Date of Award
Open Access Dissertation
Chemistry and Biochemistry
Donna A. Chen
Heterogeneous catalysis remains at the core of chemical manufacturing industries with 80-90 % of chemical processes relying on the use of catalysts. Unlike homogeneous catalyst that has extremely well-defined active sites, active sites in heterogeneous catalysis are complex and show dynamic behavior under reaction conditions, and change their structures, composition, particle size. The complexity associated with these systems has made the rational design of the catalyst a difficult problem. Highly crystalline metalorganic frameworks (MOFs) as heterogeneous catalysts present the unique opportunity to systematically modify the geometries, ensemble sizes, and compositions of highly dispersed active sites due to their tailorable, and post-modifiable pores and cavities with uniform distribution of identical catalytic species.
In this work, we report the first study of a gas-phase hydrogenation reaction catalyzed by highly dispersed metal nodes of a crystalline, bimetallic CuRhBTC(BTC3- =benzenetricarboxylate) metal-organic framework (MOF), whereas other isostructural monometallic and bimetallic MOFs has no hydrogenation activity. X-ray photoelectron spectroscopy and X-ray absorption experiments identify the oxidation state of Rh in CuRhBTC as +2, which is a Rh oxidation state that has not previously been observed for crystalline MOF metal nodes. These Rh2+ sites are active for the catalytic hydrogenation of propylene to propane at room temperature, and the MOF structure stabilizes the Rh2+ oxidation state under reaction conditions. Density functional theory calculations suggest a mechanism in which hydrogen dissociation and propylene adsorption occur at the Rh2+ sites.
Based on the activity of Rh2+ sites for alkene hydrogenation and known activity of Rh ions for hydroformylation reaction at lower pressures (≤ 1atm), the CuRhBTC was studied as a potential catalyst for ethylene hydroformylation reaction at elevated temperature and pressures. The ethylene hydroformylation studies on monometallic CuBTC, RhBTC, bimetallic CuMBTC(M=Rh, Co, Ru ions), along with Rh-standards: Rh3+/SiO2, Rh2+/SiO2, Rh/SiO2, Rh-Y zeolites showed Rh-containing MOFs along with Rh-standards exhibited activity for ethylene hydroformylation to propanal and diethyl ketone as reaction products, whereas the other monometallic and bimetallic MOFs did not show any activity. The increase in diethyl ketone product with increased Rh concentration suggests that ionic Rh sites are needed for the formation of the coupling product during hydroformylation.
One of the common strategies to achieve the desired activity of supported metal particles is tuning the particle size and interactions between the particle and the support. The growth of metal particles and associated loss of surface area is a major route of catalyst deactivation in heterogeneous catalysis. In our work, propylene hydrogenation activity was studied on small (1-2 nm) Pd particles deposited on different carbon and oxide supports using strong electrostatic adsorption (SEA). The size of the Pd particles was varied by annealing at higher temperatures and was assessed by chemisorption, scanning transmission electron microscopy (STEM), and x-ray diffraction (XRD) measurements. Our study showed that Pd particles deposited by SEA on graphene nanoplatelets (graphitic, oxGN-alfa) have higher activity than small, uniformly sized Pd particles on silica. We found that the Pd exhibits a particle size effect on SiO2 and oxGNAlfa, with smaller particles showing higher activity whereas this effect on oxVXC72 was complicated by carbon decoration.
Shakya, D. M.(2022). Modification of Active Sites in Catalytic Materials for Gas-Phase Heterogeneous Catalysis. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/6827