Structure sensitivity in Pt-catalyzed hydrodeoxygenation of multi-oxygenated lignol model compounds

Abstract

The valorization of lignin into aromatic chemicals such as benzene, toluene, and xylenes (BTX) has the potential to promote sustainability in the chemical industry. A critical step in this valorization is the removal of oxygen atoms from lignin-derived substrates by hydrodeoxygenation (HDO), typically catalyzed by supported metal catalysts. However, the structure sensitivity of supported metal catalysts for HDO has been under-explored, particularly for Pt catalysts and multi-oxygenated substrates that are representative compounds for realistic lignin/lignol feedstocks. In this work, the structure sensitivity of Pt catalysts supported on inert SiO₂ for the HDO of dihydroeugenol (DHE), exhibiting both hydroxy and methoxy functionalities, is explored by varying the Pt particle size distribution, and thus the fraction of exposed well-coordinated and under-coordinated Pt active sites. The desired HDO reactions are removal of methoxy substituents (partial HDO) and phenolic OH (complete HDO), while avoiding aromatic hydrogenation leading to undesired saturated hydrocarbons. Measurements of Pt particle size dependent reactivity demonstrate that aromatic ring hydrogenation is more effectively catalyzed by well-coordinated Pt sites located on low-index planes of Pt nanoparticles, which become the dominant exposed active site with Pt particle sizes above ∼4 nm. Pt nanoparticles of ∼2 nm exhibit high selectivity toward partial HDO via demethoxylation, forming of 4-propylphenol without significant aromatic ring hydrogenation. Interestingly, our results suggest well-coordinated Pt sites also facilitate surface tautomerization of phenol to the keto form (dienone), which is essential for complete HDO via dehydroxylation over supported metals on non-acidic supports under mild conditions (<300 °C). These findings identify a trade-off exhibited by Pt catalysts in the HDO of lignols, wherein well-coordinated Pt sites promote both complete HDO to aromatics as well as aromatic hydrogenation, while Pt catalysts exhibiting primarily under-coordinated sites selectively produce partial HDO products (i.e. phenols). The mechanistic implications related to these observations are discussed.