化学链甲烷氧化偶联载氧体催化剂Na_(2)WO_(4)/Mn_(7)SiO_(12)-SiO_(2)的研究

Study of an Na_(2)WO_(4)/Mn_(7)SiO_(12)-SiO_(2)oxygen-carrier catalyst for chemical looping-oxidative coupling of methane

通过提高Mn_(2)O_(3)-Na_(2)WO_(4)/SiO_(2)中Mn_(2)O_(3)的含量并经原位化学链甲烷氧化偶联(CL-OCM)反应活化制得了具有良好活性和选择性的Na_(2)WO_(4)/Mn_(7)SiO_(12)-SiO_(2)载氧体催化剂,在750℃、12 s甲烷停留时间、27的较低剂烷比(mCat/mCH_(4))条件下,获得了12%的CH_(4)转化率和81.5%的C2-C3选择性;进一步降低mCat/mCH_(4)到13.5时,CH_(4)转化率降至7%,但C2-C3选择性可达90%.要指出的是,C3产物中仅检测到了C3H6,其选择性约为5%.XRD(X-ray diffraction)分析表明,随着CL-OCM氧化-还原循环次数的增加,催化剂的载氧体逐渐由Mn_(2)O_(3)转变为Mn_(7)SiO_(12),同时伴随着CH_(4)转化率的逐渐下降及C2-C3选择性的不断提高直至稳定.基于上述认识,将Na_(2)WO_(4)/Mn_(2)O_(3)-SiO_(2)在800℃空气中直接焙烧,可一步制得Na_(2)WO_(4)/Mn_(7)SiO_(12)-SiO_(2)载氧体催化剂.相比于Mn_(2)O_(3),载氧体Mn_(7)SiO_(12)中的晶格氧活度较低,构建的Mn_(7)SiO_(12)■[MnSiO_(3)+MnWO_(4)]的氧化-还原循环可适度减缓晶格氧的释放速率,抑制了目标产物的深度氧化,从而获得了较高的C2-C3产物选择性,但CH_(4)转化率有所降低.上述研究结果对设计构建高效的CL-OCM载氧体催化剂具有借鉴意义.

A promising chemical looping-oxidative coupling of methane(CL-OCM)oxygen-carrier catalyst,Na_(2)WO_(4)/Mn_(7)SiO_(12)-SiO_(2),was obtained by adding extra Mn_(2)O_(3)to Mn_(2)O_(3)-Na_(2)WO_(4)/SiO_(2)and in-situ activating in the reaction stream.After experiencing an induction period,the oxygen-carrier phase transformed from Mn_(2)O_(3)to Mn_(7)SiO_(12)in association with an improvement in C2-C3 selectivity but decreased CH_(4)conversion.The Na_(2)WO_(4)/Mn_(7)SiO_(12)-SiO_(2)oxygen-carrier catalysts could also be obtained by directly calcining the Na_(2)WO_(4)/Mn_(2)O_(3)-SiO_(2)precursor at 800℃in air.At 750℃and a CH_(4)residence time of 12 s,the catalyst achieved 12%(or 7%)CH_(4)conversion and 81.5%(or 90.0%)C2-C3 selectivity using a mCat/mmCH_(4)weight ratio of 27(or 13.5).Notably,only C3H6was detected as C3 products,whose selectivity was about 5%.The CL-OCM reaction proceeded selectively through the redox cycle mode of Mn_(7)SiO_(12)■[MnSiO4+MnWO_(4)].The lattice-oxygen mobility in Mn_(7)SiO_(12)was much weaker than that in Mn_(2)O_(3),which improved C2-C3 selectivity but decreased CH_(4)conversion.Our findings provide guidance for the exploration of more advanced catalytic oxygen-carrier catalysts toward efficient CL-OCM process.