One-step synthesis of bi-functional zeolite catalyst with highly exposed octahedral Co for efficient epoxidation of bulky cycloalkenes

Abstract

Bi-functional zeolite catalyst, incorporated with both organic groups and transition metal ions, has been firstly one-step synthesized by dry gel conversion method, which is characterized by powder XRD, solid-state 27Al, 29Si, 13C MAS NMR, FT-IR, Raman, UV–vis, X-ray photoelectron spectroscopy, and so on. Notably, the SiO/SiI ratio in the synthesis gel has important influence on the coordination state and distribution of cobalt species in the hybrid zeolite. Low SiO/SiI ratio has accelerated the formation of hybrid zeolite with tetrahedral framework cobalt while high SiO/SiI ratio has accelerated the additional assembly of hybrid zeolite with octahedral extra-framework cobalt. In the catalytic epoxidation of bulky cycloalkenes with air, hybrid zeolite with both tetrahedral framework cobalt and octahedral extra-framework cobalt shows the most excellent catalytic performance and TON value, compared with pure inorganic zeolite, hybrid zeolites with only tetrahedral framework cobalt species, and hybrid zeolites with Co species introduced by ion-exchange or wet impregnation methods, which is ascribed to the synergistic roles of organic groups and cobalt species. On one hand, the highly exposed octahedral extra-framework cobalt species act as an efficient supplement of tetrahedral framework cobalt, and the interactions of accessible catalytic active sites with bulky cycloalkene molecules have been strengthened greatly. On the other hand, mercaptopropyl groups are beneficial for the formation of stable heterogeneous catalyst with lipophilic surface through anchoring the extra-framework cobalt species and modifying the surface of the zeolite, which accelerate the epoxidation reaction efficiently.