Direct synthesis of furfuryl alcohol from furfural: catalytic performance of monometallic and bimetallic Mo and Ru phosphides

Abstract

The catalytic properties of monometallic and bimetallic Ru and Mo phosphides were evaluated for their ability to selectively hydrogenate furfural to furfuryl alcohol. Monometallic MoP showed high selectivity (98%) towards furfuryl alcohol, while RuP and Ru₂P exhibited lower selectivity at comparable conversion. Bimetallic promotional effects were observed with Ru_1.0Mo_1.0P, as the pseudo-first order reaction rate constant for furfural hydrogenation to furfuryl alcohol, k₁, was at least 5× higher than MoP, RuP, and Ru₂P, while maintaining a 99% selectivity. Composition-directed catalytic studies of Ru_xMo_2−xP (0.8 < x < 1.2) provided evidence that Ru rich compositions positively influence k₁, but not the selectivity. The rate constant ratio k₁/(k₂ + k₃) for furfuryl alcohol production compared to methyl furan (k₂) and tetrahyrofurfuryl alcohol (k₃) followed the trend of Ru_1.0Mo_1.0P > Ru_1.2Mo_0.8P > MoP > Ru_0.8Mo_1.2P > RuP > Ru₂P. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to examine the configuration of adsorbed furfural on the synthesized catalysts, but the results were inconclusive and no correlation could be found with the selectivity due to the possible IR inactive surface modes with furfural adsorption. However, gas phase density functional theory calculations suggested the x = 1.0 material in Ru_xMo_2−xP (0.8 < x < 1.2) had the most favorable furfural adsorption energy. Experimentally, we also observed that the solvent greatly influenced both the conversion and selectivity, where isopropanol provided the highest selectivity to furfuryl alcohol. Finally, recycling experiments showed a 12% decrease in k₁ after 3 cycles without any regeneration, but the activity could be fully recovered through a re-reduction step.