Transesterification to biodiesel with superhydrophobic porous solid base catalysts

Article abstract of DOI:10.1002/cssc.201100166

Super combo: Superhydrophobic and porous solid bases are synthesized by co-polymerization of divinylbenzene (DVB) and 1-vinylimidazolate (VI). The materials show higher activities towards the transesterification of tripalmitin with methanol than conventional bases and VI mono-/polymer. Their performance is attributed to the synergy of superhydrophobicity combined with active VI sites. These catalysts also remain very active upon recycling.

Full text of DOI:10.1002/cssc.201100166

DOI: 10.1002/cssc.201100166
Transesterification to Biodiesel with Superhydrophobic Porous Solid Base
Catalysts
Fujian Liu,[b] Wei Li,[c] Qi Sun,[a] Longfeng Zhu,[b] Xiangju Meng,[a] Yi-Hang Guo,*[c] and Feng-Shou Xiao*[a]
The production of biodiesel from renewable resources has at-
tracted much attention due to the increasing demand for
energy.[1] One of the most important routes for producing bio-
diesel involves the transesterification of triglycerides with
short-chain alcohols, normally catalyzed by acids and bases.[2–3]
Liquid acids (e.g., H2SO4) exhibit good catalytic activities but
their environmentally unfriendly properties, such as strong cor-
rosion and difficult recyclability, severely hinder their applica-
tion.[4] Solid acids (e.g., sulfated zirconia or supported hetero-
polyacids) offer advantages such as catalyst recycling and a
high stability towards CO2 (a catalyst poison) in air, but apply-
ing these catalysts in industrial processes is difficult owing to
the relatively low activities, low degree of exposure for active
sites, and leaching of active sites.[4a,5] Base catalysts are very
active compared to acids.[3,6] Homogeneous base catalysts,
such as NaOH and KOH, are preferred in industry because of
their widespread availability and low cost, but environmental
concerns and the lack of an option to regenerate the catalysts
strongly limit their applications.[3c,6a] Solid bases have activities
similar to homogeneous base catalysts, but their sensitive
active sites, which can be poisoned by molecules such as H2O,
CO2, and fatty acids (FFAs) in crude vegetable oils, still pose a
great challenge.[6a] In these solid catalysts many factors, such
as textural parameters, wettability, and adsorption features,
strongly influence catalytic properties.[2b,3a,7] Superhydrophobic-
ity is a particularly useful catalyst property when seeking to
significantly enhance activity in transesterifications of triglycer-
ides with methanol.[7,8]
transesterification of tripalmitin as well as virgin plant oil with
methanol show that the PDVB-VI catalyst exhibits unprece-
dentedly high activities, excellent stability, and an extraordinary
ability for regeneration compared to conventional bases and
vinylimidazolate mono- and polymers.
Samples of porous PDVB-VI-n (where n is the molar ratio of
1-vinylimidazolate with divinylbenzene) were obtained by a hy-
drothermal route, at 1008C for 24 h in the presence of ethyl
acetate, methyl acetate, or THF as solvent. Nitrogen adsorp-
tion–desorption isotherms of these samples showed type IV
curves with hysteresis loops in the relative pressure region
0.7–0.9 (Figure S1, Table 1), indicating mesoporosity. Corre-
spondingly, the pore sizes were found to be in the range 20–
40 nm. Interestingly, the samples have high Brunauer–Emmett–
Teller (BET) surface areas (513–680 m2 gÀ1) and large pore vol-
umes (0.45–1.24 cm3 gÀ1), as shown in Table 1. X-ray photoelec-
tron spectroscopy (XPS) (Figure S2) and IR (Figure S3) spectra
indicated the presence of 1-vinylimidazolate groups in the
sample. Transmission electron microscopy (TEM) images clearly
confirmed the presence of hierarchical mesopores in the size
range 20–40 nm (Figure S4), and scanning electron microscopy
(SEM) images showed the rough surface of the samples
(Figure S5).
When a water droplet was dropped to contact the surface
of PDVB-VI-0.5, the contact angle was 1528 (Figure 1a), indicat-
ing that the PDVB-VI-0.5 sample is superhydrophobic. This fea-
ture is possibly related to its surface roughness, unique meso-
structure, and organic framework; similar phenomena have
been reported previously.[9] In contrast, when a droplet of
salad oil was brought into contact with the same sample, the
angle was 15.78 (Figure 1b), indicating its oleophilic features.
When a methanol droplet was dropped onto the surface the
angle was 08 (Figure 1c), indicating superwettability for metha-
nol. After making contact with methanol the volume of the
sample expanded significantly, indicating that PDVB-VI-0.5 has
excellent swelling properties (Figure S6). As a result of these
features PDVB-VI-0.5 has a very large adsorption capacity for
methanol (10.3 ggÀ1) and plant oil (ESG oil 10.8 ggÀ1, Fig-
ure S7). This implies excellent miscibility of methanol with
plant oil in PDVB-VI samples, which is a helpful feature for the
conversion of plant oil to biodiesel.[10] When a glycerol droplet
was to make contact with the surface of the PDVB-VI-0.5
sample the angle was 1358 (Figure 1d), indicating that PDVB-
VI-0.5 has good anti-wetting properties for glycerol. This fea-
ture is helpful for repelling glycerol from PDVB-VI in the trans-
esterification of tripalmitin with methanol. After quaternary
ammonization of PDVB-VI-0.5 by treatment with CH3I (sample
PDVB-VI-0.5-Q), the sample exhibited a lower contact angle for
water (1268, Figure S8), less swelling (Figure S6), a lower ad-
sorption capacity for organic compounds, and higher adsorp-
Many superhydrophobic materials, with varying composi-
tions, have been fabricated rationally, but the search for solid
base catalysts with superhydrophobic features has so far been
unsuccessful. Herein, we demonstrate a superhydrophobic and
porous solid base, PDVB-VI, obtained by co-polymerization of
divinylbenzene and 1-vinylimidazolate. Catalytic tests in the
[a] Dr. Q. Sun, Dr. X. Meng, Prof. F.-S. Xiao
Key Lab of Applied Chemistry of Zhejiang Province
Department of Chemistry, Zhejiang University (XiXi Campus)
Hangzhou 310007 (PR China)
Fax: (+86)431-85168590
E-mail: fsxiao@mail.jlu.edu.cn
[b] Dr. F. Liu, Dr. L. Zhu
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and
College of Chemistry, Jilin University
Changchun 130012 (PR China)
[c] Dr. W. Li, Prof. Y.-H. Guo
School of Chemistry, Northeast Normal University
Changchun 130024 (PR China)
Fax: (+86)431-85098705
E-mail: guoyh@nenu.edu.cn
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100166.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1059

a very low adsorption capacity
at different humidity levels. The
PDVB-VI-0.5-Q sample shows a
higher water adsorption capaci-
ty, which is the result of a
change in hydrophobicity. In
contrast, the methanol adsorp-
Table 1. Textural parameters of various solid base catalysts, and catalytic data on their use in the transesterifi-
cation of tripalmitin with methanol and other alcohols.[a]
Entry Sample
Alkaline
content
[mmolgÀ1
Textural parameters
Alcohol
Alkyl palmitate
yield [%]
t=1 h t=3 h
[b]
[b]
[c]
SBET
[m2 gÀ1
Vp
[cm3 gÀ1
Dp
]
]
]
[nm]
1
2
3
4
5
6
7
8
PDVB-VI-0.2
PDVB-VI-0.33
PDVB-VI-0.5
M-PDVB-VI-0.5[e]
T-PDVB-VI-0.12[f]
PDVB-VI-0.5-Q
PDVB-VI-0.5[g]
PDVB-VI-0.5[h]
PDVB-VI-0.5
PDVB-VI-0.5
Amberlite 400
Hydrotalcite [Mg6Al2(OH)16CO3] 30.2
PVI
0.953[d]
2.063[d]
2.822[d]
2.822[d]
0.953[d]
2.063[d]
2.822[d]
2.822[d]
2.822[d]
2.822[d]
5.18
670
594
513
486
576
680
521
510
513
513
0.28
103
–
–
–
–
–
–
–
1.13
1.241
0.815
0.915
0.454
1.0
0.813
0.811
0.815
0.815
–
0.85
–
–
–
–
24.2
38.1
19.2
26.3
methanol
methanol
methanol
methanol
99.5
99.3
99.7
99.8
–
63.1
99.2
98.9
30.1
99.7
99.9
99.6
99.2
–
tion
isotherms
(Figure 2b)
reveal a much larger adsorption
capacity for PDVB-VI-0.5 com-
pared to hydrotalcite, in good
agreement with the superwetta-
bility of PDVB-VI-0.5 for metha-
nol. Furthermore, tests for CO2
adsorption in air show that
6.83 methanol
29.1
methanol
methanol
methanol
ethanol[i]
89.3
99.7
99.3
64.3
55.1
79.8
80.7
85.5
97.3
82.4
99.9
98.1
85.2
0
24.6
26.8
19.2
19.2
–
33
–
–
–
–
–
–
9
10
11
12
13
14
15
16
17
18
19
1-butanol[j] 24.6
methanol
methanol
methanol
methanol
methanol
methanol
methanol
methanol
methanol
57.5
61.6
59.3
67.2
55.8
99.6
94.6
38.1
0
PDVB-VI-n
samples
cannot
10.625
adsorb CO2 while PDVB-VI-0.5-Q
can adsorb small amounts of
CO2 (Table S1).
VI
10.625
17.86
17.86
17.86
25.0
CaO
KOH
KOH[k]
NaOH
DVB
–
–
–
Catalytic data on the transes-
terification of tripalmitin over
various catalysts is presented in
0
–
[a] For each run, 0.05 g catalyst, 0.84 g tripalmitin, and 3.76 mL alcohol were used. [b] BET surface area and
pore volume estimated from N2 adsorption results. [c] Pore size distribution estimated from BJH model. [d] Cal-
culated from the imidazole content of the starting composition. [e] Mesoporous PDVB-PVI synthesized from
using methyl acetate as solvent. [f] Mesoporous PDVB-PVI synthesized from using THF as solvent. [g] Catalyst
recycled three times. [h] Catalyst recycled five times. [i] T=788C. [j] T=115 8C. [k] Catalyst exposed to air for 7
days.
Table 1.
Conventional
solid
bases, such as Amberlite 400,
hydrotalcite, CaO, NaOH, and
KOH, are active for this reaction.
Among these catalysts Amber-
lite 400, hydrotalcite, CaO, and
NaOH show relatively low yields
(79.8–85.2% in 3 h, entries 11, 12, 15, and 18), while KOH is
very active (99.6% in 1 h, entry 16). However, after exposure to
air the activity of KOH is reduced (94.6% in 1 h, entry 17) due
to its sensitivity to CO2. 1-vinylimidazolate monomer (VI) shows
a high yield (97.3% in 3 h, entry 14), indicating that imidazole
sites are very active towards the transesterification. After poly-
merization, the PVI sample shows a lower conversion than VI,
which is due to the better dispersion of catalytic basic sites in
the (liquid) monomer compared to the polymer. Interestingly,
the solid base catalysts co-polymerized from DVB and VI
(PDVB-VI-n, n=0.2–0.5) exhibited much higher activities than
VI monomer, considering that the number of catalytically
active sites in the PDVB-VI-n samples is much lower than that
of the VI monomer. For example, after 1 h, PDVB-VI-0.5 gave a
yield of 99.3%, while VI monomer gave a yield of 67.2%. How-
ever, after quaternary ammonization, PDVB-VI-0.5-Q sample
shows lower activity, giving the yield of 63.1%. Compared with
VI monomer, it is clear that PDVB-VI-n samples have superhy-
drophobic property. Apparently, the superhydrophobicity plays
a critical role in enhancing catalytic activity in the transesterifi-
cation. The significant reduction in yield over PDVB-VI-0.5-Q
compared to PDVB-VI-0.5 can be reasonably attributed to the
change in the hydrophobicity, because both samples have a
similar composition and similar textural parameters. Possibly,
superhydrophobicity and good oleophilicity of PDVB-VI-n are
very favorable for the miscibility of methanol with tripalmi-
tin.[10] Once a hydrophilic glycerol by-product is formed the
molecules quickly leave the hydrophobic catalysts, promoting
Figure 1. Contact angles of (A) a water droplet, (B) a salad oil droplet, (C) a
methanol droplet, and (D) a glycerol droplet on PDVB-PVI-0.5, at a contact
time of ca. 0.5 s.
tion capacities for glycerol and water (Figure S7). Interestingly,
when a droplet of ethanol or 1-butanol was dropped onto the
surface of PDVB-VI-0.5 the angles were ca. 108 or 158, respec-
tively (Figure S9), indicating that an increased chain length of
the alcohols reduces the wettability compared with methanol.
This might be due to the larger molecular diameters of ethanol
and 1-butanol, leading to slower diffusion rates into the nano-
porous PDVB-VI-0.5 sample.
Figure 2a shows water adsorption isotherms for the PDVB-
VI-0.5, PDVB-VI-0.5-Q, and hydrotalcite samples. Hydrotalcite
has a high adsorption capacity for water, while PDVB-VI-0.5 has
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lysts are very sensitive to CO2.[6a]
Moreover, the superhydropho-
bicity of the PDVB-VI samples
strongly reduces the effect of
water on recyclability.[10b]
The catalyst activity was low-
ered when methanol was re-
placed with ethanol or 1-buta-
nol, giving yields of 64.3% ethyl
palmitate and 55.1% butyl pal-
mitate after 3 h (Table 1, entries
9 and 10). Besides intrinsic cata-
lytic behavior, this phenomenon
might be attributable to the
lower wettability of the PDVB-
VI-0.5 sample for ethanol and 1-
butanol than for methanol, as
shown in Figure S9.
Figure 2. (A) Adsorption curves for water at different humidities, and (B) adsorption curves for methanol at differ-
ent relative pressures, over: (a) PDVB-VI-0.5, (b) PDVB-VI-0.5-Q, and (c) hydrotalcite.
Eruca Sativa Gars (ESG) is a
low-cost plant oil ($0.15 kg in
China)[12] that has been used as
the transesterification reaction,[11] in good agreement with the
high contact angle of glycerol on PDVB-VI-0.5 (Figure 1d).
Figure 3 shows the dependencies of the catalytic activities
of PDVB-VI-0.5, KOH, and CaO in the transesterification of tri-
palmitin with methanol versus time, giving more insight into
catalytic kinetics. For example, at t=10 min PDVB-VI-0.5 and
a feedstock for biodiesel production. ESG oil normally contains
both free fatty acids (3.6%) and water (0.08%) as impurities,[12]
which easily poison conventional basic catalysts.[11a] However,
after transesterification of ESG oil with methanol for 2 h over
PDVB-VI-0.5 the major products (Table S2) were fatty acid
methyl esters (yield 88.1%), including methyl palmitate (C16:0
,
,
6.9%), methyl stearate (C18:0, 4.1%), methyl oleate (C18:1
25.8%), methyl linoleate (C18:2, 49.2%), and methyl g-linolenate
(C18:3, 2.1%). These catalytic data indicate that the PDVB-VI-0.5
catalyst is very active towards transesterification of plant oil
with methanol; even more active than a solid acid-catalyzed
procedure reported earlier (SO42À/ZrO2-SiO2-Et hybrid catalyst,
yield 63.1% in 24 h).[13] Importantly, the imidazole sites in the
PDVB-VI-0.5 catalyst are not sensitive to free fatty acids and
water, and saponification does not appear to take place. Free
fatty acids usually consume conventional base catalysts, such
as NaOH, due to saponification. Additionally, the transesterifi-
cation of ESG oil with methanol was carried out under very
mild conditions (658C and 1 atm), which is potentially impor-
tant for the large-scale industrial production of biodiesel. In
contrast, conventional base catalysts are generally performed
at higher temperatures (130–1508C) and pressures
(1.7 MPa).[10a]
Figure 3. Catalytic activity versus time in the transesterification of tripalmitin
with methanol over (a) PDVB-VI-0.5, (b) KOH, and (c) CaO. Conditions: 0.05 g
catalyst, 0.84 g tripalmitin, 3.76 mL methanol, 658C.
In summary, superhydrophobic and porous solid base cata-
lysts are successfully synthesized by co-polymerization of di-
vinylbenzene and 1-vinylimidazolate for the first time. Catalytic
tests in the methanol transesterification of both tripalmitin and
ESG oil show that the solid base catalysts are much more
active and stable than conventional base catalysts such as
basic resin, hydrotalcite, CaO, and NaOH, which is of great im-
portance for production of clean biodiesel from low-cost re-
newable feedstocks.
KOH exhibited yields of 53.8 and 46.5%, respectively. Since the
same mass (0.05 g) of KOH has more alkaline sites (0.89 mmol)
than PDVB-VI-0.5 (0.14 mmol), these results suggest that PDVB-
VI-0.5 is a more active catalyst than KOH.
In addition, the PDVB-VI samples have superior recycling
properties. For example, after recycling 3 and 5 times in air
under atmospheric pressure, PDVB-VI-0.5 still gave yields of
99.2 and 98.9%, respectively (Table 1, Figure S10). This is due
to the stable imidazole transesterification sites in the solid cat-
alysts. Particularly, imidazole sites cannot be poisoned by CO2
in air, which is confirmed by the adsorption of CO2 on PDVB-VI
samples (Table S1). In contrast, conventional solid base cata-
ChemSusChem 2011, 4, 1059 – 1062
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Acknowledgements
This work is supported by the State Basic Research Project of
China (2009CB623507) and National Natural Science Foundation
of China (20973079).
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Keywords: catalysis
materials · polymers · renewable resources
· hydrophobic effect · mesoporous
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Received: March 28, 2011
Published online on May 30, 2011
1062
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