Selective methanol formationviaCO-assisted direct partial oxidation of methane over copper-containing CHA-type zeolites prepared by one-pot synthesis

Article abstract of DOI:10.1039/d0gc03645e

Direct conversion of methane into methanol at low temperature is a promising process for the efficient utilization of abundant methane as energy and chemical resources, and metal-loaded zeolites have been recently focused as a new candidate catalyst for this reaction. Here, catalytic CO-assisted direct conversion of methane into methanol was demonstrated over Cu-CHA zeolites. Methanol was selectively produced as a solo oxygenate, without the production of C2 oxygenates caused by CO insertion. For the preparation of the Cu-CHA zeolite catalyst, besides the conventional ion-exchange method, a one-pot synthesis approach using a Cu complex as a structure-directing agent was also applied. As both catalysts showed similar activation energies of 29-34 kJ mol^?1, the latter one was found to be more feasible resulting in a 1.5 times higher methanol production rate per gram of catalyst. The methanol production rate per Cu atom decreased with the increase of the Cu loading amount, indicating the necessity of low copper loadings for well-dispersed isolated single copper active sites, which would explain the improved result of the one-pot synthesis approach.

Full text of DOI:10.1039/d0gc03645e

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Selective methanol formation via CO-assisted
direct partial oxidation of methane over copper-
containing CHA-type zeolites prepared by one-pot
synthesis†
Cite this: Green Chem., 2021, 23,
2148
Sibel Sogukkanli, Takahiko Moteki * and Masaru Ogura
Direct conversion of methane into methanol at low temperature is a promising process for the efficient utiliz-
ation of abundant methane as energy and chemical resources, and metal-loaded zeolites have been recently
focused as a new candidate catalyst for this reaction. Here, catalytic CO-assisted direct conversion of methane
into methanol was demonstrated over Cu-CHA zeolites. Methanol was selectively produced as a solo oxyge-
nate, without the production of C2 oxygenates caused by CO insertion. For the preparation of the Cu-CHA
zeolite catalyst, besides the conventional ion-exchange method, a one-pot synthesis approach using a Cu
complex as a structure-directing agent was also applied. As both catalysts showed similar activation energies of
29–34 kJ mol−1, the latter one was found to be more feasible resulting in a 1.5 times higher methanol pro-
duction rate per gram of catalyst. The methanol production rate per Cu atom decreased with the increase of
the Cu loading amount, indicating the necessity of low copper loadings for well-dispersed isolated single
copper active sites, which would explain the improved result of the one-pot synthesis approach.
Received 27th October 2020,
Accepted 19th February 2021
DOI: 10.1039/d0gc03645e
rsc.li/greenchem
from these enzymes, numerous research studies have been
devoted towards developing synthetic catalysts and related pro-
Introduction
Methane, the main component of natural (and shale) gas, is cesses for the partial oxidation of methane using O2 as the
one of the most abundant and inexpensive forms of fossil ideal oxidant (CH4 + 1/2O2 → CH3OH) at low temperatures.
fuels and thus holds great potential to serve as a near-future Accordingly, metal-containing zeolites have emerged as prom-
source for energy and chemicals.1–3 The current industrial ising heterogeneous catalysts because of their capability to
routes from methane to valuable products require the for- host comparable metal active sites to those found in biological
mation of syngas (CO and H2) at high temperatures (>1000 K) systems. Many zeolites with different metal ions (Cu, Fe, and
and pressures (>4 MPa), which make them too costly and others) have been investigated for the direct conversion of
effective only in large-scale industries.4 The direct, low-temp- methane.10,14–19 Particular interest in this regard has been
erature, and on-site conversion of methane into liquid oxyge- focused on copper-containing zeolites that activate methane
nates such as methanol is hence a highly desirable alternative even with small concentrations of O2. Until recently, it was
green route for the efficient transportation and utilization of believed that the partial oxidation of methane over copper-con-
methane.5–7 Such a conversion is, however, extremely difficult taining zeolites could be realized only through a non-catalytic
to achieve, as it requires the cleavage of a strong C–H bond in stepwise process (i.e., chemical looping),20–24 which consists of
methane and simultaneously needs to prevent methanol from three distinct stages operated at a wide range of temperature
further oxidation to COx.8–10
and requires several hours for one turnover. However, in 2016,
In nature, methane monooxygenase enzymes (MMOs) that Narsimhan et al. broke this ground by establishing a catalytic
include iron or copper centers in active sites (soluble MMO11 process for continuous methanol production in the gas phase.
or particulate MMO,12,13 respectively) have shown their own The study used various copper-containing zeolites (MFI, BEA,
ability to oxidize methane directly and selectively to methanol MOR, FER, FAU, and CHA-types) to convert methane into
with gaseous O2 under mild conditions. Taking inspiration methanol by feeding a gas mixture of CH4, H2O, and O2 at low
temperatures (483–543 K).25 Therein, copper-containing CHA-
type zeolites (Cu-SSZ-13 and Cu-SAPO-34) with their small pore
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba,
and cage-based structure revealed better catalytic performance
Meguro-kuTokyo, 153-8505, Japan. E-mail: moteki@iis.u-tokyo.ac.jp
than those having medium and large pores. Despite this, the
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
low methanol production rates (∼4.0 × 10−2 mol molCu h−1
−1
d0gc03645e
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or ∼6.0 μmol gcat h−1 26
)
achieved so far impede the indus- tion (NaOH, 50 wt% in water, Wako) as an alkali source, N,N,N-
−1
trial implementation of this continuous catalytic process.
trimethyl-1-adamantanamine hydroxide (25 wt% TMAdaOH in
On the other hand, Shan et al. and Tang et al. have recently water, Sachem) as a crystallization organic structure-directing
developed another catalytic process for the partial oxidation of agent (OSDA), and deionized water. Moreover, Cu-TEPA solution
methane by using gaseous O2 with the assistance of CO at was prepared as a cooperative OSDA to be used with TMAdaOH
423 K in an aqueous medium.27,28 Whilst this process initially in the one-pot synthesis of CHA-type zeolites from the chemi-
proposed to employ rhodium-containing MFI-type zeolite (Rh- cals tetraethylenepentamine (TEPA, TCI) and copper sulfate
ZSM-5) as a catalyst, our group has broadened its application pentahydrate (99.9 wt% CuSO4·5H2O, Wako).
to other platinum group (Ru, Pd, Ir, and Pt) and transition
One-pot synthesis of aluminosilicate Cu-CHA materials with
different copper loadings
metal (Fe, Co, Ni, and Cu) containing MFI-type zeolites.29 It
was noted that there is a peculiar requirement for CO to be
present for the conversion of methane. And due to the pres- One-pot synthesis of aluminosilicate Cu-CHA materials
ence of CO, other oxygenates such as formic acid and acetic was performed by a procedure similar to that described else-
acid besides methanol formed in appreciable amounts. In the where33
with
reaction
gel
comprising
study,29 copper showed the highest methanol selectivity even 1.0SiO2 : 0.033Al2O3 : xNaOH : 0.2TMAdaOH : yCu-TEPA : 20H2O
though its reactivity was less than those of platinum group (where x = 0.1 and y = 0.01–0.1). Initially, Cu-TEPA as a copper
metals. Additionally, the catalyst showed a methanol pro- complex of CuSO4·5H2O and TEPA was prepared in a molar ratio
duction rate of 0.3 mol molCu h−1 (or 2.3 × 10 μmol gcat
of 1 : 1, respectively. For doing that, CuSO4·5H2O was first dis-
−1), which outperformed the previously reported chemical solved in deionized water followed by dropwise addition of
−1
−1
h
looping processes over copper-containing zeolites. Although TEPA, and the final solution was stirred for 1 h. Separately,
the CO-assisted reaction system over copper-containing zeo- aluminum hydroxide was dissolved in an aqueous mixture of
lites offers significant advantages in the conversion of TMAdaOH and NaOH. The prepared Cu-TEPA was then added
methane, it requires to be further investigated to enhance into this solution, and the mixture was further stirred for
methanol formation as well as to obtain a better understand- 10 min. After that, colloidal silica was slowly added, and the stir-
ing of its mechanism and active sites.
ring was continued for 3 h to form an aluminosilicate reactant.
As alluded to earlier, copper-containing CHA-type zeolites The resulting mixture was then transferred into a stainless-steel
with their small pore cage-based structure (composed of 8 × 8 × autoclave (no. 4749, Parr Instruments) and subjected to hydro-
12 Å cages interconnected by windows with a maximum pore thermal treatment at 423 K for 14 days. The solid products were
diameter of 3.8 Å)30 are superior catalysts for methanol pro- recovered by filtration, washed with deionized water, and dried
duction from methane compared to medium and large pore overnight at 373 K. These solids were finally calcined at 823 K
ones in either a stoichiometric stepwise or continuous catalytic with a temperature ramp of 276 K min−1 for 10 h to remove the
manner. This is due to confinement effects in the small pore of occluded organic structure-directing agents in zeolites.
the CHA structure that provides high stabilization of copper
Preparation of aluminosilicate Cu-CHA materials by liquid
ion-exchange
active species, which may expedite the conversion of methane
to methanol.31,32 In view of this fact, we herein explore the CO-
assisted direct conversion of methane into methanol over The conventional aluminosilicate CHA-type zeolite (SSZ-13) was
copper-containing aluminosilicate SSZ-13 as CHA-type zeolite synthesized from reaction gel with the following molar compo-
catalysts (Cu-CHA). Different from our previous study on the sition: 1.0SiO2 : 0.04Al2O3 : 0.012NaOH : 0.2TMAdaOH : 20H2O.
MFI structure,29 the present results demonstrate that the CHA First, aluminum hydroxide, TMAdaOH and NaOH were dissolved
structure becomes successful in suppressing the formation of in deionized water under stirring for 10 min. Then, colloidal
other oxygenates and in producing methanol simultaneously as silica was slowly added, and the solution was homogenized under
an only oxygenate with a trace of an unidentified product. continuous stirring for 30 min. Later, the reaction mixture was
Furthermore, by controlling the copper species and contents via placed in an autoclave, and heated at 433 K in an oven under
different synthetic techniques along with optimizing the reac- rotation at 20 rpm for 5 days. After that, the solid products were
tion temperature, the catalyst could achieve the highest metha- recovered, purified, dried, and calcined by the same procedures
nol production rates reported so far for any copper-containing as described above. The calcined CHA-type zeolite was then ion-
zeolites used in a catalytic process with O2 as the oxidant.
exchanged with 1 M NaNO3 solution for 24 h at room temperature
(Na-CHA-IE). The Cu-CHA material was finally prepared by twice
liquid ion-exchange of Na-CHA-IE with 0.01 M (CH3COO)2Cu
solution for 24 h at room temperature. The solid Cu-CHA material
was recovered by the same procedures as described above.
Experimental section
Materials
Characterization
The following raw materials were used as received: colloidal
silica (Ludox® AS-40, 40 wt% suspension in water, Sigma- Powder X-ray diffraction (XRD) patterns of solid products were
Aldrich) as a silica source, aluminum hydroxide (Al(OH)3, recorded using a Rigaku Rint 2000 diffractometer with CuKα
Sigma-Aldrich) as an aluminum source, sodium hydroxide solu- radiation (40 kV, 40 mA). The crystal sizes and morphologies
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of the resultant products were confirmed using a scanning room temperature. Gaseous samples (CO, CO2, and CH4) were
electron microscope (SEM, Hitachi S-4500, 5 kV). Elemental quantified using an on-line Shimadzu GC-8A gas chromato-
analysis of the products was carried out with a Thermo graphy (GC-FID) system equipped with an active carbon (2m, 4φ
Scientific iCAP-6300 inductively coupled plasma-atomic emis- × 3φ, 60/80) column and a methanizer (nickel on silica-alumina,
sion spectrometer (ICP-AES) system after dissolving the pro- Alfa-Aesar). The calculations for gaseous products were per-
ducts in hydrofluoric acid and hydrochloric acid solutions, formed based on calibration lines which were presented in a pre-
and a Jeol JSX-3100RII X-ray fluorescence (XRF) system with a vious work of our group.29 Thereafter, the aqueous suspension
Rh anode X-ray tube (50 kV). Nitrogen adsorption–desorption was filtered, and the liquid samples that evolved during the cata-
measurements were performed on
a
Quantachrome lytic reaction were analyzed using an off-line Shimadzu GC-14B
Quadrasorb-evo instrument at liquid nitrogen temperature gas chromatography (GC-FID) system with a Zebron phase ZB-1
1
with autogas pretreatment at 573 K for 3 h under vacuum. (no. 198347) capillary column and H-NMR with DANTE presa-
Diffuse reflectance (DR) UV–Vis spectra were recorded using a turation pulse. Methanol (CH3OH) was the only liquid organic
JASCO V-650 spectrometer in the 190–800 nm wavelength product that has been determined from the analyzing methods
range at a scan rate of 100 nm min−1 using BaSO4 as a refer- applied in this work, and its quantification was made based on
ence. Fourier transform infrared (FT-IR) spectroscopy was con- the calibration curve which was constructed from the solutions
ducted on a Jasco FT-IR 4100 spectrometer equipped with a with known CH3OH concentrations and their relative peak areas
mercury cadmium telluride (MCT) detector (JASCO Corp., in GC-FID (see Fig. S3†). Moreover, the catalytic reactions were
Japan). The powder sample was made into a self-standing performed with a CO partial pressure of 0.25–1.00 MPa, and
wafer and pretreated at 423 K, and transmission spectra were without the assistance of CO by a procedure similar to that
collected at room temperature under 1 atm of CO.
described above at 473 K for 3 h. Also, the reference reactions
were carried out by a procedure similar to that described above
without using O2 and CH4.
CO-assisted direct partial oxidation of methane reactions over
Cu-CHA materials
Methane oxidation reactions were carried out in a high-
pressure batch reactor. In a typical catalytic reaction, approxi-
mately 50 mg of zeolite catalyst and 5 mL of deionized water
along with a stirrer were loaded into a 50 mL stainless-steel
autoclave (Taiatsu Techno, TVS-N2-type). Initially, the reactors
were purged with pure nitrogen flow for 10 min and evacuated
with the help of a pump until the inner pressure reached
around 0.05 MPa. Pure oxygen (O2, Taiyo Nippon Sanso),
carbon monoxide (CO, Taiyo Nippon Sanso), and methane
(CH4, Taiyo Nippon Sanso) were then fed into the reactors with
gas pressures of 0.20, 0.50 and 2.00 MPa, respectively.
Following this, the reactors were immersed in preheated oil
baths at temperatures of 423, 448, and 473 K for a certain
period of time. The temperature profiles of the oil baths
during heating-up to the described temperatures are presented
in Fig. S2.† Additionally, all reactions were performed under
stirring at 500 rpm that is fast enough to confirm both homo-
geneous heat distribution and homogeneous mixing of liquid
and gas phases. After reaction for the given times, the reactors
Results and discussion
A series of Cu-CHA zeolites with different copper loadings
were prepared through either a one-pot synthesis method
using copper tetraethylenepentamine (Cu-TEPA) and 1-ada-
mantanamine hydroxide (TMAdaOH) as cooperative structure-
directing agents (SDAs) in alkali media,34 or liquid ion-
exchange of conventional aluminosilicate CHA-type zeolite.
The obtained zeolite samples were designated depending on
the preparation method used: one-pot synthesized as Cu-
CHA-T(x) and ion-exchanged as Cu-CHA-IE(x), where “x”
denotes the Cu/cage ratio as shown in Table 1. The Cu/cage
ratios were calculated based on the equation given below.
ðCu=AlÞ ꢀ 36=ððSi=AlÞ þ 1Þ
Cu=cage ¼
3
The UV-Vis spectra of the as-synthesized samples disclosed
were placed in a cold-water bath for 15 min to cool down to that the Cu-TEPA complex was retained during crystallization
Table 1 Physicochemical properties of CHA-type zeolites prepared by the one-pot synthesis method with different copper loadings or liquid ion-
exchange
Samplea
Si/Alb
Na/Alb
Cu/Alb
Al/cage
Cu (wt%)
Cu/cage
Vmicroc (cm3 g−1
)
Cu-CHA-T(0.1)
Cu-CHA-T(0.3)
Cu-CHA-T(0.5)
Cu-CHA-T(0.8)
Cu-CHA-IE(0.5)
16.8
15.8
17.8
16.8
14.0
0.2
0.3
0.2
0.2
n.a.
0.2
0.5
0.8
1.2
0.7
0.7
0.7
0.7
0.7
0.8
0.9
2.3
4.3
6.5
4.9
0.1
0.3
0.5
0.8
0.5
0.31
0.30
0.29
0.26
0.31
a Cu-CHA-T(x) denotes the samples prepared from one-pot synthesis using TMAdaOH and Cu-TEPA as cooperative structure-directing agents,
whereas Cu-CHA-IE(x) denotes the sample prepared by conventional liquid ion-exchange. The value x represents the number of Cu atoms per
cage. b Si, Al, Na, and Cu contents were determined by ICP-AES and XRF measurements. c Micropore volumes were calculated by a t-plot method.
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and was encapsulated inside Cu-CHA-T(x) samples (Fig. S6†).
All of the calcined Cu-CHA materials showed the calcined
highly crystalline and pure phase of CHA as demonstrated by
X-ray diffraction (XRD), SEM and nitrogen porosimetry (Fig. S4
and S5†). Additionally, no formation of large CuxOy nano-
particles was observed from the XRD patterns along with the
UV-vis spectra, indicating a dispersion of isolated copper
species throughout zeolite crystals.
The catalytic performance of Cu-CHA zeolites was evaluated
by CO-assisted direct conversion of methane over Cu-CHA-T
(0.5) and Cu-CHA-IE(0.5) since their identical copper loadings
allow the investigation of the effect of preparation methods on
methanol production. According to the prior literature,25–27
methane conversion reactions were initially carried out at
423 K. As seen in Fig. 1, both zeolites demonstrated the for-
mation of methanol in the presence of CO, where Cu-CHA-T
(0.5) exhibited higher catalytic performance than Cu-CHA-IE
(0.5) by yielding a methanol production rate of 2.1 × 10 μmol
Fig. 2 Methanol production rates over the Cu-CHA-T(0.5) zeolite cata-
lyst under different CO partial pressures ranging from 0.25 to 1.00 MPa
(0.20 MPa O2, 2.00 MPa CH4, 5 mL H2O, 50 mg catalyst, 423 K, 3 h).
occurred at 0.5 MPa. However, further increasing the CO
partial pressure up to 1.00 MPa resulted in a sharp decrease of
methanol formation, which might be a result of overoxidation
of CO in large amounts. Later, the reaction temperature and
time were investigated at the optimized CO partial pressure of
0.50 MPa over both Cu-CHA-T(0.5) and Cu-CHA-IE(0.5) cata-
lysts. As seen in Fig. 3(a) and (b), methanol yields of both zeo-
lites increased with the increase of the reaction temperature
from 423 to 473 K. Furthermore, when the reactions were con-
ducted at 473 K, Cu-CHA-T(0.5) and Cu-CHA-IE(0.5) zeolites
showed the highest initial methanol production rates of 5.3 ×
10 and 3.5 × 10 μmol gcat−1 h−1, respectively. To the best of our
knowledge, at the time of writing this paper, these rates rep-
resent the highest values reported so far for any copper-con-
taining zeolites from a catalytic reaction system with O2 as the
oxidant. The reason behind these high rates can be attribu-
table to the fact that elevated temperatures generally improve
the catalytic activity of reactants and catalysts,35,36 as well as
the dissolution rates of these gaseous reactants of O2, CO, and
gcat h−1 almost similar to that of Cu-ZSM-5.29 In both cases,
−1
unlike previously studied Cu-ZSM-5,29 methanol was formed
as the only liquid oxygenated product over Cu-CHA zeolites.
No other potential chemicals such as formic acid, acetic acid,
and formaldehyde, except a trace of an unidentified product,
1
were observed by either GC-FID or H-NMR analysis under our
reaction conditions, indicating unprecedented high methanol
selectivity of the CHA structure. As also shown in Fig. 1, when
the reactions were carried out without CO, only a small
amount of methanol was formed over both zeolites. These
values are comparable to each other and the values obtained
in the previous CO-free catalytic processes.25,26
To study the impact of reaction conditions on methanol
production, the CO-assisted reaction system was further inves-
tigated over Cu-CHA-T(0.5) and Cu-CHA-IE(0.5) zeolites. First,
we carried out CO partial pressure optimization studies on Cu-
CHA-T(0.5). CO pressure between 0.25 and 1.00 MPa was
applied by a procedure similar to that described above and the
results are presented in Fig. 2. As seen in Fig. 2, lower partial
pressures (0.25–0.50 MPa) resulted in increased methanol for-
mation rates, and the maximum methanol production
Fig. 3 Methanol yields (μmol gcat−1) versus the reaction time for CO-
assisted direct conversion of methane at low temperatures of 423 K
(light red), 448 K (red), and 473 K (dark red) over (a) Cu-CHA-T(0.5) and
(b) Cu-CHA-IE(0.5) zeolite catalysts (0.20 MPa O2, 0.50 MPa CO, 2.00
MPa CH4, 5 mL H2O, 50 mg catalyst).
−1
Fig. 1 Methanol production rates (μmol gcat h−1) of direct conversion
of methane over Cu-CHA-T(0.5) and Cu-CHA-IE(0.5) zeolite catalysts
with and without the assistance of CO (0.20 MPa O2, 0.50 MPa CO, 2.00
MPa CH4, 5 mL H2O, 50 mg catalyst, 423 K, 3 h).
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the water-gas shift reaction (CO + H2O → CO2 + H2 or CO +
2H2 → CH3OH) might also occur over copper-containing
zeolite catalysts at the reaction temperature between 423 K and
523 K. However, when the reference reactions were carried out
in the absence of O2 and CH4 at 473 K, no formation of CO2
and methanol could be determined by GC-FID, respectively,
denying the evolution of other reaction paths (data not
shown). Based on these results, it can be suggested that CO
oxidation is mainly responsible for CO2 formation in the CO-
assisted direct conversion system. Since CO plays a critical role
in methane activation to produce methanol with high rates, its
consumption in large amounts through such a side reaction
thwarts the efficiency of the reaction system. Therefore, it is
significant to elucidate the role of CO in the conversion of
methane into methanol, and to find an alternative substitute
that can be replaced with CO to achieve possibly much higher
production rates.
Fig. 4 Arrhenius plots for Cu-CHA-T(0.5) and Cu-CHA-IE(0.5) obtained
from methanol production rate measurements from 423 to 478 K (0.20
MPa O2, 0.50 MPa CO, 2.00 MPa CH4, 5 mL H2O, 50 mg catalyst, 3 h).
It is generally accepted that adsorption of CO can influence
CH4 into water (as shown in Fig. S7†). Based on the initial the energetics of a reaction, having a profound effect on the
methanol production rates, the activation energies (Ea) were kinetics and thermodynamics of the system, regardless of
estimated over Cu-CHA-T(0.5) and Cu-CHA-IE(0.5) zeolites. whether these surface species are necessary for that elemen-
Fig. 4 shows the Arrhenius plots within the temperature range tary step or not. In a previous work, the presence of CO as a
of 423 to 473 K, and the corresponding effective Ea values. The ligand and a reductant was proven for platinum group
calculations revealed activation energies of 34 2 and 29 2 kJ metals.29 To obtain an insight into the role of CO for copper-
mol−1 for Cu-CHA-T(0.5) and Cu-CHA-IE(0.5) zeolites, respect- containing zeolites, the zeolites were studied by FTIR spec-
ively. These similar values point out that Cu-CHA-T(0.5) and troscopy. As illustrated in Fig. 5, the coordination of CO on
Cu-CHA-IE(0.5) zeolites consisted of similar copper species as copper metal sites was confirmed over both Cu-CHA-T(0.5)
active sites. On the other hand, the amount of active sites and Cu-CHA-IE(0.5) zeolites. Two bands with maxima at 2135
within the CHA structure should be different since Cu-CHA-T and 2157 cm−1 are related to the stretching vibrations of C O
□
(0.5) zeolite showed much higher methanol production rates in carbonyl species adsorbed over two different CuI sites.
than Cu-CHA-IE(0.5) despite the fact that the same metal According to the literature, the 2135 cm−1 band corresponds to
loading and reaction conditions were applied (Fig. 1). After Cu+ in the constrained six-membered ring of the CHA struc-
24 h of reaction, methanol formation was suppressed and a ture whereas the 2157 cm−1 band corresponds to Cu+ in the
non-linear CH3OH yield was observed along with the increase larger cages of the CHA structure.37 Moreover, the 2135 cm−1
in the reaction time (Fig. 3), even though it was far below the band is generally assigned to Cu+ species near two Al sites in
theoretical equilibrium. This might be a result of catalyst de- the six-membered ring together with Na+ or proton to balance
activation and/or the decrease of reactants by unintended side the total charge, while the 2157 cm−1 band is near a single Al
reactions. Additionally, overoxidation of methanol to COx
might be another potential reason.
As it was stated previously,29 the consumption of CO to
form CO2 is the major challenge for this catalytic system over
Rh-ZSM-5. In order to evaluate this issue also for copper-con-
taining zeolites, in Fig. S8,† CO consumption and CO2 pro-
duction are presented along with the increase in reaction time
over Cu-CHA-T(0.5) and Cu-CHA-IE(0.5) zeolites at 473 K. As it
can be seen over both catalysts, the behaviour of CO consump-
tion has been found to be corresponding to that of CO2 pro-
duction. The calculated theoretical CO conversions (CO + 1/
2O2 → CO2) approached 100% upon extending the reaction
time to 24 h (i.e., theoretical maximum of CO consumption is
9.2 mmol determined by the total amount of O2 gas fed in the
batch reactor (4.6 mmol)). These results were consistent with
Fig. 5 FT-IR spectra of Cu-CHA-T(0.5) and CO-coordinated Cu-CHA-T
(0.5) (black, top two lines), and Cu-CHA-IE(0.5) and CO-coordinated
Cu-CHA-IE(0.5) (grey, bottom two lines). CO coordination was con-
the prior work performed by our group.29 Additionally, it
would be tough to estimate the total oxidation of methane into
CO2 owing to the far lower methane conversion. Apart from
ducted in the same reactor used for the catalytic test (0.50 MPa CO,
the CO oxidation reaction, other potential reactions including 50 mg catalyst, 473 K, 3 h).
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site in the six-membered ring.38 The intensities of these bands one-pot synthesis with lower copper loadings might be promis-
are also attributed to the concentration of CO-coordinated ing for further improving the catalytic activity.
copper species. As it was assumed from a synthetic point of
view, these results suggested that Cu-CHA-T(0.5) possesses
more isolated Cu+ species with CO coordination especially in
isolated Al sites of the CHA framework. This clearly indicates
Conclusions
Cu-CHA zeolites, which were prepared through either one-pot
synthesis or liquid copper ion exchange methods, exhibit
excellent performance in the CO-assisted catalytic direct
partial oxidation of methane into methanol. Methanol was
observed as the one and only oxygenate product, indicating un-
precedented high selectivity towards methanol. Methanol pro-
that one-pot synthesis using Cu-TEPA is much efficient to fab-
ricate isolated copper active species compared with the con-
ventional ion-exchange method since the encapsulation of
maximum one Cu-TEPA per cage structure could provide good
dispersion of mononuclear copper atoms in CHA-type
zeolite.39
−1
duction rates of 5.3 × 10 and 3.5 × 10 μmol gcat h−1 were
To evaluate the effect of copper contents on methanol pro-
duction, the performance of Cu-CHA-T(x) zeolites with
different copper loadings was also screened at 473 K for 3 h.
Fig. 6 shows the methanol production rates per mol copper
along with the copper amount per cage. The highest rate was
achieved at the lowest copper content (Cu/cage = 0.1), and the
rate decreased with the increase of the copper content (Fig. 6).
This result indicates that increasing copper content does not
necessarily lead to an increase in the methanol production
rates, which is totally different from the conventional copper-
oxo studies realized in either stepwise stoichiometric or con-
tinuous catalytic processes. As it is well established by us and
other groups,27–29 isolated mononuclear Rh species have
gained high importance as active sites for CO-assisted partial
oxidation of methane to methanol. In the case of copper, its
high mobility through the small pore window of CHA-type
zeolite has been discussed in the previous studies, and that
allows the formation of di- and trinuclear metal-oxo species
for CO-free methane conversion.12,16,20–26,32 In contrast, our
result strongly suggested that the formation of a multi-nuclear
cluster was not feasible for the CO-assisted methane conver-
sion, and maintaining the isolated copper site would be the
key to prepare active sites. On the basis of these speculations,
it can be suggested that Cu-CHA materials prepared through
obtained over Cu-CHA-T(0.5) and Cu-CHA-IE(0.5), respectively.
These rates represented the highest values reported so far for
any copper-containing zeolites. The content of active copper
species in Cu-CHA zeolites varied with the synthetic strategies,
among which one-pot synthesis is found to be more efficient
to fabricate isolated copper active species. This also agreed
with the gradual decrease in methanol production rates with
the increase of copper loadings, which is opposite of conven-
tional multi-nuclear copper-oxo studies. These results high-
light the necessity of low copper loadings to direct the syn-
thesis of well-dispersed isolated single copper active sites by a
one-pot synthesis method.
Author contributions
Sibel Sogukkanli: catalyst preparation, catalytic test, analysis,
writing the original draft, manuscript editing and review;
Takahiko Moteki: project administration, conceptualization,
data validation, supervision, manuscript editing and review;
Masaru Ogura: supervision, funding acquisition, project
administration, manuscript editing and review.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We would like to thank Prof. T. Okabe and Dr T. Ouchi (The
University of Tokyo) for the use of the XRF facility and Dr K.
Iyoki (The University of Tokyo) for the assistance with ICP-AES
preparations.
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Fig. 6 Effect of copper loadings of Cu-CHA-T zeolite catalysts on
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methane into methanol (0.20 MPa O2, 0.50 MPa CO, 2.00 MPa CH4,
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