Ambient methane functionalization initiated by electrochemical oxidation of a vanadium (V)-oxo dimer

The abundant yet widely distributed methane resources require efficient conversion of methane into liquid chemicals, whereas an ambient selective process with minimal infrastructure support remains to be demonstrated. Here we report selective electrochemical oxidation of CH4 to methyl bisulfate (CH3OSO3H) at ambient pressure and room temperature with a molecular catalyst of vanadium (V)-oxo dimer. This water-tolerant, earth-abundant catalyst possesses a low activation energy (10.8 kcal mol‒1) and a high turnover frequency (483 and 1336 hr−1 at 1-bar and 3-bar pure CH4, respectively). The catalytic system electrochemically converts natural gas mixture into liquid products under ambient conditions over 240 h with a Faradaic efficiency of 90% and turnover numbers exceeding 100,000. This tentatively proposed mechanism is applicable to other d0 early transition metal species and represents a new scalable approach that helps mitigate the flaring or direct emission of natural gas at remote locations.

T he wide geological distribution of natural gas resources leads to an undesirable loss of methane (CH 4 ) especially at remote locations via flaring or direct emission into the atmosphere 1,2 . One possible strategy to mitigate such an issue is to convert CH 4 into liquid chemicals at the source of emission under ambient condition with minimal reliance on an industrial infrastructure 2 . Fundamentally, this catalytic conversion requires low activation energy and high reactivity, in order to accommodate the low thermal energy and partial pressure of CH 4 substrate at ambient condition. Existing approaches of CH 4 functionalization usually operated at high pressure and/or elevated temperature [3][4][5][6][7][8][9][10][11][12][13][14][15][16] , involve metal-catalyzed reactions [3][4][5][6][7] , superacid-based activation 17 , or catalysis based on peroxo species for a free-radical chain mechanism [8][9][10] . For the metal-catalyzed reactions reported by Periana and others (Fig. 1a) [3][4][5][6] , electrophilic activation of CH 4 is followed by an oxidation process that regenerates the metal active sites, mostly precious metals including Pt and Pd. Yet, in ambient conditions, the reactivities of these electrophilic metal species seem insufficient to activate CH 4 . In the approaches based on radical chain mechanism (Fig. 1b) [8][9][10] , initiators including peroxo species yield oxygen radicals that activate CH 4 at room temperature with low activation energies 18 . But it is uneasy to achieve a sustained, selective catalytic process that generates and replenishes radical species. The challenge of balancing the reactivity and regeneration of active species call for an alternative approach for ambient CH 4 functionalization.
We propose that a controlled electrochemical generation of oxygen radicals is capable to address the above-mentioned challenge, as electrochemical redox process provides a sustained method of replenishing radical species at ambient conditions without sacrificing their high reactivities 19,20 . While d 0 early transition metal centers are not known to be directly oxidizable, we accidentally found that electrochemical oxidation of d 0 early transition metal-oxo species in 68-98% H 2 SO 4 yields cation radicals on the sulfonic ligand that selectively activate CH 4 at ambient pressure and room temperature (Fig. 1c). Here, we report d 0 vanadium (V)-oxo dimer (V 2 V,V , 1) 21 as the model catalyst for mechanistic understanding (Fig. 1d). As the tentative turnover-limiting step (TLS) uncommon for homogenous electrocatalysis, an electrochemical one-electron removal of 1 yields a cation radical (V 2 V,V •+ , 2) reactive towards CH 4 , while the catalytic cycle is fulfilled by additional electrochemical oxidation and cation radical regeneration.

Results
Discovery of ambient CH 4 activation with vanadium (V)-oxo catalyst. The d 0 vanadium (V)-oxo catalyst (1) was prepared by dissolving V 2 O 5 in 98% H 2 SO 4 . Cyclic voltammograms of 10 mM 1 in 98% H 2 SO 4 under 1-bar nitrogen gas (N 2 ) (blue), 1-bar CH 4 (red), and a blank control (black) are displayed in Fig. 2a at 25 ºC on a platinum (Pt) working electrode. A quasi-reversible peak corresponding to V V /V IV redox couple was observed with a midpoint potential E 1/2 = 0.644 V vs. Hg 2 SO 4 /Hg reference electrode, with a diffusion coefficient D = 2.18 × 10 −11 m 2 s −1 for 1 based on Randles-Savcik analysis ( Supplementary Fig. 1a, b) 22 . Additional oxidation current of 1 was observed at the electrochemical potential E > 1.6 V vs. Hg 2 SO 4 /Hg, and such an oxidation current is larger in CH 4 than in N 2 . This observation suggests that 1 can be further oxidized electrochemically and CH 4 is likely to react with the resultant oxidized species. Bulk electrolysis in 98% H 2 SO 4 under 1-bar CH 4 was conducted at a E = 2.255 V vs. Hg 2 SO 4 /Hg for 6 h with an electrode of fluorinedoped tin oxide ( Supplementary Fig. 1c). The liquid composition after electrolysis were analyzed by 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy. CH 3 OSO 3 H, which yields methanol after hydrolysis 11 , was observed at chemical shift δ = 3.34 ppm in 1 H NMR after electrolysis with 10 mM 1 under CH 4 (red in Fig. 2b). No gaseous or liquid products other than CH 3 OSO 3 H were observed as a product of CH 4 oxidation within our detection limit via NMR spectroscopy (Fig. 2b), mass spectroscopy ( Supplementary Fig. 2a, b) and gas chromatography ( Supplementary Fig. 3). Organic products were not detected in the absence of 1 under CH 4 (black), with 10 mM 1 under N 2 (blue in Fig. 2b), or at a less anodic potential (E = 1.855 V vs. Hg 2 SO 4 / Hg) ( Supplementary Fig. 2d). These data confirm that CH 4 undergoes a two-electron oxidation into CH 3 OSO 3 H, initiated by the electrochemical oxidation of 1. The absence of a well-defined redox wave preceding the current onset in Fig. 2a suggests either a homogenous electrocatalysis limited by the rate of electron transfer or the occurrence of materials deposition during the scans of cyclic voltammetry. The measurement of X-ray photoelectron spectra on a FTO electrode after 6-h electrolysis with 10 mM 1 detected no residual V signal on the electrode (Supplementary Fig. 4) despite the observed CH 3 OSO 3 H formation (1-bar CH 4 , E = 2.255 V vs. Hg 2 SO 4 /Hg). Reusing this vanadiumexposed FTO electrode for 6-h electrolysis in neat 98% H 2 SO 4 under CH 4 yielded no CH 4 activation. These results suggest a likely homogenous electrocatalysis limited by charge transfer. We also conducted isotope-labeling experiments by introducing 13 CH 4 as the substrate at 1-bar pressure. The introduction of 13 CH 4 in lieu of CH 4 of natural abundance leads to the surge of 13 CH 3 OSO 3 H signal at δ = 58.6 ppm in 13 C NMR after electrolysis (Fig. 2c). An introduction of a 50% 13 C-enriched CH 4 yields not only the same peak in 13 C NMR but also a triplet in 1 H NMR with 13 C-1 H doublet and 12 C-1 H singlet at a 1:1 ratio of integration area ( Supplementary Fig. 5). The optical absorption spectra of the solution before and after electrolysis were identical to each other ( Supplementary Fig. 2e), implying that 1 as a catalyst was regenerated after electrolysis.  Fig. 2 Electrochemical functionalization of CH 4 and natural gas mixture. a, b Cyclic voltammograms (a) and 1 H NMR spectra of liquid samples after 6-h electrolysis (b) for 10 mM 1 in 1-bar CH 4 (red), 10 mM 1 in 1-bar N 2 (blue), and blanks without 1 (black). Dashed blue, current density (j) of blue trace magnified by a factor of 10. *, internal standard acetic acid. c 13 C NMR spectra of samples before (black) and after electrolysis with 13 CH 4 (red) and CH 4 of natural abundance (blue), respectively. d Calculated TONs (red) and electric charges passed (blue) versus the duration of electrolysis. e F.E. of CH 4 functionalization in 10 mM 1 vs. electrode potential E under 1-bar N 2 (green), 1-bar CH 4 (blue) and 3-bar CH 4 (red). f Cumulative TONs for C 1 (red), C 2 (green, multiplied by a factor of 5), and C 3 products (black, multiplied by a factor of 50) as well as F.E. values of total liquid products (blue) are plotted against the duration of bulk electrolysis. TON values for CH 3  We expanded the substrate scope from CH 4 to ethane (C 2 H 6 ), propane (C 3 H 8 ), and ultimately natural gas mixture at 1-bar pressure. C 2 H 6 was oxidized to a mixture of acetic acid (CH 3 COOH) and ethyl bisulfate (C 2 H 5 OSO 3 H) ( Supplementary  Fig. 6a), whose TOF values are 297 and 235 h −1 , respectively. C 3 H 8 was converted to mostly isopropyl bisulfate (i-C 3 H 7 OSO 3 H) with trace acetone (CH 3 COCH 3 ) in a 6-h electrolysis ( Supplementary  Fig. 6b), with TOF values of 962 and 2 h −1 , respectively. One challenge in designing a process of natural gas utilization is to balance the low reactivity of the major component CH 4 with the high reactivity of minor light alkane components 24 , as in some cases the latter substrates can react 100-time faster than CH 4 25 . The similar TOF values among CH 4 , C 2 H 6 , and C 3 H 8 reported here renders 1 a suitable candidate of direct natural gas utilization at ambient conditions without much upstream separation.
Natural gas supplied to UCLA via pipeline by SoCalGas 26 was used as the substrate of electrolysis (E = 2.255 V vs. Hg 2 SO 4 /Hg) with 0.7 mM 1 in 98% H 2 SO 4 at room temperature and ambient pressure. Powered by electricity, natural gas was oxidized into organic chemicals while dioxygen in air was reduced on the counter electrode, fulfilling a partial oxidation of natural gas with air with a net reaction of CH 4 The yielded products mainly consist of CH 3 OSO 3 H from CH 4 , CH 3 COOH, and C 2 H 5 OSO 3 H from C 2 H 6 , and CH 3 COCH 3 from C 3 H 8 in the natural gas. The TONs of C 1 -C 3 products reached about 107,000, 9300, and 200 within 240 h, respectively ( Fig. 2f) with the final concentration of CH 3 OSO 3 H approaching 10 mM ( Supplementary Fig. 7). The observed one-order-ofmagnitude difference between 1 and CH 3 OSO 3 H product again supports the catalytic feature of our observation. The total F.E. of all liquid products remain stable at around 90% during electrolysis after an initial induction period (Fig. 2f, Supplementary Table 2). The linear relationship between TONs and electrolysis durations suggest that 1 remains active and is tolerant to the impurities in natural gas mixture. Previous analysis suggests that a lower H 2 SO 4 concentration in the electrolyte, ideally below 80%, is needed for industrial implementation 11 . This requirement is against the thermodynamic limit of reactions in H 2 SO 4 with SO 3 as the oxidant 11 . Yet we found that 1 remains active towards CH 4 functionalization in aqueous solution with H 2 SO 4 concentrations as low as 68% under 25 ºC and 1bar CH 4 ( Supplementary Fig. 6c), yielding a mixture of methanol and CH 3 OSO 3 H ( Supplementary Fig. 6d). The robustness under prolonged operation and the applicability in diluted H 2 SO 4 -H 2 O mixed solvent renders the catalyst potentially suitable to be employed to functionalize natural gas with minimal maintenance.
Scale-up potential over vanadium (V)-oxo electrocatalyst. The reported catalyst 1 is also capable to yield high product concentrations amenable for practical implementations. Due to the limited solubility of CH 4 in the solvent within current batch reactor 11 , electrocatalytic experiments of higher product concentrations were conducted under room temperature at 11-bar CH 4 pressure, in order to mitigate the mass transport issue of limited gas solubility (vide supra) ( Supplementary Fig. 7). A 72-h electrolysis leads to a CH 3 OSO 3 H concentration of~110 mM with F.E. = 81.2% (E = 2.376 V vs. Hg 2 SO 4 /Hg). Adding 1 M CH 3 OSO 3 H prior to the electrocatalysis under the same condition does not hinder the catalysis or decompose the pre-added CH 3 OSO 3 H. A single-pass conversion of 1% was observed in the mixed-flow electrochemical reactor, comparable to the results that lead to electrochemical reduction of CO 2 and CO at nearindustrial level 27,28 . The product concentrations reported here are higher than the ones in other electrocatalysis 5,6 , and suggest that high product concentration of electrocatalysis is attenable. There seems no observable fundamental limit for product concentrations exceeding one mole per liter, a threshold considered suitable for industrial applications 11 .
Kinetics investigation of electrochemical CH 4 functionalization. The attractive feature of catalyst 1 led us to investigate the underlying mechanism during electrolysis with CH 4 as the substrate. The current density corresponding CH 3 OSO 3 H formation (j CH4 ), a surrogate of CH 4 -activating rate, was investigated as a function of catalyst concentration [1] (Fig. 3a), the electrode potential E (Fig. 3b), the partial pressure of CH 4 (p CH4 ) (Fig. 3c), and the temperature T (Fig. 3d). A linear relationship with a slope = 1.03 ± 0.08 between log 10 (j CH4 ) and log 10 ([1]) suggests that CH 4 activation is first-order on 1 (Fig. 3a). When log 10 (j CH4 ) was plotted against E (Fig. 3b), a Tafel slope of about 120 mV dec −1 was observed before j CH4 plateaus at larger E values as CH 4 is depleted near electrode. This suggests that the first electron removal from 1 is the TLS, uncommon for homogeneous electrocatalysis (Supplementary Note 2). The overlapping points under 1 and 3-bar CH 4 pressure when E < 2.1 V vs. Hg 2 SO 4 /Hg suggest that CH 4 is not involved in the TLS or any pre-equilibrium steps. When E > 2.1 V vs. Hg 2 SO 4 /Hg, a linear relationship between log 10 (j CH4 ) and log 10 (p CH4 ) with a slope of 0.91 ± 0.07 (Fig. 3c) suggests that CH 4 is activated in a first order after the TLS. We also found that the residual current density, the difference between total current density (j total ) and j CH4 , is independent of p CH4 (Supplementary Fig. 2c). It further corroborates that no gaseous or liquid products other than CH 3 OSO 3 H were generated from CH 4 oxidation, and solvent oxidation into O 2 and possibly trace persulfate 29,30 is the only plausible side reaction (vide infra). The Arrhenius plot between ln(j CH4 ) and 1/T yields an apparent activation energy (E a ) as low as 10.8 ± 0.6 kcal mol −1 (Fig. 3d), consistent with the observed ambient reactivity. When conducting electrolysis of 1 in 98% D 2 SO 4 with CH 4 of natural isotope abundance, 2 D and 1 H NMR spectra showed no H/D exchange in the methyl group of product CH 3 OSO 3 H ( Supplementary Fig. 6e). This excludes the possible mechanism induced by an electrochemically generated superacid 17 , which should yield significant H/D exchange in the methyl group 11 . In addition, when CH 4 was exposed to 1 in 98% H 2 SO 4 with added K 2 S 2 O 8 or H 2 O 2 in the absence of electricity, CH 4 functionalization was not observed at ambient conditions ( Supplementary Fig. 6f). This illustrates that it is difficult for chemical method to sustainably generate reactive radical intermediates at room temperature, which necessitates our use of electrochemistry as proposed before. It also shows that the possible formation of peroxo species including peroxoacids is not contributing to the observed reactivity. When the electrolyte was switched from 98% H 2 SO 4 to oleum with 20% SO 3 , a 5.4:1 molar ratio between CH 3 SO 3 H and CH 3 OSO 3 H was observed after electrocatalysis ( Supplementary Fig. 8). This indicates the formation of CH 3 • radical during the catalysis, which yields CH 3 SO 3 H in the presence of SO 3 9 . Overall, our experimental data support an electrochemical catalysis of low activation energy. After a turnoverlimiting one-electron oxidation of 1, the oxidized species undergoes a first-order C-H activation in CH 4 and a formation of CH 3 • radical (Fig. 1d).
Unveiling dimer structure of vanadium (V)-oxo catalyst. Despite its ease of preparation, the structural information of 1 is not well understood. It was hypothesized to be a V 2 V,V dimer with two terminal V V ≡O moieties connected by a bridging oxo 21 . We measured 1's optical absorption (Supplementary Fig. 9a) and the 51 V NMR spectrum ( Supplementary Fig. 9b), which confirmed that 1 is different from monometallic VO 2 + species in an aqueous medium Density functional theory (DFT) calculations suggest that 1 may exist as two isomers, 1a and 1b (Fig. 4a), with a calculated energy difference of 1.2 kcal mol −1 . In an attempt to obtain the correct structure of 1 and real conformation in solution, X-ray absorption spectroscopy of V atom was conducted for 10 mM 1 in 98% H 2 SO 4 , solid V 2 O 5 , and metallic V foil (solid red, dashed blue, and dashed yellow in Fig. 4b, c, respectively). We carried out a least-square-regression analysis on X-ray absorption near-edge structure (XANES) for the threshold positions, the first peak in the derivative spectra, of VO, V 2 O 3 , VO 2 , and V 2 O 5 to determine the electronic structure and oxidation state of vanadium in 1 (Supplementary Fig. 9c) 31 . The electronic structure of vanadium in 1 remains similar to that of vanadium in V 2 O 5 , confirming the d 0 electronic structure of vanadium. The extended X-ray absorption fine structure (EXAFS) can offer coordination information of absorbing atoms by extracting the structural parameters. As shown in Fig. 4c, the absence of noticeable peaks in the region beyond 4 Å (solid red), compared with those of V 2 O 5 and V foil (dashed blue and dashed yellow, respectively), indicates that 1 is a complex homogenously dispersed in the solvent. The peak around 1.56 Å in 1's EXAFS spectrum (gray area) is attributed to the V-O bonds, following the assignment of V-O bonds in the V 2 O 5 sample. While this comparison provides some information, the general low symmetries of the vanadium-based species prevent us from gaining detailed structural information of 1 solely based on EXAFS data 32 . To this end, we conducted the fitting of 1's V K-edge EXAFS spectrum combining the structure suggested by DFT calculations (shown in Fig. 4d). It reveals that the central V atoms are penta-coordinated by O atoms with three types of V-O bond lengths (1.58, 1.68, and 1.96 Å) in the first coordinated shell, with a bridging oxo with a V-O bond length of 1.68 Å. We note that the EXAFS of 1 suggests a unique coordination environment 2.0-3.5 Å away from V atom (blue area), which is different from the monometallic VO 2 + species in aqueous medium ( Supplementary Fig. 9d). The fitting results of second shell (blue area) indicate that consistent with the predicted structure 1a, there are not only three S atoms in the second shell (2.73 and 3.13 Å) but also one V atom at the distance of 3.27 Å away from the central V atom (Supplementary Table 3). Detailed analysis is provided in the Supplementary information ( Supplementary  Fig. 9e, f). These results reveal the existence of a hypothesized structure of µ-oxo bridged V 2 V,V dimer 33 , and suggest that 1a is the structure of 1 in the solution.
Operando characterizations for mechanistic study. Additional operando characterizations were conducted to confirm a homogenous electrocatalysis and elucidate identities of immediate species. Operando Raman spectroscopy measures the vibrational spectra of chemical species near the FTO electrode at different values of E in CH 4 . No spectral changes were observed and the vibrational spectra evidently differs from solid V 2 O 5 sample (Supplementary Fig. 10 Fig. 2a). The pre-edge region also witnesses a broadening and intensity decrease of the pre-edge peak concurrent with the increase of E and the observation of electrocatalytic CH 4 functionalization ( Supplementary Fig. 11). This suggests an increase of coordination symmetry near V atom and possibly a loss of sulfonic ligand 31 . The operando EXAFS results ( Supplementary Fig. 12) also displays a decrease of average coordination number of sulfonic ligands per V atom concurrent with increasing E values. Those results imply that the bisulfate group in CH 3 OSO 3 H likely originate from the vanadium catalyst, a plausible radical rebound mechanism 35 .

Proposed electrocatalytic cycle with vanadium (V)-oxo dimer.
Combining experimental and computational results, we established a proposed catalytic cycle of 1 for CH 4 functionalization (Fig. 1d) despite its uncommon assignment of TLS that warrants additional investigation (Supplementary Note 2). A turnover-limiting electrochemical oxidation of 1a removes one electron from O 2p orbitals in the sulfonic ligand, which is calculated as the highest occupied molecular orbital (HOMO) of 1a (Fig. 4e). The resultant cation radical 2 is predicted to possess an empty frontier spin-orbital on the same O 2p orbitals (lowest unoccupied molecular orbital (LUMO) of 2 in Fig. 4e), which is postulated to initiate H-atom abstraction from CH 4 . DFT calculations predict reaction trajectory between 2 and CH 4 without significant energy barrier (Fig. 4e, Supplementary Fig. 13a). This is consistent with our experimental observation that the TLS is the one-electron oxidation of 1 other than the step of C-H activation (Supplementary Note 2). The calculated barrier-less C-H activation also helps explain the similar TOFs toward various light alkanes in the natural gas 24 . Currently, we were unable to experimentally characterize   2 and the H-atom abstraction step due to its transient nature, which will be of our research focus in the future. Yet the subsequent steps of CH 4 functionalization seems to proceed with the formation of CH 3 • and a radical rebound process 35 . This leads to a two-electron oxidation and CH 3 OSO 3 H formation with a ligand loss on a V 2 IV,V dimer (3), which will be readily re-oxidized electrochemically to regenerate 1 (Fig. 1d, Supplementary Fig. 14).
Exploration of molecular and material variants for vanadium (V)-oxo catalyst. DFT calculations of the atomic charges 36 suggest that cation radical 2 is stabilized thanks to orbital delocalization, in comparison to the scenario when one electron was removed from sulfuric acid ( Supplementary Fig. 13b, c). This is consistent with the results that the calculated ionization energies of 1a is lower by 12-14 kcal mol −1 than that of sulfuric acid. This implies that the metal-oxo centers as carrier of sulfonic ligands stabilize the electrochemically generated cation radical, at the same time maintain a cation radical reactive enough for ambient CH 4 functionalization. Other d 0 early transition-metal-oxo species can possess similar reactivities. We found that d 0 metal-oxo species, including Ti IV -oxo and Cr VI -oxo, are also electrochemically active towards functionalizing CH 4 at ambient conditions ( Supplementary Fig. 15). A more extensive survey over the first half of the Period 4 elements except Sc indicates that only Ti, V, Cr, and possibly Mn display similar reactivities (Supplementary Fig. 16). It suggests the strategy of employing d 0 early transition-metal-oxo species is generally applicable for ambient electrochemical functionalization of natural gas. As such a general trend of reactivity was not observed before, we posit our electrochemical approach may offer new perspective towards the challenge of CH 4 functionalization. Practically, a heterogeneous catalyst variant may also be desirable. While 1 is characterized as a homogenous catalyst, we found a two-dimensional layered material, VOPO 4 •2H 2 O (4) with exposed V-oxo edges 37 (Supplementary Fig. 17a, b), acts as a heterogeneous variant of 1 in 98% H 2 SO 4 ( Supplementary  Fig. 17c, d). This preliminary result suggests that even within the same metal-oxo system, the catalyst subsequently its reactivity can be tuned with additional materials design and engineering. This heterogenous variant also simplifes product separation in downstream process, thanks to the absence of vanadium in the liquid phase.

Discussion
Overall, the general tunability of catalyst composition may herald better catalysts with higher TOF, lower oxidation potential, as well as versatile design of the overall process. The ambient condition of reported catalysis facilitates the use of O 2 in ambient air as the terminal electron acceptor, as well as the use of ambient natural gas feedstock for onsite functionalization without a centralized facility. Future research will focus on possible scale-up with the exploration of optimal reaction conditions. The employment of 98% H 2 SO 4 or more diluted acids, other than oleum, mitigates the generation of excessive acid in product separation and is attractive for practical application 38 . Additional fundamental and engineering investigation, including the employment of gas diffusion electrode 39 as well as ingenious design of electrochemical reactors 28 , will further explore the possible application of converting CH 4 into commodity chemicals with minimal infrastructure support at remote locations. This will lead to the more efficient usage of green-house gases and reducing their emission into atmosphere.

Methods
Chemicals and materials. All chemicals were purchased from Sigma-Aldrich, Thermo Fisher Chemical, or VWR International, unless otherwise stated. All chemicals were used as received unless specified. Dimethyl sulfoxide-d 6 (DMSO-d 6 ) was purchased from Cambridge Isotope Laboratories, Inc. All deionized (DI) water was obtained from a Millipore Milli-Q Water Purification System. Fluorine-doped Tin Oxide (FTO) glass was purchased from Hartford Glass Incorporation. CH 4 (99.5%) was purchased from Airgas, C 2 H 6 (99%), C 3 H 8 (98%), and 13 CH 4 (99%; 99 atom% 13 C) were purchased from the Sigma-Aldrich. Natural gas mixture was obtained from the outlet in Molecular Science Building 4211, Department of Chemistry and Biochemistry, UCLA, which is supplied via pipeline by SoCalGas. SRI multiple gas analyzer #5 gas chromatograph (GC), 8610C is used to analyze the natural gas mixture. The components are 91.78 mole% CH 4 , 4.31 mole% C 2 H 6 , 0.31 mole% C 3 H 8 , 0.04 mole% n-C 4 H 10 , 0.03 mole% i-C 4 H 10 , 0.01 mole% n-C 5 H 12 , 0.01 mole% i-C 5 H 12 , and 0.81 mole% CO 2 . Unless specifically noted, reagent-grade 98% H 2 SO 4 (VWR BDH3074-3.8LP) was employed as the solvent, which contains 5 ppm of metal impurities. When needed, we also employ high-purity 98% H 2 SO 4 (Sigma-Aldrich 339741), which contains 0.3 ppm of metal impurities as shown in the product certificate.
Chemical and material characterizations. Ultraviolet-visible (UV-vis) spectra was conducted on Hewlett-Packard 8453 UV-vis spectrophotometer. Proton NMR ( 1 H-NMR) and carbon NMR ( 13 C-NMR) were recorded on a Bruker AV400 (400 MHz) spectrometer. Deuterium NMR ( 2 D-NMR) was recorded on a Bruker AV500 (500 MHz) spectrometer. Vanadium NMR ( 51 V-NMR) was performed on an Agilent DD2 600 (600 MHz) spectrometer. Powder X-ray diffraction (XRD) patterns were measured on a Panalytical X'Pert Pro X-ray Powder Diffractometer with a Cu Kα source (λ = 1.54178 Å), The intensities were recorded within the 2θ range from 10°to 60°with a voltage of 45  Catalyst preparation. Homogeneous bimetallic catalyst 1 was prepared by dissolving vanadium pentoxide (V 2 O 5 ) in 98% H 2 SO 4 with ultra-sound treatment for 6 h. Homogeneous titanium (IV)-oxo and chromium (VI)-oxo catalysts were prepared by dissolved titanyl sulfate (TiOSO 4 ) and potassium chromate (K 2 CrO 4 ) in 98% H 2 SO 4 , respectively. The heterogeneous variant 4 (VOPO 4 ·2H 2 O) was prepared based on previous literature 37, 40 . V 2 O 5 (4.8 g), H 3 PO 4 (85.5%, 26.6 mL), and H 2 O (115.4 mL) were refluxed at 110 ºC for 16 h. After gently cooling down to room temperature, the yellow precipitate in the mixture was collected by filtration and washed several times with water and acetone. The resulting sample was dried in an oven at 100 ºC for 3 h. When 4 was investigated for its electrochemical response, 4 was loaded onto a FTO electrode via a dip-coating procedure. A dispersion of 4 was prepared at a concentration of 6 mg mL −1 in 2-propanol. The yellow dispersion was ultrasonicated for 30 min until the color of the dispersion became faded. Afterwards, sodium carboxymethyl cellulose (CMC) was added into the dispersion (weight ratio of VOPO 4 ·2H 2 O: CMC = 80: 5). The mixture was stirred at 600 revolution per minute (rpm) on the heating plate to remove excess 2propanol and form a homogenous slurry, which was then dip-coated onto FTO at a loading amount of 1.9 mg cm −2 for 4.
Electrochemical characterization. All electrochemical experiments were recorded using a Gamry Instruments Reference 600+ and Interface 1000 potentiostats. Unless mentioned specifically, a three-electrode setup was applied with a Pt wire pseudo-reference electrode and a Pt counter electrode. The Pt pseudo-reference electrode was calibrated to a Hg 2 SO 4 /Hg (saturated K 2 SO 4 ) reference electrode (CH Instrument, Inc.) via the measurement of open-circuit potentials. The relationship is E(V vs. Hg 2 SO 4 /Hg) = E(V vs. Pt) + 0.755 V. The gas environment of the electrochemical cell was controlled either CH 4 (Airgas, 99.5%) or N 2 (Airgas, 99.999%), which were bubbled into the reactor at rates of 7.2 (CH 4 ) and 10 (N 2 ) standard cubic centimeters per minute (sccm) with the use of a mass flow controllers (Omega Engineering, Inc., FMA5510A). The data were reported after iR compensation. Unless noted specifically, the electrolyte is 98% H 2 SO 4 with a certain vanadium concentration of 1. Experiments of cyclic voltammetry were conducted in a single-chamber electrochemical cell with a 2-mm Pt working electrode (CH Instruments, Inc.). Bulk electrolysis with 5 mL homogeneous catalyst solution was typically conducted in a two-chamber electrochemical cell with a Nafion 117 membrane as the separator and a piece of commercial fluorine-doped tin-oxide (FTO) glass as the working electrode. Here, the FTO glass was encapsulated with Teflon tapes so that the exposed electrode is 1 × 1 cm in dimension. The solution was pre-saturated with CH 4 or N 2 for 20 min before the commencement of electrolysis. The background signal contribution of FTO glass was subtracted for the recorded data. Liquid aliquots were taken before, during and after the electrolysis for product analysis. Gaseous samples were taken manually from the outgas of the reactor, diluted with pure N 2 (1:5 ratio) for transportation purpose, and manually injected into the GC-MS. The injected permanent gas was not well separated by the column, but the MS spectra of the eluted gas peak (t~3.3 min) was capable to quantify permanent gases with a detection limit of~10 ppm. Additional measurements of gas chromatography for the gaseous effluents was conducted by the SRI multiple gas analyzer #5. The experiments at elevated pressure was conducted into a custom-designed setup utilizing a pressure vessel with two gas feedthrough and three electric feedthroughs (Parr Instrument, Series 4600, 1000 mL). In this setup, the gas pressure was controlled between 1 and 11 bar and a constant gas flow of up to 28.7 sccm was maintained during the electrolysis. The temperature of the electrolysis was maintained by an oil bath at a range between 25 and 55 ºC. For experiments using substrates other than CH 4 , the same procedure is followed except the gas flow rate is set at 10 sccm calibrated against N 2 . In the 240-h electrolysis using natural gas mixture as the substrate, the electrolyte was refilled after each aliquot sampling in order to maintain a constant electrolyte volume. Experiment with 50% 13 C-enriched CH 4 was conducted by feeding regular CH 4 and 13 CH 4 gases at equal flow rates controlled by two mass flow controller.
We found that typical reagent-grade 98% H 2 SO 4 possesses nonzero background activity toward CH 4 at high electrochemical potential ( Supplementary Fig. 16). We argue that such background activity is due to the trace metal impurity, possibly V as H 2 SO 4 is industrially synthesized by a V 2 O 5 catalyst in the contact process. The reagent-grade 98% H 2 SO 4 or oleum employed in most studies contains about 5 ppm of metal impurities (VWR BDH3074-3.8LP). When we employed high-purity 98% H 2 SO 4 (Sigma-Aldrich 339741) that only contained 0.3 ppm of metal impurities as shown in the product certificate, the observed reactivity of neat 98% H 2 SO 4 vanished ( Supplementary Fig. 16).
When the heterogeneous variant 4 was investigated for its electrochemical response, a similar procedure was followed. However, owing to the difference of solution composition, the Pt pseudo-reference electrode has a different relationship with the Hg 2 SO 4 /Hg reference electrode: E(V vs. Hg 2 SO 4 /Hg) = E(V vs. Pt) + 0.268 V.
Attempts of using chemical oxidants at ambient conditions. When CH 4 (7.2 sccm, 1 bar) was bubbled into a 98% H 2 SO 4 solution with 10 mM 1 and 10 mM K 2 S 2 O 8 for 6 h at ambient conditions, the formation of methyl bisulfate (CH 3 O-SO 3 H) as a possible product of CH 4 oxidation was not observed ( Supplementary  Fig. 6f). Similar experiment was conducted with 10 mM H 2 O 2 in lieu of K 2 S 2 O 8 . The formation of methyl bisulfate (CH 3 OSO 3 H) as a possible product of CH 4 oxidation was not observed ( Supplementary Fig. 6f), either.
Quantification of liquid products. 1 H-NMR was applied to quantify product accumulation in DMSO-d 6 using acetic acid (CH 3 COOH) as the internal standard. Totally, 0.4 mL liquid aliquots from electrolysis were mixed with 0.1 mL DMSO-d 6 prior to the measurements. Chemical shifts are reported on a parts-per-million (ppm) scale. Methyl bisulfate (CH 3 OSO 3 H) exhibits a singlet at 3.34 ppm while the singlet from acetic acid (CH 3 COOH) peak resides at 1.96 ppm. A calibration curve was constructed by determining the relative ratio of integrated area between the NMR peaks of CH 3 OSO 3 H and CH 3 COOH. Product quantification of C 2 H 6 , C 3 H 8 , and natural gas mixture follows the same protocol, except for the quantification of CH 3 COOH as a C 2 product from C 2 H 6 . The quantification of CH 3 COOH as a C 2 product was fulfilled by adding a known concentration of CH 3 OSO 3 H as an internal standard in a separate 1 H-NMR measurement.
Calculation of FE. The F.E. of bulk electrolysis was calculated based on the following equation: Here, F is the Faraday's constant, C product is the concentration of product after bulk electrolysis, V solution is the total electrolyte volume, and the overall charge is the total electric charges passed through the working electrode. The variable n in the equation is the number of electrons required in order to generate one product molecule by electrochemistry. n = 2 for the formation of methyl bisulfate (CH 3 OSO 3 H) from CH 4 . n = 2 and 6 for the formation of ethyl bisulfate (C 2 H 5 OSO 3 H) and acetic acid (CH 3 COOH) from C 2 H 6 , respectively. n = 2 and 4 for the formation of isopropyl bisulfate (i-C 3 H 7 OSO 3 H) and acetone (CH 3 COCH 3 ) from C 3 H 8 , respectively.
Calculation of TOF and TON. In the following we provide the protocols that we calculate the TOFs and TONs for the reported data, based on the methods established in prior literature 19,23 . The diffusion coefficient for 1 (D) was determined from the cyclic voltammgrams based on the Randles-Sevcik equation 22 : Here j p is the peak current density of quasi-reversible redox couple, n is the number of electrons transferred in the redox event, F is the Faraday's constant, C cat is 1's concentration, v is the scan rate, R is the gas constant, and T is the temperature of experiment. As derived from Supplementary Fig. 1a, b, D = 2.18 × 10 −7 cm 2 s −1 for species 1 in the electrolyte. The observed TOF of bulk electrolysis was determined based on the following equation 19,23 : Here, j product is the partial current density of product formation in bulk electrolysis, n is the number of electrons required to generate one product molecule, F is the Faraday's constant, C cat is the concentration of catalyst 1, D is the diffusion coefficent of catalyst (2.18 × 10 −7 cm 2 s −1 for species 1 as determined above). Similarly, the TON of bulk electrolysis was determined based on the following equation 19,23 : Here, C product is the product concentration after bulk electrolysis, V solution is the total electrolyte volume, A is the electrode area, C cat is the concentration of catalyst 1, D is the diffusion coefficent of catalyst (2.18 × 10 −7 cm 2 s −1 for species 1 as determined above), and TOF is the TOF calculated based on above protocol. On the condition of a homogenous process with molecular catalyst 1, the diffusion coefficient extracted from the V(V)/V(IV) redox couple is a reasonable approximate of the real catalytic redox couple, the vanadium(V)-oxo dimer and its one-electron-deficient cation radical, because we are unable to characterize the cation radical due to its transient nature. Here, we offer a rough estimate of the relative uncertainty of such an approximation based on our proposed mechanism. The Stokes-Einstein relationship 41 predicts that the diffusion coefficient D of similar molecules follows: D~MW −1/3 , in which MW is the molecular weight. As MW = 538.16 g mol −1 for 1, the relative uncertainty of D in our procedure is about 6% or 12%, assuming the loss of one or even two of the sulfonic ligands, respectively. As the TOF~D −123 , at most about 10% relatively uncertainly will incur in our practice.
Computational methods. All calculations were performed with Turbomole 42-52 using the M06 density functional 53 . The def2-SVP basis set was used for geometry optimizations and free energy corrections, and the def2-TZVP basis set was used for electronic energies 54 . Solvation was modeled with COSMO 55 with the dielectric constant set to 101 56 . Images were rendered using Chemcraft 57 . The natural bond orbital (NBO) analysis was used for atomic charge calculations 36 .
Details of XAS experiments. XAS, including X-ray absorption near edge spectra (XANES) and extended X-ray absorption fine structure (EXAFS), at V K-edge were collected in total-fluorescence-yield mode at ambient conditions at BL17C of National Synchrotron Radiation Center (NSRRC), Hsinchu, Taiwan. Spectra were recorded in the energy range from −100 to 600 eV, relative to that of V K-edge absorption (5465.0 eV). The XAS spectra were processed by subtracting the baseline of pre-edge and normalizing that of post-edge. EXAFS analysis was carried out using Fourier transform on k 3 -weighted EXAFS oscillations to assess the contribution of each bond pair to Fourier transform peak. The curve fitting of EXAFS spectra was conducted using the software, REX2000, with FEFF program.
The operando XAS experiments at V K-edge were conducted under the same procedure at TPS beamline 44A of NSRRC, Hsinchu, Taiwan. A three-electrode arrangement was used during the operando measurements. The electrolyte was saturated with 1-bar CH 4 , and the measurements were performed using an Autolab PGSTAT204 potentiostat (Metrohm Autolab) in a customized reactor.
Operando Raman characterization. A three-electrode setup in a home-made cell was adopted for Operando Raman spectroscopy and the electrochemical measurements. The measurements in a CH 4 -saturated electrolyte were recorded using a Raman microscopy (UniNano UNIDRON) and an Autolab PGSTAT204 potentiostat (Metrohm Autolab). A laser of 633 nm with a spot size of~1 µm 2 served as the excitation source, and the output power was 2.5 mW. A 50× objective lens was employed for operando measurements during electrolysis, while all results were obtained under an exposure duration of 3 s with the accumulation number of 60 times. 57. Chemcraft-graphical software for visualization of quantum chemistry computations. https://www.chemcraftprog.com.