60th Annual Report on Research 2015

Background

The project focuses on understanding the reaction pathways during hydrodenitrogenation (HDN) reactions of basic and non-basic N compounds on Ru clusters. Specifically, it addresses the catalytic requirements for the hydrogenation of the nitrogen ring (HYD) and hydrogenolysis of the C-N bond (CNH) of two distinct types of nitrogen compounds and establishes the correlation between rate parameters and thermodynamic properties of reactants, intermediates, and products.

Research Progress

During this reporting period, we (1) synthesized small Ru clusters with a narrow size distribution using colloidal chemistry methods, (2) characterized the Ru clusters and then probe their bulk phase transition using chemical titration and in-situ microscopic techniques, and (3) probed the reaction pathways and the kinetic dependencies during hydrogenation of pyridine and pyrrole.

Synthesis of dispersed Ru clusters with colloidal chemistry methods. We have successfully synthesized un-supported Ru clusters with a narrow size range by polyol reduction. Typically, RuCl₃ was reduced in refluxing glycol in the presence of polyvinylpyrrolidone stabilizers (Mw=55,000). The resulting Ru clusters were subsequently rinsed by acetone and re-dispersed in ethanol. Finally, the as-synthesized Ru clusters were exposed to a gas mixture with varying hydrogen sulfide and hydrogen pressures, during which Ru clusters were converted to RuS₂ clusters.

Characterization of Ru clusters with in-situ electron microscopic techniques. Ru clusters were characterized with in-situ electron microscopic techniques. This specific research task focuses on exploring the dynamic behavior of the Ru clusters as they undergo bulk oxidation and bulk sulfidation. An in-situ electron microscopy stage was developed and used for treating the Ru clusters. Gases such as O₂, H₂, or sulfur containing species were introduced to the sample while the instantaneous phase changes of the Ru clusters were captured with movie or time-dependent images. The questions that we seek to address are: (1) the phase transition dynamics of Ru clusters and (2) the relation of the bulk phase transition to the active sites on Ru during hydrodenitrogenation reactions.

Rate and selectivity assessments of hydrodenitrogenation of basic vs. non-basic compounds. The reaction pathways and mechanism for hydrodenitrogenation on sulfided Ru clusters were probed using a plug flow reactor operating under differential conditions. Pyridine and pyrrole were selected as the model compounds in these kinetic assessments.

The rates of hydrogenation and hydrogenolysis were measured. For pyridine hydrogenation, the rate dependencies of H₂, H₂S, and pyridine partial pressures were measured on 0.4 wt.% Ru/SiO₂ catalysts (510.0-1305.0 kPa H₂, 0-15.0 kPa H₂S, 0-21.0 kPa pyridine) at 623 K, pre-sulfided with a mixture of 0.15% H₂S in H₂ at 673 K for 3 h. Piperidine was the main product with over 90% carbon selectivities, while pentene, pentane, and N-pentylpiperidine were the minor products. The reaction orders with respect to hydrogen and hydrogen sulfide were found to be 1.4 and 0, respectively, and with respect to pyridine were found to vary from 1.0 to 0. The initial H insertion onto the pyridinic ring was the rate-determining step for hydrogenation. Hydrogenation rates were at least an order of magnitude larger than the hydrogenolysis of the C-N bond. Currently, we seek to probe the mechanism and kinetic requirements of C-N bond cleavage. We are in the process of constructing a kinetic model that captures both the dependence of hydrogenation and hydrogenolysis reactions. We seek to correlate the number of surface vacancy sites (determined by the sulfur chemical potential) to these rates and selectivities.

We are applying the same kinetic treatment above to explain the hydrodenitrogenation reactivities for pyrrole.

Impact on Career of PI and Students

This grant has allowed the PI to initiate new research capabilities on applying in-situ electron microscopic techniques to interrogate the bulk structures and dynamics of Ru clusters as they tether between phases. It also supports (i) undergraduate students (partially supported by departmental research fellowships), (ii) partial time of two post-doctoral candidates, and (iii) a PhD student. The PI also begins a new research program on tandem hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) with Imperial Oil, which supports two more PhD students.