Synthesis of sodium-doped mesoporous alumina and its strong base catalysis for double bond migration of olefins

Sodium-doped mesoporous alumina (Na/mesoAl2O3) has been successfully synthesized through doping sodium azide and the pyrolysis with the intention to enhance the strength of base sites (O2−) and to weaken that of acid sites (Al3+) on mesoporous alumina (mesoAl2O3). Powder XRD and pore-size distribution analysis on Na/mesoAl2O3 demonstrated that the loaded metallic sodium hardly collapsed the intrinsic mesoporosity and bulk structure of mesoAl2O3. The Na/mesoAl2O3 promoted the double bond migration of 2,3-dimethyl-1-butene even at a low reaction temperature of 275 K to afford 2,3-dimethyl-2-butene in a high yield of 81% in 4 h, indicating that Na/mesoAl2O3 has very strong base sites including superbase sites on the surface. Na/mesoAl2O3 was also successfully applied to the isomerization of α-pinene to yield β-pinene in excellent selectivities of 93–96%, while parent mesoAl2O3 and sodium-doped conventional γ-Al2O3 gave β-pinene in lower selectivities of 58% and 89%, respectively. The active strong base sites on Na/mesoAl2O3 should be a part of surface O2− ions, whose basic character is reinforced by the electron donation from the introduced sodium metals.