Activation And Conversion Of Light Alkane Over Gallium-modified ZSM-5 Zeolites As Studied By Solid-state NMR Spectroscopy

Light alkanes,such as methane,ethane and propane,have abundant reserves in nature.Catalytic conversion of light alkanes to high-valued products such as alcohols,olefins and aromatics is one of the promising routes to replace petroleum technology,and has attracted intensive interest from both industry and academia.Taking into account the chemical inertness of C-H bond,selective conversion of light alkanes at mild condition remains a tremendous challenge.Metal(Zn,Ga and Mo,etc.)modified ZSM-5 zeolites are promising catalysts in term of high activity toward direct conversion of alkanes.Numerous studies show that Ga species on Ga-modified zeolites play an important role in the activation of C-H bond and the dehydroaromatization of light alkanes.However,the reaction mechanism particularly the initial C-H bond activation and the role of active centres are still unclear due to the complexity of the metal species and the lack of effective characterization method.Gaining deep insights into the structure,location and acid properties of metal species and the reaction mechanism of alkane over active centres is helpful for the design and development of high-efficient catalysts for alkane conversion.Herein,the structural and properties of active Ga species and the reaction mechanism of light alkane over Ga-modified ZSM-5 zeolites were comprehensively studied by solid-state NMR(SSNMR)in combination with FTIR,GC-MS etc..The main contents and results of this thesis are listed as blow(1)Reaction mechanism study of methane activation over Ga-modified ZSM-5 zeolite by solid-state NMR spectroscopy.The structure of Ga species on different Ga-modified ZSM-5 samples was probed by 71Ga NMR experiments.It was found that the distribution of Ga species on Ga/ZSM-5 prepared by different methods was significantly different.Two types of Ga species,namely GaO+ species and highly dispersed GaOx,were determined on the sample prepared by impregnation method.The activation of methane on Ga/ZSM-5 was explored by using 13C and 1H MAS NMR.Four types of Ga-CH3 species along with Ga-OH groups were observed on Ga/ZSM-5,evidencing different pathways for heterolytic cleavage of C-H bond of methane.The local environment and the structure of different Ga-CH3 species were further studied by using 2D 1H-1H and 1H-13C correlation NMR experiments.Combined with DFT calculation results,four routes were proposed for the activation of methane C-H bond on highly dispersed GaOx and GaO+ species.The results provide insight into methane activation on metal-modified zeolite and are helpful for the mechanistic understanding of the chemistry of light alkanes on zeolites.(2)Probing the active sites for methane activation on Ga/ZSM-5 zeolites with solid-state NMR spectroscopy.A series of Ga/ZSM-5 catalysts were prepared with different treatment temperature.71Ga NMR and and 31P NMR of adsorbed trimethylphosphine(TMP)experiments showed that cationic Ga species are responsibility for the formation of strong Lewis acid sites,including GaO+ and GaOx clusters.Combining 13C NMR and 31P NMR experiments,the activity of Ga/ZSM-5 for methane activation was quantitatively correlated with strong Lewis acid sites GaO+ ions and GaOx species.Furthermore,the distribution of the active Ga species and the interaction between Ga species and zeolite framework were probed by using 1H-71Ga and 13C-27 Al double-resonance NMR,and the C-Al internuclear distance was measured.These results provide direct evidence for the location of the active Ga species at Br(?)nsted acid site in zeolite channel,which are supported by DFT theoretical calculation.The results provide new insight into the nature and role of Ga species in Ga-modified zeolites for the conversion of light alkanes.(3)Active sites and reaction mechanism of propane dehydroaromatization on Ga-modified ZSM-5.The structure and acidity of Ga/ZSM-5 catalysts were studied by solid-state NMR.The catalytic performance of zeolites and reaction mechanism for propane aromatization were also investigated.With the help of 71Ga NMR and 31P NMR of adsorbed TMP experiments,it was found that the sample prepared by physical mixing only contains Ga2O3 species with weak Lewis acidity,and the samples prepared by impregnation and impregnation followed by reduction-oxidation treatment contain both highly dispersed gallium clusters and cationic Ga species.Cationic Ga species was favored on Ga/ZSM-5-IE prepared by ion exchange.Both dispersed gallium clusters and cationic Ga exhibited stronger Lewis acidity.Combined with the activity tests of propane aromatization on Ga-modified ZSM-5,it was found that Ga/ZSM-5 zeolites with strong Lewis acid sites showed high catalytic activity for propane aromatization.The synergic active sites composed of Bronsted acid site and Lewis acid sites(cationic Ga and dispersed Ga species)on Ga/ZSM-5 zeolite were identified by 2D 1H-31P HETCOR NMR spectroscopy,which were correlated to the catalytic activity.Furthermore,the methyl cyclopentenyl carbocation species were clearly observed for the first time during propane aromatization reaction by using solid-state NMR,which were important intermediates for the formation of aromatic.Combined with in situ solid-state NMR,we found that the rate of generation and conversion of these ring intermediates increase significantly because of the introduction of Ga species,thus improving the aromatics selectivity.These results provide valuable information for the understanding of the mechanism of propane aromatization,and are helpful for the development and optimization of high-efficient catalysts for alkanes aromatization.