Hydrodechlorination of 1,2-dichloroethane and dichlorodifluoromethane over Ni/C catalysts:The effect of catalyst carbiding

摘要

In spite of previous reports,nickel catalysts appeared active in hydrodechlorination of 1,2-dichloroethane,producing ethylene with very high selectivity(up to 97%)at low temperatures of reaction(210-230 °C).In addition,at these temperatures,nickel deactivation by surface chloriding,observed by others,does not practically occur.The use of different nickel salts(chloride,nitrate,acetate)as catalyst precursors makes it possible to obtain different metal dispersions,reflected in variations of overall catalytic activity expressed per mass of nickel.The conditions of catalyst reduction led to a complete reduction of nickel precursor and effective removal of the counterion(Cl~-or NO_3~-).Interestingly,in the case of the chloride-and acetate-derived catalysts,the selectivity to vinyl chloride increases gradually with time on stream,at the expense of ethylene,even up to 30%.The appearance of fcc NiC_x solutions(x <= 0.1)and an hcp Ni3C phase in Ni/C catalysts used results from a massive production of ethylene,which is an efficient carbiding agent.An increasingly deeper transformation of Ni to Ni3C led to a higher selectivity to vinyl chloride.Alternatively,the Ni/C samples which were most selective toward ethylene(and characterized by a smaller Ni particle size)contained only tiny amounts of carbon in the form of NiC_x solution.The behavior of Ni/C catalysts seems to be largely regulated by the population of surface carbon species.A higher surface carbon content(implied by a higher carbon content in Ni3C than in NiC_(x<=0.1))creates an undeniable difficulty in removal of the second chlorine atom from 1,2-dichloroethane.It is suggested that,in such conditions,a new reaction route leading to vinyl chloride via a concerted elimination of HC1 is feasible.The Ni/C catalysts subjected to CC12F2 hydrodechlorination contained even higher amounts of Ni3C than the analogous samples screened in the reaction of 1,2-dichloroethane.