Two-Dimensional Metal-Organic Layers on Carbon Nanotubes to

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Functional Nanostructured Materials (including low-D carbon)

Two-Dimensional Metal-Organic Layers on Carbon Nanotubes to Overcome Conductivity Constraint in Electrocatalysis Ling Yang, Lingyun Cao, Ruiyun Huang, Zhong-Wei Hou, XiangYang Qian, Bing An, Hai-Chao Xu, Wenbin Lin, and Cheng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13356 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Applied Materials & Interfaces

Two-Dimensional Metal-Organic Layers on Carbon Nanotubes to Overcome Conductivity Constraint in Electrocatalysis Ling Yang[a], Lingyun Cao[a], Ruiyun Huang[a], Zhong-Wei Hou[a], Xiang-Yang Qian[a], Bing An[a], Hai-Chao Xu[a], Wenbin Lin[a], [b], Cheng Wang*[a] [a]

College of Chemistry and Chemical Engineering, iCHEM, State Key Laboratory of Physical

Chemistry of Solid Surface, Xiamen University, Xiamen 361005, China. [b]

Department of Chemistry, University of Chicago, 929 East 57th Street, Chicago, Illinois

60637, United States KEYWORDS:

metal–organic

layers,

metal-organic

frameworks,

electrosynthesis,

electrocatalyst, alcohol oxidation

ABSTRACT: Application of metal-organic frameworks (MOFs) in electrocatalysis is of great interest, but is limited by low electrical conductivities of most MOFs. To overcome this limitation, we constructed a two-dimensional version of MOF—metal–organic layer (MOL) on conductive multi-walled carbon nanotubes (CNTs) via facile solvothermal synthesis. The redox active MOLs supported on the CNT efficiently catalyze the electrochemical oxidation of alcohols to aldehydes and ketones. Interestingly, this CNT/MOL assembly also endowed the

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selectivity for primary vs. secondary alcohols via well designed interfacial interactions. This work opens doors towards a variety of designer electrocatalysts built from functional metalorganic frameworks.

Introduction Metal-organic frameworks (MOFs) have emerged as a powerful platform to integrate molecular entities into functional materials.1-13 MOFs can catalyze electrocatalytic reactions such as oxygen evolution,14-17 hydrogen evolution,18, 19 oxygen reduction,20-22 and CO2 reduction,23, 24 but the relatively low electrical conductivities of most MOFs limit their efficiencies and the scope of reactions. In particular, these constraints limit the use of MOFs in electrosynthesis of organic compounds. Using electricity as green, safe and atomically efficient oxidation reagents in organic synthesis has attracted much attention recently.25 For example, the catalytic oxidations of alcohols to the corresponding carbonyl compounds is a transformation of significant interest

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,

but many of the chemical oxidants used in this reaction such as chromium (VI) and manganese reagents are quite toxic. By contrast, electrochemical oxidation of alcohols can be much cleaner and much cheaper.27, 28 The electrode potential can also finetune the oxidative power. Electrode modification can further regulate electrochemistry on the surface to control selectivity and reaction rates, opening up numerous possibilities. Porous MOFs with tunable catalytic centers can be a good candidate for electrode modification. The electrical conductivity of MOFs usually relies on a hopping mechanism, in which electron or hole undergoes step-by-step jump to adjacent ligands via repeated redox reactions with concomitant counter-ion movement in the MOF channel to balance the local charges.29, 30 However, the electrolytes in organic solvents


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usually contain large tetra-alkyl-ammonium cations that have very low diffusivities in MOFs, which limit the conductivity and the overall rate of electrochemical transformation. If the MOF is non-conductive, only the layer directly attached to the electrode surface can be active in catalysis. As a result, the relatively large organic molecules need to diffuse through the thick MOF layers to reach the active layer, which further limits their conversion rates. We and others have recently constructed a two-dimensional version of MOFs, metal– organic layers (MOLs)31-41, which are 2D networks built from organic linkers and metal coordination nodes. The thinness of MOLs down to a monolayer can overcome the limitation of electrical conductivity by decreasing the transport distance of the electron and hole. In this paper, we report the growth of catalytically competent MOLs on multi-walled carbon nanotubes (CNTs) as efficient electrocatalysts for the oxidation of alcohols. The softness of the MOL thin film makes it possible to grow MOLs around CNTs as a coat. By combining the MOL with the molecular catalytic entity and the CNT electrode with a high surface area, we can significantly enhance the activity in electrochemistry and direct the reaction path towards the desired product.

Results and discussion The MOL was constructed from three-connected 4,4',4''-nitrilotribenzoate (NTB) and sixconnected Hf6(µ3-O)4(µ3-OH)4(HCO2)6 secondary building unit (SBU) to form a twodimensional network of kagome dual (kgd) topology (Supporting Information [SI], Figure S1). Treatment of HfOCl2·8H2O with H3NTB at 120 oC under solvothermal condition led to the formation of the MOL in 72% yield with a chemical formula of Hf6(µ3-O)4(µ3-OH)4(µ1-OH)2(µ1OH2)2(HCO2)4(NTB)2, as confirmed by thermogravimetric analysis (TGA) (Figure S2), infrared


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spectrum (Figure S3) and proton nuclear magnetic resonance (1H-NMR) studies of digested MOLs (Figure S4). The formate capping groups on the SBU can undergo exchange with other carboxylates for post-synthetic modification of the MOLs.

Figure 1. a) Tapping-mode AFM topography of the MOL and the height profile along the white line. b) Experimental (red line) and simulated (black line) PXRD patterns Hf-NTB-MOL. c) HRTEM image and Fourier transform (FFT) of the image of Hf-NTB-MOL. d) HAADF image of the MOL. Transmission electron microscopy (TEM) showed that the MOLs existed as wrinkled ultrathin films with an average sheet area of 0.3 × 0.3 µm2 (Figure S1e). High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field (HAADF) images showed patterns of hexagons corresponding to the Hf6 SBUs in the structure (Figure 1a, 1b). The


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distance between adjacent SBUs in the images was about 17.2 ± 0.3 Å, which matched that in the structural model (17.5 Å). Such a 2D kgd network was also confirmed by powder X-ray diffraction (PXRD) patterns, which can be indexed to (hk) reflections like other crystalline 2D materials (Figure 1c).37 The main diffraction peaks of (10) at 5.82º and (11) at 10.10º are consistent with the 2D unit cell of the structural model with a=b=17.5 Å, γ = 120o. Furthermore, atomic force microscopy (AFM) revealed the height of the MOLs to be 1.2 ± 0.2 nm (Figure 1d), which is very close to the van der Waals diameter of the Hf6 cluster in a monolayer.

Figure 2. a-c) Schematic illustrating the synthesis of catalysts 1 and 2, which are assemblies of TEMPO-CO2H or TEMPO-OPO3H2 with CNT/MOL through carboxylate-SBU and phosphateSBU linkages, respectively. a) Structures of the SBU, NTB ligand and CNTs. b) Structure of the MOL in a 2D kgd lattice on CNTs. c) 1 and 2 for electrochemical oxidation of alcohols. d) TEM image of MOLs. e) MOLs on CNTs (CNT/MOL). f) TEM image of catalyst 1.


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The CNT/MOL assembly was obtained by adding carboxyl-functionalized CNTs in the MOL synthesis (Figure 2). The carboxylate groups on the outer surface of CNTs replace some of the capping formates on the MOL SBUs, building a bridge between the CNTs and the MOL. TEM showed images of CNTs covered by the MOL films (Figure 2e). The PXRD pattern of CNT/MOL gave peaks corresponding to both the MOL and the CNTs, confirming the retention of MOL structure (Figure S5). TGA showed a two-stage weight loss with the first drop between 300–430 oC for MOL decomposition and a second drop after 430 oC due to CNT removal (Figure S2), giving weight percentages of 43.4% for MOL and 56.6% for CNTs, respectively.

Figure 3. a) The conductivity of CNT/MOL, 1 and MOL. b) Cyclic voltammetry curve of 1. c) Relationship between catalytic current and concentration of several alcohols. d) Reuse of catalyst 2. The CNT/MOL assembly increased the conductivity of MOL significantly, from 1.6×10-7 S·m-1 for MOL alone to 3.65 S·m-1 for CNT/MOL (Figure 3a, Figure S6). Cyclic voltammetry


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(CV) scan of CNT/MOL showed a pair of redox peaks of the NTB ligand at around 0.92 V (vs. Cp2Fe+/0) (Figure S7). Integration of the oxidative peak gave a charge corresponding to oxidation of 34.6% of the NTB ligand to NTB·+. The electrochemically generated NTB·+ can oxidize benzyl alcohol, as confirmed by an oxidative catalytic current in the presence of benzyl alcohol and the base 2,6-lutidine. We then loaded CNT/MOL onto a macroporous reticulated vitreous carbon (RVC, 100 PPI) as the working electrode for electrolysis at 0.96 V. Complete conversion of benzyl alcohol was observed in 12 hours, leading to the formation of benzaldehyde and benzoic acid as well as some unidentified byproducts. The low product selectivity is likely due to the high oxidation potential of NTB. To reduce the potential and improve selectivity, we attached 4-carboxy-2,2,6,6tetramethylpiperidine 1-Oxyl free radical (TEMPO-CO2H) and 4-phosphonooxy-2,2,6,6tetramethylpiperidine 1-Oxyl (TEMPO-OPO3H2) onto CNT/MOL by replacing some of the remaining formates on the MOL SBUs (Figure 2c). A mixture of TEMPO-CO2H or TEMPOOPO3H2 and CNT/MOL in DMF or H2O was heated at 60 oC for 24 h to form the CNT/MOLTEMPO-CO2- (1) and CNT/MOL-TEMPO-OPO32- (2) assemblies. TEMPO and its nitroxyl derivatives are active catalysts in electrochemical organic synthesis.42 Immobilizing the TEMPO moieties on the electrode surface are preferred for recycling expensive TEMPO and simplifying work-up. In addition, immobilized TEMPO on the working electrode also prevents reduction of oxidized TEMPO (oxoammonium ion) on the counter electrode to increase Faradaic efficiency. After the modification, the molar ratio between TEMPO-CO2H and NTB in 1 was determined by high-performance liquid chromatography (HPLC) to be 3:5, which corresponds to the attachment of 1.2 TEMPO-CO2H per Hf6 SBU (Figure S8). Similarly, the amount of TEMPO-OPO3H2 was determined to be 1.5 per SBU. The ratio between formate and NTB was


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1:3 (Figure S9), which is significantly lower than the ratio in unmodified CNT/MOL (2:1), confirming the exchange of formates by TEMPO-CO2H. TEM image and PXRD patterns confirmed retention of the CNT/MOL structure after TEMPO-CO2H modification (Figure 2f, Figure S5). Conductivity measurement of the powdery sample of 1 gave a value of 1.89 S·m-1 (Figure 3a, Figure S6). A CV study of 1 showed reversible redox peaks of TEMPO at 0.34 V (Figure 3b, Figure S10) in addition to that of NTB at 0.92 V. Integration of the oxidative half of the TEMPO peak gave a charge corresponding to 26.1% of the TEMPO-CO2H on the MOL. Table 1. Electrochemical oxidation of benzyl alcohol. [a]

Entry Deviation from above conditions Yieldb.(%) 1 none 100 c 2 twice amount of catalyst 100 3 no 2,6-lutidine 58 4 no catalyst trace 5 CNTs only trace 6 CNT/MOL only trace 7 catalyst 1 suspension only 0 8 no voltage 0 9d 0.15 equiv TEMPO only 76 10e 0.15 equiv TEMPO-CO2Me only 79 11 Catalyst 2 100 [a] Reaction conditions: 20mM benzyl alcohol, acetonitrile (8.5mL), H2O (1.5mL), 0.1M Bu4NClO4, 6.9mM 2,6-lutidine, 20mg 1 or 2 (containing 0.02 equiv. of TEMPO w.r.t. benzyl alcohol) on RVC as anode, Pt cathode, Ag / AgClO4 (10 mM in 0.1 M Bu4NClO4, CH3CN) reference electrode, 100min. [b]The yield was determined by 1H-NMR with 1,4-dinitrobenzene as a standard.

[c]

50min.

[d]

24% benzoic acid was also obtained as a byproduct.

[e]

21% benzoic acid

was also obtained as a byproduct.


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When benzyl alcohol and 2,6-lutidine were added to the electrochemical system, a catalytic oxidation current was observed at 0.34 V. Bulk electrolysis at 0.46 V with the 1 or 2 on RVC electrode gave full conversion of benzyl alcohol to benzaldehyde within 100 min at room temperature with 100% selectivity (Table 1, entry 1，entry 11, Figure S11) and 97% Faradaic efficiency. The only byproduct was H2 which was generated on the counter electrode as confirmed by GC analysis (Figure S12). When twice the amount of the catalyst was loaded, the reaction time to complete conversion was shortened by half (Table 1, entry 2, Figure S13), indicating a linear relationship between the amount of loaded catalyst and the catalytic current density. The catalytic role of TEMPO-CO2H on the MOL was further verified by control experiments with RVC (Table 1, entry 4), or RVC / CNTs (Table 1, entry 5) or RVC / CNT/MOL (Table 1, entry 6), all of which gave only trace amounts of the product. Electricity is indispensable for the oxidation reaction as no product was observed when the voltage on the working electrode was switched off (Table 1, entry 8). Moreover, 1 dispersed in solution instead of loaded on the electrode displayed no catalytic activity (Table 1, entry 7), confirming that the oxidation happened on the electrode. 2,6-lutidine had a promoting effect for the reaction as its absence led to a sharp reduction of the yield to 58% (Table 1, entry 3). 1 was reused three times, and the structure of 1 was also retained after the electrolysis (Figure S5, Figure S14). The catalytic activity with slightly reduced (conversion in the electrolysis of 100 min: from 100% in the first run to 85% in the third run), due to detachment of TEMPO-CO2H from the MOL (Figure S15). Phosphate linkage between the TEMPO and Hf6 SBU is much stronger than the carboxylate ones. The 2 is thus much more stable than 1, and can


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be reused for six times with only slight decrease in activity (conversion in the electrolysis of 100 min: from 100% in the first run to 97% in the sixth run) (Figure 3c, Figure S16). The performance of 1 and 2 was superior over a homogeneous solution of TEMPO.43 A large amount (0.15 equiv) of TEMPO (Table 1, entry 9) or TEMPO-CO2Me (Table 1, entry 10) was needed in the homogeneous system, as compared to 0.02 equivalents of TEMPO used with catalyst 1. The Faradaic efficiencies of homogeneous TEMPO solution (62%) were also lower than that of 1 (97%) probably due to the undesired TEMPO+ reduction on the counter electrode. We also explored the substrate scope of 1. For example, electrochemical oxidation of secondary aromatic alcohol 1-phenylethanol (Table S1, entry 1) afforded acetophenone in 76% yield after the electrolysis of 100 min. The catalyst can also oxidize primary aliphatic alcohols such as 1,4-butanediol (Table S1, entry 2) in 79% yield. However, when secondary aliphatic alcohols such as 4-phenyl-2-butanol (Table S1, entry 3) and isopropanol (Table S1, entry 4) were used as the substrates, no products were obtained after electrolysis at the same voltage for 100 min.


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Figure 4. a) The relationship between the steric sizes of the substitutions on the 4-position of TEMPO and the electrocatalytic oxidation yields of isopropanol. b) The selectivity of the TEMPO-based catalyst for n-proposal over isopropanol. The selective oxidation of primary alcohols in the presence of secondary alcohols can be a useful process.44 TEMPO preferentially catalyzes the oxidation of primary alcohols over secondary ones. Here, by immobilizing TEMPO-CO2H on the MOL, we even reinforce this selectivity (Figure 4a). Using a 1:1 mixture of isopropanol and n-propanol as the substrate, 1 selectively oxidized n-propanol to propanal without any detectable conversion of isopropanol (Figure 4b, Table S1, entry 8), while homogeneous TEMPO catalyst produced a mixture of propanal and acetone (Figure 4b, Table S1, entry 7). Similarly, in a 1:1 mixture of benzyl alcohol and 4-phenyl-2-butanol, 1 selectively oxidized benzyl alcohol without any conversion of 4phenyl-2-butanol (Table S1, entry 9).

Figure 5. a) Proposed catalytic cycle of electrochemical oxidation of alcohols with a TEMPObased catalyst. b) The conformational change of TEMPO resulting from the 4-substitution group


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(R-). c) Relative adduct formation constant (green) and the O-N-C6-cis-Me dihedral angle (θ) as a function of the 4-substitution group on TEMPO. d) The steric bulk of SBU on MOL induces conformational changes that disfavor adduct formation.

TEMPO-catalyzed alcohol oxidation begins by electrochemical oxidation of TEMPO to TEMPO+, which then reacts with the alcohol to form a TEMPO-alcohol adduct through an O-N bond.45 The adduct is further converted to TEMPOH and an aldehyde or ketone via Habstraction (Figure 5a, Scheme S1). Kinetic measurements showed a Langmuir-type dependence of catalytic current on substrate concentration for all alcohols (Figure 3d, Figure S17). Such a dependence is consistent with a rate-determining step of H-abstraction (collapse of the adduct, Figure 5a) and a rate law of

r=

kKn(TEMPO)ሾROHሿ 1+KሾROHሿ

(Eq. 1)

where ሾROHሿ is the alcohol concentration; k is the rate constant for H-abstraction; K is an effective equilibrium constant for TEMPO-alcohol adduct formation (see SI Section 17). Fitting of the kinetic curves of different alcohols gave similar rate constants (k values) (Table S2), but a significantly lower formation constant (K) for aliphatic secondary alcohol than other alcohols. These fitting parameters were consistent with calculations using density functional theory (DFT). Transition states for the H-abstraction were found by the synchronous transit-guided quasi-Newton method, which showed similar activation energies responsible for the k values of different alcohols (Table S4). In contrast, thermodynamic analyses of the formation constants ‫ ܭ‬gave a low value for aliphatic secondary alcohol as compared to others (Table S3). We thus attributed the selectivity against aliphatic secondary alcohol to the unfavorable thermodynamics of adduct formation.


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The adduct formation constant is related to the steric size of the substitution group (R) on the 4-position of TEMPO. DFT calculations of a series of TEMPO derivatives with alkyl substitutions of increasing sizes gave decreasing adduct formation constants with isopropanol (Figure 5b, Table S5). A large substitution group on the 4-position induces a conformational change that pushes the axial oxygen on N closer to vicinal cis-methyl groups, adding energy penalty to the adduct (Figure 5c). This is supported by a correlation between the dihedral angle of O=N-C-Me (θ, Figure 5c) and the relative adduct formation constants KR/KH (KH is the formation constant of unsubstituted TEMPO) (Figure 5b). The MOL surface is equivalent to a large substitution group on the 4-position of TEMPO, which impedes reaction with secondary aliphatic alcohols (Figure 5d). Conclusions We have developed a CNT/MOL assembly for electrocatalysis. After post-synthetic modification with TEMPO-CO2H and TEMPO-OPO3H2, the CNT/MOL-TEMPO assemblies serve as an efficient electrocatalyst for oxidation of alcohols. This work highlights a general method for assembling two-dimensional metal-organic layers (MOLs) on electrodes for efficient electrochemical conversions. Because of the versatility and molecular tunability of MOLs, we believe that we just opened a door towards many electrocatalysts that can be designed to possess distinct activities and selectivities. Experimental Materials and apparatus Reagents were commercially available and used without further purification unless otherwise indicated. Carboxyl-functionalized multi-walled carbon nanotubes (CNTs) were


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supplied by Chengdu Organic Chemistry Research Institute. Distilled water with the specific resistance of 18.2 MΩ·cm was obtained by Direct-Q 3UV. A Nicolet 6700 instrument equipped with an MCT detector was used to collect transmission IR spectrum of powder samples at a 4 cm−1 resolution. Thermogravimetric analysis (TGA) was performed in air using a Shimadzu TGA-50 equipped with an alumina pan and heated at a rate of 2°C per minute. Powder X-ray diffraction data were collected on Agilent SuperNova, Rigaku and Bruker D8 Venture diffractometers using Cu Kα radiation sources (λ = 1.54178 Å). Electron microscopy images were obtained on a Tecnai F20, JEOL 2100 High-Resolution Transmission Electron Microscopy, and JEOL 1400 with an electron acceleration energy of 200 kV. TEM samples were prepared by dispersing the samples in ethanol with a sonic bath followed by drop casting onto carbon film on Molybdenum grid. AFM images were taken on a Bruker Multimode Ⅴ. 1H NMR spectra were recorded on a Bruker NMR 500 DRX spectrometer at 500 MHz and a Bruker 400 MHz DRX spectrometer and referenced to the proton resonance resulting from incomplete deuteration of the CD3CN (δ 1.94) or DMSO-d6 (δ 2.50). The amounts of TEMPO and H3NTB in the catalysts were quantified by a SHIMADZU HPLC instrument on a wondasil C18 column using aqueous H3PO4 solution (20 mM) / CH3OH (0.25/0.75) as eluent and a UV detector (λ = 242 nm, λ = 345 nm). Synthetic procedures for MOLs The synthetic conditions for MOLs were optimized by systematically exploring the parameter space of reactant concentrations and reaction temperatures. In an optimized procedure, HfOCl2 8H2O (4mg) and H3NTB (3mg) were dissolved in a mixture of HCO2H, water, and DMF, in a molar ratio of 1.1/1/833/1005/3259 in Pyrex vial, and kept at 120 ºC for 2 days. Yellow precipitates were obtained and washed with DMF for three times (yield 72%).


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Synthetic procedures for 1 and 2 A mixture of HfOCl2 8H2O (4mg), CNT (5mg) and H3NTB (3mg) was dissolved in a mixture of DMF, HCO2H, and water in Pyrex vial, and kept at 120 ºC in a small vial for 1 day. The whole vial was mechanically shaken during the reaction to mix reactants. Black suspensions of CNT/MOL were obtained and washed with DMF for three times. Then a mixture of CNT/MOL and TEMPO-CO2H was dispersed in DMF in Pyrex vial and kept at 60 ºC for 24h. Black suspensions of 1 were obtained and washed with DMF for three times. The synthetic condition for 2 was similar to that of 1. A mixture of CNT/MOL and TEMPO-OPO3H2 was dispersed in H2O in Pyrex vial and kept at 60 ºC for 24h. Suspensions of 2 were obtained and washed with H2O for three times. Electrochemical measurements Cyclic voltammograms were recorded with a three-electrode system and an electrochemical station (CHI660E) using a glassy carbon disk working electrode (diameter, 5 mm), a Pt plate auxiliary electrode and a Ag / AgClO4 (10 mM in 0.1 M Bu4NClO4, CH3CN) reference electrode. Controlled potential electrolysis experiments were performed with a CHI-1000c electrochemical workstation. To prepare the catalyst ink, 1 was ultrasonically dispersed in a mixture of ethanol (0.48 mL) and 5% Nafion (0.02 mL) for 15 min. 10 uL of the resulting ink was pipetted onto the glassy carbon disk electrode. Under neutral conditions, 1 shows the redox peaks (black curve) of TEMPO-CO2H (Figure S10). The oxidation peak at 0.407V suggests that TEMPO was oxidized directly on the anode to give N-oxoammonium (TEMPO+). The reduction peak of TEMPO+/TEMPO at 0.281V suggests that TEMPO+ is stable within the timescale of CV. When


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benzyl alcohol was added into the electrolyte, there was no significant change in the voltammogram curve of the 1 catalyst (blue curve). When 2,6-lutidine was added as a base, the anodic current increased dramatically (red curve). The anodic current rises at the same potential as that of the redox peak of TEMPO+/TEMPO, suggesting that TEMPO+ generated via anodic oxidation could catalytically oxidize benzyl alcohol. The oxoammonium salt accepted one hydrogen from the alcohol, being converted to hydroxylamine while producing the oxidized substrate; the hydroxylamine is then electrochemically oxidized back to the nitroxyl radical and subsequently to the active oxoammonium salt. Electrical Conductivity measurements Electrical conductivity measurements were carried out on a KEITHLEY 2450 source meter. The I–V curve was measured on pellets of the samples on a four-probe resistance measurement set-up. The pellets were prepared by pressing powders of MOL, CNT/MOL and 1 into round disks. The thickness l and diameter d of each pellet was measured for the conductivity calculation. The resistance R of each pellet was then obtained from the I-V curve. The conductivity of each sample is calculated by the following equation:

σ=

4l Rπd2

(Eq. 2)

ASSOCIATED CONTENT Supporting Information. Experimental procedures; structural characteristics, electrochemical study, mechanistic study, and DFT calculations. The following files are available free of charge.


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AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENT We acknowledge funding support from the National Natural Science Foundation and Ministry of Science and Technology of the P. R. China (NSFC21671162, 2016YFA0200702, NSFC21471126), the National Thousand Talents Program of the P. R. China. REFERENCES 1. Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keeffe, M.; Yaghi, O. M., Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. Science 2002, 295, 469-472. 2. Ferey, G., Hybrid Porous Solids: Past, Present, Future. Chem. Soc. Rev. 2008, 37, 191-214. 3. Le Ouay, B.; Kitagawa, S.; Uemura, T., Opening of an Accessible Microporosity in an Otherwise Nonporous Metal-Organic Framework by Polymeric Guests. J. Am. Chem. Soc. 2017, 139, 7886-7892. 4. Xiao, D. J.; Bloch, E. D.; Mason, J. A.; Queen, W. L.; Hudson, M. R.; Planas, N.; Borycz, J.; Dzubak, A. L.; Verma, P.; Lee, K.; Bonino, F.; Crocella, V.; Yano, J.; Bordiga, S.; Truhlar, D. G.; Gagliardi, L.; Brown, C. M.; Long, J. R., Oxidation of Ethane to Ethanol by N2O in a MetalOrganic Framework with Coordinatively Unsaturated Iron (II) Sites. Nat. Chem. 2014, 6, 590595. 5. Fujita, D.; Fujita, M., Fitting Proteins into Metal Organic Frameworks. ACS Cent. Sci. 2015, 1, 352-353.


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