Researchers at the US Department of Energy’s Argonne National Laboratory have developed an unusually active form of vanadium for hydrogenation reactions. Vanadium is an inexpensive common metal that could replace some of the precious metals currently found in catalysts used in these reactions, frequently used in processing of fuels (petro- and drop-in bio-) and petrochemicals.
The vanadium catalyst exhibits unprecedented reactivity in liquid- and gas- phase alkene/alkyne hydrogenation. Catalyst poisoning experiments revealed that 100% of the V sites are active for hydrogenation. A paper on their work is published in the RSC journal Chemical Communications.
Earth-abundant and inexpensive late first-row transition metals such as Fe, Co, Ni and Cu have been extensively researched as alternative catalysts to noble metals (Ru, Rh, Pd and Pt) for the hydrogenation of unsaturated organic functional groups. While early- and mid-first row transition metals (e.g. Sc, Ti, V, Cr, Mn) have been largely studied for their role as promoter ions for hydrogenation-active metals, much less is known about how the manipulation of their coordination environment and electronic structures can impart hydrogenation reactivity. Some of the rare examples of early transition metal hydrogenation catalysts include (1) in situ reduced titanocenes and (2) a high-valent bis(imido)vanadium(V) catalyst, which operates via heterolytic hydrogenation mechanism.
Our group recently developed a series of well-defined, site- isolated first-row organo-transition metal catalysts (V3+, Cr3+, Mn2+, Co2+ and Ni2+) supported on a catechol-containing porous organic polymer (CatPOP), which were demonstrated to be active for alkyne semi-hydrogenation. … it remains unclear from this previous study whether the hydrogenation activity of these catalysts arose mainly from the low coordination of V(III) and/or from the redox-active ligand of the CatPOP support. In order to verify which of the two variables impart(s) this unprecedented reactivity, we omitted redox contributions from the solid catalyst support by replacing CatPOP with redox-inactive SiO2. Our findings showed that the SiO2, as a support, is superior to CatPOP, revealing that a redox-active ligand is not a prerequisite for the reported vanadium’s hydrogenation system. This finding establishes that site isolation can impart unprecedented reactivity, and opens up new areas for catalysis with early- and mid-first-row transition metals.
Here, we support organovanadium(III) sites on a redox- innocent bulk oxide surface, silica (SiO2), via surface organometallic chemistry (SOMC). This strategy afforded conceivably the first example of well-defined, low-coordinate organovanadium(III) catalysts on SiO2 with remarkable activity for liquid- and gas-phase hydrogenation of alkenes and alkynes under mild conditions.—Sohn et al.
Vanadium is a first-row transition metal; like its neighbors titanium and chromium, vanadium is much more abundant and cheaper than the precious metals. Unfortunately, most vanadium on its own will not work for the hydrogenation process. To make the vanadium work required a three-step process.
First, the vanadium has to be in its 3+ oxidation state, a very reactive but unstable state. Second, the vanadium had to be relatively dispersed on the surface—if the clumps of vanadium atoms were too big, they would cease to be as active. Last, the vanadium atoms had to be “low-coordinated”, which means that there would be electronic room for the target molecules to bind.
Getting single-atom vanadium into this special configuration on metal oxide surfaces is not easy. It requires the use of special synthetic techniques such as surface organometallic chemistry and atomic layer deposition. However, if we can make vanadium or another abundant metal as catalytically active as the noble metals, we can create dramatic cost savings in these very common and commercially important catalytic processes.—Max Delferro, corresponding author
The research was funded by the DOE Office of Science’s Office of Science.
H. Sohn, J. Camacho-Bunquin, R. R. Langeslay, P. A. Ignacio-de Leon, J. Niklas, O. G. Poluektov, C. Liu, J. G. Connell, D. Yang, J. Kropf, H. Kim, P. C. Stair, M. Ferrandon and M. Delferro (2017) “Isolated, well-defined organovanadium(III) on silica: single-site catalyst for hydrogenation of alkenes and alkynes”, Chem. Commun. doi: 10.1039/C7CC01876B