Effect of the grafting agent on the structure and catalytic performance of Ti-MCM-22

Abstract

Ti(IV)-incorporated MCM-22 (Ti-MCM-22) was prepared by a simple grafting method using different titanium alkoxides, including Ti(OEt)₄, Ti(OiPr)₄ and Ti(OBu)₄ onto acid-treated calcined MCM-22. The X-ray diffraction patterns and N₂ sorption isotherms confirmed the retention of the basal MWW layer structure in Ti-MCM-22. The Si/Ti molar ratios analyzed by ICP-MS increased with the size of titanium alkoxides and varied in the order of Ti(OEt)₄ < Ti(OiPr)₄ < Ti(OBu)₄ based on the same amounts of titanium alkoxides used in the grafting solutions. The UV-vis and X-ray absorption spectra revealed that both tetrahedrally and octahedrally coordinated Ti(IV) species were present on Ti-MCM-22 prepared by grafting. However, the amount of octahedral Ti(IV) decreased with the decrease in Ti content. The solid-state ²⁹Si NMR spectra indicated that most Ti(IV) species were incorporated near the T-sites on the open bottom of the supercage, which were easily accessed by the grafting agent. When applied in epoxidation of cycloalkenes, Ti-MCM-22 prepared by grafting with Ti(OEt)₄ demonstrated better catalytic activity than Ti-YNU-1 or those grafted with Ti(OiPr)₄ and Ti(OBu)₄. The best catalytic performances were obtained over Ti-MCM-22 with a Si/Ti molar ratio of ca. 100 prepared by grafting with Ti(OEt)₄. The conversions were very high and epoxide selectivities were close to 100% in cyclohexene and cyclooctene oxidation using t-butylhydrogen peroxide as the oxidant.