Addressing confinement effect in alkenes epoxidation using 'isoreticular' titanosilicate zeolite catalysts
Abstrakt
Titanosilicate zeolites are a prominent class of catalysts applied in various oxidation reactions, including epoxidation of alkenes. Numerous studies have been performed to understand the influence of Ti sites structure ('open' vs. 'closed') and their distribution between internal and external surface of zeolite on the catalyst performance, although the effect of Ti location in micropores of different sizes remained unrevealed.
Here, we report the application of ADOR synthesis strategy for the preparation of model 'isoreticular' titanosilicate zeolites with the same topology but continuously tunable porosity for addressing the influence of micropore confinement on the activity and selectivity of the catalysts in alkene epoxidation reaction. A set of 'isoreticular' Ti-IPC-n (n = 2, 4, 6, 7) zeolites were synthesized by post-synthesis of Ti-substituted UTL germanosilicate.
Structural, textural and acidic properties of the prepared catalysts were characterized by XRD, nitrogen physisorption, FTIR spectroscopy of adsorbed d(3)-acetonitrile, while their activity and selectivity were compared in the epoxidation of 1-hexene and cyclohexene as model linear and cyclic alkenes with hydrogen peroxide as oxidant. The obtained results confirmed the successful preparation of Ti-substituted IPC-n zeolites with similar total concentration of Ti sites (0.15-0.17 mmol/g).
For both 1-hexene and cyclohexene epoxidation, the specific activity of a Ti active center (TOF) gradually decreased with the micropore size in a sequence Ti-UTL (TOF(1-hexene) = 126 h(-1); TOF(cyclohexene) = 227 h(-1)) > Ti-IPC-7 (124; 143) > Ti-IPC-2 (124; 130) > Ti-IPC-6 (90; 67) > Ti-IPC-4 (64; 37). In line with reducing diffusion limitations of substrates and products, the site time yield of both targeted epoxides increases with increasing the size of the pores in 'isoreticular' zeolites reaching the maximal value for Ti-UTL (586 and 491 h(-1) for 1,2-epoxyhexane and cyclohexeneoxide, respectively).