Controlling activity of palladium-catalyzed reactions via supported materials
Date
2021-12
Authors
Naik, Pranjali Jayavant
Major Professor
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Slowing, Igor
Huang, Wenyu
Venditti, Vincenzo
Pruski, Marek
Anand, Robbyn
Committee Member
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Abstract
This dissertation investigates how two metal oxide supports (mesoporous silica and ceria) influence the activity of Pd catalyzed liquid-phase reactions. The first chapter provides a general introduction to Pd catalysts as well as a review of previous attempts to control and modify Pd properties for various catalytic applications.
Chapter 2 describes a method to create ligand-free size-controlled Pd nanoparticles by confining them in the pores of mesoporous silica nanoparticles (MSN). Surfactants and pore-expanding agents were used to fine-tune the size of pores between 2 and 5 nm. These pores successfully controlled the size of Pd nanoparticles between 2 and 4 nm as evidenced by STEM analysis and H2 pulse chemisorption. Because of the increased number of coordinatively unsaturated sites and higher back donation capacity, the Pd particles with the smallest size had the highest catalytic activity for a Suzuki-Miyaura cross-coupling reaction and the hydrogenation of phenol in the aqueous phase.
In Chapter 3, palladium supported on high surface ceria (Pd/CeO2) demonstrated to be an efficient catalyst for hydrodehalogenation (HDH) of halophenols under mild conditions. The catalyst's high HDH activity can be attributed to the formation of electron-rich phenoxide species during dissociative adsorption of the halophenol onto the ceria support. Much lower rates are obtained when the reaction proceeds directly on Pd nanoparticle without dissociative adsorption onto the support. The mechanism also involves oxidative addition of C-halogen bond into Pd followed by reductive elimination to give phenol and hydrogen halide. Various spectroscopy techniques provided evidence supporting the proposed mechanism. Comparison of HDH reactivity of different halophenols results in high activities for Cl- and Br-, moderate for F- and poor for I-. The reason for this trend is the rate determining reductive elimination of halide from the catalyst surface. For iodophenols the strong chemisorption of the halide blocks active sites and results in catalyst poisoning. However, addition of pyridine to the reaction mixture, promotes reductive elimination of the halide in the form of pyridinium iodide, which ultimately restores catalyst turnover.
Chapter 4 explores Pd/CeO2 as a catalyst for the transfer hydrodehalogenation (THD) of halophenols under mild conditions (65 °C) using isopropanol as hydrogen source. In contrast to HDH, the conversion was most efficient for fluorophenol, and reactivity decreased with halogen size suggesting that oxidative addition, which is the rate limiting step in HDH at 35 °C is not limiting under our THD conditions. THD kinetics, NMR, and temperature programmed surface reaction (TPSR) experiments indicated that oxidative addition of C-X and isopropanol oxidation compete for the same active sites on Pd. The isopropanol-derived hydride is adsorbed onto Pd sites and then used for hydrogenation of the Pd-chemisorbed halophenol. In the case of THD, the H abstraction is the rate-determining step.
Chapter 5 relates the kinetics of Pd/CeO2 catalyzed phenol hydrogenation to the binding dynamics of the substrate onto the catalytic material. To enable the characterization of the mode, kinetics and equilibria of binding of the substrate onto the catalyst, we synthesized ceria nanocubes with narrow size distribution and uniform surface termination (100). Incorporation of Pd onto the support resulted in Pd nanoparticles of size ~1.5 nm. The catalytic hydrogenation of phenol appears to involve the initial binding of the substrate to the support characterized by anisotropic motions of the weakly adsorbed molecule. This event is followed by migration to the metallic sites where the substrate has decreased mobility, likely involving flat binding on the Pd surface. Disruption of the anisotropic binding to the support by the addition of phosphate interferents leads to a proportional decrease in reaction rates indicating that substrate-support interactions are critical to catalytic activity.
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