Structural investigations of functionalized mesoporous silica-supported palladium catalyst for Heck and Suzuki coupling reactions

Abstract

FSM-16 mesoporous silica-supported mercaptopropylsiloxane Pd(II) complex, Pd-SH-FSM, has been shown to act as an active, stabile, and recyclable heterogeneous catalyst for the Heck reaction of 4-bromoacetophenone with ethyl acrylate and for the Suzuki reaction of 4-bromoanisole with phenylboronic acid. The structure of the Pd species in Pd-SH-FSM before and after the reaction was well characterized by a combination of XRD, TEM, UV–vis, Pd K-edge XANES/EXAFS, and Pd L III -edge XANES, and the results were compared with those of a amorphous silica-supported mercaptopropylsiloxane Pd(II) complex (Pd-SH–SiO 2), unmodified FSM-16-supported Pd(OAc) 2 (Pd-FSM), and previously reported heterogeneous catalysts (Pd zeolite and Pd/C). The aggregation behavior of Pd species during the reaction greatly depends on the support. On Pd-SH-FSM after Heck and Suzuki reactions, Pd(II) species coordinated to the sulfur ligands were the main Pd species together with small Pd clusters as minor species, and the catalyst was reused without marked loss in the activity. On Pd-SH–SiO 2, relatively large numbers of the Pd(II) species were converted to Pd clusters after the Suzuki reaction. On ligandless Pd-FSM, most of the Pd was aggregated to form Pd metal particles. The activity order of the recycled catalysts (Pd-SH-FSM > Pd-SH–SiO 2 > Pd-FSM) indicates that the Pd(II) complexes on the sulfur ligands and possibly the small Pd clusters are more active than Pd metal particles. It is concluded that the sulfur ligands in the size-restricted mesopore are effective for preventing the aggregation of coordinated Pd complexes, and this results in high durability and recycling characteristic of the Pd-SH-FSM. The deactivation of the Pd complex via Pd aggregation was significant for the conventional Pd catalysts; [Pd(NH 3) 4] 2+ complexes in Pd zeolite and PdO nanoclusters on Pd/C were changed to less reactive metal particles or clusters after Heck and Suzuki reactions.