Silica-anchored metal ions are observed to catalyze C-C coupling of olefins. The extent of chain growth appears to depend on the identity of metal ion and the nature of support. We aim to provide molecular level insights on catalytic performance of hydrides of group-4 metals (M = Ti4+, Zr4+, Hf4+) supported on silica and silica-alumina for ethylene oligomerization using periodic Density Functional Theory (DFT). Here, we construct representative models of monohydride (SiO)3MH and dihydride (SiO)2MH2 sites on (111) and (001) surface of β-cristobalite, a silica polymorph. We compare the kinetics and energetics of key steps along Cossee-Arlman mechanism. Further, to investigate the role of lewis acidic Al as in silica-alumina we systematically substitute a Si(+4) with Al(+3)/H(+1) in the silica model to form (SiO)2(AlOH+)MH site. Oligomerization behavior of group-4 hydrides on silica (no Al) and Zr-monohydride on 1Al/silica is compared based on turnover frequencies (TOFs) and average degree of polymerization (DOP) obtained from DFT computed kinetic parameters. We show that Zr and Hf hydrides on silica make polymer chains, while Ti has potential of more controlled chain growth. TOFs suggest that Zr-hydrides are most active for ethylene chain growth as compared to that of Ti and Hf. Presence of Al in the vicinity of Zr-monohydride (1Al/silica) does not significantly change the TOF of the site having no Al, but the chain length of the oligomers or DOP is reduced by more than an order of magnitude. These predictions qualitatively agree to experiments where reaction of ethylene over Zr-hydrides on silica-alumina produces higher fraction of C4s, followed by C6+ range hydrocarbons, unlike that on silica support. The microscopic details and reactivity comparisons in this work aid in catalyst development under different reaction conditions. This approach can be used to expand the material space by screening other metal ions for oligomerization.

Interrogation of single-site, oxide-supported group 4 metal hydrides for ethylene oligomerization by density functional theory

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.