Pd–sepiolite catalyst for Suzuki coupling reaction in water: Structural and catalytic investigations

[Pd(NH3)4]2+-exchanged sepiolite clay (Pd–sepiolite) has been applied to the catalytic Suzuki-type carbon–carbon coupling reactions of 4-bromophenol with phenylboronic acid or sodium tetraphenylborate in water. The Pd–sepiolite effectively catalyzed the reaction under mild reaction conditions (at room temperature in air). The Pd–sepiolite system exhibits higher yield than unsupported Pd(II) salts, [Pd(NH3)4]Cl2-impregnated SiO2 (Pd–SiO2), and a commercially available Pd/C consisting of Pd metal particles. The structure of Pd species in the catalysts before and after the reaction was well characterized by a combination of XRD, TEM, UV-Vis, Pd K-edge XANES/EXAFS, and Pd image-edge XANES. XAFS and TEM results confirmed the formation of metal particles after the reaction by unsupported Pd(II) salt and Pd–SiO2. In contrast, for Pd–sepiolite the change in the structure of Pd species after the reaction was not significant; the highly dispersed Pd(II) complex, present before the reaction, was still the main Pd species together with the small Pd clusters (2–7 nm) as minor species. As a result of the high stability, Pd–sepiolite was reused without losing its activity. Significantly high turnover numbers (TON = 940,000) were also attained at reflux temperature. It is suggested that Pd metal precipitation during the reaction is inhibited by a strong electrostatic interaction of sepiolite with Pd(II) species.