Interesterification of triglycerides with methyl acetate for the co-production biodiesel and triacetin using hydrotalcite as a heterogenous base catalyst

Article abstract of DOI:10.1016/j.cattod.2020.07.056

Biodiesel is a renewable fuel with several advantages over petro-based fuels and is conventionally produced by the transesterification of triglycerides with alcohols. Chemical interesterification of triglycerides is a good alternative as compared to transesterification, as it yields a value-added compound, triacetin, as a co-product instead of the already abundant commodity, glycerol. In this work, interesterification of soybean oil was carried out with methyl acetate to co-produce biodiesel and triacetin, using calcined hydrotalcite as a heterogeneous base catalyst. Several base catalysts were tested for this reaction. Calcined Mg-Al hydrotalcite (Mg:Al mole ratio – 3:1) was found as the best catalyst for achieving 95.9 % conversion of soybean oil triglycerides in 4 h and yielding 5.3 % selectivity of triacetin along with the value-added intermediates - monoacetindiglycerides and diacetinmonoglycerides, produced with a selectivity of 46 % and 48.7 %, respectively. The optimum reaction conditions were found at an oil to methyl acetate mole ratio of 1:50, catalyst loading of 0.04 g/cm³ and a temperature of 200 ^oC. The catalysts were fully characterized before and after the reaction, and were found to possess high surface area with both basic and acidic sites. The effect of various reaction conditions on the rate of reaction, conversion of triglycerides and selectivity of triacetin were studied. The concentration profiles of the reactants, intermediates and products obtained at different temperatures were used to obtain the kinetic rate constants and the activation energy of each parallel step was evaluated. Reusability studies showed that the catalyst was stable and reusable up to three cycles.

Full text of DOI:10.1016/j.cattod.2020.07.056

Journal Pre-proof
Interesterification of triglycerides with methyl acetate for the
co-production biodiesel and triacetin using hydrotalcite as a
heterogenous base catalyst
Manali S. Dhawan, Scott Calabrese Barton, Ganapati D. Yadav
PII:
S0920-5861(20)30531-9
DOI:
https://doi.org/10.1016/j.cattod.2020.07.056
CATTOD 13063
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Interesterification of triglycerides with methyl acetate for the co-production
biodiesel and triacetin using hydrotalcite as a heterogenous base catalyst
Manali S. Dhawan1,2, Scott Calabrese Barton2, Ganapati D. Yadav1*
1Department of Chemical Engineering
Institute of Chemical Technology
Nathalal Parekh Marg, Matunga, Mumbai-400 019, India
and
2Department of Chemical Engineering and Materials Science
Michigan State University
East Lansing, MI 48824
USA
Email : gdyadav@yahoo.com, gd.yadav@ictmumbai.edu.in
Tel.: +91-22-3361-1001, Fax: +91-22-3361-1020
*Author to whom correspondence should be addressed
Graphical abstract
1

Highlights
 Co-production of value-added products - biodiesel and triacetin
 Calcined Mg-Al hydrotalcite (Mg:Al – 3:1) as a reusable base catalyst
 Characterization and activity screening of various base catalysts
 Kinetic model of the interesterification reaction
 Efficient and green process
Abstract
2

Biodiesel is a renewable fuel with several advantages over petro-based fuels and is
conventionally produced by the transesterification of triglycerides with alcohols. Chemical
interesterification of triglycerides is a good alternative as compared to transesterification, as it
yields a value-added compound, triacetin, as a co-product instead of the already abundant
commodity, glycerol. In this work, interesterification of soybean oil was carried out with methyl
acetate to co-produce biodiesel and triacetin, using calcined hydrotalcite as a heterogeneous base
catalyst. Several base catalysts were tested for this reaction. Calcined Mg-Al hydrotalcite (Mg:Al
mole ratio – 3:1) was found as the best catalyst for achieving 95.5 % conversion of soybean oil
triglycerides in 4 h and yielding 5.3 % selectivity of triacetin along with the value-added
intermediates - monoacetindiglycerides and diacetinmonoglycerides, produced with a selectivity
of 46 % and 48.7 %, respectively. The optimum reaction conditions were found at an oil to
methyl acetate mole ratio of 1:50, catalyst loading of 0.04 g/cm3 and a temperature of 200 ºC.
The catalysts were fully characterized before and after the reaction, and were found to possess
high surface area with both basic and acidic sites. The effect of various reaction conditions on
the rate of reaction, conversion of triglycerides and selectivity of triacetin were studied. The
concentration profiles of the reactants, intermediates and products obtained at different
temperatures were used to obtain the kinetic rate constants and the activation energy of each
parallel step was evaluated. Reusability studies showed that the catalyst was stable and reusable
up to three cycles.
Keywords: Biodiesel; triglycerides; triacetin; hydrotalcite; interesterification; kinetics.
3

1. Introduction
In recent years, there has been great interest in the development of renewable fuels due to the
depletion of fossils fuels, increasing energy demand and environmental pollution [1,2]. Biodiesel
(fatty acid methyl esters or FAME) is one such fuel which possesses superior properties
compared to petroleum-based fuels, such as biodegradability, low harmful emissions, high flash
point and cetane number, etc. [3,4]. Biodiesel is conventionally produced by transesterification
of triglycerides with methanol using acid or base chemical catalysts or enzymes [5–7]. This
process is accompanied by the production of 10 % w/w crude glycerol as a co-product, which is
usually handled as a waste, thereby reducing the process efficiency [8–11]. Glycerol can be
employed for varied applications in numerous fields, but this requires further rigorous
purification procedures [10]. Also, the production of glycerol exceeds the current demand for
chemical production [11]. The efficiency of the biodiesel production process can be increased if
glycerol is not formed as a co-product.
One of the possibilities to utilize glycerol is to convert it into oxygenated derivatives such as
glycerol esters, namely, glycerol triacetate (triacetin), glycerol diacetate (diacetin), or glycerol
monoacetate (monoacetin) [12]. Triacetin finds application as a plasticizer and gelatinizing agent
in polymers and explosives and also as an additive in cosmetics and pharmaceutical industries
[13,14]. It has also been recognized that triacetin can be successfully used as valuable fuel
additive to biodiesel up-to 10 wt. %. The final fuel mixture thus obtained possesses improved
cold, combustion and viscosity properties, and also meets the ASTM D6451 and EN 14214
quality standards [15]. These aforementioned fuel additives are generally produced by
esterification and transesterification of glycerol with acetic acid and methyl acetate, respectively
[12,16,17]. However, these procedures require the isolation and purification of glycerol after
biodiesel synthesis. As a result, these two consecutive processes are being carried out
independently in two separate reactors.
There exists a possibility of combining the two reactions in a single one-pot reaction by
conducting interesterification reaction of triglycerides with methyl acetate, where triacetin is
formed as a valuable co-product with FAME instead of glycerol. Interesterification is highly
reversible and leads to the presence of value-added intermediate compounds,
4

monoacetindiglycerides and diacetinmonoglycerides, in high quantities in the reaction mixture
[18]. Their separation using vacuum distillation has been investigated in the literature [19,20].
This reaction has been mostly studied using chemical homogenous catalysts, enzymes or under
supercritical conditions. Lipases have been extensively studied to successfully catalyze the
interesterification of triglycerides giving nearly quantitative yield of FAME with minimum loss
in activity upon reuse. However, the quantitative production of triacetin was not explored in
detail. Enzyme catalyzed reactions offer the advantages of conducting reactions at mild
conditions and catalyst reusability but fail to achieve higher yields in short reaction times
[14,21–24]. There are reports on the co-production of biodiesel and triacetin under supercritical
conditions, but high reaction temperatures and pressures are required to achieve high yields,
which reduces the scalability of the process [25–28]. Casas et al. explored the chemical
interesterification of triglycerides with methyl acetate using potassium methoxide as a
homogeneous catalyst. This reaction can absolutely be achieved with milder reaction conditions
and lower operating cost, but it suffered lower yields due to insolubility of the catalyst in the
reaction system and high reversibility of the reaction. In a further study to solve the solubility
issues, they studied the same reaction using methanolic potassium methoxide as a catalyst, which
increased the reaction rate due to homogeneity of the process [29]. However, enhanced side
reactions were observed due to the presence of methanol, leading to the production of
monoacetin, diacetin and glycerol as side products [30]. There has been a study on the
interesterification reaction of triglycerides using tert-butoxide in tert-butanol as a catalyst which
reduced the side reactions, thereby achieving 73.2 wt. % and 16.6 wt. % content of biodiesel and
triacetin, respectively [31].
We aimed to use a heterogeneous base catalyst for the interesterification reaction which can
achieve good conversion of triglycerides and high selectivity of intermediates and products. In
our previous work, we carried out simultaneous co-production of biodiesel and glycerol
carbonate using calcined hydrotalcite as a catalyst and achieved good conversions [32]. Thus, in
the present work, we intended to use the calcined hydrotalcite as a heterogeneous base catalyst to
achieve the interesterification of triglycerides with methyl acetate to co-produce biodiesel and
triacetin. Calcination of Mg-Al hydrotalcite is well known to achieve a mixed oxide that
possesses Lewis basicity [33–35]. The effect of different Mg to Al mole ratios in the calcined
5

Mg-Al hydrotalcite catalyst was studied. These calcined Mg-Al hydrotalcites were screened with
their respective metal oxides (MgO, Al2O3) to assess their individual effect on the reaction. The
activity of calcined Mg-Al hydrotalcite supported on hexagonal mesoporous silica (CHT-HMS)
was also screened for this reaction. Additionally, it was thought to study the effect of Ca-Al
hydrotalcite on the interesterification reaction as it has served good activity for the
transesterification of vegetable oil in the literature [36–38]. The reaction kinetics was derived on
the basis of the mean concentration profile of the reactant, product and intermediate species. The
virgin and the reused catalysts were fully characterized.
2. Experimental section
2.1. Chemicals
Refined soybean oil was obtained from a local market. The average molecular weight of soybean
oil was 874 g/mol and it was of the same fatty acid composition as reported in our previous study
[32]. The following chemicals were obtained from recognized vendors and used as received:
methyl acetate, sodium hydroxide, sodium carbonate (all from S.D. Fine Chemicals Ltd,
Mumbai), magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, calcium nitrate
tetrahydrate, triacetin, methanol HPLC grade, isopropyl alcohol HPLC grade, n-hexane HPLC
grade (all from Thomas Baker, Mumbai), methyl palmitate (97%), methyl oleate (96%), methyl
stearate (99%), methyl linoleate (all from Alfa Aesar, Mumbai), hexadecyl amine (Otto Chemie
Pvt. Ltd., Mumbai), tetraethyl orthosilicate (Sigma–Aldrich, USA).
2.2. Catalyst synthesis
2.2.1. Preparation of calcined Mg-Al Hydrotalcite (Mg-Al CHT)
Hydrotalcite of varying Mg-Al ratios (1:1, 2:1, 3:1) was synthesized by a co-precipitation
method. A solution containing 3 - x moles of magnesium nitrate hexahydrate and x moles of
aluminium nitrate nonahydrate for a ratio x/3 = 0.25, 0.33, 0.5 was dissolved in 150 mL distilled
water. Solution of 6 + x moles of sodium hydroxide and 2 mol of anhydrous sodium carbonate
was prepared in 150 mL distilled water. Both the solutions were co-added under mechanical
stirring (speed of agitation = 600 rpm) to the round bottom flask (500 mL) maintained at a
temperature of 30˚C. The pH of the solution was between 9 and 10. The white precipitate of
hydrotalcite obtained was digested at 60˚C for 12 h. Hydrotalcite was washed with distilled
6

water until neutral pH and then dried at 100˚C for 24 h. Dried hydrotalcite was crushed to fine
powder and calcined at 500 ˚C for 6 h to obtain calcined Mg-Al hydrotalcite (Mg-Al CHT)
[34,39,40].
2.2.2. Preparation of calcined Ca-Al Hydrotalcite (Ca-Al HT)
Ca-Al HT of mole ratio 3:1 was prepared by co-precipitation. A solution of calcium nitrate
tetrahydrate (0.09 mol) and aluminium nitrate nonahydrate (0.03 mol) was prepared in 150 mL
distilled water. This solution was co-precipitated by co-adding it with a solution of sodium
hydroxide (0.18 mol) and anhydrous sodium carbonate (0.06 mol) prepared in 150 mL distilled
water. The rest of the procedure was followed in a similar manner as described above for Mg-Al
hydrotalcite. The Ca-Al HT catalyst was calcined at 500 ˚C for 6 h.
2.2.3. Preparation of 10 % w/w Mg-Al CHT supported on hexagonal mesoporous silica
(CHT-HMS)
HMS was prepared by a protocol reported in literature [34,40,41]. CHT-HMS of Mg-Al mole
ratio 3:1 was synthesized by co-precipitation. A solution containing magnesium nitrate
hexahydrate (0.009 mol) and aluminium nitrate nonahydrate (0.003 mol), and a solution of
sodium hydroxide (0.027 mol) and anhydrous sodium carbonate (0.008 mol) were each prepared
separately in 50 mL distilled water. To 5 g of HMS, the above solutions were added
simultaneously under mechanical stirring (600 rpm) in an oil bath maintained at 30˚C. The rest
of the procedure was followed in a similar manner as described above for Mg-Al hydrotalcite.
The fine powder was calcined at 500 ˚C for 6 h to obtain 10 % w/w CHT-HMS.
2.2.4. Preparation of MgO and Al2O3
MgO and Al2O3 were synthesized by co-precipitation procedure. A 150 ml solution of
magnesium nitrate hexahydrate (0.5 M) was co-added with a 150 ml solution of sodium
hydroxide (0.5 M) in a round bottom flask under mechanical stirring to obtain white precipitates
of Mg(OH)2. Similarly, a solution of aluminium nitrate nonahydrate (0.5 M) was co-added to the
same concentration of sodium hydroxide solution to obtain Al(OH)3. The neutralization,
washing, filtration and drying procedures were carried out in the same way as described above
(Section 2.2.1). The calcination was performed at 500 ˚C for 6 h to get MgO and Al2O3 catalyst.
7

2.3. Catalyst Characterization techniques
The fresh and reused catalyst was characterized by various techniques such as TPD, XRD, FT-
IR, SEM, N2 adsorption-desorption for BET surface area and DSC-TGA. The characterization
protocol is mentioned in ESI and can also be found in literature [43,44].
2.4.Reaction Procedure
The interesterification reactions were performed in a 100 cm3 high pressure autoclave (Amar
Equipments Pvt. Ltd., Mumbai) equipped with pitched bladed turbine impellor, temperature
controller and pressure indicator. Soybean oil triglycerides acted as a limiting reagent. Methyl
acetate was utilized as reactant as well as solvent. The control reaction was conducted using
0.008 mol soybean oil, 0.4 mol methyl acetate (1:50 molar ratio) (total volume = 40 cm3) and
catalyst loading 0.04 g/cm3 (1.6 g) at an agitation speed of 1000 rpm at 200 °C. The samples
were collected periodically using sampling valve till 4 h and the catalyst particles present in the
sample were removed by filtration.
2.5. Reaction Analysis
The conversion of soybean oil triglycerides and selectivity of intermediates as well as triacetin
was analyzed by HPLC (Agilent 1200 Infinity series) using Agilent C18 reverse phase column
(250 mm x 4.6 mm, 5 µm) with gradient elution of methanol and isopropyl alcohol-hexane (5:4,
v/v), as reported in our previous study [32] and also in literature [42]. The compounds were
analyzed using a UV detector at a wavelength 205 nm with a column temperature of 40 ºC,
injection volume of 10 µL, phase flow rate of 1.0 ml min-1 The details of the analytical
.
methodology (Table S1) and chromatogram of reaction mixture (Figure S1) are given in
supplementary information (ESI). Product confirmation was done by GC-MS (Thermo Scientific
Trace 1300 Gas Chromatograph equipped with an ISQL LT single quadrupole Mass
Spectrometer) using TG-5MS capillary column (150 mm x 0.25 mm, 0.25 µm) (Figure S2-S7).
The conversion calculations were done using soybean oil triglycerides (T) as the limiting
reactant (1). The total area under the peaks of soybean oil triglyceride, monoacetindiglycerides
(MA), diacetinmonoglycerides (DA) were used to calculate conversion and selectivity. The
differential rate equations were used to derive the total mole balance of the three consecutive
interesterification reactions. The mole balance equations and the selectivity based on area of
8

MA, DA and triacetin were used to get the concentrations of these species. The details of this
approach are mentioned in the ESI. The selectivity of triacetin was calculated by taking into
account MA, DA and triacetin (TA) (2).
0
푡
퐶
푇
−퐶
푇
퐶표푛푣푒푟푠푖표푛 =
(1)
0
퐶
푇
푡
퐶
푇퐴
푆푒푙푒푐푡푖푣푖푡푦 (푇푟푖푎푐푒푡푖푛) =
(2)
푡
푡
푡
퐶
푀퐴
+ 퐶
+ 퐶
퐷퐴 푇퐴
푡
푡
where, 퐶푇0 is the initial concentration of triglyceride and 퐶푇 , 퐶푀퐴푡, 퐶퐷퐴푡, 퐶푇퐴 are the
concentrations of the triglyceride, MA, DA and TA at any time, ‘t’.
3. Results and discussions
3.1. Catalyst characterization
3.1.1. TPD
The different Mg:Al mole ratio in the Mg-Al CHT was suspected to have the most effect on the
basicity and acidity profiles of the catalyst. Hence, TPD analysis was done for all the catalysts to
determine acidity and basicity using NH3 and CO2 as probe molecules, respectively. TPD
analysis of all the catalysts confirmed the presence of both weak basic and acidic sites (Figure 1
and 2). The basicity and acidity of Ca-Al HT and CHT-HMS, despite containing Ca and Mg to
Al in the ratio 3:1 respectively, was found to be similar to Mg-Al CHT (1:1). This means that the
basic and acidic sites of Mg-Al CHT in CHT-HMS were masked by neutral HMS. Also, Ca-Al
HT (3:1) imparts lower basicity to hydrotalcite as compared to Mg-Al CHT (3:1). The values of
the concentration of these sites are given in Table 1. It was found that with the increase in Mg-Al
mole ratio, the basicity of the Mg-Al CHT catalyst increased. Mg-Al CHT (3:1) possessed the
highest basicity. The spent catalyst showed a negligible loss in the basic and acidic sites after
reuse confirming the stability of the catalyst.
3.1.2. XRD
The XRD patterns of all the catalysts are shown in Figure 3. XRD pattern of Ca-Al HT showed a
very small peak at low ϴ value (23.2°) which correspond to the conversion of the layered
structure of Ca-Al layered double hydroxide to Ca-Al mixed oxide, and formation of amorphous
phase (JCPDS 890460). The sharp, symmetric peak at 29.6° and broad peaks at high ϴ values
9

(35-65°) indicate the formation of a crystalline calcite phase (CaCO3), which might have resulted
from the non-uniform distribution of Ca and Al cations, resulting in Ca2+ ions agglomeration
(JCPDS 72-1652). This is in accordance with the XRD results obtained by Cortez et al. [36]. 10
% w/w CHT-HMS showed a strong reflection at 21.2°, corresponding to that of amorphous SiO2
obtained after calcination of HMS. The Mg-Al particles on the surface/pores of HMS might be
too small, and thus no peak of the oxides was obtained in the XRD pattern of CHT-HMS. The
XRD pattern of Mg-Al CHT (3:1) depicted characteristic peaks of the Mg-Al oxide at 43° and
62.4° corresponding to (200) and (220) planes (JCPDS 45-0946). This shows that after
calcination, the layered double hydroxides were transformed to Mg-Al mixed oxides and the
intensity of peaks signify amorphous material. The crystallite size of Mg-Al CHT (3:1) was
calculated as 11.3 nm using the Scherrer equation. It increased to 16.6 nm after reuse. Reused
Mg-Al CHT (3:1) showed similar reflections as Mg-Al CHT (3:1) indicating the fidelity of the
catalyst.
3.1.3. FT-IR
FTIR patterns of all the catalysts are depicted in Figure 4. The FTIR spectra of Ca-Al HT
showed the presence of carbonate ions at around 1420 cm-1 and metal oxides vibrations at 874
cm-1 and 711 cm-1. The FTIR spectra of CHT-HMS showed broad band at 1069 cm-1
corresponding to Si-O-Si asymmetric stretching vibration, Si-O-Si symmetric vibration at 794
cm-1, O-Si-O bending vibrations at 470 cm-1, and a slight presence of water showing –OH
stretching and bending vibration at 3438 cm-1 and 1639 cm-1. FTIR analysis of Mg-Al CHT (3:1)
showed the presence of Mg-O bands at 426 cm-1 and Al-O at 621 cm-1. The bands of carbonates
at 1250-1500 cm-1 were of decreased intensity as a result of their loss during calcination [33].
Reused Mg-Al CHT (3:1) showed a similar FTIR pattern as that of virgin Mg-Al CHT (3:1)
along with a slight presence of H2O molecules, depicted by their stretching vibration at 3457 cm-
1 and bending vibration at 1633 cm-1. This shows that the reused catalyst retains its structure even
after reuse, which signifies the stability of the catalyst.
3.1.4. Surface area analysis
The BET surface area, pore volume and pore size of all the catalysts measured by nitrogen
adsorption-desorption method are given in Table 2. Ca-Al HT showed a type III isotherm,
characteristic of a nonporous or macroporous solid and a little H4 type of hysteresis which
10

indicated narrow slit-like pores and particles with internal void. CHT-HMS, virgin and reused
Mg-Al CHT (3:1) showed a type IV isotherm signifying a mesoporous solid but with different
types of hysteresis (Figure 5). CHT-HMS exhibited H3 type of hysteresis indicating slit-shaped
pores and plate-like particles. The low total specific surface area of CHT-HMS after loading of
Mg-Al CHT on high specific surface area HMS could be due to the blockage of the pores in
HMS, which reduces the accessibility of N2 into the pore structure. Similar findings have been
reported after loading Mg-Al CHT on HMS [41,45]. Mg-Al CHT (3:1) showed the highest
surface area with a H1 type hysteresis loop, corresponding to well defined cylindrical pores open
at both ends with good connectivity (according to IUPAC classification). Reused Mg-Al CHT
(3:1) was found to be stable as it showed similar textural features with small losses after third re-
use.
3.1.5. SEM
SEM images of the catalysts are depicted in Figure 6. The virgin as well as the reused catalyst
show agglomerated particles with irregular morphology.
3.1.6. EDAX
The composition of the elements of virgin and reused Mg-Al CHT (3:1) catalyst was determined
by EDAX and it showed the presence of Mg and Al in the same mole ratio as that taken
theoretically for the preparation of the catalyst (Table 3).
3.1.7. DSC-TGA
Thermal stability of the samples was determined by DSC-TGA analysis of the catalysts as given
in Figure 7 and 8. DSC curve of both the catalysts showed an endothermic peak at around 90 °C,
which is due to the loss of surface adsorbed water and decomposition of carbonates, thereby
contributing to the weight loss. The TGA analysis of Mg-Al CHT (3:1) showed a total weight
loss of 4.1 %, which meant that the catalyst was stable at higher temperature. Reused catalyst
also showed nearly similar weight loss (7.5 %) as that of fresh catalyst.
3.2.Interesterification of soybean oil triglycerides with methyl acetate
Scheme 1 depicts the transesterification reaction of soybean oil triglycerides (T) with methyl
acetate, which consists of three steps. Fatty acid methyl esters (F) are formed in each consecutive
step of the reaction. Intermediates – monoacetindiglycerides (MA) and diacetinmonoglycerides
(DA) are formed in step 1 and step 2, respectively, followed by formation of triacetin (TA) in
step 3. The rate constants of each reversible step are mentioned on the arrow where k1 is the rate
11

constant of first step for conversion of T to MA and k1´ is the rate constant of the reverse
reaction, and so on. The effect of various reaction parameters on the conversion of triglycerides
and selectivity of triacetin were studied and plotted as a function of time.
O
O
O
R
R
O
O
O
O
O
O
O
O
k1
k1
(Step 1)
R
CH3
O
H3C
H3C
O
F
R
O
O
R
R
+
+
+
+
+
+
'
O
H3C
M
T
MA
O
O
R
O
O
R
O
O
O
O
O
k2
H3C
CH3
O
H3C
H3C
O
F
R
O
O
(Step 2)
'
O
k2
R
CH3
O
O
H3C
M
MA
DA
O
O
CH3
O
R
O
O
O
O
O
(Step 3)
O
k3
H3C
CH3
O
H3C
H3C
O
F
R
'
O
H3C
CH3
k3
O
O
O
H3C
M
O
DA
TA
Scheme 1: Consecutive reactions in interesterification of soybean oil triglycerides with
methyl acetate
3.2.1. Efficacy of various catalysts
The interesterification reaction was studied using various base catalysts such as MgO, Alumina,
Ca:Al HT (3:1), CHT-HMS (Mg:Al=3:1), Mg-Al CHT (3:1), Mg-l CHT (2:1) and Mg-Al CHT
(1:1). The reactions were performed for 4 h with the conditions maintained at oil to methyl
acetate mole ratio of 1:50 (total volume = 40 cm3) at 200 oC with agitation speed of 1000 rpm
12

and catalyst loading of 0.04 g/cm3. All the aforementioned catalysts had remarkable activity
towards the conversion of soybean oil triglycerides but with varied selectivities towards triacetin
(Figure 9). MgO showed good activity towards the conversion of triglycerides (85.4 %), but the
selectivity of triacetin was comparatively low (2.4 %). However, the activity of alumina towards
the interesterification reaction was tremendously low with a conversion of 70.5 % and selectivity
of triacetin as 1 %. This shows that both acidic and basic sites play a role in the reaction to favor
the conversion of intermediates to triacetin. Thus, Mg-Al hydrotalcite was a good choice. CHT-
HMS showed less activity towards the reaction (conversion: 92.7 %, selectivity of triacetin: 2.8
%) which might be due to the hinderance of basic sites by HMS, as shown by XRD analysis
(Figure 3). The activity of the Mg-Al CHT catalysts with different Mg:Al mole ratios was close
to each other in terms of conversion with varying selectivities towards triacetin, in the following
order: Mg-Al CHT (3:1) (95.9 %, 5.3 %) > Mg-Al CHT (2:1) (92.2 %, 4.2 %) > Mg-Al CHT
(1:1) (89.7 %, 3.9 %). Ca:Al HT also showed appreciable activity, with 92.5 % conversion and
4.3 % selectivity of triacetin. In all cases it was observed that, higher conversion of intermediate
MA to DA, favored high selectivity to triacetin (TA). Mg-Al CHT (3:1) was found to have the
highest activity for the conversion of soybean oil triglycerides and selectivity towards triacetin.
This is expected because of the high basicity of this catalyst as compared to the other catalysts,
as seen in TPD analysis (Table 1). Hence it was used in further experimental studies and reaction
optimization.
3.2. Effect of speed of agitation
The speed of agitation was varied from 600-1000 rpm with the other parameters held constant at
catalyst loading of 0.04 g/cm3, mole ratio of oil:methyl acetate at 1: 50 and temperature 200 oC
(Figure 10). The conversion of soybean oil triglycerides and selectivity of triacetin increased
from 84 % to 95.9 % and 4.1 % to 5.3 %, respectively, when the speed of agitation was increased
from 600 to 800 rpm. There was no appreciable change in the conversion of soybean oil as well
as selectivity of triacetin when the speed was increased from 800 to 1000 rpm. This signifies that
at 1000 rpm, mass transfer resistance was minimal, and it was chosen as the agitation speed for
further experimental studies.
3.2.3. Effect of catalyst loading
13

The effect of catalyst loading was studied in the range from 0.03 to 0.05 g/cm3, keeping the other
parameters constant. The conversion of soybean oil triglycerides was found to increase
significantly from 81.2 % to 95.9 % when the catalyst loading was increased from 0.03 g/cm3 to
0.04 g/cm3. The selectivity of triacetin also followed the same trend with increase from 3.8 % to
5.3 % (Figure 11). This is due to the proportional increase in the number of catalytic sites which
can catalyze the conversion of reactants and intermediates to products with higher catalyst
loading. The optimum catalyst loading was found to be 0.04 g/cm3 as above this there was not
much notable change observed in conversion and selectivity.
3.2.4. Effect of Mole Ratio of Soybean oil triglycerides to Methyl acetate
Soybean oil to methyl acetate mole ratio was varied from 1:30 to 1:60 with the total volume and
other parameters held constant (Figure 12). Increasing mole ratio of soybean oil to methyl
acetate not only resulted in increasing conversion of soybean oil triglycerides but also favored
the reaction of intermediates with methyl acetate towards triacetin formation. The high viscosity
due to the triglycerides is compensated by using excess methyl acetate. It also acted as a solvent
to dissolve the intermediates and triacetin. The increase in the mole ratio from 1:30 to 1:50 lead
to increase in conversion from 68.6 % to 95.9 % and selectivity of triacetin also doubled.
Increase in the mole ratio from 1:50 to 1:60 did not lead to a much appreciable increase in the
conversion of soybean oil or selectivity of triacetin. Hence, mole ratio of 1:50 was used for
further reactions.
3.2.5. Effect of temperature
The effect of temperature on the interesterification reaction rate was studied in the range of 180-
210 ºC with the other variables constant (Figure 13). An increase in reaction temperature lead to
a significant increase in the rate of the interesterification reaction. Higher reaction temperatures
favoured the conversion of triglycerides and intermediates to end product, triacetin.
3.2.6. Development of kinetic model
For a fixed catalyst loading w, the rates of reaction or formation can be established.
The net rate of consumption of methyl acetate (M) by three parallel reactions is given by the
following equation:
14

−푟푀 = − 푑퐶 = 푤. { 푘1퐶푀퐶푇 + 푘2퐶푀퐶푀퐴 + 푘3퐶푀퐶퐷퐴 − 푘1 퐶퐹퐶푀퐴 + 푘2 퐶퐹퐶퐷퐴 + 푘3 퐶퐹퐶푇퐴
(1)
′
′
′
푀
(
)
(
)
}
푑푡
The rate of consumption of triglycerides (T) and the rates of formation of fatty acid methyl esters
(biodiesel) (F), monoacetindiglycerides (MA) and diacetinmonoglycerides (DA), triacetin (TA)
are as follows:
−푟푇 = − 푑퐶 = 푤. (푘1퐶푀퐶푇 − 푘1 퐶퐹퐶푀퐴)
(2)
′
푇
푑푡
푑퐶
′
′
푀퐴
푟푀퐴
=
= 푤. (푘1퐶푀퐶푇 − 푘1 퐶퐹퐶푀퐴 − 푘2퐶푀퐶푀퐴 + 푘2 퐶퐹퐶퐷퐴)
(3)
푑푡
푑퐶
′
′
′
퐹
푟퐹 =
= 푤. (푘1퐶푀퐶푇 − 푘1 퐶퐹퐶푀퐴 + 푘2퐶푀퐶푀퐴 − 푘2 퐶퐹퐶푀퐴 + 푘3퐶푀퐶퐷퐴 − 푘3 퐶퐹퐶푇퐴)
푑푡
(4)
푑퐶
′
′
퐷퐴
푟퐷퐴
푟푇퐴
=
= 푤. (푘2퐶푀퐶푀퐴 − 푘2 퐶퐹퐶퐷퐴 − 푘3퐶푀퐶퐷퐴 + 푘3 퐶퐹퐶푇퐴)
(5)
푑푡
푑퐶
′
푇퐴
=
= 푤. (푘3퐶푀퐶퐷퐴 − 푘3 퐶퐹퐶푇퐴)
(6)
푑푡
The mean concentration profile of the reactant, intermediates and product at different
temperature was used to deduce the kinetics using the integral method. The rate equations were
integrated using scipy.integrate.solve_ivp in Python [46]. The concentration of T, MA, DA and
TA at all studied temperatures and experimental time points were estimated by nonlinear least
squares fitting of the pre-exponential factor (k0) and activation energy (Ea), using a reference
temperature (T0) of 200 °C. The experimental and fitted concentration profile at 200 ºC is shown
in Figure 14. The fit was found to be good, with an R2 value of 99.96 %. The concentration
profiles at other studied temperatures are provided in the ESI (Figure S8). Rate constant (k) at
200 °C (equals pre-exponential factor as T = T0) and activation energy for each reversible
interesterification step (Scheme 1) are given in Table 4. Pre-exponential factor and activation
energy calculated at a reference temperature of 200 °C were used to calculate forward and
reverse kinetic rate constants at each studied temperature (T) using the Arrhenius equation (7).
퐸
푎
1
1
푘 = 푘0 ∗ 푒푥푝 ( 푅 (푇 − 푇))
(7)
0
15

The Arrhenius plot was made using the values of kinetic rate constants (Figure 15). The
activation energy of the first two reverse reactions i.e. conversion of MA to T and conversion of
DA to MA were found to be negative. The negative activation energy suggest that the first two
reversible reactions are non-elementary [47], and likely consist of sub-steps wherein a pre-
reactive intermediate is formed before going to final products. The overall activation energy for
the interesterification reaction can be written as the sum of individual activation energies of all
steps and it was found to be positive. Thus, increasing conversion and selectivity to MA, DA and
triacetin was found with increasing temperature. The rate constants of the first two reverse
reactions, i.e. k1´ and k2´ were found to decrease with increase in temperature as a result of
negative activation energy. This means that the reversibility of interesterification reaction
increases with decreasing temperature.
3.2.7. Catalyst reusability
Reusability studies of the catalyst Mg-Al CHT (3:1) were conducted over three cycles (Figure
16). The catalyst, filtered after the completion of reaction, was washed with methanol to remove
any adsorbed compounds. The catalyst was dried at 100 ºC for 12 h followed by calcination at
500 ºC for 6 h. The catalyst lost during filtration was replaced with fresh catalyst in each
experiment. There was a marginal decrease of 1.6 % in conversion of triglycerides and 0.5 % in
the selectivity of triacetin at the end of the 3rd reuse. The reused catalyst was also characterized
by various techniques as discussed earlier. The results show the stability and activity of the
catalyst after three reuses.
4. Conclusion
The interesterification of soybean oil triglycerides with methyl acetate to co-produce biodiesel
and triacetin was studied using various heterogeneous base catalysts. Mg-Al CHT (3:1) was
found to be the best catalyst for the conversion of triglycerides and selectivity towards triacetin.
The reaction conditions were optimized at agitation speed of 1000 rpm, a catalyst loading of 0.04
g/cm3, oil to methyl acetate mole ratio of 1:50 and a temperature of 200 oC. Increase in
temperature was found to enable the reaction towards higher triacetin formation. The kinetic
model for the system was developed using concentration data and the activation energy of each
step was calculated. The virgin and the reused catalyst were fully characterized to establish its
16

structural fidelity. The catalyst was found to be stable and reusable without any significant loss
in its activity.
Acknowledgement
Manali S. Dhawan acknowledges Prof. G. D. Yadav for the award of Ph.D. (Tech.) Research
Fellowship under his J.C. Bose National Fellowship of Department of Science and Technology,
Govt. of India and research grant from Chemical Engineering and Material Science department,
Michigan State University. G.D. Yadav acknowledges support from R.T. Mody Distinguished
Professor Endowment, Tata Chemicals Darbari Seth Distinguished Professor of Leadership and
Innovation and J.C. Bose National Fellowship of Department of Science and Technology, Govt.
of India. Manali S. Dhawan is a Dual Ph. D. degree program between Institute of Chemical
Technology, Mumbai and Michigan State University, East Lansing. USA co-supervised by G.D.
Yadav and S.C. Barton.
Author Credit Statement
Manali S. Dhawan is a Dual Ph. D. degree program between Institute of Chemical Technology,
Mumbai, India and Michigan State University, East Lansing. USA co-supervised by G.D. Yadav
and S.C. Barton.
Conflict of interest statement
The authors declare no conflict of interest.
17

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23

Figure Captions and Figures
Figure 1
CO2-TPD patterns of catalysts (a) Ca-Al HT (b) CHT-HMS (c) Mg-Al CHT (1:1)
(d) Mg-Al CHT (2:1) (e) Mg-Al CHT (3:1) (f) Reused Mg-Al CHT (3:1).
Figure 2
NH3-TPD patterns of catalysts (a) Ca-Al HT (b) CHT-HMS (c) Mg-Al CHT (1:1)
(d) Mg-Al CHT (2:1) (e) Mg-Al CHT (3:1) (f) Reused Mg-Al CHT (3:1).
Figure 3
Figure 4
Figure 5
XRD of catalysts (a) Ca-Al HT (b) CHT-HMS (c) Mg-Al CHT (3:1) (d) Reused
Mg-Al CHT (3:1).
FT-IR spectra of catalysts (a) Ca-Al HT (b) CHT-HMS (c) Mg-Al CHT (3:1) (d)
Reused Mg-Al CHT (3:1).
N2 adsorption-desorption isotherms of catalysts (a) Ca-Al HT (b) CHT-HMS (c)
Mg-Al CHT (3:1) (d) Reused Mg-Al CHT (3:1).
Figure 6
Figure 7
SEM images of (a) Mg-Al CHT (3:1) (b) Reused Mg-Al CHT (3:1).
DSC analysis of catalysts (a) Mg-Al CHT (3:1) (b) Reused Mg-Al CHT (3:1).
TGA analysis of catalysts (a) Mg-Al CHT (3:1) (b) Reused Mg-Al CHT (3:1).
Figure 8
Figure 9
Effect of different catalysts on the conversion of soybean oil triglycerides and
selectivity of intermediates and triacetin. Soybean oil: 7.0 g, soybean oil:methyl
acetate mole ratio: 1:50, speed of agitation: 1000 rpm, temperature: 200 °C,
catalyst loading: 0.04 g/cm3, total volume 40 cm3, reaction time 240 min.
Figure 10
Effect of speed of agitation on conversion of soybean oil triglycerides and
selectivity of triacetin. Soybean oil: 7.0 g, soybean oil:methyl acetate mole ratio:
1:50, temperature: 200 °C, catalyst loading: 0.04 g/cm3, total volume 40 cm3,
reaction time 240 min.
( )1000 rpm, (▲) 800 rpm, (■) 600 rpm
Figure 11
Effect of catalyst loading on conversion of soybean oil triglycerides and
selectivity of triacetin. Soybean oil: 7.0 g, soybean oil:methyl acetate mole ratio:
24

1:50, speed of agitation: 1000 rpm, temperature: 200 °C, total volume 40 cm3,
reaction time 240 min.
( ) 0.05 g/cm3, (▲) 0.04 g/cm3, (■) 0.03 g/cm3
Figure 12
Figure 13
Figure 14
Effect of soybean oil triglycerides to methyl acetate mole ratio on conversion of
soybean oil triglycerides and selectivity of triacetin. Speed of agitation: 1000 rpm,
catalyst loading: 0.04 g/cm3, temperature: 200 °C, total volume 40 cm3, reaction
time 240 min.
(×) 1:30, (■) 1:40, (▲) 1:50, ( ) 1:60
Effect of temperature on conversion of soybean oil triglycerides and selectivity of
triacetin. Soybean oil: 7.0 g, soybean oil:methyl acetate mole ratio: 1:50, speed of
agitation: 1000 rpm, catalyst loading: 0.04 g/cm3, total volume 40 cm3, reaction
time 240 min.
(×) 180 ºC, (■) 190 ºC, (▲) 200 ºC, ( ) 210 ºC
Experimental (x) and fitted (—) concentration (C) profile of soybean oil
triglycerides (T), fatty acid methyl esters (F), monoacetindiglycerides (MA),
diacetinmonoglycerides (DA) and triacetin (TA) as a function of time. Soybean
oil: 7.0 g, soybean oil:methyl acetate mole ratio: 1:50, speed of agitation: 1000
rpm, temperature: 200 °C, catalyst loading: 0.04 g/cm3, total volume 40 cm3,
reaction time 240 min.
Figure 15
Figure 16
Arrhenius plot with kinetic rate constants (k) as a function of temperature (T).
Effect of catalyst reuse on the conversion of soybean oil and selectivity of
intermediates and triacetin. Soybean oil: 7.0 g, soybean oil:methyl acetate mole
ratio: 1:50, speed of agitation: 1000 rpm, temperature: 200 °C, catalyst loading:
0.04 g/cm3, total volume 40 cm3, reaction time 240 min.
25

(f)
(e)
(d)
(c)
(b)
(a)
0
100
200
300
400
500
Temperature (°C)
Figure 1: CO2-TPD patterns of catalysts (a) Ca-Al HT (b) CHT-HMS (c) Mg-Al CHT (1:1) (d)
Mg-Al CHT (2:1) (e) Mg-Al CHT (3:1) (f) Reused Mg-Al CHT (3:1)
26

(f)
(e)
(d)
(c)
(b)
(a)
50
150
250
350
450
550
Temperature (°C)
Figure 2: NH3-TPD patterns of catalysts (a) Ca-Al HT (b) CHT-HMS (c) Mg-Al CHT (1:1) (d)
Mg-Al CHT (2:1) (e) Mg-Al CHT (3:1) (f) Reused Mg-Al CHT (3:1)
27

(d)
(c)
(b)
(a)
5
15
25
35
45
55
65
75
2 ϴ (˚)
Figure 3: XRD of catalysts (a) Ca-Al HT (b) CHT-HMS (c) Mg-Al CHT (3:1) (d) Reused Mg-Al
CHT (3:1)
28

(d)
(c)
(b)
(a)
4000
3400
2800
2200
1600
1000
400
Wavenumber (cm-1)
Figure 4: FT-IR spectra of catalysts (a) Ca-Al HT (b) CHT-HMS (c) Mg-Al CHT (3:1) (d)
Reused Mg-Al CHT (3:1)
29