Steel Slag Decorated with Calcium Oxide and Cerium Oxide as a Solid Base for Effective Transesterification of Palm Oil

Abstract

For further resource utilization of solid waste steel slag and the reduction in biodiesel production costs, this study used steel slag as a carrier to synthesize a CaO-CeO2/slag solid base catalyst for the effective transesterification of palm oil into fatty acid methyl esters (FAMEs). The synthesis involved a two-step impregnation of steel slag with nitrate of calcium and cerium and thermal activation at 800 °C for 180 min. Then, the catalysts’ textural, chemical, and CO2 temperature-programmed desorption properties were characterized. The catalytic activity depended highly on the ratio of Ca-Ce to steel slag mass; the CaO-CeO2/slag-0.8 catalyst showed outstanding performance. Characterization showed that the surface area and total basicity of the Ca-Ce/slag-0.8 catalyst were 3.66 m²/g and 1.289 mmol/g, respectively. The reactivity results showed that FAMEs obtained using 7 wt.% catalyst, 9:1 of methanol-to-palm-oil molar ratio, 180 min reaction duration, and 70 °C reaction temperature was optimum (i.e., 95.3% yield). In addition, the CaO-CeO2/slag-0.8 catalyst could be reused for at least three cycles, retaining 91.2% of FAMEs yield after n-hexane washing. Hence, the catalyst exhibits an excellent potential for cost-effective and environmentally friendly biodiesel production.

Full text

Citation: Sun, J.; Yu, H.; Zhang, P.; Qi,
G.; Chen, X.; Liang, X.; Si, H. Steel
Slag Decorated with Calcium Oxide
and Cerium Oxide as a Solid Base for
Effective Transesterification of Palm
Oil. Processes 2023,11, 1810. https://
doi.org/10.3390/pr11061810
Academic Editor: Aldo Muntoni
Received: 22 May 2023
Revised: 7 June 2023
Accepted: 12 June 2023
Published: 14 June 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
processes
Article
Steel Slag Decorated with Calcium Oxide and Cerium Oxide as
a Solid Base for Effective Transesterification of Palm Oil
Jichao Sun 1, Hewei Yu 1,*, Peisen Zhang 1, Gaoyu Qi 1, Xiuxiu Chen 2, Xiaohui Liang 2and Hongyu Si 2,*
1School of Energy and Power Engineering, Qilu University of Technology (Shandong Academy of Sciences),
Jinan 250353, China
2Energy Research Institute, Qilu University of Technology (Shandong Academy of Sciences),
Jinan 250013, China
*Correspondence: yhw@qlu.edu.cn (H.Y.); sihyusderi@163.com (H.S.)
Abstract:
For further resource utilization of solid waste steel slag and the reduction in biodiesel
production costs, this study used steel slag as a carrier to synthesize a CaO-CeO
2
/slag solid base
catalyst for the effective transesterification of palm oil into fatty acid methyl esters (FAMEs). The
synthesis involved a two-step impregnation of steel slag with nitrate of calcium and cerium and
thermal activation at 800
◦
C for 180 min. Then, the catalysts’ textural, chemical, and CO
2
temperature-
programmed desorption properties were characterized. The catalytic activity depended highly on the
ratio of Ca-Ce to steel slag mass; the CaO-CeO
2
/slag-0.8 catalyst showed outstanding performance.
Characterization showed that the surface area and total basicity of the Ca-Ce/slag-0.8 catalyst were
3.66 m
2
/g and 1.289 mmol/g, respectively. The reactivity results showed that FAMEs obtained
using 7 wt.% catalyst, 9:1 of methanol-to-palm-oil molar ratio, 180 min reaction duration, and 70
◦
C
reaction temperature was optimum (i.e., 95.3% yield). In addition, the CaO-CeO
2
/slag-0.8 catalyst
could be reused for at least three cycles, retaining 91.2% of FAMEs yield after n-hexane washing.
Hence, the catalyst exhibits an excellent potential for cost-effective and environmentally friendly
biodiesel production.
Keywords: steel slag; calcium oxide; cerium oxide; transesterification; biodiesel
1. Introduction
The energy shortage and pollutant emission crisis have prompted researchers to
develop new energy sources to replace traditional fossil fuels. Biofuel refers to solid, liquid,
or gas fuel made of or extracted from biomass, and is an essential direction in developing
and using renewable energy. Biodiesel has become one of the clean energy substitutes
for ordinary petrochemical diesel because of its reproducibility, high calorific value, and
environmental friendliness. Biodiesel is chemically classified as fatty acid methyl esters
(FAMEs) [
1
]. It is usually obtained by the transesterification of triglycerides (e.g., vegetable
oil, animal fat, microalgae oil, and waste oil) and alcohol (typically methanol) in the
presence of a catalyst (Figure 1) [2].
Catalysis is crucial to biodiesel technology. Homogeneous acid and base catalysts have
high catalytic efficiencies, but they portend recycling and pH-neutral wastewater challenges.
Solid acid catalysts can synergistically catalyze the esterification and transesterification of
waste oil. However, they usually require a much higher reaction temperature or longer
reaction time. Solid base catalysts are easy to separate, and the transesterification conditions
are mild, conducive to biodiesel’s continuous and large-scale sustainable production with
in-depth research value.
Several types of solid bases (such as alkali earth metal oxides, alkali metal-supported
catalysts, solid base catalysts supported by molecular sieves, hydrotalcite, anion-exchange
resins, and so on) can be used in transesterification catalysis to prepare biodiesel [
3
]. Popular
among them are the alkaline earth metal oxides (such as CaO, MgO, SrO, and BaO). They
Processes 2023,11, 1810. https://doi.org/10.3390/pr11061810 https://www.mdpi.com/journal/processes

Processes 2023,11, 1810 2 of 16
are solid base catalysts whose basic sites mainly come from oxygen with negative electric
lattice and hydroxyl group generated by water adsorption on the surface. CaO is considered
one of the most promising solid base catalysts for biodiesel production due to its high
basicity, availability, and economy. Ya¸sar et al. [
4
] used CaO derived from calcinated waste
eggshell to catalyze rapeseed oil and methanol under the optimized reaction conditions
of 4% catalyst, 1 h reaction time at 60
◦
C, to achieve 95.12% biodiesel yield. However, in
the reaction system with only a CaO catalyst, the leaching of Ca
2+
species into the reaction
media was a crucial problem affecting the stability of the catalyst [
5
]. Pandit et al. [
6
]
confirmed that in the catalytic transesterification of microalgal oil and methanol by CaO
catalyst, Ca
2+
leaching would reduce the availability of the active site of reactants, reducing
the conversion from 89.7% to 78.2% after three cycles.
Processes 2023, 11, x FOR PEER REVIEW 3 of 17
Figure 1. Schematic diagram of transesterification reaction.
2. Materials and Methods
2.1. Materials
The analytical grade reagents of cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O)
(>99.0% purity), calcium(II) nitrate tetrahydrate (Ca(NO3)2·4H2O) (>98.5% purity), anhy-
drous methanol (>99.5% purity), anhydrous ethanol (>99.7% purity) were purchased from
Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The chromatographic grade re-
agents n-hexane (>99.9% purity) and methyl salicylate (>99.5% purity) were supplied by
Aladdin Reagent Co., Ltd. (Shanghai, China).
Steel slag was collected from a steel mill at Yanshan City located in Hebei Province,
China. The chemical composition of the original steel slag (Table 1) was analyzed by X-
ray fluorescence (XRF). The palm oil was procured from Guangdong Province, China,
which was native to Malaysia. Its compositions were quantitatively analyzed with
myristic acid (C14:0, 1.2%), palmitic acid (C16:0, 39.51%), lauric acid (C17:0, 0.27%), stearic
acid (C18:0, 7.8%), arachidic acid (C20:0, 0.33%), oleic acid (C18:1, 39.71%), and linoleic
acid (C18:2, 11.13%). Its acid value (AV ) and saponification value (SV), seen in Table 2,
were determined through GB 5009.229-2016 and GB/T 5534-2008 criteria, respectively. Be-
sides, the palm oil’s average relative molecular weight (M, g/mol) can be estimated using
Equation (1).
M = 1000 × 3 × 56.1
𝑆𝑉 − 𝐴𝑉 (1)
Table 1. Chemical composition of the original steel slag/%.
CaO SiO2 Al2O3 MgO SO3 TiO2 P2O5 Fe2O3 MnO K2O Na2O Other
41.36 31.09 13.34 8.85 2.30 1.11 0.40 0.38 0.36 0.34 0.34 0.13
Table 2. Basic indices of the palm oil used in the current study.
Reagent AV
mgKOH/g
SV
mgKOH/g
M
g/mol
Palm oil 0.42 182.60 923.98
2.2.
Catalyst Synthesis and Evaluation of Activity
The steel slag was ground into a fine powder using a pulverizer, and sieved using a
125 µm standard sieve. Ca(NO3)2·4H2O and Ce(NO3)3·6H2O, which provided the active
site components, and the steel slag as their carriers were successively dissolved in deion-
ized water in a particular proportion and impregnated at room temperature for 120 min
using a magnetic agitator. After soaking, it was dried in a water bath at 90 °C. Afterward,
the dried catalyst precursors were placed in a muffle furnace and heated to 800 °C for 180
Figure 1. Schematic diagram of transesterification reaction.
Currently, CeO
2
is the most widely used rare earth metal oxide, capable of forming
oxygen vacancies. It has excellent catalytic oxidation performance due to its unique crystal
structure and high capacity for storing and releasing oxygen. Meanwhile, Ce exhibits
acid-base dual properties, which can improve the catalyst’s stability. Dehghani et al. [
7
]
synthesized an active and stable catalyst of CaO-loaded Ce-MCM-41 to transform waste
vegetable oil into biodiesel. A 96.8% conversion of triglycerides can be achieved at a
methanol-to-oil molar ratio of 9 with 5 wt.% of catalyst loading at 60
◦
C and for 6 h. Else-
where, Zhang et al. [
8
] prepared a novel CeO
2
@CaO catalyst via a hydrothermal method,
achieving 98% biodiesel yield after 6 h. Repeated tests indicated that >80% of biodiesel
yield could be obtained after nine reaction cycles, establishing the stability of CeO
2
@CaO.
These studies showed that catalyst stability could be improved after complexing CaO with
CeO
2
. Yet, the catalytic activity of this catalyst type should be further optimized to shorten
the reaction time.
China is the leading steel producer and supplier nation, accounting for over half of
the global steel production [
9
]. As a by-product of steel production, steel slag accounts
for 10–20% of crude steel. China’s annual output of steel slag is about 70 million tons;
the cumulative inventory exceeds 1 billion tons, and the utilization rate of steel slag is
only about 30% [
10
]. Unutilized steel slag occupies a large amount of industrial land and
causes excellent pressure on the atmosphere, soil, and water. Therefore, seeking compre-
hensive utilization of steel slag conforms to the requirement of sustainable development.
Additionally, it is a basis for developing a circular economy and building green homes.
The steel slag contains majorly CaO, SiO
2
, Al
2
O
3
, MnO, MgO, Fe
2
O
3
, P
2
O
5
, and free
calcium oxide (f-CaO) [
11
]. For resource utilization of steel slag, Liu et al. [
12
] studied the
synthesis of hydrotalcite-type mixed oxide catalysts from waste steel slag for transesterifi-
cation. Casiello et al. [
13
] verified the feasibility of steel slag as a catalyst for synthesizing
fames from soybean oil. Elsewhere, Kang et al. [
14
] prepared a novel CeO
2
-loaded porous
alkali-activated steel slag-based photocatalyst used for photocatalytic water-splitting for
hydrogen production. Lin et al. [
15
] used acid leaching–electrolyzation–calcination to treat

Processes 2023,11, 1810 3 of 16
steel slag in preparing catalytic H
2
O
2
degradation of dye wastewater as a catalyst. The
above studies fully demonstrated the effective use of steel slag in catalyst research.
The present study introduced steel slag as a carrier to combine the active components
of CaO and CeO
2
to prepare a solid base catalyst. We investigated the effect of the mass
ratio of CaO-CeO
2
to steel slag on the catalytic performance of CaO-CeO
2
/slag catalysts.
Various technologies, including N
2
adsorption–desorption, X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS), field-emission scanning electron microscopy coupled
with energy-dispersive spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy
(FTIR), and CO
2
-temperature-programmed desorption (TPD) clarified the physical and
chemical properties of the catalysts. Reaction parameters (such as catalyst dosage, methanol-
to-oil molar ratio, reaction temperature, and duration) were optimized for catalytic transes-
terification to achieve the highest biodiesel yield. Finally, the acid resistance and reusability
of the CaO-CeO
2
/slag catalyst were conducted to evaluate the catalyst’s feasibility for
biodiesel production.
2. Materials and Methods
2.1. Materials
The analytical grade reagents of cerium(III) nitrate hexahydrate (Ce(NO
3
)
3·
6H
2
O)
(>99.0% purity), calcium(II) nitrate tetrahydrate (Ca(NO
3
)
2·
4H
2
O) (>98.5% purity), anhy-
drous methanol (>99.5% purity), anhydrous ethanol (>99.7% purity) were purchased from
Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The chromatographic grade
reagents n-hexane (>99.9% purity) and methyl salicylate (>99.5% purity) were supplied by
Aladdin Reagent Co., Ltd. (Shanghai, China).
Steel slag was collected from a steel mill at Yanshan City located in Hebei Province,
China. The chemical composition of the original steel slag (Table 1) was analyzed by X-ray
fluorescence (XRF). The palm oil was procured from Guangdong Province, China, which
was native to Malaysia. Its compositions were quantitatively analyzed with myristic acid
(C14:0, 1.2%), palmitic acid (C16:0, 39.51%), lauric acid (C17:0, 0.27%), stearic acid (C18:0,
7.8%), arachidic acid (C20:0, 0.33%), oleic acid (C18:1, 39.71%), and linoleic acid (C18:2,
11.13%). Its acid value (AV) and saponification value (SV), seen in Table 2, were determined
through GB 5009.229-2016 and GB/T 5534-2008 criteria, respectively. Besides, the palm
oil’s average relative molecular weight (M, g/mol) can be estimated using Equation (1).
M=1000 ×3×56.1
SV −AV (1)
Table 1. Chemical composition of the original steel slag/%.
CaO SiO2Al2O3MgO SO3TiO2P2O5Fe2O3MnO K2O Na2O Other
41.36 31.09 13.34 8.85 2.30 1.11 0.40 0.38 0.36 0.34 0.34 0.13
Table 2. Basic indices of the palm oil used in the current study.
Reagent AV
mgKOH/g
SV
mgKOH/g
M
g/mol
Palm oil 0.42 182.60 923.98
2.2. Catalyst Synthesis and Evaluation of Activity
The steel slag was ground into a fine powder using a pulverizer, and sieved using a
125
µ
m standard sieve. Ca(NO
3
)
2·
4H
2
O and Ce(NO
3
)
3·
6H
2
O, which provided the active
site components, and the steel slag as their carriers were successively dissolved in deionized
water in a particular proportion and impregnated at room temperature for 120 min using a
magnetic agitator. After soaking, it was dried in a water bath at 90
◦
C. Afterward, the dried
catalyst precursors were placed in a muffle furnace and heated to 800
◦
C for 180 min at a

Processes 2023,11, 1810 4 of 16
5
◦
C/min heating rate to obtain the Ca-Ce/slag-xcatalyst. Here, xrepresents the mass ratio
of CaO to steel slag, calculated from the mass of Ca(NO
3
)
2·
4H
2
O. During the experiment, x
was 0.4, 0.6, 0.8, and 1.0. Here, the mass ratio of CaO to CeO2was fixed at 1:1.
The synthesized Ca-Ce/slag-xcatalyst catalyzed the transesterification of palm oil
and methanol, and the catalytic activity was evaluated. The entire experimental flow
chart is exhibited in Figure 2. A total of 20 g of palm oil was first poured into a 250 mL
three-necked flask. Then, the Ca-Ce/slag-xcatalyst and methanol (in proportion to palm
oil) were transferred to the flask. The reaction flask was placed in the microwave reactor. A
thermocouple was inserted to measure the reaction temperature, while magnetic stirring
ensured the homogeneity of the reactants. After the transesterification, the Ca-Ce/slag-x
catalyst and liquid products were separated via centrifugation. The liquid products were
transferred into a separatory funnel to realize the layering of crude biodiesel and glycerin.
Then, the crude biodiesel was dried at 105
◦
C to remove the unreacted methanol before gas
chromatographic (GC) analysis.
Processes 2023, 11, x FOR PEER REVIEW 4 of 17
min at a 5 °C/min heating rate to obtain the Ca-Ce/slag-x catalyst. Here, x represents the
mass ratio of CaO to steel slag, calculated from the mass of Ca(NO3)2·4H2O. During the
experiment, x was 0.4, 0.6, 0.8, and 1.0. Here, the mass ratio of CaO to CeO2 was fixed at
1:1.
The synthesized Ca-Ce/slag-x catalyst catalyzed the transesterification of palm oil
and methanol, and the catalytic activity was evaluated. The entire experimental flow chart
is exhibited in Figure 2. A total of 20 g of palm oil was first poured into a 250 mL three-
necked flask. Then, the Ca-Ce/slag-x catalyst and methanol (in proportion to palm oil)
were transferred to the flask. The reaction flask was placed in the microwave reactor. A
thermocouple was inserted to measure the reaction temperature, while magnetic stirring
ensured the homogeneity of the reactants. After the transesterification, the Ca-Ce/slag-x
catalyst and liquid products were separated via centrifugation. The liquid products were
transferred into a separatory funnel to realize the layering of crude biodiesel and glycerin.
Then, the crude biodiesel was dried at 105 °C to remove the unreacted methanol before
gas chromatographic (GC) analysis.
Figure 2. The experimental flow chart of catalyst synthesis and transesterification.
The FAMEs were assessed using an 8890 gas chromatograph (Agilent, California,
USA) equipped with a flame ionization detector (FID) and an HP-5 (30 m × 320 µm × 0.25
µm) column and determined by internal standardization. n-hexane was the solvent used
to prepare the GC sample, while methyl salicylate served as the internal standard. The
injector and FID temperatures were 250 and 300 °C, respectively. The programmed tem-
perature of the column oven was set as follows: start at 140 °C (keep for 4 min) and ramp
at 10 °C/min to 260 °C (keep for 12 min). High-purity nitrogen was the carrier gas with a
split ratio of 25:1. FAMEs’ yield of the products was analyzed according to the reference
method with methyl salicylate, and calculated using the following Equation (2) [16]:
FAM Es ′ y ie ld󰇛%󰇜=Σ
𝑓

𝐴

𝐴

×𝑚
𝑚
× 100% (2)
where Aester is the peak area of FAME, Ainternal is the peak area of internal standard, minternal is
the mass of methyl salicylate, msample is the mass of the sample used, and fester is the correc-
tion factor of the FAME.
2.3. Catalyst Characterization
The XPS analyses were carried out on a Thermo escalab 250Xi (Thermo Fisher, Wal-
tham, MA, USA) with Al Kα radiation (hν = 1486.6 eV). The test results were corrected
with C1s as 284.8 eV. Further, XRD spectra were obtained using a SmartLab apparatus
(Rigaku, Tokyo, Japan) to investigate the crystalline phases in the 2θ range from 5° to 90°
Figure 2. The experimental flow chart of catalyst synthesis and transesterification.
The FAMEs were assessed using an 8890 gas chromatograph (Agilent, California, USA)
equipped with a flame ionization detector (FID) and an HP-5 (30 m
×
320
µ
m
×
0.25
µ
m)
column and determined by internal standardization. n-hexane was the solvent used to
prepare the GC sample, while methyl salicylate served as the internal standard. The injector
and FID temperatures were 250 and 300
◦
C, respectively. The programmed temperature of
the column oven was set as follows: start at 140
◦
C (keep for 4 min) and ramp at 10
◦
C/min
to 260
◦
C (keep for 12 min). High-purity nitrogen was the carrier gas with a split ratio of
25:1. FAMEs’ yield of the products was analyzed according to the reference method with
methyl salicylate, and calculated using the following Equation (2) [16]:
FAMEs’ yield(%)=Σfester Aester
Ainter nal
×minter nal
msam ple
×100% (2)
where A
ester
is the peak area of FAME, A
internal
is the peak area of internal standard, m
internal
is the mass of methyl salicylate, m
sample
is the mass of the sample used, and f
ester
is the
correction factor of the FAME.
2.3. Catalyst Characterization
The XPS analyses were carried out on a Thermo escalab 250Xi (Thermo Fisher, Waltham,
MA, USA) with Al K
α
radiation (h
ν
= 1486.6 eV). The test results were corrected with
C1s as 284.8 eV. Further, XRD spectra were obtained using a SmartLab apparatus (Rigaku,
Tokyo, Japan) to investigate the crystalline phases in the 2
θ
range from 5
◦
to 90
◦
with
a step size of 0.02
◦
and at a scanning speed of 5
◦
/min. The tube voltage and electricity

Processes 2023,11, 1810 5 of 16
current were 40 kV and 30 mA, respectively. Additionally, the average crystalline size of
the catalyst was evaluated using the Debye–Scherrer equation, expressed as D = K
γ
/
β
cos
θ
,
where K is the Scherrer constant,
γ
is the wavelength of the X-ray,
β
is the half-peak width
of the diffraction peak of the measured sample, and
θ
is the Bragg angle. The catalyst’s
morphology was inspected by SUPRA
™
55 FE-SEM (Zeiss, Oberkochen, Germany) with
an accelerating voltage of 5 kV. Additionally, the elemental distribution of the catalyst was
synchronously detected by the EDS instrument. Here, the sample was sprayed with Au to
enhance its electrical conductivity. In addition, the textural properties of the catalyst were
determined by N
2
adsorption and desorption using ASAP 2460 surface area and a porosity
analyzer (Micromeritics, Norcross, Georgia, USA) at -196
◦
C. Before the adsorption test, the
sample was degassed at 100
◦
C for 3 h under a vacuum. The Brunauer–Emmer–Teller (BET)
absorption curve and the Barrett–Joyner–Halenda (BJH) model determined the catalyst’s
surface area and pore size distribution. The FTIR spectroscopy evaluated the functional
groups of the catalyst on an iS50/6700 spectrometer (Thermo Fisher Scientific, Waltham,
MA, USA) with a wavenumber of 400 to 4000 cm
−1
. The basic strength and basicity of
the catalysts were quantitated by CO
2
-TPD, using a TP-5080 (Xianquan, Tianjin, China)
chemical adsorption instrument. Briefly, helium gas (pre-treated) first purged the sample
at 300
◦
C (ramping rate of 10
◦
C/min) for 60 min before lowering it to 50
◦
C. Then, the
carrier /reference gas was switched to CO
2
and adsorbed at a flow rate of 30 mL/min for
30 min before the carrier gas was switched back to helium at 50
◦
C for 60 min. After the
baseline was stabilized, the temperature was raised to 900
◦
C at 10
◦
C/min. Ten signals
were recorded every 1 s.
3. Results
3.1. Optimization of the Ca-Ce/Slag Catalyst Synthesis
We studied the effect of the Ca-Ce-to-slag mass ratio on the catalytic capability of
the Ca-Ce/slag-xin the transesterification of palm oil and methanol under the follow-
ing conditions: catalyst dosage = 7 wt.%, methanol-to-oil molar ratio = 15, reaction
temperature = 65 ◦C
, and reaction duration = 120 min. The results are illustrated in Figure 3.
No FAMEs’ yield was obtained when steel slag was used as catalyst. A 36.7% FAMEs’ yield
was only attained when the Ca-Ce-to-slag mass ratio was 0.4, probably due to insufficient
active component content on the catalyst surface, leading to poor catalytic activity. When
the mass ratio increased from 0.6 to 0.8, the FAMEs’ yield gradually optimized (from 55.0%)
to 85.2%, attributed to available basic sites for transesterification. However, the yield
reduced significantly to 76.2% when the mass ratio was >0.8. This occurrence may be due
to excess CaO and CeO
2
agglomeration and the tethering of active sites onto clusters. Thus,
the Ca-Ce/slag-1.0 catalytic performance was severely hindered [
17
]. Borah et al. [
18
] also
observed excess loading of Zn on CaO, which decreased the conversion because of the
structural distortion of the parent catalyst. Thus, the 0.8 mass ratio of Ca-Ce-to-slag was
selected as optimum for the catalyst synthesis.
3.2. Catalyst Characterization
3.2.1. XRD Analysis
The XRD patterns of slag and Ca-Ce/slag-xcatalysts are shown in Figure 4. The slag
shows a considerably broad peak at 2
θ
of 20–40
◦
, ascribed to aromatic carbon sheets [
19
],
indicating the presence of uncombined carbon. For the Ca-Ce/slag-xcatalysts, the peaks at
2
θ
= 32.18
◦
, 37.34
◦
, 53.84
◦
, 64.12
◦
, 67.34
◦
, and 79.62
◦
are attributable to the crystal faces of
(111), (200), (220), (311), (222), and (400) for CaO, whereas the peaks at 2
θ
=28.52
◦
, 33.08
◦
,
47.48
◦
, 56.30
◦
, 59.06
◦
, 69.36
◦
, 76.68
◦
, 79.00
◦
, and 88.42
◦
are assigned to the crystal faces of
(111), (200), (220), (311), (222), (400), (331), (420), and (422) for CeO
2
[
20
]. We observed that
the diffraction peak intensity of CaO and CeO
2
increased correspondingly with the Ca and
Ce loading. Moreover, using the Debye–Scherrer equation, the average crystalline sizes
of CaO and CeO
2
in the catalyst were calculated as 55.63, 63.41, 69.89, and 14.01, 14.98,
16.47 nm for the Ca-Ce/slag-0.4 catalyst, Ca-Ce/slag-0.6, and Ca-Ce/slag-1.0 catalysts,

Processes 2023,11, 1810 6 of 16
respectively. The small average crystalline size enhances the specific surface area and pore
volume of the catalyst, thus guaranteeing the catalytic activity. For the Ca-Ce/slag-1.0
catalyst, numerous accumulations, and agglomerations of CaO and CeO
2
resulted in a large
average crystalline size, explaining the excessive addition of active Ca and Ce components
that lowered the catalyst’s activity (Figure 3).
Processes 2023, 11, x FOR PEER REVIEW 6 of 17
Figure 3. Effect of Ca-Ce-to-slag mass ratio.
3.2. Catalyst Characterization
3.2.1. XRD Analysis
The XRD paerns of slag and Ca-Ce/slag-x catalysts are shown in Figure 4. The slag
shows a considerably broad peak at 2θ of 20–40°, ascribed to aromatic carbon sheets [19],
indicating the presence of uncombined carbon. For the Ca-Ce/slag-x catalysts, the peaks
at 2θ = 32.18°, 37.34°, 53.84°, 64.12°, 67.34°, and 79.62° are aributable to the crystal faces
of (111), (200), (220), (311), (222), and (400) for CaO, whereas the peaks at 2θ =28.52°, 33.08°,
47.48°, 56.30°, 59.06°, 69.36°, 76.68°, 79.00°, and 88.42° are assigned to the crystal faces of
(111), (200), (220), (311), (222), (400), (331), (420), and (422) for CeO2 [20]. We observed that
the diffraction peak intensity of CaO and CeO2 increased correspondingly with the Ca and
Ce loading. Moreover, using the Debye–Scherrer equation, the average crystalline sizes of
CaO and CeO2 in the catalyst were calculated as 55.63, 63.41, 69.89, and 14.01, 14.98, 16.47
nm for the Ca-Ce/slag-0.4 catalyst, Ca-Ce/slag-0.6, and Ca-Ce/slag-1.0 catalysts, respec-
tively. The small average crystalline size enhances the specific surface area and pore vol-
ume of the catalyst, thus guaranteeing the catalytic activity. For the Ca-Ce/slag-1.0 cata-
lyst, numerous accumulations, and agglomerations of CaO and CeO2 resulted in a large
average crystalline size, explaining the excessive addition of active Ca and Ce components
that lowered the catalyst’s activity (Figure 3).
Figure 4. XRD paerns of the slag and Ca-Ce/slag-x catalysts.
0.40.60.81.0
0
10
20
30
40
50
60
70
80
90
100
FAMEs yield/%
Mass ratio of Ca-Ce to slag
10 20 30 40 50 60 70 80 90
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦♦
♦
♦
♦
♦
♦
♦
♦
♦
∗
∗
∗
∗
∗♦
♦
♦
♦♦♦
♦
♦
CaO
slag
Ca-Ce/slag-0.4
Ca-Ce/slag-0.8
Intensity/a.u.
2θ
/°
Ca-Ce/slag-1.0
CeO2
♦∗
Figure 3. Effect of Ca-Ce-to-slag mass ratio.
Processes 2023, 11, x FOR PEER REVIEW 6 of 17
Figure 3. Effect of Ca-Ce-to-slag mass ratio.
3.2. Catalyst Characterization
3.2.1. XRD Analysis
The XRD paerns of slag and Ca-Ce/slag-x catalysts are shown in Figure 4. The slag
shows a considerably broad peak at 2θ of 20–40°, ascribed to aromatic carbon sheets [19],
indicating the presence of uncombined carbon. For the Ca-Ce/slag-x catalysts, the peaks
at 2θ = 32.18°, 37.34°, 53.84°, 64.12°, 67.34°, and 79.62° are aributable to the crystal faces
of (111), (200), (220), (311), (222), and (400) for CaO, whereas the peaks at 2θ =28.52°, 33.08°,
47.48°, 56.30°, 59.06°, 69.36°, 76.68°, 79.00°, and 88.42° are assigned to the crystal faces of
(111), (200), (220), (311), (222), (400), (331), (420), and (422) for CeO2 [20]. We observed that
the diffraction peak intensity of CaO and CeO2 increased correspondingly with the Ca and
Ce loading. Moreover, using the Debye–Scherrer equation, the average crystalline sizes of
CaO and CeO2 in the catalyst were calculated as 55.63, 63.41, 69.89, and 14.01, 14.98, 16.47
nm for the Ca-Ce/slag-0.4 catalyst, Ca-Ce/slag-0.6, and Ca-Ce/slag-1.0 catalysts, respec-
tively. The small average crystalline size enhances the specific surface area and pore vol-
ume of the catalyst, thus guaranteeing the catalytic activity. For the Ca-Ce/slag-1.0 cata-
lyst, numerous accumulations, and agglomerations of CaO and CeO2 resulted in a large
average crystalline size, explaining the excessive addition of active Ca and Ce components
that lowered the catalyst’s activity (Figure 3).
Figure 4. XRD paerns of the slag and Ca-Ce/slag-x catalysts.
0.40.60.81.0
0
10
20
30
40
50
60
70
80
90
100
FAMEs yield/%
Mass ratio of Ca-Ce to slag
10 20 30 40 50 60 70 80 90
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
∗
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦♦
♦
♦
♦
♦
♦
♦
♦
♦
∗
∗
∗
∗
∗♦
♦
♦
♦♦♦
♦
♦
CaO
slag
Ca-Ce/slag-0.4
Ca-Ce/slag-0.8
Intensity/a.u.
2θ
/°
Ca-Ce/slag-1.0
CeO2
♦∗
Figure 4. XRD patterns of the slag and Ca-Ce/slag-xcatalysts.
3.2.2. N2Adsorption–Desorption Analysis
The microstructural properties of the slag and Ca-Ce/slag-xcatalysts are listed in
Table 3. The BET surface area of the slag, Ca-Ce/slag-0.4, and Ca-Ce/slag-0.8 catalysts are
0.27, 1.29, and 3.66 m
2
/g, respectively. These values indicate that CaO and CeO
2
determine
the BET surface area of the catalysts within a specific numerical range. Moreover, the pore
volume for the slag, Ca-Ce/slag-0.4, and Ca-Ce/slag-0.8 catalysts are 0.00049, 0.022, and
0.043 cm
3
/g, respectively. In general, the microstructural properties of Ca-Ce/slag-xare
superior to those of the steel slag carrier. However, those of Ca-Ce/slag-0.8 are better than
Ca-Ce/slag-0.4 catalyst, especially the average pore diameter (39.27 nm for the former;
22.19 nm for the latter). The molecular size of typical triglycerides is 5 nm [
21
]; hence, larger
average pore sizes are more conducive to their transportation, providing better accessibility
to the catalysts’ active sites and favoring transesterification. The N
2
adsorption–desorption
curve and pore size distribution plot of the Ca-Ce/slag-0.8 catalyst are depicted in Figure 5.

Processes 2023,11, 1810 7 of 16
The catalyst exhibits a type V isotherm, having H3-type hysteresis loops, confirming the
slit formed by the accumulation of flake particles [
22
]. The pore size distribution of the Ca-
Ce/slag-0.8 extends from 10 to 100 nm, showing the predominance of mesopores
(2–50 nm)
and macrospores (>50 nm). As previously established, these pore size distributions could
be crucial to the catalytic biodiesel production process, conducive to mass transfer and
diffusion [23].
Table 3. Microstructural properties of the slag and Ca-Ce/slag-xcatalysts.
Samples BET Surface
Area (m2/g)
Pore Volume
(cm3/g)
Average Pore
Diameter (nm)
slag 0.27 0.00049 -
Ca-Ce/slag-0.4 1.29 0.022 22.19
Ca-Ce/slag-0.8 3.66 0.043 39.27
Processes 2023, 11, x FOR PEER REVIEW 7 of 17
3.2.2. N2 Adsorption–Desorption Analysis
The microstructural properties of the slag and Ca-Ce/slag-x catalysts are listed in Ta-
ble 3. The BET surface area of the slag, Ca-Ce/slag-0.4, and Ca-Ce/slag-0.8 catalysts are
0.27, 1.29, and 3.66 m2/g, respectively. These values indicate that CaO and CeO2 determine
the BET surface area of the catalysts within a specific numerical range. Moreover, the pore
volume for the slag, Ca-Ce/slag-0.4, and Ca-Ce/slag-0.8 catalysts are 0.00049, 0.022, and
0.043 cm3/g, respectively. In general, the microstructural properties of Ca-Ce/slag-x are
superior to those of the steel slag carrier. However, those of Ca-Ce/slag-0.8 are beer than
Ca-Ce/slag-0.4 catalyst, especially the average pore diameter (39.27 nm for the former;
22.19 nm for the laer). The molecular size of typical triglycerides is 5 nm [21]; hence,
larger average pore sizes are more conducive to their transportation, providing beer ac-
cessibility to the catalysts’ active sites and favoring transesterification. The N2 adsorption–
desorption curve and pore size distribution plot of the Ca-Ce/slag-0.8 catalyst are depicted
in Figure 5. The catalyst exhibits a type V isotherm, having H3-type hysteresis loops, con-
firming the slit formed by the accumulation of flake particles [22]. The pore size distribu-
tion of the Ca-Ce/slag-0.8 extends from 10 to 100 nm, showing the predominance of mes-
opores (2–50 nm) and macrospores (>50 nm). As previously established, these pore size
distributions could be crucial to the catalytic biodiesel production process, conducive to
mass transfer and diffusion [23].
0.0 0.2 0.4 0.6 0.8 1.0
0
5
10
15
20
25
30
adsorption
desorption
10 100
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
dV/dD Pore volume (cm³/g·nm)
Pore diameter/nm
Quantity adsorbed
(
cm
3
/g STP
)
Relative pressure
(
P/P
0
)
Figure 5. N2 adsorption–desorption curve of the Ca-Ce/slag-0.8 catalyst.
Table 3. Microstructural properties of the slag and Ca-Ce/slag-x catalysts.
Samples BET Surface
Area (m2/g)
Pore Volume
(cm3/g)
Average Pore
Diameter (nm)
slag 0.27 0.00049 -
Ca-Ce/slag-0.4 1.29 0.022 22.19
Ca-Ce/slag-0.8 3.66 0.043 39.27
3.2.3. SEM-EDS Analysis
The SEM characterized the morphologies of the Ca-Ce/slag-0.8 catalyst. As illustrated
in Figure 6a, the catalyst appears as uneven and irregular strips or blocks. These strip or
block structures pile on each other, with a few accumulated slit pores in between. Figure
6b depicts the images of the catalyst’s local surface magnified 30,000×. We observed that
the catalyst’s surface contains some fine particles with pores. These pores facilitate effi-
cient contact between the reactants and the active sites of the Ca-Ce/slag-0.8 catalyst.
Figure 5. N2adsorption–desorption curve of the Ca-Ce/slag-0.8 catalyst.
3.2.3. SEM-EDS Analysis
The SEM characterized the morphologies of the Ca-Ce/slag-0.8 catalyst. As illustrated
in Figure 6a, the catalyst appears as uneven and irregular strips or blocks. These strip or
block structures pile on each other, with a few accumulated slit pores in between. Figure 6b
depicts the images of the catalyst’s local surface magnified 30,000
×
. We observed that the
catalyst’s surface contains some fine particles with pores. These pores facilitate efficient
contact between the reactants and the active sites of the Ca-Ce/slag-0.8 catalyst.
Additionally, the EDS spectrum (Figure 6c) reflects the elemental composition and
proportion of the Ca-Ce/slag-0.8 catalyst. Due to the nature of metal oxide on the steel
slag carrier of the catalyst, oxygen exhibited the highest atomic percentage in the catalyst.
The weight ratio of Ca and Ce was 34.82% and 34.47%, respectively, consistent with the
parameter setting in the catalyst preparation, thereby confirming the homogeneity of the
catalyst. Meanwhile, we identified minute amounts of Mg, Al, Si, and Cl in the catalyst,
arising from the steel slag carrier.

Processes 2023,11, 1810 8 of 16
Processes 2023, 11, x FOR PEER REVIEW 8 of 17
(a) (b)
(c)
Figure 6. The SEM-EDS images of the Ca-Ce/slag-0.8 catalyst. (a) SEM image with a 10,000× mag-
nification, (b) SEM image with a 30,000× magnification, (c) EDS element spectrum.
Additionally, the EDS spectrum (Figure 6c) reflects the elemental composition and
proportion of the Ca-Ce/slag-0.8 catalyst. Due to the nature of metal oxide on the steel slag
carrier of the catalyst, oxygen exhibited the highest atomic percentage in the catalyst. The
weight ratio of Ca and Ce was 34.82% and 34.47%, respectively, consistent with the pa-
rameter seing in the catalyst preparation, thereby confirming the homogeneity of the
catalyst. Meanwhile, we identified minute amounts of Mg, Al, Si, and Cl in the catalyst,
arising from the steel slag carrier.
3.2.4. FTIR Analysis
The change in the chemical functional groups between slag and Ca-Ce/slag-x cata-
lysts was characterized by FTIR (Figure 7). For steel slag carrier, the bands at 1435 and 875
cm−1 were assigned to the in-plane bending and out-of-plane bending of the carbonate
groups, respectively [24]. For Ca-Ce/slag-x catalysts, a new band at 3641 cm−1 belonging
to the stretching of the -OH groups can be observed in the FTIR spectra due to the water-
absorbing characteristics of the CaO and CeO2 components. Significantly, the –OH groups
of the Ca-Ce/slag-0.8 catalyst showed a sharper band, indicating more water-absorbing
potential. In addition, the bands at 800–400 cm−1 belonged to Ca–O and Ce–O vibration
bonds.
Figure 6.
The SEM-EDS images of the Ca-Ce/slag-0.8 catalyst. (
a
) SEM image with a 10,000
×
magnification, (b) SEM image with a 30,000×magnification, (c) EDS element spectrum.
3.2.4. FTIR Analysis
The change in the chemical functional groups between slag and Ca-Ce/slag-xcatalysts
was characterized by FTIR (Figure 7). For steel slag carrier, the bands at 1435 and 875 cm
−1
were assigned to the in-plane bending and out-of-plane bending of the carbonate groups,
respectively [
24
]. For Ca-Ce/slag-xcatalysts, a new band at 3641 cm
−1
belonging to the
stretching of the -OH groups can be observed in the FTIR spectra due to the water-absorbing
characteristics of the CaO and CeO
2
components. Significantly, the –OH groups of the
Ca-Ce/slag-0.8 catalyst showed a sharper band, indicating more water-absorbing potential.
In addition, the bands at 800–400 cm−1belonged to Ca–O and Ce–O vibration bonds.
Processes 2023, 11, x FOR PEER REVIEW 9 of 17
4000 3500 3000 2500 2000 1500 1000 500
1471
3641
875
425
425
875
1471
3641 425
875
Transmittance/a.u.
Wavenumber/cm
−1
slag
Ca-Ce/slag-0.4
Ca-Ce/slag-0.8
1454
Figure 7. FTIR spectra of the slag and Ca-Ce/slag-x catalysts.
3.2.5. XPS Analysis
The XPS was deduced from a chemical state of the metal species of the Ca-Ce/slag-
0.8 catalyst (Figure 8). Figure 8a shows the XPS survey with photoelectron peaks of Ce 3d,
O 1s, Ca 2p, C 1s, and Si 2p at the binding energies of 930.1, 545.1, 360.1, 298.1, and 110.1
eV, respectively, reflecting the primary elements of the catalyst. Further, high-resolution
XPS spectra of O 1s (Figure 8b) can be deconvoluted into two peaks. One peak centered at
531.4 eV corresponded to the oxygen species of surface hydroxyl or carbonate groups; the
other peak centered at 528.7 eV was ascribed to the O2- in the laice of metal oxides [25].
Moreover, the high-resolution XPS spectra in Figure 8c confirmed the presence of Ca, ev-
idenced by the bands assigned to Ca 2p 1/2 at 350.5 eV and Ca 2p 3/2 at 347.0 eV. Com-
pared to the CaO peak (Ca 2p 1/2 at 349.9 eV and Ca 2p3/2 at 346.6 eV) reported in the
literature, the binding energies of Ca 2p in the Ca-Ce/slag-0.8 catalyst shifted towards a
higher values, suggesting the interaction between Ca and other elements (such as Ce),
enhancing the stability of active components [26]. The Ce 3d spectrum of the catalyst is
shown in Figure 8d. The peaks at 916.2, 907.1, 900.4, 897.8, 888.7, and 881.9 eV are typically
assigned to the Ce4+ species. In contrast, other peaks at 902.6 and 883.9 eV were aributed
to the Ce3+ species [27]. More Ce3+ species could generate more oxygen vacancies because
of the unsaturated chemical bonds and charge imbalance [28], further increasing the active
sites and, eventually, the activity on the catalyst. Hence, the concentration ratio of
Ce3+/(Ce3+ + Ce4+) of the Ca-Ce/slag-0.8 catalyst was estimated as 27.9%, confirming its cat-
alytic performance.
1200 1000 800 600 400 200 0
Intensity/a.u.
Si 2p
Ce 3d
C 1s
Ca 2p
Binding energy/eV
O 1s
(a)
536 534 532 530 528 526
−OH/CO
3
2−
O
2−
O 1s
(b)
528.7
Intensity/a.u.
Binding energy/eV
531.4
Figure 7. FTIR spectra of the slag and Ca-Ce/slag-x catalysts.

Processes 2023,11, 1810 9 of 16
3.2.5. XPS Analysis
The XPS was deduced from a chemical state of the metal species of the Ca-Ce/slag-0.8
catalyst (Figure 8). Figure 8a shows the XPS survey with photoelectron peaks of Ce 3d,
O 1s, Ca 2p, C 1s, and Si 2p at the binding energies of 930.1, 545.1, 360.1, 298.1, and 110.1
eV, respectively, reflecting the primary elements of the catalyst. Further, high-resolution
XPS spectra of O 1s (Figure 8b) can be deconvoluted into two peaks. One peak centered at
531.4 eV corresponded to the oxygen species of surface hydroxyl or carbonate groups; the
other peak centered at 528.7 eV was ascribed to the O
2-
in the lattice of metal oxides [
25
].
Moreover, the high-resolution XPS spectra in Figure 8c confirmed the presence of Ca,
evidenced by the bands assigned to Ca 2p 1/2 at 350.5 eV and Ca 2p 3/2 at 347.0 eV.
Compared to the CaO peak (Ca 2p 1/2 at 349.9 eV and Ca 2p3/2 at 346.6 eV) reported in
the literature, the binding energies of Ca 2p in the Ca-Ce/slag-0.8 catalyst shifted towards
a higher values, suggesting the interaction between Ca and other elements (such as Ce),
enhancing the stability of active components [
26
]. The Ce 3d spectrum of the catalyst
is shown in Figure 8d. The peaks at 916.2, 907.1, 900.4, 897.8, 888.7, and 881.9 eV are
typically assigned to the Ce
4+
species. In contrast, other peaks at 902.6 and 883.9 eV were
attributed to the Ce
3+
species [
27
]. More Ce
3+
species could generate more oxygen vacancies
because of the unsaturated chemical bonds and charge imbalance [
28
], further increasing
the active sites and, eventually, the activity on the catalyst. Hence, the concentration ratio
of Ce
3+
/(Ce
3+
+ Ce
4+
) of the Ca-Ce/slag-0.8 catalyst was estimated as 27.9%, confirming
its catalytic performance.
Processes 2023, 11, x FOR PEER REVIEW 9 of 17
4000 3500 3000 2500 2000 1500 1000 500
1471
3641
875
425
425
875
1471
3641 425
875
Transmittance/a.u.
Wavenumber/cm
−1
slag
Ca-Ce/slag-0.4
Ca-Ce/slag-0.8
1454
Figure 7. FTIR spectra of the slag and Ca-Ce/slag-x catalysts.
3.2.5. XPS Analysis
The XPS was deduced from a chemical state of the metal species of the Ca-Ce/slag-
0.8 catalyst (Figure 8). Figure 8a shows the XPS survey with photoelectron peaks of Ce 3d,
O 1s, Ca 2p, C 1s, and Si 2p at the binding energies of 930.1, 545.1, 360.1, 298.1, and 110.1
eV, respectively, reflecting the primary elements of the catalyst. Further, high-resolution
XPS spectra of O 1s (Figure 8b) can be deconvoluted into two peaks. One peak centered at
531.4 eV corresponded to the oxygen species of surface hydroxyl or carbonate groups; the
other peak centered at 528.7 eV was ascribed to the O2- in the laice of metal oxides [25].
Moreover, the high-resolution XPS spectra in Figure 8c confirmed the presence of Ca, ev-
idenced by the bands assigned to Ca 2p 1/2 at 350.5 eV and Ca 2p 3/2 at 347.0 eV. Com-
pared to the CaO peak (Ca 2p 1/2 at 349.9 eV and Ca 2p3/2 at 346.6 eV) reported in the
literature, the binding energies of Ca 2p in the Ca-Ce/slag-0.8 catalyst shifted towards a
higher values, suggesting the interaction between Ca and other elements (such as Ce),
enhancing the stability of active components [26]. The Ce 3d spectrum of the catalyst is
shown in Figure 8d. The peaks at 916.2, 907.1, 900.4, 897.8, 888.7, and 881.9 eV are typically
assigned to the Ce4+ species. In contrast, other peaks at 902.6 and 883.9 eV were aributed
to the Ce3+ species [27]. More Ce3+ species could generate more oxygen vacancies because
of the unsaturated chemical bonds and charge imbalance [28], further increasing the active
sites and, eventually, the activity on the catalyst. Hence, the concentration ratio of
Ce3+/(Ce3+ + Ce4+) of the Ca-Ce/slag-0.8 catalyst was estimated as 27.9%, confirming its cat-
alytic performance.
1200 1000 800 600 400 200 0
Intensity/a.u.
Si 2p
Ce 3d
C 1s
Ca 2p
Binding energy/eV
O 1s
(a)
536 534 532 530 528 526
−OH/CO
3
2−
O
2−
O 1s
(b)
528.7
Intensity/a.u.
Binding energy/eV
531.4
Processes 2023, 11, x FOR PEER REVIEW 10 of 17
354 352 350 348 346 344 342
Ca 2p 3/2
Ca 2p 1/2
Ca 2p
347.0
350.5
(c)
Intensity/a.u.
Binding energy/eV
930 920 910 900 890 880 870
Ce
3+
Ce
3+
Ce
4+
881.9
883.9
888.7
897.8
900.4
902.6
907.1
(d) Ce 3d
Intensity/a.u.
Binding energy/eV
916.2
Figure 8. XPS spectra of the Ca-Ce/slag-0.8 catalyst. (a) XPS survey; (b) O 1s; (c) Ca 2p; (d) Ce 3d.
3.2.6. CO2-TPD Analysis
The CO2-TPD measurement studied the surface basicity of the slag and Ca-Ce/slag-x
catalysts and the results are exhibited in Figure 9 and Table 4. Typically, surface basicity
is a crucial index in determining the catalytic activity of solid base catalysts. The TPD pro-
file of the steel slag exhibited three desorption peaks, with a basicity of 0.009, 0.110, and
0.069 mmol/g during the temperature intervals of 50–530, 530–745, and 745–900 °C, re-
spectively. The total basicity of the slag added up to 0.188 mmol/g. It was challenging to
support catalytic transesterification based on this low value. The Ca-Ce/slag-0.4 and Ca-
Ce/slag-0.8 catalysts exhibited no desorption peak at 50–340 °C, demonstrating the negli-
gible weak basic sites. For the Ca-Ce/slag-0.4 catalyst, two distinct desorption peaks were
revealed at temperature intervals of 350–540 and 540–750 °C, correlating to the moderate
and strong base sites, with respective basicities of 0.235 and 0.193 mmol/g. For the Ca-
Ce/slag-0.8 catalyst, the corresponding temperature intervals of the moderate and strong
base sites were shown at 340–560 and 560–800 °C, with corresponding basicities of 0.758
and 0.509 mmol/g. With the increased additive of active components Ca and Ce, the tem-
perature range of desorption peaks moved to the high-temperature region, and the basic-
ity increased correspondingly. The total basicity of the Ca-Ce/slag-0.4 and Ca-Ce/slag-0.8
catalysts was 0.445 and 1.289 mmol/g, respectively. In summary, the adequate basicity of
the Ca-Ce/slag-0.8 catalyst can guarantee a high catalytic performance in transesterifica-
tion. This finding compares with the HNTS-La /Ca catalyst, whose total basicity is 0.809
mmol/g [29].
Figure 8. XPS spectra of the Ca-Ce/slag-0.8 catalyst. (a) XPS survey; (b) O 1s; (c) Ca 2p; (d) Ce 3d.

Processes 2023,11, 1810 10 of 16
3.2.6. CO2-TPD Analysis
The CO
2
-TPD measurement studied the surface basicity of the slag and Ca-Ce/slag-x
catalysts and the results are exhibited in Figure 9and Table 4. Typically, surface basicity
is a crucial index in determining the catalytic activity of solid base catalysts. The TPD
profile of the steel slag exhibited three desorption peaks, with a basicity of 0.009, 0.110,
and 0.069 mmol/g during the temperature intervals of 50–530, 530–745, and 745–900
◦
C,
respectively. The total basicity of the slag added up to 0.188 mmol/g. It was challenging
to support catalytic transesterification based on this low value. The Ca-Ce/slag-0.4 and
Ca-Ce/slag-0.8 catalysts exhibited no desorption peak at 50–340
◦
C, demonstrating the
negligible weak basic sites. For the Ca-Ce/slag-0.4 catalyst, two distinct desorption peaks
were revealed at temperature intervals of 350–540 and 540–750
◦
C, correlating to the
moderate and strong base sites, with respective basicities of 0.235 and 0.193 mmol/g. For
the Ca-Ce/slag-0.8 catalyst, the corresponding temperature intervals of the moderate and
strong base sites were shown at 340–560 and 560–800
◦
C, with corresponding basicities
of 0.758 and 0.509 mmol/g. With the increased additive of active components Ca and
Ce, the temperature range of desorption peaks moved to the high-temperature region,
and the basicity increased correspondingly. The total basicity of the Ca-Ce/slag-0.4 and
Ca-Ce/slag-0.8 catalysts was 0.445 and 1.289 mmol/g, respectively. In summary, the
adequate basicity of the Ca-Ce/slag-0.8 catalyst can guarantee a high catalytic performance
in transesterification. This finding compares with the HNTS-La /Ca catalyst, whose total
basicity is 0.809 mmol/g [29].
Processes 2023, 11, x FOR PEER REVIEW 11 of 17
0 200 400 600 800
0
4
8
12
0.0
1.5
3.0
4.5
0.0
0.4
0.8
1.2
1.6
Temperature/oC
Intensity/a.u.
Ca-Ce/slag-0.8
Ca-Ce/slag-0.4
slag
Figure 9. CO2-TPD profiles of the slag and Ca-Ce/slag-x catalysts.
Table 4. CO2-TPD data for the slag and Ca-Ce/slag-x catalysts.
Samples Temperature
Interval/°C
Peak
Temperature/°C
Basicity
/(mmol/g)
Total
Basicity/(mmol/g)
Slag
50–530 476 0.009
0.188 530–745 656 0.110
745–900 745 0.069
Ca-Ce/slag-0.4
50–350 206 0.017
0.445 350–540 446 0.235
540–750 649 0.193
Ca-Ce/slag-0.8
50–340 168 0.022
1.289
340–560 469 0.758
560–800 664 0.509
3.3. Catalytic Activity
3.3.1. Transesterification Parameters Optimization
Using the Ca-Ce/slag-0.8 catalyst, the transesterification of palm oil with methanol to
obtain the corresponding FAMEs’ yields was investigated by studying the reaction pa-
rameters: catalyst dosage, methanol-to-palm-oil molar ratio, reaction temperature, and re-
action duration. The optimization results are shown in Figure 10.
Figure 9. CO2-TPD profiles of the slag and Ca-Ce/slag-xcatalysts.

Processes 2023,11, 1810 11 of 16
Table 4. CO2-TPD data for the slag and Ca-Ce/slag-xcatalysts.
Samples Temperature
Interval/◦C
Peak
Temperature/◦C
Basicity
/(mmol/g)
Total Basic-
ity/(mmol/g)
Slag
50–530 476 0.009
0.188
530–745 656 0.110
745–900 745 0.069
Ca-Ce/slag-0.4
50–350 206 0.017
0.445
350–540 446 0.235
540–750 649 0.193
Ca-Ce/slag-0.8
50–340 168 0.022
1.289
340–560 469 0.758
560–800 664 0.509
3.3. Catalytic Activity
3.3.1. Transesterification Parameters Optimization
Using the Ca-Ce/slag-0.8 catalyst, the transesterification of palm oil with methanol
to obtain the corresponding FAMEs’ yields was investigated by studying the reaction
parameters: catalyst dosage, methanol-to-palm-oil molar ratio, reaction temperature, and
reaction duration. The optimization results are shown in Figure 10.
Processes 2023, 11, x FOR PEER REVIEW 12 of 17
23456789101112
0
10
20
30
40
50
60
70
80
90
100
FAMEs yield/%
Catalyst dosage/wt.%
(a)
3691215
0
10
20
30
40
50
60
70
80
90
100
FAMEs yield/%
Methanol to palm oil molar ratio
(b)
50 55 60 65 70
0
10
20
30
40
50
60
70
80
90
100
FAMEs yield/%
Reaction temperature
/
o
C
(c)
0 30 60 90 120 150 180 210 240 270
0
10
20
30
40
50
60
70
80
90
100
FAMEs yield/%
Reaction duration
/
min
(d)
Figure 10. Effects of transesterification parameters on FAMEs yield: (a) Catalyst dosage; (b) Metha-
nol-to-palm-oil molar ratio; (c) Reaction temperature; (d) Reaction duration.
The optimization study of the catalyst dosage was performed using 3, 5, 7, 9, and 11
wt.% dosages for 120 min at 65 °C with a methanol-to-palm-oil molar ratio of 15. As de-
picted in Figure 10a, the catalyst dosage dramatically affects the FAMEs yield. When the
catalyst dosage was 3 wt.%, the concentration in the reaction system was sparse, making
the reaction incomplete, achieving a mere 7.5% yield. The number of O2- anion sites is
proportional to the catalyst dosage. Higher dosages can provide more sites for adsorbing
H+ from methanol to form active centers [30], thus improving the contact of the reactants
with the active centers, ultimately improving the transesterification efficiency. So, the
FAMEs’ yield was optimized (from 32.7%) to 85.2% as the catalyst dosage ranged from 5
to 7 wt.%. However, the yield started to decrease with further catalyst dosing. Figure 10a
depicts that the FAMEs’ yield depreciated to 75.5% when the catalytic dosage was further
increased to 11 wt.%. This result could be aributed to excessive catalyst dosage causing
saponification, affecting the main reaction’s transesterification, thereby lowering the reac-
tion yield [31]. In summary, a 7 wt.% catalyst dosage was selected as optimum.
The methanol-to-palm-oil molar ratio optimization was carried out under 3, 6, 9, 12,
and 15 ratios for 120 min at 65 °C with 7 wt.% catalyst dosage. The methanol-to-oil molar
ratio is vital in transesterification, where the stoichiometric ratio of transesterification be-
tween palm oil and methanol is at least 1:3 [16]. Higher positive FAMEs’ yield can be ob-
tained by increasing methanol dosage due to the reversible nature of transesterification.
As shown in Figure 10b, when the molar ratio of methanol to palm oil is on a 3-to-9 scale,
the increase in the molar ratio was beneficial in improving the FAMEs yield, where the
FAMEs yield increased from 60.1% to 90.3%. Increasing the methanol content is conducive
Figure 10.
Effects of transesterification parameters on FAMEs yield: (
a
) Catalyst dosage; (
b
) Methanol-
to-palm-oil molar ratio; (c) Reaction temperature; (d) Reaction duration.
The optimization study of the catalyst dosage was performed using 3, 5, 7, 9, and
11 wt.% dosages for 120 min at 65
◦
C with a methanol-to-palm-oil molar ratio of 15. As

Processes 2023,11, 1810 12 of 16
depicted in Figure 10a, the catalyst dosage dramatically affects the FAMEs yield. When the
catalyst dosage was 3 wt.%, the concentration in the reaction system was sparse, making
the reaction incomplete, achieving a mere 7.5% yield. The number of O
2-
anion sites is
proportional to the catalyst dosage. Higher dosages can provide more sites for adsorbing H
+
from methanol to form active centers [
30
], thus improving the contact of the reactants with
the active centers, ultimately improving the transesterification efficiency. So, the FAMEs’
yield was optimized (from 32.7%) to 85.2% as the catalyst dosage ranged from 5 to 7 wt.%.
However, the yield started to decrease with further catalyst dosing. Figure 10a depicts that
the FAMEs’ yield depreciated to 75.5% when the catalytic dosage was further increased to
11 wt.%. This result could be attributed to excessive catalyst dosage causing saponification,
affecting the main reaction’s transesterification, thereby lowering the reaction yield [
31
]. In
summary, a 7 wt.% catalyst dosage was selected as optimum.
The methanol-to-palm-oil molar ratio optimization was carried out under 3, 6, 9,
12, and 15 ratios for 120 min at 65
◦
C with 7 wt.% catalyst dosage. The methanol-to-oil
molar ratio is vital in transesterification, where the stoichiometric ratio of transesterification
between palm oil and methanol is at least 1:3 [
16
]. Higher positive FAMEs’ yield can be
obtained by increasing methanol dosage due to the reversible nature of transesterification.
As shown in Figure 10b, when the molar ratio of methanol to palm oil is on a 3-to-9 scale, the
increase in the molar ratio was beneficial in improving the FAMEs yield, where the FAMEs
yield increased from 60.1% to 90.3%. Increasing the methanol content is conducive to
reducing the viscosity of the reaction system, thereby reducing the mass transfer resistance
between reactants, and further improving the conversion efficiency of reactants. However, a
depressing trend of FAMEs yield was observed when the molar ratio exceeded 9. Excessive
methanol lowers the contact between the catalyst and palm oil, leading to a reduced yield.
Furthermore, the reaction temperature was optimized under 50, 55, 60, 65, and 70
◦
C
at 120 min duration using a 7 wt.% catalyst dosage and a methanol-to-palm-oil molar
ratio of 9. When the temperature was held at 50
◦
C (Figure 10c), the reaction between
methanol and palm oil in the presence of the catalyst resulted in the lowest FAMEs’ yield
because the temperature was lower than that required for conversion reaction into methyl
ester molecules [
32
]. In the transesterification of macromolecular triglycerides, sufficient
temperature is needed to provide enough energy to overcome the threshold. On the
other hand, increasing the temperature can reduce the reactant viscosity and improve the
miscibility, thus decreasing the mass-transfer resistance [
33
]. The FAMEs’ yield increased
substantially from 26.3% to 90.3% by raising the reaction temperature from 50 to 65
◦
C.
Further increasing the reaction temperature to 70
◦
C improved the FAMEs yield to 92.9%.
At 70
◦
C, the molecular movement rate of the reaction system was fast enough to enhance
the effective collision chance between the reactants and the catalyst, thus obtaining a higher
FAMEs’ yield.
To optimize the reaction duration within 30–240 min, we set other conditions at their
optimized values, i.e., 7 wt.% catalyst dosage, nine methanol-to-palm-oil molar ratio, and
70
◦
C. The transesterification rate is slow, especially the solid catalysts. Palm oil and
methanol are immiscible, and the whole reaction system is a catalyst–methanol–palm oil
three-phase state, cumbersome to homogenize. At the same time, the FAMEs’ yield of
transesterification was low in the initial reaction stage due to the palm oil and methanol
needing to be mixed and diffused for a long duration to adsorb to the active center on the
catalyst. As evidenced in Figure 10d, a 16.4% FAMEs’ yield was attained after the initial
30 min of transesterification. It increased significantly to 92.9% when the reaction reached
the 120 min mark. When the reaction duration was extended to 180 min, the FAMEs’ yield
slowly enhanced to 95.3%. After that, a puny increase of 1.3% FAMEs’ yield was achieved
upon increasing the duration to 240 min. We opine that the transesterification reached
equilibrium when the reaction duration exceeded 180 min.
Table 5presents the catalytic performance in the transesterification compared with
some recent reports on solid base catalysts. These studies considered the production
conversion (or yield) and transesterification operating parameters. A >90% conversion

Processes 2023,11, 1810 13 of 16
or biodiesel yield was obtained from various types of solid base catalysts under different
transesterification conditions. Based on the scientific literature, our results revealed that
the Ca-Ce/slag-0.8 catalyst has satisfactory catalytic performance and relatively moderate
transesterification conditions compared to other heterogeneous base catalysts. Moreover,
the Ca-Ce/slag-0.8 catalyst optimizes the steel slag waste, saving preparation costs, and
making it highly applicable industrially.
Table 5. Comparison of performance efficiencies of various base catalysts.
Catalyst Oil Feedstock
Transesterification Parameters
Con. or
Yield 4/% Ref.
C. Dosage 1
/wt.%
Molar
Ratio 2
Temp 3
/◦C
Time
/min
CaO/CeO2Waste seed oil 4 9 70 90 90.14 [34]
ZnO/BiFeO3Canola oil 4 15 65 360 95.43 [35]
K2CO3/γ-Al2O3Sunflower oil 5 12 80 240 99.3 [36]
Ca-Mg-Al Sunflower oil 2.5 15 60 360 95 [30]
NaOH/Chitosan-Fe3O4Waste cooking oil 0.5 6 25 270 92 [37]
biochar/CaO-K2CO3Waste edible oil 4 18 65 200 98.83 [23]
MgO/CaO nanorods Castor oil 6 15 70 70 96.2 [38]
CuO/ZnO Waste cooking oil 5 9 65 120 93.5 [39]
Acai seed ash Soybean oil 12 18 100 60 98.5 [40]
Ca-Ce/slag-0.8 Palm oil 7 9 70 180 95.3 This study
1
C. dosage—catalyst dosage;
2
Molar ratio—alcohol-to-oil molar ratio;
3
Temp—temperature;
4
Con. or yield—
conversion or yield.
3.3.2. Effect of FFAs Content
To verify the acid resistance effect of the Ca-Ce/slag-0.8 catalyst, we added 2, 5, and
8 wt.% of oleic acid to the palm oil under optimal conditions: 7 wt.% catalyst dosage,
9 methanol-to-oil molar ratio, and reaction at 70
◦
C for 180 min. With 2 and 5 wt.% oleic
acid added to the palm oil, the FAMEs’ yield decreased from 95.3% to 84.8% and 78.9%,
respectively (Figure 11). However, a 63.1% FAMEs’ yield was still achieved even when up
to 8 wt.% oleic acid was added. We found that the Ca-Ce/slag-0.8 catalyst has specific acid
resistance in transesterification, but the oil’s free fatty acids’ content should not be higher
than 5 wt.%.
Processes 2023, 11, x FOR PEER REVIEW 14 of 17
biochar/CaO-K2CO3 Waste edible
oil 4 18 65 200 98.83 [23]
MgO/CaO nanorods Castor oil 6 15 70 70 96.2 [38]
CuO/ZnO Waste cooking
oil 5 9 65 120 93.5 [39]
Acai seed ash Soybean oil 12 18 100 60 98.5 [40]
Ca-Ce/slag-0.8 Palm oil 7 9 70 180 95.3
This
study
1 C. dosage—catalyst dosage; 2 Molar ratio—alcohol-to-oil molar ratio; 3 Temp— tem peratu re; 4 Con.
or yield—conversion or yield.
3.3.2. Effect of FFAs Content
To verify the acid resistance effect of the Ca-Ce/slag-0.8 catalyst, we added 2, 5, and
8 wt.% of oleic acid to the palm oil under optimal conditions: 7 wt.% catalyst dosage, 9
methanol-to-oil molar ratio, and reaction at 70 °C for 180 min. With 2 and 5 wt.% oleic acid
added to the palm oil, the FAMEs’ yield decreased from 95.3% to 84.8% and 78.9%, respec-
tively (Figure 11). However, a 63.1% FAMEs’ yield was still achieved even when up to 8
wt.% oleic acid was added. We found that the Ca-Ce/slag-0.8 catalyst has specific acid
resistance in transesterification, but the oil’s free fay acids’ content should not be higher
than 5 wt.%.
The Ca/slag-0.8 catalyst was synthesized under the same preparation conditions for
comparison, and its acid resistance was studied (Figure 11). With the addition of oleic acid,
the FAMEs’ yield of Ca/slag-0.8 catalyst decreased faster than that of the Ca-Ce/slag-0.8
catalyst. Only 42.4% of the FAMEs’ yield could be obtained when 8 wt.% of oleic acid was
added to the transesterification reaction. This finding indicates that the acid resistance of
Ca-Ce/slag-0.8 was more enhanced than that of Ca/slag-0.8.
0123456789
40
50
60
70
80
90
100
Ca-Ce/slag-0.8
Ca/slag-0.8
FAMEs yield/%
Oleic acid
dosage/wt.%
Figure 11. Effect of oleic acid dosage on the FAMEs yield.
3.4. Catalyst Reusability
The reusability of solid catalysts is an aractive property relative to homogeneous
catalysts. Transesterification was conducted at optimized conditions for three recycling
tests. During each cycle, the Ca-Ce/slag-0.8 catalyst was reused by processing in two dif-
ferent ways: (1) Separated from the transesterification mixture by filtration and dried at
105 °C for 5 h; (2) Separated from the transesterification mixture by centrifugation and
washed thrice with n-hexane to remove polar and non-polar adsorbed compounds before
drying at 105 °C for 5 h. As depicted in Figure 12, for the direct drying method, the Ca-
Ce/slag-0.8 catalyst exhibited good stability for the first two cycles, achieving 95.3% and
90.1% FAMEs’ yields. After that, the yield depreciated with the recycling number. Only
Figure 11. Effect of oleic acid dosage on the FAMEs yield.
The Ca/slag-0.8 catalyst was synthesized under the same preparation conditions for
comparison, and its acid resistance was studied (Figure 11). With the addition of oleic acid,
the FAMEs’ yield of Ca/slag-0.8 catalyst decreased faster than that of the Ca-Ce/slag-0.8
catalyst. Only 42.4% of the FAMEs’ yield could be obtained when 8 wt.% of oleic acid was

Processes 2023,11, 1810 14 of 16
added to the transesterification reaction. This finding indicates that the acid resistance of
Ca-Ce/slag-0.8 was more enhanced than that of Ca/slag-0.8.
3.4. Catalyst Reusability
The reusability of solid catalysts is an attractive property relative to homogeneous
catalysts. Transesterification was conducted at optimized conditions for three recycling
tests. During each cycle, the Ca-Ce/slag-0.8 catalyst was reused by processing in two
different ways: (1) Separated from the transesterification mixture by filtration and dried
at 105
◦
C for 5 h; (2) Separated from the transesterification mixture by centrifugation and
washed thrice with n-hexane to remove polar and non-polar adsorbed compounds before
drying at 105
◦
C for 5 h. As depicted in Figure 12, for the direct drying method, the
Ca-Ce/slag-0.8 catalyst exhibited good stability for the first two cycles, achieving 95.3%
and 90.1% FAMEs’ yields. After that, the yield depreciated with the recycling number.
Only 82.1% of the FAMEs’ yield was achieved in the third cycle. In contrast, the FAMEs’
yield slowly decreased from 95.3% to 93.8%, and 91.2% after the three cycles, with the one
washed with n-hexane. This observation indicated that the active site on the catalyst was
covered by organic matter, which hindered the catalytic activity of the Ca-Ce/slag-0.8. Lee
et al. [
41
] reported that the palm oil conversion was reduced to 70% after the third cycles
catalyzed by waste obtuse horn shell-derived CaO catalyst. In summary, the Ca-Ce/slag-0.8
catalyst synthesized in the present study exhibited an appreciable FAMEs’ yield.
Processes 2023, 11, x FOR PEER REVIEW 15 of 17
82.1% of the FAMEs’ yield was achieved in the third cycle. In contrast, the FAMEs’ yield
slowly decreased from 95.3% to 93.8%, and 91.2% after the three cycles, with the one
washed with n-hexane. This observation indicated that the active site on the catalyst was
covered by organic maer, which hindered the catalytic activity of the Ca-Ce/slag-0.8. Lee
et al. [41] reported that the palm oil conversion was reduced to 70% after the third cycles
catalyzed by waste obtuse horn shell-derived CaO catalyst. In summary, the Ca-Ce/slag-
0.8 catalyst synthesized in the present study exhibited an appreciable FAMEs’ yield.
Cycle 1 Cycle 2 Cycle 3
0
20
40
60
80
100
120
Direct drying
Washed with n-hexane
FAMEs yield/%
Cycle number
Figure 12. Effect of recycling of the Ca-Ce/slag-0.8 catalyst on the FAMEs’ yield.
4. Discussion
This study synthesized a newly cost-effective base catalyst of Ca-Ce/slag to generate
biodiesel from palm oil. The effect of the mass ratio of Ca-Ce to steel slag on the trans-
esterification activity was investigated. Moreover, diversified characterization techniques
were employed to analyze the catalyst properties (such as crystalline phases, microscopic
pore structure, morphology, functional groups, and surface basicity). The results showed
that the mass ratio of Ca-Ce to steel slag at 0.8 (i.e., Ca-Ce/slag-0.8 catalyst) exhibited ex-
cellent physical and chemical properties. According to the range of transesterification con-
ditions selected in this study, the optimal transesterification conditions are as follows:
methanol: oil molar ratio of 9, catalyst dosage of 7 wt.%, and reaction at 70 °C for 180 min,
yielding 95.3% of FAMEs. Moreover, the Ca-Ce/slag-0.8 catalyst exhibited stable catalytic
performance after three cycles with >90% FAMEs’ yield. Consequently, this study pro-
vides a new view on the resource utilization of steel slag and an excellent prospect for
developing low-cost solid biodiesel catalysts.
Author Contributions: Conceptualization, J.S. and H.Y.; methodology, G.Q.; software, G.Q. and
P.Z.; validation, P.Z., J.S. and X.C.; formal analysis, X.L.; investigation, H.Y.; resources, J.S.; data
curation, H.Y.; writing—original draft preparation, H.S.; writing—review and editing, J.S.; visuali-
zation, H.S.; supervision, X.C.; project administration, H.Y. and H.S; funding acquisition. All authors
have read and agreed to the published version of the manuscript.
Funding: This research was funded by Open Project of Shandong Key Laboratory of Biomass Gasi-
fication Technology [BG-KFX-08], the National Natural Science Foundation of China [52206262], the
Science, Education and Industry Training New Project of Qilu University of Technology
[2022PX091], the Natural Science Foundation of Shandong Province, China [ZR2020QE210], “20
New Universities” Funding Project of Jinan [2021GXRC053] and Undergraduate Training Program
for Innovation and Entrepreneurship Program of Shandong Province.
Data Availability Statement: The data that support the findings of this study are available within
the article.
Figure 12. Effect of recycling of the Ca-Ce/slag-0.8 catalyst on the FAMEs’ yield.
4. Discussion
This study synthesized a newly cost-effective base catalyst of Ca-Ce/slag to generate
biodiesel from palm oil. The effect of the mass ratio of Ca-Ce to steel slag on the transes-
terification activity was investigated. Moreover, diversified characterization techniques
were employed to analyze the catalyst properties (such as crystalline phases, microscopic
pore structure, morphology, functional groups, and surface basicity). The results showed
that the mass ratio of Ca-Ce to steel slag at 0.8 (i.e., Ca-Ce/slag-0.8 catalyst) exhibited
excellent physical and chemical properties. According to the range of transesterification
conditions selected in this study, the optimal transesterification conditions are as follows:
methanol: oil molar ratio of 9, catalyst dosage of 7 wt.%, and reaction at 70
◦
C for 180 min,
yielding 95.3% of FAMEs. Moreover, the Ca-Ce/slag-0.8 catalyst exhibited stable catalytic
performance after three cycles with >90% FAMEs’ yield. Consequently, this study provides
a new view on the resource utilization of steel slag and an excellent prospect for developing
low-cost solid biodiesel catalysts.

Processes 2023,11, 1810 15 of 16
Author Contributions:
Conceptualization, J.S. and H.Y.; methodology, G.Q.; software, G.Q. and P.Z.;
validation, P.Z., J.S. and X.C.; formal analysis, X.L.; investigation, H.Y.; resources, J.S.; data curation,
H.Y.; writing—original draft preparation, H.S.; writing—review and editing, J.S.; visualization, H.S.;
supervision, X.C.; project administration, H.Y. and H.S; funding acquisition. All authors have read
and agreed to the published version of the manuscript.
Funding:
This research was funded by Open Project of Shandong Key Laboratory of Biomass Gasifi-
cation Technology [BG-KFX-08], the National Natural Science Foundation of China [52206262], the
Science, Education and Industry Training New Project of Qilu University of Technology [2022PX091],
the Natural Science Foundation of Shandong Province, China [ZR2020QE210], “20 New Universities”
Funding Project of Jinan [2021GXRC053] and Undergraduate Training Program for Innovation and
Entrepreneurship Program of Shandong Province.
Data Availability Statement:
The data that support the findings of this study are available within
the article.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Woranuch, W.; Ngaosuwan, K.; Kiatkittipong, W.; Wongsawaeng, D.; Appamana, W.; Powell, J.; Rokhum, S.L.; Assabumrungrat,
S. Fine-tuned fabrication parameters of CaO catalyst pellets for transesterification of palm oil to biodiesel. Fuel
2022
,323, 124356.
[CrossRef]
2.
Li, H.; Wang, Y.; Ma, X.; Guo, M.; Li, Y.; Li, G.; Cui, P.; Zhou, S.; Yu, M. Synthesis of CaO/ZrO
2
based catalyst by using UiO–66(Zr)
and calcium acetate for biodiesel production. Renew. Energy 2022,185, 970–977. [CrossRef]
3.
Zul, N.A.; Ganesan, S.; Hamidon, T.S.; Oh, W.D.; Hussin, M.H. A review on the utilization of calcium oxide as a base catalyst in
biodiesel production. J. Environ. Chem. Eng. 2021,9, 105741. [CrossRef]
4.
Ya¸sar, F. Biodiesel production via waste eggshell as a low-cost heterogeneous catalyst: Its effects on some critical fuel properties
and comparison with CaO. Fuel 2019,255, 115821–115828. [CrossRef]
5.
Teo, S.H.; Rashid, U.; Taufiq-Yap, Y.H. Heterogeneous catalysis of transesterification of jatropha curcas oil over calcium-cerium
bimetallic oxide catalyst. RSC. Adv. 2014,4, 48836–48847. [CrossRef]
6.
Pandit, P.R.; Fulekar, M.H. Biodiesel production from microalgal biomass using CaO catalyst synthesized from natural waste
material. Renew. Energy 2019,136, 837–845. [CrossRef]
7.
Dehghani, S.; Haghighi, M.; Vardast, N. Structural/texture evolution of CaO/MCM nanocatalyst by doping various amounts
of cerium for active and stable catalyst: Biodiesel production from waste vegetable cooking oil. Int. J. Energy. Res.
2019
,43,
3779–3793. [CrossRef]
8.
Zhang, N.; Xue, H.; Hu, R. The activity and stability of CeO
2
@CaO catalysts for the production of biodiesel. RSC. Adv.
2018
,
8, 32922. [CrossRef]
9.
Shu, K.; Sasaki, K. Occurrence of steel converter slag and its high value-added conversion for environmental restoration in China:
A review. J. Clean. Prod. 2022,373, 133876. [CrossRef]
10.
Gan, Y.; Li, C.; Ke, W.; Deng, Q.; Yu, T. Study on pavement performance of steel slag asphalt mixture based on surface treatment.
Case. Stud. Constr. Mat. 2022,16, e1131. [CrossRef]
11.
Song, B.; Wang, Z.; Li, J.; Ma, Y. Preparation and electrocatalytic properties of kaolin/steel slag particle electrodes. Catal. Commun.
2021,148, 106177. [CrossRef]
12.
Liu, G.; Yang, J.; Xu, X. Synthesis of hydrotalcite-type mixed oxide catalysts from waste steel slag for transesterification of glycerol
and dimethyl carbonate. Sci. Rep-UK 2020,10, 10273. [CrossRef]
13.
Casiello, M.; Losito, O.; Aloia, A.; Caputo, D.; Fusco, C.; Attrotto, R.; Monopoli, A.; Nacci, A.; D’Accolti, L. Steel Slag as New
Catalyst for the Synthesis of Fames from Soybean Oil. Catalyst 2021,11, 619. [CrossRef]
14.
Kang, L.; Zhang, Y.J.; Zhang, L.; Zhang, K. Preparation, characterization and photocatalytic activity of novel CeO
2
loaded porous
alkali-activated steel slag-based binding material. Int. J. Hydrogen. Energy. 2017,42, 17341–17349. [CrossRef]
15.
Lin, S.; Zhang, T.; Cao, X.; Liu, X. Recovery of converter steel slag to prepare catalytic H
2
O
2
degradation of dye wastewater as a
catalyst. J. Mater. Sci-Mater. El. 2021,32, 24889–24901. [CrossRef]
16.
Gao, L.; Teng, G.; Xiao, G.; Wei, R. Biodiesel from palm oil via loading KF/Ca–Al hydrotalcite catalyst. Biomass Bioenergy
2010
,34,
1283–1288. [CrossRef]
17.
Kesserwan, F.; Ahmad, M.N.; Khalil, M.; El-Rassy, H. Hybrid CaO/Al
2
O
3
aerogel as heterogeneous catalyst for biodiesel
production. Chem. Eng. J. 2020,385, 123834. [CrossRef]
18.
Borah, M.J.; Das, A.; Das, V.; Bhuyan, N.; Deka, D. Transesterification of waste cooking oil for biodiesel production catalyzed by
Zn substituted waste egg shell derived CaO nanocatalyst. Fuel 2019,242, 345–354. [CrossRef]
19.
Ning, Y.; Niu, S. Preparation and catalytic performance in esterification of a bamboo-based heterogeneous acid catalyst with
microwave assistance. Energy Convers. Manag. 2017,153, 446–454. [CrossRef]

Processes 2023,11, 1810 16 of 16
20.
Qu, T.; Niu, S.; Zhang, X.; Han, K.; Lu, C. Preparation of calcium modified Zn-Ce/Al
2
O
3
heterogeneous catalyst for biodiesel
production through transesterification of palm oil with methanol optimized by response surface methodology. Fuel
2021
,
284, 118986. [CrossRef]
21.
Yadav, M.; Sharma, Y.C. Process optimization and catalyst poisoning study of biodiesel production from kusum oil using
potassium aluminum oxide as efficient and reusable heterogeneous catalyst. J. Clean. Prod. 2018,199, 593–602. [CrossRef]
22.
Zhang, Y.; Niu, S.; Kan, H.; Li, Y.; Lu, C. Synthesis of the SrO–CaO–Al
2
O
3
trimetallic oxide catalyst for transesterification to
produce biodiesel. Renew. Energy 2021,168, 981–990.
23.
Foroutan, R.; Mohammadi, R.; Razeghi, J.; Ramavandi, B. Biodiesel production from edible oils using algal biochar/CaO/K
2
CO
3
as a heterogeneous and recyclable catalyst. Renew. Energy 2021,168, 1207–1216. [CrossRef]
24.
Murguía-Ortiz, D.; Cordova, I.; Manriquez, M.E.; Ortiz-Islas, E.; Cabrera-Sierra, R.; Contreras, J.L.; Alcantar-Vazquez, B.;
Trejo-Rubio, M.; Vazquez-Rodriguez, J.T.; Castro, L.V. Na-CaO/MgO dolomites used as heterogeneous catalysts in canola oil
transesterification for biodiesel production. Mater. Lett. 2021,291, 129587. [CrossRef]
25.
Niu, S.; Zhang, X.; Ning, Y.; Zhang, Y.; Qu, T.; Hu, X.; Gong, Z.; Lu, C. Dolomite incorporated with cerium to enhance the stability
in catalyzing transesterification for biodiesel production. Renew. Energy 2020,154, 107–116. [CrossRef]
26.
Wang, J.; Yang, L.; Luo, W.; Yang, G.; Miao, C.; Fu, J.; Xing, S.; Fan, P.; Lv, P.; Wang, Z. Sustainable biodiesel production via
transesterification by using recyclable Ca2MgSi2O7catalyst. Fuel 2017,196, 306–313. [CrossRef]
27.
Ma, S.; Tan, H.; Li, Y.; Wang, P.; Zhu, Y. Excellent low-temperature NH
3
-SCR NO removal performance and enhanced H
2
O
resistance by Ce addition over the Cu0.02Fe0.2CeyTi1−yOx(y = 0.1, 0.2, 0.3) catalysts. Chemosphere 2019,243, 125309. [CrossRef]
28.
Ma, Y.; Li, W.; Wang, H.; Chen, J.; Wen, J.; Xu, S.; Tian, X.; Gao, L.; Hou, Z.; Zhang, Q. Enhanced performance of iron-cerium NOx
reduction catalysts by sulfuric acid treatment: The synergistic effect of surface acidity and redox capacity. Appl. Catal. A-Gen.
2021,621, 118200. [CrossRef]
29.
Lin, T.; Zhao, S.; Niu, S.; Lyu, Z.; Hu, X. Halloysite nanotube functionalized with La-Ca bimetallic oxides as novel transesterifica-
tion catalyst for biodiesel production with molecular simulation. Energy Convers. Manag. 2020,220, 113138. [CrossRef]
30.
Dahdah, E.; Estephane, J.; Haydar, R.; Youssef, Y.; Khoury, B.E.; Gennequin, C.; Aboukais, A.; Abi-Aad, E.; Aouad, S. Biodiesel
production from refined sunflower oil over Ca–Mg–Al catalysts: Effect of the composition and the thermal treatment. Renew.
Energy 2020,146, 1242–1248. [CrossRef]
31.
Li, H.; Niu, S.; Lu, C.; Liu, M.; Huo, M. Transesterification catalyzed by industrial waste—Lime mud doped with potassium
fluoride and the kinetic calculation. Energy Convers. Manag. 2014,86, 1110–1117. [CrossRef]
32.
Al-Muhtaseb, A.H.; Osman, A.I.; Jamil, F.; Mehta, N.; Al-Haj, L.; Coulon, F.; Al-Maawali, S.; Al Nabhani, A.; Kyaw, H.H.; Myint,
M.T.Z.; et al. Integrating life cycle assessment and characterisation techniques: A case study of biodiesel production utilising
waste Prunus Armeniaca seeds (PAS) and a novel catalyst. J. Environ. Manag. 2022,304, 114319. [CrossRef]
33.
Farooq, M.; Ramli, A.; Naeem, A. Biodiesel production from low FFA waste cooking oil using heterogeneous catalyst derived
from chicken bones. Renew. Energy 2015,76, 362–368. [CrossRef]
34.
Al-Muhtaseb, A.H.; Osman, A.I.; Kumar, P.S.M.; Jamil, F.; Al-Haj, L.; Al Nabhani, A.; Kyaw, H.H.; Myint, M.T.Z.; Mehta, N.;
Rooney, D.W. Circular economy approach of enhanced bifunctional catalytic system of CaO/CeO
2
for biodiesel production from
waste loquat seed oil with life cycle assessment study. Energy Convers. Manag. 2021,236, 114040. [CrossRef]
35.
Salimi, Z.; Hosseini, S.A. Study and optimization of conditions of biodiesel production from edible oils using ZnO/BiFeO
3
nano
magnetic catalyst. Fuel 2019,239, 1204–1212. [CrossRef]
36.
Junior, E.S.; Perez, V.H.; Reyero, I.; Serrano-Lotina, A.; Justo, O.R. Biodiesel production from heterogeneous catalysts based
K2CO3supported on extruded γ-Al2O3.Fuel 2019,241, 311–318. [CrossRef]
37.
Helmi, M.; Hemmati, A. Synthesis of magnetically solid base catalyst of NaOH/Chitosan-Fe
3
O
4
for biodiesel production from
waste cooking oil: Optimization, kinetics and thermodynamic studies. Energy Convers. Manag. 2021,248, 114807. [CrossRef]
38.
Abukhadra, M.R.; Mohamed, A.S.; El-Sherbeeny, A.M.; Soliman, A.T.A.; Elgawad, A.E.E.A. Sonication induced transesterification
of castor oil into biodiesel in the presence of MgO/CaO nanorods as a novel basic catalyst: Characterization and optimization.
Chem. Eng. Process. 2020,154, 108024. [CrossRef]
39.
Guo, M.; Jiang, W.; Ding, J.; Lu, J. Highly active and recyclable CuO/ZnO as photocatalyst for transesterification of waste cooking
oil to biodiesel and the kinetics. Fuel 2022,315, 123254. [CrossRef]
40.
Mares, E.K.L.; Gonçalves, M.A.; Lourenço, E.K.; Luz, P.T.S.; Filho, G.N.R.; Zamian, J.R.; Conceição, L.R.V. Acai seed ash as a novel
basic heterogeneous catalyst for biodiesel synthesis: Optimization of the biodiesel production process. Fuel
2021
,299, 120887.
[CrossRef]
41.
Lee, S.L.; Wong, Y.C.; Tan, Y.P.; Yew, S.Y. Transesterification of palm oil to biodiesel by using waste obtuse horn shell-derived CaO
catalyst. Energy Convers. Manag. 2015,93, 282–288. [CrossRef]
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