Transesterification of vegetable oil with subcritical methanol using heterogeneous transition metal oxide catalysts

Article abstract of DOI:10.1039/c2ra21666c

Transesterification of vegetable oil with subcritical methanol over newly-developed heterogeneous Mn catalysts for the production of biodiesel fuel is reported. The dense CH₃OH can improve the catalytic performance as a result of the effective removal of the reaction products with methylation of the intermediate carboxylates on the catalyst surface.

Full text of DOI:10.1039/c2ra21666c

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COMMUNICATION
Transesterification of vegetable oil with subcritical methanol using
heterogeneous transition metal oxide catalysts{
Tomoharu Oku,* Masanori Nonoguchi, Toshimitsu Moriguchi, Hiroko Izumi, Atsushi Tachibana and
Takeo Akatsuka
Received 1st August 2012, Accepted 2nd August 2012
DOI: 10.1039/c2ra21666c
can promote the transesterification of triglycerides using a contin-
uous-flow fixed-bed reactor under the subcritical condition of
methanol, 200 uC (Tc = 239.5 uC), in a heterogeneous manner
without serious catalyst leaching. These results prompted us to
investigate the transesterification of vegetable oil with supercritical
(SCF) or subcritical methanol (SubCF) as a reaction medium.
Previously, Ikariya and Oku5 reported that the use of supercritical or
Transesterification of vegetable oil with subcritical methanol over
newly-developed heterogeneous Mn catalysts for the production
of biodiesel fuel is reported. The dense CH3OH can improve the
catalytic performance as a result of the effective removal of the
reaction products with methylation of the intermediate carbox-
ylates on the catalyst surface.
subcritical methanol as
a reaction medium and a reactant
The utilization of biomass resources as a new energy source or
chemical raw materials attracts much attention because they are not
only alternatives for exhaustible resources but also support global
warming policies. One of the most widespread practical applications
of biomass is the production of biodiesel fuel (BDF), namely fatty
acid methyl esters (FAME) from vegetable oils.1 BDF is con-
ventionally produced by transesterification of vegetable oil with
methanol using a homogenous alkaline catalyst, NaOH or NaOCH3
etc., as shown in Scheme 1. Although the homogeneous catalyst
systems provide reasonable productivity, the complicated catalyst
removal process causes an increase in wastes and a serious decrease
in the product yields and quality. In order to simplify the process,
various heterogeneous metal-oxide based basic catalysts have been
developed, however, most of them have serious drawbacks such as
catalyst leaching and poor durability.2 In fact, during reactions large
amounts of Mg and Al ions were leached from MgO/Al2O3
hydrotalcite catalysts. On the other hand, since ordinary solid acid
catalysts1 required relatively harsh reaction conditions for a smooth
reaction, the undesired decomposition of glycerin and methanol led
to a serious decrease in the productivity and selectivity. Thus, the
solid acid or base catalysts suffered from a dilemma of activity/
selectivity and durability for BDF synthesis. Moreover, Yoo et al.3
has recently reported that unary metal oxide catalysts (SrO, CaO,
ZnO, TiO2 and ZrO2) can promote the transesterification of rape
seed oil with supercritical or subcritical methanol using a batch-type
reactor; but even the optimal ZnO catalyst can dissolve slightly in
FAME during a reaction. Recently, the Hillion group at the Institut
Franc¸ais du Petrole4 has reported that a mixed oxide of Zn and Al
significantly improved the catalytic performance of solid catalysts
and promoted methylation of organic compounds thanks to its
unique properties such as high diffusivity and reasonable high
solubility of organic compounds. Although the catalytic reactions
over the solid catalysts6 in SCF with unique physicochemical
properties are a promising approach to attain highly efficient
catalytic reactions, it still remains relatively unexplored. In particular,
the synthesis of BDF over solid catalysts in near-supercritical
methanol conditions is not well understood. Here we report on a
manganese heterogeneous catalyst with high activity and long
catalyst lifetime for the transesterification of vegetable oil using
subcritical methanol (subCH3OH) as a reactant and a compatible
solvent. This unique solvent can dissolve both non-polar fatty acid
esters (vegetable oil and FAME) and polar glycerin (by-product).
Firstly, we investigated the phase behavior of vegetable oil,
FAME, and CH3OH under various conditions by using a sapphire
Strategic Technology Research Center, Nippon Shokubai Co. Ltd., 5-8,
Nishi Otabi-cho, Suita, Japan. E-mail: tomoharu_oku@shokubai.co.jp;
Fax: +81 6 6317 2992; Tel: +81 6 6317 2252
{ Electronic Supplementary Information (ESI) available: experimental
details about the preparation of typical catalysts and the reaction methods
using the continuous-flow fixed-bed reactor under subcritical conditions. See
DOI: 10.1039/c2ra21666c
Scheme 1
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Table 1 Reaction of triolein with subCH3OH in a batch reactora
Yield, mol (%)
Entry Catalyst
FAME Glycerin Leaching conc.,b ppm
1
2
3
4
5
6
7
8
9
ZnTiO3
NiTiO3
CoTiO3
FeTiO3
MnTiO3
83
63
77
94
87
36
63
41
94
49
61
53
79
63
56
15
24
63
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
Mn–Al mixed oxide 94
TS-1
76
79
69
80
43
65
77
TiO2 on ZrO2
Ti0.5Zr0.5O2
HTiNbO5
CoV2O7
CeVO4
Hydrotalcitec
Ti: ,1 Zr: n.d.
Ti: ,1 Zr: n.d.
n.d.
n.d.
Ce: n.d. V: 250
Mg: 17800 Al: 6900
10
11
12
13
a
Fig. 1 Contact-time course of the product distributions over the MnTiO3
Conditions: under autogenous pressure at 200 uC for 24 h, 60 g of
triolein, 20 g of CH3OH and 2.5 g of catalyst powder in a 200 ml
stainless-steel vessel.
catalyst.
b
c
n.d. means ‘not detected’.
Exceptional
temperature is 150 uC.
cell. Visual inspection revealed that under subcritical CH3OH
conditions, 200 uC at 5 MPa, all organic compounds were dissolved,
resulting in a single liquid phase. Table 1 shows some results of
screening tests of various binary transition metal oxide catalysts7 for
the reaction of triolein with subCH3OH using a batch reactor at
200 uC and autogenous pressure. All catalysts tested exhibited good
activity toward the transesterification without any serious leaching of
the active species, except for the catalysts listed in entries 9, 10 and 13
in Table 1. Contrary to these metal oxide catalysts, the hydrotalcite
catalyst suffered from a serious leaching of Mg and Al metal ions at
milder conditions, 150 uC (entry 14, Table 1). Among them, the most
efficient catalysts were the crystalline ilmenites, FeTiO3, MnTiO3,
and the mixed oxide consisting of Mn and Al calcined at high
temperature. The MnTiO3 catalyst can promote transesterification of
variously vegetable oil, palm, rape-seed, canola and coconut oil as
listed in Table 2. Also of note, short-chain triglycerides were
convertible with a reasonably high rate to produce glycerin in good
yield. The discrepancies between the yields of FAME and glycerin in
Table 1 and 2 can be explained by the formation of mono- and
diglycerides as intermediates in such incomplete reactions.
Fig. 2 Infrared spectra of the MnTiO3 catalyst used.
When the flow ratio of palm oil/methanol = 1/1 by weight and the
liquid hourly space velocity, LHSV (ml-liquid ml-cat21 h21) is 1 h21
,
which was calculated based on the liquid flow rate of reactants per
volume of catalyst bed, the catalyst maintained its catalytic activity
for more than 1000 h.
Based on the results obtained using the batch-type reactor, we
then examined the reaction of refined palm oil with subCH3OH over
the MnTiO3 catalyst using the continuous-flow fixed-bed reactor
under the conditions of 200 uC at 5 MPa at various contact-times.
Fig. 1 shows the contact-time course dependence of the product
distributions of transesterification, indicating that the transesterifica-
tion reaction proceeded consecutively and reversibly (Scheme 1).
In order to gain further insights into the mechanism of the
catalyst, the adsorbates to the surface of the MnTiO3 catalyst were
evaluated by IR spectroscopy of the used catalyst which was washed
with butanol at room temperature and dried under vacuum. As
shown in Fig. 2, characteristic signals due to the metal carboxylate
adsorbates were observed and then the signals disappeared after
reuse in a batch reactor under otherwise identical conditions.
Manganese diacetate as a model compound of the possible catalyst
intermediates is known to readily react with alcohol to give
manganese oxide which could further react with acetic acid to
regenerate the diacetate complex.8 In fact, methylacetate was
generated in a separate experiment of a reaction of manganese
diacetate with excess methanol under atmospheric pressure and
reflux for 8 h.
Table 2 Reaction of various source oils with subCH3OH over MnTiO3
catalyst in a batch reactora
Yield, mol (%)
Vegetable oil
Conversion, mol (%)
FAME
Glycerin
Palm
99
100
100
96
81
81
76
94
87
46
52
53
69
49
Rape seed
Canola
Coconut
Triolein
a
These experimental results strongly suggested the key role of the
metal oxide catalyst during the catalysis. Firstly, the reaction of
triglyceride with the metal hydroxide on the surface would afford
diglyceride and the surface carboxylate species. The resulting
100
Conditions: under autogenous pressure at 200 uC for 24 h and the
reaction was performed on an identical scale as in Table 1.
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Scheme 2
carboxylate species bonded to the surface and released the desired
esters, FAME, by a similar reaction to the model reaction,
regenerating the metal hydroxide species (Scheme 2). Subcritical
CH3OH promotes the transesterification with the help of the
hydroxide units on the surface by effective removal of the surface
carboxylate to simultaneously generate FAME. In a similar way, the
diglyceride efficiently gives glycerin and two molecules of FAME
through the monoglyceride as shown in Scheme 1, eqn (2) and (3).
Notably, these reactions might proceed reversibly, and consequently
a small amount of the mono-glycerides intermediates remains in the
product FAME due to thermodynamic reasons.
Fig. 3 Reaction of palm oil and subCH3OH over the MnTiO3 and the
Mn–Al mixed oxide catalysts. Conditions: 15 ml of catalyst, 200 uC, 5.0
MPa, the flow ratio of palm oil to CH3OH was 1 : 1 by weight, and LHSV
of the mixed solution of reactants was 1.0 h21
.
the FAME mixtures with fresh methanol under the same conditions,
except the 2.7 h21 of LHSV, gave a 99.8 mol% yield of FAME and a
98.4 mol% yield of glycerin.
Conclusions
In conclusion, we have reported a practical synthetic process for
BDF and glycerin from the transesterification of vegetable oils using
subCH3OH as a reactant and a reaction medium over the newly-
developed heterogeneous Mn catalysts which have high activity and
a long lifetime. The quality of the co-product glycerin from this
process is high enough to use directly as a raw material for various
useful chemicals without further purification. The use of the
successive continuous-flow fixed-bed reaction system causes quanti-
tative conversion of the transesterification without any catalyst
removal processes. The present process is notable since the operation
is simple, waste-free, and produces high purity glycerin. The use of
subCH3OH contributes not only to improving the limitations of
contact and diffusion concerns by forming a single phase system, but
also to promoting the effective desorption of the carboxylate species
bonded to the catalyst surface. Further investigations of the detailed
mechanism of the transesterification of the triglycerides over the Mn-
Al mixed oxide catalyst as well as development into a commercial
scale process are now in progress.
The residual monoglycerides were further converted into glycerin
and FAME by using the bench-scale consecutive two-stage fixed-bed
reactor,4 almost achieving a quantitative reaction over the MnTiO3
catalyst. The transesterification of refined palm oil with subCH3OH
in the first reactor, under the conditions described in Table 3,
proceeded smoothly to give a mixture of FAME (91.1 mol%) and
glycerin (82.5 mol%) with 14.2 mol% of glycerides as the
intermediates at 96.7% conversion. After the excess methanol was
removed by the flash evaporator, the ester phase containing FAME
and glycerides obtained from the simple separation of immiscible
glycerin (see ESI{) reacted with fresh subCH3OH in the second
reactor to afford the desired FAME and glycerin with 99.4 mol%
and 98.4 mol% as the overall yield, respectively (Table 3).
Practical advantages in the Mn–Al mixed oxide catalyst system
were demonstrated by its high catalytic performance in terms of the
activity and durability, compared to the MnTiO3 catalyst as shown
in Fig. 3. Notably, no serious catalyst deactivation was observed
even after 5000 h operation under 200 uC at 5 MPa and the 1.0 h21
of LHSV at a flow ratio = 1/1 by weight of the refined palm oil and
methanol. At the first reactor, the yields of FAME, glycerin and
glycerides intermediates were 96.4 mol%, 96.6 mol% and 2.5 mol%,
respectively, at 99.1% conversion, and a similar second reaction of
Acknowledgements
We acknowledge the valuable discussions with Prof. Dr Takao
Ikariya, Tokyo Institute of Technology, Tokyo, and part of this
work was supported by the RITE Joint Program to Promote
Technological Development.
Table 3 Reaction of refined palm oil with subCH3OH using
continuous-flow consecutive two-stage fixed-bed reactor
a
Reactor
1st
Conditions
Result, mol (%)
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