H2O-Built Proton Transfer Bridge Enhances Continuous Methane Oxidation to Methanol over Cu-BEA Zeolite

Article abstract of DOI:10.1002/anie.202105167

Direct oxidation of methane to methanol (DMTM) is a big challenge in C₁ chemistry. We present a continuous N₂O-DMTM investigation by simultaneously introducing 10 vol % H₂O into the reaction system over Cu-BEA zeolites. Combining a D₂O isotopic tracer technique and ab initio molecular dynamics (AIMD) simulation, we for the first time demonstrate that the H₂O molecules can participate in the reaction through a proton transfer route, wherein the H₂O molecules can build a high-speed proton transfer bridge between the generated moieties of CH₃^? and OH^? over the evolved mono(μ-oxo) dicopper ([Cu-O-Cu]²⁺) active site, thereby pronouncedly boosting the CH₃OH selectivity (3.1→71.6 %), productivity (16.8→242.9 μmol g_cat^?1 h^?1) and long-term reaction stability (10→70 h) relative to the scenario of absence of H₂O. Unravelling the proton transfer of H₂O over the dicopper [Cu-O-Cu]²⁺ site would substantially contribute to highly efficient catalyst designs for the continuous DMTM.

Full text of DOI:10.1002/anie.202105167

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition Chemie
www.angewandte.org
Accepted Article
Title: H2O-Built Proton Transfer Bridge Pronouncedly Enhances
Continuous Methane Oxidation to Methanol over Cu-BEA Zeolite
Authors: Ruinian Xu, Ning Liu, Chengna Dai, Yan Li, Jie Zhang, Bin
Wu, Gangqiang Yu, and Biaohua Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202105167
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Angewandte Chemie International Edition
10.1002/anie.202105167
RESEARCH ARTICLE
H
2
O-Built Proton Transfer Bridge Enhances Continuous Methane
Oxidation to Methanol over Cu-BEA Zeolite
[
a]
[a]
[a]
[c]
[c]
[a]
Ruinian Xu, Ning Liu,* Chengna Dai, Yan Li, Jie Zhang, Bin Wu,
[
a]
[a],[b]
Gangqiang Yu, Biaohua Chen
[
a]
Dr. R. Xu, Prof. N. Liu, Prof. C. Dai, Dr. B. Wu, Dr. G. Yu, Prof. B. Chen
Faculty of Environment and Life
Beijing University of Technology
Beijing, 100124, PR China
E-mail: liuning@bjut.edu.cn
Prof. B. Chen
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering
Changzhou University
Changzhou, 213164, PR China
Dr. Y. Li, Prof. J. Zhang
[
[
b]
c]
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
Beijing, 100029, PR China
Supporting information for this article is given via a link at the end of the document.
3
+ [5a]
2+ [5b-
Abstract: Direct oxidation of methane to methanol (DMTM)
constitutes a big challenge in C chemistry. Herein, we present a
continuous N O-DMTM investigation by simultaneously introducing
0 vol% H O into the reaction system over Cu-BEA zeolites.
Combining a D O isotopic tracer technique and ab initio molecular
dynamics (AIMD) simulation, we for the first time demonstrate that the
O molecules can participate in the reaction through a proton
diverse active site motifs such as [Cu
3
(μ-O
3 2
)] , [Cu (μ-O)] ,
1
g]
)]2+[5h] and [CuO] . Two types of reaction modes of
+ [5i]
[
Cu (μ-O
2 2
2
1
2
stepwise-[5b, 5j, 6] and continuous-DMTM[5k, 7] have been reported.
2
Much higher CH OH selectivity[
3-4, 5b, 8]
can be achieved by the
3
H
2
stepwise reaction mode owing to the spatial separation of the
oxidant activation and CH oxidation steps; however, the
laboriousness of the method, and the extremely low CH OH
transfer route, wherein the H
2
O molecules can build a high-speed
-
proton transfer bridge between the generated moieties of CH
3
and
-
2+
4
OH over the evolved mono(μ-oxo) dicopper ([Cu-O-Cu] ) active site,
thereby pronouncedly boosting the CH OH selectivity (3.1 → 71.6%),
3
3
-
1
-1
productivity (16.8 → 242.9 μmol gcat h ) and long-term reaction
stability (10 → 70 h) relative to the scenario of absence of H O.
2
production capability greatly hinder its practical application.
Conversely, continuous DMTM has gradually attracted interesting
2+
2
Unravelling the proton transfer of H O over the dicopper [Cu-O-Cu]
site would substantially contribute to highly efficient catalyst designs
for the continuous DMTM.
attentions[5k, 7, 9] due to the much higher CH
OH productivity and
3
ease of operation. Román-Leshkov and coworkers performed
Introduction
pioneering work on continuous DMTM,[5k] reporting a sustained
Methane (CH
possesses huge deposit on earth.[1] The transformation of CH
into a valuable liquid chemical product, such as CH OH, is greatly
4
) being of the main component of natural gas
-1
catalytic activity (~288 h, producing 491 μmol gcat CH
3
OH) over
OH
4
Cu-ZSM-5 zeolite by using O
productivity, oxidants other than O
as the oxidant. To improve CH
2
3
O,[7a-d] and H
2 2 2
, such as N O
[7e]
3
2
appealing due to the ease of transportation and storage.[2]
However, the current industrial method (syngas route) is energy
intensive and requires high temperatures and pressures.[3]
Inspired by methane monooxygenases (MMOs) that can directly
have also been reported for continuous DMTM because they
readily produce more active oxygen species. Although many
studies have been reported on DMTM, it remains challenging to
3
obtain high CH OH productivity and selectivity, especially for
oxidize CH
of methane to methanol (DMTM) has attracted great attention[2-4]
especially using Cu-modified zeolites (Cu-Z)[5] as catalysts with
4
into CH
3
OH at room temperature, the direct oxidation
continuous DMTM which remains largely unexplored[9] and
,
necessitates a fundamental understanding of efficient catalyst
design and process optimization.
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
a
b100
9
6.5
2
2
1
1
50
00
50
00
242.9
N O+CH
2
4
N O+CH +H O
80
2
4
2
7
1.6
CO2
6
4
0
0
C2H4
1
18.9
CH OH
3
CH3OCH3
Coke
8
4.3
5
0
0
20
0
1
3.5
1
0.7
1
4.0
15.2
16.8
13.1
5
.5
3.4
3.1
0
.8
0.4
%
%
%
0
.14
Cu-BEA-
0.38
Cu-BEA-
0.60
3%
Cu-BEA-0.6%-H O
Cu-BEA-0.6%
2
Cu-BEA-
Cu-BEA-
c
71.6%
3.1%
N
2
O+CH
4
2 4 2
N O+CH +H O
H
C
O
Si Al Cu
Figure 1. Activity measurement results and schematic diagram of H
2
O-built proton transfer bridge during N
2
O-DMTM over Cu-BEA zeolites. a) CH
3
OH productivities
of Cu-BEA samples with diverse Cu loadings of 0.14, 0.38, 0.60 and 3.0 wt.% during N
2
O-DMTM in presence (red) and absence (black) of H
2
O (derived from 6 h’s
2 4 2
test of Figure S1a); reaction conditions: N O : CH : H
O : He = 30 : 15 : 10 (0) : 45 (55), GHSV = 12,000 h , T = 320 oC; b) Product selectivity of Cu-BEA-0.6%
-1
o
after long-term testing (70 h, Figure S2) in presence and absence of H
the thermogravimetric analysis (TGA) of Figures S3a-S3c). Schematic diagram of H
outstandingly favoring CH OH production.
2
O (denoted as Cu-BEA-0.6%-H
2
O and Cu-BEA-0.6%, respectively) at T = 320 C based on
O-built proton transfer bridge over the dicopper [Cu] --[Cu]+ site of Cu-BEA
+
2
3
stability (passing through 70 h’s test, accumulating ~16,000 μmol
-
1
The H
2
O has been widely accepted playing a vital role during
g
cat CH
3
OH, Figure S2) can be achieved over the best performed
sample of Cu-BEA-0.6%. Combining the D O isotopic tracer
technique and ab initio molecular dynamics (AIMD), we
demonstrate that H O can directly participate in the reaction
through a proton transfer route, wherein the H O molecules can
build a high-speed proton transfer bridge between the generated
DMTM, which however has long been recognized as a solvent to
OH,[4a, 4b, 6] until recently van Bokhoven et al.[5b]
2
sweep out CH
3
made a breakthrough demonstrating that H
2
O can directly
2
participate in the DMTM reaction to regenerate the active site of
Cu-MOR. This finding has aroused great research attention to
2
explore the specific effect of H
2
O during DMTM. For example, Liu
O can adjust the interfacial reaction
OH selectivity during DMTM
O. Inspired by these reports, we herein present a
O-DMTM investigation by simultaneously
moieties of CH
3
- and OH- over the evolved dicopper active site
et al.[10] newly reported that H
([Cu-O-Cu] ) (Figure 1c), thereby significantly promoting CH OH
2+
2
3
pathway to significantly promote CH
over CeO -Cu
continuous
introducing 10 vol% H
3
generation / desorption whilst extensively hindering the carbon
deposition. To the best of our knowledge, this is the first report
2
2
N
2
finding the proton-transfer reaction of H
2
O during DMTM.
2
O into the reaction system over Cu-BEA
Unravelling the correlation between H O-proton-transfer and
2
o
dicopper [Cu-O-Cu]+ active structure motif can substantially
contribute to other highly efficient catalyst designs for the DMTM.
zeolites at a moderate temperature (T = 320 C). Pronouncedly
-1
-1
enhanced CH
3
OH productivity (16.8 → 242.9 μmol gcat h ,
Figure 1a), selectivity (3.1 → 71.6%, Figure 1b) and long-term
2
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Angewandte Chemie International Edition
10.1002/anie.202105167
RESEARCH ARTICLE
Results and Discussion
Characterization of active site motif of Cu-BEA samples.
a
d
g
b
c
Cu 2P3/2
CuO
CuO
Cu 2P
Cu-BEA-0.6%
Cu-BEA-0.38%
Cu2+ Cations
440 nm
Cu-BEA-3%
Cu-BEA-3%
Cu-BEA-0.6%
Cu-BEA-0.38%
Cu-BEA-0.6%
Cu-BEA-0.14%
Standard CuO
Cu-BEA-0.38%
Cu-BEA-0.14%
Cu-BEA-0.14%
H-BEA
´1/3
200
300
400
500
600
1
00 200 300 400 500 600 700 800 950
945
940
935
930
o
Bingind Energy (eV)
Wavelength (nm)
T ( C)
Exp
e
f
Best Fit
SS-Ofw+ef
1
2
3
SS-Tfw
dCu-Cu = 3.16 Å
SS-Cuef
3
-Third shell
-Second shell
-First shell
3
.16
2
1
2
.75 3.00 3.25 3.50
R (Å)
Exp
Best Fit
1
.0 1.5 2.0 2.5 3.0 3.5
1.0 1.5 2.0 2.5 3.0 3.5
R (Å)
R (Å)
h
Cu-BEA-0.6%
Cu-BEA-0.6%-N2O
EPR Estimated Cu2+ ratios (%)
1
00%
-1
-
2
76.2%
-3
( e V )
G
-4
A =146.768
D
//
-5
g =2.364
/
/
-
6
23.8%
-
7
0
.0
-
1.0
-0.2
-
0.8
Dm-
-0.4
0.6
-0.6 O(eV)
-0.4
N2
-0.8
Dm H 2
O (eV) -0.2
0
.0-1.0
2
600 2800 3000 3200 3400 3600
Z-Cu2
Z-Cu-OH Z-Cu
Z-Cu-O
Z-Cu-O-Cu
Field (G)
Figure 2. Physicochemical property characterizations. a) H
2
-TPR (with standard CuO as a reference), b) XPS spectra of Cu 2p for the Cu-BEA-0.14, 0.38, 0.6 and
o
3%, c) UV-vis spectra of Cu-BEA-0.14, 0.38 and 0.6% (pretreated by N
2
O at T = 250 C) and H-BEA (taking for comparison), d) magnitude and f) imaginary part of
o
3
the FT-EXAFS spectra for the N
2
O-pretrated Cu-BEA-0.6% (T = 250 C) obtained by Fourier transforming the k χ(k) curves of Figure S5b in 2.4 - 10.8 Å, e) optimized
Cu-BEA models with a dicopper [Cu-O-Cu]+ active site (dcu-cu = 3.16 Å) being utilized for the EXAFS data fitting; g) EPR spectra of Cu-BEA-0.6% samples being
o
o
2+
2
respectively pretreated and non-pretreated by N O (T = 250 C) at a low temperature (T = -118 C); the occupation ratio of [Cu-O-Cu] was estimated based on the
Spincount calibrated module included in the Xenon software (BRUKER, Germany); as noted, 250 oC is an optimum pretreatment temperature to generate the
highest amounts of [Cu-O-Cu]2+, as illustrated by the UV-vis spectra in Figure S6, h) formation Gibbs free energies (∆G) as functions of ∆μN2O and ∆μH2O (chemical
potential) for the diverse active sites of [Cu] , [Cu-O] , [CuOH] , [Cu] --[Cu] and [Cu-O-Cu]2+ based on the ab initio thermodynamics (AIT) by DFT.
+
+
+
+
+
Cu-BEA samples with diverse Cu loadings were prepared by
conventional wet ion exchange (0.14, 0.38 and 0.6 wt.%) and
incipient-wetness impregnation (3.0 wt.%) approaches based on
low Cu loadings (0.14, 0.38 and 0.6 wt.%); whereas large
-1
amounts of CuO
x
species (357.6 µmol g , Figure S4) emerged
over Cu-BEA-3.0%.[11] Moreover, it is interesting to note that the
the commercial H-BEA (Si/Al = 12.5). The H
2
-TPR (Figure 2a)
mono(µ-O) dicopper [Cu-O-Cu]2+ can be formed after the N
O
2
combined with XPS (Figure 2b) reveals that the Cu2+ cations
constitute the major copper species on the Cu-BEA samples with
pretreatment (T = 250 C) over the Cu-BEA-0.6% by displaying a
o
characteristic UV-vis peak at 440 nm in Figure 2c.[12] Such [Cu-O-
3
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Cu]2+ structure can be further verified by the H
-TPR of N
O
2
2
2
AIMD; ii) the lowest N O-DMTM activity of Cu-BEA-3.0% is
pretreated Cu-BEA-0.6% (Figure S7a), in-situ FTIR of adsorbed
majorly related to the ready overoxidation of CH
largely formed CuO species (Figure S1e).
2
O-proton-transfer reaction uncovered by D O isotopic
4 2
into CO by the
CO (Figure S7b) and extended X-ray adsorption fine structure
x
(
EXAFS) (Figures 2d-2f). As noted, the Cu-Cu distance (dCu-Cu
=
H
2
3.16 Å, Figure 2d) derived from the Cu--Cu single shell (SS) fitting
tracer technique. During the 70 h long-term stability test (Figure
is in good agreement with the DFT (density functional theory)
optimized model (Figure 2e). This finding indicates that along with
the increase in Cu loading (0.14 → 0.6 wt.%), the neighboring
dicopper sites of [Cu] --[Cu]+ could be gradually formed, which
S2), we surprisingly found that the Cu-BEA-0.6% exhibited an
ultrahigh durability after introducing H
which however can be quickly (~10 h) deactivated for the scenario
of absence of H O. This finding is totally contrary to N O direct
dissociation reaction system, wherein the H O addition can
2
O into the reaction system,
+
2
2
can be further evolved into [Cu-O-Cu]2+ after interaction with N
O.
2
2
As noted, the monomeric [Cu]2+ cations also contribute to the
broad peak around [600-400 nm] (see Supporting Information of
Characterizations of Cu species on Cu-BEA-0.6%); and the
severely deactivate the zeolite catalysts (Figure S8) due to its
strong competitive adsorption effect that can readily transform the
2
+
- [14]
metal cation ([M] ) into an inert state ([M-OH] ). This finding
gives us a hint that the H O probably participates in some other
reaction routes rather than deactivating the catalyst sample to
dissociate into αO during O-DMTM. Moreover, as
illustrated in Figure 1b, the H O can significantly promote CH OH
+
+
existence of dicopper [Cu] --[Cu] sites cannot be totally ruled out
for the Cu-BEA-0.14% and 0.38% samples, due to the detection
limit of UV-vis. Taking the advantage of that the [Cu-O-Cu]2+
species are silent in electron paramagnetic resonance (EPR), the
2
N
2
O
N
2
2
3
related occupation ratio was further estimated by EPR (Figure 2g), selectivity (3.1 → 71.6%) and simultaneously extensively reduce
which indicates that nearly 23.8% of Cu2+ participates in the
formation of [Cu-O-Cu]2+ site, being similar to the value of the
literature report.[13] To further evaluate the thermodynamic
stabilities of the evolved dicopper sites under the reaction
condition of our present work, the ab initio thermodynamics (AIT)
analyses were conducted (Figure 1h). Obviously, the dicopper
the carbon deposition (96.5 → 13.5%). This finding implies that
O most likely transforms the deposited carbon species into
CH OH products in the N O-DMTM, although the solvent effect of
H
2
3
2
H
2
O cannot be completely eradicated.
2
To explore the specific reaction mechanism of H O during
N
2
O-DMTM,
a
temperature programmed surface reaction
O isotopic tracer experiment was further
sites ([Cu-O-Cu]2+ and [Cu] --[Cu] ) exhibit much higher
+
+
(TPRSR) based D
2
thermodynamic stability than those of the monomeric Cu sites;
conducted over the Cu-BEA-0.6% (Figures 3a-3d). As shown in
Figure 3a, along with the increasing of the temperature, the m/z
+
+
and the [Cu] --[Cu] is thermodynamically prone to evolve into
Cu-O-Cu]2+ due to much higher thermodynamic stability of [Cu-
[
signals of 19 (HOD) and 33 (CH
3
OD) gradually emerged (T > 300
O exactly can
2
+
[13]
oC), which gives us a solid evidence that the H
O-Cu] , which is also consistent with the literature report.
2
Combining the above characterization results (Figures 2a-2h)
with the activity measurement results (Figure 1a), we can deduce
participate in N
2
O-DMTM reaction. Moreover, it is interestingly
+
found that the signal of 22 (D
3
O ) gradually grew up along with
that i) the highest H
2
O-mediated N
2
O-DMTM activity of Cu-BEA-
the growing of 33 (CH
3
OD) and declining of 20 (D
O can participate in N
proton transfer route
2
O) at T = 300 -
+
+
o
0.6% can be closely related to the generated dicopper [Cu] --[Cu]
375 C. This finding indicates that the D
2
2
O-
site that possesses a much higher activity than that of the
DMTM
reaction
through
a
+
D O
3
+
monomeric [Cu] acting as the active site on the low-Cu-loaded
(
CH +N O+2D O

CH OD+HOD+N +D O).
3 2 2
Proton trasnfer
4
2
2
samples; this will be further discussed later based on the DFT and
4
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Angewandte Chemie International Edition
10.1002/anie.202105167
RESEARCH ARTICLE
a
a
b
b
the contributions of these signals have been totally subtracted
from Figure 3a. Further comparing the peak intensities of the
TPSR of D
2
O-DMTM
TPSR Without D O
2
1
9 HOD
1
9 HOD
0 D O
2
0 D O
signals of 31 (CH
3b (see Figures 3c-3d), one can find that the D
O-DMTM majorly following the D O-mediated reaction route to
generate much a higher amount of CH OD. Therefore, the
significantly promoted H O-mediated CH OH productivity of Cu-
BEA-0.6% (Figure 2a) can be closely related to the direct reaction
involvement of H O during the N O-DMTM, especially for the
O being firstly observed in present
OH) and 33 (CH
OD) derived from Figures 3a-
2
3
3
2
2
3
1 CH OH
31 CH3OH
3
2
O would lead to
200
300
400
500
600
+
N
2
2
3
3 CH OD
22 D O
3
3
3
2
3 CH OD
3
3
+
2 D O
3
2
00 300 400 500 600 700 800
200 250 300 350 400 450 500 550 600
o
o
T ( C)
T ( C)
c
c
d
d
2
3
CH3OD (33)-D2O
CH3OD (33)
CH3OH (31)-D2O
CH3OH (31)
2
2
proton-transfer reaction of H
2
work.
O
+D 2
H4
CH
3
OH (31)
2O+C
2
Mechanistic insight into H O-proton-transfer reaction
CH
H4
N
2
00 250 300 350 400 450 500 550 600
3
OD (33)
2O+C
o
T ( C)
N
based on in-situ FTIR and ab-initio (DFT and AIMD)
simulations. To shed more light on the reaction mechanism, the
in-situ FTIR was further conducted over Cu-BEA-0.6% by
simultaneously introducing N O (2vol%), CH (2vol%) (balanced
Figure 3. D
reaction based on mass spectrometry (TPSR-MS). TPSR-MS of a) D
mediated N O-DMTM (denoted as D O-DMTM) and b) N O-DMTM without D
addition displaying the m/z signals of 19 (HOD), 20 (D
CH OH) and 33 (CH OD) in the logarithmic coordinates; other MS signals of
6 (CH ), 28(N ) and 44 (N
2
O isotopic tracer result of temperature programmed surface
2
O-
2
2
2
2
O
+
2
3
O), 22 (D O) , 31
2
4
(
3
3
by He) and H
2
O (bubbled by the mixed flow of 40 mL min-1 at T =
1
4
2
2
O) are depicted in Figures S9a-S9b; for better
o
2
5 C) into a microreactor. As can be seen (Figure S11a-S11b),
+
comparison, the signals of 33 (CH
3
OD) and 22 (D
3
O ) were further depicted as
the bands at 2852 and 2964 cm-1 belonging to the v(CH
) of
3
linear coordinates, as inserted in panel (a). Comparison of the signals of c)
CH OD (m/z = 33) and d) CH OH (m/z = 31) detected in panels (a) and (b),
being respectively represented by the circle / triangle and dash lines. TPSR
conditions: 2 vol% N O, 2vol% CH balanced by He with a total flow rate of 40
O was bubbled into the reaction system (T= 25 oC) by
o
CH
3
OH[5c, 15] gradually emerged as T > 250 C indicating that the
3
3
started at T > 250 oC. Noteworthily, being
4
activation of CH
2
4
different from other literature reports,[5b, 5c, 15] no obvious bands
-
1
mL min , wherein the D
the mixed flow.
2
belonging to the v(CH
3
) of the methoxy groups (CH
3
O) (2980,
-1
2
869, 2823 cm ) can be detected, which only displays one band
2
To make a distinguishment, the TPSR with a single D O
at 2923 cm-1 [v(CH
3
)]. The in-situ FTIR with Cu-BEA-0.6% being
-1
o
feeding (bubbled by He of 40 mL min , T = 25 C) was further
o
initially pretreated by N
to CH
2
O (T = 250 C) and subsequently exposed
conducted over Cu-BEA-0.6% (Figure S10a) by displaying
+
4
was further conducted to monitor the v(OH) stretching
marginal variations for the signal of 22 (D
3
O ), which can safely
vibration (Figure S11c), which clearly displays a band at 3675 cm-
+
verify that the detected signal of 22 (D
generated from the H O-mediated N O-DMTM reaction (following
the proton-transfer-reaction route) rather than intermolecular
reactions of D O. Additionally, it should also be noted that due to
the small isotopic abundance of the atomic D in the utilized CH
gas (99.999%), which can be oxidized into D O (m/z = 20) to
3
O) in Figure 3a is mainly
1
being corresponding to v(OH) at the α-site.[15] Combining these
2
2
findings together, we can deduce that the H
DMTM would follow the radical mechanism during CH
step, wherein the CH can be initially oxidized into the radicals of
CH - and OH- over the active site. In this regard, [Cu-CH
2
O-mediated N
2
O-
4
activation
2
4
4
]+--[Cu-
3
3
2
+
2+
OH] would be formed over the evolved dicopper [Cu-O-Cu] site
further participate in the DMTM reaction producing the signals of
(
see Eqs. 1-2); similar findings concerning the generation of CH -
3
+
CH
3
OD (33), HOD (19) and D
3
O (22) as detected in Figure 3b,
5
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
-N
2
a
+CH
4
b
II
III
V
B. CH
4
Activation
- and OH-
forming CH
3
TS-I
TS-II
TS-III
TS-IV
VII
0
TS-I
.91
I
II
0
III 0.64 IV
TS-II
I
-10
-20
-30
-40
IV
A. N
2
O Dissociation
H2O Proton Transfer Route
+N
2
O
C. H
2
O Proton Transfer
I→II:
N2O Dissociation
VI
III→IV: CH4 Activation
V→VI: H3O+ and CH3OH
foramtion
+2
H
+H
2
O
2
O
c
VI→VII: H2O reforming and
CH3OH desorption
VIII
V
0.03 VI 0.05 VII
TS-III
TS-IV
V
-
CH
-
3
H
OH
V
Dicopper [Cu] --[Cu]+
+
2
O
H
C
O
Si Al Cu
VI
TS-III
VII
+
VII
iv Product desorption
i 3
C H O formation
C
TS-IV
O reforming
VI
C
iii
H
2
Cii CH OH formation
3
2+
DFT and AIMD Metadynamics Simulations over Dicopper [Cu--Cu]
H O Proton Transfer Route
2
+
Figure 4. DFT and AIMD simulation results of the H
simulated elementary steps, including N
reforming, CH
2
O-proton-transfer-route over Cu-BEA with a dicopper [Cu] --[Cu]+ site. a) DFT (I→IV) and AIMD (V→VII)
+
2
O dissociation (I→II), CH
4
activation (III→IV) and proton transfer steps of H
3
O , CH
3
OH formation (V→VI) and H
2
O
o
3
OH desorption (VI→VII) steps. b, c, Inserted panel represents the reaction Gibbs free energy diagram (T = 320 C) along with the reaction coordinate.
Metadynamics-computed free energy surface of b) 2D and c) 3D views based on AIMD. The minimal free energy reaction route is shown by gray balls connected
by a gray dash line in panel (b) and the contour lines are separated by 0.054 eV intervals.
-
and OH radicals over the dicopper [Cu-O-Cu]2+ site of Cu/Na-
An energetic diagram with the basic elementary steps of Eqs.
1-2 being simulated by DFT and H O-proton transfer steps (Eqs.
3-4) being simulated by AIMD was also inserted in Figure 4a. As
ZSM-5 have been reported;[5f] whereas the [CH
-Cu-OH] would
+
3
2
+
be formed over the monomeric [Cu] site, wherein the CH
3
OH can
be
generated following the
well-reported “rebound
can be seen, the N
2
O can initially interact with the dicopper [Cu]+-
mechanism”.[5i, 16] As for the further reaction of [Cu-CH
]+--[Cu-
-[Cu]+ site to generate αO ([Cu-O-Cu] ) by overcoming a
2+
3
+
OH] , we herein propose a H
2
O proton transfer mechanism based
relatively low Gibbs free energy barrier (∆G) of 0.91 eV (TS-I, T =
on the AIMD-based metadynamics simulations (Figure 4a and
320 oC). Subsequently, CH
4
can be readily oxidized into the
+
+
+
Movie S1), including the H
2
O proton transfer and H
3
O -mediated
radicals of [Cu-CH
3
] and [Cu-OH] (TS-II, ∆G = 0.64 eV). Taking
DMTM steps (Eqs. 3-4).
the advantage of the distance between [Cu-CH
(dCu-Cu = 4.81 Å, Figure S12a), two H O molecules can build a
high-speed proton transfer bridge connecting the generated
]+ and [Cu-OH]+
3
+
+
N
2
O(g)+[Cu] --[Cu] -Z→[Cu-O-Cu]-Z+N
2
(g)
O dissociation step) (1)
(g)→ [Cu-CH ]-[Cu-OH]-Z
CH activation step) (2)
O + OD-
Proton Transfer of H
O +OD →[Cu-CH OD]-[Cu-HOD]-Z+D
2
(N
2
[
Cu-O-Cu]-Z+CH
4
3
moieties of CH
3
- and OH- (Model V): i) the H
2
O molecule being
(
4
close to the [Cu-CH
3
]+ can be readily dissociated into the
+
-
+
-
2
D
2
O(g)→D
3
fragments of OH and H , wherein the OH can interact with [Cu-
]+ producing [Cu-CH
OH] and the H can quickly transfer to
+
+
(
2
O) (3)
CH
the neighboring H
that, the proton of the second H
3
3
+
-
+
[
Cu-CH
3
]-[Cu-OH]-Z+D
3
3
2
O
2
O forming H
3
O (TS-III, ∆G = 0.03 eV); ii) after
+
(
H
3
O -mediated DMTM) (4)
2
O molecule can migrate from the
6
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
+
H
3
O+ to [Cu-OH] regenerating H
2
O (TS-IV, ∆G = 0.05 eV); iii)
OH can be barrierlessly desorbed from
the active site to complete the whole catalytic cycle. Obviously,
the H O-built proton transfer bridge greatly favors CH OH
barrier [0.16 (TS-IV) and 0.53 (TS-V) eV, Figure S14] to a certain
finally, the product of CH
3
degree relative to the H
2
O-absence route [0.78 eV (TS-III), Figure
O-proton-transfer
S13], it is still much less efficient than the H
2
2
3
route [0.03 (TS-III) and 0.05 (TS-IV) eV, Figure 4a]. This finding
can also be quantitatively validated by the microkinetic modeling
production and desorption through overcoming two extremely low
free energy barriers of 0.03 and 0.05 eV, being respectively
corresponding to the proton transfer rates of 6.87×1012 and 4.64
2
revealing that the reaction rate of H O-proton-transfer-route
displays 8 orders of magnitude higher than that of the H
2
O-direct-
×
1012 s-1 (T = 320 oC). In addition to that, the metadynamics
computed free energy surface (FES) of the H O proton transfer
route (V→VII), being projected along the coordination numbers
CN) of [CNaH-O(H2O)] and [CNH-OH-Cu] (respectively corresponding
to the H-O bond of the first H O molecule and eventually reformed
O, Figure S12a), was further depicted in Figures 4b, 4c. As can
reaction route (8.58 ×103 versus 8.35 ×10-5 s-1, T = 320 oC).
2
Therefore, the H
2
O-mediated N
2
O-DMTM reaction would majorly
+
follow the H
2
O-proton-transfer-route over the dicopper [Cu] --
+
(
[Cu] site.
+
+
+
2
In addition to the dicopper [Cu] --[Cu] , the monomeric [Cu]
has also been detected based on the experimental
characterizations. In light of that, the related N O-DMTM reaction
mechanism was also simulated by DFT by taking both the
scenarios of absence and presence of H O into account (see
H
2
be seen, after crossing two small hills (TS-III and TS-IV), the
reaction system can readily reach the final minimum energy state
2
(
VII) which possesses the lowest free energy (G = -0.16 eV). This
finding reveals that the H O-built proton transfer bridge not only
kinetically favors CH OH production, but also thermodynamically
renders the reaction toward the CH OH formation direction. As
noted, H O being of one type of strong absorbent, the AIT
2
2
Figures S15-S16) to illustrate the diverse catalytic behaviors of
the monomeric [Cu]+ and dicopper [Cu]--[Cu] sites. It can be
unraveled that two major factors leading to much lower reaction
3
3
+
+
+
2
efficiency of monomeric [Cu] than that of dicopper [Cu] --[Cu] : i)
the monomeric [Cu]+ needs overcoming much higher N
analyses have also been conducted to investigate whether there
2
O
+
exists the complexations of H
CH
indicated that the H
2
O with Cu species of [Cu] , [Cu-
dissociation barrier [1.31 eV (TS-I), Figure S15] than that of
]+, [Cu-OH]+ and [Cu-O-Cu]2+ (see Figures S25-S26). It is
O complexations with Cu species can be
dicopper [Cu] --[Cu] [0.91 eV (TS-I), Figure 4a] to generate the
αO which plays a crucial role in the activation of CH ; ii) being
different from the scenario of dicopper [Cu] --[Cu] , wherein the
O can boost CH OH production through a proton-transfer
reaction, the H O majorly functions as the solvent during N O-
+
+
3
2
4
+
+
significantly limited under the reaction condition of present work
o
(
T = 320 C) due to the low thermodynamic stabilities. However,
H
2
3
it is also worth noting that there probably exists kinetic diffusion
hindrance effect of H O, which can kinetically hinder the internal
diffusions of N O and CH to the active site when the H O content
is higher than a certain degree (Figure S20a).
For comparison, two other types of N O-DMTM mechanisms
O-absence (Figure S13, following the rebound mechanism)
2
2
2
DMTM (Figure S16). The quantitative activity comparisons by
microkinetic modelings further validate much higher reaction
2
4
2
+
+
efficiency of the dicopper [Cu] --[Cu] , especially in the presence
2
of H O, which displays 2 orders of magnitude higher reaction rate
2
than that of monomeric [Cu] (8.58 ×10 versus 7.07 ×10 s-1, T
+
3
-5
of H
2
o
and H
2
O-direct-reaction mechanisms (Figure S14, H
2
O directly
= 320 C, Figure S17).
reacting with generated radicals of CH -
and OH ) were also
-
3
+
+
simulated by DFT over the dicopper [Cu] --[Cu] . As observed,
Conclusion
2
although the H O-direct-reaction route can also reduce the energy
7
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RESEARCH ARTICLE
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Acknowledgements
We acknowledge the final support from the National Natural
Science Foundation of China (No. 21906003, Major Program
M. H. Mahyuddin, A. Staykov, Y. Shiota, K. Yoshizawa, ACS Catal.
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Polytechnic University for the helpful discussion on AIMD
simulations; and we also acknowledge Dr. M. Wang from Peking
University for the helpful discussions on EXAFS data fitting and
2
D O isotopic tracer data analysis.
Keywords: Direct oxidation of methane to methanol • Proton
transfer • N O • Zeolite • Ab initio calculations
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RESEARCH ARTICLE
Entry for the Table of Contents
The H
2
O can directly participated in the continuous N
2
O-DMTM reaction through a proton-transfer-bridge over the dicopper [Cu-O-
Cu] site of Cu-BEA zeolite, which can significantly boost CH
3
OH production rate and enhance long-term reaction stability.
9
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