Catalyst Design And Mechanism Study For Selective Low Temperature Oxidation Of Methane To High Value-added Chemicals

Methane（CH₄）,the major component of natural gas,shale gas,methane hydrates,and coal-bed gas,is one of the most abundantly available carbon-based feedstocks.It also shows great potential to become a major energy source other than coal and oil.Meanwhile,it exhibits the great capacity of the greenhouse effect,which is about 25 time higher that that of CO₂.Therefore,it is of significance to transform methane into valuable chemical products,such as menthol,formic acid and acetic acid,which is also favorable for the alleviation of energy and resource problems caused by the depletion of crucial oil,the mitigation of greenhouse effect and creating more economic values.Indeed,methanol is a clean fuel source applied in fuel cell（FC）technology.Formic acid is widely used as a disinfecting and preservative agent.Acetic acid is one of the important chemical intermediates,which has a global demand of 6.5 million tons per year.However,the strongly chemical stability of C-H bonds(104 kcal mol^-1),lower polarizability and disaffinity of electron would make the activation of methane extremely difficult.The inertness of methane also leads to a higher reaction temperature for the catalytic oxidation process and the subsequently but undesirably enhanced over-oxidation of the value-added products of methanol,formaldehyde etc.into CO₂.In conclusion,it is still challenging for the effective and selective conversion of methane into the valuable products of oxygenated compounds under mild reaction conditions.Herein,in order to obtain higher methane conversion and product selectivity,the corresponding zeolite catalysts were constructed,and the corresponding reaction mechanism was proved by DFT and experimental characterization methods.The specific experimental results are summarized as follows:（1）Firstly,The Fe-MOR（mordenite）catalyst was prepared and applied to the low-temperature oxidation of methane,and high catalytic performance was obtained.The octahedral dimer Fe³⁺species[Fe₂（μ-O）₂]in extra framework was confirmed as the initial active site by X-ray photoelectron spectroscopy,X-ray absorption near-edge structure and extended X-ray absorption fine structure,UV-vis diffuse-reflectance spectra,and high-angle annular dark field and scanning transmission electron microscopy with the combination of DFT calculations.DFT calculations indicate that the methanol was generated via the decomposition of methyl peroxide（CH₃OOH*）intermediates on[Fe₂（μ-OH）₂O₂].The Fe-MOR catalyst can produce a high methanol selectivity of 75.6%by the presence of Cu²⁺precursor that can efficiently suppress the over-oxidation of methanol,and a high formic acid selectivity up to 81-82%by just adjusting the reaction temperature.（2）To further elucidate the effect of the structure of Fe species in Fe-MOR on the performance of methane oxidation,three kinds of Fe-MOR catalysts were prepared via different preparation method.Notably,the Fe-MOR-F catalyst prepared via freeze-drying method,showing excellent catalytic performance for the direct oxidation of methane with H₂O₂ under mild condition.The XRD,UV-vis,and XPS studies revealed that the Fe-MOR-F catalyst possess more dispersed and oligomeric iron species,which is strongly correlated with its high catalytic activity for direct methane oxidation.The DFT calculations indicated the highly dispersed monomeric and dimeric Fe species can effectively homogenize H₂O₂to active hydroxyl groups,while iron clusters can decompose hydrogen peroxide into inactive O₂.The EPR results show more·OH radicals were generated on the Fe-MOR-F catalyst,which is in line with DFT calculation results.（3）To further increase the selectivity of methanol,a series of Cu-ZSM-5 catalysts were prepared and applied in methane oxidation reaction.Notably,the single-metal[Cu₂（μ-O）]²⁺-ZSM-5 catalyst can activate methane to methanol with superior high selectivity up to 91.3%in C1 oxygenates and 12.31 mol_methanol kg^-1_cat h^-1 by avoiding over-oxidation.Density functional theory（DFT）calculations combined with in situ FT-IR and EPR demonstrated that methanol forms via direct dissociation of strongly adsorbed*CH₃OOH generated by*CH₃ spontaneously reacting with H₂O₂ rather than·OH.It was found that water and chloride play a crucial role in enhancing methanol formation.（4）A series of Rh-ZSM-5 catalysts were synthesized and used in the selective oxidation and carbonylation of methane into acetic acid using H₂O₂ and CO as oxidation and carbonylation agent,respectively.The catalytic results indicated that the presence of CO could efficiently enhance the amount of products and lead to the formation of acetic acid.Among all the Rh-contained catalysts,Rh-ZSM-5-H₂ exhibited the best catalytic performance,with the high oxygenate yield of 5.92 mol kgcat^-1 h^-1and excellent C2 product selectivity of 53.97%.EPR and the catalytic results showed that methanol was formed via the combination of CH₄and·OH species from the free radical dissociation of H₂O₂.It would further be oxidated into formic acid.Acetic acid was formed by the coupling of CH₄,·OH and CO on the Rh species.Notably,CO pressure also played a crucial role in enhancing acetic acid formation.As the pressure of CO increases,the selectivity to acetic acid increases correspondingly.The XPS,EXAFS and XANES results confirmed that the lower valence rhodium is more favorable for the formation of acetic acid.