Catalytic upgrading of biomass relies on the selective conversion of C-O bonds contained in phenolic and furanic species. Metals such as Pd or Ru supported on reducible oxides such as TiO2
have shown significant promise for these selective transformations, with enhancement in both activity and selectivity reported[1-3]. Reasons for the enhanced activity proposed in the literature include promoter effects of the metal to create defects on the support, selective cleavage at the perimeter of the metal particle, and strong metal support interactions via decoration of the metal particle. Because several potential active sites are created under reaction conditions for this family of catalysts, the true nature of the active site required for the desirable transformation is often speculated without concrete proof. Here we use a wide range of catalysts with varying metal particle size and TiO2
phase to clearly identify the active sites for each transformation via a combination of characterization and reaction kinetics. Metal particle sizes are estimated both with microscopy as well as structure insensitive reactions to probe the presence of a TiO2
overlayer on metal particles, and the resulting influence on TOF values. We demonstrate that the pathway from guaiacol to toluene follows a complex route, with the metal serving as a promoter to create Lewis acid sites on the support necessary for transalkylation, while the metal/oxide perimeter sites are responsible for phenol C-O bond cleavage [1-2]. DFT calculations confirm the experimental evidence, and a mechanism is proposed. We use catalysts synthesized with carefully controlled distances between reduced metals particles and oxide clusters separated by a controlled distance across a conductive bridge to quantify the role of the promoter effect vs. the direct metal/oxide interface on furfural conversion. Finally, we discuss the positive interaction of these perimeter sites with water, enhancing the rate of cresol conversion and catalyzing the ring rearrangement of furfural.
Boonyasuwat, S., Omotoso, T., Resasco, D.E. and Crossley, S.P., 2013. Conversion of guaiacol over supported Ru catalysts. Catalysis letters, 143(8), pp.783-791.
Omotoso, T., Boonyasuwat, S. and Crossley, S.P., 2014. Understanding the role of TiO2 crystal structure on the enhanced activity and stability of Ru/TiO 2 catalysts for the conversion of lignin-derived oxygenates. Green Chemistry,16(2), pp.645-652.
 Resasco, D.E. and Crossley, S.P., 2015. Implementation of concepts derived from model compound studies in the separation and conversion of bio-oil to fuel. Catalysis Today, 257, pp.185-199.