Cathodic reduction of 2-hydroxy-1,4-naphthoquinone (lawsone) to generate 1,2,4-trihydroxynaphthalene as regenerable reducing agent for dispersed indigo

A central chemical step in the application of vat dyes is their reduction into the alkali soluble leuco-form. In denim production the indigo dye is reduced by catalytic hydrogenation, by use of a chemical stock vat or by cathodic electron transfer in presence of soluble mediator systems. In this study the potential of 2-hydroxy-1,4-anthraquinone (lawsone) to serve as reversible redox couple for indirect cathodic reduction of dispersed indigo was investigated. Results from cyclic voltammetry, voltammetry in flow cells equipped with different cathode material and batch electrolysis experiments in presence of dispersed indigo demonstrate successful electron transfer from cathodically reduced lawsone to dispersed indigo above pH 12.5. At this pH the di-anion of 1,2,4-trihydroxynaphthaline is present in solution. At lower pH e.g. pH 11.5 the mono-anion is present, which is not able to reduce indigo molecules. For the study synthetic chemicals have been used as a model, however when the chemically equivalent natural colorants henna and indigo are used instead a new route for a fully bio-based indigo dyeing becomes available.