Shiff base complexes of zinc(II) as catalysts for biodiesel production

Article abstract of DOI:10.1016/j.molcata.2011.11.012

New zinc(II) compounds containing functionalized Schiff bases have been prepared and characterized. The complexes catalyze the transesterification of soybean oil in mild conditions, and their activity can be modulated by a fine selection of the anions and/or of the substituents on the ancillary bidentate ligand. The compounds are active in the esterification of fatty acids and could therefore also be used to produce biodiesel from waste oils.

Full text of DOI:10.1016/j.molcata.2011.11.012

Journal of Molecular Catalysis A: Chemical 353–354 (2012) 106–110
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Shiff base complexes of zinc(II) as catalysts for biodiesel production
Martino Di Serioa,b,∗, Giuseppina Carotenutoa, Maria Elena Cucciolitoa,b, Matteo Legaa,b
,
Francesco Ruffoa,b, Riccardo Tessera,b, Marco Trifuoggia
a Dipartimento di Chimica “Paolo Corradini”, Università di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, via Cintia, 80126 Napoli, Italy
b Consorzio Interuniversitario di Reattività Chimica e Catalisi, via Celso Ulpiani 27, 70126 Bari, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 July 2011
Received in revised form 9 November 2011
Accepted 10 November 2011
Available online 20 November 2011
New zinc(II) compounds containing functionalized Schiff bases have been prepared and characterized.
The complexes catalyze the transesterification of soybean oil in mild conditions, and their activity can be
modulated by a fine selection of the anions and/or of the substituents on the ancillary bidentate ligand.
The compounds are active in the esterification of fatty acids and could therefore also be used to produce
biodiesel from waste oils.
© 2011 Elsevier B.V. All rights reserved.
Dedicated to Prof. Elio Santacesaria on the
occasion of his 70th birthday.
Keywords:
Biodiesel
Lewis acid
Transesterification
Esterification
Schiff base complexes
1. Introduction
Biodiesel is produced today by the transesterification of triglyc-
erides of refined/edible type oils using methanol and an alkaline
Biodiesel (a mixture of Fatty Acid Methyl Esters, FAME) is the
second most common biofuel produced in the world and the most
common in Europe. The UE Directive 2009/28/EC stated that a
mandatory 10% minimum target has to be achieved by all Mem-
ber States for the share of biofuels in transport petrol and diesel
consumption by 2020.
Nowadays the majority of biodiesel is produced by transester-
ification with methanol of refined edible oils such as rapeseed,
sunflower, palm, soybean, etc. However, most of these raw mate-
rials do not fulfill the sustainability criteria indicated by the UE
Directive 2009/28/EC, and so they shall not be taken into account
for measuring compliance with the requirements of this Directive
concerning national targets. Instead, the waste vegetable or animal
oils widely fulfill the sustainability criteria and so are more conve-
nient from an ecological point of view. Moreover the use of waste
oil is also economically of great interest. Currently more than 85%
of the cost of biodiesel obtained from edible vegetable oils is the
cost of raw materials.
homogeneous catalyst (NaOH, NaOMe): the reaction is normally
performed at 60–80 ◦C. Glycerol and FAME are separated by set-
tling after catalyst neutralization. The crude glycerol and biodiesel
obtained are then purified.
However, homogeneous alkaline catalysts cannot be used
directly with waste oils due to the presence of large amounts of free
fatty acids (FFA) [1]; in fact, for the use of these catalysts the FFA
concentration should be less than 0.5% (w/w) [1]. Several methods
have been proposed to solve this problem [2]; one of these solu-
tions is to use either a homogeneous or heterogeneous acid catalyst
which can catalyze the esterification of FFA and transesterification
of glicerides [1–3].
The use of homogeneous Brønsted acids (mainly H2SO4) was
proposed, but because their transesterification activity is low,
high acid concentrations are necessary which lead to an increase
in the waste formation deriving from catalyst neutralization [1].
Several heterogeneous acids were also used (ion-exchange resin,
metal oxides, heteropolyacids, etc.) [1–3]. In general these cat-
alysts show good performances in esterification of FFA, but the
activity in glycerides transesterification is not satisfactory. More-
over a great number of these catalysts showed a rapid deactivation
due to leaching [4,5], fouling of catalytic surface by prod-
ucts/reagents [6] and/or changing of oxidation state of the active
species [6,7].
∗
Corresponding author at: Dipartimento di Chimica “Paolo Corradini”, Università
di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, via Cintia, 80126
Napoli, Italy. Tel.: +39 081674414.
E-mail address: diserio@unina.it (M. Di Serio).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.11.012

M. Di Serio et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 106–110
107
O2CR
O2CR
OH
RCO2
+ 3 MeOH
3 RCO2Me
+
HO
OH
Scheme 1. Transesterification of soybean oil with methanol.
A few years ago it was demonstrated that homogeneous Lewis
acid can be used for triglyceride transesterification with methanol
[8] and, in particular, that the homogeneous metal carboxylates can
be used in biodiesel production from oils with high FFA concentra-
tion [9,10].
However, three main drawbacks are linked with the latter
class of catalysts: (1) the best transesterification performances
was shown by Pb carboxylates salts; (2) the necessity to use high
reaction temperature and pressure (T > 190 ◦C, P > 20 bar); (3) the
homogeneity of the catalyst with the consequent problem of purifi-
cation of the products.
For industrial application, it is obviously necessary to select a
safer metal, e.g. zinc, which shows significant activity, likely to be
improved. For example, it was shown that the activity of metal
carboxylates catalysts increases with the carbon numbers of the
carboxylate anions [9,11].
In some cases, the use of ligands also increases the catalyst activ-
ity. For example, in the same reaction conditions, the FAME yield
was 52 or 78 using as catalyst zinc crotonate or zinc crotonate with
stoichiometric quantity of quinoline, respectively [11]. Moreover,
Schiff base–zinc complexes proved to be good catalysts for ester
methanolysis of (rac)-1-phenylethanol picolinic acid ester (PEP)
[12].
AMP (2-aminomethylpyridine) zinc complexes anchored on a
polymer showed a heterogeneous catalytic activity on the trans-
esterification reaction of various substrates by methanol at room
temperature [13]. For the latter complexes a strong effect of the
anions of the starting zinc salts was observed, too [13].
Clearly, the use of ligands and of different types of anions influ-
ences the acidity of the metal center, and, as a consequence, its
catalytic activity. However, the acidity should not be the highest
possible, as a matter of fact the interactions between the metal
atom and the incoming transesterification reagents must not be
too strong, because this would slow down the addition-release
mechanism of the reagents at the metal atom [2,14–16].
The present work fits into this context, and describes a library of
Shiff base complexes of zinc(II) with the general formula reported
in Fig. 1.
The ligands differ in the R function, which controls the stere-
oelectronic properties of the corresponding complexes, while the
additional hydroxyl group is potentially useful for anchoring the
catalysts on a solid matrix.
Moreover the complexes also differ for the anions X.
The complexes were tested in the transesterification of soybean
oil with methanol (Scheme 1), to study the influence on the catalytic
activity of the ligands and of the anions.
2. Experimental
2.1. Materials and general methods
All reagents were purchased from Aldrich and used without
further purification. THF was distilled from LiAlH4. The newly syn-
thesised complexes are reported in Table 1. Most of these were
synthesised following a general procedure (see Section 2.2.1), while
for some of them a specific procedure was adopted. Ligand 1 was
described elsewhere [17]. The compounds showed satisfactory zinc
elemental analysis, which combined to relative integrations in the
NMR spectra unequivocally assign the structure to the complexes.
NMR spectra were recorded at 200 MHz (Varian Model Gemini
spectrometer) and dmso-d6 was used as a solvent. The follow-
ing abbreviations were used for describing NMR multiplicities: s,
singlet; d, doublet; dd, double doublet; t, triplet; q, quartet; m,
multiplet; br, broad peak.
2.2. Synthetic procedures
2.2.1. General procedure for the synthesis of zinc catalysts
Pyridinecarboxaldehyde (2.00 mmol) and solid ZnX2 (X = Cl or
AcO, 2.00 mmol) were added to a suspension of p-aminophenol
(0.218 g, 2.00 mmol) in 5 ml of acetone (diethyl ether in the case
of 9). Immediate formation of a yellow–orange precipitate was
observed, which was collected by filtration, washed with acetone
and dried under vacuum (yield: >90%).
1H NMR [200 MHz, dmso-d6, (CHD2)2SO (␦ 2.55) as internal
standard]:
Zn(1)Cl2: 9.76 (s, 1H), 8.81 (d, 1H, 3
J
H–H = 5.0 Hz), 8.65 (s, 1H), 8.06
3
3
(m, 2H), 7.66 (dd, 1H, JH–H = 7.0 Hz), 7.26 (d, 2H, JH–H = 8.4 Hz),
6.75 (d, 2H).
Anal. Calcd Zn: 19.5%. Found: 18.6%.
Zn(2)Cl2: 9.82 (br, 1H), 8.80 (br, 1H), 7.95 (m, 2H), 7.45 (m, 3H),
6.87 (d, 2H, 3JH–H = 8.6 Hz), 3.34 (s, 3H).
Anal. Calcd Zn: 18.8%. Found: 17.6%.
Zn(3)Cl2: 9.74 (s, 1H), 8.50 (s, 1H), 8.05 (d, 1H, 3JH–H = 8.0 Hz), 7.81
(t, 1H, 3JH–H = 8.0 Hz), 7.67 (d, 1H), 7.27 (d, 2H, 3JH–H = 8.4 Hz), 6.78
(d, 2H).
Anal. Calcd Zn: 15.8%. Found: 15.5%.
Zn(4)Cl2: 9.62 (s, 1H), 8.49 (s, 1H), 7.82 (t, 1H, 3JH–H = 7.6 Hz), 7.68
(d, 1H), 7.27 (d, 2H, 3JH–H = 8.4 Hz), 6.91 (d, 1H), 6.80 (d, 2H), 3.91
(s, 3H).
Anal. Calcd Zn: 17.9%. Found: 16.9%.
Zn(5)Cl2: 9.61 (s, 1H), 8.65 (s, 1H), 8.11 (d, 2H, 3JH–H = 8.8 Hz), 7.96
(m, 3H), 7.30 (d, 1H, 3JH–H = 8.8 Hz), 7.05 (d, 2H), 6.81 (d, 2H), 3.81
(s, 3H).
One of the better catalysts was also tested with oil containing
high FFA concentration to check the possibility of using this cata-
lyst in the production of biodiesel from waste oils. In this case also
the catalyst was also tested in the esterification of free fatty acids
(Scheme 2).
3
Zn(6)Cl2: 9.65 (s, 1H), 8.66 (s, 1H), 8.18 (d, 2H, JH–H = 6.8 Hz),
8.10–7.90 (m, 3H), 7.56 (d, 2H), 7.32 (d, 2H, JH–H = 9.0 Hz), 6.82
3
(d, 2H).
Zn(7)Cl2: 9.68 (s, 1H), 8.99 (s, 1H), 8.74 (s, 1H), 8.60 (dd, 1H,
3JH–H = 8.0 Hz, 4JH–H = 1.8 Hz), 8.32 (dd, 1H, 3JH–H = 8.0 Hz), 8.25–8.0
(m, 3H), 7.83 (t, 1H), 7.37 (d, 2H, 3JH–H = 8.8 Hz), 6.84 (d, 1H).
Anal. Calcd Zn: 14.4%. Found: 14.4%.
N
OH
N
R
Zn
X
X
RCO2H
+
MeOH
X= Cl, AcO, CF3CO2, CF3SO3
RCO2Me
+
H2O
Fig. 1. General formula of synthetised Shiff base complexes of zinc (II).
Scheme 2. Esterification of free fatty acids.

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M. Di Serio et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 106–110
Table 1
Ligands and zinc catalysts.
coordination to the metal center, since the one pot procedure
described in Section 2.2.1 was not successful.
A solution of the zinc precursor ZnX2 (X = CF3CO2 or CF3SO3,
2.00 mmol) in 5 mL of diethyl ether was added to solid 1 (0.396 g,
2.00 mmol) suspended in 5 mL of diethyl ether. After 72 hours of
stirring the yellow precipitate was collected by filtration, washed
with diethyl ether and dried under vacuum.
Ligands
Catalysts
N
N
OH
OH
N
N
Zn(1)Cl2
1
2
1H NMR [200 MHz, dmso-d6, (CHD2)2SO (ı 2.55) as internal
standard]:
Zn(2)Cl2
Zn(3)Cl2
Zn(4)Cl2
3
Zn(1)(CF3CO2)2: 9.93 (s, 1H), 8.83 (d, 1H, JH–H = 4.4 Hz), 8.74 (s,
3
3
1H), 8.23 (t, 1H, JH–H = 7.2 Hz), 8.02 (d, 1H, JH–H = 7.4 Hz), 7.82
3
3
N
N
OH
(t, 1H, JH–H = 5.4 Hz), 7.19 (d, 2H, JH–H = 7.6 Hz), 6.74 (d, 2H,
Br
3JH–H = 8.6 Hz).
3
Anal. Calcd Zn: 13.4%. Found: 10.6%.
3
Zn(1)(CF3SO3)2: 9.93 (s, 1H), 8.82 (d, 1H, JH–H = 4.4 Hz), 8.74 (s,
N
OH
N
3
3
1H), 8.23 (t, 1H, JH–H = 7.2 Hz), 8.02 (d, 1H, JH–H = 7.4 Hz), 7.82
MeO
3
3
(t, 1H, JH–H = 5.4 Hz), 7.19 (d, 2H, JH–H = 7.6 Hz), 6.74 (d, 2H,
4
3JH–H = 8.6 Hz).
Anal. Calcd Zn: 11.6%. Found: 10.7%.
N
OH
N
Zn(5)Cl2
5
2.2.3. Preparation of Zn(8)Cl2
Complex Zn(8)Cl2 was obtained by reacting ZnCl2 with ligand 8,
which was prepared in situ by reacting 1 with triethoxysilylpropy-
lisocianate.
MeO
N
OH
N
A mixture of 1 (0.396 g, 2.0 mmol) and triethoxysilylpropyliso-
cianate (0.54 g, 2.2 mmol) was refluxed in dry THF. After 3 h solid
ZnCl2 (0.27 g, 2.0 mmol) was added and hexane was added to pre-
cipitate an orange solid, which was collected by filtration, washed
with hexane, and dried under vacuum (yield: 70%).
1H NMR [200 MHz, dmso-d6, (CHD2)2SO (␦ 2.55) as internal
standard]:
Zn(6)Cl2
Zn(7)Cl2
6
Cl
N
OH
N
O2N
7
Zn(8)Cl2: 8.77 (d, 1H, 3JH–H = 5.0 Hz), 8.66 (s, 1H), 8.05 (m, 2H),
7.77 (t, 1H, 3JH–H = 6.0 Hz), 7.61 (dd, 1H, 3JH–H = 8.0 Hz), 7.38 (d, 2H,
3JH–H = 8.6 Hz), 7.13 (d, 2H), 3.74 (q, 6H, 3JH–H = 6.9 Hz), 3.02 (q, 2H),
1.52 (m, 2H), 1.14 (t, 9H), 0.57 (m, 2H).
N
N
N
N
N
OC(O)NH(CH2)3Si(OEt)3
N
Zn(8)Cl2
8
9
1
1
1
Anal. Calcd Zn: 11.2%. Found: 9.6%.
N
N
N
N
Zn(9)Cl2
OH
2.2.4. Catalytic runs
The screening of the catalysts was performed using a series
of 5–6 small stainless steel vial reactors. Both the reagents (soy-
bean oil, FFA < 0.2%, w/w) and methanol and a defined amount of
the catalyst were introduced in each reactor. All the reactors were
then heated in a ventilated oven. The temperature of the oven was
initially fixed at 50 ◦C for 14 min and then increased at a rate of
20 ◦C/min until it reached the reaction temperature. At the end of
the reaction, the samples were quenched by putting the vials in a
cold bath. Experimental runs were also performed by adding oleic
acid to the reactants. Oleic acid was chosen as the test molecule for
simulating the behavior of FFA.
Blank runs (without catalysts) were also done taking into
account any contribution of non-catalytic mechanisms to FAME
yield and FFA conversion.
The FAME yields were determined using the H NMR technique
[18]. FFA conversion was determined by measuring the residual FFA
concentration by titration [19].
OH
OH
OH
Zn(1)(AcO)2
Zn(1)(CF3CO2)2
Zn(1)(CF3SO3)2
3
Zn(9)Cl2: 8.69 (d, 1H, JH–H = 3.5 Hz), 8.62 (s, 1H), 8.24 (t, 1H,
3JH–H = 7.6 Hz), 8.06 (d, 1H), 7.79 (dd, 1H), 4.96 (br, 1H), 3.70 (br,
4H).
Anal. Calcd Zn: 22.8%. Found: 20.1%.
Zn(1)(AcO)2: 9.90 (br, 1H), 8.80 (s, 1H), 8.70 (d, 1H, 3JH–H = 3.8 Hz),
3. Results and discussion
3
8.10 (m, 2H), 7.64 (dd, 1H, JH–H = 7.0 Hz), 7.43 (d, 2H,
3JH–H = 8.0 Hz), 6.81 (d, 2H), 1.76 (s, 6H).
Anal. Calcd Zn: 17.1%. Found: 15.4%.
3.1. Synthesis of complexes
The ligands and the zinc catalysts are reported in Table 1. The
synthesis of the complexes did not require the previous isolation of
the ligands, but could be carried out in one simple step, by reacting
in situ the amine with the appropriate pyridinecarboxaldehyde in
2.2.2. Preparation of Zn(1)(CF3CO2)2 and Zn(1)(CF3SO3)2
In the case of trifluoroacetate or trifluoromethanesulfonate
derivatives it was necessary to isolate the ligand 1 prior to its

M. Di Serio et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 106–110
109
acetone
-H2O
N
OH
N
+ H2N
OH
+
ZnX2
R
N
O
Zn
R
X
X
Scheme 3. Synthesis of the catalysts.
Table 2
the presence of a stoichiometric amount of zinc chloride or zinc
acetate (Scheme 3).
Only in some specific cases was it necessary to isolate the biden-
tate ligands prior to their coordination, as described in detail in
Section 2.
Influenceof ligands on Zn(N–Nꢀ)Cl2 catalytic activity. Reaction conditions:T = 150 ◦C;
soybean oil = 2 g; methanol = 0.88 g; Cat. = 0.30 mmol; React. time = 60 min.
Run
Catalyst
FAME yield (%)
<2%
B.1
–
B.2
B.3
B.4
B.5
B.6
B.7
B.8
B.9
1
<2%
29
57
35
9
48
76
39
84
67
59
Most of the complexes were sparingly soluble in common
organic solvents, and their NMR characterization was performed
in dmso-d6. The presence of the CH N bond was clearly shown by
the presence of the expected singlet at high frequency (ı 8.5–9).
As alluded in the Introduction, the architectures of the ligands
was dictated by at least three important factors:
ZnCl2
Zn(1)Cl2
Zn(2)Cl2
Zn(3)Cl2
Zn(4)Cl2
Zn(5)Cl2
Zn(6)Cl2
Zn(7)Cl2
Zn(8)Cl2
Zn(9)Cl2
B.10
B.11
B.12
(i) convenient synthesis and commercial accessibility;
(ii) availability of a function (in this case –OH) useful for hetero-
geneization. The feasibility of this strategy was demonstrated
by decorating Zn(1)Cl2 with a triethoxysilylpropylcarbammate
linkage. The triethoxysilyl group in the corresponding product
Zn(8)Cl2 is known to react readily with the surface of silica
affording the heterogeneized form of the catalyst (Scheme 4)
[20]
Table 3
Influence of anions on Zn(1)X2 catalytic activity. Reaction conditions: soybean
oil = 2 g; methanol = 0.88 g; Cat. = 0.30 mmol; React. time = 60 min.
Run
Catalyst
T (◦C)
FAME yield (%)
(iii) the opportunity to control the Lewis acidity of zinc, through
modulation of the electronic properties of the ligands, which
span from electron-withdrawing (e.g. 7) to electron-donating
(e.g. 4).
B.4
C.1
C.2
C.3
C.4
C.5
Zn(1)Cl2
150
150
150
150
120
120
57
93
95
51
91
44
Zn(1)(CH3CO2)2
Zn(1)(CF3CO2)2
Zn(1)(CF3SO3)2
Zn(1)(CH3CO2)2
Zn(1)(CF3CO2)2
This latter target was also pursued by using four differ-
ent anions with the following order of coordinating ability:
Cl− > CH3CO2− > CF3CO2− > CF3SO3
. As a confirmation of this
−
Table 4
trend, the spectra of Zn(1)(CF3CO2)2 and Zn(1)(CF3SO3)2 in dmso-
d6 were identical (see Section 2.2.2), which suggests that both
anions are readily displaced by solvent molecules affording
cationic solvato-species [Zn(1)(dmso)n]2+. This possibility was con-
firmed through conductivity measurements. Solutions of both
Zn(1)(CF3CO2)2 and Zn(1)(CF3SO3)2 in DMSO showed a molar
conductivity (80 S cm2 mol−1) within the range expected for elec-
trolytes [21]. As expected, in the same solvent Zn(1)Cl2 and
Zn(1)(CH3CO2)2 were found to be non-conductive.
Influence of (1) ligand of Zn(CH3CO2)2 on catalytic activity. Reaction conditions:
T = 150 ◦C; soybean oil = 2 g; methanol = 0.88 g; React. time = 60 min.
Run
Catalyst
Cat. amount (mmol)
FAME yield (%)
D.1
D.2
C.1
D.3
Zn(CH3CO2)2
Zn(CH3CO2)2
Zn(1)(CH3CO2)2
Zn(1)(CH3CO2)2
0.30
0.06
0.30
0.06
22
12
93
52
Table 5
3.2. Catalytic runs
Influence of FFA concentration on activity of Zn(CH3CO2)2 and Zn(1) (CH3CO2)2
catalytic systems. Reaction conditions: T = 150 ◦C; Oil (FFA conc., 7%) = 2 g;
Methanol = 0.88 g; Cat. = 0.06 mmol; React. time = 60 min.
The trans-esterification of soybean oil was performed by stirring
a mixture of oil, methanol and the appropriate catalysts according
to the conditions reported in Tables 2–5.
First of all, it is important to point out that in the adopted reac-
tion condition the contribution to FAME yield of the non-catalytic
reaction is very low (see, entry B.1 of Table 2).
Run
Catalyst
FFA conv. (%)
FAME yield (%)
E.1
E.2
E.3
–
<1
30
26
2
16
48
Zn(CH3CO2)2
Zn(1)(CH3CO2)2
N
) Si(OEt)
OC(O)NH(CH2 3
N
silica
-EtOH
3
Zn
Cl Cl
Zn(8)Cl2
O
N
OC(O)NH(CH2)3Si O
N
O
Zn
Cl Cl
Scheme 4. Strategy for the heterogeneization of the catalyst.

110
M. Di Serio et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 106–110
Moreover, the ligands without the presence of metal ions
4. Conclusions
showed a low activity (see, entry B.2 of Table 2).
The early experiments were designed to evaluate the influence
of the properties of the N,Nꢀ-ligands, and were carried out by study-
ing the activity of the corresponding dichlorozinc complexes (see
Table 2).
This work demonstrates that it is possible to obtain molecu-
lar species of zinc(II) capable of catalyzing the transesterification
of soybean oil in mild conditions. Their activity can be modulated
by a fine selection of the anions and/or of the substituents on the
ancillary bidentate ligand. The compounds are also active in the
esterification of fatty acids. Future work will involve the hetero-
geneization of the catalysts on solid matrices, by employing the OH
group present on the ligand scaffold, and the study of the activity
of corresponding complexes containing other Lewis acids.
The acidity of the catalytic site has an important effect on the
performance. In fact, the highest activity was offered by the nitro-
species Zn(7)Cl2, which promoted 84% (entry B.10) of conversion to
biodiesel. On the other hand, in the presence of moderate electron-
donor ligands (2 and 4, entries B.5 and B.7) the yield resulted lower
than that observed with free ZnCl2 (Table 2), although in these cases
the interference of steric effects cannot be ruled out. Instead, these
apparently prevail in the presence of bulky substituents in the ortho
position of the pyridine ring, as in Zn(3)Cl2, which promote only 9%
of conversion to biodiesel (B.6).
The effect of the anions is instead reported in Table 3. It is evident
that high conversion can be achieved through fine tuning of the
basic property of the anion. In fact, a strong Lewis base (Cl−) plau-
sibly competes with the substrate in the coordination to the metal
site, thus inhibiting its activity (57%, B.4). On the other hand, the
presence of a weak donor (CF3SO3−) excessively enhances the acid-
ity of the metal center, which is therefore deactivated by a strong
coordination of the product (51%, C.3).
Acknowledgements
Financial Support from MIPAAF (AGROPROM DM 246/2007) is
gratefully acknowledged. We also thank Mrs. Marina Tranchillo
(Tranchiggia) for technical assistance.
References
[1] E. Lotero, Y. Liu, D.E. Lopez, K. Suwannakaran, D.A. Bruce, J.G. Goodwin Jr., Ind.
Eng. Chem. Res. 44 (2005) 5353–5363.
[2] M. Di Serio, R. Tesser, L. Pengmei, E. Santacesaria, Energy & Fuels 22 (2008)
207–217.
[3] Y.C. Sharma, B. Bhaskar Singh, J. Korstad, Biofuels. Bioprod. Bioref. 5 (2011)
69–92.
[4] K. Suwannakarn, E. Lotero, J.G. Goodwin Jr., C. Lu, J. Catal. 255 (2008) 279–286.
[5] M. Di Serio, R. Tesser, L. Casale, A. D’Angelo, M. Trifuoggi, E. Santacesaria, Top.
Catal. 53 (2010) 811–819.
[6] Y.M. Park, D.W. Lee, D.K. Kim, J.S. Lee, K.Y. Lee, Catal. Today 131 (2008) 238–243.
[7] M. Di Serio, M. Cozzolino, R. Tesser, P. Patrono, F. Pinzari, B. Bonelli, E. Santace-
saria, Appl. Catal. A Gen. 320 (2007) 1–7.
[8] F.R. Abreu, D.G. Lima, E.H. Hamú, S. Einloft, J.C. Rubim, P.A.Z. Suarez, JAOCS 80
(2003) 601–604.
[9] M. Di Serio, R. Tesser, M. Dimiccoli, F. Cammarota, M. Nastasi, E. Santacesaria,
J. Mol. Cat. A Chem. 239 (2005) 111–115.
[10] X. Hou, Y. Qi, X. Qiao, G. Wang, Z. Qin, J. Wang, Korean J. Chem. Eng. 24 (2007)
311–313.
The delicate balance between these two effects is satisfied by the
−
intermediate behavior of CH3CO2 and CF3CO2−, which promote
almost quantitative conversion at 150 ◦C (C.1 and C.2). Lowering
the temperature to 120 ◦C allows to discriminate between the two
anions, disclosing the higher ability of the acetate ion (91% vs 44%,
C.4 vs C.5).
Given these premises, further catalytic runs were carried out
using acetate ions in combination with ligand 1 by varying the
conditions of catalyst loading (Table 4).
Two different concentrations (0.30 and 0.06 mmol) were used,
and the results compared with those achieved under identical con-
ditions using the simple Zn(CH3CO2)2. In all cases, the use of the
ligand significantly increased the obtained FAME yields.
A further refinement was carried out by introducing a percent-
age of FFA (Free Fatty Acids, 7%) in the oil phase, to determine
the activity of the catalyst Zn(1)(CH3COO2)2 towards the esteri-
fication reaction in comparison with Zn(CH3COO2)2 and without
the catalyst (Table 5).
This demonstrates that the Zn(1)(CH3COO2)2 catalyst combines
the significant trans-esterification activity with a certain attitude
towards FFA esterification and so it can be considered as good
candidate for the heterogeneization on a solid support (using the
procedure previously described) to obtain an heterogeneous cat-
alyst which could be used for biodiesel production from waste
oils.
[11] C.J. Chunk, Lewis Acid Catalyst Design for Transesterification of Lower Quality
Feedstock for Biodiesel Production. Ph.D. Thesis, University of Bath, UK, 2007.
[12] R. Kannappan, M. Matsumoto, J. Hallren, K.M. Nicholas, J. Mol. Catal. A Chem.
339 (2011) 72–78.
[13] D.W. Yooa, J.H. Hanb, S.H. Namb, H.J. Kimb, C. Kimb, J.K. Leea, Inorg. Chem.
Commun. 9 (2006) 654–657.
[14] G.W. Parshall, S.I. Ittel, Homogeneous Catalysis: The Applications and Chem-
istry of Catalysis by Soluble Transition Metal, 2nd ed., Wiley-Interscience, New
York, 2005.
[15] M. Di Serio, B. Apicella, G. Grieco, P. Iengo, L. Fiocca, R. Po, E. Santacesaria, J. Mol.
Catal. A Chem. 130 (1998) 233–240.
[16] L. Angiolini, D. Caretti, L. Mazzocchetti, E. Salatelli, R. Willem, M. Biesemans, J.
Organomet. Chem. 691 (2006) 3043–3052.
[17] G.N. Walker, M.A. Klett, J. Med. Chem. 9 (1966) 624–630.
[18] G. Gelbard, O. Brès, R.M. Vargas, F. Vielfaure, U.F. Schuchardt, J. Am. Oil Chem.
Soc. 72 (1995) 1239–1241.
[19] ASTM D803-82 (colourimetric method).
[20] See for example N.C. Mehendale, J.R.A. Sietsma, K.P. de Jong, C.A. van Walree,
R.J.M. Klein Gebbink, G. van Koten, Adv. Synth. Catal. 349 (2007) 2619–2630.
[21] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81–122.