Study Of Selective Oxidation Of Methane On Boron-Based Catalyst

Natural gas is a carbon resource that is much cleaner than petroleum and coal.As a benefit from the development of mining technologies for combustible ice and shale gas,the available reserves of nature gas continue to increase,leading to great attention to efficient conversion of natural gas into fuels and chemicals.The main component of natural gas is methane with a content ranged from 70%to 90%in general.Therefore,the core challenge for the efficient conversion of natural gas lies in the methane activation.In this study,we focus on methane selective oxidation and attempt to achieve conversions of methane to C1 platform molecules(i.e.methanol,formaldehyde,and CO)with high selectivities by means of mediating solid catalysts and reaction conditions.Methane has a strong C-H bond(bond energy 426 kJ/mol)and is hard to be activated.As a consequence,high temperature and metal oxide catalysts(e.g.V2O5,MoO3)are usually required for methane selective oxidation.These severe reaction conditions,however,make the aimed products(i.e.methanol,formaldehyde,and CO)readily to be further oxidized to CO2,thus rendering methane selective oxidation impractical.Inspired by recent researches of propane oxidative dehydrogenation to propene on boron-based catalysts,we found that supported B2O3 catalysts prepared by incipient wetness impregnation methods showed high selectivities to methanol,formaldehyde,and CO in methane oxidation.At an optimized reaction condition and 16%methane conversion,the sum of selectivities to methanol,formaldehyde,and CO was above 90%,irrespective of the catalyst support used or the ratio of methane to O2.Moreover,these B2O3 catalysts exhibited excellent stability during methane oxidation,indicative of great potential for their application in such reactions.We further found that the methane oxidation rates normalized by the surface area of B2O3 for the supported B2O3 catalysts were nearly independent on the support(e.g.Al2O3,SiO2,TiO2,and ZrO2)and the loading amount of B2O3(8-30wt%),suggesting that the active sites for methane oxidation located at the surface of B2O3 layers and their activity was insensitive to the thickness of the B2O3 layers.Solid-state NMR characterization unveiled that boron centers with a coordination number of three(BO3)or four(BO4)were both present on the surface of supported B2O3 catalysts,but only the BO3 structures were able to catalyze methane selective oxidation.In addition,kinetic studies showed that the formations of methanol and formaldehyde from methane oxidation were parallel reaction pathways,both of which were kinetically limited by the activation of C-H bond in methane.It is worth noting that the B2O3 catalysts directly used O2 molecules adsorbed on their surfaces as the reagent for oxidizing methane,in contrast to the involvement of lattice O-atoms in oxidation reactions on classical metal oxide catalysts that follow the Mars van Krevelen mechanism.Accordingly,we surmised that adsorbed O2 molecules had weaker oxidative ability than lattice O-atoms,accounting for the much better stability of methanol,formaldehyde,and CO products on B2O3.The performance of boron nitride(BN)catalysts in methane selective oxidation was also examined here.We found that B2O3 layers were formed in situ on the BN surfaces at the methane oxidation conditions,rendering areal rates and selectivities similar with the supported B2O3 catalysts.These results indicate that similar active sites and reaction mechanisms are involved in methane selective oxidation on boron-based catalysts.Such boron-based materials thus provide a new direction for the design of highly selective methane oxidation catalysts,and would be of significant importance for selective oxidation of methane and other alkanes.