(270g) Reaction Mechanisms Responsible for the Selective Vapor Phase Rearrangement of Furanics

The conversion of the abundant but highly reactive furfural to more stable fuel precursors is an important challenge in the conversion of biomass to higher value products. Cyclopentanone is a stable building block that can serve both as a commodity chemical and as a building block for diesel and jet fuels. The ring rearrangement of furfural to cyclopentenone in the aqueous phase in the presence of an acid catalyst is a well-studied reaction, commonly referred to as Piancatelli rearrangement, that is believed to require the presence of a condensed aqueous phase to proceed. Here we report alternative mechanisms for the conversion of furfural to cyclopentenone and cyclopentanone in a single vapor phase upgrading step using multifunctional catalysts. Direct vapor phase upgrading enables catalytic strategies that can be coupled with pyrolysis and torrefaction steps that don’t require a condensed phase to proceed, where undesirable humin formation may occur. We illustrate how selective sites present at the interface of Ru nanoparticles on a TiO₂ support can facilitate this rearrangement in the vapor phase at temperatures as high as 400°C in a flow reactor, both with model compounds and real biomass vapors. We further show how water molecules can interact with these interfacial sites to modify the chemistry, which we propose is due to a different mechanism than the established Piancatelli rearrangement. We show that acidic coadsorbates that promote this reaction in the condensed aqueous phase, such as acetic acid, inhibit the vapor phase rearrangement by competing for active sites. This behavior is expressed in the context of a kinetic model, with a mechanism proposed involving binding to the metal as well as interaction with the interfacial site. The requirement of an interfacial site for this reaction is contrasted over supports with varying acidity, such as TiO₂ vs. Al₂O₃. We compare this chemistry over Ru with that over Pd and Ni to generate an overall correlation with ring rearrangement rate with the metal-carbon bond strength.

Interfacial sites and oxide overlayers manipulate selectivity for selective C-O cleavage and structure sensitive decarbonylation reactions as well. By systematically controlling the degree of oxidation while maintaining nanoparticle size, we illustrate the importance of partially oxidized sites on the Ni surface, and how these sites can influence the rate of C-O cleavage and decarbonylation reactions.