Supported gold- and silver-based catalysts for the selective aerobic oxidation of 5-(hydroxymethyl)furfural to 2,5-furandicarboxylic acid and 5-hydroxymethyl-2-furancarboxylic acid

The sustainable synthesis of two important intermediates relevant for the production of bio-based polymers, 2,5-furandicarboxylic acid (FDCA) and 5-hydroxymethyl-2-furancarboxylic acid (HFCA), via oxidation of 5-(hydroxymethyl)furfural (HMF) was investigated using supported gold- and silver-based catalysts in water with air as the oxidant. High yields and selectivities for the production of FDCA (89%) and HFCA (≥98%) were achieved under the optimized reaction conditions with Au/ZrO2 and Ag/ZrO2 catalysts, respectively. While FDCA was mainly formed in the presence of gold catalysts at a maximum productivity of 67 molFDCA h−1 molAu−1, silver catalysts showed a remarkably high activity in aldehyde oxidation producing HFCA in almost quantitative yields with a maximum productivity of 400 molHFCA h−1 molAg−1. By variation of the reaction parameters, the Au/ZrO2 catalyst could be tuned to produce also HFCA, whereas the Ag/ZrO2 catalyst exclusively produced HFCA in a wide range of reaction parameters. The observed differences in catalyst selectivities can be taken as a starting point for further mechanistic investigation on the oxidation of HMF, contributing to a fundamental understanding of this reaction which is particularly important for establishing the production of bio-based polymers

Role of Iron on the Structure and Stability of Ni3.2Fe/Al2O3 during Dynamic CO2 Methanation for P2X Applications

An energy scenario, mainly based on renewables, requires efficient and flexible Power-to-X (P2X) storage technologies, including the methanation of CO₂. As active Ni⁰ surface sites of monometallic nickel-based catalysts are prone to surface oxidation under hydrogen-deficient conditions, we investigated iron as “protective” dopant. A combined operando X-ray absorption spectroscopy and X-ray diffraction setup with quantitative on-line product analysis was used to unravel the structure of Ni and Fe in an alloyed Ni-Fe/Al₂O₃ catalyst during dynamically driven methanation of CO₂. We observed that Fe protects Ni from oxidation and is itself more dynamic in the oxidation and reduction process. Hence, such “sacrificial” or“protective” dopants added in order to preserve the catalytic activity under dynamic reaction conditions may not only be of high relevance with respect to fine-tuning of catalysts for future industrial P2X applications but certainly also of general interest

Direct Catalytic Route to Biomass-Derived 2,5-Furandicarboxylic Acid and Its Use as Monomer in a Multicomponent Polymerization

Efficient synthesis of valuable platform chemicals from renewable feedstock is a challenging, yet essential strategy for developing technologies that are both economical and sustainable. In the present study, we investigated the synthesis of 2,5-furandicarboxylic acid (FDCA) in a two-step catalytic process starting from sucrose as largely available biomass feedstock. In the first step, 5-(hydroxymethyl)furfural (HMF) was synthesized by hydrolysis and dehydration of sucrose using sulfuric acid in a continuous reactor in 34% yield. In a second step, the resulting reaction solution was directly oxidized to FDCA without further purification over a Au/ZrO$_{2}$ catalyst with 84% yield (87% selectivity, batch process), corresponding to 29% overall yield with respect to sucrose. This two-step process could afford the production of pure FDCA after the respective extraction/crystallization despite the impure intermediate HMF solution. To demonstrate the direct application of the biomass-derived FDCA as monomer, the isolated product was used for Ugi-multicomponent polymerizations, establishing a new application possibility for FDCA. In the future, this efficient two-step process strategy toward FDCA should be extended to further renewable feedstock

Direct Catalytic Route to Biomass-Derived 2,5-Furandicarboxylic Acid and Its Use as Monomer in a Multicomponent Polymerization

Efficient synthesis of valuable platform chemicals from renewable feedstock is a challenging, yet essential strategy for developing technologies that are both economical and sustainable. In the present study, we investigated the synthesis of 2,5-furandicarboxylic acid (FDCA) in a two-step catalytic process starting from sucrose as largely available biomass feedstock. In the first step, 5-(hydroxymethyl)furfural (HMF) was synthesized by hydrolysis and dehydration of sucrose using sulfuric acid in a continuous reactor in 34% yield. In a second step, the resulting reaction solution was directly oxidized to FDCA without further purification over a Au/ZrO2 catalyst with 84% yield (87% selectivity, batch process), corresponding to 29% overall yield with respect to sucrose. This two-step process could afford the production of pure FDCA after the respective extraction/crystallization despite the impure intermediate HMF solution. To demonstrate the direct application of the biomass-derived FDCA as monomer, the isolated product was used for Ugi-multicomponent polymerizations, establishing a new application possibility for FDCA. In the future, this efficient two-step process strategy toward FDCA should be extended to further renewable feedstock

Functional Model for the [Fe] Hydrogenase Inspired by the Frustrated Lewis Pair Concept

[Fe] hydrogenase (Hmd) catalyzes the heterolytic splitting of H-2 by using, in its active site, a unique organometallic iron-guanylylpyridinol (FeGP) cofactor and, as a hydride acceptor, the substrate methenyltetrahydromethanopterin (methenyl-H4MPT+). The combination FeGP/methenyl-H4MPT+ and its reactivity bear resemblance to the concept of frustrated Lewis pairs (FLPs), some of which have been shown to heterolytically activate H2. The present work exploits this interpretation of Hmd reactivity by using the combination of Lewis basic ruthenium metalates, namely K[CpRu(CO)(2)] (KRp) and a related polymeric Cp/Ru/CO compound (Rs), with the new imidazolinium salt 1,3-bis(2,6-difluorophenyl)-2-(4-tolyl)imidazolinium bromide ([(Tol)Im(F4)]Br-+(-)) that was designed to emulate the hydride acceptor properties of methenyl-H4MPT+. Solid-state structures of [(Tol)Im(F4)]Br-+(-) and the corresponding imidazolidine H(Tol)Im(F4) reveal that the heterocycle undergoes similar structural changes as in the biological substrate. DFT calculations indicate that heterolytic splitting of dihydrogen by the FLP Rp(-)/[(Tol)Im(F4)](+) is exothermic, but the formation of the initial Lewis pair should be unfavorable in polar solvents. Consequently the combination Rp(-)/[(Tol)Im(F4)](+) does not react with H-2 but leads instead to side products from nucleophilic substitution (k = 4 x 10(-2) L mol (-1) s(-1) at room temperature). In contrast, the heterogeneous combination Rs/[(Tol)Im(F4)](+) does split H-2 heterolytically to give H(Tol)Im(F4) and HRuCp(CO)(2) (HRp) or D(Tol)Im(F4) and DRp when using D-2. The reaction has been followed by H-1/H-2 and F-19 NMR spectroscopy as well as by IR spectroscopy and reaches 96% conversion after 1 d. Formation of H(Tol)Im(F4) under these conditions demonstrates that superelectrophilic activation by protonation, which has been proposed for methenyl-H4MPT+ to increase its carbocationic character, is not necessarily required for an imidazolinium ion to serve as a hydride acceptor. This unprecedented functional model for the [Fe] hydrogenase, using a Lewis acidic imidazolinium salt as a biomimetic hydride acceptor in combination with an organometallic Lewis base, may provide new inspiration for biomimetic H-2 activation

Functional Model for the [Fe] Hydrogenase Inspired by the Frustrated Lewis Pair Concept

[Fe] hydrogenase (Hmd) catalyzes the heterolytic splitting of H₂ by using, in its active site, a unique organometallic iron-guanylylpyridinol (FeGP) cofactor and, as a hydride acceptor, the substrate methenyltetrahydromethanopterin (methenyl-H₄MPT⁺). The combination FeGP/methenyl-H₄MPT⁺ and its reactivity bear resemblance to the concept of frustrated Lewis pairs (FLPs), some of which have been shown to heterolytically activate H₂. The present work exploits this interpretation of Hmd reactivity by using the combination of Lewis basic ruthenium metalates, namely K­[CpRu­(CO)₂] (<b>KRp</b>) and a related polymeric Cp/Ru/CO compound (<b>Rs</b>), with the new imidazolinium salt 1,3-bis­(2,6-difluorophenyl)-2-(4-tolyl)­imidazolinium bromide (<b>[</b>^<b>Tol</b><b>Im</b>^<b>F4</b><b>]</b>^<b>+</b><b>Br</b>^<b>–</b>) that was designed to emulate the hydride acceptor properties of methenyl-H₄MPT⁺. Solid-state structures of <b>[</b>^<b>Tol</b><b>Im</b>^<b>F4</b><b>]</b>^<b>+</b><b>Br</b>^<b>–</b> and the corresponding imidazolidine <b>H</b>^<b>Tol</b><b>Im</b>^<b>F4</b> reveal that the heterocycle undergoes similar structural changes as in the biological substrate. DFT calculations indicate that heterolytic splitting of dihydrogen by the FLP <b>Rp</b>^<b>–</b><b>/[</b>^<b>Tol</b><b>Im</b>^<b>F4</b><b>]</b>^<b>+</b> is exothermic, but the formation of the initial Lewis pair should be unfavorable in polar solvents. Consequently the combination <b>Rp</b>^<b>–</b><b>/[</b>^<b>Tol</b><b>Im</b>^<b>F4</b><b>]</b>^<b>+</b> does not react with H₂ but leads instead to side products from nucleophilic substitution (<i>k</i> = 4 × 10^–2 L mol ^–1 s^–1 at room temperature). In contrast, the heterogeneous combination <b>Rs/[</b>^<b>Tol</b><b>Im</b>^<b>F4</b><b>]</b>^<b>+</b> does split H₂ heterolytically to give <b>H</b>^<b>Tol</b><b>Im</b>^<b>F4</b> and HRuCp­(CO)₂ (<b>HRp</b>) or <b>D</b>^<b>Tol</b><b>Im</b>^<b>F4</b> and <b>DRp</b> when using D₂. The reaction has been followed by ¹H/²H and ¹⁹F NMR spectroscopy as well as by IR spectroscopy and reaches 96% conversion after 1 d. Formation of <b>H</b>^<b>Tol</b><b>Im</b>^<b>F4</b> under these conditions demonstrates that superelectrophilic activation by protonation, which has been proposed for methenyl-H₄MPT⁺ to increase its carbocationic character, is not necessarily required for an imidazolinium ion to serve as a hydride acceptor. This unprecedented functional model for the [Fe] hydrogenase, using a Lewis acidic imidazolinium salt as a biomimetic hydride acceptor in combination with an organometallic Lewis base, may provide new inspiration for biomimetic H₂ activation

Revisiting the Synthesis and Elucidating the Structure of Potassium Cyclopentadienyldicarbonylruthenate, K[CpRu(CO)₂]

Known procedures for the synthesis of K­[CpRu­(CO)₂] (KRp) via reductive cleavage of the ruthenium dimer Rp₂ were found to be inconsistent and have thus been revisited, and a revised protocol using K­[HB­(<i>sec</i>-Bu)₃] (K-Selectride) as the reducing agent is now reported that gives yellow KRp in crystalline form in around 40% yield. The structure of KRp·THF has been determined by X-ray diffraction, representing the first crystallographic characterization of an Rp^– salt. Inevitably the reductive cleavage of Rp₂ also gives a poorly soluble black solid as an additional product, which has now been analyzed by a variety of methods, including ¹³C MAS NMR spectroscopy using ¹³CO-labeled material. The black solid has been identified as a polymeric Cp/Ru/CO compound with both bridging and terminal CO ligands in a 3:1 ratio. The present report may stimulate the use of the [CpRu­(CO)₂]⁻ (Rp^–) anion, which has been barely exploited as yet in comparison to its popular congener [CpFe­(CO)₂]⁻ (Fp^–)

Revisiting the Synthesis and Elucidating the Structure of Potassium Cyclopentadienyldicarbonylruthenate, K[CpRu(CO)₂]

Known procedures for the synthesis of K­[CpRu­(CO)₂] (KRp) via reductive cleavage of the ruthenium dimer Rp₂ were found to be inconsistent and have thus been revisited, and a revised protocol using K­[HB­(<i>sec</i>-Bu)₃] (K-Selectride) as the reducing agent is now reported that gives yellow KRp in crystalline form in around 40% yield. The structure of KRp·THF has been determined by X-ray diffraction, representing the first crystallographic characterization of an Rp^– salt. Inevitably the reductive cleavage of Rp₂ also gives a poorly soluble black solid as an additional product, which has now been analyzed by a variety of methods, including ¹³C MAS NMR spectroscopy using ¹³CO-labeled material. The black solid has been identified as a polymeric Cp/Ru/CO compound with both bridging and terminal CO ligands in a 3:1 ratio. The present report may stimulate the use of the [CpRu­(CO)₂]⁻ (Rp^–) anion, which has been barely exploited as yet in comparison to its popular congener [CpFe­(CO)₂]⁻ (Fp^–)