Multilple host–guest interactions in heterogeneous vanadium catalysts: Inorganic nanosheets modified alpha-amino acids as ligands

The catalysts using layered double hydroxide (LDH) nanosheet-modified α-amino acid anions as ligands has been proved to be easily recycled in vanadium-catalyzed asymmetric epoxidation of cinnamyl alcohol with the enantioselectivity well preserved and the yield only slightly reduced. The α-amino acids employed here include l-glutamate, l-alanine, and l-serine, which are anchored to LDH layers through monodentate electrostatic interactions in the ascending intensity while coordinated with vanadium center in the coordinating intensity of l-glutamate > l-serine > l-alanine. The stronger coordination with α-amino acid anion caused the vanadium center to be leached independently in less percentage, while the leaching of α-amino acid anion depends on the dual host–guest interactions. In nanosheet-modified l-glutamate system, all of the vanadium centers leached together with l-glutamate, but in nanosheet-modified l-alanine system, all of the vanadium centers leached independently. The electrostatic interaction of brucite-like layer with l-serine is stronger than with l-alanine, yet the stronger coordination of l-serine to vanadium caused more l-serine to be leached. The weakest electrostatic interactions between intercalated l-glutamate and LDH layer result in the visible loss of l-glutamate in the vanadium/LDH nanosheet-modified l-glutamate system, but the V-glutamate species leached into the solution was catalytically active in the epoxidation, compensating for the activity loss in the recycling experiments in spite of the higher l-glutamate leaching.