Session: General Catalysis:

Improved understanding of reaction pathways and critical steps can lead to reduced process costs for biofuels that reduce our dependence on fossil fuels. In this talk, I will describe our work towards elucidating the reaction mechanism for reductive etherification and its application for the production of a renewable diesel blendstock. Our team previously developed a single-phase catalyst for the reductive etherification reaction (palladium on niobium phosphate, Pd-NbOPO₄) and performed a technoeconomic analysis for the conversion of waste biomass into a targeted diesel bioblendstock through this chemistry. High palladium loading (5 wt%) on the Pd-NbOPO₄ catalyst was noted as an important cost-driver that impacted the minimum fuel selling price. Transmission electron microscopy of Pd-NbOPO₄ showed an average Pd nanoparticle size of 10 nm in our catalyst samples and this matched previous literature examples of +10-nm Pd particles for optimizing the reaction rate for reductive etherification. Understanding the reaction mechanism and the increased rate from large Pd crystals could inform lower Pd loading synthesis procedures and lower catalyst material costs to reduce the barriers to industry adoption. Two possible pathways for the reductive etherification reaction have been previously proposed with critical intermediate species of either a ketal or enol ether being reduced on the Pd surface. The large disparity in size and shape between these molecules could have a strong influence on the adsorption energy and the transition state energies leading to the ether product. The hydrogen gas used for reduction was replaced with deuterium gas to test the change in molecular weight between the ketal and enol ether intermediates. The purified product was analyzed by mass spectroscopy and nuclear magnetic resonance spectroscopy to identify the primary reaction intermediates. Experiments were also performed with homogeneous acid catalysts combined with a heterogeneous palladium catalyst to determine if solution-based acid chemistry is also at play. Ongoing work includes incorporation of experimental results into computational studies of the thermodynamic landscape and adsorption energies for the reductive etherification reaction on Pd-NbOPO₄.

Rajeev Surendran Assary

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