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

Abstract

Some polynuclear molybdenum complexes such as Mo 2(OAc) 4, Mo 2(OBu t) 4, [Mo 3O(CCH 3)(OAc) 6(CH 3OH) 3]Cl, and [(RhCp*) 4Mo 4O 16] 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 Mo 2(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 [Mo 3O(CCH 3)(OAc) 6(CH 3OH) 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; Mo 2 (4100) > Mo 3 (110) > Mo 4 (7.3) ⪢ Mo 7 O 24 2−(0.3), where the numbers in parentheses denote TOF (h −1). The exceeding thermal treatment above 673–723 K for the active Mo 2 and Mo 3 species causes the complete cleavage of the Mo–Mo bond to give highly dispersed MoO 3 on silica and FSM-16, which is inactive for propene metathesis. By contrast to silica and FSM-16, the Mo 2 and Mo 3 catalysts prepared on Al 2O 3, MgO and NaY are inactive for propene metathesis.