Conversion of furfural and 2-methylpentanal on Pd/SiO2 and Pd–Cu/SiO2 catalysts

The conversion of furfural (FAL) and 2-methylpentanal (MPAL) under hydrogen has been studied over silica-supported monometallic Pd and bimetallic Pd–Cu catalysts. At low space times, the conversion of MPAL yields primarily pentane (decarbonylation), but at higher space times, di-methylpentyl ether (etherification) becomes the main product. Upon addition of Cu, both the overall activity and the decarbonylation selectivity decrease while the selectivity to hydrogenation and etherification increases. In contrast to MPAL, the conversion of FAL shows no etherification products at any space time in the temperature range 210–250 °C but only produces furan via decarbonylation. It is proposed that the presence of the aromatic ring in the furfural molecule has a marked effect in inhibiting the formation of the alkoxide surface intermediate, which is required in the etherification reaction. Density Functional Theory (DFT) calculations of furfural and 2-methyl pentanal have been conducted to gain a better understanding of the differences in the molecule–surface interactions between the aldehydes and the Pd and Pd–Cu surfaces. The reaction mechanisms and the resulting selectivity towards the possible reaction paths (hydrogenation/etherification/decarbonylation) are discussed in terms of the relative stability of the η2-(C,O) and acyl surface species occurring on the different metal surfaces.