Synthesis and characterization of a CaFe2O4 catalyst for oleic acid esterification

Article abstract of DOI:10.1039/c5ra17857f

Esterification of free fatty acid (oleic acid) with ethanol over a calcium ferrite catalyst was investigated in the present study. The calcium ferrite catalyst (CaFe₂O₄) was synthesized by the sol-gel method, which exhibited high catalytic activity for esterification of oleic acid. The morphology and size (500-1000 nm) of the synthesized catalyst were observed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) was used to ensure the absence of impurities. The orthorhombic structure of calcium ferrite was exposed by X-ray diffractometry (XRD). The effects of reaction variables such as catalyst loading, methanol to acid ratio, reaction time and temperature on the conversion of fatty acids were studied. The optimum conditions for the esterification process was a molar ratio of alcohol to oleic acid at 12:1 with 5 wt% of CaFe₂O₄ at 70 °C with a reaction time of 2 h. XRD patterns of the recycled catalyst evidenced that the catalyst structure was unchanged up to the 3^rd cycle, which indicated the long life of the catalyst.

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Synthesis and characterization of CaFe2O4 catalyst for oleic acid
esterification
Huei Ruey Ong,a Md Maksudur Rahman Khan,a,* Abu Yousuf,b Nor Amalina Hussain,a and Chin Kui
Chenga
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Esterification of free fatty acid (oleic acid) with ethanol over calcium ferrite catalyst was investigated in the present study.
Calcium ferrite catalyst (CaFe2O4) was synthesized by the sol–gel method, which exhibited high catalytic activity for
esterification of oleic acid. The morphology and size (500–1000 nm) of the synthesized catalyst were observed by scanning
electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) was studied to ensure the absence of
impurities. The orthorhombic structure of calcium ferrite was exposed by X-ray diffractometry (XRD). The effects of
reaction variables such as catalyst loading, methanol to acid ratio, reaction time and temperature on the conversion of
fatty acid were studied. The optimum condition for the esterification process was molar ratio of alcohol to oleic acid at
12:1 with 5 wt% of CaFe2O4 at 70°C with reaction time of 2 h. XRD patterns of recycled catalyst evidenced that catalyst
structure was unchanged up to 3rd cycle, which indicated the long life of catalyst.
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catalysts by heterogeneous reusable catalysts is one of the
major demands of the present society. Already a number of
heterogeneous catalysts have been developed 9, 11, 12, but most
Introduction
Long chain methyl and ethyl esters derived from vegetable oil
or animal fat, known as biodiesel, is considered as a potential
alternative of diesel fuel. Theoretically, such oils and fats
should not contain more than 1% free fatty acids (FFAs) since
of them are not practicable due to either low conversion
efficiency or higher oil to alcohol molar ratio or short life time
and requirements of high reaction time and temperature.
Srilatha et al.9 used heteropoly tungstate supported on niobia
catalysts at the conditions 25% of catalyst loading, 14:1 of
methanol to palmitic acid molar ratio , 4 h of reaction time at
65°C and they achieved ~90% conversion of palmitic acid.
Besides, tungstated zirconia 7 was used to esterify lauric acid in
a well-stirred semi-batch reactor at 130 °C at atmospheric
pressure and 85% conversion was achieved by 2 h. Another
saponification of these FFAs reduces the yield of fatty acid
1-3
alkyl esters (FAAEs) in alkaline transesterification reactions
.
Saponification is an undesirable reaction since it leads the
extra cost for separation of biodiesel from glycerin and
reduces the efficiency of the alkaline catalyst 2, 4. Practically,
higher levels of FFAs (up to 20%) were detected in waste oil,
byproducts of the refining of vegetable oils, some nonedible
13
solid acid catalyst ferric-alginate was studied by with 0.16:1
oils, animal fats and oils that are presently used as a starting
2, 5,
material for biodiesel production
ferric-alginate to lauric acid mass ratio, 16:1 methanol to lauric
acid molar ratio, 3 h and they obtained 97.7% methyl laurate.
In the advanced level of solid acid catalyst study, attention has
been focused on the porosity and particle size of the catalyst.
Though Mekala et al. 14 proposed a novel pore diffusion model
where a major conversion of acidic acid occurs inside the
catalyst pore, but the internal pore diffusion resistance was
higher compared to bulk solution. Contrary, the large
molecules have limited access to the internal pores of the
6. To efficiently utilize
these low-cost feedstocks for biodiesel synthesis, a preliminary
acid-catalyzed esterification pretreatment is necessary to
reduce their FFA contents 7.
Although liquid acids such as H2SO4, HF, H3PO4 and HCl are
often used to lower the FFA level in those oils because of their
high conversion and low cost, their usage are associated with
effluent disposal problems, loss of catalyst and high equipment
cost due to the corrosive nature of acids 8-10. The replacement
of these hazardous and polluting corrosive homogeneous acid
catalyst pellets; hence the reaction occurs at the pore
15
entrance, restricting further diffusion
. On the other hand,
catalyst with high surface area can provide highest fatty acid
methyl ester (FAME) yield 16. Therefore, nano sized catalyst
has become more attractive for the esterification of FFA. Wang
17
et al. obtained ~96% conversion of waste cooking oil using
aluminumdodecatungstophosphate (AIPW) nanotube as
catalyst at 55°C, 1:34 oil/methanol ratio, 14 h of reaction time
and 3 wt% of catalyst.
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Most of the esterification of FFA studies used methanol 1, 9 as a scanning speed 1°/min. The scanning step size 0.02° was over a
short chain alcohol, now a days ethanol has become more range of 2θ = 20-80°.
rational because of some advantages over methanol, X-ray photoelectron spectroscopy (XPS) analysis was carried
18-20
propanol, butanol and some other larger chain alcohols
.
out on a Quantum 2000 scanning ESCA microprobe system
Ethanol can be derived from agricultural products and is (Physical Electronics, Inc.). The microcrystalline structure and
renewable and biologically less objectionable in the surface characteristics of the catalyst have been investigated
environment 18
.
by using Scanning Electron Microscope, SEM (Zeiss EVO MA
In the present work, we have studied the structural and 15). Sample powder was placed on an aluminum foil with
morphological properties of calcium ferrite catalysts and double-sided carbon tape for energy dispersive x-ray
analyzed its catalytic activities in order to ascertain its spectroscopic (EDS) micro-analysis to determine the elemental
application in the biodiesel field.
compositions of the catalyst using JEOL JSM-7600, USA.
Esterification of Oleic acid with ethanol
Experimental section
Catalyst preparation
The esterification reaction of oleic acid was performed in a 500
mL three necked round bottom flask and equipped with a
reflux condenser. Oleic acid (50 mL) was added into the flask
followed by a mixture of ethanol (18.5 – 166.5 mL) and
CaFe2O4 catalyst of 1, 3, 5, 7 and 8 wt% to the weight of oleic
acid. The molar ratio of ethanol to oleic acid was 2:1, 4:1, 6:1,
8:1, 12:1 and 18:1. The reaction temperature was followed as
40, 50, 60 and 70 °C. The reaction was initiated by stirring,
The catalyst was prepared by
a sol-gel method. A
stoichiometric ratio of Ca(NO3)2•4H2O and Fe(NO3)3•9H2O
were mixed in a three-neck flask with 30 % aqueous NH3
solution, and the mixture was stirred at 200 rpm and at room
temperature for 24 h and sol was obtained21, 22. The solution
was then slowly heated to 80 °C and maintained at that typically at 250 rpm, and stopped after reaction time of 10 h
for product analysis. The product solution was centrifuged and
amount of FFA was determined by titration method (ASTM
D5555). In brief, 4–5 g of samples were dispersed in
isopropanol (75 mL) and hexane (15 mL) followed by titration
against 0.25 N NaOH solution.
temperature level until the water evaporated. The resulting
brown dry gel-like slurry was calcined at 450 °C for 2 h
followed by heat treatment at 1050 °C for 10 h using the
furnace to obtained CaFe2O4 powder. Finally, CaFe2O4 powders
were ground in the mortar. To recycle the catalyst, it was
recovered by vacuum filtration unit using nylon membrane
(pore size = 0.22 µm) followed by ethanol washing. The filter
cake was dried in an oven at 100 °C, weighted and reused in
the reaction system. Additionally, the method described by
Oliveira et al., 2010 was followed to regenerate the catalyst,
where after filtrating the catalyst was washed by n-hexane
followed by drying in an oven at 100 °C. Finally the catalyst
was calcined in a muffle furnace at 300 °C for 3 h.
Characterization of spent catalysts
To reuse the catalyst, after the first cycle, it was recovered by
simple filtration, washed with ethanol, dried in an oven at 100
ºC, weighted and inserted in the reaction system again. To
improve the activity of used catalyst, they were regenerated
by calcination process. The recycled catalyst was washed a
sequence with n-hexane, dried in an oven at 100ºC and
calcined at 300ºC for 3 h, before the catalyst was placed in the
reactor, the procedure was adopted from Oliveira et al.24. XRD
patterns of the reused catalyst were analyzed.
Characterization of the catalyst
The acidity of the solid catalyst was determined by using
Hammett indicator method. Acidic strength of the catalyst was
tested using the modified method adopted from other studies
13, 23, where 0.1 g of dried catalyst was added to a test tube
and suspended in 3–5 mL of anhydrous methanol. Then, one
drop of 0.1% Hammett indicator (methyl violet, thymol blue,
and methyl orange) was added and left to equilibrate for 2 h.
Any changes in color were noted.
Results and Discussion
Catalyst characterization
Nitrogen adsorption isotherms of the fresh catalyst and the Catalyst acidity. The color change of indicators corresponding
used catalyst (after 3rd cycle) were measured at -196 °C using to their pH range which represent the acidity of the catalyst is
an ASAP 2010 apparatus (Micromeritics). Firstly, all samples presented in Table 1. The acid strength of the catalyst lies in
were outgassed at room temperature then at 200 °C to a the range between pH 1.6 and 2.8. This proves that the
pressure of <0.2 Pa for 5 h. The specific surface area was catalyst was strongly acidic and was most suitable for
determined by the Brunauer Emmett Teller (BET) method. The esterification reaction.
pore size distributions were measured by Barrett-Joyner-
Halenda (BJH) method and the total pore volume was
calculated from the amount of N2 adsorbed up to P/P0 = 0.97.
X-ray diffraction patterns of the prepared catalyst were
collected using X-ray diffractometer (Rigaku MiniFlex II)
operated at accelerating voltage of 30 kV and emission current
of 15 mA with graphite-monochromatized Cu-Kα radiation and
2 | J. Name., 2012, 00, 1-3
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Table 1 Acidity of CaFe2O4, determined by Hammett indicator
method
Indicators
Color change
pH
Observed color
(Fresh→ after 3rd
cycle)
range
Methyl
Yellow → Red
Yellow → Red
Methyl violet Blue → Yellow 0.0 – 1.6 Blue
3.2 – 4.4 Red (<3.2)
orange
Thymol blue
1.2 – 2.8 Red (<1.2)
SEM and EDS analysis. SEM was used to exhibit the
morphology and particle sizes of the samples. SEM images of
calcined CaFe2O4 samples are shown in Fig. 1a. It can be
observed that CaFe2O4 are nearly ball like spherical particles
with the average diameter of 500 - 1000 nm. The particles are
agglomerated form cluster structure and the sizes are
relatively consisted with the broad size distribution, clearly
represented in SEM analysis.
(b)
Fig. 1 (a) SEM and (b) EDS of CaFe2O4 catalyst.
Crystallites of the CaFe2O4 samples could be consisted with the
core-shell structure. The nonuniform crystalline core-shell
structures of similar nanoparticles have recently been
reported for MgFe2O4, LiNbO3, NiFe2O4 25-27. Meanwhile, the
EDS spectrum, in Fig. 1b reveals that the CaFe2O4 spherical
particles are mostly consisted with the higher purity of Ca, Fe,
and O elements. In addition concentration and annealed
temperature are significantly dominant to obtain the CaFe2O4.
The signal of O may be from contributions, first it is from the
oxygen absorbed on the surface of the sample and second is
from the sample itself.
BET analysis
0.80
0.75
0.70
0.65
0.60
0.55
Catalyst after 3rd cycle
0.50
0.45
0.40
0.35
Fresh Catalyst
0.30
0.25
0.20
0.15
0.10
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
P/P
0
Fig. 2 Linearized BET plot for fresh CaFe2O4 and after 3rd cycle
The specific surface area of the catalyst was determined by the
modelling of N2 adsorption data with BET isotherm. The
linearized BET plot is presented in Fig 2. The obtained BET
surface area, average pore diameter and specific pore volume
for the fresh catalyst, were 2.48 m2 g-1 , 96.11 Å, and 0.0078
cm3 g-1 , respectively. The recycled catalyst after 3rd cycle
showed the BET surface area, average pore diameter and
specific pore volume as 1.97 m2 g-1, 91.18 Å, and 0.0063 cm3
g-1, respectively. Around 20% decrease in the surface area in
the recycled catalyst might be due to the blockage of the pores
during the reaction and regeneration process.
(a)
XRD analysis. Crystalline structures of the CaFe2O4 particles
could be clearly understood through the prominent use of XRD
measurement. The sharp peaks from diffraction patterns show
the crystalline nature of the samples. The XRD pattern of the
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synthesized calcium ferrite powder as shown in Fig. 3 can
match well with the documented XRD data of CaFe2O4 (JCPDS
card No. 32-0168), which indicated that well crystallized
catalyst has been obtained. A typical XRD pattern of CaFe2O4
displays peaks at 2θ values of 28.68°, 32.4°, 47.68°, 56.12°,
58.96°, 69.36°, 76.56°, and 88.21° similar to those major XRD
2800
2400
2000
1600
peaks of CaFe2O4 structure as reported in the literature 28
.
The diffraction peaks could be indexed to (220), (320), (040),
(201), (121), (131), (311), (401), (170) and (322) planes. It is
observed that a few major peaks of CaFe2O4—(040), (320),
(201), (121), (241), (401) and (170) overlaps. Based on the
analysis, the crystal structure of the mechanosynthesised
product is found to be orthorhombic with the unit cell
parameters a = 9.214(3)Å , b = 10.686(3) Å and c = 3.004(4) Å.
It should be noted that these unit cell parameters are smaller
to those reported for bulk CaFe2O4, (a = 9.230 Å, b = 10.705 Å,
540
535
530
525
520
Binding energy, eV
(a)
c = 3.024 Å) 29
.
8000
7000
6000
5000
4000
3000
2000
1000
CaFe2O4
Fresh sample
After 1st run
After 2nd run
After 3rd run
80
0
20
30
40
50
60
70
2
θ,degree
(b)
1400
Fig. 3 XRD patterns for the CaFe2O4 powder.
1300
1200
1100
1000
900
XPS analysis. Chemical composition of the CaFe2O4 particles
was investigated by XPS analysis illustrated in the Fig. 4. It is
perceived that oxygen on the sample surface exists at least in
two specific forms with the following binding energies: 529.74
and 531.86 eV, as shown in Fig. 4a. The peak at 529.74 eV is
mainly assigned to the oxygen in the sample lattice to the
formation of CaFe2O4. On the other hand, the peak at 531.86
eV corresponds to oxygen in the sample surface adsorption of
(–OH).
Fig. 4 b shows the deconvoluted XPS spectrum of the two
360
355
350
345
340
335
significant valence states of Fe in the Fe 2p region. The peaks
Binding energy,eV
at the binding energies of 710.55, 711.0 and 724.25 eV are
attributed 30-34 to Fe3+. This result contributes to the chemical
existence of the formation of Fe based oxide material sample
(c)
Fig. 4 High-resolution XPS spectra of (a) O 1s, (b) Fe 2p, and (c)
Ca 2p.
structure. The high resolution Ca 2p region spectra were
deconvoluted into four peaks of the Fig. 4c, where, the peaks
of 345.73 and 349.17 eV can be attributed to Ca 2p1/2 and
consequently, 347.66 and 351.05 eV are represented as Ca
Catalytic activity
The initial catalytic activity of CaFe2O4 was slower than that of
a homogeneous sulfuric acid catalyst up to 3 h. Fig. 5
illustrated that the rate of oleic acid conversion by both
catalysts sulfuric acid and CaFe2O4 became almost same after 4
2p3/2. The peak located at 529.74 eV for O 1s in the sample
reflects to the formation of CaFe2O4 structure. Additionally,
the formation of CaFe2O4 materials is also substantiated by
EDS and XRD analysis.
4 | J. Name., 2012, 00, 1-3
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h. The maximum conversion (96%) was achieved by 4 h and
there was no significant change with an extended time of
reaction. Therefore, it would be very beneficial to replace the
hazardous sulfuric acid catalyst by recyclable solid catalyst
CaFe2O4. Moreover, it showed better conversion efficiency
under favourable conditions compared to other solid catalysts
(Table 2).
Fig. 6 Conversion of the oleic acid with diverse catalyst doses
(reaction temperature, 70°C, ethanol/oleic acid mole ratio
12:1, reaction time 4 h).
Effect of ethanol/ oleic acid molar ratio
Fig. 7 shows the effect of various ethanol/oleic acid molar
ratios (6:1, 12:1, 18:1) on the ethyl ester conversion at 5 wt%
of CaFe2O4 catalyst and 70°C reaction temperature. As seen in
Fig. 7, the ethyl ester conversion and the degree of the ethyl
ester conversion depend largely upon ethanol/oleic acid molar
ratio. The reaction was faster at the initial phase and reached a
higher final conversion after 4 h and then it became constant.
The conversion was increased from the ratio of 6:1 to 12:1 and
it was unaffected for further increase to 18:1. The low yield at
the feed ratio of 6:1 might be due to an insufficient quantity of
ethanol for nucleophilic attack on the Brønsted acid sites of
the catalyst 11. On the other hand, higher feed ratio did not
increase the yield percentage and it might happen because of
the production of water preferentially drives the hydrolysis of
Fig. 5 Catalytic activity of CaFe2O4 solid catalyst, sulphuric acid
and noncatalytic (reaction temperature, 70 °C, mass amount of
catalyst 5 wt%, ethanol/oleic acid mole ratio 12:1)
catalyzed ● H2SO4 catalyzed CaFe2O4 catalyzed.
■ non-
▲
Effect of catalyst concentration
Fig. 6 shows the effect of catalyst doses and it was examined
by dosing 1, 3, 5, 7 and 8 wt% of CaFe2O4 to oleic acid
maintaining the reaction temperature 70°C, ethanol/oleic acid
mole ratio 12:1 and a reaction time 4 h. Using 1 wt% of
catalyst, 70% conversion was achieved and the conversion
ratio increased with the increasing catalyst amount, which
could be attributed to the reason that more CaFe2O4 catalyst
would provide more active reaction sites. It was clear that the
amount of catalyst had a positive effect on the conversion
ratio of oleic acid, and the conversion ratio (93%) became
constant with catalyst amount above 5 wt%. Excess amount of
catalyst may form emulsion which increased the viscosity and
led to the formation of gels. The formation of emulsion will
FAME into oleic acid 36
.
therefore block the reaction 35
.
Fig. 7 Conversion of the oleic acid with diverse ethanol/oleic
acid molar ratio (reaction temperature 70 °C, mass amount of
catalyst 5 wt%)
■ 6:1 ● 12:1 ▲ 18:1.
Effect of reaction temperature
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The heterogeneous catalyzed esterification reaction is strongly
influenced by the reaction temperature. The effect of the
reaction temperature was studied from 40 to 70°C with
ethanol/oleic acid molar ratio 12:1 and 5 wt. % of CaFe2O4
catalyst to oleic acid as shown in Fig. 8. The conversion rate of
oleic acid was faster up to 1 h for all the batches, i.e., at 50°C ,
60°C and 70°C. Subsequently, the conversion reached to the
steady state. However, the conversion rate was increased with
the increase of temperature and the value was 95% for 70°C.
Fig. 8 also exhibits that the initial reaction rate was high, and
then the reaction rate decreased. Fig 8 displays that the initial
reaction rate was high for all temperature ranges followed by a
decrease in reaction rate leading to a plateau. In the initial
reaction periods, the reaction mixture was free from water,
which subsequently produced as the reaction progressed
Fig. 8 Conversion of the oleic acid with diverse temperature on
oleic acid conversion (mass concentration of catalyst 5 wt%,
causing the backward reaction to occur. 37
.
Initial reaction rate increased gradually with the increase of
temperature (Table inside the Fig. 8). The reaction rate
constant (k) for the esterification reaction of the oleic acid was
determined by fitting the experimental data to the first-order
reaction kinetic model. The rate constants were
calculated as 0.0077, 0.0116, 0.0148 and 0.0178 s-1
for 40, 50, 60 and 70°C respectively. The activation
energy was estimated as 24.60 kJ mole-1 from
Arrhenius plot shown in the inset of Fig. 8. The
smooth increase in rate constant with increase of temperature
suggests that the mass transfer steps were not the rate
limiting factor.
■
▲ ▼
60°C
ethanol/oleic acid mole ration 12:1) 40°C ● 50°C
70°C, Inset the Figure: Arrhenius plot and Table of initial
reaction rates.
Catalyst reactivation and recyclability
To recycle the catalyst, it was recovered by simple filtration,
washed with ethanol, dried in an oven at 100 °C, weighted and
inserted in the reaction system again. Fig. 9 shows the activity
of the calcinated and uncalcinated catalysts during multi-cycle
uses. Without calcination, the conversion drops from 96 to
76% after the first cycle and at the fourth cycle it was about
65% (Fig. 9: column Y). To improve the activity of used catalyst,
they were regenerated by calcination process. A better
performance was obtained with a sequence of washing with n-
hexane, drying in an oven at 100ºC and calcining at 300ºC for 3
h
20. In this case, the conversion was improved 13% after the
first cycle and about 9% after the second cycle, though the
improvement was insignificant after the third cycle (Fig. 9:
column X). On the other hand, XRD patterns (Fig. 3) indicated
that the synthesized particles were mainly composed of
CaFe2O4. The fresh CaFe2O4 and used CaFe2O4 catalysts had
well-crystallized structures with characteristic and symmetric
reflections. Fresh CaFe2O4 and recycled CaFe2O4 (1st to 3rd
cycles) catalysts had the same patterns, indicating that the
CaFe2O4 catalyst was stable.
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Table 2 Esterification conditions of oleic acid by recently developed solid catalysts.
Acid catalyst
Temperature,
Reaction
time, h
3.6
Alcohol:
oleic acid
22:1
Catalyst
Oleic
acid Reference
°C
doses, wt.%
conversion, %
4
1-butyl-3-methylimidazolium
tetrachloroferrite
([BMIM][FeCl4])
65
-
83.4
20
37
H3PW/ZrO2
100
4
6:1
20
88
Organophosphonic
acid- 112
10
8.8:1
14.5
77.02
functionalized silica SG–T–P
38
39
Sulfonated cation exchange 82
8
3
4
9:1
20g
1.33
5
93
60
96
resin
Sulfonated carbons Starbons- 80
300
10:1
12:1
CaFe2O4
70
Present
study
Acknowledgements
Authors gratefully acknowledge the Faculty of Chemical and
natural resources Engineering, University Malaysia Pahang for
funding to carry out this work by a research grant (Project No.:
RDU100395).
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Fig. 9 Conversion of the oleic acid with recycled catalyst
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