Unexpected activity of MgO catalysts in oxidative coupling of methane: Effects of Ca-promoter

Article abstract of DOI:10.1016/j.mcat.2021.111677

Unexpectedly, we founded very different catalytic activities of MgO in oxidative coupling of methane (OCM) depending on company that supplied the precursors, which was due to the Ca-related impurities. Accordingly, we investigated the effects of Ca-promoter on the catalytic activity of MgO catalysts. Substituting Ca into the MgO catalysts strongly influenced both the formation of oxygen vacancies and the base properties of surface lattice oxygen (O_lat), which are key factors in this reaction. The formed oxygen vacancies resulted in high methane conversion through the facile conversion of methane to methyl radical, whereas O_lat with medium basicity played an important role in producing ethylene through the facile dehydrogenation of ethane.

Full text of DOI:10.1016/j.mcat.2021.111677

Molecular Catalysis 510 (2021) 111677
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Unexpected activity of MgO catalysts in oxidative coupling of methane:
Effects of Ca-promoter
*
JeongHyun Cho, Dahye Kwon, Inchan Yang, Suna An, Ji Chul Jung
Department of Chemical Engineering, Myongji University, Yongin 17058, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
Unexpectedly, we founded very different catalytic activities of MgO in oxidative coupling of methane (OCM)
depending on company that supplied the precursors, which was due to the Ca-related impurities. Accordingly, we
investigated the effects of Ca-promoter on the catalytic activity of MgO catalysts. Substituting Ca into the MgO
catalysts strongly influenced both the formation of oxygen vacancies and the base properties of surface lattice
oxygen (Olat), which are key factors in this reaction. The formed oxygen vacancies resulted in high methane
conversion through the facile conversion of methane to methyl radical, whereas Olat with medium basicity
played an important role in producing ethylene through the facile dehydrogenation of ethane.
Oxidative coupling of methane
MgO catalyst
Ca-promoter
Lattice oxygen species
Base property
1
. Introduction
different companies (companies A and B), and we labeled the MgO
catalysts as MA100 and MB100 according to the companies. Surpris-
Recently, shale gas has attracted great attention as a promising
alternative energy source. Hence, the conversion of methane (CH ),
ingly, they exhibited different catalytic performances in OCM, even
though the catalysts were prepared under exactly the same conditions
(Fig. 1). The reasons for this variation in the catalytic performance
were confirmed through ICP-AES analysis. MA100 catalyst did not
retain any metal components, except for Mg, whereas MB100 con-
tained ca. 1 mol% of Ca in all metal components. For this reason, we
infer that the Ca content of MB100 caused the difference in the cata-
lytic performance, indicating that Ca plays a role in promoting MgO
catalysts in OCM. Accordingly, we prepared a series of MgO catalysts
with different content of Ca-promoter to investigate its effects. Then,
we employed the catalysts in OCM in a continuous-flow fixed-bed
reactor. Based on the obtained results, we intensively analyzed the role
of oxygen vacancy and the base properties of surface lattice oxygen
(Olat) on MgO to design a high-performance OCM catalyst.
4
which is the main component of the shale gas, into high-value chem-
icals is considered an important process [1–5]. Among the various
methane conversion methods, oxidative coupling of methane (OCM) to
2
produce ethane and ethylene (C -hydrocarbons) has attracted research
in heterogeneous catalysis. To commercialize OCM, however, the
chemical stability of methane and the thermodynamically favorable
formation of carbon monoxide and carbon dioxide from methane must
be overcome [6–8].
To overcome these issues, many researchers have developed various
catalysts, including lanthanide series oxides, Na-W-Mn mixed metal ox-
ides, and alkaline-earth metal oxides, and investigated their active sites in
OCM [9–13]. According to previous reports on OCM, Na
regarded as a high-performance catalyst for the OCM [14–16]. However,
the active sites of Na WO /Mn/SiO are not yet clarified owing to their
2 4 2
WO /Mn/SiO is
2
4
2
2. Experimental
structural complexity [17]. Therefore, there is a need for more funda-
mental research that is necessary to clarify the catalytically active sites for
OCM. Hence, we have investigated the catalytic active sites through
relatively simple and clear structures such as perovskite and single-oxide
catalysts [18–22].
In this study, we employed various single-oxide catalysts, such as
ZnO, CaO, BaO, and MgO to OCM, and MgO yielded interesting results.
Magnesium nitrate precursors for MgO were purchased from two
2.1. Catalyst preparation
The MXY catalysts (X = precursor supplying company (A or B), Y =
mol% of Mg in metal components, (Mg/(Mg+Ca))×100)) were prepared
through a solid-state method with varying loading amounts of the Ca-
promoter precursor. For example, MA99 represents that which was
prepared using the precursor from company A and the mol% of Mg in all
*
Corresponding author.
E-mail addresses: jcjung@mju.ac.kr, cjung98@gmail.com (J.C. Jung).
https://doi.org/10.1016/j.mcat.2021.111677
Received 20 April 2021; Received in revised form 23 May 2021; Accepted 25 May 2021
Available online 12 June 2021
2
468-8231/© 2021 Elsevier B.V. All rights reserved.

J. Cho et al.
Molecular Catalysis 510 (2021) 111677
Table 1
ICP-AES results of the prepared MXY catalysts
a
Catalyst
Mole fraction
Mg
Ca
MB100
MA100
MA99
MA90
MA0
0.99
1
0.01
0
0.99
0.90
0
0.01
0.10
1
a
Mole fractions of the metal components were calculated as the mole
ratio of the corresponding metal component (Mg or Ca) to the total metal
components (Mg+Ca) in the catalyst.
1
000 column was used to separate the reaction products. CH
hydrocarbons were detected by the FID, and CO and CO by the TCD.
The catalytic activities were then calculated using Eqs. (1)–(3), and the
yield of C -hydrocarbons was obtained by multiplying the CH conver-
sion by the C selectivity. To minimize experimental errors, we per-
4 2
and C -
2
Fig. 1. Catalytic activities of MX100 in OCM (Reaction conditions: reaction
◦
ꢀ 1
temperature = 775 C, reactant flow: 20 mL min (CH
4
1
:O
2
:N
2
= 3:1:1 (v/v/
2
4
v)), and gas hourly space velocity (GHSV) = 10,000 hꢀ
)
2
formed the activity test five times for each catalyst, and used the average
values for evaluations.
metal components (Mg+Ca) of the prepared catalyst is 99%. Mg
NO O (ACS reagent, 99%, Sigma-Aldrich (company A) and Extra
•6H
pure reagent, 98%, Yakuri Pure Chemicals (company B)) and Ca
NO O were used as the precursors for MgO catalysts and Ca-
(
3
)
2
2
moles of CH
4
consumed
CH
4
conversion =
× 100(%),
(1)
(2)
(3)
moles of CH
4
in the feed
(
3
)
2
•4H
2
promoter, respectively. The precursors were thoroughly mixed in an
2
× moles of C
2
ꢀ hydrocarbons
◦
agate mortar. Then, the mixture was calcined at 950 C for 5 h in a muffle
C
2
selectivity =
× 100(%),
moles of CH
4
consumed
furnace to obtain the MXY catalysts.
moles of CO
× 100(%),
moles of CH consumed
X
CO
X
selectivity =
2
.2. Characterization of the catalysts
4
Inductively coupled plasma atomic emission spectrometry (ICP-AES)
3
. Results and Discussion
.1. Confirmation of successful preparation of MXY catalysts
ICP-AES analysis was performed to explore the reasons for the dif-
was employed to investigate the atomic compositions of the catalysts
using an ICPS-8100 spectrometer (Shimadzu). XRD measurements were
performed using an X’pert-Pro PANalytical diffractometer (λ = 1.54056
Å) to confirm the crystal structure of the catalysts, and XRD patterns
3
◦
◦
within a 2θ range of 10 ꢀ 90 were recorded. The base properties of the
synthesized catalysts were analyzed by CO -temperature programmed
desorption (CO -TPD) measurements and the Hammett indicator
method. The CO -TPD was conducted using a BELCAT B (Microtrac
ference in the catalytic activities of MA100 and MB100, and the ob-
tained results are shown in Table 1. It was confirmed that MA100, which
is the MgO catalyst synthesized with the precursor from company A, is
composed of 100 mol% of Mg in all the metal components of the cata-
lyst. Surprisingly, MB100, the MgO catalyst synthesized with the pre-
cursor from company B, contained 1 mol% of Ca in all the metal
components of the catalyst, even though the catalyst was prepared
without an additional Ca source. It implies that Ca-related compounds
exist as impurities in the precursor purchased from company B, and this
study was triggered by the idea that impurity could affect the catalysis of
MgO in OCM. Moreover, we confirmed that the MAY catalysts were
successfully prepared with the intended mole fraction of Mg and Ca
constituents. Consequently, we assume that Ca present in MB100
resulted in the difference in catalytic performance compared to MA100
2
2
2
MRB, Japan), and 0.05 g of each catalyst was loaded into the quartz
◦
reactor of the TPD apparatus. Firstly, it was pretreated at 400 C for 1 h
ꢀ 1
under helium inflow (30 mL min ). When cooled to room temperature
ꢀ
by helium, 5% CO
at room temperature for 40 min to adsorb CO
room temperature with helium for 30 min to desorb the physically
adsorbed CO . The TPD profiles of the catalysts were recorded from
room temperature to 950 C at a heating rate of 10 C min and the
desorbed CO was detected by a thermal conductivity detector (TCD).
2
in helium (30 mL min 1) was pulsed into the reactor
2
. Finally, it was purged at
2
◦
◦
ꢀ 1
2
The Hammett indicator method was employed using four Hammett in-
dicators, including bromothymol blue (H_ = 7.3), phenolphthalein (H_
(
Fig. 1), and it is expected that the MAY catalysts would exhibit different
=
9.5), nile blue A (H_ = 10.1), and tropaeolin O (H_ = 11.1), to analyzed
activities in OCM with respect to the Ca content.
the strength of the base sites on the catalysts. These indicators were
When Ca was added into the MgO catalysts, the MXY catalysts were
expected to experience structural changes. XRD measurements were
employed to investigate the structural properties of the MXY catalysts
ꢀ 5
added to the water to prepare a 1.0 × 10 M solution, and then, each
catalyst (ca. 0.02 g) was added to the solution (ca. 2 mL).
(
Fig. 2). Using JCPDS cards, we confirmed that the MXY catalysts were
2
.3. Catalytic reaction
successfully prepared [24,25]. Interestingly, peaks that indicate the
presence of CaO were not observed in the XRD patterns of the MgO
catalysts with small Ca content (MB100, MA100, MA99). However,
peaks corresponding to CaO were observed in the catalysts with rela-
tively large Ca content (MA90), and MA0, which was prepared with only
Ca precursors, showed only CaO characteristic peaks. It is also note-
The OCM reaction was performed in a continuous-flow fixed-bed
◦
reactor. The reactor was heated to 775 C under nitrogen flow, and then,
the reactor feed was changed to the reactant flow: CH
4
:O
2
:N
2
= 3:1:1 (v/
ꢀ
1
v/v). The total flow rate of the reactant feed was set at 20 mL min and
ꢀ 1
◦
the gas hourly space velocity was 10000 h . The water vapor produced
during the reaction was removed by a cold trap. The produced gas was
periodically analyzed using an online gas chromatography system (YL-
worthy that the major peaks of MgO at 2θ = 42.96 (200) were gradually
shifted towards the left as the Ca content increased. These results show
that, in the case of catalysts with small Ca content, Mg in MgO was
substituted by Ca according to the addition of the Ca-promoter precur-
sor, whereas phase segregation from the MgO surface in form of CaO was
6
500, Younglin, Republic of Korea) equipped with a flame ionization
detector (FID) and a thermal conductivity detector (TCD). A carboxen-
2

J. Cho et al.
Molecular Catalysis 510 (2021) 111677
Fig. 2. (a) XRD patterns of the prepared MXY catalysts and (b) magnified region of XRD patterns of the prepared MXY catalysts within a 2θ range of 41 ꢀ 44◦
◦
Table 2
Table 3
d-spacing of the MXY catalysts
2
Amounts of base sites in MAY catalysts quantified from CO -TPD results
a
Catalysts
MA100
0.2102
MB100
0.2104
MA99
MA90
Catalyst
Peak area (a.u.)
Weak (35 ◦Cꢀ 200
Medium (200 ◦Cꢀ 400
Strong (400 ◦Cꢀ 800
d-spacing (nm)
0.2106
0.2105
◦
◦
◦
C)
C)
C)
MA100
MA99
MA90
MA0
2120.5
517.7
-
5844.5
4917.9
3559.9
1925.0
12621.1
66357.6
191905.1
-
-
a
Peak areas were calculated from the amounts of CO
2
adsorption in the TPD
profiles.
3
.2. Base properties of MAY catalysts
When Mg in MgO is substituted to Ca by adding a Ca-promoter, the
electronic properties of Olat bounded to metals, that is the base properties
of Olat, are expected to greatly change [35,37–39]. It is well-known that
Olat is responsible for the active sites that determine catalytic activities in
OCM [18,29,34]. Moreover, according to previous studies on catalysts for
OCM, the catalytic activities in OCM have considerable relationships with
the basicity of Olat [40]. Hence, we infer that the difference in the activity
of MXY catalysts could be explained by investigating the base properties of
Olat in the catalysts. Thus, we analyzed the base properties of Olat using
CO -TPD and the profiles are presented in Fig. 3. Herein, we divided the
2
regions into three, according to the ranges of temperatures for CO
2
◦
desorption, and the regions were designated as weak base site (35 Cꢀ 200
◦
◦
◦
◦
◦
C), medium base site (200 Cꢀ 400 C), and strong base site (400 Cꢀ 800
2
Fig. 3. CO -TPD profiles of MAY catalysts
C). In addition, we calculated the number of base sites from the peak areas
of each temperature region (Table 3).
The CO -TPD profiles of the prepared MAY catalysts were outstand-
ingly different. In detail, the peak temperatures for the CO desorption of
observed when the Ca content of the MgO catalyst increased beyond a
certain amount [26–28]. For clear confirmation, we calculated the
d-spacing using Bragg’s law (nλ = 2d sinθ). As listed in Table 2, the
values of the d-spacing increased as Ca was added, indicating a successful
substitution of Mg by Ca in the prepared catalysts. From the obtained
XRD results, we infer that substituting Mg with Ca changes the structural
properties of the catalyst surface and the properties of Olat, which
greatly influence the catalytic activities in OCM [18,29,34]. Moreover,
according to previous studies on metal oxide catalysts for OCM, adding
Ca into MgO, could result in structural defects as oxygen vacancies
2
2
the catalysts were differently presented, and the peak areas were also
different. In the case of MA100, weak and medium sites were observed, but
not strong base sites. MA99 showed all the three base sites, and the number
of base sites was increased compared to those of MA100. Meanwhile, in the
case of MA90 and MA0, both medium- and strong-base sites were observed
2
in their CO -TPD profiles. Notably, the number of strong-base sites showed
an increasing trend, but that of medium-base sites showed a volcano-
shaped curve with respect to the Ca content in the MXY catalysts. Conse-
quently, we confirmed MA99 contained the largest amounts of Olat with
weak and medium basicity and MA0 contained the largest amount of Olat
with strong basicity.
[
30–33,35], and the oxygen vacancy could facilitate the conversion of
methane to the methyl radical [15,36]. Therefore, it is believed that Ca
could serve as a promoter in MgO catalysts for OCM and MAY catalysts
exhibit different catalytic activities in OCM with respect to the Ca
content.
To further check the basicity of Olat in the MAY catalysts, we employed
the Hammett indicator method, using four indicators with different
3

J. Cho et al.
Molecular Catalysis 510 (2021) 111677
4 x 2 2
Fig. 4. Catalytic activities of MXY catalysts in OCM ((a) CH conversion (%), (b) CO selectivity (%), (c) C selectivity (%), and (d) C yield (%))
basicity (H_), including bromothymol blue (H_ = 7.3), phenolphthalein
H_ = 9.5), nile blue A (H_ = 10.1), and tropaeolin O (H_ = 11.1). The
their catalytic activities in the reaction. In other words, Ca could serve as
a promoter for MgO catalyst in OCM.
(
results are shown in Fig. S1. In particular, the prepared MAY catalysts
exhibited a completely different trend of color change. When bromothy-
mol blue and phenolphthalein were used, the colors of the solutions were
changed to blue and pink, respectively. This implies that all MAY catalysts
had H_ of 9.5 or higher. However, when the nile blue A indicator was used,
the color of the solution added MA100 could not completely change to
red, whereas that of the other solutions completely changed to red. This
shows that the H_ value of MA100 was between 9.5 and 10.1. In addition,
in the case of tropaeolin O solution, the solutions added MA99 and MA100
remained their yellow color, meanwhile, that added MA90 and MA0
completely changed to orange. This confirms that the H_ value of MA99
was between 10.1 and 11.1 and that of MA90 and MA0 was 11.1 or
higher. Based on the above results, we finally confirmed that the basicity
of the MAY catalysts increased in the order of MA100, MA99, and MA90 ≈
MA0. More importantly, the results obtained from the Hammett indicator
The methane conversion was remarkably dependent on the Ca content
of the MXY catalysts. It is well-known that oxygen in feed gases is
adsorbed onto the oxygen vacancies on the catalyst surface, and the
adsorbed oxygen (Oads) facilitates the conversion of methane to methyl
radical, which is subsequently dimerized into C
Thus, considering that the Ca-promoted MgO catalysts exhibited a higher
value of CH conversion than pure MgO, we could infer that oxygen va-
2
compounds [15,41,42].
4
cancies are developed as Ca is added to MgO catalysts. The XRD results
confirmed that Mg was substituted with an appropriate amount of Ca in
the Ca-promoted MgO, and this changed the structural properties of the
catalyst surface. In addition, when additives such as Ca are added to MgO,
lattice distortion occurs due to the difference in radius between Ca and
Mg, and such a distorted lattice possesses abundant defects like oxygen
vacancies [30–33,35]. For these reasons, MA99 and MB100, which con-
tained 1 mol% of Ca in all metal components of the prepared catalysts,
2
method are in good agreement with those of the CO -TPD analysis.
4
exhibited the highest value of CH conversion owing to their large number
Through the CO -TPD analysis and the Hammett indicator method, we
2
of oxygen vacancies.
confirmed that the MXY catalysts retained different base properties of Olat
,
Considerable changes in the CO/CO
ing to the Ca content are also noteworthy results. As the Ca content
gradually increased, the C /C ratio showed the same trend as the
yield, whereas the CO/CO ratio showed the opposite trend. Ac-
cording to previous research, Olat is responsible for converting the C
formed by the dimerization of methyl radicals to C or CO [15,42].
2 2 4 2 6
and C H /C H ratios accord-
which are expected to strongly affect their catalytic activity in OCM.
2
H
4
2 6
H
3
.3. Catalytic activities of MXY catalysts in OCM and property of surface
C
2
2
oxygen species
2 6
H
2
H
4
x
As expected, the activity of the MXY catalysts remarkably varied in
OCM with respect to the Ca content in OCM (Fig. 4). Particularly, the C
Considering that the MAY catalysts showed a different selectivity trend,
we predict that substituting Mg with Ca in Ca-promoted MgO changes
the properties of Olat. In addition, in our previous study, we obtained
2
yield over the MAY catalysts showed a volcano-shaped curve with
respect to the Ca content in the catalysts. Accordingly, MA100, which
was composed of pure MgO, exhibited poor catalytic activities in OCM,
whereas MA99 exhibited the highest catalytic performance in terms of
that electrophilic oxygen species with medium basicity converts C
[40,43], whereas the nucleophilic oxygen species with strong ba-
sicity facilitates CO production [23]. Therefore, we presumed that
adding an appropriate amount of Ca to the MgO catalyst would enrich
lat with medium basicity, whereas Olat with strong basicity would be
developed with excessive Ca content. These results are interestingly
2 6
H to
2 4
C H
x
2
C yield (ca. 14.1%). Moreover, MB100 and MA99 with similar Ca
content exhibited a similar activity. These results indicate that the Ca
content of the MXY catalysts is one of the crucial factors that determine
O
4

J. Cho et al.
Molecular Catalysis 510 (2021) 111677
Scheme 1. Proposed role of surface oxygen species in OCM
consistent with the results of the CO
2
-TPD analysis and Hammett indi-
2
highest C yield due to its well-developed oxygen vacancies and abun-
cator method. Thus, the catalysts (MB100 and MA99) that contained 1
mol% of Ca in all metal components exhibited high C selectivity due
to their large amount of Olat with medium basicity, which facilitates the
dehydrogenation of C to C . On the contrary, MA90 and MA0
showed high CO selectivity due to their abundant Olat with strong ba-
dant Olat with medium basicity. Finally, we conclude that Ca could be
one of the efficient promoters for MgO catalysts in OCM through the
2 4
H
formation of oxygen vacancies and tuning of base properties of Olat
.
2
H
6
2 4
H
2
CRediT authorship contribution statement
sicity which is well known as a strong oxidant for the production of CO
from various hydrocarbons [44]. Additionally, considering the CO
2
2
JeongHyun Cho: Conceptualization, Investigation, Data curtion,
Writing – original draft. Dahye Kwon: Investigation, Writing – original
draft, Methodology. Inchan Yang: Investigation, Visualization. Suna An:
Investigation, Visualization. Ji Chul Jung: Conceptualization, Funding
acquisition, Writing – review & editing, Supervision.
generated over MA100 without Olat with strong basicity and the results
of previous studies, we infer that CO could also be produced by con-
verting methyl radical to CO in subsequent gas-phase reactions [41].
2
2
With the relationship between the catalytic activity (methane con-
version and product selectivity) and the properties of MAY catalysts, we
propose the role of the surface oxygen species (Oads and Olat) in OCM
(Scheme 1). Oxygen in the gas phase is initially adsorbed on the oxygen
Declaration of Competing Interest
vacancies of the catalysts, and subsequently, Oads is generated. The
generated Oads converts methane to the methyl radical through a het-
erogeneous reaction, and subsequently, the formed methyl radicals are
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.
dimerized to C
with medium basicity produces C
dehydrogenation of C , and that with strong basicity is responsible for
converting C and CO to CO . Therefore, we concluded that well-
H
2 6
through a homogenous reaction. More importantly, Olat
H
2 4
by converting C through a facile
2 6
H
2 6
H
Acknowledgements
H
2 6
2
developed oxygen vacancies and abundant Olat with medium basicity
are crucial factors to designing efficient and high-performance catalysts
for OCM.
This research was supported by C1 Gas Refinery Program through
the National Research Foundation of Korea(NRF) funded by the Ministry
of Science, ICT & Future Planning (2015M3D3A1A01064908).
4
. Conclusions
Supplementary materials
We observed strange phenomena with very different catalytic ac-
Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.mcat.2021.111677.
tivities of MgO in oxidative coupling of methane (OCM). The activity
varied according to the company supplied the precursors, which is
attributed to the Ca-related impurities contained in the precursors.
Accordingly, we prepared a series of MAY catalysts (Y = mol% of Mg in
metal components, (Mg/(Mg+Ca))×100)) by a solid-state method with
varying loading amounts of Ca-promoter and thoroughly investigated
the effects of the Ca-promoter on the MgO catalysts. Interestingly, the
catalytic performance of the prepared catalysts varied with respect to
the added Ca amount. When Mg was substituted by Ca, oxygen va-
cancies were formed and the base property of the lattice oxygen species
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