Effect of the chemical composition on the catalytic performance of MgyAlOx catalysts for alcohol elimination reactions

Alcohol C-H bond activation was studied using 2-propanol elimination reactions at 533 K and atmospheric pressure on Mg-Al mixed oxides. Several oxides with Mg/Al ratios of 0.1-9.0 were prepared by thermal decomposition of coprecipitated hydrotalcite-like precursors. Acid and base properties were measured by temperature-programmed desorption of NH3 and CO2 coupled with infrared spectra of adsorbed CO2. The structural composition and homogeneity were investigated by X-ray diffraction and solid-state Al-27 NMR (nuclear magnetic resonance). Elimination reactions of 2-propanol on Mg-Al mixed oxides proceeded through reaction pathways occurring on dual acid-base sites (Mg2+-O2- or Al3+-O2-). The chemical nature and acid-base properties of the active sites as well as the catalyst bulk structure and product formation rates for elimination reactions strongly depended on the Mg/Al ratio. In Mg-rich catalysts (Mg/Al greater than or equal to 1), Al3+ replaces Mg2+ inside the MgO matrix and forms a homogeneous solid without disrupting lattice structure. Elimination reactions occurred on surface Mg2+-O2- pairs via E-1cB -like mechanisms that selectively produced the dehydrogenation product (acetone) and to a lesser extent, the dehydration product (propylene). The presence of increasing concentrations of more electronegative Al3+ cations decreased the solid average basicity and the catalytic activity. The Al-rich catalysts (Mg/Al < 1) were much more active than the Mg-rich ones, converting 2-propanol mainly to propylene via an E-2 mechanism. The shift in the dehydration reaction mechanism from E-1cB (Mg-rich catalysts) to E-2 (Al-rich catalysts) was demonstrated by the existence of two different compensation effect lines, and it was attributed to a change of the active site from Mg2+-O2- to Al3+-O2- and to significant structural modifications. Al-rich samples are structurally heterogeneous oxides that contain a separate quasi-amorphous Al2O3-like phase where dehydration takes place at high turnover rates. The surface, structural, and catalytic transition from homogeneous Mg-rich to heterogeneous Al-rich mixed oxides was related to the structural homogeneity of the parent coprecipitated precursor. Whereas a single phase of hydrotalcite-like structure with Mg2+- and Al3+ cations in close interaction was present in Mg-rich samples, the Al-rich precursors contained Mg2+ and Al3+ cations in separate hydroxide phases. (C) 2003 Elsevier Science (USA). All rights reserved.