Pd And Cu Monometallic And Single Atom Alloy Catalysts For The Hydrogenation Of Biomass Based Chemicals

Abstract

The liquid-phase selective hydrogenation ofbio-based platform molecules like furfural was studied with non-toxic Cu-based heterogeneous catalysts.The initial materials were synthesised via wet impregnation using various copper precursors (nitrate, acetate, and sulfate) at two different loadings. High Cu loading (5 wt%) led to the formation of well-defined nanoparticles, while lower loading (1 wt%) generated highly dispersed atomic and dimeric Cu species on the non-porous nano-Al2O3support. Copper sulfatederived catalysts severely reduced the selectivity of furfuryl alcohol from 94.6% to 0.8% and promoted acetalisation reactions instead. On the contrary, sulfur-free copper acetate derived catalysts were found optimal for catalysing this reaction. The research then focused on enhancing colloidally synthesised Cu catalysts by incorporating trace-amounts of Pd atoms via galvanic replacement. These materials were referred as single atom alloy catalysts (SAA), as EXAFS confirmed they were atomically dispersed Pd atoms on Cu nanoparticles. These SAA catalysts improved the furfural conversion to furfuryl alcohol compared to the monometallic catalysts, as they presented the advantages of Cu (high selectivity) and Pd (superior activity) monometallic catalysts, without the drawbacks (copper’s low activity and palladium’s poor selectivity). As a result, SAA proved to be optimal green/atom efficient catalysts. Finally, the synthesised materials were tested for the hydrogenation of crotonaldehyde. Crotonaldehyde was chosen as it lacked the directing group present in furfural (furan ring), so the catalysts can be examined when the C ═ O hydrogenation is not specifically preferred. The SAA catalysts improved the normalised catalytic activity by nineteen-fold when compared to the Pd100 benchmark catalyst, while maintaining the reactive pathway of the Cu nanoparticle host. In essence, the presence of Pd “fast-forwarded” the extent of the reaction. For the wet impregnation monometallic Cu materials, the acetate precursor catalysts (1 and 5 wt%) showed superior activity, while the 5 wt% sulfur-based was the worst.