"Connecting cation site location to alkane dehydrogenation activity in Ni/BEA catalysts"

Principal Scientist, Catalytic Carbon Transformation & Scale-up Center, NREL

Director, ChemCatBio Consortium

Hosted by: Dr. Aaron Sadow

Abstract: Our research group at NREL has been studying C-H bond activation at zeolite-supported ionic metal sites in the context of alkane dehydrogenation for upgrading to fuel-range molecules through subsequent alkene coupling. Initially, Cu(1+) sites supported at ion-exchange sites in BEA zeolite were identified as the active site for isobutane dehydrogenation through a combined experimental and computational approach. More recently, Ni-modified beta zeolite (Ni/BEA) catalysts were found to activate carbon-hydrogen bonds in isobutane. Employing a controlled synthesis of Ni/BEA enabled efficient introduction of ion-exchanged Ni(2+) sites across varying Ni loadings (0.43% - 1.8%). These catalysts exhibited site time yields for hydrogen production that increased with increasing Ni loading, contrary to the expected behavior of single-site catalysts were the site time yield would be invariant with loading. In situ spectroscopic methods indicated isolated, 4-coordinate Ni(2+) species for all investigated loadings. Quantum mechanics/molecular mechanics modelling identified two distinct Ni(2+) sites that were consistent with the structural characterization, but having different relative stabilities due to their coordination environment. Computed reaction energetics for isobutane dehydrogenation revealed that the more stable Ni(2+) species at a six-membered 4Si-2Al ring, termed “Ni-6MR”, would be less active for isobutane dehydrogenation than the less stable Ni(2+) at the five-membered 3Si-2Al ring, “Ni-5MR”. These differing local structures of the isolated cationic Ni(2+) sites in Ni/BEA accounts for the non-single-site catalytic behavior, that is, increased H₂ site time yields observed at greater Ni loadings.