AgII-Mediated Electrocatalytic Ambient CH4 Functionalization Inspired by HSAB Theory

Article abstract of DOI:10.1002/anie.202104217

Although most class (b) transition metals have been studied in regard to CH₄ activation, divalent silver (Ag^II), possibly owing to its reactive nature, is the only class (b) high-valent transition metal center that is not yet reported to exhibit reactivities towards CH₄ activation. We now report that electrochemically generated Ag^II metalloradical readily functionalizes CH₄ into methyl bisulfate (CH₃OSO₃H) at ambient conditions in 98 % H₂SO₄. Mechanistic investigation experimentally unveils a low activation energy of 13.1 kcal mol^?1, a high pseudo-first-order rate constant of CH₄ activation up to 2.8×10³ h^?1 at room temperature and a CH₄ pressure of 85 psi, and two competing reaction pathways preferable towards CH₄ activation over solvent oxidation. Reaction kinetic data suggest a Faradaic efficiency exceeding 99 % beyond 180 psi CH₄ at room temperature for potential chemical production from widely distributed natural gas resources with minimal infrastructure reliance.

Full text of DOI:10.1002/anie.202104217

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Accepted Article
Title: AgII-mediated electrocatalytic ambient CH4 functionalization
inspired by HSAB theory
Authors: Danlei Xiang, Jesus A Iñiguez, Jiao Deng, Xun Guan, Antonio
Martinez, and Chong Liu
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10.1002/anie.202104217
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RESEARCH ARTICLE
AgII-mediated electrocatalytic ambient CH4 functionalization
inspired by HSAB theory
Danlei Xiang,[a] Jesus A. Iñiguez,[a] Jiao Deng,[a] Xun Guan,[a] Antonio Martinez,[a] Chong Liu*[a,b]
[a]
D. Xiang, J. A. Iñiguez, Dr. J. Deng, X. Guan, A. Martinez, Prof. Dr. C. Liu
Department of Chemistry and Biochemistry
University of California Los Angeles
Los Angeles, California 90095, United States
E-mail: chongliu@chem.ucla.edu
[b]
Prof. Dr. C. Liu
California Nanoscience Institute
University of California Los Angeles
Los Angeles, California 90095, United States
Supporting information for this article is given via a link at the end of the document.
Abstract: Developing an efficient chemical transformation pathway of
ambient CH4 functionalization is an ongoing challenge and the key
resides on the discovery of new catalytic systems. The hard-soft acid-
base (HSAB) theory suggests that high-valent transition metals, as
soft class (b) Lewis acids based on Pearson’s classification, are
suitable candidates for CH4 activation due to methyl moiety’s relatively
low value of chemical hardness. While most of class (b) transition
metals have been studied, divalent silver (AgII), possibly due to its
reactive nature, is the only class (b) high-valent transition metal center
that is yet reported to exhibit reactivities towards CH4 activation.
Inspired by such notion, here we report that electrochemically
generated AgII metalloradical readily functionalizes CH4 into methyl
bisulfate (CH3OSO3H) at ambient conditions in 98% H2SO4.
Mechanistic investigation experimentally unveils a low activation
energy of 13.1 kcal•mol−1, a high pseudo-first-order rate constant of
CH4 activation up to 2.8×103 h−1 at room temperature and a CH4
pressure of 85 psi, and two competing reaction pathways preferable
towards CH4 activation over solvent oxidation. Reaction kinetic data
suggest a Faradaic efficiency exceeding 99% beyond 180 psi CH4 at
room temperature for potential chemical production from widely
distributed natural gas resources with minimal infrastructure reliance.
electrophilic activation[6] or
a
radical-based mechanism.[7]
Consistently, many high-valent class (b) metals in the d-block of
periodic table, including RhI, II [8] PdII, III [9] IrIII,[10] PtII, IV [11] AuI, III [12]
, , ,
,
HgII,[6b] have been reported for CH4 activation (Figure 1a). Some
of the borderline metals with intermediate chemical softness,
including MnIII,[13] CoIII,[13] RuIV, VIII [14] OsIV, VIII [15] TlIII,[6c] and PbIV,[6c,
,
,
13] are reactive towards CH4, too. Yet there is one exception, silver
(Ag). While monovalent AgI as a mild oxidant (η = 6.96; E° = 0.80
V vs normal hydrogen electrode (NHE) for Ag+ (aq.)/Ag (s))[5, 16]
may not be oxidative enough to break the C−H bond in CH4 (E° =
0.59 V vs NHE for CH3OH (l)/CH4 (g)),[2b,
16]
divalent AgII is
similarly soft (η = 6.7)[5] and possesses a AgII/AgI redox potential
(E° = ~2.5 V vs NHE for AgII/AgI in 98% H2SO4)[17] comparable to
other reported CH4-activation catalysts.[2a, 2b, 9b] Therefore, it is
intriguing that divalent AgII has not been known for CH4 activation
despite the reported AgII-based reactivities on much weaker C−H
bonds in organic synthesis.[18] The d9 electronic configuration of
AgII not only renders it a metalloradical, but also introduces the
Jahn-Teller effect in an Oh ligand field that elongates the ligand
bond in the axial position (Figure 1b).[19] Such weakly bound axial
ligands and AgII’s likely radical nature may offer an opportunity for
substrate binding and CH4 activation in a radical-based activation
pathway with low reaction barrier, leading to our hypothesis that
AgII, once continuously generated, may serve as the active
species towards ambient CH4 functionalization catalytically.
Our strategy of investigating AgII as a potential active
species towards CH4 activation includes continuous electro-
generation of reactive AgII species in an inert solvent environment
(Figure 1c). Electrochemistry offers a viable and clean route of
creating reactive intermediates for synthesis including C−H
activation.[20] Such a strategy has enabled our previous discovery
of electrocatalytic CH4 functionalization with vanadium-oxo dimer
as the homogeneous catalysts at ambient temperature and
pressure.[2a] Similarly, the strategy of electrochemical catalysis
can be also applied for the generation of AgII intermediate, which
circumvents the instability of AgII and establishes an
electrocatalytic cycle mediated by the AgII/AgI redox couple
(Figure 1c). We also sought to choose a weakly bound solvation
environment as a model system that offers labile binding sites for
substrate activation, supports the radical nature of AgII in a
possible radical-based mechanism, and mitigates side reactions
of solvent oxidation due to AgII’s reactive nature. Such
Introduction
Ambient CH4 functionalization offers a route of chemical
synthesis that taps on the vast, widely distributed natural gas
resources while mitigating the environmentally unfriendly CH4
emission into the atmosphere.[1] The key step in this process is
the activation of CH4’s C−H bond at relatively low temperature and
pressure, which demands a kinetically reactive species with a low
activation barrier.[2] One approach in search of the desirable
reactive species begins from the classic hard-soft acid-base
(HSAB) theory, initially introduced by Ralph Pearson.[3] The
theory introduces chemical hardness
η as a measure of
molecules’ electrophilicity[4] and stability in the context of Lewis
acid-base adduct. In a homolytic cleavage of CH4, the transferred
methyl moiety possesses relatively low chemical hardness (η =
4.87).[5] The softness of methyl moiety along with H atom (η =
6.42) involved in H-atom abstraction suggests that a soft, class
(b) transition-metal Lewis acid of high valence and increased
electrophilicity may be reactive towards CH4 via either
1
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Figure 1. a) Transition metals reported for direct C−H activation of homogeneous CH4 functionalization.[2b] The values of chemical hardness (η) are tabulated for
the elements’ respective oxidation states.[5] NA, not available. b) Schematics of the frontier orbitals and structure of a proposed AgII metalloradical in 98% H2SO4. L
and L’, ligands; SOMO, Singly Occupied Molecular Orbital. c) The strategy of an electrocatalytic CH4 activation in this work.
consideration leads to the choice of 98% H2SO4, which not only
minimizes possible water oxidation reaction due to its oxidative
stability[2a, 2b] but also can stabilize AgII compound at 20 ºC.[21] The
electron-withdrawing bisulfate ligand bound to AgII in 98% H2SO4
is postulated to provide access to CH4 activation in the elongated
axial position,[19a] maximize the radical nature on metal center
due to spin localization,[19b] and simplify product detection by
mitigating the additional oxidation with the formation of methyl
bisulfate (CH3OSO3H) after a presumed two-electron oxidation.
Such favorable features in conjunction with our expertise in
electrochemical CH4 activation[2a, 22] constitute our motivation for
exploring a AgII-mediated electrocatalytic CH4 functionalization in
98% H2SO4 inspired by the HSAB theory.
Here we present our demonstration of AgII-mediated
electrocatalytic CH4 activation in 98% H2SO4 at ambient
conditions. We found that electrochemically generated AgII
intermediate is capable of activating CH4 and yielding CH3OSO3H
with minimal side reactions at room temperature and ambient
pressure. The CH4-activation reactivity, measured as the partial
current density of CH3OSO3H formation jCH4, is systematically
studied as a function of the applied electrochemical potential (E),
duration of electrolysis (t), the concentration of pre-catalyst AgI
(cAg), temperature (T), and the partial pressure of CH4 (pCH4) up to
125 psi. The results support the scheme of electrocatalysis for
CH4 functionalization mediated by AgII in an EC’ mechanism[23]
(Figure 1c), in which the electrochemically generated AgII
(Hg notwithstanding), offer a favorable prospect for its utilization
in chemical production from widely distributed natural gas with
minimal infrastructure reliance.
Results and Discussion
Evidence for an electrocatalytic CH4 functionalization
Cyclic voltammograms under various pCH4 values in 98%
H2SO4 support an electrocatalytic activation of CH4 with AgI as the
pre-catalyst. Experiments of cyclic voltammetry were conducted
with 5 mM Ag2SO4 in 98% H2SO4 (cAg = 10 mM), under a single-
chamber three-electrode setup at 25 ºC (298 K) in a pre-defined
pressure (Figure S1), with fluorine-doped tin oxide (FTO) glass as
an innocent working electrode.[2a, 9d] Figure 2a shows the lack of
features in voltammogram (j-E relationship) without the addition
of Ag2SO4 (yellow trace). In N2, voltammograms of AgI solution
displayed an oxidative charge transfer peaking at Ep,a = 1.82 V (all
potentials are reported vs Hg2SO4/Hg electrode if not mentioned)
before the onset of solvent oxidation at around 2.1 V (Figure 2a
19b]
and 2b).[17,
The corresponding reduction peak of the
electrochemically generated oxidant is observed predominantly at
1.08 V, albeit a minute peak dependent on scan rate (Figure 2b
and 2c) and cAg (Figure S2) is visible at 1.52 V. The reductive peak
at 1.08 V is assigned as a direct reduction of AgII into AgI.[19b] The
secondary one at 1.52 V is suggested as the reduction of bisulfate
•
undergoes
a turnover-limiting CH4-activating step with low
radicals (HSO4 ) homogenously generated from the Ag-bound
activation energy (13.1 kcal•mol−1), high pseudo-first-order rate
constant (kobs) of 2.8×103 h−1 (pCH4 = 85 psi), and a high turnover
number (TON) of about 2.45×104 when t = 72 h and pCH4 = 125
psi. Three independent kinetic characterizations suggest that AgII-
mediated CH4 activation is kinetically favored at room
temperature over the side reaction of solvent oxidation. The
overall selectivity of the reaction, measured as the Faradaic
efficiency (FE) in electrocatalysis, was observed to be 72% when
pCH4 = 85 psi and is predicted to exceeding 99 % when pCH4 > 180
psi after overcoming the mass-transport constraint due to CH4’s
limited solubility. The discovered reactivity of electrochemically
generated AgII, in conjunction with Ag’s relatively lower cost
comparing to its more precious counterparts in class (b) metals
−
bisulfate ligand (HSO4 ) via a ligand-metal charge transfer
(LMCT),[19b, 24] which helps explain why the integrated reductive
charge is smaller than the oxidative one. We observed that the
reaction kinetic responsible to the reductive peak at 1.52 V is
sluggish since this peak is much less visible at a higher scan rate
in Figure 2b and 2c. Despite the noted complication of reductive
behavior, the single-electron oxidation peak from AgI to AgII yields
a satisfactory linear relationship in the Randles-Ševčík analysis[25]
(Figure 2c and 2d) with a diffusion coefficient of 2.8×10−7 cm2•s‒1.
Additional numerical simulation of cyclic voltammetry, whose
details are listed in Figure S3, Table S1, and Table S2, not only
2
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Figure 2. a) j-E plot when cAg = 10 mM with increasing pCH4 (red to blue traces) and the result without Ag in N2 (yellow trace, “blank in N2”). b) j-E plot when cAg = 10
mM in N2 with various scan rates (v). c) j/v1/2-E plot from results shown in (b). d) The plot of anodic peak current density (jp,a) versus v1/2 from results shown in (b)
and (c). D, the calculated diffusion coefficient. e) Enlarged j-E plot in (a) at around 1.5 V. f) j-E relationship when cAg = 3 mM with increasing pCH4. Unless noted,
room temperature, 98% H2SO4 electrolyte, v = 100 mV·s‒1, and iR-compensated.
supports the obtained value of diffusion coefficient but also
suggests a quasi-reversible electrochemical oxidation of AgI as a
desirable E step in an EC’ mechanism with a charge-transfer rate
constant (ks) of 3×10-6 cm/s. When the N2 environment was
switched to CH4, a pCH4-dependent change in the voltammograms
was observed (Figure 2a and S4). The current density of AgI
oxidation increases with higher pCH4 values, along with the
decrease of reductive current density peaking at 1.08 V. In
addition, a noticeable disappearance of the reductive peak at 1.52
V was observed in CH4 (Figure 2e). As higher pCH4 values
correlate with higher CH4 concentrations in the electrolyte, the
electrocatalytic behavior in cyclic voltammetry is limited by the
solubility of CH4 and electro-generated AgII is ambiently reactive
towards CH4.
The proposed electrocatalytic CH4 activation is
corroborated by analyzing the solution after electrolysis via 1H
nuclear magnetic resonance (NMR) spectroscopy. Room-
temperature preparative bulk electrolysis (t = 3 h) was conducted
at cAg = 10 mM in a two-chamber three-electrode setup (Figure
S5), and the composition of the resultant electrolyte in the anodic
chamber was analyzed by 1H NMR spectroscopy. Electrolysis at
E = 1.637 V and pCH4 = 15 psi (ambient pressure) leads to the
1
observed change in voltammograms is consistent with
a
emergence of a peak at chemical shift δ = 3.34 ppm in H NMR
hypothesized EC’ mechanism mediated by electro-generated AgII
(Figure 1c), in which the increased oxidative current density stems
from the regenerated AgI near the electrode after an oxidative CH4
activation. The disappearance of reductive peak at 1.52 V in CH4
suggests that CH4 competitively reacts with AgII metalloradical in
spectra (blue trace in Figure 3a), consistent with a presumed
formation of CH3OSO3H (Figure S6a). CH3OSO3H formation was
not observed either in N2 with AgI or in CH4 devoid of AgI (red and
yellow trace in Figure 3a, respectively). The amount of generated
CH3OSO3H is proportional to the amount of electric charge
passed and linear to t up to 12 h when E = 1.602 V (Figure 3b).
This suggests that the CH3OSO3H production is persistent in CH4.
Attempts of detecting gas-phase products with an online gas
chromatograph (Figure S1a) did not find any detectable CO, CO2,
and other carbon-based products (Figure S7). 13C isotope-
labeling experiment with 13CH4 in electrolysis led to a signal at δ
= 58.6 ppm in 13C NMR spectrum hence 13CH3OSO3H formation,
which was not observable under CH4 of natural isotope
abundance (Figure 3c). Those data support an electrocatalytic
−
lieu of LMCT on the HSO4 ligand, hinting the presence of a
radical-based reaction pathway. We note that at room
temperature CH4 only possesses a solubility of about 0.8 mM at
ambient pressure and less than 9 mM at the highest tested
pressure (pCH4 = 165 psi, see SI), a concentration smaller than the
cAg used in Figure 2a to 2e (cAg = 10 mM). This suggests mass
transport of CH4 may limit the catalytic response shown in Figure
2a. Evidently, when cAg = 3 mM, j-E relationship displays a more
pronounced catalytic current of CH4 activation, even at ambient
pressure (15 psi) (Figure 2f). This confirms that the
3
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Figure 3. a) 1H NMR spectra of solutions after a 3-h electrolysis with cAg = 10 mM in CH4 (blue) and N2 (red) or cAg = 0 mM in CH4 (yellow). E = 1.637 V; *, Acetic
acid as internal standard; d6-DMSO. b) The evolution of electric charges (blue) and CH3OSO3H concentration (cMHS, red) during a 12-h electrolysis. 25℃; cAg = 10
mM; E = 1.602 V. c) 13C NMR spectra of solutions after a 3-h electrolysis with cAg = 3 mM in 13CH4 (red) and CH4 of natural abundance (blue). d6-DMSO; E = 1.637
V. d) UV-vis spectra of the pre-catalyst AgI (red), electrogenerated AgII (blue), and AgO dissolved in 98% H2SO4 (yellow). The inset shows the yellow-colored
electrogenerated AgII. e) EPR spectrum of AgO in 98 % H2SO4 (77 K).
functionalization of CH4 into CH3OSO3H with minimal carbon-
based side products.
product was methanesulfonic acid (CH3SO3H) with a minute
amount of CH3OSO3H (31:1 ratio) (Figure S6c). The detection of
CH3SO3H from oleum suggests that electrolysis generates SO3-
reactive CH3•,[7a] consistent with the implication from HSAB theory
due to CH3•’s softness (Figure 1a).
Additional experimental data support
a CH4-activating
electrocatalysis mediated by AgII. Visual inspection during
electrolysis unveiled a yellow hue in solution near the FTO
electrode under potentials anodic enough to produce AgII (insets
in Figure 3d and S2), which faded away after the termination of
electrochemical oxidation. UV-vis spectroscopy recorded an
absorption peak at 364 nm with a shoulder around 700 nm (blue
trace in Figure 3d) that existed for more than 10 min after
electrolysis (Figure S8a). Such absorption features are distinct
from the stable spectrum of AgI (red trace in Figure 3d) and
identical to the one of commercially available AgO dissolved in
98% H2SO4 (yellow trace in Figure 3d and more in Figure S8b). In
the solution of AgO, a distorted octahedral AgII metalloradical
Reaction kinetics in the electrocatalysis
We evaluated the reactivity of CH4 activation as a function
of electrochemical potential E, CH4 pressure pCH4, and electrolysis
duration t. When cAg = 0.5 mM and pCH4 = 15 psi, the partial current
density (jCH4) and the Faradaic efficiency (FE) of CH3OSO3H
formation, quantified by 1H NMR (Figure S6d, Table S3), are
displayed against E in Figure 4a. CH3OSO3H formation takes
place when E > 1.5 V, concurrent with AgII generation in Figure
2b. A near-plateaued jCH4 at higher E contributes to a maximal FE
of 36.2% at 1.662 V when cAg = 0.5 mM (Figure 4a). This observed
optimum of electrocatalytic CH4 functionalization is about 600 mV
depicted in Figure 1c is confirmed via electron paramagnetic
26]
resonance (EPR) spectroscopy[19b,
with our measured
spectrum at 77 K (Figure 3e; g1 = 2.07, g2 = 2.09, g3 = 2.43). Given
the similar optical spectra, we deemed that AgII metalloradical is
generated upon electrochemical oxidation. Moreover, we found
that the solution of AgO is reactive towards CH4 ambiently and
yields CH3OSO3H, albeit with weaker reactivity (Figure S6b). This
observation supports our initial inspiration that AgII as a soft Lewis
acid is reactive towards CH4 and corroborates our proposed EC’
mechanism mediated by AgII (Figure 1c). Last, when oleum (20%
free SO3 basis) was employed as the electrolyte, the predominant
more cathodic than our previously reported vanadium-oxo
[2a]
electrocatalyst
and comparable with the Pd-based ones in
literature.[9] As the observed jCH4 plateau does not change
significantly when cAg = 10 mM (Figure S9, Table S3), it is the
mass transport of CH4 that limits both the jCH4 and FE since AgII
seems highly reactive towards CH4 and CH4’s solubility is about
0.8 mM ambiently (see SI).[27] Additional evaluation was
conducted under elevated pressures when cAg = 0.5 (entry 1 to 4)
and 3 mM (entry 5 to 10) in Figure 4b. When t = 3 h (entry 1 to 8
4
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Figure 4. a) Partial current density (jCH4, red) and Faradaic efficiencies (FE, blue) as a function of E. t = 3 h. b) Concentration of accumulated CH3OSO3H (cMHS
,
yellow), FE (blue), and turnover numbers (TONs, red) at different values of pCH4, t, and cAg. E = 1.737 V; *, Multiplied by a factor of 10. c) The plot of log10(jCH4) versus
log10(pCH4). E = 1.737 V. d) The plot of log10(jCH4) versus log10(cAg). pCH4 = 15 psi; E = 1.637 V. e) The plot of log10(jCH4) versus E. pCH4 = 15 psi. Each data point is
the average of at least two or three independent measurements. 25 ºC.
in Figure 4b), increasing pCH4 leads to increased FE and jCH4 and
results in a maximal FE of 72.1% at cAg = 0.5 mM, pCH4 = 85 psi,
and E = 1.737 V (Table S4). When pCH4 = 15 and 85 psi, our
reported data suggest a full-cell voltage of 1.07 and 1.15 V,
respectively, for an electricity-driven CH4 functionalization with the
reduction O2 into H2O as the reduction half-reaction, in
comparison to the thermodynamic driving force of 0.48 V and 0.50
V based on our calculation (Table S5). The ideal energy input,
AgII yielded ∂log10(jCH4)/∂log10(cAg) = 1.00 when pCH4 = 15 psi and
E = 1.637 V (Figure 4d, Table S8). Those results suggest that the
turnover-limiting step probably involves one equivalent of CH4 and
AgII. Plotting log10(jCH4) versus E when cAg = 10 mM and pCH4 = 15
psi yielded a Tafel slope of 70.6±4.8 mV/dec (Figure 4e, Table
S3). The obtained value of Tafel slope is in accordance with a
proposed EC’ mechanism with a continuous regeneration of AgII
(E step) and a turnover-limiting CH4-activating C’ step (Figure 1c),
which predicts a theoretical value of 60 mV/dec for Tafel slope.[28]
The obtained reaction kinetics on Ag enables us to calculate kobs,b
as the pseudo-first-order rate constant of CH4 activation,[29]
frequently dubbed as turnover frequency, based on bulk
electrolysis data (hence the subscript “b”).[2a, 30] At 25 ºC, cAg = 0.5
-1
assuming a unity of FE, is 210 and 220 kJ molCH4 (Table S6)
when pCH4 = 15 and 85 psi, respectively, in the context of a lower
-1
heating value (LHV) of 802 kJ molCH4 for CH4. Comparing the
results when cAg = 0.5 (entry 1 to 4) and 3 mM (entry 5 to 10) in
Figure 4b, higher cAg values increase CH3OSO3H concentration
(cMHS) yet lower FE values, likely due to the CH4’s limited solubility. mM, and E = 1.737 V, kobs,b reaches as high as 3.5×102 and
Extending t from 3 h to 72 h yielded cMHS > 25 mM when cAg = 3
mM (25 ºC, pCH4 = 125 psi, and E = 1.737 V; entry 9 and 10 in
Figure 4b, S10, and Table S7). While there remains much room
for optimization, the reported values of FE and cMHS illustrate the
favorable intrinsic activity and practical usability of this ambient
electrocatalytic CH4 activation.
Electrocatalytic kinetics was determined to be first-order on
both CH4 and AgII with a turnover-limiting CH4 activation and a
high value of pseudo-first-order rate constant kobs. Devoid of
mass-transport complication, jCH4 as a surrogate of the CH4-
2.8×103 h−1 when pCH4 = 15 and 85 psi, respectively (Table S4).
The values of kobs,b are on par with some of the best
9d, 11c]
electrocatalysts for CH4 activation (Table S9).[2a,
Furthermore, we calculated the effective turnover number (TON)
of CH4 activation, presented in Figure 4b. [30a, 30b] Similarly due to
CH4’s limited solubility (vide supra), higher cAg yielded lower TON
when comparing between cAg = 0.5 (entry 1 to 4) and 3 mM (entry
5 to 10) in Figure 4b. Prolonged electrolysis increases the
calculated TON values. When t = 72 h and pCH4 = 125 psi, TON
tops 2.45×104 (entry 10 in Figure 4b, Table S7) and no intrinsic
issue seems to prevent the electrocatalytic system from keeping
operation. The obtained TON value showcases the longevity of
the discovered electrocatalyst.
activation rate is found to be mostly linear with pCH4. When cAg
=
3 mM and E = 1.737 V, ∂log10(jCH4)/∂log10(pCH4) = 0.78 (Figure 4c,
Table S4). Experiments with cAg as a proxy of electrogenerated
5
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Figure 5. a) A kinetic model that includes competing AgII-based solvent oxidation (➊) and CH4 activation (➋), which were studied via steady-state bulk electrolysis
(“method 1”), temperature-dependent UV-vis absorption spectroscopy (“method 2”), and double potential step chronoamperometry (“method 3”). (b to e) The time
evolutions of AgII’s absorption spectra at 298 K (b, d) and the logarithmic of normalized absorbance at 364 nm ln(A364/A364,0) under different temperatures (c, e)
when pCH4 = 0 (b, c) and 15 psi (d, e). f) The logarithmic of pseudo-first-order rate constant for solvent oxidation (ln(ksol), blue) and CH4 activation (ln(kobs,s), red)
versus the inverse of temperature (1/T). Ea,sol and Ea,CH4, the calculated activation energies for AgII-based solvent oxidation and CH4 activation, respectively.
We found that CH4 activation is kinetically favored over the
undesirable side reaction of solvent oxidation. In 98% H2SO4
experiments of double potential step chronoamperometry[28] at
higher values of pCH4 indicate that kobs is linearly dependent on
pCH4 (“method 3” in Figure 5a). At a certain pCH4 value, E was first
poised at 2.0 V to oxidatively generate AgII population near the
electrode; then E was switched to 0.75 V at which potential the
residual AgII that had not reacted was reduced back to AgI. Right
after the potential application of 2.0 V on the electrode, current
electrogenerated AgII either activates CH4 or reacts with solvent
•
possibly via
a
HSO4 -yielding LMCT (vide supra). We
quantitatively established a reaction model that includes two
competing reaction pathways: CH4 activation and the undesirable
solvent oxidation (Figure 5a). The kobs,b calculated from bulk
electrolysis offers the first approach for elucidating the kinetics of
CH4 activation branch (“method 1” in Figure 5a). We also took
advantages of the difference in optical absorption between AgI
and AgII (Figure 3d and S2) and the equivalent identity of AgII in
the AgO solution and the post-electrolyzed AgI solution (Figure
S8, vide supra), and henceforth employed the AgII’s absorbance
at 364 nm (A364) of AgO solution to probe the transient of AgII at
different temperatures (“method 2” in Figure 5a, see SI). When
pCH4 = 0 psi, the initial decrease of A364 from AgII followed a first-
order kinetics for solvent oxidation (Figure 5b and 5c), which
possesses a rate constant ksol = 1.9 h−1 at 25 ºC and an activation
density
j was recorded under different pCH4 (Figure 6a).
Qualitatively, the higher steady-state j(t) under higher pCH4 at 2.0
V indicates a CH4-mediated electrocatalysis while the constant
steady-state j(t) at 0.75 V suggests that a Ag-based catalytic cycle
imposes no loss of Ag in the solution. If j0(t) is denoted as the
transient j(t) value when pCH4 = 0 psi, kobs,t based on such transient
measurement (hence the subscript “t”) can be mathematically
described:
ꢀ(ꢁ)
=
ꢃꢄꢁꢂꢃ,ꢄꢁ
(1)
ꢂ
ꢀꢀ(ꢁ)
The plots of j(t)/j0(t) versus t1/2 at various pCH4 values are displayed
in Figure 6b and the calculated kobs,t values are mostly linear to
pCH4 (Figure 6c, Table S11). The slope of kobs,t versus pCH4, at a
value of 2.8×10−4 psi−1•s−1, yields kcat as the second-order rate
constant of the bimolecular reaction between AgII and CH4. The
observed linear relationship between kobs,t and pCH4 offers another
evidence that the AgII-based CH4 activation is the turnover-limiting
step. Overall, the rate constants of AgII-based CH4 activation were
determined via three independent methods (Figure 5a): steady-
state bulk electrolysis (kobs,b), time-dependent absorption
energy Ea,sol = 25.7 kcal/mol (Figure 5f, Table S10). When pCH4
=
15 psi, the initial A364 decrease contributed from both solvent
oxidation and CH4 activation similarly followed a first-order
kinetics and was much faster (Figure 5d and 5e). The resultant
pseudo-first-order rate constant of CH4 activation from absorption
spectroscopy (hence the subscript “s”) kobs,s = 88 h−1 at 25 ºC with
an activation energy Ea,CH4 = 13.1 kcal/mol (Figure 5f, Table S10).
Furthermore, although our setup of UV-vis absorption
spectroscopy is unable to operate at elevated pressure,
6
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Figure 6. a) Double potential step chronoamperometry at different pCH4. E = 2.0 and 0.75 V for time t ∊ [0, 40 s] and t ∊ [40 s, 80 s], respectively; cAg = 10 mM. b)
The plot of j(t)/j0(t) versus t1/2 based on the results in (a) at different pCH4. j0(t) and j(t), current densities at time t when pCH4 = 0 and pCH4 ≠ 0, respectively. c) Pseudo-
first-order rate constant kobs,t for CH4 activation extracted from the linear region in (b) as a function of pCH4
.
spectroscopy
(kobs,s),
and
double
potential
step
Reaction mechanism and attainable Faradaic efficiency
Our results warrant mechanistic discussion in this
chronoamperometry (kobs,t). Notwithstanding the values of kobs,s
fetched when cAg > 10 mM, a satisfactory agreement was reached
at low cAg value (Table S12). The larger rate constant and the
lower value of activation energy for CH4 activation suggest that
during ambient electrocatalysis CH4 functionalization is order-of-
magnitude faster than the undesirable solvent oxidation.
a
electrocatalytic CH4 activation inspired by HSAB theory (Figure 7).
We showed that an electrochemical oxidation of AgI yields AgII
metalloradical continuously (E step and route i in Figure 7). EPR
spectroscopy supports AgII’s radical nature and the quasi-
reversible oxidative charge transfer is supported by the
successful application of Randles-Ševčík analysis and numerical
simulation.[25] Without CH4, AgII slowly undergoes solvent
oxidation (route ii in Figure 7; ksol = 1.9 h−1 and Ea,sol = 25.7
•
kcal/mol at 25 ºC), possibly via a HSO4 -yielding LMCT[19b, 31] as
evident in the reductive peak of 1.52 V in the cyclic voltammogram.
•
The resultant HSO4 is proposed to further dimerize and yield
persulfate, which eventually can disproportionate and generate
32]
O2 (Figure S11).[19b,
In CH4, a turnover-limiting reaction
between one equivalent of AgII and CH4 initiates CH4 activation
(route iii in Figure 7). The first-order reaction kinetics for both AgII
and CH4 has been supported by the near-unity values of
∂log10(jCH4)/∂log10(pCH4
)
and ∂log10(jCH4)/∂log10(cAg
)
in bulk
electrolysis, the time-dependent change of A364 for AgII, and the
experiments of double potential step chronoamperometry. The
reported pseudo-first-order rate constant kobs = 3.5×102 or 88 h−1
(kobs,b and kobs,s, respectively) at 25 ºC and pCH4 = 15 psi with Ea,CH4
= 13.1 kcal/mol. The quantitative determination of reaction
kinetics offers opportunities for additional understanding and
design of the reported electrocatalytic system.
We pondered whether AgII is directly responsible to
turnover-limiting C−H activation in CH4 (C’ step and route iii in
Figure 7) and the nature of reactions after CH4 activation during
•
catalysis (route iv and v in Figure 7). We contend that HSO4
−
generated from the LMCT of Ag-bound HSO4 has a minimal
contribution, if any, in the turnover-limiting step of AgII-initiated
CH4 activation, not only because the inclusion of HSO4 will
prevent a first-order kinetics for both AgII and CH4 but also
•
•
because HSO4 thermally generated from persulfate is reported
9b]
as inactive towards CH4.[2a,
The electrolysis in oleum with
yielded CH3SO3H suggests the presence of CH3• intermediate.[7a]
As η = 6.7, 6.42, and 4.87 for AgII, H•, and CH3•, respectively,[5]
we postulate a homolytic CH4 cleavage in the turnover-limiting
step, while we do not exclude the possibility of a Ag−CH3
intermediate from electrophilic activation. The reaction steps after
the turnover-limiting step of CH4 activation remain elusive. We
tentatively consider that the yielded CH3• can either directly react
Figure 7. a) A proposed catalytic cycle. b) Individual steps i to v in the catalysis
and the overall electrocatalytic oxidation reaction. TLS, turnover-limiting step.
7
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10.1002/anie.202104217
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RESEARCH ARTICLE
with another AgII for a consecutive two-electron oxidation (route
iv in Figure 7), or react with one HSO4• indirectly generated from
AgII to yield CH3OSO3H (route v in Figure 7). Nonetheless, as
shown by our data, the branch of CH4-activating pathway is
kinetically faster than the solvent oxidation at room temperature
and ambient pressure, paving a selective electrocatalytic CH4
activation. Assuming two consecutive AgII-mediated oxidations
for one equivalent of CH4 activation (route iii and iv in Figure 7),
the half-reaction of overall electrocatalysis is written as:
ꢍ
( )
( ) ꢇꢈꢂꢉꢊꢋꢄ
( )
(
)
ꢅꢆꢅ ꢇ + ꢆꢆꢈꢉꢅ ꢊ ꢋ⎯⎯⎯⎯⎯ꢌ ꢅꢆꢌꢉꢈꢉꢌꢆ ꢊ + 2 ꢆ ꢍꢎꢊꢏꢐꢁꢑꢒ
+2 ꢑꢎ
(2)
Last, the quantification of reaction kinetics indulges us to
predict the electrocatalysis’s ultimate performance when all of the
engineering constraints, particularly the mass transport of CH4,
are successfully addressed in
a hypothetical scenario. In
Figure 8. Calculated FE as a function of pCH4 and T based on the established
kinetic model and the measured kinetic parameters.
particular, we are interested in the FE towards CH4
functionalization. Our established kinetic model (Figure 5 and 6)
suggests that the pseudo-first-order kinetic rate constant of CH4
activation and the selectivity towards CH4 activation are both
the overall turnover. Future experiments are desired to continue
investigating the nature of reactions after the turnover-limiting CH4
activation. Engineering optimization is needed to overcome the
mass transport limitation due to CH4 limited solubility and
maximize the electrocatalysis’s selectivity at low temperature.
Moreover, the successful inquiry of CH4 catalysts initiated based
on the chemical hardness of possible reaction intermediates
encourages us to adopt such a strategy to discover more CH4-
activating catalysts, particularly with electrochemical charge
transfer as the reaction’s driving force. This work offers a new
route of ambient electrocatalytic CH4 functionalization as well as
a strategy that may be generally applicable for future catalyst
design, in order to achieve chemical production from widely
distributed natural gas resources with minimal infrastructure
reliance.
functions of CH4 pressure pCH4 and temperature T, i.e. kobs
=
kobs(pCH4, T) and FE = FE(pCH4, T). We utilized our kinetic model
shown in Figure 5a and calculated the values of FE(pCH4, T) as a
function of pCH4 and T (Figure 8, Table S13):
(
)
ꢓꢔ ꢕꢏꢐꢅ, ꢖ
ꢑ
ꢔ
ꢕ
ꢔ
ꢀ,ꢁꢂꢃ
ꢒ
ꢎ
ꢓ
ꢎ
ꢘ
ꢆꢖꢗ
(
)
ꢔ
ꢆꢖꢗ
ꢄꢁꢂꢃ,ꢄ ꢕꢏꢐꢅ, 298 ꢑ
=
(3)
ꢑ
ꢑ
ꢔ
ꢕ
ꢔ
ꢕ
ꢔ
ꢆꢖꢗ
ꢀ,ꢁꢂꢃ
ꢒ
ꢀ,ꢄꢅꢆ
ꢒ
ꢎ
ꢓ
ꢎ
ꢎ
ꢓ
ꢎ
ꢘ
ꢘ + ꢄꢃꢁꢙ 298 ꢑ
(
)
(
)
ꢄꢁꢂꢃ,ꢄ ꢕꢏꢐꢅ, 298 ꢑ
At T = 25 ºC (298K), FE(pCH4, T) is calculated as 53% at
ambient pressure (pCH4 = 15 psi). FE reaches 90% when pCH4
>
62 psi (4.3 bar), 95% when pCH4 > 85 psi (5.9 bar), and 99% when
pCH4 > 180 psi (12.4 bar) (Figure 8). This suggests that Ag-based
electrocatalysis of CH4 activation is capable of reaching near-
unity selectivity at room temperature and an accessible pCH4 that
is much lower than the syngas synthesis from steam reforming of
CH4 (650 ºC and 30 bar).[33] We also considered how reaction
temperature T affects FE (Figure 8). FE drops significantly at
higher temperature, because Ea,sol > Ea,CH4 and hence the rate of
solvent oxidation increases much faster than the one of CH4
activation. This suggests that AgII-mediated electrocatalysis is
selective and specifically suitable at lower temperature. While the
absolute rate of CH4 activation increases with temperature, it is
the competition between CH4 activation and the undesirable
solvent oxidation that dictates the selectivity. Overall, calculations
of FE(pCH4, T) suggest that the AgII-mediated electrocatalysis is
reactive and selective for low-temperature CH4 functionalization
and further engineering optimization will be constructive to reach
the electrocatalyst’s full potential.
Notes
A provisional patent has been filed under the names of C.L., D.X.,
J.A.I., J.D.. All other authors declare no competing interests.
Acknowledgements
We thank Dr. Paul Oyala at Caltech for the use of the EPR
spectrometer and Prof. Dr. Ellen Sletten at UCLA for the use of
temperature-dependent UV-vis spectrophotometer. We thank Dr.
Di Yang and Dr. Shengtao Lu for helpful discussions, and Mr.
Gary Chan for setup preparation. We thank the Molecular
Instrumentation Center lab at UCLA for sample characterizations.
J.A.I. acknowledges a Eugene V. Cota-Robles Fellowship. C.L.
acknowledges the NSF Award (CHE-1955836), startup fund from
UCLA, and the financial support of the Jeffery and Helo Zink
Endowed Professional Development Term Chair.
Conclusion
Inspired by the HSAB theory, we explored the
electrocatalytic CH4 functionalization mediated by AgII
metalloradical, the last class (b) metal whose reactivity towards
CH4 is unknown. Kinetic characterization establishes that
selective electrocatalytic CH4 activation proceeds ambiently in
98% H2SO4 with low activation energy and high kinetic rate
constant. The electrocatalytic system is unique that low reaction
temperature is preferred for high reaction selectivity, while a
reaction pressure slightly higher than ambience (6~10 bar) favors
Keywords: electrochemistry • high-valent silver • homogeneous
catalysis • HSAB theory • methane functionalization
8
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Entry for the Table of Contents
Inspired by the HSAB theory, we explored the electrocatalytic CH4 functionalization mediated by AgII metalloradical, the last class (b)
transition metal whose reactivity towards CH4 is unknown. Detailed mechanistic investigation unveils a low activation energy of 13.1
kcal•mol−1 and a high pseudo-first-order rate constant of CH4 activation up to 2.8×103 h−1 at room temperature.
10
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