Study Of Methane Selective Oxidation To Formaldehyde On Boron Nitride Catalysts

As the proved reserves of methane on the earth increase continuously,efficient conversion of methane is becoming more crucial and urgent.Selective oxidation of methane can directly yield important C1 platform molecules(e.g.formaldehyde,methanol,and CO),which is a promising pathway for the conversion of methane to more valuable chemicals/fuels.Traditional metal oxide catalysts(mainly molybdenumbased and vanadium-based catalysts)used for the selective oxidation of methane possess highly active lattice oxygen species,which typically leads to readily formation of fully oxidized CO2 from methane.Therefore,it is critical to develop catalysts of moderate oxidation ability for effective control of product selectivity in methane oxidation.In recent years,two-dimensional boron nitride material has been widely used in the oxidative dehydrogenation of alkanes to alkenes,because of their unique mild ability to catalyze oxidation reactions.Compared with traditional metal oxide catalysts,these boron nitride catalysts show much higher selectivity to alkenes and extremely low selectivity to COx.Accordingly,it is suggested that boron nitride catalysts could also exhibit excellent performance in methane selective oxidation,while limited studies have been reported for methane conversion on boron nitride catalysts.Based on the above consideration,this study attempts to systematically evaluate the performance of boron nitride catalysts in methane selective oxidation to formaldehyde and examine the relevant structure-performance relationship of the boron nitride catalysts,which is divided into three parts as shown below.These results would inspire novel strategies for breaking the bottleneck of formaldehyde yield in methane oxidation and shed light on the nature of active sites and reaction mechanism of alkane activation on such nonmetal catalysts.In the first part,the catalytic performance of boron nitride catalysts for the selective oxidation of methane and effects of pretreatment temperature,reaction temperature,space velocity,and other relevant factors were systematically examined.At～5%methane conversion,the selectivity of HCHO reached 41%,while the CO2 selectivity was below 4%,providing compelling evidence for the strong inhibition of full oxidation to CO2 on boron nitride catalysts.Moreover,it is found that boron nitride catalysts suffered deactivation in the selective oxidation of methane,because the defect sites present on the boron nitride catalysts.NiOx was further used to fill these defect sites which led to and successfull improvement of the stability of the boron nitride catalysts to a large extent.In the second part,X-ray diffraction and infrared spectroscopy characterization showed that boron oxide species formed on the surface of boron nitride serves as the active sites in methane oxidation.Kinetic data and 18O isotope experiments further unveiled that the O2 and methane reactions are activated in a concerted manner on the boron oxide centers,in which the O2 dissociation and the cleavage of the C-H bond in methane take place concurrently.In the third part,the basic additives were introduced to the boron nitride catalysts in order to improve the performance of methane oxidation.Compared with other basic oxides,MgO showed a superior effect.The addition of 1 wt.%MgO increased the methane conversion rate by about 80%,while the corresponding oxidation selectivity was not affected significantly.The results of kinetic and 18O isotope-exchange assessments implied that MgO assists the boron oxide centers in breaking the C-H bond of methane and thus lowering the activation barrier of methane oxidation.