Solvent-free oxidative esterification of furfural to 2-methyl furoate using novel copper-exchanged tungstophosphoric acid supported on montmorillonite K-10 catalyst

Abstract

The valorization of bio-based chemicals using green processes plays a vital role in the up-gradation of biofuels that are useful to reduce the demand for fossil feedstock. Furfural is a promising renewable biomass-based platform chemical, which can be catalyically transformed into many value-added chemicals. Oxidative esterification of furfural produces 2-methyl furoate, which is used as flavour and fragrance in fine chemical industries. There lies a challenge to find a cheaper and efficient catalyst for one-pot oxidative esterification of furfural. Supported heteropolyacids (HPAs) are useful for many biomass transformation reactions owing to their strong Bronsted acidity as well as high surface area, which both help promote catalytic surface reaction. In the present work, we have synthesized Cu exchanged tungstophosphoric acid (TPA) with different supports, viz. SiO2, ZrO2, and montmorillonite K-10 clay (K10) which were prepared by incipient wetness impregnation. Oxidative esterification of furfural with methyl alcohol was investigated using the aforesaid catalysts and tert-butyl hydroperoxide (TBHP) as an oxidant. 20% w/w Cu substituted TPA (abbreviated as Cu-TK10) catalyst is green, stable, active, selective, reusable, and cheaper. It was observed that the higher acidic strength and good surface area are responsible for oxidative esterification of furfural and methyl alcohol with higher selectivity. In this work, 91% conversion with 100% selectivity was achieved in 5 h at 120 °C using 1:3 molar ratio of furfural and TBHP with a catalyst loading of 0.01 g/cm3. The synthesized catalysts were analyzed by various characterization techniques such as SEM, XRD, FT-IR, BET surface area analyser, and NH3-TPD, before and after use. This is an interesting example of valorization of furfural.