Metalloenzyme-Inspired Ce-MOF Catalyst for Oxidative Halogenation Reactions

[EN] The structure of UiO-66(Ce) is formed by CeO2-x defective nanoclusters connected by terephthalate ligands. The initial presence of accessible Ce3+ sites in the as-synthesized UiO-66(Ce) has been determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR)-CO analyses. Moreover, linear scan voltammetric measurements reveal a reversible Ce4+/Ce3+ interconversion within the UiO-66(Ce) material, while nanocrystalline ceria shows an irreversible voltammetric response. This suggests that terephthalic acid ligands facilitate charge transfer between subnanometric metallic nodes, explaining the higher oxidase-like activity of UiO-66(Ce) compared to nanoceria for the mild oxidation of organic dyes under aerobic conditions. Based on these results, we propose the use of Ce-based metal-organic frameworks (MOFs) as efficient catalysts for the halogenation of activated arenes, as 1,3,5-trimethoxybenzene (TMB), using oxygen as a green oxidant. Kinetic studies demonstrate that UiO-66(Ce) is at least three times more active than nanoceria under the same reaction conditions. In addition, the UiO-66(Ce) catalyst shows an excellent stability and can be reused after proper washing treatments. Finally, a general mechanism for the oxidative halogenation reaction is proposed when using Ce-MOF as a catalyst, which mimics the mechanistic pathway described for metalloenzymes. The superb control in the generation of subnanometric CeO2-x defective clusters connected by adequate organic ligands in MOFs offers exciting opportunities in the design of Ce-based redox catalysts.This work has been supported by the Spanish Government through the "Severo Ochoa" (SEV-2016-0683, MINECO) and RTI2018-101033-B-I00 (MCIU/AEI/FEDER, UE). J. M. Salas is acknowledged for his contribution to CO-IR experiments. The Electron Microscopy Service of the UPV is also acknowledged for their help in sample characterization.Rojas-Buzo, S.; Concepción Heydorn, P.; Olloqui-Sariego, JL.; Moliner Marin, M.; Corma Canós, A. (2021). Metalloenzyme-Inspired Ce-MOF Catalyst for Oxidative Halogenation Reactions. ACS Applied Materials & Interfaces. 13(26):31021-31030. https://doi.org/10.1021/acsami.1c074963102131030132

Electrolytic synthesis of chloroacetic acids in a filter-press reactor from polychloromethanes

Electrochemical carboxylation of carbon tetrachloride and chloroform to synthesize chloroacetic acids in mild conditions has been investigated by galvanostatic electrolyses using a homemade electrochemical filter-press reactor. Undivided filter-press reactor consisting of a planar Zn cathode and a planar Al sacrificial anode has been shown to be an optimal electrochemical device for the electrocarboxylation reaction. The influence of some operative parameters, such as current density and concentration of supporting electrolyte, on the efficiency and selectivity of the synthetic process has been investigated. Highly current efficiencies and target products yields (both of them > 85%) were achieved in all cases, whereas byproducts formation was minimal.Dirección General de Ciencia y Tecnología de España. CTQ2004-0036

Understanding the potential-induced activation of a cobalt MOF electrocatalyst for the oxygen evolution reaction

Metal–organic frameworks (MOFs) are attractive porous materials for electrocatalytic applications associated with carbon-free energy storage and conversion. This type of material usually requires a post-treatment to be used as electrocatalyst. The present work comprehensively investigates the electrochemical activation of a cobalt-MOF@Nafion composite that produces outstanding electrocatalytic performance for the water oxidation reaction at neutral pH. A detailed electrochemical characterization reveals that the electroactivation of the composite requires the participation of the oxygen evolution reaction (OER) and leads to a significant increase in the electroactive population of cobalt centers. It is shown that an increase of the applied activation potential in the OER region results in a faster electroactivation of the Co-MOF without affecting the intrinsic electrocatalytic properties of the active cobalt centers, as evidenced by the unique linear correlation between the electrocatalytic OER current and the population of electroactive cobalt. In addition, at structural level, it is shown that the electrochemical activation causes the partial disruption of the Nafion adlayer, as well as morphological changes of the Co–MOF particles from a compact, rounded morphology, before electrochemical activation, to a more open and expanded structure, after electroactivation; with the concomitant increase of the number of surface–exposed cobalt centers. Interestingly, these cobalt centers retain their coordinative chemistry and their laminar distribution in the nanosheets at the nanoscale, which is consistent with the preservation of their intrinsic electrocatalytic activity after potential–induced activation. In this scenario, these results suggest that only the electroactivated cobalt centers with good accessibility to the electrolyte are electrochemically active. This work provides a better understanding of the processes and structural changes underlying the electrochemical activation at neutral pH of a Co–MOF for boosting the electrocatalytic water oxidation reaction9 página

Voltammetric study of the adsorbed thermophilic plastocyanin from Phormidium laminosum up to 90 °c

Redox thermodynamics and kinetics of plastocyanin from Phormidium laminosum, and of azurin from Pseudonomas aeruginosa, have been investigated as a function of temperature by protein film voltammetry. To this purpose, both proteins have been physisorbed on a pyrolytic graphite edge electrode. A pronounced negative shift of the plastocyanin standard potential, compared to a slight shift in the case of azurin, has been found upon increasing the temperature. Hence, significant conformational and/or solvation changes accompany the redox conversion of plastocyanin. Lower electron transfer rate constants (by c.a. one order of magnitude) and higher activation enthalpies have been found for plastocyanin as compared to azurin. The voltammetric response of azurin vanishes irreversibly at temperatures close to 60 °C, whereas the redox properties of plastocyanin remain unaltered, except for some loss of electroactive protein, after heating the electrode at temperatures as high as 90 °C.Ministerio de Economía y Competitividad CTQ 2008-00371, BFU2009-07190Junta de Andalucía P07-FQM-02492, P06-CVI-0171

Ultra-low metal loading rhodium phosphide electrode for efficient alkaline hydrogen evolution reaction

The practical production of hydrogen from water electrolyzers demands efficient electrocatalysts with maximized and optimized active sites that promote the Hydrogen Evolution Reaction (HER) at wide pH ranges. Herein, we successfully synthesized a rhodium-based nanomaterial with extremely low metal loading (2 μg/cm−2) as electrocatalyst for the HER. In particular, the material consists of carbon-supported rhodium phosphide (Rh2P) as active sites, which are partially covered with carbon patches. The so-developed nanomaterial exhibits high crystallinity, resistance to sintering, and outstanding electrocatalytic activity and operational stability in an extended pH interval. Notably, Rh2P displays specific-mass activities, ca. 2.5- and 5-fold higher than those of the benchmark 20 wt% Pt/C at an overpotential of 50 mV in acidic and alkaline media, respectively. Comparison of the electrocatalytic performance of the current Rh2P electrocatalyst with those of phosphorus-free rhodium NPs and an alternative rhodium phosphide nanomaterial, reveals that the inclusion of phosphorus atoms, the purity and crystallinity of the Rh2P phase are critical to boost the electrocatalytic HER. This is corroborated by theoretical simulations using DFT, which also prove that the presence of C-patches on Rh2P favors the H2O dissociation during HER electrocatalytic cycle and prevents phosphorous leaching. Overall, this work provides new insights for the rational design and controlled synthesis of small NPs for using as efficient electrocatalysts in hydrogen-based renewable energy devices

Halide encapsulation by dicarboxylate oxido-vanadium cage complexes

Compounds [BuN][VO(oda) Cl], 1, [BuN][VO(glut)2Cl], 2, and [BuN][VO(glut)F], 3, (oda = oxydiacetate, O(CHCOO) ; glut = glutarate, CH(CHCOO) ) were obtained by a stepwise reaction of in situ prepared [BuN]VO with HCl (or HF for 3) and then with the dicarboxylic acid X(CHCOOH) (X = O and CH), under appropriate reaction conditions. Multinuclear magnetic resonance (H, C{H}, Cl, F and V), electrochemical studies, X-ray structural determinations (single crystal and powder), thermogravimetric analyses (TGA) and Density Functional Theory (DFT) calculations were employed to characterise these polyoxovanadate complexes 1-3. They included encapsulated halide anions, two chloride ions in 1 and 2 and one fluoride ion in 3, where the shape and dimensions of the cage were governed by the halide size. The stabilizing template effect of the chloride ion towards the bowl-shaped [VO(OOCR)] fragment (i.e. the half part of 1 and 2), containing a crown-shaped {VO} subunit, or that of the fluoride ion towards the planar {VO} moiety in 3, was definitively demonstrated by DFT calculations. The HOMO composition of 1 prompted us to study the possible oxidation of the two encapsulated chloride ions toward a chlorine molecule. The electrochemical behaviors of 1-3 were thus investigated. However, the chlorine molecule in the model [VO(oda)(Cl)], 6c, was not capable to stabilise the polyoxovanadate cage [VO(oda)], 4c, according to DFT calculations.Financial support from the Junta de Andalucía (Proyecto de Excelencia FQM-7079), Ministerio de Economía y Competitividad (CTQ2014-52641-P) and Universidad de Sevilla (VI Plan Propio) is gratefully acknowledged

Voltammetric study of the adsorbed thermophilic plastocyanin from Phormidium laminosum up to 90 °c

Redox thermodynamics and kinetics of plastocyanin from Phormidium laminosum, and of azurin from Pseudonomas aeruginosa, have been investigated as a function of temperature by protein film voltammetry. To this purpose, both proteins have been physisorbed on a pyrolytic graphite edge electrode. A pronounced negative shift of the plastocyanin standard potential, compared to a slight shift in the case of azurin, has been found upon increasing the temperature. Hence, significant conformational and/or solvation changes accompany the redox conversion of plastocyanin. Lower electron transfer rate constants (by c.a. one order of magnitude) and higher activation enthalpies have been found for plastocyanin as compared to azurin. The voltammetric response of azurin vanishes irreversibly at temperatures close to 60 °C, whereas the redox properties of plastocyanin remain unaltered, except for some loss of electroactive protein, after heating the electrode at temperatures as high as 90 °C. © 2012 Elsevier B.V. All rights reserved.Peer Reviewe

Metalloenzyme-Inspired Ce-MOF Catalyst for Oxidative Halogenation Reactions

The structure of UiO-66(Ce) is formed by CeO2-x defective nanoclusters connected by terephthalate ligands. The initial presence of accessible Ce3+ sites in the as-synthesized UiO-66(Ce) has been determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR)-CO analyses. Moreover, linear scan voltammetric measurements reveal a reversible Ce4+/Ce3+ interconversion within the UiO-66(Ce) material, while nanocrystalline ceria shows an irreversible voltammetric response. This suggests that terephthalic acid ligands facilitate charge transfer between subnanometric metallic nodes, explaining the higher oxidase-like activity of UiO-66(Ce) compared to nanoceria for the mild oxidation of organic dyes under aerobic conditions. Based on these results, we propose the use of Ce-based metal-organic frameworks (MOFs) as efficient catalysts for the halogenation of activated arenes, as 1,3,5-trimethoxybenzene (TMB), using oxygen as a green oxidant. Kinetic studies demonstrate that UiO-66(Ce) is at least three times more active than nanoceria under the same reaction conditions. In addition, the UiO-66(Ce) catalyst shows an excellent stability and can be reused after proper washing treatments. Finally, a general mechanism for the oxidative halogenation reaction is proposed when using Ce-MOF as a catalyst, which mimics the mechanistic pathway described for metalloenzymes. The superb control in the generation of subnanometric CeO2-x defective clusters connected by adequate organic ligands in MOFs offers exciting opportunities in the design of Ce-based redox catalysts.This work has been supported by the Spanish Government through the “Severo Ochoa” (SEV-2016-0683, MINECO) and RTI2018-101033-B-I00 (MCIU/AEI/FEDER, UE). J. M. Salas is acknowledged for his contribution to CO-IR experiments. The Electron Microscopy Service of the UPV is also acknowledged for their help in sample characterization

Halide encapsulation by dicarboxylate oxido-vanadium cage complexes

Compounds [Bu4N]2[V8O16(oda)4⊂2Cl], 1, [Bu4N]2[V8O16(glut)4⊂2Cl], 2, and [Bu4N][V4O8(glut)2⊂F], 3, (oda = oxydiacetate, O(CH2COO)2 2−; glut = glutarate, CH2(CH2COO)2 2−) were obtained by a stepwise reaction of in situ prepared [Bu4N]VO3 with HCl (or HF for 3) and then with the dicarboxylic acid X(CH2COOH)2 (X = O and CH2), under appropriate reaction conditions. Multinuclear magnetic resonance (1 H, 13C{1 H}, 35Cl, 19F and 51V), electrochemical studies, X-ray structural determinations (single crystal and powder), thermogravimetric analyses (TGA) and Density Functional Theory (DFT) calculations were employed to characterise these polyoxovanadate complexes 1–3. They included encapsulated halide anions, two chloride ions in 1 and 2 and one fluoride ion in 3, where the shape and dimensions of the cage were governed by the halide size. The stabilizing template effect of the chloride ion towards the bowl-shaped [V4O8(OOCR)4] fragment (i.e. the half part of 1 and 2), containing a crown-shaped {V4O8} subunit, or that of the fluoride ion towards the planar {V4O8} moiety in 3, was definitively demonstrated by DFT calculations. The HOMO composition of 1 prompted us to study the possible oxidation of the two encapsulated chloride ions toward a chlorine molecule. The electrochemical behaviors of 1–3 were thus investigated. However, the chlorine molecule in the model [V8O16(oda)4⊂(Cl2)], 6c, was not capable to stabilise the polyoxovanadate cage [V8O16(oda)4], 4c, according to DFT calculations.Junta de Andalucía (Proyecto de Excelencia FQM-7079)Ministerio de Economía y Competitividad (CTQ2014-52641-P)Universidad de Sevilla (VI Plan Propio