Non-Innocent Role of the Ceria Support in Pd-Catalyzed Halophenol Hydrodehalogenation

Abstract

The hydrodehalogenation (HDH) of halophenols is efficiently catalyzed by palladium supported on high surface ceria (Pd/CeO2) under mild conditions (35 °C, 1 atm H2). A combination of NMR, DRIFTS, Raman spectroscopy and XPS studies as well as HDH kinetics of substituted halobenzenes suggest the reaction proceeds mainly via a sequence of dissociative adsorption of phenolic hydroxyl onto the support, oxidative addition of Pd into the C-halogen bond and reductive elimination to give phenol and hydrogen halide. The dissociative adsorption of -OH group onto oxygen vacancies of the ceria support results in an electron-rich intermediate that facilitates the turnover-limiting reductive elimination step. In contrast, the direct pathway catalyzed by Pd without dissociative adsorption of the reactants on the support takes place at a slower rate. The mechanistic insights gained in this study were used to modify reaction conditions for enabling HDH of recalcitrant halides such as fluorides and iodides.

Efficient Transfer Hydrodehalogenation of Halophenols Catalyzed by Pd Supported on Ceria

“…The high activity of Pd on CeO2 has been attributed to high dispersion due to strong metal-support interactions, and to activation of the hydroxyl moieties of substrates and hydrogen donors via dissociative adsorption onto oxygen vacancies of the support. [40,47,52,53] The spent Pd/CeO2 catalyst had similar properties to those of the fresh material. Its XRD pattern showed no diffractions corresponding to Pd, suggesting that the metal did not undergo aggregation during the reaction (Fig.…”

“…The high surface ceria support (194 m 2 g -1 ) was prepared by evaporation-induced selfassembly as previously reported. [9,40,47] Impregnation of the support with Pd(O2CCH3)2 via incipient wetness followed by calcination in air at 350 °C and reduction at the same temperature under flowing H2 provided the Pd/CeO2 catalyst (0.88 wt % Pd by ICP-OES).…”

“…In fact, we have previously observed that oxidative addition of C-X bonds to Pd/CeO2 can proceed even at room temperature. [53] Therefore, the reductive elimination of Pd-chemisorbed intermediates should present much higher barriers and act as the limiting factor in this reaction. Since the strength of Pd-halogen interactions increases with halogen size (Pd-F < Pd-Cl < Pd-Br < Pd-I) [61,62] it is reasonable to assume that the THD reaction involves facile oxidative addition of C-X bond to Pd and a much more difficult reductive elimination of the halide from a C-Pd-X intermediate.…”

“…[38] Previously, we reported that Pd supported on ceria (Pd/CeO2) is highly active for the hydrodehalogenation (HDH) of halophenols -including fluorophenols-under mild reaction conditions (35 °C, 1 bar H2). [9] The high HDH activity of this catalyst is attributed to the cooperativity between the redox active support and the catalytic metal. Specifically, dissociative adsorption of halophenols onto the redox active vacany sites of ceria results in electron-rich phenoxides which are much more reactive towards halide substitution by H.…”

“…A common approach for removing halogen atoms from organic compounds is catalytic hydrodehalogenation. [6][7][8][9][10][11][12][13][14] Because this process utilizes H2 as a reagent, it requires pressurized equipment and involves hazards such as high flammability or explosion. In addition, H2 is mostly generated via natural gas steam reforming that produces CO2, and therefore has an important environmental footprint.…”