Hierarchical FAU-Type Hafnosilicate Zeolite as a Robust Lewis Acid Catalyst for Catalytic Transfer Hydrogenation

FAU-type hafnosilicate zeolite with a hierarchical structure (Hf-USY) was constructed through a post-synthesis strategy containing the controlled dealumination of the commercial H-USY zeolite and the subsequent dry impregnation of the Cp2HfCl2 precursor. The incorporation of Hf ions into the zeolite framework involved the interaction between the silanol groups from framework dealumination and Cp2HfCl2 molecules. Characterization results from UV-vis and X-ray photoelectron spectroscopy verified that the incorporated Hf ions mainly existed in the tetrahedrally coordinated form in the zeolite framework. The creation of Lewis acid sites associated with Hf incorporation was confirmed by Fourier transform infrared spectroscopy with pyridine and deuterated acetonitrile adsorption. The as-synthesized Hf-USY zeolite served as a highly efficient catalyst in the transfer hydrogenation of ethyl levulinate to gamma-valerolactone, outperforming the Sn- and Zr-silicate analogues. The Hf-USY catalyst also exhibited good performance in the transfer hydrogenation of more challenging substrates such as furanic, aromatic, as well as alkene-substituted carbonyl compounds into the corresponding alcohols. The influence of Lewis acid sites, preparation parameters, as well as the hydrogen donor on the catalytic activity of the Hf-USY zeolite was investigated in detail.