Borohydride and metallic copper as a robust dehalogenation system: Selectivity assessment and system optimization

Abstract

Hydrodechlorination (HDC) using noble-metal catalysts in the presence of H-donors is a promising tool for the treatment of water contaminated by halogenated organic compounds (HOCs). Cu is an attractive alternative catalyst to noble metals since it is cheaper than Pd, Rh, or Pt and more stable against deactivation. Cu with borohydride (BH4−) as reductant (copper-borohydride reduction system; CBRS) was applied here for the treatment of saturated aliphatic HOCs. The HDC ability of CBRS was evaluated based upon product selectivities during reduction of CCl3–R compounds (R = H, F, Cl, Br, and CH3). For CHCl3, CH2Cl2, and CHCl2–CH3, the dechlorination reaction proceeds predominantly via α-elimination with initial product selectivities to CH4 and C2H6 of 84–85 mol-% and 70–72 mol-%. For CCl4, CBrCl3, CFCl3, and CCl3–CH3, stepwise hydrogenolysis dominates. CH2Cl–R compounds are formed as recalcitrant intermediates with initial selectivities of 50–72 mol-%, whereas CH4 and C2H6 are minor products with 16–35 mol-% and 30–35 mol-%. The effect of reaction conditions on product selectivities were investigated for CHCl3 as target. Solution composition, variation of reducing agents (BH4−, H* from H2) and increase of electron pressure (electric potential at Cu electrode and Fe0 as support) did not have marked influence on the selectivities (ratio of CH4 : CH2Cl2). Product selectivities for reduction of CCl3–R compounds were found to be substrate-specific rather than reductant-specific. Since the formation of halogenated by-products could not be avoided, transformation via a second reduction step was optimized by higher catalyst dose, addition of Ag, and vitamin B12 to the CBRS. Comparison between Pd and Cu based on costs, catalyst activities, selectivities, metal stability, and fate of halogenated by-products shows that the CBRS is a potent alternative to conventional HDC catalysts and can be recommended as ‘agent of choice’ for treatment of α-substituted haloalkanes in heavily contaminated waters.