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Effect of calcination temperature on biodiesel production from palm
kernel oil using a bifunctional catalyst derived from calcium carbide
residue (CCR) and anthill clay (AC)
Justina Oduwa Okhonmina
1
* and Kessington Obahiagbon
2
1,2
Department of Chemical Engineering, Faculty of Engineering, University, of Benin, Benin City, Edo
State, Nigeria.
DOI: https://doi.org/10.51583/IJLTEMAS.2026.150500239
Received: 25 May 2026; Accepted: 30 May 2026; Published: 19 June 2026
ABSTRACT
This study aims at investigating the effect of calcination temperature on biodiesel production from palm kernel
oil using a bifunctional catalyst derived from calcium carbide residue (CCR) and anthill clay (AC). The specific
objectives include to synthesize a ZnO-doped CaO/Anthill clay composite using CCR and anthill clay as
precursors, to evaluate the effect of various calcination temperatures (650 °C to 850 °C) on the catalyst’s
chemical and structural properties, to identify the optimal temperature that maximizes the density of active sites
while preventing structural degradation, and to test the catalyst's effectiveness by converting palm kernel oil
(PKO) into biodiesel and evaluating the final yield. The CCR and AC precursors were prepared and characterized
using X-ray diffraction, scanning electron microscopy, Fourier Transform Infrared spectroscopy, X-ray
fluorescence, and Brunauer-Emmett-Teller. Composites of the prepared CCR and AC were formulated in ratios
1:4, 2:3, 1:1, 3:2, and 4:1 of CCR:AC and doped with 1.5 M zinc nitrate, by wet impregnation and calcined at 3
different temperatures (650
o
C, 750
o
C, and 850
o
C), resulting in 15 samples (A1 – E3) which were used for the
production. Ratio 2:3 catalyst (D2) calcined at 750
o
C gave the highest yield (92.73%) and reusability studies
gave an overall reduction of 27.69 % in yield across 6 cycles. Its XRF analysis gave compositions of 52.1%
CAO, 10.4% SiO
2
, and 7.4% Al
2
O
3
. Its surface area, pore diameter and pore volume were 226.9m
2
/g, 2.9nm and
0.2cc/g respectively. The PKO characterization results showed the PKO’s suitability for the biodiesel production.
The produced biodiesel properties aligned with ASTM D6571 and EN 14214 standards upon characterization.
This study successfully synthesized a high-performance ZnO-doped CaO/Anthill clay composite from
sustainable materials, identifying calcination temperature as the key factor influencing catalyst activation and
structural stability and also contributes to the promotion of renewable and sustainable energy.
Keywords: Palm kernel oil, Calcium Carbide Residue, Anthill clay, Calcination Temperature, Biodiesel.
INTRODUCTION
The global shift toward sustainable energy has positioned biodiesel as a viable, carbon-neutral alternative to
traditional fossil fuels. Biodiesel is an alternate energy source which is made from biological components like
vegetable or animal fats and used cooking oils [17]. Transesterification is the most widely used technique for
producing biodiesel [12]. It involves the reaction of triglycerides with alcohol (often methanol or ethanol) in the
presence of a catalyst to produce glycerol and biodiesel [4]; [6]; [8]. While biodiesel offers lower emissions and
high biodegradability, its commercial production is often limited by high costs. Homogeneous catalysts are
largely responsible for this, as they cannot be recovered and require expensive water-washing steps. To
overcome these barriers, recent research has focused on the "circular economy" by developing heterogeneous
catalysts from industrial and environmental waste, such as calcium carbide residue (CCR) and anthill clay [11];
[17]; [19]. When heterogeneous catalysts are used in transesterification, they yield glycerin with greater purity
and lower levels of dissolved ions [15].
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The performance of these waste-derived catalysts depends heavily on the calcination temperature. This thermal
treatment is necessary to convert inactive mineral precursors into reactive oxides like CaO and ZnO. However,
finding the optimal temperature is a major challenge. If the temperature is too low, the catalyst remains
chemically inactive; if it is too high, the material undergoes sintering, which causes the internal pores to collapse
and reduces the surface area available for the reaction [2]; [14]. Identifying the precise thermal "sweet spot" is
therefore essential for creating an efficient and stable catalyst.
MATERIALS & METHODS
Collection of materials
The CCR and AC precursors were obtained from some welder’s shop and anthill respectively, in Ugbowo in
Benin City, Edo State, Nigeria. The PKO was purchased from a local vendor in Benin City also.
Catalyst preparation
According to Okhonmina et al. [17], the locally sourced precursors were beneficiated, pulverized and screened
to < 200µm and calcined at 800
o
C for 4 hours. The calcined AC was treated with 2M H
2
SO
4
, washed and dried.
It was calcined again at 800
o
C for 2 hours, 30 minutes. The precursors were combined in 5 different ratios; 1:4,
2:3, 1:1, 3:2, and 4:1 of CCR:AC. The samples were then doped with equal quantities of 1.5 M zinc nitrate at a
composite material-to-zinc nitrate ratio of 1:4 using the wet impregnation technique to produce suspensions.
These suspensions were continuously stirred on a magnetic stirrer at 80 °C for 6 h to ensure proper
homogenization. Thereafter, the mixtures were left to settle, and the excess liquid was removed through
decantation. The resulting slurries were dried in a vacuum oven at 120 °C until complete dehydration was
achieved. The dried doped composite materials, prepared in five different proportions, were subsequently
pulverized and sieved to obtain particle sizes below 200 μm. Each composite sample was then divided into three
portions and calcined in a muffle furnace at 650 °C, 750 °C, and 850 °C under static air conditions for 4 hrs,
resulting in 15 different catalyst samples (A1 – E3) which were used for the biodiesel production.
Catalyst characterization
The precursors and formulated catalyst were characterized using X-ray diffraction (XRD) analysis for crystalline
phase, Scanning electron microscopy (SEM) with Energy Dispersive X-ray (EDX) for the surface structure,
Fourier transform infrared spectroscopy (FTIR) for bond structure and interaction, X-ray fluorescence (XRF)
analysis for elemental composition, and Brunauer-Emmett-Teller (BET) analysis for surface area and pore
properties.
Characterization of palm kernel oil
The palm kernel oil was characterized to determine its suitability as feedstock for biodiesel production.
Production of biodiesel
Production of biodiesel was carried out using the 15 different catalysts at fixed input parameters of 65
o
C, 90
mins, 9:1 and 3 wt % reaction temperature, reaction time, methanol to oil ratio and catalyst loading respectively.
A one-step technique transesterification process was used for the biodiesel production, based on the bi-
functionality of the synthesized catalyst. A batch-type stirred reactor which comprised a hot plate magnetic stirrer
with a magnetic stirring bar for proper mixing to obtain uniform mixture, and a 250ml conical flask with a cork
was employed for the biodiesel production. 50g of the PKO was transferred into the reactor and heated to 65
o
C.
3 wt % of catalyst and 9:1 methanol to oil ratio mixture was added, and the mixture was left to react for 90 mins
[17], using a fixed stirring speed.
After each production, the resulting mixture was transferred into a separating funnel and left to stay overnight
for proper settling, after which the glycerol at the bottom layer was run out with the aid of the separating funnel.
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The biodiesel with the unreacted methanol mixture was washed and transferred to an oven. The unreacted
methanol evaporated while the biodiesel was stored. The transesterification process, separation process and the
produced PKO biodiesel are displayed below in figure 1 (a), (b) and (c) respectively. Equation 1 below was used
to determine the biodiesel yield.
(1)
(a) (b) (c)
Figure 1: Biodiesel production process (a) transesterification process (b) separation process (c) produced PKO
biodiesel
All experiments were conducted in triplicate to ensure the reliability and reproducibility of the results, and the
average values obtained were reported. Furthermore, the reaction conditions were carefully controlled
throughout the experimental process to minimize errors and maintain consistency in the biodiesel production
procedure.
Catalyst reusability studies
The reusability of the catalyst that gave the highest yield was evaluated through repeated transesterification
reactions under optimum conditions. After each cycle, the catalyst was recovered by filtration, washed with
methanol, and oven-dried at 80 °C for 12 hours before reuse. The recovered catalyst was reused for six
consecutive cycles, and the biodiesel yield obtained after each run was recorded.
Characterization of the produced biodiesel
The produced biodiesel was characterized to determine its properties such as acid value, FFA, saponification
value, average molecular weight, density, kinematic viscosity, moisture content, iodine value, peroxide value,
etc.
RESULTS AND DISCUSSION
X-ray diffraction (XRD) analysis
With the aid of the XRD analysis, identification of the crystalline phases was achieved and the structure and
physical properties of the precursors and composite catalyst were determined [18]. Figure 2 (a), (b) and (c)
below show the XRD pattern of the precursors and composite catalyst.
The XRD pattern of the calcined calcium carbide residue (CCR) precursor in figure 2 (a) reflects the complete
thermal transformation of its original carbonate and hydroxide phases. The pretreatment at 800 °C ensures the
full decomposition of Portlandite [Ca(OH)
2
] and Calcite [CaCO
3
] into cubic calcium oxide (CaO). This is
evidenced by sharp, high-intensity diffraction peaks at 2θ values of approximately 32.2°, 37.4°, and 53.9° [14].
The high degree of crystallinity at this temperature provides a stable source of basic active sites, which are
essential for the subsequent transesterification of triglycerides [13].
The diffraction pattern of the anthill clay (AC) precursor in figure 2 (b), also calcined at 800 °C, indicates a
structural shift toward a more reactive aluminosilicate state. The thermal energy at this level facilitates the
transition of Kaolinite into metakaolinite, characterized by the disappearance of kaolinite reflections and the
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presence of a broad amorphous halo between 2 θ = 20° and 30° [22]. The persistent sharp peak at 26.6° signifies
the stable crystalline Quartz (SiO
2
) fraction of the clay. This combination of a stable crystalline framework and
a reactive amorphous phase provides the necessary acidic sites and structural support for the composite material.
The XRD pattern of the final composite catalyst in figure 2 (c) —prepared at a 3:2 ratio of CCR to AC and
calcined at 750 °C—demonstrates the successful integration of the 1.5 M zinc nitrate dopant into the precursor
matrix. This pattern is characterized by the co-existence of sharp reflections for cubic CaO and hexagonal
wurtzite ZnO, with the latter appearing at 2 θ values of 31.8°, 34.4°, and 36.2° [3]. The secondary calcination at
750 °C serves as the critical activation stage for the doped zinc species, ensuring they are well-dispersed across
the aluminosilicate surface of the anthill clay and the basic surface of the CaO.
The presence of distinct CaO and ZnO peaks confirms the bifunctional nature of the catalyst. The basic sites
provided by the CCR-derived CaO and the acidic sites provided by the ZnO and AC framework work in synergy
to facilitate both transesterification and esterification. This 3:2 composition and 750 °C calcination temperature
avoid the structural sintering often observed at 850 °C, maintaining a high active surface area. This structural
arrangement is responsible for the peak biodiesel yield of 92.73%, as it provides the optimal electronic
environment for methanol activation and free fatty acid conversion [16].
Scanning electron microscopy (SEM) with Energy Dispersive X-ray (EDX)
The surface morphology of the precursors and composite catalyst were observed using SEM. The results
obtained at different magnifications are displayed below in figures 3, 4 and 5.
The calcined calcium carbide residue (CCR) (Figure 3) displays a highly agglomerated and irregular
morphology. The 800 °C pretreatment promotes the formation of dense crystalline clusters, providing a robust
source of basic active sites essential for initiating the transesterification process. The utilization of waste-derived
precursors for catalyst synthesis not only reduces production costs but also promotes environmental
sustainability by repurposing industrial byproducts. This approach is supported by Makarevičienė et al. [11],
who demonstrated that properly prepared catalysts from natural waste and industrial residues exhibit superior
catalytic properties for commercial-grade biodiesel conversion.
The anthill clay (AC) (Figure 4) exhibits a contrasting flaky and layered architecture. This sheet-like
arrangement, typical of thermally treated aluminosilicates, provides a porous structural scaffold. This
morphology is critical for enhancing the surface-area-to-volume ratio and facilitating the anchoring of metal
oxide dopant [22].
The composite catalyst (Figure 5) reveals a significant transition toward a more granular and textured surface
after 1.5 M zinc nitrate doping and 750 °C calcination. The images show a uniform distribution of active
nanoparticles across the aluminosilicate flakes, creating an interconnected network of micro-pores. This
morphology minimizes mass transfer resistance, allowing large triglyceride molecules to easily access active
sites, which justifies the observed peak yield of 92.73% [20]. The absence of severe particle fusion suggests that
the 750 °C treatment successfully optimized the catalyst's crystallinity without triggering the sintering-induced
structural collapse often observed at higher temperatures [10].
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Figure 3: SEM images of CCR precursor (a) x340 (b) x500
Figure 4: SEM images of THC precursor (a) x500 (b) x1000
Figure 5: SEM images of composite catalyst (a) x500 (b) x750
Fourier transform infrared (FTIR) spectroscopy analysis
The identification of different functional groups available on the precursors and composite catalyst surfaces was
carried out using Fourier transform infrared (FTIR) analysis. Figures 6 to 8 show the FTIR results of the
precursors and composite catalyst.
The spectrum of the calcined CCR (Figure 6) shows a significant reduction in the O–H band (3640 cm⁻¹),
indicating the dehydration of calcium hydroxide into Calcium Oxide (CaO). The remaining peaks at
approximately 1420 cm⁻¹ and 875 cm⁻¹ are attributed to the stretching and bending vibrations of C–O bonds
from atmospheric CO
2
adsorption on the basic CaO surface, confirming its high chemical reactivity [14].
The calcined anthill clay (AC) spectrum (Figure 7) reflects the thermal transition of kaolinite into metakaolinite.
The loss of structural hydroxyl groups at 3695 cm⁻¹ and 3620 cm⁻¹ indicates successful dehydroxylation. The
dominant band shifting toward 1050-1080 cm⁻¹ represents the reactive amorphous Si–O–Al framework, while
persistent peaks at 798 cm⁻¹ confirm the stable Quartz fraction [22].
The composite catalyst (Figure 8), subsequently calcined at 750 °C, demonstrates the successful chemical
integration of the 1.5 M zinc nitrate dopant. The spectrum is characterized by a broad, intense absorption band
in the 400-600 cm⁻¹ range, which is the direct result of metal-oxygen (M–O) vibrations for both Zn–O and Ca–
O. This overlap confirms the formation of an integrated metal-oxide-aluminosilicate matrix. The presence of
these combined functional groups validates the bifunctional nature of the catalyst, providing the acidic and basic
sites necessary for the high biodiesel yield achieved in the production phase [2]; [11].
Figure 6: FTIR spectra of CCR precursor Figure 7: FTIR spectra of AC precursor
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Figure 8: FTIR spectra of the composite catalyst
X-ray Fluorescence (XRF) Analysis
The elemental composition of the calcium carbide residue (CCR), anthill clay (AC), and the resulting composite
catalyst reveals a strategic combination of active components and structural supports. The CCR precursor is
highly rich in CaO (89.8%), establishing it as the primary source of basic active sites essential for the
transesterification reaction [1]; [9]. The AC precursor contains significant amounts of SiO₂ (46.9%) and Al₂O₃
(24.1%), characteristic of aluminosilicate clays that serve as stable, high-surface-area frameworks for catalyst
immobilization [5]; [21].
A significant chemical transformation is observed in the composite catalyst, specifically the inclusion of ZnO
(20.9%), which was not present in substantial quantities in either precursor. This
enrichment indicates the successful introduction of a zinc-based dopant during synthesis to enhance the
catalyst's bifunctionality and thermal stability [7]. The final composition effectively balances basicity from CaO
(52.1%) with structural integrity and acidic sites provided by SiO₂, Al₂O₃, and ZnO, which is a hallmark of high-
performance heterogeneous catalysts capable of achieving biodiesel yields above 90% [1]; [7]. Table 1 shows
the elemental composition of CCR precursor, AC precursor and the composite catalyst.
Table 1: Elemental composition of CCR precursor, AC precursor and composite catalyst
Component
CCR precursor
AC precursor
Concentration (%) of
composite catalyst
CaO
89.8
1.5
52.1
SiO
2
4.04
46.9
10.4
Al
2
O
3
3.9
24.1
7.4
SO
3
0.6
0.4
0.3
Fe
2
O
3
0. 4
20.6
5.2
Cl
0. 43
0.7
0.9
SnO
2
0. 25
0.0
0.3
TiO
2
0.13
3.8
0.9
CoO
0.09
0.1
0.02
BaO
0.09
0.0
0.02
SrO
0.06
0.02
0.06
CuO
0.06
0.14
0.05
Ta
2
O
5
0.05
0.06
0.2
ZrO
2
0.03
0.5
0.3
Cr
2
O
3
0.03
0.1
0.03
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Brunauer-Emmett-Teller (BET) analysis.
The surface characteristics of the precursors and composite catalyst were quantitatively ascertained using the
Brunauer-Emmett-Teller (BET) analysis. The surface area, pore diameter and pore volume determined from the
analysis are displayed in table 2 below.
The textural properties of the precursors and the resulting composite catalyst highlight a strategic structural
evolution suitable for high-efficiency catalysis. The anthill clay (AC) precursor exhibits the highest specific
surface area at 314.2 m²/g, confirming its role as a superior aluminosilicate scaffold with an inherent capacity
for high active-site dispersion [5]; [22]. Following the integration of calcium carbide residue (CCR) and the 1.5
M zinc nitrate dopant, the surface area of the composite catalyst settles at 226.9 m²/g. This reduction, alongside
a decrease in pore volume from 0.28 cc/g to 0.2 cc/g, indicates that the metal oxides successfully occupied the
AC pore network during the 750 °C calcination phase [1].
Critically, all materials maintain a consistent pore diameter within the 2.8–2.9 nm range, identifying the system
as distinctly mesoporous. This pore size is ideal for biodiesel production as it allows for the unhindered diffusion
of bulky triglyceride molecules to the active sites within the catalyst matrix, thereby minimizing mass transfer
resistance [13]. The final composite combines a robust surface area with an optimized mesoporous architecture,
ensuring the necessary site accessibility to facilitate the peak yields observed in the production tests [7]; [16].
Table 2: Surface characteristics of precursors and composite catalyst
Characterization of the oil feedstock (PKO)
The physicochemical properties of the Palm Kernel Oil (PKO) as presented in Table 3 indicate that while the oil
is a viable feedstock for biodiesel production, its high acidity necessitates the use of a specialized catalytic
system. The Acid Value of 7.3 mgKOH/g and a corresponding Free Fatty Acid (FFA) content of 3.7% exceed
WO
3
0.02
0.0
0.5
NiO
0.02
0.01
0.0
PbO
0.01
0.04
0.03
V
2
O
5
0.01
0.2
0.07
MnO
0.01
0.1
0.04
Cs
2
O
0.01
0.09
0.08
ZnO
0.01
0.04
20.9
Rb
2
O
0.00
0.01
0.01
Nb
2
O
5
0.0
0.04
0.03
P
2
O
5
0.0
0.2
0.02
MgO
0.0
0.0
0.0
K
2
O
0.0
0.31
0.03
Ag
2
O
0.0
0.04
0.05
Parameter
CCR
AC
Composite catalyst
Surface area (m
2
/g)
221.7
314.2
226.9
Pore diameter (nm)
2.9
2.8
2.9
Pore volume (cc/g)
0.20
0.28
0.2
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the typical 1% threshold for simple base-catalyzed transesterification. This level of acidity confirms the
importance of using the ZnO-doped bifunctional catalyst synthesized in this study, as the Lewis acidic sites
provided by the ZnO are essential for the simultaneous esterification of FFAs and transesterification of
triglycerides to prevent soap formation [2].
The structural and energy characteristics of the PKO are further defined by its Saponification Value of 234.2
mgKOH/g and Average Molecular Weight of 742 g/mol. These high values suggest a prevalence of short-to-
medium chain fatty acids, which are beneficial for producing biodiesel with low viscosity and high volatility.
Furthermore, the Iodine Value of 19 mg I₂/100g is notably low, indicating a high degree of saturation. This is a
positive indicator for the resulting biofuel’s oxidative stability, suggesting it will be less prone to rancidity or
sludge formation during long-term storage [13]; [19].
Physical and purity parameters such as the Density (0.90 g/cm³) and Viscosity (5.2 m²/s) align with standard
profiles for tropical vegetable oils. However, the Moisture Content of 1.09% and Peroxide Value of 16 mEq/kg
suggest a slight degree of primary oxidation and water presence, which can lead to competitive side reactions.
Despite these impurities, the overall profile confirms that the PKO is an ideal candidate for conversion into high-
quality biodiesel when processed with an optimized, thermally activated bifunctional catalyst [1].
Table 3: Physiochemical properties of PKO
Property
Value
Density (g/cm
3
)
0.90
Average molecular weight (g/mol)
742
Peroxide value (mEq/kg)
16
Acid value (mgKOH/g oil)
7.3
Free Fatty Acid (%)
3.7
Saponification value (mgKOH/g oil)
234.2
Viscosity @ 40
o
C, 30.0 RPM & 22% (m
2
/s)
5.2
Moisture content (%)
1.09
Iodine value (mg I
2
/100g oil)
19
Biodiesel production
Table 4: Results of biodiesel production for effect of calcination temperature on catalysts functionality
Catalyst
Catalyst
composition
(CCR:AC)
Calcination
temperature
(
o
C)
Reaction
temperature
(
o
C)
Reaction
time
(mins)
Methanol-
to-oil ratio
(-)
Catalyst
loading
(wt.%)
Biodiesel
yield (%)
A1
1:4
650
65
90
9
3
64.36
A2
1:4
750
65
90
9
3
70.58
A3
1:4
850
65
90
9
3
68.59
B1
2:3
650
65
90
9
3
71.44
B2
2:3
750
65
90
9
3
76.23
B3
2:3
850
65
90
9
3
74.81
C1
1:1
650
65
90
9
3
75.34
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C2
1:1
750
65
90
9
3
77.53
C3
1:1
850
65
90
9
3
76.86
D1
3:2
650
65
90
9
3
80.23
D2
3:2
750
65
90
9
3
92.73
D3
3:2
850
65
90
9
3
87.45
E1
4:1
650
65
90
9
3
79.32
E2
4:1
750
65
90
9
3
88.74
E3
4:1
850
65
90
9
3
85.23
Biodiesel production
The data in Table 4 illustrates a distinct parabolic relationship between calcination temperature and catalytic
activity across all catalyst groups (A through E). While the catalyst composition (the ratio of CCR to AC) sets
the baseline for potential yield, the thermal treatment—specifically the choice of 750 °C—acts as the critical
"activation switch" for achieving maximum efficiency.
Thermal Activation and the 750 °C Optimum
Across every series, the biodiesel yield peaks at a calcination temperature of 750 °C (e.g., A2, B2, C2, D2, and
E2). This temperature appears to be the "sweet spot" for several physicochemical transformations. At 750 °C,
the precursors undergo optimal decomposition and phase transformation—specifically the conversion of calcium
carbonates from CCR into active CaO and the integration of the zinc dopant into the metal oxide matrix.
This creates a high density of active basic sites and a well-developed pore structure that facilitates the diffusion
of large triglyceride molecules. Recent studies confirm that this temperature range is ideal for maximizing
crystallinity and surface basicity in heterogeneous calcium-based catalysts [14].
The Decline at 850 °C: Sintering and Structural Degradation
A consistent decline in yield is observed when the calcination temperature is increased further to 850 °C (e.g.,
A3, B3, C3, D3, and E3). For instance, in the D-series, the yield drops from its peak of 92.73% (D2) to 87.45%
(D3). This "thermal over-treatment" typically leads to a phenomenon known as sintering, where small catalyst
particles fuse together.
This fusion reduces the total surface area and causes "pore collapse," effectively burying active sites that were
previously accessible for the reaction. Modern characterization research highlights that excessively high
temperatures can also lead to the formation of less active mixed-metal oxides or solid-state reactions that
neutralize the basic sites necessary for methanol activation [13].
Synergistic Effect of Composition and Temperature
The table also reveals that the effect of temperature is more pronounced as the CCR (Calcium) content increases.
In Group A (1:4 ratio), the jump from 650 °C to 750 °C only improves the yield by about 6%. However, in
Group D (3:2 ratio), the same 100 °C increase results in a massive 12.5% surge in yield (from 80.23% to 92.73%).
This suggests that a higher concentration of the basic precursor requires this specific thermal energy to fully
unlock its bifunctional potential. The combination of high CCR content and 750 °C calcination provides the
most effective acidity/basicity balance for simultaneous esterification and transesterification [20].
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Influence of Calcination Temperature on Catalyst Activity and Biodiesel Yield
The biodiesel yield profiles demonstrate that 750 °C is the optimal calcination temperature for maximum
catalytic efficiency, yielding a peak of 92.73%. This temperature acts as a critical threshold, providing sufficient
thermal energy to fully activate the CaO and ZnO species while preserving the catalyst's porous structure.
Lower temperatures, such as 650 °C, result in "under-calcination," where incomplete precursor decomposition
leads to a deficit of active basic sites and lower yields (e.g., 80.23%). Conversely, increasing the temperature to
850 °C triggers thermal sintering and pore collapse, reducing the accessible surface area and causing the yield
to drop to 87.45%.
The superior performance at 750 °C ensures high site accessibility and stable crystallinity, consistent with recent
findings that identify this window as the "activation sweet spot" for bifunctional catalysts [13]; [16].
In conclusion, calcination temperature significantly influenced catalyst structure and catalytic activity. Moderate
calcination temperatures promoted decomposition of precursor species and formation of active CaO phases,
while excessive temperatures may have induced particle sintering and pore collapse, resulting in reduced surface
area and catalytic performance.
Reaction kinetics
Reaction kinetics plays an important role in biodiesel production because it determines the rate at which
triglycerides are converted into fatty acid methyl esters during the transesterification process. The reaction rate
is influenced by several factors, including reaction temperature, reaction time, methanol-to-oil ratio, catalyst
loading, and agitation speed. In heterogeneous catalysis, the availability of active basic sites, catalyst surface
area, pore structure, and mass transfer between the reactants and catalyst surface also significantly affect the
conversion efficiency. In this study, the improved biodiesel yield observed at the optimum calcination
temperature may be attributed to enhanced catalyst crystallinity, increased formation of active basic sites, and
improved dispersion of ZnO species on the catalyst surface. However, excessively high calcination temperatures
may lead to particle agglomeration and pore collapse, thereby reducing catalytic activity and biodiesel yield.
Although detailed kinetic modeling was beyond the scope of this work, the findings suggest that calcination
temperature strongly influences the catalytic behavior and transesterification efficiency of the ZnO-doped
CaO/anthill clay catalyst.
Catalyst activation mechanism
The catalytic mechanism likely involves the generation of strong basic sites from CaO and ZnO species, which
facilitate methoxide ion formation during transesterification. The anthill clay support improves catalyst
dispersion, thermal stability, and surface area, thereby enhancing accessibility of active sites.
Catalyst reusability studies
In the first run, the biodiesel yield was 92.73 %. The second, third and fourth runs produced yields of 86.93 %,
79.03 % and 76.54 %, respectively. The fifth and sixth runs recorded biodiesel yields of 70.14 %, and 65.04 %
respectively, as illustrated in Figure 9 below. A steady decline in biodiesel yield was observed with increasing
reuse cycles, amounting to an overall reduction of about 27.69 % between the first and sixth runs.
This decrease in performance may be attributed to the gradual occupation of the catalyst’s active sites by
unreacted triglycerides, which reduces the availability of active sites and consequently lowers catalytic efficiency
[23]
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Figure 9: Catalyst reusability studies results
Characterization of the produced biodiesel
The physicochemical properties of the produced biodiesel, as detailed in Table 5, demonstrate that the fuel is of
high quality and fully complies with the international ASTM D6751 and EN 14214 standards. A standout feature
of this fuel is its Cetane number of 67.44, which significantly exceeds the minimum requirements of 47 and 51,
respectively. This high value indicates superior ignition quality, which leads to smoother engine performance
and reduced noise during combustion. Furthermore, the Higher Heating Value (40.64 MJ/kg) confirms that the
biodiesel possesses an energy content comparable to conventional petroleum diesel, ensuring efficient power
generation [2].
The safety and flow characteristics of the fuel are also well within the optimized range for engine use. The Flash
point of 179 °C is substantially higher than the required minimum of 130 °C, making the biodiesel exceptionally
safe for long-term storage and transportation. Additionally, the Viscosity at 40 °C (3.6 mPa.s) falls perfectly
within the standard limits, which is critical for ensuring proper fuel atomization and preventing wear on the fuel
injection pump. The cold flow properties, represented by a Pour point of -5 °C and a Cloud point of -6 °C,
suggest that the fuel will remain liquid and functional in moderately cold climates [1]; [19].
Finally, the chemical stability and purity of the biodiesel are confirmed by its low acidity and saturation levels.
The Acid value (0.266 mgKOH/g) and Free Fatty Acid content (0.133%) are well below the corrosive thresholds,
protecting engine components from damage. Moreover, the low Iodine value (23.5 g I₂/100g) suggests a low
level of unsaturation, which significantly improves the fuel's oxidative stability and prevents the formation of
sludge during storage. Overall, these properties validate the effectiveness of the ZnO-doped CaO/Anthill clay
catalyst in producing a high-grade, stable, and environmentally friendly biofuel [13].
Table 5: Properties of biodiesel
Properties
Biodiesel
ASTM D6751
EN 14214
Colour
Golden yellow
Not specified
Not specified
Physical state at room temperature
Liquid
Not specified
Not specified
Density (g/cm
3
)
0.863
Not specified
Not specified
Acid value (mgKOH/g)
0.266
<0.5
<0.5
Free fatty acid (%)
0.133
Not specified
Not specified
Saponification value (mgKOH/g)
206.44
Not specified
Not specified
Iodine value (I
2
/100g)
23 .5
Not specified
<120
Viscosity @ 40
o
C (mPa.s)
3.6
1.9 to 6.0
3.5 to 5.0
92.73
86.93
79.03
76.54
70.14
65.04
0
20
40
60
80
100
1 2 3 4 5 6
Biodiesel Yield (%)
Number of Cycles
Catalyst Reusability Studies
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Pour point (
o
C)
-5
<0
<0
Flash point (
o
C)
179
>130
>120
Cloud point (
o
C)
-6
Not specified
Not specified
Cetane number
67.44
≥47
≥51
Higher heating value
40.64
Not specified
≥35
Aniline point
81
Not specified
Not specified
Economic feasibility and scalability of catalyst production
The catalyst precursors used in this study are naturally occurring inexpensive and abundantly available waste-
derived materials (anthill clay and calcium carbide residue), making the catalyst economically attractive for
large-scale biodiesel production. The synthesis procedure, involving calcination and impregnation, is relatively
simple and adaptable to industrial-scale operations. Furthermore, the low ZnO loading reduces overall catalyst
production cost while enhancing catalytic activity. However, detailed techno-economic analysis and pilot-scale
validation were beyond the scope of this study and should be investigated in future work.
Environmental impact and energy consumption
The use of waste-derived catalyst materials contributes to sustainable waste valorization and reduces
environmental impact. In addition, heterogeneous catalysis minimizes soap formation and wastewater generation
during biodiesel purification. Nevertheless, catalyst calcination remains energy-intensive, and future studies
should focus on reducing thermal energy requirements for sustainable large-scale production.
CONCLUSION
This study successfully achieved its primary goal of synthesizing a high-performance ZnO-doped CaO/Anthill
clay composite using calcium carbide residue and anthill clay as sustainable precursors. By evaluating a thermal
range from 650 °C to 850 °C, it was determined that the calcination temperature is the critical factor in balancing
chemical activation with structural stability. The research identified 750 °C as the optimal temperature, as it
successfully maximized the density of active sites while preventing the structural degradation and sintering
observed at higher temperatures. Testing the catalyst's effectiveness on palm kernel oil confirmed its high
efficiency, reaching a peak biodiesel yield of 92.73% with catalyst reusability evaluation giving an overall
reduction of 27.69 % in biodiesel yield across 6 cycles. These findings demonstrate that utilizing waste materials
provides a cost-effective and environmentally friendly pathway for biodiesel production. The produced biodiesel
properties aligned with ASTM D6571 and EN 14214 standards upon characterization. This study contributes to
the promotion of renewable and sustainable energy.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the Petroleum Technology Development Fund (PTDF) for supporting thi s
study, through the PTDF Professorial Chair on Renewable Energy in the Department of Chemical Engineering,
University of Benin, Nigeria.
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