Surface structural-chemical characterization of a single-site d0 heterogeneous arene hydrogenation catalyst having 100 active sites

摘要

Structural characterization of the catalytically significant sites on solid catalyst surfaces is frequently tenuous because their fraction, among all sites, typically is quite low. Here we report the combined application of solid-state ¹³C-cross-polarization magic angle spinning nuclear magnetic resonance (¹³C-CPMAS-NMR) spectroscopy, density functional theory (DFT), and Zr X-ray absorption spectroscopy (XAS) to characterize the adsorption products and surface chemistry of the precatalysts (η⁵-C5H5)2ZrR2 (R = H, CH3) and [η⁵-C5(CH3)5]Zr(CH3)3 adsorbed on Brønsted superacidic sulfated alumina (AlS). The latter complex is exceptionally active for benzene hydrogenation, with ∼100% of the Zr sites catalytically significant as determined by kinetic poisoning experiments. The ¹³C-CPMAS-NMR, DFT, and XAS data indicate formation of organozirconium cations having a largely electrostatic [η⁵-C5(CH3)5]Zr(CH3)2⁺···AlS⁻ interaction with greatly elongated Zr···OAlS distances of ∼2.35(2) Å. The catalytic benzene hydrogenation cycle is stepwise understandable by DFT, and proceeds via turnover-limiting H₂ delivery to surface [η⁵-C₅(CH₃)₅]ZrH₂(benzene)⁺···AlS⁻ species, observable by solid-state NMR and XAS.