The role of chlorine and additives on the density and strength of Lewis and Brønsted acidic sites of gamma-Al2O3 support used in oxychlorination catalysis: a FTIR study

In two recent contributions [PCCP, 12 (2010) 5605; Dalton Trans., 39 (2010) 8437] we combined in situ and in operando XANES/EXAFS, CO chemisorption and catalytic tests to elucidate the role that dopants (LiCl, KCl, CsCl, MgCl2 LaCl3) have in the nature, relative fraction, reducibility and dispersion of Cu-phases on CuCl2/-Al2O3 catalysts for C2H4 oxychlorination reaction, a key step of PVC chemistry. In the present work we extend these studies by investigating the effect that the dopants have on the nature, population and strength of surface Lewis and Brønsted sites of the support, using IR spectroscopy of adsorbed CO at liquid nitrogen temperature. The doping eliminates all the surface Lewis acidity in CsCl and KCl doped-catalysts and strongly suppresses it in the remaining cases. The increase of the strength of the Brønsted sites is remarkable in all cases but the CsCl doped one. To understand both the effect of Cl- anions and dopant cations a set of dopant free, HCl-impregnated and of Cu-free dopant-impregnated supports have been investigated. Addition of chlorine decreases the density and the strength of Lewis sites, while it increases those of the Brønsted sites. Catalytic testing of each material revealed that formation of chlorinated byproducts was directly correlated with the density of Lewis acid sites. Furthermore, an empirical correlation was found between the strength of the Brønsted acid sites of the support and the stretching frequency of CO adsorbed on the reduced fraction of the active copper chloride phase. The present study is aimed to complement the published literature on alumina, underlining the usefulness of the molecular approach made by IR spectroscopy low temperature adsorbed CO to investigate the surface of catalyst support.