Selective Oxidation of 5-Hydroxymethylfurfural by Ruthenium Supported on Reconstructed Magnesium-Aluminum Hydrotalcite under Alkali-Free Conditions

Magnesium-aluminum hydrotalcite was prepared by the hydrothermal method. By taking advantage of the “memory effect” of hydrotalcite, the calcined hydrotalcite was rehydrated, and simultaneously, ruthenium was impregnated, reduced, and supported to obtain the catalyst Ru/MgAl-RLDH. The specific surface area of the original hydrotalcite MgAl-LDH was 5.1 m2/g, while that of the reconstructed Ru4/MgAl-RLDH increased to 55.3 m2/g. The total basicity of the surface of the original hydrotalcite MgAl-LDH catalyst was 0.31 mmol/g, and all of them were weak basic sites. The total basicity of the surface of the reconstructed hydrotalcite Ru4/MgAl-RLDH catalyst was 0.45 mmol/g, and it had more abundant basic sites, including weak basic sites and moderately strong basic sites. The basic sites of the reconstructed hydrotalcite could be systematically adjusted by changing the calcination temperature during the reconstruction process. When the calcination temperature was 400°C and the ruthenium loading was 4 wt %, the Ru4/MgAl-RLDH catalyst prepared showed the highest activity. Under the optimal reaction conditions (110°C, 11 h, 1 MPa O2, and a catalyst dosage of 0.08 g), the conversion rate of HMF reached 100%, and the yield of FDCA was 80.9%. Thermogravimetric analysis confirmed that the reconstructed hydrotalcite Ru4/MgAl-RLDH had better thermal stability than the original Ru4/MgAl-LDH.