Continuous Partial Oxidation of Methane to Methanol Catalyzed by Diffusion-Paired Copper Dimers in Copper-Exchanged Zeolites

Article abstract of DOI:10.1021/jacs.9b04906

Copper-exchanged zeolites can continuously and selectively catalyze the partial oxidation of methane to methanol using only oxygen and water at low temperatures, but the genesis and nature of the active sites are currently unknown. Herein, we demonstrate that this reaction is catalyzed by a [Cu-O-Cu]²⁺ motif that forms via a hypothesized proton-aided diffusion of hydrated Cu ions within the cages of SSZ-13 zeolites. While various Cu configurations may be present and active for methane oxidation, a dimeric Cu motif is the primary active site for selective partial methane oxidation. Mechanistically, CH₄ activation proceeds via rate-determining C-H scission to form a surface-bound C₁ intermediate that can either be desorbed as methanol in the presence of H₂O/H⁺ or completely oxidized to CO₂ by gas-phase O₂. High partial oxidation selectivity can be obtained with (i) high methane and water partial pressures and (ii) maximizing Cu dimer formation by using zeolites with high Al content and low Cu loadings.

Full text of DOI:10.1021/jacs.9b04906

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Article
Continuous Partial Oxidation of Methane to Methanol Catalyzed
by Diffusion-Paired Cu Dimers in Copper-Exchanged Zeolites
Kimberly T. Dinh, Mark M. Sullivan, Karthik Narsimhan, Pedro
Serna, Randall J. Meyer, Mircea Dinca, and Yuriy Roman-Leshkov
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b04906 • Publication Date (Web): 21 Jun 2019
Downloaded from http://pubs.acs.org on June 22, 2019
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Continuous Partial Oxidation of Methane to Methanol Catalyzed by
Diffusion-Paired Cu Dimers in Copper-Exchanged Zeolites
Kimberly T. Dinh†§, Mark M. Sullivan†§, Karthik Narsimhan†, Pedro Serna±, Randall J. Meyer±,
Mircea Dincă‡, Yuriy Román-Leshkov†*
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†Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cam-
bridge, Massachusetts 02139, United States
±ExxonMobil Research and Engineering, Annandale, New Jersey 08801, United States
‡Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massa-
chusetts 02139, United States
ABSTRACT: Copper-exchanged zeolites can continuously and selectively catalyze the partial oxidation of methane to meth-
anol using only oxygen and water at low temperatures, but the genesis and nature of the active sites are currently unknown.
Herein, we demonstrate this reaction is catalyzed by a [Cu-O-Cu]2+ motif that forms via a hypothesized proton-aided dif-
fusion of hydrated Cu ions within the cages of SSZ-13 zeolites. While various Cu configurations may be present and active
for methane oxidation, a dimeric Cu motif is the primary active site for selective partial methane oxidation. Mechanistically,
CH4 activation proceeds via rate-determining C–H scission to form a surface bound C1 intermediate that can either be
desorbed as methanol in the presence of H2O/H+ or completely oxidized to CO2 by gas phase O2. High partial oxidation
selectivity can be obtained with (i) high methane and water partial pressures and (ii) maximizing Cu dimer formation by
using zeolites with high Al content and low Cu loadings.
offer significant advantages over thermochemical cycling
processes, continuous methanol production systems re-
Introduction
main largely unexplored,29 and practical applications ne-
The selective oxidation of CH4 to value-added chemicals
remains a grand challenge in catalysis.1-3 Current industrial
cessitate fundamental understanding for catalyst and pro-
cess improvement.30 Primarily, the nature and genesis of
processes to upgrade CH4 to petrochemical precursors re-
the active sites involved in catalytic C–H activation are cur-
rently unknown.31 While similarities between the stoichio-
metric and catalytic systems may exist, it is uncertain if the
well-characterized active sites of stoichiometric oxidation
systems20, 24, 32-33 are related to those active in a catalytic
process. In fact, the notable differences between the activ-
ity and water-tolerance of the active sites in Cu-exchanged
zeolites under stoichiometric versus catalytic oxidation re-
action conditions30 suggest that the active sites for the two
processes may be entirely different from each other. This
work, for the first time, details the kinetics and active site
of Cu-exchanged zeolites for the continuous catalytic par-
tial oxidation of methane to methanol using dioxygen in
the presence of water.
quire high temperatures (>1000 K) and pressures (>40 bar)
and are only economical at large scales,4 but the increasing
availability of natural gas in remote locations necessitates
the development of smaller scale CH4 conversion processes
at mild conditions.5 Iron6 and copper7-8 metalloenzymes
selectively convert CH4 to methanol directly at ambient
conditions, but these enzymes suffer from limited temper-
ature stability for industrial operation.9-10 Similar to biolog-
ical systems, small metal clusters and single metal atoms
(Fe6, 11-12, Cu13-20, Rh21, Pd22) within zeolites are capable of
converting CH4 to methanol at mild conditions. Of partic-
ular interest are Cu-exchanged zeolites that activate CH4
using only molecular oxygen as the sole oxidant.13, 18, 23-27
Although highly selective, nearly all reported oxidation
processes using Cu-exchanged zeolites operate in a cyclic,
stoichiometric manner, requiring an anhydrous catalyst re-
activation step at high-temperatures.
Herein, advanced synthesis techniques coupled with rig-
orous kinetic analysis and in situ spectroscopic characteri-
zation served to identify the active sites responsible for the
direct, steady state catalytic conversion of CH4 to methanol
in copper-exchanged zeolites. Kinetic experiments per-
formed on zeolites with the chabazite (CHA) topology,
composed of 8 x 8 x 12 Å cages interconnected by windows
with a maximum pore diameter of 3.8 Å32, uncovered a
[Cu–O–Cu]2+ dimeric motif as the active site for selective
C–H activation. Although H2O and zeolitic protons are ki-
A standout report in this context is the recent work by
Narsimhan et al. who demonstrated the first instance of a
continuous, gas phase catalytic process for the direct con-
version of CH4 to methanol.28 The system used Cu-ex-
changed zeolites to produce methanol using exclusively
CH4, H2O, and O2 at 473 K. This process featured high se-
lectivity across a wide range of zeolite topologies, albeit at
very low conversion (0.001%). Although catalytic processes
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netically inconsequential, they are necessary for desorp- selective oxidation of CH4 to CO2. The fundamental exam-
1
2
3
4
tion of CH3OH. Cu dimers form under reaction conditions
by hydrated ionic diffusion along a protonic highway, high-
lighting the importance of zeolite H+ content. Excess Cu
(Cu/cage>0.3) resulted in nanoparticle formation and un-
ination of CH4 activation kinetics, site requirements, and
conversion pathways provides a roadmap for the optimiza-
tion of low-temperature CH4-to-methanol technologies.
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Table 1. Composition and synthesis method for SSZ-13 zeolites.
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Composition
Cu Content
(wt%)
Catalyst
Cu/cage
Synthesis Method
Si/Al
8.8
8.8
8.8
8.8
8.8
8.8
8.8
Cu/Al
Na/Al
10
11
12
13
14
15
16
17
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H-CHA
-
-
-
-
-
-
-
-
-
-
2-6MRb
Cu-CHA(0.05)
Cu-CHA(0.10)
Cu-CHA(0.12)
Cu-CHA(0.20)
Cu-CHA(0.23)
Cu-CHA(0.42)
0.04
0.08
0.10
0.16
0.19
0.34
0.41
0.76
0.90
1.6
0.05
0.10
0.12
0.20
0.23
0.42
2-6MR, Cu-IEc
2-6MR, Cu-IE
2-6MR, Cu-IE
2-6MR, Cu-IE
2-6MR, Cu-IE
2-6MR, Cu-IE
1.9
3.3
d
2-6MR, NH4 , Cu-IE
NH4-Cu-CHA(0.16)
8.8
0.13
-
1.3
0.16
Cu-CHA(0.11)
23
21
0.22
0.07
0.27
-
0.92
0.33
1.0
0.11
1-6MRe
Cu-CHA(0.04)
Na-Cu-CHA(0.12)
-
0.04
0.12
1-6MR
2-6MR-Cuf
26
0.51
Cu-CHA(0.13)
Cu-CHA(0.72)
11
0.13
-
-
1.1
0.13
1-6MR
12
0.76
5.7
0.72
1-6MR, 2x Cu-IE
1Al-Cu-CHA
0Al-Cu-CHA
76
0.23
-
0.31
1.0
0.04
F-g
IWIh
∞
aCatalyst nomenclature is defined by extraframework cations present and composition where Cu-CHA(x) denotes an SSZ-13
catalyst with x Cu atoms per 8 x 8 x 12 Å cage.
b2-6MR denotes SSZ-13 synthesis with Na in the synthesis gel, following which the zeolite was ion-exchanged twice with
NH4NO3 to remove Na
cCu-IE denotes Cu was introduced via aqueous copper ion-exchange
dNH4 denotes that following ion-exchange with NH4NO3, the zeolite was not calcined to remove NH3
e1-6MR denotes SSZ-13 synthesis without Na in the synthesis gel and the direct incorporation of Cu using TEPA
f2-6MR-Cu denotes SSZ-13 synthesis with Na present in the synthesis gel and the direct incorporation of Cu using TEPA
gF- denotes SSZ-13 synthesis using a fluoride media
hIWI denotes preparation via incipient wetness impregnation of a defect-free SSZ-13
Results and Discussion
compositional details are given in Section 1 of the Support-
ing Information and summarized in Table 1. Catalysts des-
ignated as Cu-CHA(x) are copper-exchanged SSZ-13 zeo-
lites with x Cu atoms per 8 x 8 x 12 Å cage.
1. Reaction pathway for catalytic partial CH4 oxida-
tion over a Cu active site
Investigation of active site requirements for the direct
conversion of CH4 to methanol necessitates control of Cu
ion speciation, and therefore aluminum distribution,
within the zeolite. We employed select synthetic methods
to create a suite of catalysts with varied Cu content and Al
Reaction rates were measured under kinetic control (Fig-
ures S1, S2, and S3, and Table S1). Reaction conditions were
chosen to produce CH3OH, DME, and CO2 in yields large
enough to aid in reaction pathway analysis, even though
over 95% selectivity to CH3OH can be achieved via reaction
condition and catalyst choice (Figure S4). A first-order
delplot constructed from a contact-time kinetic study of a
speciation to study the effects of Cu distribution on C–H
activation.34,
35
Synthesis, characterization, and catalyst
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Figure 1. A. Product selectivity as a function of conversion. Product formation rates as a function of B. PCH4, C. PO2, and D. PH2O
variations over Cu-CHA(0.11). rTotal=rCH3OH+2rDME+rCO2, T = 543 K, 0.25 gcat, 25 - 200 sccm total flow, PCH4 = 18 kPa, PO2 = 0.09 kPa,
PH2O = 3.14 kPa, bal He except as noted.
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representative catalyst (Cu-CHA(0.11), Figure 1A) indicates
a sequential reaction pathway in which CH4 can be first
partially oxidized to CH3OH before secondary downstream
oxidation to CO2.
of C–H scission was 97 kJ mol-1 (Figure S6). This barrier,
indicative of the energetic difference between gas phase re-
actants and the transition state due to measurement in the
PCH4 first-order regime, is in close accordance with the 99
kJ mol-1 C–H activation barrier calculated by Zhao et. al.37
over Cu active sites in mordenite zeolites and other similar
computational work.37, 40 The observed rate-determining
C–H scission is in agreement with the previously reported
primary kinetic isotope effect during steady state methane-
to-methanol reactions using CD4 over Cu-ZSM-5 cata-
lysts.28
Accordingly, the total rate of C–H activation was analyzed
as the sum of all products formed (rtotal=rCH3OH+2rDME+rCO2).
Kinetic analysis revealed a near first-order dependence of
the C–H activation rate on PCH4 (Figure 1B), implying weak
binding of CH4 to the active site. The near second-order
dependence of DME formation rates on PCH4 is consistent
with a bimolecular dehydration of methanol over bare
Brønsted acid sites. C–H activation rates exhibited a near
zero-order dependence on PO2 while CO2 formation rates
exhibited a linear dependence on O2 partial pressure at PO2
< 0.1 kPa (Figure 1C), suggesting that the overoxidation
process involves direct reaction of a C1 intermediate with
either unbound O2 or with a low-coverage of O2-derived
species. The weak dependence of C–H activation on PO2 in-
dicates that the reoxidation of Cu-active sites prior to C–H
activation in the redox cycle is not rate-limiting.
Similar kinetic experiments were performed using the 14
Cu-CHA samples listed in Table 1, whose Al speciation and
Cu/Al compositions were controlled synthetically to probe
the impacts of metal loading and Cu site speciation upon
kinetics and product selectivity. All catalysts generally ex-
hibited an increasing degree of undesirable downstream
oxidation events with increasing conversion (Figure 2A) as
expected from the hypothesized sequential conversion
pathway. Catalysts with Cu/cage>0.3 exhibited lower par-
tial oxidation selectivity than expected based on sequential
conversion, suggesting that higher Cu loadings direct the
formation of active sites that are unselective towards me-
thane partial oxidation. The purely siliceous 0Al-Cu-CHA
sample, which lacked [AlO4]- T-sites necessary for direct-
ing Cu ion exchange, only contained CuOx nanoparticles
within the CHA pores. This Al-free sample displayed broad
UV-vis features in the 20,000 and 40,000 cm-1 regions that
are associated with copper oxide nanoparticles (Figure
S7).41-42 This catalyst also exhibited the highest apparent
activation energy (140 kJ mol-1) of all measured catalysts
and near 0% selectivity to CH3OH. Al-containing catalysts
with increasing catalyst Cu content above Cu/cage>0.3 also
exhibited increasing apparent activation energies, similar
UV-Vis features to those observed in 0Al-Cu-CHA (Figure
Total C–H activation rates were unaffected by H2O par-
tial pressure (Figure 1D), and the partial oxidation selectiv-
ity decreased with decreasing water partial pressure to the
limit of 0% in the absence of a H2O co-feed (Figure S5).
These results implicate a reaction pathway where a rate-
determining C–H scission event catalyzed by an oxidized
copper-oxygen species is followed by either (i) H2O-facili-
tated methanol desorption or (ii) secondary oxidation of a
C1 intermediate to CO2. Previous reports have hypothe-
sized that H2O substantially decreases the desorption en-
ergy of surface-bound methoxy species to form methanol
after initial C–H scission events.36-37
33, 38-39
Lower partial
pressures of H2O decrease the probability of methanol de-
sorption events, resulting in more frequent interactions of
activated C1 intermediates with gas phase O2 and increased
total oxidation (Figure 1D). The apparent activation energy
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S7, Table S2), and low partial oxidation selectivity, impli-
cating the onset of CuOx nanoparticle formation when ex-
ceeding a copper loading of Cu/cage=0.3.
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In addition, a sample with a low Al content (1Al-Cu-
CHA, Si/Al = 76) was synthesized to isolate, on average, 1
Al T-site for every ~3 cages (one CHA cage consists of 36 T-
sites that are each shared between three cages). This sam-
ple featured only ~50% selectivity to methanol at
Cu/cage<0.2, a sub-first order dependence on PCH4, and a
first order dependence on PO2 (Figures 2A, S9 and S10), dif-
fering significantly from all other Cu-CHA zeolites investi-
gated. Spectroscopic studies of 1Al-Cu-CHA under working
conditions indicated the presence of isolated Cu sites (vide
infra). To further illustrate the effect of Si/Al ratio, Figure
2B compares partial oxidation selectivity of four catalysts
of differing Al content and similar Cu loadings below 0.2
Cu/cage at isoconversion (0.003%). Decreasing Al content
resulted in decreased selectivity to partial oxidation (Fig-
ures 2B, S12), underscoring the importance of zeolitic H+
species in the turnover cycle (vide infra). Moreover, a fully
siliceous zeolite without ion-exchanged Cu species present
displayed 0% selectivity towards methanol (0Al-Cu-CHA,
Figure S8), suggesting that partial oxidation of methane re-
quires ion-exchanged Cu species within the zeolite.
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B
Figure 2C shows the overall site time yield across all cat-
alysts (STY; the total rate of C–H activation normalized by
Cu content) varied by a factor of 4 over a span of Cu-load-
ing that varied by a factor of 20. This minimal variation in
STY across a large range of Cu loadings in conjunction with
similar kinetic pressure dependences and energetic barri-
ers (Table S2) implies the mechanisms and energetics of C–
H activation are similar across numerous Cu-centric active
sites, excluding 1Al-Cu-CHA. Slight variations in STY and
kinetic dependences may be indicative of a small subset of
Cu species in some catalyst samples that are either inactive
for C-H activation or distinct from the selective partial ox-
idation sites present in Cu/Cage < 0.3 samples. These re-
sults highlight the necessity of moderate Cu loadings and
high Al content to direct the synthesis of well-dispersed
partial oxidation active sites.
C
Figure 2. A. Selectivity as a function of CH4 conversion for
separate catalyst samples. All catalyst properties are listed in
Table 1. B. Partial oxidation selectivity at 0.003% conversion
versus decreasing Al content for four catalyst samples. C. Cu-
normalized total product formation rates (Site time yield,
STY) as a function of Cu/cage. Reaction conditions: 0.25 gcat
543 K, 25 - 200 sccm total flow, PCH4 = 18 kPa, PO2 = 0.09 kPa,
PH2O = 3.14 kPa, bal He.
2. Effects of pretreatment, Brønsted acidity, and
NH3 on CH4 oxidation
Inspired by reports on the mobilization of ammonia-
solvated [H3N–Cu–NH3]+ species to form multinuclear cop-
per active sites in SSZ-13 at 473 K under NOx reduction con-
ditions,43-45 46-47 we applied similar reductive and oxidative
pretreatments to gain insight into the nature of the for-
mation and turnover of active sites for CH4 oxidation on
Cu-CHA(0.23). As shown in Figure 3A, oxidative and re-
ductive pretreatments had minimal effects on steady-state
rates with one notable exception: NH3 severely decreased
rates of C–H activation whether introduced during pre-
treatment or as a co-feed, and rates could not be entirely
recovered upon removal of the NH3 co-feed (Figure 3A).
These inhibitive effects can be attributed to the favorable
,
adsorption of NH3 to both Brønsted acid sites and ex-
traframework Cu ions within the zeolite pores.34, 48-50
To probe the effects of NH3 on C–H activation, we per-
formed an operando NH3 titration (Figure 3B) over Cu-
CHA(0.23) under the following conditions: (i) baseline
steady state flows (SS), (ii) NH3 co-feed with steady state
flows (SS+NH3), (iii) steady state flows after removal of the
NH3 titrant (SS post NH3), and (iv) baseline steady state
flows after heating the catalyst to 673 K and holding for >10
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h under pure helium flow (SS post-purge). Upon introduc-
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tion of an NH3 co-feed, methanol synthesis was almost en-
tirely suppressed and CO2 formation rates were substan-
tially decreased (Figure 3B, left). Removal of the NH3 co-
feed resulted in a moderate increase in CO2 formation
rates, but methanol production rates did not recover to the
original values (Figure 3B, middle). Di Iorio et al. reported
that upon adsorption of NH3 to both Brønsted acid sites
and Cu, treatment with a wet He flow at 433 K selectively
desorbed NH3 from Cu without perturbing NH3 coordi-
nated to zeolitic protons.48 Upon removal of the NH3 co-
feed in our system, the CH4/O2/H2O stream contacted the
NH3-titrated catalyst for ~1000 min at 543 K—conditions
that selectively removed NH3 only from Cu sites. This re-
sult is a strong indication that the persistent, significant
inhibition of C–H activation rates after NH3 removal was
due to the blocking of Brønsted acid sites rather than a
complete blockage of the Cu centers. An extended 12 h
treatment under flowing helium at 673 K, known to induce
NH3 desorption even from zeolitic protons,48 resulted in
full recovery of CH4 conversion rates and selectivity values
(Figure 3B, right), further evincing the deleterious effects
of proton-blocking.
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SS
post-purge
B
SS
SS+ NH3
SS post-NH3
The necessity of protons to methanol production was in-
ferred from the negligible selectivity to methanol shown in
the center of Figure 3B (SS post-NH3), conditions under
which NH3 inhibits protons but not Cu as discussed above.
Moreover, increasing the extent of NH3 titration (leading
to decreasing conversion) did not result in increasing se-
lectivity to methanol as expected from Figure 2A. Thus, we
hypothesize that the absence of H+ on Al sites impedes ei-
ther (i) the formation of selective active sites or (ii) the de-
sorption of methanol from selective active sites (if these
sites remain). A combination of these two factors is also
possible. Rapid regeneration of methanol synthesis rates
upon NH3 purging (Figure 3B, SS post-purge) and the par-
tial similarities of spectroscopic features of Cu species un-
der SS and SS post-NH3 conditions (vide infra) imply some
fraction of selective partial methane oxidation active sites
were present under SS post-NH3 conditions, implicating
the importance of H+ for CH3OH desorption.
C
Increasing
NH3
SS post-NH3
Fresh
Cu-CHA(0.23) was also characterized ex situ by FTIR
spectroscopy following exposure and subsequent removal
of an NH3 co-feed under reaction conditions (Figure 3C,
red trace, SS post-NH3). A strong signal from NH3 adsorbed
to Brønsted acid sites (3265 cm–1) was observed while a sig-
nal from NH3 adsorbed to Cu (1635 cm–1) was entirely ab-
sent (Figure 3C, peak assignment in Figure S11). Neither of
these features were observed on a fresh Cu-CHA(0.23) that
was not exposed to reaction conditions or NH3 dosing (dot-
ted trace, Fresh). Next, the SS post-NH3 Cu-CHA(0.23)
sample was subjected to a series of 0.4 µmol NH3 in He
pulses, resulting in the appearance of both Cu–NH3 signa-
tures and more prominent NH4+ features (Figure 3C), con-
firming the selective desorption of NH3 from Cu and mini-
mal desorption from Brønsted acid sites after NH3 co-feed
Figure 3. Effect of NH3 on catalytic methanol formation and
Cu speciation for Cu-CHA(0.23). A. Product formation rates
versus pretreatment conditions and NH3 co-feed as listed. B.
Effect on methanol formation. C. Ex situ FTIR of two catalyst
samples: (i) “Fresh” Cu-CHA(0.23), never exposed to reaction
conditions, only exposed to helium at 543 K, and (ii) Cu-
CHA(0.23) exposed to methanol synthesis flows, NH3 co-feed,
and removal of NH3 co-feed, before being removed from the
reactor, pelletized, placed in the IR cell, and treated with sub-
sequent in situ dosing with NH3. CH4 activation conditions
were 0.25 gcat, 543 K, 25 sccm, PCH4 = 18 kPa, PO2 = 0.09 kPa,
PH2O = 3.14 kPa, bal He. PNH3 = 0.08 kPa when co-feeding NH3.
Spectra were collected in transmission of a 10 mg, 7 mm self-
supporting catalyst wafer at 543 K.
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removal under reaction conditions. The importance of ze- and not present upon titration with NH3 (Figure S15). Li et
1
2
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8
olitic protons on C–H activation was also highlighted when
testing catalysts where framework acid sites were ion-ex-
al. calculated features in this region can be from Cu dimers
in SSZ-13.53 Definitive assignment of this band is inconclu-
sive, however, as features in this region may simply be re-
lated to reduction and oxidation of Cu ions.24 These results
fall in line with an abundant, oxidized dimeric Cu-active
site at steady state prior to a rate-determining C–H activa-
tion event.
+
changed with NH4 or Na+ (Figures S12 and S13). Both of
these ions inhibit methanol formation and appear to de-
crease the total rate of C-H activation. Although all of the
effects of NH4+ and Na+ ions on partial methane oxidation
rates and selectivity cannot be conclusively proven to be
identical, both ions cause similar effects to those observed
from NH3 co-feed-induced H+ inhibition.
NH3 co-feed (SS+NH3) resulted in the generation of a
strong Cu(I) pre-edge feature, the disappearance of both
Cu(II) pre-edge features (Figure 4C inset), the disappear-
ance of the Cu-Cu scattering signal, and a decrease in the
first shell coordination number (CN) from 3.1 ± 1.1 to 1.8 ±
0.3. These changes are consistent with the destruction of
Cu-dimers and the formation of mobile [H3N–Cu–NH3]+
species similar to those previously reported by Paolucci et
al and represented in Figure 4E.46 Removal of the NH3 co-
feed (SS post-NH3) resulted in a partial recovery in both the
Cu-O and Cu-Cu scattering signals and a partial decrease
in the intensity of the Cu(I) pre-edge feature.
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Taken together, these data show Brønsted acid site
blocking has a twofold effect: (i) near complete inhibition
of methanol formation pathways and (ii) significant inhi-
bition of either C–H or O2 activation processes. Figure 2C
shows that the rate of C–H activation is relatively invariant
across samples of varying Al content (8.8 < Si/Al < 76).
Therefore, we posit that protons are not kinetically rele-
vant to the rate-determining C–H activation step. How-
ever, given the suppression of total CH4 conversion due to
H+-inhibition, Brønsted acid sites are necessary for the
completion of a C–H activation catalytic cycle, possibly by
enabling active site formation or reoxidation.
As discussed previously, NH3 remains bound to Brønsted
acid sites under methanol synthesis conditions after re-
moval of the NH3 titrant, demonstrating that zeolitic pro-
tons are crucial for both Cu reoxidation and reformation of
Cu dimers for a significant fraction of Cu ions. In the final
step of the titration experiment, initial steady state Cu spe-
ciation was restored after purging NH3 from zeolitic pro-
tons under helium flow at 673 K and subsequently reintro-
ducing methanol synthesis flows.
The interrelation between Cu speciation, Brønsted acid
sites, and NH3 was investigated by in situ X-ray absorption
spectroscopy (XAS) during an identical NH3 titration ex-
periment to that shown in Figure 3B. The effects of Al-den-
sity within the zeolite framework were also probed using
two representative catalysts of similar Cu content: (i) high
Al content (Cu-CHA(0.11)), and (ii) low Al content (1Al-Cu-
CHA). Figures 4A and 4B show the XAS spectra of the high-
Al content Cu-CHA(0.11) during an entire NH3 titration ex-
periment; fitting of the extended X-ray absorption fine
structure (EXAFS) of Cu-CHA(0.11) upon exposure to
steady state methanol synthesis conditions agrees with a
[Cu–O–Cu]2+ motif (fits of all spectra are summarized in
Figure S14 and Table S3). The XANES spectra indicated the
presence of both Cu(I) (8983 eV51) and Cu(II) (8978 and
8987 eV51), consistent with a Cu(II)/Cu(I) redox cycle dur-
ing C–H activation. The first shell scattering peak fit to a
Cu-O coordination number (CN) of 3.1 ± 1.1 (RCu-O = 1.90 ±
0.02 Å) and the second shell scattering peak fit to a Cu-Cu
CN of 1.1 ± 0.6 (RCu-Cu = 2.94 ± 0.05 Å). These values are
consistent with a moiety where each Cu atom coordinates
to two framework O atoms or coordinating H2O molecules,
one extraframework O atom, and an additional Cu atom,
represented in Figure 4E. The observed interatomic dis-
tances for both Cu-O and Cu-Cu are also consistent with
distances calculated for Cu dimers in SSZ-13,52 yet we also
recognize that the uncertainty of the Cu-O CN does not
preclude the presence of either a Cu dimer coordinating to
2 extraframework O’s or a subset of non-dimeric Cu spe-
cies. Fitting of the second shell EXAFS peak with a T-site
resulted in a negative CN (Table S4). However, given the
overlap of Cu-T-site and Cu-Cu scattering, small contribu-
tions by a T-site cannot be ruled out. In situ UV-vis exper-
iments of Cu-CHA(0.11) display a feature centered around
37,500 cm-1 that is present under methanol synthesis flows
Figures 4C and 4D compare the effects of NH3 titration
on low- and high-Al content samples via in situ XAS under
the same methanol synthesis and NH3 titration conditions.
1Al-Cu-CHA exhibits a stronger Cu(I) pre-edge feature un-
der steady state methanol synthesis conditions versus Cu-
CHA(0.11). A fit of the EXAFS of 1Al-Cu-CHA (Figure 4D,
Figure S16, Table S5) under these same conditions provided
a Cu-O CN of 1.8 ± 0.5 (RCu-O = 1.94 ± 0.02 Å) and a smaller
Cu-Cu CN of 0.6 ± 0.3 (RCu-Cu = 2.9 ± 0.1 Å) compared to Cu-
CHA(0.11). Due to the nature of XAS as a bulk averaging
technique, a Cu-Cu coordination number less than 1 is in-
dicative of a mixture of isolated Cu-O species and multinu-
clear Cu-oxo species. An NH3 co-feed to 1Al-Cu-CHA
yielded similar results as with Cu-CHA(0.11): only Cu(I)
was observed, Cu-O scattering intensity was reduced, and
Cu-Cu scattering was eliminated. These results imply that
an increasingly sparse Al population decreases the likeli-
hood of multinuclear Cu species even at a moderate Cu/Al
loading, and that isolated Cu species cannot be facilely aer-
obically oxidized.
Because of its bulk sampling nature, EXAFS alone cannot
identify the active site for selective partial methane oxida-
tion. However, EXAFS in combination with the reactivity
measurements and NH3 titration experiments indicate a
Cu dimer active site is likely to achieve selective catalytic
partial oxidation of methane to methanol. Therefore, these
combined reactivity and in situ spectroscopic data support
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Journal of the American Chemical Society
the hypothesis that a [Cu–O–Cu]2+ motif acts as the active
site for catalytic partial CH4 oxidation in SSZ-13, the dimer
is formed at low Cu loadings (Cu/cage<0.3), and free zeo-
litic protons are critical for Cu dimer formation as evinced
from selective synthesis, spectroscopic, and NH3 titration
results.
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C
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B
D
E
Figure 4. A. XANES and B. EXAFS of Cu-CHA(0.11) and C. XANES and D. EXAFS of 1Al-Cu-CHA and Cu-CHA(0.11) under methanol
synthesis and NH3 flows. All spectra were collected at 543 K. The catalyst was pretreated in 1 kPa O2, bal He from 298 K to 543 K
at 5 K min-1 (Pretreat). At 543 K, the catalyst was exposed to methanol synthesis flows of PCH4 = 18 kPa, PO2 = 0.09 kPa, PH2O = 3.14
kPa, bal He (SS), an additional NH3 co-feed of PNH3 = 0.16 kPa (SS + NH3), and then the removal of NH3 (SS post-NH3). Following
NH3 removal, the catalyst was brought to 673 K at 6 K min-1 and held in dry He before cooling to 543 K and exposure to methanol
+
synthesis flows (SS post-purge). E. Hypothesized schematic representations of the average states of charge balancing Cu, NH4 ,
and H+ species as a function of reaction flows and treatments. Single O atoms may correspond to framework zeolite O atoms or
coordinating H2O molecule.
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Al
Cu+
Al
H
H
O
H
O
H
H
O
H
Cu+
Al
H
H+
H
HN H
H
O
H+
O
H
H
H+
H
Al
Al
O
Al
H+
Al
H
Cu+
O
H O
H
H
H
H+
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Al
H O
Al
Al
H
H+
H+
H O H
Al
Al
H+
H+
H
H
O
O
H
H
Al
Cu+
Al
H+
H O
H
H
O
+
Cu+
H
H+
Al
Al
Al
H+
Figure 5. Depiction of hypothesized H+/H2O-aided diffusion of Cu+ and NH3 inhibition within SSZ-13 to form Cu dimers relevant
to the proposed catalytic methane oxidation cycle. Single O atoms may correspond to framework zeolite O atoms or coordinating
H2O molecules
of these cages via the 8-MR windows with a minimal ener-
getic penalty.46 Psofogiannakis et al. used force field/mo-
lecular dynamics simulations to demonstrate that tetra-
hydrated Cu ions are fully desorbed from the SSZ-13 frame-
work and mobile on the picosecond timescale at tempera-
tures relevant to this work (500-700 K). Further, Paolucci
et al. calculated an energetic barrier of ~35 kJ mol-1 for the
rotation of [H3N-Cu-NH3]+ species through an 8MR win-
dow to an adjacent cage.46, 54 Assuming a Cu ion can be mo-
bilized and transferred to a proximate [AlO4]– T-site,
charge balance can be maintained by water-assisted pro-
ton hopping (Figure 5, top-left). A zeolite with a high
enough Al content, combined with the H+/H2O/Cu ion ex-
change process and Cu ion rotation between CHA cages
would effectively allow Cu ions to access the entirety of the
zeolite framework and facilitate Cu ion pairing even in low
Cu-content zeolites. This Cu ion mobility, thus, enables
formation of thermodynamically favorable Cu dimer spe-
cies in the presence of water..34, 54
3. Proposed mechanism for [Cu–O–Cu]2+ motif for-
mation
The formation of a [Cu–O–Cu]2+ species as CH4 oxida-
tion active sites is puzzling given C–H activation was ob-
served at Cu densities as low as 0.04 Cu/cage where the
probability of having two Cu ions within the same cage is
prohibitively small. Further, while the partial methane ox-
idation selectivities of Cu-CHA(0.05 and Cu-CHA(0.04)
were near unity (Figure 2A), the selectivity for methanol of
1Al-Cu-CHA was much lower despite similar Cu/cage val-
ues among the three catalysts (0.05, 0.04, and 0.04 respec-
tively), demonstrating that the active sites for selective
partial oxidation form when the zeolite Al-content is suffi-
ciently high even when the Cu loading is low. However, we
stress that the high Cu loadings lead to non-selective active
species regardless of Al-content as depicted above. In situ
NH3 titrations and concurrent spectroscopic data clearly
show that Brønsted acid sites are involved in the formation
of the [Cu–O–Cu]2+ motif. We posit that these results can
be rationalized by considering the proton-aided diffusion
of hydrated Cu ions within the CHA framework schemati-
cally represented in Figure 5; this hypothesized process is
fundamentally analogous to the putative mechanism that
allows hydrated Cu ions to diffuse into and out of CHA
cages during aqueous ion exchanges.
The low energetic barrier for this ion exchange across a
single CHA cage and the facile shifting of Cu ions and pro-
tons between neighboring cages provide a means for Cu
ions to access the entire microporous zeolite when a suffi-
ciently high Al content allows for Cu ion exchange between
nearby [AlO4]– T-sites.55 The introduction of NH3 into the
zeolite disperses Cu species as mobile [H3N–Cu–NH3]+ and
+
simultaneously blocks protons by NH4 formation. Upon
removal of NH3 co-feed, Cu ions are deposited on [AlO4]–
Each framework T-site in CHA is shared amongst three
neighboring cages, and extraframework cations (e.g., H+,
Cu+, Cu2+) coordinated to Al T-sites can readily access each
+
T-sites, but now NH4 (and not H+) ions are coordinated to
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as methanol in the presence of H2O/H+ or completely oxi-
[AlO4]– sites that lack Cu ions. These NH4+ ions are too en-
ergetically stable to desorb from [AlO4]– T-sites to facilitate
Cu ion mobility (Figure 4, Figure 5).
dized to CO2 by gas phase O2. Importantly, the [Cu–O–
Cu]2+ motif forms via the diffusion of hydrated Cu ions
along a proton-paved highway. High partial oxidation se-
lectivity can be targeted by utilizing (i) reaction conditions
of high CH4 and water partial pressure, and (ii) catalysts
containing a high density of Brønsted acid sites and mod-
erate Cu loadings (Cu/cage<0.3) to maximize the presence
of isolated dimeric Cu species. This work highlights the im-
portance of mechanistic understanding to direct both syn-
thesis-driven active site speciation and reaction condition
selection for process optimization.
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Aerobic oxidation of Cu to selectively produce methanol
necessitates dimeric species in contrast to the complete ox-
idation of both monomeric and dimeric Cu(I) achieved by
an NO2 pretreatment (Figure S17).34 Thus, catalytic CH4
partial oxidation is greatly hindered without Cu dimer for-
mation via an available ion diffusion pathway. The in-
creased presence of Cu(I) following removal of the NH3 co-
feed observed in Figure 4A (SS post-NH3) compared to the
original methanol synthesis flows (SS) is explained by the
inability of O2 to oxidize all isolated Cu(I) when the latter
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+
is trapped by NH4 (Figure 5, top-right cage of left scheme).
Methods
The fraction of isolated Cu species oxidized by O2 are
thought to account for the non-zero rate of CO2 formation
in the presence of NH3 and following NH3 removal. Only
upon complete NH3 desorption and re-introduction of
methanol synthesis flows is this significant fraction of
Cu(I) re-oxidized and C–H activation rates are restored.
This coincides with the opening of ionic diffusive channels
that result in reformation of dimeric Cu active sites.
See Supporting Information for a detailed Methods sec-
tion.
ASSOCIATED CONTENT
Supporting Information
Supporting information contains all experimental methods,
catalyst characterization, and supporting figures referenced in
the main text. This material is available free of charge via the
Internet at http://pubs.acs.org.
This theory is corroborated by the results in Figure 4C
and D of the isolated Al 1Al-Cu-CHA sample; this sample
exhibits predominantly isolated Cu-O species that cannot
diffuse due to a sparse population of [AlO4]– T-sites. In line
with this assumption, EXAFS analysis of the 1Al-Cu-CHA
sample revealed limited Cu-Cu scattering (Figure 4D). Cu
dimer formation in a zeolite with low Al content can be
accounted for by pockets of proximate Al sites, resulting
from either (i) a random Al distribution (The average Al-
Al distance at this Si/Al is 1.7 nm, close enough to permit
Cu dimer formation via the 0.9 nm maximum diffusion ra-
dius of a Cu ion46), or (ii) Al zoning formed during zeolite
synthesis as previously observed in other frameworks.56-57
Furthermore, the increased intensity of the Cu(I) pre-edge
feature of 1Al-Cu-CHA (Figure 4C) is consistent with a
greater population of isolated Cu-O species. However, par-
tial methane oxidation activity over isolated Cu species
cannot be ruled out. The Cu speciation and distinct kinet-
ics of 1Al-Cu-CHA are still under investigation. While this
mechanism is not unequivocally proven, the comprehen-
sive and complex results reported in this work can be
simply rationalized via an ionic diffusion pathway that fa-
cilitates the formation of [Cu–O–Cu]2+ motifs that are ac-
tive for selective partial oxidation of CH4.
AUTHOR INFORMATION
Corresponding Author
*yroman@mit.edu
Author Contributions
§These authors contributed equally
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support
of ExxonMobil. K.D. acknowledges the partial support
from the National Science Foundation Graduate Research
Fellowship under Grant No. 1122374. Any opinion, findings,
and conclusions or recommendations expressed in this
material are those of the author(s) and do not necessarily
reflect the views of the National Science Foundation.
MRCAT operations are supported by the Department of
Energy and the MRCAT member institutions. This re-
search used resources of the Advanced Photon Source, a
U.S. Department of Energy (DOE) Office of Science User
Facility operated for the DOE Office of Science by Argonne
National Laboratory under Contract No. DE-AC02-
06CH11357. This work made use of the MRSEC Shared Ex-
perimental Facilities at MIT, supported by the National
Science Foundation under award number DMR-14-19807.
The authors thank Z. Wang for help in ICP data collection,
J. Katsoudas, T. Wu, and S. Aryal for help with XAS data
collection, M. Stone and T. Gani for comments on the man-
uscript, and A. Stubbs and X. He for helpful discussion.
Conclusions
The reaction pathway and active site for the direct con-
version of CH4 to methanol was elucidated with kinetic ex-
periments in combination with spectroscopic techniques.
Low Cu-loadings (Cu/cage<0.3) with sufficiently high Al
content are critical to speciate a [Cu–O–Cu]2+ motif that
catalyzes selective CH4 partial oxidation while avoiding
formation of other Cu structures and CuxOy nanoclusters
that promote complete CH4 oxidation to CO2. CH4 activa-
tion proceeds via rate-determining C–H scission to form a
surface-bound C1 intermediate that can either be desorbed
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Page 10 of 11
methane to methanol. Proc. Natl. Acad. Sci. U. S. A. 2009, 106
(45), 18908-18913.
19. Grundner, S.; Luo, W.; Sanchez-Sanchez, M.; Lercher,
J. A., Synthesis of single-site copper catalysts for methane partial
oxidation. Chem. Commun. (Cambridge, U. K.) 2016, 52 (12),
2553-2556.
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