A Study On The Nature Of Solvent Effect Over Titanosilicates/H₂O₂ System And The Control Of Active-site Microenvironment

Green chemistry refers to the reduction or elimination of environmental pollution from industrial production through chemical techniques or methods.As an importa nt part of green chemistry,solvent greening is the use of safe and non-toxic solvent and the recycling of solvent.It is very important to study the nature of the solvent effect for the selection of green solvents on the basis of ensuring the reaction activity.This thesis focuses on the nature of the solvent effect in titanosilicates/H₂O₂ catalytic system,directed by the solvent greening.We try to precisely control the microenvironment of Ti active sites,in order to improve the performance of titanosilicates,based on the understanding that the solvent effect depends on the Ti active center.Finally,to achieve the high-efficiency TS/H₂O₂ catalytic system for the selective oxidation of hydrocarbons,the key factors on determining the O atom economy are discussed.And it is expected that the obtained scientific understanding and technical methods will provide a strong support for the development of a new generation of HPPO process.?1?Based on the results that,a compensation effect was observed in the H₂O₂activation reaction when the solvent was varied,we designed a series of TS-1 zeolites containing different content of Ti?OSi?₃OH active sites to explore the solvent effect.Through various characterizations and 1-hexene epoxidations under mixed solvents,it is found that the solvent effect of titanosilicate is unrelated to its topology,but depends on the state of Ti active center.The nature of the solvent effect is believed to be that the solvent could work as the proton-transfer supporter for promoting the proton transfer during the activation of H₂O₂.As the Ti active sites are Ti?OSi?₃OH species,the best proton-transfer supporter is water molecules.The used solvent affects the proton-transfer ability by adjusting the state of the water molecules.The weaker the hydrogen bonds among water molecules are,the stronger the proton-transfer ability is.As the Ti active sites are Ti?OSi?₄ species,the best proton-transfer supporter is a proton-donating solvent molecule?such as an alcohol molecule?.The strong ability of the solvent to donate proton is beneficial for proton-transfer ability.Based on the understanding of the solvent effect,we design and achieve the high-efficiency HPPO processes using methanol or tert-butanol as solvents,and it is promising to promote their potential industrial application prospects.?2?Based on the recognition that the solvent effect of titanosilicate depends on the state of Ti active center,the Ti active-site microenvironments were precisely controlled by the cation exchange or the mild alkaline treatment,and their catalytic performance s were discussed.The K⁺exchange method was used to modify the Ti-MWW zeolite,and a highly active Ti-MWW-K zeolite with the Ti-OH?KO-Si?active sites was successfully synthesized.The overall investigation of the X-ray diffraction,UV-Visible diffuse reflectance spectra,infrared spectra,and so on have shown that the K⁺modification had a negligible effect on the physicochemical property,the state of the Ti active site and the Lewis acidic strength.The deep exploration has indicated that breaking of the hydrogen-bonding interactions between O?atom of H₂O₂(H?-O?-O?-H_end)and H atom of the silanols adjacent to Ti-OH?Si-OH?HO-Ti??in Ti-MWW-K zeolite would promote the activation of H₂O₂,which was responsible for its superior catalytic oxidation activity.In addition,the modified Ti-MWW-K zeolite presents a unique solvent effect,that when the solvent is protic MeOH or t-BuOH,the improvement of 1-hexene conversion after the cationic modification is significantly higher than that in aprotic acetonitrile or acetone.It can be ascribed to the enhanced proton transfer via the breaking of hydrogen-bonding interactions between the protic MeOH and water?from the added H₂O₂ aqueous solution?through electrostatics?water's dipole interacting with K⁺?.The mild alkaline treatment was also used to modify the Ti-MWW zeolite,and a highly active Ti-MWWⁿ zeolite with the Ti-OH?HO-Si?₃ active sites was successfully synthesized.Combined with the investigations of UV-Vis,UV-Raman,IR spectra,and so on,the newly generated Ti-OH?HO-Si?₃ active sites were identified.And a receivable strategic mechanism for constructing Ti-OH?HO-Si?₃ species in titanosilicates was proposed that the Si-OH species adjacent to Ti-OH are dissolved selectively to form the new Ti-OH?HO-Si?₃ species.Besides,the Ti-MWWⁿ-K zeolite without the further removal of K after the alkaline treatment also presents a similar solvent effect as the Ti-MWW-K zeolite due to the existence of the K⁺species.?3?“Atomic economy”is the core of the selective oxidation of hydrocarbons.The key factors in determining the O atom economy were studied for achieving the high-efficiency TS/H₂O₂ system.Based on the analysis of the H₂O₂ reaction pathways,the effects of reactant,Lewis acidity of titanosilicates,the hydrogen-bonding interactions,and solvents on H₂O₂ utilization efficiency were discussed in detail.It can be concluded that the key to achieving high utilization efficiency of H₂O₂ over titanosilicates/H₂O₂system is the synergistic effect of regulating the formation of Ti-OOH species,the stability of Ti-OOH species and the transfer ability of active“O”to reactant.