Catalytic reduction of vat and sulfur dyes with hydrogen

Vat and sulfur dyes must be reduced to their leuco forms before application. Since this process is currently carried out using irreversible reducing chemicals, it involves high economic and environmental costs. Here we show that reduction can alternatively be performed in an alkaline bath at room temperature by using H2 activated by a Pd catalyst. Sulfur dyes are thus directly reduced, whereas vat dyes require an electron transfer mediator. This alternative reaction can also be driven using H2 at sub-atmospheric pressures. It further avoids all drawbacks raised by electrolytic reductions, so far suggested as alternative green processes.