Screening of zero valent mono/bimetallic catalysts and recommendation of Raney Ni (without reducing agent) for dechlorination of 4-chlorophenol

Chlorophenol (CP) is considered as environmentally hazardous material due to its acute toxicity, persistent nature and strong bioaccumulation. The dechlorination of 4-CP was investigated by using various catalysts such as bimetallic (Fe0/Cu0, Al0/Fe0), Pd/C, Raney Ni and Fe0 at room temperature. Among the catalysts studied, Raney Ni proved to be very economical and efficient catalyst that worked without the use of an external reducing agent. The dechlorination of 4-CP by Raney Ni was therefore further explored. Complete dechlorination of 4-CP (30 mg L−1) was achieved in 6 h at an optimum Raney Ni catalyst loading of 3 g L−1. The effect of triethylamine (TEA) and tripropylamine (TPA) was also investigated and it was observed that 100% dechlorination is possible in presence of 45 mg L−1 of TEA. The kinetics of dechlorination of 4-CP was investigated and found to be first order with a rate constant of 0.017 min−1 at 50 οC, and it enhances to 0.109 min−1 with addition of TEA. In the absence of a reducing agent, acidic to neutral pH favors dechlorination of 4-CP. The final product of dechlorination was estimated to be phenol by performing HPLC, LCMS and NMR analysis. Based on the results, a probable dechlorination mechanism of 4-CP is also proposed. It can be concluded that the catalytic hydrodechlorination is an effective and economical technique for dechlorination of 4-CP and it has a potential for the dechlorination of other toxic derivatives of chlorinated aromatics.