Direct oxidation of methane to methanol over Cu-zeolites at mild conditions

Article abstract of DOI:10.1016/j.mcat.2020.110886

The partial oxidation of methane to methanol over a Cu-Na-MOR catalyst is studied in this work. The reaction, performed in a fixed-bed reactor, is accomplished according to a three steps cycling process: adsorption of methane, desorption of methanol promoted by water and regeneration of the catalyst. The operating conditions of the different steps of the process have been optimized to maximize methanol yield. The regeneration using air, instead of pure oxygen, has been found to increase methanol yield in the following cycle. Optimum desorption is carried out using water concentration of 5.2 mol % and 3.04 Nm³ h^?1 kg^?1_cat. At the optimal conditions, the yield of methanol raised to 754 μmol/g Cu, corresponding to 52 % of adsorbed methane being transformed into methanol.

Full text of DOI:10.1016/j.mcat.2020.110886

MolecularCatalysis487(2020)110886
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Direct oxidation of methane to methanol over Cu-zeolites at mild conditions
Mauro Álvarez, Pablo Marín, Salvador Ordóñez*
Department of Chemical and Environmental Engineering, University of Oviedo, Faculty of Chemistry, Julián Clavería 8, 33006 Oviedo, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords:
Methane upgrade
Mordenite
Methanol
Cyclic reactors
Partial oxidation
The partial oxidation of methane to methanol over a Cu-Na-MOR catalyst is studied in this work. The reaction,
performed in a fixed-bed reactor, is accomplished according to a three steps cycling process: adsorption of
methane, desorption of methanol promoted by water and regeneration of the catalyst. The operating conditions
of the different steps of the process have been optimized to maximize methanol yield. The regeneration using air,
instead of pure oxygen, has been found to increase methanol yield in the following cycle. Optimum desorption is
carried out using water concentration of 5.2 mol % and 3.04 Nm3 h−1 kgc−at1. At the optimal conditions, the yield
of methanol raised to 754 μmol/g Cu, corresponding to 52 % of adsorbed methane being transformed into
methanol.
1. Introduction
development of a heterogeneous catalyst for this reaction has been fo-
cused on mimicking the structure of the active site found in these en-
Methanol is widely used in industry as solvent, fuel additive or
feedstock for the production of other chemicals [1–4]. The current
technology for methanol manufacturing is based on the production of
syngas from methane raw material via stream reforming. This process is
energy and capital intensive and, for this reason, the search of a process
for the direct conversion of methane to methanol is of great interest
[5–9]. However, the CeH bond on methane molecule is the strongest
among all the hydrocarbons [10,11], requiring harsh reaction condi-
tions (e.g. temperature) to activate it. At high reaction temperature,
overoxidation of methanol to carbon oxides may take place even in
presence of catalyst, making this process very challenging [12–17].
In the last years, the research in this field has been focused on the
development of a catalyst able of activating methane at low tempera-
ture and preventing further oxidation to carbon oxides. Different types
of catalysts have been proposed, which are classified as homogeneous
(e.g. [18–20]) and heterogeneous (e.g [20–22]). Heterogeneous cata-
lysts are a better option from the point of view of a future industrial
application. Indeed, the product recovery is easier, their cost is lower
and the operating conditions are mild (most homogeneous catalysts are
based on the use of strong acid conditions).
zymes. On the other side, zeolites are materials with highly ordered
internal structure, formed by parallel channels of regular size. These
materials are good candidates to host metallic centers similar to those
of pMMO [13,25].
Many zeolite topologies have been studied [26–29], but copper
exchanged mordenites (Cu-MOR) catalysts have emerged as the most
interesting ones. Their high yield towards methanol and large pores
facilitate the desorption of the products from the active centers [16,30].
Nowadays, there is still no consensus about the active site configura-
tion. Many works indicate that bis(μ-oxo)dicopper active sites are the
only ones responsible of the catalytic behavior. However, other works
suggest that mono(μ-oxo)dicopper and trinuclear copper-oxo clusters
can also be active [31,32].
The oxygen of the active site reacts with methane, leading to in-
termediate adsorbed methoxy species. At low reaction temperature,
these species are strongly absorbed on the active site, preventing other
methane molecules adsorption and stopping the reaction. If tempera-
ture is increased to promote desorption and methanol formation, the
undesired overoxidation to carbon oxides takes place [33].
To recover methanol, liquid [24] or vapor [13] water is introduced
in the reactor. The role of water on this step is still under discussion, but
many authors suggest it can be twofold. On one hand, water can dis-
place methanol from the active centers by competitive adsorption and,
on the other hand, it stabilizes the reaction intermediates [9,24].
These two steps, adsorption and desorption, are performed at 200 °C
or below to minimize undesired overoxidation reactions [6,13]. In
The direct oxidation of methane to methanol at mild and aerobic
conditions is a reaction that actually occurs in microorganisms cata-
lyzed by methane monooxygenase (MMO) enzymes. There are two
known types of MMO enzymes: soluble (sMMO) and particulate
(pMMO). These enzymes contain diiron and dicopper active centers
responsible of the activation of methane molecules [23,24]. The
⁎ Corresponding author.
E-mail address: sordonez@uniovi.es (S. Ordóñez).
https://doi.org/10.1016/j.mcat.2020.110886
Received 16 December 2019; Received in revised form 5 March 2020; Accepted 10 March 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

M. Álvarez, et al.
MolecularCatalysis487(2020)110886
Fig. 1. Flowsheet of the experimental device used in this work.
contact with water, the copper clusters are hydrolyzed and therefore
deactivated [34]. Thus, the material needs to be dehydrated and re-
oxidized at high temperature, before reuse. A cheap and available
oxidant, such as oxygen or air, would be preferred in view of a future
industrial process [23,35].
Summarizing, the overall reaction corresponds to a cyclic process
formed by three steps: (1) methane adsorption, (2) methanol desorption
and (3) oxidative regeneration of the catalyst [2,14]. This process has
been studied for the upgrading of methane emissions on remote oil
exploitations [9], which otherwise are exhausted or flared depending
on local regulations [36,37]. However this direct conversion process
brings the possibility of harnessing other emissions with lower methane
concentration, such as those related to coal mining or waste manage-
ment [38–40].
2.2. Catalyst characterization
Copper loading of the catalyst was determined by dissolving the
sample in aqua regia and analyzing the resulting liquid by ICP-MS. The
textural properties (surface area and pore volume) were determined by
nitrogen adsorption using a Micromeritics ASAP 2020 Plus (before the
analysis, the samples were degassed under vacuum at 150 °C for 10 h).
X-ray powder diffraction (XRD) analysis were performed in a Bruker
D8 Discover to obtain information about the crystallographic structure
of the zeolite before and after the ion exchange procedure.
Transmission electron microscopy (TEM) measurements were carried
out in both bright and dark field contrasts using a MET JEOL-JEM
2100 F microscope to study the dispersion of copper in the zeolite
structure.
In the present work, the reactivity of Cu-MOR catalyst for the direct
oxidation of methane to methanol has been explored. The catalyst has
been prepared via aqueous ion-exchange of a commercial Na-MOR. The
reaction studies have been performed in a stainless steel fixed-bed re-
actor loaded with 3 g of catalyst with a particle size in the range
0.355−1 mm that forms a catalytic bed of 110 mm. This is a step fur-
ther in reactor size compared to previous works from the literature
about this reaction. Thus, in these works, the reactor is considerably
smaller with much lower amounts of catalyst (up to 0.7 g [16,41]),
smaller particles sizes (up to 0.500 mm [9,42]) and bed lengths (up to
1.4 cm [14]). The present work is aimed at demonstrating that me-
thanol can be obtained from methane in quantitative amounts ac-
cording to a step-wise cycling process. For this reason, the use of a
larger reactor is of great importance, as a first step towards the scale-up
of the process. The operating conditions of the different steps have also
been optimized in order to maximize the yield of methanol.
2.3. Experimental device
The reaction was studied in a tubular stainless steel fixed-bed re-
actor (internal diameter 6.8 mm). The reactor tube was loaded with 3 g
of catalyst, maintained in a fixed position using a porous plug. The
catalytic bed had a length of 110 mm. The reactor configuration fulfills
the requirement to ensure plug flow pattern through the catalytic bed
[44]: ratio of reactor inner diameter to catalyst particle size at least 10
(in this case, it is exactly 10) and ratio of bed length to catalyst particle
size higher than 50 (in this case, it is 162). The existence of plug flow
inside the reactor is essential to prevent a bad distribution of the re-
actants and channeling.
The tube was filled with glass spheres (1 mm) upstream the catalytic
bed, in order to pre-heat the feed. The tube was surrounded by an
electrical oven, the temperature is controlled using a thermocouple
placed inside the reactor tube very close to the catalytic bed.
The flowsheet of the experimental device is depicted in Fig. 1. The
gases (methane, nitrogen, oxygen and air) are supplied from cylinders
(Air Liquide) and their flowrate is set using mass flow controllers
(Bronkhost). The reactor feed is prepared by mixing the corresponding
gases in adequate proportions. No appreciable pressure drop was ob-
served in the reactor operating at atmospheric pressure.
A syringe pump is used to feed the liquid water and mix it with the
hot nitrogen stream, causing its vaporization. To prevent the occur-
rence of water concentration pulses during the vaporization, a mixing
tank of 1 l was placed downstream the water injection point. This tank
and all the pipes from the injection point to the reactor inlet are covered
with heating tape, maintained at 150 °C to prevent condensation.
The composition of the reactor effluent can be analyzed on-line or
off-line. The on-line analysis is carried out continuously, using a mass
spectrometer (MS) OmniStar GSD 301. However, the measurements of
this kind of equipment can be easily interfered by the presence of water
(in great amount during methanol desorption).
2. Materials and methods
2.1. Catalyst preparation
Mordenite zeolite (Na-MOR, Si/Al = 6.5, CBV10A), supplied from
Zeolyst International, was used to prepare the catalyst by ion exchange
with a copper solution. The zeolite (10 g) was mixed with 0.01 M
copper (II) acetate solution (780 mL). The mixture was stirred for 24 h
at room temperature and filtered [24,43]. The precipitation of Cu(OH)2
was avoided by maintaining the pH at 5.7 [23,42]. The exchange
procedure was repeated three times to increase copper loading. After
the last exchange, the filtrate was rinsed with distilled water and dried
at 110 °C overnight. The dried catalyst was pelletized and sieved to the
desired particle size (0.355−1 mm). Finally, the catalyst is activated in
an oxygen gas flow using a temperature at 450 °C (ramp of 1 °C/min,
hold 4 h).
2

M. Álvarez, et al.
MolecularCatalysis487(2020)110886
Fig. 2. Temperature conditions and stream composition for
450ºC
three-step cycling process studied.
200ºC
200ºC
150ºC
N2 CH4 N2
N2 + H2O
N2
O2 / Air
CH4
N2
adsorpƟon
desorpƟon
acƟvaƟon
st
nd
The off-line analysis is based on the use of a cold trap that condenses
methanol and water of the reactor effluent during the desorption step.
The cold trap consists of an U-tube made of borosilicate glass and
placed inside an isopropanol/liquid nitrogen bath (temperature
−50 °C). The condensate is accumulated in the U-tube during the
desorption step. Then, the liquid sample is collected and the species
analyzed in a gas chromatograph (GC) Shimadzu GC-2010 equipped
with a CP-Sil 8CB column and a flame ionization detector (FID). Ethyl
acetate is used as internal standard. The estimated relative standard
deviation for this analytical method is 7.4 %. The non-condensable
gases leave the cold trap and are analyzed in the mass spectrometer. To
prevent condensation of water or any product, the reactor the outlet
pipes were also covered with heating tape maintained at 150 °C.
is introduced in its structure. This can be explained by the blockage of
some pores with copper oxide clusters of large size [23,42]. The
properties of the catalyst after 19 reaction cycles are similar to those of
the fresh catalyst, which means that its structure does not change
during the reaction process.
XRD measurements before and after the ion-exchange procedure are
displayed in Fig. 3. After the addition of copper, no new peaks could be
detected. This suggests that the addition of copper is in the form of
crystalline particles of very small size (less than 3 nm) or non-crystal-
line (i.e. amorphous) phase [16]. Nonetheless, a decrease in peak in-
tensity is observed after the addition of copper, which is attributed to a
decrease in crystallinity. Scherrer equation (τ = k·λ/β·cosθ) was used to
estimate this decrease: 20 % lower compared to the fresh zeolite sup-
port.
Considering that the solid only have two phases, cooper oxide the
most dense and dark, and the zeolite less dense, it is possible to qua-
litatively estimate dispersions by TEM. The larger cooper oxides crys-
tallites (Fig. 4A) are on the surface of the catalyst, while the smaller
ones seem to be inside the zeolite pores (Fig. 4B). The larger clusters are
attributed to amorphous copper oxide, since no new peaks were found
in the XRD spectra. The smaller clusters have a size in the range
1.37–2.81 nm. The activation of methane is attributed to these small
clusters, as is indicated in the bibliography [7].
2.4. Reaction tests
The reaction of direct oxidation of methane to methanol was carried
out according to a three-step cyclic process: adsorption, desorption and
regeneration [24]. Fig. 2 shows a sketch of the step and their operating
conditions, as discussed below. In the adsorption step, a methane
stream of 120 ml n.t.p./min (WHSV = 2.29 Nm3 h−1 kgcat−1) is fed at
200 °C and 1 atm for 20 min. Then, methanol desorption is promoted by
feeding a flow of water in nitrogen gas at 150 °C and 1 atm for 4 h. The
flow rates of water and nitrogen were varied in the range 0.4–0.7 g/h
and 150–220 mL n.t.p./min, respectively. Finally, the catalyst is re-
generated at oxidizing conditions using a gas flow rate of 120 ml n.t.p./
min (WHSV = 2.29 Nm3 h−1 kgcat−1) of oxygen or air at 450 °C and
1 atm (ramp to 450 °C and hold for 4 h). This temperature was selected,
because it ensures the fully dehydration of the catalyst [35] and also
maximizes the production of methanol [43]. After every step, the
system was purged with nitrogen (gas flow 120 ml n.t.p./min) to
eliminate remaining gases in the piping and bed voids.
3.2. Preliminary reaction studies
The first reaction experiments were aimed to demonstrate that the
Cu-MOR catalyst is able to catalyze the partial oxidation of methane to
methanol. The general experimental reaction procedure was detailed in
section 2.4. As explained, the reaction is accomplished in three steps:
adsorption, desorption and regeneration.
In the preliminary tests, the desorption step was carried out using a
N2 flow rate of 220 ml n.t.p./min (WHSV = 4.42 Nm3 h−1 kgc−at1) and a
water concentration of 4.5 mol %. During the 4 h of the desorption step,
a total amount of 2 g of water were introduced in the reactor, from
which 74 % were recovered as sample in the cold trap. This sample was
analyzed by GC and methanol concentration was 30 mmol/L.
Considering the sample mass (1.48 g), the amount of catalyst in the
reactor (3.12 g) and its copper loading (4.5 %), the yield of methanol
was determined: 330 μmol/g Cu. The regeneration was done with an
oxygen gas flow and using a temperature ramp of 1 °C/min up to 450 °C
(hold 4 h).
Additional tests were required to quantify the amount of methane
adsorbed on the catalyst in the adsorption step. Thus, after the ad-
sorption, the reactor was heated in an air stream of 120 ml n.t.p./min
(WHSV = 2.29 Nm3 h−1 kgcat−1) at a rate of 10 °C/min up to 450 °C.
This caused the total oxidation of the adsorbed methane to CO2, which
is analyzed on-line by MS (signal of m/z = 44) [16,24]. The CO2 signal
of the MS can be used to quantify the amount of methane previously
adsorbed on the catalyst using a calibration. The decomposition of a
sodium bicarbonate sample of known weight was used to calibrate the
MS [9]).
The reaction cycle was repeated three times for the same catalyst
batch and the operating conditions indicated above. The average yield
of methanol was a value of 327 μmol/g Cu with a relative standard
deviation of 2.5 %. These results suggest that the regeneration step is
able to restore the catalytic activity and, hence, prove that the catalyst
can be reused in the cycling process.
The performance of another batch of catalyst, prepared according to
the same methodology, was also studied. In this case, the yield of me-
thanol at the abovementioned conditions was 307 μmol/g Cu, which is
only 6 % lower than that of the first batch. Consequently, it can be
confirmed the good reproducibility of the different stages involved in
the experimental procedure (i.e. catalyst preparation, reaction testing
and sample analysis).
3. Results and discussion
3.1. Catalyst characterization results
The preparation of the catalyst was done by ion exchange on com-
mercial zeolites. The procedure was repeated three times to increase the
copper loading up to 4.5 wt% (ICP-MS) in the fresh catalyst. This value
is very similar to those reported in the bibliography for similar catalysts
(e.g. 4.3 wt% [24]). The copper content of the catalyst after being used
in 19 reaction cycles was analyzed again and a loading of 4.5 wt% was
obtained. This indicates that copper is not lost during the reaction.
According to the results presented in Table 1, the BET surface area
and micropore volume of the zeolite slightly decrease when the copper
3

M. Álvarez, et al.
MolecularCatalysis487(2020)110886
Table 1
Composition and morphological analysis (BET) of fresh and used Cu-exchanged mordenite.
Cu loading (wt. %)
Cu /Al (mol/mol)
BET surface area (m2/g)
Micropore volume (cm3/g)
Na – MOR
Cu – Na – MOR
Cu – Na – MOR (after 19 cycles)
0
4.5
4.5
0
0.54
0.54
376
359
350
0.17
0.15
0.14
Cu - MOR
Na - MOR
5
25
45
Fig. 5. Effect of regeneration gas composition and temperature ramp on me-
thanol yield. Symbols: activation with pure oxygen ( ) and activation with
synthetic air ( ).
Fig. 3. XRD spectra of Na-MOR (brown) and Cu-Na-MOR (blue). Crystallinity of
the CuNa-MOR is 20 % lower than the fresh zeolite and no new peaks related to
crystalline copper oxide clusters are observed.
Fig. 5 shows the impact of temperature ramps in the range 1–5 °C/
min on the yield of methanol obtained in the following reaction cycle.
Thereby, if the regeneration were not adequate in one cycle, methanol
yield would be reduced in the following one. An increase of the ramp to
2 °C/min has no influence on methanol yield, 311 μmol/g Cu, only 5%
lower than that obtained at 1 °C/min (i.e. close to the reproducibility
confidence interval, as discussed in the previous section). However, the
increase to a ramp of 5 °C/min has a marked negative consequence on
methanol yield, reducing it to 171 μmol/g Cu. This can be explained by
the reduction of the total regeneration time, which is reduced to 5.5 h,
half of the time than with a ramp of 1 °C/min, causing an incomplete
regeneration of the catalytic activity.
Some works [41] have reported that methanol yield decreases when
oxygen pressure used in the regeneration step is increased above 1 bar.
For this reason, a study of the impact of oxygen partial pressure below
1 bar has been proposed in the present work. In particular, the re-
generation step has been done using synthetic air (20 % oxygen in
3.3. Optimization of operating conditions
Once the catalyst has demonstrated its activity towards methanol in
the partial oxidation reaction of methane, the studies have been focused
on optimizing the operating conditions. The aim of this section is to
determine the influence of the experimental conditions of the different
steps of the process, in order to maximize the yield of methanol.
3.3.1. Optimization of the regeneration step
The regeneration step of the preliminary reaction tests was carried
out using an oxygen gas flow and a temperature ramp of 1 °C/min. Such
a low ramp increased considerably the time required for the re-
generation. In order to reduce the regeneration time, faster temperature
ramps have been considered. The final regeneration temperature and
the hold time remained identical (450 °C and 4 h, respectively).
Fig. 4. Transmission Electron Microscopy (TEM) images obtained for copper exchanged mordenite.
4

M. Álvarez, et al.
MolecularCatalysis487(2020)110886
Table 2
Summary of the optimal conditions for a reaction cycle. (Total gas flow and
WHSV at desorption step consider the totality of the gas flow, composed by the
N2 flow and the water introduced).
700
600
500
400
Adsorption
Desorption
Regeneration
Gas (mol%)
100 CH4
120
2.29
200
5.2 H2O + 96.8 N2
20 O2 + 80 N2
120
2.29
450 (1 °C/min)
240
Total Flow (mL n.t.p./min)
159
3.04
150
240
WHSV (Nm3 h−1 kgcat
Temperature (ºC)
Duration (min)
)
−1
20
2
4
6
8
Fig. 6. Effect of desorption gas composition and flow rate on methanol yield.
Symbols: 150 ml N2 n.t.p./min ( ), 190 ml N2 n.t.p./min ( ) and 220 ml N2
n.t.p./min ( ).
nitrogen) instead of pure oxygen. The results, shown in Fig. 5, indicate
that oxygen partial pressure has a marked influence on methanol yield,
an increase of 58 % with 504 μmol/g Cu (with a ramp of 1 °C/min)
being observed when using air as regenerant. A similar increase is also
obtained for the ramp of 2 °C/min, whereas at higher heating rates the
methanol yield markedly decreases.
150
250
350
450
This is an important finding, since air is a cheaper oxidant.
Consequently, the following operating conditions are found to be op-
timal for the regeneration step: air gas flow and a temperature ramp of
2 °C/min up to 450 °C. In the next studies of the present work, these
conditions will be used.
Fig. 7. Mass spectrometer CO2 signal (m/z = 44) produced when heating the
reactor (10 °C/min) in air, after the adsorption step. Black and red curves re-
present consecutive experiments.
In this section, the performance of the individual reaction steps has
been studied at the optimum conditions. First, a test has been proposed
to quantify the amount of methane adsorbed on the catalyst during the
adsorption step. This test consists of a convectional adsorption step with
methane at 200 °C, followed by a temperature programmed desorption
(rate of 10 °C/min) in an air gas flow (no water is added). Since me-
thane is adsorbed strongly, an increase of temperature is required,
which causes its oxidation to carbon dioxide. This gas is analyzed on-
line using the MS (signal m/z = 44), as shown in the curves represented
on Fig. 7. The presence of two CO2 peaks, at 210 and 270 °C suggests
that there are two different kind of active centers on the catalyst. The
CO2 peak obtained at low temperature, which is the larger one, corre-
sponds to mildly adsorbed methane; note that the adsorption step was
carried out at 200 °C and this peak is produced at 210 °C. Using the
calibration of the CO2 signal of the MS, an estimation of the amount of
adsorbed methane can be obtained: 1482 μmol/g Cu, from which
911 μmol/g Cu corresponds to the first peak and 571 μmol/g Cu to the
second. The yield of methanol obtained in the desorption step was
754 μmol/g Cu, which means that 52 % of the adsorbed methane is able
to react to methanol.
The test (adsorption and temperature-programed desorption) was
repeated twice to check the reproducibility. As depicted in Fig. 7, both
MS signals overlap completely. The reactor effluent was also analyzed
using the MS during the regeneration step. As shown in Fig. 8, a small
CO2 peak (signal m/z = 44) was produced at 200 °C. This means that a
small part of the adsorbed methane cannot be upgraded to methanol
during the desorption step. On increasing temperature as part of the
regeneration step, this methane is oxidized to CO2 and desorbed. The
amount of CO2 generated in the regeneration step is 106 μmol CO2/g
Cu, which is only 7 % of the amount of adsorbed methane. Differently
to the temperature programmed test previously discussed, where there
were two CO2 peaks, only the first peak is produced during the
3.3.2. Optimization of the desorption step
The desorption step is the part of the process where methanol is
recovered from the catalyst. This is accomplished by gaseous water in a
nitrogen flow. At low temperature (150 °C), water causes the desired
methanol desorption. This section is focused on the study of the influ-
ence of water concentration and gas flow during the desorption step.
The preliminary reactions were carried out using a N2 flow of
220 ml n.t.p./min and a water concentration of 4.5 mol%. Fig. 6 sum-
marizes the experimental results, corresponding to water concentration
in the range 3.2–7.7 mol % and N2 flow rate 150−220 mL n.t.p./min.
At 220 ml n.t.p./min, methanol yield increased to 609 μmol/g Cu, when
water concentration increased from 4.5 to 6.2 mol %. This behavior can
be related to the shift of the methanol adsorption equilibrium caused by
the increase on the water concentration.
However, the best improvement in methanol yield was observed
when the N2 flow rate was reduced to 190 or even 150 ml n.t.p./min,
both with similar results in the range 700−750 μmol/g Cu. At 150 and
190 ml n.t.p./min, the influence of water concentration is slightly dif-
ferent to that observed at 220 ml n.t.p./min. Thus, on increasing water
concentration, methanol yield increases, has a maximum at 5.2 mol%
and, then, decreases slowly. At 220 ml n.t.p./min, the maximum was
not observed, because of falling outside of the experimental region.
Considering these results, the optimal conditions for the desorption
step are: N2 flow rate of 150 ml n.t.p./min (WHSV = 3.04 Nm3 h−1
kgcat−1) and water concentration 5.2 mol %; at these conditions,
754 μmol/g Cu were produced.
3.4. Quantification of the reactor performance at the optimal operating
conditions
The optimal conditions for the operation of the reactor, determined
in the previous section, are summarized in Table 2.
5

M. Álvarez, et al.
MolecularCatalysis487(2020)110886
demonstrated that the regeneration step restored the catalytic activity
of the Cu-MOR catalyst, after three consecutive reaction cycles. How-
ever, it would be interesting to analyze the medium to long term sta-
bility of the catalytic.
H2O
The yield of methanol at the optimum operating conditions was
754 μmol/g Cu. After 18 reactions, the yield decreased to 725 μmol/g
Cu. Thus, the yield decreased 3.9 %, which is within the estimated
reproducibility of the reaction (relative standard deviation of 2.5 %, as
discussed section 3.2). In other words, the loss of catalytic activity can
be considered negligible.
CO2
4. Conclusions
The performance of a Cu-MOR catalyst used for the direct conver-
sion of methane to methanol has been investigated in a fixed-bed re-
actor. The reaction is accomplished according to a cyclic process made
of three steps: adsorption, desorption and regeneration. It has been
demonstrated that methanol can be synthetized by this method at mild
conditions (200 °C and 1 atm) and the catalytic activity is preserved
after several reaction cycles.
50
150
250
350
450
Fig. 8. Mass spectrometer signals obtained during the regeneration of the cat-
alyst for CO2 (m/z = 44, black curve) and H2O (m/z = 18, blue curve).
The operating conditions of the different steps have been optimized
in order to maximize the yield of methanol. The regeneration step is
carried out at high temperature, 450 °C, and for 4 h. On one hand, it has
been evidenced that the use of temperature ramps higher than 5 °C/min
to reach the final regeneration temperature have a negative impact on
the yield of methanol. On the other hand, the use of air during the
regeneration, instead of oxygen, produces an increase of the yield of
methanol obtained in the following cycle by 58 %. The desorption step
is highly affected by the total gas flow rate and water concentration. It
has been concluded that the optimal conditions are 3.04 Nm3 h−1
−1
kgcat
and water molar fraction 5.2 mol%.
The optimization of the reaction conditions resulted in an increase
of methanol yield from 320 μmol/g Cu to 754 μmol/g Cu. A deeper
analysis of the catalyst performance at the optimum reaction conditions
has revealed that 52 % of the adsorbed methane is actually transformed
to methanol; the rest is desorbed or oxidized to CO2 during the deso-
rption and regeneration steps.
CRediT authorship contribution statement
Fig. 9. Percentage of the CH4 adsorbed on the catalyst (1482 μmol/g Cu) that is
transformed into methanol (Blue: 754 μmol/g Cu), fully oxidized during deso-
rption (Yellow: 608 μmol/g Cu) and eliminated during the activation of the
catalyst (Green: 106 μmol/g Cu).
Mauro Álvarez: Investigation, Writing - original draft. Pablo
Marín: Methodology, Supervision, Validation. Salvador Ordóñez:
Conceptualization, Supervision, Writing - review & editing.
Declaration of Competing Interest
regeneration step. Thus, part of the mildly adsorbed methane (attrib-
uted to the 210 °C CO2 peak of the temperature programmed test) can
be in active centers which are not associated to methanol formation
and, for this reason, this methane remained the same after the (water)
desorption step at 150 °C. Fig. 8 also depicts the MS signal of m/z = 18
attributed to water. During the regeneration of the catalyst, all the
water should be desorbed and the copper active phase re-oxidized. The
maximum of the water peak is produced at 135 °C, but water is still
present in the reactor effluent up to around 300 °C. The maximum re-
generation temperature (450 °C) guarantees the fully dehydration of the
catalyst.
Considering the results previously reported a yield to methanol of
52 % was obtained at these optimal conditions, while 7 % of the me-
thane adsorbed was oxidized to CO2 during the activation step (Fig. 9).
The rest of the methane adsorbed is considered to be bonded weaker to
the active centers, being fully oxidized to CO2 during the desorption
step.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
This work was supported by the METHENERGY+ Project of the
Research Fund for Coal and Steel (EU) [grant number 754077]. Also,
authors would like to acknowledge the technical support provided by
Servicios Científico-Técnicos de la Universidad de Oviedo.
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