Selective synthesis and characterization of single-site HY zeolite-supported rhodium complexes and their use as catalysts for ethylene hydrogenation and dimerization.

摘要

Single-site Rh(CO)2, Rh(C2H4)2 and Rh(NO)2 complexes anchored on various dealuminated HY zeolites can be used as precursors for the selective surface mediated synthesis of well-defined site-isolated Rh(CO)(H)x complexes. DFT calculations and D 2 isotope exchange experiments provide strong evidence for the formation of a family of site isolated mononuclear rhodium carbonyl hydride complexes (including the first examples of RhH complexes with undissociated H2 ligands): Rh(CO)(H2), Rh(CO)(H)2, and Rh(CO)(H). The fraction of each individual complex formed varies significantly with the Si/Al ratio of the zeolite and the nature of the precursor used.;HY zeolite-supported mononuclear Rh(CO)2 complexes are very active in ethylene hydrogenation and ethylene dimerization under ambient conditions. There is strong evidence for the cooperation mechanism between mononuclear rhodium complexes and Bronsted acid sites of the zeolite support in C-C bond formation process, as well as ethane formation. Finally, it is shown that the dimerization pathway selectivity can be progressively tuned (and completely switched off) by modifying the number of Bronsted acid sites on the zeolite surface.;HY zeolite-supported mononuclear Rh(NO)2 complexes can be selectively formed upon exposure of Rh(CO)2/HY to the gas phase NO/He. They are structurally similar to Rh(CO)2/HY with Rh(I) retaining square planar geometry and nitrosyl ligands adopting a linear configuration. Rh(NO)2/HY30 is active in ethylene hydrogenation and ethylene dimerization under ambient conditions. This is the first unprecedented example of a supported transition-metal nitrosyl complex capable of performing a catalytic reaction. Moreover, this is the first example of a site-isolated Rh complex with ligands other than ethylene or carbonyl, which can catalyze both ethylene hydrogenation and dimerization. Unlike its dicarbonyl counterpart, dinitrosyl rhodium complex has a uniquely different reactivity towards ethylene and hydrogen.;The mononuclear site-isolated nature of the Rh species on both HY-supported Rh(CO)2 and Rh(NO)2 is preserved after 20 hours of continuous catalysis as evidenced by FTIR data and HAADF-STEM images of the used catalyst.;The comparison of catalytic results for Rh(CO)2/HY30 and Rh(NO) 2/HY30 is the first successful example of the precise manipulation of the ligand environment (CO and NO) around the single metal atom anchored to the solid support (on the single atom scale) and the opportunity to observe how it affects reactivity and catalytic activity, using catalytic ethylene hydrogenation and dimerization as a model reaction. This opens up a new chapter in the chemistry of supported single-site materials and demonstrates there is a pathway to truly and selectively tune the catalytic activity by changing the electron density on the metal center (as well as ligand environment).