Malonate complexes at γ-alumina surface determined by a multi-technique characterization approach and a surface complex model

Abstract

γ-Al2O3 is a transition alumina oxide used in many different applications. Its surface chemistry seems to be a key factor in the preparation of heterogeneous catalyst where γ-Al2O3 is used as a support. The modification and description of its surface are necessary to understand the role of the support on the catalyst preparation and on its performance. In this work, the modification of the surface is performed with a carboxylic diacid, malonic acid, which specifically adsorbs at the γ-Al2O3 surface and modifies the surface chemistry (surface sites density and global surface charge). The impact of the impregnation pH, malonic acid content, and drying are studied. By using a multi-technique approach, both the solid/liquid interface and the dried surface were characterized. The infrared spectroscopic information coupled with zetametry measurements indicate the formation of an inner sphere complex adsorbed via both side-on ester linkage and bridging bidentate mode involving the two carboxylic groups. A surface complexation model was built to support the experimental results and obtain complementary information. It underlines the different reactivity of the γ-Al2O3 facets and of the surface sites responsible for the adsorption. The whole (100) facet is involved in the adsorption while only the µ1-AlVI-OH and µ1-AlIV-OH sites of the (110) facet participate in the malonic acid adsorption. The model also confirms the consumption of 3–4 surface hydroxyls per adsorbed molecule, in accordance with ester linkage and bridging bidentate structure. This molecular level description of the surface chemistry is of interest in different environmental fields such as wastewater treatment and pollutant removal. In the catalysis field, the detailed surface chemistry will be very useful to understand the impact of this modified support on the catalytic performance.