Effect of Nickel Active Site Density on the Deactivation of Ni-Beta Zeolite Catalysts during Ethene Dimerization
Arunima Saxena, Ravi Joshi, Ranga Rohit Seemakurthi, Elsa Koninckx, Linda J. Broadbelt, Jeffrey Greeley, Rajamani Gounder
Abstract
Isolated Ni(II) sites supported on zeolites and other porous materials transform in situ during alkene dimerization to form active Ni(II)-alkyl centers, and their density influences the kinetic orders and mechanisms of deactivation of Ni-Beta zeolites during ethene dimerization (453 K). Ni-Beta containing high Ni site densities shows deactivation rates that are second-order in Ni, consistent with a dual-site deactivation mechanism involving the formation of unreactive Ni-alkyl-Ni intermediates, as confirmed by DFT calculations. Under the same reaction conditions, by contrast, Ni-Beta containing low Ni site densities shows deactivation rates that are first-order in Ni, consistent with a single-site deactivation mechanism reflecting inhibition by strongly bound intermediates derived from heavier alkene oligomers. On Ni-Beta containing low Ni site densities, cofeeding H2 along with ethene results in a higher number of Ni(II)-alkyl intermediates formed at initial reaction times and a concomitant change to deactivation kinetics that become second-order in Ni. These findings reveal the strong influence of the density of active Ni(II)-alkyl centers in porous supports, which depends both on material properties and reaction conditions that generate active centers in situ, on the kinetics and mechanisms of deactivation during alkene oligomerization.