This study investigates structure−reactivity relationships within branched per-and polyfluoroalkyl substances (PFASs) undergoing cobalt-catalyzed reductive defluorination reactions. Experimental results and theoretical calculations reveal correlations among the extent of PFAS defluorination, the local C−F bonding environment, and calculated bond dissociation energies (BDEs). In general, BDEs increase in the following order: tertiary C−F bonds < secondary C−F bonds < primary C−F bonds. A tertiary C−F bond adjacent to three fluorinated carbons (or two fluorinated carbons and one carboxyl group) has a relatively low BDE that permits an initial defluorination to occur. Both a biogenic cobalt−corrin complex (B 12 ) and an artificial cobalt−porphyrin complex (Co-PP) are found to catalytically defluorinate multiple C−F bonds in selected PFASs. In general, Co-PP exhibits an initial rate of defluorination that is higher than that of B 12 . Neither complex induced significant defluorination in linear perfluorooctanoic acid (PFOA; no tertiary C−F bond) or a perfluoroalkyl ether carboxylic acid (tertiary C−F BDEs too high). These results open new lines of research, including (1) designing branched PFASs and cobalt complexes that promote complete defluorination of PFASs in natural and engineered systems and (2) evaluating potential impacts of branched PFASs in biological systems where B 12 is present.

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.

Chlorate (ClO3 –) is an undesirable byproduct in the chlor-alkali process. It is also a heavily used chemical in various industrial and agricultural applications, making it a toxic water pollutant worldwide. Catalytic reduction of ClO3 – into Cl– by H2 is of great interest to both emission control and water purification, but platinum group metal catalysts are either sluggish or severely inhibited by halide anions. Here, we report on the facile preparation, robust performance, and mechanistic investigation of a MoO x –Pd/C catalyst for aqueous ClO3 – reduction. Under 1 atm H2 and room temperature, the Na2MoO4 precursor is rapidly immobilized from aqueous solution onto Pd/C as a mixture of low-valent Mo oxides. The catalyst enables complete reduction of ClO3 – in a wide concentration range (e.g., 1 μM to 1 M) into Cl–. The addition of Mo to Pd/C not only enhances the catalytic activity by >55-fold, but also provides strong resistance to concentrated salts. To probe the reaction mechanisms, we conducted a series of kinetic measurements, microscopic and X-ray spectroscopic characterizations, sorption experiments, tests with other oxyanion substrates, and a comparative study using dissolved Mo species. The catalytic sites are the reduced MoO x species (primarily MoIV), showing selective and proton-assisted reactivity with ClO3 –. This work demonstrates a great promise of using relatively abundant metals to expand the functionality of hydrogenation catalysts for environmental and energy applications.

Supported palladium (Pd) catalysts have been extensively studied for water purification applications. However, this technology is primarily challenged by the high cost of Pd and the lack of optimization of catalyst formulations. In this report, we demonstrate a convenient approach to prepare and optimize Pd catalysts for the reduction of toxic oxyanions (bromate, chlorate, and perchlorate). Water-dissolved Na2PdCl4 was quickly adsorbed in the suspension of activated carbon within 5 min and reduced into Pd0 nanoparticles in situ within another 5 min under 1 atm H2 at 20 °C. In terms of both material characterizations and reaction kinetics, the Pd catalysts prepared with the new method show no significant difference from those prepared by the conventional method (involving multiple-step high-temperature procedures) and from benchmark commercial Pd catalysts. With the very simple approach to control, evaluate, and optimize Pd content in the catalyst, we elucidate the relationships among the Pd content, Pd0 particle size, and catalytic activity. We further showcase that the precious metals in previously reported Re–Pd/C and Mo–Pd/C catalysts can be saved up to 80% without sacrificing the activity. The new and convenient catalyst preparation method will significantly enhance the cost-effectiveness of reductive catalysis technologies for water purification.

For over two decades, Pd has been the primary hydrogenation metal studied for reductive catalytic water treatment applications. Herein, we report that alternative platinum group metals (Rh, Ru, Pt and Ir) can exhibit substantially higher activity, wider substrate selectivity and variable pH dependence in comparison to Pd. Cross comparison of multiple metals and oxyanion substrates provides new mechanistic insights into the heterogeneous reactions. Activity differences and pH effects mainly originate from the chemical nature of individual metals. Considering the advantages in performance and cost, results support renewed investigation of alternative hydrogenation metals to advance catalytic technologies for water purification and other environmental applications.