Biodiesel production from soybean oil transesterification in subcritical
methanol with K
3
PO
4
as a catalyst
Jian-Zhong Yin
a,
⇑
, Zhen Ma
a
, Zi-Yang Shang
a
, Da-Peng Hu
a
, Zhi-Long Xiu
b
a
State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116024, PR China
b
School of Biological Science and Technology, Dalian University of Technology, Dalian 116024, PR China
article info
Article history:
Received 30 August 2010
Received in revised form 24 November 2011
Accepted 24 November 2011
Available online 13 December 2011
Keywords:
Biodiesel
Subcritical methanol
Catalytic transesterification
K
3
PO
4
Process enhancement
abstract
Biodiesel production from soybean oil transesterification under the presence of a heterogeneous catalyst
tri-potassium phosphate in subcritical methanol has been studied. The results showed that tri-potassium
phosphate exhibited high catalytic activity for the transesterification of soybean oil. The effects of reac-
tion variables, including the reaction temperature, the molar ratio of methanol to oil, the catalyst amount,
and the water and oleic acid concentrations in the soybean oil, on the yield of fatty acid methyl ester
(FAME or biodiesel) were systematically investigated. Under the optimal conditions, e.g., the temperature
of 220 °C and methanol to oil molar ratio of 24:1, 95.6% of the FAME yield was obtained in 30 min with
only 1 wt% K
3
PO
4
.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Biodiesel, an alternative diesel fuel, is made from renewable
biological sources such as vegetable oils and animal fats. It is bio-
degradable and nontoxic, has low emission profiles and is therefore
environmentally benign [1–6]. The most common way to the pro-
duction of biodiesel is through transesterification of vegetable oils
or animal fats with methanol under the homogeneous catalysis of
alkali [1,4] or acids. Homogeneous alkali catalysts have the advan-
tage of high reaction rate at a low temperature. However, when the
raw materials (oils or fats) contain a high percentage of free fatty
acids (FFAs) or water which occurs usually in natural vegetable oils
and animal fats, the alkali catalyst will react with the free fatty
acids to form soaps [7], as a result, decrease the production rate
of biodiesel oil. On the other hand, homogeneous acid catalysts
are very suitable for the transesterification of oils with high FFA
and water contents although the reaction rate is much lower than
the homogeneous alkali catalysts [7–9].
For the above homogenous alkali- or acid-catalyzed biodiesel
production process, the separation of the catalyst from a mixture
of reactants and products is technically difficult. To resolve this
problem, a non-catalytic biodiesel production method with super-
critical methanol (scMeOH) has been developed [2,10]. However,
this method requires extreme temperature and pressure
conditions, e.g., 350 °C and 43 MPa, and induces breakdown of
unsaturated fatty acids and trans-isomerization, leading to adverse
effects on the fluidity of the fuel at low temperatures [11,12].
Therefore, many researchers attempted to add co-solvents
[13,14] or a very small amount of catalyst [15–17] in order to re-
duce the temperature and pressure of scMeOH process.
In our previous work [15], we used a small amount of potassium
hydroxide (KOH) catalyst to enhance the efficiency of scMeOH pro-
cess. The result showed that by adding only 0.1 wt% of KOH, the
reaction temperature of the scMeOH process was decreased from
350 °C to 160 °C and the molar ratio of methanol to oil was de-
creased from 42 to 24. However, KOH is still a homogenous base
catalyst in the scMeOH process and the recovery of the catalyst,
despite a very little amount, is still a problem.
In the present work, we used tri-potassium phosphate (K
3
PO
4
)
as the catalyst to enhance the reaction rate of transesterification
of soybean oil under subcritical methanol conditions. Since K
3
PO
4
is insoluble in methanol, such a process possesses the advantage
of easy recovery of catalyst from the reaction medium, and simul-
taneously can decrease significantly the temperature and pressure
of the process. The influence of various reaction parameters, such
as reaction temperature, catalyst amount, molar ratio of methanol
to soybean oil and reaction time on the yield of fatty acid methyl
ester (FAME) was discussed.
2. Experimental
2.1. Materials
Refined soybean oil from Shanghai Fulingmen Food Co., Ltd.,
was used as the oil source for the reactions. Methanol (analytical
0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2011.11.056
⇑
Corresponding author. Tel./fax: +86 411 84986274.
E-mail address: jzyin@dlut.edu.cn (J.-Z. Yin).
Fuel 93 (2012) 284–287
Contents lists available at SciVerse ScienceDirect
Fuel
journal homepage: www.elsevier.com/locate/fuel