High surface area, nanostructured boehmite and alumina catalysts: Synthesis and application in the sustainable epoxidation of alkenes

We report a new, straightforward and inexpensive sol-gel route to prepare boehmite nanorods [gamma-AlO(OH)-NR] with an average length of 23 nm +/- 3 nm, an average diameter of 2 nm +/- 0.3 nm and a high specific surface area of 448 m(2)/g, as evidenced by TEM and N-2-physisorption, respectively. The boehmite was converted to gamma-alumina with preserved nanorod morphology (gamma-Al2O3-NR) and high surface area upon calcination either at 400 or 600 degrees C. These nanostructured materials are active and selective heterogeneous catalysts for the epoxidation of alkenes with the environmentally friendly H2O2. The best catalyst, gamma-Al2O3-NR-400, showed to be versatile in the scope of alkenes that could be converted selectively to their epoxide and displayed enhanced reusability compared to previously reported alumina catalysts. Furthermore, the catalytic performance of the material was enhanced by optimising the reaction conditions such as the solvent and the type of hydrogen peroxide source. Under the optimised reaction conditions, the gamma-Al2O3-NR-400 catalyst displayed 58% cyclooctene oxide yield after 4h of reaction at 80 degrees C with full selectivity towards the epoxide product. The correlation between the catalytic activity of these materials and their physicochemical properties such as surface area, hydrophilicity and number and type of acid sites was critically discussed based on a detailed characterisation study.