Highly selective valorisation of D-xylose from corncobs hydrolysates catalysed by C-scorpionate gold(III) complexes

Abstract

D-xylonic acid is a multifunctional, environmentally friendly, and versatile platform chemical, and one of the most promising conversion products of D-xylose for replacing several petrochemicals that is still not produced commercially. Although microbial conversion is the most widely studied process, the low efficiency and cost of these methods create opportunities for the development of chemical routes. In this work, we report for the first time the direct chemical conversion of real hemicellulose-derived pentose-rich hydrolysates into D-xylonic acid. Furthermore, a novel chemical oxidation route based on C-scorpionate gold(III) complexes was explored. Two specific complexes are used as model catalysts, the cationic [AuCl₂(κ²-Tpm)]Cl (1, Tpm = hydrotris(pyrazol-1-yl)methane, HC(C₃N₂H₃)₃) or the neutral [AuCl₂(κ²-Tpms)] complex (2, Tpms = tris(1H-pyrazol-1-yl)methane sulfonate, [SO₃C(C₃H₃N₂)₃]⁻). The catalytic operational conditions (i.e., reaction time, temperature, type and concentration of alkali and oxidant agent) were initially optimised for D-xylose solutions using cationic complex 1. Under the best conditions (100 °C, 2 mmol NaOH and 20 mmol H₂O₂) nearly 100% conversion of D-xylose and a 94% D-xylonic acid yield were achieved, with minimal formation of secondary products, when using 6 g/L D-xylose as sole carbon substrate. The direct catalytic conversion of real hemicellulosic hydrolysate, without any previous purification was possible and up to 36 g/L of D-xylose, regardless of the presence of other sugars, aliphatic acids, furans and phenolic compounds. Comparing similar catalytic conditions among model D-xylose solutions and real hydrolysates with similar titres, the later presented better performance, even though D-xylose conversion and D-xylonic acid yield decreased slightly for higher D-xylose concentrations. The catalysis was further improved by using the neutral gold(III) C-scorpionate complex 2, which further increased the D-xylose conversion and selectivity.