Aqueous Interfacial Chemistry in the Catalyst Preparation of NiMo/Al_2O_3 System by EDTA-Containing Impregnation

摘要

The present work studies the effect of organic chelating agents on the solution—support interfacial process during the catalyst preparation. EDTA—Ni(Ⅱ)—Mo(Ⅵ)—γ-Al_2O_3—H_2O is used as a representative catalyst synthesis system. It has been found that Ni(Ⅱ) adsorption increases with pH value starting from pH 6 and reaches a maximum at pH 8 without EDTA, whereas the adsorption vs pH is reversed (maximum at pH 3.5 and no adsorption at pH 9) when EDTA is introduced into this system with a molar ratio of EDTA/Ni(Ⅱ) = 1. According to ion-exchange theory, Ni(Ⅱ) possibly forms a surface ternary complex with the alumina surface sites through EDTA acting as a bridge. It has also been found that Mo(Ⅵ) adsorption decreases with pH (maximum at pH 3 and no adsorption at pH 9) in the absence of EDTA. EDTA suppresses the adsorption of Mo(Ⅵ), although the adsorption of Mo(Ⅵ) vs pH remains the same. EDTA has a higher coordinating constant with the surface Al~(3+) than do Mo(Ⅵ) anions. Free EDTA competes with Mo(Ⅵ) on the alumina surface sites. Because EDTA has a preference to chemically bond with Ni(Ⅱ), which forms a stable complex, Ni—EDTA could be selectively adsorbed on the alumina surface. On the basis of these experimental results, it can be rationalized that EDTA promotes the dispersion of both Ni(Ⅱ) and Mo(Ⅵ) on alumina. Meanwhile, EDTA also limits strong interactions between the metal ions and alumina, thereby contributing to the formation of more vigorous sites.