Quantification of copper phases, their reducibility and dispersion in doped-CuCl2/Al2O3catalysts for ethylene oxychlorination

Abstract

The comprehensive understanding of the composition, behaviour and reactivity of a catalyst used inside industrial plants is an extremely hard task that is rarely achieved. It requires the use of different spectroscopic techniques, applied under in situ or in operando conditions, and combined with the investigation of the catalyst activity. Often the operating experimental conditions are different from technique to technique and the different results must be compared with care. In the present contribution, we combined in situ XANES/EXAFS, IR spectroscopy of adsorbed CO, CO chemisorption and catalytic tests performed using a pulse reactor in depletive mode. This multitechnical approach resulted in the understanding of the role that dopants (LiCl, KCl, CsCl, MgCl₂ LaCl₃) have in the nature, relative fraction, reducibility and dispersion of Cu-phases on CuCl₂/γ-Al₂O₃ catalysts for oxychlorination reaction, a key step of the PVC chemistry. In the undoped catalyst two Cu phases coexist: Cu-aluminate and supported CuCl₂, being the latter the only active one [J. Catal., 2000, 189, 91]. EXAFS and XANES highlighted that all dopants contribute more or less efficiently in increasing the fraction of the active copper species, that reaches a value of almost 100% in the case of MgCl₂ or LaCl₃. EXAFS directly, and IR indirectly, proved that the addition of KCl or CsCl (and less efficiently of LiCl) results in the formation of mixed CuK_xCl₂₊x or CuCs_xCl₂₊x phases, so altering the chemical nature of the active phase. XANES spectroscopy indicates that addition of MgCl₂ or LaCl₃ does not affect the reducibility by ethylene (under static conditions) of the active CuCl₂ phase and that the reducibilility of the new copper-dopant mixed chloride are in the order CuCl₂ > CuLi_xCl₂₊x > CuK_xCl₂₊x > CuCs_xCl₂₊x. However, when reduction is done inside a pulse reactor, a more informative picture comes out. The last technique is able to differentiate all samples, and their ability to be reduced by ethylene resulted in the order: La- > Mg- > Li-doped > undoped > K- > Cs-doped catalyst. To understand this apparent discrepancy the dispersion of the active phase, measured by CO chemisorption, was needed: it has been found that addition of LiCl increases enormously the dispersion of the active phase, LaCl₃ significantly and MgCl₂ barely, while addition of both KCl and CsCl results in a decrease of the surface area of the active phase. The mechanism of the enhancing effect of La and Mg on catalytic activity is still not clear, but it could be associated to the modification that they induce to the support surface: the Cu is so highly dispersed that almost all is in direct contact with support surface. It is finally worth noticing that the previous EXAFS and XANES study allowed us to refer the chemisorption data to the active phase only, while the IR study allowed us to fix the Cu⁺/CO surface stoichiometry. Summarizing the use of a multidisciplinary approach has been the conditio sine qua non (mandatory condition) to understand the complex role that the different additives have on the active phase of the CuCl₂/γ-Al₂O₃ catalysts for ethylene oxychlorination.