Propene metathesis reaction on di- and trinuclear molybdenum complexes grafted on mesoporous FSM-16 and silica: Structural characterization and their catalytic performances Original Research Article

Some polynuclear molybdenum complexes such as Mo2(OAc)4, Mo2(OBut)4, [Mo3O(CCH3)(OAc)6(CH3OH)3]Cl, and [(RhCp*)4Mo4O16] grafted with silica and mesoporous channels of FSM-16 were investigated for their structural characterization and catalytic performances in propane metathesis reaction. It was demonstrated by ESR, FT-IR, and EXAFS studies that Mo2(OAc)4 is grafted on FSM-16 while keeping a dinuclear Mo–Mo unit by a partial removal of the acetate ligand at 623 K, resulting in the formation of mixed valent dinuclear species [Mo–Mo]V characteristic of an intense ESR signal (g = 1.926) and EXAFS parameters (R (Mo–Mo) = 2.11 Å; coordination number = 0.6), similar to those grafted on silica. The resulting Mo dimer species grafted on FSM-16 exhibited a high activity in propene metathesis to yield ethene and 2-butenes. On the other hand, FT-IR, TPD/MS and EXAFS studies revealed that [Mo3O(CCH3)(OAc)6(CH3OH)3]Cl on FSM-16 was transformed by pyrolysis at 573 K to Mo trinuclear species which is active for propene metathesis. The relative activities of surface-grafted Mo species grafted on FSM-16 were measured as the following order with the respect to molybdenum nuclearity of the precursor complexes; Mo2 (4100) > Mo3 (110) > Mo4 (7.3) ⪢ Mo7 O242−(0.3), where the numbers in parentheses denote TOF (h−1). The exceeding thermal treatment above 673–723 K for the active Mo2 and Mo3 species causes the complete cleavage of the Mo–Mo bond to give highly dispersed MoO3 on silica and FSM-16, which is inactive for propene metathesis. By contrast to silica and FSM-16, the Mo2 and Mo3 catalysts prepared on Al2O3, MgO and NaY are inactive for propene metathesis.