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Fig. 2 Photographs of the transformation of commercial oleifera to biodiesel
catalysed by zirconocene bis(perfluorooctanesulfonate).(a, left) At the beginning
of the reaction; (b, middle) homogeneous mixture during the reaction at 80 °C; (c,
right) upon completion, the reaction becomes heterogeneous: the upper layer
contains the product and unconsumed reactants, while the precipitate at the bottom
is catalyst 1 in its hydrated form.
Fig. 2
OH
1
(2.7 mol%)
o
+
EtOH
RCOOEt + HO
, 92%
R = CH (CH ) CH=CH(CH ) CH -
OH
commercial oleifera
80 C, 48 hrs
4
6
9.03 g
5
31 mmol ,oleic
40 mL
functional group
3
2 7
2 6
2

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Highlights:
V Air-stable and water-tolerant Zirconocene Lewis acid catalysts were synthesized.
V The catalyst is highly efficient for transformation of vegetable oils into biodiesel.
V Zirconocene behaves as a reaction-induced self-separation catalyst in the
reaction.