Fuel 300 (2021) 120947
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Full Length Article
Light olefins by methane partial oxidation using hydrated waste eggshell
as catalyst
Dirléia dos Santos Lima, Oscar W. Perez-Lopez *
Laboratory of Catalytic Processes-PROCAT, Department of Chemical Engineering, Federal University of Rio Grande do Sul (UFRGS), Ramiro Barcelos Street, 2777 CEP
90035-007 Porto Alegre, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Calcination-hydration cycles
Catalysts
Light olefins
Oxidative coupling of methane
Waste eggshell
Catalysts obtained from the residual eggshell were prepared by calcination-hydration-calcination cycles under
static atmosphere or under air flow and evaluated in the oxidative coupling of methane to obtain C2-C3 hy­
drocarbons. It was found that the calcination-hydration-calcination cycles under different conditions influence
the specific surface area, the crystallinity and the basicity of the obtained materials. The production of light
olefins from methane was observed under all samples. The best results were obtained for the hydrated sample
calcined under air flow, reaching a methane conversion of 30% and selectivity for ethylene above 53% at 800 ◦ C.
The hydrated and muffle-calcined sample showed a higher C2/CO2 ratio. These differences are attributed to the
strength of the basic sites and the presence of surface lattice oxygen.
1. Introduction
The increase in methane production from shale gas and estimates of
oil depletion have attracted global attention to methane conversion
processes, as it has significant potential for direct or indirect conversion
to chemicals and value-added fuels. The conversion of methane to
higher hydrocarbons with the formation of C–C bonds through the
interaction with oxygen is a coupling or condensation reaction [1,2].
The oxidative coupling of methane (OCM) is a promising route to
replace petroleum-based chemical processes, such as naphtha cracking,
to allow the production of C2 (C2H6 and C2H4) and C3 (C3H8 and C3H6)
hydrocarbons, which are valuable intermediates for many chemical in­
dustries [3-5]. This process involves the reaction of methane with oxy­
gen in the presence of a catalyst at elevated temperature and moderate
pressure. The OCM comprises heterogeneous catalytic processes and
homogeneous non-catalytic processes for converting methane mainly to
C2 hydrocarbons, Eq. (1) and Eq (2). However, in addition to these two
selective reactions, the oxidation of hydrocarbons to COx also takes
place [6,7].
2CH4 + 0.5 O2 → C2H6 + H2O;
(1)
C2H6 + 0.5 O2 → C2H4 + H2O;
(2)
The OCM process produces ethane and ethylene directly from
methane and has attracted considerable attention over past decades, but
due to complete oxidation reactions on the gas phase or catalyst surface,
it is still very difficult to achieve a C2 yield of over 25% in a conventional
fixed bed reactor [8,9]. This is one of the main factors that prevent its
application on an industrial scale.
Most active and selective catalysts for the oxidative coupling reac­
tion of methane are composed of two or three irreducible oxides, for
example alkali metal oxides, alkaline earth metal oxides and rare earth
metal oxides [3,10-15]. Catalysts for the OCM reaction must have oxy­
gen species on their surface, active sites for the generation of methyl
radicals and high basicity for the rapid desorption of these formed
radicals [16]. Thus, this process has been extensively investigated on
many potential oxide catalysts in order to find the most efficient, which
should have high ethylene selectivity and catalytic performance at
milder temperatures. Some of the most recently employed catalysts are
NaWMn/SiO2, Na2WO4/TiO2, Sr2TiO4, Na2WO4/SiO2, Mn/WO3/TiO2
pure or metal-modified, and LaAlO3 perovskites [16-22].
Mn2O3-Na2WO4/SiO2 is considered the most promising catalyst for
the OCM process. However, it only has better catalytic performance
above 800 ◦ C. To improve its performance at milder temperatures, Wang
et al. [19] modified this catalyst using TiO2, MgO, Ga2O3 and ZrO2.
Among the synthesized catalysts, the TiO2 modified Mn2O3-Na2WO4/
SiO2 catalyst showed significantly improved performance for the low
temperature OCM process, achieving approximately 23% CH4
* Corresponding author.
E-mail address: perez@ufrgs.br (O.W. Perez-Lopez).
https://doi.org/10.1016/j.fuel.2021.120947
Received 18 January 2021; Received in revised form 23 April 2021; Accepted 25 April 2021
Available online 6 May 2021
0016-2361/© 2021 Elsevier Ltd. All rights reserved.
D. dos Santos Lima and O.W. Perez-Lopez
Fuel 300 (2021) 120947
conversion and C2–C3 selectivity around 73% at 700 ◦ C, along with
promising stability for at least 300 h with no signs of deactivation.
Ivanova et al. [16] used four different preparation routes to syn­
thesize Sr2TiO4 with the layered perovskite structure: solprecipitation
(SP), coprecipitation (CO), citrate precursor (CT) and mechanochemical
(MA) methods with additional calcination at 1100 ◦ C. The upward trend
in CH4 conversion and C2 yield on oxidative methane coupling at
800–900 ◦ C was as follows: SP ≈ MA > CO > CT samples. The C2 yield of
the most active catalysts was measured at about 12%.
Cheng et al. [20] studied the effect of calcination temperature on the
characteristics and performance of lithium manganese catalysts sup­
ported on WO3/TiO2 acid solid for OCM. The best calcination temper­
ature obtained for the support and catalyst was 800 ◦ C, for which the
highest C2 yield reached was 16.3% at the reaction temperature of
750 ◦ C.
Even with the development of new catalyst for the OCM process, low
yields on C2 hydrocarbons are still obtained, showing that there are still
many challenges to be overcome before OCM becomes industrially
viable. Therefore, there remains a crucial need for more efficient cata­
lysts with high C2 selectivity at significant methane conversion levels
and long term thermal stability [4].
Calcium oxide (CaO), as well as several other alkali metal oxides, has
catalytic properties for various applications. This oxide can be obtained
from the calcination of eggshell, a low cost and abundant residue
composed mainly of CaCO3 [23-25]. Due to its low specific surface area,
which is an important characteristic to promote selective oxidation re­
actions, the eggshell has great potential to be applied in the OCM
process.
Eggshell has been used in transesterification processes to obtain
biodiesel [25-33], catalytic benzene oxidation [34], water purification
[35], photocatalytic reduction and bacterial inactivation [36]. Karoshi
et al. [37] used calcined eggshell as a low cost catalyst for methane
partial oxidation. Parameters including oxygen concentration, flow rate
and temperature were found to influence methane conversion and
product selectivity.
Putra et al. [38] reported that the modification of calcite by hydra­
tion–dehydration increases the presence of strong basic sites and im­
proves its properties as a catalyst for the production of biodiesel. Fresh
calcite was thermally decomposed at 800 ◦ C and refluxed in water at
60 ◦ C for 6 h, and subsequently dehydrated at 600 ◦ C. The methyl ester
content was enhanced to 93.9 wt% for catalyst obtained by calcite
hydration-dehydration, from 75.5 wt% for calcined calcite.
Risso et al. [24] reported the development of CaO catalysts derived
from eggshells, oysters and mollusks for use in soybean oil methanolysis.
Eggshells were subjected to ultrasound irradiation and shellfish shells to
calcination-hydration-calcination cycles to increase the surface area of
CaO and improve its catalytic activity. As a result of the treatments, it
was observed that 5 h of sonication reduced CaO particle size by 34%,
leading to a 56% increase in activity. Two hydration-dehydration cycles
applied to the material obtained by calcination of oyster shells provided
CaO with a specific surface area of 27 m2.g− 1, resulting in 2.5 times
higher transesterification rate than that obtained with the untreated
sample.
In a previous work [39] we prepared catalysts from eggshell by
simple thermal treatments using different calcination atmospheres and
evaluated in the oxidative coupling of methane. It was found that the
calcination atmosphere influences the specific surface area, the crys­
tallinity, and the surface basicity of the obtained materials. The best
result was obtained at 800 ◦ C for the sample calcined under air flow.
Given that the hydration-dehydration of the eggshell significantly
increases its basicity and considering that this is an important charac­
teristic of catalysts for the OCM process, in this work it was evaluated the
influence of hydration-calcination cycles during the preparation of cat­
alysts from eggshell for the light olefins production by methane partial
oxidation.
2. Experimental section
2.1. Catalyst preparation
The preparation of catalysts from chicken eggshell with hydration
step was performed in four stages: i) eggshell calcination under syn­
thetic air flow (50 mL.min− 1) or in muffle furnace at 800 ◦ C with a
heating ramp of 10 ◦ C.min− 1 for 2 h; ii) hydration of 2 g of calcined
sample in 50 mL of distilled water with constant stirring for 1 h at room
temperature; iii) drying in an oven at 80 ◦ C for 12 h and iv) calcination
of the ground and sieved sample (32–42 mesh) under synthetic air flow
(50 mL.min− 1) or in muffle furnace at 800 ◦ C for 2 h with a heating ramp
of 10 ◦ C.min− 1.
In order to evaluate the influence of hydration, samples obtained by
a single calcination (calcination in muffle or under air flow) were also
characterized and tested in the reaction.
2.2. Catalyst characterization
The X-ray diffraction (XRD) patterns were obtained in a Bruker D2
Phaser X-ray diffractometer using Cu-Kα radiation [40-43]. The average
crystallite sizes were estimated according to Scherrer’s Equation (3):
DXRD =
Kλ
βcosθ
(3)
where K = 0.9, λ is the Cu-Kα radiation wavelength (0.154 nm), β is
the line broadening at half width of most intense peak and θ is the
corresponding angle.
The specific surface area of the catalysts was obtained by N2 phys­
isorption measurements in a Quantachrome analyzer, model NOVA
4200e. The samples were previously degassed under vacuum at 300 ◦ C
for a period of 3 h. The specific surface area values were determined by
the BET multi-point method [44,45].
The basicity of the catalysts was determined by CO2 temperatureprogrammed desorption (CO2-TPD) in a SAMP3 multipurpose system
equipped with a thermal conductivity detector (TCD). The samples were
pre-treated at 800 ◦ C under helium flow. The CO2 adsorption was per­
formed at room temperature for 30 min using 30 mL.min− 1 of a mixture
consisting of 5 vol% CO2 in helium. After the adsorption, the sample was
purged with helium flow for 30 min. Then, the temperature was raised
up to 800 ◦ C with a heating ramp of 10 ◦ C.min− 1 and 30 mL.min− 1 of
helium flow [46,47].
Temperature-programmed desorption of O2 (O2-TPD) experiments
were carried out in the same multipurpose system (SAMP3) using pre­
viously calcined samples. The O2 adsorption was performed at room
temperature for 1 h using 30 mL.min− 1 of a mixture of 5% O2 in nitro­
gen. After flushing with nitrogen for 30 min, the temperature was raised
up to 850 ◦ C with a heating ramp of 10 ◦ C.min− 1 and 30 mL.min− 1 of
nitrogen flow. The desorbed O2 was monitored using a TCD detector.
Semi-quantitative chemical analysis by Energy-dispersive X-ray
spectroscopy (EDS) was performed on fresh eggshell in a Phenom
equipment, model Pro-X, using backscattered electrons at 15 kV.
2.3. Catalytic activity
The catalytic tests were carried out at atmospheric pressure, using a
quartz tubular reactor as previously described [39]. The flow rate used
in the tests was 120 mL.min− 1 with a ratio 1:2:9 of CH4: air: N2 (CH4/O2
ratio = 2.4). The tests were carried out using 0.1 g of catalyst in a
stepwise mode from 600 to 800 ◦ C [46]. Five GC analyses of 5 min were
taken at each temperature. The results were obtained from the mean of
these analyses. The stability test was performed at 800 ◦ C with GC an­
alyses performed every 15 min. The products were analyzed by on-line
gas chromatography (Varian 3600cx) with thermal conductivity detec­
tor (TCD) and flame ionization detector (FID), using a packed column
(Porapak Q) and N2 as the carrier gas.
2
D. dos Santos Lima and O.W. Perez-Lopez
Fuel 300 (2021) 120947
The selectivity for the formed hydrocarbons, CH4 conversion and O2
conversion were calculated based on the following equations:
% Selectivity of hydrocarbons =
CH 4 conversion =
O2 conversion =
Ci moles produced
∑
x100
Ci moles
CH 4 in − CH 4 out
x100
CH 4 in
Table 2
Specific surface area and crystallite size of the samples at different stages.
(4)
(5)
O2 in − O2 out
x100
O2 in
(6)
Sample
Treatment
SBET(m2/
g)
Crystallite size
(nm)a
NC
C_M
C_A
CH_M
CH_A
Not calcined
Calcination in muffle
Calcination under air flow
Calcination in muffle + Hydration
Calcination under air flow +
Hydration
Calcination in muffle + Hydration +
Calcination in muffle
Calcination under air flow +
Hydration + Calcination under air
flow
1.3
1.4
5.7
4.4
7.9
48.0b
53.6c
49.1c
53.1d
53.0d
5.0
48.8c
4.9
41.9c
CHC_MM
CHC_AA
3. Results and discussion
Table 1 presents the results of the EDS analysis carried out on the
fresh eggshell. It is observed that in addition to calcium as the major
component there are appreciable amounts of zinc and arsenic, which is
in accordance with the results reported by Mora, 2003 [48]. Considering
that all catalysts were obtained from the same sample, any effect that
may have occurred due to the presence of these elements affected all
catalysts in the same way.
Table 2 shows the specific surface area and the average crystallite
size of all samples at different stages of preparation, where the uncal­
cined eggshell (NC) was included for comparison. It was noticed that
sample calcined under air flow (C_A) shows higher surface area than
sample calcined in muffle [39]. After hydration, both samples showed a
larger specific surface area than the respective calcined samples, as also
observed by Risso et al. [24], which may be attributed to the formation
of calcium hydroxide. However, after the complete calcinationhydration-calcination cycle, no differences in surface area were
observed between both hydrated samples (CHC_MM and CHC_AA),
regardless of calcination method.
Fig. 1 shows the XRD patterns of all samples. The uncalcined sample
(NC) exhibits peaks related to CaCO3 whereas both the calcined samples,
in muffle or under air flow, present reflections of CaO. It is noticed that
the sample calcined in muffle (C_M) shows high crystallinity than sam­
ple calcined under air flow (C_A), which is in agreement with the specific
surface area and average crystallite size for these samples (Table 2).
Both hydrated samples exhibit peaks associated with the hexagonal
crystalline shape of calcium hydroxide [49], which demonstrates that
the simple condition adopted for hydration (1 h at room temperature)
was suitable for complete hydration of the samples. The sample hy­
drated after calcination in air (CH_A) showed higher crystallinity than
sample hydrated after calcination in muffle (CH_M), differently of the
respective single-calcined sample.
After the complete calcination-hydration-calcination cycle, the dif­
fractograms of the final samples showed peaks at 32.2◦ , 37.3◦ , 58.9◦ ,
64.2◦ and 67.3◦ , which correspond to the CaO phase [50,51] with a
difference in crystallinity and average crystallite size: CHC_AA <
CHC_MM.
CO2-TPD profiles in Fig. 2 shows that all samples presented only one
peak of desorption in the range between 500 and 700 ◦ C. For a better
analysis of the results, deconvolution of the CO2-TPD profiles using
Gaussian function was performed. The deconvolution results are shown
in Table 3.
a
b
c
d
Fig. 1. Characterization of the catalysts: XRD patterns.
The peaks located between 500 and 700 ◦ C indicate the presence of
strong basic sites on the CaO surface [24,26] for all samples. Among the
single-calcined samples, the sample calcined in air (C_A) presented the
largest number of basic sites, as well as a high alkaline strength since the
two peaks obtained after the deconvolution showed higher desorption
temperatures than those of the sample calcined in muffle (C_M). After
the complete calcination-hydration-calcination cycle, both the CHC_AA
and CHC_MM samples showed a greater amount of basic sites than their
respective single-calcined samples, C_A and C_M. In addition, there is
also an increase in the strength of the basic sites for both samples that
were subjected to treatment in cycles, since the desorption temperatures
were higher. Among the samples submitted to cycles, the sample
calcined in air (CHC_AA) showed a higher amount of basic sites than the
sample calcined in muffle (CHC_MM). Furthermore, the CHC_AA sample
showed a higher fraction of the stronger sites (2nd peak) than the
CHC_MM sample, 41.2% and 34.6%, respectively.
The most significant result of the hydration cycles was in the strength
of the basicity for the CHC_AA and CHC_MM samples, in which the
desorption peaks occurred at higher temperatures, indicating the pres­
ence of basic sites with greater basic strength. On the other hand, the
integrated areas related to these peaks were in the order: CHC_AA > C_A
Table 1
Mean surface composition of fresh eggshell obtained by
EDS.
Element
Composition (wt%)
Calcium
Carbon
Aluminium
Silicon
Zinc
Sulfur
Arsenic
89.3 ± 5.8
2.1 ± 0.1
1.2 ± 0.3
0.8 ± 0.2
2.3 ± 1.0
0.7 ± 0.2
3.6 ± 0.8
Determined from XRD pattern using Scherrer’s equation
Obtained at 29.4◦ for CaCO3.
Obtained at 37.3◦ for CaO.
Obtained at 34.1◦ for Ca(OH)2.
3
D. dos Santos Lima and O.W. Perez-Lopez
Fuel 300 (2021) 120947
According to literature [52,53] adsorbed oxygen is released at temper­
atures below 400 ◦ C, the peak between 400 and 700 ◦ C is related to the
desorption of surface lattice oxygen, whereas the peak at temperatures
above 700 ◦ C is related to the desorption of bulk lattice oxygen.
The two samples calcined in muffle with and without hydration, C_M
and CHC_MM respectively, showed very small desorption peaks. The
C_M sample exhibited peaks at around 400 ◦ C and 700 ◦ C, whereas
CHC_MM presents a small peak at 730 ◦ C, related to the desorption of
bulk lattice oxygen. The samples calcined under air flow presented a
very different behavior, exhibiting only a peak of desorption around
400 ◦ C but with great intensity, which is related to surface lattice oxygen
desorption [52]. The C_A sample presented a peak smaller than CHC_AA,
which indicates a low amount of surface lattice oxygen. On the other
hand, the desorption of the surface lattice oxygen from CHC_AA is
observed up to approximately 500 ◦ C, which suggests a greater inter­
action strength than in C_A.
Fig. 4 shows the CH4 and O2 conversions at different reaction tem­
peratures. Fig. 4 (a) shows that CH4 conversion increases with the
temperature for all the samples, reaching a maximum value close to 30%
at 800 ◦ C for the CHC_AA catalyst. These conversion values are similar to
the ones found for the sample without hydration and calcined under air
flow (C_A). However, examining the values obtained in the range be­
tween 650 and 750 ◦ C, a significant increase in CH4 conversion was
noted for the CHC_AA sample, indicating a positive effect of the
hydration-calcination cycles. This result may be related to the presence
of a higher percentage of basic sites with greater basic strength in
CHC_AA, as shown in Table 3. For purposes of comparison with the
homogeneous reaction, Arutyunov and Strekova [54] reported a CH4
conversion of less than 5% in the 350–450 ◦ C range in the absence of a
catalyst, whereas Liang et al. [55] reported conversions of CH4 and O2
close to 0% at a reaction temperature of 800 ◦ C, in the absence of
catalyst.
Fig. 4 (a) shows that the calcination atmosphere had a great influ­
ence on methane conversion, since samples in which the single calci­
nation (C_M) or both calcinations were performed in a muffle (CHC_MM)
showed lower CH4 conversion values than the ones calcined under air
flow for all temperatures evaluated. These results can be explained by
the O2-TPD (Fig. 3) since both samples calcined in air (C_A and CHC_AA)
showed a strong desorption peak around 400 ◦ C related to surface lattice
oxygen. The lower CH4 conversion obtained with the CHC_MM sample is
also explained by the low reactivity of oxygen present in this sample,
since its O2-TPD profile revealed the lowest amount of desorbed oxygen.
These results demonstrate that the presence of surface lattice oxygen is
more important than basicity for CH4 conversion. This same trend was
observed for O2 conversion: CHC_AA > C_A > C_M > CHC_MM, where
the highest conversion values were above 90% at 800 ◦ C.
Fig. 5 shows the ethylene, ethane and propylene selectivity for all
samples obtained from the analysis performed with the FID detector.
These results were calculated using equation (4). The production of
ethylene and propylene was favored by the increase in the reaction
temperature, indicating that these olefins are obtained by dehydroge­
nation of their respective paraffin, as demonstrated by the selectivity
results for ethane presented in Fig. 5b. In addition, it is observed that the
selectivity for ethane has the opposite behavior to the selectivity for
ethylene. Among the hydrated samples, the best results of ethylene
production were obtained for the CHC_AA sample (calcined in air flow)
with selectivity values greater than 52% at 800 ◦ C. These results were
slightly superior than the ethylene selectivity obtained with the catalyst
without hydration and calcined under synthetic air flow (C_A). The
CHC_AA catalyst showed the highest propylene selectivity at 750 ◦ C,
reaching values around 2.8%, higher than that obtained for samples
without hydration (C_A). The selectivity to propylene at 800 ◦ C was
higher for both hydrated samples (CHC_AA and CHC_MM) than for the
catalyst without hydration, reaching around 3% for CHC_MM. The high
selectivity for olefins (ethylene and propylene) presented by the catalyst
CHC_AA resulted in the lower selectivity for ethane, while an opposite
Fig. 2. Characterization of the catalysts: CO2-TPD curves.
Table 3
Deconvolution of CO2-TPD profiles for calcined samples.
Catalyst
C_A
C_M
CHC_AA
CHC_MM
Temperature (◦ C)
Relative fraction of
the total sites (%)
1st
peak
2nd
peak
1st
peak
2nd
peak
596
574
611
614
620
596
640
644
65.9
80.7
58.8
65.4
34.1
19.3
41.2
34.6
Total basic sites (mmol/
gcat)
0.0434
0.0267
0.0550
0.0405
> CHC_MM > C_M (Table 3), indicating this order in the amount of basic
sites. This result shows that the hydration cycles had a positive effect for
increasing the number of basic sites of the materials, since the hydrated
samples had a higher density of basic sites and with greater basic
strength than samples without hydration (C_A and C_M).
Fig. 3 shows the results of O2-TPD for all samples calcined at 800 ◦ C
in a muffle (C_M and CHC_MM) or under air flow (C_A and CHC_AA).
Fig. 3. O2-TPD profiles of the catalysts calcined at 800 ◦ C.
4
D. dos Santos Lima and O.W. Perez-Lopez
Fuel 300 (2021) 120947
Fig. 4. Conversion as a function of reaction temperature: a) CH4 conversion and b) O2 conversion.
Fig. 5. Selectivity obtained as a function of reaction temperature: a) to ethylene, b) to ethane and c) to propylene.
5
D. dos Santos Lima and O.W. Perez-Lopez
Fuel 300 (2021) 120947
behavior is presented by the catalyst with single calcination in muffle
(C_M). These results demonstrate that the calcination-hydrationcalcination cycles positively influence the selectivity for obtaining hy­
drocarbons, especially propylene and are higher to those obtained by
Jones et al. [56] where the highest selectivity found for C2+ products
was approximately 25% at 800 ◦ C.
Fig. 6 presents the yield of C2 − C3 products (a), as well as the yield
for propylene (b). Fig. 6(a) shows that the yield of C2-C3 increases with
temperature for all catalysts. It was observed that the highest yields were
obtained for the catalysts calcined under air flow (C_A and CHC_AA) and
among them the highest yields of C2-C3 were obtained with the CHC_AA
catalyst, mainly between 650 and 750 ◦ C. These results demonstrate that
the presence of both alkaline sites and surface lattice oxygen favors the
selective conversion of methane to ethylene [57]. On the other hand, the
propylene yield (Fig. 6(b)) was notably higher for the CHC_AA catalyst
over the entire temperature range, with an emphasis on temperatures of
700 and 750◦ C. The maximum propylene yield was obtained at 800 ◦ C
in the following order: CHC_AA > CHC_MM > C_A ≈ C_M. These results
are mainly due to the increase in basicity and basic strength that
occurred through the treatment with hydration-calcination cycles for
the CHC_MM and CHC_AA samples.
Fig. 7 shows the ratio between C2 and CO2 products obtained as a
function of the reaction temperature. The CHC_MM catalyst presented
the highest C2/CO2 ratio among all the samples. The highest C2/CO2
ratio was obtained at 750 ◦ C. These results can be attributed to the
absence of surface lattice oxygen in the CHC_MM sample, as showed in
Fig. 3. In contrast, the C_A and CHC_AA samples showed a high amount
of surface lattice oxygen which promotes the partial oxidation of
methane and methane combustion, in addition to oxidative coupling of
methane, as reported in the literature [58,59]. This leads to a lower C2/
CO2 ratio for both samples calcined under air flow (C_A and CHC_AA). It
is also worth noting that the sample CHC_MM calcined in muffle showed
the lower CH4 and O2 conversions among all the samples (Fig. 4), which
partially explains the higher C2/COx ratio obtained by this catalyst.
Fig. 8 presents the results for the stability test performed at 800 ◦ C for
Fig. 7. C2/CO2 ratio as a function of reaction temperature.
the CHC_AA and CHC_MM hydrated samples. The CH4 conversion
remained constant over the time-on-stream, with an average value of
around 28% for CHC_AA. There was a slight decline in activity for the
CHC_MM sample throughout the test, but the average value at the end of
the reaction was close to that of CHC_AA. The CHC_AA catalyst showed a
high and approximately constant O2 conversion of 91% during the re­
action time. On the other hand, the CHC_MM sample showed a slight
increase in O2 conversion values with the time-on-stream, which can be
related to oxygen deficiency on the surface, reaching a value of 88% at
the end of the test. Regarding the selectivity results for C2 and C3 hy­
drocarbons, both samples showed practically constant values
throughout the reaction. The difference was that the CHC_MM sample
showed greater selectivity in the formation of C2 and C3 olefins,
resulting in average values of 55.1% of ethylene and 3.4% of propylene.
The results obtained in this work showed that CaO catalysts with
different morphologies and crystallinity were obtained through eggshell
treatment by calcination and hydration. The basic character of initial
samples has also been considerably modified. The results of the catalytic
tests showed that among the hydrated samples the lowest methane and
oxygen conversions were obtained for the sample calcined in a static
atmosphere after hydration (CHC_MM), a result that may be linked to
the higher percentage of basic sites with the lowest basic strength pre­
sent on this catalyst. In contrast, the CHC_AA sample, where the calci­
nation after hydration was under an air flow, showed the best results for
the conversion of methane and oxygen, which can be ascribed to a larger
fraction of stronger basic sites and to the presence of surface lattice
oxygen. These results followed the same trend for the samples without
hydration cycles, where the catalyst calcined under air flow (C_A)
showed higher methane conversion than sample calcined in muffle
(C_M). On the other hand, the highest C2/CO2 ratio and the highest
selectivity for obtaining propylene (at 800 ◦ C) were obtained for the
CHC_MM sample, where the two calcinations were carried out in a static
atmosphere, resulting in a material with a higher strength of basic sites
and practically without surface lattice oxygen.
Table 4 shows a comparison of the results obtained in this work with
some reported in the literature. For this comparison, the selectivity for
ethylene values were also recalculated considering the C2 hydrocarbons
and the COx compounds formed during the reaction. Regarding methane
conversion, it is observed that the results obtained in this work are su­
perior or very close to those obtained by other authors using more
complex catalysts. This comparison demonstrates that the catalysts ob­
tained from eggshell present very interesting results, since these values
are comparable or even higher than some works in the literature that use
more complex materials.
Fig. 6. Yield of C2-C3 (ethylene and propylene) olefins (a), and C3 (propane and
propylene) yield (b), as a function of reaction temperature.
6
D. dos Santos Lima and O.W. Perez-Lopez
Fuel 300 (2021) 120947
Fig. 8. CH4 and O2 conversion (a, b) and hydrocarbons selectivity (c, d) during the stability test at 800 ◦ C. CHC_AA: (a, c); and CHC_MM: (b, d).
from the eggshell were observed. The main result of the hydration was
the increase in the total amount as well as in the strength of the basic
sites of these materials, which had a positive effect on the catalytic
properties for OCM.
The CHC_AA sample, hydrated and calcined under air flow, showed
the best results for the conversion of methane and selectivity for C2,
which can be related to the larger fraction of stronger basic sites and to
the presence of surface lattice oxygen.
The highest C2/CO2 ratio was obtained for the CHC_MM sample that
showed a higher strength of basic sites.
The results showed that it was possible to obtain simple, cheap and
selective catalysts from eggshell. The proper combination of thermal
treatment and hydration improves the catalyst activity and selectivity to
olefins. The highest methane conversion and selectivity to ethylene was
obtained at 800 ◦ C using eggshells calcined under air flow, regardless of
hydration. The highest yield for propylene was obtained between 750
and 800 ◦ C with hydrated eggshells, regardless of the thermal treatment.
Table 4
CH4 conversion and C2 selectivity comparison with some reported in the
literature.
Catalyst
T
(◦ C)
CH4/
O2
XCH4
(%)
SC2
(%)
SC2H4
(%)
Reference
CHC_MM
CHC_MM
800
800
2.4
2.4
25.3
25.3
70.3
–
31.6
48.1
CHC_AA
CHC_AA
800
800
2.4
2.4
29.2
29.2
59.8
–
29.6
53.6
SrTiO3(STO)
Sr0.8Ca0.2TiO3
Sr2TiO4_SP1
Na2WO4/Mn/
SiO2
1%Ni–TbOx/
MgO
La2Li0.5Al0.5O4
1%Y/Sm2O3
LaAlO3
800
800
800
850
3
3
4
3.5
32.5
30.4
20.8
41.0
–
–
56.3
60.0
34.9
38.2
29.5
–
This work
This
work*
This work
This
work*
[5]
[5]
[16]
[60]
800
4
17.0
22.0
–
775
800
775
3
4
3
17.0
21.0
31.0
62.0
–
14.0
40.0
23.00
9.00
[56]
[61]
[62]
[63]
Declaration of Competing Interest
* Selectivity without COx.
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.
4. Conclusions
Significant influence of the hydration-calcination cycles in the spe­
cific surface area, crystallinity and alkalinity of the materials obtained
7
D. dos Santos Lima and O.W. Perez-Lopez
Fuel 300 (2021) 120947
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9