REDOX BEHAVIOR AND DISPERSION OF SUPPORTED CHROMIUM CATALYSTS

Supported chromium catalysts with different support composition are st udied by electron spin resonance spectroscopy (ESR), thermogravimetric al analysis (TGA), and Al-27 magic angle spinning nuclear magnetic res onance (MAS-NMR). With ESR, the presence of Cr5+ (gamma-signal) and tw o types of Cr3+ (delta- and beta-signals) are observed; however, their concentrations are support-, loading-, and pretreatment-dependent. Th e Cr5+ loading increases with the alumina content of the support and c an be maximized after moderate reduction temperatures and by using rel atively low Cr loadings. Dispersed Cr3+ (delta-signal) is formed by re duction, and its amount increases with Al content of the support, Cr l oading, and reduction temperature. On silica-alumina and alumina it is stable against recalcination. Cr2O3-like clusters (beta-signal) are o bserved only on Cr/alumina after reduction and recalcination. TGA anal ysis indicates an increasing interaction strength between Cr and the s upport from silica over silica-alumina to alumina. The quenching of th e Al-27 lines by paramagnetic Cr3+ was observed by Al-27 MAS-NMR measu rements on the recalcined samples. This quenching effect was stronger on the Cr/alumina sample than on the Cr/silica-alumina samples, indica ting Cr3+ substitution in octahedral Al sites. The observed difference s in redox behavior and dispersion of Cr between the catalysts are dis cussed in terms of the support properties and in relation with previou s results obtained by diffuse reflectance spectroscopy (DRS).