Silica-supported guanidine catalyst for continuous flow biodiesel production

Article abstract of DOI:10.1039/c1gc15727b

The new silylant agent 3-(N,N′-dicyclohexylguanidine)- propyltrimethoxysilane (GPMS) was prepared straightforwardly in high yield by the simple reaction between 3-aminopropyltrimethoxysilane and dicyclohexylcarbodiimide. This new organosilane containing t

Full text of DOI:10.1039/c1gc15727b

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Volume 13 | Number 11 | November 2011 | Pages 2977–3316
ISSN 1463-9262
COVER ARTICLE
Dupont et al.
Silica-supported guanidine catalyst
for continuous flow biodiesel
production
1463-9262(2011)13:11;1-8

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PAPER
Silica-supported guanidine catalyst for continuous flow biodiesel production
Joa˜o M. Balbino,a Eliana W. de Menezes,a Edilson V. Benvenutti,b Renato Catalun˜a,a Gu¨nter Ebelinga and
Jairton Dupont*a
Received 20th June 2011, Accepted 20th July 2011
DOI: 10.1039/c1gc15727b
The new silylant agent 3-(N,N¢-dicyclohexylguanidine)-propyltrimethoxysilane (GPMS) was
prepared straightforwardly in high yield by the simple reaction between 3-aminopropyltri-
methoxysilane and dicyclohexylcarbodiimide. This new organosilane containing the basic
dicyclohexylguanidine group covalently attached on silica gel is an efficient heterogeneous catalyst
for the production of biodiesel. Conversions up to 98% were obtained for soybean oil methanolysis
at 353 K for 3 h. This catalytic system was transposed to a semi-pilot scale in a continuous flow
reactor and gave 44% conversion. The calculated activation energy of the methanolysis in this
continuous system indicated that the reaction is probably under mass transfer limitations.
solid catalysts for the methanolysis of vegetable oils. Indeed,
Introduction
guanidines are strong bases and their basicities are in the
Biodiesel has become an attractive substitute to mineral diesel
range of common inorganic bases such as alkali hydroxides and
due to its environmental benefits, in particular for those derived
carbonates.25 However, there is no direct and simple method for
from vegetable oils.1–4 Biodiesel is mainly synthesized by the
the efficient immobilization of these organic basic catalysts in
transesterification process, catalyzed by acids or bases or under
solid supports.
alcoholic supercritical conditions with high efficiency, yielding
We report a simple and efficient approach for the preparation
40–90% more energy than that used in its production.5 Indus-
of a new organosilane containing a dicyclohexylguanidine group
trially, biodiesel is usually produced by homogeneous catalysis
immobilized on silica gel and its use as heterogeneous catalyst for
in the presence of basic species, since these are much more
the production of biodiesel in both batch and semi-continuous
catalytically active and less corrosive than the acid. However,
reactors.
technological problems such as corrosion, emulsification and
separation of glycerine and water and energy consumption are
usually associated with these methodologies.7
Results and discussion
One of the possible solutions is the immobilization of the cat-
alyst on a solid support that will render the processes recyclable
with the minimum use of solvents (including water) and energy.6
It is also apparent that industrial processes resulting from im-
provement of the catalyst efficiency will be intimately associated
with reaction engineering such as continuous flow processes.
Indeed, there some interesting examples of optimized conditions
for production of biodiesel using heterogeneous catalysts,6–19 or
immobilized acids and bases in ionic liquids.12,20–22 However,
it appears that supported guanidines either in chloromethyl
polystyrene23 or on mesoporous silica24 are the most promising
The new silylant agent 3-(N,N¢-dicyclohexylguanidine)-
propyltrimethoxysilane (GPMS) was prepared straight-
forward in 94% yield by the reaction between 3-
aminopropyltrimethoxysilane and dicyclohexylcarbodiimide
(Scheme 1).
Scheme 1 Preparation of GPMS.
aLaboratory of Molecular Catalysis, Institute of Chemistry, UFRGS,
Avenida Bento Gonc¸alves, 9500, Porto Alegre, 91501-970, RS Brazil.
E-mail: Jairton.dupont@ufrgs.br; Fax: +55 51 33087204; Tel: +55 51
33086321
The supported catalyst (SiGPMS) was obtained by grafting
via simple reflux of a toluene suspension GPMS with silica gel
for 48 h. The GPMS silane amount immobilized on the catalyst
surface is 16 w/w% (0.4 mmol of GPMS silane immobilized
per gram of catalyst) could be estimated from CHN elemental
analysis.
bLaborato´rio de Qu´ımica do Estado So´lido e de Superf´ıcies, Institute of
Chemistry, UFRGS, Avenida Bento Gonc¸alves, 9500, Porto Alegre,
91501-970, RS Brazil. E-mail: benvenutti@iq.ufrgs.br; Fax: +55 51
33087204; Tel: +55 51 33087176
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Green Chem., 2011, 13, 3111–3116 | 3111
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Table 1 Textural properties of silica support and the catalyst
The covalent attachment of the guanidine fragment was
corroborated by solid-state 13C and 29Si NMR measurements
(see Fig. 1 and 2). The 29Si NMR spectrum (Fig. 1) shows
peaks at -110 and -101 ppm are characteristic of the siloxane
(Q4) and silanol (Q3) groups, respectively.24,26 The set of signals
between -75 and -55 ppm are formed by silicon atoms bonded
to carbon atoms of organic groups, thus forming predominantly
C–Si (OSi)3 and C–Si(OH)(OSi)2 species, characterized in that
order as signals T3 and T2.27,28 Therefore the silanization of the
support by the guanidine occurs via at least two covalent bonds.
This was further corroborated by 13C NMR experiment (Fig. 2).
It is clearly observed peaks at 25.6 and 33.4 ppm, characteristic
of the –CH2 and –CH groups of the ring, respectively. It has also
peaks at 45.0 ppm related to the –OCH3 and 52.0 ppm associated
with the –CH2–N groups.28
BET surface
area/m2 g-1
BJH pore
BJH pore
size/A
volume/cm3 g-1
˚
Sample
SiO2
SiGPMS
340
300
0.93
0.71
97.5
83.7
in which the silane is immobilized in a highly dispersed form.
Therefore, considering the flexibility of the GPMS side chain,
the thickness of the GPMS group coverage on the silica surface
can be estimated as around 1 nm.
The TGA and DTG curves (see Fig. 3) of catalyst show two
main weight losses: the first one occurred up to 150 ◦C due
to the water desorption and the second between 240 ◦C that is
probably related to the removal of the GPMS surface derivatives.
The high thermal stability of the SiGPMS, is evidence that the
GPMS silane was immobilized on catalyst surfaces in a covalent
way. The observed GPMS weight loss by thermal analysis of
14% corroborates the value determined by combustion analysis
(16%).
Fig. 1 29Si CP/MAS NMR spectrum for the SiGPMS catalyst.
Fig. 3 TGA and DTG curves of SiGPMS catalyst.
Transmission infrared spectra of the SiGPMS material and
pure silica matrix are presented in Fig. 4. Pure silica shows broad
bands above 3500 cm-1 due to SiO–H stretching and typical silica
overtone bands at 1630, 1870 and 1985 cm-1. The main bands
observed for SiGPMS catalyst are: band at 1450 cm-1, due to the
CH2 bending of hydrocarbon chain; band at 1340 cm-1, assigned
to –N–CH deformation of substituted amine; bands at 2850 and
2930 cm-1, attributed to the symmetric and asymmetric CH2
stretching, respectively; and a strong band with maximum near
to 1630 cm-1 that was assigned to –C N– stretching of the
guanidine group. The broad band at 3435 cm-1 is due to the
NH2 antisymmetric streching.29,30 The presence of the guanidine
infrared bands in the spectrum, after thermal treatment up to
150 ◦C in vacuum, is a further evidence that the GPMS silane is
immobilized in the silica surface in covalent way, according with
the TGA results discussed above.
Fig. 2 13C CP/MAS NMR spectrum for the SiGPMS catalyst.
The GPMS silane immobilization by grafting reaction pro-
duces only a slight decreasing in the specific surface and also
in the pore volume of porous material (see Table 1), typical of
a surface coverage process. The near 1.4 nm decrease on the
pore diameter observed for silica support (SiGPMS), after the
grafting reaction, is compatible with surface monolayer coverage
A basic strength (H_) of 15.0 £ H_ £ 17 for the SiGPMS
catalyst was determined using different acid–base indicators.31,32
The titration using 2,4-dinitroaniline indicated that SiGPMS a
basicity of 0.81 0.02 mmol g-1cat, quite similar that found for
the KF loaded on zeolite NaY activated at 673 K.32
3112 | Green Chem., 2011, 13, 3111–3116
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Table 3 Methanolysis of soybean by SiGPMS in semi-pilot scale flow
reactor at various temperaturesa
Entry
T/K
MeOH/oil
T
D
M
B
C
1
2
3
4
5
6
7
8
357
367
373
378
389
394
399
357
367
373
378
389
399
409
20
20
20
20
20
20
20
30
30
30
30
30
30
30
88.7
82.7
79.8
76.7
68.2
61.7
56.4
81.3
78.6
76.7
75.3
70.4
67.0
63.2
1.4
1.3
1.9
1.1
1.7
2.8
1.0
1.0
0.3
0.6
1.9
1.7
1.2
1.5
0.3
2.0
1.4
1.9
1.5
1.2
0.7
0.4
0.4
0.7
0.4
1.8
1.1
0.3
9.8
11.3
17.3
20.2
23.8
31.8
38.3
43.6
18.7
21.4
23.3
24.7
29.6
33.0
36.8
14.0
16.9
20.3
28.6
34.3
41.9
17.3
21.0
22.0
22.3
26.1
30.7
35.0
9
10
11
12
13
14
a M – monoglycerides. D – diglycerides. T – triglycerides. B – biodiesel
(methyl esters). C – total conversion.
Fig. 4 FTIR spectra of (a) pure silica; (b) SiGPMS catalyst, obtained
The yields and selectivities on biodiesel obtained using the
new SiGPMS catalyst are the same or even slightly superior
to those obtained by guanidine-based catalyst either under
homogeneous or heterogeneous conditions.23,24,33 Moreover, the
SiGPMS catalyst could be re-used at least 7 times without lost
in its catalytic activity or selectivity and most important it can
be used in a continuous flow processes (Fig. 5). In preliminary
studies a reactor with a total volume of 10.0 mL was loaded
with 6.0 g of the catalyst. The pressure was maintained at 50 kgf
cm-2 and fractions were collected throughout the day, for each
experiment, after stabilization of reaction temperature and the
results are summarized in Table 3.
after heating up to 150 ◦C in vacuum. The bar value is 1.0.
The SiGPMS catalyst was applied in the transesterification
reaction of soybean oil with methanol for 3 h and 50 kgf cm-2
nitrogen pressure under various reaction conditions (Table 2).
It is clear that the total conversion of soybean oil occurs at
a minimum temperature of 353.15 K and reduces down to
62.4% at 333.15 K (entries 5 and 6, Table 2). The reaction is
highly influenced by the amount of methanol used from 7.8%
at molar ratio of 6 to almost quantitative conversion at molar
ratio methanol: soybean oil of 15–20 (entries 8–11, Table 2).
The best ratio of SiGMPS : soybean oil at 353.15 K, 50 kgf
cm-2 argon pressure, molar ratio methanol : soybean of 20 for
3 h to attain conversions above 99% is a minimum of 0.50 g
SiGPMS/4.5 g of oil (1.70% m/m GPMS). Finally, for the batch
reaction conditions the use of argon pressure is not necessary
since almost the same conversion and selectivities are observed
at atmospheric pressure (compare entries 13 and 16, Table 2).
The highest conversion was obtained on the reaction per-
formed at 399 K temperature and low flow of the reactants
methanol (1.47 g h-1) and soybean oil (2.0 g h-1).
The activation energy for the methanolysis in this system was
estimated by the modified Arrhenius equation ln(%conversion) =
-Ea/RT-1 + ln A (Fig. 6). The activation energy (37 kJ mol-1)
for the reaction performed with a 20 : 1 (methanol : oil) molar
ratio is in the range of those obtained for the transesterification
of vegetable oils with basic catalysts (33–84 kJ mol-1) and it
is limited by mass transfer and catalyst activity.3,34,35 However,
at 30 : 1 (MeOH : oil) ratio the calculated activation energy
(16 kJ mol-1) is typical of heterogeneous catalytic processes that
is under mass transfer limitations.35
Table 2 Methanolysis of soybean by SiGPMS (cat.) for 3 h, 50 kgf cm-2
nitrogen pressure and under various reaction conditionsa
Entry T/K MeOH/oil Cat. (%)
T
D
M
B
C
1
2
3
4
5
6
7
8
473
423
393
373
353
333
323
353
353
353
353
353
353
353
353
353
30
30
30
30
30
30
30
20
15
10
6
20
20
20
20
20
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
2.5
1.7
1.4
0.8
1.7
0.1
0.2
0.7
1.4
0.1
1.3 0.9 97.7 99.9
1.7 1.2 96.9 99.8
2.3 3.2 93.8 99.3
2.5 1.2 94.9 98.6
0.9 3.5 95.5 99.9
Conclusions
In summary we have reported a straightforward method for
the preparation a guanidine containing a silylant agent by
the simple condensation of 3-aminopropyltrimethoxysilane and
dicyclohexylcarbodiimide. This reagent allows the covalent
attachment of the basic dicyclohexylguanidine catalyst on silica
gel that was demonstrated to be one of the most efficient and
recyclable catalytic systems for the methanolysis of soybean
under relatively mild reaction conditions. Moreover this solid
catalyst allows the development of a continuous flow processes
for the production of biodiesel. Calculations of activation energy
for the reaction in this system indicated mechanism limited by
mass transfer and catalyst activity.
37.6 5.6 1.2 55.6 62.4
42.3 6.8 3.2 47.7 57.7
1.2
56.3 3.5 2.7 37.5 43.7
70.2 2.7 1.9 25.2 29.8
92.2 1.4 1.1 5.3
1.9
0.8
5.7
4.9 1.5 92.4 98.8
9
10
11
12
13
14
15
16b
7.8
1.3 1.2 95.6 98.1
1.9 1.6 95.7 99.2
1.3 1.4 91.6 94.3
23.7 3.4 1.2 71.7 76.3
0.9 1.9 2.3 94.9 99.1
a M – monoglycerides. D – diglycerides. T – triglycerides. B – biodiesel
(methyl esters). C – total conversion. b At atmospheric pressure.
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Fig. 5 Reaction system in semi-pilot scale flow for synthesis of biodiesel. (→) Direction of flow; (1) Serpentine and (2) differential isothermal reactor
with a capacity of 10 mL. FIC: flow indicator and controller; TIC: temperature indicator and controller. TI: temperature indicator; PI: pressure
indicator.
The N2 isotherms of the catalyst and pure silica, previously
Experimental
degassed 160 ◦C under vacuum, for 3 h, were obtained using
Materials
a Tristar 3020 Micromeritics equipment. The specific surface
areas were determined by the BET multipoint method and the
average pore size was obtained by BJH method.
The 3-aminopropyltrimethoxysilane (APMS) (Alfa Aesar) and
N,N¢-dicyclohexylcarbodiimide (DCC) (Acros) were used with-
out purification. Toluene (Vetec) was used after distillation.
For infrared analysis, self-supporting disks of the samples
were heated in up to 150 ◦C for 2 h, under vacuum. The
Silica gel (Merck) with a particle size of 0.2–0.5 mm was used as
equipment used was a Shimadzu FTIR, Prestigie 21. The spectra
support for the catalyst. Commercial soybean oil and methanol
were obtained at room temperature with a resolution of 4 cm-1,
(Vetec) were used in the transesterification reactions without
with 150 cumulative scans.
further purifications.
Nuclear magnetic resonance (1H and 13C NMR) spectra of
GPMS silane were obtained in Varian spectrometer 300 MHz
using deuterated chloroform (CDCl3) as solvent and tetram-
ethylsilane (TMS) as internal standard.
Elemental analysis of the organic groups immobilized on
the support surface was carried on a CHN Perkin Elmer
M CHNS/O Analyzer, model 2400.
The basic strength (H_) and basicity, or basic sites density
(mmol g-1cat), were determined according to the methods based
on the colour change of Hammet indicators and titration with
benzoic acid, respectively.31,32 The H_ indicators used in this
study (their pKa values are given in parentheses) were: indigo
carmine (12.2), 2,4-dinitroaniline (15.0), 4-chloro-2-nitroaniline
(17.2) and 4-nitroaniline (18.4).
Solid-state 13C and 29Si NMR measurements for the SiGPMS
catalyst were performed on a Bruker 300/P spectrometer using
cross polarization and Magic angle spinning (CP/MAS). The
13C experiments were obtained using acquisition time of 0.18 ms
while 29Si measurements were made utilizing acquisition time of
0.082 ms. Frequencies of 75.5 and 59.6 MHz for carbon and
silicon, respectively, were used.
The thermogravimetric analysis of the samples was performed
under nitrogen flow on a TA Instrument system model TGA
Q5000, with a heating rate of 20 ◦C min-1, from room tempera-
ture up to 700 ◦C.
Synthesis of 3-(N,N¢-dicyclohexylguanidine)propyltrimethoxy
silane
Dicyclohexylcarbodiimide (DCC) (10.70 g. 51.9 mmol) and 3-
aminopropyltrimethoxysilane (APMS) (9.30 g. 51.9 mmol) were
mixed in dry toluene (50 mL) and kept at 100 ◦C under stirring
for 24 h. The toluene was removed under reduced pressure
yielding an amber oil (18.8 g. 94% yield) sufficiently pure for
further work. 1H NMR (300 MHz. CDCl3) d: 7.23 (singlet. 2H.
NH); 3.50 (singlet. 9H. OCH3); 3.20–3.09 (multiplet. 2H. NCH
ring); 2.93 (triplet. 2H. J = 7.2 Hz. NCH2); 1.92–1.76 (multiplet.
3114 | Green Chem., 2011, 13, 3111–3116
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12% using methyl esters standards from the correction of the
peak areas using oleic acid as internal standard. Thus, the
conversion was obtained by the equation: conversion yield =
[(AM + AD + AB)/ATotal ] ¥ 100, where AM, AD and AB are the
areas corresponding to the monoglycerides, diglycerides and
methyl esters of fatty acids, respectively, and ATotal is the sum
total of the areas, which include products obtained as well as
non-transesterified triglycerides.
In the continuous flow reactor the methanol and soybean oil
feed was controlled with flow controllers, previously calibrated
in order to adjust the flow of reagents to the reactor internal
volume (10 mL). Once the reactants, methanol and vegetable
oil, are immiscible at room temperature and pressure, they were
heated through a small diameter coil (3.2 mm i.d.) at reaction
temperature, with simultaneous pressure adjustment with the
aid of a back pressure valve, allowing homogenization of the
sample. After this step, the reaction mixture was sent through
a differential isothermal reactor containing the catalyst. The
mixture emerged from the reactor was treated and analyzed as
same as outlined above for the batch experiments.
Knowing that residence time and percentage of conversion
are small in the differential reactor, so far from equilibrium
conditions, it is possible to determine the kinetic constant
as a function of temperature. Therefore, at different reaction
temperatures tested, it was observed that an increase in the
percentage of conversion also corresponds to an increase in the
value of the rate constant, keeping valid the approximation: %
of conversion ~ k. Thus, the activation energy for the trans-
esterification of soybean oil with methanol in continuous flow
system was estimated from the modified Arrhenius equation:
ln(%conversion) = -Ea/RT-1 + ln A.
Fig. 6 Curves according to the Arrhenius equation for the transester-
ification of soybean oil in the continuous flow system with molar ratio
methanol : soybean oil equal to (a) 20 : 1 and (b) 30 : 1.
Acknowledgements
4H. CH2); 1.74–1.44 (multiplet. 8H. CH2); 1.36–1.00 (multiplet.
10H. CH2); 0.65–0.55 (multiplet. 2H. SiCH2). 13C NMR (75
MHz. CDCl3) d: 153.5 (NNC N); 77.2 (OCH3); 55.0 and 49.8
(NCH2 and NCH ring); 33.8 (NCH2CH2); 25.0. 24.7 and 24.3
(CH2 ring); 5.2 (SiCH2).
Thanks are due to CNPq (575469/2008-0), INCTCatal.,
FAPERGS and CAPES for partial financial support.
Notes and references
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