HernandezMartinez(2023) next materials

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Next Materials
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Canola oil transesterification for biodiesel production using potassium and
strontium supported on calcium oxide catalysts synthesized from oyster
shell residues
M.A. Hernández-Martínez a, J.A. Rodriguez a, G. Chavez-Esquivel b, D. Ángeles-Beltrán b,
J.A. Tavizón-Pozos c, *
a
Área Académica de Química, Universidad Autónoma del Estado de Hidalgo, Carr Pachuca-Tulancingo km. 4.5, Colonia Carboneras, Mineral de la Reforma, Hidalgo
42184, Mexico
b
Departamento de Ciencias Básicas, Universidad Autónoma Metropolitana Azcapotzalco, Av. San Pablo 180, Col. Reynosa Tamaulipas, Azcapotzalco, Ciudad de México
02200, Mexico
c
Investigadoras e Investigadores por México del CONAHCYT, Área Académica de Química, Universidad Autónoma del Estado de Hidalgo, Carr Pachuca-Tulancingo km.
4.5, Colonia Carboneras, Mineral de la Reforma, Hidalgo 42184, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords:
Biodiesel
Calcium Carbonate
Box-Behnken
Strontium
Potassium
Oyster shells
This work studied the effect of K precursor comparing KNO3 and KHCO3 and the combination with SrO, sup­
ported on CaO from oyster shell and limestone (1:1 mass ratio) for transesterification canola oil to produce
biodiesel. The loading of Sr and K were 9 wt% and were calcined at 800 ◦ C. The catalytic transesterification
evaluations were conducted at 60 ◦ C, methanol/oil of 10, 8 wt% of the catalyst for 1 h, where SrK/CaO catalysts
presented biodiesel yields above 80 %. Moreover, the Box-Behnken response surface design was used with re­
action time, methanol-to-oil ratio, and temperature as factors using SrKH/CaO catalyst. In comparison to using
KNO3, the KHCO3 increase the concentration of CaO. However, basicity and biodiesel yield were the same
regardless of the K precursor. Incorporating K into Sr/CaO increased the particle size, the basicity, and the
biodiesel yield. Furthermore, the design of experiments successfully showed an optimum methanol-to-oil ratio of
15, 46 ◦ C, and 3 h. Nonetheless, the SrK/CaO catalysts presented a reusability of only two cycles. The incor­
poration of Sr and K did not avoid the lixiviation of the active sites, but they may retard this process the catalyst
may be more active at low reaction times.
1. Introduction
In recent years, biodiesel has attracted attention due to the growing
need to decrease petroleum dependence and have sustainable, clean,
efficient, and renewable fuels with low sulfur content and aromatics.
Particularly, first-generation biodiesel is economical, biodegradable,
accessible, non-toxic, and easy to produce [1,2]. According to the In­
ternational Energy Agency, in 2020, biodiesel world production
decreased by 4 % due to the COVID-19 pandemic. Still, in 2021 and
2022, biodiesel world production recovered to a growth rate of 9 %,
producing 19.2 billion liters per year, with the European Union as the
leading producer, followed by the USA and Indonesia [3].
Biodiesel mainly contains fatty acid methyl esters (FAME) produced
throughout the transesterification reaction in presence of an alkali
catalyst such as NaOH or KOH and mild conditions such as 60 ◦ C and
atmospheric pressure [4]. The reaction performance is sensitive to
temperature, methanol-to-oil ratio, and the presence of free fatty acids
(FFA) and water. Homogeneous catalysts are highly active, cheap, cor­
rosive, and toxic. Also, they are difficult to recover and need water to
wash them out; e.g., biodiesel production from soybean oil requires 0.31
gallons of water per gallon of biodiesel [5,6]. In this sense, heteroge­
neous catalysts had appeared as a solution to reduce FFA and water
sensibility and consumption, avoid saponification side reactions, reuse
catalysts in several cycles, be economically feasible, and reduce or
eliminate their toxicity. Alkaline earth metals oxides are promising for
catalysts in transesterification reactions. CaO is the most studied mate­
rial due to its high basic sites, non-toxicity, low solubility in methanol,
and simplicity in fabrication using inexpensive materials such as
* Corresponding author.
E-mail address: [email protected] (J.A. Tavizón-Pozos).
https://doi.org/10.1016/j.nxmate.2023.100033
Received 30 March 2023; Received in revised form 17 August 2023; Accepted 22 August 2023
2949-8228/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Please cite this article as: M.A. Hernández-Martínez et al., Next Materials, https://doi.org/10.1016/j.nxmate.2023.100033
M.A. Hernández-Martínez et al.
Next Materials xxx (xxxx) xxx
limestone and calcium carbonate [7–9]. It has been accepted that the
active sites on the surface are M2+ and O2- ion pairs which work as Lewis
acid sites and Bronsted basic sites, respectively [10–13]. These sites are
closely related to the calcination temperature since it determines the
coordination defects and the Miller index of the crystals [14]. These sites
are fundamental to the activation of methanol, i.e., methanolysis, and
interactions between the carboxylic groups or double bonds of the tri­
glycerides [15–17]. CaO can be obtained from natural sources such as
eggshells, seashells, or limestones, and its calcination temperature of
CaO is usually ≥ 700 ◦ C depending on the catalytic system synthesis
[18–21]. Seashells of bivalves and crustaceans are usually considered
waste, even though CaCO3 is one of the most exploited resources glob­
ally. In this sense, reusing and generating value-added products from
seashell waste can support the sustainability of already established
processes [22]. Remarkably, oyster shells mainly comprise CaCO3,
which can be transformed into CaO at 700 ◦ C. This material is consid­
ered seafood restaurant waste and is produced in hundreds of thousands
of tons worldwide. Even if the oyster shells are used as fertilizer or
reused in oyster farms, tons are still disposed of or grounded. Therefore,
it is a cheap, accessible, and sustainable material that can be used as
catalysts [21,23,24].
As several authors have reported, the basic strength and the number
of sites of CaO can be enhanced if combined with an alkaline metal or
alkaline earth oxide [25–30]. D’Cruz et al. compared alkaline earth
oxides promoted with Li, Na, and K in the transesterification of canola
oil [31]. The CO2-TPD results showed that CO2 desorbs at higher tem­
peratures when an alkali metal is in the MgO, CaO, or BaO support. In
another study, incorporating K into CaO showed that the alkaline metal
reduces the leaching of the active species [25]. This effect was confirmed
by Olvera et al., which reported that K reduces the lixiviation compared
to Li supported on CaO. However, Li/CaO is a more basic and active
catalyst than K/CaO [17].
On the other hand, SrO has proven to be an efficient and active
material since it presents more basic strength and active site density
than Mg and CaO, making it selective to transesterification reactions
[11]. Also, its lixiviation is minimum and non-toxic compared to BaO
[32]. This means it could be reused several times, losing only 2–3 % of
its activity [33]. SrO could be combined with CaO to increase the ba­
sicity and catalytic activity. Li et al. found that through an improved
co-precipitation method developed by them, SrO/CaO catalysts (Sr/Ca
molar ratio of 0.5 and calcined at 900 ◦ C) reached a 98 % of conversion
of palm oil at 60 ◦ C, methanol to oil ratio of 9 and 30 min [34]. The
activity was attributed to forming of a SrxCa1− xO imperfect crystal
which enhanced the basicity. In another study, SrO and CaO were
combined by impregnating Sr(NO3)2 on CaO and calcined at 800 ◦ C for
5 h. The catalyst was evaluated in the jatropha oil transesterification and
showed 98 % biodiesel yield at 65 ◦ C with a methanol-to-oil ratio of 10
for 30 min in an ultrasonic-assisted reactor. Recently, SrO has also been
incorporated into eggshell CaO catalysts to convert jatropha oil [35].
This study reported that 7 mmol⋅g-1 of SrO increased the basicity of the
eggshells 2.16-fold, reaching a biodiesel yield of 88 %. However, by
using a Box-Behnken surface method, it was possible to achieve a 99 %
of biodiesel yield and keep it higher than 80 % yield for three cycles.
Moreover, they concluded that at low SrO loadings (1–7 mmol⋅g-1) the
particles are well dispersed, and there are more labile sites. In contrast
with high SrO (10 mmol⋅g-1) it could lead to aggregation of the particles
not taking advantage of them. Also, the incorporation of SrO onto CaO,
changed the texture and size of the particles, causing a decrease on the
surface area of the catalysts.
On this basis, this work studied support made by oyster shells and
limestone promoted with K and Sr to increase the basicity of the catalytic
system. Moreover, this work compares the effect of the potassium pre­
cursor on the basic character and reusability performance. Also, the BoxBehnken experiment design was used to find the optimal reaction
conditions.
2. Experimental
2.1. Materials
The reagents used in this work were limestone and oyster shells,
methanol (CH3OH, > 99 %, Avator-J.T. Baker), strontium nitrate (Sr
(NO3)2, > 99 %, Merck Sigma-Aldrich), potassium bicarbonate (KHCO3,
> 99.7 %, Merck Sigma-Aldrich), potassium nitrate (KNO3, > 99 %,
Meyer), benzene (C6H6, anhydrous, 99.8 %, Merck Sigma-Aldrich),
benzoic acid (C6H5COOH, > 99.5 %, Merck Sigma-Aldrich), sodium
hydroxide (NaOH, >98 %, Merck Sigma-Aldrich), n-hexane (CH3(CH2)
4CH3, 95 %, Merck Sigma-Aldrich), ethyl ether ((C2H5)2O, anhydrous
95–100 %, Meyer), formic acid (HCO2H, 88 %, Avator- J.T. Baker)
commercial canola oil and waste cooking oil from local market and
restaurant, TLC aluminum sheets silica gel 60 matrix W F254 s (Merck
Sigma-Aldrich).
2.2. Catalysts synthesis
Novel CaO support was synthesized from oyster shells and limestone
from a local restaurant and construction store. Oyster shells were
washed with water, crushed with a hammer into a fine powder, and
calcined at 800 ◦ C for 4 h with a 6 ◦ C/min heating ramp. Then, the oyster
shell powder was combined with lime in a 1:1 mass proportion (Fig. S1).
The solid was mixed with methanol (J. T. Baker) at 60 ◦ C with constant
agitation for 30 min. After this, the support was dried at 60 ◦ C overnight.
The resulting solid was calcined at 800 ◦ C for 4 h. Sr(NO3)2 (SigmaAldrich, > 99 %) was used as Sr precursor. In contrast, K was added
using two different precursors, KHCO3 (Sigma-Aldrich, > 99 %) and
KNO3 (Meyer, 99 %), setting two catalyst series denoted SrKH for bi­
carbonate and SrKN for nitrate. K and Sr were impregnated with a metal
loading of 9 wt% each (Fig. S2). CaO support was added into methanol
at 60 ◦ C with constant agitation. Then, the proper amount of K and Sr
precursors were added to the mixture and kept in agitation for 30 min.
Afterward, the catalysts were dried at 60 ◦ C overnight and calcined at
800 ◦ C for 4 h with a heating ramp of 6 ◦ C⋅min-1.
2.3. Catalysts characterization
The thermogravimetric and differential thermal analysis (TGA-DTA)
was performed on TA Instruments Q600 equipment. The materials were
placed in an inert atmosphere of N2 and were heated at 10 ◦ C/min from
room temperature to 800 ◦ C. Crystal phases were determined by X-ray
diffraction (DRX), carried out in a Bruker AXS diffractometer D8 using
CuKα radiation. The X-ray diffraction pattern was acquired by scanning
from 4◦ to 70◦ with a 0.019◦ step s-1. The spectra were compared with
previously cited crystallographic tables. The Hammett indicator method
measured the catalyst’s strength and basicity [36]. The indicators
employed were bromothymol blue (H_=7.2), phenolphthalein (H_=9.3),
and alizarin yellow (H_=11). 10 mg of solid material were dried at
100 ◦ C for 12 h. Then, 2 ml of indicator (1 w/v% in benzene) was added
and macerated for 2 h. The color change was registered. For the basic
sites, 12.5 mg of catalysts were dried at 100 ◦ C for 12 h and added to 25
ml of a 0.5 N benzoic acid in benzene solution. The mixture was shaken
for 30 min and macerated for 12 h. The liquid phase was separated, and
the solid was washed with 5 ml of benzene. The separated liquid phase
and the wash benzene were mixed with a 0.2 N solution of NaOH in
methanol using bromothymol blue as the indicator. FT-IR spectra of the
calcined catalysts were carried out as KBr pellets in the wavenumber
region of 500 – 4000 cm-1 on a Perkin-Elmer GX59750 apparatus with 4
cm-1 resolution. Scanning Electron Microscopy and Energy Dispersive
X-ray Spectroscopy (SEM-EDS) were used to monitor the metal disper­
sion and the elemental composition on the calcined catalyst surfaces.
JSM-7800F Ultra-High Resolution Scanning Electron Microscope
equipment was used with backscattered (COMPO), secondary electrons
(LED), 2000 and 5000x of magnification; the work distance range was
2
M.A. Hernández-Martínez et al.
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8.5 and 11.5 mm, and with compositional characteristics obtained using
EDS NORAN.
filtered and washed with n-hexane to remove the non-polar compounds.
Then, methanol was used to wash the glycerin out of the material. Then,
the catalysts were dried at 120 ◦ C and reused.
2.4. Catalysts evaluation
3. Results
The transesterification reactions were carried out with methanol and
commercial canola oil (887 g•mol-1 according to [37]) purchased in a
local market and own waste canola oil synthesized from frying food
several times. The batch reactor system comprises a 50 ml flat-bottom
round flask, a reflux condenser, a magnetic bar agitator, and a
heating-stirrer system. First, 0.66 g of the calcined catalysts were dried
at 120 ◦ C for 12 h and added to methanol under vigorous agitation (>
600 rpm) for 5 min at 60 ◦ C. Then, 7 g of canola oil was poured into the
catalysts-methanol mixture. The catalysts loading was 8 wt% to oil to
ensure the number of active sites (Fig. S3), the methanol to oil molar
ratio was changed to 10:1, 12.5:1, and 15:1, and the reaction tempera­
tures were 40, 50, and 60 ◦ C; the reaction time was 1, 2, and 3 h. After
the reaction, the products were centrifugated at 2000 rpm for 10 min to
separate the liquid phases from the solid. The liquid phases were
introduced in a separatory funnel and let separately by gravity. The
lower phase was separated, and the upper phase was mixed with 5 ml of
n-hexane (J. T. Baker) and 2 ml of deionized water to wash out the
remaining glycerol from the methyl esters produced. The n-hexane was
evaporated at 60 ◦ C, whereas the aqueous phase was discarded. The
biodiesel yield was calculated using Eq. (1).
Biodiesel Yield (%) =
mass of biodiesel generated (g)x100
mass of crude oil (g)
3.1. Characterization
3.1.1. Thermogravimetric analysis
The TGA-DTA of the oyster shell alone and CaO support (limestone +
oyster shell) is presented in Fig. 1.
The oyster shell was composed of CaCO3 since its principal loss of
weight (35 %) happened at 760 ◦ C [23]. The CaO support was composed
of a mix of Ca(OH)2 and CaCO3 due to the presence of the limestone.
Two primary weight losses were observed at 420 and 700 ◦ C, corre­
sponding to the transformation of the hydroxide (10 %) and carbonate
into CaO (25 %) [40]. The TGA-DTA results of the Sr and K-supported
catalysts are presented in Fig. 2.
SrO/CaO showed three weight losses at 420 (8 %), 600 (30 %), and
850 ◦ C (37 %), whereas KN/CaO at 410 (5 %), 670 (20 %), and 850 ◦ C
(44 %). The first signal was attributed to the decomposition of Ca(OH)2
to generate CaO. Then, Sr(NO3)2 and KNO3 decomposed between 600
and 700 ◦ C, and the last weight loss was the generation of CaO from
carbonate species [27,41,42]. Notably, KN/CaO showed that nitrate
species and CaCO3 decompose almost at the same time. The KH/CaO
catalyst had a different decomposition process. First, the weight loss at
190 ◦ C (4 %) was due to the degradation of KHCO3 into K2O [43]. The
second and the third signals at 420 (10 %) and 700 ◦ C (25 %) were the
formations of CaO from the hydroxide and carbonate species, respec­
tively. The TGA-DTA results of the bimetallic-supported catalysts are
exhibited in Fig. 3.
SrKN/CaO catalyst presented five main decomposition signals
located at 400 (7 %), 500 (10 %), 600 (25 %), 740 (33 %), and 890 ◦ C
(37 %). The first weight loss was assigned to the formation of CaO from
the calcium hydroxide, followed by the decomposition of both nitrates
deposited salts. Sr(NO3)2 and KNO3 transformed into SrO and K2O
together. The last two weight losses were attributed to the remaining
nitrates that strongly interact with calcium carbonate’s support and the
transformation of the CaCO3. However, considering the results of Sr/
CaO and KN/CaO, it is possible that a superficial strontium nitrate
fraction could be decomposed before potassium nitrate on SrKN/CaO.
SrKH/CaO catalyst showed five weight loss signals at 60 (7 %), 380 (12
%), 560 ◦ C (23 %), 670 ◦ C (32 %), and 890 ◦ C (40 %). The initial weight
loss could be due to the decomposition of KHCO3, and the second one to
the Ca(OH)2. The interactions between potassium, strontium, and cal­
cium could lead to a slight exothermic weight gain just before the second
weight loss. Hence, it could indicate a lattice rearrangement in a metallic
complex. In this sense, it is possible that Sr(NO3)2 decomposition slightly
shifted to lower temperatures compared to SrKN/CaO catalyst (600 →
560 ◦ C). Then, the fourth weight loss seems to be a combination of
strontium and calcium carbonate transformation, whereas the last signal
was only attributed to calcium carbonate. The calcium carbonate did not
fully transform into CaO at the set calcination temperature in both
catalysts.
(1)
Thin layer chromatography was achieved to confirm the presence of
biodiesel using 5 × 7 cm slabs of aluminum plate silica gel 60 W F254s
sheets (Sigma-Aldrich). The slabs were visualized with iodine spheres.
The developing solvent mixture comprised n-hexane, ethyl ether
(Meyer, 99 %), and formic acid (JT Baker, 88 %) in a 90:10:0.1 volu­
metric ratio. The canola oil reference and the reaction products were
spotted at the slab bottom and placed in the chamber with the solvent.
2.5. Optimization of the production of biodiesel
The production of biodiesel was optimized by regular Box-Behnken
surface design. The three selected variables were temperature,
methanol-to-oil molar ratio, and reaction time, whereas the biodiesel
yield was the response. The factors were chosen based on the available
literature and previous studies in our investigation group [35,38,39].
The analysis of the experimental data was carried out by Minitab 18
software. Once the design of the experiment was performed, a full
quadratic model was formulated, as shown in Eq. (2).
∑3
∑3
∑2 ∑3
Y = β0 +
β xi +
β x2 +
β xij
(2)
i=1 i
i=1 ii i
i=1
i+1 ij
Y= surface response of the biodiesel.
Xi, Xij= the variables.
β0 = the constant term.
βi, βii, βij = the coefficients of the linear parameters.
The coded and uncoded variables are shown in Table 1.
The reusability of the catalyst was performed at the theoretical
optimal conditions obtained by Box-Behnken surface design, as Section
2.4 methodology describes. After the first reaction, the catalysts were
3.1.2. X-ray diffraction
The X-ray diffractograms of the 800 ◦ C powders calcined catalysts
are presented in Fig. 4.
The CaO support showed peaks of Ca(OH)2 hexagonal phase located
at 18.03◦ (001), 28.69◦ (100), 34.09◦ (101), 47.09◦ (102), 50.82◦ (110),
54.36◦ (111), 62.62◦ (201) accordingly to PDF 44-1481 card [44,45].
This indicated that all the CaCO3 was decomposed at the set calcination
temperature. However, this material was highly hygroscopic since the
only appreciable phases were calcium hydroxide due to the air moisture
exposure. When potassium is added to the material, CaO cubic phase
diffraction peaks appeared at 32◦ (111), 37.35◦ (200), and 53.86◦ (220)
Table 1
Levels and experimental range of the chosen independent variables for BoxBehnken design of experiments.
Variables
Level and range
Time (h)
Met/oil molar ratio
Temperature (◦ C)
-1
1
10
40
0
2
12.5
50
+1
3
15
60
Coded variables
Uncoded variables
3
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Fig. 1. TGA-DTA of the oyster shell alone and the CaO support pretreated with methanol and dried at 60 ◦ C.
Fig. 2. TGA-DTA of the Sr/CaO, KN/CaO, and KH/CaO catalysts dried at 60 ◦ C.
(PDF 04-0777). These peaks appeared no matter the potassium precur­
sor. The only difference is that when KHCO3 was used, the CaO phase
increased its concentration and crystal size. Hence, this precursor
improved the formation of CaO, whereas the KNO3 did it to a minor
degree. Moreover, the presence of K increased the stability of CaO,
decreasing the appearance of Ca(OH)2 [17].
The Sr/CaO calcined catalyst showed diffraction peaks correspond­
ing to Ca(OH)2 and CaO. This catalyst also showed an orthorhombic
SrCO3 phase (PDF 05–0418) with peaks located at 25.16◦ (111), 25.8◦
(021), 36.5◦ (130), and 44.08◦ (221). This phase could be formed during
the oxidation of the Sr(NO3)2 and strong interactions with the carbonate
species. This result concurs with the TGA-DTA analysis, which showed
that above 800 ◦ C, carbonate species are still present in the lattice. The
SrK/CaO catalysts showed the presence of CaO and Ca(OH)2 diffraction
peaks similar to KH/CaO and KN/CaO catalysts. As these materials, the
one impregnated with KHCO3 promoted the formation of CaO in com­
parison to the one impregnated with KNO3. Additionally, these catalysts
showed the presence of K2Sr(CO3)2 hexagonal phase (PDF 31–1090)
with peaks located at 13.02◦ (002), 23.2◦ (102), 27.6◦ (103), 32.7◦
(104), 33.6◦ (103), 39.79◦ (006), and 41.31◦ (202). This phase was
formed during the calcination as TGA-DTA results proposed.
3.1.3. Fourier transform infrared spectroscopy
The FTIR spectra of the calcined catalysts are presented in Fig. 5. All
materials showed bands relative to the Ca(OH)2, CaCO3, and CaO pha­
ses. The sharp band at 3642 cm-1 and the wide band between 3000 and
3500 cm-1 are related to the stretching mode of -OH groups from the Ca
(OH)2 generated from the air’s moisture and the hygroscopic character
of the CaO support [46,47].
The bands located at 1447 and 874 cm-1 were assigned to the
– O, and the vibrations (ν3) from adsorbed at­
asymmetric stretch of C–
mospheric CO2 of carbonate ions on the basic sites, respectively
[47–49]. The absence of asymmetric and symmetric stretching modes of
– O (1500 – 2900 cm-1) confirms that most calcium carbonates were
C–
oxidated during calcination [50]. Nonetheless, the KN and KH/CaO
might still present CaCO3 species. Moreover, as XRD results showed, the
calcination at 800 ◦ C led to the formation of the K2Sr(CO3)2 phase on the
SrK/CaO catalysts.
4
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Next Materials xxx (xxxx) xxx
Fig. 3. TGA-DTA of the SrKN/CaO, SrKH/CaO catalysts dried at 60 ◦ C.
Fig. 4. XRD profiles of the synthesized catalyst calcinated at 800 ◦ C.
3.1.4. Hammett indicators
The basic strength and total sites of the prepared catalysts are pre­
sented in Table 2. The CaO support showed an alkali character between
7.6 and 10 of Hammett’s indicators.
The basicity results show that when Sr was incorporated into CaO
support, the basicity slightly decreased as well as the basic sites. How­
ever, when Sr and K are together on the support, the basicity increases
more than the support alone. SrKN/CaO presented 1.5 times more basic
sites than the support, whereas SrKH/CaO was 2.6 times more. This is
due to the decomposition of the supported precursor salts and the ionic
arrangement in the lattice [25]. As KHCO3 oxidizes at lower tempera­
tures than KNO3 or Sr(NO3)2, K ions might introduce CaO, promoting
the basicity of lattice oxygen.
Table 2
Basic strength and total sites of the CaO support, Sr/CaO, SrKH/CaO, and SrKN/
CaO calcined at 800 ◦ C.
Fig. 5. FTIR spectra of the Sr and K supported on CaO catalysts calcined
at 800 ◦ C.
5
Catalysts
Basicity
Total basic sites (meq⋅g-1
cat)
CaO
KN/CaO
KH/CaO
Sr/CaO
SrKN/CaO
SrKH/CaO
9.3< H_<11
10.7< H_<11
10.7< H_<11
7.2< H_<9.3
11< H_
11< H_
4.07 ± 0.4
4.66 ± 0.4
5.41 ± 0.5
2.24 ± 0.2
6.01 ± 0.6
10.59 ± 1
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3.1.5. Scanning electron microscopy
Fig. 6 shows the scanning electron microscopy (SEM) with the
configuration of secondary electrons (LED) and backscattered electrons
(COMPO) of the microstructure of the catalysts CaO, KN/CaO, KH/CaO,
Sr/CaO, SrKN/CaO, and SrKH/CaO. The micrographs showed that all
catalysts presented characteristic rock-like agglomerates with various
sizes and irregular shapes [41,51–53].
The potassium salt precursor affected the average particle size for the
KN/CaO (b1-b2) and KH/CaO catalysts (c1-c2). The average particle
size of the KN/CaO catalyst was 2.29 µm, whereas that of the KH/CaO
catalyst was 1.04 µm. The change in particle size could be related to the
transformation process during calcination since KHCO3 decomposes at
lower temperatures than KNO3, as indicated by TGA. The Sr/CaO (d1d2) showed a small average particle size of < 1 µm providing a rough
surface shape. The SrKN/CaO (e1-e2) and SrKH/CaO (f1-f2) catalysts
showed small, irregularly shaped particle agglomerations. These two
catalysts presented a growth in the particle size since it was approxi­
mately 4 µm of average length. Hence, the combination of Sr and K leads
to bigger particles than Sr and K alone. The EDS results (a3-f3) indicated
the presence of carbon from the remaining carbonates species and
possibly strongly adsorbed CO2 from the contact with air. However, the
SrKN/CaO and SrKH/CaO catalysts exhibited a high carbon concentra­
tion, possibly due to the K2Sr(CO3)2 phase formation, as shown by the
XRD and TGA results.
seems more hygroscopic than KH/CaO. In this sense, there are slightly
more basic sites when using KHCO3. However, the basic strength was
practically the same in both catalysts. Thus, both catalysts showed
similar performance. Therefore, despite the observed changes in the
characterization results, there was no significant difference in biodiesel
yield.
It was observed that strontium nitrate decreased the transformation
temperature when combined with potassium salt. However, no matter
the precursor, both catalysts generated the K2Sr(CO3)2 phase, which was
stable at 800 ◦ C [56]. Since KHCO3 decomposed at a lower temperature,
Sr(NO3)2 decreased its interaction with the support. This resulted in an
increment of CaO and a small amount of K2Sr(CO3)2. In contrast, KNO3
and Sr(NO3)2 decomposed almost simultaneously, and their interaction
was stronger, increasing the K2Sr(CO3)2 phase. The SEM results showed
that combining K and Sr caused the particle size to increase with respect
to Sr/CaO and K/CaO catalysts, i.e., the interactions between K and Sr
cause agglomerates of the particles regardless of the precursor.
The structural changes affected the basic character of the catalysts.
Since the ionic radii of K+, and Sr2+ are 152 and 132 pm, respectively,
they can be introduced in the calcium oxide, hydroxide, or carbonate
lattice [17,25]. According to the literature, the changes in the basicity
are related to the ionic distances M-O. Hence, more considerable dis­
tances lead to higher basicity [57,58]. In this sense, in Sr/CaO, the
introduction of Sr in the CaO lattice decreased the ionic lengths. This led
to more Lewis acid sites provided by exposed M2+ sites. In the bimetallic
catalysts, the ionic distances increased as K and Sr were together with
Ca. Nonetheless, as K was introduced first in the lattice when KHCO3 was
used, Sr could not decrease the ionic length, and the basic character was
augmented. On the other hand, the nitrates simultaneous decomposition
provoked the well-dispersed Sr could decrease the ionic length and the
basic sites.
3.2. Catalytic evaluation
3.2.1. Effect of Sr and K precursor
Fig. 7 shows the results of the catalytic evaluation of the canola oil
transesterification reaction using CaO, KN/CaO, KH/CaO, Sr/CaO,
SrKN/CaO, and SrKH/CaO catalysts.
The CaO support composed of oyster shells and limestone yielded 33
% after one hour of reaction. Sr/CaO showed an increment in the bio­
diesel yield reaching 73 %, even with its slightly more acidic character.
As the Brønsted basic sites carried out the methanolysis, the Lewis acid
sites absorbed the triglycerides and FFA [54]. Under this supposition, it
is quite likely that on Sr/CaO proceeded a Langmuir-Hinshelwood
mechanism which consists of five steps: methanol adsorption, triglyc­
eride adsorption on an acid site, the reaction between them, desorption
of diglyceride and desorption of biodiesel [17,54,55]. The KN/CaO and
KH/CaO presented similar biodiesel yields (75 %) due to their high al­
kali character provided by K than CaO alone. Due to their strong alkali
character, both SrK/CaO catalysts exhibited higher yields than mono­
metallic ones, reaching 80 % of biodiesel yield. Hence, the distinct K
precursor, alone or combined with Sr, did not present significant dif­
ferences in the biodiesel yield.
SrK/CaO catalysts were tested on the wasted canola oil trans­
esterification reaction presented in Table 3.
Both catalysts were active for the wasted canola oil with a biodiesel
yield above 70 %. Nonetheless, the biodiesel yield was lower than that of
fresh canola oil. As with the fresh canola oil, the potassium carbonateimpregnated catalyst was slightly higher than the potassium nitrate
catalyst. The activity decreased due to the higher FFA concentration in
the wasted oil. Hence, these catalysts can be used on wasted cooking oils
with acceptable biodiesel yields.
According to TGA-DTA results, the KH/CaO catalyst decomposes the
potassium salt and calcium carbonate before 800 ◦ C. In contrast, the
nitrate-impregnated catalysts still have CaCO3 species at this tempera­
ture. Potassium bicarbonate decomposed at low temperatures and did
not interfere in the CaO generation process since the carbonate species
were decomposed at the same temperature as the support alone. In
contrast, nitrate salts are highly stable compounds that need more en­
ergy to decompose. Moreover, it seems that KNO3 species strongly
interact with the support delaying the decomposition of carbonate
species. Hence, XRD and SEM results confirmed more CaO crystals and
larger particle sizes in KH/CaO than in KN/CaO. Moreover, KN/CaO
3.2.2. Optimization of transesterification of fresh canola oil with SrKH/
CaO
SrKH/CaO catalyst was selected to optimize the reaction system due
to its more economical potassium precursor, high basic character,
number of active sites, and biodiesel yield. A Box-Behnken design was
used to optimize the time, methanol-to-oil molar ratio, and temperature
of the transesterification of canola oil. The values of the biodiesel yield
and the coded and uncoded variables are presented in Table 4.
The quadratic equation that describes the relationship between the
biodiesel yield (response) and the independent variables (time A,
methanol/oil B, temperature C) is presented in Eq. (3).
Yield (%) = 89.17 + 0.18A + 1.17B + 1.01C − 0.43A2 − 0.45B2 − 2.53C2
+ 4.27AB − 4.16AC + 1.03BC
(3)
According to the statistical analysis of the experimental and theo­
retical data, the Student-t-test of two-tailed paired data with 95 %
confidence presented a texp value of 0.001 < tcritical 2.144. In this sense,
the results do not show significant differences. Fig. 8 shows the BoxBehnken surface and contour plots of the effect of the time (A) and
methanol-to-oil molar ratio (B) at 50 ◦ C.
It is possible to observe that the biodiesel yield may achieve two
maximums (≥ 90 %) and two minimums (≤ 85 %) in a chair-like
response surface plot when the temperature is set at 50 ◦ C. Hence, the
predicted maximums were located at 3 h and a methanol/oil=15, and
for 1 h, methanol/oil=10. The minimums were at 3 h and methanol/
oil=10, and 1 h and methanol/oil=15.
Fig. 9 exhibits the relationship between temperature (C) and the
methanol-to-oil molar ratio (B) for 2 h.
This response surface design showed one maximum biodiesel yield in
a broad zone from 46◦ to 55◦ C and a methanol-to-oil ratio of 12.5–15.
The minimum biodiesel yield is predicted at 40 ◦ C and 1 h. At this point
in the reaction, when the temperature is lower than 46 ◦ C w methanol/
6
M.A. Hernández-Martínez et al.
Next Materials xxx (xxxx) xxx
Fig. 6. SEM micrographs at 2000, and 5000x of magnification and EDS for the (a1-a3) CaO, (b1-b3) KN/CaO, (c1-c3) KH/CaO, (d1-d3) Sr/CaO, (e1-e3) SrKN/CaO
and (f1-f3) SrKH/CaO catalyst.
7
M.A. Hernández-Martínez et al.
Next Materials xxx (xxxx) xxx
The relationship between the temperature and time at a constant
methanol-to-oil molar ratio showed a minimum at 40 ◦ C and 1 h since
there is insufficient energy and time to reach yields higher than 85 %.
The maximum was from 54 ◦ C to 60 ◦ C for 1–1.5 h. However, the bio­
diesel yield decreased from 90 % to 83 % as time passed.
Optimizing at 50 ◦ C and varying the methanol-to-oil ratio and time
showed that the catalyst gets dispersed as the methanol concentration
increases, requiring more time to reach a high yield [61]. On the other
hand, when methanol concentration is low, it is more probable that both
reagents get the active sites and the yield increases. However, as the
methanol consumption and the product concentration increases, the
reversible reaction can occur in prolonged reaction times, decreasing the
biodiesel yield [62]. Also, it has been proposed that the emulsification of
methanol with biodiesel and glycerol could happen [41]. When the
temperature and methanol-to-oil ratio change and the time is set for 2 h,
the energy plays an important role. Between 46 and 55 ◦ C, the methanol
has adequate energy to get onto the active sites. At lower temperatures,
there is a lack of energy, and over 55 ◦ C, the mass transfer limitations of
the gas-liquid phase of methanol hinder the reaction yield. However,
there is enough energy to react quickly and keep the yield above 85 %.
At this temperature range, the methanol-to-oil ratio should be higher
than 12.5 to have yields near 90 % because as more methanol is in the
reaction system, the equilibrium is shifted to products. The reverse
transesterification reaction took place at long reaction times. Temper­
ature and time at a fixed methanol-to-oil ratio showed high biodiesel
yields at short intervals, and subsequent decrease could be attributed to
the quickly reached equilibrium. Moreover, it is also possible that the
hydrolysis of FAME to FFA could happen [62,63]. It was considered that
the lixiviation of the active sites would affect the biodiesel yield at long
reaction times. The lixiviation process would free the K+, Sr2+, and Ca2+
ions, contributing homogenous to the reaction. Furthermore, the Ca
matrix was unstable even with incorporating K and Sr since the surface
could generate calcium methoxides and glyceroxides. However, it oc­
curs at slower rates than CaO alone [64]. Hence, the reaction yield may
decrease or keep constant in a certain yield. On this basis, it could be
necessary to study the biodiesel yield at short times to have a clearer
reaction following.
The Box-Behnken modeling predicted that at theoretical optimal
conditions, a methanol-to-oil ratio of 15, at 46 ◦ C for 3 h, a 94 % of
biodiesel yield could be reached. In this sense, the obtained average
experimental biodiesel yield achieved to verify the model was 91 ± 3.6
%. Therefore, the model can satisfactorily predict the performance of
this set catalytic system.
The physicochemical properties of the biodiesel obtained from
canola oil at the determined optimal conditions are exposed in Table 5.
The acid number of the biodiesel was adequate, as the ASTM and
European standards indicate, whereas the saponification number was
lower than the fresh canola oil. Biodiesel density (0.878 g•cm-3)
decreased compared to canola oil and was between the American and
European standards and it. The viscosity was 5.45 mm2•s-1 which is
slightly higher for the EN14214 required parameters; nonetheless, it is
between the range mentioned by the ASTM regulation. Generally, the
biodiesel quality is acceptable according to the standards and could be
used in engines.
Fig. 7. Catalytic transesterification of canola oil at 60 ◦ C, methanol/oil=10,
8 wt% of the synthesized catalyst for 1 h.
Table 3
Comparison of the catalytic transesterification of fresh and wasted canola oil at
60 ◦ C, methanol/oil=10, 8 wt% of the synthesized catalyst for 1 h.
Fresh canola oil
Catalyst
Wasted canola oil
Biodiesel yield (%)
SrKN/CaO
SrKH/CaO
79.76 ± 3.1
80.67 ± 3.2
74.07 ± 2.6
78.21 ± 1.2
Table 4
Box-Behnken response surface design of the variables and results of the trans­
esterification of canola oil over SrKH/CaO.
Run
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Time (h)
Met/oil
Temperature (◦ C)
1 (− 1)
3 (1)
1 (− 1)
3 (1)
1 (− 1)
3 (1)
1 (− 1)
3 (1)
2 (0)
2 (0)
2 (0)
2 (0)
2 (0)
2 (0)
2 (0)
10 (− 1)
10 (− 1)
15 (1)
15 (1)
12.5 (0)
12.5 (0)
12.5 (0)
12.5 (0)
10 (− 1)
15 (1)
10 (− 1)
15 (1)
12.5 (0)
12.5 (0)
12.5 (0)
50 (0)
50 (0)
50 (0)
50 (0)
40 (− 1)
40 (− 1)
60 (1)
60 (1)
40 (− 1)
40 (− 1)
60 (1)
60 (1)
50 (0)
50 (0)
50 (0)
Biodiesel Yield (%)
Predicted
Experimental
91.21
83.03
85.01
93.91
80.86
89.54
91.20
83.24
85.04
85.32
85.00
89.40
89.17
89.17
89.17
87.56
81.74
86.30
97.57
82.41
88.74
92.00
81.68
87.14
82.47
87.85
87.31
88.89
89.46
89.16
**The coded variables are inside the parenthesis
***All experiments were carried out with a catalyst concentration of 8 wt% to
oil.
3.2.3. Catalyst reusability
Fig. 11 presents the reusability of the two SrK/CaO catalysts to
compare the stability of the potassium precursors.
It is possible to observe that the two SrK/CaO catalysts decreased
their biodiesel performance by 16.5 % in the second cycle. Moreover,
the third cycle presented meager biodiesel yields (< 10 %). The signif­
icant activity loss was attributed to the leaching of the active sites.
During the reaction, the Ca matrix is transformed into Ca(OH)2, calcium
methoxide, and calcium glyceroxide, which are significantly less alka­
line and active than CaO. Additionally, it has been proposed that each
crystalline phases of CaO react at different rates with methanol and the
oil= 10, there is insufficient energy to carry on the reaction. Suppose the
temperature is above 55 ◦ C at any methanol-to-oil ratio. In that case, the
yield slightly decreases because more methanol is a gas phase and is
more difficult to get onto the active sites [59,60]. Comparing these re­
sults with those presented in Fig. 8, it is important to notice that in this
case the maximum yield was lower than 90 % because it may need more
time to react.
Fig. 10 exhibits the relationship between temperature and time with
a methanol-to-oil ratio of 12.5.
8
M.A. Hernández-Martínez et al.
Next Materials xxx (xxxx) xxx
Fig. 8. Response surface (left) and contour (right) plots of the biodiesel yield as a function of methanol to oil molar ratio and time at 50 ◦ C.
Fig. 9. Response surface (left) and contour (right) plots of the biodiesel yield as a function of temperature and methanol-to-oil molar ratio for 2 h.
Fig. 10. Response surface (left) and contour (right) plots of the biodiesel yield as a function of temperature and time ratio with a methanol-to-oil ratio of 12.5.
9
M.A. Hernández-Martínez et al.
Next Materials xxx (xxxx) xxx
incorporation of Sr and K did not avoid the lixiviation of the active sites,
but they may retard this process, these results could indicate that the
catalyst may be more active at low reaction times, as shown by the
response surface results of the yield as a function of temperature and
time, leaving a methanol-to-oil ratio of 12.5. The produced biodiesel
quality was acceptable according to the regulations. Finally, both SrKH/
CaO and SrKN/CaO did not show good stability since the activity
decreased by more than 15 % at the second catalytic cycle as the cata­
lysts either leached or easily pulverized while stirring the reaction due to
its low mechanical resistance. However, as these catalysts are synthe­
tized from limestone, oyster shells and accessible salts, they could be
considered as sustainable materials to produce biodiesel. The deacti­
vated catalyst could be used as fertilizer since their components are
innocuous to plants.
Table 5
Properties of produced biodiesel at determined optimal conditions.
Parameter
Saponification
number
Acid number
Density (15 ◦ C)
Viscosity (40 ◦ C)
Unit
Biodiesel
Canola
oil
ASTM
D6751
EN
14214
mg
KOH•g-
96.01
112.75
-
-
≤0.5
≤0.5
0.87–0.89
1.9–6
0.86–0.9
3.5–5
1
mg
KOH•g-
0.116
0.132
g•cm-3
mm2•s-1
0.878
5.45
0.941
26.57
1
Funding
The authors have no relevant financial or non-financial interests to
disclose.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
The authors thank CONAHCYT for project 216 of Investigadoras e
Investigadores por México-CONAHCYT program. Thanks to Dr. Dwight
Roberto Acosta Najarro from IF-UNAM for his support in the SEM-EDS
measurements and Dr. Israel Ibarra for assistance in Box-Behnken
analysis. Tavizón-Pozos thanks Laura Aranda for her support and love.
Fig. 11. Reusability of the SrK/CaO synthesized catalysts at 46 ◦ C, and a
methanol-to-oil ratio of 15 for 3 h.
Appendix A. Supporting information
formed glycerol. In this sense, the activity loss is related to the crystal­
linity changes and loss [64,65]. Hence, the Sr and K incorporation may
affect this superficial reaction rate decreasing the transformation into
these mentioned phases. However, the lixiviation would not be avoided
but retarded. Besides, as this behavior was similar using both precursors,
it was concluded that there was no significant difference between them
in the stability of the catalyst. Hence, further research must study the
enhancement of these catalysts.
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.nxmate.2023.100033.
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