[Ru(N,N'-bis-(2-pyridylmethyl)-o-phenylenediamine)(XCN)2][OTf]2 (X = Me, Ph) and [Ru(2,9-di(pyrid-2'-yl)-1,10-phenanthroline)(MeCN)2][OTf]2 as Catalysts for the Hydrodeoxygenation of Biomass-Derived Substrates in Aqueous Acidic Media

Classically, the biomass conversion field has focused heavily on both process engineering and the design of effective heterogeneous catalysts for the purpose of obtaining high-value compounds from sugar-derived furanic platform chemicals. One variable that is missing from this equation is the intrinsic reactivity of the substrates that process engineers have chosen to work with. To create a catalytic biomass conversion process that is competitive with current industrial processes, both catalyst design and substrate choice must be considered. This work outlines the synthesis of two molecular ruthenium pre-catalysts featuring tetradentate nitrogen-based ligands, [Ru(BPOPD)(XCN)2][OTf]2 (X = Me, Ph) and [Ru(DPP)(MeCN)2][OTf]2. The catalytic activity of [Ru(DPP)(MeCN)2][OTf]2 was evaluated for the conversion of biomass-derived furfuryl alcohol, and furfuryl acetate to the value-added compounds 1,4-pentanediol – being the primary target compound – and cyclopentanol in aqueous, acidic reaction medium at elevated temperature and pressure. Catalytic reactions using furfuryl alcohol as a substrate were severely limited by the formation of solid resins (the product yields showing a strong negative correlation with increasing substrate concentration), with maximum yields of 1,4-pentanediol and cyclopentanol being 23% and 41% respectively. A 2-level full factorial design of experiments study with four independent variables and a centre point was carried out in two replicate sets for the conversion of furfuryl acetate by [Ru(DPP)(MeCN)2][OTf]2, showing good reproducibility between replicates, and no solid humin formation. This enabled a full statistical analysis of variable impact on product distribution and yield. The maximum yields of 1,4-pentanediol and cyclopentanol using furfuryl acetate as a substrate are 68% and 35% respectively. The decreased self-reactivity of furfuryl acetate with respect to furfuryl alcohol dramatically increases the yields of target products, but there is still a strong negative correlation of product yield to increasing substrate concentration which is highlighted by the design of experiments study. This negative correlation between target product yield and increasing substrate concentration – brought about by the intrinsic self-reactivity of the highly oxygenated compounds present in carbohydrates and their derivatives – continues to be a primary challenge towards realizing an effective transformation of biomass-derived compounds to useful chemicals or fuels.