Theoretical Study On The Mechanism Of Methane Activation Catalyzed By Metal-Supported Zeolites

Due to the difficulties in transportation as well as high cost,shale gas and natural gas mainly composed of methane cannot be effectively utilized.Therefore,it is crucial to convert methane to condensable chemicals.So far,a low-temperature conversion of methane has been widely investigated by experiments;however,the economy-saving technology has never been developed.Although the reaction conditions for methane conversion is mild,since methanol tends to be overoxided to CO2,which limits the application of such technique.In recent years,a certain kind of biological enzyme,such as methane monooxygenase(pMMO)containing a CuxOy active center,can"moderately"oxidize methane under ambient conditions,and subsequently exhibit a high selectivity of product methanol.Similarly,copper-exchanged zeolite has been developed as catalyst for methane conversion at low temperature.Nonetheless,to the best of our knowledge,the structure of active center and the reaction mechanism are on debates,so that the relationship between electronic structure and reactivity of methane activation in Cu-exchanged zeolite is still unclear.This thesis is focused on the reaction mechanism of methane conversion to methanol over Cu-containing zeolite catalysts,and reveals the structure-performance relationship between active centers and reactivity.Some instructive results were obtained as follows:(1)[Cu(?-O)Cu]2+ has been experimentally confirmed as the active center which exhibits the reactivity for methane oxidation to methanol,and heteronuclear bimetallic doping is an important strategy in the regulation of reactivity for zeolite catalysis.However,as far as we know,methane activation on bimetal catalyst has not beel investigated.Herein,based on[Cu(?-O)Cu]2+,metal Ag,Zn and Au was respectively introduced to derive the heteronuclear bimetallic active center[Cu(?-O)M]2+(M=Ag,Zn,Au),and reaction mechanism of methane conversion to methanol was further investigated by using density functional theory calculations.The results indicate that from thermodynamics and kinetics,the direct mechanism(radical mechanism)is more favorable than indirect mechanism for the partial oxidation of lmethane to methanol in the heteronuclear bimetallic active centers.Meanwhile,C-H bond cleavage is the rate-limiting step,and the ability for CH4 activation decreases in the sequence:[Cu(?-O)Ag]2+(triplet)>[Cu(?-O)Au]2+(triplet)>[Cu(?-O)Cu]2+(triplet)>[Cu(?-O)Zn]2+(doublet).It can be seen that compared with the traditional[Cu(?-O)Cu]2+,the introduction of heteronuclear metal Ag and Au can effectively improve the catalytic activity for the activation of methane C-H bond,while the introduction of Zn has the negative effect.(2)Based on[Cu(?-O)Cu]2+,the catalytic activity of another dinuclear copper species bis-[Cu2(?-O)2]2+for methane activation was further discussed.The calculated results suggest that bis-[Cu2(?-O)2]+-ZSM-5 shows low activity on methane C-H bond actiation,but electronic structure of its active site can be regulated by introducing heteronuclear metals(Ag,Zn,Au and Pd).Meanwhile,methane activation on the[CuM(?-O)2]2+-ZSM-5(M=Cu,Ag,Zn,Au,Pd)is assigned to radical mechanism,and the ability of C-H bond cleavage decreases in the sequence:[CuPd(?-O)2]2+(doublet)>[CuAu(?-O)2]2+(triplet)>[CuZn(?-O)2]2+(doublet)>[Cu2(?-O)2]2+(triplet)>[CuAg(?-O)2]2+(triplet),that is,the introduction of Zn,Au and Pd can promote the activation of methane,and the introduction of Ag inhibits the activation of methane C-H bond.(3)Based on the results of two sections above,it can be found that heteronuclear metal doping significantly affects the reactivity of methane C-H bond activation.In order to elucidate the relationship between the electronic structure and catalytic performance of catalyst,and establish the reactivity descriptors for C-H activation of methane.Herein,the methane activation over diverse copper-oxygen species was also investigated.Meanwhile,reactivity descriptor was proposed by referring to the calculation results of both heteronuclear bimetals and different copper-oxygen species:The stronger hydrogen affinity of the active site is,the lower the activation barriers of the methane C-H activation will be.(4)The bis-[Cu2(?-O)2]was introduced to MOF-808 materials,additionally,The C-H activation of methane on three kinds of N-containing ligands(Bzz,Iza,His)in Cu-MOF-808 catalysts have been investigated.The results suggest that these three catalysts all exhibit excellent catalytic activity for the activation of methane C-H bond,and MOF-808-Bzz-Cu shows the best catalytic performance.It is worth noting that,compared to the common methods(hetero-nuclear bimetallic,active center size)used to modulate the electronic structure of the active center in the Cu-zeolite catalyst,the electronic structures of active center of Cu-MOF catalyst can be adjusted by the introduction of different ligands.