Water-Involved Methane Selective Catalytic Oxidation by

doi.org/10.26434/chemrxiv.12715502.v1

Water-Involved Methane Selective Catalytic Oxidation by Dioxygen overCopper-ZeolitesLanlan Sun, Yu Wang, Chuanming Wang, Zaiku Xie, Naijia Guan, Landong Li

Submitted date: 25/07/2020 • Posted date: 27/07/2020Licence: CC BY-NC-ND 4.0Citation information: Sun, Lanlan; Wang, Yu; Wang, Chuanming; Xie, Zaiku; Guan, Naijia; Li, Landong (2020):Water-Involved Methane Selective Catalytic Oxidation by Dioxygen over Copper-Zeolites. ChemRxiv.Preprint. https://doi.org/10.26434/chemrxiv.12715502.v1

The selective oxidation of methane to methanol is a dream reaction of direct methane functionalization, whichremains a key challenge in catalysis and a hot issue of controversy. Herein, we report the water-involvedmethane selective catalytic oxidation by dioxygen over copper-zeolites. At 573 K, a state-of-the-art methanolspace-time yield of 543 mmol/molCu/h with methanol selectivity of 91 % is achieved with Cu-CHA catalyst.Temperature-programmed surface reactions with isotope labelling determine water as the dominating oxygenand hydrogen source of hydroxyl in methanol while dioxygen participates in the reaction through reducing towater. Spectroscopic analyses reveal the fast redox cycle of Cu2+-Cu+-Cu2+ during methane selectiveoxidation, which is closely related to the high catalytic activity of Cu-CHA. Density functional theorycalculations suggest that both CuOH monomer and dimer in Cu-CHA can catalyze the selective oxidation ofmethane to methanol with Cu-OOH as the key reaction intermediate, and meanwhile, various copper sitesundergo interconversion under reaction conditions.

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http://doi.org/10.26434/chemrxiv.12715502.v1

https://chemrxiv.org/authors/Lanlan_Sun/9158084

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1

Water-involved methane selective catalytic oxidation by dioxygen over

copper-zeolites

Lanlan Sun,1 Yu Wang,1 Chuanming Wang,2* Zaiku Xie,2 Naijia Guan,1,3 Landong Li1,3*

1 School of Materials Science and Engineering & National Institute for Advanced

Materials, Nankai University, Tianjin 300350, P.R. China

2 State Key Laboratory of Green Chemical Engineering and Industrial Catalysis,

SINOPEC Shanghai Research Institute of Petrochemical Technology, Shanghai 201208,

P.R. China

3 Key Laboratory of Advanced Energy Materials Chemistry of Ministry of Education,

Nankai University, Tianjin 300071, P.R. China

Abstract: The selective oxidation of methane to methanol is a dream reaction of direct

methane functionalization, which remains a key challenge in catalysis and a hot issue of

controversy. Herein, we report the water-involved methane selective catalytic oxidation

by dioxygen over copper-zeolites. At 573 K, a state-of-the-art methanol space-time

yield of 543 mmol/molCu/h with methanol selectivity of 91 % is achieved with Cu-CHA

catalyst. Temperature-programmed surface reactions with isotope labelling determine

water as the dominating oxygen and hydrogen source of hydroxyl in methanol while

dioxygen participates in the reaction through reducing to water. Spectroscopic analyses

reveal the fast redox cycle of Cu2+-Cu+-Cu2+ during methane selective oxidation, which

is closely related to the high catalytic activity of Cu-CHA. Density functional theory

calculations suggest that both CuOH monomer and dimer in Cu-CHA can catalyze the

selective oxidation of methane to methanol with Cu-OOH as the key reaction

intermediate, and meanwhile, various copper sites undergo interconversion under

reaction conditions.

2

Methane, the main component of natural gas, is an abundant fossil resource widely

distributed throughout the earth (1). In the energy-intensive industrial processes,

methane is first converted to the syngas via reforming or partial oxidation (2), and then

transformed to fuel and chemicals. By contrast, the direct functionalization of methane

appears to be more intriguing; however, it is greatly challenged by the large C-H bond

dissociation energy of 435 kJ/mol. Several strategies, for example the oxidative or

nonoxidative coupling (3-5) and the dehydroaromatization (6, 7), have been developed

for the direct transformation of methane to chemicals (8). The selective oxidation of

methane to methanol triggers persistent interests because this type of oxidative C−H

bond activation is thermodynamically favorable under relatively mild conditions. The

dream reaction of methane-to-methanol (MTM) has been accomplished with molecular

Periana catalyst (9) and its solid analogue (10) in strong acidic media via multistep

oxyfunctionalization, which unfortunately suffers from both economic and environment

concerns.

Inspired by nature methane monooxygenase (MMO) that can catalyze the oxidation

of methane by dioxygen under ambient conditions (11), the first-row transition metal

cations stabilized by zeolite matrix, for example iron-zeolites (12, 13) and

copper-zeolites (14-24), and copper in metal-organic frameworks (25, 26) have been

developed as candidate materials for MTM transformation. A stepwise strategy

consisting of i) sample activation in dioxygen at high temperature, ii) methane

introduction at low temperature and iii) product extraction with water steam, is

commonly employed, which tactfully blocks the co-existence of methane and dioxygen

and accordingly hinders the over-oxidation of methanol (16-21). However, this is a

chemical looping process and the maximum productivity of methanol is self-limited in

one complete cycle (typically a dozen hours). To establish a promising MTM

transformation, a continuous catalytic process is desired (27) and several key

requirements should be satisfied, namely i) the use of abundant and inexpensive

oxidants like dioxygen, ii) a high selectivity to methanol, iii) a sustainable methane

conversion rate, and iv) good catalyst stability during operation.

We report here the continuous production of methanol from methane selective

3

catalytic oxidation in the presence of water and dioxygen. With the elaborate optimizing

of catalytic materials and reaction conditions, high methanol selectivity and space-time

yield can be simultaneously achieved, which makes an important step forward for the

oxidative functionalization of methane. A clear picture of MTM mechanism is depicted

via the combination of spectroscopic analyses and theoretical calculations, which

clarifies the current misunderstandings and will stimulate the further development of

this dream reaction.

Catalytic performance of copper-zeolites in methane oxidation

Copper-zeolites were prepared via wet ion-exchange (see supplementary materials for

details, and Fig. S1-6 and Table S1 for preliminary characterization results) and applied

in methane catalytic oxidation in the presence of water and dioxygen under different

conditions. The reaction temperature was raised from 473 to 523~723 K to promote

methane conversion, and meanwhile, the concentration of dioxygen was regulated (from

impurities in methane and water, and extra dioxygen) to minimize byproduct carbon

dioxide production. Cu-CHA, in significant contrast to other copper-zeolites such as

Cu-MOR and Cu-MFI, exhibited remarkable catalytic activity in MTM (see catalyst and

reaction condition optimization in Fig. S7-21). The catalytic performance of Cu-CHA

was controlled by multiple factors, including but not limited to the reaction temperature,

the dioxygen concentration and the space velocity.

4

Figure 1. Steady-state methane selective catalytic oxidation to methanol. (a):

Methane conversion on Cu-CHA (square), Cu-MOR (circle) and Cu-MFI (diamond)

catalysts under different conditions. Typical reaction conditions: 0.1 g catalyst, total

flowrate= 60 mL/min; 98% CH4, 0 or 2% H2O, 50~3000 ppm O2; (b):

Time-on-stream behaviors of Cu-CHA in methane catalytic oxidation to methanol

(red column) and carbon dioxide (yellow column). Reaction conditions: 0.1 g

catalyst, total flowrate= 60 mL/min; 98% CH4, 2% H2O, 400 ppm O2, temperature=

573 K.

Experimentally, a methanol space time yield (STY) of 542 mmol/molCu/h (195

µmol/gcat/h) with methanol selectivity of 91 % was achieved at 573 K in the presence 2%

water and 400 ppm dioxygen (Fig. 1a). This STY value is over an order of

magnitude higher than previous reports on copper-zeolites in similar catalytic processes

(Table S2). High reaction temperature of 573 K is crucial to trigger the fast

Cu2+-Cu+-Cu2+ redox cycle (vide infra) to promote methane conversion, and meanwhile

523 573 623 673 723 523 573 623 673 723 523 573 623 673 723 523 573 623 673 723 523 573 623 673/ / / /0

100

200

300

400

500

600

700

ST

Y (

mm

ol/

mo

l Cu·h

)

Temperature (K)

0

20

40

60

80

100

Se

lectiv

ity (%

)

CH4-H2O-O2 (50 ppm) CH4-H2O-O2 (200 ppm) CH4-H2O-O2 (400 ppm) CH4-H2O-O2 (3000 ppm) CH4-O2 (3000 ppm) a

b

0 100 200 300 400 5000

200

400

600

800

1000

Reaction Time (h)

0

20

40

60

80

100

Pro

du

ct y

ield

(m

mo

l/m

ol C

u·h

)S

ele

ctivity

(%)

5

the extremely low dioxygen/methane ratio (<1/2000) is the key to maintain the high

selectivity toward desired product methanol. The time-on-stream behaviors of Cu-CHA

in MTM were further investigated, and both the methanol selectivity and STY were

amazingly stable in 500 hours’ operation (Fig. 1b). These data clearly demonstrate the

potential of a one-step catalytic process for the continuous production of methanol from

methane selective oxidation. Since 2 % water was employed as the reagent, methanol

aqueous solution with mass fraction of 1.2 % could be obtained from MTM (Fig. S22)

and it is valuable for further utilization.

Isotope labelling of reaction pathway

For insight into the reaction pathway of MTM, temperature-programmed surface

reactions (TPSR) with isotope labelling were performed. In the TPSR mode, methane

oxidation started at ~523 K, generating methanol as the desired product and carbon

dioxide as the byproduct on Cu-CHA in the presence of water and dioxygen (Fig. S23;

carbon dioxide was detected as the dominating product on Cu-MOR and Cu-MFI, Fig.

S24 & S25). Dihydrogen was also detected in the outlet, but its amount was lower than

methanol and slightly lagged behind methanol. In the absence of water, carbon

dioxygen was detected as the dominating product and trace methanol could be detected

at above 673 K when water was produced from methane total oxidation (Fig. S26),

indicating the key role of water in MTM. With 13CH4 as the isotopically labelled reagent,

the carbon source of methanol and carbon dioxide was definitely confirmed to be

methane (Fig. S27).

6

Figure 2. Temperature-programmed surface reactions of methane oxidation on

Cu-CHA with isotope labelling. (a): CH4-D2O-O2 system; (b): CD4-H2O-O2 system;

(c): CH4-H218O-O2 system; (d): CH4-H2O-18O2 system. Reaction conditions: 0.1 g

catalyst, total flowrate= 60 mL/min; 98% methane, 2% water, 400±50 ppm dioxygen.

Deuterium labelling experiments (D2O as reagent in Fig. 2a and CD4 as reagent in Fig.

2b) indicated that the hydrogen in the hydroxyl of methanol dominantly came from

water while the dihydrogen was apparently produced via the combination of hydrogen

abstracted from methane and dissociated from water. Subsequently, 18O labelling

experiments were performed to reveal the active oxidant in MTM. With H218O as the

isotopically labelled reagent (Fig. 2c), CH318OH was observed as the dominating

product together with significant amount of CH3OH due to the presence of non-labelled

water in the reaction system. It seems that water acted as the dominating oxidant in

methane conversion. The absence of non-labelled CO2 ruled out the direct oxidation of

methane by dioxygen, and the C18O2 and CO18O should come from the CH318OH

a b

c d

450 500 550 600 650 700 750

D2HD

CD3-OD

H2

CO2

CD3-OH

MS

Sig

na

l (a

.u.)

Temperature (K)

CD3-O-CD3 *5

CD4-H2O-O2

450 500 550 600 650 700 750

MS

Sig

na

l (a

.u.)

Temperature (K)

CO18O C18O2

CH3-OH

H218O

CO2

CH3-18OH

18O2

CH4-H2O-18O2

450 500 550 600 650 700 750

CH3-OH

CH3-OD

CO2

MS

Sig

na

l (a

.u.)

Temperature (K)

CH3-O-CH3 *5

D2HDH2

CH4-D2O-O2

450 500 550 600 650 700 750

O2

CO18O C18O2

CH3-OH

H2O *0.2

CO2

CH3-18OH

MS

Sig

na

l (a

.u.)

Temperature (K)

CH4-H218O-O2

7

reforming (major route, Equation 1) and oxidation (minor route, Fig. S28, Equation 2),

respectively. Meanwhile, the consumption of non-labelled dioxygen was observed

at >573 K, confirming the participation of dioxygen in MTM. With 18O2 as the

isotopically labelled reagent (Fig. 2d), the oxygen in methanol product was further

confirmed to be from water and the carbon dioxide byproduct mainly from methanol

reforming. The consumption of 18O2 and the formation of H218O were observed at >573

K, demonstrating the pathway of dioxygen participation in the reaction.

CH3OH + H2O → CO2 + 3H2 (1)

2CH3OH + 3O2 → 2CO2 + 4H2O (2)

Figure 3. Dihydrogen production during methane selective oxidation catalyzed

by Cu-CHA zeolite. (a): Effect of dioxygen concentration in the reaction system on

the ratio of dihydrogen to methanol in the product; (b): Effect of dioxygen

concentration in the reaction system on the ratio of dihydrogen to methanol in the

product with the secondary reforming of methanol excluded. Reaction conditions:

0.1 g catalyst, total flowrate= 60 mL/min; 98% CH4, 2% H2O 50~550 ppm O2, He

balance.

According to the TPSR results, both water and dioxygen participate in the MTM

reaction. Providing methane is oxidized by water, dihydrogen should be produced

together with methanol with stoichiometric ratio of 1 (Equation 3). However, the ratio

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

nH

2 /

nC

H3

OH

O2 concentration (ppm)

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

(nH

2 -

nC

O2*3

)/ n

CH

3O

H

O2 concentration (ppm)

ba

8

of dihydrogen to methanol was far below 1 and also depended on the concentration of

dioxygen in the reaction system (Fig. 3a), i.e. nH2/nCH3OH ratio increased with increasing

dioxygen concentration from 50 to 550 ppm (high methanol selectivity of >85% was

achieved in all cases). Considering that methanol could be further transformed to carbon

dioxide via secondary reforming (Equation 1), the dihydrogen contribution from

reforming was further deducted to give the intrinsic nH2/nCH3OH ratio in methane

selective oxidation. It is amazing to reveal that only very low amount of dihydrogen was

produced along with methanol formation, i.e. (nH2-nCO2*3)/nCH3OH < 0.1, at all dioxygen

concentrations of 50~550 ppm. That is, dihydrogen was mainly produced from

secondary methanol reforming instead of methane direct oxidation, in good agreement

with the lagged formation of dihydrogen (Fig. S23). Now, we can present a picture of

MTM pathway in the presence of water and dioxygen (shown as [O] for simplicity), as

shown in Equation 4. From the view of element balance, methane can be oxidized by

[O], but the presence of water is essential to initiate the reaction and becomes the main

source of oxygen in methanol.

CH4 + H2O → CH3OH + H2 (3)

CH4 + H2O + [O] → CH3OH + H2O (4)

Dynamic changes in copper sites during reaction

The active copper sites in Cu-zeolites for MTM have been hotly debated (14, 16-20,

28-32). The topology and composition of zeolite host show significant impacts on the

structure of cooper sites, which also undergo dynamic changes under various conditions

(33-35). Here, the dynamic changes of copper sites in CHA under reaction relevant

conditions were investigated by ultraviolet-visible-near infrared (UV-Vis-NIR)

spectroscopy. The spectrum of Cu-CHA (473 K in He, Fig. S29) showed clear d-d

transitions of Cu2+ (3d9) at ~12000 cm-1 and ligand-to-metal charge transfer (LMCT)

transitions of isolated Cu2+ (O2-Cu2+→O-Cu+) at 50000~45000 cm-1 (36-38). The

Cu-CHA sample was then treated in different atmospheres at increasing temperature

from 473 to 773 K to demonstrate the dynamic changes of copper sites (Fig. S30).

9

Treating in oxygen or water resulted in a slight increase in the intensity of band at

12000 cm-1 and noticeable shifts in the band at 40000~30000 cm-1, probably due to the

reconstruction and creation of a small quantify of isolated Cu2+ species. Treating in

methane resulted in significant declines in the intensities of bands at 12000 cm-1 and

50000~45000 cm-1, due to the reduction of Cu2+ in methane. The charge transfer

transitions of Cu2+ and Cu+ were highly overlapped and their interpretation might be

misleading, while the decline in the intensity of band at 12000 cm-1 clearly revealed the

reduction of isolated Cu2+ to Cu+ since no d-d transitions occur for Cu+ with fully

occupied d shell (37). Interestingly, the Cu+ could be readily oxidized back to Cu2+ in

H2O-O2 at 573 K (Fig. S31). That is, a redox cycle of Cu2+-Cu+-Cu2+ in Cu-CHA might

involve in the reaction system of CH4-H2O-O2.

Figure 4. UV-Vis-NIR spectroscopic analyses of copper sites in Cu-CHA zeolite.

(a): In situ UV-Vis-NIR spectra of Cu-CHA sample under different atmospheres at

473~623 K. Conditions: 25 mg catalyst, total flowrate= 15 mL/min; 0 or 98% CH4, 0

or 2% H2O, 0 or 400 ppm O2, He balance; (b): Dynamic changes in the intensities of

absorption band at ~12000 cm-1.

a

b

50000 40000 30000 20000 10000

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4

Wavenumber (cm-1)

Inte

nsi

ty (

a.u

.)

He

473 K

50000 40000 30000 20000 10000

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4

Wavenumber (cm-1)

Inte

nsi

ty (

a.u

.)

He

523 K

50000 40000 30000 20000 10000

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4

Wavenumber (cm-1)

Inte

nsi

ty (

a.u

.)

He

573 K

50000 40000 30000 20000 10000

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4

Wavenumber (cm-1)

Inte

nsi

ty (

a.u

.)

He

623 K

0 20 40 60 80 100

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4

Time-on-stream (min)

No

rma

lize

d I

nte

nsi

ty (

a.u

.)

He

473 K

0 20 40 60 80 100

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4


No

rma

lize

d I

nte

nsi

ty (

a.u

.)

He

573 K

0 20 40 60 80 100

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2OCH4


No

rma

lize

d I

nte

nsi

ty (

a.u

.)

He

623 K

0 20 40 60 80 100

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4


No

rma

lize

d I

nte

nsi

ty (

a.u

.)

He

523 K

10

The dynamic changes of copper species in Cu-CHA under different atmospheres at a

constant temperature were then focused. At low temperatures of 473 and 523 K, the

introduction of dioxygen (CH4-H2O-O2) resulted in the oxidation of Cu+ species (those

existed in parent Cu-CHA due to the auto-reduction of Cu2+ in helium) to Cu2+, while

the Cu2+ species were quite stable in H2O or CH4-H2O (Fig. 4a). At high temperatures

of 573 and 623 K, the dynamic changes of copper species between the isolated Cu2+ and

Cu+ under different atmospheres were clearly demonstrated (Fig. 4b). Typically,

isolated Cu2+ species in Cu-CHA were quickly (partially) reduced by methane to Cu+,

which could not be oxidized back to Cu2+ simply by water. On the other hand, the

introduction of trace dioxygen resulted in the fast re-oxidation of Cu+ to Cu2+.

According to these results, isolated copper species should exist in the form of stable

Cu2+ in Cu-CHA in CH4-H2O-O2 at 473~623 K and the fast redox cycle of

Cu2+-Cu+-Cu2+ should involve in MTM reaction at 573~623 K. This should be the key

reason for the much higher MTM activity achieved in this study in comparison with

literature reports under slightly different reaction conditions (23, 24). Besides, no

fingerprint absorption features for dicopper species in the region of 20000~25000 cm-1

(28, 29) were observed under all employed conditions.

11

Figure 5. FTIR spectroscopic analyses of Cu-CHA zeolite. (a): In situ FTIR spectra

of Cu-CHA sample under different atmospheres at 573 K. Conditions: 25 mg catalyst,

total flowrate= 15 mL/min; 0 or 98% CH4, 0 or 2% H2O, 0 or 400 ppm O2, He balance;

(b): Dynamic changes in the intensities of selected FTIR bands; (c): Steady-state FTIR

spectra of Cu-CHA sample under different atmospheres with isotope labelling at 573 K.

Conditions: 25 mg catalyst, total flowrate= 15 mL/min; 0 or 98% methane, 0 or 2%

water, 0 or 400±50 ppm dioxygen.

For more information on Cu-CHA for MTM, in situ FTIR spectroscopic analyses

were performed at 573 K. As shown in Fig. 5a, several IR bands at 3730, 3655, 3605

and 3580 cm-1 were observed for Cu-CHA in flowing helium, due to Si-OH, Cu(II)-OH,

two types of Si-OH-Al species, respectively (34, 39). The Si-OH was very stable under

different atmospheres and regarded as a reference for quantitative analyses (Fig. 5b).

Feeding methane to Cu-CHA resulted in the quick appearance of IR band at 2155 cm-1

due to the formation of carbonyl group on Cu+ (40), and the carbonyl group could be

employed as in situ probe to titrate Cu+ centers. The intensity of IR band at 2155 cm-1

decreased gradually with the introduction of water (CH4-H2O) and disappeared with the

3800 3700 3600 2200 2100

21

55

35

80

36

05

36

55

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4

Wavenumber (cm-1)

He

37

30

3700 3600 2800 2700 2600 2100

21

55

21

05

CH4-H2O-O2

CH4-H2O

CH4

H218O

CH4

35

70

35

95

36

45

13CH4

O2

18O2

36

55

37

30

35

80

36

05

Wavenumber (cm-1)

0 20 40 60 80 100

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4


No

rma

lize

d I

nte

nsi

ty (

a.u

.)He

3655 cm-1

0 20 40 60 80 100

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4


No

rma

lize

d I

nte

nsi

ty (

a.u

.)

He

3730 cm-1

0 20 40 60 80 100

H2O

CH4-H2O

CH4-H2O-O2CH4-H2O

CH4


No

rma

lize

d I

nte

nsi

ty (

a.u

.)

He

2155 cm-1

0 20 40 60 80 100

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4


No

rma

lize

d I

nte

nsi

ty (

a.u

.)

He

3605 cm-1

0 20 40 60 80 100

H2O

CH4-H2O

CH4-H2O-O2

CH4-H2O

CH4


No

rma

lize

d I

nte

nsi

ty (

a.u

.)

He

3580 cm-1

a

c

b

12

further introduction of dioxygen (CH4-H2O-O2). Removal of the O2 from CH4-H2O-O2

stream resulted in the reappearance of Cu+ centers. These results agree well with the

dynamic changes of copper sites as revealed by in situ UV-Vis-NIR spectroscopy (Fig.

4). Meanwhile, a noticeable increase in the intensity of Brønsted acid sites in the

six-membered rings (3580 cm-1) was observed upon methane activation, following the

possible pathway of Equation 5 (Z: Zeolite Si-O-Al site). To confirm the hypothesis,

CD4 was fed to Cu-CHA and the formation of deuterated Brønsted acid sites at 2640

cm-1 could be identified (Fig. S32). With the introduction of water (CH4-H2O), the

intensities of IR bands due to the Brønsted acid sites in the six-membered rings (3580

cm-1) and Cu(II)-OH (3655 cm-1) increased synchronously due to the hydrolysis of bare

copper sites (Equation 6).

Z2 − Cu2+ + CH4 → Z − [Cu − CH3]+ + Z − H+ (5)

Z2 − Cu2+ + H2O → Z − [Cu − OH]+ + Z − H+ (6)

The steady-state FTIR spectra of Cu-CHA sample under different atmospheres were

then recorded. As shown in Fig. 5c, treating in isotope-labelled 18O2 did not bring about

significant changes in the structure of Cu-CHA, excluding the gas-phase oxygen isotope

exchange by dioxygen. In contrast, treating in H218O resulted in the oxygen isotope

exchange in Si-OH-Al (from 3605 and 3580 cm-1 to 3595 and 3570 cm-1, respectively)

and Cu(II)-OH groups (from 3655 cm-1 to 3645 cm-1) with νO-H/ν18O-H of ~1.003. That is,

the active hydroxyls can undergo fast isotope exchange with water. Feeding methane to

the 18O-exchange Cu-CHA led to the formation of 18O-carboxyl group (band at 2105

cm-1, νC-O/νC-18O =1.024) on Cu+ sites, in accordance with the appearance of

13C-carboxyl group at 2107 cm-1 upon feeding 13CH4 to Cu-CHA (νC-O/ν13C-O =1.023).

Reaction mechanism of methane-to-methanol

Following nature particulate MMO (pMMO), the dicopper sites in zeolites have been

proposed for methane oxidation to methanol via a chemical looping process. However,

it has been recently argued that the pMMO contains only mononuclear copper centers

(41). Experimentally, there were no signs of dicopper sites in our Cu-CHA under all

13

conditions employed (Fig. 4a). The dicopper sites, even if might exist under certain

conditions, would undergo hydrolysis to monomeric cooper species in excess water. A

series of Cu-CHA samples with different copper loadings were further prepared and

their specific activity was compared. As shown in Fig. S33, the site specific methanol

yields were in the similar high level of 400~600 mmol/molCu/h at copper loadings of

0.6~2.3 wt.% while the methanol yield decreased to <300 mmol/molCu/h with further

increasing copper loading to 3.5 wt.%. In this context, it is rational to propose

monomeric cooper species, i.e. Cu(II)-OH observed by FTIR spectroscopy (Fig. 5), as

the catalytically active sites for MTM, although the formation of dicopper sites could

not be fully excluded.

a

b c

-300

-200

-100

0

100

-80

-292

-2

-101

-145

2

-48

6

-95

TS-DTS-CTS-B

+CH4

-210

-204

-42

-16

64

M10

M9

M8

M7

M6

M5

M4

M1

M3

M2

+CH4

-H2O

-CH

3O

H

+ O

2

-CH

3O

H

+H2 O

En

erg

y (

kJ/

mo

l)

TS-A

-26

TS-A TS-DTS-B TS-C

M3 M7 M9M5 M6M4

-300

-200

-100

0

+CH4

-204

-306

-31

-90

-64

-13

-95

-74

-CH

3O

H

D5

TS-F

D4

D3

TS-E

D2

D1

En

erg

y (

kJ/

mo

l)

TS-E TS-F

D5D1 D2

Cu-Monomer Cu-Dimer

TS-A

TS-B

TS-C

TS-D

TS-E

TS-F

M2M1

M3

M4

M5

M6

M7

M8

M9

D1 D2

D3

D4

D5

D0M0

14

Figure 6. Reaction mechanism from DFT calculations. (a): Proposed reaction

mechanism for the methane oxidation to methanol reaction on Cu-CHA involving

CuOOH intermediate; (b): Energy profile of methane selective oxidation to methanol

catalyzed by CuOH monomer in CHA at 0 K; (c): Energy profile of methane selective

oxidation to methanol catalyzed by CuOH dimer in CHA at 0 K.

Finally, the complete reaction mechanism for MTM conversion was proposed from

spin-polarized density functional theory (DFT) calculations to rationalize the above

experimental observations. As shown in Fig. 6a, CuOH monomer or dimer was set as

the intrinsic active sites for MTM conversion. Regarding the structure evolution and the

energy variation at CuOH monomer (Fig. 6b, Fig. S34), the activation of methane

(TS-A, 90 kJ/mol) and the subsequent adsorption of water resulted in the formation of

mobilized CH3Cu[OH2]2 cation in the cage of CHA zeolite (M3). Dioxygen molecule

was demonstrated to chemically adsorb on Cu+ center to form four-coordinated Cu

complex (M4). Interestingly, the framework-bound methoxide (ZCH3) could be

obtained via demethylation (TS-B) accompanied by the formation of two-coordinated

H2OCuO2 complexes (M5). Methanol could then be produced by the typical

methylation (TS-C) between methoxide and adjacent H2O, leaving H atom to H2OCuO2.

The energy barriers in both steps were 101 and 50 kJ/mol, respectively. The adsorbed

dioxygen was found to play an important role to maintain the coordination structure of

Cu complex and facilitate the formation of methanol from water. Framework-bound

CuOOH or hydrated CuOOH (M7) was identified as the key intermediate in the MTM

reaction pathway catalyzed by CuOH. The CuOOH was also active for methane (TS-D,

99 kJ/mol) to produce methanol and regenerate CuOH active center. Dioxygen served

as the explicit oxidant for C-H activation in these steps. In the case of CuOH dimer

evolution (Fig. 6c, Fig. S35), one CuOH was assumed to follow the aforementioned

CuOH monomer pathway, generating CuOOH in close proximity to another CuOH site

in zeolites. If appropriate in distance, more stable binuclear Cu complex (D2) could be

formed. The feasible breaking of O-OH bond in CuOOH (TS-E, 82 kJ/mol) led to the

formation of immobilized OHCuOCuOH complex (D3). The activation of methane by

15

OHCuOCuOH (TS-F) only needed to overcome a low barrier of ~59 kJ/mol to produce

methanol and to regenerate CuOH dimer. Comparing the kinetics in Cu monomer and

dimer pathways (TS-D versus TS-F), it seemed that HOCuOCuOH might be more

active than CuOOH for methane activation. Herein, we would like to highlight that

CuOH and the previous proposed active species like Cu(II), CuOCu, CuOOCu could be

readily interconverted by the participation of methane or water (Fig. 6a, Figs. S36-39).

The proposed reaction pathway originated from CuOH provides an alternative to

produce methanol without dihydrogen (Fig. 3b) and reveals the direct role of dioxygen

in MTM conversion. In the experiments, the fast redox cycle of Cu2+-Cu+-Cu2+ during

MTM reaction was clearly identified (Fig. 4) and the presence of Cu+ was proved to be

necessary for the chemisorption of dioxygen by theoretical calculations (Fig. 6). That is,

the fast redox cycle of Cu2+-Cu+-Cu2+ is the origination of the high activity of Cu-CHA

under our reaction conditions. According to theoretical calculations, the oxygen in

methanol might come from both water and dioxygen (see dioxygen labelling in Fig. 6a),

while the fast oxygen isotope exchange between CuOH and water (Fig. 5c) made water

as the apparent oxygen source of methanol (Fig. 2c, 2d). Water is known as a must for

the methane-to-methanol conversion while it plays an important role in the catalytic

route by hydrolyzing the Cu-O bond, which resulted in the formation of mobilized

CH3Cu[OH2]2 cations from framework-bound species and the transformation of

dicopper species (CuOCu, CuOOCu) to monomeric cooper species (CuOH).

Conclusions

A catalytic route of methane selective oxidation to methanol in the presence of water

and dioxygen was demonstrated, and the specific roles of water and dioxygen were

clearly explicated. With the elaborate optimizing of catalyst and reaction parameters, a

state-of-the-art methanol space-time yield of 543 mmol/molCu/h with methanol

selectivity of 91 % was achieved at 573 K with a Cu-CHA catalyst. The reaction

pathway of MTM was established by kinetic analyses with isotope labelling. The

apparent oxygen and hydrogen source of hydroxyls in methanol was determined to be

water while dioxygen indeed participated in the reaction through reducing to water,

16

resulting in the unique process of water-involved methane oxidation by dioxygen. The

fast redox cycle of Cu2+-Cu+-Cu2+ was identified by in situ spectroscopy under certain

reaction conditions and revealed to be closely related to the high catalytic activity of

Cu-CHA.

On the basis of all the experiment fragments, a detailed mechanism of MTM was

interpreted via density functional theory calculations. Both CuOH monomer and dimer

in Cu-CHA could catalyze the selective oxidation of methane to methanol, with

CuOOH as the key intermediate. The previous proposed active species like Cu(II),

CuOH, CuOCu and CuOOCu could be readily interconverted with the participation of

methane or water under reaction conditions. These findings have addressed most

misconceptions in MTM reaction over copper-zeolites (42) and will make a major step

forward in methane oxidative functionalization.

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Acknowledgments

We acknowledge the National Natural Science Fund of China (21722303, 21673295,

20

21421001) and 111 Project (B12015, B18030) for supporting the work.

Author Contributions

L.S. and Y.W. conducted material preparations, performance tests and spectroscopic

analyses. C.W. and Z.X. contributed to theoretical calculations and directed the

theoretical section. N.G. analyzed the data and provided helpful discussions. L.L.

directed and supervised the project. L.S., C.W. and L.L. prepared the manuscript.

Competing Interests

The authors declare no competing financial interests.

Additional information

Supplementary Materials is available for this paper, including Materials and Methods,

Figures S1-S39, Tables S1-S2.

Materials & Correspondence should be addressed to C.W. or L.L.

download fileview on ChemRxivManuscript.pdf (2.03 MiB)



1

Supplementary Materials for

Water-involved methane selective catalytic oxidation by dioxygen

over copper-zeolites

Lanlan Sun, Yu Wang, Chuanming Wang*, Zaiku Xie, Naijia Guan, Landong Li*

Correspondence to:

[email protected] (C. Wang) & [email protected] (L. Li)

This PDF file includes:

Materials and Methods

Figs. S1 to S39

Tables S1 to S2

mailto:[email protected]

mailto:[email protected]

2

Materials and Methods

Zeolite host: CHA (H-SSZ-13, SiO2/Al2O3=22), MFI (H-ZSM-5, SiO2/Al2O3=25) and

MOR (H-modenite, SiO2/Al2O3=23) from Shandong Qilu Huaxin High-Tech Co. Ltd.

and used as received.

Gases and chemicals: Methane (>99.995%), helium (>99.999%) and dioxygen

(>99.995%) from Air Liquide (China) Co. Ltd.; Copper actetate monohydrate

(analytical reagent), i.e. Cu(CH3COO)2·H2O, from Alfa Aesar (China) Chemical Co.

Ltd.

Isotope reagents: D2O (99.9% enrichment), H218O (97% enrichment), 18O (98%

enrichment), CD4 (99% enrichment) and 13CH4 (99% enrichment) from ISOTEC

Laboratories Inc.

Preparation of Cu-zeolites via wet ion exchange

Cu-zeolite samples were prepared via repeated wet ion exchange under controlled pH

value. In a typical experiment, 0.5 g zeolite host was placed in the three-necked flask

and exchanged with 30 mL 0.01 M Cu(CH3COO)2 for 6 h with pH value kept at 5±0.2

(unless otherwise stated) under stirring. The resulting solid was filtrated and thoroughly

washed. The final product was dried at 353 K overnight and calcined in flowing air at

773 K for 6 h.

Characterization of Cu-zeolites

The chemical compositions of Cu-zeolites were analyzed on an IRIS Advantage

inductively coupled plasma atomic emission spectrometer (ICP-AES).

The X-ray diffraction (XRD) patterns of Cu-zeolite samples were recorded on a

Bruker D8 diffractometer using Cu-Kα radiation (λ= 0.1541 nm) at a scanning rate of

6 o/min in the region of 2θ = 5-50o.

The textual properties of Cu-zeolites were determined by argon adsorption-

desorption isotherms at 87 K collected on a Quantachrome iQ-MP gas adsorption

analyzer. The total surface areas were calculated via the Brunauer Emmett Teller (BET)

equation and the micropore properties were determined using the t-plot method.

3

Scanning electron microscopy (SEM) images of Cu-zeolite samples were obtained

on a JSM-7500F electron microscope.

Transmission electron microscopy (TEM) images of Cu-zeolite samples under study

were acquired on a FEI Tecnai G2 F20 electron microscope. High angle annular dark

filed scanning transmission electron microscopy (HAADF-STEM) images were

acquired on a FEI Talos electron microscope. The element mapping analysis was

performed under HAADF-STEM mode using a FEI built-in energy dispersive spectrum.

FTIR spectra of Cu-zeolite samples were collected on a Bruker Tensor 27

spectrometer in the diffuse reflectance mode with Harrick Praying Mantis setup and a

liquid nitrogen cooled high sensitivity mercury-cadmium-telluride detector. The in situ

diffuse reflectance FTIR spectroscopic analyses under different reaction conditions

were performed in a Harrick CHC-CHA-3 chamber.

UV-vis-NIR spectra of Cu-zeolite samples were collected on a PerkinElmer Lambda

750 UV/VIS/NIR spectrometer in the diffuse reflectance mode with a Harrick Praying

Mantis setup. The in situ UV-vis-NIR spectroscopic analyses under different reaction

conditions were performed in a HVC-DRM-5 chamber.

The hydrogen temperature-programmed reduction (H2-TPR) of Cu-zeolites was

performed on a Quantachrome ChemBET 3000 chemisorption analyzer. In a typical

experiment, Cu-zeolite sample of ca. 0.1 g was calcined in dry air at 673 K for 1 h and

cooled to 323 K in flowing Ar. H2-TPR profile was recorded in flowing 10%H2/Ar at a

heating rate of 10 K/min from 323 to 1023 K.

Steady-state methane catalytic oxidation to methanol

The selective catalytic oxidation of methane was performed on a fixed-bed micro-

reactor at ambient pressure. Typically, catalyst sample of 0.1 g (sieve fraction 250-400

μm) was placed in the quartz reactor and the reactant gas mixture containing methane,

water and dioxygen was fed to the quartz reactor at designated temperature. The total

flow rate was controlled at 60 mL/min, corresponding to a gas hourly space velocity

(GHSV) of 30,000 /h. The reaction outlet was on-line analyzed by a gas chromatograph

(SHIMADZU GC-2014) equipped with a thermal conductivity detector (TCD, with one

4

MS-13X packed column and two Porapak N packed columns) and a flame ionization

detector (FID, with one Plot Q capillary column). Methane, carbon monoxide, carbon

dioxide and dihydrogen were analyzed by TCD, while methane, C1 oxygenates and

C2+ hydrocarbons were analyzed by FID (argon as an internal standard, methane as a

link between TCD and FID). The concentration of methanol (also for carbon monoxide,

carbon dioxide, etc.) was quantitatively determined by external standard method (see

below for typical calibration curves) and the yield of methanol (𝑌CH3OH) was obtained

via volume conversion. The methanol selectivity was determined by normalization:

𝑆CH3OH(%) = [CH3OH]outlet

∑ Product∗ 100% , (byproducts below the detect limitation of gas

chromatograph were ignored and the possible coke deposition was fully excluded even

after long-term running). The conversion of methane was calculated as: 𝐶CH4 (%) =

𝑌CH3OH

𝑆CH3OH∗ 100%.

Temperature-programmed surface reactions of methane catalytic oxidation

The temperature-programmed surface reactions (TPSR) were also performed in the

fixed-bed reactor. In a typical experiment, sample of ca. 0.1 g was placed in the quartz

reactor and the reactant gas mixture was fed to the quartz reactor at 60 mL/min at 393

K. After the reaction outlet reached steady, the TPSR was initiated by heating up from

393 to 793 K at a rate of 5 K/min. The reaction outlet was on-line analyzed by a mass

spectrometer (Pfeiffer Omnistar GSD 320). The m/z values were carefully selected for

target reagents and products (see table below), and the overlapped fractions and

0E+00 1E-10 2E-10 3E-100

1000

2000

3000

4000

Pe

ak

Are

a (

a.u

.)

nmethanol

0.0E+00 1.0E-09 2.0E-09 3.0E-090

300

600

900

1200

Pe

ak

Are

a (

a.u

.)

nCarbon dioxide

5

background signals were subtracted. The presence of formaldehyde, formic acid,

methyl formate and C2+ hydrocarbons could be fully excluded by gas chromatograph

and mass spectrometer (below the detection limit).

The m/z values for chemicals employed in this study and their relative intensity

H2 2 (100%), 1(1%) HD 3 (100%), 2 (1%), 1 (1%)

D2 4 (100%), 2(1%) O2 32 (100%), 16 (22%)

O18O 34 (100%), 18 (22%), 16 (22%) 18O2 36 (100%), 18 (22%)

H2O 18 (100%), 17 (21%), 16 (1%) D2O 20 (100%), 18 (21%), 16 (1%)

H218O 20 (100%), 19(21%), 18 (1%) CH3OH 31 (100%), 32 (74%), 29 (43%)

CH3OD 32 (100%), 33 (74%), 29 (43%) CD3OH 33 (100%), 35 (74%), 30 (40%)

CD3OD 34 (100%), 36 (71%), 30 (50%) CH318OH 33 (100%), 34 (74%), 31 (43%)

13CH3OH 32 (100%), 33 (74%), 30 (43%) CO2 44 (100%), 28 (50%), 16 (50%)

CO18O 46 (100%) C18O2 48 (100%), 30 (50%), 18 (50%)

13CO2 45 (100%), 29 (50%), 18 (50%) CH3-O-CH3 45 (100%), 46 (61%), 29 (39%)

CD3-O-CD3 50 (100%), 52 (61%), 31 (39%) 13CH3-O-13CH3 47 (100%), 48 (61%), 30 (39%)

Indicative m/z value shown in red; Relative intensity shown in parentheses.

Computational methods and modeling

All spin-polarized DFT calculations were performed using VASP package (1). The

projector augmented wave (PAW) was used to describe electron-ion interaction with

the plane wave basis set kinetic energy cutoff equal to 400 eV (2,3). The Bayesian error

estimation functional with van der Waals (vdW) correlation (BEEF-vdW) was used (4).

The sampling of Brillouin zone was only with Г point (5). The dimer method was

utilized to locate all transition states (6). A force threshold of 0.02 eV/Å was employed

for structure optimization of all intermediates and transition states.

The Cu-CHA structure were represented by 36T hexagonal cell. All atoms in the cell

were allowed to relax with the lattice constants (13.72, 13.72, 14.86 Å) being fixed.

The harmonic frequency calculations employed a partial Hessian vibrational analysis

6

(PHVA). The zero point energies (ZPE), enthalpies, entropies, and Gibbs free energies

were then calculated from harmonic frequencies.

References

(1) G. Kresse, J. Furthmuller, Phys. Rev. B, 1996, 54, 11169-11186.

(2) P. E. Blochl, Phys. Rev. B, 1994, 50, 17953-17979.

(3) G. Kresse, D. Joubert, Phys. Rev. B, 1999, 59, 1758-1775.

(4) J. Wellendorff, K. T. Lundgaard, A. Møgelhøj, V. Petzold, D. D. Landis, J. K.

Nørskov, T. Bligaard, K. W. Jacobsen, Phys. Rev. B, 2012, 85, 235149.

(5) H. J. Monkhorst, J. D. Pack, Phys. Rev. B, 1976, 13, 5188-5192.

(6) G. Henkelman, H. Jonsson, J. Chem. Phys., 1999, 111, 7010-7022.

7

Figures & Tables

Fig S1. XRD patterns of Cu-zeolites under study

Fig S2. Low-temperature argon adsorption-desorption isotherms of

Cu-zeolites under study.

10 20 30 40

Cu-CHA

Cu-MFI

Inte

nsi

ty (

a.u

.)

2 Theta [deg.]

Cu-MOR

0.0 0.2 0.4 0.6 0.8 1.0

Ar

ad

sorb

ed

(S

TP

) (c

m3/

g-1

)

Cu-MOR

Relative Pressure (P/P0)

Cu-MFI

Cu-CHA

8

Fig S3. SEM images of Cu-zeolites under study

Cu-CHA

Cu-MFI

Cu-MOR

9

Fig S4. TEM images of Cu-zeolites with corresponding element

mapping analyses

Cu-CHA SiO

Al Cu

Cu-MFI

Al

Si

Cu

O

Cu-MOR

Al Cu

SiO

10

Fig S5. UV-Vis-NIR spectra of Cu-zeolites under ambient conditions

Fig S6. Hydrogen temperature-programmed reduction profiles of Cu-

zeolites under study

50000 40000 30000 20000 10000

Cu-MOR

Cu-MFIIn

ten

sity

(a

.u.)

Wavenumber (cm-1)

Cu-CHA

400 600 800 1000

Cu-MOR

Cu-MFI

Inte

nsi

ty (

a.u

.)

Temperature (K)

Cu-CHA

11

Fig S7. Time-on-stream behaviors of methane hydration on Cu-CHA catalyst. Reaction

conditions: 0.1 g catalyst, total flowrate= 60 mL/min; 98% methane, 2% H2O, 50 ppm O2.

Methanol (Red column) and carbon dioxide (Dark Yellow column) shown as products.

0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

20

40

60

80

100

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

12




0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

13




0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

14




0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

800

1600

2400

3200

4000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

800

1600

2400

3200

4000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

15


conditions: 0.1 g catalyst, total flowrate= 60 mL/min; 98% methane, 2% helium, 3000 ppm O2.


0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

16

Fig S12. Time-on-stream behaviors of methane hydration on Cu-MOR catalyst. Reaction



0 100 200 300 400 5000

20

40

60

80

100

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

17



Methanol (Red column) and carbon dioxide (Dark Yellow column), shown as products.

0 100 200 300 400 5000

20

40

60

80

100

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

400

800

1200

1600

2000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

18




0 100 200 300 400 5000

20

40

60

80

100

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

400

800

1200

1600

2000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

19




0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

2000

4000

6000

8000

10000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

2800

5600

8400

11200

14000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

20

Fig S16. Time-on-stream behaviors of methane hydration on Cu-MFI catalyst. Reaction



0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

21




0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

22




0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

400

800

1200

1600

2000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

400

800

1200

1600

2000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

23




0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

200

400

600

800

1000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

1600

3200

4800

6400

8000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

2000

4000

6000

8000

10000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

0 100 200 300 400 5000

1600

3200

4800

6400

8000

T= 723 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

24


conditions: 0.1 g catalyst, total flowrate= 60 mL/min; 98% methane, 2% helium, 3000 ppm O2.


0 100 200 300 400 5000

200

400

600

800

1000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 523 K

0 100 200 300 400 5000

2000

4000

6000

8000

10000

T= 573 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

2000

4000

6000

8000

10000

T= 623 K

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

0 100 200 300 400 5000

2000

4000

6000

8000

10000

ST

Y (

mm

ol/

mo

l Cu· h

)

Reaction time (min)

T= 673 K

25

Fig S21. Effect of gas hourly space velocity on methanol yield and

selectivity from methane oxidation over Cu-CHA catalyst. Reaction

conditions: 0.1 g catalyst, temperature = 573 K, total flowrate= 5~100

mL/min; 98% methane, 2% H2O, 400 ppm O2.

Fig S22. Typical gas chromatograms for the on-line monitor and off-line analysis of

the products from methane selective oxidation over Cu-CHA

10000 20000 30000 400000

100

200

300

400

500

600

700

Se

lectiv

ity (%

)S

TY

(m

mo

l/m

ol C

u·h

)

GHSV (/h)

0

20

40

60

80

100

On-line monitor Off-line analysis

26

Fig S23. Temperature-programmed

surface reaction of methane oxidation on

Cu-CHA. Reaction conditions: 0.1 g

catalyst, total flowrate= 60 mL/min; 98%

CH4, 2% H2O, 400 ppm O2.



Cu-MOR. Reaction conditions: 0.1 g


CH4, 2% H2O, 400 ppm O2.

450 500 550 600 650 700 750

H2

CO2

CH3-OH

MS

Sig

na

l (a

.u.)

Temperature (K)

CH3-O-CH3 *5

450 500 550 600 650 700 750

H2

CO2

CH3-OH

MS

Sig

na

l (a

.u.)

Temperature (K)

CH3-O-CH3 *5

27



Cu-MFI. Reaction conditions: 0.1 g


CH4, 2% H2O, 400 ppm O2.





CH4, 2% O2.

450 500 550 600 650 700 750

H2

CO2

CH3-OH

MS

Sig

na

l (a

.u.)

Temperature (K)

CH3-O-CH3 *5

450 500 550 600 650 700 750

H2

CO2

CH3-OH

MS

Sig

na

l (a

.u.)

Temperature (K)

H2O *0.2

28





13CH4, 2% H2O, 400 ppm O2.


surface reaction of methanol oxidation on


catalyst, total flowrate= 60 mL/min; 200

ppm CH3OH, 400±50 ppm 18O2, He

balance

450 500 550 600 650 700 750

H2

13CO2

13CH3-OH

MS

Sig

na

l (a

.u.)

Temperature (K)

13CH3-O-13CH3 *5

450 500 550 600 650 700 750

H2

CO18OC18O2

CH3OCH3

H218O

CO2

MS

Sig

na

l (a

.u.)

Temperature (K)

CH3OH

29

Fig S29. UV-Vis-NIR spectrum of Cu-CHA recorded in flowing helium at

473 K. The region of d-d transitions, charge transfer transitions and LMCT

transitions shown for reference.

Fig S30. In situ UV-Vis-NIR spectra of Cu-CHA treated in different atmospheres at

increasing temperature from 473 to 773 K (color changes from light to dark).

Conditions: 25 mg catalyst, total flowrate= 15 mL/min; 98% CH4, or 2% H2O, or 400

ppm O2, He balance

50000 40000 30000 20000 10000

LM

CT

charge tranfer

transitions

Inte

nsi

ty (

a.u

.)

Wavenumber (cm-1)

d-d

transitions

50000 40000 30000 20000 10000

Inte

nsi

ty (

a.u

.)

Wavenumber (cm-1)

CH4

50000 40000 30000 20000 10000

Inte

nsi

ty (

a.u

.)

Wavenumber (cm-1)

O2

50000 40000 30000 20000 10000

Inte

nsi

ty (

a.u

.)

Wavenumber (cm-1)

H2O

30

Fig S31. In situ UV-Vis-NIR spectra of methane-reduced Cu-CHA

treated in H2O or H2O-O2 (2% H2O, 400 ppm O2 in He) at 573 K

Fig S32. In situ FTIR spectra of CD4 adsorption on Cu-CHA at 573 K and the

dynamic changes in the intensity of selected IR bands during CD4 adsorption.

Conditions: 25 mg catalyst, total flowrate= 5 mL/min; 10% CD4 in He.

50000 40000 30000 20000 10000

H2O-O2

H2O

Wavenumber (cm-1)

Inte

nsi

ty (

a.u

.)

CH4

0 20 40 60 80 100

3580 & 2640 cm-1

3580 cm-1

2640 cm-1


No

rma

lize

d I

nte

nsi

ty (

a.u

.)

3800 3700 3600 2800 2700 2600 2500

CD4

36

55

26

90 26

60

27

50

37

30

35

8036

05

26

40

Wavenumber (cm-1)

He

31

Fig S33. Impacts of copper loading on the site and mass specific

activity of Cu-CHA samples in methane selective oxidation to

methanol. Reaction conditions: 0.1 g catalyst, total flowrate= 60

mL/min; 98% CH4, 2% H2O, 400 ppm O2, temperature= 573 K.

Fig S34. Energy profile of methane to methanol reaction catalyzed

by CuOH monomer in CHA at 573 K.

0.5 1.0 1.5 2.0 2.5 3.0 3.50

100

200

300

400

500

600

700

Cu Loading (wt.%)

Pro

du

ctiv

ity

(m

mo

l/m

ol/

h)

0

50

100

150

200

250

Pro

du

ctiv

ity

(

mo

l/g

/h

)

-200

-100

0

100

200

-155

-88-84

206

89

5873

221

165

211

121

89

114

4225

TS-DTS-C

TS-B

TS-A

En

erg

y (

kJ/

mo

l)

M10M9

M8

M7M6

M5

M4

M1

M3

M2

32

Fig S35. Energy profile of methane to methanol reaction catalyzed

by CuOH dimer in CHA at 573 K.

Fig S36. Energy profile of methane activation by Cu(II) in CHA.

-200

-100

0

100

-155-160

113

32

15

66

-11-8

TS-F

TS-E

D5

D4

D3

D2D1

En

erg

y (

kJ/

mo

l)

33

Fig S37. Energy profile of CuOCu conversion to CuOH dimer in

CHA.

Fig S38. Energy profile of methane activation by CuOCu in CHA.

34

Fig S39. Energy profile of CuOOCu conversion to CuOOH and

CuOH in CHA.

Table S1 Physicochemical properties of Cu-zeolite samples under study

Sample Zeolite host Si/Al a Cu (%) Cu/Al a SBET (m2/g)

Cu-CHA H-SSZ-13 11.0 2.30 0.23 621

Cu-MOR H-mordenite 10.9 2.72 0.27 445

Cu-MFI H-ZSM-5 12.8 2.37 0.27 426

a: molar ratio, determined by ICP analyses

35

Table S2 Comparison of samples and processes for the selective oxidation of methane to

methanol

Sample Process Productivity Sel.

(%) STY c Ref.

Site a Mass b

pMMO i) Duroquinol as electron

donor; ii) 318 K in CH4 1 h 19506 1380 / 19506

Nature 2010,

464, 115

Cu-MFI i) 723 K in O2; ii) 448 K in CH4;

iii) RT in 1:1 H2O/CH3CN

27 8.9 98

/ JACS 2005, 127,

1394 Cu-MOR 20 11.3 /

Cu-MOR i) 723 K in O2; ii) 473 K in CH4;

iii) RT in H2O 19 13 98 /

Chem. Common.

2012, 48, 404

Fe-MFI 548 K in CH4-N2O-H2O 2 h 270 95 62 135 J. Catal. 2014,

318, 14

Cu-CHA i) 723 K in O2; ii) 473 K in CH4;

iii) 473 K in H2O

60 31 95

/ Chem. Common.

2015, 51, 4447 Cu-AEI 90 36 /

Cu-MOR i) 723 K in He; ii) 473 K in CH4;

iii) 408 K in H2O 310 160 80 /

Nat. Commun.

2015, 6, 7546

Cu-CHA

483 K in CH4-O2-H2O

/ / / / 6.1 ACS Cent. Sci.

2016, 2, 424 Cu-Na-MFI 108 h 238 88 70.6 2.2

Cu-H-MFI 288 h 1498 242 / 5.2

Cu-MOR i) 723 K in He; ii) 473 K in CH4;

iii) 408 K in H2O 350 151 96 /

Chem. Common.

2016, 52, 2553

Cu-CHA 573 K in CH4-N2O-H2O / / / 12.5 98 Chem. Common.

2016, 52, 13401 543 K in CH4-N2O-H2O 23 h 1288 644 15.8 56

Cu-MOR i)723 K in He; ii) 473 K in CH4

(36 bar); iii) 473 K in H2O 140 103.3 / /

Angew. Chem.

2016, 128, 5557

Cu-MOR i) 673 K in He; ii) 473 K in CH4

(7 bar); iii) 473 K in H2O (7 bar) 204 / 97 /

Science 2017,

356, 523

Cu-MOR

i) 723 K in O2; ii) 423 K in CH4;

iii) 408 K in H2O 215 67 / /

Chem. Common.

2017, 53, 4116 i) 873 K in N2O; ii) 423 K in CH4;

iii) 408 K in H2O 311 97 / /


iii) 473 K in H2O 200 125 90 /

JACS 2017, 139,

14961


iii) 473 K in H2O 50 28.1 95 /

ACS Catal.

2017, 7, 4291

36

Cu-NU

-1000

i) 473 K in O2; ii) 423 K in CH4;

iii) 408 K in H2O 11 17.7 45 /

JACS 2017, 139,

10294

Cu-MOR i) 673 K in O2; ii) 473 K in CH4

(7 bar); iii) 473 K in H2O (7 bar) 316 59 98 /

Angew. Chem.

2018, 57, 8906


iii) 408 K in H2O 470 170 89 /

JACS 2018, 140,

15270

Cu/SiO2 i) 1073 K in O2; ii) 473 K in CH4;

iii) 478 K in H2O 36.8 11.5 / /

ACS Catal.

2018, 8, 5721

Cu-MOF

-808

i) 423 K in N2O; ii) 423 K in CH4;

iii) 423 K in H2O 23 71.8 / /

JACS 2018, 140,

18208


iii) 473 K in H2O 420 155 90 /

ACS Catal.

2019, 9, 5308

Cu-CHA 543 K in CH4-O2-H2O 20 h 520 156 53 26 JACS 2019, 141,

11641

Cu/Al2O3 i) 973 K in air; ii) 473 K in CH4;

iii) 298 K in H2O 110 8.3 83 /

Angew. Chem.

2019, 131, 9946

Cu-CHA 573 K in CH4-H2O-O2 500 h 271000 97260 91 542 This work

a: mmolCH3OH/molMe; b: µmolCH3OH/gcat;

c: mmolCH3OH/molMe/h

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Water-Involved Methane Selective Catalytic Oxidation by