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

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, CFCl_3, 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.