Supported PdAu Catalysts for the Selective Reduction of Unsaturated Carbon-Carbon Bonds in the Presence of Aromatic Rings

Title: Supported PdAu Catalysts for the Selective Reduction of Unsaturated Carbon-Carbon Bonds in the Presence of Aromatic Rings

Thomas J. Schwartz, Spencer D. Lyman, Ali Hussain Motagamwala, James A. Dumesic

Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, WI 53706

Abstract:

A promising strategy for the production of biorenewable chemicals from biomass uses heterogeneous catalysts to upgrade biologically-derived platform intermediates.¹  One such platform is the class of 2-pyrones produced by polyketide biosynthesis.²  In the present work, we show that the 2-pyrone 4-hydroxycoumarin (4HC) can be used as a starting material for the synthesis of a variety of high-value chemicals, including pharmaceutical building blocks, fragrances, and chemical intermediates.  These target molecules must retain the aromatic ring present in 4HC (see Figure 1).  Accordingly, this presentation will focus on the design of catalysts for the selective reduction of unsaturated carbon-carbon bonds in the presence of aromatic groups.

Figure 1. Conversion of 4-hydroxycoumarin (4HC) to 4-hydroxydihydrocoumarin (4HDHC), coumarin, and dihydrocoumarin (DHC).  The conversion of lactones, such as coumarin and DHC, to their corresponding lactams has been reported in the literature.³

Supported palladium catalysts are effective for the hydrogenation of 4HC.  Selectivities of 88% to 4HDHC and 6% to DHC at 74% 4HC conversion were obtained using a 2 wt% Pd/SiO₂ catalyst.  However, in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy performed in a specially configured batch reactor reveals that, at high conversions, monometallic Pd catalysts partially reduce the aromatic ring in 4HC.  The rate of over-hydrogenation is dependent on the size of the Pd nanoparticles.  Notably, 10-15 nm Pd particles are more active for over-hydrogenation than are 2 nm Pd particles.  In contrast, the application of a bimetallic PdAu/SiO₂ catalyst leads to 97% selectivity to 4HDHC and 3% selectivity to DHC at 63% 4HC conversion.  Over-hydrogenated products were not detected when using this catalyst.  Furthermore, the selective production of DHC can be achieved using a bifunctional PdAu/SiO₂-Al₂O₃ catalyst containing both metal and Brønsted acid sites.

1.       Schwartz, T.J.; O'Neill, B.J.; Shanks, B.H.; Dumesic, J.A. ACS Catal. 2014, 4, 2060.

2.       Chia, M.; Schwartz, T.J.; Shanks, B.H.; Dumesic, J.A. Green Chem.2012, 14, 1850.

3.       Ito, M.; Ooshida, T. Japanese Patent JP4164603B2.