Grafting of Lanthanide Complexes on Silica Surfaces: A Theoretical Investigation

Abstract

Grafting catalysts on a surface leads to heterogeneous catalysts with well-defined active sites. However, the grafting mode of a lanthanum complex onto silica remains unknown. To shed light on this grafting reaction, different studies have been achieved in the framework of density functional theory. The silica substrate hydroxylated at 700 degrees C has been simulated both by molecular and periodic models. The created molecular models are in agreement with the rigidity of the ligand, the surface density of silanol groups, and the different spectroscopic data of a silica surface partially dehydroxylated at 700 degrees C. Two possible models of surface have henceforth been considered: the first one with one isolated silanol and the second one with two vicinal silanols linked by a siloxane bridge. The thermodynamics of a grafting reaction of lanthanum catalysts on these models has also been investigated. This reaction leads to thermodynamically stable structures that reveal different types of grafting: monografted, bigrafted, or bigrafted after breaking of a Si-O-Si bridge. Similarly to experimental approaches, coordination of triphenylphosphine oxide (O=PPh(3)) has also been considered as a probe of the grafting mode. A good agreement between the theoretical and the experimental spectroscopic values has systematically been found, but none of the grafting modes seem to be more relevant. Accordingly, it is necessary to consider in subsequent studies that all grafting modes coexist, increasing the difficulty to theoretically investigate multistep reactions.