Direct synthesis and characterization of highly ordered cobalt substituted KIT-5 with 3D nanocages for cyclohexene epoxidation

Highly ordered cobalt incorporated KIT-5 silica (Co-KIT-5) with different Co contents and a well-ordered three-dimensional cage type porous structure were prepared for the first time by using Pluronic F127 as structure directing agent at different molar water to hydrochloric acid (nH2O/nHClnH2O/nHCl) ratio. The amount of Co content in the silica framework of KIT-5 can be finely controlled with a simple adjustment of the nH2O/nHClnH2O/nHCl ratio as it controls the concentration of the H+ ions in the synthesis gel. It has been found that the nH2O/nHClnH2O/nHCl ratio of 463 is the best condition to obtain Co-KIT-5 with a high Co content. The obtained materials were characterized by various techniques such as powder X-ray diffraction (XRD), N2 adsorption studies, field emission high resolution scanning electron microscopy (FE-HRSEM), high resolution transmission electron microscopy (HRTEM), ultraviolet–visible diffused reflectance (UV–Vis DRS), electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS). Characterization results revealed that Co atom can be introduced in the silica framework without affecting the structural order and the textural parameters of the samples. ESR, XPS and UV–Vis DR spectra confirmed that the Co atoms are indeed occupy the tetrahedral coordination with the silica framework of KIT-5. The catalytic performance of Co-KIT-5 with different Co contents in the cyclohexene epoxidation has been investigated using TBHP/H2O2 as oxidants and acetonitrile as a solvent. Co-KIT-5 exhibited a high catalytic performance with TBHP as oxidant and remained inactive when H2O2 was used. The effect of various reaction parameters such as reaction time, reaction temperature, and reactant feed ratio and oxidant, affecting the catalytic activity of Co-KIT-5 has also been studied. Among the catalysts studied, Co-KIT-5-0.90 was found to be the best catalyst, affording a high conversion of cyclohexene. In addition, the catalyst was found to be highly stable and can be reused several times without affecting its catalytic activity under the optimized reaction conditions.

Herein we report on the synthesis of Co-KIT-5 materials with a high loading of cobalt using the self-assembly of the metal precursors and the polymeric surfactant. The nH2O/nHClnH2O/nHCl ratio in the synthesis mixture was tuned to control the amount of metal loading in the silica framework of KIT-5. The prepared materials were tested in the liquid-phase epoxidation of cyclohexene and found that the catalyst is highly active, stable and can be recyclable for several times.Figure optionsDownload as PowerPoint slideHighlights
► First report on the preparation of cobalt substituted KIT-5.
► Controlling of the Co content in KIT-5 by tuning water to HCl ratio.
► Showed higher activity and stability for epoxidation of cyclohexene.