Chemical-looping Process Oriented Engineering Of The Efficient Oxygen-carrier Ocm Catalysts And Insight Into The Reaction Chemistry

Oxidative coupling of methane to ethene is an important project in the field of C1energy catalysis.Many challenges exist in the CH₄-O₂ cofeeding reaction mode,for example,regulating the reaction（i.e.,the low C₂-C₃ selectivity）,removing the reaction heat and eliminating the explosion hazard.The selectivity enhancement and establishment of intrinsically safe OCM process is urgently required toward its commercial exploration.Chemical looping-oxidative coupling of methane（CL-OCM）permits the redox reactions to proceed in two individual reactors（coupling reactor and regenerator）.In coupling reactor,CH₄ is converted to products（such as C₂H₄,C₂H₆,C₃H₆,CO_x and H₂O）by the oxidized catalyst,followed by the transportation（such as via moving-bed mode）of reduced catalyst to regenerator,where the reduced catalyst is regenerated by air to finish an integral cycle.Obviously,the intrinsically safe CL-OCM process makes it possible to further improve the C₂-C₃ selectivity and effectively manage/utilize the reaction heat but the implementation of CL-OCM depends on the breakthrough of active/selective lattice oxygen carrying capacity of the catalyst.Herein,we suggested a catalyst tailoring strategy by chemically coupling“Mn²⁺（?）Mn³⁺”cycle with“oxygen storage materials（OSM）”toward efficient CL-OCM process.To reach this goal,the present dissertation studied the effects of the composition,phase,grain size and content of Ce O₂-based,Mn-based perovskite oxygen storage materials on the O₂ activation and CH₄ conversion,which are synergistically catalyzed by“Mn²⁺（?）Mn³⁺”cycle with Na₂WO₄.Finally,we revealed the mechanism of lattice oxygen evolution,concerning the cycle of“Mn²⁺（?）Mn³⁺”,and the selectivity modulation mechanism of Na₂WO₄.The research content and main results are summarized as follows:Part 1:Mn₂O₃-Na₂WO₄/Ce_xZr_1-xO₂ and Na₂WO₄/FeMnO₃ catalysts for CL-OCM reaction.The Mn₂O₃-Na₂WO₄/Ce_xZr_1-xO₂ catalysts are not suitable for CL-OCM reaction due to the low lattice oxygen availability but show high C₂-C₃ selectivity.Meanwhile,FeMnO₃ is reported to have high active lattice oxygen carrying capacity for CH₄conversion,high lattice oxygen availability and good redox stability but prefers catalytic CH₄ combustion.We wonder whether it is possible to modulate the selectivity of the lattice oxygen in FeMnO₃?To check this idea,the Na₂WO₄/Mn₂O₃-Fe₂O₃precursor（prepared with the method of physical mixing）was tested and showed induction stage at the first 7 reaction-regeneration cycles with a little decrease of CH₄conversion,then moved into the steady stage with 20%CH₄ conversion and 80%C₂-C₃ selectivity until to the 50th reaction-regeneration cycle at 800 ^oC,CH₄ residence time of 6 s and catalyst/CH₄ weight ratio of 13.5.XRD,Raman and EDX characterizations validate that the phase evolution of oxygen carriers from Mn₂O₃-Fe₂O₃ mixture to FeMnO₃ leads to the formation of induction stage.Higher calcination temperature results into deeper formation of FeMnO₃ and the Na₂WO₄/FeMnO₃catalyst is gained via calcining the Na₂WO₄/Mn₂O₃-Fe₂O₃ precursor at 1000 ^oC,showing 20%CH₄ conversion and 80%C₂-C₃ selectivity with no induction stage detected,thus validates the Na₂WO₄/FeMnO₃ catalyst as an effective CL-OCM catalyst.The 10-time scaling-up performance test of such catalyst at the same reaction conditions remains 20%CH₄ conversion with 79%C₂-C₃ selectivity.The XRD and Raman characterizations over the completely reduced catalyst reveal that the lattice oxygen evolution of the Na₂WO₄/FeMnO₃ catalyst establishes the CL-OCM reaction via the redox cycle of“FeMnO₃（?）[Mn Fe₂O₄+Mn O]”.Part 2:The selectivity modulation mechanism of FeMnO₃ and Na₂WO₄.The selectivity modulation mechanism of Na₂WO₄/FeMnO₃ remains not clear,though the CL-OCM reaction follows the MVK mechanism.The temperature-programmed surface reaction,H₂-TPR,O₂-TPD-MS and TGA were combined with DFT calculations over the FeMnO₃ and Na₂WO₄/FeMnO₃,to investigate the lattice oxygen evolution activity and the relationship between lattice oxygen evolution activity and CH₄ activation/selective conversion.The XRD results over the catalysts,calcined at different temperatures and undergoing continuous reduction by CH₄,indicate that the formation of FeMnO₃ makes it slower for CH₄ to reduce Mn³⁺to Mn²⁺.The CH₄-TPSR-MS results reveal that the conversion of CH₄ relies on the release of lattice oxygen in FeMnO₃ and the slower reduction of Mn³⁺by CH₄ is attributed to the weakened ability of lattice oxygen for oxidative activation of CH₄,linked with FeMnO₃ formation;the H₂-TPR,O₂-TPD-MS and TGA results validate that the weakened ability of lattice oxygen for oxidative activation of methane is due to the lowered lattice oxygen migration activity in FeMnO₃,which prevents C₂-C₃ products from deep oxidation;DFT calculations indicate that the[WO₄]sites,introduced by Na₂WO₄,not only show the superior ability of adsorbing CH₃*species formed from CH₄,but also facilitate the CH₃*species to desorb into gas phase as CH₃·radicals,which is paramount for the improvement of the C₂-C₃ selectivity.Part 3:Na₂WO₄/TiO₂-Mn₂O₃ catalyst for low-temperature CL-OCM reaction.The catalyst/CH₄ weight ratio of CL-OCM can be greatly lowered over Na₂WO₄/FeMnO₃ catalyst due to its high lattice oxygen availability,but its relatively low lattice oxygen migration activity leads to the effective reaction temperature as high as 800 ^oC.From the view of industrial application,it is of great significance to lower the reaction temperature via catalyst tailoring strategy.Our group previously reported that the low-temperature initiation of“Mn₂O₃（?）MnTiO₃”chemical cycle is a trigger for the TiO₂ doped Na₂WO₄-Mn₂O₃/Si O₂ catalyst toward low-temperature OCM reaction.The calculation results indicate the thermodynamic superiority of“2CH₄+4TiO₂+2Mn₂O₃→C₂H₄+4MnTiO₃+2H₂O”over“2CH₄+6Mn₂O₃→C₂H₄+4Mn₃O₄+2H₂O”.In accordance,a series of Na₂WO₄/TiO₂-Mn₂O₃ catalysts with varied Mn/Ti weight ratio were prepared with the method of physical mixing and were tested in the CL-OCM reaction.The preferred catalyst of Na₂WO₄/2Mn1Ti demonstrates 19.5%CH₄ conversion with 80%C₂-C₃ selectivity at 740 ^oC,CH₄ residence time of 6 s,catalyst/CH₄ weight ratio of 13.5 and is stable for at least 50 reaction-regeneration cycles.In combining the above reaction results with the XRD,O₂-TPD-MS and CH₄-TPSR-MS characterization results,it is clear that the catalysts with Mn/Ti weight ratio<2/1（including Mn/Ti=1/9,3/7,5/5,2/1）show better low-temperature CL-OCM performance than those with Mn/Ti ratio>2/1（including Mn/Ti=7/3,9/1）.Notably,the former catalysts possess relatively less available lattice oxygen and work via“[TiO₂+Mn₂O₃]（?）MnTiO₃”cycle,whereas the latter catalysts have more available lattice oxygen and show a different lattice oxygen evolution process via“Mn₂O₃（?）Mn₃O₄”cycle.The Na₂WO₄/2Mn1Ti catalyst shows outstanding low-temperature CL-OCM performance against the others.The O₂-TPD-MS and CH₄-TPSR-MS results reveal that the low-temperature“[TiO₂+Mn₂O₃]（?）MnTiO₃”cycle is the source of the superb low-temperature CL-OCM performance of the Na₂WO₄/2Mn1Ti;DFT calculations indicate that the[WO₄]sites of the Na-WO₄/Mn₂O₃（222）structure show stronger ability to adsorb CH₃*species,whose formation is facilitated by the synergistic interaction between Na₂WO₄ and Mn₂O₃,than Mn sites and favor the CH₃*species to desorb into gas phase as CH₃·radical instead of continuous oxidative dehydrogenation of CH₃*,thereby leading to the improvement of C₂-C₃ selectivity.In contrast,over the Mn₂O₃（222）structure without[WO₄]sites,the CH₃*species is firmly adsorbed by the Mn sites and is easily to be further dehydrated.Part 4:Mn₂O₃-Na₂WO₄/Ce_xZr_1-xO₂ catalysts for low-temperature OCM.Although the Mn₂O₃-Na₂WO₄/Ce_xZr_1-xO₂ catalysts are not suitable for CL-OCM reaction,they show good activity and selectivity in the low-temperature CH₄/O₂cofeeding OCM reaction.The light-off temperature and OCM performance are subject to the Ce/Zr molar ratio and Mn₂O₃-Na₂WO₄ doping.The Mn-W-Na/Ce_0.15Zr_0.85O₂catalyst with Ce/Zr molar ratio of 0.15/0.85 demonstrates 25%CH₄ conversion with67%C₂-C₃ selectivity at 660 ^oC,CH₄/O₂ molar ratio of 5,atmospheric pressure.The O₂-TPO results over the catalysts reduced by CH₄ at 850 ^oC validate that the surface oxygen vacancies could activate O₂ to form oxygen species at around 480 ^oC,showing correlation with the low-temperature OCM performance.The O₂-TPD results demonstrate that the catalysts with different Ce/Zr molar ratios have different kinds and amount of surface oxygen species.The CH₄-pulse experiments were carried out using the catalysts after surface oxygen species desorption at different temperatures,showing that the oxygen species desorbing at 450-700 ^oC are active and selective for CH₄conversion to C₂-C₃.In-situ O₂ adsorption FT-IR experiments were performed on the Mn-W-Na/Ce_0.15Zr_0.85O₂ catalyst after surface oxygen species desorption at different temperatures,showing that the oxygen species desorbing at 450-700 ^oC are identified as superoxide species（O₂^-）.The kinds and amount of different oxygen species are modulated by the Ce/Zr molar ratio-related oxygen vacancies amount.According to the gradual transformation of O₂ by oxygen vacancies via the process of O₂→O₂^-→O^-→O^2-,the Ce_0.15Zr_0.85O₂ with less oxygen vacancies shows relatively slower rate to transform the superoxide species（O₂^-）to lattice oxygen species(O^2-)and the formed O₂^-is stabilized by Mn₂O₃-Na₂WO₄,thus shows higher C₂-C₃ selectivity.In contrast,the Mn₂O₃-Na₂WO₄ modification cannot substantially lower the transfer rate of O₂^-to O^2-for the catalysts with higher Ce content because of their too more oxygen vacancies.It is thus not surprising that the catalysts with high Ce content are not selective for the OCM to form C₂-C₃ products.