Indian Journal of Chemistry
Vol. 50A, August 2011, pp. 1017-1025
Lanthanum and zinc incorporated hydrotalcites as solid base catalysts for
biodiesel and biolubricants production
R Rahul, Jitendra K Satyarthi & D Srinivas*
Catalysis Division, National Chemical Laboratory, Pune 411 008, India
Email: [email protected]
Received 24 June 2011; revised and accepted 15 July 2011
Mg-Al hydrotalcites doped with varying amounts of lanthanum and zinc ions (10 – 30 mol %) have been prepared by
co-precipitation method and used, after calcination at 873 K, as solid base catalysts for transesterification of soybean oil
with methanol (producing biodiesel) and n-octanol (producing biolubricants). The catalyst with 20 mol % of lanthanum
shows the highest transesterification activity (soybean oil conversion = 100 % and biodiesel yield = 95 %) at 423 K in 4 h.
Catalytic activity varies in proportion with the basicity of the catalysts.
Keywords: Catalysis, Solid base catalysts, Hydrotalcites, Doped hydrotalcites, Lanthanum, Zinc, Transesterification,
Vegetable oils, Biodiesel, Biolubricants
With increasing concern about the energy crisis as
well as the need for clean environment, the need for
sustainable and eco-friendly processes and products,
especially in the area of energy and fuels, is gaining
more and more importance. Biodiesel is an example
of such a promising product, which is non-toxic, biodegradable and a renewable fuel comprising mono
methyl or ethyl esters of long-chain fatty acids,
derived from vegetable oils and animal fat by the
transesterification with methanol or ethanol1.
Biodiesel helps in reducing the air pollution as it
produces lower amounts of SOx, COx, HC and
particulate matter. Conventionally, biodiesel is
produced using homogeneous catalysts such NaOH,
KOH or their methoxides or carbonates2. However,
the issue with the homogeneous processes is removal
of catalyst after the reaction, which requires
additional steps like neutralization and water washing
to get the final product (fatty acid methyl ester and
glycerol). This downstream process generates
significant amounts of inorganic salt byproducts and
waste water. Glycerol produced in this process is of
low quality and needs expensive purification steps to
produce a pharmaceutical grade product. Due to these
problems heterogeneous catalysts are preferred which
make the catalyst separation easier and provide a
higher quality biodiesel and glycerol1. Several
heterogeneous acid and base catalysts have been
reported3-12. Since base catalysts are 3000 times more
active than acid catalysts, greater interest has been
focused on the development of efficient, solid bases
which can replace NaOH or KOH in the biodiesel
manufacturing process6-12.
Hydrotalcites (anionic clays, HT) of general
formula [M2+(1-x)M3+x(OH)2]x+(An-)x/n).yH2O are an
interesting class of solids whose acid-base properties
can be tailored by varying the constituent metal ions
and their composition6. Their structure is akin to
brucite-like layers, in which a divalent metal cation is
located in the centre of oxygen octahedron. Partial
isomorphous substitution of trivalent ions for divalent
ions (0.2 ≤ x ≤ 0.4) results in a positive charge on the
layers. Organic or inorganic anions are intercalated
between the layers in order to maintain charge
balance. Water of crystallization is also generally
found in the interlayer galleries. Hydrotalcites find
applications as additives in polymers, as precursors to
magnetic materials and in biology and medicine13-18.
Hydrotalcites, as such or after thermal treatment, have
been used as catalysts in several organic transformations including transesterification of vegetable oils
and fats19-26. Loading of KI, KF and K2CO3
(20-100 wt %) enhanced the basicity and thereby the
catalytic activity of hydrotalcites27-29. Studies on Fe,
Co, La and Li doped hydrotalcites have also been
reported30-35.
We report herein studies on lanthanum and zinc
(10-30 mol %) incorporated Mg-Al hydrotalcites for
1018
INDIAN J CHEM, SEC A, AUGUST 2011
transesterification of soybean oil with methanol and
n-octanol. While the reaction with methanol produces
biodiesel, that with n-octanol yields biolubricants.
Materials and Methods
Preparation of the catalysts
Hydrotalcites (with Mg/Al molar ratio = 2) were
prepared by co-precipitation method6,13. In a typical
procedure, aqueous solutions (100 mL) of magnesium
and aluminum nitrates (Merck, India) were prepared
and taken in two separate burettes. The solutions were
simultaneously added to a beaker containing 50 mL of
deionized water; pH was maintained at 10 using a
mixture of NaOH (1.2 M) and Na2CO3 (0.1 M)
solutions. The nitrates solutions were added over 2 h
with stirring. At the end, the slurry was transferred
to a Teflon-lined stainless steel autoclave and aged
at 363 K for 24 h. The solid formed was separated
by filtration, washed thoroughly with deionized
water until all the sodium ions were removed and the
filtrate became neutral. It was then dried at 373 K
for 24 h. The catalyst obtained was labeled as
HT-as-synthesized. It was then calcined at 873 K for
6 h and the material obtained after calcination was
designated as HT-calcined.
Catalysts with varying compositions of lanthanum
(10, 20 and 30 mol % with respect to Al) and zinc
(10, 20 and 30 mol % with respect to Mg) were
prepared in a similar manner. Nitrates of lanthanum
and zinc were used as precursors. The modified
catalysts, thus prepared, are denoted as HT-M X,
where M stands for La or Zn and X represents the
mole percentage of that element.
Characterization of the catalysts
The metal ion composition was determined on an
Inductively Couple Plasma-Optical Emission Spectrometer (Spectro Arcos ICP-OES). A known quantity of
calcined hydrotalcites was dissolved in a minimum
amount of 2 N HNO3 and made up to the required
volume before analysis. X-ray diffraction (XRD)
patterns of the powder samples were recorded on an
X’Pert Pro (Philips) diffractometer using Cu-Kα
radiation and a proportional counter detector. Thin
films of the samples were prepared on XRD glass
plates and analysed. Diffractographs were recorded in
the 2θ range of 5 – 80°, with a scan rate of 2°/min and
step size of 0.02°. Unit cell parameters (a and c) were
estimated from the diffraction patterns (2θ and interplanar spacing d). The specific surface area (SBET) of
the samples was determined from N2 adsorptiondesorption measurements conducted at 77 K using a
NOVA 1200 Quanta Chrome equipment. Prior to N2
adsorption, the samples were evacuated at 373 K (in
the case of as-synthesized) or 573 K (in the case of
calcined) for several hours. Thermal analysis of the
samples was done on a Seiko DTA-TG 320 instrument, under air flow (50 cm3/min), at a ramp rate of
10 K/min, in the temperature range 308-1090 K.
X-ray photoelectron spectra (XPS) of the samples
were acquired on a VG Microtech Multilab ESCA
3000 with Mg-Kα radiation (hν = 1253.6 eV). Base
pressure in the analysis chamber was maintained at
3 – 6 × 10-10 mbar. The peak corresponding to carbon
1s (at 284.2 eV) was taken as the reference in
estimating the binding energy (BE) values of various
elements in the catalyst. The error in BE values is
± 0.1 eV. The measurements included a survey
spectrum and the core-level spectra, which were
analyzed following the standard procedures. The
diffuse reflectance UV-visible (DRUV-vis) spectra
were recorded on a Shimadzu UV-2550 spectrophotometer using BaSO4 as the reference.
Basicity measurements
Basicity of the catalysts was determined using
Hammett indicators and by benzoic acid titration
method21,26,36. In the case of the former, 25 mg of the
calcined catalyst was shaken with 1 mL solution of
Hammett indicator in methanol and left to equilibrate.
Colour of the catalyst was noted. The following
Hammett indicators were used: Neutral red
(pKa = 6.8), phenol red (pKa =7.4), phenolphthalein
(pKa = 8.2), nile blue (pKa = 9.7), 4-nitroaniline
(pKa = 18.4). The base strength is quoted as being
stronger than the weakest indicator which exhibits a
colour change.
In benzoic acid titration method, 0.1 g of the
catalyst was suspended in 2 mL of phenolphthalein
indictor solution (0.01 mg/mL toluene). It was then
stirred for 0.5 h and titrated against 0.01 M benzoic
acid in toluene solution until the pink colour changed
to colourless. The total basicity was, thus, determined.
In other experiments for determining soluble
basicity, 0.1 g of the catalyst was suspended in 10 mL
of distilled water. It was stirred for 1 h and filtered.
The filtrate was titrated against 0.01 M solution of
benzoic acid in methanol using phenolphthalein
(0.01 % in methanol) as the indicator. End point was
noted when the pink colour of the solution changed to
colourless.
RAHUL et.al.: CATALYTIC ACTIVITY OF La/Zn DOPED HYDROTALCITES FOR BIODIESEL/ BIOLUBRICANTS PRODUCTION
Reaction procedure
In a typical transesterification reaction37, soybean
oil (5 g), alcohol (5-20 moles times excess of oil) and
catalyst (1-10 wt % of oil) were charged into a
Teflon-lined stainless steel autoclave (100 mL). The
autoclave was placed in a rotating hydrothermal
reactor (Hiro Co., Japan; 50 rpm). The reaction was
conducted at 353-423 K for 1-6 h. At the end of the
reaction, the autoclaves were allowed to cool to
298 K. The catalyst was separated from the reaction
mixture by centrifugation and filtration. In the
experiments with methanol, two separate liquid layers
were formed, the lower layer being the unreacted
methanol + glycerol and the upper one being fatty
acid alkyl esters. The solubility of lower alcohols is
negligibly small in fatty acid alkyl esters and oils at
room temperature. In experiments with n-octanol only
a single layer was detected. The unreacted alcohol
was removed by vacuum distillation. Now the
reaction mixture contained transesterified product
(fatty acid alkyl esters), glycerol and unreacted oil.
The contents were taken in a separating funnel and
petroleum ether (20 mL) was added. The oil and the
transesterified product readily went into the ether
layer; glycerol remained as a separate layer. Glycerol
was removed. The fatty acid esters and unconverted
oil were recovered by distilling out the ether portion.
The products were analyzed by HPLC (Perkin Elmer
Series 200) equipped with an ELSD detector and
reverse phase, C-18 Spheri-5 column (length =
250 mm, i.d. = 4.6 mm and particle size = 5 µm).
1
H nuclear magnetic resonance (NMR) spectroscopy
1019
(Bruker AV 200 MHz) was also used to identify the
product and estimate FAME yield38. The soybean oil
procured from the local market had the following
fatty acid composition: palmitic acid (C16.0) – 12 %,
stearic acid (C18.0) – 3 %, oleic acid (C18.1) – 27 %,
linoleic acid (C18.2) – 52 %, linolenic (C18.3) – 6 wt %.
The fatty acid (FFA) level was < 0.4 wt %.
Results and Discussion
Characterization of the catalysts
Figures 1 and 2 show the corresponding powder
X-ray diffractograms of as-synthesized and calcined
forms of “neat” and La/Zn substituted Mg-Al
hydrotalcites. The as-synthesized hydrotalcite
(HT-as-synthesized) showed characteristic reflections
at 11.6°, 23.5°, 35.0°, 39.6°, 47.1°, 60.7° and 62.1°
arising from (003), (006), (012), (015), (018), (110)
and (113) planes, respectively. The XRD profile of
HT-as-synthesized is consistent with the structure of
[Mg4Al2(OH)12(CO3)(H2O)3]0.5 (hexagonal system,
rhombo-centered lattice, space group R3m (JCPDS
70-2151))19-26. Absence of reflections at 18.5° and
20.5° and 15.2° and 30.2° reveal that the material
is pure and doesn’t contain impurity phases
like MgAl2(OH)8 and Mg5(CO3)4(OH)2.4H2O,
respecttively19-26. From the peak positions of (003)
and (006) reflections, the basal spacing between
the layers (c = 3/2[d(003)+ 2d(006)]) was calculated.
The reflection corresponding to (110) was used to
determine the unit cell parameter, a, where a = 2d(110).
Values of these parameters (Table 1) are in good
agreement with those reported by others19-26.
Fig. 1 – X-ray diffraction profiles of (a) as-synthesized and (b) calcined forms of lanthanum incorporated hydrotalcites. [Asterisk
indicates the peaks due to MgO and A2O3].
INDIAN J CHEM, SEC A, AUGUST 2011
1020
Fig. 2 – X-ray diffraction profiles of (a) as-synthesized and (b) calcined forms of zinc incorporated hydrotalcites. [Asterisk indicates the
peaks due to MgO and A2O3].
Table 1
– Elemental composition, structural, textural and electronic properties of hydrotalcites
Sample
Unit cell parameters
(XRD, nm)
a
c
HT as-synthesized
HT calcined
HT-La 20 as-synthesized
HT-La 10 calcined
HT-La 20 calcined
HT-La 30 calcined
HT-Zn 10 as-synthesized
HT-Zn 20 as-synthesized
HT-Zn 10 calcined
HT-Zn-20 calcined
HT-Zn 30 calcined
0.304
2.269
0.306
2.322
0.305
0.305
Specific surface
area (SBET)
2.268
2.267
55
97
55
111
98
144
42
62
175
84
147
Binding energy –
O 1s (XPS, eV)
530.9
530.6
530.5
530.5
Comp.: molar ratio from ICP-OESa
Mg:Al = 1.8 (2)
Mg:Al = 1.8 (2)
Mg:(Al+La) = 1.9 (2), Al:La = 5.5 (4)
Mg:(Al+La) = 1.7 (2), Al:La = 9.8 (9)
Mg:(Al+La) = 1.9 (2), Al:La = 5.5 (4)
Mg:(Al+La) = 1.6 (2), Al:La = 2.5 (2.3)
(Mg+Zn):Al =1.5 (2), Mg:Zn = 21.7 (9)
(Mg+Zn):Al =1.5 (2), Mg:Zn = 10.6 (4)
(Mg+Zn):Al =1.5 (2), Mg:Zn = 21.7 (9)
(Mg+Zn):Al =1.5 (2), Mg:Zn = 10.6 (4)
(Mg+Zn):Al =1.3 (2), Mg:Zn = 5.5 (2)
a
Theoretical (nominal) molar ratio is given in parentheses.
Lanthanum incorporation affected the crystal
structure of HT. Intensity of the XRD peaks
decreased with increasing La content from 10 to
30 mol % of Al (Fig. 1). Formation of a separate
La2O3/La(OH)3 phase was detected even at 10 mol %
of La. This phase is more abundant when the amount
of La increases to 30 mol %. Although the oxidation
state of Al and La is the same (+3), the effective
ionic radius of La is nearly twice (Shannon ionic
radius of La = 0.103 nm) as that of Al (0.054 nm).
Hence, a major part of La would most likely
reside in the interlayer space of HT as a separate
oxide/hydroxide phase or on the external surface
of HT crystallites or as independent segregated
phases. This explains the decrease in the intensity of
HT peaks and observation of additional reflections
in the XRD of HT-La as-synthesized samples.
Mg and Zn have the same oxidation state (+2)
and similar effective ionic radius (0.072 nm for
Mg2+ and 0.074 nm for Zn2+). Formation of no
separate ZnO/Zn(OH)2 phase was observed even
at 30 mol % of Zn (Fig. 2) indicating that Zn should
be in the framework of HT. With increasing
lanthanum loading, the unit cell parameter, c,
corresponding to interlayer separation, increased
from 2.27 to 2.32 nm. No such increase in c-value
was observed when Zn2+ ions were incorporated.
This corresponds to reduced coulombic interaction
RAHUL et.al.: CATALYTIC ACTIVITY OF La/Zn DOPED HYDROTALCITES FOR BIODIESEL/ BIOLUBRICANTS PRODUCTION
1021
Table 2 – Thermal analysis of as-synthesized hydrotalcites
Catalyst
298-393
HT
HT-La 10
HT-La 20
HT-La 30
HT-Zn 10
HT-Zn 20
HT-Zn 30
3.5
4.6
3.9
4.0
2.2
2.8
2.2
Weight loss (%) in the temp. range (K) =
393-523
523-603
603-873
14.5
11.7
10.0
10.0
14.6
15.8
16.2
7.3
4.3
6.8
6.9
8.2
8.6
5.6
between the brucite-like layers and the interlayer
anions (CO32-) with increasing La3+ ion concentration.
Upon calcination at 873 K, the hydrotalcite is
decomposed and converted into a mixed oxide phase.
The peaks marked by asterisk in the traces of calcined
samples in Figs 1 and 2 correspond to those of Al2O3
and MgO. In addition to Mg(Al)La oxide, formation
of La2O2CO3 phase (JCPDS 48-1113) was also
detected in HT-La-calcined samples. In the case of
HT-Zn-calcined samples, the formation of Mg(Al)Zn
mixed oxide with X-ray reflections at 35.3°, 57.4° and
63° (JCPDS 76-2464) was detected.
The chemical compositions of the as-synthesized
and calcined HT materials were estimated using
ICP-OES technique (Table 1). In “neat” HT, the
Mg:Al molar ratio was estimated to be 1.8:1 which
is very close to the input or nominal value of 2:1.
The amount of La in final HT-La material is nearly
the same as the nominal value. However, only less
than half of the input Zn is incorporated in the final
product (Table 1). N2 physisorption measurements
reveal that upon calcination the specific surface
area of samples increased two-folds. With La
incorporation the surface area had increased still
further (Table 1).
In thermogravimetric analysis, the decomposition
of hydrotalcites occurred in three stages via
(i) desorption of loosely bound surface water (in the
temperature range 298 – 393 K), (ii) desorption of
interlayer structural water (393 – 523 K) and
(iii) dehydroxylation and decarboxylation of hydrotalcite framework (523 – 873 K). The total weight
loss is between 42 – 48 % and decreased in the order:
HT > HT-La > HT-Zn (Table 2).
“Neat and calcined” hydrotalcites were UV silent.
The calcined HT-La samples, on the other hand,
showed an intense absorption band at 205 nm due to
O2- ↔ La3+ charge transfer transition. Pure La2O3
showed this band at slightly higher wavelength
20.7
23.5
21.2
21.1
16.2
13.3
12.4
Total weight loss (%)
873-1273
2.1
1.7
2.7
2.5
3.7
4.6
5.8
48.2
45.9
44.6
44.5
44.8
45.1
42.1
Fig. 3 – 27Al MAS NMR spectra of fresh and spent hydrotalcites.
[1, HT as-synthesized; 2, HT calcined; 3, HT-La 20 calcined
(fresh); 4, HT-La 20 calcined (spent)].
(215 nm). HT-Zn calcined samples showed broad
charge transfer bands in the region 200 – 350 nm.
Intensity of these bands increased with increasing
metal (La and Zn) content in the sample.
HT-as-synthesized showed a sharp intense27Al MAS
NMR peak at 9 ppm corresponding to Al in an
octahedral coordination with a small quadrupolar
splitting constant (Fig. 3). The calcined samples,
showed two asymmetric broad signals at 67.2 and
8.4 ppm for “neat” HT, at 69.5 and 9.6 ppm for
HT-La 20 and at 64.0 and 8.5 ppm for spent
HT-La 20 (Fig. 3). While the signal at 8.4 – 9.6 ppm
is attributed to Al in octahedral coordination site, that
at 64.0 – 69.5 ppm corresponds to Al in a tetrahedral
coordination environment. The broadness of the peak
is due to non-zero quadrupolar coupling interactions.
These peaks for calcined samples are consistent with
the presence of Mg(Al) mixed oxides39,40. The surface
properties of HT-La-calcined materials were studied
by XPS (Table 1). The peak arising from O 1s core
level is very sharp and occurs at 530.5 eV. This peak
INDIAN J CHEM, SEC A, AUGUST 2011
1022
Table 3 – Methanolysis of soybean oil over calcined hydrotalcites. [React. cond.: Catalyst = 5 wt. % of oil, soybean oil = 5 g;
oil:methanol = 1:15 (mol/mol), temp. = 423 K, reaction time = 4 h]
Catalyst
HT
HT-La 10
HT La 20
HT-La 30
HT-Zn 10
HT-Zn 20
HT-Zn 30
a
Oil conversion
(wt. %)
15.4
81.2
100
90.1
8.6
22.5
23.3
TG
84.6
18.8
0.01
9.9
91.4
77.5
76.7
Product distribution (wt. %)a
DG
MG
12.3
0.9
12.5
14.7
1.5
3.5
7.6
12.2
6.9
0
17.4
1.2
16.2
1.0
FAME
2.1
54.1
95.0
70.3
1.7
3.8
5.9
TG = triglycerides, DG = diglycerides, MG = monoglycerides and FAME = fatty acid methyl esters.
for HT-calcined occurs at 530.9 eV. The decrease in
O 1s binding energy in the case of HT-La 20 indicates
a higher effective negative charge on the surface
oxygen atoms in HT-La 20. This leads to an increase
in the electron donating ability, and thereby the
increased basicity, of the surface oxygen atoms of
HT-La 20. In other words, La incorporation enhances
the basicity of the mixed oxide materials.
Basicity of the materials was estimated using
Hammett indicators and by titration with standard
benzoic acid21,26,36. The following Hammett indicators
were used: neutral red (pKa = 6.8), phenol red
(pKa = 7.4), phenolphthalein (pKa = 8.2), nile blue
(pKa = 9.7) and 4-nitroaniline (pKa = 18.4). With all
these indicators, except for 4-nitroaniline, a change
in colour was observed. Hence, the strength of the
basicity of hydrotalcite catalysts used in this
investigation is in the range: 9.7 < pKa > 18.4. Despite
its limitations, this method has been widely used to
determine the surface basicity of solid catalysts. In the
titration with benzoic acid and phenolphthalein
(pKa = 8.2) as indicator, it was found that the
lanthanum containing materials possess higher
number of basic sites than the “neat,” calcined
hydrotalcite (0.34 mmol/g). The material containing
20 mol % La has a higher number of basic sites
(0.42 mmol/g) than those with 10 (0.37 mmol/g) and
30 (0.35 mmol/g) mol % La. The basicity of HT-La
materials is significantly higher than that of pure
La2O3 (0.02 mmol/g). Decrease in basicity was
observed in the case of a Zn-incorporated hydrotalcite
(0.27 mmol/g) (Table 3).
Catalytic activity
The calcined materials showed enhanced catalytic
activity as compared to the as-synthesized
hydrotalcites. Blank experiments, without any
Fig. 4 – Kinetics plot for transesterification of soybean oil with
methanol over HT-La 20 calcined. [React. cond.: Catalyst = 5 wt.
% of soybean oil; oil = 5 g; soybean oil:methanol (molar ratio) =
1:15; temp. = 373 K. Curve 1, TG conv.; 2, TG; 3, DG, 4, MG; 5,
FAME. TG = triglycerides; DG = diglycerides; MG = monoglycerides; FAME = fatty acid methyl esters].
catalyst, revealed that the contribution due to noncatalytic reaction is significantly low at the chosen
reaction conditions (temp. = 423 K, autogeneous
pressure). Table 3 lists the catalytic activity data of
different calcined hydrotalcites. The lanthanum
incorporated hydrotalcites exhibited the highest
catalytic activity with soybean oil conversion in the
range of 81 – 100 % and with fatty acid methyl ester
(FAME) yields of 54-95 %. Oil conversion increased
with La content up to 20 mol % and above which it
decreased. This variation in catalytic activity is in line
with the basicity of these materials. As expected
the Zn containing material having lower basicity
exhibited lower catalytic activity. Figure 4 presents
the kinetic plot of transesterification reaction over
calcined HT-La 20 catalyst. Transesterification of
RAHUL et.al.: CATALYTIC ACTIVITY OF La/Zn DOPED HYDROTALCITES FOR BIODIESEL/ BIOLUBRICANTS PRODUCTION
triglycerides is a three-step, consecutive, reversible
reaction wherein in the first step, the triglyceride (TG)
is converted into diglyceride (DG), which is then in
the second and third steps is converted sequentially
into monoglyceride (MG) and glycerol (G), respecttively, producing one mole of FAME at each step.
Stoichiometrically, one mole of triglyceride requires
three moles of methanol to produce three moles of
FAME and one mole of glycerol. Excess methanol
would drive the equilibrium towards the end-products,
i. e., FAME and glycerol. The kinetic plot of
transesterification of soybean oil over HT-La 20
(Fig. 4) was found to be consistent with this 3-step
equilibrium process. The kinetics was also monitored
by 1H-NMR spectroscopy (Fig. 5) and FAME yield
was calculated using the peak at δ = 3.67 ppm (due to
–OCH3) and δ = 2.30 ppm (due to α-CH2 protons)38.
The FAME yields determined from NMR and HPLC
are in good agreement.
The catalyst (HT-La 20 calcined) is active
(oil conversion = 74.4 % and FAME yield = 65.9 %)
even at temperatures as low as 353 K (Table 4).
However, with increasing temperature the activity
increased and near complete conversion of
triglycerides (99.9 %) with a FAME yield of 97.5 %
Table 4
was achieved at 393 K. The molar ratio of oil to
methanol has a marked effect. Catalytic activity
increased with increasing methanol content and
approached a complete conversion of soybean oil at
373 K with an oil-to-methanol molar ratio of 1:20.
Vegetable oil conversion increased with increasing
amount of the catalyst approaching a maximum
conversion at about 5 – 10 wt % of catalyst with
respect to oil. All these parameter optimization
Fig. 5 – 1H-NMR spectrum of the reaction mixture in
transesterification of soybean oil with methanol over HT-La 20
calcined as a function of time. [React. cond.: Catalyst = 5 wt. % of
soybean oil; oil = 5 g; soybean oil:methanol (molar ratio) = 1:15;
temp. = 373 K].
– Influence of reaction parameters on methanolysis of soybean oil over calcined HT-La 20
Effect of temp.
[React. cond.: Catalyst = 5 wt. % of oil; soybean oil = 5 g; oil:methanol = 1:15 (mol/mol); reaction time = 2 h]
Temp. (K)
Oil conv. (wt. %)
Product distribution (wt. %)a
TG
DG
MG
353
74.4
25.6
6.7
2.5
373
95.9
4.1
2.4
4.5
393
99.9
0.1
0.5
1.8
423
99.9
0.1
0.4
5.1
FAME
65.9
89.0
97.5
94.5
Effect of oil:methanol molar ratio
[React. cond.: Catalyst = 5 wt. % of oil, soybean oil = 5 g, reaction temp. = 373 K, reaction time = 2 h]
Molar ratio
Oil conv. (wt. %)
TG
DG
MG
1:5
15.3
84.7
10.5
1.1
1 : 10
63.1
36.9
14.7
6.5
1 : 15
95.9
4.1
2.4
4.5
1 : 20
99.6
0.4
1.4
2.6
FAME
3.7
42.0
89.0
93.5
Effect of the amount of catalyst
[React. cond.: Soybean oil = 5 g; oil:methanol = 1:15 (mol/mol); reaction temp. = 373 K; reaction time = 2 h]
Catalyst (wt. % oil)
Oil conv. (wt. %)
TG
DG
MG
0
0.6
99.4
0.6
0
1
14.5
85.6
8.9
1.0
3
43.3
56.7
13.3
3.2
5
95.9
4.1
2.4
4.5
10
98.6
1.4
0.5
0.8
FAME
0
4.5
26.9
89.0
97.3
a
1023
TG = triglycerides; DG = diglycerides; MG = monoglycerides; FAME = fatty acid methyl esters.
INDIAN J CHEM, SEC A, AUGUST 2011
1024
Table 5 – Catalyst reusability studies for methanolysis and octanolysis of soybean oil over calcined HT-La 20.
[React. cond.: Catalyst = 5 wt.% of oil; soybean oil = 5 g; molar ratio of oil:alcohol = 1:15 (for methanol) and 1:9 (for octanol),
temp. = 423 K (for methanol) and 463 K (for octanol); react. time = 4 h]
Number of recycles
0
1
2
Oil conversion
(wt. %)
100
52.8
16.6
TG
Product distribution (wt. %)
DG
MG
With methanol
0.01
47.2
83.5
1.5
18.0
11.1
3.5
4.1
0.5
FAME or FAOE
94.9
30.7
4.9
With octanol
0
92.8
7.2
4.7
88.1
1
79.6
20.4
3.7
76.0
2
55.1
44.9
2.2
52.8
a
TG = triglycerides; DG = diglycerides; MG = monoglycerides; FAME = fatty acid methyl esters; FAOE = fatty acid octyl esters.
studies reveal that the lanthanum containing
hydrotalcites are more active even at low
temperatures. This observation agrees well with the
reports by other workers30-32. The higher basicity of
these La catalysts is responsible for their efficient
catalytic activity at low temperatures. Solid catalysts
active at moderate temperature and pressure are
desirable, as the material of construction and
processing costs of biodiesel would be low.
The calcined HT-La 20 catalyst was subjected to
reusability studies, wherein at the end of each
experiment, the catalyst was separated from the
product by filtration, washed with methanol and
activated at 873 K for 6 h. As seen from Table 5, the
catalytic activity (vegetable oil conversion) drops
down from 100 to 16.6 % at the end of 2nd recycle.
The possible causes for the activity loss were
examined by characterizing the spent catalyst. The
XRD of fresh and spent HT-La 20 reveals that the
structure of the catalyst is intact even after reuse. No
shift in position, changes in linewidth and intensity
were observed. A similar conclusion is also drawn
from 27Al MAS NMR studies (Fig. 3). Chemical
composition of the fresh and spent (1st and 2nd
recycle) catalysts were determined by ICP-OES.
No change was found in the composition even after
the reuse of the catalyst.
The soluble basicity of the catalyst was determined
by titration method. In this method, 0.1 g of the
catalyst was suspended in 10 ml of distilled water. It
was stirred for 1 h and filtered. The filtrate was
titrated against 0.01 M solution of benzoic acid in
methanol using phenolphthalein (0.01 % in methanol)
as indicator. It was found that the soluble basicity due
to leaching of metal ions into the solution was
negligible. However, presence of some amount of
organic matter on catalyst surface was detected
by CH analysis. It is therefore concluded that the
non-reusability of the catalyst is not due to structural
or metal loss but due to adsorption of polar organic
molecules on the catalyst surface making the basic
active sites not available to reactant molecules.
Calcination of the used catalyst at still higher
temperatures (973 K) regenerated the active sites and
the original activity (soybean oil conversion = 100 %
and FAME selectivity = 94 %) was regained.
Transesterification of vegetable oil with octanol
yields biolubricants (Table 5)37. HT-La 20 was highly
active and high yields of biolubricant were obtained.
Even in this case, a decrease in catalytic activity was
observed in recycle studies. But the extent of the
activity loss was lower than that observed with the
more hydrophilic methanol (Table 5).
In another experiment, fatty acid methyl ester
procured from a commercial source was
transesterified with n-octanol over HT-La-20 catalyst.
High conversion of FAME (94.5 %) was achieved at
463 K over 8 h with 100 % selectivity for fatty acid
octyl esters. The catalyst could be reused with little
loss in activity. The conversion was 93.1 % in the first
recycle and 89.5 % in the second recycle. To the best
of our knowledge, this is the first report on the
application of HT-La catalysts in the production of
biolubricants.
Conclusions
The catalytic activity of calcined Mg-Al
hydrotalcites doped with varying amounts of
lanthanum and zinc ions (10 – 30 mol %) was
investigated for transesterification of soybean oil
with methanol (producing fatty acid methyl esters,
FAME biodiesel) and n-octanol (producing fatty acid
RAHUL et.al.: CATALYTIC ACTIVITY OF La/Zn DOPED HYDROTALCITES FOR BIODIESEL/ BIOLUBRICANTS PRODUCTION
octyl esters, FAOE biolubricants). The catalyst
with 20 mol % lanthanum (HT-La 20) showed
the highest transesterification activity (soybean oil
conversion = 100 % and FAME yield = 95 % at
423 K in 4 h). Catalytic activity varied in proportion
with the basicity of the catalysts. While the structure
and metal composition was intact, a loss in catalytic
activity was observed, which is attributed to
deposition of the byproduct (glycerol) molecules on
the surface, making the active sites not available
to the reaction. This loss in activity was found to
be higher in reactions with methanol than that with
long-chain alcohols like n-octanol. Activity of the
used catalyst can be regained by calcining the catalyst
at higher temperatures.
Acknowledgement
JKS acknowledges the Council of Scientific and
Industrial Research (CSIR), New Delhi for the award
of Senior Research Fellowship.
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