Study On The Process Conditions And Properties Of CH₃OH Synthesis From CH₄ Direct Conversion At Low Temperature By Using CeO₂/CuO_x Catalysts

At present,the chemical utilization of CH₄ is CO₂ reforming of CH₄ or partial oxidation of CH₄ to syngas,then the chemical products such as CH₃OH are obtained from syngas.However,the process has several disadvantages,such as high reaction temperature（⁷00℃）,long conversion route,high requirements on equipment,high investment and operation costs.Therefore,there are effective methods for direct conversion and utilization of methane by changing traditional ideas,preparing new catalysts,designing new reaction pathways or processes.In the early stage,CH₃OH was synthesized from CH₄-O₂-H₂O over CeO₂/Cu₂O/Cu（111）model catalyst in a batch reactor at 177℃.The selectivity of CH₃OH is about 70%,and no other oxygenates in the liquid phase was observed.However,the catalyst was obtained by vapor deposition method,which is high cost for preparation.Therefore,in the paper,the CeO₂/CuO_x（0≤x≤1）catalysts were prepared by conventional catalyst preparation method,which was used to catalyse methanol synthesis from CH₄ direct coversion at low temperature in a continuous reactor.The reaction conditions and catalyst properties are studied.The conclusions are obtained by using BET,XRD,H₂-TPR,XPS,SEM and activity data:（1）The optimal process conditions for CH₃OH synthesis from CH₄ direct conversion at low temperature are as follows:the reaction temperature is 180℃;the molar ratio of CH₄:O₂:H₂O is 20:2:8.It’s found that methanol yield and catalyst stability over CeO₂/CuO catalyst after reduction are better than that over CeO₂/Cu₂O and CeO₂/Cu catalysts.The highest CH₃OH yield is 0.14μmol·g^-1_cat·h^-1 over CeO₂/CuO with 1 mol%CeO₂,and the stability could be maintained for 8 h.（2）There is no obvious correlation between catalyst performances and its specific surface area,pore volume or pore diameter.（3）The interaction between Cu₂O species and CeO₂ is stronger than that between CuO species and CeO₂.The weak interaction between CuO and CeO₂leading to catalyst deactivation during the reaction.（4）Low valence copper species are oxidized to CuO during the reaction.Adequate CeO₂ content（0.5 and 1 mol%）enhances the interaction between low valence copper species and CeO₂,then,restrain the oxygenation of lower valence copper species.（5）The interaction between Cu₂O species and CeO₂ improves CH₃OH yield and the stability of catalyst.（6）Particle agglomeration after the reaction is a major reason for the catalyst deactivation.