Theoretical Study Of Methane-to-Methanol Conversion Over Metal-Exchanged FER Zeolite Catalyst

Being a primary component of natural gas,methane has abundant natural reserves,and its rational utilization is important for alleviating energy pressure and mitigating the greenhouse effect,etc.The conversion of methane into other high value-added chemicals is one of the important catalysis research areas,where methanol has a higher commercial value and a broader application market than other conversion products.However,direct catalytic oxidation of methane to methanol is extremely challenging because methanol is much more reactive than methane,resulting in unfavorable overoxidation.In this field,metal-exchange zeolites are one of the most promising and studied materials,due to their ability to stabilize the active sites while preventing over-oxidization of the product methanol.However,current methane conversion rate and methanol yield still do not meet the requirements for industrial applications.Further development of more active and more productive metal-exchange zeolites requires a deep understanding of the structural characteristics and structure-property relationships of such catalysts,as well as finding suitable zeolite topology types for supporting active sites.In this paper,the capacity of metal-exchange FER zeolite in catalytic methane-to-methanol conversion was investigated using density functional theory method.Firstly,two categories of metal-exchange active sites were constructed on the FER zeolite.The first category includes mono-,bi-and trinuclear[Cu_x（μ-O）_y]ⁿ⁺type copper-oxo sites,while the second category includes Fe-,Co-and Ni-doped heterometallic binuclear[Cu（μ-O）M]²⁺type sites.Studying these two categories of active site helps to understand the effect of the coordination environment of FER zeolites on the catalytic capacity of their supported active sites.The electronic structure characteristics of these two categories of active sites were analyzed in order to understand their behavior during methane activation.The intermediate and transition states of the above two categories of active sites catalyzed methane-to-methanol conversion were calculated,and the relationship between the energy barriers of rate-determining step and the electronic structure characteristics of the active site was also investigated.Calculation results show that for[Cu_x（μ-O）_y]ⁿ⁺type sites,the energy barriers of rate-determining step are low and favor the methane activation;while for[Cu（μ-O）M]²⁺type sites,the heterometallic doping leads to an increase in the energy barriers of rate-determining step.Calculation results also show that for[Cu_x（μ-O）_y]ⁿ⁺type sites,the energy barriers of rate-determining step showed a significant negative correlation with the magnitude of the spin density of bridging O atoms of the active site;while for[Cu（μ-O）M]²⁺type sites,the[Cu（μ-O）M]²⁺site does not fit this negative correlation.The projected density of states analysis revealed that the reactions of the two categories of active sites adopt different electron transfer mechanisms.For[Cu_x（μ-O）_y]ⁿ⁺type sites,the electrons on the C-H bond of methane are mainly transferred to the bridging O atom during reaction;while for[Cu（μ-O）M]²⁺type sites,the electrons on the C-H bond of methane are mainly transferred to the transition metal atom during reaction.A conclusion was drawn from the comparison of these two categories of active sites.The spin density of the bridging O atom reflects the ease of the reaction only when the acceptor orbital for electron transfer is mainly located on the bridging O atom;and when the acceptor orbital is mainly located on the transition metal,it is necessary to discuss the ease of the reaction according to the actual hybridization status of the transition metal 3d orbitals.Finally,a special case of an abnormally high energy of the triplet intermediate of the[Cu（μ-O）Co]²⁺site is explained according to the exchange-enhanced reactivity principle.The importance of the hybridization status of the transition metal 3d orbitals in reducing the energy of reactive intermediates is emphasized by further comparison of the triplet[Cu（μ-O）Co]²⁺site with the triplet[Cu₂（μ-O）]²⁺site.In summary,the results of this paper indicate that FER zeolite are highly active in the reaction with methane when supporting[Cu_x（μ-O）_y]ⁿ⁺type sites,providing theoretical support for further development of the potential of this topology in experiments;while the reaction activity decreases when supporting[Cu（μ-O）M]²⁺type sites.However,due to their high stability,such sites facilitate the inhibition of competitive formation of other inactive species,and provide implications for designing strategies to improve methanol selectivity,etc.