Kinetics and mechanism study on the preparation of 4,4′-diaminostilbene-2,2′-disulfonic acid by reduction of 4,4′-dinitrostilbene-2,2′-disulfonic acid with zero-valent iron

This research is aimed at improving the understanding of the reduction of 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNS) by zero-valent iron for the manufacture of 4,4′-diaminostilbene-2,2′-disulfonic acid (DSD). The formation of 5-amino-2-formylbenzene sulfonic acid (ABAS) during the process indicates that oxidation with the cleavage of CC double bond does occur besides the principle nitro-group reduction processes. The kinetics of the reduction of DNS and the formation of DSD are pseudo-first order. Typical kobs and kobs′ for the reduction of DNS and formation of DSD are 0.10898 ± 0.01841 min−1 and 0.07167 ± 0.00213 min−1, respectively (with 20 g/L, 60–80 mesh and the BET surface area of 0.815 m2 g−1 zero-valent iron, at 40 rpm mixing rate, 24 °C, neutral unbuffered conditions). Surface-area normalized rate constant (kSA) for the reduction of DNS is 0.4012 L m2 h−1. The effects of Fe0 concentration, rate of mixing and temperature on the kSA were investigated, and the results indicate that the rates of reaction were controlled by mass transfer.