Toward net-zero sustainable aviation fuel with wet waste-derived volatile fatty acids.

With the increasing demand for net-zero sustainable aviation fuels (SAF), new conversion technologies are needed to process waste feedstocks and meet carbon reduction and cost targets. Wet waste is a low-cost, prevalent feedstock with the energy potential to displace over 20% of US jet fuel consumption; however, its complexity and high moisture typically relegates its use to methane production from anaerobic digestion. To overcome this, methanogenesis can be arrested during fermentation to instead produce C2 to C8 volatile fatty acids (VFA) for catalytic upgrading to SAF. Here, we evaluate the catalytic conversion of food waste-derived VFAs to produce n-paraffin SAF for near-term use as a 10 vol% blend for ASTM "Fast Track" qualification and produce a highly branched, isoparaffin VFA-SAF to increase the renewable blend limit. VFA ketonization models assessed the carbon chain length distributions suitable for each VFA-SAF conversion pathway, and food waste-derived VFA ketonization was demonstrated for >100 h of time on stream at approximately theoretical yield. Fuel property blending models and experimental testing determined normal paraffin VFA-SAF meets 10 vol% fuel specifications for "Fast Track." Synergistic blending with isoparaffin VFA-SAF increased the blend limit to 70 vol% by addressing flashpoint and viscosity constraints, with sooting 34% lower than fossil jet. Techno-economic analysis evaluated the major catalytic process cost-drivers, determining the minimum fuel selling price as a function of VFA production costs. Life cycle analysis determined that if food waste is diverted from landfills to avoid methane emissions, VFA-SAF could enable up to 165% reduction in greenhouse gas emissions relative to fossil jet.

Applying Dynamic Strain on Thin Oxide Films Immobilized on a Pseudoelastic Nickel-Titanium Alloy.

Direct alteration of material structure/function through strain is a growing area of research that has allowed for novel properties of materials to emerge. Tuning material structure can be achieved by controlling an external force imposed on materials and inducing stress-strain responses (i.e., applying dynamic strain). Electroactive thin films are typically deposited on shape or volume tunable elastic substrates, where mechanical loading (i.e., compression or tension) can affect film structure and function through imposed strain. Here, we summarize methods for straining n-type doped titanium dioxide (TiO2) films prepared by a thermal treatment of a pseudo-elastic nickel-titanium alloy (Nitinol). The main purpose of the described methods is to study how strain affects electrocatalytic activities of metal oxide, specifically hydrogen evolution and oxygen evolution reactions. The same system can be adapted to study the effect of strain more broadly. Strain engineering can be applied for optimization of a material function, as well as for design of adjustable, multifunctional (photo)electrocatalytic materials under external stress control.

Stabilization of reactive Co4O4 cubane oxygen-evolution catalysts within porous frameworks.

A major challenge to the implementation of artificial photosynthesis (AP), in which fuels are produced from abundant materials (water and carbon dioxide) in an electrochemical cell through the action of sunlight, is the discovery of active, inexpensive, safe, and stable catalysts for the oxygen evolution reaction (OER). Multimetallic molecular catalysts, inspired by the natural photosynthetic enzyme, can provide important guidance for catalyst design, but the necessary mechanistic understanding has been elusive. In particular, fundamental transformations for reactive intermediates are difficult to observe, and well-defined molecular models of such species are highly prone to decomposition by intermolecular aggregation. Here, we present a general strategy for stabilization of the molecular cobalt-oxo cubane core (Co4O4) by immobilizing it as part of metal-organic frameworks, thus preventing intermolecular pathways of catalyst decomposition. These materials retain the OER activity and mechanism of the molecular Co4O4 analog yet demonstrate unprecedented long-term stability at pH 14. The organic linkers of the framework allow for chemical fine-tuning of activity and stability and, perhaps most importantly, provide "matrix isolation" that allows for observation and stabilization of intermediates in the water-splitting pathway.

Oxygen-atom transfer chemistry and thermolytic properties of a di-tert-butylphosphate-ligated Mn4O4 cubane.

[Mn4O4{O2P(OtBu)2}6] (1), an Mn4O4 cubane complex combining the structural inspiration of the photosystem II oxygen-evolving complex with thermolytic precursor ligands, was synthesized and fully characterized. Core oxygen atoms within complex 1 are transferred upon reaction with an oxygen-atom acceptor (PEt3), to give the butterfly complex [Mn4O2{O2P(OtBu)2}6(OPEt3)2]. The cubane structure is restored by reaction of the latter complex with the O-atom donor PhIO. Complex 1 was investigated as a precursor to inorganic Mn metaphosphate/pyrophosphate materials, which were studied by X-ray absorption spectroscopy to determine the fate of the Mn4O4 unit. Under the conditions employed, thermolyses of 1 result in reduction of the manganese to Mn(II) species. Finally, the related butterfly complex [Mn4O2{O2P(pin)}6(bpy)2] (pin = pinacolate) is described.