Abstract
Ti(IV) incorporated layered stacking MWW zeolites with containing similar porous structure of ERB-1 and MCM-22 precursors (Ti-MCM-22 and Ti-ERB-1) and ERB-1 precursor as starting material subsequently further prepared micro-/mesoporous silica MCM-36 by silicates pillars (Ti-MCM-36) through a simple grafting method using different titanium alkoxides, including Ti(OEt)4, Ti(OiPr)4 and Ti(OBu)4 onto the post-treated calcined MWW. The X-ray diffraction patterns and N2 sorption isotherms confirmed the retention of 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. After acidizing with 2 M HNO3 concentration on Ti-MCM-22 due to the purpose of removed extra-framework TiO2 species, UV-Vis observation presents that 1-butanol has a well-retained Ti-anchored activity rather than ethanol as grafting solvent. Solid state 29Si NMR spectra indicated that most Ti(IV) were incorporated near the T-sites on the open bottom of the supercage, which were easily accessed by grafting agent. When applied in epoxidation of cycloalkenes, Ti-MCM-22 prepared by grafting with Ti(OEt)4 demonstrated better catalytic activities than Ti-YNU-1 or those grafting with Ti(OiPr)4 and Ti(OBu)4. The best catalytic performances were obtained over Ti-MCM-22 with 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 cyclooctane oxidation using t-butylhydrogen peroxide as the oxidant. ERB-1 precursor as initial material follows the step-by-step experimental procedure to derive micro-/mesoporous silica MCM-36. Ti(IV)-incorporated MCM-36 (Ti-MCM-36) was prepared by a simple grafting method using different titanium alkoxides, including Ti(OEt)4, Ti(OiPr)4 and Ti(OBu)4 in various solvents, including ethanol, 1-butanol, and toluene. The X-ray diffraction patterns and N2 sorption isotherms confirmed the retention of the basal MWW layer structure in Ti-MCM-36. 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, while the solvent had little influence on the Ti loading. UV–vis and XANES spectra showed that titanium species were mainly tetrahedral coordinated Ti(IV) and small amount of titania clusters when grafting in ethanol and 1-butanol, while large bulky titania crystallites were formed in toluene. Ti-MCM-36 prepared by grafting with Ti(OEt)4 in 1-butanol demonstrated better catalytic activities in epoxidation of olefins, including cyclohexene, cyclooctene and dicyclopentadiene (DCPD) than Ti-YNU-1 or those grafted with Ti(OiPr)4 and Ti(OBu)4. In DCPD oxidation using t-butylhydrogen peroxide (TBHP) as the oxidant, about 100% DCPD conversionand 95% diepoxide selectivity could be achieved using TBHP/DCPD molar ratio of 3.5.
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