Hydrodeoxygenation (HDO) is an important step in the upgrading of the complex oxygenates that are produced via pyrolysis or depolymerization of both the cellulose and lignin fractions of biomass. While group 10 metals (Ni, Pd, Pt) have high reforming activity under some conditions, they tend to promote both C-O and undesirable C-C bond scission. In the case of oxygenated polyaromatics produced from lignin, they also promote hydrogenation of the aromatic rings resulting in the production of lower value products. Alloying a group 10 metal with a second more oxyphilic metal (e.g. adding Fe to Ni or Zn to Pt) has been shown to produce catalysts that have high selectivity for the desirable C-O bond hydrogenolysis while simultaneously not promoting hydrogenation of aromatic rings. Although this enhanced selectivity has been observed experimentally, the mechanism by which the second metal alters the selectivity is poorly understood. Our work has therefore been focused on obtaining a mechanistic understanding of these alloy effects. In particular we have been using model catalysts consisting of Pt(111) single crystal surfaces upon which controlled amounts of Zn have been vapor deposited. In this talk we will describe our most recent TPD and HREELS studies of how Zn addition to Pt affects the electivity for C-O versus C-C bond cleavage in lignin-derived model compounds including, benzaldehyde, anisole, and guaiacol. Our results show that both electronic and ensemble effects are important. Zn addition is found to weaken the interaction of aromatic rings with the Pt(111) surface causing the bonding configuration to change from one where the ring is parallel to the surface to one where the ring is tilted away from the surface. C-O functionalities in these molecules are also shown to interact with adjacent Pt-Zn sites with the oxygen bonded to the Zn and the C to the Pt, a bonding configuration that facilitates C-O bond cleavage. Reactivity studies obtained using carbon-supported PtZn catalysts and a high pressure liquid flow reactor will also be presented allowing comparisons to be made between the model catalysts and their high surface area analogs.

Mechanistic studies of the HDO of lignin-derived oxygenates on PtZn: From single crystals to high surface area catalysts

253rd National Meeting (2017)

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