ChemSusChem

Cofactor Regeneration: Second Nature for Silicon Nanowires The cover image illustrates the successful regeneration of NAD(P)H cofactors by using silicon nanowires under visible light, mimicking natural photosynthesis. The efficient light harvesting and utilization of solar energy is a key factor in natural photosynthesis, maintaining the viability of green plants and photosynthetic bacteria by converting solar energy into chemical energy. In natural photosynthesis, photoexcited electrons from photosynthetic reaction centers are passed along to regenerate reducing power in the form of NAD(P)H cofactors from their oxidized form. The Communication by Chan Beum Park and colleagues at the Korea Advanced Institute of Science and Technology and MIT, reported on page 2129, demonstrates that hydrogen-terminated silicon nanowires possess a high photocatalytic ability that is suitable for visible-light-driven regeneration of NAD(P)H and redox enzymatic synthesis towards biocatalyzed artificial photosynthesis.

Sweet success of electron: Glycerol from biofuel industries can be valorised through green and sustainable electrochemical processes (fuel or electrolysis cell) for cogenerating energy or hydrogen and value-added chemicals (i.e., dihydroxyacetone, hydroxypyruvate, mesoxalate, etc.). Kinetics, selectivity and reaction mechanism strongly depend on the composition and structure of the catalyst and on the applied potential. Pt-free catalysts as active as Pt have been developed for the controlled oxidation of glycerol.

Say no to O: The catalytic deoxygenation of biomass-derived feedstocks (sugars, sugar alcohols) is a promising pathway towards a sustainable chemical production economy. The development of efficient synthetic protocols for oxygen removal is highly desirable to derive chemical building blocks and fuels. A major challenge is selection/design of effective catalysts. This Highlight appraises the hurdles and opportunities for deoxygenation of biomass feedstocks.

Down to the wires: A highly efficient artificial photosynthetic system employing hydrogen-terminated silicon nanowires is reported. The nanowires enable a cascading electron transfer from electron donor to NAD⁺ via a rhodium-based electron mediator. Approximately 80 % of NADH is photoregenerated from NAD⁺ by the nanowires during 2 h of light irradiation, and successfully coupled with the photoenzymatic synthesis of L-glutamate.

A molecular catalyst system containing only earth-abundant elements is combined with g-C₃N₄ as photosensitizer. The in situ generated complexes serve as catalysts for visible-light-driven H₂ production in aqueous solution, and do not require organic solvent or a Brønsted acid. H₂ evolution from the Ni-based catalyst system can be maintained for over 60 h, demonstrating that the combination is a promising approach to improve the lifetimes of molecular catalysts towards photochemical H₂ production.

Noble causes: Cellulose is effectively converted into methanol, propylene, and ethylene glycol over Cu-based catalysts. Overall yields of above 93 %, together with 63 % yield of C₁–C₃ compounds, can be reached over simple noble-metal-free systems, opening new opportunities for the sustainable and cost-efficient valorization of cellulose.

Da-ddy, DDQ: By using catalytic amounts of DDQ combined with MnO₂ as oxidant, an efficient oxidative CO coupling of benzylic sp³ CH bonds with carboxylic acids affords a series of carboxylic esters in 70–98 % yields. A wide range of functional groups and various carboxylic acids are tolerated. The reaction involves both CH functionalization and CO bond formation.

Water, a powerful ally: A Mn^III complex with an N₄O-coordinating ligand and a deprotonated water molecule as sixth ligand is chemically oxidized by a Ce^IV one-electron oxidant. The results indicate catalytic oxygen atom transfer to alkenes, with the oxygen atom originating from a water molecule. These findings demonstrate the possibility of using water as oxygen source to perform the oxidation of organic substrates.

If you′ve got it, use it: Malic acid is converted into dimethyl malonate by a direct, one-pot process. The process is cyanide- and halide-free. Phosphovanadomolybdates serve as bifunctional catalysts, effecting the oxidative decarboxylation and esterification in a consecutive manner. Oxidative CC bond cleavage first forms hemiacetals. The results serve as example for the production of valuable chemicals by fully utilizing the oxygen atoms and basic structure inherent to biomass products.

Chromite anodes in LWO PC-SOFC: Chemically compatible LSCN/LWO symmetric cells show the high protonic conductivity and p-type electronic conductivity of LSCN in reducing atmospheres. This allows to avoid blending with other protonic or electronic materials and enlarges the surface area. An R_p of 2.68 Ω cm² at 750 °C and performance in LF-limited processes demonstrate the potential of LSCN as anode material.

Thumbs up for porous graphene! Single-layer-graphene-assembled 3D hexaporous carbon is prepared by catalyst-free pyrolysis of poly(4-styrenesulfonic acid-co-maleic acid) sodium salt. The material has a hierarchical structure of mesoporous as well as microporous graphene with hexagonal nanopores of uniform size and shape. The results show that the sample is highly conducting in aqueous electrolyte, with an extremely high surface area of 1720 m² g⁻¹ and an enhanced capacitance of 154 F g⁻¹.

An order of enzymes, hold the cells: For chemical and fuel production from biomass, chemical methods are often supplemented by biological means, such as microbial systems. It is demonstrated here that microbial reaction cascades can be redesigned and simplified to an extent that cell-free bulk production of chemicals and fuels from biomass becomes feasible. Specifically, glucose is converted to ethanol and to isobutanol by using only six and eight enzymes respectively and one cofactor. Larger alcohols such as isobutanol are known to poison microbial systems.

What a wait for solid state: 2,5-Dibromo-3,4-propylenedioxythiophene (DBProDOT), a new monomer that is a solid at room temperature, has been synthesized to produce poly(3,4-propylenedioxythiophene) (ssPProDOT). I₂-free solid-state dye-sensitized solar cells with ssPProDOT exhibit exceptional long-term stability (>1500 h) with an efficiency of 3.5 % at 100 mW cm⁻².

Silylamine molecular liquids reversibly react with CO₂ to form the corresponding ammonium carbamates (reversible ionic liquids). The molecular liquid is easily regenerated quantitatively from the reversible ionic liquid upon heating. The ability to tune bulk properties by making structural changes presents an opportunity for optimization by developing a system that balances conversion with viscosity and minimizes the energy required for regeneration.

Test boosts performance: The significant increase of the energy storage capacity upon three-eletrode tests observed in oxygen- and nitrogen+oxygen-rich carbons in an acidic electrolyte is attributed to two different mechanisms: For oxygen-rich carbons (CA, CB), the double-layer capacitance is amplified because of the removal of O-I and O-II bulky oxygen-containing functionalities, whereas for nitrogen+oxygen-rich (CAU, CBU) carbons, the pseudocapacitance is enhanced, a result of the increased proportion of pyridone N-5 nitrogen atoms.

Creating gateways for electrons: The number of hydrogen evolution sites on Ga₂O₃-based photocatalysts is optimized in a continuously flushed water-splitting set-up equipped with on-line gas analysis. A fast sequential photodeposition method is established by monitoring the amount of hydrogen evolved from the sacrificial agent methanol.

Sphere of influence: We report the valorization of two natural biopolymers as highly efficient, reversible CO₂ adsorbents. Pyrolysis leads to graphitic carbons that adsorb amounts of CO₂ at 0 °C and atmospheric pressure double that of the currently best CO₂ adsorbents (per mass unit). In addition, the materials also exhibit higher adsorption capacity per volume unit, a parameter that is more relevant for industrial applications where the limiting factor is the volume of adsorption towers.

Solid improvement: Cellulose-derived carbon catalysts with SO₃H, COOH, and phenolic OH groups are used for the efficient catalytic conversion of fructose into 5-hydroxymethylfurfural in an ionic liquid (see picture).

Plant-oil-based antibacterial coatings: Antibacterial soybean-oil-based cationic polyurethane coatings have been successfully prepared from five different amino polyols. The structure of these amino polyols affects the mechanical properties, thermal stability, and antibacterial properties of the coatings. These coatings exhibit antibacterial properties to both gram-positive and gram-negative bacteria, with best antibacterial activity against a structural mutant of Salmonella minnesota possessing a severely truncated outer membrane structure.

Thermolytic sugars from biomass: Using a simple acid pretreatment, the catalytic activity of naturally occurring alkali and alkaline earth metals in lignocellulosic biomass is dramatically reduced, allowing the purely thermal production of sugars from biomass. The optimal amount of acid for the thermal depolymerization of biomass to sugars is proportional to the amount of alkali and alkaline earth metals inherently contained in the biomass feedstock.

CO₂selective adsorbent: The 8-membered-ring small-pore zeolite ZK-5, ion-exchanged with different cations, is investigated as a CO₂ selective adsorbent. Despite its high Si/Al ratio (4.7) ZK-5 can adsorb CO₂ at 10 % v/v in nitrogen with high working capacity as well as high CO₂/N₂ selectivity. Mg-ZK-5 is identified as an adsorbent with low isosteric heat of adsorption and excellent properties for adsorption-driven pressure swing processes.

Coming up for air: Supported gold nanoparticles enable the aerobic oxidation of a less reactive alcohol, 1-octanol, to directly produce octanoic acid and octyl octanoate under base-free conditions. The product distributions can be tuned by the proper selection of metal oxide supports. The results may enable greener processes for the production of high-value carboxylic acids and esters.

The derivatization of methyl ricinoleate followed by ring-closing metathesis (RCM) or sequential RCM/hydrogenation and RCM/cross-metathesis lead to a variety of biosourced compounds of interest for fine chemistry and polymer synthesis. 3,6-Dihydropyran, and α,β-unsaturated lactone derivatives have been prepared with concomitant production of polymer precursors. Tetrahydropyran and lactone derivatives as well as valuable monomers in particular polyamide precursors have also been prepared.

Recovery of Ionic Liquids: Solutions of ionic liquids (ILs) and amines are separated into aqueous two-phase systems in the presence of CO₂, in which the upper phase is ammonium-salt-rich and the lower phase is IL-rich. Thus, the ILs in aqueous solutions can be efficiently recovered, and the amines can be regenerated by heating and bubbling Ar or N₂ through the salt-rich phase. The recovery efficiency of the ILs is as high as 99 %.

The other side of membranes: The use of tailor-made anion-exchange membranes in reverse electrodialysis is studied. Membranes with thinner film thickness achieve the highest power density because of their lower area resistance and high permselectivity (see figure). This is, to the best of our knowledge, the first example of a reverse electrodialysis stack using tailor-made anion-exchange membranes prepared in a more environmentally friendly and safer way relative to conventional methods.

Methane storage: Newly developed nanoporous carbon monoliths have superior advantages over traditional activated-carbon powder. In particular, nanoporous carbon monoliths can provide an explicit clue to solve the damping of adsorption and desorption kinetics owing to the diffusion-drive obstacle.

Mechanism of catalyst deactivation: Electrocatalytic oxygen reduction at the air cathode in silicon–air batteries critically impacts battery performance. We describe a mechanism of air-electrode deactivation: upon discharge, the MnO₂ catalyst is modified by the formation of of a MnF₂ surface layer formation that is catalytically inactive for the oxygen reduction reaction and limits the discharge capacity. The ability for this deactivation layer to form is greatly impacted by the water content of the electrolyte.