Transfer hydrogenation of phenol on supported Pd catalysts using formic acid as an alternative hydrogen source

Palladium nanoparticles with size smaller than 10 nm were loaded on several supports including activated carbon (AC), MIL-101, TiO₂, Al ₂O₃, and TiO₂-activated carbon composites (TiO₂-AC). These catalysts showed high activity in liquid phase hydrogenation of phenol. Their catalytic performances in transfer hydrogenation of phenol with formic acid under mild conditions (T = 50 °C and P < 5 bar) were also tested for the purpose of in situ upgrading of bio-oil. The activity followed the trend of Pd/AC > Pd/TiO₂-AC > Pd/MIL-101 > Pd/TiO₂ > Pd/Al₂O₃. When 400 μL of formic acid was introduced into 10 mL of 0.25 M aqueous phenol solution, phenol can be fully hydrogenated on 200 mg of Pd/AC catalyst at 50 °C within 4 h, with 80% selectivity to cyclohexanone. The high activity of Pd/AC catalyst in hydrogen transfer makes it possible for in situ upgrading of bio-oil under mild conditions by converting unstable components (such as HCOOH and phenol) to stable ones (mainly cyclohexanone) which can then be potentially introduced into current refinery. In addition, the effect of formic acid on the hydrogenation activity of supported Pd catalysts was also reported. Presence of formic acid significantly decreased the hydrogenation activity of Pd supported on MIL-101 and oxides (TiO₂ and Al₂O₃), probably due to the competitive adsorption of molecular formic acid with phenol on a single Pd site.