60th Annual Report on Research 2015

In the first fiscal year (09/2014-08/2015) supported by the ACS PRF grant PRF# 54262-ND4, my group achieved a significant accomplishment: for the first time, we demonstrated that the 3D structures of reaction intermediates which are too fast for NMR to measure can be determined by our vibrational cross angle method.

The reaction system investigated in this work is the generation of hydrogen gas by decomposing formic acid, catalyzed by a ruthenium PN³-pincer complex (1 in scheme  1. Formic acid, a major product of biomass processing and a formal adduct of H₂ and CO₂, has recently attracted considerable research interest. It is considered a potentially renewable liquid supply and storage material for hydrogen. A sustainable cycle can be envisioned using formic acid to supply hydrogen. To store hydrogen, hydrogen and CO₂ are added together to form formic acid. To release hydrogen, formic acid is decomposed into hydrogen and CO₂. The hydrogen storage density of formic acid is relatively high, 53g H₂/L, suitable for automobile and portable applications. Therefore, even before formic acid can be produced from renewable sources with a reasonable cost, it is already very attractive to use formic acid as hydrogen source for many applications.

The decomposition of formic acid with the presence of catalyst 1 is illustrated in scheme  1. When catalyst 1 is added into either the pure formic acid or its solutions, hydrogen immediately liberates from the solution. 2 was proposed to be the key intermediate structure for the reaction. However, it is too transient for NMR to resolve. Using the vibrational cross angle we developed, we have determined its structure, shown in fig.1. Based on the determined intermediate structure and NMR and DFT calculation results, we have elucidated the reaction mechanism of this catalyst, displayed in scheme 2. According to this mechanism, we have developed a very efficient catalytic system based on the catalyst to decompose formic acid into hydrogen plus CO₂ with a turn over number >1,000,000, of which the lifetime is long enough for many practical applications. Based on this catalytic system, we have built the first self-sustainable formic acid hydrogen fuel cell model car (12W). Two manuscripts from this project are under preparation. Patent applications for the intellectual properties invented from this study are also under preparation.

This model system study allows us to explore all aspects of our method in determining reaction intermediate structures, which will be applied to the studies of other catalytic systems including the Fe-nonheme C-H activation catalysts in the near future.

Scheme  1. The structures of the N3-pincer catalyst (1) and the major intermediate (2) of the decomposition of formic acid catalyzed by 1.

Figure 1. (a) The difference (Er) between experimental and calculated vibrational cross-angles for possible conformations of intermediate 2 with different Ru-O1-C2-O2 and C1-Ru-O1-C2 dihedral angles. The z-axis is the value of Er shown in a color map. The minimum Er value is marked with a white box and the exact dihedral angles are displayed in (b) where the yellow bonds show the dihedral angle C1-Ru-O1-C2 and the green bonds show the dihedral angle Ru-O1-C2-O2. The t-butyl groups are omitted for clarity. (c) The difference (Er) between experimental and calculated vibrational cross-angles for possible conformations of intermediate 2 with different Ru-O1-C2 bond angles. (d)&(e) The structure of intermediate 2 determined. The t-butyl groups have been omitted for clarity.

Scheme 2. Computed mechanism for the dehydrogenation of formic acidcatalyzed by PN³ Ru pincer complex. The free energies (in red) are mass-balanced and relative to Cat. + HCOOH.