Analysis of kinetics and reaction pathways in the aqueous-phase hydrogenation of levulinic acid to form γ-Valerolactone over Ru/C

Reaction pathways and kinetics governing the Ru-catalyzed hydrogenation of levulinic acid (LA) in the aqueous phase to form γ-valerolactone (GVL) were considered in a packed bed reactor. GVL can be produced by two distinct hydrogenation pathways; however, over Ru/C at temperatures below 423 K, it forms exclusively via intramolecular esterification of 4-hydroxypentanoic acid (HPA). Over Ru/C, reasonable hydrogenation rates of LA to HPA were observed at near-ambient temperatures (e.g., 0.08 s^-1 at 323 K), but GVL selectivities are poor (<5%) under these conditions. Apparent barriers for LA hydrogenation and HPA esterification are 48 and 70 kJ mol^-1, respectively, and GVL selectivity improves at higher temperatures alongside increasing mass transfer limitations in 45-90 μm catalyst particles. Reactivity and selectivity trends in LA hydrogenation below 343 K are well-described by an empirical kinetic model capturing sequential hydrogenation and esterification. Coupling stacked beds of Ru/C and Amberlyst-15 delivers high GVL yields (∼80%) at near ambient temperatures (323 K) and practical residence times.