Application of parahydrogen for mechanistic investigations of heterogeneous catalytic processes

Abstract

Parahydrogen-induced polarization technique (PHIP), based on the pairwise addition of molecular hydrogen to a substrate, was successfully applied to obtain novel information on the mechanisms of heterogeneous catalytic hydrogenation, hydrodesulfurization, and oligomerization processes. In particular, the PHIP effects were observed upon hydrogenation with parahydrogen catalyzed by the immobilized neutral complexes of rhodium and iridium, which confirms the similarity in the mechanisms of homogeneous and heterogeneous hydrogenation for such systems. In the study of acetylene oligomerization, a significant NMR signal enhancement was revealed for a number of C₄ oligomers, with the enhancement levels by far exceeding that observed in hydrogenation of carbon-carbon triple bonds. The mechanistic features of heterogeneous hydrogenation of a number of six-membered cyclic hydrocarbons over supported metal catalysts were investigated, and their hydrogenation scheme based on the pairwise addition of molecular hydrogen was proposed. Furthermore, the PHIP technique revealed that heterogeneous hydrodesulfurization of thiophene mainly proceeds via hydrogenation followed by a C—S bond cleavage. A significant enhancement of sensitivity in combination with characteristic line shapes of NMR signals make the PHIP method a unique and highly informative tool for the investigation of heterogeneous catalytic processes.