Highly Active Palladium/Activated Carbon Catalysts for Heck Reactions: Correlation of Activity, Catalyst Properties, and Pd Leaching

Abstract

A variety of palladium on activated carbon catalysts differing in Pd dispersion, Pd distribution, Pd oxidation state, and water content were tested in Heck reactions of aryl bromides with olefins. The optimization of the catalyst (structure ± activity relationship) and reaction conditions (temperature, solvent, base, and Pd loading) allowed Pd/C catalysts with very high activity for Heck reactions of unactivated bromobenzene (turnover number (TON) % 18 000, turnover frequency (TOF) up to 9000, Pd concentrations down to 0.005 mol %) to be developed. High Pd dispersion, low degree of reduction, sufficient content of water, and uniform Pd impregnation are criteria for the most active system. The catalysts combine high activity and selectivity under ambient conditions (air and moisture), easy separation (filtration), and quantitative recovery of palladium. De-termination of Pd in solution after and during the reaction, and catalyst characterization before and after the reaction (transmission electron microscopy (TEM), X-ray diffraction (XRD)), indicate dissolution/reprecipitation of palladium during the reaction. The Pd concentration in solution is highest at the beginning of the reaction and is a minimum (< 1 ppm) at the end of the reaction. Palladium leaching correlates significantly with the reaction parameters.

Preparation and Characterization of Pd Modified TiO2 Nanofiber Catalyst for Carbon–Carbon Coupling Heck Reaction

“…These results agree with Shmidt and Mametova's observation of the oxidative attack of the halide to the metal crystallites, yielding directly Pd(II) in solution [32]. As indicated in previous literature reports, presence of Pd(II) ions in the reaction mixture indicates the Pd/TiO 2 catalyst is quasi-homogenous [11]. Comparable results were obtained by Nasrollahzadeh and coworkers in 2014.…”

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“…Leaching of palladium species from the catalyst support is one of the most commonplace issues for deactivation of catalyst in liquid phase reactions [33][34][35] .…”

Preparation and properties of cotton stalk carbon/gold nanoparticles composite

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