Improving the production of maleic acid from biomass: TS-1 catalysed aqueous phase oxidation of furfural in the presence of γ-valerolactone†
The production of di-acids from biomass, i.e. maleic acid (MAc), can be improved by combining the utilization of solvents and solid catalysts. The results reported here demonstrate that GVL, a renewable solvent, allows not only the production of furfural from biomass at high yield, but also its direct upgrading to valuable MAc using H2O2 as the oxidant and TS-1 as the catalyst. In addition, the incorporation of GVL results in higher MAc yields (70% at 70 °C) than those obtained in conventional aqueous medium (54%) and importantly, the presence of GVL prevents the deactivation of the catalysts by suppressing the deposition of insoluble heavy by-products over the surface and cavities of TS-1 (according to catalyst characterization by chemical and thermogravimetric analysis, and Raman and X-ray photoelectron spectroscopy). The activity and stability of the catalyst, using furfural directly obtained from biomass (e.g., processing corn cobs with GVL–H2O mixtures), showed that the catalyst could be reused for 17 runs without detectable deactivation, whereas in water the deactivation was already evident in the 8th run. The final purification of the product is facilitated by the low solubility of Na maleate in GVL. A precipitation protocol based on NaOH addition was developed to effectively separate MAc from the GVL–H2O reaction medium, leading to 87% of the initial MAc being recovered as a precipitate from the rest of the diacids. The reduction of separation steps and the increased catalyst activity and stability decrease the complexity of the process and facilitate the sustainable production of renewable chemicals.