Isostructural metal-carboxylates MIL-100(M) and MIL-53(M) (M: V, Al, Fe and Cr) as catalysts for condensation of glycerol with acetone

The synthesis of solketal from acetone and glycerol (I) was investigated in the presence of the isostructural MOFs of the families MIL-100(M) and MIL-53(M) (M = V, Al, Fe and Cr) and mixed MIL-53(Al,V) (Al/V – 100/0, 75/25, 50/50, 25/75 and 0/100 atom/atom). The main products were a five-membered solketal (2,2-dimethyl-1,3-dioxane-4-methanol, (II)) and a six-membered acetal (2,2-dimethyl-dioxane-5-ol, (III)). It was demonstrated that the reaction rate and isomer selectivity depend on different parameters such as the type of metal ion, the length of the M-O bond, the rate constant for the exchange of the water molecules from the first coordination sphere of a metal ion and the value of the zero point of the surface charge (pH_PZC). Investigation of mixed MIL-53(Al,V) shows that the reaction rate and selectivity towards (II) increase with increasing V³⁺ content in MIL-53(Al,V). V-containing MOFs possess a high activity and selectivity at 25 °C. The efficiencies of MIL-100(V) and MIL-47(V) were higher than those of H₂SO₄, SnCl₂ and p-toluenesulfonic acid at 25 °C. The MIL-100(V) catalyst showed good reusability for 4 catalyst recycles.