Influence of additives in defining the active phase of the ethylene oxychlorination catalyst

Abstract

The understanding, at the atomic level, of the role played by additives (dopants or promoters) in the chemistry of an industrial catalyst is a very complex and difficult task. We succeeded in this goal for the ethylene oxychlorination catalyst (CuCl₂/γ-Al₂O₃), used to produce dichloroethane, a key intermediate of the polyvinyl chloride chemistry (PVC). Among the most used additives for both fluid and fixed beds technologies (LiCl, KCl, CsCl, MgCl₂, LaCl₃, CeCl₄) we have been able to highlight that KCl, and CsCl, forming in reaction conditions a mixed phase with CuCl₂, strongly modify the catalyst behaviour. In particular, these additives are able to displace the rate determining step from the CuCl oxidation (undoped catalyst) to the CuCl₂ reduction. This results from the decrease of the rate of the latter reaction, thus the overall activity of the system. For all remaining additives the rate determining step remains the CuCl oxidation, as for the undoped catalyst. These results have been obtained coupling the catalyst activity monitored with a pulse reactor working in both non-depletive and depletive modes with time resolved XANES spectroscopy performed under in operando conditions (i.e. coupled with mass spectrometry). Formation of CuK_xCl₂₊x and CuCs_xCl₂₊x mixed phases has been proved monitoring the Cu(II) d–d transitions with UV-Vis spectrometer and the CO stretching frequency of carbon monoxide adsorbed on reduced catalyst by in situ IR spectroscopy. Finally, of high relevance is the observation that the fully oxidized catalyst is inactive. This unexpected evidence highlight the role of coordinatively unsaturated Cu(I) species in adsorbing ethylene on the catalyst surface indicating that copper, in the working catalyst, exhibits a (I)/(II) mixed valence state.