The Proximity Effect Of The Active Components In Na-Mn-W/SiO₂ Catalyst On Oxidative Coupling Of Methane

C2 hydrocarbon is the vital feedstock for chemical industry,which mainly depends on petroleum cracking.Industrialization of oxidative coupling of methane(OCM)reaction to obtain C2 hydrocarbons is of great significance,which will replace the traditional petroleum cracking routine.Among all the developed catalysts,Na-Mn-W/SiO2 is becoming the most promising system which has a high CH4 conversion,high C2 selectivity and an excellent time-on-stream stability.OCM is a typical example for a heterogeneous-homogeneous reaction,in which homogeneous has a great contribution to the whole performance.And Na-Mn-W/SiO2 could have a variable composition.Therefore it deserves to research the relationship between the interaction of Na,Mn,W and the effective distance of three active elements on catalytic results.Mix catalysts samples are synthesized according to different combination of three elements and different preparation methods.In Chapter 3,Na-Mn(0.78wt%Na-2wt%Mn/SiO2)and Na-W(5wt%Na2WO4/SiO2)samples has C2 yield of 3.82%and 5.94%respectively.And the randomly mixed sample(MM 13)has a siginificant mix-enhaned results in macro-scale,C2 yield raising two times to 12.48%.And Na element play a vital role in this phenomenon.According to a series of catalytic bed tests,it could be found that this mix-enhanced effect relates to the interface area between different catalyst particles and their sequence.In Chapter 4,the effect of homogeneous reaction on OCM is explored by adjusting the reaction conditions such as GHSV and CH4/O2 ratio.The Catalyst provided by Shell Company(2wt%Na2WO4-2wt%Mn/SiO2)has a stable performance in most of scope.While Na-W and Na-Mn samples show a rapid decrease in CH4 conversion and a steady increase in C2 selectivity with increasing GHSV and CH4/O2 ratio.However,the most obvious discrepancy is that there is a obvious products distribution on MM 13 sample,which is probably steming from the change of intermediates' reaction pathway by the generated interface.And the effective activation and consumption of O2 is an important precondition for reaching an excellent performance.In the XRD pattens of Na-Mn-W/SiO2 system catalysts,the major phases are Na2WO4,Mn2O3 and a-cristobalite,and quartz would additionally appear with different extent after OCM reaction.According to XPS results,W and Na elements are mainly enriched on the surface with Na/W ratio larger than 5,while Mn element was uniformly distributed.The surface structure of Shell catalyst could be divided into two types,a dense large grain and a porous cluster packing with a uniform distribution of active elements.While all the prepared samples were an aggregation of distorted nanoparticles with a larger agglomerates of Mn rich phase,which suggest that a suitable SiO2 supporter and preparation conditions are also important for catalytic performance.