Selective Hydrogenation of Unsaturated Carbon-Carbon Bonds in Aromatic-Containing Platform Molecules

The combination of chemical and biological catalysis enables the production from biomass of coumarin and dihydrocoumarin (DHC), opening new routes to the formation of fine chemicals and pharmaceutical building blocks. Each of these products requires the hydrogenation of 4-hydroxycoumarin (4HC) to 4-hydroxydihydrocoumarin (4HDHC), which, in turn, requires the reduction of an unsaturated C-C bond in the presence of an aromatic ring. Using in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, we show that reaction at 348 K over monometallic Pd catalysts leads to the partial reduction of the aromatic ring in 4HC, obtaining 93% selectivity for C=C bond hydrogenation at 82% 4HC conversion and with a low turnover frequency (TOF). Decreasing the Pd dispersion from 70% to 6% not only leads to an increase in the rate of 4HC hydrogenation, but it also leads to an increase in the rate of overhydrogenation. However, the formation of bimetallic PdAu nanoparticles inhibits the overhydrogenation reaction while also doubling the TOF to a value of 6 ks(-1) for 4HDHC production. A bimetallic PdAu catalyst supported on SiO2 leads to 97% selectivity for C=C bond hydrogenation at 86% 4HC conversion, while an acidic support such as amorphous silica-alumina can be used to produce DHC directly from 4HC.