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            This project aims at synthesizing new nanostructured catalysts composed of metallic nanoparticles immobilized on basic supports like poly(4-vinylpyridine) or magnesium oxide. We expect the resulting nanostructures on these solids to be capable of promoting the heterolytic activation of hydrogen and the ionic hydrogenation of aromatic and heteroaromatic compounds of relevance to the manufacture of cleaner fossil fuels. Although such ionic mechanisms are well know in solution, they are extremely rare on solid surfaces.

During the second year of the project a material containing 10% Ru (average size 3.1 nm) on poly(4-vinylpyridine), characterized during the first period of the grant, was optimized as a catalyst for the hydrogenation of a variety of aromatic and nitrogen-heteroaromatic molecules; it was also found that the catalyst can be re-used several times without any appreciable loss of activity. The hydrogenation rates for e.g. quinoline were strongly sensitive to solvent polarity and to the addition of external acid or base, in agreement with an ionic hydrogenation mechanism, while the rates of toluene hydrogenation were insensitive to the same factors. This suggests the existence of two different hydrogenation sites on the catalyst, one specific for polar N-heteroaromatics and capable of heterolytic hydrogen splitting (type A) and a second one specific for benzenoid aromatics, probably acting by a conventional homolytic hydrogen splitting mechanism (type B), see Fig. 1. Further evidence for the dual-site nature of this catalyst was obtained by the selective poisoning of type B sites caused by small amounts of added thiophene, which had no effect on type A sites.

Fig. 1. Model of dual-site structure of a Ru/PVPy hydrogenation catalyst

            Similar catalysts containing Ni and Pd nanoparticles immobilized on poly(4-vinylpyridine) were synthesized and characterized by TEM and XRD, and were also found to be efficient for the hydrogenation of N-heteroaromatic compounds like quinoline under moderate reaction conditions, but  not for reducing simple aromatic molecules like toluene. This suggests that the Ni and Pd catalysts only display type A sites, capabe of hydrogenating polar N-heteroaromatics.

A second generation of catalysts displaying very high hydrogenation activities is now being developed using Ru nanoparticles imbedded in MgO as the basic support.  The best catalytic performance has been achieved with a 1% Ru/MgO material, which hydrogenates toluene to methylcyclohexane at 120 ^oC and 50 atm H₂ in THF with a TOF of 1500 h^-1 and quinoline to 1,2,3,4-tetrahydroquinoline at 150 ^oC and 50 atm H₂ with a TOF of 12,000 h^-1, probably the highest activity observed so far for the hydrogenation of quinoline.

Fig. 2. Selective hydrogenation of quinoline by Ru/MgO

An analogous solventless reaction maintains a TOF of ca. 8500 and the catalyst shows no sign of deactivation after reducing 50,000 mol of quinoline per mol Ru. Full characterization of these materials, as well as optimization of the reaction parameters and mechanistic studies is in progress.