Chlorine-catalysed pyrolysis of 1,2-dichloroethane. Part 1.—Experimental results and proposed mechanism

Abstract

Experimental studies of the pyrolysis and chlorination of 1,2-C₂H₄Cl₂ in the presence of small proportions of chlorine, or of chlorine plus nitric oxide, confirm that the main propagating steps between 520 and 620 K are Ċl + C₂H₄Cl₂(DCE) [graphic omitted] Ċ₂H₃Cl₂(R)+ HCl, Ċ₂H₃Cl₂ [graphic omitted] C₂H₃Cl(VC)+Ċl, Cl₂+Ċ₂H₃Cl₂ [graphic omitted] C₂H₃Cl₃(TCE)+Ċl. The initial rates (d[VC]/dt)₀ and (d[TCE]/dt)₀ decrease together as the vessels age, but k₂/k₃=γ remains constant at constant p_DCE^˙γ increases as p_DCE is increased, or if inert gases are added, as expected from unimolecular behaviour of k₂, and the Arrhenius parameters of γ increase together as p_DCE is increased. γ^(p) is evaluated for several ranges of p_DCE at five temperatures to allow study of k^(p)₂ by unimolecular theory in Part 2. The addition of VC lowers (d[VC]/dt)₀ through reaction (– 2), and the results are used to evaluate k_–2/k₁;k_–2 is also pressure-dependent.

In vessels with fresh surfaces, d[VC]/dt is proportional to p_DCE×p^0.5_Cl2 for low p_Cl2 and is independent of added inert gases; in aged vessels, the order in p_DCE becomes 0.62, very close to the dependence of γ^(p) on p_DCE. Calculations of [R]₀ and [Ċ]₀ from the observed rates and rate constants point to an initiation step Cl₂ S → CIS +Ċl and termination by the reverse reaction in fresh-surfaced vessels, where S is a surface site; with aged surfaces, the results point to the same initiation step combined with termination by R + SCl → C₂H₃Cl₃+ S especially at lower temperatures.