Manganese‐containing redox catalysts for selective hydrogen combustion under a cyclic redox scheme

Abstract

Selective hydrogen combustion (SHC) in the presence of light hydrocarbons was demonstrated with a series of Mn‐containing mixed oxide redox catalysts in the context of a chemical looping‐oxidative dehydrogenation scheme. Unpromoted and 20 wt % Na2WO4‐promoted Mg6MnO8, SrMnO3, and CaMnO3 exhibited varying SHC capabilities at temperatures between 550 and 850°C. Reduction temperature of unpromoted redox catalysts increased in the order Mg6MnO8 < SrMnO3 < CaMnO3. Promotion with 20 wt % Na2WO4 resulted in more selective redox catalysts capable of high‐temperature SHC. XPS analysis revealed a correlation between suppression of near‐surface Mn and SHC selectivity. Na2WO4/CaMnO3 showed steady SHC performance (89% H2 conversion, 88% selectivity) at 850°C over 50 redox cycles. In series with a Cr2O3/Al2O3 ethane dehydrogenation catalyst, Na2WO4/CaMnO3 combusted 84% of H2 produced while limiting COx yield below 2%. The redox catalysts reported can be suitable for SHC in a cyclic redox scheme for the production of light olefins from alkanes. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3141–3150, 2018

Reduction Kinetics of Perovskite Oxides for Selective Hydrogen Combustion in the Context of Olefin Production

“…While these results are promising, the CL-ODH performance depends strongly on the selection of metal oxide materials for the redox catalyst, as non-selective combustion by lattice oxygen would result in CO x formation and loss of valuable ethylene or ethane. [14,17] In both previously cited examples, the propensity of the redox catalyst towards selective hydrogen combustion (over hydrocarbon combustion) enabled high ethylene yield and mitigated the thermodynamic constraints on the C 2 H 6 /C 2 H 4 /H 2 equilibrium. [10,13,15] Selectivity towards hydrogen combustion (S H ) is ultimately one of the most important considerations for redox catalyst development.…”

“…Recent work within the chemical looping community showcased improved olefin yields from the conversion of ethane (via CL-ODH) and n-hexane feedstocks resulting from hydrogen combustion by highly selective redox catalysts. [9,10,[15][16][17]31] In these works, the alkali tungstate Na 2 WO 4 emerged as a beneficial chemical promoter capable of enhancing selectivity towards hydrogen combustion in multiple systems, including rock salt-structured Mg 6 MnO 8 and perovskite-structured CaMnO 3 and SrMnO 3 . [9,10,17,31] In summary, a number of promising redox catalysts have been discovered for the SHC reaction, and their effects on olefin yields have been well described.…”

“…[9,10,[15][16][17]31] In these works, the alkali tungstate Na 2 WO 4 emerged as a beneficial chemical promoter capable of enhancing selectivity towards hydrogen combustion in multiple systems, including rock salt-structured Mg 6 MnO 8 and perovskite-structured CaMnO 3 and SrMnO 3 . [9,10,17,31] In summary, a number of promising redox catalysts have been discovered for the SHC reaction, and their effects on olefin yields have been well described. However, despite the multitude of studies demonstrating the SHC properties of various metal oxides via reaction testing, a rigorous investigation of the process variables affecting selective hydrogen combustion has not been attempted.…”

“…In the present study, we comprehensively examined the selective hydrogen combustion properties of a promising SHC redox catalyst, the perovskite oxide CaMnO 3 (with and without a Na 2 WO 4 promoter phase) [17,31] , through a combination of fixed-bed reaction testing and thermogravimetric (TG) experiments. We endeavor to quantify the reduction rates of the solid redox catalyst by three gaseshydrogen, ethylene, and ethaneas a function of dynamic process variables.…”

“…ethylene, propylene); moreover, the addition of 20 wt% Na 2 WO 4 to CaMnO 3 (forming Na 2 WO 4 /CaMnO 3 ), has been shown to significantly increase the selectivity of base CaMnO 3 through creation of a Na-and W-rich surface layer which suppresses surface-bound oxygen species. [17,31] The promising performance of CaMnO 3 and Na 2 WO 4 /CaMnO 3 at temperatures around 650°C potentially makes the redox catalysts suitable for a sequential process featuring ethane dehydrogenation followed by selective hydrogen combustion (EDH+SHC). Using the proposed EDH+SHC process as a case study, we endeavor in the present work to quantitatively explain the high selectivity of CaMnO 3 and Na 2 WO 4 /CaMnO 3 by studying their reduction kinetics in the presence of H 2 , C 2 H 4 , and C 2 H 6 .…”