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

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 C-13-cross-polarization magic angle spinning nuclear magnetic resonance (C-13-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 (eta(5)-C5H5)(2)ZrR2 (R = H, CH3) and [eta(5)-C-5(CH3)(5)]Zr(CH3)(3) adsorbed on Bronsted superacidic sulfated alumina (AlS). The latter complex is exceptionally active for benzene hydrogenation, with similar to 100% of the Zr sites catalytically significant as determined by kinetic poisoning experiments. The C-13-CPMAS-NMR, DFT, and XAS data indicate formation of organozirconium cations having a largely electrostatic [eta(5)-C-5(CH3)(5)]Zr(CH3)(2)(+)center dot center dot center dot AlS- interaction with greatly elongated Zr center dot center dot center dot O-AlS distances of similar to 2.35(2) angstrom. The catalytic benzene hydrogenation cycle is stepwise understandable by DFT, and proceeds via turnover-limiting H-2 delivery to surface [eta(5)-C-5(CH3)(5)]ZrH2(benzene)(+)center dot center dot center dot AlS- species, observable by solid-state NMR and XAS.