Construction of Zeolite Framework-Anchored Rh-(O-Zn)x Sites for Ethylene Hydroformylation

Abstract

Zeolite-confined Rh-based catalysts have emerged as promising heterogeneous candidates for olefin hydroformylation. However, they face challenges of reactant- and product-induced Rh leaching and aggregation. Herein, zeolite framework-anchored Rh^δ+-(O-Zn)_x sites were designed and are shown to have remarkable activity and stability for gas-phase ethylene hydroformylation. The bimetallic catalysts were synthesized by coencapsulating Rh and Zn species into Silicalite-1 zeolite, and the Rh^δ+-(O-Zn)_x sites were in situ constructed during the induction period of the hydroformylation process through the interaction between mobile Rh-carbonyl species and framework ≡SiOZn-O(H). The change of the Zn/Rh molar ratio significantly affects the dispersion of Rh and the proportion of highly active Rh^δ+. The optimal 0.2Rh@Zn₃-S-1 catalyst achieves a propanal turnover frequency as high as 148 h^-1 at 363 K and shows no sign of deactivation during the 40 h test. In contrast, zinc-free 0.2Rh@S-1 suffers rapid deactivation due to Rh aggregation. In situ Fourier transform infrared (FTIR) spectroscopy reveals that the transfer desorption of propanal from Rh to Zn-O contributes to the redispersion of Rh during the construction of Rh^δ+-(O-Zn)_x structures. Moreover, the observed HRh(CO)₂ species together with the enrichment of Rh^δ+-propionyl intermediates on the catalyst indicates that the hydrogenation of acyl species is the rate-limiting step of ethylene hydroformylation, which is further supported by kinetic analysis. This study presents a new strategy for designing stable and efficient gas-phase ethylene hydroformylation catalysts using zeolite-anchored metal species as inorganic ligands for Rh^δ+ centers and provides insights into the hydroformylation mechanism occurring on the bimetallic sites.