Redox behavior and dispersion of supported chromium catalysts

Supported chromium catalysts with different support composition are studied by electron spin resonance spectroscopy (ESR), thermogravimetrical analysis (TGA), and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS-NMR). With ESR, the presence of Cr³⁺ (γ-signal) and two types of Cr³⁺ (δ- and β-signals) are observed; however, their concentrations are support-, loading-, and pretreatment-dependent. The Cr⁵⁺ loading increases with the alumina content of the support and can be maximized after moderate reduction temperatures and by using relatively low Cr loadings. Dispersed Cr³⁺ (δ-signal) is formed by reduction, and its amount increases with Al content of the support, Cr loading, and reduction temperature. On silica-alumina and alumina it is stable against recalcination. Cr₂O₃-like clusters (β-signal) are observed only on Cr/alumina after reduction and recalcination. TGA analysis indicates an increasing interaction strength between Cr and the support from silica over silica-alumina to alumina. The quenching of the ²⁷Al lines by paramagnetic Cr³⁺ was observed by ²⁷Al MAS-NMR measurements on the recalcined samples. THis quenching effect was stronger on the Cr/alumina sample than on the Cr/silica-alumina samples, indicating Cr³⁺ substitution in octahedral Al sites. The observed differences in redox behavior and dispersion of Cr between the catalysts are discussed in terms of the support properties and in relation with previous results obtained by diffuse reflectance spectroscopy (DRS).