Immobilisation of L-proline onto mixed-linker zirconium MOFs for heterogeneous catalysis of the aldol reaction

Novel heterogeneous catalysts based on zirconium sulfonated metal-organic framework (MOF)-supported L-proline has been developed for the asymmetric aldol reaction. In this study, a chiral zirconium-based sulfonated MOF-proline was assembled through the solvothermal reaction of ZrCl₄ with a combination of benzene-1,4-dicarboxylic acid (H₂bdc) and sodium 4-carboxy-2-sulfobenzoate (H₂bdc-SO₃Na) in the presence of L-proline. The MOF showed excellent crystallinity, high porosity and thermal stability, making it an attractive catalyst for the aldol reaction since the bifunctionality can be enhanced to act both as Brønsted acid and Lewis base active sites. The reaction between acetone and two different aldehydes was investigated and the catalyst showed good activity and enantioselectivity, especially in the reaction with 4-nitrobenzaldehyde (83 % enantiomeric excess and 85 % conversion). The reusability of the catalyst was evaluated by organic solvent nanofiltration membranes. After three successive aldol reactions, the enantiomeric excess and conversion reaction of the catalyst showed only slight decreases, confirming good recyclability. Moreover, scale-up of the organic solvent nanofiltration membrane operation is relatively simple, promoting ease of operation on an industrial scale.