Date of Degree

6-2014

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor(s)

Roberto Sanchez-Delgado

Subject Categories

Chemistry | Mechanics of Materials | Nanoscience and Nanotechnology

Keywords

Hydrogenation, Ionic mechanism, N-heteroaromatic, Palladium, Ruthenium, S-heteroaromatic

Abstract

We developed a series of catalysts, composed of metal nanoparticles immobilized on basic supports for the hydrogenation of heteroaromatics of relevance to cleaner fossil fuels and biodiesel, and for the dehydrogenation of heteroaromatics of relevance to hydrogen storage in organic liquids. Our catalyst design involves nanostructured catalysts composed of metal particles immobilized on basic supports capable of ionic mechanism that may avoid catalyst poisoning and enhance catalytic activity.

We prepared a new catalyst composed of Pd nanoparticles immobilized on MgO by NaBH4 reduction of Na2PdCl4 in methanol in the presence of the support. TEM measurements revealed well-dispersed 1.7 nm Pd particles attached to MgO, also characterized by XPS, XRD and hydrogen pulse chemisorption measurements. The new catalyst is efficient for the hydrogenation of the heterocyclic ring of quinolines, as well as for the mild reduction of a variety of alkenes representative of fuel components, and the partial saturation of biodiesel.

In the second part, we switched our attention to ruthenium nanoparticles, with the aim of achieving higher activity and broader reactivity, supported on functionalized carbon nanotubes, of higher surface area. Our second catalyst was prepared by first attaching pyridine groups to the surface of the nanotubes, and then depositing the metal particles by NaBH4 reduction of RuCl3.3H2O. TEM, XRD and XPS analysis indicate the presence of 1.7 nm Ru(0) particles attached mainly to the surface of the nanotubes. The Ru/py-CNTs results in an unprecedentedly high activity for the selective hydrogenation of N-heteroaromatic compounds, with respect to other reported systems. The activity of this catalyst was extended also to plain aromatic compounds under mild conditions and to the challenging S-heteroaromatics under more forcing conditions.

Lastly, we evaluated the efficacy of the catalyst for the dehydrogenation of 1,2,3,4-tetrahydroquinoline, which was achieved with reasonable turnover frequencies under moderate conditions. The hydrogenation/dehydrogenation reactions thus establish a cyclic process of possible utility in hydrogen storage in organic liquids. We screened other substrate pairs that may be adequate for hydrogen storage, by computational study of the thermodynamics of dehydrogenation, which allowed us to identify other N-heterocycles that may prove useful in future studies.

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