Analysis of Kinetics and Reaction Pathways in the Aqueous-Phase Hydrogenation of Levulinic Acid To Form γ-Valerolactone over Ru/C

Abstract

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.