The exceptional activity of a phosphazenium hydroxide catalyst incorporated onto silica in the transesterification of tributyrin with methanol

Article abstract of DOI:10.1039/b903969d

A phosphazenium hydroxide catalyst incorporated onto silica showed exceptional activity in the transesterification of tributyrin with methanol and could be used repeatedly without suffering any appreciable deactivation. The Royal Society of Chemistry 2009

Full text of DOI:10.1039/b903969d

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
The exceptional activity of a phosphazenium hydroxide catalyst
incorporated onto silica in the transesterification of tributyrin
with methanol
a
a
b
c
Mi-Young Kim, Gon Seo,* O Zoon Kwon and Duk Rye Chang
Received (in Cambridge, UK) 25th February 2009, Accepted 16th March 2009
First published as an Advance Article on the web 17th April 2009
DOI: 10.1039/b903969d
A phosphazenium hydroxide catalyst incorporated onto silica
showed exceptional activity in the transesterification of tributyrin
with methanol and could be used repeatedly without suffering
any appreciable deactivation.
The high electron density on the phosphazene moiety is
responsible for its high catalytic activity. Phosphazenes and
phosphazenium compounds incorporated onto divinylbenzene
cross-linked polystyrene show a high activity in various organic
11
reactions. These phosphazene catalysts can be repeatedly used
because of their high stability.
Biodiesel produced by the transesterification of triglyceride
with a lower alcohol is a carbon-neutral fuel without the
net emission of CO2 and also a sustainable transportation
In this Communication, we have incorporated phosphazenium
iodide onto silica by reacting silica with the adduct of
phosphazene and (3-iodopropyl)trimethoxysilane. The further
treatment of phosphazenium iodide with sodium hydroxide
produces an effective phosphazenium hydroxide catalyst
1
–4
fuel without using petroleum sources.
Homogeneous base
catalysts, such as potassium and sodium hydroxides, are mainly
employed in the transesterification of vegetable oil and animal
fat with methanol because of their high activity and con-
venience. However, the post-treatment of biodiesel products
to neutralize the remaining base and wash out salts are
relatively expensive, thereby lowering the feasibility of bio-
diesel. Although several heterogeneous catalysts, such as
zeolites, alkali earth oxides and hydrotalcites, have been applied
as base catalysts for transesterification, their low activities and
2
incorporated onto silica (PzOH/SiO ). This catalyst shows a
comparable activity with sodium hydroxide in terms of its
turnover frequency (TOF) in the transesterification of
tributyrin with methanol. Moreover, the catalyst retains its
high activity in the transesterification reaction, even after
repeated use, further enhancing its feasibility for application
to biodiesel production as a commercial catalyst.
2
–5
inconvenient pre-treatments limit their application. Lipase
incorporated onto mesoporous silica has also been extensively
studied because of its high activity, despite its relatively high
Scheme 1 shows the preparation process for the PzOH/SiO
2
catalyst. Firstly, phosphazene 1, with one n-butyl substituent,
was prepared from the reaction of hexamethylphosphoramide
(99%, Aldrich) and phosgene, followed subsequently by a
reaction with n-butylamine (B98.5%, Yakury) and potassium
t-butoxide (99%, TCI). The further reaction of 1 with
(3-iodopropyl)trimethoxysilane (B97%, Fluka) produced
phosphazenium iodide 2, with n-butyl and trimethoxysilyl
substituents. 2 was easily incorporated onto silica (Merck,
6
cost. Heterogeneous catalysts have many advantages: their
easy separation from biodiesel products simply by filtering,
their capability for repeated use and their high handling safety
without causing corrosion.
Phosphazenes are extremely strong bases that are effectively
used in several organic reactions as catalysts. Phosphazene
bases efficiently accelerate the aza-Henry reaction of ketimines
2
ꢀ1
Silica 60, 0.040–0.063 mm, SBET = 476 m g ) by refluxing in
toluene. Phosphazenium hydroxide incorporated onto silica
hydrated under ambient conditions, 3, was obtained by
treating 2 with 0.1 M NaOH dissolved in methanol (99.8%,
Aldrich; MeOH).
7
compared to trimethyl guanidine. Phosphazene bases also
show the best performance among various organic bases in
Suzuki–Miyaura coupling reactions for the synthesis of
8
dictyomedins. Moreover, the use of phosphazene catalysts
has enabled the water-free alcoholysis of vegetable fatty esters
9
A thermogravimetric (TG) curve of 3, recorded in an
ambient atmosphere, indicates the amount of phosphazenium
hydroxide incorporated onto the silica, because the oxidative
in water-free conditions.
We have already reported the preparation method for a
phosphazenium chloride catalyst incorporated onto silica and
1
0
its exceptional activity in the chlorination of organic acids.
a
School of Applied Chemical Engineering and The Research Institute
for Catalysis, Chonnam National University, 300 Yongbong-Dong,
Buk-Gu, Gwangju 500-757, Korea. E-mail: gseo@chonnam.ac.kr;
Fax: +82 62 530 1899; Tel: +82 62 530 1876
Samchully Pharmaceutical Co., Ltd, Sihwa Industrial
Complex 1Na-802, 1241-2, Jeongwang-Dong, Siheung-Si,
Gyeonggi-Do 429-450, Korea. E-mail: ozkwon@scp.co.kr;
Fax: +82 31 448 3001; Tel: +82 31 448 1403
b
c
Korea Institute of Industrial Technology, 1110-9 Oryong-Dong,
Buk-Gu, Gwangju 500-480, Korea. E-mail: drchang@kitech.re.kr;
Fax: +82 62 600 6179; Tel: +82 600 6130
2
Scheme 1 Preparation of the PzOH/SiO catalyst incorporated
onto silica.
3
110 | Chem. Commun., 2009, 3110–3112
This journal is ꢁc The Royal Society of Chemistry 2009

View Article Online
1
4,15
of CO
The band at 2362 cm remained after evacuation, even at
00 1C, demonstrating the very strong basicity of the PzOH
catalyst.
The transesterification of tributyrin (99%, Aldrich; TriB)
2
adsorbed onto the cationic species, respectively.
ꢀ1
2
with methanol (99.8%, Aldrich; MeOH) was carried out in an
autoclave.w The conversion denoted the percentage of TriB
consumed with respect to that charged as a reactant. The
selectivity for methyl butyrate was defined as the percentage of
methyl butyrate among the produced esters.
The conversion of TriB gradually increased with increasing
catalyst loading, as shown in Fig. 3. The proportional increase
in conversion with increasing catalyst loading indicated that
the transesterification over the PzOH/SiO2 catalyst was
controlled by the surface catalytic reaction at low catalyst
loadings. The amount of catalyst required for complete
transesterification also varied with the ratio of MeOH to TriB,
as expected. A high MeOH concentration in the reactant
induced a high TriB conversion.
13
31
Fig. 1
2
C and P MAS NMR spectra of the PzOH/SiO catalyst.
2
The activity of the PzOH/SiO catalyst was represented by
its TOF in order to compare its activity to that of sodium
hydroxide. Table 1 lists the conversion, TOF and selectivity
obtained in the transesterification reaction over the
removal of carbon, hydrogen, oxygen and phosphorus reduces
the weight of the PzOH/SiO . The weight loss at 200–600 1C of
obtained from the TG curve suggested that 3.0 wt% of the
phosphazenium hydroxide was incorporated, equivalent to
.10 mmol of phosphazenium hydroxide per gram of silica.
This value is in a good agreement with the amount of
2
3
2
PzOH/SiO and NaOH catalysts. The loading levels of these
catalysts were controlled to obtain similar levels of conversion
and selectivity. The TOF of TriB was calculated based on the
numbers of phosphazenium hydroxide and sodium hydroxide
molecules. The conversion and TOF decreased with decreasing
MeOH : TriB molar ratio over both catalysts. The strong
coordination of TriB with hydroxide ions resulted in low
conversions at low MeOH : TriB molar ratios. However, the
0
ꢀ1
phosphazenium hydroxide, 0.14 mmol g , determined from
the exchange capacity of hydroxide by a titration using 0.01 N
1
2
HCl and thymol blue indicator.
1
3
31
The C and P magic angle spinning (MAS) nuclear
magnetic resonance (NMR) spectra of the phosphazenium
hydroxide catalyst indicate that the phosphazene moiety is
perfectly preserved, even in its incorporated state, as shown in
2
TOF was 0.16, 0.036 and 0.018 over the PzOH/SiO catalyst,
compared to 0.23, 0.066 and 0.088 over the NaOH catalyst at
MeOH : TriB ratios of 30 : 1, 12 : 1 and 6 : 1, respectively.
The TOF over both catalysts decreased with decreasing
MeOH: TriB ratio. The TOF over the PzOH/SiO2 catalyst
was as low as one-fifth that over the NaOH catalyst under
severe conditions of MeOH : TriB = 6 : 1, but this value
indicated that the PzOH/SiO2 catalyst was very active and
comparable to the homogeneous sodium hydroxide catalyst.
1
3
Fig. 1.
The strong basicity of the PzOH/SiO2 catalyst induced
strong adsorption of CO onto it, as shown in Fig. 2. The
2
CO2 adsorbed at 50 1C showed three absorption bands at
ꢀ1
2
310, 2343 and 2362 cm , which were attributed to the
asymmetric vibration mode of weakly bound CO , the asymmetric
OCO stretching of gaseous CO and the asymmetric stretching
2
2
At MeOH: TriB = 12: 1, the activity of the PzOH/SiO
2
catalyst, as represented by its TOF, was about half that of the
sodium hydroxide catalyst, even though it was heterogeneous
and maintained the advantages of separation and repeated use.
Fig. 2 The IR spectra of CO
The catalyst was evacuated at 200 1C before exposure to CO
0 Torr and 50 1C. The IR spectra were recorded under evacuation
2
adsorbed onto the PzOH/SiO
2
catalyst.
2
at
3
Fig. 3 Transesterification of TriB with methanol over the PzOH/SiO
catalyst at 60 1C for 2 h with different levels of catalyst loading.
2
while increasing the temperature of the sample wafer.
This journal is ꢁc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 3110–3112 | 3111

View Article Online
2
Table 1 Transesterification of TriB with MeOH over the PzOH/SiO and NaOH catalystsw
ꢀ1
Catalyst
Loading/mg
25
MeOH : TriB ratio
Repeated use
Conversion (%)
TOF/s
Selectivity (%)
PzOH/SiO
2
30 : 1
12 : 1
6 : 1
30 : 1
12 : 1
6 : 1
—
—
—
—
—
—
—
1
81
72
36
83
71
32
99
100
94
98
87
0.160
0.036
0.018
0.230
0.066
0.088
—
—
—
—
—
68
72
50
59
70
35
97
99
92
94
85
0
0
0
0
1
1
00
00
NaOH
0.1
0
0
0
0
0
0
.3
.1
PzOH/SiO
2
1000
30 : 1
0
0
0
0
0
0
0
0
00
00
00
00
00
00
00
00
2
3
4
a
Reaction: TriB = 0.005 mol, reflux for 2 h.
The activity of the PzOH/SiO
2
catalyst in the transesterifi-
used. After the transesterification, the products were analyzed using a
gas chromatograph (Donam, DS6200) equipped with a HP-5 capillary
column and an FID detector. The response factors for TriB, dibutyrin,
monobutyrin and methyl butyrate were determined through the multi-
point calibration method using dibutylether (99.3%, Aldrich) as an
cation was maintained over its repeated use, as shown in
Table 1. The conversion also remained high with repeated
use, without the requirement for further treating with hydroxide
or rinsing with solvent. This result demonstrates the high
stability of the catalyst under these reaction conditions. The
selectivity for methyl butyrate was high over the catalyst,
suggesting that further purification of products to remove
partially converted TriBs is unnecessary.
2
internal standard. For the repeated use of the PzOH/SiO catalyst, the
vial containing the products and catalyst was centrifuged for 5 min.
After decanting the products, new reactants were charged into the vial,
and the transesterification was performed by following the same
procedure described above.
1
M. Marchetti, V. U. Miguel and A. F. Errazu, Renewable Sustainable
Energy Rev., 2007, 11, 1300.
2 Y. Xi and R. J. Davis, J. Catal., 2008, 254, 190.
2
The PzOH/SiO catalyst exhibited an exceptional activity in
the transesterification of TriB with methanol, even in its
incorporated state on the silica. Although the TOF of the
transesterification was affected by the MeOH : TriB molar
´
3
M. J. Ramos, A. Casas, L. Rodrı
Appl. Catal., A, 2008, 346, 79.
I. N. Martyanov and A. Sayari, Appl. Catal., A, 2008, 339, 45.
´
guez, R. Romero and A. Perez,
´
4
ratio, the activity of the PzOH/SiO
2
catalyst was about one-
5 G. J. Suppes, M. A. Dasari, E. J. Doskocil, P. J. Mankidy and
M. J. Goff, Appl. Catal., A, 2004, 257, 213.
fifth that of the homogeneous sodium hydroxide catalyst at the
lowest MeOH : TriB molar ratio. The PzOH/SiO2 catalyst
continued to preserve its advantages as a heterogeneous
catalyst in terms of its separation and the capability for its
repeated use. The removal of methanol and methyl butyrate
via distillation from the filtrates offers the promise of providing
water-free glycerin without emitting any waste water. The
6
7
M. Shakeri and K. Kawakami, Catal. Commun., 2008, 10, 165.
N. K. Pahadi, H. Ube and M. Terada, Tetrahedron Lett., 2007, 48,
8
700.
8 M. Ebisawa, M. Ueno, Y. Oshima and Y. Kondo, Tetrahedron
Lett., 2007, 48, 8918.
9
M. C. Alarco
A, 2008, 346, 52.
10 K.-S. Kim, J.-H. Kim and G. Seo, Chem. Commun., 2003, 372.
n, A. Corma, S. Iborra and J. P. Gomez, Appl. Catal.,
´ ´
1
1
1
1 R. M. Wehmeyer, Method for preparing and using supported
phosphazenium catalysts, Eur. Pat., 1 294 794 B1 (15 March
exceptional activity and high convenience of the PzOH/SiO
2
catalyst raises hope for the wider application of incorporated
phosphazenium hydroxide catalysts in the important field of
biodiesel production.
2
006).
2 The amount of phosphazenium hydroxide incorporated was
determined from the added amount of 0.01 N HCl at the point
of the color change from yellow to blue..
This work was supported by a Korea Research Foundation
Grant (MOEHRD) (KRF-2007-412-J02001).
13
31
3
C and P MAS NMR spectra of the phosphazenium hydroxide
catalyst were recorded on an FT-NMR spectrometer (Varian, Unity
Solid Inova VB 200 MHz system) at a spinning rate of 5 kHz.
13
31
C and P NMR spectra were referenced to hexamethylbenzene
and phosphoric acid, respectively. Spectral data for compound 3:
Notes and references
1
3
w Four small vials charged with the reactants (8.36 ml) and catalyst
were installed in the stirring shaft of an autoclave preheated at 60 1C.
The transesterification was started by stirring the vials at 200 rpm for
C NMR: d = 49.2 (C
(C and C ) and 11.9 (C
d
and C ), 30.8 (C
z
), 37.6 (N–(CH
3
)
2
c
), 20.6
31
b
y
a
and C ); P NMR: d = 43.4.
x
14 T. Montanari and G. Busca, Vib. Spectrosc., 2008, 46, 45.
15 J. C. Lavalley, Catal. Today, 1996, 27, 377.
2
h. Various molar ratios of MeOH : TriB (6 : 1, 12 : 1 and 30 : 1) were
3
112 | Chem. Commun., 2009, 3110–3112
This journal is ꢁc The Royal Society of Chemistry 2009