Effect of Promoters on Manganese-Containing Mixed Metal Oxides for Oxidative Dehydrogenation of Ethane via a Cyclic Redox Scheme

Abstract

Ethylene is an important building block in the chemical industry; state of the art ethylene production (steam cracking) has multiple drawbacks, including high energy consumption, coke formation, and significant CO 2 and NO x emissions. We propose a chemical looping oxidative dehydrogenation (CL-ODH) process to convert ethane into ethylene in a two-step, cyclic redox scheme. In this process, lattice oxygen in a metal oxide based redox catalyst is used to combust the hydrogen formed in ethane dehydrogenation, thereby enhancing ethylene formation while retarding coke formation. The oxygen-deprived redox catalyst is subsequently regenerated with air, releasing heat to balance the overall heat requirement. CL-ODH can realize a reduction of over 80% in primary energy consumption and pollutant emissions. The key to this process is an efficient redox catalyst with high selectivity and facile oxygen transport. Previously we determined that oxides with an Mg 6 MnO 8 structure allow high lattice oxygen mobility and satisfactory oxygen-carrying capacity for the proposed redox reactions. However, unpromoted Mg 6 MnO 8 exhibits poor ethylene selectivity, producing primarily CO 2 . In the current study, we examine the effects of various sodium-containing promoters on Mg 6 MnO 8 CL-ODH activity and mechanism. Sodium tungstate promoted Mg 6 MnO 8 was the most effective redox catalyst, showing an ethylene selectivity of 89.2% and yield of 68.2%, a significant improvement of thermal cracking (38.9% yield). Temperature-programmed reaction (TPR) experiments indicate that the reaction proceeds via gas-phase ethane thermal cracking in parallel with selective hydrogen combustion on the redox catalyst surface. XPS analysis indicates that the decreased ethane/ethylene oxidation activity on the sodium tungstate promoted redox catalysts results from the suppression of near-surface Mn 4+ . This is due to a combination of decreased surface manganese content and reduction in average Mn oxidation state. The suppression of Mn 4+ results in a decrease in electrophilic surface oxygen species, inhibition of ethylene combustion, and enhanced ethylene yield.

Modular‐scale ethane to liquids via chemical looping oxidative dehydrogenation: Redox catalyst performance and process analysis

“…The decrease in selectivity is mainly attributable to the formation of additional CO 2 . The higher conversion does not result in higher C3+ products, in contrast to our previously reported high‐temperature (>800°C) systems . This can be attributed to facile deep oxidation of easier‐to‐activate higher hydrocarbons combined with the relatively low operating temperature, which limits C3+ formation through gas‐phase reactions.…”

“…The products were analyzed with an Agilent 7890 gas chromatograph with flame ionization detector for hydrocarbon quantification and He and Ar thermal conductivity detectors for permanent gas analysis. Further details with respect to the experimental setup can be found in our previous publications . The results at 750°C and gas hourly space velocity (GHSV) of 600/h (WHSV of 0.8/h) and 120/h (WHSV 0.016/h) were used for modeling as the base conversion (Base) and high conversion (High) cases, respectively.…”

“…The redox catalyst, which functions as both a catalyst and a mass separation agent, significantly intensifies the ethylene production by facilitating: (a) ethane conversion; (b) reactive oxygen separation; and (c) autothermal ODH operation with simplified product separation . We have previously reported multiple high‐performance CL‐ODH oxygen carriers that provide excellent selectivity and single‐pass olefin yield . Modeling work shows that, relative to conventional steam cracking, CL‐ODH reduces the energy consumption and CO 2 emissions by up to 82% .…”

Composite structure and enhanced photocatalytic activity in Mn–Co–Ni–O/LaMnO3 microparticles

“…These porous channels will not only make the active surface accessible to the TC molecules for photocatalytic degradation, but also facilitate the efficient transfer of the photogenerated carriers in two different structures during photocatalysis [20]. In addition, the growth of LM significantly reduces the proportion of electrophilic Mn 4+ ions in the composite material, which is more favorable for redox in photocatalytic reactions [9], thus the degradation of TC under the visible light by the S5 sample is enhanced. This result is consistent with the previous study that the enhanced degradation of phenol by cryptomelane can be obtained by an increase in the ratio of Mn 3+ /Mn 4+ ions [7].…”

“…Yao et al observed that Co-Mn oxides with large surface area have higher activity than the individual Co and Mn oxides for volatile organic compounds degradation due to the better redox property of Co-Mn material [8]. It can be concluded that polymetallic transition metal oxides possess better photocatalytic performance than that of the monometallic oxide because of the multiple coordination numbers and oxidation states [9]. As a typical mixed transition metal oxide, Mn-Co-Ni-O spinel has multiple applications due to its low cost and environmental safety, such as thermistors, uncooled infrared detectors and magnetic materials [10].…”