Hafnosilicate microspheres as effective catalysts for the conversion of dihydroxyacetone to ethyl lactate

Transition-metal silicates are recognized as effective solid acid catalysts to produce value-added compounds from bio-based platform chemicals. One crucial feature in such catalytic transformations is the isomorphic substitution of transition metal into the tetrahedral coordination environment of SiO₂ matrices. This substitution generates the acidic sites which represent the catalytically active sites. Herein, the preparation of hafnium-doped mesoporous SiO₂ (hafnosilicates) is disclosed, and these materials are explored as catalysts for the efficient conversion of dihydroxyacetone (DHA) to ethyl lactate (EL). The one-step catalyst preparation is based on the aerosol-assisted sol-gel (AASG) process and leads to spherical mesoporous silicates with a homogeneous insertion of isolated Hf – as single sites into the SiO₂ matrix, as indicated by XPS. These sites display high Lewis acidity (as evidenced by Pyridine-IR) and this translates into good catalytic performance. Moreover, pH adjustment via the controlled addition of triethanolamine before the aerosol process is shown as a practical way to improve the textural properties, the degree of Hf insertion into the SiO₂ matrix (Lewis acidity), and therefore the catalytic activity. Under optimized reaction conditions (5 mL of 0.4 M DHA, 140 °C, 6 h, 200 mg of catalyst), complete conversion of DHA to EL was observed with a yield higher than 45%.