Heteropolyacid supported on Zr-Beta zeolite as an active catalyst for one-pot transformation of furfural to gamma-valerolactone

Abstract

A novel bifunctional catalyst that enables an efficient one-pot conversion of furfural into gamma-valerolactone (GVL) has been developed by anchoring heteropolyacid (HPA) on Zr-Beta zeolite. The catalysts were prepared by a post-synthesis procedure, which consists of the dealumination of Al-Beta, incorporation of Zr into the beta framework through solid-state ion-exchange and impregnation of the HPA. Zr-Beta is used as a Lewis acid catalyst to catalyze the transfer hydrogenation of furfural and levulinic acid/ester using 2-propanol as a hydrogen donor. To deal with the inability of Zr-Beta to catalyze the hydrolytic ring-opening of furans toward GVL, phosphotungstic acid (HPW) and silicotungstic acid (HSiW) were introduced to the Zr-Beta as Bronsted acid sites. The characterization of the catalysts using XRD, UV-vis and XPS as well as TPD of ammonia and FT-IR spectroscopy of the adsorbed pyridine revealed that the HPA/Zr-Beta possesses both isolated Lewis and Bronsted acid sites. When they were applied to the one-pot cascade conversion of furfural, the initial activity of the HPA/Zr-Beta toward GVL production were 2-3 times greater than that for Zr-Beta due to the enhanced hydrolytic ring-opening of the furans promoted by the added Bronsted acidity. Especially, HPW loaded Zr-Beta demonstrated a remarkable GVL yield of similar to 70% at 433 K after 24 h due to its high thermal stability and stronger Bronsted acidity, and its activity far surpasses that of the conventional Sn-Al-Beta zeolite (similar to 40%). Overall, this study demonstrates that an incorporation of HPA into Lewis acid Sn- or Zr-Beta zeolites is an effective strategy to create isolated Lewis and Bronsted acid sites within a single catalyst, thereby allowing the selective cascade catalysis for the cost-effective production of high-value chemicals.