Studies On The Reactivity Of Intermediates In Ethanol Conversion Over Acidic Zeolites

With the continuous depletion of fossil energy and the worsening energy crisis,the development of clean and low-carbon renewable energy has become one of the important ways to achieve sustainable energy development.Ethanol,as a clean and economical renewable energy source,has gained considerable attention from researchers due to its increasing production in recent years.Zeolite-catalyzed ethanol-to-hydrocarbons（ETH）is an essential method for generating high-value chemicals like low-carbon olefins,aromatics,and gasolines from renewable sources.This method not only reduces the dependence on petroleum feedstocks,but also is a promising way to replace or supplement fossil energy sources.On this account,ETH process has recently attracted extensive attentions from both academia and industry,and is important for promoting sustainable energy development.Current research on ETH process focuses on catalyst development and modification,the understanding of mechanism has yet to be established,and there is still a lack of knowledge about the reaction intermediates and their reactivity.Therefore,a comprehensive study of the reactivity of key intermediates in ETH process can not only enrich the understanding of the reaction mechanism,but also provide guidance for the design of new catalysts and optimization of the catalytic reaction process.In this work,the intermediates during ETH process on zeolites have been investigated by combining selective isotopic labeling,solid-state nuclear magnetic resonance technique（ss NMR）,gas chromatography-mass spectrometry（GC-MS）analysis and theoretical calculations,and the specific results are as follows:1.Identification of intermediates during the initial stage of ETH reaction.By combining selective ¹³C-labeling with ss NMR techniques,we have isolated,and identified intermediates formed during the initial stage of ethanol conversion on H-Y and H-ZSM-5 zeolites.With a stop-flow protocol,we have successfully prepared the surface ethoxy species,which were further characterized by 2D ¹³C?¹³C J-refocused INADEQUATE NMR technique.Moreover,we identified the other C₂ intermediates of ethanol dehydration on zeolites（i.e.,diethyl ether and triethyloxonium ion）and identified their ¹³C chemical shifts.These results not only enrich the understanding of the reaction intermediates during the initial stage of ethanol conversion,but also provide the basis for species identification for the further studies on the mechanism of ETH process.2.Unraveling the role of surface ethoxy species in C?C bond formation.Through a comprehensive approach that combines ss NMR,GC-MS analysis,and density functional theory（DFT）theoretical calculations,we have conducted an in-depth investigation into the role of surface ethoxy species on H-Y zeolite during ETH process.By the combination of isotopic labeling and ss NMR,surface ethoxy species were observed as the key intermediate during the transformation of ethene over H-Y zeolite for the first time.Furthermore,n-butane was identified as the initial C₄-product via ¹³C isotopic tracer experiments.On this basis,DFT theoretical calculations comfirmed the crucial role of surface ethoxy species in the initial C?C bond formation.These remarkable findings not only shed light on the reactivity of surface ethoxy species,but also reveal the mechanism of initial C?C bond formation during the ETH process.3.Studies on the deprotonation and hydride transfer of surface ethoxy species.Using propane as a probe molecule,we investigated the reactivity of surface ethoxy species and elucidated the reaction pathways of surface ethoxy species at the atomic level by analyzing the initial products.Firstly,through the application of solid-state NMR,we identified ethene and ethane as the initial products on H-Y zeolite and H-ZSM-5 zeolite,respectively.Then,we combined the selective ¹³C and ²H isotopic labeling with GC-MS analysis to trace the sources of the ¹³C and ²H atoms in the initial products.This allows us to determine the precise pathway of the initial reaction.On this basis,with the help of DFT theoretical calculations,we proposed the detailed mechanisms of deprotonation and hydride transfer reactions of surface ethoxy species,thus revealing the carbenium-ion-like nature of surface ethoxy species for the first time.4.Elucidation of the effect of zeolite acidity on the reactivity of surface ethoxy species.Based on the aforementioned findings,we selected a series of MFI zeolites（H-[B]-ZSM-5,Silicalite-1,and H-ZSM-5 zeolites with different Si/Al ratios）as working catalysts,and investigated the effect of the acidity on the reactivity of surface ethoxy species by analyzing the initial reaction of surface ethoxy species with propane.Based on the detailed characterizations of the zeolite acidity,we observed that both the strength and amounts of the Br?nsted acid sites affected the reactivity of surface ethoxy species:both the stronger acid strength and the higher amount of Br?nsted acid sites promoted the hydride transfer reaction.These insights allowed us to establish a quantitative relationship between the acidity of zeolites and the reactivity of surface ethoxy species,obtained by plotting the ethane/ethene ratio of the products as a function of the acid amount of the zeolites.Furthermore,these results are expected to provide theoretical guidance for the selection and optimization of catalysts in the ETH process.