Study Of Low-temperature Selective Oxidation Of Methane To Methanol

Selective oxidation of methane to form methanol as a platform molecule is one of the most ideal pathways for efficiently utilizing nature gas resources.Over-oxidation of methane to CO2,however,is thermodynamically more favorable than partial oxidation of methane to methanol,formaldehyde,or CO,making the selective oxidation of methane to methanol a crucial challenge for the studies on catalytic conversions of methanol.Inspired by the structures of methane monooxygenases from biological systems,metal oxide clusters constructed in the molecule-size voids of zeolites enable the activation of the C-H bond in methane stoichiometrically at mild conditions,and the resulting methoxide species can be further converted to methanol via hydrolysis.In these novel reaction processes,the correlations between the structure of the metal oxide clusters and their activity for methane oxidation,together with the effects of the confining voids in zeolites on the stability of the active metal oxide clusters and the activation of the C-H bond in methane,however,are lack of fundamental understanding.In this study,the activity of methane oxidation was examined for zeolite-supported metal oxide catalysts with different frameworks(i.e.MOR,MFI,BEA,CHA,FAU,and MCM-41)and metal centers(i.e.Cu,Ni,Co,Cr,Zn,Mn,Fe,Ru,and Pd),among which CuOx clusters on MOR showed much higher activity than the others.In addition,the ion-exchange method led to Cu-MOR catalysts with better methanol yields than those prepared by other methods(e.g.incipient wetness impregnation,hydrothermal synthesis,and solid-phase ball milling method).Here we combined separate exchanges of Na+ and Co2+ ions with protons to determine the concentrations of Br(?)nsted acid sites located at single Al3+ sites(i.e.Al-OH-(Si-O)n?3-Al)and those at Al3+ pair sites(i.e.Al-OH-(Si-O)n?2-Al-OH)for a given zeolite.The dependence of the exchanged amounts of these two kinds of Br(?)nsted acid sites on the added amount of Cu2+ ions during the preparation of Cu-MOR catalysts indicates that the Cu2+ ions prefer to exchange with the protons on single Al3+ sites over those on Al3+ pair sites;when these sites are all occupied,spare Cu2+ ions then adsorb on extra framework Al3+ sites.The assessment of methane oxidation activity on the Cu-MOR catalysts showed that the turnover number of methane per Cu atom increased with increasing the Cu loading until a certain Cu amount,and then decreased as the Cu loading increased further.Such critical Cu amounts were determined by the Si/Al ratio of the zeolite and also the concentration of Al3+ pair sites in the zeolite.These results suggest that the CuOx clusters formed from the Cu2+ ions resided at the Al3+ pair sites are more reactive in methane oxidation than those derived from the Cu2+/ions bound to the single Al3+ sites or the framework Al3+ sites.Moreover,the formation of those CuOx clusters on the Al3+pair sites and the single Al3+ sites were observed in situ by ultraviolet-visible spectroscopy,and a good correlation between the concentration ratios of these two CuOx cluster species and the methane oxidation activity was found.The above study systematically examines the effects of Al distribution on the methane oxidization activity of zeolite-supported metal oxide clusters,and thus provides a basis for further understanding the stabilization of active metal oxide clusters by voids of different zeolitic frameworks and also such confinement effects for C-H activation transition states,reaction intermediates,and products.These results would apparently contribute to a rational design of more active catalysts for low-temperature selective oxidation of methane.