On the development of chrome-free tanning agents: an advanced Trojan horse strategy using ‘Al–Zr-oligosaccharides’ produced by the depolymerization and oxidation of biomass

Abstract

This work addresses the manufacturing of a pioneering chrome-free tanning agent, consisting of ‘Al–Zr-oligosaccharides’ produced from biomass. First, a microwave-assisted AlCl₃–H₂O₂–H₂O reaction system was used for the simultaneous degradation and oxidation of hemicellulose in corncob. The obtained aqueous solution containing ‘Al-oligosaccharides’ was then combined with Zr to produce water-soluble ‘Al–Zr-oligosaccharides’. These species behaved as a ‘Trojan horse’, allowing the uniform and controlled penetration of Al and Zr into the leather, avoiding their accumulation on the surface. The effects of the reaction temperature and the AlCl₃ and H₂O₂ concentrations used during the synthesis were systematically analyzed. Increases in the temperature and AlCl₃ concentration improved the hemicellulose conversion, while the use of high H₂O₂ concentrations augmented the amounts of –CHO/–COOH functionalities in the oligosaccharides, with both phenomena being paramount for the efficient formation of ‘Al–Zr-oligosaccharides’. However, more severe reaction conditions led to the formation of an undesirable species, negatively influencing the tanning performance and chromaticity of the tanning agent. Given this, these parameters were optimized, and it was found that it is possible to convert up to 72 wt% of the hemicellulose present in corncob into an ‘advanced Trojan horse tanning agent’ contained in a colorless water solution, conducting the synthesis at 129 °C with a solvent comprising 56 mM AlCl₃ and 2.6 vol% H₂O₂. The application of this solution for leather tanning provided leather with a shrinkage temperature as high as 87 °C, thus meeting the restrictive requirements of the leather industry. Therefore, these results are a landmark achievement in this field and represent a step forward to a sustainable and renewable leather industry.