Modification of bio-based β-diketone from wheat straw wax: synthesis of polydentate lipophilic super-chelators for enhanced metal recovery

Abstract

Bio-derived lipophilic polydentate chelators have been synthesized and tested for their chelating ability using a range of metal salts of Cu, Co, Ni, Fe, and Cr. These novel molecules were produced by the Michael addition reaction of 14,16-hentriacontanedione, isolated from wheat straw wax, with methyl acrylate or bio-derived dimethyl itaconate via microwave heating. The Michael adducts could either be used directly as esters or be hydrolysed to their acid form. Critically, the creation of additional binding sites via the carboxylate moieties leads to an enhanced metal uptake over both a non-renewable commercially available lipophilic β-diketone (dibenzoylmethane) and the unmodified hentriacontane-14,16-dione, for the chelation of Fe(III), Cr(III) and Ni(II). The modified β-diketone containing a single carboxylic acid functionality was able to extract 167 mg L⁻¹ of Fe(III) from an FeCl₃ solution with no pH adjustment. In comparison, no chelation was observed with dibenzoylmethane, while unmodified hentriacontane-14,16-dione was able to extract 81 mg L⁻¹. The modified chelators containing one and two ester carboxylates extracted 255 and 305 mg L⁻¹ Cr(III) from a solution of CrCl₃ at pH 5–6, 238 mg L⁻¹ was extracted by the unmodified β-diketone whilst no extraction was observed using dibenzoylmethane. This suggest some minor contribution or positive effect to chelation due to neighbouring ester groups. The chelator containing two carboxylic acid groups (tetra-dentate when combined with the diketone) was the most proficient in this study for removal of Ni from an NiCl₂ solution (140 mg L⁻¹). It was also found that at higher pH almost quantitative extraction was achieved using the polydentate chelators.